Natural and Induced Mitochondrial Phosphate Carrier Loss: DIFFERENTIAL DEPENDENCE OF MITOCHONDRIAL METABOLISM AND DYNAMICS AND CELL SURVIVAL ON THE EXTENT OF DEPLETION.

The relevance of mitochondrial phosphate carrier (PiC), encoded by SLC25A3, in bioenergetics is well accepted. However, little is known about the mechanisms mediating the cellular impairments induced by pathological SLC25A3 variants. To this end, we investigated the pathogenicity of a novel compound heterozygous mutation in SLC25A3 First, each variant was modeled in yeast, revealing that substituting GSSAS for QIP within the fifth matrix loop is incompatible with survival on non-fermentable substrate, whereas the L200W variant is functionally neutral. Next, using skin fibroblasts from an individual expressing these variants and HeLa cells with varying degrees of PiC depletion, PiC loss of ∼60% was still compatible with uncompromised maximal oxidative phosphorylation (oxphos), whereas lower maximal oxphos was evident at ∼85% PiC depletion. Furthermore, intact mutant fibroblasts displayed suppressed mitochondrial bioenergetics consistent with a lower substrate availability rather than phosphate limitation. This was accompanied by slowed proliferation in glucose-replete medium; however, proliferation ceased when only mitochondrial substrate was provided. Both mutant fibroblasts and HeLa cells with 60% PiC loss showed a less interconnected mitochondrial network and a mitochondrial fusion defect that is not explained by altered abundance of OPA1 or MFN1/2 or relative amount of different OPA1 forms. Altogether these results indicate that PiC depletion may need to be profound (\u3e85%) to substantially affect maximal oxphos and that pathogenesis associated with PiC depletion or loss of function may be independent of phosphate limitation when ATP requirements are not high

Exclusive neuronal expression of SUCLA2 in the human brain

SUCLA2 encodes the ATP-forming subunit (A-SUCL-) of succinyl-CoA ligase, an enzyme of the citric acid cycle. Mutations in SUCLA2 lead to a mitochondrial disorder manifesting as encephalomyopathy with dystonia, deafness and lesions in the basal ganglia. Despite the distinct brain pathology associated with SUCLA2 mutations, the precise localization of SUCLA2 protein has never been investigated. Here we show that immunoreactivity of A-SUCL- in surgical human cortical tissue samples was present exclusively in neurons, identified by their morphology and visualized by double labeling with a fluorescent Nissl dye. A-SUCL- immunoreactivity co-localized >99% with that of the d subunit of the mitochondrial F0-F1 ATP synthase. Specificity of the anti-A-SUCL- antiserum was verified by the absence of labeling in fibroblasts from a patient with a complete deletion of SUCLA2. A-SUCL- immunoreactivity was absent in glial cells, identified by antibodies directed against the glial markers GFAP and S100. Furthermore, in situ hybridization histochemistry demonstrated that SUCLA2 mRNA was present in Nissl-labeled neurons but not glial cells labeled with S100. Immunoreactivity of the GTP-forming subunit (G-SUCL-) encoded by SUCLG2, or in situ hybridization histochemistry for SUCLG2 mRNA could not be demonstrated in either neurons or astrocytes. Western blotting of post mortem brain samples revealed minor G-SUCL- immunoreactivity that was however, not upregulated in samples obtained from diabetic versus non-diabetic patients, as has been described for murine brain. Our work establishes that SUCLA2 is expressed exclusively in neurons in the human cerebral cortex

Two transgenic mouse models for beta subunit components of succinate-CoA ligase yielding pleiotropic metabolic alterations

Succinate-CoA ligase is a heterodimer enzyme composed of Suclg1 -alpha- and a substrate-specific Sucla2 or Suclg2 -beta- subunit yielding ATP or GTP, respectively. In humans, the deficiency of this enzyme leads to encephalomyopathy with, or without methylmalonyl aciduria, in addition to resulting in mitochondrial DNA depletion. We generated mice lacking either one Sucla2 or Suclg2 allele. Sucla2 heterozygote mice exhibited tissue- and age-dependent decreases in Sucla2 expression associated with decreases in ATP-forming activity, but rebound increases in cardiac Suclg2 expression and GTP-forming activity. Bioenergetic parameters including substrate-level phosphorylation were not different between wild type and Sucla2 heterozygote mice unless a submaximal pharmacological inhibition of succinate-CoA ligase was concomitantly present. mtDNA contents were moderately decreased, but blood carnitine esters were significantly elevated. Suclg2 heterozygote mice exhibited decreases in Suclg2 expression but no rebound increases in Sucla2 expression or changes in bioenergetic parameters. Surprisingly, deletion of one Suclg2 allele in Sucla2 heterozygote mice still led to a rebound but protracted increase in Suclg2 expression, yielding double heterozygote mice with no alterations in GTP-forming activity or substrate-level phosphorylation, but more pronounced changes in mtDNA content and blood carnitine esters, and an increase in succinate dehydrogenase activity. We conclude that a partial reduction in Sucla2 elicits rebound increases in Suclg2 expression which is sufficiently dominant to overcome even a concomitant deletion of one Suclg2 allele, pleiotropically affecting metabolic pathways associated with succinate-CoA ligase. These results as well as the availability of the transgenic mouse colonies will be of value in understanding succinate-CoA ligase deficiency

Localization of SUCLA2 and SUCLG2 subunits of succinyl CoA ligase within the cerebral cortex suggests the absence of matrix substrate-level phosphorylation in glial cells of the human brain.

We have recently shown that the ATP-forming SUCLA2 subunit of succinyl-CoA ligase, an enzyme of the citric acid cycle, is exclusively expressed in neurons of the human cerebral cortex; GFAP- and S100-positive astroglial cells did not exhibit immunohistoreactivity or in situ hybridization reactivity for either SUCLA2 or the GTP-forming SUCLG2. However, Western blotting of post mortem samples revealed a minor SUCLG2 immunoreactivity. In the present work we sought to identify the cell type(s) harboring SUCLG2 in paraformaldehyde-fixed, free-floating surgical human cortical tissue samples. Specificity of SUCLG2 antiserum was supported by co-localization with mitotracker orange staining of paraformaldehyde-fixed human fibroblast cultures, delineating the mitochondrial network. In human cortical tissue samples, microglia and oligodendroglia were identified by antibodies directed against Iba1 and myelin basic protein, respectively. Double immunofluorescence for SUCLG2 and Iba1 or myelin basic protein exhibited no co-staining; instead, SUCLG2 appeared to outline the cerebral microvasculature. In accordance to our previous work there was no co-localization of SUCLA2 immunoreactivity with either Iba1 or myelin basic protein. We conclude that SUCLG2 exist only in cells forming the vasculature or its contents in the human brain. The absence of SUCLA2 and SUCLG2 in human glia is in compliance with the presence of alternative pathways occurring in these cells, namely the GABA shunt and ketone body metabolism which do not require succinyl CoA ligase activity, and glutamate dehydrogenase 1, an enzyme exhibiting exquisite sensitivity to inhibition by GTP