A role for alternative splicing in circadian control of exocytosis and glucose homeostasis.

The circadian clock is encoded by a negative transcriptional feedback loop that coordinates physiology and behavior through molecular programs that remain incompletely understood. Here, we reveal rhythmic genome-wide alternative splicing (AS) of pre-mRNAs encoding regulators of peptidergic secretion within pancreatic ß cells that are perturbed in Clock-/- and Bmal1-/- ß-cell lines. We show that the RNA-binding protein THRAP3 (thyroid hormone receptor-associated protein 3) regulates circadian clock-dependent AS by binding to exons at coding sequences flanking exons that are more frequently skipped in clock mutant ß cells, including transcripts encoding Cask (calcium/calmodulin-dependent serine protein kinase) and Madd (MAP kinase-activating death domain). Depletion of THRAP3 restores expression of the long isoforms of Cask and Madd, and mimicking exon skipping in these transcripts through antisense oligonucleotide delivery in wild-type islets reduces glucose-stimulated insulin secretion. Finally, we identify shared networks of alternatively spliced exocytic genes from islets of rodent models of diet-induced obesity that significantly overlap with clock mutants. Our results establish a role for pre-mRNA alternative splicing in ß-cell function across the sleep/wake cycle.

A genome-wide CRISPR screen identifies CALCOCO2 as a regulator of beta cell function influencing type 2 diabetes risk.

Identification of the genes and processes mediating genetic association signals for complex diseases represents a major challenge. As many of the genetic signals for type 2 diabetes (T2D) exert their effects through pancreatic islet-cell dysfunction, we performed a genome-wide pooled CRISPR loss-of-function screen in a human pancreatic beta cell line. We assessed the regulation of insulin content as a disease-relevant readout of beta cell function and identified 580 genes influencing this phenotype. Integration with genetic and genomic data provided experimental support for 20 candidate T2D effector transcripts including the autophagy receptor CALCOCO2. Loss of CALCOCO2 was associated with distorted mitochondria, less proinsulin-containing immature granules and accumulation of autophagosomes upon inhibition of late-stage autophagy. Carriers of T2D-associated variants at the CALCOCO2 locus further displayed altered insulin secretion. Our study highlights how cellular screens can augment existing multi-omic efforts to support mechanistic understanding and provide evidence for causal effects at genome-wide association studies loci.

Characterization of Erg K+ channels in alpha- and beta-cells of mouse and human islets.

Voltage-gated eag-related gene (Erg) K(+) channels regulate the electrical activity of many cell types. Data regarding Erg channel expression and function in electrically excitable glucagon and insulin producing cells of the pancreas is limited. In the present study Erg1 mRNA and protein were shown to be highly expressed in human and mouse islets and in alpha-TC6 and Min6 cells alpha- and beta-cell lines, respectively. Whole cell patch clamp recordings demonstrated the functional expression of Erg1 in alpha- and beta-cells, with rBeKm1, an Erg1 antagonist, blocking inward tail currents elicited by a double pulse protocol. Additionally, a small interference RNA approach targeting the kcnh2 gene (Erg1) induced a significant decrease of Erg1 inward tail current in Min6 cells. To investigate further the role of Erg channels in mouse and human islets, ratiometric Fura-2 AM Ca(2+)-imaging experiments were performed on isolated alpha- and beta-cells. Blocking Erg channels with rBeKm1 induced a transient cytoplasmic Ca(2+) increase in both alpha- and beta-cells. This resulted in an increased glucose-dependent insulin secretion, but conversely impaired glucagon secretion under low glucose conditions. Together, these data present Erg1 channels as new mediators of alpha- and beta-cell repolarization. However, antagonism of Erg1 has divergent effects in these cells; to augment glucose-dependent insulin secretion and inhibit low glucose stimulated glucagon secretion.

A glucose-dependent spatial patterning of exocytosis in human ß-cells is disrupted in type 2 diabetes.

Impaired insulin secretion in type 2 diabetes (T2D) is linked to reduced insulin granule docking, disorganization of the exocytotic site, and an impaired glucose-dependent facilitation of insulin exocytosis. We show in ß-cells from 80 human donors that the glucose-dependent amplification of exocytosis is disrupted in T2D. Spatial analyses of granule fusion, visualized by total internal reflection fluorescence (TIRF) microscopy in 24 of these donors, demonstrate that these are non-random across the surface of ß-cells from donors with no diabetes (ND). The compartmentalization of events occurs within regions defined by concurrent or recent membrane-resident secretory granules. This organization, and the number of membrane-associated granules, is glucose-dependent and notably impaired in T2D ß-cells. Mechanistically, multi-channel Kv2.1 clusters contribute to maintaining the density of membrane-resident granules and the number of fusion 'hotspots', while SUMOylation sites at the channel N- (K145) and C-terminus (K470) determine the relative proportion of fusion events occurring within these regions. Thus, a glucose-dependent compartmentalization of fusion, regulated in part by a structural role for Kv2.1, is disrupted in ß-cells from donors with type 2 diabetes.

Targeting of voltage-gated K+ and Ca2+ channels and soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins to cholesterol-rich lipid rafts in pancreatic alpha-cells: effects on glucagon stimulus-secretion coupling.

Pancreatic alpha-cells secrete glucagon in response to low glucose to counter insulin actions, thereby maintaining glucose homeostasis. The molecular basis of alpha-cell stimulus-secretion coupling has not been fully elucidated. We investigated the expression of voltage-gated K(+) (K(V)) and Ca(2+) (Ca(V)) channels, and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in pancreatic alpha-cells and examined their targeting to specialized cholesterol-rich lipid rafts. In alpha-cells, we detected the expression of K(V)4.1/4.3 (A-type current), K(V)3.2/3.3 (delayed rectifier current), Ca(V)1.2 (L-type current), Ca(V)2.2 (N-type current), and the SNARE (synaptosomal-associated protein of 25 kDa, syntaxin 1A, and vesicle-associated membrane protein 2) and SNARE-associated proteins (Munc-13-1 and Munc-18a). We also detected caveolin-2, a structural protein of cholesterol-rich lipid rafts. Of these proteins, caveolin-2, K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins (syntaxin 1A, synaptosomal-associated protein of 25 kDa, and vesicle-associated membrane protein 2) target to lipid raft domains on alpha-cell plasma membranes. Disruption of lipid rafts by depletion of membrane cholesterol with methyl-beta-cyclodextrin decreased the association of K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins with lipid rafts. This resulted in inhibition of A-type K(V) currents and enhancement of glucagon secretion from alpha-cells. Consistently, capacitance measurements of exocytosis of single alpha-cells showed enhanced exocytosis after membrane cholesterol depletion. Taken together, our results demonstrate the association of K(V)4, Ca(V)1.2, and SNARE proteins with lipid rafts in pancreatic alpha-cells. Glucagon secretion from alpha-cells is regulated by lipid rafts, and the dissociation of SNARE proteins from cholesterol-rich lipid raft domains enhances glucagon secretion.

STEAP4 expression in human islets is associated with differences in body mass index, sex, HbA1c, and inflammation.

OBJECTIVE: STEAP4 (six-transmembrane epithelial antigen of the prostate 4) is a metalloreductase that has been shown previously to protect cells from inflammatory damage. Genetic variants in STEAP4 have been associated with numerous metabolic disorders related to obesity, including putative defects in the acute insulin response to glucose in type 2 diabetes. PURPOSE: We examined whether obesity and/or type 2 diabetes altered STEAP4 expression in human pancreatic islets. METHODS: Human islets were isolated from deceased donors at two medical centers and processed for quantitative polymerase chain reaction. Organ donors were selected by status as non-diabetic or having type 2 diabetes. Site 1 (Edmonton): N = 13 type 2 diabetes donors (7M, 6F), N = 20 non-diabetic donors (7M, 13F). Site 2 (Virginia): N = 6 type 2 diabetes donors (6F), N = 6 non-diabetic donors (3M, 3F). RESULTS: STEAP4 showed reduced islet expression with increasing body mass index among all donors (P < 0.10) and non-diabetic donors (P < 0.05) from Site 1; STEAP4 showed reduced islet expression among type 2 diabetes donors with increasing hemoglobin A1c. Islet STEAP4 expression was also marginally higher in female donors (P < 0.10). Among type 2 diabetes donors from Site 2, islet insulin expression was reduced, STEAP4 expression was increased, and white blood cell counts were increased compared to non-diabetic donors. Islets from non-diabetic donors that were exposed overnight to 5 ng/ml IL-1ß displayed increased STEAP4 expression, consistent with STEAP4 upregulation by inflammatory signaling. CONCLUSIONS: These findings suggest that increased STEAP4 mRNA expression is associated with inflammatory stimuli, whereas lower STEAP4 expression is associated with obesity in human islets. Given its putative protective role, downregulation of STEAP4 by chronic obesity suggests a mechanism for reduced islet protection against cellular damage.

N-acyl Taurines and Acylcarnitines Cause an Imbalance in Insulin Synthesis and Secretion Provoking ß Cell Dysfunction in Type 2 Diabetes.

The processes contributing to ß cell dysfunction in type 2 diabetes (T2D) are uncertain, largely because it is difficult to access ß cells in their intact immediate environment. We examined the pathophysiology of ß cells under T2D progression directly in pancreatic tissues. We used MALDI imaging of Langerhans islets (LHIs) within mouse tissues or from human tissues to generate in situ-omics data, which we supported with in vitro experiments. Molecular interaction networks provided information on functional pathways and molecules. We found that stearoylcarnitine accumulated in ß cells, leading to arrest of insulin synthesis and energy deficiency via excessive ß-oxidation and depletion of TCA cycle and oxidative phosphorylation metabolites. Acetylcarnitine and an accumulation of N-acyl taurines, a group not previously detected in ß cells, provoked insulin secretion. Thus, ß cell dysfunction results from enhanced insulin secretion combined with an arrest of insulin synthesis.

Acyl coenzyme A esters differentially activate cardiac and beta-cell adenosine triphosphate-sensitive potassium channels in a side-chain length-specific manner.

Recent evidence demonstrates that long-chain acyl coenzyme A esters (CoAs) activate cardiac and beta-cell plasma-membrane (pmK(ATP)) adenosine triphosphate (ATP)-sensitive potassium channels. In this study, we have investigated the differential effects of acyl CoAs of short and medium side-chain length on cardiac and beta-cell pmK(ATP) isoforms. At the single-channel level, the addition of acyl CoAs of differing side-chain length (2 to 16 carbons) to the inside face of membrane patches from ventricular myocytes caused varying increases in pmK(ATP) channel open probability proportional to increases in acyl side-chain length (20 mumol/L acetyl CoA: 310% +/- 90%, 20 mumol/L decanoyl CoA: 570% +/- 150%). A similar dependence of activation on side-chain length was observed in recombinant pmK(ATP) channels (SUR2A/Kir6.2) with full activation of current requiring both the acyl and CoA moieties in the esterified form. We found the recombinant beta-cell K(ATP) channel (SUR1/Kir6.2) to be much less sensitive to medium-chain acyl CoAs (decanoyl CoA: 124% +/- 15% v 231% +/- 25% in SUR2A/Kir6.2), suggesting a role for the cardiac sulfonylurea receptor, SUR2A, in the molecular mechanism of activation by these compounds. We propose that fatty acid metabolism, and the resultant generation of acyl CoAs of varying side-chain length, may be an important regulator of cellular excitability via interactions with the K(ATP) channel.

Functional plasticity of the human infant ß-cell exocytotic phenotype.

Our understanding of adult human ß-cells is advancing, but we know little about the function and plasticity of ß-cells from infants. We therefore characterized islets and single islet cells from human infants after isolation and culture. Although islet morphology in pancreas biopsies was similar to that in adults, infant islets after isolation and 24-48 hours of culture had less insulin staining, content, and secretion. The cultured infant islets expressed pancreatic and duodenal homeobox 1 and several (Glut1, Cav1.3, Kir6.2) but not all (syntaxin 1A and synaptosomal-associated protein 25) markers of functional islets, suggesting a loss of secretory phenotype in culture. The activity of key ion channels was maintained in isolated infant ß-cells, whereas exocytosis was much lower than in adults. We examined whether a functional exocytotic phenotype could be reestablished under conditions thought to promote ß-cell differentiation. After a 24- to 28-day expansion and maturation protocol, we found preservation of endocrine markers and hormone expression, an increased proportion of insulin-positive cells, elevated expression of syntaxin 1A and synaptosomal-associated protein 25, and restoration of exocytosis to levels comparable with that in adult ß-cells. Thus, human infant islets are prone to loss of their exocytotic phenotype in culture but amenable to experimental approaches aimed at promoting expansion and functional maturation. Control of exocytotic protein expression may be an important mechanism underlying the plasticity of the secretory machinery, an increased understanding of which may lead to improved regenerative approaches to treat diabetes.

Islet cholesterol accumulation due to loss of ABCA1 leads to impaired exocytosis of insulin granules.

OBJECTIVE: The ATP-binding cassette transporter A1 (ABCA1) is essential for normal insulin secretion from ß-cells. The aim of this study was to elucidate the mechanisms underlying the impaired insulin secretion in islets lacking ß-cell ABCA1. RESEARCH DESIGN AND METHODS: Calcium imaging, patch clamp, and membrane capacitance were used to assess the effect of ABCA1 deficiency on calcium flux, ion channel function, and exocytosis in islet cells. Electron microscopy was used to analyze ß-cell ultrastructure. The quantity and distribution of proteins involved in insulin-granule exocytosis were also investigated. RESULTS: We show that a lack of ß-cell ABCA1 results in impaired depolarization-induced exocytotic fusion of insulin granules. We observed disturbances in membrane microdomain organization and Golgi and insulin granule morphology in ß-cells as well as elevated fasting plasma proinsulin levels in mice in the absence of ß-cell ABCA1. Acute cholesterol depletion rescued the exocytotic defect in ß-cells lacking ABCA1, indicating that elevated islet cholesterol accumulation directly impairs granule fusion and insulin secretion. CONCLUSIONS: Our data highlight a crucial role of ABCA1 and cellular cholesterol in ß-cells that is necessary for regulated insulin granule fusion events. These data suggest that abnormalities of cholesterol metabolism may contribute to the impaired ß-cell function in diabetes.

Investigation of transport mechanisms and regulation of intracellular Zn2+ in pancreatic alpha-cells.

During insulin secretion, pancreatic alpha-cells are exposed to Zn(2+) released from insulin-containing secretory granules. Although maintenance of Zn(2+) homeostasis is critical for cell survival and glucagon secretion, very little is known about Zn(2+)-transporting pathways and the regulation of Zn(2+) in alpha-cells. To examine the effect of Zn(2+) on glucagon secretion and possible mechanisms controlling the intracellular Zn(2+) level ([Zn(2+)](i)), we employed a glucagon-producing cell line (alpha-TC6) and mouse islets where non-beta-cells were identified using islets expressing green fluorescent protein exclusively in beta-cells. In this study, we first confirmed that Zn(2+) treatment resulted in the inhibition of glucagon secretion in alpha-TC6 cells and mouse islets in vitro. The inhibition of secretion was not likely via activation of K(ATP) channels by Zn(2+). We then determined that Zn(2+) was transported into alpha-cells and was able to accumulate under both low and high glucose conditions, as well as upon depolarization of cells with KCl. The nonselective Ca(2+) channel blocker Gd(3+) partially inhibited Zn(2+) influx in alpha-TC cells, whereas the L-type voltage-gated Ca(2+) channel inhibitor nitrendipine failed to block Zn(2+) accumulation. To investigate Zn(2+) transport further, we profiled alpha-cells for Zn(2+) transporter transcripts from the two families that work in opposite directions, SLC39 (ZIP, Zrt/Irt-like protein) and SLC30 (ZnT, Zn(2+) transporter). We observed that Zip1, Zip10, and Zip14 were the most abundantly expressed Zips and ZnT4, ZnT5, and ZnT8 the dominant ZnTs. Because the redox state of cells is also a major regulator of [Zn(2+)](i), we examined the effects of oxidizing agents on Zn(2+) mobilization within alpha-cells. 2,2'-Dithiodipyridine (-SH group oxidant), menadione (superoxide generator), and SIN-1 (3-morpholinosydnonimine) (peroxynitrite generator) all increased [Zn(2+)](i) in alpha-cells. Together these results demonstrate that Zn(2+) inhibits glucagon secretion, and it is transported into alpha-cells in part through Ca(2+) channels. Zn(2+) transporters and the redox state also modulate [Zn(2+)](i).

Identification and pharmacological characterization of sarcolemmal ATP-sensitive potassium channels in the murine atrial HL-1 cell line.

Activation of ATP-sensitive potassium (KATP) channels is known to have cardioprotective effects during periods of ischemia and reperfusion, making these channels important targets for clinical drug discovery. Using electrophysiological techniques we identify KATP channels in a mouse atrial cell line (HL-1). HL-1 KATP channels exhibited a concentration-dependent inhibition by ATP (IC50 = 23.3 +/- 3.2 microM), a unitary single-channel conductance of 55 pS, and sensitivity to the isoform-specific KATP channel opener P1075 and inhibitor HMR1098. Adenoviral infection of a dominant-negative Kir6.2 subunit significantly reduced the P1075-sensitive sarcKATP current. Taken together, the data indicate that HL-1 KATP channels are composed of sulfonylurea receptor isoform SUR2A coupled to the pore-forming Kir6.2 subunit--the molecular makeup of sarcKATP channels found in native cardiac myocytes. Pharmacological activation of HL-1 cell KATP channels also resulted in action potential shortening. Using the membrane potential-sensitive dye DiBac4(3), we demonstrated that the sarcKATP channel opener P1075 (20 microM) produced a concentration-dependent hyperpolarization of a monolayer of HL-1 cells that could be reversed by channel inhibition with HMR1098 (20 microM). We conclude that the HL-1 cells are an excellent cell line for studying cardiac sarcKATP channels, and these cells may also provide an important tool for the testing of novel pharmacological modulators of KATP channels in fluorescence-based assays.

Basigin (CD147) is the target for organomercurial inhibition of monocarboxylate transporter isoforms 1 and 4: the ancillary protein for the insensitive MCT2 is EMBIGIN (gp70).

Translocation of monocarboxylate transporters MCT1 and MCT4 to the plasma membrane requires CD147 (basigin) with which they remain tightly associated. However, the importance of CD147 for MCT activity is unclear. MCT1 and MCT4 are both inhibited by the cell-impermeant organomercurial reagent p-chloromercuribenzene sulfonate (pCMBS). Here we demonstrate by site-directed mutagenesis that removal of all accessible cysteine residues on MCT4 does not prevent this inhibition. pCMBS treatment of cells abolished co-immunoprecipitation of MCT1 and MCT4 with CD147 and enhanced labeling of CD147 with a biotinylated-thiol reagent. This suggested that CD147 might be the target of pCMBS, and further evidence for this was obtained by treatment of cells with the bifunctional organomercurial reagent fluorescein dimercury acetate that caused oligomerization of CD147. Site-directed mutagenesis of CD147 implicated the disulfide bridge in the Ig-like C2 domain of CD147 as the target of pCMBS attack. MCT2, which is pCMBS-insensitive, was found to co-immunoprecipitate with gp70 rather than CD147. The interaction between gp70 and MCT2 was confirmed using fluorescence resonance energy transfer between the cyan fluorescent protein- and yellow fluorescent protein-tagged MCT2 and gp70. pCMBS strongly inhibited lactate transport into rabbit erythrocytes, where MCT1 interacts with CD147, but not into rat erythrocytes where it interacts with gp70. These data imply that inhibition of MCT1 and MCT4 activity by pCMBS is mediated through its binding to CD147, whereas MCT2, which associates with gp70, is insensitive to pCMBS. We conclude that ancillary proteins are required to maintain the catalytic activity of MCTs as well as for their translocation to the plasma membrane.