Abstract 2173: Nkx2.5 is Required for Development of the Outflow Tract Through Regulation of Semaphoring 3C Expression and Neural Crest Cells Migration
Embryonic development of heart involves a delicate interaction of a core group of transcription factors. Nkx2.5 is an evolutionally conserved homeodomain-containing transcription factor which is highly expressed in the early heart progenitor cells in both primary and secondary heart fields. The global knockout of Nkx2.5 causes early embryonic lethality and precludes the assessment of its functions in later stages of development as well as specific subpopulations of the cells. To address the temporal and spatial functions of Nkx2.5 in the secondary heart field, we generated its conditional knockout (CKO) in the specific lineage originating from Islet-1 (Isl1) expressing progenitors by Isl1-IRES-Cre mediated removal of the floxed allele of Nkx2.5. Fate mapping, using Isl1-IRES-Cre and R26R-lacZ reporter mouse lines, demonstrated that Isl1-expressing cells contribute to the majority of the outflow tract and distal portion of right ventricle as well as patches of cardiomyocytes throughout the rest of the ventricles and atria. CKO neonates showed progressive cyanosis and expired within the 1st postnatal day. Gross inspection and histological analysis of the heart and great vessels demonstrated persistent truncus arteriosus, interruption of aortic arch and membranous type ventricular septal defects. Indian ink injection at embryonic day 10.5 showed a significant narrowing in the proximal portion of the 4th aortic arch. In situ hybridization indicates a significant reduction in plexin A2 demonstrating the arrest in migration of cardiac neural crest cells to the outflow tract of CKO mice. Semaphoring 3C expression, which provides the cue for the migration of the cardiac neural crest cells into the outflow tract, was markedly reduced in proximal out flow tract and cardiomyocytes of CKO embryos. This study demonstrates for the first time that Nkx2.5 is required for the proper septation of the outflow tract and development of great arteries through regulation of expression of semaphorin 3C in the myocardial cells which provides the signaling cues for the migration of the cardiac neural crest cells.