Thymopoietin, a thymic polypeptide, specifically interacts at neuronal nicotinic alpha-bungarotoxin receptors

Structural and functional heterogeneity of nicotinic receptors

Three gene families of the ligand-gated ion channel gene superfamily encode proteins which bind cholinergic ligands: (1) nicotinic acetylcholine receptors (AChRs) from skeletal muscle, (2) AChRs from neurons, and (3) neuronal alpha-bungarotoxin-binding proteins (alpha BgtBPs). AChRs from muscles and nerves function as ACh-gated cation channels, but alpha BgtBPs do not appear to function in this way. A family of neuronal AChR subtypes has been characterized using monoclonal antibodies and cDNA probes. Neuronal AChRs exhibit sequence homologies with muscle AChRs, but differ in subunit composition, pharmacological and electrophysiological properties, and, in some cases, apparent functional roles. The genes that encode the subunits of the various purified AChR subtypes have been determined in several cases. Histological localization of AChR subunit mRNAs by in situ hybridization and of subunit proteins by immunohistochemistry is being conducted with increasing resolution. The subunit structure of alpha BgtBP is uncertain, but cDNAs have been identified for two subunits. Sequences of these cDNAs reveal that alpha BgtBPs are members of the ligand-gated ion channel gene family, and suggest that they could function as gated cation channels. Biochemical and molecular genetic approaches to studies of neuronal AChRs and related proteins are merging to provide a detailed description of a complex family of AChRs widely dispersed throughout the nervous system, which are probably important to many activities of the nervous system, but whose functional roles are not yet well characterized.

Modulation of nicotinic acetylcholine receptor function in bovine adrenal chromaffin cells by thymopentin

Thymopentin is a five amino acid peptide which corresponds to amino acids 32-36 of the thymic polypeptide thymopoietin. Previous work had shown that TP-5 could modulate responsiveness at the muscle-type nicotinic receptor. The present studies show that neuronal nicotinic receptor function, measured as radiolabelled noradrenaline release from bovine adrenal medullary cells, was attenuated by thymopentin. The pentapeptide exhibited specificity for nicotinic receptor evoked release, since thymopentin did not affect potassium-stimulated secretion of [3H]noradrenaline. It appears, therefore, that thymopentin antagonizes catecholamine release by specifically modulating nicotinic receptor responsiveness.

The effects of synthetic thymopoietin on binding of alpha-bungarotoxin to rat neural membranes

Chemically synthesized bovine thymopoietin I (TPO-I) and thymopoietin II (TPO-II) were evaluated as inhibitors of 125I alpha-Bungarotoxin binding to rat brain neural membranes and found to be over 1,000 x less potent (IC50 = 10 microM) than reported for thymopoietin isolated from bovine thymus.

Thymopoietin, a thymic polypeptide, prevents nicotinic agonist-induced morphological changes in neonatal muscle cells in culture

Thymopoietin, a polypeptide hormone isolated from thymus and involved in immune function, potently inhibited [125I]alpha-bungarotoxin binding to neonatal muscle cells in culture (IC50 = 3.8 nM) and blocked carbachol-stimulated 22Na uptake with an IC50 of 1.9 +/- 0.2 nM and 23 +/- 7 nM at a half-maximal and maximal concentration of carbachol, respectively. Studies were subsequently done to evaluate potential long-term functional consequences of this interaction of thymopoietin at the nicotinic receptor. Exposure (1-3 days) of neonatal muscle cells in culture to nicotine (3 x 10(-6) M) or carbachol (1 x 10(-6) M) resulted in a decline in myotube branching and a decrease in myotube length. Thymopoietin did not appreciably alter myotube morphology on its own; however, it prevented the effects of nicotine and carbachol on muscle cell morphology at concentrations (1-10 nM) which corresponded well to those with which thymopoietin interacted at the receptor. The action of alpha-bungarotoxin on the myotubes was very similar to that of thymopoietin. These studies suggest that the endogenously occurring polypeptide, thymopoietin, has the potential to modulate muscle cell morphology through an interaction at the nicotinic receptor.

Thymopoietin, a polypeptide ligand for the alpha-bungarotoxin binding site in brain: an autoradiographic study

Thymopoietin, a 48-49-amino acid polypeptide present in the thymus gland, was investigated as a potential ligand for the neuronal nicotinic alpha-bungarotoxin binding site in rat brain. Binding of [125I]alpha-bungarotoxin to whole rat brain sections was inhibited by thymopoietin in a concentration-dependent manner with an IC50 of 30.0 +/- 8.2 nM as compared to 1.1 +/- 0.3 nM for alpha-bungarotoxin. However, at concentrations of thymopoietin of up to 1 microM, [3H]nicotine binding to high affinity sites was not inhibited. Thysplenin, a polypeptide with considerable homology to thymopoietin did not affect [125I]alpha-bungarotoxin binding. These results suggest that thymopoietin selectively interacts with the nicotinic alpha-bungarotoxin binding site labelled by [125I]alpha-bungarotoxin rather than the neuronal nicotinic receptor(s) labelled by [3H]nicotine. Autoradiographic studies revealed that 1 microM thymopoietin almost completely inhibited [125I]alpha-bungarotoxin binding in all brain regions. Computer-assisted image analysis of displacement curves was performed on various brain areas rich in alpha-bungarotoxin binding, such as the dorsal endopiriform nucleus, fields 1 and 2 of Ammon's horn, the polymorph cell layer of the dentate gyrus and cortical layers 4 and 5. Thymopoietin inhibited [125I]alpha-bungarotoxin binding with similar potency in all these regions, suggesting that it interacted at the same site in the different brain areas. The IC50 values averaged over the six regions were 24.6 +/- 2.8 nM for thymopoietin and 1.2 +/- 0.2 nM for alpha-bungarotoxin. These results show that thymopoietin specifically interacted with the alpha-bungarotoxin site with a similar potency in different brain regions. It is suggested that thymopoietin represents a selective ligand for alpha-bungarotoxin binding sites in brain.

Thymopoietin, a thymic polypeptide, potently interacts at muscle and neuronal nicotinic alpha-bungarotoxin receptors

Current studies suggest that several distinct populations of nicotinic acetylcholine (ACh) receptors exist. One of these is the muscle-type nicotinic receptors with which neuromuscular nicotinic receptor ligands and the snake toxin alpha-bungarotoxin interact. alpha-Bungarotoxin potently binds to these nicotinic receptors and blocks their function, two characteristics that have made the alpha-toxin a very useful probe for the characterization of these sites. In neuronal tissues, several populations of nicotinic receptors have been identified which, although they share a nicotinic pharmacology, have unique characteristics. The alpha-bungarotoxin-insensitive neuronal nicotinic receptors, which may be involved in mediating neuronal excitability, bind nicotinic agonists with high affinity but do not interact with alpha-bungarotoxin. Subtypes of these alpha-toxin-insensitive receptors appear to exist, as evidenced by findings that some are inhibited by neuronal bungarotoxin whereas others are not. In addition to the alpha-bungarotoxin-insensitive sites, alpha-bungarotoxin-sensitive neuronal nicotinic receptors are also present in neuronal tissues. These latter receptors bind alpha-bungarotoxin with high affinity and nicotinic agonists with an affinity in the microM range. The function of the nicotinic alpha-bungarotoxin receptors are as yet uncertain. Thymopoietin, a polypeptide linked to immune function, appears to interact specifically with nicotinic receptor populations that bind alpha-bungarotoxin. Thus, in muscle tissue where alpha-bungarotoxin both binds to the receptor and blocks activity, thymopoietin also potently binds to the receptor and inhibits nicotinic receptors-mediated function. In neuronal tissues, thymopoietin interacts only with the nicotinic alpha-bungarotoxin site and not the alpha-bungarotoxin-insensitive neuronal nicotinic receptor population. These observations that thymopoietin potently and specifically interacts with nicotinic alpha-bungarotoxin-sensitive receptors in neuronal and muscle tissue, together with findings that thymopoietin is an endogenously occurring agent, could suggest that this immune-related polypeptide represents a ligand for the alpha-bungarotoxin receptors. The function of thymopoietin at the alpha-bungarotoxin receptor is as yet uncertain; however, a potential trophic, as well as other roles are suggested.

Molecular cloning of the bovine thymopoietin gene and its expression in different calf tissues: evidence for a predominant expression in thymocytes

Thymopoietin (TP), a 49 amino acid peptide, is regarded as a thymic hormone, secreted specifically by some epithelial cells in the thymic stroma and exerting a multitude of effects on maturation and function of T lineage cells. As part of our study on the molecular biology of TP, we isolated cDNA clone coding for a bovine TP precursor and used it as a probe to analyze the presence of mRNA coding for TP in different tissues. The cDNA clone reported here is 1.1 kb long and contains an open reading frame (ORF) of 741 bp which corresponds to 247 amino acids. The 147 bp coding for the entire bovine TP are at the 5' end of the ORF. A DNA fragment coding for amino acids 1-42 of bovine TP was used as a probe to look for hybridizable RNA sequences, extracted from various calf tissues, by the S1 nuclease protection method. Our results indicate that the TP gene is expressed predominantly in lymphatic tissues. Lymphatic tissues with the highest levels observed were thymocytes and not thymic stroma. Lower, but still significant, amounts were present in tonsils, neck lymph nodes, and small intestine (probably because of its lymphatic part--the Peyer's patches), whereas cultured thymic stromal cells, spleen tissue and peripheral blood mononuclear cells displayed a low level of TP mRNA. The TP gene expression in all other (non-lymphatic) tissues tested, was weak, barely detectable or virtually absent. However, the cerebellum could be singled out with relatively strong expression of TP mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for thymopoietin and thymopoietin/alpha-bungarotoxin/nicotinic receptors within the brain

Thymopoietin, a polypeptide hormone of the thymus that has pleiotropic actions on the immune, endocrine, and nervous systems, potently interacts with the neuromuscular nicotinic acetylcholine receptor. Thymopoietin binds to the nicotinic alpha-bungarotoxin (alpha-BGT) receptor in muscle and, like alpha-BGT, inhibits cholinergic transmission at this site. Evidence is given that radiolabeled thymopoietin similarly binds to a nicotinic alpha-BGT-binding site within the brain and does so with the characteristics of a specific receptor ligand. Thus specific binding to neuronal membranes was saturable, of high affinity (Kd = 8 nM), linear with increased tissue concentration, and readily reversible; half-time was approximately 5 min for association and 10 min for dissociation. Binding of 125I-labeled thymopoietin was displaced not only by unlabeled thymopoietin but also by alpha-BGT and the nicotinic receptor ligands d-tubocurarine and nicotine; various other receptor ligands (muscarinic, adrenergic, and dopaminergic) did not affect binding of 125I-labeled thymopoietin. Thymopoietin was shown by ELISA to be present in brain extracts, displacement curves of thymus and brain extracts being parallel to the standard thymopoietin curve, and Western (immuno) blot identified in brain and thymus extracts a thymopoietin-immunoreactive polypeptide of the same molecular mass as purified thymopoietin polypeptide. We conclude that thymopoietin and thymopoietin-binding sites are present within the brain and that the receptor for thymopoietin is the previously identified nicotinic alpha-BGT-binding site of neuronal tissue.

Brain alpha-bungarotoxin binding protein cDNAs and MAbs reveal subtypes of this branch of the ligand-gated ion channel gene superfamily

alpha-Bungarotoxin (alpha Bgt) is a potent, high-affinity antagonist for nicotinic acetylcholine receptors (AChRs) from muscle, but not for AChRs from neurons. Both muscle and neuronal AChRs are thought to be formed from multiple homologous subunits aligned around a central cation channel whose opening is regulated by ACh binding. In contrast, the exact structure and function of high-affinity alpha Bgt binding proteins (alpha BgtBPs) found in avian and mammalian neurons remain unknown. Here we show that cDNA clones encoding alpha BgtBP alpha 1 and alpha 2 subunits define alpha BgtBPs as members of a gene family within the ligand-gated ion channel gene superfamily, but distinct from the gene families of AChRs from muscles and nerves. Subunit-specific monoclonal antibodies raised against bacterially expressed alpha BgtBP alpha 1 and alpha 2 subunit fragments reveal the existence of at least two different alpha BgtBP subtypes in embryonic day 18 chicken brains. More than 75% of all alpha BgtBPs have the alpha 1 subunit, but no alpha 2 subunit, and a minor alpha BgtBP subtype (approximately 15%) has both the alpha 1 and alpha 2 subunits.

Thymopoietin interacts at the alpha-bungarotoxin site of and induces process formation in PC12 pheochromocytoma cells

Thymopoietin, a polypeptide isolated from thymus and involved in immune regulation, potently inhibited [125I]alpha-bungarotoxin binding in both pheochromocytoma (PC12) cells in culture (IC50 of 3.9 nM) and in PC12 cell membranes (IC50 of 2.2 nM). The degree of inhibition produced by thymopoietin was similar to that observed with alpha-bungarotoxin; in contrast, nicotinic receptor ligands affected alpha-bungarotoxin binding only at micromolar concentrations, in agreement with previous work. Binding of thymopoietin was reversible. Studies with PC12 cell membranes suggested that the interaction between alpha-bungarotoxin and thymopoietin at the receptor was competitive. The effect of thymopoietin was subsequently assessed on various morphological characteristics of PC12 cells in culture. Exposure of the cells to the polypeptide resulted in neurite extension, which was evident as early as 1-2 days in culture and was maximal after 4-6 days; this response was observed with concentrations of thymopoietin as low as 10(-8) M. Nerve growth factor also induced neurite extension in PC12 cells; however, the effects of nerve growth factor were qualitatively and quantitatively distinct from those which occurred with thymopoietin. Moreover, a monoclonal antibody to nerve growth factor completely prevented the nerve growth factor-induced process formation without affecting the thymopoietin-induced response. On the other hand, alpha-bungarotoxin resulted in the formation of processes which appeared morphologically similar to those induced by thymopoietin, although alpha-bungarotoxin appeared less potent than the thymic polypeptide. The effect of thymopoietin appeared to be specific; thysplenin, a polypeptide with approximately 80% homology with thymopoietin, did not elicit process formation. The thymopoietin-induced effect was reversed upon removal of the polypeptide from the culture medium. These results show that thymopoietin, a polypeptide endogenous to mammalian systems, potently interacted at the alpha-bungarotoxin site in a neuronal cell line. Furthermore, thymopoietin could elicit process formation in PC12 cells, suggesting that it may be a neuronotrophic factor.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.

Phorbol esters and d-tubocurarine up-regulate alpha-bungarotoxin sites in chromaffin cells in culture via distinct mechanisms

Previous work had shown that nicotinic antagonists resulted in a marked up-regulation of alpha-bungarotoxin sites in chromaffin cells in culture. The present experiments were done to determine the intracellular mechanism(s) whereby nicotinic antagonists might mediate their effects on these receptors. Chromaffin cells were cultured for three days with various concentrations of 4 beta-phorbol 12-myristate 13-acetate, an agent which affects protein kinase C by mimicking the actions of diacylglycerol. The phorbol ester resulted in a dose-dependent increase in alpha-bungarotoxin binding which was maximal with 100 nM 4 beta-phorbol 12-myristate 13-acetate. This increase in binding appeared to be due to an increase in the maximal number of alpha-bungarotoxin sites. Time dependence studies showed that the effect of the phorbol was undetectable with incubations of 24 h or less and appeared to plateau by 72-96 h. A similar increase in toxin binding was also observed with 4 beta-phorbol 12,13-dibutyrate. On the other hand, an inactive analog of 4 beta-phorbol 12-myristate 13-acetate had no significant effect on binding. D-Sphingosine, an inhibitor of protein kinase C, was able to partially block the phorbol ester-induced increase in toxin binding while polymyxin B, another protein kinase C inhibitor, completely prevented the up-regulation of the alpha-bungarotoxin sites. Carbachol and nicotine prevented this enhancement of toxin binding in the presence of 4 beta-phorbol 12-myristate 13-acetate. Although the phorbol ester resulted in an increase in toxin binding, acetylcholine-evoked catecholamine secretion from chromaffin cells in culture was decreased, indicating a dissociation between the functional nicotinic acetylcholine receptor population and the alpha-bungarotoxin sites. To determine whether agents which affect protein kinase C can alter the up-regulation of alpha-bungarotoxin sites by d-tubocurarine, 4 beta-phorbol 12-myristate 13-acetate was added to the cells in combination with the nicotinic antagonist. The up-regulation of toxin binding sites induced by d-tubocurarine was additive with that induced by the phorbol and was not affected by polymyxin B. Thus, the results would suggest that there are at least two mechanisms by which alpha-bungarotoxin binding sites can be regulated. One is mediated via an interaction at nicotinic receptors, while the other occurs in response to phorbol esters and thus may be mediated by protein kinase C. Interestingly, although the molecular mechanisms resulting in alpha-bungarotoxin receptor up-regulation differ, both the d-tubocurarine- and the phorbol ester-induced increases were prevented by nicotinic receptor ligands.