Mechanisms for low density lipoprotein receptor internalization

Abstract

Low density lipoprotein receptor (LDLR) internalization clears cholesterol-laden LDL particles from circulation in humans. Defective LDLR trafficking that causes elevated serum cholesterol levels have been associated with genetic diseases, such as familial hypercholesterolemia (FH) and autosomal recessive hypercholesterolemia (ARH). However, our understanding of the mechanisms underlying LDLR trafficking remains incomplete. To identify factors critical to LDLR trafficking, we pursued a genome-wide RNAi screen using Caenorhabditis elegans LRP-1/megalin as a model for LDLR transport. In this screen, we identified an unanticipated requirement for the clathrin-binding endocytic adaptor epsin1 in LDLR endocytosis. Epsin1 depletion reduced LDLR internalization rates in mammalian cells, similar to that observed following clathrin depletion. Genetic and biochemical analyses of epsin in C. elegans and mammalian cells uncovered a requirement for the ubiquitin-interaction motif (UIM) as critical for receptor transport. As the epsin UIM promotes the internalization of some ubiquitinated receptors, I predicted LDLR ubiquitination as necessary for endocytosis. However, engineered ubiquitination-impaired LDLR mutants showed modest internalization defects that were further enhanced with epsin1 depletion, demonstrating epsin1-mediated LDLR endocytosis is independent of receptor ubiquitination. Furthermore, I examined requirement of the ability of ubiquitin-binding for epsin UIM function in LDLR internalization. Genetic analyses using ubiquitin-binding-impaired EPN-1 mutants demonstrated that EPN-1 binding to ubiquitinated protein is not required for LRP-1 internalization in C. elegans, revealing an undefined regulatory role of epsin UIM in LDLR internalization. Since clathrin knockdown resulted in 50% reduction in LDLR internalization, I suspected that LDLR can be internalized via an alternative mechanism. The major pathway mediated by caveolae invagination was tested and the results indicated that LDLR can undergo either caveolae- or clathrin-mediated endocytosis. Finally, I provide evidence that epsin1-mediated LDLR uptake occurs independently of either of the two documented internalization motifs (FxNPxY or HIC) encoded within the LDLR cytoplasmic tail, indicating an additional mechanism for LDLR.