Salivary proline-rich proteins: biochemistry, molecular biology, and regulation of expression

Salivary Gland Adaptation to Dietary Inclusion of Hydrolysable Tannins in Boars

Simple Summary

Tannins have traditionally been avoided in animal nutrition due to their anti-nutritive effects. However, recent studies reported hydrolysable tannins as beneficial additives that have antimutagenic, anticarcinogenic, antidiarrheal, and antiulcerogenic effects on animals. In a study testing the inclusion of hydrolysable tannins as a potential nutritive factor to reduce boar taint in entire males, significant enlargement of the parotid glands (parotidomegaly) was observed. In this study, we aimed to determine the morphological and immunohistochemical basis for the observed parotidomegaly. We discovered that enlargement of glandular lobules and acinar area, an increased ratio between the nucleus and cytoplasm of serous cells, and increased excretion of proline-rich proteins (PRPs) were characteristic of the experimental group that received the highest dietary tannin intake. The mandibular salivary gland, on the other hand, did not show significant morphological changes among the experimental groups. This suggests increased functional activity of the parotid salivary glands as the first and most important line of defense against high dietary tannin and its potential negative effects.

Abstract

The ingestion of hydrolysable tannins as a potential nutrient to reduce boar odor in entire males results in the significant enlargement of parotid glands (parotidomegaly). The objective of this study was to characterize the effects of different levels of hydrolysable tannins in the diet of fattening boars (n = 24) on salivary gland morphology and proline-rich protein (PRP) expression at the histological level. Four treatment groups of pigs (n = 6 per group) were fed either a control (T0) or experimental diet, where the T0 diet was supplemented with 1% (T1), 2% (T2), or 3% (T3) of the hydrolysable tannin-rich extract Farmatan^®. After slaughter, the parotid and mandibular glands of the experimental pigs were harvested and dissected for staining using Goldner’s Trichrome method, and immunohistochemical studies with antibodies against PRPs. Morphometric analysis was performed on microtome sections of both salivary glands, to measure the acinar area, the lobular area, the area of the secretory ductal cells, and the sizes of glandular cells and their nuclei. Histological assessment revealed that significant parotidomegaly was only present in the T3 group, based on the presence of larger glandular lobules, acinar areas, and their higher nucleus to cytoplasm ratio. The immunohistochemical method, supported by color intensity measurements, indicated significant increases in basic PRPs (PRB2) in the T3 and acidic PRPs (PRH1/2) in the T1 groups. Tannin supplementation did not affect the histo-morphological properties of the mandibular gland. This study confirms that pigs can adapt to a tannin-rich diet by making structural changes in their parotid salivary gland, indicating its higher functional activity.

Effects of spaceflight on the mouse submandibular gland

OBJECTIVE

This study was conducted to determine if the morphology and biochemistry of the mouse submandibular gland is affected by microgravity and the spaceflight environment.

DESIGN

Tissues from female mice flown on the US space shuttle missions Space Transportation System (STS)-131 and STS-135 for 15 and 13 d, respectively, and from male mice flown on the 30 d Russian Bion-M1 biosatellite, were examined using transmission electron microscopy and light and electron microscopic immunohistochemistry.

RESULTS

In contrast to the parotid gland, morphologic changes were not apparent in the submandibular gland. No significant changes in protein expression, as assessed by quantitative immunogold labeling, occurred in female mice flown for 13-15 d. In male mice, however, increased labeling for salivary androgen binding protein alpha (in acinar cell secretory granules), and epidermal growth factor and nerve growth factor (in granular convoluted duct cell granules) was seen after 30 d in space.

CONCLUSION

These results indicate that spaceflight alters secretory protein expression in the submandibular gland and suggest that the sex of the animals and the length of the flight may affect the response. These findings also show that individual salivary glands respond differently to spaceflight. Saliva contains proteins secreted from salivary glands and is easily collected, therefore is a useful biofluid for general medical analyses and in particular for monitoring the physiology and health of astronauts.

Glycan recognition at the saliva - oral microbiome interface

The mouth is a first critical interface where most potentially harmful substances or pathogens contact the host environment. Adaptive and innate immune defense mechanisms are established there to inactivate or eliminate pathogenic microbes that traverse the oral environment on the way to their target organs and tissues. Protein and glycoprotein components of saliva play a particularly important role in modulating the oral microbiota and helping with the clearance of pathogens. It has long been acknowledged that glycobiological and glycoimmunological aspects play a pivotal role in oral host-microbe, microbe-host, and microbe-microbe interactions in the mouth. In this review, we aim to delineate how glycan-mediated host defense mechanisms in the oral cavity support human health. We will describe the role of glycans attached to large molecular size salivary glycoproteins which act as a first line of primordial host defense in the human mouth. We will further discuss how glycan recognition contributes to both colonization and clearance of oral microbes.

Dynamic Proteome Alteration and Functional Modulation of Human Saliva Induced by Dietary Chemosensory Stimuli

Saliva flow measurements and SDS-PAGE separation of human whole saliva freshly collected after oral stimulation with citric acid (sour), aspartame (sweet), iso-α-acids (bitter), mono sodium l-glutamate (umami), NaCl (salty), 6-gingerol (pungent), hydroxy-α-sanshool (tingling), and hydroxy-β-sanshool (numbing), followed by tryptic digestion, nano-HPLC-MS/MS, and label-free protein quantitation demonstrated a stimulus- and time-dependent influence of the dietary chemosensates on salivation and the salivary proteome composition. Gene ontology enrichment analysis showed evidence for stimulus-induced alterations of the saliva proteome to boot an efficient molecular defense network of the oral cavity, e.g., 6-gingerol increased salivary lactoperoxidase activity, catalyzing the oxidation of thiocyanate to produce the antimicrobial and antifungal hypothiocyanate, from 0.37 ± 0.02 to 0.91 ± 0.05 mU/mL 45 s after stimulation. In comparison, oral citric acid stimulation induced an increase of myeloperoxidase activity, catalyzing the chloride oxidation to generate antimicrobial hypochloride in saliva, from 0.24 ± 0.04 to 0.70 ± 0.1 mU/mL as well as an increase of salivary levels of lysozyme, exhibiting antimicrobial activity on Gram-positive bacteria, from 6.0-10 to 100-150 μg/mL. Finally, microbial growth experiments clearly demonstrated for the first time that the increase of the salivary lysozyme abundance upon oral citric acid stimulation translates into an enhanced biological function, that is an almost complete growth inhibition of the two lysozyme-sensitive Gram-positive bacteria tested.

Reactivation of peroxidase activity in human saliva samples by polyphenols

OBJECTIVES

The enzyme lactoperoxidase (LPO), which is released into several body fluids like saliva, is an essential part to maintain the oral bacterial homeostasis by catalysing the oxidation of thiocyanate (SCN^-) to hypo-thiocyanite (^-OSCN). The formation of unreactive redox intermediates (like Compound II) leads to a decreased pseudo-halogenating enzyme activity, which is associated with a higher risk for oral infections. According to former studies with bovine LPO selected flavonoids were tested in respect to their potential to reactivate the enzymatic activity in a more physiological, human salivary system.

DESIGN

Saliva samples from healthy donors were collected and characterized by using several gel staining methods and immunoblotting. Afterwards kinetic measurements were performed by applying the TNB-assay to evaluate the pseudo-halogenating salivary peroxidase (SAPX) activity. The measurements were performed in the presence of excess H₂O₂ to simulate pro-inflammatory conditions. Moreover selected flavonoids or an ethanolic extract of Tormentillae rhizoma were applied to test their regenerating effect on the LPO-derived ^-OSCN production.

RESULTS

Despite the complex protein composition of the collected saliva samples, an SAPX-derived pseudo-halogenating activity could be identified. The ^-OSCN regenerating effects of the tested polyphenols were completely comparable to previous in vitro experiments with bovine LPO. Thus, we could show that phenolic substances are suitable to regenerate the peroxidase activity in human saliva samples after H₂O₂-induced inactivation.

CONCLUSION

The studies provide new insights into the effect of pharmaceutical relevant polyphenols on salivary peroxidase activity and thus, suggest this enzyme as a new target for the prevention and therapy of oral inflammatory diseases.

Salivary proline-rich protein may reduce tannin-iron chelation: a systematic narrative review

Background

Tannins are often cited for antinutritional effects, including chelation of non-heme iron. Despite this, studies exploring non-heme iron bioavailability inhibition with long-term consumption have reported mixed results. Salivary proline-rich proteins (PRPs) may mediate tannin-antinutritional effects on non-heme iron bioavailability.

Aim

To review evidence regarding biochemical binding mechanisms and affinity states between PRPs and tannins, as well as effects of PRPs on non-heme iron bioavailability with tannin consumption in vivo.

Methods

Narrative systematic review and meta-analysis. Common themes in biochemical modeling and affinity studies were collated for summary and synthesis; data were extracted from in vivo experiments for meta-analysis.

Results

Thirty-two studies were included in analysis. Common themes that positively influenced tannin-PRP binding included specificity of tannin-PRP binding, PRP and tannin stereochemistry. Hydrolyzable tannins have different affinities than condensed tannins when binding to PRPs. In vivo, hepatic iron stores and non-heme iron absorption are not significantly affected by tannin consumption (d = −0.64-1.84; −2.7-0.13 respectively), and PRP expression may increase non-heme iron bioavailability with tannin consumption.

Conclusions

In vitro modeling suggests that tannins favor PRP binding over iron chelation throughout digestion. Hydrolyzable tannins are not representative of tannin impact on non-heme iron bioavailability in food tannins because of their unique structural properties and PRP affinities. With tannin consumption, PRP production is increased, and may be an initial line of defense against tannin-non-heme iron chelation in vivo. More research is needed to compare competitive binding of tannin-PRP to tannin-non-heme iron complexes, and elucidate PRPs’ role in adaption to non-heme iron bioavailability in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-017-0197-z) contains supplementary material, which is available to authorized users.

A study of procyanidin binding to Histatin 5 using Electrospray Ionization Tandem Mass Spectrometry (ESI-MS/MS) and molecular simulations

Tannins act as antioxidants, anticarcinogens, cardio-protectants, anti-inflammatory and anti-microbial agents and bind to salivary peptides by hydrophilic and hydrophobic mechanisms. Electrospray Ionization Mass Spectrometry (ESI-MS) has been used to assess both hydrophilic and hydrophobic components of noncovalent binding in protein complexes. In the present study, direct infusion Electrospray-Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ES-FTICR MS) is used to assess relative binding affinities of procyanidin tannin stereoisomers for salivary peptides arising from aqueous solutions. The condensed tannins procyanidin B1, B2, B3, and B4 demonstrate significantly different binding affinities for the salivary peptide Histatin 5. Rigid docking combined with molecular dynamics optimization is used to investigate procyanidin-Histatin 5 binding mechanisms and as a basis to rationalize trends found in the corresponding ES-FTICR MS experiments. The relative binding affinities of the four procyanidin rotamers are different in the gas and liquid phases. The simulation results indicate that many of the same contact points are made in both phases, but there is a increase in strong electrostatic interactions and an decrease in π-π contacts upon transfer from the liquid to the gas phase. The simulations reveal that the tannin interactions can make close contacts with a variety of amino acid residues on the peptide.

Hydrolysable tannin fed to entire male pigs affects intestinal production, tissue deposition and hepatic clearance of skatole

The effect of adding hydrolysable tannins to the diet of fattening boars was studied. Performance, reproductive organ weights, salivary gland morphology, boar taint compounds and skatole metabolism were evaluated. At 123 days of age and 52 ± 6 kg liveweight, 24 Landrace × Large White boars were assigned within a litter to four treatment groups: control (T0 fed mixture with 13.2 MJ/kg, 17.5% crude proteins) and three experimental diets for which the T0 diet was supplemented with 1%, 2% and 3% of hydrolysable tannin-rich extract (T1, T2 and T3, respectively). Pigs were kept individually with ad libitum access to feed and water and slaughtered at 193 days of age and 122 ± 10 kg liveweight. Adding hydrolysable tannins to the diet had no negative effect on growth performance at 1% and 2%, whereas the 3% inclusion reduced feed intake and resulted in an adaptive response of the salivary glands (particularly parotid gland hypertrophy). Relative to T0, fat tissue skatole concentration was increased in the T1 group, but was similar in T2 and T3. Across treatments tissue skatole concentrations were proportional to the activity of hepatic CYP450. The results indicate the potential of tannin supplementation to reduce boar taint although further investigations are needed in order to establishing optimal dosage.

Flexible and accessible workflows for improved proteogenomic analysis using the Galaxy framework

Proteogenomics combines large-scale genomic and transcriptomic data with mass-spectrometry-based proteomic data to discover novel protein sequence variants and improve genome annotation. In contrast with conventional proteomic applications, proteogenomic analysis requires a number of additional data processing steps. Ideally, these required steps would be integrated and automated via a single software platform offering accessibility for wet-bench researchers as well as flexibility for user-specific customization and integration of new software tools as they emerge. Toward this end, we have extended the Galaxy bioinformatics framework to facilitate proteogenomic analysis. Using analysis of whole human saliva as an example, we demonstrate Galaxy’s flexibility through the creation of a modular workflow incorporating both established and customized software tools that improve depth and quality of proteogenomic results. Our customized Galaxy-based software includes automated, batch-mode BLASTP searching and a Peptide Sequence Match Evaluator tool, both useful for evaluating the veracity of putative novel peptide identifications. Our complex workflow (approximately 140 steps) can be easily shared using built-in Galaxy functions, enabling their use and customization by others. Our results provide a blueprint for the establishment of the Galaxy framework as an ideal solution for the emerging field of proteogenomics.

Salivary biomarkers of physical fatigue as markers of sleep deprivation

STUDY OBJECTIVE

Determine whether a salivary biomarker of physical fatigue, referred to as the fatigue biomarker index (FBI), can discriminate a control group from a sleep deprived group when saliva is collected under controlled conditions. The study expands on previous work examining changes in the composition of saliva during periods of prolonged exercise.

METHODS

Thirty (30) young adults (14 Control [CON]; 16 Sleep Deprived [SDEP]) were monitored for mood state (Profile of Mood States [POMS]), cognitive performance (Stroop Color-Conflict Tests), and salivary biomarkers of physical fatigue over a 48-h period with sampling at 3-h intervals. Trials lasted from 06:00 on day 1 (time = -3 h) to 09:00 on day 3 (time = 48 h). Levels of salivary biomarkers were calculated from liquid chromatography-mass spectrometry (LC-MS) data. Statistical comparisons were made using Wilcoxon rank sum tests with a Bonferroni correction to limit type 1 error. Receiver-operator characteristic (ROC) analysis was used to evaluate the ability of the various parameters to distinguish the SDEP population from the CON population.

RESULTS

Longitudinal analysis demonstrated significant between-group differences in all three parameters. ROC analysis demonstrated that cognitive performance tests and salivary biomarkers of physical fatigue distinguish the SDEP population from the CON population.

CONCLUSIONS

A previously identified salivary biomarker of physical fatigue may provide an alternative method for discriminating sleep deprived from rested individuals. The salivary biomarker of physical fatigue holds promise as an objective measure of sleep deprivation, perhaps eventually removing the reliance on self-reported sleep diaries and/or repeated polysomnographs for longitudinal tracking of sleep quality and/or diagnosis of sleep disorders.

Aggregation of the salivary proline-rich protein IB5 in the presence of the tannin EgCG

In the mouth, proline-rich proteins (PRP), which are major components of stimulated saliva, interact with tannins contained in food. We report in vitro interactions of the tannin epigallocatechin gallate (EgCG), with a basic salivary PRP, IB5, studied through electrospray ionization mass spectrometry (ESI-MS), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS). In dilute protein (IB5) solutions of low ionic strength (1 mM), the proteins repel each other, and the tannins bind to nonaggregated proteins. ESI-MS experiments determine the populations of nonaggregated proteins that have bound various numbers of tannin molecules. These populations match approximately the Poisson distribution for binding to n = 8 sites on the protein. MS/MS experiments confirm that complexes containing n = 1 to 8 EgCG molecules are dissociated with the same energy. Assuming that the 8 sites are equivalent, we calculate a binding isotherm, with a binding free energy Δμ = 7.26RT(a) (K(d) = 706 μM). In protein solutions that are more concentrated (0.21 mM) and at higher ionic strength (50 mM, pH 5.5), the tannins can bridge the proteins together. DLS experiments measure the number of proteins per aggregate. This number rises rapidly when the EgCG concentration exceeds a threshold (0.2 mM EgCG for 0.21 mM of IB5). SAXS experiments indicate that the aggregates have a core-corona structure. The core contains proteins that have bound at least 3 tannins and the corona has proteins with fewer bound tannins. These aggregates coexist with nonaggregated proteins. Increasing the tannin concentration beyond the threshold causes the transfer of proteins to the aggregates and a fast rise of the number of proteins per aggregate. A poisoned growth model explains this fast rise. Very large cationic aggregates, containing up to 10,000 proteins, are formed at tannin concentrations (2 mM) slightly above the aggregation threshold (0.2 mM).

Lacrimal proline rich 4 (LPRR4) protein in the tear fluid is a potential biomarker of dry eye syndrome

Dry eye syndrome (DES) is a complex, multifactorial, immune-associated disorder of the tear and ocular surface. DES with a high prevalence world over needs identification of potential biomarkers so as to understand not only the disease mechanism but also to identify drug targets. In this study we looked for differentially expressed proteins in tear samples of DES to arrive at characteristic biomarkers. As part of a prospective case-control study, tear specimen were collected using Schirmer strips from 129 dry eye cases and 73 age matched controls. 2D electrophoresis (2DE) and Differential gel electrophoresis (DIGE) was done to identify differentially expressed proteins. One of the differentially expressed protein in DES is lacrimal proline rich 4 protein (LPRR4). LPRR4 protein expression was quantified by enzyme immune sorbent assay (ELISA). LPRR4 was down regulated significantly in all types of dry eye cases, correlating with the disease severity as measured by clinical investigations. Further characterization of the protein is required to assess its therapeutic potential in DES.

Orosensory stimulation effects on human saliva proteome

Saliva flow induced by 6-gingerol (pungent), hydroxy-α/β-sanshools (tingling), and citric acid (sour) was measured, and the time-dependent changes in the whole saliva proteome were analyzed by means of 2D-PAGE, followed by tryptic in-gel digestion and MALDI-TOF-MS peptide mass fingerprint analysis. The proteins showing significantly decreased abundance after oral 6-gingerol stimulation were identified as glutathione S-transferase P, the heat shock protein β-1, the heat shock 70 kDa protein 1, annexin A1, and cytoplasmic β-actin, whereas prolactin inducible proteins (PIP), short palate, lung and nasal epithelium carcinoma-associated protein 2 (SPLUNC2), zinc-α-2-glycoproteins (Zn-α-GP), and carbonic anhydrase VI (CAVI) were found with increased abundance. As the effects of this study were observed instantaneously upon stimulation, any proteome modulation is very likely to result from the release of proteins from preformed vesicles and not from de novo synthesis. The elevated levels of SPLUNC2, Zn-α-GP, and CAVI might be interpreted to trigger innate protective mechanisms in mucosal immunity and in nonimmune mucosal defense and might play an important role during the initial stage of inflammation.

Gene expression profiles of the three major salivary glands in rats

The protein components of saliva reflect the condition of the whole body as well as the salivary glands. The aim of this study is to characterize the gene expression profiles in each of the rat major salivary glands-the parotid, submandibular, and sublingual glands. Gene expression was analyzed using DNA microarrays, and observed differences in expression of representative genes were confirmed by quantitative, real-time polymerase chain reaction. Among the glands, the contribution to the high expression of genes encoding various proteins, specifically mucin 10, proline-rich glycoproteins, proline-rich protein 2, proline-rich proteoglycans, cystatin 10, amylase, deoxyribonuclease I, and von Ebner's gland protein, was significantly greater in the parotid gland than the other glands. The submandibular and sublingual glands had similar gene expression profiles that differed from profile of the parotid gland. For example, the genes encoding mucin 19 and ovomacroglobulin were highly expressed only in the submandibular and sublingual glands. In summary, we characterized gene expression in the rat major salivary glands and provided basic information on salivary gland marker proteins.

Ability of a salivary intrinsically unstructured protein to bind different tannin targets revealed by mass spectrometry

Astringency is thought to result from the interaction between salivary proline-rich proteins (PRP) that belong to the intrinsically unstructured protein group (IUP), and tannins, which are phenolic compounds. IUPs have the ability to bind several and/or different targets. At the same time, tannins have different chemical features reported to contribute to the sensation of astringency. The ability of both electrospray ionization mass spectrometry and tandem mass spectrometry to investigate the noncovalent interaction occurring between a human salivary PRP, IB5, and a model tannin, epigallocatechin 3-O-gallate (EgCG), has been reported. Herein, we extend this method to study the effect of tannin chemical features on their interaction with IB5. We used five model tannins, epigallocatechin (EgC), epicatechin 3-O-gallate (ECG), epigallocatechin 3-O-gallate (EgCG), procyanidin dimer B2 and B2 3'-O-gallate, which cover the main tannin chemical features: presence of a gallate moiety (galloylation), the degree of polymerization, and the degree of B ring hydroxylation. We show the ability of IB5 to bind these tannins. We report differences in stoichiometries and in stability of the IB5•1 tannin complexes. These results demonstrate the main role of hydroxyl groups in these interactions and show the involvement of hydrogen bonds. Finally, these results are in line with sensory analysis, by Vidal et al. (J Sci Food Agric 83:564-573, 2003) pointing out that the chain length and the level of galloylation are the main factors affecting astringency perception.

On approaches to the functional restoration of salivary glands damaged by radiation therapy for head and neck cancer, with a review of related aspects of salivary gland morphology and development

Radiation therapy for cancer of the head and neck can devastate the salivary glands and partially devitalize the mandible and maxilla. As a result, saliva production is drastically reduced and its quality adversely altered. Without diligent home and professional care, the teeth are subject to rapid destruction by caries, necessitating extractions with attendant high risk of necrosis of the supporting bone. Innovative techniques in delivery of radiation therapy and administration of drugs that selectively protect normal tissues can reduce significantly the radiation effects on salivary glands. Nonetheless, many patients still suffer severe oral dryness. I review here the functional morphology and development of salivary glands as these relate to approaches to preventing and restoring radiation-induced loss of salivary function. The acinar cells are responsible for most of the fluid and organic material in saliva, while the larger ducts influence the inorganic content. A central theme of this review is the extent to which the several types of epithelial cells in salivary glands may be pluripotential and the circumstances that may influence their ability to replace cells that have been lost or functionally inactivated due to the effects of radiation. The evidence suggests that the highly differentiated cells of the acini and large ducts of mature glands can replace themselves except when the respective pools of available cells are greatly diminished via apoptosis or necrosis owing to severely stressful events. Under the latter circumstances, relatively undifferentiated cells in the intercalated ducts proliferate and redifferentiate as may be required to replenish the depleted pools. It is likely that some, if not many, acinar cells may de-differentiate into intercalated duct-like cells and thus add to the pool of progenitor cells in such situations. If the stress is heavy doses of radiation, however, the result is not only the death of acinar cells, but also a marked decline in functional differentiation and proliferative capacity of all of the surviving cells, including those with progenitor capability. Restoration of gland function, therefore, seems to require increasing the secretory capacity of the surviving cells, or replacing the acinar cells and their progenitors either in the existing gland remnants or with artificial glands.

Interaction of copper and human salivary proteins

Interaction of taste molecules with saliva is the first step in the flavor perception process. Saliva is assumed to influence copper-induced sensation by controlling the copper solubility or causing astringency via binding of proteins with copper. This study was performed to identify the nature of copper-protein interactions in relation to the sensory perception of copper. Saliva was treated with CuSO4 x 5H2O at levels of 0, 2.5, 5, 10, 20, or 40 mg/L, and changes in salivary proteins were analyzed using high performance liquid chromatography (HPLC) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Protein peaks that showed changes in HPLC were characterized with SDS-PAGE. HPLC analysis revealed that copper treatment up to 40 mg/L decreased several proteins, including the dominant peak, by 70%. This peak was composed of alpha-amylase, a secretory component, and basic proline-rich proteins. SDS-PAGE results showed that salivary proteins of molecular weight 29 kDa and 33 kDa precipitated when copper was added at concentrations > or =10 mg/L. This study provides biochemical information for understanding perception mechanisms of copper sensation.

Influence of the glycosylation of human salivary proline-rich proteins on their interactions with condensed tannins

Binding of condensed tannins to salivary proteins is supposed to be involved in their astringency. First, complexes arising from the interaction of saliva from two individuals and tannins were studied. Then interaction mixture models containing purified saliva proteins were developed. The highest polymerized tannins predominantly precipitated together with the salivary proteins. Electrophoresis of proteins in combination with thiolysis analysis of tannins indicated proline-rich protein (PRP)-polyphenol complexes in precipitated fractions and also in the soluble ones with individual differences. Individual salivas exhibiting different protein patterns were discriminated with regard to their ability to interact with tannins. From binding studies with purified classes of salivary proteins, interactions were shown to depend on the nature of the protein, in particular on their glycosylation state. For low concentrations of tannins, glycosylated PRP-tannin interactions led to complexes that remained soluble, whereas those arising from nonglycosylated PRP-tannin interactions were precipitated. This finding could indicate that under physiological conditions, complexes involving glycosylated proteins maintain part of the lubrication of the oral cavity, whereas tannin trapping leads to a lower astringency perception.

Salivary Proteins as a Defense Against Dietary Tannins

Tannins, a diverse group of water-soluble phenolics with high affinity to proteins, are widely distributed in various parts of plants, and have negative effects in herbivores after ingestion. Some mammalian species are thought to counteract tannins by secreting tannin-binding salivary proteins (TBSPs). Several types of TBSPs are found in the saliva of laboratory animals, livestock, and wildlife. Among them, proline-rich proteins (PRPs) and histatins are effective precipitators of tannins. It is widely accepted that, at the least, PRPs act as a first line of defense against tannins. Many observations support this idea: in vitro affinity of PRPs to tannins is far higher than that of other proteins such as bovine serum albumin; complexes formed between PRPs and tannins are stable even under the conditions in the stomach and intestine; and PRP production is induced by ingesting tannins. It is believed that species that usually ingest tannins as part of their natural diets produce high levels of PRPs, whereas species not exposed to tannins produce little or no PRPs. This hypothesis is generally supported, although studies on TBSPs in wildlife are limited. This work stresses the importance of gathering basic information on such items as the characteristics of unidentified TBSPs, and seasonal and geographical variations in PRP production.

Proteome analysis of glandular parotid and submandibular-sublingual saliva in comparison to whole human saliva by two-dimensional gel electrophoresis

The secretions of the salivary parotid and submandibular-sublingual (SMSL) glands constitute the main part of whole human saliva (WS) in which proline-rich proteins (PRPs) and mucins represent dominant groups. Although proteome analysis had been performed on WS, no identification of PRPs or mucins by 2-DE and MS was achieved in WS and no comprehensive analysis of both glandular secretions is available so far. The aim of this study was to compare the protein map of WS to parotid and SMSL secretions for the display of PRPs and mucins. WS and glandular secretions were subjected to 2-DE and spots were analyzed by MALDI-MS. New components identified in WS were cyclophilin-B and prolyl-4-hydroxylase. Also acidic and basic PRPs as well as the proline-rich glycoprotein (PRG) could now be mapped in WS. Acidic PRPs were found equally in parotid and SMSL secretions, whereas basic PRPs and PRG were found primarily in parotid secretion. Salivary mucin MUC7 was identified in SMSL secretion. Thus, the more abundant proteins of WS can be explained mainly by mixed contributions of parotid and SMSL secretions with only few components remaining that may be derived from local sources in the oral cavity.

Intrinsically unstructured proteins evolve by repeat expansion

The proportion of the genome encoding intrinsically unstructured proteins increases with the complexity of organisms, which demands specific mechanism(s) for generating novel genetic material of this sort. Here it is suggested that one such mechanism is the expansion of internal repeat regions, i.e., coding micro- and minisatellites. An analysis of 126 known unstructured sequences shows the preponderance of repeats: the percentage of proteins with tandemly repeated short segments is much higher in this class (39%) than earlier reported for all Swiss-Prot (14%), yeast (18%) or human (28%) proteins. Furthermore, prime examples, such as salivary proline-rich proteins, titin, eukaryotic RNA polymerase II, the prion protein and several others, demonstrate that the repetitive segments carry fundamental function in these proteins. In addition, their repeat numbers show functionally significant interspecies variation and polymorphism, which underlines that these regions have been shaped by intense evolutionary activity. In all, the major point of this paper is that the genetic instability of repetitive regions combined with the structurally and functionally permissive nature of unstructured proteins has powered the extension and possible functional expansion of this newly recognized protein class.

Messenger RNA profiling: a prototype method to supplant conventional methods for body fluid identification

Conventional methods of body fluid identification use a variety of labor-intensive, technologically diverse techniques that are performed in a series, not parallel, manner and are costly in terms of time and sample. Theoretically, the identification of a body fluid may be made by determining a sufficient number of mRNAs that are expressed exclusively in cells that collectively comprise that body fluid. Advantages of an mRNA-based approach, compared to conventional biochemical methods of analysis, include greater specificity, simultaneous and semi-automatic analysis through a common assay format, improved timeliness, decreased sample consumption and compatibility with DNA extraction methodologies. In this report, we demonstrate that RNA is stable in biological stains and can be recovered in sufficient quantity and quality for analysis. Messenger RNA from the housekeeping genes S15, beta-actin and GAPDH was detected in blood, semen and saliva stains using a sensitive reverse transcriptase-polymerase chain reaction assay (RT-PCR). Additionally, we have identified a number of candidate tissue-specific genes, statherin, histatin 3, PRB1, PRB2 and PRB3 that may be useful for the positive identification of saliva. Messenger RNAs from these genes were detectable in saliva stains but not in blood or semen stains. Collectively these findings constitute the basis of a prototype RNA based assay system that may eventually supplant conventional methods for body fluid identification.

Tandem mass spectrometry for structural characterization of proline-rich proteins: application to salivary PRP-3

Proline-rich proteins (PRPs), including collagens, complement 1q, and salivary PRPs, are unusually difficult to sequence by mass spectrometry, due to the high efficiency of cleavage at the amide bond on the N-terminal of proline residues and the consequently low relative abundance of fragment arising from cleavages at other amide bonds. To fully characterize these proteins by mass spectrometry, specialized approaches to fragmentation are needed for the peptides with high proline content. Our work reported herein focused on the analysis of the set of peptides derived by tryptic cleavage of the salivary protein PRP-3. Two methods of fragmentation were compared: Collision-induced dissociation (CID) and electron capture dissociation (ECD). The data obtained demonstrated that ECD spectra of peptides containing more than 30% proline residues are simpler and easier to interpret than are CID spectra of those peptides. Factors that limit the two methods of fragmentation include the complexity of information contained in the CID spectra and the low efficiency of ECD processes. A complementary approach using both decomposition methods provides more complete and interpretable sequence information and yielded >93% sequence coverage for the 11-kDa PRP-3 isolated from human saliva.

The oral medicine of tooth wear

This review illustrates, through a series of case histories, how oral medicine insights aid the diagnosis and management of patients with excessive tooth wear. The cases reviewed are drawn from the records of 500 southeast Queensland patients referred to the author over a 12 year period. Patients most at risk of dental erosion have work and sports dehydration, caffeine addiction, gastro-oesophageal reflux, asthma, diabetes mellitus, hypertension or other systemic diseases or syndromes that predispose to xerostomia. Saliva protects the teeth from the extrinsic and intrinsic acids which cause dental erosion. Erosion, exacerbated by attrition and abrasion, is the main cause of tooth wear. These cases illustrate that teeth, oral mucosa, salivary glands, skin and eyes should be examined for evidence of salivary hypofunction and attendant medical conditions. Based on comprehensive oral medicine, dietary analyses and advice, it would seem patients need self-management plans to deal with incipient chronic tooth wear. The alternative is the expensive treatment of pain, occlusal damage and pulp death required to repair the effects of acute severe tooth wear.

Transcriptional regulation of salivary proline-rich protein gene expression

Mechanisms governing gene expression and regulation in eukaryotes are remarkably complex. The results from in vivo transgenic and in vitro transfection studies designed to identify cis-element(s) and trans-factor(s) associated with the salivary proline-rich proteins (PRPs) gene expression are utilized as a paradigm to discuss the regulation of salivary-specific gene expression. Particular attention is given to the molecular mechanism(s) underlying the salivary PRP R15 gene regulation. In rodents, the PRPs are selectively expressed in the acinar cells of salivary glands, and are inducible by the beta-agonist isoproterenol as well as by dietary tannins. The results from a series of experiments using chimeric reporter constructs containing different lengths of the R15 distal enhancer region, their mutations, and various expressing constructs are analyzed and discussed. These data suggest that the inducible nuclear orphan receptor NGFI-B may participate in the regulation of salivary acinar cell-specific and inducible expression of the rat R15 gene via three distinct distal NGFI-B sites. Taken together, a model for the induction of R15 gene expression by isoproterenol is proposed. However, the exact molecular basis of this NGFI-B-mediated transactivation of cAMP-regulated R15 expression remains to be established.

Regulation of salivary-gland-specific gene expression

The results from in vivo transgenic and in vitro transfection studies designed to identify cis-element(s) and transfactor(s) governing the salivary proline-rich proteins (PRPs), amylase, and parotid secretory protein (PSP) gene expression are utilized as a paradigm to discuss the regulation of salivary-specific gene expression. Particular attention is given to the molecular mechanism(s) underlying the salivary PRP R15 gene regulation. In rodents, the PRPs are selectively expressed in the acinar cells of salivary glands, and are inducible by the beta-agonist isoproterenol and by dietary tannins. The results from a series of experiments using chimeric reporter constructs containing different lengths of the R15 distal enhancer region, their mutations, and various expressing constructs are analyzed and discussed. These data suggest that the inducible nuclear orphan receptor NGFI-B may participate in the regulation of salivary acinar-cell-specific and inducible expression of the rat R15 gene via three distinct distal NGFI-B sites. Taken together, a model for the induction of R15 gene expression by Ipr is proposed. However, the exact molecular basis of this NGFI-B-mediated transactivation of cAMP-regulated R15 expression remains to be established.

Proline-rich-protein promoters direct LacZ expression to the granular convoluted tubular cells of the submandibular gland in adult transgenic mice

The ability of two mouse PRP gene promoters to direct the expression of the bacterial lacZ reporter gene was tested in transgenic mice. Transgenes A1-lacZ and C1-lacZ consisted of 8.2 kb A1 and 7.8 kb C1 PRP promoters respectively fused to the lacZ coding sequence. A1 and C1 are two A-type PRP genes isolated from the inbred SWR mice, which show the same gene structure and similar sequence to the closely related MP2 and M14 PRP genes previously cloned from outbred CD-1 mice. We here show that both A1-lacZ and C1-lacZ transgenes have very similar expression patterns: (1) they expressed the lacZ gene in all 14 established transgenic lines under normal (non-stimulated) conditions; (2) the expression was restricted to the granular convoluted tubular cells of the submandibular glands; (3) the expression was developmentally regulated beginning at sexual maturation and lasting to at least 1.5 years of age; and (4) expression in some lines was probably influenced by sex hormones, since higher expression was found in males than in females. A1-lacZ and C1-lacZ are the first transgenes derived from the PRP/GRP (glutamine/glutamic acid-rich protein) gene superfamily to be expressed in the granular convoluted tubular cells (with known endocrine functions), rather than in the acinar cells (with mainly exocrine functions) of the submandibular glands.

Developmental expression of a tandemly repeated, proline-and glutamine-rich amino acid motif on hyphal surfaces on Candida albicans

cDNA sequences encoding a cell wall protein have been isolated from the opportunistic pathogen, Candida albicans, an organism that can cause serious disease in immunocompromised patients such as those with AIDS. The cDNA encodes a peptide that is largely composed of an acidic, repeated motif 10 amino acids in length that is rich in proline and glutamine residues. The cDNA gene product was found to be present on hyphal surfaces by immunofluorescence assays using monospecific antisera raised to the recombinant protein produced in Pichia pastoris. The hyphae-specific surface location was also seen on organisms colonizing the gastrointestinal mucosa of mice, indicating that the antigen is produced and developmentally regulated during growth in host tissues. The cDNA clone hybridized to an abundant messenger RNA 2.3 kilobases in size that was present in hyphal but not yeast forms. These studies demonstrate that the bud-hypha transition is accompanied by the de novo synthesis of proteins that are targeted to hyphal surfaces. The primary sequence of the unique amino acid motif shares features with surface proteins of other lower eukaryotic microorganisms and with host acidic salivary proline-rich proteins.

Expression of a functional rat salivary cystatin S polypeptide in Escherichia coli

Cystatin S is a cysteine proteinase inhibitor that is transiently expressed during rat submandibular gland development and can be induced by isoproterenol in the adult. A cDNA for rat cystatin S which included the entire coding sequence of the secreted cystatin was cloned. A coding region of the cystatin gene was amplified by polymerase chain reaction and cloned into the pGEX-2T expression vector. The chimeric plasmid was transformed into Escherichia coli, and protein expression was induced by isopropyl-beta-D-thiogalactopyranoside. The expressed protein was purified from insoluble inclusion bodies after solubilization with urea and fast protein liquid chromatography on a MonoQ column. The purified recombinant cystatin reacted with antibodies to cystatin S purified from rat submandibular glands and showed an amino-terminal amino acid sequence identical to that of rat cystatin S. The recombinant protein exhibited papain inhibition activity comparable to natural cystatin. This was a successful expression and purification of a functionally and immunologically reactive recombinant cystatin from E. coli, an approach which will be used later towards generating recombinant variants to study the binding and functional domains of this cysteine protease inhibitor.

Sialochemistry: a diagnostic tool?

Saliva has proven to be a discriminating element in forensic arenas, an effective indicator of acute diseases of salivary glands, and a promising probe for drug monitoring. With the advent of sensitive immunochemical assays, the compositional profile of human salivary secretions has been expanded considerably. Thus, the establishment of a range of "normal values" for a variety of "intrinsic" and "extrinsic" salivary components represented the initial step to use saliva as a diagnostic tool of oral health status. Unfortunately, numerous cross-sectional studies have shown a wide individual variation in the salivary composition of healthy populations, thus precluding its use as a diagnostic chair-side test for the screening of the most common chronic oral diseases (e.g. caries and periodontal disease). A possible explanation may arise from the wide functional versatility of salivary molecules. For instance, it has been recognized recently that in addition to its digestive properties, salivary amylase may modulate bacterial colonization, whereas histatins are not only antifungal but also bactericidal. Thus, low levels of already known antimicrobial salivary molecules (e.g., secretory IgA, lactoferrin, and lysozyme) could be compensated with higher concentrations of other molecules with antimicrobial activity, such as amylase and histatins. Consequently, for caries and periodontal diseases, longitudinal sialochemical studies may yield more insight than cross-sectional studies.