Effects of neurotrophins on embryonic retinal outgrowth

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

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

Therapeutic targeting of diabetic retinal neuropathy as a strategy in preventing diabetic retinopathy

Diabetes causes a panretinal neurodegeneration herein termed diabetic retinal neuropathy, which manifests in the retina early and progresses throughout the disease. Clinical manifestations include changes in the ERG, perimetry, dark adaptation, contrast sensitivity and colour vision which correlate with laboratory findings of thinning of the retinal neuronal layers, increased apoptosis in neurons and activation of glial cells. Possible mechanisms include oxidative stress, neuronal AGE accumulation, altered balance of neurotrophic factors and loss of mitohormesis. Retinal neural damage precedes and is a biologically plausible cause of retinal vasculopathy later in diabetes, and this review suggests that strategies to target it directly could prevent diabetes induced blindness. The efficacy of fenofibrate in reducing retinopathy progression provides a possible proof of concept for this approach. Strategies which may target diabetic retinal neuropathy include reducing retinal metabolic demand, improving mitochondrial function with AMPK and Sirt1 activators or providing neurotrophic support with neurotrophic supplementation.

Primary retinal cultures as a tool for modeling diabetic retinopathy: an overview

Experimental models of diabetic retinopathy (DR) have had a crucial role in the comprehension of the pathophysiology of the disease and the identification of new therapeutic strategies. Most of these studies have been conducted in vivo, in animal models. However, a significant contribution has also been provided by studies on retinal cultures, especially regarding the effects of the potentially toxic components of the diabetic milieu on retinal cell homeostasis, the characterization of the mechanisms on the basis of retinal damage, and the identification of potentially protective molecules. In this review, we highlight the contribution given by primary retinal cultures to the study of DR, focusing on early neuroglial impairment. We also speculate on possible themes into which studies based on retinal cell cultures could provide deeper insight.

Nerve growth factor in diabetic retinopathy: beyond neurons

Diabetic retinopathy (DR), a major ocular complication of diabetes, is a leading cause of blindness in US working age adults with limited treatments. Neurotrophins (NTs), a family of proteins essential for growth, differentiation and survival of retinal neurons, have emerged as potential players in the pathogenesis of DR. NTs can signal through their corresponding tropomyosin kinase related receptor to mediate cell survival or through the p75 neurotrophin receptor with the co-receptor, sortilin, to mediate cell death. This review focuses on the role of NGF, the first discovered NT, in the development of DR. Impaired processing of proNGF has been found in ocular fluids from diabetic patients as well as experimental models. Evidence from literature and our studies support the notion that NTs appear to play multiple potential roles in DR, hence, understanding their contribution to DR may lead to promising therapeutic approaches for this devastating disease.

Truncated TrkB: beyond a dominant negative receptor

BDNF activates trkB receptors to regulate neuronal survival, differentiation, and proliferation. Mutations in the BDNF gene, altered BDNF expression, and altered trkB expression are associated with degenerative and psychiatric disorders. The full-length trkB receptor (trkB.tk(+)) undergoes autophosphorylation to activate intracellular signaling pathways. The truncated trkB receptor (trkB.t1) is abundantly expressed in the brain but lacks the catalytic tyrosine kinase domain. TrkB.t1 is a dominant-negative receptor that inhibits trkB.tk(+) signaling. While this is an important function of trkB.t1, it is only one of its many functions. TrkB.t1 sequesters and translocate BDNF, induces filopodia and neurite outgrowth, stimulates intracellular signaling cascades, regulates Rho GTPase signaling, and modifies cytoskeletal structures. TrkB.t1 is an active signaling molecule with regulatory effects on neurons and astrocytes.

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

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

Neurotrophic factors and neural prostheses: potential clinical applications based upon findings in the auditory system

Spiral ganglion neurons (SGNs) are the target cells of the cochlear implant, a neural prosthesis designed to provide important auditory cues to severely or profoundly deaf patients. The ongoing degeneration of SGNs that occurs following a sensorineural hearing loss is, therefore, considered a limiting factor in cochlear implant efficacy. We review neurobiological techniques aimed at preventing SGN degeneration using exogenous delivery of neurotrophic factors. Application of these proteins prevents SGN degeneration and can enhance neurite outgrowth. Furthermore, chronic electrical stimulation of SGNs increases neurotrophic factor-induced survival and is correlated with functional benefits. The application of neurotrophic factors has the potential to enhance the benefits that patients can derive from cochlear implants; moreover, these techniques may be relevant for use with neural prostheses in other neurological conditions.

Neurotrophic rationale in glaucoma: A TrkA agonist, but not NGF or a p75 antagonist, protects retinal ganglion cellsin vivo

Glaucoma is a major cause of vision impairment, which arises from the sustained and progressive apoptosis of retinal ganglion cells (RGC), with ocular hypertension being a major risk or co-morbidity factor. Because RGC death often continues after normalization of ocular hypertension, growth factor-mediated protection of compromised neurons may be useful. However, the therapeutic use of nerve growth factor (NGF) has not proven effective at delaying RGC death in glaucoma. We postulated that one cause for the failure of NGF may be related to its binding to two receptors, TrkA and p75. These receptors have distinct cellular distribution in the retina and in neurons they induce complex and sometimes opposing activities. Here, we show in an in vivo therapeutic model of glaucoma that a selective agonist of the pro-survival TrkA receptor was effective at preventing RGC death. RGC loss was fully prevented by combining the selective agonist of TrkA with intraocular pressure-lowering drugs. In contrast, neither NGF nor an antagonist of the pro-apoptotic p75 receptor protected RGCs. These results further a neurotrophic rationale for glaucoma.

Normal eye-specific patterning of retinal inputs to murine subcortical visual nuclei in the absence of brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) is a preferred ligand for a member of the tropomyosin-related receptor family, trkB. Activation of trkB is implicated in various activity-independent as well as activity-dependent growth processes in many developing and mature neural systems. In the subcortical visual system, where electrical activity has been implicated in normal development, both differential survival, as well as remodeling of axonal arbors, have been suggested to contribute to eye-specific segregation of retinal ganglion cell inputs. Here, we tested whether BDNF is required for eye-specific segregation of visual inputs to the lateral geniculate nucleus and the superior colliculus, and two other major subcortical target fields in mice. We report that eye-specific patterning is normal in two mutants that lack BDNF expression during the segregation period: a germ-line knockout for BDNF, and a conditional mutant in which BDNF expression is absent or greatly reduced in the central nervous system. We conclude that the availability of BDNF is not necessary for eye-specific segregation in subcortical visual nuclei.

Neurotrophin-3 and TrkC in the frog visual system: changes after axotomy

Neurotrophins are potent regulators of the survival of different neuronal populations in the CNS. Little is known of the immunodistribution of neurotrophin-3 (NT-3) and tyrosine kinase C (TrkC) receptor in the frog visual system, which can successfully regenerate and recover vision after injury. In this study we show that both NT-3 and TrkC are present in the frog retina and tectum, and that their distribution changes after optic nerve transection. Both NT-3 and TrkC are present in the ganglion cell layer, inner nuclear layer, nerve fiber layer and outer plexiform layer, and in Müller cells of control retinas. Quantification of identified RGCs shows that there are only small changes in the proportion, or intensity, of NT-3 immunostained cells surviving after axotomy and regeneration. Müller cell staining, however, is increased. TrkC staining in the retina does not change after axotomy. In the tectum, NT-3 immunoreactivity is present in the retinorecipient layer 9, and in radial processes of neurons and ependymoglia. TrkC is present in ependymoglia and in tectal neurons. After axotomy or colchicine treatment fewer NT-3-immunoreactive processes are present in layer 9 and there is decreased staining of tectal neurons. These data are consistent with the hypothesis that NT-3 is synthesized in the retina and anterogradely transported to the tectum. TrkC immunostaining, on the other hand, increases in tectal cells after optic nerve transection, suggesting that it may be regulated by the supply of NT-3 from the retina.

Neurotrophins are not required for normal embryonic development of olfactory neurons

Neurons of the vertebrate olfactory epithelium (OE) regenerate continuously throughout life. The capacity of these neurons to regenerate and make new and precise synaptic connections in the olfactory bulb provides a useful model to study factors that may control or mediate neuronal regeneration. Expression and in vitro studies have suggested potential roles for the neurotrophins in the olfactory system. To directly examine whether neurotrophins are required for olfactory neuron development, we characterized in vivo the role of the neurotrophins in the primary olfactory system. For this, we generated mutant mice for TrkA, TrkB, TrkC, and also for BDNF and NT3 together with P2-IRES-tau-LacZ trangenic mice. Histochemical staining for beta-galactosidase at birth allowed in vivo analysis of the P2 subpopulation of olfactory neurons as well as their projections to the olfactory bulb. Our data indicate that Trk signaling is not required for normal embryonic development of the olfactory system.

Characterization of a transformed rat retinal ganglion cell line

The purpose of the present study was to establish a rat retinal ganglion cell line by transformation of rat retinal cells. For this investigation, retinal cells were isolated from postnatal day 1 (PN1) rats and transformed with the psi2 E1A virus. In order to isolate retinal ganglion cells (RGC), single cell clones were chosen at random from the transformed cells. Expression of Thy-1 (a marker for RGC), glial fibrillary acidic protein (GFAP, a positive marker for Muller cells), HPC-1/syntaxin (a marker for amacrine cells), 8A1 (a marker for horizontal and ganglion cells) and neurotrophins was studied using reverse transcriptase-polymerase chain reaction (RT-PCR), immunoblotting and immunocytochemistry. One of the retinal cell clones, designated RGC-5, was positive for Thy-1, Brn-3C, Neuritin, NMDA receptor, GABA-B receptor, and synaptophysin expression and negative for GFAP, HPC-1, and 8A1, suggesting that it represented a putative RGC clone. The results of RT-PCR analysis were confirmed by immunocytochemistry for Thy-1 and GFAP. Upon further characterization by immunoblotting, the RGC-5 clone was positive for Thy-1, negative for GFAP, 8A1 and syntaxin. RGC 5 cells were also positive for the expression of neurotrophins and their cognate receptors. To establish the physiological relevance of RGC-5, the effects of serum/trophic factor deprivation and glutamate toxicity were analyzed to determine if these cells would undergo apoptosis. The protective effects of neurotrophins on RGC-5 after serum deprivation was also investigated. Apoptosis was studied by terminal deoxynucleotidyl transferase-mediated fluoresceinated dUTP nick end labeling (TUNEL). Serum deprivation resulted in apoptosis and supplementation with both BDNF and NT-4 in the growth media, protected the RGC-5 cells from undergoing apoptosis. On differentiation with succinyl concanavalin A (sConA), RGC-5 cells became sensitive to glutamate toxicity, which could be reversed by inclusion of ciplizone (MK801). In conclusion, a transformed rat retinal cell line, RGC-5, has certain characteristics of retinal ganglion cells based on Thy-1 and Brn-3C expression and its sensitivity to glutamate excitotoxicity and neurotrophin withdrawal. These cells may be valuable in understanding of retinal ganglion cell biology and physiology including in vitro manipulations in experimental models of glaucoma.