Transplantation of retinal pigment epithelium and photoreceptors generated concomitantly via small molecule-mediated differentiation rescues visual function in rodent models of retinal degeneration

Comparison of retinal degeneration treatment with four types of different mesenchymal stem cells, human induced pluripotent stem cells and RPE cells in a rat retinal degeneration model

BACKGROUND

Retinal degeneration (RD) is a group of disorders on irreversible vision loss. Multiple types of stem cells were used in clinical trials for RD treatment. However, it remains unknown what kinds of stem cells are most effective for the treatment. Therefore, we investigated the subretinal transplantation of several types of stem cells, human adipose-derived stem cells (hADSCs), amniotic fluid stem cells (hAFSCs), bone marrow stem cells (hBMSCs), dental pulp stem cells (hDPSCs), induced pluripotent stem cell (hiPSC), and hiPSC-derived retinal pigment epithelium (RPE) cells for protection effects, paracrine effects and treatment efficiency in an RD disease model rats.

METHODS

The generation and characterization of these stem cells and hiPSC-derived RPE cells were performed before transplantation. The stem cells or hiPSC-derived RPE cell suspension labelled with CellTracker Green to detect transplanted cells were delivered into the subretinal space of 3-week-old RCS rats. The control group received subretinal PBS injection or non-injection. A series of detections including fundus photography, optomotor response (OMR) evaluations, light-dark box testing, electroretinography (ERG), and hematoxylin and eosin (HE) staining of retinal sections were conducted after subretinal injection of the cells.

RESULTS

Each stem cell, hiPSC-derived RPE cell or PBS (blank experiment) was successfully transplanted into at least six RCS rats subretinally. Compared with the control rats, RCS rats subjected to subretinal transplantation of any stem cells except hiPSCs showed higher ERG waves (p < 0.05) and quantitative OMR (qOMR) index values (hADSCs: 1.166, hAFSCs: 1.249, hBMSCs: 1.098, hDPSCs: 1.238, hiPSCs: 1.208, hiPSC-RPE cells: 1.294, non-injection: 1.03, PBS: 1.06), which indicated better visual function, at 4 weeks post-injection. However, only rats that received hiPSC-derived RPE cells maintained their visual function at 8 weeks post-injection (p < 0.05). The outer nuclear layer thickness observed in histological sections after HE staining showed the same pattern as the ERG and qOMR results.

CONCLUSIONS

Compared to hiPSC-derived RPE cells, adult and fetal stem cells yielded improvements in visual function for up to 4 weeks post-injection; this outcome was mainly based on the paracrine effects of several types of growth factors secreted by the stem cells. Patients with RD will benefit from the stem cell therapy.

2,2',4,4'-tetrabromodiphenyl ether causes depigmentation in zebrafish larvae via a light-mediated pathway

Polybrominated diphenyl ethers (PBDEs) are organic pollutants widely detected in various environmental media due to their high persistence and bioaccumulation. PBDE-induced visual impairment and neurotoxicity were previously demonstrated using zebrafish (Danio rerio) models, and recent research reported the phenotypic depigmentation effect of PBDEs at high concentrations on zebrafish, but whether those effects are still present at environment-relevant levels is still unclear. Herein, we performed both phenotypic examination and mechanism investigation in zebrafish embryos (48 hpf) and larvae (5 dpf) about their pigmentation status when exposing to PBDE congener BDE-47 (2,2',4,4'-tetrabrominated diphenyl ether) at levels from 0.25 to 25 μg/L. Results showed that low-level BDE-47 can restrain the relative melanin abundance of zebrafish larvae to 70.47% (p < 0.05) and 61.54% (p < 0.01) respectively under 2.5 and 25 μg/L BDE-47 compared with control, and the thickness of retinal pigment epithelium (RPE) remarkably reduced from 571.4 nm to 350.3 nm (p < 0.001) under 25 μg/L BDE-47 exposure. We also observed disrupted expressions of melanin synthesis genes and disorganized mitfa differentiation patterns based on Tg(mifta:EGFP), as well as visual impairment resulting from thinner RPE. Considering both processes of visual development and melanin synthesis are highly sensitive to ambient light conditions, we prolonged the light regime of maintaining zebrafish larvae from 14 hours light versus 10 hours dark (14L:10D) to 18 hours light versus 6 hours dark (18L:6D). Lengthening photoperiod successfully rescued the fluorescent level of mitfa in zebrafish epidermis and most gene expressions associated with melanin synthesis under 25 μg/L BDE-47 exposure to the normal level. In conclusion, our work reported the effects of low-level PBDEs on melanin production using zebrafish embryos and larvae, and identified the potential role of a light-mediated pathway in the neurotoxic mechanism of PBDEs.

The Potential and Application of iPSCs in Gene and Cell Therapy for Retinopathies and Optic Neuropathies

This review focuses on in vitro modeling of diseases and the development of therapeutic strategies using iPSCs for the two most common types of optical pathologies: hereditary neuropathies and retinopathies. Degeneration of retinal ganglion cells and the subsequent optic nerve atrophy leads to various types of neuropathies. Damage to photoreceptor cells or retinal pigment epithelium cells causes various retinopathies. Human iPSCs can be used as a model for studying the pathological foundations of diseases and for developing therapies to restore visual function. In recent years, significant progress has also been made in creating ganglionic and retinal organoids from iPSCs. Different research groups have published data pertaining to the potential of using iPSCs for the modeling of optic neuropathies such as glaucoma, Leber hereditary optic neuropathy, etc., including in the development of therapeutic approaches using gene editing tools.

Reprogramming iPSCs to study age-related diseases: Models, therapeutics, and clinical trials

The unprecedented rise in life expectancy observed in the last decades is leading to a global increase in the ageing population, and age-associated diseases became an increasing societal, economic, and medical burden. This has boosted major efforts in the scientific and medical research communities to develop and improve therapies to delay ageing and age-associated functional decline and diseases, and to expand health span. The establishment of induced pluripotent stem cells (iPSCs) by reprogramming human somatic cells has revolutionised the modelling and understanding of human diseases. iPSCs have a major advantage relative to other human pluripotent stem cells as their obtention does not require the destruction of embryos like embryonic stem cells do, and do not have a limited proliferation or differentiation potential as adult stem cells. Besides, iPSCs can be generated from somatic cells from healthy individuals or patients, which makes iPSC technology a promising approach to model and decipher the mechanisms underlying the ageing process and age-associated diseases, study drug effects, and develop new therapeutic approaches. This review discusses the advances made in the last decade using iPSC technology to study the most common age-associated diseases, including age-related macular degeneration (AMD), neurodegenerative and cardiovascular diseases, brain stroke, cancer, diabetes, and osteoarthritis.

Stem cell therapy in retinal diseases

Alteration of the outer retina leads to various diseases such as age-related macular degeneration or retinitis pigmentosa characterized by decreased visual acuity and ultimately blindness. Despite intensive research in the field of retinal disorders, there is currently no curative treatment. Several therapeutic approaches such as cell-based replacement and gene therapies are currently in development. In the context of cell-based therapies, different cell sources such as embryonic stem cells, induced pluripotent stem cells, or multipotent stem cells can be used for transplantation. In the vast majority of human clinical trials, retinal pigment epithelial cells and photoreceptors are the cell types considered for replacement cell therapies. In this review, we summarize the progress made in stem cell therapies ranging from the pre-clinical studies to clinical trials for retinal disease.

hPSC-derived RPE transplantation for the treatment of macular degeneration

Macular degeneration (MD) is a group of diseases characterized by irreversible and progressive vision loss. Patients with MD suffer from severely impaired central vision, especially elderly people. Currently, only one type of MD, wet age-related macular degeneration (AMD), can be treated with anti-vascular endothelium growth factor (VEGF) drugs. Other types of MD remain difficult to treat. With the advent of human pluripotent stem cells (hPSCs) and their differentiation into retinal pigmented epithelium (RPE), it is promising to treat patients with MD by transplantation of hPSC-derived RPE into the subretinal space. In this review, the current progress in hPSC-derived RPE transplantation for the treatment of patients with MD is described from bench to bedside, including hPSC differentiation into RPE and the characterization and usage of hPSC-derived RPE for transplantation into patients with MD.

Retinitis Pigmentosa: Novel Therapeutic Targets and Drug Development

Retinitis pigmentosa (RP) is a heterogeneous group of hereditary diseases characterized by progressive degeneration of retinal photoreceptors leading to progressive visual decline. It is the most common type of inherited retinal dystrophy and has a high burden on both patients and society. This condition causes gradual loss of vision, with its typical manifestations including nyctalopia, concentric visual field loss, and ultimately bilateral central vision loss. It is one of the leading causes of visual disability and blindness in people under 60 years old and affects over 1.5 million people worldwide. There is currently no curative treatment for people with RP, and only a small group of patients with confirmed RPE65 mutations are eligible to receive the only gene therapy on the market: voretigene neparvovec. The current therapeutic armamentarium is limited to retinoids, vitamin A supplements, protection from sunlight, visual aids, and medical and surgical interventions to treat ophthalmic comorbidities, which only aim to slow down the progression of the disease. Considering such a limited therapeutic landscape, there is an urgent need for developing new and individualized therapeutic modalities targeting retinal degeneration. Although the heterogeneity of gene mutations involved in RP makes its target treatment development difficult, recent fundamental studies showed promising progress in elucidation of the photoreceptor degeneration mechanism. The discovery of novel molecule therapeutics that can selectively target specific receptors or specific pathways will serve as a solid foundation for advanced drug development. This article is a review of recent progress in novel treatment of RP focusing on preclinical stage fundamental research on molecular targets, which will serve as a starting point for advanced drug development. We will review the alterations in the molecular pathways involved in the development of RP, mainly those regarding endoplasmic reticulum (ER) stress and apoptotic pathways, maintenance of the redox balance, and genomic stability. We will then discuss the therapeutic approaches under development, such as gene and cell therapy, as well as the recent literature identifying novel potential drug targets for RP.

Novel Approaches in the Drug Development and Delivery Systems for Age-Related Macular Degeneration

The number of patients with ocular disorders has increased due to contributing factors such as aging populations, environmental changes, smoking, genetic abnormalities, etc. Age-related macular degeneration (AMD) is one of the common ocular disorders which may advance to loss of vision in severe cases. The advanced form of AMD is classified into two types, dry (non-exudative) and wet (exudative) AMD. Although several therapeutic approaches are explored for the management of AMD, no approved therapy can substantially slow down the progression of dry AMD into the later stages. The focus of researchers in recent times has been engaged in developing targeted therapeutic products to halt the progression and maintain or improve vision in individuals diagnosed with AMD. The delivery of anti-VEGF agents using intravitreal therapy has found some success in managing AMD, and novel formulation approaches have been introduced in various studies to potentiate the efficacy. Some of the novel approaches, such as hydrogel, microspheres, polymeric nanoparticles, liposomes, implants, etc. have been discussed. Apart from this, subretinal, suprachoroidal, and port delivery systems have also been investigated for biologics and gene therapies. The unmet potential of approved therapeutic products has contributed to several patent applications in recent years. This review outlines the current treatment options, outcomes of recent research studies, and patent details around the novel drug delivery approach for the treatment of AMD.

New Aspects on the Treatment of Retinopathy of Prematurity: Currently Available Therapies and Emerging Novel Therapeutics

Retinopathy of prematurity (ROP) is a rare proliferative ocular disorder in preterm infants. Because of the advancements in neonatal care, the incidence of ROP has increased gradually. Now, ROP is one of the leading causes of blindness in children. Preterm infants with immature retinal development are exposed to supplemental oxygen inside an incubator until their cardiopulmonary system is adequately developed. Once they are returned to room air, the relatively low oxygen level stimulates various angiogenesis factors initiating retinal neovascularization. If patients with ROP are not offered adequate and timely treatment, they can experience vision loss that may ultimately lead to permanent blindness. Although laser therapy and anti-vascular endothelial growth factor agents are widely used to treat ROP, they have limitations. Thus, it is important to identify novel therapeutics with minimal adverse effects for the treatment of ROP. To date, various pharmacologic and non-pharmacologic therapies have been assessed as treatments for ROP. In this review, the major molecular factors involved in the pathogenesis of ROP, currently offered therapies, therapies under investigation, and emerging novel therapeutics of ROP are discussed.

Newer therapeutic options for inherited retinal diseases: Gene and cell replacement therapy

Inherited retinal diseases (IRD) are genotypically and phenotypically varied disorders that lead to progressive degeneration of the outer retina and the retinal pigment epithelium (RPE) eventually resulting in severe vision loss. Recent research and developments in gene therapy and cell therapy have shown therapeutic promise in these hitherto incurable diseases. In gene therapy, copies of a healthy gene are introduced into the host cells via a viral vector. Clinical trials for several genes are underway while treatment for RPE65 called voretigene neparvovec, is already approved and commercially available. Cell therapy involves the introduction of stem cells that can replace degenerated cells. These therapies are delivered to the target tissues, namely the photoreceptors (PR) and RPE via subretinal, intravitreal, or suprachoroidal delivery systems. Although there are several limitations to these therapies, they are expected to slow the disease progression and restore some visual functions. Further advances such as gene editing technologies are likely to result in more precise and personalized treatments. Currently, several IRDs such as retinitis pigmentosa, Stargardt disease, Leber congenital amaurosis, choroideremia, achromatopsia, and Usher syndrome are being evaluated for possible gene therapy or cell therapy. It is important to encourage patients to undergo gene testing and maintain a nationwide registry of IRDs. This article provides an overview of the basics of these therapies and their current status.

Administration of Melatonin in Diabetic Retinopathy Is Effective and Improves the Efficacy of Mesenchymal Stem Cell Treatment

Stem cell transplantation is a promising therapeutic technique for the treatment of a variety of diseases; nevertheless, stem cell therapy may not always work as well as it could. The goal of this study was to test the hypothesis that employing a powerful antioxidant like melatonin improves stem cell transplantation success and potentiates stem cell function in the therapy of diabetic retinopathy. For this purpose, 50 adult male rats were divided into the following: control group: this group received 0.5 ml of 0.1 M of sodium citrate buffer (pH = 4.5) (intraperitoneal (I.P.)). The confirmed diabetic rats were divided into 4 groups: diabetic group: confirmed diabetic rats received no treatments with a regular follow of the blood glucose profile for 8 weeks; melatonin group: confirmed diabetic rats received melatonin (5 mg/kg/day); stem cell group: the confirmed diabetic rats were given intravitreal injection of stem cells (2 μl cell suspension of stem cells (3 × 10⁴ cells/μl)); and melatonin+stem cell group: confirmed diabetic rats received melatonin (5 mg/kg/day), orally once daily for 8 weeks, and 2 μl cell suspension of stem cells (3 × 10⁴ cells/μl) was carefully injected into the vitreous cavity. Our results showed that administration of melatonin and/or stem cell restored the retinal oxidative/antioxidant redox and reduced retinal inflammatory mediators. Coadministration of melatonin and stem cells enhanced the number of transplanted stem cells in the retinal tissue and significantly reduced retinal BDEF, VEGF, APOA1, and RBP4 levels as compared to melatonin and/or stem alone. We may conclude that rats treated with melatonin and stem cells had their retinal oxidative/antioxidant redox values restored to normal and their histological abnormalities reduced. These findings support the hypothesis that interactions with the BDEF, VEGF, APOA1, and RBP4 signaling pathways are responsible for these effects.

Molecular Mechanisms of Retinal Pigment Epithelium Dysfunction in Age-Related Macular Degeneration

The retinal pigment epithelium (RPE), situated upon Bruch’s membrane, plays multiple roles in the ocular system by interacting with photoreceptors and. Therefore, dysfunction of the RPE causes diseases related to vision loss, such as age-related macular degeneration (AMD). Despite AMD being a global cause of blindness, the pathogenesis remains unclear. Understanding the pathogenesis of AMD is the first step for its prevention and treatment. This review summarizes the common pathways of RPE dysfunction and their effect in AMD. Potential treatment strategies for AMD based on targeting the RPE have also been discussed.

Interventional Strategies for Parkinson Disease: Can Neural Precursor Cells Forge a Path Ahead?

Neural precursor cells (NPCs), derived from pluripotent stem cells (PSCs), with their unique ability to generate multiple neuronal and glial cell types are extremely useful for understanding biological mechanisms in normal and diseased states. However, generation of specific neuronal subtypes (mature) from NPCs in large numbers adequate for cell therapy is challenging due to lack of a thorough understanding of the cues that govern their differentiation. Interestingly, neural stem cells (NSCs) themselves are in consideration for therapy given their potency to form different neural cell types, release of trophic factors, and immunomodulatory effects that confer neuroprotection. With the recent COVID-19 outbreak and its accompanying neurological indications, the immunomodulatory role of NSCs may gain additional significance in the prevention of disease progression in vulnerable populations. In this regard, small-molecule mediated NPC generation from PSCs via NSC formation has become an important strategy that ensures consistency and robustness of the process. The development of the mammalian brain occurs along the rostro-caudal axis, and the establishment of anterior identity is an early event. Wnt signaling, along with fibroblast growth factor and retinoic acid, acts as a caudalization signal. Further, the increasing amount of epigenetic data available from human fetal brain development has enhanced both our understanding of and ability to experimentally manipulate these developmental regulatory programs in vitro. However, the impact on homing and engraftment after transplantation and subsequently on therapeutic efficacy of NPCs based on their derivation strategy is not yet clear. Another formidable challenge in cell replacement therapy for neurodegenerative disorders is the mode of delivery. In this Perspective, we discuss these core ideas with insights from our preliminary studies exploring the role of PSC-derived NPCs in rat models of MPTP-induced Parkinson's disease following intranasal injections.

Cell Ferroptosis: New Mechanism and New Hope for Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a leading cause of inherited retinal degeneration, with more than 60 gene mutations. Despite the genetic heterogenicity, photoreceptor cell damage remains the hallmark of RP pathology. As a result, RP patients usually suffer from reduced night vision, loss of peripheral vision, decreased visual acuity, and impaired color perception. Although photoreceptor cell death is the primary outcome of RP, the underlying mechanisms are not completely elucidated. Ferroptosis is a novel programmed cell death, with characteristic iron overload and lipid peroxidation. Recent studies, using in vitro and in vivo RP models, discovered the involvement of ferroptosis-associated cell death, suggesting a possible new mechanism for RP pathogenesis. In this review, we discuss the association between ferroptosis and photoreceptor cell damage, and its implication in the pathogenesis of RP. We propose that ferroptotic cell death not only opens up a new research area in RP, but may also serve as a novel therapeutic target for RP.

Stem Cell Therapy for Retinal Degeneration: The Evidence to Date

There is a rise in the number of people who have vision loss due to retinal diseases, and conventional therapies for treating retinal degeneration fail to repair and regenerate the damaged retina. Several studies in animal models and human trials have explored the use of stem cells to repair the retinal tissue to improve visual acuity. In addition to the treatment of age-related macular degeneration (AMD) and diabetic retinopathy (DR), stem cell therapies were used to treat genetic diseases such as retinitis pigmentosa (RP) and Stargardt’s disease, characterized by gradual loss of photoreceptor cells in the retina. Transplantation of retinal pigment epithelial (RPE) cells derived from embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) have shown promising results in improving retinal function in various preclinical models of retinal degeneration and clinical studies without any severe side effects. Mesenchymal stem cells (MSCs) were utilized to treat optic neuropathy, RP, DR, and glaucoma with positive clinical outcomes. This review summarizes the preclinical and clinical evidence of stem cell therapy and current limitations in utilizing stem cells for retinal degeneration.

A defined subset of clonal retinal stem cell spheres is biased to RPE differentiation

Retinal stem cells (RSCs) are rare pigmented cells found in the pigmented ciliary layer of the mammalian retina. Studies show that RSCs can replicate to maintain the stem cell pool and produce retinal progenitors that differentiate into all retinal cell types. We classified RSCs based on their level and distribution of pigment into heavily pigmented (HP), lightly pigmented (LP), and centrally pigmented (CP) spheres. We report that CP spheres are capable of generating large cobblestone lawns of retinal pigment epithelial (RPE) cells. The other clonal sphere types (HP and LP) primarily produce cells with neural morphology and fewer RPE cells. The RSCs are homogeneous, but their downstream progenitors are different. We found that CP spheres contain highly proliferative populations of early RPE progenitors that respond to proliferative signals from the surrounding non-pigmented cells. HP and LP spheres contain late RPE progenitors which are not affected by proliferative signals.

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Three types of clonal retinal stem cell spheres form from the same single stem cell

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Centrally pigmented spheres contain populations of early RPE progenitors

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Heavily and lightly pigmented spheres contain populations of late RPE progenitors

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Downstream RPE progenitors are different due to extrinsic and intrinsic factors