Molecular cloning of Frizzled-10, a novel member of the Frizzled gene family

Cadherins and catenins, Wnts and SOXs: embryonic patterning in Xenopus

Wnt signaling plays a critical role in a wide range of developmental and oncogenic processes. Altered gene regulation by the canonical Wnt signaling pathway involves the cytoplasmic stabilization of beta-catenin, a protein critical to the assembly of cadherin-based cell-cell adherence junctions. In addition to binding to cadherins, beta-catenin also interacts with transcription factors of the TCF-subfamily of HMG box proteins and regulates their activity. The Xenopus embryo has proven to be a particularly powerful experimental system in which to study the role of Wnt signaling components in development and differentiation. We review this literature, focusing on the role of Wnt signaling and interacting components in establishing patterns within the early embryo.

Isolation of Xenopus frizzled-10A and frizzled-10B genomic clones and their expression in adult tissues and embryos

Frizzled genes, encoding WNT receptors, play key roles in cell fate determination. Here, we isolated two Xenopus frizzled genes (Xfz10A and Xfz10B), probably reflecting pseudotetraploidy in Xenopus. Xfz10A (586 amino acids) and Xfz10B (580 amino acids) both encoded by a single exon, consisted of the N-terminal cysteine-rich domain, seven transmembrane domains, and the C-terminal Ser/Thr-X-Val motif. Xfz10A and Xfz10B were 97.0% identical at the amino acid level, and Xfz10B was 100% identical to previously reported Xfz9, yet Xfz10A was 85.3% and 62.4% identical to FZD10 and FZD9, respectively. Xfz10 mRNA appeared as 3.4 kb in adult tissues and embryos. RT-PCR analyses revealed the expression of more Xfz10A mRNA in stomach, kidney, eye, skeletal muscle, and skin, and more Xfz10B mRNA in heart and ovary, but in embryos, two mRNAs were equally expressed from the blastula stage with their peak expression at the late gastrula stage. The main site of Xfz10 mRNA expression was neural fold at the neurula stage and the dorsal region of midbrain, hindbrain, and spinal cord at the tadpole stage. These results suggest that Xfz10 has important roles in neural tissue formation.

Alternative splicing of the WNT-2B/WNT-13 gene

Secreted glycoprotein WNTs play important roles in carcinogenesis and development. We have previously reported molecular cloning of WNT-2B/WNT-13. Here, we have isolated a novel WNT-2B isoform (WNT-2B2), in addition to the original WNT-2B isoform (WNT-2B1). WNT-2B1 and WNT-2B2 are completely different in the 5'-UTR and in the N-terminal part of the coding region. The N-terminal hydrophobic domain is contained in WNT-2B1, but not in WNT-2B2. WNT-2B1 and WNT-2B2 share the WNT-core domain, and show 87.0% amino-acid identity. We have determined the structure of the WNT-2B gene. The WNT-2B1 mRNA consists of exons 1, 2, and 4-7, while the WNT-2B2 mRNA consists of exons 3-7. WNT-2B2 was expressed in fetal brain, fetal lung, fetal kidney, caudate nucleus, testis, glioblastoma cell lines A172, SW1783, gastric cancer cell lines MKN28, MKN74, and cervical cancer cell line HeLa S3. WNT-2B1 expression level was relatively higher in fetal brain and fetal lung than in other tissues or cell lines expressing WNT-2B2. These results indicate that the WNT-2B1 and WNT-2B2 mRNAs are transcribed due to alternative splicing with distinct expression profile. This is the first report on the WNT isoforms derived from the same gene due to alternative splicing.

Molecular cloning and characterization of human FGF-20 on chromosome 8p21.3-p22

The fibroblast growth factors (FGFs) play important roles in morphogenesis, angiogenesis, tissue remodeling, and carcinogenesis. Human FGF-20 has been cloned and characterized in this study. FGF-20 encodes a 211-amino-acid polypeptide with the FGF-core domain. A strong hydrophobic region was found in the FGF-core domain of FGF-20; however, no typical N-terminal signal sequence was found in FGF-20, just as in FGF-9 and FGF-16. Total amino acid identities are as follows: FGF-20 vs FGF-9, 71.6%; FGF-20 vs FGF-16, 66.2%; FGF-9 vs FGF-16, 72.4%. Phylogenic analysis indicated that FGF-20, FGF-9, and FGF-16 constitute a subfamily among the FGF family. FGF-20 mRNA of 2.4 kb in size was detected in colon cancer cell line SW480 by Northern blot analysis. Lower levels of FGF-20 mRNA were detected in human fetal tissues and primary cancers by cDNA-PCR. The nucleotide sequence of FGF-20 cDNA is split into three parts in the human genome sequence of the chromosome 8p21.3-p22 region (Accession No. AB020858). These results indicate that the FGF-20 gene, located on human chromosome 8p21.3-p22, consists of three exons. Compared with the nucleotide sequence of FGF-20 cDNA determined in this study, one nucleotide deletion and one nucleotide substitution in the putative coding region were identified in human genome sequence AB020858.

Identification, gene structure, and expression of human frizzled-3 (FZD3)

We report the identification, genomic structure, chromosomal localization, and expression analysis of human frizzled-3 (FZD3), a 7-transmembrane receptor belonging to the frizzled family. The cDNA obtained from adult human brain shows 91% identity at the nucleotide level and 98% at the amino acid level to mouse frizzled-3 (fzd3). The FZD3 locus is located on chromosome 8p21, spans 48 Kb and its coding sequence is distributed in 6 exons intercalated by 5 introns. FZD3 is expressed in all analyzed human tissues, with quantitatively higher expression in the CNS and in urogenital structures.

Characterization of avian frizzled genes in cranial placode development

To determine the possible role of Wnt signaling in cranial placode development, we have cloned several chick frizzled genes, a family of putative Wnt receptor molecules, and analyzed their expression during chick embryogenesis. Chick frizzled-2 (cFz-2) and frizzled-7 (cFz-7) are expressed broadly in cranial ectoderm, tissue that is competent to express markers of the trigeminal placode (Stark et al., 1997. Development 124, 4287-4295; Baker et al., 1999. Development 126, 147-156). In addition, cFz-2 and cFz-7 are uniquely expressed in other cranial placodes, including the olfactory, lens, and otic placodes. Chick frizzled-1 (cFz-1) is expressed in the lens, otic placode and, along with cFz-7, in epibranchial placodes. Each frizzled gene expressed in the otic placode displays a unique domain of expression: cFz-1 transcripts are detected in the medial wall of the vesicle, cFz-2 in the rostral rim of the vesicle, and cFz-7 in the lateral half of the vesicle. Other chick frizzled family members cloned that do not show striking expression in cranial placodes include frizzled-4 (cFz-4), frizzled-8 (cFz-8), frizzled-9 (cFz-9), and frizzled-10 (cFz-10). A brief summary of their expression is given, along with a brief summary of non-placodal expression of cFz-1, cFz-2, and cFz-7. In all, frizzled genes show dynamic expression at key times during embryonic development, particularly in the cranial placodes.

Molecular cloning and genomic structure of human frizzled-3 at chromosome 8p21

WNT receptors encoded by the Frizzled genes are implicated in carcinogenesis as well as in embryonic development. Human Frizzled-3 (FZD3) gene, encoding seven-transmembrane receptor with the N-terminal cysteine-rich domain, has been cloned and characterized. Expression of the FZD3 mRNAs was investigated by using three FZD3 specific probes: HF3S1, corresponding to the 5'-UTR and a part of the coding region; HF3S2, corresponding to a part of the coding region; HF3S3, corresponding to the 3'-UTR. HF3S1 and HF3S2 hybridized to the 14.0-, 9.0-, 4.0- and 1.8-kb FZD3 mRNA, while HF3S3 hybridized to the 14.0-, 9.0-, and 4.0-kb FZD3 mRNA. The 14. 0-kb FZD3 mRNA was the major transcript in fetal brain and adult cerebellum, while the 1.8-kb FZD3 mRNA was the major transcript in adult pancreas, and many cancer cell lines examined. The 1.8-kb FZD3 mRNA, alternatively polyadenylated by the internal AATAAA signal in the coding region, is predicted to encode the truncated FZD3 protein lacking the region through the second extracellular loop to the C-terminal tail, and might function as the transmembrane-type antagonist for WNTs. The FZD3 gene consists of 8 exons, and has been mapped to human chromosome 8p21.

Sequence, expression, and location of zebrafish frizzled 10

Members of the frizzled gene family encode seven-pass transmembrane proteins that function in the interpretation and reception of Wnt-mediated cell-cell communication events. To investigate frizzled function in early zebrafish development, we isolated the maternally contributed frizzled 10 (fz10) gene and localized it to linkage group 8 using radiation hybrid mapping. The cloned zebrafish fz10 is closely related to the fz10 group from other organisms. Zygotic expression of fz10 is observed in the posterior tail mesenchyme, dorsal neural tube, and different parts of the brain.

Molecular cloning and genomic structure of the betaTRCP2 gene on chromosome 5q35.1

Beta-catenin, IkappaBalpha, and HIV Vpu are recruited to the ubiquitin-proteasome degradation pathway by betaTRCP, one of the components of the ubiquitin ligase complex. betaTRCP2, a related gene of betaTRCP, was cloned and characterized. Three isoforms, betaTRCP2A, betaTRCP2B, and betaTRCP2C, were identified. All of these betaTRCP2 isoforms consist of an F-box and seven WD repeats. Human betaTRCP2A shows 86% total amino acid identity with human betaTRCP. betaTRCP2 mRNA of 4.5 kb in size was detected almost ubiquitously. Sequence analyses on betaTRCP2 genomic clones revealed that the betaTRCP2 gene consists of at least 14 exons. Exons 1 and 4-14 are shared among all betaTRCP2 isoforms. betaTRCP2A of 508 amino acids lacks exons 2 and 3, betaTRCP2B of 529 amino acids contains exon 3, and betaTRCP2C of 542 amino acids contains exon 2. These results indicate that three betaTRCP2 isoforms are transcribed due to alternative splicing. The betaTRCP2 gene has been mapped to human chromosome 5q35.1 by fluorescence in situ hybridization.

Identification of chick frizzled-10 expressed in the developing limb and the central nervous system

We have identified chick frizzled (Fz)-10, encoding a Wnt receptor, and examined the expression pattern during embryogenesis. Fz-10 is expressed in the region posterior to the Hensen's node at stage 6. Fz-10 expression is detected in the dorsal domain of the neural tube and the central nervous system of the developing embryo. In the developing limb, Fz-10 expression starts at stage 18 in the posterior-dorsal region of the distal mesenchyme, and gradually expands to the anterior-distal region. Fz-10 is also expressed in the feather bud and branchial arch. Implantation of Sonic hedgehog (Shh)-expressing cells into the anterior margin of the limb bud resulted in the induction of Fz-10 expression in anterior-dorsal mesenchyme.

Molecular cloning and characterization of human Frizzled-4 on chromosome 11q14-q21

The WNT receptors, encoded by the Frizzled genes, are implicated in a variety of cellular processes such as cell fate determination, cell polarity control, and malignant transformation. Human Frizzled-4 (FZD4) cDNAs have been cloned and characterized. FZD4 spans a total of 7392 nucleotides and encodes a 537-amino-acid protein with the N-terminal cysteine-rich domain, seven transmembrane domains, and the C-terminal S/T-X-V motif. The FZD4 mRNA of 7.7 kb in size were detected almost ubiquitously in normal human tissues and larger amounts in fetal kidney, adult heart, skeletal muscle, and ovary. Among cancer cell lines, the FZD4 mRNA level was higher in HeLa S3. The FZD4 gene has been mapped to human chromosome 11q14-q21. FZD4 is homologous to FZD9 and FZD10, and overall amino acid identity is as follows: FZD4 vs FZD9, 51.6%; FZD4 vs FZD10, 51.2%; FZD9 vs FZD10, 65.7%. FZD4 consists of two exons, while FZD9 and FZD10 consist of a single exon. FZD4 might belong to rather the independent FZD subfamily than the FZD9-FZD10 subfamily.