Role and regulation of heme iron acquisition in Gram-negative pathogens

MINI REVIEW ARTICLE published: 08 October 2013 doi: 10.3389/fcimb.2013.00055 CELLULAR AND INFECTION MICROBIOLOGY Role and regulation of heme iron acquisition in gram-negative pathogens Laura J. Runyen-Janecky* Department of Biology, University of Richmond, Richmond, VA, USA Edited by: Frédéric J. Veyrier, Institut Pasteur, France Reviewed by: Erin R. Murphy, Ohio University Heritage College of Osteopathic Medicine, USA Zehava Eichenbaum, Georgia State University, USA *Correspondence: Laura J. Runyen-Janecky, Department of Biology, Gottwald Science Center, University of Richmond, Richmond, VA 23173, USA e-mail: Bacteria that reside in animal tissues and/or cells must acquire iron from their host. However, almost all of the host iron is sequestered in iron-containing compounds and proteins, the majority of which is found within heme molecules. Thus, likely iron sources for bacterial pathogens (and non-pathogenic symbionts) are free heme and heme-containing proteins. Furthermore, the cellular location of the bacterial within the host (intra or extracellular) influences the amount and nature of the iron containing compounds available for transport. The low level of free iron in the host, coupled with the presence of numerous different heme sources, has resulted in a wide range of high-affinity iron acquisition strategies within bacteria. However, since excess iron and heme are toxic to bacteria, expression of these acquisition systems is highly regulated. Precise expression in the correct host environment at the appropriate times enables heme iron acquisitions systems to contribute to the growth of bacterial pathogens within the host. This mini-review will highlight some of the recent findings in these areas for gram-negative pathogens. Keywords: heme, hemin, hem, hemoglobin, iron, pathogens, regulation, Fur INTRODUCTION Almost all living organisms require iron for growth. One notable exception is the Lyme disease pathogen, Borrelia burgdorferi, which uses manganese in place of iron (Posey and Gherardini, 2000). Iron is critical for a wide range of cellular functions; however, high levels of iron are toxic because iron catalyzes the formation of reactive oxygen species, and iron acquisition by cells is highly regulated as a result. In the complex interaction between human host and bacterium, iron plays a critical role. Free ferric (Fe3+ ) iron is poorly soluble in aerobic conditions at neutral pHs; however, ferrous (Fe2+ ) iron is much more soluble. Additionally, the host sequesters free iron in iron binding proteins (such as ferritin, transferrin, lactoferrin) and in heme and hemoproteins to prevent iron toxicity and to withhold nutrients from pathogens, thereby limiting pathogen growth. Thus, free iron is not readily available to the bacterial pathogen inside the host. Pathogens have evolved numerous mechanisms to capture this limited supply of free iron and iron from host iron proteins. Since the type of iron available varies depending on the location of the pathogen within the human host and since pathogens occupy a wide variety of host niches, there is a diversity of iron acquisition mechanisms employed by both intracellular and extracellular pathogens. This mini-review focuses on acquisition of iron in gram-negative pathogens from one of the most abundant sources—host heme. AVAILABILITY OF HEME AND HEME-CONTAINING MOLECULES IN THE HUMAN HOST Approximately 70% of the iron in the human body is within heme, a heterocyclic organic ring called porphryin covalently bound to one ferrous iron atom (Bridges and Seligman, 1995). Frontiers in Cellular and Infection Microbiology Heme is critical for functions including oxygen transport, enzymatic reactions, and cellular respiration. Heme is synthesized in almost all human cell types (the majority in erythroid cells, and to a lesser extent in hepatocytes) and can be obtained from the diet (reviewed in Hamza and Dailey, 2012). Heme is an essential biomolecule; however, excess free heme is toxic to cells due to its lipophilic nature, lipid peroxidation capacity, and ability to catalyze the production of reactive oxygen species (reviewed in Anzaldi and Skaar, 2010). Thus, over 95% of the heme is bound to proteins (hemoproteins), the majority of which are intracellular (Bridges and Seligman, 1995). The intracellular free heme pool is approximately 0.1 μM, which is less than 0.1% of total cellular heme (Granick et al., 1975). The majority of heme in the human body (∼67%) is in hemoglobin, which is primarily found in erythrocytes (Bridges and Seligman, 1995). Other major hemoproteins include myoglobin and cytochromes. Recently, additional hemoproteins have been described, including cytoglobin and neuroglobin, which appear to play a role in oxygen homeostasis/oxygen stress (Liu et al., 2012b; Watanabe et al., 2012; Storz et al., 2013). Additional heme binding proteins exist that are most likely important in scaffolds for synthesis and scavenging heme. The existence of heme chaperones for incorporating heme into apo-hemoproteins has been proposed, but such proteins have yet to been identified in humans (Severance and Hamza, 2009). All of these proteins represent potential heme sources for intracellular pathogens. Although the majority is intracellular, limited amounts of heme can be found extracellular and thus available to extracellular pathogens. One of the major locations for extracellular heme is in blood hemoglobin (estimated to be 80–800 nM in serum) www.frontiersin.org October 2013 | Volume 3 | Article 55 | 1 Runyen-Janecky Heme iron acquisition by pathogens (Schryvers and Stojiljkovic, 1999). Hemoglobin from lysed erythrocytes is bound by haptoglobin for eventual recycling by macrophage and hepatocytes (Tolosano et al., 2010). Free heme, from damaged hemoglobin, is bound by serum hemopexin and, to a lesser extent, serum albumin. In the gut, dietary heme may be bioavailable to bacteria, either free or complexed with hemopexin. Heme levels are thought to be low in the respiratory track; however, since the heme auxotroph Haemophilus influenzae can live in this environment, there must be enough heme to support bacterial growth (Fournier et al., 2011). The urogenital track has varying amounts of heme: the bladder, urethra, and male genital track likely have low heme levels; however, there may be high heme levels in the female urogenital track during menses (Schryvers and Stojiljkovic, 1999). Finally, even in environments where heme is typically low, heme and hemoproteins are released by cells damaged during infection. BACTERIAL HEME TRANSPORTERS AND LIBERATION OF IRON FROM HEME Host microenvironments that have potential heme sources have selected for bacteria with high-affinity heme transport systems which locate and transport heme into the bacterial cell. Heme auxotrophs can use the intact heme for insertion into bacterial hemoproteins. Additionally for both heme prototrophs and autotrophs alike, the iron can be extr (...truncated)