Apoptosis is a genetically controlled pathway of cell suicide, or programmed cell death. Unlike necrotic cell death it does not lead to an induction of the inflammatory process and so can be used to control cell numbers throughout an organism on a very large scale. The immune system is one that constantly utilises apoptosis to maintain homeostasis. Of the millions of B and T lymphocytes produced every day, only a minute proportion live to fulfil their defensive role. Those that recognise self-antigens must be eliminated before they are able to cause harm, and those that do not adequately recognise foreign antigens are of little use and must also be removed. These cells are ordered to die in a way that causes their host no harm – this is apoptosis
The precise order of events is still controversial, but the broad picture is fairly well understood. The process is triggered by a variety of upstream events such as lack of necessary growth factors, exposure to UV light, or damage to DNA. Following such a damaging stimulus the cell initiates a cascade of events which lead to the activation of caspases. These cysteinyl-aspartyl proteases cleave a range of substrates within the cell resulting in the characteristic packing of the cell contents into membranous vesicles that are rapidly phagocytosed by scavenger cells.
The precise order of events is still controversial, but the broad picture is fairly well understood. The process is triggered by a variety of upstream events such as lack of necessary growth factors, exposure to UV light, or damage to DNA. Following such a damaging stimulus the cell initiates a cascade of events which lead to the activation of caspases. These cysteinyl-aspartyl proteases cleave a range of substrates within the cell resulting in the characteristic packing of the cell contents into membranous vesicles that are rapidly phagocytosed by scavenger cells.
I am using the binding (B) subunit of Shiga-like toxin 1 (SLT-1) as a tool to study the mechanism of apoptosis in human B lymphocytes. SLT-1 is a heterodimeric toxin consisting of a catalytic A chain non-covalently associated with a homopentameric B chain. The B chain pentamer is responsible for binding holotoxin to the cell surface where it interacts with Gb3, a class of glycosphingolipid. This binding is the first obligatory step that leads to toxin endocytosis. In many cell lines, retrograde transport of toxin to the endoplasmic reticulum occurs. From here the reduced A chain gains access to the cytosol to irreversibly inactivate 28S ribosomal RNA which in turn leads to a cessation of protein synthesis and necrotic cell death.
The B chain pentamer by itself is also able to induce death of cells to which it binds, but in this case the mechanism is via apoptosis. My research will investigate apoptosis of B cells triggered by SLT B chain, and aims to identify the natural ligand binding to Gb3 of human B lymphocytes
Rose Jarvis 2001