HOMEODOMAIN FACTOR NKX2-5 CONTROLS LEFT RIGHT ASYMMETRIC EXPRESSION OF BHLH GENE EHAND DURING MURINE HEART DEVELOPMENT/

One of the first morphological manifestations of left/right (L/R) asym metry in mammalian embryos is a pronounced rightward looping of the li near heart tube. The direction of looping is thought to be controlled by signals from an embryonic L/R axial system. We report here that mor phological L/R asymmetry in the murine heart first became apparent at the linear tube stage as a leftward displacement of its caudal aspect. Beginning at the same stage, the basic helix-loop-helix (bHLH) factor gene eHand was expressed in a strikingly left-dominant pattern in myo cardium, reflecting an intrinsic molecular asymmetry. In hearts of emb ryos lacking the homeobox gene Nkx2-5, which do not loop, left-sided e Hand expression was abolished. However, expression was unaffected in S c1(-/-) hearts that loop poorly because of hematopoietic insufficiency , and was right-sided in hearts of inv/inv embryos that display situs inversus. The data predict that eHand expression is enhanced in descen dants of the left heart progenitor pool as one response to inductive s ignaling from the L/R axial system, and that eHand controls intrinsic morphogenetic pathways essential for looping. One aspect of the intrin sic response to L/R information falls under Nkx2-5 homeobox control.