Elucidating Acetate Metabolism: Identification of Transporters and Enzymes Required for Acetate Utilization in the Fungal Pathogen Cryptococcus Neoformans

Cryptococcus neoformans is the leading cause of fungal meningitis world-wide. While exposure to this environmental sporophyte is common during childhood, those who are immune compromised are at risk of infection. Following inhalation, this basidiomycetous fungus subsequently colonizes other organs though hematogenous dissemination, eventually crossing the blood brain barrier and colonizing the brain where it causes as cryptococcal meningitis. Changes in the availability of carbon sources stemming from the movement from soil to the lungs induce changes in fungal metabolism. Specifically, alveolar macrophages, which present a first line of defense against infection, provide a glucose-/amino acid-poor environment. As such, the use of alternate carbon sources such as acetate and lactate may be needed to establish pulmonary infection. The focus of this study is the utilization of the non-preferred carbon source acetate. Following the generation of acetyl-CoA in the cytosol, Cryptococcus possesses two parallel pathways for import of acetyl-CoA into the mitochondria. The first is the carnitine shuttle which imports acetyl groups via acetyl-carnitine. We have identified three genes which could encode for carnitine acetyltransferases, CNAG_00537, CNAG_006551 and CNAG_05042, and a gene for the acyl-carnitine translocase ACUH (CNAG_00499). The carnitine shuttle has not been characterized in C. neoformans and its role in virulence is unreported. The second pathway, the glyoxylate shunt, imports glyoxylate cycle intermediates into the mitochondria in exchange for metabolites required for gluconeogenesis. We have identified two potential transporters, AcuL (CNAG_02288) and Dic (CNAG_04712). Our characterization of the C. neoformans deletion mutants of the genes encoding for components of the acetate utilization pathway reveal differences to the pathways previously described from Saccharomyces cerevisiae and Candida albicans. This suggests that mechanisms of alternative carbon source utilization are not conserved across fungi. Based on the requirement of ACUL, CNAG_00537, and CNAG_006551 for virulence, and their divergence from or complete lack of human orthologs, we propose these enzymes and transporters could be targets for improved antifungal drugs