New molecular methods to study gene functions in Candida infections

Systems biology of fungal infection

Elucidation of pathogenicity mechanisms of the most important human-pathogenic fungi, Aspergillus fumigatus and Candida albicans, has gained great interest in the light of the steadily increasing number of cases of invasive fungal infections. A key feature of these infections is the interaction of the different fungal morphotypes with epithelial and immune effector cells in the human host. Because of the high level of complexity, it is necessary to describe and understand invasive fungal infection by taking a systems biological approach, i.e., by a comprehensive quantitative analysis of the non-linear and selective interactions of a large number of functionally diverse, and frequently multifunctional, sets of elements, e.g., genes, proteins, metabolites, which produce coherent and emergent behaviors in time and space. The recent advances in systems biology will now make it possible to uncover the structure and dynamics of molecular and cellular cause-effect relationships within these pathogenic interactions. We review current efforts to integrate omics and image-based data of host-pathogen interactions into network and spatio-temporal models. The modeling will help to elucidate pathogenicity mechanisms and to identify diagnostic biomarkers and potential drug targets for therapy and could thus pave the way for novel intervention strategies based on novel antifungal drugs and cell therapy.

Reporter gene assays in Candida albicans

Reporter systems are used in Candida albicans in three major experimental areas. These include gene expression, promoter analysis, and protein expression/localization. Heterologous expression in C. albicans is either not effective or inefficient due to the alternative codon usage in Candida, particularly CTG. Consequently, several reporter genes have been constructed by optimizing codons for expression in Candida. The reporter systems include lacZ, luciferase, and GFP. Generally, PCR site directed mutagenesis has been used to construct the modified reporter. Reporter gene vectors are not commercially available for Candida, but they can normally be requested from the laboratories that developed the constructs.

Genetically regulated filamentation contributes to Candida albicans virulence during corneal infection

Candida albicans is a commensal fungus of the normal flora yet causes opportunistic infection following trauma or surgery and during immunosuppression. C. albicans virulence factors include morphogenesis into invasive filaments, adherence to host cells, and secretion of proteases. This study evaluated the role of fungal hyphal extension in experimental C. albicans keratitis using genetically altered yeast strains. Scarified corneas of adult BALB/c mice were topically inoculated with wild-type (SC5314) or 10 transposon-induced mutant strains of C. albicans and monitored for 4 days post inoculation (PI). In vitro growth kinetics and the yeast strains' ability to bud into pseudohyphae or hyphae were also compared. The wild-type human isolate had a high degree of virulence in the murine cornea, and four fungal strains deficient in genes regulating adherence or encoding membrane proteins did not significantly differ from the parental strain (P>0.3). Five yeast strains deficient in genes involved in filamentation resulted in fully or partially attenuated keratomycosis (P<0.0001). The overall growth kinetics of wild-type and mutant strains were similar in rich media (P>0.9), but mutants with deficient morphogenesis had reduced filamentation in vitro. Phenotypic switching from yeasts to filamentous forms facilitates the establishment and progression of experimental corneal disease by C. albicans.

Expression of interleukin-8 and its receptor IL-8RA in chronic hyperplastic candidosis

INTRODUCTION

Neutrophils are the main opponents of Candida albicans in chronic hyperplastic candidosis. They migrate from the circulation to the epithelium where they form microabscesses. We therefore hypothesized that the neutrophil chemokine interleukin-8 (IL-8) might play a role in the neutrophil-Candida interaction.

METHODS

Biopsies from patients with chronic hyperplastic candidosis (n = 10) were stained using the avidin-biotin-peroxidase complex protocol for IL-8 and IL-8 receptor A and were compared to healthy control mucosa (n = 3). A set of C. albicans agar sections was similarly analysed.

RESULTS

In chronic hyperplastic candidosis lesions IL-8 was strongly expressed in both vascular endothelium and mucosal epithelium. Many resident and immigrant inflammatory cells, including intraepithelial neutrophils, were IL-8 receptor A positive. In addition, IL-8 (or an analogue) was found in the candidal mother cell in chronic hyperplastic candidosis and in agar, whereas the tips of the hyphae expressed IL-8 receptor A (or an analogue).

CONCLUSION

IL-8 may play a role in the recruitment of neutrophils from the vascular compartment to the epithelial microabscesses. C. albicans may have developed an ability to sense IL-8. The IL-8 ligand-receptor interaction may help to direct the growth of the IL-8-receptor-containing tips of the hyphae away from the IL-8-producing candidal cell body (a centrifugal growth pattern to facilitate host tissue penetration). Later, this ability might help to keep the vulnerable hyphal tips away from areas with high concentrations of host IL-8 and candidacidal neutrophils. We suggest that this phenomenon, in contrast to chemotropism, is named chemophobia.

The functional basis of mycophenolic acid resistance in Candida albicans IMP dehydrogenase

Candida albicans is an important fungal pathogen of immunocompromised patients. In cell culture, C. albicans is sensitive to mycophenolic acid (MPA) and mizoribine, both natural product inhibitors of IMP dehydrogenase (IMPDH). These drugs have opposing interactions with the enzyme. MPA prevents formation of the closed enzyme conformation by binding to the same site as a mobile flap. In contrast, mizoribine monophosphate, the active metabolite of mizoribine, induces the closed conformation. Here, we report the characterization of IMPDH from wild-type and MPA-resistant strains of C. albicans. The wild-type enzyme displays significant differences from human IMPDHs, suggesting that selective inhibitors that could be novel antifungal agents may be developed. IMPDH from the MPA-resistant strain contains a single substitution (A251T) that is far from the MPA-binding site. The A251T variant was 4-fold less sensitive to MPA as expected. This substitution did not affect the k(cat) value, but did decrease the K(m) values for both substrates, so the mutant enzyme is more catalytically efficient as measured by the value of k(cat)/K(m). These simple criteria suggest that the A251T variant would be the evolutionarily superior enzyme. However, the A251T substitution caused the enzyme to be 40-fold more sensitive to mizoribine monophosphate. This result suggests that A251T stabilizes the closed conformation, and this hypothesis is supported by further inhibitor analysis. Likewise, the MPA-resistant strain was more sensitive to mizoribine in cell culture. These observations illustrate the evolutionary challenge posed by the gauntlet of chemical warfare at the microbial level.