Caveolae mediated lipid uptake and trafficking in health and disease

Abstract

Caveolae are 70-100 nm sized plasma membrane invaginations creating a unique lipid environment that are particularly abundant in adipocytes, myocytes and endothelial cells. Structurally, caveolae are formed by a protein coat of Caveolins and Cavins. Additionally, several membrane binding proteins such as Syndapin2, EHD2 and dynamin are important regulators of caveolae function and behavior. In the past, caveolae were found to be involved in the cellular lipid uptake. Mice lacking caveolae revealed an impaired lipid metabolism resulting in reduced weight gain and lipodystrophy. Contrary, raised numbers of caveolae resulted in increased cellular lipid uptake leading to large adipose tissue depots. As caveolae are mobile membrane domains which can get endocytosed as small vesicles, it was proposed that caveolae are able to migrate to lipid droplets to facilitate lipid trafficking. However, up to date it is not understood how caveolae migrate from the plasma membrane, what triggers the trafficking and what happens mechanistically at the lipid droplet coat. By using genetically modified mouse embryonic fibroblasts, 3T3-L1 adipocytes and mouse models we investigate the involvement of caveolae trafficking in cellular lipid metabolism. We showed that changing the rate of caveolar endocytosis in vivo results in altered lipid uptake and consequently lipid droplet sizes. Thereby, either an increased caveolae internalization or raised numbers of caveolae lead to an increased fatty acid uptake and larger lipid droplets. Furthermore, platinum replica electron microscopy revealed that the fatty acid treatment itself triggers caveolae detachment from the plasma membrane. Detailed confocal microscopy and structured illumination microscopy (SIM) in live cells showed that caveolae re-locate to lipid droplets after fatty acid treatment. Importantly, this migration can be prevented when caveolae endocytosis is inhibited either by overexpressing the caveolae stabilizer EHD2 or a non-hydrolyzing dynamin mutant. The classic endocytic pathway involves endosomal trafficking followed by lysosomal degradation. However, caveolae trafficking to lipid droplets was not impaired in cells treated with endo- or lysosomal inhibitors indicating a rather direct trafficking from the plasma membrane to lipid droplets. Additionally, we showed the involvement of the fatty acid binding receptor CD36 in this process. Loss of CD36 completely abolished the caveolae dependent fatty acid uptake. Taken together, here we present novel insights in caveolae mediated lipid uptake and trafficking. Notably, this precisely regulated pathway is impaired in obesity. By analyzing adipose tissue obtained from obese patients or mouse models we observed an impaired caveolae plasma membrane stabilization. Detailed electron microscopy investigation of murine obese tissue showed increased numbers of caveolae detached from the plasma membrane indicating enhanced caveolar endocytosis and consequently raised lipid uptake. These results give novel insights in the underlying molecular changes helping to better understand obesity. Future studies are essential to investigate in detail the involvement of caveolae mediated lipid metabolism in obesity and other metabolic challenging diseases.