Chitin and ?(1?3) glucan synthases in the protoplastic entomophthorales

Human Pathogenic Entomophthorales

The pathogenic entomophthoralean fungi cause infection in insects and mammalian hosts. Basidiobolus and Conidiobolus species can be found in soil and insect, reptile, and amphibian droppings in tropical and subtropical areas. The life cycles of these fungi occur in these environments where infecting sticky conidia are developed. The infection is acquired by insect bite or contact with contaminated environments through open skin. Conidiobolus coronatus typically causes chronic rhinofacial disease in immunocompetent hosts, whereas some Conidiobolus species can be found in immunocompromised patients. Basidiobolus ranarum infection is restricted to subcutaneous tissues but may be involved in intestinal and disseminated infections. Its early diagnosis remains challenging due to clinical similarities to other intestinal diseases. Infected tissues characteristically display eosinophilic granulomas with the Splendore-Höeppli phenomenon. However, in immunocompromised patients, the above-mentioned inflammatory reaction is absent. Laboratory diagnosis includes wet mount, culture serological assays, and molecular methodologies. The management of entomophthoralean fungi relies on traditional antifungal therapies, such as potassium iodide (KI), amphotericin B, itraconazole, and ketoconazole, and surgery. These species are intrinsically resistant to some antifungals, prompting physicians to experiment with combinations of therapies. Research is needed to investigate the immunology of entomophthoralean fungi in infected hosts. The absence of an animal model and lack of funding severely limit research on these fungi.

The cell wall: a carbohydrate armour for the fungal cell

The cell wall is composed of a polysaccharide-based three-dimensional network. Considered for a long time as an inert exoskeleton, the cell wall is now seen as a dynamic structure that is continuously changing as a result of the modification of culture conditions and environmental stresses. Although the cell wall composition varies among fungal species, chemogenomic comparative analysis have led to a better understanding of the genes and mechanisms involved in the construction of the common central core composed of branched beta1,3 glucan-chitin. Because of its essential biological role, unique biochemistry and structural organization and the absence in mammalian cells of most of its constitutive components, the cell wall is an attractive target for the development of new antifungal agents. Genomic as well as drug studies have shown that the death of the fungus can result from inhibition of cell wall polysaccharide synthases. To date, only beta1,3 glucan synthase inhibitors have been launched clinically and many more targets remain to be explored.

Differential expression of chitin synthase (CHS) and glucan synthase (FKS) genes correlates with the formation of a modified, thinner cell wall in in vivo-produced Beauveria bassiana cells

During infection (in vivo), the entomopathogenic fungus Beauveria bassiana produces yeast-like cells that are surrounded by modified cell walls. These modifications have been related to the fungus ability to limit recognition by the host defense system. The composition of the in vivo cell wall was analyzed using a combination of cytochemical and molecular techniques. The in vivo cell walls still contained both chitin and 1,3-beta-glucan, but they were significantly thinner than in vitro cell walls (50-60 nm versus 100-160 nm, respectively). The difference in cell wall thickness was correlated with transcriptional regulation of cell wall-related genes: quantitative RT-PCR reactions demonstrated that B. bassiana chitin synthase (CHS) and glucan synthase (FKS) genes are down regulated in vivo. These analyses indicate that in vivo-triggered phenotypic modifications, including cell wall adjustments, are controlled by molecular mechanisms that include regulation of gene expression at the transcriptional level.

Cloning of two chitin synthase gene fragments from a protoplastic entomophthorale

Chitin synthase expression was studied in spontaneously produced protoplasts and in hyphal bodies of the Entomophthorale species Entomophaga aulicae. The absence of wall in protoplasts was correlated to an absence of chitin synthase. Two chitin synthase activities with different biochemical characteristics have been detected in the hyphal bodies. Two chitin synthase gene fragments EaCHS1 and EaCHS2 of 600 bp were obtained using PCR amplification of genomic DNA. Their amino acid sequences showed 75% identity. Compared with other fungal chitin synthases, they belong to class II. EaCHS1 and EaCHS2 were used to probe total RNA from E. aulicae hyphal bodies and protoplasts. A single transcript of 2.4 kb hybridized only with EaCHS1 in protoplasts and hyphal bodies.

Evasion of host defense by in vivo-produced protoplast-like cells of the insect mycopathogen Beauveria bassiana

In vivo cells (hyphal bodies) of the hyphomycetous insect pathogen Beauveria bassiana collected from host Spodoptera exigua larval hemolymph were osmotically sensitive and lacked a well-defined cell wall. In light and electron microscope studies, a galactose-specific lectin purified from S. exigua hemolymph, concanavalin A (specific for alpha-mannose), and a polyclonal antibody to B. bassiana cell walls all bound to surfaces of in vitro-produced B. bassiana blastospores; however, none of these probes labelled the thin layer of extracellular material covering the plasma membranes of hyphal bodies. These cells were observed freely circulating in S. exigua hemolymph at 36 h postinfection, although immunocompetent hemocytes were known to be present. Additionally, association of hyphal bodies with hemocytes in monolayers was significantly less than for opsonized in vitro blastospores or submerged conidia. The absence of antigenically important galactomannan components on in vivo cells may therefore allow these cells to escape recognition and phagocytosis. Lack of structural components (e.g., chitin, as evidenced by the absence of binding of wheat germ agglutinin) may also be important with respect to evasion of host cellular defense mechanisms. Production of wall material resumed 48 to 60 h postinfection and therefore may coincide with loss of phagocytic capabilities of the hemocytes due to immunosuppressive effects of fungal metabolites. The protoplast-like cells may be formed by the action of hydrolytic enzymes in the hemocytes or by inhibition of fungal cell wall synthetases.

Biosynthesis of beta-glucans in fungi

Glucans are the most abundant polysaccharides present in fungi. The present review provides updated information on the structure and synthesis of beta-glucans in fungal cells. Synthesis of these polymers made up of B1,3 chains with a variable degree of B1,6 branching involves several reactions: initiation, chain elongation and branching, of which the most studied one is the elongation step. This reaction, catalyzed by the so-called glucan synthetases, utilizes UDPG as sugar donor. Properties of glucan synthetases are extremely variable depending on the fungal species, and their developmental stage. Because of the importance of these polysaccharides it is anticipated that comprehension of their mechanism of synthesis, is important for the understanding of cell wall assembly and cell growth and morphogenesis, as well as for the design of specific antifungal drugs.