Project description:Mitochondrial redox homeostasis is essential for cellular metabolism and development. To investigate the consequences of disrupting redox homeostasis in this organelle in a metazoan organism, we generated a double mutant lacking mitochondrial glutathione reductase (gsr-1a) and thioredoxin reductase (trxr-2) genes in Caenorhabditis elegans. While gsr-1a or trxr-2 single mutants are phenotypically normal, double gsr-1a trxr-2 mutants displayed small body size, gonadal migration defects, reduced brood size, and prolonged egg-laying period, without developmental delay or lethality. Transcriptomic analysis revealed strong induction of stress and detoxification genes, prominently those regulated by the transcription factor ATFS-1, indicating activation of the mitochondrial unfolded protein response (UPRmt). Consistent with this, gsr-1a trxr-2 worms exhibited constitutive ATFS-1 nuclear localization and robust hsp-6::GFP expression. Triple gsr-1a trxr-2; atfs-1 mutants were nonviable, demonstrating that UPRmt activation is essential under mitochondrial redox stress. Despite the induction of a stress response at the transcriptional level, gsr-1a trxr-2 double mutants maintained normal respiration, ATP and ROS production, and were not more resistant to oxidative or pathogen stressors. In turn, mitochondrial morphology was altered in a tissue-specific manner, with elongated networks in muscle cells while more fragmented in hypodermis. These changes were independent of mitophagy but sensitive to disruption of mitochondrial fission or fusion machinery, highlighting the importance of dynamic remodeling of the mitochondrial network for survival in animals with impaired mitochondrial redox homeostasis. Functionally, gsr-1a trxr-2 mutants showed impaired motility, reduced calcium uptake upon carbachol stimulation, enhanced hypodermal wound repair, and decreased fertilization efficiency associated with lower muscle exopher production. Overall, our data show that mitochondrial GSR-1a and TRXR-2 act redundantly to preserve redox balance in this organelle. Their simultaneous loss triggers a constitutive ATFS-1–dependent UPRmt that sustains viability but compromises growth, fertility, and muscle performance, revealing mitochondrial redox control as a core determinant of organismal homeostasis.

Project description:The mitochondrial unfolded protein response (UPR mt ) is regulated by the bZIP protein ATFS-1 which promotes mitochondrial protein homeostasis (proteostasis) and mitochondrial biogenesis in Caenorhabditis elegans . Upon mitochondrial perturbation, the ATFS-1-dependent transcriptional program promotes gene expression, leading to mitochondrial recovery. Conversely, atfs-1 -deletion worms harbor dysfunctional mitochondria, are developmentally impaired, and short-lived. However, atfs-1 -deletion worms develop to adults suggesting the presence of other signaling pathways that promote mitochondrial function and biogenesis in the absence of atfs-1 . We hypothesized that additional transcription factors regulate, or promote, mitochondrial function in the absence of atfs-1 . Here, we screened for transcription factors that could reduce the decline in mitochondrial function in the atfs-1 mutants when inhibited. Here, we demonstrate that inhibition of the nuclear hormone receptor NHR-180 re-establishes a functional mitochondrial network in atfs-1(null) worms, increases mtDNA content, and improves the developmental rate of wildtype worms. NHR-180 increases transcription of genes required for cytosolic protein synthesis in response to mitochondrial perturbation. Inhibition of the S6 kinase homolog, rsks-1 , in atfs-1(null) worms leads to a recovery of the mitochondrial network and mtDNA content consistent with nhr-180 regulating expression of protein synthesis components. Consistent with the observations in C. elegans , S6 kinase inhibition also increased mitochondrial biogenesis in mammalian atf5 -knockout cells that harbor severely impaired mitochondria. Intriguingly, nhr-180 or S6 kinase inhibition also rescues mitochondrial dysfunction caused by mutations in multiple genes required for oxidative phosphorylation. Combined, these studies suggest that increased protein synthesis contributes to the mitochondrial dysfunction caused by perturbations in OXPHOS gene expression and suggest a relatively straightforward approach to reducing the impact of mitochondrial dysfunction.

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Caenorhabditis elegans

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