Regulation of Cell survival by Cell / Extracellular Matrix Adhesion.
Our primary interest is how adhesion mediated signalling regulates epithelial cell survival. Cells require signals from the extracellular matrix (ECM) to provide them with positional information regarding their context within a tissue. Not only is this information required for cells to proliferate and differentiate, but also the absence of positional cues results in apoptosis, a genetically defined programme to remove damaged, displaced or surplus cells. Precise regulation of this process is critical to cell function and its perturbation is seen in many diseases.
Apoptosis is controlled by the Bcl-2 family of proteins. These form pores that release a variety of factors from mitochondria (e.g. cytochrome c, Smac/DIABLO) required for initiating cell death. Permeabilisation of the outer mitochondrial membrane (OMM) is, therefore, the central event in the apoptotic programme and its correct regulation is vital. We have found that adhesion initiated signals regulate the Bcl-2 protein, Bax. In adherent cells Bax is sequestered in the cytosol in a latent conformation. Following inhibition of adhesion signalling, it translocates to the OMM where it undergoes a conformational change and assembles into multimolecular complexes thought to form the pores responsible for OMM permeabilisation. Bax function is thus regulated at a number of levels, firstly at the level of its sub cellular localisation, and then following translocation where it undergoes activation and assembly. This later step appears to be a critical step when a cell actually commits to the apoptotic pathway, and may act as a survival "check point".
Current work is following a number of complimentary approaches. We are defining the molecular signalling pathway by which Bax subcellular localisation is controlled, which requires adhesion activated protein kinases. Secondly, we are determining the molecular basis by which Bax switches between inactive and active conformations, examining how interactions between Bax and possible chaperones influence its conformation, and in particular how specific domains of Bax regulate its targeting. Thirdly, we are defining the molecular interactions associated with cell commitment to apoptosis. We are using a combination of molecular and cell bioogical methods, including live cell imaging and FRET. It is hoped that this work will illuminate the processes by which cell fate decisions are made at a molecular level.