Validating Photoreceptor Replacement Therapy in a Severe LCA Mouse Model

Retinal degeneration is a leading cause of irreversible blindness, with its onset and severity varying widely across disorders. Among the most severe is Leber congenital amaurosis (LCA), a rare genetic condition that leads to the early and rapid degeneration of photoreceptors, with cone cells often completely lost by the age of three in human patients. Due to this early loss, even second-order neurons like bipolar cells are affected during their developmental window, resulting in abnormal synapse formation and inner retinal remodeling. These downstream effects create major obstacles for restorative therapies.

In a recent publication, Christopher Procyk and colleagues from the lab of Rachael Pearson at King’s College London, explored whether human cone photoreceptors (hCones), derived from pluripotent stem cells, can be functionally integrated into an LCA model with advanced degeneration. Building on prior work demonstrating hCone transplant success in the milder rd1 mouse model (1), this study tested whether such therapy can be effective even in an end-stage scenario. The findings indicate that hCone transplantation stimulates structural remodeling and partial restoration of visual function, even in a model with extreme inner retinal disruption.

Transplantation Assessment in the Aipl1−/− Model of LCA

To test therapeutic viability under severe conditions, researchers used Aipl1−/− mice, a well-established model that recapitulates the early and aggressive photoreceptor loss characteristic of LCA. These mice exhibit nearly complete cone loss by postnatal day 60, accompanied by extensive inner retinal remodeling, including synaptic disorganization and reactive gliosis.

At three months of age—well past the point of photoreceptor loss—Aipl1−/− mice received subretinal transplants of 500,000 GFP-labeled hCones per eye. At six months (three months post-transplantation), retinas were assessed for structural integration, synapse formation, and visual function.

Characteristics of the pre-transplanted Retinal Landscape

Immunohistochemical analysis before transplantation revealed a complete absence of cone photoreceptors in the mid-central retina. In response to this loss, bipolar and horizontal cells exhibited pronounced dendritic retraction, with horizontal cells additionally showing abnormal morphology. In contrast, inner retinal neurons such as amacrine and retinal ganglion cells (RGCs) appeared structurally intact, suggesting a degree of preservation despite the extensive outer retinal degeneration.

Notably, Müller glia exhibited upregulated expression of glial fibrillary acidic protein (GFAP), a hallmark of reactive gliosis and scarring. Synaptic markers were also significantly downregulated—RIBEYE (presynaptic) and mGluR6 (postsynaptic) were nearly undetectable in the outer retina, indicating disrupted ribbon synapse formation. This confirmed the retina’s highly degenerative state, setting a high bar for potential therapeutic rescue.

Host Neurons Remodel in Response to hCone Transplantation

After transplantation, hCones formed multilayered clusters of 10–15 layers adjacent to the inner nuclear layer (INL). These donor cells expressed photoreceptor-specific proteins and showed robust integration.

Host bipolar and horizontal cells extended dendrites into the donor grafts. Remarkably, some rod-specific ON-bipolar cells even migrated into the donor cell mass. Müller glia also infiltrated the transplanted area, suggesting functional interaction similar to their native role in maintaining retinal homeostasis and recycling visual pigments.

Crucially, hCones upregulated RIBEYE. This upregulation coincided with the re-expression of mGluR6 on the dendrites of host bipolar cells, indicating early stages of synaptogenesis. Although it remains unclear whether these are fully mature ribbon synapses, the presence of these markers, along with functional responses described below, suggests active glutamatergic signal transmission.

Restored Visual Responses: Evidence from Multiple Modalities

While full-field electroretinograms (ERGs) did not yield detectable signals in either transplanted or untreated Aipl1−/− mice, this was not unexpected. ERGs assess global retinal function, but the transplanted region represented only a small fraction of the retina of approximately 10% – similar in scale to the human macula.

In contrast, localized micro-electroretinograms (mERGs) detected reproducible light-evoked responses in the transplant region, confirming functional donor-host connectivity. These responses were weaker than those recorded in Gnat1−/− mice (rod-dysfunctional, cone-only control), but still clearly distinguishable from untreated Aipl1−/− mice.

Spike sorting, an analysis technique that isolates and identifies the firing patterns of individual retinal ganglion cells (RGCs), revealed a significant increase in light-responsive units following transplantation, particularly for ON responses. However, ON-OFF responses remained underrepresented compared to control mice, suggesting that certain aspects of light processing are still compromised. This may reflect incomplete or maladaptive synaptic wiring between transplanted hCones and host retinal circuits.

Latency measurements showed that transplanted mice had similar response times to cone-only controls under daylight-like conditions. This suggests that signal transmission pathways were not only reactivated, but also operated within normal physiological windows.

Light-Driven Head Tracking Behavior is Partially Restored

To test visually guided behavior, researchers used Striatech’s OptoDrum, which assesses optokinetic head tracking in response to moving gratings. Visual acuity improved significantly in transplanted mice relative to controls.

More importantly, 8 out of 15 mice in the transplant group demonstrated consistent head-tracking behavior, evidence of restored transmission to the brain of relevant visual function. While not all mice responded, this is a substantial outcome given the model’s severity. Researchers note that this response rate could maybe be enhanced with stronger light stimuli, as the OptoDrum setup has irradiance limited to standard computer monitors.

Conclusion: Even Late-Stage Retinas Can Regain Function

This study expands the application possibilities of photoreceptor replacement, showing that hCones can survive, integrate, and function even in a severely degenerated retina. Inner retinal neurons retain sufficient plasticity to establish new synapses with transplanted cells, despite prior loss of native inputs and significant remodeling.

While it remains to be confirmed whether the formed synapses are structurally canonical ribbon synapses, the glutamatergic transmission and behavioral improvements argue in favor of meaningful recovery. Future work could refine stimulus protocols (e.g., graded light levels or naturalistic lighting) to better evaluate ON-OFF dynamics and other complex features of vision.

By validating hCone therapy in both the milder rd1 and the severe Aipl1−/− models, this research strongly supports its broader clinical potential. Even in cases once thought too advanced for intervention, photoreceptor transplantation may offer a viable path to vision restoration.

Blog author: Emilia Kawecka, Technical University of Munich, Student Assistant at Striatech

Original article: Christopher A. Procyk et al., Human cone photoreceptor transplantation stimulates remodeling and restores function in AIPL1 model of end-stage Leber congenital amaurosis, Stem Cell Reports, 2025.

Citation: 1. “Restoration of visual function in advanced disease after transplantation of purified human pluripotent stem cell-derived cone photoreceptors” Ribeiro, Joana et al., Cell Reports, 2021.