Retinal ganglion cell response to nerve growth factor in the regenerating and intact visual system of the goldfish (Carassius auratus)

Regulation of Stem Cell Properties of Müller Glia by JAK/STAT and MAPK Signaling in the Mammalian Retina

In humans and other mammals, the neural retina does not spontaneously regenerate, and damage to the retina that kills retinal neurons results in permanent blindness. In contrast to embryonic stem cells, induced pluripotent stem cells, and embryonic/fetal retinal stem cells, Müller glia offer an intrinsic cellular source for regenerative strategies in the retina. Müller glia are radial glial cells within the retina that maintain retinal homeostasis, buffer ion flux associated with phototransduction, and form the blood/retinal barrier within the retina proper. In injured or degenerating retinas, Müller glia contribute to gliotic responses and scar formation but also show regenerative capabilities that vary across species. In the mammalian retina, regenerative responses achieved to date remain insufficient for potential clinical applications. Activation of JAK/STAT and MAPK signaling by CNTF, EGF, and FGFs can promote proliferation and modulate the glial/neurogenic switch. However, to achieve clinical relevance, additional intrinsic and extrinsic factors that restrict or promote regenerative responses of Müller glia in the mammalian retina must be identified. This review focuses on Müller glia and Müller glial-derived stem cells in the retina and phylogenetic differences among model vertebrate species and highlights some of the current progress towards understanding the cellular mechanisms regulating their regenerative response.

Nerve growth factor protects against palmitic acid-induced injury in retinal ganglion cells

Accumulating evidence supports an important role for nerve growth factor (NGF) in diabetic retinopathy. We hypothesized that NGF has a protective effect on rat retinal ganglion RGC-5 cells injured by palmitic acid (PA), a metabolic factor implicated in the development of diabetes and its complications. Our results show that PA exposure caused apoptosis of RGC-5 cells, while NGF protected against PA insult in a concentration-dependent manner. Additionally, NGF significantly attenuated the levels of reactive oxygen species (ROS) and malondialdehyde (MDA) in RGC-5 cells. Pathway inhibitor tests showed that the protective effect of NGF was completely reversed by LY294002 (PI3K inhibitor), Akt VIII inhibitor, and PD98059 (ERK1/2 inhibitor). Western blot analysis revealed that NGF induced the phosphorylation of Akt/FoxO1 and ERK1/2 and reversed the PA-evoked reduction in the levels of these proteins. These results indicate that NGF protects RGC-5 cells against PA-induced injury through anti-oxidation and inhibition of apoptosis by modulation of the PI3K/Akt and ERK1/2 signaling pathways.

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

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

Upregulation of retinal transglutaminase during the axonal elongation stage of goldfish optic nerve regeneration

Fish CNS neurons can repair their axons following nerve injury, whereas mammalian CNS neurons cannot regenerate, and become apoptotic within 1-2 weeks after the nerve lesion. One explanation for these differences is that one, or several molecules are upregulated in fish CNS neurons during nerve regeneration, and this same molecule is downregulated in mammalian CNS neurons before the development of apoptosis caused by nerve injury. A molecule satisfying these criteria might successfully rescue and repair the mammalian CNS neurons. In this study, we looked for such a candidate molecule from goldfish retinas. Transglutaminase derived from goldfish retina (TG(R)) was characterized as a regenerating molecule after optic nerve injury. A full-length cDNA for TG(R) was isolated from the goldfish retinal cDNA library prepared from axotomized retinas. Levels of TG(R) mRNA and protein increased only in the retinal ganglion cells (RGCs) between 10 and 40 days after optic nerve transection. Recombinant TG(R) protein enhanced neurite outgrowth from adult fish RGCs in culture. Specific interference RNA and antibodies for TG(R) inhibited neurite outgrowth both in vitro and in vivo. In contrast, the level of TG(R) protein decreased in rat RGCs within 1-3 days after nerve injury. Furthermore, the addition of recombinant TG(R) to retinal cultures induced striking neurite outgrowth from adult rat RGCs. These molecular and cellular data strongly suggest that TG(R) promotes axonal elongation at the surface of injured RGCs after optic nerve injury.

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.

Increased levels of TrkA in the regenerating retinal ganglion cells of fish

Retinal ganglion cells of the fish have the spontaneous capacity to regenerate after nerve crush, a phenomenon known to be facilitated by nerve growth factor (NGF). We have studied the high-affinity NGF receptor TrkA, during the regeneration of the tench (Tinca tinca L.) optic nerve, using immunocytochemical techniques. TrkA-like immunoreactivity increased during the regeneration of the retinal ganglion cells. The increase is followed by a change in the subcellular distribution from perinuclear in control cells to cytoplasmic and perinuclear in regenerating ones. This increase was observed when antibodies against the extracellular domain of TrkA were used; no changes in TrkA-like immunoreactivity were observed with antibodies against the intracellular domain of TrkA. We thus conclude that modulation of TrkA is involved in the regeneration of fish retinal ganglion cells.

Brain-derived neurotrophic factor gene expression in the developing zebrafish

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of polypeptides that includes NGF, NT-3, NT-4/5 and NT-6. Although neurotrophins are known to be expressed in teleost fishes little is known about their functions in the development of these vertebrates. We are therefore studying BDNF in the zebrafish, Danio rerio. The structure of zebrafish BDNF mRNA was established using PCR and cDNA cloning. The encoded BDNF was 91% identical to mammalian BDNF. Southern blot analysis revealed a unique BDNF gene. Northern blot analysis detected two heterogeneous populations of BDNF transcripts centered at 1.6 and 2 kb. BDNF transcripts were first measurable 24 h post-fertilization (pf). Their abundance relative to total transcripts increased 6-fold between 1 day and 3 days pf and again 2-fold by 7 days pf. In situ hybridization analyses of 4-day-old larvae revealed BDNF transcripts in the retina, brain, otic vesicle, pectoral fin and the hair cells of the neuromast. The early onset and cellular sites of expression suggest that BDNF functions in nervous system and fin development in the zebrafish.

The conservation of neurotrophic factors during vertebrate evolution

Neurotrophic factors are a family of extracellular ligands that affect the differentiation, survival (by inhibition of apoptosis) and maintenance of function of neuronal cells in vertebrates. The family includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5). The survival specificities of NGF and BDNF for different classes of chick neurons are maintained from the fish to the mammalian proteins, implying a conserved interaction with neuronal cell surface receptors (of the Trk family). However, the quantitative effect of a fish neurotrophin can differ significantly from that of the mammalian orthologue.

Fibroblast growth factor induces proliferating cell nuclear antigen-immunoreactive cells in goldfish retina

New rod photoreceptors are added to mature teleost retinas throughout life by regulated proliferation of rod precursor cells (RPCs). In this study, candidate regulators of RPC proliferation, acidic and basic fibroblast growth factors (aFGF and bFGF; 0.1 microgram/eye), interleukin-6 (IL-6; 0.1 microgram) and phytohaemagglutinin (HA15; 1.0 microgram), were injected intravitreally into one eye of goldfish (body length 5-6 cm), and mitotic RPCs in both retinas were detected and counted 3-50 days later by immunohistochemistry for proliferating cell nuclear antigen (PCNA). Retinal integrity after treatment was assessed by immunohistochemistry for tyrosine hydroxylase (TH) and other retinal antigens. All the agents applied altered the density of PCNA-immunoreactive (ir) cells in the outer and inner nuclear layers (ONL and INL) in both retinas as soon as 2-3 days after unilateral injection. Initially (2-20 days after injection), particularly in the treated retina, PCNA-ir cells appeared in clusters accompanied by various numbers of scattered individual cells, but subsequently the clusters of PCNA-ir cells disappeared while the density of singly distributed cells increased until 30 days after injection. At the doses given, these effects were most striking with aFGF and bFGF and less with IL-6 and HA15. In radial cryosections, other cellular elements immunoreactive to markers such as TH, serotonin, neuropeptide Y, substance P, glutamine synthetase, glial fibrillary acidic protein and protein kinase C, were found normal in terms of morphology. In addition, a monoclonal antibody (NN-2) was found to label some non-neuronal structures (macrophages, microglia and blood vessels) inside and outside the retina intoxicated with 6-hydroxydopamine, a few NN-2-ir cells being PCNA-positive. However, clustered PCNA-ir and marginal neuroblast cells were NN-2-negative. These results indicate that FGFs may play an important role in stimulating the proliferation of RPCs, for example, in the regeneration of fish retinas following neurotoxic destruction.

Brain-derived neurotrophic factor is more highly conserved in structure and function than nerve growth factor during vertebrate evolution

Mammalian nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are members of a protein family with perfectly conserved domains arranged around the cysteine residues thought to stabilize an invariant three-dimensional scaffold in addition to distinct sequence motifs that convey different neuronal functions. To study their structural and functional conservation during evolution, we have compared NGF and BDNF from a lower vertebrate, the teleost fish Xiphophorus, with the mammalian homologues. Genomic clones encoding fish NGF and BDNF were isolated by cross-hybridization using probes from the cloned mammalian factors. Fish NGF and BDNF were expressed by means of recombinant vaccinia viruses, purified, and their neuronal survival specificities for different classes of neurons were found to mirror those of the mammalian factors. The half-maximal survival concentration for chick sensory neurons was 60 pg/ml for both fish and mammalian purified recombinant BDNF. However, the activity of recombinant fish NGF on both chick sensory and sympathetic neurons was 6 ng/ml, 75-fold lower than that of mouse NGF. The different functional conservation of NGF and BDNF is also reflected in their structures. The DNA-deduced amino acid sequences of processed mature fish NGF and BDNF showed, compared to mouse, 63% and 90% identity, respectively, indicating that NGF had reached an optimized structure later than BDNF. The retrograde extrapolation of these data indicates that NGF and BDNF evolved at strikingly different rates from a common ancestral gene about 600 million years ago. By RNA gel blot analysis NGF mRNA was detected during late embryonic development; BDNF was present in adult brain.

Identification of high- and low-affinity NGF receptors during development of the chicken central nervous system

In order to study regulation of the nerve growth factor (NGF) receptor during embryogenesis in chick brain, we have used affinity crosslinking of tissues with 125I-NGF. NGF interacts with high- and low-affinity receptors; high-affinity receptors are required for the majority of NGF's actions. Most measurements of receptor levels do not distinguish between high- and low-affinity forms of the receptor. We have used the lipophilic crosslinking agent HSAB to identify the high-affinity, functional receptor during development of the chicken central nervous system. A peak of expression during Embryonic Days 5-10 was detected in all regions of the chicken central nervous system, but, shortly after birth, only the cerebellar region displays significant levels of NGF receptor protein. The time course of expression confirms the dramatic regulation of the NGF receptor gene during defined embryonic periods. The detection of high-affinity NGF receptors in brain and neural retina provides strong evidence that NGF is involved in essential ontogenetic events in the development of the chicken central nervous system.

Differential effects of axotomy on substance P-containing and nicotinic acetylcholine receptor-containing retinal ganglion cells: time course of degeneration and effects of nerve growth factor

The time course of degeneration of chick retinal ganglion cells was examined with Nissl stains and immunohistochemical methods for detection of substance P-like immunoreactive and nicotinic acetylcholine receptor immunoreactive neurons. Small lesions were made in the retinae, adjacent to the optic nerve head, and were subsequently sectioned parallel to the vitreal surface, permitting direct comparison of normal and axotomized retinal ganglion cells distal to the site of axon damage. At four and six days after surgery, a large number of degenerating cells with clear cytoplasm and pyknotic nuclei were seen. After eight, 10 and 14 days, many retinal ganglion cells displayed a chromatolytic response with dispersed Nissl granules, eccentric nuclei and the cells appeared crenulated. The number of apparently normal neurons in the ganglion cell layer in the axotomized region was reduced by about 50% six days following surgery, by about 70% on the 10th day and by about 75% on the 17th day. The remaining neurons in the ganglion cell layer were identified as displaced amacrine cells. From day 2 onwards, increased numbers of glial cells were present in the optic fibre, ganglion cell and inner plexiform layers. Many glial cells were enlarged and displayed extensive cytoplasmic processes, while others showed mitotic activity. Somata and proximal dendrites of retinal ganglion cells were intensely stained for substance P-like immunoreactivity at two and four days following surgery. At six, eight and 10 days, staining intensity was markedly reduced though still evident and at 14 and 17 days, substance P-like immunoreactivity had virtually disappeared. The persistence of limited substance P-like immunoreactive ganglion cells 10 days after surgery indicates that these cells have a relatively protracted response to axotomy. Nicotinic acetylcholine receptor-like immunoreactivity in the ganglion cells at two and four days following axotomy was substantially reduced. The majority of faintly stained nicotinic acetylcholine receptor-like immunoreactive ganglion cells, as visualized in counterstained sections, did not exhibit pyknosis in the immediate period following axotomy. Double label studies demonstrated that substance P-like immunoreactive ganglion cells were distinct from the nicotinic acetylcholine receptor-like immunoreactive ganglion cells. In a second set of experiments, nerve growth factor was then placed into the vitreous humor following intra-retinal axotomy. The somata, dendrites and proximal axons of lesioned substance P-like immunoreactive ganglion cells in these retinae were more intensely stained for a longer period of time and appeared more robust than cells from untreated retinae.(ABSTRACT TRUNCATED AT 400 WORDS)

Survival of purified embryonic chick retinal ganglion cells in the presence of neurotrophic factors

In a search for neurotrophic factors (NTFs) regulating retinal ganglion cell (RGC) death in the chick embryo we have used purified and cultured RGCs. Purification of RGCs from embryonic day 10 was achieved by employing the "panning" method (Silverstein and Chun: Soc Neurosci Abstr 13:1054, 1987). The obtained neuron population consisted of 97% RGCs as demonstrated by retrograde labeling with a fluorescence dye. RGCs were cultured at low density in a chemically defined medium and short-term survival (24 hr) was determined. In the absence of NTFs, less than 3% of the RGCs survived. In the presence of various crude or purified NTFs (eye, brain, and tectum extracts; glial-conditioned medium; ciliary neurotrophic factor [CNTF]; nerve growth factor [NGF]) 31% to 52% of the RGCs were maintained. The effects of NGF and CNTF were not additive. Neither acidic nor basic fibroblast growth factor was able to maintain RGCs in culture. Our results, obtained with a culture system which allowed the analysis of direct trophic actions, suggest that NGF and CNTF may be NTFs for overlapping subpopulations of chick RGCs.

A morphometric study of the retinal ganglion cell response to optic nerve severance in the frog Rana pipiens

This study examines the cell body response to axotomy of retinal ganglion cells in the frog Rana pipiens. Cell soma sizes were measured in carefully matched regions of Nissl-stained wholemounted retinae after either nerve crush, nerve cut with stump separation, nerve crush with intraocular nerve growth factor (NGF) or nerve cut with NGF applied to the proximal stump. The state of axonal regeneration was also assessed in each case by anterograde transport of HRP. Following nerve crush axons crossed the lesion by 7 days, reached the chiasma by 14 days and entered the tectum around 20-30 days. The normally evenly stained ganglion cells exhibited granular Nissl staining at 7 and 10 days but very little change in soma size. From 10 to 28 days the mean retinal ganglion cell area increased by 102% and maintained this size until at least 75 days. By 102 days soma size had nearly returned to normal. A population of displaced amacrine cells retained a normal appearance and soma size throughout regeneration. Following nerve cut and stump separation the retinal ganglion cells were slightly more reactive in appearance at 7 days after crush but otherwise the soma reaction developed in a similar manner. Axon tracing revealed no extension beyond the lesion site in these animals and therefore the state of axonal growth did not affect the early soma response. NGF applied at the time of the lesion had no detectable effect on the soma reaction. Although many retinal ganglion cells re-establish contact with visual centres after axotomy in the frog, a considerable proportion die. This contrasts with both the goldfish, where all cells regenerate successfully, and various mammals, where none do so and all retinal ganglion cells die. All retinal ganglion cells in the frog undergo reactive changes similar to those of goldfish and there is no sign of the cell shrinkage seen in mammals. Therefore the cell death in frog would appear to be different from that in mammalian retina but similar to that of mammalian peripheral nerve in which chromatolysis generally preceeds death.

Effect of different optic nerve lesions on retinal ganglion cell death in the frog Rana pipiens

Following optic nerve crush in various species of frog, a proportion of the retinal ganglion cells re-establishes functional contact with the optic tectum. However, as much as 50% of the retinal ganglion cells die during this process. The determinants of an individual ganglion cell's fate have not been established. In this study of Rana pipiens, cell survival after optic nerve crush was compared with that after nerve cut followed by stump separation, a procedure that considerably delayed entry of optic axons to the brain. It was also ascertained, in the case of delayed ingrowth, whether application of nerve growth factor immediately after lesion influenced the cell death process. This study confirmed that retinal ganglion cell death is a relatively late event in regeneration, because in several animals where anterograde HRP labeling demonstrated regenerating axons within the tectum, no cell death had occurred. There was no statistically significant difference in cell death at 75 days after lesion between animals receiving nerve crush and those receiving nerve cut with stump separation, even though most crush animals had regenerated a complete visual projection, whereas most nerve cut animals had not. The application of NGF did not influence the level of cell death at 75 days after lesion. These results suggest that contact of optic axons with the optic tract or tectum is not necessary for retinal ganglion cell death to occur. However, this does not necessarily mean that contact with the brain is not involved with cell death during regeneration following nerve crush because it is possible that the mechanisms of cell death are different when axons are prevented from regenerating. Further investigations are therefore required to establish the reasons for this cell death.

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

A neurotrophic factor derived from goldfish brain: characterization and purification

Previous studies carried out in our laboratory have demonstrated that goldfish brain contains substances that promote neurite extension from regenerating retinae in culture. Fractionation of the brain extract by molecular sieving chromatography revealed the presence of several molecular species, including two peaks that have neurotrophic activity, representing low-molecular-weight substances. One peak was eluted (P-a) with an apparent molecular weight of about 13 kDa and was designated substratum neurite extension factor (SNEF) because it retained its neurotrophic activity when adsorbed onto the substratum. This recovered Sephadex fraction (P-a) when applied in vivo intraocularly caused an earlier capacity of the corresponding retinae to sprout in vitro. Thus, at 3 and 5 days after injury the neuritic growth indices from the factor-treated retinae were of 0.9 +/- 0.2 and 2.8 +/- 0.5, respectively, as compared with indices of 0.3 +/- 0.1 and 0.9 +/- 0.2, respectively, in retinae of injured but nontreated nerves. The factor was further purified by two steps of HPLC (ion exchange followed by reversed phase). The results showed that it is an acidic glycoprotein with an apparent molecular weight of 10 kDa.

Cell death of asynaptic neurons in regenerating spinal cord

The weakly electric fish Sternarchus albifrons possesses a unique class of asynaptic neurons, the electromotor neurons, whose axons constitute the electric organ. The cell bodies of origin of the electrocyte axons are located in the spinal cord. Both spinal cord and electromotor neurons ( electrocytes ) regenerate after amputation of the tail. Sternarchus spinal cords which have regenerated for 1 or more years show a progression in number of perikarya of electromotor neurons along the rostro-caudal axis. The most recently regenerated region of the cord is at the caudal end, which consists of a tube of ependyma. Progressing rostrally along regenerated spinal cord from the caudal end, numerous cells are generated and large numbers of electromotor neurons differentiate. The maximum number of electromotor neurons per transverse section of regenerated cord is five times higher than in normal mature cord. Rostral to this, the number of electromotor neurons decreases gradually to the normal number near the transition zone (the border with unregenerated cord). As the more rostral regenerated cord has presumably had a longer period of regeneration, we conclude that excess numbers of electromotor neurons are generated initially, and that subsequently the number of these neurons is decreased by cell death. This conclusion is supported by the fact that younger regenerates (2-4 months) have larger-than-normal numbers of perikarya of electromotor neurons extending up to the transition zone (Anderson and Waxman , 1981). No evidence of migration or depletion of electromotor neurons from unregenerated cord rostral to the amputation has been observed. Since the axons of the electromotor neurons in Sternarchus do not normally form any synapses, this study provides evidence that factors other than synaptic competition must be responsible for determining cell death during regeneration of these spinal neurons.

Extract from brain stimulates neurite outgrowth from fetal rat retinal explants

Explants from rat fetal retina were placed in culture and assayed for fiber outgrowth. In contrast to results obtained with lower vertebrates, nerve growth factor (NGF) does not seem to play a role in this system: NGF is not able to stimulate fiber outgrowth and antibodies to NGF do not block the spontaneously occurring fiber outgrowth. However, an extract prepared from pig brain is able to stimulate fiber outgrowth in a dose-dependent manner. It is suggested that such an extract can be used as a source of putative neurotrophic factors exhibiting in the mammalian central nervous system (CNS) an action similar to that of NGF in the peripheral nervous system (PNS) of mammals and in the CNS of lower vertebrates like fishes and amphibia.

Nerve growth factor stimulates neurite outgrowth from goldfish retinal explants: the influence of a prior lesion

Goldfish (Carassius auratus) retinal explants, whose ganglion cells were 'primed' in vivo by an optic nerve crush at varying intervals prior to culture, were found to respond by enhanced neurite outgrowth to low (ng/ml) concentrations of nerve growth factor (NGF). This in vitro response was dose-dependent and specific for NGF. The spontaneous fiber outgrowth which normally occurs in vitro in response to optic nerve lesion without exogenous NGF in the medium could be reduced by approximately 80% with administration of NGF antiserum. These observations strongly indicate the formation of an NGF-like molecule in the goldfish retina. The magnitude and sensitivity of the NGF response was dependent on the post-crush interval, referred to as days post-axotomy (DPA). Without a prior crush there was no response unless explants remained in culture 1-2 weeks before NGF treatment. NGF elicited the greatest increase in neurite outgrowth when administered to 7 DPA explants. With increasing intervals after axotomy the response decreased until by 35 DPA none could be elicited.

Effect of nerve growth factor on regeneration of goldfish optic axons

Axonal outgrowth following a crush of the goldfish optic nerve was enhanced if nerve growth factor (NGF) was administered by intraocular injection or by local application to the lesion site. Various forms of NGF (beta, 2.5S and 7S) were effective, producing a 20-40% decrease in the time required for recovery of the startle reaction to a bright light. A corresponding increase in axonal outgrowth was revealed by histological examination of the optic nerves. The effect produced by a single intraocular injection given at the time of the lesion was not further increased by subsequent injections. Up to 14 days after the lesion, the size of the retinal ganglion cell bodies and the incidence of nucleoli detectable by light microscopy were not affected by the NGF treatment.

Factor(s) from goldfish brain induce neuritic outgrowth from explanted regenerating retinas

The extensive outgrowth from explanted goldfish retina in vitro stimulated by the axotomy of the optic nerve several days prior to the explantation, enables a more direct examination of the events which accompany optic nerve regeneration. Thus far neuritic outgrowth has been obtained in serum-containing medium. In the present report we demonstrate for the first time that factor(s) from adult goldfish brain can support neuritic outgrowth from retinal explant and replace the serum. This finding suggests that goldfish neuronotrophic factor(s) are active on retinal ganglia cells. Under the same experimental conditions nerve growth factor from submaxillary gland did not exhibit outgrowth stimulatory activity.

Regenerating goldfish retinal explants: induction and maintenance of neurites by conditioned medium from cells originated in the nervous system

Fiber outgrowth from goldfish regenerating retinas can be induced by conditioned medium of cloned cells which originated in the nervous system, i.e. glioma and neuroblastoma. Dilution of the released factor(s) was required to achieve optimal effect; high concentrations are detrimental. The fibers can be maintained for at least 2 weeks in vitro, and reach a length of several millimeters. This system may provide a means to purify and characterize neurotrophic factors involved in nerve regeneration.

Growth from regenerating goldfish retinal cultures in the absence of serum or hormonal supplements: tissue extract effects

The minimal requirements for the regeneration of optic nerve fibers in vitro were established in a serum-free retinal explant preparation. This serum-free preparation was developed as a prerequisite for testing the growth-promoting activity of tissue extracts prepared from the primary target of regenerating fibers. Explants taken from goldfish retinas 14 days after a prior optic nerve crush were capable of long-term survival and regenerated neurite outgrowth without serum or hormonal supplements. Serum-free conditions for explant outgrowth required only a basic Leibovitz (L-15) media containing 0.6% methyl cellulose (MC). Explants were also capable of neurite outgrowth in L-15 media alone when culture dishes were preplated with MC. MC treatment permitted both the regeneration of neurites in serum-free L-15 and a significant increase in the rate and extent of neurite outgrowth when combined with 10% fetal calf serum (FCS). Explants grown in L-15 with both MC and FCS produced a 2.5-fold increase in the length of neurite outgrowth over MC alone and a 1.5-fold increase in the length of neurite outgrowth over FCS alone. MC activity which permitted minimal serum-free regeneration and optimal serum supplemented regeneration was determined to be substrate related. Retinas were dissociated to determine if ganglion cells, like the intact explant, were capable of survival and neurite regeneration in serum-free conditions. These cells survived and extended long neurites when grown in L-15 with FCS or with FCS and MC, but they did not survive in serum-free L-15 with MC. The minimal serum-free conditions for explant survival and neurite regeneration were used as a model system to test the growth-promoting activity of crude tissue extracts prepared from the goldfish brain. Extracts prepared from the primary target region, the optic tectum, stimulated a significant 2.5-fold increase in the length of regenerating neurites. The optic tectal extract (OTex) stimulated outgrowth with significantly high specific activity when compared with extracts of identical protein concentrations prepared from the cerebellum (Cex). At a minimal protein concentration of 150 micrograms/ml, the OTex stimulated a 1.5-fold increase in neurite outgrowth above Cex. These results indicated that a serum-free culture preparation had been established for optic nerve regeneration. This culture system has proven to be an extremely sensitive bioassay model without the masking effect of a serum supplement. Serum-free cultures may be used in further studies to determine the role neurotrophic factors may play in a widely used model of successful central nervous system (CNS) regeneration.

Increased activity of ornithine decarboxylase in goldfish following optic nerve crush

Ornithine decarboxylase (ODC) activity increased in goldfish retina, brain, and kidney several days following unilateral intraorbital optic nerve crush. Activity in both retinas followed a similar time course, an elevation being first detectable 3 days following crush, maximal on day 5 and returning to control levels by day 7. Increases were significantly higher in the retina on the lesioned side than in control retina. If one eye was surgically removed, a more prolonged elevation of ODC activity was seen in the remaining eye than was observed in control retinas following contralateral nerve crush. Sham operation or behavioral stress secondary to aversive electrical shock did not lead to significant increases in ODC activity in brain or retina. Explants of retinas whose optic nerve had been crushed 10--14 days prior to explantation, as well as those derived from the unlesioned side, showed increases in ODC activity following addition of beta-NGF to the medium. The response was greater in post-crush retinas than in control retinas. These results taken together suggest an increased responsiveness of the regenerating retina to stimuli that lead to increased ODC activity. While the nature of the in vivo signal(s) may be complex, it is inferred that degenerative changes in the distal stump play a significant role.

Ornithine decarboxylase activity in retinal explants of goldfish undergoing optic nerve regeneration

Retinal explants cultured in the presence of fetal calf serum (FCS) exhibit an increase in the activity of ornithine decarboxylase which is maximal several hours after explantation. The measured activity of this enzyme is higher in explants of retinas whose optic nerve had been crushed several days previously (post-crush, PC) than in control (normal, N) retinas. The addition of rabbit antibodies against nerve growth factor (NGF) to the incubation medium does not block this stimulatory effect of FCS, a result suggesting that factors other than NGF present in FCS are responsible for the observed stimulation. An inhibitor of ODC synthesis, diaminopropane (DAP), and an irreversible inhibitor of ODC activity, alpha-DL-difluoromethylornithine (alpha-DFMO), each suppressed the FCS-stimulated ODC activity when added to the culture medium. Since FCS addition also promotes neuritic outgrowth from PC goldfish retinal explants, we explored the possible relationship of the stimulated ODC activity and the ability of explants to extend neurites. Concentrations of DAP or alpha-DFMO that block ODC activity also suppress neuritic outgrowth. Possible non-specific actions of the drugs unrelated to the block of ODC are examined. While the increases in OCD activity seen in PC and N goldfish retinas explanted into FCS-containing medium may be a requisite, they cannot be sufficient to support neuritic outgrowth. Intrinsic changes in the retinal explant secondary to crush of its optic nerve as well as factor(s) present in FCS that may be unrelated to the stimulation of ODC activity also appear necessary for neuritic outgrowth.