Increased expression of sAnk1.5 does not predispose to Type 2 Diabetes in transgenic mice

Skeletal muscle represents about 40% of the body mass and is the site where the major part of blood glucose is disposed following insulin stimulation. Due to this critical role, skeletal muscle dysfunctions often result in the development of systemic metabolic diseases. Type 2 Diabetes (T2D) is the most common chronic metabolic disorder, representing nearly 90% of the overall diabetes cases. T2D is characterized by insulin resistance followed by reduced insulin release from pancreatic b-cells, resulting in high glucose concentration in bloodstream and glucose intolerance. T2D is a multifactorial disorder, as its onset has both genetic and environmental origins. Genome Wide Association Studies have identified hundreds of single nucleotide polymorphisms (SNPs) associated to T2D susceptibility, in the human genome. Interestingly, several of these SNPs were identified in the ANK1 locus, although these SNPs were found in regions neither coding nor endowed with a regulatory activity. However, two recent independent studies identified a novel SNP in the internal promoter of the ANK1 gene, which drives the expression of sAnk1.5, a striated muscle-specific small ANK1 isoform. The sAnk1.5 protein is localized on the sarcoplasmic reticulum (SR) membrane, in skeletal muscle fibers, and interacts with Obscurin, a giant protein of the sarcomere. This interaction stabilizes the SR and guarantees the close apposition of this organelle around the contractile apparatus. The ANK1 internal promoter carrying the C/C variant displays higher transcriptional activity with respect to the T/T variant. Accordingly, skeletal muscle biopsies of individuals carrying the C/C genotype showed higher levels of both sAnk1.5 mRNA and protein compared to those carrying the T/T genotype. The aim of this thesis was to investigate whether sAnk1.5 overexpression in skeletal muscle might predispose to T2D susceptibility. Accordingly, we generated a transgenic mouse model with the coding sequence of the murine sAnk1.5 under the transcriptional control of the skeletal muscle-specific rat myosin light chain promoter. In these transgenic mice, protein levels of sAnk1.5 were increased up to 50% in skeletal muscles with respect to wild type mice. Basal glucose levels, glucose and insulin tolerance were monitored over a period of 12-months. In addition, 2-months old mice were fed with a high fat diet for twelve weeks. The results obtained did not reveal significant differences in glucose and insulin disposal between transgenic and wild type mice. In conclusion, our results, show that sAnk1.5 overexpression does not appear to predispose to a pre-diabetic or diabetic condition