Tau aggregation results in an impaired ability to counteract microtubule destabilization via specific combinatorial tau phosphorylation patterns

Tau phosphorylation is a defining feature of Alzheimer’s disease, yet it also plays an important physiological role in regulating functional interactions with microtubules (MTs), particularly during normal neuronal development. While individual tau phosphorylation sites have been well-studied, how combinatorial phosphorylation impacts tau’s behavior in both contexts is poorly understood. We hypothesized that contrasting developmental and pathological models of tau phosphorylation would yield insights into mechanisms of impaired neuronal resilience in tauopathies. We use transgenic PS19 tau mice to demonstrate that increases in individual tau phospho-epitopes with aging do not correlate with their enrichment in tau aggregates in these mice. Using seeded aggregation of tau in neurons, we observe that tau aggregate formation does not necessarily result in MT destabilization; instead, aggregation diminishes tau’s ability to counteract destabilization when MTs are externally perturbed. We identify that combinations of tau phospho-signatures are responsive to MT destabilization, and phosphorylation at residues S235 and S262 as particularly critical for regulation of interactions with MTs. Phosphorylation of these residues was associated both with significantly reduced binding to MTs, as well as impaired phosphatase response in aggregates. As a whole, this work provides mechanistic insight into phospho-regulation of tau function and prompts reconsideration of the canonical view that tau phosphorylation precedes MT destabilization in disease. Rather, we provide compelling evidence that phosphorylation of tau may be a response to changes in MT stability and that cell death may result from aggregated tau’s inability to function in this manner.