Defined Tau Phosphospecies Differentially Inhibit Fast Axonal Transport Through Activation of Two Independent Signaling Pathways

Abstract

Tau protein is subject to phosphorylation by multiple kinases at more than 80 different sites. Some of these sites are associated with tau pathology and neurodegeneration, but other sites are modified in normal tau as well as in pathological tau. Although phosphorylation of tau at residues in the microtubule-binding repeats is thought to reduce tau association with microtubules, the functional consequences of other sites are poorly understood. The AT8 antibody recognizes a complex phosphoepitope site on tau that is detectable in a healthy brain but significantly increased in Alzheimer’s disease (AD) and other tauopathies. Previous studies showed that phosphorylation of tau at the AT8 site leads to exposure of an N-terminal sequence that promotes activation of a protein phosphatase 1 (PP1)/glycogen synthase 3 (GSK3) signaling pathway, which inhibits kinesin-1-based anterograde fast axonal transport (FAT). This finding suggests that phosphorylation may control tau conformation and function. However, the AT8 includes three distinct phosphorylated amino acids that may be differentially phosphorylated in normal and disease conditions. To evaluate the effects of specific phosphorylation sites in the AT8 epitope, recombinant, pseudophosphorylated tau proteins were perfused into the isolated squid axoplasm preparation to determine their effects on axonal signaling pathways and FAT. Results from these studies suggest a mechanism where specific phosphorylation events differentially impact tau conformation, promoting activation of independent signaling pathways that differentially affect FAT. Implications of findings here to our understanding of tau function in health and disease conditions are discussed.

Frontotemporal Lobar Dementia Mutant Tau Impairs Axonal Transport through a Protein Phosphatase 1γ-Dependent Mechanism

“…Based on our findings, the bulk of this disruption occurred through a specific PAD-and PP1γdependent mechanism. The negative impact on retrograde FAT is similar to that of phosphomimics at select sites in tau (e.g., S422, S199 and S202) that we previously tested in isolated squid axoplasm (Tiernan et al, 2016;Morris et al, 2020), suggesting that some pathogenic tau forms may also disrupt pathways involved in the regulation of retrograde FAT and cellular processes dependent on this direction of transport (e.g., neurotrophic factor signaling).…”

Tau: A Signaling Hub Protein

“…The highly dynamic conformational flexibility of tau, differentially impacted by phosphorylation of specific residues, might confer upon this protein its ability to work as a signaling hub and activate and/or modulate different signaling pathways (Smith et al, 2006;Kang et al, 2020). Collectively, these and other studies suggest a complex relationship between specific phosphorylation events in tau and their net impact on tau conformation, as well as the potential co-existence of additional biologically active domains in tau that, like PAD, may promote activation of other signaling pathways (Morris et al, 2020).…”

“…TNT1/2 antibodies also robustly label tau inclusions in several non-AD tauopathies including progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Pick's disease, and chronic traumatic encephalopathy (CTE; Kanaan et al, 2011Kanaan et al, , 2012Combs et al, 2016;Cox et al, 2016;Tiernan et al, 2016;Combs and Kanaan, 2017). Moreover, pathological forms of tau known to inhibit FAT including oligomers, AT8 phospho-tau, and tau phosphorylated at Ser422 are all recognized by TNT1 and TNT2 antibodies in the context of human tauopathies (Kanaan et al, 2011(Kanaan et al, , 2012(Kanaan et al, , 2016Patterson et al, 2011;Combs et al, 2016;Cox et al, 2016;Tiernan et al, 2016;Morris et al, 2020). Overall, this body of work strongly supports a mechanism linking several specific pathological tau species to aberrant PAD exposure, abnormal activation of a PP1-GSKβ pathway, and deficits in FAT.…”

Prion-like strain effects in tauopathies

“…Intracellular tau is primarily located in the axons and to a lesser extent in somatodendritic compartments such as the cell membrane (Arrasate et al 2000 ), mitochondria (Li et al 2016 ), and nucleus (Wang et al 1993 ). Depending on its cellular localization, tau also plays important roles in axonal transport (Rodríguez-Martín et al 2013 ; Lacovich et al 2017 ; Morris et al 2021 ), nucleic acid protection (Sultan et al 2011 ; Violet et al 2014 ), synaptic plasticity (Spires-Jones and Hyman 2014 ), and neuronal maturation (Fiock et al 2020 ).

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