December 1, 1999;
Involvement of the small GTPases XRhoA and XRnd1 in cell adhesion and head formation in early Xenopus development.
family of small GTPases regulates a variety of cellular functions, including the dynamics of the actin cytoskeleton
, cell adhesion, transcription, cell growth and membrane trafficking. We have isolated the first Xenopus homologs of the Rho
-like GTPases RhoA
and examined their potential roles in early Xenopus development. We found that Xenopus Rnd1
) is expressed in tissues undergoing extensive morphogenetic changes, such as marginal zone cells involuting through the blastopore
, somitogenic mesoderm
formation and neural crest cells. XRnd1
also causes a severe loss of cell adhesion in overexpression experiments. These data and the expression pattern suggest that XRnd1
regulates morphogenetic movements by modulating cell adhesion in early embryos. Xenopus RhoA
) is a potential XRnd1
antagonist, since overexpression of XRhoA
increases cell adhesion in the embryo
and reverses the disruption of cell adhesion caused by XRnd1
. In addition to the potential roles of XRnd1
in the regulation of cell adhesion, we find a role for XRhoA
in axis formation. When coinjected with dominant-negative BMP receptor (tBR) in the ventral
side of the embryo
causes the formation of head
structures resembling the phenotype seen after coinjection of wnt inhibitors with dominant-negative BMP receptor. Since dominant-negative XRhoA
is able to reduce the formation of head
structures, we propose that XRhoA
activity is essential for head
formation. Thus, XRhoA
may have a dual role in the embryo
by regulating cell adhesion properties and pattern formation.
[+] show captions
Fig. 3. Whole-mount in situ hybridization analysis of XRnd1 and XRhoA transcripts. (A-I) XRnd1, (J-O) XRhoA. (A,C,K,L) Dorsal is right. (D,F,H,I,M,N,O) Anterior is left. (A) Animal (left) and vegetal (right) view of early gastrula (stage 10) embryos. XRnd1is expressed
in the ectoderm and dorsal blastopore lip. The arrowhead indicates the location of the dorsal blastopore lip. (B) Vegetal view of a midgastrula (stage 11) embryo. Note the high level of XRnd1
expression around the blastopore (arrow). (C) Mid-gastrula (stage
10.5) embryo that was cut along the midline. The arrowhead indicates the location of the dorsal blastopore. XRnd1 expression persists in the involuted mesoderm. (D) Dorsal view of an early
neurula (stage 13) embryo. XRnd1 expression is absent from the midline. (E) Histological cross section of the same embryo as in D, which reveals high level of XRnd1 expression in the somitogenic
mesoderm (SM), but not in the notochord (No). (F) Parasagittally cut midneurula (stage 20) embryo showing the high level of XRnd1 expression in unsegmented somitogenic mesoderm and expression in
newly formed somites (arrowheads). (G) Anterior transverse section of a mid-neurula stage embryo (stage 20) revealing XRnd1 expression in cranial neural crest cells (NC) and somites (So).
(H) Dorsal view of an early tailbud stage (stage 22) embryo. Note XRnd1 expression in newly formed somites (arrows) and cranial neural crest cells (arrowhead). (I) Dorsolateral view of a tailbud stage (stage 27) embryo. XRnd1 expression now remains only in the posterior region of the somitogenic mesoderm (So). (J) XRhoA expression: vegetal (left) and animal (right) view of gastrula (stage 11) embryos. XRhoA expression is present in the ectoderm and marginal zone. (K,L) two halves of a gastrula (stage 10.5) embryo
probed for XRhoA (K) and cerberus (L) to demonstrate the presence of XRhoA expression in the organizer region. Note that embryos were cut along the midline prior to in situ hybridization analysis. The
image of the embryo in K was horizontally flipped to allow for easier comparison with L. Small white arrows indicate the boundary between involuted mesoderm and overlying ectoderm. The black arrowhead shows the position of the dorsal blastopore lip and the
small arrow indicates the ventral blastopore lip. Note that both cerberus and RhoA are expressed in the ventral marginal region. XRhoA is also expressed in the ectoderm, although the surface staining appears weaker in K due to lighting conditions used for
photography. (M) Sagittally sectioned neurula embryo (stage 13) showing the expression of XRhoA in the ectoderm and mesoderm. (N) Enlargement of the boxed area in M; Me, mesoderm; Ec, ectoderm. (O) Tailbud stage (stage 23) embryo. XRhoA expression is enriched in the head and to some extent in the tail region.
rhoa ( ras homolog family member A ) gene expression in Xenopus laevis embryos, NF stage 23, as assayed by in situ hybridization, lateral view, anterior left, dorsal up.