Dynamics of insulin secretion
Biphasic insulin secretion
Insulin release from pancreatic β cells in response to glucose is characterized by biphasic kinetics: an initial component (1st phase), which develops rapidly but lasts only a few minutes, followed by a sustained component (2nd phase) [26–28] (Figure 7.3(a)). It has been thought that the biphasic response of insulin secretion reflects primarily the dynamics of spatially and functionally distinct insulin granules. The prevailing hypothesis is that the 1st phase of insulin secretion is attributable to fusion of predocked granules from a readily releasable pool (RRP) that accounts for less than 5% of total granules, while the 2nd phase involves recruitment of granules from a more distant reserve pool (RP) that accounts for the great majority of total granules [27–30]. A recent study using total internal reflection fluorescence microscopy (TIRFM) found three distinct modes of insulin granule exocytosis, based on dynamics of insulin granules: a mode comprising predocked granules that are immediately fused to the plasma membrane by stimulation (called old face), another mode comprising granules that are newly recruited by stimulation and immediately fused to the plasma membrane (a docking state can hardly be detected by TIRFM) (called restless newcomer), and a third mode comprising granules that are newly recruited by stimulation, but are first paused or docked and then fused to the plasma membrane (called resting newcomer) . In this model, a RRP responsible for the 1st phase is located away from the plasma membrane but is yet immediately releasable, and both 1st and 2nd phases of insulin granule exocytosis involve the restless newcomer (Figure 7.3(a)). Glucose-induced F-actin-remodeling has recently been shown to be involved in mediating the 2nd phase of insulin secretion  (Figure 7.3(a)).
Insulin granule exocytosis in pancreatic β cells, like synaptic vesicle exocytosis in neurons, involves several processes, including granule recruitment to the plasma membrane, docking of granules at the plasma membrane, priming of fusion machinery, and fusion of granules with the plasma membrane. However, the kinetics of exocytosis is ultrafast (a few milliseconds) in synaptic vesicles of the neuron and slow (a few hundred milliseconds) in large-dose core granules of the pancreatic β cell [33,34]. SNARE proteins critical for synaptic vesicle exocytosis are expressed in pancreatic β cells and β-cell lines . This exocytotic machinery, including the t-SNAREs syntaxin 1 and SNAP-25 as well as the v-SNARE synaptobrevin/VAMP-2, function similarly in insulin granule exocytosis [33,34]. Syntaxin 1 and SNAP-25 form a cluster along the plasma membrane of pancreatic β cells . The rise in [Ca2+]i triggers the formation of the SNARE complex from Syntaxin 1, SNAP-25, and VAMP-2, which promotes membrane fusion [19,34]. The SNARE complex is responsible for induction of insulin granule exocytosis in response to glucose .
In neurons, SNARE proteins interact with many vesicle associated proteins including Sec1/Munc18 (SM) protein, Munc13, synaptotagmins, and complexin . SM protein and Munc13 promote the assembly of SNARE proteins, and synaptotagmin and complexin control Ca2+-dependent triggering of exocytosis . SM protein associates with the closed form of syntaxins, and the closed form is presumed to prevent participation in SNARE complexes [36,37]. In pancreatic β cells Munc18-1 and Munc18c are involved in the regulation of the 1st and 2nd phases of GIIS, respectively, doing so by promoting localization of insulin granules to the plasma membrane . Munc13 mediates synaptic vesicle priming by stabilizing the open conformation of Syntaxin 1, thereby allowing the formation of SNARE complexes [39,40]. In pancreatic β cells, Munc13-1 plays an essential role in the priming step in insulin granule exocytosis through its interaction with the Rab3 effector Rim2α . Munc13-1 also mediates both 1st and 2nd phases of GIIS .
The small G-protein Rab family comprises more than 60 members [43,44]. Among them, Rab3 and Rab27a are associated with insulin granules of pancreatic β cells [45,46]. Both Rab3 and Rab27a are localized to insulin granules and function through interaction with their effector proteins Rim2α and granuphilin, respectively [41,47–49]. Rim2α plays critical roles in docking and priming steps through its interaction with Rab3 And Munc13-1, respectively . The interaction of granuphilin with Syntaxin 1A/Munc18-1 is also important for docking of insulin granules to the plasma membrane .
Modulation of insulin secretion by various intracellular signals
GIIS is modulated by various nutrients and hormonal and neuronal inputs (Figure 7.1). Most of these hormones and neurotransmitters exert their effects on insulin secretion by binding to their specific surface receptors, which are members of the superfamily of trimeric G-protein-coupled receptors (GPCRs) . Based on the properties of G-proteins, GPCRs are subdivided into different functional classes, primarily Gs, Gq/11, and Gi -protein-coupled receptors (Table 7.1). G-proteins are linked to specific signaling pathways that have multiple effects on β-cell function in the modulation of GIIS. Gs and Gq/11-proteins potentiate GIIS and may have several other beneficial effects on β-cell function . Gi-protein exerts an inhibitory effect on GIIS . GIIS also has been shown to be modulated by the insulin signaling pathway in rodents in vivo and islets isolated from rodents and humans [51–54]. As stimulation of insulin secretion is an essential pharmacologic strategy for treatment of type 2 diabetes, GPCR and the insulin signaling pathways in pancreatic β cells offer many attractive drug targets.