Role of clusterin in Alzheimer´s disease

Alzheimers disease is the most common cause of dementia with an estimated prevalence of 30 million cases worldwide. Due to the expected life span increase the number is thought to quadruple till 2050 making AD likely to become one of the most important public health issues. This is even more dramatic considering that there is no effective treatment available. The essential clinical feature of this disease is a gradual loss in cognitive functions starting with the loss of recent memory leading over a period of 7 10 years to loss of autonomy and finally to death. At the microscopic level this disease is characterized by extracellular amyloid deposits containing the A-peptide which is derived from amyloid precursor protein by proteolytic processing. A small fraction of cases is caused by a genetic problem which leads to an increase of A production. The majority of cases, however, is of unknown etiology and is characterized by a slow increase of deposits during life time. Genetic association studies suggest that variants of the Clusterin gene are associated with an increased risk to develop this disease. 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. Clusterins 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. Clusterin also interacts with A and can be found in amyloid plaques from Alzheimers patients. Our recent discovery that ApoER2 and VLDL receptor bind clusterin prompted the idea that these receptors which are expressed by many neurons might be involved in the clearance of these aggregates. The aim of this project is to prove whether this hypothesis that ApoER2 and VLDL receptor are involved in keeping the balance between production and removal of A complexed to clusterin. Furthermore, it is planned to study the function of the clusterin variants which are linked to this pathology. The results of this project are expected to shed light on grossly missing molecular and mechanistic aspects of the progression of this devastating disease and to open new avenues for the development of novel therapeutic strategies.

Clusterin is a protein which is ubiquitously expressed in mammals. Its function(s) however, are still enigmatic. Recent discoveries suggest that Clusterin might be involved in the pathology of late onset Alzheimer's disease. One theory claims that Clusterin forms soluble complexes with A peptides preventing A from aggregating. Extracellular depositions of A aggregates are a hallmark of this devastating disease. In this project we wanted to test whether ApoER2 and VLDL receptor might be able to bind such A/Clusterin complexes thereby removing them from the extracellular space targeting them for lysosomal degradation. During the first year of this project we evaluated basic principles of this hypothesis had to realize that we were not able to prove them. Thus we turned our attention to projects which were always running in parallel and which are centered around our long standing interest in studying certain aspects of the Reelin signaling pathway. The above mentioned receptors ApoER2 and VLDLR are part of the Reeling signaling pathway which guides newly generated neurons during embryonic brain development from their birth place to their final position in the forebrain and other parts of the brain. Reelin binding to ApoER2 and VLDL receptor leads to phosphorylation of Dab1 and subsequently to the ultimate cell responses required for the correct positioning of newly generated neurons. The mechanisms responsible for Dab1-phosphorylation are not completely understood but are mediated by receptor clustering and involve members of the src family of non-receptor tyrosine kinases. At this stage of the project we established a novel method at MPL to study protein/protein interactions at the subcellular level at an hitherto unprecedented resolution. These methods are time-resolved fluorescence anisotropy imaging (TR-FAIM) and fluorescence lifetime imaging microscopy (FLIM) both based upon "Förster resonance energy transfer". Based upon the unexpected results we were able to redefine the initial step in the canonical Reelin pathway which is started by full length Reelin via increasing preformed receptor clusters at the cell surface of responding neurons. In addition we discovered a second pathway elicited by the central fragment of Reelin which does not start with rearranging the cluster size of the receptor. The second part of the project was centered on a novel Reelin-independent function of Dab1. We discovered Dab1 as adapter protein for EGF receptor being part of a hitherto unknown signaling pathway elicited by binding of EGF to its cognate receptor. This pathway could play a role in maintaining cell homeostasis in the epithelium of the small intestine.