Tau is a microtubule-associated protein that is classically thought to play a role in stabilizing microtubules and the pathological accumulation of tau protein is a hallmark of several diseases collectively known as tauopathies, including Alzheimer’s disease (AD). Despite the clear implications for tau playing a critical role in tauopathies, many questions regarding its deposition in disease and mechanisms of toxicity remain unanswered. This dissertation was aimed at addressing two key questions in the field. 1) Does tau deposition occur first in the axons of affected neurons before proceeding to the somatodendritic compartment? 2) Does pathological modification of tau cause abnormalities in the ability of tau to modulate protein phosphatase 1 (PP1)? A long-held hypothesis on the progressive deposition of tau pathology in AD is that pathological tau accumulates first in axons of neurons and then progresses back into the cell bodies to form neurofibrillary tangles, however, studies have not directly analyzed this relationship in human tissue. In the early phases of tau deposition, both AT8 phosphorylation and exposure of the amino terminus of tau occur in tauopathies, and these modifications are linked to mechanisms of synaptic and axonal dysfunction. Here, the hippocampus of 44 well-characterized human samples from cases ranging between non-demented and mild cognitively impaired were examined for AT8 phosphorylation, amino terminus exposure, and amyloid- (Aβ) pathology in the axons and neuronal cell bodies within strata containing the CA3-Schaffer collateral and dentate granule-mossy fiber pathways. We show that tau pathology first appears in the axonal compartment of affected neurons in the absence of observable tau pathology in the corresponding cell bodies and independent of the presence of Aβ pathologies. Using the axonal marker, SMI-312, we confirmed that the majority of tau pathology-positive neuropil threads were axonal in origin. These results support the hypothesis that AT8 phosphorylation and PAD exposure are early pathological events and that the deposition of tau pathology occurs first in the axonal compartment prior to observable pathology in the cell bodies of affected neuronal pathways. The functional implications of AT8 and PAD-exposed tau deposition early in the axons of affected neurons is important because of a recently identified mechanism where these pathogenic forms of tau activate a PP1-dependent signaling pathway and lead to disruption of axonal functions. However, the connection between tau and PP1 was not defined. Here, we performed detailed studies on the interaction between tau and PP1 and subsequent effects on PP1 activity. Wild-type tau interacts with and activates PP1α and γ, but shows little to no interaction with PP1β, and this effect depends primarily on the microtubule binding repeats in tau. Additionally, AT8 tau increased the interactions with and activity of PP1γ, while deletion of PAD in the presence of AT8 reduced this interaction. These results suggest that tau’s function likely extends beyond stabilizing microtubules to include regulation of PP1 signaling cascades, and disease-associated tau phosphorylation may alter this function. Collectively, this work suggests forms of pathological tau, such as AT8 phospho-tau, that alter PP1 signaling and disrupt axonal function deposit in the axons of affected hippocampal neurons early during disease pathogenesis and prior to their appearance in the somatodendritic compartment of neurons.
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