Chronic and age-dependent effects of the spongiform neurodegeneration-associated MGRN1 E3 ubiquitin ligase on mitochondrial homeostasis

Spongiform encephalopathy is an intriguing yet poorly understood neuropathology characterized by vacuoles, demyelination, and gliosis. It is observed in patients with prion disease, primary mitochondrial disease, HIV-1 infection of the brain, and some inherited disorders, but the underlying mechanism of disease remains unclear. The brains of mice lacking the MGRN1 E3 ubiquitin ligase develop vacuoles by 9 months of age. MGRN1-dependent ubiquitination has been reported to regulate mitofusin 1 and GP78, suggesting MGRN1 may have a direct effect on mitochondrial homeostasis. Here, we demonstrate that some MGRN1 localizes to mitochondria, most likely due to N-myristoylation, and mitochondria in cells from Mgrn1 null mutant mice display fragmentation and depolarization without recruitment of the parkin E3 ubiquitin ligase. The late onset of pathology in the brains of Mgrn1 null mutant mice suggests that a further, age-dependent effect on mitochondrial homeostasis may be required to trigger vacuolation. Parkin protein and mRNA levels showed a significant decline in the brains of Mgrn1 null mutant mice by 12 months of age. To test whether loss of parkin triggers vacuolation through a synergistic effect, we generated Mgrn1; parkin double mutant mice. By 1 month of age, their brains demonstrated more severe mitochondrial dysfunction than Mgrn1 null mutants, but there was no effect on the age-of-onset of spongiform neurodegeneration. Expression of the ATF4 transcription factor, a key regulator of the mitochondrial stress response, also declined in the brains of aged Mgrn1 null mutant mice. Together, the data presented here indicate that loss of MGRN1 has early, direct effects on mitochondrial homeostasis and late, indirect effects on the ability of cells to respond to mitochondrial stress.