Global oral health inequalities: the view from a research funder.

Despite impressive worldwide improvements in oral health, inequalities in oral health status among and within countries remain a daunting public health challenge. Oral health inequalities arise from a complex web of health determinants, including social, behavioral, economic, genetic, environmental, and health system factors. Eliminating these inequalities cannot be accomplished in isolation of oral health from overall health, or without recognizing that oral health is influenced at multiple individual, family, community, and health systems levels. For several reasons, this is an opportune time for global efforts targeted at reducing oral health inequalities. Global health is increasingly viewed not just as a humanitarian obligation, but also as a vehicle for health diplomacy and part of the broader mission to reduce poverty, build stronger economies, and strengthen global security. Despite the global economic recession, there are trends that portend well for support of global health efforts: increased globalization of research and development, growing investment from private philanthropy, an absolute growth of spending in research and innovation, and an enhanced interest in global health among young people. More systematic and far-reaching efforts will be required to address oral health inequalities through the engagement of oral health funders and sponsors of research, with partners from multiple public and private sectors. The oral health community must be "at the table" with other health disciplines and create opportunities for eliminating inequalities through collaborations that can harness both the intellectual and financial resources of multiple sectors and institutions.

Substrate-induced conformational changes and dynamics of UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferase-2.

O-Glycan biosynthesis is initiated by the transfer of N-acetylgalactosamine (GalNAc) from a nucleotide sugar donor (UDP-GalNAc) to Ser/Thr residues of an acceptor substrate. The detailed transfer mechanism, catalyzed by the UDP-GalNAc polypeptide:N-acetyl-alpha-galactosaminyltransferases (ppGalNAcTs), remains unclear despite structural information available for several isoforms in complex with substrates at various stages along the catalytic pathway. We used all-atom molecular dynamics simulations with explicit solvent and counterions to study the conformational dynamics of ppGalNAcT-2 in several enzymatic states along the catalytic pathway. ppGalNAcT-2 is simulated both in the presence and in the absence of substrates and reaction products to examine the role of conformational changes in ligand binding. In multiple 40-ns-long simulations of more than 600 ns total run time, we studied systems ranging from 45,000 to 95,000 atoms. Our simulations accurately identified dynamically active regions of the protein, as previously revealed by the X-ray structures, and permitted a detailed, atomistic description of the conformational changes of loops near the active site and the characterization of the ensemble of structures adopted by the transferase complex on the transition pathway between the ligand-bound and ligand-free states. In particular, the conformational transition of a functional loop adjacent to the active site from closed (active) to open (inactive) is correlated with the rotameric state of the conserved residue W331. Analysis of water dynamics in the active site revealed that internal water molecules have an important role in enhancing the enzyme flexibility. We also found evidence that charged side chains in the active site rearrange during site opening to facilitate ligand binding. Our results are consistent with the single-displacement transfer mechanism previously proposed for ppGalNAcTs based on X-ray structures and mutagenesis data and provide new evidence for possible functional roles of certain amino acids conserved across several isoforms.

Cloning and characterization of a ninth member of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family, ppGaNTase-T9.

We have cloned, expressed and characterized the gene encoding a ninth member of the mammalian UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) family, termed ppGaNTase-T9. This type II membrane protein consists of a 9-amino acid N-terminal cytoplasmic region, a 20-amino acid hydrophobic/transmembrane region, a 94-amino acid stem region, and a 480-amino acid conserved region. Northern blot analysis revealed that the gene encoding this enzyme is expressed in a broadly distributed manner across many adult tissues. Significant levels of 5- and 4.2-kilobase transcripts were found in rat sublingual gland, testis, small intestine, colon, and ovary, with lesser amounts in heart, brain, spleen, lung, stomach, cervix, and uterus. In situ hybridization to mouse embryos (embryonic day 14.5) revealed significant hybridization in the developing mandible, maxilla, intestine, and mesencephalic ventricle. Constructs expressing this gene transiently in COS7 cells resulted in no detectable transferase activity in vitro against a panel of unmodified peptides, including MUC5AC (GTTPSPVPTTSTTSAP) and EA2 (PTTDSTTPAPTTK). However, when incubated with MUC5AC and EA2 glycopeptides (obtained by the prior action of ppGaNTase-T1), additional incorporation of GalNAc was achieved, resulting in new hydroxyamino acid modification. The activity of this glycopeptide transferase is distinguished from that of ppGaNTase-T7 in that it forms a tetra-glycopeptide species from the MUC5AC tri-glycopeptide substrate, whereas ppGaNTase-T7 forms a hexa-glycopeptide species. This isoform thus represents the second example of a glycopeptide transferase and is distinct from the previously identified form in enzymatic activity as well as expression in embryonic and adult tissues. These findings lend further support to the existence of a hierarchical network of differential enzymatic activity within the diversely regulated ppGaNTase family, which may play a role in the various processes governing development.

A revolution in biomedical assessment: the development of salivary diagnostics.

Since the early 1900s, saliva has proven to be a noninvasive medium from which to measure a wide range of hormones, pharmaceuticals, and antibodies. It has also proven to be a convenient source of host and microbial DNA. As we enter the era of genomic medicine, increasing use of salivary diagnostics will help catalyze a shift from disease diagnosis to health surveillance. However, with the advances in this technology comes the additional obligation to ensure the privacy and rights of patients.

The association of basic proline-rich peptides from human parotid gland secretions with caries experience.

To address whether there are associations between the peptide composition of human parotid saliva and dental decay (caries) experience, we have characterized the peptides from parotid ductal saliva collected from nine adults who have remained free from dental caries (mean age = 59.2; Decayed Missing Filled Surfaces index [DMFS] = 0) and nine individuals who have experienced caries (mean age = 51.2; mean DMFS = 38.4). Ethanol-soluble peptides were size-fractionated on columns of Bio-Gel P-2; the salivary peptides derived from caries-susceptible subjects appeared larger than those found in the saliva of caries-free subjects. Peptides were then resolved into 19 species by cation exchange HPLC. Sequence analysis identified 18 peptides that appear to be proteolytic cleavage products of the basic proline-rich proteins IB-4, IB-5, IB-7, IB-8b, and P-B. The peptides that were more abundant in saliva obtained from the caries-free group differed from those isolated from the caries-susceptible group. The median peptide concentration of one possible precursor protein, IB-7, was found to be higher in saliva collected from caries-free individuals than in that from caries-susceptible individuals. Although differences were found in the phenotypes of proline-rich proteins expressed by these groups of caries-free and caries-susceptible subjects, no statistically significant associations were observed among proline-rich phenotypes and the level of any peptide. Collectively, our results indicate that proteolytic processing of parotid salivary proteins differs among individuals who have remained caries-free and those who have experienced dental decay.

Diverse spatial expression patterns of UDP-GalNAc:polypeptide N-acetylgalactosaminyl-transferase family member mRNAs during mouse development.

Cell migration and adhesion during embryonic development are complex processes which likely involve interactions among cell-surface carbohydrates. While considerable work has implicated proteoglycans in a wide range of developmental events, only limited attention has been directed towards understanding the 7role(s) played by the related class of mucin-type O-glycans. The initial step of mammalian mucin-type O-glycosylation is catalyzed by a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGaNTases). The spatial expression patterns of the messenger RNAs of seven ppGaNTase family members were investigated from gastrulation through organogenesis stages of mouse development. The seven glycosyltransferases were expressed in unique patterns during embryogenesis. ppGaNTase-T1, -T2, -T4, and -T9 were expressed more ubiquitously than ppGaNTase-T3, -T5, and -T7. Organ systems with discrete accumulation patterns of ppGaNTase family members include the gastrointestinal tract (intestine, liver, stomach, submandibular gland), nervous system (brain, eye), lung, bone, yolk sac, and developing craniofacial region. The pattern in the craniofacial region included differential expression by family members in developing mandible, teeth, tongue and discrete regions of the brain including the pons and migratory, differentiating neurons. Additionally, ppGaNTase-T5 accumulates in a subset of mesenchymal cells at the ventral-most portions of the E12.5 maxilla and mandible underlying the dental lamina. The unique spatiotemporal expression of the different ppGaNTase family members during development suggests unique roles for each of these gene products.

Characterization of a UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase that displays glycopeptide N-acetylgalactosaminyltransferase activity.

We report the cloning, expression, and characterization of a novel member of the mammalian UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) family that transfers GalNAc to a GalNAc-containing glycopeptide. Northern blot analysis revealed that the gene encoding this enzyme, termed ppGaNTase-T6, is expressed in a highly tissue-specific manner. Significant levels of transcript were found in rat and mouse sublingual gland, stomach, small intestine, and colon; trace amounts were seen in the ovary, cervix, and uterus. Recombinant constructs were expressed transiently in COS7 cells but demonstrated no transferase activity in vitro against a panel of unmodified peptides, including GTTPSPVPTTSTTSAP (MUC5AC). However, when incubated with the total glycosylated products obtained by action of ppGaNTase-T1 on MUC5AC (mainly GTT(GalNAc)PSPVPTTSTT(GalNAc)SAP), additional incorporation of GalNAc was achieved, resulting in new hydroxyamino acids being modified. The MUC5AC glycopeptide failed to serve as a substrate for ppGaNTase-T6 after modification of the GalNAc residues by periodate oxidation and sodium borohydride reduction, indicating a requirement for the presence of intact GalNAc. This suggests that O-glycosylation of multisite substrates may proceed in a specific hierarchical manner and underscores the potential complexity of the processes that regulate O-glycosylation.

Structure-function analysis of the UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase. Essential residues lie in a predicted active site cleft resembling a lactose repressor fold.

Mucin-type O-glycosylation is initiated by a family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGaNTases). Based on sequence relationships with divergent proteins, the ppGaNTases can be subdivided into three putative domains: each putative domain contains a characteristic sequence motif. The 112-amino acid glycosyltransferase 1 (GT1) motif represents the first half of the catalytic unit and contains a short aspartate-any residue-histidine (DXH) or aspartate-any residue-aspartate (DXD)-like sequence. Secondary structure predictions and structural threading suggest that the GT1 motif forms a 5-stranded parallel beta-sheet flanked by 4 alpha-helices, which resembles the first domain of the lactose repressor. Four invariant carboxylates and a histidine residue are predicted to lie at the C-terminal end of three beta-strands and line the active site cleft. Site-directed mutagenesis of murine ppGaNTase-T1 reveals that conservative mutations at these 5 positions result in products with no detectable enzyme activity (D156Q, D209N, and H211D) or <1% activity (E127Q and E213Q). The second half of the catalytic unit contains a DXXXXXWGGENXE motif (positions 310-322) which is also found in beta1,4-galactosyltransferases (termed the Gal/GalNAc-T motif). Mutants of carboxylates within this motif express either no detectable activity, 1% or 2% activity (E319Q, E322Q, and D310N, respectively). Mutagenesis of highly conserved (but not invariant) carboxylates produces only modest alterations in enzyme activity. Mutations in the C-terminal 128-amino acid ricin-like lectin motif do not alter the enzyme's catalytic properties.

Cloning and expression of a novel, tissue specifically expressed member of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family.

We report the cloning and expression of the fifth member of the mammalian UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) family. Degenerate polymerase chain reaction amplification and hybridization screening of a rat sublingual gland (RSLG) cDNA library were used to identify a novel isoform termed ppGaNTase-T5. Conceptual translation of the cDNA reveals a uniquely long stem region not observed for other members of this enzyme family. Recombinant proteins expressed transiently in COS7 cells displayed transferase activity in vitro. Relative activity and substrate preferences of ppGaNTase-T5 were compared with previously identified isoforms (ppGaNTase-T1, -T3, and -T4); ppGaNTase-T5 and -T4 glycosylated a restricted subset of peptides whereas ppGaNTase-T1 and -T3 glycosylated a broader range of substrates. Northern blot analysis revealed that ppGaNTase-T5 is expressed in a highly tissue-specific manner; abundant expression was seen in the RSLG, with lesser amounts of message in the stomach, small intestine, and colon. Therefore, the pattern of expression of ppGaNTase-T5 is the most restricted of all isoforms examined thus far. The identification of this novel isoform underscores the diversity and complexity of the family of genes controlling O-linked glycosylation.

Isoform-specific O-glycosylation by murine UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase-T3, in vivo.

Multiple isoforms of UDP-GalNAc:polypeptide N-acetylgalactosaminyl- transferase (ppGaNTase) have been cloned and expressed from a variety of organisms. In general, these isoforms display different patterns of tissue-specific expression, but exhibit overlapping substrate specificities, in vitro . A peptide substrate, derived from the sequence of the V3 loop of the HIV gp120 protein (HIV peptide), has previously been shown to be glycosylated in vitro exclusively by the ppGaNTase-T3 (Bennett et al. , 1996). To determine if this isoform-specificity is maintained in vivo , we have examined the glycosylation of this substrate when it is expressed as a reporter peptide (rHIV) in a cell background (COS7 cells) which lacks detectable levels of the ppGaNTase-T3. Glycosylation of rHIV was greatly increased by coexpression of a recombinant ppGaNTase-T3. Overexpression of ppGaNTase-T1 yielded only partial glycosylation of the reporter. We have also determined that the introduction of a proline residue at the +3 position flanking the potential glycosylation site eliminated ppGaNTase-T3 selectivity toward rHIV observed both in vivo and in vitro .

Chromosomal organization and expression analysis of two distinct genes encoding glutamine/glutamic acid-rich proteins.

GRP-Ca and GRP-Cb are two genes that encode glutamine/glutamic acid-rich proteins of the rat. These genes are very similar in structure and sequence, differing only within an approx. 90 bp segment of exon 3. We have used distinct oligonucleotide probes to unambiguously distinguish GRP-Ca and GRP-Cb gene expression. The two genes are expressed to relatively equivalent levels only in the submandibular gland. Chronic daily exposure to the beta-adrenergic agonist, isoprenaline, resulted in a statistically significant decrease in GRP-Ca expression, with no effect on GRP-Cb, in contrast with previous reports. Furthermore it was determined by PCR analysis of both submandibular-gland cDNA and genomic DNA that the GRP-Cb gene shows interanimal variability in the number of 69 bp tandem repeats found within exon 3; GRP-Cb genes were shown to contain four, five or six repeats. GRP-Ca shows no such variability, containing only five tandem repeats in all animals examined. The two genes were localized to within 450 kb of one another at q43-q44 of rat chromosome 4 using somatic cell hybrid analysis, pulsed-field gel analysis and fluorescent in situ hybridization.

cDNA cloning and expression of a novel UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase.

The cDNA for a fourth member of the mammalian UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family, termed ppGaNTase-T4, has been cloned from a murine spleen cDNA library and expressed transiently in COS7 cells as a secreted functional enzyme. Degenerate primers, based upon regions that are conserved among the known mammalian members of the enzyme family (ppGaNTase-T1, -T2, and -T3) and three Caenorhabditis elegans homologues (ppGaNTase-TA, -TB, and -TC), were used in polymerase chain reactions to identify and clone this new isoform. Substrate preferences for recombinant murine ppGaNTase-T1 and ppGaNTase-T4 isozymes were readily distinguished. ppGaNTase-T1 glycosylated a broader range of synthetic peptide substrates; in contrast, the ppGaNTase-T4 preferentially glycosylated a single substrate among the panel of 11 peptides tested. Using Northern blot analysis, a ppGaNTase-T4 message of 5.5 kilobases was detectable in murine embryonic tissues, as well as the adult sublingual gland, stomach, colon, small intestine, lung, cervix, and uterus with lower levels detected in kidney, liver, heart, brain, spleen, and ovary. Thus, the pattern of expression for ppGaNTase-T4 is more restricted than for the three previously reported isoforms of the enzyme. The variation in expression patterns and substrate specificities of the ppGaNTase enzyme family suggests that differential expression of these isoenzymes may be responsible for the cell-specific repertoire of mucin-type oligosaccharides on cell-surface and secreted O-linked glycoproteins.

Biosynthesis of a low-molecular-mass rat submandibular gland mucin glycoprotein in COS7 cells.

We have examined the biosynthesis of a low-molecular-mass mucin from rat submandibular gland (RSMG) expressed recombinantly in COS7 tissue culture cells, focusing primarily on the addition of carbohydrate to the protein core of the mucin. We find evidence for N-linked glycosylation, but this modification is not required for secretion of the mucin. Similarly, although the recombinant RSMG mucin, like its native counterpart, contains large amounts of O-linked carbohydrate, chain extension beyond the initial O-linked GalNAc moiety is not required for secretion. We have identified partially glycosylated mucin by a combination of metabolic pulse-chase and lectin precipitations of the biosynthetic intermediates. Our results suggest that the addition of GalNAc to threonine and serine in the RSMG mucin does not occur simultaneously, as has been described for other O-glycosylated proteins.

Identification of essential histidine residues in UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase-T1.

UDP-N-acetyl-d-galactosamine:polypeptide N-acetylgalactosaminyltransferases (ppGaNTases) catalyse the initial step of mucin-type O-glycosylation. The activity of bovine ppGaNTase-T1 isoenzyme was inhibited by diethyl pyrocarbonate (DEPC) modification. Activity was partially restored by hydroxylamine treatment, indicating that one of the reactive residues was a histidine. The transferase was protected against DEPC inactivation when UDP-GalNAc and EPO-G, a peptide pseudo-substrate PPDAAGAAPLR, were simultaneously present, while presence of EPO-G alone did not alter DEPC inactivation. However, inclusion of UDP-GalNAc alone potentiated DEPC-inhibition of the enzyme, suggesting that UDP-GalNAc binding changes the accessibility or reactivity of an essential histidine residue. Deletion of the first 56 amino acids (including one hisitidine residue) yielded a fully active secreted form of the bovine ppGaNTase-T1 enzyme. Each of the 14 remaining histidines in the enzyme were mutated to alanine, and the recombinant mutants were recovered from COS7 cells. The mutation of histidine residues His211-->Ala and His344-->Ala resulted in recombinant proteins with no detectable enzymic activity. A significant decrease in the initial rate of GalNAc transfer to the substrate was observed with mutants His125-->Ala and His341-->Ala (1% and 6% of wild-type activity respectively). Mutation of the remaining ten histidine residues yielded mutants that were indistinguishable from the wild-type enzyme. Mutagenesis and SDS/PAGE analysis of all N-glycosylation sequons revealed that positions N-95 and N-552 are occupied by N-linked sugars in COS7 cells. Ablation of either site did not perturb enzyme biosynthesis or enzyme activity.

Charge distribution of flanking amino acids inhibits O-glycosylation of several single-site acceptors in vivo.

From surveys of known O-glycosylation sites and in vitro glycosylation assays with synthetic peptide acceptors, it appears that the presence of charged amino acids near serine/threonine residues reduces the likelihood of O-glycosylation by UDP-GalNAc polypeptide:N-acetylgalactosaminyltransferases (ppGaNTases). Previously, we demonstrated that the in vivo O-glycosylation of a sequence derived from a known glycosylation site of human von Willebrand factor (PHMAQVTVGPGL) was markedly reduced when charged residues were substituted at position -1 and +3 relative to the single threonine. In contrast, acidic residues at positions -2, +1, and +2 had no effect (Nehrke et al., 1996), suggesting that charge distribution but not charge density was important. To determine whether the charge distribution effect on O-glycosylation is limited to a specific sequence context or restricted to unique isoforms of ppGaNTase, we have analyzed the in vivo O-glycosylation of six secreted recombinant reporter proteins in three different cell backgrounds. The differential presence of known ppGaNTase transcripts was determined in each cell type by Northern blot analysis. Each reporter, which contains a single site of O-glycosylation, was O-glycosylated in a cell-background-specific manner; digestion with O-glycanase and alpha-N-acetylgalactosaminidase following mild acid hydrolysis suggested that simple type II core structures were acquired. However, in COS7 cells, one reporter peptide acquired glycosaminoglycans in preference to mucin-type O-glycans. Regardless of cell background or the reporter examined, the substitution of glutamic acid residues at positions -1 and +3 markedly diminished the level of mucin-type O-glycosylation. Charge distribution would appear, therefore, to play a more general role in determining the extent to which solitary O-glycosylation sites are modified.