Munc13-1 Deficiency Reduces Insulin Secretion and Causes Abnormal Glucose Tolerance

Abstract

Munc13-1 is a diacylglycerol (DAG) receptor that is essential for synaptic vesicle priming. We recently showed that Munc13-1 is expressed in rodent and human islet ␤-cells and that its levels are reduced in islets of type 2 diabetic humans and rat models, suggesting that Munc13-1 deficiency contributes to the abnormal insulin secretion in diabetes. To unequivocally demonstrate the role of Munc13-1 in insulin secretion, we studied heterozygous Munc13-1 knockout mice (؉/؊), which exhibited elevated glucose levels during intraperitoneal glucose tolerance tests with corresponding lower serum insulin levels. Munc13-1؉/؊ mice exhibited normal insulin tolerance, indicating that a primary islet ␤-cell secretory defect is the major cause of their hyperglycemia. Consistently, glucosestimulated insulin secretion was reduced 50% in isolated Munc13-1 ؉/؊ islets and was only partially rescued by phorbol ester potentiation. The corresponding alterations were minor in mice expressing one allele of a Munc13-1 mutant variant, which does not bind DAG (H567K/؉). Capacitance measurements of Munc13-1 ؉/؊ and Munc13-1 H567k/؉ islet ␤-cells revealed defects in granule priming, including the initial size and refilling of the releasable pools, which become accentuated by phorbol ester potentiation. We conclude that Munc13-1 plays an important role in glucosestimulated insulin secretion and that Munc13-1 deficiency in the pancreatic islets as occurs in diabetes can reduce insulin secretion sufficient to cause abnormal glucose homeostasis. Diabetes 55: [1421][1422][1423][1424][1425][1426][1427][1428][1429] 2006 N ormal glucose-stimulated insulin secretion (GSIS) from pancreatic islets follows a biphasic pattern (1). The first phase is a robust release lasting ϳ5-10 min and is triggered by ATPsensitive K ϩ channel-dependent Ca 2ϩ entry into the pancreatic ␤-cells. This is followed by a second phase and sustained release, which is regulated by second messengers including diacyglycerol (DAG), cAMP, and permissive elevated levels of Ca 2ϩ (2). In patients with type 2 diabetes, this pattern of secretion is perturbed, resulting in an abolished first phase and a blunted second phase (2). It is generally accepted that the biphasic secretion pattern is contributed by the release of spatially and functionally distinct insulin granules. The morphologically docked granule pool at the plasma membrane contains Ͻ10% of the total of Ͼ10,000 insulin granules per cell, and only a fraction (20 -30%) of the docked pool is primed and fusion competent for immediate release (3). It is the release of this docked and primed granule pool that contributes to the first phase of insulin secretion (2-4). One of the aims of the present study was to identify the proteins that prime insulin granules for release.As in neurons (5), exocytosis in pancreatic islet ␤-cells is regulated by the SNARE (soluble N-ethylmaleimidesensitive factor attachment protein [SNAP] receptor) proteins VAMP, SNAP-25, and syntaxin-1A (6). These SNARE proteins form a ternary complex capable of ...

Doc2β Is a Novel Munc18c-interacting Partner and Positive Effector of Syntaxin 4-mediated Exocytosis

“…In contrast, overexpression of Doc2␤ enhanced both basal and glucose-stimulated VAMP2 association with Syntaxin 4. The mechanism behind this could lie in the ability of Doc2␤ to compete Munc18c away from Syntaxin 4 in the absence of stimulus, but could also be mediated by its interaction with other molecules such as Munc13-1, which has been demonstrated to influence both first phase and second phase insulin secretion in studies using islet cells isolated from Munc13-1 heterozygous knockout mice (50).…”

Lysosomal acid lipase and lipophagy are constitutive negative regulators of glucose-stimulated insulin secretion from pancreatic beta cells

“…Candidates include long-chain fatty acyl CoAs, perhaps via protein acylation or modulation of ion channels [43], and diacylglycerol, albeit most probably acting via mechanisms independent of protein kinase C [44,45]. These metabolites can be derived from breakdown of TG.…”

Fatty Acid Signaling in the β-Cell and Insulin Secretion

“…While it seems likely that FFA signaling via FFAR1 involves increases in intracellular Ca 2ϩ , the mechanisms by which TG/FFA cycling provides signals for insulin secretion are poorly understood and are likely to be multiple. As discussed above, they may involve protein acylation (3,100), the DAG receptor Munc-13 (56), and atypical C-kinase enzymes activated by LC-CoA (101). The lipase enzymes involved remain to be determined.…”

“…DAG, the levels of which rise in the ␤-cell in response to glucose (36,52), not only activate protein kinase C, which is implicated in insulin secretion (53), but also bind to the C 1 domain of the synaptic vesicle priming protein Munc-13 (54), which has recently been shown to be important for normal insulin secretion (55,56). Thus, heterozygous Munc-13-1 ϩ/Ϫ mice show reduced GSIS, both in vivo and in isolated islets (56), such that Munc-13 is a prime candidate as an effector molecule in ␤-cell lipid signaling. TG/FFA will also affect membrane glycerophospholipid metabolism, which could influence secretion via alteration of membrane physicochemical properties (57).…”