Tau-tubulin kinase 1 enhances prefibrillar tau aggregation and motor neuron degeneration in P301L FTDP-17 tau-mutant mice

Tau-tubulin kinase-1 (TTBK1) phosphorylates microtubule-associated protein tau at specific serine/threonine residues found in paired helical filaments (PHFs), and its expression is up-regulated in the brain in Alzheimer disease, suggesting its role in tauopathy pathogenesis. To understand the effects of TTBK1 on tauopathy in vivo, we have developed bigenic mice overexpressing full-length TTBK1 and the P301L tau mutant. The bigenic mice show enhanced tau phosphorylation at multiple sites (AT8, 12E8, PHF-1, and pS422), tauC3-immunoreactive tau fragmentation, and accumulation of tau aggregates in cortical and hippocampal neurons at 12-13 mo of age. However, the phosphorylated tau aggregates were predominantly sarkosyl soluble and migrated in the light sucrose density fraction after discontinuous sucrose gradient ultracentrifugation, which suggests that they form small oligomers. The bigenic mice show significant locomotor dysfunction as determined by both rotorod and grip strength tests, as well as enhanced loss of motor neurons in the L4-L5 spinal cord. This neuronal dysfunction and degeneration was associated with increased levels of tau oligomers, cyclin-dependent protein kinase 5 activators p35 and p25, and pY216 phosphorylated glycogen synthase kinase 3-β. These data suggest that TTBK1 up-regulation enhances tau phosphorylation and oligomerization, whose toxicity results in enhanced neurodegeneration and locomotor dysfunction in a tauopathy animal model.