The Role of Proline Catabolism in Candida albicans Pathogenesis

Candida albicans is an opportunistic fungal pathogen that has evolved in close association with human hosts. Pathogenicity is linked to an array of virulence characteristics expressed in response to environmental cues and that reflect the requirement to take up and metabolize nutrients available in the host. Metabolism generates the energy to support the bioenergetic demands of infectious growth, including the ability to reversibly switch morphologies from yeast to filamentous hyphal forms. Amino acids are among the most versatile nutrients available in the hosts as they can serve as both carbon and nitrogen sources, be transformed to key metabolic intermediates, or utilized to modulate extracellular pH via deamination forming ammonia. Of the proteinogenic amino acids, proline is unique in having a secondary amine covalently locked within an imine ring. Accumulating evidence implicates proline catabolism as being critical in the pathogenesis of many human diseases, ranging from bacterial and parasitic infections to cancer progression. This work focuses on the role of proline catabolism on C. albicans pathogenesis. Paper I describes how proline induces filamentous growth in C. albicans. Hyphal growth is induced by an increase in intracellular ATP, a positive regulator of the Ras1/cAMP/PKA pathway. Proline is a direct substrate for ATP production, its catabolism in the mitochondria by proline oxidase (Put1) and Δ-pyrroline-5-carboxylate (P5C) dehydrogenase (Put2) leads to the generation of FADH2 and NADH, respectively. Arginine and ornithine induce filamentous growth due to being catabolized to proline. Strikingly, mitochondrial proline catabolism is essential for hyphal growth and escape from macrophages. Paper II documents that proline catabolism is an important regulator of reactive oxygen species (ROS) homeostasis in C. albicans. When cells depend on proline as an energy source, the activities of the two catabolic enzymes Put1 and Put2 must operate in synchrony; perturbation of these highly regulated catabolic steps exerts deleterious effects on growth. Cells lacking PUT2 exhibit increased sensitivity to exogenous proline. This sensitivity is linked to ROS generation, likely due to the accumulation of the toxic intermediate P5C. Consistently, a put2-/mutant is avirulent in Drosophila and in a 3D skin infection model, and hypovirulent in neutrophils and a systemic murine infection model. Paper III shows that the enzymatic step directly downstream of Put2, the deamination of glutamate to α-ketoglutarate catalyzed by glutamate dehydrogenase (Gdh2), releases the ammonia responsible for the alkalization of the extracellular environment when C. albicans cells grow in the presence of amino acids. Cells lacking GDH2 do not alkalinize the medium. Alkalization is thought to induce hyphal growth in cells engulfed by macrophages. Surprisingly, filamentous growth of gdh2-/cells is not impaired in filament-inducing media, or importantly, in situ in the phagosome of primary murine macrophages. Thus, alkalization is not a requisite for filamentous growth within macrophages. The results demonstrate that under physiologically relevant host conditions, proline catabolism is important for C. albicans pathogenesis. Further studies are warranted to determine the applicability of this pathway as a potential target for therapeutic approaches aimed at combating this major fungal pathogen.