Steady-State Levels of Miro1 Linked to Phosphorylation at Serine 156 and Mitochondrial Respiration in Dopaminergic Neurons.

Miro1 has emerged as an interesting target to study Parkinson's disease-relevant pathways since it is a target of PINK1 and Parkin. Miro1 is a mitochondrial GTPase with the primary function of facilitating mitochondrial movement, and its knockout in mice is postnatally lethal. Here, we investigated the effect of the artificial RHOT1/Miro1 S156A mutation since it is a putative PINK1 phosphorylation site shown to be involved in Miro1 degradation and mitochondrial arrest during mitophagy. We gene-edited a homozygous phospho-null Miro1 S156A mutation in induced pluripotent stem cells to study the mutation in human dopaminergic neurons. This mutation causes a significant depletion of Miro1 steady-state protein levels and impairs further Miro1 degradation upon CCCP-induced mitophagy. However, mitochondrial mass measured by Tom20 protein levels, as well as mitochondrial area, are not affected in Miro1 S156A neurons. The mitochondria are slightly lengthened, which is in line with their increased turnover. Under basal conditions, we found no discernable effect of the mutation on mitochondrial movement in neurites. Interestingly, the S156A mutation leads to a significant reduction of mitochondrial oxygen consumption, which is accompanied by a depletion of OXPHOS complexes III and V. These effects are not mirrored by Miro1 knockdown in neuroblastoma cells, but they are observed upon differentiation. Undifferentiated Miro1 S156A neural precursor cells do not have decreased Miro1 levels nor OXPHOS complexes, suggesting that the effect of the mutation is tied to development. In mature dopaminergic neurons, the inhibition of Miro1 Ser156 phosphorylation elicits a mild loss of mitochondrial quality involving reduced mitochondrial membrane potential, which is sufficient to induce compensatory events involving OXPHOS. We suggest that the mechanism governing Miro1 steady-state levels depends on differentiation state and metabolic demand, thus underscoring the importance of this pathway in the pathobiology of Parkinson's disease.