Fox (forkhead) genes are involved in the dorso-ventral patterning of the Xenopus mesoderm.

Fox (forkhead/winged helix) genes encode a family of transcription factors that are involved in embryonic pattern formation, regulation of tissue specific gene expression and tumorigenesis. Several of them are transcribed during Xenopus embryogenesis and are important for the patterning of ectoderm, mesoderm and endoderm. We have isolated three forkhead genes that are activated during gastrulation and play an important role in the dorso-ventral patterning of the mesoderm. XFKH1 (FoxA4b), the first vertebrate forkhead gene to be implicated in embryonic pattern formation, is expressed in the Spemann-Mangold organizer region and later in the embryonic notochord. XFKH7, the Xenopus orthologue of the murine Mfh1(Foxc2), is expressed in the presomitic mesoderm, but not in the notochord or lateral plate mesoderm. Finally, XFD-13'(FoxF1b)1 is expressed in the lateral plate mesoderm, but not in the notochord or presomitic mesoderm. Expression pattern and functional experiments indicate that these three forkhead genes are involved in the dorso-ventral patterning of the mesoderm.

Differential expression of non-muscle myosin heavy chain genes during Xenopus embryogenesis.

Class II non-muscle myosins are implicated in diverse biological processes such as cytokinesis, cellularization, cell shape changes and gastrulation. Two distinct non-muscle myosin heavy chain genes have been reported in all vertebrates: non-muscle myosin heavy chain-A (NMHC-A) and -B (NMHC-B). We report here the isolation of the Xenopus homolog of NMHC-A and present a comparative analysis of the developmental and spatial expression patterns of NMHC-A and the previously isolated NMHC-B to address the role of NMHCs in Xenopus development. A 7.5 kb NMHC-A mRNA is present, maternally in unfertilized eggs and throughout embryogenesis, as well as in all adult tissues examined. An additional 8.3 kb zygotic transcript for NMHC-A is also detected, but only during embryonic stages. Whole mount in situ hybridization with tailbud stage embryos shows that NMHC-A mRNA is predominantly expressed in the epidermis, whereas NMHC-B mRNA is expressed in the somites, brain, eyes and branchial arches. Interestingly, the expression of NMHC-B in developing somites is gradually restricted to the center of each somite as differentiation proceeds. DAPI nuclear staining demonstrated that NMHC-B mRNA is colocalized with the nuclei or perinuclear area. In animal cap experiments, treatment with activin A or ectopic expression of Xbra and an activated form of Xlim1 markedly up-regulates NMHC-B as well as muscle actin mRNAs and slightly down-regulates NMHC-A mRNA, consistent with NMHC-B expression in the somitic muscle and NMHC-A expression in the epidermis.

Cloning of the cDNA encoding a myosin heavy chain B isoform of Xenopus nonmuscle myosin with an insert in the head region.

The complete amino acid sequence of Xenopus laevis nonmuscle myosin heavy chain B (MHC-B) has been deduced from overlapping cDNA clones isolated from an XTC cell library. RNA blots of various developmental stages, adult tissues, and XTC cells detect a single transcript of 7.5 kb which is expressed at similar levels throughout development. MHC-B mRNA was detected in XTC cells, heart, lung, spleen, and brain, at lower levels in ovary, testis, pancreas, stomach, liver, and eye, but not in kidney and skeletal muscle. Protein expression in adult tissues, as detected by immunoblot analysis, correlates well with mRNA expression. In chickens and humans, a fraction of the mRNA encoding the MHC-B isoform was found previously to contain a 10-amino acid insert at amino acid 211 near the ATP-binding site. As reported elsewhere, in the chicken this insert-bearing isoform is nervous system-specific. The Xenopus sequence shows a 16-amino acid insertion at the same position; 7 of 16 residues are identical to those in the chicken and human insertion, and these identical residues include a consensus target sequence for cyclin-p34cdc2 kinase. In contrast to chicken, all frog tissues and embryonic stages tested contained the insert-bearing form, and no evidence for a non-insert-bearing MHC-B isoform was found in Xenopus.

The Drosophila fsh locus, a maternal effect homeotic gene, encodes apparent membrane proteins.

The maternal effect gene fsh is involved in the establishment of segments and the specification of their identities; the progeny of mutant females are missing portions of thoracic and abdominal segments, and may have homeotic transformations of third thoracic segments to second thoracic segments. The fsh locus interacts synergistically with loci such as Ubx and trx in the production of homeotic transformations. We have characterized cDNA clones corresponding to the major fsh transcripts expressed in ovaries and early embryos, and to a pupal transcript. The expression of fsh transcripts in ovaries is restricted to the germline; in developing embryos, transcripts are found throughout the cytoplasm. The different ovarian/embryonic transcripts (7.6 and 5.9 kb) are generated by use of alternative polyadenylation and splice sites. These transcripts encode two large predicted proteins of 110 and 205 kDa that have unusual amino acid compositions: 40% of the residues are glycine, alanine, or serine, and there are several regions of homopolymers and simple sequence repeats. Hydropathy analysis indicates that these proteins span the membrane. We suggest that the expression of fsh proteins in the membrane of the embryo is required for proper functioning of genes such as Ubx in the specification of segmental identity.