Investigating the role of alternative splicing in the islets of Langerhans in developing diabetes.

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Transcriptome analysis of Islets of Langerhans derived from mice that differ in their susceptibility to develop diabetes, like the diabetes susceptible new zealand obese mouse (NZO) and the diabetes resistant mouse B6-ob/ob, revealed significant expression differences between the two mouse strains and the discovery of diabetes genes. We propose that differential gene expression is not the only contributor to diabetes resistance and susceptibility and that diverse protein isoforms resulting from splicing can influence the functionality of islets. The goal of this proposal is to study and understand the role of alternative splicing in the onset of diabetes. Therefore, RNA-seq data from isolated islets of male NZO and B6-ob/ob mice were analyzed and 730 genes with alternative splicing events identified. The results from this preliminary analysis shall be further studied in three major working plans: 1. Alternative splicing isoforms that might contribute to a dysfunction of the islets of Langerhans shall be identified and characterized. It will be analyzed how alternative isoforms differ from the original isoform, if functional domains or intracellular localization of the proteins are altered and how this can affect fundamental processes like insulin secretion. Overall NZO exons exhibit a larger skipping rate resulting in 40% frameshift and 55% frame preserving variants representing potentially altered proteins. 2. Genetic variants that might affect alternative splicing shall be identified. The analysis of RNAseq data from the two different mouse strains revealed a high number of single nucleotide polymorphisms (SNPs) that potentially contribute to the observed splicing pattern. Therefore, the localization of these SNPs and a potential impact on regulatory elements shall be further analyzed. Identified SNPs with impact on alternative splicing will be compared with human diabetes-associated SNPs from GWAS data in order to translate our results to human data. 3. It shall be analyzed how the loss of a neuronal signature in the islets of Langerhans can be associated with the onset of diabetes. Cells from islets show a similar expression pattern like neuronal cells and express neuronal enriched splicing factors like Srrm4 (serine/arginine repetitive matrix 4). Srrm4 is lower expressed in islets from NZO mice and known SRRM4 regulated exons show higher skipping. Therefore, it will be analyzed how these exons can alter proteins and thereby β-cell functionality with regard on processes like proliferation, apoptosis and insulin secretion.