Endonuclease G, mitochondrial is an enzyme that in humans is encoded by the ENDOG gene. [5] [6] This protein primarily participates in caspase-independent apoptosis via DNA degradation when translocating from the mitochondrion to nucleus under oxidative stress. [7] As a result, EndoG has been implicated in cancer, aging, and neurodegenerative diseases such as Parkinson's disease (PD). Regulation of its expression levels thus holds potential to treat or ameliorate those conditions. [7] [8]
The enzyme encoded by this gene is a member of the conserved DNA/RNA non-specific ββα-Me-finger nuclease family and possesses a unique site selectivity of poly(dG).poly(dC) sequences in double-stranded DNA. The protein is initially synthesized as an inactive 33-kDa precursor. This precursor is activated by proteolytic cleavage of the mitochondrial targeting sequence, thus producing a mature 28-kDa enzyme that is translocated to the mitochondrial intermembrane space, where it forms an active homodimer. [9] [10] [11] The H-N-N motif (His-141, Asn-163, Asn-172) is crucial for the protein's catalytic function and substrate specificity, and the His-141 amino acid is necessary for magnesium coordination. The amino acid Asn-251 also appears to be catalytic, and Glu-271 appears to be another magnesium ligand, but both are located far from the H-N-N motif and, thus, their interactions are unclear. [11]
The protein encoded by this gene is a nuclear encoded endonuclease that is localized in the mitochondrial intermembrane space. [6] [12] The encoded protein is widely distributed among animals and cleaves DNA at GC tracts. This protein is capable of generating the RNA primers required by DNA polymerase gamma to initiate replication of mitochondrial DNA. [6] In some apoptotic pathways, EndoG is released from the mitochondrion and migrates to the nucleus, where it degrades chromatin with the help of other nuclear proteins. [7] [9] [11] In one such pathway, caspase-independent apoptosis, the E3 ligase C-terminal of Hsc-70 interacting protein (CHIP), a regulator of EndoG expression, functions as a protective mechanism against oxidative stress. Under normal conditions, EndoG remains bound to Hsp70 and CHIP; however, when undergoing oxidative stress, EndoG dissociates from Hsp70 and CHIP and translocates to the nucleus, where it degrades DNA to effect apoptosis. Therefore, maintaining low levels of EndoG could prevent cell death caused by stress conditions. [13] In epithelial cells, the nuclear localization and proapoptotic function of EndoG leads it to play a role in cell senescence. [10] In addition to DNA degradation, EndoG also stimulates inhibitors of apoptosis proteins (IAPs) to target proteins for proteasomal degradation. [14]
The Endonuclase G enzyme is an important constituent in apoptotic signaling and oxidative stress, most notably as part of the mitochondrial death pathway and cardiac myocyte apoptosis signaling. [15] Programmed cell death is a distinct genetic and biochemical pathway essential to metazoans. An intact death pathway is required for successful embryonic development and the maintenance of normal tissue homeostasis. Apoptosis has proven to be tightly interwoven with other essential cell pathways. The identification of critical control points in the cell death pathway has yielded fundamental insights for basic biology, as well as provided rational targets for new therapeutics a normal embryologic processes, or during cell injury (such as ischemia-reperfusion injury during heart attacks and strokes) or during developments and processes in cancer, an apoptotic cell undergoes structural changes including cell shrinkage, plasma membrane blebbing, nuclear condensation, and fragmentation of the DNA and nucleus. This is followed by fragmentation into apoptotic bodies that are quickly removed by phagocytes, thereby preventing an inflammatory response. [16] It is a mode of cell death defined by characteristic morphological, biochemical and molecular changes. It was first described as a "shrinkage necrosis", and then this term was replaced by apoptosis to emphasize its role opposite mitosis in tissue kinetics. In later stages of apoptosis the entire cell becomes fragmented, forming a number of plasma membrane-bounded apoptotic bodies which contain nuclear and or cytoplasmic elements. The ultrastructural appearance of necrosis is quite different, the main features being mitochondrial swelling, plasma membrane breakdown and cellular disintegration. Apoptosis occurs in many physiological and pathological processes. It plays an important role during embryonal development as programmed cell death and accompanies a variety of normal involutional processes in which it serves as a mechanism to remove "unwanted" cells.
The BNIP3 pathway involves mitochondrial release and nuclear translocation of the endonuclease G. [17] [18] It is not clear, however, that how BNIP3 interacts with mitochondria. It has been shown that BNIP3 interacts with the voltage-dependent anion channel (VDAC) to directly induce mitochondrial release and nuclear translocation of EndonucleaseG. Data has identified VDAC as an interacting partner of BNIP3 and provide direct evidence to support that EndoG is a mediator of the BNIP3 cell death pathway. [19] Most notably, Enodnuclease G is pivotal during oxidative stress by ischemia-reperfusion injury, specifically in the myocardium as part of a heart attack (also known as ischemic heart disease). Ischemic heart disease, which results from an occlusion of one of the major coronary arteries, is currently still the leading cause of morbidity and mortality in western society. [20] [21] During ischemia reperfusion, ROS release substantially contribute to the cell damage and death via a direct effect on the cell as well as via apoptotic signals. More recently, Endonuclease G is considered a determinant of cardiac hypertrophy. A link has been established between Endonuclease G and mitochondrial function during cardiac hypertrophy, partly through the effects of Endo G on Mfn2 and Jp2, and revealed a role for Endonuclease G in the crosstalk between the processes controlled by Mfn2 and Jp2 in maladaptive cardiac hypertrophy. [22]
Previous studies reported greater efficacy of anticancer drugs when used in conjunction with high EndoG levels. Thus, regulators of EndoG, such as CHIP, could serve as therapeutic targets for oxidative stress-induced cell death in cancer and aging. [13] Through its association with cell senescence in epithelial cells, EndoG may also contribute to age-related vascular diseases such as arteriosclerosis. [10] Similarly, myonuclear localization of EndoG is correlated with atrophied aging skeletal muscle, leading to increased apoptotic signaling and muscle mass loss. EndoG has also been implicated in Parkinson's disease (PD), as it induces DNA fragmentation in neurons when translocated from the mitochondria to nuclei. This mechanism involves the kynurenine pathway and the permeability transition pore; as such, targeting molecules in this pathway could prevent EndoG-mediated cell death and effectively help treat PD in patients. [8] Similarly, EndoG knockdown in mice mitigated injurious insults; thus, therapeutic strategies to inhibit or silence EndoG could help protect tissues during injury and disease. So far, two EndoG inhibitors, PNR-3-80 (5-((1-(2-naphthoyl)-5-chloro-1H-indol-3-yl)methylene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione) and PNR-3-82 (5-((1-(2-naphthoyl)-5-methoxy-1H-indol-3-yl)methylene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione, have been tested and confirmed. [9]
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Apoptosis is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay. The average adult human loses between 50 and 70 billion cells each day due to apoptosis. For an average human child between eight and fourteen years old, each day the approximate loss is 20 to 30 billion cells.
In chemistry and biology, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (O2), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (O2H), superoxide (O2-), hydroxyl radical (OH.), and singlet oxygen. ROS are pervasive because they are readily produced from O2, which is abundant. ROS are important in many ways, both beneficial and otherwise. ROS function as signals, that turn on and off biological functions. They are intermediates in the redox behavior of O2, which is central to fuel cells. ROS are central to the photodegradation of organic pollutants in the atmosphere. Most often however, ROS are discussed in a biological context, ranging from their effects on aging and their role in causing dangerous genetic mutations.
Apoptosis regulator BAX, also known as bcl-2-like protein 4, is a protein that in humans is encoded by the BAX gene. BAX is a member of the Bcl-2 gene family. BCL2 family members form hetero- or homodimers and act as anti- or pro-apoptotic regulators that are involved in a wide variety of cellular activities. This protein forms a heterodimer with BCL2, and functions as an apoptotic activator. This protein is reported to interact with, and increase the opening of, the mitochondrial voltage-dependent anion channel (VDAC), which leads to the loss in membrane potential and the release of cytochrome c. The expression of this gene is regulated by the tumor suppressor P53 and has been shown to be involved in P53-mediated apoptosis.
Apoptosis inducing factor is involved in initiating a caspase-independent pathway of apoptosis by causing DNA fragmentation and chromatin condensation. Apoptosis inducing factor is a flavoprotein. It also acts as an NADH oxidase. Another AIF function is to regulate the permeability of the mitochondrial membrane upon apoptosis. Normally it is found behind the outer membrane of the mitochondrion and is therefore secluded from the nucleus. However, when the mitochondrion is damaged, it moves to the cytosol and to the nucleus. Inactivation of AIF leads to resistance of embryonic stem cells to death following the withdrawal of growth factors indicating that it is involved in apoptosis.
Bcl-2 homologous antagonist/killer is a protein that in humans is encoded by the BAK1 gene on chromosome 6. The protein encoded by this gene belongs to the BCL2 protein family. BCL2 family members form oligomers or heterodimers and act as anti- or pro-apoptotic regulators that are involved in a wide variety of cellular activities. This protein localizes to mitochondria, and functions to induce apoptosis. It interacts with and accelerates the opening of the mitochondrial voltage-dependent anion channel, which leads to a loss in membrane potential and the release of cytochrome c. This protein also interacts with the tumor suppressor P53 after exposure to cell stress.
Superoxide dismutase 2, mitochondrial (SOD2), also known as manganese-dependent superoxide dismutase (MnSOD), is an enzyme which in humans is encoded by the SOD2 gene on chromosome 6. A related pseudogene has been identified on chromosome 1. Alternative splicing of this gene results in multiple transcript variants. This gene is a member of the iron/manganese superoxide dismutase family. It encodes a mitochondrial protein that forms a homotetramer and binds one manganese ion per subunit. This protein binds to the superoxide byproducts of oxidative phosphorylation and converts them to hydrogen peroxide and diatomic oxygen. Mutations in this gene have been associated with idiopathic cardiomyopathy (IDC), premature aging, sporadic motor neuron disease, and cancer.
BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 is a protein found in humans that is encoded by the BNIP3 gene.
Diablo homolog (DIABLO) is a mitochondrial protein that in humans is encoded by the DIABLO gene on chromosome 12. DIABLO is also referred to as second mitochondria-derived activator of caspases or SMAC. This protein binds inhibitor of apoptosis proteins (IAPs), thus freeing caspases to activate apoptosis. Due to its proapoptotic function, SMAC is implicated in a broad spectrum of tumors, and small molecule SMAC mimetics have been developed to enhance current cancer treatments.
DNA damage-inducible transcript 3, also known as C/EBP homologous protein (CHOP), is a pro-apoptotic transcription factor that is encoded by the DDIT3 gene. It is a member of the CCAAT/enhancer-binding protein (C/EBP) family of DNA-binding transcription factors. The protein functions as a dominant-negative inhibitor by forming heterodimers with other C/EBP members, preventing their DNA binding activity. The protein is implicated in adipogenesis and erythropoiesis and has an important role in the cell's stress response.
Apoptosis-inducing factor 1, mitochondrial is a protein that in humans is encoded by the AIFM1 gene on the X chromosome. This protein localizes to the mitochondria, as well as the nucleus, where it carries out nuclear fragmentation as part of caspase-independent apoptosis.
Dynamin-1-like protein is a GTPase that regulates mitochondrial fission. In humans, dynamin-1-like protein, which is typically referred to as dynamin-related protein 1 (Drp1), is encoded by the DNM1L gene and is part of the dynamin superfamily (DSP) family of proteins.
28S ribosomal protein S29, mitochondrial, also known as death-associated protein 3 (DAP3), is a protein that in humans is encoded by the DAP3 gene on chromosome 1. This gene encodes a 28S subunit protein of the mitochondrial ribosome (mitoribosome) and plays key roles in translation, cellular respiration, and apoptosis. Moreover, DAP3 is associated with cancer development, but has been observed to aid some cancers while suppressing others.
ADP/ATP translocase 4 (ANT4) is an enzyme that in humans is encoded by the SLC25A31 gene on chromosome 4. This enzyme inhibits apoptosis by catalyzing ADP/ATP exchange across the mitochondrial membranes and regulating membrane potential. In particular, ANT4 is essential to spermatogenesis, as it imports ATP into sperm mitochondria to support their development and survival. Outside this role, the SLC25AC31 gene has not been implicated in any human disease.
Voltage-dependent anion-selective channel protein 2 is a protein that in humans is encoded by the VDAC2 gene on chromosome 10. This protein is a voltage-dependent anion channel and shares high structural homology with the other VDAC isoforms. VDACs are generally involved in the regulation of cell metabolism, mitochondrial apoptosis, and spermatogenesis. Additionally, VDAC2 participates in cardiac contractions and pulmonary circulation, which implicate it in cardiopulmonary diseases. VDAC2 also mediates immune response to infectious bursal disease (IBD).
Voltage-dependent anion-selective channel protein 3 (VDAC3) is a protein that in humans is encoded by the VDAC3 gene on chromosome 8. The protein encoded by this gene is a voltage-dependent anion channel and shares high structural homology with the other VDAC isoforms. Nonetheless, VDAC3 demonstrates limited pore-forming ability and, instead, interacts with other proteins to perform its biological functions, including sperm flagella assembly and centriole assembly. Mutations in VDAC3 have been linked to male infertility, as well as Parkinson's disease.
Mitophagy is the selective degradation of mitochondria by autophagy. It often occurs to defective mitochondria following damage or stress. The process of mitophagy was first described over a hundred years ago by Margaret Reed Lewis and Warren Harmon Lewis. Ashford and Porter used electron microscopy to observe mitochondrial fragments in liver lysosomes by 1962, and a 1977 report suggested that "mitochondria develop functional alterations which would activate autophagy." The term "mitophagy" was in use by 1998.
Necroptosis is a programmed form of necrosis, or inflammatory cell death. Conventionally, necrosis is associated with unprogrammed cell death resulting from cellular damage or infiltration by pathogens, in contrast to orderly, programmed cell death via apoptosis. The discovery of necroptosis showed that cells can execute necrosis in a programmed fashion and that apoptosis is not always the preferred form of cell death. Furthermore, the immunogenic nature of necroptosis favors its participation in certain circumstances, such as aiding in defence against pathogens by the immune system. Necroptosis is well defined as a viral defense mechanism, allowing the cell to undergo "cellular suicide" in a caspase-independent fashion in the presence of viral caspase inhibitors to restrict virus replication. In addition to being a response to disease, necroptosis has also been characterized as a component of inflammatory diseases such as Crohn's disease, pancreatitis, and myocardial infarction.
ADP/ATP translocase 2 is a protein that in humans is encoded by the SLC25A5 gene on the X chromosome.
Parthanatos is a form of programmed cell death that is distinct from other cell death processes such as necrosis and apoptosis. While necrosis is caused by acute cell injury resulting in traumatic cell death and apoptosis is a highly controlled process signalled by apoptotic intracellular signals, parthanatos is caused by the accumulation of Poly(ADP ribose) (PAR) and the nuclear translocation of apoptosis-inducing factor (AIF) from mitochondria. Parthanatos is also known as PARP-1 dependent cell death. PARP-1 mediates parthanatos when it is over-activated in response to extreme genomic stress and synthesizes PAR which causes nuclear translocation of AIF. Parthanatos is involved in diseases that afflict hundreds of millions of people worldwide. Well known diseases involving parthanatos include Parkinson's disease, stroke, heart attack, and diabetes. It also has potential use as a treatment for ameliorating disease and various medical conditions such as diabetes and obesity.
Kidney ischemia is a disease with a high morbidity and mortality rate. Blood vessels shrink and undergo apoptosis which results in poor blood flow in the kidneys. More complications happen when failure of the kidney functions result in toxicity in various parts of the body which may cause septic shock, hypovolemia, and a need for surgery. What causes kidney ischemia is not entirely known, but several pathophysiology relating to this disease have been elucidated. Possible causes of kidney ischemia include the activation of IL-17C and hypoxia due to surgery or transplant. Several signs and symptoms include injury to the microvascular endothelium, apoptosis of kidney cells due to overstress in the endoplasmic reticulum, dysfunctions of the mitochondria, autophagy, inflammation of the kidneys, and maladaptive repair.
