IDH3A

Abstract: Alterations in the expression and activity of the transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator-1α (ppargc1a or PGC-1α) have been reported in multiple movement disorders, yet it is unclear how a lack of PGC-1α impacts transcription and function of the cerebellum, a region with high PGC-1α expression. We show here that mice lacking PGC-1α exhibit ataxia in addition to the previously described deficits in motor coordination. Using q-RT-PCR in cerebellar homogenates from PGC-1α -/- mice, we measured expression of 37 microarray-identified transcripts upregulated by PGC-1α in SH-SY5Y neuroblastoma cells with neuroanatomical overlap with PGC-1α or parvalbumin (PV), a calcium buffer highly expressed by Purkinje cells. We found significant reductions in transcripts with synaptic (complexin1, Cplx1; Pacsin2), structural (neurofilament heavy chain, Nefh), and metabolic (isocitrate dehydrogenase 3a, Idh3a; neutral cholesterol ester hydrolase 1, Nceh1; pyruvate dehydrogenase alpha 1, Pdha1; phytanoyl-CoA hydroxylase, Phyh; ubiquinol-cytochrome c reductase, Rieske iron-sulfur polypeptide 1, Uqcrfs1) functions. Using conditional deletion of PGC-1α in PV-positive neurons, we determined that 50% of PGC-1α expression and a reduction in a subset of these transcripts could be explained by its concentration in PV-positive neuronal populations in the cerbellum. To determine whether there were functional consequences associated with these changes, we conducted stereological counts and spike rate analysis in Purkinje cells, a cell type rich in PV, from PGC-1α -/- mice. We observed a significant loss of Purkinje cells by six weeks of age, and the remaining Purkinje cells exhibited a 50% reduction in spike rate. Together, these data highlight the complexity of PGC-1α’s actions in the central nervous system and suggest that dysfunction in multiple cell types contribute to motor deficits in the context of PGC-1α deficiency.

Abstract: Mitochondrial encephalopathies are a heterogeneous group of disorders which generally carries a grave prognosis. Using exome sequencing, we identified a homozygous mutation, Pro-304-His in the IDH3A gene, in a patient suffering from infantile encephalopathy with peripheral and autonomic nervous system involvement. Mammalian isocitrate dehydrogenase (IDH) 3 is a heterotetramer of 2alfa, 1beta, and 1gamma subunits, and IDH3A encodes the alfa subunit of the mitochondrial NAD+-dependent IDH. Here we show that in contrast to wild-type human IDH3A, the human IDH3A which harbor the p.Pro304His mutation does not complement the yeast Δidh1/Δidh2 growth defect on ethanol-acetate. We therefore propose that homozygosity for the p.Pro304His mutation is deleterious for mitochondrial NAD+-specific IDH3A activity in human. IDH3A now joins the list of TCA cycle-related proteins, which includes ACO2, DLD, SLC25A19, FH, and succinate dehydrogenase subunits, all associated with neurological disorders.

Abstract: The mitochondrial protease OMI (also known as HtrA2) has been implicated in Parkinson's Disease (PD) and deletion or protease domain point mutations have shown profound neuropathologies in mice. A beneficial role by OMI, in preserving cell viability, is assumed to occur via the avoidance of dysfunctional protein turnover. However relatively few substrates for mitochondrial Omi are known. Here we report our identification of three novel mitochondrial substrates that impact metabolism and ATP production. Using a dual proteomic approach we have identified three interactors based upon ability to bind to OMI, and/or to persist in the proteome after OMI activity has been selectively inhibited. One candidate, the chaperone HSPA8, was common to each independent study. Two others (PDHB subunit and IDH3A subunit) did not appear to bind to OMI, however persisted in the mito-proteome when OMI was inhibited. Pyruvate dehydrogenase (PDH) and isocitrate dehydrogenase (IDH) are two key Kreb's cycle enzymes that catalyse oxidative decarboxylation control points in mitochondrial respiration. We verified both PDHB and IDH3A co-immunoprecipitate with HSPA8 and after elution, were degraded by recombinant HtrA2 in vitro. Additionally our gene expression studies, using rotenone (an inhibitor of Complex I) showed Omi expression was silenced when pdhb and idh3a were increased when a sub-lethal dose was applied. However higher dose treatment caused increased Omi expression and decreased levels of pdhb and idh3a transcripts. This implicates mitochondrial OMI in a novel mechanism relating to metabolism.