Characterization of antibodies against major fish CNS myelin proteins: immunoblot analysis and immunohistochemical localization of 36K and IP2 proteins in trout nerve tissue

Evolution of myelin ultrastructure and the major structural myelin proteins

Myelin sheaths, as the specialized tissue wrapping the nerve fibers in the central and peripheral nervous systems (CNS and PNS), are responsible for rapid conduction of electrical signals in these fibers. We compare the nerve myelin sheaths of different phylogenetic origins-including mammal, rodent, bird, reptile, amphibian, lungfish, teleost, and elasmobranch-with respect to periodicities and inter-membrane separations at their cytoplasmic and extracellular appositions, and correlate these structural parameters with biochemical composition. P0 glycoprotein and P0-like proteins are present in PNS of terrestrial species or land vertebrates (Tetrapod) and in CNS and PNS of aquatic species. Proteolipid protein (PLP) is a major component only in the CNS myelin of terrestrial species and is involved in compaction of the extracellular apposition. The myelin structures of aquatic garfish and lungfish, which contain P0-like protein both in CNS and PNS, are similar to those of terrestrial species, indicating that they may be transitional organisms between water and land species. This article is part of a Special Issue entitled SI: Myelin Evolution.

Appearance of Myelin proteins during vertebrate evolution

Myelin, defined as an arrangement of spirally fused unit membranes, is an acquisition of vertebrates and first appeared during evolution in Gnathostomata. In all species studied PNS and CNS myelins contain the myelin-associated glycoprotein (MAG) and the myelin basic protein (MBP). Throughout phylogeny PNS myelin is characterized by the major P(0) glycoprotein which is called IP in fishes. The PNS myelin proteins did not evolve further except for the addition of P(2) protein from reptiles onward. In Elasmobranchii and Chondrostei, PNS and CNS myelin proteins are similar. CNS myelin of actinopterygian fishes possesses a 36,000 Da protein (36K) in addition to P(0)-like IP glycoproteins. In tetrapod CNS myelin, P(0) is replaced by the proteolipid protein (PLP) and the Wolfgram protein (WP). Of particular interest in a transitional phylogenetic sense are the lungfish Protopterus, carrying glycosylated PLP (g-PLP) but no P(0), 36K or WP, and the bichir Polypterus, showing simultaneous presence of P(0), 36K and PLP. These results indicate that myelin proteins could be valuable molecular markers in establishing vertebrate phylogenetic relationships and in reconstructing the fish-tetrapod transition.

Major isoform of zebrafish P0 is a 23.5 kDa myelin glycoprotein expressed in selected white matter tracts of the central nervous system

The zebrafish mpz gene, encoding the ortholog of mammalian myelin protein zero, is expressed in oligodendrocytes of the zebrafish central nervous system (CNS). The putative gene product, P0, has been implicated in promoting axonal regeneration in addition to its proposed structural functions in compact myelin. We raised novel zebrafish P0-specific antibodies and established that P0 is a 23.5 kDa glycoprotein containing a 3 kDa N-linked carbohydrate moiety. P0 was localized to myelin sheaths surrounding axons, but was not detected in the cell bodies or proximal processes of oligodendrocytes. Many white matter tracts in the adult zebrafish CNS were robustly immunoreactive for P0, including afferent visual and olfactory pathways, commissural and longitudinal tracts of the brain, and selected ascending and descending tracts of the spinal cord. P0 was first detected during development in premyelinating oligodendrocytes of the ventral hindbrain at 48 hours postfertilization (hpf). By 72 hpf, short segments of longitudinally oriented P0-immunoreactive myelinating axons were seen in the hindbrain; expression in the spinal cord, optic pathways, hindbrain commissures, midbrain, and peripheral nervous system followed. The mpz transcript was found to be alternatively spliced, giving rise to P0 isoforms with alternative C-termini. The 23.5 kDa isoform was most abundant in the CNS, but other isoforms predominated in the myelin sheath surrounding the Mauthner axon. These data provide a detailed account of P0 expression and demonstrate novel P0 isoforms, which may have discrete functional properties. The restriction of P0 immunoreactivity to myelin sheaths indicates that the protein is subject to stringent intracellular compartmentalization, which likely occurs through posttranslational mechanisms.

Myelin structure and composition of myelinated tissue in the African lungfish

To analyze myelin structure and the composition of myelinated tissue in the African lungfish(Protopterus dolloi), we used a combination of ultrastructural and biochemical techniques. Electron microscopy showed typical multilamellar myelin: CNS sheaths abutted one another, and PNS sheaths were separated by endoneurial collagen. The radial component, prominent in CNS myelin of higher vertebrates, was suggested by the pattern of staining but was poorly organized. The lipid and myelin protein compositions of lungfish tissues more closely resembled those of teleost than those of higher vertebrates (frog, mouse). Of particular note, for example, lungfish glycolipids lacked hydroxy fatty acids. Native myelin periodicities from unfixed nerves were in the range of those for higher vertebrates rather than for teleost fish. Lungfish PNS myelin had wider inter-membrane spaces compared with other vertebrates, and lungfish CNS myelin had spaces that were closer in value to those in mammalian than to amphibian or teleost myelins. The membrane lipid bilayer was narrower in lungfish PNS myelin compared to other vertebrates, whereas in the CNS myelin the bilayer was in the typical range. Lungfish PNS myelin showed typical compaction and swelling responses to incubation in acidic or alkaline hypotonic saline. The CNS myelin, by contrast, did not compact in acidic saline but did swell in the alkaline solution. This lability was more similar to that for the higher vertebrates than for teleost.

Zebrafish myelination: a transparent model for remyelination?

There is currently an unmet need for a therapy that promotes the regenerative process of remyelination in central nervous system diseases, notably multiple sclerosis (MS). A high-throughput model is, therefore, required to screen potential therapeutic drugs and to refine genomic and proteomic data from MS lesions. Here, we review the value of the zebrafish (Danio rerio) larva as a model of the developmental process of myelination, describing the powerful applications of zebrafish for genetic manipulation and genetic screens, as well as some of the exciting imaging capabilities of this model. Finally, we discuss how a model of zebrafish myelination can be used as a high-throughput screening model to predict the effect of compounds on remyelination. We conclude that zebrafish provide a highly versatile myelination model. As more complex transgenic zebrafish lines are developed, it might soon be possible to visualise myelination, or even remyelination, in real time. However, experimental outputs must be designed carefully for such visual and temporal techniques.

Characterization of the shark myelin Po protein

Myelin, the insulating sheath made by extensive plasma membrane wrapping, is dependent on the presence of highly adhesive molecules that keep the two sides of the membrane in tight contact. The Po glycoprotein (Po) is the major component of the peripheral nervous system (PNS) myelin of mammals. The exact role that Po protein has played in the evolution of myelin is still unclear, but several phylogenetic observations suggest that it is a crucial component in the development of myelin as a multi-lamellar membrane structure. Sharks, which appeared in the fossil record about 400 million years ago, are the first fully myelinated organisms. In this study we investigated the expression pattern of shark myelin Po to suggest a way it might have played a role in the evolution of myelin in the central nervous system. We found that sharks have more than two isoforms (32, 28 and 25 kD), and that some of these might not be fully functional because they lack the domains known for Po homophilic adhesion.

Myelin Structure and Composition in Zebrafish

To establish a standard for genotype/phenotype studies on the myelin of zebrafish (Danio rerio), an organism increasingly popular as a model system for vertebrates, we have initiated a detailed characterization of the structure and biochemical composition of its myelinated central and peripheral nervous system (CNS; PNS) tissues. Myelin periods, determined by X-ray diffraction from whole, unfixed optic and lateral line nerves, were approximately 153 and approximately 162 Angstrom, respectively. In contrast with the lability of PNS myelin in higher vertebrates, zebrafish lateral line nerve myelin exhibited structural stability when exposed to substantial changes in pH and ionic strength. Neither optic nor lateral line nerves showed swelling at the cytoplasmic apposition in CaCl(2)-containing Ringer's solution, in contrast with nerves from other teleost and elasmobranch fishes. Zebrafish optic nerve showed greater stability against changes in NaCl and CaCl(2) than lateral line nerve. The nerves from zebrafish having mutations in the gene for myelin basic protein (mbpAla2Thr and mbpAsp25Val) showed similar myelin periods as the wildtype (WT), but gave approximately 20% less compact myelin. Analysis of proteins by SDS-PAGE and Western blotting identified in both CNS and PNS of WT zebrafish two orthologues of myelin P0 glycoprotein that have been characterized extensively in trout--intermediate protein 1 (24 kDa) and intermediate protein 2 (28 kDa). Treatment with endoglycosidase-F demonstrated a carbohydrate moiety of approximately 7 kDa, which is nearly threefold larger than for higher vertebrates. Thin-layer chromatography for lipids revealed a similar composition as for other teleosts. Taken together, these data will serve as a baseline for detecting changes in the structure and/or amount of myelin resulting from mutations in myelin-related genes or from exogenous, potentially cytotoxic compounds that could affect myelin formation or stability.

The 36K protein of zebrafish CNS myelin is a short-chain dehydrogenase

Previous studies identified homologues to mammalian myelin genes expressed in the teleost central nervous system (CNS), including myelin basic protein (MBP), protein zero (P0), and a member of the proteolipid protein family, DM20. In addition, an uncharacterized 36-kDa (36K) protein is a major component of teleost myelin, but is not a major component of myelin in other species. In the present study, we sought to better understand myelin proteins and myelination in one teleost, zebrafish, by molecular characterization of the zebrafish 36K protein. Purified zebrafish CNS myelin was isolated and the amino acid sequences of peptides present in the 36-kDa band were determined by mass spectrometry. These sequences matched a previously uncharacterized EST in The Institute for Genome Research (TIGR) zebrafish database that is related to the short-chain dehydrogenase/reductase (SDR) protein family. In vitro expression of the zebrafish 36K cDNA in Neuro 2a cells resulted in a protein product that was recognized by a 36K polyclonal antibody. The zebrafish 36K mRNA and protein expression patterns were determined and correlated to other known myelin gene expression profiles. In addition, we determined by in situ hybridization that a human 36K homologue (FLJ13639) is expressed in oligodendrocytes and neurons in the adult human cortex. This study identified a major myelin protein in zebrafish, 36K, as a member of the SDR superfamily; an expression pattern similar to other myelin genes was demonstrated.

Molecular cloning, tissue expression, and partial characterization of the major fish CNS myelin protein 36k

A full-length cDNA clone encoding the major structural protein of trout CNS myelin 36K was isolated and sequenced. The deduced amino acid sequence did not reveal a putative transmembrane domain and exhibited no structural homology with any of the known myelin proteins. 36K instead shared characteristic structural elements with enzymes of the short-chain dehydrogenase family. The highest similarity in the database (60%), however, was obtained with a human protein of unknown function. By Northern blotting, a single mRNA species of about 2 kb was identified, which was expressed in brain tissue but not in liver. By in situ hybridization, a selective labeling of myelinating glial cells in the trout CNS but not in the PNS was revealed. The developmental appearance of the 36K transcript closely coincided with a period of active myelin deposition in most regions of the trout brain. As a first step in elucidating the structural and biochemical role of 36K for myelin formation and maintenance, we have overexpressed it in Escherichia coli as a soluble His-tag fusion protein and purified it in high yield by Ni+-chelated affinity chromatography. By SDS-PAGE, a single band of the expected molecular size was revealed, which heavily cross-reacted with polyclonal antibodies generated against the native protein. The results of circular dichroism spectroscopy are compatible with a betaalphabeta-barrel structure (Rossman fold), confirming the results of computer-assisted secondary structure predictions.

Structure, heterologous expression, and adhesive properties of the P(0)-like myelin glycoprotein IP1 of trout CNS

The IP1 protein of trout CNS myelin as well as an IP1/P(0) chimeric protein were stably expressed in CHO cells. Successful targeting of the recombinant proteins to the membrane surface was verified by immunofluorescence staining. Full-length expression of IP1 could be confirmed by Western blot analysis of proteins extracted from stably transfected CHO-cells. The adhesive properties of IP1 were studied by an in vitro aggregation assay in which microscopic examination was combined with electronic particle counting. While IP1 conveyed only a weak increase in cell aggregation of transfected CHO cells, the IP1/P0 chimera was much more effective. In the presence of specific antibodies, cell aggregation was strongly reduced. The adhesive properties of P(0)-like proteins are discussed considering recent crystallographic data on the atomic structure of the extracellular domain of mammalian P(0).

Partial characterization of the 5'-flanking region of trout IP: a Po-like gene containing a PLP-like promoter

The IP gene of trout encodes two Po-like glycoproteins which are expressed by oligodendrocytes in the fish CNS. A 679 bp fragment of its 5'-flanking region was isolated from a genomic library and sequenced. The transcription start point was determined 124 bp upstream the ATG initiator codon by primer extension analysis. Apart from a modified TATA-box and an inverted CCAAT-box located at canonical distances from the transcription start site several eucaryotic cis-acting regulatory elements were identified in the 679 bp upstream region, including an AP-1 binding site, a brain specific Sp1 motif, a cyclic AMP responsive element and a consensus sequence for POU homeodomain protein binding. The occurence of respective DNA-binding proteins for Sp1, AP-1 and POU in the nuclei of trout oligodendrocyte progenitor cells was verified by gel retardation experiments. Functional activity of various subfragments of the 679 bp upstream region was demonstrated by CAT reporter gene analysis. A computer-assisted sequence alignment of the trout IP 5'-flanking end with the corresponding region of the mammalian PLP gene promoter revealed four sites of high homology, while similarity with the mammalian Po gene promotor was low. The results are discussed with respect to the phylogenetic shift from Po-like proteins to PLP during evolution of the vertebrate CNS myelin sheath.

Adaptive plasticity of Xenopus glial cells in vitro and after CNS fiber tract lesions in vivo

Xenopus oligodendrocytes and aspects of their differentiation were analyzed in vitro and in vivo using cell- and stage-specific antibodies. Undifferentiated oligodendrocytes were derived from optic nerves or spinal cords. They divided in vitro, were of elongated shape, were glial fibrillary acidic protein and O4 positive, transiently exhibited several antigens including HNK-1 and L1, and promoted axon growth as do Schwann cells. With forskolin they differentiated and, much like myelin-forming oligodendrocytes in the intact optic nerve and spinal cord, they expressed sets of advanced myelin markers. These advanced myelin markers disappeared from the regenerating optic nerve 4 weeks after lesion. The optic nerve instead was populated by cells with radial processes and somata in the center of the nerve; among them were cells and processes that were O4 positive and that are suspected to represent undifferentiated oligodendrocytes. Where processes of these cells reached to the retinal axons in the nerve's periphery, advanced myelin markers typical of differentiated oligodendrocytes reappeared 8 weeks after lesion. These glial changes did not occur in the absence of retinal axons. Thus, the apparent capability of Xenopus oligodendrocytes to adapt to the transient absence, reappearance, and regenerative state of the axons enables them to contribute to central nervous system fiber tract repair. This occurs in the lesioned optic nerve but not in the spinal cord, where no such glial changes were observed and where axons fail to regenerate.

Cloning and expression of the proteolipid protein DM20 cDNA from the brain of the rainbow trout, Oncorhynchus mykiss

The myelin sheath in higher vertebrates consists predominantly of proteolipid protein (PLP) and its smaller isoform DM20. Mutations in the PLP gene produces several neurological disorders such as Pelizaeus-Merzbacher disease and the rumpshaker phenotype in mice. This paper describes the cloning and expression of DM20 from the brain of Rainbow trout. We have isolated a nearly full-length cDNA clone containing 1835 bp that codes for a protein of 258 amino acids. Trout DM20 shows extensive homology with DM20 from higher vertebrates and includes the four hydrophobic regions that are believed to span the myelin membrane. The DM20 transcript is expressed throughout the central nervous system of the trout but appears at its highest levels in the spinal cord and medulla oblongata. The transcript is expressed at very low levels on hatching day but increases 179-fold by the 5th week. Contrary to higher vertebrates, there is no switch to the PLP transcript in maturing trout. Moreover, the rsh mutation (186 Thr to Ile) that produces the rumpshaking neurological disorder in mice has no effect in trout.

NADPH-diaphorase histochemistry reveals oligodendrocytes in the rainbow trout (teleosts)

We studied the distribution of NADPH-diaphorase (NADPHd) in the brain of the rainbow trout. Not only neurons but also tanycytes and oligodendrocytes showed NADPHd positivity. Interestingly, staining of oligodendrocytes was delicate and revealed fine characteristics of these cells, comparable to those revealed by classical 'oligodendrocyte methods' in mammals (such as the rich branching of processes and their association with myelin sheaths). NADPHd histochemistry indicated that trout oligodendrocytes are heterogeneous as regards cell size, shape and number of processes, and that there is a positive correlation between the size of axons and that of the associated oligodendrocytes. NADPHd histochemistry thus appears to be a useful method for the study of oligodendrocyte populations in bony fishes. Furthermore, this is the first report of NADPHd activity in oligodendrocytes of any vertebrate.

Glial repair at the lesion site in regenerating goldfish spinal cord: an immunohistochemical study using species-specific antibodies

We have used fish-specific antibodies to show that repair in regenerating goldfish spinal cord is accompanied by the recovery of the astrocytic environment and restoration of the central canal. Astrocyte processes trailed the regenerated axons bridging the new cord, suggesting that they are not needed for axonal regrowth.

Growth cones of regenerating retinal axons contact a variety of cellular profiles in the transected goldfish optic nerve

Following optic nerve transection in goldfish, retinal axons regenerate. To determine what the growth cones use as a substrate for their growth, regenerating growth cones were labeled by horseradish peroxidase (HRP) application to the retina 5-6 days after intraorbital optic nerve section (ONS) and identified at 10-11 days after ONS in the brain sided (distal) portion of the optic nerve in thick and serial ultrathin sections. Leading growth cones (n = 5) were found in intimate contact with a variety of elements: with myelin fragments alone, with myelin fragments and glial cells, and with the basal lamina of the glia limitans and the surface of a fibroblast outside the boundary of previous fascicles. In ultrathin sections of conventionally treated regenerating optic nerves, (unlabeled) axon profiles--in addition to myelin fragments--were seen to be in contact with an astrocyte and an oligodendrocyte, suggesting that the growth cones of these axons may have been associated with those cells. The data suggest that leading growth cones of regenerating axons may be capable of growing along myelin fragments and on a wide variety of cellular surfaces in the goldfish optic nerve.

Similarities and differences between fish oligodendrocytes and Schwann cells in vitro

In light of the striking differences between oligodendrocytes of the optic nerve/tract of adult goldfish and their mammalian counterparts, a further characterization of goldfish oligodendrocytes was performed. A comparison with Schwann cells was included because fish optic nerve/tract-derived oligodendrocytes bear remarkable similarities to this type of glial cell. Fish optic nerve/tract-derived oligodendrocytes that had differentiated into 04 and 6D2-positive cells and thus expressed early myelin marker molecules were found to incorporate BrdU and to divide in vitro over a period of weeks. For the induction of more advanced markers of myelinogenesis such as the CNS-specific myelin protein 36K, oligodendrocytes required extensive contact with axons. Other agents, such as fetal calf or carp serum, substrate components, or forscolin failed, however, to induce 36K expression. 04/6D2-positive oligodendrocytes could be distinguished from fish 6D2-positive Schwann cells derived from cranial nerves by their antigenic phenotype: Schwann cells but not oligodendrocytes exhibited the low affinity NGF receptor. While both cell types carry the cell adhesion molecules NCAM, E 587 antigen, and the L2/HNK-1 epitope, only Schwann cells possess a further adhesion molecule, Neurolin.

Myelin repair by Schwann cells in the regenerating goldfish visual pathway: regional patterns revealed by X-irradiation

In the regenerating goldfish optic nerves, Schwann cells of unknown origin reliably infiltrate the lesion site forming a band of peripheral-type myelinating tissue by 1-2 months, sharply demarcated from the adjacent new CNS myelin. To investigate this effect, we have interfered with cell proliferation by locally X-irradiating the fish visual pathway 24h after the lesion. As assayed by immunohistochemistry and EM, irradiation retards until 6 months formation of new myelin by Schwann cells at the lesion site, and virtually abolishes oligodendrocyte myelination distally, but has little or no effect on nerve fibre regrowth. Optic nerve astrocyte processes normally fail to re-infiltrate the lesion, but re-occupy it after irradiation, suggesting that they are normally excluded by early cell proliferation at this site. Moreover, scattered myelinating Schwann cells also appear in the oligodendrocyte-depleted distal optic nerve after irradiation, although only as far as the optic tract. Optic nerve reticular astrocytes differ in various ways from radial glia elsewhere in the fish CNS, and our observations suggest that they may be more permissive to Schwann cell invasion of CNS tissue.

Major glycoproteins in carp CNS myelin: homology to P0 protein with HNK-1/L2 carbohydrate epitope

We studied the myelin protein profiles of carp from a phylogenetic point of view. The carp central nerve myelin contained two reactive bands, 28 and 25 kDa, demonstrated with anti-bovine P0 antibody. Their molecular weights are slightly different from those of two positive bands found in carp peripheral myelin. The N-terminal amino acid sequences of these four positive bands were identical to one another and showed high homology with those of mammalian P0 protein, suggesting that carp central myelin contains the P0-like protein. Lectin binding analysis revealed that carbohydrate structure of the P0-like proteins in carp central myelin is similar to those in peripheral myelin of carp and other vertebrates. Further, the carp P0-like glycoproteins, like the P0 proteins of other vertebrates, reacted with antibodies that recognize the HNK-1/L2 carbohydrate epitope. We conclude that the major structural glycoprotein in central myelin of the carp is homologous to P0 protein in peripheral myelin of other higher classes of vertebrates.

Characterization of basic proteins from goldfish myelin

Myelin basic protein (MBP) from common goldfish (Carassius auratus) myelin was extracted with dilute mineral acid. Immunological cross-reactivity of the goldfish MBP, with polyclonal antisera raised against bovine MBP, suggested that the goldfish protein has epitopes for these antibodies. It also reacted with a monoclonal antibody specific for a seven amino acid epitope (130-137) conserved in the MBP of most mammalian species. To characterize the charge heterogeneity of this protein, we iodinated the protein with 125I and chromatographed it on a carboxymethyl cellulose-52 column together with a nonlabeled acid soluble fraction prepared from human white matter as a carrier protein. All of the goldfish protein was recovered in the unbound fraction, demonstrating that it was less cationic than the carrier protein (human MBP). We have also examined the urea alkaline gel profile of the goldfish MBP together with the human C-1, C-2, C-3, C-4, and C-8 components. The results from these experiments indicated that this MBP extracted from goldfish brain myelin lacked the microheterogeneity that is associated with MBPs from higher vertebrates. The MBPs from goldfish myelin were separated into their isoforms by reversed-phase HPLC. Amino acid compositions were determined for both the 17- and 14-kDa goldfish proteins. Amino acid analysis revealed similarities with the compositions of other MBPs; however, the serine content in both the 17- and 14-kDa proteins was higher than that of the human C-1, the mouse C-1 protein, and the shark proteins. The HPLC-purified 14-kDa goldfish protein was chemically cleaved with CNBr for partial sequence analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelination of regenerated axons in goldfish optic nerve by Schwann cells

This study uses immunohistochemistry and EM to examine the site of injury in goldfish optic nerve during axonal regeneration. Within seven days of nerve crush axons begin to regrow and a network of GFAP+ reactive astrocytes appears in the nerve on either side of the injury. However, the damaged area remains GFAP-. By 42 days after nerve crush, the sheaths of new axons acquire myelin marker 6D2, and the crush area becomes populated by a mass of longitudinally-orientated S-100+ cells. Ultrastructurally, the predominant cells in the crush area bear a strong resemblance to peripheral nerve Schwann cells; they display a one-to-one association with myelinated axons, have a basal lamina and are surrounded by collagen fibres. It is proposed that these cells are Schwann cells which enter the optic nerve as a result of crush, where they become confined to the astrocyte-free crush area.

In vitro differentiation of trout oligodendrocytes: evidence for an A2B5-positive origin

The molecular differentiation of oligodendrocytes derived from larval trout brain was studied in dissociated cell cultures using a range of cell type and stage specific antibodies. By double-labeling immunostaining using A2B5 antibodies in conjunction with antibodies against the myelin glycoproteins IP1 and IP2 evidence was obtained that oligodendrocytes of trout in vitro originate from A2B5+ precursor cells, which in terms of morphology closely resemble 0-2A progenitors of the mammalian CNS. Most surprisingly these cells did not differentiate in vitro beyond the level of IP2 expression, which signifies the initial step of oligodendroglial development in vivo. Hence it appears that in trout oligodendrocytes the initiation of the developmental program is intrinsically regulated, whereas further maturation of the cells requires appropriate environmental stimulation.

Developmental expression of myelin proteins by oligodendrocytes in the CNS of trout

Using immunohistochemical techniques, the pattern of cytoplasmic staining and the temporal order of expression of 5 major myelin components of oligodendrocytes were studied in the developing central nervous system of trout. The two myelin glycoproteins, IP1 and IP2, in the cytoplasm of glial cells showed a granular pattern of immunostaining, whereas the 36K protein was homogeneously distributed. Analysis of freshly dissociated cells during early stages of myelinogenesis revealed a constant chronological sequence of expression of myelin proteins by the oligodendrocytes: glycoprotein IP2 was the first protein to appear during glial development together with the galactocerebroside GalC at stage 28 followed by the 36K at stage 30 and finally IP1 at stage 32. The deposition of myelin proteins into the nascent myelin sheath occurred in the same chronological order as their expression by oligodendrocytes. Moreover myelin basic protein, which was not detectable in glial cells, on tissue sections was found to appear in parallel with IP2.

Cell cultures enriched in oligodendrocytes from the central nervous system of trout in terms of phenotypic expression exhibit parallels with cultured rat Schwann cells

Oligodendrocytes were isolated from the white matter of young trout by Percoll density centrifugation of enzymatically dissociated tissue and cultured on poly-D-lysine-coated petri dishes. Using antisera recognizing myelin-specific compounds of fish CNS (36K, IP2) up to 72% of the isolated cells could be identified as oligodendrocytes with an average yield of 4 x 10(6) cells per gram of wet tissue. Taken in culture, the cells rapidly regenerated their processes and soon acquired a morphology closely resembling mammalian oligodendrocytes in vitro. On the other hand, in terms of phenotypic expression, interesting parallels were revealed with the known in vitro behavior of Schwann cells: Galactocerebroside, which in mammalian oligodendrocytes is persistently expressed over longer periods of time in vitro, rapidly disappeared from the surface of cultured trout oligodendrocytes. In contrast, the fish CNS myelin glycoprotein IP2, which like IP1 is immunologically related to the major myelin product of Schwann cells, P0, was continuously expressed over several weeks in culture. Two other myelin protein constituents, 36K and IP1, transiently declined in vitro, but later on fully reappeared in the glial cells of trout. The present cell culture system offers an experimental model for studying in vitro the factors underlying oligodendroglial regeneration and remyelination in the fish CNS.

Electrophoretic characterization and immunoblot analysis of the proteins from the myelin-like light membrane fraction of shrimp ventral nerve (Penaeus duorarum)

1. The proteins of the light membrane fraction (LMF) from the ventral nerve of the pink shrimp (Penaeus duorarum) were separated by SDS gel electrophoresis and analysed by staining and immunoblotting. 2. Shrimp LMF carried four major proteins with apparent molecular weights of Mr = 21,500, 40,000, 78,000, 85,000 and four minor components (Mr = 36,000, 41,500, 43,000, 50,000). 3. None of these proteins bound Concanavalin A. 4. The four major proteins showed no reaction with antisera against six vertebrate myelin proteins. Only the minor Mr = 50,000 component was weakly recognized by the antibodies against mammalian myelin P0 protein.

Monospecific antiserum against 5'-nucleotidase from Torpedo electric organ: immunocytochemical distribution of the enzyme and its association with Schwann cell membranes

The cellular and subcellular distribution of 5'-nucleotidase in tissues of the electric ray Torpedo marmorata has been investigated by means of an antiserum raised against the native enzyme purified from the electric organ. As revealed by immunohistochemistry the enzyme is associated with the surface of the axons of the electric nerves and of spinal nerves. Using the post-embedding colloidal gold technique at the electron-microscopical level 5'-nucleotidase could be located at the plasma membrane of the Schwann cells including the myelin and the fine processes covering the terminal axon ramifications. Also the perineurial sheath of the axons inside the electric organ is 5'-nucleotidase positive. The plasma membrane of the axon and the terminal axon region or the postsynaptic membrane do not contain 5'-nucleotidase. Immunoprecipitation studies using polyacrylamide beads suggest that the ecto-Ca2+- or -Mg2+-adenosine 5'-triphosphatase previously ascribed to synaptosomes of the Torpedo electric organ is not associated with the same membranes as 5'-nucleotidase. Within the electric organ the dorsal plasma membrane of the electroplaque cell, blood capillaries and the connective tissue layer surrounding the columns of electroplaque cells also bind the antibodies. In central nervous tissue solely blood vessels show immunofluorescence. Within the electric lobe both the surface of the electromotor neurons as well as the myelinated axons giving rise to the electric nerve are negative. This also applies to the axons of the optic nerve suggesting that the antiserum is Schwann cell specific, and does not bind to a potential oligodendroglial 5'-nucleotidase. In peripheral tissue the surface of skeletal muscle fibres as well as that of individual myofibrils bind the anti-5'-nucleotidase antibodies. Our results demonstrate that the Schwann cell plasma membrane, including myelin, contains 5'-nucleotidase and that one can distinguish by means of a specific antiserum between Schwann cell and oligodendroglia plasma membranes. The functional significance of the association of 5'-nucleotidase with Schwann cells along the entire surface of axons including the synaptic region as well as with other parts of the electric tissue is discussed regarding its catalytic activity and also the possibility that this surface glycoprotein may be involved in mediating cellular interactions.

Immunological evidence for the presence of myelin-related integral proteins in the CNS of hagfish and lamprey

Antibodies against myelin proteins were utilized in the analysis of total particulate material from the brains of the agnathan hagfish and lamprey. Immunoblotting revealed in both species the presence of bands at 50,000 dalton that reacted with anti-bovine PNS-P0 antibodies. Single bands of 34,000 dalton and 51,000 dalton were immunodetected with anti-trout CNS-36K antibodies in lamprey and hagfish, respectively. Antibodies against mammalian myelin basic protein (MBP) and proteolipid protein (PLP) were not recognized. In spite of the lack of multilayered myelin in agnatha, the presence of myelin-related integral proteins suggests that agnathan glial cells have already acquired the capacity to synthesize some proteins that are similar to typical myelin proteins. This represents a crucial evolutionary step towards myelination.

Antigenic sites common to major fish myelin glycoproteins (IP) and to major tetrapod PNS myelin glycoprotein (Po) reside in the amino acid chains

The major myelin glycoproteins in the CNS and PNS of trout (IP) were enzymatically deglycosylated with endoglycosidase F (Endo F) and examined by electro-immunoblotting. Following carbohydrate removal and loss of concanavalin A affinity each of the four IP components underwent a similar reduction in molecular size, corresponding to approximately 3,000 daltons. Immunological cross-reactivities with anti-bovine Po or anti-trout IP2 antibodies, were however fully retained by the Endo F cleavage products. This strongly implies that the antigenic sites shared by the mammalian Po protein and the various intermediate glycoproteins of trout CNS and PNS are located in the protein portion. Immunoblot analysis of the PNS myelin proteins from various species of the major vertebrate classes with anti-trout IP2 antiserum revealed striking differences in the immunological properties of the individual Po components which were not detected when anti-bovine Po antiserum was used as a probe.

A glycosylated proteolipid protein is common to CNS myelin of recent lungfish (Ceratodidae, Lepidosirenidae)

1. Myelin proteins from the CNS of recent lungfish (Lepidosiren paradoxa, Protopterus dolloi, Neoceratodus forsteri) were separated and analysed by staining and immunoblotting. 2. All species showed a glycosylated component (g-PLP) that cross-reacted with antibodies against tetrapod proteolipid protein (PLP), indicating phylogenetic relationships with amphibia. 3. Actinopterygian IP or teleostean 36k components were not detectable in lungfish CNS myelin. 4. The identical size of g-PLPs from Lepidosiren and Protopterus (Mr = 29,000) underlines the close relationship of the Lepidosirenidae. The smaller size of g-PLP from the ceratodidan Neoceratodus forsteri (Mr = 27,500) pointed to an earlier diversion.

Central nervous system myelin of teleosts: comparative electrophoretic analysis of its proteins by staining and immunoblotting

CNS myelin was isolated from 24 teleostean fishes and the proteins were analyzed by staining and immunoblotting. All species showed a 36 K protein, two or more glycosylated hydrophobic intermediate protein (IP) components and several myelin basic protein bands (BP). The 36 K protein was specific for teleostean fishes. The IP and BP components displayed substantial variations in their proportions as well as in molecular sizes when comparing the different teleosts. This contrasts with CNS myelin proteins which appear more stable in terrestrial vertebrates.