What We Do

Every blood vessel in the body is lined with a specialized layer of polarized cells known as endothelium. An essential function of the endothelial monolayer is the regulation of barrier integrity, which prevents the leakage of plasma and proteins out of the circulation while still permitting the flux of nutrients and immune cells to target tissues.

In principle, permeability of the endothelial monolayer can reflect contributions from leaking between endothelial cells (paracellular leak) and through individual endothelial cells (transcellular leak, or transcytosis). It is widely accepted that paracellular leak predominates during inflammatory states such as sepsis and acute lung injury. Accordingly, by far the majority of research on endothelial permeability has focused on this route of endothelial permeability: the methods of study are relatively straight-forward and there is obvious relevance to human disease. In contrast, the contribution of transcytosis to overall endothelial permeability is relatively obscure, particularly in the setting of inflammation. This is largely due to technical difficulties in distinguishing transcellular permeability from intercellular gaps, particularly in a dynamic and quantifiable way. In addition, endothelial cells grown in culture appear to lose the ability to perform transcytosis as they are passaged. Much of the initial work on transcytosis used electron microscopy of animal tissues, an expensive and often a mostly descriptive endeavour. Transcytosis (at least in the apical to basal direction) is best described for the plasma protein albumin and is mediated by caveolae, small vesicles that bud off from the apical endothelial surface and release their cargo at the basal membrane. This process requires the protein caveolin-1 and the large GTPase dynamin; the latter is thought to mediate the scission of internalized caveolae from the apical plasmalemma.

Figure 1. Two routes of endothelial permeability

My lab is interested in both routes of endothelial permeability and how they are related.

We study paracellular leak during inflammation; for instance, using the influenza A virus as a model pathogen, we investigate how the virus induces lung endothelial permeability to cause pulmonary edema, a characteristic clinical feature of severe influenza infections in humans. We have reported effects of the virus on lung endothelial viability and on tight junction integrity; interestingly, at least some of the effect of the virus on endothelial barrier integrity is independent of viral replication and involves degradation of the tight junction constituent claudin-5. It is also worth noting that systemic microvascular permeability (i.e. in all organs) is a feature of sepsis that leads to hypotension, organ edema and potentially multiorgan failure. Remarkably, there are no treatments for microvascular leak so identifying and testing potential endothelial barrier-enhancing compounds is another major area of interest for my lab.

Figure 2. Influenza-induced lung endothelial leak. Note the proximity of the lung endothelium to the alveolar epithelium

Another area of study in the lab is the contribution of endothelial transcytosis to the overall permeability of the endothelium to macromolecules. For example, the circulating hormone insulin must leave the vascular lumen in order to exert its effects on critical downstream tissues such as fat or muscle. While insulin delivery is rate-limiting for its metabolic action on tissues, it has been difficult to distinguish the roles of capillary recruitment and bona fide transendothelial transport of insulin in this process. We are currently determining whether insulin transcytosis is rate-limiting for its action in skeletal muscle and adipose tissue.   

Interestingly, endothelial transcytosis underlies the first stage of atherosclerosis. Accumulation of LDL-derived cholesterol under the arterial endothelium triggers an inflammatory reaction that culminates in luminal narrowing and eventually an unstable arterial plaque. However, how the LDL gets under the endothelium is poorly understood. Autopsy studies on young individuals dying of non-cardiac causes reveal a healthy, continuous endothelial layer overlying cholesterol deposits and the average LDL particle is too large to pass through intact cell-cell junctions; thus, LDL is likely to cross the endothelium by transcytosis. The canonical model of LDL receptor-initiated endocytosis does not explain LDL accumulation in the arterial intima. We are currently using our transcytosis assays and ex vivo perfusion models to investigate this important process and have reported an unexpected role for the scavenger receptor SR-BI in LDL transcytosis.

The work in the lab is centred on live cell imaging that is complemented by traditional biochemical and molecular biology approaches. The lab is located in the Keenan Research Centre for Biomedical Science, St. Michael's Hospital (a tertiary care teaching hospital).  Dr. Lee and the hospital are affiliated both with the University of Toronto and with Ryerson University.