Molecular mechanism(s) of cholesterol-effect on APP and BACE1 metabolism - the two key proteins of Alzheimer's disease
The DAAD-MZOS bilateral project between Prof. Jochen Walter (University of Bonn) and Dr. Silva Katusic Hecimovic (Rudjer Boskovic Institute) addresses the mechanism(s) of cholesterol-effect in the pathogenesis of Alzheimer's disease (AD). Accumulation/aggregation of amyloid-beta peptide (Abeta) in the brain is considered to be a central event in AD pathogenesis. Abeta is derived through cleavage of the amyloid precursor protein (APP) by proteases beta- (BACE1) and gamma-secretase. Both proteases are considered to be drug targets for AD, but clinically suitable inhibitors have been difficult to find. Alzheimer’s disease thus remains a global health problem with no adequate therapies. It is, therefore, essential to identify alternative strategies to treat AD. Recent evidence increasingly implicates that Abeta formation can be modulated by cholesterol levels suggesting that altered cholesterol metabolism may trigger the pathogenic processes leading to AD. However, the molecular mechanism(s) that would explain how cholesterol metabolism modulates the pathogenesis of AD have been largely unknown. The link between altered cholesterol metabolism and AD has been recently revealed in a rare inherited lysosomal storage disorder Niemann-Pick-type-C (NPC), which is caused by loss-of-function mutations in NPC1/2 genes (NPC1 mutations being the most common). Interestingly, NPC-disease is characterized by cellular cholesterol accumulation, progressive neurodegeneration and increased levels of APP-C-terminal fragments (APP-CTFs) and Abeta, one of the key pathological features of AD. Additionally, NPC1 depletion (NPC1+/-) in AD-transgenic mouse model (PS1xAPP) enhances the progression of AD by increasing Abeta accumulation. More recently, altered expression of NPC1 mRNA/protein were reported in AD brains indicating a bidirectional link between NPC1 (dys)function and Alzheimer’s disease. The NPC-disease may thus present an innovative model to study the molecular mechanism(s) of cholesterol-mediated Abeta accumulation/aggregation in which a defect in a single gene involved in cholesterol trafficking (NPC1) causes AD-like phenotype. In our recent results, we provide evidence for an indirect mechanism of cholesterol-effect upon NPC1-loss-of-function on APP processing. Using NPC1-/- cells we demonstrated that increased Abeta in NPC disease is mediated by cholesterol-dependent increased processing of APP by BACE1 (Malnar et al. Biochim.Biophys.Acta. 2010;1802:682-91) probably due to decreased expression of APP at the cell surface and its increased partitioning into cholesterol-rich membranes - lipid rafts (Kosicek et al. Biochem.Biophys.Res.Commun. 2010;393:404-9). Using biochemical, molecular biological and cell biological techniques we will further test the mechanistic role of cholesterol-accumulation on increased BACE1-cleavage of APP and increased Abeta levels. We will test the hypotheses that alterations in cholesterol metabolism (due to NPC1 dysfunction) contribute to accumulation of Abeta and the pathogenesis of AD by modulating endocytic trafficking of BACE1 and/or by modulating lysososomal/autophagic function. To test this we will use both NPC non-neuronal and neuronal animal cellular models generated by shRNA lentiviral knockdown approach. We believe that these studies will identify a unique and specific cholesterol-mediated molecular pathway/trigger involved in the metabolism of APP and BACE1 and will provide clues for development of novel therapies against this devastating disorder.