- Email: [email protected]
Coordinated control of both insulin secretion and insulin action through calpain-10-mediated regulation of exocytosis?
Molecular Genetics and Metabolism 91 (2007) 305–307 www.elsevier.com/locate/ymgme
Commentary
Coordinated control of both insulin secretion and insulin action through calpain-10-mediated regulation of exocytosis? Mark D. Turner
*
Centre for Diabetes and Metabolic Medicine, Institute of Cell and Molecular Science, Barts and The London Queen Mary’s School of Medicine and Dentistry, University of London, London E1 2AT, United Kingdom Received 26 April 2007; accepted 27 April 2007 Available online 7 June 2007
Abstract Calpain-10 was first identified through a genome scan seeking to identify diabetes predisposition genes. Both genetic and functional data has since indicated that calpain-10 has an important role in insulin resistance and intermediate phenotypes, including those associated with adipocytes and skeletal muscle. Evidence presented in this issue by Brown, Yeaman, and Walker utilizes siRNA technology to specifically knock down calpain-10 expression, and suggests that calpain-10 facilitates GLUT4 translocation through effects on the distal secretory pathway. Calpain-10 is also an important molecule in the pancreatic b-cell, where it has been shown to regulate exocytosis through partial proteolysis of a member of the secretory granule fusion machinery. In addition, calpain-10 has also been implicated in reorganization of the actin cytoskeleton that accompanies both GLUT4 vesicle translocation and insulin secretion. Taken together, these findings provide fresh hope for the development of novel diabetic treatments, utilizing either pharmacological activators that specifically target calpain-10, or through targeted calpain-10 gene therapy. Therapeutic intervention in this way could simultaneously enhance both insulin secretion and insulin action. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Calpain-10; Type 2 diabetes; Glucose homeostasis; Skeletal muscle; Pancreatic b-cell
Introduction Diabetes affects approximately 4% of the adult population worldwide, with 90% of all diabetics having type 2 diabetes (T2D). Although T2D is characterized by defects of insulin action, hepatic glucose output, and insulin secretion, the biochemistry underlying diabetogenesis has yet to be elucidated. Glucose homeostasis is regulated by glucose uptake (into adipose tissue and skeletal muscle) and insulin secretion (from pancreatic b-cells), both processes which involve exocytotic vesicle fusion and dynamic cytoskeletal reorganization that facilitates vesicle trafficking. Calpain-10 is a member of the calpain family of cysteine proteases, and was first identified as a susceptibility
*
Fax: +44 20 7882 2186. E-mail address: [email protected]
1096-7192/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ymgme.2007.04.019
gene for T2D by a genome scan [1]. Diabetic subjects homozygous for the disease-associated calpain-10 UCSNP43 G-allele genotype correlated with a reduction in calpain-10 mRNA level in skeletal muscle compared to heterozygotes [2], and similarly there was reduction in calpain-10 protein in diabetic skeletal muscle [3]. In the current issue Brown, Yeaman, and Walker specifically knock down calpain-10 expression to determine calpain-10 action on glucose uptake in skeletal muscle, and suggest that calpain-10 facilitates GLUT4 translocation through effects on the distal secretory pathway. Although the actual molecular machinery proteolysed by calpain-10 to trigger GLUT4 vesicle translocation is currently unknown, there are however strong similarities to calpain-10 action in pancreatic b-cells. This gives rise to the possibility that calpain-10 might coordinately control glucose homeostasis through parallel actions in multiple cell types.
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M.D. Turner / Molecular Genetics and Metabolism 91 (2007) 305–307
Function of calpain-10 in exocytosis Soluble N-ethyl maleimide sensitive fusion protein attachment receptors (SNAREs) have been recognized as key components of protein complexes that drive membrane fusion in vesicular transport [4]. Moreover, the processes of GLUT4-containing vesicle translocation and insulin secretory granule exocytosis both utilize SNARE molecules, or their associated homologues. As GLUT4 translocation is an analogous process to that of exocytosis, this raises the prospect that calpain-10 might control glucose homeostasis through a shared mechanism of action in multiple cell types. Calpains are a family of structurally similar cysteine proteases which often function to partially proteolyse substrates, thereby modifying rather than terminating protein function. The first clue that the calpain family might be involved in SNARE-mediated exocytotic fusion came from the report of proteolytic cleavage of the SNARE molecules, syntaxin-1, VAMP-2, and SNAP-25, following addition of activated exogenous calpain to alveolar epithelial cell lysates [5]. SNAP-23, a homologue of SNAP-25, was subsequently reported to be specifically cleaved by l-calpain in intact activated platelets [6], and calpain inhibition has also been shown to block platelet secretion [7]. SNAREs form helical structures that together form helical bundles between granules/vesicles (v-SNAREs) and target membranes (t-SNAREs) to facilitate membrane fusion [8–10]. Specifically, SNAREs facilitate regulated secretion by docking granules to specific areas of plasma membrane where syntaxin-1 and SNAP-25 bind to the Lc-type Ca2+ channel [11,12]. This ensures that the bound granule complex is exposed to the high levels of Ca2+ found immediately beneath the inner mouth of the open Ca2+-channel, an ideal environment for the operation of a Ca2+-dependent protease. Upon b-cell membrane depolarization the Ca2+-channel opens, activating calpain-10, which in turn then proteolyses SNAP-25 in a region adjacent to the N-terminus [13]. As this area of the molecule contains the syntaxin-binding domain [8], proteolysis in this region would almost certainly lead to conformational changes within the entire SNARE complex. Removal of SNARE complex from the membrane interface then frees the way for membrane–membrane contact and exocytotic fusion. Function of calpain-10 in cytoskeletal rearrangement The cytoskeleton plays a critical role in both glucosestimulated intracellular protein trafficking and exocytosis of insulin-containing secretory granules. Calpains often function to cleave cytoskeletal protein, thereby providing an additional possible role for the involvement of calpain in vesicle trafficking, and thus glucose homeostasis. Previous studies have demonstrated that calpain-10 is required for insulin-responsive GLUT4 vesicle trafficking and glucose uptake in 3T3-L1 adipocytes [14]. Specifically, both pharmacological calpain inhibition and the overexpression of a calpain-10 antisense vector blocked the actin
reorganization that is required for insulin-stimulated GLUT4 translocation to the plasma membrane in adipocytes. Inhibition of calpain activity has also been shown to impair glucose uptake into skeletal muscle [15], although this study was reliant upon the use of pharmacological inhibitors which are notorious for their lack of specificity amongst cysteine proteases. However, the current study by Brown, Yeaman, and Walker utilizes siRNA technology to specifically knock down calpain-10 expression in skeletal muscle. Given the similarity between the function of calpain-10 reported in adipocytes and the current report in this issue, it is highly likely that calpain-10 interacts with the same biochemical machinery on the cytoskeleton of both cell types. Interestingly, in b-cells only the 54 kDa isoform of calpain-10 has been shown to be associated with the cytoskeleton [16]. As this is the same isoform that was shown to associate with and cleave the t-SNARE SNAP-25 [13], it is tempting to speculate that calpain-10 might regulate insulin secretion as part of a single complex at the plasma membrane. Support for this hypothesis comes from the finding that there is dynamic interaction between t-SNAREs and F-actin during glucose-stimulated insulin secretion [17]. Therefore, activation of this one isoform of calpain-10 could result in simultaneous actions on both the exocytotic machinery and the cytoskeleton. Conclusion Following the seminal report by Graeme Bell and colleagues [1] first linking calpain-10 with diabetes predisposition, subsequent research initially focussed primarily on the role of gene polymorphisms in disease predisposition in various ethnic groups. However, specific biological actions of calpain-10 in cells and tissues involved in glucose homeostasis have now begun to emerge [18,19]. The current report by Brown, Yeaman, and Walker is the first to specifically identify calpain-10 as the calpain family member facilitating GLUT4 translocation in skeletal muscle cells. Importantly, they show that calpain-10 does not regulate glycogen synthesis or insulin signalling, but instead appears to exert a direct effect on glucose uptake through actions in the late secretory pathway. As calpain-10 is known to act in this way to regulate insulin secretory granule exocytosis, this suggests that the two cell types are likely to control glucose homeostasis through parallel processes. In terms of clinical impact, this provides for the possibility of developing high efficacy treatments that would improve glycaemic control in diabetic patients with impaired calpain-10 expression or function, providing benefit through simultaneous improvements to both insulin secretion and insulin action. Acknowledgment Diabetes UK for financial support (BDA:RD03/ 0002689).
M.D. Turner / Molecular Genetics and Metabolism 91 (2007) 305–307
References [1] Y. Horikawa, N. Oda, N.J. Cox, X. Li, M. Orho-Melander, M. Hara, Y. Hinokio, T.H. Lindner, H. Mashima, P.E.H. Schwarz, L. del Bosque-Plata, Y. Horikawa, Y. Oda, I. Yoshiuchi, S. Colilla, K.S. Polonsky, S. Wei, P. Concannon, N. Iwasaki, J. Schulze, L.J. Baier, C. Bogardus, L. Groop, E. Boerwinkle, C.L. Hanis, G.I. Bell, Genetic variation in the gene encoding calpain-10 is associated with type 2 diabetes mellitus, Nat. Genet. 26 (2000) 163–175. [2] L.J. Baier, P.A. Permana, X. Yang, R.E. Pratley, R.L. Hanson, G.-Q. Shen, D. Mott, W.C. Knowler, N.J. Cox, Y. Horikawa, N. Oda, G.I. Bell, C. Bogardus, A calpain-10 gene polymorphism is associated with reduced muscle mRNA levels and insulin resistance, J. Clin. Invest. 106 (2000) R69–R73. [3] X. Yang, R.E. Pratley, L.J. Baier, Y. Horikawa, G.I. Bell, C. Bogardus, P.A. Permana, Reduced skeletal muscle calpain-10 transcript level is due to a cumulative decrease in major isoforms, Mol. Genet. Metab. 73 (2001) 111–113. [4] R. Jahn, R.H. Scheller, SNARES—engines for membrane fusion, Nat. Rev. Mol. Cell Biol. 7 (2006) 631–643. [5] U.-J.P. Zimmerman, S.K. Malek, L. Liu, H.L. Li, Proteolysis of synaptobrevin syntaxin, and SNAP-25 in alveolar epithelial type II cells, IUBMB Life 48 (1999) 453–458. [6] T.W. Rutledge, S.W. Whiteheart, SNAP-23 is a target for calpain cleavage in activated platelets, J. Biol. Chem. 277 (2002) 37009–37015. [7] K. Croce, R. Flaumenhaft, M. Rivers, B. Furie, B.C. Furie, I.M. Herman, D.A. Potter, Inhibition of calpain blocks platelet secretion, aggregation, and spreading, J. Biol. Chem. 274 (1999) 36321–36327. [8] E.R. Chapman, S. An, N. Barton, R. Jahn, SNAP-25, a t-SNARE which binds to both syntaxin and synaptobrevin via domains that may form coiled coils, J. Biol. Chem. 269 (1994) 27427–27432. [9] P.I. Hanson, R. Roth, H. Morisaki, R. Jahn, J.E. Heuser, Structure and conformational changes in NSF and its membrane receptor complexes visualized by quick-freeze/deep-etch electron microscopy, Cell 90 (1997) 523–535.
307
[10] R.C. Lin, R.H. Scheller, Structural organization of the synaptic exocytosis core complex, Neuron 19 (1997) 1087–1094. [11] O. Wiser, M.K. Bonnet, D. Atlas, Functional interaction of syntaxin and SNAP-25 with voltage-sensitive L- and N-type Ca2+ channels, EMBO J. 15 (1996) 4100–4110. [12] O. Wiser, M. Trus, A. Hernandez, E. Renstrom, S. Barg, P. Rorsman, D. Atlas, The voltage sensitive Lc-type Ca2+ channel is functionally coupled to the exocytotic machinery, Proc. Natl. Acad. Sci. USA 96 (1999) 248–253. [13] C. Marshall, G.A. Hitman, C.J. Partridge, A. Clark, H. Ma, T.R. Shearer, M.D. Turner, Evidence that an isoform of calpain-10 is a regulator of exocytosis in pancreatic b-cells, Mol. Endocrinol. 19 (2005) 213–224. [14] D.S. Paul, A.W. Harmon, C.P. Winston, Y.M. Patel, Calpain facilitates GLUT4 vesicle translocation during insulin-stimulated glucose uptake in adipocytes, Biochem. J. 376 (2003) 625–632. [15] L.J. Logie, A.E. Brown, S.J. Yeaman, M. Walker, Calpain inhibition and insulin action in cultured human muscle cells, Mol. Genet. Metab. 85 (2005) 54–60. [16] M.D. Turner, F.K. Fulcher, B.T. Smith, C.R. Velasquez, E. Aganna, C.J. Partridge, G.A. Hitman, A. Clark, Y.M. Patel, Calpain facilitates actin reorganization during glucose-stimulated insulin secretion, Biochem. Biophys. Res. Commun. 352 (2007) 650–655. [17] D.C. Thurmond, C. Gonelle-Gispert, M. Furukawa, P.A. Halban, J.E. Pessin, Glucose-stimulated insulin secretion is coupled to the interaction of actin with the t-SNARE (target membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein) complex, Mol. Endocrinol. 17 (2003) 732–742. [18] M.D. Turner, P.G. Cassell, G.A. Hitman, Calpain-10: from genome search to function, Diabetes Metab. Res. Rev. 21 (2005) 505–514. [19] M. Ridderstrale, H. Parikh, L. Groop, Calpain 10 and type 2 diabetes: are we getting closer to an explanation? Curr. Opin. Clin. Nutr. Metab. Care 8 (2005) 361–366.
Commentary
Coordinated control of both insulin secretion and insulin action through calpain-10-mediated regulation of exocytosis? Mark D. Turner
*
Centre for Diabetes and Metabolic Medicine, Institute of Cell and Molecular Science, Barts and The London Queen Mary’s School of Medicine and Dentistry, University of London, London E1 2AT, United Kingdom Received 26 April 2007; accepted 27 April 2007 Available online 7 June 2007
Abstract Calpain-10 was first identified through a genome scan seeking to identify diabetes predisposition genes. Both genetic and functional data has since indicated that calpain-10 has an important role in insulin resistance and intermediate phenotypes, including those associated with adipocytes and skeletal muscle. Evidence presented in this issue by Brown, Yeaman, and Walker utilizes siRNA technology to specifically knock down calpain-10 expression, and suggests that calpain-10 facilitates GLUT4 translocation through effects on the distal secretory pathway. Calpain-10 is also an important molecule in the pancreatic b-cell, where it has been shown to regulate exocytosis through partial proteolysis of a member of the secretory granule fusion machinery. In addition, calpain-10 has also been implicated in reorganization of the actin cytoskeleton that accompanies both GLUT4 vesicle translocation and insulin secretion. Taken together, these findings provide fresh hope for the development of novel diabetic treatments, utilizing either pharmacological activators that specifically target calpain-10, or through targeted calpain-10 gene therapy. Therapeutic intervention in this way could simultaneously enhance both insulin secretion and insulin action. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Calpain-10; Type 2 diabetes; Glucose homeostasis; Skeletal muscle; Pancreatic b-cell
Introduction Diabetes affects approximately 4% of the adult population worldwide, with 90% of all diabetics having type 2 diabetes (T2D). Although T2D is characterized by defects of insulin action, hepatic glucose output, and insulin secretion, the biochemistry underlying diabetogenesis has yet to be elucidated. Glucose homeostasis is regulated by glucose uptake (into adipose tissue and skeletal muscle) and insulin secretion (from pancreatic b-cells), both processes which involve exocytotic vesicle fusion and dynamic cytoskeletal reorganization that facilitates vesicle trafficking. Calpain-10 is a member of the calpain family of cysteine proteases, and was first identified as a susceptibility
*
Fax: +44 20 7882 2186. E-mail address: [email protected]
1096-7192/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ymgme.2007.04.019
gene for T2D by a genome scan [1]. Diabetic subjects homozygous for the disease-associated calpain-10 UCSNP43 G-allele genotype correlated with a reduction in calpain-10 mRNA level in skeletal muscle compared to heterozygotes [2], and similarly there was reduction in calpain-10 protein in diabetic skeletal muscle [3]. In the current issue Brown, Yeaman, and Walker specifically knock down calpain-10 expression to determine calpain-10 action on glucose uptake in skeletal muscle, and suggest that calpain-10 facilitates GLUT4 translocation through effects on the distal secretory pathway. Although the actual molecular machinery proteolysed by calpain-10 to trigger GLUT4 vesicle translocation is currently unknown, there are however strong similarities to calpain-10 action in pancreatic b-cells. This gives rise to the possibility that calpain-10 might coordinately control glucose homeostasis through parallel actions in multiple cell types.
306
M.D. Turner / Molecular Genetics and Metabolism 91 (2007) 305–307
Function of calpain-10 in exocytosis Soluble N-ethyl maleimide sensitive fusion protein attachment receptors (SNAREs) have been recognized as key components of protein complexes that drive membrane fusion in vesicular transport [4]. Moreover, the processes of GLUT4-containing vesicle translocation and insulin secretory granule exocytosis both utilize SNARE molecules, or their associated homologues. As GLUT4 translocation is an analogous process to that of exocytosis, this raises the prospect that calpain-10 might control glucose homeostasis through a shared mechanism of action in multiple cell types. Calpains are a family of structurally similar cysteine proteases which often function to partially proteolyse substrates, thereby modifying rather than terminating protein function. The first clue that the calpain family might be involved in SNARE-mediated exocytotic fusion came from the report of proteolytic cleavage of the SNARE molecules, syntaxin-1, VAMP-2, and SNAP-25, following addition of activated exogenous calpain to alveolar epithelial cell lysates [5]. SNAP-23, a homologue of SNAP-25, was subsequently reported to be specifically cleaved by l-calpain in intact activated platelets [6], and calpain inhibition has also been shown to block platelet secretion [7]. SNAREs form helical structures that together form helical bundles between granules/vesicles (v-SNAREs) and target membranes (t-SNAREs) to facilitate membrane fusion [8–10]. Specifically, SNAREs facilitate regulated secretion by docking granules to specific areas of plasma membrane where syntaxin-1 and SNAP-25 bind to the Lc-type Ca2+ channel [11,12]. This ensures that the bound granule complex is exposed to the high levels of Ca2+ found immediately beneath the inner mouth of the open Ca2+-channel, an ideal environment for the operation of a Ca2+-dependent protease. Upon b-cell membrane depolarization the Ca2+-channel opens, activating calpain-10, which in turn then proteolyses SNAP-25 in a region adjacent to the N-terminus [13]. As this area of the molecule contains the syntaxin-binding domain [8], proteolysis in this region would almost certainly lead to conformational changes within the entire SNARE complex. Removal of SNARE complex from the membrane interface then frees the way for membrane–membrane contact and exocytotic fusion. Function of calpain-10 in cytoskeletal rearrangement The cytoskeleton plays a critical role in both glucosestimulated intracellular protein trafficking and exocytosis of insulin-containing secretory granules. Calpains often function to cleave cytoskeletal protein, thereby providing an additional possible role for the involvement of calpain in vesicle trafficking, and thus glucose homeostasis. Previous studies have demonstrated that calpain-10 is required for insulin-responsive GLUT4 vesicle trafficking and glucose uptake in 3T3-L1 adipocytes [14]. Specifically, both pharmacological calpain inhibition and the overexpression of a calpain-10 antisense vector blocked the actin
reorganization that is required for insulin-stimulated GLUT4 translocation to the plasma membrane in adipocytes. Inhibition of calpain activity has also been shown to impair glucose uptake into skeletal muscle [15], although this study was reliant upon the use of pharmacological inhibitors which are notorious for their lack of specificity amongst cysteine proteases. However, the current study by Brown, Yeaman, and Walker utilizes siRNA technology to specifically knock down calpain-10 expression in skeletal muscle. Given the similarity between the function of calpain-10 reported in adipocytes and the current report in this issue, it is highly likely that calpain-10 interacts with the same biochemical machinery on the cytoskeleton of both cell types. Interestingly, in b-cells only the 54 kDa isoform of calpain-10 has been shown to be associated with the cytoskeleton [16]. As this is the same isoform that was shown to associate with and cleave the t-SNARE SNAP-25 [13], it is tempting to speculate that calpain-10 might regulate insulin secretion as part of a single complex at the plasma membrane. Support for this hypothesis comes from the finding that there is dynamic interaction between t-SNAREs and F-actin during glucose-stimulated insulin secretion [17]. Therefore, activation of this one isoform of calpain-10 could result in simultaneous actions on both the exocytotic machinery and the cytoskeleton. Conclusion Following the seminal report by Graeme Bell and colleagues [1] first linking calpain-10 with diabetes predisposition, subsequent research initially focussed primarily on the role of gene polymorphisms in disease predisposition in various ethnic groups. However, specific biological actions of calpain-10 in cells and tissues involved in glucose homeostasis have now begun to emerge [18,19]. The current report by Brown, Yeaman, and Walker is the first to specifically identify calpain-10 as the calpain family member facilitating GLUT4 translocation in skeletal muscle cells. Importantly, they show that calpain-10 does not regulate glycogen synthesis or insulin signalling, but instead appears to exert a direct effect on glucose uptake through actions in the late secretory pathway. As calpain-10 is known to act in this way to regulate insulin secretory granule exocytosis, this suggests that the two cell types are likely to control glucose homeostasis through parallel processes. In terms of clinical impact, this provides for the possibility of developing high efficacy treatments that would improve glycaemic control in diabetic patients with impaired calpain-10 expression or function, providing benefit through simultaneous improvements to both insulin secretion and insulin action. Acknowledgment Diabetes UK for financial support (BDA:RD03/ 0002689).
M.D. Turner / Molecular Genetics and Metabolism 91 (2007) 305–307
References [1] Y. Horikawa, N. Oda, N.J. Cox, X. Li, M. Orho-Melander, M. Hara, Y. Hinokio, T.H. Lindner, H. Mashima, P.E.H. Schwarz, L. del Bosque-Plata, Y. Horikawa, Y. Oda, I. Yoshiuchi, S. Colilla, K.S. Polonsky, S. Wei, P. Concannon, N. Iwasaki, J. Schulze, L.J. Baier, C. Bogardus, L. Groop, E. Boerwinkle, C.L. Hanis, G.I. Bell, Genetic variation in the gene encoding calpain-10 is associated with type 2 diabetes mellitus, Nat. Genet. 26 (2000) 163–175. [2] L.J. Baier, P.A. Permana, X. Yang, R.E. Pratley, R.L. Hanson, G.-Q. Shen, D. Mott, W.C. Knowler, N.J. Cox, Y. Horikawa, N. Oda, G.I. Bell, C. Bogardus, A calpain-10 gene polymorphism is associated with reduced muscle mRNA levels and insulin resistance, J. Clin. Invest. 106 (2000) R69–R73. [3] X. Yang, R.E. Pratley, L.J. Baier, Y. Horikawa, G.I. Bell, C. Bogardus, P.A. Permana, Reduced skeletal muscle calpain-10 transcript level is due to a cumulative decrease in major isoforms, Mol. Genet. Metab. 73 (2001) 111–113. [4] R. Jahn, R.H. Scheller, SNARES—engines for membrane fusion, Nat. Rev. Mol. Cell Biol. 7 (2006) 631–643. [5] U.-J.P. Zimmerman, S.K. Malek, L. Liu, H.L. Li, Proteolysis of synaptobrevin syntaxin, and SNAP-25 in alveolar epithelial type II cells, IUBMB Life 48 (1999) 453–458. [6] T.W. Rutledge, S.W. Whiteheart, SNAP-23 is a target for calpain cleavage in activated platelets, J. Biol. Chem. 277 (2002) 37009–37015. [7] K. Croce, R. Flaumenhaft, M. Rivers, B. Furie, B.C. Furie, I.M. Herman, D.A. Potter, Inhibition of calpain blocks platelet secretion, aggregation, and spreading, J. Biol. Chem. 274 (1999) 36321–36327. [8] E.R. Chapman, S. An, N. Barton, R. Jahn, SNAP-25, a t-SNARE which binds to both syntaxin and synaptobrevin via domains that may form coiled coils, J. Biol. Chem. 269 (1994) 27427–27432. [9] P.I. Hanson, R. Roth, H. Morisaki, R. Jahn, J.E. Heuser, Structure and conformational changes in NSF and its membrane receptor complexes visualized by quick-freeze/deep-etch electron microscopy, Cell 90 (1997) 523–535.
307
[10] R.C. Lin, R.H. Scheller, Structural organization of the synaptic exocytosis core complex, Neuron 19 (1997) 1087–1094. [11] O. Wiser, M.K. Bonnet, D. Atlas, Functional interaction of syntaxin and SNAP-25 with voltage-sensitive L- and N-type Ca2+ channels, EMBO J. 15 (1996) 4100–4110. [12] O. Wiser, M. Trus, A. Hernandez, E. Renstrom, S. Barg, P. Rorsman, D. Atlas, The voltage sensitive Lc-type Ca2+ channel is functionally coupled to the exocytotic machinery, Proc. Natl. Acad. Sci. USA 96 (1999) 248–253. [13] C. Marshall, G.A. Hitman, C.J. Partridge, A. Clark, H. Ma, T.R. Shearer, M.D. Turner, Evidence that an isoform of calpain-10 is a regulator of exocytosis in pancreatic b-cells, Mol. Endocrinol. 19 (2005) 213–224. [14] D.S. Paul, A.W. Harmon, C.P. Winston, Y.M. Patel, Calpain facilitates GLUT4 vesicle translocation during insulin-stimulated glucose uptake in adipocytes, Biochem. J. 376 (2003) 625–632. [15] L.J. Logie, A.E. Brown, S.J. Yeaman, M. Walker, Calpain inhibition and insulin action in cultured human muscle cells, Mol. Genet. Metab. 85 (2005) 54–60. [16] M.D. Turner, F.K. Fulcher, B.T. Smith, C.R. Velasquez, E. Aganna, C.J. Partridge, G.A. Hitman, A. Clark, Y.M. Patel, Calpain facilitates actin reorganization during glucose-stimulated insulin secretion, Biochem. Biophys. Res. Commun. 352 (2007) 650–655. [17] D.C. Thurmond, C. Gonelle-Gispert, M. Furukawa, P.A. Halban, J.E. Pessin, Glucose-stimulated insulin secretion is coupled to the interaction of actin with the t-SNARE (target membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein) complex, Mol. Endocrinol. 17 (2003) 732–742. [18] M.D. Turner, P.G. Cassell, G.A. Hitman, Calpain-10: from genome search to function, Diabetes Metab. Res. Rev. 21 (2005) 505–514. [19] M. Ridderstrale, H. Parikh, L. Groop, Calpain 10 and type 2 diabetes: are we getting closer to an explanation? Curr. Opin. Clin. Nutr. Metab. Care 8 (2005) 361–366.