Receptor Tyrosine Kinase B (trkB) and Photoreceptor Function, an Update Using Amacrine and RPE Cell-Specific trkB Knockout Mice

Abstract

Abstract: : Purpose: Elimination of trkB in the mouse retina interferes with normal developmental maturation of rod photoreceptors. Quantitative analysis of electroretinograms (ERG), measured in postnatal day 16 (P16) trkB knockout (trkB^-/-) mice, demonstrated the absence of a detectable b-wave, in the presence of an a-wave and an anatomically intact inner retina (Rohrer et al., 1999). Mouse rods do not express TrkB receptors, thus this indirect effect could be mediated by retinal pigment epithelial (RPE), Müller, horizontal and/or amacrine (AC) cells, all of which express TrkB receptors. We wished to further analyze the potential contribution of trkB-expressing cells to rod development and function, by creating and analyzing cell-specific trkB knockout mice. Methods: Mice carrying a floxed trkB gene (trkB^fbz/fbz; Xu et al., 2001; Rohrer, 2001) were crossed with those expressing Cre-recombinase under a cell-specific promoter (dopaminergic AC (DA-AC): tyrosine hydroxylase (TH) and RPE: tyrosinase (Tyr)). Resulting littermates (P16-30) were analyzed using scotopic single flash-ERGs. Light sensitivity and kinetics of a- and b-waves were analyzed and compared to previously obtained data from trkB^-/- animals. Immunohistochemistry was performed to monitor successful Cre-recombinase activity. Results: (A) ERG analysis of trkB^fbz/fbz and trkB^fbz/fbz x Tyr-Cre littermates revealed comparable ERG amplitudes and kinetics. (B) On the other hand, eliminating trkB from DA-ACs (trkB^fbz/fbz x TH-Cre) resulted in minimal TH-levels in those cells, levels which were comparable to those found in trkB^-/- animals. The comparison with trkB^fbz/fbz littermates revealed a significant increase in the a-wave threshold and reduced amplitudes (p<0.01). Interestingly, the trkB^fbz/fbz x TH-Cre ERGs at medium light intensities (8.5 x 10⁹ photons/mm²) were reminiscent of the b-waveless trkB^-/- ERG; whereas at the highest light intensity measured (3.4 x 10¹¹ photons/mm²), small and extremely slow b-waves could be recorded. Conclusion: Results from the cell-specific trkB knockout mice demonstrate that TrkB-signaling in RPE cells does not contribute to the rod phenotype observed in the trkB^-/- mouse. They do, however, support the hypothesis that dopamine is important for the transmission of light responses from photoreceptors to inner retinal neurons.

Keywords: electroretinography: non-clinical • dopamine • growth factors/growth factor receptors