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Munc18b is a major mediator of insulin exocytosis in rat pancreatic ?-cells.

ABSTRACT: Sec1/Munc18 proteins facilitate the formation of trans-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes that mediate fusion of secretory granule (SG) with plasma membrane (PM). The capacity of pancreatic ?-cells to exocytose insulin becomes compromised in diabetes. ?-Cells express three Munc18 isoforms of which the role of Munc18b is unknown. We found that Munc18b depletion in rat islets disabled SNARE complex formation formed by syntaxin (Syn)-2 and Syn-3. Two-photon imaging analysis revealed in Munc18b-depleted ?-cells a 40% reduction in primary exocytosis (SG-PM fusion) and abrogation of almost all sequential SG-SG fusion, together accounting for a 50% reduction in glucose-stimulated insulin secretion (GSIS). In contrast, gain-of-function expression of Munc18b wild-type and, more so, dominant-positive K314L/R315L mutant promoted the assembly of cognate SNARE complexes, which caused potentiation of biphasic GSIS. We found that this was attributed to a more than threefold enhancement of both primary exocytosis and sequential SG-SG fusion, including long-chain fusion (6-8 SGs) not normally (2-3 SG fusion) observed. Thus, Munc18b-mediated exocytosis may be deployed to increase secretory efficiency of SGs in deeper cytosolic layers of ?-cells as well as additional primary exocytosis, which may open new avenues of therapy development for diabetes.

SUBMITTER: Lam PP

PROVIDER: S-EPMC3712044 | biostudies-literature | 2013 Jul

REPOSITORIES: biostudies-literature

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Munc18b is a major mediator of insulin exocytosis in rat pancreatic β-cells.

Lam Patrick P L PP Ohno Mitsuyo M Dolai Subhankar S He Yu Y Qin Tairan T Liang Tao T Zhu Dan D Kang Youhou Y Liu Yunfeng Y Kauppi Maria M Xie Li L Wan Wilson C Y WC Bin Na-Rhum NR Sugita Shuzo S Olkkonen Vesa M VM Takahashi Noriko N Kasai Haruo H Gaisano Herbert Y HY

Diabetes 20130219 7

Sec1/Munc18 proteins facilitate the formation of trans-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes that mediate fusion of secretory granule (SG) with plasma membrane (PM). The capacity of pancreatic β-cells to exocytose insulin becomes compromised in diabetes. β-Cells express three Munc18 isoforms of which the role of Munc18b is unknown. We found that Munc18b depletion in rat islets disabled SNARE complex formation formed by syntaxin (Syn)-2 and Syn-3. ...[more]

PMID: 23423569

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Project description:Tissue-specific autoimmunity occurs when selected antigens presented by susceptible alleles of the major histocompatibility complex are recognized by T cells. However, the reason why certain specific self-antigens dominate the response and are indispensable for triggering autoreactivity is unclear. Spontaneous presentation of insulin is essential for initiating autoimmune type 1 diabetes in non-obese diabetic mice1,2. A major set of pathogenic CD4 T cells specifically recognizes the 12-20 segment of the insulin B-chain (B:12-20), an epitope that is generated from direct presentation of insulin peptides by antigen-presenting cells3,4. These T cells do not respond to antigen-presenting cells that have taken up insulin that, after processing, leads to presentation of a different segment representing a one-residue shift, B:13-214. CD4 T cells that recognize B:12-20 escape negative selection in the thymus and cause diabetes, whereas those that recognize B:13-21 have only a minor role in autoimmunity3-5. Although presentation of B:12-20 is evident in the islets3,6, insulin-specific germinal centres can be formed in various lymphoid tissues, suggesting that insulin presentation is widespread7,8. Here we use live imaging to document the distribution of insulin recognition by CD4 T cells throughout various lymph nodes. Furthermore, we identify catabolized insulin peptide fragments containing defined pathogenic epitopes in β-cell granules from mice and humans. Upon glucose challenge, these fragments are released into the circulation and are recognized by CD4 T cells, leading to an activation state that results in transcriptional reprogramming and enhanced diabetogenicity. Therefore, a tissue such as pancreatic islets, by releasing catabolized products, imposes a constant threat to self-tolerance. These findings reveal a self-recognition pathway underlying a primary autoantigen and provide a foundation for assessing antigenic targets that precipitate pathogenic outcomes by systemically sensitizing lymphoid tissues.

Dataset Information

Munc18b is a major mediator of insulin exocytosis in rat pancreatic ?-cells.

Publications

Munc18b is a major mediator of insulin exocytosis in rat pancreatic β-cells.

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