Alzheimer's disease (AD) is pathologically characterized by the accumulation of -amyloid peptides (A) generated via sequential proteolysis of amyloid precursor protein (APP). Cleavage of APP by ?- and - secretases releases the entire ectodomain, leaving behind membrane bound C-terminal fragments (CTF) capable of mediating intracellular signaling until they are further processed by ?-secretase. In order to activate in a constitutive manner putative signaling associated with APP-CTF, we have designed a membrane-tethered APP cytoplasmic domain (mAICD). We found that accumulation of APP-CTFs generated by processing of APP or expression of mAICD (but not AICD) results in adenylate cyclase-dependent activation of PKA, inhibition of GSK3, and enhanced axondendritic arborization in primary cortical neurons. We identified a novel interaction between APP intracellular domain and the heterotrimeric G-protein subunit G?S. By mutagenesis of the interaction motif identified within APP as well as expression of a dominant negative G?S mutant, we demonstrated that interaction with G?S and subsequent G?S coupling to adenylate cyclase are essential for membrane-bound APP intracellular domain-induced neurite outgrowth. Moreover, our preliminary results indicate that mutation of a previously described G?O binding motif of APP-CTF also reduces dendritic outgrowth. Thus, by analogy to other G-protein coupled receptors, it is possible that G- protein-mediated signaling through APP-CTF involves stochastic or simultaneous binding of G?O and G?S on the same APP molecule allowing dynamic regulation of APP function in neuronal morphology and neuronal dysfunction. Interestingly, APP processing and A production is a highly regulated process under the control of a number of phosphorylation events that could be affected by G-protein coupled receptor signaling cascades. Based on these findings, we hypothesize that spatiotemporal signaling of APP cytoplasmic domain with G?S/G?O-proteins could selectively affect axodendritic development and impact on AD pathogenesis. In order to investigate this hypothesis, we propose to examine (1) if axodendritic localization of -CTF is regulated by G- protein interactions;(2) characterize how -CTFG-protein interactions selectively affect axodendritic signaling;and (3) elucidate how spatiotemporal localization of APP-CTF affects full-length APP processing and A production. Altogether, our study provides clear evidence that APP intracellular domain has a non- transcriptional role in regulating neurite outgrowth through its membrane association via cAMP-dependent signaling and GSK3 inhibition - two processes that are known to have a role in memory consolidation, A production and Tau-associated pathology. Our investigation will address the importance of a previously unrecognized intracellular signaling pathway associated with APP-CTF. A better understanding of APP-CTF and its associated signaling partners might provide important insights into the cellular mechanisms by which APP-CTF affects synaptic function and A production, thus have an impact on AD pathogenesis.
Our recent investigation identified a novel functional coupling of amyloid precursor protein C-terminal fragment with GaS-protein at the neuronal membrane, which elicits a dual response in cultured neurons - PKA activation and GSK3 inhibition. Clear evidence in the literature suggests that these dual-signaling events would favor axodendritic development and also impact on neuronal function. Using fluorescent chimeric proteins and FRET probes for intracellular signaling, we propose to elucidate spatiotemporal regulation of amyloid precursor protein C-terminal fragment-associated signaling in neurons and its significance to neuronal function and dysfunction.
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