SID1 encodes the enzyme whose function represents the committed step in siderophore biosynthesis and strains deficient in Sid1 are unable
to produce siderophores and unable to grow on iron-depleted media. In both the G186A and G217B backgrounds loss of siderophore production impairs intramacrophage growth and modestly decreases virulence in vivo. While siderophore production is conserved in both strains, G217B has a greater reliance on this virulence mechanism since siderophore MS275 deficiency reduces lung infection to a greater degree in this background than its loss in G186A (Hilty et al., 2011). G217B also utilizes iron acquisition mechanisms that depend on the vacuolar ATPase and an extracellular glutathione-dependent iron reductase. The VMA1 gene encodes the V-ATPase catalytic subunit A required for vacuolar acidification. Mutation of this gene severely reduces Histoplasma virulence in macrophages and in mice (Hilty et al., 2008). Supplementation with iron restores intramacrophage Small molecule library growth of the vma1 mutant linking the vacuolar ATPase to iron homeostasis. G217B yeast secrete a gamma-glutamyltransferase (Ggt1) which catalyzes a two-step glutathione-dependent reaction to reduce iron to its ferrous state (Zarnowski et al., 2008).
Loss of this iron reductase activity reduces the virulence of Histoplasma yeast in cultured macrophages although the importance of this function in vivo has yet to be determined. The relative contributions of each of these iron acquisition mechanisms to Histoplasma pathogenesis are becoming clear for G217B with the creation of mutants and RNAi lines that lack these factors. However, parallel studies of Vma1- and Ggt1-deficient G186A
yeast are lacking. The finding that siderophore production is more important for G217B than G186A virulence suggests different, and perhaps compensatory, mechanisms for iron acquisition and storage may be in operation among the different clades. In support of this, the G186A genome, but not that of G217B, contains the FET3 and FTR1 genes that encode for components of a high-affinity iron transport system (Hilty et al., 2011). Thus, while iron acquisition is an essential virulence requirement shared by Histoplasma strains, the molecular mechanisms to achieve this are specific Celecoxib to the different Histoplasma phylogenetic groups. The adhesins used by Histoplasma to gain entry into host macrophages have only been determined for G217B to date. It has been assumed that G217B and G186A use common factors for binding to host cells. For G217B yeast, cell-surface localized Hsp60 acts as the adhesin that mediates attachment of yeast cells to CD18-family complement receptors on macrophages (Long et al., 2003; Habich et al., 2006). For binding to dendritic cells, a different adhesin-receptor pair is used; G217B yeast cells utilize cell surface-localized cyclophilin A to bind to host VLA-5 (Gomez et al., 2008).