Development of stem cell therapy to restore photopic vision

Hereditary retinal disease and age related macular degeneration (AMD) are major causes of irreversible blindness in Europe and involve the loss of the light sensitive cone and rod photoreceptor cells in the retina. Inherited retinal dystrophies affect 1 in 3,000, and age related macular degeneration (AMD) affects 1 in 10 people over the age of 60. The lack of effective treatments for these conditions means there is a requirement to develop new therapies. The replacement of lost photoreceptors by cell transplantation is one possible approach, but transplanted cells need to make functional connections with the host retina. We have recently demonstrated that transplantation of immature photoreceptor cells (precursors) from the developing retina into mouse models of retinal disease results in the integration of new rod photoreceptors that restore low light vision in mice with a non progressive form of inherited blindness. As human vision is dependent upon cone photoreceptors, which provide high acuity daylight and colour vision, as well as rod photoreceptors, which provide low light vision, cone transplantation is likely to be essential for clinical application. In late stage retinitis pigmentosa, despite loss of most rods, patients have useful vision until the last remaining cones degenerate. Furthermore, cone transplantation is critical for optimal retinal repair in patients with AMD as this condition involves degeneration only of the cone-rich macula region of the retina. Since the retina consists of only 5% cones we might be able to restore significant function with the replacement of relatively few cones. In certain instances patients with macular degeneration can fixate and create new pseudo-fovea following training and this might be enhanced if, by transplantation, cone density could be increased outside the degenerate macula.

In this project we will determine the conditions for effective transplantation of cone photoreceptors and the restoration of daylight vision in animal models of degenerative retinal disease, in order to provide the framework for developing similar approaches to treat human disease. We will conduct the following investigations to achieve this goal. (i) We will perform experiments to identify important interactions that may limit the number of new photoreceptors connecting after transplantation and will modify conditions to improve transplantation into the degenerating retina. We will study rod transplantation in order to develop improved protocols for transplanting cones (ii) We will identify and manipulate the genetic controls that may limit cone integration and use our acquired knowledge of how to effectively isolate and transplant rod precursors in order to achieve optimal integration of new functional cone cells. (iii) We will determine the synaptic connectivity of new cone and rod cells as, in order to restore vision, it is necessary for transplanted photoreceptors to form connections with cells of the inner retina (bipolar cells) and we need to know whether this occurs in the degenerating environment. (iv) We will transplant photoreceptor precursors generated from stem cell lines (embryonic stem cell lines) and examine whether these are as good as the cells isolated from the developing retina, which we use in our proof of principle experiments. This is essential as to translate our findings in mice to humans we need to show that photoreceptor precursors from a renewable source, such as embryonic stem cells can effectively restore vision. We will progress from studies involving transplantation of photoreceptor precursors derived from developing mouse retinae, to studies involving transplantation of human stem cell-derived cone precursors that might be useful for clinical application. Each programme component will inform the next step, providing the best opportunity to develop a successful long-term strategy for clinical application.

Retinal degenerations leading to loss of photoreceptors are the leading cause of untreatable blindness in Europe. Currently there are no treatments that restore lost photoreceptors and visual function. There is clearly a need for new therapeutic approaches. In 2006 we discovered that photoreceptor precursors can be effectively transplanted into the adult retina and recently we have been able to provide the first definitive evidence of restoration of rod-mediated vision, following transplantation into Gnat1-/- animals, a model of congenital stationary night blindness. Our research to date has established principles of effective rod photoreceptor transplantation. We propose that retinal repair by photoreceptor transplantation is one of the most feasible types of CNS repair as new photoreceptors need only make short, single synaptic connections to the inner retinal circuitry to contribute to visual function. As humans rely on cone and not rod vision, cone transplantation is likely to be the most effective strategy for clinical application. We are now seeking to prove the concept that it is possible to restore photopic vision in mouse models of human retinal degenerative diseases by cell transplantation based on our proven strategy for rod transplantation. The following inter-related investigations will be conducted: (i) modify environmental conditions to enhance cell integration and migration (ii) manipulate the genetic controls that limit cone integration, including transplantation of Nrl-/- and Nr2e3-/- cells, lineage tracing and using genetics fluorescent reporters (iv) examine retinal circuitry of transplanted cells using new synaptic labeling methodologies that utilise engineered viral vectors to transduce cells through connecting synapses. (v) transplant photoreceptor precursors generated from ES/iPS cells and demonstrate that that they are functional. This programme aims to provide the framework for developing stem cell therapy to treat human retinal disease.

Hereditary retinal disease and age related macular degeneration (AMD) leading to photoreceptor loss are the leading causes of untreatable blindness in the UK. Inherited retinal degenerations affect 1 in 3000 of the population whilst AMD affects 1 in 10 people over the age of 60 and its prevalence is increasing. AMD is responsible for over 50% of blind registrations in the UK in patients over 65 years of age. At present, there are no available treatments that offer anything more than a temporary slowing of disease and even then to only a minority of patients. The increasing socio-economic burden of AMD provides an urgent imperative for developing new therapies. We have recently discovered that transplantation of immature photoreceptors improves rod vision in mouse models of retinal degeneration. The proposed programme aims to build on our discovery and to develop stem cell therapy for the treatment of retinal diseases and restoration of photopic vision. By the end of this 5 year programme we aim to have established robust proof-of-concept for stem cell derived photoreceptor transplantation that will support the development of a clinical trial in patients with macular degeneration.