EXPRESSION OF PAX-3 IS INITIATED IN THE EARLY NEURAL PLATE BY POSTERIORIZING SIGNALS PRODUCED BY THE ORGANIZER AND BY POSTERIOR NONAXIAL 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 ne ural plate, We present several lines of evidence that support a model where Pax-3 expression is initiated by signals that posteriorize the n euroaxis, and then secondarily restricted dorsally in response to dors al-ventral patterning signals. First, in chick and Xenopus gastrulae t he onset of Pax-3 expression occurs in regions fated to become posteri or CNS, Second, Hensen's node and posterior non-axial mesoderm which u nderlies the neural plate induce Pax-3 expression when combined with p resumptive anterior neural plate explants. In contrast, presumptive an terior neural plate explants are not competent to express Pax-3 in res ponse to dorsalizing signals from epidermal-ectoderm. Third, in a hete rospecies explant recombinant assay with Xenopus animal caps (ectoderm ) as a responding tissue, late, but not early, Hensen's node induces P ax-3 expression, Chick posterior non-axial mesoderm also induces Pax-3 , provided that the animal caps are neuralized by treatment with noggi n, Finally we show that the putative posteriorizing factors, retinoic acid and bFGF, induce Pax-3 in neuralized animal caps. However, blocki ng experiments with a dominant-inhibitory FGF receptor and a dominant- inhibitory retinoic acid receptor suggest that Pax-3 inductive activit ies arising from Hensen's node and posterior non-axial mesoderm do not strictly depend on FGF or retinoic acid.