Intracellular Trafficking Regulators for the Atherogenic Lipoproteins LDL and Lp(a)

Abstract

Elevated levels of low-density lipoprotein (LDL) and its derivative lipoprotein(a) (Lp(a)) in the bloodstream are directly implicated in the development and progression of atherosclerotic cardiovascular diseases (ASCVD). LDL homeostasis is mainly regulated by the LDL receptor (LDLR), which is expressed predominantly on the cell surface of hepatocytes where it binds and removes free LDL particles from circulation. Endocytosed LDLR may then either be recycled back to the cell surface or diverted to lysosomes for its degradation. LDLR recycling is therefore critical for efficient LDL uptake. Endocytic recycling is a complex process involving several trafficking steps and many potential molecular mediators. Here I characterized the role of a small GTPase, RAB10, in the trafficking of LDLR. I found that RAB10 depletion inhibits LDLR recycling from endocytic recycling compartment (ERC), thereby reducing surface LDLR abundance and cellular LDL uptake. Further investigation showed that RAB10 also promotes recycling of another cell surface receptor, the transferrin receptor (TFR), but from a different endocytic compartment. Our findings suggest that RAB10 impacts recycling of LDLR and TFR by promoting vesicle trafficking from different intracellular compartments. Although Lp(a) is another casual risk factor for ASCVD, there are no specific Lp(a)-lowering treatments, and their development is limited by a gap in knowledge regarding Lp(a) regulation. Unlike the well-known receptor mediated pathway for LDL homeostasis, the mechanism(s) for cellular uptake of Lp(a) are poorly understood, and several receptors, including LDLR, have been individually studied with conflicting data. To address this gap, we applied a high-throughput whole-genome CRISPR screen to interrogate the regulators of Lp(a) endocytosis in HuH7 cells. Surprisingly, LDLR and other known regulators of LDLR, including SCAP, MBTPS2, MYLIP were among the top hits in the screen. No other receptors, including those previously proposed to serve as Lp(a) receptors, exhibited a functional influence on Lp(a) uptake in this screen. Our results suggest that the LDL receptor is the primary mediator of Lp(a) uptake in hepatocytes. Collectively, the work done in this thesis increases our understanding of receptor mediated trafficking of LDL and Lp(a).