Brain-derived neurotrophic factor signalling in adult pig retinal ganglion cell neurite regeneration in vitro

Short-Term Hyperuricemia Leads to Structural Retinal Changes That Can be Reversed by Serum Uric Acid Lowering Agents in Mice

Purpose

Metabolic disorders have been implicated in ocular diseases, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR). Recently, hyperuricemia (HUA) has been proposed as another risk factor for AMD, although no cause-and-effect experimental data have been published. In this study, we investigated whether HUA would initiate AMD or related retinal damages in hyperuricemic mice.

Methods

HUA was induced in male ICR mice by dietary supplements of uric acid and oxonic acid potassium salt, with or without treatments by allopurinol or benzbromarone for various durations. Serum uric acid and angiotensin II concentrations were measured by enzyme-linked immunosorbent assay (ELISA) at regular intervals. The retinal damages were assessed by hematoxylin and eosin staining, immunostaining, and TUNEL assay. The cause-and-effect of HUA was compared among the study groups.

Results

The results showed that the total thickness of photoreceptor inner and outer segments, as well as the thickness of the photoreceptor outer segment alone, were reduced under HUA. Furthermore, HUA elevated serum angiotensin II, which indicated activation of the renin–angiotensin system (RAS), leading to higher matrix metalloproteinase-2 (MMP-2) expression, and glial activation in the ganglion cell layer. HUA also led to the reduction of retinal pigment epithelium gap junction protein connexin-43 and apoptosis. Uric acid lowering agents, allopurinol or benzbromarone, were effective in ameliorating the impairments.

Conclusions

HUA may pose as a causative factor of retinal injuries. The reduction of serum uric acid may reduce the detrimental effects caused by HUA.

Low-Level Laser Therapy with 670 nm Alleviates Diabetic Retinopathy in an Experimental Model

Purpose:

To evaluate the effects of low-level laser therapy (LLLT) on the retina with diabetic retinopathy (DR).

Methods:

Eight Wistar rats were used as a control group, and 64 rats were injected intraperitoneally with 55 mg/kg of streptozotocin to induce diabetes and served as a diabetic group. After the establishment of the DR, the rats were separated into (a) 32 rats with DR; did not receive any treatment, (b) 32 rats with DR were exposed to 670 nm LLLT for 6 successive weeks (2 sessions/week). The retinal protein was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, total antioxidant capacity (TAC), hydrogen peroxide (H₂O₂), and histological examination.

Results:

LLLT improved retinal proteins such as neurofilament (NF) proteins (200 KDa, 160 KDa, and 86 KDa), neuron-specific enolase (NSE) (46 KDa). Moreover, the percentage changes in TAC were 46.8% (P < 0.001), 14.5% (P < 0.01), 4.8% and 1.6% (P > 0.05), and in H₂O_2, they were 30% (P < 0.001), 25% (P < 0.001), 20% (P < 0.01), and 5% (P > 0.05) after 1, 2, 4, and 6 weeks, compared with the control. DR displayed swelling and disorganization in the retinal ganglion cells (RGCs) and photoreceptors, congestion of the capillaries in the nerve fiber layer, thickening of the endothelial cells’ capillaries, and edema of the outer segment of the photoreceptors layer. The improvement of the retinal structure was achieved after LLLT.

Conclusion:

LLLT could modulate retinal proteins such as NSE and NFs, improve the RGCs, photoreceptors, and reduce the oxidative stress that originated in the retina from diabetes-induced DR.

Pigment Epithelium-Derived Factor (PEDF) Receptors Are Involved in Survival of Retinal Neurons

The demise of retinal ganglion cells (RGCs) is characteristic of diseases of the retina such as glaucoma and diabetic or ischemic retinopathies. Pigment epithelium-derived factor (PEDF) is a multifunctional secreted protein that mediates neuroprotection and inhibition of angiogenesis in the retina. We have studied expression and regulation of two of several receptors for PEDF, patatin-like phospholipase 2 gene product/PEDF-R and laminin receptor (LR), in serum-starved RGC under normoxia and hypoxia and investigated their involvement in the survival of retinal neuronal cells. We show that PEDF-R and LR are co-expressed in RGC and R28 retinal precursor cells. Expression of both receptors was enhanced in the presence of complex secretions from retinal glial (Müller) cells and upregulated by VEGF and under hypoxic conditions. PEDF-R- and LR-knocked-down cells demonstrated a markedly attenuated expression of anti-apoptotic Bcl-2 family members (Bcl-2, Bcl-xL) and neuroprotective mediators (PEDF, VEGF, BDNF) suggesting that both PEDF-R and LR mediate pro-survival effects of PEDF on RGC. While this study does not provide evidence for a differential survival-promoting influence of either PEDF-R or LR, it nevertheless highlights the importance of both PEDF receptors for the viability of retinal neurons.

Neuritin-overexpressing transgenic mice demonstrate enhanced neuroregeneration capacity and improved spatial learning and memory recovery after ischemia-reperfusion injury

Acute ischemia-reperfusion (IR)-induced brain injury is further exacerbated by a series of slower secondary pathogenic events, including delayed apoptosis due to neurotrophic factor deficiency. Neuritin, a neurotrophic factor regulating nervous system development and plasticity, is a potential therapeutic target for treatment of IR injury. In this study, Neuritin-overexpressing transgenic (Tg) mice were produced by pronuclear injection and offspring with high overexpression used to generate a line with stable inheritance for testing the neuroprotective capacity of Neuritin against transient global ischemia (TGI). Compared to wild-type mice, transgenic mice demonstrated reduced degradation of the DNA repair factor poly [ADP-ribose] polymerase 1 (PARP 1) in the hippocampus, indicating decreased hippocampal apoptosis rate, and a greater number of surviving hippocampal neurons during the first week post-TGI. In addition, Tg mice showed increased expression of the regeneration markers NF-200, synaptophysin, and GAP-43, and improved recovery of spatial learning and memory. Our findings exhibited that the window of opportunity of neural recovery in Neuritin transgenic mice group had a tendency to move ahead after TGI, which indicated that Neuritin can be used as a potential new therapeutic strategy for improving the outcome of cerebral ischemia injury.

Cerebral Immunohistochemical Characterization of the H2S and the Oxytocin Systems in a Porcine Model of Acute Subdural Hematoma

The hydrogen sulfide (H₂S) and the oxytocin/oxytocin receptor (OT/OTR) systems interact in trauma and are implicated in vascular protection and regulation of fluid homeostasis. Acute brain injury is associated with pressure-induced edema formation, blood brain barrier disruption, and neuro-inflammation. The similarities in brain anatomy: size, gyrencephalic organization, skull structure, may render the pig a highly relevant model for translational medicine. Cerebral biomarkers for pigs for pathophysiological changes and neuro-inflammation are limited. The current study aims to characterize the localization of OT/OTR and the endogenous H₂S producing enzymes together with relevant neuro-inflammatory markers on available porcine brain tissue from an acute subdural hematoma (ASDH) model. In a recent pilot study, anesthetized pigs underwent ASDH by injection of 20 mL of autologous blood above the left parietal cortex and were resuscitated with neuro-intensive care measures. After 54 h of intensive care, the animals were sacrificed, the brain was removed and analyzed via immunohistochemistry. The endogenous H₂S producing enzymes cystathionine-ɤ-lyase (CSE) and cystathionine-β-synthase (CBS), the OTR, and OT were localized in neurons, vasculature and parenchyma at the base of sulci, where pressure-induced injury leads to maximal stress in the gyrencephalic brain. The pathophysiological changes in response to brain injury in humans and pigs, we show here, are comparable. We additionally identified modulators of brain injury to further characterize the pathophysiology of ASDH and which may indicate future therapeutic approaches.

microRNAs in sudden hearing loss

OBJECTIVES

To compare the circulating microRNA (miRNA) expression profiles between sudden sensory neural hearing loss (SSNHL) patients and age-matched normal hearing controls.

STUDY DESIGN

Prospective cohort multi-center study.

METHODS

Patients presenting within 28 days of onset of SSNHL were prospectively recruited along with contemporaneous age-matched controls. Pooled sera of four patient (n = 09, mean age = 53.0 years; 07, 55.0; 10, 52.9; 10, 51.6) and two control (09, 51.2 and 03, 50.0) groups were assessed using a TaqMan Low Density Array. The patients' sera were also divided into two pools, untreated (04, 57.7) and treated (32, 52.6) for additional analysis. miRNA expression level was derived from cycle threshold (Ct) values normalized to a global mean. Inter-group mean Ct differences with fold changes ≥2.0 and ≤0.5 at P < .05 were considered significant. Bioinformatic databases were used to identify putative target mRNAs or validated genes and their functional annotations.

RESULTS

Thirty-six SSNHL patients (mean age 53.0 ± standard deviation (SD) 15.2 years) and 12 controls (50.9 ± 11.9) were studied. Eight miRNAs hsa-miR-590-5p/ -186-5p/ -195-5p/ -140-3p/ -128-3p/ -132-3p/ -375-3p, and -30a-3p were identified as significantly differentially expressed in SSNHL patients. Most of these miRNAs were abundantly identified in the nervous system and the putative target messenger RNAs (mRNAs) were enriched in signaling pathways such as phosphatidyl inositol 3 kinase/protein Kinase B (PI3K/Akt), Ras and mitogen-activated protein kinase (MAPK).

CONCLUSION

These findings suggest the possible cellular signaling pathways that underlie the disruption of auditory signal transmission in SSNHL.

LEVEL OF EVIDENCE

2 Laryngoscope, 130:E416-E422, 2020.

Safety threshold of intravitreal clonidine in rabbit's eyes

AIM

To determine the safe dose of intravitreal clonidine (IVC), a potential drug for neuroprotection and angiogenesis inhibition in rabbits.

METHODS

A total of 28 rabbits were divided into four groups. Three groups received IVC with concentrations of 15 (Group A), 25 (Group B), and 50 (Group C) µg/0.1 mL and the control group (Group D) received 0.1 mL balanced salt solution (BSS). To investigate IVC safety, electroretinography (ERG) was performed at baseline, then at 1, 4 and 8wk after injection. After last ERG, all rabbits were euthanized, their eyes were enucleated and subjected to routine histopathological evaluation, immunohistochemistry for glial fibrillary acidic protein (GFAP) and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) test.

RESULTS

Based on ERG, histopathology, GFAP and TUNEL assay findings, 15 µg IVC was determined as the safe dose in rabbit eyes. While, the results of routine histopathology and TUNEL assay were unremarkable in all groups, toxic effects attributed to 25 and 50 µg IVC were demonstrated by ERG and GFAP tests.

CONCLUSION

Totally 15 µg clonidine is determined as the safe dose for intravitreal injection in rabbits. Contribution of IVC in neuroprotection and inhibition of angiogenesis deserve more studies.

Neuroprotection in Glaucoma

Glaucoma is a degenerative optic neuropathy characterized by retinal ganglion cell (RGC) loss and visual field defects. It is known that in some glaucoma patients, death of RGCs continues despite intraocular pressure (IOP) reduction. Neuroprotection in the field of glaucoma is defined as any treatment, independent of IOP reduction, which prevents RGC death. Glutamate antagonists, ginkgo biloba extract, neurotrophic factors, antioxidants, calcium channel blockers, brimonidine, glaucoma medications with blood regulatory effect and nitric oxide synthase inhibitors are among compounds with possible neuroprotective activity in preclinical studies. A few agents (such as brimonidine or memantine) with neuroprotective effects in experimental studies have advanced to clinical trials; however the results of clinical trials for these agents have not been conclusive. Nevertheless, lack of compelling clinical evidence has not prevented the off-label use of some of these compounds in glaucoma practice. Stem cell transplantation has been reported to halt experimental neurodegenerative disease processes in the absence of cell replacement. It has been hypothesized that transplantation of some types of stem cells activates multiple neuroprotective pathways via secretion of various factors. The advantage of this approach is a prolonged and targeted effect. Important concerns in this field include the secretion of unwanted harmful mediators, graft survival issues and tumorigenesis. Neuroprotection in glaucoma, pharmacologically or by stem cell transplantation, is an interesting subject waiting for broad and multidisciplinary collaborative studies to better clarify its role in clinical practice.

Mesenchymal stem cells secrete brain-derived neurotrophic factor and promote retinal ganglion cell survival after traumatic optic neuropathy

OBJECTIVE

The study aimed to investigate whether intravitreal injection of mesenchymal stem cells (MSCs) cultivated in vitro could increase the number of survived retinal ganglion cells (RGCs) after traumatic optic neuropathy and sought to identify potential mechanisms underlying such growth.

METHODS

The right eye of 24 cats in the MSC transplantation group accepted intravitreal injection of MSCs, and the other 24 cats in the phosphate buffer (PBS) control group received isotonic saline after traumatic optic neuropathy. The RGCs' survival rate in separated retinal and brain-derived neurotrophic factor (BDNF) expression were observed by Dil labeling and Enzyme-Linked Immuno Sorbent Assay (ELISA), respectively, at 3, 7, 14, and 28 days after transplantation.

RESULTS

Quantitative analysis showed that RGCs were significantly attenuated at 3, 7, 14, and 28 days after transplantation in both areas of the retina (peripheral, P7d = 0.0011, P14d < 0.001, P28d < 0.001; central, P3d = 0.0437, P7d = 0.0067, P14d < 0.001, P28d < 0.001). Mean density of RGCs in the MSC transplantation group was significantly higher than that of the PBS control group after 14 days of treatment (P < 0.001). The homogenates BDNF (hBDNF) in the MSC transplantation group was obviously higher than that in the PBS control group at 14 and 28 days (P < 0.05).

CONCLUSIONS

The MSCs transplanted into the retina of cats can slow down RGC apoptosis and steadily express BDNF. The MSC-mediated neuroprotection after optic nerve injury may be related to BNDF.

Glia-neuron interactions in the mammalian retina

The mammalian retina provides an excellent opportunity to study glia-neuron interactions and the interactions of glia with blood vessels. Three main types of glial cells are found in the mammalian retina that serve to maintain retinal homeostasis: astrocytes, Müller cells and resident microglia. Müller cells, astrocytes and microglia not only provide structural support but they are also involved in metabolism, the phagocytosis of neuronal debris, the release of certain transmitters and trophic factors and K(+) uptake. Astrocytes are mostly located in the nerve fibre layer and they accompany the blood vessels in the inner nuclear layer. Indeed, like Müller cells, astrocytic processes cover the blood vessels forming the retinal blood barrier and they fulfil a significant role in ion homeostasis. Among other activities, microglia can be stimulated to fulfil a macrophage function, as well as to interact with other glial cells and neurons by secreting growth factors. This review summarizes the main functional relationships between retinal glial cells and neurons, presenting a general picture of the retina recently modified based on experimental observations. The preferential involvement of the distinct glia cells in terms of the activity in the retina is discussed, for example, while Müller cells may serve as progenitors of retinal neurons, astrocytes and microglia are responsible for synaptic pruning. Since different types of glia participate together in certain activities in the retina, it is imperative to explore the order of redundancy and to explore the heterogeneity among these cells. Recent studies revealed the association of glia cell heterogeneity with specific functions. Finally, the neuroprotective effects of glia on photoreceptors and ganglion cells under normal and adverse conditions will also be explored.

Brain-derived neurotrophic factor is a novel target gene of the has-miR-183/96/182 cluster in retinal pigment epithelial cells following visible light exposure

Light-induced retinal injury is clinically and experimentally well-documented. It may be categorized into three types: Photothermal, photomechanical and photochemical injuries. To date, the variation in the hsa-miR-183/96/182 cluster and its potential target genes in human primary retinal pigment epithelial (RPE) cells, following visible light exposure, has not been reported. In the present study, RPE cells were exposed to 4 h of constant visible light. The expression of the hsa-miR-183/96/182 cluster was determined using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and its potential target genes were investigated. Additionally, hsa-miR-183, hsa-miR-96, hsa-miR-182 and has-miR-183/96/182 mimics were designed and synthesized in vitro, and transfected into the RPE cells. Subsequently, the expression of brain-derived neurotrophic factor (BDNF) mRNA and protein was measured, using RT-qPCR and western blotting, respectively. The regulation of miRNAs to the BDNF gene were then validated using a dual luciferase reporter gene assay system. The expression of hsa-miR-183, hsa-miR-96 and hsa-miR-182 significantly increased in RPE cells following 4 h of visible light exposure, compared with RPE cells that had been exposed to dark conditions (P<0.01). Following RPE cell transfection with mimics, BDNF mRNA and protein expression in the RPE cells was significantly downregulated compared with control RPE cells (P<0.05, P<0.01, respectively). Similarly, the ratio of Renilla luciferase/firefly luciferase significantly decreased in the RPE cells of the mimic + wild type (WT) group compared with cells of the psiCHECK(TM)-2 (a vector lacking the sequence of the BDNF gene), wild type and mimic + mutation groups (P<0.05, P<0.01). The present study suggests that BDNF is a target gene of the has-miR-183-96-182 cluster in RPE cells. The present study suggests an underlying protective mechanism against retinal light injury and may provide a novel target for the prevention and treatment of light-induced retinal injury.

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.

Characterization of the effects of retinal pigment epithelium-conditioned media on porcine and aged human retina

BACKGROUND

Retinal pigment epithelium (RPE) cells produce neurotrophic factors that rescue photoreceptors from degeneration. Previously, we showed that conditioned medium (CM) from fetal vs adult RPE cells resulted in significantly better porcine retinal preservation, and possessed significantly higher levels of hepatocyte growth factor (HGF) and pigment epithelium-derived factor (PEDF). This study aimed to further describe the effects of human fetal RPE-CM on porcine and aged human retina, and to characterize its effects biochemically.

METHODS

RPE-CM was harvested from passage-2 fetal RPE, 7 days after passage, 24-hours after exposure to basal medium. After culture in RPE-CM, porcine retinal morphology was assessed with confocal microscopy. The effects of RPE-CM on porcine and aged human retina survival were assessed by cytotoxicity and apoptosis biochemical assays. To characterize RPE-CM biochemically, effects of heating, digesting with proteinase-K, dilution, concentration, and fractionation were tested. Recombinant proteins and neutralizing antibodies were used to identify proteins that might contribute to the salutary effects of RPE-CM on porcine retina.

RESULTS

Culturing porcine retina in RPE-CM significantly preserved outer nuclear layer width and the number of nuclei in cross-section, and significantly decreased photoreceptor axon retraction. RPE-CM decreased porcine retinal death by 17-34 % (p<0.05) compared to basal medium. Human retina from age-related macular degeneration (AMD) and non-AMD donors responded similarly after culture in RPE-CM. Heating, proteinase-K digestion, and dilution significantly diminished RPE-CM-mediated preservation of porcine retina, whereas concentrating RPE-CM significantly enhanced its preservation of porcine retina. Molecular cut filtration identified retina-preserving activity in the 3-100 kDa filtrate. PEDF or HGF at 90 % receptor occupancy significantly improved retinal preservation over 48 h of culture compared to basal medium. Neutralizing PEDF in RPE-CM decreased its ability to reduce retinal apoptosis by 23-27 % (p<0.05).

CONCLUSION

RPE-CM reduced biochemically and histologically measured degeneration in porcine retinae. This effect was concentration-dependent, and can be attributed to a protein component(s) in a 3-100 kDa molecular cut fraction. Human retina (including non-AMD and AMD Caucasian and non-AMD African-American) responds to culture in RPE-CM similarly to porcine retina. Receptor occupancy calculations and retinal viability data indicate that PEDF may be one of the components that contribute to retina preservation by RPE-CM.

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.

TRPC6: an underlying target for human glaucoma

Glaucoma is one of the leading causes of visual impairment and blindness worldwide. Of known risk factors for glaucoma, an increased in intraocular pressure is most highly correlated with glaucomatous damage. Irrespective of the cause, apoptosis of the retinal ganglion cells is the eventual outcome. It is widely accepted that glaucoma is a neurodegenerative disease that is strongly correlated with central nervous system disorders, such as Alzheimer's disease. These two disorders also share some similarities in pathogenic mechanisms. Recent studies suggest that the transient receptor potential canonical 6 channel could work together with brain-derived neurotrophic factor to promote neuron survival in brain and retina. In this study, we propose that transient receptor potential canonical 6 may contribute to the pathogenesis of human glaucoma and become a potential therapeutic target.

Glial scar size, inhibitor concentration, and growth of regenerating axons after spinal cord transection

A mathematical model has been formulated in accordance with cell chemotaxis and relevant experimental data. A three-dimensional lattice Boltzmann method was used for numerical simulation. The present study observed the effects of glial scar size and inhibitor concentration on regenerative axonal growth following spinal cord transection. The simulation test comprised two parts: (1) when release rates of growth inhibitor and promoter were constant, the effects of glial scar size on axonal growth rate were analyzed, and concentrations of inhibitor and promoters located at the moving growth cones were recorded. (2) When the glial scar size was constant, the effects of inhibitor and promoter release rates on axonal growth rate were analyzed, and inhibitor and promoter concentrations at the moving growth cones were recorded. Results demonstrated that (1) a larger glial scar and a higher release rate of inhibitor resulted in a reduced axonal growth rate. (2) The axonal growth rate depended on the ratio of inhibitor to promoter concentrations at the growth cones. When the average ratio was < 1.5, regenerating axons were able to grow and successfully contact target cells.

N-acetyl serotonin derivatives as potent neuroprotectants for retinas

N-acetylserotonin (NAS) is synthesized from serotonin by arylalkylamine N-acetyltransferase (AANAT), which is predominantly expressed in the pineal gland and retina. NAS activates TrkB in a circadian manner and exhibits antidepressant effects in a TrkB-dependent manner. It also enhances neurogenesis in hippocampus in sleep-deprived mice. Here we report the identification of NAS derivatives that possess much more robust neurotrophic effects with improved pharmacokinetic profiles. The compound N-[2-(5-hydroxy-1H-indol-3-yl)ethyl]-2-oxopiperidine-3-carboxamide (HIOC) selectively activates TrkB receptor with greater potency than NAS. It potently protects retinas from light-induced retinal degeneration (LIRD), which is tightly coupled with pronounced TrkB activation in retinas. Pharmacokinetic studies demonstrate that this compound is stable in serum and liver microsomes. It can pass the blood-brain barrier and blood-retinal barrier. Hence, HIOC is a good lead compound for further drug development for treating retinal degenerative diseases.

Ras/p38 and PI3K/Akt but not Mek/Erk signaling mediate BDNF-induced neurite formation on neonatal cochlear spiral ganglion explants

Neurotrophins participate in regulating the survival, differentiation, and target innervation of many neurons, mediated by high-affinity Trk and low-affinity p75 receptors. In the cochlea, spiral ganglion (SG) neuron survival is strongly dependent upon neurotrophic input, including brain-derived neurotrophic factor (BDNF), which increases the number of neurite outgrowth in neonatal rat SG in vitro. Less is known about signal transduction pathways linking the activation of neurotrophin receptors to SG neuron nuclei. In particular, the p38 and cJUN Kinase (JNK), mitogen-activated protein kinase (MAPK) pathways, which participate in JNK signaling in other neurons, have not been studied. We found that inhibition of Ras, p38, phosphatidyl inositol 3 kinase (PI3K) or Akt signaling reduced or eliminated BDNF mediated increase in number of neurite outgrowth, while inhibition of Mek/Erk had no influence. Inhibition of Rac/cdc42, which lies upstream of JNK, modestly enhanced BDNF induced formation of neurites. Western blotting implicated p38 and Akt signaling, but not Mek/Erk. The results suggest that the Ras/p38 and PI3K/Akt are the primary pathways by which BDNF promotes its effects. Activation of Rac/cdc42/JNK signaling by BDNF may reduce the formation of neurites. This is in contrast to our previous results on NT-3, in which Mek/Erk signaling was the primary mediator of SG neurite outgrowth in vitro. Our data on BDNF agree with prior results from others that have implicated PI3K/Akt involvement in mediating the effects of BDNF on SG neurons in vitro, including neuronal survival and neurite extension. However, the identification of p38 and JNK involvement is entirely novel. The results suggest that neurotrophins can exert opposing effects on SG neurons, the balance of competing signals influencing the generation of neurites. This competition could provide a potential mechanism for the control of neurite number during development.

Peptides derived from the solvent-exposed loops 3 and 4 of BDNF bind TrkB and p75(NTR) receptors and stimulate neurite outgrowth and survival

Brain-derived neurotrophic factor (BDNF) is critically involved in modeling the developing nervous system and is an important regulator of a variety of crucial functions in the mature CNS. BDNF exerts its action through interactions with two transmembrane receptors, either separately or in concert. BDNF has been implicated in several neurological disorders, and irregularities in BDNF function may have severe consequences. Administration of BDNF as a drug has thus far yielded few practicable results, and the potential side effects when using a multifunctional protein are substantial. In an effort to produce more specific compounds without side effects, small peptides mimicking protein function have been developed. The present study characterized two mimetic peptides, Betrofin 3 and Betrofin 4, derived from the BDNF sequence. Both Betrofins bound the cognate BDNF receptors, TrkB and p75(NTR), and induced neurite outgrowth and enhanced neuronal survival, probably by inducing signaling through tha Akt and MAPK pathways. Distinct, charged residues within the Betrofin sequences were identified as important for generating the neuritogenic response, which was also inhibited when BDNF was added together with either Betrofin, indicating partial agonistic effects of the peptides. Thus, two peptides derived from BDNF induced neurite outgrowth and enhanced neuronal survival, probably through binding to BDNF receptors.

Prolonged blockade of VEGF receptors does not damage retinal photoreceptors or ganglion cells

It has recently been reported that relatively short-term inhibition of vascular endothelial growth factor (VEGF) signaling can cause photoreceptor cell death, a potentially clinically important finding since VEGF blockade has become an important modality of treatment of ocular neovascularization and macular edema. However, in a set of studies in which we achieved extended and complete blockage of VEGF-induced vascular leakage through retinal expression of a VEGF binding protein, we did not observe any toxicity to retinal neurons. To follow-up on these apparently discrepant findings, we designed a set of experiments with the kinase inhibitor SU4312, which blocks phosphorylation of VEGF receptors, to look directly for evidence of VEGF inhibition-related retinal toxicity. Using transgenic mice with sustained expression of VEGF in photoreceptors, we determined that periocular injection of 3 microg of SU4312 every 5 days markedly suppressed subretinal neovascularization, indicating effective blockade of VEGF signaling. Wild-type mice given periocular injections of 5 microg of SU4312 every 5 days for up to 12 weeks showed normal scotopic and photopic electroretinograms (ERGs), no TUNEL stained cells in the retina, and no reduction in outer nuclear layer thickness. Incubation of cultured ganglion cells or retinal cultures containing photoreceptors with high doses of SU4312 did not reduce cell viability. These data suggest that blocking VEGF signaling in the retina for up to 12 weeks does not damage photoreceptors nor alter ERG function and should reassure patients who are receiving frequent injections of VEGF antagonists for choroidal and retinal vascular diseases.

Synergistic effects of osteonectin and brain-derived neurotrophic factor on axotomized retinal ganglion cells neurite outgrowth via the mitogen-activated protein kinase-extracellular signal-regulated kinase 1/2 pathways

Our previous study identified osteonectin (ON) in a screen of factors made by Schwann cells (SCs) which promoted peripheral and central neurons survival and neuritogenesis, however, the mechanisms of ON promoting effects are largely unknown. In the present study, we investigated the effects of ON-deficient SC-conditioned medium (SCCM) and molecular mechanisms of ON, in regulating retinal ganglion cells (RGCs) survival and neurite outgrowth. Neonatal rat RGCs and SCs were purified by immunopanning technique. RGC survival and neuritogenesis reduced significantly when treated with either ON-null mice SCCM or ON-immunodepleted (IP) SCCM (P<0.05). In contrast to wild type SCCM, in the presence of a tyrosine kinase receptor (Trk) inhibitor (K252a), ON-null mice SCCM-induced neuritogenesis were further reduced by 24%. The Trk-mediated signaling pathways became more sensitive to K252a inhibition in the absence of ON. We also showed the synergistic effects of ON and brain-derived neurotrophic factor (BDNF) in promoting RGCs growth and the involvement of ON in two major neurotrophin-mediated signaling pathways, PI-3K-Akt and MAPK-Erk1/2. ON alone activated Akt phosphorylation and increased survival. Blockage of TrkB signalling pathway by TrkB-Fc chimera (BDNF scavenger) or K252a in ON-treated cultures reduced Akt-P level significantly. This suggests that ON induces BDNF synthesis and secretion from RGCs. The enhancement of neuritogenesis and Erk1/2 phosphorylation by ON in BDNF-treated cultures further demonstrate the signaling pathways responsible for the synergistic effect of ON on BDNF-induced neurite outgrowth. To the best of our knowledge, this is the first report showing the synergistic effects of ON on classical neurotrophins which participate in the same signalling pathways in regulating RGC neurite outgrowth.

Treatment of stroke with a synthetic liver X receptor agonist, TO901317, promotes synaptic plasticity and axonal regeneration in mice

In this study, we tested the hypothesis that TO901317 promotes synapse plasticity and axonal regeneration after stroke. Adult male C57BL/6J mice were subjected to middle cerebral artery occlusion (MCAo) and treated with or without TO901317 starting 24 h after MCAo daily for 14 days. Axonal damage and regeneration were evaluated by immunostaining. TO901317 significantly increased synaptophysin expression and axonal regeneration, as well as decreased the expressions of amyloid betaA4 precursor protein and Nogo receptor (NgR) in the ischemic brain. To test whether TO901317 regulates the phosphorylation of phosphatidylinositol 3-kinase (p-PI3K) and Akt (p-Akt) activity in the ischemic brain, MCAo mice were treated with or without TO901317 starting 24 h after MCAo daily for 4 days and were then killed at 5 days after MCAo. TO901317 treatment significantly increased p-PI3K and p-Akt activity, but did not increase total PI3K expression in the ischemic brain. Using primary cortical neuron (PCN) culture, TO901317 significantly increased synaptophysin expression, p-PI3K activity, and decreased NgR expression compared with nontreated controls. TO901317 also significantly increased neurite outgrowth, and inhibition of the PI3K/Akt pathway by LY294002 decreased neurite outgrowth in both controls and TO901317-treated groups in cultured hypoxic PCN. These data indicate that TO901317 promotes synaptic plasticity and axonal regeneration, and that PI3K/Akt signaling activity contributes to neurite outgrowth.

Neurotrophic effect of a novel TrkB agonist on retinal ganglion cells

PURPOSE

Retinal ganglion cells (RGCs) die in glaucoma and virtually all optic neuropathies. Recently, novel tropomyosin-related kinase B (TrkB) monoclonal antibodies have been shown to activate TrkB receptors and exert neuroprotective and neurotrophic effects. In the present study, the authors examined the ability of one of them, 29D7, to elicit RGC survival and neurite growth both in culture and in vivo.

METHODS

RGCs from postnatal day (P)3 to P4 Sprague-Dawley rats were isolated by sequential immunopanning using a monoclonal antibody to Thy1. RGCs were cultured in serum-free defined medium in 96-well plates. RGC viability was assessed after 1 to 3 days by MTT assay. Activation of TrkB and downstream signaling molecules was confirmed by Western blot analysis. Intravitreal injections of 29D7 were performed after optic nerve axotomy, and subsequent RGC survival was quantified using beta-III tubulin immunostaining. Regeneration was assessed using retrograde fluorogold tracing in an optic nerve-peripheral nerve graft model.

RESULTS

Similar to brain-derived neurotrophic factor (BDNF), the 29D7 antibody strongly promoted RGC survival and neurite growth in vitro compared with medium alone or control IgG. Forskolin, which weakly supported RGC survival on its own, potentiated the effect of 29D7. Intravitreal injection of 29D7 enhanced RGC survival but not regeneration in vivo 2 weeks after optic nerve injury.

CONCLUSIONS

Together, these findings demonstrate the potential for antibody-mediated TrkB agonism as a potential therapeutic approach to enhance RGC survival after optic nerve injury. Further studies are needed to elucidate the mechanistic differences between this TrkB agonist and BDNF.

In vitro characterization of micropatterned PLGA-PHBV8 blend films as temporary scaffolds for photoreceptor cells

In developed countries the aging population faces increasing risks of blinding retinal diseases, for which there are few effective treatments available. Photoreceptor transplantation represents one approach, but generally results have been disappointing. We hypothesize that micropatterned biodegradable poly(L-lactic acid-co-glycolic acid)/poly(hydroxybutyrate-co-hydroxyvaleric acid) (PLGA-PHBV8) blend films could deliver photoreceptor cells in a more organized manner than bolus injections. Blending of PLGA and PHBV8 was used to optimize the degradation rate of the temporary template. At the end of 8 weeks, for both thin and thick films of PLGA-PHBV8 a 50% decrease of their initial weight with increasing water uptake was observed. When photoreceptor cells were seeded onto micropatterned PLGA-PHBV8 films with parallel grooves (21- and 42-microm-wide grooves and 20 microm ridge width and depth), the cells preferred laminin-deposited grooves to ridges and expressed rod- and cone-specific markers such as rhodopsin and arrestin. A loss in photoreceptor viability of 50% was observed after 7 days in culture. The effects of either retinal pigment epithelium (RPE)-derived or Muller glial cell-derived conditioned media or bFGF on the survival of photoreceptor cells seeded on PLGA-PHBV8 films were investigated. Addition of either RPE- and Muller-conditioned media increased statistically (p < 0.01) the viability of photoreceptor cells after 7 days of incubation. Our results suggest that such biodegradable micropatterned PLGA-PHBV8 blend films have a potential to deliver photoreceptor cells to the subretinal space and ensure laminar organization and maintenance of differentiation, and that incorporation of intrinsic factors within the scaffold would enhance the survival rate of transplanted cells.

Tissue Engineering Applied to the Retinal Prosthesis: Neurotrophin-Eluting Polymeric Hydrogel Coatings

Several groups are developing visual prostheses to aid patients with vision loss. While these devices have shown some success in the clinic, they are severely limited by poor resolution, and in many cases have as few as 15 electrodes. Pixel density is poor because high stimulation thresholds require large electrodes to minimize charge density that would otherwise damage the electrode and tissue. A significant contributor to high stimulation threshold requirements is poor biocompatibility. We investigated the application of one system popular in tissue engineering, drug-releasing hydrogels, as a mechanism to improve the tissue-electrode interface. Hydrogels studied (i.e., PEGPLA photocrosslinkable polymers) released neurotrophic factors (i.e., BDNF) known to promote neuron survival and neurite extension in the retina. Hydrogels were examined in co-culture with retinal explants for 7 and 14 days, at which time neurite extension and neurite density was measured. Neurite extension was enhanced in samples exposed to BDNF-releasing hydrogels at 7 days; however, these increases were absent by day 14 suggesting declining drug release. Thus, PEGPLA hydrogels are excellent candidates for short-term (< 14 day) acute release of therapeutic factors in the retina, but will require additional modifications for application with neural prostheses. Additionally, these results suggest that the effects of neurotrophic factors are short-lived in the absence of additional support cues, and tissue engineering systems employing such factors may only produce transient benefits to the patient.

Melatonin inhibitory effect on cAMP accumulation in the chick retina development

During vertebrate neurodevelopment, neuritogenesis and synaptogenesis are modulated by intracellular cAMP rises. Melatonin, which is implicated in neuronal differentiation, mainly inhibits this pathway. Here, an investigation about the profile of this effect during the vertebrate neurodevelopment is reported. In the embryonic chick retinas at days 8, 12, 14, 16 and at 2 days post-hatched (E8, E12, E14, E16 and PH), those control embryonic retinas incubated only with the phosphodiesterase inhibitor at days corresponding to commencement of neuronal differentiation (E8, E12) and PH, presented cAMP levels inhibited by melatonin. While the cAMP accumulation stimulated by forskolin was inhibited in the embryonic retinas at all testing days. Neither the unselective antagonist N-acetyl-2-benziltryptamine (luzindole) nor the selective Mel(1b) antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) blocked the melatonin concentration-dependent inhibitory effect on cAMP accumulation in the retinas initiating differentiation (E7-E9), suggesting a tight binding between melatonin and their receptors. However, 4-P-PDOT competitively reverted the melatonin effect on cAMP stimulated by forskolin during synaptogenesis stages. Together, the melatonin effect on cAMP levels in chick retina, which is mainly through melatonin receptors, is depending on the developmental period observed, probably taking part in the mechanisms surrounding the melatonin action on neuronal differentiation.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

Adult retinal neuronal cell culture

Despite a relatively long history, general knowledge is not widespread that adult neurons can be maintained in cell culture for fairly extended periods of time. Within the central nervous system, this capacity seems to be particularly well developed in the retina, although it is still not clear whether this property is due to physical reasons (spatial configuration, simple connections) or to more fundamental differences (molecular composition, physiological function). Irrespective of the reasons, in vitro model systems are useful for investigating physiological and pathological processes occurring in mature retina. The authors argue that the numerous molecular changes undergone during maturation (modifications in ion channels and receptors, apoptotic pathways and growth factor effects) should be taken into account when using in vitro approaches to study processes involved in photoreceptor and ganglion cell degeneration, and hence that more classical methods relying on embryonic or newborn tissue should be interpreted with caution. A number of examples are given where the use of adult retinal neuronal culture may be especially informative: neurite regeneration, neuroprotection assays and pathogenic mechanisms; and areas of further research that should be explored: cell transplantation.

BDNF production by olfactory ensheathing cells contributes to axonal regeneration of cultured adult CNS neurons

Olfactory ensheathing cells (OECs) are the main glial cell type that populates mammalian olfactory nerves. These cells have a great capacity to promote the regeneration of axons when transplanted into the injured adult mammalian CNS. However, little is still known about the molecular mechanisms they employ in mediating such a task. Brain-derived neurotrophic factor (BDNF) was identified as a candidate molecule in a genomic study that compared three functionally different OEC populations: Early passage OECs (OEC Ep), Late passage OECs (OEC Lp) and the OEC cell line TEG3 [Pastrana, E., Moreno-Flores, M.T., Gurzov, E.N., Avila, J., Wandosell, F., Diaz-Nido, J., 2006. Genes associated with adult axon regeneration promoted by olfactory ensheathing cells: a new role for matrix metalloproteinase 2. J. Neurosci. 26, 5347-5359]. We have here set out to determine the role played by BDNF in the stimulation of axon outgrowth by OECs. We compared the extracellular BDNF levels in the three OEC populations and show that it is produced in significant amounts by the OECs that can stimulate axon regeneration in adult retinal neurons (OEC Ep and TEG3) but it is absent from the extracellular medium of OEC Lp cells which lack this capacity. Blocking BDNF signalling impaired axonal regeneration of adult retinal neurons co-cultured with TEG3 cells and adding BDNF increased the proportion of adult neurons that regenerate their axons on OEC Lp monolayers. Combining BDNF with other extracellular proteins such as Matrix Metalloproteinase 2 (MMP2) further augmented this effect. This study shows that BDNF production by OECs plays a direct role in the promotion of axon regeneration of adult CNS neurons.

Brain‐Derived Neurotrophic Factor (BDNF) Acts Primarily via the JAK/STAT Pathway to Promote Neurite Growth in the Major Pelvic Ganglion of the Rat: Part 2

INTRODUCTION

Surgical and radiation therapies of bladder and prostate cancers may damage cavernous nerves and cause erectile dysfunction (ED). We previously showed that brain-derived neurotrophic factor (BDNF) could restore erectile function in a neurogenic ED rat model. We now investigated the signaling mechanism of BDNF in major pelvic ganglia (MPG) explants.

AIM

To identify the signaling mechanism that mediates the neurotrophic effect of BDNF in cultured MPG.

METHODS

Major pelvic ganglia was isolated from male rats for immunohistochemistry and immunofluorescence staining to locate BDNF receptors, pan-neurotrophin 75 (p75), tropomyosin-related kinase B (TrkB), and tropomyosin-related kinase C (TrkC). The dorso-caudal region of MPG was treated with BDNF to determine the optimal dosage for promoting neurite growth. Specific kinase inhibitors AG490, KT5720, LY294002, and U0126 were then used to treat MPG either alone or prior to BDNF treatment. The treated MPG was examined for neurite growth and for expression and phosphorylation of JAK2, STAT1, and STAT3 by Western blot analysis.

MAIN OUTCOME MEASURES

Lengths of neurite growth from MPG were measured to quantify the effects of BDNF and to identify specific signaling pathways. Ratios of phosphorylated vs. unphosphoryated proteins of JAK2, STAT1, and STAT2 in control and treated MPG were determined to confirm JAK/STAT as the principal signaling pathway.

RESULTS

Tropomyosin-related kinase B and TrkC were localized to neurons whereas p75 to perineuronal satellite glial cells (SGC). The optimal dosage of BDNF for promoting MPG neurite growth was between 25 and 50 ng/mL. Among the four specific kinase inhibitors, AG490 was the strongest in suppressing MPG neurite growth as well as BDNF-induced phosphorylation of JAK2, STAT1, and STAT3.

CONCLUSIONS

In rat MPG, TrkB and TrkC were expressed in neurons, whereas p75 in SGC. Optimal BDNF dosage for promoting MPG neurite growth was between 25 and 50 ng/mL. BDNF promotes MPG neurite growth primarily by activating the JAK/STAT pathway.

Retinal growth hormone is an anti-apoptotic factor in embryonic retinal ganglion cell differentiation

Cells of the neural retina in the chick embryo undergo several waves of apoptosis during development, including peaks at approximately embryonic day (ED) 7 and 12. Prominent among the cells involved in these phases of cell death are the retinal ganglion cells (RGCs). We have previously shown that growth hormone (GH) is expressed in the neural retina, and particularly, in the RGCs. Here we study the ability of GH to rescue retinal cells from apoptosis, both in vitro and in vivo. When retinas from embryos at ED 6-8 are explanted on collagen gels, the application of recombinant GH, at 10(-6)m, significantly reduced the incidence of apoptotic cells in the cultures as judged by terminal deoxynucleotide transferase-mediated dUTP-biotin nick end labelling (TUNEL). GH was delivered to neural retinas in ovo, by microinjection into the eye cup at ED 2. When these embryos were examined at ED 6-8, no reduction in cell death was observed below the normal low control levels. However, when antibodies to GH were microinjected, the incidence of cell death increased significantly at ED 6, providing evidence that in vivo immunoneutralization of endogenous GH results in triggering of apoptotic signaling pathways. Evidence that RGCs are a particular target of this neuroprotective effect of GH was provided by examination of cultures enriched for RGCs by immunopanning. In serum-free culture, RGCs, identified by anti-Islet 1 immunolabelling, were found to be susceptible to the effect of GH immunoneutralization, which approximately quadrupled the incidence of apoptosis in the cultures. We propose that GH is a naturally occurring autocrine and/or paracrine neuroprotective agent in the developing retina which is involved in the regulation of retinal cell numbers during early embryogenesis.

Extracellular signal-regulated kinases 1/2 are required for adult retinal ganglion cell axon regeneration induced by fibroblast growth factor-2

The intracellular signaling mechanisms used by neurotrophic factors to promote axon growth in the mature, injured central nervous system are not well understood. Here we investigated the signaling cascades that control fibroblast growth factor-2 (FGF-2)-mediated retinal ganglion cell (RGC) axon extension in vivo. For this purpose, a novel adeno-associated virus (AAV) was used to deliver the FGF-2 gene to RGCs, providing a sustained source of this neurotrophic factor. FGF-2 gene transfer led to an approximately ten-fold increase in the number of axons that extended past the lesion site compared with control nerves. Axon growth correlated with FGF-2-induced activation of the extracellular signal-regulated kinases 1/2 (Erk1/2), but not phosphoinositide 3-kinase or protein kinase C. Pharmacological inhibition of Erk1/2 activation resulted in an approximately 80% decrease in the number of axons that regenerated in the injured optic nerve. Our data demonstrate that the Erk1/2 pathway is an essential signaling component in FGF-2-mediated axon regeneration in the mature, injured visual system.

Neuroprotective effects of erythropoietin on glutamate and nitric oxide toxicity in primary cultured retinal ganglion cells

Erythropoietin receptor (EpoR) is expressed in the central nervous system (CNS), however, no clear consensus has been obtained whether Epo acts as a prosurvival factor in neurons. Because retinal ganglion cell (RGC) death is a common cause of reduced visual function in several ocular diseases, we explored whether Epo might potentially be beneficial in protecting RGCs from glutamate and nitric oxide (NO)-induced cytotoxicity, using isolated RGCs by a two-step panning method. Brain-derived neurotrophic factor (BDNF) was used as a positive control. EpoR mRNA was expressed in isolated RGCs, and EpoR protein was expressed on the RGCs in the normal and ischemic retinas. Epo had less potential to improve the survival of primary RGCs in serum-free medium than BDNF. In these cells, BDNF, but not Epo, downregulated the expression of Bim, a proapoptotic Bcl-2 family member that plays a key role in cytokine-mediated cell survival, suggesting a possible mechanism for this difference. When RGCs were cultured with glutamate or an NO-generating reagent, the survival of RGCs was compromised, and Bcl-2 expression was decreased in these cells. Both Epo and BDNF significantly reduced RGC death induced by glutamate and NO. In agreement with this, these factors reversed the Bcl-2 expression. These findings suggest that Epo may be a potent neuroprotective therapeutic agent for the treatment of ocular diseases that are characterized by RGC death.

Extracellular signal-regulated kinase 1/2 mediates survival, but not axon regeneration, of adult injured central nervous system neurons in vivo

Neurotrophins play important roles in the response of adult neurons to injury. The intracellular signaling mechanisms used by neurotrophins to regulate survival and axon growth in the mature CNS in vivo are not well understood. The goal of this study was to define the role of the extracellular signal-regulated kinases 1/2 (Erk1/2) pathway in the survival and axon regeneration of adult rat retinal ganglion cells (RGCs), a prototypical central neuron population. We used recombinant adeno-associated virus (AAV) to selectively transduce RGCs with genes encoding constitutively active or wild-type mitogen-activated protein kinase kinase 1 (MEK1), the upstream activator of Erk1/2. In combination with anterograde and retrograde tracing techniques, we monitored neuronal survival and axon regeneration in vivo. MEK1 gene delivery led to robust and selective transgene expression in multiple RGC compartments including cell bodies, dendrites, axons and targets in the brain. Furthermore, MEK1 activation induced in vivo phosphorylation of Erk1/2 in RGC bodies and axons. Quantitative analysis of cell survival demonstrated that Erk1/2 activation promoted robust RGC neuroprotection after optic nerve injury. In contrast, stimulation of the Erk1/2 pathway was not sufficient to induce RGC axon growth beyond the lesion site. We conclude that the Erk1/2 pathway plays a key role in the survival of axotomized mammalian RGCs in vivo, and that activation of other signaling components is required for axon regeneration in the growth inhibitory CNS environment.