Candida albicans is an opportunistic human fungal pathogen that causes invasive infections in immunocompromised individuals. Despite the high anticandidal activity among the echinocandins (ECNs), a first-line therapy, resistance remains an issue.

Fungal diseases largely affect human and animal health and dramatically diminish food crop yields [1]. Among fungi, systemic Candida infections are the second or third most common pathogens isolated from blood cultures in the USA [2]. Candida albicans is still the predominant Candida species, causing up to 50% of candidemia despite an increase in diversity of Candida species isolated from clinical samples [3]. In healthy individuals, C. albicans is a harmless inhabitant of mucosal surfaces throughout the body. However, in immune-compromised individuals, C. albicans can become a dangerous pathogen, causing severe or even fatal infections. In this review, we summarize recent data linking the reduced susceptibility of C. albicans cells to mainline echinocandin (ECN) drugs to aneuploidies of chromosomes 5 (Ch5) and Ch2. Evolution of ECN resistance and mechanisms influencing susceptibilityThe ECN drugs caspofungin, anidulafungin, and micafungin that are recommended as frontline therapy for candidiasis have few adverse actions and drug-drug interactions [4]. ECN drugs kill C. albicans cells by inhibiting glucan synthase, thus interfering with biosynthesis of the cell wall. Unlike well-studied multiple resistance mechanisms to fluconazole, another common anticandidal from the azole class, there is only one generally recognized mechanism of clinical resistance to drugs from the ECN class. This mechanism involves point mutations in the essential FKS1 gene (orf19.2929) encoding a catalytic subunit of the 1,3-β-glucan synthase complex. Mutations are clustered in two "hotspot" regions, HS1 and HS2, encompassing residues from 641 to 649, and from 1,345 to 1,365, respectively [5]. Mutations in these regions cause dramatic elevation of C. albicans minimum inhibitory concentration (MIC) values to ECNs and reduce the sensitivity of the glucan synthase to up to 3,000-fold, the concentration of caspofungin inhibiting 50% of enzymatic activity (IC50) [6].However, it has now become obvious that C. albicans possesses mechanisms independent of FKS1 mutations that can decrease susceptibility to ECNs, although these "alternative" mechanisms do not confer clinical resistance. These "alternative" mechanisms have been brought to light by dozens of clinical isolates of Candida species that display a wide range of increased MIC values for ECNs, including some at or below the MIC breakpoints, but, importantly, without canonical FKS1 mutations [7-10]. Consistent with these observations, several laboratories found that mutants lacking FKS1 mutations, but displaying (albeit relatively modest) 2 to 8 fold increases of MIC, can be easily generated in vitro on agar plates supplemented with caspofungin. While FKS1 mutations leading to resistance can also arise in vitro, these are typically rare [11][12][13]. Furthermore, while Cowen and colleagues observed evolution of ECN

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are present ubiquitously at the cell surface in all eukaryotes. They play a crucial role in the interaction of the cell with its external environment, allowing the cell to receive signals, respond to challenges, and mediate adhesion. In yeast and fungi, they also participate in the structural integrity of the cell wall and are often essential for survival. Roughly four decades after the discovery of the first GPI-APs, this review provides an overview of the insights gained from studies of the GPI biosynthetic pathway and the future challenges in the field. In particular, we focus on the biosynthetic pathway in Saccharomyces cerevisiae, which has for long been studied as a model organism. Where available, we also provide information about the GPI biosynthetic steps in other yeast/ fungi. Although the core structure of the GPI anchor is conserved across organisms, several variations are built into the biosynthetic pathway. The present Review specifically highlights these variations and their implications. There is growing evidence to suggest that several phenotypes are common to GPI deficiency and should be expected in GPI biosynthetic mutants. However, it appears that several phenotypes are unique to a specific step in the pathway and may even be species-specific. These could suggest the points at which the GPI biosynthetic pathway intersects with other important cellular pathways and could be points of regulation. They could be of particular significance in the study of pathogenic fungi and in identification of new and specific antifungal drugs/ drug targets. © 2018 IUBMB Life, 70(5):355-383, 2018.

Candida albicans is a prevalent fungal pathogen of humans. Understanding the development of decreased susceptibility to ECN drugs of this microbe is of substantial interest, as it is viewed as an intermediate step allowing the formation of FKS1 resistance mutations. We used six previously characterized mutants that decreased caspofungin susceptibility either by acquiring aneuploidy of chromosome 5 (Ch5) or by aneuploidy-independent mechanisms. When we exposed these caspofungin-adapted mutants to caspofungin again, we obtained 60 evolved mutants with further decreases in caspofungin susceptibility, as determined with CLSI method. We show that the initial adaptation to caspofungin is coupled with the adaptation to other ECNs, such as micafungin and anidulafungin, in mutants with no ploidy change, but not in aneuploid mutants, which become more susceptible to micafungin and anidulafungin. Furthermore, we find that the initial mechanism of caspofungin adaptation determines the pattern of further adaptation as parentals with no ploidy change further adapt to all ECNs by relatively small decreases in susceptibility, whereas aneuploid parentals adapt to all ECNs, primarily by large decrease in susceptibilities. Our data suggest that either distinct or common mechanisms can govern adaptation to different ECNs.

CaGpi14 is the catalytic subunit of the first mannosyltransferase that is involved in the glycosylphosphatidylinositol (GPI) biosynthetic pathway in Candida albicans. We show that CaGPI14 is able to rescue a conditionally lethal gpi14 mutant of Saccharomyces cerevisiae, unlike its mammalian homologue. The depletion of this enzyme in C. albicans leads to severe growth defects, besides causing deficiencies in GPI anchor levels. In addition, CaGpi14 depletion results in cell wall defects and upregulation of the cell wall integrity response pathway. This in turn appears to trigger the osmoticstress dependent activation of the HOG1 pathway and an upregulation of HOG1 as well as its downstream target, SKO1, a known suppressor of expression of hyphae-specific genes. Consistent with this, mutants of CaGPI14 are unable to undergo hyphal transformations in different hyphae-inducing media, under conditions that produce abundant hyphae in the wild-type cells. Hyphal defects in the CaGPI14 mutants could not be attributed either to reduced protein kinase C activation or to defective Ras signalling in these cells but appeared to be driven by perturbations in the HOG1 pathway.