Fig. 6

Isl1 and retinoids repress nkx2.2b expression. (A-D) Retinoic acid (RA) treatment blocks nkx2.2b expression. (A,B) Lateral views of 14s control embryo incubated in 10% DMSO showing nkx2.2b, as well as emx1/krox20 (inset in A) and olig2 (inset in B) expression. (C,D) Treatment with 100 nM (C) and 100 µM all-trans RA (D), results in a strong downregulation of nkx2.2b expression in the head (C) and a complete block of nkx2.2b in the trunk (D). Note rostral expansion of krox20 in rhombomere 5 (inset in C) and up-regulated olig2 expression in the trunk (inset in D) indicative for RA induced posteriorization, as described previously (Begemann and Meyer, 2001). (E-J) Interference with endogenous retinoid synthesis by DEAB treatment up-regulates nkx2.2b. (E-G) Expression of nkx2.2b in head (E) and trunk (transversal section in F), emx1/krox20 in head (inset in E) and olig2 in the trunk (transversal section in G) of control embryos treated with 10% DMSO. (H-J) Treatment of embryos with 10 µM DEAB results in the expansion of nkx2.2b expression in the head (H) and trunk (I) and a reduction of olig2 (J). Change of krox20 expression (inset in H) confirms anteriorization as described previously (Begemann et al., 2004). (K-Q) Ectopic isl1 expression inhibits nkx2.2b activation in the trunk. Expression of nkx2.2b (K,N), foxa2 (L,O,P) and Znp-1 (M,Q) in a non-injected control embryo (K-M) and in an embryo injected with 70 pg isl1 RNA at the 18s stage (N-Q). In isl1 injected embryos, foxa2 is reduced to the MFP (O) or completely lost (N,P). In some areas, the ventral most neural tube cells express Znp-1 (arrow in Q). (R) Schematic model of interactions in the ventral neural tube restricting nkx2.2b expression to the LFP. RA, signalling induced by retinoids derived from paraxial mesoderm. Scale bar in (H) represents 100 µm (for A-H), in (J and Q) 20 µm (for F,G,I,J,L,M,O-Q), and in (N) 100 µm (for K,N).