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

Abstract

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.

Figure 1

The ‘Ying and Yang’ of saliva.

Figure 2

Interactions of salivary proteins with the microbiota.

Figure 3. Relative abundances of protein and peptide species in human saliva

Most biomarkers presently considered for diagnostic use are derived from plasma and are less abundant by several orders of magnitude than the major intrinsic proteins of saliva. The values indicated by bars are good-faith estimates of the author, based on numerous literature sources (too many to cite them all here). The error bars indicate the widely varying measurements reported. For better comparison, concentrations were converted to percentage of total salivary protein. Comprehensive reviews are available [,,,–116]. PRP: Proline-rich protein; PSP-SPLUNC: Parotid secretory protein-short palate, lung and nasal epithelium clone 2; SAG: Salivary agglutinin; SC: Secretory component; S-IgA: Secretory IgA; SLPI: Secretory leukocyte peptidase inhibitor.