14.5 Cell Death and Replacement [in progress]

Cell death can occur in response to environmental stressors, severe cell damage, or as a part of normal cell functioning. In multicellular organisms, cell death does not have to be lethal to the organism itself, as long as those cells can be replaced (e.g., via mitosis). This chapter focuses on types of cell death in animal cells (e.g., Figure 14.15), and the mechanisms and signalling pathways involved.

Figure 14.15 Cell death through necrosis (cell death due to damage) and apoptosis (programmed cell death). [source? Probably need to replace (if copyright issues) or credit]

Types of cell death

Cell death may be a controlled process initiated by a multicellular organism, or can occur in an uncontrolled or unregulated manner. Necrosis is a type of cell death that is uncontrolled and unplanned when cells are exposed to extreme stress or damage (Figure 14.15). During necrosis, cells swell and burst, releasing their contents into the surrounding tissue. This can cause inflammation and damage to nearby cells and can also activate the immune system in animals. Apoptosis, also known as programmed cell death, is a regulated process that eliminates unnecessary or damaged cells in multicellular organisms (Figure 14.15). In animal cells, apoptosis is a tightly controlled process that involves a sequence of molecular events. Apoptosis can be triggered by internal signals such as DNA damage, or external signals such as the presence of a pathogen or toxin. Autophagy, the physiological process of a cell degrading its own components, may also play a role in cell death. It is still unclear whether the role of autophagy in cell death are simply a consequence of attempting to prevent death or an important cause in the cell death process.

Apoptosis: programmed cell death in animal cells

During apoptosis, the cell undergoes several distinct morphological changes in five major steps (Figure 14.16). Once a cell has been programmed for cell death, the chromosomes condense, and the cytoplasm shrinks. Subsequently, DNA (“laddering”), the nucleus and the cell undergo fragmentation. These apoptotic bodies are then engulfed via phagocytosis and digested by neighbouring cells or specialized immune cells. Apoptosis makes it possible to remove cells that are abnormal and those that are no longer necessary for our development. As humans, programmed cell death not only allows for the formation of complex structures such as our hands, but it also helps eliminate probable cancer cells while helping to maintain homeostasis within our body.

Figure 14.16 The five steps of apoptosis in animal cells, beginning with cell shrinkage, followed by fragmentation, and ending with phagocytosis. [copyright – need to replace]

Mechanisms of apoptosis

Apoptosis relies heavily on a variety of cellular components, most notably caspases. Caspases are a family of enzymes that cleave target proteins. Non-apoptotic cells contain inactive procaspases that are activated by proteolytic cleavage, a reaction that hydrolyzes peptide bonds forming smaller subunits (Figure 14.17). These smaller subunits are initiator caspases (IC), which cleave and activate effector caspases (Figure 14.17). Effector caspases cleave target proteins, also by proteolytic cleavage. The cleavage of these target proteins initiate a series of changes (listed in Table 14.1), which promote controlled cell death.

Figure 14.17 Connections between procaspase, initiator caspases, effector caspases, and cell death. [potentially need to remake to better reflect the relationship between initiator and effector caspases, add targets – i.e., get rid of Table below]

Table 14.1 Targets of effector caspases and consequences in cell death. [would be better in a figure]

Apoptosis: signalling pathways

Apoptosis can be initiated through two pathways. The intrinsic pathway is induced by intracellular signalling in response to cellular changes. It activates pro-apoptotic proteins, resulting in signalling cascades that lead to cell death. The extrinsic pathway is activated by the absence or presence of extracellular signalling molecules.

Intrinsic Pathway

Under normal functioning, mechanisms are in place to inhibit pro-apoptotic pathways. BAX and BAK are proteins on the mitochondrial outer membrane (MOM) that activate apoptosis, but the Bcl-2 protein inhibits their activity (Figure 14.18). Furthermore, initiator caspases are typically inhibited by XIAP (Figure 14.18).

Figure 14.18 Normal functioning in non-apoptotic cells. Bcl-2 inhibits pro-apoptotic proteins BAK and BAX, and IC are inhibited by XIAP. [needs to be redrawn to put cyt C and SMAC in MIM, XIAP and caspases in cytosol. Abbreviations, colour-coding should be defined in caption. Perhaps best thing would be to combine this figure and the following figure so that the full process is represented.]

During periods of cell stress, BH3 is activated, inhibiting Bcl-2 (Figure 14.19). Consequently, BAX and BAK are activated, allowing for the permeabilization of the MOM (Figure 14.19). This releases proteins from the mitochondrial inner membrane (MIM), including cytochrome-c and SMAC (Figure 14.19). Cytochrome-c binds with the protein APAF1, creating the apoptosome complex that cleaves procaspase 9, generating IC9 (Figure 14.19). At the same time, SMAC inhibits XIAP, allowing IC9 to cleave and activate EC, resulting in cell death (Figure 14.19).

Figure 14.19 Apoptotic pathway under cell stress. BH3 inhibits anti-apoptotic protein Bcl-2, activating BAK and BAX. Cytochrome-c and APAF1 form the apoptosome complex which cleaves procaspases. SMAC inhibits XIAP, activating IC, leading to cell death. [same comments as for previous figure]

Two examples of cell stress that initiate apoptosis include persistent DNA damage and increases in intracellular Ca²⁺ concentrations. When a cell experiences DNA damage, the p53 protein becomes active, and inhibits G₁ Cdk-cyclin, an enzyme responsible for DNA replication and the transition from the G₁ to S phase, so DNA repair can be attempted (Figure 14.20). If DNA repair is unsuccessful, p53 initiates apoptosis by activating Puma. This inhibits Bcl-2, and apoptosis proceeds as described above (Figure 14.20). In cases of persistent stress, intracellular Ca²⁺ concentrations will increase, causing mitochondrial swelling. This releases cytochrome-c, creating the apoptosome complex, resulting in apoptosis (Figure 14.20).

Figure 14.20 Examples of intracellular changes that stimulate intrinsic apoptosis signalling pathways. When there is irreparable DNA damage, PUMA inhibits Bcl-2, allowing the apoptotic pathway to proceed. In stressful environments, calcium concentration in the cell increases, releasing cytochrome-c and initiating apoptosis. [define abbreviations]

Extrinsic Pathway

The extrinsic pathway of apoptosis can be controlled by both survival factors and death receptors. Survival factors are signalling molecules such as growth factors and hormones that activate signalling transduction pathways that inhibit apoptosis. Survival factor signalling molecules, like growth factors and hormones, activate the Akt enzyme, which phosphorylates and deactivates the pro-apoptosis protein Bad (Figure 14.21). Without survival factors, Bad is no longer phosphorylated and is able to inhibit Bcl-2, allowing apoptosis to proceed (Figure 14.21). Therefore, in the presence of a survival factor, normal cells survive, but in the absence of a survival factor, normal cells undergo apoptosis.

Figure 14.21 The impact of survival factors on extrinsic apoptosis signalling. Without survival factors, BAD is no longer phosphorylated and can inhibit Bcl-2, initiating apoptosis. [define abbreviations]

Death receptors are also part of the extrinsic pathway and involve signaling molecules such as FAS ligand, TNF𝛂, and TRAIL that trigger apoptosis. These molecules bind to death receptors, activating initiator caspases 8 and/or 10, which then activate effector caspases 3 and 7, as well as other pro-apoptotic proteins such as BID (Figure 14.22). The death receptor pathway is involved in various physiological processes, such as cytotoxic T-cell activity and development/embryology.

Figure 14.22 The impact of death receptors on extrinsic apoptosis signalling. When a death receptor binds with specific signalling molecules, caspases are activated, initiating apoptosis. [define abbreviations]