Effects of brain-derived neurotrophic factor on the development of NADPH-diaphorase/nitric oxide synthase-positive amacrine cells in the rodent retina

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

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

Neuroprotective and restorative effects of the brain-derived neurotrophic factor in retinal diseases

Brain-derived neurotrophic factor (BDNF) is a neurotrophin proposed to be implicated in ameliorating the course of some neurodegenerative disorders. Given the fact that retina is considered as an out-pouching of the central nervous system, its related diseases have long been suggested to receive protective influence from this signaling molecule. The role of BDNF in retinal neurorestoration, neuroprotection and oxidative stress has extensively been tested over the past two decades. Nonetheless, almost the entire related literature root in animal studies and clinical research on this topic is lacking. Although much of the evidence have validated the protective properties of BDNF against various retinal cell diseases, bringing such insights into clinical context would depend on further well-designed research. The present review is an attempt to categorize and discuss the available evidence with regard to the BDNF and retinal diseases.

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.

Targeted delivery of brain-derived neurotrophic factor for the treatment of blindness and deafness

Neurodegenerative causes of blindness and deafness possess a major challenge in their clinical management as proper treatment guidelines have not yet been found. Brain-derived neurotrophic factor (BDNF) has been established as a promising therapy against neurodegenerative disorders including hearing and visual loss. Unfortunately, the blood–retinal barrier and blood–cochlear barrier, which have a comparable structure to the blood–brain barrier prevent molecules of larger sizes (such as BDNF) from exiting the circulation and reaching the targeted cells. Anatomical features of the eye and ear allow use of local administration, bypassing histo-hematic barriers. This paper focuses on highlighting a variety of strategies proposed for the local administration of the BDNF, like direct delivery, viral gene therapy, and cell-based therapy, which have been shown to successfully improve development, survival, and function of spiral and retinal ganglion cells. The similarities and controversies for BDNF treatment of posterior eye diseases and inner ear diseases have been analyzed and compared. In this review, we also focus on the possibility of translation of this knowledge into clinical practice. And finally, we suggest that using nanoparticulate drug-delivery systems may substantially contribute to the development of clinically viable techniques for BDNF delivery into the cochlea or posterior eye segment, which, ultimately, can lead to a long-term or permanent rescue of auditory and optic neurons from degeneration.

Functional regulation of neuronal nitric oxide synthase expression and activity in the rat retina

In the nervous system within physiological conditions, nitric oxide (NO) production depends on the activity of nitric oxide synthases (NOSs), and particularly on the expression of the neuronal isoform (nNOS). In the sensory systems, the role of NO is poorly understood. In this study, we identified nNOS-positive cells in the inner nuclear layer (INL) of the rat retina, with distinct characteristics such as somata size, immunolabeling level and location. Employing mathematical cluster analysis, we determined that nNOS amacrine cells are formed by two distinct populations. We next investigated the molecular identity of these cells, which did not show colocalization with calbindin (CB), choline acetyltransferase (ChAT), parvalbumin (PV) or protein kinase C (PKC), and only partial colocalization with calretinin (CR), revealing the accumulation of nNOS in specific amacrine cell populations. To access the functional, circuitry-related roles of these cells, we performed experiments after adaptation to different ambient light conditions. After 24h of dark-adaptation, we detected a subtle, yet statistically significant decrease in nNOS transcript levels, which returned to steady-state levels after 24h of normal light-dark cycle, revealing that nNOS expression is governed by ambient light conditions. Employing electron paramagnetic resonance (EPR), we demonstrated that dark-adaptation decreases NO production in the retina. Furthermore, nNOS accumulation changed in the dark-adapted retinas, with a general reduction in the inner plexiform layer. Finally, computational analysis based on clustering techniques revealed that dark-adaptation differently affected both types of nNOS-positive amacrine cells. Taken together, our data disclosed functional regulation of nNOS expression and activity, disclosing new circuitry-related roles of nNOS-positive cells. More importantly, this study indicated unsuspected roles for NO in the sensory systems, particularly related to adaptation to ambient demands.

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.

Amacrine cell gene expression and survival signaling: differences from neighboring retinal ganglion cells

PURPOSE. To describe how developing amacrine cells and retinal ganglion cells (RGCs) differ in survival signaling and global gene expression. METHODS. Amacrine cells were immunopurified and processed for gene microarray analysis. For survival studies, purified amacrine cells were cultured at low density in serum-free medium, with and without peptide trophic factors and survival pathway inhibitors. The differences in gene expression between amacrine cells and RGCs were analyzed by comparing the transcriptomes of these two cell types at the same developmental ages. RESULTS. The amacrine cell transcriptome was very dynamic during development. Amacrine cell gene expression was remarkably similar to that of RGCs, but differed in several gene ontologies, including polarity- and neurotransmission-associated genes. Unlike RGCs, amacrine cell survival in vitro was independent of cell density and the presence of exogenous trophic factors, but necessitated Erk activation via MEK1/2 and AKT signaling. Finally, comparison of the gene expression profile of amacrine cells and RGCs provided a list of polarity-associated candidate genes that may explain the inability of amacrine cells to differentiate axons and dendrites as RGCs do. CONCLUSIONS. Comparison of the gene expression profile between amacrine cells and RGCs may improve our understanding of why amacrine cells fail to differentiate axons and dendrites during retinal development and of what makes amacrine cells differ in their resistance to neurodegeneration. Switching RGCs to an amacrine cell-like state could help preserve their survival in neurodegenerative diseases like glaucoma, and amacrine cells could provide a ready source of replacement RGCs in such optic neuropathies.

Setting the pace for retinal development: environmental enrichment acts through insulin-like growth factor 1 and brain-derived neurotrophic factor

Environmental enrichment strongly affects visual system maturation both at retinal and cortical levels. Which molecular pathways are activated by an enriched environment (EE) to regulate visual system development has not been clarified. Here, we show that early [postnatal day 1 (P1) to P7] insulin-like growth factor 1 (IGF-1) injections in the eyes of non-EE rat pups mimic EE effects both in increasing BDNF levels in the retinal ganglion cell layer at P10 and in determining a more adult-like retinal acuity, assessed with pattern electroretinogram at P25. Blocking IGF-1 action in EE animals during the same early postnatal time window by injecting the IGF-1 receptor antagonist JB1 prevents EE effects both on BDNF expression and on retinal acuity maturation. Reducing BDNF expression in the retina of IGF-1-treated rats prevents IGF-1 effects on retinal acuity development. Finally, we show that tyrosine hydroxylase (TH) expression is increased in the retina of P10 EE and IGF-1-treated rats and that blocking TH expression in EE animals prevents EE from affecting retinal acuity development. Thus, early levels of IGF-1 play a key role in mediating EE effects on retinal development through an action that requires BDNF and involves dopaminergic amacrine cell network.

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.

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.

The importance of transgene and cell type on the regeneration of adult retinal ganglion cell axons within reconstituted bridging grafts

When grafted onto the cut optic nerve, chimeric peripheral nerve (PN) sheaths reconstituted with adult Schwann cells (SCs) support the regeneration of adult rat retinal ganglion cell (RGC) axons. Regrowth can be further enhanced by using PN containing SCs transduced ex vivo with lentiviral (LV) vectors encoding a secretable form of ciliary neurotrophic factor (CNTF). To determine whether other neurotrophic factors or different cell types also enhance RGC regrowth in this bridging model, we tested the effectiveness of (1) adult SCs transduced with brain-derived neurotrophic factor (BDNF) or glial cell line-derived neurotrophic factor (GDNF), and (2) fibroblasts (FBs) genetically modified to express CNTF. SCs transduced with LV-BDNF and LV-GDNF secreted measurable and bioactive amounts of each of these proteins, but reconstituted grafts containing LV-BDNF or LV-GDNF transduced SCs did not enhance RGC survival or axonal regrowth. LV-BDNF modified grafts did, however, contain many pan-neurofilament immunolabeled axons, many of which were also immunoreactive for calcitonin gene-related peptide (CGRP) and were presumably of peripheral sensory origin. Nor-adrenergic and cholinergic axons were also seen in these grafts. There were far fewer axons in LV-GDNF engineered grafts. Reconstituted PN sheaths containing FBs that had been modified to express CNTF did not promote RGC viability or regeneration, and PN reconstituted with a mixed population of SCs and CNTF expressing FBs were less effective than SCs alone. These data show that both the type of neurotrophic factor and the cell types that express these factors are crucial elements when designing bridging substrates to promote long-distance regeneration in the injured CNS.

Retinal functional development is sensitive to environmental enrichment: a role for BDNF

Retina has long been considered less plastic than cortex or hippocampus, the very sites of experience-dependent plasticity. Now, we show that retinal development is responsive to the experience provided by an enriched environment (EE): the maturation of retinal acuity, which is a sensitive index of retinal circuitry development, is strongly accelerated in EE rats. This effect is present also in rats exposed to EE up to P10, that is before eye opening, suggesting that factors sufficient to trigger retinal acuity development are affected by EE during the first days of life. Brain derived neurotrophic factor (BDNF) is precociously expressed in the ganglion cell layer of EE with respect to non-EE rats and reduction of BDNF expression in EE animals counteracts EE effects on retinal acuity. Thus, EE controls the development of retinal circuitry, and this action depends on retinal BDNF expression.

Dark-rearing-induced reduction of GABA and GAD and prevention of the effect by BDNF in the mouse retina

Gamma-aminobutyric acid (GABA) is an important retinal neurotransmitter. We studied the expression of GABA, glutamate decarboxylase 65 (GAD65) and GAD67 by immunocytochemistry and Western blot, in the retinas of control and dark-reared C57BL/6J black mice. This study asked three questions. First, is visual input necessary for the normal expression of GABA, GAD65 and GAD67? Second, can the retina recover from the effects of dark-rearing if returned to a normal light-dark cycle? Third, does BDNF prevent the influence of dark-rearing on the expression of GABA and GAD? At postnatal day 10 (P10), before eye opening, GABA immunoreactivity was present in the ganglion cell layer (GCL), in the innermost rows of the inner nuclear layer (INL) and throughout the inner plexiform layer (IPL) of control and dark-reared retinas. In P30 control retinas, GABA immunoreactivity showed similar patterns to those at P10. However, in P30 dark-reared retinas, the density of GABA-immunoreactive cells was lower in both the INL and GCL than in control retinas. In addition, visual deprivation retarded GABA immunoreactivity in the IPL. Western blot analysis showed corresponding differences in the levels of GAD65 but not of GAD67 expression between control and dark-rearing conditions. In our study, dark-rearing effects were reversed when the mice were put in normal cyclic light-dark conditions for 2 weeks. Moreover, dark-reared retinas treated with BDNF showed normal expression of both GABA and GAD65. Our data indicate that normal expression of GABA and GAD65 is dependent on visual input. Furthermore, the data suggest that BDNF controls this dependence.

Effects of brain-derived neurotrophic factor on cell survival, differentiation and patterning of neuronal connections and Muller glia cells in the developing retina

The aim of the present study was to determine the influence of brain-derived neurotrophic factor (BDNF) on survival, phenotype differentiation and network formation of retinal neurons and glia cells. To achieve a defined concentration and constant level of BDNF over several days, experiments were performed in an organotypic culture of the developing rat retina. After 6 days in vitro, apoptosis in the different cell layers was determined by TUNEL staining and cell-type-specific antibodies were used to identify distinct neuronal cell types and Müller cells. Cultured retinas treated with BDNF (100 ng BDNF/mL medium) were compared with untreated as well as with age-matched in vivo retinas. Quantitative morphometry was carried out using confocal microscopy. BDNF promoted the in vitro development and differentiation of the retina in general, i.e. the number of cells in the nuclear layers and the thickness of the plexiform layers were increased. For all neurons, the number of cells and the complexity of arborizations in the synaptic layers were clearly up-regulated by BDNF. In control cultures, the synaptic stratification of cone bipolar cells within the On- and Off-layer of the inner plexiform layer was disturbed and a strong reactivity of Müller cell glia was observed. These effects were not present in BDNF-treated cultures. Our data show that BDNF promotes the survival of retinal interneurons and plays an important role in establishing the phenotypes and the synaptic connections of a large number of neuronal types in the developing retina. Moreover, we show an effect of BDNF on Müller glia cells.

Brain-derived neurotrophic factor regulates expression of vasoactive intestinal polypeptide in retinal amacrine cells

Brain-derived neurotrophic-factor (BDNF) is expressed in the retina and controls the development of subtypes of amacrine cells. In the present study we investigated the effects of BDNF on amacrine cells expressing vasoactive intestinal polypeptide (VIP). Rats received three intraocular injections of BDNF on postnatal days (P) 16, 18, and 20. The animals were sacrificed on P22, P40, P60, P80, and P120, and VIP expression in their retinas was detected by immunohistochemistry (P22, P40) and by radioimmunoassay (RIA; P22, P40, P60, P80, P120) to assess the time course of BDNF effects on VIP. A significant increase in the density of VIP-positive amacrine cells was detected in BDNF-treated retinas, and VIP concentration was up-regulated by 150% both at P22 and at P40 with respect to untreated controls. VIP concentration then slowly declined in the treated retinas over a period of 3 months; however, a statistically significant increase of 50% was still detectable on P120. The impact of endogenous BDNF on the regulation of VIP expression in the retina was analyzed in mice homozygous for a targeted deletion of the BDNF gene locus (bdnf-/-). VIP immunohistochemistry revealed a marked reduction of VIP-positive amacrine cells and of VIP-immunopositive processes in the inner plexiform layer of the BDNF knockout mice. Mice lacking BDNF expressed only 5% of the VIP protein in their retinas compared with the retinas of wild-type mice as measured by RIA. Our data show that BDNF is a major regulator of VIP expression in retinal amacrine cells and exerts long-lasting effects on VIP content.

Nitric oxide acts in a positive feedback loop with BDNF to regulate neural progenitor cell proliferation and differentiation in the mammalian brain

Nitric oxide (NO) is believed to act as an intercellular signal that regulates synaptic plasticity in mature neurons. We now report that NO also regulates the proliferation and differentiation of mouse brain neural progenitor cells (NPCs). Treatment of dissociated mouse cortical neuroepithelial cluster cell cultures with the NO synthase inhibitor L-NAME or the NO scavenger hemoglobin increased cell proliferation and decreased differentiation of the NPCs into neurons, whereas the NO donor sodium nitroprusside inhibited NPC proliferation and increased neuronal differentiation. Brain-derived neurotrophic factor (BDNF) reduced NPC proliferation and increased the expression of neuronal NO synthase (nNOS) in differentiating neurons. The stimulatory effect of BDNF on neuronal differentation of NPC was blocked by L-NAME and hemoglobin, suggesting that NO produced by the latter cells inhibited proliferation and induced neuronal differentiation of neighboring NPCs. A similar role for NO in regulating the switch of neural stem cells from proliferation to differentiation in the adult brain is suggested by data showing that NO synthase inhibition enhances NPC proliferation and inhibits neuronal differentiation in the subventricular zone of adult mice. These findings identify NO as a paracrine messenger stimulated by neurotrophin signaling in newly generated neurons to control the proliferation and differentiation of NPC, a novel mechanism for the regulation of developmental and adult neurogenesis.

Cell death in the inner nuclear layer of the retina is modulated by BDNF

Developing amacrine cells in the vertebrate retina undergo naturally-occurring cell death which is accentuated by the early removal of retinal ganglion cells. We show that providing BDNF or decreasing endogenous BDNF via competitive binding with soluble TrkB receptors in a whole-retina culture assay modulates the frequency of dying cells in the amacrine cell layer. Ganglion cells synthesize BDNF, and amacrine cells express TrkB receptors, suggesting a likely signaling mechanism.

Nitric oxide modulates retinal ganglion cell axon arbor remodeling in vivo

Nitric oxide (NO) has been postulated to act as an activity-dependent retrograde signal that can mediate multiple aspects of synaptic plasticity during development. In the visual system, a role for NO in activity-dependent structural modification of presynaptic arbors has been proposed based on NO's ability to prune inappropriate projections and segregate axon terminals. However, evidence demonstrating that altered NO signaling does not perturb ocular dominance map formation leaves unsettled the role of NO during the in vivo refinement of visual connections. To determine whether NO modulates the structural remodeling of individual presynaptic terminal arbors in vivo we have: 1. Used NADPH-diaphorase histochemistry to determine the onset of NO synthase (NOS) expression in the Xenopus visual system. 2. Used in vivo time-lapse imaging to examine the role of NO during retinal ganglion cell (RGC) axon arborization. We show that NOS expression in the target optic tectum is developmentally regulated and localized to neurons that reside in close proximity to arborizing RGC axons. Moreover, we demonstrate that perturbations in tectal NO levels rapidly and significantly alter the dynamic branching of RGC arbors in vivo. Tectal injection of NO donors increased the addition of new branches, but not their stabilization in the long term. Tectal injection of NOS inhibitors increased the dynamic remodeling of axonal arbors by increasing branch addition and elimination and by lengthening pre-existing branches. Thus, these results indicate that altering NO signaling significantly modifies axon branch dynamics in a manner similar to altering neuronal activity levels (Cohen-Cory, 1999). Consequently, our results support a role for NO during the dynamic remodeling of axon arbors in vivo, and suggest that NO functions as an activity-dependent retrograde signal during the refinement of visual connections.