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

Vimentin and desmin of a cartilaginous fish, the shark Scyliorhinus stellaris: sequence, expression patterns and in vitro assembly

In the shark Scyliorhinus stellaris we have biochemically identified and cDNA-cloned orthologs of human vimentin and desmin, SstV and SstD, as deduced from immunoblotting and sequence alignment with teleost, frog and human vimentin and desmin, respectively. This allowed us to further clarify the relationship of previously identified lower vertebrate intermediate filament proteins to mammalian vimentin and desmin. Immunofluorescence microscopy with antibodies H5 and VIM13.2 showed vimentin expression in shark eye and brain and absence in epithelia, which resembles the situation in higher vertebrates. In addition, SstV is expressed in many mesenchymal cell types which corresponds to the case in terrestrial vertebrates but strongly differs from teleosts. Surprisingly, shark interstitial cells, including fibroblasts, express neither SstV nor keratins but other as yet unidentified intermediate filament proteins as deduced from their reactivity with antibody IFA. In vitro assembly studies of recombinant SstV revealed a temperature optimum for uncompromised filament assembly of 15 degrees C. At 18 degrees C, but more pronounced at 21 degrees C and 24 degrees C, which is notably above the animal's inherent preferred environmental temperature, both, SstV and SstD assemble into thick and inflexible fibers. Thus, environmental temperature apparently is, as a general principle, a driving force for the fine tuning of protein primary structure and eventually 3D structure.

Zebrafish vimentin: molecular characterization, assembly properties and developmental expression

To provide a basis for the investigation of the intermediate filament (IF) protein vimentin in one of the most promising experimental vertebrate systems, the zebrafish (Danio rerio), we have isolated a cDNA clone of high sequence identity to and with the characteristic features of human vimentin. Using this clone we produced recombinant zebrafish vimentin and studied its assembly behaviour. Unlike other vimentins, zebrafish vimentin formed unusually thick filaments when assembled at temperatures below 21 degrees C. At 37 degrees C few filaments were observed, which often also terminated in aggregated masses, indicating that its assembly was severely disturbed at this temperature. Between 21 and 34 degrees C apparently normal IFs were generated. By viscometry, the temperature optimum of assembly was determined to be around 28 degrees C. At this temperature, zebrafish vimentin partially rescued, in mixing experiments, the temperature-dependent assembly defect of trout vimentin. Therefore it is apparently able to "instruct" the misorganized trout vimentin such that it can enter normal IFs. This feature, that assembly is best at the normal body temperature of various species, puts more weight on the assumption that vimentin is vital for some aspects of generating functional adult tissues. Remarkably, like in most other vertebrates, zebrafish vimentin appears to be an abundant factor in the lens and the retina as well as transiently, during development, in various parts of the central and peripheral nervous system. Therefore, promising cell biological investigations may now be performed with cells involved in the generation of the vertebrate eye and brain, and, in particular, the retina. Moreover, the power of genetics of the zebrafish system may be employed to investigate functional properties of vimentin in vivo.

Restricted expression of the neuronal intermediate filament protein plasticin during zebrafish development

In the adult goldfish visual pathway, expression of the neuronal intermediate filament (nIF) protein plasticin is restricted to differentiating retinal ganglion cells (RGCs) at the margin of the retina. Following optic nerve injury, plasticin expression is elevated transiently in all RGCs coincident with the early stages of axon regeneration. These results suggest that plasticin may be expressed throughout the nervous system during the early stages of axonogenesis. To test this hypothesis, we analyzed plasticin expression during zebrafish (Danio rerio) neuronal development. By using immunocytochemistry and in situ hybridization, we found that plasticin is expressed in restricted subsets of early zebrafish neurons. Expression coincides with axon outgrowth in projection neurons that pioneer distinct axon tracts in the embryo. Plasticin is expressed first in trigeminal, Rohon-Beard, and posterior lateral line ganglia neurons, which are among the earliest neurons to initiate axonogenesis in zebrafish. Plasticin is expressed also in reticulospinal neurons and in caudal primary motoneurons. Together, these neurons establish the first behavioral responses in the embryo. Plasticin expression also coincides with initial RGC axonogenesis and progressively decreases after RGC axons reach the tectum. At later developmental stages, plasticin is expressed in a subset of the cranial nerves. The majority of plasticin-positive neurons are within or project axons to the peripheral nervous system. Our results suggest that plasticin subserves the changing requirements for plasticity and stability during axonal outgrowth in neurons that project long axons.

Plasticin, a newly identified neurofilament protein, is preferentially expressed in young retinal ganglion cells of adult goldfish

The adult goldfish retina and optic nerve display continuous growth, plasticity, and the capacity to regenerate throughout the animal's life. The intermediate filament proteins in this pathway are different from those in adult mammalian nerves, which do not continuously grow or normally regenerate. One novel intermediate filament protein of the goldfish visual pathway is plasticin, which is synthesized in ganglion cells and transported into the optic nerve. Using specific polyclonal antibodies raised against a plasticin fusion protein, we investigated the distribution of this protein in the normal retina and nerve and in the retina and nerve following optic nerve crush. In the normal pathway, plasticin was localized predominantly to the axons of very young ganglion cells; however, there was considerable immunoreactivity in older axons as they approach the chiasm. In addition, following optic nerve crush, all ganglion cell somata and their axons proximal to the crush site became equally immunoreactive. The results suggest that plasticin may contribute to axonal growth, plasticity, and regeneration.

Differential expression of keratins in goldfish optic nerve during regeneration

The goldfish visual pathway, unlike the visual pathway of higher vertebrates, retains continuous growth and development throughout life and is capable of functional regeneration. The structure and expression of proteins that support the physiological attributes of this system are of interest. Glial cells in this pathway express keratins as the predominant intermediate filament proteins rather than the expected glial fibrillary acidic protein. Previously we identified and characterized cDNA clones representing two type I keratins from the goldfish optic nerve, GK48 and GK49. The GK48 protein is the type I keratin partner to the type II keratin ON3, while the GK49 protein is expressed in a different cell type. Here, we extend our studies on the expression of mRNA for the GK48, GK49, and ON3 proteins at the early stages of optic nerve regeneration. RNase protection assays show that at 10 days post-crush, there is no overall change in levels of mRNA for these proteins as compared to uncrushed control nerves and nerves from unoperated fish. In addition, we show by in situ hybridization that the GK49 protein shows no changes in its distribution of mRNA in the optic nerve after crush. In contrast, the levels of GK48 and ON3 mRNA are greatly reduced within the crush zone. However, these two mRNAs are differentially expressed at different time points during regeneration, with GK48 mRNA appearing in the crush zone before ON3. These results indicate that the mRNA for the GK48 and ON3 proteins are differentially regulated during regeneration and that these two proteins are expressed in a different cell type from the GK49 protein.

The return of phosphorylated and nonphosphorylated epitopes of neurofilament proteins to the regenerating optic nerve of Xenopus laevis

Neurofilament proteins of mammalian axotomized peripheral axons, which regenerate effectively, resemble those of embryonic axons. However, injured centrally projecting mammalian axons, which fail to regenerate, have very different neurofilament compositions than during development. If changes in neurofilament composition after injury reflect the ability of axotomized neurons to regenerate effectively, then the neurofilaments of centrally projecting axons that can regenerate should more closely resemble those of developing axons. In this study, the neurofilament compositions of injured optic axons of the frog, Xenopus laevis, were examined, since these axons can regenerate a fully functional projection. Antibodies to phosphorylated and nonphosphorylated forms of neurofilament proteins that had been used previously to study the neurofilament composition of newly developing X. laevis optic axons were used in immunocytochemical studies to examine the return of neurofilaments to the optic nerve after an intraorbital nerve crush. Intraocularly injected wheat germ agglutinin conjugated to horseradish peroxidase was used to label the regenerating axons independently of their neurofilaments. Neurofilament immunoreactivities disappeared rapidly from crushed axons during the first week after surgery. By nine days after surgery, antibodies to nonphosphorylated forms of middle (NF-M) and low molecular weight (NF-L) neurofilament proteins and the Xenopus neuronal intermediate filament protein (XNIF) began to stain the nerve just beyond the lesion. By this time, however, growing axonal terminals had reached the optic chiasm. Antibodies to phosphorylated epitopes of NF-M began to stain axons at 15 days, just as growing axons began to arrive at the optic tectum. Nonphosphorylated high molecular weight neurofilament protein (NF-H) began to appear in axons between 18 and 21 days after surgery. Thus, the reappearance of neurofilaments during optic axon regeneration resembled the general pattern seen during development. The chief difference between development and regeneration was that neurofilament epitopes took longer to emerge during regeneration. One possibility is that cues encountered along the optic pathway influence the neurofilament composition of retinal ganglion cell axons. Then, the greater distances travelled by regenerating axons could account for the longer time taken for their neurofilament compositions to mature.

Complex expression of keratins in goldfish optic nerve

Keratins are the predominant intermediate filament proteins in the nonneuronal cells of the goldfish optic nerve. At least three different keratin pairs are expressed in this tissue, indicating an unexpected complexity. Expression of the type II keratin ON3 in goldfish optic nerve astrocytes predicts the expression of a type I keratin partner. Here we report the cDNA sequence and predicted amino acid sequence of two type I keratins from the goldfish optic nerve, designated GK48 and GK49. The GK48 protein is the goldfish equivalent of mammalian keratin 18 (K18) and is the most likely type I keratin partner to the ON3 protein. The GK49 protein is similar to the GK50 protein, a type I keratin characterized previously from the goldfish optic nerve. The GK48 and ON3 mRNAs are expressed in a variety of goldfish tissues, whereas the expression of GK49 mRNA has a more limited expression. In addition, in situ hybridization experiments show that the expression of the GK48 and ON3 mRNAs are evenly distributed throughout the optic nerve, while the GK49 mRNA is expressed along longitudinal lines. These results show that there is a diversity of keratin expression within different cell types in the goldfish optic nerve.

Axon dependent glial changes during optic fiber regeneration in the goldfish

During optic fiber regeneration in the goldfish, astrocytes in the visual system undergo a number of changes. These include hypertrophy of cell processes, increased reactivity with anti-intermediate filament antisera, and expression of cytoskeletal antigens not usually seen in these cells. In the present study, I have asked how much of this response might be due to interactions of glial cells with regenerating optic axons. Animals with and without a retina (regenerating and nonregenerating animals, respectively) had their optic nerve crushed and were then examined at various postoperative times with immunohistochemical methods. Three major differences between these two groups of animals were observed. First, in nonregenerating animals the crush lesion is not repopulated by immunoreactive glial cells while in regenerating animals it is. Second, the nature of the glial hypertrophy in the optic nerve is different in regenerating and nonregenerating animals. Finally, there is marked submeningeal swelling in regenerating nerves that is absent from nonregenerating nerves. Thus, these three aspects of the cellular response to optic nerve crush in the goldfish--wound healing, optic nerve gliosis, and non-neural cellular responses--appear to depend on interactions between regenerating optic axons and astrocytes or other non-neuronal cells of the visual paths for their expression.

Cloning of a type I keratin from goldfish optic nerve: differential expression of keratins during regeneration

We report the cDNA sequence and predicted amino acid sequence of a novel type I keratin, designated as GK50, and show that keratin expression in the goldfish optic nerve is highly complex. The GK50 protein is one of at least three type I keratins expressed in goldfish optic nerve based on both antibody reactivity and blot-binding to the type II keratin ON3. After optic nerve crush in situ hybridization shows a localized increase in GK50 mRNA expression in the crush zone. This is in contrast to ON3 mRNA which shows a localized increase that is limited to the proximal and distal margins of the crush zone, suggesting a diversity of keratin expression in different cell types of the goldfish optic nerve.

Plasticin, a novel type III neurofilament protein from goldfish retina: increased expression during optic nerve regeneration

The goldfish visual pathway displays a remarkable capacity for continued development and plasticity. The intermediate filament proteins in this pathway are unexpected and atypical, suggesting these proteins provide a structure that supports growth and plasticity. Using a goldfish retina lambda gt10 library, we have isolated a full-length cDNA clone that encodes a novel type III intermediate filament protein. The mRNA for this protein is located in retinal ganglion cells, and its level dramatically increases during optic nerve regeneration. The protein is transported into the optic nerve within the slow phase of axonal transport. We have named this protein plasticin because it was isolated from a neuronal pathway well known for its plasticity.

Expression of neuronal intermediate filament proteins ON1 and ON2 during goldfish optic nerve regeneration: effect of tectal ablation

Goldfish retinal explants were used to study optic tectum participation in the regulation of intermediate filament protein synthesis in retinal ganglion cells during optic nerve regeneration. Retinas were explanted at various times after removal of the contralateral optic tectum. The synthesis of the intermediate filament proteins ON1 and ON2 in the cultures was quantitated by labeling with [35S]methionine, followed by two-dimensional gel electrophoresis, autoradiography, and densitometry. Neuritic growth from the explants was quantitated based on fiber length and density. In retinal explants placed in culture after 23 days of optic nerve regeneration, the synthesis of ON1 and ON2 was reduced when the tectum had been ablated. In contrast, synthesis of these proteins in explants placed in culture at an earlier stage of regeneration was not affected by tectal ablation. At all time points tested, neuritic outgrowth from retinal explants was stimulated by tectal ablation. These findings indicate that the synthesis of the ON1 and ON2 intermediate filament proteins during regeneration is not directly regulated by axonal volume. Further, our findings suggest that interaction between growing axons and tectum is important for sustained expression of these proteins during the later stages of optic nerve regeneration.

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.

A polyclonal antibody to goldfish neuronal 145 kDa intermediate filament protein

A polyclonal antibody raised to a protein from goldfish optic tectum recognises, immunohistochemically, axons throughout normal goldfish visual pathway. In goldfish with injured optic nerve, this antibody recognises degenerating neuronal debris as well as regenerating fibres. On immunoblot, the antibody recognises, primarily, a neuronal intermediate filament protein in the region of 145 kDa. Such an antibody should prove useful in studies pertaining to goldfish visual pathway.

Expression of glial fibrillary acidic protein (GFAP) in goldfish optic nerve following injury

By using an antibody to goldfish glial fibrillary acidic protein (GFAP), the reaction of goldfish optic nerve to injury has been studied by immunoblotting and immunohistochemical methods. Goldfish optic nerve, which normally lacks GFAP immunoreactivity (Nona et al.: Glia, 2:189-200, 1989), expresses GFAP following injury. This immunoreactivity, which is observed as early as 10 days after crush and which is still evident at 30 days after crush, all but disappears by 150 days after crush. Since it is well established that functional restoration of synaptic connections and the recovery of vision takes place in goldfish following optic nerve injury, our results indicate that reactive astrocytes do not represent an impediment to regeneration in goldfish visual system.

An epithelium-type cytoskeleton in a glial cell: astrocytes of amphibian optic nerves contain cytokeratin filaments and are connected by desmosomes

In higher vertebrates the cytoskeleton of glial cells, notably astrocytes, is characterized (a) by masses of intermediate filaments (IFs) that contain the hallmark protein of glial differentiation, the glial filament protein (GFP); and (b) by the absence of cytokeratin IFs and IF-anchoring membrane domains of the desmosome type. Here we report that in certain amphibian species (Xenopus laevis, Rana ridibunda, and Pleurodeles waltlii) the astrocytes of the optic nerve contain a completely different type of cytoskeleton. In immunofluorescence microscopy using antibodies specific for different IF and desmosomal proteins, the astrocytes of this nerve are positive for cytokeratins and desmoplakins; by electron microscopy these reactions could be correlated to IF bundles and desmosomes. By gel electrophoresis of cytoskeletal proteins, combined with immunoblotting, we demonstrate the cytokeratinous nature of the major IF proteins of these astroglial cells, comprising at least three major cytokeratins. In this tissue we have not detected a major IF protein that could correspond to GFP. In contrast, cytokeratin IFs and desmosomes have not been detected in the glial cells of brain and spinal cord or in certain peripheral nerves, such as the sciatic nerve. These results provide an example of the formation of a cytokeratin cytoskeleton in the context of a nonepithelial differentiation program. They further show that glial differentiation and functions, commonly correlated with the formation of GFP filaments, are not necessarily dependent on GFP but can also be achieved with structures typical of epithelial differentiation; i.e., cytokeratin IFs and desmosomes. We discuss the cytoskeletal differences of glial cells in different kinds of nerves in the same animal, with special emphasis on the optic nerve of lower vertebrates as a widely studied model system of glial development and nerve regeneration.

Organization of astrocytes in the visual pathways of the goldfish: an immunohistochemical study

We have used antisera directed against glial cytoskeletal proteins to examine the distribution and organization of astrocytes in the visual pathways of the goldfish. We describe two different types of cells, which may be distinguished by their unique cytoskeletal proteins. Antibodies raised against a 48 Kd optic nerve protein react with stellate astrocytes in the optic nerve but virtually no glial cells in the brain (although blood vessels and the meninges in the brain were stained). The optic nerve astrocytes form a dense meshwork of processes through which the optic fibers pass. The intraorbital and intracranial segments of the nerve are divided into fascicles, each bounded by a glia limitans, which extend across the optic chiasm. Astroglial cells in the brain bind antibodies raised against a 50 Kd brain cytoskeletal protein. These antibodies show a very limited cross-reactivity with optic nerve cells. Brain astrocytes have filiform profiles and most appear to be deployed as radial glia. The glial fabric of the brain, as revealed by these antibodies, is far more loosely woven than that of the optic nerve. There is a sharp boundary between the two types of glial cells, immediately behind the optic chiasm. Glial processes in the optic tracts arise from cells in the preoptic area, whereas those in the optic tectum arise from cells that reside locally. In the optic tract, a glia limitans was often difficult to discern, whereas in the tectum one was always evident and composed of endfeet at the pial extremities of radial glial processes. These findings are discussed both in the context of previous observations by other workers as well as with regard to their possible functional implications.

A type II keratin is expressed in glial cells of the goldfish visual pathway

The predominant intermediate filament proteins of the goldfish visual pathway consist of neuronal and non-neuronal isoelectric variants (58 kd). We have isolated a cDNA clone for the glial intermediate filament protein (ON3) from an optic nerve expression library. The predicted amino acid sequence of this clone reveals that it codes for a type II keratin representing the goldfish equivalent of mammalian keratin K8. K8 has been shown to be associated with embryogenesis and development. Unlike the mammalian visual system, the goldfish visual pathway displays a remarkable capacity for functional regeneration. The expression of K8, a protein not usually expressed in glial cells but shown to be associated with development, in the goldfish optic nerve may be involved with the processes of growth and regeneration in the goldfish visual pathway.

Glial fibrillary acidic protein (GFAP) from goldfish: its localisation in visual pathway

An intermediate filament fraction, isolated from goldfish brain, contains a prominent protein having a molecular weight of 51 kDa. In normal goldfish visual pathway, this protein is present in tectum and tract, but not in optic nerve. A polyclonal antibody raised to this protein clearly labels ependymal glial profiles in tectum and parallel processes in the tract, whereas optic nerve is unlabelled; Müller fibres in the retina are also labelled. A similar, but less prominent, pattern of staining is observed with antibodies, raised elsewhere, against glial fibrillary acidic protein from human and porcine. These results suggest that the 51 kDa protein is a GFAP, demonstrate the heterogeneity of astrocytes in goldfish visual pathway, and are consistent with the idea that GFAP is well conserved in vertebrate phylogeny.

Localization of cytokeratins in tissues of the rainbow trout: fundamental differences in expression pattern between fish and higher vertebrates

Using a panel of antibodies against different cytokeratins in immunofluorescence microscopy on frozen tissue sections and two-dimensional gel electrophoresis of cytoskeletal proteins from these tissues, we have studied the tissue distribution of cytokeratins in a fish, the rainbow trout Salmo gairdneri. We have distinguished at least 14 different cytokeratin polypeptides in only a limited number of tissues, thus demonstrating the great complexity of the cytokeratin pattern in a fish species. The simplest cytokeratin pattern was that present in hepatocytes, comprising one type-II (L1) and two type-I (L2, L3) polypeptides that appear to be related to mammalian cytokeratins 8 and 18, respectively. Two or all three cytokeratins of this group were also identified in several other epithelial tissues, such as kidney. Epithelia associated with the digestive tract contained, in addition, other major tissue-specific cytokeratins, such as components D1-D3 (stomach, intestine and swim bladder) and B1 and B2 (biliary tract). With the exception of D1, all these polypeptides were also found in a cultured cell line (RTG-2). Epidermal keratinocytes contained D1 and six other major cytokeratins, termed E1-E6. The most complex cytokeratin pattern was that found in the gill epithelium. Surprisingly, antibodies specific for cytokeratins of the L1-L3 group also reacted with certain cell-sheet-forming tissues that are not considered typical epithelia and in higher vertebrates express primarily, if not exclusively, vimentin. Such tissues were (a) endothelia, including the pillar cells of the "gill filaments", (b) scale-associated cells, and (c) the ocular lens epithelium, and also several nonepithelial cell types, such as (d) fibroblasts and other mesenchymal cells, (e) chondrocytes, (f) certain vascular smooth muscle cells, and (g) astroglial cells of the optic nerve. The differences between the patterns of cytokeratin expression in this fish species and those of higher vertebrates are discussed. It is concluded that the diversity of cytokeratins has already been established in lower vertebrates such as fish, but that the tissue-expression pattern of certain cytokeratins has been restricted during vertebrate evolution. We discuss the value of antibodies specific for individual cytokeratin polypeptides as marker molecules indicating cell and tissue differentiation in fish histology, embryology, and pathology.

Studies of the early stages of optic axon regeneration in the goldfish

We have studied the early stages (4-14 days) of axonal regeneration following intraorbital optic nerve crush in the goldfish. We used 3H-proline autoradiography to anterogradely label and visualize the growing axons and wheat germ agglutinin-conjugated horseradish peroxidase (WGA:HRP) for retrograde labeling to determine the cells of origin of the earliest projections. The first retinal ganglion cells (RGCs) that could be retrogradely filled from the optic tract, following optic nerve crush, were in the central retina and were seen at 8 days postoperative. More peripheral cells were only labeled with longer postcrush survival periods. Thus, the first axons to regenerate past the lesion were from central RGCs. The axons of these cells extended into the cranial nerve stump between 4 and 5 days postcrush and entered the nerve as a fascicle, which travelled just beneath its surface. Studies of nerve cross sections from animals at 5-8 days postoperative demonstrated that initial outgrowth was not confined to any particular locale within the nerve although the early fibers appeared to avoid its temporal aspect. When the regenerating axons reached the optic tract they remained in fascicles but left the surface to run along the medial, deep portion of the tract, immediately adjacent to the diencephalon and pretectum. The positions occupied by the earliest-regenerating axons in the optic nerve were variable and not always appropriate for their central retinal origin. However, the abrupt change in growth trajectory as the fibers entered the optic tract brought them into the areas of the visual paths that are occupied by central axons in intact animals. We suggest that this change in position is related to both changes in the structural organization of the intracranial visual paths and to possible axon guidance signals in the region of the nerve-tract juncture.

Distribution of specific intermediate-filament proteins in the goldfish retina

The intermediate-filament proteins expressed in the goldfish retina were investigated by immunohistochemistry and by immunoblotting. Polyclonal antibodies that previously had been raised against the goldfish optic nerve neurofilament (ON1 and ON2) and glial filament (ON3 and ON4) proteins were used in this study. Anti-ON1/ON2 antiserum reacted on a retinal immunoblot with two proteins having molecular weights and isoelectric points corresponding to those of ON1 and ON2. Histologically, the most pronounced anti-ON1/ON2 reactivity was observed in the ganglion cell layer of the goldfish retina. The anti-ON3/ON4 antiserum reacted with a single protein on a retinal immunoblot. This protein had a molecular weight and isoelectric point which corresponded to the goldfish optic nerve glial filament proteins. This anti-serum labeled horizontal cells in retina sections. Three previously unidentified goldfish visual-pathway intermediate-filament proteins sharing a molecular weight of 60K were observed on two-dimensional gels of retinal cytoskeletal proteins and on retinal immunoblots which were probed with a monoclonal antibody which recognizes an epitope common to all intermediate filament proteins. The possible existence of homologs of mammalian GFAP and vimentin in the goldfish retina was also explored. Antibodies directed against mammalian GFAP and vimentin labeled the Müller fibers and the cone horizontal cells, respectively. However, immunoblot analysis and a comparison of the two-dimensional gel electrophoresis patterns of goldfish retinal and rat spinal cord cytoskeletal proteins demonstrated a lack of goldfish proteins identical to the mammalian intermediate-filament proteins.

Phosphorylation of proteins in normal and regenerating goldfish optic nerve

Within 6 h after radiolabeled phosphate was injected into the eye of goldfish, labeled acid-soluble and acid-precipitable material began to appear in the optic nerve and subsequently also in the lobe of the optic tectum, to which the optic axons project. From the rate of appearance of the acid-precipitable material, a maximal velocity of axonal transport of 13-21 mm/day could be calculated, consistent with fast axonal transport group II. Examination of individual proteins by two-dimensional gel electrophoresis revealed that approximately 20 proteins were phosphorylated in normal and regenerating nerves. These ranged in molecular weight from approximately 18,000 to 180,000 and in pI from 4.4 to 6.9. Among them were several fast transported proteins, including protein 4, which is the equivalent of the growth-associated protein GAP-43. In addition, there was phosphorylation of some recognizable constituents of slow axonal transport, including alpha-tubulin, a neurofilament constituent (NF), and another intermediate filament protein characteristic of goldfish optic axons (ON2). At least some axonal proteins, therefore, may become phosphorylated as a result of the axonal transport of a phosphate carrier. Some of the proteins labeled by intraocular injection of 32P showed changes in phosphorylation during regeneration of the optic axons. By 3-4 weeks after an optic tract lesion, five proteins, including protein 4, showed a significant increase in labeling in the intact segment of nerve between the eye and the lesion, whereas at least four others (including ON2) showed a significant decrease. When local incorporation of radiolabeled phosphate into the nerve was examined by incubating nerve segments in 32P-containing medium, there was little or no labeling of the proteins that showed changes in phosphorylation during regeneration. Segments of either normal or regenerating nerves showed strong labeling of several other proteins, particularly a group ranging in molecular weight from 46,000 to 58,000 and in pI from 4.9 to 6.4. These proteins were presumably primarily of nonneuronal origin. Nevertheless, if degeneration of the axons had been caused by removal of the eye 1 week earlier, most of the labeling of these proteins was abolished. This suggests that phosphorylation of these proteins depends on the integrity of the optic axons.

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.