Expression of brain-derived neurotrophic factor and of its functional receptor in neonatal and adult rat retina

Brain-Derived Neurotrophic Factor-Mediated Neuroprotection in Glaucoma: A Review of Current State of the Art

Retinal ganglion cells (RGCs) are neurons of the visual system that are responsible for transmitting signals from the retina to the brain via the optic nerve. Glaucoma is an optic neuropathy characterized by apoptotic loss of RGCs and degeneration of optic nerve fibers. Risk factors such as elevated intraocular pressure and vascular dysregulation trigger the injury that culminates in RGC apoptosis. In the event of injury, the survival of RGCs is facilitated by neurotrophic factors (NTFs), the most widely studied of which is brain-derived neurotrophic factor (BDNF). Its production is regulated locally in the retina, but transport of BDNF retrogradely from the brain to retina is also crucial. Not only that the interruption of this retrograde transport has been detected in the early stages of glaucoma, but significantly low levels of BDNF have also been detected in the sera and ocular fluids of glaucoma patients, supporting the notion that neurotrophic deprivation is a likely mechanism of glaucomatous optic neuropathy. Moreover, exogenous NTF including BDNF administration was shown reduce neuronal loss in animal models of various neurodegenerative diseases, indicating the possibility that exogenous BDNF may be a treatment option in glaucoma. Current literature provides an extensive insight not only into the sources, transport, and target sites of BDNF but also the intracellular signaling pathways, other pathways that influence BDNF signaling and a wide range of its functions. In this review, the authors discuss the neuroprotective role of BDNF in promoting the survival of RGCs and its possible application as a therapeutic tool to meet the challenges in glaucoma management. We also highlight the possibility of using BDNF as a biomarker in neurodegenerative disease such as glaucoma. Further we discuss the challenges and future strategies to explore the utility of BDNF in the management of glaucoma.

Melatonin protects inner retinal neurons of newborn mice after hypoxia-ischemia

Retinopathy of prematurity is a vision-threatening disease associated with retinal hypoxia-ischemia, leading to the death of retinal neurons and chronic neuronal degeneration. During this study, we used the oxygen-induced retinopathy mice model to mimic retinal hypoxia-ischemia phenotypes to investigate further the neuroprotective effect of melatonin on neonatal retinal neurons. Melatonin helped maintain relatively normal inner retinal architecture and thickness and preserve inner retinal neuron populations in avascular areas by rescuing retinal ganglion and bipolar cells, and horizontal and amacrine neurons, from apoptosis. Meanwhile, melatonin recovered visual dysfunction, as reflected by the improved amplitudes and implicit times of a-wave, b-wave, and oscillatory potentials. Additionally, elevated cleaved caspase-3 and Bax protein levels and reduced Bcl-2 protein levels in response to hypoxia-ischemia were diminished after melatonin treatment. Moreover, melatonin increased BDNF and downstream phospho-TrkB/Akt/ERK/CREB levels. ANA-12, a TrkB receptor antagonist, antagonized these melatonin actions and reduced melatonin-induced neuroprotection. Furthermore, melatonin rescued the reduction in melatonin receptor expression. This study suggests that melatonin exerted anti-apoptotic and neuroprotective effects in inner retinal neurons after hypoxia-ischemia, at least partly due to modulation of the BDNF-TrkB pathway.

Age Related Response of Neonatal Rat Retinal Ganglion Cells to Reduced TrkB Signaling in vitro and in vivo

During development of retinofugal pathways there is naturally occurring cell death of at least 50% of retinal ganglion cells (RGCs). In rats, RGC death occurs over a protracted pre- and early postnatal period, the timing linked to the onset of axonal ingrowth into central visual targets. Gene expression studies suggest that developing RGCs switch from local to target-derived neurotrophic support during this innervation phase. Here we investigated, in vitro and in vivo, how RGC birthdate affects the timing of the transition from intra-retinal to target-derived neurotrophin dependence. RGCs were pre-labeled with 5-Bromo-2'-Deoxyuridine (BrdU) at embryonic (E) day 15 or 18. For in vitro studies, RGCs were purified from postnatal day 1 (P1) rat pups and cultured with or without: (i) brain derived neurotrophic factor (BDNF), (ii) blocking antibodies to BDNF and neurotrophin 4/5 (NT-4/5), or (iii) a tropomyosin receptor kinase B fusion protein (TrkB-Fc). RGC viability was quantified 24 and 48 h after plating. By 48 h, the survival of purified βIII-tubulin immunopositive E15 but not E18 RGCs was dependent on addition of BDNF to the culture medium. For E18 RGCs, in the absence of exogenous BDNF, addition of blocking antibodies or TrkB-Fc reduced RGC viability at both 24 and 48 h by 25-40%. While this decrease was not significant due to high variance, importantly, each blocking method also consistently reduced complex process expression in surviving RGCs. In vivo, survival of BrdU and Brn3a co-labeled E15 or E18 RGCs was quantified in rats 24 h after P1 or P5 injection into the eye or contralateral superior colliculus (SC) of BDNF and NT-4/5 antibodies, or serum vehicle. The density of E15 RGCs 24 h after P1 or P5 injection of blocking antibodies was reduced after SC but not intraretinal injection. Antibody injections into either site had little obvious impact on viability of the substantially smaller population of E18 RGCs. In summary, most early postnatal RGC death in the rat involves the elimination of early-born RGCs with their survival primarily dependent upon the availability of target derived BDNF during this time. In contrast, late-born RGC survival may be influenced by additional factors, suggesting an association between RGC birthdate and developmental death mechanisms.

Brain-derived Neurotrophic Factor in the Aqueous Humor of Glaucoma Patients

BACKGROUND

Brain-derived neurotrophic factor (BDNF) may be involved in the pathogenesis of glaucoma. BDNF concentrations reported in previous studies have varied widely, and the concentration of BDNF in aqueous humor is unknown. In this study, BDNF concentrations in the aqueous humor of glaucoma patients and control patients were measured with ELISA kits.

METHODS

This prospective, observational study examined BDNF levels in aqueous humor in 62 eyes of 43 patients who underwent cataract surgery or trabeculectomy (11 glaucoma patients and 32 non-glaucoma cataract patients as controls). BDNF concentrations were examined by 4 different enzyme-linked immunosorbent assay (ELISA) techniques.

RESULTS

The mean ± SD patient age was 72.0 ± 10.1 (range 35 to 87) years. Two of the techniques detected no BDNF in aqueous humor in any samples (n=3 and n=9, respectively); the average value was less than zero. An ultrasensitive ELISA kit did not yield reliable measurements. Finally, in an even more sensitive ELISA (Simoa-HD1), performed by an outside contractor, 25 (54.3%) eyes were below the detection limit, including 20 (55.6%) control and 5 (50%) glaucoma cases. For eyes with detectable BDNF, the overall BDNF concentration was 0.158 pg/mL (n=21): 0.196 pg/mL (n=16) in controls and 0.034 pg/mL (n=5) in glaucoma cases.

CONCLUSIONS

BDNF level in aqueous humor varies widely.

Serum Brain-Derived Neurotrophic Factor in Glaucoma Patients in Japan: An Observational Study

PURPOSE

The aim of this study was to measure serum levels of brain-derived neurotrophic factor (BDNF) in Japanese patients with primary open angle glaucoma (POAG) and normal tension glaucoma (NTG).

METHODS

This was a prospective observational study of serum BDNF levels in 78 patients who underwent cataract surgery or trabeculectomy (27 glaucoma patients and 51 non-glaucoma cataract patients as controls). Patient age was 68.8 ± 11.1 years (mean ± standard deviation; range 35-86 years). The numbers of patients with POAG and NTG were 16 and 11, respectively. POAG was diagnosed by intraocular pressure measurement, gonioscopy, optic nerve head change, and presence of a visual field defect.

RESULTS

Serum BDNF concentration was significantly lower in the glaucoma group (including both POAG and NTG) than in the control group (7.2 ± 3.6 ng/mL vs. 12.2 ± 9.3 ng/mL, p=0.004). Serum BDNF concentration was lower in early glaucoma than in moderate glaucoma. There was no correlation between serum BDNF concentration and age. When patients with NTG and POAG were compared, serum BDNF concentration was lower in the former. Serum BDNF concentration was not significantly correlated with glaucoma parameters, including optical coherence tomography and visual field defects.

CONCLUSION

This is the first study to investigate serum BDNF concentration in glaucoma patients in Japan. Future studies should evaluate the role of BDNF as a potential biomarker of glaucoma.

Target-Derived Neurotrophic Factor Deprivation Puts Retinal Ganglion Cells on Death Row: Cold Hard Evidence and Caveats

Glaucoma and other optic neuropathies are characterized by axonal transport deficits. Axonal cargo travels back and forth between the soma and the axon terminus, a mechanism ensuring homeostasis and the viability of a neuron. An example of vital molecules in the axonal cargo are neurotrophic factors (NTFs). Hindered retrograde transport can cause a scarcity of those factors in the retina, which in turn can tilt the fate of retinal ganglion cells (RGCs) towards apoptosis. This postulation is one of the most widely recognized theories to explain RGC death in the disease progression of glaucoma and is known as the NTF deprivation theory. For several decades, research has been focused on the use of NTFs as a novel neuroprotective glaucoma treatment. Until now, results in animal models have been promising, but translation to the clinic has been highly disappointing. Are we lacking important knowledge to lever NTF therapies towards the therapeutic armamentarium? Or did we get the wrong end of the stick regarding the NTF deprivation theory? In this review, we will tackle the existing evidence and caveats advocating for and against the target-derived NTF deprivation theory in glaucoma, whilst digging into associated therapy efforts.

Increased levels of synaptic proteins involved in synaptic plasticity after chronic intraocular pressure elevation and modulation by brain-derived neurotrophic factor in a glaucoma animal model

The dendrites of retinal ganglion cells (RGCs) synapse with the axon terminals of bipolar cells in the inner plexiform layer (IPL). Changes in the RGC dendrites and synapses between the bipolar cells in the inner retinal layer may critically alter the function of RGCs in glaucoma. The present study attempted to discover changes in the synapse using brain-derived neurotrophic factor (BDNF) after glaucoma induction by chronic intraocular pressure elevation in a rat model. Immunohistochemical staining revealed that the BDNF-injected group had a significant increase in the level of synaptophysin, which is a presynaptic vesicle protein, in the innermost IPL compared with the phosphate-buffered saline (PBS)-injected group. SMI-32, which is a marker of RGCs, was colocalized with synaptophysin in RGC dendrites, and this colocalization significantly increased in the BDNF-injected group. After the induction of glaucoma, the BDNF-injected group exhibited increases in the total number of ribbon synapses, as seen using electron microscopy. Expression of calcium/calmodulin-dependent protein kinase II (CaMKII), cAMP-response element binding protein (CREB) and F-actin, which are key molecules involved in synaptic changes were upregulated after BDNF injection. These initial findings show the capability of BDNF to induce beneficial synaptic changes in glaucoma.

Summary: Application of BDNF increased the expression of synaptic vesicle proteins in the inner retina via the p-Akt, CaMKII and CREB pathways, increasing F-actin in RGC dendrites.

The expression of adenosine receptors changes throughout light induced retinal degeneration in the rat

The modulation of adenosine receptors, A1 (A1R) and A2A (A2AR), is neuroprotective in different models of retinal injury. In order to understand the processes underlying retinal degeneration, we studied the expression of adenosine receptors in the retinas of control and continuously illuminated (CI) rats by qRT-PCR, Western blot (WB) and immunohistochemistry (IHC). Significant increases of A1R, A2AR, and A2BR mRNAs at 1, 5, and 7 days of CI (P < 0.0001) were observed by qRT-PCR. Also, a significant increase of A3R mRNA was detected after 5 and 7 days of CI. WB studies showed a significant rise of A1R on day 1 of CI and on days 5 and 7 (P < 0.0001), while A2AR increase was seen from 2 days of CI on (P < 0.001). After 1 day of CI, A1R immunoreactivity (A1R-IR) increased in ganglion cell layer, inner nuclear layer, and in both the outer and inner plexiform layers. After 2 days of CI, the A1R-IR went back to control levels. After 5 days of CI, a second rise in A1R, which persisted until 7 days of CI, was measured (P < 0.0001). A significant rise of A2aR immunoreactivity was also observed at day 2 of CI at GCL and INL and subsided at days 5 and 7 (P < 0.0001). The observed up-regulation of A1R after 1 day of CI, corresponds with the peak of oxidative stress; while the rise of A2aR at day 2 of CI, coincides with the massive apoptosis of photoreceptors. We postulate that an early modulation of adenosine receptors could delay or prevent the degeneration of photoreceptors.

BDNF Polymorphism: A Review of Its Diagnostic and Clinical Relevance in Neurodegenerative Disorders

Brain-derived neurotrophic factor (BDNF) has a unique role in the neuronal development, differentiation, and survival in the developing and adult nervous system. A common single-nucleotide polymorphism in the pro-region of the human BDNF gene, resulting in a valine to methionine substitution (Val66Met), has been associated with the susceptibility, incidence, and clinical features of several neurodegenerative disorders. Much research has been dedicated to evaluating the effects of polymorphism in the past decade, and functional effects of this genetic variation. A better understanding of how this naturally occurring polymorphism associates with or influences physiology, anatomy, and cognition in both healthy and diseased adults in neurodegenerative conditions will help understand neurochemical mechanisms and definable clinical outcomes in humans. Here we review the role and relevance of the BDNF Val66Met polymorphism in neurodegenerative diseases, with particular emphasis on glaucoma, multiple sclerosis (MS), Alzheimer’s disease (AD) and Parkinson’s disease (PD). Several controversies and unresolved issues, including small effect sizes, possible ethnicity, gender, and age effects of the BDNF Val66Met are also discussed with respect to future research.

Relationship between Sigma-1 receptor and BDNF in the visual system

Glaucoma is an incurable optic neuropathy characterized by dysfunction and death of retinal ganglion cells (RGCs). Brain derived neurotrophic factor (BDNF) is an essential neurotrophin that supports RGC function and survival. Despite BDNF's importance, our knowledge of molecular mechanisms that modulate BDNF processing and secretion is incomplete. Sigma-1 receptor (S1R) is associated with increased BDNF in hippocampus and with BDNF secretion by brain-derived astrocytes and neuronal cell lines. Much less is known about the relationship between S1R and BDNF in the visual system. Here, we examine how S1R activation and deletion alter expression of mature BDNF (mBDNF) and proBDNF in retina and cultured optic nerve head (ONH) astrocytes. For S1R activation, the S1R agonist (+)-pentazocine (PTZ, 0.5 mg/kg) was administered by intraperitoneal injection to C57BL/6J mice, 3 times per week, for 5 weeks. Expression of proBDNF and mBDNF was also examined in S1R knockout and age-matched C57BL/6J mice. In vitro, cultured ONH astrocytes were treated with 3 μM PTZ for 24 h followed by collection of media and ONH astrocyte lysates. Results showed that treatment with (+)-PTZ increased mBDNF protein in both retina and hippocampus. In contrast, S1R deletion was associated with retinal mBDNF deficits. In ONH astrocytes S1R agonist (+)-PTZ significantly increased levels of secreted BDNF and proBDNF in cell lysates. These findings support a role for S1R in the modulation of BDNF levels within the retina and optic nerve head. Treatment with S1R agonists might provide benefit in diseases such as glaucoma by increasing BDNF levels from endogenous sources.

Glaucoma: Biological Trabecular and Neuroretinal Pathology with Perspectives of Therapy Innovation and Preventive Diagnosis

Glaucoma is a common degenerative disease affecting retinal ganglion cells (RGC) and optic nerve axons, with progressive and chronic course. It is one of the most important reasons of social blindness in industrialized countries. Glaucoma can lead to the development of irreversible visual field loss, if not treated. Diagnosis may be difficult due to lack of symptoms in early stages of disease. In many cases, when patients arrive at clinical evaluation, a severe neuronal damage may have already occurred. In recent years, newer perspective in glaucoma treatment have emerged. The current research is focusing on finding newer drugs and associations or better delivery systems in order to improve the pharmacological treatment and patient compliance. Moreover, the application of various stem cell types with restorative and neuroprotective intent may be found appealing (intravitreal autologous cellular therapy). Advances are made also in terms of parasurgical treatment, characterized by various laser types and techniques. Moreover, recent research has led to the development of central and peripheral retinal rehabilitation (featuring residing cells reactivation and replacement of defective elements), as well as innovations in diagnosis through more specific and refined methods and inexpensive tests.

Synaptic Dysfunction in Alzheimer's Disease and Glaucoma: From Common Degenerative Mechanisms Toward Neuroprotection

Alzheimer’s disease (AD) and glaucoma are two distinct multifactorial neurodegenerative diseases, primarily affecting the elderly. Common pathophysiological mechanisms have been elucidated in the past decades. First of all both diseases are progressive, with AD leading to dementia and glaucoma inducing blindness. Pathologically, they all feature synaptic dysfunction with changes of neuronal circuitry, progressive accumulation of protein aggregates such as the beta amyloid (Aβ) and intracellular microtubule inclusions containing hyperphosphorylated tau, which belongs to microtubule associated protein family. During an early phase of degeneration, both diseases are characterized by synaptic dysfunction and changes of mitogen-activated protein kinases (MAPK). Common degenerative mechanisms underlying both diseases are discussed here, along with recent results on the potential use of the visual system as a biomarker for diagnosis and progression of AD. Common neuropathological changes and mechanisms in AD and glaucoma have facilitated the transfer of therapeutic strategies between diseases. In particular, we discuss past and present evidence for neuroprotective effects of brain-derived neurotrophic factor (BDNF).

The promise of stem cell-based therapeutics in ophthalmology

The promising role of cellular therapies in the preservation and restoration of visual function has prompted intensive efforts to characterize embryonic, adult, and induced pluripotent stem cells for regenerative purposes. Three main approaches to the use of stem cells have been described: sustained drug delivery, immunomodulation, and differentiation into various ocular structures. Studies of the differentiation capacity of all three types of stem cells into epithelial, neural, glial and vascular phenotypes have reached proof-of-concept in culture, but the correction of vision is still in the early developmental stages, and the requirements for effective in vivo implementation are still unclear. We present an overview of some of the preclinical findings on stem-cell rescue and regeneration of the cornea and retina in acute injury and degenerative disorders.

Role of BDNF/TrkB pathway in the visual system: Therapeutic implications for glaucoma

INTRODUCTION

Neuroprotective therapeutics are needed to treat glaucoma, an optic neuropathy that results in death of retinal ganglion cells (RGCs).

AREAS COVERED

The BDNF/TrkB pathway is important for RGC survival. Temporal and spatial alterations in the BDNF/TrkB pathway occur in development and in response to acute optic nerve injury and to glaucoma. In animal models, BDNF supplementation is successful at slowing RGC death after acute optic nerve injury and in glaucoma, however, the BDNF/TrkB signaling is not the only pathway supporting long term RGC survival.

EXPERT COMMENTARY

Much remains to be discovered about the interaction between retrograde, anterograde, and retinal BDNF/TrkB signaling pathways in both neurons and glia. An ideal therapeutic agent for glaucoma likely has several modes of action that target multiple mechanisms of neurodegeneration including the BDNF/TrkB pathway.

Glaucoma -state of the art and perspectives on treatment

Glaucoma is a chronic optic neuropathy characterized by progressive damage to the optic nerve, death of retinal ganglion cells and ultimately visual field loss. It is one of the leading causes of irreversible loss of vision worldwide. The most important trigger of glaucomatous damage is elevated eye pressure, and the current standard approach in glaucoma therapy is reduction of intraocular pressure (IOP). However, despite the use of effective medications or surgical treatment leading to lowering of IOP, progression of glaucomatous changes and loss of vision among patients with glaucoma is common. Therefore, it is critical to prevent vision loss through additional treatment. To implement such treatment(s), it is imperative to identify pathophysiological changes in glaucoma and develop therapeutic methods taking into account neuroprotection. Currently, there is no method of neuroprotection with long-term proven effectiveness in the treatment of glaucoma. Among the most promising molecules shown to protect the retina and optic nerve are neurotrophic factors. Thus, the current focus is on the development of safe and non-invasive methods for the long-term elevation of the intraocular level of neurotrophins through advanced gene therapy and topical eye treatment and on the search for selective agonists of neurotrophin receptors affording more efficient neuroprotection.

Retinal ganglion cell apoptotic pathway in glaucoma: Initiating and downstream mechanisms

Apoptosis of retinal ganglion cells (RGCs) in glaucoma causes progressive visual field loss, making it the primary cause of irreversible blindness worldwide. Elevated intraocular pressure and aging, the main risk factors for glaucoma, accelerate RGC apoptosis. Numerous pathways and mechanisms were found to be involved in RGC death in glaucoma. Neurotrophic factors deprivation is an early event. Oxidative stress, mitochondrial dysfunction, inflammation, glial cell dysfunction, and activation of apoptotic pathways and prosurvival pathways play a significant role in RGC death in glaucoma. The most important among the involved pathways are the MAP-kinase pathway, PI-3 kinase/Akt pathway, Bcl-2 family, caspase family, and IAP family.

Rescue of retinal function by BDNF in a mouse model of glaucoma

Vision loss in glaucoma is caused by progressive dysfunction of retinal ganglion cells (RGCs) and optic nerve atrophy. Here, we investigated the effectiveness of BDNF treatment to preserve vision in a glaucoma experimental model. As an established experimental model, we used the DBA/2J mouse, which develops chronic intraocular pressure (IOP) elevation that mimics primary open-angle glaucoma (POAG). IOP was measured at different ages in DBA/2J mice. Visual function was monitored using the steady-state Pattern Electroretinogram (P-ERG) and visual cortical evoked potentials (VEP). RGC alterations were assessed using Brn3 immunolabeling, and confocal microscope analysis. Human recombinant BDNF was dissolved in physiological solution (0.9% NaCl); the effects of repeated intravitreal injections and topical eye BDNF applications were independently evaluated in DBA/2J mice with ocular hypertension. BDNF level was measured in retinal homogenate by ELISA and western blot. We found a progressive decline of P-ERG and VEP responses in DBA/2J mice between 4 and 7 months of age, in relationship with the development of ocular hypertension and the reduction of Brn3 immunopositive RGCs. Conversely, repeated intravitreal injections (BDNF concentration = 2 µg/µl, volume = 1 µl, for each injection; 1 injection every four days, three injections over two weeks) and topical eye application of BDNF eye-drops (12 µg/µl, 5 µl eye-drop every 48 h for two weeks) were able to rescue visual responses in 7 month DBA/2J mice. In particular, BDNF topical eye treatment recovered P-ERG and VEP impairment increasing the number of Brn3 immunopositive RGCs. We showed that BDNF effects were independent of IOP reduction. Thus, topical eye treatment with BDNF represents a promisingly safe and feasible strategy to preserve visual function and diminish RGC vulnerability to ocular hypertension.

Neuronal cell death in the inner retina and the influence of vascular endothelial growth factor inhibition in a diabetic rat model

To inhibit vascular changes in diabetic retinopathy, inhibiting vascular endothelial growth factor (VEGF) has become a mainstay of the treatment of diabetic retinopathy. However, its effects on neuronal cells remain to be elucidated. We aimed to evaluate the effect of VEGF inhibition on neuronal cells in a streptozotocin-induced diabetic rat retina. VEGF inhibition was performed by intravitreal VEGF-A antibody injection. After anti-VEGF treatment, apoptosis in retinal ganglion cells (RGCs) increased, and novel apoptosis in amacrine and bipolar cells of the inner nuclear layer was observed by TUNEL staining. Phosphorylated Akt expression was significantly higher in RGCs but was decreased in neuronal cells of the inner nuclear layer after anti-VEGF treatment by Western blot analysis and immunohistochemical staining. These results demonstrate that VEGF inhibition significantly increased RGC apoptosis and neuronal cell apoptosis in the inner nuclear layer of a diabetic retina, which seems to consist primarily of amacrine and bipolar cells. The phosphorylated Akt pathway, which plays a neuroprotective role via VEGF, was significantly affected by VEGF inhibition in the inner nuclear layer, suggesting that neurotrophic factor deprivation is the main mechanism for neuronal cell death after inhibiting VEGF. The results of this study show that inhibiting VEGF may have detrimental effects on the apoptosis of neuronal cells in the inner layers of the diabetic retina.

Brain derived neurotrophic factor in the retina of the teleost N. furzeri

BDNF plays an important role in the development and maintenance of visual circuitries in the retina and brain visual centers. In adulthood, BDNF signaling is involved in neural protection and regeneration of retina. In this survey, we investigated the expression of BDNF in the retina of adult Nothobranchius furzeri, a teleost fish employed for age research. After describing the retina of N. furzeri and confirming that the structure is organized in layers as in all vertebrates, we have studied the localization of BDNF mRNA and protein throughout the retinal layers. BDNF mRNA is detectable in all layers, whereas the protein is lacking in the photoreceptors. The occurrence of BDNF provides new insights on its role in the retina, particularly in view of age-related disease of retina.

Mapping kainate activation of inner neurons in the rat retina

Kainate receptors mediate fast, excitatory synaptic transmission for a range of inner neurons in the mammalian retina. However, allocation of functional kainate receptors to known cell types and their sensitivity remains unresolved. Using the cation channel probe 1-amino-4-guanidobutane agmatine (AGB), we investigated kainate sensitivity of neurochemically identified cell populations within the structurally intact rat retina. Most inner retinal neuron populations responded to kainate in a concentration-dependent manner. OFF cone bipolar cells demonstrated the highest sensitivity of all inner neurons to kainate. Immunocytochemical localization of AGB and macromolecular markers confirmed that type 2 bipolar cells were part of this kainate-sensitive population. The majority of amacrine (ACs) and ganglion cells (GCs) showed kainate responses with different sensitivities between major neurochemical classes (γ-aminobutyric acid [GABA]/glycine ACs > glycine ACs > GABA ACs; glutamate [Glu]/weakly GABA GCs > Glu GCs). Conventional and displaced cholinergic ACs were highly responsive to kainate, whereas dopaminergic ACs do not appear to express functional kainate receptors. These findings further contribute to our understanding of neuronal networks in complex multicellular tissues.

What can we learn about stroke from retinal ischemia models?

Retinal ischemia is a very useful model to study the impact of various cell death pathways, such as apoptosis and necrosis, in the ischemic retina. However, it is important to note that the retina is formed as an outpouching of the diencephalon and is part of the central nervous system. As such, the cell death pathways initiated in response to ischemic damage in the retina reflect those found in other areas of the central nervous system undergoing similar trauma. The retina is also more accessible than other areas of the central nervous system, thus making it a simpler model to work with and study. By utilizing the retinal model, we can greatly increase our knowledge of the cell death processes initiated by ischemia which lead to degeneration in the central nervous system. This paper examines work that has been done so far to characterize various aspects of cell death in the retinal ischemia model, such as various pathways which are activated, and the role neurotrophic factors, and discusses how these are relevant to the treatment of ischemic damage in both the retina and the greater central nervous system.

Agonistic effect of polyunsaturated fatty acids (PUFAs) and its metabolites on brain-derived neurotrophic factor (BDNF) through molecular docking simulation

Background

Brain-derived neurotrophic factor (BDNF) is a potent neurotrophic factor that is implicated in the regulation of food intake and body weight. Polyunsaturated fatty acids (PUFAs) localised in cell membranes have been shown to alter the levels of BDNF in the brain, suggesting that PUFAs and BDNF could have physical interaction with each other. To decipher the molecular mechanism through which PUFAs modulates BDNF’s activity, molecular docking was performed for BDNF with PUFAs and its metabolites, with 4-Methyl Catechol as a control.

Results

Inferring from molecular docking studies, lipoxin A4 (LXA4), and a known anti-inflammatory bioactive metabolite derived from PUFAs, with a binding energy of −3.98 Kcal/mol and dissociation constant of 1.2mM showed highest binding affinity for BDNF in comparison to other PUFAs and metabolites considered in the study. Further, the residues Lys 18, Thr 20, Ala 21, Val 22, Phe 46, Glu 48, Lys 50, Lys 58, Thr 75, Gln 77, Arg 97 and Ile 98 form hot point motif, which on interaction enhances BDNF’s function.

Conclusion

These results suggest that PUFAs and their metabolites especially, LXA4, modulate insulin resistance by establishing a physical interaction with BDNF. Similar interaction(s) was noted between BDNF and resolvins and protectins but were of lesser intensity compared to LXA4.

Shp-2 regulates the TrkB receptor activity in the retinal ganglion cells under glaucomatous stress

Tropomyosin-receptor-kinase B (TrkB receptor) activation plays an important role in the survival of retinal ganglion cells (RGCs). This study reports a novel finding that, SH2 domain-containing phosphatase-2 (Shp-2) binds to the TrkB receptor in RGCs and negatively regulates its activity under glaucomatous stress. This enhanced binding of TrkB and Shp2 is mediated through caveolin. Caveolin 1 and 3 undergo hyper-phosphorylation in RGCs under stress and bind to the Shp2 phosphatase. Shp2 undergoes activation under glaucomatous stress conditions in RGCs in vivo with a concurrent loss of TrkB activity. Inhibiting the Shp2 phosphatase restored TrkB activity in cells exposed to excitotoxic and oxidative stress. Collectively, these findings implicate a molecular basis of Shp2 mediated TrkB deactivation leading to RGC degeneration observed in glaucoma.

Stem cell-based delivery of brain-derived neurotrophic factor gene in the rat retina

As an alternative to a viral vector, the application of stem cells to transfer specific genes is under investigation in various organs. Using this strategy may provide more effective method to supply neurotrophic factor to the neurodegenerative diseases caused by neurotrophic factor deprivation. This study investigated the possibility and efficacy of stem cell-based delivery of the brain-derived neurotrophic factor (BDNF) gene to rat retina. Rat BDNF cDNA was transduced into rat bone marrow mesenchymal stem cells (rMSCs) using a retroviral vector. Its incorporation into the experimental rat retina and the expression of BDNF after intravitreal injection or subretinal injection were detected by real-time PCR, western blot analysis, and immunohistochemical staining. For the incorporated rMSCs, retinal-specific marker staining was performed to investigate the changes in morphology and the characteristics of the stem cells. Transduction of the rMSCs by retrovirus was effective, and the transduced rMSCs expressed high levels of the BDNF gene and protein. The subretinal injection of rMSCs produced rMSC migration and incorporation into the rat retina (about 15.7% incorporation rate), and retinal BDNF mRNA and protein expression was increased at 4 weeks after transplantation. When subretinal injection of rMSCs was applied to axotomized rat retina, it significantly increased the expression of BDNF until 4 weeks after transplantation. Some of the transplanted rMSCs exhibited morphological changes, but the retinal-specific marker stain was not sufficient to indicate whether neuronal differentiation had occurred. Using mesenchymal stem cells to deliver the BDNF gene to the retina may provide new treatment for glaucoma.

Protecting the retinal neurons from glaucoma: lowering ocular pressure is not enough

The retina is theater of a number of biochemical reactions allowing, within its layers, the conversion of light impulses into electrical signals. The axons of the last neuronal elements, the ganglion cells, form the optic nerve and transfer the signals to the brain. Therefore, an appropriate cellular communication, not only within the different retinal cells, but also between the retina itself and the other brain structures, is fundamental. One of the most diffuse pathologies affecting retinal function and communication, which thus reverberates in the whole visual system, is glaucoma. This insidious disease is characterized by a progressive optic nerve degeneration and sight loss which may finally lead to irreversible blindness. Nevertheless, the progressive nature of this pathology offers an opportunity for therapeutic intervention. To better understand the cellular processes implicated in the development of glaucoma useful to envision a targeted pharmacological strategy, this manuscript first examines the complex cellular and functional organization of the retina and subsequently identifies the targets sensitive to neurodegeneration. Within this context, high ocular pressure represents a key risk factor. However, recent literature findings highlight the concept that lowering ocular pressure is not enough to prevent/slow down glaucomatous damage, suggesting the importance of combining the hypotensive treatment with other pharmacological approaches, such as the use of neuroprotectants. Therefore, this important and more novel aspect is extensively considered in this review, also emphasizing the idea that the neuroprotective strategy should be extended to the entire visual system and not restricted to the retina.

Neurotrophic factors and the regeneration of adult retinal ganglion cell axons

The adult central nervous system (CNS) has only a limited capacity to regenerate axons after injury. This is due to a number of factors including the presence of extrinsic inhibitory factors that limit plasticity, lack of effective trophic support, and intrinsic changes in neuronal responsiveness. In this review, we describe the expression and role of neurotrophins in retinal ganglion cells (RGCs) during development and adulthood, and the receptors and miscellaneous signaling systems that influence axonal regeneration after injury. The impact of exogenous neurotrophic factors on adult RGCs injured at different sites in the visual pathway is described for several modes of delivery, including recombinant factors, viral vectors, cell transplantation, as well as combinatorial treatments involving other pharmacotherapeutic agents. Indirect, off-target effects of neurotrophic factors on RGC axonal regeneration are also considered. There remain unresolved issues relating to optimal delivery of neurotrophic factors, and we emphasize the need to develop safe, reliable methods for the regulation of exogenous supply of these factors to the injured CNS.

A cell-permeable phosphine-borane complex delays retinal ganglion cell death after axonal injury through activation of the pro-survival extracellular signal-regulated kinases 1/2 pathway

The reactive oxygen species (ROS) superoxide has been recognized as a critical signal triggering retinal ganglion cell (RGC) death after axonal injury. Although the downstream targets of superoxide are unknown, chemical reduction of oxidized sulfhydryls has been shown to be neuroprotective for injured RGCs. On the basis of this, we developed novel phosphine-borane complex compounds that are cell permeable and highly stable. Here, we report that our lead compound, bis (3-propionic acid methyl ester) phenylphosphine borane complex 1 (PB1) promotes RGC survival in rat models of optic nerve axotomy and in experimental glaucoma. PB1-mediated RGC neuroprotection did not correlate with inhibition of stress-activated protein kinase signaling, including apoptosis stimulating kinase 1 (ASK1), c-jun NH2-terminal kinase (JNK) or p38. Instead, PB1 led to a striking increase in retinal BDNF levels and downstream activation of the extracellular signal-regulated kinases 1/2 (ERK1/2) pathway. Pharmacological inhibition of ERK1/2 entirely blocked RGC neuroprotection induced by PB1. We conclude that PB1 protects damaged RGCs through activation of pro-survival signals. These data support a potential cross-talk between redox homeostasis and neurotrophin-related pathways leading to RGC survival after axonal injury.

WITHDRAWN: Reprint of: Mechanisms of retinal ganglion cell injury and defense in glaucoma

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.exer.2010.04.002. The duplicate article has therefore been withdrawn.

Expression and cell localization of brain-derived neurotrophic factor and TrkB during zebrafish retinal development

Brain-derived neurotrophic factor (BDNF) signaling through TrkB regulates different aspects of neuronal development, including survival, axonal and dendritic growth, and synapse formation. Despite recent advances in our understanding of the functional significance of BDNF and TrkB in the retina, the cell types in the retina that express BDNF and TrkB, and the variations in their levels of expression during development, remain poorly defined. The goal of the present study is to determine the age-dependent changes in the levels of expression and localization of BDNF and TrkB in the zebrafish retina. Zebrafish retinas from 10 days post-fertilization (dpf) to 180 dpf were used to perform PCR, Western blot and immunohistochemistry. Both BDNF and TrkB mRNAs, and BDNF and full-length TrkB proteins were detected at all ages sampled. The localization of these proteins in the retina was very similar at all time points studied. BDNF immunoreactivity was found in the outer nuclear layer, the outer plexiform layer and the inner plexiform layer, whereas TrkB immunoreactivity was observed in the inner plexiform layer and, to a lesser extent, in the ganglion cell layer. These results demonstrate that the pattern of expression of BDNF and TrkB in the retina of zebrafish remains unchanged during postembryonic development and adult life. Because TrkB expression in retina did not change with age, cells expressing TrkB may potentially be able to respond during the entire lifespan of zebrafish to BDNF either exogenously administered or endogenously produced, acting through paracrine mechanisms.

Mechanisms of retinal ganglion cell injury and defense in glaucoma

Glaucoma is a disease in which retinal ganglion cells (RGCs) die leading ultimately to blindness. Over the past decade and a half, information has begun to emerge regarding specific molecular responses of the retina to conditions of elevated intraocular pressure (IOP). It is now clear that the state of the RGC in glaucoma depends on a balance of pro-survival and pro-death pathways in the retina and details of these responses are still being worked out. In this review, we will discuss the evidence supporting the involvement of specific apoptotic cascades as well as the insults that trigger RGC apoptosis. In addition, we will present evidence supporting the existence of endogenous protective mechanisms as well as exogenous neuroprotective strategies.

Perinatal systemic inflammatory response syndrome and retinopathy of prematurity

Fetal and neonatal inflammation is associated with several morbidities of prematurity. Its relationship to retinopathy of prematurity (ROP) has not been investigated. Our objective was to determine the relationship between cytokine levels and ROP in the first 3 postnatal wks. Data for this study were derived from the NICHD Cytokine Study. Dried blood spots (DBS) were obtained from infants <1000 g on days 0-1, 3 +/- 1, 7 +/- 2, 14 +/- 3, and 21 +/- 3. Infants were classified into three groups-no, mild, and severe ROP. Multiplex Luminex assay was used to quantify 20 cytokines. Temporal profiles of cytokines were evaluated using mixed-effects models after controlling for covariates. Of 1074 infants enrolled, 890 were examined for ROP and 877 included in the analysis. ROP was associated with several clinical characteristics on unadjusted analyses. Eight cytokines remained significantly different across ROP groups in adjusted analyses. IL-6 and IL-17 showed significant effects in early time periods (D0-3); TGF-beta, brain-derived neurotrophic factor (BDNF), and regulated on activation, normal T cell expressed and secreted (RANTES) in later time periods (D7-21) and IL-18, C-reactive protein (CRP), and neurotrophin-4 (NT-4) in both early and later time periods. We conclude that perinatal inflammation may be involved in the pathogenesis of ROP.

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.

A rat model for studying neural stem cell transplantation

AIM

The goal of this project was to develop a rat model for neural stem cell (NSC) transplantation studies in which NSCs were modified with brain-derived neurotrophic factor (BDNF) genes that may permit extensive and reliable analysis of the transplants.

METHODS

NSCs were cultured and purified by limiting dilution assay in vitro and infected with recombinant retrovirus pLXSN-BDNF (BDNF-NSCs) and retrovirus pLXSN (p-NSCs). The expression of BDNF genes in transgenic and control NSC groups was measured by FQ-PCR and ELISA assays. NSCs were then transplanted into the subretinal space of normal rat retinas in four groups, which included NSCs alone, BDNF-NSCs, phosphate buffered saline (PBS) control, and normal control. Survival, migration, and differentiation of donor cells in host retinas were observed with optical coherence tomography (OCT), Heidelberg retina angiograph (HRA), and immunohistochemistry, respectively.

RESULTS

The results obtained by FQ-PCR demonstrated that the copy numbers of BDNF gene templates from BDNF-NSCs were the highest among the four groups (P<0.05). Consistent with the results of FQ-PCR, BDNF protein level from the supernatant of the BDNF-NSCs group was much higher than that of the other two groups (P<0.05) as suggested by the ELISA assays. HRA and OCT showed that graft cells could successfully survive. Immunohistochemical analysis revealed that transplanted BDNF-NSCs could migrate in the host retinas and differentiate into glial cells and neurons three months after transplantation.

CONCLUSION

BDNF promotes NSCs to migrate and differentiate into neural cells in the normal host retinas.

Sustained delivery of NT-3 from lens fiber cells in transgenic mice reveals specificity of neuroprotection in retinal degenerations

Several neurotrophic factors (NTFs) are effective in protecting retinal photoreceptor cells from the damaging effects of constant light and slowing the rate of inherited photoreceptor degenerations. It is currently unclear whether, if continuously available, all NTFs can be protective for many or most retinal degenerations (RDs). We used transgenic mice that continuously overexpress the neurotrophin NT-3 from lens fibers under the control of the alphaA-crystallin promoter to test for neuroprotection in light-damage experiments and in four naturally occurring or transgenically induced RDs in mice. Lens-specific expression of NT-3 mRNA was demonstrated both by in situ hybridization in embryos and by reverse-transcriptase polymerase chain reaction (RT-PCR) in adult mice. Furthermore, NT-3 protein was found in abundance in the lens, ocular fluids, and retina by enzyme-linked immunosorbent assay (ELISA) and immunocytochemistry. Overexpression of NT-3 had no adverse effects on the structure or function of the retina for up to at least 14 months of age. Mice expressing the NT-3 transgene were protected from the damaging effects of constant light to a much greater degree than those receiving bolus injections of NT-3. When the NT-3 transgene was transferred into rd/rd, Rds/+, Q344ter mutant rhodopsin or Mertk knockout mice, overexpression of NT-3 had no protective effect on the RDs in these mice. Thus, specificity of the neuroprotective effect of NT-3 is clearly demonstrated, and different molecular mechanisms are inferred to mediate the protective effect in light-induced and inherited RDs.

Kainic acid intraocular injections during the postnatal critical period induce plastic changes in the visual system

Changes in the retino-collicular projection and in the number of optic nerve (ON) axons in adult rats were analyzed after partial loss of retinal ganglion cells (RGCs), induced by intravitreal injections of kainic acid (KA) on postnatal days 2-3 (P2-P3) or 10-12 (P10-P12). KA injected at P2-P3 decreased the volume of the adult contralateral superior colliculus (SC) and the density of the retino-collicular contralateral projection, but maintained the neonatal pattern in the ipsilateral projection from the un-injected eye. ON axon number was significantly increased in the un-injected eye but decreased in the KA-injected eye. Thus, restriction of the ipsilateral retino-collicular projection and RGC death in the un-injected eye are modified by KA at P2-P3, during the postnatal critical period, but not at P10-P12, after it is over. We suggest that, in the SC contralateral to the KA-injected eye, the disappearance of axon terminals belonging to RGC killed by KA would decrease competition between ipsilateral and contralateral terminals, thus contributing to maintaining the neonatal pattern in the ipsilateral retino-collicular projection. The reduction in RGC death in the un-injected eye could also be related to the disappearance of RGC terminals in the contralateral SC, which would have increased neurotrophic factor availability.

Protein kinase C regulates the expression of M1 receptors and BDNF in rat retinal cells

Protein kinase C (PKC) plays a key role in cellular events including proliferation, survival and differentiation. Our previous study showed the effect of phorbol 12-myristate 13-acetate (PMA), a PKC activator, inducing a decrease in retinal cells proliferation. This effect was mediated by muscarinic type 1 receptors (M1) activation and brain derived neurotrophic factor (BDNF) treatment also induced a decrease in cell proliferation. Based on these results we analyzed the expression of either M1 receptors or BDNF following PMA treatment of retinal cell cultures. Our data demonstrated that PMA induced a decrease in both protein expressions after 48 h in culture. However, after 45 min, PMA induced a transient increase in BDNF expression and a decrease in M1 receptors expression. Analyzing the expression of M1 receptors and BDNF during the postnatal development in vivo, we observed a decrease in both proteins. Taken together our results suggest the involvement of PKC in the control of M1 expression in retinal cells.

Effects of optic nerve injury, glaucoma, and neuroprotection on the survival, structure, and function of ganglion cells in the mammalian retina

Glaucoma is an optic neuropathy that originates with pressure-induced damage to the optic nerve. This results in the retrograde degeneration of ganglion cells in the retina, and a progressive loss of vision. Over the past several years, a number of studies have described the structural and functional changes that characterize ganglion cell degeneration in the glaucomatous eye, and following optic nerve injury. In addition, a variety of different strategies for providing neuroprotection to the injured retina have been proposed. Many of these are based on the use of brain-derived neurotrophic factor (BDNF), a particularly potent neuroprotectant in the mammalian eye and the basis of our research in this area. Of particular importance is the fact that BDNF not only promotes ganglion cell survival following damage to the optic nerve, but also helps to preserve the structural integrity of the surviving neurons, which in turn results in enhanced visual function. The studies presented here describe these attributes, and serve as the foundation for ongoing work that suggests a need to think beyond the eye in the development of future treatment strategies.

The neurobiology of cell death in glaucoma

Glaucoma is an optic neuropathy in which the optic nerve axons are damaged, resulting in death of retinal ganglion cells (RGCs). The primary region of damage is thought to be the optic nerve head (ONH), with the lateral geniculate nucleus (LGN) and optic radiations to the visual cortex being secondarily affected. Neurotrophin deprivation resulting from optic nerve injury is thought to cause RGCs to die by apoptosis by inhibition of cell survival pathways. However, disruption of retrograde axonal transport is not the only mechanism associated with optic nerve damage and RGC death, and thus, an additional mechanism of injury is likely to be involved in glaucomatous optic neuropathy.

Retinal TrkB receptors regulate neural development in the inner, but not outer, retina

BDNF signaling through its TrkB receptor plays a pivotal role in activity-dependent refinement of synaptic connectivity of retinal ganglion cells. Additionally, studies using TrkB knockout mice have suggested that BDNF/TrkB signaling is essential for the development of photoreceptors and for synaptic communication between photoreceptors and second order retinal neurons. Thus the action of BDNF on refinement of synaptic connectivity of retinal ganglion cells could be a direct effect in the inner retina, or it could be secondary to its proposed role in rod maturation and in the formation of rod to bipolar cell synaptic transmission. To address this matter we have conditionally eliminated TrkB within the retina. We find that rod function and synaptic transmission to bipolar cells is not compromised in these conditional knockout mice. Consistent with previous work, we find that inner retina neural development is regulated by retinal BDNF/TrkB signaling. Specifically we show here also that the complexity of neuronal processes of dopaminergic cells is reduced in conditional TrkB knockout mice. We conclude that retinal BDNF/TrkB signaling has its primary role in the development of inner retinal neuronal circuits, and that this action is not a secondary effect due to the loss of visual signaling in the outer retina.

Brain-derived neurotrophic factor (BDNF)-like immunoreactivity localization in the retina and brain of Cichlasoma dimerus (Teleostei, Perciformes)

Brain-derived neurotrophic factor (BDNF) is a neurotrophin involved in the development and maintenance of vertebrate nervous systems. Although there were several studies in classical animal models, scarce information for fish was available. The main purpose of this study was to analyze the distribution of BDNF in the brain and retina of the cichlid fish Cichlasoma dimerus. By immunohistochemistry we detected BDNF-like immunoreactive cells in the cytoplasm and the nuclei of the ganglion cell layer and the inner nuclear layer of the retina. In the optic tectum, BDNF-like immunoreactivity was detected in the nucleus of neurons of the stratum periventriculare and the stratum marginale and in neurons of the intermediate layers. In the hypothalamus we found BDNF-like immunoreactivity mainly in the cytoplasm of the nucleus lateralis tuberis and the nucleus of the lateral recess. To confirm the nuclear and cytoplasm localization of BDNF we performed subcellular fractionation, followed by Western blot, detecting a 39 kDa immunoreactive-band corresponding to a possible precursor form of BDNF in both fractions. BDNF-like immunoreactivity was distributed in areas related with photoreception (retina), the integration center of retinal projections (optic tectum) and the control center of background and stress adaptation (hypothalamus). These results provide baseline anatomical information for future research about the role of neurotrophins in the adult fish central nervous system.

Retrograde axonal transport obstruction of brain-derived neurotrophic factor (BDNF) and its TrkB receptor in the retina and optic nerve of American Cocker Spaniel dogs with spontaneous glaucoma

OBJECTIVE

To determine the degree of retrograde optic nerve axonal transport obstruction at the scleral lamina cribosa level by examining levels of brain-derived neurotrophic factor (BDNF) and its tyrosine kinase receptor type B (TrkB) in dogs presenting with high intraocular pressure.

ANIMALS STUDIED

A total of 10 eyes, four normal and six glaucomatous eyes, from normal and affected American Cocker Spaniels with primary glaucoma were studied. All eyes were assessed by neuro-ophthalmic examination, tonometry, gonioscopy, slit-lamp biomicroscopy, and indirect ophthalmoscopy prior to enucleation.

METHODS

Immunocytochemistry analysis was performed to evaluate BDNF and TrkB receptor expression in retina and optic nerve in normal and glaucomatous dogs.

RESULTS

In all normal eyes BDNF immunostaining was detected in the cytoplasm of retinal ganglion cells (RGC), inner plexiform layer (IPL), inner nuclear layer (INL), nerve fiber layer (NFL), optic nerve head cells, and lamina cribosa cells. In all glaucomatous eyes BDNF was more evident in RGC, NFL and lamina cribosa cells. TrkB receptor was detected in the cytoplasm of RGC, NFL and ONH bundles in all normal eyes, and in a more intense pattern in all glaucomatous eyes.

CONCLUSIONS

BDNF retrograde axonal transport is substantially inhibited by intraocular pressure elevation. TrkB accumulation at the ONH in glaucoma suggests a role for neurotrophin deprivation in the pathogenesis of RGC death in canine glaucoma, as well as a possible paracrine and/or autocrine signaling within the lamina cribosa. Neurotrophin signaling may regulate more than neuronal development, survival and differentiation. BDNF neurotrophin and its TrkB receptor expression by lamina cribosa cells and ONH astrocytes in glaucomatous eyes may help to determine the role of these cells as a paracrine source in terms of retinal ganglion cell survival, during episodes of elevated intraocular pressure.

Expression of brain-derived neurotrophic factor in cholinergic and dopaminergic amacrine cells in the rat retina and the effects of constant light rearing

Brain-derived neurotrophic factor (BDNF) regulates many aspects of neuronal development, including survival, axonal and dendritic growth and synapse formation. Despite recent advances in our understanding of the functional significance of BDNF in retinal development, the retinal cell types expressing BDNF remains poorly defined. The goal of the present study was to determine the localization of BDNF in the mammalian retina, with special focus on the subtypes of amacrine cells, and to characterize, at the cellular level, the effects of constant light exposure during early postnatal period on retinal expression of BDNF. Retinas from 3-week-old rats reared in a normal light cycle or constant light were subjected to double immunofluorescence staining using antibodies to BDNF and retinal cell markers. BDNF immunoreactivity was localized to ganglion cells, cholinergic amacrine cells and dopaminergic amacrine cells, but not to AII amacrine cells regardless of rearing conditions. Approximately 75% of BDNF-positive cells in the inner nuclear layer were cholinergic amacrine cells in animals reared in a normal lighting condition. While BDNF immunoreactivity in ganglion cells and cholinergic amacrine cells was significantly increased by constant light rearing, which in dopaminergic amacrine cells was apparently unaltered. The overall structure of the retina and the density of ganglion cells, cholinergic amacrine cells and AII amacrine cells were unaffected by rearing conditions, whereas the density of dopaminergic amacrine cells was significantly increased by constant light rearing. The present results indicate that cholinergic amacrine cells are the primary source of BDNF in the inner nuclear layer of the rat retina and provide the first evidence that cholinergic amacrine cells may be involved in the visual activity-dependent regulation of retinal development through the production of BDNF. The present data also suggest that the production or survival of dopaminergic amacrine cells is regulated by early visual experience.

Effects of neurotrophins on the survival and regrowth of injured retinal neurons

The focus of this short review is the role of certain neurotrophins and their receptors on the survival and regrowth of retinal ganglion cells (RGCs) whose axons are damaged in the optic nerve. Initial experiments in our laboratory documented patterns of RGC death after axotomy. Subsequent studies were designed to investigate the distribution of high-affinity neurotrophin receptors in neurons and glial cells of the retina and optic nerve. This information was used both in vitro and in vivo to study the effects of specific trophic molecules on the survival and regrowth of injured RGCs. During the course of experiments involving neurotrophin administration, an endogenous source of trophic support--independent of the exogenous administration of growth factors--was found within the eye. Several experiments were subsequently undertaken to define further this survival effect and determine its nature and source within the eye. Finally, anatomical techniques that help visualize fine axonal processes within the retina have provided insights into the specific effects of neurotrophins on the growth and branching of injured CNS axons.

Environmental enrichment effects on development of retinal ganglion cell dendritic stratification require retinal BDNF

A well-known developmental event of retinal maturation is the progressive segregation of retinal ganglion cell (RGC) dendrites into a and b sublaminae of the inner plexiform layer (IPL), a morphological rearrangement crucial for the emergence of the ON and OFF pathways. The factors regulating this process are not known, although electrical activity has been demonstrated to play a role. Here we report that Environmental Enrichment (EE) accelerates the developmental segregation of RGC dendrites and prevents the effects exerted on it by dark rearing (DR). Development of RGC stratification was analyzed in a line of transgenic mice expressing plasma-membrane marker green fluorescent protein (GFP) under the control of Thy-1 promoter; we visualized the a and b sublaminae of the IPL by using an antibody selectively directed against a specific marker of cholinergic neurons. EE precociously increases Brain Derived Neurotrophic Factor (BDNF) in the retina, in parallel with the precocious segregation of RGC dendrites; in addition, EE counteracts retinal BDNF reduction in DR retinas and promotes a normal segregation of RGC dendrites. Blocking retinal BDNF by means of antisense oligos blocks EE effects on the maturation of RGC dendritic stratification. Thus, EE affects the development of RGC dendritic segregation and retinal BDNF is required for this effect to take place, suggesting that BDNF could play an important role in the emergence of the ON and OFF pathways.

Specific alterations of tyrosine hydroxylase immunopositive cells in the retina of NT-4 knock out mice

To assess the effect of NT-4 deprivation on maturation of retinal circuitry, we investigated a mouse with targeted deletion of the gene encoding nt-4 (nt-4(-/-)). In particular, we studied neurons immunostained by an antibody recognizing tyrosine hydroxylase (TH), the rate limiting enzyme for dopamine (DA) synthesis. We found that TH immunopositive processes were altered in the retina of nt-4(-/-). Alteration of TH immunopositive processes in nt-4(-/-) mice resulted in changes of DA turnover, as assessed by high-pressure liquid chromatography measurements. These findings suggest that retinal NT-4 plays a role in the morphological maturation of dopaminergic retinal cells.