s2fieldsofstudy: Biology Chemistry

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Natural Peptides in Drug Discovery Targeting Acetylcholinesterase

Acetylcholinesterase-inhibitory peptide has gained much importance since it can inhibit acetylcholinesterase (AChE) and increase the availability of acetylcholine in cholinergic synapses, enhancing cholinergic transmission in pharmacological treatment of Alzheimer’s disease (AD). Natural peptides have received considerable attention as biologically important substances as a source of AChE inhibitors. These natural peptides have high potential pharmaceutical and medicinal values due to their bioactivities as neuroprotective and neurodegenerative treatment activities. These peptides have attracted great interest in the pharmaceutical industries, in order to design potential peptides for use in the prophylactic and therapy purposes. Some natural peptides and their derivatives have high commercial values and have succeeded in reaching the pharmaceutical market. A large number of peptides are already in preclinical and clinical pipelines for treatment of various diseases. This review highlights the recent researches on the various natural peptides and future prospects for AD management.

Introduction
Acetylcholinesterase (AChE) (E.C.3.1.1.7) is a serine hydrolase which catalyzes the hydrolysis of neurotransmitter acetylcholine into choline and acetic acid [1,2]. According to cholinergic neurotransmission, acetylcholinesterase (AChE) inhibition would increase the levels of acetylcholine in the brain, thus improving cholinergic synapses in Alzheimer's disease (AD) patients [3][4][5][6]. The impairment of cognitive functions and the behavioral disruptions that affect patients with AD are associated with deficiencies in cortical excitability, particularly cholinergic neurotransmission. Disturbances of acetylcholine metabolism have been detected in the AD patient brains with higher levels of AChE expression in decomposition of acetylcholine and deterioration in activity of choline acetyltransferase (ChAT), an enzyme which is responsible for the synthesis of acetylcholine [7][8][9].
AD is a progressive neurodegenerative disorder with complex multifactorial pathogenesis, memory impairment, and damaged cognitive skills, including attention, spatial orientation, judgment, language disorder, motoric problem, and loss in healthy aging [10][11][12]. The main hallmark of AD patient brain is characterized by extracellular senile plaques containing amyloid-β peptides (Aβ) deposits, neurofibrillary tangles, progressive synapse loss, and severe oxidative stress [13,14]. Aβ-induced oxidative stress is triggered by protein oxidation, lipid peroxidation, free radical formation, neuronal cell death via apoptotic pathway, DNA oxidation leading to DNA damage both in mitochondria and nucleus [15][16][17][18]. The most common form of AD syndrome is dementia. Dementia is a clinical syndrome characterized by progressive decline in cognitive function as well as behavioral disorders beyond the normal aging [19,20]. It is estimated that AD represents approximately 60-80% of all dementia cases [21,22]. Dementia mainly affects elderly and it is presumed that, by 2050, more than 115 million people suffer dementia, which majority contributed by AD [23]. In fact, most of the AD symptoms were related to loss of cholinergic function in the basal forebrain, the discovery of AChE inhibition is important for the treatment of AD [24].
One of the best approach in devastating AD is based on accelerating decomposition of available of acetylcholine which mainly improves the pathological symptom [25,26]. Inhibition of AChE is a promising strategy to develop novel and causal therapeutics in AD treatment [27,28]. AChE inhibitors are one of the most intensively probed categories of compounds in seeking an effective treatment of AD [29,30]. Another member of the cholinesterase (ChE) family is butyrylcholinesterase (BChE), which is also considered as promising target for AD drug discovery since the BChE activity increases in the late stages of AD. A previous study reported that BChE inhibitors could act in a similar way as well as AChE inhibitors, in particular to slow down cognitive decline [31]. AD drugs used recently as AChE inhibitors include tacrine, rivastigmine, galantamine, metrifonate, and donepezil [32,33]. However, these drugs may lead to injurious gastrointestinal side effects such as nausea, vomiting, diarrhea, and anorexia [33][34][35][36]. Currently, there is no cure for AD and these drugs only can temporarily improve symptoms or slow the progression of AD [37,38]. Thus, current approaches to the treatment of AD targeting AChE need be developed.
The other safer sources that can be used as AChE inhibitors are obtained from natural peptides. These peptides can be found in various sources, such as protein hydrolysates. Protein hydrolysate is a good source of nutrients for health due to it contains a high natural peptide compounds. Protein hydrolysates are well-known as mixtures of polypeptides, oligopeptides, and amino acids that are manufactured from protein sources using partial hydrolysis [39][40][41]. The advantages of consuming protein hydrolysate containing natural peptides due to its small molecular size thus easily absorbed in the digestive tract and quickly delivered into the bloodstream reaching muscle tissue, immune cells, internal organs, and brain cells. Protein hydrolysate is also known as food protein-derived peptides. Animal experiments prove that protein-derived peptides in the form of bovine casein-derived peptide (CH-3) are capable of preventing cognitive decline in Alzheimer's disease of mice model with a decreasing of inflammation and oxidative stress. The suppressive effect of CH-3 is mainly due to the tripeptide: methionine-lysine-proline (MKP). Hence, this study suggests that CH-3 has therapeutic potential for preventing cognitive impairment in AD [42]. Moreover, a previous study also reported that 2,5-Diketopiperazine (DKP), also known as cyclic dipeptides which contain cyclo(-Phe-Phe), have received considerable attention as biologically important molecules as a source of AChE inhibitory peptide [43,44]. DKP can be synthesized from the N-terminus of amino acid residues of a natural linear peptide and can be found in whey protein hydrolysates.
There is an abundance of natural peptides (NPs) which are known as specific protein fragments and the majority have positive impacts on neurodegenerative preventives and treatments [45][46][47][48][49]. To date, more than 1500 different NPs have been studied as functional biomolecules including neuropeptides in a database called 'Biopep' [50,51]. NPs display drug-like or neurodegenerative treatment activities as neuropeptides have been reported in many previous studies [52][53][54][55]. Most natural neuropeptides were isolated using bioassays based on physiological impacts as well as light responses, spontaneous potentials responsible for the rhythmic contraction of the gut, motor patterns, or heartbeat frequency changes [56][57][58]. Neuropeptides noticed in a fraction form as biologically functional molecules, have been identified and isolated from various natural sources, and their activities were investigated in many medical and pharmaceutical purposes [59][60][61][62]. Here, we present a review which is mainly concerned with NPs targeting AChE in the context of different natural source matrices.

Neuropeptides from Plant Sources
Plants are rich sources of pharmacologically bioactive agents and some studies of Portugal's medicinal plants showed great results for AChE inhibition with the species Melissa officinalis, Mentha suaveolens, Laurus nobilis, Hypericum undulatum, and Sanguisorba minor [63,64]. These plants were used in traditional medicines, in particular, for the treatment of nerve dysfunction or related problems over three decades [64]. Recently, hemp seed peptides targeting AChE-inhibitory enzymatic hydrolysates were obtained using six distinct proteases (pepsin, pepsin-pancreatin, alcalase, papain, flavourzyme, and thermoase) at a range of concentrations (1-4%). The activity of hemp seed peptides showed that about 1% pepsin hemp seed peptides had high activity of AChE inhibition with IC 50 value was 6 × 10 −3 mg/mL compared to IC 50 value was within 8-11.6 × 10 −3 mg/mL from other enzymatic hemp seed peptides. The analysis of mass spectrometry exhibited that most of the peptide sizes in all the hydrolysate forms were less than 1000 Da: 244-1009 Da for the pepsin hemp seed peptides, 246-758 Da for the papain hemp seed peptides, and 246-607 Da for the alcalase hemp seed peptides. The pepsin hemp seed peptides showed higher AChE-inhibitory effects which might be based on the enhanced synergistic effects from a longer peptide size range when compared to the alcalase and papain hemp seed peptides which had a smaller size range [53].
A more recent study following Tsai et al. [65] was the identification of the polysaccharide-peptide complex in AChE inhibitory activities from the mushroom, Cordyceps militaris (CPSP) and characterization of AChE inhibitory properties. The covalent bond of polysaccharide-peptide complexes are attributed to O-glycosidic linkages in CPSP-F1 and N-glycosidic linkages in CPSP-F2 and CPSP-F3. The three polymers of CPSP-F1, CPSP-F2, and CPSP-F3 were extracted and separated by ultrasound-assisted extraction and diethylaminoethanol (DEAE)-Sepharose CL-6B column chromatography. Polysaccharide-peptide complexes were identified by DEAE-Sepharose CL-6B column chromatography followed by high-performance gel-filtration chromatography. The results showed that CPSP-F1 and CPSP-F2 had half maximal inhibitory concentrations of 32.2 ± 0.2 mg/mL and 5.3 ± 0.0 mg/mL, respectively.
Meanwhile, the fruit peptides of Ziziphus jujuba, known as the jujube or Chinese date, a species of Ziziphus in the buckthorn family Rhamnaceae, are being consumed worldwide due to their health benefits, as both food and herbal medicine. As herbal medicine, one of the main functions of jujube is to relax the brain by calming down the mind and improving sleep quality. Jujube is known to possess neuroprotective activities, including protecting neuronal cells against neurotoxin stress, stimulating neuronal differentiation, increasing expression of neurotrophic factors, and promoting memory and learning [66]. A study showed that the Zizyphus jujube fruit exerts neurobiological effects, especially antioxidant and anticholinesterase activities [67]. The fruit peptide of jujube was introduced as a new peptide and named Snakin-Z, and consists of 31 amino acids with a sequence: CARLNCVPKGTSGNTETCPCYASLHSCRKYG. The Snakin-Z activity showed considerable inhibition against AChE. The half maximal inhibitory concentration (IC 50 ) value of Snakin-Z against AChE was 0.58 ± 0.08 mg/mL. This peptide has 80% enzyme inhibitory activity on AChE at 1.5 mg/mL. The Snakin-Z also had high antioxidant activity (IC 50 = 0.75 ± 0.09 mg/mL). Thus, it is suggested that Snakin-Z might be beneficial for AD treatment [68]. Another beneficial part of the jujube is its seed. Recent findings indicate that the seed of the jujube ameliorates an Aβ-induced synaptic long-term potentiation (LTP) deficit in the hippocampal tissue through BDNF/TrkB signaling. This phenomenon is induced by a regulatory effect of jujube seed on the post-translation modification of the brain-derived neurotrophic factor (BDNF) [69].
The fermentation of some edible plants are also popular in the discovery of new AChE inhibitors. A traditional Chinese salt-fermented soybean, known as commercial Douchi, has been determined to have acetylcholinesterase (AChE) inhibitory activity. The results indicated that nineteen samples of commercial Douchi showed various extents of AChE inhibitory activities. The IC 50 value of AChE inhibitory activities of Douchi extracts ranged from 0.040 to 2.319 mg/mL. Douchi extracts using Aspergillus exhibited significantly higher AChE inhibitory activity than using Mucor and Bacillus [70].
The other fermentation based product is sufu (fermented tofu), a traditional Chinese food, that has been manufactured by fermenting soybean (yellow soybean & black soybean). Sufu was reported as having AChE inhibitory activities. The bioactivities of anti-AChE from 15 brands of commercial sufu samples were taken from various regions in China, and self-produced sufu produced by fermentation with Actinomucor elegans 3.118, was successfully determined. The results indicated that sufu extract performed significant inhibition activity toward AChE in vitro. It proved that sufu no. 5 had the strongest inhibitory activity (IC 50 = 0.191 mg/mL), while the prefermented sufu showed the highest AChE inhibitory activity during sufu production. According to this result, it might be possible that Chinese sufu could be considered in the treatment of Alzheimer's disease patients [71]. The source of fermented milks cultured with various probiotic strains could improve amyloid precursor protein (APP) metabolism in Alzheimer's disease. Fermented milk cultured with Lactobacillus helveticus IDCC 3801 induced a strong decrease in the incidence of Alzheimer's disease but also ameliorated scopolamine-induced amnesia. This also suggests that the extract from fermented milk with L. helveticus IDCC 3801 may improve APP metabolism and memory deficit [72].
Rice bran is one of bioactive food-derived peptides source and possesses the capability to enhance positive health by decreasing chronic complication risks including obesity and aging diseases such as AD. The high demand of rice bran in the global food market has supported extensive research on natural peptides that are used both in prevention and treatment of AD. Fractions of peptides from rice bran have been identified and characterized for potential inhibitory activities against AD. This peptide showed nearly 45% reduction in cell cytotoxicity on amyloid-induced neuronal cells. Hence, rice bran peptides could possibly to be used as natural nutraceuticals to help with the management of AD [73,74].

Neuropeptides from Marine Sources
Approximately 70% of the Earth's surface is covered by saltwater oceans which occupy more than 90% of the biosphere [75,76]. Oceans are a rich source of bioactive compounds with diverse structures and functions as well as potential therapeutic agents [75,76]. In recent years, many bioactive compounds have been extracted from various marine animals like sea snails, sponges, soft corals, marine fungus, marine algae, and marine microbes [77][78][79][80][81]. The exploration of new metabolites from marine organisms has become popular, and approximately 10,000 bioactive compounds have been successfully isolated including polysaccharides, enzymes/protein, minerals, vitamins, polyunsaturated fatty acids (PUFA), alkaloids, phenols, terpenoids, and natural peptides [82][83][84][85][86][87][88][89][90][91]. Natural peptides generally have 3 to 20 amino acid residues, with their functions dependent on the composition and sequence length of amino acids. Natural peptides with a low molecular weight have an escalated feasibility of permeability through the gut wall without any disruption, hence employing their biological and functional activities [92,93].
Natural peptides were first discovered and isolated from marine organisms in the 1960s. After the discovery of neurotoxins, studies on marine peptides were done, vastly in the 1980s [62]. Approximately more than 2000 natural peptides have been isolated, representing 1/10 of the total bioactive compounds obtained from marine organisms [94]. Thus, there is abundant amount of published articles relating to marine-derived peptide isolation. Since then, the explorations of marine neuropeptides have continued with the purpose to assure their application in treatment or prevention of neurodegenerative diseases.

Cone Snail Conotoxins
Conotoxins are neuropeptides isolated from venom of sea snail or cone snails (Conus sp.) which normally consist of 10 to 30 amino acid (A.A.) residues with one or more disulfide bonds. Conotoxins have a variety of AA compositions and reaction mechanisms, but most of them have not been determined. The majority of conotoxins modulate the activity of ion channels. However, some conotoxins are also capable of inhibiting cholinesterase. Over the last few decades, conotoxins have been the subject of pharmacological interest. The venom of ω-conotoxin from marine cone snails has been proven to be a valuable device in neuroscience, employing a key role in the the identification and characterization of Voltage-dependent calcium channel (VGCC) subtypes [95,96].
Other evidence of conotoxin mechanisms in the inhibition of cholinesterases can be found in insect and mammalian nervous systems, in particular, presynaptic muscarinic receptors on the liberation of acetylcholine from striatal cholinergic neurons were investigated. The electrically stimulated release of [3H]-acetylcholine has been examined from rat striatal slices and its inhibition by carbachol are affected by specific inhibitors of voltage-operated calcium channels of the N-type by ω-conotoxin GVIA. The generated release of [3H]-acetylcholine was decreased by ω-conotoxin GVIA, indicating that the N-type channel was involved in the release of some neurotransmitters. The N-type channels were responsible for approximately two thirds of neurotransmitters release. The release was >97% blocked by ω-toxins. In the imaging experiment on brain slices, where cholinesterases had been inhibited by paraoxon, the inhibition of [3H]-acetylcholine release by endogenous acetylcholine accumulated in the tissue could be exhibited by the escalation of the release after atropine addition. The inhibition was high in slices with functional N-type channels. Thus, it was concluded that the N-type calcium channel may contribute to the stimulation of acetylcholine release in rat striatum [97].
In a recent study, Minic et al. showed that the potential of venom toxins for the inhibition of protein receptors in the human body was implicated in the pathophysiology of AD. AChE has been identified as a promising target for treatment of cognitive impairments associated with Alzheimer's by applying its enzyme inhibitors. A conotoxin has been reported playing a role as an effective inhibitor of acetylcholinesterase. The in silico study of a certain conotoxin has been designed with acetylcholinesterase, and may open aspects in binding affinity and specificity improvement. Docking studies were performed to structurally determine the binding interactions involved in the binding pocket of venom toxins with acetylcholinesterase occupied by each toxin in complex with acetylcholinesterase. The binding mode between each toxin and AChE was similar to conotoxin and AChE. Hence, it is a great prospect to investigate conotoxins as potential reversible and irreversible inhibitors of acetylcholinesterase for the treatment of the cognitive disorder of Alzheimer's disease [98].

AChEIs from Sponges
Previous studies reported that aqueous and organic extracts of tropical marine sponges have biological activities including sponge peptides towards AChE. Approximately 66 extracts were obtained and screened from 35 marine sponge species from the Caribbean Sea (Curaçao) and from 8 marine sponge species from the Great Barrier Reef (Lizard Island). The extracts were dissolved in aqueous and organic solvents then tested for anti-AChE activities. The most interesting findings were acquired from extracts of Pandaros acanthifolium, Neofibularia nolitangere, Verongula rigida, Topsentia ophiraphidites, and Ircinia felix. The inhibition activities against AChE from some sponges were generally found to be weak. However, the methanolic extract of T. ophiraphidites showed stronger anti-AChE activity compared to other sponge extracts [99].
Furthermore, the assessment activities of acetylcholinesterase inhibitors (AChEIs) of 134 extracts obtained from 45 species of marine sponges have been reported by Beedessee and coworkers [100]. Microplate and Thin-layer chromatography (TLC) assays had revealed potent activities of acetylcholinsterase inhibitors in two ethyl acetate sponge extracts: Pericharax heteroraphis and Amphimedon navalis. Further investigated extracts were found to possess inhibitory kinetics by exhibiting mixed competitive/noncompetitive inhibition. However, the pure peptides, such as AChEIs from marine sponges, are still difficult to explore. Thus, the potential of marine sponges as a source of AChEIs are essential in future, particularly the purification and characterization of solitary bioactive compounds for their therapeutic applications in combating neurodegenerative diseases.
Turk et al. reported that approximately 33 deep-sea Antarctic marine sponges were successfully screened in ethanolic extracts and they showed strong inhibition toward acetylcholinesterase. The most eminent AChE activities were associated with the extracts from sponges belonging to the genus Latrunculia. While most of the AChE inhibition activities associate with known secondary metabolites, some other immensely strong acetylcholinesterase inhibitors still associate to the unknown compounds [101].

Marine Bacterial Peptides
A huge variety of bioactive compounds can be extracted from marine bacteria. This means that the field of marine bioactive compounds, such as peptides, is continuously expanding. Recent advances in biological techniques enable a high level interest in natural peptides being developed. An overview of the discovery of new marine bacterial peptides with potential biological activities against AChE has been widely explored.
Bacterial associates of marine soft corals and sponges with anticholinesterase activities were extensively reported in the previous studies. Approximately 887 marine bacteria were screened in a microplate-based assay for the potential inhibitions towards acetylcholinesterase, and around 140 marine bacteria (15.8%) were found to inhibit acetylcholinesterase found in the electric eel enzyme. The majority of the active bacterial isolates were associated bacteria with soft corals and marine sponges. The maximum inhibition was found about 54% by a bacterial strain M18SP4P, isolated from the marine sponge Fasciospongia cavernosa. Based on its phenotypic properties and 16S rDNA sequencing, the strain was identified as Bacillus subtilis. This result showed that acetylcholinesterase inhibitors are quite common found in marine bacteria [102].
Fengwu et al. [103] reported that isolated endophytic bacteria which were screened from oysters showed acetylcholinesterase inhibitory activity. Out of eight strains endophytic bacteria separated from oyster, one of them had AChE inhibitory activity, which was identified as Bacillus subtilis, the inhibition rate was 22.4%, and it increased 3.3-fold at 37 • C for 78 h incubation.

Marine Fungus Peptides
Marine fungi, particularly those associated with marine animals and plants, appear to be a uniquely rich resource for secondary metabolites, such as antibiotic, antitumor, antiinflammatory, antidiabetes, and anti-acetycholinesterase agents [104][105][106][107]. A study conducted by Wu et al. [107] reported that Talaromycesone A, talaroxanthenone, and AS-186c were isolated from the culture broth and mycelia of a marine fungus Talaromyces sp. strain LF458, displayed potent acetylcholinesterase inhibitory activities with IC 50 values were 0.00749, 0.00161, and 0.0026 M, respectively. All isolated compounds were subjected to bioactivity assays.

Neuropeptides from Terrestrial Venomous Animal
Venomous animals are widely spread all over the globe. Several terrestrial vertebrates (reptiles, birds, insects, and mammals) are also venomous [108,109]. Venomous animals permanently or periodically exploit self-generated substances for self-defense or capture prey that are toxic to other species and cause death, even in small doses [110,111]. Several venomous animals have special venom glands to produce their venom, while others contain toxic substances in various body parts. A number of animals have a specialized organ used for introducing their venom into prey's body. Major arthropods (scorpions, bees, and wasps) have multicellular glands attached to the stinging apparatus. In most terrestrial venomous, animals such as spiders, snakes, and millipedes, their venom glands are connected to their mouthparts and the venom is spurted into the body of prey through prick or bite. The most studied of the venomous terrestrial animals are snakes, spiders, and scorpions [112,113]. It is known that animal venoms are complex mixtures with compositions depending on the species [114].
Bee venom obtained from stepwise fractionation with methanol, acetone, polyacrylamide, Sephadex gels, and anion exchangers has been reported to contain the polypeptides minimine and melittin. One of these polypeptides has a molecular weight of around 6000 Da. This polypeptide was successfully inhibited growth of third-instar larvae with a LD 50 dose value of approximately 0.005 µg.
On the other hand, these polypeptides were also reported to have in vitro anti-acetylcholinesterase activity [115].
The Taiwan snake (Bungarus multicinctus) possesses venom called α-bungarotoxin, a three-finger α-neurotoxin that has action against the acetylcholine receptor (AChR). The remarkable three-finger toxin fold has maximally thrived to use a distinct combination of functional groups to generate an armor of certain targets of AChR subtypes [116]. Powerful inhibition of mammalian acetylcholinesterase was detected in the venom of the green mamba snake, Dendroaspis anyusticeps. The compound responsible for inhibiting AChE was isolated and purified by gel filtration on Sephadex G-50, followed by ion exchange chromatography on Bio-Rex 70 and SP Sephadex C-25, respectively. These substances are known as polypeptides, named fasciculins [117].

Neuropeptides from Venomous Amphibian
In general, amphibians have one or more efficacious neuropeptides in their dorsal skin secretions. The most common neuropeptide belongs to amphibians of the frog genera, called caerulein, with peptide sequence: [pEQDY(SO 3 )TGWMDF-NH 2 ]. Caerulein has various biomedical activities at nanomolar concentrations. The penetration of caerulein is known to be able to affect smooth muscle contraction, reduce blood pressure, have gastrin-like activity, change satiety-related sedation, and thermoregulation. The Australian frogs from genus Litoria have the capability to modify caeruleins in different seasons. The Uperoleia genus of toadlets can produce a number of tachykininand bombesin-type neuropeptides, including uperolein [pEPDPNAFYGLM-NH2], while froglets of the genus Crinia can produce various disulfide-containing neuropeptides that cause smooth muscle contraction at very small concentrations (nanomolar size), such as assigniferin 1 peptide, RLCIPYIIPC-OH. These peptides can be used in the study of drug-induced pancreatitis as biomarkers of autophagy and pancreatic cell apoptosis; autophagy malfunction is now considered a major contributor to Alzheimer's disease and other neurodegenerative disorders [118,119].
Amphibian defensive skin secretions are known to have a plethora of biologically-active peptides that are structural and functional analogs of vertebrate neuropeptides. Wang et al. reported that the amino acid sequences of two invertebrate neuropeptide analogs are IPPQFMRF amide (IF-8 amide) and EGDEDEFLRF amide (EF-10 amide) isolated from the defensive skin secretions of two different species of African hyperoliid frogs: Kassina maculata and Phylictimantis verrucosus [120]. These neuropeptides represent the C-terminal tetrapeptide amides of both novel peptides: -FMRF amide and -FLRF amide, reflecting the canonical structural motifs of the FMRF amide-related peptides (FaRPs) which are recognized as the ubiquitous invertebrate neuropeptide family. FaRPs show the potential targets of the defensive arsenal of amphibian tegumental secretions to invertebrate predators, and the possibility of more novel peptides from amphibians may represent more potent vertebrate neuropeptides.
Siano et al. reported that the amphibian skin peptide found with sequence: TKPTLLGLPLGAG PAAGPGKR-NH 2 possessed a significant AChE. The present work demonstrates that subtraction of the size of the peptide origin could potentially form new compounds with significant cholinesterase inhibition activity [121].

Strategies for the Rational Design of Peptide as Therapeutic Leads
Peptide therapeutics are exclusively associated with lower production complexity compared with protein-based biopharmaceuticals. This is of course beneficial, considering the low cost of drug production, due to generally approaching those of small molecules. Therefore, peptides are in the sweet spot between small molecules and biopharmaceuticals in many ways. Natural peptides are frequently not immediately suitable for use as therapy because of intrinsic weaknesses, including poor chemical and physical stabilities, and a short plasma half-life. Some of these weaknesses have been successfully resolved through the conventional design of therapeutic peptides. Besides conventional natural peptide design, the wide range of peptide technologies has been presenting new opportunities and future directions within the peptide field. These include multifunctional peptides in cell penetration, as well as conjugation of peptide drugs and effective technologies focusing on selection routes of administration [122,123]. Peptides have thrived as highly potent signal transduction molecules, employing powerful physiological effects. Peptides are generally categorized by a relatively short turning plasma half-life, as well as physicochemical properties for their use in biomedical and pharmaceutical fields. Therefore, conventional rational design of peptide therapeutics has focused on rectifying techniques to relieve susceptibilities [123]. Several modified natural peptides with activity against acetylcholinesterase of Alzheimer's disease are shown on Table 1.
Peptide toxicity has been evaluated mostly by investigating the hemolysis activity toward human red blood cells [124]. The Melittin peptide is known to be high cytolytic in bioassays. Its action is beneficial to block the electrostatic interactions that most peptides need to interact with microbes [125]. Toxicity assays should be conducted under conditions mimicking the environment that the peptides will confront in the host cells [126]. In selected examples, the toxicity assays have been performed in various in vitro mammalian cell cultures and in vivo animal model viabilities in the presence of peptides [127,128]. Based on peptide stability, extracts obtained from different mammalian tissues or commercially available proteases are well fit to determine peptide susceptibility to molecular target degradation and may affect the peptide activities [129,130].
The decreasing number of approved drugs produced by the pharmaceutical industry, which is accompanied by the increasing expense of R&D, is causing a high demand for alternative approaches to increase pharmaceutical R&D productivity. This circumstance has supported a revival in peptide research as potential drug candidates. New synthetic strategies for confining metabolism and alternative routes of administration have appeared in recent years and produced a large number of peptide-based drugs that are now being marketed. Peptide-based drug discovery will be important for answering new therapeutic challenges [131].

Peptide Design by Chemical Synthesis
Since Bruce Merrifield invented the methods of solid-phase peptide synthesis (SPSS) in the early 1960s, an increasing number of research groups have focused on peptide synthesis. However, the classical step-by-step procedures for peptide synthesis still has limitations, such as decreasing final product purity after passing through several coupling steps. It is understandable that a challenging method for the activation of a peptide terminal group needs protecting groups, both t-butoxycarbonyl (Boc) and 9-fluorenylmethoxycarbonyl (Fmoc). Besides Boc and Fmoc, novel amino acid protecting groups and new techniques have been largely introduced to improve the quality and quantity of peptide products. The condensation of the peptide fragment method was a popular method for peptide production in the 1980s. In contrast, the racemization rate and overall reaction still have difficulties to be resolved. In overcoming these issues, Kent and coworkers introduced the peptide coupling revolution by utilizing the chemoselective reaction of unprotected peptide segments, namely native chemical ligation. Later on, a large number of studies have focused on the development of propitious ligating techniques including the well-known click reactions, ligation of peptide hydrazides, and ligation of 5-oxaproline with α-ketoacid-hydroxylamine [132][133][134].
Another promising strategy in rational peptide drug design is to improve the physicochemical properties of natural peptides, which often have a tendency to aggregate and sometimes poorly dissolve in water. As expected from chemical design strategies, the development of streamlined techniques other than chemical conversion were highly needed for eco-friendly peptide synthesis to avoid aggregation, such methods include the peculation of hydrophobic darns, which can be achieved by substituting or catalyzing N-methylation of the incorporated amino acids. If solubility issues arise in certain peptide drug candidates, the regular basis zone is on molecular charge allocation and the isoelectric point (pI) of the peptides, which are related to favorable pH and purification to obtain the desired final product [123].
The chemical optimization strategy of a therapeutic peptide is based on the structure-activity relationship and/or quantitative structure-activity relationship studies of newly synthesized peptide derivatives, with the objective to improve bioavailability, limit biodegradation, reduce elimination, and increase affinity or selectivity to its target or receptor. The main concern for the low oral bioavailability of peptide drugs are presystemic enzymatic degradation and poor penetration of the intestinal mucosa. According to Lipinski's rule of five evaluations, some peptides may be poor candidates to shift from the gastrointestinal tract to the circulatory system due to their unfavorable physicochemical properties [135,136].
Many efforts have been done regarding enhancing drug delivery by designing drug substances as prodrugs to improve drug penetration via transcellular pathways to treat patients. The other alternative route goes through the paracellular pathway by modulating the intercellular junction protein clusters. It is presumed that this approach has a remarkable future prospect in oral delivery of large molecules such as peptides, proteins, and oligonucleotides. Moreover, this approach has the potential to deliver large molecules to the brain as diagnostic and therapeutic agents for brain diseases such as Alzheimer's disease [137][138][139].

Peptide Design by Molecular Biological Approach
Bioactive peptides can be acquired immediately from living organisms by the hydrolysis of proteins or by chemical synthesis and these approaches are not cost-effective. A biological expression system using genetic techniques based on a fusion technology would be a more efficient method for the production of bioactive peptides. However, the molecular biology research has led to the availability of amino acid sequence data determined by the cloning and cDNA sequencing of genes [140].
The production of natural neuropeptides in competent host cells has been studied by Rosenfeld and coworkers. They used alternative processing of the RNA transcribed from the appeared calcitonin gene to obtain a messenger RNA production in neural tissue distinct from that in parafollicular cells as called thyroid cells. The thyroid mRNA encodes a precursor to the hormone calcitonin whereas that in neural tissues generates a designed neuropeptide, referred to as calcitonin gene-related peptide (CGRP). This approach allows the application of recombinant DNA technology to analyze the complex neurobiological systems in the absence of previous structural and functional information from living organisms [141].

Peptide Product by Designed Enzymatic Degradation
Enzymatic hydrolysis of proteins is catalyzed by proteases, which cleave peptide bonds between two amino acids consuming a molecule of water per bond cleaved. Hence, the continuous cleavage of peptide bonds breaks down proteins into products of lower molecular weight such as peptones, peptides, and amino acids. Independently of the type of food protein, the enzymatic hydrolysis process commonly comprises the following stages. Grinding the raw material and homogenization in water (or buffer), temperature equilibration, and pH adjustment to the optimum values of the enzyme employed, followed by enzyme addition. Recently, ultrasonic assisted hydrolysis was evaluated with the purpose of facilitating the production of low molecular weight peptides. Upon completion of the reaction, the enzyme needs to be inactivated by heating or pH adjustment. Alternatively, continuous membrane reactor, where the enzyme is continuously recycled to the reaction tank, might also be used in order to stop the reaction and save enzyme costs.
Afterwards, the digested material, which contains the bioactive peptides, is separated from the precipitate and lipids by several methods: centrifugation or decantation, fractionated, and further stabilized by spray-drying. In utilizing an appropriate enzyme and having good control over processing conditions including pH, temperature, enzyme/protein ratio, and time, are critical points for the production of protein hydrolysates with good properties. Eventually, these process variables determine the extent of the hydrolysis reaction for a protein-enzyme system. This is normally indicated by the degree of hydrolysis (DH), which is defined as the percentage of peptide bonds cleaved. There are several methods to determine the DH such as pH-stat, trinitrobenzenesulfonic acid (TNBS), O-phthaldialdehyde (OPA), trichloroacetic acid soluble nitrogen (SN-TCA), and formol titration methods.
Several novel AChE-inhibitory peptides were discovered and their amino acid sequences were elucidated. Hemp seed proteins (HSPs) were enzymatically hydrolyzed and the released peptides were investigated as potential therapeutic agents. Membrane isolated HSPs (mHPI) were the most soluble with >60% solubility at pH 3-9 when compared to 27% solubility of isoelectric pH precipitated proteins (iHPI). However, iHPI formed emulsions with smaller oil droplet sizes (<1 µm) while mHPI formed bigger oil droplets. The iHPI was subjected to enzymatic hydrolysis using different concentrations (1-4%) of six proteases including pepsin, pancreatin, flavourzyme, thermoase, papain, and alcalase, to produce various HSP hydrolysates (HPHs). Some of HPHs amino acid sequences were identified to successfully produce and confirm the in vitro AChE-inhibitory activities. Subsequently, it could be used as a prospective model for future design that may serve as potent AD therapeutic agents [142].

Future Perspectives of Virtual Screening on Natural Peptides
In recent years, virtual screening has been used to assist the binding models suggestion between bioactive natural products with their molecular targets systematically [153,154]. A combination of pharmacophore modeling, virtual screening, and molecular docking studies provides information to identify and design AChE inhibitors with higher selectivity [155]. In a previous study, a retrospectively validated structure-based virtual screening (SBVS) protocol to identify potent AChE inhibitors has been developed [156]. The construction and the retrospective validation of the SBVS protocol [156] made use of the enhanced version of the database of useful decoys (DUD-E) [157]. The decoys in DUD-E were selected from the ZINC database [158,159]. Therefore, we were tempted to apply the validated SBVS protocol [156] to identify potent AChE inhibitors from natural products collected in the newest version of the ZINC database, ZINC15 [159].
In total, 190,090 natural products from ZINC15 were subjected in the screening campaign. Lipinski's rule of five for drug likeness [160] was used as the first filter to discard nondruggable compounds in this campaign [161,162]. The filter resulted in 79,711 compounds that remained to be further screened virtually using the retrospectively validated SBVS protocol [156]. Only 23 compounds were identified as potent AChE inhibitors. Thence, the overall hit rate was 0.01%, which was considered as very low. Nevertheless, these identified potent compounds could serve as the pivotal compounds in the development of drug for AD [163][164][165] since the database for the retrospective validation which was compiled by Mysinger et al. [157] defined potent AChE inhibitors as inhibitors "with affinities (IC 50 , EC 50 , Ki, Kd, and log variants thereof) of 1 µM or better". The following are the ZINC code [159] of the hit compounds: ZINC000253412911, ZINC000253411436, ZINC000095919360, ZINC000095486033, ZINC000085549645, ZINC000085626729, ZINC000085629900, ZINC000085772672, ZINC000085772673, ZINC000085641926, ZINC000085597045, ZINC000085595326, ZINC000014443683, ZINC000008791438, ZINC000008765420, ZINC000002123767, ZINC000002104243, ZINC000002159521, ZINC000071318273, ZINC000034074522, ZINC000049601562, ZINC000038993374 and ZINC000038661945. Notably, similar to peptides, nine out of the 23 hits contained an amide bond (Table 2). On the other hand, the attempts to screen all virtual 400 dipeptides and 8000 tripeptides by employing the same SBVS protocol resulted in zero hits. Therefore these nine compounds are of considerable and timely interest. Short molecular dynamics (MD) simulations of enzyme-inhibitor using NAMD [166] by employing the SBVS results of the compounds of interest as the starting points have been performed. These MD simulations were performed to prioritize the compounds and to analyze the plausible AChE-ligand binding interactions.
The results indicate that compounds ZINC000002159521 and ZINC000038661945 are potential to be further tested since their minimum total energy were lower than the minimum total energy of AChE without ligand at MD simulations 0 to 200 ps (Table 2). ZINC000002159521 and ZINC000038661945 are potential to be further tested since their minimum total energy were lower than the minimum total energy of AChE without ligand at MD simulations 0 to 200 ps (Table 2). ZINC000002159521 and ZINC000038661945 are potential to be further tested since their minimum total energy were lower than the minimum total energy of AChE without ligand at MD simulations 0 to 200 ps (Table 2). ZINC000002159521 and ZINC000038661945 are potential to be further tested since their minimum total energy were lower than the minimum total energy of AChE without ligand at MD simulations 0 to 200 ps (Table 2). ZINC000002159521 and ZINC000038661945 are potential to be further tested since their minimum total energy were lower than the minimum total energy of AChE without ligand at MD simulations 0 to 200 ps ( Table 2).  Figure 1 shows the interactions of compound ZINC000002159521 at its lowest total energy from the MD simulations as a representative to examine the AChE-ligand binding interactions. The need of an aromatic moiety in the ligand to bind to AChE binding pocket [156] is clearly seen in Figure 1. The aromatic chromenone moiety from ZINC000002159521 is directly surrounded by aromatic residues in the AChE binding pocket, i.e.: W 84 , Y 130 , F 330 , and Y 442 . On the other subpocket, the phenol moiety from ZINC000002159521 is located adjacent to W 279 and F 290 . Notably, the O carbonyl from the amide bond in ZINC000002159521 serves as hydrogen bond acceptor for Y 121 . This indicates the importance of amide bond for inhibitor to bind to AChE. Another identified hydrogen bond is the bond between O hydroxyl from the phenol moiety in ZINC000002159521 as hydrogen bond acceptor to S 286 by employing a water molecule for bridging the bond. These insights from molecular modeling approaches could provide significant helps in the discovery and design novel AChE inhibitors.


s2fieldsofstudy: Biology Psychology

text:
Sex specific effects of pre-pubertal stress on hippocampal neurogenesis and behaviour

Experience of traumatic events in childhood is linked to an elevated risk of developing psychiatric disorders in adulthood. The neurobiological mechanisms underlying this phenomenon are not fully understood. The limbic system, particularly the hippocampus, is significantly impacted by childhood trauma. In particular, it has been hypothesised that childhood stress may impact adult hippocampal neurogenesis (AHN) and related behaviours, conferring increased risk for later mental illness. Stress in utero can lead to impaired hippocampal synaptic plasticity, and stress in the first 2–3 weeks of life reduces AHN in animal models. Less is known about the effects of stress in the post-weaning, pre-pubertal phase, a developmental time-point more akin to human childhood. Therefore, we investigated persistent effects of pre-pubertal stress (PPS) on functional and molecular aspects of the hippocampus. AHN was altered following PPS in male rats only. Specifically males showed reduced production of new neurons following PPS, but increased survival in the ventral dentate gyrus. In adult males, but not females, pattern separation and trace fear conditioning, behaviours that rely heavily on AHN, were also impaired after PPS. PPS also increased the expression of parvalbumin-positive GABAergic interneurons in the ventral dentate gyrus and increased glutamic acid decarboxylase 67 expression in the ventral hilus, in males only. Our results demonstrate the lasting effects of PPS on the hippocampus in a sex- and time-dependent manner, provide a potential mechanistic link between PPS and later behavioural impairments, and highlight sex differences in vulnerability to neuropsychiatric conditions after early-life stress.

Introduction
Childhood adversity has repeatedly been identified as a risk factor for mental and physical illness 1 . However, neurobiological mechanisms underpinning this relationship are not fully understood. Stressful experiences activate the hypothalamic-pituitary-adrenal (HPA) axis, resulting in the release of corticosteroid hormones. These hormones traverse the blood-brain barrier, providing a pathway through which stress can alter brain development and leave individuals susceptible to a range of psychiatric disorders later in life. The limbic system, especially the hippocampus, is predicted to display considerable sensitivity to stress during early life; it contains high levels of glucocorticoid receptors, and is undergoing significant structural and functional changes as it matures 2,3 . Accordingly, childhood maltreatment is consistently associated in humans with reduced hippocampal volume, particularly in the dentate gyrus and Cornu Ammonis 3 (CA3) subfields 1,[4][5][6][7] . Hippocampal-dependent behaviours, such as pattern recognition, spatial memory and verbal declarative memory, are also impaired in human sufferers of childhood maltreatment [8][9][10] . Although informative, studies in humans do not allow extensive investigation into the underlying mechanisms of the impact of early-life stress on the hippocampus to be explored.
In rodents, stress early in life can produce significant alterations in the adult hippocampus. Stress in utero can lead to impaired hippocampal synaptic plasticity, and stress in the first 2-3 weeks of life reduces adult hippocampal neurogenesis (AHN) in the dentate gyrus and brain-derived neurotrophic factor expression [11][12][13] . Less is known about the effects of stress in the post-weaning, pre-pubertal phase, a developmental time-point more akin to human childhood 14 . Studies to date have revealed that pre-pubertal stress (PPS) impairs synaptic plasticity in the dorsal hippocampus, alters the expression of corticosteroid receptors and impairs performance in a hippocampal-dependent contextual fear task in males [15][16][17] .
AHN is believed to play a crucial role in a number of hippocampal-dependent cognitive processes, including contextual fear responses, trace fear conditioning and pattern separation [18][19][20][21] . Pattern separation is a computational process that keeps similar input patterns distinct, allowing similar memories to remain separate 22 . The more comparable the memories, the harder it is to 'pattern separate' them. Experimentally, pattern separation can be assessed through analysing the ability to spatially discriminate between objects placed relatively far apart (easy condition, spatial locations do not overlap greatly) or close together (difficult condition, spatial overlap and potential for interference greater) 20,21,23 . This function is attributed to the dentate gyrus, through the unique action of young adult-born neurons [20][21][22]24,25 . These neurons also play a specific role in fear conditioning-reductions in AHN using either antimitotic agents or transgenic models specifically impairs fear learning only when a delay is interposed between the fear-associated and aversive stimuli, known as trace conditioning [26][27][28] . AHN is exquisitely sensitive to a range of environmental factors, notably stress, physical exercise and environmental enrichment 19 . Therefore, the first aim of this study was to determine whether PPS would decrease AHN and impair behaviours that rely on AHN, specifically, pattern separation and trace conditioning.
A number of intracellular pathways and extrinsic factors regulate AHN 29,30 . GABA signalling plays a particularly important role in AHN, regulating division of neural stems cells and modulating survival of newborn neurons 31,32 . Therefore, the second aim of this study was to investigate the consequences of PPS on parvalbumin (PV) + GABAergic (γ-aminobutyric acid) interneurons and glutamic acid decarboxylase 67 (GAD67) expression to elucidate a mechanistic link between PPS and adult hippocampal function.

Animals
Animal experiments adhered to the European regulations on animal experimentation (Directive 2010/63/EU) and the UK Home Office Animals (Scientific Procedures) Act 1986. Male and female Lister-hooded rats were bred in-house at Cardiff University from adult pairs (Charles River) for this study. Litters were weaned on postnatal day 21 (PND 21), and housed in same-sex cages (32 cm × 50 cm × 21 cm) with littermates. Males and females were housed in the same room, food and water were provided ad libitum unless stated otherwise, cages were lined with wood shavings, a cardboard tube and wooden stick were provided as enrichment and light was maintained on a 12:12-h light/dark cycle (not reverse). Between 9 and 13 animals per group and sex were used in each experiment, and separate cohorts of animals were used for each behavioural experiment and for immunohistochemistry. Sample sizes were based on power calculations from previous studies using the same animal model and similar behavioural analyses 16,17 .

Pre-pubertal stress
On PND 25-27, half of the litters experienced a shortterm PPS protocol that has been described previously 16,33,34 , and was originally described by Jacobson-Pick and Richter-Levin 35 . This took place in a designated room, separate from the holding room. On PND 25, between 9:00 a.m. and 13.00 p.m., animals were given a 10 min swim stress in an opaque swimming tank (25 cm high, 34 cm diameter), 12 L capacity filled with 6 L of 25 ± 1°C water. PND 26 involved restraint stress: between 9:00 a.m. and 13:00 p.m. animals were placed into plastic restraint tubes (15 cm length, 5 cm diameter) for three sessions of 30 min, separated by 30 min breaks in the home cages. The final stressor on PND 27 involved placing animals onto an elevated platform (15 × 15 cm 2 , 115 cm high) for three 30-min sessions, separated by 60 min in the home cage between 9:00 a.m. and 5:00 p.m. Animals were then returned to their home cages and holding rooms and left undisturbed (aside from cage cleaning) until early adulthood (PND 60).
Litters were alternately allocated to experimental groups (PPS or control) by order of birth. A maximum of three animals per litter were used in each experiment, and a minimum of five litters per group (PPS or control) were generated to minimise effects of pseudoreplication 36 . The litter of origin was accounted for in all statistical analyses.

BrdU administration
Forty-four rats (male: 10 control, 13 PPS; female: 10 control, 11 PPS) were injected intraperitoneally on PND 60-65 with bromodeoxyuridine (BrdU, 200 mg/kg in 0.9% sterile saline solution) and killed by transcardial perfusion with 0.01 M phosphate-buffered saline (PBS) and 4% paraformadelhyde (PFA) under anaesthesia 24 h later to assess baseline rates of cell proliferation and neurogenesis in the dentate gyrus. Brains were left in PFA overnight (4°C ), and transferred to 30% sucrose solution for cryoprotection. Coronal 30-µm sections were cut through the entire hippocampal extent on a freezing microtome (Leica RM2245) and placed into a solution of cryoprotectant for storage at −20°C until immunohistochemical analysis.

BrdU and DCX
Sections were washed between each step for 3 × 5 min in 0.01 M Tris-buffered saline (TBS, pH 7.4) and all steps were carried out at room temperature, unless otherwise specified. One in every 12 sections was denatured in 45°C, 1 M HCl for 30 min, followed by incubation in blocking solution (0.3% Triton-X in 0.01 M TBS (TTBS), 2% donkey serum) for 60 min, rat anti-BrdU (1:100 in blocking solution, OBT0030, ABD Serotec, UK) for 48 h at 4°C and Alexa Fluor 488 (5 µg/ml in TTBS, donkey anti-rat, Life Technologies, UK) for 2 h in the dark. Sections were then incubated with goat anti-DCX (1:100 in blocking solution, SC8066, Santa Cruz Biotechnology, UK) for 24 h at 4°C to identify newly differentiated immature neurons, Alexa Fluor 647 (10 µg/ml in TTBS, donkey anti-goat, Life Technologies, UK) for 2 h in the dark. Sections were then exposed to DAPI (4′,6-diamidino-2-phenylindole; 1:3000 in TBS, D9542, Sigma UK) for 5 min. Washed sections were then mounted onto glass microscope slides and coverslipped with fluorescence mounting medium (S3023, Dako, UK). The number of cells double labelled with BrdU and DCX were counted in the supra-and infrapyramidal blades of each dentate gyrus in the dorsal (Bregma −1.72 to −5.28 mm) and ventral (Bregma −5.28 to −6.72 mm) portions of the hippocampus according to the Atlas of Paxinos and Watson (2009). The total number of double-labelled cells was estimated by multiplying the total number counted for each area (dorsal or ventral, supra-or infrapyramidal blade) by 12 37 . Counts and total volume of the hippocampus was analysed using Zen Blue (Carl Zeiss Microscopy, Germany).

Parvalbumin and DCX
All steps were carried out at room temperature, unless otherwise specified. One in every 12 sections was blocked in blocking solution (1% Triton-X in 0.01 M PBS, 10% donkey serum) for 2 h, goat anti DCX (1:100, SC8066, Santa Cruz) and mouse anti-parvalbumin (1:1000, P3088, Sigma-Aldrich) in 0.1% Triton-X, and 0.2% donkey serum in 0.01 M PBS overnight at 4°C. Sections were washed in 0.1% Triton-X, 0.2% donkey serum in 0.01 M PBS at least three times before incubation with donkey anti-goat Alexa 647 (10 µg/ml in 0.01 M PBS, Life Technologies UK) and donkey anti-mouse Alexa 555 (1:1000 in 0.01 M PBS, Life Technologies UK) for 2 h in the dark. Sections were washed with 0.01 M PBS and incubated for 10 min in the dark with DAPI stain (1:3000 in 0.01 M PBS, D9542, Sigma UK) for nuclei staining. Sections were washed twice more in 0.01 M PBS before being mounted on standard microscopy slides using Mowiol aqueous mounting medium (Sigma, UK) and standard cover slips. Sections were imaged using an epifluorescent microscope (Leica DM6000B Upright Timelapse System with Leica Application Suite Advanced Fluorescence 3.0.0 build 8134 software, Leica Microsystems). The number of cells were quantified by manual counting in the supra-and infrapyramidal blades of the dorsal (−2.28 to -5.04 mm) and ventral (−5.16 to −6.48 mm) dentate gyrus. The area was calculated for each region and the number of immunopositive cells quantified through visual counting, giving a cell count per mm 2 . The analysis was performed using Image J, with the plugin 'Cell Counter'.

GAD67
All steps were carried out at room temperature, unless otherwise specified. Sections were washed between each step for 3 × 30 min in blocking solution (0.2% Triton-X in 0.01 M PBS (TTBS) containing 5% donkey serum, 2.5% bovine serum albumin). One in every 12 sections was incubated in blocking solution for 160 min, GAD67 antibody (1:500 in blocking solution, MAB5406, Millipore, UK) for 48 h at 4°C and Alexa 647 (1:1000 in blocking solution, donkey anti-mouse, Life Technologies, UK) for 150 min in the dark. Sections were then placed into DAPI (1:200 in 0.01 M PBS, D9542, Sigma, UK) for 10 min in the dark before being mounted onto glass slides as above. The number of cells labelled with GAD67 were counted through the entirety of the dentate gyrus (infraand suprapyramidal blades), CA1, CA2 and CA3 and hilus in the ventral (Bregma 5.28 to −6.72 mm) and dorsal (Bregma −1.72 to −5.28 mm) portions of the hippocampus, according to the Atlas of Paxinos and Watson (2009). The number of cells per measured area was obtained using Zen Blue (Zeiss).

Pattern separation
Animals Pattern separation ability was assessed using a modified spontaneous location recognition paradigm 20,21 .
Apparatus Testing took place in a black plastic circular arena (45 cm high x 75 cm diameter) with wood shavings covering the floor. The arena was placed in the centre of a dimly lit room, and three proximal spatial cues were placed equidistant around the arena to aid spatial orientation. Objects used were tin cans (7.5 cm × 7.5 cm × 11 cm) and small beer bottles (5.5 cm × 5.5 cm × 15 cm) with the labels removed. Blu-tack was used to fix them to the floor of the arena, and they and the arena were cleaned with 70% ethanol solution between trials. A video camera was suspended directly above the arena and trials were recorded for later analysis.
Behaviour Animals freely explored the arena for 10 min a day for 5 days. On day 6, objects were placed into the arena. Objects were tin cans for half of the animals from each group and sex, and bottles for the remaining animals. Animals were tested in two conditions-easy (large spatial separation, LS) and difficult (small spatial separation, SS). Three identical objects, A1, A2 and A3 were placed 12.5 cm away from the walls of the arena, 25 cm from the centre with either A2 and A3 at 120°(LS) or 50°(SS) apart ( Fig. 1). Animals explored the arena for 10 min (sample phase). After 24 h, animals were returned for 5 min (choice phase). The arena now contained two new objects, A4 and A5, identical in appearance to those used in the sample phase. A4 was in the same (familiar) location as A1, and A5 was placed equidistant to the previous locations of A2 and A3 (novel spatial location, Fig. 1). Rodents display an innate desire to explore novel objects and objects in novel locations over familiar ones 20,38 , thus animals will explore A5 in preference to A1. It will be harder to recognise A5 in the novel location in the SS condition than in the LS condition because SS places greater demand on pattern separation abilities than LS 20,21 . The amount of time animals spent exploring objects in the sample and choice phases was assessed from video recordings by an observer blind to the group. A discrimination ratio was then calculated 20,38 : time exploring novel location object À time exploring familiar location object ð Þ total exploration time : Half of the animals from each group and sex were tested in the LS first then the SS, the other half experienced SS Fig. 1 Apparatus used for pattern separation. Animals explored three objects, A1, A2 and A3, in a circular 75 cm arena for 10 min on day 1. The objects were placed 12.5 cm from the perimeter. A2 and A3 were either separated by a a large (52 cm) or b small distance (20.5 cm). c After 24 h, animals were re-exposed to the arena, A1 was replaced with an identical object, A4 and objects A2 and A3 were replaced with A5, placed equidistance to the position of A2 and A3 then LS. Exploration was counted as the rat having its head directed at the object (2 cm or less), or touching the object with its nose. Sitting on or climbing over the object was not counted.

Fear conditioning
Animals Animals were trained in trace, delay or control protocols at PND 60-100. One hundred and thirty rats (male: 33 control, 36 PPS; female: 30 control, 31 PPS) were used, with sample sizes of 9-13 per protocol per sex. Three days before testing, animals were handled daily and taken to the testing room for habituation to transport and handling.
Apparatus Testing took place in two identical standard modular test chambers (32 cm × 25.5 cm × 27 cm, Sandown Scientific, UK) with grid floors (19 stainless-steel rods, 1 cm apart) underneath which was a stainless-steel pan. The ceiling, front and back walls of the boxes were clear plexiglass, the sides walls stainless steel, and the chamber was enclosed within a sound-attenuating chamber with a ventilation fan providing a background noise of 63 dB and a video camera attached to the inside of the door. The grid floor was connected to a shock generator, and there was a speaker attached to the inside of the sound-attenuating chamber. The boxes were connected to a computer and shock, light, sound and video generation were controlled by a computer. One box (context one, C1) was scented with a drop of lavender oil (Botanics Aromatherapy Pure Essential Oil, Boots, UK) placed onto a tissue in the pan, and contained an infrared (IR) light bar to allow filming as this box was always dark. In the second box (context two, C2), the pan was filled with wood shavings and the walls and ceiling decorated with black stars on a white background, and this box was always light (via houselight in the test chamber). Boxes were cleaned with ethanol wipes between animals, and sawdust/ lavender scent replaced. Half of the animals from each group and sex were trained in C1 and the other half in C2, and all trials were video recorded for later analysis. The conditioned stimulus (CS) was 15 s, 75 dB white noise and the unconditioned stimulus (US) was 0.5 s, 0.5 mA scrambled footshock in all protocols.
Behavior Training. Animals were placed individually into chambers for 120 s. Animals were then trained in one of three protocols: trace protocol (TP)-rats received 10 CS-US pairings, with a 30 s stimulus free trace interval between the offset of the CS and the onset of the US, delay protocol (DP)-10 CS-US pairings were presented, here the CS was followed immediately by the US and control protocol (CP)-animals received 10 presentations of the CS and 10 presentations of the US, which were explicitly unpaired. Intertrial intervals (ITI) were 312s (+/-62 s) for trace and delay protocols, 156s (+/-31s) for the control protocol. ITI length is positively correlated with freezing to later presentations of the CS in trace conditioning, and negatively associated with freezing to a context 39 . This length of ITI was selected as optimal for assessing freezing to cue and context in both trace and delay protocols 39 .
Context recall 1. Animals were returned to their training chamber 24 h later for 10 min to assess contextual fear responses. Cue recall. Forty-eight hours after training, animals trained in C1 were placed into C2 and vice versa to reduce contextual associations and maximise responding to the CS. A plastic insert was placed over the bars of the floor to aid the discrimination. Animals received a 120 s acclimation period, followed by 360 s of CS the 240 s post -CS.
Context recall 2. Seventy-two hours after training, animals were returned to their training chamber for 10 min to assess contextual fear responses.
Freezing during training (baseline, post -shock and trace interval) and recall was analysed from video recordings by an observer blind to group. Freezing was defined as immobility with the exception of movement required for respiration, and was sampled every 10 s.

Statistical analysis
All data were checked for homogeneity of variance and normality, and transformed when these conditions were not met. Data were analysed in JMP (statistical software, SAS Institute, Cary, NC, USA) using generalised linear models, with experimental treatment (control or PPS), sex, separation distance (pattern separation only), protocol (trace, delay or control, fear conditioning only) fitted as factors. Litter was nested within group and added as a random factor to account for litter of origin and animal was nested within litter and added as a random factor to account for multiple measurements on the same animal.

Results
Neurogenesis PPS resulted in a 46% decrease in cell proliferation in the suprapyramidal blade of the ventral dentate gyrus in males only (F 1,8.81 = 4.9, p = 0.05, Fig. 2a), as measured by the number of co-labelled BrdU and DCX cells. There were no changes in the infrapyramidal blade or in females (F 1,7.541 = 0.28, p = 0.62). Cell survival (as measured by DCX-labelled neurons) was increased by 65% in supraand infrapyramidal blades of the ventral dentate gyrus after PPS in males (F 1,9.29 = 14.21, p = 0.004, Fig. 2b

Pattern separation
Animals from both groups spent an equal amount of time exploring all three objects in the sample phase (F 1,8.5 = 0.01, p = 0.91). During the choice phase for LS, all animals spent more time exploring the novel object (A5) than the familiar one (A4) (one-sample t test: control males: t = 3.62, p = 0.005; PPS males: t = 3.05, p = 0.01; control females: t = 3.12, p = 0.01; PPS females: t = 3.18, p = 0.01), and both sexes were able to discriminate between objects equally well during LS (F 1,4.46 = 0.07, p = 0.81, Fig. 3a), demonstrating that PPS did not affect pattern separation in the easy condition. During the choice phase for SS, only control males spent more time exploring the novel object than the familiar one (onesample t test: control males: t = 4.38, = 0.001), and control males were the only group able to discriminate between novel and familiar objects in SS (F 1,37.55 = 9.3, p = 0.006, Fig. 3b). Thus, control and stressed females were equally unable to distinguish between the novel and familiar objects in the SS condition, and stressed males showed an impairment in spatial pattern separation in the more difficult SS condition when compared to control males.

Fear conditioning: context
Results are shown from the first 90 s of each period only. Twenty-four hours after conditioning, male and female control animals experiencing CP and TP demonstrated robust levels of contextual freezing (Fig. 4a, data shown for males only). As expected, contextual freezing was markedly decreased in control DP animals where stronger associations to the CS are formed, resulting in decreased contextual freezing 39 . However, PPS resulted in increased contextual freezing in DP males (F 2,198.3 = 3.24, p = 0.04, Fig. 4a) but not females (F 2,24.49 = 0.67, p = 0.52). There were no differences between protocols or stressed and control animals in contextual freezing responses 72 h post conditioning (males: F 2,24.76 = 0.75, p = 0.48; females: F 2,30.65 = 2.12, p = 0.14).

Fear conditioning: cue recall
Forty-eight hours after conditioning cue recall was performed. In males, PPS reduced conditioned freezing  during CS presentation in the TP (F 2,198.9 = 3.6, p = 0.03, Fig. 4b). In females, PPS increased freezing during CS presentation in the DP (F 2,185 = 3.37, p = 0.04, Fig. 4c). Overall, DP animals showed a greater freezing response to the CS than CP and TP animals, and TP froze more than CP animals (males: F 2,198.9= = 3.6, p < 0.0001; females: F 2,24.24 = 25, p < 0.0001). This is expected as DP animals make stronger CS-US associations than TP animals, who make stronger associations than controls. There was no effect of PPS on post CS freezing, but overall, TP froze more than DP animals (males: F 2,197.1 = 3.48, p = 0.03; females: F 2,27.08 = 4.2, p = 0.03).
To investigate whether differences between the groups were present during encoding, we analysed freezing responses for 60 s after presentation of the 10 US (shocks) during conditioning, and during the 30 s 'trace' gap for TP animals (after presentation of CS, sound). All animals, males and females, froze more after US presentation than at baseline (F 10,631.1 = 28.2, p < 0.0001), and TP animals froze more after CS presentation from CS 2 (F 10,222 = 17.44, p < 0.0001). PPS resulted in lower levels of freezing in male TP animals both after US (F 1,11.14 = 7.06, p = 0.02) and CS presentation (F 1,10.93 = 10.74, p = 0.007, Fig. 4d).

GABA and GABAergic interneurons
PPS increased the expression of GABAergic (PV+) interneurons in the infrapyramidal but not suprapyramidal blade of the ventral hippocampus of males only (F 1,8.27 = 6.83, p = 0.03, Fig. 5a). There were no differences in females (F 1,7.948 = 3.2, p = 0.1). GAD67 was increased in the ventral but not dorsal hilus in stressed males (F 1,311 = 18.3, p < 0.0001, Fig. 5b). Furthermore, following PPS, there was a significant positive correlation between the number of DCX+ and PV+ cells in the suprapyramidal blade of the dentate gyrus only (r = 0.64, p = 0.04).

Summary
The present study demonstrates that PPS has a profound effect on the structure and function of the adult Fig. 4 Fear responses in PPS and control animals exposed to three different conditioning protocols. a PPS males experiencing the delay protocol demonstrated enhanced levels of contextual freezing when compared to their control counterparts in a 24 h recall test. b PPS trace protocol males froze significantly less than controls to representation of the CS 48 h after conditioning. c PPS delay protocol females froze more than controls to representation of the CS 48 h after conditioning. d PPS males experiencing the trace protocol froze less post CS and post US on the conditioning day when compared to controls. Con control animals, PPS pre-pubertal stress animals, CP control protocol, TP trace protocol, DP delay protocol. *p < 0.05, **p < 0.01. Error bars represent 1SE male hippocampus. Specifically, we found that PPS decreased production but enhanced survival of newborn neurons in the ventral dentate gyrus, and impaired pattern separation and trace fear conditioning-behaviours that depend on neurogenesis. We also found evidence of altered GABA signalling following PPS-expression of PV + interneurons was increased in the ventral dentate gyrus and GAD67 increased in the ventral hilus. Due to GABA's role in maintaining quiescence of neural progenitor cells and promoting survival of newborn neurons, this provides a potential molecular mechanism underlying alterations in AHN after PPS.

Neurogenesis
We predicted that PPS would reduce proliferation (BrdU/DCX co-expressing cells) and survival (DCX only) of newborn neurons, as prenatal and early postnatal stressors (first 1-3 weeks of life) robustly decrease AHN in adult males 40,41 . In the present study, PPS decreased proliferation, but resulted in increased survival of newborn neurons, and these changes were specific to the ventral dentate gyrus in males. Thus, similar to prenatal and early postnatal stressors, PPS results in decreased cell proliferation in the male dentate gyrus 42,43 . However, the importance of timing is highlighted, as in contrast to other early-life stress paradigms, PPS increases cell survival. Delineating the contrasting mechanisms of dysfunction produced by stress at different early-life time points is crucial to producing effective interventions.
Altered neurogenesis is robustly associated with impairments in hippocampal-dependent behaviours, especially those with a temporal or spatial component 44 . Therefore, the second aim of the present study was to determine the consequences of PPS on two hippocampaldependent behaviours, trace conditioning and pattern separation.

Trace and delay fear conditioning
We previously demonstrated that PPS results in impaired hippocampal-dependent contextual fear conditioning responses in male animals 16 . Although an intact hippocampus is necessary for this task (at least in one trial studies) 45 , other brain regions, such as the amygdala, are also required 46,47 . Therefore, we used a trace/delay fear protocol to more precisely dissect the effects of PPS on the trace manipulation which is thought to rely on AHN.
Males subjected to PPS displayed lower conditioned fear responses to the CS but only after training with the trace protocol. This is likely due to deficits on encoding, as lower levels of freezing were observed in PPS trace protocol males after the second presentation of CS and US during training. Thus, PPS selectively impacts hippocampal function in males but not females. Hippocampal lesions impair responses to the CS in trace but not delay protocols, whereas other regions are equally important in both [47][48][49] . Furthermore, the acquisition of trace fear is impaired following ablation of AHN using opotogenetic or infusion methods, yet delay fear responses remain intact 28,45,50 . The relative contributions of dorsal and ventral hippocampus to trace conditioning are not entirely clear. One study found that bilateral infusions of the AMPA/kainate glutamate receptor antagonist CNQX into the dorsal dentate gyrus disrupts expression of trace fear conditioning (ventral inactivation was not assessed) 50 , whereas others reveal inactivation of the ventral, but not dorsal, hippocampus impairs acquisition and expression of trace fear 51 .
The effects of increased newborn neuronal survival on trace memories are unknown, but increased neurogenesis disrupts contextual fear memories 52,53 . Together, our data suggest that PPS-induced alterations in neurogenesis could underlie the impairments in trace fear conditioning observed in males in the present study. Males who had experienced PPS froze more to contextual cues 24 h after training in the delay condition. The impact of PPS on contextual fear conditioning selectively in males is consistent with our previous findings, although here the direction of effect is different 16 . There are methodological differences between the studies which likely account for the disparity in results-a single presentation of the US in our previous work compared to the present study which used 10 CS-US pairings. Ablation of AHN results in impaired contextual fear responses in single but not multiple trial conditioning 54 . The enhanced contextual freezing in delay conditioning we measured may be modulated by other neural substrates including the amygdala, which is known to be overactive after multiple CS-US pairings in this model 34 . Females froze more to representation of the cue during the delay protocol only after PPS, another behaviour which relies more heavily on the amygdala 55 .

Pattern separation
AHN is necessary for successful spatial pattern separation, particularly when stimuli are of increasing similarity 56 . We showed that animals were able to discriminate between stimuli when they were presented at large separation distances, but only control males successfully completed the task at the smaller, more difficult, separation distances. Deficits in pattern separation are particularly evident in psychiatric disorders with a strong hippocampal phenotype, such as post-traumatic stress disorder, schizophrenia and major depressive disorder 57 . To date, no studies have considered the impact of earlylife stress on pattern separation. This study provides novel evidence for the role of PPS in disrupted pattern separation ability in adult life, and again points to a role between disrupted AHN and impaired hippocampal-dependent behaviour.
Typically, the dorsal hippocampus has been associated with impairments in cognitive and spatial behaviour (such as pattern separation) and the ventral hippocampus with emotional responses (such as those associated with fear conditioning) 44 . However, the situation is likely more complex, and the dorsal hippocampus has been shown to play a role in trace fear responses, especially with long trace intervals 58 . Presently, the relative contributions of dorsal and ventral hippocampus to the pattern separation task used in the present study are unknown. However, studies using pattern separation for reward value suggest a crucial role for the ventral dentate gyrus 59 , and one study found expression of the immediate-early gene zif268 in both ventral and dorsal dentate gyrus when rats were required to change between different navigational strategies in a plus maze 60 . Further work is required to delineate the relative contributions of dorsal and ventral hippocampus to the pattern separation task used in the current study. Here, we see impairments in both emotional (trace fear) and spatial (pattern separation) tasks correlated with altered neurogenesis in the ventral hippocampus.

GABAergic interneurons
Neural stem cells, the substate for neurogenesis, are largely quiescent, a state maintained through tonic GABA current from PV (PV+) interneurons 61 . We found that PPS increased PV+ interneurons in the ventral dentate gyrus, and increased GAD67, the rate-limiting enzyme in GABA production, in the ventral hilus in males specifically. This suggests a mechanism whereby GABA could be responsible for supressing generation of adult-born neurons in the ventral hippocampus in males following PPS.
GABA promotes survival of newborn neurons 61,62 . This may explain why although generation of new neurons was decreased, survival of DCX-expressing neurons, a population aged 24 h-2 weeks, was markedly increased after PPS in the male ventral dentate gyrus, especially as we find a positive correlation between the number of PV+ and DCX+ cells in the ventral dentate gyrus. Taken together, altered GABA signalling through increased numbers of PV+ interneurons may underlie aberrant neurogenesis and behaviour observed after PPS. Altered GABA signalling has been previously found in models of PPS, with levels of α1-3 subunits of the ionotropic GABA A receptor altered in a region-specific manner 63 .

Sex differences
Sex-based differences in the development of neuropsychiatric disorders abound, and have been found previously in our PPS model 16,64,65 . In the present study, PPS produces fewer changes in females than males. Specifically, PPS enhanced freezing to representation of the CS after a delay protocol only in females, with no other changes evident. This complements research by Toledo-Rodriguez and Sandi 66 , who found that PPS on PND 28-30 impaired extinction to a tone CS in adult males but not females. Interestingly, this study found increased fear responses to a tone in adolescent males, and reduced contextual freezing in adolescent females after PPS, highlighting the importance of timing when assessing the long-term outcomes of early-life stress.
PPS decreased the production yet increased the survival of newborn neurons in the infrapyramidal blade of the ventral dentate gyrus of males, with no effect in females. This mirrors the adult situation, where males demonstrate greater sensitivity to the effects of stress hormones, showing more extensive suppression of neural stem cell proliferation in the dentate gyrus 67 . However, adults also show decreased survival of newborn neurons in response to stress, which is in contrast to the increased survival observed in the present study. Prenatal stressors almost universally result in decreased generation and survival of newborn neurons in both males and females, whereas postnatal stress in the first 1-2 weeks of life produces more variable results 40 . Age of assessment is also vital: males experiencing early postnatal stress tend to have enhanced neurogenesis at PND 21, which switches to suppression as the animals age (as measured by DCXpositive cells). In contrast, females display decreased neurogenesis at PND 21, and this effect disappears in adulthood (DCX-positive cells). Less is known about the age-and sex-specific effects of this early prenatal stress on cell proliferation. Our study demonstrates that PPS produces a decrease in proliferation (measured by BrdU) with an increase in survival (DCX-positive cells) in adult males, with no effects apparent in females, marking the prepubertal phase as a time-point with unique consequences for AHN.
Sex differences were also seen in the pattern separation task, male and female rats performed equally well when the task was easy, but only control males were able to complete the difficult phase. We previously found male animals were superior in a spatial navigation task (water maze) 16 , and a number of other studies reveal a male advantage in spatial reference; however, others find no difference 68 . The fact that control females could not complete the more difficult pattern separation means assessing the effects of PPS on females in this task was not possible, and alternative tasks are needed to probe further female responses to PPS.
Expression of PV+, GABAergic interneurons and GAD67 were altered in males experiencing PPS, but remained unchanged in females. Prenatal stress decreases PV+ interneurons and GAD67 expression in the frontal cortex and hippocampus in early and adult life in males 69,70 , and alters the expression of GABA receptor subunits in a region-and sex-dependent manner 71 . PPS has previously been shown to decrease the expression of GABA A receptor α1 subunit in the hippocampus of adult males, and elevation is associated with stress resilience 72,73 , but little is known of the effects in females. Exposure to a more extended PPS paradigm (spanning PND 28-42) resulted in reduced messenger RNA levels of GAD67 in the central nucleus of the amygdala in male rats 74 , whereas a shorter protocol (PND 28-30) was insufficient to elicit such changes. This suggests that the exact timing and duration of early-life stress are likely crucial in determining later outcomes. Our results add to the literature demonstrating sex differences in response to PPS. In the present study, males display greater responses to PPS, but previous work demonstrates that females are sensitive in different ways. For example, females display learning impairments, exhibit greater compulsivity in a delay discounting task and have reduced preference for sweet solutions (anhedonia) 75,76 . Furthermore, males and females react similarly regarding certain behaviours: both sexes display heightened anxiety and altered cognitive bias after PPS 17,33,77 . In humans sex differences are also observed: associations between childhood maltreatment and incidents of anxiety and depression are higher in women than men, whereas risk for substance use disorder show the opposite pattern 78,79 . This suggests that males and females are distinct in their vulnerabilities to the experience of stress during development. Understanding these differences is crucial in the advancement of appropriate interventions and treatments for psychiatric illness associated with experience of childhood maltreatment.