Title: Carbonic Anhydrase and CO2 Sensing during Cryptococcus neoformans Growth, Differentiation, and Virulence
Abstract: The gas carbon dioxide (CO2) plays a critical role in microbial and mammalian respiration, photosynthesis in algae and plants, chemoreception in insects, and even global warming [1Hetherington A.M. Raven J.A. The biology of carbon dioxide.Curr. Biol. 2005; 15: 406-410Abstract Full Text Full Text PDF Scopus (17) Google Scholar, 2Enserink M. What mosquitoes want: secrets of host attraction.Science. 2002; 298: 90-92Crossref PubMed Scopus (22) Google Scholar, 3Bidani A. Crandall E.D. Velocity of CO2 exchanges in the lungs.Annu. Rev. Physiol. 1988; 50: 639-652Crossref PubMed Google Scholar, 4Suh G.S. Wong A.M. Hergarden A.C. Wang J.W. Simon A.F. Benzer S. Axel R. Anderson D.J. A single population of olfactory sensory neurons mediates an innate avoidance behaviour in Drosophila.Nature. 2004; 431: 854-859Crossref PubMed Scopus (393) Google Scholar, 5Badger M.R. Price G.D. CO2 concentrating mechanisms in cyanobacteria: molecular components, their diversity and evolution.J. Exp. Bot. 2003; 54: 609-622Crossref PubMed Scopus (538) Google Scholar]. However, how CO2 is transported, sensed, and metabolized by microorganisms is largely not understood. For instance, CO2 is known to induce production of polysaccharide capsule virulence determinants in pathogenic bacteria and fungi via unknown mechanisms [6Zaragoza O. Fries B.C. Casadevall A. Induction of capsule growth in Cryptococcus neoformans by mammalian serum and CO2.Infect. Immun. 2003; 71: 6155-6164Crossref PubMed Scopus (127) Google Scholar, 7Granger D.L. Perfect J.R. Durack D.T. Virulence of Cryptococcus neoformans. Regulation of capsule synthesis by carbon dioxide.J. Clin. Invest. 1985; 76: 508-516Crossref PubMed Scopus (229) Google Scholar, 8Makino S. Sasakawa C. Uchida I. Terakado N. Yoshikawa M. Cloning and CO2-dependent expression of the genetic region for encapsulation from Bacillus anthracis.Mol. Microbiol. 1988; 2: 371-376Crossref PubMed Scopus (62) Google Scholar]. Therefore, we studied CO2 actions in growth, differentiation, and virulence of the basidiomycetous human fungal pathogen Cryptococcus neoformans. The CAN2 gene encoding β-carbonic anhydrase in C. neoformans was found to be essential for growth in environmental ambient conditions but dispensable for in vivo proliferation and virulence at the high CO2 levels in the host. The can2Δ mutant in vitro growth defect is largely attributable to defective fatty acid synthesis. CO2 was found to inhibit cell-cell fusion but not filamentation during sexual reproduction. The can2 mutation restored early mating events in high CO2 but not later steps (fruiting body formation, sporulation), indicating a major role for carbonic anhydrase and CO2/HCO3− in this developmental cascade leading to the production of infectious spores. Our studies illustrate diverse roles of an ancient enzyme class in enabling environmental survival of a ubiquitous human pathogen.