Anti-MuSK autoantibodies block binding of collagen Q to MuSK

• Published in: Neurology Vol. 77; no. 20; pp. 1819 - 1826
• Main Authors: KAWAKAMI, Y, ITO, M, HIRAYAMA, M, SAHASHI, K, OHKAWARA, B, MASUDA, A, NISHIDA, H, MABUCHI, N, ENGEL, A. G, OHNO, K
• Format: Journal Article
• Language: English
• Published: Hagerstown, MD Lippincott Williams & Wilkins 15-11-2011
• Subjects: Acetylcholinesterase - metabolism; Animals; Antibodies, Blocking - pharmacology; Autoantibodies - pharmacology; Binding Sites, Antibody; Biological and medical sciences; Collagen - antagonists & inhibitors; Collagen - metabolism; Medical sciences; Mice; Mice, Knockout; Muscle Proteins - antagonists & inhibitors; Muscle Proteins - metabolism; Myasthenia Gravis, Autoimmune, Experimental - chemically induced; Myasthenia Gravis, Autoimmune, Experimental - immunology; Myasthenia Gravis, Autoimmune, Experimental - metabolism; Neurology; Pneumology; Receptor Protein-Tyrosine Kinases - antagonists & inhibitors; Receptor Protein-Tyrosine Kinases - immunology; Receptor Protein-Tyrosine Kinases - metabolism; Receptors, Cholinergic - immunology; Receptors, Cholinergic - metabolism; Tumors of the respiratory system and mediastinum; Autoimmunity; Autoantibody; Nervous system diseases; Collagen; Glycoprotein
• ISSN: 0028-3878; 1526-632X
• DOI: 10.1212/wnl.0b013e318237f660

Muscle-specific receptor tyrosine kinase (MuSK) antibody-positive myasthenia gravis (MG) accounts for 5%-15% of autoimmune MG. MuSK mediates the agrin-signaling pathway and also anchors the collagenic tail subunit (ColQ) of acetylcholinesterase (AChE). The exact molecular target of MuSK-immunoglobulin G (IgG), however, remains elusive. As acetylcholine receptor (AChR) deficiency is typically mild and as cholinesterase inhibitors are generally ineffective, we asked if MuSK-IgG interferes with binding of ColQ to MuSK.
We used 3 assays: in vitro overlay of the human ColQ-tailed AChE to muscle sections of Colq-/- mice; in vitro plate-binding assay to quantitate binding of MuSK to ColQ and to LRP4; and passive transfer of MuSK-IgG to mice.
The in vitro overlay assay revealed that MuSK-IgG blocks binding of ColQ to the neuromuscular junction. The in vitro plate-binding assay showed that MuSK-IgG exerts a dose-dependent block of MuSK binding to ColQ by but not to LRP4. Passive transfer of MuSK-IgG to mice reduced the size and density of ColQ to ∼10% of controls and had a lesser effect on the size and density of AChR and MuSK.
As lack of ColQ compromises agrin-mediated AChR clustering in Colq-/- mice, a similar mechanism may lead to AChR deficiency in MuSK-MG patients. Our experiments also predict partial AChE deficiency in MuSK-MG patients, but AChE is not reduced in biopsied NMJs. In humans, binding of ColQ to MuSK may be dispensable for clustering ColQ, but is required for facilitating AChR clustering. Further studies will be required to elucidate the basis of this paradox.