The transcription factor Sp1 plays a crucial role in dok-7 gene expression.

Dok-7 is a cytoplasmic activator of the muscle-specific receptor tyrosine kinase MuSK, and these two proteins are essential for neuromuscular junction (NMJ) formation. Mutations of the human DOK7 gene underlie a limb-girdle type of congenital myasthenic syndrome, a group of disorders characterized by NMJ synaptopathy. Because MuSK governs postsynaptic specialization of NMJs and controls where the NMJ forms in the skeletal muscle, it is crucial to appropriately regulate when and where Dok-7 is expressed to activate MuSK. However, the mechanisms underlying expression of the dok-7 gene remain unclear. Here, we show that two Sp1 consensus sequences in the mouse dok-7 5'-flanking region are necessary for dok-7 gene expression in muscle cells. We further demonstrate that the transcription factor Sp1 activates dok-7 gene expression through interaction with these two Sp1 sites. Taken together, these results indicate that Sp1 plays a crucial role in the regulation of the dok-7 gene.

Autoantibodies to low-density lipoprotein receptor-related protein 4 in myasthenia gravis.

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction, where acetylcholine receptor (AChR), muscle-specific kinase (MuSK), and low-density lipoprotein (LDL) receptor-related protein 4 (Lrp4) are essential. About 80% and 0% to 10% of patients with generalized MG have autoantibodies to AChR and MuSK, respectively, but pathogenic factors are elusive in others. Here we show that a proportion of AChR antibody-negative patients have autoantibodies to Lrp4. These antibodies inhibit binding of Lrp4 to its ligand and predominantly belong to the immunoglobulin G1 (IgG1) subclass, a complement activator. These findings together indicate the involvement of Lrp4 antibodies in the pathogenesis of AChR antibody-negative MG.

Mutations causing DOK7 congenital myasthenia ablate functional motifs in Dok-7.

Dok-7 is a cytoplasmic activator of muscle-specific receptor-tyrosine kinase (MuSK). Both Dok-7 and MuSK are required for neuromuscular synaptogenesis. Mutations in DOK7 underlie a congenital myasthenic syndrome (CMS) associated with small and simplified neuromuscular synapses likely due to impaired Dok-7/MuSK signaling. The overwhelming majority of patients with DOK7 CMS have at least one allele with a frameshift mutation that causes a truncation in the COOH-terminal region of Dok-7 and affects MuSK activation. Dok-7 has pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains in the NH2-terminal moiety, both of which are indispensable for MuSK activation in myotubes, but little is known about additional functional elements. Here, we identify a chromosome region maintenance 1-dependent nuclear export signal (NES) in the COOH-terminal moiety and demonstrate that the NES-mediated cytoplasmic location of Dok-7 is essential for regulating the interaction with MuSK in myotubes. The NH2-terminal PH domain is responsible for the nuclear import of Dok-7. We also show that the Src homology 2 target motifs in the COOH-terminal moiety of Dok-7 are active and crucial for MuSK activation in myotubes. In addition, CMS-associated missense mutations found in the PH or PTB domain inactivate Dok-7. Together, these findings demonstrate that, in addition to the NH2-terminal PH and PTB domains, the COOH-terminal NES and Src homology 2 target motifs play key roles in Dok-7/MuSK signaling for neuromuscular synaptogenesis. Ablation or disruption of these functional elements in Dok-7 probably underlies the neuromuscular junction synaptopathy observed in DOK7 CMS.