Structural insights into the molecular mechanisms of myasthenia gravis and their therapeutic implications

Abstract

The nicotinic acetylcholine receptor (nAChR) is a major target of autoantibodies in myasthenia gravis (MG), an autoimmune disease that causes neuromuscular transmission dysfunction. Despite decades of research, the molecular mechanisms underlying MG have not been fully elucidated. Here, we present the crystal structure of the nAChR α1 subunit bound by the Fab fragment of mAb35, a reference monoclonal antibody that causes experimental MG and competes with ~65% of antibodies from MG patients. Our structures reveal for the first time the detailed molecular interactions between MG antibodies and a core region on nAChR α1. These structures suggest a major nAChR-binding mechanism shared by a large number of MG antibodies and the possibility to treat MG by blocking this binding mechanism. Structure-based modeling also provides insights into antibody-mediated nAChR cross-linking known to cause receptor degradation. Our studies establish a structural basis for further mechanistic studies and therapeutic development of MG.DOI: http://dx.doi.org/10.7554/eLife.23043.001

Receptor autoimmunity: diagnostic and therapeutic implications

“…Muscle-type nicotine AChR, neurotransmitter member of the ligand-gated ion channels family, is a pentameric molecule located in the in the middle of the post-synaptic membrane, with 5 subunits (α1 2 βδ and ε) in the adult muscle [36,37]: the acetylcholine binds the receptor at the interface of the α-δ and the α-ε subunits. In the α1 subunit, between the amino acids 67-70 is located the main immunogenic region that plays an important role on the pathogenesis of MG [38].…”

Antibody‐induced crosslinking and cholesterol‐sensitive, anomalous diffusion of nicotinic acetylcholine receptors

“…A single IgG monoclonal antibody molecule can crosslink two nAChR molecules (Tzartos and Starzinski‐Powitz ). Recent X‐ray structural studies have confirmed that a single mAb35 molecule is able to bind two α1 subunits from two adjacent monomers – thereby crosslinking them – whereas it does not bind to the two α1 subunits within a single nAChR monomer (Noridomi et al ). Moreover, antibodies directed against the MIR of the receptor have been reported to form oligomeric complexes larger than the dimer (Conti‐Tronconi et al ; Tzartos et al ).…”

Structure-Based Approaches to Antigen-Specific Therapy of Myasthenia Gravis

“…Given their established myasthenogenic role, extensive efforts have been put into characterizing the interactions between MG autoantibodies and nAChR using biochemical [45,46,[48][49][50][51][52][53], structural [22,[54][55][56], and modeling approaches [57]. More recently, the first crystal structures of human (pdb code: 5HBT) and mouse (pdb code: 5HBV) nAChR ECD bound by the Fab fragment of an EAMG autoantibody, Fab35 were determined [58]. Both crystal structures are very similar, so the discussion here will focus mainly on the human complex (pdb code: 5HBT).…”

“…As mentioned above, nAChR α1 is just one subunit of the nAChR pentamer and is intrinsically unstable, making the expression of wild type nAChR α1 ECD in stable soluble form very challenging. However, as discussed earlier in this chapter, crystallography studies of nAChR α1 ECD in recent years have accumulated extensive experience and knowledge in designing strategic mutations to improve the stability and expression level of nAChR α1 ECD protein while preserve the binding of MIR-directed MG autoantibodies [22,31,58] These progresses will greatly facilitate the approach to using engineered antigen chimera to specially inhibit and eliminate nAChR-specific B cells for MG treatment.…”