1. Institute of Metabolic Physiology , Heinrich Heine University , D-40225 Düsseldorf , Germany
2. Institute for Vascular and Islet Cell Biology , German Diabetes Center (DDZ), Leibniz Center for Diabetes Research at Heinrich Heine University , D-40225 Düsseldorf , Germany
3. German Center for Diabetes Research (DZD e.V.), Helmholtz Zentrum München , D-85764 Neuherberg , Germany