The role of caveolae and caveolin-1 in the regulation of endothelial endocytosis/transcytosis

Abstract

Caveolar vesicles are specialized membrane compartments found in most differentiated cell types. They can exist as single 50--100 nm omega-shaped invaginations or as a variety of variously shaped aggregates. Caveolae have been implicated in various cellular processes including signal transduction, cell cycle regulation, calcium homeostasis, maintenance of the cytoskeletal architecture, endocytosis, and transcytosis.;The role of endothelial cell caveolae in the uptake and transport of macromolecules from the blood-space to the interstitial-space remains controversial. To address this issue directly, we employed caveolin-1 gene knock-out mice that lack caveolin-1 protein expression and caveolae organelles. Here, we show that endothelial cell caveolae are required for the efficient uptake and transport of a known caveolar ligand, i.e. albumin, in vivo. Our results indicate that gold-conjugated albumin remains in the blood vessel lumen and is not endocytosed by Cav-1 deficient lung endothelial cells; in contrast, gold-conjugated albumin was concentrated and internalized by lung endothelial cell caveolae in wild-type mice, as expected. Also, little or no uptake of radio-iodinated albumin was observed in the aortic segments from Cav-1 deficient mice, while aortic segments from wild-type mice showed robust uptake that was time- and temperature-dependent and competed by cold albumin. We conclude that endothelial cell caveolae are required for the efficient uptake and transport of albumin from the blood to the interstitium.;Interestingly, these mice are viable, but show a loss of endothelial caveolae and striking defects in caveolae-mediated endocytosis. Thus, a compensatory mechanism must be operating in these mice. One possible compensatory response would be an increase in the para-cellular pathway, resulting in increased microvascular permeability. To address this possibility we injected 125I albumin directly into the circulatory system. Our results indicate that iodinated BSA is removed from the circulatory system of Cav-1 deficient mice at a substantially faster rate than from wild type mice. As loss of caveolin-1 expression results in constitutive activation of eNOS activity, we also examined whether these increases in microvascular permeability are NO-dependent. Interestingly, treatment with L-NAME (a well-established NOS-inhibitor) successfully reversed the microvascular hyper-permeability phenotype of Cav-1 knock-out mice. Based on these findings we conclude that caveolin-1 plays a dual regulatory role in controlling microvascular permeability: (i) first, as structural protein that is required for caveolae formation and caveolar transcytosis; and (ii) second, as tonic inhibitor of eNOS activity to negatively regulate the para-cellular pathway.