Functional Analysis of small G protein during cardiac early morphogenesis

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To understand the mechanism of normal myofibrillogenesis in vivo, we have reported 3D observation of myofibril formation, cell-cell and cell-matrix adhesions, and tyrosine phosphorylation during looping of whole-mounted chicken embryonic heart. We found that formation and arrangement of myofibrils develops in close association with changing cell-cell and cell-matrix adhesions. In this study, we performed 3D observation of focal adhesion kinase (FAK) and paxillin immunolocalization before and during looping (4- to 15-somite stages) in the same manner. Before myofibrillogenesis (4 to 7-somite stages), faint and particulate pattern of FAK and paxillin was randomly expressed at the bottom of the inner myocardial cell layer facing the cardiac jelly. During initial stages of myofibrillogenesis (8 to 9-somite stages), fibrillar pattern of FAK and paxillin appeared at termini of stress fiber-like thick actin bundles which were recognized as an initial stage of myofibrillogensis. During active stages of myofibrillogenesis (10-13-somite stages), paxillin was also localized at Z-bands of immature striated myofibrils, while FAK was not co-localized with Z-bands. At the 14-15 somite stages or later, the expression of FAK and paxillin diminished especially at sites of mature striated myofibrils. These results suggests that paxillin is involved in the initial arrangement and sarcomere formation of cardiac myofibrils, while FAK is involved in the initial arrangement of myofibrils.