XB-ART-40502J Cell Sci 2009 Nov 15;122Pt 22:4049-61. doi: 10.1242/jcs.031948.
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Xenopus delta-catenin is essential in early embryogenesis and is functionally linked to cadherins and small GTPases.
Catenins of the p120 subclass display an array of intracellular localizations and functions. Although the genetic knockout of mouse delta-catenin results in mild cognitive dysfunction, we found severe effects of its depletion in Xenopus. delta-catenin in Xenopus is transcribed as a full-length mRNA, or as three (or more) alternatively spliced isoforms designated A, B and C. Further structural and functional complexity is suggested by three predicted and alternative translation initiation sites. Transcript analysis suggests that each splice isoform is expressed during embryogenesis, with the B and C transcript levels varying according to developmental stage. Unlike the primarily neural expression of delta-catenin reported in mammals, delta-catenin is detectable in most adult Xenopus tissues, although it is enriched in neural structures. delta-catenin associates with classical cadherins, with crude embryo fractionations further revealing non-plasma-membrane pools that might be involved in cytoplasmic and/or nuclear functions. Depletion of delta-catenin caused gastrulation defects, phenotypes that were further enhanced by co-depletion of the related p120-catenin. Depletion was significantly rescued by titrated p120-catenin expression, suggesting that these catenins have shared roles. Biochemical assays indicated that delta-catenin depletion results in reduced cadherin levels and cell adhesion, as well as perturbation of RhoA and Rac1. Titrated doses of C-cadherin, dominant-negative RhoA or constitutively active Rac1 significantly rescued delta-catenin depletion. Collectively, our experiments indicate that delta-catenin has an essential role in amphibian development, and has functional links to cadherins and Rho-family GTPases.
PubMed ID: 19843587
PMC ID: PMC2776500
Article link: J Cell Sci
Species referenced: Xenopus laevis
Genes referenced: cdh1 cdh2 cdh3 ctnnb1 ctnnd1 fn1 h4c4 rac1 rho rho.2 rhoa sult4a1 zbtb33
Morpholinos: ctnnd1 MO1 ctnnd1 MO2 ctnnd2 MO1 ctnnd2 MO2 sult4a1 MO1 zbtb33 MO1
Article Images: [+] show captions
|Fig. 3. Spatial expression of Xenopus -catenin. (A) As viewed in animal versus vegetal regions, whole-mount in situ RNA hybridization detects -catenin mRNA signals in the ectoderm regions of blastula (subpanels A-C) and gastrula (subpanel D) embryos. At neurulation (subpanel E), the anterior and dorsal neural regions displayed the most apparent signals. Embryos at tadpole stages (subpanel F) showed a distinctive staining pattern in tissues of neural derivation such as brain, eye vesicle, ear vesicle, branchial arches (higher magnification in subpanel G) and spinal cord as well as somites (higher magnification in subpanel H). Subpanels I-L are cross-section views of paraffin-fixed embryos from corresponding stages. Sense probe hybridization was processed in parallel as negative controls (subpanels M-O). (B) RT-PCR analyses detect -catenin transcripts in all adult Xenopus tissues examined, with stronger expression in brain, nerve, muscle and skin. (C) Immunoblotting using an N-terminus-directed antibody detected three -catenin isoforms migrating at approximately 160, 130 and 100 kDa. The 130 and 100 kDa isoforms are ubiquitously present, whereas the 160 kDa appears to be brain specific. An antibody directed against the -catenin C-terminus reacts with the 160 kDa and 130 kDa isoforms in brain.|
|Figure 2. enopus δ-catenin temporal expression profiles. (A) Schematic diagram of δ-catenin alternative splicing events, and the PCR primers used to resolve them. The nucleotide lengths of PCR products are indicated in parentheses. (B) RT-PCR analyses indicate δ-catenin transcripts are deposited maternally and expressed throughout early embryonic stages. Both long (a,b,c) and short (a′,b′,c′) splicing variants were detected, with b′ and c′ having increased expression following neurulation. (C) Immunoblotting confirms δ-catenin protein expression throughout Xenopus embryogenesis. Antibodies directed against Xenopus amino acids 83-521 (N-terminal domain) recognize a δ-catenin isoform migrating at approximately 100 kDa; antibodies directed against Xenopus amino acids 1297-1314 (C-terminal domain) react mainly with a 100 kDa doublet, with reactivity additionally appearing at 130 kDa (marked with an asterisk) and 160 kDa (not shown, but see Fig. 3C). The 130 and 160 kDa bands are most evident following immunoprecipitation (results not shown).|
|Figure 4. Xenopus δ-catenin associates with classical cadherins and displays a significant non-membrane-associated fraction. Endogenous δ-catenin complexes were immunoprecipitated using a C-terminus-directed antibody (1297-1314), and immunoblotted using antibodies direct against cadherins. Positive co-immunoprecipitation results suggest an association of δ-catenin with C-cadherin in gastrulating embryos (A), and with E-cadherin (B) and N-cadherin (C) in neurulation stage embryos. Immunoglobulin heavy chain (IgG H.C.) bands are included to reflect the specific versus negative control antibody input. (D) Crude membrane fractionations of gastrula embryos followed by immunoblot analyses indicate the predominant localization (∼70%) of endogenous δ-catenin within non-plasma-membrane pools. As expected, GAPDH is almost exclusively evident in the non-plasma-membrane pools. C-cadherin predominantly resides within the plasma-membrane pool (65%), with the remaining fraction likely to reflect associations with non-sedimenting vesicular stores, endoplasmic reticulum or Golgi.|
|Figure 5. Antisense morpholino depletion of endogenous δ-catenin results in developmental defects. (A) Schematic diagram of the morpholino-based strategy, including the relative positions of PCR diagnostic primers. MO 6 targets δ-catenin RNA at the splice junction between intron 5 and exon 6, whereas MO 9 targets the junction between intron 8 and exon 9. MO 6 produces exon 6 elimination, with predicted codon frame-shift and early polypeptide termination. In addition to the normal transcript, RT-PCR confirmed (via DNA sequencing) the expected alteration in pre-RNA splicing (using oligos F1+R2; marked with an asterisk). Likewise, MO 9 results in alternative splicing, exon elimination and translational termination (PCR product marked with a single apostrophe was confirmed to be skipping of exon 9 and part of exon 8. An additional partial exon 10 deletion is indicated with a double apostrophe). δ-catenin transcription and mRNA stability did not appear to be significantly altered (oligos D and C, see also Fig. 2A), with histone H4 serving as an internal control. (B) Immunoblotting of gastrula stage embryo extracts confirmed the reduction of δ-catenin protein following morpholino injection. Non-specific bands (labeled with asterisks) and β-actin act as loading controls. Numbers indicate relative band intensities normalized to the uninjected control. (C) δ-catenin knockdown results in developmental phenotypes, including significant delays in blastopore closures and gastrulation defects (upper right panel). Although most embryos outwardly appear to recover from these effects and complete blastopore closure, the majority of MO-9-injected embryos were developmentally arrested during early tailbud or tadpole stages and subsequently died. For surviving embryos, abnormalities were again outwardly evident, particularly at tadpole stages, including shortened anterior-posterior axes, smaller craniofacial skeletons and eyes, malformed gut and edema (lower panel). Control embryos injected with standard morpholino displayed no obvious phenotypes (upper left and middle panel). P-values indicate statistical significance.|
|Figure 6. Fig. 6. δ-catenin-knockdown phenotypes are rescued by overexpression of either δ-catenin or p120-catenin. (A) The specificity of δ-catenin-depletion phenotypes were verified through rescue with select constructs. Exogenous and titrated FL (full-length) δ-catenin (see also supplementary material Fig. 4A,B) as well as M434 δ-catenin (initiated with the fourth potential translation start site, see also Fig. 1) largely rescued blastopore closure defects arising from endogenous δ-catenin depletion. Titrated levels of p120-catenin, but not β-catenin, also reproducibly displayed significant rescuing activity. (B) Co-injection of δ-catenin and p120-catenin morpholino, each at subphenotypic doses, produces enhanced phenotypic effects.(C) Use of a depletion-rescue strategy suggests that the PDZ-binding motif of δ-catenin is dispensable for the rescuing capacity of δ-catenin in blastopore closure. By contrast, the δ-catenin N-terminus or armadillo domain in isolation failed to rescue δ-catenin depletion. (D) Schematic presentation of various rescue constructs with a summary of their rescuing effects (asterisk indicates data from Fig. 7D). For all panels P-values indicate statistical significances.|
|Figure 7. δ-catenin depletion leads to reduced cadherin functions. (A) Immunoblotting shows that δ-catenin depletion reproducibly leads to reduced C-, E- and N-cadherin levels, whereas p120-catenin and β-catenin levels are not significantly altered.(B) Calcium-dependant adhesive functions were decreased in δ-catenin-depleted naive ectoderm cells. Following calcium removal from ectoderm explants, note the larger cell aggregates remaining after control MO injection. (C) Using an in vitro assay with the extracellular domain of E-cadherin tethered to a solid substrate (chamber glass), cadherin-mediated adhesion is decreased in naive ectoderm cells depleted of δ-catenin. By contrast, a similar assay that uses tethered fibronectin, did not resolve changes in cell attachment (presumably integrin mediated). (D) A titrated dose of exogenous C-cadherin significantly rescues blastopore closure defects induced by depletion of endogenous δ-catenin. By contrast, a δ-catenin mutant construct lacking armadillo repeats 1-5, and failing to co-immunoprecipitate with C-cadherin (supplementary material Fig. 5A) showed minimal rescuing effects. For all panels P-values indicate statistical significance.|
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Abu-Elneel, A delta-catenin signaling pathway leading to dendritic protrusions. 2008, Pubmed