Identification of the docked granule pool responsible for the first phase of glucose-stimulated insulin secretion.

Abstract

The mechanisms underlying the first phase of glucose-stimulated insulin release, the deterioration of which marks the early stages of both type 1 and type 2 diabetes, are essentially unknown. Among many hypotheses, one holds that the first phase is due to a readily releasable pool of insulin-containing granules. We used current knowledge of the mechanisms of exocytosis and the proteins involved in docking granules at the plasma membrane to test this hypothesis. A docked pool of readily releasable granules was identified by immunoprecipitation of the plasma membrane protein syntaxin with a specific antibody and by co-immunoprecipitation of soluble N-ethylmaleimide-sensitive factor attachment protein-25 (SNAP-25) and the granule proteins synaptobrevin and synaptotagmin. The four SNARE proteins co-immunoprecipitated each other, thus identifying the core complex associated with docked granules. Using co-immunoprecipitation as a marker for docked granules, we found that the docked pool was rapidly discharged during the first phase of glucose-stimulated insulin release and refilled during the second phase. Other secretagogues also released the pool, whereas the physiological inhibitor norepinephrine blocked its release. Further studies on the nature of this pool of granules should shed light on the causes of its deterioration in the early stages of diabetes and the reasons for deficient insulin release.

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

“…Also, human β-cell-exocytosis does not plateau for depolarizations up to 500 ms (Braun et al, , 2009, again arguing against pool depletion. However, this does not means that depletion of the RRP is irrelevant for biphasic insulin secretion, indeed there is good evidence for a role of a finite pool of granules (Daniel et al, 1999;Henquin et al, 2002;Olofsson et al, 2002), but rather that short depolarizations of increasing lengths do not show evidence of a smaller IRP. Finally, our results confirmed that cAMP-raising agents such as forskolin increase the efficacy of Ca 2+ on exocytosis.…”

“…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.…”

Syntaxin 4 and Synip (Syntaxin 4 Interacting Protein) Regulate Insulin Secretion in the Pancreatic β HC-9 Cell

“…This pool of insulin granules has been referred as the readily releasable pool, and it appears to account for the first phase of stimulusmediated insulin secretion (13). This initial release process requires the functional interactions of the t-SNARE 1 proteins, syntaxin 1 and SNAP25, with the v-SNARE protein, VAMP2.…”

Arginine vasopressin-induced glucagon release: interaction with glucose and cyclic AMP-dependent protein kinase

“…Glucagon release was measured under perifusion condition (Kikuchi et al, 1974;Daniel et al, 1999). Forskolin (10 |oM) enhanced the effect of AVP (100 riM) on glucagon release (a peak of -3 fold and -7 fold of the baseline level for AVP and the combination of AVP and forskolin, respectively) (Fig.…”

“…Glucagon release was measured under perifusion condition as previously described (Kikuchi et al, 1974;Daniel et al, 1999). Briefly, InRlG9 cells were grown in custommade coverslips, which were placed in a perifusion chamber.…”