Expression of TrkB receptor isoforms in the developing avian visual system

Abstract

The expression of novel TrkB receptor transcripts has been characterized to understand the potentially diverse roles of brain-derived neurotrophic factor (BDNF) in the developing avian visual system. In situ localization with an extracellular domain probe common to all TrkB transcripts labeled a sub-population of large retinal ganglion cells as well as many associated visual nuclei, including the neuronal layers within the tectum that receive retinal innervation. Because of the potential for structurally and functionally distinct receptors derived from the TrkB gene locus, cDNA cloning and reverse transcription-PCR analysis were used to further analyze receptor isoform expression in the retina and tectum. Receptor isoforms were sequenced that contained a deletion of the N terminus, a deletion in the putative ligand-binding domain, or a deletion in the cytoplasmic juxtamembrane (JM) domain. Two novel JM insertion sequences also were identified, one of which exhibits weak homology to beta-actin and was found in both kinase-containing (TK+) and kinase deletion (KD) receptor isoforms. In the developing retina, TK+ receptor mRNA is upregulated during the period of retinal ganglion cell (RGC) death, consistent with the proposed role of BDNF as a tectal-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 expressed in both the retina and tectum, it is conceivable that TrkB also mediates autocrine/paracrine signaling within these structures or anterograde retinotectal trophic support.

A TrkB/EphrinA Interaction Controls Retinal Axon Branching and Synaptogenesis

“…Use of forskolin results in an increase in cAMP levels in retinal axons, which correlates with an enhanced long-term survival via an increased surface localization of TrkB (MeyerFranke et al, 1995(MeyerFranke et al, , 1998. Retinal ganglion cells (RGCs) were identified by staining for markers expressed on retinal axons such as TrkB, Brn3A, and Thy-I (Sheppard et al, 1991;Xiang et al, 1995;Garner et al, 1996). Routinely, those cells/axons with the longest axons were positive for these markers.…”

“…1). The electroporated cells were plated at low density on laminin/merosin-coated dishes and were cultured for the next 4 d. RGCs and their axons were identified by staining for markers expressed on retinal axons such as TrkB, Brn3A, and Thy-I (Sheppard et al, 1991;Xiang et al, 1995;Garner et al, 1996). Cells with the longest axons stained positive for these markers.…”

Contributions of the Optic Tectum and the Retina as Sources of Brain-Derived Neurotrophic Factor for Retinal Ganglion Cells in the Chick Embryo

“…Several isthmic and diencephalic nuclei that innervate the SGC or SGFS depend on an intact retinotectal circuitry during development (Ostrach and Mathers, 1979;Wizenmann and Thanos, 1990;Clarke, 1992;Page et al, 1993), and many of them express the BDNF receptor trkB during development (Garner et al, 1996), including the relevant stages during the third week of incubation for the isthmo-optic nucleus (ION; von Bartheld et al, 1996b) and the parvocellular isthmic nucleus (Ipc; R. Williams, Karolinska Institute, personal communication). Therefore, BDNF produced by tectal neurons may be transported anterogradely or retrogradely and used by isthmic or thalamic nuclei.…”

“…The retinal concentration of BDNF after injection was ϳ100 -500 times higher (total BDNF ϭ 5-10 ng) than the concentration of endogenous BDNF at this age (total BDNF/retina Ϸ 50 pg) ; J. E. Johnson, Wake Forest University, personal communication). Alternative explanations of BDNF immunoprecipitation with trkC antibodies include the possibilities that heterodimers of exogenous BDNF and endogenous NT-3 form that bind to trkC (Jungbluth et al, 1994;Philo et al, 1994), that trk and p75 may coprecipitate (Huber and Chao, 1995), or that the trkC antibody may recognize certain isoforms of trkB (Garner et al, 1996) although it does not recognize the predominant form of trkB (Lefcort et al, 1996).…”

“…Most retinal ganglion cells (RGC) degenerate after target removal or optic stalk transection in chicken (Hughes and LaVelle, 1975;Vanselow et al, 1990). RGC can be maintained by brain-derived neurotrophic factor (BDN F) in vitro (Johnson et al, 1986;Rodríguez-Tébar et al, 1989;Cohen-Cory and Fraser, 1994), and cells in the RGC layer express the BDNF receptor trkB (Okazawa et al, 1993(Okazawa et al, , 1995Perez and Caminos, 1995;Rickman and Brecha, 1995;Cohen-Cory et al, 1996;Garner et al, 1996;Hallböök et al, 1996). The expression of BDNF mRNA in target regions and its upregulation by physiological input further support the view that BDNF derived from the target plays a role in the maintenance of RGC (Castrén et al, 1992;Cohen-Cory and Fraser, 1994;Schoups et al, 1995) (for review, see von Bartheld, 1998a), but the retrograde transport of BDNF from the tectum to the retina has not been shown in the developing visual system.…”