Researchers from the National Eye Institute (NEI), part of the National Institutes of Health generated a “disease-in-a-dish” model to study the disease. The findings are published in Communications Biology.
“This new model of a rare eye disease is a terrific example of translational research, where collaboration among clinical and laboratory researchers advances knowledge not by small steps, but by leaps and bounds,” said Michael F. Chiang, M.D., director of the NEI, part of the National Institutes of Health.
L-ORD is a rare, dominantly inherited disorder, meaning that it can occur when there is an abnormal gene from one parent. L-ORD is caused by a mutation in the gene that encodes the protein CTRP5.
Symptoms, including difficulty seeing in the dark and loss of central vision, usually appear around age 50 to 60. As L-ORD progresses, cells in the retinal pigment epithelium (RPE), a layer of tissue that nourishes the retina’s light-sensing photoreceptors, shrink and die. Loss of RPE leads to the loss of photoreceptors and in turn, to loss of vision.
The investigators were led by Kapil Bharti, Ph.D., who directs the NEI Ocular and Stem Cell Translational Research Section, and Kiyoharu (Josh) Miyagishima, Ph.D. and Ruchi Sharma, Ph.D., staff scientists in the section and leading authors of the study. They developed a laboratory model that uses induced pluripotent stem cells developed from skin (fibroblasts) to make RPE.
The patient-derived RPE shared key characteristics of the disorder in humans, including deposits of apolipoprotein E near the tissue, and abnormal secretions of vascular endothelial growth factor, a protein that stimulates blood vessel growth.
The RPE cells also were dysmorphic, or deformed. By contrast, RPE from the unaffected siblings appeared normal. The researchers also found that the patient-derived RPE secreted far less of the mutant and the non-mutant CTRP5 protein compared with the models made from the unaffected siblings.
Using a computer modeling technique, they showed that mutant CTRP5 was less likely to bind with cell receptors that help fine-tune fat metabolic regulation. Less receptor binding in turn leads to chronic activation of AMP-activated protein kinase (AMPK), a key regulator of energy homeostasis and fat metabolism.
They theorized that when AMPK is chronically activated, it becomes less sensitive to imbalances in energy demand and nutrient supply. When metabolic imbalances run unchecked, they alter lipid metabolism, which explains how apolipoprotein E accumulates as deposits near the RPE layer.
Testing that theory, the researchers chemically inhibited the chronically activated AMPK in the patient-derived RPE model and found fewer deposits of apolipoprotein E, and less abnormal secretion of vascular endothelial growth factor.
Next, using the patient-derived RPE model, they tested two potential treatment strategies: A gene therapy approach to encourage expression of normal CTRP5 in the RPE model, and the use of the diabetes drug metformin, which appears to modulate AMPK activity, re-sensitizing it to changes in cellular energy status. Both strategies prevented signs of L-ORD in RPE models.
“Importantly, we now have two potential strategies to disrupt the L-ORD disease process. While gene therapy may be years away, metformin is a drug that’s long been used to treat diabetes,” said Bharti, who with NEI collaborators is planning a clinical trial to test the drug in people with L-ORD.
Although L-ORD disease is rare, it shares similarities with other retinal degenerations like age-related macular degeneration, a leading cause of vision loss. The model developed for this study may prove helpful in understanding such age-related disease changes in the RPE.
Retinal degeneration is one of the major reasons for vision loss, and stem cell therapy has been extensively investigated to repair and regenerate damaged retinal cells. Several types of stem cells have been tested in preclinical and clinical trials to understand their efficiency in reversing retinal degeneration.
To date, human embryonic stem cells (hESCs)-, induced pluripotent stem cells (iPSCs)-derived RPE cells, mesenchymal stem cells (MSCs) and retinal progenitor cells (RPCs) have been tested in addition to paracrine factors and exosomes derived from MSCs.
Conventional therapies for retinal diseases slow the progression of the diseases; however, the long-term benefit is achieved by repairing and regenerating the damaged retinal tissue. Moreover, since the retina does not have intrinsic regenerative properties, stem cell therapies have been sought to repair and regenerate the damaged retina.1
Several preclinical and clinical studies have demonstrated that transplantation of stem cells and factors derived from stem cells produce clinically measurable improvement. This review will discuss the different stem cells utilized to treat retinal diseases and the clinical benefits and challenges in utilizing stem cells to treat retinal degeneration.
The etiology of retinal degenerative diseases includes genetic and non-genetic factors leading to the loss of photoreceptor cells and eventually the RPE cells. Age-related macular degeneration (AMD) is one of the most common forms of vision loss, which might either be due to degradation of RPE cells (dry AMD) or choroidal neovascularization (wet AMD). Wet AMD is treated with anti-VEGF therapy, which can lead to clinical improvement in vision.2,3
In contrast, fewer therapeutic options are available to improve the vision in patients with dry AMD. Retinitis pigmentosa (RP) occurs due to autosomal4 or X-linked mutations, which contribute to the degeneration of photoreceptors leading to vision loss. FDA-approved therapeutic interventions for RP include gene therapy for patients with a biallelic mutation in RPE655 and retinal prosthesis for late-stage RP.6
Diabetic retinopathy (DR) is caused due to chronic hyperglycemia and is treated with anti-VEGF to limit the neovascularization at the proliferative stage or late-stage DR.7 Although these conventional therapies have improved the disease prognosis, repeated administration is required to diminish the disease progression, and gene therapy is applicable only for those patients with vision loss due to specific mutations.
Stem Cells for Retinal Diseases
Stem cells were tested in several clinical trials, and the approaches include transplantation of undifferentiated stem cells, pre-differentiated stem cells, or stem cell-derived factors. Several studies and clinical trials have utilized RPE cells derived from hESCs or iPSCs, and MSCs derived from various tissue sources and tested their retinal regenerative potential. Here, we have summarized and analyzed the potential of each cell type for the treatment of retinal disorders.
Preclinical Studies with Stem Cells
ESCs, due to their extensive proliferative and differentiation potential, have been used as a cell source to treat various degenerative diseases, including retinal degeneration. Subretinal transplantation of hESC-derived RPE cells in a preclinical mouse model of AMD showed no tumor growth with the transplanted cells detected at the injection site seven months after injection,8 and some injected cells formed an RPE monolayer above the native layer.9 In a similar study, RPCs derived from hESCs integrated into the mouse ganglion cell layer (GCL), expressed retinal ganglion cells (RGCs) marker Brn3a,10 and outer nuclear layer (ONL) thickness increased in the injected animals.11 In a study involving non-human primates, subretinal transplantation of hESC-derived retinal organoids was well tolerated and the transplanted cells integrated into the retinal layer in the injury site created by laser ablation.12
iPSCs, similar to ESCs, have pluripotent differentiation ability but without ethical concerns. The human iPSC-derived retina was transplanted into the subretinal space of monkeys with laser-induced retinal injury and immune-deficient rats with RP. The transplanted cells integrated into the rat retina and formed synaptic connections with the host bipolar cells. In the monkey model, the transplanted cells integrated into the host retinal layer and improved electroretinogram (ERG) and visual guided saccade (VGS) scores were observed.13
Similarly, in an RP mouse model, subretinal transplantation of iPSC-derived RPE spheroids delayed thinning of retinal ONL, increased pigment epithelium-derived factor (PEDF) levels, reduced the number of apoptotic cells as well as microglial infiltration in the retina.14
In Royal College of Surgeons (RCS) rats with an inherited mutation of MER proto-oncogene tyrosine kinase (MERTK) gene as a model of retinal degeneration, subretinal transplantation of iPSC-derived RPE cells significantly rescued visual function as measured by optokinetic tracking thresholds (OKT).
None of the animals showed abnormal proliferation or teratoma formation; however, the graft was compromised in two animals due to inflammatory response.15 Interestingly, co-transplantation of RPCs and RPE cells derived from iPSCs was superior to transplanting individual cell types, it resulted in better visual response and preservation of ONL in a rat model of retinal degeneration.16
Further, in an animal model of RP, subretinally transplanted iPSC-derived CRX-expressing photoreceptor precursors engrafted at the inner nuclear layer (INL). The transplanted cells expressed the pan cone marker, Arrestin 3, indicating further maturation.17
In a preclinical study with rats and pigs, iPSCs obtained from AMD patient CD34+ cells, when differentiated into RPE cells, integrated and rescued the retinal degeneration. In this study, the authors found that, compared to suspension cells, ten times fewer RPE cells were required to achieve the same therapeutic effect when transplanted as a monolayer.
Whereas RPE cells transplanted as cell suspension failed to integrate into the rat RPE layer, poly (lactic-co-glycolic acid) (PLGA) based scaffold facilitated the integration of transplanted RPE patch into the rat Bruch’s membrane.18 Stem cell-based therapies have also been explored as an option to treat retinal ischemic injuries with abnormal endothelial progenitor cells (EPCs) prevalent in diabetic patients. hiPSC-derived endothelial cells alleviated oxygen-induced retinal injury in mouse models and reduced pathological vaso-obliteration and neovascular tufts.19
MSCs have been studied extensively for their potential in the treatment of several retinal disorders. Here, we have discussed some recent reports that utilized MSCs in the preclinical models of retinal degeneration. Human dental-pulp-derived MSCs (DP-MSCs) on intravitreal transplantation improved the retinal function in a rat model of retinal degeneration,20 and rat bone marrow-derived MSCs (BM-MSCs) rescued the ONL thickness by enhancing autophagy.21
Intravitreal injection of umbilical cord-derived MSCs (UC-MSCs), and the exosomes derived from UC-MSCs suppressed inflammatory response, retinal damage and improved the visual functions in a mouse model of retinal injury.22
Injection of mouse BM-MSCs or paracrine factors derived from BM-MSCs into the anterior ocular chamber induced proliferation of progenitor cells in the ciliary body and promoted ocular regeneration and repair in a glaucoma mouse model.23 Similarly, conditioned media (CM) from BM-MSCs containing the paracrine factors significantly reduced the intraocular pressure (IOP) and protected the host RGCs.24
Intravenous injection of UC-MSCs reduced diabetes-associated microvascular leakage in the retina by upregulating the expression of tight junction protein occludin.25 In a streptozotocin-induced diabetic mouse model of DR, intravitreal injection of adipose tissue-derived MSCs (AD-MSCs) increased intraocular levels of neurotrophic factors and prevented the loss of RGCs.26
Several studies have analyzed the potential of MSC-derived factors, cells, and engineered MSCs to repair the damaged retina. Extracellular vesicles derived from human BM-MSCs significantly protected RGCs and prevented retinal nerve fiber layer thinning in a preclinical rat model of glaucoma.27
Injection of the stromal fraction of adipose tissue, which is enriched with pericytes, decreased vascular leakage, apoptosis and improved the “b” wave amplitude in a DR mouse model.28 Similar reductions in vascular leakage and improvements in visual acuity were observed when CM derived from human AD-MSCs were intravitreally injected in Ins2Akita mouse model of DR.29 Murine BM-MSCs genetically modified to produce neurotrophin-4 preserved the retinal bioelectrical activity in the injured retina and completely restored the laminated organization of the outer retina in an RP animal model.30
Similarly, BM-MSCs genetically engineered to express C-X-C chemokine receptor type 4,31 or PEDF32 significantly reduced the retinal damage, reduced the level of pro-inflammatory cytokines, and restored the retinal structure and function in DR disease models. Interestingly, the administration of neural stem cells derived from UC-MSCs significantly improved the vision and survival of RGCs in diabetic rats.33
refgerene link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8327474/
More information: AMPK modulation ameliorates dominant disease phenotype of CTRP5 variant in retinal degeneration, Communications Biology (2021). DOI: 10.1038/s42003-021-02872-x