Twist and shout

Snail regulates p21(WAF/CIP1) expression in cooperation with E2A and Twist

Snail, a zinc-finger transcriptional repressor, is essential for mesoderm and neural crest cell formation and epithelial-mesenchymal transition. The basic helix-loop-helix transcription factors E2A and Twist have been linked with Snail during embryonic development. In this study, we examined the role of Snail in cellular differentiation through regulation of p21(WAF/CIP1) expression. A reporter assay with the p21 promoter demonstrated that Snail inhibited expression of p21 induced by E2A. Co-expression of Snail with Twist showed additive inhibitory effects. Deletion mutants of the p21 promoter revealed that sequences between -270 and -264, which formed a complex with unidentified nuclear factor(s), were critical for E2A and Snail function. The E2A-dependent expression of the endogenous p21 gene was also inhibited by Snail.

Induction of chondrocyte growth arrest by FGF: transcriptional and cytoskeletal alterations

The effect of fibroblast growth factor (FGF) on mature chondrocytes, the cells responsible for axial skeletal development, is growth attenuation rather than stimulation. This singular response has been linked to signaling via FGF receptor 3 (FGFR3), partly because mutations causing chronic FGFR3 activation lead to various human disorders of bone growth. In order to study how FGF inhibits growth, we analyzed its effect on a rat chondrocyte-derived cell line. We show that the FGF-induced growth arrest occurs at the G1 phase,accompanied by profound changes in gene expression and cytoskeletal organization. Within minutes of binding, FGF induces tyrosine kinase activity in the focal substrate adhesions where it colocalizes with vinculin. Upon FGF stimulation, FGFR3 is selectively removed from the focal adhesions, which is followed by their disassembly and disruption of the organized cytoskeleton. Multiple genes are induced following FGF stimulation in chondrocytes, which has been shown by DNA array screening and confirmed for some by immunoblotting. These genes include regulators of cell differentiation and proliferation such as c-jun, JunD, cyclin-D1, NFκB1 and of plasma-membrane microdomain morphology, such as ezrin. The transcription factor Id1 is downregulated, consistent with the cells' exit from the mitotic cycle. Moreover, following FGF stimulation, levels of FGFR3 mRNA and protein decline, as does downstream signaling through the MAPK pathway. The importance of this FGFR3-mediated on-off control is illustrated in transgenic mice expressing mutant, hyperactive FGFR3, where abnormally high levels of NFκB are expressed throughout their bone growth-plates. A working model is presented of the signaling network involved in regulating FGF-induced chondrocyte differentiation and receptor downregulation.

Xrel3 is required for head development in Xenopus laevis

The Rel/NF-kappa B gene family encodes a large group of transcriptional activators involved in myriad differentiation events, including embryonic development. We have shown previously that Xrel3, a Xenopus Rel/NF-kappa B-related gene, is expressed in the forebrain, dorsal aspect of the mid- and hindbrain, the otocysts and notochord of neurula and larval stage embryos. Overexpression of Xrel3 causes formation of embryonic tumours. We now show that Xrel3-induced tumours and animal caps from embryos injected with Xrel3 RNA express Otx2, Shh and Gli1. Heterodimerisation of a C-terminally deleted mutant of Xrel3 with wild-type Xrel3 inhibits in vitro binding of wild-type Xrel3 to Rel/NF-kappa B consensus DNA sequences. This dominant interference mutant disrupts Shh, Gli1 and Otx2 mRNA patterning and inhibits anterior development when expressed in the dorsal side of zygotes, which is rescued by co-injecting wild-type Xrel3 mRNA. In chick development, Rel activates Shh signalling, which is required for normal limb formation; Shh, Gli1 and Otx2 encode important neural patterning elements in vertebrates. The activation of these genes in tumours by Xrel3 overexpression and the inhibition of their expression and head development by a dominant interference mutant of Xrel3 indicates that Rel/NF-kappa B is required for activation of these genes and for anterior neural patterning in Xenopus.

Overexpression of a novel Xenopus rel mRNA gene induces tumors in early embryos

The Rel family of transcriptional activators form a large diverse group of proteins that are involved in the activation of genes involved in immunity, development, apoptosis and cancer. So far, none of the rel genes cloned in mammals appear to be required for embryonic development. We have cloned and characterized a cDNA from an embryonic cDNA library that encodes a novel Xenopus Rel protein, called Xrel3. Xrel3 is a member of the cRel subfamily and is most closely related to but distinct from other Xenopus Rel members. The expression of Xrel3 mRNA was investigated using Northern analysis, RNase protection assay, reverse transcriptase-linked polymerase chain reaction and in situ hybridization. Messages are present maternally and are slightly enriched in the equatorial region of the blastula stage embryo. At gastrulation, the accumulation of Xrel3 messages declines to undetectable levels but then increases after neurulation. In situ RNA hybridization was used to determine the spatial location of Xrel3 messenger RNA in embryos. Messages are localized to the developing forebrain, dorsal mid-hindbrain region, the inner ear primordium, or otocyst, and in the notochord. Overexpression by microinjection of Xrel3 RNA induced tumors in the developing embryo that appeared after gastrulation. The location of the tumors depended on the location of the injection site. These results suggest that Xrel3 might have a generalized role in regulation of cell differentiation in the embryo.

Somatic mosaicism for Friedreich's ataxia GAA triplet repeat expansions in the central nervous system

Most patients with Friedreich's ataxia (FRDA) carry expanded GAA repeats in both homologues of the frataxin gene on chromosome 9. We determined the size of the GAA repeats in autopsied samples from the CNS of six FRDA patients. We observed heterogeneity of repeat sizes in different CNS regions, indicative of extensive mitotic instability. Samples from the same CNS subdivision (e.g., cortex, thalamus) contained a similar mixture of alleles, suggesting that the pattern of repeat size mosaicism reflects the developmental history of each sample. Regional differences in repeat size could not account for the characteristic distribution of pathology in FRDA, which appears instead to be related to the pattern of frataxin expression.