Candida glabrata's Genome Plasticity Confers a Unique Pattern of Expressed Cell Wall Proteins

Abstract

Candida glabrata is the second most common cause of candidemia, and its ability to adhere to different host cell types, to microorganisms, and to medical devices are important virulence factors. Here, we consider three characteristics that confer extraordinary advantages to C. glabrata within the host. (1) C. glabrata has a large number of genes encoding for adhesins most of which are localized at subtelomeric regions. The number and sequence of these genes varies substantially depending on the strain, indicating that C. glabrata can tolerate high genomic plasticity; (2) The largest family of CWPs (cell wall proteins) is the EPA (epithelial adhesin) family of adhesins. Epa1 is the major adhesin and mediates adherence to epithelial, endothelial and immune cells. Several layers of regulation like subtelomeric silencing, cis-acting regulatory regions, activators, nutritional signaling, and stress conditions tightly regulate the expression of many adhesin-encoding genes in C. glabrata, while many others are not expressed. Importantly, there is a connection between acquired resistance to xenobiotics and increased adherence; (3) Other subfamilies of adhesins mediate adherence to Candida albicans, allowing C. glabrata to efficiently invade the oral epithelium and form robust biofilms. It is noteworthy that every C. glabrata strain analyzed presents a unique pattern of CWPs at the cell surface.

Keywords: Candida glabrata; adherence; cell wall proteins; clinical isolates; fluconazole resistance; genome plasticity; subtelomeric silencing; virulence.

Figure 1

Map of the right subtelomeric region of chromosome E (E_-R) where the EPA1, EPA2, and EPA3 cluster is localized. EPA1 expression is negatively regulated by the cis-acting element called Negative Element (NE, orange rectangle) shown as an orange T arrow, which depends on yKu70 and yKu80 proteins (yKu, brown circle). The dashed orange arrow indicates the genetic requirement of yKu for the activity of NE. EPA2 is induced under oxidative stress and requires Yap1 (purple) and Skn7 (dark blue) transcription factors. EPA3 is repressed by the protosilencer Sil2126 (dark gray rectangle), indicated by the black T arrow from Sil2126 to EPA3 promoter. EPA1, EPA2, and EPA3 are subject to another layer of global regulation called subtelomeric silencing (indicated by a horizontal black triangle below the chromosome and a black T arrow from the telomere to EPA1p). Silencing propagates from the telomere (T) and depends on the SIR complex (Sir2, Sir3, and Sir4) (2, 3, and 4), Rap1 (yellow circle), Rif1 (blue), and yKu proteins. EPA3 expression responds also to osmotic stress and glucose starvation.

Figure 2

EPA6 and EPA7 promoters are negatively regulated by their respective negative elements (NEs). (A top) Schematic representation of the subtelomeric EPA6 gene and its downstream NE. EPA6 is repressed by subtelomeric silencing (indicated by a black horizontal triangle). (A bottom) Activity of the promoter of EPA6 is negatively regulated by its NE. (B top) Schematic representation of the subtelomeric EPA7 gene and its downstream NE. EPA7 is repressed by subtelomeric silencing (indicated by a black horizontal triangle). (B bottom) Activity of the promoter of EPA7 is negatively regulated by its NE. Replicative plasmids containing EPA6 or EPA7 promoter fused with YFP were transformed into Candida glabrata wild-type strain. One plasmid contained a heterologous 3′ UTR from the HIS3 gene. A second plasmid contained the cognate EPA6 or EPA7 3′ UTR containing its NE. Stationary phase cultures of each of these strains were diluted into fresh rich media and fluorescence was measured by FACS every two hours. Results are the mean of three biological repetitions. Statistical analysis was performed using 2 way ANOVA Bonferroni posttests. * means p < 0.05, ** p < 0.01 and *** p < 0.001.