Programmed cell-death (or PCD) is death Death is the termination of the biological functions that define a living organism. The word refers both to a particular process and to the condition that results thereby. The nature of the latter has been for millennia a central concern of the world's religious traditions and of philosophical enquiry. Belief in some kind of afterlife or rebirth of a cell The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, in any form, mediated by an intracellular program.^[1] In contrast to necrosis Necrosis is the premature death of cells and living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death. While apoptosis often provides beneficial effects to the organism, necrosis is almost always, which is a form of cell-death that results from acute tissue Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues injury and provokes an inflammatory Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process. Inflammation is not a synonym for infection. Even in cases where inflammation is response, PCD is carried out in a regulated process A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation which generally confers advantage during an organism's life-cycle A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction . For example, a complex life cycle of Fasciola hepatica includes three different multicellular generations: 1) "adult" hermaphroditic; 2) sporocyst; 3). PCD serves fundamental functions during both plant Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, and metazoa Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also (multicellular animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also) tissue development.

Types

• Apoptosis Apoptosis is the process of programmed cell death (PCD) that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, loss of cell membrane asymmetry and attachment, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA or Type I cell-death.
• Autophagic In cell biology, autophagy, or autophagocytosis, is a catabolic process involving the degradation of a cell's own components through the lysosomal machinery. It is a tightly-regulated process that plays a normal part in cell growth, development, and homeostasis, helping to maintain a balance between the synthesis, degradation, and subsequent or Type II cell-death. (Cytoplasmic The cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the contents of the cell nucleus are not part of the cytoplasm and are instead called the nucleoplasm. Also in eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the: characterized by the formation of large vacuoles A vacuole is a membrane organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including various enzymes in solution, though in certain cases they may contain solids which have been which eat away organelles In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer in a specific sequence prior to the nucleus In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes being destroyed.)^[2]

Besides these two types of PCD, other pathways have been discovered.^[3] Called "non-apoptotic programmed cell-death" (or "caspase Caspases, or cysteine-aspartic proteases or cysteine-dependent aspartate-directed proteases are a family of cysteine proteases, which play essential roles in apoptosis , necrosis and inflammation-independent programmed cell-death" or "necrosis-like programmed cell-death"), these alternative routes to death are as efficient as apoptosis and can function as either backup mechanisms or the main type of PCD.

Other forms of programmed cell death include anoikis, almost identical to apoptosis except in its induction; cornification, a form of cell death exclusive to the eyes; excitotoxicity Excitotoxicity is the pathological process by which nerve cells are damaged and killed by glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate such as the NMDA receptor and AMPA receptor are overactivated. Excitotoxins like NMDA and kainic acid which bind to these receptors, as well as and Wallerian degeneration Wallerian degeneration is a process that results when a nerve fiber is cut or crushed, in which the part of the axon separated from the neuron's cell nucleus degenerates. This is also known as anterograde degeneration.

Plant cells undergo particular processes of PCD which are similar to autophagic cell death. However, some common features of PCD are highly conserved in both plants and metazoa.

History

The concept of "programmed cell-death" was used by Lockshin & Williams^[4] in 1964 in relation to insect Insects are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living tissue development, around eight years before "apoptosis" was coined. Since then, PCD has become the more general of these two terms.

PCD has been the subject of increasing attention and research efforts. This trend has been highlighted with the award of the 2002 Nobel Prize in Physiology or Medicine The Nobel Prize in Physiology or Medicine is awarded once a year by the Swedish Karolinska Institute. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in Physics, Chemistry, Literature, Peace, and Physiology or Medicine since 1901. The first Nobel Prize in Physiology or to Sydney Brenner Sydney Brenner, CH FRS is a South African biologist and a 2002 Nobel prize in Physiology or Medicine laureate, shared with H. Robert Horvitz and John Sulston (United Kingdom The United Kingdom of Great Britain and Northern Ireland[note 7] is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of the island of Ireland, and many small islands. Northern Ireland is the only part of the UK with a land), H. Robert Horvitz H. Robert Horvitz is an American biologist best known for his research on the nematode worm Caenorhabditis elegans. He is currently at the Massachusetts Institute of Technology where he is Professor of Biology and a member of the McGovern Institute for Brain Research. He is also an Investigator of the Howard Hughes Medical Institute. In 2000 he (US) and John E. Sulston Sir John Edward Sulston, FRS is a British biologist and the 2002 Nobel prize in Physiology or Medicine laureate (UK).^[5]

Programmed cell-death in plant tissue

Programmed cell death in plants has a number of molecular similarities to animal apoptosis Apoptosis is the process of programmed cell death (PCD) that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, loss of cell membrane asymmetry and attachment, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA, but it also has differences, most obviously the presence of a cell wall A cell wall is a tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion and the lack of an immune system An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own healthy cells and tissues in order to function which removes the pieces of the dead cell. Instead of an immune response, the dying cell synthesizes substances to break itself down and places them in a vacuole A vacuole is a membrane bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been which ruptures as the cell dies.^[6]

In "APL regulates vascular tissue identity in Arabidopsis Arabidopsis is a genus in the family Brassicaceae. They are small flowering plants related to cabbage and mustard. This genus is of great interest since it contains thale cress (Arabidopsis thaliana), one of the model organisms used for studying plant biology and the first plant to have its entire genome sequenced. Changes in thale cress are",^[7] Bonke and colleagues state that one of the two long-distance transport systems in vascular plants Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the ferns, clubmosses, flowering plants, conifers and other gymnosperms. Scientific names for the group include Tracheophyta and Tracheobionta, but neither name is very widely used.[, xylem In vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek ξυλον , "wood", and indeed the best-known xylem tissue is wood, though it is found throughout the plant. Its basic function is to transport water but it also transports some, consists of several cell-types "the differentiation of which involves deposition of elaborate cell-wall A cell wall is a tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion thickenings and programmed cell-death." The authors emphasize that the products of plant PCD play an important structural role.

Basic morphological and biochemical features of PCD have been conserved in both plant and animal kingdoms In biology, kingdom or regnum is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany. The complete sequence of ranks is life, domain, kingdom, phylum, class, order, family, genus, and species.^[8] It should be noted, however, that specific types of plant cells carry out unique cell-death programs. These have common features with animal apoptosis—for instance, nuclear DNA Nuclear DNA, nuclear deoxyribonucleic acid , is DNA contained within a nucleus of eukaryotic organisms. In most cases it encodes more of the genome than the mitochondrial DNA and is passed sexually rather than matrilineally. Nuclear DNA is the most common DNA used in forensic examinations degradation—but they also have their own peculiarities, such as nuclear In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes degradation being triggered by the collapse of the vacuole A vacuole is a membrane bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been in tracheary Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast has broken down and disappeared. The presence of elements of the xylem.^[9]

Janneke Balk and Christopher J. Leaver, of the Department of Plant Sciences, University of Oxford The University of Oxford , located in the English city of Oxford, is the oldest surviving university in the English-speaking world and is regarded as one of the world's leading academic institutions. Although the exact date of foundation remains unclear, there is evidence of teaching there as far back as the 11th century. The University grew, carried out research on mutations Mutations are changes in the DNA sequence of a cell's genome and are caused by radiation, viruses, transposons and mutagenic chemicals, as well as errors that occur during meiosis or DNA replication. They can also be induced by the organism itself, by cellular processes such as hypermutation in the mitochondrial genome of sun-flower Sunflowers are annual plants native to the Americas, that possess a large inflorescence (flowering head) cells. Results of this research suggest that mitochondria In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 10 micrometers (μm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of play the same key role in vascular plant PCD as in other eukaryotic A eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the cells.^[10]

PCD in pollen prevents inbreeding

During pollination Pollination is the process by which pollen is transferred in plants, thereby enabling fertilization and sexual reproduction. Pollen grains, which contain the male gametes to where the female gamete(s) are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself. The receptive part of the carpel is called a, plants enforce self-incompatibility (SI) as an important means to prevent self-fertilization Reproduction is the biological process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: sexual and asexual. Research on the corn poppy (Papaver rhoeas) has revealed that proteins Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the pistil The gynoecium is a term with several meanings in botanical usage. In reference to mosses; liverworts; and hornworts, gynoecium refers to a cluster of archegonia and any associated modified leaves or stems present on a gametophyte shoot. The more common use of gynoecium, however, is to refer to the ovule-producing part of a flower. The gynoecium in on which the pollen Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants. When pollen lands lands, interact with pollen and trigger PCD in incompatible (ie. self) pollen. The researchers, Steven G. Thomas and Veronica E. Franklin-Tong, also found that the response involves rapid inhibition of pollen-tube The pollen tube of most seed plants acts as a conduit to transport sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms (conifers and gnetophytes). In other gymnosperms (Ginkgo and cycads) the pollen tube is involved only in nutrient uptake from growth, followed by PCD.^[11]

Programmed cell death in slime molds

The social slime mold Dictyostelium discoideum has the peculiarity of adopting either a predatory amoeba-like behavior in its unicellular form, or coalescing into a mobile slug-like form when dispersing the spores which will give birth to the next generation.^[12]

The stalk is composed of dead cells which have undergone a type of PCD that shares many features of an autophagic cell-death: massive vacuoles forming inside cells, a degree of chromatin condensation, but no DNA-fragmentation.^[13] The structural role of the residues left by the dead cells is reminiscent of the products of PCD in plant tissue.

D. discoideum is a slime mold, part of a branch which may have emerged from eukaryotic ancestors about a billion years before the present. They apparently emerged after the ancestors of green-plants and the ancestors of fungi and animals had differentiated. But in addition to their place in the evolutionary tree, the fact that PCD has been observed in the humble, simple, six-chromosome D. discoideum has additional significance: it permits the study of a developmental PCD path which does not depend on the caspases which are characteristic of apoptosis.^[14]

Evolutionary origin of PCD

Biologists had long suspected that mitochondria originated from bacteria which had been incorporated as endosymbionts ("living together inside") of larger eukaryotic cells. It was Lynn Margulis who from 1967 on championed this theory, which has since become widely accepted.^[15] The most convincing evidence for this theory is the fact that mitochondria possess their own DNA and are equipped with genes and replication apparatus.

This evolutionary step would have been more than risky for the primitive eukaryotic cells which began to engulf the energy-producing bacteria and conversely, a perilous step for the ancestors of mitochondria which began to invade their proto-eukaryotic hosts. This process is still evident today, between human white blood-cells and bacteria. Most of the time, invading bacteria are destroyed by the white blood-cells; however, it is not uncommon for the chemical warfare waged by prokaryotes to succeed, with the consequence known as infection by its resulting damage.

One of these rare evolutionary events, about two billion years before the present, made it possible for certain eukaryotes and energy-producing prokaryotes not only to coexist, but to mutually benefit from their symbiosis.^[16]

Mitochondriate eukaryotic cells live poised between life and death, because mitochondria still retain their repertoire of molecules which can trigger cell suicide.^[17] This process has now been evolved to happen only when programmed.^{[citation needed]} Given certain signals to cells (such as feedback from neighbors, stress or DNA-damage), mitochondria release caspase activators which trigger the cell-death inducing biochemical cascade. As such, the cell-suicide mechanism is now crucial to all of our lives.

Programmed death of entire organisms

References

1. ^ Engelberg-Kulka H, Amitai S, Kolodkin-Gal I, Hazan R (2006). "Bacterial Programmed Cell Death and Multicellular Behavior in Bacteria". PLoS Genetics 2 (10): e135. doi:10.1371/journal.pgen.0020135. http://genetics.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pgen.0020135.
2. ^ Schwartz LM, Smith SW, Jones ME, Osborne BA (1993). "Do all programmed cell deaths occur via apoptosis?". PNAS 90 (3): 980–4. doi:10.1073/pnas.90.3.980. PMID 8430112. ;and, for a more recent view, see Bursch W, Ellinger A, Gerner C, Fröhwein U, Schulte-Hermann R (2000). "Programmed cell death (PCD). Apoptosis, autophagic PCD, or others?". Annals of the New York Academy of Sciences 926: 1–12. PMID 11193023.
3. ^ Kroemer G, Martin SJ (2005). "Caspase-independent cell death". Nature Medicine 11 (7): 725–30. doi:10.1038/nm1263. PMID 16015365.
4. ^ Lockshin RA, Williams CM (1964). "Programmed cell death—II. Endocrine potentiation of the breakdown of the intersegmental muscles of silkmoths". Journal of Insect Physiology 10 (4): 643–649. doi:10.1016/0022-1910(64)90034-4. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T3F-49N9JTD-BN&_user=10&_coverDate=08%2F31%2F1964&_alid=509668351&_rdoc=1&_fmt=summary&_orig=search&_cdi=4945&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=244d74b599c3c6e6979401175da87303.
5. ^ "The Nobel Prize in Physiology or Medicine 2002". The Nobel Foundation. 2002. http://nobelprize.org/nobel_prizes/medicine/laureates/2002/index.html. Retrieved 2009-06-21.
6. ^ Collazo C, Chacón O, Borrás O (2006). "Programmed cell death in plants resembles apoptosis of animals". Biotecnología Aplicada 23: 1–10. http://elfosscientiae.cigb.edu.cu/PDFs/BA/2006/23/1/BA002301RV001-010.pdf.
7. ^ Bonke M, Thitamadee S, Mähönen AP, Hauser MT, Helariutta Y (2003). "APL regulates vascular tissue identity in Arabidopsis". Nature 426 (6963): 181–6. doi:10.1038/nature02100. PMID 14614507.
8. ^ Solomon M, Belenghi B, Delledonne M, Menachem E, Levine A (1999). "The involvement of cysteine proteases and protease inhibitor genes in the regulation of programmed cell death in plants". The Plant Cell 11 (3): 431–44. doi:10.2307/3870871. PMID 10072402. See also related articles in The Plant Cell Online
9. ^ Ito J, Fukuda H (2002). "ZEN1 is a key enzyme in the degradation of nuclear DNA during programmed cell death of tracheary elements". The Plant Cell 14 (12): 3201–11. doi:10.1105/tpc.006411. PMID 12468737.
10. ^ Balk J, Leaver CJ (2001). "The PET1-CMS mitochondrial mutation in sunflower is associated with premature programmed cell death and cytochrome c release". The Plant Cell 13 (8): 1803–18. doi:10.1105/tpc.13.8.1803. PMID 11487694.
11. ^ Thomas SG, Franklin-Tong VE (2004). "Self-incompatibility triggers programmed cell death in Papaver pollen". Nature 429 (6989): 305–9. doi:10.1038/nature02540. PMID 15152254.
12. ^ Crespi B, Springer S (2003). "Ecology. Social slime molds meet their match". Science 299 (5603): 56–7. doi:10.1126/science.1080776. PMID 12511635.
13. ^ Levraud JP, Adam M, Luciani MF, de Chastellier C, Blanton RL, Golstein P (2003). "Dictyostelium cell death: early emergence and demise of highly polarized paddle cells". Journal of Cell Biology 160 (7): 1105–14. doi:10.1083/jcb.200212104. PMID 12654899.
14. ^ Roisin-Bouffay C, Luciani MF, Klein G, Levraud JP, Adam M, Golstein P (2004). "Developmental cell death in dictyostelium does not require paracaspase". Journal of Biological Chemistry 279 (12): 11489–94. doi:10.1074/jbc.M312741200. PMID 14681218.
15. ^ de Duve C (1996). "The birth of complex cells". Scientific American 274 (4): 50–7. doi:10.1038/scientificamerican0496-50. PMID 8907651.
16. ^ Dyall SD, Brown MT, Johnson PJ (2004). "Ancient invasions: from endosymbionts to organelles". Science 304 (5668): 253–7. doi:10.1126/science.1094884. PMID 15073369.
17. ^ Chiarugi A, Moskowitz MA (2002). "Cell biology. PARP-1--a perpetrator of apoptotic cell death?". Science 297 (5579): 200–1. doi:10.1126/science.1074592. PMID 12114611.

See also

External links

• Apoptosis and Cell Death Labs
• International Cell Death Society
• The Bcl-2 Family Database

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