Localization of GAD-like immunoreactivity in the pancreas and stomach of the rat and mouse

The Role of α-Cells in Islet Function and Glucose Homeostasis in Health and Type 2 Diabetes

Pancreatic α-cells are the major source of glucagon, a hormone that counteracts the hypoglycemic action of insulin and strongly contributes to the correction of acute hypoglycemia. The mechanisms by which glucose controls glucagon secretion are hotly debated, and it is still unclear to what extent this control results from a direct action of glucose on α-cells or is indirectly mediated by β- and/or δ-cells. Besides its hyperglycemic action, glucagon has many other effects, in particular on lipid and amino acid metabolism. Counterintuitively, glucagon seems also required for an optimal insulin secretion in response to glucose by acting on its cognate receptor and, even more importantly, on GLP-1 receptors. Patients with diabetes mellitus display two main alterations of glucagon secretion: a relative hyperglucagonemia that aggravates hyperglycemia, and an impaired glucagon response to hypoglycemia. Under metabolic stress states, such as diabetes, pancreatic α-cells also secrete GLP-1, a glucose-lowering hormone, whereas the gut can produce glucagon. The contribution of extrapancreatic glucagon to the abnormal glucose homeostasis is unclear. Here, I review the possible mechanisms of control of glucagon secretion and the role of α-cells on islet function in healthy state. I discuss the possible causes of the abnormal glucagonemia in diabetes, with particular emphasis on type 2 diabetes, and I briefly comment the current antidiabetic therapies affecting α-cells.

Multiple autoimmunity, type 1 diabetes (T1DM), autoimmune thyroiditis and thyroid cancer: is there an association? A case report and literature review

Type 1 diabetes mellitus (T1DM) is characterized by selective autoimmune destruction of pancreatic b-cells, resulting in insulin deficiency. Associated autoimmune disorders, such as celiac disease, autoimmune thyroiditis, and gastritis, can coexist in patients with T1DM. These disorders are characterized by the presence of antibodies against tissue transglutaminase (anti-tTG-IgA), thyroglobulin, and thyroid peroxidase (anti-TG, anti-TPO), as well as antibodies against gastric parietal cells. Children with T1DM may also develop organ-specific multiple autoimmunity, with the coexistence of one or more autoimmune disorders. Furthermore, there is a lot of controversy regarding the role of thyroid autoimmunity in the pathogenesis of thyroid cancer. We present a child with T1DM and multiple autoimmunity including autoimmune Hashimoto's thyroiditis (HT), who developed thyroid cancer. The literature on the prevalence of associated autoimmunity in children with T1DM and the prevalence, pathogenesis, and timely diagnosis of thyroid cancer among patients with HT is also reviewed.

Postnatal development of the endocrine pancreas in mice lacking functional GABAB receptors

Adult mice lacking functional GABAB receptors (GABAB1KO) have glucose metabolism alterations. Since GABAB receptors (GABABRs) are expressed in progenitor cells, we evaluated islet development in GABAB1KO mice. Postnatal day 4 (PND4) and adult, male and female, GABAB1KO, and wild-type littermates (WT) were weighed and euthanized, and serum insulin and glucagon was measured. Pancreatic glucagon and insulin content were assessed, and pancreas insulin, glucagon, PCNA, and GAD65/67 were determined by immunohistochemistry. RNA from PND4 pancreata and adult isolated islets was obtained, and Ins1, Ins2, Gcg, Sst, Ppy, Nes, Pdx1, and Gad1 transcription levels were determined by quantitative PCR. The main results were as follows: 1) insulin content was increased in PND4 GABAB1KO females and in both sexes in adult GABAB1KOs; 2) GABAB1KO females had more clusters (<500 μm(2)) and less islets than WT females; 3) cluster proliferation was decreased at PND4 and increased in adult GABAB1KO mice; 4) increased β-area at the expense of the α-cell area was present in GABAB1KO islets; 5) Ins2, Sst, and Ppy transcription were decreased in PND4 GABAB1KO pancreata, adult GABAB1KO female islets showed increased Ins1, Ins2, and Sst expression, Pdx1 was increased in male and female GABAB1KO islets; and 6) GAD65/67 was increased in adult GABAB1KO pancreata. We demonstrate that several islet parameters are altered in GABAB1KO mice, further pinpointing the importance of GABABRs in islet physiology. Some changes persist from neonatal ages to adulthood (e.g., insulin content in GABAB1KO females), whereas other features are differentially regulated according to age (e.g., Ins2 was reduced in PND4, whereas it was upregulated in adult GABAB1KO females).

Immunohistochemical study on GABAergic system in salivary glands

Gamma-aminobutyric acid (GABA) and its receptors are found in the central nervous system and several peripheral tissues. The purpose of this study was to determine the expression and distribution of GABA and glutamate decarboxylase (GAD), a GABA biosynthetic enzyme, in rat salivary gland. Western blot and real time quantitative RT-PCR revealed that GAD67 was the major isoform of GAD in the salivary glands. Furthermore, both GABA and GAD were detected around the acinar cells in the submandibular glands by immunohistochemical analysis. When both sympathetic and parasympathetic nerves related to the submandibular glands were denervated, the immunoreactivities of GABA and GAD were dramatically depressed, and levels of GAD67 and GABA significantly decreased. However, no morphological changes in the glands were observed after denervation. These results indicate that GAD67 is present around acinar cells in the salivary glands, and suggest that the GABAergic system in the glands is closely related to the autonomic nervous system.

Insulin as a physiological modulator of glucagon secretion

Glucose homeostasis is regulated primarily by the opposing actions of insulin and glucagon, hormones that are secreted by pancreatic islets from beta-cells and alpha-cells, respectively. Insulin secretion is increased in response to elevated blood glucose to maintain normoglycemia by stimulating glucose transport in muscle and adipocytes and reducing glucose production by inhibiting gluconeogenesis in the liver. Whereas glucagon secretion is suppressed by hyperglycemia, it is stimulated during hypoglycemia, promoting hepatic glucose production and ultimately raising blood glucose levels. Diabetic hyperglycemia occurs as the result of insufficient insulin secretion from the beta-cells and/or lack of insulin action due to peripheral insulin resistance. Remarkably, excessive secretion of glucagon from the alpha-cells is also a major contributor to the development of diabetic hyperglycemia. Insulin is a physiological suppressor of glucagon secretion; however, at the cellular and molecular levels, how intraislet insulin exerts its suppressive effect on the alpha-cells is not very clear. Although the inhibitory effect of insulin on glucagon gene expression is an important means to regulate glucagon secretion, recent studies suggest that the underlying mechanisms of the intraislet insulin on suppression of glucagon secretion involve the modulation of K(ATP) channel activity and the activation of the GABA-GABA(A) receptor system. Nevertheless, regulation of glucagon secretion is multifactorial and yet to be fully understood.

Differential expression of gamma-aminobutyric acid receptor A (GABA(A)) and effects of homocysteine

BACKGROUND

gamma-Aminobutyric acid (GABA) is a known inhibitory neurotransmitter in the mammalian central nervous system, and homocysteine (Hcy) behaves as an antagonist for GABA(A) receptor. Although the properties and functions of GABA(A) receptors are well studied in mouse neural tissue, its presence and significance in non-neural tissue remains obscure. The aim of the present study was to examine the expression of GABA(A) receptor and its subunits in non-neural tissue.

METHODS

The mice were analyzed. The presence of GABA(A) receptor and its subunits was evaluated using Western blot and reverse transcription polymerase chain reaction.

RESULTS

We report that GABA(A) receptor protein is abundant in the renal medulla, cortex, heart, left ventricle, aorta and pancreas. Low levels of GABA(A) receptor protein were detected in the atria of the heart, right ventricle, lung and stomach. The mRNA protein expression of GABA(A) receptor subunit shows that alpha1, beta1, beta3 and gamma1 subunits are present only in brain. The mRNA protein expression levels of GABA(A) receptor alpha2, alpha6, beta2 and gamma3 subunits were highly expressed in brain compared to other tested tissue, while GABA(A) receptor gamma2 subunit was expressed only in brain and kidney. Treatment of microvascular endothelial cells with Hcy decreased GABA(A) receptor protein level, which was restored to its baseline level in the presence of GABA(A) receptor agonist, muscimol. The distribution of GABA(A) and GABA(B) receptors in wild type mice was determined and tissue-specific expression patterns were found showing that several receptor subtypes were also expressed in the central nervous system.

CONCLUSIONS

Hcy, a GABA(A) agonist, was found to decrease GABA(A) expression levels. These data enlarge knowledge on distribution of GABA receptors and give novel ideas of the effects of Hcy on different organs.

Type B gamma-aminobutyric acid receptors modulate the function of the extracellular Ca2+-sensing receptor and cell differentiation in murine growth plate chondrocytes

Extracellular calcium-sensing receptors (CaRs) and metabotropic or type B gamma-aminobutyric acid receptors (GABA-B-Rs), two closely related members of family C of the G protein-coupled receptor superfamily, dimerize in the formation of signaling and membrane-anchored receptor complexes. We tested whether CaRs and two GABA-B-R subunits (R1 and R2) are expressed in mouse growth plate chondrocytes (GPCs) by PCR and immunocytochemistry and whether interactions between these receptors influence the expression and function of the CaR and extracellular Ca(2+)-mediated cell differentiation. Both CaRs and the GABA-B-R1 and -R2 were expressed in the same zones of the growth plate and extensively colocalized in intracellular compartments and on the membranes of cultured GPCs. The GABA-B-R1 co-immunoprecipitated with the CaR, confirming a physical interaction between the two receptors in GPCs. In vitro knockout of GABA-B-R1 genes, using a Cre-lox recombination strategy, blunted the ability of high extracellular Ca(2+) concentration to activate phospholipase C and ERK1/2, suppressed cell proliferation, and enhanced apoptosis in cultured GPCs. In GPCs, in which the GABA-B-R1 was acutely knocked down, there was reduced expression of early chondrocyte markers, aggrecan and type II collagen, and increased expression of the late differentiation markers, type X collagen and osteopontin. These results support the idea that physical interactions between CaRs and GABA-B-R1s modulate the growth and differentiation of GPCs, potentially by altering the function of CaRs.

Complex Formation with the Type B γ-Aminobutyric Acid Receptor Affects the Expression and Signal Transduction of the Extracellular Calcium-sensing Receptor

We co-immunoprecipitated the Ca(2+)-sensing receptor (CaR) and type B gamma-aminobutyric acid receptor (GABA-B-R) from human embryonic kidney (HEK)-293 cells expressing these receptors and from brain lysates where both receptors are present. CaRs extensively co-localized with the two subunits of the GABA-B-R (R1 and R2) in HEK-293 cell membranes and intracellular organelles. Coexpressing CaRs and GABA-B-R1s in HEK-293 cells suppressed the total cellular and cell surface expression of CaRs and inhibited phospholipase C activation in response to high extracellular [Ca(2+)] ([Ca(2+)](e)). In contrast, coexpressing CaRs and GABA-B-R2s enhanced CaR expression and signaling responses to raising [Ca(2+)](e). The latter effects of the GABA-B-R2 on the CaR were blunted by coexpressing the GABA-B-R1. Coexpressing the CaR with GABA-B-R1 or R2 enhanced the total cellular and cell surface expression of the GABA-B-R1 or R2, respectively. Studies with truncated CaRs indicated that the N-terminal extracellular domain of the CaR participated in the interaction of the CaR with the GABA-B-R1 and R2. In cultured mouse hippocampal neurons, CaRs co-localized with the GABA-B-R1 and R2. CaRs and GABA-B-R1s also co-immunoprecipitated from brain lysates. The expression of the CaR was increased in lysates from GABA-B-R1 knock-out mouse brains and in cultured hippocampal neurons with their GABA-B-R1 genes deleted in vitro. Thus, CaRs and GABA-B-R subunits can form heteromeric complexes in cells, and their interactions affect cell surface expression and signaling of CaR, which may contribute to extracellular Ca(2+)-dependent receptor activation in target tissues.

GABA-synthesizing enzyme, GAD67, from dermal fibroblasts: evidence for a new skin function

Glutamate decarboxylase (GAD) catalyzes the synthesis of gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, from glutamate. An expression of GAD protein has been reported for brain and pancreas, but not for skin. In this study, we present evidence that GAD67 mRNA and protein are expressed in mouse skin and in human dermal fibroblasts. The expression of GAD67 gene is weaker in aged mouse than the young one. To further explore the function of GAD in skin, we have examined a potential role(s) of GABA in human dermal fibroblasts. We have observed that GABA stimulates the synthesis of hyaluronic acid (HA) and enhances the survival rate of the dermal fibroblasts when fibroblasts are exposed to H(2)O(2) an oxidative stress agent. Also observed were lowering the levels of HA and collagen in the embryonic skin from GAD67 deficient mouse as compared to those from the wild-type (WT) mouse. In this study, we have presented the evidences that GAD67 is localized in the dermis and is potentially involved in variety of skin activities.

Peripheral GABAB agonists stimulate gastric acid secretion in mice

1 We characterized the effects of intravenous GABA and preferential GABAA (muscimol), GABAB (R-baclofen and SKF-97541) and GABAC agonists (imidazole-4-acetic acid) on gastric acid secretion in urethane-anesthetized mice implanted with a gastric cannula, and determined the role of vagal cholinergic mechanisms, and gastrin and somatostatin by using peptide immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. 2 The selective GABA(B) agonists R-baclofen (0.1-3 mg kg(-1), i.v.) and SKF-97541 (0.01-0.3 mg kg(-1), i.v.) induced a dose-related stimulation of gastric acid secretion. SKF-97541 was about 10 times more potent than R-baclofen stimulating gastric acid secretion. Neither GABA (0.1-100 mg kg(-1), i.v.) nor muscimol (0.1-3 mg kg(-1)) nor imidazole-4-acetic acid (0.1-10 mg kg(-1)) affected basal gastric acid secretion. 3 Stimulatory effects of SKF-97541 (0.1 mg kg(-1), i.v.) were blocked by the selective GABAB antagonist, 2-hydroxysaclofen, cholinergic blockade with atropine, subdiaphragmatic vagotomy or gastrin immunoneutralization. 4 Somatostatin immunoneutralization or SSTR2 blockade with PRL-2903 enhanced the secretory response to SKF-97541 (0.1 mg kg(-1), i.v.) by 78 and 105%, respectively. 5 In SSTR2 knockout mice, SKF-97541 (0.1 mg kg(-1), i.v.) increased basal gastric acid secretion by 48%. Neither GABA nor muscimol nor imidazole-4-acetic acid modified basal gastric acid secretion in SSTR2 knockout mice. 6 These results indicate that, in mice, stimulation of GABAB receptors increases gastric acid secretion through vagal- and gastrin-dependent mechanisms. Somatostatin implication might be secondary to the release of gastrin and the increase in gastric luminal acidity.

Histamine and histidine decarboxylase are hallmark features of ECL cells but not G cells in rat stomach

The oxyntic mucosa of the rat stomach is rich in ECL cells which produce and secrete histamine in response to gastrin. Histamine and the histamine-forming enzyme histidine decarboxylase (HDC) have been claimed to occur also in the gastrin-secreting G cells in the antrum. In the present study, we used a panel of five HDC antisera and one histamine antiserum to investigate whether histamine and HDC are exclusive to the ECL cells. By immunocytochemistry, we could show that the ECL cells were stained with the histamine antiserum and all five HDC antisera. The G cells, however, were not stained with the histamine antiserum, but with three of the five HDC antisera. Thus, histamine and HDC coexist in the ECL cells (oxyntic mucosa) but not in G cells (antral mucosa). Western blot analysis revealed a typical pattern of HDC-immunoreactive bands (74, 63 and 54 kDa) in oxyntic mucosa extracts with all five antisera. In antral extracts, immunoreactive bands were detected with three of the five HDC antisera (same as above); the pattern of immunoreactivity differed from that in oxyntic mucosa. Food intake of fasted rats or treatment with the proton pump inhibitor omeprazole raised the HDC activity and the HDC protein content of the oxyntic mucosa but not of the antral mucosa; the HDC activity in the antrum was barely detectable. We suggest that the HDC-like immunoreactivity in the antrum represents a cross-reaction with non-HDC proteins and conclude that histamine and HDC are hallmark features of ECL cells but not of G cells.

Gene expression and localization of GABA(C) receptors in neurons of the rat gastrointestinal tract

The effects of GABA in the CNS are mediated by three different GABA receptors: GABA(A), GABA(B) and GABA(C) receptors. GABA(A) and GABA(B) receptors, but not yet GABA(C) receptors, have been demonstrated in the enteric nervous system, where GABA has been proposed to be a transmitter. The purpose of this study was to determine whether GABA(C) receptors are present and thus may play a role in mediating the effects of GABA in the myenteric plexus of the rat gastrointestinal tract. We examined the expression of the three known GABA(C) receptor subunits, rho1, rho2 and rho3, in the rat duodenum, ileum and colon using the reverse transcriptase-polymerase chain reaction. We determined the localization of GABA(C) receptors in the myenteric plexus of these regions using two different antisera directed against GABA(C) receptor subunits. The polymerase chain reaction revealed that all three subunits were expressed in the gastrointestinal tract. When the layers of the intestine were separated and the layer containing myenteric neurons was assayed, the rho3 subunit was found in the ileum and colon, whereas rho1 was expressed in the duodenum and weakly in the colon and rho2 was expressed in the ileum. Immunocytochemistry revealed numerous labeled neurons in the myenteric plexus of each region. Colocalization showed that a large proportion of calbindin plus calretinin immunoreactive neurons (intrinsic primary afferent neurons) were immunoreactive for the GABA(C) receptor, and that 56% of nitric oxide synthase immunoreactive neurons (inhibitory motor neurons) exhibited the receptor. These results indicate that GABA(C) receptors of differing subunit compositions are expressed by neurons in the rat gastrointestinal tract. The effects of GABA on intrinsic sensory and on inhibitory motor neurons are likely to be mediated in part through GABA(C) receptors.

Human fibroblasts ubiquitously express glutamic acid decarboxylase 65 (GAD 65): possible effects of connective tissue inflammation on GAD antibody titer

BACKGROUND

Type 1 diabetes is caused by a destruction of pancreatic beta cells due to autoimmunity. Autoantibody against glutamic acid decarboxylase (GAD) 65 expressed in pancreatic beta cells is widely used as a predictive marker for pancreatic destruction. In this study, we hypothesized that if certain cells in periodontal tissues could express GAD, then it may influence GAD antibody titer.

METHODS

We used: 1) reverse transcription-polymerase chain reaction (PCR) analysis to detect GAD 65 mRNA in various cells; 2) nucleotide sequencing analysis to confirm that amplified PCR product is the gene encoding GAD; and 3) Western blotting to determine the expression of GAD 65 protein in human gingival fibroblasts. Immunohistochemical staining of GAD 65 protein in normal and inflamed gingiva was performed to examine the potential influence of periodontal inflammation on GAD 65 expression. GAD antibody titer in sera of periodontal patients as well as healthy subjects was measured to determine if periodontal patients could develop autoantibody against GAD 65.

RESULTS

Cultured human gingival, periodontal, and dermal fibroblasts and mesangial cells expressed GAD mRNA. Nucleotide sequencing analyses confirmed the amplified PCR product as GAD 65. Western immunoblotting analyses and immunohistochemical staining revealed that the GAD 65 protein was expressed in vitro and in vivo. The expression of GAD 65 in inflamed tissue was higher than that in normal tissues. Two of 62 periodontal patients without diabetes showed an increased antibody titer against GAD 65, while none of the systemically healthy subjects showed an increased antibody titer against this antigen.

CONCLUSIONS

We concluded that periodontal inflammation may result in higher levels of GAD and influence GAD antibody titer, and, hence, affect diabetic diagnosis based upon GAD antibody production.

Electrophysiological effects of GABA on cat pancreatic neurons

In mammalian peripheral sympathetic ganglia GABA acts presynaptically to facilitate cholinergic transmission and postsynaptically to depolarize membrane potential. The GABA effect on parasympathetic pancreatic ganglia is unknown. We aimed to determine the effect of locally applied GABA on cat pancreatic ganglion neurons. Ganglia with attached nerve trunks were isolated from cat pancreata. Conventional intracellular recording techniques were used to record electrical responses from ganglion neurons. GABA pressure microejection depolarized membrane potential with an amplitude of 17.4 +/- 0.7 mV. Electrically evoked fast excitatory postsynaptic potentials were significantly inhibited (5.4 +/- 0.3 to 2.9 +/- 0.2 mV) after GABA application. GABA-evoked depolarizations were mimicked by the GABA(A) receptor agonist muscimol and abolished by the GABA(A) receptor antagonist bicuculline and the Cl(-) channel blocker picrotoxin. GABA was taken up and stored in ganglia during preincubation with 1 mM GABA; beta-aminobutyric acid application after GABA loading significantly (P < 0.05) increased depolarizing response to GABA (15.6 +/- 1.0 vs. 7.8 +/- 0.8 mV without GABA preincubation). Immunolabeling with antibodies to GABA, glial cell fibrillary acidic protein, protein gene product 9.5, and glutamic acid decarboxylase (GAD) immunoreactivity showed that GABA was present in glial cells, but not in neurons, and that glial cells did not contain GAD, whereas islet cells did. The data suggest that endogenous GABA released from ganglionic glial cells acts on pancreatic ganglion neurons through GABA(A) receptors.

Immunocytochemical evidence suggesting that diamine oxidase catalyzes biosynthesis of gamma-aminobutyric acid in antropyloric gastrin cells

gamma-Aminobutyric acid (GABA) is a neurotransmitter that also occurs in a few non-neuronal cell types, where it may serve as a paracrine modulator. GABA is biosynthesized from glutamate by glutamate decarboxylase (GAD) and from putrescine via diamine oxidase (DAO). GAD is demonstrable in several GABA-positive cell types but is undetectable in the GABA-containing gastrin cells and somatostatin cells of the antropyloric mucosa of the stomach. Using two antisera raised against synthetic peptides corresponding to two different regions of rat DAO, we now demonstrate strong reactivity for DAO in gastrin-positive cells of the rat antropyloric mucosa, whereas somatostatin-positive cells as well as other structures of the antrum are unreactive. Western blotting analysis of antrum and colon demonstrate that both antisera react with a single band of 85 kD, consistent with the predicted molecular weight of DAO. Expression of DAO mRNA in the antrum is demonstrated by reverse transcriptase polymerase chain reaction (RT-PCR). Our results strongly indicate that gastrin cells produce GABA via DAO-catalyzed oxidation of putrescine, and experimental data moreover suggest that the biosynthesis of GABA is regulated by the prandial state. Because GABA modulates release of somatostatin, these results point to a new mechanism of paracrine interaction between gastrin cells and somatostatin cells.

Innervation of the gastric mucosa

A plethora of neuronal messengers ("classical" transmitters, gaseous messengers, amino acid transmitters, and neuropeptides) are capable of mediating or modulating gastric functions. Accordingly, the stomach is richly innervated. Gastric nerves are either intrinsic to the gastric wall, i.e., they have their cell bodies in the intramural ganglia and thus belong to the enteric nervous system, or they reach the stomach from outside, originating in the brainstem, in sympathetic ganglia, or in sensory ganglia. Topographically, the nerve fibers in the stomach reach all layers from the most superficial portions of the gastric glands to the outer smooth muscle layer. This wide distribution implies that virtually all different cell types may be reached by neuronal messengers. Within the gastric mucosa endocrine and paracrine cells (e.g., gastrin cells, ECL cells, somatostatin cells), exocrine cells (parietal cells, chief cells, mucous cells), smooth muscle cells, and stromal cells are regulated by neuronal messengers. The sensory innervation, responding to capsaicin, plays an important role in mucosal protection, and in ulcer healing. Presumably also other nerves are involved and a plasticity in the neuropeptide expression has been demonstrated at the margin of gastric ulcers. Taken together, available data indicate a complex interplay between hormones, paracrine messengers and neuronal messengers, growth factors and cytokines in the regulation of gastric mucosal activities such as secretion, local blood flow, growth, and restitution after damage.

Autoradiographic distribution of radioactivity from (14)C-GABA in the mouse

We investigated the distribution of radioactivity from (14)C-labeled gamma-aminobutyric acid (GABA) in the mouse by in vivo autoradiography to clarify the tissues that show GABA uptake and/or GABA binding. Male mice were injected intravenously with (14)C-GABA in both the absence and presence of an excess of unlabeled GABA, baclofen and isoguvacine. Whole-body autoradiography of (3)H-baclofen, a GABA(B) receptor agonist was also performed. At short intervals after (14)C-GABA injection ( 3 and 6 minutes), very high radioactivity was detected in the kidney cortex, liver, pineal gland, hypophysis, median eminence of the hypothalamus, and cervical ganglion. The hyaline cartilage and glandular part of the stomach showed moderate radioactivity. In the presence of an excess amount of unlabeled GABA, radioactivity in most of tissues decreased significantly, but no significant difference in radioactivity was observed in the presence of baclofen and isoguvacine, agonists of GABA(A) and GABA(B) receptors, respectively. Autoradiography of (3)H-baclofen showed that the kidney had high level of radioactivity, whereas the activity in other tissues and organs was similar or lower than in the blood except for the content of the urinary bladder and the pancreas at 15 minutes after injection. These data indicate that radioactivity from incorporated (14)C-GABA into a variety of cells is much higher than that from bound (14)C-GABA to the receptor sites. Our results suggest that GABA can be quickly localized in many organs of the mouse body after 3 minutes following injection, and GABA may serve multiple functions in those organs.

GABA(A) receptor subunit messenger RNA expression in the enteric nervous system of the rat: implications for functional diversity of enteric GABA(A) receptors

GABAergic neurons occur in the myenteric plexus and submucosa and their innervations of the gut, where GABA stimulates motor neurons, and non-neural cells via "central type" GABA(A) receptors. These receptors occur on half of the neurons in the rat intestine. The GABA(A) receptor is a ligand-gated chloride channel constructed from different subunit families (alpha, beta, gamma, delta, epsilon). In rat these exist as subtypes, alpha1-6, beta1-3, gamma1-3 and delta, defining the clinically relevant pharmacological features of GABA(A) receptors. However, the identity, distribution, and abundance of enteric GABA(A) receptor subunits are unknown. To identify and map the regional expression of GABA(A) receptor subunit messenger RNAs in the enteric nervous system, we assayed enteric RNA from the ileum of Sprague-Dawley rats by reverse transcription-polymerase chain reaction for alpha1-6, beta 1-3, gamma1-3, and delta subunit messenger RNAs. Subunit messenger RNA localization, was probed by in situ hybridization. Reverse transcription-polymerase chain reaction analysis of RNA from myenteric and submucosal nerve layers revealed the expression alpha1, alpha3, beta2, beta3, gamma1 and gamma3 subunit messenger RNAs. Little alpha4 and alpha6 and no alpha2, beta1, gamma2 or delta subunit messenger RNA were detected. In situ hybridization revealed that transcripts for alpha1, alpha3, alpha5 and beta2 subunits occur in both myenteric and submucous ganglia. However, beta3 messenger RNA was found only in myenteric plexus. The gamma1 subunit messenger RNA was also restricted to the cells in the myenteric plexus while gamma3 was found in cells of both nerve layers. In this study of the subunit messenger RNA expression profile of GABA(A) receptors within the enteric nerve layers we show an abundant, diverse and widespread distribution that is unique in comparison to the CNS. The distinctive and heterogeneous distribution of enteric GABA(A) subunits may be important in the integration of neural control of gut function.

Control of autoimmune diabetes in NOD mice by GAD expression or suppression in beta cells

Glutamic acid decarboxylase (GAD) is a pancreatic beta cell autoantigen in humans and nonobese diabetic (NOD) mice. beta Cell-specific suppression of GAD expression in two lines of antisense GAD transgenic NOD mice prevented autoimmune diabetes, whereas persistent GAD expression in the beta cells in the other four lines of antisense GAD transgenic NOD mice resulted in diabetes, similar to that seen in transgene-negative NOD mice. Complete suppression of beta cell GAD expression blocked the generation of diabetogenic T cells and protected islet grafts from autoimmune injury. Thus, beta cell-specific GAD expression is required for the development of autoimmune diabetes in NOD mice, and modulation of GAD might, therefore, have therapeutic value in type 1 diabetes.

Glutamic acid decarboxylase autoantibodies in saliva of children with type 1 diabetes

The aim of this study was to investigate the presence of glutamic acid decarboxylase (GAD) autoantibodies in the saliva of children with type 1 diabetes. A total of 30 diabetic children were used in this study while 80 healthy individuals of similar age were used as controls. Total saliva and peripheral blood was collected from all diabetic children and controls. Determination of GAD autoantibodies was performed using an enzyme immunoassay (Varelisa) which is based on a recombinant human GAD65. The results of this study showed that GAD autoantibodies were found to be present in 21 out of 30 diabetic serum samples, while they were present in all samples of diabetic saliva. In contrast, they were absent from serum and saliva of healthy controls. Since GAD is considered one of the basic antigenic targets of the autoimmune process in type 1 diabetes in the pancreatic beta cells, its presence in the saliva may be relevant as a predictive marker.

Double-label immunofluorescence study of glutamic acid decarboxylase in the fetal and adult ovine pancreas by light and confocal microscopy: evidence for predominant beta-cell coexpression

Glutamic acid decarboxylase (GAD) is present in the central nervous system and in several nonneuronal tissues including the pancreatic islets. There are two isoforms with molecular weights of 65 kDa (GAD65) and 67 kDa (GAD67). The cellular specificity of the two molecular forms of GAD and their levels within the mammalian islets may be species-dependent, being coexpressed in both beta and in non-beta cells. We have examined the ovine pancreas, from the adult and fetal stages of late gestation, for the expression of GAD65 within the islet cells by double-label immunofluorescence light and confocal microscopy. In the adult tissue, GAD65 was colocalized in a majority of the beta cells (> 95%), with only a few glucagon and somatostatin cells (< 5%) showing immunolocalization. During the fetal stages GAD65 also showed a similar predominant beta-cell coexpression. The enzyme was also detected in a few fetal glucagon (< 5%) but not somatostatin cells. In the degenerating large fetal islets, GAD65 was also observed in the majority of the residual beta cells. These results demonstrate that in the ovine pancreas GAD65 is expressed during fetal development and is predominantly beta-cell-restricted. This pattern of expression is maintained during adult life. However, the physiological role of pancreatic GAD and/or its biosynthetic product, gamma-aminobutyric acid, in islet function in the sheep and in other ruminants remains unclear.

Transcription and translation of two glutamate decarboxylase genes in the ileum of rat, mouse and guinea pig

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter, synthesised from glutamate by glutamate decarboxylase (GAD), in the central nervous system. Two forms of GAD, designated GAD 65 and GAD 67, are encoded by distinct genes and have been demonstrated in the mammalian brain. GABA has been postulated to be synthesised in neurons of the enteric nervous system (ENS), but evidence for its role as an enteric neurotransmitter is equivocal. We therefore aimed to determine whether GAD 65 and GAD 67 messenger RNAs (mRNAs) and proteins were expressed in the ileum of mice, rats and guinea pigs. Using an RNase protection assay, both GAD 65 and GAD 67 mRNAs were detected in the rodent small intestine. Antisera specific for GAD 65 or GAD 67, used in immunoblot analyses, revealed GAD 65-like and GAD 67-like immunoreactivity in rat and guinea pig ileum. Anti-GAD 65 antisera detected a major band of 65 kDa. Anti-GAD 67 antisera detected a major band of 55 kDa, which probably represented a breakdown product, and a minor band of 67 kDa. Analysis of immunoblot extracts of rat and guinea pig ileum revealed more GAD 67-like than GAD 65-like immunoreactivity. GAD enzymatic activity was high in the rat and guinea-pig brain, and low in the whole and dissected ileum. These results demonstrate that both GAD 65 and GAD 67 genes are transcribed and translated in the ileum of three rodent species and lend indirect support to the postulate that GABA is synthesised by neurons of the ENS and intestinal endocrine cells.

gamma-Aminobutyric acid (GABA) metabolism in mammalian neural and nonneural tissues

4-Aminobutyric acid (GABA), a major inhibitory neurotransmitter of mammalian central nervous system, is found in a wide range of organisms, from prokaryotes to vertebrates. GABA is widely distributed in nonneural tissue including peripheral nervous and endocrine systems. GABA acts on GABAA and GABAB receptors. GABAA receptors are ligand-gated chloride channels modulated by a variety of drugs. GABAB receptors are essentially presynaptic, usually coupled to potassium or calcium channels, and they function via a GTP binding protein. In neural and nonneural tissues, GABA is metabolized by three enzymes--glutamic acid decarboxylase (GAD), which produces GABA from glutamic acid, and the catabolic enzymes GABA-transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH). Production of succinic acid by SSADH allows entry of the GABA carbon skeleton into the tricarboxylic acid cycle. Alternate sources of GABA include putrescine, spermine, spermidine and ornithine, which produce GABA via deamination and decarboxylation reactions, while L-glutamine is an additional source of glutamic acid via deamination. GAD from mammalian brain occurs in two molecular forms, GAD65 and GAD67 (referring to subunit relative molecular weight (Mr) in kilodaltons). These different forms of GAD are the product of different genes, differing in nucleotide sequence, immunoreactivity and subcellular localization. The presence and characteristics of GAD have been investigated in a wide variety of nonneural tissues including liver, kidney, pancreas, testis, ova, oviduct, adrenal, sympathetic ganglia, gastrointestinal tract and circulating erythrocytes. In some tissues, one form (GAD65 or GAD67) predominates. GABA-T has been located in most of the same tissues, primarily through histochemical and/or immunochemical methods; GABA-T is also present in a variety of circulating cells, including platelets and lymphocytes. SSADH, the final enzyme GABA catabolism, has been detected in some of the tissues in which GAD and GABA-T have been identified, although the presence of this enzyme has not been in mammalian pancreas, ova, oviduct, testis or sympathetic ganglia.

Transient expression of glutamate decarboxylase and gamma-amino butyric acid in embryonic lens fibers of the rat

We have determined the localization and developmental expression of glutamate decarboxylase (GAD67) in the rat lens. Immunofluorescence experiments showed that GAD67 was transiently expressed in the nuclear fiber cells of the lens between embryonic days (E) 15 and 20, with maximal immunostaining occurring on E17 and E18. gamma-amino butyric acid (GABA) co-localized with GAD67 in the embryonic nuclear fiber cells. Reverse transcription-polymerase chain reaction (RT-PCR) tests showed that at least three alternatively spliced forms of GAD67 mRNA, including mRNAs with and without the I80 and the I86 insert, were transiently co-expressed with GAD67 in the embryonic lens. The major GAD67 protein in the lens was 67 kDa. We conclude that enzymatically active GAD67 is transiently expressed in the lens nuclear fiber cells of the embryonic rat. The transient expression is regulated by transcriptional and/or posttranscriptional processes. We speculate on the basis of possible common gene regulatory elements for glutamate and ornithine decarboxylases and the involvement of these enzymes with polyamine synthesis, that the transient expression of GAD67 may be connected to nuclear and/or DNA breakdown during lens fiber cell differentiation.

Baclofen, a gamma-aminobutyric acid-b receptor agonist, delays diabetes onset in the non-obese diabetic mouse

Glutamic acid decarboxylase (GAD) is the enzyme responsible for the synthesis of gamma-aminobutyric acid (GABA). GAD has been identified as a 64-kDa antigen expressed in pancreatic beta-cells, to which autoantibodies are generated prior to the onset of type 1 (insulin-dependent) diabetes mellitus. GAD may therefore be an initiating factor in beta-cell destruction. We administered baclofen, a GABA-B receptor agonist, to non-obese diabetic (NOD) mice in an attempt to down-regulate GAD expression and thereby reduce the incidence of diabetes. Twenty-four female NOD mice were given baclofen in their drinking water at a final dose of 50 mg/kg body weight daily from weaning to 30 weeks of age. Twenty-four sex- and litter-matched mice were used as controls. At 30 weeks there was no difference in the incidence of diabetes in the treated group compared with the controls. However, there was a significant delay in the onset of diabetes in the treated group (P < 0.001, parallelism test). The degree of insulitis and the GAD activity in the pancreas per mg of protein were unchanged by baclofen treatment with respect to controls. These results suggest that baclofen may be effective in delaying diabetes onset in NOD mice by stimulating GABA activity, as this neurotransmitter, localised in the islets, may modulate insulin secretion and the antigen expression associated with it.

Immunohistochemical and histochemical evidence for the presence of noradrenaline, serotonin and gamma-aminobutyric acid in chief cells of the mouse carotid body

The immunohistochemical study revealed tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), phenylethanolamine N-methyltransferase (PNMT), serotonin, glutamate decarboxylase (GAD) and gamma-aminobutyric acid (GABA) immunoreactivities in the mouse carotid body. TH and DBH immunoreactivities were found in almost all chief cells and a few ganglion cells, and in relatively numerous varicose nerve fibers of the carotid body. The histofluorescence microscopy showed catecholamine fluorescence in almost all chief cells. However, no PNMT immunoreactivity was observed in the carotid body. Serotonin, GAD and GABA immunoreactivities were also seen in almost all chief cells of the carotid body. From combined immunohistochemistry and fluorescence histochemistry, catecholamine and serotonin or catecholamine and GABA were colocalized in almost all chief cells. Thus, these findings suggest that noradrenaline, serotonin and GABA may be synthesized and co-exist in almost all chief cells of the mouse carotid body and may play roles in chemoreceptive functions.

Colocalization of gamma-aminobutyrate and gastrin in the rat antrum: an immunocytochemical and in situ hybridization study

BACKGROUND/AIMS

The inhibitory neurotransmitter gamma-aminobutyrate (GABA) has been shown to coexist with insulin in pancreatic beta-cells. We have presently investigated whether GABA also colocalizes with gastrin in G cells in rat antral mucosa.

METHODS

Three alternative approaches were used: (1) gastrin in situ hybridization and GABA immunocytochemistry on consecutive cryostat sections; (2) GABA immunocytochemistry and gastrin immunocytochemistry on adjacent semithin and ultrathin sections; and (3) double-immunogold labeling of GABA and gastrin in the same ultrathin section.

RESULTS

Colocalization of GABA and gastrin was observed with each of the three approaches. In the double-immunogold labeled cells, the G-cell granules displayed a high gold-particle density indicating gastrin and a low particle density indicating GABA, whereas the converse was true for the extragranular cytoplasmic matrix. The gold-particle ratios between these compartments were 11 (for gastrin) and 0.36 (for GABA), respectively. GABA labeling was also observed in two other antral endocrine cell types, classified by morphological criteria as somatostatin producing D cells and serotonin producing ECn cells.

CONCLUSIONS

This is the first direct demonstration of GABA in gastrointestinal G cells. Our findings suggest that GABA may have a paracrine function in the stomach mucosa, analogous to its presumed role in the pancreatic islets.

Ultrastructural cytochemistry of acetylcholinesterase enzyme activity in pancreatic tissue transplants in rats

The distribution of acetylcholinesterase (AChE) at the ultrastructural level was investigated in normal and in pancreatic fragments transplanted for 56 days into the anterior eye chamber of heterologous rats using enzyme cytochemical methods. Acetylcholinesterase reaction products were seen on the basal surface of the acinar cells in normal pancreas. Acetylcholinesterase enzyme activity was also detected on the axolemma of the surviving nerve fibres. This enzyme reaction product forms alternating thick and thin bands on the axolemma. Some of these AChE-positive nerve fibres accompany blood vessels that also survive after transplantation. AChE were seen in cytoplasm adjacent to the surviving alpha and pancreatic polypeptide cells. We conclude that the ability of some neurons and cells to produce and or store acetylcholinesterase is still retained after transplantation of pancreatic tissue into the anterior eye chamber of rats.

Gamma-aminobutyric acid (GABA) immunoreactivity in the mouse adrenal gland

Gamma-aminobutyric acid (GABA) immunoreactivity was revealed by immunocytochemistry in the mouse adrenal gland at the light and electron microscopic levels. Groups of weakly or faintly GABA immunoreactive chromaffin cells were often seen in the adrenal medulla. By means of immunohistochemistry combined with fluorescent microscopy, these GABA immunoreactive chromaffin cells showed noradrenaline fluorescence. The immunoreaction product was seen mainly in the granular cores of these noradrenaline cells. These results suggest the co-existence of GABA and noradrenaline within the chromaffin granules. Sometimes thick or thin bundles of GABA immunoreactive nerve fibers with or without varicosities were found running through the cortex directly into the medulla. In the medulla, GABA immunoreactive varicose nerve fibers were numerous and were often in close contact with small adrenaline cells and large ganglion cells; a few, however, surrounded clusters of the noradrenaline cells, where membrane specializations were formed. Single GABA immunoreactive nerve fibers, and thin or thick bundles of the immunoreactive varicose nerve fibers ran along the blood vessels in the medulla. The immunoreaction deposits were observed diffusely in the axoplasm and in small agranular vesicles of the GABA immunoreactive nerve fibers. Since no ganglion cells with GABA immunoreactivity were found in the adrenal gland, the GABA immunoreactive nerve fibers are regarded as extrinsic in origin.

Human autoantibodies react with glutamic acid decarboxylase antigen in human and rat but not in mouse pancreatic islets

The presence of one of the major targets for autoantibodies in Type 1 (insulin-dependent) diabetes mellitus, the enzyme glutamic acid decarboxylase, was studied in human, rat and mouse pancreatic tissue using immunoprecipitation and immunohistochemical techniques. Immunoprecipitation of glutamic acid decarboxylase was attempted with lysates of [35S]-methionine-labelled rat or mouse pancreatic islets using two different glutamic acid decarboxylase antisera, one mouse monoclonal antibody raised against the 65 kDa isoform of the enzyme, sera from six patients with Type 1 diabetes, one patient with stiff-man syndrome and sera from 19 non-obese diabetic mice. The same sera were used for immunoperoxidase staining of cryosections of human, rat or mouse pancreas. Using patient sera glutamic acid decarboxylase was detected by immunoprecipitations from isolated rat islets but not from islets of five different mouse strains tested, including the non-obese diabetic mouse. When using the non-obese diabetic mouse sera, glutamic acid decarboxylase could not be detected in either rat or mouse tissue. Immunoperoxidase staining demonstrated high levels of glutamic acid decarboxylase in human and rat pancreatic islets but low levels in mouse islets. Direct measurements of enzyme activity showed glutamic acid decarboxylase to be present in mouse islets at a level of about 40% of that in rat islets, and subsequent Western blot analyses indicated that mouse islets express the 67 kDa isoform, whereas as in rat islets both the 67 and 65 kDa isoforms are present.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroepithelial cells in the rat spinal cord express glutamate decarboxylase immunoreactivity in vivo and in vitro

It is unknown whether neuroepithelial cells in the mammalian central nervous system express neurotransmitter-synthesizing enzymes. In this study, expression of glutamate decarboxylase (GAD), the gamma-aminobutyric acid (GABA)-synthesizing enzyme, was examined in proliferative cells and postmitotic neuroblasts in embryonic rat spinal cord. Immunostaining coronal sections of the embryonic spinal cord with K2 antiserum, which recognizes GAD proteins encoded by the GAD67 gene, revealed intensely stained neuroepithelial cells in the basal plate at embryonic day (E) 13, in the intermediate plate between E 13-16, and last seen in the alar plate at E 16. Nissl counterstaining demonstrated that a small number of these GAD-immunoreactive cells adjacent to the neural tube lumen were mitotic. The ventral-to-dorsal gradient of GAD expression in precursor cells and postmitotic neuroblasts correlates anatomically and temporally with the sequential generation of motoneurons, commissural neurons, and interneurons in the dorsal horn. Some of these GAD-immunoreactive neuroepithelial cells may re-enter the mitotic cycle, while others are postmitotic neuroblasts presumably migrating to the intermediate zone to differentiate into young neurons. Double-immunostaining cells acutely dissociated from E 11-18 spinal cords with K2 and anti-bromodeoxyuridine antisera, following a bromodeoxyuridine pulse in vivo, revealed considerable numbers of DNA-synthesizing cells immunoreactive for GAD. The absolute number of double-stained cells peaked during E 12-15, coinciding with terminal cell division in most spinal neurons. These observations suggest that spinal neuronal precursors can synthesize GAD-related proteins prior to, or during, the terminal cell cycle. Although GAD immunoreactivity revealed by K2 antiserum was detected in proliferative cells and in migrating postmitotic neuroblasts, GABA immunoreactivity was never detectable in these cells. These early embryonic GAD-immunoreactive neuroepithelial cells may either synthesize levels of GABA that cannot be detected immunocytochemically, and/or express enzymatically inactive GAD-related proteins.

Human monoclonal islet cell antibodies from a patient with insulin-dependent diabetes mellitus reveal glutamate decarboxylase as the target antigen

The autoimmune phenomena associated with destruction of the beta cell in pancreatic islets and development of type 1 (insulin-dependent) diabetes mellitus (IDDM) include circulating islet cell antibodies. We have immortalized peripheral blood lymphocytes from prediabetic individuals and patients with newly diagnosed IDDM by Epstein-Barr virus transformation. IgG-positive cells were selected by anti-human IgG-coupled magnetic beads and expanded in cell culture. Supernatants were screened for cytoplasmic islet cell antibodies using the conventional indirect immunofluorescence test on cryostat sections of human pancreas. Six islet cell-specific B-cell lines, originating from a patient with newly diagnosed IDDM, could be stabilized on a monoclonal level. All six monoclonal islet cell antibodies (MICA 1-6) were of the IgG class. None of the MICA reacted with human thyroid, adrenal gland, anterior pituitary, liver, lung, stomach, and intestine tissues but all six reacted with pancreatic islets of different mammalian species and, in addition, with neurons of rat cerebellar cortex. MICA 1-6 were shown to recognize four distinct antigenic epitopes in islets. Islet cell antibody-positive diabetic sera but not normal human sera blocked the binding of the monoclonal antibodies to their target epitopes. Immunoprecipitation of 35S-labeled human islet cell extracts revealed that a protein of identical size to the enzyme glutamate decarboxylase (EC 4.1.1.15) was a target of all MICA. Furthermore, antigen immunotrapped by the MICA from brain homogenates showed glutamate decarboxylase enzyme activity. MICA 1-6 therefore reveal glutamate decarboxylase as the predominant target antigen of cytoplasmic islet cell autoantibodies in a patient with newly diagnosed IDDM.