Cellular Apoptosis and Organ Injury in Sepsis: A Review

APOPTOSIS: PHYSIOLOGICAL CELL DEATH Apoptosis, as a biological phenomenon, is readily identifiable by several characteristic features. It characteristically affects scattered single cells, not groups of contiguous cells as in necrosis, with the dying cell undergoing a relatively ordered form of cell death. This physiological cell death is characterized by cell shrinkage, cellular crenation, cytoplasmic and chromatin condensation, and internucleosomal DNA fragmentation (1). Changes in membrane glycosylation and lipid profiles, and alteration in expression of surface receptors have been observed. The apoptotic cells are rapidly phagocytosed and degraded by neighbouring cells or resident macrophages without an inflammatory response. This mechanism prevents the release of the phlogistic contents of cells and avoids the possibility of neighbouring host cell injury. The process differs significantly from cell death by necrosis or lysis where cells release their contents into the surrounding tissues and perpetuate the local inflammatory response. A glossary of terms pertinent to this review is included as an appendix. APOPTOSIS: MORPHOLOGICAL EVENTS During apoptosis, the dying cell undergoes a series of profound structural changes. The earliest event observed by electron microscopy is condensation of chromatin to form sharply circumscribed, uniformly dense, cresentic masses that abut the nuclear envelope (2). Nucleolar changes include the dispersal of peripheral nucleolar chromatin to form aggregates in the centre of the nucleus. Simultaneously with the nuclear changes, apoptotic cells detach from neighbouring cells, and specialized surface structures such as microvilli appear. Cell volume decreases, cell density increases, cytoplasmic organelles compact, and convolution of the cell and nuclear outline becomes evident (1) (Fig. 1). Cytoplasmic changes include cytoskeletal filament aggregation, clumping of ribosomal particles, and rearrangement of rough endoplasmic reticulum to form a series of concentric whorls (3). Cytoplasmic and nuclear condensation is followed by the production of numerous membrane protuberances at the plasma membrane that subsequently separate with sealing of the plasmalemma to form membrane-bound apoptotic bodies of varying sizes with condensed cytoplasm and crowded, intact cytoplasmic organelles. The production of apoptotic bodies is a late occurrence in the apoptotic process and is observed extensively in vitro, but less commonly in vivo. This observation emphasises the rapidity of the apoptotic process whereby apoptotic cells are rapidly phagocytosed in vivo prior to apoptotic body formation (4). Phagocytosis is mediated by adjacent epithelial cells, mononuclear phagocytes, or tumor cells. Once phagocytosed, apoptotic bodies are degraded by lysosomal enzymes derived from the ingesting cell. Rapid phagocytosis of apoptotic cells in vivo before their secondary degeneration helps explain the absence of inflammation associated with apoptosis. Apoptotic bodies that escape phagocytosis lose their integrity after an hour or so, resulting in swelling, loss of density, membrane rupture, and organelle disruption and dispersal referred to as secondary necrosis (Fig. 1).Fig. 1: Morphological aspects of cell death by oncosis and apoptosis. Necrosis can occur after both forms of cell death. A normal cell is shown at the top. 1a, Swelling. 1b, Blebbing, vacuolization, and increased permeability. 2a, Shrinkage and pyknosis. 2b, Budding and karyohexis. 2c, Apoptotic bodies. 3, Necrotic changes (shrinkage, coagulation, and karyolysis) occurring after rupture of a cell surface bleb in oncosis, or secondarily due to failure of apoptotic bodies to be phagocytosed in apoptosis. Adapted from Majno and Joris, 1995.APOPTOSIS: BIOCHEMICAL EVENTS Cytoskeletal and membrane alterations Cell shrinkage and apoptotic body formation require significant changes in both the cytoskeleton and plasma membrane lipid bilayer. Cytoskeletal changes include tissue transglutaminase activation, microtubule disruption, α-fodrin (non-erythroid spectrin) and actin cleavage, and a requirement for actin polymerization. These changes facilitate membrane budding and play a role in the maintenance of plasma membrane integrity in apoptotic cells (5). Membrane changes include redistribution of phosphatidylserine from its normal location on the inner leaf of the plasma membrane lipid bilayer to the outer leaf, exposure of surface sugar residues from loss of membrane sialic acid, and loss of expression of surface markers such as FcγRIII (CD16), complement regulatory molecules (CD45 and CD59), and adhesion molecules (CD11/CD18). These membrane changes are believed to play a role in the recognition and eventual phagocytosis of apoptotic cells (6). Cell shrinkage Condensation of the cytoplasmic space resulting in cell shrinkage appears to be a universal characteristic of apoptosis (7). This change is thought to be consequent to net movement of water out of the cell due to vesicles budding from the endoplasmic reticulum and Golgi apparatus fusing with the plasma membrane with release of their contents into the extracellular space. A role for active ion efflux has been implicated in cell shrinkage with active efflux of Na+ and K+ ions through the Na+,K+ ATPase pump and Ca2+-dependent channel (8,9). Apoptotic DNA degradation Apoptotic cells display dramatic changes in the nucleus, including chromatin condensation and margination. A further nuclear-associated event during apoptosis is the degradation of DNA into 180- to 200-bp oligonucleosomal fragments. These fragments form a ladder-type pattern when subjected to agarose gel electrophoresis, a feature that is now one of the biochemical hallmarks of apoptosis (10). However, it should be noted that apoptotic internucleosomal DNA fragmentation is not universal, although higher molecular weight fragments (50–300 Kbp) have been reported in certain cell types immediately proceeding or in the absence of oligonucleosomal fragmentation (11). A number of putative endonucleases have been proposed, including, DNase I, DNase II, NUC-18, as well as other novel endonucleases including the caspase-activated deoxyribonuclease (12). The fact that mRNA of these endonucleases is expressed in only a limited number of human tissues suggests that other enzymes may participate in the degradation of DNA during apoptosis. Regulators of apoptosis It is well established that cell and are it is now that of cell death is as and in the maintenance of tissue Apoptotic cell death is by and the death can be by that physiological forms of cell death a of and the that a and event for the of cell death not the on of the by and of the and for normal cell and a to as a of cell death (Fig. The of of these an for in the or of cell death cell death have been and a number of have been shown to be or and the of apoptosis is by the of are to the cell and the of are to cell 1: and of cell death in and The of and and and The and apoptotic and apoptosis should of a cell to apoptosis and the the of the cell. Adapted from and the of a death can be by from the cells The that certain cells are with cellular apoptotic by cells are to for the of a limited of extracellular cell are relatively as a of for a and cell death. A review of the and and of apoptosis The of and nuclear with can to from or is a in both cell and apoptosis This is through an of the by from this the of a such as or of a of apoptosis and in its the by in apoptosis. It is now believed that apoptosis may the fact that the and apoptosis are the this of cell the apoptotic by the cell. are by neighbouring cells, and this that cells for and the of cell. This has for as or are to and cellular the of and cell in the absence of apoptosis and a for in (Fig. of the and death this not apoptosis but as a to apoptotic the absence of the cell to an apoptotic the of and to cell may cells in through of although other Adapted from and tumor The is a that a role in the cellular to DNA to or loss of are the in human and that of tumor is as as of in DNA a cell DNA to occur prior to further in the event of DNA of the apoptotic the mechanism whereby apoptosis is several have that apoptosis by of such as and of such as and of the (Fig. However, apoptosis can by apoptosis of and is not for cell death both and of apoptotic cell death have been reported these that the role of may be as a of DNA and the of the cellular cell or apoptosis. and death expression receptors to the tumor necrosis is by extracellular A number of death receptors to the have been including and this of a cytoplasmic the death receptors to the apoptotic of these receptors by a apoptosis by The a that with the of the The that the death receptors are molecules to the to and to and to and The for is further are in the of the death is a in apoptosis. The of death that apoptosis a of to the of apoptotic cell death. It that an can apoptosis in certain and that it may the death through an or alterations in death may have in cell death in human to be in cell death in and The and of the significant to the and the that may play a role in apoptosis (Fig. a that after for human of this for a role of as of apoptosis from the fact that of apoptosis and the of such as apoptosis have been shown to by apoptosis, to However, display an normal that may be in the a mechanism through the apoptotic process can be However, is now that suggests that although may certain characteristic biochemical and of apoptosis, cells that have a and have been with have their or and are to by a mechanism not readily identifiable as apoptosis The human The of a cell to apoptosis is by of the the of (Fig. The of cell death is the apoptotic of cells and of has been shown to cellular by apoptosis by a of including and in of the has been shown to in with a expression in peripheral in not a to a of apoptosis regulatory of the have been including apoptotic and and apoptotic and of the are of with other through a of and A is with the of death to a or death to an apoptotic cell death is appears to have a number of in the cellular to an apoptotic including as an ion a membrane and as an or (Fig. A of the putative of of the in apoptosis. such as to at to apoptosis. may form ion and ion appears to as an or by other apoptotic out of the to to to with other may as a membrane by the of the and loss of the membrane A death for in of such as with and its may to or ion channel to its to the or appears to have ion channel and may its role by the loss of and an that has been associated with both and apoptosis are of a fact by the of in human their through of the cells to through of the cell the to cellular of has been shown to apoptosis The is a thought to play a role during cell The forms of the of and have been reported to of their through of apoptosis on to on the and a of the a of by and can cell from apoptosis by has been shown to be in to extracellular the of cells to apoptosis. referred to as and are of apoptosis and cell in the absence of are now referred to as of has the number of and that cells the of the a apoptotic and apoptosis are by the of This suggests that cell types be to the tissues their and of the of may be a to apoptosis. cells and cells have been shown to be on such as and in The requirement of cells for may be in and of cells during and after an response. a such as and are both and at in have been shown to and apoptosis in and in the inflammatory have been shown to and that both and may cell of a local in be to apoptosis in the cell with a to the regulatory evident in apoptosis, it is not that a and of molecules are of an apoptotic response. molecular have been implicated in the apoptotic including and The of and biochemical observed and cell types in the of apoptosis has to that these may a or of the apoptotic process The of a may explain of apoptosis such as can apoptosis by a of have the as the of the apoptotic process to the of apoptosis. the apoptotic process can be into several but the in the apoptotic process is and are the or in the and are and the is in the characteristic and biochemical of apoptosis The of apoptosis, a process with regulatory into although to in a in the apoptotic and in apoptosis for apoptosis the number of has been an in of the death and in the release of apoptotic of the death by its This has with at the and of the death is now is that to cell death. or on the death the cell and the apoptotic and in the of for and of apoptosis. and apoptotic are the and the of to of the and of its cytoplasmic death a of into a apoptotic have been (Fig. The characterized the death with its death and its death of the of such as the of the cell to apoptotic is by with to form a that of the apoptotic a series of are include of These enzymes can in a of including of and changes, including of and of The of in apoptosis is both cell and on the of the cell. of has been shown during apoptosis of peripheral that of apoptosis in these cells. of in and prevents apoptosis and has been observed during The of on apoptosis is a number of including the cell the and of and the of of has been shown to be in A event in apoptosis in cell is the of resulting in a in ions to play a role in apoptosis in certain cell with of extracellular or apoptosis in these cells has been shown to have of in the apoptotic including of enzymes such as tissue and chromatin and However, in the of alteration in cellular not to be for the of apoptosis, the possibility that alterations in in certain cells occur as a of apoptosis. apoptotic cell death is not of has been shown to apoptosis in certain cells such as in a has been observed cell the of apoptosis by a of and has been as a apoptotic However, the requirement for production is not as production is not evident during of apoptosis in cell that the of the cell may be during apoptosis without of through a mechanism can apoptosis in the or of that are not the of apoptosis in the in apoptosis have been production disruption have been shown to and release of the that may play a role in of These the that is a but not in the apoptotic A The of to appears to be a in apoptosis suggests that a and a of to the of in apoptosis. groups have the as this putative of the apoptotic and a are during apoptosis and are of the apoptotic process The of the of has of of apoptosis is is a with a are to and are to (Fig. appears to be the of the apoptotic by and and in and of has been shown to with and to form a an with the of mediated by These into the of and in the regulatory of A of by the and a an and prevents of by A death for of with it from in with from the and can subsequently resulting in the the role of in and of have on the universal requirement of in apoptosis. These of by and prior to loss of of at the and the fact that certain cell that DNA and not have a can be to by apoptosis These that the role of the as a may not in cell and that an of in the apoptotic by one death may be to its cell apoptosis to as This the possibility that may in be a in apoptotic The that apoptosis to the of by in The that certain in in to a in cellular apoptosis, for the failure of is an and it that the of apoptosis occurring in is and to and the of have been shown to apoptotic in tissue cells and their These include increased and secondary to increased production of associated with and the of in the and is one of the in the observed in a it has been implicated in the increased apoptosis in a of tissue types that during It is of apoptosis in cells and The of is to in plasma and at higher it a role in cell and and of at these for tissue and However, these of are when only to of receptors on cells are and it is for this that when in as in tissue and in to increased apoptosis. the apoptotic of may be by and mediated by both of in in is a with in and by and to by other cell are by one but is the form in vivo. of with a of to of to in and increased tissue and a These are not as profound as with but for significant and The of and are and the production of the that for the of the of is the for of with have in is associated with cell apoptosis and an in This mechanism appears to be for apoptosis, as well has been reported to cell apoptosis it significant of apoptosis of apoptosis is associated with and this may be for by a of other and such as and a number of of the The has been shown to the of such as and cellular apoptosis The associated with in of such as of the have been reported to apoptosis in and through the in of the observed in cell death is that apoptosis and this is with increased production by these cells and apoptosis are in biological and are a characteristic of in to has been implicated in the of and the of apoptosis. DNA changes in the form of and that may apoptotic cell death. play a role in the cell death occurring in cell including cells cells cells cells cells and cells such as and are the their to a on have been shown to apoptosis through of is a that is after and and is by has been implicated in apoptosis in numerous cell including and cells and may be in apoptosis and apoptosis the the and of of these is a of the outer of have that of or in a in and death. of a active lipid A a and an in the outer membrane of It with a of cell types the receptors and receptors and to cells the of in This cellular to and is an to the in the of of in and in of due to its to in cells receptors profound and release from macrophages of a of the It has that a number of and can a in and have in in that is one of molecules that The of has been to explain identifiable of can be in as as of from that is increased in with This of the into the from where and of cellular apoptotic is one mechanism by this that apoptosis or is a of and is associated with This may in be to of the resident of The associated with is mediated by of these by macrophages may and their to the of tissue has been shown to apoptosis in macrophages by as as This profound when to the of as the of are of apoptosis of this it has been reported that apoptosis may for the increased inflammatory and associated with in the of have shown that is of cell death in macrophages of and that this apoptosis on an for one of the failure occurring in the of has been to cell apoptosis. has been shown to of mRNA in a and in and cells. in mRNA and the has been with increased apoptosis have that cell apoptosis in a and and it rapidly changes associated with a on expression of the and the these the of on apoptosis with and that apoptosis in a is a phenomenon, as DNA fragmentation electron and This of apoptosis by is by from other have that apoptosis a that is of and that may a novel to the death at in that not apoptosis in from a of are by in human expression of the and the in of these is on and of have shown that is to cell death in but may and apoptosis. of apoptosis by but is an that the of the of on be the occurring in from local loss of integrity at with out into is not the only of apoptosis. in the outer of are of cellular apoptosis. has shown that apoptosis in a cell through and in an epithelial cell with a molecular other apoptosis, and host vivo that apoptosis vivo of the of on cellular apoptosis of the on apoptosis on and in vivo apoptosis in the and other in to that of DNA fragmentation in the of by gel and This mediated in by and as and DNA fragmentation with these in but in to apoptosis in and apoptosis are with the of both prevents apoptosis. These are to increased of and from has been as an event to and is associated with significant from to the or of from and are characterized by a inflammatory and it is that apoptosis significantly to in this It has been that a in with of may be the for has that may cells to apoptosis. that apoptosis in the but that this This with increased and expression observed in the that the of be by the of through the of cell apoptosis the event the of from and of in a to the role of apoptosis in DNA fragmentation in but in and epithelial cells the of such have to apoptosis as a with before and after of has been shown to and other apoptotic tissue and to in a of The has been shown to be in of a of to significantly the of at and in separate this to an of in occurring in a The that the mechanism of this may be through the of that is by this a series of that that and cells as the and this in cell apoptosis a of exposure cells are a in the of an host these may pertinent on the of and have from the to the the may be a in the of This that epithelial or integrity is is that is by increased apoptosis, but is that epithelial cell apoptosis, the of a from has been as a for the of epithelial cell apoptosis. after of of to the epithelial apoptotic at to after such as these may for the of although to be in this The event in is to volume and associated with of and cellular The and may in from A has a of on apoptotic as a of in vivo. after from and of a apoptotic by a in with associated in expression of the of such it is that to may play a role in has a role for in the observed in both and to a in inflammation of and are a that failure is not to apoptosis or that apoptosis is of However, these are in the and the of that in vivo has profound on cellular apoptosis of such from well it can be that tissue cell apoptosis occur in of apoptosis in active human have been is a of this in has been in with of significant apoptotic occurring in of have been in as with without The of to with of in this the in the of these in to of The as a of apoptosis and apoptosis has been in a of to This increased apoptosis occurring in peripheral mononuclear cells of an from increased significantly after and for a of prior to to The in mononuclear cells to increased apoptotic as by and chromatin condensation and DNA in in after reported increased of nuclear in with is a cell death not to with and and These by with increased of and from to apoptosis as to when with This suggests that apoptosis in may in significant tissue of epithelial and apoptosis in with and The tissue from with and with from undergoing and microscopy the nuclear and for active and that epithelial and apoptosis in the of the of with the of apoptosis. apoptosis associated with a in of that apoptotic loss of epithelial cells may to and that increased apoptosis may a the apoptotic cell death in with and A of of and with a of in and by and DNA gel from apoptosis in of of and of of in and a in of with both of increased apoptosis. of tissues of apoptosis. for active a significant in in The that this is the to of dying from of this and from this be the by further be It the single of a role for apoptosis in However, it not apoptosis is or to the host in these This the the of apoptotic in the of This review of apoptosis and its to the of and pertinent to the human the of should in but and should However, the of from to the is has been the at a molecular for of at a in This of thought to in the of as a form of from are to human at have the in although in the of at and groups A and one of in and in the it that to of the less the have the of this a the of apoptosis are cell the on apoptotic of single at these is to be of the apoptotic occurring cells and their for not or to that apoptosis. this only the as an the this has with apoptosis as a of However, apoptosis the in a to that such a mechanism to from inflammatory are by that from with active and from with apoptosis is to this process is by the of to the of with to and phagocytosis of This the of to the of has on apoptosis, epithelial cell apoptosis, apoptosis, such a the and reported on the of of a to to in tissues and However, the from this be It is that the of and a the of in a of in tissue is have to and such as these before can apoptotic as a This and for and in the of the occurrence of cellular apoptosis in has been to this is of and may be the in the of these The of human it that require a both and on cell and and the pertinent apoptotic in the may is the of cellular apoptosis, or is cellular apoptosis the of