9.9Signals of Cell Death
There are signals designed to lead cells to death. In nematodes, some cells are genetically destined to die during the developmental process (see the Column in 10.3). In such cells, the nucleus is condensed following the disappearance of the nuclear envelope, blebs are formed in the cytoplasm, and the cell shrinks and eventually dies - a characteristic process from which the phenomenon derives its name, apoptosis (apo means “away” and ptosis means “falling” in Greek). The signal transduction in apoptosis is transduceed by a chain reaction involving the protease known as caspase. As shown in Figure. 9-11, the caspase chain reaction may be initiated by the release of the protein known as cytochrome c from mitochondria to the cytoplasm or by the activation of the Fas receptor on the plasma membrane by extracellular signaling molecules mediating apoptosis. The latter mechanism is effective for killing cells infected with viruses.
The dysfunction of apoptosis reactions, which allows cells that are supposed to die to remain alive, is one of the causes of cancer, and is also known to exacerbate immune disorders and other diseases.
Fig. 9-11. Apoptotic signals leading to cell death
Apoptosis takes place as a result of a chain reaction involving caspase (a protease). It may occur as a result of the release of a protein known as cytochrome c from mitochondria in the cell or the activation of a receptor known as Fas on the cell surface.