Hypoxia-inducible factor induces cysteine dioxygenase and promotes cysteine homeostasis in Caenorhabditis elegans.

Dedicated genetic pathways regulate cysteine homeostasis. For example, high levels of cysteine activate cysteine dioxygenase, a key enzyme in cysteine catabolism in most animal and many fungal species. The mechanism by which cysteine dioxygenase is regulated is largely unknown. In an unbiased genetic screen for mutations that activate cysteine dioxygenase (cdo-1) in the nematode Caenorhabditis elegans, we isolated loss-of-function mutations in rhy-1 and egl-9, which encode proteins that negatively regulate the stability or activity of the oxygen-sensing hypoxia inducible transcription factor (hif-1). EGL-9 and HIF-1 are core members of the conserved eukaryotic hypoxia response. However, we demonstrate that the mechanism of HIF-1-mediated induction of cdo-1 is largely independent of EGL-9 prolyl hydroxylase activity and the von Hippel-Lindau E3 ubiquitin ligase, the classical hypoxia signaling pathway components. We demonstrate that C. elegans cdo-1 is transcriptionally activated by high levels of cysteine and hif-1. hif-1-dependent activation of cdo-1 occurs downstream of an H2S-sensing pathway that includes rhy-1, cysl-1, and egl-9. cdo-1 transcription is primarily activated in the hypodermis where it is also sufficient to drive sulfur amino acid metabolism. Thus, the regulation of cdo-1 by hif-1 reveals a negative feedback loop that maintains cysteine homeostasis. High levels of cysteine stimulate the production of an H2S signal. H2S then acts through the rhy-1/cysl-1/egl-9 signaling pathway to increase HIF-1-mediated transcription of cdo-1, promoting degradation of cysteine via CDO-1.

Proteins are large molecules in our cells that perform various roles, from acting as channels through which nutrients can enter the cell, to forming structural assemblies that help the cell keep its shape. Proteins are formed of chains of building blocks called amino acids. There are 20 common amino acids, each with a different 'side chain' that confers it with specific features. Cysteine is one of these 20 amino acids. Its side chain has a 'thiol' group, made up of a sulfur atom and a hydrogen atom. This thiol group is very reactive, and it is an essential building block of enzymes (proteins that speed up chemical reactions within the cell), structural proteins and signaling molecules. While cysteine is an essential amino acid for the cell to function, excess cysteine can be toxic. The concentration of cysteine in animal cells is tightly regulated by an enzyme called cysteine dioxygenase. This enzyme is implicated in two rare conditions that affect metabolism, where the product of cysteine dioxygenase is a key driver of disease severity. Additionally, cysteine dioxygenase acts as a tumor suppressor gene, and its activity becomes blocked in diverse cancers. Understanding how cysteine dioxygenase is regulated may be important for research into these conditions. While it has been shown that excess cysteine drives the production and activity of cysteine dioxygenase, how the cell detects high levels of cysteine remained unknown. Warnhoff et al. sought to resolve this question using the roundworm Caenorhabditis elegans. First, the scientists demonstrated that, like in mammals, high levels of cysteine drive the production of cysteine dioxygenase in C. elegans. Next, the researchers used an approach called an unbiased genetic screening to find genes that induce cysteine dioxygenase production when they are mutated. These experiments revealed that the protein HIF-1 can drive the production of cysteine dioxygenase when it is activated by a pathway that senses hydrogen sulfide gas. Based on these results, Warnhoff et al. propose that high levels of cysteine lead to the production of hydrogen sulfide gas that in turn drives the production of cysteine dioxygenase via HIF-1 activation of gene expression. The results reported by Warnhoff et al. suggest that modulating HIF-1 signaling could control the activity of cysteine dioxygenase. This information could be used in the future to develop therapies for molybdenum cofactor deficiency, isolated sulfite oxidase deficiency and several types of cancer. However, first it will be necessary to demonstrate that the same signaling pathway is active in humans.

Hypoxia-inducible factor induces cysteine dioxygenase and promotes cysteine homeostasis in Caenorhabditis elegans.

Dedicated genetic pathways regulate cysteine homeostasis. For example, high levels of cysteine activate cysteine dioxygenase, a key enzyme in cysteine catabolism in most animal and many fungal species. The mechanism by which cysteine dioxygenase is regulated is largely unknown. In an unbiased genetic screen for mutations that activate cysteine dioxygenase (cdo-1) in the nematode C. elegans, we isolated loss-of-function mutations in rhy-1 and egl-9, which encode proteins that negatively regulate the stability or activity of the oxygen-sensing hypoxia inducible transcription factor (hif-1). EGL-9 and HIF-1 are core members of the conserved eukaryotic hypoxia response. However, we demonstrate that the mechanism of HIF-1-mediated induction of cdo-1 is largely independent of EGL-9 prolyl hydroxylase activity and the von Hippel-Lindau E3 ubiquitin ligase, the classical hypoxia signaling pathway components. We demonstrate that C. elegans cdo-1 is transcriptionally activated by high levels of cysteine and hif-1. hif-1-dependent activation of cdo-1 occurs downstream of an H2S-sensing pathway that includes rhy-1, cysl-1, and egl-9. cdo-1 transcription is primarily activated in the hypodermis where it is also sufficient to drive sulfur amino acid metabolism. Thus, the regulation of cdo-1 by hif-1 reveals a negative feedback loop that maintains cysteine homeostasis. High levels of cysteine stimulate the production of an H2S signal. H2S then acts through the rhy-1/cysl-1/egl-9 signaling pathway to increase HIF-1-mediated transcription of cdo-1, promoting degradation of cysteine via CDO-1.

Obtaining the necessary molybdenum cofactor for sulfite oxidase activity in the nematode Caenorhabditis elegans surprisingly involves a dietary source.

Molybdenum cofactor (Moco) is a prosthetic group necessary for the activity of four unique enzymes, including the essential sulfite oxidase (SUOX-1). Moco is required for life; humans with inactivating mutations in the genes encoding Moco-biosynthetic enzymes display Moco deficiency, a rare and lethal inborn error of metabolism. Despite its importance to human health, little is known about how Moco moves among and between cells, tissues, and organisms. The prevailing view is that cells that require Moco must synthesize Moco de novo. Although, the nematode Caenorhabditis elegans appears to be an exception to this rule and has emerged as a valuable system for understanding fundamental Moco biology. C. elegans has the seemingly unique capacity to both synthesize its own Moco as well as acquire Moco from its microbial diet. However, the relative contribution of Moco from the diet or endogenous synthesis has not been rigorously evaluated or quantified biochemically. We genetically removed dietary or endogenous Moco sources in C. elegans and biochemically determined their impact on animal Moco content and SUOX-1 activity. We demonstrate that dietary Moco deficiency dramatically reduces both animal Moco content and SUOX-1 activity. Furthermore, these biochemical deficiencies have physiological consequences; we show that dietary Moco deficiency alone causes sensitivity to sulfite, the toxic substrate of SUOX-1. Altogether, this work establishes the biochemical consequences of depleting dietary Moco or endogenous Moco synthesis in C. elegans and quantifies the surprising contribution of the diet to maintaining Moco homeostasis in C. elegans.

moc-6/MOCS2A is necessary for molybdenum cofactor synthesis in C. elegans.

Molybdenum cofactor (Moco) is an essential prosthetic group that mediates the activity of 4 animal oxidases and is required for viability. Humans with mutations in the genes encoding Moco-biosynthetic enzymes suffer from Moco deficiency, a neonatal lethal inborn error of metabolism. Caenorhabditis elegans has recently emerged as a useful and tractable genetic discovery engine for Moco biology. Here, we identify and characterize K10D2.7/moc-6, the C. elegans ortholog of human MOCS2A, a sulfur-carrier protein essential for Moco synthesis. Using CRISPR/Cas9 gene editing, we generate 3 null mutations in K10D2.7/moc-6 and with these alleles genetically demonstrate that K10D2.7/moc-6 is necessary for endogenous Moco synthesis in C. elegans.