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

Abstract

Haems are metalloporphyrins that serve as prosthetic groups for various biological processes including respiration, gas sensing, xenobiotic detoxification, cell differentiation, circadian clock control, metabolic reprogramming and microRNA processing [1][2][3][4] . With a few exceptions, haem is synthesized by a multistep biosynthetic pathway comprising defined intermediates that are highly conserved throughout evolution 5 . Despite our extensive knowledge of haem biosynthesis and degradation, the cellular pathways and molecules that mediate intracellular haem trafficking are unknown. The experimental setback in identifying haem trafficking pathways has been the inability to dissociate the highly regulated cellular synthesis and degradation of haem from intracellular trafficking events 6 . Caenorhabditis elegans and related helminths are natural haem auxotrophs that acquire environmental haem for incorporation into haemoproteins, which have vertebrate orthologues 7 . Here we show, by exploiting this auxotrophy to identify HRG-1 proteins in C. elegans, that these proteins are essential for haem homeostasis and normal development in worms and vertebrates. Depletion of hrg-1, or its paralogue hrg-4, in worms results in the disruption of organismal haem sensing and an abnormal response to haem analogues. HRG-1 andCorrespondence and requests for materials should be addressed to I.H. (hamza@umd.edu). Supplementary Information is linked to the online version of the paper at www.nature.com/nature.Author Contributions Experimental design and execution were as follows: worm experiments and microarrays, A.R., A.U.R., M.K. and I.H.; mammalian experiments, A.R., A.U.R., M.T., C.H., S.U., M. HRG-4 are previously unknown transmembrane proteins, which reside in distinct intracellular compartments. 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. Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1. These findings reveal conserved pathways for cellular haem trafficking in animals that define the model for eukaryotic haem transport. Thus, uncovering the mechanisms of haem transport in C. elegans may provide insights into human disorders of haem metabolism and reveal new drug targets for developing anthelminthics to combat worm infestations.In animals, the terminal enzyme in haem synthesis, ferrochelatase, is located on the matrix side of the inner mitochondrial membrane 8 . Most newly synthesized haem must be transported through mitochondrial membranes to haemoproteins found in distinct intracellular membrane compartments 6 . Haem synthesis is regulated at multiple steps by effectors including iron, haem and oxygen to prevent the uncoordinated accumulation of haem or its precursors 5 . C. elegans is a haem auxotroph and is therefore a unique genetic animal model in which to identify the molecule...

Differential regulation of iron homeostasis during human macrophage polarized activation

“…It is still unknown whether the inflammatory response affects the feline leukemia virus, subgroup C, receptor that exports heme from macrophages [15] and whether the heme transporter HRG1 proteins play a role in macrophage iron metabolism [16]. On the other hand, the downregulation of HCP-1, a folate transporter and heme carrier protein that transports iron out of the endosome, may impair iron recycling and contribute to greater iron sequestration within inflammatory macrophages [14].…”

Regulation of heme biosynthesis and transport in metazoa

“…In the worm intestine, HRG-1 is responsible for mobilizing heme out of the endosomal-lysosomal organelles, whereas its paralog HRG-4 imports dietary heme through the apical surface of intestinal cells. The heme transport activities of C. elegans HRG-1 and HRG-4 have been verified in worms, Xenopus oocytes, and yeast [70,83]. In a similar way, the homolog of HRG-4 in Leishmania amazonensis, Leishmania Heme Response-1 (LHR-1), mediates heme uptake in the parasites [84].…”

“…In a similar way, the homolog of HRG-4 in Leishmania amazonensis, Leishmania Heme Response-1 (LHR-1), mediates heme uptake in the parasites [84]. HRG-1, HRG-4, and LHR-1 are all transcriptionally up-regulated under heme-limiting conditions [70,84].…”

“…HRG-1 and its mammalian homologs also transport heme within the cell. As discussed above, C. elegans HRG-1 transports heme from the endosomal-lysosomal organelles to the cytosol within the intestinal cells [70]. Similarly, mammalian HRG1 transports heme from the phagolysosomes to the cytosol of the reticuloendothelial macrophages [75].…”

“…It remains to be tested whether the low affinity heme transport activity of this folate transporter has physiological relevance. By using the model organism Caenorhabditis elegans, Rajagopal et al [70] identified a high-affinity heme importer-heme responsive gene-1 (HRG-1). Because C. elegans and related nematodes are unable to synthesize heme, they completely rely on food for heme nutrition.…”