Wnt proteins regulate acetylcholine receptor clustering in muscle cells

In the study we systematically studied the effect of all 19 different Wnts in mammals on acetylcholine receptor (AChR) cluster formation. We identified five Wnts (Wnt9a, Wnt9b, Wnt10b, Wnt11, and Wnt16) that are able to stimulate AChR clustering, of which Wnt9a and Wnt11 are expressed abundantly in developing muscles. Using Wnt9a and Wnt11 as example, we demonstrated that Wnt induction of AChR clusters was dose-dependent and non-additive to that of agrin, suggesting that Wnts may act via similar pathways to induce AChR clusters. We provide evidence that Wnt9a and Wnt11 bind directly to the extracellular domain of MuSK, to induce MuSK dimerization and subsequent tyrosine phosphorylation of the kinase. In addition, Wnt-induced AChR clustering requires LRP4.These results identify Wnts as new players in AChR cluster formation, which act in a manner that requires both MuSK and LRP4, revealing a novel function of LRP4.The neuromuscular junction (NMJ) is a cholinergic synapse that exhibits a high degree of subcellular specialization characteristic of chemical synapses [1,2]. Its formation is regulated by factors from motoneurons. For example, neural agrin binds LRP4, a member of the low-density lipoprotein receptor (LDLR) family, and subsequently activates the tyrosine kinase MuSK [3-7], leading to the clustering of AChR through mediator proteins including cytoskeletal protein α-actinin [2,8]. Interestingly, muscle fiber prepatterning or aneural AChR cluster formation in the advance of innervation requires MuSK and LRP4 but not agrin, whereas nerve-induced AChR clusters require all [5,7,9]. These observations suggest that MuSK may be regulated by agrin-independent, yet unidentified ligand(s).Wnt is a family of secreted glycoproteins that play a critical role in development [10]. Wnt signals through a receptor complex consisting of Frizzled (Fz) receptor and LRP5/6 [11]. Fz interacts the adapter protein dishevelled (Dvl) to activate intracellular canonical and non-canonical