Axodendritic signaling of APP-CTF

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Alzheimer's disease (AD) is pathologically characterized by the accumulation of ß-amyloid peptides (Aß) generated via sequential proteolysis of amyloid precursor protein (APP). Cleavage of APP by α- and ß- secretases releases the entire ectodomain, leaving behind membrane bound C-terminal fragments (CTF) capable of mediating intracellular signaling until they are further processed by γ-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 GSK3ß, and enhanced axondendritic arborization in primary cortical neurons. We identified a novel interaction between APP intracellular domain and the heterotrimeric G-protein subunit GαS. 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 GαS and subsequent GαS 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 GαO 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 GαO and GαS on the same APP molecule allowing dynamic regulation of APP function in neuronal morphology and neuronal dysfunction. Interestingly, APP processing and Aß 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 GαS/GαO-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 ß-CTF is regulated by G- protein interactions; (2) characterize how ß-CTFG-protein interactions selectively affect axodendritic signaling; and (3) elucidate how spatiotemporal localization of APP-CTF affects full-length APP processing and Aß 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 GSK3ß inhibition - two processes that are known to have a role in memory consolidation, Aß 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 Aß production, thus have an impact on AD pathogenesis.