Immunology & Infectious Diseases

Judith Rhodes

Judith Rhodes, PhD

Pathology & Lab Med

Professor

Research Summary

The long-term goal of the research in Dr. Rhodes’ laboratory is to understand the pathogenesis of fungal infections. The model disease being studied is invasive aspergillosis, an often fatal infection of immunocompromised patients caused by the ubiquitous organism Aspergillus fumigatus.

The Rhodes lab is currently studying three different, but related, signal transduction pathways to determine their roles in the pathogenesis of invasive infections. Two of these pathways were initially identified as up-regulated genes during growth of A. fumigatus on mammalian cell monolayers. The first candidate gene from this screen is a member of the ras family of GTPases called rheb. The gene rhbA appears to be involved in nutrient sensing, especially nitrogen sensing. Mutants in which the wild type gene has been depleted have reduced growth on poor nitrogen sources when compared with the wild type, reduced virulence, and the mutants are hypersensitive to the drug rapamycin, which is an inhibitor of TOR kinase. Based on our studies, we placed RhbA in the TOR pathway, upstream from TOR, which has subsequently been confirmed in other systems. The regulators of rheb in the model organisms Saccharomyces cerevisiae and Schizosaccharomyces pombe appear to be different, so we are testing to see if Aspergillus follows one of these paradigms. We are investigating the proteins that interact with RhbA using recombinant rheb in in vitro label-transfer experiments in an effort to resolve this question.

Protein kinase A signaling has been shown to be involved in stress response and virulence in a number of fungal pathogens of plants and animals, and the regulatory subunit transcript was another candidate gene identified in our screen. Our initial studies on PKA activity in A. fumigatus have shown that the sensitivity to exogenous cAMP, the molecule responsible for activating PKA, is linked to the process called carbon catabolite repression. The end result of this is that the ability of A. fumigatus to respond to increases in cAMP is dependent upon on the carbon source present in the medium, and presumably in the environment. Mutants that we have constructed that lack functional copies of the regulatory or the C1 catalytic subunit of PKA are being used to study the role of PKA in stress response, since we hypothesize that initiation of infection in a mammalian host would be a stressful situation for the fungus. As part of this investigation we are working to define the phosphoproteome of PKA in A. fumigatus, so that we may better understand this regulatory network.

Our interest in cAMP-regulated signaling pathways has led to a study of the rasA and rasB genes in A. fumigatus. These genes appear to be responsible for early steps in the developmental pathway that leads to conidiation and the spatial patterning in hyphal growth. Since these processes are crucial for the initiation of infection, we would like to understand these processes and how they are regulated. In our initial studies we used dominant negative and dominant active alleles of both ras genes, but we have now made deletion mutants. Mutants lacking rasB have remarkable hyperbranching phenotypes, in which groups of branches may sprout from a single site. Localization of GFP-tagged versions of the ras genes is being used to determine the role that compartmentalization plays in controlling the functions of the proteins.

From these studies, we hope to develop a more complete picture of how A. fumigatus able to deal with the stresses involved in the transition from growth in environmental niches to growth in the mammalian lung, how the invasive growth pattern is initiated and maintained, and what the nutritional cues are that allow this organism to be so nutritionally versatile.

Recent Publications

Fuller KK, Zhao W, Askew DS, Rhodes JC. Deletion of the protein kinase Aregulatory subunit leads to deregulation of mitochondrial activation and nuclear duplication in Aspergillus fumigatus. Eukaryot Cell. 2009 Mar;8(3):271-7. Epub2009 Jan 5. PubMed PMID: 19124579; PubMed Central PMCID: PMC2653251.

Fortwendel JR, Fuller KK, Stephens TJ, Bacon WC, Askew DS, Rhodes JC.Aspergillus fumigatus RasA regulates asexual development and cell wall integrity.Eukaryot Cell. 2008 Sep;7(9):1530-9. Epub 2008 Jul 7. PubMed PMID: 18606827;PubMed Central PMCID: PMC2547073.

Rhodes JC. Aspergillus fumigatus: growth and virulence. Med Mycol. 2006 Sep;44Suppl 1:S77-81. Review. PubMed PMID: 17050423.

Zhao W, Panepinto JC, Fortwendel JR, Fox L, Oliver BG, Askew DS, Rhodes JC.Deletion of the regulatory subunit of protein kinase A in Aspergillus fumigatusalters morphology, sensitivity to oxidative damage, and virulence. Infect Immun. 2006 Aug;74(8):4865-74. PubMed PMID: 16861675; PubMed Central PMCID: PMC1539607.

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