Parthanatos (derived from the Greek Θάνατος, "Death") is a form ofprogrammed cell death that is distinct from other cell death processes such asnecrosis andapoptosis. While necrosis is caused by acute cell injury resulting in traumatic cell death and apoptosis is a highly controlled process signalled by apoptoticintracellular signals, parthanatos is caused by the accumulation of Poly(ADP ribose) (PAR) and the nuclear translocation ofapoptosis-inducing factor (AIF) frommitochondria.[1] Parthanatos is also known asPARP-1 dependent cell death. PARP-1 mediates parthanatos when it is over-activated in response to extremegenomic stress and synthesizes PAR which causesnuclear translocation of AIF.[2] Parthanatos is involved in diseases that afflict hundreds of millions of people worldwide. Well known diseases involving parthanatos includeParkinson's disease,stroke,heart attack, anddiabetes.[citation needed] It also has potential use as a treatment for ameliorating disease and various medical conditions such as diabetes andobesity.[citation needed]
The term parthanatos was not coined until a review in 2009.[1] The word parthanatos is derived fromThanatos, the personification of death in Greek mythology.
Parthanatos was first discovered in a 2006 paper by Yu et al. studying the increased production of mitochondrialreactive oxygen species (ROS) byhyperglycemia.[3] This phenomenon is linked with negative effects arising from clinical complications ofdiabetes andobesity.
Researchers noticed that high glucose concentrations led to overproduction ofreactive oxygen species and rapid fragmentation ofmitochondria. Inhibition of mitochondrialpyruvate uptake blocked the increase of ROS, but did not prevent mitochondrial fragmentation. After incubating cells with the non-metabolizablestereoisomer L-glucose, neither reactive oxygen species increase nor mitochondrial fragmentation were observed. Ultimately, the researchers found that mitochondrial fragmentation mediated by the fission process is a necessary component for high glucose-induced respiration increase and ROS overproduction.[citation needed]
Extended exposure to high glucose conditions are similar to untreated diabetic conditions, and so the effects mirror each other. In this condition, the exposure creates a periodic and prolonged increase in ROS production along with mitochondrial morphology change. Ifmitochondrial fission was inhibited, the periodic fluctuation of ROS production in a high glucose environment was prevented. This research shows that when cell damage to the ROS is too great, PARP-1 will initiate cell death.[citation needed]
Poly(ADP-ribose) polymerase-1 (PARP-1) is a nuclear enzyme that is found universally in alleukaryotes and is encoded by the PARP-1 gene. It belongs to the PARP family, which is a group of catalysts that transferADP-ribose units from NAD (nicotinamide dinucleotide) to protein targets, thus creating branched or linear polymers.[4] The majordomains of PARP-1 impart the ability to fulfill its functions. These protein sections include theDNA-binding domain on theN-terminus (allows PARP-1 to detect DNA breaks), the automodification domain (has aBRCA1 C terminus motif which is key for protein-protein interactions), and acatalytic site with the NAD+-fold (characteristic of mono-ADP ribosylating toxins).[1]
Normally, PARP-1 is involved in a variety of functions that are important for cellhomeostasis such as mitosis. Another of these roles isDNA repair, including the repair of base lesions and single-strand breaks.[5] PARP-1 interacts with a wide variety of substrates includinghistones,DNA helicases, high mobility group proteins,topoisomerases I and II, single-strand break repair factors,base-excision repair factors, and severaltranscription factors.[1]
PARP-1 accomplishes many of its roles through regulating poly(ADP-ribose) (PAR). PAR is apolymer that varies in length and can be either linear or branched.[6] It is negatively charged which allows it to alter the function of the proteins it binds to eithercovalently or non-covalently.[1] PAR binding affinity is strongest for branched polymers, weaker for long linear polymers and weakest for short linear polymers.[7] PAR also binds selectively with differing strengths to the different histones.[7] It is suspected that PARP-1 modulates processes (such asDNA repair,DNA transcription, andmitosis) through the binding of PAR to its target proteins.
The parthanatos pathway is activated byDNA damage caused bygenotoxic stress orexcitotoxicity.[8] This damage is recognized by the PARP-1 enzyme which causes anupregulation in PAR. PAR causes translocation ofapoptosis-inducing factor (AIF) from the mitochondria to thenucleus where it induces DNA fragmentation and ultimatelycell death.[9] This general pathway has been outlined now for almost a decade. While considerable success has been made in understanding the molecular events in parthanatos, efforts are still ongoing to completely identify all of the major players within the pathway, as well how spatial and temporal relationships between mediators affect them.
Extreme damage of DNA causing breaks and changes inchromatin structure have been shown to induce the parthanatos pathway.[8] Stimuli that causes the DNA damage can come from a variety of different sources.Methylnitronitrosoguanidine, analkylating agent, has been widely used in several studies to induce the parthanatos pathway.[10][11][12] A noted number of other stimuli or toxic conditions have also been used to cause DNA damage such asH2O2,NO, andONOO− generation (oxygenglucose deprivation).[10][13][14]
The magnitude, length of exposure, type of cell used, and purity of the culture, are all factors that can influence the activation of the pathway.[15] The damage must be extreme enough for the chromatin structure to be altered. This change in structure is recognized by the N-terminalzinc-finger domain on the PARP-1 protein.[16] The protein can recognize both single and double strand DNA breaks.
Once the PARP-1 protein recognizes the DNA damage, itcatalyzespost-transcriptional modification of PAR.[9] PAR will be formed either as a branched or linear molecule. Branching and long-chain polymers will be more toxic to the cell than simple short polymers.[17] The more extreme the DNA damage, the more PAR accumulates in the nucleus. Once enough PAR has accumulated, it will translocate from the nucleus into thecytosol. One study has suggested that PAR can translocate as a free polymer,[17] however translocation of a protein-conjugated PAR cannot be ruled out and is in fact a topic of active research.[8] PAR moves through the cytosol and enters the mitochondria through depolarization.[9] Within the mitochondria, PAR binds directly to theAIF which has a PAR polymer binding site, causing the AIF to dissociate from the mitochondria.[18] AIF is then translocated to the nucleus where it induces chromatin condensation and large scale (50Kb) DNA fragmentation.[9] How AIF induces these effects is still unknown. It is thought that an AIF associated nuclease (PAAN) that is currently unidentified may be present.[8] Human AIF have a DNA binding site[10] that would indicate that AIF binds directly to the DNA in the nucleus directly causing the changes. However, as mice AIF do not have this binding domain and are still able to undergo parthanatos,[19] it is evident that there must be another mechanism involved.
PAR, which is responsible for the activation of AIF, is regulated in the cell by theenzyme poly(ADP-ribose) glycohydrolase (PARG). After PAR is synthesized by PARP-1, it is degraded through a process catalyzed by PARG.[20] PARG has been found to protect against PAR-mediated cell death[9] while its deletion has increased toxicity through the accumulation of PAR.[9]
Before the discovery of the PAR and AIF pathway, it was thought that the overactivation of PARP-1 lead to over consumption ofNAD+.[21] As a result of NAD+ depletion, a decrease of ATP production would occur, and the resulting loss of energy would kill the cell.[22][23] However it is now known that this loss of energy would not be enough to account for cell death. In cells lackingPARG, activation of PARP-1 leads to cell death in the presence of ample NAD+.[24]
Parthanatos is defined as a unique cell death pathway fromapoptosis for a few key reasons. Primarily, apoptosis is dependent on thecaspase pathway activated bycytochrome c release, while the parthanatos pathway is able to act independently of caspase.[8] Furthermore, unlike apoptosis, parthanatos causes large scale DNA fragmentation (apoptosis only produces small scale fragmentation) and does not formapoptotic bodies.[25]
While parthanatos does share similarities withnecrosis, is also has several differences. Necrosis is not a regulated pathway and does not undergo any controlled nuclear fragmentation. While parthanatos does involve loss of cell membrane integrity likenecrosis, it is not accompanied by cell swelling.[26]
| Parthanatos | Apoptosis | Necrosis | |
|---|---|---|---|
| Chromatin Condensation | Yes | Yes | No |
| Nuclear fragmentation | Yes | Yes | No |
| Apoptotic bodies | No | Yes | No |
| Mitochondrial Swelling | No | Sometimes | Yes |
| Membrane Blebbing | No | Yes | Yes, late |
| Caspase Dependent | No | Yes | Sometimes |
| Regulated Pathway | Yes | Yes | No |
The PAR enzyme was originally connected to neural degradation pathways in 1993. Elevated levels ofnitric oxide (NO) have been shown to causeneurotoxicity in samples of rat hippocampalneurons.[27] A deeper look into the effects of NO on neurons showed that nitric oxides cause damage to DNA strands; the damage in turn elicits PAR enzyme activity that leads to further degradation and neuronal death. PAR- blockers halted thecell death mechanisms in the presence of elevated NO levels.[27]
PARP activity has also been linked to the neurodegenerative properties of toxin inducedParkinsonism. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is aneurotoxin that has been linked to neurodegeneration and development of Parkinson Disease-like symptoms in patients since 1983. The MPTP toxin's effects were discovered when four people were intravenously injecting thetoxin that they produced inadvertently when trying to street-synthesise the merpyridine (MPPP) drug.[28] The link between MPTP and PARP was found later when research showed that the MPTP effects on neurons were reduced inmutated cells lacking the PARP gene.[29] The same research also showed highly increased PARP activation indopamine producing cells in the presence of MPTP.
Alpha-synuclein is a protein that binds to DNA and modulatesDNA repair.[30] A key feature ofParkinson's disease is the pathologic accumulation and aggregation of alpha-synuclein. In theneurons of individuals with Parkinson's disease, alpha-synuclein is deposited as fibrils in intracytoplasmic structures referred to asLewy bodies. Formation of pathologic alpha-synuclein is associated with activation ofPARP1, increased poly(ADP) ribose generation and further acceleration of pathologic alpha-synuclein formation.[31] This process can lead to cell death by parthanatos.[31]
Parthanatos, as a cell death pathway, is being increasingly linked to severalsyndromes connected with specific tissue damage outside of thenervous system. This is highlighted in the mechanism ofstreptozotocin (STZ) induceddiabetes. STZ is a chemical that is naturally produced by the human body. However, in high doses, STZ has been shown to produce diabetic symptoms by damagingpancreatic β cells, which are insulin-producing.[32] The degradation of β cells by STZ was linked to PARP in 1980 when studies showed that a PAR synthesis inhibitor reduced STZ's effects on insulin synthesis. Inhibition of PARP causes pancreatic tissue to sustaininsulin synthesis levels, and reduce β cell degradation even with elevated STZ toxin levels.[33]
PARP activation has also been preliminarily connected witharthritis,[34]colitis,[35] andliver toxicity.[36]
The multi-step nature of the parthanatos pathway allows for chemical manipulation of its activation and inhibition for use in therapy. This rapidly developing field seems to be currently focused on the use of PARP blockers as treatments for chronically degenerative illnesses. This culminated in 3rd generation inhibitors such asmidazoquinolinone andisoquinolindione currently going to clinical trials.[8]
Another path for treatments is to recruit the parthanatos pathway to induceapoptosis intocancer cells, however no treatments have passed the theoretical stage.[8]