Transport of sulfatides towards myelin. Effect of colchicine, monensin and calcium on their intracellular translocation

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

Role of rRAB22b, an oligodendrocyte protein, in regulation of transport of vesicles from trans Golgi to endocytic compartments

Intracellular membrane trafficking plays an essential role in the biogenesis and maintenance of myelin. Members of the Rab protein family are important components of the systems that regulate intracellular vesicle transport. We examine the function of rRab22b, a novel rat Rab protein cloned from an oligodendrocyte cDNA library, by visualizing and identifying in living Hela cells the organelles that contain rRab22b. Our results show that rRab22b is present in the trans Golgi/TGN and endocytic compartments. Trafficking of membranes from trans Golgi to endocytic compartments takes place via small tubulo vesicular organelles containing rRab22b. The formation of vesicles in the trans Golgi also appears to be regulated by rRab22b. Additionally, our results suggest that rRab22b controls the transport of vesicles from the trans Golgi to endocytic compartments that localize in oligodendrocyte processes. That rRab22b is involved in the transport of certain proteins from trans Golgi to myelin is suggested by the evidence that certain proteins being targeted to the plasma membrane are first transported from trans Golgi to endocytic compartments.

Cytoskeletal reorganization during the formation of oligodendrocyte processes and branches

During oligodendrocyte development, signals relevant to process formation must be transduced into appropriate changes in cytoskeletal organization. We have explored how microtubules and microfilaments interact during the outgrowth and branching of oligodendrocyte processes in culture. We observed that microfilaments are enriched in the peripheral region beneath the plasma membrane and constitute the major cytoskeletal element at the leading edge of the process, which is also enriched in the B-isoform of the non-muscle myosin II heavy chain. Microtubules form a dense bundle within the process and splay before extending into the leading edge and branches, following tracks laid by microfilaments. Pharmacologic disruption of microfilaments and microtubules compromised normal process outgrowth and branching. However, microtubules rapidly reinvaded most processes after removal of both antimicrotubule and antimicrofilament drugs, but the reinvasion was severely compromised if the antimicrofilament drug was retained. These results are consistent with the hypothesis that microfilaments guide the local reorganization of microtubules for the elongation of oligodendrocyte processes and the formation of new branches.

Molecular analysis of the monomeric GTP-binding proteins of oligodendrocytes

Vesicle transport plays an important role in the formation of myelin. Transport of proteins, including proteolipid protein and myelin associated glycoprotein, from their site of synthesis in the endoplasmic reticulum in the perikaryon of the oligodendrocytes, to myelin, takes place via carrier vesicles. The mechanisms that regulate vesicle transport in oligodendrocytes are largely unknown. The presence of monomeric GTP-binding proteins in myelin and oligodendrocytes suggested the hypothesis that these proteins participate in the regulation of vesicle transport. In an attempt to identify the Rab and Rho GTP-binding proteins present in oligodendrocytes, a cDNA library specific for these proteins was generated using a reverse transcriptase-polymerase chain reaction (RT-PCR) approach. Twelve different clones containing sequences that coded for members of the Rab and Rho families of GTP-binding proteins were isolated. This group includes Rab1, -1b, -2, -5b, -5c, -7, -8, -12, -14, -23 and Rho A. One additional clone revealed a novel cDNA sequence. Analysis of the effector loop motif indicated that this sequence encodes for a member of the Rab family. We refer to this new sequence as Rab0. Comparison of Rab0 with the most similar rat Rab sequences, Rab 14 and Rab 22, and with a recently cloned human Rab22b, showed a 71%, 72% and 94% identity, respectively. By RT-PCR analysis the Rab0 mRNA was found to be mainly expressed in oligodendrocytes and to a lesser extent in oligodendrocyte precursors, astrocytes and microglia. Moreover, the highest levels of Rab0 mRNA were observed in areas of the brain that are heavily myelinated. Rab0 mRNA was also detected in other tissues such as kidney, liver, skeletal muscle. These data provide initial evidence regarding signal transduction pathways that regulate intracellular transport in oligodendrocytes.

Vesicular transport of myelin proteolipid and cerebroside sulfates to the myelin membrane

The possibility that cerebroside sulfates and myelin proteolipid (PLP) could be simultaneously located in transport vesicles destined to be assembled in myelin was investigated in the brain of 20 day old rats. The brain was homogenized and fractionated according to Burkart et al. (J Biol Chem 257:3151-3156, 1982) to obtain a microsomal fraction that was further subfractionated in a linear sucrose density gradient following the procedure of Siegrist et al. (J Neurochem 33:497-504, 1979) to obtain a vesicular fraction which has been shown to transport cerebroside sulfates (Burkart et al., as above). This fraction was associated with acid hydrolase activity and had a lipid composition different from that of myelin and microsomal fractions. Studied by slab gel electrophoresis, dot blot, and Western blot analysis, using a highly specific anti-PLP antibody, it was found to contain myelin PLP. In view of previous findings of several laboratories including our own, the presence of myelin proteolipid in a vesicular fraction which is related to the transport of cerebroside sulfates gives further support to the hypothesis that the delivery of both constituents to the myelin membrane could be associated.

Cultured oligodendrocytes metabolize a fluorescent analogue of sulphatide; inhibition by monensin

It has been suggested that oligodendrocytes can actively phagocytose myelin debris during active myelination or after injury and experimental demyelination. Therefore, we have used a fluorescent analogue (N-lissamine rhodaminyl-(12-aminododecanoyl) cerebroside 3-sulphate) to study the metabolic fate of sulphatide, a galactosphingolipid that is highly enriched in myelin membranes. The fluorescent sulphatide was incorporated in small unilamellar vesicles and administered to cultured oligodendrocytes. The association of the lipid probe to the cells in culture was saturable in time and with the concentration of the probe. The processes of association, internalization and subcellular distribution were followed by confocal scanning laser microscopy and appeared to be very rapid. Within 20 min a marked perinuclear staining was seen. After prolonged incubation the fluorescence distributed gradually over the cytoplasm and into cellular branches along structures suggestive of cytoskeletal elements. Lipid analysis demonstrated that ceramide was the major metabolite present in the cells but galactosylceramide, sphingomyelin and free fatty acid were also detected. In the culture medium only free fatty acid and sphingomyelin were found. Monensin did not affect the cellular association and internalization of the fluorescent sulphatide but markedly reduced its conversion to metabolic products. These results indicate that exogenous sulphatide is targeted to the Golgi apparatus prior to its lysosomal degradation.

Alteration of intracellular traffic by monensin; mechanism, specificity and relationship to toxicity

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of protein synthesis during CNS myelination produces focal accumulations of membrane vesicles in oligodendrocytes

Optic nerves of Xenopus tadpoles were exposed to cycloheximide to identify changes that occur during CNS myelin membrane formation when protein synthesis is inhibited. Groups of stage 51-56 tadpoles were immersed in either 10 or 20 micrograms ml-1 cycloheximide, and at specified times between 12 and 18 h after initial immersion tadpoles were killed and their optic nerves prepared for ultrastructural analysis. As early as 12 h there were alterations in oligodendrocytes from treated animals compared with control animals. The number of polyribosomes in the perikarya and cell processes was greatly reduced and the rough endoplasmic reticulum was disorganized. Mitochondria and microtubules were normal in appearance. Many oligodendroglial tongue processes at the inner margin of the myelin sheath were enlarged, occasionally indented the axon and were filled with vesicular profiles. Vesicles were noted in other cytoplasmic regions of oligodendrocytes and focal changes in the lamellar structure of myelin were found in paranodal regions. The internodal portions of the myelin sheath, axons and astrocytes appeared normal. Polyacrylamide gels of optic nerves showed that the incorporation of 35S-methionine into polypeptides had been almost completely inhibited by treatment with cycloheximide. These observations suggest that cycloheximide, by inhibiting synthesis of myelin proteins, alters the ability of oligodendrocytes to incorporate membrane components into CNS myelin sheaths.

Inhibition of the synthesis of glycosphingolipids affects the translocation of proteolipid protein to the myelin membrane

Brain slices obtained from young rats were incubated with different radioactive precursors, in the presence and absence of L-cycloserine (an inhibitor of the synthesis of sphingosine) in order to explore the possibility that transport of proteolipids--and specifically of the major myelin proteolipid PLP--to the myelin membrane could be coupled to the transport of cerebrosides or sulfatides. At a concentration of 0.15 mM L-cycloserine, the incorporation of [3H] glycine into total proteins, proteolipid apoproteins (APL), PLP, and myelin basic proteins (MBP) of the total homogenate was unaffected by the presence of the inhibitor, whereas the incorporation of [3H] serine into glycosphingolipids decreased markedly. Under similar incubation conditions, the entry of labeled APL and of PLP into the myelin membranes in the presence of L-cycloserine decreased markedly (50%) in comparison to controls. Entry of MBP was not affected by the inhibitor. These results indicate that when synthesis of glycosphingolipids is inhibited by L-cycloserine, thus decreasing the availability of cerebrosides and sulfatides, the translocation of PLP to myelin is disrupted, suggesting that its transport through the oligodendroglial cell could be coupled to the transport of glycosphingolipids and, most probably, of sulfatides.