Studies on Islet Amyloid Polypeptide Aggregation : From Model Organism to Molecular Mechanisms

• Main Author: Schultz, Sebastian
• Format: Doctoral Thesis
• Language: English
• Published: Linköpings universitet, Cellbiologi 2011
• Subjects: Medical and Health Sciences; Medicin och hälsovetenskap
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-70094; http://nbn-resolving.de/urn:isbn:9789173930994

The proper folding of a protein into its defined three--‐dimensional structure is one of the many fundamental challenges a cell encounters. A number of tightly controlled pathways have evolved to assist in the proper folding of a protein, but also to aid in the removal of misfolded proteins. Despite the presence of these pathways accumulation of misfolded proteins can still occur. Amyloid deposits consist of misfolded proteins with a characteristic highly ordered fibrillar structure that will exert affinity for the amyloid dye Congo red and has a unique X-ray diffraction pattern. Currently 27 different proteins have been identified as amyloid forming proteins in human, however the exact role of amyloid in the pathogenesis of the connected disease is most often unclear. Islet amyloid is made up of the beta cell derived hormone islet amyloid polypeptide (IAPP) and is associated with the development of type 2 diabetes. Propagation of IAPP-fibrils is believed to be one important cause of the pancreatic beta cell death detected in patients with type 2 diabetes. IAPP is a naturally occurring polypeptide hormone stored and secreted together with insulin. IAPP and insulin arise from posttranslational processing of their biological inactive precursors proIAPP and proinsulin. In addition to human, cat and monkey IAPP will form amyloid deposits in conditions resembling human type 2 diabetes. However, IAPP from mouse and rat do not form amyloid as a result of the differences in amino acid sequence. My main research goal was to establish a unique model system suitable to study the effects of proIAPP and IAPP aggregation. I selected Drosophila melanogaster due to its many suitable characteristics as a model organism and its superior genetic toolbox. I have demonstrated that over--‐expression of hproIAPP and hIAPP in the central nervous system (CNS) results in aggregate formation in the brain and neighbouring fat body. Consistent with previous studies, expression of mIAPP does not result in the formation of aggregates. To investigate the intracellular effects of hproIAPP and hIAPP aggregation on a specific population of neurons, we targeted the expression of these peptides specifically to 16 neurons in the brain, the pdf- neurons. These pdf-neurons are divided into 2 clusters of 8 cells per brain hemisphere. First I showed that expression of aggregation prone hIAPP and hproIAPP resulted in significant death of the 8 cells, whereas expression of mIAPP had no such effect. In efforts to pinpoint the mechanisms behind the observed cell death I demonstrated that hproIAPP and hIAPP both pass the ERs quality control for protein folding and that the initiated cell death does not occur through classical apoptosis. Instead, selective autophagy is activated by hIAPP and hproIAPP. This activation counteracts the usually neuro-protective effects of autophagy and contributes to cell death. Strikingly, I also showed that Aâ, the amyloid protein implicated in Alzheimer’s disease, does not exhibit any intracellular toxicity when expressed in pdf-cells. This supports the existence of separate toxic pathways for different amyloid proteins.