Caiyong Chen

Iron and Porphyrin Trafficking in Heme Biogenesis

Iron is an essential element for diverse biological functions. In mammals, the majority of iron is enclosed within a single prosthetic group: heme. In metazoans, heme is synthesized via a highly conserved and coordinated pathway within the mitochondria. However, iron is acquired from the environment and subsequently assimilated into various cellular pathways, including heme synthesis. Both iron and heme are toxic but essential cofactors. How is iron transported from the extracellular milieu to the mitochondria? How are heme and heme intermediates coordinated with iron transport? Although recent studies have answered some questions, several pieces of this intriguing puzzle remain unsolved.

Cellular and mitochondrial iron homeostasis in vertebrates

Iron plays an essential role in cellular metabolism and biological processes. However, due to its intrinsic redox activity, free iron is a potentially toxic molecule in cellular biochemistry. Thus, organisms have developed sophisticated ways to import, sequester, and utilize iron. The transferrin cycle is a well-studied iron uptake pathway that is important for most vertebrate cells. Circulating iron can also be imported into cells by mechanisms that are independent of transferrin. Once imported into erythroid cells, iron is predominantly consumed by the mitochondria for the biosynthesis of heme and iron sulfur clusters. This review focuses on canonical transferrin-mediated and the newly discovered, non-transferrin mediated iron uptake pathways, as well as, mitochondrial iron homeostasis in higher eukaryotes. This article is part of a Special Issue entitled: Cell Biology of Metals.

Heme metabolism and erythropoiesis.

Purpose of reviewHeme biosynthesis requires a series of enzymatic reactions that take place in the cytosol and the mitochondria as well as the proper intercellular and intracellular trafficking of iron. Heme can also be acquired by intestinal absorption and intercellular transport. The purpose of this review is to highlight recent work on heme and iron transport with an emphasis on their relevance in erythropoiesis. Recent findingsWhereas the enzymes responsible for heme biosynthesis have been identified, transport mechanisms for iron, heme, or heme synthesis intermediates are only emerging. Recent studies have shed light on how these molecules are transported among various cellular compartments, as well as tissues. Much of this progress can be attributed to the use of model organisms such as S. cerevisiae, C. elegans, D. rerio, and M. musculus. Genetic studies in these models have led to the identification of several new genes involved in heme metabolism. Although our understanding has greatly improved, it is highly likely that other regulators exist and additional work is required to characterize the pathways by which heme and iron are transported within the erythron. SummaryThe identification of heme and iron transport mechanisms will improve our understanding of blood development and provide new insight into human blood disorders.

An Intercellular Heme-Trafficking Protein Delivers Maternal Heme to the Embryo during Development in C. elegans

Extracellular free heme can intercalate into membranes and promote damage to cellular macromolecules. Thus it is likely that specific intercellular pathways exist for the directed transport, trafficking, and delivery of heme to cellular destinations, although none have been found to date. Here we show that Caenorhabditis elegans HRG-3 is required for the delivery of maternal heme to developing embryos. HRG-3 binds heme and is exclusively secreted by maternal intestinal cells into the interstitial fluid for transport of heme to extraintestinal cells, including oocytes. HRG-3 deficiency results either in death during embryogenesis or in developmental arrest immediately post-hatching-phenotypes that are fully suppressed by maternal but not zygotic hrg-3 expression. Our results establish a role for HRG-3 as an intercellular heme-trafficking protein.

The mTORC1/4E-BP pathway coordinates hemoglobin production with L-leucine availability

Inadequate leucine uptake by maturing red blood cells limits the production of hemoglobin. Red cells need leucine to make hemoglobin Inhibitors of the protein complex mTORC1 are used in the clinic, but they can cause anemia. Limitation of nutrients, such as amino acids, inhibits mTORC1 activity, which reduces protein synthesis by enhancing the activity of the translation inhibitor protein 4E-BP. The oxygen-carrying protein complex hemoglobin in red blood cells contains globin proteins, which have a particularly high percentage of leucine residues, leading Chung et al. to investigate the role of mTORC1 in hemoglobin production. Deficiency or inhibition of LAT3, an uptake transporter for l-leucine, prevented hemoglobin production in zebrafish and mouse red blood cells. Inadequate l-leucine uptake prevented the translation of globin-encoding transcripts. Thus, red cell function critically depends on leucine availability. In multicellular organisms, the mechanisms by which diverse cell types acquire distinct amino acids and how cellular function adapts to their availability are fundamental questions in biology. We found that increased neutral essential amino acid (NEAA) uptake was a critical component of erythropoiesis. As red blood cells matured, expression of the amino acid transporter gene Lat3 increased, which increased NEAA import. Inadequate NEAA uptake by pharmacologic inhibition or RNAi-mediated knockdown of LAT3 triggered a specific reduction in hemoglobin production in zebrafish embryos and murine erythroid cells through the mTORC1 (mammalian target of rapamycin complex 1)/4E-BP (eukaryotic translation initiation factor 4E–binding protein) pathway. CRISPR-mediated deletion of members of the 4E-BP family in murine erythroid cells rendered them resistant to mTORC1 and LAT3 inhibition and restored hemoglobin production. These results identify a developmental role for LAT3 in red blood cells and demonstrate that mTORC1 serves as a homeostatic sensor that couples hemoglobin production at the translational level to sufficient uptake of NEAAs, particularly l-leucine.

Heme utilization in the Caenorhabditis elegans hypodermal cells is facilitated by Heme Responsive Gene-2

Background: C. elegans acquires environmental heme through specific trafficking machinery. Results: Heme-responsive gene-2 (HRG-2) is a heme-binding, type I membrane protein specifically expressed in the hypodermis; cytochrome distribution is abnormal in HRG-2-deficient worms. Conclusion: HRG-2 facilitates heme utilization in the hypodermis. Significance: Regulation of heme homeostasis by an HRG-2 prototype could be a general mechanism employed by metazoans. The roundworm Caenorhabditis elegans is a heme auxotroph that requires the coordinated actions of HRG-1 heme permeases to transport environmental heme into the intestine and HRG-3, a secreted protein, to deliver intestinal heme to other tissues including the embryo. Here we show that heme homeostasis in the extraintestinal hypodermal tissue was facilitated by the transmembrane protein HRG-2. Systemic heme deficiency up-regulated hrg-2 mRNA expression over 200-fold in the main body hypodermal syncytium, hyp 7. HRG-2 is a type I membrane protein that binds heme and localizes to the endoplasmic reticulum and apical plasma membrane. Cytochrome heme profiles are aberrant in HRG-2-deficient worms, a phenotype that was partially suppressed by heme supplementation. A heme-deficient yeast strain, ectopically expressing worm HRG-2, revealed significantly improved growth at submicromolar concentrations of exogenous heme. Taken together, our results implicate HRG-2 as a facilitator of heme utilization in the Caenorhabditis elegans hypodermis and provide a mechanism for the regulation of heme homeostasis in an extraintestinal tissue.

Regulation of heme biosynthesis and transport in metazoa

Heme is an iron-containing tetrapyrrole that plays a critical role in regulating a variety of biological processes including oxygen and electron transport, gas sensing, signal transduction, biological clock, and microRNA processing. Most metazoan cells synthesize heme via a conserved pathway comprised of eight enzyme-catalyzed reactions. Heme can also be acquired from food or extracellular environment. Cellular heme homeostasis is maintained through the coordinated regulation of synthesis, transport, and degradation. This review presents the current knowledge of the synthesis and transport of heme in metazoans and highlights recent advances in the regulation of these pathways.

Corrigendum: Mitochondrial Atpif1 regulates haem synthesis in developing erythroblasts

This corrects the article DOI: 10.1038/nature11536

Erratum: Corrigendum: Mitochondrial Atpif1 regulates haem synthesis in developing erythroblasts

This corrects the article DOI: 10.1038/nature11536

Erratum: Mitochondrial Atpif1 regulates haem synthesis in developing erythroblasts (Nature (2012) 491 (608-612) doi:10.1038/nature11536)

This corrects the article DOI: 10.1038/nature11536