intrinsic pathway and extrinsic pathway of apoptosis.
The Intrinsic Pathway: Cell Death Starts Inside
As discussed above, apoptosis can be initiated by internal events (i.e., "intrinsic pathway") involving the disruption of mitochondria and the specific enzyme cytochrome C, in turn leading to the downstream activation of caspases. Before we continue, let’s quickly review mitochondrial structure and function. Mitochondria are called the “powerhouses of the cell” because they are the site where most of the ATP is generated via aerobic metabolism. The following figure (3.9) shows that an outer membrane surrounds each mitochondrion. The inner membrane is folded into the matrix as cristae. F1 particles, which are ATPase enzymes, are localized on the cristae and oriented into the matrix.
Figure 3.9. The structure of a mitochondrion.
This design is very effective because it organizes the metabolic functions of the mitochondria as shown in the following figure (Figure 3.10). Thus glycolysis (anaerobic metabolism) generates ATP in the cytoplasm as well as providing the substrate molecules for aerobic metabolism, leading to ATP production within the mitochondria. This occurs via tightly linked events: the Krebs cycle in the matrix
and the electron transport chain in the cristae, coupled with ATP synthesis via F1 ATPases bound to the cristae.
Figure 3.10. Compartmentalization within a mitochondrion.
Most readers will have learned the details behind mitochondrial structure and function in high school and introductory biology books. But what is important here is the presence of cytochrome C in the electron transport chain and its role in apoptosis (Figure 3.11).
Figure 3.11. Apoptosis initiated by mitochondrial damage.
Typically this molecule is tightly bound within the cristae. But, as shown in Figure 3.11, when mitochondria are disrupted, the cytochrome C located on the inner mitochondrial membrane gets released allowing it to bind to and activate caspase 9. Similarly Apaf-1 (apoptotic activating factor-1) is bound to mitochondria via Bcl-2 (beta-cell lymphoma-2). Bcl-2 is found in a number of cancers where it suppressed apoptosis. Mitochondrial disruption leads to the release of Apaf-1 which also binds to caspase 9, activating it. This is just one example of many intrinsic pathways.
Death Receptors and the Extrinsic Pathway
Alternatively, surface receptors can be activated by specific ligands that bind to “death receptors” (i.e., “Extrinsic Pathway”). Death receptors are members of the tumor necrosis factor (TNF)/nerve growth factor (NGF) receptor superfamily. They make up a subfamily characterized by the intracellular death domain (DD). The extrinsic pathway is typically mediated by immune cells to initiate intracellular signaling and the downstream activation of relevant caspases. Some work suggests both intrinsic and extrinsic pathways mediate apoptosis during oogenesis and likely of aging eggs after fertilization.
The diagram presented in Figure 3.12 shows some of the signaling events that are initiated when tumor necrosis factor alpha (TNFα) leads to apoptosis. It should be noted that TNFα also mediates other signaling pathways involved in normal cellular functions. For TNF-mediated apoptosis the acronym TRAIL (tumor-necrosis factor-related apoptosis inducing ligand) is used to specify this function versus the factor’s other roles.
Figure 3.12. The signaling events initiated by TNFα binding.
The binding of TNFα to its receptor (TNF-receptor or TNFR) makes the receptor’s intracellular death domain available for binding to TRADD (TNFR-associated death domain). TRADD is an adaptor that in turn directs the binding of FADD (Fas-associated death domain), another adaptor that mediates the binding of pro-caspase-8 to this multiprotein complex. This leads to the proteolytic processing of the inactive pro-caspase-8 into the active caspase-8 enzyme. Caspase-8 is an initiator caspase that in turn proteolytically activates several other caspases. The activated caspases-3,6 and 7 are effector caspases that proteolytically digest a number of target proteins, ultimately leading to apoptosis. There are a number of other apoptosis-specific pathways, each of which involves unique sets of adaptor proteins and caspases and each of which is designed to direct apoptosis at a specific place or time in human development or other aspects of cell function.
Chapter 4
Egg Differentiation and Genetic Abnormalities
The egg is a differentiated cell type. It is specialized in many ways: to receive the sperm during fertilization, to supply the majority of the cytoplasm for early development, to provide half of the genome for the zygote and to provide information to initiate the events of early development. The egg specializes early and subsequently undergoes unequal cell divisions during meiosis (releasing smaller polar bodies) so that this differentiated egg cytoplasm is not greatly diminished. One aspect of egg structure is the presence of various "envelopes" that surround it. As we will see, these cellular and non-cellular (extracellular matrix) components are critical to the survival and fertilization of the egg.
The Egg is a Differentiated Cell
Compared to other cells in the body, the egg is a very large, essentially round cell. The growth and differentiation phases occur simultaneously during prophase I of meiosis. At this stage the nucleus is called a germinal vesicle. The germinal vesicle is a very specialized nucleus. For example it contains a minimal version of "lampbrush chromosomes", common to other species, that are actively involved in gene transcription. The egg itself is surrounded by egg coats which consists of cells (cumulus oophorus; corona radiata) and zona pellucida (protein “shell”) as will be detailed in Chapter 6 when we discuss its role in fertilization. The egg also contains many specialized organelles in its cytoplasm. Cortical granules (see Chapter 6) align adjacent to the egg cell membrane in anticipation of fertilization. Adjacent to the nucleus are the annulate lamellae. The annulate lamellae consist of parallel stacks of nuclear envelope-like membranes that lie adjacent to the nucleus which may give rise to them. Evidence indicates that the annulate lamellae are essential for the formation of the pronuclei during fertilization.
Many animals have large store of yolk but this is not the case in humans. The human egg has a minimal amount of yolk. Interestingly, yolk proteins made in liver and are transported to egg via the blood. Yolk proteins are taken into growing oocytes via receptor-mediated endocytosis. The small amount of yolk is due to the fact that the developing embryo only needs internal nutrients until implantation. At that time it obtains nutrients from the maternal body via the placental relationship.
Meiotic Divisions
Human gametes, like those of mammals and most other animals, are haploid (i.e., contain only half the amount of somatic DNA). This is because the diploid state will be re-established at fertilization when the haploid sperm and haploid egg fuse to produce the diploid zygote. The goal of meiosis is to reduce the diploid state to haploid via two meiotic divisions. The problem with meiosis during oogenesis is the egg has gone through a period of significant growth and differentiation. During
