Understanding the role of DNAJC30 and ISCU in mitochondrial homeostasis

Abstract

Williams-Beuren Syndrome (WBS) is a rare multisystem genomic disorder characterized by mild to moderate cognitive impairment, unique personality traits, distinctive facial features, cardiovascular problems, and infantile hypercalcemia. Previous studies have shown that it is caused by a hemizygous deletion of chromosomal locus 7q11.23, also known as Williams-Beuren syndrome critical region (WBSCR). The WBSCR spans approximately 28 genes. However, except for elastin, the genotype-phenotype correlation has not been established for other genes belonging to WBSCR. The WBSCR also includes DNAJC30, an intronless gene which encodes a type III J protein (Hsp40 family). Bioinformatic analysis suggested that DNAJC30 localizes to mitochondria. Therefore, we investigated the role of DNAJC30 in mitochondrial homeostasis and development of WBS. In the present study, we demonstrated the mitochondrial localization and ATPase stimulating activity of DNAJC30. We observed robust stimulation of the ATPase activity of mtHsp70 by DNAJC30 in vitro. Further, DNAJC30 was found to localize to the inner mitochondrial membrane with its N-terminal facing the mitochondrial matrix. In addition, downregulation of DNAJC30 led to loss of mitochondrial membrane potential, increase in mitochondrial superoxide levels and overall increase in cellular ROS levels. Based on these findings, we propose that the above cellular defects associated with depletion or loss of DNAJC30 may result in mitochondrial dysfunction and subsequently lead to the development of Williams-Beuren syndrome. Mammalian Fe-S cluster biogenesis involves a highly conserved scaffold protein, ISCU. ISCU plays a central role in de novo Fe-S cluster assembly. In addition, ISCU also participates in the transfer of Fe-S clusters to target apoproteins in coordination with the chaperone machinery. A homozygous intronic (7044G-C) mutation in ISCU is known to cause hereditary myopathy with lactic acidosis. This mutation is shown to interfere with the normal splicing of the ISCU gene, thereby significantly reducing its mRNA and protein levels. A compound heterozygous mutation in ISCU is also associated with hereditary myopathy with lactic acidosis. This mutation introduces G50E substitution in ISCU protein, thereby significantly affecting protein activity. Both these mutations in ISCU are associated with a significant reduction in the activities of Fe-S proteins aconitase and succinate dehydrogenase, highlighting the significance of ISCU in mammalian Fe-S cluster biogenesis. Therefore, this study was aimed at mapping critical residues of scaffold protein ISCU and understanding their importance in ISCU function. In the present study, we characterized a critical residue of ISCU and investigated its importance in ISCU function. It was observed that His42 residue plays an important role in ISCU function, since the H42N mutation led to a significant decrease in the interaction between ISCU and NFS1. In addition, the H42N mutation was accompanied by significantly reduced activities of Fe-S proteins aconitase and respiratory complex II. Further, the H42N mutation resulted in enhanced mitochondrial ROS levels and iron accumulation, highlighting the importance of His42 residue of ISCU in normal Fe-S cluster biogenesis and mitochondrial function.