Candida albicans Hyphal Formation and the Expression of the Efg1-Regulated Proteinases Sap4 to Sap6 Are Required for the Invasion of Parenchymal Organs

Abstract

The ability to change between yeast and hyphal cells (dimorphism) is known to be a virulence property of the human pathogen Candida albicans. The pathogenesis of disseminated candidosis involves adhesion and penetration of hyphal cells from a colonized mucosal site to internal organs. Parenchymal organs, such as the liver and pancreas, are invaded by C. albicans wild-type hyphal cells between 4 and 24 h after intraperitoneal (i.p.) infection of mice. In contrast, a hypha-deficient mutant lacking the transcription factor Efg1 was not able to invade or damage these organs. To investigate whether this was due to the inability to undergo the dimorphic transition or due to the lack of hypha-associated factors, we investigated the role of secreted aspartic proteinases during tissue invasion and their association with the different morphologies of C. albicans. Wild-type cells expressed a distinct pattern of SAP genes during i.p. infections. Within the first 72 h after infection, SAP1, SAP2, SAP4, SAP5, SAP6, and SAP9 were the most commonly expressed proteinase genes. Sap1 to Sap3 antigens were found on yeast and hyphal cells, while Sap4 to Sap6 antigens were predominantly found on hyphal cells in close contact with host cells, in particular, eosinophilic leukocytes. Mutants lacking EFG1 had either noticeably reduced or higher expressed levels of SAP4 to SAP6 transcripts in vitro depending on the culture conditions. During infection, efg1 mutants had a strongly reduced ability to produce hyphae, which was associated with reduced levels of SAP4 to SAP6 transcripts. Mutants lacking SAP1 to SAP3 had invasive properties indistinguishable from those of wild-type cells. In contrast, a triple mutant lacking SAP4 to SAP6 showed strongly reduced invasiveness but still produced hyphal cells. When the tissue damage of liver and pancreas caused by single sap4, sap5, and sap6 and double sap4 and -6, sap5 and -6, and sap4 and -5 double mutants was compared to the damage caused by wild-type cells, all mutants which lacked functional SAP6 showed significantly reduced tissue damage. These data demonstrate that strains which produce hyphal cells but lack hypha-associated proteinases, particularly that encoded by SAP6, are less invasive. In addition, it can be concluded that the reduced virulence of hypha-deficient mutants is not only due to the inability to form hyphae but also due to modified expression of the SAP genes normally associated with the hyphal morphology.

Phylogeny and evolution of the aspartyl protease family from clinically relevant Candida species

“…albicans SAP1-3 and SAP4-6 gene families are not pseudogenes that have subsisted to purifying selection and concerted evolution, but apparently have acquired new and specific functions in nitrogen metabolism, switching, and cell adherence (Cassone et al 2002, Felk et al 2002, Ripeau et al 2002, Schofield et al 2003. C. tropicalis has only one gene (SAPT4) related to the C. albicans SAP1-3 and SAP4-6 families.…”

HIV-1 anti-retroviral drug effect on the C. albicans hyphal growth rate by a Bio-Cell Tracer system

“…Inhibition of ergosterol biosynthesis in C. albicans causes a variety of functional alterations in the cell membrane such as permeability changes, leakiness and disruptive interactions with non-sterol and lipid components. Ergosterol biosynthesis is more sensitive to azoles in mycelial cultures than yeast cultures, and this observation has been used to justify the efficacy of azoles in vivo (13,18,35).…”

“…The introduction of highly active antiretroviral therapy (HAART) including PI has been accompanied by a reduction in the frequency of many of the secondary infections caused by HIV infection, including oral lesions (2,(8)(9)(10)12,19). Infection by C. albicans generally involves adherence and colonization of superficial tissues (13,22,24). During this process, budding yeast cells are able to transform to hyphae and penetrate into the deep tissue (29).…”

“…These structural alterations were attributed to depletion of ergosterol (32). Hyphal-deficient mutants are known to be avirulent in infections (13,24). C. albicans extracellular proteolytic activity due to secreted aspartic proteases has been purposed as putative virulence factor during the tissue invasion process by hyphal cells.…”

“…Production of extracellular proteases in Candida was first reported by Staib, 1969 (22,33). C. albicans has the ability to secrete proteases facilitating the invasion of mucosal tissue (13). Declining incidence of oropharyngeal candidoses and opportunistic infections over recent years can be attributed to the use of highly active anti-retroviral therapy (HAART), *Corresponding Author.…”