Initial steps of wall protoplast regeneration in Candida albicans

Cryo-Electron Tomography of Candida glabrata Plasma Membrane Proteins

Fungal plasma membrane proteins have long been recognized as targets for the development of antifungal agents. Despite recent progress in experimental approaches and computational structural predictions, our knowledge of the structural dynamics and spatial distribution of these membrane proteins in the context of their native lipid environment remains limited. By applying cryo-electron tomography (cryoET) and subtomogram analysis, we aim to characterize the structural characteristics and spatial distribution of membrane proteins present in Candida glabrata plasma membranes. This study has resulted in the identification of the membrane-embedded structure of the fungal H+-ATPase, Pma1. Tomograms of the plasma membrane revealed that Pma1 complexes are heterogeneously distributed as hexamers that cluster into distinct membrane microdomains. This study characterizes fungal membrane proteins in the native cellular landscape and highlights the unique potential of cryoET to advance our understanding of cellular biology and biological systems.

Interactions between Cellulose and (1,3;1,4)-β-glucans and Arabinoxylans in the Regenerating Wall of Suspension Culture Cells of the Ryegrass Lolium multiflorum

Plant cell walls (PCWs) form the outer barrier of cells that give the plant strength and directly interact with the environment and other cells in the plant. PCWs are composed of several polysaccharides, of which cellulose forms the main fibrillar network. Enmeshed between these fibrils of cellulose are non-cellulosic polysaccharides (NCPs), pectins, and proteins. This study investigates the sequence, timing, patterning, and architecture of cell wall polysaccharide regeneration in suspension culture cells (SCC) of the grass species Lolium multiflorum (Lolium). Confocal, superresolution, and electron microscopies were used in combination with cytochemical labeling to investigate polysaccharide deposition in SCC after protoplasting. Cellulose was the first polysaccharide observed, followed shortly thereafter by (1,3;1,4)-β-glucan, which is also known as mixed-linkage glucan (MLG), arabinoxylan (AX), and callose. Cellulose formed fibrils with AX and produced a filamentous-like network, whereas MLG formed punctate patches. Using colocalization analysis, cellulose and AX were shown to interact during early stages of wall generation, but this interaction reduced over time as the wall matured. AX and MLG interactions increased slightly over time, but cellulose and MLG were not seen to interact. Callose initially formed patches that were randomly positioned on the protoplast surface. There was no consistency in size or location over time. The architecture observed via superresolution microscopy showed similarities to the biophysical maps produced using atomic force microscopy and can give insight into the role of polysaccharides in PCWs.

Antibiofilm Activity on Candida albicans and Mechanism of Action on Biomembrane Models of the Antimicrobial Peptide Ctn[15-34]

Ctn[15–34], the C-terminal fragment of crotalicidin, an antimicrobial peptide from the South American rattlesnake Crotalus durissus terrificus venom, displays remarkable anti-infective and anti-proliferative activities. Herein, its activity on Candida albicans biofilms and its interaction with the cytoplasmic membrane of the fungal cell and with a biomembrane model in vitro was investigated. A standard C. albicans strain and a fluconazole-resistant clinical isolate were exposed to the peptide at its minimum inhibitory concentration (MIC) (10 µM) and up to 100 × MIC to inhibit biofilm formation and its eradication. A viability test using XTT and fluorescent dyes, confocal laser scanning microscopy, and atomic force microscopy (AFM) were used to observe the antibiofilm effect. To evaluate the importance of membrane composition on Ctn[15–34] activity, C. albicans protoplasts were also tested. Fluorescence assays using di-8-ANEPPS, dynamic light scattering, and zeta potential measurements using liposomes, protoplasts, and C. albicans cells indicated a direct mechanism of action that was dependent on membrane interaction and disruption. Overall, Ctn[15–34] showed to be an effective antifungal peptide, displaying antibiofilm activity and, importantly, interacting with and disrupting fungal plasma membrane.

Identification of whole pathogenic cells by monoclonal antibodies generated against a specific peptide from an immunogenic cell wall protein

We selected the immunogenic cell wall ß-(1,3)-glucosyltransferase Bgl2p from Candida albicans as a target protein for the production of antibodies. We identified a unique peptide sequence in the protein and generated monoclonal anti- C. albicans Bgl2p antibodies, which bound in particular to whole C. albicans cells.

Collection of proteins secreted from yeast protoplasts in active cell wall regeneration

The yeast cell wall is a dynamic and complex matrix of polysaccharides (glucans, mannans, and chitin), proteins and minor amounts of lipids that isolate the yeast from the extracellular medium, protecting it against osmotic and physical injuries. Removal of this essential structure for cell integrity and viability by controlled enzymatic digestion in an iso-osmotic medium brings about protoplast formation. When yeast protoplasts are incubated in an osmotically stabilized liquid nutrient medium, cell wall precursors are secreted into the culture medium to de novosynthesize the cell wall. During the early stages of the regeneration process of protoplast walls, many wall protein precursors (presumably structural proteins along with remodeling and cross-linking enzymes) are shed into the extracellular medium but not covalently incorporated into the nascent cell wall, intriguingly enabling their easy isolation and solubilization. We have developed a method to collect proteins secreted from yeast protoplasts in active cell wall regeneration under conditions that are suitable for subsequent proteomic analyses. This procedure circumvents some of the troubles intrinsically related to other extraction protocols of cell wall proteins, such as chemical or enzymatic modifications, and poor quality in protein resolution and identification because of linkages to glucan/chitin residues. It further offers a valuable model system to understand how the de novocell wall biosynthesis occurs in the yeast cell or how the yeast cell wall participates in morphogenesis.

Genomic response programs of Candida albicans following protoplasting and regeneration

Transcription profiling of Candida albicans cells responding to the elimination of the wall (protoplasts) and posterior regeneration was explored. DNA microarrays were used to measure changes in the expression of 6039 genes, and the upregulated genes during regeneration at 28 degrees C were assigned to fourteen categories. A total of 407 genes were upregulated during the process, of which 144 reached a maximum after 1 h. MKC1, a gene encoding a member of the regulatory pathway involved in cell wall integrity was overexpressed. Time-dependent expression divided the genes into 40 clusters. Clusters 1-19 were highly expressed initially (time 0) and downregulated following incubation, whereas transcription of the genes grouped into clusters 20-40 showed the opposite behaviour. These results suggest that the first clusters group genes permitting the cell adaptation to a sub-optimal environment due to removal of the wall, whereas the second group represents genes required for protoplasts regeneration after shifted to optimal conditions from 4 to 28 degrees C.

Variations in mRNA transcript levels of cell wall-associated genes of Saccharomyces cerevisiae following spheroplasting

mRNA transcript levels of 38 genes from Saccharomyces cerevisiae were investigated during attempted spheroplast regeneration. Many of the genes selected are involved in cell wall biosynthesis. Spheroplasts did not regenerate into osmotically competent cells during the experiment. However, at a mRNA level, the quantities of transcripts were altered between the experimental and control populations. KRE11, EGT2 and MSS10 had their transcript levels increased by more than 10-fold during attempted spheroplast regeneration. A further six genes, FLO1, TIR1, SED1, HKR1, YGR189 and MUC1, showed transcript level increases of at least 5-fold. Five genes showed a change in transcript levels from an undetectable level to a detectable level: SKT5, KRE1, KRE5, SEC53 and DHS1. PMT2 showed a rapid decrease in mRNA levels followed by an increase to the basal level. Thus, cell stress genes, biosynthetic genes and some glycosylphosphatidylinositol-anchored cell wall proteins have their transcript levels increased in regenerating spheroplasts, but their transcription was not sufficient to initiate the replacement of the cell wall in liquid medium.