Sci. Aging Knowl. Environ., 8 December 2004
Vol. 2004, Issue 49, p. re9
[DOI: 10.1126/sageke.2004.49.re9]


Poly(ADP-Ribosyl)ation, PARP, and Aging

Sascha Beneke, and Alexander Bürkle

The authors are in the Molecular Toxicology Group, Department of Biology, Box X911, University of Konstanz, D-78457 Konstanz, Germany. E-mail: alexander.buerkle{at} (A.B.)

Key Words: centrosome • DNA repair • genomic stability • PARP • poly(ADP-ribosyl)ation • telomere

Abstract: Poly(ADP-ribose) polymerases (PARPs) catalyze the poly(ADP-ribosyl)ation of proteins. This posttranslational modification, as generated by the DNA damage-activated enzymes PARP-1 and -2, has long been known to be involved in DNA repair. Correlative data have suggested an association between DNA damage-induced poly(ADP-ribosyl)ation and mammalian longevity, and this link has recently been strengthened by the discovery of interactions between PARP-1 and the Werner syndrome protein. Emerging additional members of the PARP family display different cellular localizations and are involved in diverse processes such as the regulation of telomere or centrosome function, thereby providing further, independent links between poly(ADP-ribosyl)ation and the aging process.

Introduction Back to Top

Aging is the time-dependent general decline in physiological function of an organism, which is accompanied by a progressively increased risk of morbidity and mortality (1). At the cellular level, the aging process is associated with cumulative damage inflicted on biological macromolecules, including lipids, proteins, and nucleic acids. This damage impairs macromolecular activity, compromising cellular function and eventually leading to an increased risk to the health of the whole organism. Naturally occurring sources of such molecular damage include various forms of radiation and oxidants such as reactive oxygen species (ROS). ROS are produced endogenously during normal cellular processes such as biochemical detoxification reactions or phagocyte activation, although the mitochondrial respiratory chain is responsible for most of the ROS present in a cell (2) (and see "The Two Faces of Oxygen" and Kristal Perspective). Because of the steady turnover of RNA, proteins, and lipids, the effects of damage on cellular metabolism will be minor, but damaged DNA is especially critical to the survival of a cell, of course. If DNA damage is not repaired, the "best" outcome (from the viewpoint of the affected cell) is that the cell will survive with its altered genetic information, and a mutation will eventually be passed on to succeeding generations. Alternatively, the mutational load may be so severe as to lead to cell death. Impaired maintenance of DNA is referred to as genomic instability, and the results can range from subtle single-base substitutions to large chromosomal aberrations, such as translocations, deletions, amplifications, sister-chromatid exchanges (SCEs), and aneuploidy (an abnormal number of chromosomes). Any of these events can jeopardize cell survival, tissue homeostasis, and the organism's survival. Thus, to maintain the integrity of genetic information important for survival, several sophisticated DNA repair mechanisms have evolved, and DNA lesions produced by ROS are mainly repaired by the base excision repair pathway (described further below; see figure 1 in Sinclair Perspective).

The Cellular Poly(ADP-Ribosyl)ation System Back to Top

Poly(ADP-ribosyl)ation is a posttranslational protein modification first described in the 1960s (3), carried out by a family of enzymes referred to as poly(ADP-ribose) polymerases or PARPs. Poly(ADP-ribosyl)ation is an immediate cellular response to genotoxic insults induced by ionizing radiation, alkylating agents, and oxidative stress [for a review, see (4)]. It is catalyzed mainly by the 113-kD enzyme PARP-1, the founding member of the PARP family (Fig. 1). NAD+ serves as a substrate for the reaction and is cleaved to give nicotinamide and an ADP-ribosyl moiety. The latter is attached to glutamate or aspartate residues on a wide variety of acceptor proteins, and this is followed by additional ADP-ribosyl transfer cycles onto existing mono- or oligo-ADP-ribosyl adducts. This process leads to the production of chains involving up to 200 ADP-ribosyl units, as well as the formation of branches on growing ADP-ribose polymers (Fig. 2). The resulting treelike structure carries many negative charges and profoundly alters the function of the modified protein. The main target (or "acceptor") protein is PARP-1 itself, but the polymer is also attached to several additional proteins in the cell, such as histones, topoisomerases, and high-mobility-group (HMG) proteins. Poly(ADP-ribosyl)ation is a transient modification, because it is rapidly reversed by the enzyme poly(ADP-ribose) glycohydrolase [PARG (5)]. The half-life of the polymer is therefore very short--on the order of 1 min under conditions of DNA damage--and the presence of polymer is tightly linked to the occurrence of DNA strand breaks.

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Fig. 1. Human PARP-1 has a three-domain structure. Numbers indicate the amino acids bordering distinct domains. FI and FII are the two zinc fingers present in PARP-1, with C denoting cysteine and H histidine. NLS denotes the bipartite nuclear localization signal.


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Fig. 2. Schematic view of the biosynthesis of poly(ADP-ribose). NAD+ is cleaved into nicotinamide (NAM, orange) and an ADP-ribosyl (blue) residue by PARP (magenta), and the ADP-ribose moieties are attached to an acceptor protein (green).

PARP-1 activity is responsible for approximately 90% of the total poly(ADP-ribose) formation in a living cell. But in the past few years, several other PARPs have been discovered based on sequence homology, and although these other PARPs collectively account for only 10% of cellular poly(ADP-ribosyl)ation, some of these proteins have been shown to be important regulators of cellular functions. For example, PARP-2 is activated by DNA strand breaks like PARP-1 (6), and its activity was discovered in PARP-1 knockout mouse cells displaying significant poly(ADP-ribosyl)ation after genotoxic stress (7). PARP-2 knockout mice do not display any obvious spontaneous phenotype, but, like PARP-1-null mice, they are DNA repair-compromised after challenge with ionizing radiation or alkylating agents. PARP-1/PARP-2 double-knockout mice exhibit embryonic lethality (8), pointing to an important function of PARPs during development that can be masked by functional redundancy. Furthermore, both PARP-1 (9) and PARP-3 (10) are detected at centrioles and centrosomes, which to some extent seem to be regulated by poly(ADP-ribosyl)ation (discussed below).

PARP-1 is one of the first proteins to be cleaved in the execution phase of apoptosis (11). This cleavage may fulfill several functions. First, it may be that cleavage of PARP-1 by apoptotic proteases (caspase 3 or 7) inhibits DNA repair either by abrogating DNA damage signaling through separating the catalytic C terminus of PARP-1 from its DNA binding domain (DBD), or by blocking the access of other DNA repair enzymes to DNA strand breaks introduced by endonucleases in the course of apoptosis (as the PARP-1 DBD sticks to DNA ends), or both. Second, excessive PARP-1 activity consumes a substantial amount of energy, and so PARP-1 inactivation would conserve energy needed for the completion of apoptosis.

Telomeres are another important cellular structure that can be influenced by poly(ADP-ribosyl)ation. Tankyrases (TNKS-1 and TNKS-2, also termed PARP-5) as well as PARP-1 and PARP-2 are involved in telomere maintenance (12-16). Tankyrases have also been shown to take part in vesicle trafficking by interaction with IRAP [insulin-responsive amino peptidase (17)], a protein resident in endocytic vesicles also bearing the glucose transporter GLUT4, which upon insulin stimulation is inserted into the plasma membrane.

Finally, another poly(ADP-ribose) polymerase that has been investigated in some detail is PARP-4, the so-called vault PARP (18). Vault particles are large cytoplasmic multicomponent structures, and although their function in normal cells is unclear at present, they may be involved in multidrug resistance of cancer cells. As is evident from human genome sequence searches, there appear to be a number of additional PARP isoforms awaiting elucidation of their functional relevance.

PARPs in DNA Repair Back to Top

As noted above, both PARP-1 and PARP-2 are activated by DNA strand breaks. During the process of base excision repair, these two enzymes are thought to regulate the accessibility of nicked DNA to other repair enzymes.

In base excision repair, modified bases are first recognized by specific DNA glycosylases; they are then excised and the abasic DNA strand is cut. After this, one of two subpathways is employed: short-patch (SP) or long-patch (LP) base excision repair. During the main SP repair pathway, DNA polymerase {beta} (Pol {beta}) removes the abasic deoxyribose-phosphate moiety and inserts a single nucleotide. DNA ligase III then reseals the interrupted strand. Both Pol {beta} and ligase III bind to the scaffold protein XRCC1, which also binds PARP-1 and PARP-2. During LP base excision repair, DNA polymerases {epsilon}, {delta}, or {beta} synthesize a short stretch of new DNA, thus displacing the old damaged strand. Flap endonuclease 1 (FEN1) cleaves the resulting overhang, and the DNA is resealed by DNA ligase I (19).

It is believed that PARP-1 (and perhaps PARP-2) attracts other repair proteins to the side of damage, via poly(ADP-ribose) formation, and orchestrates the decision about which route of repair to choose. PARP-1 and PARP-2 knockout mice both show severe defects in DNA repair, as well as an increased sensitivity to challenge with ionizing radiation or alkylating agents. A recent paper postulates that the poly(ADP-ribosyl)ation system is always engaged but is not essential for the execution of the base excision repair process in vitro, suggesting that it is not the PARPs themselves but their poly(ADP-ribosyl)ated products that are required for effective DNA repair (20). A plausible mechanism for the contribution of the poly(ADP-ribosyl)ation system that was suggested many years ago (21) and has recently gained strong support (22, 23) is that it brings about local chromatin relaxation, either by covalent modification of histones or by noncovalent interaction of histones with a poly(ADP-ribose)-modified PARP. Alternatively, poly(ADP-ribose) may be required to orchestrate other events involved in DNA repair, such as regulation of the cell cycle, DNA replication, or transcription machineries. Indeed, it has been shown that tumor suppressor protein p53, the "guardian of the genome," is not only a substrate for direct modification by poly(ADP-ribose) but can bind to the polymer in a noncovalent fashion (24-28). In PARP-1-depleted cells, the concentration of the DNA-polymerase {alpha}/primase protein complex is reduced and DNA replication is affected (27, 28). Additionally, transcription of some genes is disturbed in the absence of PARP-1, and it is noteworthy that the enzyme has been linked to the general transcription machinery, facilitating accurate transcription initiation [PARP-1 has been isolated independently as an RNA polymerase II transcription factor, TFII-C (29)].

PARP-1 has also been shown to interact with DNA-dependent protein kinase (DNA-PK), the initial activator of the nonhomologous end-joining DNA repair pathway (30). DNA-PK is a heterotrimeric protein kinase consisting of two regulatory subunits, Ku70 and Ku80, and a catalytic subunit, DNA-PKcs. It belongs to the family of DNA damage-activated phosphatidylinositol 3-kinase-like protein kinases, which also comprises the proteins ATM (ataxia telangiectasia mutated) and ATR (ATM related). DNA-PK binds to and is activated by double-strand DNA breaks and interacts with several other proteins, including p53 (31), PARP-1 (30), and WRN [the product of the Werner syndrome gene; see below (32)]. DNA-PK is important for DNA repair by the nonhomologous end-joining pathway and formation of the B and T cell repertoire, as well as telomere capping. Loss of DNA-PK function by a spontaneous mouse mutation led to the development of severe combined immunodeficiency syndrome [for a review, see (33)]. PARP-1 is phosphorylated by DNA-PK and in turn can modify the catalytic subunit of DNA-PK.

The cellular functions of poly(ADP-ribosyl)ation under conditions of genotoxic stress have been investigated by inhibiting PARP activity through treatment of cells or organisms with low-molecular-weight compounds (mainly NAD+ analogs), by expression of PARP-1 antisense RNA or of dominant negative derivatives of PARP-1, or by genetic means. For example, expression of the PARP-1 DBD greatly sensitizes cells to DNA damage induced by {gamma} irradiation or alkylating agents (34). The DBD behaves as a dominant negative mutant, competing with wild-type PARP for DNA strand breaks and thus inhibiting DNA damage-dependent poly(ADP-ribosyl)ation. As a consequence, cells undergo apoptosis more readily, and the rate of mutations (35) and gene amplification (36) is increased in surviving cells. Conversely, overexpression of full-length PARP-1 (37) leads to a greater abundance of poly(ADP-ribose) in cells and less genomic instability after challenge with genotoxic agents.

Such studies have established that, in proliferating cells exposed to low levels of genotoxic stress, poly(ADP-ribosyl)ation contributes to recovery from cytotoxicity. Genomic instability is suppressed, as assessed by measurement of several biological markers such as chromosomal aberrations, gene amplification, SCEs, or mutagenesis, establishing PARP-1 and PARP-2 as cytoprotective factors. Increased concentrations of poly(ADP-ribose) lead to greater genomic stability, whereas inhibiting the poly(ADP-ribosyl)ation system renders cells more susceptible to the effects of DNA damage such as induction of mutations, gene amplifications, or apoptosis. Last but not least, severe damage to DNA can trigger "overshooting" by PARP-1, leading to excessive NAD+ consumption, adenosine triphosphate depletion, and subsequent cell death owing to energy depletion (38). This phenomenon can be achieved by either acute overactivation or sustained activity of PARP, coupled with inadequate regeneration capacity for NAD+. This suicidal overactivation of PARP has been reported in several nonproliferative cell types in vivo as well as in cell culture, including (i) pancreatic islet cells undergoing destruction in type I diabetes induced by ROS, nitric oxide (NO), or its metabolites, or by the drug streptozotocin; (ii) neurons destroyed after ischemia-reperfusion damage, where excessive production of ROS and NO also takes place; (iii) the death of dopaminergic neurons in the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a drug inducing ROS formation specifically in this type of neuron)-induced model of Parkinson's disease; (iv) cell death in the postischemic heart; and (v) the death of endothelial cells, leading to vascular dysfunction. On the one hand, impaired PARP activity leads to less effective repair of DNA damage, and on the other hand PARP overactivity leads to cell destruction. Either extreme may lead to cell and tissue malfunction, one of the hallmarks of aging.

Poly(ADP-Ribosyl)ation and Mammalian Life Span Back to Top

Several years ago, we described a correlation between poly(ADP-ribosyl) formation and the life span of mammalian species, comparing the maximally achievable PARP activity in permeabilized blood mononuclear cells (MNCs) of 13 mammalian species (39). We found a strong positive correlation between the polymer synthesis capacity per cell and the longevity of a species, with the difference between the shortest- (rat) and the longest-lived species (human) being fivefold. This difference was not due to different PARP-1 protein concentrations, as measured by Western blotting, which suggests that the differences in activity were a result of subtle variations in primary structure, the overall protein sequence homology between rat and human PARP-1 being about 94% (40). When rat and human PARP-1 were expressed in the baculovirus system, analysis of the enzymic automodification reaction revealed that human PARP-1 displayed a poly(ADP-ribosyl)ation capacity that was twice that of the rat enzyme (41). Although these data demonstrated that there is indeed a primary structure-dependent difference in PARP-1 activity as a function of species, this could not fully account for the difference seen in our comparative analyses of MNCs. It may be that accessory proteins modulate PARP activity after DNA damage within the cell.

We also showed that lymphoblastoid cell lines established using peripheral blood samples from French centenarians showed a significantly higher poly(ADP-ribosyl)ation capacity than control cell lines (42), as measured by specific enzyme activity (cellular PARP-1 activity divided by the amount of cellular PARP-1 protein). In a DNA sequencing approach, we identified four new polymorphisms in the human PARP-1 coding region, but none of them was associated with human longevity in our sample (43). This result again suggests that hypothetical accessory factors are important for the regulation of PARP-1 activity.

PARPs and Cellular Processes Involved in Aging Back to Top

As noted above, PARPs are also implicated in the aging process through an involvement in various cellular systems other than DNA repair.

PARPs and telomeres

Telomeres play an important role in defining the "life span" of human cells in culture (36) (and see Hornsby Perspective). Normal somatic cells isolated from the human body, such as fibroblasts, are able to undergo only a limited number of cell divisions (dependent on the age of the donor) before they reach the so-called "Hayflick limit," cease dividing, and become senescent, exhibiting phenotypic changes such as cellular flattening and expression of a senescence-associated {beta}-galactosidase. Such cells are arrested in the G1 phase of the cell cycle, distinct from quiescent cells arrested in G0. After acquiring the senescent phenotype, cells are still viable and can be maintained in culture for several months (see "More Than a Sum of Our Cells").

The telomeres of primary cells shorten with each division owing to the "end-replication problem" (44), because conventional DNA polymerases need a free 3' hydroxyl group for DNA synthesis. This is usually provided by the activity of the DNA polymerase {alpha}/primase complex, which synthesizes an initial RNA oligonucleotide primer. During lagging strand synthesis on linear duplex DNA, the most distally located primer is not replaced, leading to a loss of at least 25 bases from telomeres for each round of DNA replication. Endogenous oxidative DNA damage makes an additional contribution to telomere erosion in replicating fibroblasts, and this can be strongly accelerated by DNA damage induced by ROS or alkylating agents from exogenous sources [for a review, see (45)], further linking DNA damage to the aging process. Telomeric DNA in mammals comprises repeats of TTAGGG hexanucleotides, ending in a 3' single-strand overhang. The telomere is covered by dimers of the telomeric repeat binding factors TRF-1 and TRF-2, and TRF-2 is responsible for the putative t-loop structure adopted by telomere ends (46, 47). The termini of telomeres are folded back, and the free single strand invades the adjacent double-stranded region of DNA, thus building a ternary DNA complex. This unusual arrangement probably shields the telomere from recognition by the DNA double-strand break repair machinery.

In germline cells and tissue stem cells, including their highly proliferative progeny, the multicomponent enzyme telomerase counteracts telomeric sequence loss (48). It is thought that the telomeric t-loop structure has to be unfolded before telomerase can access the telomere and elongation can take place. This is achieved by inhibiting binding of TRF-1 and/or TRF-2. At least four members of the PARP family--TNKS-1 and TNKS-2; PARP-1 and PARP-2--are localized to telomeric DNA and are able to modify TRF-1 and TRF-2 respectively, thus inhibiting their DNA binding activity (14, 49). In TNKS-1-overexpressing cells, for example, this leads to an increase in telomere length. Because the PARP activity of tankyrases inhibits the binding of TRF-1 to the telomere, telomerase activity is facilitated, and the cell can avoid senescence caused by telomere erosion. Very recently, Dynek and Smith used a small interfering RNA-based approach to reduce TNKS-1 activity, which led to mitotic arrest of cells by blocked segregation of telomeres (16).

The involvement of PARP-1 and PARP-2 connects DNA repair to telomere length regulation. PARP-1 has been found to coimmunoprecipitate with TRF-2 in HeLa cell extracts and has been shown to modify it in vitro (15). PARP-2 interacts with TRF-2 in ALT (alternative lengthening of telomeres) cells within nuclear structures referred to as PML (pro-myelocytic leukemia) bodies, and modifies and inhibits TRF-2 in vitro (ALT cells use a telomerase-independent mechanism of telomere lengthening, probably involving DNA recombination events taking place in PML bodies). In this context, PARP-2 knockout mice display a higher frequency of unprotected telomeres and increased chromosomal instability (14).

PARP and WRN protein

There are several rare genetic disorders that induce premature aging in humans, one being Werner syndrome (WS), which is caused by a mutation in the WRN gene (42) (and see "Of Hyperaging and Methuselah Genes"). The WRN protein is a member of the RecQ helicase family, possessing an additional exonuclease activity, and forms part of the DNA replication complex, cooperating with several DNA replication- and repair-related proteins (32, 50-55). Recently, it has been shown that PARP-1 and WRN cooperate functionally in preventing carcinogenesis in vivo, because mice lacking both PARP-1 and WRN genes exhibit an increased frequency of chromosomal aberrations and earlier onset of tumor formation as compared to WRN-mutated mice (50). In WS cells, poly(ADP-ribosyl)ation after DNA damage by hydrogen peroxide and methyl methane sulfonate (single-strand DNA break inducers), but not by bleomycin (a double-strand break inducer), is impaired, pointing to a loss of repair capacity dependent on the repair pathway involved (56). There is also a link to telomeres and DNA repair, as WRN interacts with TRF-2 (57, 58) and FEN-1, as well as with the Ku70 subunit of DNA-PK (32), the same protein complex with which PARP-1 associates (30, 32).

PARP and mitosis

Errors in mitotic chromosomal segregation are one pathway leading to aneuploidy and represent a step toward tumor formation. Inaccurate chromosome segregation can result if multipolar rather than bipolar mitotic spindles are formed. For proper bipolar spindle formation, the centrosomes, which organize the poles of the spindle in most animal cells, must duplicate once and only once per cell division cycle. In this context, it is noteworthy that PARP-1, PARP-3, and probably TNKS-1 as well (59) are involved in regulating centrosomes and centrioles (microtubule-based structures that form part of the centrosome). PARP-3 is localized at the daughter centriole and interacts with PARP-1 at this location (10). PARP-1 itself has been shown to modify centrosomal proteins, and inhibition of its activity leads to hyperamplification of centrosomes, giving rise to aberrant cell division (9). Furthermore, PARP-1 binds to and modifies several centromere-organizing proteins such as Bub3, CenpA, and CenpB (60).

PARP and inflammation

Ultraviolet damage is important in the aging of skin, inducing signaling through many cytokine receptors [for a review, see (61)]. Nuclear factor kappa B (NF-{kappa}B), a master transcription factor in cytokine signaling, is tightly coupled with inflammatory responses [for a review, see (62)]. As revealed by experiments on PARP-1 knockout mice, NF-{kappa}B is dependent on the presence of PARP-1 protein, which is probably acting as an accessory transcriptional coactivator (63-65). In the absence of PARP-1, NF-{kappa}B fails to induce the inflammatory cascade, thus rendering PARP-1 knockout animals resistant to septic shock and a variety of other pathophysiological conditions (66-70). In this context, PARP-1 is important for regulation of the immune response but might also be viewed as a "pro-aging" factor because it is complicit in tissue damage involving inflammation.

Concluding Remarks Back to Top

PARP proteins are crucial players in the cellular responses to various kinds of insult to genomic DNA, be it oxidative DNA damage, telomere erosion, or improper segregation of chromosomes. PARPs appear to provide cells and organisms with very versatile tools to fight various kinds of threat to genomic integrity, thus keeping in check aging-related malfunctions such as cancer formation and cell loss, and consequently may contribute to "healthy aging."

In this review, we have brought together some quite disparate lines of evidence supporting an involvement of members of the PARP family in the aging process (Fig. 3). First, because genotoxic stress (mainly induced by ROS) is believed to be the major driving force of cellular aging, mechanisms that counteract it or reverse its consequences are important for genetic integrity. A key pathway for eliminating oxidative DNA damage, spontaneously formed abasic sites, or single-strand DNA breaks is DNA base excision repair, and the activity of this repair pathway seems to be facilitated by PARP-1 and PARP-2. Indeed, the capacity of blood MNCs to synthesize poly(ADP-ribose) shows a positive correlation with the life span of mammalian donor species. Second, the maintenance of telomere length is very important in allowing replicating human cells to avoid cellular senescence and replicative crisis. Two PARPs, TNKS-1 and -2, serve as regulators of telomere length, modifying TRF-1 and alleviating its negative influence on telomerase activity; PARP-1 and PARP-2 can modify TRF-2 and thus "open up" the telomeric t-loop structure. Third, PARPs can interact with gene products such as the WRN protein that serve to prevent premature aging and retard age-related diseases. Fourth, because several PARPs are components of the mitotic apparatus and appear to have a regulatory function governing chromosome segregation, they may also counteract genomic instability indirectly. Fifth, by interacting with important cell cycle regulators such as p53, PARPs [and likely their reaction product poly(ADP-ribose)] take an active part in DNA damage surveillance and the regulation of cell division (Fig. 4). Finally, inflammatory processes are induced by or themselves mediate the formation of ROS. Inflammation is strictly reliant on the activity of the NF-{kappa}B transcription factor system, and this depends on the role of PARP-1 protein as a transcriptional coactivator.

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Fig. 3. Schematic overview of PARP proteins and their possible involvement in aging processes.


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Fig. 4. Summary of the interactions of PARP-1 and TNKS-1 and -2 with other protein partners.

PARPs are clearly versatile agents, participating in many cellular regulatory tasks. They have been reported to be present in every cellular compartment, including mitochondria (71), although the latter localization is subject to debate. Poly(ADP-ribose) polymers translocate from the nucleus after genotoxic stress, and it has been shown that the polymer is the main if not the only trigger for the release of apoptosis-inducing factor from mitochondria [for a review, see (72)]. One function of PARG is the degradation of polymer to counteract this signal for apoptosis.

Although poly(ADP-ribose) polymers and PARP-1 have been known since 1964, their essential functions have not yet been fully elucidated. The emergence of even more PARPs with partially overlapping functions makes the situation more complex, but it also broadens the spectrum of possible ways for cells to use the basal function that all PARPs have in common, namely poly(ADP-ribosyl)ation. Furthermore, we should keep in mind that all ADP-ribose polymers may not be created equal: Short and long, unbranched and highly branched polymers might well have different effects on cell signaling, repair, and survival. Although this year brings the 40th anniversary of the discovery of poly(ADP-ribosyl)ation, the chapter is unlikely to be closed soon.

December 8, 2004
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Citation: S. Beneke, A. Bürkle, Poly(ADP-Ribosyl)ation, PARP, and Aging. Sci. Aging Knowl. Environ. 2004 (49), re9 (2004).

Poly(ADP-ribosyl)ation by PARP-1: `PAR-laying' NAD+ into a nuclear signal.
M. Y. Kim, T. Zhang, and W. L. Kraus (2005)
Genes & Dev. 19, 1951-1967
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