Identification of the dialysable serum inducer of germ-tube formation in Candida albicans

Abstract

Yeast cells of Candida albicans are induced by serum at 37 6C to produce germ tubes, the first step in a transition from yeast to hyphal growth. Previously, it has been shown that the active component is not serum albumin but is present in the dialysable fraction of serum. In this study, serum induction of germ-tube formation is shown to occur even in the presence of added exogenous nitrogen sources and is therefore not signalled by nitrogen derepression. The active component in serum was purified by ion-exchange, reverse-phase and size-exclusion chromatography from the dialysable fraction of serum and was identified by NMR to be D-glucose. Enzymic destruction of glucose, using glucose oxidase, demonstrated that D-glucose was the only active component in these fractions. Induction of germ-tube formation by D-glucose required a temperature of 37 6C and the pH optimum was between pH 7?0 and 8?0. D-Glucose induced germ-tube formation in a panel of clinical isolates of C. albicans. Although D-glucose is the major inducer in serum, a second non-dialysable, trichloroacetic acid precipitable inducer is also present. However, whereas either 1?4 % (v/v) serum or an equivalent concentration of D-glucose induced 50 % germ-tube formation, the non-dialysable component required a 10-fold higher concentration to induce 50 % germ-tube formation. Serum is, therefore, the most effective induction medium for germ-tube formation because it is buffered at about pH 8?5 and contains two distinct inducers (glucose and a non-dialysable component), both active at this pH.

Environmental Sensing and Signal Transduction Pathways Regulating Morphopathogenic Determinants of Candida albicans

“…However, deletion of either CDC25 or RAS1 affected the glucose-and serum-induced cAMP signal, consistent with mediation of the glucoseinduced cAMP increase in C. albicans by the Cdc25-Ras1 branch of the glucose response pathway and probably not via the Gpr1-Gpa2 branch (193). Addition of serum results in a rapid response, similar to that seen with glucose, supporting a recent finding that glucose is the main factor in serum affecting Candida morphogenesis (135). Two important gaps remain in our understanding of the upstream components of the cAMP-PKA pathway in C. albicans: (i) the identity of the receptor activating the Cdc25 guanine nucleotide exchange protein and (ii) the nature of the ligand for Gpr1.…”

Regulatory Circuitry Governing Fungal Development, Drug Resistance, and Disease

“…For decades, the component of serum responsible for triggering morphogenesis remained elusive. For some time, the glucose in serum was suggested to be responsible for morphogenesis (243); however, it was recently shown that bacterial peptidoglycans in serum trigger hyphal growth by directly activating the adenylyl cyclase Cyr1 (635). RPMI and M199 media also stimulate yeast-to-hypha morphogenesis at elevated temperatures and are thought to act through a common pathway with serum.…”

Calcineurin Is Required forCandida albicansTo Survive Calcium Stress in Serum

“…Early work suggested that serum had a lethal effect on C. albicans and that this activity was heat stable and dialyzable (30), but it was later demonstrated that the reduced numbers of CFU measured by these experiments were attributable to increased aggregation of Candida cells rather than to a lethal effect of serum (10). This serum-induced aggregation is related to serum stimulation of the yeast-tohypha transition, and though the search for the switching stimulus in serum has been one of the most enduring questions in Candida pathogenesis (4,17,25,36,46), no obvious culprit has been definitively identified yet. Recent studies reveal that serum does become profoundly toxic to C. albicans when calcineurin is either inhibited or mutated (7,44), findings reminiscent of the early C. albicans-serum work.…”