Laminin modification subretinal bio-scaffold remodels retinal pigment epithelium-driven microenvironment in vitro and in vivo

Pig Models in Retinal Research and Retinal Disease

The pig has been used as a large animal model in biomedical research for many years and its use continues to increase because induced mutations phenocopy several inherited human diseases. In addition, they are continuous breeders, can be propagated by artificial insemination, have large litter sizes (on the order of mice), and can be genetically manipulated using all of the techniques that are currently available in mice. The pioneering work of Petters and colleagues set the stage for the use of the pig as a model of inherited retinal disease. In the last 10 years, the pig has become a model of choice where specific disease-causing mutations that are not phenocopied in rodents need to be studied and therapeutic approaches explored. The pig is not only used for retinal eye disease but also for the study of the cornea and lens. This review attempts to show how broad the use of the pig has become and how it has contributed to the assessment of treatments for eye disease. In the last 10 years, there have been several reviews that included the use of the pig in biomedical research (see body of the review) that included information about retinal disease. None directly discuss the use of the pig as an animal model for retinal diseases, including inherited diseases, where a single genetic mutation has been identified or for multifactorial diseases such as glaucoma and diabetic retinopathy. Although the pig is used to explore diseases of the cornea and lens, this review focuses on how and why the pig, as a large animal model, is useful for research in neural retinal disease and its treatment.

Micromolded honeycomb scaffold design to support the generation of a bilayered RPE and photoreceptor cell construct

Age-related macular degeneration (AMD) causes blindness due to loss of retinal pigment epithelium (RPE) and photoreceptors (PRs), which comprise the two outermost layers of the retina. Given the small size of the macula and the importance of direct contact between RPE and PRs, the use of scaffolds for targeted reconstruction of the outer retina in later stage AMD and other macular dystrophies is particularly attractive. We developed microfabricated, honeycomb-patterned, biodegradable poly(glycerol sebacate) (PGS) scaffolds to deliver organized, adjacent layers of RPE and PRs to the subretinal space. Furthermore, an optimized process was developed to photocure PGS, shortening scaffold production time from days to minutes. The resulting scaffolds robustly supported the seeding of human pluripotent stem cell-derived RPE and PRs, either separately or as a dual cell-layered construct. These advanced, economical, and versatile scaffolds can accelerate retinal cell transplantation efforts and benefit patients with AMD and other retinal degenerative diseases.

Biomaterials used for tissue engineering of barrier-forming cell monolayers in the eye

Cell monolayers that form a barrier between two structures play an important role for the maintenance of tissue functionality. In the anterior portion of the eye, the corneal endothelium forms a barrier that controls fluid exchange between the aqueous humor of the anterior chamber and the corneal stroma. This monolayer is central in the pathogenesis of Fuchs endothelial corneal dystrophy (FECD). FECD is a common corneal disease, in which corneal endothelial cells deposit extracellular matrix that increases the thickness of its basal membrane (Descemet's membrane), and forms excrescences (guttae). With time, there is a decrease in endothelial cell density that generates vision loss. Transplantation of a monolayer of healthy corneal endothelial cells on a Descemet membrane substitute could become an interesting alternative for the treatment of this pathology. In the back of the eye, the retinal pigment epithelium (RPE) forms the blood-retinal barrier, controlling fluid exchange between the choriocapillaris and the photoreceptors of the outer retina. In the retinal disease dry age-related macular degeneration (dry AMD), deposits (drusen) form between the RPE and its basal membrane (Bruch's membrane). These deposits hinder fluid exchange, resulting in progressive RPE cell death, which in turn generates photoreceptor cell death, and vision loss. Transplantation of a RPE monolayer on a Bruch's membrane/choroidal stromal substitute to replace the RPE before photoreceptor cell death could become a treatment alternative for this eye disease. This review will present the different biomaterials that are proposed for the engineering of a monolayer of corneal endothelium for the treatment of FECD, and a RPE monolayer for the treatment of dry AMD.

Baicalein limits osteoarthritis development by inhibiting chondrocyte ferroptosis

Osteoarthritis (OA) is a common degenerative disease of articular cartilage, and there is currently no effective treatment. Baicalein, a flavonoid extracted from plants of the Scutellaria genus, has frequently been used in the past as an anti-inflammatory and anti-allergic therapy. This study investigated the effect of baicalein on OA development. For in vivo study, a destabilization of the medial meniscus (DMM)-induced OA model was established in 8-week-old wild-type (WT) and AMPKα-knockout (KO) mice, while an in vitro study was performed using chondrocytes in an OA microenvironment induced by interleukin-1β (IL-1β) exposure. We found that baicalein alleviated OA development in vivo and exerted a chondroprotective effect in vitro by suppressing chondrocyte ferroptosis. Baicalein reduced OA-related pain sensitivity by inhibiting ferroptosis of chondrocytes in OA mice. Baicalein also facilitated AMPK holoenzyme assembly, stability, and activity and suppressed ferroptosis by inducing AMPKα phosphorylation in chondrocyte. In addition, AMPKα preserved nuclear factor erythroid 2-related factor 2(Nrf2) abundance in chondrocytes and induced Nrf2 into nucleus by promoting Keap1 degradation. Meanwhile, Nrf2 increased expression of heme oxygenase-1(HO-1) to inhibit chondrocyte lipid ROS. Taken together, these results showed that baicalein alleviated OA development by improving the activity of AMPK/Nrf2/HO-1 signaling to inhibit chondrocyte ferroptosis, revealing baicalein to be a potential therapeutic strategy for OA.

Recognizing the Differentiation Degree of Human Induced Pluripotent Stem Cell-Derived Retinal Pigment Epithelium Cells Using Machine Learning and Deep Learning-Based Approaches

Induced pluripotent stem cells (iPSCs) can be differentiated into mesenchymal stem cells (iPSC-MSCs), retinal ganglion cells (iPSC-RGCs), and retinal pigmental epithelium cells (iPSC-RPEs) to meet the demand of regeneration medicine. Since the production of iPSCs and iPSC-derived cell lineages generally requires massive and time-consuming laboratory work, artificial intelligence (AI)-assisted approach that can facilitate the cell classification and recognize the cell differentiation degree is of critical demand. In this study, we propose the multi-slice tensor model, a modified convolutional neural network (CNN) designed to classify iPSC-derived cells and evaluate the differentiation efficiency of iPSC-RPEs. We removed the fully connected layers and projected the features using principle component analysis (PCA), and subsequently classified iPSC-RPEs according to various differentiation degree. With the assistance of the support vector machine (SVM), this model further showed capabilities to classify iPSCs, iPSC-MSCs, iPSC-RPEs, and iPSC-RGCs with an accuracy of 97.8%. In addition, the proposed model accurately recognized the differentiation of iPSC-RPEs and showed the potential to identify the candidate cells with ideal features and simultaneously exclude cells with immature/abnormal phenotypes. This rapid screening/classification system may facilitate the translation of iPSC-based technologies into clinical uses, such as cell transplantation therapy.

New Prospects for Retinal Pigment Epithelium Transplantation

ABSTRACT

Retinal pigment epithelium (RPE) transplants rescue photoreceptors in selected animal models of retinal degenerative disease. Early clinical studies of RPE transplants as treatment for age-related macular degeneration (AMD) included autologous and allogeneic transplants of RPE suspensions and RPE sheets for atrophic and neovascular complications of AMD. Subsequent studies explored autologous RPE-Bruch membrane-choroid transplants in patients with neovascular AMD with occasional marked visual benefit, which establishes a rationale for RPE transplants in late-stage AMD. More recent work has involved transplantation of autologous and allogeneic stem cell-derived RPE for patients with AMD and those with Stargardt disease. These early-stage clinical trials have employed RPE suspensions and RPE monolayers on biocompatible scaffolds. Safety has been well documented, but evidence of efficacy is variable. Current research involves development of better scaffolds, improved modulation of immune surveillance, and modification of the extracellular milieu to improve RPE survival and integration with host retina.

Scaffold free retinal pigment epithelium sheet engineering using modified alginate-RGD hydrogel

Tissue-specific extracellular matrix (ECM) plays a critical role in cell survival and homeostasis, which are particularly essential for directing differentiation of different complex tissues such as retina. However, ECM maintenance should be considered to design an effective therapeutic strategy for retina regeneration. To achieve this, cell sheet engineering has emerged as a growing approach to closely reconstruct basal membrane of cells through a scaffold-free manner. Several irreversible sight-threatening diseases are characterized by the dysfunction and lose of retinal pigment epithelium (RPE), leading to vision loss and eventually total blindness in patients. According to impressive developments in achievement of RPE from human embryonic stem cells (hESCs), we obtained RPE cells without any extrinsic factors in a co-culture system, and cultured them on a temporary alginate hydrogel substrate. Subsequently, Arg-Gly-Asp (RGD) peptide was superficially immobilized on the upper layer of hydrogel to improve cell attachment before harvesting sheet layer. RPE cell sheet layer was released by treating pre-seeded hydrogels with sodium citrate as a calcium chelating agent and characterized in both in vitro and in vivo models. RPE sheets formed tight junction and expressed high levels of retina structural markers such as ZO-1, Bestrophin and Collagen type IV. One week after in vivo transplantation of RPE sheet, cells survived in the subretinal space, indicating that our harvesting method is non-invasive. To sum up, we introduced a unique scaffold-free method for RPE cell sheet engineering, which can find potential use for future therapeutic purposes.

Nano-Biomaterials for Retinal Regeneration

Nanoscience and nanotechnology have revolutionized key areas of environmental sciences, including biological and physical sciences. Nanoscience is useful in interconnecting these sciences to find new hybrid avenues targeted at improving daily life. Pharmaceuticals, regenerative medicine, and stem cell research are among the prominent segments of biological sciences that will be improved by nanostructure innovations. The present review was written to present a comprehensive insight into various emerging nanomaterials, such as nanoparticles, nanowires, hybrid nanostructures, and nanoscaffolds, that have been useful in mice for ocular tissue engineering and regeneration. Furthermore, the current status, future perspectives, and challenges of nanotechnology in tracking cells or nanostructures in the eye and their use in modified regenerative ophthalmology mechanisms have also been proposed and discussed in detail. In the present review, various research findings on the use of nano-biomaterials in retinal regeneration and retinal remediation are presented, and these findings might be useful for future clinical applications.

HO-1-mediated ferroptosis as a target for protection against retinal pigment epithelium degeneration

Oxidative stress-mediated retinal pigment epithelium (RPE) degeneration plays a vital role in retinal degeneration with irreversible visual impairment, most notably in age-related macular degeneration (AMD), but a key pathogenic factor and the targeted medical control remain controversial and unclear. In this work, by sophisticated high-throughput sequencing and biochemistry investigations, the major pathologic processes during RPE degeneration in the sodium iodate-induced oxidative stress model has been identified to be heme oxygenase-1 (HO-1)-regulated ferroptosis, which is controlled by the Nrf2–SLC7A11–HO-1 hierarchy, through which ferrous ion accumulation and lethal oxidative stress cause RPE death and subsequently photoreceptor degeneration. By direct knockdown of HO-1 or using HO-1 inhibitor ZnPP, the specific inhibition of HO-1 overexpression has been determined to significantly block RPE ferroptosis. In mice, treatment with ZnPP effectively rescued RPE degeneration and achieved superior therapeutic effects: substantial recovery of the retinal structure and visual function. These findings highlight that targeting HO-1-mediated RPE ferroptosis could serve as an effectively retinal-protective strategy for retinal degenerative diseases prevention, including AMD.

Tissue Engineering Strategies for Retina Regeneration

The retina is a complex and fragile photosensitive part of the central nervous system which is prone to degenerative diseases leading to permanent vision loss. No proven treatment strategies exist to treat or reverse the degenerative conditions. Recent investigations demonstrate that cell transplantation therapies to replace the dysfunctional retinal pigment epithelial (RPE) cells and or the degenerating photoreceptors (PRs) are viable options to restore vision. Pluripotent stem cells, retinal progenitor cells, and somatic stem cells are the main cell sources used for cell transplantation therapies. The success of retinal transplantation based on cell suspension injection is hindered by limited cell survival and lack of cellular integration. Recent advances in material science helped to develop strategies to grow cells as intact monolayers or as sheets on biomaterial scaffolds for transplantation into the eyes. Such implants are found to be more promising than the bolus injection approach. Tissue engineering techniques are specifically designed to construct biodegradable or non-degradable polymer scaffolds to grow cells as a monolayer and construct implantable grafts. The engineered cell construct along with the extracellular matrix formed, can hold the cells in place to enable easy survival, better integration, and improved visual function. This article reviews the advances in the use of scaffolds for transplantation studies in animal models and their application in current clinical trials.

Nanocarriers, Progenitor Cells, Combinational Approaches, and New Insights on the Retinal Therapy

Progenitor cells derived from the retinal pigment epithelium (RPECs) have shown promise as therapeutic approaches to degenerative retinal disorders including diabetic retinopathy, age-related macular degeneration and Stargardt disease. However, the degeneration of Bruch's membrane (BM), the natural substrate for the RPE, has been identified as one of the major limitations for utilizing RPECs. This degeneration leads to decreased support, survival and integration of the transplanted RPECs. It has been proposed that the generation of organized structures of nanofibers, in an attempt to mimic the natural retinal extracellular matrix (ECM) and its unique characteristics, could be utilized to overcome these limitations. Furthermore, nanoparticles could be incorporated to provide a platform for improved drug delivery and sustained release of molecules over several months to years. In addition, the incorporation of tissue-specific genes and stem cells into the nanostructures increased the stability and enhanced transfection efficiency of gene/drug to the posterior segment of the eye. This review discusses available drug delivery systems and combination therapies together with challenges associated with each approach. As the last step, we discuss the application of nanofibrous scaffolds for the implantation of RPE progenitor cells with the aim to enhance cell adhesion and support a functionally polarized RPE monolayer.

Tissue engineering approaches towards the regeneration of biomimetic scaffolds for age-related macular degeneration

Age-related macular degeneration (AMD) is the third major cause of blindness in people aged above 60 years. It causes dysfunction of the retinal pigment epithelium (RPE) and leads to an irreversible loss of central vision. The present clinical treatment options are more palliative in controlling the progression of the disease and do not functionally restore the degenerated RPE monolayer and photoreceptors. Currently, the clinical transplantation of RPE cells has shown poor engraftment potential due to the absence of an intact Bruch's membrane in AMD patients, thereby the vision is unable to be restored completely. Although tissue engineering strategies target the development of Bruch's membrane-mimetic substrates, the challenge still lies in the development of an ultrathin, biologically and mechanically equivalent membrane to restore visual acuity. Further, existing limitations such as cellular aggregation, surgical complications including retinal tissue damage, tissue rejection, disease transmission, inferior mechanical strength, and the loss of vision over time demand the search for an ideal strategy to restore the functional RPE. Hence, this review aims to provide insights into various approaches, from conventional cell therapy to 3D bioprinting, and their unmet challenges in treating AMD by outlining the pathophysiology of AMD and the host tissue response with respect to injury, treatment and preclinical animal models.

Cell-Based Therapies for Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of blindness worldwide. The pathogenesis of AMD involves dysfunction and loss of the retinal pigment epithelium (RPE), a monolayer of cells that provide nourishment and functional support for the overlying photoreceptors. RPE cells in mammals are not known to divide, renew or regenerate in vivo, and in advanced AMD, RPE loss leads to degeneration of the photoreceptors and impairment of vision. One possible therapeutic approach would be to support and replace the failing RPE cells of affected patients, and indeed moderate success of surgical procedures in which relatively healthy autologous RPE from the peripheral retina of the same eye was transplanted under the retina in the macular area suggested that RPE replacement could be a means to attenuate photoreceptor cell loss. This prompted exploration of the possibility to use pluripotent stem cells (PSCs) as a potential source for "healthy and young" RPE cells for such cell-based therapy of AMD. Various approaches ranging from the use of allogeneic embryonic stem cells to autologous induced pluripotent stem cells are now being tested within early clinical trials. Such PSC-derived RPE cells are either injected into the subretinal space as a suspension, or transplanted as a monolayer patch upon scaffold support. Although most of these approaches are at early clinical stages, safety of the RPE product has been demonstrated by several of these studies. Here, we review the concept of cell-based therapy of AMD and provide an update on current progress in the field of RPE transplantation.

Biotechnology and Biomaterial-Based Therapeutic Strategies for Age-Related Macular Degeneration. Part II: Cell and Tissue Engineering Therapies

Age-related Macular Degeneration (AMD) is an up-to-date untreatable chronic neurodegenerative eye disease of multifactorial origin, and the main causes of blindness in over 65 y.o. people. It is characterized by a slow progression and the presence of a multitude of factors, highlighting those related to diet, genetic heritage and environmental conditions, present throughout each of the stages of the illness. Current therapeutic approaches, mainly consisting on intraocular drug delivery, are only used for symptoms relief and/or to decelerate the progression of the disease. Furthermore, they are overly simplistic and ignore the complexity of the disease and the enormous differences in the symptomatology between patients. Due to the wide impact of the AMD and the up-to-date absence of clinical solutions, Due to the wide impact of the AMD and the up-to-date absence of clinical solutions, different treatment options have to be considered. Cell therapy is a very promising alternative to drug-based approaches for AMD treatment. Cells delivered to the affected tissue as a suspension have shown poor retention and low survival rate. A solution to these inconveniences has been the encapsulation of these cells on biomaterials, which contrive to their protection, gives them support, and favor their retention of the desired area. We offer a two-papers critical review of the available and under development AMD therapeutic approaches, from a biomaterials and biotechnological point of view. We highlight benefits and limitations and we forecast forthcoming alternatives based on novel biomaterials and biotechnology methods. In this second part we review the preclinical and clinical cell-replacement approaches aiming at the development of efficient AMD-therapies, the employed cell types, as well as the cell-encapsulation and cell-implant systems. We discuss their advantages and disadvantages and how they could improve the survival and integration of the implanted cells.

First steps for the development of silk fibroin-based 3D biohybrid retina for age-related macular degeneration (AMD)

Age-related macular degeneration is an incurable chronic neurodegenerative disease, causing progressive loss of the central vision and even blindness. Up-to-date therapeutic approaches can only slow down he progression of the disease.

OBJECTIVE

Feasibility study for a multilayered, silk fibroin-based, 3D biohybrid retina.

APPROACH

Fabrication of silk fibroin-based biofilms; culture of different types of cells: retinal pigment epithelium, retinal neurons, Müller and mesenchymal stem cells ; creation of a layered structure glued with silk fibroin hydrogel.

MAIN RESULTS

In vitro evidence for the feasibility of layered 3D biohybrid retinas; primary culture neurons grow and develop neurites on silk fibroin biofilms, either alone or in presence of other cells cultivated on the same biomaterial; cell organization and cellular phenotypes are maintained in vitro for the seven days of the experiment.

SIGNIFICANCE

3D biohybrid retina can be built using silk silkworm fibroin films and hydrogels to be used in cell replacement therapy for AMD and similar retinal neurodegenerative diseases.

Bruch's membrane pathology: A mechanistic perspective

Bruch's membrane, an extracellular matrix located between the retinal pigment epithelium and the choroid, plays a vital role as structural and functional support to the retinal pigment epithelium. Dysfunction of Bruch's membrane in both age-related macular degeneration and other ocular diseases is caused mostly by extracellular matrix degeneration, deposit formation, and angiogenesis. Although these factors are dealt in greater detail with respect to the cells that are degenerated such as the retinal pigment epithelium and the endothelial cells, the pathology involving the Bruch's membrane is often underrated. Since in most of the macular degenerations early degenerative changes are also observed in the Bruch's membrane, addressing only the cellular component without the underlying membrane will not yield an ideal clinical benefit. This review aims to discuss the factors and the mechanisms affecting the integrity of the Bruch's membrane, which would aid in developing an effective therapy for these pathologies.

A Systematic Review on Transplantation Studies of the Retinal Pigment Epithelium in Animal Models

The retinal pigment epithelium (RPE) and the adjacent light-sensitive photoreceptors form a single functional unit lining the back of the eye. Both cell layers are essential for normal vision. RPE degeneration is usually followed by photoreceptor degeneration and vice versa. There are currently almost no effective therapies available for RPE disorders such as Stargardt disease, specific types of retinitis pigmentosa, and age-related macular degeneration. RPE replacement for these disorders, especially in later stages of the disease, may be one of the most promising future therapies. There is, however, no consensus regarding the optimal RPE source, delivery strategy, or the optimal experimental host in which to test RPE replacement therapy. Multiple RPE sources, delivery methods, and recipient animal models have been investigated, with variable results. So far, a systematic evaluation of the (variables influencing) efficacy of experimental RPE replacement parameters is lacking. Here we investigate the effect of RPE transplantation on vision and vision-based behavior in animal models of retinal degenerated diseases. In addition, we aim to explore the effect of RPE source used for transplantation, the method of intervention, and the animal model which is used.

METHODS

In this study, we systematically identified all publications concerning transplantation of RPE in experimental animal models targeting the improvement of vision (e.g., outcome measurements related to the morphology or function of the eye). A variety of characteristics, such as species, gender, and age of the animals but also cell type, number of cells, and other intervention characteristics were extracted from all studies. A risk of bias analysis was performed as well. Subsequently, all references describing one of the following outcomes were analyzed in depth in this systematic review: a-, b-, and c-wave amplitudes, vision-based, thickness analyses based on optical coherence tomography (OCT) data, and transplant survival based on scanning laser ophthalmoscopy (SLO) data. Meta-analyses were performed on the a- and b-wave amplitudes from electroretinography (ERG) data as well as data from vision-based behavioral assays.

RESULTS

original research articles met the inclusion criteria after two screening rounds. Overall, most studies were categorized as unclear regarding the risk of bias, because many experimental details were poorly reported. Twenty-three studies reporting one or more of the outcome measures of interest were eligible for either descriptive (thickness analyses based on OCT data; n = 2) or meta-analyses. RPE transplantation significantly increased ERG a-wave (Hedges' g 1.181 (0.471-1.892), n = 6) and b-wave (Hedges' g 1.734 (1.295-2.172), n = 42) amplitudes and improved vision-based behavior (Hedges' g 1.018 (0.826-1.209), n = 96). Subgroup analyses revealed a significantly increased effect of the use of young and adolescent animals compared to adult animals. Moreover, transplanting more cells (in the range of 105 versus in the range of 104) resulted in a significantly increased effect on vision-based behavior as well. The origin of cells mattered as well. A significantly increased effect was found on vision-based behavior when using ARPE-19 and OpRegen® RPE.

CONCLUSIONS

This systematic review shows that RPE transplantation in animal models for retinal degeneration significantly increases a- and b- wave amplitudes and improves vision-related behavior. These effects appear to be more pronounced in young animals, when the number of transplanted cells is larger and when ARPE-19 and OpRegen® RPE cells are used. We further emphasize that there is an urgent need for improving the reporting and methodological quality of animal experiments, to make such studies more comparable.

Establishing Liposome-Immobilized Dexamethasone-Releasing PDMS Membrane for the Cultivation of Retinal Pigment Epithelial Cells and Suppression of Neovascularization

Age-related macular degeneration (AMD) is the eye disease with the highest epidemic incidence, and has great impact on the aged population. Wet-type AMD commonly has the feature of neovascularization, which destroys the normal retinal structure and visual function. So far, effective therapy options for rescuing visual function in advanced AMD patients are highly limited, especially in wet-type AMD, in which the retinal pigmented epithelium and Bruch's membrane structure (RPE-BM) are destroyed by abnormal angiogenesis. Anti-VEGF treatment is an effective remedy for the latter type of AMD; however, it is not a curative therapy. Therefore, reconstruction of the complex structure of RPE-BM and controlled release of angiogenesis inhibitors are strongly required for sustained therapy. The major purpose of this study was to develop a dual function biomimetic material, which could mimic the RPE-BM structure and ensure slow release of angiogenesis inhibitor as a novel therapeutic strategy for wet AMD. We herein utilized plasma-modified polydimethylsiloxane (PDMS) sheet to create a biomimetic scaffold mimicking subretinal BM. This dual-surface biomimetic scaffold was coated with laminin and dexamethasone-loaded liposomes. The top surface of PDMS was covalently grafted with laminin and used for cultivation of the retinal pigment epithelial cells differentiated from human induced pluripotent stem cells (hiPSC-RPE). To reach the objective of inhibiting angiogenesis required for treatment of wet AMD, the bottom surface of modified PDMS membrane was further loaded with dexamethasone-containing liposomes via biotin-streptavidin linkage. We demonstrated that hiPSC-RPE cells could proliferate, express normal RPE-specific genes and maintain their phenotype on laminin-coated PDMS membrane, including phagocytosis ability, and secretion of anti-angiogenesis factor PEDF. By using in vitro HUVEC angiogenesis assay, we showed that application of our membrane could suppress oxidative stress-induced angiogenesis, which was manifested in decreased secretion of VEGF by RPE cells and suppression of vascularization. In conclusion, we propose modified biomimetic material for dual delivery of RPE cells and liposome-enveloped dexamethasone, which can be potentially applied for AMD therapy.

Printable Graphene Oxide Micropatterns for a Bio-Subretinal Chip

Recently, implantable artificial subretinal chips using electronic components have replaced photoreceptors to serve as the most feasible treatment for retinal diseases. As such a chip that is meant to be implanted and used for very long periods, growing retinal cells on it to improve the electrical stimulation efficiency and attraction of neuronal elements remains a challenge. Here, an inkjet printing technology is employed to create graphene oxide (GO) micropatterns onto microelectrodes of a photovoltaic-powered implantable retinal chip. These GO micropatterns allow human retinal pigment epithelium (RPE) cells to specially attach and grow in each microelectrode. In addition, the cell proliferation, viability, and tight junction of RPE cells are improved during culturing. The development of a simple surface-coating technology would pave the way for the development of the first fully integrated and encapsulated retinal prostheses with biocompatible on-chip microelectrodes for long-term implantation, which could be effectively applied in retina tissue engineering and therapy.

Development of a Graphene Oxide-Incorporated Polydimethylsiloxane Membrane with Hexagonal Micropillars

Several efforts have been made on the development of bioscaffolds including the polydimethylsiloxane (PDMS) elastomer for supporting cell growth into stable sheets. However, PDMS has several disadvantages, such as intrinsic surface hydrophobicity and mechanical strength. Herein, we generated a novel PDMS-based biomimetic membrane by sequential modifications of the PMDS elastomer with graphene oxide (GO) and addition of a hexagonal micropillar structure at the bottom of the biomembrane. GO was initially homogenously mixed with pure PDMS and then was further coated onto the upper surface of the resultant PDMS. The elastic modulus and hydrophilicity were significantly improved by such modifications. In addition, the development of hexagonal micropillars with smaller diameters largely improved the ion permeability and increased the motion resistance. We further cultured retinal pigment epithelial (RPE) cells on the surface of this modified PDMS biomembrane and assayed its biocompatibility. Remarkably, the GO incorporation and coating exhibited beneficial effect on the cell growth and the new formation of tight junctions in RPE cells. Taken together, this GO-modified PDMS scaffold with polyhexagonal micropillars may be utilized as an ideal cell sheet and adaptor for cell cultivation and can be used in vivo for the transplantation of cells such as RPE cells.

Sheets of human retinal progenitor transplants improve vision in rats with severe retinal degeneration

Loss of photoreceptors and other retinal cells is a common endpoint in retinal degenerate (RD) diseases that cause blindness. Retinal transplantation is a potential therapy to replace damaged retinal cells and improve vision. In this study, we examined the development of human fetal retinal sheets with or without their retinal pigment epithelium (RPE) transplanted to immunodeficient retinal degenerate rho S334ter-3 rats. Sheets were dissected from fetal human eyes (11-15.7 weeks gestation) and then transplanted to the subretinal space of 24-31 d old RD nude rats. Every month post surgery, eyes were imaged by high-resolution spectral-domain optical coherence tomography (SD-OCT). SD-OCT showed that transplants were placed into the subretinal space and developed laminated areas or rosettes, with clear development of plexiform layers first seen in OCT at 3 months post surgery. Several months later, as could be expected by the much slower development of human cells compared to rat cells, transplant photoreceptors developed inner and later outer segments. Retinal sections were analyzed by immunohistochemistry for human and retinal markers and confirmed the formation of several retinal subtypes within the retinal layers. Transplant cells extended processes and a lot of the cells could also be seen migrating into the host retina. At 5.8-8.6 months post surgery, selected rats were exposed to light flashes and recorded for visual responses in superior colliculus, (visual center in midbrain). Four of seven rats with transplants showed responses to flashes of light in a limited area of superior colliculus. No response with the same dim light intensity was found in age-matched RD controls (non-surgery or sham surgery). In summary, our data showed that human fetal retinal sheets transplanted to the severely disturbed subretinal space of RD nude rats develop mature photoreceptors and other retinal cells, integrate with the host and induce vision improvement.

Scaffolds for retinal pigment epithelial cell transplantation in age-related macular degeneration

In several retinal degenerative diseases, including age-related macular degeneration, the retinal pigment epithelium, a highly functionalized cell monolayer, becomes dysfunctional. These retinal diseases are marked by early retinal pigment epithelium dysfunction reducing its ability to maintain a healthy retina, hence making the retinal pigment epithelium an attractive target for treatment. Cell therapies, including bolus cell injections, have been investigated with mixed results. Since bolus cell injection does not promote the proper monolayer architecture, scaffolds seeded with retinal pigment epithelium cells and then implanted have been increasingly investigated. Such cell-seeded scaffolds address both the dysfunction of the retinal pigment epithelium cells and age-related retinal changes that inhibit the efficacy of cell-only therapies. Currently, several groups are investigating retinal therapies using seeded cells from a number of cell sources on a variety of scaffolds, such as degradable, non-degradable, natural, and artificial substrates. This review describes the variety of scaffolds that have been developed for the implantation of retinal pigment epithelium cells.

Induced pluripotent stem cell-based therapy for age-related macular degeneration

INTRODUCTION

In age-related macular degeneration (AMD), stem cells could possibly replace or regenerate disrupted pathologic retinal pigment epithelium (RPE), and produce supportive growth factors and cytokines such as brain-derived neurotrophic factor.  Induced pluripotent stem cells (iPSCs)-derived RPE was first subretinally transplanted in a neovascular AMD patient in 2014. Areas covered: Induced PSCs are derived from the introduction of transcription factors to adult cells under specific cell culture conditions, followed by differentiation into RPE cells. Induced PSC-derived RPE cells exhibit ion transport, membrane potential, polarized VEGF secretion and gene expression that is similar to native RPE. Despite having similar in vitro function, morphology, immunostaining and microscopic analysis, it remains to be seen if iPSC-derived RPE can replicate the myriad of in vivo functions, including immunomodulatory effects, of native RPE cells.  Historically, adjuvant RPE transplantation during CNV resections were technically difficult and complicated by immune rejection. Autologous iPSCs are hypothesized to reduce the risk of immune rejection, but their production is time-consuming and expensive.  Alternatively, allogenic transplantation using human leukocyte antigen (HLA)-matched iPSCs, similar to HLA-matched organ transplantation, is currently being investigated. Expert opinion: Challenges to successful transplantation with iPSCs include surgical technique, a pathologic subretinal microenvironment, possible immune rejection, and complications of immunosuppression.