TCF7L2 Regulates Late Events in Insulin Secretion From Pancreatic Islet β-Cells

Abstract

OBJECTIVEPolymorphisms in the human TCF7L2 gene are associated with reduced insulin secretion and an increased risk of type 2 diabetes. However, the mechanisms by which TCF7L2 affect insulin secretion are still unclear. We define the effects of TCF7L2 expression level on mature β-cell function and suggest a potential mechanism for its actions.RESEARCH DESIGN AND METHODSTCF7L2 expression in rodent islets and β-cell lines was altered using RNAi or adenoviral transduction. β-Cell gene profiles were measured by quantitative real-time PCR and the effects on intracellular signaling and exocytosis by live cell imaging, electron microscopy, and patch clamp electrophysiology.RESULTSReducing TCF7L2 expression levels by RNAi decreased glucose- but not KCl-induced insulin secretion. The glucose-induced increments in both ATP/ADP ratio and cytosolic free Ca2+ concentration ([Ca2+]i) were increased compared with controls. Overexpression of TCF7L2 exerted minor inhibitory effects on glucose-regulated changes in [Ca2+]i and insulin release. Gene expression profiling in TCF7L2-silenced cells revealed increased levels of mRNA encoding syntaxin 1A but decreased Munc18–1 and ZnT8 mRNA. Whereas the number of morphologically docked vesicles was unchanged by TCF7L2 suppression, secretory granule movement increased and capacitance changes decreased, indicative of defective vesicle fusion.CONCLUSION—TCF7L2 is involved in maintaining expression of β-cell genes regulating secretory granule fusion. Defective insulin exocytosis may thus underlie increased diabetes incidence in carriers of the at-risk TCF7L2 alleles.

Translating TCF7L2: from gene to function

“…In addition, Tcf7l2 knockdown reduced the expression of Munc18-1 (also known as Stxbp1) and increased the expression of Stx1a (which encodes syntaxin 1a), both of which encode key players in the control of insulin exocytosis. Interestingly both Munc18-1 and Stx1a contain a putative TCF7L2 binding site in their 5' untranslated regions, suggesting a direct regulatory role of TCF7L2 [13]. It should be noted, however, that levels of TCF7L2 mRNA [10] and TCF7L2 protein [15] are increased, not decreased, in human islets from type 2 diabetic donors.…”

“…• Decreased beta cell mass Decreased beta cell proliferation following TCF7L2 knockdown [12] Decreased 'maximal' insulin secretion-response to arginine at 28 mmol/l glucose [10] • Impaired insulin processing or release Altered SLC30A8, Munc18-1 and Stx1A expression in mice with Tcf7l2 knockdown [13] Increased proinsulin:insulin ratio in rs7903146 T allele carriers [9,17] Decreased 'maximal' insulin secretion-response to arginine at 28 mmol/l glucose [10] • Impaired GLP1 signalling in beta cells [9,11] Response to GLP1 impaired to greater extent than that to i.v. glucose [10] • Decreased glucagon secretion reducing beta cell insulin secretion [9] Needs replicating…”

“…Reducing TCF7L2 protein production using siRNA led to a small reduction in glucose-induced insulin secretion and a much greater reduction in GLP1-induced secretion [12,13]. In the first study [12], knockdown of TCF7L2 in human islets decreased beta cell proliferation, and overexpression of TCF7L2 had a protective effect against glucose-and cytokine-induced apoptosis, suggesting a role in regulation of beta cell mass.…”

“…What remains uncertain is whether the striking incretin effect reflects a GLP1/GIP-specific effect, or whether this is a generalised defect in insulin processing and secretion. In favour of the latter, TCF7L2 expression affects the direct effect of glucose both in vitro [12,13] and in vivo [16]; there is an increase in the proinsulin:insulin ratio in carriers of the risk allele [17], suggesting a defect in insulin processing; and the 'maximum' insulin secretory capacity, as assessed by the response to arginine at a constant plasma glucose concentration of 28 mmol/l, is reduced in risk allele carriers with impaired glucose tolerance or type 2 diabetes [10]. It would be interesting to see how the insulin secretory response to tolbutamide is affected in islets with reduced TCF7L2 expression, or, indeed, in humans carrying the TCF7L2 risk allele, as this may help further address this question.…”

Wnt4 antagonises Wnt3a mediated increases in growth and glucose stimulated insulin secretion in the pancreatic beta-cell line, INS-1

“…Subsequent studies implicated β-cell dysfunction as the underlying cause of diabetes in patients with the TCF7L2 risk variant [2][3][4][5] and have suggested that Wnt signalling plays an important role in both pancreatic development and in the function of mature pancreatic islets [6]. The activation of the canonical Wnt pathway, by ligands such as Wnt3a, has been of particular interest as it can increase β-cell proliferation, decrease apoptosis and improve glucose stimulated insulin secretion [6][7][8][9][10][11][12]. However there is now increasing evidence that non-canonical Wnt ligands such as Wnt4 can antagonise the signalling mediated by Wnt3a and may play an equally important role in β-cell function [13][14][15].…”