Carbonic anhydrase and CO2 sensing during Cryptococcus neoformans growth, differentiation, and virulence

Nov 21 2005

By using pathogenic fungi as model systems for understanding fungal diseases, two groups of researchers are reporting new work that offers insight into how carbon dioxide (CO₂) governs the morphogenic changes that allow pathogenic fungi to survive in different environments and invade the human body, and they provide new evidence for how CO₂ sensing and metabolism utilize evolutionarily conserved enzymes to control the growth and sexual reproduction of pathogenic microbes.

The two studies are reported by Joseph Heitman and colleagues at Duke University Medical Center and by Fritz Mühlschlegel of the University of Kent, Jochen Buck at Cornell University, and their collaborators.

How organisms sense and respond to CO₂ at the cellular level is not fully understood, but it is of great importance in understanding the biology of microbes, plants, and animals alike. For example, CO₂ levels govern the detection of prey by female mosquitoes, control respiration in mammals, and of course play a critical regulatory role in photosynthesis in algae and plants; in addition, CO₂ sensing and transport are involved in many cellular processes and virulence attributes of diverse pathogenic bacteria and fungi--in both B. anthracis, which causes Anthrax, and C. neoformans, which causes meningitis, CO >₂ induces the production of sugar-based capsules that surround and protect the invading cell from the host during infection.

In their new work, Mühlschlagel and colleagues studied the function of CO₂ sensing in two major human fungal pathogens, C. albicans and C. neoformans. Both cause life-threatening, invasive infections in immunocompromised patients--for example, those infected with HIV or undergoing bone-marrow transplantation. The two fungi, which are distantly related in evolution, have different attributes governing their virulence in humans. For C. albicans, a transition between different morphological forms ("yeast" and "filamentous" forms) plays a major role, whereas for C. neoformans, synthesis of a polysaccharide capsule is key.

In the bodies of mammals, the CO₂ concentration is more than 150-fold higher (5%) than it is in atmospheric air (0.033%). Consequently, C. albicans and C. neoformans are exposed to dramatically elevated CO₂ concentrations when causing systemic disease. In their research, the authors identify CO₂ as a physiological signal that induces the pathogenic filamentous transition in C. albicans; they also demonstrate that an ancient group of enzymes called adenylyl cyclases are the so-called chemosensors mediating both the CO₂ -dependent filamentation in C. albicans and the capsule biosynthesis in C. neoformans. The authors go on to show that CO₂ sensing in C. albicans is essential for superficial (skin) infections, in which yeast must be able to grow despite significantly lowered CO₂ levels present at the skin surface. Based on their findings, the authors conclude that CO₂ sensing is a vital mediator of fungal virulence in different host environments--for example, at different sites within the body.

In a related paper, Joseph Heitman and colleagues, also using C. neoformans as a model system for understanding fungal disease, provide new evidence that CO₂ sensing and metabolism govern growth, sexual reproduction, and virulence of this pathogenic microbe.

In this work, the researchers investigated the CO₂-sensing mechanism of C. neoformans. This pathogen normally infects the human host (a high-CO₂ environment) from the air (low CO₂ in the course of causing deadly fungal meningitis. The authors found that an enzyme called carbonic anhydrase (CA) plays a critical role in the yeast's growth in ambient air; the CA enzyme accomplishes this by providing bicarbonate substrates for fatty-acid biosynthesis and other cellular processes. In contrast, the CA enzyme is dispensible for the yeast's survival in the high- CO₂ environment of the host. Another major finding of the work is that for C. neoformans, high CO₂ blocks sexual differentiation, which is known to be a key process for generating infectious spores, and the CA enzyme plays a central role in the process. Together with the accompanying work by Mühlschlegel and colleagues, this study implicates a key role for CO₂ in growth and differentiation in the fungal kingdom.

The identification of a link between CO₂ sensing and fungal virulence may have broad implications for the study of microbial pathogens and the role of adenylyl cyclase in virulence trait regulation by CO₂. Furthermore, it may in time facilitate the development of a new class of antimicrobial agents.