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Muscle-specific kinase (MuSK) is essential for the formation, function, and preservation of neuromuscular synapses. Activation of MuSK by a MuSK agonist antibody may stabilize or improve the function of the neuromuscular junction (NMJ) in patients with disorders of the NMJ, such as congenital myasthenia (CM). Here, we generated and characterized ARGX-119, a first-in-class humanized agonist monoclonal antibody specific for MuSK, that is being developed for treatment of patients with neuromuscular diseases. We performed in vitro ligand-binding assays to show that ARGX-119 binds with high affinity to the Frizzled-like domain of human, nonhuman primate, rat, and mouse MuSK, without off-target binding, making it suitable for clinical development. Within the Fc region, ARGX-119 harbors L234A and L235A mutations to diminish potential immune-activating effector functions. Its mode of action is to activate MuSK, without interfering with its natural ligand neural Agrin, and cluster acetylcholine receptors in a dose-dependent manner, thereby stabilizing neuromuscular function. In a mouse model of DOK7 CM, ARGX-119 prevented early postnatal lethality and reversed disease relapse in adult Dok7 CM mice by restoring neuromuscular function and reducing muscle weakness and fatigability in a dose-dependent manner. Pharmacokinetic studies in nonhuman primates, rats, and mice revealed a nonlinear PK behavior of ARGX-119, indicative of target-mediated drug disposition and in vivo target engagement. On the basis of this proof-of-concept study, ARGX-119 has the potential to alleviate neuromuscular diseases hallmarked by impaired neuromuscular synaptic function, warranting further clinical development. Editor's summary: Muscle-specific kinase (MuSK) is required for the formation and maintenance of neuromuscular junctions, the synaptic connections between motor neurons and muscle fibers. Impairments in MuSK signaling cause certain forms of neuromuscular disease such as congenital myasthenic syndrome and myasthenia gravis. Here, Vanhauwaert et al. present the development of ARGX-119, an agonist antibody for MuSK. ARGX-119 activated MuSK in vitro across different species, including humans, and showed no off-target binding. ARGX-119 was safe in healthy mice and rescued motor function, prolonged survival, and reversed disease relapse in a mouse model of congenital myasthenia. These data support further development of ARGX-119 for neuromuscular diseases. —Daniela Neuhofer [ABSTRACT FROM AUTHOR]

In the Guillain-Barre syndrome subform acute motor axonal neuropathy (AMAN), Campylobacter jejuni enteritis triggers the production of anti-ganglioside Abs (AGAbs), leading to immune-mediated injury of distal motor nerves. An important question has been whether injury to the presynaptic neuron at the neuromuscular junction is a major factor in AMAN. Although disease modeling in mice exposed to AGAbs indicates that complement-mediated necrosis occurs extensively in the presynaptic axons, evidence in humans is more limited, in comparison to the extensive injury seen at nodes of Ranvier. We considered that rapid AGAb uptake at the motor nerve terminal membrane might attenuate complement-mediated injury. We found that PC12 rat neuronal cells rapidly internalized AGAb, which were trafficked to recycling endosomes and lysosomes. Consequently, complement-mediated cytotoxicity was attenuated. Importantly, we observed the same AGAb endocytosis and protection from cytotoxicity in live mouse nerve terminals. AGAb uptake was attenuated following membrane cholesterol depletion in vitro and ex vivo, indicating that this process may be dependent upon cholesterol-enriched microdomains. In contrast, we observed minimal AGAb uptake at nodes of Ranvier, and this structure thus remained vulnerable to complement-mediated injury. These results indicate that differential endocytic processing of AGAbs by different neuronal and glial membranes might be an important modulator of site-specific injury in acute AGAb-mediated Guillain-Barre syndrome subforms and their chronic counterparts., Introduction The Guillain-Barre syndromes (GBSs) are acute, immune-mediated neuropathies affecting the peripheral nervous system (PNS), usually triggered by preceding infectious events including Campylobacter jejuni enteritis. In the acute motor axonal [...]

Anti-GM1 ganglioside autoantibodies are used as diagnostic markers for motor axonal peripheral neuropathies and are believed to be the primary mediators of such diseases. However, their ability to bind and exert pathogenic effects at neuronal membranes is highly inconsistent. Using human and mouse monoclonal anti-GM1 antibodies to probe the GM1-rich motor nerve terminal membrane in mice, we here show that the antigenic oligosaccharide of GM1 in the live plasma membrane is cryptic, hidden on surface domains that become buried for a proportion of anti-GM1 antibodies due to a masking effect of neighboring gangliosides. The cryptic GM1 binding domain was exposed by sialidase treatment that liberated sialic acid from masking gangliosides including GD1a or by disruption of the live membrane by freezing or fixation. This cryptic behavior was also recapitulated in solid-phase immunoassays. These data show that certain anti-GM1 antibodies exert potent complement activation-mediated neuropathogenic effects, including morphological damage at living terminal motor axons, leading to a block of synaptic transmission. This occurred only when GM1 was topologically available for antibody binding, but not when GM1 was cryptic. This revised understanding of the complexities in ganglioside membrane topology provides a mechanistic account for wide variations in the neuropathic potential of anti-GM1 antibodies., Introduction The sialic acid--containing glycosphingolipids known as gangliosides are concentrated in plasma membrane microdomains, where they modulate the topological organization and function of membrane proteins (1), (2). Their oligosaccharide head [...]