PROJECT SUMMARY / ABSTRACT This proposal aims to determine the function of ABHD5, a protein that is essential for the catabolism of triacylglycerol in humans, as well as in other animals, worms, and plants. Loss of function mutations in, or deletion of, ABHD5 blocks the hydrolysis of triacylglycerol, which accumulates, and leads to ichthyosis, steatosis, myopathy, and other alterations. ABHD5 has been reported to activate two enzymes: ATGL, the main triacylglycerol lipase in cells, and PNPLA1, which catalyzes the synthesis of acylceramides in skin. In addition, ABHD5 has been proposed to regulate ATGL activity and thus, triacylglycerol lipolysis and storage in cells. However, the mechanism of action of ABHD5 remains undefined. Moreover, several lines of evidence suggest that ABHD5 is not a direct activator of ATGL: We have shown that overexpression of ABHD5 in mouse adipose tissue, which expresses high levels of ATGL, does not increase lipolysis. In addition, in ATGL knock-out mice, simultaneous knock-down of ABHD5 increases liver TAG further, indicating that even in the absence of ATGL, ABHD5 regulates TAG amounts, thus implying a mechanism independent of ATGL. This suggests that ABHD5 does not directly activate ATGL, but rather that ABHD5 catalyzes a different reaction in triacylglycerol lipolysis. This project proposes to conduct enzymology studies using purified ABHD5, ATGL, and catalytically dead mutant forms, to determine the enzymatic function of ABHD5, its substrates and products, and the regulation ATGL. In addition, it proposes lipidology studies to show how the proposed ABHD5 function explains the lipid alterations described in cells lacking functional ABHD5. These studies are expected to elucidate the function of ABHD5, refine the understanding of the enzymatic reactions in the lipolytic pathway, and shed light on its regulation. This new knowledge will change the current understanding of lipolysis and provide new insights into obesity and related diseases. This research will be conducted by a team composed primarily of undergraduate students. It will allow the students to gain experience in biomedical research, support research by the principal investigator, and promote collaborations that will strengthen the research environment at Brooklyn College.