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

Visual stimulation and brain-derived neurotrophic factor (BDNF) have protective effects in experimental autoimmune uveoretinitis

AIMS

To investigate the therapeutic potential of visual stimulation (VS) and BDNF in murine experimental autoimmune uveoretinitis (EAU).

MAIN METHODS

Mice were immunized by subcutaneous injection of interphotoreceptor retinoid-binding protein in Freund's complete adjuvant and intravenous injection of pertussis toxin, and were then exposed to high-contrast VS 12 h/day (days 1-14 post-immunization). EAU severity was assessed by examining clinical score, visual acuity, inflammatory markers, and immune cells in the retina. The transcriptome of activated retinal cells was determined by RNA-seq using RNA immunoprecipitated in complex with phosphorylated ribosomal protein S6. The retinal levels of protein products of relevant upregulated genes were quantified. The effect of BDNF on EAU was tested in unstimulated mice by its daily topical ocular administration (days 8-14 post-immunization).

KEY FINDINGS

VS attenuated EAU development and decreased the expression of pro-inflammatory cytokines/chemokines and numbers of immune cells in the retina (n = 10-20 eyes/group for each analysis). In activated retinal cells of control mice (n = 30 eyes/group), VS upregulated genes encoding immunomodulatory neuropeptides, of which BDNF and vasoactive intestinal peptide (VIP) also showed increased mRNA and protein levels in the retina of VS-treated EAU mice (n = 6-10 eyes/group for each analysis). In unstimulated EAU mice, BDNF treatment mimicked the protective effects of VS by modulating the inflammatory and stem cell properties of Müller cells (n = 5 eyes/group for each analysis).

SIGNIFICANCE

VS effectively suppresses EAU, at least through enhancing retinal levels of anti-inflammatory and neuroprotective factors, VIP and BDNF. Our findings also suggest BDNF as a promising therapeutic agent for uveitis treatment.

Thymoquinone Attenuates Retinal Expression of Mediators and Markers of Neurodegeneration in a Diabetic Animal Model

BACKGROUND

Diabetic retinopathy (DR) is a slow eye disease that affects the retina due to a long-standing uncontrolled diabetes mellitus. Hyperglycemia-induced oxidative stress can lead to neuronal damage leading to DR.

OBJECTIVE

The aim of the current investigation is to assess the protective effects of thymoquinone (TQ) as a potential compound for the treatment and/or prevention of neurovascular complications of diabetes, including DR.

METHODS

Diabetes was induced in rats by the administration of streptozotocin (55 mg/kg intraperitoneally, i.p.). Subsequently, diabetic rats were treated with either TQ (2 mg/kg i.p.) or vehicle on alternate days for three weeks. A healthy control group was also run in parallel. At the end of the treatment period, animals were euthanized, and the retinas were collected and analyzed for the expression levels of brain-derived neurotrophic factor (BDNF), tyrosine hydroxylase (TH), nerve growth factor receptor (NGFR), and caspase-3 using Western blotting techniques in the retina of diabetic rats and compared with the normal control rats. In addition, dichlorofluorescein (DCF) levels in the retina were assessed as a marker of reactive oxygen species (ROS), and blood-retinal barrier breakdown (BRB) was examined for vascular permeability. The systemic effects of TQ treatments on glycemic control, kidney and liver functions were also assessed in all groups.

RESULTS

Diabetic animals treated with TQ showed improvements in the liver and kidney functions compared with control diabetic rats. Normalization in the levels of neuroprotective factors, including BDNF, TH, and NGFR, was observed in the retina of diabetic rats treated with TQ. In addition, TQ ameliorated the levels of apoptosis regulatory protein caspase-3 in the retina of diabetic rats and reduced disruption of the blood-retinal barrier, possibly through a reduction in reactive oxygen species (ROS) generation.

CONCLUSION

These findings suggest that TQ harbors a significant potential to limit the neurodegeneration and retinal damage that can be provoked by hyperglycemia in vivo.

In vitro Model Systems for Studies Into Retinal Neuroprotection

Therapy development for neurodegenerative diseases of the retina constitutes a major unmet medical need, and this may be particularly relevant for inherited diseases of the retina, which are largely untreatable to this day. Therapy development necessitates appropriate models to improve the understanding of the underlying degenerative mechanisms, as well as for the testing and evaluation of novel treatment approaches. This review provides an overview of various in vitro model systems used to study retinal neuroprotection. The in vitro methods and technologies discussed range from primary retinal cell cultures and cell lines, to retinal organoids and organotypic retinal explants, to the cultivation of whole eyeballs. The advantages and disadvantages of these methods are compared and evaluated, also in view of the 3R principles (i.e., the refinement, reduction, and replacement of live animal testing), to identify suitable in vitro alternatives for in vivo experimentation. The article further expands on the use of in vitro models to test and evaluate neuroprotective treatments and to aid the development of retinal drug delivery systems. Among the pharmacological agents tested and characterized in vitro are such that interfere with aberrant cyclic guanosine monophosphate (cGMP) -signaling or such that inhibit the activities of poly (ADP-ribose) polymerase (PARP), histone deacetylases (HDAC), calpain-type proteases, as well as unfolded protein response-related stress. We then introduce nanoparticle-based drug delivery systems and discuss how different in vitro systems may be used to assess their efficacy in the treatment of retinal diseases. The summary provides a brief comparison of available in vitro models and relates their advantages and limitations to the various experimental requirements, for instance, for studies into disease mechanisms, novel treatments, or retinal toxicity. In many cases, combinations of different in vitro models may be required to obtain a comprehensive view of the efficacy of a given retinal neuroprotection approach.

7,8-Dihydroxyflavone alleviates apoptosis and inflammation induced by retinal ischemia-reperfusion injury via activating TrkB/Akt/NF-kB signaling pathway

Retinal ischemia-reperfusion injury (RIRI) is of common occurrence in retinal and optic nerve diseases. The BDNF/TrkB signaling pathway has been examined to be neuroprotective in RIRI. In this study, we investigated the role of a potent selective TrkB agonist 7,8-dihydroxyfavone (DHF) in rat retinas with RIRI. Our results showed that RIRI inhibited the conversion of BDNF precursor (proBDNF) to mature BDNF (mBDNF) and increased the level of neuronal cell apoptosis. Compared with RIRI, DHF+RIRI reduced proBDNF level and at the same time increased mBDNF level. Moreover, DHF administration effectively activated TrkB signaling and and downstream Akt and Erk signaling pathways which increased nerve cell survival. The combined effects of mBDNF/proBDNF increase and TrkB signaling activation lead to reduction of apoptosis level and protection of retinas with RIRI. Moreover, it was also found that astrocytes labeled by GFAP were activated in RIRI and NF-kB mediated the increased expressions of inflammatory factors and these effects were partially reversed by DHF administration. Besides, we also used RNA sequencing to analyze the differently expressed genes (DEGs) and their enriched (Kyoto Encyclopedia of Genes and Genomes) KEGG pathways between Sham, RIRI, and DHF+RIRI. It was found that 1543 DEGs were differently expressed in RIRI and 619 DEGs were reversed in DHF+RIRI. The reversed DEGs were typically enriched in PI3K-Akt signaling pathway, Jak-STAT signaling pathway, NF-kB signaling pathway, and Apoptosis. To sum up, the DHF administration alleviated apoptosis and inflammation induced by RIRI via activating TrkB signaling pathway and may serve as a promising drug candidate for RIRI related ophthalmopathy.

In Vitro and In Vivo Proteomic Comparison of Human Neural Progenitor Cell-Induced Photoreceptor Survival

Retinal degenerative diseases lead to blindness with few treatments. Various cell-based therapies are aimed to slow the progression of vision loss by preserving light-sensing photoreceptor cells. A subretinal injection of human neural progenitor cells (hNPCs) into the Royal College of Surgeons (RCS) rat model of retinal degeneration has aided in photoreceptor survival, though the mechanisms are mainly unknown. Identifying the retinal proteomic changes that occur following hNPC treatment leads to better understanding of neuroprotection. To mimic the retinal environment following hNPC injection, a co-culture system of retinas and hNPCs is developed. Less cell death occurs in RCS retinal tissue co-cultured with hNPCs than in retinas cultured alone, suggesting that hNPCs provide retinal protection in vitro. Comparison of ex vivo and in vivo retinas identifies nuclear factor (erythroid-derived 2)-like 2 (NRF2) mediated oxidative response signaling as an hNPC-induced pathway. This is the first study to compare proteomic changes following treatment with hNPCs in both an ex vivo and in vivo environment, further allowing the use of ex vivo modeling for mechanisms of retinal preservation. Elucidation of the protein changes in the retina following hNPC treatment may lead to the discovery of mechanisms of photoreceptor survival and its therapeutic for clinical applications.

Ex-vivo-examination of ultrastructural changes in organotypic retina culture using near-infrared imaging and optical coherence tomography

Optical coherence tomography (OCT) dramatically changed the way of diagnostic assessment in retinal diseases during the last years. Using this technique in-vivo in-depth analysis of the retina and its layers is possible. Since animal research is changing by intrinsic and extrinsic pressure to animal-(in-vivo)-free methods, we adapted OCT-measurements to organotypic cultures. An easy to use protocol was generated to assess standardized OCT assessments in organotypic culture. First, two custom-made devices need to be made to change any commercially available OCT for examinations in humans into a device allowing ex-vivo analyses of organotypic culture. The modification is feasible within seconds. After OCT measurement of the ex-vivo tissues, quantitative evaluation of the retinas were performed via ImageJ software. OCT pictures of ex-vivo retinas were obtained for time periods of seven days and the thickness of retinal tissue was evaluated. The reproducibility of the pictures and measurements was very high (SD < 15%). In conclusion, an easy to use protocol for the investigation of different effects on retinal cultures with commercially available OCT devices was successfully established.

Neurodegeneration in diabetic retina and its potential drug targets

Diabetic retinopathy (DR) is one of the major complications of diabetes causing vision loss and blindness worldwide. DR is widely recognized as a neurodegenerative disease as evidenced from early changes at cellular and molecular levels in the neuronal component of the diabetic retina, which is further supported by various retinal functional tests indicating functional deficits in the retina soon after diabetes progression. Diabetes alters the level of a number of neurodegenerative metabolites, which increases influx through several metabolic pathways which in turn induce an increase in oxidative stress and a decrease in neurotrophic factors, thereby damage retinal neurons. Loss of neurons may implicate in vascular pathology, a clinical signs of DR observed at later stages of the disease. Here, we discuss diabetes-induced potential metabolites known to be detrimental to neuronal damage and their mechanism of action. In addition, we highlight important neurotrophic factors, whose level have been found to be dysregulated in diabetic retina and may damage neurons. Furthermore, we discuss potential drugs and strategies based on targeting diabetes-induced metabolites, metabolic pathways, oxidative stress, and neurotrophins to protect retinal neurons, which may ameliorate vision loss and vascular damage in DR.

Neurodegeneration as a primary change and role of neuroprotection in diabetic retinopathy

Diabetic retinopathy (DR) was earlier recognized as a vascular disease, but nowadays, it is considered as a neurovascular disorder. Neuronal death is the primary change which leads to various vascular changes which are visible to an ophthalmologist. But these changes are feature of an advanced disease and can affect vision at any moment of time. There are various evidences which suggests that glutamate excitotoxicity, hyperhomocysteinemia, kynurenic acid, and erythro-poietin plays important role in causation of retinal ganglionic cell apoptosis in diabetic patients. Adaptive optics, a new imaging technique, also showed that loss of photoreceptors (specialized neurons) is the early change in diabetic retinopathy. These changes suggest DR as a neurovascular disorder. Neuroprotective agents also showed good results in delaying progression of DR especially memantine, insulin receptor activation, and neurotrophic factors. More research in this field will help us to find novel therapeutic measures for DR, which can delay or even stop progression of DR at a very early stage.

BDNF protects retinal neurons from hyperglycemia through the TrkB/ERK/MAPK pathway

Diabetic retinopathy (DR) is one of the most common diabetic eye diseases and a leading cause of blindness. It is characterized by changes in the blood vessels of the retina. The pathogenesis of DR is complex and to date, the precise mechanisms involved remain unclear. Previous studies have reported that DR is associated with neurodegeneration and that apoptosis may occur in diabetic retinas. In the present study, retinal neurons under conditions of hyperglycemia were used as a model to study apoptosis in diabetic retinas. Retinal neurons exposed to hyperglycemia exhibited high levels of apoptosis. Brain‑derived neurotrophic factor (BDNF), a member of the neurotrophin family, was effective in protecting retinal neurons from hyperglycemia in vitro. BDNF promoted neuronal cell survival in a concentration‑dependent manner. In addition, BDNF was demonstrated to promote the expression of tropomyosin‑related kinase B (TrkB) and elevate the phosphorylation levels of TrkB and ERK in retinal neurons exposed to hyperglycemia. The results of the present study demonstrated that BDNF may protect retinal neurons from hyperglycemia via the TrkB/ERK/MAPK pathway and provides novel insights into the pathogenesis of DR.

Neurodegeneration and neuroprotection in diabetic retinopathy

Diabetic retinopathy is widely considered to be a neurovascular disease. This is in contrast to its previous identity as solely a vascular disease. Early in the disease progression of diabetes, the major cells in the neuronal component of the retina consist of retinal ganglion cells and glial cells, both of which have been found to be compromised. A number of retinal function tests also indicated a functional deficit in diabetic retina, which further supports dysfunction of neuronal cells. As an endocrinological disorder, diabetes alters metabolism both systemically and locally in several body organs, including the retina. A growing body of evidences indicates increased levels of excitotoxic metabolites, including glutamate, branched chain amino acids and homocysteine in cases of diabetic retinopathy. Also present, early in the disease, are decreased levels of folic acid and vitamin-B12, which are potential metabolites capable of damaging neurons. These altered levels of metabolites are found to activate several metabolic pathways, leading to increases in oxidative stress and decreases in the level of neurotrophic factors. As a consequence, they may damage retinal neurons in diabetic patients. In this review, we have discussed those potential excitotoxic metabolites and their implications in neuronal damage. Possible therapeutic targets to protect neurons are also discussed. However, further research is needed to understand the exact molecular mechanism of neurodegeneration so that effective neuroprotection strategies can be developed. By protecting retinal neurons early in diabetic retinopathy cases, damage of retinal vessels can be protected, thereby helping to ameliorate the progression of diabetic retinopathy, a leading cause of blindness worldwide.

Controlled adhesion and growth of long term glial and neuronal cultures on Parylene-C

This paper explores the long term development of networks of glia and neurons on patterns of Parylene-C on a SiO₂ substrate. We harvested glia and neurons from the Sprague-Dawley (P1–P7) rat hippocampus and utilized an established cell patterning technique in order to investigate cellular migration, over the course of 3 weeks. This work demonstrates that uncontrolled glial mitosis gradually disrupts cellular patterns that are established early during culture. This effect is not attributed to a loss of protein from the Parylene-C surface, as nitrogen levels on the substrate remain stable over 3 weeks. The inclusion of the anti-mitotic cytarabine (Ara-C) in the culture medium moderates glial division and thus, adequately preserves initial glial and neuronal conformity to underlying patterns. Neuronal apoptosis, often associated with the use of Ara-C, is mitigated by the addition of brain derived neurotrophic factor (BDNF). We believe that with the right combination of glial inhibitors and neuronal promoters, the Parylene-C based cell patterning method can generate structured, active neural networks that can be sustained and investigated over extended periods of time. To our knowledge this is the first report on the concurrent application of Ara-C and BDNF on patterned cell cultures.

Human retinal development in an in situ whole eye culture system

Phenotypic characterization of human retinogenesis may be facilitated by use of the tissue culture system paradigm. Traditionally, most culture protocols involve isolation of retinal tissue and/or cells, imposing various degrees of trauma, which in many cases leads to abnormal development. In this paper, we present a novel culture technique using whole embryonic eyes to investigate whether the retina in situ can develop normally in vitro. All procedures were carried out in accordance with the Declaration of Helsinki. Human embryos were obtained from elective abortions with the informed consent of the women seeking abortion. A total of 19 eyes were enucleated. The ages of the embryonic retinas were 6-7.5 weeks. Eyecups from 2 eyes were fixed immediately, to be used as controls. The remaining 17 eyes were placed on culture plates and divided into 3 groups kept for 7 (n = 4), 14 (n = 7) and 28 (n = 6) days in vitro (DIV). After fixation, the specimens were processed for hematoxylin and eosin staining, immunohistochemistry and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Antibodies against recoverin (rods and cones), protein kinase C (PKC; rod bipolar cells) and vimentin (Müller cells) were used. TUNEL was used to detect apoptotic cells. In hematoxylin- and eosin-stained sections, the control retinas displayed a neuroblast cell layer (NBL) and an inner marginal zone. Specimens kept 7-14 DIV had a similar appearance, while 28-day specimens consisted of an NBL with almost no marginal zone. Thirteen of the 17 cultured retinas displayed completely normal lamination without rosettes or double folds. Pyknotic cells were found at the inner margin of the retinas, and the proportion of these cells increased with time in vitro. TUNEL staining revealed a few scattered cells in 7-DIV specimens, and the amount of stained cells in the inner part of the retinas progressively increased in 14- and 28-DIV specimens. Vimentin labeling showed cells arranged in a vertical pattern in all retinas. Labeling with recoverin revealed photoreceptors in 4 of the retinas kept for 14 DIV, and in all retinas kept for 28 DIV. After 28 DIV, 2 of the eyes labeled with PKC contained rod bipolar cells with minimal axons. Here we showed that human embryonic retinas can be kept in culture in situ within the eye for at least 4 weeks. Abnormal lamination is not as frequent as in isolated full-thickness retinas, indicating that physical and biochemical contact with surrounding tissues is vital for proper development. Several types of the retina-specific neuronal and glial cells were seen to differentiate according to the in vivo schedule. The results are important for future studies of retinal development, and the technique can also be used for testing the effects of various drugs on the immature retina.

Functional and structural assessment of retinal sheet allograft transplantation in feline hereditary retinal degeneration

PURPOSE

To investigate whether sheets of fetal retinal allografts can integrate into the dystrophic Abyssinian cat retina with progressive rod cone degeneration.

METHODS

Fetal retinal sheets (cat gestational day 42), incubated with BDNF microspheres, were transplanted to the subretinal space of four cats at an early disease stage. Cats were studied by fundus examinations, bilateral full-field flash ERGs, and indocyanine green and fluorescein angiograms up to 4 months following surgery. E42 donor and transplanted eyes were analyzed by histology and immunohistochemistry for retinal markers.

RESULTS

Funduscopy and angiography showed good integration of the transplants in two of four cats, including extension of host blood vessels into the transplant and some scarring in the host. In these two, transplants were found in the subretinal space with laminated areas, with photoreceptor outer segments in normal contacts with the host retinal pigment epithelium. In some areas, transplants appeared to be well-integrated within the host neural retina. Neither of these two cats showed functional improvement in ERGs. In the other two cats, only remnants of donor tissue were left. Transplants stained for all investigated cellular markers. No PKC immunoreactivity was detected in the fetal donor retina at E42, but developed in the 4-month-old grafts.

CONCLUSIONS

Fetal sheet transplants can integrate well within a degenerating cat retina and develop good lamination of photoreceptors. Functional improvement was not demonstrated by ERG in cats with well-laminated grafts. Transplants need to be further evaluated in cat host retinas with a more advanced retinal degeneration using longer follow-up times.

Progenitor cell-derived factors enhance photoreceptor survival in rat retinal explants

Explantation of postnatal rat retinas is associated with degenerative events that show morphological similarities to human retinal degenerative disorders. The most evident morphological features are photoreceptor apoptosis involving caspase-3 and Müller cell activation. The purpose of the present study was to determine the content of protective factors in rat retinal progenitor cells and analyze the influence of the identified factors on the survival of photoreceptor cells and retinal gliosis. Tissue inhibitors of matrix metalloproteinase-1 (TIMP-1) and vascular endothelial growth factor (VEGF) were identified as putative beneficial factors, and their combined effect was examined in rat retinal explant cultures. Photoreceptor apoptosis was estimated by cell counts of cleaved caspase-3 and caspase-12 immunolabeled as well as TUNEL labeled cells. TIMP-1 and VEGF in combination significantly suppressed photoreceptor apoptosis involving caspase-3 activation. Cell counts of caspase-12 and TUNEL labeled photoreceptors showed no significant difference between the experiment and control retinas. TIMP-1 and VEGF appeared to have no effect on Müller cell activation as measured by GFAP and Ki-67 immunohistochemistry. Our data suggest that TIMP-1 and VEGF in combination promote the survival of photoreceptor cells in rat retinal explants, possibly by affecting a caspase-3 signaling pathway.

Neural cell death is induced by neutralizing antibody to nerve growth factor: an in vivo study

The central nervous system (CNS) of vertebrates originates from neuroepithelial cells located within the embryonic neural tube. Coincidental with the processes of proliferation, migration and differentiation in the developing CNS, cell death is also a major phenomenon during normal development. The investigation of neural cell death in development has focused on the role of target-derived survival factors such as nerve growth factor (NGF). In this study, the effects of anti-NGF antibody on neural cell death in the cerebral cortex have been investigated. Injection of anti-NGF antibody into the cisterna magnum of mouse pups increased the number of neural cell deaths and resulted in thinning of the cerebral cortex compared with a control group. It is concluded that endogenous NGF is essential for cortical cell survival in the cerebral cortex of the newborn mouse. Moreover, this method may be applied to the other factors and different CNS regions, allowing identification of molecules and signals involved in neural cell survival.

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.

BDNF-treated retinal progenitor sheets transplanted to degenerate rats: improved restoration of visual function

The aim of this study was to evaluate the functional efficacy of retinal progenitor cell (RPC) containing sheets with BDNF microspheres following subretinal transplantation in a rat model of retinal degeneration. Sheets of E19 RPCs derived from human placental alkaline phosphatase (hPAP) expressing transgenic rats were coated with poly-lactide-co-glycolide (PLGA) microspheres containing brain-derived neurotrophic factor (BDNF) and transplanted into the subretinal space of S334ter line 3 rhodopsin retinal degenerate rats. Controls received transplants without BDNF or BDNF microspheres alone. Visual function was monitored using optokinetic head-tracking behavior. Visually evoked responses to varying light intensities were recorded from the superior colliculus (SC) by electrophysiology at 60days after surgery. Frozen sections were studied by immunohistochemistry for photoreceptor and synaptic markers. Visual head tracking was significantly improved in rats that received BDNF-coated RPC sheets. Relatively more BDNF-treated transplanted rats (80%) compared to non-BDNF transplants (57%) responded to a "low light" intensity of 1cd/m2 in a confined SC area. With bright light, the onset latency of SC responses was restored to a nearly normal level in BDNF-treated transplants. No significant improvement was observed in the BDNF-only and no surgery transgenic control rats. The bipolar synaptic markers mGluR6 and PSD-95 showed normal distribution in transplants and abnormal distribution of the host retina, both with or without BDNF treatment. Red-green cones were significantly reduced in the host retina overlying the transplant in the BDNF-treated group. In summary, BDNF coating improved the functional efficacy of RPC grafts. The mechanism of the BDNF effects--either promoting functional integration between the transplant and the host retina and/or synergistic action with other putative humoral factors released by the RPCs--still needs to be elucidated.

Workgroup report: incorporating in vitro alternative methods for developmental neurotoxicity into international hazard and risk assessment strategies

This is the report of the first workshop on Incorporating In Vitro Alternative Methods for Developmental Neurotoxicity (DNT) Testing into International Hazard and Risk Assessment Strategies, held in Ispra, Italy, on 19-21 April 2005. The workshop was hosted by the European Centre for the Validation of Alternative Methods (ECVAM) and jointly organized by ECVAM, the European Chemical Industry Council, and the Johns Hopkins University Center for Alternatives to Animal Testing. The primary aim of the workshop was to identify and catalog potential methods that could be used to assess how data from in vitro alternative methods could help to predict and identify DNT hazards. Working groups focused on two different aspects: a) details on the science available in the field of DNT, including discussions on the models available to capture the critical DNT mechanisms and processes, and b) policy and strategy aspects to assess the integration of alternative methods in a regulatory framework. This report summarizes these discussions and details the recommendations and priorities for future work.

Temporal changes in the expression of some neurotrophins in spinal cord transected adult rats

Functional recovery of neurons in the spinal cord after physical injury is essentially abortive in clinical cases. As neurotrophins had been reported to be responsible, at least partially, for the lesion-induced recovery of spinal cord, it is not surprising that they have become the focus of numerous studies. Studies on endogenous neurotrophins, especially the three more important ones, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in injured spinal cord might provide some important clues in clinical treatment. Here we investigate the immunohistological expression of the above three factors at lower thoracic levels of the spinal cord as well as changes in the motor functions of the adult rat hindlimbs after cord transection. The injured rats were allowed to survive 3, 7, 14 and 21 days post operation (dpo). Flaccid paralysis was seen at 3 dpo following cord transection, however, hindlimb function showed partial recovery from 7 dpo to 21 dpo. The numbers of NGF, BDNF and NT-3 immunopositive neurons and their optical densities all increased in the lesion-induced cord. The immuno-expression of NGF and BDNF peaked at 7 dpo, while that of NT-3 peaked at 7 dpo and remained so at least up to 14 dpo. These results suggested that neurotrophins might play essential roles in functional recovery of after spinal cord injury, but the time points for the expression of the three factors differed somewhat.

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

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

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.

NMDA receptor activation modulates programmed cell death during early post-natal retinal development: a BDNF-dependent mechanism

Glutamate is a classical excitotoxin of the central nervous system (CNS), but extensive work demonstrates neuroprotective roles of this neurotransmitter in developing CNS. Mechanisms of glutamate-mediated neuroprotection are still under scrutiny. In this study, we investigated mediators of glutamate-induced neuroprotection, and tested whether this neurotransmitter controls programmed cell death in the developing retina. The protective effect of N-methyl-d-aspartate (NMDA) upon differentiating cells of retinal explants was completely blocked by a neutralizing antibody to brain-derived neurotrophic factor (BDNF), but not by an antibody to neurotrophin-4 (NT-4). Consistently, chronic activation of NMDA receptor increased the expression of BDNF and trkB mRNA, as well as BDNF protein content, but did not change the content of NT-4 mRNA in retinal tissue. Furthermore, we showed that in vivo inactivation of NMDA receptor by intraperitoneal injections of MK-801 increased natural cell death of specific cell populations of the post-natal retina. Our results show that chronic activation of NMDA receptors in vitro induces a BDNF-dependent neuroprotective state in differentiating retinal cells, and that NMDA receptor activation controls programmed cell death of developing retinal neurons in vivo.

Electrical synapse formation disrupts calcium-dependent exocytosis, but not vesicle mobilization

Electrical coupling exists prior to the onset of chemical connectivity at many developing and regenerating synapses. At cholinergic synapses in vitro, trophic factors facilitated the formation of electrical synapses and interfered with functional neurotransmitter release in response to photolytic elevations of intracellular calcium. In contrast, neurons lacking trophic factor induction and electrical coupling possessed flash-evoked transmitter release. Changes in cytosolic calcium and postsynaptic responsiveness to acetylcholine were not affected by electrical coupling. These data indicate that transient electrical synapse formation delayed chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and synchronized, vesicular release. Despite the inability to release neurotransmitter, neurons that had possessed strong electrical coupling recruited secretory vesicles to sites of synaptic contact. These results suggest that the mechanism by which neurotransmission is disrupted during electrical synapse formation is downstream of both calcium influx and synaptic vesicle mobilization. Therefore, electrical synaptogenesis may inhibit synaptic vesicles from acquiring a readily releasable state. We hypothesize that gap junctions might negatively interact with exocytotic processes, thereby diminishing chemical neurotransmission.