Whole-Brain Mapping of the Expression Pattern of T1R2, a Subunit Specific to the Sweet Taste Receptor

Artificial sweeteners and Type 2 Diabetes Mellitus: A review of current developments and future research directions

While artificial sweeteners are Generally Regarded as Safe (GRAS), the scientific community remains divided on their safety status. The previous assumption that artificial sweeteners are inert within the body is no longer valid. Artificial sweeteners, known for their high intense sweetness and low or zero calories, are extensively used today in food and beverage products as sugar substitutes and are sometimes recommended for weight management and Type 2 Diabetes Mellitus (T2DM) patients. The general omission of information about the concentration of artificial sweeteners on market product labels makes it challenging to determine the amounts of artificial sweeteners consumed by people. Despite regulatory authorization for their usage, such as from the United States Food and Drug Administration (FDA), concerns remain about their potential association with metabolic diseases, such as T2DM, which the artificial sweeteners were supposed to reduce. This review discusses the relationship between artificial sweetener consumption and the risk of developing T2DM. With the increasing number of recent scientific studies adding to the debate on this subject matter, we assessed recent literature and up-to-date evidence. Importantly, we highlight future research directions toward furthering knowledge in this field of study.

Aspartame enhances the scavenging activity of mice to low-dose Escherichia coli infection via strengthening macrophage phagocytosis caused by sweet taste receptor activation

Aspartame is the most common artificial sweetener and a famous sweet-taste receptor agonist. Macrophages are essential in the antibacterial system to maintain the stability of the intestinal environment. Recently, the sweet taste receptor has been found in macrophages. However, the effects of aspartame on macrophage phagocytosis in the gastrointestinal tract are little known. The current study sought to assess the influence of aspartame intake on the scavenging activity of mice to low-dose Escherichia coli infection and related mechanisms. Firstly, no inflammatory response or pathological injury was observed in the intestines of mice after oral administration of aspartame (25-100 mg/kg, i.g.) for 2 weeks. Subsequently, aspartame intake was found to enhance the scavenging activity of mice to low-dose E. coli infection. Similarly, aspartame dose-dependent strengthened the ability of RAW264.7 cells to phagocytose GFP-E.coli J96. Further mechanism evaluation reflected that aspartame could enhance macrophage phagocytosis, migration, and rearrangement via PLCβ-2/Ca2+/PKCβ/Rho A/ROCK1 pathway caused by sweet taste receptor activation. In conclusion, the present study, for the first time, demonstrated that aspartame could enhance the scavenging activity of mice to low-dose E. coli infection via strengthening macrophage phagocytic function through activating sweet taste receptor. It is then suggested that aspartame may affect the antibacterial activity of human gastrointestinal macrophages, and further studies need to validate these effects.

Mechanisms and Functions of Sweet Reception in Oral and Extraoral Organs

The oral detection of sugars relies on two types of receptor systems. The first is the G-protein-coupled receptor TAS1R2/TAS1R3. When activated, this receptor triggers a downstream signaling cascade involving gustducin, phospholipase Cβ2 (PLCβ2), and transient receptor potential channel M5 (TRPM5). The second type of receptor is the glucose transporter. When glucose enters the cell via this transporter, it is metabolized to produce ATP. This ATP inhibits the opening of KATP channels, leading to cell depolarization. Beside these receptor systems, sweet-sensitive taste cells have mechanisms to regulate their sensitivity to sweet substances based on internal and external states of the body. Sweet taste receptors are not limited to the oral cavity; they are also present in extraoral organs such as the gastrointestinal tract, pancreas, and brain. These extraoral sweet receptors are involved in various functions, including glucose absorption, insulin release, sugar preference, and food intake, contributing to the maintenance of energy homeostasis. Additionally, sweet receptors may have unique roles in certain organs like the trachea and bone. This review summarizes past and recent studies on sweet receptor systems, exploring the molecular mechanisms and physiological functions of sweet (sugar) detection in both oral and extraoral organs.

Effects of non-nutritive sweeteners on growth and intestinal health by regulating hypothalamic RNA profile and ileum microbiota in guinea pigs

BACKGROUND

Non-nutritive sweeteners (NNS) are commonly used in sweetened foods and beverages; however their role in metabolic regulation is still not clear. In this experiment, we used guinea pigs as an animal model to study the effect of NNS on body growth and intestinal health by modifying gut microbiota and hypothalamus-related proteins.

RESULTS

For a 28-day feeding experiment a total of 40 guinea pigs were randomly divided into four groups, one control (CN) group and three treatments, in which three NNS were added to the diet: rebaudioside A (RA, 330 mg kg-1), sodium saccharin (SS, 800 mg kg-1), and sucralose (TGS, 167 mg kg-1), respectively. The TGS group exhibited significantly reduced food consumption in comparison with the CN group (P < 0.05) whereas the RA group showed increased food consumption in comparison with the CN group (P < 0.05). Notably, Taste receptor type 1 subunit 2 (T1R2) expression in the hypothalamus was significantly higher in the RA group than in the CN group (P < 0.05). The mRNA expressions of appetite-stimulated genes arouti-related neuropeptide (AGRP), neuropeptide Y (NPY), and thyroid stimulating hormone (TSHB) were significantly higher than those in the CN group (P < 0.05) but mRNA expressions of appetite-suppressed genes tryptophan hydroxylase 2(THP2) were significantly lower in the TGS group (P < 0.05). Furthermore, NNS in the guinea pig diets (RA, SS, TGS) significantly increased the relative abundance of Muribaculaceae but decreased the relative abundance of Clostridia_vadin BB60 in comparison with the CN group (P < 0.05). We also found that dietary supplementation with RA also significantly altered the relative abundance of Lactobacillus.

CONCLUSION

Our finding confirmed that dietary supplementation with RA and TGS affected body growth and intestinal health by modulating hypothalamic RNA profiles and ileum microbiota, suggesting that NNS should be included in guinea-pig feeding. © 2024 Society of Chemical Industry.

Comprehensive mapping of Epithelial Na+ channel α expression in the mouse brain

Epithelial sodium channel (ENaC) is responsible for regulating Na+ homeostasis. While its physiological functions have been investigated extensively in peripheral tissues, far fewer studies have explored its functions in the brain. Since our limited knowledge of ENaC's distribution in the brain impedes our understanding of its functions there, we decided to explore the whole-brain expression pattern of the Scnn1a gene, which encodes the core ENaC complex component ENaCα. To visualize Scnn1a expression in the brain, we crossed Scnn1a-Cre mice with Rosa26-lsl-tdTomato mice. Brain sections were subjected to immunofluorescence staining using antibodies against NeuN or Myelin Binding Protein (MBP), followed by the acquisition of confocal images. We observed robust tdTomato fluorescence not only in the soma of cortical layer 4, the thalamus, and a subset of amygdalar nuclei, but also in axonal projections in the hippocampus and striatum. We also observed expression in specific hypothalamic nuclei. Contrary to previous reports, however, we did not detect significant expression in the circumventricular organs, which are known for their role in regulating Na+ balance. Finally, we detected fluorescence in cells lining the ventricles and in the perivascular cells of the median eminence. Our comprehensive mapping of Scnn1a-expressing cells in the brain will provide a solid foundation for further investigations of the physiological roles ENaC plays within the central nervous system.

The Effects of Artificial Sweeteners on Intestinal Nutrient-Sensing Receptors: Dr. Jekyll or Mr. Hyde?

The consumption of artificial and low-calorie sweeteners (ASs, LCSs) is an important component of the Western diet. ASs play a role in the pathogenesis of metabolic syndrome, dysbiosis, inflammatory bowel diseases (IBDs), and various inflammatory conditions. Intestinal nutrient-sensing receptors act as a crosstalk between dietary components, the gut microbiota, and the regulation of immune, endocrinological, and neurological responses. This narrative review aimed to summarize the possible effects of ASs and LCSs on intestinal nutrient-sensing receptors and their related functions. Based on the findings of various studies, long-term AS consumption has effects on the gut microbiota and intestinal nutrient-sensing receptors in modulating incretin hormones, antimicrobial peptides, and cytokine secretion. These effects contribute to the regulation of glucose metabolism, ion transport, gut permeability, and inflammation and modulate the gut-brain, and gut-kidney axes. Based on the conflicting findings of several in vitro, in vivo, and randomized and controlled studies, artificial sweeteners may have a role in the pathogenesis of IBDs, functional bowel diseases, metabolic syndrome, and cancers via the modulation of nutrient-sensing receptors. Further studies are needed to explore the exact mechanisms underlying their effects to decide the risk/benefit ratio of sugar intake reduction via AS and LCS consumption.

Saccharin and aspartame excite rat retinal neurons

Retinal sensitivity to a variety of artificial sweeteners was tested by monitoring changes in internal free calcium in isolated retinal neurons using Fluo3. Several ligands, including aspartame and saccharin elevated internal free calcium. The effects of these ligands were mediated by both ligand-gated membrane channels and G-protein coupled receptors. We explored the receptors responsible for this phenomenon. Surprisingly, mRNA for subunits of the sweet taste receptor dimer (T1R2 and T1R3) were found in retina. Interestingly, knockdown of T1R2 reduced the response to saccharin but not aspartame. But TRPV1 channel antagonists suppressed the responses to aspartame. The results indicate that artificial sweeteners can increase internal free calcium in the retinal neurons through multiple pathways. Furthermore, aspartame reduced the b-wave, but not the a-wave, of the electroretinogram, indicating disruption of communication between photoreceptors and second order neurons.

Taste Receptors beyond Taste Buds

Taste receptors are responsible for detecting their ligands not only in taste receptor cells (TRCs) but also in non-gustatory organs. For several decades, many research groups have accumulated evidence for such “ectopic” expression of taste receptors. More recently, some of the physiologic functions (apart from taste) of these ectopic taste receptors have been identified. Here, we summarize our current understanding of these ectopic taste receptors across multiple organs. With a particular focus on the specialized epithelial cells called tuft cells, which are now considered siblings of type II TRCs, we divide the ectopic expression of taste receptors into two categories: taste receptors in TRC-like cells outside taste buds and taste receptors with surprising ectopic expression in completely different cell types.