Rapid Proliferation Compensates for Defective Filamentation in Candida albicans Pathogenesis

Abstract

A recent study (Dunker et al.) has shown that a Candida albicans mutant, defective for filamentation, is fully virulent due to rapid cellular proliferation in host tissues. These findings challenge the current paradigm in C. albicans pathogenesis and suggest that defects in one virulence property can be compensated for by enhancements in another.

C. albicans is the most frequently isolated human fungal pathogen and the fourth leading cause of hospital-acquired bloodstream infections in the USA. [1]. Mortality rates for these infections are approximately 40% [2], and a wide variety of immunocompromised individuals, including cancer patients on chemotherapy, organ transplant recipients, and AIDS patients, are susceptible. For many years, the ability of C. albicans to undergo a reversible morphological transition from single budding yeast cells to hyphal filaments, which can facilitate tissue invasion and macrophage lysis, has been considered a ‘hallmark’ virulence trait. Initially, this hypothesis was supported by the finding that mutants locked in either the filamentous or yeast form were highly defective for virulence in a mouse disseminated model of candidiasis [3,4]. A subsequent study demonstrated that allowing a yeast-locked C. albicans strain to form filaments at different time points during a disseminated infection was sufficient to promote virulence [5]. One limitation of these studies, however, is that they depended on transcriptional regulators to manipulate C. albicans morphology, so it was unclear whether the observed results were due to morphology per se or expression of other genes unrelated to morphology. However, a subsequent large-scale functional genomics study showed that most mutants defective for the yeast–filament transition were attenuated for virulence, and a previous report demonstrated that deletion of HGC1, which encodes a cyclin-related protein directly involved in filamentation, results in a strain that is defective for virulence [6,7].

Although filamentation is clearly important for C. albicans virulence, is it actually required? A recent study by Dunker et al. suggests that filamentation is not an absolute requirement for virulence, at least under certain conditions [8]. This study used the C. albicans eed1Δ/Δ mutant, which is defective for filament extension [9]; EED1 is also important for adhesion, dissemination, and escape from epithelial cells, and is induced in biofilms and under high cell density. Consistent with previous work, the eed1Δ/Δ mutant showed reduced damage to macrophages as well as to renal, hepatic, and oral epithelial cells in vitro and was attenuated for virulence in an intraperitoneal infection model [8]. Also consistent with previous findings, the eed1Δ/Δ mutant demonstrated reduced translocation across the gastrointestinal (GI) tract in immunocompetent mice. Unexpectedly, however, in a mouse model of systemic candidiasis, this mutant showed equivalent virulence to that of a wild-type strain at medium inoculum sizes and enhanced virulence at low inoculum sizes. Mice infected with the mutant had a delayed renal cytokine response but showed increased infiltration of immune cells at later infection time points. Interestingly, enhanced virulence of eed1Δ/Δ yeast cells was associated with rapid proliferation and high organ fungal burdens, particularly at late time points of infection. Several lines of evidence are presented to suggest that rapid proliferation occurs as a result of better metabolic adaptation, including the observation that the eed1Δ/Δ mutant shows enhanced growth on physiological carbon sources and in kidney homogenates [8]. The finding that enhanced virulence occurs specifically in the systemic infection model, where there is more opportunity for rapid proliferation, also supports this hypothesis. These findings are significant because they suggest that, under the appropriate conditions, virulence defects of filament-defective C. albicans strains can be compensated for by rapid proliferation of yeast cells as a result of enhanced metabolic adaptation (Figure 1).

Figure 1.

Rapid Cellular Proliferation Can Compensate for Defective Filamentation in Candida albicans Pathogenesis. In a wild-type C. albicans strain (left) the ability to undergo a reversible morphological transition from yeast to filamentous cells plays an important role in determining virulence potential (thick arrow). The eed1Δ/Δ mutant (right) is defective for filamentation but maintains virulence potential by rapid proliferation of yeast cells (thick arrow), most likely as a consequence of metabolic adaptation. Other independent mechanisms, which have not yet been identified, could also possibly function to maintain virulence potential of the eed1Δ/Δ mutant (broken line) (adapted from [12]).

The results of Dunker et al. [8] are consistent with those of a previous report by Noble et al. in which screening of a homozygous deletion library that covered about 11% of the C. albicans genome identified 115 mutants that were defective for infectivity (based on kidney fungal burden in a mouse systemic model) [10]; interestingly, about half of these mutants were not defective for the yeast–filament transition, suggesting that morphogenesis is not an absolute requirement for C. albicans pathogenicity. This study also showed that several of these mutants were specifically required for glucosylceramide biosynthesis and highly attenuated for virulence in a mouse systemic model [10]. Although the exact mechanism by which glucosylceramide biosynthesis promotes C. albicans virulence is unclear, maintenance of membrane integrity remains a possibility. Importantly, both the Dunker et al. and Noble et al. studies paint a more complex picture of C. albicans pathogenesis, suggesting that many factors, in addition to filamentation, play a role in ultimately determining virulence potential [8,10].

While the C. albicans yeast–filament transition is known to be an important virulence property, this appears to be more the exception, rather than the rule, for several other highly successful fungal pathogens. Candida glabrata and Histoplasma capsulatum, for example, grow primarily in the yeast form during infection, and a recent study has shown that two non-albicans Candida strains, genetically engineered to undergo strong filamentation, have significantly reduced fungal burdens in a mouse systemic model [11]. The findings of Dunker et al. are also consistent with the notion that other virulence properties not related to filamentation, such as rapid cellular proliferation, can play more important roles in fungal pathogenesis. This study suggests that C. albicans has retained an ability to use these virulence properties, even in cases where filamentation is not an option, and highlights the strong adaptability of this pathogen [8]. Based on these findings, it is likely that other properties, in addition to rapid cellular proliferation, can also compensate for attenuated C. albicans virulence that occurs as a consequence of defective filamentation. Such properties could also be mediated by Eed1 (Figure 1), and determining the precise biochemical function of this factor would be useful in this regard. To date, only very few mutants from the Noble et al. screen that are attenuated for pathogenesis but not defective for filamentation have been characterized [10], and additional studies that focus on such mutants may also shed more light on novel properties that can promote virulence of C. albicans and other fungal pathogens. Ultimately, gaining a more comprehensive and integrated understanding of the balance among various virulence properties that contribute to fungal pathogenesis will help to inform the development of more effective antifungal strategies.