Fungal cells change shape in response to environmental stimuli, and these morphogenic transitions drive pathogenesis and niche adaptation. For example, dimorphic fungi switch between yeast and hyphae in response to changing temperature. The basidiomycete Cryptococcus neoformans undergoes an unusual morphogenetic transition in the host lung from haploid yeast to large, highly polyploid cells termed Titan cells. Titan cells influence fungal interaction with host cells, including through increased drug resistance, altered cell size, and altered Pathogen Associated Molecular Pattern exposure. Despite the important role these cells play in pathogenesis, understanding the environmental stimuli that drive the morphological transition, and the molecular mechanisms underlying their unique biology, has been hampered by the lack of a reproducible in vitro induction system. Here we demonstrate reproducible in vitro Titan cell induction in response to environmental stimuli consistent with the host lung. In vitro Titan cells exhibit all the properties of in vivo generated Titan cells, the current gold standard, including altered capsule, cell wall, size, high mother cell ploidy, and aneuploid progeny. We identify the bacterial peptidoglycan subunit Muramyl Dipeptide as a serum compound associated with shift in cell size and ploidy, and demonstrate the capacity of bronchial lavage fluid and bacterial co-culture to induce Titanisation. Additionally, we demonstrate the capacity of our assay to identify established (cAMP/PKA) and previously undescribed (USV101) regulators of Titanisation in vitro. Finally, we investigate the Titanisation capacity of clinical isolates and their impact on disease outcome. Together, these findings provide new insight into the environmental stimuli and molecular mechanisms underlying the yeast-to-Titan transition and establish an essential in vitro model for the future characterization of this important morphotype.

Background: Ca2+ signals, elicited by cues from the oocyte and female tract, regulate human sperm behavior.Results: CatSper channel activation (flagellum) and Ca2+ store mobilization (neck) caused similar [Ca2+]i elevation but induced functionally different behaviors.Conclusion: Sperm motility pattern is determined by the site of Ca2+ mobilization.Significance: Selection of Ca2+ signaling components and/or regulation of their availability for activation controls human sperm behavior.

[Ca2 +]i signalling is a key regulatory mechanism in sperm function. In mammalian sperm the Ca2 +-permeable plasma membrane ion channel CatSper is central to [Ca2 +]i signalling, but there is good evidence that Ca2 + stored in intracellular organelles is also functionally important. Here we briefly review the current understanding of the diversity of Ca2 + stores and the mechanisms for the regulation of their activity. We then consider the evidence for the involvement of these stores in [Ca2 +]i signalling in mammalian (primarily human) sperm, the agonists that may activate these stores and their role in control of sperm function. Finally we consider the evidence that membrane Ca2 + channels and stored Ca2 + may play discrete roles in the regulation of sperm activities and propose a mechanism by which these different components of the sperm Ca2 +-signalling apparatus may interact to generate complex and spatially diverse [Ca2 +]i signals.