Structure and biological functions of fungal cerebrosides

Anais da Academia Brasileira de Ciências (2004) 76(1): 67-84 (Annals of the Brazilian Academy of Sciences) ISSN 0001-3765 www.scielo.br/aabc Structure and biological functions of fungal cerebrosides ELIANA BARRETO-BERGTER, MARCIA R. PINTO and MARCIO L. RODRIGUES Instituto de Microbiologia Professor Paulo de Góes, Departamento de Microbiologia Geral Universidade Federal do Rio de Janeiro, Cidade Universitária, CCS, Bl. I, Ilha do Fundão 21941-590 Rio de Janeiro, RJ, Brasil Manuscript received on October 20, 2003; accepted for publication on October 22, 2003; presented by L. R. Travassos ABSTRACT Ceramide monohexosides (CMHs, cerebrosides) are glycosphingolipids composed of a hydrophobic ceramide linked to one sugar unit. In fungal cells, CMHs are very conserved molecules consisting of a ceramide moiety containing 9-methyl-4,8-sphingadienine in amidic linkage to 2-hydroxyoctadecanoic or 2-hydroxyhexadecanoic acids, and a carbohydrate portion consisting of one residue of glucose or galactose. 9-Methyl 4,8-sphingadienine-containing ceramides are usually glycosylated to form fungal cerebrosides, but the recent description of a ceramide dihexoside (CDH) presenting phytosphingosine in Magnaporthe grisea suggests the existence of alternative pathways of ceramide glycosylation in fungal cells. Along with their unique structural characteristics, fungal CMHs have a peculiar subcellular distribution and striking biological properties. In Pseudallescheria boydii, Candida albicans, Cryptococcus neoformans, Aspergillus nidulans, A. fumigatus, and Schizophyllum commune, CMHs are apparently involved in morphological transitions and fungal growth. The elucidation of structural and functional aspects of fungal cerebrosides may therefore contribute to the design of new antifungal agents inhibiting growth and differentiation of pathogenic species. Key words: glucosylceramide, cerebrosides, glycosphingolipids, fungal pathogens, antifungal therapy. INTRODUCTION The frequency of fatal mycoses associated with immunosuppression has increased in the last two decades (Dromer and Dupont 1996). Despite the clinical relevance of fungal infections, however, the current antifungal therapy is ineffective in several cases. For over four decades, the principal target of antifungal therapy has been ergosterol in the fungal cell membrane. Although this has proven to be a successful and relatively selective antifungal target, reports of resistance and intolerance to currently available antifungal agents are increasing. In this context, the search for novel antifungal agents has Correspondence to: Eliana Barreto-Bergter E-mail: been greatly stimulated. Glycosphingolipids (GSLs) consist of a ceramide (N -acylsphingosine) moiety linked to a glycan chain of variable length and structure. These molecules have been implicated in many fundamental cellular processes including growth, differentiation, and morphogenesis. GSLs may also modulate cell signaling by controlling the assembly and specific activities of plasma membrane proteins (Hakomori 1993, Kasahara and Sanai 2000). Several natural product inhibitors of sphingolipid biosynthesis have been discovered in recent years (Georgopapadakou 2000), and some of them exhibit a potent and selective antifungal activity. The roles of fungal monohexosylceramides An Acad Bras Cienc (2004) 76 (1) 68 ELIANA BARRETO-BERGTER, MARCIA R. PINTO and MARCIO L. RODRIGUES (CMHs, cerebrosides) elucidated in the last three years suggests that a new target for antifungal therapy may emerge (Rodrigues et al. 2000, Pinto et al. 2002, Levery et al. 2002). Cerebrosides are neutral glycosphingolipids that contain a monosaccharide, normally glucose or galactose, in 1-ortho-betaglycosidic linkage with the primary alcohol of an N acyl sphingoid (ceramide). In plants the monosaccharide is normally glucose and the sphingoid usually phytosphingosine. In animals, the monosaccharide is usually galactose, though this may vary with the tissue and the sphingoid is usually sphingosine or dihydrosphingosine. Since cerebrosides contain one sugar unit, they are also called ceramide monohexosides (CMHs), differing from gangliosides in that the latter contain at least one sialic acid residue. CMHs also differ from globosides in that these glycolipids contain multiple sugar moieties, whereas cerebrosides only contain one. TABLE I Fungal CMHs: ceramide* and carbohydrate moieties. Fungal specie A. fumigatus A. versicolor A. flavus P. boydii Fusarium sp F. oxysporum F. solani P. brasiliensis H. capsulatum C. neoformans C. albicans M. grisea S. schenckii Major fatty acid C18:1(OH) Sugar Glucose/ Galactose C16:0(OH) C18:0(OH) C16:0(OH) C18:0(OH) C18:1(OH) C18:0(OH) C18:1(OH) C18:0(OH) C18:1(OH) C18:0(OH) C18:0(OH) C18:1(OH) C18:0(OH) C18:1(OH) Glucose Glucose Glucose Glucose Glucose Glucose Glucose Glucose/ Galactose STRUCTURAL ASPECTS OF FUNGAL CEREBROSIDES *9-Methyl-4,8-sphingadienine is the long chain base composing CMHs have been widely detected in fungal cells (reviewed by Warnecke and Heinz 2003). The current literature indicates that cerebrosides seem to be present in almost all fungal species studied so far, with Saccharomyces cerevisiae representing a wellknown exception. Fungal cerebrosides are much conserved structures, in which modifications include different sites of unsaturation as well as the varying length of fatty acid residues in the ceramide moiety (Table I). Fungal CMHs contain a ceramide moiety with 9-methyl-4, 8-sphingadienine in amidic linkage to 2-hydroxyoctadecanoic or 2-hydroxyhexadecanoic acids, and a carbohydrate portion consisting of one residue of glucose or galactose. Exceptionally, cerebrosides from S. kluyveri have an extremely rare trihydroxy sphingoid base as a unique feature (Takakuwa et al. 2002). The long chain base 9-methyl-4,8-sphingadiene was first described in monohexosylceramides from Aspergillus oryzae (Fujino and Ohnishi 1977) and was subsequently isolated from Schizophyllum commune (Kawai and Ikeda 1982), from the plant the ceramide unit of all the molecules described in this table. An Acad Bras Cienc (2004) 76 (1) pathogen Fusicoccum amygdali (Ballio et al. 1979), and the edible fungi Clitocybe geotrope and C. nebularis (Fodegal et al. 1986). CMHs were further characterized in lipid extracts from the fungal species Aspergillus fumigatus (Toledo et al. 1999, VillasBoas et al. 1994a), A. niger (Levery et al. 2000), A. versicolor (Villas-Boas et al. 1994a), Acremonium chrysogenum (Sakaki et al. 2001), Amanita muscaria (Weiss and Stiller 1972), A. rubescens (Weiss and Stiller 1972), Candida albicans (Matsubara et al. 1987), C. deformans (Mineki et al. 1994), C. utilis (Wagner and Zofcsik 1966a, Wagner and Zofcsik 1966b), Colletotrichum gloeosporioides (Da Silva et al. unpublished), Cryptococcus neoformans (Rodrigues et al. 2000), Fonsecaea pedrosoi (Nimrichter et al. unpublished), Fusarium sp (Duarte et al. 1998), Ganoderma lucidum (Mizushina et (...truncated)