Stem cells for neuroprotection in glaucoma

Addressing neurodegeneration in glaucoma: Mechanisms, challenges, and treatments

Glaucoma is the leading cause of irreversible blindness globally. The disease causes vision loss due to neurodegeneration of the retinal ganglion cell (RGC) projection to the brain through the optic nerve. Glaucoma is associated with sensitivity to intraocular pressure (IOP). Thus, mainstay treatments seek to manage IOP, though many patients continue to lose vision. To address neurodegeneration directly, numerous preclinical studies seek to develop protective or reparative therapies that act independently of IOP. These include growth factors, compounds targeting metabolism, anti-inflammatory and antioxidant agents, and neuromodulators. Despite success in experimental models, many of these approaches fail to translate into clinical benefits. Several factors contribute to this challenge. Firstly, the anatomic structure of the optic nerve head differs between rodents, nonhuman primates, and humans. Additionally, animal models do not replicate the complex glaucoma pathophysiology in humans. Therefore, to enhance the success of translating these findings, we propose two approaches. First, thorough evaluation of experimental targets in multiple animal models, including nonhuman primates, should precede clinical trials. Second, we advocate for combination therapy, which involves using multiple agents simultaneously, especially in the early and potentially reversible stages of the disease. These strategies aim to increase the chances of successful neuroprotective treatment for glaucoma.

Towards stem cell-based neuronal regeneration for glaucoma

Glaucoma is a neurodegenerative disease as a leading cause of global blindness. Retinal ganglion cell (RGC) apoptosis and optic nerve damage are the main pathological changes. Patients have elevated intraocular pressure and progressive visual field loss. Unfortunately, current treatments for glaucoma merely stay at delaying the disease progression. As a promising treatment, stem cell-based neuronal regeneration therapy holds potential for glaucoma, thereby great efforts have been paid on it. RGC regeneration and transplantation are key approaches for the future treatment of glaucoma. A line of studies have shown that a variety of cells can be used to regenerate RGCs, including embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), mesenchymal stem cells (MSCs), and retinal progenitor cells (RPCs). In this review, we overview the current progress on the regeneration of pluripotent stem cell-derived RGCs and outlook the perspective and challenges in this field.

Neuroprotective effects of BDNF and GDNF in intravitreally transplanted mesenchymal stem cells after optic nerve crush in mice

AIM

To assess the neuro-protective effect of bone marrow mesenchymal stem cells (BMSCs) on retinal ganglion cells (RGCs) following optic nerve crush in mice.

METHODS

C56BL/6J mice were treated with intravitreal injection of PBS, BMSCs, BDNF-interference BMSCs (BIM), and GDNF-interference BMSCs (GIM) following optic nerve crush, respectively. The number of surviving RGCs was determined by whole-mount retinas and frozen sections, while certain mRNA or protein was detected by q-PCR or ELISA, respectively.

RESULTS

The density (cell number/mm²) of RGCs was 410.77±56.70 in the retina 21d after optic nerve crush without any treatment, compared to 1351.39±195.97 in the normal control (P<0.05). RGCs in BMSCs treated eyes was 625.07±89.64/mm², significantly higher than that of no or PBS treatment (P<0.05). While RGCs was even less in the retina with intravitreal injection of BIM (354.07+39.77) and GIM (326.67+33.37) than that without treatment (P<0.05). BMSCs injection improved the internal BDNF expression in retinas.

CONCLUSION

Optic nerve crush caused rust loss of RGCs and intravitreally transplanted BMSCs at some extent protected RGCs from death. The effect of BMSCs and level of BDNF in retinas are both related to BDNF and GDNF expression in BMSCs.

Human neural progenitor cells decrease photoreceptor degeneration, normalize opsin distribution and support synapse structure in cultured porcine retina

Retinal neurodegenerative disorders like retinitis pigmentosa, age-related macular degeneration, diabetic retinopathy and retinal detachment decrease retinal functionality leading to visual impairment. The pathological events are characterized by photoreceptor degeneration, synaptic disassembly, remodeling of postsynaptic neurons and activation of glial cells. Despite intense research, no effective treatment has been found for these disorders. The current study explores the potential of human neural progenitor cell (hNPC) derived factors to slow the degenerative processes in adult porcine retinal explants. Retinas were cultured for 3 days with or without hNPCs as a feeder layer and investigated by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), immunohistochemical, western blot and quantitative real time-polymerase chain reaction (qRT-PCR) techniques. TUNEL showed that hNPCs had the capacity to limit photoreceptor cell death. Among cone photoreceptors, hNPC coculture resulted in better maintenance of cone outer segments and reduced opsin mislocalization. Additionally, maintained synaptic structural integrity and preservation of second order calbindin positive horizontal cells was also observed. However, Müller cell gliosis only seemed to be alleviated in terms of reduced Müller cell density. Our observations indicate that at 3 days of coculture, hNPC derived factors had the capacity to protect photoreceptors, maintain synaptic integrity and support horizontal cell survival. Human neural progenitor cell applied treatment modalities may be an effective strategy to help maintain retinal functionality in neurodegenerative pathologies. Whether hNPCs can independently hinder Müller cell gliosis by utilizing higher concentrations or by combination with other pharmacological agents still needs to be determined.

Current perspective of neuroprotection and glaucoma

Glaucoma is the second leading cause of blindness worldwide and is most notably characterized by progressive optic nerve atrophy and advancing loss of retinal ganglion cells (RGCs). The main concomitant factor is the elevated intraocular pressure (IOP). Existing treatments are focused generally on lowering IOP. However, both RGC loss and optic nerve atrophy can independently occur with IOP at normal levels. In recent years, there has been substantial progress in the development of neuroprotective therapies for glaucoma in order to restore vital visual function. The present review intends to offer a brief insight into conventional glaucoma treatments and discuss exciting current developments of mostly preclinical data in novel neuroprotective strategies for glaucoma that include recent advances in noninvasive diagnostics going beyond IOP maintenance for an enhanced global view. Such strategies now target RGC loss and optic nerve damage, opening a critical therapeutic window for preventative monitoring and treatment.

Stem cell approaches to glaucoma: from aqueous outflow modulation to retinal neuroprotection

Long-term pharmacological management of glaucoma currently relies on self-administered drugs to regulate intraocular pressure (IOP). A number of approaches using stem cells have recently shown promise as potential future treatment strategies complementary to IOP lowering. Several sources of endogenous stem cells have been identified in the eye, some of which may be able to repair the damaged trabecular meshwork and restore functional regulation of aqueous outflow. Neural and mesenchymal stem cells secrete growth factors which provide neuroprotective effects, reducing loss of retinal ganglion cells (RGCs) in animal models. In the future, stem cells may even replace RGCs to reform functional connections between the eye and the brain, although the complexity of such a repair task is formidable. With advances in biomaterial cell scaffolds and concurrent efforts in other neural systems, stem cell therapies are becoming a realistic option for treating multiple eye diseases, and despite ongoing challenges, there are reasons for optimism that stem cells may play a role in the treatment of human glaucoma in the future.

Neuroprotective therapies in glaucoma: I. Neurotrophic factor delivery

Glaucoma is a neurodegenerative eye disease that causes permanent blindness at the progressive stage and the number of people affected worldwide is expected to reach over 79 million by 2020. Currently, glaucoma management relies on pharmacological and invasive surgical treatments mainly by reducing the intraocular pressure (IOP), which is the most important risk factor for the progression of the visual field loss. Recent research suggests that neuroprotective or neuroregenerative approaches are necessary to prevent retinal ganglion cells (RGCs) loss and visual impairment over time. Neuroprotection is a new therapeutic strategy that attempts to keep RGCs alive and functional. New gene and cell therapeutics encoding neurotrophic factors (NTFs) are emerging for both neuroprotection and regenerative treatments for retinal diseases. This article briefly reviews the role of NTFs in glaucoma and the potential delivery systems.

Neuroprotective effects of intravitreally transplanted adipose tissue and bone marrow-derived mesenchymal stem cells in an experimental ocular hypertension model

BACKGROUND AIMS

The purpose of this study was to investigate the neuroprotective effects of bone marrow bone marrow-derived and adipose tissue-derived mesenchymal stromal cells (MSCs) that were intravitreally transplanted in an experimental ocular hypertension (OHT) model.

METHODS

An OHT rat model was generated by means of intracameral injection of hyaluronic acid into the anterior chamber. MSCs labeled with green fluorescence protein were transplanted intravitreally 1 week after OHT induction. At the end of the second and fourth weeks, retinal ganglion cells were visualized with the use of a flat-mount retina method and were evaluated by means of immunofluorescence staining against green fluorescence protein, vimentin, CD105, and cytokines (interleukin [IL]-1Ra, prostaglandin E2 receptor, IL-6, transforming growth factor-β1, interferon-γ and tumor necrosis factor-α).

RESULTS

The retinal ganglion cell numbers per area were significantly improved in stem cell-treated OHT groups compared with that in the non-treated OHT group (P < 0.05). The results of immunohistochemical analyses indicated that a limited number of stem cells had integrated into the ganglion cell layer and the inner nuclear layer. The number of cells expressing proinflammatory cytokines (interferon-γ and tumor necrosis factor-α) decreased in the MSC-transferred group compared with that in the OHT group after 4 weeks (P < 0.01). On the other hand, IL-1Ra and prostaglandin E2 receptor expressions were increased in the rat bone marrow-derived MSC group but were more significant in the rat adipose tissue-derived MSC group (P < 0.01).

CONCLUSIONS

After intravitreal transplantation, MSCs showed a neuroprotective effect in the rat OHT model. Therefore, MSCs promise an alternative therapy approach for functional recovery in the treatment of glaucoma.

Stem cell therapy for glaucoma: possibilities and practicalities

Glaucoma is a progressive, neurodegenerative, optic neuropathy in which currently available therapies cannot always prevent, and do not reverse, vision loss. Stem cell transplantation may provide a promising new avenue for treating many presently incurable degenerative conditions, including glaucoma. This article will explore the various ways in which transplantation of stem or progenitor cells may be applied for the treatment of glaucoma. We will critically discuss the translational prospects of two cell transplantation-based treatment modalities: neuroprotection and retinal ganglion cell replacement. In addition, we will identify specific questions that need to be addressed and obstacles to overcome on the path to clinical translation, and offer insight into potential strategies for approaching this goal.

Identification of retinal ganglion cell neuroprotection conferred by platelet-derived growth factor through analysis of the mesenchymal stem cell secretome

The development of neuroprotective strategies to attenuate retinal ganglion cell death could lead to novel therapies for chronic optic neuropathies such as glaucoma. Intravitreal transplantation of mesenchymal stem cells slows retinal ganglion cell death in models of optic nerve injury, but the mechanism of action remains unclear. Here we characterized the neuroprotective effects of mesenchymal stem cells and mesenchymal stem cell-derived factors in organotypic retinal explant culture and an in vivo model of ocular hypertensive glaucoma. Co-culture of rat and human bone marrow-derived mesenchymal stem cells with retinal explants increased retinal ganglion cell survival, after 7 days ex vivo, by ∼2-fold and was associated with reduced apoptosis and increased nerve fibre layer and inner plexiform layer thicknesses. These effects were not demonstrated by co-culture with human or mouse fibroblasts. Conditioned media from mesenchymal stem cells conferred neuroprotection, suggesting that the neuroprotection is mediated, at least partly, by secreted factors. We compared the concentrations of 29 factors in human mesenchymal stem cell and fibroblast conditioned media, and identified 11 enriched in the mesenchymal stem cell secretome. Treatment of retinal explants with a cocktail of these factors conferred retinal ganglion cell neuroprotection, with factors from the platelet-derived growth factor family being the most potent. Blockade of platelet-derived growth factor signalling with neutralizing antibody or with small molecule inhibitors of platelet-derived growth factor receptor kinase or downstream phosphatidylinositol 3 kinase eliminated retinal ganglion cell neuroprotection conferred by mesenchymal stem cell co-culture. Intravitreal injection of platelet-derived growth factor -AA or -AB led to profound optic nerve neuroprotection in vivo following experimental induction of elevated intraocular pressure. These data demonstrate that mesenchymal stem cells secrete a number of neuroprotective proteins and suggest that platelet-derived growth factor secretion in particular may play an important role in mesenchymal stem cell-mediated retinal ganglion cell neuroprotection. Furthermore, platelet-derived growth factor may represent an independent target for achieving retinal ganglion cell neuroprotection.

Application of human persistent fetal vasculature neural progenitors for transplantation in the inner retina

Persistent fetal vasculature (PFV) is a potentially serious developmental anomaly in human eyes, which results from a failure of the primary vitreous and the hyaloid vascular systems to regress during development. Recent findings from our laboratory indicate that fibrovascular membranes harvested from subjects with PFV contain neural progenitor cells (herein called NPPFV cells). Our studies on successful isolation, culture, and characterization of NPPFV cells have shown that they highly express neuronal progenitor markers (nestin, Pax6, and Ki67) as well as retinal neuronal markers (β-III-tubulin and Brn3a). In the presence of retinoic acid and neurotrophins, these cells acquire a neural morphological appearance in vitro, including a round soma and extensive neurites, and express mature neuronal markers (β-III-tubulin and NF200). Further experiments, including real-time qRT-PCR to quantify characteristic gene expression profiles of these cells and Ca(2+) imaging to evaluate the response to stimulation with different neurotransmitters, indicate that NPPFV cells may resemble a more advanced stage of retinal development and show more differentiation toward inner retinal neurons rather than photoreceptors. To explore the potential of inner retinal transplantation, NPPFV cells were transplanted intravitreally into the eyes of adult C57BL/6 mice. Engrafted NPPFV cells survived well in the intraocular environment in presence of rapamycin and some cells migrated into the inner nuclear layer of the retina 1 week posttransplantation. Three weeks after transplantation, NPPFV cells were observed to migrate and integrate in the inner retina. In response to daily intraperitoneal injections of retinoic acid, a portion of transplanted NPPFV cells exhibited retinal ganglion cell-like morphology and expressed mature neuronal markers (β-III-tubulin and synaptophysin). These findings indicate that fibrovascular membranes from human PFV harbor a population of neuronal progenitors that may be potential candidates for cell-based therapy for degenerative diseases of the inner retina.

Towards retinal ganglion cell regeneration

Traumatic optic nerve injury and glaucoma are among the leading causes of incurable vision loss across the world. What is worse, neither pharmacological nor surgical interventions are significantly effective in reversing or halting the progression of vision loss. Advances in cell biology offer some hope for the victims of optic nerve damage and subsequent partial or complete visual loss. Retinal ganglion cells (RGCs) travel through the optic nerve and carry all visual signals to the brain. After injury, RGC axons usually fail to regrow and die, leading to irreversible loss of vision. Various kinds of cells and factors possess the ability to support the process of axon regeneration for RGCs. This article summarizes the latest advances in RGC regeneration.

Neuroprotection in glaucoma

Glaucoma is a neurodegenerative disease characterized by loss of retinal ganglion cells and their axons. Recent evidence suggests that intraocular pressure (IOP) is only one of the many risk factors for this disease. Current treatment options for this disease have been limited to the reduction of IOP; however, it is clear now that the disease progression continues in many patients despite effective lowering of IOP. In the search for newer modalities in treating this disease, much data have emerged from experimental research the world over, suggesting various pathological processes involved in this disease and newer possible strategies to treat it. This review article looks into the current understanding of the pathophysiology of glaucoma, the importance of neuroprotection, the various possible pharmacological approaches for neuroprotection and evidence of current available medications.

Glaucoma: genes, phenotypes, and new directions for therapy

Glaucoma, a leading cause of blindness worldwide, is characterized by progressive optic nerve damage, usually associated with intraocular pressure. Although the clinical progression of the disease is well defined, the molecular events responsible for glaucoma are currently poorly understood and current therapeutic strategies are not curative. This review summarizes the human genetics and genomic approaches that have shed light on the complex inheritance of glaucoma genes and the potential for gene-based and cellular therapies that this research makes possible.

Adult retinal stem cells revisited

Recent advances in retinal stem cell research have raised the possibility that these cells have the potential to be used to repair or regenerate diseased retina. Various cell sources for replacement of retinal neurons have been identified, including embryonic stem cells, the adult ciliary epithelium, adult Müller stem cells and induced pluripotent stem cells (iPS). However, the true stem cell nature of the ciliary epithelium and its possible application in cell therapies has now been questioned, leaving other cell sources to be carefully examined as potential candidates for such therapies. The need for identification of the ontogenetic state of grafted stem cells in order to achieve their successful integration into the murine retina has been recognized. However, it is not known whether the same requirements may apply to achieve transplant cell integration into the adult human eye. In addition, the existence of natural barriers for stem cell transplantation, including microglial accumulation and abnormal extracellular matrix deposition have been demonstrated, suggesting that several obstacles need to be overcome before such therapies may be implemented. This review addresses recent scientific developments in the field and discusses various strategies that may be potentially used to design cell based therapies to treat human retinal disease.

Neurotrophic factor delivery as a protective treatment for glaucoma

Glaucoma is a progressive optic neuropathy and a major cause of visual impairment worldwide. Neuroprotective therapies for glaucoma aim to ameliorate retinal ganglion cell degeneration through direct or indirect action on these neurons. Neurotrophic factor (NTF) delivery is a key target for the development of potential neuroprotective glaucoma treatments. This article will critically summarize the evidence that NTF deprivation and/or dysfunction plays a role in the pathogenesis of glaucoma. Experimental support for the neuroprotective potential of NTF supplementation in animal models of glaucoma will be reviewed, in particular for brain-derived neurotrophic factor, ciliary neurotrophic factor, and glial cell line-derived neurotrophic factor. Finally, the challenges of clinical translation will be considered with an emphasis on the most promising NTF delivery strategies including slow-release drug delivery, gene therapy, and cell transplantation.

Bayesian adaptive estimation of the contrast sensitivity function: the quick CSF method

The contrast sensitivity function (CSF) predicts functional vision better than acuity, but long testing times prevent its psychophysical assessment in clinical and practical applications. This study presents the quick CSF (qCSF) method, a Bayesian adaptive procedure that applies a strategy developed to estimate multiple parameters of the psychometric function (A. B. Cobo-Lewis, 1996; L. L. Kontsevich & C. W. Tyler, 1999). Before each trial, a one-step-ahead search finds the grating stimulus (defined by frequency and contrast) that maximizes the expected information gain (J. V. Kujala & T. J. Lukka, 2006; L. A. Lesmes et al., 2006), about four CSF parameters. By directly estimating CSF parameters, data collected at one spatial frequency improves sensitivity estimates across all frequencies. A psychophysical study validated that CSFs obtained with 100 qCSF trials ( approximately 10 min) exhibited good precision across spatial frequencies (SD < 2-3 dB) and excellent agreement with CSFs obtained independently (mean RMSE = 0.86 dB). To estimate the broad sensitivity metric provided by the area under the log CSF (AULCSF), only 25 trials were needed to achieve a coefficient of variation of 15-20%. The current study demonstrates the method's value for basic and clinical investigations. Further studies, applying the qCSF to measure wider ranges of normal and abnormal vision, will determine how its efficiency translates to clinical assessment.

Identification of barriers to retinal engraftment of transplanted stem cells

PURPOSE

Intraocular stem cell transplantation may be therapeutic for retinal neurodegenerative diseases such as glaucoma via neuronal replacement and/or neuroprotection. However, efficacy is hindered by extremely poor retinal graft integration. The purpose was to identify the major barrier to retinal integration of intravitreally transplanted stem cells, which was hypothesized to include the cellular and/or extracellular matrix (ECM) components of the inner limiting membrane (ILM).

METHODS

Mesenchymal stem cells (MSCs) were cocultured on the vitreal surface of retinal explants. Retinal MSC migration was compared between control explants and explants in which portions of the ILM were removed by mechanical peeling; the inner basal lamina was digested with collagenase; and glial cell reactivity was selectively modulated with alpha-aminoadipic acid (AAA). In vivo, the MSCs were transplanted after intravitreal AAA or saline injection into glaucomatous rat eyes.

RESULTS

Retinal MSC migration correlated positively with the amount of peeled ILM, whereas enzymatic digestion of the basal lamina was robust but did not enhance MSC entry. In contrast, AAA treatment suppressed glial cell reactivity and facilitated a >50-fold increase in MSC migration into retinal explants. In vivo analysis showed that AAA treatment led to a more than fourfold increase in retinal engraftment.

CONCLUSIONS

The results demonstrated that the ECM of the inner basal lamina is neither necessary nor sufficient to prevent migration of transplanted cells into the neural retina. In contrast, glial reactivity was associated with poor graft migration. Targeted disruption of glial reactivity dramatically improved the structural integration of intravitreally transplanted cells.

Transplantation prospects for the inner retina

Transplantation of stem or progenitor cells is an attractive new approach for treating neurodegenerative conditions of the central nervous system, which aims to protect or replace neurons and improve function. Proof of principle has already been shown in the retina that photoreceptors may be replaced by transplantation of neural progenitor cells. However, the task of retinal ganglion cell replacement is much more complex, as new cells will need to establish complex connections within the retina and also extend axons to precise targets in the brain. Although progress has been made in this field, it is likely that neuroprotective clinical applications will be established more quickly. Our laboratory has focused on the intraocular transplantation of cells to treat inner retinal disease, either by neuronal replacement or neuroprotection of existing cells. We have investigated the efficacy and effects of transplanting a variety of cell types, including human Müller stem cells (MIO-M1), oligodendrocyte precursor cells (OPCs), and bone marrow-derived mesenchymal stromal cells (MSCs) in a rat model of experimentally induced glaucoma. We also have developed and characterized a novel in vitro organotypic retinal explant culture system for exploring the methods of enhancing the efficacy of cell transplantation for the inner retina. In this review, we discuss the potentially beneficial effects of intraocular cell injections, identify current shortcomings of retinal stem cell therapy, and suggest directions for future research.