Difference between revisions of "2009 Group 2 Project"
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ER-mediated pathways have recently been linked to caspase activation. It is thought that the members of the [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=596&ordinalpos=3&itool=EntrezSystem2.PEntrez.Gene.Gene_ResultsPanel.Gene_RVDocSum Bcl-2] family that either inhibit or enhance apoptosis bind to the membrane of the ER (1). The response to abnormal events within the ER, such as a build up of misfolded proteins or the alteration in the homeostasis of calcium signalling, results in stress on the ER (3), which has been shown to initiate signalling for apoptosis. [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=120329&ordinalpos=4&itool=EntrezSystem2.PEntrez.Gene.Gene_ResultsPanel.Gene_RVDocSum Caspase-12] is said to be activated by these stresses, and if over expression occurs, it can lead to the activation of other pathways (3). These pathways are not fully understood, but there are suggestions that activation of procaspase-9 and the formation of apoptosomes could be avoided via these pathways – directly activating mitochondria-mediated cytochrome c release via the activation of the Bax (3).
ER-mediated pathways have recently been linked to caspase activation. It is thought that the members of the [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=596&ordinalpos=3&itool=EntrezSystem2.PEntrez.Gene.Gene_ResultsPanel.Gene_RVDocSum Bcl-2] family that either inhibit or enhance apoptosis bind to the membrane of the ER (1). The response to abnormal events within the ER, such as a build up of misfolded proteins or the alteration in the homeostasis of calcium signalling, results in stress on the ER (3), which has been shown to initiate signalling for apoptosis. [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=120329&ordinalpos=4&itool=EntrezSystem2.PEntrez.Gene.Gene_ResultsPanel.Gene_RVDocSum Caspase-12] is said to be activated by these stresses, and if over expression occurs, it can lead to the activation of other pathways (3). These pathways are not fully understood, but there are suggestions that activation of procaspase-9 and the formation of apoptosomescould be avoided via these pathways – directly activating mitochondria-mediated cytochrome c release via the activation of the Bax (3).
===Granzyme –A and –B mediated pathways===
===Granzyme –A and –B mediated pathways===
Revision as of 13:37, 27 May 2009
- 1 Introduction
- 2 Definition
- 3 What Happens During Apoptosis?
- 4 Why Does Apoptosis Occur?
- 5 Brief History Of Apoptosis
- 6 References
- 7 Signaling
- 8 Apoptosis Signalling Diagram
- 9 Regulations of the apoptosis
- 10 The morphology of apoptosis
- 11 Timeline
- 12 Design model of apoptosis
- 13 Apoptosis from a Clinical Point of View
- 14 References
- 15 2009 Group Projects
In all living organisms there is a harmonious balance between the synthesis of new cells and the destruction of existing cells under normal conditions. This balance is particularly vital in maintaining both proper function and structure within the organism. The complex events that encompass these processes take place in response to specific and controlled signals. In this paper, focus is placed upon the process of “Programmed Cell Death” (PCD), more commonly known as Apoptosis, and the significance of this process in maintaining cell population. Mechanisms, abnormalities and future implications of research in Apoptotic Cell Death will also be explored.
The word Apoptosis is derived from a word of ancient Greek origin: απóπóτσισ. This literally means “falling off” or “falling away” as in leaves from a tree in autumn which incidentally also involves apoptosis. During Apoptosis, cells are described as “programmed” to self-destruct or commit “cellular suicide”, as opposed to “cellular homicide”, characteristic of Necrotic Cell Death (another form of cell death). The term Apoptosis was first used by John Kerr in 1972, in reference to the morphological presentation of ‘‘apoptotic bodies’’, characteristic of what Glucksmann described as developmental cell death in 1951. Years of research in cell biology have defined apoptosis as a physiological process that occurs via an extrinsic or intrinsic (mitochondrial) pathway. Both of these pathways involve activation of the Caspase Cascade, which eventuates in the death of the cell. This process is an essential part of normal physiological function and as a result each day billions of cells in the human body die via apoptosis, particularly blood cells and those in epithelia lining organs such as the intestine.
What Happens During Apoptosis?
This process of Apoptosis requires specific chemical and biological conditions that activate what is known as the Caspase Cascade. Specialized proteins including Bax and Bcl-2 are the primary intracellular signaling proteins involved in activating the Caspase Cascade that induces apoptosis. The various biochemical mechanisms that characterize the Caspase Cascade ultimately eventuate in death of the cell. Apoptosis is an ATP-dependant intracellular process that may be induced by numerous different stimuli including infectious and injurious agents, for example viruses and mutagenic agents respectively. However it is important to note that not all cells present the same sensitivity to stimuli. Upon receiving signals from such stimuli the cell self-instructs itself to die. Apoptosis includes a variety of highly orchestrated events; some that work in parallel with others and some that work in sequence to others. These events will be discussed in detail below, however in brief they include the initiation of the caspase cascade, cell shrinkage, protein fragmentation, chromatin condensation, DNA degradation and plasma membrane blebbing, finally followed by rapid engulfment of the deceased cell by neighbouring phagocytic cells.
Why Does Apoptosis Occur?
Apoptosis occurs as a normal process in most living organisms, including humans. In humans it is an integral component of normal development of the embryo, maturation and cell turnover and has also been implicated in a range of pathologically significant conditions, particularly cancer and autoimmune disease. Apoptosis most commonly occurs as a homeostatic process which is carried out by cells of living organisms as a means of eliminating unwanted cells and to maintain cell population in tissue. Unwanted cells can be described as those that have been exposed to injurious or infectious agents rendering them damaged and potentially dangerous to the rest of the organism. This defense mechanism can occur as a response to viral or bacterial infection, damage to DNA, for example by mutagenic agents, genetic disorders, as in autoimmune disease and various other injurious agents. A vast amount of apoptosis can occur during the developmental stages of an organism. A common example is the formation of the human hand during embryological development. In its initial stages the hand begins as a paddle-like structure, however in order for digitization to occur the inter-digital cells must undergo apoptosis resulting in the formation of a normal human hand. Another frequent example is the development of a tadpole into a frog. In the immune system cells infected by a virus may recognise the infectious agent triggering the process of apoptosis. Apoptosis acts to destroy the cell to prevent the virus from replicating and infecting further cells of the organism. Although apoptosis occurs under normal physiological conditions it may also occur pathologically in an abnormal disease state. Apoptosis has been implicated in many chronic disease processes such as those present in autoimmune diseases, for example rheumatoid arthritis, and neurodegenerative diseases including Multiple Sclerosis and Parkinson’s disease which is thought to be associated with too much apoptosis. On the contrary, too little apoptosis is of particular significance in the formation of cancer, essentially due to mutations in cancer cells that prevent them from undergoing apoptosis. Apoptosis has also been observed in plants, particularly higher plants. The xylem in plants consists of spaces that result from the death of cells that previously occupied those spaces, comparable to the digitization of the hand during embryological development.
Brief History Of Apoptosis
- 1842: Carl Vogt described that cells die during normal vertebrate development.
- 1951: Glucksmann wrote an influential review on apoptosis and re-sparked early interest in the field of apoptosis research.
- 1972: John Kerr first coined the term Apoptosis. John Kerr. He noticed apoptosis in hepatocytes but had originally termed this process “shrinkage necrosis” due to the observed shrinkage of the dying hepatocytes. Further to this, Kerr recognized that unlike necrosis, no inflammation was present hypothesised that the process being observed was separate from necrosis. He then helped confirm this hypothesis using electron microscopy to identify and describe morphological differences between necrosis and apoptosis
- 1976: Programmed Cell Death was first observed in the nematode Caenorhabditis elegans by J.E. Sulston and described as a “defined pattern of cell deaths”.
- 1980: Wyllie discovered that DNA degredation and nuclear chromatin condensation was characteristic of apoptosis and that these events where associated with intracellular, endogenous signals.
- 1982 - 1986: Horvitz et al. discovered that cell death occurred as a natural process during development of the nematode Caenorhabditis elegans. It was discovered that the worm began with 1090 somatic cells before reaching a mature state after which 131 died through apoptosis, leaving the hermaphrodite worm with a total of 959 somatic cells. Identification of genetic pathways dedicated to this programmed death of cells and the production the first cell death mutants (ced-1, ced-2) occurred soon after. These were milestone discoveries that sparked unparalleled interest in the field of apoptosis.
- 1988: Vaux et al. first identified a component of the apoptosis mechanism - Bcl-2; an important cell death inhibitor.
- 1991: Yonish et al. determined that p53 was able to induce apoptosis (in myeloid leukaemic cells).
- 1994: Birnbaum et al. identified an apoptosis inhibiting gene which in turn lead to the identification of IAPs and their specific roles.
- 1994 - Current: many researchers identified genes and proteins involved in the induction and inhibition of apoptosis, regulation of the process and the many receptors and pathways involved. Some of these important apoptotic factors include IAPs (Inhibitor Apoptotic Proteins), TNF (tumor necrosis factor), caspases (intracellular proteolytic enzymes) and bax (pro-apoptotic intracellular enzyme).
- Current: Much research is currently being conducted in the area of Apoptosis, however our aim to better understand and determine the process and factors involved, is being combined with the modern focus directed towards the therapeutic applications of our knowledge of Apoptosis, particularly factors that induce and inhibit the process and how these may be implicated in pathologies such as cancer, AIDS, autoimmune disease and neurodegenerative disease. Although copious amounts of information have been found in relation to apoptotic cell death, it has been established through many unanswered questions in the area that further research is highly warranted.
 Lawen A. (2003). BioEssays. Apoptosis - an introduction, 25:888–896.
 Vinay Kumar et al. (2008) Robbins Basic Pathology. (8th Edition). Philadelphia, Pa.: Saunders/Elsevier.
 Kerr JFR, Wyllie AH, Currie AR. Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Br J Cancer 1972;26:239–571.
 Vaux DL. (2002). Cell Death Differentiation. Apoptosis timeline, 9:349–354.
 Glucksmann A (1951) Cell deaths in normal vertebrate ontogeny. Biol. Rev. 26: 59±86.
 William K. Purves et al. (2004) Life - The Science of Biology. (7th Edition). Gordonsville, VA 22942 U.S.A.: VHPS/W.H. Freeman & Co.
 Bruce Alberts et al. (2002). Molecular Biology of the Cell. (5th Edition). New York, USA.: Garland Science.
 Kimball’s Biology Pages (2008). Apoptosis. Retrieved on May 20th, 2009 from source http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Apoptosis.html
 Elmore S. (2007). Toxicologic Pathology. Apoptosis: A Review of Programmed Cell Death, 35:495-516.
 Robert H. Horvitz (1999). Why Does Programmed Cell Death or Apoptosis Occur? Retrieved on May 16th, 2009 from source http://www.scientificamerican.com/article.cfm?id=why-does-programmed-cell
 Peter ME, Heufelder AE, Hengartner MO. (1997). Proceedings of the National Academy of Science of the United States of America. Advances in apoptosis research, 94(24):12736-7.
 Sulston JE. (1976). Philosophical Translations of the Royal Society of London. Series B, Biological Sciences. Post-embryonic development in the ventral cord of Caenorhabditis elegans, 275(938):287-97.
 Wyllie AH. (1980). Nature. Glucocorticoid-induced thymocyte apoptosis is associated with endogenous endonuclease activation, 284(5756):555-6.
 Horvitz HR, Ellis HM and Sternberg PW (1982) Programmed cell death in nematode development. Neurosci. Comment. 1: 56 ± 65.
 Hengartner MO, Horvitz HR. (1994). Philosophical Translations of the Royal Society of London. Series B, Biological Sciences. The ins and outs of programmed cell death during C. elegans development, 345(1313):243-6
 Ellis HM, Horvitz HR. (1986). Cell. Genetic control of programmed cell death in the nematode C. elegans. 44(6):817-29.
 Horvitz RH. (1999). Cancer Research. Genetic Control of Programmed Cell Death in the Nematode Caenorhabditis elegans, 59, 1701s-1706s.
 Vaux DL, Cory S and Adams JM (1988). Nature. Bcl-2 gene promotes haemopoietic cell survival and cooperates with c-myc to immortalize pre-B cells, 355: 440 ± 442.
 Yonish RE, Resnitzky D, Lotem J, Sachs L, Kimchi A and Oren M. (1991). Nature. Wildtype p53 induces apoptosis of myeloid leukaemic cells that is inhibited by interleukin-6, 353: 345 ± 347.
 Birnbaum MJ, Clem RJ and Miller LK. (1994). Journal of Virology. An apoptosis inhibiting gene from a nuclear polyhedrosis virus encoding a polypeptide with Cys/His sequence motoif, 68: 2521 ± 2528.
 Diamantisa A, Magiorkinisa E, Sakorafasb GH, Androutsosc G. (2008). Onkologie. A Brief History of Apoptosis: From Ancient to Modern Times, 31:702–706.
 Formigli L, Conti A, Lippi D. (2004). Minerva Medica. "Falling leaves": a survey of the history of apoptosis, 95(2):159-64.
Apoptosis signalling is a multi-pathway mechanism that results in the convergence of the activation of caspases (a family of cystiene proteases). These pathways are activated by either extracellular or intracellular stimuli. The caspases are activated by four major pathways; these include Mitochondrial-mediated pathway, death receptor-mediated pathway, granzyme-A and granzyme-B-mediated pathway, and Endoplasmic Reticulum (ER) - mediated pathway (1).
Death receptor-mediated pathway
Death receptor-mediated pathways are an extracellular activated pathway. The death receptors Fas and the tumour necrosis factor receptor TNFR are found on the surface of the plasma membrane. They are activated by death ligands. The death ligands Fas Ligand FasL and tumour necrosis factor alpha TNF-α bind to the receptor – initiating the cascade. Death receptors are single transmembrane-spanning proteins; Fas and TNFR-1 receptors have a cytoplasmic tail which is known as the death domain (DD) (fig 1) (3). After binding, the death receptors undergo trimerisation, which signals for the death inducing signalling complex (DISC). It consists of the recruitment of adaptor proteins, procaspase-8, and either Fas associated death domains (FADD) or TNF receptor associated death domain (TNFADD). Procaspase-8 cleavage of the pro-domain activates caspases-8 to its pro-apoptotic form (1), which then activates Caspase-3 (4). Caspases-3 protein can be activated in the death receptor-mediated pathway or the mitochondria-mediated pathway, and initiates DNA cleavage when activated. Caspase-3 cleaves a series of proteins responsible for maintaining cell life and structure. Including DNA metabolism substrates, such as poly(ADP-ribosome)polymerase PARP which facilitates DNA repair, cytoskeleton substrates, nuclear lamins, gelsolin, and fodrin (2,4). Caspase activation can be cross-linked, which demonstrates the signalling complexities of apoptosis. Caspase-8 can also act on the mitochondria-mediated pathway – amplifying the cascade of reactions (4). Bid is a proapoptotic member of the Bcl-2 family (1) that is activated by Caspase-8. It is located within the cytosol and, in the presence of caspase-8, can cleave to form tBid (truncated Bid). tBid can translocate to mitochondria – activating Bax, effects the functioning on the mitochondria and results in cytochrome c release (1).
Mitochondria-mediated pathways are an intracellular activated pathway, which is activated from the release of cytochrome c as a result of increased membrane permeability of the mitochondria itself (1). The release of cytochrome c occurs from both the inner mitochondrial membrane and the outer mitochondrial membrane (3). The release of cytochrome c into the cytosol results in the formation of apoptosomes. These wheel-like complexes are formed by the binding of apoptotic protease activating factor 1 Apaf-1 and caspase-9 (1). Conformational changes occur within the complex, which cleaves and in turn activates caspase-9, which then activates caspase-3. Caspase-3 will cleave the target proteins, initiating cellular degradation.
ER-mediated pathways have recently been linked to caspase activation. It is thought that the members of the Bcl-2 family that either inhibit or enhance apoptosis bind to the membrane of the ER (1). The response to abnormal events within the ER, such as a build up of misfolded proteins or the alteration in the homeostasis of calcium signalling, results in stress on the ER (3), which has been shown to initiate signalling for apoptosis. Caspase-12 is said to be activated by these stresses, and if over expression occurs, it can lead to the activation of other pathways (3). These pathways are not fully understood, but there are suggestions that activation of procaspase-9 and the formation of apoptosomes could be avoided via these pathways – directly activating mitochondria-mediated cytochrome c release via the activation of the Bax (3).
Granzyme –A and –B mediated pathways
Granzyme –A and –B mediated pathways are intracellular mediated pathways that release granzyme A (Grz A) and granzyme B (Grz B). Granzymes are serine proteases, which act very similarly to caspases. These pathways can be either independent or caspase-dependent pathways (1). Grz A is a slow acting independent pathway, which activates an ER associated complex known as SET. As a result of the reduction of reactive oxygen species (ROS) and mitochondrial membrane, signalling for the translocation of the complex into the cell’s nucleus is induced. Once the complex is within the cell’s nucleus, Grz A cleaves the complex – degrading the chromosomal DNA (4). Grz B is a depended-caspase pathway and is released from cytotoxic T Lymphocytes (CTL). It binds to cell surface receptors before being endocytosed into the cytosol with the help of a pore-forming protein perforin (1). Grz B triggers apoptosis in two ways; by cleaving Bid or by directly activating caspase-3. Grz B truncates Bid to tBid, which increases the membrane permeability of the mitochondria, consequently releasing cytochrome c. Cytochrome c actives caspase-9 (as described above) and downstream activation of caspase-3 occurs (4).
The mechanisms for apoptosis signalling are complex, and a lot is still not fully understood. There are a number of pathways which cross link, which enable the amplification of the apoptosis cascade. The diagram below demonstrates this.
Apoptosis Signalling Diagram
1. Wang ZB, Liu YQ, Cui YF. (2005). Pathways to caspase activation. Cell Bio Int. 29:489-496.
2. Stasser A, O'Connor Liam, and Dixit VM. (2000). Apoptosis Signalling. Annu.Rev.Biochem. 69:217-245
3. Czerski L, Nunez G. (2004).Apoptosome formation and caspase activation: is it different in the heart?. J Mol Cell Cardio. 37:643-652.
4. Chávez-Galán L, Arenas-Del Angel MC, Zenteno E, Chávez R and Lascurain R. (2009). Cell Death Mechanisms Induced by Cytotoxic Lymphocytes. Cellular & Molecular Immunology. 6(1):15-25.
Regulations of the apoptosis
Since apoptosis is leading to the irreversable damage of the cell due to the spontaneously activated caspases, it is crucial that caspase activations are tightly regulated. These regulation may delay or stop the apoptosis process so that it gives cell a time to repair the damage and return to their normal state. This is achieved by the adjustment of the availability of key anti- and pro- apoptotic molecules.: Regulation of apoptosis mechanisms are complex and still in progress of research, however following topics are the major regulators of the apoptosis.
Bcl-2 is the major regulator protein family of apoptosis via the mitochondrial mediated apoptosis pathway.There are around 25 different family members known to date, and both proapoptotic and antiapoptotic regulating proteins exist in this family. The activity and overall levels of pro- and anti-apoptotic families are under tight transcriptional and post-translational control. In healthy cells anti-apoptotic protein levels are high, whereas pro-apoptotic proteins are virtually absent or conformationally restrained. However, upon exposure to death signal this balance shifts in favour of pro-apoptotic proteins that are rapidly induced or dislodged. 
- Bcl-2, Bcl-XL, Bcl-W, Mcl-1 and Al/Bft-1 are major antiapoptotic proteins of the Bcl-2 family.
- Bcl-2 antiapoptotic proteins prevent mitochondrial changes; antiapoptotic family directly binding to and neutralizing the killer proteins(Bad), thus guard the mitochondrial membrane from pore formation. Formation of the pore may lead to release of cytochrome c and initiation of apoptosis.
- Bcl-2 family proteins(Bcl-XL,Bcl-2) can bind to the CED homolog Apaf-1 preventing its interaction with cytochrome c, thus preventing apoptosis.
- Antiapoptotic family can arrest the cell cycle mainly in G0 and G1 phase,it gives cell a chance to repaire the damage.
- Proteins such as Bax, Bak, and Mtd/Bok are the main proteins of proapoptotic family which have autonomous cytodestructive activity.
- They are able to bind Bcl-2, Bcl-XL, and prevent their antiapoptotic activity.
- They have similar structural to the pore-forming proteins. It enables them to form channels in the intracellular membranes and induce the release of cytochrome c from mitochondria.
- The induction of proapoptotic family protein is initiated after death stimuli. And p53 protein is also one of the initiator
ADEDs family proteins
In the death receptor mediated pathway, adaptor protein interacts with death receptor and initiate apoptosis. One of ADEDs(Death effector domain proteins) proteins, cFlip, is a homolog of caspases-8 and -10 that contains DEDs but lacks proteolytic activity. cFlip and other antiapoptotic DED family proteins compete with caspases for binding to Fadd/Mort-1, thus functioning as a transdominant inhibitors of these caspases involved in TNF family cytokine signaling. 
Inhibitor of apoptosis protein(IAP) family proteins constitute a novel group of apoptosiss suppressors that are conserved throughout animal evolution. Although the mechanisms of these proteins suppress cell death are not well understood, the only clearly identified activity for these proteins is as inhibitors of caspases. Specifically, several of the human IAP family proteins have been reported to directly bind and potently inhibit particular members of the caspase family of cell death proteases.:
Interestingly, not all caspases are targets of IAP suppression. To date, only caspases-3, -7 and -9 have been reported to be bound and inhhibited by IAPs. Caspases -1, -6, -8, and -10 are not. The significance of this selective inhibition of caspases by IAPs is that the inhibited caspases operate in the distal portions of the apoptotic proteolytic cascades, with some (such as caspases-3 and -7) functiooning as the ultimate effectors of apoptosis that cleave the various proteins responsible for cell death.
Ubiquitin is the protein which is often regarded as a tag for the 'waste product', it is now recognized it influences multiple biological processes, and it is also involved in regulation of the apoptosis. Ubiquitin is conjugated onto the protein and it directly influence the protein function. Moreover ubiquilated protein is recognised by Ub receptors and it leads to either degradation or culminating of the protein. With this mechanisms
- They control levels of many pro- and anti-apoptotic molecules
- induce apoptosis by promoting IAPs family degradation
- Ubiquitin may also limit the activation of caspases via ubiquitin (Ub)-dependent modifications.
Guardian of the genome 'p53' maintains the genetic integrity by recognition of DNA damage and regulation of the apoptosis. It found very low amount in normal cell. Following the DNA damages and death stimuli, it is activated and performs its action.
- When the damage is reversible, it arrests cell cycle and gives cell a time to repair.
- However if the damage is irreversible, it initiate the apoptosis through a mitochondrial pathway. Upon its activation, p53 binds to the outer mitochondrial membrane and induces permeabilization. It results release of cytochrome c which lead to caspase activation.
Click here for a general overview of the p53 genome.
The morphology of apoptosis
The morphogenesis of apoptosis are characterised within the nucleus of the cell’s compartment – in which, it illustrates four cardinal traits: volume reduction, chromatin condensation, recognition by phagocytic cells and phagocytosis.
- Cell shrinkage
Osmosis is considered to be the offset in the commencement of the apoptotic pathway, whereby, due to the influx of sodium ions (Na+)
there is lost of intracellular fluid selectively.
Therefore, it illustrates in an abrupt onset of cellular shrinkage and leads to the induction of:
- Increase of the cell’s density, in which, leads to the compression of the cytoplasmic organelles - the endoplasmic reticulum becomes dilated and the convolution of the cell and nuclear outlines becomes evident.
- Deterioration of the cell’s specialised membrane integrity, such as, the loss of microvilli, junctional complexes or desmosomes.
- Lost of extracellular signaling between adjacent cells, as a result, it dissociates from neighboring cells.
This process allows the cell to reorganise its peripheral focal adhesions, in which, is also allows the cellular structure to adopt a more rounded morphological appearance.
- Lost of plasma membrane integrity
The outer structural appearance of the cell formulates protrusions from the surface of its plasma membrane - known as blebs - the formation of blebs are due to the separation of the plasma membrane from the cytoskeleton of the cell.
The process of blebbing continues until the final molecular event of pyknosis – the condensation of the nuclear chromatin which takes place within the cell’s physical compartment.
- Chromatin condensation
The activation of condensation initiates at the periphery of the nuclear membrane, forming a dense crescent ring-like structure.
This is connected with the nuclear envelope, and later throughout the process, it is followed by clumping & margination of the chromatin structures, towards the inner nuclear membrane.
Throughout the intermediate stage of pyknosis, specific contents of the cell undergoes cytoskeletal differentiation:
- present of cytoplasmic shrinkage
- nuclear matrix & the lamina endures degradation – causes a loss in the structural integrity & allows the formation of small osmophilic granules to aggregate throughout the whole nucleus.
- Compression of the osmophilic granules causes pressuring against the chromatin structures, in which, it generates a stress response adjacent to other constituents within the cell and causes retention of the protein fibrillar core.
- DNA fragmentation
Progressively, with abundant cytoplasm condensed within the cell, it forms Extensive blebbing from the cell’s plasma membrane, as a result,
it leads to the final stages of pyknosis & the initiation of karyorrheix. Initiation of karyorrheix leads to:
- Degradation of the cell's chromosomal DNA, whereby, chromosomal digestion results first in
large (30-50 and 200-300kbp) DNA fragments and,
- Nucleus of the cell undergoing internucleosomal fragmentation of the DNA, in which, the enzyme endonuclease is responsible for cleaving off the double stranded DNA.
- Disintegration of the cell's nucleus, degrades into small granular spheres termed Apoptotic Bodies.
The nucleus are surrounded by a nuclear membrane, containing variability amount of the cell's constituent (such as condensed cytoplasm, cytosolic elements, parts of the nuclei & crowded intact cytoplasmic organelles.) 
|Structural shrinkage of the cell||Cell Membrane permeability|
|Blebbing from the surface of the cell's plasma membrane||Vacuole in the cytoplasm filled with cellular remanant|
|Cell disintegrates into apoptotic bodies||Diluation of the cellular organelles|
|Phagocytosis by macrophages||Loss of membrane integrity|
|Total cell lysis|
Apoptotic bodies are engulfed by phagocytosis of adjacent cells - conducted by professional phagocytes of macrophages & parenchymal cells.
One of the integral morphological aspect of apoptosis is in the phagocytosis process, whereby, the process
of uptake & degradation of the apoptotic bodies is rapid due to, it prevent leakage of contents from the apoptotic bodies,
as a result it dissembles the mechanical pathway of apoptosis from eliciting an inflammatory response in the host.
Once phagocytosed, the apoptotic bodies are degraded by lysosomal enzymes derived from the ingesting cells and
therefore eventually being reduced to unrecognisable residual bodies.
The morphology of necrosis
Necrosis is termed by the series of changes that accompany cell death, whereby, it occurs as a result of unintentional traumatic injury,
whether thermal, chemical, or anoxic.
Necrotic cells are unable to maintain their membrane integrity & results in leakage of contents and thus leads to eliciting an inflammatory response in the host.
Initially, the hallmark of necrosis is cellular swelling, in which, the cells fail in exchanging cellular fluid from the intracellular & extracellular regions, as a result, it cells cannot gain energy of ATP for mechanical function and leads to the physiological response of cellular swelling.
In-addition, the morphology of the cell membrane becomes permeable throughout the cellular death phase.
There is a change in the cellular content of the cell with the loss of density, the organelles may dilute and ribosomes dissociates from adjacent endoplasmic reticulum, and finally the lost of membrane integrity leads to rupture and lysis of the cellular content into the surrounding environment, causing an pathological inflammatory response.
Schematic drawing illustrating the main events of the biochemical & physiological pathway of apoptosis.
--Daniel Lee 16:54, 3 May 2009 (EST)
Design model of apoptosis
Apoptosis from a Clinical Point of View
As we now know that apoptosis follows a series of tightly regulated steps, it’s important to consider the implications of malfunctions and irregularities. Interruptions and disruptions to the system are caused by many different human pathologies, many of which are by an unknown mechanism. The need to understand the causal effect of these different abnormalities is essential in the potential treatment of these diseases. Diseases which target this system include; autoimmune diseases, neurodegenerative diseases, cancer and others.
- There are two ways in which this utilises the apoptotic pathway;
- Inhibiting apoptosis in the clearing of B or T cells resulting in tissue destruction
- Inhibiting apoptosis in the destruction of cells displaying autoantigens on their cell surface (Blumenthal et al., 2004)
- An example with Alzheimer’s disease
- It has been found that the amyloid plaques (APP) are a substrate for caspase 3 resulting in defective protein and therefore reducing the inhibition of apoptosis (Robertson et al., 1999)
- Also the cleavage of APP by caspases 6, 8 and 9 generates the peptide C31, a potent inducer of apoptosis (Chandra et al., 2000)
- These factors play a pivotal role in the over-active apoptosis and hence the increase neurodegeneration as seen in Alzheimer’s disease
- The use of capsase inhibition is proving to be effective in the protection against some neurodegenerative diseases, as well as from stroke, myocardial infarction, hepatic injury, spinal cord injury and many others.
- An example with IDN-6556, a selective and irreversible inhibitor of specific caspases used in liver injury. It shows potential in the application for liver transplants, as it helps restore many of the livers enzyme levels (Schulze-Osthoff et al., 2005)
- Cancer is almost apoptosis resistant
- Roughly half of all cancers affect p53 function, which is usually pro-apoptotic (Fadeel et al., 2005)
- iASPP (inhibitory apoptosis-stimulating protein of p53) has been found to be up-regulated in various cancers such as breast carcinomas (Fadeel et al., 2005)
- suppression of iASPP has been effective in inducing p53-dependent apoptosis – possibly a new strategy for the treatment of tumours expressing the wild-type p53 (O’Neil et al., 2003)
- Survivin is a protein of the IAP family which is expressed in cancers of the breast, colon, lung, pancreas and prostate. The inactivation of such a protein greatly increase the chance of therapeutic intervention (Altieri, 2003)
- Haematopoietic malignancies such as multiple myeloma and leukaemia are associated with a high ratio of Bcl-2 – BAX. By decreasing the concentrations of Bcl-2 and increasing BAX, the susceptibility of apoptosis occurring is increased in the cancerous cell. (Bentley et al., 1996; Wright el at., 1996)
- Chemotherapeutic drugs induce apoptosis with the aim of affecting the malignant cells more than the normal cells, unfortunately, some cancer cells are chemoresistant and many healthy cells are destroyed. The combination of these drugs with G-CSF, an anti-apoptotic factor for neutrophils, greatly decreases the apoptotic affects on neutrophils and hence the possibility of infection accompanying the chemotherapy (Nikolaizik et al., 1999; Yousefi et al., 2004). This combination therapy is also used with fibroblast growth factor, which reduces the gastrointestinal side-effects, and p53 inhibitors, which reduce normal tissue damage (Fadeel et al., 2005).
With the increasing understanding of what goes wrong with the apoptotic pathway comes the improved clinical effectiveness of the therapeutic intervention and prevention strategies. The future direction for the treatment of these various conditions lies predominantly in the improvement in selective targeting. The need to adapt and alter treatments specifically for the individual by the use of varying combination therapies is a huge focus, as there is an essential need to target the affected areas, whilst minimising the damage to normal areas. There has been a massive advancement in the understanding of the diseases affecting this fundamental pathway over the last 50 years, and there is still a long but optimistic future.
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2009 Group Projects
--Mark Hill 14:02, 19 March 2009 (EST) Please leave these links to all group projects at the bottom of your project page.
Group 1 Meiosis | Group 2 Cell Death - Apoptosis | Group 3 Cell Division | Group 4 Trk Receptors | Group 5 The Cell Cycle | Group 6 Golgi Apparatus | Group 7 Mitochondria | Group 8 Cell Death - Necrosis | Group 9 Nucleus | Group 10 Cell Shape