Apoptosis Review

Mechanism of Cell Death

There are many different mechanisms of cellular death including, but not limited to, necrosis, autophagy and apoptosis, each of which have different triggers and different pathomorphologies. Necrosis is an inflammatory process of cell death that is usually caused by extracellular damage, but research has also shown that the cell may initiate the process in order to induce inflammatory and/or reparative processes in the organism (Zong and Thompson, 2006). Autophagy, or “self-eating,” is a non-inflammatory, self-destructive cellular process in which the cell removes misfolded proteins, clears damaged organelles, and essentially degrades itself (Glick, Barth and Macleod, 2010). Apoptosis is an ATP-requiring self-destructive cellular process characterized by blebbing, the process by which the cell breaks neatly into membrane-bound apoptotic bodies, without causing an inflammatory response nor damaging surrounding cells (Edinger and Thompson, 2004). This page will review the mechanisms by which apoptosis is regulated See Table 1 from Inoue and Tani in 2014 for more distinctions between the different cell death pathways.

What is apoptosis?

Apoptosis, “falling off,” is also known as programmed cell death, and is important to many processes including regulation of cellular populations, immune system development and function, hormone-dependent atrophy, embryonic development, and chemically induced cell death (Elmore, 2007). In contrast to other forms of cellular death, which are a result of outside influences on the cell, apoptosis is essentially cellular suicide. The mechanism of apoptosis is unique from other forms of cellular death because of the extensive plasma membrane blebbing that takes place. Blebbing of the membrane into neat apoptic bodies prevents inflammation by packaging away cellular components rather than spilling them out into the intracellular matrix, as in necrosis (Elmore, 2007). One of the most characteristic features of apoptosis is the process of pyknosis, or karyopyknosis, in which the chromatin in the cell condenses. Pyknosis is accompanied by overall cell shrinkage causing the cytoplasm to become dense and the organelles more tightly packed (Elmore, 2007) as seen in the video below. 

In addition to cell shrinkage and blebbing, an important feature of apoptosis is the expression of phosphatidylserine on the outside of the cellular membrane as an “eat-me” signal for macrophages and other phagocytic cells. This promotes the clearing of the cellular debris resulting from the apoptic process and prevents an inflammatory response (Inoue and Tani, 2014).

What causes apoptosis?

Figure 1. The molecular signaling pathways involved in apoptosis.

There are two main apoptic pathways involved in programmed cell death: the extrinsic/death receptor pathway and the intrinsic/mitochondrial pathway, and a third pathway in which the cell is killed via the T-cell mediated perforin/granzyme pathway. While each pathway begins differently, they all converge on one “execution” pathway which involves the cleavage of caspase-3. The caspase proteins are essential to apoptosis, but will not be discussed here as they are focused on in the "Caspases" section of the website.

The extrinsic pathway is initiated by transmembrane receptor-mediated interactions involving receptors in the tumor necrosis factor (TNF) superfamily. Once a ligand has bound to a death receptor, cytoplasmic adaptor proteins are recruited and bind to the corresponding death receptors and initiate a cascade which results in the activation of the execution pathway. One example of the extrinsic pathway is the FasL/FasR model which can be seen in Figure 2 below.

Figure 2. Summary of the FasL/FasR pathway of apoptosis.

The intrinsic pathway, as the name implies, is mediated by intracellular signaling, and is initiated by the mitochondria in response to intracellular signals. These intracellular signals may manifest as a lack of growth factors, hormones or cytokines necessary to keep the brakes on the apoptic process, or intracellular damage from radiation, toxins, viral infections, etc. All of these apoptic triggers cause an opening of the mitochondrial permeability transition (MPT) pore, loss of mitochondrial transmembrane potential, and release of two groups of pro-apoptotic proteins. The first group released acts to activate the caspase-dependent mitochondrial pathway (see caspase section), and consists of cytochrome c, Smac/DIABLO, and the serine protease HtrA2/Omi. Notice the role that cytochrome c plays in activating APAF1 and procaspase-9 in the figure below.

Figure 3. Extrinsic (left) and intrinsic (right) pathways of apoptosis.

The second release of pro-apoptotic proteins is a late event that happens only after the cell is committed to dying, and consists of AIF (apoptosis inducing factor), endonuclease G, and CAD (caspase-activated DNase). These AIF and endonuclease G constitute the “caspase-independent effectors” seen in the figure above. All of them translocate to the nucleus, where they collectively promote DNA fragmentation and chromatin condensation (Stage I condensation). CAD later moves to the nucleus and promotes DNA fragmentation and chromatin condensation, but it must first be cleaved by caspase-3 (Stage II condensation).

All of these mitochondrial apoptotic events are mediated by the Bcl-2 family of proteins, which govern mitochondrial membrane permeability and can be either pro-apoptotic or anti-apoptotic, and a whole host of other cellular signaling molecules (see Figure 1).

The perforin-granzyme B pathway is mediated by cytotoxic T-cells, in which the transmembrane protein perforin allows the passage of the serine protease granzyme B into the target cell. Once inside the cell, granzyme B can activate the caspase-mediated pathway of apoptosis as well as the mitochondrial pathway via cleavage of Bid and induction of cytochrome c release.

*section reference to Elmore, 2007

Why is apoptosis important?

Apoptosis plays an important role not only in maintaining homeostasis and cellular turnover, but also in development and the prevention of disease. Apoptosis is used in organismal development to form structures, such as in the embryonic development of mouse paws. The whole paw forms in a sort of “spade-like” structure and then the cells making up the tissue between the digits undergo apoptosis to allow separation of individual digits (Molecular Biology of the Cell, 4^th ed.).

Figure 4. Development of mouse paws via apoptosis.

Now think of intrinsic apoptosis as a cellular safety switch. When something goes wrong in the cell, there is a lack of appropriate growth factors or irrepairable DNA damage occurs, the brakes are removed from the apoptotic cascade and the cell sacrifices itself without harming any other cells around it. Without this cellular check, diseases like cancer would be more common and more devastating, as is the case in patients with mutations in the important apoptosis regulator p53. See Rivlin et al, 2011 for more information about p53. 

Summary Video Yay!