JoséM. Musacchio

GGF/Neuregulin Is a Neuronal Signal That Promotes the Proliferation and Survival and Inhibits the Differentiation of Oligodendrocyte Progenitors

We show that GGF/neuregulin is a mitogen for prooligodendrocytes (O4+/O1- cells), oligodendrocytes (O4+/O1+ cells), and type-2 astrocytes. Heregulin beta 1, another neuregulin isoform, is also mitogenic. The proliferative effect of glial growth factor (GGF) does not require, but is greatly potentiated by, serum factors. GGF also promotes the survival of pro-oligodendrocytes under serum-free conditions. High levels of GGF reversibly inhibit the differentiation and lineage commitment of oligodendrocyte progenitors and, in differentiated cultures, result in loss of O1 and myelin basic protein expression. All three erbB receptors are expressed by progenitors and are activated by GGF; the relative abundance of these receptors changes during differentiation. Finally, cortical neurons release a soluble mitogen for pro-oligodendrocytes that is specifically blocked by antibodies to GGF. These results implicate the neuregulins in the neuronal regulation of oligodendrocyte progenitor proliferation, survival, and differentiation.

Cloning and characterization of R-PTP-kappa, a new member of the receptor protein tyrosine phosphatase family with a proteolytically cleaved cellular adhesion molecule-like extracellular region.

We describe a new member of the receptor protein tyrosine phosphatase family, R-PTP-kappa, cDNA cloning predicts that R-PTP-kappa is synthesized from a precursor protein of 1,457 amino acids. Its intracellular domain displays the classical tandemly repeated protein tyrosine phosphatase homology, separated from the transmembrane segment by an uncharacteristically large juxta-membrane region. The extracellular domain of the R-PTP-kappa precursor protein contains an immunoglobulin-like domain and four fibronectin type III-like repeats, preceded by a signal peptide and a region of about 150 amino acids with similarity to the Xenopus A5 antigen, a putative neuronal recognition molecule (S. Takagi, T. Hsrata, K. Agata, M. Mochii, G. Eguchi, and H. Fujisawa, Neuron 7:295-307, 1991). Antibodies directed against the intra- and extracellular domains reveal that the R-PTP-kappa precursor protein undergoes proteolytic processing, following which both cleavage products remain associated. By site-directed mutagenesis, the likely cleavage site was shown to be a consensus sequence for cleavage by the processing endopeptidase furin, located in the fourth fibronectin type III-like repeat. In situ hybridization analysis indicates that expression of R-PTP-kappa in the central nervous system is developmentally regulated, with highest expression seen in actively developing areas and, in the adult, in areas capable of developmental plasticity such as the hippocampal formation and cerebral cortex. The mouse R-PTP-kappa gene maps to chromosome 10, at approximately 21 centimorgans from the centromere.

The expression of a novel receptor-type tyrosine phosphatase suggests a role in morphogenesis and plasticity of the nervous system

Analysis of the localization of receptor-type protein tyrosine phosphatase-beta (RPTP-beta) by in situ hybridization and immunocytochemistry indicates that it is predominantly expressed in the developing central nervous system (CNS). RPTP-beta is highly expressed in radial glia and other forms of glial cells that play an important role during development. The immunoreactivity localizes to the radial processes of these cells, which act as guides during neuronal migration and axonal elongation. The pattern of RPTP-beta expression changes with the progression of glial cell differentiation. In the adult, high levels of RPTP-beta are seen in regions of the brain where there is continued neurogenesis and neurite outgrowth. The spatial and temporal patterns of RPTP-beta expression suggest that this receptor phosphatase plays a role in morphogenesis and plasticity of the nervous system.

THREE FORMS OF RPTP-BETA ARE DIFFERENTIALLY EXPRESSED DURING GLIOGENESIS IN THE DEVELOPING RAT BRAIN AND DURING GLIAL CELL DIFFERENTIATION IN CULTURE

In situ hybridization and Northern analysis demonstrate that the three splicing variants of RPTP‐β have different spatial and temporal patterns of expression in the developing brain. The 9.5‐kb and 6.4‐kb transcripts, which encode transmembrane protein tyrosine phosphatases with different extracellular domains, are predominantly expressed in glial progenitors located in the subventricular zone (SVZ). The 8.4‐kb transcript, which encodes a secreted chondroitin sulfate proteoglycan (phosphacan), is expressed at high levels by more mature glia that have migrated out of the SVZ. The three transcripts are also differentially expressed in glial cell cultures; O2A progenitors express high levels of the 9.5‐ and 8.4‐kb transcript, whereas type 1 astrocyte progenitors predominantly express the 6.4‐kb transcript. C6 gliomas also express high levels of the 6.4‐kb transcript. Treating C6 cells with the differentiating agent dibutyryl cyclic‐AMP (DBcAMP), induces a decrease in the 6.4‐kb transcript and a corresponding increase in the 8.4‐kb transcript. O2A cells grown in the presence of basic fibroblast growth factor (bFGF) and platelet‐derived growth factor (PDGF) remain highly proliferative and undifferentiated, and continue to express high levels of RPTP‐β. However, when O2A cells are grown in conditions that induce oligodendrocyte differentiation, there is a marked decrease in the expression of the transmembrane forms of RPTP‐β, as determined by immunofluorescence. These results demonstrate that RPTP‐β expression is regulated during glial cell differentiation and suggest that the different forms of RPTP‐β perform distinct functions during brain development.

Computer-assisted modeling of multiple dextromethorphan and sigma binding sites in guinea pig brain

Computer-assisted, simultaneous analysis of self- and cross-displacement experiments demonstrated the existence of several binding sites in guinea pig brain for dextromethorphan, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP), and 1,3-di-o-tolyl guanidine (DTG). Dextromethorphan binds with high affinity to two sites (R1 Kd 50-83 and R2 Kd 8-19 nM) and with low affinity to two additional sites (R3 and R4). (+)-3-PPP binds to one high-affinity (R1 Kd 24-36 nM), to one intermediate-affinity (R3 Kd 210-320 nM), and to two (R2 and R4) low-affinity sites. DTG binds with almost identical high affinity to two different sites (R1 Kd 22-24 and R3 Kd 13-16 nM). These results confirm that dextromethorphan, (+)-3-PPP, and DTG bind to the common DM1/sigma 1 site (R1). The binding of DTG to two different sites with identical affinities precludes the use of this compound as a specific marker for sigma receptors. Besides, haloperidol displaces labeled ligands from both high-affinity DTG sites (R1 and R3) with high affinity. Thus, haloperidol sensitivity should not be used as the single criterion to identify a putative receptor. The resolution of these novel sites also may provide new insights into the multiple effects of antipsychotic drugs. In addition, this investigation has important implications regarding the methods that must be applied to characterize multiple binding sites and their relations with putative receptors.

Fading and tachyphylaxis to the contractile effects of substance P in the guinea-pig ileum

Substance P (SP) caused an immediate and vigorous contraction of the longitudinal smooth muscle layer of the guinea-pig ileum. The contractile response to SP, unlike that to acetylcholine or histamine was not maintained but faded to baseline levels in about 6 min. When 0.3-1.0 nM SP was added the fading time was shorter than 6 min and tachyphylaxis did not develop. Higher concentrations of SP produced fading times of about 6 min that could not be increased even by adding extremely high concentrations of the peptide, up to 1800 nM. Short fading times and the lack of development of tachyphylaxis are the result of the rapid adsorption and/or metabolism of SP. The addition of exogenous peptidases such as pronase, chymotrypsin and an extract of black widow spider venom gland dramatically increased the rate of degradation of SP, shortened the fading response and blocked the development of tachyphylaxis. Tetrodotoxin and atropine reduced the fading time by 25%, while eserine increased its duration several-fold; these findings are consistent with the existence of a cholinergic nerve component in the mediation of some of the effects of SP receptor and, in part, to adsorption and metabolism of the peptide. The magnitude of the tachyphylaxis to SP was proportional to the concentration of the desensitizing dose of the peptide and was specific to SP and to the related peptide physalaemin; no cross-tachyphylaxis towards other agents was found.

SKF 525-A and cytochrome P-450 ligands inhibit with high affinity the binding of [3H]dextromethorpan and σ ligands to guinea pig brain.

The DM1/sigma 1 site binds dextromethorphan (DM) and sigma receptor ligands. The broad binding specificity of this site and its peculiar subcellular distribution prompted us to explore the possibility that this site is a member of the cytochrome P-450 superfamily of enzymes. We tested the effects of the liver microsomal monooxygenase inhibitor SKF 525-A (Proadifen), and other P-450 substrates on the binding of [3H]dextromethorphan, [3H]3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine and (+)-[3H]1,3-Di-o-tolyl-guanidine ([3H]DTG) to the guinea pig brain. SKF 525-A, l-lobeline and GBR-12909 inhibited the binding of the three labeled ligands with nM affinity. Each drug has identical nM Ki values for the high-affinity site labeled by the three ligands. This indicated that they displaced the labeled ligands from the common DM1/sigma 1 site. Debrisoquine and sparteine, prototypical substrates for liver debrisoquine 4-hydroxylase, displayed Ki values of 9-13 and 3-4 microM respectively against the three labeled ligands. These results, the broad specificity of the DM1/sigma 1 binding site, and its peculiar subcellular distribution, raises the possibility that this binding site is a member of the cytochrome P-450 superfamily of isozymes, rather than a neurotransmitter receptor. These findings may have important implications for the understanding of the therapeutic, side effects and toxicity of several neurotropic drugs.

Caramiphen: a non-opioid antitussive with potent anticonvulsant properties in rats☆

The effect of the non-opioid antitussive caramiphen was studied in the rat maximal electroshock test. Caramiphen produced a dose- and time-dependent blockade of tonic hindlimb extension and was nearly twice as potent as the prototypical anticonvulsant drug diphenylhydantoin. Pretreatment with a subthreshold-effective dose of caramiphen potentiated the anticonvulsant effects of diphenylhydantoin, lowering its ED50 33-fold. The anticonvulsant effects of caramiphen were not associated with its cholinolytic activity since (a) its anticonvulsant effects were not antagonized by physostigmine and (b) the more potent cholinolytic atropine was only weakly effective against maximal electroshock convulsions when tested at doses 25 times the minimally effective dose of caramiphen. Anticonvulsant effects of caramiphen were associated with minimal behavioral impairment. The results demonstrate that caramiphen is a potent anticonvulsant against generalized convulsions and, like other non-opioid antitussives, will enhance the anticonvulsant properties of diphenylhydantoin. It is suggested that the anticonvulsant effects of caramiphen result from specific binding to brain receptors labelled by the non-opioid antitussive dextromethorphan, and that the interactions with diphenylhydantoin involve allosteric interactions between the different binding sites.

Effects of haloperidol and reduced haloperidol on binding to σ sites

The s.c. administration of a single dose of 0.1 mg/kg of reduced haloperidol to guinea pigs produced a marked inhibition of the binding of [3H]dextromethorphan and [3H3-(3-hydroxyphenyl))-N-(n-prperidine ([3H](+)-3-PPP) to brain. The inhibition was still evident 10 days later, and it was accompanied by residual brain levels of reduced haloperidol, and much lower levels of haloperidol. Scatchard and computer-assisted analysis demonstrated that the inhibition was due to a reduction in the number of binding sites without changes in the affinity. In the rat, haloperidol and reduced haloperidol also produced a rapid inhibition of binding to σ sites. Interestingly, the brain of the reduced haloperidol-treated rats contained both haloperidol and reduced haloperidol, but the levels of reduced haloperidol in the haloperidol-treated rats were undetectable. However, the inhibition observed was of comparable magnitude, indicating that the haloperidol remaining in the brain is also inhibitory. In vitro experiments showed that the inhibition produced by haloperidol and reduced haloperidol and appparently competitive, but when brain membranes were preincubated with either drug, the inhibition was noncompetitive. By contrast, the inhibition produced by dextromethorphan was always competitive. Moreover, the inhibition produced by haloperidol and reduced haloperidol could not be reversed by washing. This investigation strongly suggests that the inhibition observed after the administration of haloperidol or reduced haloperidol is not a classic agonist-induced receptor down-regulation. The results indicated that the inhibition produced is a complex phenomenon, and suggest the formation of a slowly reversible or irreversible complex with reduced haloperidol or haloperidol.

Processing of enkephalin precursors by chromaffin granule enzymes

Subcellular localization studies indicate that the enzyme activities which cleave enkephalins from larger polypeptides are located in both membranous and soluble components of the chromaffin granules and not in the lysosomes. Cleavage of endogenous precursors produced methionine enkephalin [Met-E], leucine enkephalin [Leu-E], and Met-E-Arg6. Cleavage of synthetic peptide E produced Leu-E, Met-E, and Met-E-Arg6. The pH optimum for enkephalin production is pH 5.7. Dithiothreitol prevents the inhibition of enkephalin conversion produced by p-chloromecurobenzoate. Studies with peptide E indicate that cleavage appears to occur at pairs of basic amino acid residues. The presence of enkephalin producing enzymes with the precursors and the products in the chromaffin granules could be important in the elucidation of the factors that regulate enkephalin biosynthesis.