Phosphorylation of the proapoptotic BH3-only protein bid primes mitochondria for apoptosis during mitotic arrest.

Mitosis is a moment of exquisite vulnerability for a metazoan cell. Failure to complete mitosis accurately can lead to aneuploidy and cancer initiation. Therefore, if the exit from mitosis is delayed, normal cells are usually removed by apoptosis. However, how failure to complete mitosis activates apoptosis is still unclear. Here, we demonstrate that a phosphorylated form of the BH3-only protein Bid regulates apoptosis if mitotic exit is delayed. Bid is phosphorylated on serine 66 as cells enter mitosis, and this phosphorylation is lost during the metaphase-to-anaphase transition. Cells expressing a nonphosphorylatable version of Bid or a BH3-domain mutant were resistant to mitotic-arrest-induced apoptosis. Thus, we show that Bid phosphorylation primes cells to undergo mitochondrial apoptosis if mitotic exit is delayed. Avoidance of this mechanism may explain the selective pressure for cancer cells to undergo mitotic slippage.

Bcl-2 proteins and mitochondria--specificity in membrane targeting for death.

The localization and control of Bcl-2 proteins on mitochondria is essential for the intrinsic pathway of apoptosis. Anti-apoptotic Bcl-2 proteins reside on the outer mitochondrial membrane (OMM) and prevent apoptosis by inhibiting the activation of the pro-apoptotic family members Bax and Bak. The Bcl-2 subfamily of BH3-only proteins can either inhibit the anti-apoptotic proteins or directly activate Bax or Bak. How these proteins interact with each other, the mitochondrial surface and within the OMM are complex processes we are only beginning to understand. However, these interactions are fundamental for the transduction of apoptotic signals to mitochondria and the subsequent release of caspase activating factors into the cytosol. In this review we will discuss our knowledge of how Bcl-2 proteins are directed to mitochondria in the first place, a crucial but poorly understood aspect of their regulation. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

FoxG1 and TLE2 act cooperatively to regulate ventral telencephalon formation.

FoxG1 is a conserved transcriptional repressor that plays a key role in the specification, proliferation and differentiation of the telencephalon, and is expressed from the earliest stages of telencephalic development through to the adult. How the interaction with co-factors might influence the multiplicity and diversity of FoxG1 function is not known. Here, we show that interaction of FoxG1 with TLE2, a Xenopus tropicalis co-repressor of the Groucho/TLE family, is crucial for regulating the early activity of FoxG1. We show that TLE2 is co-expressed with FoxG1 in the ventral telencephalon from the early neural plate stage and functionally cooperates with FoxG1 in an ectopic neurogenesis assay. FoxG1 has two potential TLE binding sites: an N-terminal eh1 motif and a C-terminal YWPMSPF motif. Although direct binding seems to be mediated by the N-terminal motif, both motifs appear important for functional synergism. In the neurogenesis assay, mutation of either motif abolishes functional cooperation of TLE2 with FoxG1, whereas in the forebrain deletion of both motifs renders FoxG1 unable to induce the ventral telencephalic marker Nkx2.1. Knocking down either FoxG1 or TLE2 disrupts the development of the ventral telencephalon, supporting the idea that endogenous TLE2 and FoxG1 work together to specify the ventral telencephalon.

How adhesion signals reach a mitochondrial conclusion--ECM regulation of apoptosis.

A fundamental aspect in metazoans is the ability of a cell to recognise its positional context within a tissue. This is important in both development and homeostasis, where cell proliferation, differentiation and apoptosis are strictly controlled to form and maintain tissues. Much information has been generated on how cells receive and interpret adhesion-mediated signals. The non-receptor tyrosine kinase, Fak (focal adhesion kinase) has received much attention with regard to adhesion mediated signalling, including its role in survival. Survival signals are required to suppress the default pathway of apoptosis. The ultimate outcome of apoptotic signalling is the release of factors from the mitochondria into the cytosol. How the defined signalling pathways that control apoptosis converge on the mitochondria is an area with many unresolved questions.

Bid binding to negatively charged phospholipids may not be required for its pro-apoptotic activity in vivo.

Bid is a ubiquitous pro-apoptotic member of the Bcl-2 family that has been involved in a variety of pathways of cell death. Unique among pro-apoptotic proteins, Bid is activated after cleavage by the apical caspases of the extrinsic pathway; subsequently it moves to mitochondria, where it promotes the release of apoptogenic proteins in concert with other Bcl-2 family proteins like Bak. Diverse factors appear to modulate the pro-apoptotic action of Bid, from its avid binding to mitochondrial lipids (in particular, cardiolipin) to multiple phosphorylations at sites that can modulate its caspase cleavage. This work addresses the question of how the lipid interactions of Bid that are evident in vitro actually impact on its pro-apoptotic action within cells. Using site-directed mutagenesis, we identified mutations that reduced mouse Bid lipid binding in vitro. Mutation of the conserved residue Lys157 specifically decreased the binding to negatively charged lipids related to cardiolipin and additionally affected the rate of caspase cleavage. However, this lipid-binding mutant had no discernable effect on Bid pro-apoptotic function in vivo. The results are interpreted in relation to an underlying interaction of Bid with lysophosphatidylcholine, which is not disrupted in any mutant retaining pro-apoptotic function both in vitro and in vivo.

FAK engages multiple pathways to maintain survival of fibroblasts and epithelia: differential roles for paxillin and p130Cas.

Different cell types interpret their distinct extracellular matrix (ECM) environments to bring about specific cell fate decisions, and can differentiate or undergo apoptosis depending on their local adhesive interactions. Apoptosis in response to an inappropriate ECM environment is termed ;anoikis', or homelessness. Several studies, utilising a variety of cell types, have indicated a common, crucial role for focal adhesion kinase (FAK) in suppressing anoikis. A wide range of different integrins can activate FAK, raising the question of how cell type specific effects are regulated. In this study, we have used a constitutively active form of FAK to examine the mechanism of FAK-mediated survival signalling in cell types from distinct embryonic lineages that show differing sensitivities to anoikis. We demonstrate that both fibroblasts and epithelial cells prevent anoikis through FAK activation. We show that FAK activates multiple downstream pathways in order to suppress anoikis. However FAK regulates survival through a more restricted set of pathways in the more anoikis-sensitive epithelial cells. Furthermore, we identify a novel role for paxillin in apoptosis suppression.