Sterol-response pathways mediate alkaline survival in diverse fungi

Abstract

33The ability for cells to maintain homeostasis in the presence of extracellular stress is 34 essential for their survival. Stress adaptations are especially important for microbial pathogens 35 to respond to rapidly changing conditions, such as those encountered during the transition from 36 the environment to the infected host. Many fungal pathogens have acquired the ability to quickly 37 adapt to changes in extracellular pH to promote their survival in the various micro-environments 38 encountered during a host infection. For example, the fungal-specific Rim/Pal alkaline response 39 pathway has been well characterized in many fungal pathogens, including Cryptococcus 40 neoformans. However, alternative mechanisms for sensing and responding to host pH have yet 41 to be extensively studied. Recent observations from a genetic screen suggest that the C. 42neoformans sterol homeostasis pathway is required for growth at elevated pH. This work 43 explores interactions among mechanisms of membrane homeostasis, alkaline pH tolerance, 44and Rim pathway activation. We find that the sterol homeostasis pathway is necessary for 45 growth in an alkaline environment, and that an elevated pH is sufficient to induce Sre1 46 activation. This pH-mediated activation of the Sre1 transcription factor is linked to the 47 biosynthesis of ergosterol, but is not dependent on Rim pathway signaling, suggesting that 48 these two pathways are responding to alkaline pH independently. Furthermore, we discover that 49 C. neoformans is more susceptible to membrane-targeting antifungals in alkaline conditions 50 highlighting the impact of micro-environmental pH on the treatment of invasive fungal infections. 51Together, these findings further connect membrane integrity and composition with the fungal pH 52 response and pathogenesis. 53Diverse cell types, from simple unicellular microorganisms to complex multicellular 55 eukaryotes, interpret alterations in extracellular pH as a common signal for changes in the 56 external environment. Pathogenic microorganisms are often uniquely exposed to wide 57 fluctuations in pH as they move between various micro-environments in the human host. Among 58 these, fungi that cause invasive fungal infections (IFIs) have acquired the ability to rapidly adapt 59 to changes in extracellular pH to promote their survival during an infection. The shift of a fungal 60 pathogen from an acidic external environment to the neutral/alkaline pH of the mammalian host 61 is associated with the activation of the fungal-specific Rim/Pal signaling pathway, triggering 62 cellular changes important for survival in these new conditions. These changes include 63 alterations in the cell wall, often accompanied by larger morphological transitions that promote 64 host colonization. In the common fungal pathogen Candida albicans, pH-directed cellular 65 responses includes the ability to transition between yeast-like growth and invasive hyphal forms 66 (1-3). The opportunistic human fungal pathogen and basidiomycete yeast Cryptococcus 67 ne...

Cryptococcus neoformans Mar1 function links mitochondrial metabolism, oxidative stress, and antifungal tolerance

“…All samples were pelleted, flash frozen on dry ice, and lyophilized overnight. RNA was prepared and sequenced as previously described with some modifications (Brown et al, 2020). RNA was isolated using the Qiagen RNeasy Plant minikit with on-column DNase digestion (Qiagen, Valencia, CA).…”