Click here to close
Hello! We notice that you are using Internet Explorer, which is not supported by Xenbase and may cause the site to display incorrectly.
We suggest using a current version of Chrome,
FireFox, or Safari.
???displayArticle.abstract???
BACKGROUND: The morphogenetic events of early vertebrate development generally involve the combined actions of several populations of cells, each engaged in a distinct behavior. Neural tube closure, for instance, involves apicobasal cell heightening, apical constriction at hingepoints, convergent extension of the midline, and pushing by the epidermis. Although a large number of genes are known to be required for neural tube closure, in only a very few cases has the affected cell behavior been identified. For example, neural tube closure requires the actin binding protein Shroom, but the cellular basis of Shroom function and how it influences neural tube closure remain to be elucidated.
RESULTS: We show here that expression of Shroom is sufficient to organize apical constriction in transcriptionally quiescent, naive epithelial cells but not in non-polarized cells. Shroom-induced apical constriction was associated with enrichment of apically localized actin filaments and required the small GTPase Rap1 but not Rho. Endogenous Xenopus shroom was found to be expressed in cells engaged in apical constriction. Consistent with a role for Shroom in organizing apical constriction, disrupting Shroom function resulted in a specific failure of hingepoint formation, defective neuroepithelial sheet-bending, and failure of neural tube closure.
CONCLUSIONS: These data demonstrate that Shroom is an essential regulator of apical constriction during neurulation. The finding that a single protein can initiate this process in epithelial cells establishes that bending of epithelial sheets may be patterned during development by the regulation of expression of single genes.
shroom3 (shroom family member 3) expression in Xenopus laevis embryo, assayed by in situ hybridization, NF stage 13, dorsal view, anterior up.
shroom3 (shroom family member 3) expression in Xenopus laevis embryo, assayed by in situ hybridization, NF stage 15, dorsal view, anterior up.
shroom3 (shroom family member 3) expression in Xenopus laevis embryo, assayed by in situ hybridization, NF stage 17, dorsal view, anterior up.
Xenopus shroom3 expression assayed by in situ hybridization. Dorsal view of stage 18 embryo.
Figure 5. Shroom Activity Is Required for Xenopus Neurulation (A) Xshroom is expressed exclusively in the neural plate during neural tube closure and then is maintained in the nasal and otic placodes (arrowhead and arrows, respectively). Dorsal view; anterior at top. Stages shown are 13, 15, 17, 18, and 20.(B) Expression of Shrm754–1108 disrupts anterior neural tube closure as evidenced by morphology (left), pax-3 expression (middle), and epidermal keratin expression (right). Embryos are shown at tailbud stage in dorsal views with anterior at top; neighboring schematics depict representative cross-sections.(C) Pax-3 expression also reveals consistent, but less-penetrant, defects in spinal neural tube closure. Dorsal view; anterior at left.(D) Shrm754–1108-induced neural tube defects are dose-dependent (N > 70 for each category).
Figure 6. Disruption of Xshroom Splicing Blocks Neural-Tube Closure (A) RT-PCR confirms defective splicing in MO-injected embryos. Lane 1: control, uninjected stage 15 cDNA, Lanes 2 and 3: two samples of MO-injected stage 15 cDNA. Lane 4: Control, uninjected stage 17 cDNA. Lane 5: MO-injected stage 17 cDNA. For each sample, five embryos were pooled.(B) Control stage 22 embryo stained for pax-3 to visualize the dorsal neural folds.(C and D) Embryos injected with the Xshroom MO display severe defects in anterior neuraltube closure.(E) Control embryo, anterior view.(F) Bilaterally injected embryo displays a total lack of hingepoints or anterior neural folds.(F′) Mini-ruby lineage tracer indicates region of delivery of the MO in (H).(G) Control embryo, anterior view.(H) Embryo injected unilaterally with Xshroom-MO. The hingepoint forms normally on the uninjected side (arrow) but is missing on the injected (right) side.(H′) Mini-ruby lineage tracer indicates region of delivery of the MO in (F).
Figure 7. Shroom Is Required for Hingepoint Formation. (A) During amphibian neurulation, the anterior neural plate bends at two paired, lateral hingepoints. (B) High-magnification view of Xshroom expression revealed by in situ hybridization. Xshroom is expressed diffusely throughout the anterior neural plate but is most strongly expressed in the forming hingepoints (The dorsal aspect of the anterior neural plate at stage 18 is shown). (C) Paired hingepoints (arrows) are evident by morphology in the anterior neural plate of control embryos (left embryo). Dorsal view, anterior at top.(D) Actin staining reveals accumulated apical actin in hingepoints of control embryos (arrows).(E) In embryos injected unilaterally with Shrm754–1108 mRNA, no hingepoint forms on the injected (right) side. (F) Actin-staining confirms the absence of a hingepoint on the Shrm754–1108-injected side of experimental embryos (right).
Figure 1. Shroom Induces Apical Constriction in Epithelial Cells
(A) The blastula assay used in this study. Individual Xenopus blastomeres at the 4 cell stage undergo cell division without transcription 25, 26 and 27 and through asymmetric divisions give rise to both outer, polarized epithelial cells and inner, nonpolarized cells 20 and 24. Injection at the 4 cell stage delivers mRNA (red) to individual blastomeres, and subsequent cell divisions result in the distribution of injected mRNA (red) into neighboring polarized (gray) and nonpolarized (white) cells.
(B) Animal view of Xenopus blastulae; Shroom expression induces pigment concentrations associated with reduced apical cell surface area as compared to that of neighboring cells.
(C) Cross-sections of Shroom-expressing cells reveal the conversion of cuboidal cells into wedge-shaped cells. (B′ and C′) Schematics of cells in (B) and (C). (D) Apical surface areas of cells expressing Shroom are reduced by approximately 50% when these cells are compared to neighboring cells or control cells. Data shown are mean area (in pixels) ± SEM. (Details of quantitation and statistical analysis are presented in the legend to Movie 1 in the Supplemental Data). (E) Structure/function analysis of Shroom in Xenopus blastulae. All deletion constructs were efficiently expressed in MDCK cells (not shown).
Figure 2. Shroom Induces Apical Constriction in Mammalian Cells
(A) MDCK cells transiently transfected to express ShrmL exhibit apical constriction; lower panel shows a cross-section of the cells reconstructed from optical sections. Shroom-expressing cells, visualized by anti-Shroom antibody, take on a wedge shape.
(A′) Same cells as in panel (A), but showing the tight junction marker ZO-1 to highlight the reduced apical surface area in Shroom-expressing cells as compared to nonexpressing cells. Cross-sections demonstrate the normal apical localization of ZO-1.
(A′) Merge of panels (A) and (A′) (green = Shroom; red = ZO-1).
(B) High-magnification cross-section of MDCK cells stained for ZO-1 to mark the apical surface.
(B′) Same cells as in panel (B), but showing the anti-Shroom antibody. The Shroom-expressing cell is wedge-shaped; Shroom protein is restricted to the apical surface (arrowhead).
(B′) Same cells as in panels (B) and (B′), but showing concentrated, ectopic f-actin at the apical surface of the Shroom-expressing cell (arrowhead).
(C) Merged view of cells expressing the alternative splice form of Shroom lacking the PDZ domain (ShrmS). The cells are stained with anti-Shroom (green) and ZO-1 (red). The ShrmS-expressing cell is wedge-shaped.
(D and E). Merged views of cells expressing Shroom deletion constructs, processed as for (C). Deletion constructs do not induce wedging.
Figure 3. Shroom Induces Apically Restricted Actin Accumulation
(A) Cells on the surface of control Xenopus blastulae display only junctional actin.
(B) Shroom-expressing cells display ectopic actin accumulation (arrowheads).
(C) Coexpression of Shrm754–1108 inhibits the ectopic actin accumulation but does not affect the junctional actin.
(D) Cross-section of control blastula (bright field).
(D′) No actin enrichment is seen in control cross-sections.
(E) Pigment concentrations are apparent in cross-sections of Shroom-expressing blastulae.
(E′) Each pigment concentration in Shroom-expressing blastulae is associated with accumulated actin exclusively at the apical surface (arrowheads). No actin accumulation is observed at the basal surface of the polarized cells (arrow).
(F) Bright-field cross-section of a Shroom-expressing blastula.
(F′) Actin (red) accumulates only at the apical surface of the outer, epithelial cells.
(F′) Visualization of GFP (green) from co-injected mRNA confirms delivery of Shroom mRNA to both inner and outer cells. In injected embryos, pigment, but not actin, is often observed below the surface, probably because of invagination associated with apical constriction
Figure 4. Blastula Assays Suggest Molecular Mechanisms for Shroom Function. (A) Phenotypes used for scoring embryos in panels (B)–(G). Scoring of phenotypes was carried out at stage 7. Graphs indicate the percent of embryos that display each phenotype for each treatment.
(B) Shroom-induced apical constriction was not inhibited by coexpression of RhoA-N19.
(C) Shroom-induced pigment concentration was inhibited by hRap1A-N17 in a dose-dependent manner.
(D) Shroom activity was inhibited by XRap1A-N17 in a dose-dependent manner. Wild-type XRap1A did not inhibit Shroom activity (not shown).
(E) Shroom activity was inhibited by Ras-N17 in a dose-dependent manner. Reversion of the pigment phenotype by Rap1A-N17 and Ras-N17 was very robust when scored at stage 7, prior to the onset of zygotic transcription [25]. In some cases, the reversion was less apparent after the MBT, probably because of new transcription of the GTPases or their cofactor GAPs and GEFs.
(F) Shroom activity was inhibited by Rap1GAP in a dose-dependent manner.
(G) Shrm754–1108 inhibits Shroom-induced pigment concentration in a dose-dependent manner.
Figure 8. Failure of Hingepoint Formation Results in Defective Bending of the Neuroepithelium (A) In cross-section, bending at the hingepoint (arrow) is apparent on the uninjected side; the elevating neural plate is concave. On the Shrm754–1108-injected side, no hingepoint has formed; the elevated neural plate remains convex.
(A′) Actin staining of cross-sections confirms that apical actin is accumulated on the uninjected side (left) and is most intense at the hingepoint (arrow); apical actin is diffuse on the injected side (right).
shroom3 (shroom family member 3) expression in Xenopus laevis embryo, assayed by in situ hybridization, NF stage 20, dorsal view, anterior up.
