Adenosine deaminase of cultured brain cells

Two types of adenosine deaminase (EC 3.5.4.4) were found in cultured cells of central-nervous-system origin. The predominant and more active enzyme was obtained in soluble form from the cytosol of mouse neuroblastoma (N-18), neonatal hamster astrocytes (NN), human oligodendroglioma (HOL) and human astrocytoma (Cox Clone). Particulate adenosine deaminase was probably associated with the plasma membrane. When radioactive adenosine was added to superfusates of monolayer cultures it was rapidly converted into inosine and hypoxanthine. The metabolic conversion required adenosine uptake by the cells, a probable transition through the intracellular ATP pool(s) and a rapid excretion into the superfusate of the catabolic products. We discuss the evidence that points to adenosine and its derivatives as neurohumoral modulators of central-nervous-system function.

Variability in the binding of anti-MAG and anti-SGPG antibodies to target antigens in demyelinating neuropathy and IgM paraproteinemia

Densitometry of immunostained Western blots or thin layer chromatograms and enzyme-linked immunosorbent assays (ELISAs) were used to compare the relative strengths of IgM binding to myelin-associated glycoprotein (MAG), P0 glycoprotein, peripheral myelin protein-22 (PMP-22), sulfate-3-glucuronyl paragloboside (SGPG), and other potential target antigens in a series of eleven patients with sensory or sensorimotor demyelinating neuropathy and IgM paraproteinemia. The IgM from all patients exhibited reactivity with both MAG and SGPG, and there was a statistically significant correlation between the overlay assays and ELISAs for measuring the strength of IgM binding to MAG and to SGPG. However, the data revealed variations in the relative strengths with which the antibodies bound to the potential target antigens and heterogeneity in their fine specificities. First, there was a poor correlation between the strength of binding to MAG and to SGPG, respectively. Second, reactivity with MAG or SGPG in a few of the patients was only detected by one of the two assay systems. Third, about one-third of the patients' IgM absolutely required the sulfate on SGPG for reactivity, whereas the others retained some reactivity after removal of the sulfate. Fourth, IgM from two of the patients exhibited unusually strong reactivity with the proteins of compact myelin, P0 and PMP22. These relative differences in strengths of antibody binding to the potential antigens were compared with the patients' clinical presentations and with their responses to intravenous immunoglobulin (IVIg) therapy in a clinical trial in which they participated. For the most part, these variations did not correlate with clinical presentation, which was relatively homogeneous in this series of patients. However, an inverse relationship was noted between degree of reactivity to MAG by ELISA and response to IVIg. Two of the patients who responded had only mild elevations of IgM antibodies to nerve glycoconjugates and exhibited some unusual immunochemical and clinical characteristics in comparison to the other patients. The results demonstrate differences in the relative strengths with which anti-MAG and anti-SGPG IgM antibodies from different patients bind to potential neural target antigens which may affect pathogenic mechanisms and response to therapy.

The myelin-associated glycoprotein of the peripheral nervous system in trembler mutants contains increased alpha 2-3 sialic acid and galactose

The myelin-associated glycoprotein (MAG) exhibits an abnormally high apparent molecular weight in sciatic nerve, but not in brain, of dysmyelinating trembler mutants (Inuzuka et al.: J Neurochem 44:793-797, 1985). Antibodies to the large and small isoforms of MAG (L- and S-MAG) and probes for oligosaccharide structure were used to determine if this was due to overexpression of L-MAG or increased glycosylation. Nerves from both control and trembler 36-day-old mice contained primarily S-MAG with only traces of L-MAG. The distribution of the two isoforms appeared normal in trembler mice, and both isoforms exhibited the higher apparent molecular weight. Lectin binding showed that, in contrast to brain in which most glycoproteins contain primarily alpha 2-3 linked sialic acid, most glycoproteins of both control and trembler nerve contained primarily alpha 2-6 linked sialic acid. Lectin binding and glycosidase treatments demonstrated that the higher molecular weight of MAG in trembler nerves was due to an increased content of alpha 2-3 linked sialic acid and galactose. The abnormal glycosylation of MAG in trembler mutants may contribute to the myelin pathology.

Exfoliation of the beta-adrenergic receptor and the regulatory components of adenylate cyclase by cultured rat glioma C6 cells

Cultured rat glioma C6 cells exfoliate membrane vesicles which have been termed 'exosomes' into the culture medium. The exosomes contained both stimulatory and inhibitory GTP-binding components of adenylate cyclase (the stimulatory, Gs, and the inhibitory, Gi, regulatory components) and beta-adrenergic receptors but were devoid of adenylate cyclase activity. It was therefore apparent that the catalytic component of adenylate cyclase was either not exfoliated or was inactivated during the exfoliation process. The presence of Gs or Gi in the exosomes was detected by ADP ribosylation using [alpha-32P]NAD in the presence of cholera or pertussis toxins, respectively. The exosomal concentration of each of the two components was estimated to be about one fifth of that of the cell membrane when expressed on a per mg protein basis. Exosomal Gs was almost as active as the membrane-derived Gs in its ability to reconstitute NaF- and guanine nucleotide-stimulated adenylate cyclase activity in membranes of S49 cyc- cells, which lack a functional Gs. The ability of exosomal Gs to reconstitute isoproterenol-stimulated activity, however, was much lower than that of membrane Gs. The density of beta-adrenergic receptors in the exosomes was much less than that found in the membranes. Although the exosomal receptors bound the antagonist iodocyanopindolol with the same affinity as receptors from the cell membrane, the affinity for the agonist isoproterenol was 13- to 18-fold lower in the exosomes. In addition, this affinity was not modulated by GTP in the exosomes. Thus, exfoliated beta-adrenergic receptors seem to be impaired in their ability to couple to and activate Gs. This was directly tested by coupling the receptors to a foreign adenylate cyclase using membrane fusion. The fusates were then assayed for agonist-stimulated activity. While significant stimulation of the acceptor adenylate cyclase was obtained using C6 membrane receptors, the exosomal receptors were completely inactive. Thus during exfoliation, there appear to be changes in the components of the beta-adrenergic-sensitive adenylate cyclase that results in a nonfunctional system in the exosomes.

Synaptic effects of the synthetic pyrethroid resmethrin in rat brain in vitro

Resmethrin (30 microM) induced release of transmitters was not affected by manipulation of the Na+ current with either choline or tetrodotoxin agents which readily reversed the effects of veratridine, deltamethrin and cypermethrin. Resmethrin (I50: 2.2 microM) inhibited the ATP dependent uptake of Ca2+ but deltamethrin and cypermethrin were much less effective. Resmethrin also displaced Ca2+ from crude synaptosomal membranes. The release promoting effects of resmethrin in rat brain in vitro are better explained by its effects on Ca2+ rather than through a specific effect on the Na+ channel. In contrast, the effects of deltamethrin and cypermethrin promote transmitter release by a Na+ dependent process.

Mn2+-stimulated ATPase in rat brain

Divalent cation ATPases were prepared from rat brain synaptic vesicles, synaptosomal plasma membranes, and plasma membranes from the brain stem and sciatic nerve and tested for optimal stimulation by Mn2+, Mg2+, or Ca2+. ATPase in the synaptic vesicle subfraction was optimally stimulated by Mn2+. All plasma membrane preparations were optimally stimulated by Mg2+. Separate Mn2+ and Mg2+ ATPases could not be distinguished by either chemical inactivation or substrate preference criteria. Mn2+ stimulated ATPase in the micromolar range and it is suggested that Mn2+ interaction with ATPase may be of physiological and/or toxicological importance by being related to the cellular metabolism of this element.

Exfoliation of membrane ecto-enzymes in the form of micro-vesicles

Cultures from various normal and neoplastic cell lines exfoliated vesicles with 5'-nucleotidase activity which reflected the ecto-enzyme activity of the parent monolayer culture. The ratio of 5'-nucleotidase to ATPase activity in the microvesicles indicated that cellular ecto-ATPase was conserved in the exfoliative process. Phospholipids of the microvesicles contained significantly increased amounts of sphingomyelin and total polyunsaturated fatty acids. It was concluded that the shedded vesicles constituted a select portion of the plasma membrane. Examination by electron microscopy showed the vesicles had an average diameter of 500 to 1000 nm and often contained a second population of vesicles about 40 nm in diameter. As much as 70% of the plasma membrane ecto-5'-nucleotidase activity of a culture was released into the medium over a 24-h period. Phosphoesterhydrolases from C-6 glioma or N-18 neuroblastoma microvesicles dephosphorylated cell surface constituents when in contact with monolayer cultures. Exfoliated membrane vesicles may serve a physiologic function; it is proposed that they be referred to as exosomes.

Selective chemical modification of plasma membrane ectoenzymes

As part of an investigation of the organization of cell surface macromolecular assemblies, we have treated intact central nervous system cells with chemical probes which react convalently with proteins and aminophospholipids. Selective alterations of the enzymatic activities of ecto-ATPases, ecto-5'-nucleotidases and cholinesterases were obtained under appropriate reaction conditions. The cross-linking reagent, 1,5-difluoro-2,4-dinitrobenzene, was a potent inactivator of ecto-ATPase of C6 glioblastoma, IMR-32 neuroblastoma and of a primary rat astroblast cell line (RB). Ecto-5'-nucleotidase and acetylcholinesterase were less sensitive to difluorodinitrobenzene. 1-Fluoro-2,4-dinitrobenzene at concentrations which inactivated ecto-ATPase had little effect on ecto-5'-nucleotidase. Conversely, 2,4,6-trinitrobenzenesulfonic acid was a potent inactivator of ecto-5'-nucleotidase but had no effect on ecto-ATPase. The difluorodinitrobenzene inactivation of ecto-ATPase and of ecto-5'-nucleotidase as well as the fluorodinitrobenzene inactivation of ecto-ATPase could be prevented by the presence of the appropriate substrates in the reaction medium. In the presence of protecting nucleotide substrates, a decrease in reactivity with proteins and lipids was observed when the isotopic probe fluorodinitro[3H]-benzene was used.

A comparative study of brain Ca2+-ATPases

1. Particulate brain ATPases from various vertebrates were optimally activated by Ca2+, Mg2+ or Mn2+. 2. Specific enzyme activity with AT32P as substrate was low in lower vertebrates and increased on the evolutionary scale. 3. The properties of the brain ATPases suggested that most of the activity was associated with plasma membrane ecto-ATPase.

On the activation of plasma membrane ecto-enzymes by treatment with neuraminidase

It had been proposed that sialyl-residues on the surface of the cell control the activity of certain plasma membrane ecto-enzymes. We have tested the effects of several established (or presumptive) ecto-enzymes in tissue cultures of CNS-derived cells. Application of neuraminidases to cultured mouse neuroblastoma (N-18), neonatal Syrian hamster astrocytes (NN), human astrocytoma (Cox clone) and two lines of primary mouse astroblasts failed to change the activity of ecto-ATPase and 5'-nucleotidase. Only two of the seven neuraminidase preparations produced marked or moderate increases in inorganic pyrophosphatase, p-nitrophenylphosphatase and cholinesterase. We have concluded that the stimulation of these enzymes was not due to removal of sialyl-residues. We suggest that contaminants (haemolysins?) in neuraminidase preparations of Clostridium perfringens increased membrane permeability and facilitated substrate-product translocation.

Lipoprotein synthesis. I. rat plasma lipoprotein composition and synthesis from radioactive precursors

The in vivo synthesis of rat plasma lipoproteins was studied by the use of isotopic protein and lipid precursors. Labelled amino acids, palmitic acid and tripalmitin were administered by stomach tube and the radioactivity in the plasma lipoproteins was determined following preparative ultracentrifugal isolation at densities of 1.006, 1.019, 1.063 and 1.21 g/ml.In response to triglyceride feeding, amino acid composition of the high density lipoprotein changed little, but in the low density lipoproteins proportionality in the amino acid pattern was changed as reflected by increases and decreases in certain amino acids.Isotopic amino acids were not incorporated in proportion to the relative abundance with which they occurred in the lipoproteins. Triglyceride feeding markedly stimulated isotope utilization, especially in the low density fractions. Methionine, though only present in small amounts, was extensively utilized and it is suggested that this amino acid may play a significant role in the synthesis of lipoproteins, other than the role of a methyl donor for phosphatidylcholine.