Transplanted human cones incorporate into the retina and function in a murine cone degeneration model

Abstract

Once human photoreceptors die, they do not regenerate, thus, photoreceptor transplantation has emerged as a potential treatment approach for blinding diseases. Improvements in transplant organization, donor cell maturation, and synaptic connectivity to the host will be critical in advancing this technology for use in clinical practice. Unlike the unstructured grafts of prior cell-suspension transplantations into end-stage degeneration models, we describe the extensive incorporation of induced pluripotent stem cell (iPSC) retinal organoid–derived human photoreceptors into mice with cone dysfunction. This incorporative phenotype was validated in both cone-only as well as pan-photoreceptor transplantations. Rather than forming a glial barrier, Müller cells extended throughout the graft, even forming a series of adherens junctions between mouse and human cells, reminiscent of an outer limiting membrane. Donor-host interaction appeared to promote polarization as well as the development of morphological features critical for light detection, namely the formation of inner and well-stacked outer segments oriented toward the retinal pigment epithelium. Putative synapse formation and graft function were evident at both structural and electrophysiological levels. Overall, these results show that human photoreceptors interacted readily with a partially degenerated retina. Moreover, incorporation into the host retina appeared to be beneficial to graft maturation, polarization, and function.

Human photoreceptor cell transplants integrate into human retina organoids

“…Live imaging in culture showed that transplant and host HRO come in direct contact (Fig. 1E4) as reported for in vivo animal studies (14). After a few days, transplant and host were fixed together and analyzed by histology.…”

“…Previous studies in mice and our data in HROs show that cell processes of host MG extend into and form contact with PR transplants, supporting the notion that MG provide a permissive and supportive (56) rather than repressive environment. PR transplants in HRO at the timepoint studied are mostly unordered, and previous studies in mice showed a comparable histological phenotype at an early stage of transplant integration (14). However, such phenotype is also reminiscent of some retinal pathologies.…”

“…In the present work, GFAP was observed within the integrated transplants. In previous animal studies, GFAP was also expressed in the PR transplant area (14). Thus, it will be interesting to determine if some level or functions of gliosis might support or limit transplant integration.…”

“…1A). Using previously established protocols and human pluripotent stem cell lines, we generated HROs (14,21,39). Cone PRs derived from such HROs at day 200 were recently shown to be most suitable for transplantation studies in mice (14).…”

“…Whereas mouse photoreceptors integrate in smaller numbers, if at all, into the host retina (11)(12)(13), human photoreceptors, particularly cones, have recently been shown to structurally integrate in large clusters and/or contact host second-order neurons and Müller glia (14,15). While exchange of genetic and protein material has been shown to frequently occur in mouse-to-mouse photoreceptor transplants (11)(12)(13)(16)(17)(18), this phenomenon was not observed in human transplants into rodent models (14,(19)(20)(21). Hence, animal studies support the possibility of beneficial cell replacement therapy, posing the questions how to effectively translate findings to the clinic, and which patients at what stage of pathology would profit most.…”