Developmental outflow tract abnormalities of Jag1-deficient mice are associated with abnormal ventricular activation and desynchronized contraction.
The Notch signaling pathway is an evolutionarily conserved intercellular communication mechanism essential for mammalian embryonic development. Mutations in the human Jagged1 (Jag1) gene, which encodes a ligand of the Notch receptor, cause Alagille syndrome--an autosomal dominant disorder frequently associated with congenital heart diseases (CHDs) such as Tetralogy of Fallot. To investigate the role of Jag1 in cardiac development, we generated Jag1flox/flox Islet1-cre+ mice with a conditional deletion of the Jag1 gene in the cardiac outflow tract. Mice carrying this targeted deletion exhibited severe cardiac malformations characteristic of Tetralogy of Fallot. The predominant defect observed was a double outlet right ventricle (DORV), in which both the aorta and pulmonary trunk arise from the right ventricle. This abnormality was consistently associated with a ventricular septal defect (VSD), present in 100% of homozygous mutants. Additional defects included abnormalities in the morphology of atrioventricular and semilunar valves, most commonly presenting as myxomatous mitral valves or altered leaflet numbers. Since Islet1 is also expressed in the sinoatrial and atrioventricular nodes, we employed optical mapping to visualize the cardiac conduction system. Analysis of E14.5 embryos and adult mice revealed altered activation patterns. While control hearts displayed a mature apex-to-base activation with conduction through both bundle branches, mutant embryos exhibited abnormal activation initiating exclusively from the left ventricle, indicating right bundle branch block (RBBB). In adult heterozygotes, electrical activation was asynchronous and often originated from ectopic sites, particularly in the posterior ventricular wall, deviating from the normal apical initiation observed in controls. For functional analysis, we employed high-resolution ultrasound (Vevo imaging) on adult heterozygotes, as homozygotes did not survive postnatally. Most hemodynamic parameters showed no significant changes, suggesting early compensatory mechanisms to maintain cardiac output under compromised conditions. However, speckle-tracking strain analysis identified localized contractile defects and mechanical dyssynchrony, particularly affecting the anterior wall. In summary, our study demonstrates that conditional deletion of Jag1 leads to both morphological and electrophysiological abnormalities in the heart. These defects were evident in both homozygous and heterozygous embryos, with adult heterozygotes displaying persistent electrophysiological and mechanical alterations. The data suggest that disruption of Jag1-dependent signaling contributes to the pathogenesis of Tetralogy of Fallot and affects both the structural development and electrical function of the heart.