Phosphoinositide breakdown in isolated myelin is stimulated by GTP analogues and calcium

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.

The phosphoinositide signaling cycle in myelin requires cooperative interaction with the axon

Previous studies on the origin of myelin phosphoinositides involved in signaling mechanisms indicated axon to myelin transfer of phosphatidylinositol followed by myelin-localized incorporation of axon-derived phosphate groups into phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5-bisphosphate. This is in agreement with other studies showing the presence of phosphorylating activity in myelin that converts phosphatidylinositol into the mono-and diphospho derivatives. It was also found that the second messenger, inositol 1,4,5-trisphosphate, is hydrolyzed to inositol 1,4-bisphosphate by a myelin-localized enzyme. The present study was undertaken to determine the locus of the remaining reactions leading to formation of free inositol and completion of the cycle by resynthesis of phosphatidylinositol. The latter reaction was found to occur preferentially in isolated axons, and to a limited extent if at all in myelin. On the other hand, hydrolytic reactions which sequentially convert inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, inositol 1-phosphate, and free inositol were found to occur more prominently in myelin. Thus, restoration of phosphoinositides following signal-induced breakdown of PIP2 in myelin is seen as requiring metabolic interplay between myelin and axon.

Biochemistry of demyelination

The myelin sheath, a lipid-rich multilamellar membrane of relative stability, both insulates and enhances conduction in nerve axons. A notable feature of myelin-specific proteins, in particular myelin basic protein, is their susceptibility to proteolytic activity and their encephalitogenicity, which induces inflammatory demyelination in the CNS. The final common pathway of myelin breakdown in vivo is well documented and there is evidence that myelin disruption can be mediated directly by soluble (circulating) factors and for following receptor-driven phagocytosis by macrophages. However the exact mechanism(s) of demyelination in multiple sclerosis is still unresolved, both antigen-specific and--non-specific events having the potential to generate the myelinolytic process.

Characterization of guanylyl cyclase in purified myelin

This study was undertaken to characterize the enzymatic properties of the particulate guanylyl cyclase previously shown to be present at a high level of activity in purified rat brain myelin. Significant activation was achieved by both Lubrol-PX and Triton X-100, the latter being somewhat more effective. A pH optimum of 7.8 was observed, compared to 7.4 for microsomes. Employing 1.2 mM GTP with 1% Triton X-100, linearity of response was observed up to 60 min and approximately 1.2 mg of myelin protein. Kinetic analysis revealed Km values of 0.258mM and 0.486mM for myelin and microsomes, respectively, similar values being obtained by Lineweaver-Burke analysis or Direct Linear Plot. Vmax values were 20 and 266 pmol/mg protein/min for myelin and microsomes, respectively. Washing of the myelin with 0.5 M NaCl or 0.1% Na taurocholate did not remove a significant amount of guanylyl cyclase activity, indicating the enzyme to be intrinsic to the myelin sheath.

Phosphorylation of myelin protein: recent advances

Since it was first described 25 years ago, phosphorylation has come to be recognized as a widespread and dynamic post-translation modification of myelin proteins. In this review, the phosphorylation characteristics of myelin basic protein, protein zero (P0), myelin-associated glycoprotein and 2'3' cyclic nucleotide 3'-phosphodiesterase are summarized. Emphasis is placed on recent advances in our knowledge concerning the protein kinases involved and the sites of phosphorylation in the amino acid sequences, where known. The possible roles of myelin protein phosphorylation in modulating myelin structure, the process of myelin assembly and mediation of signal transduction events are discussed.

ADP-ribosyltransferase activity in myelin membranes isolated from human brain

An ADP-ribosyltransferase has been identified in compact myelin and in several white matter fractions which contain less compact myelin, fractionated on the basis of increasing protein/lipid ratios. One fraction the P3A contained the greatest activity although the activity in compact myelin was only slightly less. The ADP-ribosyltransferase activity of solubilized myelin was stimulated by increasing amounts of GTP gamma S and was specific for the beta-isomer of NAD. Although ADP-ribosylation was demonstrated with the heterotrimeric G proteins in the 40-50 kDa range, the substrate for the ADP-ribosyltransferase in the 20 kDa range was identified as MBP. ADP-ribosyltransferase; myelin basic protein; signal transduction.

Isoprenylated proteins in myelin

Incubation of rat brainstem slices with [3H]-mevalonate ([3H]MVA) in the presence of lovastatin resulted in the incorporation of label into three groups of myelin-associated proteins with molecular masses of 47, 21-27, and 8 kDa, as revealed on sodium dodecyl sulfate-polyacrylamide rod gel electrophoresis. Although the gel patterns of [3H]MVA-derived prenylated proteins were similar, the relative level of 3H incorporated into each protein species differed between myelin and the brainstem homogenate. Immunoprecipitation studies identified the 47-kDa prenylated protein as a 2'-3'-cyclic nucleotide phosphodiesterase, whereas the 8-kDa protein proved to be the gamma subunit of membrane-associated guanine nucleotide regulatory protein. The 3H-labeled 21-27-kDa group in myelin corresponds to the molecular mass of the extensive Ras-like family of monomeric GTP-binding proteins known to be prenylated in other tissues. Increase in lovastatin concentration resulted in reduced levels of [3H]MVA-labeled species in myelin and concomitantly increased levels in the cytosol. A cold MVA chase restored to normality the appearance of [3H]MVA-labeled proteins in myelin. Furthermore, a high lovastatin concentration in the brainstem slice incubation mixture altered the appearance of newly synthesized nonprenylated myelin proteins, including proteolipid protein and the 17-kDa subspecies of myelin basic protein. Because other myelin proteins were unaffected by the high lovastatin concentration, restricting the availability of MVA in myelin-forming cells may selectively alter processes required for myelinogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanosine-5'-(3-O-thio)triphosphate-mediated stimulation of phosphoinositidase C in solubilized rat peripheral nerve myelin and its alteration in streptozotocin-induced diabetes

The regulation of phosphoinositidase C (PIC) activity by guanosine-5'-(3-O-thio)triphosphate (GTP gamma S) was characterized in a cholate-solubilized peripheral myelin-enriched fraction from rat sciatic nerve. The GTP analog maximally enhanced PIC-catalyzed hydrolysis of exogenous phosphatidylinositol-4,5-bisphosphate (PIP2) in a dose-dependent manner only within a narrow range of cholate concentrations. Maximal stimulation was attained at 0.6 microM GTP gamma S and could be completely prevented by 1 microM guanosine-5'-(2-O-thio)diphosphate. Neither adenylyl-imidodiphosphate nor adenosine triphosphate (ATP) enhanced PIC activity. Carbamoylcholine (1 mM) added together with GTP gamma S increased the extent of PIP2 hydrolysis over that elicited by GTP gamma S alone and this stimulation was blocked by the muscarinic receptor antagonist, atropine (50 microM). In detergent-solubilized myelin preparations from streptozotocin-induced diabetic rats, a higher concentration of the guanine nucleotide analog was required to achieve stimulation comparable to that obtained with corresponding preparations from normal animals. These results suggest that sciatic nerve myelin possesses muscarinic receptors coupled via a GTP-binding protein to PIC and that this system can be reconstituted in detergent-solubilized extracts. It is possible that the function of G proteins in cell signaling is impaired in experimental diabetic neuropathy.

Guanylyl cyclase activity in rat brain myelin and white matter

Purified myelin from rat brainstem was found to have an appreciable level of guanylyl cyclase activity, as seen in the formation of 3',5'-cyclic GMP from [3H]GTP at a rate approximately 45% that of whole brainstem. Freshly isolated myelin from pooled rat brainstems was incubated with GTP in an appropriate mixture. This gave rise to 29.9 +/- 3.6 pmol of 3',5'-cyclic GMP/mg of protein/min measured by HPLC and a similar result (26.7 +/- 2.6 pmol/mg/min) with 125I-3',5'-cyclic GMP radioimmunoassay. The latter method applied to the reaction product from whole brainstem gave a value of 56.6 +/- 3.4 pmol/mg/min. In analyzing brainstem products by HPLC we observed in most trials concurrent formation of a second radiolabeled product that comigrated with 2',3'-cyclic GMP but that, on further examination, proved not to be that product. Its identity remains unknown.

Oligodendroglial lineage cells express neuroligand receptors

This study was designed to determine whether cells of the oligodendroglial lineage express neuroligand receptors linked to Ca2+ mobilization. Intracellular Ca2+ levels were monitored with a video-based imaging system and cells were characterized with immunocytochemical markers. O-2A progenitor cells (A2B5+/GFAP-) and mature oligodendroglia (GC+/MBP+) responded to norepinephrine, glutamate, ATP, and histamine with increased intracellular Ca2+ levels. As O-2A progenitor cells differentiated into mature oligodendroglia, there was an increase in the percentage of cells that responded to ATP and histamine with an increase in intracellular Ca2+ levels. Both O-2A progenitor cells and mature oligodendroglia were pharmacologically heterogeneous with respect to their ability to respond to neuroligands with an increase in intracellular Ca2+. Treatment with bradykinin, carbachol, and substance P also increased intracellular Ca2+ levels in O-2A progenitor cells and mature oligodendroglia. Whereas the percentage of cells that responded to bradykinin and substance P increased with differentiation of O-2A progenitor cells into mature oligodendroglia, the trend was reversed with respect to the percentage of cells responding to carbachol. These results suggest that cells of the oligodendroglial lineage exhibit neuroligand receptors linked to Ca2+ mobilization and that the ability of these cells to respond to neuroligands is developmentally regulated.

Hydrolysis of inositol trisphosphate by purified rat brain myelin

Highly purified rat brain myelin was found to hydrolyze inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, but subsequent hydrolysis of the latter, characteristic of whole brainstem, did not occur. Inositol 1,4,5-trisphosphate 5-phosphatase in myelin was approximately 33% of the level in microsomes and 127% that of the cytosolic fraction from brainstem. The myelin and microsomal enzymes had similar properties, as follows: activation by saponin, requirement for Mg2+ and similar Kact (0.16 and 0.13 mM), Km (8.7 +/- 2.5 and 7.0 +/- 1.0 microM), and pH optima (6.6-6.8). Vmax values were 11.2 +/- 1.0 and 26.3 +/- 2.0 nmol/mg/min for myelin and microsomes, respectively. A possible role for this enzyme in phosphoinositide-mediated signal transduction within myelin and its subcompartments is discussed.

Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca2+ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.

Phorbol ester-mediated stimulation of phospholipase D activity in sciatic nerve from normal and diabetic rats

Evidence for the presence of phospholipase D activity in sciatic nerve was obtained by incubation of 32P-prelabeled nerve segments in the presence of ethanol and measurement of [32P]phosphatidylethanol (PEth) formation expressed as a fraction of total phospholipid radioactivity. PEth synthesis was enhanced with increasing concentrations of ethanol (100 mM-2 M). 4-beta-Phorbol dibutyrate (100 nM-1 microM) stimulated PEth formation up to twofold in a time- and dose-dependent manner. The stimulatory effect evoked by 100 nM phorbol ester was completely abolished by Ro 31-8220 (compound 3), a selective protein kinase C inhibitor. Efforts to identify the phospholipid precursor of PEth were unsuccessful, suggesting this product arises from a small discrete precursor pool. On subcellular fractionation of nerve, the ratio of basal and 4-beta-phorbol dibutyrate-stimulated phospholipase D activity recovered in a myelin-enriched fraction, compared with a nonmyelin fraction, was 0.5 when results are expressed as a percentage of total phospholipid radioactivity. This ratio rises to 1.2 if the results are calculated assuming only phosphatidylcholine and phosphatidylethanolamine are potential precursors. The results suggest that myelin is a major locus of phospholipase D activity. Nerve from streptozotocin-induced diabetic and control animals displayed the same basal phospholipase D activity, but the enzyme in diabetic nerve was stimulated to a greater extent by a suboptimal concentration of 4-beta-phorbol dibutyrate. These results support the conclusion that protein kinase C modulates phospholipase D activity in nerve and suggest that in diabetic nerve the enzyme activation mechanism may possess increased sensitivity.

Activity and distribution of phosphoinositidase C in rat sciatic nerve

The hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) by rat sciatic nerve cytosolic phosphoinositidase C [phosphoinositide-specific phospholipase C (PIC)] was studied at neutral pH and at ionic concentrations that approximate intracellular conditions. The principal water-soluble product formed was shown to be inositol trisphosphate by anion exchange chromatography. The maximum hydrolysis rate (2.5 nmol/min/mg protein) was achieved at less than 100 nM Ca2+. Hydrolysis was markedly increased to 15 nmol/min/mg protein by inclusion of K+ in the reaction mixture. In the presence of 200 mM K+, the optimum Ca2+ was increased to approximately 600 nM. Higher Ca2+ concentrations progressively inhibited PIP2 hydrolysis. Mg2+ also inhibited the reaction, but the presence of equimolar amounts of ATP and Mg2+ had no effect. Appreciable degradation of phosphatidylinositol-4-phosphate (PIP) also occurred in the nanomolar Ca2+ range, whereas breakdown of phosphatidylinositol (PI) required millimolar Ca2+. The presence of PIP but not PI inhibited PIP2 hydrolysis. Upon subcellular fractionation of nerve, more than 50% of recovered PIC activity was in the cytosol and about 20% was located in a myelin-enriched fraction. Using PIP2 as substrate, PIC activities in nerves from normal and streptozotocin-induced diabetic animals were not different. However, the myelin-associated enzyme from diabetic animals was more labile to freezing and thawing.

Axon-myelin transfer of phospholipids and phospholipid precursors. Labeling of myelin phosphoinositides through axonal transport

Previous studies have provided evidence for axon-to-myelin transfer of intact lipids and lipid precursors for reutilization by myelin enzymes. Several of the lipid constituents of myelin showed significant contralateral/ipsilateral ratios of incorporated radioactivity, indicative of axonal origin, whereas proteins and certain other lipids did not participate in this transfer-reutilization process. The present study will examine the labeling of myelin phosphoinositides by this pathway. Both 32PO4 and [3H]inositol were injected monocularly into 7-9-wk-old rabbits and myelin was isolated 7 or 21 days later from pooled optic tracts and superior colliculi. In total lipids 32P counts of the isolated myelin samples showed significant contralateral/ipsilateral ratios as well as increasing magnitude of contralateral-ipsilateral differences during the time interval. Thin-layer chromatographic isolation of the myelin phosphoinositides revealed significant 32P-labeling of these species, with PIP and PIP2 showing time-related increases. This resembled the labeling pattern of the major phospholipids from rabbit optic system myelin in a previous study and suggested incorporation of axon-derived phosphate by myelin-associated enzymes. The 32P label in PI, on the other hand, remained constant between 7 and 21 days, suggesting transfer of intact lipid. This was supported by the labeling pattern with [3H]inositol, which also showed no increase over time for PI. These results suggest axon-myelin transfer of intact PI followed by myelin-localized incorporation of axon-derived phosphate groups into PIP and PIP2. The general topic of axon-myelin transfer of phospholipids and phospholipid precursors is reviewed.

Detection of G proteins in purified bovine brain myelin

Following a previous report on detection of muscarinic receptors in myelin with the implied presence of G proteins, we now demonstrate by more direct means the presence of such proteins and their quantification. Using [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP gamma S) as the binding ligand, purified myelin from bovine brain was found to contain approximately half the binding activity of whole white matter (138 +/- 9 vs. 271 +/- 18 pmol/mg of protein). Scatchard analysis of saturation binding data revealed two slopes, a result suggesting at least two binding populations. This binding was inhibited by GTP and its analog but not by 5'-adenylylimidodiphosphate [App(NH)p], GMP, or UTP. Following sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) of myelin proteins and blotting on nitrocellulose, [alpha-32P]GTP bound to three bands in the 21-27-kDa range in a manner inhibited by GTP and GTP gamma S but not App(NH)p. ADP-ribosylation of myelin with [32P]NAD+ and cholera toxin labeled a protein of 43 kDa, whereas reaction with pertussis toxin labeled two components of 40 kDa. Cholate extract of myelin subjected to chromatography on a column of phenyl-Sepharose gave at least three major peaks of [35S]GTP gamma S binding activity. SDS-PAGE and immunoblot analyses of peak I indicated the presence of Go alpha, Gi alpha, and Gs alpha. Further fractionation of peak II by diethyl-aminoethyl-Sephacel chromatography gave one [35S]GTP gamma S binding peak with the low-molecular-mass (21-27 kDa) proteins and a second showing two major protein bands of 36 and 40 kDa on SDS-PAGE.(ABSTRACT TRUNCATED AT 250 WORDS)