Neonatal β cell development in mice and humans is regulated by calcineurin/NFAT

Little is known about the mechanisms governing neonatal growth and maturation of organs. Here we demonstrate that calcineurin/Nuclear Factor of Activated T?cells (Cn/NFAT) signaling regulates neonatal pancreatic development in mouse and human islets. Inactivation of calcineurin b1 (Cnb1) in mouse islets impaired dense core granule biogenesis, decreased insulin secretion, and reduced cell proliferation and mass, culminating in lethal diabetes. Pancreatic β cells lacking Cnb1 failed to express genes revealed to be direct NFAT targets required for replication, insulin storage, and secretion. In contrast, glucokinase activation stimulated Cn-dependent expression of these genes. Calcineurin inhibitors, such as tacrolimus, used for human immunosuppression, induce diabetes. Tacrolimus exposure reduced Cn/NFAT-dependent expression of factors essential for insulin dense core granule formation and secretion and neonatal β cell proliferation, consistent with our genetic studies. Discovery of conserved pathways regulating β cell maturation and proliferation suggests new strategies for controlling β cell growth or replacement in human islet diseases.     

Regulating insulin signaling and beta-cell function through IRS proteins

Diabetes mellitus is a complex disorder that arises from various causes, including dysregulated glucose sensing and impaired insulin secretion (maturity onset diabetes of youth, MODY), autoimmune-mediated beta-cell destruction (type 1), or insufficient compensation for peripheral insulin resistance (type 2). Type 2 diabetes is the most prevalent form that usually occurs at middle age; it afflicts more than 30 million people over the age of 65, but is appearing with greater frequency in children and adolescents. Dysregulated insulin signaling exacerbated by chronic hyperglycemia promotes a cohort of systemic disorders--including dyslipidemia, hypertension, cardiovascular disease, and female infertility. Understanding the molecular basis of insulin resistance can prevent these disorders and their inevitable progression to type 2 diabetes.     

Protein kinase-mediated regulation of calcineurin through the phosphorylation of modulatory calcineurin-interacting protein 1

Calcineurin is a serine/threonine protein phosphatase that plays a critical role in many physiologic processes such as T-cell activation, skeletal myocyte differentiation, and cardiac hypertrophy. We previously showed that active MEKK3 is capable of stimulating calcineurin/nuclear factor of activated T-cells (NFAT) signaling in cardiac myocytes through phosphorylation of modulatory calcineurin-interacting protein 1 (MCIP1). However, the protein kinases that function downstream of MEKK3 to mediate MCIP1 phosphorylation and the mechanism of MCIP1-mediated calcineurin regulation have not been defined. Here, we show that MEK5 and big MAP kinase 1 (BMK1) function downstream of MEKK3 in a signaling cascade that induces calcineurin activity through phosphorylation of MCIP1. Genetic studies showed that BMK1-deficient mouse lung fibroblasts failed to mediate MCIP1 phosphorylation and activate calcineurin/NFAT in response to angiotensin II, a potent NFAT activator. Conversely, restoring BMK1 to the deficient cells restored angiotensin II-mediated calcineurin/NFAT activation. Thus, using BMK1-deficient mouse lung fibroblast cells, we provided the genetic evidence that BMK1 is required for angiotensin II-mediated calcineurin/NFAT activation through MICP1 phosphorylation. Finally, we discovered that phosphorylated MCIP1 dissociates from calcineurin and binds with 14-3-3, thereby relieving its inhibitory effect on calcineurin activity. In summary, our findings reveal a previously unrecognized essential regulatory role of mitogen-activated protein kinase signaling in calcineurin activation through the reversible phosphorylation of a calcineurin-interacting protein, MCIP1.     

Pancreatic beta-cell growth and survival--a role in obesity-linked type 2 diabetes?

Obesity-linked type 2 diabetes is a disease of insulin resistance combined with pancreatic beta-cell dysfunction. Although a role for beta-cell mass in the pathogenesis of obesity-linked type 2 diabetes has recently gained prominence, the idea is still being developed. It is proposed that in early obesity an increase in beta-cell mass and function might compensate for peripheral insulin resistance. However, as time and/or the severity of the obesity continue, there is decay in such adaptation and the beta-cell mass becomes inadequate. This, together with beta-cell dysfunction, leads to the onset of type 2 diabetes. It is becoming evident that elements in insulin and insulin growth factor (IGF)-1 signal-transduction pathways are key to regulating beta-cell growth. Current evidence indicates that interference of insulin signaling in obesity contributes to peripheral insulin resistance. This article examines whether a similar interference of IGF-1 signaling in the beta-cell could hinder upregulation of beta-cell mass and/or function, resulting in a failure to compensate for insulin resistance.     

Transmitted beta-cell dysfunction as a cause for type 2-diabetes

The pathways that control insulin release and regulate pancreatic beta-cell mass are crucial on the development of type 2 diabetes mellitus. Maturity-onset diabetes of the young comprises a number of single-gene disorders affecting beta-cell development and/or function. A genetic basis for the more common forms of type 2 diabetes which affect adults in developed as well as many developing countries is less clear cut. It is also characterized by abnormal beta-cell function. Appropriate inbred rodent models are an essential tool for the identification of genes and environmental factors that increase the risk of type 2 diabetes. The informations available from studies in the Goto-Kakizaki (GK) rat are here reviewed in such a perspective. This model was obtained by selective breeding of individuals with mild glucose intolerance from a non-diabetic Wistar rat colony. Heritability of defective beta-mass and beta-cell function in GK model is proposed to reflect the complex interactions of three pathogenic players: (1) three independent loci containing genes causating impaired insulin secretion; (2) gestational metabolic (hyperglycaemic) impairment inducing a programming of endocrine pancreas (decreased beta-cell mass) which is transmitted to the next generation; (3) secondary (acquired) loss of beta-cell differentiation due to chronic exposure to hyperglycaemia (glucotoxicity). A better understanding of the mechanisms involved in the failure of beta-cell function in the GK model will lead to identification of new therapeutic targets for both the prevention and treatment of type 2 diabetes.     

活化的T细胞核内因子（NFAT）的调节及其在神经系统中的功能

活化的T细胞核内因子（nuclear factor of activated T-cells, NFAT）作为细胞信号转导通路中的一类重要的转录因子参与细胞功能的调节. NFAT的活化主要是通过细胞内钙/钙调神经磷酸酶（Ca2+/calcineurin）的刺激启动,它脱磷酸后发生核转位并与DNA的特定序列结合,同时通过与其它转录因子的协同作用,调节目的基因的特定表达. NFAT在免疫系统中所调节的基因表达已经得到了充分的研究. 近年实验研究发现,NFAT的转录因子家族在脊椎动物的神经系统中也发挥着非常重要的作用. 本文综述了NFAT家族蛋白的分类、结构、磷酸酶与激酶对其出入核的调节及在神经系统中的研究进展,使得能够更加全面地认识calcineurin/NFAT信号通路的作用. 此外,由于环孢菌素A（cyclosporin A）等药物在神经系统应用的局限性,对于NFAT调节深入研究,也将为筛选或者开发更为高效、低毒药物提供新的思路.     

Maturity-onset diabetes of the young

Maturity-onset diabetes of the young (MODY) is a subtype of noninsulin dependent diabetes mellitus (NIDDM). It is characterized by an early age of onset and autosomal dominant mode of inheritance. These features and the availability of large multigenerational pedigrees make MODY useful for genetic studies of diabetes. In the large, 5-generational RW pedigree, MODY is tightly linked to genetic markers on chromosome 20q. Affected subjects in this family show abnormalities of carbohydrate metabolism varying from impaired glucose tolerance (IGT) to severe diabetes. Approximately 30% of diabetic subjects become insulin requiring and vascular complications occur. MODY is also linked to the glucokinase gene on chromosome 7p and many different mutations associated with MODY have been identified in this gene. MODY due to mutations in the glucokinase gene is a relatively mild form of diabetes with mild fasting hyperglycemia and IGT in the majority. It is rarely insulin requiring and rarely has vascular complications. Clinical studies indicate that the genetic or primary defect in MODY is characterized by deranged and deficient insulin secretion and not by insulin resistance and that there are quantitative and qualitative differences in insulin secretory defects which differentiate subjects with MODY due to glucokinase mutations from those with mutations in the gene on chromosome 20q. These differences correlate with the severity of diabetes between these two genetic forms of MODY.     

Genetic deficiency of glycogen synthase kinase-3beta corrects diabetes in mouse models of insulin resistance

Despite treatment with agents that enhance beta-cell function and insulin action, reduction in beta-cell mass is relentless in patients with insulin resistance and type 2 diabetes mellitus. Insulin resistance is characterized by impaired signaling through the insulin/insulin receptor/insulin receptor substrate/PI-3K/Akt pathway, leading to elevation of negatively regulated substrates such as glycogen synthase kinase-3beta (Gsk-3beta). When elevated, this enzyme has antiproliferative and proapoptotic properties. In these studies, we designed experiments to determine the contribution of Gsk-3beta to regulation of beta-cell mass in two mouse models of insulin resistance. Mice lacking one allele of the insulin receptor (Ir+/-) exhibit insulin resistance and a doubling of beta-cell mass. Crossing these mice with those having haploinsufficiency for Gsk-3beta (Gsk-3beta+/-) reduced insulin resistance by augmenting whole-body glucose disposal, and significantly reduced beta-cell mass. In the second model, mice missing two alleles of the insulin receptor substrate 2 (Irs2-/-), like the Ir+/- mice, are insulin resistant, but develop profound beta-cell loss, resulting in early diabetes. We found that islets from these mice had a 4-fold elevation of Gsk-3beta activity associated with a marked reduction of beta-cell proliferation and increased apoptosis. Irs2-/- mice crossed with Gsk-3beta+/- mice preserved beta-cell mass by reversing the negative effects on proliferation and apoptosis, preventing onset of diabetes. Previous studies had shown that islets of Irs2-/- mice had increased cyclin-dependent kinase inhibitor p27(kip1) that was limiting for beta-cell replication, and reduced Pdx1 levels associated with increased cell death. Preservation of beta-cell mass in Gsk-3beta+/- Irs2-/- mice was accompanied by suppressed p27(kip1) levels and increased Pdx1 levels. To separate peripheral versus beta-cell-specific effects of reduction of Gsk3beta activity on preservation of beta-cell mass, mice homozygous for a floxed Gsk-3beta allele (Gsk-3(F/F)) were then crossed with rat insulin promoter-Cre (RIP-Cre) mice to produce beta-cell-specific knockout of Gsk-3beta (betaGsk-3beta-/-). Like Gsk-3beta+/- mice, betaGsk-3beta-/- mice also prevented the diabetes of the Irs2-/- mice. The results of these studies now define a new, negatively regulated substrate of the insulin signaling pathway specifically within beta-cells that when elevated, can impair replication and increase apoptosis, resulting in loss of beta-cells and diabetes. These results thus form the rationale for developing agents to inhibit this enzyme in obese insulin-resistant individuals to preserve beta-cells and prevent diabetes onset.     

Rat maternal diabetes impairs pancreatic beta-cell function in the offspring

It has been shown that maternal diabetes increases the risk for obesity, glucose intolerance, and Type 2 diabetes mellitus in the adult life of the offspring. Mechanisms for these effects on the offspring are not well understood, and little information is available to reveal the mechanisms. We studied the effect of maternal diabetes on beta-cell function in the offspring of streptozotocin (STZ)-induced diabetic rat mothers (STZ-offspring). STZ-offspring did not become glucose intolerant up to 15 wk of age. At this age, however, insulin secretion was significantly impaired, as measured by in vivo and in vitro studies. Consistent with these changes, islet glucose metabolism and some important glucose metabolic enzyme activities were reduced. No significant changes were found in islet morphological analysis. These data indicate that beta-cell function is impaired in adult STZ-offspring; these changes may contribute to the development of type 2 diabetes mellitus in adulthood.     

A field of myocardial-endocardial NFAT signaling underlies heart valve morphogenesis

The delicate leaflets that make up vertebrate heart valves are essential for our moment-to-moment existence. Abnormalities of valve formation are the most common serious human congenital defect. Despite their importance, relatively little is known about valve development. We show that the initiation of heart valve morphogenesis in mice requires calcineurin/NFAT to repress VEGF expression in the myocardium underlying the site of prospective valve formation. This repression of VEGF at E9 is essential for endocardial cells to transform into mesenchymal cells. Later, at E11, a second wave of calcineurin/NFAT signaling is required in the endocardium, adjacent to the earlier myocardial site of NFAT action, to direct valvular elongation and refinement. Thus, NFAT signaling functions sequentially from myocardium to endocardium within a valvular morphogenetic field to initiate and perpetuate embryonic valve formation. This mechanism also operates in zebrafish, indicating a conserved role for calcineurin/NFAT signaling in vertebrate heart valve morphogenesis.