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Chapter 30 : Signaling Pathways in the Dimorphic Human Fungal Pathogen Penicillium marneffei
Category: Fungi and Fungal Pathogenesis
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This chapter focuses on the heterotrimeric G proteins and their role in growth and morphogenesis in the thermally dimorphic human pathogen Penicillium marneffei. Signaling is terminated when the intrinsic GTPase activity of the α subunit leads to the hydrolysis of GTP to GDP, thus leading to the reassociation of the α subunit and the βγ complex. There are three a subunits encoding genes identified in P. marneffei: gasA, gasB, and gasC. The gasA and gasC genes play distinct roles in P. marneffei morphogenesis yet show some overlap. Two major differences may explain this difference in regulation in P. marneffei compared to Saccharomyces cerevisiae and Ustilago maydis. First, the latter organisms are both predominantly yeasts that undergo a yeast-to-hypha transition while P. marneffei is predominantly hyphal and switches between a hyphal and yeast form. Second, the inducing signal(s) for dimorphic switching for the latter organisms is chemical in nature while the signal for P. marneffei is temperature, a physical signal. A growing number of genes encoding α subunits have been cloned from various fungal species, and their biological functions have been investigated. These studies have demonstrated the importance and the complexity of G protein signaling in the regulation of key processes of fungal life cycles, such as dimorphic switching, mating, asexual development, and pathogenicity.
P. marneffei life cycle. A diagram of the P. marneffei life cycle, showing the various vegetative growth and developmental stages, is presented. The solid circles represent nuclei in cells. The filamentous phase exhibited at 25°C is shown above the dashed line and is characterized by hyphal growth and the asexual development program which produces conidia. The yeast phase expressed at 37°C is shown below the dashed line and is characterized by the arthroconidiation program, yeast growth, and division by fission. The dimorphic switch which links the yeast and hyphal growth phases in response to temperature is shown. (From reference 2 with permission.)
Heterotrimeric G protein signaling. Heterotrimeric G proteins are trimers whose function depends on the ability to dissociate into an α monomer and a βγ dimer. G proteins are generally coupled to 7-TM receptors. When GDP is bound, the α subunit associates with the βγ subunit to form an inactive complex that binds to the receptor. On binding of the signal to the receptor, the α subunit becomes activated and exchanges GDP for GTP, thus dissociating from the receptor and the β complex. Both the α and βγ subunits can trigger downstream signaling pathways. RGS proteins accelerate the GTP hydrolysis rate of the Gα subunit, thus driving the GTP-bound α subunit back to the inactive GDP-bound form.
Signaling by the gasA-encoded α subunit. GasA signaling at 25°C is likely to involve both a cAMP-PKA pathway and a MAPK pathway in the negative regulation of asexual development and the expression of the core regulatory factors BrlA and AbaA. GasA also plays a minor positive role in the regulation of secondary metabolic pathways. However, it has no apparent role during yeast cell morphogenesis at 37°C. The signal/ligand for this process is not defined. The degree of regulation of each process is indicated by the thickness of the line.
Toxicological studies of GasA signaling. The predicted signaling pathway through GasA to the cAMP-PKA pathway, based on genetic and physiological studies using chemical inhibitors and analogues as well as known pathways in related fungi, is shown. Adenylate cyclase (AC), phosphodiesterase (PDE), and PKA with its catalytic (C) and regulatory (R) subunits are illustrated, as is the action of H-89, theophylline, and dbcAMP on the cAMP-PKA pathway. The effect of these toxicological agents on growth and development for the wild type (GasA) and the strains carrying either a gasA dominant activating (GasAACT), dominant negative (GasANEG), or deletion (∆GasA) allele is presented. The studies support the involvement of a cAMP-PKA pathway in GasA signaling.
Control of conidial germination. Conidial germination in P. marneffei is controlled by the action of the α-subunit-encoding gasC, the Ras GTPase-encoding rasA, and the small Rho-type (CDC42) GTPase-encoding cflA genes. Dominant activated gasCACT or dominant activated cflAACT alleles result in precocious germination under inducing conditions (i.e., presence of a carbon source). Conidial germination is delayed in a ∆gasC strain or in strains carrying the dominant negative cflANEG allele or dominant negative or dominant activated rasAACT /rasANEG alleles. The rasA gene is upstream of cflA based on genetic interaction studies.
Proposed model for signal transduction pathways regulating germination, asexual development, and secondary metabolism in P. marneffei. The α subunit GasC plays a major role in the regulation of germination. GasC also regulates the production of secondary metabolites and, to a lesser extent, asexual development. The action of the subunit in GasC signaling is unclear, and it is proposed that GasC acts through the cAMP-PKA pathway. The signals and receptors linked to this G protein are unknown. The degree of regulation of each process is indicated by the thickness of the line.
Proposed model for G protein signaling in P. marneffei. The α subunit GasA signals via the cAMP-PKA pathway and a MAPK pathway to regulate asexual development and, to a lesser extent, secondary metabolism. GasC plays a major role in the regulation of germination. GasC also regulates secondary metabolites and, to a lesser extent, asexual development and is proposed to signal through the cAMP-PKA pathway. The overlap between GasA and GasC function may reflect the fact that they may share a subunit. The action of the α subunit GasB is unknown. The degree of regulation of each process is indicated by the thickness of the line.