Molecular and cell-based approaches for neuroprotection in glaucoma

CD3ζ-Mediated Signaling Protects Retinal Ganglion Cells in Glutamate Excitotoxicity of the Retina

Excessive levels of glutamate activity could potentially damage and kill neurons. Glutamate excitotoxicity is thought to play a critical role in many CNS and retinal diseases. Accordingly, glutamate excitotoxicity has been used as a model to study neuronal diseases. Immune proteins, such as major histocompatibility complex (MHC) class I molecules and their receptors, play important roles in many neuronal diseases, while T-cell receptors (TCR) are the primary receptors of MHCI. We previously showed that a critical component of TCR, CD3ζ, is expressed by mouse retinal ganglion cells (RGCs). The mutation of CD3ζ or MHCI molecules compromises the development of RGC structure and function. In this study, we investigated whether CD3ζ-mediated molecular signaling regulates RGC death in glutamate excitotoxicity. We show that mutation of CD3ζ significantly increased RGC survival in NMDA-induced excitotoxicity. In addition, we found that several downstream molecules of TCR, including Src (proto-oncogene tyrosine-protein kinase) family kinases (SFKs) and spleen tyrosine kinase (Syk), are expressed by RGCs. Selective inhibition of an SFK member, Hck, or Syk members, Syk or Zap70, significantly increased RGC survival in NMDA-induced excitotoxicity. These results provide direct evidence to reveal the underlying molecular mechanisms that control RGC death under disease conditions.

Fractal Phototherapy in Maximizing Retina and Brain Plasticity

The neuroplasticity potential is reduced with aging and impairs during neurodegenerative diseases and brain and visual system injuries. This limits the brain's capacity to repair the structure and dynamics of its activity after lesions. Maximization of neuroplasticity is necessary to provide the maximal CNS response to therapeutic intervention and adaptive reorganization of neuronal networks in patients with degenerative pathology and traumatic injury to restore the functional activity of the brain and retina.Considering the fractal geometry and dynamics of the healthy brain and the loss of fractality in neurodegenerative pathology, we suggest that the application of self-similar visual signals with a fractal temporal structure in the stimulation therapy can reactivate the adaptive neuroplasticity and enhance the effectiveness of neurorehabilitation. This proposition was tested in the recent studies. Patients with glaucoma had a statistically significant positive effect of fractal photic therapy on light sensitivity and the perimetric MD index, which shows that methods of fractal stimulation can be a novel nonpharmacological approach to neuroprotective therapy and neurorehabilitation. In healthy rabbits, it was demonstrated that a long-term course of photostimulation with fractal signals does not harm the electroretinogram (ERG) and retina structure. Rabbits with modeled retinal atrophy showed better dynamics of the ERG restoration during daily stimulation therapy for a week in comparison with the controls. Positive changes in the retinal function can indirectly suggest the activation of its adaptive plasticity and the high potential of stimulation therapy with fractal visual stimuli in a nonpharmacological neurorehabilitation, which requires further study.

Molecular Mechanisms of Glaucoma Pathogenesis with Implications to Caveolin Adaptor Protein and Caveolin-Shp2 Axis

Glaucoma is a common retinal disorder characterized by progressive optic nerve damage, resulting in visual impairment and potential blindness. Elevated intraocular pressure (IOP) is a major risk factor, but some patients still experience disease progression despite IOP-lowering treatments. Genome-wide association studies have linked variations in the Caveolin1/2 (CAV-1/2) gene loci to glaucoma risk. Cav-1, a key protein in caveolae membrane invaginations, is involved in signaling pathways and its absence impairs retinal function. Recent research suggests that Cav-1 is implicated in modulating the BDNF/TrkB signaling pathway in retinal ganglion cells, which plays a critical role in retinal ganglion cell (RGC) health and protection against apoptosis. Understanding the interplay between these proteins could shed light on glaucoma pathogenesis and provide potential therapeutic targets.

Subtype-specific survival and regeneration of retinal ganglion cells in response to injury

Retinal ganglion cells (RGCs) are a heterogeneous population of neurons that function synchronously to convey visual information through the optic nerve to retinorecipient target areas in the brain. Injury or disease to the optic nerve results in RGC degeneration and loss of visual function, as few RGCs survive, and even fewer can be provoked to regenerate their axons. Despite causative insults being broadly shared, regeneration studies demonstrate that RGC types exhibit differential resilience to injury and undergo selective survival and regeneration of their axons. While most early studies have identified these RGC types based their morphological and physiological characteristics, recent advances in transgenic and gene sequencing technologies have further enabled type identification based on unique molecular features. In this review, we provide an overview of the well characterized RGC types and identify those shown to preferentially survive and regenerate in various regeneration models. Furthermore, we discuss cellular characteristics of both the resilient and susceptible RGC types including the combinatorial expression of different molecular markers that identify these specific populations. Lastly, we discuss potential molecular mechanisms and genes found to be selectively expressed by specific types that may contribute to their reparative capacity. Together, we describe the studies that lay the important groundwork for identifying factors that promote neural regeneration and help advance the development of targeted therapy for the treatment of RGC degeneration as well as neurodegenerative diseases in general.

Construction and analysis of mRNA, lncRNA, and transcription factor regulatory networks after retinal ganglion cell injury

Retinal ganglion cell (RGC) injury is a critical pathological feature of several optic neurodegenerative diseases. The regulatory mechanisms underlying RGC injury remain poorly understood. Recent evidence has highlighted the important roles of long noncoding RNAs (lncRNAs) in degenerative neuropathy but few studies have focused on lncRNAs associated with RGC injury. In this study, we analyzed dysregulated lncRNAs associated with RGC injury, their potential regulatory functions, and the molecular mechanisms underlying the regulation of lncRNAs and transcription factors (TFs). We analyzed lncRNA and mRNA profiles in the GSE142881 dataset associated with RGC injury and identified 1049 differentially expressed genes (DEGs), with 18 differentially expressed (DE) TFs among 883 DE mRNAs and 312 DE lncRNAs. The predicted DE lncRNAs and DE mRNAs were used to construct a lncRNA-mRNA co-expression network. Functional enrichment analysis was performed to explore the functions of the lncRNAs and mRNAs. The co-expression network between DE lncRNAs and DE mRNAs was highly enriched in inflammatory and immune-related pathways, indicating that they play role in the process of RGC injury. Among the DE mRNAs, we screened 18 DE TFs, including activating transcription factor 3 (ATF3), associated with RGC injury. Co-expression analysis predicted that 13 lncRNAs were potential binding targets of ATF3. The screening of the potential targets of these 13 lncRNAs showed that they were also significantly enriched in functional pathways associated with inflammation and apoptosis. After analysis, we constructed the mRNA-ATF3-lncRNA regulatory network after RGCs injury. In summary, we identified the gene module associated with immune and inflammatory responses after optic nerve injury and constructed a regulatory network of lncRNA-TF-mRNA. The results indicate that lncRNAs, by binding to TFs, can regulate downstream genes and function during RGC injury. The results provide a foundation for further studies of the mechanism of RGC injury and provide insight into the clinical diagnosis and investigation direction of neurodegenerative diseases such as traumatic optic neuropathy and glaucoma.

Neurotrophic Factors in Glaucoma and Innovative Delivery Systems

Glaucoma is a neurodegenerative disease and a worldwide leading cause of irreversible vision loss. In the last decades, high efforts have been made to develop novel treatments effective in inducing protection and/or recovery of neural function in glaucoma, including neurotrophic factors (NTFs). These approaches have shown encouraging data in preclinical setting; however, the challenge of sustained, targeted delivery to the retina and optic nerve still prevents the clinical translation. In this paper, the authors review and discuss the most recent advances for the use of NTFs treatment in glaucoma, including intraocular delivery. Novel strategies in drug and gene delivery technology for NTFs are proving effective in promoting long-term retinal ganglion cells (RGCs) survival and related functional improvements. Results of experimental and clinical studies evaluating the efficacy and safety of biodegradable slow-release NTF-loaded microparticle devices, encapsulated NTF-secreting cells implants, mimetic ligands for NTF receptors, and viral and non-viral NTF gene vehicles are discussed. NTFs are able to prevent and even reverse apoptotic ganglion cell death. Nevertheless, neuroprotection in glaucoma remains an open issue due to the unmet need of sustained delivery to the posterior segment of the eye. The recent advances in intraocular delivery systems pave the way for possible future use of NTFs in clinical practice for the treatment of glaucoma.

Cell therapy assisted autotransplantation of olfactory tract into the optic nerve: A potential treatment for optic neuropathy

Optic neuropathy is an invaliding pathology with diverse clinical manifestation and varying causes. Current understanding of etiopathological aspects of optical neuropathy does not provide an effective treatment protocol. In this article we discuss existing treatment methods, and their effectiveness, evaluated depending on disease etiology. The olfactory tract is a source of olfactory ensheating cells, whose unique properties can have treatment potential in correction of nerve degeneration. Transplantation of an olfactory tract graft into the damaged optic nerve is a technically achievable intervention, though anatomical limitations exist in the proposed surgical access. Optic nerve defects can also be potentially treated with axon growth stimulating therapy (Zymosan and CTP-cAMP). Optic neuropathy can be potentially cured by autotransplantation of a portion of the olfactory tract. Neuroanatomical and histomorphological aspects of olfactory tract autotransplantation into the damaged optic nerve are provided. Feasibility, technical and anatomical features, potential setbacks and limitations are discussed. Anatomical limitations exist, but with current neurosurgical technology can be overcome. Regenerative potential of olfactory tract glial cells plays an important role in nerve restoration and can play a crucial role in further understanding of nerve degeneration treatment.

Neuroprotection: A versatile approach to combat glaucoma

In most retinal diseases, neuronal loss is the main cause of vision loss. Neuroprotection is the alteration of neurons and/or their environment to encourage the survival and function of the neurons, especially in environments that are deleterious to the neuronal health. The area of neuroprotection progresses with a therapeutically-based hope of improving vision and clinical outcomes for patients through the developments in neurotrophic therapy, antioxidative therapy, anti-excitotoxic, anti-ischemic, anti-inflammatory, and anti-apoptotic care. In this review, we summarize the various neuroprotection strategies for the treatment of glaucoma, genetics of glaucoma and the role of various nanoplatforms in the treatment of glaucoma.

Differentiation of soluble aqueous humor metabolites in primary open angle glaucoma and controls

We report an analysis of the aqueous humor (AH) metabolome of primary open angle glaucoma (POAG) in comparison to normal controls. The AH samples were obtained from human donors [control (n = 35), POAG (n = 23)]. The AH samples were subjected to one-dimensional ¹H nuclear magnetic resonance (NMR) analyses on a Bruker Avance 600 MHz instrument with a 1.7 mM NMR probe. The same samples were then subjected to isotopic ratio outlier analysis (IROA) using a Q Exactive orbitrap mass spectrometer after chromatography on an Accela 600 HPLC. Clusterfinder Build 3.1.10 was used for identification and quantification based on long-term metabolite matrix standards. In total, 278 metabolites were identified in control samples and 273 in POAG AH. The metabolites identified were fed into previously reported proteome and genome information and the OmicsNet interaction network generator to construct a protein-metabolite interactions network with an embedded protein-protein network. Significant differences in metabolite composition in POAG compared to controls were identified indicating potential protein/gene pathways associated with these metabolites. These results will expand our previous understanding of the impeded AH metabolite composition, provide new insight into the regulation of AH outflow, and likely aid in future AH and trabecular meshwork multi-omics network analyses.

The Susceptibility of Retinal Ganglion Cells to Optic Nerve Injury is Type Specific

Retinal ganglion cell (RGC) death occurs in many eye diseases, such as glaucoma and traumatic optic neuropathy (TON). Increasing evidence suggests that the susceptibility of RGCs varies to different diseases in an RGC type-dependent manner. We previously showed that the susceptibility of several genetically identified RGC types to N-methyl-D-aspartate (NMDA) excitotoxicity differs significantly. In this study, we characterize the susceptibility of the same RGC types to optic nerve crush (ONC). We show that the susceptibility of these RGC types to ONC varies significantly, in which BD-RGCs are the most resistant RGC type while W3-RGCs are the most sensitive cells to ONC. We also show that the survival rates of BD-RGCs and J-RGCs after ONC are significantly higher than their survival rates after NMDA excitotoxicity. These results are consistent with the conclusion that the susceptibility of RGCs to ONC varies in an RGC type-dependent manner. Further, the susceptibilities of the same types of RGCs to ONC and NMDA excitotoxicity are significantly different. These are valuable insights for understanding of the selective susceptibility of RGCs to various pathological insults and the development of a strategy to protect RGCs from death in disease conditions.

A Multidisciplinary Consensus for Clinical Care and Research Needs for Sturge-Weber Syndrome

BACKGROUND

Sturge-Weber syndrome is a neurocutaneous disorder associated with port-wine birthmark, leptomeningeal capillary malformations, and glaucoma. It is associated with an unpredictable clinical course. Because of its rarity and complexity, many physicians are unaware of the disease and its complications. A major focus moving ahead will be to turn knowledge gaps and unmet needs into new research directions.

METHODS

On October 1-3, 2017, the Sturge-Weber Foundation assembled clinicians from the Clinical Care Network with patients from the Patient Engagement Network of the Sturge-Weber Foundation to identify our current state of knowledge, knowledge gaps, and unmet needs.

RESULTS

One clear unmet need is a need for consensus guidelines on care and surveillance. It was strongly recommended that patients be followed by multidisciplinary clinical teams with life-long follow-up for children and adults to monitor disease progression in the skin, eye, and brain. Standardized neuroimaging modalities at specified time points are needed together with a stronger clinicopathologic understanding. Uniform tissue banking and clinical data acquisition strategies are needed with cross-center, longitudinal studies that will set the stage for new clinical trials. A better understanding of the pathogenic roles of cerebral calcifications and stroke-like symptoms is a clear unmet need with potentially devastating consequences.

CONCLUSIONS

Biomarkers capable of predicting disease progression will be needed to advance new therapeutic strategies. Importantly, how to deal with the emotional and psychological effects of Sturge-Weber syndrome and its impact on quality of life is a clear unmet need.

A Promising Tool in Retina Regeneration: Current Perspectives and Challenges When Using Mesenchymal Progenitor Stem Cells in Veterinary and Human Ophthalmological Applications

Visual impairment is a common ailment of the current world population, with more exposure to CCD screens and fluorescent lighting, approximately 285 billion people suffer from this deficiency and 13% of those are considered clinically blind. More common causes for visual impairment include age-related macular degeneration (AMD), glaucoma and diabetic retinopathy (Zhu et al. Molecular Medicine Reports, 2015; Kolb et al. 2007; Machalińska et al. Current Eye Research, 34(9),748-760, 2009) among a few. As cases of retinal and optic nerve diseases rise, it is vital to find a treatment, which has led to investigation of the therapeutic potential of various stem cells types (Bull et al. Investigative Opthalmology & Visual Science, 50(9), 4244, 2009; Bull et al. Investigative Opthalmology & Visual Science, 49(8), 3449, 2008; Yu et al. Biochemical and Biophysical Research Communications, 344(4), 1071-1079, 2006; Na et al. Graefe's Archive for Clinical and Experimental Ophthalmology, 247(4), 503-514, 2008). In previous studies, some of the stem cell variants used include human Muller SCs and bone marrow derived SCs. Some of the regenerative potential characteristics of mesenchymal progenitor stem cells (MSCs) include their multilineage differentiation potential, their immunomodulatory effects, their high proliferative activity, they can be easily cultured in vitro, and finally their potential to synthesize and secrete membrane derived vesicles rich in growth factors, mRNA and miRNA which possibly aid in regulation of tissue damage regeneration. These facts alone, explain why MSCs are so widely used in clinical trials, 350 up to date (Switonski, Reproductive Biology, 14(1), 44-50, 2014). Animal studies have demonstrated that sub-retinal transplantation of MSCs delays retinal degeneration and preserves retinal function through trophic response (Inoue et al. Experimental Eye Research, 85(2), 234-241, 2007). Umbilical cord derived MSCs (UC/MSCs) have also been shown to contain neuroprotective features of ganglion cells in rat studies (Zwart et al. Experimental Neurology, 216(2), 439-448, 2009). This review aims to present current MSC therapies in practice, as well as their retinal regeneration potential in animal models, and their innovative prospects for treatment of human retinal diseases.

Advances in experimental optic nerve regeneration

PURPOSE OF REVIEW

Recent advances in experimental studies of optic nerve regeneration to better understand the pathophysiology of axon regrowth and provide insights into the future treatment of numerous optic neuropathies.

RECENT FINDINGS

The optic nerve is part of the central nervous system and cannot regenerate if injured. There are several steps that regenerating axons of retinal ganglion cells (RGCs) must take following optic nerve injury that include: maximizing the intrinsic growth capacity of RGCs, overcoming the extrinsic growth-inhibitory environment of the optic nerve, and optimizing the reinnervation of regenerated axons to their targets in the brain. Recently, some degree of experimental optic nerve regeneration has been achieved by factors associated with inducing intraocular inflammation, providing exogenous neurotrophic factors, reactivating intrinsic growth capacity of mature RGCs, or by modifying the extrinsic growth-inhibitory environment of the optic nerve. In some experiments, regenerating axons have been shown to reinnervate their central targets in the brain.

SUMMARY

Further approaches to the combination of aforementioned treatments will be necessary to develop future therapeutic strategy to promote ultimate regeneration of the optic nerve and functional vision recovery after optic nerve injury.

Role of nuclear factor (erythroid-derived 2)-like 2 in the age-resistant properties of the glaucoma trabecular meshwork

Glaucoma is a major cause of irreversible blindness. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) regulates the expression of numerous antioxidants within cells and is therefore a focus of current ophthalmic research. To determine the roles of Nrf2 in mediating the glaucoma trabecular meshwork (GTM), the present study evaluated the levels of Nrf2 expression in GTM and human trabecular meshwork (HTM) cells by reverse-transcription-quantitative polymerase chain reaction and western blot analysis. It was principally observed that Nrf2 expression was downregulated in GTM cells. In addition, to determine the influence of Nrf2 on the apoptosis and proliferation of GTM and HTM cells, transfection assays and western blotting were performed to evaluate the expression of apoptosis-related proteins. The results of the current study indicated that Nrf2 may promote viability and reduce apoptosis in GTM and HTM cells. Collectively, these data suggest that Nrf2 may be a novel therapeutic target to treat glaucoma.

Oral Delivery of a Synthetic Sterol Reduces Axonopathy and Inflammation in a Rodent Model of Glaucoma

Glaucoma is a group of optic neuropathies associated with aging and sensitivity to intraocular pressure (IOP). The disease is the leading cause of irreversible blindness worldwide. Early progression in glaucoma involves dysfunction of retinal ganglion cell (RGC) axons, which comprise the optic nerve. Deficits in anterograde transport along RGC axons to central visual structures precede outright degeneration, and preventing these deficits is efficacious at abating subsequent progression. HE3286 is a synthetic sterol derivative that has shown therapeutic promise in models of inflammatory disease and neurodegenerative disease. We examined the efficacy of HE3286 oral delivery in preventing loss of anterograde transport in an inducible model of glaucoma (microbead occlusion). Adult rats received HE3286 (20 or 100 mg/kg) or vehicle daily via oral gavage for 4 weeks. Microbead occlusion elevated IOP ~30% in all treatment groups, and elevation was not affected by HE3286 treatment. In the vehicle group, elevated IOP reduced anterograde axonal transport to the superior colliculus, the most distal site in the optic projection, by 43% (p = 0.003); HE3286 (100 mg/kg) prevented this reduction (p = 0.025). HE3286 increased brain-derived neurotrophic factor (BDNF) in the optic nerve head and retina, while decreasing inflammatory and pathogenic proteins associated with elevated IOP compared to vehicle treatment. Treatment with HE3286 also increased nuclear localization of the transcription factor NFκB in collicular and retinal neurons, but decreased NFκB in glial nuclei in the optic nerve head. Thus, HE3286 may have a neuroprotective influence in glaucoma, as well as other chronic neurodegenerations.

Using genetic mouse models to gain insight into glaucoma: Past results and future possibilities

While all forms of glaucoma are characterized by a specific pattern of retinal ganglion cell death, they are clinically divided into several distinct subclasses, including normal tension glaucoma, primary open angle glaucoma, congenital glaucoma, and secondary glaucoma. For each type of glaucoma there are likely numerous molecular pathways that control susceptibility to the disease. Given this complexity, a single animal model will never precisely model all aspects of all the different types of human glaucoma. Therefore, multiple animal models have been utilized to study glaucoma but more are needed. Because of the powerful genetic tools available to use in the laboratory mouse, it has proven to be a highly useful mammalian system for studying the pathophysiology of human disease. The similarity between human and mouse eyes coupled with the ability to use a combination of advanced cell biological and genetic tools in mice have led to a large increase in the number of studies using mice to model specific glaucoma phenotypes. Over the last decade, numerous new mouse models and genetic tools have emerged, providing important insight into the cell biology and genetics of glaucoma. In this review, we describe available mouse genetic models that can be used to study glaucoma-relevant disease/pathobiology. Furthermore, we discuss how these models have been used to gain insights into ocular hypertension (a major risk factor for glaucoma) and glaucomatous retinal ganglion cell death. Finally, the potential for developing new mouse models and using advanced genetic tools and resources for studying glaucoma are discussed.

Is there a relationship between brain-derived neurotrophic factor for driving neuronal auditory circuits with onset of auditory function and the changes following cochlear injury or during aging?

Brain-derived neurotrophic factor, BDNF, is one of the most important neurotrophic factors acting in the peripheral and central nervous system. In the auditory system its function was initially defined by using constitutive knockout mouse mutants and shown to be essential for survival of neurons and afferent innervation of hair cells in the peripheral auditory system. Further examination of BDNF null mutants also revealed a more complex requirement during re-innervation processes involving the efferent system of the cochlea. Using adult mouse mutants defective in BDNF signaling, it could be shown that a tonotopical gradient of BDNF expression within cochlear neurons is required for maintenance of a specific spatial innervation pattern of outer hair cells and inner hair cells. Additionally, BDNF is required for maintenance of voltage-gated potassium channels (KV) in cochlear neurons, which may form part of a maturation step within the ascending auditory pathway with onset of hearing and might be essential for cortical acuity of sound-processing and experience-dependent plasticity. A presumptive harmful role of BDNF during acoustic trauma and consequences of a loss of cochlear BDNF during aging are discussed in the context of a partial reversion of this maturation step. We compare the potentially beneficial and harmful roles of BDNF for the mature auditory system with those BDNF functions known in other sensory circuits, such as the vestibular, visual, olfactory, or somatosensory system.

Critical neuroprotective roles of heme oxygenase-1 induction against axonal injury-induced retinal ganglion cell death

Although axonal damage induces significant retinal ganglion cell (RGC) death, small numbers of RGCs are able to survive up to 7 days after optic nerve crush (NC) injury. To develop new treatments, we set out to identify patterns of change in the gene expression of axonal damage-resistant RGCs. To compensate for the low density of RGCs in the retina, we performed retrograde labeling of these cells with 4Di-10ASP in adult mice and 7 days after NC purified the RGCs with fluorescence-activated cell sorting. Gene expression in the cells was determined with a microarray, and the expression of Ho-1 was determined with quantitative PCR (qPCR). Changes in protein expression were assessed with immunohistochemistry and immunoblotting. Additionally, the density of Fluoro-gold-labeled RGCs was counted in retinas from mice pretreated with CoPP, a potent HO-1 inducer. The microarray and qPCR analyses showed increased expression of Ho-1 in the post-NC RGCs. Immunohistochemistry also showed that HO-1-positive cells were present in the ganglion cell layer (GCL), and cell counting showed that the proportion of HO-1-positive cells in the GCL rose significantly after NC. Seven days after NC, the number of RGCs in the CoPP-treated mice was significantly higher than in the control mice. Combined pretreatment with SnPP, an HO-1 inhibitor, suppressed the neuroprotective effect of CoPP. These results reflect changes in HO-1 activity to RGCs that are a key part of RGC survival. Upregulation of HO-1 signaling may therefore be a novel therapeutic strategy for glaucoma.

Magnetic nanoparticles as intraocular drug delivery system to target retinal pigmented epithelium (RPE)

One of the most challenging efforts in drug delivery is the targeting of the eye. The eye structure and barriers render this organ poorly permeable to drugs. Quite recently the entrance of nanoscience in ocular drug delivery has improved the penetration and half-life of drugs, especially in the anterior eye chamber, while targeting the posterior chamber is still an open issue. The retina and the retinal pigment epithelium/choroid tissues, located in the posterior eye chamber, are responsible for the majority of blindness both in childhood and adulthood. In the present study, we used magnetic nanoparticles (MNPs) as a nanotool for ocular drug delivery that is capable of specific localization in the retinal pigmented epithelium (RPE) layer. We demonstrate that, following intraocular injection in Xenopus embryos, MNPs localize specifically in RPE where they are retained for several days. The specificity of the localization did not depend on particle size and surface properties of the MNPs used in this work. Moreover, through similar experiments in zebrafish, we demonstrated that the targeting of RPE by the nanoparticles is not specific for the Xenopus species.

Ciliary neurotrophic factor and fibroblast growth factor increase the speed and number of regenerating axons after optic nerve injury in adult Rana pipiens

Neurotrophins such as ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF) and growth factors such as fibroblast growth factor (FGF-2) play important roles in neuronal survival and in axonal outgrowth during development. However, whether they can modulate regeneration after optic nerve injury in the adult animal is less clear. The present study investigates the effects of application of these neurotrophic factors on the speed, number, and distribution of regenerating axons in the frog Rana pipiens after optic nerve crush. Optic nerves were crushed and the factors, or phosphate-buffered saline, were applied to the stump or intraocularly. The nerves were examined at different times after axotomy, using anterograde labeling with biotin dextran amine and antibody against growth-associated protein 43. We measured the length, number, and distribution of axons projecting beyond the lesion site. Untreated regenerating axons show an increase in elongation rate over 3 weeks. CNTF more than doubles this rate, FGF-2 increases it, and BDNF has little effect. In contrast, the numbers of regenerating axons that have reached 200 μm at 2 weeks were more than doubled by FGF-2, increased by CNTF, and barely affected by BDNF. The regenerating axons were preferentially distributed in the periphery of the nerve; although the numbers of axons were increased by neurotrophic factor application, this overall distribution was substantially unaffected.

The potential of stem cell research for the treatment of neuronal damage in glaucoma

Stem cell research offers a wide variety of approaches for the advancement of our understanding of basic mechanisms of neurodegeneration and tissue regeneration and for the discovery and development of new therapeutic strategies to prevent and restore neuronal cell loss. Similar to most other regions of our central nervous system, degenerative diseases of the retina lead to the loss of neurons, which are not replaced. Recent work in animals has provided proof-of-concept evidence for the restoration of photoreceptor cells by cell transplantation and neuronal cell replacement by regeneration from endogenous cell sources. However, efficient therapeutic prevention of neuronal cell loss has not been achieved. Moreover, successful cell replacement of retinal neurons in humans, including that of ganglion cells, remains a major challenge. Future successes in the discovery and translation of neuroprotective drug and gene therapies and of cell-based regenerative therapies will depend on a better understanding of the underlying disease pathomechanisms. Existing stem cell and cell-reprogramming technologies offer the potential to generate human retina cells, to develop specific human-cell-based retina disease models, and to open up novel therapeutic strategies. Further, we might glean substantial knowledge from species that can or cannot regenerate their neuronal retina, in the search for new therapeutic approaches. Thus, stem cell research will pave the way toward clinical translation. In this review, I address some of the major possibilities presently on offer and speculate about the power of stem cell research to gain further insights into the pathomechanisms of retinal neurodegeneration (with special emphasis on glaucoma) and to advance our therapeutic options.

Clinical trials for glaucoma neuroprotection are not impossible

PURPOSE OF REVIEW

To discuss issues that affect development of new glaucoma treatments known as neuroprotection.

RECENT FINDINGS

There are barriers to neuroprotection trials for glaucoma.

SUMMARY

With design features discussed in the review, neuroprotection trials can be effectively carried out.

The insulin-like growth factor 1 receptor is essential for axonal regeneration in adult central nervous system neurons

Axonal regeneration is an essential condition to re-establish functional neuronal connections in the injured adult central nervous system (CNS), but efficient regrowth of severed axons has proven to be very difficult to achieve. Although significant progress has been made in identifying the intrinsic and extrinsic mechanisms involved, many aspects remain unresolved. Axonal development in embryonic CNS (hippocampus) requires the obligate activation of the insulin-like growth factor 1 receptor (IGF-1R). Based on known similarities between axonal growth in fetal compared to mature CNS, we decided to examine the expression of the IGF-1R, using an antibody to the βgc subunit or a polyclonal anti-peptide antibody directed to the IGF-R (C20), in an in vitro model of adult CNS axonal regeneration, namely retinal ganglion cells (RGC) derived from adult rat retinas. Expression of both βgc and the β subunit recognized by C20 antibody were low in freshly isolated adult RGC, but increased significantly after 4 days in vitro. As in embryonic axons, βgc was localised to distal regions and leading growth cones in RGC. IGF-1R-βgc co-localised with activated p85 involved in the phosphatidylinositol-3 kinase (PI3K) signaling pathway, upon stimulation with IGF-1. Blocking experiments using either an antibody which neutralises IGF-1R activation, shRNA designed against the IGF-1R sequence, or the PI3K pathway inhibitor LY294002, all significantly reduced axon regeneration from adult RGC in vitro (∼40% RGC possessed axons in controls vs 2-8% in the different blocking studies). Finally, co-transfection of RGC with shRNA to silence IGF-1R together with a vector containing a constitutively active form of downstream PI3K (p110), fully restored axonal outgrowth in vitro. Hence these data demonstrate that axonal regeneration in adult CNS neurons requires re-expression and activation of IGF-1R, and targeting this system may offer new therapeutic approaches to enhancing axonal regeneration following trauma.

Hydrocortisone stimulates neurite outgrowth from mouse retinal explants by modulating macroglial activity

PURPOSE

There is mounting evidence that retinal ganglion cells (RGCs) require a complex milieu of trophic factors to enhance cell survival and axon regeneration after optic nerve injury. The authors' goal was to examine the contribution of components of a combination of hormones, growth factors, steroids, and small molecules to creating a regenerative environment and to determine if any of these components modulated macroglial behavior to aid in regeneration.

METHODS

Postnatal day 7 mouse retinal explants embedded in collagen were used as an in vitro model of neurite regeneration. Explants were treated with the culture supplements fetal bovine serum, N2, and G5 and a mixture of G5 and N2 components, designated enhanced N2 (EN2). Explants were evaluated for neurite outgrowth over 7 days in culture. The effects of each treatment were also evaluated on cultured RGCs purified by Thy1 immunopanning. Immunohistochemistry and qPCR analysis were used to evaluate differences in gene expression in the explants due to different treatments.

RESULTS

EN2 stimulated significant neurite outgrowth from explants but not from purified RGCs. Elimination of hydrocortisone (HC) from EN2 reduced the mean neurites per explant by 37%. EN2-treated explants demonstrated increased expression of Gfap, Glul, Glt1, Cntf, Pedf, and VegfA compared with explants treated with EN2 without HC. Subsequent experiments showed that increased expression of Cntf and Glul was critical to the trophic effect of HC.

CONCLUSIONS

These data suggest that the HC in EN2 indirectly contributed to neurite outgrowth by activating macroglia to produce neurotrophic and neuroprotective molecules.

Four steps to optic nerve regeneration

The failure of the optic nerve to regenerate after injury or in neurodegenerative disease remains a major clinical and scientific problem. Retinal ganglion cell (RGC) axons course through the optic nerve and carry all the visual information to the brain, but after injury, they fail to regrow through the optic nerve and RGC cell bodies typically die, leading to permanent loss of vision. There are at least 4 hurdles to overcome in preserving RGCs and regenerating their axons: 1) increase RGC survival, 2) overcome the inhibitory environment of the optic nerve, 3) enhance RGC intrinsic axon growth potential, and 4) optimize the mapping of RGC connections back into their targets in the brain.

Current and emerging medical therapies in the treatment of glaucoma

INTRODUCTION

Glaucoma is a disease of the eye in which the optic nerve and retinal ganglion cells (RGCs) are injured, leading to the loss of the peripheral visual field and eventually to profound vision loss and blindness. Glaucoma is usually characterized by an increase in intraocular pressure (IOP), which is treated with ocular hypotensive drugs. However, both RGC apoptosis and optic nerve atrophy, due to glaucoma, can occur independently of IOP.

AREAS COVERED

This review discusses several current and emerging treatments for glaucoma. Current research is updating the known properties of a number of drugs now used to treat glaucoma. Some drugs may offer neuroprotection, not only reducing vision loss, but restoring injured or compromised RGCs and optic nerve cells. Several molecules now under development aim to lower IOP primarily by enhancing aqueous drainage through conventional pathways of the trabecular meshwork and Schlemm's canal. Gene transfer models are being investigated, and a murine-derived neurotrophic growth factor (NGF) seems to offer the promise of actually restoring visual function in some patients. Drugs that are already widely used are being re-branded in preservative-free formulations.

EXPERT OPINION

The ultimate goal in glaucoma research is to find new compounds that will not only normalize IOP, but also arrest or even reverse apoptotic damage to the optic nerve and RGCs to slow the rate of progression of the disease so that it will not interfere with the patient's ability to see and his/her quality of life. This should be obtained with affordable costs, minimal side effects and a reasonable schedule.

Early gene expression changes in the retinal ganglion cell layer of a rat glaucoma model

PURPOSE

To identify patterns of early gene expression changes in the retinal ganglion cell layer (GCL) of a rodent model of chronic glaucoma.

METHODS

Prolonged elevation of intraocular pressure (IOP) was produced in rats by episcleral vein injection of hypertonic saline (N = 30). GCLs isolated by laser capture microdissection were grouped by grading of the nerve injury (<25% axon degeneration for early injury; >25% for advanced injury). Gene expression was determined by cDNA microarray of independent GCL RNA samples. Quantitative PCR (qPCR) was used to further examine the expression of selected genes.

RESULTS

By array analysis, 533 GCL genes (225 up, 308 down) were significantly regulated in early injury. Compared to only one major upregulated gene class of metabolism regulation, more were downregulated, including mitochondria, ribosome, proteasome, energy pathways, protein synthesis, protein folding, and synaptic transmission. qPCR confirmed an early upregulation of Atf3. With advanced injury, 1790 GCL genes were significantly regulated (997 up, 793 down). Altered gene categories included upregulated protein synthesis, immune response, and cell apoptosis and downregulated dendrite morphogenesis and axon extension. Of all the early changed genes, 50% were not present in advanced injury. These uniquely affected genes were mainly associated with upregulated transcription regulation and downregulated protein synthesis.

CONCLUSIONS

Early GCL gene responses to pressure-induced injury are characterized by an upregulation of Atf3 and extensive downregulation in genes associated with cellular metabolism and neuronal functions. Most likely, these changes represent those specific to RGCs and are thus potentially important for enhancing RGC survival in glaucoma.

Nanoparticles for retinal gene therapy

Ocular gene therapy is becoming a well-established field. Viral gene therapies for the treatment of Leber's congentinal amaurosis (LCA) are in clinical trials, and many other gene therapy approaches are being rapidly developed for application to diverse ophthalmic pathologies. Of late, development of non-viral gene therapies has been an area of intense focus and one technology, polymer-compacted DNA nanoparticles, is especially promising. However, development of pharmaceutically and clinically viable therapeutics depends not only on having an effective and safe vector but also on a practical treatment strategy. Inherited retinal pathologies are caused by mutations in over 220 genes, some of which contain over 200 individual disease-causing mutations, which are individually very rare. This review will focus on both the progress and future of nanoparticles and also on what will be required to make them relevant ocular pharmaceutics.

CYP1B1, a developmental gene with a potential role in glaucoma therapy

The association of CYP1B1 gene alterations in primary congenital glaucoma individuals has been known for about a decade. Recent evidence has shown the involvement of CYP1B1 mutations in a number of forms of glaucoma and anterior segment disorders. This suggests a wide role for CYP1B1 in ocular physiology. Histochemical studies of eyes from individuals with primary congenital glaucoma revealed abnormalities in the anterior chamber angle, the region between the cornea and the iris, containing the trabecular meshwork. The cells of the trabecular meshwork serve as a filter to allow drainage of the aqueous humour, the fluid formed by the ciliary body that fills the anterior chamber. Mutations in CYP1B1 that affect its activity have frequently been shown to influence development of the trabecular meshwork, and it is thought that CYP1B1, a monooxygenase, acts to form or degrade some endobiotic compound that is necessary for proper development of the filtering structures. The rapidly developing area of stem cell research suggests a potential therapeutic approach for glaucomas resulting from deleterious mutations in CYP1B1, that is, the transfer of stem cells, differentiated to a specific lineage, containing wild-type CYP1B1 to specific regions of the eye, where they will develop into normal cells of that region and rectify the defect.

Combined application of BDNF to the eye and brain enhances ganglion cell survival and function in the cat after optic nerve injury

PURPOSE

To determine whether application of BDNF to the eye and brain provides a greater level of neuroprotection after optic nerve injury than treatment of the eye alone.

METHODS

Retinal ganglion cell survival and pattern electroretinographic responses were compared in normal cat eyes and in eyes that received (1) a mild nerve crush and no treatment, (2) a single intravitreal injection of BDNF at the time of the nerve injury, or (3) intravitreal treatment combined with 1 to 2 weeks of continuous delivery of BDNF to the visual cortex, bilaterally.

RESULTS

Relative to no treatment, administration of BDNF to the eye alone resulted in a significant increase in ganglion cell survival at both 1 and 2 weeks after nerve crush (1 week, 79% vs. 55%; 2 weeks, 60% vs. 31%). Combined treatment of the eye and visual cortex resulted in a modest additional increase (17%) in ganglion cell survival in the 1-week eyes, a further significant increase (55%) in the 2-week eyes, and ganglion cell survival levels for both that were comparable to normal (92%-93% survival). Pattern ERG responses for all the treated eyes were comparable to normal at 1 week after injury; however, at 2 weeks, only the responses of eyes receiving the combined BDNF treatment remained so.

CONCLUSIONS

Although treatment of the eye alone with BDNF has a significant impact on ganglion cell survival after optic nerve injury, combined treatment of the eye and brain may represent an even more effective approach and should be considered in the development of future optic neuropathy-related neuroprotection strategies.

Early cellular signaling responses to axonal injury

Background

We have used optic nerve injury as a model to study early signaling events in neuronal tissue following axonal injury. Optic nerve injury results in the selective death of retinal ganglion cells (RGCs). The time course of cell death takes place over a period of days with the earliest detection of RGC death at about 48 hr post injury. We hypothesized that in the period immediately following axonal injury, there are changes in the soma that signal surrounding glia and neurons and that start programmed cell death. In the current study, we investigated early changes in cellular signaling and gene expression that occur within the first 6 hrs post optic nerve injury.

Results

We found evidence of cell to cell signaling within 30 min of axonal injury. We detected differences in phosphoproteins and gene expression within the 6 hrs time period. Activation of TNFα and glutamate receptors, two pathways that can initiate cell death, begins in RGCs within 6 hrs following axonal injury. Differential gene expression at 6 hrs post injury included genes involved in cytokine, neurotrophic factor signaling (Socs3) and apoptosis (Bax).

Conclusion

We interpret our studies to indicate that both neurons and glia in the retina have been signaled within 30 min after optic nerve injury. The signals are probably initiated by the RGC soma. In addition, signals activating cellular death pathways occur within 6 hrs of injury, which likely lead to RGC degeneration.