Gilbert de Murcia

Poly(ADP-ribose): novel functions for an old molecule

The addition to proteins of the negatively charged polymer of ADP-ribose (PAR), which is synthesized by PAR polymerases (PARPs) from NAD+, is a unique post-translational modification. It regulates not only cell survival and cell-death programmes, but also an increasing number of other biological functions with which novel members of the PARP family have been associated. These functions include transcriptional regulation, telomere cohesion and mitotic spindle formation during cell division, intracellular trafficking and energy metabolism.

XRCC1 Is Specifically Associated with Poly(ADP-Ribose) Polymerase and Negatively Regulates Its Activity following DNA Damage

ABSTRACT Poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30 ) is a zinc-finger DNA-binding protein that detects and signals DNA strand breaks generated directly or indirectly by genotoxic agents. In response to these breaks, the immediate poly(ADP-ribosyl)ation of nuclear proteins involved in chromatin architecture and DNA metabolism converts DNA damage into intracellular signals that can activate DNA repair programs or cell death options. To have greater insight into the physiological function of this enzyme, we have used the two-hybrid system to find genes encoding proteins putatively interacting with PARP. We have identified a physical association between PARP and the base excision repair (BER) protein XRCC1 (X-ray repair cross-complementing 1) in theSaccharomyces cerevisiae system, which was further confirmed to exist in mammalian cells. XRCC1 interacts with PARP by its central region (amino acids 301 to 402), which contains a BRCT (BRCA1 C terminus) module, a widespread motif in DNA repair and DNA damage-responsive cell cycle checkpoint proteins. Overexpression of XRCC1 in Cos-7 or HeLa cells dramatically decreases PARP activity in vivo, reinforcing the potential protective function of PARP at DNA breaks. Given that XRCC1 is also associated with DNA ligase III via a second BRCT module and with DNA polymerase β, our results provide strong evidence that PARP is a member of a BER multiprotein complex involved in the detection of DNA interruptions and possibly in the recruitment of XRCC1 and its partners for efficient processing of these breaks in a coordinated manner. The modular organizations of these interactors, associated with small conserved domains, may contribute to increasing the efficiency of the overall pathway.

Poly(ADP-ribose) polymerase: a molecular nick-sensor

Poly(ADP-ribose) polymerase (PARP) participates in the intricate network of systems developed by the eukaryotic cell to cope with the numerous environmental and endogenous genotoxic agents. Cloning of the PARP gene has allowed the development of genetic and molecular approaches to elucidate the structure and function of this abundant and highly conserved enzyme.

PARP-2, A Novel Mammalian DNA Damage-dependent Poly(ADP-ribose) Polymerase

Poly(ADP-ribosylation) is a post-translational modification of nuclear proteins in response to DNA damage that activates the base excision repair machinery. Poly(ADP-ribose) polymerase which we will now call PARP-1, has been the only known enzyme of this type for over 30 years. Here, we describe a cDNA encoding a 62-kDa protein that shares considerable homology with the catalytic domain of PARP-1 and also contains a basic DNA-binding domain. We propose to call this enzyme poly(ADP-ribose) polymerase 2 (PARP-2). The PARP-2 gene maps to chromosome 14C1 and 14q11.2 in mouse and human, respectively. Purified recombinant mouse PARP-2 is a damaged DNA-binding protein in vitro and catalyzes the formation of poly(ADP-ribose) polymers in a DNA-dependent manner. PARP-2 displays automodification properties similar to PARP-1. The protein is localized in the nucleusin vivo and may account for the residual poly(ADP-ribose) synthesis observed in PARP-1-deficient cells, treated with alkylating agents or hydrogen peroxide.

Resistance to endotoxic shock as a consequence of defective NF‐κB activation in poly (ADP‐ribose) polymerase‐1 deficient mice

Poly (ADP‐ribose) polymerase‐1 is a nuclear DNA‐binding protein that participates in the DNA base excision repair pathway in response to genotoxic stress in mammalian cells. Here we show that PARP‐1‐deficient cells are defective in NF‐κB‐dependent transcription activation, but not in its nuclear translocation, in response to TNF‐α. Treating mice with lipopolysaccharide (LPS) resulted in the rapid activation of NF‐κB in macrophages from PARP‐1+/+ but not from PARP‐1−/− mice. PARP‐1‐deficient mice were extremely resistant to LPS‐induced endotoxic shock. The molecular basis for this resistance relies on an almost complete abrogation of NF‐κB‐dependent accumulation of TNF‐α in the serum and a down‐regulation of inducible nitric oxide synthase (iNOS), leading to decreased NO synthesis, which is the main source of free radical generation during inflammation. These results demonstrate a functional association in vivo between PARP‐1 and NF‐κB, with consequences for the transcriptional activation of NF‐κB and a systemic inflammatory process.

Poly(ADP-ribose) polymerase-1: what have we learned from the deficient mouse model?

Poly (ADP-ribose) polymerase (113 kDa; PARP-1) is a constitutive factor of the DNA damage surveillance network developed by the eukaryotic cell to cope with the numerous environmental and endogenous genotoxic agents. This enzyme recognizes and is activated by DNA strand breaks. This original property plays an essential role in the protection and processing of the DNA ends as they arise in DNA damage that triggers the base excision repair (BER) pathway. The generation, by homologous recombination, of three independent deficient mouse models have confirmed the caretaker function of PARP-1 in mammalian cells under genotoxic stress. Unexpectedly, the knockout strategy has revealed the instrumental role of PARP-1 in cell death after ischemia-reperfusion injury and in various inflammation process. Moreover, the residual PARP activity found in PARP-1 deficient cells has been recently attributed to a novel DNA damage-dependent poly ADP-ribose polymerase (62 kDa; PARP-2), another member of the expanding PARP family that, on the whole, appears to be involved in the genome protection. The present review summarizes the recent data obtained with the three PARP knockout mice in comparison with the chemical inhibitor approach.

Functional interaction between PARP-1 and PARP-2 in chromosome stability and embryonic development in mouse

The DNA damage‐dependent poly(ADP‐ribose) polymerases, PARP‐1 and PARP‐2, homo‐ and heterodimerize and are both involved in the base excision repair (BER) pathway. Here, we report that mice carrying a targeted disruption of the PARP‐2 gene are sensitive to ionizing radiation. Following alkylating agent treatment, parp‐2−/−‐derived mouse embryonic fibroblasts exhibit increased post‐replicative genomic instability, G2/M accumulation and chromosome mis‐segregation accompanying kinetochore defects. Moreover, parp‐1−/−parp‐2−/− double mutant mice are not viable and die at the onset of gastrulation, demonstrating that the expression of both PARP‐1 and PARP‐2 and/or DNA‐dependent poly(ADP‐ribosyl) ation is essential during early embryogenesis. Interestingly, specific female embryonic lethality is observed in parp‐1+/−parp‐2−/− mutants at E9.5. Meta phase analyses of E8.5 embryonic fibroblasts highlight a specific instability of the X chromosome in those females, but not in males. Together, these results support the notion that PARP‐1 and PARP‐2 possess both overlapping and non‐redundant functions in the maintenance of genomic stability.

Role of XRCC1 in the Coordination and Stimulation of Oxidative DNA Damage Repair Initiated by the DNA Glycosylase hOGG1

XRCC1 participates in DNA single strand break and base excision repair (BER) to preserve genetic stability in mammalian cells. XRCC1 participation in these pathways is mediated by its interactions with several of the acting enzymes. Here, we report that XRCC1 interacts physically and functionally with hOGG1, the human DNA glycosylase that initiates the repair by BER of the mutagenic oxidized base 8-oxoguanine. This interaction leads to a 2- to 3-fold stimulation of the DNA glycosylase activity of hOGG1. XRCC1 stimulates the formation of the hOGG1 Schiff-base DNA intermediate without interfering with the endonuclease activity of APE1, the second enzyme in the pathway. On the contrary, the stimulation in the appearance of the incision product seems to reflect the addition of the effects of XRCC1 on the two first enzymes of the pathway. The data presented support a model by which XRCC1 will pass on the DNA intermediate from hOGG1 to the endonuclease APE1. This results in an acceleration of the overall repair process of oxidized purines to yield an APE1-cleaved abasic site, which can be used as a substrate by DNA polymerase β. More importantly, the results unveil a highly coordinated mechanism by which XRCC1, through its multiple protein-protein interactions, extends its orchestrating role from the base excision step to the resealing of the repaired DNA strand.

Structure and function of poly(ADP-ribose) polymerase

Poly(ADP-ribose) polymerase (PARP) participates in the intricate network of systems developed by the eukaryotic cell to cope with the numerous environmental and endogenous genetoxic agents. Cloning of the PARP gene has allowed the development of genetic and molecular approaches to elucidate the structure and the function of this abundant and highly conserved enzyme. This article summarizes our present knowledge in this field.

Parp-1 protects homologous recombination from interference by Ku and Ligase IV in vertebrate cells

Parp‐1 and Parp‐2 are activated by DNA breaks and have been implicated in the repair of DNA single‐strand breaks (SSB). Their involvement in double‐strand break (DSB) repair mediated by homologous recombination (HR) or nonhomologous end joining (NHEJ) remains unclear. We addressed this question using chicken DT40 cells, which have the advantage of carrying only a PARP‐1 gene but not a PARP‐2 gene. We found that PARP‐1−/− DT40 mutants show reduced levels of HR and are sensitive to various DSB‐inducing genotoxic agents. Surprisingly, this phenotype was strictly dependent on the presence of Ku, a DSB‐binding factor that mediates NHEJ. PARP‐1/KU70 double mutants were proficient in the execution of HR and displayed elevated resistance to DSB‐inducing drugs. Moreover, we found deletion of Ligase IV, another NHEJ gene, suppressed the camptothecin of PARP‐1−/− cells. Our results suggest a new critical function for Parp in minimizing the suppressive effects of Ku and the NHEJ pathway on HR.