Phosphorylation but Not Oligomerization Drives the Accumulation of Tau with Nucleoporin Nup98

Abstract

Tau is a neuronal protein that stabilizes axonal microtubules (MTs) in the central nervous system. In Alzheimer’s disease (AD) and other tauopathies, phosphorylated Tau accumulates in intracellular aggregates, a pathological hallmark of these diseases. However, the chronological order of pathological changes in Tau prior to its cytosolic aggregation remains unresolved. These include its phosphorylation and detachment from MTs, mislocalization into the somatodendritic compartment, and oligomerization in the cytosol. Recently, we showed that Tau can interact with phenylalanine-glycine (FG)-rich nucleoporins (Nups), including Nup98, that form a diffusion barrier inside nuclear pore complexes (NPCs), leading to defects in nucleocytoplasmic transport. Here, we used surface plasmon resonance (SPR) and bio-layer interferometry (BLI) to investigate the molecular details of Tau:Nup98 interactions and determined how Tau phosphorylation and oligomerization impact the interactions. Importantly, phosphorylation, but not acetylation, strongly facilitates the accumulation of Tau with Nup98. Oligomerization, however, seems to inhibit Tau:Nup98 interactions, suggesting that Tau-FG Nup interactions occur prior to oligomerization. Overall, these results provide fundamental insights into the molecular mechanisms of Tau-FG Nup interactions within NPCs, which might explain how stress-and disease-associated posttranslational modifications (PTMs) may lead to Tau-induced nucleocytoplasmic transport (NCT) failure. Intervention strategies that could rescue Tau-induced NCT failure in AD and tauopathies will be further discussed.

Tau liquid–liquid phase separation: At the crossroads of tau physiology and tauopathy

“…In the case of self-coacervation of full-length tau, hydrophobic interactions may be weak enough to be ignored compared to the electrostatic interactions. Hence, no effect of 1,6-hexanediol, an aliphatic alcohol that can dissolve hydrophobic interaction-mediated condensates, was observed on selfcoacervation (Hochmair et al, 2022;Najafi et al, 2021). However, when the electrostatic force weakens, the impact of hydrophobic interactions becomes more evident, as observed in the case of phase separation of hyperphosphorylated tau (Wegmann et al, 2018).…”

“…Before droplet generation, tau monomers (R Mono ~10 nm) seem to coexist with tau clusters. The Wegmann group observed electrostatic interaction-dependent spontaneous formation of mesoscopic clusters (R Clust ~100-200 nm) of full-length tau below C sat (~50-100 μM) (Ambadipudi et al, 2017;Hochmair et al, 2022;Wegmann et al, 2018), which disappeared under phase separation enabling conditions in vitro. The formation of these clusters and their size depended on tau concentration in the media (Hochmair et al, 2022).…”

Nuclear pore dysfunction and disease: a complex opportunity

“…At the cellular level, tauopathies are characterized by the accumulation of tau protein aggregates in the brain [ 113 ]. In healthy neurons, tau binds to microtubules and stabilizes the cytoskeleton [ 11 , 113 , 114 ], and the affinity of tau for microtubules is regulated by its phosphorylation [ 113 ]. In several tauopathies, specific sites on tau become hyperphosphorylated, impairing the interaction between tau and microtubules, leading to microtubule destabilization and tau mislocalization [ 9 , 11 , 14 , 113 , 114 ].…”

“…In AD, tau hyperphosphorylation is correlated with the severity of neurodegeneration [ 9 , 113 ]. When tau is hyperphosphorylated, it interacts with the central channel Nups, Nup62, and Nup98, leading to their co-aggregation with tau in the cytoplasm and subsequent defects in NCT [ 9 , 11 , 14 ]. Nup98 cytoplasmic mislocalization occurs in other tauopathies as well, including FTD, CBD, and progressive supranuclear palsy (PSP) [ 12 ].…”

Nucleoporin 98 mislocalization is a common feature in primary tauopathies

“…Though the current study does not look into the mechanism behind the Nup98 mislocalization, a direct interaction of pathological phospho-tau and Nup98 could be the cause of mislocalization of Nup98 in these tauopathies based on the fact that there exists a correlation between the number of AT8-positive neurons and neurons with abnormal nuclear and cytoplasmic localization of Nup98, combined with the evidence of interaction between Nup98 and tau in Alzheimer’s disease. 7 The mislocalization of Nup98 suggests that the NCT in these tauopathies is compromised. The disruption of the NCT caused by an interaction between phospho-tau and Nup98, a common feature in tauopathies, provides an opportunity to perhaps come up with a common therapeutic strategy to combat diverse primary neuropathies.…”