Tau phosphorylation and PAD exposure in regulation of axonal growth

Abstract

Introduction: Tau is a microtubule associated phosphoprotein found principally in neurons. Prevailing dogma continues to define microtubule stabilization as the major function of tau in vivo, despite several lines of evidence suggesting this is not the case. Most importantly, tau null mice have deficits in axonal outgrowth and neuronal migration while still possessing an extensive microtubule network. Instead, mounting evidence suggests that tau may have a major function in the regulation of fast axonal transport (FAT) through activation of neuronal signaling pathways. Previous studies identified a phosphatase activating domain (PAD) at the tau N-terminal that is normally sequestered, but is constitutively exposed in tauopathies. When exposed, the PAD activates a signaling cascade involving PP1 and GSK3β which affects cellular functions including release of cargo from kinesin. Furthermore, we discovered that PAD exposure can be regulated by a single phosphorylation at T205. Exposure of the PAD is an early event in multiple tauopathies and a major contributing factor to neurodegeneration associated with tau hyperphosphorylation. However, effects of tau PAD exposure on anterograde FAT raised the interesting possibility that this pathway may be a mechanism for physiological regulation of cargo delivery through site-specific phosphorylation of tau and transient activation of PP1 and GSK3β. Significantly, there is already evidence of local control of PP1 and GSK3β at sites which require cargo delivery.Methods: To investigate this hypothesis, first we evaluated cellular localization of tau PAD exposure, pT205 tau phosphorylation, and active GSK3β in primary hippocampal neurons during development. Second, we analyzed the axonal outgrowth of tau knockout neurons following transfection with full length hTau40-WT, hTau40-ΔPAD, or hTau40-T205A.Results and Discussion: The results presented here suggest that transient activation of a PP1-GSK3β signaling pathway through locally regulated PAD exposure is a mechanism for cargo delivery, and thereby important for neurite outgrowth of developing neurons.

The γ-Adducin 1–357 fragment promotes tau pathology

“…Building upon this foundation, our current study demonstrates that γ-adducin 1–357 fragment enhances tau phosphorylation. It has been reported that over phosphorylation of tau also affects neurite outgrowth ( Morris and Brady, 2022 ). Thus, γ-adducin 1–357 fragment may not only directly influence the formation of actin filaments, but also interfere with neurite outgrowth by enhancing tau phosphorylation.…”

Axon-Autonomous Effects of the Amyloid Precursor Protein Intracellular Domain (AICD) on Kinase Signaling and Fast Axonal Transport

“…Interestingly, our data show that axonal GSK3 is not activated by AICD, nor does this kinase mediate its inhibitory effect on FAT. This is of particular interest, as prior studies have documented GSK3β activation in AICD-overexpressing mice [17,18,43] and AD-affected brain tissues [3]. One possible explanation for these different effects of AICD on different pathways in distinct experimental settings is that some effects are restricted or markedly more prominent on specific subcellular compartments.…”

“…For example, phosphorylation of tau at specific sites can promote activation of a phosphatase-dependent pathway leading to GSK3 activation [36,[44][45][46]. Studies showing AICD-dependent activation of GSK3β or protein kinase A (PKA) all relied on analysis of whole-brain or whole-cell lysates [16][17][18]43,47]. In those studies, the impact of AICD was reported to depend on kinase activation as well as nuclear signaling, and modulation of NMDAR and L-type Ca 2+ channels, which are lacking in the axoplasm [15,16,[48][49][50][51].…”

“…Disruption of kinase signaling in neurons may also have additional effects. For example, studies using whole-brain or whole-cell lysates, which do not distinguish between effects in soma and cellular processes or terminals, described additional kinases that were activated by transgenic expression of AICD [17,18,43]. This may reflect the different molecular composition and environment of specific neuronal subcompartments other than axons, where AICD would disrupt other pathways.…”

Biochemical approaches to assess the impact of post-translational modifications on pathogenic tau conformations using recombinant protein

“…In fact, several modifications cause the adoption of PAD-exposed conformations and/or cause toxicity through PAD-dependent mechanisms, including specific phosphorylation modifications, multimeric aggregates, and mutant forms of tau associated with inherited tauopathies [ 64–66 , 92–95 ]. Furthermore, mimicking phosphorylation of tau at the S199/S202/T205 and PHF-1 epitopes (S396/S404) makes the paperclip structure more compact [ 51 ].…”