They have identified the protein, MG53, as a therapeutic target intended for treating metabolic disorders. the energy metabolism in mammalian cells. In response to elevated blood glucose levels, pancreatic beta cells located in the islets of Langerhans secrete insulin. The secreted insulin triggers the uptake of glucose in adipose and muscle tissue. Standard medical diagnosis of diabetes, insulin resistance, and other disorders from the energy metabolism include the testing EMD638683 of the glucose-insulin regulatory system [1, 2, 3], e. g., by an oral or intravenous glucose tolerance test. Initially, insulin binds and activates the insulin receptor (IR) located as a homodimer in the membrane of the cell. Upon a very fast binding event of insulin to the extracellular binding site from the IR homodimer [4, 5], a slower rate-limiting phase occurs that manifests with a conformational change in the complex [6]. The binding of insulin to the IR EMD638683 triggers autophosphorylation from the intracellular IRGI domain, and the autophosphorylation initiates a signaling cascade intended for the regulation of glucose uptake [7, 8]. The cell maintains the capability Rabbit polyclonal to PROM1 to tight regulation by recycling of the phosphorylated IR homodimer. The recovery of the IRGI is accomplished either by the dissociation of insulin from the IR or via an internalization from the entire complex (endocytosis), which moves the complex into the cytoplasm. In the cytoplasm, the insulin dissociates from the IRGI, and the dissociated insulin is degraded [9]. After dissociation of insulin, the IR becomes dephosphorylated and either earnings back to the membrane or is degraded, seeFigure 1 . The recycling of the internalized IR is a critical step for the regulation of the energy metabolism and subject of intensive experimental and theoretical studies. == Figure 1 . == The processes of insulin-dependent activation and recycling from the IR. First, insulin binds to the IRGI. Afterwards, the IR gets autophosphorylated. Two alternative processes describe the dephosphorylation from the IR: (1) The IRGI can dephosphorylate on the plasma membrane by the dissociation of insulin; (2) The IRGI can be internalized into the cytoplasm. There, the insulin is degraded, and the IR is transported back to the plasma membrane. The processes of dephosphorylation on the plasma membrane and in the cytoplasm are both catalyzed by the enzyme PTP1B. Since the binding of insulin and insulin analogs to the IRGI is of great importance to pharmaceutical applications, extensive experimental research [4, 5, 6, 10, 11, 12, 13, 14] continues to be focused on this complex extracellular mechanism. The engagement of insulin with the receptor displays allosteric properties, such as unfavorable cooperativity and insulin dependence of the dissociation rate. Several groups [15, 16, 17, 18, 19, 20, 21] have proposed mechanisms intended for the binding process. Molecular processes inside the cell are much more difficult to be experimentally accessed. Only little is known, how the IR transmembrane domains transfer signals across the membrane and how the receptor becomes activated. A broad range of experimental methods [22, 23, 24, 25, 26] provides quantitative data, but currently, the experimental view on the molecular details of the life cycle of the IRGI inside the cell remains still unclear. The various theoretical models for the IR recycling do not converge on a unique, consensus network topology. The modeling from the detailed topology of the life cycle from the IR remains problematic due to many still unknown biochemical and structural properties. Several mathematical models for the glucose-insulin regulatory system have been developed over the past 50 years; for a review see [27]. One of the first EMD638683 models [28, 29, 30] from the IR recycling is the two-component ordinary differential equation (ODE) system of Quon & Campfield [31]. Their model is able to reproduce experimental data of ligand-induced down- and up-regulation of receptors as well as the initial spontaneous display from the surface IRs for cell cultures of BC3H-1 myocytes. Extended and more detailed models have been.