Fred K. Hagen

The Proteomes of Human Parotid and Submandibular/Sublingual Gland Salivas Collected as the Ductal Secretions

Saliva is a body fluid with important functions in oral and general health. A consortium of three research groups catalogued the proteins in human saliva collected as the ductal secretions: 1166 identifications--914 in parotid and 917 in submandibular/sublingual saliva--were made. The results showed that a high proportion of proteins that are found in plasma and/or tears are also present in saliva along with unique components. The proteins identified are involved in numerous molecular processes ranging from structural functions to enzymatic/catalytic activities. As expected, the majority mapped to the extracellular and secretory compartments. An immunoblot approach was used to validate the presence in saliva of a subset of the proteins identified by mass spectrometric approaches. These experiments focused on novel constituents and proteins for which the peptide evidence was relatively weak. Ultimately, information derived from the work reported here and related published studies can be used to translate blood-based clinical laboratory tests into a format that utilizes saliva. Additionally, a catalogue of the salivary proteome of healthy individuals allows future analyses of salivary samples from individuals with oral and systemic diseases, with the goal of identifying biomarkers with diagnostic and/or prognostic value for these conditions; another possibility is the discovery of therapeutic targets.

Proteomic Analysis of Human Parotid Gland Exosomes by Multidimensional Protein Identification Technology (MudPIT)

Human ductal saliva contributes over a thousand unique proteins to whole oral fluids. The mechanism by which most of these proteins are secreted by salivary glands remains to be determined. The present study used a mass spectrometry-based, shotgun proteomics approach to explore the possibility that a subset of the proteins found in saliva are derived from exosomes, membrane-bound vesicles of endosomal origin within multivesicular endosomes. Using MudPIT (multidimensional protein identification technology) mass spectrometry, we catalogued 491 proteins in the exosome fraction of human parotid saliva. Many of these proteins were previously observed in ductal saliva from parotid glands (265 proteins). Furthermore, 72 of the proteins in parotid exosomes overlap with those previously identified as urinary exosome proteins, proteins which are also frequently associated with exosomes from other tissues and cell types. Gene Ontology (GO) and KEGG pathway analyses found that cytosolic proteins comprise the largest category of proteins in parotid exosomes (43%), involved in such processes as phosphatidylinositol signaling system, calcium signaling pathway, inositol metabolism, protein export, and signal transduction, among others; whereas the integral plasma membrane proteins and associated/peripheral plasma membrane proteins (26%) were associated with extracellular matrix-receptor interaction, epithelial cell signaling, T-cell and B-cell receptor signaling, cytokine receptor interaction, and antigen processing and presentation, among other biological functions. In addition, these putative saliva exosomal proteins were linked to specific diseases (e.g., neurodegenerative disorders, prion disease, cancers, type I and II diabetes). Consequently, parotid glands secrete exosomes that reflect the metabolic and functional status of the gland and may also carry informative protein markers useful in the diagnosis and treatment of systemic diseases.

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:polypeptideN-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 controllingO-linked glycosylation.

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 thatO-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:polypeptideN-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 acidglycosyltransferase 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 β-sheet flanked by 4 α-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 β-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 β1,4-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.

Initiation of protein O glycosylation by the polypeptide GalNAcT-1 in vascular biology and humoral immunity.

ABSTRACT Core-type protein O glycosylation is initiated by polypeptide N-acetylgalactosamine (GalNAc) transferase (ppGalNAcT) activity and produces the covalent linkage of serine and threonine residues of proteins. More than a dozen ppGalNAcTs operate within multicellular organisms, and they differ with respect to expression patterns and substrate selectivity. These distinctive features imply that each ppGalNAcT may differentially modulate regulatory processes in animal development, physiology, and perhaps disease. We found that ppGalNAcT-1 plays key roles in cell and glycoprotein selective functions that modulate the hematopoietic system. Loss of ppGalNAcT-1 activity in the mouse results in a bleeding disorder which tracks with reduced plasma levels of blood coagulation factors V, VII, VIII, IX, X, and XII. ppGalNAcT-1 further supports leukocyte trafficking and residency in normal homeostatic physiology as well as during inflammatory responses, in part by providing a scaffold for the synthesis of selectin ligands expressed by neutrophils and endothelial cells of peripheral lymph nodes. Animals lacking ppGalNAcT-1 are also markedly impaired in immunoglobulin G production, coincident with increased germinal center B-cell apoptosis and reduced levels of plasma B cells. These findings reveal that the initiation of protein O glycosylation by ppGalNAcT-1 provides a distinctive repertoire of advantageous functions that support vascular responses and humoral immunity.

Targeting Aberrant Glutathione Metabolism to Eradicate Human Acute Myelogenous Leukemia Cells

Background: Eradication of primary human leukemia cells represents a major challenge. Therapies have not substantially changed in over 30 years. Results: Using normal versus leukemia specimens enriched for primitive cells, we document aberrant regulation of glutathione metabolism. Conclusion: Aberrant glutathione metabolism is an intrinsic property of human leukemia cells. Significance: Interventions based on modulation of glutathione metabolism represent a powerful means to improve therapy. The development of strategies to eradicate primary human acute myelogenous leukemia (AML) cells is a major challenge to the leukemia research field. In particular, primitive leukemia cells, often termed leukemia stem cells, are typically refractory to many forms of therapy. To investigate improved strategies for targeting of human AML cells we compared the molecular mechanisms regulating oxidative state in primitive (CD34+) leukemic versus normal specimens. Our data indicate that CD34+ AML cells have elevated expression of multiple glutathione pathway regulatory proteins, presumably as a mechanism to compensate for increased oxidative stress in leukemic cells. Consistent with this observation, CD34+ AML cells have lower levels of reduced glutathione and increased levels of oxidized glutathione compared with normal CD34+ cells. These findings led us to hypothesize that AML cells will be hypersensitive to inhibition of glutathione metabolism. To test this premise, we identified compounds such as parthenolide (PTL) or piperlongumine that induce almost complete glutathione depletion and severe cell death in CD34+ AML cells. Importantly, these compounds only induce limited and transient glutathione depletion as well as significantly less toxicity in normal CD34+ cells. We further determined that PTL perturbs glutathione homeostasis by a multifactorial mechanism, which includes inhibiting key glutathione metabolic enzymes (GCLC and GPX1), as well as direct depletion of glutathione. These findings demonstrate that primitive leukemia cells are uniquely sensitive to agents that target aberrant glutathione metabolism, an intrinsic property of primary human AML cells.

Specificity of O-glycosylation by bovine colostrum UDP-GalNAc: polypeptide ?-N-acetylgalactosaminyltransferase using synthetic glycopeptide substrates

The factors determining glycosylation of mucin type glycoproteins are not well understood. In the present work, we investigated the role of the peptide moiety and of the presence of O-glycan chains on O-glycosylation by UDP-GalNAc: polypeptide α-N-acetylgalactosaminyl-transferase (ppGalNAc-T). We used purified ppGalNAc-T from bovine colostrum and a series of synthetic glycopeptide and peptide substrates most of which contained sequences derived from the tandem repeat region of MUC2 mucin. The rate of incorporation of GalNAc into Thr was significantly greater than toward Ser residues. The presence of one or two GalNAc-Thr moieties in the substrate significantly reduced enzyme activity, and this effect was more pronounced when the disaccharide Galβ1–3GalNAc was present. Thus the sequential attachment of a second GalNAc residue in the vicinity of a pre-existing GalNAc-Thr or Galβ1–3GalNAc-Thr occurs at a slower rate than primary glycosylation of carbohydrate-free peptide. Analysis of products by HPLC showed that the enzyme was selective in glycosylating peptides or glycopeptides with the PTTTPIST sequence in that the preferred primary glycosylation site was the third Thr from the aminoterminal end; secondary glycosylation depended on the site of the primary glycosylation. Negatively but not positively charged amino acids on the carboxy-terminal side of the putative secondary glycosylation site resulted in high activity suggesting charge-charge interactions of substrates with the enzyme. These studies indicate that O-glycosylation by bovine colostrum ppGalNAc-T is a selective process dependent on both the amino acid sequence and prior glycosylation of peptide substrates.

Quantitative Analysis of Age Specific Variation in the Abundance of Human Female Parotid Salivary Proteins

Human saliva is a protein-rich, easily accessible source of potential local and systemic biomarkers to monitor changes that occur under pathological conditions; however, little is known about the changes in abundance associated with normal aging. In this study, we performed a comprehensive proteomic profiling of pooled saliva collected from the parotid glands of healthy female subjects, divided into two age groups 1 and 2 (20-30 and 55-65 years old, respectively). Hydrophobic charge interaction chromatography was used to separate high- from low-abundance proteins prior to characterization of the parotid saliva using multidimensional protein identification technology (MudPIT). Collectively, 532 proteins were identified in the two age groups. Of these proteins, 266 were identified exclusively in one age group, while 266 proteins were common to both groups. The majority of the proteins identified in the two age groups belonged to the defense and immune response category. Of note, several defense related proteins (e.g., lysozyme, lactoferrin and histatin-1) were significantly more abundant in group 2 as determined by G-test. Selected representative mass spectrometric findings were validated by Western blot analysis. Our study reports the first quantitative analysis of differentially regulated proteins in ductal saliva collected from young and older female subjects. This study supports the use of high-throughput proteomics as a robust discovery tool. Such results provide a foundation for future studies to identify specific salivary proteins which may be linked to age-related diseases specific to women.

Cloning and sequence homology of a rat UDP-GalNAc:polypeptideN-acetylgalactosaminyltransferase

A UDP-GalNAc:polypeptideN-acetylgalactosaminyltransferase (polypeptide GalNAc transferase) cDNA was amplified from rat sublingual, submandibular and parotid glands, brain, skeletal muscle, and liver, using the polymerase chain reaction (PCR) and sequences derived from bovine polypeptide GalNAc transferase-Type 1 (polypeptide GalNAc transferase-T1). The transcripts encoding the rat sublingual gland and bovine enzymes were 91% identical in nucleotide sequence, except in their 5′ and 3′ untranslated regions. The enzymes encoded by the rat and bovine cDNAs were 559 amino acids in length and were virtually identical (98% amino acid sequence identity and 99.5% homologous overall). Northern blot analysis indicates that the polypeptide GalNAc transferase-T1 transcripts are expressed in many tissues but at widely differing levels. Although the amino acid sequence of polypeptide GalNAc transferase-T1 is conserved among mammals, the pattern of tissue expression varies between rats and humans. For example, the steady-state level of polypeptide GalNAc transferase-T1 transcript is quite low in lung relative to other rat tissues, whereas high expression of this transcript is detected in human lung. Therefore, we surmise that isoforms of polypeptide GalNAc transferase must exist and that isoforms are expressed in a tissue-dependent fashion. Searches of the GenBank database have revealed homologous sequences for several isoforms derived from several human tissues. In addition, hypothetical proteins fromC. elegans also display strong homology; evidence suggests six ancestral isoforms of polypeptide GalNAc transferases may exist inC. elegans.