Slow-channel transgenic mice: a model of postsynaptic organellar degeneration at the neuromuscular junction.

The slow-channel congenital myasthenic syndrome (SCCMS) is a dominantly inherited disorder of neuromuscular transmission characterized by delayed closure of the skeletal muscle acetylcholine receptor (AChR) ion channel and degeneration of the neuromuscular junction. The identification of a series of AChR subunit mutations in the SCCMS supports the hypothesis that the altered kinetics of the endplate currents in this disease are attributable to inherited abnormalities of the AChR. To investigate the role of these mutant AChR subunits in the development of the synaptic degeneration seen in the SCCMS, we have studied the properties of the AChR mutation, epsilonL269F, found in a family with SCCMS, using both in vitro and in vivo expression systems. The mutation causes a sixfold increase in the open time of AChRs expressed in vitro, similar to the phenotype of other reported mutants. Transgenic mice expressing this mutant develop a syndrome that is highly reminiscent of the SCCMS. Mice have fatigability of limb muscles, electrophysiological evidence of slow AChR ion channels, and defective neuromuscular transmission. Pathologically, the motor endplates show focal accumulation of calcium and striking ultrastructural changes, including enlargement and degeneration of the subsynaptic mitochondria and nuclei. These findings clearly demonstrate the role of this mutation in the spectrum of abnormalities associated with the SCCMS and point to the subsynaptic organelles as principal targets in this disease. These transgenic mice provide a useful model for the study of excitotoxic synaptic degeneration.

Desensitization of mutant acetylcholine receptors in transgenic mice reduces the amplitude of neuromuscular synaptic currents.

While the slow onset of desensitization of nicotinic acetylcholine receptors (AChRs), relative to the rate of acetylcholine removal, excludes this kinetic state from shaping synaptic responses in normal neuromuscular transmission, its role in neuromuscular disorders has not been examined. The slow-channel congenital myasthenic syndrome (SCCMS) is a disorder caused by point mutations in the AChR subunit-encoding genes leading to kinetically abnormal (slow) channels, reduced miniature endplate current amplitudes (MEPCs), and degeneration of the postsynaptic membrane. Because of this complicated picture of kinetic and structural change in the neuromuscular junction, it is difficult to assess the importance of the multiple factors that may be responsible for the reduced endplate current amplitudes, and ultimately the clinical syndrome. In order to address this we have used a transgenic mouse model for the SCCMS that has slow AChR ion channels and reduced endplate responsiveness in the absence of any of the degenerative changes. We found that the reduction in MEPC amplitudes in these mice could not be explained by either reduced AChR number or by reduced AChR channel conductance. Rather, we found that the mutant AChRs in situ manifested an activity-dependent reduction in sensitivity that caused diminished MEPC and endplate current amplitude with nerve stimulation. This observation demonstrates that the basis for the reduction in MEPC amplitudes in the SCCMS may be multifactorial. Moreover, these findings demonstrate that, under conditions that alter their rate of desensitization, the kinetic properties of nicotinic AChRs can control the strength of synaptic responses.