Muscle-specific kinase myasthenia gravis IgG4 autoantibodies cause severe neuromuscular junction dysfunction in mice.

Abstract

Myasthenia gravis is a paralytic disorder with autoantibodies against acetylcholine receptors at the neuromuscular junction. A proportion of patients instead has antibodies against muscle-specific kinase, a protein essential for acetylcholine receptor clustering. These are generally of the immunoglobulin-G4 subclass and correlate with disease severity, suggesting specific myasthenogenic activity. However, immunoglobulin-G4 subclass antibodies are generally considered to be 'benign' and direct proof for their pathogenicity in muscle-specific kinase myasthenia gravis (or other immunoglobulin-G4-associated disorders) is lacking. Furthermore, the exact electrophysiological synaptic defects caused at neuromuscular junctions by human anti-muscle-specific kinase autoantibodies are hitherto unknown. We show that purified immunoglobulin-G4, but not immunoglobulin-G1-3, from patients with muscle-specific kinase myasthenia gravis binds to mouse neuromuscular junctions in vitro, and that injection into immunodeficient mice causes paralysis. Injected immunoglobulin-G4 caused reduced density and fragmented area of neuromuscular junction acetylcholine receptors. Detailed electrophysiological synaptic analyses revealed severe reduction of postsynaptic acetylcholine sensitivity, and exaggerated depression of presynaptic acetylcholine release during high-rate activity, together causing the (fatigable) muscle weakness. Intriguingly, compensatory transmitter release upregulation, which is the normal homeostatic response in acetylcholine receptor myasthenia gravis, was absent. This conveys extra vulnerability to neurotransmission at muscle-specific kinase myasthenia gravis neuromuscular junctions. Thus, we demonstrate that patient anti-muscle-specific kinase immunoglobulin-G4 is myasthenogenic, independent of additional immune system components, and have elucidated the underlying electrophysiological neuromuscular junction abnormalities.

• Authors: Rinse Klooster, Jaap J Plomp, Maartje G Huijbers, Erik H Niks, Kirsten R Straasheijm, Frank J Detmers, Pim W Hermans, Kevin Sleijpen, Aad Verrips, Mario Losen, Pilar Martinez-Martinez, Marc H De Baets, Silvère M van der Maarel, Jan J Verschuuren
• Journal: Brain : a journal of neurology (Brain) Vol. 135 Issue Pt 4 Pg. 1081-101 (Apr 2012) ISSN: 1460-2156 [Electronic] England
• PMID: 22396395 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Autoantibodies
• Immunoglobulin G
• Receptors, Cholinergic
• MUSK protein, human
• Receptor Protein-Tyrosine Kinases

Topics

• Action Potentials (drug effects)
• Adult
• Animals
• Autoantibodies (blood)
• Disease Models, Animal
• Electromyography
• Female
• Humans
• Immunoglobulin G (adverse effects, blood)
• Male
• Mice
• Mice, Inbred C57BL
• Mice, SCID
• Microscopy, Electron, Transmission
• Middle Aged
• Motor Endplate (drug effects, physiopathology)
• Muscle Contraction (drug effects)
• Muscle Strength (drug effects, physiology)
• Myasthenia Gravis (blood, complications, immunology, therapy)
• Neural Conduction (drug effects, physiology)
• Neuromuscular Junction (drug effects, pathology, physiopathology, ultrastructure)
• Neuromuscular Junction Diseases (complications, pathology)
• Plasmapheresis (methods)
• Receptor Protein-Tyrosine Kinases (immunology)
• Receptors, Cholinergic (immunology)
• Young Adult