Cell Biology

Apoptosis is the process of programmed cell death that is a normal component of the development of multicellular organisms. Cells die in response to a variety of stimuli and during apoptosis they do so in a controlled, regulated fashion. This makes apoptosis distinct from another form of cell death called necrosis in which uncontrolled cell death leads to lysis of cells, inflammatory responses and, potentially, to serious damage to organism. Apoptosis, by contrast, is a process in which cells play an active role in their own death. Therefore, apoptosis is often referred to as cell suicide. Apoptosis is needed to destroy cells that represent a threat to the integrity of the organism. These cells may be infected with viruses, with DNA damage, or cancer cells. Apoptotic cells display distinctive morphology during the apoptotic process. These changes include cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation. Apoptosis can be induced in cells through a number of mechanisms, which may originate either extracellularly or intracellularly. Extrinsic signals may be the binding of death inducing ligands to cell surface receptors called death receptors, as well as hormones, growth factors, nitric oxide or cytokines. Apoptosis can be initiated following intrinsic signals that are produced following cellular stress, including heat, radiation, nutrient deprivation, viral infection, and so on. The sensitivity of cells to any of these stimuli can vary depending on a number of factors such as the expression of pro- and anti-apoptotic proteins (e.g. the Bcl-2 proteins or the inhibitor of apoptosis proteins), the severity of the stimulus and the stage of the cell cycle.

Apoptotic adaptor proteins play a critical role in regulating pro- and anti-apoptotic signaling pathways following activation of the death receptors. Adaptor proteins, such as FADD (Fas-associated death domain) and TRADD (TNF receptor-associated death domain), are recruited to ligand-activated, oligomerized death receptors to mediate apoptotic signaling pathways. Apoptotic adaptors associate with the cytoplasmic portion of the death receptors through a domain common to both known as the death domain (DD).

Following association with the ligand-bound receptor, the adaptor proteins recruit pro-caspase-8 or pro-caspase-10 by way of their death effector domains (DED) to form the DISC (death-inducing signaling complex). Formation of the DISC initiates a caspase signaling cascade that ultimately induces apoptosis. Other adaptor proteins mediate apoptotic signaling through distinct mechanisms. CRADD/RAIDD (caspase and RIP adaptor with a death domain), and PIDD (p53-induced protein with a death domain) are two adaptor proteins that associate with pro-caspase-2 to form the PIDDosome following DNA damage. Formation of this complex leads to the cleavage and activation of caspase-2.

DAXX (Fas death domain-associated-xx) and RIP1 (receptor interacting protein 1) are two additional adaptor proteins containing a death domain that are recruited to Fas or to TNF RI respectively, upon ligand binding. These two proteins are involved in mediating caspase-independent apoptotic signaling pathways. Like TNF RI and Fas, TRAIL receptors (TRAIL R1/DR4 or TRAIL R2/DR5) and DR3 also contain intracellular death domains and can induce apoptosis by way of adaptor-mediated caspase-8 or caspase-10 recruitment. In addition to promoting apoptosis, ligand binding to TNF RI (TNF-alpha), or DR3 (TWEAK) can cause the intracellular adaptor proteins to recruit TRAF-2 (TNF Receptor-associated factor 2). TRAF-2 signaling subsequently leads to the activation of NFkB which induces the expression of anti-apoptotic genes such as FLIP and Bcl-2.

The cell cycle is an ordered set of events, culminating in cell growth and division. The cell cycle of eukaryotes can be divided in two brief periods: interphase, during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA, and the mitosis (M) phase, during which the cell splits itself into two distinct cells, often called daughter cells. By studying molecular events in cells, interphase is divided into three stages, G1, S, and G2. Thus the cell cycle consists of four phases: G1, S, G2, M.G1 phase is from the end of the previous M phase until the beginning of DNA synthesis, and G stands for gap. During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. This phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication.

Early work in frog and invertebrate embryos suggested that cell cycle events are triggered by the activity of a biochemical oscillator centered on cyclin-CDK complexes. The cyclin/CDK complexes induce two processes, duplication of centrosomes and DNA during interphase, and mitosis. The roles of individual cyclins were tested by adding recombinant proteins to cyclin- biologidepleted extracts. Cyclin E supports DNA replication and centrosome duplication, cyclin A supports both of these processes and mitosis, and cyclin B supports mitosis alone.