Figure 7

Distantly related NTR domains mimic the activity of Sfrp1 _NTR with different efficiencies. (a-j) Brightfield views of embryos injected with different full-length or chimerical mRNAs as indicated. Insets in (c,d,f,g) correspond to embryos processed for double in situ hybridization for rx3 (red) and foxA2 (blue). Note that injections of Sfrp1 (b), Sfrp2 (c, and inset) lead to similar expansion of anterior structures compared to control embryos (a), while Sfrp3 has a very weak anteriorizing effect (d) observed only in 4% of the injected embryos (Table 1; inset in (d) shows an embryo injected with a high dose (500 ng/μl) of Sfrp3 mRNA). Similarly, Sfrp3 _NTR induces a weak anteriorisation at a low frequency (embryo shown in (g); Table 1), whereas the distantly related NTR motif from Netrin-1 (i) induces an expansion of the forebrain as observed with Sfrp1_NTR. Note that the functionality of the NTR domain depends on an intact tertiary structure, since cysteine to serine mutations in Sfrp1_{NTR-C 177S;C 180S}and Netrin-1_{NTR-C 471S;C 475S}constructs (h,j) induce a total or partial loss of the effect. See Table 1. Scale bar: 0.1 mm.