Improving functional connectivity following transplantation of cone photoreceptors

Hereditary retinal disease and age-related macular degeneration (AMD) are major causes of irreversible blindness in the UK. Inherited retinal dystrophies affect 1 in 2,500, usually during childhood or early adulthood, while AMD affects 1:3 over the age of 75. The number of number of people in the UK affected by sight loss is set to double, to 4 million, by 2050 and sight loss is estimated to cost the UK economy £28bn a year, directly and indirectly. At present, we lack effective treatments for these conditions and there is an urgent requirement to develop new therapies. Both conditions involve the loss of the light sensitive cone and rod photoreceptor cells in the retina. Photoreceptor replacement aims to restore vision by the transplantation of healthy cells, ideally derived from a renewable source. Once transplanted these cells must form new connections (synapses) with their target cells, called bipolar cells, within the host retina. Restoring functional connectivity following transplantation is an ambitious goal for CNS repair. Nonetheless, the macula, which is crucial for high acuity daylight vision occupies a small area and relatively few functional photoreceptor cells may be required to achieve useful vision, so even low efficiency cone photoreceptor transplantation may result in clinical benefit. Stem cell biology has seen extraordinary progress in the past decade and we, and others, now have the ability to generate of large numbers of transplantable photoreceptors from a variety of stem cell sources. While there are some reported indications of new connections being formed between transplanted photoreceptors and host bipolar cells, achieving robust functional synaptic connectivity remains a significant challenge, particularly in advanced retinal disease, where the retina can undergo many, often inhibitory, changes. We have new and exciting data that demonstrates the feasibility of rescuing visual function (mouse models of) advanced retinal disease by transplantation of human stem cell-derived photoreceptors. Most importantly, this rescue does indeed appear to be mediated by the formation of new synaptic connections between the donor and host neurons. In this project, we will establish the full extent of synaptic connections following transplantation of stem cell-derived cone photoreceptors that can be achieved used current methods. We then seek to develop new methods to further improve functional connectivity in order to restore daylight vision in animal models of advanced degenerative retinal disease. We will conduct the following investigations to achieve this goal. We will (i) perform experiments to establish the extent of functional connectivity between transplanted cells and the host eye using current reported protocols, (ii) identify important interactions that may limit the number of new functional connections made after transplantation and develop strategies to improve connectivity and restore vision. Together, these experiments using both murine and human embryonic stem cell-derived donor cells and rodent models of advanced retinal disease will provide the framework for us to move to developing similar approaches to treat human disease.

Retinal diseases due to photoreceptor degeneration are the leading cause of untreatable blindness in industrialised countries. We seek to develop a new clinical therapy by transplanting healthy photoreceptors to replace the cells that are lost. Whilst CNS repair is an ambitious goal, photoreceptor transplantation is one of the most feasible types of repair, as transplanted cells only need to make short-distance synaptic connections to host bipolar cells in order to connect to the remaining circuitry and relatively few new functional cones may be required for useful vision. Towards our aim, we have developed robust protocols that generate large numbers of bona fide photoreceptors from mouse and human pluripotent stem cell (PSC) sources and shown that transplantation of healthy photoreceptors into models of partial degeneration can rescue vision. However, an unequivocal demonstration of functional connectivity in models of advanced degeneration has yet to be achieved despite its clinical importance; the first clinical trials of photoreceptor transplantation will be in patients with near-complete photoreceptor loss. We have continued to improve our transplantation approach and now have important data demonstrating rescue of retinal function in a mouse model of advanced retinal disease following the transplantation of human PSC-derived cones. Moreover, the data strongly indicate that this is mediated by synaptic connectivity between donor and host. Here, we will characterise the extent of functional connectivity that can be achieved in advanced disease using current methods. We have developed a number of tools and will use synaptic tracing, electrophysiology and behavioural tests to assess function from cell to behaviour. Next, we seek to develop methods to improve donor/host connectivity by manipulating the host environment and by altering the ability of donor cells to respond to cues (positive & negative) within the host and to assess their ability to improve vision.

Potential beneficiaries: This programme stands to be of significant benefit to academic scientists, patients affected by retinal degeneration and broader society as well as Industry and policy makers.

Vision and neuroscientists: This work will raise international knowledge and understanding of how donor cell transplantation restores function and of new techniques that enhance transplantation efficiencies and neural connectivity within host retinal tissue. The publication of this work in high ranking journals and its presentation to international conference audiences will open up the research for further discussion and collaboration, to support the adoption, independent validation and further adaption of these techniques across vision research and into other disciplines of neuroscience and replacement/regeneration fields.

Patients and the public: In the UK, 86% of people value their sight more than any other sense and over 80% of UK severe/sight impairment certifications are due to AMD, glaucoma, hereditary retinal disorders, diabetic eye disease, optic atrophy and cerebrovascular disease, all conditions which stand to benefit from a regenerative medicines treatment approach, such as cell transplantation. As demonstrated by the James Lind Alliance Priority Setting Partnership for Sight-loss and Vision and through the hundreds of public enquires we receive annually, the potential for stem cell-based therapies has captured the public's imagination, with many people believing these technologies represent the best hope for people with severe and rapidly progressing vision loss. Many with sight loss experience significant impact on their emotional and mental well-being. Through our interactions with patients we know that scientific misreporting and a lack of access to reliable and accurate information about latest ophthalmic research contributes to patient's feelings of isolation and abandonment, making our commitment to public engagement a fundamental component of this work.

Industry: It is increasingly critical to attract Industry interest to provide the financial resources to facilitate the movement of fundamental research from the academic setting into the translational and clinical space. This project has highly significant potential to attract future Industry investment as current interest in stem cell-based therapies will only pay-off in the long run should appropriate molecular strategies to improve functional connectivity between donor and host be identified. Within the wider context of our programme, we are investing in the development of post-docs with the necessary skills and expertise to develop these strategies, an investment that would not be commercially viable for Industry to make. Through RRA's past success in the spin-out gene therapy company, Athena Vision, (now Meira GTx PLC) with whom we continue to collaborate on the implementation of clinical trials for five patented therapies, we offer Industry an experienced partner with a proven ability to deliver research for rapid translation into early phase human studies.

Policy makers: This programme is at the forefront of eye and vision research. We have a track record of delivering advancements in techniques and understanding in the field and both applicants are regularly called upon to share expert testimony to Government-commissioned workshops and All Party Parliamentary Groups. This programme of research will continue to feed into the high-quality evidence base needed to justify current and future upward trends in funding and support for vision research as well as feed into broader government need within the current economic climate to maintain the UK's reputation and a world leader in innovative health research, and to continue to attract future national and international investment.