A Review of the Salivary Proteome and Peptidome and Saliva-derived Peptide Therapeutics

Role of Anthocyanins in the Interaction between Salivary Mucins and Wine Astringent Compounds

Wine astringency is a very complex sensation whose complete mechanism has not been entirely described. Not only salivary proline-rich proteins (PRPs) are involved in its development; salivary mucins can also play an important role. On the other hand, it has been described that anthocyanins can interact with PRPs, but there is no information about their potential role on the interactions with mucins. In this work, the molecular interactions between salivary mucins (M) and different wine phenolic compounds, such as catechin (C), epicatechin (E) and quercetin 3-β-glucopyranoside (QG), as well as the effect of the anthocyanin malvidin 3-O-glucoside (Mv) on the interactions with mucins, were assessed by isothermal titration calorimetry (ITC). Results showed that the interaction between anthocyanin and mucins is stronger than that of both flavanols analyzed, since the affinity constant values were 10 times higher for anthocyanin than for catechin, the only flavanol showing interaction in binary assay. Moreover, at the concentration at which polyphenols are usually found in wine, flavonols seem not to be involved in the interactions with mucins. These results showed, for the first time, the importance of wine anthocyanins in the mechanisms of astringency involving high-molecular-weight salivary proteins like mucins.

The Post-Translational Modifications of Human Salivary Peptides and Proteins Evidenced by Top-Down Platforms

In this review, we extensively describe the main post-translational modifications that give rise to the multiple proteoforms characterized to date in the human salivary proteome and their potential role. Most of the data reported were obtained by our group in over twenty-five years of research carried out on human saliva mainly by applying a top-down strategy. In the beginning, we describe the products generated by proteolytic cleavages, which can occur before and after secretion. In this section, the most relevant families of salivary proteins are also described. Next, we report the current information concerning the human salivary phospho-proteome and the limited news available on sulfo-proteomes. Three sections are dedicated to the description of glycation and enzymatic glycosylation. Citrullination and N- and C-terminal post-translational modifications (PTMs) and miscellaneous other modifications are described in the last two sections. Results highlighting the variation in the level of some proteoforms in local or systemic pathologies are also reviewed throughout the sections of the manuscript to underline the impact and relevance of this information for the development of new diagnostic biomarkers useful in clinical practice.

Artificial saliva precipitation index (ASPI): An efficient evaluation method of wine astringency

Astringency is one of the most important organoleptic characteristics of red wines, and its intensity evaluation method has been the focus of research in recent years. An artificial saliva system was developed to establish an accurate and reliable evaluation method for the astringency intensity of dry red wines based on saliva precipitation index (SPI). To achieve this, five key protein families, which presented high reactivities and sensitivities in protein-tannin binding reactions, were selected from human whole saliva. The concentrations of the five proteins (proline-rich protein, α-amylase, lactoferrin, lysozyme, and albumin) and pH were optimized using response surface methodology based on the human salivary conditions to simulate the real salivary environment. The artificial saliva precipitation index method was applied to 60 commercial dry red wines and it exhibited a high correlation (Coef_ASPI = 0.94) with the sensory scores, indicating better performance than the traditional SPI method and other analytical approaches.

Evaluation of the Suitability of Dried Saliva Spots for In-Depth Proteome Analyses for Clinical Applications

Previously, we performed nontargeted proteome analysis using dried blood spots (DBSs) that are widely used in newborn screening for the clinical diagnosis of congenital genetic diseases and immunodeficiency. We have developed an efficient and simple pretreatment method for DBSs that can detect more than 1000 proteins. To complement proteins that are difficult to detect via DBS analysis with less invasive alternative body fluids, we conducted this study to investigate the proteins detected from dried saliva spots (DSSs) using single-shot LC-MS/MS, which is practical in clinical settings. We also clarified whether DSSs have the same advantages as DBSs, and we investigated the influence of saliva collection conditions and the storage environment on their protein profile. As a result, we detected approximately 5000 proteins in DSSs and whole saliva, and we concluded that they were sufficient to complement the proteins lacking in the blood analysis. DSSs could be used as an alternative tool to DBSs for detecting the presence of causative proteins.

Untangling Mucosal Drug Delivery: Engineering, Designing, and Testing Nanoparticles to Overcome the Mucus Barrier

Mucus is a complex viscoelastic gel and acts as a barrier covering much of the soft tissue in the human body. High vascularization and accessibility have motivated drug delivery to various mucosal surfaces; however, these benefits are hindered by the mucus layer. To overcome the mucus barrier, many nanomedicines have been developed, with the goal of improving the efficacy and bioavailability of drug payloads. Two major nanoparticle-based strategies have emerged to facilitate mucosal drug delivery, namely, mucoadhesion and mucopenetration. Generally, mucoadhesive nanoparticles promote interactions with mucus for immobilization and sustained drug release, whereas mucopenetrating nanoparticles diffuse through the mucus and enhance drug uptake. The choice of strategy depends on many factors pertaining to the structural and compositional characteristics of the target mucus and mucosa. While there have been promising results in preclinical studies, mucus-nanoparticle interactions remain poorly understood, thus limiting effective clinical translation. This article reviews nanomedicines designed with mucoadhesive or mucopenetrating properties for mucosal delivery, explores the influence of site-dependent physiological variation among mucosal surfaces on efficacy, transport, and bioavailability, and discusses the techniques and models used to investigate mucus-nanoparticle interactions. The effects of non-homeostatic perturbations on protein corona formation, mucus composition, and nanoparticle performance are discussed in the context of mucosal delivery. The complexity of the mucosal barrier necessitates consideration of the interplay between nanoparticle design, tissue-specific differences in mucus structure and composition, and homeostatic or disease-related changes to the mucus barrier to develop effective nanomedicines for mucosal delivery.

Pilot study of oral fluid and plasma meropenem and piperacillin concentrations in the intensive care unit

BACKGROUND

Therapeutic drug monitoring (TDM) of β-lactam antibiotics may be used to optimize dosing for patients in the intensive care unit (ICU). A noninvasive matrix such as oral fluid may be interesting in selected patient groups. We compared the oral fluid concentrations of piperacillin and meropenem with the respective unbound and total concentrations in plasma. A secondary objective was to evaluate feasibility of the collection of oral fluid samples in this specific patient population.

METHODS

The study included 20 non-intubated ICU patients, age 22 to 77 y, receiving piperacillin or meropenem via continuous intravenous infusion. The standard protocol consisted of collecting a paired plasma-oral fluid sample for 3 consecutive days. Oral fluid was obtained from the patients using a standardized procedure by spitting in a plastic container after 2 min of gathering oral fluid in the mouth.

RESULTS

Antibiotic concentrations of piperacillin and meropenem are measurable, albeit very low, in unstimulated oral fluid of ICU patients. For piperacillin, a poor correlation was found between oral fluid and both total and unbound plasma concentrations (Spearman's correlation coefficients (Rs) 0.46 and 0.48 respectively). For meropenem this correlation was better (Rs for oral fluid versus total and unbound plasma meropenem concentration 0.92 and 0.93 respectively). Dispersion of antibiotic concentrations was greater in oral fluid than in blood. Collecting oral fluid samples was difficult in non-intubated ICU patients.

CONCLUSIONS

Oral fluid from non-intubated ICU patients, obtained through a standardized procedure, cannot be recommended as an alternative matrix for quantitative meropenem or piperacillin TDM.

Interaction between salivary proteins and cork phenolic compounds able to migrate to wine model solutions

This work reports the study of the interaction of human salivary proteins (SP) with phenolic compounds that migrate from cork stoppers to wine. This study yields valuable data to understand the influence that these compounds may have on the sensory perception of wine from an astringency perspective. For that, three cork fractions containing the phenolic compounds that migrate in greater amounts from cork to model wine solutions were selected. Fraction M1 contains gallic acid, protocatechuic acid, vanillin and protocatechuic aldehyde; fraction M2 comprises essentially gallic acid and ellagic acid, as well as castalagin and dehydrocastalagin; and fraction M3 contains the two isomeric ellagitannins castalagin and vescalagin. The reactivity of each fraction towards SP was M3 > M2 > M1. Within M3 fraction, castalagin showed a higher ability to precipitate SP (mainly aPRPs, statherin and P-B peptide) comparatively to vescalagin. In M1 fraction, caffeic and sinapic acids were the compounds with the highest interaction with SP, mainly cystatins. In addition, there also seems to be a matrix effect (presence of other compounds) that could be affecting these interactions.

Salivary Metabolomics: From Diagnostic Biomarker Discovery to Investigating Biological Function

Metabolomic profiling of biofluids, e.g., urine, plasma, has generated vast and ever-increasing amounts of knowledge over the last few decades. Paradoxically, metabolomic analysis of saliva, the most readily-available human biofluid, has lagged. This review explores the history of saliva-based metabolomics and summarizes current knowledge of salivary metabolomics. Current applications of salivary metabolomics have largely focused on diagnostic biomarker discovery and the diagnostic value of the current literature base is explored. There is also a small, albeit promising, literature base concerning the use of salivary metabolomics in monitoring athletic performance. Functional roles of salivary metabolites remain largely unexplored. Areas of emerging knowledge include the role of oral host-microbiome interactions in shaping the salivary metabolite profile and the potential roles of salivary metabolites in oral physiology, e.g., in taste perception. Discussion of future research directions describes the need to begin acquiring a greater knowledge of the function of salivary metabolites, a current research direction in the field of the gut metabolome. The role of saliva as an easily obtainable, information-rich fluid that could complement other gastrointestinal fluids in the exploration of the gut metabolome is emphasized.

Short Cationic Peptidomimetic Antimicrobials

The rapid growth of antimicrobial resistance against several frontline antibiotics has encouraged scientists worldwide to develop new alternatives with unique mechanisms of action. Antimicrobial peptides (AMPs) have attracted considerable interest due to their rapid killing and broad-spectrum activity. Peptidomimetics overcome some of the obstacles of AMPs such as high cost of synthesis, short half-life in vivo due to their susceptibility to proteolytic degradation, and issues with toxicity. This review will examine the development of short cationic peptidomimetics as antimicrobials.

Thermally Assisted Generation of Protein-Poly(ethylene sodium phosphate) Conjugates with High Mineral Affinity

Protein therapeutics has recently attracted interest in various medical treatments. However, the structure and function preservation in proteins under physiological conditions is still an important issue and reliable immobilization techniques are required. In this study, the thermally assisted complexation of proteins with amphiphilic polyphosphoesters is proposed as a new methodology for their durability improvement. Amphiphilic cholesterol-terminated poly(ethylene sodium phosphate) (CH-PEP·Na) was synthesized via the organocatalytic ring-opening polymerization of 2-methoxy-2-oxo-1,3,2-dioxaphospholane initiated by cholesterol as the hydrophobic molecule and followed by demethylation and neutralization. For the protein nanocarrier preparation, a complex of the amphiphilic CH-PEP·Na with bovine serum albumin (BSA) was formed through the hydrophobic interactions between the lipophilic moieties of the protein and the cholesteryl groups of the CH-PEP·Na chains, which were induced by thermal treatment at 90 °C. The resulting complex size ranged between 27 and 51 nm, as confirmed by dynamic light scattering. The complexes dispersed in an aqueous medium exhibited a high stability in size for up to 1 month of storage. CH-PEP·Na efficiently inhibited the thermal aggregation and sedimentation of BSA, unlike poly(ethylene sodium phosphate) (PEP·Na) and cholesterol-terminated poly(ethylene glycol) (CH-PEG). In addition, CH-PEP·Na was able to protect the complexed BSA against proteolytic digestion and the BSA-CH-PEP·Na complexes well adsorbed onto hydroxyapatite even in the presence of BSA (5.5 g/dL). Hence, thermally induced protein-CH-PEP·Na complexes can be a potential tool for the development of bone and dental applications.

Extensive Characterization of the Human Salivary Basic Proline-Rich Protein Family by Top-Down Mass Spectrometry

Human basic proline-rich proteins and basic glycosylated proline-rich proteins, encoded by the polymorphic PRB1-4 genes and expressed only in parotid glands, are the most complex family of adult salivary proteins. The family includes 11 parent peptides/proteins and more than 6 parent glycosylated proteins, but a high number of proteoforms with rather similar structures derive from polymorphisms and post-translational modifications. 55 new components of the family were characterized by top-down liquid chromatography-mass spectrometry and tandem-mass platforms, bringing the total number of proteoforms to 109. The new components comprise the three variants P-H S₁ → A, P-Ko P₃₆ → S, and P-Ko A₄₁ → S and several of their naturally occurring proteolytic fragments. The paper represents an updated reference for the peptides included in the heterogeneous family of proteins encoded by PRB1/PRB4. MS data are available via ProteomeXchange with the identifier PXD009813.

Top-down proteomic profiling of human saliva in multiple sclerosis patients

Multiple sclerosis is a chronic disease of the central nervous system characterized by inflammation, demyelination and neurodegeneration which is of undetermined origin. To date a single diagnostic test of multiple sclerosis does not exists and novel biomarkers are demanded for a more accurate and early diagnosis. In this study, we performed the quantitative analysis of 119 salivary peptides/proteins from 49 multiple sclerosis patients and 54 healthy controls by a mass spectrometry-based top-down proteomic approach. Statistical analysis evidenced different levels on 23 proteins: 8 proteins showed lower levels in multiple sclerosis patients with respect to controls and they were mono- and di-oxidized cystatin SN, mono- and di-oxidized cystatin S1, mono-oxidized cystatin SA and mono-phosphorylated statherin. 15 proteins showed higher levels in multiple sclerosis patients with respect to controls and they were antileukoproteinase, two proteoforms of Prolactin-Inducible Protein, P-C peptide (Fr.1-14, Fr. 26-44, and Fr. 36-44), SV1 fragment of statherin, cystatin SN Des_1-4, cystatin SN P₁₁ → L variant, and cystatin A T₉₆ → M variant. The differences observed between the salivary proteomic profile of patients suffering from multiple sclerosis and healthy subjects is consistent with the inflammatory condition and altered immune response typical of the pathology. Data are available via ProteomeXchange with identifier PXD009440.

SIGNIFICANCE

To date a single diagnostic test of multiple sclerosis does not exist, and diagnosis is based on multiple tests which mainly include the analysis of cerebrospinal fluid. However, the need for lumbar puncture makes the analysis of cerebrospinal fluid impractical for monitoring disease activity and response to treatment. The possible use of saliva as a diagnostic fluid for oral and systemic diseases has been largely investigated, but only marginally in multiple sclerosis compared to other body fluids. Our study demonstrates that the salivary proteome of multiple sclerosis patients differs considerably compared to that of sex and age matched healthy individuals and suggests that some differences might be associated with the different disease-modifying therapy used to treat multiple sclerosis patients.

Salivary Biomarkers in Systemic Sclerosis Disease

Scleroderma or systemic sclerosis (SSc) is frequently detected at an advanced stage due to diagnosis difficulties. Salivary biomarkers, if existing, could be used for predictive diagnosis of this disease. Human saliva contains a large number of proteins that can be used for diagnosis and are of great potential in clinical research. The use of proteomic analysis to characterize whole saliva (WS) in SSc has gained an increasing attention in the last years and the identification of salivary proteins specific for SSc could lead to early diagnosis or new therapeutic targets. This review will present an overview about the use of WS in SSc studies. The proteomic technologies currently used for global identification of salivary proteins in SSc, as well as the advantages and limitations for the use of WS as a diagnostic tool, will be presented.

Interactions between wine phenolic compounds and human saliva in astringency perception

Astringency is a complex perceptual phenomenon involving several sensations that are perceived simultaneously. The mechanism leading to these sensations has been thoroughly and controversially discussed in the literature and it is still not well understood since there are many contributing factors. Although we are still far from elucidating the mechanisms whereby astringency develops, the interaction between phenolic compounds and proteins (from saliva, oral mucosa or cells) seems to be most important. This review summarizes the recent trends in the protein-phenol interaction, focusing on the effect of the structure of the phenolic compound on the interaction with salivary proteins and on methodologies based on these interactions to determine astringency.

Influence of saliva on the oral microbiota

Saliva plays a major role in determining the composition and activity of the oral microbiota, via a variety of mechanisms. Molecules, mainly from saliva, form a conditioning film on oral surfaces, thus providing receptors for bacterial attachment. The attached cells use saliva components, such as glycoproteins, as their main source of nutrients for growth. Oral bacteria work sequentially and in a concerted manner to catabolize these structurally complex molecules. Saliva also buffers the pH in the biofilm to around neutrality, creating an environment which is conducive to the growth of many oral bacteria that provide important benefits to the host. Components of the adaptive and innate host defences are delivered by saliva, and these often function synergistically, and at sublethal concentrations, so a complex relationship develops between the host and the resident microbiota. Dysbiosis can occur rapidly if the flow of saliva is perturbed.

The Interactions of CPP-ACP with Saliva

The repair of early dental caries lesions has been demonstrated by the application of the remineralisation technology based on casein phosphopeptide-stabilised amorphous calcium phosphate complexes (CPP-ACP). These complexes consist of an amorphous calcium phosphate mineral phase stabilised and encapsulated by the self-assembly of milk-derived phosphopeptides. During topical application of CPP-ACP complexes in the oral cavity, the CPP encounters the enamel pellicle consisting of salivary proteins and peptides. However the interactions of the CPP with the enamel salivary pellicle are not known. The studies presented here reveal that the predominant peptides of CPP-ACP complexes do interact with specific salivary proteins and peptides of the enamel pellicle, and provide a mechanism by which the CPP-ACP complexes are localised at the tooth surface to promote remineralisation.

A Study of the Variation in the Salivary Peptide Profiles of Young Healthy Adults Acquired Using MALDI-TOF MS

A cross-sectional observational study was conducted to evaluate the inter-individual variation in the MALDI-TOF MS peptide profiles of unstimulated whole saliva in a population of 268 systemically healthy adults aged 18-30 yr (150 males and 118 females) with no apparent caries lesions or periodontal disease. Using Spectral Clustering, four subgroups of individuals were identified within the study population. These subgroups were delimited by the pattern of variation in 9 peaks detected in the 2-15 kDa m/z range. An Unsupervised Feature Selection algorithm showed that P-C peptide, a 44 residue-long salivary acidic proline-rich protein, and three of its fragments (Fr. 1-25, Fr. 15-35 and Fr. 15-44) play a central role in delimiting the subgroups. Significant differences were found in the salivary biochemistry of the subgroups with regard to lysozyme and chitinase, two enzymes that are part of the salivary innate defense system (p < 0.001). These results suggest that MALDI-TOF MS salivary peptide profiles may relate information on the underlying state of the oral ecosystem and may provide a useful reference for salivary disease biomarker discovery studies.

Cross-species comparison of mammalian saliva using an LC-MALDI based proteomic approach

Despite the importance of saliva in the regulation of oral cavity homeostasis, few studies have been conducted to quantitatively compare the saliva of different mammal species. Aiming to define a proteome signature of mammals' saliva, an in-depth SDS-PAGE-LC coupled to MS/MS (GeLC-MS/MS) approach was used to characterize the saliva from primates (human), carnivores (dog), glires (rat and rabbit), and ungulates (sheep, cattle, horse). Despite the high variability in the number of distinct proteins identified per species, most protein families were shared by the mammals studied with the exception of cattle and horse. Alpha-amylase is an example that seems to reflect the natural selection related to digestion efficacy and food recognition. Casein protein family was identified in all species but human, suggesting an alternative to statherin in the protection of hard tissues. Overall, data suggest that different proteins might assure a similar role in the regulation of oral cavity homeostasis, potentially explaining the specific mammals' salivary proteome signature. Moreover, some protein families were identified for the first time in the saliva of some species, the presence of proline-rich proteins in rabbit's saliva being a good example.

Interaction between red wine procyanidins and salivary proteins: effect of stomach digestion on the resulting complexes

Some tannin(T)/SP complexes were disrupted by gastric digestion, leading to T release. This was seen for complexes formed with tannin mono-, di- and trimers. In contrast, complexes made by tetra- and pentamers were more resistant to gastric digestion.

Salivary peptidomic as a tool to disclose new potential antimicrobial peptides

Antimicrobial peptides (AMPs) and peptidomimetics are among the new generation of antibiotics due to their broad spectrum of activity towards pathogenic agents. Considering the fact that the oral cavity is a relevant entryway for pathogenic microorganisms, it must be armed with several defenses in order to maintain homeostasis. Thus, we aimed at disclosing potential AMPs in saliva from humans, pigs and mice. We enriched salivary peptides by filtration and then separated and identified multiple peptides by nanoHPLC-MALDI-TOF/TOF. Using this approach, we identified 45 different peptides from mice's saliva and 94 from pig's saliva. Almost all peptides from pig and mice saliva were new assignments, encouraging the application of this technique to other mammals' saliva. The potential antimicrobial activity was then calculated in silico with an available CAMP's algorithm. Eight peptides from pig's basic proline-rich protein, as well as 6 peptides resulting from human's P-B peptide, 1 from P-C peptide fragmentation and 1 from statherin were found to have potential antimicrobial activity. Therefore, the peptides GPPPQGGRPQG and RFGYGYGPYQPVPEQPLYPQ were synthetically obtained and their antimicrobial properties evaluated in vitro for different bacterial strains. These showed to exert an antibacterial effect in Staphylococcus aureus. This report validates the prospection of mammal's saliva to find new alternatives to antibiotics.

BIOLOGICAL SIGNIFICANCE

With the growing problem of antibiotic resistance, the identification of new routes in antibiotic therapy is on the scientific agenda worldwide. Antimicrobial peptides (AMPs) and peptidomimetics are among the new generation of antibiotics due to their broad spectrum of activity towards pathogenic agents and owing to a less probable microbial resistance that can arise from their use. Thus, we explored the saliva from humans, pigs and mice to identify potential AMPs using a peptidomic approach. At the end, eight peptides from pig's basic proline-rich protein, six peptides resulting from human's P-B peptide, one from P-C peptide fragmentation and one from statherin were found to have potential antimicrobial activity. From this, the peptides GPPPQGGRPQG and RFGYGYGPYQPVPEQPLYPQ were tested showing to have antibacterial effect in S. aureus which highlights the potential of mammal's saliva to find new alternatives to antibiotics.

Top-down analytical platforms for the characterization of the human salivary proteome

Comprehensive analysis and characterization of the human salivary proteome is an important step towards the possible use of saliva for diagnostic and prognostic purposes. The contribution of the different sources to whole saliva, and the evaluation of individual variability and physiological modifications have been investigated by top-down proteomic approaches, disclosing the faceted and complex profile of the human salivary proteome. All this information is essential to develop saliva protein biomarkers. In this Review the major results obtained in the field by top-down platforms, and the improvements required to allow a more complete picture, will be discussed.

New procyanidin B3-human salivary protein complexes by mass spectrometry. Effect of salivary protein profile, tannin concentration, and time stability

Several factors could influence the tannin-protein interaction such as the human salivary protein profile, the tannin tested, and the tannin/protein ratio. The goal of this study aims to study the effect of different salivas (A, B, and C) and different tannin concentrations (0.5 and 1 mg/mL) on the interaction process as well as the complex's stability over time. This study is focused on the identification of new procyanidin B3-human salivary protein complexes. Thus, 48 major B3-human salivary protein aggregates were identified regardless of the saliva and tannin concentration tested. A higher number of aggregates was found at lower tannin concentration. Moreover, the number of protein moieties involved in the aggregation process was higher when the tannin concentration was also higher. The selectivity of the different groups of proteins to bind tannin was also confirmed. It was also verified that the B3-human salivary protein complexes formed evolved over time.

Off-line hyphenation of boronate affinity monolith-based extraction with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for efficient analysis of glycoproteins/glycopeptides

Boronate affinity materials have attracted increasing attentions as sample enrichment platforms for glycoproteomic analysis in recent years. However, most of the boronate affinity materials that have already employed for proteomic analysis are suffering from apparent disadvantages, such as alkaline pH for binding, weak affinity, and relatively poor selectivity. Benzoboroxoles are a unique class of boronic acids which have showed excellent binding properties for the recognition of cis-diol-containing compounds. Recently, a 3-carboxy-benzoboroxole-functionalized monolithic column had been reported and it had exhibited the best selectivity and affinity as well as the lowest binding pH among all reported boronate affinity monolithic columns. In this study, an off-line hyphenation of this boronate affinity monolithic column-based extraction with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was developed and the powerfulness of this hyphenated approach in the analysis of glycoproteins and glycopeptides in complex samples was investigated. The approach was first applied to the analysis of glycopeptides in the tryptic digest of horseradish peroxidase (HRP). Totally 22 glycopeptides were identified. To the best of our knowledge, this is the best performance among all the boronic acid-functionalized materials. We further employed this approach to the analysis of intact proteins in human saliva. Totally 6 intact glycoproteins were successfully identified. As comparison, when the samples were analyzed without extraction, only a few glycopeptides were identified from the tryptic digest of HRP while no glycoproteins were found from the saliva samples.

Comparative adherence of Candida albicans and Candida dubliniensis to human buccal epithelial cells and extracellular matrix proteins

Candida albicans and Candida dubliniensis are very closely related pathogenic yeast species. Despite their close relationship, C. albicans is a far more successful colonizer and pathogen of humans. The purpose of this study was to determine if the disparity in the virulence of the two species is attributed to differences in their ability to adhere to human buccal epithelial cells (BECs) and/or extracellular matrix proteins. When grown overnight at 30°C in yeast extract peptone dextrose, genotype 1 C. dubliniensis isolates were found to be significantly more adherent to human BECs than C. albicans or C. dubliniensis genotypes 2-4 (P < 0.001). However, when the yeast cells were grown at 37°C, no significant difference between the adhesion of C. dubliniensis genotype 1 and C. albicans to human BECs was observed, and C. dubliniensis genotype 1 and C. albicans adhered to BECs in significantly greater numbers than the other C. dubliniensis genotypes (P < 0.001). Using surface plasmon resonance analysis, C. dubliniensis isolates were found to adhere in significantly greater numbers than C. albicans to type I and IV collagen, fibronectin, laminin, vitronectin, and proline-rich peptides. These data suggest that C. albicans is not more adherent to epithelial cells or matrix proteins than C. dubliniensis and therefore other factors must contribute to the greater levels of virulence exhibited by C. albicans.

Top-down platform for deciphering the human salivary proteome

Proteomic platforms can be classified in bottom-up strategies, which analyze the sample after proteolytic digestion, and top-down strategies, which analyze the intact naturally occurring proteome. Bottom-up platforms are high-throughput because they can investigate a large number of proteins, regardless of their dimension. Nonetheless, information on post-translational modifications (PTMs) can be lost, especially those regarding naturally occurring cleavages and alternative splicing. Top-down platforms cannot cover vast proteomes, however, they can disclose subtle structural variations occurring during protein maturation and allow label-free relative quantifications in an unlimited number of samples. A repertoire of 256 masses belonging to naturally occurring proteins and peptides consistently detected by RP-HPLC-ESI-MS analysis of the acidic soluble fraction of human whole saliva is presented in this study. Of them, 233 have been identified, while 23 are still pending for the definitive characterization. The present review reports average and mono-isotopic masses of the peptides and proteins detected, RP-HPLC elution times, PTMs, origin and quali-quantitative variations observed in several physiological and pathological conditions. The information reported can be a reference for users of top-down RP-HPLC-ESI-MS proteomic platforms applied to the study of the human salivary proteome as well as of other human bodily fluids.

An evolutionary perspective of mammal salivary peptide families: cystatins, histatins, statherin and PRPs

Saliva's role in the oral cavity, such as lubrication, protection of tissues and antimicrobial action is a reflex of its composition, among which are several peptide families like statherin, histatins, proline rich proteins (PRPs) and some members of the cystatin family. These peptides present different evolutionary pathways being in the case of histatin, statherin and PRP families restricted to few millions and comprising few species when compared with cystatins, where duplication occurred at more than 650 mya. Though the recognized relevance of phylogenetic approaches to disclose relationships among different species, information on the salivary proteins that allow the association between peptide families-related structure and function in the oral cavity is scarce. In the present study, the four major salivary peptides classes are reviewed considering the few known phylogenetic studies focusing on their evolution among mammals. New perspectives and challenges for future and multidisciplinary experimental works are drawn.

One decade of salivary proteomics: current approaches and outstanding challenges

Efforts have been made in the last decade towards the complete characterization of saliva proteome using gel-based and gel-free approaches. The combination of these strategies resulted in the increment of the dynamic range of saliva proteome, which yield in the identification of more than 3,000 different protein species. Comparative protein profiling using isotope labeling and label free approaches has been used for the identification of novel biomarkers for oral and related diseases. Although progresses have been made in saliva proteome characterization, the comparative profiling in different pathophysiological conditions is still at the beginning if compared to other bodily fluids. The potential biomarkers identified so far lack specificity once common differentially expressed proteins were detected in the saliva of patients with distinct diseases. In addition, recent research works focused on saliva peptidome profiling already allowed a better understanding of peptides' physiological role in oral cavity. This review provides an overview of the major achievements in saliva proteomics giving emphasis to methodological concerns related with saliva collection, treatment and analysis, as well as the main advantages and pitfalls underlying salivary proteomic strategies and potential clinical outcomes.

The scientific exploration of saliva in the post-proteomic era: from database back to basic function

The proteome of human saliva can be considered as being essentially completed. Diagnostic markers for a number of diseases have been identified among salivary proteins and peptides, taking advantage of saliva as an easy-to-obtain biological fluid. Yet, the majority of disease markers identified so far are serum components and not intrinsic proteins produced by the salivary glands. Furthermore, despite the fact that saliva is essential for protecting the oral integuments and dentition, little progress has been made in finding risk predictors in the salivary proteome for dental caries or periodontal disease. Since salivary proteins, and in particular the attached glycans, play an important role in interactions with the microbial world, the salivary glycoproteome and other post-translational modifications of salivary proteins need to be studied. Risk markers for microbial diseases, including dental caries, are likely to be discovered among the highly glycosylated major protein species in saliva. This review will attempt to raise new ideas and also point to under-researched areas that may hold promise for future applicability in oral diagnostics and prediction of oral disease.

Carbohydrates inhibit salivary proteins precipitation by condensed tannins

Condensed tannins are a group of polyphenols that are associated with the astringency sensation, as they readily interact and precipitate salivary proteins. As this interaction is affected by carbohydrates, the aim of this work was to study the effect of some carbohydrates used in the food industry [arabic gum (AG), pectin, and poligalacturonic acid (PGA)] on the salivary proteins/grape seed procyanidins interaction. This was assessed monitoring the salivary proteins that remain soluble in the presence of condensed tannins with the addition of carbohydrates (HPLC) and analysis of the respective precipitates (SDS-PAGE). The results show that pectin was the most efficient in inhibiting protein/tannin precipitation, followed by AG and PGA. The results suggest that pectin and PGA exert their effect by formation of a ternary complex protein/polyphenol/carbohydrate, while AG competes with proteins for tannin binding (competition mechanism). The results also point out that both hydrophilic and hydrophobic interactions are important for the carbohydrate effects.

From the salivary proteome to the OralOme: comprehensive molecular oral biology

OBJECTIVES

There have been several efforts to identify the protein components of saliva. Some of these studies were conducted in healthy individuals and other in individuals with different oral and systemic disorders. However, a resource compiling and reviewing all of the proteins identified in proteomic studies is still lacking. The aim of this project is to develop such a resource.

DESIGN

The proteins identified by proteomic studies were compiled and all information concerning them was manually curated according to "IPI History search" and UniProt. Proteins were classified according to gene ontology using PANTHER. The involvement of each protein in disease was scrutinized using DAVID and a classification into protein disease classes was performed.

RESULTS

This survey of proteins in the oral cavity lead to the identification of 3397 non-redundant proteins, 605 altered in pathological conditions and 51 present only in disease. These proteins originate from different sources: 3115 from saliva, 990 from oral mucosa and 1929 from plasma. All protein sources contribute with different numbers and types of proteins to identical functions. However, each source produces specific proteins. Examples of the use of this proteomics database of saliva included the analysis of the changes in the proteome associated with periodontitis and a survey of systemic disease potential biomarkers in saliva.

CONCLUSION

The database generated with this work and the information therein stands as a resource for investigators/clinicians studying the oral biology, searching for molecular disease markers, developing diagnostic and prognostic tests, and contributing to the discovery of new therapeutic agents.

Susceptibility to dental caries and the salivary proline-rich proteins

Early childhood caries affects 28% of children aged 2–6 in the US and is not decreasing. There is a well-recognized need to identify susceptible children at birth. Caries-free adults neutralize bacterial acids in dental biofilms better than adults with severe caries. Saliva contains acidic and basic proline-rich proteins (PRPs) which attach to oral streptococci. The PRPs are encoded within a small region of chromosome 12. An acidic PRP allele (Db) protects Caucasian children from caries but is more common in African Americans. Some basic PRP allelic phenotypes have a three-fold greater frequency in caries-free adults than in those with severe caries. Early childhood caries may associate with an absence of certain basic PRP alleles which bind oral streptococci, neutralize biofilm acids, and are in linkage disequilibrium with Db in Caucasians. The encoding of basic PRP alleles is updated and a new technology for genotyping them is described.

Diagnostic potential of saliva: current state and future applications

BACKGROUND

Over the past 10 years, the use of saliva as a diagnostic fluid has gained attention and has become a translational research success story. Some of the current nanotechnologies have been demonstrated to have the analytical sensitivity required for the use of saliva as a diagnostic medium to detect and predict disease progression. However, these technologies have not yet been integrated into current clinical practice and work flow.

CONTENT

As a diagnostic fluid, saliva offers advantages over serum because it can be collected noninvasively by individuals with modest training, and it offers a cost-effective approach for the screening of large populations. Gland-specific saliva can also be used for diagnosis of pathology specific to one of the major salivary glands. There is minimal risk of contracting infections during saliva collection, and saliva can be used in clinically challenging situations, such as obtaining samples from children or handicapped or anxious patients, in whom blood sampling could be a difficult act to perform. In this review we highlight the production of and secretion of saliva, the salivary proteome, transportation of biomolecules from blood capillaries to salivary glands, and the diagnostic potential of saliva for use in detection of cardiovascular disease and oral and breast cancers. We also highlight the barriers to application of saliva testing and its advancement in clinical settings.

SUMMARY

Saliva has the potential to become a first-line diagnostic sample of choice owing to the advancements in detection technologies coupled with combinations of biomolecules with clinical relevance.