Heme homeostasis is regulated by the conserved and concerted functions of HRG-1 proteins

Hemes are essential but potentially cytotoxic cofactors that participate in critical and diverse biological processes. Although the pathway and intermediates for heme biosynthesis have been well defined, the intracellular networks which mediate heme trafficking remain unknown. Caenorhabditis elegans and related helminths are natural heme auxotrophs requiring environmental heme for growth and development. We exploited this auxotrophy to identify HRG-1 and HRG-4 in C. elegans and show that they are essential for heme homeostasis and normal vertebrate development. We demonstrate that heme deficiency upregulates expression of hrg-4 and its evolutionarily conserved paralog hrg-1. Depletion of either HRG-1 or HRG-4 in worms results in disruption of organismal heme sensing and abnormal response to heme analogs. HRG-1 and HRG-4 are novel transmembrane proteins that bind heme and have evolutionarily conserved functions. Transient knockdown of hrg-1 in zebrafish leads to hydrocephalus, yolk tube malformations, and, most strikingly, profound defects in erythropoiesis - phenotypes that are fully rescued by worm HRG-1. These findings reveal unanticipated and conserved pathways for cellular heme trafficking in animals that defines the paradigm for eukaryotic heme transport. Uncovering the mechanisms of heme transport in C. elegans will provide novel insights into human disorders of heme metabolism and generate unique anthelmintics to combat worm infestations.

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Rajagopal A, Rao AU, Amigo J, Tian M, Upadhyay SK, Hall C, Uhm S, Mathew MK, Fleming MD, Paw BH, Krause M, Hamza I