Cellular Aging: Cell Death

Cell senescence is distinct from what is properly called cell death. Cells can, of course, encounter violent situations in which proteins precipitate, membranes are ruptured, or their access to energy sources is destroyed. In these situations, cells typically lose the ability to maintain their volumes against osmotic forces, and they swell and rupture (technically, lyse), spilling their contents and provoking an inflammatory response. This process is called either necrosis or oncosis, and is seen in acute situations such as infarct (a region of tissue suddenly deprived of blood flow, as when a clot lodges in a small artery or arteriole), severe chemical toxicity, and extreme thermal damage.

Necrotic cells, generated in an uncontrolled manner, create many problems for an organism because of inflammation and because of the leakage of potentially dangerous chemicals or enzymes. Also, cells lysing from infection may spew out viruses or other pathogens. As a protective mechanism, therefore, organisms can preempt such deaths by invoking a much more biological and controlled response, known as apoptosis, or programmed cell death. These forms of death are a sort of cell suicide, in which cells self-destruct in a controlled and contained manner. All cells carry within themselves the capacity to self-destruct, but are normally restrained from doing so. If this restraint is removed when a cell is challenged, it will default to the self-destruct mode and, assuming that the challenge is not so severe that the cell becomes necrotic, it will undergo this physiological form of death.

The term programmed cell death derived originally from developmental and embryonic observations, and it emphasizes the idea that specific genes regulate the death of cells. Many of these genes have now been recognized and are described below. In developmental situations, death frequently, if surprisingly, requires the synthesis of new proteins, perhaps including those involved in killing the cell. The morphology is more often than not apoptosis. However, apoptosis does not necessarily require protein synthesis and is usually not programmed (meaning that the sequence of death is coded in the genes) except in the generic sense that it was preprogrammed into the cell and simply required release or activation.

The apoptotic cell is recognized by several characteristics. It is a rounded, blebbing cell, with limited permeability of its cell membrane. In an active process, it has moved a component of the inner cell membrane, phosphatidyl serine (PS), to the external surface. The exposed PS will serve as a signal to the phagocytes that will consume it. The chromatin (the complex of protein, DNA, and RNA that can be strained, rendering the chromosome—Greek for colored body—visible) coalesces in the nucleus and frequently marginates, or condenses, along one side of the nuclear membrane. The DNA then fragments into pieces that are multiples of 180 base pairs (nucleosomal fragments), detectable by electrophoresis or by TUNEL (Terminal deoxyUridine Nucleotide End Labeling) in situ. There are, however, many gray areas, and some cells may display intermediate patterns.

There are many entry points into apoptosis, and there are several variants, depending on tissue type and history. Three of these variants may be summarized as follows:

Caspase-dependent apoptosis. A chief effector of apoptosis is caspase-3, a highly specific protease. Proteases are enzymes that digest proteins. They are divided into several categories, with one classification referring to an amino acid in the enzyme that is essential for its activity. Caspases contain an essential cysteine, contributing the "c" in the name of the enzyme. Also, most proteases recognize a specific amino acid sequence in the substrate and cut the substrate protein at that site. Caspases identify a sequence of four amino acids terminating in aspartic acid. Thus the name caspase is derived from Cysteine ASPartyl protease. Very few proteins have the appropriate sequences, but those that do, and are thus destroyed by caspase-3, include cytoskeletal components that maintain the shape of the cell, enzymes necessary for synthesizing messenger RNA, and enzymes needed for repair of DNA damage. Caspase-3 exists in cells as an inactive proenzyme, with its activity blocked by extra amino acids added during its synthesis. This extra sequence begins with a site recognized by other caspases. Thus, other caspases remove this sequence and activate the enzyme. The active enzyme can also activate other pro-caspase-3 molecules (autoactivation). This process is well described by Earnshaw, Martins, and Kaufmann (1999).

Fas-dependent apoptosis. In many situations, particularly in the immune system, the number of cells is tightly regulated. Cell number has to be increased rapidly to fight an infection and reduced again after the infection subsides. Failure to precisely control numbers may result in autoimmune reactions, in which the body makes antibodies to its own proteins, generating life-threatening inflammations, or to the loss of too many cells, leading to increased susceptibility to infection or an inability to conquer an infection. Thus, mechanisms for cell death are very elaborate in the immune system, though the control mechanisms are used by other cells as well. Many of these cells carry on their surface one member of a family of closely related proteins. One of the most common proteins is called Fas, after an activity first recognized by immunologists. Fas can bind the protein Fas Ligand, which itself may either circulate in the blood or be attached to another cell. Fas bound to Fas Ligand can also attach to one or two similarly linked Fas molecules, forming dimers (two molecules linked) or trimers (three molecules). All of the Fas molecules stretch across the cell membrane to the intracellular side. The dimer or trimer forms interact with other proteins on the inside of the cell in a complex reaction that ultimately results in the freeing of pro-caspase-8 from an inactive bound form. This caspase-8 becomes activated and activates caspase-3, leading to apoptosis. Other receptors in the Fas Ligand family include those binding tumor-necrosis factor, and all members of the family contain similar amino acid sequences and structures, including a region important for the activation of caspases called the death domain.

Fas-independent apoptosis. Apoptosis may also be activated by mechanisms independent of the Fas-FasL pathway. For instance, any of a number of mechanisms may damage mitochondria, leading to the depolarization of the mitochondria, opening of a charge-dependent pore (the mitochondrial membrane permeability transition pore), and leakage of cytochrome c and other mitochondrial components into the cytoplasm. The cytochrome c displaces an inhibitor from pro-caspase-9, allowing its activation, whereupon it activates caspase-3.

Caspase-independent cell death. Some cell deaths, most typically those of large, cytoplasm-rich cells or postmitotic cells, do not rely heavily on caspases. They therefore display a somewhat different morphology from that described below and exhibit rather an autophagic morphology. In autophagy—literally, self-eating—the bulk of the cytoplasm is destroyed in large lysosomal vesicles (autophagosomes) before the morphology becomes more classically apoptotic.