Evaluation of the Role of TOK Channels in Candida albicans

Abstract

Fungal disease affects billions of people globally each year. They range from mild superficial infections to serious systemic diseases. The mortality rate associated with invasive fungi is above 40 %, killing over 1.6 million people annually, more than tuberculosis and malaria combined. Currently, there are only four main classes of antifungal treatments, all of which are prone to issues of resistance, toxicity, high associated costs. Therefore, developing new strategies to tackle fungal pathogens is urgently required. Two-pore domain Outwardly-rectifying K+ (TOK) channels are potassium selective plasma membrane ion channel proteins unique to fungi and are thought to be the main contributors to major outward K+ currents. At present, the physiological role of TOK channels remains elusive, but evidence suggests that their activation leads to reduced fungal viability and virulence, making them an attractive potential therapeutic target for future antimycotic agents. This project aimed to decipher the role of TOK channels in Candida albicans with a view to validate them as viable therapeutic targets to treat fungal diseases. Heterologous expression of cloned C. albicans TOK (CaTOK) channels in Xenopus laevis oocytes were used to assess basic pharmacology by two-electrode voltage clamp electrophysiology. In addition a range of microbiological assays, utilising wild-type and genetically modified strains of the model yeast, C. albicans, were used to evaluate CaTOKs influence on fungal growth, cell viability and morphology. Electrophysiological assay identified the quorum sensing molecule farnesol as a pharmacological activator, and tetraethylammonium and Ba2+ ions as inhibitors of the CaTOK channel. However, there were no observable differences in growth, viability or morphology of Candida albicans strains or their ability to form biofilms where CaTOK channel functional activity had been genetically or pharmacologically altered. Explanations for the lack of physiological impact of modulating CaTOK in fungi are critically discussed, but despite a range of circumstantial evidence in the literature the CaTOK channel was not validated a suitable target for future antifungal therapies.

Date of Award	10 May 2023
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Anthony Lewis (Supervisor) & Roger Draheim (Supervisor)