Evaluation of AAV-Mediated Expression of Chop2-GFP in the Marmoset Retina

PURPOSE: Converting inner retinal neurons to photosensitive cells by expressing channelrhodopsin-2 (ChR2) offers a novel approach for treating blindness caused by retinal degenerative diseases. In the present study, the recombinant adeno-associated virus serotype 2 (rAAV2)-mediated expression and function of a fusion construct of channelopsin-2 (Chop2) and green fluorescent protein (GFP) (Chop2-GFP) were evaluated in the inner retinal neurons in the common marmoset Callithrix jacchus. METHODS: rAAV2 vectors carrying ubiquitous promoters were injected into the vitreous chamber. Expression of Chop2-GFP and functional properties of ChR2 were examined by immunocytochemical and electrophysiological methods 3 months after injection. RESULTS: The percentage of Chop2-GFP-expressing cells in the ganglion cell layer was found to be retinal region- and animal age-dependent. The highest percentage was observed in the far-peripheral region. Chop2-GFP expression was also found in the foveal and parafoveal region. In the peripheral retina in young animals with high viral concentrations, the expression of Chop2-GFP was observed in all major classes of retinal neurons, including all major types of ganglion cells. The morphologic properties of Chop2-GFP-positive cells were normal for at least 3 months, and ChR2-mediated light responses were demonstrated by electrophysiological recordings. CONCLUSIONS: The rAAV2-mediated expression of ChR2 was observed in the inner retinal neurons in the marmoset retina through intravitreal delivery. The marmoset could be a valuable nonhuman primate model for developing ChR2-based gene therapy for treating blinding retinal degenerative diseases.