• retinal ganglion cells
• retina
• rd10 mutant
• photoreceptor degeneration
• neural remodeling
• GFP
• retinitis pigmentosa
• transneuronal effects

Retinitis Pigmentosa (RP), a family of inherited diseases leading to progressive photoreceptor death, is one of the major causes of blindness in the world, with no cure yet.
Albeit the primary cause of the disease is, typically, a defect in a photoreceptor-specific gene, it has been shown that the degeneration of rods and cones triggers remodeling and secondary death of inner retinal neurons, and particularly of bipolar and horizontal cells. Aim of this project is to test the hypothesis that, as an effect of photoreceptor progressive loss, concomitant changes also occur in retinal ganglion cells (RGCs), the last neurons of the retinal visual pathway and the only exit of retinal information to higher brain centers.
We assessed the retention of morphology, overall architecture and survival rate of RGCs in a mouse model of RP, at various stages of the disease progression. Specifically, we generated a transgenic mouse, the rd10/Thy1-GFP mutant, by crossing GFP-M mice, in which GFP is expressed in a small population of RGCs of various types, and rd10 mice, a recent model of autosomal recessive RP. Rd10 mice carry a missense mutation of the beta-subunit of the rod-specific phosphodiesterase gene, causing a typical rod-cone degeneration with a peak at postnatal day 24 (P24). The expression of GFP in a small number of RGCs in the retina of the mutant allowed the detailed study of the fine structure of these neurons at various ages.
By combining immunocytochemistry, confocal microscopy and analytic morphometry, we studied RGCs in a total of 50 whole mounted retinas of 3 age groups (3, 7 and 9 months of age, thus past the complete degeneration of photoreceptors). A number of 572 RGCs were identified and grouped according to the classification of Sun et al. (2002a). In particular, 4 parameters were taken into account to identify RGCs: the diameter of the dendritic tree, the diameter of the body, the mean stratification depth within the inner plexiform layer and the typical shape of the dendritic arborization. Five RGCs of the same type, at each time point, were drawn three dimensionally with a computer assisted image analyzer. The neuronal tracings were mathematically evaluated to obtain some parameters highly indicative of dendritic tree complexity and fine architecture: the total dendritic length, the total number of nodes and the dendritic tree area. Eight different types of RGCs were analyzed and drawn (for a total of 164 cells), including type A (the largest), B (the smallest) and C (medium sized), both ON and OFF.
We found a remarkable preservation of the structural complexity of all the types of RGCs studied, up to 9 months of age, even in the occurrence of major remodeling among second order neurons. In addition, at 9 months of age, the survival of cells in the GCL appeared comparable to the wt counterpart. Finally, still at 9 months, injections of cholera toxin in the eyes demonstrated the presence of anterograde axonal transport of RGCs to the lateral geniculate nucleus and to the superior colliculus. Yet, at the same age, we detected a decrease of 30% in the density of retinal blood vessels providing nourishment to ganglion cells.
The remarkable, long term preservation of RGC fine structure, survival and capability of anterograde axonal transport in the retina of rd10 mutant mice, despite blood vessel impoverishment and second order neurons remodeling and degeneration, opens perspectives to therapy for RP based on ganglion cells preservation. In particular, the strategy of implanting epiretinal prostheses, directly stimulating ganglion cells, to restore vision in RP patients could be applied successfully in those cases in which ganglion cells are still viable, such in the case of the rd10 mutation.