Morphogenesis in Candida albicans

Abstract

Candida albicans is termed a dimorphic fungus because it proliferates in either a yeast form or a hyphal form. The switch between these forms is the result of a complex interplay of external and internal factors and is coordinated in part by polarity-regulating proteins that are conserved among eukaryotic cells. However, yeast and hyphal cells are not the only morphological states of C. albicans. The opaque form required for mating, the pseudohyphal cell, and the chlamydospore represent distinct cell types that form in response to specific genetic or environmental conditions. In addition, hyperextended buds can form as a result of various cell cycle-related stresses. Recent studies are beginning to shed light on some of the molecular controls regulating the various morphogenetic forms of this fascinating human pathogen.

Figure 1

Distinct morphological forms of C. albicans. In yeast-form growth a blastospore buds off a new cell, resulting in two discrete cells. The separated spindle pole bodies elaborate spindles that separate the chromosomes across the mother-daughter junction defined by a septin band (green). Polarized growth is defined by a crescent-shaped polarisome at the tip of the growing bud. In pseudohyphal growth the nuclear division also crosses the mother-daughter junction defined by a septin band, and polarized growth is characterized by the polarisome. The cells themselves are more elongated than during yeast growth, and the cells remain attached after cytokinesis. Hyphal growth is defined by both a polarisome and a Spitzenkorper at the tip of the growing hyphae. The nucleus divides within the elongating germ tube across the region that defines the eventual septum and then one nucleus migrates back to the mother cell and the other moves farther into the elongating germ tube. Opaque-form cells are capable of responding to mating pheromone by elongating a mating projection, or shmoo. The nucleus migrates into the projection and, after fusion of the mating projection to a cell of the opposite mating type, undergoes a Kar3p-mediated fusion with the other nucleus. Chlamydospores are formed at the end of suspensor cells. They have a thicker cell wall and are larger than blastospores; the nucleus divides within the suspensor cell and then the daughter nucleus migrates into the chlamydospore across a septin structure. The final chlamydospore has an elaborate septin-derived substructure. See text for details.