Hemizygous deletion of Tbk1 worsens neuromuscular junction pathology in TDP-43 transgenic mice

Mutations in the genes TARDBP (encoding the TDP-43 protein) and TBK1 can cause familial ALS. Neuronal cytoplasmatic accumulations of the misfolded, hyperphosphorylated RNA-binding protein TDP-43 are the pathological hallmark of most ALS cases and have been suggested to be a key aspect of ALS pathogenesis. Pharmacological induction of autophagy has been shown to reduce mutant TDP-43 aggregates and alleviate motor deficits in mice. TBK1 is exemplary for several other ALS genes that regulate autophagy. Consequently, we employed double mutant mice with both a heterozygous Tbk1 deletion and transgenic expression of human TDP-43G298S to test the hypothesis that impaired autophagy reduces intracellular clearance of an aggregation-prone protein and enhances toxicity of mutant TDP-43. The heterozygous deletion of Tbk1 did not change expression or cellular distribution of TDP-43 protein, motor neuron loss or reactive gliosis in the spinal cord of double-mutant mice at the age of 19 months. However, it aggravated muscle denervation and, albeit to a small and variable degree, motor dysfunction in TDP-43 G298S transgenic mice, as similarly observed in the SOD1 G93A transgenic mouse model for ALS before. Conclusively, our findings suggest that TBK1 mutations can affect the neuromuscular synapse.