INVOLVEMENT OF WNT1 AND PAX2 IN THE FORMATION OF THE MIDBRAIN-HINDBRAIN BOUNDARY IN THE ZEBRAFISH GASTRULA

The secreted signalling molecule encoded by the wnt1 gene and the pair ed box-containing pax2 gene are thought to play an integral role in pa tterning the zebrafish rostral nervous system. Using a double-label an alysis, we compare the expression patterns of wnt1 RNA and pax2 protei n during zebrafish embryogenesis to determine whether they were expres sed in identical or overlapping patterns in individual embryos. During gastrulation, wnt1 RNA was detected in a pattern similar but not iden tical to the pax2 protein. Later, wnt1 and pax2 co-localize to the mid brain-hindbrain boundary. Exogenous retinoic acid, a teratogen that is known to affect the formation of the midbrain-hindbrain boundary, has a profound affect on both wnt1 and pax2 expression at gastrulation. F urthermore, when pax2 is overexpressed in zebrafish embryos, the wnt1 pattern of expression expands ventrally in the prospective rostral neu roepithelium. Despite the widespread and random distribution of exogen ous pax2 RNA, it alone is unable to induce wnt1 expression in other ec topic sites. These results are consistent with the coordinate expressi on of wnt1 and pax2 being in a pathway responsible for establishing th e midbrain-hindbrain boundary and support the earlier interpretation t hat pax2 may regulate wnt1 expression [Krauss et al., 1992], although only in a subset of embryonic cells. These data suggest that a predisp osition for the regionalization of the central nervous system exists a t gastrulation. (C) 1995 Wiley-Liss, Inc.