Salivary Proteome Patterns Affecting Human Salt Taste Sensitivity

Multi-attribute temporal descriptive methods in sensory analysis applied in food science: A systematic scoping review

Among descriptive sensory evaluation methods, temporal methods have a wide audience in food science because they make it possible to follow perception as close as possible to the moment when sensations are perceived. The aim of this work was to describe 30 years of research involving temporal methods by mapping the scientific literature using a systematic scoping review. Thus, 363 research articles found from a search in Scopus and Web of Science from 1991 to 2022 were included. The extracted data included information on the implementation of studies referring to the use of temporal methods (details related to subjects, products, descriptors, research design, data analysis, etc.), reasons why they were used and the conclusions they allowed to be drawn. Metadata analysis and critical appraisal were also carried out. A quantitative and qualitative synthesis of the results allowed the identification of trends in the way in which the methods were developed, refined, and disseminated. Overall, a large heterogeneity was noted in the way in which the temporal measurements were carried out and the results presented. Some critical research gaps in establishing the validity and reliability of temporal methods have also been identified. They were mostly related to the details of implementation of the methods (e.g., almost no justification for the number of consumers included in the studies, absence of report on panel repeatability) and data analysis (e.g., prevalence of use of exploratory data analysis, only 20% of studies using confirmatory analyses considering the dynamic nature of the data). These results suggest the need for general guidelines on how to implement the method, analyze and interpret data, and report the results. Thus, a template and checklist for reporting data and results were proposed to help increase the quality of future research.

Effect of salivary gland removal on taste preference in mice

To evaluate the effect of decreased salivary secretion on taste preference, we investigated taste preference for five basic tastes by a 48 h two-bottle preference test using a mouse model (desalivated mice) that underwent surgical removal of three major salivary glands: the parotid, submandibular, and sublingual glands. In the desalivated mice, the avoidance behaviors for bitter and salty tastes and the attractive behaviors for sweet and umami tastes were significantly decreased. We confirmed that saliva is necessary to maintain normal taste preference. To estimate the cause of the preference changes, we investigated the effects of salivary gland removal on the expression of taste-related molecules in the taste buds. No apparent changes were observed in the expression levels or patterns of taste-related molecules after salivary gland removal. When the protein concentration and composition in the saliva were compared between the control and desalivated mice, the protein concentration decreased and its composition changed after major salivary gland removal. These results suggest that changes in protein concentration and composition in the saliva may be one of the factors responsible for the changes in taste preferences observed in the desalivated mice.

A Catalog of Coding Sequence Variations in Salivary Proteins' Genes Occurring during Recent Human Evolution

Saliva houses over 2000 proteins and peptides with poorly clarified functions, including proline-rich proteins, statherin, P-B peptides, histatins, cystatins, and amylases. Their genes are poorly conserved across related species, reflecting an evolutionary adaptation. We searched the nucleotide substitutions fixed in these salivary proteins' gene loci in modern humans compared with ancient hominins. We mapped 3472 sequence variants/nucleotide substitutions in coding, noncoding, and 5'-3' untranslated regions. Despite most of the detected variations being within noncoding regions, the frequency of coding variations was far higher than the general rate found throughout the genome. Among the various missense substitutions, specific substitutions detected in PRB1 and PRB2 genes were responsible for the introduction/abrogation of consensus sequences recognized by convertase enzymes that cleave the protein precursors. Overall, these changes that occurred during the recent human evolution might have generated novel functional features and/or different expression ratios among the various components of the salivary proteome. This may have influenced the homeostasis of the oral cavity environment, possibly conditioning the eating habits of modern humans. However, fixed nucleotide changes in modern humans represented only 7.3% of all the substitutions reported in this study, and no signs of evolutionary pressure or adaptative introgression from archaic hominins were found on the tested genes.

Pungency Perception and the Interaction with Basic Taste Sensations: An Overview

The perception of pungency can be attributed to the combination of pain and heat, and it has critical impacts on food flavor and food consumption preferences. Many studies have reported a variety of pungent ingredients with different Scoville heat units (SHU), and the mechanism of pungent perception was revealed in vivo and in vitro. The worldwide use of spices containing pungent ingredients has led to an increasing awareness of their effects on basic tastes. However, the interaction between basic tastes and pungency perception based on structure-activity relationship, taste perception mechanism and neurotransmission lacks review and summary, considering its brighter prospects in food flavor. Thus, in this review, common pungency substances and pungency evaluation methods, and the mechanism of pungency perception is presented, and the interaction between basic tastes and pungency perception and the possible factors of their interaction are reviewed in detail. Pungent stimuli are mainly transduced through transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential fixed hormone isoform (TRPA1) activated by stimulants. Using modern detection techniques combined with sensory standards, different substances produce different degrees of pungent stimulation, ranging from 10⁴ to 10⁷ SHU/g. Pungent stimuli can affect taste receptor or channel protein conformation and regulate taste bud cell sensitivity by producing neurotransmission products. The products of neurotransmission and taste receptor cell activation in turn act on taste perception. When there are simultaneous effects of taste perception, pungency stimulation may enhance the perception of salty at a certain concentration, with a mutual inhibition effect with sour, sweet, and bitter taste, while its interaction with umami taste is not obvious. However, due to the complexity of perception and the uncertainty of many perceptual receptors or channels, the current studies of interactions are still controversial. Based on the understanding of the mechanism and influencing factors, the availability of pungency substances is proposed in the perspective of food industry in order to achieve new development.

The role of saliva in taste and food intake

Saliva is well-described in oral food processing, but its role in taste responsiveness remains understudied. Taste stimuli must dissolve in saliva to reach their receptor targets. This allows the constituents of saliva the opportunity to interact with taste stimuli and their receptors at the most fundamental level. Yet, despite years of correlational data suggesting a role for salivary proteins in food preference, there were few experimental models to test the role of salivary proteins in taste-driven behaviors. Here we review our experimental contributions to the hypothesis that salivary proteins can alter taste function. We have developed a rodent model to test how diet alters salivary protein expression, and how salivary proteins alter diet acceptance and taste. We have found that salivary protein expression is modified by diet, and these diet-induced proteins can, in turn, increase the acceptance of a bitter diet. The change in acceptance is in part mediated by a change in taste signaling. Critically, we have documented increased detection threshold, decreased taste nerve signaling, and decreased oromotor responding to quinine when animals have increases in a subset of salivary proteins compared to control conditions.

The role of saliva in taste dysfunction among cancer patients: Mechanisms and potential treatment

Two of the highest prevalent symptoms reported by cancer patients are taste dysfunction (17.6-93%) and dry mouth/xerostomia (40.4-93%). While it has been hypothesized that reduced saliva may impair taste function, few studies investigate the co-occurrence of taste and dry mouth symptoms in cancer patients. This review provides a summary of the physiological relationship between saliva and taste, focusing on taste transduction mechanism, regulation of the taste sensitivity, and protection of taste receptor cells, including the impact of cancer treatments and malignancy on saliva function, salivary components, and the mechanisms in which it can negatively impact the taste function. Here, the authors present a scoping review of the recent literature reporting on the association between taste dysfunction and dry mouth in cancer patients, including reports of non-pharmaceutical liposomal agents or drugs taken to improve dry mouth symptoms that also assess taste dysfunction. Considering the complexities of cancer and cancer treatment, understanding the physiological relationship between saliva and taste function may provide important insight into identifying treatments for alleviating taste dysfunction and dry mouth symptoms. There are substantial research gaps given the limited studies assessing the co-occurrence of taste loss and dry mouth and inconsistencies in the assessment of these symptoms. Clinical studies examining taste dysfunction will provide a foundational groundwork that will help understand the relationship between taste and saliva. Considering the increased rates in survivorship and the significant negative impact of taste dysfunction on quality of life, more research is needed to reduce the suffering of cancer patients.

A secret of salivary secretions: Multimodal effect of saliva in sensory perception of food

Saliva plays multifunctional roles in oral cavity. Even though its importance for the maintenance of oral health has long been established, the role of saliva in food perception has attracted increasing attention in recent years. We encourage researchers to discover the peculiarity of this biological fluid and aim to combine the data concerning all aspects of the saliva influence on the sensory perception of food. This review presents saliva as a unique material, which modulates food perception due to constant presence of saliva in the mouth and thanks to its composition. Therefore, we highlight the salivary components that contribute to these effects. Moreover, this review is an attempt to structure the effects of saliva on perception of different food categories, where the mechanisms of salivary impact in perception of liquid, semi-solid, and solid foods are revealed. Finally, we emphasize that the large inter-individual variability in salivary composition and secretion appear to contribute to the fact that everyone experiences food in their own way. Therefore, the design of the sensory studies should consider the properties of volunteers' saliva and also carefully monitor the experimental conditions that affect salivary composition and flow rate.

Repeated exposure to epigallocatechin gallate solution or water alters bitterness intensity and salivary protein profile

Polyphenols, bitter and astringent compounds present in many healthy foods, induce varied sensory responses across individuals. These differences in liking and flavor intensity may be attributable, in part, to differences in saliva. In the current study, we tested the effect of repeated consumption of a bitter polyphenol (epigallocatechin gallate, EGCG) solution on perceived bitterness intensity and salivary protein composition. We hypothesized exposure to EGCG would cause an increase in concentrations of salivary proteins that inhibit bitterness of polyphenols. We also hypothesized that participants with higher habitual polyphenol, specifically the flavanols, intake would experience less bitterness from EGCG solutions than those with low habitual intake, and that the high flavanol consumers would be more resistant to salivary alterations. We also tested whether bovine milk casein, a food analog for salivary proteins that may suppress bitterness, would decrease bitterness intensity of the EGCG solution and mitigate effects of the intervention. Participants (N = 37) in our crossover intervention adhered to two-week periods of daily bitter (EGCG) or control (water) solution consumption. Bitterness intensity ratings and citric acid-stimulated saliva were collected at baseline and after each exposure period. Results indicate that bitterness intensity of the EGCG solution decreased after polyphenol (bitter EGCG) exposure compared to control (water) exposure. Casein addition also decreased bitterness intensity of the EGCG solution. While there was not a significant overall main effect of baseline flavanol intake on solution bitterness, there was an interaction between intervention week and baseline flavanol intake. Surprisingly, the higher flavanol intake group rated EGCG solutions as more bitter than the low and medium intake groups. Of proteins relevant to taste perception, several cystatins changed in saliva in response to the intervention. Interestingly, most of these protein alterations occurred more robustly after the control (water) exposure rather than the bitter (EGCG) exposure, suggesting that additional factors not quantified in this work may influence salivary proteins. Thus, we confirm in this study that exposure to bitterness suppresses ratings of bitterness over time, but more work needs to establish the causal factors of how diet influences salivary proteins.

Quantitative analysis of taste disorder in COVID-19 patients, the hypersensitivity to salty quality

Recently, many of the studies have illustrated that the new pandemic SARS-CoV-2 can affect Central Nervous System through the olfactory bulb. In addition to investigating anosmia or hyposmia induced by this virus, a quantitative analysis was needed to clarify the taste and smell disorder of the new coronavirus. The four basic taste quality with five concentrations for sweet, sour, bitter, and salty were administered to 75 subjects divided into three groups: COVID-19 patients with taste disorder, COVID-19 patients without taste disorder, and control group.

The results indicated the increment of sweet (2.68 ± 0.14), sour (3.34 ± 0.12) and bitter (3.39 ± 0.2) thresholds in COVID-19 patients with taste disorder in comparison with patients without taste disorder that the threshold were: 2 ± 0.16, 2.11 ± 0.2 and 2.55 ± 0.5 for sweet, sour, and bitter respectively. On the other hand, the patients inversely showed a significant decrease in the salty taste threshold (0.51 ± 0.03) compared to COVID-19 positive control groups (1.11 ± 0.11).

Additionally, despite taste disorder in almost all of the patients with smell deficiency, only 30% of cases with taste disorder reported smell deficiency. It may be concluded that some of the taste disorders in patients with COVID-19 disorder could be associated with taste receptors dysfunction or the spread of infection to the cranial nerves responsible for the conduction of tastes sensation.

Differences in Salivary Proteins as a Function of PROP Taster Status and Gender in Normal Weight and Obese Subjects

Taste plays an important role in processes such as food choices, nutrition status and health. Salivary proteins contribute to taste sensitivity. Taste reduction has been associated with obesity. Gender influences the obesity predisposition and the genetic ability to perceive the bitterness of 6-n-propylthiouracil (PROP), oral marker for food preferences and consumption. We investigated variations in the profile of salivary proteome, analyzed by HPLC-ESI-MS, between sixty-one normal weight subjects (NW) and fifty-seven subjects with obesity (OB), based on gender and PROP sensitivity. Results showed variations of taste-related salivary proteins between NW and OB, which were differently associated with gender and PROP sensitivity. High levels of Ps-1, II-2 and IB-1 proteins belonging to basic proline rich proteins (bPRPs) and PRP-1 protein belonging to acid proline rich proteins (aPRPs) were found in OB males, who showed a lower body mass index (BMI) than OB females. High levels of Ps-1 protein and Cystatin SN (Cyst SN) were found in OB non-tasters, who had lower BMI than OB super-tasters. These new insights on the role of salivary proteins as a factor driving the specific weight gain of OB females and super-tasters, suggest the use of specific proteins as a strategic tool modifying taste responses related to eating behavior.

Protein Ingredient in Saliva on Oral Dryness Patients Caused by Calcium Blocker

Oral dryness as a side effect of certain drugs is increasing. The aim of this study was to examine the change of the protein ingredient in saliva of oral dryness patients caused by calcium blocker. Six patients taking calcium blocker and six healthy elderly were enrolled. Unstimulated salivary flow rate, protein concentration, and flow rate of protein were measured and compared between the patients taking calcium blocker and healthy elderly. iTRAQ (Isobaric Tag for Relative and Absolute Quantitation) proteomic analysis was performed to extract the salivary protein changed in patient taking calcium blocker, and the intensities of Western blotting products were quantified (unpaired t-test). Unstimulated salivary flow rate was significantly lower on patients taking calcium blocker (p < 0.01). Protein concentration tended to be higher and the flow rate of protein tended to be lower on patients. As the result of iTRAQ proteomic analysis, calmodulin-like protein 3, glutathione S-transferase P, and keratin type I cytoskeletal 13 increased characteristically in patient taking calcium blocker, and the expression in calmodulin-like protein 3 was significantly larger (p < 0.01). The results of this study indicated that calmodulin-like protein 3 increased in patients taking calcium blocker and could be a salivary biomarker for oral dryness caused by calcium blocker.

Agonists of Orally Expressed TRP Channels Stimulate Salivary Secretion and Modify the Salivary Proteome

Natural compounds that can stimulate salivary secretion are of interest in developing treatments for xerostomia, the perception of a dry mouth, that affects between 10 and 30% of the adult and elderly population. Chemesthetic transient receptor potential (TRP) channels are expressed in the surface of the oral mucosa. The TRPV1 agonists capsaicin and piperine have been shown to increase salivary flow when introduced into the oral cavity but the sialogogic properties of other TRP channel agonists have not been investigated. In this study we have determined the influence of different TRP channel agonists on the flow and protein composition of saliva. Mouth rinsing with the TRPV1 agonist nonivamide or menthol, a TRPM8 agonist, increased whole mouth saliva (WMS) flow and total protein secretion compared with unstimulated saliva, the vehicle control mouth rinse or cinnamaldehyde, a TRPA1 agonist. Nonivamide also increased the flow of labial minor gland saliva but parotid saliva flow rate was not increased. The influence of TRP channel agonists on the composition and function of the salivary proteome was investigated using a multi-batch quantitative MS method novel to salivary proteomics. Inter-personal and inter-mouth rinse variation was observed in the secreted proteomes and, using a novel bioinformatics method, inter-day variation was identified with some of the mouth rinses. Significant changes in specific salivary proteins were identified after all mouth rinses. In the case of nonivamide, these changes were attributed to functional shifts in the WMS secreted, primarily the over representation of salivary and nonsalivary cystatins which was confirmed by immunoassay. This study provides new evidence of the impact of TRP channel agonists on the salivary proteome and the stimulation of salivary secretion by a TRPM8 channel agonist, which suggests that TRP channel agonists are potential candidates for developing treatments for sufferers of xerostomia.

Does ENaC Work as Sodium Taste Receptor in Humans?

Taste reception is fundamental for the proper selection of food and beverages. Among the several chemicals recognized by the human taste system, sodium ions (Na⁺) are of particular relevance. Na⁺ represents the main extracellular cation and is a key factor in many physiological processes. Na⁺ elicits a specific sensation, called salty taste, and low-medium concentrations of table salt (NaCl, the common sodium-containing chemical we use to season foods) are perceived as pleasant and appetitive. How we detect this cation in foodstuffs is scarcely understood. In animal models, such as the mouse and the rat, the epithelial sodium channel (ENaC) has been proposed as a key protein for recognizing Na⁺ and for mediating preference responses to low-medium salt concentrations. Here, I will review our current understanding regarding the possible involvement of ENaC in the detection of food Na⁺ by the human taste system.

Bitter-Induced Salivary Proteins Increase Detection Threshold of Quinine, But Not Sucrose

Exposures to dietary tannic acid (TA, 3%) and quinine (0.375%) upregulate partially overlapping sets of salivary proteins which are concurrent with changes in taste-driven behaviors, such as rate of feeding and brief access licking to quinine. In addition, the presence of salivary proteins reduces chorda tympani responding to quinine. Together these data suggest that salivary proteins play a role in bitter taste. We hypothesized that salivary proteins altered orosensory feedback to bitter by decreasing sensitivity to the stimulus. To that end, we used diet exposure to alter salivary proteins, then assessed an animal's ability to detect quinine, using a 2-response operant task. Rats were asked to discriminate descending concentrations of quinine from water in a modified forced-choice paradigm, before and after exposure to diets that alter salivary protein expression in a similar way (0.375% quinine or 3% TA), or 1 of 2 control diets. Control animals received either a bitter diet that does not upregulate salivary proteins (4% sucrose octaacetate), or a nonbitter diet. The rats exposed to salivary protein-inducing diets significantly decreased their performance (had higher detection thresholds) after diet exposure, whereas rats in the control conditions did not alter performance after diet exposure. A fifth group of animals were trained to detect sucrose before and after they were maintained on the 3% TA diet. There was no significant difference in performance, suggesting that these shifts in threshold are stimulus specific rather than task specific. Taken together, these results suggest that salivary proteins reduce sensitivity to quinine.

Comparison of salivary proteome of children with different sensitivities for bitter and sweet tastes: association with body mass index

BACKGROUND/OBJECTIVES

Oral sensorial perception is a key aspect in food choices and knowing the mechanisms modulating such perception is of major importance in the context of child obesity, which is reaching high rates in Mediterranean countries. Salivary proteome has been linked to taste sensitivity in adults. The aim of this study was to search for differences in salivary proteomes of children with different bitter or sweet taste sensitivities and to assess if these potential differences are associated with their body mass index percentile (BMI percentile).

SUBJECTS/METHODS

387 children aged 8-9 years old were assessed for BMI percentile and classified according to their sensitivity to bitter and sweet tastes, according to their caffeine and sucrose detection thresholds, respectively. Saliva protein composition was compared among taste sensitivity groups, taking into account BMI percentile and gender, using gel-based proteomics approaches, coupled to mass spectrometry for protein identification.

RESULTS

Among the salivary proteins related to bitter taste sensitivity, higher levels of cystatins were observed in bitter-sensitive children, in the case of those of normal weight, and in bitter low-sensitive, in the case of overweight children. For sweetness, the relationship between saliva and taste perception was also dependent on BMI percentile, with several proteins (including salivary cystatins) differing between taste sensitivity groups, with disparities arising between normal-weight and overweight children. Cystatin isoforms A, B and SA were observed to be considerably increased in saliva from obese children.

CONCLUSIONS

Salivary proteome is related with sensitivities to bitter and sweet tastes in children, but the association is dependent on BMI percentile and gender.

Oral sensations and secretions

Sensations experienced in the mouth influence food choices, both immediately and in the long term. Such sensations are themselves influenced by experience with flavors, the chemical environment of the mouth, genetics of receptors for flavors, and individual behavior in the chewing of food. Gustation, the sense of taste, yields information about nutrients, influences palatability, and feeds into the human body's preparation to receive those nutrients. Olfaction, the sense of smell, contributes enormously to defining and identifying food flavors (and is experienced even after placing food inside the mouth). Another vital component of food flavor is texture, which contributes to palatability, especially if a food's texture violates a person's expectations. Next, chemesthesis is the sense of chemically induced irritancy and temperature, for example spiciness and stinging. All of these sensations are potentially modified by saliva, the chemical and physical media of the mouth. As a person experiences the culmination of these oral sensations, modified through an individual's own unique saliva, the flavors in turn influence both what and how a person eats.

Main effects of human saliva on flavour perception and the potential contribution to food consumption

Whole saliva is a mixture composed by the secretions of the major and minor salivary glands and the crevicular fluid, bacteria, cells and food debris. Its properties (flow and composition) are highly intra- and inter-individually dependent and reflect the health status of individuals. Saliva plays a key role in the eating process and on the perception of flavour. Flavour corresponds to the combined effect of taste sensations, aromatics and chemical feeling factors evoked by food in the oral cavity. It is a key determinant of food consumption and intake. This review summarises the evidence about the role of saliva in flavour perception and its potential contribution to food intake. All in all, evidence on the relationships between salivary parameters and both food perception and feeding behaviour is presented. This review emphasises that new studies accounting for the effect of salivary constituents on flavour alterations due to diseases (i.e. cancer, obesity and diabetes) are lacking and are expected in the incoming years.

Current Status and Future Perspectives in Flavor Research: Highlights of the 11th Wartburg Symposium on Flavor Chemistry & Biology

The 11th Wartburg Symposium on Flavor Chemistry & Biology, held at the hotel "Auf der Wartburg" in Eisenach, Germany, from June 21 to 24 in 2016, offered a venue for global exchange on cutting-edge research in chemistry and biology of odor and taste. The focus areas were (1) functional flavor genomics and biotechnology, (2) flavor generation and precursors, (3) new approaches and precursors, (4) new approaches and technologies, (5) new molecules and structure/activity relationships, (6) food-borne bioactives and chemosensory health prevention, and (7) chemosensory reception, processing, and perception. Selected from more than 250 applicants, 160 distinguished scientists and rising stars from academia and industry from 24 countries participated in this multidisciplinary event. This special issue comprises a selection of 33 papers from oral presentations and poster contributions and is prefaced by this symposium introduction to carve out essential achievements in odor and taste chemistry and to share future research perspectives.