Iron gathering by zoopathogenic fungi

FEMS Immunology and Medical Microbiology 40 (2004) 95^100 www.fems-microbiology.org MiniReview Iron gathering by zoopathogenic fungi Dexter H. Howard Department of Microbiology, Immunology and Molecular Genetics, David Ge¡en School of Medicine at UCLA, Los Angeles, CA 90095, USA Received 21 July 2003; received in revised form 8 October 2003; accepted 8 October 2003 Abstract Iron is a metal required by most microorganisms and is prominently used in the transfer of electrons during metabolism. The gathering of iron is, then, an essential process and its fulfilment becomes a crucial pathogenetic event for zoopathogenic fungi. Iron is rather unavailable because it occurs on the earth’s surface in its insoluble ferric form in oxides and hydroxides. In the infected host iron is bound to proteins such as transferrin and ferritin. Solubilization of ferric iron is the major problem confronting microorganisms. This process is achieved by two major mechanisms: ferric reduction and siderophore utilization. Ferric reductase is frequently accompanied by a copper oxidase transport system. There is one example of direct ferric iron transport apparently without prior reduction. Ferric reduction may also be accomplished by low molecular mass compounds. Some fungi have evolved a process of iron acquisition involving the synthesis of iron-gathering compounds called siderophores. Even those fungi that do not synthesize siderophores have developed permeases for transport of such compounds formed by other organisms. Fungi can also reductively release iron from siderophores and transport the ferrous iron often by the copper oxidase transport system. There is a great diversity of iron-gathering mechanisms expressed by pathogenic fungi and such diversity may be found even in a single species. 2 2003 Published by Elsevier B.V. on behalf of the Federation of European Microbiological Societies. Keywords : Iron; Ferrireductase; Siderophore ; Zoopathogenic fungus 1. Introduction Iron is required by most microorganisms. The metal has two readily available ionization states and is often used as a cofactor in oxidation^reduction reactions. The selection of iron for this role from among other capable transition metals may be related to the fact that it is the second most abundant metal (after aluminum) in the earth’s crust and thus was abundantly available in the prebiotic world [1]. The early atmosphere of those times must have been reductive and ferrous iron was probably ‘present in the earliest Archaean environment’ [2]. With the advent of oxygen and an aerobic atmosphere iron was converted to the ferric form and combined into insoluble compounds (oxides and hydroxides) [3^5]. Iron gathering thus involves two processes : solubilization of the insoluble ferric form from environmental sources or from high a⁄nity binding E-mail address : (D.H. Howard). proteins in a host, and transport of the metal across the fungal membrane. This review will cover the means developed by fungi to accomplish these two goals. Earlier I had prepared a review of the acquisition, transport, and storage of iron by pathogenic fungi [6]. Inspired by the invitation to prepare a minireview on this topic, I have refreshed the data by noting recent publications and I have reanalyzed from a di¡erent standpoint some of the work previously reviewed. The major focus will be on the diversity of iron-gathering methods among zoopathogenic fungi. 2. Ferrireduction 2.1. Cryptococcus neoformans C. neoformans is a basidiomycetous yeast [7] that causes meningoencephalitis in immunocompromised patients. At one time it was a leading cause of death in AIDS patients. Its prevalence in developed nations has been markedly reduced by the advent of e¡ective antiretroviral and antifungal therapy. However, in many areas where that ther- 0928-8244 / 03 / $22.00 2 2003 Published by Elsevier B.V. on behalf of the Federation of European Microbiological Societies. doi:10.1016/S0928-8244(03)00301-8 FEMSIM 1641 9-2-04 First published online 6 November 2003 96 D.H. Howard / FEMS Immunology and Medical Microbiology 40 (2004) 95^100 2.2. Candida albicans This yeast is a commensal organism that occurs in the gastrointestinal tract and on the oral and vaginal mucosae [7]. From these locations it takes opportunistic advantage of immunocompromised individuals. In a recent study of iron transport by C. albicans two genes, CaFTR 1 and CaFTR 2, were detected [15]. The genes were identi¢ed as homologues to the iron permease gene FTR 1 of S. cerevisiae. Although screened for under conditions of iron limitation, the highest amount of the CaFTR 1 transcript was expressed under conditions of iron limitation while the greatest amount of CaFTR 2 transcript was detected under conditions of iron repletion. The Ftr 1 function requires a ferrous oxidase Fet 3. The CaFTR 1 gene was required for iron acquisition by C. albicans in ironde¢cient environments in vitro and in vivo. Strains of C. albicans in which the CaFTR 1 gene was deleted were essentially avirulent [15]. A multicopper oxidase gene from C. albicans has been cloned and characterized [16]. However, a null mutant strain was not reduced in pathogenicity in a mouse model of candidiasis [16]. In S. cerevisiae the Sc Fet 3 multicopper ferroxidase requires the activity of a membrane copper permease and an intracellular copper transporting P-type ATPase, Sc Ccc 2 [17]. However, the deletion of the CaCCC 2 transporter gene in C. albicans did not result in reduced virulence of the strain and an alternative pathway involving hemin has been suggested [17]. 2.3. Geotrichum candidum This fungus is a rare pathogen of humans but a rather common phytopathogen in citrus fruits [18]. G. candidum is a yeast that reproduces by ¢ssion rather than budding [7]. Thus it resembles Schizosaccharomyces pombe in its method of growth and reproduction but appears to vary from that fungus in its iron-gathering methods [9,18]. G. candidum does not appear to form siderophores [18]. Instead, iron uptake is mediated by two iron-regulated transport systems. One system was speci¢c for either ferric or ferrous iron, while the other was speci¢c for ferrioxamine B-mediated iron uptake [18]. The Km values for ferric and ferrous ions were identical (3 WM). Experiments were designed to determine if changes in the valence form of iron occurred prior to transport. A ferric speci¢c chelator, ethylenediamine-di(o-hydroxyphenylacetic acid) (EDDHA), caused 60% inhibition of Fe(III) uptake whereas the ferrous trapping reagents ferrozine and dipyridyl were not e¡ective or resulted in only slight inhibition of Fe(III) uptake. In contrast, ferrozine and dipyridyl caused 80% and 50% inhibition of Fe(II) transport, respectively, while EDDHA was only slightly inhibitory. The work with the speci¢c chelators led the authors to state that ferric ion ‘‘may not be reduced prior to its penetration into the cells, at least during the transport period’’ [18]. More (...truncated)