EXPRESSION OF THE LIM CLASS HOMEOBOX GENE XLIM-1 IN PRONEPHROS AND CNS CELL LINEAGES OF XENOPUS-EMBRYOS IS AFFECTED BY RETINOIC ACID AND EXOGASTRULATION

The LIM class homeobox gene Xlim-1 is expressed in Xenopus embryos in the lineages leading to (i) the notochord, (ii) the pronephros, and (i ii) certain cells of the central nervous system (CNS). In its first ex pression phase, Xlim-1 mRNA arises in the Spemann organizer region, ac cumulates in prechordal mesoderm and notochord during gastrulation, an d decays in these tissues during neurula stages except that it persist s in the posterior tip of the notochord. In the second phase, expressi on in lateral mesoderm begins at late gastrula, and converges to the p ronephros at tailbud stages. Expression in a central location of the n eural plate also initiates at late gastrula, expands anteriorly and po steriorly, and becomes established in the lateral regions of the spina l cord and hindbrain at tailbud stages. Thus Xlim-1 expression precede s morphogenesis, suggesting that it may be involved in cell specificat ion in these lineages. Enhancement of Xlim-1 expression by retinoic ac id (RA) was first detectable in the dorsal mesoderm at initial gastrul a. During gastrulation and early neurulation, RA strongly enhanced Xli m-1 expression in all three lineages and also expanded its expressing domains; this overexpression correlated well with RA phenotypes such a s enlarged pronephros and hindbrain-like structure. Exogastrulation re duced Xlim-1 expression in the lateral mesoderm and ectoderm but not i n the notochord, suggesting that the second phase of Xlim-1 expression requires mesoderm/ectoderm interactions. RA treatment of exogastrulae did not revert this reduction.