Xenopus muscle‐specific kinase: molecular cloning and prominent expression in neural tissues during early embryonic development

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Abstract
A muscle‐specific receptor tyrosine kinase, designated MuSK, mediates agrin‐induced aggregation of acetylcholine receptors at the vertebrate neuromuscular junction. cDNAs encoding Xenopus MuSK were isolated from embryonic cDNA libraries. The full‐length MuSK cDNA encodes for a polypeptide of 948 amino acids and possesses the features unique to mammalian MuSK, including four Ig‐like domains, C6 box, transmembrane region and an intracellular tyrosine kinase domain. Interestingly, Xenopus MuSK also contains a kringle domain similar to that previously reported for Torpedo MuSK. The overall amino acid sequence identity of Xenopus MuSK with mammalian MuSK is ≈ 65%. Northern blot analysis demonstrated the presence of three MuSK transcripts (≈ 1 kb, ≈ 3 kb and ≈ 7 kb) which were differentially expressed during development. The expression of the ≈ 7 kb MuSK transcript remained as the predominant species in adult tissues, e.g. skeletal muscle, spleen and lung. Immunocytochemical analysis with a MuSK‐specific antibody revealed that Xenopus MuSK was colocalized with AChRs at neuromuscular junctions as well as in spontaneous acetylcholine receptor hot spots of cultured muscle cells. In situ hybridization revealed prominent expression of MuSK transcripts in neural tissues and myotomal muscle during the period of neurulation and synaptogenesis. The MuSK transcript detected at abundant levels in the central nervous system (CNS) was localized to the brain, spinal cord and eye vesicles during early embryonic development. In addition, the MuSK protein in the developing eye was found to be prominently expressed during embryonic stages of 32 and 35. These findings raise an intriguing possibility that, in addition to the known function in the formation of the neuromuscular junctions, MuSK may be involved in neural development.