Heterogeneity in glutamic acid decarboxylase expression among single rat pancreatic beta cells

Abstract

Pancreatic beta cells contain the enzyme glutamic acid decarboxylase (GAD) that catalyses the biosynthesis of the neurotransmitter g-aminobutyric acid (GABA) [1,2]. Two non-allelic genes probably derived from a common ancestral gene encode GAD. The two GAD isoforms, named GAD65 and GAD67 according to their approximate molecular weight in kDa, share a similar structure, except in the first 95 amino acids [3]. Despite their homology, there are structural and functional differences between them that are reflected by their intracellular compartmentalization and the degree of saturation with the coenzyme pyridoxal phosphate (PLP) [2±4].Although the physiological significance of GAD and GABA in beta-cell function is not entirely clear, the presence of antibodies directed against GAD65 have been implicated in the pathogenesis of Type I (insulin-dependent) diabetes mellitus and Stiff-Man Abstract Aims/hypothesis. An isoform of glutamic acid decarboxylase, (GAD)65 has been identified as a pancreatic beta-cell autoantigen in Type I (insulin dependent) diabetes mellitus. We investigated the expression of GAD isoforms among single rat beta cells in culture, under different conditions and the correlation between GAD65 expression and insulin secretion-rate. Results. Independent of culture conditions, 100 % of fresh and cultured beta cells express GAD67. In contrast, considerable heterogeneity in GAD65 expression among single beta cells was observed. After 2 days in culture in 2.6 mmol/l glucose, only 24 % of the beta cells express GAD65. This percentage increases to 39 % in 5.6 mmol/l glucose and to 54 % and 56 % in 11.6 mmol/l and 20.6 mmol/l glucose, respectively. Moreover, reducing glucose concentration from 11.6 to 2.5 mmol/l for 2 days, reduces GAD65 expression by nearly 30 %. After 11 days in culture with 11.6 mmol/l glucose, 50 % of beta cells continue expressing GAD65, this percentage is further increased to nearly 75 % by including either nerve growth factor or dibutyryl cyclic AMP or both in the culture medium. When beta cells are challenged for 1 h with 20.6 mmol/l glucose, 67 % respond forming insulin-immunoplaques. More than two-thirds of insulin-secretors are GAD65-positive, in contrast to only 11 % of the non-secreting cells. Moreover, 87 % of beta cells that have a high insulin secretory rate express GAD65. Conclusion/interpretation. These results show that the most active beta cells, which secrete more insulin, also express GAD65 and that manipulating extracellular glucose may modify the expression of the enzyme and possibly the autoimmune attack in Type I diabetes. [Diabetologia (1999

Modulation of Glucocorticoid-Induced GAD Expression in Pancreatic β-Cells by Transcriptional Activation of the GAD67 Promoter and Its Possible Effect on the Development of Diabetes

“…In humans, the rate of GAD expression is increased in pancreatic ␤-cells at the postnatal stage but is decreased in adulthood (46). Similarly, the expression of GAD in the islets of adult rats is probably at a maximal level and thus was used as a marker of ␤-cell maturity in rats (47). Taken together, the rate of GAD expression in ␤-cells seems to be correlated with age-dependent ␤-cell maturation.…”

? Tranzitornyy? neonatal'nyy diabet vsledstvie duplikatsii v 6-y khromosome [dup(6)(q24.2q24.2) de novo]

“…Анализ результатов этих исследований на моделях животных и у человека позво лил выявить такие потенциально важные антигены, как инсулин, глютаминовая кислота декарбоксилазы (GAD) и тирозинфосфотаза протеина IA-2, гликопротеиды (ганг-лиозид Gm2-1), а также такие компоненты клеток, как ту-булин и актин [14,25,35,39].…”

“…Среди популяции Р-клеток обнаружены большие клет ки, образующие бляшки, которые отвечают за большую часть секретируемого инсулина и представляют самый низкий порог его секреции в ответ на изменения внекле точных концентраций глюкозы [21,39]. Эта субпопуляция Р-клеток, обеспечивающая высокий уровень секреции ин сулина, была названа LP-клетками.…”

“…Эти последние субпопуляции клеток будут отвечать за «выживание» Р-клеток [38]. Однако отсутствие LP-клеток делает островки неспособными реагировать на повышен ные концентрации глюкозы в плазме, что может привес ти к манифестации диабета [39]. Таким образом, понима ние механизмов регуляции экспрессии GAD-65 может оказать помощь в уменьшении аутоиммунной агрессии против р-клеток.…”

Streptozotocin Upregulates GAD67 Expression in MIN6N8a Mouse Beta Cells

“…Degli et al suggested that accumulation of glutamate upon perturbation of GABA network, produced by nutrient or environmental stress, could up-regulate expression of GAD [17]. High glucose concentrations were also shown to increase GAD expression in cultured islets [18][19][20]. GAD expression varies also during ontogeny.…”