The incidence of fungal infection is increasing and yet available antifungal drugs are limited, some are toxic, and drug resistant strains are emerging. We have elucidated a conserved signal transduction cascade that controls virulence of Cryptococcus neoformans, the leading cause of fungal meningitis. The central element of this virulence pathway is the calcium-calmodulin activated protein phosphatase calcineurin, which is the molecular target of the immunosuppressive antifungal drugs cyclosporin A and FK506. C. neoformans mutants lacking either the catalytic A or the regulatory B subunit of calcineurin are inviable at 37 degrees C and other stress conditions and, as a consequence, are avirulent in animal models. In studies supported by this award, we identified: 1) the calcineurin B regulatory subunit and calmodulin, 2) the calcineurin binding protein (Cbpl) that is a conserved regulator or effector and which is the founding member of a protein family conserved from fungi to humans, and 3) the novel C2 domain protein Cts1 that may function as a downstream effector of the calcineurin signaling pathway to promote cell wall biogenesis and growth at 37degrees C. In parallel we discovered that calcineurin is required for virulence of Candida albicans, the most common human fungal pathogen. C. albicans cnb1/cnb1 mutants lacking the calcineurin B regulatory subunit are severely attenuated in animal models. Yet, in contrast to C. neoformans calcineurin is not required for growth of C. albicans at 37 degrees C. Instead, calcineurin is necessary for C. albicans to survive and proliferate in serum. These studies illustrate how a conserved signaling cascade has been co-opted to control virulence of two divergent fungal pathogens by unique molecular mechanisms. Here we propose to delineate this molecular virulence cascade in both C. neoformans and C. albicans. Importantly, this pathway can be targeted for therapeutic intervention using non-immunosuppressive calcineurin inhibitors that retain antifungal activity and synergistic drug combinations that we have discovered.
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