A defect in ATP-citrate lyase links acetyl-CoA production, virulence factor elaboration and virulence in Cryptococcus neoformans

The interaction of Cryptococcus neoformans with phagocytic cells of the innate immune system is a key step in disseminated disease leading to meningoencephalitis in immunocompromised individuals. Transcriptional profiling of cryptococcal cells harvested from cell culture medium or from macrophages found differential expression of metabolic and other functions during fungal adaptation to the intracellular environment. We focused on the ACL1 gene for ATP-citrate lyase, which converts citrate to acetyl-CoA, because this gene showed elevated transcript levels in macrophages and because of the importance of acetyl-CoA as a central metabolite. Mutants lacking ACL1 showed delayed growth on medium containing glucose, reduced cellular levels of acetyl-CoA, defective production of virulence factors, increased susceptibility to the antifungal drug fluconazole and decreased survival within macrophages. Importantly, acl1 mutants were unable to cause disease in a murine inhalation model, a phenotype that was more extreme than other mutants with defects in acetyl-CoA production (e.g. an acetyl-CoA synthetase mutant). Loss of virulence is likely due to perturbation of critical physiological interconnections between virulence factor expression and metabolism in C.neoformans. Phylogenetic analysis and structural modelling of cryptococcal Acl1 identified three indels unique to fungal protein sequences; these differences may provide opportunities for the development of pathogen-specific inhibitors.