The formation of the neuromuscular junction (NMJ) is orchestrated by the muscle-specific receptor tyrosine kinase MuSK and by neural agrin, an extracellular activator of MuSK. We previously showed that the MuSK-interacting protein Dok-7 is essential for neuromuscular synaptogenesis, although the mechanisms by which Dok-7 regulates MuSK activity and promotes synapse formation have been unclear. Here, we show that Dok-7 directly interacts with the cytoplasmic portion of MuSK and activates the receptor tyrosine kinase, and that neural agrin requires Dok-7 to activate MuSK. In vivo overexpression of Dok-7 increased MuSK activation and promoted NMJ formation. Furthermore, Dok-7 was required for the localization of MuSK in the central region of muscle, which is essential for the correct formation of NMJs in this region. These observations indicate that Dok-7 positively regulates neuromuscular synaptogenesis by controlling MuSK activity, its distribution, and its responsiveness to neural agrin.

The quantum Hall effect is a macroscopic quantum phenomenon in a two-dimensional electron system. The two-dimensional electron system in SrTiO3 has sparked a great deal of interest, mainly because of the strong electron correlation effects expected from the 3d orbitals. Here we report the observation of the quantum Hall effect in a dilute La-doped SrTiO3-two-dimensional electron system, fabricated by metal organic molecular-beam epitaxy. The quantized Hall plateaus are found to be solely stemming from the low Landau levels with even integer-filling factors, ν=4 and 6 without any contribution from odd ν's. For ν=4, the corresponding plateau disappears on decreasing the carrier density. Such peculiar behaviours are proposed to be due to the crossing between the Landau levels originating from the two subbands composed of d orbitals with different effective masses. Our findings pave a way to explore unprecedented quantum phenomena in d-electron systems.

Ferroelectric polarization and metallic conduction are two seemingly irreconcilable properties that cannot normally coexist in a single system, as the latter tends to screen the former. Polar metals, however, defy this rule and have thus attracted considerable attention as a new class of ferroelectrics exhibiting novel properties. Here, we fabricate a new polar metal film based on the typical ferroelectric material BaTiO3by combining chemical doping and epitaxial strain induced by a substrate. The temperature dependences of the c-axis lattice constant and the second harmonic generation intensity of La-doped BaTiO3films indicate the existence of polar transitions. In addition, through La doping, films become metallic at the polar phase, and metallicity enhancement at the polar state occurs in low-La-doped films. This intriguing behaviour is effectively explained by our first-principles calculations. Our demonstration suggests that the carrier doping to ferroelectric material with epitaxial strain serves as a new way to explore polar metals.