Neural protein 1B236/myelin-associated glycoprotein (MAG) defines a subgroup of the immunoglobulin superfamily

We have reviewed the structure and properties of the neural protein 1B236/MAG. This molecule consists largely of five Ig-like domains separated from its carboxyl terminal tail by a single membrane-spanning region. Two forms of the protein differ in the length and sequence of the carboxyl terminus: these are encoded by alternatively spliced mRNAs that are differentially expressed during postnatal neural development. The Ig-like domains of 1B236/MAG are unusual in having structural similarities to Ig V domains but with short Cys-Cys distances characteristic of C domains. Several other Ig-like molecules exhibit this structural feature, including the cell adhesion molecule N-CAM, which is most closely related in sequence to 1B236/MAG. We have proposed 1B236/MAG as the prototype for this subgroup of the Ig family and offer a model for this type of Ig domain structure. 1B236/MAG probably acts as a cell adhesion molecule to mediate interactions between cells in a fashion similar to that proposed for N-CAM. In particular, 1B236/MAG may be involved in interactions between myelinating oligodendrocytes or Schwann cells and axons or between adjacent layers of myelin membrane during the process of myelin compaction. It is most likely that the homophilic or heterophilic interactions of 1B236/MAG occur through binding to the Ig-like domains. The structure of 1B236/MAG is therefore quite consistent with its proposed function and may serve as the model for this class of cell-cell interaction molecules. One would predict, for example, that the neuron-glia cell adhesion molecule Ng-CAM, also known as NILE or L1 (Bock et al. 1985, Friedlander et al. 1985), which mediates interactions between neurons and glial cells, would have a very similar structure to those of N-CAM and 1B236/MAG. In addition, the carboxyl terminal tails of the 1B236/MAG proteins may also be involved in interactions with cytoskeletal components, during membrane vesicle transport through the glial cytoplasm during myelination or through neuronal axoplasm or cytoplasm. The availability of full-length cDNA clones of 1B236/MAG mRNAs with the ability to express these products in vitro will enable the structure and interactions of 1B236/MAG to be tested in detail.

Splice site selection in the proteolipid protein (PLP) gene transcript and primary structure of the DM-20 protein of central nervous system myelin

Proteolipid protein (PLP) is the major myelin membrane protein of the central nervous system. We have isolated a copy of an alternatively spliced PLP gene transcript from a mouse brain cDNA library that was screened for PLP-related sequences. The encoded 241-amino acid protein differs from PLP by an internal deletion of 35-amino acid residues (116-150) from the major hydrophilic domain. This PLP variant is identical with the DM-20 protein of myelin, previously described as a brain-specific myelin component and known to be related to PLP. We determined the corresponding nucleotide sequence of the rat PLP gene and found that DM-20 mRNA results when a second 5' splice site, located 105 nucleotides within the third exon of the primary PLP transcript, is utilized in precursor mRNA (pre-mRNA) splicing. This demonstrates that alternative 5' splice site selection can determine the protein product of a cellular gene. DM-20 mRNA is expressed in rat brain with approximately 50% abundance relative to PLP mRNA and appears to be developmentally coregulated.

Two forms of 1B236/myelin-associated glycoprotein, a cell adhesion molecule for postnatal neural development, are produced by alternative splicing

The structures of two rat brain-specific 1B236 mRNAs, alternative splice products from a single gene regulated differently during postnatal brain development, were deduced from full-length cDNA clones. The 626- and 582-amino acid-long encoded proteins are indistinguishable from two forms of myelin-associated glycoprotein, a cell adhesion molecule involved in axonal-glial and glial-glial interactions in postnatal brain development, particularly in myelination. The two proteins share a single membrane-spanning domain and a glycosylated N terminus but differ in the structures of their C termini. The N terminus consists of five domains related in sequence to each other and to immunoglobulin-like molecules, especially the neural cell adhesion molecule N-CAM, suggesting a common structure for cell adhesion molecules.

Two forms of 1B236/myelin-associated glycoprotein, a cell adhesion molecule for postnatal neural development, are produced by alternative splicing

The structures of two rat brain-specific 1B236 mRNAs, alternative splice products from a single gene regulated differently during postnatal brain development, were deduced from full-length cDNA clones. The 626- and 582-amino acid-long encoded proteins are indistinguishable from two forms of myelin-associated glycoprotein, a cell adhesion molecule involved in axonal-glial and glial-glial interactions in postnatal brain development, particularly in myelination. The two proteins share a single membrane-spanning domain and a glycosylated N terminus but differ in the structures of their C termini. The N terminus consists of five domains related in sequence to each other and to immunoglobulin-like molecules, especially the neural cell adhesion molecule N-CAM, suggesting a common structure for cell adhesion molecules.

Jimpy mutant mouse: a 74-base deletion in the mRNA for myelin proteolipid protein and evidence for a primary defect in RNA splicing

The mouse mutant jimpy carries an X chromosome-linked recessive gene defect that affects the formation of myelin in the central nervous system. To understand the molecular basis of the jimpy mutation, we have examined the expression of mRNAs encoding myelin proteolipid protein (PLP). PLP mRNAs were detectable in jimpy brain RNA at 21 days after birth but were severely reduced in abundance compared to wild-type littermates. Nucleotide sequence analysis of cDNA clones for PLP mRNA, isolated from a cDNA library of jimpy brain poly(A)+ RNA, revealed that the PLP mRNA expressed in jimpy contained a deletion of 74 nucleotides with respect to the wild-type sequence. This deletion causes a frameshift in the open reading frame resulting in an altered carboxyl terminus for jimpy PLP. Probes specific for the deleted sequence, however, hybridize with equal efficiency to genomic DNA from jimpy and wild-type littermates, suggesting that the defect in the jimpy PLP mRNA is generated by aberrant RNA processing rather than by deletion of genomic sequences. We conclude that a mutation in the gene for PLP that leads to an incorrectly spliced RNA transcript is the primary defect of this genetic disorder.

Phenylarsine oxide inhibits agonist-induced changes in photolabeling but not agonist-induced desensitization of the beta-adrenergic receptor

In the human lymphocyte, desensitization of the beta-adrenergic receptor-adenylate cyclase complex is associated with sequestration of the receptor as well as a change in photolabeling of beta-receptor proteins. Thus, desensitization of the lymphocyte beta-adrenergic receptor-adenylate cyclase system is associated with a selective reduction in the photoaffinity labeling of an Mr approximately equal to 55,000 beta-adrenergic receptor-binding site as compared to an Mr approximately equal to 68,000 beta-adrenergic receptor-binding moiety. In order to examine the relationship between sequestration and reduction in labeling of the Mr approximately equal to 55,000 peptide, we have studied the effect of phenylarsine oxide (an inhibitor of beta-receptor sequestration in astrocytoma cells) on agonist-induced desensitization of the beta-adrenergic receptor-adenylate cyclase system in circulating lymphocytes. Incubation of cells with phenylarsine oxide prior to exposure to agonists did not block the consequent reduction in isoproterenol-stimulated adenylate cyclase activity. However, sequestration of the receptor, as assessed by a decrease in accessibility of beta-adrenergic receptors on intact cells to hydrophilic receptor ligands, is blocked by phenylarsine oxide. Thus, the agonist-induced reduction in binding of the hydrophilic beta-adrenergic receptor ligand CGP-12177 was blocked by phenylarsine oxide (without phenylarsine oxide, 57 +/- 6% of control, with phenylarsine oxide, 97 +/- 3% of control). Photolabeling studies with [125I]iodocyanopindolol diazirine revealed that phenylarsine oxide pretreatment also blocked the selective loss in labeling of the Mr approximately equal to 55,000 beta-adrenergic receptor protein. These data suggest that agonist-induced alterations in the photolabeling pattern of the lymphocyte beta-adrenergic receptor that occur with desensitization closely parallel the apparent sequestration of beta-adrenergic receptors but can be dissociated from the initial desensitization phenomenon.

Structural characterization of a heterogeneous family of rat brain mRNAs

A large heterogeneous family of RNAs derived from a single rat gene contains members that differ from each other at one or more of three positions. Their 5' ends are nested and transcription can begin at 22 or more sites covering 265 nucleotides. Many of the 5' ends are detectable only in brain RNAs, and even 5' ends common with other tissues appear with different absolute and relative abundances in brain RNA. The central portions of the RNAs are of two forms, differing only by the presence or absence of 17 nucleotides; these forms are probably produced by alternative splicing. Polyadenylation occurs at either of two sites. This complicated family of 88 RNAs encodes two novel putative proteins that differ at their C termini.

Nucleotide sequences of two mRNAs for rat brain myelin proteolipid protein

The 3200 and 1600 nucleotide mRNAs encoding rat brain proteolipid protein (PLP), the major protein component of central nervous system myelin, are heterogeneous at their 5' ends, differ in their 3' polyadenylation sites, and are transcribed from a single gene. The mRNAs, which first appear postnatally, encode identical 277 amino acid proteins that are 99% identical to the bovine protein sequence. Thus, PLP has been highly conserved during mammalian evolution. A single amino-terminal methionine is removed post-translationally, indicating that PLP does not require a signal peptide sequence for insertion into the myelin membrane. Mouse and monkey utilize the 3200 but not the 1600 nucleotide mRNA, suggesting that there is no functional necessity for two sizes of rat PLP mRNAs.

Effects of L-carnitine on exercise tolerance in chronic stable angina: a multicenter, double-blind, randomized, placebo controlled crossover study

L-carnitine was studied in forty-four men with stable chronic angina in a multicenter, double-blind, randomized, placebo controlled crossover trial. A cycloergometer exercise test was performed after a 10-day wash-out with placebo and at the end of each 4-week treatment period with either L-carnitine (1 g twice daily) or placebo. The mean (+/- SD) exercise work load showed an increase after L-carnitine compared to placebo (102.73 +/- 22.23 and 97.05 +/- 22.77 watts respectively, p = 0.001), as did the watts to onset of angina (95.7 +/- 24.07 and 87.44 +/- 24.67, p = 0.000). On the contrary, the ST segment depression was reduced by L-carnitine compared to placebo both at the maximum work load (1.40 +/- 0.90 and 1.69 +/- 0.82 mm, p = 0.05) and at the maximum work load common to L-carnitine and placebo (1.24 +/- 0.90 and 1.66 +/- 0.79 mm, p = 0.005). 22.7% of the patients became free of angina with L-carnitine and 9.1% with placebo. Resting and exercise blood pressure, heart-rate and double product were unaffected by L-carnitine. 1 patient decided to discontinue the trial because of gastric pyrosis while taking the active drug. The results of this study show that treatment with L-carnitine increases exercise tolerance and reduces ECG indices of ischemia in stable effort-induced angina.

Use of specific antibody for rapid clearance of circulating blood background from radiolabeled tumor imaging proteins

A major problem that arises when radiolabeled serum proteins are used for tumor imaging is the presence of a large amount of circulating background activity that persists for several days. This delays imaging for at least 2 days following injection and necessitates computer subtraction of simulated background (second radiopharmaceutical injection) which introduces artifacts that are difficult to control. We propose here the injection of specific antibody immediately before imaging as an alternate way of reducing blood background through clearance of the immune complex by the liver. 111In-alkyl human transferrin and IgG were injected IV in BALB/c tumor mice, and followed in 18 h by anti-human transferrin and anti-human IgG antibody IV. Two hours later, the tumor and organ distribution of activity was compared with control mice not receiving antibody. 111In-transferrin blood activity was reduced to 1/48 of control with no decrease in tumor concentration: as a result, the tumor to blood ratio increased from 1.4:1 to 78:1. 111In-IgG blood activity was reduced to 1/17 of control, again with no decrease in tumor. The tumor to blood ratios increased from 0.7:1 to 17:1. The liver picked up most of the blood activity with none of the complex going to spleen, bone marrow, or kidney. Dog experiments showed clearance of blood was 90% complete in less than 15 min following antibody injection. Simultaneous scintillation images showed complete clearance of activity from the heart and great vessels in the chest and neck, and over the abdomen, with a concomitant increase in liver activity but no increase in spleen, kidney, or bone marrow activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain-specific genes have identifier sequences in their introns

The 82-nucleotide identifier (ID) sequence is present in the rat genome in 1-1.5 X 10(5) copies and in cDNA clones of precursors of brain-specific mRNAs. One brain-specific gene contains more than one ID sequence in its introns. There is an excess of ID sequences to brain genes, and some ID sequences appear to have been inserted as mobile elements into other genetic locations. Therefore, brain genes contain ID sequences in their introns, but not all ID sequences are located in brain gene introns. A brain ID consensus sequence has been obtained by comparing 8 ID nucleotide sequences.

Endogenous and exogenous ocular and systemic silver deposition

The ocular distribution of particulate silver after prolonged ingestion and the topical application of an organic silver compound (argyrol) are described and compared with the distribution after long-term occupational contact with refined silver. The deposits appear to be inert and to cause no functional visual deficit.

Meige disease: striatal dopaminergic preponderance

A dopamine agonist (apomorphine) and a cholinomimetic drug (physostigmine) were administered to five patients with blepharospasm and oromandibular dystonia (Meige disease). The effects of haloperidol and levodopa were also assessed. Apomorphine lessened and physostigmine aggravated the facial dyskinesias in all patients, while placebo injections had no consistent effect. Levodopa did not modify the symptoms, but haloperidol attentuated the facial dystonia. Dysfunction of the basal ganglia, characterized by a state of striatal dopamine preponderance, probably underlies the dystonic spasms in Meige disease. The prominent cholinergic enhancement of facial dyskinesias may distinguish this disorder pharmacologically from tardive dyskinesia, a differentiation which has practical therapeutic implications.

Characteristics of individual repetitive sequence families in the sea urchin genome studied with cloned repeats

Cloned repetitive sequences from the S. purpuratus genome a few hundred to approximately 1000 nucleotides long were used to investigate the characteristics of individual repetitive sequence families. They wer terminally labeled by the kinase procedure and reacted with sheared S. purpuratus DNA. Repetition frequencies were measured for 26 individual families and were found to vary from a few to several thousand copies per genome. Estimates of sequence divergence were made for 18 cloned repeat families by measuring thermal stability of the heteroduplexes formed between the genomic DNA and the cloned fragments, compared with that of the renatured cloned fragments. The difference was less than 4 degrees C for three of the 18 families, and less than 10 degress C for 13 of the 18 families. These 13 repetitive sequence families lack any detectable highly divergent sequence relatives, and the results reported are shown not to change when the renaturation criterion is lowered below 55 degrees C in 0.18 M Na+. Five of the 18 cloned families displayed greater sequence divergence. The average sequence divergence of the total short repetitive sequence fraction of S. purpuratus DNA was found to match closely the average of the divergences of the cloned repeat sequences.