Quantifiable In Vivo Imaging Biomarkers of Retinal Regeneration by Photoreceptor Cell Transplantation

Subretinal microglia support donor photoreceptor survival in rd1 mice

PURPOSE

To investigate the potential relationship between subretinal microglia and transplanted donor photoreceptors.

METHODS

Photoreceptor precursors were transplanted into wild-type mice and rd1 mice by trans-scleral injection. Immunohistochemistry was employed to detect microglia and macrophages. PlX5622 feed was used to achieve microglia depletion and microglia repopulation. RNA-seq and qPCR were utilized to evaluate gene expression. Confocal microscopy was used to observe the interaction between microglia and donor photoreceptors.

RESULTS

Donor photoreceptors survived in rd1 mice but not in wild-type mice after trans-scleral injection. The microglial cells closely interacted with donor cells. While donor cells failed to survive in rd1 mice after microglia depletion, they could survive following microglia repopulation. The RNA-seq analysis showed a pro-neurodevelopmental effect of sub-retinal microglia/RPE tissue in rd1 mice.

CONCLUSIONS

Subretinal microglia supported donor photoreceptor survival in rd1 mice.

Human iPSC-derived photoreceptor transplantation in the cone dominant 13-lined ground squirrel

Several retinal degenerations affect the human central retina, which is primarily comprised of cones and is essential for high acuity and color vision. Transplanting cone photoreceptors is a promising strategy to replace degenerated cones in this region. Although this approach has been investigated in a handful of animal models, commonly used rodent models lack a cone-rich region and larger models can be expensive and inaccessible, impeding the translation of therapies. Here, we transplanted dissociated GFP-expressing photoreceptors from retinal organoids differentiated from human induced pluripotent stem cells into the subretinal space of damaged and undamaged cone-dominant 13-lined ground squirrel eyes. Transplanted cell survival was documented via noninvasive high-resolution imaging and immunohistochemistry to confirm the presence of human donor photoreceptors for up to 4 months posttransplantation. These results demonstrate the utility of a cone-dominant rodent model for advancing the clinical translation of cell replacement therapies.

Transpupillary-Guided Trans-Scleral Transplantation of Subretinal Grafts in a Retinal Degeneration Mouse Model

Transplantation of photoreceptor cells and retinal pigment epithelial (RPE) cells provide a potential therapy for retinal degeneration diseases. Subretinal transplantation of therapeutic donor cells into mouse recipients is challenging due to the limited surgical space allowed by the small volume of the mouse eye. We developed a trans-scleral surgical transplantation platform with direct transpupillary vision guidance to facilitate the subretinal delivery of exogenous cells in mouse recipients. The platform was tested using retinal cell suspensions and three-dimensional retinal sheets collected from rod-rich Rho::EGFP mice and cone-rich OPN1LW-EGFP;NRL-/- mice, respectively. Live/dead assay showed low cell mortality for both forms of donor cells. Retinal grafts were successfully delivered into the subretinal space of a mouse model of retinal degeneration, Rd1/NS, with minimum surgical complications as detected by multimodal confocal scanning laser ophthalmoscope (cSLO) imaging. Two months post-transplantation, histological staining demonstrated evidence of advanced maturation of the retinal grafts into 'adult' rods and cones (by robust Rho::EGFP, S-opsin, and OPN1LW:EGFP expression, respectively) in the subretinal space. Here, we provide a surgical platform that can enable highly accurate subretinal delivery with a low rate of complications in mouse recipients. This technique offers precision and relative ease of skill acquisition. Furthermore, the technique could be used not only for studies of subretinal cell transplantation but also for other intraocular therapeutic studies including gene therapies.

Progress in Modeling Neural Tube Development and Defects by Organoid Reconstruction

It is clear that organoids are useful for studying the structure as well as the functions of organs and tissues; they are able to simulate cell-to-cell interactions, symmetrical and asymmetric division, proliferation, and migration of different cell groups. Some progress has been made using brain organoids to elucidate the genetic basis of certain neurodevelopmental disorders. Such as Parkinson's disease and Alzheimer's disease. However, research on organoids in early neural development has received insufficient attention, especially that focusing on neural tube precursors. In this review, we focus on the recent research progress on neural tube organoids and discuss both their challenges and potential solutions.

Retinal magnification factors at the fixation locus derived from schematic eyes with four individualized surfaces

Retinal magnification factors (RMFs) allow the conversion of angles to lengths in retinal images. In this work, we propose paraxial and non-paraxial RMF calculation methods that incorporate the individual topography and separation of the anterior and posterior surfaces of the cornea and crystalline lens, assuming homogeneous ocular media. Across 34 eyes, the two RMF methods differ by 0.1% on average, due to surface tilt, decenter, and lack of rotational symmetry in the non-paraxial modeling, which results in up to 2.2% RMF variation with retinal meridian. Differences with widely used individualized RMF calculation methods are smallest for eyes with ∼24 mm axial length, and as large as 7.5% in a 29.7 mm long eye (15D myope). To better model the capture of retinal images, we propose the tracing of chief rays, instead of the scaling of posterior nodal or principal distances often used in RMF definitions. We also report that RMF scale change is approximately proportional to both refractive error and axial separation between the ophthalmoscope's exit pupil and the eye's entrance pupil, resulting in RMF changes as large as 13% for a 1cm displacement in a 15D myopic eye. Our biometry data shows weak correlation and statistical significance between surface radii and refractive error, as well as axial length, whether considering all eyes in the study, or just the high myopes, defined as those with refractive error sphere equivalent ≤ -4D. In contrast, vitreous thicknesses show a strong correlation (r ≤ -0.92) and significance (p ≤ 10-13) with refractive error when considering all eyes or just high myopes (r ≤ -0.95; p ≤ 10-5). We also found that potential RMF change with depth of cycloplegia and/or residual accommodation is smaller than 0.2%. Finally, we propose the reporting of individual ocular biometry data and a detailed RMF calculation method description in scientific publications to facilitate the comparison of retinal imaging biomarker data across studies.

The Prospects for Retinal Organoids in Treatment of Retinal Diseases

Retinal degeneration (RD) is a significant cause of incurable blindness worldwide. Photoreceptors and retinal pigmented epithelium are irreversibly damaged in advanced RD. Functional replacement of photoreceptors and/or retinal pigmented epithelium cells is a promising approach to restoring vision. This paper reviews the current status and explores future prospects of the transplantation therapy provided by pluripotent stem cell-derived retinal organoids (ROs). This review summarizes the status of rodent RD disease models and discusses RO culture and analytical tools to evaluate RO quality and function. Finally, we review and discuss the studies in which RO-derived cells or sheets were transplanted. In conclusion, methods to derive ROs from pluripotent stem cells have significantly improved and become more efficient in recent years. Meanwhile, more novel technologies are applied to characterize and validate RO quality. However, opportunity remains to optimize tissue differentiation protocols and achieve better RO reproducibility. In order to screen high-quality ROs for downstream applications, approaches such as noninvasive and label-free imaging and electrophysiological functional testing are promising and worth further investigation. Lastly, transplanted RO-derived tissues have allowed improvements in visual function in several RD models, showing promises for clinical applications in the future.

A Surgical Kit for Stem Cell-Derived Retinal Pigment Epithelium Transplants: Collection, Transportation, and Subretinal Delivery

Transplantation of stem cell-derived retinal pigment epithelium (RPE) cells is a promising potential therapy for currently incurable retinal degenerative diseases like advanced dry age-related macular degeneration. In this study, we designed a set of clinically applicable devices for subretinal implantation of RPE grafts, towards the overarching goal of establishing enabling technologies for cell-based therapeutic approaches to regenerate RPE cells. This RPE transplant kit includes a custom-designed trephine for the production of RPE transplants, a carrier for storage and transportation, and a surgical device for subretinal delivery of RPE transplants. Cell viability assay confirmed biocompatibility of the transplant carrier and high preservation of RPE transplants upon storage and transportation. The transplant surgical device combines foldable technology that minimizes incision size, controlled delivery speed, no fluid reflux, curved translucent tip, usability of loading and in vivo reloading, and ergonomic handle. Furthermore, the complementary design of the transplant carrier and the delivery device resulted in proper grasping, loading, and orientation of the RPE transplants into the delivery device. Proof-of-concept transplantation studies in a porcine model demonstrated no damage or structural change in RPE transplants during surgical manipulation and subretinal deployment. Post-operative assessment confirmed that RPE transplants were delivered precisely, with no damage to the host retina or choroid, and no significant structural change to the RPE transplants. Our novel surgical kit provides a comprehensive set of tools encompassing RPE graft manufacturing to surgical implantation rendering key enabling technologies for pre-clinical and clinical phases of stem cell-derived RPE regenerative therapies.

Characterization and allogeneic transplantation of a novel transgenic cone-rich donor mouse line

OBJECTIVES

Cone photoreceptor transplantation is a potential treatment for macular diseases. The optimal conditions for cone transplantation are poorly understood, partly because of the scarcity of cones in donor mice. To facilitate allogeneic cone photoreceptor transplantation studies in mice, we aimed to create and characterize a donor mouse model containing a cone-rich retina with a cone-specific enhanced green fluorescent protein (EGFP) reporter.

METHODS

We generated OPN1LW-EGFP/NRL-/- mice by crossing NRL-/- and OPN1LW-EGFP mice. We characterized the anatomical phenotype of OPN1LW-EGFP/NRL-/- mice using multimodal confocal scanning laser ophthalmoscopy (cSLO) imaging, immunohistology, and transmission electron microscopy. We evaluated retinal function using electroretinography (ERG), including 465 and 525 nm chromatic stimuli. Retinal sheets and cell suspensions from OPN1LW-EGFP/NRL-/- mice were transplanted subretinally into immunodeficient Rd1 mice.

RESULTS

OPN1LW-EGFP/NRL-/- retinas were enriched with OPN1LW-EGFP+ and S-opsin+ cone photoreceptors in a dorsal-ventral distribution gradient. Cone photoreceptors co-expressing OPNL1W-EGFP and S-opsin significantly increased in OPN1LW-EGFP/NRL-/- compared to OPN1LW-EGFP mice. Temporal dynamics of rosette formation in the OPN1LW-EGFP/NRL-/- were similar as the NRL-/- with peak formation at P15. Rosettes formed preferentially in the ventral retina. The outer retina in P35 OPN1LW-EGFP/NRL-/- was thinner than NRL-/- controls. The OPN1LW-EGFP/NRL-/- ERG response amplitudes to 465 nm stimulation were similar to, but to 535 nm stimulation were lower than, NRL-/- controls. Three months after transplantation, the suspension grafts showed greater macroscopic degradation than sheet grafts. Retinal sheet grafts from OPN1LW-EGFP/NRL-/- mice showed greater S-opsin + cone survival than suspension grafts from the same strain.

CONCLUSIONS

OPN1LW-EGFP/NRL-/- retinae were enriched with S-opsin+ photoreceptors. Sustained expression of EGFP facilitated the longitudinal tracking of transplanted donor cells. Transplantation of cone-rich retinal grafts harvested prior to peak rosette formation survived and differentiated into cone photoreceptor subtypes. Photoreceptor sheet transplantation may promote greater macroscopic graft integrity and S-opsin+ cone survival than cell suspension transplantation, although the mechanism underlying this observation is unclear at present. This novel cone-rich reporter mouse strain may be useful to study the influence of graft structure on cone survival.

Outer Retinal Cell Replacement: Putting the Pieces Together

Retinal degenerative diseases (RDDs) affecting photoreceptors (PRs) are one of the most prevalent sources of incurable blindness worldwide. Due to a lack of endogenous repair mechanisms, functional cell replacement of PRs and/or retinal pigmented epithelium (RPE) cells are among the most anticipated approaches for restoring vision in advanced RDD. Human pluripotent stem cell (hPSC) technologies have accelerated development of outer retinal cell therapies as they provide a theoretically unlimited source of donor cells. Human PSC-RPE replacement therapies have progressed rapidly, with several completed and ongoing clinical trials. Although potentially more promising, hPSC-PR replacement therapies are still in their infancy. A first-in-human trial of hPSC-derived neuroretinal transplantation has recently begun, but a number of questions regarding survival, reproducibility, functional integration, and mechanism of action remain. The discovery of biomaterial transfer between donor and PR cells has highlighted the need for rigorous safety and efficacy studies of PR replacement. In this review, we briefly discuss the history of neuroretinal and PR cell transplantation to identify remaining challenges and outline a stepwise approach to address specific pieces of the outer retinal cell replacement puzzle.