Complete assignment of the 1H- and 13C-n.m.r. spectra of the O-deacetylated glucuronoxylomannan from Cryptococcus neoformans serotype B

The structure of a C. neoformans polysaccharide motif recognized by protective antibodies: A combined NMR and MD study

Cryptococcus neoformans is a fungal pathogen responsible for cryptococcosis and cryptococcal meningitis. The C. neoformans' capsular polysaccharide and its shed exopolysaccharide function both as key virulence factors and to protect the fungal cell from phagocytosis. Currently, a glycoconjugate of these polysaccharides is being explored as a vaccine to protect against C. neoformans infection. In this study, NOE and J-coupling values from NMR experiments were consistent with a converged structure of the synthetic decasaccharide, GXM10-Ac3, calculated from MD simulations. GXM10-Ac3 was designed as an extension of glucuronoxylomannan (GXM) polysaccharide motif (M2) which is common in the clinically predominant serotype A strains and is recognized by protective forms of GXM-specific monoclonal antibodies. The M2 motif is a hexasaccharide with a three-residue α-mannan backbone, modified by β-(1→2)-xyloses (Xyl) on the first two mannoses (Man) and a β-(1→2)-glucuronic acid (GlcA) on the third Man. Combined NMR and MD analyses reveal that GXM10-Ac3 adopts an extended structure, with Xyl/GlcA branches alternating sides along the α-mannan backbone. O-acetyl esters also alternate sides and are grouped in pairs. MD analysis of a twelve M2-repeating unit polymer supports the notion that the GXM10-Ac3 structure is uniformly represented throughout the polysaccharide. This derived GXM model displays high flexibility while maintaining a structural identity, yielding insights to further explore intermolecular interactions between polysaccharides, interactions with anti-GXM mAbs, and the cryptococcal polysaccharide architecture.

The structure of a C. neoformans polysaccharide motif recognized by protective antibodies: A combined NMR and MD study

Cryptococcus neoformans is a fungal pathogen responsible for cryptococcosis and cryptococcal meningitis. The C. neoformans capsular polysaccharide and shed exopolysaccharide functions both as a key virulence factor and to protect the fungal cell from phagocytosis. Currently, a glycoconjugate of these polysaccharides is being explored as a vaccine to protect against C. neoformans infection. In this combined NMR and MD study, experimentally determined NOEs and J-couplings support a structure of the synthetic decasaccharide, GXM10-Ac3, obtained by MD. GXM10-Ac3 was designed as an extension of glucuronoxylomannan (GXM) polysaccharide motif (M2) which is common in the clinically predominant serotype A strains and is recognized by protective forms of GXM-specific monoclonal antibodies. The M2 motif is characterized by a 6-residue α-mannan backbone repeating unit, consisting of a triad of α-(1→3)-mannoses, modified by β-(1→2)-xyloses on the first two mannoses and a β-(1→2)-glucuronic acid on the third mannose. The combined NMR and MD analyses reveal that GXM10-Ac3 adopts an extended structure, with xylose/glucuronic acid branches alternating sides along the α-mannan backbone. O-acetyl esters also alternate sides and are grouped in pairs. MD analysis of a twelve M2-repeating unit polymer supports the notion that the GXM10-Ac3 structure is uniformly represented throughout the polysaccharide. This experimentally consistent GXM model displays high flexibility while maintaining a structural identity, yielding new insights to further explore intermolecular interactions between polysaccharides, interactions with anti-GXM mAbs, and the cryptococcal polysaccharide architecture.

Diversity among strains of Cryptococcus neoformans var. gattii as revealed by a sequence analysis of multiple genes and a chemotype analysis of capsular polysaccharide

We demonstrated the diversity of Cryptococcus neoformans var. gattii strains by a sequence analysis of multiple genes: (i) the intergenic spacer (IGS) 1 and 2 regions of the rRNA gene; (ii) the internal transcribed spacer (ITS) region, including 5.8S of the rRNA gene; (iii) TOP1 (topoisomerase); and (iv) CAP59. In these studies, we compared C. neoformans var. gattii with varieties grubii, and neoformans of C. neoformans. Phylogenetic analysis indicated that both C. neoformans var. grubii and C neoformans var. neoformans are monophyletic, but C. neoformans var. gattii showed polyphyletic. C. neoformans var. gattii can be divided into three phylogenetic groups, I, II, and III, with high bootstrap support. Phylogenetic group I contains serotype B and C strains, and groups II and III include serotype B strains. Because the serotype B strains of C. neoformans var. gattii exhibited more genetic divergence, the serological characteristics and chemotypes of their capsular polysaccharide were further investigated. No remarkable difference among the serotype B strains was found in the reactivities to factor serum 5, which is specific for serotype B. The NMR spectra of the capsular polysaccharide from serotype B strains could be divided into three characteristic patterns, but the chemical shifts were very similar. These results suggested that the serotype B strain of C. neoformans var. gattii has more genetic diversity than the serotype C strain of C. neoformans var. gattii or the varieties grubii and neoformans of C. neoformans, but there was no correlation between genotype and chemotype.

Assimilation of xylose, mannose, and mannitol for synthesis of glucuronoxylomannan of Cryptococcus neoformans determined by 13C nuclear magnetic resonance spectroscopy

Cryptococcus neoformans NIH 409 was cultured in a defined medium containing D-[1-13C]xylose (Xyl), D-[1-13C]mannose (Man), or D-[1-13C]mannitol as the sole carbon source. The distribution of 13C in the Man, Xyl, glucuronic acid (GlcA), and O-acetyl constituents of native and de-O-acetylated glucuronoxylomannan (GXM) was determined by one-dimensional 13C nuclear magnetic resonance spectroscopy. The carbon chain of Man was incorporated intact into GXM since 13C was observed only in carbon 1 of Man, GlcA, and Xyl. The carbon chain of mannitol was incorporated intact into GXM since 13C was observed only in carbons 1 and 6. This was expected since mannitol has an axis of symmetry. The carbon chain of Xyl was identified only in carbons 1 and 3 of Man, GlcA, and Xyl. This pattern of labeling is consistent with the assimilation of Xyl through the pentophosphate pathway.

Cryptococcus neoformans chemotyping by quantitative analysis of 1H nuclear magnetic resonance spectra of glucuronoxylomannans with a computer-simulated artificial neural network

The complete assignment of the proton chemical shifts obtained by nuclear magnetic resonance (NMR) spectroscopy of de-O-acetylated glucuronoxylomannans (GXMs) from Cryptococcus neoformans permitted the high-resolution determination of the total structure of any GXM. Six structural motifs based on an alpha-(1-->3)-mannotriose substituted with variable quantities of 2-O-beta- and 4-O-beta-xylopyranosyl and 2-O-beta-glucopyranosyluronic acid were identified. The chemical shifts of only the anomeric protons of the mannosyl residues served as structure reporter groups (SRG) for the identification and quantitation of the six triads present in any GXM. The assigned protons for the mannosyl residues resonated at clearly distinguishable positions in the spectrum and supplied all the information essential for the assignment of the complete GXM structure. This technique for assigning structure is referred to as the SRG concept. The SRG concept was used to analyze the distribution of the six mannosyl triads of GXMs obtained from 106 isolates of C. neoformans. The six mannosyl triads occurred singularly or in combination with one or more of the other triads. The identification and quantitation of the SRG were simplified by using a computer-simulated artificial neural network (ANN) to automatically analyze the SRG region of the one-dimensional proton NMR spectra. The occurrence and relative distribution of the six mannosyl triads were used to chemotype C. neoformans on the basis of subtle variations in GXM structure determined by analysis of the SRG region of the proton NMR spectrum by the ANN. The data for the distribution of the six SRGs from GXMs of 106 isolates of C. neoformans yielded eight chemotypes, Chem1 through Chem8.

Structure of the 13C-enriched O-deacetylated glucuronoxylomannan of Cryptococcus neoformans serotype A determined by NMR spectroscopy

The complete assignment of 1H and 13C chemical shifts for 99% uniformly 13C-labeled O-deacetylated glucuronoxylomannan (GXM) of Cryptococcus neoformans serotype A isolate 9759-Mu-1 was accomplished by the analysis of HCCH-TOCSY and HCCH-COSY spectra. The sequence of the glycosyl residues was determined by a GHMBC experiment using 20% uniformly 13C-labeled GXM; GXM was prepared by a novel procedure that insured the virtual exclusion of adjacent 13C-labeled carbon atoms. For each residue in the GXM of 9759-Mu-1 we determined its linkage position, its anomeric configuration, and its position in the repeating sequence as follows: [sequence: see text]

Synthesis of a pentasaccharide corresponding to the repeating unit of the exopolysaccharide from Cryptococcus neoformans serovar D

The assembly of the pentasaccharide repeating unit of the exopolysaccharide from Cryptococcus neoformans serovar D (i.e. 1) is described. The glucuronic acid residue in 1 is introduced as a glucopyranoside and oxidized in a later stage of the synthesis. Thus, iodonium ion-assisted glycosylation of the partially protected methyl mannopyranoside 11 with ethylthio donor 14 gave, after selective deprotection, disaccharide 18. Elongation of the latter with D-glucopyranoside 35 gave trisaccharide 36. Subsequent protective group manipulations yielded the acceptor 37. Condensation of disaccharide donor 31 with trisaccharide acceptor 37 yielded pentasaccharide 38. Protective group manipulations of 38 afforded 42, the glucoside of which was oxidized to yield the corresponding glucuronide 44. Hydrogenolysis of 44 gave the target pentasaccharide 1.

Structure of the O-deacetylated glucuronoxylomannan from Cryptococcus neoformans Cap70 as determined by 2D NMR spectroscopy

Cryptococcus neoformans, an opportunistic pathogen, is the fourth leading cause of death among AIDS patients. The yeast's capsule is a major virulence factor, and serotype is related to the chemical structure of glucuronoxylomannan (GXM), its capsular polysaccharide. The GXM from Cap70, a hypocapsular mutant of serotype D isolate B-3501, was investigated by chemical analysis and 2D NMR spectroscopy. The assignment of 1H and 13C chemical shifts for the O-deacetylated polysaccharide was accomplished from the analysis of DQF-COSY, TOCSY, and gradient-enhanced HSQC spectra. The sequence and linkage positions of glycosyl residues were determined by NOESY and ROESY spectra. Two repeating polysaccharide components were identified as having the following structures in approximately equal proportions: [formula: see text] It is not known if these repeating units comprise a single or two separate polymer chains. Pentasaccharide 2 has been known to be the major GXM polymer of B-3501 and other serotype D isolates. Hexasaccharide 1 is identified for the first time although it has subsequently been identified in other C. neoformans isolates. The presence of 1 in the GXM of Cap70 is consistent with the extra xylose found relative to that in isolate B-3501. The mannose:xylose:glucuronic acid:O-acetyl molar ratio of Cap70 GXM is 3.00:1.73:0.78:1.75, while the same ratio for B-3501 and other serotype D isolates is approximately 3.00:1.00:0.80:1.75. Methylation analysis confirmed that the GXM of Cap70 contains unsubstituted, monosubstituted (2-linked), and disubstituted (2- and 4-linked) mannose in a ratio of 0.87:1.75:0.38. Dot blot immunoassay indicates that Cap70 is a serotype D isolate like its parent strain.

Structure of the O-deacetylated glucuronoxylomannan from Cryptococcus neoformans serotype C as determined by 2D 1H NMR spectroscopy

The primary structure of the O-deacetylated capsular glucuronoxylomannan (GXM) isolated from Cryptococcus neoformans serotype C was investigated by 2D NMR spectroscopy. Assignment of the 1H NMR chemical shifts for the polysaccharide was accomplished from the analysis of DQF-COSY, TOCSY, NOESY and/or ROESY spectra of three isolates (298, 34, and 401). These isolates contain the same polysaccharide glycosyl residues but in different proportions. The serotype C GXM consists of two repeating polysaccharide units that have the following structures: [formula: see text] It is not known if these repeating units comprise a single or two separate polymer chains. The relative amounts of the more highly branched octasaccharide 1 in the isolates studied (i.e., approximately 75% in isolate 34, 50% in isolate 298, and 25% in isolate 401) can be used to explain the serological specificity of these isolates with C. neoformans factor sera, as was previously determined by ELISA in this laboratory. The octasaccharide 1 component is the one previously postulated as the structure of the serotype C GXM although definitive placement of the beta-Xyl-(1-->4) residues had previously not been determined. The heptasaccharide 2 component is uniformly found as the repeating unit in the polysaccharide from serotype B isolates. Additionally, GXM 401 was found to contain a small amount of the hexasaccharide repeating unit usually attributed to serotype A GXM.

Polysaccharide antigens of the capsule of Cryptococcus neoformans

The major significance of the capsular polysaccharide of C. neoformans is its role in potentiating opportunistic infections by the yeast. It has the ability to exert a broad spectrum of influences on the immune response, from activation of phagocytic cells and complement components of the alternative pathway, to the induction of specific antibody, T-suppressor cells, DTH responses, and cytokines (51). These biological properties along with the serotype specificities are all determined by the physical properties and chemical structures of the polysaccharide antigens that compose the capsule. There is evidence not only for an association of lethal infections with serotype A in patients with advanced AIDS (34, 56), but also for a role for the capsule in directly influencing the infection of CD4+ cells by HIV (57). Together, these phenomena raise intriguing questions about the possible connection between the chemistry of these capsular antigens and cryptococcal infections in AIDS patients. One speculation is that AIDS creates the optimal physiological conditions for the establishment and spread of cryptococcosis. It has been observed that during the progression of AIDS there is a shift towards a T-2 response (14). This could lead to conditions that would inhibit the cellular immune responses that block dissemination of cryptococcal infections. Thus, an important consideration in the application of vaccine or immune modulation therapies in the treatment of cryptococcosis in AIDS victims would be the design of vaccines that could boost the T-1 immune response. It has been shown that the form and dose of an antigenic challenge can influence the induction of a T-1 or T-2 immune response (61). Recently, Murphy has reported that gamma interferon and interleukin 2 are up-regulated in the spleens of mice that produce anticryptococcal TDH and TAMP cells in response to immunogenic doses of cryptococcal culture filtrate antigen given with Freund's complete adjuvant (49). Perhaps purified cryptococcal antigens (e.g., MP) conjugated to an appropriate carrier or adjuvant could be used in therapeutic strategies to limit cryptococcosis in immunocompromised individuals. Future investigations of virulence and pathogenicity in the context of defined polysaccharide antigens from encapsulated strains of C. neoformans will contribute to a better understanding of the regulation of cryptococcal infection and immunity at the cellular and molecular levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural variability in the glucuronoxylomannan of Cryptococcus neoformans serotype A isolates determined by 13C NMR spectroscopy

Cryptococcus neoformans, the etiologic agent of cryptococcal meningoencephalitis, produces glucuronoxylomannan (GXM) as the major capsule component. Purified GXMs obtained from eight serotype A isolates of C. neoformans were treated by ultrasonic irradiation and then O-deacetylated prior to their comprehensive chemical analysis by GLC, GLC-MS, and 13C NMR spectroscopy. The average xylose: mannose: glucuronic acid molar ratio of the eight isolates is 1.96 +/- 0.25: 3.00: 0.58 +/- 0.10. Methylation analyses and 13C NMR spectroscopy show a general structure for GXM that is comprised of a linear (1----3)-alpha-D-mannopyranan substituted with beta-D-GlcpA and with beta-D-Xylp at O-2. Variable quantities of unsubstituted (1----3)-alpha-D-Manp were observed between the eight isolates studied. In several isolates some of the (1----3)-alpha-D-Manp residues are disubstituted with beta-D-GlcpA at O-2 and with beta-D-Xylp at O-4; this type of substitution was not previously thought to occur in serotype A isolates. Heterogeneity, between isolates, in the disposition of the substituents along the mannopyranan backbone was revealed by 13C NMR spectroscopy. The eight isolates, and three isolates previously studied, were each assigned to one of four distinct groups based on the 13C NMR chemical shifts of the anomeric carbons. Six of the eleven isolates gave identical spectra (Group I). The six major anomeric resonances from Group I were assigned to specific glycosidic linkages present in GXM. The remaining five isolates gave more complex spectra that are indicative of additional linkages and comprise the remaining three groups. Three of these five isolates contain substantial amounts of linkages previously thought to be distinctive of serotypes B and C, i.e., Manp residues that are 4-O-glycosylated with beta-D-Xylp. Methylation analyses only predicted an average repeating unit, whereas 13C NMR spectroscopy demonstrated that GXM from each isolate may be categorized into four groups by the occurrence of distinct sequences of carbohydrate residues.