A New Mouse Model in which the Proximal Tandem Ig Element of Titin is Truncated (IGKO) - Assessment of Diastolic Function

Abstract

Titin is a giant protein that functions as a molecular spring and contributes to the passive diastolic stiffness of the left ventricle (LV). Titin's spring region consists of three sequence elements: the tandem Ig, the N2B and the PEVK elements. Mouse models exist in which either of the latter two elements (N2B KO, PEVK KO) has been deleted, but considering that these elements are also likely to play roles in signaling, we made a model in which we deleted 2/3 of the proximal tandem Ig element (IGKO), a region that is not known to contain signaling functions. Homozygous KO mice survive to adulthood and are fertile with no change in LV Weight to Body Weight (LVW/BW) ratio (3.67±0.07mg/g WT vs 3.51±0.04mg/g IgKO). A custom titin exon microarray confirmed the deletion of the expected exons with no other changes. Gel electrophoresis showed that the IGKO expresses an 88kDa smaller titin isoform but no other changes in isoform expression. Immuno-electron microscopy indicates that the remaining spring elements extend to a higher degree than in WT myocardium. Mechanical measurements in cardiomyocytes (see also poster by Methawasin) and myofibers show that the deletion increases passive stiffness in titin in a sarcomere length dependence manner, without adaptations in the passive stiffness of the extracellular matrix. Initial isolated intact heart data and in-vivo pressure volume measurement suggest that the IGKO has increased stiffness in-vivo. Further phenotypic characterization is currently underway. In summary, shortening the proximal tandem Ig element of titin produces a diastolic stiffness phenotype.