TBK1 deficiency leads to TDP-43 pathology driven by endo-lysosomal dysfunction in human-induced motor neurons

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by motor neuron loss accompanied by cytoplasmic localization of TDP-43 proteins and their insoluble accumulations. Over the past decades, the development of whole-genome sequencing and whole-exome sequencing has substantially advanced our understanding of the genetic etiology of ALS. Haploinsufficiency of TBK1 has been found to associate with or cause ALS by recent exome sequencing. However, the exact mechanism of this connection remains elusive. Here, we generated a human stem cell model harboring loss-of-function mutations of TBK1 by gene editing. We found that TBK1 deficiency was sufficient to cause TDP-43 pathology in human-induced motor neurons. In addition, TBK1 interacted with endosomes and was required for normal endosomal maturation and subsequent lysosomal acidification. Surprisingly, TDP-43 pathology resulted from the dysfunctional endo-lysosomal pathway rather than the impaired autophagosome formation mechanism. Restoring TBK1 levels in TBK1 deficient cells re-sustained endo-lysosomal function and TDP-43 homeostasis and maintained motor neuron physiological functions. Notably, using patient-derived motor neurons, we found that haploinsufficiency of TBK1 sensitized neurons to lysosomal stress, and chemical regulators of the endo-lysosomal pathway rescued the neurodegenerative process. Together, our results revealed that the mechanisms of TBK1 in maintaining TDP-43 and motor neuron homeostasis and modulating endosomal maturation might be able to rescue neurodegenerative disease phenotypes caused by TBK1 deficiency.