Basic protein in brain myelin is phosphorylated by endogenous phospholipid-sensitive Ca2+-dependent protein kinase

Interleukin-2 receptors and interleukin-2-mediated signaling in myelin: activation of diacylglycerol kinase and phosphatidylinositol 3-kinase

Myelin was previously shown to possess neurotransmitter and cytokine receptors that trigger well-defined signaling mechanisms within the multilamellar structure. The present study reveals the presence of an interleukin-2 (IL-2) receptor in isolated mouse CNS myelin that responds to recombinant mouse IL-2 by activating diacylglycerol kinase (DAGK) and phosphoinositide 3-kinase (PI3K); additional evidence suggests participation by protein tyrosine kinase. Activation of myelin DAGK by IL-2 occurred in brain stem tissue mince and was blocked by chelerythrin chloride, indicating an essential role for myelin-localized protein kinase C. Two inhibitors of PI3K, wortmannin and LY294002, blocked endogenous PI3K as well as that enhanced by IL-2. Activation of PI3K by IL-2 was also blocked by tyrphostin A25, a selective inhibitor of PTK, suggesting activation of the latter by IL-2 is upstream to PI3K activation. This reaction resulted in tyrosine phosphorylation of a protein tentatively identified as the p85 subunit of PI3K. Developmental changes were noted in that receptor density and signaling activity were robust during the period of rapid myelination and declined rapidly thereafter.

Glial cells as targets for cytotoxic immune mediators

Oligodendrocytes and Schwann cells are the glia principally responsible for the synthesis and maintenance of myelin. Damage may occur to these cells in a number of conditions, but perhaps the most studied are the idiopathic inflammatory demyelinating diseases, multiple sclerosis in the CNS, and Guillain-Barré syndrome and its variants in the peripheral nervous system (PNS). This article explores the effects on these cells of cytotoxic immunological and inflammatory mediators: similarities are revealed, of which perhaps the most important is the sensitivity of both Schwann cells and oligodendrocytes to many such agents. This area of research is, however, characterised and complicated by numerous and often very substantial inter-observer discrepancies. Marked variability in cell culture techniques, and in assays of cell damage and death, provide artifactual explanations for some of this variability; true inter-species differences also contribute. Not the least important conclusion centres on the limited capacity of in vitro studies to reveal disease mechanisms: cell culture findings merely illustrate possibilities which must then be tested ex vivo using human tissue samples affected by the relevant disease.

Mechanisms of damage to myelin and oligodendrocytes and their relevance to disease

Oligodendrocytes synthesize and maintain myelin in the central nervous system (CNS). Damage may occur to these cells in a number of conditions, including infections, exposure to toxins, injury, degeneration, or autoimmune disease, arising both in the course of human disease and in experimental animal models of demyelination and dysmyelination; multiple sclerosis is the commonest human demyelinating disorder. Conventional classical accounts of the pathology of this and other myelin diseases have given great insights into their core features, but there remain considerable uncertainties concerning the timing, means and cause(s) of oligodendrocyte and myelin damage. At present, therapeutic efforts largely concentrate on immune manipulation and damage limitation, an approach that has produced only modest effects in multiple sclerosis. One reason for this must be the limited understanding of the mechanisms underlying cell damage - clearly, successful therapeutic strategies for preserving the oligodendrocyte-myelin unit must depend on knowledge of how oligodendrocyte damage and death occurs. In this review, mechanisms of oligodendrocyte and myelin damage are considered, and attempts made to relate them to disease processes, clinical and experimental. The hallmarks of different cell death processes are described, and oligodendrocyte-myelin injury by cellular and soluble mediators is discussed, both in vitro and invivo. Recent developments concerning the pathological involvement of oligodendrocytes in neurodegenerative disease are summarized. Finally, these neuropathological and applied neurobiological observations are drawn together in the context of multiple sclerosis.

Oligodendrocytes utilize a matrix metalloproteinase, MMP-9, to extend processes along an astrocyte extracellular matrix

Matrix metalloproteinases (MMPs), the key effectors of extracellular matrix remodeling, have been demonstrated to regulate the extension of neurites from neuronal cell bodies. In this report we have addressed the hypothesis that oligodendrocytes (OLs) may utilize a similar mechanism in extending their processes during the initial phase of myelination. Furthermore, given our previous findings linking protein kinase C (PKC) to the OL process outgrowth, we tested the postulate that this signal transduction pathway may regulate MMPs and thus the process outgrowth phenotype. We demonstrate that in response to pharmacologic activators of PKC, cultured human OLs augment their process extension with a concomitant increase in the activity of an MMP, MMP-9, as measured by gelatin zymography. Similarly, the phorbol ester-enhanced process extension and increased MMP-9 activity were both inhibited by calphostin C, a selective PKC inhibitor. Also, MMP inhibitors such as 1,10-phenanthroline and synthetic dipeptides that inactivate the MMP catalytic site negated the 4beta-phorbol-12,13-dibutyrate (PDB)-mediated process extension, further supporting the key role of MMPs in process extension in vitro. Finally, the elevation of MMP-9 protein expression in the mouse corpus callosum, a tissue rich in OL and myelin, coincided with the previously documented temporal increase in myelination that occurs postnatally. Taken together, these data suggest that MMP-9 constitutes an important mediator of OL process outgrowth, and that this protease in turn can be regulated by PKC. The results are relevant not only to the initial steps of myelination during development, but also to the attempted remyelination that has been shown to occur in pathologic conditions such as MS.

Multivalent ganglioside and sphingosine conjugates modulate myelin protein kinases

Gangliosides, added exogenously at concentrations of 10-100 microM, inhibit intrinsic protein kinase activities in purified rat brain myelin. Multivalent neoganglioproteins--gangliosides covalently attached, via their lipid moieties, to bovine serum albumin--were much more potent, inhibiting myelin protein phosphorylation half-maximally at a concentration of 100 nM. Different ganglioside conjugates varied 10-fold in inhibitory potency; GT1b-conjugates being the most potent and GM3-conjugates being the least. Conjugates of ganglioside oligosaccharides, lacking the lipid moiety, did not inhibit myelin protein phosphorylation, whereas conjugates of sphingosine inhibited nearly as potently as GT1b conjugates. Conjugate-mediated inhibition of myelin protein phosphorylation was due to inhibition of a protein serine kinase activity rather than activation of a phosphatase activity. We conclude that (i) clustered gangliosides or sphingosine are potent myelin protein kinase inhibitors, and (ii) sphingolipid metabolism is not required for myelin protein kinase inhibition. In contrast to their effects on myelin protein phosphorylation, ganglioside conjugates stimulated phosphorylation of a presumptive axon membrane protein. The data support the conclusion that gangliosides and other sphingolipids, when appropriately clustered, are potent modulators of central nervous system protein phosphorylation.

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.

In vivo phosphorylation of 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP): CNP in brain myelin is phosphorylated by forskolin- and phorbol ester-sensitive protein kinases

2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) was phosphorylated in vivo, in brain slices and in a cell free system. Phosphoamino acid analysis of immunoprecipitated CNP labeled in vivo and in brain slices revealed phosphorylation of phosphoserine (94%) and phosphothreonine (5%) residues. Phosphorylation of CNP increased by 3-fold after brain slices were incubated with forskolin. Similarly, incubation of isolated myelin with [gamma-32]ATP with cAMP (5 microM) and cAMP (5 microM)+catalytic unit of cAMP dependent protein kinase dramatically increased CNP2 phosphorylation by 4- and 6-fold, respectively. It is feasible that CNP2 was predominantly phosphorylated on serine and/or threonine residues of the amino terminal peptide of CNP2, and this phosphorylation was catalyzed by protein kinase A. Phosphorylation of CNP1 and CNP2 increased 2-fold by incubating brain slices with phorbol ester. Forskolin and phorbol ester increased the phosphorylation of single, but distinct, CNP peptides. We present the first biochemical evidence that CNP2, on a protein mass basis, is far more heavily phosphorylated than CNP1, suggesting there are more phosphorylation sites on CNP2 than CNP1 and that at least one site is located on the 20-amino acid terminus of CNP2 and that it is likely a PKA site.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

Protein kinase C in rat brain myelin

It has been suggested that phosphorylation of myelin basic protein (MBP) in CNS is catalyzed by protein kinase C (PKC). In order to demonstrate that PKC in the myelin phosphorylates MBP, PKC was partially purified from rat CNS myelin by solubilization with Triton X-100 followed by a DEAE-cellulose column. MBP and histone III-S were phosphorylated in the presence of Ca2+ and phospholipid by rat myelin PKC. High voltage electrophoresis revealed that the phosphoamino acids in MBP by this kinase was serine residue, which is known to be the amino acid phosphorylated by PKC. The activity of PKC extracted from myelin was inhibited by the addition of psychosine to the incubation mixture. To confirm the presence of PKC molecule and to identify the isoform of PKC in the myelin, the solubilized myelin fraction was applied on SDS-PAGE, transferred to a nitrocellulose sheet and stained with anti-PKC monoclonal antibodies. Rat CNS myelin contained the PKC of about 80 kDa (intact PKC), and no proteolytic fragments were observed. PKC isozymes in myelin were type II and III. A developmental study from 14 to 42 postnatal days showed that PKC activity in CNS myelin seemed to parallel the deposition of myelin protein.

Isolation and characterization of periaxolemmal and axolemmal enriched membrane fractions from the rat central nervous system

In this report, we describe the fractionation of crude axolemmal fractions from rat lower brainstem into subfractions enriched in markers for either periaxolemmal myelin or axolemma. These subfractions were isolated on density gradients as bands layering on 0.8M and 1.0M sucrose. Both subfractions consisted of unilamellar vesicles. Relative to myelin purified from the same starting material, the 0.8M subfraction was enriched in MAG, CNPase, carbonic anhydrase and Na+, K+ ATPase but was extremely low in PLP and MBP. In addition, this fraction exhibited a protein profile distinct from myelin. The 1.0M fraction was also highly enriched in Na+, K+ ATPase and had an overall composition similar to the 0.8M subfraction. However, it differed from the 0.8M subfraction by being low in MAG, CNPase, and carbonic anhydrase, but enriched in voltage-dependent Na+ channel, axon-specific fodrin, and MAP-1B. Based on these characteristics we concluded that the 0.8M and 1.0M subfractions were highly enriched in periaxolemmal myelin and axolemmal membrane, respectively. Plasmolipin10 was unique with equally high levels in myelin and in the 0.8M and 1.0M subfractions. Both subfractions were enriched, relative to myelin, in the alpha subunit of the GTP binding protein, Go, and the alpha subunit common to all G proteins, GA/1. Electrophysiology with membrane subfractions fused to lipid bilayers showed that both membranes contained sets of K+ and Cl- channels, which based on channel sizes and open times, are largely distinct from one another.

Vestibular Compensation Affects Endogenous Phosphorylation of Frog Brain Proteins

The effect of unilateral labyrinthectomy followed by the process of vestibular compensation on the incorporation of radioactive phosphate into frog brain proteins was investigated. Phosphoproteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by autoradiography. The present data show that unilateral labyrinthectomy affects the incorporation of 32P into various frog brain proteins. In particular, the phosphorylation of a 20-kDa protein appeared enhanced during early stages of vestibular compensation (4-12 days). This 20-kDa protein was shown to be immunologically related to myelin basic protein and its phosphorylation was regulated by an endogenous calcium/calmodulin-dependent protein kinase. These data might indicate that in addition to neuronal components, components of glial origin are also involved in biochemical events that lead to functional recovery after neuronal lesions.

Muscarinic cholinergic receptor-mediated phosphoinositide metabolism in peripheral nerve

Few receptor-mediated phenomena have been detected in peripheral nerve. In this study, the ability of the muscarinic cholinergic receptor agonist carbamylcholine to enhance phosphoinositide (PPI) breakdown in sciatic nerve was investigated by measuring the accumulation of inositol phosphates. Rat sciatic nerve segments were prelabeled with myo-[3H]inositol and then incubated either with or without carbamylcholine in the presence of Li+. [3H]Inositol monophosphate ([3H]IP) accumulation contained most of the radioactivity in inositol phosphates, with [3H]inositol bisphosphate ([3H]IP2) and [3H]inositol trisphosphate ([3H]IP3) accounting for 7-8% and 1-2% of the total, respectively. In the presence of 100 microM carbamylcholine, [3H]IP accumulation increased by up to 150% after 60 min. The 50% effective concentration for the response was determined to be 20 microM carbamylcholine and stimulated IP generation was abolished by 1 microM atropine. Enhanced accumulation of IP2 and IP3 was also observed. Determination of the pA2 values for the muscarinic receptor antagonists atropine (8.9), pirenzepine (6.5), AF-DX 116 (11-[[2-[(diethylamino)methyl]-1-piperidinyl] acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one) (5.7), and 4-diphenylacetoxy-N-methylpiperidinemethiodide (4-DAMP) (8.6) strongly suggested that the M3 muscarinic receptor subtype was predominantly involved in mediating enhanced PPI degradation. Following treatment of nerve homogenates and myelin-rich fractions with pertussis toxin and [32P]NAD+, the presence of an ADP-ribosylated approximately 40-kDa protein could be demonstrated. The results indicate that peripheral nerve contains key elements of the molecular machinery needed for muscarinic receptor-mediated signal transduction via the phosphoinositide cycle.

Age-dependent decrease of process formation by cultured oligodendrocytes is augmented by protein kinase C stimulation

The proportion of cultured rat oligodendrocytes (OL) that extended processes of over three soma diameter in length is dependent on the age of the animals from which the brains were derived; up to 70% of neonatal OL attained this criterion within 3 days, and this proportion progressively decreased with advancing ages of the animals (1, 3, and 6 months). The lower extent of process formation from older rat OL could be augmented, and indeed to equal neonatal levels, by treatment of cells with phorbol esters that stimulate protein kinase C: 4 beta-phorbol-12,13-dibutyrate (PDB) and phorbol-12-myristate-13-acetate (PMA). Enhancement of process formation by PDB and PMA was also observed for cultured adult human and bovine OL. For adult OL from all three species, a phorbol ester that binds but that does not activate protein kinase C, 4 alpha-phorbol-12,13-didecanoate, did not result in enhancement of process formation. Selectively to biologically active phorbol esters was shown by the inability of a wide range of growth factors to promote process extension. Immunohistochemical analyses indicate that the type III isozyme of protein kinase C predominates in cultured OL; the apparent intensity of immunoreactive PKC was not different between controls or cultures treated for 12 days with PDB, suggesting that the persistent presence of PDB might not have down-regulated the enzyme, in contrast to other cell types. We propose that stimulation of protein kinase C is critical to the triggering of process formation by cultured OL in vitro.

Central nervous system myelin: structure, function, and pathology

Multiple sclerosis (MS) and a number of related distinctive diseases are characterized by the active degradation of central nervous system (CNS) myelin, an axonal sheath comprised essentially of proteins and lipids. These demyelinating diseases appear to arise from complex interactions of genetic, immunological, infective, and biochemical mechanisms. While circumstances of MS etiology remain hypothetical, one persistent theme involves recognition by the immune system of myelin-specific antigens derived from myelin basic protein (MBP), the most abundant extrinsic myelin membrane protein, and/or another equally susceptible myelin protein or lipid component. Knowledge of the biochemical and physical-chemical properties of myelin proteins and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to the understanding of how and why these antigens become selected during the development of MS. This review focuses on current understanding of the molecular basis underlying demyelinating disease as it may relate to the impact of the various protein and lipid components on myelin morphology; the precise molecular architecture of this membrane as dictated by protein-lipid and lipid-lipid interactions; and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

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

Purified myelin from rat brainstem, prelabeled in vivo by intracerebral injection of [3H]myoinositol, showed enhanced breakdown of phosphoinositides on treatment with 5'-guanylylimidodiphosphate [Gpp-(NH)p] and Ca2+. Concentration variation of the former in the presence of Ca2+ showed a dose-dependent release of inositol 1,4-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3), while inositol 1-phosphate (IP) release was erratic. Concentration-dependent release of IP2 and IP3 was also observed with Ca2+ as the variable in the presence of Gpp(NH)p. Carbachol, when present, did not enhance the stimulatory effect of Gpp(NH)p alone. Addition of diphosphoglycerate during incubation enhanced IP3 at the expense of IP2, suggesting the presence of IP3 phosphatase in myelin.

Cleavage of the P0 glycoprotein of the rat peripheral nerve myelin: tentative identification of cleavage site and evidence for the precursor-product relationship

The incubation of sciatic nerve slices in Krebs Ringer bicarbonate (KRB) buffer (pH 7.4) at 37 degrees C, or the incubation of freshly isolated myelin in ammonium bicarbonate buffer (pH 8), resulted in the generation of a 24 kDa protein with a concomitant decrease of P0 protein. The conversion of P0 into 24 kDa protein was blocked by heating isolated myelin at 100 degrees C for 5 min suggesting that the reaction is enzyme mediated. Inclusion of the protease inhibitors and chelating agent to isolated myelin did not prevent the formation of 24 kDa protein. Similarly, addition of CaCl2 to isolated myelin did not accentuate the formation of 24 kDa protein suggesting that the conversion of P0 into 24 kDa protein may not be due to Ca2+ activated protease. It is postulated that the formation of 24 kDa protein may be due to neutral protease and/or metalloproteinase associated with the PNS myelin. 24 kDa protein was purified and characterized. The N-terminal sequence of 1-17 amino acid residues of 24 kDa protein was identical to P0. 24 kDa protein was immunostained and immunoprecipitated with anti-P0 antiserum indicating the immunological similarities between P0 and 24 kDa protein. Labeling of 24 kDa protein with [35S]methionine provided evidence that P0 may be in all probability cleaved between Met-168 and Met-193. Further studies were carried out to demonstrate that 24 kDa protein was phosphorylated, glycosylated and acylated like P0. Phosphorylation of 24 kDa protein in the nerve slices was increased five-fold by phorbol esters and phosphoserine was the only phosphoamino acid identified after partial acid hydrolysis of 24 kDa protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Accumulation of galactosylsphingosine (psychosine) does not interfere with phosphorylation and methylation of myelin basic protein in the twitcher mouse

In attempts to elucidate mechanisms of demyelination in the twitcher mouse (Twi), phosphorylation and methylation of myelin basic protein (MBP) were examined in the brainstem and spinal cord of this species. Phosphorylation of MBP in isolated myelin by an endogenous kinase and an exogenous [32P]ATP was not impaired and protein kinase C activity in the brain cytosol was not reduced. When the methylation of an arginine residue of MBP was examined in slices of the brainstem and spinal cord, using [3H]methionine as a donor of the methyl groups, no difference was found between Twi and the controls. Radioactivity of the [3H] methionine residue of MBP of Twi was also similar to that of the controls. Thus, accumulation of psychosine in Twi does not interfere with the activity of endogenous kinase, methylation of MBP, and the synthesis and transport of MBP into myelin membrane.

2',3'cyclic nucleotide-3'-phosphodiesterase in peripheral nerve myelin is phosphorylated by a phorbol ester-sensitive protein kinase

2',3' cyclic nucleotide-3'-phosphodiesterase (CNP) is phosphorylated in the peripheral nervous system after immunoprecipitation of myelin proteins radiolabeled in vivo, in nerve slices and in a cell-free system. Only radiolabeled phosphoserine was detected after partial acid hydrolysis of immunoprecipitated CNP. Two major phosphopeptides were resolved by two dimensional electrophoresis-chromatography after digestion with trypsin of CNP phosphorylated in the nerve slices. Phosphorylation of CNP was not stimulated a) by forskolin in the nerve slices and b) after incubation of purified nerve myelin with cAMP. However, CNP phosphorylation was increased after incubation of PNS myelin with catalytic unit of protein kinase A. Phosphorylation of the central nervous system myelin CNP was dramatically stimulated by cAMP. These results suggest that PKA may be absent from peripheral nerve myelin or CNP may not be accessible to this enzyme in the PNS. Incubation of nerve slices with phorbol 12 myristate-13-acetate caused a marked increase in the phosphorylation of CNP. These results provide strong evidence that CNP is phosphorylated in the PNS and its phosphorylation in vivo is in all probability regulated by protein kinase C.

Identification of MARPP (14-20), morphine- and cyclic AMP-regulated phosphoproteins of 14-20 kDa, as myelin basic proteins: evidence for their acute and chronic regulation by morphine in rat brain

MARPP(14-20), morphine- and cyclic AMP-regulated phosphoproteins of 14, 17-18, and 20 kDa, were identified originally by one-dimensional electrophoresis as a group of proteins whose state of phosphorylation was decreased by acute morphine and increased by chronic morphine in the rat locus coeruleus. We now show that MARPP(14-20) represent myelin basic proteins based on biochemical and immunochemical criteria. First, MARPP(14-20) were found to have isoelectric points of about 11 based on their migration on non-equilibrium pH gradient electrophoresis. Second, MARPP(14-20) were greatly enriched in myelin fractions of brain, and were not detectable, or present at very low levels, in other subcellular fractions of brain. Third, analysis of phosphorylated MARPP(14-20) by one- and two-dimensional gel electrophoresis demonstrated precise comigration with immunolabeled myelin basic proteins. In contrast, MARPP(14-20) were distinguished from histones, another group of low molecular weight, highly basic phosphoproteins, in these subcellular fractionation and immunochemical studies. Finally, we confirmed using two-dimensional electrophoresis, that changes observed previously by one-dimensional electrophoresis in the phosphorylation of MARPP(14-20) in response to acute and chronic morphine, and to acute forskolin, occur in myelin basic proteins. It was also found that changes in the state of phosphorylation of myelin basic proteins in response to chronic morphine occur without a change in the total amount of the proteins as determined by immunoblot analysis. The results demonstrate that the phosphorylation of myelin basic proteins is regulated by morphine in the nervous system, and raise the possibility that regulation of these proteins contributes to mechanisms underlying some of the acute and chronic actions of opiates in brain.

Characteristics of Ca2+/calmodulin- and Ca2+/phosphatidylserine-stimulated phosphoproteins in rat striatum

The characteristics of endogenous Ca2+/calmodulin (CaM)- and Ca2+/phosphatidylserine (PS)-stimulated phosphorylated proteins in the striatum of rat were partially determined and compared in this study. The Ca2+/CaM-dependent phosphoproteins were associated with serine and threonine residues. The sensitivity of these proteins for phosphorylation by Ca2+/CaM was not affected by pretreatment of tissue with Ca2+ chelating agent, EGTA or with non-ionic detergent, Triton X-114. Triton X-114 phase separation experiments revealed that these Ca2+/CaM-dependent phosphoproteins were partitioned in the detergent rich phase suggesting that they are integral proteins of the striatal membrane. On the other hand, the Ca2+/PS-dependent phosphorylated proteins were primarily associated with the serine residue. Phosphorylation of these proteins by Ca2+/PS were abolished after the treatment with EGTA or Triton X-114. These results suggest that Ca2+/PS-dependent striatal phosphoproteins are biochemically unstable in maintaining their state of phosphorylation.

Multiple sclerosis brain immunoglobulins stimulate myelin basic protein degradation in human myelin: a new cause of demyelination

Membrane-bound proteolysis may be implicated in the pathogenesis of demyelinating disorders including multiple sclerosis (MS). We previously found that the extent of myelin basic protein (MBP) degradation by the calcium-activated neutral protease did not differ for isolated human control myelin or MS myelin. Hence we suggested that, if involved in demyelination, the myelin neutral protease must be activated in vivo by an increased availability of free calcium. The postulate was therefore tested that immunoglobulin (Ig) binding to myelin results in activation of the myelin neutral protease, possibly through release of free calcium from calcium-binding sites of myelin. Isolated myelin from the brains of controls and patients with MS were incubated with purified Igs eluted from the brains of patients with MS or controls and degradation of MBP was assessed by quantitative electroimmunoblotting. Such degradation was significantly greater in myelin incubated in the presence of MS Igs than in myelin incubated without added Igs or in the presence of control Igs. Furthermore, the degree of MBP degradation in myelin incubated with control Igs was similar to that observed in myelin incubated without added Igs. Accordingly, it is suggested that Ig in MS brain potentiates myelin breakdown. Moreover activation of membrane-bound proteolysis by Ig binding to myelin appears to represent a hitherto undescribed pathway for demyelination in MS.

Protein phosphorylation systems in postmortem human brain

Protein phosphorylation systems regulated by cyclic adenosine 3',5'-monophosphate (cyclic AMP), or calcium in conjunction with calmodulin or phospholipid/diacylglycerol, have been studied by phosphorylation in vitro of particulate and soluble fractions from human postmortem brain samples. One-dimensional or two-dimensional gel electrophoretic protein separations were used for analysis. Protein phosphorylation catalyzed by cyclic AMP-dependent protein kinase was found to be highly active in both particulate and soluble preparations throughout the human CNS, with groups of both widely distributed and region-specific substrates being observed in different brain nuclei. Dopamine-innervated parts of the basal ganglia and cerebral cortex contained the phosphoproteins previously observed in rodent basal ganglia. In contrast, calcium/phospholipid-dependent and calcium/calmodulin-dependent protein phosphorylation systems were less prominent in human postmortem brain than in rodent brain, and only a few widely distributed substrates for these protein kinases were found. Protein staining indicated that postmortem proteolysis, particularly of high-molecular-mass proteins, was prominent in deeply located, subcortical regions in the human brain. Our results indicate that it is feasible to use human postmortem brain samples, when obtained under carefully controlled conditions, for qualitative studies on brain protein phosphorylation. Such studies should be of value in studies on human neurological and/or psychiatric disorders.

Evidence for the presence of diacylglycerol kinase in rat brain myelin

The previous demonstration that incubation of brain slices with [32P]phosphate brings about rapid labeling of phosphatidic acid in myelin suggests that the enzyme involved should be present in this specialized membrane. DAG kinase (ATP:1,2-diacyglycerol 3-phosphotransferase, E.C. 2.7.1.107) is present in rat brain homogenate at a specific activity of 2.5 nmol phosphatidic acid formed/min/mg protein, while highly purified myelin had a much lower specific activity (0.29 nmol/min/mg protein). Nevertheless, the enzyme appears to be intrinsic to this membrane since it can not be removed by washing with a variety of detergents or chelating agents, and it could not be accounted for as contamination by another subcellular fraction. Production of endogenous, membrane-associated, diacylglycerol (DAG) by PLC (phospholipase C) treatment brought about translocation from soluble to particulate fractions, including myelin. Another level of control of activity involves inactivation by phosphorylation; a 10 min incubation of brain homogenate with ATP resulted in a large decrease in DAG kinase activity in soluble, particulate and myelin fractions.

Tumor promoters accentuate phosphorylation of PO: evidence for the presence of protein kinase C in purified PNS myelin

The effects of carbon tetrachloride, methylene chloride and chloroform on phosphorylation of PO was examined. The results of the dose response curve revealed that carbon tetrachloride (0.67%), methylene chloride (2%) and chloroform (1%) induced phosphorylation of PO by approximately 4, 6, and 12-fold, respectively. PO was found to be phosphorylated on the serine residue, and the phosphorylation of the serine residue was markedly increased when PO was phosphorylated in the presence of these compounds. Since tumor promoters, carbon tetrachloride and chloroform, have been shown to activate protein kinase C in platelets it is postulated that the increased phosphorylation of PO may result from the activation of myelin associated protein kinase C. The presence of phospholipid sensitive Ca2+-dependent protein kinase (protein kinase C) in in purified nerve myelin was demonstrated by increased phosphorylation of PO in the presence of Ca2+ and phosphatidylserine.

Relationship of ATP turnover, polyphosphoinositide metabolism, and protein phosphorylation in sciatic nerve and derived peripheral myelin subfractions from normal and streptozotocin diabetic rats

Sciatic nerve from streptozotocin-induced diabetic rats has previously been shown to incorporate more 32P into phosphatidylinositol-4,5-bisphosphate (PIP2) and the principal myelin proteins than normal nerve. In the present study, labeling of ATP and PIP2 was compared. Using nerve segments, [gamma-32P]ATP specific activity reached a plateau after incubation for 4 h with [32P]orthophosphate, whereas the specific activity of [32P]PIP2 rose much more slowly and was still increasing after 8 h. The rate of disappearance of radioactivity from prelabeled ATP was biphasic, with 75% being lost within 30 min and the remainder declining much more slowly for several hours thereafter. In contrast, no decrease in prelabeled PIP2 radioactivity could be detected for up to 4 h. The kinetics of ATP metabolism were not appreciably different for normal and diabetic nerve. However, after incubation with [32P]orthophosphate for 2 h, the specific activity of PIP2 was 50-120% higher in diabetic nerve. This phenomenon, therefore, cannot be ascribed to altered specific activity of the ATP precursor pool. Greater labeling of PIP2 in 32P-labeled diabetic nerve was present in purified myelin isolated using a simple discontinuous sucrose density gradient, but not in a "nonmyelin" fraction. When nerve homogenate was fractionated on a more complex gradient, three myelin-enriched subfractions were obtained which were heterogeneous as judged by morphological appearance, protein profile, and lipid metabolic activity. The proportion of total lipid radioactivity accounted for by PIP2 was elevated in all the subfractions relative to the homogenate. As compared to myelin subfractions from normal nerve, an increased percentage of 32P in PIP2 was obtained only in the major myelin subfraction from diabetic nerve. The phosphorylation of P0 relative to the other myelin proteins was also enhanced in this subfraction in nerve from diabetic animals.

Phosphorylation of myelin basic protein in intact oligodendrocytes: inhibition by galactosylsphingosine and cyclic AMP

We have previously shown that cyclic AMP (cAMP) inhibits the protein kinase C (PKC)-mediated phosphorylation of myelin basic protein (MBP) in cultured oligodendrocytes (OLGs). Recently, it has been demonstrated that the long chain base sphingosine inhibits PKC by competing PKC effectors (diacylglycerol and phorbol esters) for a binding site on the kinase (Hannun and Bell: Science 235: 670-674, 1987). In this report we define further the mechanism by which cAMP inhibits MBP phosphorylation by comparing the effects of cAMP with that of galactosylsphingosine (psychosine), a potential catabolite of galactocerebroside, the major OLG glycosphingolipid. We identify the consequences of psychosine treatment and PKC down-regulation on OLG morphology and electrophysiology and discuss their relevance. Our results in intact ovine oligodendrocytes are consistent with a mechanism in which cAMP inhibits MBP phosphorylation by interfering with the release of diacylglycerol (DAG) from phosphatidylinositol. First, the effects of cAMP on MBP phosphorylation are reversed with exogenous TPA; and second, cAMP inhibits the incorporation of 1-[14C]arachidonate into DAG and specifically inhibits the turnover (as judged by 32PO4 3-incorporation) of phosphatidylinositol. Psychosine inhibits MBP phosphorylation, and its action can be reversed by TPA suggesting a mechanism of inhibition similar to that described for other systems. In addition, psychosine has profound effects on OLG morphology; it disintegrates OLG processes while leaving the cell soma intact. Stable hyperpolarized resting potentials were obtained following psychosine treatment, but there was a 66% decrease in membrane capacitance indicating a significant decrement in membrane surface area. The morphological changes induced by psychosine are reversible and can be eliminated by removing the drug but not by the addition of TPA. Whether inhibition of PKC by psychosine plays any role in process dissolution remains an unanswered question. However, current evidence suggests that a PKC-independent mechanism may be at play. This investigation in conjunction with our previous work emphasizes a role for the interregulation of protein kinase A (PKA) and PKC in the control of OLG somal vs. myelin components. This may have significant implications for central nervous system myelin assembly.

Immune mechanisms in the pathogenesis of demyelinating diseases

The loss of myelin which characterises many human and experimental demyelinating diseases, among them multiple sclerosis, is thought to be immune mediated, but the precise mechanisms responsible remain unknown despite intense research. Normally, myelin in the central nervous system (CNS) is protected from systemic immune responses by the blood brain barrier, which separates nervous tissue from the peripheral circulation. Here we review evidence suggesting that an understanding of the demyelinating disorders may be helped by considering their immune pathogenesis in two stages. The first is damage to the blood brain barrier; this appears to be cell mediated, and allows infiltration into the CNS of other immune effectors. These include complement and also macrophages, which together may mediate the second stage, injury to the myelin/oligodendrocyte complex.

Developmental changes in protein phosphorylation in chicken forebrain. II. Calmodulin stimulated phosphorylation

The development of calmodulin stimulated protein phosphorylation, with particular reference to calmodulin-stimulated protein kinase II (CMK II), was investigated in 3 subcellular fractions of chicken forebrain: cytosol (S3), crude synaptic plasma membranes (P2-M) and occluded cytosol (P2-S). Changes in the level of calmodulin-stimulated phosphorylation of endogenous proteins occurred over a protracted time course and were not complete until after day 52 post-hatching. By day 15 post-hatching, calmodulin-stimulated phosphoproteins characteristic of embryonic fractions had all disappeared and those characteristic of adult tissue were present but not necessarily at their mature levels. The levels of CMK II were estimated from the autophosphorylation of the alpha-subunit which was the only phosphoprotein present at 53,000 Da in the 3 fractions. Overall, calmodulin-stimulated phosphorylation and CMK II levels were low in embryonic brain and high in adult brain but two specific changes in CMK II were observed during development: (1) although CMK II concentrations increased in both membrane and cytosolic fractions until day 23 the kinase was predominantly cytoplasmic (approximately 75%) until day 23, after which it became increasingly membrane bound so that by day 52 post-hatching the majority of CMK II was present in the synaptic membrane fraction, and (2) the relative concentrations of the alpha- and beta-subunits changed from an alpha:beta-value of approximately 1:1 in the 19 day embryo to approximately 1:2 by 15 days post-hatch after which no further change was seen. The occurrence of major changes in the calmodulin stimulated protein phosphorylation system for up to 6-8 weeks after synapse formation is completed in the forebrain, provides further support for the existence of a synapse maturation phase of neuronal differentiation which is distinct from synapse formation. This phase involves only a specific subset of the developmental changes occurring in the calmodulin-stimulated phosphorylation system.

Regulation of protein kinase C activity by various lipids

Protein kinase C has recently attracted considerable attention because of its importance in the control of cell division, cell differentiation, and signal transduction across the cell membrane. The activity of this enzyme is altered by several lipids such as diacylglycerol, free fatty acids, lipoxins, gangliosides, and sulfatides. These lipids may interact with protein kinase C either directly or through calcium ions and produce their regulatory effect (activation or inhibition) on the activities of the enzymes phosphorylated by this kinase. These processes widen our perspective of the regulation of intercellular and intracellular communication.

Phosphatidic acid and phosphoinositide turnover in myelin and its stimulation by acetylcholine

Brain slices obtained from the forebrains of adult female rats were incubated with [32P]phosphate and [3H]glycerol for 60 min, and lipids extracted and analyzed by TLC. The 32P in brain slice lipids was primarily in polyphosphoinositides, phosphatidylinositol (PI), and phosphatidate (PA). Distribution of the 32P-labeled lipids in isolated myelin was biased toward PA, 38%, relative to 16% in whole tissue slice lipids. About 33% of the total labeled PA in brain slices was accounted for by that in myelin. On a per milligram protein basis, PA labeling in myelin is about 2.5-fold greater than that of whole brain slice. Since incorporation of [3H]glycerol (indicative of synthesis by the de novo synthetic pathway) was at very low levels, we conclude that [32P]phosphate entered into myelin PA primarily through a pathway involving phospholipase C activity. Much of the production of PA relates to hydrolysis of phosphoinositides, yielding diacylglycerol which is then phosphorylated within myelin. The distribution of label among the inositol-containing lipids suggests that only a fraction of the myelin polyphosphoinositides serve as substrate for rapid diglyceride production. In the presence of 10 mM acetylcholine (ACh) there was a 20-60% stimulation of [32P]phosphate incorporation into PA and PI of brain slice lipids and purified myelin. Stimulation by ACh was blocked by atropine. The observed increase in the 32P/3H ratio, relative to controls, indicated that for both total lipids and myelin lipids there was selective stimulation of a phospholipase C-dependent cycle relative to de novo biosynthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical localization of protein kinase C in developing brain tissue and in primary neuronal cultures

Antisera to protein kinase C (PKC) have been used to examine the presence and distribution of the enzyme in developing cerebellar cortex of postnatal rat and in cultures of rat sympathetic ganglia. In the cerebellar cortex of 2-,4-, and 6-day old rats, immunostaining was observed in areas of early-forming presynaptic terminals and growth cones. No staining was evident in the cortical proliferative zone. Beginning at 10 days postnatal, nuclear staining, not apparent at earlier stages, was prominent in Purkinje cells. In neuronal cultures of dissociated rat sympathetic ganglia, PKC was immunolocalized in cell bodies and bundles of neuronal processes. Immunoreactivity was particularly striking in growth cones of extending neurites and in axonal varicosities. These results suggest a role for PKC in neuronal growth following cell proliferation and in synaptic function. The appearance of nuclear staining in later developmental stages suggests that the enzyme may be involved in the promotion and maintenance of the differentiated state of neurons.

Stimulation of phosphoinositide hydrolysis in myelin by muscarinic agonist and potassium

Slices of rat brainstem that had been prelabeled by in vivo injection of [3H]inositol were stimulated with carbachol in the presence of lithium and changes measured in the radioactivity of inositol lipids and water-soluble inositol phosphates. For the latter, significant increases were seen for inositol mono- and bisphosphate but not inositol trisphosphate. Analysis of whole tissue phosphoinositides revealed significantly reduced radioactivity in phosphatidylinositol and phosphatidylinositol 4-phosphate, whereas myelin showed decreases in those as well as phosphatidylinositol 4,5-bisphosphate. These effects were blocked by atropine. Stimulation of the tissue slices with elevated K+ resulted in increased formation of inositol phosphate and decreased radioactivity in phosphatidylinositol. The effect was not blocked by atropine and in the presence of this agent, which reduced background reaction, all 3 phosphoinositides showed significant K+-induced loss of label. Elevated K+ and carbachol thus function through different mechanisms in this system. Carbachol is believed to affect myelin phosphoinositides through direct interaction with muscarinic receptors which were recently shown to be present in this membrane.

A phorbol ester-sensitive kinase catalyzes the phosphorylation of P0 glycoprotein in myelin

The proposed structural protein of peripheral nerve myelin, P0, has been shown to have several covalent modifications. In addition to being glycosylated, sulfated, and acylated, P0 is phosphorylated, with the intracellular site of this latter addition being in question. By employing nerve injury models that exhibit different levels of P0 biosynthesis in the absence and presence of myelin assembly, we have examined the cellular location of P0 phosphorylation. It is demonstrated that there is comparable P0 phosphorylation in both normal and crush-injured adult rat sciatic nerves, although the level of biosynthesis of P0 differs between these myelin maintaining and actively myelinating nerve models, respectively. The glycoprotein does not appear to be phosphorylated readily in the transected adult sciatic nerve, a preparation in which P0 biosynthesis is observed but that lacks myelin membrane. These observations suggest that the modification is not associated with the biosynthesis or maturation of P0 in the endoplasmic reticulum or Golgi, but that it instead occurs after myelin assembly. That P0 phosphorylation occurs in the normal nerve even when translation is inhibited by cycloheximide treatment lends further support to this conclusion. P0 is shown to be phosphorylated on one or more serine residues, with all or most of the phosphate group(s) being labile as evidenced by pulse-chase analysis. Addition of a biologically active phorbol ester, 12-O-tetradecanoylphorbol-13-acetate or 4 beta-phorbol 12,13-dibutyrate, substantially increases the extent of [32P]orthophosphate incorporation into the glycoprotein of normal and crushed nerve but not transected nerve. Biologically inactive 4 alpha-phorbol 12,13-didecanoate has no effect on P0 phosphorylation. Similarly, the addition of the cyclic AMP analog 8-bromo-cyclic AMP causes no appreciable changes in P0 labeling. These findings indicate that the phorbol ester-sensitive enzyme, protein kinase C, may be responsible for the phosphorylation of P0 within the myelin membrane.

Chloroform markedly stimulates the phosphorylation of myelin basic proteins

We have investigated the effect of chloroform on the phosphorylation of myelin basic proteins because tumor-promoting agents such as phorbol esters and chloroform are known to enhance the activity of protein kinase C. We report that the presence of chloroform, at a concentration known to enhance protein kinase C activity, stimulated the phosphorylation of myelin basic proteins 15-17 fold over control conditions. The phosphorylation of a 50 kiloDalton myelin protein was also stimulated but to a lesser extent. The concentration of chloroform required for the maximal phosphorylation of myelin basic proteins and the 50 kiloDalton protein was approximately 2% (v/v).

Covalent linkage of phosphoinositides to myelin basic protein: in vitro incorporation of [32P] phosphoinositides to myelin basic protein

We have previously reported that the covalent attachment of phosphoinositides to myelin basic protein (MBP) occurs both in vivo and in vitro [Smith, R. A. et al. (1986) Biochemistry 25:2677-2681; Biochemistry 25:2682-2686; and Biochem. Biophys. Res. Comm. 316:426-432]. Phosphoinositidation of MBP was also detected when [32P] phosphoinositides were incubated with myelin pretreated with Triton X-100 and EGTA. Less than 10% of this covalent linkage of phosphoinositides to MBP survived after acidic treatment (0.1 N HCl at 37 degrees C for 10 min). MBP is predicted to lack sufficient hydrophobicity to bind to membranes as shown by analysis of its amino acid sequence for hydrophobic regions and thus its phosphoinositidation may provide an anchor for this purpose.

Differential effect of ganglioside GM1 on rat brain phosphoproteins: potentiation and inhibition of protein phosphorylation regulated by calcium/calmodulin and calcium/phospholipid-dependent protein kinases

The monosialoganglioside GM1 displays complex effects on protein phosphorylation of rat cerebral cortex membrane preparations. The exogenous ganglioside at a concentration of 350 microM in absence of calcium only stimulated the phosphorylation of a protein of MW = 64,000. In presence of 1 mM calcium a twofold effect is observed irrespective of the phosphoprotein considered. In particular there is an enhancement of 32P incorporation in four major phosphoproteins of MW = 160,000, 140,000, 64,000 and 50,000 in presence of GM1 compared with that observed with calcium alone. The maximal stimulating effect is achieved with a ganglioside concentration of 35 microM. This effect is inhibited by the addition of 100 microM trifluoperazine (TFP), a phenothiazine known to inhibit calmodulin and protein kinase-C activities. These four proteins represent the major substrates for the calcium/calmodulin-dependent protein kinase with the MW = 64,000 and 50,000 proteins co-migrating with the autophosphorylated subunits of this enzyme. In addition, the ganglioside inhibited the phosphorylation of three proteins with MW = 86,000, 20,000 and 14,000. The electrophoretic properties of these phosphoproteins are similar to the autophosphorylated form of protein kinase-C and to the rat myelin basic proteins, respectively. The effect of the ganglioside on their phosphorylation is not influenced by TFP. Finally, a protein with an apparent molecular weight of 46,000 shows also an increased phosphorylation in presence of GM1. The reported results indicate that exogenous GM1 can have profound effects on different kinases such as the calcium/calmodulin dependent protein kinase, the protein kinase-C and also some unknown calcium-independent protein kinases.

Phosphorylation by protein kinase C of a synthetic heptapeptide bearing a lysine residue on the C terminal side of serine

A peptide, Ala-Ser-Gly-Ser-Phe-Lys-Leu, which corresponds to Ala103-Leu109 of Hl histone, was synthesized and tested as substrate for protein kinase C. The serine residue at position 4 was phosphorylated specifically. Another peptide lacking the lysine at position 6 was not phosphorylated by the same enzyme, indicating the importance of that basic residue as the recognition site for protein kinase C.

Endogenous basic protein phosphatases in the brain myelin

Direct treatment of brain myelin with freezing/thawing in 0.2 M 2-mercaptoethanol stimulated the endogenous myelin phosphatase activity manyfold when 32P-labeled phosphorylase a was used as a substrate, a result indicating that an endogenous myelin phosphatase is a latent protein phosphatase. When myelin was treated with Triton X-100, this endogenous latent phosphatase activity was further stimulated 2.5-fold. Diethylaminoethyl-cellulose and Sephadex G-200 chromatography of solubilized myelin revealed a pronounced peak of protein phosphatase activity stimulated by freezing/thawing in 0.2 M 2-mercaptoethanol and with a molecular weight of 350,000, which is characteristic of latent phosphatase 2, as previously reported. Moreover, endogenous phosphorylation of myelin basic protein (MBP) in brain myelin was completely reversed by a homogeneous preparation of exogenous latent phosphatase 2. By contrast, under the same conditions, endogenous phosphorylation of brain myelin was entirely unaffected by ATP X Mg-dependent phosphatase and latent phosphatase 1, although both enzymes are potent MBP phosphatases. Together, these findings clearly indicate that a high-molecular-weight latent phosphatase, termed latent phosphatase 2, is the most predominant phosphatase responsible for dephosphorylation of brain myelin.

Oligodendrocyte adhesion activates protein kinase C-mediated phosphorylation of myelin basic protein

When isolated adult oligodendrocytes adhere to a substratum myelinogenesis occurs. Investigation of the mechanism by which this happens indicated that the oligodendrocyte-substratum interaction activated protein kinase C-dependent phosphorylation of myelin basic protein and promoted the synthesis of myelin basic protein. In addition, when agents that activate protein kinase C (second messenger diacylglycerol or a tumor-promoting phorbol ester) were added to nonattached oligodendrocytes, they mimicked the influence of the substratum by inducing phosphorylation of myelin basic protein; and reagents that increase cellular adenosine 3', 5'-monophosphate (cyclic AMP) inhibited phosphorylation of myelin basic protein. Thus, at least in vitro, the interaction between oligodendrocytes and the substratum may mediate myelinogenic events, and phosphorylation of myelin basic protein may be an early requirement in the sequence of steps that ultimately results in myelin formation.

In vitro carboxymethylation of myelin basic protein

During a study to find natural substrate proteins of carboxymethylation, myelin basic protein was found to be a good substrate. The two protein carboxymethylases were purified partially using myelin basic protein as a substrate. These two enzymes may be identical with protein carboxymethylase I and II, which have been found to methylate gamma-globulin. The Km of myelin basic protein (25 microM) was very small compared with other substrates. The activities of the two carboxymethylases were high in the rat brain in comparison to the other rat organs. The activity increased during the period of myelination in the rat brain. These findings suggest that carboxymethylation of myelin basic protein may play an important role in myelination.

Phosphorylation of soluble pig epidermal proteins by endogenous calcium-activated, phospholipid-dependent protein kinase

Endogenous calcium-activated, phospholipid-dependent protein kinase phosphorylates pig epidermal protein. Pig epidermis was homogenized and centrifuged at 30,000 X g for 30 min. The supernatant was incubated with or without calcium and phospholipid. A 97 kD soluble protein from pig epidermis was phosphorylated in the presence of calcium and phosphatidylserine. The phosphorylation reaction occurred immediately. With the use of two-dimensional polyacrylamide gel electrophoresis, it was shown that the 97 kD fragment was a basic protein, and that several small proteins were also phosphorylated. The characterization of these proteins is yet to be undertaken.

In vivo phosphorylation of myelin basic proteins: age-related differences in 32P incorporation

Myelin basic proteins (MBPs) are phosphoproteins of central and peripheral nervous system myelin. We studied the phosphorylation of mouse MBPs in vivo at three different stages of development (12, 30, and 50 days) and found age-related differences in the incorporation of 32P into MBPs. At all ages studied, significant amounts of 32P were found in the MBPs as early as 1 min after intracranial injection of isotope. Incorporation of radioactive phosphate into MBPs proceeded rapidly and the resultant specific radioactivity (SA) of 32P-labeled MBPs appeared to be related to the SA of the acid-soluble phosphate pool of myelin. Changes in the SA of the myelin acid-soluble phosphate pool were observed in a 30 min time course of labeling in vivo in 50-day mice. Coincident changes were observed in the SA of the MBPs. Similar but less pronounced changes were seen in the SA of the polyphosphoinositides (PPIs) indicating that the turnover of the PPI phosphate groups is slower than the MBP phosphates or that the PPI phosphates are drawn from additional or different pools than the MBP phosphates. The phosphorylation of MBPs in developmentally related myelin fractions is investigated in a comparison paper (J. B. Ulmer and P. E. Braun (1986) Dev. Biol. 117, 502-510).

Subcellular distribution of a calmodulin-dependent protein kinase activity in rat cerebral cortex during development

The postnatal development of calmodulin-stimulated phosphorylation of endogenous proteins, in particular the autophosphorylated subunits of the calmodulin-stimulated protein kinase II, were investigated in subcellular fractions of rat cerebral cortex. The major subunit had a mol. wt. of 53,000 Da (designated 50 kDa) and the minor one a mol. wt. of 63,000 Da (designated 60 kDa). The 50-kDa subunit was found to be the only significant phosphoprotein in each fraction and throughout development at its molecular weight. However, the 60-kDa subunit was found to comigrate with other phosphoproteins that accounted for up to 15% of the radioactivity at this molecular weight and which differed between the fractions. 50-kDa autophosphorylation was found to be 3-fold greater in cytoplasmic fractions at day 10 and by adults was evenly distributed between membrane and cytoplasmic fractions. A similar pattern was also found for the total calmodulin-stimulated phosphorylation. Changes in autophosphorylation activity of the 50-kDa subunit were found to represent changes in kinase activity rather than alterations in phosphatase activity. In the membrane, this change was shown to be due to changes in the amount of enzyme. Although in the adult autophosphorylation activity is evenly distributed between membrane and soluble fractions, when differences in phosphatase activity and lack of autophosphorylation activity of the majority of post-synaptic density-associated kinase is taken into account, it is clear that the vast majority of the enzyme is membrane-bound. Phosphorylation of endogenous substrates paralleled the development of 50-kDa subunit autophosphorylation, most of which occurred between day 14 and day 30, a period which follows the most rapid phase of synaptogenesis. This pattern was different from that of the phosphorylation of myelin basic protein and two substrates of the calcium-phospholipid-dependent protein kinase. There was also a change in the ratio of autophosphorylation activity of the 50-kDa and 60-kDa subunits during development which appears to be due to a change in the amount of the subunits themselves. This ratio was the same in all fractions at any one age. We suggest that this change is due to the existence of at least two developmentally regulated isoenzymes in the cortex.