Anti-acetylcholinesterase antibodies associate with ocular myasthenia gravis

https://doi.org/10.1016/j.jneuroim.2009.11.004Get rights and content

Abstract

In MG, anti-AChR or anti-MuSK abs impair neuromuscular transmission. Partial inhibition of AChE can ameliorate symptoms, while a complete block causes a cholinergic blockade. We found anti-AChE abs in 115/240 MG patients, with no correlation with sex, age at onset, thymus pathology, presence of anti-AChR or anti-MuSK antibodies. We found a correlation with the ocular form of the disease, and with milder forms of MG not requiring immunosuppressants; moreover, when we considered only those patients who were off AChEI therapy, we found that ocular patients were positive for anti-AChE abs, while generalized patients were negative. According to an experimental model, we hypothesize that anti-AChE abs could contribute to ptosis through an inhibition of the sympathetic innervation of the tarsal muscle.

Introduction

Acetylcholinesterase (AChE) belongs to a family of closely homologous proteins, that include other serine esterases like carboxylesterase, cholesterol esterase and lipase, as well as non-catalytic proteins like the C-terminal domain of thyroglobulin and two Drosophila proteins, glutactin and neurotactin, that are involved in cellular interactions (Krejci et al., 1991).

Vertebrates AChE, and the related butyrylcholinesterase (BChE), hydrolyze acetylcholine (ACh). While the mechanisms of choline ester hydrolysis are basically the same in both enzymes (Sussman et al., 1991), they have different tissue distributions: AChE is particularly represented in the central nervous system (CNS), at the neuromuscular junction (NMJ) and in red blood cells, whereas BChE is enriched in the liver and heart (Jbilo et al., 1994). At the molecular level, AChE have different isoforms, depending on alternative splicing and association with non-catalytic proteins: these splice variants possess the same catalytic domain but differ in the small C-terminal domain (Massoulié, 2002). The AChER variant produces a soluble monomer which is found in embryonic tissues and in cell cultures (Li et al., 1991), and can be upregulated in the mouse brain during stress (Kaufer et al., 1998), while the AChEH variant produces a GPI-anchored dimer which is mainly expressed in blood cells (Li et al., 1991). The AChET variant is the only type of subunit expressed in the brain and muscles of adult mammals (Legay et al., 1995), and produces various oligomers including monomers, dimers, and tetramers, as well as collagen- and hydrophobic-tailed hetero-oligomers: the collagen-tailed asymmetric form of AChE is expressed at the NMJ, especially in fast-twitch muscles, and contains a specific collagen subunit, ColQ, which strongly anchors the enzyme to synaptic basal lamina and, through perlecan and dystroglycan, to muscle cell subsarcolemmal cytoskeletal scaffold, so that catalytic subunits can be released only by collagenase treatment (Cartaud et al., 2004, Krejci et al., 1997); in the CNS the predominant form is a tetramer that can be solubilized by detergents (Gennari et al., 1987) and is covalently linked by disulfide bonds to a 20 kDa hydrophobic subunit, PRiMA, which anchors AChE to cell membranes (Perrier et al., 2002).

At the NMJ, AChE is located in the synaptic cleft where it hydrolyzes ACh molecules released by the nerve terminal, ending acetylcholine receptor (AChR) activation (Van der Kloot and Molgó, 1994): because of this important function, AChE inhibition is the target of potent neurotoxic agents including anaesthetic drugs, insecticides, snake venoms and chemical weapons. Autoimmune myasthenia gravis (MG) (Vincent et al., 2000) and congenital myasthenic syndromes (Engel and Sine, 2005) are characterized by impaired neuromuscular transmission. In MG, anti-AChR autoantibodies (abs) (Bartoccioni et al., 1980) induce AChR loss and reduce efficiency of neuromuscular transmission, resulting in fluctuating skeletal muscle weakness and fatigue. The treatment with AChE inhibitory drugs (AChEI), like neostigmine and pyridostigmine, increases ACh availability at the NMJ, with improved neuromuscular transmission and short-term relief of muscle weakness (Punga and Stålberg, 2009). AChE essential function in maintaining a normal neuromuscular transmission is reinforced by clinical and experimental data: patients with mutations in ColQ (Engel and Sine, 2005, Mihaylova et al., 2008), as well as AChE knockout mice (Mouisel et al., 2006), have a myasthenic syndrome with no resistance to fatigue because of prolonged AChR activation.

Anti-AChE abs are present in individuals with autoimmune diseases like systemic lupus erythematosus, rheumatoid arthritis and Graves' disease with thyroid autoantibodies (Weetman et al., 1988, Geen et al., 2004). Because of sequence homology in the C-terminal domain between AChE and thyroglobulin (Tg), it has been postulated that a cross reaction of anti-Tg abs with AChE of extraocular muscles can have a role in Graves' ophthalmopathy (Ludgate et al., 1986, Mappouras et al., 1995). Experimental data in rats show that anti-AChE abs can induce an autoimmune, complement-mediated preganglionic sympathectomy with long lasting ptosis (Brimijoin and Lennon, 1990, Tang et al., 1999) and permanent dysautonomia (Brimijoin et al., 1993). The presence and role of anti-AChE abs in MG patients have been poorly investigated: apart from two case reports in the '80s (Phillips et al., 1981, Livneh et al., 1988), there is only a paper examining a small group of MG patients (Mappouras et al., 1995).

In the present study we examined serum anti-AChE immunoreactivity in 240 individuals with MG, evaluating the correlation with clinical and immunological parameters.

Section snippets

Clinical data

The study included 240 myasthenic patients followed in our Institution since 1982; all subjects gave informed consent to be included in the study. The diagnosis of MG was based on the association of clinical signs of fluctuating weakness with fatigability and at least two of the following criteria: unequivocal improvement after i.v. edrophonium/i.m. neostigmine injection, electrophysiological signs of abnormal neuromuscular transmission (either a decrement greater than 11% in compound muscle

Results

We tested 240 MG patients for anti-AChE immunoreactivity. We fixed the cut off for positivity at 0.211 O.D., which was the mean optical density value + 2 standard deviations (SD) obtained from 80 sera of sex and age matched healthy blood donors (HBD). Anti-AChE abs were positive in 115 (48%) of 240 patients tested. Table 2 shows the anti-AChE immunoreactivity in relation to clinical features of patients, disease severity and anti-AChR/anti-MuSK immunoreactivity. As shown in the table, we

Discussion

A normal neuromuscular transmission needs the presence of post-synaptic AChRs which are activated by the pre-synaptically released ACh; a reduction in released ACh (as observed in Eaton–Lambert syndrome) or in AChR (as seen in MG) produces weakness and fatigability. A normal AChE function is necessary for neuromuscular transmission as well, as any interference with its activity (by neurotoxic agents, or because of genetically determined deficiency) can induce muscle paralysis through prolonged

Acknowledgements

This work was supported by MIUR grants to C.P., A.E. and E.B.

References (34)

  • E. Bartoccioni et al.

    Myasthenia gravis, thymectomy, and antiacetycholine receptor antibody

    J. Neurol.

    (1980)
  • S. Brimijoin et al.

    Autoimmune preganglionic sympathectomy induced by acetylcholinesterase antibodies

    Proc. Natl. Acad. Sci. USA

    (1990)
  • A. Cartaud et al.

    MuSK is required for anchoring acetylcholinesterase at the neuromuscular junction

    J. Cell. Biol.

    (2004)
  • A. Evoli et al.

    Ocular myasthenia: diagnostic and therapeutic problems

    Acta Neurol. Scand.

    (1988)
  • A. Evoli et al.

    Response to therapy in myasthenia gravis with anti-MuSK antibodies

    Ann. N.Y. Acad. Sci.

    (2008)
  • J. Geen et al.

    The prevalence of anti-acetylcholinesterase antibodies in autoimmune disease

    Autoimmunity

    (2004)
  • K. Gennari et al.

    Tetrameric detergent-soluble acetylcholinesterase from human caudate nucleus: subunit composition and number of active sites

    J. Neurochem.

    (1987)
  • Cited by (0)

    1

    These authors contributed equally to the present paper.

    View full text