Boyce, Alex Andrianopoulos; Fungal dimorphism: The ability of pathogenic fungi to switch between a multicellular hyphal and unicellular yeast growth is a tightly regulated process known as dimorphic switching. Dimorphic switching requires the fungus to sense and respond to the host environment and is essential for pathogenicity.
This review will focus on the role of dimorphism in fungi commonly called thermally dimorphic Phosphorelay sporulation asexual reproduction, which switch to a yeast growth form during infection. This group of phylogenetically diverse ascomycetes includes Talaromyces marneffei recently renamed from Penicillium marneffeiBlastomyces dermatitidis teleomorph Ajellomyces dermatitidisCoccidioides species C. This review will explore both the signalling pathways regulating the morphological transition and the transcriptional responses necessary for intracellular growth.
The physiological requirements of yeast cells during infection will also be discussed, highlighting recent advances in the understanding of the role of iron and calcium acquisition during infection.
Fungi generate a variety of cellular morphologies to facilitate adaptation and colonization of new environmental niches. Highly polarized multicellular hyphae or unicellular yeast cells are the most commonly utilized cellular morphologies. However, fungi are also capable of producing specialized cell types with unique cellular morphologies during developmental pathways such as sexual or asexual reproduction.
For fungi that cause disease, infections are usually initiated by the inhalation of dormant spores generally asexual conidiaproduced outside of the host during the differentiation of the hyphal growth form asexual or sexual development.
In the lungs, host innate immune cells such as macrophages and neutrophils recognize these Specific fungal cell-wall components termed pathogen-associated molecular patterns PAMPs are recognized via membrane-associated pattern recognition receptors PRRs on macrophage membranes.
The phagocytes deploy mechansims to destroy the fungal cell through the phagolysosomal system by generating damaging reactive oxygen species ROSproducing various hydrolytic enzymes and restricting nutrients Fig.
Phagocytes of the host immune system destroy fungal cells through the phagolysosomal pathway. In the lungs of an immunocompetent host, the fungal infectious propagules usually asexual spores are recognized by host innate immune cells such as macrophages and neutrophils. Once recognized, fungal cells are phagocytosed into an early phagosome.
Lysosomes fuse with the late phagosome to produce the phagolysosome, a compartment with low pH and which contains hydrolytic enzymes to further damage the fungal cell. The MHC complex displays peptide fragments from the destroyed fungal cell for recognition by T cells. The activated macrophage releases cytokines to stimulate other cells of the immune system. Fungal cells may survive within the macrophage by the neutralization or adaptation to ROS and RNS production, by preventing the release of cytokines and by inducing genes allowing the acquisition of iron and calcium.
Some fungi have developed adaptations to circumvent the effectiveness of these host defence responses. A number of these fungi reside within phagocytotic cells of the host where they are shielded from the rest of the immune system. As protracted hyphal growth within phagocytes would lead to cell rupture, thus exposing the fungus to the host immune system, a number of fungi switch from the multicellular hyphal growth form found in the environment to a unicellular yeast growth "Phosphorelay sporulation asexual reproduction" in a process known as dimorphic switching Fig.
Other dimorphic use the yeast cell form to avoid phagocytosis and the cytotoxic environment of the phagolysosomal system; instead, they are adapted to tolerating the adaptive immune responses. Thus, dimorphic switching allows for the colonization of unique environmental niches within the host and the failure to switch almost always attenuates pathogenicity in these fungi.
In other dimorphic fungi which exist predominately as a yeast vegetative growth form outside the host, such as the plant pathogen Ustilago maydis and the human pathogen Candida albican s, the dimorphic switch from a yeast to a filamentous growth form can Phosphorelay sporulation asexual reproduction tissue penetration during infection Liu ; Nadal, Garcia-Pedrajas and Gold ; "Phosphorelay sporulation asexual reproduction," Rapala-Kozik and Kozik This review will focus on the role of dimorphism in pathogenic fungi which switch to a yeast growth form during infection.
Cellular morphologies of dimorphic human fungal pathogens.
The growth morphologies of the dimorphic ascomycetes Histoplasma capsulatum Ajellomyces capsulatumBlastomyces dermatitidis Ajellomyces dermatitidisTalaromyces marneffei Penicillium marneffeiCoccidioides immitis, Paracoccidioides brasiliensis and Sporothrix schenckii Ophiostoma schenckii. For a number of these fungi, the yeast form serves to accommodate intracellular growth within host phagocytes.
This group of fungi, with the exception of "Phosphorelay sporulation asexual reproduction." Despite this, their impact has only recently Phosphorelay sporulation asexual reproduction appreciated and consequently incidence data lags behind that of other microbes, including fungi such as C. The importance of these pathogens has lead to renewed interest in understanding how the key determinant of pathogenicity, dimorphic switching, is controlled and how these fungi cause disease.
Identification of genes required during the dimorphic switching process has occurred predominantly using targeted candidate gene or genome-wide expression approaches. Techniques to genetically manipulate these fungi have been developed, including RNA interference and Agrobacterium -mediated transformation, but site-specific gene deletion and mutation is limited either due to non-integration of introduced DNA or non-homologous DNA integration.
The exception is T. Despite these limitations, significant progress has been made in understanding the molecular mechanisms controlling dimorphic switching. This review will explore both the signalling pathways regulating the morphological transition to the yeast growth form and the transcriptional and morphological processes required for intracellular growth in this group of fungal pathogens.
Phosphorelay sporulation asexual reproduction particular, recent advances in understanding the role of iron and calcium acquisition during infection will be highlighted. Where appropriate, reference will be made to model fungi where there is a deeper understanding of the molecular mechanisms that impinge on homologous morphogenetic processes and these will be related to what is known in the dimorphic fungi. A number of signalling pathways have been identified that induce the dimorphic switch Fig.
Since Phosphorelay sporulation asexual reproduction growth also necessitates survival through the macrophage phagolysosomal system, these signalling pathways also frequently coregulate processes important for adaptation to this environment, such as adaptation to oxidative stress.
These pathways include two-component and heterotrimeric G protein signalling systems as well as Ras and cAMP signalling and the downstream mitogen-activated protein kinase MAPK signalling cascades Fig. The colours indicate characterized roles in dimorphism: The HK perceives the environmental stimulus and is autophosphorylated at a conserved histidine in the kinase domain.
The HPt protein transfers the phosphate to an aspartate residue in the RR.
Ras the Rho GTPase Cdc42, which in Phosphorelay sporulation asexual reproduction to controlling actin-mediated polarized growth, regulates the p21 activated kinases Ste20 and Cla4 to signal via MAPK pathways and regulate cellular division. CnaA dephosphorylates the CrzA transcription factor to allow entry into the nucleus to activate genes controlling processes such as cell-wall synthesis, germination of spores conidiaion homeostasis, pH adaptation, polarity and conidiophore development.
In bacteria, two-component systems comprise a membrane-associated histidine kinase HK and a cytoplasmic response regulator RR. However, despite this expansion, the phosphorylation signal is still transmitted to only two RRs, orthologous to Ssk1 and Skn7, via a single HPt, orthologous to Ypd1.
DRK1 mutants of B. Deletion of the T. Deletion of either slnA or drkA in T. Thus, these class III and VI HHKs are essential for morphogenesis and adaptation to high osmotic and oxidative stress environments, such as those encountered in the intracellular environment of the host macrophage. Mutations in DRK1 in B. The integrity of the cell wall is crucial for surviving both ROS and hydrolysis in the host macrophage. Similar to the non-dimorphic fungi Botrytis cinerea and A.
Importantly, the DRK1 orthologues in B. Recent protein profiling has shown that "Phosphorelay sporulation asexual reproduction" is a highly abundant, differentially expressed protein in the yeast growth phase of S.
Therefore, the DrkA-regulated two-component signalling pathway plays a conserved, essential role during the initiation of the dimorphic switch. It is now important to identify the downstream components of these pathways in order to understand the specific roles they play in morphogenesis, osmoregulation and cell-wall integrity.
Thus, the role of these signalling complexes, and their receptors that may sense the trigger to effect dimorphic switching does not appear to be highly conserved amongst the dimorphic fungi.
Ras protein function requires farnesylation for correct membrane association, and the addition of farnesyltransferase inhibitors to P. Therefore, a heterotrimeric G protein and Ras signalling pathway clearly influence dimorphic switching in P.
Host or temperature-derived signals trigger dimorphic switching and are likely to be transmitted via signalling pathways, ultimately culminating in changes in gene expression. Many transcriptome-based approaches have identified phase yeast or hyphal specific genes but little is known to date about how the expression of these genes is controlled. The RYP1—4 transcription factors from H.
RYP1 mutants also inappropriately produce vegetative spores conidia in liquid cultures suggesting that Ryp1 regulates multiple developmental processes Nguyen and Sil Comparison of the transcriptional profile of wild-type H. Ryp1 shows homology to a family of transcriptional regulators that function as master regulators of fungal morphogenesis. Interestingly, despite this highly conserved recognition site, comparison of target genes for each regulator suggests that there is very little overlap.
Ryp2 and Ryp3 have homology to the velvet family of developmental regulators first identified in A. Ryp2 is orthologous to A. Orthologues of these global regulators of morphogenesis are conserved across the fungi kingdom and it is clear that they are intimately linked to the regulation of morphogenetic processes such as dimorphic switching and asexual development, and processes such as secondary metabolism that are intimately associated with morphogenesis and differentiation.
Phosphorelay sporulation asexual reproduction remains to be elucidated is the link between the signalling pathways known to regulate the dimorphic transition and these core transcription factors. The switch from hyphal to yeast growth requires changes in polarized growth and cellular morphology.
The Rho GTPases Cdc42, Rac and Rho are members of the Ras GTPase superfamily that play conserved roles in regulating morphogenesis by controlling actin-mediated polarized growth and signalling pathways required for morphological responses Fig. Cdc42 is also required for recruitment of actin and polarisome components to the sites of polarized growth reviewed in Johnson RNAi-mediated silencing of P.
RNAi cdc42 knockdown strains were more efficiently phagocytosed by macrophages and displayed decreased pathogenicity, demonstrating that the large, multibudded state of P.
Phosphorelay sporulation asexual reproduction, CflB is required for the germination of conidia in macrophages Boyce, Schreider and Andrianopoulos The Ste20 orthologue in T. This is supported by the dual roles of the T.
This link has also been suggested for H. Interestingly, neither of these fungi have an abaA orthologue. Furthermore, pakB is strongly upregulated in T. This implies that PakB is part of a signalling pathway that responds to host cell inductive signals, not temperature.
Whether this mechanism is conserved in other intracellular pathogens such as H. The correct execution of budding division is also essential for yeast morphogenesis.
Septins are GTP-binding cytoskeleton components which form heterooligomeric complexes, such as filaments and rings, that are essential for cellular division.
Deletion of the gene encoding the type II myosin in T. One common survival strategy used by dimorphic fungi is to rapidly remodel the cell wall during infection in order to prevent recognition by phagocytic cell PRRs. Consistent with this, in P. Expression of the Chs3 chitin synthase is induced in P. Thus, masking or obscuring cell-wall components detected by the Phosphorelay sporulation asexual reproduction is a common mechanism of avoiding the full induction of the host's defence mechanisms.
In the phagosomal system, this is accompanied by the release of hydrolytic enzymes and toxic metabolites into the phagosome and lowering of the phagolysomal pH to aid in pathogen killing. required for oxidative stress resistance were recently found to be components of the extracellular proteome of H. Expression of genes encoding superoxide dismutase is also induced when P.
Fungal phosphorelay systems consist of one or several hybrid HKs, In this fungus, asexual reproduction (conidiation) is induced by environmental signals such. Induction of asexual sporulation, sample processing, and brlA Northern blot.
up to 50 asexual reproductive spores, or conidiospores [1, 2]. Conidiospores .