Retinal ganglion cell response to axotomy and nerve growth factor antiserum treatment in the regenerating visual system of the goldfish (Carassius auratus): an in vivo and in vitro analysis

Biologic Drugs as Analgesics for the Management of Low Back Pain and Sciatica

Objective

To discuss the current knowledge on the impact of commonly used biologic agents (i.e., anti–tumor necrosis factor–alpha [anti-TNF-α] and anti–nerve growth factor [anti-NGF]) in the management of low back pain with or without sciatica.

Methods

A narrative literature review of studies investigating the use of biologic agents for the management of low back pain and sciatica was conducted. We searched MEDLINE and EMBASE for English language publications. A hand-search of reference lists of relevant studies was also performed.

Results

Although some observational studies showed that inhibition of TNF-α reduced pain and improved function, randomized controlled trials and a meta-analysis failed to demonstrate the superiority of anti-TNF-α over placebo in this regard. Anti-TNF-α, however, reduced the risk of having invasive procedures such as discectomy and radicular block in cases of sciatica. Conversely, controlled studies showed moderate pain reduction and mild functional improvement with anti-NGF administration, but the side effect profile of anti-NGF was unfavorable compared with placebo.

Conclusions

Overall, anticytokine treatments have limited efficacy in patients with chronic low back pain with or without sciatica. However, larger and better-designed studies may need to be performed in specific patient subpopulations. Low back pain is particularly disabling in younger patients. This group therefore represents a potential target population for investigating the effectiveness of anticytokine therapies, especially where other pharmacological and nonpharmacological management strategies have failed.

Axonal Regeneration of Fish Optic Nerve after Injury

Since Sperry's work in the 1950s, it has been known that the central nervous system (CNS) neurons of lower vertebrates such as fish and amphibians can regenerate after axotomy, whereas the CNS neurons of mammals become apoptotic after axotomy. The goldfish optic nerve (ON) is one of the most studied animal models of CNS regeneration. Morphological changes in the goldfish retina and tectum after ON transection were first researched in the 1970s-1980s. Many biochemical studies of neurite outgrowth-promoting substances were then carried out in the 1980s-1990s. Many factors have been reported to be active substances that show increased levels during fish ON regeneration, as shown by using various protein chemistry techniques. However, there are very few molecular cloning techniques for studying ON regeneration after injury. In this review article, we summarize the neurite outgrowth-promoting factors reported by other researchers and describe our strategies for searching for ON regenerating molecules using a differential hybridization technique in the goldfish visual system. The process of goldfish ON regeneration after injury is very long. It takes about half a year from the start of axonal regrowth to complete restoration of vision. The process has been classified into three stages: early, middle and late. We screened for genes with increased expression during regeneration using axotomized goldfish retinal and tectal cDNA libraries and obtained stage-specific cDNA clones that were upregulated in the retina and tectum. We further discuss functional roles of these molecules in the regeneration processes of goldfish ON.

Neurotrophins and their receptors in the tench retina during optic nerve regeneration

To understand the role of neurotrophins in the visual system, we investigated the distribution of both neurotrophins and their receptors within the retina of a fish that has the capacity to spontaneously regenerate its optic nerve axons after lesion. Intact retinas and retinas from tench, whose optic nerve had been crushed, were analyzed by immunohistochemistry and in situ hybridization. Trk receptors were mainly immunolocalized in cells of the inner nuclear and ganglion cell layers, a distribution coincident with that of their mRNAs. Nerve growth factor (NGF) immunoreactivity was detected exclusively in Müller cell processes, and brain-derived neurotrophic factor (BDNF) was found in both neuronal bodies and Müller cell processes. Neurotrophin-3 (NT-3) was detected in most of the cell nuclei, and neurotrophin-4/5 (NT-4/5) was localized in fibers and in a few cells in the inner retina. An increase in both TrkA protein and mRNA was detected during axonal regeneration within the retinal ganglion cell layer, reaching a maximum 30 days postcrush and returning to normal levels by day 90, when optic nerve regeneration is almost completed in this fish. None of the other neurotrophins and receptors showed appreciable changes. The heterogeneous distribution patterns of neurotrophins and their receptors in fish retina, their differences from the distribution observed in other species, and the TrkA changes after optic nerve crush suggest an important role for these molecules in the normal physiology of the fish retina and during the regeneration process.

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.

SR 57746A attenuates cytostatic drug-induced reduction of neurite outgrowth in co-cultures of rat dorsal root ganglia and Schwann cells

A co-culture system of intact rat dorsal root ganglia (DRG) with Schwann cells was used to evaluate the potential neurotrophic activity of SR 57746A. Neuritogenesis from DRG was measured with an image analysis system following exposure to different concentrations of SR 57746A. Neurite outgrowth of intact DRG was increased by SR 57746A and this was more obvious in the presence of co-cultured Schwann cells. The neuroprotective properties of SR 57746A were studied in co-cultures of DRG and Schwann cells, in which neuritogenesis was reduced by the cytostatic drugs cisplatin, vincristine and taxol. It was found that neurite outgrowth from DRG treated with cisplatin (3 micrograms/ml) and 10 microM SR 57746A for 3 days was significantly higher than after treatment with cisplatin alone. Similarly, neuritogenesis from DRG treated with taxol (0.01 microgram/ml) or vincristine (0.5 ng/ml) in combination with 10 microM SR 57746A was significantly increased compared to treatment with taxol or vincristine alone. When intact DRG were incubated in vitro with 3 micrograms/ml cisplatin and without Schwann cells, 10 microM SR 57746A also had a neuroprotective effect. These data suggest that SR 57746A has neuroprotective potential and that this effect does not depend solely on the presence of Schwann cells.

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.

Enhancement of peripheral nerve regeneration

Numerous factors external to the nerve cell can support and enhance nerve regeneration after injury. The definition of these factors and the elucidation of their mechanisms of action are the central goals of much contemporary neurobiologic research. This research will hopefully lead to the discovery of factors that will prove to be therapeutically beneficial for patients with either peripheral nervous system (PNS) injury or central nervous system (CNS) injury. This article reviews the biology of the regeneration response of the nerve to injury and discusses many of the factors that enhance nerve growth. Finally, the nerve guide or nerve regeneration chamber model for the evaluation of putative nerve regeneration enhancing agents in vivo is also discussed.

Growth factors in the eye

In this review we report the distribution and functional significance of growth factors in the eye. Representatives of the major growth factor families are found in the eye: fibroblast growth factor, insulin and insulin-like growth factor, transforming growth factor-beta, platelet-derived growth factor, nerve growth factor, epidermal growth factor and colony-stimulating factor. There are numerous examples of their actions on ocular tissues in vitro and in some cases in vivo. The findings presented clearly illustrate that a growth factor can elicit different responses depending on the context of its action; the cell type involved, the concentration of the growth factor and the presence or absence of other growth factors can all influence the cellular response both quantitatively and qualitatively. The results of these studies in the eye are of general significance to our understanding of the role of growth factors in biological processes.

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.

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.

Embryonic and regenerating Xenopus retinal fibers are intrinsically different

Growth and guidance behavior of Xenopus embryonic (ER) (optic vesicle stage 25/26) and regenerating retinal fibers (stage 47/50 newly regenerating NR, and actively regenerating RR, respectively) have been studied in vitro on a variety of substrates in serum-free media. RR retinas receive a prior conditioning lesion 12-14 days before explantation while NR retinas are explanted immediately after axotomy. The substrates include plastic (UN), polylysine (PL), polyornithine (PO), laminin (LM), fibronectin (FN), and collagen type I (CO). Two kinds of experimental situations were tested, one in which substrates were derivatized to plastic as a planar surface, while the second involved the addition of a substrate as a soluble supplement to dishes derivatized with PL. A neurite growth index (NGI), based on density of neurite outgrowth and axon lengths, is determined for each fiber type on all substrates. Embryonic and regenerating fibers are phenotypically different fiber types; each displays a specific "substrate preference profile" (SPP), reflecting differential growth on each substrate. ER neurites grow equally well on all planar substrates, including plastic, but do not grow on CO (SPP, LM = FN = PL = PO = UN greater than CO). Both NR and RR neurites show distinct substrate preferences, but RR neurites grow more vigorously (SPP, LM greater than CO greater than PL = PO greater than FN). In media supplemented with LM, FN or CO, the SPPs showed little change but the neurite bundle patterns were qualitatively different. Only regenerating neurites display clockwise growth in laminin (LM) and fibronectin (FN)-supplemented media. Under no conditions do embryonic fibers exhibit this pattern which suggests that embryonic and regenerating retinal fibers also differ in cytoskeletal organization. Evidence of intrinsic growth differences in vitro suggest that embryonic and regenerating retinal fibers may not respond to identical guidance cues during in vivo development and regeneration of retinotectal connections.

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.

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.