| 71. |
- Sanchez, Gonzalo, et al.
(författare)
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The β-cell primary cilium is an autonomous Ca2+ compartment for paracrine GABA signaling
- 2023
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Ingår i: Journal of Cell Biology. - : Rockefeller University Press. - 0021-9525 .- 1540-8140. ; 222:1
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Tidskriftsartikel (refereegranskat)abstract
- The primary cilium is an organelle present in most adult mammalian cells that is considered as an antenna for sensing the local microenvironment. Here, we use intact mouse pancreatic islets of Langerhans to investigate signaling properties of the primary cilium in insulin-secreting β-cells. We find that GABAB1 receptors are strongly enriched at the base of the cilium, but are mobilized to more distal locations upon agonist binding. Using cilia-targeted Ca2+ indicators, we find that activation of GABAB1 receptors induces selective Ca2+ influx into primary cilia through a mechanism that requires voltage-dependent Ca2+ channel activation. Islet β-cells utilize cytosolic Ca2+ increases as the main trigger for insulin secretion, yet we find that increases in cytosolic Ca2+ fail to propagate into the cilium, and that this isolation is largely due to enhanced Ca2+ extrusion in the cilium. Our work reveals local GABA action on primary cilia that involves Ca2+ influx and depends on restricted Ca2+ diffusion between the cilium and cytosol.
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| 72. |
- Shuai, Hongyan
(författare)
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Studies of cAMP and Ca2+ signaling in pancreatic islet cells
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The blood glucose-lowering and -elevating hormones insulin and glucagon are released from the pancreatic islet β- and α-cells, respectively. The intracellular messengers Ca2+ and cAMP have central roles in controlling the secretion of both hormones, but the underlying mechanisms are incompletely understood. A powerful approach to gain further insight is to study the messengers in individual cells within pancreatic islets, provided that each cell can be identified. To facilitate such studies, adenoviral vectors were generated for expression of fluorescent proteins controlled by the insulin and preproglucagon promoters, as well as the somatostatin and pancreatic polypeptide promoters that identify the other two major islet cell types, δ- and PP-cells. Recordings of cAMP and Ca2+ concentration changes with fluorescent reporters demonstrated that cells expressing identification markers responded as expected to well-known stimuli and modulators of the two messengers. Glucose-induced Ca2+ oscillations in β-cells were found to be synchronized with those in δ-cells, and two subpopulations of α-cells with different Ca2+ regulation by glucose were identified. Mouse and human β-cells responded to the insulinotropic hormones glucagon, GIP and GLP-1 with elevations of cAMP. Most α-cells reacted similarly to GIP, whereas only a subpopulation – larger among human than mouse α-cells - responded to glucagon and GLP-1. The GLP-1-receptor antagonist exendin-(9-39) suppressed both GLP-1- and glucagon-induced cAMP elevations in β-cells. Since exendin-(9-39) did not antagonize glucagon receptors, glucagon apparently activates GLP-1 receptors in β-cells. Even in the absence of glucagon/GLP-1, exendin-(9-39) reduced cAMP increases obtained by glucose stimulation or elevation of Ca2+. This effect was attributable to constitutive GLP-1-receptor activity rather than paracrine effects. Exendin-(9-39) also inhibited glucose-induced insulin release, highlighting the importance of cAMP formation in nutrient-stimulated secretion. Simultaneous recordings of cAMP and Ca2+ showed a complex and variable interrelationship between the messengers and the cAMP precursor ATP in β-cells. Depolarization-induced Ca2+ increases inhibited forskolin-, IBMX- and GLP-1-induced cAMP elevations. This cAMP lowering in part reflected suppression of the Ca2+-sensitive activity of adenylyl cyclases AC5 and 6, but also autocrine signaling induced by Ca2+-triggered exocytosis of insulin and adenine nucleotides, whose receptors activate phosphodiesterases and inhibit adenylyl cyclases, respectively.
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| 73. |
- Somanath, Sangeeta, et al.
(författare)
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High extracellular glucose inhibits exocytosis through disruption of syntaxin 1A-containing lipid rafts
- 2009
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Ingår i: Biochemical and Biophysical Research Communications - BBRC. - : Elsevier BV. - 0006-291X .- 1090-2104. ; 389:2, s. 241-246
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Tidskriftsartikel (refereegranskat)abstract
- Diabetes is characterized by high blood glucose which eventually impairs the secretion of insulin. Glucose directly affects cholesterol biosynthesis and may in turn affect cellular structures that depend on the sterol, including lipid rafts that help organize the secretory apparatus. Here, we investigated the long-term effects of glucose upon lipid rafts and secretory granule dynamics in pancreatic beta-cells. Raft fractions, identified by the presence of GM1 and flotillin, contained characteristically high levels of cholesterol and syntaxin 1A, the t-SNARE which tethers granules to the plasma membrane. Seventy-two hours exposure to 28mM glucose resulted in approximately 30% reduction in membrane cholesterol, with consequent redistribution of raft markers and syntaxin 1A throughout the plasma membrane. Live cell imaging indicated loss of syntaxin 1A from granule docking sites, and fewer docked granules. In conclusion, glucose-mediated inhibition of cholesterol biosynthesis perturbs lipid raft stability, resulting in a loss of syntaxin 1A from granule docking sites and inhibition of insulin secretion.
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| 74. |
- Timerman, A P, et al.
(författare)
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Selective binding of FKBP12.6 by the cardiac ryanodine receptor
- 1996
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Ingår i: Journal of Biological Chemistry. - 1083-351X .- 0021-9258. ; 271:34, s. 20385-20391
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Tidskriftsartikel (refereegranskat)abstract
- The calcium release channels (CRC)/ryanodine receptors of skeletal (Sk) and cardiac (C) muscle sarcoplasmic reticulum (SR) are hetero-oligomeric complexes with the structural formulas (ryanodine recepter (RyR)1 protomer)4(FKBP12)4 and (RyR2 protomer)4(FKBP12.6)4, respectively, where FKBP12 and FKBP12.6 are isoforms of the 12-kDa receptor for the immunosuppressant drug FK506. The sequence similarity between the RyR protomers and FKBP12 isoforms is 63 and 85%, respectively. Using 35S-labeled FKBP12 and 35S-labeled FKBP12.6 as probes to study the interaction with CRC, we find that: 1) analogous to its action in skeletal muscle sarcoplasmic reticulum (SkMSR), FK506 (or analog FK590) dissociates FKBP12.6 from CSR; 2) both FKBP isoforms bind to FKBP-stripped SkMSR and exchange with endogenously bound FKBP12 of SkMSR; and 3) by contrast, only FKBP12. 6 exchanges with endogenously bound FKBP12.6 or rebinds to FKBP-stripped CSR. This selective binding appears to explain why the cardiac CRC is isolated as a complex with FKBP12.6, whereas the skeletal muscle CRC is isolated as a complex with FKBP12, although only FKBP12 is detectable in the myoplasm of both muscle types. Also, in contrast to the activation of the channel by removal of FKBP from skeletal muscle, no activation is detected in CRC activity in FKBP-stripped CSR. This differential action of FKBP may reflect a fundamental difference in the modulation of excitation-contraction coupling in heart versus skeletal muscle.
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| 75. |
- Wiser, O, et al.
(författare)
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The voltage sensitive Lc-type Ca2+ channel is functionally coupled to the exocytotic machinery
- 1999
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Ingår i: Proceedings of the National Academy of Sciences. - 1091-6490 .- 0027-8424. ; 96:1, s. 248-253
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Tidskriftsartikel (refereegranskat)abstract
- Although N- and P-type Ca2+ channels predominant in fast-secreting systems, Lc-type Ca2+ channels (C-class) can play a similar role in certain secretory cells and synapses. For example, in retinal bipolar cells, Ca2+ entry through the Lc channels triggers ultrafast exocytosis, and in pancreatic beta-cells, evoked secretion is highly sensitive to Ca2+. These findings suggest that a rapidly release pool of vesicles colocalizes with the Ca2+ channels to allow high Ca2+ concentration and a tight coupling of the Lc channels at the release site. In binding studies, we show that the Lc channel is physically associated with synaptotagmin (p65) and the soluble N-ethylmaleimide-sensitive attachment proteins receptors: syntaxin and synaptosomal-associated protein of 25 kDa. Soluble N-ethylmaleimide-sensitive attachent proteins receptors coexpressed in Xenopus oocytes along with the Lc channel modify the kinetic properties of the channel. The modulatory action of syntaxin can be overcome by coexpressing p65, where at a certain ratio of p65/syntaxin, the channel regains its unaltered kinetic parameters. The cytosolic region of the channel, Lc753-893, separating repeats II-III of its alpha1C subunit, interacts with p65 and "pulls" down native p65 from rat brain membranes. Lc753-893 injected into single insulin-secreting beta-cell, inhibits secretion in response to channel opening, but not in response to photolysis of caged Ca2+, nor does it affect Ca2+ current. These results suggest that Lc753-893 competes with the endogenous channel for the synaptic proteins and disrupts the spatial coupling with the secretory apparatus. The molecular organization of the Lc channel and the secretory machinery into a multiprotein complex (named excitosome) appears to be essential for an effective depolarization evoked exocytosis.
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| 76. |
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| 77. |
- Xie, Beichen, et al.
(författare)
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Plasma Membrane Phosphatidylinositol 4,5-Bisphosphate Regulates Ca2+-Influx and Insulin Secretion from Pancreatic beta Cells
- 2016
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Ingår i: CELL CHEMICAL BIOLOGY. - : Elsevier BV. - 2451-9448 .- 2451-9456. ; 23:7, s. 816-826
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Tidskriftsartikel (refereegranskat)abstract
- Insulin secretion from pancreatic beta cells is regulated by the blood glucose concentration and occurs through Ca2+-triggered exocytosis. The activities of multiple ion channels in the beta cell plasma membrane are required to fine-tune insulin secretion in order to maintain normoglycemia. Phosphoinositide lipids in the plasma membrane often gate ion channels, and variations in the concentration of these lipids affect ion-channel open probability and conductance. Using light-regulated synthesis or depletion of plasma membrane phosphatidylinositol 4,5-bisphosphate (PI[4,5]P-2), we found that this lipid positively regulated both depolarization- and glucose-triggered Ca2+ influx in a dose-dependent manner. Small reductions of PI(4,5)P-2 caused by brief illumination resulted in partial suppression of Ca2+ influx that followed the kinetics of the lipid, whereas depletion resulted in marked inhibition of both Ca2+ influx and insulin secretion.
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| 78. |
- Yeh, Yu-Te, et al.
(författare)
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Restoration of PITPNA in Type 2 diabetic human islets reverses pancreatic beta-cell dysfunction
- 2023
-
Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- Defects in insulin processing and granule maturation are linked to pancreatic beta-cell failure during type 2 diabetes (T2D). Phosphatidylinositol transfer protein alpha (PITPNA) stimulates activity of phosphatidylinositol (PtdIns) 4-OH kinase to produce sufficient PtdIns-4-phosphate (PtdIns-4-P) in the trans-Golgi network to promote insulin granule maturation. PITPNA in beta-cells of T2D human subjects is markedly reduced suggesting its depletion accompanies beta-cell dysfunction. Conditional deletion of Pitpna in the beta-cells of Ins-Cre, Pitpnaflox/flox mice leads to hyperglycemia resulting from decreasing glucose-stimulated insulin secretion (GSIS) and reducing pancreatic beta-cell mass. Furthermore, PITPNA silencing in human islets confirms its role in PtdIns-4-P synthesis and leads to impaired insulin granule maturation and docking, GSIS, and proinsulin processing with evidence of ER stress. Restoration of PITPNA in islets of T2D human subjects reverses these beta-cell defects and identify PITPNA as a critical target linked to beta-cell failure in T2D.
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| 79. |
- Yin, Peng, 1982-
(författare)
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Architecture and function of the insulin granule secretion machinery
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Insulin is released into the blood stream to normalize elevated blood glucose, for example after a meal. The hormone is synthesized by β-cells in the endocrine pancreas, and stored in small vesicles, known as secretory granules, until required. When glucose is elevated, these granules undergo regulated exocytosis and thereby secrete the hormone. The primary trigger for this is a glucose-dependent elevation in cytosolic Ca2+, which enters the cell through voltage-gated Ca2+ channels. Glucose-stimulated insulin secretion follows a biphasic timecourse, with a rapid 1st phase that lasts for a few minutes, followed by a slowly developing sustained 2nd phase. Compromised 1st phase secretion is an early sign of developing type-2 diabetes. Biphasic secretion is thought reflect the vastly different probabilities of individual insulin granules, but direct evidence for this is still lacking. In this thesis, high resolution TIRF microscopy was used to identify rate limiting steps for insulin granule exocytosis in health and in type-2 diabetes, and to understand these steps at the molecular level. It is shown that granule docking is critical for sustained insulin secretion. In β-cells from type-2 diabetic donors, docking is compromised and no longer responsive to glucose. Expression analysis in a large donor cohort suggests that this is due to decreased expression of proteins involved in the docking step. One of these proteins, the SNARE protein syntaxin-1, is well-known to cluster at the site of docked granules, which initiates the formation of functional release sites. Analysis using a series of syntaxin-1 mutations indicates that this clustering depends on specific features in its N-terminal Habc domain and involves binding of the S/M protein munc-18. The data suggest that the closed conformation of syntaxin-1 mediates the interaction between granule and plasma membrane. Finally, it is shown that voltage-gated L-type Ca2+ channels are slowly recruited to the sites of docked granules, which depends on interaction with the granule priming factor Munc13. This arrangement leads to localized the Ca2+ influx near a subset of the docked granules, which dramatically increases their release probability. Importantly, the interaction between Ca2+ channels and granules fails in type-2 diabetic β cells. In summary, the thesis highlights the importance of the spatial organization of the secretory machinery for adequate insulin secretion, and suggests that defects in this process partly underlie the disturbed blood glucose regulation in type-2 diabetes.
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| 80. |
- Yin, Peng, 1982-, et al.
(författare)
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Syntaxin clusters at secretory granules in a munc18-bound conformation
- 2018
-
Ingår i: Molecular Biology of the Cell. - 1059-1524 .- 1939-4586. ; 29:22, s. 2700-2708
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Tidskriftsartikel (refereegranskat)abstract
- Syntaxin (stx)-1 is an integral plasma membrane protein that is crucial for two distinct steps of regulated exocytosis, docking of secretory granules at the plasma membrane and membrane fusion. During docking, stx1 clusters at the granule docking site, together with the S/M protein munc18. Here we determined features of stx1 that contribute to its clustering at granules. In live insulin-secreting cells, stx1 and stx3 (but not stx4 or stx11) accumulated at docked granules, and stx1 (but not stx4) rescued docking in cells expressing botulinum neurotoxin-C. Using a series of stx1 deletion mutants and stx1/4 chimeras, we found that all four helical domains (Ha, Hb, Hc, SNARE) and the short N-terminal peptide contribute to recruitment to granules. However, only the Hc domain confers specificity, and it must be derived from stx1 for recruitment to occur. Point mutations in the Hc or the N-terminal peptide designed to interfere with binding to munc18-1 prevent stx1 from clustering at granules, and a mutant munc18 deficient in binding to stx1 does not cluster at granules. We conclude that stx1 is recruited to the docking site in a munc18-1-bound conformation, providing a rationale for the requirement for both proteins for granule docking.
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