EXPRESSION OF TRKB RECEPTOR ISOFORMS IN THE DEVELOPING AVIAN VISUAL-SYSTEM

The expression of novel TrkB receptor transcripts has been characteriz ed to understand the potentially diverse roles of brain-derived neurot rophic factor (BDNF) in the developing avian visual system. In situ lo calization with an extracellular domain probe common to all TrkB trans cripts labeled a subpopulation of large retinal ganglion cells as well as many associated visual nuclei, including the neuronal layers withi n the tectum that receive retinal innervation. Because of the potentia l for structurally and functionally distinct receptors derived from th e TrkB gene locus, cDNA cloning and reverse transcription-PCR analysis were used to further analyze receptor isoform expression in the retin a and tectum. Receptor isoforms were sequenced that contained a deleti on of the N terminus, a deletion in the putative ligand-binding domain , or a deletion in the cytoplasmic juxtamembrane (JM) domain. Two nove l JM insertion sequences also were identified, one of which exhibits w eak homology to beta-actin and was found in both kinase-containing (TK +) and kinase deletion (KD) receptor isoforms. In the developing retin a, TK+ receptor mRNA is upregulated during the period of retinal gangl ion cell (RGC) death, consistent with the proposed role of BDNF as a t ectal-derived survival factor for RGCs. However, the expression of TK transcripts in the tectum indicates that this structure also contains cells responsive to BDNF throughout development. Because BDNF is expr essed in both the retina and tectum, it is conceivable that TrkB also mediates autocrine/paracrine signaling within these structures or ante rograde retinotectal trophic support.