Bitter Taste Receptors (T2Rs) are Sentinels that Coordinate Metabolic and Immunological Defense Responses

Abstract

In addition to being responsible for bitter taste, type 2 taste receptors (T2Rs) regulate endocrine, behavioral, and immunological responses. T2R agonists include indicators of incoming threats to metabolic homeostasis, pathogens, and irritants. This review will provide an overview of T2R-regulated processes throughout the body that function defensively. We propose a broader definition of T2Rs as chemosensory sentinels that monitor toxic, metabolic, and infectious threats and initiate defensive responses.

Graphical Abstract

Introduction to T2Rs

Type 2 taste receptors (T2Rs) are G-protein coupled receptors (GPCRs) that were first identified in the mouth as bitter taste receptors. In humans, the T2R gene family consists of 25 functional receptor-encoding genes, many of which are polymorphic. Since their discovery, T2Rs have been identified in tissues throughout the body [1, 2]. The sensory experience of bitter taste alerts animals that the potential food or drink in the mouth contains toxins or other threats. The intensity of bitterness determines whether the food is ingested or rejected. In this way, T2Rs are paramount to the rejection of potentially harmful substances, which protects against ingesting toxins and potential poisoning. T2R agonists comprise a set of diverse compounds that range from common plant compounds, e.g., alkaloids, to bacterial quorum sensing molecules. Beyond the oral cavity, the detection of these compounds by T2Rs triggers protective responses that include regulatory metabolic reactions in defense of homeostasis and classic immune reactions in defense against infection. T2Rs function as chemosensors that monitor the environment and trigger responses to a wide range of threats: metabolic dysregulation, toxins, and infections. We propose that T2Rs function throughout the body as sentinels that monitor environmental challenges and coordinate defensive endocrine, behavioral, and immunological responses.

Endocrine Responses

Metabolic homeostasis is necessary to maintain health and prevent disease. Although themselves not directly sensing ingested sugars, T2Rs nonetheless play an important role in the regulation of blood glucose homeostasis and possibly thyroid hormone levels, which regulate overall metabolic tone. These responses play protective and defensive roles when the body is challenged with metabolic dysregulation, which can lead to life threatening diseases.

Gastrointestinal tract and glucose homeostasis.

Although the consumption of food is essential for life, it also presents a significant challenge to the homeostasis of blood nutrient levels, especially the consumption of carbohydrates [3]. Glucose must be transported out of the blood to prevent hyperglycemia and, in the extreme, coma. Without signals from the mouth that glucose (or starch) has been ingested, the body has little opportunity to anticipate and prepare for the absorption of glucose into the blood. This is the major role of the cephalic-phase insulin response: to release anticipatory insulin in response to consuming sugars, preparing for the influx of glucose into the blood [4].

One metabolic role of T2Rs is in the enteroendocrine cells (EECs) of the gastrointestinal tract. EECs in the gut that express T2Rs orchestrate the release of glucagon like peptide-1 (GLP-1) in response to bitter taste stimuli consumed with food (see Figure 1) [5–8]. GLP-1 helps strengthen the pancreatic insulin response to the absorption of insulin-dependent nutrients, such as glucose. This reaction complements the endocrine role of taste tissue, which triggers the cephalic-phase insulin response. The main sources of glucose prior to agricultural grain production and sugar processing were starchy roots and tubers, which would only weakly generate a cephalic-phase insulin response, especially among those with low salivary amylase levels [9, 10]. Before domestication and selective breeding, many dietary tubers, for example potatoes, cassava, and yams, contained bitter-tasting T2R agonists, which could signal the ingestion of starch in the absence of glucose or sucrose. Absent high levels of free sugar in the mouth and following metabolism of the ingested starch, there would be a large influx of glucose into the bloodstream without strong anticipatory signaling. We propose that because T2R agonists are associated with carbohydrates in these plants, T2R-triggered GLP-1 release from EECs defends against the disruption of blood glucose homeostasis when T1R and metabolic signaling is weak. By stimulating GLP-1 release, T2Rs take on a defensive role against the dysglycemic “assault” that ingested starches mount against our blood sugar homeostasis.

Figure 1. Endocrine Homeostasis Defense Responses by T2Rs.

Enteroendocrine cells (EECs) in the small intestine express type 2 taste receptors or bitter taste receptors (T2Rs). T2R agonists in the gastrointestinal tract activate T2Rs and induce the secretion of glucagon-like peptide-1 (GLP-1) and cholecystokinin (CCK). GLP-1 triggers insulin release from the pancreas, which lowers blood glucose. CCK and GLP-1 reduce appetite and food intake through vagal nerve signaling and GLP-1 can act on the brain. Additionally, T2R agonists in the stomach inhibit motilin secretion, reducing hunger and gastric motility. In the thyroid, T2R agonists inhibit responses to TSH (thyroid-stimulating hormone), resulting in decreased tri-iodothyronine (T3) and thyroxine (T4), the active forms of thyroid hormone.

As rates of type 2 diabetes mellitus (T2DM) rapidly increase in the United States and worldwide, it is necessary to find methods of controlling the incretin response that is severely dysregulated with this disease. In the Roman Empire, bitter “digestif” drinks were used to prevent lethargy and improve digestion after large meals, possibly by regulating the incretin response [11]. In lab settings, T2R agonists, specifically quinine and berberine, significantly lower postprandial blood glucose by increasing both GLP-1 and insulin release [12, 13]. Chronic activation of T2Rs has also been shown to increase glucose tolerance and suppress a wide range of inflammatory markers in diet-induced obese mice [6]. A specific mutation in T2R9 has also been associated with increased risk of T2DM [14]. Dysregulation of glucose homeostasis and inflammation are hallmarks of T2DM and the possibility that chronic treatment with bitter taste stimuli can improve these systems needs further investigation.

Thyroid Gland and thyroid hormone regulation.

An additional role of T2Rs in the defense of endocrine homeostasis is via thyroid hormones. The thyroid and its hormones influence overall resting metabolic rate [15]. T2Rs found in the thyroid repress the normal responses of thyrocytes to thyroid-stimulating hormone (TSH) by decreasing calcium release and iodide efflux (see Figure 1). A polymorphism in T2R42, expressed in the thyroid, is associated with hyperthyroidism and elevated circulating levels of thyroid hormone [16]. Though the importance of T2Rs in thyroid regulation has not been explored deeply, there is evidence that T2Rs may play a role in the regulation of thyroid hormone levels and, therefore, the determination of basal metabolism.

Behavioral Responses

T2Rs play an important role in the regulation of taste-related behavioral responses such as spitting out, vomiting, coughing, and sneezing. T2Rs play an important role in the regulation of these behaviors, and all serve to defend the body from pathogens, toxins, or irritants. A wellrecognized function of T2Rs in the oral cavity is to trigger the rejection of strongly bitter-tasting toxins. Consuming bitter compounds can stimulate expectoration (spitting), rejection, and vomiting (see Figure 2) [17]. These behavioral responses to bitter stimuli protect animals from ingesting toxins or foods contaminated with pathogens. Similarly, the Inclusive Behavioral Immune System (IBIS) is the first line of defense against pathogens and infection, and includes sickness-associated behaviors, such as lying down, self-isolation, and anorexia [18]. Relatedly, intense bitter taste induces these sickness-associated behaviors. The behavioral responses that T2Rs generate comprise the initial defenses against consuming a dangerous foods and beverages.

Figure 2. Behavioral Defense Functions of T2Rs.

The Inclusive Behavioral Immune System increases resting behaviors associated with feelings of illness triggered by intense bitterness. Pathogens, toxins, and irritants that are T2R agonists can induce sneezing, coughing, and bronchodilation in the airways. These responses both protect against bronchoconstriction and actively expel the irritants out of the airways. In the stomach, T2R agonists induce vomiting, delay gastric emptying, and disrupt normal gastric rhythms. In the gastrointestinal tract, T2R-expressing cells accelerate the speed of bowel movements, enhance fluid secretion into the gastrointestinal lumen, and cause diarrhea. These mechanisms aim to speed the passage of possible pathogens through the GI tract for a lower chance of infection. In the urethra, T2R agonists induce urination and enhanced emptying of the bladder, discussed further in Immunological Responses.

Upper Airways.

In the upper airways, T2Rs can trigger behavioral responses that serve functions for the IBIS. In the nose, T2Rs in solitary chemosensory cells induce coughing and sneezing (see Figure 2) [19]. This behavioral response serves to clear both pathogens and irritants from the airways.

Lower Airways.

T2Rs exhibit a protective function in the lower airways by stimulating bronchodilation. Bronchoconstrictive conditions like asthma can be fatal if the airways cannot be dilated. T2R agonists can induce rapid and reversible bronchodilation by reducing reactivity to stimuli like histamine [20–24]. Stimulation of T2Rs in ciliated epithelial cells in the bronchi increases ciliary beating, a mechanism to clear irritants from the lungs [25]. Both of these responses protect against bronchoconstriction and irritation.

Gastrointestinal Tract.

Nausea and vomiting are strong sensory and behavioral responses, respectively, that decrease future consumption of the food associated with them [26]. T2R agonists can additionally decrease food intake and appetite by delaying gastric emptying and disrupting normal gastric rhythms [26–30]. This increases satiety and reduces the size of meals and the speed at which food is consumed, which has the potential to reduce the dumping of nutrients into the intestine, protecting homeostasis. Furthermore, it has been extensively reported that specific T2R agonists suppress appetite and decrease nutrient intake in subsequent meals while increasing satiation. Thus, T2Rs are behavioral regulators of food intake. Diarrheal responses to intestinal pathogens are an additional behavioral response that will be discussed below in the Immunology Section.

Immunological Responses

T2Rs also serve as regulatory sentinels for the immune system. T2Rs have been investigated recently for their roles in immune responses throughout the body. In contrast to the behavioral and endocrine responses, the T2R-regulated immune responses defend the body against infection-causing pathogens and maintain the balance of microbiota. T2Rs have been identified in both solitary chemosensory cells (SCCs) and macrophages in mucosal tissues that form an interface between the external and internal environment of an animal. T2Rs can be activated by several quorum sensing molecules produced by Gram negative bacteria during biofilm formation. By sensing the levels of quorum sensing molecules and other microbial byproducts, T2Rs monitor the presence of pathogens in a manner that resembles toll-like receptors [31]. Throughout these interface tissues, T2Rs initiate innate immune responses to pathogens and are essential to fending off infection.

Upper Airways.

T2Rs perform immune functions in the sinonasal cavity, where they are expressed in SCCs [32]. T2R38 is activated by a wide range of bacterial compounds, including the quorum-sensing molecules acyl homoserine lactones (AHL) and quinolones [31]. By monitoring and responding to the presence of quorum sensing molecules in the sinonasal cavity, T2Rs help maintain the balance of the nasal microbiota and prevent biofilm formation [33]. Activation of T2Rs by AHLs increases ciliary beat, nitric oxide production, and stimulates the secretion of antimicrobial peptides (AMPs) [34–36]. All of these responses form a complete innate immune reaction to defend against sinonasal infection and rhinosinusitis. The importance of T2Rs as immune sentinels in the sinuses is reinforced by studies which show that polymorphisms in T2R38 are associated with increased frequency and severity of chronic rhinosinusitis [34, 36–39].

Lower Airways.

In addition to coordinating protective asthma responses in the lungs, T2Rs begin innate immune responses in the lung epithelia in reaction to bacterial quorum sensing molecules. T2R activation induces bactericidal levels of nitric oxide production [34]. Tracheal tuft cells (a type of SCC) begin a type-2 immune response in reaction to T2R activation, similar to that of gastrointestinal tuft cells, as explained below [40]. Nitric oxide in the lungs also stimulates phagocytosis in lung macrophages, amplifying the immune response begun by lung epithelial cells. It was recently demonstrated that T2R activation in human lung macrophages stimulates production of nitric oxide and increases phagocytosis, an effect amplified by the nitric oxide production in lung epithelial cells [41, 42]. These data support the importance of T2Rs in the pulmonary immune response to bacterial infection.

Gingiva.

The oral gingiva also utilizes T2Rs in a similar manner to the sinonasal cavity, where T2Rs coordinate immune responses to Gram-negative bacteria and protect against accelerated alveolar bone loss, which results in loss of teeth [43]. Increased activation of T2Rs with denatonium benzoate increases the expression of antimicrobial peptides and reduces the severity of periodontitis. Conversely, when components of the canonical T2R signal transduction pathway (e.g., the G-protein α-gustducin) are removed, the gingival microbiota becomes pathogenic and accelerates alveolar bone loss [43].

Gastrointestinal Tract.

Diarrhea is widely associated with bitter medicines. For example many malarial treatments have this outcome and many bitter Ayurvedic herbs are classified as cathartics, used to accelerate bowel movements [44]. Bitter compounds activate T2Rs in the cells lining the intestinal lumen and consequently draw fluid into the intestinal lumen by inducing anion secretion [45]. T2Rs cause diarrhea and fluid secretion into the intestinal lumen, eliciting a protective flushing out of parasitic infections that generated the T2R agonists.

T2Rs are present in gastrointestinal tuft, goblet, and Paneth cells [2, 46–49]. In tuft cells, activation of T2Rs initiates a type 2 immune response, known for its feed-forward circuit that protects against parasitic infection [50]. This response is characterized by expression of interleukins-13 and −25, as well as proliferation of tuft and goblet cells [51]. Bitter agonists induce this reaction from the large intestinal epithelium, assisting with the elimination of parasites from the intestine.

Paneth cells and goblet cells differ from tuft cells in that their immune functions are bacterially directed. Paneth cells secrete a variety of antimicrobial peptides (AMPs), while goblet cells produce mucins and protect the intestinal epithelium. The presence of T2Rs and their signaling pathways in these cells suggests that T2Rs induce immune responses in Paneth and goblet cells [46, 47]. The immune responses from Paneth and goblet cells may also regulate the gut microbiome [8, 52, 53]. These intestinal colonists must be tended to maintain probiotic species and to cull pro-pathogenic species. T2Rs may play a role in this process.

T2Rs are involved with the regulation of intestinal inflammation and have been linked to gastrointestinal inflammatory diseases like Crohn’s disease and colitis. Loss of function of T2R signaling is associated with enhanced inflammation. Specifically, knockout of the T2Rassociated G-protein, α-gustducin, significantly worsens gut inflammation and symptoms of colitis in mice [54, 55].

Urethra.

T2Rs are present along with the ‘taste’ signal transduction pathway in brush cells found in the urethra. T2Rs function as immune sentinels here by monitoring microbial products that are T2R agonists and inducing the detrusor muscle reflex through the release of acetylcholine [56, 57]. This reflex enhances emptying of the bladder and assists with the elimination of bacteria from the urethra. This is another example of T2R-regulated innate immune responses involving flushing of pathogens from the body.

In all, T2Rs are increasingly reported as chemoreceptors central to immune functions throughout the body. This is particularly true of tissues that form an interface between the animal and the external environment.

Conclusion

As the functions of T2Rs are identified in diverse tissues, they are increasingly characterized as defensive or protective. In the defense of endocrine homeostasis, T2Rs help regulate the incretin responses and thyroid hormone production. T2Rs also coordinate diverse immune responses in tissues frequently exposed to pathogens. T2Rs are generalized sentinels of threats to our health and function to defend the body. These protective responses occur in metabolic, immunologic, and behavioral systems. We conclude that T2Rs function as defensive regulatory chemoreceptors that coordinate immunological, behavioral, and metabolic defense responses.

There are several future directions for the study of T2Rs, which have been identified in many additional tissues, from thymus, to dermis, to heart. The purpose of T2Rs in these tissues is unclear, but warrants exploration. Although mechanistic work has been undertaken with the role of T2Rs in responses to helminth infections in the intestine, the possible interactions among T2Rs and the microbiome is an exciting area awaiting investigation.

Figure 3. Immunological Defense Functions of T2Rs.

Solitary chemosensory cells (SCCs) in the sinonasal cavity and gingiva express T2Rs. Activation of T2Rs in these locations triggers the release of antimicrobial peptides like ßdefensin, production of nitric oxide, and induces reflexes like sneezing. In the nose, ciliary beat is increased, quickening the clearance of mucus. In the lower airways (trachea, bronchi, and bronchioles) T2R agonists, such as bacterial quorum sensing molecules (QSMs), trigger nitric oxide production, tuft and goblet cell hyperplasia, phagocytosis, and inflammation reduction. T2R-expressing tuft cells in the gastrointestinal tract begin a type-2 immune response when stimulated with T2R agonists. This response includes secretion of IL-25, activating type 2 innate lymphoid cells which promote tuft and goblet cell proliferation, as well as eosinophil activation. Paneth cells in the GI tract release antimicrobial peptides in response to T2R agonists. In the urethra, brush cells express T2Rs. When QSMs are present in the urethra, the detrusor muscle reflex is induced, enhancing emptying of the bladder and clearing pathogens from the urethra.