April 26, 2005;
Planar cell polarity genes regulate polarized extracellular matrix deposition during frog gastrulation.
The noncanonical wnt/planar cell polarity (PCP) pathway  regulates the mediolaterally (planarly) polarized cell protrusive activity and intercalation that drives the convergent extension movements of vertebrate gastrulation , yet the underlying mechanism is unknown. We report that perturbing expression of Xenopus PCP genes, Strabismus
(Xstbm), Frizzled (Xfz7
), and Prickle
), disrupts radially polarized fibronectin
fibril assembly on mesodermal tissue
surfaces, mediolaterally polarized motility, and intercalation. Polarized motility is restored in Xpk
-perturbed explants but not in Xstbm- or Xfz7
-perturbed explants cultured on fibronectin
surfaces. The PCP complex, including Xpk
, first regulates polarized surface assembly of the fibronectin
matrix, which is necessary for mediolaterally polarized motility, and then, without Xpk
, has an additional and necessary function in polarizing motility. These results show that the PCP complex regulates several cell polarities (radial, planar) and several processes (matrix deposition, motility), by indirect and direct mechanisms, and acts in several modes, either with all or a subset of its components, during vertebrate morphogenesis.
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Whole-Embryo Phenotypes and Fibronectin Fibril Distribution in the Dorsal Axis
(A) Increasing severity of whole-embryo phenotype with amount of Xstbm overexpression.
(B and C) Increasing disruption of fibrillar fibronectin ECM organization with increasing amounts of Xstbm overexpression. (B) Projection of en face confocal sections (shown in schematic at left) shows increasing width of notochord field (asterisk) and uneven lateral boundaries with somitic mesoderm (arrowheads). (C) Projection of transverse sections (shown in schematic at left) shows that increasing amounts of Xstbm result in increasing amounts of fibril assembly throughout the notochord (asterisk) no longer restricted laterally by the somitic mesoderm (arrowheads). (D–F) Expression of dominant-negative Xstbm δPDZ-B and Xpk δLim-PET and overexpression of Xfz7 and Xpk produce severe whole-embryo phenotypes similar to severe Xstbm phenotypes (D), disrupt both planar (E) (en face projections of confocal z sections) and radial organization (F) (transverse projection of confocal z sections) of fibronectin ECM throughout the dorsal axial mesoderm (asterisk, notochord; arrowhead, lateral boundary of notochord and somitic mesoderm). Scalebars indicate 50 microns. Scale of (B) and (E) are the same, as are (C) and (F). ant, anterior; pos, posterior; dor, dorsal; ven, ventral; so, somitic mesoderm.
Cell Intercalation in Three-Notochord Explants
(A) Procedure for making three-notochord explants. Xstbm, Xpk, or Xfz7 mRNA with green tracer (GFP or gap43-GFP) or red tracer (Alexa594 dextran) was coinjected into two dorsal blastomeres of the 4-cell-stage embryo. Notochord sectors (without epithelium) were dissected from injected embryos at stage 10.
(B and C) Three-notochord sectors from different embryos were grafted together and cultured on either the BSA (B)- or fibronectin (C)-coated glass, exposing the deep cells to two fluorescent color time-lapse imaging. Control explants were made from embryos injected with GFP or gap43-GFP and Alexa594 dextran. The time elapsed is indicated at the bottom right. (B) On BSA, cells in control explants adopt bipolar shapes and intercalate deeply into the neighbor notochord to make a single big notochord. Xpk-overexpressing explants converged weakly. Xstbm- and Xfz7-overexpressed cells remain round and the explants never converge. (C) On fibronectin, cells in the central notochord of control explants adopt bipolar shapes and intercalate deeply. Likewise, cells within Xpk-overexpressing explants become bipolar and intercalate. In contrast, Xstbm- and Xfz7-overexpressed cells remain round and do not intercalate as in (B). ml, mediolateral.
Cell Protrusions along the Fibronectin Substrate in Explants Overexpressing Xpk or Xstbm
Xpk or Xstbm mRNA was injected into two dorsal blastomeres of the 4-cell-stage embryo. An mRNA encoding a plasma membrane localizing GFP (gap43-GFP) was injected into several dorsal blastomeres at stage 7 to allow visualization of cell protrusions and cell shapes. Dorsal open-faced explants were dissected from the injected embryos at stage 10 and cultured on the fibronectin-coated glass exposing the deep cells to confocal time-lapse imaging. The protrusions were detected on the surface on labeled cells facing the fibronectin substrate (red) and cell shapes were observed 5 μm deeper into the explant (green). (A) In control explants, gap43-GFP-labeled cells adopt bipolar shapes with protrusions at their mediolateral ends. (B) Cells in Xstbm-overexpressing explants remain isodiametric, do not adopt bipolar shapes, and extend protrusions in all directions. (C) Cells in Xpk-overexpressing explants show similar bipolar shapes and mediolaterally directed protrusions. Arrowheads show lamelliform protrusions along the fibronectin substrate. (A′, B′, and C′) Analysis of protrusive activity in (A), (B), and (C), respectively. Rose diagrams show the normalized frequency of protrusions from five cells in the explant shown in (A), eight cells in the explant shown in (B), and seven cells shown in the explant shown in (C) directed into 30° bins representing 360° around the cell’s center.
A Two-Step Model for Function of the PCP Complex in Cell Polarization during Convergent Extension Viewed in the Transverse Section
The first step (blue arrow, center) involves radial polarization of the mesodermal cells (red shading) and the associated, polarized secretion of fibronectin extracellular matrix (green) at the outer surfaces of the mesodermal cells, between the mesoderm and the overlying neural (dark gray) and underlying endodermal (light gray) epithelial tissues. Tissue interactions not yet described between one or both of these epithelial tissues and the mesoderm (black arrows, center, top) may induce this radial polarization of the mesodermal cells. As a consequence, in a polarized fashion at their outer ends, the cells assemble a fibronectin matrix, which is at the interfaces between the mesenchymal mesoderm and these adjacent epithelial tissues (green, center). In the second step (magenta arrow, center), the cells polarize mediolaterally (red shading), enabling them to produce their characteristic mediolateral intercalation behavior and, thus, convergent extension. Perturbation of Prickle function on the one hand (Pk, center left) and Stbm or Fz on the other (Stbm, Fz, center right) result in failure (light blue arrows) of radial polarization and assembly of disorganized, unpolarized matrix (nonsurface; green). Subsequently, mediolateral polarization also fails, and the tissue thickens (because of failure of radial intercalation) and fails to converge and extend (light magenta arrows). Culture of Pk-perturbed explants on a planar, exogenous fibronectin substrate rescues mediolateral polarization (far left), whereas it does not rescue Stbm- or Fz-perturbed explants (far right). These results argue that the PCP complex, including Prickle, is necessary for the first step (blue arrows) of radial polarization of the cells and, directly or indirectly, for polarized, surface assembly of fibronectin matrix, but the second step (magenta arrows) requires the polarized, surface matrix and the PCP complex but not Prickle.