BDNF in the development of the visual system of Xenopus

DSCAM is differentially patterned along the optic axon pathway in the developing Xenopus visual system and guides axon termination at the target

BACKGROUND

The Xenopus retinotectal circuit is organized topographically, where the dorsal-ventral axis of the retina maps respectively on to the ventral-dorsal axis of the tectum; axons from the nasal-temporal axis of the retina project respectively to the caudal-rostral axis of the tectum. Studies throughout the last two decades have shown that mechanisms involving molecular recognition of proper termination domains are at work guiding topographic organization. Such studies have shown that graded distribution of molecular cues is important for topographic mapping. However, the complement of molecular cues organizing topography along the developing optic nerve, and as retinal axons cross the chiasm and navigate towards and innervate their target in the tectum, remains unknown. Down syndrome cell adhesion molecule (DSCAM) has been characterized as a key molecule in axon guidance, making it a strong candidate involved in the topographic organization of retinal fibers along the optic path and at their target.

METHODS

Using a combination of whole-brain clearing and immunohistochemistry staining techniques we characterized DSCAM expression and the projection of ventral and dorsal retinal fibers starting from the eye, following to the optic nerve and chiasm, and into the terminal target in the optic tectum in Xenopus laevis tadpoles. We then assessed the effects of DSCAM on the establishment of retinotopic maps through spatially and temporally targeted DSCAM knockdown on retinal ganglion cells (RGCs) with axons innervating the optic tectum.

RESULTS

Highest expression of DSCAM was localized to the ventral posterior region of the optic nerve and chiasm; this expression pattern coincides with ventral fibers derived from ventral RGCs. Targeted downregulation of DSCAM expression on ventral RGCs affected the segregation of medial axon fibers from their dorsal counterparts within the tectal neuropil, indicating that DSCAM plays a role in retinotopic organization.

CONCLUSION

These findings together with previous studies demonstrating cell-autonomous roles for DSCAM during the development of pre- and postsynaptic arbors in the Xenopus retinotectal circuit indicates that DSCAM exerts multiple roles in coordinating axon targeting and structural connectivity in the developing vertebrate visual system.

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.

Artificial cells drive neural differentiation

We report the construction of artificial cells that chemically communicate with mammalian cells under physiological conditions. The artificial cells respond to the presence of a small molecule in the environment by synthesizing and releasing a potent protein signal, brain-derived neurotrophic factor. Genetically controlled artificial cells communicate with engineered human embryonic kidney cells and murine neural stem cells. The data suggest that artificial cells are a versatile chassis for the in situ synthesis and on-demand release of chemical signals that elicit desired phenotypic changes of eukaryotic cells, including neuronal differentiation. In the future, artificial cells could be engineered to go beyond the capabilities of typical smart drug delivery vehicles by synthesizing and delivering specific therapeutic molecules tailored to distinct physiological conditions.

Neuroanatomical distribution and functions of brain-derived neurotrophic factor in zebrafish (Danio rerio) brain

Brain-derived neurotrophic factor (BDNF) is an extensively studied protein that is evolutionarily conserved and widely distributed in the brain of vertebrates. It acts via its cognate receptors TrkB and p75^NTR and plays a central role in the developmental neurogenesis, neuronal survival, proliferation, differentiation, synaptic plasticity, learning and memory, adult hippocampal neurogenesis, and brain regeneration. BDNF has also been implicated in a plethora of neurological disorders. Hence, understanding the processes that are controlled by BDNF and their regulating mechanisms is important. Although, BDNF has been thoroughly studied in the mammalian models, contradictory effects of its functions have been reported on several occasions. These contradictory effects may be attributed to the sheer complexity of the mammalian brain. The study of BDNF and its associated functions in a simpler vertebrate model may provide some clarity about the effects of BDNF on the neurophysiology of the brain. Keeping that in mind, this review aims at summarizing the current knowledge about the distribution of BDNF and its associated functions in the zebrafish brain. The main focus of the review is to give a comparative overview of BDNF distribution and function in zebrafish and mammals with respect to distinct life stages. We have also reviewed the regulation of bdnf gene in zebrafish and discussed its role in developmental and adult neurogenesis.

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.

Spinal Plasticity and Behavior: BDNF-Induced Neuromodulation in Uninjured and Injured Spinal Cord

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophic factor family of signaling molecules. Since its discovery over three decades ago, BDNF has been identified as an important regulator of neuronal development, synaptic transmission, and cellular and synaptic plasticity and has been shown to function in the formation and maintenance of certain forms of memory. Neural plasticity that underlies learning and memory in the hippocampus shares distinct characteristics with spinal cord nociceptive plasticity. Research examining the role BDNF plays in spinal nociception and pain overwhelmingly suggests that BDNF promotes pronociceptive effects. BDNF induces synaptic facilitation and engages central sensitization-like mechanisms. Also, peripheral injury-induced neuropathic pain is often accompanied with increased spinal expression of BDNF. Research has extended to examine how spinal cord injury (SCI) influences BDNF plasticity and the effects BDNF has on sensory and motor functions after SCI. Functional recovery and adaptive plasticity after SCI are typically associated with upregulation of BDNF. Although neuropathic pain is a common consequence of SCI, the relation between BDNF and pain after SCI remains elusive. This article reviews recent literature and discusses the diverse actions of BDNF. We also highlight similarities and differences in BDNF-induced nociceptive plasticity in naïve and SCI conditions.

Early development and function of the Xenopus tadpole retinotectal circuit

The retinotectal circuit is the major component of the amphibian visual system. It is comprised of the retinal ganglion cells (RGCs) in the eye, which project their axons to the optic tectum and form synapses onto postsynaptic tectal neurons. The retinotectal circuit is relatively simple, and develops quickly: Xenopus tadpoles begin displaying retinotectal-dependent visual avoidance behaviors by approximately 7-8 days post-fertilization, early larval stage. In this review we first provide a summary of the dynamic development of the retinotectal circuit, including the microcircuitry formed by local tectal-tectal connections within the tectum. Second, we discuss the basic visual avoidance behavior generated specifically by this circuit, and how this behavior is being used as an assay to test visual system function.

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.

Netrin-1 directs dendritic growth and connectivity of vertebrate central neurons in vivo

BACKGROUND

Netrins are a family of extracellular proteins that function as chemotropic guidance cues for migrating cells and axons during neural development. In the visual system, netrin-1 has been shown to play a key role in retinal ganglion cell (RGC) axon growth and branching at the target, where presynaptic RGC axons form partnerships with the dendrites of tectal neurons. However, the signals that guide the connections between RGC axons and their postsynaptic partners are yet unknown. Here, we explored dynamic cellular mechanisms by which netrin-1 influences visual circuit formation, particularly those that impact postsynaptic neuronal morphology and connectivity during retinotectal wiring.

RESULTS

Time-lapse in vivo imaging of individual Xenopus laevis optic tectal neurons co-expressing tdTomato and PSD95-GFP revealed rapid remodeling and reorganization of dendritic arbors following acute manipulations in netrin-1 levels. Effects of altered netrin signaling on developing dendritic arbors of tectal neurons were distinct from its effects on presynaptic RGC axons. Within 4 h of treatment, tectal injection of recombinant netrin-1 or sequestration of endogenous netrin with an UNC-5 receptor ectodomain induced significant changes in the directionality and orientation of dendrite growth and in the maintenance of already established dendrites, demonstrating that relative levels of netrin are important for these functions. In contrast, altering DCC-mediated netrin signaling with function-blocking antibodies induced postsynaptic specialization remodeling and changed growth directionality of already established dendrites. Reducing netrin signaling also decreased avoidance behavior in a visually guided task, suggesting that netrin is essential for emergent visual system function.

CONCLUSIONS

These in vivo findings together with the patterns of expression of netrin and its receptors reveal an important role for netrin in the early growth and guidance of vertebrate central neuron dendritic arbors. Collectively, our studies indicate that netrin shapes both pre- and postsynaptic arbor morphology directly and in multiple ways at stages critical for functional visual system development.

Intraocular BDNF promotes ectopic branching, alters motility and stimulates abnormal collaterals in regenerating optic fibers

A great deal of effort has been invested in using trophic factors and other bioactive molecules to promote cell survival and axonal regeneration in the adult central nervous system. Far less attention has been paid to investigating potential effects that trophic factors may have that might interfere with recovery. In the visual system, BDNF has been previously reported to prevent regeneration. To test if BDNF is inherently incompatible with regeneration, BDNF was given intraocularly during optic nerve regeneration in the adult goldfish. In vivo imaging and anatomical analysis of selectively labeled axons were used as a sensitive assay for effects on regeneration within the tectum. BDNF had no detectable inhibitory effect on the ability of axons to regenerate. Normal numbers of axons regenerated into the tectum, exhibited dynamic growth and retractions similar to controls, and were able to navigate to their correct target zone in the tectum. However, BDNF was found to have additional effects that adversely affected the quality of regeneration. It promoted premature branching at ectopic locations, diminished the growth rate of axons through the tectum, and resulted in the formation of ectopic collaterals. Thus, although BDNF has robust effects on axonal behavior, it is, nevertheless, compatible with axonal regeneration, axon navigation and the formation of terminal arbors.

Localization of BDNF expression in the developing brain of zebrafish

The brain-derived neurotrophic factor (BDNF) gene is expressed in differentiating and post-mitotic neurons of the zebrafish embryo, where it has been implicated in Huntington's disease. Little is known, however, about the full complement of neuronal cell types that express BDNF in this important vertebrate model. Here, we further explored the transcriptional profiles during the first week of development using real-time quantitative polymerase chain reaction (RT-qPCR) and whole-mount in situ hybridization (WISH). RT-qPCR results revealed a high level of maternal contribution followed by a steady increase of zygotic transcription, consistent with the notion of a prominent role of BDNF in neuronal maturation and maintenance. Based on WISH, we demonstrate for the first time that BDNF expression in the developing brain of zebrafish is structure specific. Anatomical criteria and co-staining with genetic markers (shh, pax2a, emx1, krox20, lhx2b and lhx9) visualized major topological domains of BDNF-positive cells in the pallium, hypothalamus, posterior tuberculum and optic tectum. Moreover, the relative timing of BDNF transcription in the eye and tectum may illustrate a mechanism for coordinated development of the retinotectal system. Taken together, our results are compatible with a local delivery and early role of BDNF in the developing brain of zebrafish, adding basic knowledge to the study of neurotrophin functions in neural development and disease.

A role for the polarity complex and PI3 kinase in branch formation within retinotectal arbors of zebrafish

The developing zebrafish retinotectal arbors make many trial branches with synapses but most are retracted. With NMDA blockers, branches are withdrawn at a higher rate, and a synapse on a branch not only stabilizes that branch, but biases new branches to form nearby. Here, we tested whether new branch formation requires the polarity complex, which is essential for organizing the cytoskeleton in initial axon formation. The complex (PAR3, PAR6, and atypical protein kinase C [aPKC]) is downstream of phosphatidyl-inositol-3-kinase (PI3K), and its aPKC could be activated by retrograde arachidonic acid synaptic signaling. DiO-labeled arbors in zebrafish were imaged on day 3 (before treatment) and 1-2 days after treatment to suppress or inhibit PAR3, PAR6, or PI3K. Intraocular antisense (AS) oligos to PAR3 or PAR6 both severely limited branch addition, which was most evident in arbors with few branches before treatment. As a result of the inability to branch, arbor segments grew longer than in controls. Both PI3K inhibition (LY294002) and AS suppression of PI3Kα and PI3Kδ isoforms likewise limited branch addition but also decreased growth, as the sum of segment lengths was below normal after 2 days. Both the results support the idea that the polarity complex and PI3K participate in arbor branch formation. The PKC inhibitor Go6983 also severely restricted branch addition and growth, as did bisindolyl-maleimide and calphostin C reported previously, consistent with PKCζ, but not PKCµ, participation. These experiments suggest a mechanism whereby activity signaling could affect the branching of retinotectal arbors.

Lycium barbarum (wolfberry) reduces secondary degeneration and oxidative stress, and inhibits JNK pathway in retina after partial optic nerve transection

Our group has shown that the polysaccharides extracted from Lycium barbarum (LBP) are neuroprotective for retinal ganglion cells (RGCs) in different animal models. Protecting RGCs from secondary degeneration is a promising direction for therapy in glaucoma management. The complete optic nerve transection (CONT) model can be used to study primary degeneration of RGCs, while the partial optic nerve transection (PONT) model can be used to study secondary degeneration of RGCs because primary degeneration of RGCs and secondary degeneration can be separated in location in the same retina in this model; in other situations, these types of degeneration can be difficult to distinguish. In order to examine which kind of degeneration LBP could delay, both CONT and PONT models were used in this study. Rats were fed with LBP or vehicle daily from 7 days before surgery until sacrifice at different time-points and the surviving numbers of RGCs were evaluated. The expression of several proteins related to inflammation, oxidative stress, and the c-jun N-terminal kinase (JNK) pathways were detected with Western-blot analysis. LBP did not delay primary degeneration of RGCs after either CONT or PONT, but it did delay secondary degeneration of RGCs after PONT. We found that LBP appeared to exert these protective effects by inhibiting oxidative stress and the JNK/c-jun pathway and by transiently increasing production of insulin-like growth factor-1 (IGF-1). This study suggests that LBP can delay secondary degeneration of RGCs and this effect may be linked to inhibition of oxidative stress and the JNK/c-jun pathway in the retina.

Primary neuron culture for nerve growth and axon guidance studies in zebrafish (Danio rerio)

Zebrafish (Danio rerio) is a widely used model organism in genetics and developmental biology research. Genetic screens have proven useful for studying embryonic development of the nervous system in vivo, but in vitro studies utilizing zebrafish have been limited. Here, we introduce a robust zebrafish primary neuron culture system for functional nerve growth and guidance assays. Distinct classes of central nervous system neurons from the spinal cord, hindbrain, forebrain, and retina from wild type zebrafish, and fluorescent motor neurons from transgenic reporter zebrafish lines, were dissociated and plated onto various biological and synthetic substrates to optimize conditions for axon outgrowth. Time-lapse microscopy revealed dynamically moving growth cones at the tips of extending axons. The mean rate of axon extension in vitro was 21.4±1.2 µm hr⁻¹ s.e.m. for spinal cord neurons, which corresponds to the typical ∼0.5 mm day⁻¹ growth rate of nerves in vivo. Fluorescence labeling and confocal microscopy demonstrated that bundled microtubules project along axons to the growth cone central domain, with filamentous actin enriched in the growth cone peripheral domain. Importantly, the growth cone surface membrane expresses receptors for chemotropic factors, as detected by immunofluorescence microscopy. Live-cell functional assays of axon extension and directional guidance demonstrated mammalian brain-derived neurotrophic factor (BDNF)-dependent stimulation of outgrowth and growth cone chemoattraction, whereas mammalian myelin-associated glycoprotein inhibited outgrowth. High-resolution live-cell Ca²⁺-imaging revealed local elevation of cytoplasmic Ca²⁺ concentration in the growth cone induced by BDNF application. Moreover, BDNF-induced axon outgrowth, but not basal outgrowth, was blocked by treatments to suppress cytoplasmic Ca²⁺ signals. Thus, this primary neuron culture model system may be useful for studies of neuronal development, chemotropic axon guidance, and mechanisms underlying inhibition of neural regeneration in vitro, and complement observations made in vivo.

Under pressure: cellular and molecular responses during glaucoma, a common neurodegeneration with axonopathy

Glaucoma is a complex neurodegenerative disorder that is expected to affect 80 million people by the end of this decade. Retinal ganglion cells (RGCs) are the most affected cell type and progressively degenerate over the course of the disease. RGC axons exit the eye and enter the optic nerve by passing through the optic nerve head (ONH). The ONH is an important site of initial damage in glaucoma. Higher intraocular pressure (IOP) is an important risk factor for glaucoma, but the molecular links between elevated IOP and axon damage in the ONH are poorly defined. In this review and focusing primarily on the ONH, we discuss recent studies that have contributed to understanding the etiology and pathogenesis of glaucoma. We also identify areas that require further investigation and focus on mechanisms identified in other neurodegenerations that may contribute to RGC dysfunction and demise in glaucoma.

Differential effects of unfolded protein response pathways on axon injury-induced death of retinal ganglion cells

Loss of retinal ganglion cells (RGCs) accounts for visual function deficits after optic nerve injury, but how axonal insults lead to neuronal death remains elusive. By using an optic nerve crush model that results in the death of the majority of RGCs, we demonstrate that axotomy induces differential activation of distinct pathways of the unfolded protein response in axotomized RGCs. Optic nerve injury provokes a sustained CCAAT/enhancer binding homologous protein (CHOP) upregulation, and deletion of CHOP promotes RGC survival. In contrast, IRE/XBP-1 is only transiently activated, and forced XBP-1 activation dramatically protects RGCs from axon injury-induced death. Importantly, such differential activations of CHOP and XBP-1 and their distinct effects on neuronal cell death are also observed in RGCs with other types of axonal insults, such as vincristine treatment and intraocular pressure elevation, suggesting a new protective strategy for neurodegeneration associated with axonal damage.

Reelin is required for class-specific retinogeniculate targeting

Development of visual system circuitry requires the formation of precise synaptic connections between neurons in the retina and brain. For example, axons from retinal ganglion cells (RGCs) form synapses onto neurons within subnuclei of the lateral geniculate nucleus (LGN) [i.e., the dorsal LGN (dLGN), ventral LGN (vLGN), and intergeniculate leaflet (IGL)]. Distinct classes of RGCs project to these subnuclei: the dLGN is innervated by image-forming RGCs, whereas the vLGN and IGL are innervated by non-image-forming RGCs. To explore potential mechanisms regulating class-specific LGN targeting, we sought to identify differentially expressed targeting molecules in these LGN subnuclei. One candidate targeting molecule enriched in the vLGN and IGL during retinogeniculate circuit formation was the extracellular matrix molecule reelin. Anterograde labeling of RGC axons in mutant mice lacking functional reelin (reln(rl/rl)) revealed reduced patterns of vLGN and IGL innervation and misrouted RGC axons in adjacent non-retino-recipient thalamic nuclei. Using genetic reporter mice, we further demonstrated that mistargeted axons were from non-image-forming, intrinsically photosensitive RGCs (ipRGCs). In contrast to mistargeted ipRGC axons, axons arising from image-forming RGCs and layer VI cortical neurons correctly targeted the dLGN in reln(rl/rl) mutants. Together, these data reveal that reelin is essential for the targeting of LGN subnuclei by functionally distinct classes of RGCs.

Developmental regulation of axon branching in the vertebrate nervous system

During nervous system development, axons generate branches to connect with multiple synaptic targets. As with axon growth and guidance, axon branching is tightly controlled in order to establish functional neural circuits, yet the mechanisms that regulate this important process are less well understood. Here, we review recent advances in the study of several common branching processes in the vertebrate nervous system. By focusing on each step in these processes we illustrate how different types of branching are regulated by extracellular cues and neural activity, and highlight some common principles that underlie the establishment of complex neural circuits in vertebrate development.

E3 ligase Nedd4 promotes axon branching by downregulating PTEN

Regulated protein degradation via the ubiquitin-proteasome system (UPS) plays a central role in building synaptic connections, yet little is known about either which specific UPS components are involved or UPS targets in neurons. We report that inhibiting the UPS in developing Xenopus retinal ganglion cells (RGCs) with a dominant-negative ubiquitin mutant decreases terminal branching in the tectum but does not affect long-range navigation to the tectum. We identify Nedd4 as a prominently expressed E3 ligase in RGC axon growth cones and show that disrupting its function severely inhibits terminal branching. We further demonstrate that PTEN, a negative regulator of the PI3K pathway, is a key downstream target of Nedd4: not only does Nedd4 regulate PTEN levels in RGC growth cones, but also, the decrease of PTEN rescues the branching defect caused by Nedd4 inhibition. Together our data suggest that Nedd4-regulated PTEN is a key regulator of terminal arborization in vivo.

Distinct roles for Robo2 in the regulation of axon and dendrite growth by retinal ganglion cells

Guidance factors act on the tip of a growing axon to direct it to its target. What role these molecules play, however, in the control of the dendrites that extend from that axon's cell body is poorly understood. Slits, through their Robo receptors, guide many types of axons, including those of retinal ganglion cells (RGCs). Here we assess and contrast the role of Slit/Robo signalling in the growth and guidance of the axon and dendrites extended by RGCs in Xenopus laevis. As Xenopus RGCs extend dendrites, they express robo2 and robo3, while slit1 and slit2 are expressed in RGCs and in the adjacent inner nuclear layer. Interestingly, our functional data with antisense knockdown and dominant negative forms of Robo2 (dnRobo2) and Robo3 (dnRobo3) indicate that Slit/Robo signalling has no role in RGC dendrite guidance, and instead is necessary to stimulate dendrite branching, primarily via Robo2. Our in vitro culture data argue that Slits are the ligands involved. In contrast, both dnRobo2 and dnRobo3 inhibited the extension of axons and caused the misrouting of some axons. Based on these data, we propose that Robo signalling can have distinct functions in the axon and dendrites of the same cell, and that the specific combinations of Robo receptors could underlie these differences. Slit acts via Robo2 in dendrites as a branching/growth factor but not in guidance, while Robo2 and Robo3 function in concert in axons to mediate axonal interactions and respond to Slits as guidance factors. These data underscore the likelihood that a limited number of extrinsic factors regulate the distinct morphologies of axons and dendrites.

Does elevated intraocular pressure reduce retinal TRKB-mediated survival signaling in experimental glaucoma?

Reduced retrograde transport of neurotrophins (NT) and their receptors has been hypothesized to contribute directly to retinal ganglion cell (RGC) loss in glaucoma. However, strategies of supplementing NT and NT receptors have failed to avert ultimate RGC death in experimental glaucoma. This study examines the response of major components of the NT system and their interacting proteins in a rat glaucoma model. Unilateral chronic intraocular pressure (IOP) elevation was produced by episcleral vein injection of hypertonic saline (N = 99). Retinas were collected and grouped by extent of optic nerve injury. Quantitative reverse transcription PCR, western blot analysis and immunohistochemistry were used to determine mRNA and protein levels and protein localization. Out of three RGC-specific Brn3 proteins (Brn3a, b, and c), only Brn3a was significantly downregulated at the message level to 35 +/- 4% of fellow values with the severest nerve injury. With IOP elevation, no significant alterations were found in retinal mRNA levels for BDNF, NGF, NT-4/5 or NT-3. The abundance of mature retinal BDNF protein was not significantly affected by elevated IOP, while proBDNF protein decreased linearly with increasing injury grade (r(2) = 0.50). In retinas with the severest nerve injury, TrkB and TrkC receptor mRNA levels significantly declined to 67 +/- 9% and 44 +/- 5% of fellow values, respectively. However, the levels of TRKB protein and its phosphorylated form were unchanged. Message level for p75(NTR) was linearly upregulated up to 219 +/- 26% with increasing injury (r(2) = 0.46), but no alteration was detected at protein level. The mRNA expression of p75(NTR) apoptosis adaptor proteins NADE, NRIF, and Lingo1 were significantly downregulated in retinas with the greatest nerve injury. A positive correlation was found between injury extent and message levels for Jun (r(2) = 0.23) as well as Junb (r(2) = 0.27), and RGC labeling of activated JUN protein increased. Atf3 mRNA levels demonstrated a positive linear correlation to the extent of injury (r(2) = 0.53), resulting in a nearly five-fold increase (482 +/- 76%) in eyes with the greatest nerve damage. Among downstream pro-survival signaling components, Erk5 mRNA expression was linearly upregulated (r(2) = 0.32) up to 157 +/- 15% of fellow values in retinas with the severest nerve injury (p < 0.01). A slight positive correlation was found between NF-kappaB message levels and injury extent (r(2) = 0.12). Bcl-xl mRNA levels in the most severely injured retinas were significantly reduced to 83 +/- 7% by elevated IOP exposure. Message levels for Erk1/2, Akt1-3 or Bcl2 appeared unaffected. Elevated IOP did not alter mRNA levels of pro-apoptotic Bim, Bax, or p53. This study demonstrates that elevated IOP exposure does not result in a dramatic decrease in retinal levels of either BDNF or its receptor, TrkB. It shows that the responses of NT pathways to elevated IOP are complex, particularly with regard to the role of p75(NTR) and Atf3. A better understanding of the roles of these proteins in IOP-induced injury is likely to suggest informed strategies for neuroprotection in glaucoma.

Neurotrophin roles in retinal ganglion cell survival: lessons from rat glaucoma models

The neurotrophin (NT) hypothesis proposes that the obstruction of retrograde transport at the optic nerve head results in the deprivation of neurotrophic support to retinal ganglion cells (RGC) leading to apoptotic cell death in glaucoma. An important corollary to this concept is the implication that appropriate enhancement of neurotrophic support will prolong the survival of injured RGC indefinitely. This hypothesis is, perhaps, the most widely recognized theory to explain RGC loss resulting from exposure of the eye to elevated intraocular pressure (IOP). Recent studies of NT signaling using rat glaucoma models, have examined the endogenous responses of the retina to pressure exposure as well as studies designed to augment NT signaling in order to rescue RGC from apoptosis following pressure-induced injury. The examination of these studies in this review reveals a number of consistent observations and provides direction for further investigations of this hypothesis.

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.

Growth dynamics and morphology of regenerating optic fibers in tectum are altered by injury conditions: an in vivo imaging study in goldfish

The dynamic behavior of axons in systems that normally regenerate may provide clues for promoting regeneration in humans. When the optic nerve is severed in adult goldfish, all axons regenerate back to the tectum to reestablish accurate connections. In adult mammals, regeneration can be induced in optic and other axons but typically few fibers regrow and only for short distances. These conditions were mimicked in the adult goldfish by surgically deflecting 10-20% of optic fibers from one tectum into the opposite tectum which was denervated of all other optic fibers by removing its corresponding eye. At 21-63 days, DiI was microinjected into retina to label a few fibers and the fibers were visualized in the living fish for up to 5-7 h. The dynamic behavior and morphology of these regenerating deflected fibers were analyzed and compared to those regenerating following optic nerve crush. At 3-4 weeks, deflected fibers were found to form more branches and to maintain many more branches than crushed fibers. Although both deflected and crushed fibers exhibited stochastic growth and retraction, deflected fibers spent more time growing but grew for less distance. At 2 months, both deflected and crushed fibers became much more stable. These results show that the morphology and behavior of fibers regenerating into the same target tissue can be substantially altered by the injury conditions, that is, they show state-dependent plasticity. The morphology and behavior of the deflected fibers suggest they were impaired in their capacity to grow to their correct targets.

TGFbeta ligands promote the initiation of retinal ganglion cell dendrites in vitro and in vivo

Each type of neuron develops a unique morphology critical to its function, but almost all start with the basic plan of one long axon and multiple short, branched dendrites. Though extrinsic signals are known to direct many steps in the development of neuronal structure, little is understood about the initiation of processes, particularly dendrites. We find that Xenopus retinal ganglion cells (RGCs) explanted early will extend axons and not dendrites in dissociated cultures. If RGCs develop longer in vivo prior to culturing, many now extend dendrite-like processes in vitro, suggesting that an extrinsic factor is required to stimulate dendrite initiation. Members of the transforming growth factor beta (TGFbeta) superfamily, bone morphogenetic protein 2 (BMP2), and growth and differentiation factor 11 (GDF11), can signal cultured RGCs to form dendrites. Furthermore, TGFbeta ligands have an endogenous role: blocking BMP/GDF signaling with a secreted antagonist or inhibitory receptors reduces the number of primary dendrites extended in vivo.

5-HT2A receptors are differentially expressed in bullfrog and rat retinas: a comparative study

Expression of 5-hydroxytryptamine (5-HT) 2A receptor (5-HT2A) was studied in bullfrog and rat retinas by immunocytochemistry. In the bullfrog retina, 5-HT2A-immunoreactivity was observed in both the outer and inner plexiform layers (OPL and IPL). Double labeling experiments further showed that 5-HT2A was expressed in Müller cells stained by GFAP. Labeling for 5-HT2A was strong in the somata and endfeet and relatively weak in the major processes and fine branchets of Müller cells. In contrast, 5-HT2A immunoreactivity was hardly detected in the rat retina, and no rat Müller cells were labeled. Furthermore, immunocytochemical assay demonstrated that labeling for 5-HT was present in amacrine cells and displaced amacrine cells in the inner retina of bullfrog, but not in the rat retina. These results suggest that 5-HT may modulate retinal information processing via activating 5-HT2A expressed in neuronal and glial elements in bullfrog, but that such modulation is unlikely to occur in rat.

Cell-autonomous TrkB signaling in presynaptic retinal ganglion cells mediates axon arbor growth and synapse maturation during the establishment of retinotectal synaptic connectivity

BDNF contributes to the activity-dependent establishment and refinement of visual connectivity. In Xenopus, BDNF applications in the optic tectum influence retinal ganglion cell (RGC) axon branching and promote synapse formation and stabilization. The expression patterns of BDNF and TrkB suggest that BDNF specifically regulates the maturation of RGC axons at the target. It is possible, however, that BDNF modulates retinotectal synaptic connectivity by differentially influencing presynaptic RGC axons and postsynaptic tectal cells. Here, we combined single-cell expression of a dominant-negative TrkB-enhanced green fluorescent protein (GFP) fusion protein with confocal microscopy imaging in live Xenopus tadpoles to differentiate between presynaptic and postsynaptic actions of BDNF. Disruption of TrkB signaling in individual RGCs influenced the branching and synaptic maturation of presynaptic axon arbors. Specifically, GFP-TrkB.T1 overexpression increased the proportion of axons with immature, growth cone-like morphology, decreased axon branch stability, and increased axon arbor degeneration. In addition, GFP-TrkB.T1 overexpression reduced the number of red fluorescent protein-synaptobrevin-labeled presynaptic specializations per axon terminal. In contrast, overexpression of GFP-TrkB.T1 in tectal neurons did not alter synaptic number or the morphology or dynamic behavior of their dendritic arbors. Electron microscopy analysis revealed a significant decrease in the number of mature synaptic profiles and in the number of docked synaptic vesicles at retinotectal synapses made by RGC axons expressing GFP-TrkB.T1. Together, our results demonstrate that presynaptic TrkB signaling in RGCs is a key determinant in the establishment of visual connectivity and indicate that changes in tectal neuron synaptic connectivity are secondary to the BDNF-elicited enhanced stability and growth of presynaptic RGCs.

Brain-derived neurotrophic factor selectively regulates dendritogenesis of parvalbumin-containing interneurons in the main olfactory bulb through the PLCgamma pathway

Molecular mechanisms of neurotrophin signaling on dendrite development and dynamics are only partly understood. To address the role of brain-derived neurotrophic factor (BDNF) in the morphogenesis of GABAergic neurons of the main olfactory bulb, we analyzed mice lacking BDNF, mice carrying neurotrophin-3 (NT3) in the place of BDNF, and TrkB signaling mutant mice with a receptor that can activate phospholipase Cgamma (PLCgamma) but is unable to recruit the adaptors Shc/Frs2. BDNF deletion yielded a compressed olfactory bulb with a significant loss of parvalbumin (PV) immunoreactivity in GABAergic interneurons of the external plexiform layer. Dendrite development of PV-positive interneurons was selectively attenuated by BDNF since other Ca2+ -binding protein-containing neuron populations appeared unaffected. The deficit in PV-positive neurons could be rescued by the NT3/NT3 alleles. The degree of PV immunoreactivity was dependent on BDNF and TrkB recruitment of the adaptor proteins Shc/Frs2. In contrast, PLCgamma signaling from the TrkB receptor was sufficient for dendrite growth in vivo and consistently, blocking PLCgamma prevented BDNF-dependent dendrite development in vitro. Collectively, our results provide genetic evidence that BDNF and TrkB signaling selectively regulate PV expression and dendrite growth in a subset of neurochemically-defined GABAergic interneurons via activation of the PLCgamma pathway.

TrkB/BDNF signaling regulates photoreceptor progenitor cell fate decisions

Neurotrophins, via activation of Trk receptor tyrosine kinases, serve as mitogens, survival factors and regulators of arborization during retinal development. Brain-derived neurotrophic factor (BDNF) and TrkB regulate neuronal arborization and survival in late retinal development. However, TrkB is expressed during early retinal development where its functions are unclear. To assess TrkB/BDNF actions in the early chick retina, replication-incompetent retroviruses were utilized to over-express a dominant negative truncated form of TrkB (trunc TrkB), or BDNF and effects were assessed at E15. Clones expressing trunc TrkB were smaller than controls, and proliferation and apoptosis assays suggest that decreased clone size correlated with increased cell death when BDNF/TrkB signaling was impaired. Analysis of clonal composition revealed that trunc TrkB over-expression decreased photoreceptor numbers (41%) and increased cell numbers in the middle third of the inner nuclear layer (INL) (23%). Conversely, BDNF over-expression increased photoreceptor numbers (25%) and decreased INL numbers (17%). Photoreceptors over-expressing trunc TrkB demonstrated no increase in apoptosis nor abnormalities in lamination suggesting that TrkB activation is not required for photoreceptor cell survival or migration. These studies suggest that TrkB signaling regulates commitment to and/or differentiation of photoreceptor cells from retinal progenitor cells, identifying a novel role for TrkB/BDNF in regulating cell fate decisions.

BDNF increases synapse density in dendrites of developing tectal neurons in vivo

Neuronal connections are established through a series of developmental events that involve close communication between pre- and postsynaptic neurons. In the visual system, BDNF modulates the development of neuronal connectivity by influencing presynaptic retinal ganglion cell (RGC) axons. Increasing BDNF levels in the optic tectum of Xenopus tadpoles significantly increases both axon arborization and synapse density per axon terminal within a few hours of treatment. Here, we have further explored the mechanisms by which BDNF shapes synaptic connectivity by imaging tectal neurons, the postsynaptic partners of RGCs. Individual neurons were co-labeled with DsRed2 and a GFP-tagged postsynaptic density protein (PSD95-GFP) to visualize dendritic morphology and postsynaptic specializations simultaneously in vivo. Immunoelectron microscopy confirmed that PSD95-GFP predominantly localized to ultrastructurally identified synapses. Time-lapse confocal microscopy of individual, double-labeled neurons revealed a coincident, activity-dependent mechanism of synaptogenesis and axon and dendritic arbor growth, which is differentially modulated by BDNF. Microinjection of BDNF into the optic tectum significantly increased synapse number in tectal neuron dendritic arbors within 24 hours, without significantly influencing arbor morphology. BDNF function-blocking antibodies had opposite effects. The BDNF-elicited increase in synapse number complements the previously observed increase in presynaptic sites on RGC axons. These results, together with the timescale of the response by tectal neurons, suggest that the effects of BDNF on dendritic synaptic connectivity are secondary to its effects on presynaptic RGCs. Thus, BDNF influences synaptic connectivity in multiple ways: it enhances axon arbor complexity expanding the synaptic territory of the axon, while simultaneously coordinating synapse formation and stabilization with individual postsynaptic cells.

Enhanced Neurotrophin Synthesis and Molecular Differentiation in Non-Transformed Human Retinal Progenitor Cells Cultured in a Rotating Bioreactor

One approach to the treatment of retinal diseases, such as retinitis pigmentosa, is to replace diseased or degenerating cells with healthy cells. Even if all of the problems associated with tissue transplant were to be resolved, the availability of tissue would remain an ongoing problem. We have previously shown that transformed human retinal cells can be grown in a NASA-developed horizontally rotating culture vessel (bioreactor) to form three-dimensional-like structures with the expression of several retinal specific proteins. In this study, we have investigated growth of non-transformed human retinal progenitors (retinal stem cells) in a rotating bioreactor. This rotating culture vessel promotes cell-cell interaction between similar and dissimilar cells. We cultured retinal progenitors (Ret 1-4) alone or as a co-culture with human retinal pigment epithelial cells (RPE, D407) in this system to determine if 3D structures can be generated from non-transformed progenitors. Our second goal was to determine if the formation of 3D structures correlates with the upregulation of neurotrophins, basic fibroblast growth factor (bFGF), transforming growth factor alpha (TGFalpha), ciliary neurotrophic factor (CNTF), and brain-delivered neurotrophic factor (BDNF). These factors have been implicated in progenitor cell proliferation, commitment, differentiation, and survival. We also investigated the expression of the following retinal specific proteins in this system: neuron specific enolase (NSE); tyrosine hydroxylase (TH); D(2)D(3), D(4) receptors; protein kinase-C alpha (PKCalpha), and calbindin. The 3D structures generated were characterized by phase and scanning transmission electron microscopy. Retinal progenitors, cultured alone or as a co-culture in the rotating bioreactor, formed 3D structures with some degree of differentiation, accompanied by the upregulation of bFGF, CNTF, and TGFalpha. Brain-derived neurotrophic factor, which is expressed in vivo in RPE (D407), was not expressed in monolayer cultures of RPE but expressed in the rotating bioreactor-cultured RPE and retinal progenitors (Ret 1-4). Upregulation of neurotrophins was noted in all rotating bioreactor-cultured cells. Also, upregulation of D(4) receptor, calbindin, and PKCalpha was noted in the rotating bioreactor-cultured cells. We conclude that non-transformed retinal progenitors can be grown in the rotating bioreactor to form 3D structures with some degree of differentiation. We relied on molecular and biochemical analysis to characterize differentiation in cells grown in the rotating bioreactor.

Brain-derived neurotrophic factor modulates the dopaminergic network in the rat retina after axotomy

Dopaminergic cells in the retina express the receptor for brain-derived neurotrophic factor (BDNF), which is the neurotrophic factor that influences the plasticity of synapses in the central nervous system. We sought to determine whether BDNF influences the network of dopaminergic amacrine cells in the axotomized rat retina, by immunocytochemistry with an anti-tyrosine hydroxylase (TH) antiserum. In the control retina, we found two types of TH-immunoreactive amacrine cells, type I and type II, in the inner nuclear layer adjacent to the inner plexiform layer (IPL). The type I amacrine cell varicosities formed ring-like structures in contact with AII amacrine cell somata in stratum 1 of the IPL. In the axotomized retinas, TH-labeled processes formed loose networks of fibers, unlike the dense networks in the control retina, and the ring-like structures were disrupted. In the axotomized retinas treated with BDNF, strong TH-immunoreactive varicosities were present in stratum 1 of the IPL and formed ring-like structures. Our data suggest that BDNF affects the expression of TH immunoreactivity in the axotomized rat retina and may therefore influence the retinal dopaminergic system.

Brain-derived neurotrophic factor is expressed in a gradient in the superior colliculus during development of the retinocollicular projection

Abstract Theoretical models of topographic map formation have postulated a gradient of attractant in addition to a gradient of repulsion in the target. In species where many axons grow past their correct positions initially, it has also been argued that a parallel gradient of attractant or branching signal is required to ensure collateral formation at the correct position (O'Leary et al., 1999). Brain-derived neurotrophic factor (BDNF) is a known attractant and promotes branching of retinal axons. We have examined its distribution in the superior colliculus and that of its receptor, trkB, in the retina, using immunohistochemistry and in situ hybridization, respectively, during the development of the topographic retinocollicular projection in the wallaby, a marsupial mammal. The number of glial endfeet expressing BDNF at the surface of the colliculus was found to be in a high caudal-to-low rostral gradient during the time when the retinocollicular projection was developing. When the projection was mature the rostrocaudal gradient had disappeared and the number of detectable endfeet expressing BDNF was very low. Messenger RNA for TrkB was expressed in the retinal ganglion cell layer throughout the time when the retinocollicular projection was developing, with no difference in expression across the nasotemporal axis of the retina. The low rostral to high caudal distribution of BDNF in glial endfeet supports the idea that it is providing a parallel gradient of attractant or branching signal in the colliculus.

Rapid BDNF-induced retrograde synaptic modification in a developing retinotectal system

In cultures of hippocampal neurons, induction of long-term synaptic potentiation or depression by repetitive synaptic activity is accompanied by a retrograde spread of potentiation or depression, respectively, from the site of induction at the axonal outputs to the input synapses on the dendrites of the presynaptic neuron. We report here that rapid retrograde synaptic modification also exists in an intact developing retinotectal system. Local application of brain-derived neurotrophic factor (BDNF) to the Xenopus laevis optic tectum, which induced persistent potentiation of retinotectal synapses, led to a rapid modification of synaptic inputs at the dendrites of retinal ganglion cells (RGCs), as shown by a persistent enhancement of light-evoked excitatory synaptic currents and spiking activity of RGCs. This retrograde effect required TrkB receptor activation, phospholipase Cgamma activity and Ca2+ elevation in RGCs, and was accounted for by a selective increase in the number of postsynaptic AMPA-subtype glutamate receptors at RGC dendrites. Such retrograde information flow in the neuron allows rapid regulation of synaptic inputs at the dendrite in accordance to signals received at axon terminals, a process reminiscent of back-propagation algorithm for learning in neural networks.

Spontaneous retinal activity modulates BDNF trafficking in the developing chick visual system

Both neuronal activity and neurotrophin signaling play critical roles in normal CNS development. This study examined whether spontaneous retinal activity (SRA) also governs the axonal transport of endogenous brain-derived neurotrophic factor (BDNF) protein within the developing chick visual system. In previous work, we have found that during the normal period of SRA, retinal BDNF protein levels decrease by about 50% while BDNF mRNA levels remain elevated. Here, we show that the blockade of SRA with tetrodotoxin (TTX), or the blockade of axonal transport with colchicine, both reversed the normal mismatch between retinal BDNF mRNA and protein. The axonal transport of retinal-derived BDNF in segments of the optic nerve as well as tectal-derived BDNF protein transported in segments of the optic tract were both significantly reduced after very brief periods of activity blockade. These results suggest that normal SRA plays a role in regulating the axonal transport of endogenous BDNF protein.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

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

Brain-derived neurotrophic factor (BDNF) has been implicated in stimulating retinal ganglion cell (RGC) survival and axonal regeneration in rodent animal models in vivo and in vitro, but very little data are available on neurotrophin effects in higher mammals. We hence analysed BDNF signalling in primary cultures of adult pig RGC. As detected by immunohistochemistry, HPLC analysis and RT-PCR, BDNF protein and mRNA were present within pig retina in vivo and in vitro, where it may be involved in baseline RGC neuritogenesis. Initial dose-response studies established optimal effects were induced by 20 ng/ml BDNF, leading to an approximately threefold increase in neurite length. We analysed the respective contributions of phosphatidyl inositol 3 kinase (PI3K) and mitogen activated protein kinase (MAPK) cascades to BDNF-induced neurite regeneration. Addition of either the PI3K inhibitor wortmannin or the MAPK inhibitor U0126 blocked 50-100% BDNF-induced neurite elongation; U0126 also significantly reduced neurite regeneration below untreated control levels. The trk receptor inhibitor K252a had no observable effect on neurite regeneration or morphology. These data hence demonstrate that BDNF is a potent stimulator of neurite growth in RGC prepared from an adult large mammal retina, and that at least two signalling pathways are causally involved. BDNF-based therapy may be of potential use in treating RGC degeneration in humans.

Effects of prolonged delivery of brain-derived neurotrophic factor on the fate of neural stem cells transplanted into the developing rat retina

One of the major roles of brain-derived neurotrophic factor (BDNF) is to promote the differentiation and support the survival of neurons in the central nervous system. The objective of the present study was to evaluate the effect of BDNF on the fate of adult rat hippocampus-derived neural stem cells (AHPCs) transplanted into the developing rat retina. Immunohistochemical analysis showed a significant increase in the ratio of grafted AHPCs stained for MAP2ab (P<0.05) and a marked decrease in the ratio of nestin-positive grafted cells in the slow-releasing BDNF group compared with the control group. The respective changes in the ratios of MAP5 and GFAP-positive grafted cells were comparable for the two groups. The results reported here suggest a potentially beneficial role for extended delivery of BDNF in the differentiation of grafted neural stem cells, which may lead to a novel modification of stem cell transplantation.

TrkB receptor signaling regulates developmental death dynamics, but not final number, of retinal ganglion cells

We investigated the effects of endogenous neurotrophin signaling on the death-survival of immature retinal ganglion cells (RGCs) in vivo. Null mutation of brain-derived neurotrophic factor [(BDNF) alone or in combination with neurotrophin 4 (NT4)] increases the peak rate of developmental RGC death as compared with normal. Null mutation of NT4 alone is ineffective. Null mutation of the full-length trkB (trkBFL) receptor catalytic domain produces a dose-dependent increase in the peak RGC death rate that is negatively correlated with retinal levels of trkBFL protein and phosphorylated (activated) trkBFL. Depletion of target-derived trkB ligands by injection of trkB-Fc fusion protein into the superior colliculus increases the peak rate of RGC death compared with trkA-Fc-treated and normal animals. Adult trkBFL+/- mice have a normal number of RGCs, despite an elevated peak death rate of immature RGCs. Thus, target-derived BDNF modulates the dynamics of developmental RGC death through trkBFL activation, but BDNF/trkB-independent mechanisms determine the final number of RGCs.

Sperry and Hebb: oil and vinegar?

The interface between so-called activity-dependent and activity-independent mechanisms of circuit development is discussed here in light of recent findings that question the role of activity in brain development. This debate is presented simplistically here in terms of Sperry's chemoaffinity hypothesis versus Hebb's rules of correlation-based synaptic change, which are often presented as being mutually exclusive - much like oil and vinegar.

Fibroblast growth factors redirect retinal axons in vitro and in vivo

Growth factors have been shown previously to participate in the process of axon target recognition. We showed that fibroblast growth factor receptor (FGFR) signaling is required for Xenopus laevis retinal ganglion cell (RGC) axons to recognize their major midbrain target, the optic tectum [neuron 17 (1996), 245]. Therefore, we have hypothesized that a change in expression of a fibroblast growth factor (FGF) at the entrance of the optic tectum, the border between the diencephalon and mesencephalon, may serve as a signal to RGC axons that they have reached their target. To determine whether RGC axons can sense changes in FGF levels, we asked whether they altered their behavior upon encountering an ectopic source of FGF. We found that in vivo RGC growth cones avoided FGF-misexpressing cells along their path, and that FGF-2 directly repelled RGC growth cones in an in vitro growth cone turning assay. These data support the idea that RGC axons can sense changes in FGF levels, and as such provide a mechanism by which FGFR signaling is involved in RGC axon target recognition.

Determinants of the exclusion zone in dopaminergic amacrine cell mosaics

A fundamental organizing feature of the retina is the presence of regularly spaced distributions of neurons, yet we have little knowledge of how this patterning emerges during development. Among these retinal mosaics, the spatial organization of the dopaminergic amacrine cells is unique: using nearest-neighbor and Vornoi domain analysis, we found that the dopaminergic amacrine cells were neither randomly distributed, nor did they achieve the regularity documented for other retinal cell types. Autocorrelation analysis revealed the presence of an exclusion zone surrounding individual dopaminergic amacrine cells and modeling studies confirmed this organization, as the mosaic could be simulated by a minimal distance spacing rule defined by a broad set of parameters. Experimental studies determined the relative contributions of tangential dispersion, fate determination, and cell death in the establishment of this exclusion zone. Clonal boundary analysis and simulations of proximity-driven movement discount tangential dispersion, while data from bcl-2 overexpressing mice rule out feedback-inhibitory fate-deterministic accounts. Cell death, by contrast, appears to eliminate dopaminergic amacrine cells that are within close proximity, thereby establishing the exclusion zone surrounding individual cells and in turn creating their mosaic regularity.

Possible involvement of a fibroblast growth factor 9 (FGF9)-FGF receptor-3-mediated pathway in adult pig retinal ganglion cell survival in vitro

The expression and potential roles of fibroblast growth factors (FGF) and their cognate FGF receptors (FGFR) in adult mammalian retinal ganglion cells (RGC) are poorly known. We show that FGFR-3 and FGFR-4 are especially pronounced on RGC and amacrine cell bodies in adult pig inner retinae both in vivo and in vitro. Western blotting revealed distinct profiles for each receptor. Expression of each FGFR and effects of the preferred ligand for FGFR-3, FGF9, upon RGC survival and neurite outgrowth were examined in primary retinal cell cultures: whereas there was no stimulation of neuritogenesis, RGC survival was promoted in a dose-dependent manner (ED(50) approximately 500 pg/ml, mean maximal increase of 60%) and could be completely blocked by addition of FGF9 neutralising antibody. Experiments with three additional FGF (FGF1, FGF2, and FGF4) showed no stimulation of RGC survival above control levels. Taken together, these data suggest that the ligand-receptor couple FGF9-FGFR-3 may function to promote survival of adult mammalian RGC, and their application might be beneficial in retinal degenerative diseases such as glaucoma.

Changes in brain-derived neurotrophic factor and trkB receptor in the adult Rana pipiens retina and optic tectum after optic nerve injury

In this study we used immunocytochemistry to investigate the distribution of brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase (trkB) in retina and optic tectum of the frog Rana pipiens during regeneration after axotomy. We also measured changes in BDNF mRNA in retina and tectum. Retrograde labeling was used to identify retinal ganglion cells (RGCs) prior to quantification of the BDNF immunoreactivity. In control animals, BDNF was found in the majority of RGCs and displaced amacrine cells and in some cells in the inner nuclear layer (INL). After axotomy, BDNF immunoreactivity was reduced in RGCs but increased in the INL. BDNF mRNA levels in the retina remained high before and after axotomy. Three months after axotomy, after reconnection to the target, the staining intensity of many of the surviving RGCs had partially recovered. In the control tectum, BDNF staining was present in ependymoglial cells and in neurons throughout layers 4, 6, 8, and 9. After axotomy, BDNF staining in tectal neurons became more intense, even though mRNA synthesis was transiently down-regulated. In control retinas, trkB receptor immunostaining was present in most RGCs; no significant changes were observed after axotomy. In control tectum, trkB was detected only in ependymoglial cells. After axotomy, many neuronal cell bodies were transiently labeled. Our data are consistent with the hypothesis that a considerable fraction of the BDNF normally present in RGCs is acquired from their targets in the tectum. However, there are also intraretinal sources of BDNF that could contribute to the survival of RGCs.

BDNF overexpression increases dendrite complexity in hippocampal dentate gyrus

There is increasing evidence that brain-derived neurotrophic factor (BDNF) modulates synaptic and morphological plasticity in the developing and mature nervous system. Plasticity may be modulated partially by BDNF's effects on dendritic structure. Utilizing transgenic mice where BDNF overexpression was controlled by the beta-actin promoter, we evaluated the effects of long-term overexpression of BDNF on the dendritic structure of granule cells in the hippocampal dentate gyrus. BDNF transgenic mice provided the opportunity to investigate the effects of modestly increased BDNF levels on dendrite structure in the complex in vivo environment. While the elevated BDNF levels were insufficient to change levels of TrkB receptor isoforms or downstream TrkB signaling, they did increase dendrite complexity of dentate granule cells. These cells showed an increased number of first order dendrites, of total dendritic length and of total number of branch points. These results suggest that dendrite structure of granule cells is tightly regulated and is sensitive to modest increases in levels of BDNF. This is the first study to evaluate the effects of BDNF overexpression on dendrite morphology in the intact hippocampus and extends previous in vitro observations that BDNF influences synaptic plasticity by increasing complexity of dendritic arbors.

Local and target-derived brain-derived neurotrophic factor exert opposing effects on the dendritic arborization of retinal ganglion cells in vivo

The dendritic and axonal arbors of developing retinal ganglion cells (RGCs) are exposed to two sources of BDNF: RGC dendrites are exposed to BDNF locally within the retina, and RGC axons are exposed to BDNF at the target, the optic tectum. Our previous studies demonstrated that increasing tectal BDNF levels promotes RGC axon terminal arborization, whereas increasing retinal BDNF levels inhibits RGC dendritic arborization. These results suggested that differential neurotrophic action at the axon versus dendrite might be responsible for the opposing effects of BDNF on RGC axonal versus dendritic arborization. To explore this possibility, we examined the effects of altering BDNF levels at the optic tectum on the elaboration of RGC dendritic arbors in the retina. Increasing tectal BDNF levels resulted in a significant increase in dendritic branching, whereas neutralizing endogenous tectal BDNF with function-blocking antibodies significantly decreased dendritic arbor complexity. Thus, RGC dendritic arbors react in opposing manners to retinal- versus tectal-derived BDNF. Alterations in retinal BDNF levels, however, did not affect axon terminal arborization. Thus, RGC dendritic arborization is controlled in a complementary manner by both local and target-derived sources of BDNF, whereas axon arborization is modulated solely by neurotrophic interactions at the target. Together, our results indicate that developing RGCs modulate dendritic arborization by integrating signals from discrete sources of BDNF in the eye and brain. Differential integration of spatially discrete neurotrophin signals within a single neuron may therefore finely tune afferent and efferent neuronal connectivity.