A mitochondrial protein, Bit1, mediates apoptosis regulated by integrins and Groucho/TLE corepressors

A delicate balance of signals regulates cell survival. One set of these signals is derived from integrin-mediated cell adhesion to the extracellular matrix (ECM). Loss of cell attachment to the ECM causes apoptosis, a process known as anoikis. In searching for proteins involved in cell adhesion-dependent regulation of anoikis, we identified Bit1, a mitochondrial protein that is released into the cytoplasm during apoptosis. Cytoplasmic Bit1 forms a complex with AES, a small Groucho/transducin-like enhancer of split (TLE) protein, and induces cell death with characteristics of caspase-independent apoptosis. Cell attachment to fibronectin counteracts the apoptotic effect of Bit1 and AES. Increasing Bit1 expression enhances anoikis, while suppressing the expression reduces it. Thus, we have elucidated an integrin-controlled pathway that is, at least in part, responsible for the cell survival effects of cell-ECM interactions.     

Bowline, a novel protein localized to the presomitic mesoderm, interacts with Groucho/TLE in Xenopus

Cells in the prospective somite of Xenopus laevis embryos rotate in an orchestrated manner to form a segregated somite. The prospective somite boundaries are prepatterned by gene expressions in the unsegmented presomitic mesoderm (PSM). However, the roles of polarized gene expression in this boundary formation are not well elucidated. Here we identified a novel gene, bowline, which localizes to the anterior halves of S-II, III in the PSM of X. laevis. Bowline associated with corepressor XGrg-4, a Xenopus homolog of Groucho/TLE protein. A WRPW tetrapeptide motif in Bowline was prerequisite for coprecipitation with XGrg-4 and for downregulation of X-Delta-2 by bowline RNA injection. This study indicates that Bowline is a novel protein interacting with Groucho/TLE and may play a role in somitogenesis in X. laevis.     

Involvement of co-repressors Groucho and CtBP in the regulation of single-minded in Drosophila

Dorso-ventral patterning results in the establishment of the two germ layers in the Drosophila embryo, mesoderm and mesectoderm, that are separated by a strip of cells giving rise to the mesectoderm and eventually to the ventral midline. The mesectoderm is specified by the expression of single-minded (sim) which is activated through the concerted action of Dorsal and Twist in addition to a Notch signal. In the mesoderm, sim is repressed by Snail together with the co-repressor C-terminal binding protein (CtBP). Here, we address the involvement of the two co-repressors CtBP and Groucho (Gro) in repression of sim in the neuroectoderm. It was shown earlier that sim is restricted in the neuroectoderm with help of Suppressor of Hairless [Su(H)] and Hairless. Using the female sterile technique, we generated germ line clones deficient for Gro, CtBP or Hairless and assayed sim mRNA relative to snail mRNA expression. We show that sim repression requires both co-repressors Gro and CtBP to be fully repressed in the neuroectoderm, suggesting that a repression complex is assembled including Su(H) and Hairless as was shown for other Notch target genes before. Moreover, our work implies that Gro is important for the repression of sim specifically within the mesoderm anlagen, indicating that Snail and CtBP are insufficient to entirely silence sim in this germ layer.