Miga Mediated Endoplasmic Reticulum–mitochondria Contact Sites Regulate Neuronal Homeostasis

Endoplasmic reticulum(ER)–mitochondria contact sites(ERMCSs)are dynamic contact regions between the ER and mitochondria,which are crucial for multiple cellular processes such as calcium signaling,lipid transport,mitochondrial dynamics,and autophagy.Although ERMCSs have drawn highly attention,the molecular organization,functions,regulation of ERMCSs,and the physiological roles of altered ERMCSs are not fully understood in higher eukaryotes.Abnormal ERMCSs have been associated with multiple human neurological diseases.For example,dramatic increase of ERMCSs had been observed in the brains of the Alzheimer disease patients.However,the evidence is still lacking regarding whether the increase of ERMCSs causes the disease conditions.In this study,we found that Miga markedly increases ERMCSs and causes severe neurodegeneration upon overexpression in fly eyes.The photoreceptor cells were totally lost in the 30 days old flies with Miga overexpression.We found that mitochondrial outer membrane located protein Miga interacts with an ER protein Vap33 through its FFAT-like motif.An amyotrophic lateral sclerosis(ALS)disease related Vap33 mutation considerably reduces its interaction with Miga.The overexpression of Miga lacking FFAT-like motif or knocking down Vap33 expression while overexpression of Miga at the same time did not increase ERMCSs nor cause eye degeneration,suggesting Miga induced eye degeneration depends on ERMCSs formation.Consistent with these observations,the overexpression of PTPIP51,a protein mediates ERMCSs in mammals,or an artificial ER-mitochondrial tether enhanced ERMCSs formation and cause severe eye degeneration.It indicates that the proper ERMCSs and the distance between ER and mitochondria are critical for neuronal homeostasis.Moreover,we found that multiple serine/threonine residues of Miga were phosphorylated.The phosphorylation of the serine/threonine residues inside and nearby FFAT-like motif was required for Miga's interaction with Vap33 and Miga-mediated ERMCSs formation.The interaction between Miga and Vap33 further enhance the hyperphosphorylation of Miga in several clusters upstream of FFAT-like motif.The phosphorylation of these sites were not required for ERMCSs formation but could fine-tuned Miga activity.We further identified that Ca MKII and CKI are required for the hyperphosphorylation of Miga.Miga has two orthologs in mammals: Miga1 and Miga2.We found that MIGA2 binds to VAPA/B and mediates MERCSs formation in culture cells and play similar roles as Miga.In summary,we identified Miga as a novel component in ERMCSs and provided direct evidence that excessive ERMCSs lead to neurodegeneration.This study shed new lights on the molecular mechanisms underlying neurodegeneration diseases.