Date Published: January 9, 2014
Publisher: Public Library of Science
Author(s): Maurizio Del Poeta, Leonardo Nimrichter, Marcio L. Rodrigues, Chiara Luberto, Joseph Heitman.
Sphingolipids have recently emerged as key regulators of pathogenicity in a variety of fungi. Glucosylceramide is a sphingolipid important for fungal cell division, alkaline tolerance, hyphal formation, and spore germination and, thus, for the regulation of fungal virulence. Present in many fungi, including yeasts, molds, and in dimorphic fungi, fungal glucosylceramide exhibits a characteristic structure synthesized by fungal specific enzymes. Thus, it is envisioned as an important fungal target for compounds with specific and broad-spectrum activity.
Glucosylceramide (GlcCer) is a sugar sphingolipid composed of a sphingoid backbone, a fatty acid, and a glucose moiety (Figure 1). Notably, GlcCer is found in plants, fungi, and animals and absent in bacteria and in some eukaryotes such as the baker yeast Saccharomyces cerevisiae. In spite of the presence of GlcCer in most organisms, its synthetic pathway and molecular structure varies significantly –, resulting in the occurrence of rather unique GlcCer molecular species in different organisms. The final reaction of GlcCer synthesis is catalyzed by the glucosylceramide synthase enzyme (GCS), which transfers a glucose moiety from uridine 5-diphosphate (UDP)-glucose onto the C1 hydroxyl group of ceramide via an oxygen-glycosidic bond (Figure 2).
Fungal GlcCer has a very unique chemical structure compared to GlcCers in mammals and plants.
Recent in vitro studies have uncovered a role for GlcCer/Gcs1 in the regulation of alkali tolerance of C. neoformans, providing a potential mechanism through which this lipid might affect C. neoformans virulence in mice. In fact, it was observed that the Δgcs1 strain fails to replicate and grow when the fungal cells are shifted from an acidic to a neutral/alkaline environment. Since the in vitro neutral/alkali condition mimics the host extracellular environment encountered by C. neoformans during infection (alveoli and blood stream), it is suggested that it is indeed the inability of the Δgcs1 strain to grow in these conditions that determines the failure of this strain to survive in the host extracellular environment, allowing the host to mount an effective immune response and clear the infection .
The molecular mechanism by which methylated GlcCer regulates fungal alkali tolerance is still unknown. On the other hand, the fact that in neutral/alkaline conditions GlcCer rapidly concentrates at the plasma membrane, cell wall, and bud neck from the intracellular compartment  and that, in its absence, there is no budding  suggests that, in these specific extracellular conditions, the presence of GlcCer at the bud neck and in its vicinity becomes important to allow progression through the cell cycle. The presence of methylated GlcCer might be necessary to maintain the proper curvature of the membranes at the bud neck so that the bud can initiate and grow to become the daughter cell. In addition, most of the buds present in the Δgcs1 and Δsmt1 cells exposed to alkaline pH are small, suggesting that GlcCer, and particularly methylated GlcCer, may be important for the production of bud membranes.
As discussed, GlcCer is present and enriched in the membrane of vesicles that can cross the cell wall, reaching the extracellular space. The presence of these GlcCer-enriched vesicles in the extracellular space might have an important role in pathogenicity of C. neoformans and in the modulation of the host response to the fungus. Indeed, these extracellular vesicles are loaded with proteins associated with virulence of C. neoformans40,41 and can impair macrophage functions . Additionally, GlcCer is antigenic, and antibodies against GlcCer have been detected during human cryptococcal infections , . Thus, exposure of GlcCer on the surface of the vesicles might also induce a host antibody response. Interestingly, administration of antibodies against GlcCer protects immunocompetent mice from developing a lethal meningitis , but their role in protecting immunocompromised hosts, such as HIV-infected patients, is less clear . Even if antibodies against GlcCer would exert a protective role only in condition of immunocompetency, it is proposed that these antibodies may be useful as passive immunization against the infection caused by Cryptococcus gattii and certain dimorphic fungi, such as Histoplasma capsulatum, Blastomyces dermatiditis and Coccidioides immitis, which can also afflict immunocompetent individuals.
All the evidence/observations discussed support a critical role for the synthesis of GlcCer in pathogenicity of C. neoformans. Furthermore, since GlcCer may influence virulence of many yeasts, molds, dimorphic fungi, and Pneumocystis spp., anti-GlcCer therapy might have a very broad applicability.