Histoplasma capsulatum Depends on De Novo Vitamin Biosynthesis for Intraphagosomal Proliferation

Abstract

During infection of the mammalian host, Histoplasma capsulatum yeasts survive and reside within macrophages of the immune system. Whereas some intracellular pathogens escape into the host cytosol, Histoplasma yeasts remain within the macrophage phagosome. This intracellular Histoplasma-containing compartment imposes nutritional challenges for yeast growth and replication. We identified and annotated vitamin synthesis pathways encoded in the Histoplasma genome and confirmed by growth in minimal medium that Histoplasma yeasts can synthesize all essential vitamins with the exception of thiamine. Riboflavin, pantothenate, and biotin auxotrophs of Histoplasma were generated to probe whether these vitamins are available to intracellular yeasts. Disruption of the RIB2 gene (riboflavin biosynthesis) prevented growth and proliferation of yeasts in macrophages and severely attenuated Histoplasma virulence in a murine model of respiratory histoplasmosis. Rib2-deficient yeasts were not cleared from lung tissue but persisted, consistent with functional survival mechanisms but inability to replicate in vivo. In addition, depletion of Pan6 (pantothenate biosynthesis) but not Bio2 function (biotin synthesis) also impaired Histoplasma virulence. These results indicate that the Histoplasma-containing phagosome is limiting for riboflavin and pantothenate and that Histoplasma virulence requires de novo synthesis of these cofactor precursors. Since mammalian hosts do not rely on vitamin synthesis but instead acquire essential vitamins through diet, vitamin synthesis pathways represent druggable targets for therapeutics.

Riboflavin and pantothenic acid biosynthesis are crucial for iron homeostasis and virulence in the pathogenic mold Aspergillus fumigatus

“…To confirm gene deletion-specific phenotypes, mutant strains were complemented ( C strains) by re-integration of functional gene copies. Notably, we also attempted to generate a riboflavin auxotroph by deletion of Afu2g16360 , the ortholog of Histoplasma capsulatum rib2 , which is essential for riboflavin biosynthesis in this dimorphic fungal species [23]. Surprisingly, deletion of Afu2g16360/rib2 did not result in riboflavin auxotrophy in A. fumigatus (data not shown), most likely because of the presence of paralogous genes (Supplementary Figure 1A).…”

“…Lung CFU enumeration and histology show complete clearance of the fungus from the lungs 48 h after infection. Riboflavin auxotrophy also leads to avirulence in Candida albicans [6], Histoplasma capsulatum [23] and in many species of gram-negative bacteria including Mycobacteria that lack a riboflavin uptake system and are therefore totally dependent on endogenous biosynthesis [44]. Therefore, this pathway is particularly suitable for the development of antimicrobials.…”

Pulmonary histoplasmosis in a immunocompetent patient: A case report and literature review

“…If the patient has strong cellular immunity, then macrophages, epithelial cells and lymphocytes can eliminate the fungi. However, in immunocompromised individuals, Histoplasma capsulatum disseminates to a variety of organs, including the bones, spleen, liver, adrenal glands and mucocutaneous membranes, resulting in progressive disseminated histoplasmosis (21). …”

Thermally Dimorphic Human Fungal Pathogens—Polyphyletic Pathogens with a Convergent Pathogenicity Trait

“…Cite this article as Cold Spring Harb Perspect Med 2015;5:a019794 factors involved in iron acquisition and homeostasis (Hilty et al 2008(Hilty et al , 2011Hwang et al 2008Hwang et al , 2012, intracellular parasitism (Sebghati et al 2000;Edwards et al 2011b;Isaac et al 2013), cell wall a-glucan synthesis (Rappleye et al 2004;Marion et al 2006;Edwards et al 2011a), vitamin acquisition (Garfoot et al 2014), and superoxide detoxification (Youseff et al 2012;Holbrook et al 2013). In addition to these molecular genetic approaches, a number of genomics and proteomics approaches have been used to determine genes whose expression is enriched in yeast or hyphae (Hwang et al 2003;Nguyen and Sil 2008;Beyhan et al 2013;Edwards et al 2013), and the H. capsulatum secreted proteome (Albuquerque et al 2008;Holbrook et al 2011), which is presumed to include novel virulence factors.…”