A mechanism in agrin signaling revealed by a prevalent Rapsyn mutation in congenital myasthenic syndrome

Abstract

Neuromuscular junction is a synapse between motoneurons and skeletal muscles, where acetylcholine receptors (AChRs) are concentrated to control muscle contraction. Studies of this synapse have contributed to our understanding of synapse assembly and pathological mechanisms of neuromuscular disorders. Nevertheless, underlying mechanisms of NMJ formation was not well understood. To this end, we took a novel approach – studying mutant genes implicated in congenital myasthenic syndrome (CMS). We showed that knock-in mice carrying N88K, a prevalent CMS mutation of Rapsyn (Rapsn), died soon after birth with profound NMJ deficits. Rapsn is an adapter protein that bridges AChRs to the cytoskeleton and possesses E3 ligase activity. In investigating how N88K impairs the NMJ, we uncovered a novel signaling pathway by which Agrin-LRP4-MuSK induces tyrosine phosphorylation of Rapsn, which is required for its self-association and E3 ligase activity. Our results also provide insight into pathological mechanisms of CMS.

APC2CDH1negatively regulates agrin signaling by promoting the ubiquitination and proteolytic degradation of DOK7

“…50 Recent studies also revealed that the scaffold protein Rapsyn has a RING domain with E3 ligase activity, and it catalyzes the neddylation of AChRs which is critical for AChRs clustering. 68,69 Although the APC/C E3 ligase is known as an anaphase-promoting complex that regulates cell cycle transition, 27,29 it is also expressed and plays a role in postmitotic cells. 30,33,34,58,70 Although APC2 is reported to regulate invertebrate neuromuscular synapse size, 30 vertebrate NMJs are cholinergic and regulated by different molecular pathways.…”

Myasthenia Gravis With Antibodies Against Muscle Specific Kinase: An Update on Clinical Features, Pathophysiology and Treatment

“…Following MuSK-DOK7 activation, several other adaptor proteins are phosphorylated and recruited, in particular Crk and Crk-L (Hallock et al, 2010 ), as well as other kinases, such as Abl (Finn et al, 2003 ), Scr (Mittaud et al, 2001 ), and the GTPases Rac1 and Rho which induce cytoskeleton modifications contributing to the maturation of AChR clusters (Weston et al, 2003 ; Bai et al, 2018 ). The whole cascade converges into the phosphorylation of the AChR subunits and rapsyn; rapsyn is a structural protein that self-aggregates providing a scaffold that anchors AChRs with the actin cytoskeleton—through the mediation of microtubule actin cross-linking factor 1 (MACF1)—to form the mature clusters (Borges and Ferns, 2001 ; Lee et al, 2009 ; Zuber and Unwin, 2013 ; Oury et al, 2019 ; Xing et al, 2019 ; Figure 1C ).…”

Dissecting the Extracellular Complexity of Neuromuscular Junction Organizers

“…While it is clear that Rapsyn is activated after MuSK dimerization, the underlying mechanisms remain poorly understood. A very recent report indicates that agrin:LRP4-mediated MuSK activation induces tyrosine phosphorylation of Rapsyn, which is required for its self-association and E3 ligase activity (Xing et al, 2019). This reveals an intriguing scenario where the initial trigger of the signaling cascade (MuSK) directly induces a multitude of cellular events, including the ultimate activation of the protein that clusters AChRs on the post-synaptic membrane.…”