Compelling evidence suggests that oligomeric Aβ plays a crucial neurotoxic and synaptotoxic role in Alzheimer’s disease (AD), and that hyperphosphorylation of the microtubule associated protein (MAP) tau mediates or facilitates Aβ toxicity. The nature of the link between Aβ and tau in causing AD has however remained largely unexplained, casting doubt on the amyloid hypothesis itself. In neurons, control of microtubule dynamics and tubulin modifications that accumulate on stable microtubules is necessary for multiple homeostatic and regulated functions, including long-distance transport and synaptic activity. Thus, regulation of the ratio between stable and dynamic microtubules is crucial to avoid disease. To date, almost nothing is known about whether induction of hyperstable and modified microtubules is a primary activity of Aβ that contributes to tau hyperphosphorylation and synaptotoxicity. We have preliminary data that detyrosinated tubulin is enriched in hippocampal tissue of AD patients and animal models of AD, and that accumulation of detyrosinated tubulin may induce tau hyperphosphorylation in primary neurons. In addition, we found that acute incubation of primary neurons with oligomeric Aβ1-42 generated detyrosinated MTs by transient microtubule hyperstabilization. Inhibition of the formin mDia1, a positive regulator of microtubule stability, suppressed this activity, affected tau hyperphosphorylation and rescued synaptotoxicity induced by Aβ in vitro. The overall objective of this proposal is to test the paradigm- shifting hypothesis that oligomeric Aβ acutely induces hyperstable detyrosinated microtubules through the activation of mDia1, and that tubulin detyrosination contributes to tau hyperphosphorylation as part of a negative feedback loop to maintain appropriate levels of dynamic and unmodified microtubules. In this proposal, we will characterize the nature of this microtubule hyperstabilization in neurites and at synaptic sites, and investigate whether APP and integrin signaling pathways are required for this Aβ-driven microtubule activity. In addition, we will test the role of mDia1 in mediating Aβ-synaptotoxicity in vivo, and examine the molecular mechanisms by which mDia1-synaptotoxicity occurs. Our proposal relies on a multidisciplinary effort to test a pathogenic role for formin-mediated regulation of microtubule stability by Aβ and the involvement of tubulin detyrosination in the induction of tau hyperphosphorylation and neuronal injury. Our studies will test a unifying theory for the pathogenesis of AD and examine the role for mDia1 and possibly other formins as potential targets in drug therapies aimed at rescuing Aβ and phospho-...