| Cryptococcus neoformans is an invasive human fungal pathogen that causes fatal meningoencephalitis primarily in immunocompromised patients. For my thesis work, I characterized the conserved SREBP pathway in C. neoformans and its role in cryptococcal disease. Mammalian SREBPs are membrane-bound transcription factors that are proteolytically activated in response to decreased sterol concentrations. Upon activation, SREBPs regulate genes important for the synthesis and uptake of cholesterol. In the fission yeast Schizosaccharomyces pombe, the SREBP ortholog Sre1 is activated in response to both decreased sterols and low oxygen concentrations. Under low oxygen, Sre1 activates the hypoxic transcriptional response, including enzymes required for several oxygen-dependent biosynthetic pathways. Biochemical analysis of C. neoformans Sre1 protein demonstrated that it is also cleaved and activated under low sterol and low oxygen conditions. Furthermore, Sre1 is an important regulator of hypoxie sterol synthesis and metal homeostasis. Interestingly, Sre1 is required for normal pathogenesis in mice. Upon infection, Sre1 mutant C. neoformans cells reach the brain effectively but fail to proliferate in the brain tissue.;Mammalian SREBP is cleaved and activated by two sequential proteolytic events, catalyzed by the Site-1 and Site-2 proteases. STP1 is required for the proteolytic cleavage of Sre1 and is also required for virulence in mice. Sre1 positively regulates sterol biosynthesis, and consequently Sre1 pathway mutants are hypersensitive to inhibitors of the sterol biosynthetic pathway, including the azole class of antifungals. Excitingly, azoles are fungicidal to Sre1 and Stp1 mutants, suggesting that inhibitors of the Sre1 pathway may act synergistically with existing antifungals to kill C. neoformans cells. |
