Dystrobrevin and dystrophin: An interaction through coiled-coil motifs

Dystrobrevin, a dystrophin-related and -associated protein, has been proposed to be important in the formation and maintenance of the neuromuscular junction. Dystrobrevin coprecipitates with both the acetylcholine receptor complex as well as the dystrophin glycoprotein complex. Although the nature of dystrobrevin’s association with the dystrophin glycoprotein complex remains unclear, it is known that dystrobrevin binds directly to the syntrophins, a heterologous group of dystrophin-associated proteins. Using the yeast two-hybrid system to identify protein–protein interactions, we present evidence for the heterodimerization of dystrobrevin directly with dystrophin. The C terminus of dystrobrevin binds specifically to the C terminus of dystrophin. We further refined this site of interaction to these proteins’ homologous coiled-coil motifs that flank their respective syntrophin-binding sites. We also show that the interaction between the dystrobrevin and dystrophin coiled-coil domains is specific and is not due to a nonspecific coiled-coil domain interaction. From the accumulated evidence of protein–protein interactions presented here and elsewhere, we propose a partially revised model of the organization of the dystrophin-associated glycoprotein complex.

Real-time quantitative measurement of autocrine ligand binding indicates that autocrine loops are spatially localized

Autocrine ligands are important regulators of many normal tissues and have been implicated in a number of disease states, including cancer. However, because by definition autocrine ligands are synthesized, secreted, and bound to cell receptors within an intrinsically self-contained “loop,” standard pharmacological approaches cannot be used to investigate relationships between ligand/receptor binding and consequent cellular responses. We demonstrate here a new approach for measurement of autocrine ligand binding to cells, using a microphysiometer assay originally developed for investigating cell responses to exogenous ligands. This technique permits quantitative measurements of autocrine responses on the time scale of receptor binding and internalization, thus allowing investigation of the role of receptor trafficking and dynamics in cellular responses. We used this technique to investigate autocrine signaling through the epidermal growth factor receptor by transforming growth factor alpha (TGFα) and found that anti-receptor antibodies are far more effective than anti-ligand antibodies in inhibiting autocrine signaling. This result indicates that autocrine-based signals can operate in a spatially restricted, local manner and thus provide cells with information on their local microenvironment.

A decrease of reelin expression as a putative vulnerability factor in schizophrenia

Postmortem prefrontal cortices (PFC) (Brodmann’s areas 10 and 46), temporal cortices (Brodmann’s area 22), hippocampi, caudate nuclei, and cerebella of schizophrenia patients and their matched nonpsychiatric subjects were compared for reelin (RELN) mRNA and reelin (RELN) protein content. In all of the brain areas studied, RELN and its mRNA were significantly reduced (≈50%) in patients with schizophrenia; this decrease was similar in patients affected by undifferentiated or paranoid schizophrenia. To exclude possible artifacts caused by postmortem mRNA degradation, we measured the mRNAs in the same PFC extracts from γ-aminobutyric acid (GABA)A receptors α1 and α5 and nicotinic acetylcholine receptor α7 subunits. Whereas the expression of the α7 nicotinic acetylcholine receptor subunit was normal, that of the α1 and α5 receptor subunits of GABAA was increased when schizophrenia was present. RELN mRNA was preferentially expressed in GABAergic interneurons of PFC, temporal cortex, hippocampus, and glutamatergic granule cells of cerebellum. A protein putatively functioning as an intracellular target for the signal-transduction cascade triggered by RELN protein released into the extracellular matrix is termed mouse disabled-1 (DAB1) and is expressed at comparable levels in the neuroplasm of the PFC and hippocampal pyramidal neurons, cerebellar Purkinje neurons of schizophrenia patients, and nonpsychiatric subjects; these three types of neurons do not express RELN protein. In the same samples of temporal cortex, we found a decrease in RELN protein of ≈50% but no changes in DAB1 protein expression. We also observed a large (up to 70%) decrease of GAD67 but only a small decrease of GAD65 protein content. These findings are interpreted within a neurodevelopmental/vulnerability “two-hit” model for the etiology of schizophrenia.

Tonic nicotinic modulation of serotoninergic transmission in the spinal cord

The spinal serotoninergic projection from the raphe magnus has been shown to modulate nociceptive inputs, and activation of this projection mediates nicotine-elicited analgesia. Here, we investigate the interactions between cholinergic and serotoninergic systems in the spinal cord, by conducting serotonin [5-hydroxytryptamine (5-HT)] efflux experiments on mouse spinal slices. At least three spinal populations of nicotinic receptors are distinguished that affect 5-HT release. The first could be directly located on serotoninergic terminals, is insensitive to nanomolar concentrations of methyllicaconitine (MLA), and may be subjected to a basal (not maximal) cholinergic tone. The second is tonically and maximally activated by endogenous acetylcholine, insensitive to nanomolar concentrations of MLA, and present on inhibitory neurons. The last is also present on inhibitory neurons but is sensitive to nanomolar concentrations of MLA and not tonically activated by acetylcholine. Multiple nicotinic acetylcholine receptor populations thus differentially exert tonic or not tonic control on 5-HT transmission in the spinal cord. These receptors may be major targets for nicotine effects on antinociception. In addition, the presence of a tonic nicotinic modulation of 5-HT release indicates that endogenous acetylcholine plays a role in the physiological regulation of descending 5-HT pathways to the spinal cord.

A β-hairpin structure in a 13-mer peptide that binds α-bungarotoxin with high affinity and neutralizes its toxicity

Snake-venom α-bungarotoxin is a member of the α-neurotoxin family that binds with very high affinity to the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. The structure of the complex between α-bungarotoxin and a 13-mer peptide (WRYYESSLEPYPD) that binds the toxin with high affinity, thus inhibiting its interactions with AChR with an IC50 of 2 nM, has been solved by 1H-NMR spectroscopy. The bound peptide folds into a β-hairpin structure created by two antiparallel β-strands, which combine with the already existing triple-stranded β-sheet of the toxin to form a five-stranded intermolecular, antiparallel β-sheet. Peptide residues Y3P, E5P, and L8P have the highest intermolecular contact area, indicating their importance in the binding of α-bungarotoxin; W1P, R2P, and Y4P also contribute significantly to the binding. A large number of characteristic hydrogen bonds and electrostatic and hydrophobic interactions are observed in the complex. The high-affinity peptide exhibits inhibitory potency that is better than any known peptide derived from AChR, and is equal to that of the whole α-subunit of AChR. The high degree of sequence similarity between the peptide and various types of AChRs implies that the binding mode found within the complex might possibly mimic the receptor binding to the toxin. The design of the high-affinity peptide was based on our previous findings: (i) the detection of a lead peptide (MRYYESSLKSYPD) that binds α-bungarotoxin, using a phage-display peptide library, (ii) the information about the three-dimensional structure of α-bungarotoxin/lead-peptide complex, and (iii) the amino acid sequence analysis of different AChRs.

Real-time detection of the surface delivery of newly synthesized membrane proteins.

Newly synthesized membrane proteins travel from the Golgi complex to the cell surface in transport vesicles. We have exploited the ion channel properties of the nicotinic acetylcholine receptor (AChR) to observe in real time the constitutive delivery of newly synthesized AChR proteins to the plasma membrane in cultured muscle cells. Whole-cell voltage clamp was employed to monitor the current fluctuations induced by carbamylcholine upon the insertion into the plasma membrane of newly synthesized AChRs, following release from a 20 degrees C temperature block. We find that the transit of vesicles to the cell surface occurs within a few minutes after release of the block. The time course of electrical signals is consistent with many of the fusion events being instantaneous, although some appear to reveal the flickering of a fusion pore. AChR-containing vesicles can fuse individually or as conglomerates. Intracellular application of guanosine 5'-[gamma-thio]triphosphate inhibits the constitutive traffic of AChRs in most cells. Individual exocytotic vesicles carry between 10 and 300 AChR molecules, suggesting that AChRs may be packed extremely densely.

Serotonin regulates mouse cranial neural crest migration.

Serotonergic agents (uptake inhibitors, receptor ligands) cause significant craniofacial malformations in cultured mouse embryos suggesting that 5-hydroxytryptamine (serotonin) (5-HT) may be an important regulator of craniofacial development. To determine whether serotonergic regulation of cell migration might underly some of these effects, cranial neural crest (NC) explants from embryonic day 9 (E9) (plug day = E1) mouse embryos or dissociated mandibular mesenchyme cells (derived from NC) from E12 embryos were placed in a modified Boyden chamber to measure effects of serotonergic agents on cell migration. A dose-dependent effect of 5-HT on the migration of highly motile cranial NC cells was demonstrated, such that low concentrations of 5-HT stimulated migration, whereas this effect was progressively lost as the dose of 5-HT was increased. In contrast, most concentrations of 5-HT inhibited migration of less motile, mandibular mesenchyme cells. To investigate the possible involvement of specific 5-HT receptors in the stimulation of NC migration, several 5-HT subtype-selective antagonists were used to block the effects of the most stimulatory dose of 5-HT (0.01 microM). Only NAN-190 (a 5-HT1A antagonist) inhibited the effect of 5-HT, suggesting involvement of this receptor. Further evidence was obtained by using immunohistochemistry with 5-HT receptor antibodies, which revealed expression of the 5-HT1A receptor but not other subtypes by migrating NC cells in both embryos and cranial NC explants. These results suggest that by activating appropriate receptors 5-HT may regulate migration of cranial NC cells and their mesenchymal derivatives in the mouse embryo.

Unmasking the functions of the chromaffin cell α7 nicotinic receptor by using short pulses of acetylcholine and selective blockers

Methyllycaconitine (MLA), α-conotoxin ImI, and α-bungarotoxin inhibited the release of catecholamines triggered by brief pulses of acetylcholine (ACh) (100 μM, 5 s) applied to fast-superfused bovine adrenal chromaffin cells, with IC50s of 100 nM for MLA and 300 nM for α-conotoxin ImI and α-bungarotoxin. MLA (100 nM), α-conotoxin ImI (1 μM), and α-bungarotoxin (1 μM) halved the entry of 45Ca2+ stimulated by 5-s pulses of 300 μM ACh applied to incubated cells. These supramaximal concentrations of α7 nicotinic receptor blockers depressed by 30% (MLA), 25% (α-bungarotoxin), and 50% (α-conotoxin ImI) the inward current generated by 1-s pulses of 100 μM ACh, applied to voltage-clamped chromaffin cells. In Xenopus oocytes expressing rat brain α7 neuronal nicotinic receptor for acetylcholine nAChR, the current generated by 1-s pulses of ACh was blocked by MLA, α-conotoxin ImI, and α-bungarotoxin with IC50s of 0.1 nM, 100 nM, and 1.6 nM, respectively; the current through α3β4 nAChR was unaffected by α-conotoxin ImI and α-bungarotoxin, and weakly blocked by MLA (IC50 = 1 μM). The functions of controlling the electrical activity, the entry of Ca2+, and the ensuing exocytotic response of chromaffin cells were until now exclusively attributed to α3β4 nAChR; the present results constitute the first evidence to support a prominent role of α7 nAChR in controlling such functions, specially under the more physiological conditions used here to stimulate chromaffin cells with brief pulses of ACh.

Monoclonal antibodies reveal receptor specificity among G-protein-coupled receptor kinases.

Guanine nucleotide-binding regulatory protein (G protein)-coupled receptor kinases (GRKs) constitute a family of serine/threonine kinases that play a major role in the agonist-induced phosphorylation and desensitization of G-protein-coupled receptors. Herein we describe the generation of monoclonal antibodies (mAbs) that specifically react with GRK2 and GRK3 or with GRK4, GRK5, and GRK6. They are used in several different receptor systems to identify the kinases that are responsible for receptor phosphorylation and desensitization. The ability of these reagents to inhibit GRK- mediated receptor phosphorylation is demonstrated in permeabilized 293 cells that overexpress individual GRKs and the type 1A angiotensin II receptor. We also use this approach to identify the endogenous GRKs that are responsible for the agonist-induced phosphorylation of epitope-tagged beta2- adrenergic receptors (beta2ARs) overexpressed in rabbit ventricular myocytes that are infected with a recombinant adenovirus. In these myocytes, anti-GRK2/3 mAbs inhibit isoproterenol-induced receptor phosphorylation by 77%, while GRK4-6-specific mAbs have no effect. Consistent with the operation of a betaAR kinase-mediated mechanism, GRK2 is identified by immunoblot analysis as well as in a functional assay as the predominant GRK expressed in these cells. Microinjection of GRK2/3-specific mAbs into chicken sensory neurons, which have been shown to express a GRK3-like protein, abolishes desensitization of the alpha2AR-mediated calcium current inhibition. The intracellular inhibition of endogenous GRKs by mAbs represents a novel approach to the study of receptor specificities among GRKs that should be widely applicable to many G-protein-coupled receptors.

Phenotypic knockout of the high-affinity human interleukin 2 receptor by intracellular single-chain antibodies against the alpha subunit of the receptor.

The experimental manipulation of peptide growth hormones and their cellular receptors is central to understanding the pathways governing cellular signaling and growth control. Previous work has shown that intracellular antibodies targeted to the endoplasmic reticulum (ER) can be used to capture specific proteins as they enter the ER, preventing their transport to the cell surface. Here we have used this technology to inhibit the cell surface expression of the alpha subunit of the high-affinity interleukin 2 receptor (IL-2R alpha). A single-chain variable-region fragment of the anti-Tac monoclonal antibody was constructed with a signal peptide and a C-terminal ER retention signal. Intracellular expression of the single-chain antibody was found to completely abrogate cell surface expression of IL-2R alpha in stimulated Jurkat T cells. IL-2R alpha was detectable within the Jurkat cells as an immature 40-kDa form that was sensitive to endoglycosidase H, consistent with its retention in a pre- or early Golgi compartment. A single-chain antibody lacking the ER retention signal was also able to inhibit cell surface expression of IL-2R alpha although the mechanism appeared to involve rapid degradation of the receptor chain within the ER. These intracellular antibodies will provide a valuable tool for examining the role of IL-2R alpha in T-cell activation, IL-2 signal transduction, and the deregulated growth of leukemic cells which overexpress IL-2R alpha.

The chemokine monocyte chemoattractant protein-1 induces functional responses through dimerization of its receptor CCR2

Cytokines interact with hematopoietin superfamily receptors and stimulate receptor dimerization. We demonstrate that chemoattractant cytokines (chemokines) also trigger biological responses through receptor dimerization. Functional responses are induced after pairwise crosslinking of chemokine receptors by bivalent agonistic antichemokine receptor mAb, but not by their Fab fragments. Monocyte chemoattractant protein (MCP)-1-triggered receptor dimerization was studied in human embryonic kidney (HEK)-293 cells cotransfected with genes coding for the CCR2b receptor tagged with YSK or Myc sequences. After MCP-1 stimulation, immunoprecipitation with Myc-specific antibodies revealed YSK-tagged receptors in immunoblotting. Receptor dimerization also was validated by chemical crosslinking in both HEK-293 cells and the human monocytic cell line Mono Mac 1. Finally, we constructed a loss-of-function CCR2bY139F mutant that acted as a dominant negative, blocking signaling through the CCR2 wild-type receptor. This study provides functional support for a model in which the MCP-1 receptor is activated by ligand-induced homodimerization, allowing discussion of the similarities between bacterial and leukocyte chemotaxis.

Modulation of nicotinic acetylcholine receptors by strychnine

Strychnine, a potent and selective antagonist at glycine receptors, was found to inhibit muscle (α1β1γδ, α1β1γ, and α1β1δ) and neuronal (α2β2 and α2β4) nicotinic acetylcholine receptors (AcChoRs) expressed in Xenopus oocytes. Strychnine alone (up to 500 μM) did not elicit membrane currents in oocytes expressing AcChoRs, but, when applied before, concomitantly, or during superfusion of acetylcholine (AcCho), it rapidly and reversibly inhibited the current elicited by AcCho (AcCho-current). Although in the three cases the AcCho-current was reduced to the same level, its recovery was slower when the oocytes were preincubated with strychnine. The amount of AcCho-current inhibition depended on the receptor subtype, and the order of blocking potency by strychnine was α1β1γδ > α2β4 > α2β2. With the three forms of drug application, the Hill coefficient was close to one, suggesting a single site for the receptor interaction with strychnine, and this interaction appears to be noncompetitive. The inhibitory effects on muscle AcChoRs were voltage-independent, and the apparent dissociation constant for AcCho was not appreciably changed by strychnine. In contrast, the inhibitory effects on neuronal AcChoRs were voltage-dependent, with an electrical distance of ≈0.35. We conclude that strychnine regulates reversibly and noncompetitively the embryonic type of muscle AcChoR and some forms of neuronal AcChoRs. In the former case, strychnine presumably inhibits allosterically the receptor by binding at an external domain whereas, in the latter case, it blocks the open receptor-channel complex.

Low density lipoprotein receptor-negative mice expressing human apolipoprotein B-100 develop complex atherosclerotic lesions on a chow diet: No accentuation by apolipoprotein(a)

We have generated mice with markedly elevated plasma levels of human low density lipoprotein (LDL) and reduced plasma levels of high density lipoprotein. These mice have no functional LDL receptors [LDLR−/−] and express a human apolipoprotein B-100 (apoB) transgene [Tg(apoB+/+)] with or without an apo(a) transgene [Tg(apoa+/−)]. Twenty animals (10 males and 10 females) of each of the following four genotypes were maintained on a chow diet: (i) LDLR−/−, (ii) LDLR−/−;Tg(apoa+/−), (iii) LDLR−/−;Tg(apoB+/+), and (iv)LDLR−/−;Tg(apoB+/+);Tg(apo+/−). The mice were killed at 6 mo, and the percent area of the aortic intimal surface that stained positive for neutral lipid was quantified. Mean percent areas of lipid staining were not significantly different between the LDLR−/− and LDLR−/−;Tg(apoa+/−) mice (1.0 ± 0.2% vs. 1.4 ± 0.3%). However, the LDLR−/−;Tg(apoB+/+) mice had ≈15-fold greater mean lesion area than the LDLR−/− mice. No significant difference was found in percent lesion area in the LDLR−/−;Tg(apoB+/+) mice whether or not they expressed apo(a) [18.5 ± 2.5%, without lipoprotein(a), Lp(a), vs. 16.0 ± 1.7%, with Lp(a)]. Histochemical analyses of the sections from the proximal aorta of LDLR−/−;Tg(apoB+/+) mice revealed large, complex, lipid-laden atherosclerotic lesions that stained intensely with human apoB-100 antibodies. In mice expressing Lp(a), large amounts of apo(a) protein colocalized with apoB-100 in the lesions. We conclude that LDLR−/−; Tg(apoB+/+) mice exhibit accelerated atherosclerosis on a chow diet and thus provide an excellent animal model in which to study atherosclerosis. We found no evidence that apo(a) increased atherosclerosis in this animal model.

Activation of nicotinic receptor-induced postsynaptic responses to luteinizing hormone-releasing hormone in bullfrog sympathetic ganglia via a Na+-dependent mechanism

Nicotine at very low doses (5–30 nM) induced large amounts of luteinizing hormone-releasing hormone (LHRH) release, which was monitored as slow membrane depolarizations in the ganglionic neurons of bullfrog sympathetic ganglia. A nicotinic antagonist, d-tubocurarine chloride, completely and reversibly blocked the nicotine-induced LHRH release, but it did not block the nerve-firing-evoked LHRH release. Thus, nicotine activated nicotinic acetylcholine receptors and produced LHRH release via a mechanism that is different from the mechanism for evoked release. Moreover, this release was not caused by Ca2+ influx through either the nicotinic receptors or the voltage-gated Ca2+ channels because the release was increased moderately when the extracellular solution was changed into a Ca2+-free solution that also contained Mg2+ (4 mM) and Cd2+ (200 μM). The release did not depend on Ca2+ release from the intraterminal Ca2+ stores either because fura-2 fluorimetry showed extremely low Ca2+ elevation (≈30 nM) in response to nicotine (30 nM). Moreover, nicotine evoked LHRH release when [Ca2+] elevation in the terminals was prevented by loading the terminals with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid and fura-2. Instead, the nicotine-induced release required extracellular Na+ because substitution of extracellular NaCl with N-methyl-d-glucamine chloride completely blocked the release. The Na+-dependent mechanism was not via Na+ influx through the voltage-gated Na+ channels because the release was not affected by tetrodotoxin (1–50 μM) plus Cd2+ (200 μM). Thus, nicotine at very low concentrations induced LHRH release via a Na+-dependent, Ca2+-independent mechanism.

Plasmodium falciparum domain mediating adhesion to chondroitin sulfate A: A receptor for human placental infection

Malaria during the first pregnancy causes a high rate of fetal and neonatal death. The decreasing susceptibility during subsequent pregnancies correlates with acquisition of antibodies that block binding of infected red cells to chondroitin sulfate A (CSA), a receptor for parasites in the placenta. Here we identify a domain within a particular Plasmodium falciparum erythrocyte membrane protein 1 that binds CSA. We cloned a var gene expressed in CSA-binding parasitized red blood cells (PRBCs). The gene had eight receptor-like domains, each of which was expressed on the surface of Chinese hamster ovary cells and was tested for CSA binding. CSA linked to biotin used as a probe demonstrated that two Duffy-binding-like (DBL) domains (DBL3 and DBL7) bound CSA. DBL7, but not DBL3, also bound chondroitin sulfate C (CSC) linked to biotin, a negatively charged sugar that does not support PRBC adhesion. Furthermore, CSA, but not CSC, blocked the interaction with DBL3; both CSA and CSC blocked binding to DBL7. Thus, only the DBL3 domain displays the same binding specificity as PRBCs. Because protective antibodies present after pregnancy block binding to CSA of parasites from different parts of the world, DBL-3, although variant, may induce cross-reactive immunity that will protect pregnant women and their fetuses.