Apoptosis is the process of programmed cell death that occurs in every multicellular organism and plays a crucial role in shaping organs during development and controlling homeostasis and proper function of various tissues in adult organisms, including the human reproductive system. Unlike necrosis, apoptosis is an energy-requiring and well co-ordinated process that results in the formation of apoptotic bodies that are engulfed by the neighboring cells or macrophages without causing an inflammatory response.
The default pathway of ovarian follicles is to undergo apoptosis, as only ~400 follicles ovulate during a female’s reproductive life, while the rest of the developing follicles become atretic and die during folliculogenesis. During the fetal period, the main cell type undergoing apoptosis are the germ cells. Oocytes that fail to become surrounded by somatic granulosa cells during primordial follicle formation degenerate and undergo apoptotic demise. In the postnatal ovary, apoptosis is prominent in the granulosa cells of the growing follicles during the cyclic recruitment. Furthermore, apoptosis is responsible for corpus luteum regression if pregnancy does not occur (104).
22 Regulation of ovarian apoptosis
Gonadotropins are important regulators of postnatal ovarian apoptosis. In vivo rodent studies (105, 106) and in vitro studies utilizing the cultured follicles (107) have demonstrated that the decrease in gonadotropin levels causes follicular atresia, while the early apoptotic follicles can be rescued by exogenous gonadotropins. Locally produced paracrine growth factors (e.g. Insulin-like growth factor 1, IGF1; Epidermal growth factor, EGF; Basic fibroblast growth facor, FGF; and Interleukin-1 , IL-1 ) as well as hormones (e.g. estrogen and progesterone) also play a role in the regulation of ovarian apoptosis by acting as prosurvival factors of granulosa cells and inhibiting apoptosis (107-111).
In addition to the aforementioned prosurvival factors, two cellular apoptotic pathways, the extrinsic and intrinsic pathways, also regulate ovarian apoptosis. The extrinsic pathway is activated by binding of extracellular protein ligands to the proapoptotic death receptors (DR) located on the cell surface, whereas the intrinsic pathway (also termed as mithocondrial pathway) is activated in response to intracellular signals, including cellular stress and DNA damage. Both of these pathways lead to the activation of cystein-aspartic protease (caspase) cascade. Caspases are proteases that execute the cellular processes during apoptosis. After the apoptotic signal, the initiator caspases (caspase-2, -8, -9, -10) are activated, which in turn activate the downstream effector caspases (caspase-3, -6, -7).
These effector caspases cleave various cellular proteins leading to the characteristic morphological changes of apoptotic cell, including blebbing of the plasma membrane, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation (reviewed in (112)). An overview of the extrinsic and intrinsic pathways is presented in Figure 4.
BCL2 protein family members are important mediators of intrinsic apoptotic pathway.
These proteins are devided into anti- and pro-apoptotic factors based on their function.
Anti-apoptotic members of BCL2 family include BCL2, BCL-XL, BCL-W, A1, and Mcl-1, while pro-apoptotic members include BAX, BAK, BOK, BID, BAD, PUMA, and NOXA (112). The balance between these factors sets the threshold of apoptosis for intrinsic pathway.
One of the extracellular ligands that activate the extrinsic apoptosis pathway is Tumor Necrosis Factor (TNF)-Related Apoptosis Inducing Ligand (TRAIL) that belongs to the TNF superfamily. TRAIL acts through its receptors DR4 R1) and DR5 (TRAIL-R2), whose activation leads to caspase activation and apoptosis (113). In addition to its death receptors, TRAIL is also capable of binding to two decoy receptors DcR1 (TRAIL-R3) and DcR2 (TRAIL-R4), which are lacking the intracellular death domain (114, 115), and thus modulate the TRAIL pathway activity by competing the ligand binding with DR4 and DR5 (116).
TRAIL and its receptors are widely expressed in variety of tissues, including liver, lung, prostate, and myometrium (117). In addition to its ability to induce apoptosis,
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TRAIL has also been shown to have other functions, including the control of hematopoiesis, prevention of autoimmunity, and regulation of endothelial cell physiology (118-120). In human fetal ovary, TRAIL and its receptors DR5 and DcR2 are expressed both in oocytes and granulosa cells, whereas in postnatal ovary, TRAIL, DR5, as well as both DcR1 and DcR2, but not DR4, are expressed in oocytes and granulosa cells of small primary and secondary follicles (121). TRAIL and its receptors are also expressed in porcine ovaries, where the expression of TRAIL has been shown to increase and the expression of DcR1 to decrease during follicular atresia (122, 123). Furthermore, TRAIL has been shown to induce the apoptosis of primary cultured porcine granulosa cells in vitro, and eliminating the DcR1 from these cells results in increased number of apoptotic cells (123). These findings suggest that TRAIL has apoptosis-inducing activity in granulosa cell, and that decoy receptors can inhibit this ability.
Figure 4 Apoptotic signaling pathways. Extrinsic pathway: the binding of extracellular death ligand to its plasmamembrane receptor (death receptor) activates the intracellular Fas-associated protein with death domain (FADD). FADD, in turn, recruits caspase-8, which activates the effector caspases (caspase-3, -6, -7) leading to cell death.
Intrinsic pathway: death stimulus (e.g. cellular stress or DNA damage) induces the release of cytochrome c from mitochondria. Cytochrome c catalyzes the oligomerization of Apoptosis protease activating factor-1 (Apaf-1), which recruits and promotes the activation of procaspase-9. This, in turn, activates caspase-3, -6, and -7 leading to apoptosis. B cell lymphoma 2 (BCL2) inhibits the intrinsic apoptotic pathway by controlling the mitochondrial membrane permeability and thus preventing the release of cytochrome c. Activation of extrinsic pathway can also trigger the intrinsic pathway through activation of BH3 interacting domain death agonist (BID), which in turn causes the release of cytochrome c.
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