Sulfite Alters the Mitochondrial Network in Molybdenum Cofactor Deficiency.

Anna-Theresa Mellis,Joshua Kohl,Albert L. Misko,Guenter Schwarz,Juliane Roeper

doi:10.3389/fgene.2020.594828

Anna-Theresa Mellis, Joshua Kohl + Show 3 more

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https://doi.org/10.3389/fgene.2020.594828

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Abstract

Molybdenum cofactor deficiency (MoCD) is an autosomal recessive disorder belonging to the large family of inborn errors in metabolism. Patients typically present with encephalopathy and seizures early after birth and develop severe neurodegeneration within the first few weeks of life. The main pathomechanism underlying MoCD is the loss of function of sulfite oxidase (SO), a molybdenum cofactor (Moco) dependent enzyme located in mitochondrial intermembrane space. SO catalyzes the oxidation of sulfite (SO32–) to sulfate (SO42–) in the terminal reaction of cysteine catabolism, and in the absence of its activity, sulfurous compounds such as SO32–, S-sulfocysteine, and thiosulfate accumulate in patients. Despite growing evidence that these compounds affect neuronal and mitochondrial function, the molecular basis of neuronal dysfunction and cell death in MoCD is still poorly understood. Here we show that mitochondria are severely affected by the loss of SO activity. SO-deficient mouse embryonic fibroblasts display reduced growth rates and impaired ATP production when cultured in galactose, which is an indicator of mitochondrial dysfunction. We also found that mitochondria in SO-deficient cells form a highly interconnected network compared to controls while displaying a slight decrease in motility and unchanged mitochondrial mass. Moreover, we show that the mitochondrial network is directly influenced by SO32–, as a moderate elevation of SO32– lead to the formation of an interconnected mitochondrial network, while high SO32– levels induced fragmentation. Finally, we found a highly interconnected mitochondrial network in MoCD patient-derived fibroblasts, similar to our findings in mouse-derived fibroblasts. We therefore conclude that altered mitochondrial dynamics are an important contributor to the disease phenotype and suggest that MoCD should be included among the mitochondrial disorders.

Highlights

Mitochondrial sulfite oxidase (SO) catalyzes the terminal step in the catabolism of cysteine and methionine, the oxidation of toxic sulfite (SO32−) to sulfate (SO42−), thereby passing two electrons to cytochrome c (Johnson and Rajagopalan, 1979)
Since previous studies indicate that SO32− impacts mitochondrial function (Vincent et al, 2004; Zhang et al, 2004; Grings et al, 2014), we first tested whether SO-deficient mouse embryonic fibroblast (MEF) cells (Figure 1A) generated from Suox−/− mice (Kohl et al unpublished results) exhibited signs of mitochondrial damage
We cultured WT and Suox−/− MEFs in either glucoseor galactose-containing medium, thereby forcing the cells to rely on oxidative phosphorylation (OXPHOS) as the main energy source in absence of glucose

Summary

Introduction

Mitochondrial sulfite oxidase (SO) catalyzes the terminal step in the catabolism of cysteine and methionine, the oxidation of toxic sulfite (SO32−) to sulfate (SO42−), thereby passing two electrons to cytochrome c (Johnson and Rajagopalan, 1979). SO may be impaired by mutations in the SUOX gene, leading to isolated sulfite oxidase deficiency (ISOD). SO may be compromised by mutations in the molybdenum cofactor (Moco) biosynthetic genes (MOCS1, MOCS2, MOCS3, GPHN), thereby leading to Moco deficiency (MoCD) and loss of the Moco dependent SO activity (Kohl et al, 2018). The most prominent biochemical hallmark of MoCD and ISOD is the accumulation of SO32− and the sulfite-cysteine adduct S-sulfocysteine in patient urine or plasma (Mudd et al, 1967). The disorders are distinguishable on the biochemical level due to the accumulation of xanthine and hypoxanthine, substrates of xanthine oxidase, another Moco enzyme, and diminished levels of uric acid in MoCD, but not ISOD (Schwarz et al, 2009; Schwahn et al, 2015)

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