TrkB Receptor Delivered by Adeno–Associated Viruses Delays Photoreceptor Degeneration in Rhodopsin Knockout Mice

Abstract

Abstract: : Purpose: Brain–derived neurotrophic factor (BDNF) binds its cognate receptor tyrosine kinase B (TrkB) and promotes neuronal survival. Rod photoreceptors do not constitutively express the TrkB receptor. The lack of TrkB expression in photoreceptors may limit their responses to increased levels of BDNF in the degenerating retina, hence compromising their survival. We hypothesize that increased expression of TrkB in photoreceptors can promote their survival in animal models of retinitis pigmentosa. This study aimed to test this hypothesis using the rhodopsin knockout (opsin–/–) mice. Methods: Adeno–associated viruses (AAV) were made to express a truncated form of TrkB (TrkB.t) under the transcriptional control of CMV promoter. The TrkB was tagged with either FLAG or GFP to distinguish from endogenous TrkB. The opsin–/– mice were subretinally injected with AAV.TrkB in one eye and AAV.EGFP or sham injection in the contralateral eye at age 2 postnatal weeks. Retinal structures were examined histologically 3 month of postinjection and the thickness of outer nuclear layer (ONL) was analyzed morphometrically. Results: Histological examination showed photoreceptor preservation in the temporal region of the retina which had been exposed to AAV–TrkB. There was no apparent rescue in the nasal (unexposed) region of the same eye and the eyes treated with AAV.EGFP or sham injection. Morphometric analysis showed that the number of rows of nuclei in the ONL was significantly greater in AAV.TrkB–injected regions (3.1±;1.7 rows) compared to the noninjected nasal retinas or the corresponding regions of the contralateral eye (1.2±;0.7, p=0.016). Although photoreceptor nuclei were preserved in AAV.TrkB–injected areas, outer segments of photoreceptors were still absent, and rod and cone ERG amplitudes were not significantly different between AAV.TrkB injected and control eyes. Conclusions: Here we demonstrated a rescue effect of TrkB receptor in the degenerative retina. It should be possible to use this paradigm to assess the ability of TrkB to rescue photoreceptors in other animal models of retinal degeneration.

Keywords: gene transfer/gene therapy • photoreceptors • retinal degenerations: cell biology