London [UK], June 5 : For the first time, researchers have identified a distinct molecular mechanism underlying the early phases of programmed cell death, or apoptosis, a process which plays an essential role in cancer prevention.
Dr Luke Clifton from the STFC ISIS Neutron and Muon Source (ISIS) in Oxfordshire led the project co-directed by Professor Gerhard Grobner at the University of Ume and collaborators at the European Spallation Source in Sweden. This is the most current in a series of research partnerships by this team, which is looking at the biological proteins that cause apoptosis.
Apoptosis is essential for human life, and its disruption can cause cancerous cells to grow and not respond to cancer treatment. In healthy cells, it is regulated by two proteins with opposing roles known as Bax and Bcl-2.
The soluble Bax protein is responsible for the clearance of old or diseased cells, and when activated, it perforates the cell mitochondrial membrane to form pores that trigger programmed cell death. This can be offset by Bcl-2, which is embedded within the mitochondrial membrane, where it acts to prevent untimely cell death by capturing and sequestering Bax proteins.
In cancerous cells, the survival protein Bcl-2 is overproduced, leading to uninhibited cell proliferation. While this process has long since been understood to be important to the development of cancer, however, the precise role of Bax and the mitochondrial membrane in apoptosis has been unclear until now.
Dr Luke Clifton, STFC ISIS Neutron and Muon Source scientist and co-lead author, explains: "This work has both advanced our knowledge of fundamental mammalian cell processes and opened exciting possibilities for future research. Understanding what things look like when cells work properly is an important step to understanding what goes wrong in cancerous cells and so this could open doors to possible treatments."
The team used a technique known as neutron reflectometry (conducted using the advanced ISIS Surf and Offspec instruments) which enabled them to study how Bax interacts with lipids in the mitochondrial membrane. This was built on their previous studies of membrane-bound Bcl-2.
Using neutron reflectometry on SURF and OFFSPEC, they were able to study in real time the way that the protein interacts with lipids present in the mitochondrial membrane, during the initial stages of apoptosis. By employing deuterium-isotope labelling, they determined for the first time that when Bax creates pores, it extracts lipids from the mitochondrial membrane to form lipid-Bax clusters on the mitochondrial surface.
By using time-resolved neutron reflectometry in combination with surface infrared spectroscopy in the ISIS bio lab, they were able to see that this pore creation occurred in two stages. Initial fast adsorption of Bax onto the mitochondrial membrane surface was followed by a slower formation of membrane-destroying pores and Bax-lipid clusters, which occurred simultaneously. This slower perforation process occurred on timescales of several hours, comparable to cell death in vivo.
This is the first time that scientists have found direct evidence of the involvement of mitochondrial lipids during membrane perturbing in cell death initiated by Bax proteins.
Dr Luke Clifton continued, "As far as we can tell, this mechanism by which Bax initiates cell death is previously unseen. Once we know more about the interplay between Bax and Bcl-2 and how it relates to this mechanism, we'll have a more complete picture of a process that is fundamental to human life. This work really shows the capabilities of neutron reflectometry in structural studies on membrane biochemistry."
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