Nerve growth factor modulation of retinal ganglion cell physiology

Molecular pathways in experimental glaucoma models

Glaucoma is a complex and progressive disease that primarily affects the optic nerve axons, leading to irreversible vision loss. Although the exact molecular mechanisms underlying glaucoma pathogenesis are not fully understood, it is believed that except increased intraocular pressure, a combination of genetic and environmental factors play a role in the development of the disease. Animal models have been widely used in the study of glaucoma, allowing researchers to better understand the underlying mechanisms of the disease and test potential treatments. Several molecular pathways have been implicated in the pathogenesis of glaucoma, including oxidative stress, inflammation, and excitotoxic-induced neurodegeneration. This review summarizes the most important knowledge about molecular mechanisms involved in the glaucoma development. Although much research has been done to better understand the molecular mechanisms underlying this disease, there is still much to be learned to develop effective treatments and prevent vision loss in those affected by glaucoma.

Neurotrophins and Trk Neurotrophin Receptors in the Retina of Adult Killifish (Nothobranchius guentheri)

Specific subpopulations of neurons in nerve and sensory systems must be developed and maintained, and this is accomplished in significant part by neurotrophins (NTs) and the signaling receptors on which they act, called tyrosine protein kinase receptors (Trks). The neurotrophins-tyrosine protein kinase receptors (NTs/Trks) system is involved in sensory organ regulation, including the visual system. An NTs/Trks system alteration is associated with neurodegeneration related to aging and diseases, including retinal pathologies. An emergent model in the field of translational medicine, for instance, in aging study, is the annual killifish belonging to the Nothobranchius genus, thanks to its short lifespan. Members of this genus, such as Nothobranchius guentheri, and humans share a similar retinal stratigraphy. Nevertheless, according to the authors' knowledge, the occurrence and distribution of the NTs/Trks system in the retina of N. guentheri has never been investigated before. Therefore, the present study aimed to localize neurotrophin BDNF, NGF, and NT-3 and TrkA, TrkB, and TrkC receptors in the N. guentheri retina using the immunofluorescence method. The present investigation demonstrates, for the first time, the occurrence of the NTs/Trks system in N. guentheri retina and, consequently, the potential key role of these proteins in the biology and survival of the retinal cells.

Optic nerve diseases and regeneration: How far are we from the promised land?

The retinal ganglion cells (RGCs) are the sole output neurons that connect information from the retina to the brain. Optic neuropathies such as glaucoma, trauma, inflammation, ischemia and hereditary optic neuropathy can cause RGC loss and axon damage, and lead to partial or total loss of vision, which is an irreversible process in mammals. The accurate diagnoses of optic neuropathies are crucial for timely treatments to prevent irrevocable RGCs loss. After severe ON damage in optic neuropathies, promoting RGC axon regeneration is vital for restoring vision. Clearance of neuronal debris, decreased intrinsic growth capacity, and the presence of inhibitory factors have been shown to contribute to the failure of post-traumatic CNS regeneration. Here, we review the current understanding of manifestations and treatments of various common optic neuropathies. We also summarise the current known mechanisms of RGC survival and axon regeneration in mammals, including specific intrinsic signalling pathways, key transcription factors, reprogramming genes, inflammation-related regeneration factors, stem cell therapy, and combination therapies. Significant differences in RGC subtypes in survival and regenerative capacity after injury have also been found. Finally, we highlight the developmental states and non-mammalian species that are capable of regenerating RGC axons after injury, and cellular state reprogramming for neural repair.

NGF Prevents Loss of TrkA/VEGFR2 Cells, and VEGF Isoform Dysregulation in the Retina of Adult Diabetic Rats

Among the factors involved in diabetic retinopathy (DR), nerve growth factor (NGF) and vascular endothelial growth factor A (VEGFA) have been shown to affect both neuronal survival and vascular function, suggesting that their crosstalk might influence DR outcomes. To address this question, the administration of eye drops containing NGF (ed-NGF) to adult Sprague Dawley rats receiving streptozotocin (STZ) intraperitoneal injection was used as an experimental paradigm to investigate NGF modulation of VEGFA and its receptor VEGFR2 expression. We show that ed-NGF treatment prevents the histological and vascular alterations in STZ retina, VEGFR2 expression decreased in GCL and INL, and preserved the co-expression of VEGFR2 and NGF-tropomyosin-related kinase A (TrkA) receptor in retinal ganglion cells (RGCs). The WB analysis confirmed the NGF effect on VEGFR2 expression and activation, and showed a recovery of VEGF isoform dysregulation by suppressing STZ-induced VEGFA₁₂₁ expression. Reduction in inflammatory and pro-apoptotic intracellular signals were also found in STZ+NGF retina. These findings suggest that ed-NGF administration might favor neuroretina protection, and in turn counteract the vascular impairment by regulating VEGFR2 and/or VEGFA isoform expression during the early stages of the disease. The possibility that an increase in the NGF availability might contribute to the switch from the proangiogenic/apoptotic to the neuroprotective action of VEGF is discussed.

Phase 1b Randomized Controlled Study of Short Course Topical Recombinant Human Nerve Growth Factor (rhNGF) for Neuroenhancement in Glaucoma: Safety, Tolerability, and Efficacy Measure Outcomes

PURPOSE

No approved therapies directly target retinal ganglion cells (RGCs) for neuroprotection or neuroenhancement in glaucoma. Recombinant human nerve growth factor (rhNGF) has been shown to promote RGC survival and function in animal models of optic neuropathy. Here we evaluate the safety, tolerability, and efficacy of short-term, high-dose rhNGF eye drops versus placebo in a cohort of glaucoma patients.

DESIGN

This was a prospective, phase 1b, single-center, randomized, double-masked, vehicle-controlled, parallel-group study.

METHODS

This study was designed to assess safety and tolerability as well as short-term neuroenhancement of structure and function (clinicaltrials.gov NCT02855450). A total of 60 open-angle glaucoma patients were randomized 40:20 to receive either 180 μg/mL rhNGF or vehicle control eye drops in both eyes, 3 times daily for 8 weeks, with a 24-week post-treatment follow-up. One eye was officially selected as the study eye, although both eyes were studied and dosed. Primary endpoints were safety, as assessed by adverse events, and tolerability, as assessed by patient-reported outcomes. Secondary outcome measures included best corrected visual acuity (BCVA), Humphrey visual field, electroretinograpy (ERG), and optical coherence tomography (OCT) of retinal nerve fiber layer (RNFL) thickness at baseline, after 8 weeks of treatment, and at 4 and 24 weeks after treatment (12 and 32 weeks total).

RESULTS

Of the 60 randomized patients, 23 were female (38%) and the average age was 66.1 years. Through week 32, there were no treatment-related serious adverse events, including no unexpectedly severe progression of optic neuropathy, no adverse events affecting ocular function or pressure, and no drug-related systemic toxicity. Topical high-dose rhNGF was tolerated well, with a low level of symptom burden mainly eliciting periocular ache (in 52% of treated group and 5% of placebo group) and only 3 patients (7.5%) discontinuing treatment because of discomfort, of whom 1 patient (2.5%) prematurely withdrew from the study. There were no statistically significant differences in global indices of Humphrey visual field and no meaningful differences in total, quadrant, or clock-hour mean RNFL thickness between the groups, although both of these function and structure measures showed nonsignificant trends toward significance in favor of rhNGF. Real-world participant data was used to generate an estimate of cohort size needed to power subsequent studies.

CONCLUSIONS

Use of rhNGF is safe and tolerable in a topical 180-μg/mL formulation. Although no statistically significant short-term neuroenhancement was detected in this trial, given the strong effects of NGF in preclinical models and the trends detected in this study, analysis for efficacy in a neuroprotection trial is warranted. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

DZNep protects against retinal ganglion cell death in an NMDA-induced mouse model of retinal degeneration

Epigenetic gene enhancer of zeste homolog-2 (Ezh2) is reported to be associated with ocular neurodegenerative diseases; however, its underlying mechanism is poorly understood. The present study aimed to determine the role of 3-deazaneplanocin A (DZNep), which inhibits the transcription of Ezh2 by reducing the trimethylation of histone 3 lysine 27 (H3K27me3), in a retinal ganglion cell (RGC) degeneration model. Retinal damage was caused by intravitreal injection of N-methyl-D-aspartate (NMDA). DZNep and the vehicle control were intravitreally applied immediately post-NMDA injection. The severity of retinal damage was evaluated by immunofluorescence and terminal deoxyribonucleotide transferase (TdT)-mediated dUTP nick end labeling (TUNEL) staining, and retinal function was determined by electroretinogram (ERG). The transcriptome was examined by RNA sequencing and quantitative PCR (qPCR). Microglial cells were detected by immunohistochemistry. DZNep significantly prevented the cell death in the ganglion cell layer (GCL) and inner nuclear layer (INL) induced by NMDA. DZNep preserved the ERG b- and a-wave amplitudes and the b/a ratio in NMDA-treated mice. Moreover, RNA sequencing and qPCR revealed that neuroprotective genes were upregulated and played an important role in preserving retinal cells. In addition, DZNep inhibited the NMDA-induced activation of microglial cells. Our results suggest that H3K27me3 controls RGC survival at the transcriptional and epigenetic levels. The absence of H3K27me3 deposition upregulates neuroprotective genes to protect RGCs. Therefore, DZNep, which inhibits Ezh2 activity, could be a novel therapeutic treatment for ocular neurodegenerative diseases.

Nerve Growth Factor-Based Therapy in Alzheimer's Disease and Age-Related Macular Degeneration

Alzheimer’s disease (AD) is an age-associated neurodegenerative disease which is the most common cause of dementia among the elderly. Imbalance in nerve growth factor (NGF) signaling, metabolism, and/or defect in NGF transport to the basal forebrain cholinergic neurons occurs in patients affected with AD. According to the cholinergic hypothesis, an early and progressive synaptic and neuronal loss in a vulnerable population of basal forebrain involved in memory and learning processes leads to degeneration of cortical and hippocampal projections followed by cognitive impairment with accumulation of misfolded/aggregated Aβ and tau protein. The neuroprotective and regenerative effects of NGF on cholinergic neurons have been largely demonstrated, both in animal models of AD and in living patients. However, the development of this neurotrophin as a disease-modifying therapy in humans is challenged by both delivery limitations (inability to cross the blood–brain barrier (BBB), poor pharmacokinetic profile) and unwanted side effects (pain and weight loss). Age-related macular degeneration (AMD) is a retinal disease which represents the major cause of blindness in developed countries and shares several clinical and pathological features with AD, including alterations in NGF transduction pathways. Interestingly, nerve fiber layer thinning, degeneration of retinal ganglion cells and changes of vascular parameters, aggregation of Aβ and tau protein, and apoptosis also occur in the retina of both AD and AMD. A protective effect of ocular administration of NGF on both photoreceptor and retinal ganglion cell degeneration has been recently described. Besides, the current knowledge about the detection of essential trace metals associated with AD and AMD and their changes depending on the severity of diseases, either systemic or locally detected, further pave the way for a promising diagnostic approach. This review is aimed at describing the employment of NGF as a common therapeutic approach to AMD and AD and the diagnostic power of detection of essential trace metals associated with both diseases. The multiple approaches employed to allow a sustained release/targeting of NGF to the brain and its neurosensorial ocular extensions will be also discussed, highlighting innovative technologies and future translational prospects.

Nerve Growth Factor Biodelivery: A Limiting Step in Moving Toward Extensive Clinical Application?

Nerve growth factor (NGF) was the first-discovered member of the neurotrophin family, a class of bioactive molecules which exerts powerful biological effects on the CNS and other peripheral tissues, not only during development, but also during adulthood. While these molecules have long been regarded as potential drugs to combat acute and chronic neurodegenerative processes, as evidenced by the extensive data on their neuroprotective properties, their clinical application has been hindered by their unexpected side effects, as well as by difficulties in defining appropriate dosing and administration strategies. This paper reviews aspects related to the endogenous production of NGF in healthy and pathological conditions, along with conventional and biomaterial-assisted delivery strategies, in an attempt to clarify the impediments to the clinical application of this powerful molecule.

Why chitosan could be apt candidate for glaucoma drug delivery - An overview

Most of the people in the world are affected by glaucoma, which leads to irreversible blindness. Several patient friendly treatments are available, nevertheless medications lack an easy and efficient way of sustained delivery. To make the delivery with enhanced bioavailability, biodegradable and non-biodegradable polymers-based drug carriers are explored. However, ocular drug delivery issues have not been resolved yet due to less adhesiveness, poor penetration ability, pH, and temperature dependent burst releases. Chitosan is found to be effective for ocular drug delivery due to excellent physio-chemical properties in terms of overcoming the existing issues. In this review, we aim to highlight why it has been chosen and the holy grail for ocular drug delivery. Besides, we have comprehensively reviewed recent patents on chitosan as a platform for ocular drug delivery and future perspectives on factors, lacunae and challenges that need to be addressed for better ocular delivery methods for glaucoma management.

Astragalus membranaceus Injection Protects Retinal Ganglion Cells by Regulating the Nerve Growth Factor Signaling Pathway in Experimental Rat Traumatic Optic Neuropathy

Activation of the nerve growth factor (NGF) signaling pathway is a potential method of treatment for retinal ganglion cell (RGC) loss due to traumatic optic neuropathy (TON). The present study aimed to explore the biological effects of injecting Astragalus membranaceus (A. mem) on RGCs in an experimental TON model. Adult male Wistar rats were randomly divided into three groups: sham-operated (SL), model (ML), and A. mem injection (AL). The left eyes of the rats were considered the experimental eyes, and the right eyes served as the controls. AL rats received daily intraperitoneal injections of A. mem (3 mL/kg), whereas ML and SL rats were administered the same volume of normal saline. The TON rat model was induced by optic nerve (ON) transverse quantitative traction. After two-week administration, the number of RGCs was determined using retrograde labeling with Fluoro-Gold. The protein levels of NGF, tyrosine kinase receptor A (TrkA), c-Jun N-terminal protein kinase (JNK), JNK phosphorylation (p-JNK), and nuclear factor kappa-B (NF-κB) were assessed using western blotting. The levels of p75 neurotrophin receptor (p75^NTR) and NF-κB DNA binding were examined using real-time PCR and an electrophoretic mobility shift assay. In addition, the concentrations of JNK and p-JNK were assessed using an enzyme-linked immunosorbent assay. Results. The number of RGCs in ML was found to be significantly decreased (P < 0.01) relative to both AL and SL, together with the downregulation of NGF (P < 0.01), TrkA (P < 0.05), and NF-κB (P < 0.01); upregulation of p75^NTR mRNA (P < 0.01); and increased protein levels of JNK (P < 0.05) and p-JNK (P < 0.05). Treatment using A. mem injection significantly preserved the density of RGCs in rats with experimental TON and markedly upregulated the proteins of NGF (P < 0.01), TrkA (P < 0.05), and NF-κB (P < 0.01) and downregulated the mRNA level of p75^NTR(P < 0.01), as well as the proteins of JNK (P < 0.05) and p-JNK (P < 0.01). Thus, A. mem injection could reduce RGC death in TON induced by ON transverse quantitative traction by stimulating the NGF signaling pathway.

Topical delivery of nerve growth factor for treatment of ocular and brain disorders

Neurotrophins are a family of proteins that support neuronal proliferation, survival, and differentiation in the central and peripheral nervous systems, and are regulators of neuronal plasticity. Nerve growth factor is one of the best-described neurotrophins and has advanced to clinical trials for treatment of ocular and brain diseases due to its trophic and regenerative properties. Prior trials over the past few decades have produced conflicting results, which have principally been ascribed to adverse effects of systemic nerve growth factor administration, together with poor penetrance of the blood-brain barrier that impairs drug delivery. Contrastingly, recent studies have revealed that topical ocular and intranasal nerve growth factor administration are safe and effective, suggesting that topical nerve growth factor delivery is a potential alternative to both systemic and invasive intracerebral delivery. The therapeutic effects of local nerve growth factor delivery have been extensively investigated for different ophthalmic diseases, including neurotrophic keratitis, glaucoma, retinitis pigmentosa, and dry eye disease. Further, promising pharmacologic effects were reported in an optic glioma model, which indicated that topically administered nerve growth factor diffused far beyond where it was topically applied. These findings support the therapeutic potential of delivering topical nerve growth factor preparations intranasally for acquired and degenerative brain disorders. Preliminary clinical findings in both traumatic and non-traumatic acquired brain injuries are encouraging, especially in pediatric patients, and clinical trials are ongoing. The present review will focus on the therapeutic effects of both ocular and intranasal nerve growth factor delivery for diseases of the brain and eye.

Neuroprotective Strategies for Retinal Ganglion Cell Degeneration: Current Status and Challenges Ahead

The retinal ganglion cells (RGCs) are the output cells of the retina into the brain. In mammals, these cells are not able to regenerate their axons after optic nerve injury, leaving the patients with optic neuropathies with permanent visual loss. An effective RGCs-directed therapy could provide a beneficial effect to prevent the progression of the disease. Axonal injury leads to the functional loss of RGCs and subsequently induces neuronal death, and axonal regeneration would be essential to restore the neuronal connectivity, and to reestablish the function of the visual system. The manipulation of several intrinsic and extrinsic factors has been proposed in order to stimulate axonal regeneration and functional repairing of axonal connections in the visual pathway. However, there is a missing point in the process since, until now, there is no therapeutic strategy directed to promote axonal regeneration of RGCs as a therapeutic approach for optic neuropathies.

Potential of Müller Glia for Retina Neuroprotection

Müller glia constitute the main glial cells of the retina. They are spatially distributed along this tissue, facilitating their close membrane interactions with all retinal neurons. Müller glia are characterized by their active metabolic functions, which are neuroprotective in nature. Although they can become reactive under pathological conditions, leading to their production of inflammatory and neurotoxic factors, their main metabolic functions confer neuroprotection to the retina, resulting in the promotion of neural cell repair and survival. In addition to their protective metabolic features, Müller glia release several neurotrophic factors and antioxidants into the retinal microenvironment, which are taken up by retinal neurons for their survival. This review summarizes the Müller glial neuroprotective mechanisms and describes advances made on the clinical application of these factors for the treatment of retinal degenerative diseases. It also discusses prospects for the use of these cells as a vehicle to deliver neuroprotective factors into the retina.

Topical recombinant human Nerve growth factor (rh-NGF) is neuroprotective to retinal ganglion cells by targeting secondary degeneration

Optic neuropathy is a major cause of irreversible blindness worldwide, and no effective treatment is currently available. Secondary degeneration is believed to be the major contributor to retinal ganglion cell (RGC) death, the endpoint of optic neuropathy. Partial optic nerve transection (pONT) is an established model of optic neuropathy. Although the mechanisms of primary and secondary degeneration have been delineated in this model, until now how this is influenced by therapy is not well-understood. In this article, we describe a clinically translatable topical, neuroprotective treatment (recombinant human nerve growth factor, rh-NGF) predominantly targeting secondary degeneration in a pONT rat model. Topical application of rh-NGF twice daily for 3 weeks significantly improves RGC survival as shown by reduced RGC apoptosis in vivo and increased RGC population in the inferior retina, which is predominantly affected in this model by secondary degeneration. Topical rh-NGF also promotes greater axonal survival and inhibits astrocyte activity in the optic nerve. Collectively, these results suggest that topical rh-NGF exhibits neuroprotective effects on retinal neurons via influencing secondary degeneration process. As topical rh-NGF is already involved in early clinical trials, this highlights its potential in multiple indications in patients, including those affected by glaucomatous optic neuropathy.

Neuro-protection and neuro-regeneration of the optic nerve: recent advances and future directions

PURPOSE OF REVIEW

Optic neuropathies refer to a collection of diseases in which retinal ganglion cells (RGCs), the specialized neuron of the retina whose axons make up the optic nerve, are selectively damaged. Blindness secondary to optic neuropathies is irreversible as RGCs do not have the capacity for self-renewal and have a limited capacity for self-repair. Numerous strategies are being developed to either prevent further RGC degeneration or replace the cells that have degenerated. In this review, we aim to discuss known limitations to regeneration in central nervous system (CNS), followed by a discussion of previous, current, and future strategies for optic nerve neuroprotection as well as approaches for neuro-regeneration, with an emphasis on developments in the past two years.

RECENT FINDINGS

Neuro-regeneration in the CNS is limited by both intrinsic and extrinsic factors. Environmental barriers to axon regeneration can be divided into two major categories: failure to clear myelin and formation of glial scar. Although inflammatory scars block axon growth past the site of injury, inflammation also provides important signals that activate reparative and regenerative pathways in RGCs. Neuroprotection with neurotrophins as monotherapy is not effective at preventing RGC degeneration likely secondary to rapid clearance of growth factors. Novel approaches involve exploiting different technologies to provide sustained delivery of neurotrophins. Other approaches include application of anti-apoptosis molecules and anti-axon retraction molecules. Although stem cells are becoming a viable option for generating RGCs for cell-replacement-based strategies, there are still many critical barriers to overcome before they can be used in clinical practice. Adjuvant treatments, such as application of electrical fields, scaffolds, and magnetic field stimulation, may be useful in helping transplanted RGCs extend axons in the proper orientation and assist with new synapse formation.

SUMMARY

Different optic neuropathies will benefit from neuro-protective versus neuro-regenerative approaches. Developing clinically effective treatments for optic nerve disease will require a collaborative approach that not only employs neurotrophic factors but also incorporates signals that promote axonogenesis, direct axon growth towards intended targets, and promote appropriate synaptogenesis.

Neuroprotective effects of brain-derived neurotrophic factor against amyloid beta 1-40-induced retinal and optic nerve damage

Brain-derived neurotrophic factor (BDNF) could be considered a potential neuroprotective therapy in amyloid beta (Aβ)-associated retinal and optic nerve degeneration. Hence, in this study we investigated the neuroprotective effect of BDNF against Aβ1-40-induced retinal and optic nerve injury. In this study, exposure to Aβ1-40 was associated with retinal and optic nerve injury. TUNEL staining showed significant reduction in the apoptotic cell count in the BDNF-treated group compared with Aβ1-40 group. H&E-stained retinal sections also showed a striking reduction in neuronal cells in the ganglion cell layer (GCL) of retinas fourteen days after Aβ1-40 exposure. By contrast, number of retinal cells was preserved in the retinas of BDNF-treated animals. After Aβ1-40 exposure, visible axonal swelling was observed in optic nerve sections. However, the BDNF-treated group showed fewer changes in optic nerve; axonal swelling was less frequent and less marked. In the present study, exposure to Aβ was associated with oxidative stress, whereas levels of retinal glutathione (GSH), superoxide dismutase (SOD) and catalase were significantly increased in BDNF-treated than in Aβ1-40-treated rats. Both visual object recognition tests using an open-field arena and a Morris water maze showed that BDNF improved rats' ability to recognise visual cues (objects with different shapes) after Aβ1-40 exposure, thus demonstrating that the visual performance of rats was relatively preserved following BDNF treatment. In conclusion, intravitreal treatment with BDNF prevents Aβ1-40-induced retinal cell apoptosis and axon loss in the optic nerve of rats by reducing retinal oxidative stress and restoring retinal BDNF levels.

Therapeutic Potential of Mesenchymal Stem Cells and Their Secretome in the Treatment of Glaucoma

Glaucoma represents a group of progressive optic neuropathies characterized by gradual loss of retinal ganglion cells (RGCs), the neurons that conduct visual information from the retina to the brain. Elevated intraocular pressure (IOP) is considered the main reason for enhanced apoptosis of RGCs in glaucoma. Currently used therapeutic agents are not able to repopulate and/or regenerate injured RGCs and, therefore, are ineffective in most patients with advanced glaucoma. Accordingly, several new therapeutic approaches, including stem cell-based therapy, have been explored for the glaucoma treatment. In this review article, we emphasized current knowledge regarding molecular and cellular mechanisms responsible for beneficial effects of mesenchymal stem cells (MSCs) and their secretome in the treatment of glaucoma. MSCs produce neurotrophins and in an exosome-dependent manner supply injured RGCs with growth factors enhancing their survival and regeneration. Additionally, MSCs are able to generate functional RGC-like cells and induce proliferation of retinal stem cells. By supporting integrity of trabecular meshwork, transplanted MSCs alleviate IOP resulting in reduced loss of RGCs. Moreover, MSCs are able to attenuate T cell-driven retinal inflammation providing protection to the injured retinal tissue. In summing up, due to their capacity for neuroprotection and immunomodulation, MSCs and their secretome could be explored in upcoming clinical studies as new therapeutic agents for glaucoma treatment.

The Krüppel-Like Factor Gene Target Dusp14 Regulates Axon Growth and Regeneration

Purpose

Adult central nervous system (CNS) neurons are unable to regenerate their axons after injury. Krüppel-like transcription factor (KLF) family members regulate intrinsic axon growth ability in vitro and in vivo, but mechanisms downstream of these transcription factors are not known.

Methods

Purified retinal ganglion cells (RGCs) were transduced to express exogenous KLF9, KLF16, KLF7, or KLF11; microarray analysis was used to identify downstream genes, which were screened for effects on axon growth. Dual-specificity phosphatase 14 (Dusp14) was further studied using genetic (siRNA, shRNA) and pharmacologic (PTP inhibitor IV) manipulation to assess effects on neurite length in vitro and survival and regeneration in vivo after optic nerve crush in rats and mice.

Results

By screening genes regulated by KLFs in RGCs, we identified Dusp14 as a critical gene target limiting axon growth and regeneration downstream of KLF9's ability to suppress axon growth in RGCs. The KLF9-Dusp14 pathway inhibited activation of mitogen-activated protein kinases normally critical to neurotrophic signaling of RGC survival and axon elongation. Decreasing Dusp14 expression or disrupting its function in RGCs increased axon growth in vitro and promoted survival and optic nerve regeneration after optic nerve injury in vivo.

Conclusions

These results link intrinsic and extrinsic regulators of axon growth and suggest modulation of the KLF9-Dusp14 pathway as a potential approach to improve regeneration in the adult CNS after injury.

Nerve growth factor regulates endothelial cell survival and pathological retinal angiogenesis

The mechanism underlying vasoproliferative retinopathies like retinopathy of prematurity (ROP) is hypoxia‐triggered neovascularisation. Nerve growth factor (NGF), a neurotrophin supporting survival and differentiation of neuronal cells may also regulate endothelial cell functions. Here we studied the role of NGF in pathological retinal angiogenesis in the course of the ROP mouse model. Topical application of NGF enhanced while intraocular injections of anti‐NGF neutralizing antibody reduced pathological retinal vascularization in mice subjected to the ROP model. The pro‐angiogenic effect of NGF in the retina was mediated by inhibition of retinal endothelial cell apoptosis. In vitro, NGF decreased the intrinsic (mitochondria‐dependent) apoptosis in hypoxia‐treated human retinal microvascular endothelial cells and preserved the mitochondrial membrane potential. The anti‐apoptotic effect of NGF was associated with increased BCL2 and reduced BAX, as well as with enhanced ERK and AKT phosphorylation, and was abolished by inhibition of the AKT pathway. Our findings reveal an anti‐apoptotic role of NGF in the hypoxic retinal endothelium, which is involved in promoting pathological retinal vascularization, thereby pointing to NGF as a potential target for proliferative retinopathies.

The Role of Endogenous Neuroprotective Mechanisms in the Prevention of Retinal Ganglion Cells Degeneration

Retinal neurons are not able to undergo spontaneous regeneration in response to damage. A variety of stressors, i.e., UV radiation, high temperature, ischemia, allergens, and others, induce reactive oxygen species production, resulting in consecutive alteration of stress-response gene expression and finally can lead to cell apoptosis. Neurons have developed their own endogenous cellular protective systems. Some of them are preventing cell death and others are allowing functional recovery after injury. The high efficiency of these mechanisms is crucial for cell survival. In this review we focus on the contribution of the most recently studied endogenous neuroprotective factors involved in retinal ganglion cell (RGC) survival, among which, neurotrophic factors and their signaling pathways, processes regulating the redox status, and different pathways regulating cell death are the most important. Additionally, we summarize currently ongoing clinical trials for therapies for RGC degeneration and optic neuropathies, including glaucoma. Knowledge of the endogenous cellular protective mechanisms may help in the development of effective therapies and potential novel therapeutic targets in order to achieve progress in the treatment of retinal and optic nerve diseases.

Protective effects of ciliary neurotrophic factor on the retinal ganglion cells by injure of hydrogen peroxide

AIM

To explore the effect of ciliary neurotrophic factor (CNTF) on retinal ganglion cell (RGC)-5 induced by hydrogen peroxide (H₂O₂).

METHODS

After cell adherence, RGC-5 culture medium was changed to contain different concentrations of H₂O₂ from 50 to 150 µmol/L at four time points (0.5, 1, 1.5 and 2h) to select the concentration and time point for H₂O₂ induced model. Two different ways of interventions for injured RGC-5 cells respectively were CNTF as an addition in the culture medium or recombinant lentiviral plasmid carrying CNTF gene transfecting bone mesenchymal stem cells (BMSCs) for co-culture with RGC-5.

RESULTS

Compared to the control group, H₂O₂ led to RGC-5 death closely associated with concentrations and action time of H₂O₂ and we chose 125 µmol/L and 2h to establish the H₂O₂-induced model. While CNTF inhibited the loss of RGC-5 cells obviously with a dose-dependent survival rate. Nevertheless two administration routes had different survival rate yet higher rate in recombinant lentiviral plasmid group but there were no statistically significant differences.

CONCLUSION

Both the two administration routes of CNTF have effects on RGC-5 cells induced by H₂O₂. If their own advantages were combined, there may be a better administration route.

Regulation of Brain-Derived Neurotrophic Factor and Growth Factor Signaling Pathways by Tyrosine Phosphatase Shp2 in the Retina: A Brief Review

SH2 domain-containing tyrosine phosphatase-2 (PTPN11 or Shp2) is a ubiquitously expressed protein that plays a key regulatory role in cell proliferation, differentiation and growth factor (GF) signaling. This enzyme is well expressed in various retinal neurons and has emerged as an important player in regulating survival signaling networks in the neuronal tissues. The non-receptor phosphatase can translocate to lipid rafts in the membrane and has been implicated to regulate several signaling modules including PI3K/Akt, JAK-STAT and Mitogen Activated Protein Kinase (MAPK) pathways in a wide range of biochemical processes in healthy and diseased states. This review focuses on the roles of Shp2 phosphatase in regulating brain-derived neurotrophic factor (BDNF) neurotrophin signaling pathways and discusses its cross-talk with various GF and downstream signaling pathways in the retina.

Bioactivity-guided isolation of neuritogenic triterpenoids from the leaves of Ilex latifolia Thunb

Ilex latifolia Thunb is a traditional Chinese tea and herbal medicine. In this study, one new triterpene saponin (1) and six known triterpenoids (2-7) were isolated from the methanol extract of I. latifolia using a PC12 cell bioassay system. The structures and stereochemistry of these compounds were elucidated using spectroscopic methods and chemical derivatization. This new triterpene saponin (1) was characterized as an ursolic type acid with a 19α-hydroxyl and a trisaccharide moiety at C-3. Compound 1 significantly promoted the neurite outgrowth in PC12 cells by 52% at 10 μM, whereas compounds 2-7 showed less neuritogenic activity. Structure activity relationship studies indicated that introducing a trisaccharide moiety at C-3 is important for the neuritogenic activity, but the sugar group at C-28 decreased this activity. In addition, compound 1 increased the neurite outgrowth length in primary cortical neuron cells of mice and also exhibited a neuronal protection effect on H₂O₂-damaged PC12 cells at optimum concentrations.

Nerve Growth Factor: Early Studies and Recent Clinical Trials

Since its discovery, nerve growth factor (NGF) has long occupied a critical role in developmental and adult neurobiology for its many important regulatory functions on the survival, growth and differentiation of nerve cells in the peripheral and central nervous system. NGF is the first discovered member of a family of neurotrophic factors, collectively indicated as neurotrophins, (which include brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin 4/5). NGF was discovered for its action on the survival and differentiation of selected populations of peripheral neurons. Since then, an enormous number of basic and human studies were undertaken to explore the role of purified NGF to prevent the death of NGF-receptive cells. These studies revealed that NGF possesses important therapeutic properties, after topical administration, on human cutaneous pressure ulcer, corneal ulcers, glaucoma, retinal maculopathy, Retinitis Pigmentosa and in pediatric optic gliomas and brain traumas. The aim of this review is to present our previous, recent and ongoing clinical studies on the therapeutic properties of NGF.

Recombinant Human Nerve Growth Factor Treatment Promotes Photoreceptor Survival in the Retinas of Rats with Retinitis Pigmentosa

PURPOSE

Increasing evidence suggests that nerve growth factor (NGF) exerts protective effects against retinal degeneration in animal models of retinitis pigmentosa (RP). This study aims at investigating the effects of intravitreal injection of recombinant human NGF (rhNGF) on retinal photoreceptors apoptosis in an animal model of RP, the Royal College of Surgeons (RCS) rats.

METHODS

Thirty-six RCS rats were treated with intravitreal injection of rhNGF or murine NGF (mNGF) or vehicle at 20 postnatal days (pd) and sacrificed at 40 pd. The eyes were enucleated and evaluated by histology, flow cytometric analysis for rhodopsin expression, Western blot for TrkA and activated (phosphorylated) TrkA (pTrkA) levels, and TUNEL assay for apoptosis' detection.

RESULTS

RCS rats showed a significant retinal degeneration associated with cell apoptosis at 40 pd when compared to wild-type animals. Histology showed that rhNGF intravitreal treatment significantly increased retinal thickness when compared to untreated eyes. Photoreceptors' number evaluated by flow cytometry was significantly increased in both intravitreal rhNGF- and mNGF-treated groups when compared to untreated eyes. This protective effect was associated with an increase in TrkA and activated pTrkA levels and an inhibition of apoptosis. Intravitreal NGF injection was well tolerated and did not show clinical and histological signs of adverse effects.

CONCLUSIONS

Intravitreal rhNGF injection proved safe and effective in favoring retinal cell survival in RCS rats. This is the first report showing that the novel rhNGF already proved safe in a phase I study exerts a biologic effect similar to the well-characterized mNGF-induced retinal protection. These results may trigger further studies to investigate rhNGF administration for the treatment of progressive degenerative retinal disorders such as retinitis pigmentosa.

Exploring Serum Levels of Brain Derived Neurotrophic Factor and Nerve Growth Factor Across Glaucoma Stages

Purpose

To investigate the serum levels of Brain Derived Neurotrophic Factor (BDNF) and Nerve Growth Factor (NGF) in patients affected by primary open angle glaucoma with a wide spectrum of disease severity compared to healthy controls and to explore their relationship with morphological and functional glaucoma parameters.

Materials and Methods

45 patients affected by glaucoma at different stages and 15 age-matched healthy control subjects underwent visual field testing, peripapillary retinal nerve fibre layer thickness measurement using Spectral Domain Optical Coherence Tomography and blood collection for both neurotrophins detection by Enzyme-Linked Immunosorbent Assay. Statistical analysis and association between biostrumental and biochemical data were investigated.

Results

Serum levels of BDNF in glaucoma patients were significantly lower than those measured in healthy controls (261.2±75.0 pg/ml vs 313.6±79.6 pg/ml, p = 0.03). Subgroups analysis showed that serum levels of BDNF were significantly lower in early (253.8±40.7 pg/ml, p = 0.019) and moderate glaucoma (231.3±54.3 pg/ml, p = 0.04) but not in advanced glaucoma (296.2±103.1 pg/ml, p = 0.06) compared to healthy controls.

Serum levels of NGF in glaucoma patients were significantly lower than those measured in the healthy controls (4.1±1 pg/mL vs 5.5±1.2 pg/mL, p = 0.01). Subgroups analysis showed that serum levels of NGF were significantly lower in early (3.5±0.9 pg/mL, p = 0.0008) and moderate glaucoma (3.8±0.7 pg/ml, p<0.0001) but not in advanced glaucoma (5.0±0.7 pg/ml, p = 0.32) compared to healthy controls.

BDNF serum levels were not related to age, visual field mean deviation or retinal nerve fibre layer thickness either in glaucoma or in controls while NGF levels were significantly related to visual field mean deviation in the glaucoma group (r² = 0.26, p = 0.004).

Conclusions

BDNF and NGF serum levels are reduced in the early and moderate glaucoma stages, suggesting the possibility that both factors could be further investigated as potential circulating biomarkers for the early detection of glaucoma.

NGF/anti-VEGF combined exposure protects RCS retinal cells and photoreceptors that underwent a local worsening of inflammation

PURPOSE

Our previous study highlighted the potential nerve growth factor (NGF) effect on damaged photoreceptors from a rat model of spontaneous Retinitis Pigmentosa (RP). Herein, we tested the combined NGF/anti-vascular endothelial growth factor (αVEGF) effect on cultured retinal cells isolated from Royal College of Surgeons (RCS) rats receiving an intravitreal VEGF injection (iv-VEGF) to exacerbate retinal inflammation/neovascularization.

METHODS

RCS (n = 75) rats were equally grouped as untreated (n = 25), iv-saline (single saline intravitreal injection; n = 25) and iv-VEGF (single VEGF intravitreal injection; n = 25). Morphological and biochemical analysis or in vitro stimulations with the biomolecular investigation were carried out on explanted retinas. Isolated retinal cells were treated with NGF and αVEGF, either alone or in combination, for 6 days and cells were harvested for morphological and biomolecular analyses.

RESULTS

Infiltrating inflammatory cells were detected in iv-VEGF exposed RCS retinas, indicative of exacerbated inflammation and neovascularization. In cell cultures, NGF/αVEGF significantly increased retinal cell survival as well as rhodopsin expression and neurite outgrowth in photoreceptors. Particularly, NGF/αVEGF upregulated Bcl-2 mRNA, downregulated Bax mRNA, upregulated trkA^NGFR mRNA and finally upregulated both NGF mRNA and protein.

CONCLUSIONS

These data confirm and extend our previous findings on NGF-photoreceptor crosstalk, highlighting that the NGF/αVEGF combination might be an interesting approach for improving neuroprotection of RCS retinal cells and likewise photoreceptors in the presence of neovascularization. Further studies are required to translate this in vitro approach into clinical practice.

Inhibition of BDNF-AS Provides Neuroprotection for Retinal Ganglion Cells against Ischemic Injury

Background: Brain-derived neurotrophic factor (BDNF) protects retinal ganglion cells against ischemia in ocular degenerative diseases. We aimed to determine the effect of BDNF-AS on the ischemic injury of retinal ganglion cells. Methods: The levels of BDNF and BDNF-AS were measured in retinal ganglion cells subjected to oxygen and glucose deprivation. The lentiviral vectors were constructed to either overexpress or knock out BDNF-AS. The luciferase reporter gene assay was used to determine whether BDNF-AS could target its seed sequence on BDNF mRNA. The methyl thiazolyl tetrazolium assay was used to determine cell viability, and TUNEL staining was used for cell apoptosis. Results: The levels of BDNF-AS were negatively correlated with BDNF in ischemic retinal ganglion cells. BDNF-AS directly targeted its complementary sequences on BDNF mRNA. BDNF-AS regulated the expression of BDNF and its related genes in retinal ganglion cells. Down-regulation of BDNF-AS increased cell viability and decreased the number of TUNEL-positive retinal ganglion cells under oxygen and glucose deprivation conditions. Conclusion: Inhibition of BDNF-AS protected retinal ganglion cells against ischemia by increasing the levels of BDNF.

Discovery of Molecular Therapeutics for Glaucoma: Challenges, Successes, and Promising Directions

Glaucoma, a heterogeneous ocular disorder affecting ∼60 million people worldwide, is characterized by painless neurodegeneration of retinal ganglion cells (RGCs), resulting in irreversible vision loss. Available therapies, which decrease the common causal risk factor of elevated intraocular pressure, delay, but cannot prevent, RGC death and blindness. Notably, it is changes in the anterior segment of the eye, particularly in the drainage of aqueous humor fluid, which are believed to bring about changes in pressure. Thus, it is primarily this region whose properties are manipulated in current and emerging therapies for glaucoma. Here, we focus on the challenges associated with developing treatments, review the available experimental methods to evaluate the therapeutic potential of new drugs, describe the development and evaluation of emerging Rho-kinase inhibitors and adenosine receptor ligands that offer the potential to improve aqueous humor outflow and protect RGCs simultaneously, and present new targets and approaches on the horizon.

Imbalance of the Nerve Growth Factor and Its Precursor: Implication in Diabetic Retinopathy

Diabetic retinopathy is the leading cause of blindness in working age in US and worldwide. Neurotrophins including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) are known to be essential for growth, differentiation and survival of neurons in the developing and mature retina. Nevertheless, a growing body of evidence supports an emerging role of neurotrophins in retinal diseases and in particular, diabetic retinopathy. Neurotrophins are initially synthesized in a pro-form and undergo proteolytic cleavage to produce the mature form that activates two distinctive receptors, the tyrosine kinase tropomycin receptor (Trk) and, to lesser extent, the common low affinity p75 neurotrophin receptor (p75^NTR). Despite tight glycemic and metabolic control, many diabetic patients continue to experience progressive retinal damage. Understanding the molecular events involved in diabetic retinopathy is extremely important to identify novel therapeutic strategies to halt the disease progression. Diabetes induces imbalance in neurotrophins by increasing its proform, which is associated with upregulation of the p75^NTR receptor in the retina. A growing body of evidence supports a link between the imbalance of pro-neurotrophins and early retinal inflammation, neuro-and microvascular degeneration. Therefore, examining changes in the levels of neurotrophins and its receptors might provide a therapeutically beneficial target to combat disease progression in diabetic patients. This commentary aims to highlight the impact of diabetes-impaired balance of neurotrophins and in particular, the NGF and its receptors; TrkA and p75^NTR in the pathology of DR.

MicroRNA target prediction in glaucoma

Glaucoma is a progressive optic neuropathy and is one of the leading causes of blindness in the industrialized countries. The aim of this study is to investigate microRNA (miRNA) regulation in glaucoma and other neurodegenerative diseases, that share similar pathways, by means of in silico approaches such as bibliographic search and access to bioinformatic resources. First of all, data mining was carried out on Human miRNA Disease Database (HMDD) and miR2Disease databases. Then, predictions of deregulated miRNAs were carried out accessing to microrna.org database. Finally, the potential combinatorial effect of miRNAs, on regulation of biochemical pathways, was studied by an enrichment analysis performed by DIANA-miRPath v.2.0. We found, from literature search, 8 deregulated miRNAs in glaucoma and 9 and 23 in age-related macular degeneration (AMD) and Alzheimer's disease (AD), respectively. One miRNA is commonly deregulated in glaucoma and AMD (miR-23a). Two miRNAs (miR-29a, miR-29b) are common to glaucoma and AD, and four miRNAs were identified to be commonly deregulated in AMD and AD (miR-9, miR-21, miR-34a, miR-146a). The match of the miRNA common to glaucoma and the other two neurodegenerative diseases (AMD and AD) did not generate any output. Enrichment of information has been reached through miRNAs prediction: 88 predicted miRNAs are common to glaucoma and AMD, 19 are common to glaucoma and AD, and 9 are common to AMD and AD. Indeed, predicted miRNAs common to the three neurodegenerative diseases are nine (miR-107, miR-137, miR-146a, miR-181c, miR-197, miR-21, miR-22, miR-590, miR-9). DIANA-miRPath predicted that those nine miRNAs might regulate pathways involved in inflammation. The findings hereby obtained provide a valuable hint to assess deregulation of specific miRNA, as potential biomarkers and therapeutic targets, in glaucoma and other neurodegenerative diseases by means of preclinical and clinical studies.

NGF in Early Embryogenesis, Differentiation, and Pathology in the Nervous and Immune Systems

The physiology of NGF is extremely complex, and although the study of this neurotrophin began more than 60 years ago, it is far from being concluded. NGF, its precursor molecule pro-NGF, and their different receptor systems (i.e., TrkA, p75NTR, and sortilin) have key roles in the development and adult physiology of both the nervous and immune systems. Although the NGF receptor system and the pathways activated are similar for all types of cells sensitive to NGF, the effects exerted during embryonic differentiation and in committed mature cells are strikingly different and sometimes opposite. Bearing in mind the pleiotropic effects of NGF, alterations in its expression and synthesis, as well as variations in the types of receptor available and in their respective levels of expression, may have profound effects and play multiple roles in the development and progression of several diseases. In recent years, the use of NGF or of inhibitors of its receptors has been prospected as a therapeutic tool in a variety of neurological diseases and injuries. In this review, we outline the different roles played by the NGF system in various moments of nervous and immune system differentiation and physiology, from embryonic development to aging. The data collected over the past decades indicate that NGF activities are highly integrated among systems and are necessary for the maintenance of homeostasis. Further, more integrated and multidisciplinary studies should take into consideration these multiple and interactive aspects of NGF physiology in order to design new therapeutic strategies based on the manipulation of NGF and its intracellular pathways.

Sterols from Mytilidae show anti-aging and neuroprotective effects via anti-oxidative activity

For screening anti-aging samples from marine natural products, K6001 yeast strain was employed as a bioassay system. The active mussel extract was separated to give an active sterol fraction (SF). SF was further purified, and four sterol compounds were obtained. Their structures were determined to be cholesterol (CHOL), brassicasterol, crinosterol, and 24-methylenecholesterol. All compounds showed similar anti-aging activity. To understand the action mechanism involved, anti-oxidative experiments, reactive oxygen species (ROS) assays, and malondialdehyde (MDA) tests were performed on the most abundant compound, CHOL. Results indicated that treatment with CHOL increases the survival rate of yeast under oxidative stress and decreases ROS and MDA levels. In addition, mutations of uth1, skn7, sod1, and sod2, which feature a K6001 background, were employed and the lifespans of the mutations were not affected by CHOL. These results demonstrate that CHOL exerts anti-aging effects via anti-oxidative stress. Based on the connection between neuroprotection and anti-aging, neuroprotective experiments were performed in PC12 cells. Paraquat was used to induce oxidative stress and the results showed that the CHOL and SF protect the PC12 cells from the injury induced by paraquat. In addition, these substance exhibited nerve growth factor (NGF) mimic activities again confirmed their neuroprotective function.

Dynamic nature of the p75 neurotrophin receptor in response to injury and disease

Neurotrophins and their respective tropomyosin related kinase (Trk) receptors (TrkA, TrkB, and TrkC) and the p75 neurotrophin receptor (p75(NTR)) play a fundamental role in the development and maintenance of the nervous system making them important targets for treatment of neurodegenerative diseases. Whereas Trk receptors are directly activated by specific neurotrophins, the p75(NTR) is a multifunctional receptor that exerts its effects via heterodimeric interactions with TrkA, TrkB, TrkC, sortilin or the Nogo receptor to regulate a wide array of cellular functions. By partnering with different receptors the p75(NTR) regulates binding of mature versus pro-neurotrophins and activation of different signaling pathways with outcomes ranging from growth and survival to cell death. While the developmental downregulation of the p75(NTR) has raised questions regarding its role in the mature nervous system, recent data have revealed widespread expression of low levels, a role in synaptic plasticity and adult neurogenesis and upregulation in response to injury or disease. Studies are needed to better understand these processes, particularly in the damaged nervous system, but will be complicated by expression of p75(NTR) on immune cells including macrophages and microglia that are intimately involved in disease and repair processes. Recent approaches that regulate p75(NTR) function with small non-peptide ligands have demonstrated potent neuroprotection in models of injury and neurodegenerative diseases that highlight the importance of the p75(NTR) as a therapeutic target. Future studies hold the promise of revealing a wealth of information on the multifaceted actions of the p75(NTR) that will inform the design of new neurotrophin-based therapies.