Functions of Clusterin mediated by ApoER2 and VLDL receptor

Clusterin is an enigmatic glycoprotein which is ubiquitously expressed in mammals. Unravelling its biological function(s) proved to be very complicated because of its involvement in many seemingly unrelated biological processes. Clusterin`s multi-functionality is caused by its ability to interact with many different proteins. A chaperone activity is in fact one common underlying feature mediating some of its diverse functions. Clusterin forms stable and soluble complexes with so-called "client proteins" preventing them from forming toxic aggregates and promoting their clearance by receptor-mediated endocytosis. Another feature is its signalling activity playing a role in apoptosis and cell survival. Our discovery that ApoER2 and VLDL receptor bind clusterin might turn out to be an important step to characterize the functions of clusterin at the molecular and cellular level. ApoER2 and VLDL receptors are known to be key players in the Reelin signalling pathway which is indispensable for the development of laminated structures in the brain. This pathway is interconnected with pathways regulating apoptosis and cell survival and, therefore could be the missing link to understand clusterin`s function as a signalling molecule. Since both receptors are enocytosis competent, they might also be involved in clusterin`s function as chaperon to remove "client proteins" from locations where deposition of these proteins leads to pathological processes such as Alzheimer`s disease. This projects aims at defining the signalling pathway(s) of clusterin supported by ApoER2 and VLDLR. We propose that a pathway similar to the Reelin-pathway is triggered by clusterin and that ApoER2 and VLDL receptor are involved in the cellular uptake of clusterin/protein complexes by receptor mediated endocytosis. The results of this project are expected to shed light on grossly missing molecular and mechanistic aspects of the pleiotropic functions of clusterin and to have an important impact on our understanding of Alzheimer`s disease and other maladies where clusterin plays a role.

ApoER2 and VLDL receptor are members of the LDL receptor family and are part of a signaling cascade which orchestrates correct development of laminated structures of the central nervous system. This signaling pathway is elicited by Reelin an extracellular matrix protein which is secreted from specialized neurons during embryonic development of the brain. Reelin binds to ApoER2 and VLDL receptor which induces phosphorylation of Dab1 an intracellular adapter protein bound to the intracellular domains of both receptors. Besides Reelin thrombospondin, which is also present in certain areas of the brain, can elicit a similar pathway by binding to ApoEr2 and VLDL receptor. Before this project was conceived and started we knew from our previous work on development of the chicken oocyte, where VLDL receptor plays a key role in the growth of this cell, that clusterin might be another potential ligand for both receptors. The main question addressed in this project was therefor to determine whether clusterin is indeed a physiological ligand for both receptors in the mammalian brain and whether this binding has a function in the development of the mammalian brain.During this project we could demonstrate that clusterin binds to ApoER2 and VLDL receptor with a similar affinity as Reelin. Binding of clusterin to both receptors indeed elicits a Reelin-like signal by inducing Dab1 phosphorylation. Furthermore we discovered that clusterin is necessary for proliferation of neuroblasts within the subventricular zone of the developing brain and promotes chain formation of these cells which is necessary for this cells to migrate to their final destination in the mature cortex. In addition to this exiting findings we observed that both receptors are able to endozytose clusterin. This effect might open up a novel avenue to treat Alzheimers disease. This idea is nor followed up by a new project sponsored by the FWF.