The accumulation of A?1-42 peptide fragments of the amyloid precursor protein (APP) and neurofibrillary tangles (NFTs) are the cellular hallmarks of Alzheimer's disease (AD). However, the relationship between APP metabolism and hyperphosphorylation of the microtubule associated protein tau, the main component of NFTs, is poorly understood. London APP (V717I) is the most common familial AD (fAD) variant of APP, and neurons derived from induced pluripotent stem cells (iPSCs) from patients harboring this mutation have been shown to possess increased A?1-42 levels, disrupted APP intracellular localization, and accumulation of hyphosphorylated tau. We hypothesize that a pathogenic feature of disrupted APP metabolism and accumulation of A?1-42 peptide in AD is to activate pathways that inhibit microtubule dynamics while inducing tubulin post-translational modifications, and that these changes trigger a cellular stress response that leads to increased tau protein phosphorylation as an attempt to restore normal microtubule behavior. In support of this model, we have compelling evidence that: 1) stabilization of neuronal dynamic microtubules by the formin mDia1 contributes to oligomeric A?1- 42 synaptotoxicity in a APP dependent manner, and inhibition of microtubule dynamics and induction of tubulin post-translational modifications alone promotes tau hyperphosphorylation and tau-dependent synaptotoxicity; 2) tubulin post-translational modifications preferentially accumulate in pyramidal neurons with low levels of phosphorylated tau in post-mortem tissue from AD patients but not in age-matched healthy controls, suggesting that inhibition of microtubule dynamics may occur early during disease progression and promote tau hyperphosphorylation; 3) preliminary analyses of human cortical neurons derived from a London APP (V717I) knocked in iPSC line show increased levels of detyrosinated tubulin at the onset of phosphorylated tau accumulation, suggesting that premature aging of the MT cytoskeleton also occurs in this human cellular model of fAD. Our pilot study will investigate whether microtubule dynamics and tubulin post-translational modifications are affected in axons/dendrites and synapses relative to phosphorylated tau accumulation in cortical neurons derived from London APP (V717I) mutation knocked-in iPSC lines. We will further test the role of the formin mDia1 in modulating the microtubule cytoskeleton, and the increase in phosphorylated tau observed in this model of fAD. By the completion of this project, we should obtain the first comprehensive characterization of the behavior and modifications of the neuronal microtubule cytoskeleton relative to phosphorylated tau accumulation in a cellular model of fAD, as well as preliminary evidence addressing whether formin-mediated stabilization of dynamic microtubules and induction of tubulin post-translational modifications are early pathogenic features and contributing factors to phospho-tau accumulation in fAD.
Our studies will test a microtubule-based unifying hypothesis for the pathogenesis of Alzheimer's disease (AD) and examine the pathogenic role of the formin mDia1, a member of a class of cytoskeleton regulators that may be targeted in therapies aimed at rescuing both disrupted APP processing and phospho-tau toxicity in AD. This work will pave the way for analysis of early changes in microtubule behavior from other familial AD mutations and provide a new paradigm for the role of the microtubule cytoskeleton and formin function in the onset of tau pathology in AD, and possibly other neurodegenerative conditions.
