Myosins IIA and IIB have distinct biochemical properties, with IIB having a higher duty ratio, consistent with a role in maintaining cortical tension by cross-linking the cortical actin cytoskeleton (Kovacs et al., 2003;Rosenfeld et al., 2003;Wang et al., 2003). myosin IIB is required for resistance to deformation (stiffness) in the neural plate, indicating that the cytoskeleton-organizing roles of this protein translate in regulation of the biomechanical properties of the neural plate at the tissue-level. Keywords:Myosin IIB, Xenopus, SSE15206 morphogenesis, convergent extension, neural plate, neural tube closure, actin, elastimeter == INTRODUCTION == The vertebrate neural tube forms from an initially flat, short and broad neural plate which then undergoes dramatic morphogenetic events that not only bend and roll it into a tube, but also greatly narrow and lengthen it along its antero-posterior axis (Jacobson and Gordon, 1976;Schoenwolf and Smith, 1990). This narrowing and lengthening of the neural tissue, commonly referred to as convergent extension (CE), is important for neural tube closure, as it brings the neural folds toward the midline, where they will meet and fuse (Wallingford and Harland, 2002). In a urodele (tailed) amphibian,Taricha torosus, neural CE is largely dependent on CE of the underlying dorsal mesoderm (Jacobson and Gordon, 1976). The anuran (tailless) amphibianXenopus laevis, however, shows SSE15206 a form of neural CE that is mechanically independent of the underlying mesoderm (Elul et al., 1997;Keller and Danilchik, 1988;Keller et al., 1992a;Keller et al., 1992b). In sandwich explants, this mode of neural CE is induced and maintained by planar organizer signals without continued vertical signaling from the underlying mesoderm (Keller et al., 1992b). In isolated deep neural plate explants, it occurs by mediolateral cell intercalation Rabbit Polyclonal to DNA Polymerase lambda of the deep mesenchymal cells of the double layeredXenopusneural plate, using a bipolar protrusive activity like that seen in the intercalating presumptive mesodermal cells (Elul et al., 1997). In contrast, with normal, continued vertical interactions with the underlying mesoderm, this bipolar mode is replaced with a monopolar, medially directed protrusive activity that is more efficient in producing neural cell intercalation and CE (Elul and Keller, 2000;Ezin et al., 2003). This medially-directed protrusive activity is dependent on unknown signals emanating from the midline tissues of notochord and the overlying notoplate (Ezin et al., 2003;Ezin et al., 2006). CE of the neural plate, like that of the dorsal mesoderm, is dependent on the vertebrate non-canonical Wnt/planar cell polarity (PCP) pathway (Wallingford and Harland, 2001;Wallingford and Harland, 2002). However, the mechanism by which the neural cells are polarized, how they produce the forces that drive their intercalation, and the relative contribution of the autonomous neural CE movements to the forces that SSE15206 shape the neural plate and roll it into a tube remain unresolved. Medial movement of the neural folds involves bending of the neural plate. This process is driven largely by shape changes in neuroepithelial cells, which undergo apical constriction and assume a wedge or bottle-cell morphology, thus causing the neural sheet to bend (Burnside and Jacobson, 1968;Schoenwolf and Smith, 1990). The actin-binding protein Shroom is a key regulator of apical constriction duringXenopusanterior neural tube closure (Haigo et al., 2003), and in cultured MDCK cells, Shroom causes apical constriction by regulating the apical positioning of a contractile actomyosin network (Hildebrand, 2005). Myosin II is a motor protein that binds to actin filaments and moves along them by hydrolyzing ATP, and also serves as an actin cross-linker (Laevsky and Knecht, 2003;Xu et al., 2001). Vertebrates have at least two genes encoding two different non-muscle myosin II heavy chain proteins – MHC-A, MHC-B (Katsuragawa et al., 1989;Kawamoto and Adelstein, 1991;Kelley et al., 1995). A third isoform, MHC-C, has been identified in mouse (Golomb et al., 2004). MHC-A is the predominant isoform expressed inXenopusearly embryonic cells (Kelley et al., 1996), but MHC-B is up-regulated in dorsal tissues that undergo CE (Bhatia-Dey et al., 1998) SSE15206 and myosin IIB is necessary for CE of the dorsal axial and SSE15206 paraxial mesoderm and blastopore closure inXenopus(Skoglund et al., 2008). Myosins IIA and IIB have distinct biochemical properties, with IIB having a higher duty ratio, consistent with a role in maintaining cortical tension by cross-linking the cortical actin cytoskeleton (Kovacs et al., 2003;Rosenfeld et al., 2003;Wang et al., 2003). In the mouse, MHC-B plays an important role in neural development, particularly in the migration of certain groups of neurons (Ma et al., 2004;Ma et al., 2006;Tullio et al., 2001), and also in growth cone extension (Bridgman et al., 2001;Brown and Bridgman, 2003). Here we examine the role of myosin.