Miro1-mutant iPSC-derived Neurons reveal novel functions of Miro1 in mitochondrial respiration and dopamine handling

Abstract

The mitochondrial outer membrane protein Miro1 is essential for neuronal development and maintenance, and recent research indicates that Miro1 might further be implicated in Parkinson’s disease relevant neurodegenerative pathways. Miro1 is targeted by and interacts with several proteins connected to familial Parkinson’s disease, such as PINK1 and Parkin (Hsieh et al. 2016; Kazlauskaite et al. 2014; Klosowiak et al. 2016; Shaltouki et al. 2018; Wang and Schwarz 2009; Weihofen et al. 2009). Additionally, rare variants in the encoding gene RHOT1 were identified in sporadic Parkinson’s disease patients (Berenguer-Escuder et al. 2019; Grossmann et al. 2019). To better understand Miro1 involvement in Parkinson’s disease relevant pathways, I gene-edited three Miro1 mutations into previously established, healthy human induced pluripotent stem cells (Marrone et al. 2018): heterozygous R272Q [sporadic Parkinson’s disease (Grossmann et al. 2019)], homozygous S156A [preventing PINK1 phosphorylation (Wang et al. 2011)], and heterozygous K572R [preventing pSer65 Parkin ubiquitination (Kazlauskaite et al. 2014; Klosowiak et al. 2016)]. To investigate these mutations in a disease-relevant model, I differentiated neural precursor cell intermediates derived from the gene-edited stem cells into mid-brain specific dopaminergic neurons. Analysis of the putative PINK1 phosphorylation site Ser156 (Wang et al. 2011) in neurons reveals that Miro1 S156A causes a reduction of Miro1 protein levels without affecting gene expression but impairing its degradation upon mitochondrial depolarization. This is accompanied by loss of mitochondrial membrane potential resulting in reduced mitochondrial respiration. These findings revealed a correlation between Miro1 and the level of respiratory chain complexes in differentiated, postmitotic cells. Miro1 R272Q found in a sporadic patient (Grossmann et al. 2019) shares the reduction of mitochondrial respiration, but links to mitochondrial calcium instead of membrane potential. Additionally, mitochondria are more fragmented and show partial loss of cristae structure. The data point towards Miro1 R272Q disrupting the interaction of Miro1 with the mitochondrial calcium uniporter to modulate mitochondrial calcium uptake thus altering cytosolic calcium handling. Consequently, decreased 6 neurotransmitter uptake and monoamine oxidase activity alter dopamine handling in neurons. Inhibition of Parkin-mediated ubiquitination of Miro1 (Kazlauskaite et al. 2014; Klosowiak et al. 2016) in K572R neurons results in enlarged and fragmented mitochondria. This is concomitant with reduced membrane potential and increased turnover while Miro1 degradation and mitophagy are unaffected. Taken together, these novel isogenic Miro1 models shed light on domain-specific functions of Miro1. Miro1 S156A highlights the relationship between Miro1 protein levels and oxidative phosphorylation in different cell types and specific vulnerability of post-mitotic neurons. In contrast, a putative mechanism in neurons for the diseaseassociated Miro1 mutation R272Q involves mitochondrial calcium handling linking to dopamine homeostasis. The gene-edited stem cells generated in this study serve as powerful platform to investigate Miro1 function in any cell type and the research with differentiated neurons provides new insight into Miro1 function in Parkinson’s disease relevant pathways.