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Vertebrate internal organs display invariant left-right (L-R) asymmetry. A signalling cascade that sets up L-R asymmetry has recently been identified (reviewed in [1]). On the right side of Hensen's node, activin represses Sonic hedgehog (Shh) expression and induces expression of the genes for the activin receptor (ActRIIa) and fibroblast growth factor-8 (FGF8) [2] [3]. On the left side, Shh induces nodal expression in lateral plate mesoderm (LPM); nodal in turn upregulates left-sided expression of the bicoid-like homeobox gene Pitx2 [4] [5] [6]. Here, we found that the homeobox gene NKX3.2 is asymmetrically expressed in the anterior left LPM and in head mesoderm in the chick embryo. Misexpression of the normally left-sided signals Nodal, Lefty2 and Shh on the right side, or ectopic application of retinoic acid (RA), resulted in upregulation of NKX3.2 contralateral to its normal expression in left LPM. Ectopic application of FGF8 on the left side blocked NKX3.2 expression, whereas the FGF receptor-1 (FGFR-1) antagonist SU5402, implanted on the right side, resulted in bilateral NKX3.2 expression in the LPM, suggesting that FGF8 is an important negative determinant of asymmetric NKX3.2 expression. NKX3.2 expression was also found to be asymmetric in the mouse LPM but, unlike in the chick, it was expressed in the right LPM. In the inversion of embryonic turning (inv) mouse mutant, which has aberrant L-R development, NKX3.2 was expressed predominantly on the left side. Thus, NKX3.2 transcripts accumulate on opposite sides of mouse and chick embryos although, in both the mouse and chick, NKX3.2 expression is controlled by the L-R signalling pathways.

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