However, this PTB-dependent stable complex formation of Dok-7 with MuSK is
not prerequisite for Dok-7-mediated activation of MuSK in the heterologous cells or even in cultured C2C12 myoblasts. Interestingly, in addition to the PTB domain, the entire COOH-terminal region, but not PH domain, is also dispensable for MuSK activation in these cells. However, when myotubes were fully differentiated from C2C12 myoblasts, both the PTB domain and the COOH-terminal region were indispensable for MuSK activation and subsequent AChR clustering. The data suggests that a negative regulatory mechanism preventing MuSK activation is established Inhibitors,research,lifescience,medical upon differentiation from myoblasts into myotubes. Note that C2C12 myotube differentiation is accompanied by increasing expression Inhibitors,research,lifescience,medical of MuSK and Dok-7 (14). To counteract the hypothetical negative regulation, Dok-7 may need to be stably complexed with MuSK via the PTB domain and may also need an as yet unidentified function of the COOH-terminal moiety. For example, trace phosphorylation of MuSK in myotubes might allow physical interaction with Dok-7, in turn facilitating dimerization and/or conformational changes in MuSK that are necessary for its sustained activation in myotubes. It has been reported that an Inhibitors,research,lifescience,medical adaptor protein SH2-B, which has the PH and SH2 domains, binds via the SH2 domain to multiple receptor PTKs including insulin receptor (IR) and NGF receptor (TrkA).
In addition, the forced expression of IR and SH2-B in CHO cells enhanced IR-mediated signaling upon stimulation with insulin; however, it did not affect IR activity in the absence of insulin (18). Inhibitors,research,lifescience,medical Similarly, the forced expression of SH2-B in PC12 cells enhanced TrkA-mediated signaling upon NGF treatment, but again it did not affect TrkA activity in the absence of NGF (19). By contrast, Dok-7 does not ON-01910 nmr require Agrin to activate MuSK in myotubes, and furthermore, Dok-7 does not require the PTB domain, which is essential for stable binding with MuSK, to activate MuSK in 293T cells (14). Given that
Agrin requires Dok-7 Inhibitors,research,lifescience,medical to activate MuSK at least in cultured myotubes, Dok-7 appears to be a cytoplasmic activator of MuSK rather than a signal enhancer of it. Increase of expression of both MuSK and Dok-7 upon differentiation of myoblasts into myotubes may trigger the Dok-7-mediated activation heptaminol of MuSK in the central region of the developing skeletal muscle, where preferential expression of AChR, MuSK and Dok-7 together with aneural, Agrin-independent AChR clustering are observed (14). Then, Agrin and Dok-7 may cooperate to induce full activation of MuSK to orchestrate NMJ formation. Since patients with NMJ disorders due to genetic mutations of DOK7 (see below) often only present with symptoms at least 18 months after birth, it suggests that Dok-7-mediated activation is essential not only for NMJ formation but also for its maintenance (20–22). This seems to be consistent with the postsynaptic localization of Dok-7 at fully formed NMJ in adult mice.