The homeobox gene it NKX3.2 is a target of left–right signalling and is expressed on opposite sides in chick and mouse embryos

Abstract

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.

The splanchnic mesodermal plate directs spleen and pancreatic laterality,and is regulated byBapx1/Nkx3.2

“…3E) are expressed at the extreme lateral domain of the SMP and in the underlying mesenchyme. Thus, there is a conversion in the sidedness of the Bapx1 expression in the mouse and by E9.5 is in accordance with the expression pattern of the chick (Schneider et al, 1999).…”

“…A previous report predicted that Bapx1 has an early role in LR asymmetry as the gene is expressed in a side-specific manner (Schneider et al, 1999). Paradoxically, Bapx1 mesodermal expression is highest in the left LPM in chick and the right LPM in mouse.…”

The evolution and conservation of left-right patterning mechanisms

“…Instead, caronte, a cerberus/Dan-related growth factor, is active in the left paraxial mesoderm. Curiously, the homeobox transcription factor Nkx3.2 is asymmetrically expressed in the mouse and chick LPM, but on opposite sides [right in mouse versus left in chick (Schneider et al, 1999)]. It remains to be seen whether and how these chick-specific asymmetries relate to the node rotation observed in chick.…”

Required, tissue-specific roles for Fgf8 in outflow tract formation and remodeling

“…Fgf8 functions upstream of nodal and Pitx2c in embryonic LR axis determination (Boettger et al, 1999;Fischer et al, 2002;Kioussi et al, 2002;Liu et al, 2002;Meyers and Martin, 1999;Schneider et al, 1999;Welsh and O'Brien, 2000). We examined Pitx2 expression to assess whether the…”