Newly differentiated pancreatic β cells lack proper insulin secretion profiles of mature function... more Newly differentiated pancreatic β cells lack proper insulin secretion profiles of mature functional β cells. The global gene expression differences between paired immature and mature β cells have been studied, but the dynamics of transcriptional events, correlating with temporal development of glucose-stimulated insulin secretion (GSIS), remain to be fully defined. This aspect is important to identify which genes and pathways are necessary for β-cell development or for maturation, as defective insulin secretion is linked with diseases such as diabetes. In this study, we assayed through RNA sequencing the global gene expression across six β-cell developmental stages in mice, spanning from β-cell progenitor to mature β cells. A computational pipeline then selected genes differentially expressed with respect to progenitors and clustered them into groups with distinct temporal patterns associated with biological functions and pathways. These patterns were finally correlated with experim...
Genetic and acquired abnormalities contribute to pancreatic β-cell failure in diabetes. Transcrip... more Genetic and acquired abnormalities contribute to pancreatic β-cell failure in diabetes. Transcription factors Hnf4α (MODY1) and FoxO1 are respective examples of these two components, and are known to act through β-cell-specific enhancers. However, their relationship is unclear. Here we show by genome-wide interrogation of chromatin modifications that FoxO1 ablation in mature β-cells leads to increased selection of FoxO1 enhancers by Hnf4α. To model the functional significance we generated single and compound knockouts of FoxO1 and Hnf4α in β-cells. Single knockout of either gene impaired insulin secretion in mechanistically distinct fashions. Surprisingly, the defective β-cell secretory function of either single mutant in hyperglycemic clamps and isolated islets treated with various secretagogues, was completely reversed in double mutants. Gene expression analyses revealed the reversal of β-cell dysfunction with an antagonistic network regulating glycolysis, including β-cell “disall...
Tetraspanin7-mediated inhibition of β-cell voltage-dependent Ca2+ channels regulates glucose-stim... more Tetraspanin7-mediated inhibition of β-cell voltage-dependent Ca2+ channels regulates glucose-stimulated insulin secretion. Glucose-stimulated insulin secretion (GSIS) is regulated by Ca2+ influx into pancreatic β-cells through voltage-dependent Ca2+ channels (VDCCs). As some tetraspanin (TSPAN) transmembrane proteins control Ca2+ handling they may also modulate GSIS. TSPAN7 is the most abundant TSPAN in mouse and human islets. TSPAN7 autoantibodies are also found in type 1 diabetic serum, suggesting a link between TSPAN7 and diabetes. The aim herein was to determine if TSPAN7 controls β-cell Ca2+ handling and insulin secretion. Immunofluorescence imaging of mouse and human pancreatic slices indicated that TSPAN7 is highly expressed in β- and α-cells. shRNA-mediated TSPAN7 knockdown (KD) in mouse and human β-cells augmented glucose-stimulated Ca2+ influx (by 32.3±9.5% and 33.8±15.1% respectively) as well as KCl-stimulated Ca2+ influx (45 mM; 34.5±9.6%, human). Mouse β-cell Ca2+ oscillation frequency was also accelerated by TSPAN7 KD (by 29.4±10.1%). As TSPANs interact with and regulate VDCCs, we examined the effect of TSPAN7 on VDCC currents. TSPAN7 KD increased peak L-type VDCC currents in mouse and human β-cells (by 4.1±1.3 pA/pF and 2.0±0.9 pA/pF respectively). Heterologous expression of TSPAN7 with L-type VDCCs in HEK293 cells also decreased VDCC currents (CaV1.2: 4.3±1.8 pA/pF; CaV1.3: 17.5±5.4 pA/pF) and reduced KCl-stimulated (50 mM) Ca2+ influx (CaV1.2: 51.0±7.1%; CaV1.3: 28.4±7.5%). Furthermore, TSPAN7 coimmunoprecipitated with CaV1.2 and CaV1.3 from primary human β-cells and when heterologously expressed in HEK293 cells. Finally, TSPAN7 KD in human β-cells increased basal (5.6 mM glucose; 8.3±5.5 ng*hr-1) and stimulated (45 mM KCl+14 mM glucose; 20.4±17.1 ng*hr-1) insulin secretion. These findings reveal that TSPAN7 modulation of β-cell L-type VDCCs limits glucose-stimulated Ca2+ influx and insulin secretion. Disclosure M. Dickerson: None. P. Dadi: None. R.B. Butterworth: None. D. Jacobson: None. Funding American Diabetes Association (1-17-IBS-024 to D.J.); National Institutes of Health (T32DK101003, DK097392, DK115620); Vanderbilt University Medical Center Diabetes Research and Training Center (P60DK20593)
Calcium influx into pancreatic α-cells through voltage-dependent calcium channels (VDCCs) is requ... more Calcium influx into pancreatic α-cells through voltage-dependent calcium channels (VDCCs) is required for glucagon (GCG) secretion and the mechanisms that mediate this process become defective during the pathogenesis of diabetes. Potassium channels are key regulators of membrane potential (Vm), and thus, modulate VDCC activity. However, the role of calcium-activated potassium (KCa) channels in α-cell calcium handling and GCG secretion have not been determined. The aim of this study was to investigate the importance of KCa channels to α-cell electrical excitability, calcium handling, and GCG secretion. α-cells display a KCa current, which is partially inhibited by blocking small conductance KCa (SK) or big conductance KCa (BK) channels. Inhibition of P/Q-type VDCCs, sarco/endoplasmic reticulum (ER) calcium-ATPases (SERCAs), and KATP channels also partially inhibit this current. It is likely that calcium influx through VDCCs triggers calcium-induced calcium release (CICR) from ER calcium stores activating α-cell KCa channels. The inhibition of SK channels transiently increases calcium influx into α-cells at 1 mM glucose; however, over time intracellular calcium decreases. This may be due to voltage-dependent inactivation of α-cell VDCCs. Interestingly, SK channel inhibition reduces GCG secretion at 1 mM glucose while SST secretion is unaffected. Thus, it is likely that this is an intrinsic α-cell effect. These findings demonstrate that α-cells possess a KCa current and that its activation is important for α-cell electrical activity, Ca2+ handling, and GCG secretion. Disclosure M. Dickerson: None. M.K. Altman: None. P. Dadi: None. N.C. Vierra: None. D. Jacobson: None.
In type 2 diabetic patients glucagon secretion becomes dysregulated under elevated glucose condit... more In type 2 diabetic patients glucagon secretion becomes dysregulated under elevated glucose conditions worsening hyperglycemia. However, the mechanisms of glucose inhibition of glucagon secretion remain incompletely understood. Therefore, we sought to uncover the glucose dependent mechanisms that modulate pancreatic α-cell glucagon secretion. One mechanism that regulates α-cell Ca2+ influx in a glucose dependent manner is store-operated Ca2+ entry (SOCE). Therefore, in our studies we used a small molecule (AnCoA4) that inhibits the STIM1/Orai1 protein interaction and prevents store-operated Ca2+ channel activation. In whole mouse islet α-cells inhibition of SOCE with AnCoA4 reduced Ca2+ influx in 1 mM glucose (8.4% ± 1.1%). We went on to test the influence of SOCE on glucose modulation of glucagon secretion, in 1 mM glucose AnCoA4 reduced glucagon secretion in mouse pancreatic islets (53.5% ± 11%). However, there was no significant change in glucose inhibition of glucagon secretion by inhibition of SOCE at 11 mM glucose. To follow-up on this, we used transgenic mice without STIM1 in α-cells and showed that AnCoA4 does not influence Ca2+ fluctuations in knockout islet α-cells. This proves in our hands that AnCoA4 works selectively. Additionally, we used transgenic GCAMP3 mice without STIM1 in α-cells for whole islet confocal studies to measure SOCE in 1 mM and 11 mM glucose conditions. We found that in whole islet α-cells without functional STIM1, Ca2+ influx due to SOCE was reduced in 1 mM glucose. Next, we wanted to test this molecule in human islet calcium homeostasis studies. Human dispersed α-cells show inhibition of SOCE with AnCoA4 treatment in 1 mM glucose conditions. Thus, our findings identify that inhibition of STIM1/Orai1 reduces SOCE in low glucose conditions leading to decreased α-cell glucagon secretion. Disclosure M.K. Altman: None. P. Dadi: None. D. Jacobson: None.
During Glucose-Stimulated Insulin Secretion (GSIS), β-cells exhibit oscillations in cytosolic Ca ... more During Glucose-Stimulated Insulin Secretion (GSIS), β-cells exhibit oscillations in cytosolic Ca 2+ ([Ca 2+ ] c ) and endoplasmic reticulum (ER) Ca 2+ ([Ca 2+ ] ER ), which are regulated by the two-pore domain K+ channel, TALK-1. TALK-1 is the most abundant β-cell K + channel and it controls Ca 2+ homeostasis by modulating K + permeability at both the plasma and ER membrane. However, how TALK-1 modulates ER stress and β-cell failure during the pathogenesis of diabetes has not been determined. To understand how TALK-1 controls β-cell function under diabetic conditions, we assessed how a mutation in TALK-1 (ND) which causes neonatal diabetes, impacts TALK-1 channel activity and [Ca 2+ ] ER handling. While plasmallemal TALK-1 activity was inhibited with the ND mutation, [Ca 2+ ] ER storage was significantly reduced by the ND mutation compared to control TALK-1. This suggests that TALK-1 ND enhances TALK-1 activity on the ER membrane leading to reduced [Ca 2+ ] ER storage. Reducing [Ca 2+ ] ER enhances ER stress, which would be expected to cause β-cell dysfunction. Indeed, TALK-1 ND significantly reduces β-cell insulin secretion compared to control. To further investigate the role of TALK-1 we developed a β-cell-specific TALK-1 KO mouse model (β-TALK1-KO). On a high fat diet, β-TALK1-KO mice show improved glucose tolerance compared to controls. Mitochondrial calcium ([Ca 2+ ] mito ) was also examined because it is modulated by [Ca 2+ ] ER ; we found that β-TALK1-KO mice have increased [Ca 2+ ] mito . As reactive oxygen species (ROS) production is a byproduct of mitochondrial respiration, we investigated if TALK-1 affects ROS production. TALK1 KO islets were found to have reduced ROS production compared to wild type islets. Together, these data indicate that increased TALK-1 channel activity reduces [Ca 2+ ] ER , [Ca 2+ ] mito , and increases ROS; suggesting that TALK-1 plays an important role in modulating the β-cell responses to stress associated with diabetic conditions. Disclosure S.M. Graff: None. P. Dadi: None. C.E. Ibsen: None. M. Dickerson: None. K.L. Jordan: None. D. Jacobson: None.
Abstract Maturity-onset diabetes of the young (MODY) is a monogenic form of diabetes. Exome seque... more Abstract Maturity-onset diabetes of the young (MODY) is a monogenic form of diabetes. Exome sequencing of a non-obese four-generation family with autosomal dominant non-ketotic antibody-negative diabetes, who were negative for mutations in known MODY genes, identified a novel coding variant in KCNK16 (NM_001135105: c.341T>C, p.Leu114Pro) affecting a highly conserved base and predicted to be damaging. KCNK16 encodes TALK-1, a pancreatic β-cell potassium channel which modulates β-cell excitability, controlling Ca2+ influx and insulin secretion. Activation of TALK-1 reduces insulin secretion. A coding gain-of-function variant in KCNK16 is associated with type 2 diabetes in multi-ethnic genome-wide association studies. Notably, TALK-1 is not sensitive to sulfonylureas. HEK-293 cells were transfected with either wild-type or Leu114Pro TALK-1 constructs using a CMV promoter and including P2A mCherry to enable confirmation of successful transfection. Whole cell cross-membrane voltage clamping, with voltage ramped from -120mV to 60mV, demonstrated a drastic increase in current with mutant TALK-1 compared to wild-type, consistent with marked gain-of-function and likely to reduce insulin secretion. Our data suggest that KCNK16 may be a novel MODY gene. If confirmed, this has therapeutic implications not only for these individuals but potentially for novel management of type 2 diabetes. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.
American Journal of Physiology-Endocrinology and Metabolism
Pancreatic α-cells exhibit oscillations in cytosolic Ca2+ (Ca2+c), which control pulsatile glucag... more Pancreatic α-cells exhibit oscillations in cytosolic Ca2+ (Ca2+c), which control pulsatile glucagon (GCG) secretion. However, the mechanisms that modulate α-cell Ca2+c oscillations have not been elucidated. As β-cell Ca2+c oscillations are regulated in part by Ca2+-activated K+ (Kslow) currents, this work investigated the role of Kslow in α-cell Ca2+ handling and GCG secretion. α-Cells displayed Kslow currents that were dependent on Ca2+ influx through L- and P/Q-type voltage-dependent Ca2+ channels (VDCCs) as well as Ca2+ released from endoplasmic reticulum stores. α-Cell Kslow was decreased by small-conductance Ca2+-activated K+ (SK) channel inhibitors apamin and UCL 1684, large-conductance Ca2+-activated K+ (BK) channel inhibitor iberiotoxin (IbTx), and intermediate-conductance Ca2+-activated K+ (IK) channel inhibitor TRAM 34. Moreover, partial inhibition of α-cell Kslow with apamin depolarized membrane potential ( Vm) (3.8 ± 0.7 mV) and reduced action potential (AP) amplitude (1...
Islet β cells from newborn mammals exhibit high basal insulin secretion and poor glucose-stimulat... more Islet β cells from newborn mammals exhibit high basal insulin secretion and poor glucose-stimulated insulin secretion (GSIS). Here we show that β cells of newborns secrete more insulin than adults in response to similar intracellular Ca concentrations, suggesting differences in the Ca sensitivity of insulin secretion. Synaptotagmin 4 (Syt4), a non-Ca binding paralog of the β cell Ca sensor Syt7, increased by ∼8-fold during β cell maturation. Syt4 ablation increased basal insulin secretion and compromised GSIS. Precocious Syt4 expression repressed basal insulin secretion but also impaired islet morphogenesis and GSIS. Syt4 was localized on insulin granules and Syt4 levels inversely related to the number of readily releasable vesicles. Thus, transcriptional regulation of Syt4 affects insulin secretion; Syt4 expression is regulated in part by Myt transcription factors, which repress Syt4 transcription. Finally, human SYT4 regulated GSIS in EndoC-βH1 cells, a human β cell line. These fi...
Cystic fibrosis-related (CF-related) diabetes (CFRD) is an increasingly common and devastating co... more Cystic fibrosis-related (CF-related) diabetes (CFRD) is an increasingly common and devastating comorbidity of CF, affecting approximately 35% of adults with CF. However, the underlying causes of CFRD are unclear. Here, we examined cystic fibrosis transmembrane conductance regulator (CFTR) islet expression and whether the CFTR participates in islet endocrine cell function using murine models of β cell CFTR deletion and normal and CF human pancreas and islets. Specific deletion of CFTR from murine β cells did not affect β cell function. In human islets, CFTR mRNA was minimally expressed, and CFTR protein and electrical activity were not detected. Isolated CF/CFRD islets demonstrated appropriate insulin and glucagon secretion, with few changes in key islet-regulatory transcripts. Furthermore, approximately 65% of β cell area was lost in CF donors, compounded by pancreatic remodeling and immune infiltration of the islet. These results indicate that CFRD is caused by β cell loss and intr...
We used mice lacking Abcc8, a key component of the β-cell KATP-channel, to analyze the effects of... more We used mice lacking Abcc8, a key component of the β-cell KATP-channel, to analyze the effects of a sustained elevation in the intracellular Ca(2+) concentration ([Ca(2+)]i) on β-cell identity and gene expression. Lineage tracing analysis revealed the conversion of β-cells lacking Abcc8 into PP-cells, but not to α- or δ-cells. RNA-Seq analysis of FACS-purified Abcc8(-/-) β-cells confirmed an increase in Ppy gene expression, and revealed altered expression of over 4,200 genes, many of which are involved in Ca(2+)-signaling, the maintenance of β-cell identity, and cell adhesion. The expression of S100a6 and S100a4, two highly up-regulated genes, is closely correlated with membrane depolarization, suggesting their use as markers for an increase in [Ca(2+)]i Moreover, a bioinformatics analysis predicts that many of the dysregulated genes are regulated by common transcription factors, one of which, Ascl1, was confirmed to be directly controlled by Ca(2+) influx in β-cells. Interestingly,...
Glucose-stimulated insulin secretion (GSIS) relies on β-cell Ca2+ influx, which is modulated by t... more Glucose-stimulated insulin secretion (GSIS) relies on β-cell Ca2+ influx, which is modulated by the two-pore-domain K+ (K2P) channel, TALK-1. A gain-of-function polymorphism in KCNK16, the gene encoding TALK-1, increases risk for developing type-2 diabetes. While TALK-1 serves an important role in modulating GSIS, the regulatory mechanism(s) that control β-cell TALK-1 channels are unknown. Therefore, we employed a membrane-specific yeast two-hybrid (MYTH) assay to identify TALK-1-interacting proteins in human islets, which will assist in determining signaling modalities that modulate TALK-1 function. Twenty-one proteins from a human islet cDNA library interacted with TALK-1. Some of these interactions increased TALK-1 activity, including intracellular osteopontin (iOPN). Intracellular OPN is highly expressed in β-cells and is upregulated under pre-diabetic conditions to help maintain normal β-cell function; however, the functional role of iOPN in β-cells is poorly understood. We fou...
The two-pore-domain potassium (K2P) channel TREK-2 serves to modulate plasma membrane potential i... more The two-pore-domain potassium (K2P) channel TREK-2 serves to modulate plasma membrane potential in dorsal root ganglia c-fiber nociceptors, which tunes electrical excitability and nociception. Thus, TREK-2 activators are considered a potential therapeutic target for treating pain; however, there are currently no selective pharmacological tools for TREK-2 channels. Here we report the identification of the first TREK-2 selective activators using a high-throughput fluorescence-based thallium (Tl(+)) flux screen (HTS). An initial pilot screen with a bioactive lipid library identified 11-deoxy Prostaglandin F2α as a potent activator of TREK-2 channels (EC50 ~0.294 μM), which was utilized to optimize the TREK-2 Tl(+) flux assay (Z'=0.752). A HTS was then performed with 76,575 structurally diverse small molecules. Many small molecules that selectively activate TREK-2 were discovered. As these molecules were able to activate single TREK-2 channels in excised membrane patches, they are l...
Newly differentiated pancreatic β cells lack proper insulin secretion profiles of mature function... more Newly differentiated pancreatic β cells lack proper insulin secretion profiles of mature functional β cells. The global gene expression differences between paired immature and mature β cells have been studied, but the dynamics of transcriptional events, correlating with temporal development of glucose-stimulated insulin secretion (GSIS), remain to be fully defined. This aspect is important to identify which genes and pathways are necessary for β-cell development or for maturation, as defective insulin secretion is linked with diseases such as diabetes. In this study, we assayed through RNA sequencing the global gene expression across six β-cell developmental stages in mice, spanning from β-cell progenitor to mature β cells. A computational pipeline then selected genes differentially expressed with respect to progenitors and clustered them into groups with distinct temporal patterns associated with biological functions and pathways. These patterns were finally correlated with experim...
Genetic and acquired abnormalities contribute to pancreatic β-cell failure in diabetes. Transcrip... more Genetic and acquired abnormalities contribute to pancreatic β-cell failure in diabetes. Transcription factors Hnf4α (MODY1) and FoxO1 are respective examples of these two components, and are known to act through β-cell-specific enhancers. However, their relationship is unclear. Here we show by genome-wide interrogation of chromatin modifications that FoxO1 ablation in mature β-cells leads to increased selection of FoxO1 enhancers by Hnf4α. To model the functional significance we generated single and compound knockouts of FoxO1 and Hnf4α in β-cells. Single knockout of either gene impaired insulin secretion in mechanistically distinct fashions. Surprisingly, the defective β-cell secretory function of either single mutant in hyperglycemic clamps and isolated islets treated with various secretagogues, was completely reversed in double mutants. Gene expression analyses revealed the reversal of β-cell dysfunction with an antagonistic network regulating glycolysis, including β-cell “disall...
Tetraspanin7-mediated inhibition of β-cell voltage-dependent Ca2+ channels regulates glucose-stim... more Tetraspanin7-mediated inhibition of β-cell voltage-dependent Ca2+ channels regulates glucose-stimulated insulin secretion. Glucose-stimulated insulin secretion (GSIS) is regulated by Ca2+ influx into pancreatic β-cells through voltage-dependent Ca2+ channels (VDCCs). As some tetraspanin (TSPAN) transmembrane proteins control Ca2+ handling they may also modulate GSIS. TSPAN7 is the most abundant TSPAN in mouse and human islets. TSPAN7 autoantibodies are also found in type 1 diabetic serum, suggesting a link between TSPAN7 and diabetes. The aim herein was to determine if TSPAN7 controls β-cell Ca2+ handling and insulin secretion. Immunofluorescence imaging of mouse and human pancreatic slices indicated that TSPAN7 is highly expressed in β- and α-cells. shRNA-mediated TSPAN7 knockdown (KD) in mouse and human β-cells augmented glucose-stimulated Ca2+ influx (by 32.3±9.5% and 33.8±15.1% respectively) as well as KCl-stimulated Ca2+ influx (45 mM; 34.5±9.6%, human). Mouse β-cell Ca2+ oscillation frequency was also accelerated by TSPAN7 KD (by 29.4±10.1%). As TSPANs interact with and regulate VDCCs, we examined the effect of TSPAN7 on VDCC currents. TSPAN7 KD increased peak L-type VDCC currents in mouse and human β-cells (by 4.1±1.3 pA/pF and 2.0±0.9 pA/pF respectively). Heterologous expression of TSPAN7 with L-type VDCCs in HEK293 cells also decreased VDCC currents (CaV1.2: 4.3±1.8 pA/pF; CaV1.3: 17.5±5.4 pA/pF) and reduced KCl-stimulated (50 mM) Ca2+ influx (CaV1.2: 51.0±7.1%; CaV1.3: 28.4±7.5%). Furthermore, TSPAN7 coimmunoprecipitated with CaV1.2 and CaV1.3 from primary human β-cells and when heterologously expressed in HEK293 cells. Finally, TSPAN7 KD in human β-cells increased basal (5.6 mM glucose; 8.3±5.5 ng*hr-1) and stimulated (45 mM KCl+14 mM glucose; 20.4±17.1 ng*hr-1) insulin secretion. These findings reveal that TSPAN7 modulation of β-cell L-type VDCCs limits glucose-stimulated Ca2+ influx and insulin secretion. Disclosure M. Dickerson: None. P. Dadi: None. R.B. Butterworth: None. D. Jacobson: None. Funding American Diabetes Association (1-17-IBS-024 to D.J.); National Institutes of Health (T32DK101003, DK097392, DK115620); Vanderbilt University Medical Center Diabetes Research and Training Center (P60DK20593)
Calcium influx into pancreatic α-cells through voltage-dependent calcium channels (VDCCs) is requ... more Calcium influx into pancreatic α-cells through voltage-dependent calcium channels (VDCCs) is required for glucagon (GCG) secretion and the mechanisms that mediate this process become defective during the pathogenesis of diabetes. Potassium channels are key regulators of membrane potential (Vm), and thus, modulate VDCC activity. However, the role of calcium-activated potassium (KCa) channels in α-cell calcium handling and GCG secretion have not been determined. The aim of this study was to investigate the importance of KCa channels to α-cell electrical excitability, calcium handling, and GCG secretion. α-cells display a KCa current, which is partially inhibited by blocking small conductance KCa (SK) or big conductance KCa (BK) channels. Inhibition of P/Q-type VDCCs, sarco/endoplasmic reticulum (ER) calcium-ATPases (SERCAs), and KATP channels also partially inhibit this current. It is likely that calcium influx through VDCCs triggers calcium-induced calcium release (CICR) from ER calcium stores activating α-cell KCa channels. The inhibition of SK channels transiently increases calcium influx into α-cells at 1 mM glucose; however, over time intracellular calcium decreases. This may be due to voltage-dependent inactivation of α-cell VDCCs. Interestingly, SK channel inhibition reduces GCG secretion at 1 mM glucose while SST secretion is unaffected. Thus, it is likely that this is an intrinsic α-cell effect. These findings demonstrate that α-cells possess a KCa current and that its activation is important for α-cell electrical activity, Ca2+ handling, and GCG secretion. Disclosure M. Dickerson: None. M.K. Altman: None. P. Dadi: None. N.C. Vierra: None. D. Jacobson: None.
In type 2 diabetic patients glucagon secretion becomes dysregulated under elevated glucose condit... more In type 2 diabetic patients glucagon secretion becomes dysregulated under elevated glucose conditions worsening hyperglycemia. However, the mechanisms of glucose inhibition of glucagon secretion remain incompletely understood. Therefore, we sought to uncover the glucose dependent mechanisms that modulate pancreatic α-cell glucagon secretion. One mechanism that regulates α-cell Ca2+ influx in a glucose dependent manner is store-operated Ca2+ entry (SOCE). Therefore, in our studies we used a small molecule (AnCoA4) that inhibits the STIM1/Orai1 protein interaction and prevents store-operated Ca2+ channel activation. In whole mouse islet α-cells inhibition of SOCE with AnCoA4 reduced Ca2+ influx in 1 mM glucose (8.4% ± 1.1%). We went on to test the influence of SOCE on glucose modulation of glucagon secretion, in 1 mM glucose AnCoA4 reduced glucagon secretion in mouse pancreatic islets (53.5% ± 11%). However, there was no significant change in glucose inhibition of glucagon secretion by inhibition of SOCE at 11 mM glucose. To follow-up on this, we used transgenic mice without STIM1 in α-cells and showed that AnCoA4 does not influence Ca2+ fluctuations in knockout islet α-cells. This proves in our hands that AnCoA4 works selectively. Additionally, we used transgenic GCAMP3 mice without STIM1 in α-cells for whole islet confocal studies to measure SOCE in 1 mM and 11 mM glucose conditions. We found that in whole islet α-cells without functional STIM1, Ca2+ influx due to SOCE was reduced in 1 mM glucose. Next, we wanted to test this molecule in human islet calcium homeostasis studies. Human dispersed α-cells show inhibition of SOCE with AnCoA4 treatment in 1 mM glucose conditions. Thus, our findings identify that inhibition of STIM1/Orai1 reduces SOCE in low glucose conditions leading to decreased α-cell glucagon secretion. Disclosure M.K. Altman: None. P. Dadi: None. D. Jacobson: None.
During Glucose-Stimulated Insulin Secretion (GSIS), β-cells exhibit oscillations in cytosolic Ca ... more During Glucose-Stimulated Insulin Secretion (GSIS), β-cells exhibit oscillations in cytosolic Ca 2+ ([Ca 2+ ] c ) and endoplasmic reticulum (ER) Ca 2+ ([Ca 2+ ] ER ), which are regulated by the two-pore domain K+ channel, TALK-1. TALK-1 is the most abundant β-cell K + channel and it controls Ca 2+ homeostasis by modulating K + permeability at both the plasma and ER membrane. However, how TALK-1 modulates ER stress and β-cell failure during the pathogenesis of diabetes has not been determined. To understand how TALK-1 controls β-cell function under diabetic conditions, we assessed how a mutation in TALK-1 (ND) which causes neonatal diabetes, impacts TALK-1 channel activity and [Ca 2+ ] ER handling. While plasmallemal TALK-1 activity was inhibited with the ND mutation, [Ca 2+ ] ER storage was significantly reduced by the ND mutation compared to control TALK-1. This suggests that TALK-1 ND enhances TALK-1 activity on the ER membrane leading to reduced [Ca 2+ ] ER storage. Reducing [Ca 2+ ] ER enhances ER stress, which would be expected to cause β-cell dysfunction. Indeed, TALK-1 ND significantly reduces β-cell insulin secretion compared to control. To further investigate the role of TALK-1 we developed a β-cell-specific TALK-1 KO mouse model (β-TALK1-KO). On a high fat diet, β-TALK1-KO mice show improved glucose tolerance compared to controls. Mitochondrial calcium ([Ca 2+ ] mito ) was also examined because it is modulated by [Ca 2+ ] ER ; we found that β-TALK1-KO mice have increased [Ca 2+ ] mito . As reactive oxygen species (ROS) production is a byproduct of mitochondrial respiration, we investigated if TALK-1 affects ROS production. TALK1 KO islets were found to have reduced ROS production compared to wild type islets. Together, these data indicate that increased TALK-1 channel activity reduces [Ca 2+ ] ER , [Ca 2+ ] mito , and increases ROS; suggesting that TALK-1 plays an important role in modulating the β-cell responses to stress associated with diabetic conditions. Disclosure S.M. Graff: None. P. Dadi: None. C.E. Ibsen: None. M. Dickerson: None. K.L. Jordan: None. D. Jacobson: None.
Abstract Maturity-onset diabetes of the young (MODY) is a monogenic form of diabetes. Exome seque... more Abstract Maturity-onset diabetes of the young (MODY) is a monogenic form of diabetes. Exome sequencing of a non-obese four-generation family with autosomal dominant non-ketotic antibody-negative diabetes, who were negative for mutations in known MODY genes, identified a novel coding variant in KCNK16 (NM_001135105: c.341T>C, p.Leu114Pro) affecting a highly conserved base and predicted to be damaging. KCNK16 encodes TALK-1, a pancreatic β-cell potassium channel which modulates β-cell excitability, controlling Ca2+ influx and insulin secretion. Activation of TALK-1 reduces insulin secretion. A coding gain-of-function variant in KCNK16 is associated with type 2 diabetes in multi-ethnic genome-wide association studies. Notably, TALK-1 is not sensitive to sulfonylureas. HEK-293 cells were transfected with either wild-type or Leu114Pro TALK-1 constructs using a CMV promoter and including P2A mCherry to enable confirmation of successful transfection. Whole cell cross-membrane voltage clamping, with voltage ramped from -120mV to 60mV, demonstrated a drastic increase in current with mutant TALK-1 compared to wild-type, consistent with marked gain-of-function and likely to reduce insulin secretion. Our data suggest that KCNK16 may be a novel MODY gene. If confirmed, this has therapeutic implications not only for these individuals but potentially for novel management of type 2 diabetes. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.
American Journal of Physiology-Endocrinology and Metabolism
Pancreatic α-cells exhibit oscillations in cytosolic Ca2+ (Ca2+c), which control pulsatile glucag... more Pancreatic α-cells exhibit oscillations in cytosolic Ca2+ (Ca2+c), which control pulsatile glucagon (GCG) secretion. However, the mechanisms that modulate α-cell Ca2+c oscillations have not been elucidated. As β-cell Ca2+c oscillations are regulated in part by Ca2+-activated K+ (Kslow) currents, this work investigated the role of Kslow in α-cell Ca2+ handling and GCG secretion. α-Cells displayed Kslow currents that were dependent on Ca2+ influx through L- and P/Q-type voltage-dependent Ca2+ channels (VDCCs) as well as Ca2+ released from endoplasmic reticulum stores. α-Cell Kslow was decreased by small-conductance Ca2+-activated K+ (SK) channel inhibitors apamin and UCL 1684, large-conductance Ca2+-activated K+ (BK) channel inhibitor iberiotoxin (IbTx), and intermediate-conductance Ca2+-activated K+ (IK) channel inhibitor TRAM 34. Moreover, partial inhibition of α-cell Kslow with apamin depolarized membrane potential ( Vm) (3.8 ± 0.7 mV) and reduced action potential (AP) amplitude (1...
Islet β cells from newborn mammals exhibit high basal insulin secretion and poor glucose-stimulat... more Islet β cells from newborn mammals exhibit high basal insulin secretion and poor glucose-stimulated insulin secretion (GSIS). Here we show that β cells of newborns secrete more insulin than adults in response to similar intracellular Ca concentrations, suggesting differences in the Ca sensitivity of insulin secretion. Synaptotagmin 4 (Syt4), a non-Ca binding paralog of the β cell Ca sensor Syt7, increased by ∼8-fold during β cell maturation. Syt4 ablation increased basal insulin secretion and compromised GSIS. Precocious Syt4 expression repressed basal insulin secretion but also impaired islet morphogenesis and GSIS. Syt4 was localized on insulin granules and Syt4 levels inversely related to the number of readily releasable vesicles. Thus, transcriptional regulation of Syt4 affects insulin secretion; Syt4 expression is regulated in part by Myt transcription factors, which repress Syt4 transcription. Finally, human SYT4 regulated GSIS in EndoC-βH1 cells, a human β cell line. These fi...
Cystic fibrosis-related (CF-related) diabetes (CFRD) is an increasingly common and devastating co... more Cystic fibrosis-related (CF-related) diabetes (CFRD) is an increasingly common and devastating comorbidity of CF, affecting approximately 35% of adults with CF. However, the underlying causes of CFRD are unclear. Here, we examined cystic fibrosis transmembrane conductance regulator (CFTR) islet expression and whether the CFTR participates in islet endocrine cell function using murine models of β cell CFTR deletion and normal and CF human pancreas and islets. Specific deletion of CFTR from murine β cells did not affect β cell function. In human islets, CFTR mRNA was minimally expressed, and CFTR protein and electrical activity were not detected. Isolated CF/CFRD islets demonstrated appropriate insulin and glucagon secretion, with few changes in key islet-regulatory transcripts. Furthermore, approximately 65% of β cell area was lost in CF donors, compounded by pancreatic remodeling and immune infiltration of the islet. These results indicate that CFRD is caused by β cell loss and intr...
We used mice lacking Abcc8, a key component of the β-cell KATP-channel, to analyze the effects of... more We used mice lacking Abcc8, a key component of the β-cell KATP-channel, to analyze the effects of a sustained elevation in the intracellular Ca(2+) concentration ([Ca(2+)]i) on β-cell identity and gene expression. Lineage tracing analysis revealed the conversion of β-cells lacking Abcc8 into PP-cells, but not to α- or δ-cells. RNA-Seq analysis of FACS-purified Abcc8(-/-) β-cells confirmed an increase in Ppy gene expression, and revealed altered expression of over 4,200 genes, many of which are involved in Ca(2+)-signaling, the maintenance of β-cell identity, and cell adhesion. The expression of S100a6 and S100a4, two highly up-regulated genes, is closely correlated with membrane depolarization, suggesting their use as markers for an increase in [Ca(2+)]i Moreover, a bioinformatics analysis predicts that many of the dysregulated genes are regulated by common transcription factors, one of which, Ascl1, was confirmed to be directly controlled by Ca(2+) influx in β-cells. Interestingly,...
Glucose-stimulated insulin secretion (GSIS) relies on β-cell Ca2+ influx, which is modulated by t... more Glucose-stimulated insulin secretion (GSIS) relies on β-cell Ca2+ influx, which is modulated by the two-pore-domain K+ (K2P) channel, TALK-1. A gain-of-function polymorphism in KCNK16, the gene encoding TALK-1, increases risk for developing type-2 diabetes. While TALK-1 serves an important role in modulating GSIS, the regulatory mechanism(s) that control β-cell TALK-1 channels are unknown. Therefore, we employed a membrane-specific yeast two-hybrid (MYTH) assay to identify TALK-1-interacting proteins in human islets, which will assist in determining signaling modalities that modulate TALK-1 function. Twenty-one proteins from a human islet cDNA library interacted with TALK-1. Some of these interactions increased TALK-1 activity, including intracellular osteopontin (iOPN). Intracellular OPN is highly expressed in β-cells and is upregulated under pre-diabetic conditions to help maintain normal β-cell function; however, the functional role of iOPN in β-cells is poorly understood. We fou...
The two-pore-domain potassium (K2P) channel TREK-2 serves to modulate plasma membrane potential i... more The two-pore-domain potassium (K2P) channel TREK-2 serves to modulate plasma membrane potential in dorsal root ganglia c-fiber nociceptors, which tunes electrical excitability and nociception. Thus, TREK-2 activators are considered a potential therapeutic target for treating pain; however, there are currently no selective pharmacological tools for TREK-2 channels. Here we report the identification of the first TREK-2 selective activators using a high-throughput fluorescence-based thallium (Tl(+)) flux screen (HTS). An initial pilot screen with a bioactive lipid library identified 11-deoxy Prostaglandin F2α as a potent activator of TREK-2 channels (EC50 ~0.294 μM), which was utilized to optimize the TREK-2 Tl(+) flux assay (Z'=0.752). A HTS was then performed with 76,575 structurally diverse small molecules. Many small molecules that selectively activate TREK-2 were discovered. As these molecules were able to activate single TREK-2 channels in excised membrane patches, they are l...
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Papers by PRASANNA DADI