Adeno-associated virus serotype-9 mediated retinal outer plexiform layer transduction is mainly through the photoreceptors.

Due to its high ocular transduction, low immune clearance and capability to bypass the brain blood barrier, adeno-associated virus-9 (AAV9) has been regarded as a promising vector for retinal disease gene therapy. We recently demonstrated that AAV9 efficiently transduces the retinal outer plexiform layer (OPL). The OPL consists of synapses formed between axons of the rod and cone photoreceptors (cell bodies in the outer nuclear layer, ONL) and dendrites of bipolar and horizontal cells (cell bodies in the inner nuclear layer, INL). It is not clear whether AAV9 transduces the OPL through the photoreceptors in the ONL or through bipolar and horizontal cells in the INL. To map the subcelluar pathway(s) involved in AAV9-mediated OPL transduction, we delivered subretinally AAV9.CMV.eGFP, an AAV vector carrying the enhanced green fluorescent protein gene (eGFP, 1 x 10(10) viral genome particles in microliter), to young (21-day-old) and adult (2- to 3-month-old) C57BL/6 mice. Four weeks after subretinal injection, eGFP expression was examined on retinal cryosections. PSD95 (postsynaptic density protein, a marker for photoreceptor terminals), CtBP2 (C-terminal binding protein 2, a marker for the photoreceptor synaptic ribbon), PKCalpha (protein kinase Calpha, a marker for rod bipolar cells), and calbindin (a marker for horizontal cells) were localized by immunofluorescence staining. In AAV9 infected retina, eGFP expression was seen in the retinal pigment epithelia, photoreceptor inner segments, ONL, OPL, Müller cells in the INL, inner plexiform layer and ganglion cell layer. Interestingly, eGFP expression co-localized with PSD95 and CtBP2, but not with PKCalpha and calbindin. Our results suggest that AAV9 transduces the photoreceptor side of the synapses in the OPL rather than the dendrites of bipolar and horizontal cells.