The presenilins (PS1 and PS2) have only recently been isolated and identified as major familial Alzheimer's disease (FAD) genes, consequently little is known about their normal biological role or about how the presenilin mutations lead to the AD-associated increase in the levels of amyloid beta (ABeta). To learn more about the function of PS1 and PS2, and about cellular pathways that they have in common, such as those involving the pathogenic increased production of ABeta, we have focused on the characterization of protein(s) that interact with domains that are homologous in the two proteins. The C-terminus is such a homologous domain and a recent collaborative effort has resulted in the identification of a novel protein that interacts with the C-terminal of both PS1 and PS2. This protein shares significant homology with a family of neuronal Ca2+-binding proteins, therefore we have named this novel Ca2+-binding, presenilin-interacting protein calsenilin. We have found that calsenilin is a bone fide Ca2+-binding protein that is expressed specifically in the brain. Hydropathy analyses indicating that calsenilin lacks a membrane spanning domain(s) are supported by our immunofluorescence studies which indicate that calsenilin is localized in the cytoplasm of transfected cells. Interestingly, when PS2 and calsenilin are co-expressed in cells, the subcellular localization of calsenilin appears to redistribute and co-localize with PS2 to the ER and Golgi. This finding is compatible with current models of presenilin topology, which predict that the C-terminal domain is located at the cytoplasmic face of the ER/Golgi membranes, and indicates that the two proteins can interact in vivo. Finally, our preliminary data indicate that the expression of calsenilin results in alterations of the normal cleavage pattern of PS2. In this proposal we have outlined experiments designed to provide information about the basic biological properties of calsenilin. In addition, we will confirm and extend our preliminary findings, which indicate that the expression of calsenilin effects the processing of PS2, and we will determine if calsenilin has the ability to mediate the effect that the presenilin mutations have on APP metabolism and ABeta generation. We will also assess whether these properties, or the calsenilin-PS2 interaction, are altered by the presence of FAD-associated mutations in the presenilins. The experiments described in this application should supply fundamental but critical information about the basic cell biology of calsenilin and its interaction with PS2. This information will be a valuable asset for understanding presenilin biology, as well understanding presenilin-associated Ca2+-signaling pathways in the brain. Because calsenilin interacts with both of the presenilins, the elucidation of its role should provide information about pathways shared by the two proteins, such as those involved in the AD-associated increases in ABeta. Interestingly, recent data indicates that FAD-associated increases are dependent of the presence of the C-terminal domain (the domain that interacts with calsenilin) therefore, this proposal may provide a platform on which to ultimately base effective investigational and therapeutic strategies as well as future studies.
