Raising cyclic AMP (cAMP) in the islet ?-cells shows considerable therapeutic potential in the treatment of type 2 diabetes mellitus (T2DM). Elevating cAMP potentiates glucose-stimulated insulin secretion, enhances insulin synthesis to maintain insulin stores and protects ?-cell mass through effects upon ?-cell proliferation and ?-cell survival. The cAMP signal is transduced through two signaling molecules, the cAMP-dependent protein kinase (PKA) and EPAC.
The aim of this project is to use in vivo mouse models that manipulate PKA activity to understand the physiological role of ?-cell PKA activity. Mouse strains that express a Cre-Lox-dependent constitutively active PKA subunit (caPKA) and a dominant negative PKA subunit (dnPKA) will be crossed to a strain expressing the inducible Cre-recombinase specifically in the islet ?-cells (MIP-CreER) to generate mice with increased ?-cell PKA activity (?-caPKA) and inhibited ?-cell PKA activity (?-dnPKA). Experiments controlled using littermate mice will address three specific aims: 1. Determine whether activation PKA in ?-cells enhances insulin secretion &glucose clearance. 2. Determine whether reduced ?-cell PKA activity decreases insulin secretion and ?-cell mass. 3. Determine whether altering ?-cell function affects glucose sensitivity.
In aims 1 and 2 PKA activity will be altered by administering tamoxifen at 10 weeks of age and the mice characterized during the 11th week.
Aim 3 is a longer term study, in which tamoxifen will be administered at 10 weeks of age and the development of insulin resistance followed, or insulin resistance will be established before tamoxifen is administered. In vivo and in vitro studies (using isolated islets) in aims 1 and 2, will determine how PKA activity affects insulin secretion, insulin synthesis and ?-cell mass and the role that altered ?-cell PKA activity plays in the response of insulin secretion to incretin hormones. Studies performed in aims 1 and 2 will also determine whether there are potential adverse consequences of activating ?-cell PKA, such as increased risk of hypoglycemia, an impaired counter-regulatory response or an increased of unregulated ?-cell mass expansion.
Aims 1 and 2 will also explore the role of ?-cell PKA activity in the mechanisms of action of incretin hormones to potentiate insulin secretion.
In aim 3, mice will be fed a high fat diet or a standard chow control diet to determine the role of ?-cell PKA activity in the progression to insulin resistance. These studies will determine whether increasing ?-cell PKA activity can prevent and/or reverse insulin resistance and whether decreasing PKA activity reduced ?-cell viability and/or function that leads to the development of T2DM. These experiments will test the novel hypothesis that alterations in ?-cell function can be a major contributing factor to peripheral insulin sensitivity. Overall, these experiments will determine the role that PKA activity in islet ?- cells play in regulating ?-cell function and mass and the effects that this has upon whole body glucose disposal.
The cyclic AMP signaling pathway shows considerable potential in treating diabetes by preserving the ability of the beta-cells of the islets of Langerhans to secrete insulin and in patenting beta-cell mass. This project explores the physiological role of the main cAMP signaling molecule in ?-cells PKA activity within the ?-cells of the pancreatic islets of Langerhans. The project uses of novel reagents to study the physiological consequences of altering the activity of ?-cell PKA. Manipulation of beta-cell PKA activity will provide greater understanding of the basic science behind the therapeutic potential of this signaling pathway. It will also reveal potential pitfalls of harnessing this pathway for the treatment of diabetes.
