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Abstract
Cryptococcus neoformans is a ubiquitous free-living soil yeast. It is also an opportunistic pathogen that causes about 223,100 cases of cryptococcal meningitis per year, resulting in over 180,000 deaths. The success of this pathogen lies in its ability to adapt to the host physiological conditions. One major obstacle C. neoformans needs to overcome to be pathogenic is growth at host temperature levels (>37˚C). The transcription factor Crz1 is known to be important for thermotolerance, and its subcellular trafficking likely contributes to its function. During my graduate study, I investigated the molecular basis of Crz1’s function and subcellular localization. I found that Crz1 localization dynamically responds to changes in various environmental conditions, and this is dependent on several intrinsically disordered domains within the protein. Another critical but uninvestigated difference between the natural niche of C. neoformans and the mammalian host is the concentration of CO2: there is 125 times more CO2 in the host than in the ambient air. We recently found that clinical isolates from patients are generally more tolerant to high levels of CO2 than isolates from the environment. Much of my dissertation research was focused on determining the genetic basis of CO2 adaptation and tolerance in C. neoformans. By utilizing a forward genetic screen of gene deletion mutants, QTL mapping, congenic strains, and experimental evolution, I have concluded that: (1) regulation of thermotolerance and CO2 tolerance is tightly linked, (2) the RAM (Regulator of ACE2 and morphogenesis) pathway is critical for integrating multiple stress response pathways to regulate both CO2 tolerance and thermotolerance, (3) CO2 tolerance is a complex trait controlled by multiple genetic loci and their interactions, (4) response to increased CO2 levels is likely regulated post-transcriptionally, and (5) CO2-sensitive C. neoformans strains can become CO2-tolerant after in vivo passage or in vitro passage through high levels of CO2. Research to define the molecular and cellular responses induced from host stress is critical for understanding C. neoformans pathogenesis and for developing treatments as CO2 also greatly affects antifungal drug efficacy.