Muscle‐Specific Kinase Signaling and Its Therapeutic Potential

The function of the neuromuscular junction (NMJ) is compromised in many neuromuscular diseases (NMDs) such as autoimmune or congenital myasthenia gravis (MG), amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), and muscular dystrophies. The NMJ contains muscle-specific kinase (MuSK), which is a critical regulator of NMJ integrity and function. Activating the MuSK signaling cascade may have therapeutic potential in several of these NMDs that are characterized by impaired neuromuscular communication. The MuSK signaling cascade consists of different components and can be activated with interventions at different levels. In the past years, different therapeutic strategies using an engineered recombinant agrin comprised of the C-terminal fragment of the protein (mini-agrin), gene therapy of key proteins in this pathway, agonist MuSK antibodies, and SRC homology 2 domain-containing phosphotyrosine phosphatase 2 (SHP2) inhibitors have been further developed for this purpose. Each of these strategies engages distinct signaling components: mini-agrin, both as recombinant protein and gene therapy, enhances agrin-Lrp4-MuSK interaction; Dok7 gene therapy amplifies MuSK phosphorylation; Lrp4 gene therapy enhances agrin responsiveness; MuSK agonist antibodies bypass upstream defects and promote downstream signaling; SHP2 inhibitors prolong the duration of active MuSK signaling. These therapeutic strategies have ameliorated NMJ integrity and function in several preclinical models of MG, motor neuron diseases, and muscular dystrophies. In this review, we highlight MuSK signaling as a possible therapeutic target, describe the therapeutic efficacy of intervention in MuSK signaling in different NMDs, and present an outlook on future clinical development.