The Regulation of PCSK9 Structure and Function Through Lipoprotein Interactions

Abstract

Proprotein convertase subtilisin / kexin type 9 (PCSK9) is a negative regulator of the low-density lipoprotein receptor, and PCSK9 inhibition has become an important cholesterol-lowering therapeutic strategy. PCSK9 also associates with LDL particles, and evidence suggests that the activity of PCSK9 may be regulated by LDL binding. We have investigated the biochemistry of the interaction between PCSK9 and lipoproteins. Through mutagenesis and in-vitro binding assays, we found conserved motifs in the PCSK9 N-terminus that play a role in LDL binding. Through secondary structure studies using circular dichroism and computational modelling, we determined that the N-terminal region of the PCSK9 prodomain undergoes an environment-dependent structural shift that affects the ability of PCSK9 to bind LDL. We also found that the commonly found loss-of-function polymorphism R46L is capable of modulating this structural shift. Importantly, we found a surface-exposed region of the PCSK9 prodomain that maps a cluster of gain-of-function mutations (L108R, S127R, and D129G) that severely disrupt LDL binding. Through gel shift assays and density gradient centrifugation, we observed that PCSK9 shows remodeling-dependent ability to bind different classes of lipoprotein particles in vitro, binding strongly to LDL and IDL but showing barely detectable association to VLDL. Further, in human plasma, we found that lipoprotein-bound populations of PCSK9 shifted in response to differences in lipoprotein profiles between normolipidemic and hypercholesterolemic or hypertriglyceridemic subjects. Overall, elucidation of how lipoproteins regulate PCSK9 activity will reveal new targets for designing cholesterol-lowering therapeutics.