Natural and Induced Mitochondrial Phosphate Carrier Loss

Abstract

Edited by F. Anne StephensonThe 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 ϳ60% was still compatible with uncompromised maximal oxidative phosphorylation (oxphos), whereas lower maximal oxphos was evident at ϳ85% 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.

Mitochondrial copper and phosphate transporter specificity was defined early in the evolution of eukaryotes

“…Here, we directly addressed this issue by focusing on Cu and phosphate transport, which, in mammals, is mediated by the single MCF transporter SLC25A3. Multiple studies clearly connect SLC25A3 to phosphate transport, and mutations in SLC25A3 lead to skeletal muscle myopathy and heart disease in humans ( Boulet et al, 2018 ; Seifert et al, 2016 ; Bhoj et al, 2015 ; Kwong et al, 2014 ; Mayr et al, 2011 ; Mayr et al, 2007 ) and cardiac hypertrophy in mice ( Kwong et al, 2014 ). MEFs derived from the heart-specific Slc25a3 knockout mouse exhibit clear COX and SOD1 defects that can be rescued by overexpression of a Slc25a3 cDNA or addition of Cu ( Boulet et al, 2018 ).…”

“…SLC25A3 is essential in mammals as the homozygous deletion is embryonic lethal. While mammals do express two SLC25A3 isoforms, isoform A is expressed primarily in heart and skeletal muscle whereas isoform B is expressed in all tissues ( Fiermonte et al, 1998 ; Seifert et al, 2016 ; Kwong et al, 2014 ). Therefore, it is unlikely that the isoforms provide the functional redundancy that would be afforded via gene duplication or retention of MIR1 .…”

Control and Regulation of Substrate Selection in Cytoplasmic and Mitochondrial Catabolic Networks. A Systems Biology Analysis

“…A remarkable and rather unexpected result is the relatively large control exerted by the mitochondrial PiC over β-oxidation, and its broad influence throughout central catabolism ( Figure 3 – 5 , 7 ). Deficiencies in the human mitochondrial PiC (SLC25A3, OMIM 600370), leading to homozygous-lethality, have been associated with inborn errors of metabolism 1 ( Mayr et al, 2007 ), and its impact on mitochondrial and cellular energetics is just starting to be recognized ( Seifert et al, 2016 ). The rather unexplored impact of the PiC deserves further research.…”

Molecular regulation of MCU: Implications in physiology and disease

“…Patients harboring frame-shift mutation (c.158–9A→G and c.215G→A) in the gene have been identified and present with hypertrophic cardiomyopathy, muscular hypotonia and lactic acidosis [66–68]. Assessment of skin fibroblasts from control and PiC mutation revealed a decrease in oxygen consumption capacity in the mutants compared to the control [69] Although the fundamental requirement of Pi for calcium uptake and ox-phos was long established, a direct involvement of Pi in Ca 2+ uptake in vivo has not been shown. An attempt to answer this question was made in cardiac specific knock-out of slc25A3.…”