(Dys)regulation of insulin secretion

Contact persons: Dr. Ilka Wilhelmi, Dr. Thilo Speckmann

Funding: SFB958; Deutsche Forschungsgemeinschaft (DFG)

Insulin secretion from pancreatic beta cells is a highly dynamic and regulated process that is essential for maintaining glucose homeostasis. An impaired insulin release is a major factor in the pathogenesis of type 2 diabetes and results in elevated blood glucose concentrations. Insulin biosynthesis is initiated via preproinsulin in the rough endoplasmatic reticulum. Removal of the signal peptide results in proinsulin that is transported through the trans-Golgi network and sorted into immature secretory granules. These immature granules become acidic and proinsulin undergoes proteolytic cleavage resulting in the formation of insulin and C-peptide. Insulin granules are large dense-core vesicles and released when blood glucose levels increase. Glucose enters the beta cell via specific glucose transporters and is metabolized leading to a rise in ATP levels and the subsequent closure of KATP channels. The resulting membrane depolarization causes voltage-dependent Ca2+ channels to open, which triggers the fusion of insulin vesicles with the plasma membrane and the release of insulin. The number of proteins involved in the different cellular steps required for the formation and movements of insulin granules and for their fusion with the plasma membrane is steadily growing. However, the spatiotemporal coordination of the different processes underlying efficient insulin release is still poorly understood.   

The aim this research project is to investigate specific scaffolding proteins located at the plasma membrane (ITSN1/2) and the trans-Golgi (GOPC) and to evaluate their role to integrate and orchestrate processes required for efficient insulin secretion.

Intersectin 1 and 2 (ITSN1/2) are large and related scaffolds which have intensively been studied in regards to synaptic vesicle recycling. They have been implicated to be involved in vesicle exocytosis and endocytosis and to interact with actin regulators. Both proteins are also expressed in beta cells and ITSN1 is lower abundant in islets of diabetes patients. Our preliminary analysis in Itsn1/2 double knockout mice revealed an affected insulin secretion during glucose tolerance tests. Double knockout mice were used because of the high degree of similarity of ITNS1 and ITSN2 which are likely to compensate for each other. In addition, the expression of ITSN1 and ITSN2 is suppressed in beta cells which indeed affects insulin secretion. Currently, rescue experiments are performed in order to clarify the mechanistic basis for their influence on insulin release.

GOPC (Golgi-associated PDZ and coiled-coil motif-containing and scaffolding protein) is located at the trans-Golgi and was shown to regulate intracellular transport of proteins. We identified GOPC to interact with the small GTPase ARFRP1 and demonstrated that both proteins are required for correct sorting of the SNARE protein SNAP25 to the plasma membrane of beta cells. In the absence of either ARFRP1 or GOPC, SNAP25 is degraded and insulin secretion markedly reduced. According to our data, we hypothesize that ARFRP1 recruits GOPC to the Golgi which appears to be essential for the correct sorting of SNAP25 to the plasma membrane.