DEVELOPMENTAL REGULATION OF ISLET-1 MESSENGER-RNA EXPRESSION DURING NEURONAL DIFFERENTIATION IN EMBRYONIC ZEBRAFISH

Islet-1 (Isl-1) is a LIM domain/ homeodomain-type transcription regula tor that has been originally identified as an insulin gene enhancer bi nding protein. Isl-1 is also expressed by subsets of neurons in the ce ntral nervous system of rat and chick embryos. We have cloned the Isl- 1 cDNA from zebrafish and examined its expression pattern using in sit u hybridization to whole-mount embryos. Isl-1 mRNA first appears immed iately after gastrulation in the polster, the cranial ganglia, and in Rohon-Beard neurons and ventromedial cells of the spinal cord. The exp ression by the ventromedial cells is segmentally repeated and becomes restricted to the one or two cells slightly anterior to the segment bo rders. Double staining by in situ hybridization and an antibody which stains most axons suggested that these segmentally distributed cells m ay be either the rostral primary motoneuron (RoP) or middle primary mo toneuron (MiP). This raises a possibility that Isl-1 may be involved d uring determination of subtype identities of the primary motoneurons. Furthermore, the specific Isl-1 mRNA expression in the spinal cord is under the control of the somites, since mutant embryo with defective s omite failed to maintain this pattern. (C) 1994 Wiley-Liss, Inc.