Antisera against an acetylcholine receptor alpha 3 fusion protein bind to ganglionic but not to brain nicotinic acetylcholine receptors

Autoimmune autonomic ganglionopathy: Ganglionic acetylcholine receptor autoantibodies

Autoimmune Autonomic Ganglionopathy (AAG) is a rare immune-mediated disease of the autonomic nervous system. The incidence of AAG is unknown and diagnosis is often difficult due to the multicompartmental nature of the autonomic nervous system - sympathetic, parasympathetic and enteric components - with variable severity and number of components affected. Diagnostic confidence is increased when ganglionic acetylcholine receptor (gnACHR) autoantibodies are detected. Three gnACHR autoantibody diagnostic assays have been described (two binding assays, one receptor immunomodulation assay), but cross-validation between assays is limited. The prevalence of gnACHR autoantibodies in AAG is not known, with application of different clinical and laboratory criteria in the few studies of AAG cohorts and large retrospective laboratory studies of positive gnACHR autoantibodies lacking adequate clinical characterisation. Furthermore, the rate of unexpected gnACHR autoantibody positivity in conditions without overt autonomic dysfunction (false positive results) adds to the complexity of their interpretation. We review the pathophysiology of gnACHR autoantibodies and assays for their detection, with immunomodulation and high titer radioimmunoprecipitation results likely offering better AAG disease identification.

Evaluation of commercially available antibodies and fluorescent conotoxins for the detection of surface ganglionic acetylcholine receptor on the neuroblastoma cell line, IMR-32 by flow cytometry

Commercially available antibodies that bind to the human muscle acetylcholine receptor (ACHR) have been validated previously for flow cytometric use (Keefe et al., 2009; Leite et al., 2008; Lozier et al., 2015). Despite a multitude of commercially available antibodies to other nicotinic ACHRs, validation in a wide variety of immunoassay formats is lacking; when studied, a large proportion of these antibodies have been deemed not fit for most research purposes (Garg and Loring, 2017). We have recently described a flow cytometric immunomodulation assay for the diagnosis of Autoimmune Autonomic Ganglionopathy (AAG) (Urriola et al., 2021) that utilises the monoclonal antibody mab35(Urriola et al., 2021) which is specific for ganglionic ACHR (gnACHR) that contain α3 subunits (Vernino et al., 1998). Other fluorescent ligands for α3-gnACHR have not been validated for flow cytometric use. We investigated 7 commercially sourced antibodies and 3 synthetic fluorescent novel conotoxins purported to specifically bind to the extracellular domains of the gnACHR, and compared the results to staining by mab35, using flow cytometry with the neuroblastoma cell line IMR-32. We also evaluated the degree of non-specific binding by depleting the cell membrane of the relevant acetylcholine receptor with a pre-incubation step involving the serum from a patient with Autoimmune Autonomic Ganglionopathy containing pathogenic antibodies to the ganglionic acetylcholine receptor. None of the assessed conotoxins, and only one antibody (mab35) was found to perform adequately in flow cytometric staining of the native ganglionic acetylcholine receptor.

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.

Experimental autoimmune autonomic neuropathy

Antibodies specific for the neuronal ganglionic nicotinic acetylcholine receptor (nAChR) are found in high titer in serum of patients with subacute autonomic failure. This clinical disorder is known as autoimmune autonomic neuropathy (AAN). Rabbits immunized with a neuronal nAChR alpha3 subunit fusion protein produce ganglionic nAChR antibodies and develop autonomic failure (experimental AAN, or EAAN). We used quantitative measures of autonomic function to demonstrate that this animal model of neuronal nAChR autoimmunity recapitulates the cardinal autonomic features of AAN in humans. The severity of dysautonomia in the rabbit ranges from isolated cardiovagal impairment to severe panautonomic failure with fixed mydriasis, gastroparesis, dry eyes, impaired heart rate variability, hypotension, and low plasma catecholamines. The severity of autonomic failure correlates with serum antibody levels. Immunohistochemical staining of superior cervical ganglia and myenteric plexus neurons demonstrates intact presynaptic nerve terminals and intact postsynaptic neurons containing cytoplasmic nAChR, but lacking surface nAChR. These findings define the autonomic physiology and histopathology of this novel animal model and support the concept that AAN in humans is a disorder of ganglionic cholinergic synaptic transmission caused by ganglionic nAChR antibodies.

Immunization with neuronal nicotinic acetylcholine receptor induces neurological autoimmune disease

Neuronal nicotinic AChRs (nAChRs) are implicated in the pathogenesis of diverse neurological disorders and in the regulation of small-cell lung carcinoma growth. Twelve subunits have been identified in vertebrates, and mutations of one are recognized in a rare form of human epilepsy. Mice with genetically manipulated neuronal nAChR subunits exhibit behavioral or autonomic phenotypes. Here, we report the first model of an acquired neuronal nAChR disorder and evidence for its pertinence to paraneoplastic neurological autoimmunity. Rabbits immunized once with recombinant alpha3 subunit (residues 1-205) develop profound gastrointestinal hypomotility, dilated pupils with impaired light response, and grossly distended bladders. As in patients with idiopathic and paraneoplastic autoimmune autonomic neuropathy, the severity parallels serum levels of ganglionic nAChR autoantibody. Failure of neurotransmission through abdominal sympathetic ganglia, with retention of neuronal viability, confirms that the disorder is a postsynaptic channelopathy. In addition, we found ganglionic nAChR protein in small-cell carcinoma lines, identifying this cancer as a potential initiator of ganglionic nAChR autoimmunity. The data support our hypothesis that immune responses driven by distinct neuronal nAChR subtypes expressed in small-cell carcinomas account for several lung cancer-related paraneoplastic disorders affecting cholinergic systems, including autoimmune autonomic neuropathy, seizures, dementia, and movement disorders.

Neuronal nicotinic acetylcholine receptor subunit knockout mice: physiological and behavioral phenotypes and possible clinical implications

Nicotinic acetylcholine receptors (nAChRs) in the muscle, autonomic ganglia, and brain are targets for pharmacologically administered nicotine. Several of the subunits that combine to form neuronal nicotinic receptors have been deleted by knockout or mutated by knockin in mice using homologous recombination. We will review the biochemical, pharmacological, anatomical, physiological, and behavioral phenotypes of mice with genetically altered neuronal nAChR subunits. Clinically relevant mutations in nAChR genes will also be discussed. In addition, some of the signal transduction pathways activated through nAChRs will be described in order to delineate the longer-term changes that might result from persistent activation or inactivation of nAChRs. Genetically manipulated mice have greatly increased our understanding of the subunit composition and physiological properties of nAChRs in vivo. In addition, these mice have provided a model system to determine the molecular basis for many of the pharmacological actions of nicotine on neurotransmitter release and behavior. Genetic manipulations in mice have also elucidated the role of nAChR subunits in various disease states, and suggest several avenues for drug development.

Myasthenia gravis, thymoma, intestinal pseudo-obstruction, and neuronal nicotinic acetylcholine receptor antibody

Intestinal pseudo-obstruction occurs rarely in patients with myasthenia gravis (MG) and thymoma. The etiology of the intestinal pseudo-obstruction remains to be elucidated, although an autoimmune mechanism is postulated. We present the first report of neuronal nicotinic acetylcholine receptor (AChR)-specific antibody in a patient with seropositive MG, malignant thymoma, and intestinal pseudo-obstruction. This finding provides evidence that intestinal pseudo-obstruction associated with thymoma and possibly other neoplasms may be related to antibodies against the neuronal nicotinic receptors at autonomic ganglia.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Human ciliary neurotrophic factor: a structure-function analysis

Ciliary neurotrophic factor (CNTF) promotes survival in vitro and in vivo of several neuronal cell types including sensory and motor neurons. The primary structure of CNTF suggests it to be a cytosolic protein with strong similarity to the alpha-helical cytokine family which is characterized by a bundle of four anti-parallel helices. CNTF exerts its activity via complexation with CNTF receptor (CNTF-R). This complex consists of a CNTF-binding protein (CNTF-R) and two proteins important for signal transduction [gp130 and leukaemia inhibitory factor receptor (LIF-R)]. We have shortened the cDNA coding for CNTF at both the 5' and the 3' end and expressed the truncated proteins in bacteria. Biological activities of the protein preparations were determined by their ability to induce proliferation of BAF/3 cells that were stably transfected with CNTF-R, gp130 and LIF-R cDNAs. CNTF proteins with 14 amino acid residues removed from the N-terminus were biologically active whereas the removal of 23 amino acids resulted in an inactive protein. In addition, 18 amino acid residues could be removed from the C-terminus of the CNTF protein without apparent loss of bioactivity, but further truncation at the C-terminus yielded biologically inactive proteins. The introduction of two point mutations into the CNTF protein at a site that presumably interacts with one of the two signal-transducing proteins resulted in a CNTF mutant with no measurable bioactivity. In addition, a model of the three-dimensional structure of human CNTF was constructed using the recently established structural co-ordinates of the related cytokine, granulocyte colony-stimulating factor. CD spectra of CNTF together with our mutational analysis and our three-dimensional model fully support the view that CNTF belongs to the family of alpha-helical cytokines. It is expected that our results will facilitate the rational design of CNTF mutants with agonistic or antagonistic properties.

Expression of neuronal nicotinic acetylcholine receptor subunit genes in the rat autonomic nervous system

In the autonomic nervous system efferent signals are relayed in sympathetic and parasympathetic ganglia. Fast synaptic transmission between pre- and postsynaptic neurons is achieved by neuronal nicotinic acetylcholine receptors (nAChRs). There is still little known about the subunit composition of these receptors. Establishing the subunit composition of native neuronal nAChRs is important for the understanding of their functional properties both in vivo and after expression in heterologous expression systems. We have combined in situ hybridization and autoradiography to detect the presence of mRNAs encoding subunits of neuronal nAChRs in sympathetic and parasympathetic ganglia. Inspection of the autoradiographs showed that the hybridization signal of five riboprobes (alpha 3, alpha 4-1, alpha 7, beta 2 and beta 4) was significantly higher than the unspecific signal obtained with sense riboprobes. The distribution of alpha 7 was tissue-dependent: alpha 7 riboprobe binding was detected in the neurons of the superior cervical ganglion, adrenal medulla and ciliary ganglion. In contrast, the alpha 7 hybridization signal was found only in a small fraction (1-3%) of the neurons of the sphenopalatine and otic ganglia. Our results are consistent with the idea that alpha 3 mRNA expression levels are somewhat higher than those of alpha 7, alpha 4-1, beta 2 and beta 4.

Recombinant soluble human interleukin-6 receptor. Expression in Escherichia coli, renaturation and purification

The recombinant soluble human interleukin-6 receptor (srhIL-6R) was expressed in Escherichia coli as a non-glycosylated protein comprising the first 339 amino acids after the signal peptide. The protein accumulated within the cells as insoluble protein aggregates (inclusion bodies). After solubilization, 10% of the denatured srhIL-6R could be renaturated by an in vitro folding procedure using L-arginine and the glutathione-redox system. The native receptors were purified to near homogeneity by affinity chromatography on an IL-6-Sepharose column. The functional features of the recombinant soluble receptor were further analysed. A part of the extracellular domain (amino acids 145-345) of the human interleukin-6 receptor (IL-6R) was expressed in E. coli and the purified protein was used to raise antibodies in rabbits. Characterization of the antiserum obtained indicated that an epitope of 13 amino acids close to the transmembrane region is needed for recognition by the antibodies. Since the antiserum obtained did not interfere with IL-6 binding, it could be used to establish a cell-free IL-6-binding assay, In this assay, the srhIL-6R bound IL-6 with an affinity of Kd = 1.5 nM as measured by Scatchard-plot analysis. When 125I-IL-6 was chemically cross-linked to the purified srhIL-6R and analyzed by SDS/PAGE, several 125I-IL-6-containing bands were detected, indicating the possible existence of a multimeric structure of the natural IL-6/IL-6R complex. The srhIL-6R was shown to exhibit biological activity, i.e. it stimulated acute-phase protein synthesis in the recently established human hepatoma cell line HepG2-IL-6 which does not express the IL-6-binding subunit of the IL-6R complex on the cell surface.

Neurons assemble acetylcholine receptors with as many as three kinds of subunits while maintaining subunit segregation among receptor subtypes

A family of genes encoding neuronal acetylcholine receptor (AChR) subunits has been identified and cloned from vertebrates. Expression studies have implied that as few as one or two kinds of subunits may be sufficient to construct neuronal AChRs and that multiple pair-wise combinations of the gene products are capable of generating functional receptors. We show here that a class of AChRs with a predominantly synaptic location on neurons contains receptors having at least three types of subunits and that the subunits are encoded by the alpha 3, beta 4, and alpha 5 AChR genes. In addition, we show that a class of extrasynaptic AChRs on the same neurons contains the alpha 7 subunits but lacks the alpha 3, beta 4, and alpha 5 subunits. The results demonstrate that native AChRs on neurons are more complex in composition than previously appreciated and suggest that constraints on subunit interactions limit the kinds of receptor species produced.

The pRSET family of T7 promoter expression vectors for Escherichia coli

A family of eight T7 promoter-based expression plasmids is presented. These are high-copy-number vectors featuring translational start and stop elements and a multiple cloning site (polylinker) with eleven unique restriction sites in all six reading frames. Depending on the cloning strategy used, recombinant proteins may contain either short vector-encoded fusion fragments or no fusion fragments at all. Following promoter induction, proteins are usually produced at a high level.

The fall and rise of neuronal alpha-bungarotoxin binding proteins

Although neuronal [125I]-alpha-bungarotoxin binding proteins are similar in many respects to muscle nicotinic acetylcholine receptors, their functional significance has eluded researchers for the past fifteen years. Over this period, their status became increasingly doubtful, as almost all attempts failed to demonstrate that alpha-bungarotoxin could block neuronal nicotinic responses. Recently, these enigmatic proteins have been cloned and expressed in oocytes, and have been examined afresh in their native state. As Paul Clarke explains, it is time to recognize neuronal alpha-bungarotoxin binding proteins as distinct members of the nicotinic acetylcholine receptor gene family, even if perhaps they do not function quite like other members.

The alpha 5 gene product assembles with multiple acetylcholine receptor subunits to form distinctive receptor subtypes in brain

The acetylcholine receptor (AChR) alpha 5 gene has been classified as a member of the AChR gene family based on sequence homology. Expression studies, however, have yet to identify a function for the alpha 5 gene product or even to demonstrate an interaction with known AChR subunits. We report here that the alpha 5 gene product is identical to the 49 kd protein previously found on immunoblots of AChRs purified from brain and ciliary ganglia. In brain the alpha 5 gene product is present both in alpha 3- and in alpha 4-based receptor subtypes, while in the ganglion it is found in an alpha 3-based receptor subtype concentrated in postsynaptic membrane. Immunoprecipitation experiments with subunit-specific monoclonal antibodies indicate that some native AChRs are likely to have at least three kinds of subunits, with two being of the alpha type. These findings support new views about the construction of AChRs in neurons.

Epitope mapping of polyclonal and monoclonal antibodies against two alpha-bungarotoxin-binding alpha subunits from neuronal nicotinic receptors

Recently, cDNAs for alpha subunits of two different neuronal alpha-bungarotoxin-binding proteins (alpha BgtBP) were isolated from chick brain, designated alpha BgtBP alpha 1 and alpha BgtBP alpha 2. These are now also referred to as subunits alpha 7 and alpha 8, respectively. Expression studies in Xenopus oocytes have indicated that alpha 7 subunits are able to form cation channels that are sensitive to nicotinic ligands, and therefore represent bona fide nicotinic acetylcholine receptor subunits. Polyclonal and monoclonal antibodies (mAbs) have been produced against: (i) affinity-purified chick brain alpha BgtBP; and (ii) fusion proteins containing the unique cytoplasmic sequences alpha 7(327-412) and alpha 8(293-435). Here, synthetic overlapping peptides corresponding to their deduced amino acid sequences are used to map the epitopes recognized by the different antibodies. The polyclonal response to affinity-purified alpha BgtBPs and the fusion proteins indicates that sequence segments 290-420 of both subunits contain several major and minor epitopes. mAbs selected for their ability to bind both native and denatured alpha BgtBPs isolated from chick brain also recognize subunit-specific sequential epitopes within the sequence segment 290-420. The epitopes recognized by the mAbs correspond to the minor epitopes defined using antisera. The mAbs characterized in these studies will provide useful probes for further studies of alpha BgtBP structure and histological localization.

Functional contribution of neuronal AChR subunits revealed by antisense oligonucleotides

Although multiple related genes encoding nicotinic acetylcholine receptor (AChR) subunits have been identified, how each of these subunits contributes to AChRs in neurons is not known. Sympathetic neurons express four classes of AChR channels and six AChR subunit genes (alpha 3, alpha 4, alpha 5, alpha 7, beta 2, and beta 4). The contribution of individual subunits to AChR channel subtypes in these neurons was examined by selective deletion with antisense oligonucleotides. An alpha 3 antisense oligonucleotide decreased the number and altered the properties of the normally expressed ACh-activated channels. The remaining AChR channels have distinct biophysical and pharmacological properties that indicate an important functional contribution of the alpha 7 subunit.

GABAA receptor subtypes immunopurified from rat brain with alpha subunit-specific antibodies have unique pharmacological properties

The unique cytoplasmic loop regions of the alpha 1, alpha 2, alpha 3, and alpha 5 subunits of the GABAA receptor were expressed in bacterial and used to produce subunit-specific polyclonal antisera. Antibodies immobilized on protein A-Sepharose were used to isolate naturally occurring alpha-specific populations of GABAA receptors from rat brain that retained the ability to bind [3H]muscimol, [3H]flunitrazepam, [3H]Ro15-1788, and [125I]iodo-clonazepam with high affinity. Pharmacological characterization of these subtypes revealed marked differences between the isolated receptor populations and was generally in agreement with the reported pharmacological profiles of GABAA receptors in cells transiently transfected with alpha 1 beta 1 gamma 2, alpha 2 beta 1 gamma 2, alpha 3 beta 1 gamma 2, and alpha 5 beta 1 gamma 2 combinations of subunits. Additional subtypes were also identified that bind [3H]muscimol but do not bind benzodiazepines with high affinity. The majority of GABAA receptor oligomers contains only a single type of alpha subunit, and we conclude that alpha 1, alpha 2, alpha 3, and alpha 5 subunits exist in vivo in combination with the beta subunit and gamma 2 subunit.

Expression of nicotinic acetylcholine receptor subtypes in brain and retina

Neuronal nicotinic acetylcholine receptors (AChRs) are composed of two types of subunits: ACh-binding (termed alpha 2, alpha 3, alpha 4 ...) and structural (termed beta 2, beta 3, beta 4 ...). AChR subtypes composed of combinations of subunits of these two types encoded by several related genes are expressed in different parts of the nervous system, where they presumably serve different functional roles. Here we identify the ACh-binding subunit of the most prominent chicken brain AChR subtype by N-terminal amino acid sequence and show that it corresponds to the alpha 4 gene. Previously we identified the structural subunit for this AChR subtype from chicken brain as beta 2 by N-terminal amino acid sequence. Thus, this identifies both genes which encode subunits of the major nicotinic AChR subtype in avian brains. By immunoprecipitation, immunohistochemistry, and northern blot analysis we show that alpha 3 (or a very closely related sequence) is expressed at low levels in the brain and relatively high levels in the retina, while alpha 4 is expressed at high levels in the brain and lower levels in the retina. This differential expression indicates that alpha 3-containing 'ganglionic-type' AChRs may be an important AChR subtype in avian retina.

Determination of amino acids critical to the main immunogenic region of intact acetylcholine receptors by in vitro mutagenesis

The main immunogenic region (MIR) of the acetylcholine receptor (AChR) is the target for the majority of high-affinity autoantibodies produced in myasthenia gravis patients. Some monoclonal antibodies (mAbs) to the MIR bind specifically, but with low affinity, to synthetic AChR alpha subunit peptides with the sequence alpha 67-76. Studies of synthetic peptides suggest that amino acids alpha 68 and alpha 71 may be especially important to the antigenic structure of the MIR. We have studied the contribution of amino acids alpha 68 and alpha 71 to the antigenicity of the MIR on intact AChR by replacing alpha 68 (N) and alpha 71 (D) of Torpedo AChR alpha with alpha 68 (D) and alpha 71 (K) by site-directed mutagenesis, expressing the mutated transcripts in Xenopus oocytes along with wild-type Torpedo beta, gamma and delta subunits, and analyzing the expressed AChR for the binding of mAbs to the MIR. These mutations of the MIR greatly diminished binding of mAbs to the MIR. Thus, both alpha 68 and alpha 71 are critical to the antigenicity of the MIR in intact AChRs.

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.