Gene name - Death caspase-1
Cytological map position -
Function - protease
Keyword(s) - apoptosis - programmed cell death - an effector caspase
Symbol - Dcp-1
Genetic map position -
Classification - ICE/CED-3 protease
Cellular location - cytoplasmic
|Recent literature||Sudmeier, L. J., Howard, S. P. and Ganetzky, B. (2015). A Drosophila model to investigate the neurotoxic side effects of radiation exposure. Dis Model Mech 8: 669-677. PubMed ID: 26092528
Children undergoing cranial radiation therapy (CRT) for CNS malignancies are at increased risk for neurological deficits later in life. Using Drosophila as a model, wild-type third-instar larvae were irradiated with single doses of gamma-radiation, and the percentage that survived to adulthood was determined. Motor function of surviving adults was examined with a climbing assay, and longevity was assessed by measuring lifespan. Neuronal cell death was assayed by using immunohistochemistry in adult brains. Irradiating late third-instar larvae at a dose of 20 Gy or higher impaired the motor activity of surviving adults. A dose of 40 Gy or higher resulted in a precipitous reduction in the percentage of larvae that survive to adulthood. A dose-dependent decrease in adult longevity was paralleled by a dose-dependent increase in activated Death caspase-1 (Dcp1) in adult brains. Survival to adulthood and adult lifespan were more severely impaired with decreasing larval age at the time of irradiation. Differences in genotype confered phenotypic differences in radio-sensitivity for developmental survival and motor function. This work demonstrates the usefulness of Drosophila to model the toxic effects of radiation during development.
Death caspase-1 (Dcp-1) is the first known Drosophila member of the caspase family of ICE/CED-3 proteases thought to play a role in apoptosis or programmed cell death. ICE was originally described as the cysteine protease required for cleavage of pro-interleukin-1ß in order to generate the active cytokine. CED-3 is a C. elegans cell death gene, with homology to mammalian ICE (Yuan, 1993). The term caspase is based on two catalytic properties of these enzymes. The "c" refers to a cysteine protease mechanism, and "aspase" refers to the group's ability to cleave aspartic acid, the most distinctive catalytic feature of this protease family. Each of these enzymes is synthesized as a proenzyme, proteolytically activated to form a heterodimeric catalytic domain. To date, ten homologs in humans have been discovered (Alnemri, 1996).
Three apoptotic activators (Reaper, Wrinkled/Head involution defective and Grim) have been identified in Drosophila. All three possess death domains, identifying them as proteins that act as mediators between different signaling pathways and the cell death program. The products of these genes appear to activate one or more caspases, because cell killing by Reaper, HID and Grim is blocked by the baculovirus protein p35, a specific inhibitor of caspases (Song, 1997 and references).
Dcp-1 is also capable of inducing cell death. The gene was expressed in several mammalian cell lines. Cells expressing Dcp-1 display the typical apoptotic morphology, such as condensed, rounded cell morphology and severe membrane blebing. A cell-free apoptosis system was used to investigate apoptosis-like nuclear events. In this system, Dcp-1 treatment results in fragmentation of chromosomal DNA that displays the characteristic apoptotic DNA ladder. It appears that Dcp-1 is able to engage at least part of the apoptotic program in mammalian cells (Song, 1997).
No significant abnormalities in the pattern of cell death are seen in dcp-1 mutants. Either there is sufficient Dcp-1 protein encoded maternally, or there are additional caspases performing a redundent function. However, dcp-1 mutation causes lethality during larval stages. Although most of the dcp-1 mutants die before the third instar larval stage, some reach that stage and display several abnormalities. Mutant larvae lack imaginal discs and gonads. In addition, they have fragile trachea. However, the most prominent phenotype found in these larvae is the presence of melanotic tumors located in various parts of the body. Melanotic tumors can result from either the overproliferation of blood cells or from an immune response toward abnormal cells and tissues in the larva. In dcp-1 mutants, there is no evidence for hyperplasia of the lymph glands or overproliferation of blood cells. This suggests an immune reaction toward abnormal tissues or cells, possible resulting from a defect in the ability to carry out cell death (Song, 1997).
The cytoplasmic region of Fas, a mammalian death factor receptor, shares a limited homology with Reaper, an apoptosis-inducing protein in Drosophila. Expression in Drosophila cells of either the Fas cytoplasmic region (FasC) or reaper causes cell death. The death process induced by FasC or reaper is inhibited by crmA or p35, suggesting that in both cases the death process is mediated by caspase-like proteases. Both Ac-YVAD aldehyde and Ac-DEVD aldehyde, specific inhibitors of caspase 1- and caspase 3-like proteases, respectively, inhibited the FasC-induced death of Drosophila cells. However, the cell death induced by Reaper is inhibited by Ac-DEVD aldehyde, but not by Ac-YVAD aldehyde. A caspase 1-like protease activity that preferentially recognizes the YVAD sequence gradually increases in the cytosolic fraction of the FasC-activated cells, whereas the caspase 3-like protease activity recognizing the DEVD sequence is observed in the Reaper-activated cells. Partial purification and biochemical characterization of the proteases indicates that there are at least three distinct caspase-like proteases in Drosophila cells that are differentially activated by FasC and Reaper. The conservation of the Fas-death signaling pathway in Drosophila cells, which is distinct from that for Reaper, may indicate that cell death in Drosophila is controlled not only by the Reaper suicide gene, but also by a Fas-like killer gene (Kondo, 1997a).
Increasing evidence reveals that a subset of proteins participates in both the autophagy and apoptosis pathways, and this intersection is important in normal physiological contexts and in pathological settings. This shows that the Drosophila effector caspase, Drosophila caspase 1 (Dcp-1), localizes within mitochondria and regulates mitochondrial morphology and autophagic flux. Loss of Dcp-1 leads to mitochondrial elongation, increased levels of the mitochondrial adenine nucleotide translocase stress-sensitive B (SesB), increased adenosine triphosphate (ATP), and a reduction in autophagic flux. Moreover, SesB was found to suppresses autophagic flux during midoogenesis, identifying a novel negative regulator of autophagy. Reduced SesB activity or depletion of ATP by oligomycin A rescues the autophagic defect in Dcp-1 loss-of-function flies, demonstrating that Dcp-1 promotes autophagy by negatively regulating SesB and ATP levels. Furthermore, it was found that pro-Dcp-1 interacts with SesB in a nonproteolytic manner to regulate its stability. These data reveal a new mitochondrial-associated molecular link between nonapoptotic caspase function and autophagy regulation in vivo (DeVorkin, 2014).
The results reveal that starvation-induced autophagic flux occurs in both midstage egg chambers that have not entered the degeneration process as well as in those that are undergoing cell death. Furthermore, it was found that the effector caspase Dcp-1 is required for autophagic flux in degenerating midstage egg chambers in addition to its role in cell death. One mechanism of Dcp-1-induced autophagic flux is mediated through SesB. In humans, there are four mitochondrial ANT isoforms, each with a tissue-specific distribution and different roles in apoptosis. Adenine nucleotide translocase family ANT1 and ANT3 were proposed to be proapoptotic, whereas ANT2 and ANT4 were shown to be antiapoptotic (Brenner, 2011). However, the roles of mammalian ANT proteins in autophagy have yet to be characterized. The data show that reduced Dcp-1 leads to increased levels of SesB protein in fed and starvation conditions during Drosophila oogenesis and in Drosophila cultured cells. No significant change was observed in SesB transcript levels in fed conditions or after 4 h of starvation, but a significant increase was observed in cells after 2 h of starvation. This finding suggests that a transcription-related mechanism may play some role in the observed cellular response but is not sufficient to account for all of the observed changes in protein levels. Although Dcp-1 does not cleave SesB, the proform of Dcp-1 interacts with SesB, and it is predicted that this interaction regulates the stability of SesB. It was also found that SesB is required to suppress autophagic flux during midoogenesis even under nutrient-rich conditions, and reduction of SesB in Dcp-1Prev1 flies rescues the autophagic defect after starvation. This is the first study showing that an ANT functions as a negative regulator of autophagy (DeVorkin, 2014).
The Drosophila genome encodes seven caspases, and to date, only the initiator caspase Dronc and the effector caspase Drice have been shown to localize to the mitochondria (Dorstyn, 2002). In mammalian cells, caspases have been detected at the mitochondria during apoptosis; however, the role of caspases at the mitochondria, especially under nonapoptotic conditions, is poorly understood. The current results demonstrate that Dcp-1 localizes to the mitochondria where it functions to maintain the mitochondrial network morphology. Under nutrient-rich conditions, nondegenerating midstage egg chambers from Dcp-1Prev1 flies contained mitochondria that appeared elongated and overly connected, and ovaries contained increased ATP levels, indicating that Dcp-1 normally functions to negatively regulate mitochondrial dynamics and ATP levels. Consistent with these findings, overexpression of the caspase inhibitor p35 in the amnioserosa suppressed the transition of mitochondria from a tubular to a fragmented state during delamination, further suggesting that inhibition of caspases hinders normal mitochondrial dynamics (DeVorkin, 2014).
Dcp-1 acts to finely tune the apoptotic process, and cell death only occurs when caspase activity reaches a certain apoptotic threshold. Effector caspases involved in nonapoptotic processes may be restricted in time or space to regulate caspase activity. As Dcp-1 functions not only in autophagy and apoptosis but also at the mitochondria to regulate mitochondrial morphology and ATP levels, one question that remains is to how the activity of Dcp-1 is regulated. As Dcp-1 has autocatalytic activity, perhaps Dcp-1 is sequestered in mitochondria to prevent its full activation. Mitochondrial localized mammalian pro-Caspase 3 and 9 are S-nitrosylated in their catalytic active site, leading to the inhibition of their activity. Perhaps mitochondrial Dcp-1 is also S-nitrosylated, serving to limit Dcp-1's activity. In addition, mammalian Hsp60 and Hsp10 were shown to interact with mitochondrial localized pro-Caspase 3 in which they function to accelerate pro-Caspase 3 activation after the induction of apoptosis. Perhaps Dcp-1 associates with Drosophila Hsp60 or Hsp10 in the mitochondria to regulate its mitochondrial related functions. However, further studies are required to identify upstream regulators of Dcp-1 that regulate its mitochondrial, autophagic, and apoptotic functions (DeVorkin, 2014).
Effector caspases are the main executioners of apoptotic cell death; however, it is becoming increasingly evident that caspases have nonapoptotic functions in differentiation, proliferation, cytokine production, and cell survival. For example, Caspase 3 was shown to regulate tumor cell repopulation in vitro and in vivo, and it was also shown to be required for skeletal muscle and macrophage differentiation. In Drosophila, the initiator caspase Dronc maintains neural stem cell homeostasis by binding to Numb in a noncatalytic, nonapoptotic manner to regulate its activity (Ouyang, 2011). In addition, Dcp-1 is required for neuromuscular degeneration in a nonapoptotic manner (Keller, 2011). The current results show that Dcp-1 also has a nonapoptotic role during oogenesis, in which it is required to maintain mitochondrial physiology under basal conditions. Loss of Dcp-1 alters this physiology, leading to increased SesB and ATP levels that in part prevent the induction of autophagic flux after starvation. These data support the notion that caspases play a much more diverse role than previously known and that the underlying mechanisms should be better understood to appreciate the full impact of apoptosis pathway modulation for treatment in human pathologies (DeVorkin, 2004).
Caspases are synthesized as inactive proenzymes that are proteolytically processed to form the active heterodimer consisting of a 10 kD and 20 kD subunit. The consensus sequence regulating this cleavage is found in the expected region of Dcp-1. The prodomain is only 33 amino acids (Song, 1997).
date revised: 15 December 97
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