Possible New Strategies for the Treatment of Congenital Hyperinsulinism.

Objective: Congenital hyperinsulinism (CHI) is a rare disease characterized by persistent hypoglycemia as a result of inappropriate insulin secretion, which can lead to irreversible neurological defects in infants. Poor efficacy and strong adverse effects of the current medications impede successful treatment. The aim of the study was to investigate new approaches to silence β-cells and thus attenuate insulin secretion.

Acyl-Ghrelin Influences Pancreatic β-Cell Function by Interference with K Channels.

The aim for this study was to elucidate how the hypothalamic hunger-inducing hormone acyl-ghrelin (AG), which is also produced in the pancreas, affects β-cell function, with particular attention to the role of ATP-sensitive K (K) channels and the exact site of action of the hormone. AG hyperpolarized the membrane potential and decreased cytoplasmic calcium concentration [Ca] and glucose-stimulated insulin secretion (GSIS). These effects were abolished in β-cells from SUR1-knockout (KO) mice.

Approved LXR agonists exert unspecific effects on pancreatic β-cell function.

Novel agonists of the nuclear liver-X-receptor (LXR) are designed to treat metabolic disorders or cancer. The rationale to develop these new drugs is based on promising results with established LXR agonist like T0901317 and GW3965. LXRα and LXRβ are expressed in β-cells, and expression is increased by T0901317.

ATP binding without hydrolysis switches sulfonylurea receptor 1 (SUR1) to outward-facing conformations that activate K channels.

Neuroendocrine-type ATP-sensitive K (K) channels are metabolite sensors coupling membrane potential with metabolism, thereby linking insulin secretion to plasma glucose levels. They are octameric complexes, (SUR1/Kir6.2), comprising sulfonylurea receptor 1 (SUR1 or ABCC8) and a K-selective inward rectifier (Kir6.

TGR5 Activation Promotes Stimulus-Secretion Coupling of Pancreatic β-Cells via a PKA-Dependent Pathway.

The Takeda-G-protein-receptor-5 (TGR5) mediates physiological actions of bile acids. Since it was shown that TGR5 is expressed in pancreatic tissue, a direct TGR5 activation in β-cells is currently postulated and discussed. The current study reveals that oleanolic acid (OLA) affects murine β-cell function by TGR5 activation.

ATP mediates a negative autocrine signal on stimulus-secretion coupling in mouse pancreatic β-cells.

Purpose: The role of ATP, which is secreted by pancreatic β-cells, is still a matter of debate. It has been postulated that extracellular ATP acts as a positive auto- or paracrine signal in β-cells amplifying insulin secretion. However, there is rising evidence that extracellular ATP may also mediate a negative signal.

Energy depletion and not ROS formation is a crucial step of glucolipotoxicity (GLTx) in pancreatic beta cells.

We have shown previously that genetic or pharmacological deletion of K channels protect against beta cell dysfunction induced by reactive oxygen species (ROS). Since it is assumed that glucolipotoxicity (GLTx) causes ROS production, we aimed to evaluate whether suppression of K channel activity can also prevent beta cell damage evoked by GLTx. We used an in vitro model of GLTx and measured distinct parameters of stimulus-secretion coupling.

Atrial Natriuretic Peptide Affects Stimulus-Secretion Coupling of Pancreatic β-Cells.

Atrial natriuretic peptide (ANP) influences glucose homeostasis and possibly acts as a link between the cardiovascular system and metabolism, especially in metabolic disorders like diabetes. The current study evaluated effects of ANP on β-cell function by the use of a β-cell-specific knockout of the ANP receptor with guanylate cyclase activity (βGC-A-KO). ANP augmented insulin secretion at the threshold glucose concentration of 6 mmol/L and decreased K single-channel activity in β-cells of control mice but not of βGC-A-KO mice.

The LXR Ligand T0901317 Acutely Inhibits Insulin Secretion by Affecting Mitochondrial Metabolism.

The role of liver X receptor (LXR) in pancreatic β-cell physiology and pathophysiology is still unclear. It has been postulated that chronic LXR activation in β-cells induces lipotoxicity, a key step in the development of β-cell dysfunction, which accompanies type 2 diabetes mellitus. In most of these studies, the LXR ligand T0901317 has been administered chronically in the micromolar range to study the significance of LXR activation.

The Angiotensin-(1-7)/Mas Axis Improves Pancreatic β-Cell Function in Vitro and in Vivo.

The angiotensin-converting enzyme 2/angiotensin (Ang)-(1-7)/Mas axis of the renin-angiotensin system often opposes the detrimental effects of the angiotensin-converting enzyme/Ang II/Ang II type 1 receptor axis and has been associated with beneficial effects on glucose homeostasis, whereas underlying mechanisms are mostly unknown. Here we investigate the effects of Ang-(1-7) and its receptor Mas on β-cell function. Isolated islets from Mas-deficient and wild-type mice were stimulated with Ang-(1-7) or its antagonists and effects on insulin secretion determined.

Evidence against a Ca(2+)-induced potentiation of dehydrogenase activity in pancreatic beta-cells.

Pancreatic beta-cells respond to an unchanging stimulatory glucose concentration with oscillations in membrane potential (Vm), cytosolic Ca(2+) concentration ([Ca(2+)]c), and insulin secretion. The underlying mechanisms are largely ascertained. Some particular details, however, are still in debate.

Mitochondrial succinate dehydrogenase is involved in stimulus-secretion coupling and endogenous ROS formation in murine beta cells.

Aims/hypothesis: Generation of reduction equivalents is a prerequisite for nutrient-stimulated insulin secretion. Mitochondrial succinate dehydrogenase (SDH) fulfils a dual function with respect to mitochondrial energy supply: (1) the enzyme is part of mitochondrial respiratory chains; and (2) it catalyses oxidation of succinate to fumarate in the Krebs cycle. The aim of our study was to elucidate the significance of SDH for beta cell stimulus-secretion coupling (SSC).

Human Islets Exhibit Electrical Activity on Microelectrode Arrays (MEA).

This study demonstrates for the first time that the microelectrode array (MEA) technique allows analysis of electrical activity of islets isolated from human biopsies. We have shown before that this method, i.e.

Role of FXR in β-cells of lean and obese mice.

We have recently shown that the bile acid (BA) taurochenodeoxycholate (TCDC) acutely stimulates insulin secretion via activation of the farnesoid X receptor (FXR). Aims of the current investigation were to discriminate between nongenomic (≤1 h) and genomic effects (24-48 h) of BAs on β-cells and to evaluate whether FXR can modulate the adverse effects of a high-fat diet (HFD). TCDC (500 nM) as well as glycine-conjugated and unconjugated CDC (chenodeoxycholate) increased insulin secretion in acute incubations but did not evoke additional effects after 1-2 days of preincubation.

Long-term culture and functionality of pancreatic islets monitored using microelectrode arrays.

Extracellular recording of the glucose-induced electrical activity of mouse islets of Langerhans on microelectrode arrays (MEAs) is an innovative and powerful tool to address beta-cell (patho-)physiology. In a dual approach we tested whether this technique can detect concentration-dependent drug effects as well as characterize alterations in beta-cell activity during prolonged culture. First we established conditions that allow long-term investigation of beta-cell function by recording electrical activity.

The significance of the nuclear farnesoid X receptor (FXR) in β cell function.

Bile acids (BAs) are important signaling molecules that are involved in the regulation of their own metabolism, lipid metabolism, energy expenditure and glucose homeostasis. The nuclear farnesoid X receptor (FXR) and the G-protein-coupled TGR-5 are the most prominent BA receptors. FXR is highly expressed in liver and activation of liver FXR profoundly affects glucose homeostasis.

Glitazones exert multiple effects on β-cell stimulus-secretion coupling.

Earlier studies suggest that glitazones exert beneficial effects in patients with type 2 diabetes by directly affecting insulin secretion of β-cells, besides improving the effectiveness of insulin in peripheral tissues. The effects of glitazones on stimulus-secretion coupling (SSC) are poorly understood. We tested the influence of troglitazone and pioglitazone on different parameters of SSC, including insulin secretion (radioimmunoassay), cell membrane potential, various ion currents (patch-clamp), mitochondrial membrane potential (ΔΨ), and cytosolic Ca(2+) concentration (fluorescence).

H₂O₂ lowers the cytosolic Ca²⁺ concentration via activation of cGMP-dependent protein kinase Iα.

The cGMP-dependent protein kinase I (cGKI) is a key mediator of cGMP signaling, but the specific functions of its two isoforms, cGKIα and cGKIβ, are poorly understood. Recent studies indicated a novel cGMP-independent role for cGKIα in redox sensing. To dissect the effects of oxidative stress on the cGKI isoforms, we used mouse embryonic fibroblasts and vascular smooth muscle cells (VSMCs) expressing both, one, or none of them.

Bile acids acutely stimulate insulin secretion of mouse β-cells via farnesoid X receptor activation and K(ATP) channel inhibition.

Type 2 diabetes mellitus is associated with alterations in bile acid (BA) signaling. The aim of our study was to test whether pancreatic β-cells contribute to BA-dependent regulation of glucose homeostasis. Experiments were performed with islets from wild-type, farnesoid X receptor (FXR) knockout (KO), and β-cell ATP-dependent K(+) (K(ATP)) channel gene SUR1 (ABCC8) KO mice, respectively.

Rapid functional evaluation of beta-cells by extracellular recording of membrane potential oscillations with microelectrode arrays.

The membrane potential (V (m)) of beta-cells oscillates at glucose concentrations between ~6 and 25 mM, i.e. burst phases with action potentials alternate with silent interburst phases generating so-called slow waves.

Activation of the AMP-activated protein kinase enhances glucose-stimulated insulin secretion in mouse β-cells.

The AMP-activated protein kinase (AMPK) is one of the key players in cellular energy regulation adapting cellular demands to nutritional and metabolic variations. Oral antidiabetic drugs like metformin and glitazones (thiazolidinediones) are known to stimulate this enzyme. Besides their established action on peripheral organs including liver and muscles, it has been claimed that these drugs may affect β-cell function.

BK channels affect glucose homeostasis and cell viability of murine pancreatic beta cells.

Aims/hypothesis: Evidence is accumulating that Ca(2+)-regulated K(+) (K(Ca)) channels are important for beta cell function. We used BK channel knockout (BK-KO) mice to examine the role of these K(Ca) channels for glucose homeostasis, beta cell function and viability.

Methods: Glucose and insulin tolerance were tested with male wild-type and BK-KO mice.

Oxidative stress and beta-cell dysfunction.

Diabetes mellitus type 1 and 2 (T1DM and T2DM) are complex multifactorial diseases. Loss of beta-cell function caused by reduced secretory capacity and enhanced apoptosis is a key event in the pathogenesis of both diabetes types. Oxidative stress induced by reactive oxygen and nitrogen species is critically involved in the impairment of beta-cell function during the development of diabetes.

Electrophysiology of islet cells.

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells.

Suppression of KATP channel activity protects murine pancreatic beta cells against oxidative stress.

The enhanced oxidative stress associated with type 2 diabetes mellitus contributes to disease pathogenesis. We previously identified plasma membrane-associated ATP-sensitive K+ (KATP) channels of pancreatic beta cells as targets for oxidants. Here, we examined the effects of genetic and pharmacologic ablation of KATP channels on loss of mouse beta cell function and viability following oxidative stress.