Cryptococcus neoformans Senses CO ₂ through the Carbonic Anhydrase Can2 and the Adenylyl Cyclase Cac1

Abstract

Cryptococcus neoformans, a fungal pathogen of humans, causes fatal meningitis in immunocompromised patients. Its virulence is mainly determined by the elaboration of a polysaccharide capsule surrounding its cell wall. During its life, C. neoformans is confronted with and responds to dramatic variations in CO 2 concentrations; one important morphological change triggered by the shift from its natural habitat (0.033% CO 2 ) to infected hosts (5% CO 2 ) is the induction of capsule biosynthesis. In cells, CO 2 is hydrated to bicarbonate in a spontaneous reaction that is accelerated by carbonic anhydrases. Here we show that C. neoformans contains two ␤-class carbonic anhydrases, Can1 and Can2. We further demonstrate that CAN2, but not CAN1, is abundantly expressed and essential for the growth of C. neoformans in its natural environment, where CO 2 concentrations are limiting. Structural studies reveal that Can2 forms a homodimer in solution. Our data reveal Can2 to be the main carbonic anhydrase and suggest a physiological role for bicarbonate during C. neoformans growth. Bicarbonate directly activates the C. neoformans Cac1 adenylyl cyclase required for capsule synthesis. We show that this specific activation is optimal at physiological pH.

Molecular Mechanisms of Acid-Base Sensing by the Kidney

“…28 Indeed, sAC is associated with CO 2 sensing in fungi and bacteria. 29,30 Despite its name, sAC is present in both soluble and particulate cellular fractions and is thought to mediate cAMP-dependent processes at various intracellular locations, including mitochondria, nuclei, and centrioles. sAC is expressed in many kidney tubules, including proximal tubules, thick ascending limbs of Henle, distal convoluted tubules, and collecting ducts, in which high levels of V-ATPase are also found.…”

Evolution of carbonic anhydrases in fungi

“…Deletion of the C. albicans nce103 gene resulted in a lethal phenotype under low CO 2 -conditions, whereas in C. neoformans only can2, encoding the major CA, was shown to be essential under ambient air conditions. Heterologous expression of can2 from C. neoformans completely restored the high CO 2 -requiring phenotypes of S. cerevisiae and E. coli (Bahn et al 2005;Mogensen et al 2006). In the Wlamentous ascomycete Sordaria macrospora, three active -CA isoforms (CAS1, CAS2 and CAS3, carbonic anhydrase of Sordaria) have been characterized.…”

Carbonic anhydrases in fungi

“…In both fungi, adenylyl cyclase (AC) was shown to transmit the CO 2 signal in a CA-dependent manner (Bahn et al, 2005;Klengel et al, 2005;Mogensen et al, 2006). Like the mammalian and bacterial soluble ACs (Chen et al, 2000), the catalytic domains of C. albicans and C. neoformans ACs were shown to be activated by bicarbonate in vitro (Klengel et al, 2005;Mogensen et al, 2006).…”

“…All these genes encode b-class CAs and are able to rescue CA deletion strains of E. coli or S. cerevisiae (Bahn et al, 2005;Mogensen et al, 2006). The crystal structure of C.…”