An osmotic-sensitive taurine pool is localized in rat pancreatic islet cells containing glucagon and somatostatin

Molecular determinants and intracellular targets of taurine signalling in pancreatic islet β-cells

AIM

Despite its abundance in pancreatic islets of Langerhans and proven antihyperglycemic effects, the impact of the essential amino acid, taurine, on islet β-cell biology has not yet received due consideration, which prompted the current studies exploring the molecular selectivity of taurine import into β-cells and its acute and chronic intracellular interactions.

METHODS

The molecular aspects of taurine transport were probed by exposing the clonal pancreatic BRIN BD11 β-cells and primary mouse and human islets to a range of the homologs of the amino acid (assayed at 2-20 mM), using the hormone release and imaging of intracellular signals as surrogate read-outs. Known secretagogues were employed to profile the interaction of taurine with acute and chronic intracellular signals.

RESULTS

Taurine transporter TauT was expressed in the islet β-cells, with the transport of taurine and homologs having a weak sulfonate specificity but significant sensitivity to the molecular weight of the transporter. Taurine, hypotaurine, homotaurine, and β-alanine enhanced insulin secretion in a glucose-dependent manner, an action potentiated by cytosolic Ca2+ and cAMP. Acute and chronic β-cell insulinotropic effects of taurine were highly sensitive to co-agonism with GLP-1, forskolin, tolbutamide, and membrane depolarization, with an unanticipated indifference to the activation of PKC and CCK8 receptors. Pre-culturing with GLP-1 or KATP channel inhibitors sensitized or, respectively, desensitized β-cells to the acute taurine stimulus.

CONCLUSION

Together, these data demonstrate the pathways whereby taurine exhibits a range of beneficial effects on insulin secretion and β-cell function, consistent with the antidiabetic potential of its dietary low-dose supplementation.

Evaluation of Surface Treatments of PDMS Microfluidic Devices for Improving Small-Molecule Recovery with Application to Monitoring Metabolites Secreted from Islets of Langerhans

Microfluidic devices are becoming an important tool for bioanalysis with applications including studying cell secretion, cell growth, and drug delivery. Small molecules such as drugs, cell products, or nutrients may partition into polydimethylsiloxane (PDMS), a commonly used material for microfluidic devices, potentially leading to poor recovery or inaccurate delivery of such chemicals. To decrease small-molecule partitioning, surface and bulk PDMS treatments have been developed; however, these have been tested on few analytes, or their biocompatibility are unknown. Studies often focus on one analyte, whereas a diversity of chemicals are of interest and possibly affected. In this study, 11 device treatments are tested and applied to 21 biologically relevant small molecules with a variety of chemical structures. Device treatments are characterized using water contact angle measurements and evaluated by measuring recovery of the 21 target analytes using liquid chromatography-mass spectrometry. 1,5-Dimethyl-1,5-diazaundecamethylene polymethobromide (polybrene), a positively charged polymer, produced the least hydrophilic surface and was found to provide the best recovery with most of the analytes having >50% recovery and up to 92% recovery; however, recovery varied by analyte highlighting the importance of analyte diversity rather than targeting a single analyte in evaluating treatments. A polybrene-treated device was applied to investigate secretion from pancreatic islets, which are micro-organs involved in glucose homeostasis and diabetes. Islets secrete small molecules that have been shown to modulate the secretion of islets' main functional products, glucose-regulating hormones. The polybrene treatment enabled the detection of 20 target analytes from islets-on-chip during isosmotic and hypo-osmotic glucose perfusions and resulted in detection of more significant secretion changes compared to untreated PDMS.

NKCC transport mediates the insulinotropic effects of taurine and other small neutral amino acids

AIMS

Despite its high concentration in pancreatic islets of Langerhans and broad range of antihyperglycemic effects, the route facilitating the import of dietary taurine into pancreatic β-cell and mechanisms underlying its insulinotropic activity are unclear. We therefore studied the impact of taurine on beta-cell function, alongside that of other small neutral amino acids, L-alanine and L-proline.

MAIN METHODS

Pharmacological profiling of insulin secretion was conducted using clonal BRIN BD11 β-cells, the impact of taurine on the metabolic fate of glucose carbons was assessed using NMR and the findings were verified by real-time imaging of Ca²⁺ dynamics in the cytosol of primary mouse and human islet beta-cells.

KEY FINDINGS

In our hands, taurine, alanine and proline induced secretory responses that were dependent on the plasma membrane depolarisation, import of Ca²⁺, homeostasis of K⁺ and Na⁺ as well as on cell glycolytic and oxidative metabolism. Taurine shifted the balance between the oxidation and anaplerosis towards the latter, in BRIN BD11 beta-cells. Furthermore, the amino acid signalling was significantly attenuated by inhibition of Na⁺-K⁺-Cl^- symporter (NKCC).

SIGNIFICANCE

These data suggest that taurine, like L-alanine and L-proline, acutely induces glucose-dependent insulin-secretory responses by modulating electrogenic Na⁺ transport, with potential role of intracellular K⁺ and Cl^- in the signal transduction. The acute action delineated would be consistent with antidiabetic potential of dietary taurine supplementation.

Taurine rescues pancreatic β-cell stress by stimulating α-cell transdifferentiation

The semi-essential ubiquitous amino acid taurine has been shown to alleviate obesity and hyperglycemia in humans; however, the pathways underlying the antidiabetic actions have not been characterized. We explored the effect of chronic taurine exposure on cell biology of pancreatic islets, in degenerative type 1-like diabetes. The latter was modeled by small dose of streptozotocin (STZ) injection for 5 days in mice, followed by a 10-day administration of taurine (2% w/v, orally) in the drinking water. Taurine treatment opposed the detrimental changes in islet morphology and β-/α-cell ratio, induced by STZ diabetes, coincidentally with a significant 3.9 ± 0.7-fold enhancement of proliferation and 40 ± 5% reduction of apoptosis in β-cells. In line with these findings, the treatment counteracted an upregulation of antioxidant (Sod1, Sod2, Cat, Gpx1) and downregulation of islet expansion (Ngn3, Itgb1) genes induced by STZ, in a pancreatic β-cell line. At the same time, taurine enhanced the transdifferentiation of α-cells into β-cells by 2.3 ± 0.8-fold, echoed in strong non-metabolic elevation of cytosolic Ca²⁺ levels in pancreatic α-cells. Our data suggest a bimodal effect of dietary taurine on islet β-cell biology, which combines the augmentation of α-/β-cell transdifferentiation with downregulation of apoptosis. The dualism of action, stemming presumably from the intra- and extracellular modality of the signal, is likely to explain the antidiabetic potential of taurine supplementation.

The role of GABA in islet function

Gamma aminobutyric acid (GABA) is a non-proteinogenic amino acid and neurotransmitter that is produced in the islet at levels as high as in the brain. GABA is synthesized by the enzyme glutamic acid decarboxylase (GAD), of which the 65 kDa isoform (GAD65) is a major autoantigen in type 1 diabetes. Originally described to be released via synaptic-like microvesicles or from insulin secretory vesicles, beta cells are now understood to release substantial quantities of GABA directly from the cytosol via volume-regulated anion channels (VRAC). Once released, GABA influences the activity of multiple islet cell types through ionotropic GABAA receptors and metabotropic GABAB receptors. GABA also interfaces with cellular metabolism and ATP production via the GABA shunt pathway. Beta cells become depleted of GABA in type 1 diabetes (in remaining beta cells) and type 2 diabetes, suggesting that loss or reduction of islet GABA correlates with diabetes pathogenesis and may contribute to dysfunction of alpha, beta, and delta cells in diabetic individuals. While the function of GABA in the nervous system is well-understood, the description of the islet GABA system is clouded by differing reports describing multiple secretion pathways and effector functions. This review will discuss and attempt to unify the major experimental results from over 40 years of literature characterizing the role of GABA in the islet.

Endogenous Taurine Downregulation Is Required for Renal Injury in Salt-Sensitive Hypertensive Rats via CBS/H2S Inhibition

Although taurine is known to exert an antihypertensive effect, it is unclear whether it is involved in the mechanism for hypertension-related target organ injury. To reveal the role of endogenous taurine in renal injury formation during salt-sensitive hypertension and clarify its mechanisms, both salt-sensitive Dahl rats and salt-resistant SS-13BN rats were fed a high-salt diet (8% NaCl) and given 2% taurine for 6 weeks. Rat systolic blood pressure (SBP) was measured by the tail-cuff method and artery catheterization. Kidney ultrastructure was observed under an electron microscope. Taurine content and mRNA and protein levels of taurine synthases, cysteine dioxygenase type 1 (CDO1) and cysteine sulfinic acid decarboxylase (CSAD), were decreased in Dahl rats fed a high-salt diet. However, taurine supplementation and the resulting increase in renal taurine content reduced the increased SBP and improved renal function and structural damage in high-salt diet-fed Dahl rats. In contrast, taurine did not affect SS-13BN SBP and renal function and structure. Taurine intervention increased the renal H₂S content and enhanced cystathionine-β-synthase (CBS) expression and activity in Dahl rats fed a high-salt diet. Taurine reduced the renin, angiotensin II, and aldosterone contents and the levels of oxidative stress indices in Dahl rat renal tissues but increased antioxidant capacity, antioxidant enzyme activity, and protein expression. However, taurine failed to achieve this effect in the renal tissue of SS-13BN rats fed a high-salt diet. Pretreatment with the CBS inhibitor HA or renal CBS knockdown inhibited H₂S generation and subsequently blocked the effect of taurine on renin, superoxide dismutase 1 (SOD1), and superoxide dismutase 2 (SOD2) levels in high-salt-stimulated Dahl renal slices. In conclusion, the downregulation of endogenous taurine production resulted in a decrease in the renal CBS/H₂S pathway. This decrease subsequently promoted renin-angiotensin-aldosterone system (RAAS) activation and oxidative stress in the kidney, ultimately contributing to renal injury in salt-sensitive Dahl rats.

Taurine supplementation in high-fat diet fed male mice attenuates endocrine pancreatic dysfunction in their male offspring

Obesity in fathers leads to DNA damage and epigenetic changes in sperm that may carry potential risk factors for metabolic diseases to the next generation. Taurine (TAU) supplementation has demonstrated benefits against testicular dysfunction and pancreatic islet impairments induced by obesity, but it is not known if these protective actions prevent the propagation of metabolic disruptions to the next generation; as such, we hypothesized that paternal obesity may increase the probability of endocrine pancreatic dysfunction in offspring, and that this could be prevented by TAU supplementation in male progenitors. To test this, male C57Bl/6 mice were fed on a control diet (CTL) or a high-fat diet (HFD) without or with 5% TAU in their drinking water (CTAU and HTAU) for 4 months. Subsequently, all groups of mice were mated with CTL females, and the F1 offspring were identified as: CTL-F1, CTAU-F1, HFD-F1, and HTAU-F1. HFD-fed mice were normoglycemic, but glucose intolerant and their islets hypersecreted insulin. However, at 90 days of age, HFD-F1 offspring displayed normal glucose homeostasis and adiposity, but reduced glucose-induced insulin release. HFD-F1 islets also exhibited β- and α-cell hypotrophy, and lower δ-cell number per islet. Paternal TAU supplementation prevented the decrease in glucose-induced insulin secretion and normalized β-cell size and δ-cell number, and increased α-cell size/islet in HTAU-F1 mice. In conclusion, HFD consumption by male founders decreases β-cell secretion and islet-cell distribution in their offspring. TAU attenuates the deleterious effects of paternal obesity on insulin secretion and islet-cell morphology in F1 offspring.

Taurine Increases Insulin Expression in STZ-Treated Rat Islet Cells In Vitro

This research aims at figure out the effects and the pathway of taurine on insulin in islet cells cultured in vitro treated by STZ. In the experiment, islet cells were isolated from pancreatic tissue by in situ perfusion with collagenase V. The pancreatic islet cells, maintained in RPMI 1640 culture medium were divided into six groups: C: control, E: supplemented with 10 mmol/L of taurine, group M, T1, T2 and T3 was treated with STZ (0.5 mmol/L), at the same time, taurine were added in group T1,T2 and T3 for 30 min, and then culture medium were collected by centrifugation and then insulin levels were detected by radioimmunoassay, the cells were then rinsed with Hanks, and 0,10, 0, 5, 10, 20 mmol/L of taurine in group C, E, M, T1, T2 and T3 were added for 24 h respectively. Total RNA was extracted, then insulin gene and its transcription regulator such as PDX-1, NeuroD1 were amplified by semi-quantitative RT-PCR. The results showed that, the release of insulin from islet cells treated by STZ could be inhibited by taurine, gene expression of insulin, PDX-1 and NeuroD1 in STZ group decreased significantly, which were up-regulated by taurine administration. In conclusion, taurine exerts a certain degree of protective and reconstructive effects on islet cells treated by STZ.

Regulation of glucose and lipid metabolism by the pancreatic and extra-pancreatic actions of taurine

The beneficial actions of L-taurine (Tau) against glucose intolerance, obesity, type 2 diabetes (T2D), and non-alcoholic fat liver disease (NAFLD) have been linked to its antioxidant and anti-inflammatory effects, which ameliorate tissue insulin sensitivity. Importantly, there are several lines of evidence that indicate a direct action of Tau on the endocrine pancreas to regulate the secretion and paracrine actions of insulin, glucagon, and somatostatin. Furthermore, Tau can also ameliorate glucose metabolism through the enhancement of insulin signaling. However, some of the benefits of Tau upon intermediary metabolism may manifest via considerable antagonism of the action of insulin. Therefore, this review discusses the mechanisms of action by which Tau may regulate endocrine pancreatic morphofunction, and glucose and lipid homeostasis.

Novel protective role of curcumin and taurine combination against experimental hepatocarcinogenesis

Hepatocarcinogenesis is a prerequisite to hepatocellular carcinoma (HCC), which is one of the most common cancers among humans. Therefore, it is important to search for agents that protect against hepatocarcinogenesis. The present study aimed to investigate the protective effects of a combination of taurine and curcumin against experimental hepatocarcinogenesis induced by diethyl nitrosamine (DENA) in a rat model. A total of 100 rats were divided into eight groups. Eight weeks following DENA injection and treatment with curcumin and taurine, the rats were sacrificed to obtain blood and hepatic tissue samples for the evaluation of various markers and histopathological observations. Serum levels of interleukin-2 (IL-2), interferon-γ (IFN-γ), α-fetoprotein (AFP) and α-L-fucosidase (AFU) were determined. Rats injected with DENA for eight weeks showed a high percentage of malignant changes in hepatic tissues, as well as a significant increases in the serum levels of AFP and AFU and significant reductions in the serum levels of IL-2 and IFN-γ. Treatment with curcumin and taurine markedly reduced the extent of malignant changes in the rat liver tissues, with their liver tissues showing patterns similar to that of the normal control rats. In addition, this combination resulted in normal serum levels of IL-2, IFN-γ, AFP and AFU. The results of the present study suggested that a combination of curcumin and taurine may be a novel prophylactic agent against hepatocarcinogenesis in high-risk groups exposed to chemical hepatocarcinogens.

A Glycine-Insulin Autocrine Feedback Loop Enhances Insulin Secretion From Human β-Cells and Is Impaired in Type 2 Diabetes

The secretion of insulin from pancreatic islet β-cells is critical for glucose homeostasis. Disrupted insulin secretion underlies almost all forms of diabetes, including the most common form, type 2 diabetes (T2D). The control of insulin secretion is complex and affected by circulating nutrients, neuronal inputs, and local signaling. In the current study, we examined the contribution of glycine, an amino acid and neurotransmitter that activates ligand-gated Cl(-) currents, to insulin secretion from islets of human donors with and without T2D. We find that human islet β-cells express glycine receptors (GlyR), notably the GlyRα1 subunit, and the glycine transporter (GlyT) isoforms GlyT1 and GlyT2. β-Cells exhibit significant glycine-induced Cl(-) currents that promote membrane depolarization, Ca(2+) entry, and insulin secretion from β-cells from donors without T2D. However, GlyRα1 expression and glycine-induced currents are reduced in β-cells from donors with T2D. Glycine is actively cleared by the GlyT expressed within β-cells, which store and release glycine that acts in an autocrine manner. Finally, a significant positive relationship exists between insulin and GlyR, because insulin enhances the glycine-activated current in a phosphoinositide 3-kinase-dependent manner, a positive feedback loop that we find is completely lost in β-cells from donors with T2D.

Taurine supplementation ameliorates glucose homeostasis, prevents insulin and glucagon hypersecretion, and controls β, α, and δ-cell masses in genetic obese mice

Taurine (Tau) regulates β-cell function and glucose homeostasis under normal and diabetic conditions. Here, we assessed the effects of Tau supplementation upon glucose homeostasis and the morphophysiology of endocrine pancreas, in leptin-deficient obese (ob) mice. From weaning until 90-day-old, C57Bl/6 and ob mice received, or not, 5% Tau in drinking water (C, CT, ob and obT). Obese mice were hyperglycemic, glucose intolerant, insulin resistant, and exhibited higher hepatic glucose output. Tau supplementation did not prevent obesity, but ameliorated glucose homeostasis in obT. Islets from ob mice presented a higher glucose-induced intracellular Ca(2+) influx, NAD(P)H production and insulin release. Furthermore, α-cells from ob islets displayed a higher oscillatory Ca(2+) profile at low glucose concentrations, in association with glucagon hypersecretion. In Tau-supplemented ob mice, insulin and glucagon secretion was attenuated, while Ca(2+) influx tended to be normalized in β-cells and Ca(2+) oscillations were increased in α-cells. Tau normalized the inhibitory action of somatostatin (SST) upon insulin release in the obT group. In these islets, expression of the glucagon, GLUT-2 and TRPM5 genes was also restored. Tau also enhanced MafA, Ngn3 and NeuroD mRNA levels in obT islets. Morphometric analysis demonstrated that the hypertrophy of ob islets tends to be normalized by Tau with reductions in islet and β-cell masses, but enhanced δ-cell mass in obT. Our results indicate that Tau improves glucose homeostasis, regulating β-, α-, and δ-cell morphophysiology in ob mice, indicating that Tau may be a potential therapeutic tool for the preservation of endocrine pancreatic function in obesity and diabetes.

Taurine supplementation increases K(ATP) channel protein content, improving Ca2+ handling and insulin secretion in islets from malnourished mice fed on a high-fat diet

Pancreatic β-cells are highly sensitive to suboptimal or excess nutrients, as occurs in protein-malnutrition and obesity. Taurine (Tau) improves insulin secretion in response to nutrients and depolarizing agents. Here, we assessed the expression and function of Cav and KATP channels in islets from malnourished mice fed on a high-fat diet (HFD) and supplemented with Tau. Weaned mice received a normal (C) or a low-protein diet (R) for 6 weeks. Half of each group were fed a HFD for 8 weeks without (CH, RH) or with 5% Tau since weaning (CHT, RHT). Isolated islets from R mice showed lower insulin release with glucose and depolarizing stimuli. In CH islets, insulin secretion was increased and this was associated with enhanced KATP inhibition and Cav activity. RH islets secreted less insulin at high K(+) concentration and showed enhanced KATP activity. Tau supplementation normalized K(+)-induced secretion and enhanced glucose-induced Ca(2+) influx in RHT islets. R islets presented lower Ca(2+) influx in response to tolbutamide, and higher protein content and activity of the Kir6.2 subunit of the KATP. Tau increased the protein content of the α1.2 subunit of the Cav channels and the SNARE proteins SNAP-25 and Synt-1 in CHT islets, whereas in RHT, Kir6.2 and Synt-1 proteins were increased. In conclusion, impaired islet function in R islets is related to higher content and activity of the KATP channels. Tau treatment enhanced RHT islet secretory capacity by improving the protein expression and inhibition of the KATP channels and enhancing Synt-1 islet content.

Delineation of glutamate pathways and secretory responses in pancreatic islets with β-cell-specific abrogation of the glutamate dehydrogenase

The amino acid profile and the secretory responses of glutamate dehydrogenase (GDH)-deficient β-cells are characterized. This study shows that GDH is essential for both insulin release and net glutamate synthesis evoked by glucose. Adding cellular glutamate restored the full development of glucose-stimulated insulin secretion, showing the requirement for permissive glutamate levels.

In pancreatic β-cells, glutamate dehydrogenase (GDH) modulates insulin secretion, although its function regarding specific secretagogues is unclear. This study investigated the role of GDH using a β-cell–specific GDH knockout mouse model, called βGlud1^−/−. The absence of GDH in islets isolated from βGlud1^–/– mice resulted in abrogation of insulin release evoked by glutamine combined with 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid or l-leucine. Reintroduction of GDH in βGlud1^–/– islets fully restored the secretory response. Regarding glucose stimulation, insulin secretion in islets isolated from βGlud1^–/– mice exhibited half of the response measured in control islets. The amplifying pathway, tested at stimulatory glucose concentrations in the presence of KCl and diazoxide, was markedly inhibited in βGlud1^–/– islets. On glucose stimulation, net synthesis of glutamate from α-ketoglutarate was impaired in GDH-deficient islets. Accordingly, glucose-induced elevation of glutamate levels observed in control islets was absent in βGlud1^–/– islets. Parallel biochemical pathways, namely alanine and aspartate aminotransferases, could not compensate for the lack of GDH. However, the secretory response to glucose was fully restored by the provision of cellular glutamate when βGlud1^–/– islets were exposed to dimethyl glutamate. This shows that permissive levels of glutamate are required for the full development of glucose-stimulated insulin secretion and that GDH plays an indispensable role in this process.

Taurine supplementation: involvement of cholinergic/phospholipase C and protein kinase A pathways in potentiation of insulin secretion and Ca2+ handling in mouse pancreatic islets

Taurine (TAU) supplementation increases insulin secretion in response to high glucose concentrations in rodent islets. This effect is probably due to an increase in Ca2+ handling by the islet cells. Here, we investigated the possible involvement of the cholinergic/phospholipase C (PLC) and protein kinase (PK) A pathways in this process. Adult mice were fed with 2% TAU in drinking water for 30 d. The mice were killed and pancreatic islets isolated by the collagenase method. Islets from TAU-supplemented mice showed higher insulin secretion in the presence of 8.3 mm-glucose, 100 μm-carbachol (Cch) and 1 mm-3-isobutyl-1-methyl-xanthine (IBMX), respectively. The increase in insulin secretion in response to Cch in TAU islets was accompanied by a higher intracellular Ca2+ mobilisation and PLCβ2 protein expression. The Ca2+ uptake was higher in TAU islets in the presence of 8.3 mm-glucose, but similar when the islets were challenged by glucose plus IBMX. TAU islets also showed an increase in the expression of PKAα protein. This protein may play a role in cation accumulation, since the amount of Ca2+ in these islets was significantly reduced by the PKA inhibitors: N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline sulfonamide (H89) and PK inhibitor-(6-22)-amide (PKI). In conclusion, TAU supplementation increases insulin secretion in response to glucose, favouring both influx and internal mobilisation of Ca2+, and these effects seem to involve the activation of both PLC-inositol-1,4,5-trisphosphate and cAMP-PKA pathways.

Taurine supplementation enhances nutrient-induced insulin secretion in pancreatic mice islets

BACKGROUND

Taurine (TAU), a naturally occurring sulfur-containing amino acid, is found at high concentrations in plasma and mammalian tissues and regulates osmolarity, ion channel activity, and glucose homeostasis. Several reports have shown that physiological plasma TAU levels seem to be important for adequate beta (beta)-cell function and insulin action, since low concentrations of TAU in the plasma have been reported in the pre-diabetic and diabetic states.

METHODS

Glucose tolerance and insulin sensitivity were investigated in mice supplemented with 2% (w/v) TAU in their drinking water for 30 days, as well as the insulin secretion from isolated islets stimulated by glucose or L-leucine.

RESULTS

TAU-supplemented mice demonstrated improved glucose tolerance and higher insulin sensitivity, compared to controls (CTL). In addition, their islets secreted more insulin in response to high concentrations of glucose and L-leucine. L-[U-(14)C]leucine oxidation was higher in TAU than in CTL islets, whereas D-[U-(14)C]glucose oxidation, ATP levels, glucose transporter (GLUT) 2 and glucokinase (GCK) protein expressions were similar in both types of islets. The L-type beta(2) subunit voltage-sensitive Ca(2+) channel protein, as well as (45)Ca uptake, were significantly higher in TAU-supplemented than CTL islets. In addition, islets from TAU-supplemented mice secreted more glucagon than CTL islets at low glucose.

CONCLUSIONS

TAU supplementation improves glucose tolerance and insulin sensitivity in mice, as well as insulin secretion from isolated islets. The latter effect seems to be, at least in part, dependent on a better Ca(2+) handling by the islets.

Stimulation by D-glucose of 36Cl- efflux from prelabeled rat pancreatic islets

D-glucose was previously reported to cause a concentration-related decrease in the 36Cl- content of prelabeled islets prepared from ob/ob mice, a current animal model of inherited obesity. From these findings, it was inferred that the hexose stimulates Cl- efflux from islet cells and that such an increase in Cl- permeability may partly mediate glucose-induced depolarization of insulin-producing cells. The aim of the present study was to investigate the possible extension of these findings to islets prepared from normal rats by measuring the changes evoked by increasing concentrations of D-glucose in 36Cl- outflow itself from prelabeled isolated islets. After 60 min preincubation at 37 degrees C in the presence of 3 mM D-glucose and 36Cl- (75 microCi/mL), the islets were incubated for 8-10 min at 37 degrees C in the presence of increasing concentrations of the hexose (3-20 mM). The changes in 36Cl- outflow during incubation indicated that D-glucose, in excess of a threshold concentration close to 5 mM, indeed increases effluent radioactivity from the prelabeled islets. It is proposed, therefore, that a gating of volume-sensitive anion channels in glucose-stimulated insulin-producing islet cells participates in the depolarization of the plasma membrane recorded in the range of insulinotropic concentrations of the hexose.

Taurine supplement in early life altered islet morphology, decreased insulitis and delayed the onset of diabetes in non-obese diabetic mice

AIMS/HYPOTHESIS

We hypothesised that nutritional taurine, which is important for the development of the endocrine pancreas and reduces cytokine-induced apoptosis in pancreatic beta cells, would prevent or delay the onset of autoimmune diabetes, if given early in life to the non-obese diabetic (NOD) mouse.

METHODS

Pregnant NOD mice received a diet supplemented with taurine throughout gestation or until weaning, and the pancreas of the offspring was examined using immunohistochemistry. This was done at postnatal day 14 and after 8 weeks (assessment of insulitis). The animals were also monitored until they became diabetic.

RESULTS

At 14 days, pancreatic islet mass was significantly greater in animals treated with taurine than in controls. This finding was associated with a greater incidence of islet cell proliferation and a lower incidence of apoptosis. At age 8 weeks the number of islets manifesting insulitis was reduced by more than half, and the area of insulitis was reduced by 90%. Taurine treatment delayed the mean onset time of diabetes from 18 to 30 weeks in females, and from 30 to 38 weeks in males, while 20% of treated females remained free of diabetes after one year.

CONCLUSIONS/INTERPRETATION

Taurine supplementation in early life altered islet development, reduced insulitis and delayed the onset of diabetes in NOD mice.

Conversion into GABA (gamma-aminobutyric acid) may reduce the capacity of L-glutamine as an insulin secretagogue

We have carried out a detailed examination of L-glutamine metabolism in rat islets in order to elucidate the paradoxical failure of L-glutamine to stimulate insulin secretion. L-Glutamine was converted by isolated islets into GABA (gamma-aminobutyric acid), L-aspartate and L-glutamate. Saturation of the intracellular concentrations of all of these amino acids occurred at approx. 10 mmol/l L-glutamine, and their half-maximal values were attained at progressively increasing concentrations of L-glutamine (0.3 mmol/l for GABA; 0.5 and 1.0 mmol/l for Asp and Glu respectively). GABA accumulation accounted for most of the 14CO2 produced at various L-[U-14C]glutamine concentrations. Potentiation by L-glutamine of L-leucine-induced insulin secretion in perifused islets was suppressed by malonic acid dimethyl ester, was accompanied by a significant decrease in islet GABA accumulation, and was not modified in the presence of GABA receptor antagonists [50 micromol/l saclofen or 10 micromol/l (+)-bicuculline]. L-Leucine activated islet glutamate dehydrogenase activity, but had no effect on either glutamate decarboxylase or GABA transaminase activity, in islet homogenates. We conclude that (i) L-glutamine is metabolized preferentially to GABA and L-aspartate, which accumulate in islets, thus preventing its complete oxidation in the Krebs cycle, which accounts for its failure to stimulate insulin secretion; (ii) potentiation by L-glutamine of L-leucine-induced insulin secretion involves increased metabolism of L-glutamate and GABA via the Krebs cycle (glutamate dehydrogenase activation) and the GABA shunt (2-oxoglutarate availability for GABA transaminase) respectively, and (iii) islet release of GABA does not seem to play an important role in the modulation of the islet secretory response to the combination of L-leucine and L-glutamine.

Is taurine beneficial in reducing risk factors for diabetes mellitus?

Taurine is a semiessential amino acid, and its deficiency is involved in retinal and cardiac degenerations. In recent years, it was found that diabetes mellitus (DM) is associated with taurine, and many in vivo experimental studies showed that taurine administration is able to reduce the alterations induced by DM in the retina, lens, and peripheral nerve, although its effects on diabetic kidney are dubious. Interestingly, long-term taurine supplementation reduces the mortality rate in diabetic rats. The mechanisms by which taurine exerts beneficial effects in DM are discussed below. Recently, it has been suggested that taurine deficiency may alter the endocrine pancreas "fetal programming," increasing the risk of insulin resistance in adult life. The bulk of experimental data suggests that taurine administration could be useful in the treatment of type 1 DM and in the prevention of insulin resistance.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Effect of maternal low-protein diet and taurine on the vulnerability of adult Wistar rat islets to cytokines

AIMS/HYPOTHESIS

A maternal low-protein diet has been shown to induce an increased susceptibility of fetal islets to cytokines, but this effect can be avoided by maternal taurine supplementation. Here, we question whether these effects persist until adulthood in the offspring, despite the animal having a normal diet after weaning.

METHODS

Pregnant Wistar rats received a diet of either 20% or 8% protein (control [C group] and recuperated [R group] respectively), which was or was not supplemented with taurine (control treated with taurine [CT group] and recuperated treated with taurine [RT group] respectively) during gestation and lactation. When the female offspring reached adulthood, an OGTT was performed. In a second stage, islets were isolated from these offspring, then pretreated or not with taurine, and subsequently treated with cytokines.

RESULTS

Fasting glycaemia was higher (p<0.05) and insulinaemia was lower (p<0.01) in the R group than in the C group. Taurine supplementation decreased insulinaemia in the CT group and tended to increase it in the RT group. After the OGTT, glycaemia in R animals was not different from that in the C group, despite a blunted insulin response (p<0.05) which was restored by taurine. Supplementation in C-group mothers led to a weak glucose intolerance. In vitro, more apoptotic cells were observed in R islets after cytokines treatment (p<0.01). The addition of taurine to the culture medium in the R and C groups protected the islets from the cytokines (p<0.01). Maternal taurine supplementation decreased the sensitivity of islets in the RT group (p<0.01), but increased sensitivity in the CT group (p<0.01).

CONCLUSIONS/INTERPRETATION

The increased vulnerability of islets to cytokines due to a restriction of protein during fetal development was still evident when the offspring reached adulthood. The low-protein diet also induced hyperglycaemia in the presence of lower insulinaemia. Taurine supplementation protected adult islets of the R group from cytokine toxicity and restored the insulinaemia. However, unnecessary supplementation of taurine could have detrimental effects.

Regulated exocytosis of GABA-containing synaptic-like microvesicles in pancreatic beta-cells

We have explored whether γ-aminobutyric acid (GABA) is released by regulated exocytosis of GABA-containing synaptic-like microvesicles (SLMVs) in insulin-releasing rat pancreatic β-cells. To this end, β-cells were engineered to express GABA_A-receptor Cl⁻-channels at high density using adenoviral infection. Electron microscopy indicated that the average diameter of the SLMVs is 90 nm, that every β-cell contains ∼3,500 such vesicles, and that insulin-containing large dense core vesicles exclude GABA. Quantal release of GABA, seen as rapidly activating and deactivating Cl⁻-currents, was observed during membrane depolarizations from −70 mV to voltages beyond −40 mV or when Ca²⁺ was dialysed into the cell interior. Depolarization-evoked GABA release was suppressed when Ca²⁺ entry was inhibited using Cd²⁺. Analysis of the kinetics of GABA release revealed that GABA-containing vesicles can be divided into a readily releasable pool and a reserve pool. Simultaneous measurements of GABA release and cell capacitance indicated that exocytosis of SLMVs contributes ∼1% of the capacitance signal. Mathematical analysis of the release events suggests that every SLMV contains 0.36 amol of GABA. We conclude that there are two parallel pathways of exocytosis in pancreatic β-cells and that release of GABA may accordingly be temporally and spatially separated from insulin secretion. This provides a basis for paracrine GABAergic signaling within the islet.

Insulin secretion profiles are modified by overexpression of glutamate dehydrogenase in pancreatic islets

AIMS/HYPOTHESIS

Glutamate dehydrogenase (GDH) is a mitochondrial enzyme playing a key role in the control of insulin secretion. However, it is not known whether GDH expression levels in beta cells are rate-limiting for the secretory response to glucose. GDH also controls glutamine and glutamate oxidative metabolism, which is only weak in islets if GDH is not allosterically activated by L-leucine or (+/-)-2-aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH).

METHODS

We constructed an adenovirus encoding for GDH to overexpress the enzyme in the beta-cell line INS-1E, as well as in isolated rat and mouse pancreatic islets. The secretory responses to glucose and glutamine were studied in static and perifusion experiments. Amino acid concentrations and metabolic parameters were measured in parallel.

RESULTS

GDH overexpression in rat islets did not change insulin release at basal or intermediate glucose (2.8 and 8.3 mmol/l respectively), but potentiated the secretory response at high glucose concentrations (16.7 mmol/l) compared to controls (+35%). Control islets exposed to 5 mmol/l glutamine at basal glucose did not increase insulin release, unless BCH was added with a resulting 2.5-fold response. In islets overexpressing GDH glutamine alone stimulated insulin secretion (2.7-fold), which was potentiated 2.2-fold by adding BCH. The secretory responses evoked by glutamine under these conditions correlated with enhanced cellular metabolism.

CONCLUSIONS/INTERPRETATION

GDH could be rate-limiting in glucose-induced insulin secretion, as GDH overexpression enhanced secretory responses. Moreover, GDH overexpression made islets responsive to glutamine, indicating that under physiological conditions this enzyme acts as a gatekeeper to prevent amino acids from being inappropriate efficient secretagogues.

Taurine and taurine-deficiency in the perinatal period

Taurine, a non-protein sulfur amino-acid, is the most abundant free amino-acid in the body and plays an important role in several essential biological processes. Apart from its role in cholesterol degradation, it acts as neurotransmitter, and has a function as osmoregulator and antioxidant in most body tissues. During pregnancy, taurine accumulates in the maternal tissues, to be released in the perinatal period to the fetus via the placenta and to the newborn via the maternal milk. It is accumulated especially in the fetal and neonatal brain. Low maternal taurine levels result in low fetal taurine levels. Taurine-deficiency in the mother leads to growth retardation of the offspring, and to impaired perinatal development of the central nervous system and of the endocrine pancreas. The adult offspring of taurine-deficient mothers display signs of impaired neurological function, impaired glucose tolerance and vascular dysfunction; they may develop gestational diabetes and transmit the effects to the next generation. This transgeneration effect of taurine-deficiency in the perinatal period fits into the concept of fetal origin of adult disease.