Apoptosis: Programed Cell Death. Mechanisms of Action and Its Significance as a Biological Phenomenon

Apoptosis: Programmed Cell Death

This review article attempts to look at just some of the complex biochemical networks and mechanisms involved in the process of apoptosis, including not only the intrinsic and extrinsic pathways, but also how apoptotic cells undergo morphological changes. Furthermore it will show its importance as a biological phenomenon whilst also looking at the effects of defective apoptosis, both excessive and insufficient apoptosis which can lead to uncontrolled cell proliferation.

11th September 2011

Introduction

Apoptosis is the process of programmed cell death in multicellular organisms for various purposes through biochemical and characteristic morphological changes in the cell in response to both intra- and extracellular stimuli . At this point it is important to define what is meant by 'cell death'. The death of a cell is most commonly defined as an irreversible loss of plasma membrane integrity .

In human adults a remarkable 50-70 billion cells die via apoptosis every day, and the around the same number is replaced each day as part of the body's natural processes . The cells which die will have done so through one of two, very different, paths. The first is necrosis, a form of premature cell death caused by external factors (e.g. Infection, toxins and trauma). This in contrast to the second method of cell death, which is necrosis, is almost always detrimental and often fatal, whereas apoptosis is mostly beneficial to the organism (as shown by Figure 1 ). So apoptosis is a mode of cell death which occurs in 'normal' physiological conditions. Furthermore in apoptosis, the cell is actively involved in its death; hence apoptosis is sometimes referred to as cell suicide, whereas in necrosis the targeted cell is not an active participant in its death. A significant difference between apoptosis and necrosis is that the cells which die due to necrosis do not send out the same chemical signals or "find me" signals that cells undergoing apoptosis do, so that phagocytes, which are white blood cells of the immune system, cannot locate the dead cells and engulf them, which in turn leads to a build-up of dead tissue .

Comparing the two forms of cell death is a useful method in which to show the characteristics of apoptosis. One such difference is that necrosis begins by impairing the cell's ability to maintain homeostasis, which is the ability to regulate its internal environment and keep a stable, constant condition. This leads to an influx of water and extracellular ions which in turn causes the both the intracellular organelles, such as mitochondria, and the entire cell to swell and rupture (a process known as cell lysis). Due to the plasma membrane breakdown the cellular contents are released into the extracellular fluid (See Figure 2 ) and onto surrounding cells which may result in an inflammatory response in the body . This is due to the macrophage (a specialised phagocyte) releasing proinflammatory cytokines such as tumour necrosis factor α (TNFα). In contrast, cells undergoing apoptosis show morphological and biochemical features which result in no inflammatory immune response, due to the release of anti-inflammatory cytokines such as transforming growth factor β (TGFβ) into the surrounding area . After a cell has been induced to undergo apoptosis, it starts to shrink following the cleavage of the lamins and actin filaments of the cytoskeleton by caspase enzymes, which makes irregular bulges in the plasma membrane, known as blebs .

Meanwhile the nucleus undergoes pyknosis, which is the irreversible condensation of chromatin in the nucleus. This leads to the disassembly of the nuclear envelope and the fragmentation of the nuclear DNA, called karyorrhexis; this is one of the hallmarks of apoptosis . In this process, the cleavage of the chromosomal DNA into 180bp nucleosomal units is accomplished by an enzyme called CAD (caspase-activated DNase). Caspases are cysteine-based proteases which are essential to the process of apoptosis, including the protein cleavages in the cytoskeleton, which is responsible for the morphological and biochemical changes that kill the cells. The DNA fragmentation by CAD at this stage is a very clear example of how caspase activation causes characteristic features of apoptosis, because in healthy cells CAD is inhibited by ICAD (inhibitor of CAD) forming a complex with it. But during apoptosis caspase-3, which is a member of the caspase family which plays a central role in the execution of apoptosis , cleaves the ICAD at two positions which allows the CAD to degrade the chromosomal DNA within the nucleus .

Then the cell membrane continues to bulge due to blebbing until these bulges separate from the cell taking cytoplasm within it, forming apoptotic bodies, which is a form which allows for their removal either by macrophages or surrounding cells by a process known as phagocytosis. It is important to note that apoptotic cells are mostly, almost exclusively, by macrophages. Caspases again, plays a significant role in apoptosis, during this process of membrane blebbing. ROCK1, which is a substrate of caspase-3, is known to phosphorylate the cytoskeletal proteins, myosin and actin. Normally ROCK1 is regulated by Rho GTPase, which is described as a "molecular switch" for playing its role in apoptosis , but the cleavage by caspase-3 removes this regulatory domain and allows ROCK1 to be active. This aberrantly activated ROCK1 intensively phosphorylates myosin chains, leading to membrane blebbing . In addition to the 2 examples of actions taken by caspase-3, caspase-activation is also involved in the cells' production of the "find me" and "eat me" signals which are sensed by receptors on the phagocytes, which shall be discussed later on .

Phagocytosis not only avoids the damaging consequences of cell necrosis but also allows the organic components of the dead cell to be recycled by the cell that ingests it. This process is carried out by complex cell machinery which directs the phagocytised apoptotic bodies to lysosomes, which contain enzymes, and degrade their cellular components into basic biochemical building blocks: amino acids, nucleotides, fatty acids and monosaccharaides . These phagocytic cells are responsible for clearing the apoptotic cells from tissues in a clean and tidy fashion that avoids many of the problems associated with necrotic cell death. In order to promote their phagocytosis by macrophages, apoptotic cells often undergo plasma membrane changes that trigger the macrophage response .

But why is apoptosis necessary? Firstly, programmed cell death is as imperative as