Cryptococcal Titan Cell Formation Is Regulated by G-Protein Signaling in Response to Multiple Stimuli

Abstract

The titan cell is a recently described morphological form of the pathogenic fungus Cryptococcus neoformans. Occurring during the earliest stages of lung infection, titan cells are 5 to 10 times larger than the normal yeast-like cells, thereby resisting engulfment by lung phagocytes and favoring the persistence of infection. These enlarged cells exhibit an altered capsule structure, a thickened cell wall, increased ploidy, and resistance to nitrosative and oxidative stresses. We demonstrate that two G-protein-coupled receptors are important for induction of the titan cell phenotype: the Ste3a pheromone receptor (in mating type a cells) and the Gpr5 protein. Both receptors control titan cell formation through elements of the cyclic AMP (cAMP)/protein kinase A (PKA) pathway. This conserved signaling pathway, in turn, mediates its effect on titan cells through the PKA-regulated Rim101 transcription factor. Additional downstream effectors required for titan cell formation include the G 1 cyclin Pcl103, the Rho104 GTPase, and two GTPase-activating proteins, Gap1 and Cnc1560. These observations support developing models in which the PKA signaling pathway coordinately regulates many virulence-associated phenotypes in diverse human pathogens.

Titan Cell Production Enhances the Virulence of Cryptococcus neoformans

“…Strains of Cryptococcus neoformans var. grubii used in this study were KN99␣ (wild type) (30), CDX18 (gpr4⌬ gpr5⌬) (34,45), LHO31-1 (otc1⌬) (32), and LHO17 (gpr4⌬ GPR5). The LHO17 strain was isolated from the progeny of matings between CDX18 and KN99a.…”

Fungal Morphogenesis

“…Titan cells have polyploid nuclei, and undergo nuclear reduction to generate haploid progeny by unknown mechanisms (Zaragoza and Nielsen 2013). The process of titan cell formation is dependent on many central signaling proteins that control many aspects of cellular differentiation and morphogenesis, including pheromone receptors, PKA, the Rim101 transcription factor, GTPase-activating proteins Gap1 and Cnc1560, G 1 cyclin Pcl103, and Rho104 GTPase (Okagaki et al 2011).…”

Titan Cells: Barriers to Phagocytosis

“…Typically, Cryptococcal spores are inhaled into the lung, where they lodge in alveoli and establish pulmonary infections. Engulfment of normal cryptococcal cells by alveolar macrophages leads to formation of Titan cells via cell enlargement (up to 10 times) which includes cell cycle arrest or increase in DNA content (tetraploidy or octaploidy) [1,2]. Titan cell formation is induced by cAMP/PKA pathway, which helps in maintaining their virulence through thick anti-phagocytic polysaccharide capsule and melanin pigment [1].…”

“…Titan cell formation is induced by cAMP/PKA pathway, which helps in maintaining their virulence through thick anti-phagocytic polysaccharide capsule and melanin pigment [1]. Ste3a pheromone receptor and Gpr5 protein through cAMP/PKA pathway controls the Titan cell formation mediated by activation of Rim101 transcription factor [2].…”