Association of axonally transported proteins with goldfish brain myelin fractions

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.

Study of a floating fraction obtained during preparation of myelin from degenerating goldfish optic tract

A floating fraction that layers on top of 0.25 sucrose has been obtained during the preparation of myelin from intact and 9 day degenerating goldfish optic tracts. The proportion of total tract protein isolated in floating fraction rises from 6.6% to 11.0% during degeneration. This increase is paralleled by a morphologically observed splitting of myelin lamellae. Floating fraction contains all of the major myelin proteins but shows a 40% increase in the proportion of basic protein and a 2-3 fold decrease in the proportion of IP proteins (intermediate molecular weight glycoproteins) and a 36 Kd (X) protein. The lipid to protein ratio is slightly higher in floating fraction than myelin. Lipid composition is characterized by 1/2-1/3 the myelin levels of galactolipids and twofold increased levels of triglycerides and cholesterol esters. Electron microscopy of floating fraction shows a mixture of myelin fragments with few lamellae and single membrane fragments. Taken together the results indicate that floating fraction in the degenerating goldfish optic tract is at least partially derived from the breakdown of myelin.

Immunoblot identification of 13.5 kilodalton myelin basic protein in goldfish brain myelin

Myelin isolated from goldfish brain shows a multilamellar structure with a major dense line and two intraperiod lines. Sodium dodecyl sulfate gel electrophoresis revealed that the protein profile of goldfish brain myelin is distinctly different from that of rat brain myelin. No protein migrating to the position of proteolipid protein or DM-20 was seen in goldfish myelin. Goldfish acclimated to 5 degrees, 15 degrees, and 30 degrees C showed no qualitative differences in myelin proteins. The 13.5 kD protein in goldfish brain myelin and brain homogenate was intensely immunostained with the antiserum to human basic protein by the immunoblot technique. In contrast, none of the proteins of goldfish myelin were immunostained with antiproteolipid protein serum; however, both proteolipid protein and DM-20 of rat brain myelin were immunostained. The significance of the synthesis of myelin proteins by astrocytes in the goldfish brain is discussed.

Biochemical characterization of the central nervous system myelin proteins of the rainbow trout, Salmo gairdneri

Central nervous system myelin isolated from the rainbow trout (Salmo gairdneri) displays a very low median density on zonal gradient centrifugation, banding at approximately 0.32 M sucrose. Its proteins consist of a 36 K (36,000 mol.wt.) component, two Concanavalin A-reactive intermediate proteins IP1 (23,000 mol.wt.) and IP2 (26,200 mol.wt.), and two basic proteins BP1 and BP2, of which the latter co-migrates with rat SBP while BP1 is of slightly smaller size. The trout myelin proteins electrofocus at pH positions similar to those of their mammalian counterparts. Immunoblotting shows that antibodies against rat PNS myelin P0 glycoprotein are bound by IP1 and IP2, but not by 36K. None of the trout myelin proteins react with anti-rat CNS myelin proteolipid protein (PLP) antiserum. The basic proteins BP1 and BP2 bind strongly to antibodies directed against human myelin basic protein. In vivo injection of tritiated fucose or palmitate leads to radiolabeling of IP1 and IP2. Under autolytic in situ conditions the appearance of a glycosylated 20,000 mol.wt. component (IP0) is noted, with parallel reduction of both IP1 and IP2, indicating sequence homologies between IP1 and IP2. The 36K protein is not affected by autolysis.

Vectorial discharge and localization of nascent polypeptides in rough endoplasmic reticulum of developing neuronal perikarya of rat brain cortex

Rough endoplasmic reticulum (RER) prepared from bulk-isolated neuronal perikarya of rat brain cortex of different postnatal ages was found to be active in vectorial discharge of nascent proteins through the membrane; this activity increased with the increasing age of animals and reached maximal values in adults. RER isolated from whole cortical tissue (containing all cell types) exhibited vectorial release only up to 18 days of age; the preparation from adult animals was essentially devoid of secretory activity. Controlled proteolysis of various preparations suggested that in neuronal RER of 8-day-old rats the proportion of nascent proteins operationally retained in the intravesicular space was about twice that retained by cortical preparations. For the purpose of comparison, these parameters were studied also in liver RER.

Comparison of the major proteins of shark myelin with the proteins of higher vertebrates

On gel electrophoresis in dodecyl sulphate solutions shark CNS myelin showed four bands close in mobility to the proteolipid protein of bovine CNS myelin. They had apparent molecular weights of 21,000, 26,000, 27,000, and 31,500. Unlike bovine proteolipid protein, all of these shark proteins were shown to be glycosylated by staining gels with the periodate-Schiff reagent. Amino acid analyses of the polypeptides eluted from polyacrylamide gels indicated a high content of apolar amino acids and a composition approximating that of the Po protein of bovine peripheral nervous system (PNS) myelin, rather than that of the CNS proteolipid protein. The shark polypeptide of apparent molecular weight 31,500 was obtained by elution from dodecyl sulphate gels and antibodies raised against it in rabbits. By probing of electroblots with this antiserum the four shark CNS bands were shown to share common determinants with each other, with a major shark PNS protein and with sheep and chicken major PNS glycoproteins (Po). The binding of antibody was unaffected by deglycosylation of the shark CNS polypeptides with anhydrous hydrogen fluoride. Together, these results appeared to establish that shark CNS myelin contains four proteins that are closely related to a major shark PNS protein and to the Po protein of higher species.

Protein analysis of myelin isolated from the CNS of fish: developmental and species comparisons

The protein composition of myelin isolated from the CNS of four different fish species (trout, goldfish), eel, Tilapia) was analysed by SDS-polyacrylamide gel electrophoresis and compared with that of pig and rat brain. Thereby the following features were found typical to the myelin of fish: (1) a basic protein of particular low molecular weight, (2) the entire absence of Wolfgram protein, (3) the appearance of an additional major component of medium molecular weight (around 36,000) and (4) a strong Con A-affinity exhibited by the intermediate proteins, being most clearly discernible in trout and eel myelin. During development of the trout brain, in particular, a myelin fraction could first be isolated from the brainstem at 14 days after hatching and the myelin yield steadily increased during the first year of life. In tectum and cerebellum main myelin accumulation was stated during the third and seventh month. Concomitantly marked changes in the relative distribution of major myelin proteins were observed: especially the IP1-protein showed a marked increase during the first half year, while the relative amount of the 36-K protein was gradually declining during the same time.

Rapidly transported proteins released from nerve and isolated with myelin

An in vitro preparation was used to determine if rapidly transported radiolabeled protein which is isolated with the myelin fraction of frog sciatic nerves is similar to rapidly transported radiolabeled protein released from sciatic nerves. Sciatic nerves along with their eighth and ninth dorsal root ganglia were placed in modified Warburg flasks. The center well of the flasks contained 100 microCi [3H]leucine. The preparation was incubated for 24 h, the sciatic nerve removed and the myelin isolated. The solution in which the sciatic nerve portion of the preparation incubated was removed, dialyzed and lyophilized. The radiolabeled proteins isolated from myelin and those released from the nerve were separated on 10% SDS polyacrylamide gels. Results of this separation revealed that a radiolabeled protein of 100,000 daltons was isolated with myelin and released from sciatic nerves. Incubating isolated myelin in 10 mM EDTA for 14-16 h brought about the release of a protein of 100,000 daltons.

Central nervous system myelin proteins and glycoproteins in vertebrates: a phylogenetic study

CNS myelin was isolated by a conventional method from a wide range of vertebrate classes and analyzed by SDS-PAGE for proteins (Coomassie blue) and glycoproteins (concanavalin A (Con-A)-peroxidase). Mammalian, avian and reptilian myelin shared similar protein patterns (basic protein, BP; intermediate protein, DM-20; proteolipid protein, PLP; Wolfgram protein, W). Amphibians lacked DM-20 but were characterized by specific activities of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) higher than those of the other classes examined. The Con A-binding profiles were similar in the high molecular weight (HMW) regions of the classes listed above, while the typical myelin proteins in the low molecular weight (LMW) regions were devoid of Con A-binding properties. In teleost myelin a putative BP band ran well ahead of rat small basic protein (SBP), whereas the region corresponding to rat PLP was covered by several closely spaced bands, most of which bound Con A. In elasmobranch myelin, apart from bands corresponding to BP, Con A-binding glycoproteins were detected migrating in the region of rat DM-20 and PLP as well as with mammalian PNS P0 protein. Cyclostomates yielded only very small amounts of material in the myelin preparation and displayed undifferentiated Coomassie blue- and Con A-binding in the HMW region, while typical LMW myelin proteins were absent. These results demonstrate that CNS myelin from bony and cartilaginous fishes is characterized by containing several major Con A-binding proteins of low molecular weight. This is in striking contrast to myelin from phylogenetically higher classes.

Study of myelin purity in relation to axonal contaminants

Axonal remnants are considered a probable source of contamination of isolated myelin in view of the relatively tight axon-glial intercellular junction. Using the rabbit optic system to label specifically axonal components, we have found the levels of such contaminants to depend on the myelin isolation procedure, the tissue source, and the nature of the contaminant. A procedure employing repetitive treatments with EGTA was found to be highly effective in removing proline-labeled axonal proteins, the estimated upper limit of such contamination being approximately 0.6-1.2% of the myelin protein. The standard isolation procedure of Norton and Poduslo, supplemented with an additional discontinuous gradient step, proved equally effective in removing rapidly transported proteins from myelin isolated from the superior colliculus or lateral geniculate body. When the optic tract was the source, however, the EGTA procedure proved more effective in removing both rapidly and slowly transported proteins. Axonal gangliosides labeled with N-[3H] acetylmannosamine were efficiently removed by both procedures, adding support to the proposition that gangliosides detected in isolated myelin are intrinsic to that membrane.

Labelling by axonal transport of myelin-associated proteins in the rabbit visual pathway

After intraocular injections of [3H]leucine, six regions of the visual pathway of adult rabbit were used to study the spatio-temporal pattern of the slow anterograde axonal transport of radioactive proteins associated with the particulate fraction, the water-soluble fraction and the myelin fraction. Unlike other fractions, myelin-associated labelled proteins represented a time-constant (for a given region) percentage of total tissue radioactivity. This percentage increased from the first half to the second half of the optic nerve and remained high in the chiasma and tract. The peak specific radioactivity of myelin decreased in the same direction. Myelin proteins were separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and the labelling patterns obtained in different regions and at different survival times were compared. At the peak of myelin radioactivity of a given region the label was typically associated with four protein bands, L1, L2, L3 and L4, of 40000, 44000, 62000, and 68000 mol.wts. respectively. The basic protein, the proteolipid protein and the W1 component (mol.wt. 51000-53000) of the Wolfgram proteins were not significantly labelled. The radioactivity associated with the W2 component (mol.wt 60000) of the Wolfgram proteins could be derived from the closely migrating L3 component. At shorter survival times no clear labelling pattern could be detected. At longer survival times radioactivity was almost totally localized around band L3. The results presented underline the importance of choosing appropriate experimental conditions to obtain a consistent labelling pattern of myelin-associated proteins and to investigate the possible mechanism responsible for this phenomenon.

Differential turnover of axonally transported glycoproteins

Metabolic turnover of axonally transported glycoproteins has been examined in membranous and soluble subfractions of goldfish optic tectum following intraocular injection of [3H]fucose. Radioactivity in total transported glycoproteins reached a maximum in the tectum after 24-30 hr, then declined with a half-life of approximately 20 days. Radioactivity in the total membranous subfraction declined with a similar half-life of 20-21 days while radioactivity in the soluble fraction showed a significantly shorter half-life of approximately seven days. Various sized glycopeptides derived from the membranous subfraction showed differential rates of loss of radioactivity with the lower molecular weight nondialyzable molecules displaying the most rapid turnover. In contrast, the glycopeptides derived from the soluble fraction showed relatively uniform rates of turnover. The results are discussed in the context of metabolic compartmentalization between membranous and soluble glycoproteins and among the carbohydrate chains of the membranous molecules.

Similar polypeptide composition of fast-transported proteins in rat motor and sensory axons

SDS-polyacrylamide gel electrophoresis was used to characterize labeled proteins transported in rat motor and sensory axons after application of 3H-leucine to the neuron cell bodies. Two types of experiments were performed: first, transported protein accumulating proximal to a ligature placed on the sciatic nerve was analyzed; second, the segment of sciatic nerve nearest to the "wavecrest" of transported protein travelling down the nerve was analyzed. In both cases, no significant differences in peak position or amplitude were found in gels containing labeled proteins from motor or sensory axons. This may mean that the majority of fast-transported protein is involved in an axonal function common to the two types of neuron.

Release of protein from axons during rapid axonal transport: an in vitro preparation

An in vitro system from the frog was used to study fast axonal transport and determine if transported protein is released from the axons. This preparation included the eighth and ninth dorsal root ganglia with their roots, sciatic nerve and gastrocnemius muscle. The preparation was placed in three-compartment chamber with each compartment separated by a silicone grease barrier. The dorsal root ganglia were incubated in [14C]leucine for 5 h in compartment A. The labeled protein was transported down the axon from compartment A to compartment B. The sciatic nerve in compartment B was superfused with frog Ringer. This solution was collected in hourly samples and dialyzed to remove unincoprorated leucine before counting. Incubating the ganglia in 100 microng/ml cycloheximide in frog Ringer blocked the release of labeled protein from the axon. Superfusing compartment B with solution containing 100 microng/ml cycloheximide inhibited axonal and Schwann cell protein synthesis, but did not block the release of labeled protein. It was concluded that the labeled protein released into the superfusing solution was synthesized in the ganglia and transported to the axon before release. SDS acrylamide gels were used to separate the labeled proteins. Sectioning the gels in 2 mm slices and determining the radioactivity showed that 80-85% of the counts were contained in two fast moving bands.

Studies on the control of myelinogenesis. II. Evidence for neuronal regulation of myelin production

Tritiated thymidine has been used as a nuclear marker to trace the origin of Schwann cells, sited in the distal stump of a severed unmyelinated nerve, which are able to elaborate myelin around axons regenerating from an anastomosed proximal stump of a severed myelinated nerve. Two types of cross-anastomosis experiments were performed in young, adult rats: (1) the proximal stump of a myelinated sternohyoid nerve was labeled (5 mCi/kg body weight) selectively over a 4-day period of predetermined maximal thymidine uptake and two days later, after flushing the animal repeatedly with cold thymidine, the unmyelinated cervical sympathetic trunk was transected and its unlabeled distal stump linked to the proximal stump of the labeled sternohyoid nerve; (2) the distal stump of an unmyelinated cervical sympathetic trunk was labeled selectively over a 5-day period of predetermined maximal uptake and two days later, after flushing with cold thymidine, the myelinated sternohyoid nerve was severed and its unlabeled proximal stump linked to the labeled distal stump of the cervical sympathetic trunk. The fate of the labeled cells in each type of anastomosis was determined 3 weeks later by autoradiography and liquid scintillation spectrometry. In the first type, a small amount of label had migrated from proximal stumps but labeled Schwann cells were not found in successfully anastomosed distal stumps. In the second type, labeled Schwann cells were seen in the cervical sympathetic trunk in association with myelinated and non-myelinated axons regenerating from the sternohyoid nerve. These data suggest that the presence or absence of myelin formation by a Schwann cell is controlled by some property of the axon with which it is associated. Putative mechanisms underlying neuronal control of myelinogenesis are discussed.

Multipotentiality of Schwann cells in cross-anastomosed and grafted myelinated and unmyelinated nerves: quantitative microscopy and radioautography

Cross-anastomoses and autogenous grafts of unmyelinated and myelinated nerves were examined by electron microscopy and radioautography to determine if Schwann cells are multipotential with regard to their capacity to produce myelin or to assume the configuration seen in unmyelinated fibres. Two groups of adult white mice were studied. (A) In one group, the myelinated phrenic nerve and the unmyelinated cervical sympathetic trunk (CST) were cross-anastomosed in the neck. From 2 to 6 months after anastomosis, previously unmyelinated distal stumps contained many myelinated fibres while phrenic nerves joined to proximal CSTs became largely unmyelinated. Radioautography of distal stumps indicated that proliferation of Schwann cells occurred mainly in the first few days after anastomosis but was also present to a similar extent in isolated stumps. (B) In other mice, CSTs were grafted to the myelinated sural nerves in the leg. One month later, the unmyelinated CSTs became myelinated and there was no radioautographic indication of Schwann cell migration from the sural nerve stump to the CST grafts. Thus, Schwann cell proliferation in distal stumps is an early local response independent of axonal influence. At later stages, axons from the proximal stumps cause indigenous Schwann cells in distal stumps from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously myelinated nerves become associated with unmyelinated fibres. Consequently, the regenerated distal nerve resembled the proximal stump. It is suggested that this change is possible because Schwann cells which divide after nerve injury reacquire the developmental multipotentiality which permits them to respond to aoxonal influences.

Incorporation of newly formed lecithin into peripheral nerve myelin

Radioactive choline was used to study the metabolism and movement of choline-containing phospholipids in peripheral nerve myelin of adult mice. Incorporation at various times after intraperitoneal injection was measured in serial segments of sciatic nerve as well as in myelin isolated from those segments. At no time (1 h to 35 days) could a proximal-distal difference in the extent of labeling be demonstrated. This finding suggests that incorporation of precursor choline phospholipids into nerve membranes is a local event, with little contribution from the neuronal perikaryon via axoplasmic transport. Autoradiographic investigations were undertaken to elucidate the pattern of movement of radioactive choline-labeled phospholipids, predominantly lecithin, into the myelin sheaths of the sciatic nerve. A sequence of autoradiographs was prepared from animals sacrificed between 20 min and 35 days after a microinjection of precursor directly into the nerve. Analysis of these autoradiograms revealed that labeling is initially concentrated in the Schwann cell cytoplasm. Later, the label moves first into the outer regions of the myelin sheaths and is eventually distributed evenly throughout the inner and outer layers of the sheath. At no time is there a build-up of label in the axon. The rate of uptake of precursor and subsequent redistribution of lecithin into the myelin were also examined in frog sciatic nerve (18 degrees C). Both uptake and redistribution processes were considerably slower in the cold-blooded animal.

Isolation of axonally transported glycoproteins with goldfish visual system myelin

[3H]fucose labeled glycoproteins which are rapidly transported in the goldfish optic nerve have been isolated with purified preparations of goldfish optic tectal myelin. SDS acrylamide gels of myelin proteins isolated one day after intraocular injection of fucose show a broad distribution of radioactivity among high molecular weight proteins. At progressively longer times after injection there is a shift in the distribution of radioactivity with the buildup of a peak of label with electrophoretic mobility corresponding to a molecular weight of approximately 50,000. There is no corresponding peak of radioactivity in gels of total tectal membranes. Gels of myelin labeled through intracranial injections of fucose also show no single prominent peak of radioactivity. Results are discussed with reference to possible mechanisms for the shift in radioactivity patterns on gels and the possible functional significance of the myelin-associated glycoproteins.

Association of proteins undergoing slow axonal transport with goldfish visual system myelin

[3H]Proline derived radioactivity which is slowly transported in the goldfish optic nerve has been isolated in association with purified preparations of optic tectal myelin. Acrylamide gel analysis reveals that most of the slowly transported radioactivity is distributed among the high molecular weight myelin proteins with a major peak comigrating with a band (approximately 42,000 daltons) which is prominent in goldfish brain myelin. A similar peak is not seen in gels of whole synaptosome protein. Relatively small but measureable amounts of radioactivity comigrate with the basic and proteolipid proteins. A low density myelin subfraction is 3-5 fold lower in specific radioactivity (disint./min/mug protein) than a corresponding high density fraction and shows particularly low specific radioactivities in the basic and proteolipid bands. The results are discussed with reference to the possible adherence of specific axonal membrane fragments as well as other potential sources of myelin label.