Expression patterns of taste receptor type 1 subunit 3 and α-gustducin in the mouse testis during development

Taste receptor T1R3 regulates testosterone synthesis via the cAMP-PKA-SP1 pathway in testicular Leydig cells

Taste receptor type 1 subunit 3 (T1R3) is a G protein-coupled receptor encoded by the TAS1R3 gene that can be specifically activated by certain sweeteners or umami agents for sweet/umami recognition. T1R3 is a potential target for regulating male reproduction. However, studies on the impact of non-nutritive sweeteners on reproduction are limited. In the present study, we evaluated the impact of the non-nutritive sweeteners (saccharin sodium, sucralose and acesulfame-K) on testosterone synthesis in testicular Leydig cells of Xiang pigs by comparing the relative abundance of mRNA transcripts and protein expression of T1R3, steroidogenic related factors, and intracellular cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), as well as testosterone levels using Western blotting, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA). To clarify the specific mechanism, a dual luciferase assay was used to uncover the relationship between the transcription factors and steroidogenic enzyme. The acute intratesticular injection of a typical non-nutritive sweeteners was conducted to verify this impact in mouse. The results showed that saccharin sodium not only enhanced T1R3 expression in Leydig cells of Xiang pigs, but also caused significant increases in testosterone, cAMP, PKA, phosphorylation of specificity protein 1 (p-SP1), total protein of specificity protein 1 (SP1), steroidogenic acute regulatory protein (StAR), and 3β-hydroxysteroid dehydrogenase type 1 (3β-HSD1) (P < 0.05). Similarly, treatment of Leydig cells with sucralose and acesulfame-K also increased testosterone level, protein expression of T1R3, 17-α-hydroxylase/17, 20-lyase (CYP17A1), and 3β-HSD1 (P < 0.05). Treatment with SQ22536 (an adenylate cyclas inhibitor) or H89 (a PKA inhibitor) significantly reduced saccharin sodium-induced protein levels of p-SP1, StAR, CYP17A1, and 3β-HSD1 (P < 0.05). In addition, a dual luciferase assay further demonstrated that SP1 significantly increased the promoter activity of CYP17A1 (P < 0.05). When mouse testes were injected with saccharin sodium, T1R3, p-SP1, CYP17A1, and 3β-HSD1 were upregulated, leading to a significant testicular increase in testosterone and cAMP levels (P < 0.05). These results suggest a mechanism by which the taste receptor T1R3 regulates testosterone production, and this mechanism may be linked to the cAMP-PKA pathway. Understanding the interrelationship between T1R3 and the cAMP-PKA-SP1 pathway contributes to clarify the regulatory mechanisms of male reproduction.

Taste receptor type 1 member 3 is required for the fertility of male mice

Male infertility is a global health concern. However, its underlying pathophysiology remains unclear. Taste receptor type 1 member 3 (TAS1R3) is highly expressed in the testes, indicating its potential involvement in male fertility. Using wild-type and Tas1r3 knockout (KO) mice, we investigated whether TAS1R3 modulates male reproductive function. Tas1r3 KO mice exhibited reduced male fertility compared to WT mice, with fewer live pups per litter and a delayed first litter. Testicular transcriptome analysis indicated suppressed PKA/CREB/StAR signaling-mediated testosterone synthesis in Tas1r3 KO mice. In silico single-cell RNA sequencing revealed considerably higher Tas1r3 expression in Leydig cells than in other testicular cell subtypes. An in vitro study validated that Tas1r3 knockdown downregulated the expression of Creb1 and steroidogenic genes in Leydig cells. Our results suggest that testicular TAS1R3 is intricately involved in male reproduction via the PKA/CREB/StAR signaling pathway, highlighting its potential as a promising target for addressing male infertility.

Sweet taste receptors play roles in artificial sweetener-induced enhanced urine output in mice

Sweet taste receptors found in oral and extra oral tissues play important roles in the regulation of many physiological functions. Studies have shown that urine volume increases during the lifetime exposure to artificial sweeteners. However, the detailed molecular mechanism and the general effects of different artificial sweeteners exposure on urine volume remain unclear. In this study, we investigated the relationship between urinary excretion and the sweet taste receptor expression in mice after three artificial sweeteners exposure in a higher or lower concentration via animal behavioral studies, western blotting, and real-time quantitative PCR experiment in rodent model. Our results showed that high dose of acesulfame potassium and saccharin can significantly enhance the urine output and there was a positive correlation between K+ and urination volume. The acesulfame potassium administration assay of T1R3 knockout mice showed that artificial sweeteners may affect the urine output directly through the sweet taste signaling pathway. The expression of T1R3 encoding gene can be up-regulated specifically in bladder but not in kidney or other organs we tested. Through our study, the sweet taste receptors, distributing in many tissues as bladder, were indicated to function in the enhanced urine output. Different effects of long-term exposure to the three artificial sweeteners were shown and acesulfame potassium increased urine output even at a very low concentration.

Sweet Taste Receptor T1R3 Expressed in Leydig Cells Is Closely Related to Homeostasis of the Steroid Hormone Metabolism Profile

Taste receptor type 1 subunit 3 (T1R3) is initially expressed in mammal tongue for recognition and response of sweet/umami tastants and is critical to nutrient absorption, even endocrine. In this study, down-regulation of related steroidogenic enzymes such as StAR, 3β-HSD, CYP17A1, and 17β-HSD with the decrease of T1R3 expression was found in Leydig cells treated by a T1R3 inhibitor (lactisole). The abundances of progesterone, 17a-hydroxyprogesterone, androstenedione, testosterone, and deoxycorticosterone were down-regulated by 2.3, 3.5, 1.4, 1.6, and 2.2 times, respectively, after T1R3 inhibition. In addition, opposite results were found in saccharin sodium treatment. T1R3 activation contributed to intracellular cyclic adenosine monophosphate (cAMP) accumulation (14.41 ± 0.58 vs 20.21 ± 0.65) and increased testosterone (20.31 ± 3.49 vs 50.01 ± 7.44) and steroidogenic metabolite levels. Coadministration of human chorionic gonadotropin and saccharin sodium resulted in elevating the testosterone and cAMP levels and enhancing the expression levels of steroidogenic-related factors. Similarly, intratesticular injection of lactisole and saccharin sodium further confirmed that T1R3 inhibition/activation affected the expression of related steroidogenic enzymes and the testosterone levels in mice. The above findings suggest that T1R3 plays a role in testicular steroidogenesis.

Taste receptors affect male reproduction by influencing steroid synthesis

For the male genetic materials to reach and fertilize the egg, spermatozoa must contend with numerous environmental changes in a complex and highly sophisticated process from generation in the testis, and maturation in the epididymis to capacitation and fertilization. Taste is an ancient chemical sense that has an essential role in the animal's response to carbohydrates in the external environment and is involved in the body's energy perception. In recent years, numerous studies have confirmed that taste signaling factors (taste receptor families 1, 2 and their downstream molecules, Gα and PLCβ2) are distributed in testes and epididymis tissues outside the oral cavity. Their functions are directly linked to spermatogenesis, maturation, and fertilization, which are potential targets for regulating male reproduction. However, the specific signaling mechanisms of the taste receptors during these processes remain unknown. Herein, we review published literature and experimental results from our group to establish the underlying signaling mechanism in which the taste receptor factors influence testosterone synthesis in the male reproduction.

Drinking Non-nutritive Sweetness Solution of Sodium Saccharin or Rebaudioside a for Guinea Pigs: Influence on Histologic Change and Expression of Sweet Taste Receptors in Testis and Epididymis

Saccharin sodium and rebaudioside A are extensively used as non-nutritive sweeteners (NNSs) in daily life. NNSs elicit a multitude of endocrine influences on animals, differing across species and chemically distinct sweeteners, whose exposure induce activation of sweet taste receptors in oral and extra-oral tissues with consequences of metabolic changes. To evaluate the influence of NNSs on histologic change and expression of sweet taste receptors in testis and epididymis of young male guinea pigs, thirty 4-week-old male guinea pigs with body weight 245.73 ± 6.02 g were randomly divided into five groups (n = 6) and received normal water (control group) and equivalent sweetness low dose or high dose of sodium saccharin (L-SS, 1.5 mM or H-SS, 7.5 mM) or rebaudioside A (L-RA, 0.5 mM or H-RA, 2.5 mM) solution for 28 consecutive days. The results showed that the relative testis weight in male guinea pig with age of 56 days represented no significant difference among all groups; in spite of heavier body weight in L-SS and H-RA, NNS contributes no significant influence on serum testosterone and estradiol level. Low-dose 0.5 mM rebaudioside A enhanced testicular and epididymal functions by elevating the expressions of taste receptor 1 subunit 2 (T1R2) and gustducin α-subunit (GNAT3), and high-dose 7.5 mM sodium saccharin exerted adverse morphologic influences on testis and epididymis with no effect on the expression of T1R2, taste receptor 1 subunit 2 (T1R3), and GNAT3. In conclusion, these findings suggest that a high dose of sodium saccharin has potential adverse biologic effects on the testes and epididymis, while rebaudioside A is a potential steroidogenic sweetener for enhancing reproductive functions.

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Longitudinal Expression of Testicular TAS1R3 from Prepuberty to Sexual Maturity in Congjiang Xiang Pigs

Simple Summary

Taste receptor type 1 subunit 3 (T1R3), a sweet/umami taste receptor, is widely expressed from the tongue to the testis, and testis expression is associated with male sterility. In Congjiang Xiang pigs, T1R3 is expressed in elongating/elongated spermatids and Leydig cells in a stage-dependent manner during postnatal development and the spermatogenic cycle. T1R3 may contribute to regulation of spermatid differentiation and Leydig cell function, and may therefore help limit the incidence of various male reproductive pathologies.

Abstract

Testicular expression of taste receptor type 1 subunit 3 (T1R3), a sweet/umami taste receptor, has been implicated in spermatogenesis and steroidogenesis in mice. We explored the role of testicular T1R3 in porcine postnatal development using the Congjiang Xiang pig, a rare Chinese miniature pig breed. Based on testicular weights, morphology, and testosterone levels, four key developmental stages were identified in the pig at postnatal days 15–180 (prepuberty: 30 day; early puberty: 60 day; late puberty: 90 day; sexual maturity: 120 day). During development, testicular T1R3 exhibited stage-dependent and cell-specific expression patterns. In particular, T1R3 levels increased significantly from prepuberty to puberty (p < 0.05), and expression remained high until sexual maturity (p < 0.05), similar to results for phospholipase Cβ2 (PLCβ2). The strong expressions of T1R3/PLCβ2 were observed at the cytoplasm of elongating/elongated spermatids and Leydig cells. In the eight-stage cycle of the seminiferous epithelium in pigs, T1R3/PLCβ2 levels were higher in the spermatogenic epithelium at stages II–VI than at the other stages, and the strong expressions were detected in elongating/elongated spermatids and residual bodies. The message RNA (mRNA) levels of taste receptor type 1 subunit 1 (T1R1) in the testis showed a similar trend to levels of T1R3. These data indicate a possible role of T1R3 in the regulation of spermatid differentiation and Leydig cell function.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

The neuroscience of sugars in taste, gut-reward, feeding circuits, and obesity

Throughout the animal kingdom sucrose is one of the most palatable and preferred tastants. From an evolutionary perspective, this is not surprising as it is a primary source of energy. However, its overconsumption can result in obesity and an associated cornucopia of maladies, including type 2 diabetes and cardiovascular disease. Here we describe three physiological levels of processing sucrose that are involved in the decision to ingest it: the tongue, gut, and brain. The first section describes the peripheral cellular and molecular mechanisms of sweet taste identification that project to higher brain centers. We argue that stimulation of the tongue with sucrose triggers the formation of three distinct pathways that convey sensory attributes about its quality, palatability, and intensity that results in a perception of sweet taste. We also discuss the coding of sucrose throughout the gustatory pathway. The second section reviews how sucrose, and other palatable foods, interact with the gut-brain axis either through the hepatoportal system and/or vagal pathways in a manner that encodes both the rewarding and of nutritional value of foods. The third section reviews the homeostatic, hedonic, and aversive brain circuits involved in the control of food intake. Finally, we discuss evidence that overconsumption of sugars (or high fat diets) blunts taste perception, the post-ingestive nutritional reward value, and the circuits that control feeding in a manner that can lead to the development of obesity.

Influences of non‐nutritive sweeteners on ovarian and uterine expression of T1R2 and T1R3 in peripubertal female guinea pigs

The underlying mechanism of taste receptor type 1 subunit 2 (T1R2) and taste receptor type 1 subunit 3 (T1R3) in the hormonal and reproductive system is still elusive. A low or a high dose of sweetness equivalent to that sodium saccharin (SS, 1.5 or 7.5 mM) and rebaudioside A (RA, 0.5 or 2.5 mM) was administered to young female guinea pigs for 28 consecutive days from the age of 28 days. Our results indicated that the sweet taste receptor subunit T1R2 was markedly expressed in the ovary and uterus of guinea pigs, whereas the T1R3 protein was expressed at a lower level. We elucidated that low-dose (1.5 mM) SS increased body and ovary weight associated with elevated ovarian expression of T1R2 in guinea pigs, unlike the high-dose (7.5 mM) SS, which suppressed the ovarian expression of T1R2 and resulted in certain adverse effects on ovarian and uterine morphology. Furthermore, high-dose (2.5 mM) RA increased the number of corpus luteum and elevated uterine expression of T1R2, whereas low-dose (0.5 mM) RA induced increased secretion of serum progesterone. Therefore, our findings suggest that we should pay more attention to the potential adverse effects, including increases in ovary weight, morphology changes, and increased progesterone that result from the dose-dependent regulation of T1R2 by non-nutritive sweeteners (NNS) in the ovaries and uteri of peripubertal females.

Sweet taste receptor agonists alter ovarian functions and ovarian cycles in aged mice

Saccharine sodium and rebaudioside A are low-calorie sweeteners, and the biologic effects of these sweeteners in rat ovaries are related to the activity of sweet taste receptors. Data on the impact and regulatory mechanisms underlying such sweeteners on the reproduction of aged animals are currently lacking. In the present study we assessed how the consumption of sweeteners affects the ovarian cycle, ovulation, biochemical indices, and other biologic functions. Thirty-six 1-year-old mice were randomly divided into 3 groups: a control (C) group receiving regular water, a saccharin sodium group receiving a 7.5 mM solution, and the rebaudioside A group receiving a 2.5 mM solution for 30 days. We observed no significant changes in body weights in any group. However, uterine weight in the rebaudioside A group significantly increased in diestrus, and we recorded a significant increase in the percentage of abnormal estrous cycles and the number of corpora lutea in the treatment groups. TUNEL staining and Immunoreactivity for the apoptosis-inducing factor (AIF) confirmed apoptosis in granulosa cells, oocyte, and corpus luteum. Serum glucose increased significantly in both treatment groups and there was a significant increase in cholesterol in the rebaudioside A group. Furthermore, the saccharin sodium-treated group exhibited elevated serum progesterone levels compared with the other groups. In conclusion, sweeteners manifested deleterious effects on reproductive indices in aged mice.

Mammalian Taste Cells Express Functional Olfactory Receptors

The peripheral taste and olfactory systems in mammals are separate and independent sensory systems. In the current model of chemosensation, gustatory, and olfactory receptors are genetically divergent families expressed in anatomically distinct locations that project to disparate downstream targets. Although information from the 2 sensory systems merges to form the perception of flavor, the first cross talk is thought to occur centrally, in the insular cortex. Recent studies have shown that gustatory and olfactory receptors are expressed throughout the body and serve as chemical sensors in multiple tissues. Olfactory receptor cDNA has been detected in the tongue, yet the presence of physiologically functional olfactory receptors in taste cells has not yet been demonstrated. Here we report that olfactory receptors are functionally expressed in taste papillae. We found expression of olfactory receptors in the taste papillae of green fluorescent protein-expressing transgenic mice and, using immunocytochemistry and real-time quantitative polymerase chain reaction experiments, the presence of olfactory signal transduction molecules and olfactory receptors in cultured human fungiform taste papilla (HBO) cells. Both HBO cells and mouse taste papilla cells responded to odorants. Knockdown of adenylyl cyclase mRNA by specific small inhibitory RNA and pharmacological block of adenylyl cyclase eliminated these responses, leading us to hypothesize that the gustatory system may receive olfactory information in the periphery. These results provide the first direct evidence of the presence of functional olfactory receptors in mammalian taste cells. Our results also demonstrate that the initial integration of gustatory and olfactory information may occur as early as the taste receptor cells.

Extraoral Taste Receptor Discovery: New Light on Ayurvedic Pharmacology

More and more research studies are revealing unexpectedly important roles of taste for health and pathogenesis of various diseases. Only recently it has been shown that taste receptors have many extraoral locations (e.g., stomach, intestines, liver, pancreas, respiratory system, heart, brain, kidney, urinary bladder, pancreas, adipose tissue, testis, and ovary), being part of a large diffuse chemosensory system. The functional implications of these taste receptors widely dispersed in various organs or tissues shed a new light on several concepts used in ayurvedic pharmacology (dravyaguna vijnana), such as taste (rasa), postdigestive effect (vipaka), qualities (guna), and energetic nature (virya). This review summarizes the significance of extraoral taste receptors and transient receptor potential (TRP) channels for ayurvedic pharmacology, as well as the biological activities of various types of phytochemical tastants from an ayurvedic perspective. The relative importance of taste (rasa), postdigestive effect (vipaka), and energetic nature (virya) as ethnopharmacological descriptors within Ayurveda boundaries will also be discussed.