Functional evaluation of inhibition of autonomic transmitter release by autoantibody from Lambert-Eaton myasthenic syndrome

An updated advance of autoantibodies in autoimmune diseases

Autoantibodies are abnormal antibodies which are generated by pathogenic B cells when targeting an individual's own tissue. Autoantibodies have been identified as a symbol of autoimmune disorders and are frequently considered a clinical marker of these disorders. Autoimmune diseases, including system lupus erythematosus and rheumatoid arthritis, consist of a series of disorders that share some similarities and differences. They are characterized by chronic, systemic, excessive immune activation and inflammation and involve in almost all body tissues. Autoimmune diseases occur more frequently in women than men due to hormonal impacts. In this review we systemically introduce and summarize the latest advances of various autoantibodies in multiple autoimmune diseases.

Upregulation of L-type calcium channels in colonic inhibitory motoneurons of P/Q-type calcium channel-deficient mice

Enteric inhibitory motoneurons use nitric oxide and a purine neurotransmitter to relax gastrointestinal smooth muscle. Enteric P/Q-type Ca2+ channels contribute to excitatory neuromuscular transmission; their contribution to inhibitory transmission is less clear. We used the colon from tottering mice (tg/tg, loss of function mutation in the α1A pore-forming subunit of P/Q-type Ca2+ channels) to test the hypothesis that P/Q-type Ca2+ channels contribute to inhibitory neuromuscular transmission and colonic propulsive motility. Fecal pellet output in vivo and the colonic migrating motor complex (ex vivo) were measured. Neurogenic circular muscle relaxations and inhibitory junction potentials (IJPs) were also measured ex vivo. Colonic propulsive motility in vivo and ex vivo was impaired in tg/tg mice. IJPs were either unchanged or somewhat larger in tissues from tg/tg compared with wild-type (WT) mice. Nifedipine (L-type Ca2+ channel antagonist) inhibited IJPs by 35 and 14% in tissues from tg/tg and WT mice, respectively. The contribution of N- and R-type channels to neuromuscular transmission was larger in tissues from tg/tg compared with WT mice. The resting membrane potential of circular muscle cells was similar in tissues from tg/tg and WT mice. Neurogenic relaxations of circular muscle from tg/tg and WT mice were similar. These results demonstrate that a functional deficit in P/Q-type channels does not alter propulsive colonic motility. Myenteric neuron L-type Ca2+ channel function increases to compensate for loss of functional P/Q-type Ca2+ channels. This compensation maintains inhibitory neuromuscular transmission and normal colonic motility.

Presynaptic effects of immunoglobulin G from patients with Lambert-Eaton myasthenic syndrome: Their neutralization by intravenous immunoglobulins

Intravenous immunoglobulin (IVIg) treatment improves muscle strength in Lambert-Eaton myasthenic syndrome (LEMS), but its specific mode of action is unknown. We have delineated its mode of action on neuromuscular blocking properties of LEMS IgG. The effect of sera and purified IgG from six patients with LEMS on evoked quantal release was investigated after direct application to the motor nerve terminal by the perfused macro-patch-clamp electrode in mouse hemidiaphragms. The effect of LEMS IgG was analyzed alone and after coincubation with different concentrations of IVIg or its Fab fragments. All LEMS sera and purified LEMS IgG fractions taken before IVIg treatment inhibited evoked quantal release in a dose-dependent manner. When LEMS IgG was coincubated with a therapeutic IVIg preparation, presynaptic inhibitory activity of LEMS IgG was diminished in a dose-dependent fashion. Monovalent Fab fragments were as effective in neutralizing the activity of LEMS IgG as whole IVIg. These direct neutralizing effects of IVIg may explain its therapeutic efficacy.

Antibody-mediated gastrointestinal dysmotility in scleroderma

BACKGROUND & AIMS

Defects in enteric excitatory neurotransmission have been proposed to underlie the gastrointestinal dysmotility associated with scleroderma (systemic sclerosis). This study investigated whether patients with scleroderma produce antibodies that inhibit M3-muscarinic or neurokinin receptor-mediated intestinal contractions, either directly or via an effect on L-type voltage-gated calcium channels (VGCCs).

METHODS

Responses of mouse colon longitudinal muscle to stimulation by the muscarinic agonist carbachol (1-300 micromol/L) and neurokinin-1 and -2 receptor agonists were measured in the absence and presence of serum (2%) or immunoglobulin G (IgG) (0.3-1.0 mg/mL) from patients with scleroderma, those with other autoimmune disorders, and healthy controls. The role of L-type VGCCs in carbachol- and tachykinin-evoked contractions was assessed using nicardipine.

RESULTS

M3-muscarinic receptor-mediated contractions were inhibited by Ig fractions from 7 of 9 patients with scleroderma (limited and diffuse forms), 4 of 4 patients with primary Sjögren's syndrome, and 3 of 3 patients with secondary Sjögren's syndrome. Ig fractions from healthy controls did not inhibit the M3-muscarinic receptor-mediated contractions. Inhibition by Ig was concentration-dependent; a maximum inhibition of approximately 40% occurred at 0.6 mg/mL IgG. Both M3-muscarinic and neurokinin receptor-mediated contractions were L-type VGCC dependent. Patient sera had no effect on responses to neurokinin receptor stimulation, demonstrating the lack of antibodies inhibiting L-type VGCCs.

CONCLUSIONS

Functional antibodies specifically inhibiting M3-muscarinic receptor-mediated enteric cholinergic neurotransmission may provide a pathogenic mechanism for the gastrointestinal dysfunction seen in patients with scleroderma.

Role of anti-calcium channel and anti-receptor autoantibodies in autonomic dysfunction in Sjögren's syndrome

Auto-antibodies cross-reacting with L-type voltage-gated calcium channels (VGCCs) have been described in primary Sjögren's syndrome (pSS), and may mediate the cardiac defects in neonates born to mothers with pSS. L-type VGCCs are also present in autonomically innervated tissues. Therefore, the aim of this project was to investigate a role for anti-VGCC antibodies and antibodies to alpha(1)-adrenoceptors or P(2X)-purinoceptors in the autonomic dysfunction that occurs in pSS. Contraction of the sympathetically innervated vas deferens in response to stimulation of the muscle by an alpha(1)-adrenoceptor agonist (phenylephrine) or a P(2X)-purinoceptor agonist (alpha,beta-methylene ATP) was measured in the absence and presence of 2% serum. Contractions produced by phenylephrine and by alpha,beta-methylene ATP were abolished by nicardipine, demonstrating that they are coupled to calcium influx through L-type VGCCs. Serum from patients with pSS or from healthy controls did not significantly alter the L-type channel-dependent responses of smooth muscle to agonist stimulation. We therefore conclude that pSS serum does not contain autoantibodies that functionally inhibit L-type VGCCs, alpha(1)-adrenoceptors or P(2X)-purinoceptors in smooth muscle and that such autoantibodies cannot explain the autonomic dysfunction in pSS.

Autonomic dysfunction in Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder characterized by muscle weakness and autonomic dysfunction. Recent ex vivo and in vitro studies demonstrate that autoantibodies to the P/Q-subtype of voltage-gated calcium channel inhibit transmitter release from parasympathetic, sympathetic, and enteric neurons, a mechanism likely to underlie the widespread autonomic dysfunction in LEMS. This review summarizes clinical studies characterizing the autonomic symptoms and signs in LEMS and the effectiveness of treatment in alleviating these symptoms. Serological assays and in vitro pharmacologic and electrophysiologic studies are also discussed.

Identification of the amino acids on a neuronal glutamate receptor recognized by an autoantibody from a patient with paraneoplastic syndrome

Autoantibodies from a patient with paraneoplastic disease were identified previously to bind to the glutamate receptor (GluR) subunit GluR5 and to function as potential allosteric modulators of receptor activity (Gahring et al. [1995] Mol Med 1:245-253). In the present study we have used deletion mapping and mutagenesis to define the residues in GluR5 bound by this autoreactivity. The autoantibody contact residues include residues K497, N508, K510, E512, and to a lesser extent Q507. Residues 507-512 confer autoantibody specificity of the autoreactivity to GluR5. These residues have been shown in crystallographic studies (Armstrong et al. [1998] Nature 395:913-917) to participate in a loop structure, whereas residue K497 is located on a beta-strand. Notably, this binding spans tyrosine 504, a residue important in forming the agonist-binding site. We propose that autoantibody binding of essential residues in this GluR5 autoantigenic region defines a subunit-specific allosteric regulatory site on neuronal glutamate receptors and suggests how receptor dysfunction and region-specific neuronal death in the brain can progress in certain autoimmune neurological diseases.

Lambert-Eaton Myasthenic Syndrome

Weakness and autonomic dysfunction in Lambert-Eaton myasthenic syndrome (LEMS) can be partially or fully controlled by 3,4-Diaminopyridine. Intravenous immunoglobulin or plasma exchange (PE) plasmapheresis) provides short-term improvement in severely affected patients. In those at risk from paraneoplastic LEMS (cigarette smokers), an intensive search for lung cancer should be undertaken, and specific tumor therapy instituted that likely will improve the neurologic deficit. Prednisolone (1.5 mg per kg of body weight administered on alternate days, maximum dosage is 100 mg) is indicated in those with paraneoplastic or nonparaneoplastic LEMS who fail to respond sufficiently to symptomatic treatment. The addition of azathioprine or cyclosporine is indicated as corticosteroid sparing medications in nonparaneoplastic LEMS. When remission or optimal improvement is judged to be present, prednisolone should be tapered to the minimum dose that effectively controls symptoms. If full withdrawal is achieved, azathioprine dose reduction is similarly initiated. In nonparaneoplastic LEMS patients failing to respond to azathioprine after 1 to 2 years of therapy, physicians should consider substituting cyclosporine.

Neurogenic bladder in Lambert-Eaton myasthenic syndrome and its response to 3,4-diaminopyridine

Autonomic dysfunction, as well as neuromuscular involvement, is a common manifestation of Lambert-Eaton myasthenic syndrome (LEMS). Dry mouth and impotence have been described as typical features of autonomic dysfunction, but neurogenic bladder is infrequent or subclinical in LEMS. We report a patient with neurogenic bladder secondary to LEMS whose condition responded to 3,4-diaminopyridine (3,4-DAP). In this patient's serum, results of repeated measurement with P/Q-type VGCC antibodies proved positive, but not with N-type VGCC and synaptotagmin antibodies. A review of the literature turned up a few patients with voiding dysfunction related to LEMS, but no urodynamic studies were done on these patients. Ours is the first case in which 3,4-DAP was efficacious in treating LEMS and neurogenic bladder. Responses of 3,4-DAP in urodynamic studies suggest that in this LEMS patient neurogenic bladder was caused by defective neurotransmission both in the autonomic detrusor and skeletal abdominal muscles.

Autoantibodies in paraneoplastic neurological syndrome

Paraneoplastic neurological syndrome is a rare disorder caused by the secondary effects of cancer and is thought to be immune-mediated. A high titer of autoantibodies in the patient's serum and cerebrospinal fluid, directed against both neurons and tumor, have been detected in some forms of this syndrome. These autoantibodies are considered the result of an immunological response to tumor and may cross-react with cells of the nervous system, causing neuronal damage. Specific forms of this syndrome are often associated with specific antineuronal antibodies and tumors. The onset of neurological symptoms and detection of these antibodies often precede the diagnosis of the tumor; therefore, detection of these antibodies greatly assists the diagnosis of this syndrome and prompts investigations for the underlying tumor. The pathogenicity of these antineuronal antibodies has been proven in only a few cases, such as that of anti-voltage gated calcium-channel antibodies in Lambert-Eaton myasthenic syndrome. The selective involvement of specific types of neurons has not been fully elucidated. The target spectrum of some of these antineuronal antibodies correlates well with the neurological symptoms, but that of others is wider than expected from the symptoms. Interesting evidence has suggested that these antionconeuronal antibodies can suppress tumor growth. The discovery of new antibodies and characterization of target molecules have been reported with advances in the field of molecular biology. A more detailed understanding of the relationship between the cancer and the neural involvement from the molecular biological standpoint may lead to rational tumor therapy and elucidation of the mechanism of neuronal death. Here, major clinical forms with well-known antineuronal antibodies and specific tumors are reviewed; for each antineuronal antibody, the target antigens and its putative role in the pathogenesis of this syndrome are described.