Novel aspects of the molecular mechanisms controlling insulin secretion

Abstract

Pancreatic β-cells secrete insulin by Ca2+ -dependent exocytosis of secretory granules. β-cell exocytosis involves SNARE (soluble NSF-attachment protein receptor) proteins similar to those controlling neurotransmitter release and depends on the close association of L-type Ca 2+ channels and granules. In most cases, the secretory granules fuse individually but there is ultrastructural and biophysical evidence of multivesicular exocytosis. Estimates of the secretory rate in β-cells in intact islets indicate a release rate of ∼15 granules per β-cell per second, 100-fold higher than that observed in biochemical assays. Single-vesicle capacitance measurements reveal that the diameter of the fusion pore connecting the granule lumen with the exterior is ∼1.4 nm. This is considerably smaller than the size of insulin and membrane fusion is therefore not obligatorily associated with release of the cargo, a feature that may contribute to the different rates of secretion detected by the biochemical and biophysical measurements. However, small molecules like ATP and GABA, which are stored together with insulin in the granules, are small enough to be released via the narrow fusion pore, which accordingly functions as a molecular sieve. We finally consider the possibility that defective fusion pore expansion accounts for the decrease in insulin secretion observed in pathophysiological states including long-term exposure to lipids.

Newcomer insulin secretory granules as a highly calcium-sensitive pool

“…Moreover, PKC is well known to have effects on the submembrane actin barrier because of activation of proteins involved in the remodeling of the actin network such as MARCKS (42). Such actin remodeling allows granules to arrive at the cell membrane and is important for second-phase insulin secretion (24,43,60). The assumption of HCSP stabilization is most easily explained by changed properties of SNAP-25 because of phosphorylation, which might reduce the rate of complete SNARE complex formation and hence full docking.…”

“…It has been suggested that biphasic insulin release could be a result not of vesicle kinetics but of the biphasic Ca 2ϩ pattern (24,45). To test this hypothesis, we simulated the burst-like protocol with varying lengths of the first phase of depolarization (Fig.…”

Mathematical modeling and statistical analysis of calcium-regulated insulin granule exocytosis in β-cells from mice and humans

“…Using the latter method, we and others have suggested that granules can be divided into different functional pools prior to release [see e.g. Eliasson et al (2008)]. Granules docked and primed at the plasma membrane belong to the readily releasable pool (RRP).…”

“…The majority of the granules belong to a reserve pool (RP) and these needs to be transported, docked and primed prior to release. On a cellular level it has been suggested that first phase insulin secretion is equivalent to release of granules from the IRP/RRP and second phase insulin secretion correspond to the release of granules from the RP (Daniel et al, 1999;Eliasson et al, 2008). Interestingly, first phase insulin secretion is lacking in patients with type 2 diabetes (Hosker et al, 1989) why studies investigating the control of the IRP/RRP is of great importance.…”

Bulk-like endocytosis plays an important role in the recycling of insulin granules in pancreatic beta cells

“…Therefore, it is well accepted that type II diabetes involves a perturbation of the exocytotic machinery of pancreatic beta cells (Eliasson et al, 2008). However, whether changes in the membrane recycling process participate in such pathological processes has not been investigated.…”