An essential role for retinoid signaling in anteroposterior neural patterning

Xenopus hindbrain patterning requires retinoid signaling

We have asked how posterior neural tissue is patterned in Xenopus by assaying the involvement of endogenous retinoic acid (RA) in this process and by using the labial Hox gene, HoxD1, as a posterior marker. Although RA is able to inhibit anterior gene expression and activate expression of more posterior genes, the normal role of retinoids in anteroposterior (A/P) patterning is unclear. HoxD1 is an early posterior neurectodermal marker, expressed from midgastrula with a later anterior expression limit in the future hindbrain. We previously showed that HoxD1 was induced as an immediate early response to retinoic acid in naive ectoderm (animal caps). Here, we use a truncated RARalpha2.2 receptor (RARDelta) to dominantly interfere with retinoid signaling. In embryos injected with RARDelta expression of HoxD1 is eliminated. Conjugates of ectoderm and dorsolateral mesoderm show that retinoid receptors are required in the ectoderm for HoxD1 induction. Further, expression of Krox-20 in r3 and r5 of the presumptive hindbrain is compressed into a single stripe that suggests elimination of r5. RARalpha2.2 expression almost precisely overlaps that of HoxD1, suggesting that this receptor may normally activate HoxD1. Expression of neither more anterior genes including cement gland, forebrain, and midbrain markers nor a more posterior spinal cord marker is affected by RARDelta. These data suggest that the posterior hindbrain is the region of the nervous system most sensitive to retinoid loss. Finally, we compare the ability of RA and fibroblast growth factor (FGF) to posteriorize isolated anterior neurectoderm and show that both factors can act directly on this substrate. RA acts in a more anterior domain than does FGF; however, neither factor is equivalent to the natural posteriorizing capacity of the posterior mesoderm. We propose that endogenous retinoid and FGF signals pattern largely nonoverlapping regions along the A/P axis and that posterior neural patterning requires multiple inducers.

Patterning of the embryo along the anterior-posterior axis: the role of the caudal genes

Patterning along the anterior-posterior axis takes place during gastrulation and early neurulation. Homeobox genes like Otx-2 and members of the Hox family have been implicated in this process. The caudal genes in Drosophila and C. elegans have been shown to determine posterior fates. In vertebrates, the caudal genes begin their expression during gastrulation and they take up a posterior position. By injecting sense and antisense RNA of the Xenopus caudal gene Xcad-2, we have studied a number of regulatory interactions among homeobox genes along the anterior-posterior axis. Initially, the Xcad-2 and Otx-2 genes are mutually repressed and, by late gastrulation, they mark the posterior- or anterior-most domains of the embryo, respectively. During late gastrulation and neurulation, Xcad-2 plays an additional regulatory function in relation to the Hox genes. Hox genes normally expressed anteriorly are repressed by Xcad-2 overexpression while those normally expressed posteriorly exhibit more anterior expression. The results show that the caudal genes are part of a posterior determining network which during early gastrulation functions in the subdivision of the embryo into anterior head and trunk domains. Later in gastrulation and neurulation these genes play a role in the patterning of the trunk region.

Specification of the zebrafish nervous system by nonaxial signals

The organizer of the amphibian gastrula provides the neurectoderm with both neuralizing and posteriorizing (transforming) signals. In zebrafish, transplantations show that a spatially distinct transformer signal emanates from tissues other than the organizer. Cells of the germring (nonaxial mesendoderm) posteriorized forebrain progenitors when grafted nearby, resulting in an ectopic hindbrain-like structure; in contrast, cells of the organizer (axial mesendoderm) caused no posterior transformation. Local application of basic fibroblast growth factor, a candidate transformer in Xenopus, caused malformation but not hindbrain transformation in the forebrain. Thus, the zebrafish gastrula may integrate spatially distinct signals from the organizer and the germring to pattern the neural axis.

Expression of Pax-3 is initiated in the early neural plate by posteriorizing signals produced by the organizer and by posterior non-axial mesoderm

Pax-3 is a paired-type homeobox gene that is specifically expressed in the dorsal and posterior neural tube. We have investigated inductive interactions that initiate Pax-3 transcript expression in the early neural plate. We present several lines of evidence that support a model where Pax-3 expression is initiated by signals that posteriorize the neuraxis, and then secondarily restricted dorsally in response to dorsal-ventral patterning signals. First, in chick and Xenopus gastrulae the onset of Pax-3 expression occurs in regions fated to become posterior CNS. Second, Hensen's node and posterior non-axial mesoderm which underlies the neural plate induce Pax-3 expression when combined with presumptive anterior neural plate explants. In contrast, presumptive anterior neural plate explants are not competent to express Pax-3 in response to dorsalizing signals from epidermal-ectoderm. Third, in a heterospecies explant recombinant assay with Xenopus animal caps (ectoderm) as a responding tissue, late, but not early, Hensen's node induces Pax-3 expression. Chick posterior non-axial mesoderm also induces Pax-3, provided that the animal caps are neuralized by treatment with noggin. Finally we show that the putative posteriorizing factors, retinoic acid and bFGF, induce Pax-3 in neuralized animal caps. However, blocking experiments with a dominant-inhibitory FGF receptor and a dominant-inhibitory retinoic acid receptor suggest that Pax-3 inductive activities arising from Hensen's node and posterior non-axial mesoderm do not strictly depend on FGF or retinoic acid.