Key points Adeno‐associated viral vector was used to elevate the expression of muscle specific kinase (MuSK) and rapsyn (a cytoplasmic MuSK effector protein) in the tibialis anterior muscle of wild‐type and dystrophic (mdx) mice. In mdx mice, enhanced expression of either MuSK or rapsyn ameliorated the acute loss of muscle force associated with strain injury. Increases in sarcolemmal immunolabelling for utrophin and β‐dystroglycan suggest a mechanism for the protective effect of MuSK in mdx muscles. MuSK also caused subtle changes to the structure and function of the neuromuscular junction, suggesting novel roles for MuSK in muscle physiology and pathophysiology. Abstract Muscle specific kinase (MuSK) has a well‐defined role in stabilizing the developing mammalian neuromuscular junction, but MuSK might also be protective in some neuromuscular diseases. In the dystrophin‐deficient mdx mouse model of Duchenne muscular dystrophy, limb muscles are especially fragile. We injected the tibialis anterior muscle of 8‐week‐old mdx and wild‐type (C57BL10) mice with adeno‐associated viral vectors encoding either MuSK or rapsyn (a cytoplasmic MuSK effector protein) fused to green fluorescent protein (MuSK‐GFP and rapsyn‐GFP, respectively). Contralateral muscles injected with empty vector served as controls. One month later mice were anaesthetized with isoflurane and isometric force‐producing capacity was recorded from the distal tendon. MuSK‐GFP caused an unexpected decay in nerve‐evoked tetanic force, both in wild‐type and mdx muscles, without affecting contraction elicited by direct electrical stimulation of the muscle. Muscle fragility was probed by challenging muscles with a strain injury protocol consisting of a series of four strain‐producing eccentric contractions in vivo. When applied to muscles of mdx mice, eccentric contraction produced an acute 27% reduction in directly evoked muscle force output, affirming the susceptibility of mdx muscles to strain injury. mdx muscles overexpressing MuSK‐GFP or rapsyn‐GFP exhibited significantly milder force deficits after the eccentric contraction challenge (15% and 14%, respectively). The protective effect of MuSK‐GFP in muscles of mdx mice was associated with increased immunolabelling for utrophin and β‐dystroglycan in the sarcolemma. Elevating the expression of MuSK or rapsyn revealed several distinct synaptic and extrasynaptic effects, suggesting novel roles for MuSK signalling in muscle physiology and pathophysiology.
Myasthenia gravis is a muscle weakness disease characterized by autoantibodies that target components of the neuromuscular junction, impairing synaptic transmission. The most common form of myasthenia gravis involves antibodies that bind the nicotinic acetylcholine receptors in the postsynaptic membrane. Many of the remaining cases are due to antibodies against muscle specific tyrosine kinase (MuSK). Recently, autoantibodies against LRP4 (another component of the MuSK signaling complex in the postsynaptic membrane) were identified as the likely cause of myasthenia gravis in some patients. Fatiguing weakness is the common symptom in all forms of myasthenia gravis, but muscles of the body are differentially affected, for reasons that are not fully understood. Much of what we have learnt about the immunological and neurobiological aspects of the pathogenesis derives from mouse models. The most widely used mouse models involve either passive transfer of autoantibodies, or active immunization of the mouse with acetylcholine receptors or MuSK protein. These models can provide a robust replication of many of the features of the human disease. Depending upon the protocol, acute fatiguing weakness develops 2 - 14 days after the start of autoantibody injections (passive transfer) or might require repeated immunizations over several weeks (active models). Here we review mouse models of myasthenia gravis, including what they have contributed to current understanding of the pathogenic mechanisms and their current application to the testing of therapeutics.
In the mdx mouse model of Duchenne muscular dystrophy, muscle fibres are fragile and prone to injury and degeneration. Compared to wild-type mice, muscles of mdx mice also develop less specific force (contractile force/cross-sectional area). We recently reported that injecting adeno-associated viral vector encoding muscle specific kinase (AAV-MuSK) into muscles of mdx mice increased utrophin expression and made the muscles more resistant to acute stretch-induced injury. Here we injected AAV-MuSK unilaterally into the tibialis anterior muscle of mdx mice at a younger age (4 weeks), and recorded contraction force from the muscles in situ at 12 weeks of age. Compared to contralateral empty-vector control muscles, muscles injected with AAV-MuSK produced 28% greater specific force (P = 0.0005). They did not undergo the compensatory hypertrophy that normally occurs in muscles of mdx mice. Injection of AAV encoding rapsyn (a downstream effector of MuSK signalling) caused no such improvement in muscle strength. Muscles injected with AAV-MuSK displayed a 10% reduction in the number of fibres with centralized nuclei (P = 0.0015). Our results in mdx mice suggest that elevating the expression of MuSK can reduce the incidence of muscle fibre regeneration and improve the strength of dystrophin-deficient muscles.
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