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. 2010 Dec;18(12):2057-63.
doi: 10.1038/mt.2010.149. Epub 2010 Jul 13.

Restoration of Cone Vision in the CNGA3-/- Mouse Model of Congenital Complete Lack of Cone Photoreceptor Function

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Free PMC article

Restoration of Cone Vision in the CNGA3-/- Mouse Model of Congenital Complete Lack of Cone Photoreceptor Function

Stylianos Michalakis et al. Mol Ther. .
Free PMC article

Abstract

Congenital absence of cone photoreceptor function is associated with strongly impaired daylight vision and loss of color discrimination in human achromatopsia. Here, we introduce viral gene replacement therapy as a potential treatment for this disease in the CNGA3(-/-) mouse model. We show that such therapy can restore cone-specific visual processing in the central nervous system even if cone photoreceptors had been nonfunctional from birth. The restoration of cone vision was assessed at different stages along the visual pathway. Treated CNGA3(-/-) mice were able to generate cone photoreceptor responses and to transfer these signals to bipolar cells. In support, we found morphologically that treated cones expressed regular cyclic nucleotide-gated (CNG) channel complexes and opsins in outer segments, which previously they did not. Moreover, expression of CNGA3 normalized cyclic guanosine monophosphate (cGMP) levels in cones, delayed cone cell death and reduced the inflammatory response of Müller glia cells that is typical of retinal degenerations. Furthermore, ganglion cells from treated, but not from untreated, CNGA3(-/-) mice displayed cone-driven, light-evoked, spiking activity, indicating that signals generated in the outer retina are transmitted to the brain. Finally, we demonstrate that this newly acquired sensory information was translated into cone-mediated, vision-guided behavior.

Figures

Figure 1
Figure 1
Restoration of cone-mediated electroretinogram (ERG) in treated CNGA3−/− mice. Traces are from representative animals, box plots summarize the group data. (a) Single flash ERG. Scotopic rod system response (top): No difference between the treated eye (TE), untreated eye (UE), and the wild-type (wt) eye. Mixed rod/cone system response (center): amplitude increase in the TE relative to the UE indicative for cone system function improvement. Photopic conditions (traces bottom left, corresponding box plot bottom right): substantial restoration of cone system function. (b) Scotopic 6 Hz flicker ERG intensity series. Left column: traces obtained at three selected stimulus intensities; right column: corresponding box plots. Top: no difference in the rod-specific response (1/2 size to facilitate comparison) between TE and UE, no cone system contribution (confirmed by rho−/−). Center: responses to an intermediate stimulus. The waveform difference between TE and UE (red versus black trace) is due to the time lag of the cone response (grey trace) relative to the rod response (black trace). Bottom: responses to a cone-specific bright stimulus. The difference between TE and UE is clearly visible. (c) Functional benefit from treatment as a function of ERG flicker frequency (flash intensity 3 cds/m²). Red (TE) and black (UE) traces represent amplitude data, the blue trace represents the functional benefit (FB) in percent calculated as (TE − UE)/TE amplitudes. At low frequencies up to about 2–3 Hz, the responses are dominated by the rod system, and at about 5 Hz and above, by the cone system, where the benefit of restoring cone functionality is maximal.
Figure 2
Figure 2
Establishment of cone cyclic nucleotide-gated (CNG) channel and restoration of opsin expression in treated CNGA3−/− cones. (a–c) Cone-specific expression of CNGA3 (red) in a treated area of a CNGA3−/− retina. (b,c) Co-labeling with the cone marker peanut agglutinin (PNA) indicates presence of rescued CNGA3 in cone outer segments (COS). (b) Merged image (PNA, green; CNGA3, red). (c) CNGA3 signal alone. (d–j) CNGA3 replacement restores expression and correct localization of visual cascade proteins. (d) Recovery of CNGB3 expression in COS in treated retina. (e) Absence of the CNGB3 subunit in untreated COS. Specific normalization of short wavelength opsin (f,i) and medium wavelength opsin (g,j) in COS of injected areas of the retina. (h) Representative image showing cone opsin (here medium wavelength) mislocalization in an age-matched untreated CNGA3−/− retina (arrows point to mislocalized opsin within the synapses). Bars = 20 µm in a–h; 100 µm in i–j. Horizontal bars in a–d and f–h) mark the outer-to-inner segment border. In a,d–e, and h–j nuclei are stained with Hoechst dye (blue).
Figure 3
Figure 3
Establishment of a functional visual cascade and delay of degeneration in treated CNGA3−/− cones. (a) Compiled overview image of a treated CNGA3−/− retina stained for cGMP (green) and CNGA3 (red). Cone photoreceptors outside of the injected region (dorsal) contain high levels of cGMP. Expression of CNGA3 protein normalizes cGMP. (b) Higher magnification image of the treatment-border area from another treated CNGA3−/− retina stained for cGMP (green) and CNGA3 (red). (c) Cone-specific cGMP accumulation (green) in the CNGA3−/− retina at postnatal day 4 (8 days before eye opening). (d,e) All cGMP-positive (d, green) cells in a 12-day-old CNGA3−/− retina are positive for the cone marker (e) glycogen phosphorylase (GlyPh, magenta). (f) Untreated mice show marked elevation of GFAP-positive (green) stress fibers in inner retinal layers. (g) Treatment keeps GFAP stress fibers at low levels. Note that Müller cell end feet within the neurofilanent layer are GFAP-positive in treated and untreated mice. (h,i) The treatment preserves a high number of cones within the ventral retina. Retinal slices of age-matched treated and untreated CNGA3−/− mice were stained with the cone marker peanut agglutinin (PNA, green) and anti-CNGA3 (red). Bar = 100 µm in a,h, and i; 20 µm in b–g. In c,h, and i, nuclei are stained with Hoechst dye (blue). cGMP, cyclic guanosine monophosphate; GFAP, glial fibrilary acid protein; INL, inner nuclear layer; IPL, inner plexiform layer; IS, (photoreceptor) inner segments; NFL, neurofilament layer; ONL, outer nuclear layer; OPL, outer plexiform layer; OS, (photoreceptor) outer segments.
Figure 4
Figure 4
Gene replacement therapy restores responsiveness of ganglion cells to photopic stimuli in CNGA3−/− mice. (a,b) Spike trains of different types of ganglion cells from (a) treated and (b) untreated CNGA3−/− mice. Spikes were measured in response to periodic flashes of light at three different intensity levels (left). For each condition, responses to 10 successive presentations are shown. Stimulus phase is indicated at the bottom. The spike trains obtained from the treated mice show reliable response patterns for all applied light intensities. By contrast, ganglion cells from untreated retinas do not respond to light flashes at the highest light level, which corresponds to photopic conditions. Instead, some cells at this light level display spontaneous activity, which is not locked to the stimulus presentation. (c,d) Sample voltage traces recorded from the ON cells shown in a and b, respectively.
Figure 5
Figure 5
Gene replacement therapy enables cone-mediated visual processing in CNGA3−/− mice. (a) Treated mice display cone-mediated vision in a behavioral test. Mice were trained to associate a red-colored cue with a stable visible platform (acquisition). Subsequently, the mice had to discriminate between two visible platforms (discrimination), a stable platform (positioned next to the red cue = correct choice) and a platform that sank when a mouse climbed onto it (positioned next to a green cue = incorrect choice). The graph shows the mean percentage of correct choices for six trials during the discrimination test. The dotted line indicates the chance level. Statistical significance of differences from comparisons with wild type is shown on top of bars (**P < 0.01; ns, nonsignificant). (b) Representative swim paths of wild-type (wt), CNGA3/CNGB1 double knockout (DKO), untreated CNGA3−/− (untreated), and treated CNGA3−/− (treated) mice.

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