Microscopy image of the roundworm C. elegans. Image credit: Warnhoff et al. (CC BY 4.0)
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.
