LAUSR

Significance Mitochondrial dysfunction has been associated with many age-dependent neurodegenerative disorders such as Parkinson’s and Alzheimer’s disease, yet understanding how a neuron maintains a pool of functional mitochondria in sufficient density and location throughout axons in vivo still remains enigmatic. Through an unbiased in vivo forward genetic screen, we identified an endosomal sorting complexes required for transport component, TSG101, as a modulator of mitochondrial number and size in neurons and provide evidence to place TSG101 as a negative regulator of mitochondrial biogenesis. We further find that TSG101 does not contribute to Parkin/Pink1-mediated mitophagy and discovered that macroautophagy regulators ATG1 and ATG6 are dispensable in axonal mitochondrial number regulation. Through in vivo screening, it is possible to unravel translatable mechanisms of axonal mitochondrial maintenance. There is a tight association between mitochondrial dysfunction and neurodegenerative diseases and axons that are particularly vulnerable to degeneration, but how mitochondria are maintained in axons to support their physiology remains poorly defined. In an in vivo forward genetic screen for mutants altering axonal mitochondria, we identified tsg101. Neurons mutant for tsg101 exhibited an increase in mitochondrial number and decrease in mitochondrial size. TSG101 is best known as a component of the endosomal sorting complexes required for transport (ESCRT) complexes; however, loss of most other ESCRT components did not affect mitochondrial numbers or size, suggesting TSG101 regulates mitochondrial biology in a noncanonical, ESCRT-independent manner. The TSG101-mutant phenotype was not caused by lack of mitophagy, and we found that autophagy blockade was detrimental only to the mitochondria in the cell bodies, arguing mitophagy and autophagy are dispensable for the regulation of mitochondria number in axons. Interestingly, TSG101 mitochondrial phenotypes were instead caused by activation of PGC-1ɑ/Nrf2-dependent mitochondrial biogenesis, which was mTOR independent and TFEB dependent and required the mitochondrial fission–fusion machinery. Our work identifies a role for TSG101 in inhibiting mitochondrial biogenesis, which is essential for the maintenance of mitochondrial numbers and sizes, in the axonal compartment.