Do salivary proline-rich proteins counteract dietary hydrolyzable tannin in laboratory rats?

Sex differences in diet-mediated salivary protein upregulation

Our lab previously established that repeated exposure to a bitter diet can increase salivary protein (SP) expression, which corresponds to an increase in acceptance of the bitter stimulus. However, this work was exclusively in male rodents, here we examine sex differences. We found that there are no differences in SP expression (experiment 1) or quinine diet acceptance (experiment 2) across stage of estrous cycle. Yet, males and females differ in feeding behaviors, SP expression, and responses to a quinine diet (experiment 3). On a quinine diet, males accepted the diet much faster than females. Males displayed a compensatory increase in meal number as meal size and rate of feeding decreased with initial exposure to a quinine diet, whereas females decreased meal size and rate of feeding with no compensation in meal number. There were sex differences in SP expression at day 14 of quinine exposure but these were gone by day 24. Both sexes increased acceptance of quinine in a brief access taste test after the feeding trial concluded. These data suggest that males and females have different patterns of bitter diet acceptance, but extended exposure to quinine diet still results in altered bitter taste responding and changes in SP profiles in females.

Salivary proteins rescue within-session suppression and conditioned avoidance in response to an intragastric quinine infusion

A subset of salivary proteins (SPs) upregulates in response to a quinine-containing diet. The presence of these SPs then results in decreased bitter taste responding and taste nerve signaling. Bitter taste receptors in the oral cavity are also found in the stomach and intestines and contribute to behaviors that are influenced by post-oral signaling. It has been previously demonstrated that after several pairings of post-orally infused bitter stimuli and a neutral flavor, animals learn to avoid the flavor that was paired with gastric bitter, this is referred to as conditioned avoidance. Furthermore, animals will decrease licking of a neutral solution within a test session, when licking is paired with an intragastric bitter infusion; this has been described as within-session suppression. We used these paradigms to test the role of SPs in behaviors influenced by post-oral signaling. In both paradigms, the animal is given a test solution directly into the stomach (with or without quinine, and with or without SPs), and the infusions are self-administered by licking to a neutral solution (Kool-Aid). Quinine successfully conditioned a flavor avoidance, but, in a separate trial, we were unable to detect conditioning in the presence of SPs from donor animals. Likewise, quinine was able to suppress licking within the conditioned suppression paradigm, but the effect of the bitter was blocked in the presence of saliva containing SPs. Together, these data suggest that behaviors driven by post-oral signaling can be altered by SPs.

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.

Polyphenols and Lactobacillus reuteri in oral health

Oral health is one of the necessary preludes to the overall quality of life. Several medical procedures and therapies are available to treat oral diseases in general and periodontal diseases in particular, yet caries, periodontitis, oral cancer, and oral infections remain a global concern. Natural molecules, with their anti-oxidant, anti-inflammatory, and anti-microbic properties, are one of the main sources of oral health and dental health care, and should be supplemented to exploit their beneficial effects. A possible way to improve the intake of these molecules is adhering to a diet that is rich in fruits, vegetables, and probiotics, which has many beneficial properties and can improve overall health and wellbeing. The Mediterranean diet, in particular, provides several beneficial natural molecules, mainly because of the precious nutrients contained in its typical ingredients, mainly plant-based (olives, wine, citrus fruits, and many more). Its beneficial effects on several diseases and in increasing the overall wellbeing of the population are currently being studied by physicians. Among its nutrients, polyphenols (including, among other molecules, lignans, tannins, and flavonoids) seem to be of outmost importance: several studies showed their anticariogenic properties, as well as their effects in decreasing the incidence of non-communicable diseases. Therefore, plant-derived molecules - such as polyphenols - and probiotics - such as Lactobacillus reuteri - have shown a significant potential in treating and curing oral diseases, either alone or in combination, owing to their antioxidant and antimicrobial properties, respectively.

Metabolic Enabling and Detoxification by Mammalian Gut Microbes

The longstanding interactions between mammals and their symbionts enable thousands of mammal species to consume herbivorous diets. The microbial communities in mammals degrade both plant fiber and toxins. Microbial toxin degradation has been repeatedly documented in domestic ruminants, but similar work in wild mammals is more limited due to constraints on sampling and manipulating the microbial communities in these species. In this review, we briefly describe the toxins commonly encountered in mammalian diets, major classes of biotransformation enzymes in microbes and mammals, and the gut chambers that house symbiotic microbes. We next examine evidence for microbial detoxification in domestic ruminants before providing case studies on microbial toxin degradation in both foregut- and hindgut-fermenting wild mammals. We end by discussing species that may be promising for future investigations, and the advantages and limitations of approaches currently available for studying degradation of toxins by mammalian gut microbes. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Interactions between Salivary Proteins and Dietary Polyphenols: Potential Consequences on Gastrointestinal Digestive Events

The present review documents the current knowledge and hypotheses on how polyphenols-saliva interactions may modulate the bioaccessibility or bioavailability of nutrients and highlights research prospects in the field. After an updated description of the different classes of dietary polyphenols and their modifications by food processing or digestion, an overview of interactions between salivary proteins and polyphenols (with an emphasis on tannins) is provided. In vitro studies show that the solubility of salivary protein-tannin complexes in gastric conditions depends on the degree of tannin polymerization, while complexes are partly solubilized by bile salts. Salivary proteins-polyphenols interactions may affect digestive processes. For example, polyphenols can bind to and inhibit salivary amylase, with downstream consequences on starch digestion. Some salivary proteins (PRPs) prevent tannin-induced reduced protein digestibility, probably through binding tannins before they interact with digestive proteases. Salivary proteins may also act as scavenger molecules to limit the intestinal uptake of tannins.

Interference of dietary polyphenols with potentially toxic amino acid metabolites derived from the colonic microbiota

Each day, varying amounts of undigested or partially digested proteins reach the colon where they are metabolized by the microbiota, resulting in the formation of compounds such as ammonia, p-cresol, skatole, phenol, indole, and hydrogen sulfide (H₂S). In farm animals, the excessive production of these metabolites can affect the quality of meat and milk and is a source of contaminating emissions from animal manure. In humans, their accumulation is potentially harmful, and it has been proposed that they could be involved in the development of pathologies such as colorectal cancer and ulcerative colitis, among others. This review assesses the evidence supporting the use of dietary polyphenols to reduce the production of these metabolites. Most studies have used condensed (proanthocyanidins) or hydrolyzable (ellagitannins and gallotannins) tannins, and have been carried out in farm animals. Several show that the administration of tannins in pigs, chicken, and ruminants decreases the levels of ammonia, p-cresol, skatole, and/or H₂S, improving meat/milk quality and reducing manure odor. Direct application of tannins to manure also decreases ammonia emissions. Few studies were carried out in rats and humans and their results confirm, to a lesser extent, those reported in farm animals. These effects would be due to the capacity of tannins to trap ammonia and H₂S, and to modify the composition of the microbiota, reducing the bacterial populations producing metabolites. In addition, PACs prevent p-cresol and H₂S-induced alterations on intestinal cells in vitro. Tannins, therefore, appear as an interesting tool for improving the quality of animal products, human health, and the harmful emissions associated with breeding.

Megaherbivore browsers vs. tannins: is being big enough?

Megaherbivores have been of particular interest to scientists because of the physiological and ecological challenges associated with their extreme body size. Yet, one question that has seldom been explored is how browsing megaherbivores cope with plant secondary metabolites (PSMs), such as tannins, found in their food. It is possible that the sheer body size of these megaherbivores allows them to ingest tannins with no deleterious effects. However, it is plausible that megaherbivores must rely on other mechanisms to cope with PSMs, such as the production of salivary tannin-binding proteins. Thus, we aimed to determine whether megaherbivore browsers produce tannin-binding proteins to further reduce the consequences of ingesting a tannin-rich diet. Using a series of laboratory assays, we explored whether elephants, black rhinoceros, and giraffe had tannin-binding proteins in their saliva. We tested for the presence of proline-rich proteins in the saliva using two different approaches: (1) SDS-PAGE using Laemmli's (Laemmli, Nature 227:680-685, 1970) destaining method, and (2) comparative SDS-PAGE gels using Beeley et al.'s (Beeley et al. Electrophoresis 12:493-499, 1991) method for staining and destaining to probe for proline-rich proteins. Then, to test for the tannin-binding affinity of their saliva, we performed an inhibition assay. We did not observe proline-rich proteins in any of the megaherbivore species, but they did have other protein(s) in their saliva that have a high tannin-binding affinity. Our results highlight that, despite their large body sizes, and their abilities to tolerate low-quality food, browsing megaherbivores have likely evolved tannin-binding proteins as a way of coping with the negative effects of tannins.

Are there phylogenetic differences in salivary tannin-binding proteins between browsers and grazers, and ruminants and hindgut fermenters?

While feeding, mammalian browsers (primarily eat woody plants) encounter secondary metabolites such as tannins. Browsers may bind these tannins using salivary proteins, whereas mammalian grazers (primarily eat grasses that generally lack tannins) likely would not. Ruminant browsers rechew their food (ruminate) to increase the effectiveness of digestion, which may make them more effective at binding tannins than nonruminants. Few studies have included a sufficient number of species to consider possible scaling with body mass or phylogenetic effects on salivary proteins. Controlling for phylogeny, we ran inhibition radial diffusion assays of the saliva of 28 species of African herbivores that varied in size, feeding strategy, and digestive system. We could not detect the presence of salivary proline-rich proteins that bind tannins in any of these species. However, using the inhibition radial diffusion assay, we found considerable abilities to cope with tannins in all species, albeit to varying degrees. We found no differences between browsers and grazers in the effectiveness of their salivary proteins to bind to and precipitate tannins, nor between ruminants and nonruminants, or scaling with body mass. Three species bound all tannins, but their feeding niches included one browser (gray duiker), one mixed feeder (bush pig), and one grazer (red hartebeest). Five closely related species of small ruminant browsers were very effective in binding tannins. Megaherbivores, considered generalists on account of their large body size, were capable of binding tannins. However, the grazing white rhinoceros was almost as effective at binding tannins as the megaherbivore browsers. We conclude, contrary to earlier predictions, that there were no differences in the relative salivary tannin-binding capability that was related to common ancestry (phylogeny) or to differences in body size.

Altering salivary protein profile can decrease aversive oromotor responding to quinine in rats

Bitter taste is often associated with toxins, but accepting some bitter foods, such as green vegetables, can be an important part of maintaining a healthy diet. It has previously been shown that animals exposed to quinine upregulate a set of salivary proteins (SPs), and those with upregulated SPs have increased rates of feeding on a quinine diet as well as increased brief-access licking to and higher detection thresholds for quinine. These studies suggest that SPs alter orosensory feedback; however, they rely on SPs upregulated by diet exposure and cannot control for the role of learning. Here, we use taste reactivity to determine if SPs can alter bitter taste in animals with no previous bitter diet experience. First, saliva with proteins stimulated by injections of isoproterenol and pilocarpine was collected from anesthetized rats; this "donor saliva" was analyzed for protein concentration and profile. Bitter-naïve rats were implanted with oral catheters and infused with taste stimuli dissolved in saliva that contained all of the SPs from the donors, saliva that was filtered of SPs, water, or artificial saliva. Their orofacial movements were recorded and quantified. We found that presence of quinine increased movements associated with aversive stimuli, but adding SPs to the infusion was sufficient to reduce aversive oromotor responding to quinine. The effect was dependent on the total protein concentration of the saliva, as protein concentration increased aversive responses decreased. Additionally, infusions of whole saliva altered aversive responding to quinine, but not other stimuli (citric acid, NaCl, sucrose). Our work suggests that effect of these SPs is specific and the presence of SPs is sufficient to decrease aversive orosensory feedback to bitter stimuli.

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.

Altering salivary protein profile can increase acceptance of a novel bitter diet

Bitter taste is often associated with toxins, but accepting some bitter foods, such as green vegetables, can be an important part of maintaining a healthy diet. In rats and humans, repeated exposure to a bitter stimulus increases acceptance. Repeated exposure allows an individual the opportunity to learn about the food's orosensory and postingestive effects. It also alters the salivary protein (SP) profile, which in turn alters taste signaling. We have hypothesized that altering the salivary proteome plays a role in the increased acceptance after repeated exposure. Here we test this and attempt to disentangle the contribution of learning during dietary exposure from the contribution of SPs in increased acceptance of bitter diet. Dietary exposure to quinine or tannic acid and injection of isoproterenol (IPR) result in similar salivary protein profiles. Here we used either the bitter stimulus tannic acid or IPR injection to upregulate a subset of SPs before exposing animals to a novel diet containing quinine (0.375%). Control animals received either a control diet before being exposed to quinine, or a diet containing sucrose octaacetate, a compound that the animals avoid but does not alter SP profiles. The treatments that alter SP expression increased rate of feeding on the quinine diet compared to the control treatments. Additionally, tannic acid exposure altered intake and meal size of the quinine diet. These data suggest that SPs, not just learning about bitter food, increase acceptance of the bitter diet.

Pentagalloyl Glucose and Its Functional Role in Vascular Health: Biomechanics and Drug-Delivery Characteristics

Pentagalloyl glucose (PGG) is an elastin-stabilizing polyphenolic compound that has significant biomedical benefits, such as being a free radical sink, an anti-inflammatory agent, anti-diabetic agent, enzymatic resistant properties, etc. This review article focuses on the important benefits of PGG on vascular health, including its role in tissue mechanics, the different modes of pharmacological administration (e.g., oral, intravenous and endovascular route, intraperitoneal route, subcutaneous route, and nanoparticle based delivery and microbubble-based delivery), and its potential therapeutic role in vascular diseases such as abdominal aortic aneurysms (AAA). In particular, the use of PGG for AAA suppression and prevention has been demonstrated to be effective only in the calcium chloride rat AAA model. Therefore, in this critical review we address the challenges that lie ahead for the clinical translation of PGG as an AAA growth suppressor.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Salivary tannin-binding proteins are a pervasive strategy used by the folivorous/frugivorous black howler monkey

Dietary tannins can affect protein digestion and absorption, be toxic, and influence food selection by being astringent and bitter tasting. Animals that usually ingest tannins may regularly secrete tannin-binding salivary proteins (TBSPs) to counteract the negative effects of tannins or TBSPs production can be induced by a tannin-rich diet. In the wild, many primates regularly eat a diet that contains tannin-rich leaves and unripe fruit and it has been speculated that they have the physiological ability to cope with dietary tannins; however, details of their strategy remains unclear. Our research details the salivary protein composition of wild and zoo-living black howler monkeys (Alouatta pigra) feeding on natural versus manufactured low-tannin diets, and examines differences in TBSPs, mainly proline-rich proteins (PRPs), to determine whether production of these proteins is dependent on the tannin content of their food. We measured the pH, flow rate, and concentration of total protein and trichloroacetic acid soluble proteins (an index of PRPs) in saliva. Howler monkeys produced slightly alkaline saliva that may aid in the binding interaction between tannin and salivary proteins. We used gel electrophoresis to describe the salivary protein profile and this analysis along with a tannin-binding assay allowed us to detect several TBSPs in all individuals. We found no differences in the characteristics of saliva between wild and zoo-living monkeys. Our results suggest that black howler monkeys always secrete TBSPs even when fed on foods low in tannins. This strategy of constantly using this salivary anti-tannin defense enables them to obtain nutrients from plants that sometimes contain high levels of tannins and may help immediately to overcome the astringent sensation of their food allowing howler monkeys to eat tanniferous plants.

Salivary proteins alter taste-guided behaviors and taste nerve signaling in rat

Taste stimuli are normally dissolved in saliva prior to interacting with their respective receptor targets. There are hundreds of proteins in saliva, and it has been hypothesized that these proteins could interact with either taste stimuli or taste receptors to alter taste signaling and diet acceptance. However, the impact of these proteins on feeding has been relatively unexplored using rodent models. We have developed a novel technique for saliva collection that allows us to link salivary protein expression with feeding behavior. First, we monitored the microstructure of rats' feeding patterns on a 0.375% quinine diet (Q-diet) while tracking changes in salivary protein expression. We found 5 protein bands were upregulated by diet exposure to Q-diet and upregulation of a subset of these bands were statistically related to increased diet acceptance, including changes in behavioral measures that are thought to represent both orosensory and postingestive signaling. In a second experiment, we measured the licking to a range of quinine solutions (0.01-1.0mM) before and after the animals were exposed to a tannic acid diet that altered salivary protein expression. Rats found the quinine solutions less aversive after salivary protein altering diets. In a third experiment we recorded the response of the chorda tympani (CT) nerve while delivering quinine solutions (0.3-30mM) to the front of the tongue dissolved in either "donor saliva" containing salivary proteins or donor saliva which has had the salivary proteins removed. Donor saliva was collected from a separate group of animals using isoproterenol and pilocarpine. The samples containing salivary proteins resulted in lower nerve responses than those without salivary proteins. Together these data suggest that salivary proteins are capable of altering taste-guided behaviors and taste nerve signaling.

Long-Term Dose-Response Condensed Tannin Supplementation Does Not Affect Iron Status or Bioavailability

Background: Repeated phytic acid consumption leads to iron absorption adaptation but, to the best of our knowledge, the impact of repeated tannin consumption has not yet been established. Salivary proline-rich proteins (PRPs) may improve iron absorption by precipitating tannins. Objectives: This study aimed to determine the effect of long-term, dose-response condensed tannin supplementation on iron bioavailability and status and to assess the effect of salivary proteins on iron bioavailability during prolonged condensed tannin consumption. A secondary objective was to assess astringency as a potential marker for adaptation to tannins and iron bioavailability. Methods: Eleven nonanemic women were enrolled in a double-blind 3-dose crossover trial. Three (1.5, 0.25, or 0.03 g) condensed tannin supplements were consumed 3 times/d for 4 wk in random order, with 2-wk washouts in between. Meal challenges were employed before and after supplementation to assess iron bioavailability, iron status, salivary PRP changes, and astringency. Results: Tannin supplementation in any dose did not change iron bioavailability at any dose (P > 0.82) from weeks 0 to 4. Hemoglobin (P = 0.126) and serum ferritin (P = 0.83) were unchanged by tannin dose from weeks 0 to 4. There were significant correlations among tannin supplementation and iron bioavailability, basic proline-rich proteins (bPRPs) (r = 0.366, P = 0.003), and cystatin production (r = 0.27, P = 0.03). Astringency ratings did not change significantly within or between tannin doses (P > 0.126), but there were negative relations among bPRP (r < -0.32, P < 0.21), cystatin production (r < -0.2, P < 0.28), and astringency ratings. Conclusions: Condensed tannin consumption did not affect iron bioavailability or status regardless of the supplementation period in premenopausal nonanemic women. Correlation analyses suggest that bPRPs and cystatins are associated with improved iron bioavailability and that lower ratings of astringency may predict improved iron absorption with repeated tannin consumption.

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

Background

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

Aim

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

Effects of dietary tannic acid on the growth, hepatic gene expression, and antioxidant enzyme activity in Brandt's voles (Microtus brandti)

This study was designed to investigate the physiological and biochemical responses of Brandt's voles to the persistent presence of dietary tannic acid. The diet for animals in the experimental group was supplemented with 3% dietary tannic acid for 5weeks. The control group received a commercial lab chow. No significant differences were detected in body weight, organ (heart, kidney, and liver) weights, and organ parameters between animals from two groups. However, voles in the experimental group had significantly higher daily food intake, increased contents of proline and histidine in saliva and feces after protein hydrolysis, and elevated hepatic expression of transferrin than the control. Our results suggested the existence of adaptive strategies developed in Brandt's voles to overcome the adverse effects of dietary tannic acid. (1) Food consumption was increased to satisfy their nutritional demands. (2) The secretion of tannic-acid-binding salivary proteins was promoted. (3) The absorption of iron was enhanced. These alterations contributed to neutralize the negative effects of tannic acid and maintain body mass in animals supplemented with tannic acid. As the result of the consumption of tannic acid, hepatic expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase was significantly decreased, while the overall potential of the antioxidant system, characterized by increased hepatic enzymatic activities of catalase and glutathione peroxidase, was enhanced. Our results also implied the involvement of tannic acid in the regulation of lipid metabolism and oxidative stress in voles.

Seed traits and taxonomic relationships determine the occurrence of mutualisms versus seed predation in a tropical forest rodent and seed dispersal system

Although many studies have been carried out on plant-animal mutualistic assemblages, the roles of functional traits and taxonomy in determining both whether interactions involve mutualisms or predation and the structure of such assemblages are unclear. We used semi-natural enclosures to quantitatively assess the interaction strengths between seeds of 8 sympatric tree species and 4 rodent species in a tropical forest in Xishuangbanna, Yunnan, Southwest China. We found 2 clusters of species in the seed-rodent network represented by 2 genera in the Fagaceae (Castanopsis, Lithocarpus). Compared to seeds of 3 Castanopsis species, seeds with heavy weight, hard coat or caloric content (including 3 Lithocarpus species) were eaten less and more frequently hoarded by rodents. In turn, hoarded seeds showed less predation and more mutualism with rodents. Our results suggest that seed traits significantly affected the hoarding behavior of rodents, and, consequently, the occurrence of mutualisms and predation as well as assemblage structure in the plant-animal seed dispersal system. Taxonomically-related species with similar seed traits as functional groups belong to the same substructures in the assemblage. Our results indicate that both seed traits and taxonomic relationships may simplify thinking about seed dispersal systems by helping to elucidate whether interactions are likely to be dominated by predation or mutualism.

Induction of salivary proteins modifies measures of both orosensory and postingestive feedback during exposure to a tannic acid diet

There are hundreds of proteins in saliva. Although it has long been hypothesized that these proteins modulate taste by interacting with taste receptors or taste stimuli, the functional impact of these proteins on feeding remains relatively unexplored. We have developed a new technique for saliva collection that does not interfere with daily behavioral testing and allows us to explore the relationship between feeding behavior and salivary protein expression. First, we monitored the alterations in salivary protein expression while simultaneously monitoring the animals' feeding behavior and meal patterns on a custom control diet or on the same diet mixed with 3% tannic acid. We demonstrated that six protein bands increased in density with dietary tannic acid exposure. Several of these bands were significantly correlated with behaviors thought to represent both orosensory and postingestive signaling. In a follow-up experiment, unconditioned licking to 0.01–3% tannic acid solutions was measured during a brief-access taste test before and after exposure to the tannic acid diet. In this experiment, rats with salivary proteins upregulated found the tannin solution less aversive (i.e., licked more) than those in the control condition. These data suggest a role for salivary proteins in mediating changes in both orosensory and postingestive feedback.

Comparative digestive physiology

In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose). The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Many species respond to higher food intake by flexibly increasing digestive compartment size. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K+ rather than Na+ as a counter ion). Paracellular absorption is important in many birds. Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption.

Interspecific differences in tannin intakes of forest-dwelling rodents in the wild revealed by a new method using fecal proline content

Mammalian herbivores adopt various countermeasures against dietary tannins, which are among the most widespread plant secondary metabolites. The large Japanese wood mouse Apodemus speciosus produces proline-rich salivary tannin-binding proteins in response to tannins. Proline-rich proteins (PRPs) react with tannins to form stable complexes that are excreted in the feces. Here, we developed a new method for estimating the tannin intake of free-living small rodents, by measuring fecal proline content, and applied the method to a field investigation. A feeding experiment with artificial diets containing various levels of tannic acid revealed that fecal proline content was clearly related to dietary tannin content in three species (A. speciosus, Apodemus argenteus, and Myodes rufocanus). We then used fecal proline content to estimate the tannin intakes of these three forest-dwelling species in a forest in Hokkaido. In the autumn, estimated tannin intakes increased significantly in the Apodemus species, but not in M. rufocanus. We speculated that an increase in tannin intake during autumn may result from consumption of tannin-rich acorns. This hypothesis was consistent with population fluctuation patterns of the three species, which were well-synchronized with acorn abundance for the Apodemus species but not for M. rufocanus.

Tannins in plant-herbivore interactions

Tannins are the most abundant secondary metabolites made by plants, commonly ranging from 5% to 10% dry weight of tree leaves. Tannins can defend leaves against insect herbivores by deterrence and/or toxicity. Contrary to early theories, tannins have no effect on protein digestion in insect herbivores. By contrast, in vertebrate herbivores tannins can decrease protein digestion. Tannins are especially prone to oxidize in insects with high pH guts, forming semiquinone radicals and quinones, as well as other reactive oxygen species. Tannin toxicity in insects is thought to result from the production of high levels of reactive oxygen species. Tannin structure has an important effect on biochemical activity. Ellagitannins oxidize much more readily than do gallotannins, which are more oxidatively active than most condensed tannins. The ability of insects to tolerate ingested tannins comes from a variety of biochemical and physical defenses in their guts, including surfactants, high pH, antioxidants, and a protective peritrophic envelope that lines the midgut. Most work on the ecological roles of tannins has been correlative, e.g., searching for negative associations between tannins and insect performance. A greater emphasis on manipulative experiments that control tannin levels is required to make further progress on the defensive functions of tannins. Recent advances in the use of molecular methods has permitted the production of tannin-overproducing transgenic plants and a better understanding of tannin biosynthetic pathways. Many research areas remain in need of further work, including the effects of different tannin types on different types of insects (e.g., caterpillars, grasshoppers, sap-sucking insects).

Seed size, more than nutrient or tannin content, affects seed caching behavior of a common genus of Old World rodents

Scatter-hoarding rodents play an important dispersal role for many large-seeded plants. Seed traits affect their foraging behavior; however, it is difficult to isolate their effects because of the covariance among traits. Here, we used artificial seeds to partition the effects of seed size, tannin content, and nutrient content on scatter-hoarding rodents in a natural pine forest in Northwest Yunnan, China. Apodemus, a common genus of Old World rodents, consistently consumed small seeds in situ but removed medium-sized seeds (1.2-2.5 cm in diameter) and transported bigger seeds farther. Seed nutrient and tannin contents also significantly influenced rodents' behavior, but response varied substantially between years. Rodent behavior did involve some aspects of multivariate optimization. Our results strongly indicate that seed size is a decisive factor for scatter-hoarding rodents in the choice between seed predation and dispersal, while nutrient and tannin content played a less consistent role, possibly responding to confounding factors in the community. This result also has important implications for seed production by trees, which can improve the probability of long-distance dispersal of high-quality offspring by simply making them larger. The ability to tease apart the relative influence of different seed traits on the behavior of predators provides powerful insight into this important coevolutionary dynamic.

Anti-cancer, anti-diabetic and other pharmacologic and biological activities of penta-galloyl-glucose

1, 2, 3, 4, 6-penta-O-galloyl-beta-D-glucose (PGG) is a polyphenolic compound highly enriched in a number of medicinal herbals. Several in vitro and a handful of in vivo studies have shown that PGG exhibits multiple biological activities which implicate a great potential for PGG in the therapy and prevention of several major diseases including cancer and diabetes. Chemically and functionally, PGG appears to be distinct from its constituent gallic acid or tea polyphenols. For anti-cancer activity, three published in vivo preclinical cancer model studies with PGG support promising efficacy to selectively inhibit malignancy without host toxicity. Potential mechanisms include anti-angiogenesis; anti-proliferative actions through inhibition of DNA replicative synthesis, S-phase arrest, and G(1) arrest; induction of apoptosis; anti-inflammation; and anti-oxidation. Putative molecular targets include p53, Stat3, Cox-2, VEGFR1, AP-1, SP-1, Nrf-2, and MMP-9. For anti-diabetic activity, PGG and analogues appear to improve glucose uptake. However, very little is known about the absorption, pharmacokinetics, and metabolism of PGG, or its toxicity profile. The lack of a large quantity of highly pure PGG has been a bottleneck limiting in vivo validation of cancer preventive and therapeutic efficacies in clinically relevant models.

Saliva characteristics and individual sensitivity to phenolic astringent stimuli

Astringency sensation is due to interactions between salivary proteins and phenols and is based on an increased-friction mechanism. Modifications to the profile of salivary proteins and their concentration could affect tannin/protein reactions and hence the intensity of perceived astringency. Salivary characteristics of 65 subjects were compared after abstention from phenol-containing food and immediately after ingestion of tannic acid. The effect of stimulation on saliva characteristics was expressed in terms of D value, computed as the arithmetic difference between values found in saliva samples from the 2 conditions. Based on D values, subjects were clustered in two groups. Cluster 1 (Cl1, 53 cases) was characterized by low D values thus indicating that the basal saliva condition was quickly restored in these subjects. Cluster 2 (Cl2) was composed of 12 subjects whose basal salivary condition was not quickly restored, particularly in terms of salivary protein concentration and profile and saliva haze-forming capacity. Sensory data showed that subjects capable of maintaining constant saliva characteristics were less sensitive to astringent stimuli than subjects in which the same stimulations induced significant saliva modifications. The results suggest that a large proportion of the population are able to maintain their salivary protein concentration and simultaneously intercept and inactivate dietary tannins.

Bioaccessibility of phenols in common beans ( Phaseolus vulgaris L.) and iron (Fe) availability to Caco-2 cells

Samples of common and biofortified beans ( Phaseolus vulgaris ), both raw and cooked (autoclaved at 120 degrees C for 20 min) were analyzed for their polyphenol composition. Polyphenols were identified via HPLC-UV/diode array detection. Cooking favored the extraction of polyphenols without the need of a hydrolysis step, a fact that is of interest because this is the usual form in which beans are consumed. The main differences between white and colored beans were the presence of free kaempferol (13.5-29.9 microg g(-1)) and derivatives (kaempferol-3-O-glucoside) (12.5-167.5 microg g(-1)), only in red and black beans. An in vitro digestion (pepsin, pH2; pancreatin-bile extract, pH 7) was applied to beans to estimate bioaccessibility of individual polyphenols. Kaempferol from seed coats exhibited high bioaccessibility (45.4-62.1%) and a potent inhibitor effect on Fe uptake at concentrations ranging from 0.37 to 1.30 microM. Caco-2 cell ferritin formation was used to evaluate Fe uptake. Cell Fe uptake was significant only from white beans.

Iron bioavailability to piglets from red and white common beans (Phaseolus vulgaris)

Polyphenols in foods may chelate dietary Fe and lower its bioavailability. Concentrations of phenols are higher in red beans than in white beans. The aim of this study was to compare iron bioavailabilities from red and white beans in a piglet hemoglobin repletion model. Fe deficient cross bred piglets (Hampshire x Landrace x Yorkshire) were used. Nutritionally balanced diets (except for Fe) were formulated to contain 50% precooked, dehydrated beans (either small red or Great Northern white). At age 5 weeks, the piglets were assigned to two groups and fed diets containing either red or white beans for 4 weeks. Weight and hemoglobin (Hb) concentrations were monitored weekly. Feed intakes were measured daily. Hemoglobin repletion efficiency (HRE) was calculated as the gain in total body hemoglobin Fe (Hb-Fe) divided by Fe intake. Hb concentrations, Hb-Fe gains, and HRE were not different between the groups at any time point ( p > 0.05). HRE values in the red bean group were 50% in the first week and 30% over the entire 4 week period. In the white bean group, they were 56 and 26%, respectively. Proline-rich protein mRNA concentrations in parotid glands were higher in the red bean group compared to the white bean group. These results show that iron bioavailabilities from red and white beans are similar and suggest that pigs adapt to the inhibitory effects of polyphenols on iron absorption by increasing the secretion of protective proline-rich proteins in the saliva.

Differential apoptosis by gallotannin in human colon cancer cells with distinct p53 status

Gallotannin (GT), a plant polyphenol, has shown anticarcinogenic activities in several animal models including colon cancer. In our previous study, we showed that GT inhibits 1,2-dimethylhydrazine-induced colonic aberrant crypt foci and tumors in Balb/c mice, thus supporting a role for GT as a chemopreventive agent in colon cancer. However, at the molecular level, GT's mechanism of chemoprevention is still unclear. In this study, we aim at identifying GT's potential molecular mechanisms of action in in vitro studies. We show that GT differentially inhibits the growth of two isogenic HCT-116 (p53+/+, p53-/-) human colon cancer cells versus normal human intestinal epithelial cells (FHs 74Int). DNA flow cytometric analysis showed that GT induced S-phase arrest in both HCT-116 cell lines. Cell-cycle arrest in p53 (+/+) cells was associated with an increase in p53 protein levels and p21 transcript and protein levels. The inhibition of cell-cycle progression of HCT-116 p53 (+/+) cells by GT correlated with a reduction in the protein levels of cyclin D(1), pRb, and the Bax/Bcl-2 ratio. Although GT did not induce apoptosis in p53 (+/+) cells, a significant induction of apoptosis was observed in p53 (-/-) cells as shown by TUNEL staining and flow cytometry analysis. Apoptosis induction in p53 (-/-) cells was associated with a significant increase in Bax/Bcl-2 protein levels. Our results demonstrate that GT inhibits the growth of HCT-116 colon cancer cells in a p53-independent manner but exhibits differential sensitivity to apoptosis induction in HCT-116 cells with distinct p53 status.

Protein binding and astringent taste of a polymeric procyanidin, 1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranose, castalagin, and grandinin

The objective of the present investigation was to examine the oral astringency and protein-binding activity of four structurally well-defined tannins, namely, procyanidin [epicatechin16(4-->8)catechin], pentagalloyl glucose (1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranose), castalagin, and grandinin, representing the three main structural categories of tannins, the proanthocyanidins, the gallotannins, and the ellagitannins. Astringency threshold and dos/response were determined by the half-tongue test using a trained human panel. Protein-binding stoichiometry and relative affinity were determined using radioiodinated bovine serum albumin in precipitation or competitive binding assays. Procyanidin and pentagalloyl glucose were perceived as highly astringent compounds and had relatively steep dose/response curves, but castalagin and grandinin had a lower mass threshold for detection. In vitro, procyanidin was the most effective protein-precipitating agent and grandinin the least. Increasing the temperature increased protein precipitation by the hydrolyzable tannins, especially grandinin. All four polyphenols had higher relative affinities for proline-rich proteins than for bovine serum albumin.

Role of Tannin-Binding Salivary Proteins and Tannase-Producing Bacteria in the Acclimation of the Japanese Wood Mouse to Acorn Tannins

We studied the defense mechanisms against the negative effects of tannins in acorns by using the Japanese wood mouse (Apodemus speciosus) and acorns of a Japanese deciduous oak Quercus crispula, which contain 9.9% tannins on a dry weight basis. For the experiment, we allocated 26 wood mice into two groups: acclimated (N = 12) and nonacclimated (N = 14). Mice in the nonacclimated group were fed only acorns for 10 d after 4 wk of receiving a tannin-free diet. In contrast, mice in the acclimated group received ca. 3 g acorns daily in addition to the tannin-free diet for the first 4 wk, then they were fed only acorns for 10 d. Body weight, food intake, and digestibility were monitored. In addition, the amount of salivary proline-rich proteins (PRPs) and abundance of tannase-producing bacteria (TPB) in the feces of mice were measured. Of the 14 mice in the nonacclimated group, 8 died, whereas only 1 of the 12 in the acclimated group died. During the first 5 d of feeding acorns only, mice in the nonacclimated group lost, on average, 17.5% of their body mass, while those in the acclimated group lost only 2.5%. Food intake, dry matter digestibility, and nitrogen digestibility were higher in the acclimated group than in the nonacclimated group. The results indicate that wood mice can mitigate the negative effects of tannins by acclimation. Path analysis revealed that increased secretion of PRPs and abundance of Lactobacillus type of TPB might explain the acclimation to tannins.

Application of Pharmacological Approaches to Plant–Mammal Interactions

The dominant theory in the field of mammalian herbivore-plant interactions is that intake, and therefore tolerance, of plant secondary metabolites (PSMs) is regulated by mechanisms that reduce absorption and increase detoxification of PSMs. Methods designed by pharmacologists to measure detoxification enzyme activity, metabolite excretion, and most recently, drug absorption, have been successfully applied by ecologists to study PSM intake in a variety of mammalian study systems. Here, we describe several pharmacological and molecular techniques used to investigate the fate of drugs in human that have potential to further advance knowledge of mammalian herbivore-plant interactions.

Salivary Proteins as a Defense Against Dietary Tannins

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

Woodrat (Neotoma) herbivores maintain nitrogen balance on a low-nitrogen, high-phenolic forage, Juniperus monosperma

The acquisition of adequate quantities of nitrogen is a challenge for herbivorous vertebrates because many plants are in low nitrogen and contain secondary metabolites that reduce nitrogen digestibility. To investigate whether herbivores maintain nitrogen balance on plant diets low in nitrogen and high in secondary compounds, we studied the effect of juniper (Juniperus monosperma) ingestion on the nitrogen balance of two species of herbivorous woodrats (Neotoma stephensi and N. albigula). These woodrat species feed on the foliage of juniper: N. stephensi is a juniper specialist, whereas N. albigula is a generalist that incorporates some juniper in its diet. Based on the nitrogen contents of the natural diets of these woodrats, we predicted that the generalist would be in negative nitrogen balance on a juniper diet whereas the specialist would not be affected. We found that both species of woodrat had low-nitrogen requirements (334.2 mg N/kg0.75/day) and that a diet of 50% juniper did not result in negative nitrogen balance for either species. However, excretion patterns of nitrogen were altered; on the 50% juniper diet, fecal nitrogen losses increased approximately 38% and urinary nitrogen losses were half that of the control diet. The results suggest that absorption and detoxification of juniper secondary compounds may be more important for restricting juniper intake by the generalist than nitrogen imbalance.