Glucose potentiates β-cell function by inducing Tph1 expression in rat islets

Serotonin signaling to regulate energy metabolism: a gut microbiota perspective

Serotonin is one of the most potent gastrointestinal, peripheral, and neuronal signaling molecules and plays a key role in regulating energy metabolism. Accumulating evidence has shown the complex interplay between gut microbiota and host energy metabolism. In this review, we summarize recent findings on the role of gut microbiota in serotonin metabolism and discuss the complicated mechanisms by which serotonin, working in conjunction with the gut microbiota, affects total body energy metabolism in the host. Gut microbiota affects serotonin synthesis, storage, release, transport, and catabolism. In addition, serotonin plays an indispensable role in regulating host energy homeostasis through organ crosstalk and microbe-host communication, particularly with a wide array of serotonergic effects mediated by diverse serotonin receptors with unique tissue specificity. This fresh perspective will help broaden the understanding of serotonergic signaling in modulating energy metabolism, thus shedding light on the design of innovative serotonin-targeting strategies to treat metabolic diseases.

Genetic Polymorphism of Zinc Transporter-8 Gene (SLC30A8), Serum Zinc Concentrations, and Proteome Profiles Related to Type 2 Diabetes in Elderly

Background and Aims: Older adults are particularly susceptible to type 2 diabetes mellitus (T2DM) due to factors such as age-related insulin resistance, decreased physical activity, and deficiency of micronutrients, especially zinc. Studies have suggested that the risk allele of the zinc transporter 8 gene (SLC30A8) single-nucleotide poly-morphism (SNP) rs13266634 may contribute to T2DM susceptibility in addition to the complex protein interactions and alterations in the protein expressions and modifications associated with T2DM. This study was implemented to study the associations between SLC30A8 polymorphism, serum zinc levels, and the profiles of proteins differentially expressed in nondiabetic (n = 116) and prediabetic/diabetic (n = 149) subjects. Methods: SNP genotyping using TaqMan® assay and proteomic analysis by LC-MS/MS were performed in each group. Results: The results showed a higher risk of diabetes in individuals with the risk genotype CC accompanied by a low serum zinc level than in those with other genotypes. Profiles of proteins differentially expressed between the groups were identified and shown to be particularly associated with zinc-related functions, zinc transporter 8, and glucose metabolism. Proteins exclusively expressed in prediabetes/diabetes were assigned to a Reactome pathway related to zinc transporter and insulin processing. Conclusions: Our findings suggest that individuals carrying at least one copy of SLC30A8 rs13266634 accompanied by a low serum zinc level might be susceptible to T2DM, which could be due to alterations in insulin signaling and zinc metabolism. Understanding this relationship deepens our understanding of the genetic and molecular mechanisms underlying T2DM risk, offering potential targets for therapeutic intervention and prevention strategies.

The emerging roles of neuroactive components produced by gut microbiota

BACKGROUND

As a multifunctional ecosystem, the human digestive system contains a complex network of microorganisms, collectively known as gut microbiota. This consortium composed of more than 1013 microorganisms and Firmicutes and Bacteroidetes are the dominant microbes. Gut microbiota is increasingly recognized for its critical role in physiological processes beyond digestion. Gut microbiota participates in a symbiotic relationship with the host and takes advantage of intestinal nutrients and mutually participates in the digestion of complex carbohydrates and maintaining intestinal functions.

METHOD AND RESULT

We reviewed the neuroactive components produced by gut microbiota. Interestingly, microbiota plays a crucial role in regulating the activity of the intestinal lymphatic system, regulation of the intestinal epithelial barrier, and maintaining the tolerance to food immunostimulating molecules. The gut-brain axis is a two-way communication pathway that links the gut microbiota to the central nervous system (CNS) and importantly is involved in neurodevelopment, cognition, emotion and synaptic transmissions. The connections between gut microbiota and CNS are via endocrine system, immune system and vagus nerve.

CONCLUSION

The gut microbiota produces common neurotransmitters and neuromodulators of the nervous system. These compounds play a role in neuronal functions, immune system regulation, gastrointestinal homeostasis, permeability of the blood brain barrier and other physiological processes. This review investigates the essential aspects of the neurotransmitters and neuromodulators produced by gut microbiota and their implications in health and disease.

Serotonin transporter imaging agent as a probe for β-cells of pancreas

OBJECTIVE

Diabetes mellitus (DM) is one of the major diseases in the world. Nuclear medicine imaging may be able to detect functional status of pancreatic β cells in vivo, which might elucidate the pathological mechanisms of diabetes and develop individualized treatment plans. In this study, we evaluated the ability of [125I]ADAM, a serotonin transporter (SERT) imaging agent, as a probe for detecting pancreatic β-cell mass (BCM).

METHODS

In vitro cell studies were evaluated in INS-1 cells (rat islet β cell line). Biodistribution studies were performed in male normal Sprague-Dawley rats and alloxan-induced type 1 diabetes mellitus (T1DM) rats. Distribution and expression of SERT protein in pancreas of rats were also measured by immunofluorescence staining and Western blot.

RESULTS

In vitro cell studies showed that the concentration of [125I]ADAM associated with the INS-1 cells was increased gradually with incubation time, and the SERT specific inhibitor, escitalopram, exhibited the inhibitory effect on this interaction. Biodistribution studies also showed that the uptake of [125I]ADAM in the pancreas of normal rats was decreased in the presence of escitalopram. However, in the T1DM rat model with a significant β cells reduction, the uptake of pancreas was increased when compared with the control. Through immunofluorescence staining and Western blot, it was found that both the endocrine and exocrine cells of the normal pancreas expressed SERT protein, and the level of SERT protein in the exocrine cells was higher than islets. In the diabetic state, the expression of SERT in the exocrine cells was further increased.

CONCLUSIONS

The SERT imaging agent, [125I]ADAM, at the present form will not be suitable for imaging β cells, specifically because there were extraordinarily high non-specific signals contributing from the exocrine cells of pancreas. In addition, we noticed that the level of SERT expression was abnormally elevated in the diabetic state, which might provide an unexpected target for studying the pathological mechanisms of diabetes.

Exploring the serotonin-probiotics-gut health axis: A review of current evidence and potential mechanisms

Modulatory effects of serotonin (5-Hydroxytryptamine [5-HT]) have been seen in hepatic, neurological/psychiatric, and gastrointestinal (GI) disorders. Probiotics are live microorganisms that confer health benefits to their host. Recent research has suggested that probiotics can promote serotonin signaling, a crucial pathway in the regulation of mood, cognition, and other physiological processes. Reviewing the literature, we find that peripheral serotonin increases nutrient uptake and storage, regulates the composition of the gut microbiota, and is involved in mediating neuronal disorders. This review explores the mechanisms underlying the probiotic-mediated increase in serotonin signaling, highlighting the role of gut microbiota in the regulation of serotonin production and the modulation of neurotransmitter receptors. Additionally, this review discusses the potential clinical implications of probiotics as a therapeutic strategy for disorders associated with altered serotonin signaling, such as GI and neurological disorders. Overall, this review demonstrates the potential of probiotics as a promising avenue for the treatment of serotonin-related disorders and signaling of serotonin.

Delineating mouse β-cell identity during lifetime and in diabetes with a single cell atlas

Although multiple pancreatic islet single-cell RNA-sequencing (scRNA-seq) datasets have been generated, a consensus on pancreatic cell states in development, homeostasis and diabetes as well as the value of preclinical animal models is missing. Here, we present an scRNA-seq cross-condition mouse islet atlas (MIA), a curated resource for interactive exploration and computational querying. We integrate over 300,000 cells from nine scRNA-seq datasets consisting of 56 samples, varying in age, sex and diabetes models, including an autoimmune type 1 diabetes model (NOD), a glucotoxicity/lipotoxicity type 2 diabetes model (db/db) and a chemical streptozotocin β-cell ablation model. The β-cell landscape of MIA reveals new cell states during disease progression and cross-publication differences between previously suggested marker genes. We show that β-cells in the streptozotocin model transcriptionally correlate with those in human type 2 diabetes and mouse db/db models, but are less similar to human type 1 diabetes and mouse NOD β-cells. We also report pathways that are shared between β-cells in immature, aged and diabetes models. MIA enables a comprehensive analysis of β-cell responses to different stressors, providing a roadmap for the understanding of β-cell plasticity, compensation and demise.

Serotonin in the regulation of systemic energy metabolism

Serotonin is a well‐known neurotransmitter that is synthesized from the amino acid, tryptophan. To date, more than 14 different serotonin receptors have been discovered; they exist universally in our body and enable diverse biological functions in different organs. Central serotonin regulates mood and behavior, and impacts the systemic energy balance by decreasing appetite. A number of drugs that modulate central serotonin function (e.g., fenfluramine, sibutramine and lorcaserin) were approved and used as anti‐obesity drugs, but then later withdrawn due to adverse cardiovascular and carcinogenic effects. Over the past decade, the role of peripheral serotonin in regulating systemic energy metabolism has been extensively explored using tissue‐specific knockout animal models. By inhibiting the action of serotonin in liver and adipose tissues, hepatic steatosis was improved and lipid accumulation was mitigated, respectively. Recent findings show that modulation of the serotonergic system is a promising therapeutic target for metabolic diseases. This review summarizes the role of serotonin in regulating energy metabolism in different organs, and discusses the potential of serotonin modulation for treating metabolic diseases.

Chronically Elevated Exogenous Glucose Elicits Antipodal Effects on the Proteome Signature of Differentiating Human iPSC-Derived Pancreatic Progenitors

The past decade revealed that cell identity changes, such as dedifferentiation or transdifferentiation, accompany the insulin-producing β-cell decay in most diabetes conditions. Mapping and controlling the mechanisms governing these processes is, thus, extremely valuable for managing the disease progression. Extracellular glucose is known to influence cell identity by impacting the redox balance. Here, we use global proteomics and pathway analysis to map the response of differentiating human pancreatic progenitors to chronically increased in vitro glucose levels. We show that exogenous high glucose levels impact different protein subsets in a concentration-dependent manner. In contrast, regardless of concentration, glucose elicits an antipodal effect on the proteome landscape, inducing both beneficial and detrimental changes in regard to achieving the desired islet cell fingerprint. Furthermore, we identified that only a subgroup of these effects and pathways are regulated by changes in redox balance. Our study highlights a complex effect of exogenous glucose on differentiating pancreas progenitors characterized by a distinct proteome signature.

A Comparison of the Gluco-Regulatory Responses to High-Intensity Interval Exercise and Resistance Exercise

High-intensity interval exercise and resistance exercise both effectively lower blood glucose; however, it is not clear whether different regulatory mechanisms exist. This randomised cross-over study compared the acute gluco-regulatory and the physiological responses of high-intensity interval exercise and resistance exercise. Sixteen (eight males and eight females) recreationally active individuals, aged (mean ± SD) 22 ± 7 years, participated with a seven-day period between interventions. The high-intensity interval exercise trial consisted of twelve, 30 s cycling intervals at 80% of peak power capacity and 90 s active recovery. The resistance exercise trial consisted of four sets of 10 repetitions for three lower-limb exercises at 80% 1-RM, matched for duration of high-intensity interval exercise. Exercise was performed after an overnight fast, with blood samples collected every 30 min, for two hours after exercise. There was a significant interaction between time and intervention for glucose (p = 0.02), which was, on average (mean ± SD), 0.7 ± 0.7 mmol∙L⁻¹ higher following high-intensity interval exercise, as compared to resistance exercise. Cortisol concentration over time was affected by intervention (p = 0.03), with cortisol 70 ± 103 ng∙mL⁻¹ higher (p = 0.015), on average, following high-intensity interval exercise. Resistance exercise did not induce the acute rise in glucose that was induced by high-intensity interval exercise and appears to be an appropriate alternative to positively regulate blood glucose.

Deregulated Serotonin Pathway in Women with Morbid Obesity and NAFLD

Non-alcoholic fatty liver disease (NAFLD) extends from simple steatosis (SS) to non-alcoholic steatohepatitis (NASH). Peripheral serotonin (5-HT) has become as an important regulator of different metabolic pathways. 5-HT has been related to obesity and lipid accumulation in the liver. The objective of this study was to assess the relationship between the 5-HT signaling pathway and the degree of NAFLD, as well as to investigate whether peripheral 5-HT levels are related to the hepatic and jejunal mRNA abundance of serotonin receptors (HTR) in a cohort of women with morbid obesity (MO) and NAFLD. ELISA was used to quantify the serum 5-HT from normal-weight subjects (n = 26) and patients with MO (n = 58). We used RTq-PCR analysis to evaluate the relative expression of HTR in women with MO with normal liver (n = 22), SS (n = 21), and NASH (n = 15). The 5-HT was diminished in women with MO under a hypocaloric diet, regardless of the presence of NAFLD. Additionally, we report a negative correlation of 5-HT levels with metabolic syndrome criteria, suggesting that serotonin may have a protective role in obesity. Additionally, the hepatic expression of HTR2A and HTR2B were decreased in women with MO and NAFLD, but no significant differences in the HTR jejunal expression according to the presence of NAFLD were found.

Histone deacetylase inhibition by MS-275 potentiates glucose-stimulated insulin secretion without affecting glucose oxidation

AIMS

The preservation of pancreatic beta-cell function is crucial for the treatment of type 2 diabetes. Inhibition of class I histone deacetylase (HDAC) has been proved to protect beta-cells from palmitate- or cytokine-induced apoptosis and increase insulin secretion. However, the underlying molecular mechanism is unclear.

MAIN METHODS

Rat islets were isolated for insulin secretion, real-time PCR, RNA- sequencing, ChIP-PCR, and oxygen consumption rate analysis after treated with the HDAC1 and HDAC3 inhibitor MS-275.

KEY FINDINGS

MS-275 pretreatment significantly potentiated insulin secretion from rat islets. RNA-sequencing revealed that multiple signaling pathways were involved in MS-275-regulated islet function. Cacna1g and Adcy1 in calcium and cAMP signaling pathways were up-regulated in MS-275-treated islets, which was validated by real-time PCR. The expressions of the two genes displayed a similar increase in islets isolated from mice treated with MS-275. Knockdown of HDAC1 elevated Cacna1g and Adcy1 expressions in islets. ChIP-sequencing analysis showed that the pan-HDAC inhibitor sodium butyrate increased H3K27 acetylation level in the upstream region of Adcy1 and the promoter region of Cacna1g. ChIP-PCR revealed a similar result in MS-275-treated rat islets. However, MS-275 had minor effect on glucose-induced oxygen consumption rate in rat islets. Unlike glucose, MS-275 did not alter the expressions of glucose-sensitive genes such as Glut2 and Gck, but elevated intracellular Ca²⁺ concentration in beta-cells.

SIGNIFICANCE

Our findings support the notion that MS-275-potentiated insulin secretion is involved in calcium and cAMP signaling-mediated gene expressions independent of glucose oxidation. Therefore, HDAC inhibition may serve as a therapeutic strategy for type 2 diabetes.

Protein acetylation derepresses Serotonin Synthesis to potentiate Pancreatic Beta-Cell Function through HDAC1-PKA-Tph1 signaling

Rationale: Protein acetylation is tightly linked to transcriptional control and energy metabolism. However, the role of protein acetylation in islet function remains enigmatic. This study aims to determine how protein acetylation controls β-cell function and explore the underlying mechanism. Methods: The gene-expression profiles were analyzed for rat islets in response to two histone deacetylase (HDAC) inhibitors. Insulin secretion, tryptophan hydroxylase 1 (Tph1) expression, and serotonin synthesis of rat islets were detected after HDAC inhibitor treatment both in vivo and ex vivo. β-cell-specific Tph1-overexpressing transgenic rats and β-cell-specific Tph1 knockout mice were constructed to evaluate the role of Tph1 in β-cell function. The deacetylation of PKA in β-cells by HDAC1 was investigated by adenoviral infection, immunoprecipitation, and western blot. Results: Inhibition of HDACs greatly potentiated pancreatic β-cell function and reprogrammed transcriptional landscape of islets. Among the commonly up-regulated genes by two pan-HDAC inhibitors, Tph1 displayed the most prominent change. Specifically, inhibition of HDAC1 and HDAC3 by MS-275 strongly promoted Tph1 expression and endogenous serotonin synthesis in rat islets, concomitantly with enhanced insulin secretory capacity in vivo and ex vivo. β-cell-specific Tph1-overexpressing transgenic rats exhibited improved glucose tolerance and amplified glucose-stimulated insulin secretion. On the contrary, β-cell-specific Tph1 knockout mice displayed glucose intolerance and impaired insulin secretion with aging. Moreover, depletion of Tph1 in β-cells abrogated MS-275-induced insulin hypersecretion. Overexpression of HDAC1, not HDAC3, inhibited Tph1 transcriptional activity and decreased MS-275-stimulated Tph1 expression. Mechanistically, HDAC1 deacetylated PKA catalytic subunit and decreased its activity, resulting in Tph1 transcriptional repression. The acetylation mimetic K62Q mutant of PKA increased its catalytic activity. HDAC1 inhibition exerted a synergistic effect with cAMP/PKA signal on Tph1 expression. Conclusions: The present findings highlight a novel role of HDAC1-PKA-Tph1 signaling in governing β-cell functional compensation by derepressing serotonin synthesis.

The ever-changing roles of serotonin

Serotonin (5-HT) has traditional roles as a key neurotransmitter in the central nervous system and as a regulatory hormone controlling a broad range of physiological functions. Perhaps the most classically-defined functions of 5-HT are centrally in the control of mood, sleep and anxiety and peripherally in the modulation of gastrointestinal motility. A more recently appreciated role for 5-HT has emerged, however, as an important metabolic hormone contributing to glucose homeostasis and adiposity, with a causal relationship existing between circulating 5-HT levels and metabolic diseases. Almost all peripheral 5-HT is derived from specialised enteroendocrine cells, called enterochromaffin (EC) cells, located throughout the length of the lining of the gastrointestinal tract. EC cells are important luminal sensory cells that can detect and respond to an array of ingested nutrients, as well as luminal gut microbiota and their associated metabolites. Intriguingly, the interaction between gut microbiota and EC cells is dynamic in nature and has strong implications for host physiology. In this review, we discuss the traditional and modern functions of 5-HT and highlight an emerging pathway by which gut microbiota influences host health. Serotonin, also known as 5-hydroxytryptamine (5-HT), is an important neurotransmitter, growth factor and hormone that mediates a range of physiological functions. In mammals, serotonin is synthesized from the essential amino acid tryptophan by the rate-limiting enzyme tryptophan hydroxylase (TPH), for which there are two isoforms expressed in distinct cell types throughout the body. Tph1 is mainly expressed by specialized gut endocrine cells known as enterochromaffin (EC) cells and by other non-neuronal cell types such as adipocytes (Walther et al., 2003). Tph2 is primarily expressed in neurons of the raphe nuclei of the brain stem and a subset of neurons in the enteric nervous system (ENS) (Yabut et al., 2019). As 5-HT cannot readily cross the blood-brain barrier, the central and peripheral pools of 5-HT are anatomically separated and as such, act in their own distinct manners (Martin et al., 2017c). In this review we discuss the peripheral roles of serotonin, with particular focus on the interaction of gut-derived serotonin with the gut microbiota, and address emerging evidence linking this relationship with host homeostasis.

Beta cell identity changes with mild hyperglycemia: Implications for function, growth, and vulnerability

OBJECTIVE

As diabetes develops, marked reductions of insulin secretion are associated with very modest elevations of glucose. We wondered if these glucose changes disrupt beta cell differentiation enough to account for the altered function.

METHODS

Rats were subjected to 90% partial pancreatectomies and those with only mild glucose elevations 4 weeks or 10 weeks after surgery had major alterations of gene expression in their islets as determined by RNAseq.

RESULTS

Changes associated with glucose toxicity demonstrated that many of the critical genes responsible for insulin secretion were downregulated while the expression of normally suppressed genes increased. Also, there were marked changes in genes associated with replication, aging, senescence, stress, inflammation, and increased expression of genes controlling both class I and II MHC antigens.

CONCLUSIONS

These findings suggest that mild glucose elevations in the early stages of diabetes lead to phenotypic changes that adversely affect beta cell function, growth, and vulnerability.

Arsenic modifies serotonin metabolism through glucuronidation in pancreatic β-cells

In arsenic-endemic regions of the world, arsenic exposure correlates with diabetes mellitus. Multiple animal models of inorganic arsenic (iAs, as As³⁺) exposure have revealed that iAs-induced glucose intolerance manifests as a result of pancreatic β-cell dysfunction. To define the mechanisms responsible for this β-cell defect, the MIN6-K8 mouse β-cell line was exposed to environmentally relevant doses of iAs. Exposure to 0.1-1 µM iAs for 3 days significantly decreased glucose-induced insulin secretion (GIIS). Serotonin and its precursor, 5-hydroxytryptophan (5-HTP), were both decreased. Supplementation with 5-HTP, which loads the system with bioavailable 5-HTP and serotonin, rescued GIIS, suggesting that recovery of this pathway was sufficient to restore function. Exposure to iAs was accompanied by an increase in mRNA expression of UDP-glucuronosyltransferase 1 family, polypeptide a6a (Ugt1a6a), a phase-II detoxification enzyme that facilitates the disposal of cyclic amines, including serotonin, via glucuronidation. Elevated Ugt1a6a and UGT1A6 expression levels were observed in mouse and human islets, respectively, following 3 days of iAs exposure. Consistent with this finding, the enzymatic rate of serotonin glucuronidation was increased in iAs-exposed cells. Knockdown by siRNA of Ugt1a6a during iAs exposure restored GIIS in MIN6-K8 cells. This effect was prevented by blockade of serotonin biosynthesis, suggesting that the observed iAs-induced increase in Ugt1a6a affects GIIS by targeting serotonin or serotonin-related metabolites. Although it is not yet clear exactly which element(s) of the serotonin pathway is/are most responsible for iAs-induced GIIS dysfunction, this study provides evidence that UGT1A6A, acting on the serotonin pathway, regulates GIIS under both normal and pathological conditions.