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Granuphilin molecularly docks insulin granules to the fusion machinery.

Gomi H, Mizutani S, Kasai K, Itohara S, Izumi T - J. Cell Biol. (2005)

Bottom Line: The Rab27a effector granuphilin is specifically localized on insulin granules and is involved in their exocytosis.Here we show that the number of insulin granules morphologically docked to the plasma membrane is markedly reduced in granuphilin-deficient beta cells.The enhanced secretion in mutant beta cells is correlated with a decrease in the formation of the fusion-incompetent syntaxin-1a-Munc18-1 complex, with which granuphilin normally interacts.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Molecular Endocrinology and Metabolism, Institute for Molecular and Cellular Regulation, Gunma University, Gunma 371-8512, Japan.

ABSTRACT
The Rab27a effector granuphilin is specifically localized on insulin granules and is involved in their exocytosis. Here we show that the number of insulin granules morphologically docked to the plasma membrane is markedly reduced in granuphilin-deficient beta cells. Surprisingly, despite the docking defect, the exocytosis of insulin granules in response to a physiological glucose stimulus is significantly augmented, which results in increased glucose tolerance in granuphilin- mice. The enhanced secretion in mutant beta cells is correlated with a decrease in the formation of the fusion-incompetent syntaxin-1a-Munc18-1 complex, with which granuphilin normally interacts. Furthermore, in contrast to wild-type granuphilin, its mutant that is defective in binding to syntaxin-1a fails to restore granule docking or the protein level of syntaxin-1a in granuphilin- beta cells. Thus, granuphilin not only is essential for the docking of insulin granules but simultaneously imposes a fusion constraint on them through an interaction with the syntaxin-1a fusion machinery. These findings provide a novel paradigm for the docking machinery in regulated exocytosis.

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In vivo phenotypes of Grn knockout mice. Each measurement was performed in age-matched (8- to 33-wk-old) Grn+/Y (open squares or bars) and Grn−/Y (closed circles or bars) male mice. (A) Body weight change after the weaning (mean ± SD). (B) Blood glucose concentrations during an i.p. glucose tolerance test (n = 11 for Grn+/Y mice; n = 13 for Grn−/Y mice). *, P < 0.05; **, P < 0.01; ***, P < 0.002. (C) Plasma insulin concentrations before and 30 min after a glucose load (n = 8 for 0 min; n = 9 for 30 min). (D) Percentage of starting blood glucose concentration during an i.p. insulin tolerance test (n = 5 for Grn+/Y mice; n = 6 for Grn−/Y mice). Pancreatic weight (E) and total insulin content in the pancreas (F) of 20-wk-old Grn+/Y and Grn−/Y mice (n = 6). Results are provided as mean ± SEM.
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fig2: In vivo phenotypes of Grn knockout mice. Each measurement was performed in age-matched (8- to 33-wk-old) Grn+/Y (open squares or bars) and Grn−/Y (closed circles or bars) male mice. (A) Body weight change after the weaning (mean ± SD). (B) Blood glucose concentrations during an i.p. glucose tolerance test (n = 11 for Grn+/Y mice; n = 13 for Grn−/Y mice). *, P < 0.05; **, P < 0.01; ***, P < 0.002. (C) Plasma insulin concentrations before and 30 min after a glucose load (n = 8 for 0 min; n = 9 for 30 min). (D) Percentage of starting blood glucose concentration during an i.p. insulin tolerance test (n = 5 for Grn+/Y mice; n = 6 for Grn−/Y mice). Pancreatic weight (E) and total insulin content in the pancreas (F) of 20-wk-old Grn+/Y and Grn−/Y mice (n = 6). Results are provided as mean ± SEM.

Mentions: We first examined in vivo the phenotypes that might be affected by potential insulin secretion defects. Granuphilin- male mice showed normal development and were fertile, with no apparent abnormalities in general appearance or behavior. However, their weight showed a reduction of ∼10% at all ages examined (Fig. 2 A), which may be related to the mild growth defect seen in loss-of-function mutants of bitesize, the Drosophila melanogaster granuphilin homologue (Serano and Rubin, 2003). We found that the blood glucose levels examined in fasting mice or after a glucose load were significantly lower in mutant mice (Fig. 2 B). Plasma insulin concentrations after a glucose load were slightly but not significantly higher (Fig. 2 C), although it is generally difficult to measure them accurately from a limited sample size in mice. Insulin tolerance tests showed comparable insulin sensitivity between the mutant and control mice (Fig. 2 D). There was no change in the pancreas tissue weight or in the total insulin content (Fig. 2, E and F). These findings indicate that granuphilin- mice have enhanced glucose tolerance without signs of increased insulin sensitivity in peripheral tissues.


Granuphilin molecularly docks insulin granules to the fusion machinery.

Gomi H, Mizutani S, Kasai K, Itohara S, Izumi T - J. Cell Biol. (2005)

In vivo phenotypes of Grn knockout mice. Each measurement was performed in age-matched (8- to 33-wk-old) Grn+/Y (open squares or bars) and Grn−/Y (closed circles or bars) male mice. (A) Body weight change after the weaning (mean ± SD). (B) Blood glucose concentrations during an i.p. glucose tolerance test (n = 11 for Grn+/Y mice; n = 13 for Grn−/Y mice). *, P < 0.05; **, P < 0.01; ***, P < 0.002. (C) Plasma insulin concentrations before and 30 min after a glucose load (n = 8 for 0 min; n = 9 for 30 min). (D) Percentage of starting blood glucose concentration during an i.p. insulin tolerance test (n = 5 for Grn+/Y mice; n = 6 for Grn−/Y mice). Pancreatic weight (E) and total insulin content in the pancreas (F) of 20-wk-old Grn+/Y and Grn−/Y mice (n = 6). Results are provided as mean ± SEM.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2171228&req=5

fig2: In vivo phenotypes of Grn knockout mice. Each measurement was performed in age-matched (8- to 33-wk-old) Grn+/Y (open squares or bars) and Grn−/Y (closed circles or bars) male mice. (A) Body weight change after the weaning (mean ± SD). (B) Blood glucose concentrations during an i.p. glucose tolerance test (n = 11 for Grn+/Y mice; n = 13 for Grn−/Y mice). *, P < 0.05; **, P < 0.01; ***, P < 0.002. (C) Plasma insulin concentrations before and 30 min after a glucose load (n = 8 for 0 min; n = 9 for 30 min). (D) Percentage of starting blood glucose concentration during an i.p. insulin tolerance test (n = 5 for Grn+/Y mice; n = 6 for Grn−/Y mice). Pancreatic weight (E) and total insulin content in the pancreas (F) of 20-wk-old Grn+/Y and Grn−/Y mice (n = 6). Results are provided as mean ± SEM.
Mentions: We first examined in vivo the phenotypes that might be affected by potential insulin secretion defects. Granuphilin- male mice showed normal development and were fertile, with no apparent abnormalities in general appearance or behavior. However, their weight showed a reduction of ∼10% at all ages examined (Fig. 2 A), which may be related to the mild growth defect seen in loss-of-function mutants of bitesize, the Drosophila melanogaster granuphilin homologue (Serano and Rubin, 2003). We found that the blood glucose levels examined in fasting mice or after a glucose load were significantly lower in mutant mice (Fig. 2 B). Plasma insulin concentrations after a glucose load were slightly but not significantly higher (Fig. 2 C), although it is generally difficult to measure them accurately from a limited sample size in mice. Insulin tolerance tests showed comparable insulin sensitivity between the mutant and control mice (Fig. 2 D). There was no change in the pancreas tissue weight or in the total insulin content (Fig. 2, E and F). These findings indicate that granuphilin- mice have enhanced glucose tolerance without signs of increased insulin sensitivity in peripheral tissues.

Bottom Line: The Rab27a effector granuphilin is specifically localized on insulin granules and is involved in their exocytosis.Here we show that the number of insulin granules morphologically docked to the plasma membrane is markedly reduced in granuphilin-deficient beta cells.The enhanced secretion in mutant beta cells is correlated with a decrease in the formation of the fusion-incompetent syntaxin-1a-Munc18-1 complex, with which granuphilin normally interacts.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Molecular Endocrinology and Metabolism, Institute for Molecular and Cellular Regulation, Gunma University, Gunma 371-8512, Japan.

ABSTRACT
The Rab27a effector granuphilin is specifically localized on insulin granules and is involved in their exocytosis. Here we show that the number of insulin granules morphologically docked to the plasma membrane is markedly reduced in granuphilin-deficient beta cells. Surprisingly, despite the docking defect, the exocytosis of insulin granules in response to a physiological glucose stimulus is significantly augmented, which results in increased glucose tolerance in granuphilin- mice. The enhanced secretion in mutant beta cells is correlated with a decrease in the formation of the fusion-incompetent syntaxin-1a-Munc18-1 complex, with which granuphilin normally interacts. Furthermore, in contrast to wild-type granuphilin, its mutant that is defective in binding to syntaxin-1a fails to restore granule docking or the protein level of syntaxin-1a in granuphilin- beta cells. Thus, granuphilin not only is essential for the docking of insulin granules but simultaneously imposes a fusion constraint on them through an interaction with the syntaxin-1a fusion machinery. These findings provide a novel paradigm for the docking machinery in regulated exocytosis.

Show MeSH
Related in: MedlinePlus