Characterization of the immunologic responses to human in vivo acquired enamel pellicle as a novel means to investigate its composition

Mass spectrometry-based proteomic approaches for salivary protein biomarkers discovery and dental caries diagnosis: A critical review

Dental caries is a multifactorial chronic disease resulting from the intricate interplay among acid-generating bacteria, fermentable carbohydrates, and several host factors such as saliva. Saliva comprises several proteins which could be utilized as biomarkers for caries prevention, diagnosis, and prognosis. Mass spectrometry-based salivary proteomics approaches, owing to their sensitivity, provide the opportunity to investigate and unveil crucial cariogenic pathogen activity and host indicators and may demonstrate clinically relevant biomarkers to improve caries diagnosis and management. The present review outlines the published literature of human clinical proteomics investigations on caries and extensively elucidates frequently reported salivary proteins as biomarkers. This review also discusses important aspects while designing an experimental proteomics workflow. The protein-protein interactions and the clinical relevance of salivary proteins as biomarkers for caries, together with uninvestigated domains of the discipline are also discussed critically.

The composition of the dental pellicle: an updated literature review

Background

The dental pellicle is a thin layer of up to several hundred nm in thickness, covering the tooth surface. It is known to protect the teeth from acid attacks through its selective permeability and it is involved in the remineralization process of the teeth. It functions also as binding site and source of nutrients for bacteria and conditioning biofilm (foundation) for dental plaque formation.

Methods

For this updated literature review, the PubMed database was searched for the dental pellicle and its composition.

Results

The dental pellicle has been analyzed in the past years with various state-of-the art analytic techniques such as high-resolution microscopic techniques (e.g., scanning electron microscopy, atomic force microscopy), spectrophotometry, mass spectrometry, affinity chromatography, enzyme-linked immunosorbent assays (ELISA), and blotting-techniques (e.g., western blot). It consists of several different amino acids, proteins, and proteolytic protein fragments. Some studies also investigated other compounds of the pellicle, mainly fatty acids, and carbohydrates.

Conclusions

The dental pellicle is composed mainly of different proteins, but also fatty acids, and carbohydrates. Analysis with state-of-the-art analytical techniques have uncovered mainly acidic proline-rich proteins, amylase, cystatin, immunoglobulins, lysozyme, and mucins as main proteins of the dental pellicle. The pellicle has protective properties for the teeth. Further research is necessary to gain more knowledge about the role of the pellicle in the tooth remineralization process.

Evaluation of the time-dependent efficacy of commercial denture or orthodontic appliance cleansers: An in vitro study

The aim of this study was to evaluate the antimicrobial, stain and protein removal efficacy of denture or orthodontic appliance cleansers using in vitro test methods. Experimental cleansers were applied for experimental time in each evaluation method. To evaluate the microorganism removal efficacy, C. albicans and S. mutans removal rate was calculated from the specimen surface. Stain and protein removal rate was calculated using the spectrophotometer. Experimental cleansers significantly affected the microorganism removal rate for both C. albicans and S. mutans, as well as the stain and protein removal rates, at each experimental time (p<0.05). As the application time increased, the stain and protein removal rates of all experimental cleansers significantly increased (p<0.05). The present study provided in vitro evaluation methods to assess the efficacy of denture or orthodontic appliance cleansers. Also, manufacturers and researchers can predict clinical outcomes and ensure proper hygiene management of dentures or orthodontic appliances.

Functional biomedical materials derived from proteins in the acquired salivary pellicle

In the oral environment, the acquired salivary pellicle (ASP) on the tooth surface comprises proteins, glycoproteins, carbohydrates, and lipids. The ASP can specifically and rapidly adsorb on the enamel surface to provide effective lubrication, protection, hydration, and remineralisation, as well as be recognised by various bacteria to form a microbial biofilm (plaque). The involved proteins, particularly various phosphoproteins such as statherins, histatins, and proline-rich proteins, are vital to their specific functions. This review first describes the relationship between the biological functions of these proteins and their structures. Subsequently, recent advances in functional biomedical materials derived from these proteins are reviewed in terms of dental/bone therapeutic materials, antibacterial materials, tissue engineering materials, and coatings for medical devices. Finally, perspectives and challenges regarding the rational design and biomedical applications of ASP-derived materials are discussed.

Identification of in vivo Pellicle Constituents by Analysis of Serum Immune Responses

Human acquired enamel pellicle is composed of molecules that selectively adsorb from saliva onto tooth surfaces and provides a protective interface between the tooth enamel and the oral environment. To identify the micro-amounts of components present in pellicle, we immunized mice with in vivo-formed human acquired enamel pellicle and analyzed the serum immune responses. Selective reactivities of the serum (OD > 1.0 above background) against albumin, amylase, carbonic anhydrase II, sIgA, IgG, IgM, lactoferrin, lysozyme, proline-rich proteins, statherin, histatin 1, and mucous glycoprotein 1 were observed. We further confirmed the presence of proline-rich proteins, lactoferrin, lysozyme, and carbonic anhydrase II by probing in vivo pellicle with specific polyclonal anti-sera. The polyclonal antibody approach provided a powerful method for the identification of various pellicle proteins, including some which show mineral homeostasis or antimicrobial activity.

Salivary mucin 19 glycoproteins: innate immune functions in Streptococcus mutans-induced caries in mice and evidence for expression in human saliva

Saliva functions in innate immunity of the oral cavity, protecting against demineralization of teeth (i.e. dental caries), a highly prevalent infectious disease associated with Streptococcus mutans, a pathogen also linked to endocarditis and atheromatous plaques. Gel-forming mucins are a major constituent of saliva. Because Muc19 is the dominant salivary gel-forming mucin in mice, we studied Muc19(-/-) mice for changes in innate immune functions of saliva in interactions with S. mutans. When challenged with S. mutans and a cariogenic diet, total smooth and sulcal surface lesions are more than 2- and 1.6-fold higher in Muc19(-/-) mice compared with wild type, whereas the severity of lesions are up to 6- and 10-fold higher, respectively. Furthermore, the oral microbiota of Muc19(-/-) mice display higher levels of indigenous streptococci. Results emphasize the importance of a single salivary constituent in the innate immune functions of saliva. In vitro studies of S. mutans and Muc19 interactions (i.e. adherence, aggregation, and biofilm formation) demonstrate Muc19 poorly aggregates S. mutans. Nonetheless, aggregation is enhanced upon adding Muc19 to saliva from Muc19(-/-) mice, indicating Muc19 assists in bacterial clearance through formation of heterotypic complexes with salivary constituents that bind S. mutans, thus representing a novel innate immune function for salivary gel-forming mucins. In humans, expression of salivary MUC19 is unclear. We find MUC19 transcripts in salivary glands of seven subjects and demonstrate MUC19 glycoproteins in glandular mucous cells and saliva. Similarities and differences between mice and humans in the expression and functions of salivary gel-forming mucins are discussed.

Proteomic evaluation of acquired enamel pellicle during in vivo formation

Acquired enamel pellicle (AEP) is a protein film that forms on the enamel surface of teeth by selective adsorption of proteins and peptides present in the mouth. This protein film forms the interface between enamel and the damage oral biofilm, which modulates the attachment of bacteria found in oral biofilm. The overall goal of this study was to gain insight into the biological formation of the human in vivo AEP. This study hypothesized that AEP is created by the formation of successive protein layers, which consist of initial binding to enamel and subsequent protein-protein interactions. This hypothesis was examined by observing quantitative and qualitative changes in pellicle composition during the first two hours of AEP formation in the oral cavity. Quantitative mass spectrometry approaches were used to generate an AEP protein profile for each time-point studied. Relative proteomic quantification was carried out for the 50 proteins observed in all four time-points. Notably, the abundance of important salivary proteins, such as histatin 1, decrease with increasing of the AEP formation, while other essential proteins such as statherin showed constant relative abundance in all time-points. In summary, this is the first study that investigates the dynamic process to the AEP formation by using proteomic approaches. Our data demonstrated that there are significant qualitative and quantitative proteome changes during the AEP formation, which in turn will likely impact the development of oral biofilms.

Protein adhesion on dental surfaces-a combined surface analytical approach

Protein adsorption is a field of huge interest in a number of application fields. Information on protein adhesion is accessible by a variety of methods. However, the results obtained are significantly influenced by the applied technique. The objective of this work was to understand the role of adhesion forces (obtained by scanning force spectroscopy, SFS) in the process of protein adsorption and desorption. In SFS, the protein is forced to and retracted from the surface, even under unfavorable conditions, in contrast to the natural situation. Furthermore, adhesion forces are correlated with adhesion energies, neglecting the entropic part in the Gibbs enthalpy. In this context, dynamic contact angle (DCA) measurements were performed to identify the potential of this method to complement SFS data. In DCA measurements, the protein diffuses voluntarily to the surface and information on surface coverage and reversibility of adsorption is obtained, including entropic effects (conformational changes and hydrophobic effect). It could be shown that the surface coverage (by DCA) of bovine serum albumin on dental materials correlates well with the adhesion forces (by SFS) if no hydrophobic surface is involved. On those, the entropic hydrophobic effect plays a major role. As a second task, the reversibility of the protein adsorption, i.e., the voluntary desorption as studied by DCA, was compared to the adhesion forces. Here, a correlation between low adhesion forces and good reversibility could be found as long as no covalent bonds were involved. The comparative study of DCA and SFS, thus, leads to a more detailed picture of the complete adsorption/desorption cycle.

Identification of salivary mucin MUC7 binding proteins from Streptococcus gordonii

BACKGROUND

The salivary mucin MUC7 (previously known as MG2) can adhere to various strains of streptococci that are primary colonizers and predominant microorganisms of the oral cavity. Although there is a growing interest in interaction between oral pathogens and salivary mucins, studies reporting the specific binding sites on the bacteria are rather limited. Identification and characterization of the specific interacting proteins on the bacterial cell surface, termed adhesins, are crucial to further understand host-pathogen interactions.

RESULTS

We demonstrate here, using purified MUC7 to overlay blots of SDS-extracts of Streptococcus gordonii cell surface proteins, 4 MUC7-binding bands, with apparent molecular masses of 62, 78, 84 and 133 kDa from the Streptococcus gordonii strain, PK488. Putative adhesins were identified by in-gel digestion and subsequent nanoLC-tandem mass spectrometry analysis of resultant peptides. The 62 kDa and 84 kDa bands were identified as elongation factor (EF) Tu and EF-G respectively. The 78 kDa band was a hppA gene product; the 74 kDa oligopeptide-binding lipoprotein. The 133 kDa band contained two proteins; alpha enolase and DNA-directed RNA polymerase, beta' subunit. Some of these proteins, for example alpha enolase are expected to be intracellular, however, flow cytometric analysis confirmed its location on the bacterial surface.

CONCLUSION

Our data demonstrated that S. gordonii expressed a number of putative MUC7 recognizing proteins and these contribute to MUC7 mucin binding of this streptococcal strain.

Identification of Lys-Pro-Gln as a novel cleavage site specificity of saliva-associated proteases

The nonsterile environment of the oral cavity facilitates substantial proteolytic processing, not only of resident salivary proteins but also of dietary proteins. To gain insight into whole saliva enzymatic processes, the in vivo generated peptides in this oral fluid were subjected to nano-flow liquid chromatography electrospray ionization tandem mass spectrometry. The 182 peptides identified were predominantly derived from acidic and basic proline-rich proteins, statherin, and histatins. The proteolytic cleavages in the basic proline-rich proteins occurred preferentially after a Gln residue with predominant specificity for the tripeptide Xaa-Pro-Gln, where Xaa in the P(3) position was mostly represented by Lys. Using the synthetic substrates Lys-Pro-Gln-pNA and Gly-Gly-Gln-pNA, the overall K(m) values were determined to be 97 +/- 7.7 and 611 +/- 28 microm, respectively, confirming glutamine endoprotease activity in whole saliva and the influence of the amino acids in positions P(2) and P(3) on protease recognition. The pH optimum of Lys-Pro-Gln-pNA hydrolysis was 7.0, and the activity was most effectively inhibited by antipain and 4-(2-aminoethyl) benzenesulfonyl fluoride, was metal ion-dependent, and not inhibited by cysteine protease inhibitors. A systematic evaluation of enzyme activities in various exocrine and nonexocrine contributors to whole saliva revealed that the glutamine endoprotease is derived from dental plaque and likely microbial in origin. The P(1) site being occupied by a Gln residue is a nonarchetype with respect to known proteases and indicates the presence of novel glutamine-specific endoprotease(s) in oral fluid.

Peptide profile of human acquired enamel pellicle using MALDI tandem MS

The present study proposes a strategy for human in vivo acquired enamel pellicle (AEP) peptidome characterisation based on sequential extraction with guanidine and TFA followed by MALDI-TOF/TOF identification. Three different nanoscale analytical approaches were used: samples were subjected to tryptic digestion followed by nano-HPLC and mass spectrometry (MS and MS/MS) analysis. Undigested samples were analysed by LC-MS (both linear and reflector modes) and LC-MS/MS analysis, and samples were subjected to nano-HPLC followed by on-plate digestion and mass spectrometry (MS and MS/MS) analysis. The majority of the identifications corresponded to peptide/protein fragments of salivary protein, belonging to the classes: acidic PRPs, basic PRPs, statherin, cystatins S and SN and histatin 1 (all also identified in intact form). Overall, more than 90 peptides/proteins were identified. Results clearly show that peptides with acidic groups are enriched in the TFA fraction while peptides with no acidic or phosphate groups are prevalent on the guanidine extract. Also, phosphorylated peptides were observed mainly on the TFA fraction. Fragments present in the AEP show a predominance of cleavage points located at Arg, Tyr and Lys residues. Obtained data suggest that proteolytic activity could influence AEP formation and composition.

Peptidomic analysis of human acquired enamel pellicle

Human acquired enamel pellicle is the result of a selective interaction of salivary proteins and peptides with the tooth surface. In the present work, the characterization of the peptides as well as the type of interactions established with the enamel surface was performed. Peptides from in vivo bovine enamel implants in the human oral cavity were sequentially extracted using guanidine and trifluoroacetic acid solutions and the fractions obtained were analysed by LC-MS and LC-MS/MS. Based on the LC-MS data, six phosphorylated peptides were identified in an intact form, strongly adsorbed to the enamel surface. Data from the LC-MS/MS analyses allowed us to identified 30 fragment peptides non-covalently bonded to enamel [basic proline-rich proteins, histatins (1 and 3) and acidic proline-rich protein classes]. The tandem mass spectrometry experiments showed the existence of a pattern of amide bond cleavage for the different identified peptide classes suggesting a selective proteolytic activity. For histatins, a predominance of cleavage at Arg, Lys and His residues was observed, while for basic proline-rich proteins, cleavage at Arg and Pro residues prevailed. In the case of acidic proline-rich proteins, a clearly predominance of cleavage of the Gln-Gly amide bond was evident.

HPLC–MS characterization of cyclo-statherin Q-37, a specific cyclization product of human salivary statherin generated by transglutaminase 2

In the present study the analytical potential of HPLC-MS/MS was utilized for the structural characterization of a post-translational modification of statherin. Human salivary statherin (M(av)5380.0 +/- 0.3 Da) is transformed by the action of transglutaminase 2 into a cyclic derivative with an average molecular mass of 5363.0 +/- 0.3 Da. The intra-molecular bridge is generated by the loss of an ammonia molecule between the unique Ione-pair donating nucleophile Lys-6 and one acceptor among the seven glutamine residues of statherin. Digestion of the cyclic derivative with chymotrypsin, proteinase K, and carboxypeptidase Y, monitored by HPLC-electrospray ionization-ion trap-mass spectrometric analysis, demonstrated that cyclization involved almost specifically Gln-37 (> 95%), with the percentage of Gln-39 implicated in the cross-linkiing being less than 5%. The main derivative was named cyclostatherin Q37. Guineapig transglutaminase 2 showed high affinity for statherin in vitro (Km = 0.65 +/- 0.06 microM). Cyclo-statherin was detected in vivo by HPLC-electrospray ionization ion trap-mass spectrometry analysis of whole human saliva and it accounted for about 1% of total statherin. Detection of cyclo-statherin in whole saliva is suggestive of a putative role of this molecule in the formation of the "oral protein pellicle".

Relationship between the ability of oral streptococci to interact with platelet glycoprotein Ibalpha and with the salivary low-molecular-weight mucin, MG2

The oral streptococci Streptococcus sanguinis, Streptococcus gordonii and Streptococcus oralis are common aetiological agents of infective endocarditis, and their ability to adhere to and induce the aggregation of platelets is thought to be a virulence trait. The platelet glycoprotein GPIbalpha has been implicated as the adhesion receptor for S. sanguinis and S. gordonii, but it is not known if this is the case for S. oralis and other species. The aim of this study was to determine the GPIbalpha-interactive capability of a range of oral streptococci and to determine the relationship between this capability and their ability to interact with the salivary constituents that they would encounter in their normal habitat. All platelet-adhesive S. sanguinis strains and most S. gordonii strains adhered in a GPIbalpha-dependent manner, but strains of S. oralis, Streptococcus cristatus, Streptococcus parasanguinis and Streptococcus mitis had no direct affinity for platelets. Those strains that were able to bind GPIbalpha also bound to the low-molecular-weight submandibular salivary mucin, MG2, and this interaction was sialic acid-dependent. The data suggest that S. sanguinis and S. gordonii may be efficient colonizers of platelet vegetations because of their adaptation to recognize sialylated salivary mucins. In contrast, S. oralis does not interact with platelets and so is likely to colonize vegetations through an as yet unidentified mechanism.

Enzymes in the acquired enamel pellicle

The acquired pellicle is a biofilm, free of bacteria, covering oral hard and soft tissues. It is composed of mucins, glycoproteins and proteins, among which are several enzymes. This review summarizes the present state of research on enzymes and their functions in the dental pellicle. Theoretically, all enzymes present in the oral cavity could be incorporated into the pellicle, but apparently enzymes are adsorbed selectively onto dental surfaces. There is clear evidence that enzymes are structural elements of the pellicle. Thereby they exhibit antibacterial properties but also facilitate bacterial colonization of dental hard tissues. Moreover, the immobilized enzymes are involved in modification and in homeostasis of the salivary pellicle. It has been demonstrated that amylase, lysozyme, carbonic anhydrases, glucosyltransferases and fructosyltransferase are immobilized in an active conformation in the pellicle layer formed in vivo. Other enzymes, such as peroxidase or transglutaminase, have been investigated in experimental pellicles. Despite the depicted impact of enzymes on the formation and function of pellicle, broader knowledge on their properties in the in vivo-formed pellicle is required. This might be beneficial in the development of new preventive and diagnostic strategies.

Electron microscopic detection of salivary alpha-amylase in the pellicle formed in situ

Immunological and biochemical analyses have shown that alpha-amylase is an essential component of the acquired pellicle. After adsorption, this enzyme might act as a receptor for bacterial adherence. However, data indicating that amylase is bound to the pellicle surface in vivo and thus available for adhering bacteria are rare. Therefore, the present study focused on alpha-amylase within the pellicle formed in situ, using gold-immunolabeling electron microscopic techniques. Pellicles were formed by intra-oral exposure of enamel specimens for 30 and 120 min in six subjects. The results obtained by transmission electron microscopy indicate that amylase was randomly distributed in the pellicle layer without any preferential localization within the pellicle. Thus, salivary alpha-amylase might be considered as an important structural component that is even involved in the early stages of pellicle formation. The findings of field emission in-lens scanning electron microscopy provided evidence that the enzyme is located on the pellicle surface. It could be concluded that alpha-amylase might act as a receptor for bacterial adherence to the pellicle in vivo.

Ellipsometry analysis of the in vitro adsorption of tea polyphenols onto salivary pellicles

The adsorption of components from black tea and of purified tea polyphenols onto a whole unstimulated salivary pellicle-like protein layer, formed in vitro on hydroxyapatite discs, was studied by in situ ellipsometry. It was found that components from black tea and the purified polyphenols epicatechin-3-gallate (ECG), epigallocatechin-3-gallate (EGCG) and theaflavin readily adsorbed onto the pellicle. Further investigations showed that under the experimental conditions of this study, no black tea- or purified polyphenol-modified pellicles were eluted by either phosphate buffer or sodium dodecyl sulphate rinses. Therefore, black tea and its polyphenol components are indicated to have a profound effect on in vitro pellicle modification. Similar effects were observed for tannic acid.

Identification of early microbial colonizers in human dental biofilm

AIMS

To elucidate the first colonizers within in vivo dental biofilm and to establish potential population shifts that occur during the early phases of biofilm formation.

METHODS AND RESULTS

A 'checkerboard' DNA-DNA hybridization assay was employed to identify 40 different bacterial strains. Dental biofilm samples were collected from 15 healthy subjects, 0, 2, 4 and 6 h after tooth cleaning and the composition of these samples was compared with that of whole saliva collected from the same individuals. The bacterial distribution in biofilm samples was distinct from that in saliva, confirming the selectivity of the adhesion process. In the very early stages, the predominant tooth colonizers were found to be Actinomyces species. The relative proportion of streptococci, in particular Streptococcus mitis and S. oralis, increased at the expense of Actinomyces species between 2 and 6 h while the absolute level of Actinomyces remained unaltered. Periodontal pathogens such as Tannerella forsythensis(Bacteroides forsythus), Porphyromonas gingivalis and Treponema denticola as well as Actinobacillus actinomycetemcomitans were present in extremely low levels at all the examined time intervals in this healthy group of subjects.

CONCLUSION

The data provide a detailed insight into the bacterial population shifts occurring within the first few hours of biofilm formation and show that the early colonizers of the tooth surface predominantly consist of beneficial micro-organisms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The early colonizers of dental plaque are of great importance in the succession stages of biofilm formation and its overall effect on the oral health of the host.

In vitro hydroxyapatite adsorbed salivary proteins

In spite of the present knowledge about saliva components and their respective functions, the mechanism(s) of pellicle and dental plaque formation have hitherto remained obscure. This has prompted recent efforts on in vitro studies using hydroxyapatite (HA) as an enamel model. In the present study salivary proteins adsorbed to HA were extracted with TFA and EDTA and resolved by 2D electrophoresis over a pH range between 3 and 10, digested, and then analysed by MALDI-TOF/TOF mass spectrometry and tandem mass spectrometry. Nineteen different proteins were identified using automated MS and MS/MS data acquisition. Among them, cystatins, amylase, carbonic anhydrase, and calgranulin B, were identified.

Dental plaque as a microbial biofilm

New technologies have provided novel insights into how dental plaque functions as a biofilm. Confocal microscopy has confirmed that plaque has an open architecture similar to other biofilms, with channels and voids. Gradients develop in areas of dense biomass over short distances in key parameters that influence microbial growth and distribution. Bacteria exhibit an altered pattern of gene expression either as a direct result of being on a surface or indirectly as a response to the local environmental heterogeneity within the biofilm. Bacteria communicate via small diffusible signalling molecules (e.g. competence-stimulating peptide, CSP; autoinducer 2); CSP induces both genetic competence and acid tolerance in recipient sessile cells. Thus, rates of gene transfer increase in biofilm communities, and this is one of several mechanisms (others include: diffusion-reaction, neutralization/inactivation, slow growth rates, novel phenotype) that contribute to the increased antimicrobial resistance exhibited by bacteria in biofilms. Oral bacteria in plaque do not exist as independent entities but function as a co-ordinated, spatially organized and fully metabolically integrated microbial community, the properties of which are greater than the sum of the component species. A greater understanding of the significance of dental plaque as a mixed culture biofilm will lead to novel control strategies.

Statherin is an in vivo pellicle constituent: identification and immuno-quantification

Recently, we demonstrated that anti-statherin monoclonal antibodies could be generated upon immunisation of mice with in vivo formed human acquired enamel pellicle, indicating that statherin is a constituent of pellicle. To gain insight in the in vivo adsorption behaviour of statherin we tested the abundance of statherin in pellicle and investigated the relationship between statherin and protein levels in salivary secretions and pellicle using a capture ELISA. Statherin levels were approximately 20-fold higher in parotid and submandibular-sublingual secretions than in cleared whole saliva supernatant or pellicle, suggesting the rapid degradation of statherin in the oral cavity. A strong positive correlation was observed between statherin and protein levels in pellicle but not in saliva indicating that statherin and protein adsorption to pellicle are related processes. This indicates that statherin represents the integral part of proteins that constitute the pellicle structure and may play a key role in its formation.