Cell wall integrity signalling in human pathogenic fungi

Agents that activate the High Osmolarity Glycerol pathway as a means to combat pathogenic molds

Treatment of invasive fungal infections often fails due to the limited number of therapeutic options. In this study, we have analyzed the impact of agents activating the High Osmolarity Glycerol (HOG) pathway on molds that cause infections in humans and livestock. We found that agents like fludioxonil and iprodione, have a clear anti-fungal activity against pathogenic Aspergillus, Lichtheimia, Rhizopus and Scedosporium species. Only A. terreus turned out to be resistant to fludioxonil, even though it is sensitive to iprodione and able to adapt to hyperosmotic conditions. Moreover, the A. terreus tcsC gene can fully complement an A. fumigatus ΔtcsC mutant, thereby also restoring its sensitivity to fludioxonil. The particular phenotype of A. terreus is therefore likely to be independent of its TcsC kinase. In a second part of this study, we further explored the impact of fludioxonil using A. fumigatus as a model organism. When applied in concentrations of 1-2μg/ml, fludioxonil causes an immediate growth arrest and, after longer exposure, a quantitative killing. Hyphae respond to fludioxonil by the formation of new septa and closure of nearly all septal pores. Mitosis occurs in all compartments and is accompanied by a re-localization of the NimA kinase to the cytoplasm. In the swollen compartments, the massive extension of the cell wall triggers a substantial reorganization resulting in an enhanced incorporation of chitin and, most strikingly, a massive loss of galactomannan. Hence, HOG-activating agents have dramatic cell biological consequences and may represent a valuable, future element in the armory that can be used to combat mold infections.

Identification of Genes in Candida glabrata Conferring Altered Responses to Caspofungin, a Cell Wall Synthesis Inhibitor

Candida glabrata is an important human fungal pathogen whose incidence continues to rise. Because many clinical isolates are resistant to azole drugs, the drugs of choice to treat such infections are members of the echinocandin family, although there are increasing reports of resistance to these drugs as well. In efforts to better understand the genetic changes that lead to altered responses to echinocandins, we screened a transposon-insertion library of mutants for strains to identify genes that are important for cellular responses to caspofungin, a member of this drug family. We identified 16 genes that, when disrupted, caused increased tolerance, and 48 genes that, when disrupted, caused increased sensitivity compared to the wild-type parental strain. Four of the genes identified as causing sensitivity are orthologs of Saccharomyces cerevisiae genes encoding proteins important for the cell wall integrity (CWI) pathway. In addition, several other genes are orthologs of the high affinity Ca²⁺ uptake system (HACS) complex genes. We analyzed disruption mutants representing all 64 genes under 33 different conditions, including the presence of cell wall disrupting agents and other drugs, a variety of salts, increased temperature, and altered pH. Further, we generated knockout mutants in different genes within the CWI pathway and the HACS complex, and found that they too exhibited phenotypes consistent with defects in cell wall construction. Our results indicate that small molecules that inhibit the CWI pathway, or that the HACS complex, may be an important means of increasing the efficacy of caspofungin.

Aspergillus fumigatus MADS-Box Transcription Factor rlmA Is Required for Regulation of the Cell Wall Integrity and Virulence

The Cell Wall Integrity (CWI) pathway is the primary signaling cascade that controls the de novo synthesis of the fungal cell wall, and in Saccharomyces cerevisiae this event is highly dependent on the RLM1 transcription factor. Here, we investigated the function of RlmA in the fungal pathogen Aspergillus fumigatus. We show that the ΔrlmA strain exhibits an altered cell wall organization in addition to defects related to vegetative growth and tolerance to cell wall-perturbing agents. A genetic analysis indicated that rlmA is positioned downstream of the pkcA and mpkA genes in the CWI pathway. As a consequence, rlmA loss-of-function leads to the altered expression of genes encoding cell wall-related proteins. RlmA positively regulates the phosphorylation of MpkA and is induced at both protein and transcriptional levels during cell wall stress. The rlmA was also involved in tolerance to oxidative damage and transcriptional regulation of genes related to oxidative stress adaptation. Moreover, the ΔrlmA strain had attenuated virulence in a neutropenic murine model of invasive pulmonary aspergillosis. Our results suggest that RlmA functions as a transcription factor in the A. fumigatus CWI pathway, acting downstream of PkcA-MpkA signaling and contributing to the virulence of this fungus.