Rho Kinase II Phosphorylation of the Lipoprotein Receptor LR11/SORLA Alters Amyloid-β Production

Abstract

LR11, also known as SorLA, is a mosaic low-density lipoprotein receptor that exerts multiple influences on Alzheimer disease susceptibility. LR11 interacts with the amyloid-β precursor protein (APP) and regulates APP traffic and processing to amyloid-β peptide (Aβ). The functional domains of LR11 suggest that it can act as a cell surface receptor and as an intracellular sorting receptor for trans-Golgi network to endosome traffic. We show that LR11 over-expressed in HEK293 cells is radiolabeled following incubation of cells with [(32)P(i)]orthophosphate. Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) was used to discover putative LR11 interacting kinases. Rho-associated coiled-coil containing protein kinase (ROCK) 2 was identified as a binding partner and a candidate kinase acting on LR11. LR11 and ROCK2 co-immunoprecipitate from post-mortem human brain tissue and drug inhibition of ROCK activity reduces LR11 phosphorylation in vivo. Targeted knockdown of ROCK2 with siRNA decreased LR11 ectodomain shedding while simultaneously increasing intracellular LR11 protein level. Site-directed mutagenesis of serine 2206 in the LR11 cytoplasmic tail reduced LR11 shedding, decreased LR11 phosphorylation in vitro, and abrogated LR11 mediated Aβ reduction. These findings provide direct evidence that LR11 is phosphorylated in vivo and indicate that ROCK2 phosphorylation of LR11 may enhance LR11 mediated processing of APP and amyloid production.