C1q enhances cone photoreceptor survival in a mouse model of autosomal recessive retinitis pigmentosa

Abstract

Retinitis pigmentosa (RP) is a degenerative retinal disease involving progressive loss of rod and cone photoreceptor function. It represents the most common form of registered blindness among the working aged populations of developed countries. Given the immense genetic heterogeneity associated with this disease, parameters influencing cone photoreceptor survival (preservation of daytime vision) that are independent of primary mutations are exceedingly important to identify from a therapeutic standpoint. Here we identify C1q, the primary component of the classical complement pathway, as a cone photoreceptor neuronal survival factor.

Figure 1

Cone ERG responses from wild-type (WT), C1qa−/−, Rho−/−, and Rho−/−C1qa−/− mice. The timing of the major positive deflection (the b-wave) is 52 ms in each tracing. (a) Cone ERG responses from C1qa−/− and WT mice at 12 weeks of age. (b) Cone ERG rsponses were still evident in Rho−/− mice (left panel), with average readings of 33.48 μV (n=55). However, in C1qa−/−Rho−/− mice (right panel), these readings were decreased to an average of 19.86 μV at 12 weeks of age (n=34). (c) The decrease in cone-isolated ERG was highly significant, with ^**P≤0.0026. Cone ERG responses are representative of nine different animals per group.

Figure 2

Comparative retinal histology in WT, Rho−/−, and Rho−/−C1qa−/− mice. (a) The presence and pattern of cone photoreceptors were analyzed in WT, Rho−/−, and C1qa−/−Rho−/− mice. Although there was positive peanut agglutinin staining in Rho−/− and C1qa−/−Rho−/− mice at 12 weeks of age, the pattern and distribution of staining appeared radically different in C1qa−/−Rho−/− mice when compared with Rho−/− mice. (b) Retinal cryosections from 12-week-old mice were stained with an antibody specific for blue-sensitive opsin. Strong immunoreactivity was observed in the WT sections, staining blue cone photoreceptors in the central and peripheral aspects of the retina, and clearly showing the distribution of cone photoreceptors in the mouse retina (red: blue-sensitive opsin; blue: DAPI-nuclei). Although not as widespread, positive immunoreactivity for blue-sensitive opsin was also evident in Rho−/− mice. However, in C1qa−/−Rho−/− mice, strong immunoreactivity in cryosections for blue-sensitive opsin was not evident.

Figure 3

Quantitative analysis of levels of C1q transcript in normal and Rho−/− mice. Levels of C1qa transcript became significantly upregulated in the retinas of Rho−/− mice at 30 days when compared with WT mice of the same age. Upregulation continued up to and including 90 days (n=3 mice per group, and results representative of three replicate experiments).

Figure 4

Statistical analysis of cone ERG responses from C3−/−Rho−/− and Rho−/− male and female mice at 3 months of age (P=0.319; n=20 per group). The data showed no significant differences between both groups of animals, indicating that cone photoreceptor function was essentially the same in both experimental cohorts. Data was analyzed as mean±SD with significance represented by P<0.05.