Brain-derived neurotrophic factor and TrkB tyrosine kinase receptor gene expression in zebrafish embryo and larva

The genes that encode the neurotrophin family of secreted polypeptides and the Trk family of high affinity neurotrophin transmembrane protein tyrosine kinase receptors are induced at the time of neurogenesis in mammals and ar e known to play critical roles in nervous system development. We show here that in contrast to mammals, the genes encoding the neurotrophin brain-deri ved neurotrophic factor (BDNF) and the neurotrophin receptor TrkB are expre ssed throughout embryonic development in the zebrafish, At the embryonic st ages preceding transcription of endogenous genes all cells contain BDNF tra nscripts and immunoreactive BDNF and the trkB transcripts lack the region t hat encodes a kinase domain. As development proceeds, progressively fewer c ells contain BDNF transcripts and by the time of neurogenesis the trkB tran scripts encode a kinase-domain. In the 4-day-old larva, a small subset of s pecialized sensory cells on the surface and cells in deeper structures incl uding the gill arches, fin, and cloaca express the BDNF gene at high levels in a promoter-specific fashion. This progressive restriction of BDNF gene expression must involve an extinction of BDNF gene transcription in some an d induction of high levels of transcription in a promoter-specific fashion in other calls. (C) 2001 ISDN. Elsevier Science Ltd. All rights reserved.