Regulated Exocytosis of GABA-containing Synaptic-like Microvesicles in Pancreatic β-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 GABAA-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

SUR1 Regulates PKA-independent cAMP-induced Granule Priming in Mouse Pancreatic B-cells.

Measurements of membrane capacitance were applied to dissect the cellular mechanisms underlying PKA-dependent and -independent stimulation of insulin secretion by cyclic AMP. Whereas the PKA-independent (Rp-cAMPS–insensitive) component correlated with a rapid increase in membrane capacitance of ~80 fF that plateaued within ~200 ms, the PKA-dependent component became prominent during depolarizations >450 ms. The PKA-dependent and -independent components of cAMP-stimulated exocytosis differed with regard to cAMP concentration dependence; the Kd values were 6 and 29 µM for the PKA-dependent and -independent mechanisms, respectively. The ability of cAMP to elicit exocytosis independently of PKA activation was mimicked by the selective cAMP-GEFII agonist 8CPT-2Me-cAMP. Moreover, treatment of B-cells with antisense oligodeoxynucleotides against cAMP-GEFII resulted in partial (50%) suppression of PKA-independent exocytosis. Surprisingly, B-cells in islets isolated from SUR1-deficient mice (SUR1-/- mice) lacked the PKA-independent component of exocytosis. Measurements of insulin release in response to GLP-1 stimulation in isolated islets from SUR1-/- mice confirmed the complete loss of the PKA-independent component. This was not attributable to a reduced capacity of GLP-1 to elevate intracellular cAMP but instead associated with the inability of cAMP to stimulate influx of Cl- into the granules, a step important for granule priming. We conclude that the role of SUR1 in the B cell extends beyond being a subunit of the plasma membrane KATP-channel and that it also plays an unexpected but important role in the cAMP-dependent regulation of Ca2+-induced exocytosis

Increase in cellular glutamate levels stimulates exocytosis in pancreatic β-cells

AbstractGlutamate has been implicated as an intracellular messenger in the regulation of insulin secretion in response to glucose. Here we demonstrate by measurements of cell capacitance in rat pancreatic β-cells that glutamate (1 mM) enhanced Ca2+-dependent exocytosis. Glutamate (1 mM) also stimulated insulin secretion from permeabilized rat β-cells. The effect was dose-dependent (half-maximum at 5.1 mM) and maximal at 10 mM glutamate. Glutamate-induced exocytosis was stronger in rat β-cells and clonal INS-1E cells compared to β-cells isolated from mice and in parental INS-1 cells, which correlated with the expressed levels of glutamate dehydrogenase. Glutamate-induced exocytosis was inhibited by the protonophores FCCP and SF6847, by the vacuolar-type H+-ATPase inhibitor bafilomycin A1 and by the glutamate transport inhibitor Evans Blue. Our data provide evidence that exocytosis in β-cells can be modulated by physiological increases in cellular glutamate levels. The results suggest that stimulation of exocytosis is associated with accumulation of glutamate in the secretory granules, a process that is dependent on the transgranular proton gradient

Ringing the dinner bell for insulin: muscarinic M3 receptor activity in the control of pancreatic beta cell function.

In this issue of Cell Metabolism, Gautam et al. (2006) show that pancreatic beta cell M3 muscarinic acetylcholine receptors control insulin secretion. Their results highlight the role of the M3 receptor subtype in integrating nervous stimuli with metabolic control of insulin secretion and glucose homeostasis

Involvement of Protein Kinase C-ϵ in Inositol Hexakisphosphate-induced Exocytosis in Mouse Pancreatic β-Cells

Inositolhexakisphosphate (InsP6) plays a pivotal role in the pancreatic β-cell stimulus-secretion coupling. We have used capacitance measurements to study the effects of InsP6 on Ca2+-dependent exocytosis in single mouse pancreatic β-cells. In the presence of inhibitors of the protein phosphatase calcineurin to block endocytosis, intracellular application of InsP6 produced a dose-dependent stimulation of exocytosis, and half-maximal effect was observed at 22 μm. The stimulatory effect of InsP6 was dependent on protein kinase C (PKC) activity. Antisense oligonucleotides directed against specific PKC isoforms (α, βII, δ, ϵ, ξ) revealed the involvement of PKC-ϵ in InsP6-induced exocytosis. Furthermore, expression of dominant negative PKC-ϵ abolished InsP6-evoked exocytosis, whereas expression of wild-type PKC-ϵ led to a significant stimulation of InsP6-induced exocytosis. These data demonstrate that PKC-ϵ is involved in InsP6-induced exocytosis in pancreatic β-cells

Endoplasmic reticulum stress and pancreatic beta-cell death

In pancreatic beta-cells, the endoplasmic reticulum (ER) is an important cellular compartment for insulin biosynthesis, which accounts for half of the total protein production in these cells. Protein flux through the ER must be carefully monitored to prevent dysregulation of ER homeostasis and stress. ER stress elicits a signaling cascade known as the unfolded protein response (UPR), which influences both life and death decisions in cells. beta-cell loss is a pathological component of both type 1 and type 2 diabetes, and recent findings suggest that ER stress is involved. In this review, we address the transition from the physiological ER stress response to the pathological response, and explore the mechanisms of ER stress-mediated beta-cell loss during the progression of diabetes

ER stress: The link between pancreatic beta cell dysfuntion and death

In pancreatic β cells, the endoplasmic reticulum (ER) is an important cellular compartment for insulin biosynthesis, which accounts for half of total protein production in these cells. Protein flux through the ER must be carefully monitored to prevent dysregulation of ER homeostasis and stress. ER stress elicits a signaling cascade known as the unfolded protein response (UPR) which functions in influencing both cellular life and death decisions. β cell loss is a pathological component of both type 1 and type 2 diabetes, and recent findings suggest that ER stress is involved. A dysfunctional UPR and uncontrolled ER stress may be the tipping point between β cell dysfunction and the progression to cell death. In this review, we address the transition from the physiological ER stress response to the pathological response and explore the mechanisms of ER stress-mediated β cell loss during the progression of diabetes