Warren Lee

Warren Lee

Associate Professor, University of Toronto

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.

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.

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. In vitro work and early work in dogs suggested that insulin delivery, which includes crossing the microvascular endothelium, is rate-limiting. However, the relative contributions of capillary recruitment and regulated transendothelial insulin transport (transcytosis) to insulin delivery have been unclear. Thus, one area of study is insulin transcytosis.

We also use similar approaches (live cell imaging, ex vivo perfusion, animal models) in the study of atherosclerosis, since the accumulation of LDL cholesterol under the arterial endothelium constitutes the first step in the disease. Using these methods, we have reported an unexpected role for the SR-BI receptor and for ALK1 in LDL transcytosis.