5 TRK RECEPTORS IN THE ZEBRAFISH

Using a homology-based cloning strategy we have identified five member s of the Trk family in the zebrafish Danio rerio. They are homologous to the three mammalian Trk receptors in their conserved intracellular kinase regions and the organization of their extracellular regions. Th e five trk. genes are differentially expressed in the developing brain , spinal cord, cranial ganglia, and retina. Full-length forms of three of the trk genes are expressed when neurons pioneer the major axon tr acts, whereas the two other trk genes have a later onset of expression . Truncated transcripts and forms containing an extracellular juxtamem brane region insert were found. The degree of sequence variation and e xpression differences within the family suggest that each of the five zebrafish Trk receptors have a functionally distinct role. These findi ngs demonstrate that the vertebrate Trk family is larger than previous ly appreciated. (C) 1995 Academic Press, Inc.