As tempting as it might be to imagine blood vessels as a network of garden hoses carrying vital agents around the body, the reality is much more subtle and complex. Although blood travels readily within veins and arteries, other biochemical constituents pass right through the walls of these same conduits.

Among the most significant of these constituents is cholesterol, the lipid that is associated with the intake and processing of fats. When cholesterol builds up under the endothelium, the inner lining of blood vessel walls, it causes them to become stiff and inhibits their function. The result is atherosclerosis, a fundamental cause of the vascular ruptures responsible for a stroke and the vascular occlusions that lead to heart attacks.

While medical researchers would like to learn how to stop cholesterol deposits from forming in this way, until now they have had no practical means of studying the process. Tracking gold-tagged cholesterol samples injected into test animals proved to be extraordinarily inefficient. Likewise, bench-top models made from membranes seeded with endothelial cells provide no information about whether material is travelling between those cells or through them. 

The problem was largely abandoned until recently, when University of Toronto biochemist Warren Lee and his colleagues took advantage of a microscopy method developed in the 1950s. That method, called total internal reflection fluorescence (TIRF), employs the evanescent wave that penetrates into a surface when light reflects off it. Although this penetration is only on the order of nanometres, if the area contains fluorescent compounds, it will light up for imaging. 

When this technique is applied to live cells that have been injected with fluorescent cholesterol, TIRF makes it possible to see where this material is going in real time. “You’re actually getting a video of cholesterol moving through coronary artery endothelial cells,” says Lee. In a paper published in Cardiovascular Research, he and a number of other investigators from Toronto and Montreal describe how TIRF revealed the unexpected role of a molecular receptor that facilitates transcytosis, the ability of cholesterol to move through endothelial cells. “We’re actually looking at the bottom of each cell on a single cell basis,” Lee says, noting that this capability opens up entirely new prospects for finding molecular targets to prevent or treat atherosclerosis. “I can finally try to manipulate and understand transcytosis.”