A, Representative images showing the attenuation of AChR endocytosis by agrin-MuSK-rapsyn signaling. Muscle cells were either treated with agrin or distinct forms of GFP-MuSK or GFP-Rapsyn (full-length: FL; truncated form: TR) were overexpressed in muscle cells. For clarity, the boxed regions were magnified twofold and are shown in the insets. B, Quantitative analysis of the number of BBQ-labeled AChR vesicles formed in response to the activation of agrin signaling or the overexpression of its downstream molecules MuSK and rapsyn. Data are mean ± SEM; n = 172, control; 151, agrin; 97, GFP-MuSK FL; 101, GFP-MuSK TR; 65, GFP-Rapsyn FL; 98, GFP-Rapsyn TR; *p<0.001 (Student’s t test).