Hematology from the Ground Up: Lessons from C. elegans

Heme is a vital cofactor responsible for diverse biological functions such as gas synthesis and sensing, circadian clock control, microRNA processing, and xenobiotic detoxification. Heme synthesis occurs in the mitochondria in eukaryotes. Free heme is a hydrophobic and cytotoxic macrocycle. How then is heme transported through cellular membranes and organelles? What are the mechanisms for incorporating heme into specific hemoproteins that reside in the cytoplasm, peroxisomes, mitochondria, secretory pathway, and nucleus? Our work with the invertebrate animal model Caenorhabditis elegans has demonstrated that this roundworm is exceptional because it does not synthesize heme but rather utilizes environmental heme to manufacture heme-containing proteins that have human homologs. Genomic screens in C. elegans identified several novel genes which we termed Heme Responsive Genes (HRGs). HRG-1, the first metazoan heme importer/transporter, is a permease with four transmembrane domains, and is conserved in vertebrates. Depletion of hrg-1 paralogs in worms causes disruption of organismal heme sensing. Zebrafish with lower levels of hrg1 reveals hydrocephalus, yolk tube malformations, and profound defects in erythropoiesis. Hrg1 is strongly expressed in macrophages of the reticuloendothelial system in human and mouse tissues and is upregulated in a time- and dose-dependent manner by either erythrophagocytosis (EP) or induction of hemolytic anemia in mice. Hrg1 traffics to the phagolysosomal membranes during EP and depletion of Hrg1 in mouse macrophages causes attenuation of heme transport from the phagolysosomal compartment, concomitant with suppression of heme degradation and iron export. Importantly, studies using yeast, zebrafish, and mouse macrophages reveal that specific missense polymorphisms in Hrg1 are defective in heme transport. To uncover additional molecules in organismal heme homeostasis, we conducted a genome-wide RNAi screen in live, transgenic heme-sensor worms. The screen identified several hundred new candidates that integrate signals from different tissues to regulate systemic heme homeostasis. Our findings represent major discoveries in heme trafficking and establish a paradigm for heme transport in animals.