Adult onset pan-neuronal human tau tubulin kinase 1 expression causes severe cerebellar neurodegeneration in mice
The kinase TTBK1 is predominantly expressed in the central nervous system and has been implicated in neurodegenerative diseases including Alzheimer’s disease, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis through its ability to phosphorylate the proteins tau and TDP-43. Mutations in the closely related gene TTBK2 cause spinocerebellar ataxia, type 11. However, it remains unknown whether altered TTBK1 activity alone can drive neurodegeneration. In order to characterize the consequences of neuronal TTBK1 upregulation in adult brains, we have generated a transgenic mouse model with inducible pan-neuronal expression of human TTBK1. We find that these inducible TTBK1 transgenic mice (iTTBK1 Tg) exhibit motor and cognitive phenotypes, including decreased grip strength, hyperactivity, limb-clasping, and spatial memory impairment. These behavioral phenotypes occur in conjunction with progressive weight loss, neuroinflammation, and severe cerebellar degeneration with Purkinje neuron loss. Phenotype onset begins weeks after TTBK1 induction, culminating in average mortality around 7 weeks post induction. The iTTBK1 Tg animals lack any obvious accumulation of pathological tau or TDP-43, indicating that TTBK1 expression drives neurodegeneration in the absence of detectable pathological protein deposition. In exploring TTBK1 functions, we identified the autophagy related protein GABARAP to be a novel interacting partner of TTBK1 and show that GABARAP protein levels increase in the brain following induction of TTBK1. These iTTBK1 Tg mice exhibit phenotypes reminiscent of spinocerebellar ataxia, and represent a new model of cerebellar neurodegeneration.