Overexpression of Rapsyn in Rat Muscle Increases Acetylcholine Receptor Levels in Chronic Experimental Autoimmune Myasthenia Gravis

Pilar Martínez-Martínez^*, Mario Losen^*, Hans Duimel, Peter Frederik, Frank Spaans, Peter Molenaar^¶, Angela Vincent^|| and Marc H. De Baets^*^,**

From the Department of Neurology,^* Research Institute Brain and Behaviour, and the Department of Pathology, Electron Microscopy Unit, University of Maastricht, Maastricht, The Netherlands; European Graduate School of Neuroscience, Maastricht, The Netherlands; the Departments of Clinical Neurophysiology and Neurology,^*^* Maastricht University Hospital, Maastricht, The Netherlands; the Department of Molecular Cell Biology,^¶ Neurophysiology Group, Leiden University Medical Centre, Leiden, The Netherlands; and the Neurosciences Group,|| Weatherall Institute of Molecular Medicine, The John Radcliffe Hospital, Oxford, United Kingdom

The primary autoantigen in myasthenia gravis, the acetylcholinereceptor (AChR), is clustered and anchored in the postsynapticmembrane of the neuromuscular junction by rapsyn. Previously,we found that overexpression of rapsyn by cDNA transfectionprotects AChRs in rat muscles from antibody-mediated loss inpassive transfer experimental autoimmune myasthenia gravis (EAMG).Here, we determined whether rapsyn overexpression can reduceor even reverse AChR loss in muscles that are already damagedby chronic EAMG, which mimics the human disease. Active immunizationagainst purified AChR was performed in female Lewis rats. Rapsynoverexpression resulted in an increase in total muscle membraneAChR levels, with some AChR at neuromuscular junctions but muchof it in extrasynaptic membrane regions. At the ultrastructurallevel, most endplates in rapsyn-treated chronic EAMG musclesshowed increased damage to the postsynaptic membrane. Althoughrapsyn overexpression stabilized AChRs in intact or mildly damagedendplates, the rapsyn-induced increase of membrane AChR enhancedautoantibody binding and membrane damage in severe ongoing disease.Thus, these results show the complexity of synaptic stabilizationof AChR during the autoantibody attack. They also indicate thatthe expression of receptor-associated proteins may determinethe severity of autoimmune diseases caused by anti-receptorantibodies.