Dissecting the role of the scaffold proteins intersectin 1 and intersectin 2 in insulin secretion
Final Report Abstract
The secretion of insulin by pancreatic β-cells is essential for glucose homeostasis. Defects in insulin release are a major factor in the pathogenesis of type 2 diabetes (T2D), a rising health problem. Insulin is stored in large granules to be released upon increased glucose concentrations. Insulin secretion requires a well-orchestrated sequence of events comprising granule transport to the plasma membrane, remodeling of the local actin cytoskeleton, granule docking and finally fusion which has to be followed by endocytosis, potentially to retrieve critical components for subsequent release events. Although our knowledge of the proteins that influence insulin secretion is constantly growing, many questions remain unanswered. For example, βcells express the two scaffold proteins ITSN1 and ITSN2, which play an important role in the endo- and exocytosis of synaptic vesicles in neurons. With regard to β-cells, however, it is still completely unclear which function they fulfil. Since both proteins are reduced in T2D patients, it is of particular interest to elucidate to what extent this alters the functionality of β-cells. Therefore, in this project we investigated how the loss of ITSN1 and ITSN2 affects insulin secretion. ITSN1 and ITSN2 have a matching domain structure, but can both exist in a short and a long splice isoform. Only the long isoform contains a GEF module for the activation of actin regulators such as Cdc42. In β-cells, ITSN1 is mainly present in the short isoform, while ITSN2 also occurs as the long isoform. As we were able to show, this difference has a strong influence on the function of the respective proteins in β-cells. The loss of ITSN1 led to reduced insulin secretion in mice, while the loss of ITSN2 was associated with increased insulin secretion. In line with this, our electron microscopic examination of β-cells from ITSN1- or ITSN2-deficient mice also revealed striking differences. In the absence of ITSN1, many insulin granules lacked the typical dark interior, which is due to the crystalline form of the zinc-insulin complex. Such changes have also been observed upon the loss of insulin-processing enzymes. In ITSN2- deficient β-cells, on the other hand, fewer insulin granules were found close to the plasma membrane. This suggests that ITSN1 is needed in β-cells to ensure the production and/or complex formation in the insulin granules, perhaps because ITSN1 influences the sorting of factors involved in this process. ITSN2, on the other hand, appears to be required to prevent insulin granules from being released in excessive numbers, perhaps by influencing the regulation of the actin cytoskeleton, which is thought to act as a barrier to the release of insulin granules.
