Retinal degeneration is the leading cause of untreatable blindness, affecting one out of 3,000 people in developed countries. Loss of cone photoreceptors, which are largely responsible for daylight and color vision, is particularly devastating. Although retinal degeneration cannot currently be treated, one of the prospective therapies is retinal cell transplantation. A group of scientists led by Jane Sowden, Ph.D., of the University College London (UCL) is working toward that end. Using marked retinal cells from a unique reporter mouse, Sowden and her team (Sowden et al. 2010) demonstrated that both transplanted rods and cones can integrate into the retinas of both normal mice and mice with congenital blindness that models Leber congenital amaurosis (LCA).
Transplanted photoreceptor precursors integrate into sighted mice
First, Sowden and her colleagues confirmed that green fluorescent protein (GFP) expression in B6.SJL-Tg(Crx-GFP,-ALPP)1Clc/J (007066) mice – hereafter called CrxGFP mice – faithfully labels the retinal cells that differentiate into rods and cones. This allowed the UCL team to track retinal cells from these mice after they are transplanted in recipient mice. The team then sorted and transplanted GFP-expressing cells from CrxGFP retinas in various stages of embryonic and postnatal development into the sub-retinal space of adult C57BL/6J (B6J, 000664) mice. By doing this, the team could determine if transplanted cells from a particular stage of retinal development are more likely to integrate into recipient mouse retinas. Three weeks post-transplantation, GFP-expressing cells with typical rod and cone morphologies had integrated into the outer nuclear layer of the recipient B6J retinas. Sowden and her colleagues found that approximately 10 times more of the transplanted cells integrate if they originate from postnatal retinas. Regardless of the developmental stage of the CrxGFP retinas, however, at least 100-fold more rod than cone precursors integrate, although cone precursor integration improves if the transplanted cells are from embryonic retinas. Although the majority of CrxGFP cells integrate as rods, immunostaining revealed that they are actually cone precursors and would develop into cones in the native CrxGFP retinas. The team performed the same transplantation experiments in immature pre-wean B6J pups and obtained similar results, indicating that the maturity of the recipient retinal environment does not affect photoreceptor precursor integration.
Transplanted photoreceptor precursors integrate into blind mice
The UCL team then tested the ability of CrxGFP retinal cells to integrate into two mouse models of LCA – strains B6;129-Crb1rd8/J (004852) and B6;129S6-Gucy2etm1Gar/J (004953). In humans, mutations in CRB1 cause LCA and retinitis pigmentosa. In mice, Crb1 deficiency causes progressive retinal degeneration, resulting in blindness. In humans, mutations in GUCY2D, the homologue of the mouse Gucy2e gene, cause type 1a LCA, characterized by rod-cone degeneration. In mice, Gucy2e-deficiency causes cone dystrophy. Rods are not as affected.
Sowden's team found that transplanted embryonic CrxGFP retinal cells integrate well (again, mostly as rods with typical rod morphologies) into the outer nuclear layer of Crb1-deficient mice. In fact, Crb1rd8/rd8 retinas appear to integrate new photoreceptors better than do normal-sighted B6J mice. Transplanted embryonic CrxGFP retinal cells also integrate well into Gucy2e-deficient mice. However, cone cell integration is 13 times higher in these mice than in B6J mice, suggesting that the loss of both cones and the normal photoreceptor type ratio favors cone migration and integration.
Although the recipient mice in these experiments did not integrate enough rods and cones to allow the UCL team to detect an increase in eye function by electroretinogram, the team clearly demonstrated that transplantation and integration of rods and cones is possible. The integrated cells exhibit all the biochemical and structural features of normal photoreceptor cells and integrate into the recipient retina in the normal 35 rod:1 cone ratio. The Sowden team has set the stage. With more research, better understanding and improved technologies, the longstanding goal of using retinal cell transplantation to treat retinal degeneration may one day be a reality.
Reference
Lakowski J, Baron M, Bainbridge J, Barber AC, Pearson RA, Ali RR, Sowden JC. 2010. Cone and rod photoreceptor transplantation in models of the childhood retinopathy Leber congenital amaurosis using flow-sorted Crx-positive donor cells. Hum Mol Genet 19(23):4545-59.