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Caspases (cysteine-aspartic proteases, cysteine aspartases or cysteine-dependent aspartate-directed proteases) are a family of
protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s playing essential roles in programmed cell death. They are named caspases due to their specific
cysteine protease Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Discovered by Gopal ...
activity – a cysteine in its active site nucleophilically attacks and cleaves a target protein only after an
aspartic acid Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...
residue. As of 2009, there are 12 confirmed caspases in humans and 10 in mice, carrying out a variety of cellular functions. The role of these enzymes in programmed cell death was first identified in 1993, with their functions in
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...
well characterised. This is a form of programmed cell death, occurring widely during development, and throughout life to maintain cell
homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
. Activation of caspases ensures that the cellular components are degraded in a controlled manner, carrying out cell death with minimal effect on surrounding tissues. Caspases have other identified roles in programmed cell death such as
pyroptosis Pyroptosis is a highly inflammatory form of lytic programmed cell death that occurs most frequently upon infection with intracellular pathogens and is likely to form part of the antimicrobial response. This process promotes the rapid clearance of va ...
, necroptosis and
PANoptosis PANoptosis is an inflammatory cell death pathway. Consideration of the totality of biological effects from cell death in multiple studies has led to the conceptualization of PANoptosis, a unique innate immune inflammatory cell death pathway regulate ...
. These forms of cell death are important for protecting an organism from stress signals and pathogenic attack. Caspases also have a role in inflammation, whereby it directly processes pro-inflammatory cytokines such as pro- IL1β. These are signalling molecules that allow recruitment of
immune cells White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mul ...
to an infected cell or tissue. There are other identified roles of caspases such as cell proliferation,
tumour A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
suppression, cell differentiation, neural development and axon guidance and ageing. ''Caspase deficiency'' has been identified as a cause of tumour development. Tumour growth can occur by a combination of factors, including a mutation in a cell cycle gene which removes the restraints on cell growth, combined with mutations in apoptopic proteins such as caspases that would respond by inducing cell death in abnormally growing cells. Conversely, over-activation of some caspases such as caspase-3 can lead to excessive programmed cell death. This is seen in several neurodegenerative diseases where neural cells are lost, such as Alzheimer's disease. Caspases involved with processing inflammatory signals are also implicated in disease. Insufficient activation of these caspases can increase an organism's susceptibility to infection, as an appropriate immune response may not be activated. The integral role caspases play in cell death and disease has led to research on using caspases as a drug target. For example, inflammatory caspase-1 has been implicated in causing autoimmune diseases; drugs blocking the activation of Caspase-1 have been used to improve the health of patients. Additionally, scientists have used caspases as cancer therapy to kill unwanted cells in tumours.


Functional classification of caspases

Most caspases play a role in programmed cell death. These are summarized in the table below. The enzymes are sub classified into three types: Initiator, Executioner and Inflammatory. Note that in addition to apoptosis, caspase-8 is also required for the inhibition of another form of programmed cell death called necroptosis. Caspase-14 plays a role in epithelial cell keratinocyte differentiation and can form an epidermal barrier that protects against dehydration and UVB radiation.


Activation of caspases

Caspases are synthesised as inactive
zymogen In biochemistry, a zymogen (), also called a proenzyme (), is an inactive precursor of an enzyme. A zymogen requires a biochemical change (such as a hydrolysis reaction revealing the active site, or changing the configuration to reveal the activ ...
s (pro-caspases) that are only activated following an appropriate stimulus. This post-translational level of control allows rapid and tight regulation of the enzyme. Activation involves dimerization and often oligomerisation of pro-caspases, followed by cleavage into a small subunit and large subunit. The large and small subunit associate with each other to form an active heterodimer caspase. The active enzyme often exists as a heterotetramer in the biological environment, where a pro-caspase dimer is cleaved together to form a heterotetramer.


Dimerisation

The activation of initiator caspases and inflammatory caspases is initiated by dimerisation, which is facilitated by binding to adaptor proteins via protein–protein interaction motifs that are collectively referred to as
death fold The death fold is a tertiary structure motif commonly found in proteins involved in apoptosis or inflammation-related processes. This motif is commonly found in domains that participate in protein–protein interactions leading to the forma ...
s. The death folds are located in a structural domain of the caspases known as the pro-domain, which is larger in those caspases that contain death folds than in those that do not. The pro-domain of the intrinsic initiator caspases and the inflammatory caspases contains a single death fold known as
caspase recruitment domain Caspase recruitment domains, or caspase activation and recruitment domains (CARDs), are interaction motifs found in a wide array of proteins, typically those involved in processes relating to inflammation and apoptosis. These domains mediate t ...
(CARD), while the pro-domain of the extrinsic initiator caspases contains two death folds known as
death effector domain The death-effector domain (DED) is a protein interaction domain found only in eukaryotes that regulates a variety of cellular signalling pathways. The DED domain is found in inactive procaspases (cysteine proteases) and proteins that regulate casp ...
s (DED). ''Multiprotein complexes'' often form during caspase activation. Some activating multiprotein complexes includes: *The ''
death-inducing signaling complex The death-inducing signaling complex or DISC is a multi-protein complex formed by members of the death receptor family of apoptosis-inducing cellular receptors. A typical example is FasR, which forms the DISC upon trimerization as a result of i ...
'' (DISC) during extrinsic apoptosis *The '' apoptosome'' during intrinsic apoptosis *The '' inflammasome'' during pyroptosis


Cleavage

Once appropriately dimerised, the Caspases cleave at inter domain linker regions, forming a large and small subunit. This cleavage allows the active-site loops to take up a conformation favourable for enzymatic activity. ''Cleavage of Initiator and Executioner caspases'' occur by different methods outlined in the table below. * Initiator caspases auto-proteolytically cleave whereas Executioner caspases are cleaved by initiator caspases. This hierarchy allows an amplifying chain reaction or cascade for degrading cellular components, during controlled cell death.


Some roles of caspases


Apoptosis

Apoptosis is a form of programmed cell death where the cell undergoes morphological changes, to minimize its effect on surrounding cells to avoid inducing an immune response. The cell shrinks and condenses - the
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is co ...
will collapse, the nuclear envelope disassembles and the DNA fragments up. This results in the cell forming self-enclosed bodies called 'blebs', to avoid release of cellular components into the
extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
medium. Additionally, the cell membrane phospholipid content is altered, which makes the dying cell more susceptible to phagocytic attack and removal. ''Apoptopic caspases are subcategorised as:'' # ''Initiator Caspases'' (
Caspase 2 Caspase 2 also known as CASP2 is an enzyme that, in humans, is encoded by the ''CASP2'' gene. ''CASP2'' orthologs have been identified in nearly all mammals for which complete genome data are available. Unique orthologs are also present in b ...
, Caspase 8, Caspase 9,
Caspase 10 Caspase-10 is an enzyme that, in humans, is encoded by the ''CASP10'' gene. This gene encodes a protein that is a member of the cysteine-aspartic acid protease (caspase) family. Sequential activation of caspases plays a central role in the exec ...
) # ''Executioner Caspases'' ( Caspase 3,
Caspase 6 Caspase-6 is an enzyme that in humans is encoded by the ''CASP6'' gene. ''CASP6'' orthologs have been identified in numerous mammals for which complete genome data are available. Unique orthologs are also present in birds, lizards, lissamphibi ...
and Caspase 7) Once initiator caspases are activated, they produce a chain reaction, activating several other executioner caspases. Executioner caspases degrade over 600 cellular components in order to induce the morphological changes for apoptosis. ''Examples of caspase cascade during apoptosis:'' # ''Intrinsic apoptopic pathway:'' During times of cellular stress, mitochondrial ''
cytochrome Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of ...
'' c is released into the cytosol. This molecule binds an adaptor protein ( APAF-1), which recruits initiator Caspase-9 (via CARD-CARD interactions). This leads to the formation of a Caspase activating multiprotein complex called the '' Apoptosome.'' Once activated, initiator caspases such as Caspase 9 will cleave and activate other executioner caspases. This leads to degradation of cellular components for apoptosis. # ''Extrinsic apoptopic pathway:'' The caspase cascade is also activated by extracellular ligands, via cell surface Death Receptors. This is done by the formation of a multiprotein Death Inducing Signalling Complex (DISC) that recruits and activates a pro-caspase. For example, the Fas Ligand binds the FasR receptor at the receptor's extracellular surface; this activates the death domains at the cytoplasmic tail of the receptor. The adaptor protein FADD will recruit (by a Death domain-Death domain interaction) pro-Caspase 8 via the DED domain. This FasR, FADD and pro-Caspase 8 form the Death Inducing Signalling Complex (DISC) where ''Caspase-8 is activated.'' This could lead to either downstream activation of the intrinsic pathway by inducing mitochondrial stress, or direct activation of Executioner Caspases (Caspase 3, Caspase 6 and Caspase 7) to degrade cellular components as shown in the adjacent diagram.


Pyroptosis

Pyroptosis is a form of programmed cell death that inherently induces an immune response. It is morphologically distinct from other types of cell death – cells swell up, rupture and release pro-inflammatory cellular contents. This is done in response to a range of stimuli including microbial infections as well as heart attacks (myocardial infarctions). Caspase-1, Caspase-4 and Caspase-5 in humans, and Caspase-1 and Caspase-11 in mice play important roles in inducing cell death by pyroptosis. This limits the life and proliferation time of intracellular and extracellular pathogens.


Pyroptosis by caspase-1

Caspase-1 activation is mediated by a repertoire of proteins, allowing detection of a range of pathogenic ligands. Some mediators of Caspase-1 activation are: NOD-like Leucine Rich Repeats (NLRs), AIM2-Like Receptors (ALRs), Pyrin and
IFI16 Gamma-interferon-inducible protein Ifi-16 (Ifi-16) also known as interferon-inducible myeloid differentiation transcriptional activator is a protein that in humans is encoded by the ''IFI16'' gene. Function This gene encodes a member of the ...
. These proteins allow caspase-1 activation by forming a multiprotein activating complex called Inflammasomes. For example, a NOD Like Leucine Rich Repeat NLRP3 will sense an efflux of potassium ions from the cell. This cellular ion imbalance leads to oligomerisation of NLRP3 molecules to form a multiprotein complex called the NLRP3 inflammasome. The pro-caspase-1 is brought into close proximity with other pro-caspase molecule in order to dimerise and undergo auto-proteolytic cleavage. Some pathogenic signals that lead to Pyroptosis by Caspase-1 are listed below: * ''DNA in the host cytosol'' binds to AIM2-Like Receptors inducing Pyroptosis * ''Type III secretion system apparatus from bacteria'' bind NOD Like Leucine Rich Repeats receptors called NAIP's (1 in humans and 4 in mice) ''Pyroptosis by Caspase-4 and Caspase-5 in humans and Caspase-11 in mice'' These caspases have the ability to induce direct pyroptosis when
lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O- antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the out ...
(LPS) molecules (found in the cell wall of gram negative bacteria) are found in the cytoplasm of the host cell. For example, Caspase 4 acts as a receptor and is proteolytically activated, without the need of an inflammasome complex or Caspase-1 activation. A crucial downstream substrate for pyroptopic caspases is
Gasdermin D Gasdermin D (GSDMD) is a protein that in humans is encoded by the ''GSDMD'' gene on chromosome 8. It belongs to the gasdermin family which is conserved among vertebrates and comprises six members in humans, GSDMA, GSDMB, GSDMC, GSDMD, GSDME ...
(GSDMD)


Role in inflammation

Inflammation is a protective attempt by an organism to restore a homeostatic state, following disruption from harmful stimulus, such as tissue damage or bacterial infection. Caspase-1, Caspase-4, Caspase-5 and Caspase-11 are considered 'Inflammatory Caspases'. * ''Caspase-1'' is key in activating pro-inflammatory
cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in au ...
s; these act as signals to immune cells and make the environment favourable for immune cell recruitment to the site of damage. Caspase-1 therefore plays a fundamental role in the innate immune system. The enzyme is responsible for processing cytokines such as pro-ILβ and pro-IL18, as well as secreting them. * ''Caspase-4 and -5 in humans, and Caspase-11'' ''in mice'' have a unique role as a receptor, whereby it binds to LPS, a molecule abundant in gram negative bacteria. This can lead to the processing and secretion of IL-1β and IL-18 cytokines by activating Caspase-1; this downstream effect is the same as described above. It also leads to the secretion of another inflammatory cytokine that is not processed. This is called pro-IL1α. There is also evidence of an inflammatory caspase, caspase-11 aiding cytokine secretion; this is done by inactivating a membrane channel that blocks IL-1β secretion * Caspases can also induce an inflammatory response on a transcriptional level. There is evidence where it promotes transcription of ''nuclear factor-κB ( NF-κB''), a transcription factor that assists in transcribing inflammatory cytokines such as
IFN Interferons (IFNs, ) are a group of signaling proteins made and released by host cells in response to the presence of several viruses. In a typical scenario, a virus-infected cell will release interferons causing nearby cells to heighten thei ...
s, TNF, IL-6 and IL-8. For example, Caspase-1 activates Caspase-7, which in turn cleaves the poly (ADP) ribose – this activates transcription of NF-κB controlled genes.


Discovery of caspases

H. Robert Horvitz initially established the importance of caspases in apoptosis and found that the ''ced-3'' gene is required for the cell death that took place during the development of the
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...
''
C. elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
''. Horvitz and his colleague Junying Yuan found in 1993 that the protein encoded by the ced-3 gene is cysteine protease with similar properties to the
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
ian interleukin-1-beta converting enzyme (ICE) (now known as caspase 1). At the time, ICE was the only known caspase. Other mammalian caspases were subsequently identified, in addition to caspases in organisms such as fruit fly ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...
''. Researchers decided upon the nomenclature of the caspase in 1996. In many instances, a particular caspase had been identified simultaneously by more than one laboratory; each would then give the protein a different name. For example, caspase 3 was variously known as CPP32, apopain and Yama. Caspases, therefore, were numbered in the order in which they were identified. ICE was, therefore, renamed as caspase 1. ICE was the first mammalian caspase to be characterised because of its similarity to the nematode death gene ced-3, but it appears that the principal role of this enzyme is to mediate inflammation rather than cell death.


Evolution

In animals apoptosis is induced by caspases and in fungi and plants, apoptosis is induced by arginine and lysine-specific caspase like proteases called metacaspases. Homology searches revealed a close homology between caspases and the caspase-like proteins of ''Reticulomyxa'' (a unicellular organism). The phylogenetic study indicates that divergence of caspase and metacaspase sequences occurred before the divergence of eukaryotes.


See also

*
Apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...
* Apoptosome * Bcl-2 * Emricasan * Metacaspase *
Paracaspase Paracaspases (human: MALT1) are members of the C14 family of cysteine proteases. Paracaspases are proteins related to caspases present in animals and slime mold, in contrast to metacaspases, which are present in plants, fungi, and "protists". The ...
*
Pyroptosis Pyroptosis is a highly inflammatory form of lytic programmed cell death that occurs most frequently upon infection with intracellular pathogens and is likely to form part of the antimicrobial response. This process promotes the rapid clearance of va ...
* The Proteolysis Map * Programmed cell death


Notes


References


External links

*
Apoptosis Video
Demonstrates a model of a caspase cascade as it occurs in vivo.

Kimball's Biology Pages. Simple explanation of the mechanisms of apoptosis triggered by internal signals (bcl-2), along the caspase-9, caspase-3 and caspase-7 pathway; and by external signals (FAS and TNF), along the caspase 8 pathway. Accessed 25 March 2007.
Apoptosis & Caspase 7
PMAP-animation *
Tumors Beware
(from Beaker Blog) {{Fas apoptosis signaling pathway EC 3.4.22 Programmed cell death Apoptosis Proteases Caspases