Neuronal and Neuroendocrine Expression oflim3,a LIM Class Homeobox Gene, Is Altered in Mutant Zebrafish with Axial Signaling Defects

Abstract

LIM class homeobox genes code for a family of transcriptional regulators that encode important determinants of cell lineage and cell type specificity. Thelim3gene from the zebrafish,Danio rerio,is highly conserved in sequence and expression pattern compared to its homologs in other vertebrates. In this paper we report immunocytochemical analysis of Lim3 protein expression in the pituitary, pineal, hindbrain, and spinal cord of the embryo, revealing an asymmetrical, lateral and late program of pituitary development in zebrafish, distinct from the pattern in other vertebrates. We studied Lim3 expression inno tail, floating head,andcyclopsmutant embryos, all of which have midline defects, with special reference to spinal cord differentiation where Lim3 marks mostly motoneurons.cyclopsembryos showed essentially normal Lim3 expression in the hindbrain and spinal cord despite the absence of the floor plate, whileno tailmutant embryos, which lack a differentiated notochord, displayed an excess of Lim3-expressing cells in a generally normal pattern. In contrast, Lim3-positive cells largely disappeared from the posterior spinal cord infloating headmutants, except in patches that correlated with remnants of apparent floor plate cells. These results support the view that either notochord or floor plate signaling can specify Lim3-positive motoneurons in the spinal cord.