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Endocytosis is controlled by a well-orchestrated molecular machinery, where the individual players as well as their precise interactions are not fully understood. We now show that syndapin I/PACSIN 1 is expressed in pancreatic β cells and that its knockdown abrogates β cell endocytosis leading to disturbed plasma membrane protein homeostasis, as exemplified by an elevated density of L-type Ca2+ channels. Intriguingly, inositol hexakisphosphate (InsP6) activates casein kinase 2 (CK2) that phosphorylates syndapin I/PACSIN 1, thereby promoting interactions between syndapin I/PACSIN 1 and neural Wiskott–Aldrich syndrome protein (N-WASP) and driving β cell endocytosis. Dominant-negative interference with endogenous syndapin I/PACSIN 1 protein complexes, by overexpression of the syndapin I/PACSIN 1 SH3 domain, decreases InsP6-stimulated endocytosis. InsP6 thus promotes syndapin I/PACSIN 1 priming by CK2-dependent phosphorylation, which endows the syndapin I/PACSIN 1 SH3 domain with the capability to interact with the endocytic machinery and thereby initiate endocytosis, as exemplified in β cells. [ABSTRACT FROM AUTHOR]

The voltage-gated Ca (Ca) channel acts as a key player in β cell physiology and pathophysiology. β cell Ca channels undergo hyperactivation subsequent to exposure to type 1 diabetic (T1D) serum resulting in increased cytosolic free Ca concentration and thereby Ca-triggered β cell apoptosis. The present study was aimed at revealing the subtypes of Ca1 channels hyperactivated by T1D serum as well as the biophysical mechanisms responsible for T1D serum-induced hyperactivation of β cell Ca1 channels. Patch-clamp recordings and single-cell RT-PCR analysis were performed in pancreatic β cells from Ca1 channel knockout and corresponding control mice. We now show that functional Ca1.3 channels are expressed in a subgroup of islet β cells from Ca1.2 knockout mice (Ca1.2). T1D serum enhanced whole-cell Ca currents in islet β cells from Ca1.3 knockout mice (Ca1.3). T1D serum increased the open probability and number of functional unitary Ca1 channels in Ca1.2 and Ca1.3 β cells. These data demonstrate that T1D serum hyperactivates both Ca1.2 and Ca1.3 channels by increasing their conductivity and number. These findings suggest Ca1.2 and Ca1.3 channels as potential targets for anti-diabetes therapy. [ABSTRACT FROM AUTHOR]

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K, Na, Ca and Cl across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K efflux through ATP-sensitive K (K) channels, the voltage-gated Ca (Ca) channel-mediated Ca influx and K efflux through voltage-gated K (K) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The K channel-mediated K efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca influx through Ca1 channels, a major type of β cell Ca channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K efflux mediated by K2.1 delayed rectifier K channels, a predominant form of β cell K channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the Ca channel-mediated Ca entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights K channel-, Ca1 channel- and K2.1 channel-mediated ion fluxes. [ABSTRACT FROM AUTHOR]

Apolipoprotein CIII (ApoCIII) not only serves as an inhibitor of triglyceride hydrolysis but also participates in diabetes-related pathological events such as hyperactivation of voltage-gated Ca (Ca) channels in the pancreatic β cell. However, nothing is known about the molecular mechanisms whereby ApoCIII hyperactivates β cell Ca channels. We now demonstrate that ApoCIII increased Ca1 channel open probability and density. ApoCIII enhanced whole-cell Ca currents and the Ca1 channel blocker nimodipine completely abrogated this enhancement. The effect of ApoCIII was not influenced by individual inhibition of PKA, PKC, or Src. However, combined inhibition of PKA, PKC, and Src counteracted the effect of ApoCIII, similar results obtained by coinhibition of PKA and Src. Moreover, knockdown of β1 integrin or scavenger receptor class B type I (SR-BI) prevented ApoCIII from hyperactivating β cell Ca channels. These data reveal that ApoCIII hyperactivates β cell Ca1 channels through SR-BI/β1 integrin-dependent coactivation of PKA and Src. [ABSTRACT FROM AUTHOR]

Transthyretin (TTR) is a functional protein in the pancreatic β-cell. It promotes insulin release and protects against β-cell death. We now demonstrate by ligand blotting, adsorption to specific magnetic beads, and surface plasmon resonance that TTR binds to glucose-regulated proteins (Grps)78, 94, and 170, which are members of the endoplasmic reticulum chaperone family, but Grps78 and 94 have also been found at the plasma membrane. The effect of TTR on changes in cytoplasmic free Ca concentration ([Ca]) was abolished if the cells were treated with either dynasore, a specific inhibitor of dynamin GTPase that blocks clathrin-mediated endocytosis, or an antibody against Grp78, that prevents TTR from binding to Grp78. The conclusion is that TTR binds to Grp78 at the plasma membrane, is internalized into the β-cell via a clathrin-dependent pathway, and that this internalization is necessary for the effects of TTR on β-cell function. [ABSTRACT FROM AUTHOR]

The stereochemistry of the inositol backbone provides a platform on which to generate a vast array of distinct molecular motifs that are used to convey information both in signal transduction and many other critical areas of cell biology. Diphosphoinositol phosphates, or inositol pyrophosphates, are the most recently characterized members of the inositide family. They represent a new frontier with both novel targets within the cell and novel modes of action. This includes the proposed pyrophosphorylation of a unique subset of proteins. We review recent insights into the structures of these molecules and the properties of the enzymes which regulate their concentration. These enzymes also act independently of their catalytic activity via protein–protein interactions. This unique combination of enzymes and products has an important role in diverse cellular processes including vesicle trafficking, endo- and exocytosis, apoptosis, telomere length regulation, chromatin hyperrecombination, the response to osmotic stress, and elements of nucleolar function. [ABSTRACT FROM AUTHOR]

Pancreatic beta cell damage caused by pro-inflammatory cytokines interleukin-1β (IL-1β), interferon-γ (IFNγ) and tumor necrosis factor-α (TNFα) is a key event in the pathogenesis of type 1 diabetes. The suppressor of cytokine signaling-1 (SOCS-1) blocks IFNγ-induced signaling and prevents diabetes in the non-obese diabetic mouse. Here, we investigated if SOCS-1 overexpression in primary beta cells provides protection from cytokine-induced islet cell dysfunction and death. We demonstrate that SOCS-1 does not prevent increase in NO production and decrease in glucose-stimulated insulin secretion in the presence of IL-1β, IFNγ, TNFα. However, it decreases the activation of caspase-3, -8 and -9, and thereby, promotes a robust protection from cytokine-induced beta cell death. Our data suggest that SOCS-1 overexpression may not be sufficient in preventing all the biological activities of IFNγ in beta cells. In summary, we show that interference with IFNγ signal transduction pathways by SOCS-1 inhibits cytokine-stimulated pancreatic beta cell death. [ABSTRACT FROM AUTHOR]