Candida albicans ?Candida albicans is the major human fungal pathogen, causing tenacious superficial mycoses, as well as life-threatening systemic mycoses. Superficial infections of the skin and mucosal layers are common, such as “soor” in the oral cavity and vaginal candidiasis, which most women acquire at least once in their lifetimes. Systemic infections, in which multiple internal organs are affected, are of increasing importance in immunocompromised patients. Life expectancy of patients with systemic candidiasis and mycoses in general is poor, since antifungal therapies are limited. But why does C. albicans cause disease, unlike related harmless yeasts such as baker´s yeast? What are the molecular mechanisms, which contribute to its virulence and allow it to colonize and persist in the human body? |
yeast forming hypha |
|
Thick-walled chlamydospores arising on pseudohyphae
Rod/"opaque"-form (left) and yeast/"white"-form (right) |
An amazing variety of cell shapesC. albicans has an amazing flexibility to alter its cellular forms and current data strongly suggest that this ability to change is of significant importance for its virulence. C. albicans is able to grow unicellularly in a yeast form and multiply by budding, very similar to baker´s yeast. In the presence of inducing compounds such as serum yeast cells rapidly develop true hyphae, which expand by continuous apical growth. In this growth form C. albicans resembles molds such as Aspergillus or Penicillium. Hyphal filaments allow C. albicans to firmly anchor in host cells and to penetrate endothelial and epithelial layers. In certain environments the pathogen grows as pseudohyphae, characterized by elongated yeast cells, which propagate by unipolar budding and remain attached, thus giving this growth form a “beads on a chain”-appearance. Some isolates of C. albicans are able to spontaneously switch between a “white”, typical yeast-cell form, and an “opaque”-form, which appears as short rods. The reasons and functions of this “phenotypic switching” are intensely investigated at the present time. Finally, in certain environments C. albicans is able to form thick-walled chlamydospores, which are of yet unknown importance for its biology and virulence. |
|
Signalling pathways How are external environmental cues translated into the cellular program of differentiation that models a yeast form into a hypha? Current views state that several signalling pathways act in concert to trigger this morphological switch. Hypha induction by serum or certain sugars present in host cell membranes, such as N-acetylglucosamine, critically requires a protein kinase A (PKA) signalling pathway. A key regulator of morphogenesis, Efg1p, appears to get activated by PKA and to set the stage for the induction of hypha-specific genes, while repressing yeast-specific genes. In addition, a “mitogen-activated protein” (MAP)-kinase pathway also contributes to the development of true hyphae. Since hypha formation only occurs at body temperature, at low cell density and at neutral pH, pathways must exist that convey these external signals; in fact, components of a pH-pathway have been established recently. But which are the targets of these signalling pathways and how do they operate to allow the reprogramming of cellular activities in a sequential manner? |
|
|
|
Efg1p-a key regulator of morphogenesis In our laboratory we have identified Efg1p, which is one of the key regulators of morphogenesis in C. albicans. Efg1p is a transcription factor of the bHLH-type and its sequence indicates homology to transcription factors conserved in fungi (APSES-proteins), which all are involved in the interconversion of a spherical cell into a filament, such as yeast-hypha or hypha-spore transition. In C. albicans the presence of Efg1p is of paramount importance to allow hyphal morphogenesis in most inducing media. Also, chlamydospore formation is blocked in efg1 null mutants. Finally, lowering intracellular Efg1p-levels generates an “opaque”-like rod form, while EFG1 overexpression completely switches an “opaque”-form back into the “white”-typical yeast form; thus, Efg1p is also involved in phenotypic switching. In our current research we analyze the molecular events, which activate Efg1p and study the processes triggered by Efg1p. Components upstream of Efg1p include two PKA isoforms, which appear to have specific functions depending on the environment. Downstream components are under intense investigation at present in our group. In this search and analyses we take advantage of the knowledge of the complete C. albicans genomic sequence and the availability of DNA arrays containing all C. albicans genes (about 6000). |
|
Cell surface events The structure of the fungal cell surface determines the processes occurring during the contact with human host cells, such as adhesion, phagocytosis and penetration. We are in the process of analysing the PMT gene family of C. albicans, which encodes mannosyltransferases modifying secreted proteins by short mannose chains at serine or threonine residues (O-glycosylation). The PMT family encodes 5 Pmt isoforms, which appear to have different target specificities. Interestingly, the lack of the Pmt1-isoform not only blocks adhesion and abolishes virulence, but also leads to defective hypha morphogenesis and supersensitivity to antifungal drugs. But which protein targets are modified by specific Pmt proteins? One target appears to be a component of the secretory machinery (Sec20p), which we are studying in detail for this reason. To study the events occurring during the contact of C. albicans with host cells we are setting up model host cell systems, such as artificial monolayers of endothelial cells and epithelial cells (Caco-2), as well as a skin model. Interestingly, we recently discovered that siderophore uptake (siderophores are small iron-chelating compounds secreted by microorganisms) is crucial for the invasion of the skin model, while it is not essential once the fungus has entered the blood stream (presumably because heme and other iron compounds are present there). Thus, C. albicans may have to adapt to different host cells and situations and this wide repertoire of responses may determine overall virulence, rather than a single dominant virulence factor. |
Candida hypha leaving infected endothelial cell |
|
Applied aspects Much of the recent interest in Candida is due to the need for effective new antifungal compounds. Our basic research on virulence traits of C. albicans reveals reactions necessary for fungal pathogenicity and viability; such reactions may be considered potential targets for novel antifungal compounds. In some cases, such as processes involving the Efg1p regulator and the Pmt proteins described above, homologous components are missing in human cells; therefore, inhibition by a potential antifungal compound should be very selective to only attack the fungal invader and not cause side-effects in humans. |
|
|
Funding Our current research is funded by the following grants:
Candida links
|
budding yeast form
pseudohypha
