Importance of ß-Catenin in glucose and energy homeostasis.

In settings of increased insulin demand, failure to expand pancreatic ß-cells mass leads to diabetes. Genome-wide scans of diabetic populations have uncovered several genes associated with susceptibility to type 2 diabetes and a number of them are part of the Wnt signaling. ß-Catenin, a Wnt downstream effector participates in pancreatic development, however, little is known about its action in mature ß-cells. Deletion of ß-Catenin in Pdx1 pancreatic progenitors leads to a decreased ß-cell mass and impaired glucose tolerance. Surprisingly, loss of ß-catenin made these mice resistant to high fat diet because of their increased energy expenditure and insulin sensitivity due to hyperactivity. The complexity of this phenotype was also explained in part by ectopic expression of Cre recombinase in the hypothalamus. Our data implicates ß-Catenin in the regulation of metabolism and energy homeostasis and suggest that Wnt signaling modulates the susceptibility to diabetes by acting on different tissues.

mTORC1 signaling and regulation of pancreatic ß-cell mass.

The capacity of ß cells to expand in response to insulin resistance is a critical factor in the development of type 2 diabetes. Proliferation of ß cells is a major component for these adaptive responses in animal models. The extracellular signals responsible for ß-cell expansion include growth factors, such as insulin, and nutrients, such as glucose and amino acids. AKT activation is one of the important components linking growth signals to the regulation of ß-cell expansion. Downstream of AKT, tuberous sclerosis complex 1 and 2 (TSC1/2) and mechanistic target of rapamycin complex 1 (mTORC1) signaling have emerged as prime candidates in this process, because they integrate signals from growth factors and nutrients. Recent studies demonstrate the importance of mTORC1 signaling in ß cells. This review will discuss recent advances in the understanding of how this pathway regulates ß-cell mass and present data on the role of TSC1 in modulation of ß-cell mass. Herein, we also demonstrate that deletion of Tsc1 in pancreatic ß cells results in improved glucose tolerance, hyperinsulinemia and expansion of ß-cell mass that persists with aging.

Decreased IRS signaling impairs beta-cell cycle progression and survival in transgenic mice overexpressing S6K in beta-cells.

OBJECTIVE: The purpose of this study was to evaluate the role of the S6K arm of mammalian target of rapamycin complex 1 (mTORC1) signaling in regulation of ß-cell mass and function. Additionally, we aimed to delineate the importance of in vivo S6K activation in the regulation of insulin signaling and the extent to which alteration of insulin receptor substrate (IRS) signaling modulates ß-cell mass and function. RESEARCH DESIGN AND METHODS: The current experiments describe the phenotype of transgenic mice overexpressing a constitutively active form of S6K under the control of the rat insulin promoter. RESULTS: Activation of S6K signaling in these mice improved insulin secretion in the absence of changes in ß-cell mass. The lack of ß-cell mass expansion resulted from decreased G(1)-S progression and increased apoptosis. This phenotype was associated with increased p16 and p27 and decreased Cdk2 levels. The changes in cell cycle were accompanied by diminished survival signals because of impaired IRS/Akt signaling. CONCLUSIONS: This work defines the importance of S6K in regulation of ß-cell cycle, cell size, function, and survival. These experiments also demonstrate that in vivo downregulation of IRS signaling by TORC1/S6K induces ß-cell insulin resistance, and that this mechanism could explain some of the abnormalities that ultimately result in ß-cell failure and diabetes in conditions of nutrient overload.

mTORC1 activation regulates beta-cell mass and proliferation by modulation of cyclin D2 synthesis and stability.

Growth factors, insulin signaling, and nutrients are important regulators of beta-cell mass and function. The events linking these signals to the regulation of beta-cell mass are not completely understood. The mTOR pathway integrates signals from growth factors and nutrients. Here, we evaluated the role of the mTOR/raptor (mTORC1) signaling in proliferative conditions induced by controlled activation of Akt signaling. These experiments show that the mTORC1 is a major regulator of beta-cell cycle progression by modulation of cyclin D2, D3, and Cdk4 activity. The regulation of cell cycle progression by mTORC1 signaling resulted from modulation of the synthesis and stability of cyclin D2, a critical regulator of beta-cell cycle, proliferation, and mass. These studies provide novel insights into the regulation of cell cycle by the mTORC1, provide a mechanism for the antiproliferative effects of rapamycin, and imply that the use of rapamycin could negatively impact the success of islet transplantation and the adaptation of beta-cells to insulin resistance.

Disruption of Tsc2 in pancreatic beta cells induces beta cell mass expansion and improved glucose tolerance in a TORC1-dependent manner.

Regulation of pancreatic beta cell mass and function is a major determinant for the development of diabetes. Growth factors and nutrients are important regulators of beta cell mass and function. The signaling pathways by which these growth signals modulate these processes have not been completely elucidated. Tsc2 is an attractive candidate to modulate these processes, because it is a converging point for growth factor and nutrient signals. In these experiments, we generated mice with conditional deletion of Tsc2 in beta cells (betaTsc2(-/-)). These mice exhibited decreased glucose levels and hyperinsulinemia in the fasting and fed state. Improved glucose tolerance in these mice was observed as early as 4 weeks of age and was still present in 52-week-old mice. Deletion of Tsc2 in beta cells induced expansion of beta cell mass by increased proliferation and cell size. Rapamycin treatment reversed the metabolic changes in betaTsc2(-/-) mice by induction of insulin resistance and reduction of beta cell mass. The reduction of beta cell mass in betaTsc2(-/-) mice by inhibition of the mTOR/Raptor (TORC1) complex with rapamycin treatment suggests that TORC1 mediates proliferative and growth signals induced by deletion of Tsc2 in beta cells. These studies uncover a critical role for the Tsc2/mTOR pathway in regulation of beta cell mass and carbohydrate metabolism in vivo.

Generation of a reporter mouse line expressing Akt and EGFP upon Cre-mediated recombination.

The serine-threonine kinase Akt regulates multiple biological processes. An important strategy to study Akt signaling in different tissues is targeted activation of this pathway in vivo. The current studies describe the generation of a mouse model that combines a double reporter system with activation of a constitutively active form of Akt1 (caAkt) in a Cre-dependent manner. Before Cre recombination, these mice express LacZ during development as well as in most adult tissues. After Cre-mediated excision of the LacZ reporter, functionality of the transgene was demonstrated by expression of the caAkt mutant along with the second reporter, EGFP in different pancreatic compartments and in the nervous system. This animal model provides a critical reagent for assessing the effects of Akt activation in specific tissues. The lineage-tracing properties provide a useful tool to study the role of Akt signaling in regulation of differentiation programs during development and plasticity of mature tissues.

Controls for lung dendritic cell maturation and migration during respiratory viral infection.

Dendritic cells are ideally suited to orchestrate the innate and adaptive immune responses to infection, but we know little about how these cells respond to infection with common respiratory viruses. Paramyxoviral infections are the most frequent cause of serious respiratory illness in childhood and are associated with an increased risk of asthma. We therefore used a high-fidelity mouse model of paramyxoviral respiratory infection triggered by Sendai virus to examine the response of conventional and plasmacytoid dendritic cells (cDCs and pDCs, respectively) in the lung. We found that pDCs are scarce at baseline but become the predominant population of lung dendritic cells during infection. This recruitment allows for a source of IFN-alpha locally at the site of infection. In contrast, cDCs rapidly differentiate into myeloid cDCs and begin to migrate from the lung to draining lymph nodes within 2 h after viral inoculation. These events cause the number of lung cDCs to decrease rapidly and remain decreased at the site of viral infection. Maturation and migration of lung cDCs depends on Ccl5 and Ccr5 signals because these events are significantly impaired in Ccl5(-/-) and Ccr5(-/-) mice. cDCs failure to migrate to draining lymph nodes in Ccl5(-/-) or Ccr5(-/-) mice is associated with impaired up-regulation of CCR7 that would normally direct this process. Our results indicate that pDCs and cDCs respond distinctly to respiratory paramyxoviral infection with patterns of movement that should serve to coordinate the innate and adaptive immune responses, respectively.