Flik, a chick follistatin-related gene, functions in gastrular dorsalisation/neural induction and in subsequent maintenance of midline sonic hedgehogsignalling

We have targetted the chick gene Flik with antisense oligodeoxynucleotide t reatment at gastrular stages, when it is expressed in organiser-derived str uctures of the midline (K. Patel et al., 1996, Dev. Biol. 178, 327-342). A specific syndrome of deficient axial patterning and holoprosencephaly is pr oduced. Most aspects of this syndrome can be understood as due to attenuati on of dorsalising and neural-inducing signals during gastrulation,followed by failure to maintain the later signals from chordamesoderm/neural midline that pattern the mesodermal and neural cross sections during subsequent st ages. Anatomical effects are first apparent at early neurula stages and cor respond with what might be expected from a reduced counteraction of the ven tralising Bone morphogenetic protein (BMP) pathway at the earlier stages, c oupled with inadequate Sonic hedgehog (Shh) signalling subsequently. Delay in the clearing of BMP-4 RNA expression from the presumptive neural region at gastrulation is indeed seen, though chordin RNA expression within organi ser derivatives remains normal, Subsequently, specific attenuation of chord amesoderm and neural midline Shh expression is observed. Brief preincubatio n of stage 4 chick blastoderms in supernatant from Xenopus oocytes that hav e been injected with Flik RNA prolongs and enhances the competence of their peripheral epiblast to respond to neural inductive signals from grafted He nsen's nodes. This effect specifically mimics that recently observed using mu g/ml solutions of recombinant Follistatin (D. J. Connolly et al., 1999, Int. J. Dev. Biol., in press), further suggesting that Flik protein might a ct in vivo by somehow modulating activity of signalling pathways through BM P or other TGF beta-related Ligands, We discuss the significance of the obs ervations in relation to recent ideas about neural induction, about possibl e redundancy in gene action, and about subsequent patterning of the axial c ross section,suggesting that a Flik function in autocrine/paracrine mainten ance of later midline Shh signalling represents a role of the gene separate from that in primary dorsalisation/neural induction. (C) 1999 Academic Pre ss.