Enhanced labeling of a retinal protein during regeneration of optic nerve in goldfish

In memoriam: Bernard W. Agranoff (1926-2022)

This is an obituary to Bernard William Agranoff M.D., who died on October 21, 2022. One of the leading figures in the field of neuroscience, he studied the biochemistry of the brain and was the first to show that long-term memory formation required protein synthesis. He made numerous other seminal discoveries related to the fields of inositol lipids and cell signaling, neuroplasticity, and brain imaging and mentored a large cadre of trainees.

zRICH, a protein induced during optic nerve regeneration in zebrafish, promotes neuritogenesis and interacts with tubulin

Mammals do not regenerate axons in their central nervous system (CNS) spontaneously. This phenomenon is the cause of numerous medical conditions after damage to nerve fibers in the CNS of humans. The study of the mechanisms of nerve regeneration in other vertebrate animals able to spontaneously regenerate axons in their CNS is essential for understanding nerve regeneration from a scientific point of view, and for developing therapeutic approaches to enhance nerve regeneration in the CNS of humans. RICH proteins are a novel group of proteins implicated in nerve regeneration in the CNS of teleost fish, yet their mechanisms of action are not well understood. A number of mutant versions of the zebrafish RICH (zRICH) protein were generated and characterized at biochemical and cellular levels in our laboratory. With the aim of understanding the effects of RICH proteins in neuronal axon outgrowth, stable transfectants derived from the neuronal model PC12 cell line expressing zRICH Wild-Type or mutant versions of zRICH were studied. Results from differentiation experiments suggest that RICH proteins enhance neuronal plasticity by facilitating neurite branching. Biochemical co-purification results have demonstrated that zRICH binds to the cytoskeletal protein tubulin. The central domain of the protein is sufficient for tubulin binding, but a mutant version of the protein lacking the terminal domains, which cannot bind to the plasma membrane, was not able to enhance neurite branching. RICH proteins may facilitate axon regeneration by regulating the axonal cytoskeleton and facilitating the formation of new neurite branches.

Heterogeneous nuclear ribonucleoprotein K, an RNA-binding protein, is required for optic axon regeneration in Xenopus laevis

Axotomized optic axons of Xenopus laevis, in contrast to those of mammals, retain their ability to regenerate throughout life. To better understand the molecular basis for this successful regeneration, we focused on the role of an RNA-binding protein, heterogeneous nuclear ribonucleoprotein (hnRNP) K, because it is required for axonogenesis during development and because several of its RNA targets are under strong post-transcriptional control during regeneration. At 11 d after optic nerve crush, hnRNP K underwent significant translocation into the nucleus of retinal ganglion cells (RGCs), indicating that the protein became activated during regeneration. To suppress its expression, we intravitreously injected an antisense Vivo-Morpholino oligonucleotide targeting hnRNP K. In uninjured eyes, it efficiently knocked down hnRNP K expression in only the RGCs, without inducing either an axotomy response or axon degeneration. After optic nerve crush, staining for multiple markers of regenerating axons showed no regrowth of axons beyond the lesion site with hnRNP K knockdown. RGCs nonetheless responded to the injury by increasing expression of multiple growth-associated RNAs and experienced no additional neurodegeneration above that normally seen with optic nerve injury. At the molecular level, hnRNP K knockdown during regeneration inhibited protein, but not mRNA, expression of several known hnRNP K RNA targets (NF-M, GAP-43) by compromising their efficient nuclear transport and disrupting their loading onto polysomes for translation. Our study therefore provides evidence of a novel post-transcriptional regulatory pathway orchestrated by hnRNP K that is essential for successful CNS axon regeneration.

A vulnerable period of colchicine toxicity during goldfish optic nerve regeneration

The effects of intraocular (i.o.) administration of the alkaloid colchicine on visual recovery following axotomy of the goldfish optic nerve were investigated. Under the experimental conditions used, control goldfish recovered vision, measured behaviorally, within 5-7 weeks of retro-orbital optic nerve crush. Fish injected i. o. with 0.1 microg of colchicine within 3 days of optic nerve crush (post-crush; PC) recovered vision after some delay relative to control fish, while injection with colchicine between 7 and 14 days PC produced a much more profound inhibition of recovery of vision, in most cases a complete block for the duration of the study (98 days). Further evidence for a delayed susceptibility of the regenerating optic nerve to colchicine following crush was reflected in a suppression of neurite outgrowth normally seen in explanted retinal tissue taken from PC goldfish. In addition, retrograde transport of the fluorescent dye 4-(4-didecylaminostyryl)-N-methylpyridinium iodide from the optic tectum to the retina as a measure of axonal continuity revealed substantially less labeling following i.o. administration of colchicine 1 week PC when compared to retinas from fish receiving colchicine at the time of optic nerve crush. Histological sections of the retina showed no evidence of residual retinal damage resulting from the colchicine injections or from interactions of axotomy and the drug administration. These results indicate a period of increased vulnerability of the regenerating visual system to the toxic effects of i.o. administered colchicine, beginning 3-5 days PC, and remaining until regenerating optic nerve fibers have begun to reach the tectum. While colchicine has many known effects on nerve function, it is proposed that the delayed susceptibility to disruption of regeneration observed in these experiments is largely, if not entirely, attributable to a colchicine-induced accumulation of tubulin heterodimers, which are known to block microtubule assembly and to participate in a feedback inhibition of tubulin synthesis. Thus, it is during the maximal induction of tubulin synthesis and of microtubule formation which normally occurs several days following axotomy that colchicine has its greatest effect. The results suggest that colchicine may be especially neurotoxic during neural development and regeneration.

gRICH68 and gRICH70 are 2',3'-cyclic-nucleotide 3'-phosphodiesterases induced during goldfish optic nerve regeneration

Biochemical characterization of changes in gene expression that accompany optic nerve regeneration has led to the identification of proteins that may play key roles in the regeneration process. In this report, a cDNA encoding gRICH70, a novel isoform of the regeneration-induced gRICH68 protein, has been identified and characterized in goldfish. Both gRICH68 and gRICH70 show significant homology (34-36%) to mammalian 2',3'-cyclic-nucleotide 3'-phosphodiesterases (CNPases), hence the name goldfish regeneration-induced CNPase homolog (gRICH). The predicted 431-amino acid gRICH70 protein is 88% homologous to gRICH68, and the retinal mRNA for gRICH70 is coordinately induced with gRICH68 mRNA during optic nerve regeneration. Enzymatic analysis of recombinant proteins confirms that both gRICH proteins possess CNPase activity. Despite the relatively limited sequence homology, the kinetic constants obtained suggest that both gRICH proteins are at least as efficient as recombinant mouse CNP1 in catalyzing the hydrolysis of 2',3'-cAMP. Immunoprecipitation studies indicate that gRICH proteins are responsible for the majority of the CNPase activity detected in regenerating goldfish retinas. The evidence presented demonstrates that gRICH68 and gRICH70 correspond to a previously described doublet of acidic proteins that are selectively induced in the goldfish retina during optic nerve regeneration. Thus, CNPase enzyme activity is implicated for the first time in the process of nerve regeneration.

Wortmannin blocks goldfish retinal phosphatidylinositol 3-kinase and neurite outgrowth

The goldfish retina has been used extensively for the study of nerve regeneration. A role for phosphatidylinositol 3-kinase (PI3K) in neurite outgrowth from goldfish retinal explants has been examined by means of wortmannin (WT), a selective inhibitor of the enzyme. The presence of PI3K in retinal extracts was determined by means of immunoprecipitation as well as by an in vitro assay system for catalytic activity. The relative amount of the p85 subunit of PI3K detected by western blot in the retina following optic nerve crush was unchanged. WT inhibited goldfish brain PI3K activity at concentrations as low as 10(-9) M, approximating that reported for inhibition of mammalian PI3K's. Daily addition of 10(-8) M WT to retinal explants, activated by prior crush of the optic nerve, significantly inhibited neurite outgrowth during a 7 day in vitro culture period, while a single addition of WT to freshly explanted retina had no effect on neurite outgrowth. These results suggest that a PI3K-mediated process may be critical for nerve regrowth.

Isolation of cDNA clones encoding RICH: a protein induced during goldfish optic nerve regeneration with homology to mammalian 2',3'-cyclic-nucleotide 3'-phosphodiesterases

Using data derived from peptide sequencing of p68/70, a protein doublet induced during optic nerve regeneration in goldfish, we have isolated cDNAs that encode RICH (regeneration-induced CNPase homolog) from a goldfish regenerating retina cDNA library. The predicted RICH protein comprises 411 amino acids, possesses a pI of 4.48, and shows significant homology to the mammalian myelin marker enzyme 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNPase; EC 3.1.4.37). The mRNA encoding RICH was demonstrated, by both Northern blot analysis and RNase protection assays, to be induced as much as 8-fold in regenerating goldfish retinas at 20 days after nerve crush. Analysis of total RNA samples from various tissues showed a broad distribution of RICH mRNA, with the highest levels observed in gravid ovary. The data obtained strongly suggest that RICH is identical or very similar to p68/70. The molecular cloning of RICH provides the means for a more detailed analysis of its function in nerve regeneration. Additionally, the homology of RICH and CNPase suggests that further investigation may provide additional insight into the role of these proteins in the nervous system.

Rates of protein synthesis in the regenerating hypoglossal nucleus: effects of testosterone treatment

Rates of protein synthesis (ICPSleu) along the entire rostral to caudal extent of the hypoglossal nucleus were determined in adult, female rats with the quantitative autoradiographic L-[1-14C]leucine method two and five weeks after unilateral hypoglossal axotomy with and without chronic treatment with testosterone. Rates of protein synthesis were increased on the axotomized side, and the increases were greater in the rostral portion of the nucleus at both time points examined. The effects of axotomy on ICPSleu were less at five weeks post-axotomy than at two weeks. In spite of the fact that testosterone has been shown to accelerate both the rate of outgrowth of regenerating cranial motor nerves (Kujawa et al., J. Neurosci. 11:3898-3906, 1991) and the recovery of function (Kujawa et al., Exp. Neurol. 105:80-85, 1989) and to attenuate the loss of neurons (Yu et al., Exp. Neurol. 80:349-360, 1983) there were no effects of testosterone on 1CPSleu in the hypoglossal nucleus in either sham-operated or axotomized rats.

Homeobox genes are expressed in the retina and brain of adult goldfish

The goldfish (Carassius auratus) visual pathway displays continuous growth and plasticity throughout life. Since homeobox genes are important transcriptional regulators in development, we searched for homeobox genes in the adult goldfish retina and brain. Using the PCR, we discovered a repertoire of homeobox sequences expressed in these tissues. In addition to isolating homeodomain sequences found in the vertebrate Hox gene clusters, a sequence identical to the chicken CHox7 homeodomain was characterized. Furthermore, a sequence with significant homologies to the Xenopus XIHbox8 and leech Htr-A2 homeodomains was identified, and these sequences may define an additional class of homeodomain. Finally, a sequence belonging to the paired class (prd) of homeodomains is reported. Homeobox gene expression in the adult goldfish retina and brain may be associated with the persistent developmental features of these tissues.

Changes in chromatin proteins during optic nerve regeneration in the goldfish

Regeneration of the goldfish optic nerve involves massive changes in the structure and pattern of macro-molecular synthesis in the retinal ganglion cells. To explore the mechanisms that underlie these events, we investigated the changes in chromatin proteins during the course of regeneration. Three major retinal chromatin proteins, two with apparent molecular weights of 58 kDa (C1 and C2) and one at 51 kDa (C3), all having isoelectric points around 5.5, showed a fourfold increase in their synthesis and/or accumulation by 14 days of regeneration. Synthesis of C1 and C3 decreased by day 32, the time at which the axons have grown back to the optic tectum and have formed many of their synapses; synthesis of C2 remained high through day 32. All three proteins bound to DNA-cellulose and required high salt concentrations (0.2-0.5 M KCl) to be eluted. C1 and C2 had similar proteolytic digestion patterns and reacted with monoclonal antibodies that recognize the goldfish intermediate filament proteins of the ON complex. The proteins identified here could be involved in structural alterations in the chromatin, or might serve as transcription factors to regulate gene expression during nerve regeneration.

Changes in cytoskeletal protein synthesis following axon injury and during axon regeneration

Injury to the axons of facial motoneurons stimulates increases in the synthesis of actin, tubulins, and GAP-43, and decreases in the synthesis of neurofilament proteins: mRNA levels change correspondingly. In contrast to this robust response of peripheral neurons to axotomy, injured central nervous system neurons show either an attenuated response that is subsequently aborted (rubrospinal neurons) or overall decreases in cytoskeletal protein mRNA expression (corticospinal and retinal ganglion neurons). There is evidence that these changes in synthesis are regulated by a variety of factors, including loss of endoneurially or target-derived trophic factors, positive signals arising from the site of injury, changes in the intraaxonal turnover of proteins, and substitution of target-derived trophic support by factors produced by glial cells. It is concluded that there is, as yet, no coherent explanation for the upregulation or downregulation of any of the cytoskeletal proteins following axotomy or during regeneration. In considering the relevance of these changes in cytoskeletal protein synthesis to regeneration, it is emphasized that they are unlikely to be involved in the initial outgrowth of the injured axons, both because transit times between cell body and injury site are too long, and because sprouting can occur in isolated axons. Injury-induced acceleration of the axonal transport of tubulin and actin in the proximal axon is likely to be more important in providing the cytoskeletal protein required for initial axonal outgrowth. Subsequently, the increased synthesis and transport velocity for actin and tubulin increase the delivery of these proteins to support the increased volume of the maturing regenerating axons. Reduction in neurofilament synthesis and changes in neurofilament phosphorylation may permit the increased transport velocity of the other cytoskeletal proteins. There is little direct evidence that alterations in cytoskeletal protein synthesis are necessary for successful regeneration, nor are they sufficient in the absence of a supportive environment. Nevertheless, the correlation that exists between a robust cell body response and successful regeneration suggests that an understanding of the regulation of cytoskeletal protein synthesis following axon injury must be a part of any successful strategy to improve the regenerative capacity of the central nervous system.

Target-dependent regulation of retinal nicotinic acetylcholine receptor and tubulin RNAs during optic nerve regeneration in goldfish

A fundamental issue in central nervous system development regards the effect of target tissue on the differentiation of innervating neurons. We address this issue by characterizing the role the retinal ganglion cell target, i.e., the optic tectum, plays in regulating expression of tubulin and nicotinic acetylcholine receptor genes in regenerating retinal ganglion cells. Tubulins are involved in axonal growth, whereas nicotinic acetylcholine receptors mediate communication across synapses. Retinal ganglion cell axons were induced to regenerate by crushing the optic nerve. Following crush, there was a rapid increase in alpha-tubulin RNAs (3 days), which preceded the increase in nicotinic acetylcholine receptor RNAs (10-15 days). Both classes of RNAs approached control levels by the time retinotectal synapses and functional recovery were restored (4-6 weeks). If the optic nerve was repeatedly crushed or its target ablated, tubulin RNAs remained elevated, and the increase in receptor RNAs that would otherwise be seen 2 weeks after a single nerve crush did not occur. The interaction of retinal ganglion cell axons with their targets in the optic tectum appears, then, to exert a suppressive effect on the RNA encoding a cytoskeletal protein, tubulin, and an inductive effect on RNAs encoding nicotinic acetylcholine receptors involved in synaptic communication.

Neuronal intermediate filament expression during neurite outgrowth from explanted goldfish retina: effect of retinoic acid

Regulation of the goldfish neuronal intermediate filament proteins ON1 and ON2 was investigated in a retinal explant system. The synthesis of these proteins in explanted retina decreased with increasing time in culture, despite continuing neurite outgrowth. Thus, ON1/ON2 neurofilament expression is regulated independently from neurite outgrowth. During regeneration of the goldfish optic nerve in vivo, the expression of these proteins increased during the later phase of the process, when growing axons make contact with the optic tectum. The declining synthesis of ON1 and ON2 during neurite outgrowth in culture suggests that factors extrinsic to the retina are necessary to support synthesis of these proteins. Treating retinal explants with retinoic acid stimulated the synthesis of the ON1/ON2 proteins in a dose-dependent manner. This stimulation was effective during a period of declining synthesis of the ON1/ON2 proteins, restoring their synthesis towards initial levels of expression. These results show that retinoic acid serves as a modulator of neurofilament expression in this in vitro model of nerve regeneration.

Changes in alpha-tubulin and actin gene expression during optic nerve regeneration in frog retina

The optic nerve of the bullfrog was transected and the regeneration process was investigated. We previously reported that alpha-tubulin mRNA in the retina increased to a maximum 1-2 h after optic nerve transection with no specific change in actin mRNA. In the present investigation, we examined the long-term effect of optic nerve transection. Northern blot analysis revealed that alpha-tubulin mRNA increased again gradually after the rapid and transient increase and actin mRNA increased to a maximum at 7 days (more than twofold compared to the control retinas). The period during which actin mRNA reaches a maximal increase almost corresponds to the time lag between the axotomy and the initiation of axonal outgrowth. The main cytoskeletons of neuronal growth cones have been shown to consist of actin-containing microfilaments. Therefore, the transient increase of actin mRNA may have a relationship to the initial outgrowth of axons. On the other hand, the rapid and transient increase of alpha-tubulin mRNA observed in our previous studies is probably one of the initial responses of retinal ganglion cells to the axotomy, and the gradual increase in alpha-tubulin mRNA observed in this study can probably be interpreted as provision of the structural materials necessary for axonal elongation.

Phosphorylation of superior cervical ganglion proteins during regeneration

The incorporation of radioactive phosphate into proteins of both normal and regenerating ganglia of the sympathetic nervous system of the rat is reported. The incorporation reactions were carried out in vitro by incubating homogenates of excised ganglia with [gamma-32P]ATP under various conditions. It was found that incorporation of phosphate into proteins of regenerating ganglia in the molecular mass range 10,000-100,000 daltons increased up to 40% over incorporation into proteins from control ganglia during the first 3 days following injury and returned to control levels after 14 days. Analysis of the proteins by two-dimensional electrophoresis revealed that only few, i.e., less than 20, became radioactively labelled in homogenates of superior cervical ganglia in the presence of Ca2+, and even fewer in the presence of cyclic AMP. Furthermore, all these proteins fell within a narrow pI range of 4-6. The growth-associated protein, variously designated GAP-43, B-50, F-1, and pp46, has an enhanced level of expression and phosphorylation in regenerating ganglia compared with controls at day 3. Injury also caused consistently higher levels of incorporation into two other proteins with molecular masses at positions 55,000 and 85,000 and pI values of 5.1 and 4.5, respectively; the former protein most probably is beta-tubulin. The fact that both proteins are found in the 15,000 g pellet after the tissue has been solubilized in 0.5% nonionic detergent indicates that they may indeed by components of filament assemblies. Thus, the results suggest that protein phosphorylation is a mechanism involved in cytoskeletal function in regenerating nerve.

Characterization of a goldfish antigen during development and regeneration of the visual system

Normal, regenerating, and developing optic nerves of the goldfish were studied utilizing a monoclonal antibody (mAb) 1E1T which has specificity for Müller cells in the retina, radial glial cells in the tectum, and non-neuronal cells in the optic nerve. Sections of the normal optic nerve revealed longitudinally oriented chains of non-neuronal cells, that were 4-8 cells long. The number of chains in the normal nerve was very few. In addition, short acellular septa, probably the connective tissue septa, were also labeled with mAb 1E1T. Sections of crushed optic nerves showed an increase in the antigen recognized by mAb 1E1T within the septa and new septa were now visualized. Furthermore, the existing septa were longer and extended the length of the optic nerve. The formation and elongation of the septa occurred as early as 3 day postcrush. Between 3 and 11 d postcrush, there was heavy labeling of the septa and a large accumulation of non-neuronal cells at the crush site. At 3 months postcrush, the accumulation of non-neuronal cells labeled by mAb 1E1T were no longer visible but heavy labeling of the septa was still apparent.

Expression of a 48-kilodalton growth-associated protein in the goldfish retina

One of the most striking molecular correlates of optic nerve regeneration in the goldfish is the increased labeling of a 48 kilodalton (kD) acidic protein that is conveyed to the developing nerve endings from the retina by rapid axonal transport. The present study examined the biosynthesis and molecular characteristics of this protein. Retinas derived either from intact controls or from goldfish undergoing optic nerve regeneration (10-14 days postcrush) were pulse-labeled with [3H]proline or [35S]methionine, followed by subcellular fractionation and analysis of protein synthesis patterns by two-dimensional gel electrophoresis and fluorography. Synthesis of the 48-kD acidic protein (termed here GAP-48) was detected only in retinas that were undergoing axonal regeneration. Pulse-chase labeling experiments demonstrated that the protein undergoes a post-translational modification that requires 15-20 min. This processing could be selectively blocked by tunicamycin, an inhibitor of protein N-glycosylation. The protein was also found to incorporate low levels of phosphate in vitro. Thus, the differential appearance of GAP-48 in regenerating axons might be regulated either at the level of gene expression or by selective posttranslational processing in retinal ganglion cells. By the criteria of molecular weight, isoelectric point, anomalous migration properties on sodium dodecyl sulfate-polyacrylamide gels, phosphorylation, subcellular distribution, and the pattern of digestion products generated by Staphylococcus aureus V8 protease, GAP-48 appears to be equivalent to the B-50 (F-1) phosphoprotein of the mammalian brain.

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.

Elevated levels of retinal neurofilament mRNA accompany optic nerve regeneration

RNA isolated from goldfish retinas before and during optic nerve regeneration, when translated in vitro, directed the synthesis of neurofilament proteins that are normally found in high levels in the optic nerve. The major neurofilament proteins of the goldfish optic nerve comprise a group of four isoelectric variants of molecular weight 58,000 (58K) which we have identified previously as ON1-ON4. The levels of ON1 and ON2 within the optic nerve had been shown to decrease shortly after optic nerve crush and then increase to precrush levels during the regeneration process. Employing two-dimensional electrophoretic analysis of in vitro translation products and immunoprecipitations with antibodies specific for the ON proteins and an anti-intermediate filament monoclonal antibody, we show that ON1 and ON2 are encoded by mRNA synthesized in the retinas. The synthesis of ON3 and ON4 by retina RNA was undetected. This confirms data from previous ex vivo experiments that indicated that ON1 and ON2 are of neuronal origin whereas ON3 and ON4 are nonneuronal. ON1 and ON2 synthesis increases dramatically during optic nerve regeneration to levels 10- and 30-fold over precrush levels, respectively. In addition to ON1 and ON2, the synthesis of a previously unidentified 52K protein is observed at relatively high levels 20 and 32 days after optic nerve crush, but is unobserved before regeneration. Thus, optic nerve regeneration can be correlated with specific changes in intermediate filament gene expression within the retina.

Immunohistochemical localization of intermediate filament proteins of neuronal and nonneuronal origin in the goldfish optic nerve: specific molecular markers for optic nerve structures

The predominant proteins (58K) of the intermediate filament complex in the goldfish visual pathway consist of a series of isoelectric variants. Previous biochemical studies have shown that proteins ON1 and ON2 are of neuronal origin, whereas ON3 and ON4 are of nonneuronal origin. Polyclonal antibodies, purified by affinity chromatography, that are specific for ON1 and ON2 or ON3 and ON4 have been used to localize histologically the ON proteins within the normal and crushed optic nerve. Anti-ON1/ON2 antiserum presented a pattern consistent with intraaxonal staining. A nonneuronal staining pattern was observed with anti-ON3/ON4 antiserum. The two patterns were distinct from and complementary to each other. The data suggest that ON3 and ON4 represent a novel glial fibrillary acidic protein. The results are discussed in terms of the function of these proteins in development, plasticity, and regeneration.

Homology and diversity between intermediate filament proteins of neuronal and nonneuronal origin in goldfish optic nerve

The predominant intermediate filament proteins of the goldfish optic nerve have molecular weights of 58K. They can be separated into a series of four major isoelectric variants of neuronal (ON1 and ON2) and nonneuronal (ON3 and ON4) origin. The extent of homology between the goldfish 58K intermediate filament proteins themselves and to rat optic nerve vimentin and glial fibrillary acidic protein (GFAP) was investigated. Unlabeled and [32P]orthophosphate-labeled proteins were subjected to partial hydrolysis by V8 protease, chymotrypsin, and CNBr. The results show that the goldfish intermediate filament proteins share with vimentin and GFAP a 40K chymotrypsin-resistant core fragment. Phosphorylated moieties appear to be located outside the core region since they are preferentially cleaved off by chymotrypsin and not found associated with the 40K core. In addition, the goldfish ON proteins contain the antigenic site within the core that is common to most intermediate filaments. V8 or CNBr digestion indicates that many fragments that are common to ON1 and ON2 are clearly distinct from fragments that are common to ON3 and ON4. In addition, structural variability is observed between the goldfish intermediate filament proteins and vimentin and GFAP. The results are discussed in terms of intermediate filament structure and their possible role in nerve growth.

Alterations in mRNA translation products associated with regenerative responses in the retina

Translation products of mRNA from retinas of goldfish optic nerve (representing a regenerative CNS) and adult rabbit optic nerve (representing a nonregenerative CNS which can be induced to express regenerative characteristics) were examined by one- and two-dimensional gel electrophoresis. Translation products from retinas of the regenerating goldfish optic nerve included polypeptides barely detectable in the translation products of mRNA derived from retinas of uninjured controls. Some of these polypeptides, of apparent molecular weights 24-28, 43-49, 60, and 65 kilodaltons can be considered as growth-associated polypeptides described in other regenerative and developing systems. The induction of regeneration-associated characteristics in the injured adult rabbit optic nerve, "implanted" with diffusible substances from nonneuronal cells of regenerative or growing nerve, is reflected by changes in the mRNA translation products of the retina. Among such translation products are those of the following molecular weights: 16-18, 28, 32-35, 43-47, and 56-60 kilodaltons, and some higher-molecular-weight species.

Regenerating fish optic nerves and a regeneration-like response in injured optic nerves of adult rabbits

Regeneration of fish optic nerve (representing regenerative central nervous system) was accompanied by increased activity of regeneration-triggering factors produced by nonneuronal cells. A graft of regenerating fish optic nerve, or a "wrap-around" implant containing medium conditioned by it, induced a response associated with regeneration in injured optic nerves of adult rabbits (representing a nonregenerative central nervous system). This response was manifested by an increase of general protein synthesis and of selective polypeptides in the retinas and by the ability of the retina to sprout in culture.

Changes in protein content of goldfish optic nerve during degeneration and regeneration following nerve crush

After the goldfish optic nerve was crushed, the total amount of protein in the nerve decreased by about 45% within 1 week as the axons degenerated, began to recover between 2 and 5 weeks as axonal regeneration occurred, and had returned to nearly normal by 12 weeks. Corresponding changes in the relative amounts of some individual proteins were investigated by separating the proteins by two-dimensional gel electrophoresis and performing a quantitative analysis of the Coomassie Brilliant Blue staining patterns of the gels. In addition, labelling patterns showing incorporation of [3H]proline into individual proteins were examined to differentiate between locally synthesized proteins (presumably produced mainly by the glial cells) and axonal proteins carried by fast or slow axonal transport. Some prominent nerve proteins, ON1 and ON2 (50-55 kD, pI approximately 6), decreased to almost undetectable levels and then reappeared with a time course corresponding to the changes in total protein content of the nerve. Similar changes were seen in a protein we have designated NF (approximately 130 kD, pI approximately 5.2). These three proteins, which were labelled in association with slow axonal transport, may be neurofilament constituents. Large decreases following optic nerve crush were also seen in the relative amounts of alpha- and beta-tubulin, which suggests that they are localized mainly in the optic axons rather than the glial cells. Another group of proteins, W2, W3, and W4 (35-45 kD, pI 6.5-7.0), which showed a somewhat slower time course of disappearance and were intensely labelled in the local synthesis pattern, may be associated with myelin. A small number of proteins increased in relative amount following nerve crush. These included some, P1 and P2 (35-40 kD, pIs 6.1-6.2) and NT (approximately 50 kD, pI approximately 5.5), that appeared to be synthesized by the glial cells. Increases were also seen in one axonal protein, B (approximately 45 kD, pI approximately 4.5), that is carried by fast axonal transport, as well as in two axonal proteins, HA1 and HA2 (approximately 60 and 65 kD respectively, pIs 4.5-5.0), that are carried mainly by slow axonal transport. Other proteins, including actin, that showed no net changes in relative amount (but presumably changed in absolute amount in direct proportion to the changes in total protein content of the nerve), are apparently distributed in both the neuronal and nonneuronal compartments of the nerve.

Substances originating from the optic nerve of neonatal rabbit induce regeneration-associated response in the injured optic nerve of adult rabbit

We have recently shown that cell bodies of an injured optic nerve of adult rabbit can be induced to express regeneration-associated response by external signals derived from nonneuronal cells of regenerating nerves of lower vertebrates. In this study it is shown that even substances derived from a nonregenerating mammalian system also can trigger such a regenerative response. Thus, substances derived from intact nerves of neonatal rabbits and of adult rabbits, to a lesser extent, were active in triggering a regeneration-associated response, whereas substances derived from injured nerves of adult rabbit were not. However, if subsequent to the injury the nerve was implanted with silicone tube containing medium conditioned by neonatal optic nerves, the substances derived from the implanted injured nerve were active. Thus, it appears that the ability of a periaxonal environment to provide triggering substances correlates with axonal growth. Therefore, we named these substances "growth-associated triggering factors" (GATFs). It is suggested that mammalian cells are unable to express a regenerative response after an injury due to the failure of their nonneuronal cells to produce regeneration-triggering substances. This disability may be circumvented by an appropriate implantation procedure.

Role of nerve growth factor in the adult dorsal root ganglia neuron and its response to injury

The response of dorsal root ganglia (DRG) neurons to NGF deprivation and to axotomy was examined in adult guinea pigs. The success of NGF deprivation by means of an autoimmune approach was monitored by the measurement of serum antibody titer levels against guinea pig NGF with the standard bioassay for NGF activity. That the antibody produced NGF deprivation was confirmed by histologic evidence of neuronal atrophy and apparent cell loss in sections of the superior cervical ganglia (SCG) and by marked decreases (65-80%) of SCG neurotransmitter-synthesizing enzyme activity levels. By using the autoimmune approach a new source of guinea pigs was found which consistently produced high titers of cross-reacting anti-NGF antibodies. Experiments were designed to examine the response of the sensory neuron to injury while chronically deprived of NGF. Total neuronal counts in the sixth lumbar DRG 98 days after sciatic nerve crush showed no difference between NGF-deprived and control ganglia. Measurement of the size spectrum of DRG neurons showed evidence of atrophy of the NGF-deprived neurons in both the uninjured and axotomized side compared to respective controls. The mean volume of uninjured sensory neurons measured in the NGF-deprived guinea pigs was decreased 27.7% (P less than .05) compared with that of control guinea pigs. The degree of regeneration 6 days following a nerve crush was the same in NGF-deprived sensory neurons and in controls when measured by the "pinch test" and by isotope-labeled axonal transport studies.(ABSTRACT TRUNCATED AT 250 WORDS)

58,000 dalton intermediate filament proteins of neuronal and nonneuronal origin in the goldfish visual pathway

A group of proteins in the goldfish optic nerve with a molecular weight of 58K daltons was analyzed by two-dimensional gel electrophoresis. Results show that the proteins are differentially phosphorylated and found exclusively in a cytoskeletal-enriched fraction. The proteins from this fraction can be reconstituted into typical intermediate filament structures, as shown by electron microscopy. Two components which are of neuronal origin are transported within the slow phase of transport. The 58K proteins are the most abundant proteins in the optic nerve, and they are distinct from actin and tubulin. It was concluded that they are intermediate filament proteins. Cytoskeletal preparations of rat spinal cord, rat optic nerve, and goldfish optic nerve were compared by one-dimensional gel electrophoresis. The rat spinal cord contains glial fibrillary acidic protein (GFAP), and the rat optic nerve contains vimentin and GFAP, in addition to the neurofilament triplet. A typical mammalian neurofilament triplet is not detected in the goldfish optic nerve, while the major cytoskeletal constituent is a 58K band which coelectrophoreses with vimentin in the rat optic nerve by one-dimensional gel electrophoresis.

Regulation of mRNA levels for microtubule proteins during nerve regeneration

The molecular regulation of tubulin synthesis was investigated in the regenerating goldfish retina. Previous in vivo studies pointed to an increase in tubulin synthesis in the retina during regeneration of the injured goldfish optic nerve. Using labeled cDNA probes, we showed that this increase occurs as a result of enhanced tubulin mRNA levels. Analysis of labeled in vivo products revealed enhanced beta 2-tubulin synthesis accompanied by an increase in the level of the low-Mr microtubule-associated proteins identified as TAU factors. The results are discussed with respect to the possible involvement of these proteins in the process of nerve regeneration.

In vitro protein synthesis in the goldfish retinotectal pathway during regeneration: evidence for specific axonal proteins of retinal origin in the optic nerve

Four proteins with molecular weights of 58,000 can be separated as a linear array by two-dimensional gel electrophoresis. They are highly concentrated in the goldfish optic nerve and are designated as ON1, ON2, ON3, and ON4. Proteins ON1 and ON2 are undetectable in the optic nerve after disconnection and their concentration is gradually restored during regeneration. In vitro incubations of retinas, optic nerves, or tecta in the presence of [35S]methionine indicate that proteins ON1 and ON2 are of retinal origin. The labeling rate of these proteins in the retina increases fourfold after optic nerve crush whereas the overall labeling rate in the retina remains largely constant. Their synthesis cannot be detected in tissues devoid of retinal ganglion cells. This is consistent with the view that ON1 and ON2 are synthesized by retinal ganglion cells and are consequently of neuronal origin in the optic nerve. In contrast, similar experiments indicate that ON3 and ON4 are of nonneuronal origin. They are synthesized in the optic nerve in the absence of retinal ganglion cells.

Properties of endogenous, membrane-associated sialidase activity (N-acetylneuraminidase) of the goldfish visual system

The endogenous sialidase (N-acetylneuraminidase) activity of membranes prepared from goldfish retina and optic tectum displays characteristics similar to those reported for neural plasma membrane sialidases of other organisms. Endogenous membrane sialidase activity was found to be optimal at ph 4.0, and maximal release was obtained at 37-50 degrees C, above which temperature thermal instability of the preparations was observed. Optic nerve crush, which results in regeneration of retinal ganglion cell axons, did not result in significant changes in measured endogenous membrane sialidase activity in either the retina or the optic tectum. Enzymatic hydrolysis of membrane sialoglycolipid (ganglioside) accounted for about 70% of the total sialic acid released. Ganglioside GM1 accumulated as the major lipid product in both retina and tectum, indicating that the inner sialosylgalactosyl linkage in the ganglio oligosaccharide series was resistant to hydrolysis by the endogenous enzyme.

Changes in the amounts of cytoskeletal proteins within the perikarya and axons of regenerating frog motoneurons

Changes in the amounts of tubulin, actin, and neurofilament polypeptides were found in regenerating motoneurons of grass frogs during the period of axonal elongation. Ventral roots 9 and 10 were transected unilaterally about 7 mm from the spinal cord. 35 d later, [3H]colchicine binding had decreased in the proximal stumps to approximately one-half of contralateral control values, well before the regenerating motor axons had reinnervated skeletal muscles of the hind limb. [3H]colchicine binding did not change significantly in the operated halves of the 9th and 10th spinal cord segments over a 75-d period. The relative amounts of actin, tubulin, and neurofilament polypeptides in the operated ventral roots were measured by quantitative densitometry of stained two-dimensional electrophoretic gels. Alpha-tubulin, beta-tubulin, and the 68,000 molecular weight subunit of neurofilaments (NF68) decreased within the transected ventral roots to 78%, 57%, and less than 15% of control values, respectively. The amount of actin increased to 132% of control values within the operated ventral roots, although this change was not statistically significant. Opposite changes were found within motoneuronal cell bodies isolated from the spinal cord. The relative amounts of alpha-tubulin, beta-tubulin and NF68 within axotomized perikarya increased, respectively, to 191%, 146%, and 144% of that in control perikarya isolated from the contralateral side of the spinal cord. Thus, the changes in NF68 and tubulin did not occur uniformly throughout the injured cells. The possible structural and functional consequences of these changes are discussed.

Protein synthesis and transport in the regenerating goldfish visual system

The nature of the proteins synthesized in the goldfish retina and axonally transported to the tectum during optic nerve regeneration has been examined. Electrophoretic analysis of labeled soluble retinal proteins by fluorography verified our previous observation of a greatly enhanced synthesis of the microtubule subunits. In addition, labeling of a tubulin-like protein in the retinal particulate fraction was also increased during regeneration. Like soluble tubulin, the particulate material had an apparent MW of 53-55K and could be tyrosylated in the presence of cycloheximide and [3H]tyrosine. Comparison of post-crush and normal retinal proteins by two-dimensional gel electrophoresis also revealed a marked enhancement in the labeling of two acidic 68-70K proteins. Analysis of proteins slowly transported to the optic tectum revealed changes following nerve crush similar to those observed in the retina, with enhanced labeling of both soluble and particulate tubulin and of 68-70K polypeptides. the most striking change in the profile of rapidly transported protein was the appearance of a labeled 45k protein which was barely detectable in control fish.

Protein synthesis and fast axonal transport in regenerating goldfish retinal ganglion cells

To characterize the fast component of axonal transport in regenerating goldfish optic axons, the incorporation of L-2,3-[3H]proline into newly-synthesized proteins in the cell bodies of the retinal ganglion cells and the amount of transported labeled protein were determined at 2-36 days after cutting the optic tract. Both the incorporation and the amount of transported protein had doubled by 10 days after the lesion and continued to increase to about 5 times normal at 15 days, a time when a large proportion of the regenerating axon population had reached the optic tectum. Near-normal levels were recovered by 36 days. In contralateral control neurons, the incorporation of L-2,3-[3H]proline was unchanged from normal throughout, whereas the amount of labeled transported protein entering control axons was decreased by 55% at 2 and 10 days after the testing lesion, returning to normal by 15 days. An increase in fast transport velocity was seen in the regenerating axons beginning at 10 days after the lesion. However, a similar velocity increase was also seen in the contralateral control axons and in undamaged axons following removal of the cerebral hemispheres. Therefore, the velocity increase was not a specific consequence of axotomy.

Protein synthesis and axonal transport during nerve regeneration

Protein synthesis and axonal transport have been studied in regenerating peripheral nerves. Sciatic nerves of bullfrogs were unilaterally crushed or cut. The animals were killed 1, 2, or 4 weeks later, and 8th and 9th dorsal root ganglia removed together with sciatic nerves and dorsal roots. The ganglia were selectively labeled in vitro with [35S]-methionine. Labeled proteins, in dorsal root ganglia and rapidly transported to ligatures placed on the sciatic nerves and dorsal roots, were analyzed by two-dimensional polyacrylamide gel electrophoresis. Qualitative analysis of protein patterns revealed no totally new proteins synthesized or rapidly transported in regenerating nerves. However, quantitative comparison of regenerating and contralateral control nerves revealed significant differences in abundance for some of the proteins synthesized in dorsal root ganglia, and for a few of the rapidly transported proteins. Quantitative analysis of rapidly transported proteins in both the peripheral processes (spinal nerves) and central processes (dorsal roots) revealed similar changes despite the fact that the roots were undamaged. The overall lack of drastic changes seen in protein synthesis and transport suggests that the neuron in its program of normal maintenance synthesizes and supplies most of the materials required for axon regrowth.

Myelination and remyelination in the regenerating visual system of the goldfish

The remyelination of regenerated optic axons was investigated in goldfish following either optic nerve crush or ouabain retinal intoxication. Axons grown after nerve crushing acquire thinner myelin sheaths than axons originating from reconstituted ganglion cells. If axons of reconstituted ganglion cells are crushed and allowed to regenerate, the subsequent myelination is weaker than that of control axons not interrupted by crushing, but stronger than that of axons of preexisting retinal ganglion cells. The present results suggest that a neuron is capable of inducing a normally developed myelin sheath when its axon contacts an oligodendrocyte the first time, whereas a neuron whose axon contacts an oligodendrocyte and the second time is not capable of forming a normal myelin sheath in the adult animal. The present results also support the notion that the oligodendrocyte requires a neuronal signal for myelin sheath formation.

Uridine metabolism in the goldfish retina during optic nerve regeneration: cell-free preparations

The activities of uridine kinase (EC 2.7.1.48), uridine monophosphate (UMP) kinase (EC 2.7.1.3.14), and uridine diphosphate (UDP) kinase (EC 2.7.4.6) were measured in retinal high-speed supernatant fractions following unilateral optic nerve crush in the goldfish. The enzyme activities followed a similar time course, with initial increases 2-3 days following nerve crush, peak activity at 4 days, and a gradual return to basal levels by day 21. The magnitude of the stimulation on day 4 was about 35% in each case. Activities of two enzymes of intermediary metabolism, pyruvate kinase (EC 2.7.1.40) and lactic dehydrogenase (EC 1.1.1.27), were not altered, indicating that the coordinate increases in nucleoside and nucleotide kinase activities were specific responses to the nerve injury. The increased labeling could not be explained by altered phosphohydrolytic activities. The nature of the enhancement was further studied in UDP kinase, the most active of the kinases examined. Neither low-molecular-weight components nor substrate availability could account for the observed increase in UDP kinase in the 4 day post-crush retinas. The Km for UDP was unaltered, and a mixing experiment did not support the possibility that stimulatory or inhibitory factors played a role. The enhancement of UDP kinase activity was blocked by injection of actinomycin D following nerve crush. The results suggest that the observed increases in enzymes of uridine metabolism result from their increased formation following nerve crush.

Uridine metabolism in the goldfish retina during optic nerve regeneration: whole retina studies

Accumulation of radioactivity from [3H]uridine in incubations of whole goldfish retinas is increased in the ipsilateral retina during a period of regeneration that follows unilateral optic nerve crush. Brief incubations to investigate the nature of enhanced labeling of the acid-soluble fraction showed a peak uptake 4 days following crush, with a gradual decrease to control levels by 21 days following crush. That nucleoside uptake may not mediate the effect is supported by the observation that the rate of uptake of 5'-deoxyadenosine, a nonmetabolizable nucleoside analog, is the same in post-crush (PC) and normal (N) retinal incubations. Following brief incubations of PC and N retinas with [3H]uridine, there is enhanced labeling in PC retinas relative to N retinas of recovered UMP, UDP, UTP, and uridine nucleotide sugars, whereas recovery of labeled uridine itself is slightly decreased. The results suggest that the increased accumulation of radioactivity in PC retinas following incubation with uridine reflects an increase in the activities of retinal uridine kinase and uridine nucleotide kinases.

Immunocytochemical localization of (Na+, K+)-ATPase in the goldfish optic nerve

Antiserum to the catalytic subunit of goldfish brain (Na+, K+)-ATPase has been employed at the electron microscopic level by means of the peroxidase-antiperoxidase immunohistochemical method. In optic nerve, antigenic sites are restricted to the nodes of Ranvier. No reaction product is detected in underlying internodal neurolemma. Outgrowing neurites for cultured retinal explants devoid of glial ensheathment exhibit a continuous distribution of the enzyme subunit. Antibodies against eel electroplax (Na+, K+)-ATPase cross-react with the goldfish brain enzyme and show a similar immunocytochemical distribution pattern.

Changes in synthesis of specific proteins following axotomy: detection with two-dimensional gel electrophoresis

Changes in protein synthesis during development and following axotomy were analyzed by two-dimensional gel electrophoresis. The two major postganglionic nerves emerging from the superior cervical sympathetic ganglia (SCSG) of adult rats were either cut or crushed unilaterally. At intervals ranging from 1 to 112 days after surgery both SCSG were removed and incubated for 1 hr in the presence of 14C-leucine. Proteins were extracted and subjected to two-dimensional electrophoretic separation and autoradiography. With this technique, proteins are separated on the basis of isoelectric point and molecular weight. Also, intact SCSG from 1, 2, 7, and 14 day old rats were labeled and analyzed. It was found that a minority of the separated proteins exhibited some detectable change in relative rate of synthesis following axotomy. Actin exhibited a slight (less than 20%) increase in relative synthesis rate while tubulin did not change significantly. There were small but significant differences in the protein patterns following nerve crush, as opposed to nerve cut. Comparison of protein synthesis patterns from developing rat SCSG with those from intact and from axotomized adult SCSG failed to demonstrate any marked similarity between the developmental and the axotomy patterns.

Fine structural changes in nerve cell bodies of the adult rabbit dorsal motor vagal nucleus during axon reaction

Counts of neuronal nucleoli were made in the dorsal motor vagal nucleus (DMV) of the adult rabbit 10, 18, 70 and 90 days following unilateral cervical vagotomy. The structural characteristics of nerve cell bodies in the DMV were studied electron microscopically 2--90 days after cervical vagotomy. The nucleolar counts indicated that 20% of the large DMV neurones had disappeared ipsilateral to the operation 10 days postoperatively (p.o.), 65% 18 days p.o. and 70% 70 and 90 days p.o. No loss of small neurones was found. Large neurones ipsilateral to the operation showed nuclear displacement, infoldings of the nuclear membrane and disappearance of granular endoplasmic reticulum beginning 4 days p.o. and being prominent 6--18 days p.o. At the peak of the response, 10--18 days p.o., reacting neurones showed nucleolar condensation and vacuolation, the appearance of intranuclear electron-dense particles, extensive accumulation of intracytoplasmic lipid droplets, increased numbers of microtubules and neurofilaments, focal mitochondrial aggregates, and widespread mitochondrial degeneration. Ten to 21 days p.o. degenerating neurones were observed. After 30 days p.o. survival a partial recovery of surviving large DMV neurones seemed to have taken place. The findings are interpreted as indications of distubed protein metabolism, oxidative metabolism and intraneuronal transport in the axotomized DMV neurones. The unique response of these neurones compared to previously studied peripherally projecting neurones is emphasized.