However, it is important to note that other forms of programmed cell death have been described and other forms of programmed cell death may yet be discovered Formigli et al.
Apoptosis occurs normally during development and aging and as a homeostatic mechanism to maintain cell populations in tissues. Apoptosis also occurs as a defense mechanism such as in immune reactions or when cells are damaged by disease or noxious agents Norbury and Hickson, Although there are a wide variety of stimuli and conditions, both physiological and pathological, that can trigger apoptosis, not all cells will necessarily die in response to the same stimulus.
Irradiation or drugs used for cancer chemotherapy results in DNA damage in some cells, which can lead to apoptotic death through a p53 -dependent pathway. Some hormones, such as corticosteroids, may lead to apoptotic death in some cells e. Some cells express Fas or TNF receptors that can lead to apoptosis via ligand binding and protein cross-linking.
Other cells have a default death pathway that must be blocked by a survival factor such as a hormone or growth factor. There is also the issue of distinguishing apoptosis from necrosis, two processes that can occur independently, sequentially, as well as simultaneously Hirsch, ; Zeiss, At low doses, a variety of injurious stimuli such as heat, radiation, hypoxia and cytotoxic anticancer drugs can induce apoptosis but these same stimuli can result in necrosis at higher doses.
Light and electron microscopy have identified the various morphological changes that occur during apoptosis Hacker, During the early process of apoptosis, cell shrinkage and pyknosis are visible by light microscopy Kerr et al.
With cell shrinkage, the cells are smaller in size, the cytoplasm is dense and the organelles are more tightly packed. Pyknosis is the result of chromatin condensation and this is the most characteristic feature of apoptosis. On histologic examination with hematoxylin and eosin stain, apoptosis involves single cells or small clusters of cells. The apoptotic cell appears as a round or oval mass with dark eosinophilic cytoplasm and dense purple nuclear chromatin fragments Figure 1.
Electron microscopy can better define the subcellular changes. Early during the chromatin condensation phase, the electron-dense nuclear material characteristically aggregates peripherally under the nuclear membrane although there can also be uniformly dense nuclei Figures 2A, 2B. Figure 1A is a photomicrograph of a section of exocrine pancreas from a B6C3F1 mouse.
The arrows indicate apoptotic cells that are shrunken with condensed cytoplasm. The nuclei are pyknotic and fragmented. Note the lack of inflammation. Within the interstitial space there are apoptotic cells with condensed cytoplasm, condensed and hyperchromatic chromatin and fragmented nuclei long arrows.
Admixed with the apoptotic bodies are macrophages, some with engulfed apoptotic bodies arrowheads Howden et al. Figure 1C is a photomicrograph of normal thymus tissue from a control Sprague—Dawley rat for comparison. Figure ID illustrates sheets of apoptotic cells in the thymus from a rat that was treated with dexamethasone to induce lymphocyte apoptosis.
Under physiological conditions, apoptosis typically affects single cells or small clusters of cells. The majority of lymphocytes are apoptotic although there are a few interspersed cells that are morphologically normal and most likely represent lymphoblasts or macrophages. The apoptotic lymphocytes are small and deeply basophilic with pyknotic and often-fragmented nuclei. Macrophages are present with engulfed cytoplasmic apoptotic bodies arrows.
The lymphocytes are closely packed, have large nuclei and scant cytoplasm. Figure 2B is a TEM of apoptotic thymic lymphocytes in an early phase of apoptosis with condensed and peripheralized chromatin. The cytoplasm is beginning to condense and the cell outlines are irregular.
The arrow indicates a fragmented section of nucleus and the arrowhead most likely indicates an apoptotic body that seems to contain predominantly cytoplasm without organelles or nuclear material. Macrophages or other adjacent healthy cells subsequently engulf the apoptotic bodies.
For these reasons, apoptosis does not incite an inflammatory reaction. Figure 2D is a TEM of a section of thymus with lymphocytes in various stages of apoptosis. The large cell in the center of the photomicrograph is a macrophage with engulfed intracytoplasmic apoptotic bodies.
The organelle integrity is still maintained and all of this is enclosed within an intact plasma membrane. These bodies are subsequently phagocytosed by macrophages, parenchymal cells, or neoplastic cells and degraded within phagolysosomes Figure 2D. The tingible bodies are the bits of nuclear debris from the apoptotic cells. There is essentially no inflammatory reaction associated with the process of apoptosis nor with the removal of apoptotic cells because: 1 apoptotic cells do not release their cellular constituents into the surrounding interstitial tissue; 2 they are quickly phagocytosed by surrounding cells thus likely preventing secondary necrosis; and, 3 the engulfing cells do not produce anti-inflammatory cytokines Savill and Fadok, ; Kurosaka et al.
The alternative to apoptotic cell death is necrosis, which is considered to be a toxic process where the cell is a passive victim and follows an energy-independent mode of death. But since necrosis refers to the degradative processes that occur after cell death, it is considered by some to be an inappropriate term to describe a mechanism of cell death.
Oncosis is therefore used to describe a process that leads to necrosis with karyolysis and cell swelling whereas apoptosis leads to cell death with cell shrinkage, pyknosis, and karyorrhexis.
Although the mechanisms and morphologies of apoptosis and necrosis differ, there is overlap between these two processes. For example, two factors that will convert an ongoing apoptotic process into a necrotic process include a decrease in the availability of caspases and intracellular ATP Leist et al. Whether a cell dies by necrosis or apoptosis depends in part on the nature of the cell death signal, the tissue type, the developmental stage of the tissue and the physiologic milieu Fiers et al.
Using conventional histology, it is not always easy to distinguish apoptosis from necrosis, and they can occur simultaneously depending on factors such as the intensity and duration of the stimulus, the extent of ATP depletion and the availability of caspases Zeiss, Necrosis is an uncontrolled and passive process that usually affects large fields of cells whereas apoptosis is controlled and energy-dependent and can affect individual or clusters of cells.
Necrotic cell injury is mediated by two main mechanisms; interference with the energy supply of the cell and direct damage to cell membranes. Some of the major morphological changes that occur with necrosis include cell swelling; formation of cytoplasmic vacuoles; distended endoplasmic reticulum; formation of cytoplasmic blebs; condensed, swollen or ruptured mitochondria; disaggregation and detachment of ribosomes; disrupted organelle membranes; swollen and ruptured lysosomes; and eventually disruption of the cell membrane Kerr et al.
This loss of cell membrane integrity results in the release of the cytoplasmic contents into the surrounding tissue, sending chemotatic signals with eventual recruitment of inflammatory cells. Because apoptotic cells do not release their cellular constituents into the surrounding interstitial tissue and are quickly phagocytosed by macrophages or adjacent normal cells, there is essentially no inflammatory reaction Savill and Fadok, ; Kurosaka et al.
It is also important to note that pyknosis and karyorrhexis are not exclusive to apoptosis and can be a part of the spectrum of cytomorphological changes that occurs with necrosis Cotran et al. Table 1 compares some of the major morphological features of apoptosis and necrosis. Many of the genes that control the killing and engulfment processes of programmed cell death have been identified, and the molecular mechanisms underlying these processes have proven to be evolutionarily conserved Metzstein et al.
Until recently, apoptosis has traditionally been considered an irreversible process with caspase activation committing a cell to death and the engulfment genes serving the purpose of dead cell removal. However, the uptake and clearance of apoptotic cells by macrophages may involve more than just the removal of cell debris.
Hoeppner et al. Reddien et al. Moreover, mutations in engulfment genes alone allowed the survival and differentiation of some cells that were otherwise destined to die via apoptosis Reddien et al.
These findings suggest that genes that mediate corpse removal can also function to actively kill cells. In other words, the engulfing cells may act to ensure that cells triggered to undergo apoptosis will die rather than recover after the initial stages of death.
In vertebrates, there is some evidence of a potential role for macrophages in promoting the death of cells in some tissues. Elimination of macrophages in the anterior chamber of the rat eye resulted in the survival of vascular endothelial cells that normally undergo apoptosis Diez-Roux and Lang, Other studies have demonstrated that inhibition of macrophages can disrupt the remodeling of tissues in the mouse eye or in the tadpole tail during regression; two processes that involve apoptosis Lang and Bishop, ; Little and Flores, Geske and coworkers demonstrated that early pinduced apoptotic cells can be rescued from the apoptotic program if the apoptotic stimulus is removed.
Their research suggests that DNA repair is activated early in the p induced apoptotic process and that this DNA repair may be involved in reversing the cell death pathway in some circumstances. The mechanisms of apoptosis are highly complex and sophisticated, involving an energy-dependent cascade of molecular events Figure 3. To date, research indicates that there are two main apoptotic pathways: the extrinsic or death receptor pathway and the intrinsic or mitochondrial pathway.
However, there is now evidence that the two pathways are linked and that molecules in one pathway can influence the other Igney and Krammer, There is an additional pathway that involves T-cell mediated cytotoxicity and perforin-granzyme-dependent killing of the cell. The extrinsic, intrinsic, and granzyme B pathways converge on the same terminal, or execution pathway. This pathway is initiated by the cleavage of caspase-3 and results in DNA fragmentation, degradation of cytoskeletal and nuclear proteins, cross-linking of proteins, formation of apoptotic bodies, expression of ligands for phagocytic cell receptors and finally uptake by phagocytic cells.
The granzyme A pathway activates a parallel, caspase-independent cell death pathway via single stranded DNA damage Martinvalet et al. Schematic representation of apoptotic events. Each requires specific triggering signals to begin an energy-dependent cascade of molecular events. Each pathway activates its own initiator caspase 8, 9, 10 which in turn will activate the executioner caspase However, granzyme A works in a caspase-independent fashion. The execution pathway results in characteristic cytomorphological features including cell shrinkage, chromatin condensation, formation of cytoplasmic blebs and apoptotic bodies and finally phagocytosis of the apoptotic bodies by adjacent parenchymal cells, neoplastic cells or macrophages.
Apoptotic cells exhibit several biochemical modifications such as protein cleavage, protein cross-linking, DNA breakdown, and phagocytic recognition that together result in the distinctive structural pathology described previously Hengartner, Caspases are widely expressed in an inactive proenzyme form in most cells and once activated can often activate other procaspases, allowing initiation of a protease cascade. Some procaspases can also aggregate and autoactivate.
This proteolytic cascade, in which one caspase can activate other caspases, amplifies the apoptotic signaling pathway and thus leads to rapid cell death. Caspases have proteolytic activity and are able to cleave proteins at aspartic acid residues, although different caspases have different specificities involving recognition of neighboring amino acids. Once caspases are initially activated, there seems to be an irreversible commitment towards cell death.
To date, ten major caspases have been identified and broadly categorized into initiators caspase-2,-8,-9, , effectors or executioners caspase-3,-6,-7 and inflammatory caspases caspase-1,-4,-5 Cohen, ; Rai et al. Extensive protein cross-linking is another characteristic of apoptotic cells and is achieved through the expression and activation of tissue transglutaminase Nemes et al.
Another biochemical feature is the expression of cell surface markers that result in the early phagocytic recognition of apoptotic cells by adjacent cells, permitting quick phagocytosis with minimal compromise to the surrounding tissue.
Although externalization of phosphatidylserine is a well-known recognition ligand for phagocytes on the surface of the apoptotic cell, recent studies have shown that other proteins are also be exposed on the cell surface during apoptotic cell clearance. These include Annexin I and calreticulin. Annexin V is a recombinant phosphatidylserine-binding protein that interacts strongly and specifically with phosphatidylserine residues and can be used for the detection of apoptosis Van Engeland et al.
Calreticulin is a protein that binds to an LDL-receptor related protein on the engulfing cell and is suggested to cooperate with phosphatidylserine as a recognition signal Gardai et al.
The adhesive glycoprotein, thrombospondin-1, can be expressed on the outer surface of activated microvascular endothelial cells and, in conjunction with CD36, caspaselike proteases and other proteins, induce receptor-mediated apoptosis Jimenez et al.
The extrinsic signaling pathways that initiate apoptosis involve transmembrane receptor-mediated interactions. These involve death receptors that are members of the tumor necrosis factor TNF receptor gene superfamily Locksley et al. This death domain plays a critical role in transmitting the death signal from the cell surface to the intracellular signaling pathways. In these models, there is clustering of receptors and binding with the homologous trimeric ligand.
Upon ligand binding, cytplasmic adapter proteins are recruited which exhibit corresponding death domains that bind with the receptors. FADD then associates with procaspase-8 via dimerization of the death effector domain. At this point, a death-inducing signaling complex DISC is formed, resulting in the auto-catalytic activation of procaspase-8 Kischkel et al.
Once caspase-8 is activated, the execution phase of apoptosis is triggered. Another point of potential apoptosis regulation involves a protein called Toso, which has been shown to block Fas-induced apoptosis in T cells via inhibition of caspase-8 processing Hitoshi et al.
Table 2 lists the major extrinsic pathway proteins with common abbreviations and some of the alternate nomenclature used for each protein. However, they are also able to exert their cytotoxic effects on tumor cells and virus-infected cells via a novel pathway that involves secretion of the transmembrane pore-forming molecule perforin with a subsequent exophytic release of cytoplasmic granules through the pore and into the target cell Trapani and Smyth, The serine proteases granzyme A and granzyme B are the most important component within the granules Pardo et al.
Reports have also shown that granzyme B can utilize the mitochondrial pathway for amplification of the death signal by specific cleavage of Bid and induction of cytochrome c release Barry and Bleackley, ; Russell and Ley, However, granzyme B can also directly activate caspase In this way, the upstream signaling pathways are bypassed and there is direct induction of the execution phase of apoptosis.
It is suggested that both the mitochondrial pathway and direct activation of caspase-3 are critical for granzyme B-induced killing Goping et al. Recent findings indicate that this method of granzyme B cytotoxicity is critical as a control mechanism for T cell expansion of type 2 helper T Th2 cells Devadas et al. Moreover, findings indicate that neither death receptors nor caspases are involved with the T cell receptor-induced apoptosis of activated Th2 cells because blocking their ligands has no effect on apoptosis.
On the other hand, Fas-Fas ligand interaction, adapter proteins with death domains and caspases are all involved in the apoptosis and regulation of cytotoxic Type 1 helper cells whereas granzyme B has no effect. Granzyme A is also important in cytotoxic T cell induced apoptosis and activates caspase independent pathways.
This DNAse has an important role in immune surveillance to prevent cancer through the induction of tumor cell apoptosis. Therefore, inactivation of this complex by granzyme A most likely also contributes to apoptosis by blocking the maintenance of DNA and chromatin structure integrity. The intrinsic signaling pathways that initiate apoptosis involve a diverse array of non-receptor-mediated stimuli that produce intracellular signals that act directly on targets within the cell and are mitochondrial-initiated events.
The stimuli that initiate the intrinsic pathway produce intracellular signals that may act in either a positive or negative fashion. Negative signals involve the absence of certain growth factors, hormones and cytokines that can lead to failure of suppression of death programs, thereby triggering apoptosis.
In other words, there is the withdrawal of factors, loss of apoptotic suppression, and subsequent activation of apoptosis. Other stimuli that act in a positive fashion include, but are not limited to, radiation, toxins, hypoxia, hyperthermia, viral infections, and free radicals.
All of these stimuli cause changes in the inner mitochondrial membrane that results in an opening of the mitochondrial permeability transition MPT pore, loss of the mitochondrial transmembrane potential and release of two main groups of normally sequestered pro-apoptotic proteins from the intermembrane space into the cytosol Saelens et al.
These proteins activate the caspase-dependent mitochondrial pathway. The clustering of procaspase-9 in this manner leads to caspase-9 activation. The second group of pro-apoptotic proteins, AIF, endonuclease G and CAD, are released from the mitochondria during apoptosis, but this is a late event that occurs after the cell has committed to die.
Endonuclease G also translocates to the nucleus where it cleaves nuclear chromatin to produce oligonucleosomal DNA fragments Li et al. AIF and endonuclease G both function in a caspase-independent manner. CAD is subsequently released from the mitochondria and translocates to the nucleus where, after cleavage by caspase-3, it leads to oligonucleosomal DNA fragmentation and a more pronounced and advanced chromatin condensation Enari et al.
The control and regulation of these apoptotic mitochondrial events occurs through members of the Bcl-2 family of proteins Cory and Adams, The tumor suppressor protein p53 has a critical role in regulation of the Bcl-2 family of proteins, however the exact mechanisms have not yet been completely elucidated Schuler and Green, The Bcl-2 family of proteins governs mitochondrial membrane permeability and can be either pro-apoptotic or anti-apoptotic.
To date, a total of 25 genes have been identified in the Bcl-2 family. These proteins have special significance since they can determine if the cell commits to apoptosis or aborts the process. It is thought that the main mechanism of action of the Bcl-2 family of proteins is the regulation of cytochrome c release from the mitochondria via alteration of mitochondrial membrane permeability.
A few possible mechanisms have been studied but none have been proven definitively. Mitochondrial damage in the Fas pathway of apoptosis is mediated by the caspase-8 cleavage of Bid Li et al.
Serine phosphorylation of Bad is associated with , a member of a family of multifunctional phosphoserine binding molecules. When Bad is phosphorylated, it is trapped by and sequestered in the cytosol but once Bad is unphosphorylated, it will translocate to the mitochondria to release cytochrome C Zha, et al.
Bad can also heterodimerize with Bcl-Xl or Bcl-2, neutralizing their protective effect and promoting cell death Yang et al. When not sequestered by Bad, both Bcl-2 and Bcl-Xl inhibit the release of cytochrome C from the mitochondria although the mechanism is not well understood. Reports indicate that Bcl-2 and Bcl-XL inhibit apoptotic death primarily by controlling the activation of caspase proteases Newmeyer et al.
There is evidence that overexpression of either Bcl-2 or Bcl-Xl will down-regulate the other, indicating a reciprocal regulation between these two proteins. Puma and Noxa are two members of the Bcl2 family that are also involved in pro-apoptosis. Puma plays an important role in p53 -mediated apoptosis. It was shown that, in vitro, overexpression of Puma is accompanied by increased BAX expression, BAX conformational change, translocation to the mitochondria, cytochrome c release and reduction in the mitochondrial membrane potential Liu et al.
Noxa is also a candidate mediator of p53 -induced apoptosis. Studies show that this protein can localize to the mitochondria and interact with anti-apoptotic Bcl-2 family members, resulting in the activation of caspase-9 Oda et al.
Since both Puma and Noxa are induced by p53 , they might mediate the apoptosis that is elicited by geno-toxic damage or oncogene activation. The Myc oncoprotein has also been reported to potentiate apoptosis through both p53 -dependent and -independent mechanisms Meyer et al. Further elucidation of these pathways should have important implications for tumorigenesis and therapy. Table 3 lists the major intrinsic pathway proteins with common abbreviations and some of the alternate nomenclature used for each protein.
The extrinsic and intrinsic pathways both end at the point of the execution phase, considered the final pathway of apoptosis. It is the activation of the execution caspases that begins this phase of apoptosis.
Execution caspases activate cytoplasmic endonuclease, which degrades nuclear material, and proteases that degrade the nuclear and cytoskeletal proteins. Caspase-3 is considered to be the most important of the executioner caspases and is activated by any of the initiator caspases caspase-8, caspase-9, or caspase Caspase-3 specifically activates the endonuclease CAD.
Caspase-3 also induces cytoskeletal reorganization and disintegration of the cell into apoptotic bodies. Gelsolin, an actin binding protein, has been identified as one of the key substrates of activated caspase Gelsolin will typically act as a nucleus for actin polymerization and will also bind phosphatidylinositol biphosphate, linking actin organization and signal transduction. Caspase-3 will cleave gelsolin and the cleaved fragments of gelsolin, in turn, cleave actin filaments in a calcium independent manner.
This results in disruption of the cytoskeleton, intracellular transport, cell division, and signal transduction Kothakota et al. Phagocytic uptake of apoptotic cells is the last component of apoptosis.
Phospholipid asymmetry and externalization of phosphatidylserine on the surface of apoptotic cells and their fragments is the hallmark of this phase. Although the mechanism of phosphatidylserine translocation to the outer leaflet of the cell during apoptosis is not well understood, it has been associated with loss of aminophospholipid translocase activity and nonspecific flip-flop of phospholipids of various classes Bratton et al. Research indicates that Fas, caspase-8, and caspase-3 are involved in the regulation of phosphatidylserine externalization on oxidatively stressed erythrocytes however caspase-independent phosphatidylserine exposure occurs during apoptosis of primary T lymphocytes Ferraro-Peyret et al.
The appearance of phosphotidylserine on the outer leaflet of apoptotic cells then facilitates noninflammatory phagocytic recognition, allowing for their early uptake and disposal Fadok et al. This process of early and efficient uptake with no release of cellular constituents, results in essentially no inflammatory response. Table 4 lists the major proteins in the execution pathway with common abbreviations and some of the alternate nomenclature used for each protein.
The role of apoptosis in normal physiology is as significant as that of its counterpart, mitosis. It demonstrates a complementary but opposite role to mitosis and cell proliferation in the regulation of various cell populations. It is estimated that to maintain homeostasis in the adult human body, around 10 billion cells are made each day just to balance those dying by apoptosis Renehan et al.
And that number can increase significantly when there is increased apoptosis during normal development and aging or during disease. Apoptosis is critically important during various developmental processes. As examples, both the nervous system and the immune system arise through overproduction of cells. This initial overproduction is then followed by the death of those cells that fail to establish functional synaptic connections or productive antigen specificities, respectively Nijhawan et al.
Apoptosis is also necessary to rid the body of pathogen-invaded cells and is a vital component of wound healing in that it is involved in the removal of inflammatory cells and the evolution of granulation tissue into scar tissue Greenhalgh, Dysregulation of apoptosis during wound healing can lead to pathologic forms of healing such as excessive scarring and fibrosis.
Apoptosis is also needed to eliminate activated or auto-aggressive immune cells either during maturation in the central lymphoid organs bone marrow and thymus or in peripheral tissues Osborne, Additionally, apoptosis is central to remodeling in the adult, such as the follicular atresia of the postovulatory follicle and post-weaning mammary gland involution, to name a couple of examples Tilly, ; Lund et al.
Furthermore, as organisms grow older, some cells begin to deteriorate at a faster rate and are eliminated via apoptosis. One theory is that oxidative stress plays a primary role in the pathophysiology of age-induced apoptosis via accumulated free-radical damage to mitochondrial DNA Harman, ; Ozawa, It is clear that apoptosis has to be tightly regulated since too little or too much cell death may lead to pathology, including developmental defects, autoimmune diseases, neurodegeneration, or cancer.
Some conditions feature insufficient apoptosis whereas others feature excessive apoptosis. In fact, suppression of apoptosis during carcinogenesis is thought to play a central role in the development and progression of some cancers Kerr et al.
There are a variety of molecular mechanisms that tumor cells use to suppress apoptosis. Tumor cells can acquire resistance to apoptosis by the expression of anti-apoptotic proteins such as Bcl-2 or by the down-regulation or mutation of pro-apoptotic proteins such as Bax. The expression of both Bcl-2 and Bax is regulated by the p53 tumor suppressor gene Miyashita, Certain forms of human B cell lymphoma have overexpression of Bcl-2, and this is one of the first and strongest lines of evidence that failure of cell death contributes to cancer Vaux et al.
Another method of apoptosis suppression in cancer involves evasion of immune surveillance Smyth et al. In order to evade immune destruction, some tumor cells will diminish the response of the death receptor pathway to FasL produced by T cells. This has been shown to occur in a variety of ways including down-regulation of the Fas receptor on tumor cells. Other mechanisms include expression of nonfunctioning Fas receptor, secretion of high levels of a soluble form of the Fas receptor that will sequester the Fas ligand or expression of Fas ligand on the surface of tumor cells Cheng et al.
Alterations of various cell signaling pathways can result in dysregulation of apoptosis and lead to cancer. The p53 tumor suppressor gene is a transcription factor that regulates the cell cycle and is the most widely mutated gene in human tumorigenesis Wang and Harris, Tumorigenesis can occur if this system goes awry.
If the p53 gene is damaged, then tumor suppression is severely reduced. The p53 gene can be damaged by radiation, various chemicals, and viruses such as the Human papillomavirus HPV. People who inherit only one functional copy of this gene will most likely develop Li—Fraumeni syndrome, which is characterized by the development of tumors in early adulthood Varley et al.
The ATM gene encodes a protein kinase that acts as a tumor suppressor. As mentioned previously, p53 then signals growth arrest of the cell at a checkpoint to allow for DNA damage repair or can cause the cell to undergo apoptosis if the damage cannot be repaired. Other cell signaling pathways can also be involved in tumor development.
In addition to regulation of apoptosis, this pathway regulates other cellular processes, such as proliferation, growth, and cytoskeletal rearrangement Vivanco and Sawyers, In addition to cancer, too little apoptosis can also result in diseases such as autoimmune lymphoproliferative syndrome ALPS Worth et al.
This occurs when there is insufficient apoptosis of auto-aggressive T cells, resulting in multiple autoimmune diseases. An overproliferation of B cells occurs as well, resulting in excess immunoglobulin production, leading to autoimmunity. Some of the common diseases of ALPS include hemolytic anemia, immune-mediated thrombocytopenia, and autoimmune neutropenia. The different types of this condition are caused by different mutations. Type 1A results from a mutation in the death domain of the Fas receptor, Type 1B results from a mutation in Fas ligand and Type 2 results from a mutation in caspase, reducing its activity.
Excessive apoptosis may also be a feature of some conditions such as autoimmune diseases, neurodegenerative diseases, and ischemia-associated injury. Autoimmune deficiency syndrome AIDS is an example of an autoimmune disease that results from infection with the human immunodeficiency virus HIV Li et al.
The virus is subsequently internalized into the T cell where the HIV Tat protein is thought to increase the expression of the Fas receptor, resulting in excessive apoptosis of T cells. Excessive apoptosis is also thought to play an important role in various ischemia-associated injuries. One example is myocardial ischemia caused by an insufficient blood supply, leading to a decrease in oxygen delivery to, and subsequent death of, the cardiomyocytes.
Although necrosis does occur, overexpression of BAX has been detected in ischemic myocardial tissue and therapy aimed at reducing apoptosis has shown some success in reducing the degree of tissue damage Hochhauser et al. One hypothesis is that the damage produced by ischemia is capable of initiating apoptosis but if ischemia is prolonged, necrosis occurs. If energy production is restored, as with reperfusion, the apoptotic cascade that was initiated by ischemia may proceed Freude et al.
Although the extent to which apoptosis is involved in myocardial ischemia remains to be clarified, there is clear evidence that supports a role for this mode of cell death.
There are many pathological conditions that feature excessive apoptosis neurodegenerative diseases, AIDS, ischemia, etc. As our understanding of the field evolves, the identification and exploitation of new targets remains a considerable focus of attention Nicholson, The IAP family of proteins is perhaps the most important regulators of apoptosis due to the fact that they regulate both the intrinsic and extrinsic pathways Deveraux and Reed, Eight human IAP proteins have now been identified although XIAP X-linked mammalian inhibitor of apoptosis protein and survivin remain the better-known members Silke et al.
It is the main process by which organs maintain cell mass and at the same time eliminate excess and aged cells that have lost their functional importance. The typical morphological signs of apoptosis cellular shrinkage, membrane blebbing, nuclear condensation and fragmentation are the final results of a complex biochemical cascade of events, some of which are inextricably linked to the process of differentiation.
Studies that analyze all stages of this cascade, rather than the final morphological stages of apoptotic death, are essential in order that specific link s between differentiation and apoptosis are appreciated. This review outlines the main stages of the apoptosis cascade together with current methods for their morphological visualization.
Starting with a receptors and ligands known to induce apoptosis, we continue with b early initiator stages of apoptosis, and c proteins regulating and potentially inhibiting further progression of the cascade, into d irreversible execution stages of the cascade, and finally d the morphological events of apoptotic death. For each stage we present those aspects of the biochemical background that are morphologically relevant, together with proven methods for their visualization.
We offer technical advice at each stage based upon our experience of studying differentiation and apoptosis in human placental trophoblast. This is a preview of subscription content, access via your institution. Rent this article via DeepDyve. All the changes that occur during metamorphosis, including more In adult tissues, cell death exactly balances cell division.
If this were not so, the tissue would grow or shrink. If part of the liver is removed in an adult rat, for example, liver cell proliferation increases to make up the loss. Conversely, if a rat is treated with the drug phenobarbital—which stimulates liver cell division and thereby liver enlargement —and then the phenobarbital treatment is stopped, apoptosis in the liver greatly increases until the liver has returned to its original size, usually within a week or so.
Thus, the liver is kept at a constant size through the regulation of both the cell death rate and the cell birth rate. In this short section , we describe the molecular mechanisms of apoptosis and its control.
In the final section, we consider how the extracellular control of cell proliferation and cell death contributes to the regulation of cell numbers in multicellular organisms. Cells that die as a result of acute injury typically swell and burst. They spill their contents all over their neighbors—a process called cell necrosis —causing a potentially damaging inflammatory response.
By contrast, a cell that undergoes apoptosis dies neatly, without damaging its neighbors. The cell shrinks and condenses. The cytoskeleton collapses, the nuclear envelope disassembles, and the nuclear DNA breaks up into fragments.
Most importantly, the cell surface is altered, displaying properties that cause the dying cell to be rapidly phagocytosed, either by a neighboring cell or by a macrophage a specialized phagocytic cell, discussed in Chapter 24 , before any leakage of its contents occurs Figure This not only avoids the damaging consequences of cell necrosis but also allows the organic components of the dead cell to be recycled by the cell that ingests it.
Cell death. These electron micrographs show cells that have died by A necrosis or B and C apoptosis. The cells in A and B died in a culture dish, whereas the cell in C died in a developing tissue and has been engulfed by a neighboring cell. The intracellular machinery responsible for apoptosis seems to be similar in all animal cells.
This machinery depends on a family of proteases that have a cysteine at their active site and cleave their target proteins at specific aspartic acids. They are therefore called caspases. Caspases are synthesized in the cell as inactive precursors, or procaspases, which are usually activated by cleavage at aspartic acids by other caspases Figure A.
Once activated, caspases cleave, and thereby activate, other procaspases, resulting in an amplifying proteolytic cascade Figure B. Some of the activated caspases then cleave other key proteins in the cell. Some cleave the nuclear lamins, for example, causing the irreversible breakdown of the nuclear lamina ; another cleaves a protein that normally holds a DNA -degrading enzyme a DNAse in an inactive form, freeing the DNAse to cut up the DNA in the cell nucleus.
In this way, the cell dismantles itself quickly and neatly, and its corpse is rapidly taken up and digested by another cell. The caspase cascade involved in apoptosis. A Each suicide protease is made as an inactive proenzyme procaspase , which is usually activated by proteolytic cleavage by another member of the caspase family.
As indicated, two of the cleaved fragments more Activation of the intracellular cell death pathway, like entry into a new stage of the cell cycle, is usually triggered in a complete, all-or-none fashion.
The protease cascade is not only destructive and self-amplifying but also irreversible, so that once a cell reaches a critical point along the path to destruction, it cannot turn back. All nucleated animal cells contain the seeds of their own destruction, in the form of various inactive procaspases that lie waiting for a signal to destroy the cell.
It is therefore not surprising that caspase activity is tightly regulated inside the cell to ensure that the death program is held in check until needed.
How are procaspases activated to initiate the caspase cascade? A general principle is that the activation is triggered by adaptor proteins that bring multiple copies of specific procaspases, known as initiator procaspases, close together in a complex or aggregate. In some cases, the initiator procaspases have a small amount of protease activity, and forcing them together into a complex causes them to cleave each other, triggering their mutual activation.
In other cases, the aggregation is thought to cause a conformational change that activates the procaspase. Within moments, the activated caspase at the top of the cascade cleaves downstream procaspases to amplify the death signal and spread it throughout the cell see Figure B.
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