Abstract
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 (>85%) 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.
Highlights
The relevance of mitochondrial phosphate carrier (PiC), encoded by SLC25A3, in bioenergetics is well accepted
Using skin fibroblasts from an individual expressing these variants and HeLa cells with varying degrees of PiC depletion, PiC loss of ϳ60% was still compatible with uncompromised maximal oxidative phosphorylation, whereas lower maximal oxphos was evident at ϳ85% PiC depletion
In primary fibroblasts harboring the compound heterozygous PiC mutation and one potentially functional allele, PiC protein abundance was substantially lower, and the cells were not phenotypically normal; the most dramatic phenotype was a proliferation time that was approximately half the control rate and an inability to divide when supplied with only mitochondrial substrates
Summary
PiC, mitochondrial phosphate carrier; oxphos, oxidative phosphorylation; PiC-A, PiC isoform A; PiC-B, PiC isoform B; Ctrl, control; JO2, O2 consumption; PA-GFP, photoactivatable green fluorescent protein; RPA, region of photoactivation; MFN1, mitofusin 1; MFN2, mitofusin 2; kd, knockdown; FCCP, carbonyl cyanide p-trifluoromethoxyphenylhydrazone; mtDsRed, mitochondrial matrix-targeted DsRed; mtPAGFP, mitochondrial matrix-targeted photoactivatable GFP; PiC mutant, individual expressing the compound heterozygous PiC mutation; MAS, mitochondrial assay solution; Hum, humanized; ANOVA, analysis of variance. We study the impact of PiC depletion on energy metabolism and mitochondrial dynamics
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