Regulation of Intravascular Triglyceride Hydrolysis - PROJECT SUMMARY/ABSTRACT Title of Project: “Regulation of Intravascular Triglyceride Hydrolysis” My objective is to address persistent problems in the intravascular triglyceride (TG) metabolism field. This topic is important because inefficient processing of TG-rich lipoproteins (TRLs) results in higher plasma levels of atherogenic lipoproteins and increased risk of coronary heart disease (CHD). Also, current treatment strategies are limited. My plans are rooted in our laboratory’s discoveries. We discovered an endothelial cell (EC) protein, GPIHBP1, that binds lipoprotein lipase (LPL) and shuttles it into capillaries; we discovered that some of that LPL detaches from GPIHBP1 and enters the EC glycocalyx where it is active in TRL processing; we discovered that LPL is active as a monomer; we solved the structure of the LPL–GPIHBP1 complex; we discovered LPL and GPIHBP1 mutations that cause chylomicronemia by disrupting the GPIHBP1–LPL interface; and we discovered that chylomicronemia can be caused by GPIHBP1 autoantibodies. We have elucidated the regulation of LPL. We showed that GPIHBP1’s acidic domain stabilizes LPL’s catalytic domain; we showed that an LPL regulator, ANGPTL4, catalyzes the unfolding of LPL’s catalytic domain; we showed that APOC2 binds to sequences anchoring LPL’s lid and stabilizes LPL conformation; and we showed that APOA5 preserves LPL levels in capillaries by suppressing the ability of the ANGPTL3/8 complex to detach LPL from capillaries. We uncovered heterogeneity in LPL and GPIHBP1 expression in EC subsets and heterogeneity in LPL binding to the glycocalyx of capillaries and larger blood vessels. Our progress has resulted from NHLBI support, investments in recombinant proteins and monoclonal antibodies, application of advanced biophysical methods, and introducing new scientists to the field and driving productive collaborations. Our discoveries have transformed textbook models for intravascular lipolysis but have also highlighted persistent questions in the field. During the next seven years, we will answer these questions. We will identify APOA5 sequences that are important for suppressing the activity of the ANGPTL3/8 complex and use those findings to create therapeutics to preserve LPL levels in capillaries and reduce plasma levels of atherogenic lipoproteins. We will investigate whether ANGPTL3/8– mediated LPL detachment from cells results from LPL unfolding and whether the ANGPTL3/8 complex preferentially detaches LPL from the glycocalyx of capillary ECs. We will determine, by X-ray crystallography, whether APOC2 activates LPL by opening its lid. We will define the transcriptional basis for heterogeneity in LPL and GPIHBP1 expression in EC subsets, and we will determine why LPL binds avidly to the glycocalyx of heart capillaries but not to the glycocalyx of larger blood vessels or capillaries of the brain. We will investigate whether TRL processing in the choroid plexus plays an accessory role in delivering lipids to the central nervous system. We are uniquely positioned—with recombinant proteins, a trove of mAbs, expertise in biophysical methods and structural biology, and highly experienced collaborators—to address these issues. Addressing persistent questions in intravascular lipolysis will transform the field and uncover new strategies for preventing CHD.
