Abstract
Iron-sulfur (Fe-S) clusters are versatile cofactors involved in regulating multiple physiological activities, including energy generation through cellular respiration. Initially, the Fe-S clusters are assembled on a conserved scaffold protein, iron-sulfur cluster scaffold protein (ISCU), in coordination with iron and sulfur donor proteins in human mitochondria. Loss of ISCU function leads to myopathy, characterized by muscle wasting and cardiac hypertrophy. In addition to the homozygous ISCU mutation (g.7044G→C), compound heterozygous patients with severe myopathy have been identified to carry the c.149G→A missense mutation converting the glycine 50 residue to glutamate. However, the physiological defects and molecular mechanism associated with G50E mutation have not been elucidated. In this report, we uncover mechanistic insights concerning how the G50E ISCU mutation in humans leads to the development of severe ISCU myopathy, using a human cell line and yeast as the model systems. The biochemical results highlight that the G50E mutation results in compromised interaction with the sulfur donor NFS1 and the J-protein HSCB, thus impairing the rate of Fe-S cluster synthesis. As a result, electron transport chain complexes show significant reduction in their redox properties, leading to loss of cellular respiration. Furthermore, the G50E mutant mitochondria display enhancement in iron level and reactive oxygen species, thereby causing oxidative stress leading to impairment in the mitochondrial functions. Thus, our findings provide compelling evidence that the respiration defect due to impaired biogenesis of Fe-S clusters in myopathy patients leads to manifestation of complex clinical symptoms.
Highlights
Muscle-specific deficiency of iron-sulfur (Fe-S) cluster scaffold protein (ISCU) leads to myopathy
Myopathy-associated Glycine 50 Residue of ISCU Is Critical for Cell Viability—In humans, the synthesis of the Fe-S cluster utilizes the mitochondrial scaffold protein ISCU as a platform for the complex interactions between various intervening proteins (Fig. 1A)
Upon quantification of mean fluorescence intensity values, a 52% reduction in the overall mitochondrial mass was observed in the G50E mutant in comparison with the Wild type (WT) ISCU (Fig. 2J). These findings indicate that the G50E mutation in ISCU results in the reduced biogenesis of mitochondria along with loss of membrane potential
Summary
Muscle-specific deficiency of iron-sulfur (Fe-S) cluster scaffold protein (ISCU) leads to myopathy. Because Fe-S proteins play a critical role in a wide range of cellular activities, a mutation in different components of the synthesis machinery disrupts the process of Fe-S cluster biogenesis and is associated with multiple pathological conditions in humans. A progressive myopathy associated with early onset of severe muscle weakness, extreme exercise intolerance, and cardiomyopathy has been reported in some patients These patients were compound heterozygous for the common intronic splice mutation (g.7044G3 C) on one allele, leading to truncated protein and a novel (c.149G3 A) missense mutation in exon 3 on the other allele. Our findings highlight that the G50E mutation leads to severe growth defects, compromised Fe-S cluster-containing enzyme activity, sensitivity to oxidative stress, increased cellular reactive oxygen species (ROS), elevated iron level, and reduced interaction of scaffold protein with its interacting partners, contributing significantly toward mitochondrial myopathy. At the protein level, the G50E mutation was found to form a higher order oligomeric structure that probably reduces the functionality of the protein
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