Intersection of phosphate transport, oxidative stress and TOR signalling in Candida albicans virulence

Abstract

Phosphate is an essential macronutrient required for cell growth and division. Pho84 is the major high-affinity cell-surface phosphate importer of Saccharomyces cerevisiae and a crucial element in the phosphate homeostatic system of this model yeast. We found that loss of Candida albicans Pho84 attenuated virulence in Drosophila and murine oropharyngeal and disseminated models of invasive infection, and conferred hypersensitivity to neutrophil killing. Susceptibility of cells lacking Pho84 to neutrophil attack depended on reactive oxygen species (ROS): pho84-/- cells were no more susceptible than wild type C. albicans to neutrophils from a patient with chronic granulomatous disease, or to those whose oxidative burst was pharmacologically inhibited or neutralized. pho84-/- mutants hyperactivated oxidative stress signalling. They accumulated intracellular ROS in the absence of extrinsic oxidative stress, in high as well as low ambient phosphate conditions. ROS accumulation correlated with diminished levels of the unique superoxide dismutase Sod3 in pho84-/- cells, while SOD3 overexpression from a conditional promoter substantially restored these cells’ oxidative stress resistance in vitro. Repression of SOD3 expression sharply increased their oxidative stress hypersensitivity. Neither of these oxidative stress management effects of manipulating SOD3 transcription was observed in PHO84 wild type cells. Sod3 levels were not the only factor driving oxidative stress effects on pho84-/- cells, though, because overexpressing SOD3 did not ameliorate these cells’ hypersensitivity to neutrophil killing ex vivo, indicating Pho84 has further roles in oxidative stress resistance and virulence. Measurement of cellular metal concentrations demonstrated that diminished Sod3 expression was not due to decreased import of its metal cofactor manganese, as predicted from the function of S. cerevisiae Pho84 as a low-affinity manganese transporter. Instead of a role of Pho84 in metal transport, we found its role in TORC1 activation to impact oxidative stress management: overexpression of the TORC1-activating GTPase Gtr1 relieved the Sod3 deficit and ROS excess in pho84-/- null mutant cells, though it did not suppress their hypersensitivity to neutrophil killing or hyphal growth defect. Pharmacologic inhibition of Pho84 by small molecules including the FDA-approved drug foscarnet also induced ROS accumulation. Inhibiting Pho84 could hence support host defenses by sensitizing C. albicans to oxidative stress.

Hyperactive TORC1 sensitizes yeast cells to endoplasmic reticulum stress by compromising cell wall integrity

“…Conversely, reduced TORC1 signaling in oma1 Δ strains resulted in attenuated TORC1 signaling and increased virulence in Candida albicans . Thus, the amplitude of TORC1 signaling emerges as an important determinant of the capacity of C. albicans cells to withstand stress such as oxidative stress and perhaps ER stress, thus impacting its virulence and pathogenicity.…”

Phosphate in Virulence of Candida albicans and Candida glabrata

“…Cells lacking Pho84 show virulence attenuation in murine and Drosophila models of infection [75]. These cells are defective in hyphal growth and hypersensitive to killing by human whole blood and by neutrophils [75].…”

“…Cells lacking Pho84 show virulence attenuation in murine and Drosophila models of infection [75]. These cells are defective in hyphal growth and hypersensitive to killing by human whole blood and by neutrophils [75]. Releasing superoxide anion in the oxidative burst is a major antimicrobial killing mechanism of neutrophils; pho84 null mutant cells were not hypersensitive to neutrophils from a patient with chronic granulomatous disease which lack the capacity to generate an oxidative burst [75].…”

“…These cells are defective in hyphal growth and hypersensitive to killing by human whole blood and by neutrophils [75]. Releasing superoxide anion in the oxidative burst is a major antimicrobial killing mechanism of neutrophils; pho84 null mutant cells were not hypersensitive to neutrophils from a patient with chronic granulomatous disease which lack the capacity to generate an oxidative burst [75]. Accordingly cells lacking Pho84 poorly tolerate pharmacologic superoxide generators compared to wild type [75].…”

“…Releasing superoxide anion in the oxidative burst is a major antimicrobial killing mechanism of neutrophils; pho84 null mutant cells were not hypersensitive to neutrophils from a patient with chronic granulomatous disease which lack the capacity to generate an oxidative burst [75]. Accordingly cells lacking Pho84 poorly tolerate pharmacologic superoxide generators compared to wild type [75]. Measurement of intracellular reactive oxygen species (ROS) showed that both during extrinsic oxidative stress, and in unstressed normal growth conditions, pho84 null mutant cells contain more ROS than wild type [75].…”