Inter-organ signalling by HRG-7 promotes systemic haem homeostasis.

Growing evidence in vertebrates predicts that cellular haem levels in animals are maintained not only by a cell's internal capacity for haem synthesis in a cell-autonomous manner, but also by an inter-organ haem trafficking network through cell-non-autonomous regulation. Using Caenorhabditis elegans, a genetically and optically amenable animal model for visualizing haem-dependent signalling, we show that HRG-7, a protein with homology to aspartic proteases, mediates inter-organ signalling between the intestine and extra-intestinal tissues. Intestinal HRG-7 functions as a secreted signalling factor during haem starvation in extra-intestinal tissues and is regulated through a DBL-1, homologous to BMP5, dependent signal from neurons. Given the evidence that vertebrate homologues exist for each of the components of the HRG-7-mediated signalling pathway, it is conceivable that the cell-non-autonomous signalling framework that we uncovered in C. elegans may have functional relevance for inter-organ regulation of iron and haem metabolism in humans.