Purification of rat 2',3'-cyclic nucleotide 3'-phosphodiesterase

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.

Structure and chromosomal localization of the human 2?,3?-cyclic nucleotide 3?-phosphodiesterase gene

Human brain cDNA clones for the myelin associated enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) have been isolated and sequenced. The only 5' untranslated region (UTR) sequence found was that of a human CNPII mRNA, with no direct evidence for a CNPI mRNA. Human CNPase cDNAs were used to isolate genomic clones containing the human CNPase gene which is 9 kb long. Four exons were identified, separated by three introns, and the sequence of each exon and intron/exon boundary has been established. The polymerase chain reaction (PCR) was used to detect the presence of the human CNPase gene in DNA from a panel of rodent/human somatic cell hybrids. By this means the human CNPase gene was mapped to chromosome 17. In situ hybridization of a human CNPase genomic clone to metaphase chromosomes further localized this gene to chromosomal band 17q21.

Glutamine synthetase expression in rat oligodendrocytes in culture: regulation by hormones and growth factors

Glutamine synthetase (GS, EC 6.3.1.2.) has long been considered as a protein specific for astrocytes in the brain, but recently GS immunoreactivity has been reported in oligodendrocytes both in mixed primary glial cell cultures and in vivo. We have investigated its expression and regulation in "pure" oligodendrocyte cultures. "Pure" oligodendrocyte secondary cultures were derived from newborn rat brain primary cultures enriched in oligodendrocytes as described by Besnard et al. (1987) and were grown in chemically defined medium. These cultures contain more than 90% galactocerebroside-positive oligodendrocytes and produce "myelin" membranes (Fressinaud et al., 1990) after 6-10 days in subcultures (30-35 days, total time in culture). The presence of GS in oligodendrocytes from both primary glial cell cultures and "pure" oligodendrocyte cultures was confirmed by double immunostaining with a rabbit antisheep GS and guinea pig antirat brain myelin 2', 3'-cyclic nucleotide 3'-phosphodiesterase. In "pure" oligodendrocyte cultures, about half of cells were labeled with anti-GS antibody. Furthermore, on the immunoblot performed with a rabbit antisheep GS, the GS protein in "pure" oligodendrocyte secondary cultures was visualized as a single band with an apparent molecular mass of about 43 kDa. In contrast, two protein bands for GS were observed in cultured astrocytes. On the immunoblot performed with a rabbit antichick GS, two immunopositive protein bands were observed: a major one migrating as the purified adult chick brain GS and a minor one with a lower molecular mass. Two similar immunoreactive bands were also observed in pure rat astrocyte cultures. Compared to pure rat astrocyte cultures, "pure" oligodendrocyte cultures of the same age displayed an unexpectedly high GS specific activity that could not be explained by astrocytic contamination of the cultures (less than 5%). As for cultured astrocytes, treatment of oligodendrocyte cultures with dibutyryl-adenosine 3':5'-cyclic monophosphate, triiodothyronine, or hydrocortisone increased significantly GS specific activity. Interestingly, epidermal growth factor, basic fibroblast growth factor, and platelet-derived growth factor that increase the GS activity in astrocytes do not affect this activity in oligodendrocytes. Thus we confirm the finding of Warringa et al. (1988) that GS is also expressed in oligodendrocytes. We show that its activity is regulated similarly in astrocytes and oligodendrocytes by hormones, but that it is regulated differently by growth factors in these two cell types.

cDNA cloning and amino acid sequence of human brain 2',3'-cyclic-nucleotide 3'-phosphodiesterase

A cDNA of 1762 base pairs was obtained from a cDNA library of human brain by immunoscreening, and the nucleotide sequence of the cDNA was determined. The complete amino acid sequence of human 2',3'-cyclic-nucleotide 3'-phosphodiesterase was deduced from the nucleotide sequence of the cDNA. Human enzyme was found to contain 401 amino acids including initiation methionine and have a molecular weight of 45,098. RNA blot hybridization revealed a single mRNA band at the position of about 3000 bases. DNA blot hybridization suggested that a single-copy 2',3'-cyclic-nucleotide 3'-phosphodiesterase gene exists per haploid genome.

Release of proteins from the surface of bovine central nervous system myelin by salts and phospholipases

Incubation of bovine CNS myelin with phospholipase C from Bacillus cereus under conditions that lead to extensive phospholipid degradation caused 10% of the myelin protein to be released from the membrane. The myelin basic protein (MBP) was a major component of the dissolved protein. Comparable incubations with phospholipase C from Clostridium perfringens, phosphatidylinositol-specific phospholipase C from Staphylococcus aureus, or cabbage phospholipase D removed little MBP. However, concentrations of sodium chloride near 1 M and concentrations of divalent metal ions between 50 and 600 mM released typically 9-12% of the total myelin protein, with MBP again as the predominant component. Repetitive washing with calcium chloride solutions resulted in dissolution of over 90% of the MBP. When myelin was incubated in 1.0 M sodium chloride or 25 mM calcium chloride, the MBP was cleaved largely into two major peptides with apparent molecular weights near 14,000 and 12,000, but with 200 mM or higher concentrations of calcium chloride most of this protein remained intact. With appropriate manipulation of the ionic milieu, it is thus possible to remove most of this extrinsic protein from the myelin surface under relatively mild conditions. The conditions that release the protein suggest that it is held at the membrane surface by ionic interactions.

Monoclonal antibody production to human and bovine 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase): high-specificity recognition in whole brain acetone powders and conservation of sequence between CNP1 and CNP2

Monoclonal antibodies against human and bovine 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) were generated by fusing FOX-NY myeloma cells with spleen cells from RBF/Dn mice previously immunized with the purified brain antigens. The enzyme isolated from bovine brain was quite basic, with an isoelectric point of 9.71 and both the bovine and human enzymes consisted of a closely spaced doublet at approximately 44 and 46 kDa on SDS-PAGE. Six monoclonals were were identified as strongly recognizing the enzyme on both ELISA plates and on immunoblots of whole brain protein. Four monoclonals very weakly cross-reacted with guinea pig myelin basic protein. In contrast with two previous reports, some of our monoclonal antibodies did immunostain 2 or 3 protein bands in peripheral nerve, two bands closely corresponding to those immunostained in central nervous system (CNS) myelin, the Wolfgram protein fraction and in acetone powders of whole brain. Each of the 6 monoclonals reacting strongly on immunoblots recognized the enzyme in from 2 to 5 of the species examined (human, bovine, rat, mouse and rabbit). In addition, all 6 monoclonals that immunostained the enzyme in whole brain, myelin and Wolfgram protein immunoblots recognized both CNP1 (44 kDa) and CNP2 (46 kDa). The two closely spaced protein bands observed on SDS-PAGE and previously stained on immunoblots of CNS CNPase using polyvalent rabbit anti-bovine CNPase antisera, and now different monoclonal antibodies, appear to be immunologically related and to contain highly conserved sequences.

Molecular cloning of the myelin specific enzyme 2',3'-cyclic-nucleotide 3'-phosphohydrolase

We describe the isolation of cDNA clones for bovine brain 2',3'-cyclic-nucleotide 3'-phosphohydrolase (CNPase, EC 3.1.4.37), the third most abundant protein in central nervous system myelin. The cDNA encodes the complete protein (400 amino acids) and hybridizes to a major size species of mRNA in bovine brain tissue, approx. 2.7 kb in size. CNPase mRNA levels do not appear to be affected in quaking dysmyelinating mutant mice. The sequence reveals probable sites for CNPase phosphorylation by cAMP-dependent protein kinase and a region of homology with haemocyanin.

Susceptibility of the Wolfgram proteins and stability of 2',3'-cyclic nucleotide 3'-phosphodiesterase of rat brain myelin to limited proteolytic digestion

The susceptibility of proteins in the myelin membrane to proteases was studied. Lyophilized rat brain myelin suspended in water was subjected to controlled proteolytic digestion with pure trypsin (N-tosyl-L-phenylalanine chloromethyl ketone treated, 5 units/mg of myelin), and proteins remaining in the pellet were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Under these conditions, large basic protein (LBP) was completely hydrolyzed in 5-10 min, proteolipid proteins remained largely intact until 60 min, whereas Wolfgram protein (WP) was progressively degraded from 10 min onward with the simultaneous appearance of a new protein band with a molecular weight of 35K. A similar pattern was obtained on treatment with chymotrypsin or subtilisin. The 35K protein band was shown to be derived from WP by its immunological cross-reactivity with WP antibodies. Western blot analysis showed that 35K protein is the only major breakdown product of WP under these conditions. Treatment with higher concentrations of trypsin (greater than 20 units/mg of myelin) resulted in the degradation of all myelin proteins. Essentially all the 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) activity was observed in the myelin pellet after controlled or drastic digestion with trypsin. It is concluded that the major fragment of WP (35K) is located in the hydrophobic milieu of the bilayer, relatively inaccessible to trypsin, whereas a portion (20K) of the WP is exposed to the cytoplasmic side (major dense line), like LBP, and that peptide fragments (less than 14K) that remained in the myelin membrane lipid bilayer after trypsin digestion could exhibit CNP activity.

Identification of 2',3'-cyclic nucleotide 3'-phosphodiesterase in bovine adrenal medulla

The adrenal medulla contains an enzyme which catalyzes the hydrolysis of 2',3'-cAMP to 2'-AMP. For the parameters which have been examined, the adrenal medulla 2',3'-cAMP phosphodiesterase appears to be similar to brain 2',3'-cyclic nucleotide 3'-phosphodiesterase (also commonly referred to as CNPase). The apparent Km of the adrenal medulla CNPase for 2',3'-cAMP is 0.88 mM. The enzyme activity is unaltered by either EDTA, MgCl2 or CaCl2 in the presence or absence of calmodulin. The apparent molecular weight is 102,500 daltons. The function of the enzyme in either the brain or the adrenal medulla is, at the present time, unknown.

A monoclonal antibody raised to corpus callosum extract reacts with 2',3'-cyclic nucleotide 3'-phosphohydrolase

Monoclonal antibody against 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) was generated by fusing mouse myeloma cells with spleen cells from BALB/c mice immunized with delipidated white matter from rat corpus callosum. The antibody was characterized by solid-phase radioimmunoassay, immunoblot of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), immunoprecipitation from C6 glioma cells, and indirect immunofluorescence staining of monolayer cultures containing oligodendrocytes. The monoclonal antibody bound specifically to an intracellular antigen of oligodendrocytes, but not to Schwann cells, astrocytes, neurons, or fibroblast cytoplasm. The immunoblot of SDS-PAGE of CNS myelin showed that the antibody identified two protein bands at 48,000 and 50,000 molecular weight. These proteins were not identified in peripheral nervous system myelin. The monoclonal antibody immunoprecipitated CNP enzyme activity from extracts of C6 glioma cells. This monoclonal antibody should prove useful in further study of this myelin-specific enzyme in CNS myelin and in cells responsible for myelin production.

Biosynthesis and insertion of Wolfgram protein into optic nerve membranes

Antibodies against pig brain Wolfgram protein (WP) were prepared and utilized in the analysis of WP biosynthesis in membranes from optic nerves of 20 day-old rats. Newly synthesized WP appeared rapidly (less than 5 min) in myelin and in a non-myelin microsome fraction and accumulated in both thereafter. Monensin did not affect the insertion of WP in either membrane fraction. These results are consistent with biosynthesis of WP on free ribosomes.

Developmental expression of 2',3'-cyclic nucleotide 3'-phosphohydrolase in dissociated fetal rat brain cultures and rat brain

The development in primary dissociated rat brain cultures of 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) activity, the accumulation of CNP protein, and the number of cells accumulating this protein have been quantitatively determined as a function of time in culture. Parallel determinations have been made for the first two parameters for developing rat brain. The developmental profile of CNP enzymatic activity and amount of CNP protein in culture paralleled that observed in rat brain, in which the period of most active development occurred 7-25 days after birth. Mean CNP activities of 5.6 and 8.1 mumol/min/mg total protein were recorded for the cultures and rat brain, respectively, at their maximal levels. The corresponding mean values for the CNP protein accumulation were calculated to be 138 and 150 pmol/mg total protein, respectively. Thus maximal specific activities of the CNP protein were estimated to be about 800 and 1,000 mumol/min/mg CNP protein for culture and rat brain enzyme, respectively. Approximately three million cells expressing CNP appeared in the cultures per dissociated fetal rat brain seeded. Each CNP+ oligodendrocyte in culture had an average CNP activity of 3.2 pmol/min, and an average CNP protein content of 0.09 fmol (5.4 X 10(7) molecules), values which remained nearly constant during the course of development. Two principal conclusions are drawn from these data. First, the dissociated fetal brain culture system reproduces rather accurately the temporal developmental pattern of CNP expression occurring in the rat brain, but some important quantitative differences occur which suggest the need for additional environmental stimuli missing in these cultures. Second, the quantitative increases in CNP specific activity and amount of CNP protein occurring during oligodendrocyte differentiation in these cultures are primarily the result of increases in the number of CNP+ cells present which upon differentiation express very quickly, via an off-on regulation, steady-state levels of the enzyme.

An investigation of 2':3'-cyclic nucleotide 3'-phosphodiesterase (EC 3.1.4.37, CNP) in peripheral blood elements and CNS myelin

Activity of 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNP) of human erythrocyte membranes was determined in the presence of various brain CNP inhibitory compounds. Also, the hydrolysis of 2':3'-cAMP and 2':3'-cCMP by CNP of human platelets and lymphocytes was confirmed by thin layer chromatography and CNP activity was measured in lymphocytes, platelets, erythrocytes and CNS myelin. Human erythrocyte CNP activity was reduced 75 percent by the organomercurial p-chloromercuriphenyl sulfonate (1 X 10(-4) M), 46 percent by thimerosal (1 X 10(-4) M) and 35 percent by cupric chloride (1 X 10(-3) M). The 2'-AMP or 2'-CMP isomer was produced, exclusively, by the hydrolysis of 2':3'-cAMP or 2':3'-cCMP, respectively, by CNP of human lymphocytes and platelets and indicates a CNP-like activity is not only present in erythrocytes and the central and peripheral nervous systems, but also platelets and lymphocytes. CNP activities of human erythrocytes, human human and rat lymphocytes and human platelets were less than 4 percent of the activity of human and bovine CNS myelin.

The localization of 2':3'-cyclic nucleotide 3'-phosphodiesterase in bovine cerebrum by immunofluorescence

The distribution of 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in fixed-tissue sections and Percoll-isolated cells (oligodendrocytes) of bovine cerebrum was studied by immunofluorescent techniques. Fluorescence in tissue sections exposed to CNP antisera was restricted to oligodendrocyte-like cells. Also, Percoll-isolated cells (oligodendrocytes) were positively labeled with CNP antisera. This study has shown that CNP may be used as an antigenic marker for oligodendrocytes.

Immunochemical identification of 2', 3'-cyclic nucleotide 3'-phosphodiesterase in central and peripheral nervous system myelin, the Wolfgram protein fraction, and bovine oligodendrocytes

Myelin proteins and the total Wolfgram protein fraction were isolated from the CNS of several mammalian species and characterized with rabbit anti-bovine 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP) antisera after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoretic transfer to nitrocellulose membranes. The corresponding CNP proteins cross-reacted across all species examined, suggesting that the CNP amino acid sequence was fairly well conserved in all six species. The same corresponding proteins were also identified immunochemically in the crude total Wolfgram protein fraction in the region of the W1 myelin protein, thus further supporting and extending two different previous reports indicating a relationship between CNP and the W1 protein. In addition to these CNS enzyme sources, peripheral nervous system CNP (rabbit and rat sciatic nerve) was also recognized by these same rabbit anti-bovine (CNS) CNP antisera. CNP was also detected in freshly isolated delipidated bovine oligodendrocyte membranes. These results suggest that rabbit anti-bovine CNP antisera may be of use in localization and structural studies of this enzyme in several different species and will permit clear identification of CNP in oligodendrocytes and their isolated membrane fractions.

Biochemical study of heterosis for brain myelin content in mice

Heterosis (hybrid vigor) for brain myelin content has been examined in detail in (C57BL/6J x DBA/2J)F1 hybrid mice at 17 days of age. The amount of myelin isolated from the F1 hybrid brain is greater than that isolated from either parental strain. In addition, the total protein content in the myelin of the three genotypes showed the following trend: F1 greater than DBA greater than C57. However, no discernible differences in myelin protein compositions could be detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis of the whole brain for several myelin-associated constituents such as GM1 ganglioside, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase), 5'-nucleotidase, and carbonic anhydrase indicated that heterosis exists for these components. No heterosis was found for such nonmyelin constituents as gangliosides GD1a, GT, GQ, RNA, DNA, and choline acetyltransferase. A developmental study of the whole brain CNPase indicated that the heterotic effect was greatest during the most active period of myelination (17-30 days). We conclude that the heterotic effect is specific for myelin content and is probably the result of an accelerated myelin synthesis. The heterotic effect should have great potential as a new model for studying aspects of myelinogenesis.

Carbonic anhydrase, 5'-nucleotidase, and 2',3'-cyclic nucleotide-3'-phosphodiesterase activities in oligodendrocytes, astrocytes, and neurons isolated from the brains of developing rats

The activities of three myelin-associated enzymes, carbonic anhydrase, 5'-nucleotidase, and 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNP), were measured in oligodendrocytes, neurons, and astrocytes isolated from the brain of rats 10, 20, 60, and 120 days old. The carbonic anhydrase specific activity in oligodendrocytes was three- to fivefold higher than that in brain homogenates at each age, and, at all the ages, low activities of this enzyme were measured in neurons and astrocytes. The oligodendrocytes and astrocytes from the brains of rats at all ages had higher activities of the membrane-bound enzyme 5'-nucleotidase than was observed in neurons. In oligodendrocytes from 10- and 20-day-old rats, the 5'-nucleotidase activity was two-to threefold the activity in the homogenates (i.e., relative specific activity = 2.0-3.0), and the relative specific activity of this enzyme in the oligodendrocytes declined to less than 1.0 at the later ages, concomitant with the accumulation of 5'-nucleotidase in myelin. The CNP activity was always higher in oligodendrocytes than in neurons, but not appreciably different from that in astrocytes from 20 days of age onward. The relative specific activity of CNP was highest in the oligodendrocytes from 10-day-old rats but was lower, at all ages, than we had observed in bovine oligodendrocytes. These enzyme activities in oligodendroglia are quite different in amount and developmental pattern from those reported previously for myelin.

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

Phosphorylation of myelin basic protein (MBP) in rat or rabbit brain myelin was markedly stimulated by Ca2+, and this reaction was not essentially augmented by exogenous phosphatidylserine or calmodulin or both. Solubilization of myelin with 0.4% Triton X-100 plus 4 mM EGTA, with or without further fractionation, showed that Ca2+-dependent phosphorylation of MBP required phosphatidylserine, but not calmodulin. DEAE-cellulose chromatography of solubilized myelin revealed a pronounced peak of protein kinase activity stimulated by a combination of Ca2+ and phosphatidylserine; a protein kinase stimulated by Ca2+ plus calmodulin was not detected. These findings clearly indicate an involvement of phospholipid-sensitive Ca2+-dependent protein kinase in phosphorylation of brain MBP, although a possible role for the calmodulin-sensitive species of Ca2+-dependent protein kinase in this reaction could not be excluded or established. Phosphorylation of MBP in solubilized rat myelin catalyzed by the phospholipid-sensitive enzyme was inhibited by adriamycin, palmitoylcarnitine, trifluoperazine, melittin, polymyxin B, and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7).

Inhibition of 2',3'-cyclic nucleotide 3'-phosphodiesterase activity from bovine, human and guinea pig brain

The purified oligodendrocyte enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) obtained from bovine, human and guinea pig brain was inhibited by theophylline, caffeine, cupric chloride and organomercurials using a spectrophotometric assay and 1 mM 2',3'- cyclic cytidylate as the substrate. The observed inhibition by theophylline, caffeine and organomercurials was reversible.