Accepted Manuscript

Targeting poly(ADP-ribose)polymerase1 in neurological diseases: a promising

trove for new pharmacological interventions to enter clinical translation

Chandra Shekhar Sriram, Ashok Jangra, Eshvendar Reddy Kasalaa, Lakshmi

Narendra Bodduluru, Babul Kumar Bezbaruah

PII: S0197-0186(14)00157-0

DOI: http://dx.doi.org/10.1016/j.neuint.2014.07.001

Reference: NCI 3609

To appear in: Neurochemistry International

Received Date: 4 January 2014

Revised Date: 2 July 2014

Accepted Date: 4 July 2014

Please cite this article as: Sriram, C.S., Jangra, A., Kasalaa, E.R., Bodduluru, L.N., Bezbaruah, B.K., Targeting

poly(ADP-ribose)polymerase1 in neurological diseases: a promising trove for new pharmacological interventions

to enter clinical translation, Neurochemistry International (2014), doi: http://dx.doi.org/10.1016/j.neuint.

2014.07.001

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Title: Targeting poly(ADP-ribose)polymerase1 in neurological diseases: a promising trove for

new pharmacological interventions to enter clinical translation

Authors names: Chandra Shekhar Sriram a*, Ashok Jangra a, Eshvendar Reddy Kasalaa a,

Lakshmi Narendra Bodduluru a, Babul Kumar Bezbaruah a,b.

Affiliations:

a. Department of Pharmacology & Toxicology,

National Institute of Pharmaceutical Education and Research (NIPER),

III Floor, Guwahati Medical College, Narkachal Hilltop,

Bhangagarh, Guwahati, Assam, India, PIN: 781032.

b. Department of Pharmacology, III Floor, Guwahati Medical College,

Narkachal Hilltop, Bhangagarh, Guwahati, Assam, India, PIN: 781032.

*Corresponding author:

Chandra Shekhar Sriram

NIPER, Department of Pharmacology & Toxicology,

III Floor, Guwahati Medical College,

Narkachal Hilltop, Guwahati, Assam, India, PIN: 781032.

Email: srshekhar@yahoo.com

Mobile no: +91-9508818861.

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Abstract:

The highly conserved abundant nuclear protein poly(ADP-ribose)polymerase1 (PARP1)

functions at the centre of cellular stress responses and is mainly implied in DNA damage

repairing mechanism. Apart from its involvement in DNA damage repair, it does sway multiple

vital cellular processes such as cell death pathways, cell aging, insulator function, chromatin

modification, transcription, and mitotic apparatus function. Since brain is the principal organ

vulnerable to oxidative stress and inflammatory responses, upon stress encounters robust DNA

damage can occur and intense PARP1 activation may occur that may lead to various CNS

diseases. In the context of soaring interest towards PARP1 as a therapeutic target for newer

pharmacological interventions, here in the present review, we are attempting to give a silhouette

of the role of PARP1 in the neurological diseases and the potential of its inhibitors to enter

clinical translation, along with its structural and functional aspects.

Abbreviations

PARP1, poly(ADP-ribose)polymerase1; PARylation, poly(ADP-ribosyl)ation; PARG;

poly(ADP-ribose) glycohydrolase, TBI, traumatic brain injury; XRCC1, X-ray repair cross-

complementing protein1; SHM, somatic hypermutation; IGC, immunoglobulin gene conversion;

CSR, class switch recombination, MCAO, middle cerebral artery occlusion; ALS, amyotrophic

lateral sclerosis; DSBs, double stranded breaks.

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Table of contents:

1. Introduction

2. PARP1 structural and functional aspects

2.1. PARylation in response to stress induced-DNA damage

2.2. PARylation in response to programmed-DNA damage

2.3. Regulation of PARylation homeostasis

2.4. Implications of PARP1 in cellular processes

3. Development of PARP1 inhibitors

4. PARP1 in neurological diseases

4.1. PARP1 in stroke

4.2. PARP1 in traumatic brain injury (TBI)

4.3. Other central nervous system implications of PARP1 4.3.1. PARP1 in neurodegenerative diseases 4.3.2. PARP1 in neuro-inflammatory diseases

5. Conclusion

1. Introduction

It’s been 50 years since the discovery of the vital nuclear protein poly(ADP-ribose)polymerase1

(PARP1), that catalyzes poly(ADP-ribosyl)ation (PARylation) (Chambon et al., 1963).

PARylation is the process whereby a linear or multibranched polymer of ADP-ribose units

(termed PAR for poly(ADP-ribose)) covalently attached to Glu, Lys or Asp residues of acceptor

proteins (heteromodification) or onto PARP1 itself (automodification) (Pleschke et al., 2000).

Over the years the enzymes responsible for PARylation are also known by many terms such as

ADP-ribosyltransferases (ADPRTs), poly(ADP-ribose)synthetases (PARS) and ADP-

ribosyltransferase Diphtheria toxin-like (ARTD), however recently to describe PARPs more

accurately, a new nomenclature has been proposed, that names PARP1 as ADP-

riboxyltransferase1 (ART1) (Hottiger et al., 2010). Often PARP1 is also referred as a guardian

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angel, perpetrator of apoptotic cell death or international molecule of mystery (Jeggo et al., 1998;

Chiarugi and Moskowitz, 2002; Woodhouse and Dianov, 2008). Presence of a conserved

catalytic domain “PARP signature” motif is the common feature of the PARP family (PARP1 is

100% conserved in vertebrates), which is the active site (Ruf et al., 1996). Although, since long

back it’s been studied in the perspective of DNA damage finding and repair, PARP1 has more

recently been related to the regulation of chromatin organization and transcription, DNA

imprinting and methylation, chromosome organization, and insulator activity. While a 17

member superfamily of PARPs has been identified, however PARP1 is the most abundant

isoform and in the brain it accounts for more than 90% of the PARylation, this alludes to its

significance in the central nervous system (Pieper et al., 2000).

2. PARP1 structural and functional aspects

Among the 17 members super family of PARPs (EC 2.4.2.30), PARP1 is the most abundant and

the founding member with modulator structure. The crystal structure of PARP1 can help to

understand the biological functions more precisely. While many X-ray crystallography studies

were undergone for unravelling the crystal structures of the PARP family, recently Langelier and

group have determined the x-ray crystal structure of human PARP1 domains Zn1, Zn3, and

WGR-CAT bound to a DNA double-strand break (Langelier et al., 2012). The enzyme’s

structure has three main domains: an N-terminal DNA-binding domain (DBD), a central

automodification domain and a C-terminal catalytic domain (Kameshita et al., 1984) (Fig. 1).

The DBD contains three zinc-fingers (Zn1, Zn2, and Zn3) which have different roles in DNA

binding, interdomain cooperation, chromatin compaction and protein–protein interactions

(Langelier et al., 2011; Ali et al., 2012). In the central automodification domain glutamate,

aspartate and lysine residues serve as putative acceptors for auto (ADP-ribosyl)ation (Altmeyer

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et al., 2009), a leucine zipper motif mediates homo or heterodimerization and a breast cancer-

associated protein C-terminal (BRCT) motif mediates protein–protein interactions. Adjacent to

the catalytic domain a tryptophan, glycine, and arginine-rich WGR (WGR in single-letter code)

domain has also been revealed to be needed for DNA-damage induced PAR production

(Langelier et al., 2012). The catalytic domain contains the active site known as ‘‘PARP

signature’’ sequence which is required for the catalysis of PAR synthesis (Ruf et al., 1996).

Collectively, the structural and functional domains of PARP1 present the activities essential for

the wide range of functions of PARP1 in the nucleus.

2.1. PARylation in response to stress induced-DNA damage

Intracellular and extracellular toxic stresses cause DNA damage. If the resultant genome damage

from the lesions of stresses, not repaired or incorrectly repaired, that may cause mutations and

chromosomal anomalies, diseases and cell death (Pizarro et al., 2009). To defend their genome

against the detrimental consequences of the lesions, each cell has sophisticated cellular networks

to perceive the DNA damage, locate its presence and promulgate the proper mending pathway.

Among the core molecular mechanisms which control the mending pathways, PARylation

catalyzed by Poly(ADP-ribose) polymerase1 (PARP1) implicated in stress-induced DNA

damage signalling and genome integrity, but also in diverse physiological conditions initiated by

developmentally programmed DNA breakage (Wielckens et al., 1983; Kreimeyer et al., 1984;

Ménissier-de Murcia et al., 1989; Spagnolo et al., 2012).

The appearances of nicks in DNA molecule activate PARP1 and increase the cellular

PAR levels up to 500-fold (Benjamin and Gill, 1980). Apart from DNA damage PARP1 could be

activated by special non-B DNA structures such as bent, cruciform DNA or stably unpaired

DNA regions (Lonskaya et al., 2005). Furthermore, potential posttranslational modifications

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such as acetylation or ADP-ribosylation (Loseva et al., 2010; Mao et al., 2011) and

phosphorylation of PARP1 by certain protein kinases have also found to activate PARP1

(Cohen-Armon et al., 2007). In addition a latest study has found that S-sulfhydration of MEK1

(mitogen activated protein (MAP)/ERK (extracellular signal-regulated kinase) Kinase1) by

endogenous H2S, leads to PARP1 activation (Zhao et al., 2014). PARP1 locates nicks on the

DNA and then formation of a large number of discrete foci in the nucleus takes place and

thereby the PARP1 and PAR molecules will be confined to the break site without spreading into

the flanking chromatin regions (El�Khamisy et al., 2003; Bryant et al., 2009). Initial recruitment

of PARP1 mediates by the DNA-binding domain. PARP1 activation and localized poly(ADP-

ribose) synthesis then generates binding sites for a second wave of PARP1 recruitment and for

the rapid accumulation of the loading platform, X-ray repair cross-complementing protein 1

(XRCC1) at repair sites (Mortusewicz et al., 2007). The oxidized form of cellular XRCC1 could

be vital for PARP1-mediated DNA damage responses (Horton et al., 2013).

PARP1 appropriately shapes the chromatin landscape for efficient repair, by

showing antagonistic effects on chromatin plasticity: loosening versus compaction. PARP1 can

covalently ADP-ribosylate lysines in the amino-terminal tails of the core histones H1, H2A,

H2B, H3 and H4 likely promoting a local chromatin opening at DNA damage sites, which in turn

facilitates the access to the repair machinery (Messner et al., 2010). Indeed, in a recent study the

significance of PARP1 in chromatin decondensation was re-established, in which kinase-

mediated changes in nucleosome conformation triggered chromatin decondensation via

PARylation (Thomas et al., 2014). However, in striking contrast to its function in chromatin

opening, more recent findings provided evidence for a role of PARP1 and its activity in

triggering chromatin compaction. Indeed, a study had identified a PAR-mediated enrichment of

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the macrodomain containing histone macroH2A1.1 onto pulsed-laser microirradiation sites that

was associated with chromatin compaction as revealed by increasing histone and DNA staining

density (Timinszky et al., 2009). Another study demonstrated that PARP1 is the major

contributor to the recruitment of chromodomain helicase DNA-binding protein 4 (CHD4, also

known as Mi-2β) which is an integral component of the nucleosome remodeling deacetylase

(NuRD) that combines chromatin remodelling activity with histone deacetylase and demethylase

properties important for gene silencing (Polo et al., 2010).

PARP1 binds to the damaged DNA through the second zinc finger domain, forms

homodimers and catalyses the breakage of NAD+ into ADP-ribose and nicotinamide (Fig. 2). It

then uses ADP-ribose to synthesize branched nucleic acid like polymer, called PAR, which then

forms covalent bonds with the Glu, Lys or Asp residues of nuclear acceptor proteins like

histones, topoisomerase, endonucleases, etc., and exerts its activity (Pleschke et al., 2000). The

high negative charge of PAR dramatically affects the function of target proteins, leading to

electrostatic repulsion among histone proteins and DNA, a process implicated in chromatin

remodelling, DNA repair and transcriptional regulation. Furthermore in striking contrast with its

building property, PARylation can also promote the timely controlled protein disassembly

largely through the PARylation of the targeted protein. This disassembly is a critical step for the

switch to the next step in DNA repair. An interesting example is the dissociation of the histone

chaperone facilitator of chromatin transcription (FACT) from damaged chromatin upon

PARylation of its subunit Spt16 (Heo et al., 2008).

The degree of the PARylation in response to DNA damage largely depends on the nature

and amount of DNA breaks produced. For low levels of DNA damage, PARP1 activity favours

repair and survival by interacting with DNA repair enzyme cascade, such as such as XRCC1 and

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DNA-dependent protein. Moderate DNA damage leads to apoptotic cell death, during which

PARP1 will be cleaved into two fragments by caspases. Cleavage of PARP1 is assumed to foil

the activation of PARP1 by DNA damage and thereby it prevents cells from pathological

consequences such as necrosis of cells. For extensive DNA injury as observed during

ischemia/reperfusion and inflammatory conditions, the massive production of PAR ultimately

causes cell-death via at least two distinct mechanisms: energy-failure induced necrosis or

apoptosis-inducing factor (AIF) dependent apoptosis (Kauppinen, 2007) (Fig.3).

2.2. PARylation in response to programmed-DNA damage

Programmed DNA double stranded breaks (DSBs) arise as intermediates during

spermatogenesis, retroviral integration, and somatic recombination in B and T lymphocytes or

apoptotic cell death. Many DNA repair proteins, which participate in repairing exogenous DNA

damage, can also play a key role in repairing programmed DNA damage.

Germ cells experience through programmed DSBs during meiosis and raise genetic

diversity via recombination and crossover procedures between homologous chromosomes, which

need proficient repair for proper chromosomal separation. Studies in primary rat spermatocytes

have determined the PARP1 catalyzed PAR synthesis (Tramontano et al., 2007) and the

synthesis was activated by protein-bound DSBs, which were produced by topoisomerase II beta

(topoisomerase IIβ) in order to remove DNA supercoiling (Meyer-Ficca et al., 2011b). From the

recent research findings, it is evident that mouse knockout for either PARP1, poly(ADP-ribose)

glycohydrolase (PARG) (110 kDa isoform) or both display abnormal shaping of the sperm

nucleus indicating defective spermatid elongation, and the accumulation of sperm DNA strand

breaks ultimately leading to subfertility (Meyer-Ficca et al., 2009; Meyer-Ficca et al., 2011a).

Similarly, PARP1 was shown to be indispensable in the maintenance of chromosome integrity

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throughout meiosis in female germ cells (Yang, 2009). Thus, PARP1 function is being longed-

for more as a key regulator through germ cell development (Tramontano et al., 2007).

Similarly, several lines of evidence support a general role of PARP1 in DNA damage

repair activities in facilitating the integration of the viral genome into the host DNA, a critical

step in the productive infection by the retrovirus (Gäken et al., 1996; Kameoka et al., 2005).

Subsequent reports indicated that PARP1 was not strictly required in this HIV-1 gene expression

or replication (Siva and Bushman, 2002; Ariumi et al., 2005). Further work is certainly needed to

establish the exact role of PARP1 in retroviral genome integration with host DNA.

PARP1 is also found to be involved in the diversification and assembly of B cell

receptor. During immune responses, mature B cells diversify their antigen receptors through

somatic hypermutation (SHM), immunoglobulin gene conversion (IGC) and class switch

recombination (CSR) to produce high affinity antibodies endowed with specific effector

functions. SHM and CSR require the expression of activation-induced cytidine deaminase (AID)

(Muramatsu et al., 2000), an enzyme that deaminates cytidines in DNA and that generates U:G

mismatches in Ig genes (Petersen-Mahrt et al., 2002; Rada et al., 2002). The functional analyses

of PARP1 in the mutagenic repair of AID-produced lesions in the Ig genes have shown that the

BRCT domain of PARP1 is necessary to initiate a significant proportion of the mutagenic repair

specific to diversifying antibody genes (Paddock et al., 2010). Additionally, the BRCT domain

was found to interact with Ku70, Ku80 and DNA-dependent protein kinase catalytic subunit

(DNA-PKCS) and the concomitant inactivation of PARP1 and Ku70 or PARP1 and DNA ligase

IV rescued totally the IGC phenotype (Paddock et al., 2011) showing that mutagenic pathways

predominate even in the absence of PARP1. Thus, it can be conferred that PARP1 inhibits Ku70

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function and the recruitment of DNA ligase IV to allow other mutagenic pathways to achieve

gene conversion.

SHM introduces point mutations in the variable region of Ig genes, thereby increases

antibody affinity for antigen. PARP1 may play an indirect role in SHM by favouring mutagenic

over error-free DNA repair (Paddock et al., 2010). The involvement of PARP1 in IGC has been

clearly demonstrated using the chicken DT40 B cell line, a widely used cellular model that

constitutively diversifies the variable regions by IGC and SHM in vitro. Inactivation of PARP1

or disruption of its BRCT domain in DT40 cells dramatically hampered IGC (Paddock et al.,

2010).

Mature B cells diversify their antigen receptors via CSR endowing antibodies with

varying effector activities. CSR is the progression by which B cells can yield antibodies with

similar variable regions, yet have varying functions accredited to the heavy chain constant

region. Throughout CSR, this constant region can be altered from the original IgH to any of the

other isotypes: IgM to IgA, IgE, or IgG (Kataoka et al., 1980). Early studies with PARP1

inhibitors propose that PARP1 acts as an inhibitor of CSR, possibly representing its role as a

negative regulator of the progression (Shockett and Stavnezer, 1993). However, more recent

studies have assayed PARP1 function in the CSR-proficient CH12 B lymphoma cell line and

primary splenic B cells, which exhibited PARylation of PARP1, when CSR was induced (Robert

et al., 2009). PARP1-/- mice exhibit abnormal levels of basal immunoglobulins, as well as varied

population of plasma and antigen-specific plasma cells (Ambrose et al., 2009) representing that

although these cells theoretically mature there are deficits in their activity. Incompetent repair of

the programmed DNA damage may vary efficient CSR, keeping B cells from experiencing

through the normal sequence of antibody production.

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Nuclear factor kappa B (NFκB) is one of the most key transcription factors given its

association in almost all steps of B cell progression (Kim et al., 2003). Recently PAR synthesis

was found to be critical in orchestrating signalling cascades that lead to NFκB activation in

reaction to environmental DNA injury, which makes it possible that PARylation also platforms

the programmed DNA damage-activated intrinsic NFκB signalosome in B cells (Stilmann et al.,

2009). Therefore, one can speculate that PARP1 could give a direct relationship and fill up the

gaps in knowing both programmed DNA damage/repair and NFκB in B cell development,

differentiation, and maturation.

2.3. Regulation of PARylation homeostasis

Efficient repair of DNA damage needs temporally-controlled reversion of the post-translational

modifications to proceed to the next stage. The control over PARP1 activity is recognized

through diverse mechanisms. The best considered mechanism is the down-regulation of enzyme

function through auto-PARylation (Kawaichi et al., 1981). Phosphorylation of PARP1 by protein

kinase C also produces enzyme inhibition (Bauer et al., 1992). Poly-ADP-ribose glycohydrolase

(PARG), nucleoside diphosphate X (NUDIX) family of proteins and ADP-ribosyl hydrolase3

(ARH3) are very crucial enzymes to eliminate, degrade, and recycle PAR chains (Fig. 3).

Especially PARG has both endo and exo glycosidase actions and breaks the glycosidic bonds

between the ADP-ribose units, as a result quickly hydrolyzing most of ADP-ribose polymers on

PARylated proteins (Miwa et al., 1974; Formentini et al., 2009; Barkauskaite et al., 2013;

Mashimo et al., 2013). In addition, the macrodomain-containing proteins (MDCPs), such as

MacroD1, MacroD2, and C6orf130, were newly revealed to be the enzymes that fully undo the

effect of PARP1 by hydrolyzing the terminal ADP moiety (Jankevicius et al., 2013; Rosenthal et

al., 2013). The degradation of PAR is a quick procedure; the PAR half-life is roughly estimated

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to be less than one minute in cells (Miwa et al., 1974). Owing to such a fast turnover rate,

continuous activation of PARP1 leads to a significant decrease in cellular NAD+ levels. As a

result, the process of resynthesis of NAD+ depletes cellular ATP levels leading to neuronal cells

death (Alano et al., 2010). Therefore, a tight control of the metabolic yield and recycling of PAR

is essential for physiological harmony. The importance of controlled catabolism of PAR chains

under pathophysiological conditions, particularly during the DNA damage reaction, is shown by

recent studies with micro-irradiation presenting efficient recruitment of MacroD2 and PARG

isoforms to DNA damage foci (Mortusewicz et al., 2011; Jankevicius et al., 2013). In support of

this perception, removal of the nuclear PARG isoform in embryonic stem cells and mice notably

increased their sensitivity to alkylating agents and γ-irradiation (Min et al., 2010). Thus in the

context of all the above findings, one can confer that PARP1, NUDIX, PARG, as well as other

PAR modifying enzymes act harmoniously to maintain a tight regulation of the metabolic yield

and recycling of PAR, hence getting rapid and competent control of multiple cellular processes

in a spatially and temporally controlled way.

2.4. Implications of PARP1 in cellular processes

PARylation is found to be implied in cellular processes such as, DNA damage detection

(Langelier et al., 2011), locating DNA damage, DNA damage repair and maintenance of

genomic integrity (El�Khamisy et al., 2003; Mortusewicz et al., 2008; Bryant et al., 2009; Gagné

et al., 2012), modifying the chromatin landscape for efficient DNA repair (Messner et al., 2010),

participating in the expression of various proteins such as NFκB, AP-1, Oct1, HIF-1α,

nitricoxide synthase (iNOS), intercellular adhesion molecule 1 (ICAM-1), TNF-α and major

histocompatibility complex class II (Otsuka et al., 1991; Yang et al., 2000; Martin-Oliva et al.,

2006; Verheugd et al., 2013), regulating telomerase activity and aging (La Torre et al., 2013),

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regulation of cytoskeletal organization (Uchida et al., 2002) and promoting recruitment of novel

repair factors from the RNA processing pathway, such as the splicing regulator RNA binding

motif protein, X-linked (RBMX) or the RNA binding protein NONO (Adamson et al., 2012).

PARP1 hyperactivation can turn as a transient energy producing process in cellular glucose

deprivation (Buonvicino et al., 2013), additionally Poly(ADP-ribose) polymer could work as an

emergency resource of energy spent by the base-excision machinery to synthesize ATP (Oei and

Ziegler, 2000), and may also serve as a indication for protein breakdown in oxidatively injured

cells (Wang et al., 2012).

Besides to the above implications, PARP1 activity dramatically impacts on mitochondrial

function and structure (Virág et al., 1998). The cells with extensive DNA damage or damage that

is not repaired, PARP1 remains activated, leading to continued NAD+ depletion and further ATP

consumption in order to resynthesize NAD+ (Berger et al., 1983). Continued NAD+ depletion has

also been shown to induce a rapid mitochondrial dysfunction, which was followed by a collapse

in mitochondrial potential, and the release of AIF and cytochrome c (Formentini et al., 2009;

Alano et al., 2010). Later, upstream events triggering PARP1 mediated mitochondrial

dysfunction and downstream mediators have also been identified: receptor-interacting protein 1

(RIP1) and tumor necrosis factor receptor-associated factor 2 (TRAF2) mediated signalling was

found to be responsible for PARP1-mediated necrotic cell death and c-Jun N-terminal kinase

(JNK) was identified as a downstream mediator (Xu et al., 2006). Recently, the mitochondrial

energy failure has been shown to be a direct consequence of PARP1 hyper activation. A latest

study has found that the PARP1 product PAR becomes catabolized to AMP via the action of

PARG and nucleoside diphosphate-X (NUDIX) hydrolases (Formentini et al., 2009). The

accumulated AMP then serves to compete with ADP for binding to the adenine nucleotide

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transporter, abrogating energy production by the mitochondria and further contributing to the

“energy crisis.” The resulting escalation of energy crisis would culminate to programmed

necrosis (Formentini et al., 2009). Further, it is also implied that, PARP1 modulates Nrf2-

dependent transcription by forming complexes with the antioxidant response element (ARE)

within the promoter region of Nrf2 target genes and upregulate the transcriptional activity of

Nrf2 (Wu et al., 2014).

3. Development of PARP1 inhibitors

Most of the PARP1 inhibitors are competitive inhibitors and act by blocking the binding of

NAD+ to the catalytic domain of the enzyme owing to their structural similarity with NAD+.

Another mode of PARP1 inhibition was unravelled in a recent study. According to the new mode

of action, PARP1 inhibitors trap target proteins at the sites of DNA damage and form PARP1-

protein DNA complexes, which are highly toxic to cells because they block DNA replication

(Murai et al., 2012).

It is noteworthy that the majority of the presently available drugs do not discriminate

between PARP1 and other PARP species. Mostly PARP1 inhibitors interact with the

nicotinamide binding site of the catalytic domain, which is a highly conserved region that

permits cross-selectivity with other PARPs having homologous catalytic domains. Since most of

the drugs have varied selectivity among PARPs, understanding of biological effects can show

difficulties (Wahlberg et al., 2012). Accordingly, the differences between adverse effects and

therapeutic effects resulting from pan-PARP inhibition compared to selective PARP1 inhibition

are not well understood. It is recently begun to know how various PARP inhibitors affect each

PARP function, and whether additional therapeutic advantages result from specific PARP1

inhibition remains to be detected. The use of selective drugs will be very important in

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interpreting each PARPs function. For an instance, the selectivity of drugs between PARP1,

PARP2, and PARP3 is especially required to clarify roles in reaction to DNA damage.

Modalities for screening the group of PARPs have become more prevalent, which will help to

accelerate the development of selective PARP1 inhibitors. On the way to PARP1 selective

inhibitors, most attempts will be likely continue to target on the modifications of the

nicotinamide-based inhibitors. The acceptor sites among PARPs contain varying amount of

diversity, which could guide the selectivity of modifying target proteins. Also targeting non-

catalytic domains of PARP could bring a route to achieving selectivity for PARP1 inhibitors.

Specific inhibitors could have sensible importance in decreasing isoform-selective adverse

effects (e.g. the detrimental effects of PARP2 ablation on the pancreas could be prevented by the

use of a PARP1-specific inhibitor that protects the pancreas (Bai et al., 2011)). In order to

improve the selectivity and potency newly prepared PARP1 inhibitors have been intended to

bind the nicotinamide binding site and adjacent site. The best selectivity accomplished for

PARP2 versus PARP1 is 60 fold, or for PARP1 versus PARP2 is, 10 fold an amount that perhaps

cannot offer enough selectivity in cells or in vivo (Szántó et al., 2012).

Significant research effort has been dedicated to create selective PARP1 inhibitors. The

quest for the discovery of new drugs having strong propensity towards PARP1 had identified

several newer classes of drugs such as 5-oxo-2,4,5,6-tetrahydro-1H-thiopyrano[3,4-c] quinoline-

9-carboxamide derivatives (Park et al., 2011), quinazolin-4(3H)-ones (Kulkarni et al., 2012),

benzoxazinones (Gangloff et al., 2013), disaccharide nucleoside derivatives (Efremova et al.,

2013), imidazo[4,5-c]pyridinecarboxamide (Zhu et al., 2013), 7-Azaindole-1-carboxamides

(Cincinelli et al., 2014) and isoquinolinone-based tricycles (Chen et al., 2014). Many PARP1

inhibitors have shown promising results in neurological preclinical studies, which could enter

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clinical translation, such as MP-124 (Matsuura et al., 2011), HYDAMTIQ (Moroni et al., 2012),

PJ34 (Haddad et al., 2013). However, presently a Phase-1 clinical study of PARP1 inhibitor, JPI-

289 is underway for stroke (ClinicalTrials.gov).

4. PARP1 in neurological diseases

Cellular stress responses are exhibited through a sequence of regulatory procedures that take

place at the genomic, transcriptional, post-transcriptional, translational and post-translational

levels. Reactive oxidative species cause DNA damage and consequently induce PARP1

activation to repair the damaged DNA (Krietsch et al., 2012). PARP1 has been implicated in

neuronal pathology, as the brain is highly susceptible to oxidative stress due to its high oxygen

consumption, modest antioxidant defences, its lipid rich constitution, which provides readily

available substrates for oxidation, reducing the potential of certain neurotransmitters and the

presence of redox-catalytic metals such as iron and copper. Many research findings are

supporting the significant role of PARP1 in neurological diseases (Domercq et al., 2013; Jangra

et al., 2013; Martire et al., 2013; Gueguen et al., 2014; Shetty et al., 2014).

A gamut of mechanisms is implied in the neuroprotection offered by PARP1 inhibition (Fig. 4).

They are inhibition of NO and ROS production, excitotoxicity and cell necrosis due to

excitotoxin-induced calcium overload (Cosi et al., 1994; Mandir et al., 2000; Du et al., 2003;

Hamby et al., 2007), and inhibition of pro-inflammatory cytokine mediators production

(Kauppinen et al., 2009; Scalia et al., 2013). In addition, PARP1 inhibition brings suppression of

brain edema (Strosznajder et al., 2003), suppression of high mobility group box 1 (HMGB1)

release from the damaged neurons (Davis et al., 2012), inhibition of matrix metalloproteinase

activation (Kauppinen and Swanson, 2005) and protection against the breakdown of blood–brain

barrier (Lenzsér et al., 2007). PARP1 inhibition also produces neuroregeneration (Kauppinen et

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al., 2009), represses the translocation of cell death factor AIF (Culmsee et al., 2005), and inhibits

an independent death signal, free PAR formation (Andrabi et al., 2011). Furthermore, ADP-

ribose produced by PARP1 can activate transient receptor potential melastatin2 (TRPM2)

receptors, leading to amplified intracellular calcium concentrations and cell death (Fonfria et al.,

2004; Yang et al., 2006). PARP1 inhibition could foil ADP-ribose production and subsequent

activation of TRPM2 receptors. Besides to the above mechanisms, PARP1 inhibition could also

be neuroprotective via activation of Akt, which produces significant cytoprotective effects by

phosphorylating multiple apoptosis-regulatory proteins (Wang et al., 2007) (Fig. 4).

4.1. PARP1 in stroke

Probably stroke was the first indication beyond cancer, wherein a significant role of PARP1 was

discovered. Experimentally ischemia/reperfusion injury of the brain simulated in the laboratory

by predisposing primary neuronal cultures to glutamate or its agonists, or to various reactive

oxygen species, NO donors, peroxynitrite or by combined oxygen-glucose deprivation.

Glutamate was found to produce a rapid increase in PARP1 activity in cerebellar granule cells

cultures and primary cortical cultures (Cosi et al., 1994). PARP1 activity was almost tripled in a

dose dependent manner in a rat brain nuclear extract culture, upon the addition of DNA that had

been preincubated with NO (Nitric oxide) (Zhang et al., 1994). All these observations confer the

over activity of PARP1 in stroke models.

In addition, PARP1 inhibitors have been shown to be protective in the brain injury induced by

glutamate and chemical compounds that generate NO (Zhang et al., 1994). Similarly, a study of

primary cortical cultures from PARP1-/- mice were found resistant to toxicity of NMDA as well

as to the neurotoxicity elicited by combined oxygen-glucose deprivation (Eliasson et al., 1997).

Infarct volume was reduced in PARP1-/- mice after transient middle cerebral artery occlusion

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(MCAO) (Eliasson et al., 1997; Endres et al., 1997). Further studies that showing increased

PARylation in the ischemic/reperfused brain have given the pathophysiological relevance to the

above observations (Endres et al., 1998a; Endres et al., 1998b).

In some other studies, NAD+ levels were depleted and ADP-ribose formation was raised

following focal ischemia in wild-type tissue, but on the contrary, in PARP-/- tissue, neither the

poly (ADP-ribose) formation, nor the depletion of NAD+ levels was observed (Eliasson et al.,

1997; Endres et al., 1997). However, infarct volume was reduced in transient MCAO of PARP1-

/- mice (Eliasson et al., 1997; Endres et al., 1997). PARP1 inhibitor treatment in rats, 2 hours

before and 1 hour after MCAO, reduced the infarct volume, but not DNA damage (Giovannelli

et al., 2002). Further, PARP1-/- mice have shown protection from MCAO, and protection from

MCAO was lost upon the viral transfection of wild-type PARP1 (Goto et al., 2002). Similar to

the above observations, in a separate study PARP1-/- neuron cultures were found to be prevented

from hypoglycemic neuronal death and cognitive impairment, induced by glucose deprivation

(Suh et al., 2003).

Further a potent PARP1 inhibitor, PJ34 was synthesized, which is approximately, 10000

times more potent than prototypical inhibitor, 3-aminobenzamide. PJ34 exerted amelioration of

stroke damage following MCAO and reperfusion in mice and rats with significant therapeutic

window (4-6 hrs after the onset of ischemia in the MCAO). It is also reduced neuronal damage

following oxygen and glucose deprivation of in vitro neuronal cortical cultures (Abdelkarim et

al., 2001). Further studies of exploration of PJ34 activity, have found that, it reduces the

transcription of the genes encoding TNF-α, IL-6, ICAM-1 and E-selectin following cerebral

ischemia in mice (Haddad et al., 2006), its administration as late as 8 hours after transient

ischemia-reperfusion has a large protective effect on hippocampal CA1 survival in rats (Hamby

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et al., 2007) and shows anti-hemorrhagic and neuroprotective effects (Haddad et al., 2008).

Treatment of rats, several days after ischemia, with PJ34 enhanced long-term neuronal survival

and neurogenesis by reducing inflammation (Kauppinen et al., 2009). PJ34 makes Recombinant

tissue-type plasminogen activator (rt-PA) safer in experimental mice ischemic stroke and also

improves the neurological functions of rt-PA (Haddad et al., 2013). PARP1 inhibition could

produce long lasting neurological functional improvements (6-12 weeks) (Kauppinen et al.,

2009; Moroni et al., 2012).

Several other studies have also established the possible protective role of

PARP1inhibition in ischemic stroke. For an instance, administration of PARP1 inhibitor, 3-

aminobenzamide reduced ischemia-induced cerebral infarction, functional deficits and

inflammatory responses in neonatal brain (Ducrocq et al., 2000). 3-aminobenzamide also

produced long-term (28 days) improvement in neurological status and reduced infarct volume of

rat MCAO (Ding et al., 2001). PARP1 inhibition through administration of DPQ was increased

resistance to glucose deprivation in neuron cultures (Suh et al., 2003). Intake of INO-1001 in

Sprague-Dawley rats exerted regulation of ischemic nuclear translocation of AIF along with the

neuroprotective activity (Komjáti et al., 2004). PARP1 inhibitors, FR247304, NU1025, ONO-

1924H and imidazobenzodiazepine derivatives have exerted neuroprotective activity in vivo

experimental models of cerebral ischemia, additionally both FR247304 and NU1025 have

exhibited neuroprotective efficacy in vitro experimental models also (Ferraris et al., 2003;

Iwashita et al., 2004b; Kamanaka et al., 2004; Kaundal et al., 2006). Similarly, KCL-440 with

high blood-brain barrier permeability and DR2313 with free radical scavenging activity have

exerted neuroprotective effects, through PARP1 inhibition (Ikeda et al., 2005; Nakajima et al.,

2005). In addition, HYDAMTIQ, a potent PARP1 inhibitor, conferred robust neuroprotection of

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MCAO in Sprague–Dawley rats and long-lasting (up to 3 months) improvement of post-stroke

neurological deficits (Moroni et al., 2012). Similarly, in a latest study, cilostazol pre-treatment

has shown a protective effect on PARP1/AIF mediated apoptotic pathway in the rat MCAO

model (Ba et al., 2014).

Stroke and stroke related mortality may be influenced by gender and age. It is supported by the

fact that, even though the incidence of stroke patients over age 75 is same in men and women,

but mortality is nearly double in the women group. Many studies are strongly supporting the

association of PARP1 in the pathogenesis of stroke-associated neuroinjury is strongly dependent

on the gender of the animal (Eliasson et al., 1997; Hagberg et al., 2004; McCullough et al., 2005;

Liu et al., 2011). A recent study is also corroborating the gender discrepancy, in which PARP1

removal prohibited the stroke-induced loss in NAD+ in males, but worsened NAD+ loss in

PARP1-/- females. Preventing NAD+ loss with nicotinamide reduced infarct in wild type males

and PARP1 knockout mice of both sexes, with no e�ect in wild type females (Siegel and

McCullough, 2013).

Probably a recent major breakthrough in PARP1-Stroke research is some fruitful

observations from Matsuura and colleagues study. The particular study reported the findings of

the PARP1 inhibitor, MP-124 in cynomolgus and rhesus monkeys, which are subjected to

permanent or transient ischemic stroke models. The study evaluated the efficacy of MP-124, in

cerebral infarcts and neurological deficits. It also divulged different doses, timings of

administration and potential gender differences. MP-124 conspicuously reduced cerebral infarct

volumes and, neurological deficits as examined at 28 h after permanent occlusion in a dose-

dependent manner (at different doses, 0.3, 1 and 3 mg/kg/h intravenous infusion). Maximal

reduction of infarct volume found to be up to 64%. In this study in contrast to several rodent

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studies, the ameliorative effect of MP-124 was observed in female as well as male monkeys.

Besides having attractive therapeutic window PARP1 inhibitor’s activity was also gender

independent. In the study it was also found that treatment with MP-124 at 3 and 6 h post-MCAO

remained effective in significantly ameliorating not only the neurological deficits but also the

infarct volume (Matsuura et al., 2011).

The crucial part of any pharmacological research work is the extension of the proof of

concept of efficacy from rodent models to large animal models, including non-human primates.

The bitter experience of the past with futile clinical trials in patients with stroke made, inevitable

to re-examine the process by which drug candidates are translated to human stroke trials. To

improvise the drugs translation from preclinical studies to human trials, a conference of

academicians and industry representatives known as Stroke Therapy Academic Industry

Roundtable (STAIR) has came up with a certain set of recommendations for preclinical

development of acute ischemic stroke therapies and declared the need for: a) testing in rodent

models, followed by gyrencephalic species; b) defining the therapeutic time window in a well-

characterized model; c) using both permanent and transient occlusion models; d) using blinded,

physiologically-controlled reproducible studies; and e) measuring both histological and

functional outcomes assessed acutely and long-term (Ford and Lee, 2011). In the context of the

STAIR’s recommendations, one can comfortably emphasize that the study by Matsuura and

colleagues seems to satisfy these criteria, and may represent a progressive event in the clinical

translation of PARP1 inhibitors for stroke therapy.

Another ingredient generally needed for translating a new therapy into human trials is

proof that the pathway in question is relevant in human disease. There are several lines of human

evidence implicating PARP1 in stroke. For example, activation of PARP1 was observed in the

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post-mortem brain sections of the patients who have died from stroke (Love et al., 2000;

Sairanen et al., 2009). Another intriguing fact, the highest PARP1 immunoreactivity was seen in

the periinfarct area. Consistent with the results of the preclinical studies, nuclear PARP1 showed

a good correlation with the degree of neuronal necrosis (Sairanen et al., 2009). In the milieu of

the above observations one can confer that, the prospects for the clinical translation of PARP1

inhibitors for the experimental therapy of stroke are particularly strong.

4.2. PARP1 in traumatic brain injury (TBI)

The prevailing preclinical and clinical observations are strongly supporting the translational

capabilities of PARP1 in traumatic brain injury also. DNA breakage was reported after traumatic

brain injury (TBI) in many studies (LaPlaca et al., 1999; Satchell et al., 2003). Subsequently,

peroxynitrite production and PARylation was found to be concentrated within the vicinity of

necrosis in the traumatically injured neuronal tissues (Scott et al., 1999; Besson et al., 2003).

PARP1 activation was found to incept within 30 min after the trauma and its activation persisted

for 3 days (LaPlaca et al., 1999; Besson et al., 2003). Further in a separate study it was found

that, prolonged pattern of PARP1 activation was maintained by unrepaired DNA single-strand

breakage (Satchell et al., 2003).

Likewise in stroke, in some studies of PARP1 deficient brain trauma models of mice also, a

remarkable degree of protection was observed (Whalen et al., 1999). Treatment with PARP1

inhibitors such as 3-aminobenzamide, GPI-6150, INH2BP and INO-1001 was found to be

neuroprotective in rodent traumatic brain injury (Satchell et al., 2003; Besson et al., 2005; Clark

et al., 2007). In these studies, it was also established that the protective effect of PARP1

inhibition on neurological function lasts up to 21 days (Besson et al., 2003; Satchell et al., 2003).

Further in a study, a selective PARP1 inhibitor, PJ34 was found to improve the efficacy of neural

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stem cell transplantation in experimental brain trauma (Lacza et al., 2003). Besides to this in

another study, PJ34 reduced cell death of primary cortical neurons exposed to N-Methyl-N-

Nitro-N-Nitrosoguanidine (MNNG), a powerful inducer of AIF-dependent cell death. Systemic

administration of PJ34 starting as late as 24 h after controlled cortical impact produced

improvement of motor function recovery in mice with TBI (Stoica et al., 2014). In spinal cord

injury also the efficacy of different PARP1 inhibitors was observed (Genovese et al., 2005).

Apart from the attenuation of the early stage neuroinjury, PARP1 inhibition in brain

trauma models also reduced the extent of further neuroinflammatory response (d’Avila et al.,

2012). Increased PARylation of important mitochondrial proteins (mtPARP) TBI was observed

in some other study, which could affect cellular energy stores by both depleting NAD+ and

decreasing ATP production via inhibition of electron transport, which finally can lead to cell

death and dysfunction (Lai et al., 2008). In a recent study supplementation of NAD+ in rodent

TBI model was found to ameliorate the brain damage (Won et al., 2012).

PARP1 activation was also observed in human neurotrauma. Its activity was exhibited in neurons

of pericontusional tissue of patients suffering from severe TBI and an increase in the quantity of

PAR-modified proteins were found in cerebrospinal fluid from children and infants after TBI

(Ang et al., 2003; Fink et al., 2008). Apart from the expression of PARP1 in human TBI, a

number of polymorphisms have been implied to affect outcome after TBI. Sarnaik group has

revealed that after intense TBI in humans, diverse PARP1 polymorphisms are linked with

neurological outcome and indirect method of enzyme activity (Sarnaik et al., 2010). In the

particular study, DNA from 191 adult patients with intense TBI was assayed for four tagging

single nucleotide polymorphisms (tSNPs) related to haplotype blocks spanning the PARP1 gene.

Classification as the favourable or poor outcome was based on Glasgow Outcome Scale score

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assigned at 6 months. The different polymorphisms were correlated with the quantity of PAR-

modified proteins in cerebrospinal fluid (CSF). In a multiple logistic regression analysis

controlling for age, initial Glasgow Coma Scale score, and gender, the AA genotype of SNP

rs3219119, which tags a haplotype block spanning the automodification and catalytic domains of

the PARP 1 gene, was an independent predictor of favourable neurologic outcome. Another SNP

(rs2271347), which tags a haplotype block spanning the automodification and catalytic domains

of the PARP 1 gene, correlated with the amount of PARylated proteins in the cerebrospinal fluid,

but did not correlate with the clinical outcome (Sarnaik et al., 2010).

From the observations of Sarnaik and colleagues study we can deduce that after severe brain

trauma in humans, a PARP1 polymorphism in the automodification-catalytic domain is linked

with neurological outcome, while a polymorphism in the promoter region is linked with

poly(ADP-ribose) modified protein level. The latter may correspond to a genotype and

phenotype relationship between PARP1 polymorphism in the promoter region and enzyme

activity. The study has also found gender differences, which are in line with other preclinical

studies (McCullough et al., 2005; Liu et al., 2011; Siegel and McCullough, 2013). Namely,

males were 2.62 times more likely to have PAR levels above the median than were females after

a comparable degree of brain trauma, indicating that female gender in humans may attenuate the

degree of PARP1 activation (Sarnaik et al., 2010).

On the whole, in the view of the entire data, one can say that the case for the clinical

translation of PARP1 inhibitors for the experimental therapy of neurotrauma is almost as strong

as the case for stroke. Similar to stroke neurotrauma has limited therapeutic options. The

duration of PARP1 inhibitor therapy is expected to be short and PARP1 inhibitor can be given

intravenously. In contrast to stroke the onset of neurotrauma is generally well defined, and the

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patients are hospitalized in a rapid fashion. Altogether neurotrauma presents a potential

indication for the future clinical introduction of PARP1 inhibitors soon.

4.3. Other central nervous system implications of PARP1

The implication of PARP1 in additional indications of central nervous system is supported by

several lines of preclinical data (Iwashita et al., 2004a; Iwashita et al., 2004c; Wang et al., 2007;

Burguillos et al., 2011; Czapski et al., 2013). For easier understanding, we would like to present

these additional central nervous system implications of PARP1 in two headings,

neurodegenerative and neuroinflammatory diseases.

4.3.1. PARP1 in neurodegenerative diseases

To be frank the exploration of PARP1 implication in the other central nervous system diseases,

compared to stroke and traumatic brain injury, was a little bit late. PARP1 activation was

demonstrated in 1-methyl-4-phenyl-1, 2, 4, 5-tetrahydropyridine (MPTP) induced Parkinson’s

disease (PD) model, using both PARP1 inhibitors and PARP1 deficient mice (Cosi and Marien,

1999; Mandir et al., 1999). In separate studies of PARP1 inhibitors, FR255595 and benzamide,

were found to be neuroprotective in MPTP induced neurotoxicity (Iwashita et al., 2004c;

Yokoyama et al., 2010). Another PARP1 inhibitor FR261529 was also found to produce dose

dependant amelioration of methamphetamine induced dopamine depletion and its metabolites

(Iwashita et al., 2004a). Implication of p53 and AIF was extended in a separate study (Mandir et

al., 2002). Further in a study, PARP1 inhibition was found to reduce cytotoxicity in human H4

neuroglioma cells and rat ventral mesencephalic culture, induced by alpha-synuclein and MPP+

(1-methyl-4-phenylpyridinium) respectively (Outeiro et al., 2007). Inhibition of PARP1 via gene

deletion or drug inhibition reversed behavioural deficits and saved against dopamine neuron

death in AIMP2 transgenic mice (Lee et al., 2013). In a similar study, PARP1 gene deletion

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completely blocked 6-hydroxydopamine-induced dopaminergic neurodegeneration and related

PD-like symptoms (Kim et al., 2013).

An immunohistochemical study of substantia nigra from the autopsy material of PD patients

exhibited an increase in the expression of PARP1 (Soós et al., 2004). Similarly, a case control

association study comprising of 146 Parkonson’s patients 161 controls from Northern Spain, had

found that variations in the regulatory region of PARP1 gene might modify the risk for PD

(Infante et al., 2007). Despite the previous evidence, in separate study, which contained an

Italian cohort composed of 600 PD patients and 592 healthy controls, revealed that PARP1 is not

a susceptibility gene for PD in their population, suggesting the further call for the studies with

large sample population (Brighina et al., 2011).

Further, PARP1 activation was also found to exhibit in cadaveric brain sections from

patients who died of Alzheimer’s disease (AD) (Love et al., 2000), implying the vital role of

PARP1 in AD. Also in another study PARP1 deficiency in mice, reduced cognitive and motor

deficits after TBI (Whalen et al., 1999). Additionally, widespread expression of PARP1,

especially in parietal cortex and cerebellum, was observed in cadaveric brain samples of patients

who are died of amyotrophic lateral sclerosis (ALS) (Kim et al., 2004). Further in a latest study,

recruitment of the ALS-associated protein fused in sarcoma (FUS)/translocated in sarcoma

(TLS) to sites of oxidative DNA damage was found to be dependent on PARP1 (Rulten et al.,

2014). From the recent studies it is also evident that, PARP1 is implied in amyloid beta peptide-

induced neuronal damage, suggesting that its inhibition could ameliorate the beta amyloid

induced neuronal damage (Strosznajder et al., 2012; Bayrakdar et al., 2013; Martire et al., 2013).

A case and control study from a Taiwanese population comprising of 120 AD patients and 111

healthy controls, had revealed that PARP1 gene is highly associated with AD susceptibility and

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might contribute to a critical mechanism that mediates cell survival or death as a response to

cytotoxic stress (Liu et al., 2010).

Conversely, PARP1 activity and the PARylation of proteins have been shown to be

essential for changes in synaptic plasticity related to memory stabilization in the mouse

hippocampus (Fontán-Lozano et al., 2010), while PARP1 activation is also required for chronic

neuronal plasticity in mice (Goldberg et al., 2009). Learning provokes an increase in the

PARylation of histone H1 in brain regions relevant for learning and memory such as the

hippocampus (Fontán-Lozano et al., 2010). Changes and modulation of gene expression needed

for the consolidation of objective recognition memory require PARP1 activation in the

hippocampus, and PARP1 is also implied in the changes of gene expression required for long-

lasting synaptic plasticity (Fontán-Lozano et al., 2010). In the similar pattern, a study exploring

expression of PARP1 in transgenic (Tg)-ArcSwe mouse model for human AD, revealed that

PARP1 expression was generally stable across the lifespan of both Tg-ArcSwe and WT mice,

with the exception of a distinctly lower expression in the hippocampus of both mouse types at12

months compared to the other brain regions (Lillenes et al., 2013). All these observations confer

call for further studies in this area of research.

4.3.2. PARP1 in neuro-inflammatory diseases

Multiple sclerosis is the most studied among the neuroinflammatory diseases. Experimental

allergic encephalomyelitis (EAE) is considered as an adequate preclinical model of multiple

sclerosis. Several studies involving murine models of EAE, have demonstrated the protective

effects of PARP1 inhibitors in multiple sclerosis (Chiarugi, 2002; Cavone et al., 2011) and the

protection was eventually corroborated in a primate model in the marmoset. A study suggests

that astrocytes may be particularly important participants in this aspect of multiple sclerosis

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(Kauppinen and Swanson, 2005). In a study comprising of mouse and rat models of meningitis,

PARP1 inhibition produced decrease in inflammatory mediator production, improved survival

and CNS function (Koedel et al., 2002). The results from the PARP1 deficient models of EAE

were found conflicting in nature. In a study PARP1 deficiency actually accelerated the onset and

severity of the disease (Selvaraj et al., 2009), while in some other study PARP1 deficient mice

protected (Farez et al., 2009), suggesting further call for the studies in these models.

Furthermore, PARP1 inhibitors 3-aminobenzamide and DPQ were also found to be protective in

vivo and in vitro epilepsy models, possibly due to the inhibition of apoptosis and activation of

Akt cell survival signalling (Wang et al., 2007; Yang et al., 2013).

5. Conclusion

Intense severity and limited clinical options are making neurological indications more

complicated to handle. However, as the treatment with PARP1 inhibitors showing promising

results for neurological indications in multiple preclinical studies, paving the way to establish

PARP1 as a potential therapeutic target for neurological indications (Kauppinen et al., 2009;

Matsuura et al., 2011; Haddad et al., 2012; Moroni et al., 2012). As the fundamental role of

PARP1 is in DNA-damage repairing, extensive PARP1 inhibition may leave cells with

greater number of DNA anomalies which may increase the risk of genomic instability.

Surviving neurons with DNA damage might be dysfunctional and thus later on undergo

apoptosis. Additionally, the long term PARP1 inhibition might have detrimental effects beyond

the genetic stability. Furthermore, presently available drugs are not extremely specific for

PARP1. To encounter these limitations associated with PARP1 inhibition some more

studies are still required to corroborate the safety of the therapeutic approach and to find

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specific PARP1 inhibitors with limited adverse effects to ensure the safety and efficacy of the

therapy.

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Fig. 1 Schematic diagram showing the structural and functional domains of PARP1.

PARP1 contains four major domains: (a) an amino-terminal DNA binding domain (amino acids

1–372) containing two zinc finger motifs (Zn1, Zn2), a zinc binding domain (Zn3), and a nuclear

localization signal (NLS), (b) an automodification domain (amino acids 372–524) containing a

BRCA1 C-terminus (BRCT) motif, (c) a WGR (Trp–Gly–Arg) motif (amino acids 525–643),

and (d) a carboxyl-terminal catalytic domain (amino acids 653–1014) containing the a-helical

PARP regulatory domain (PRD) and the highly conserved PARP signature motif (Sig), which

defines the PARP family of proteins.

Fig. 2 Diagram representing the homeostasis of ADP-ribosylation.

Poly(ADP-ribose) (PAR) polymers are synthesized by PARP1 using nicotinamide adenine

dinucleotide (NAD+) as substrate with the subsequent release of nicotinamide (NAM). Nuclear

proteins are acceptors of the PAR, which results in their covalent modification. However, the

covalent modification of proteins by the transfer of ADP-ribose residues is only transient due to

the rapid action of group of enzymes, poly(ADP-ribose) glycohydrolase (PARG), ADP-ribosyl

hydrolase3 (ARH3), nucleoside diphosphate linked to another moiety X (NUDIX) and

macrodomain-containing proteins (MDCPs), which catalyzes the hydrolysis of these polymers

into free ADP-ribose (ADPR) units.

Fig. 3 The strength of DNA-damaging stimuli determines the fate of cells: survival, apoptosis, or

necrosis.

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1. In the case of a small degree DNA damage, poly(ADP-ribosylation) facilitates DNA repair

and thus survival. 2. Moderate genotoxic stimulus activates the p53-dependent (or possibly

independent) apoptotic pathway. 3. Most severe DNA damage may cause excessive PARP1

activation, depleting cellular NAD/ATP stores. NAD/ATP depletion blocks apoptosis and results

in necrosis. The inhibition of PARP1 in cells entering cell survival route inhibits repair and thus

diverts cells to apoptotic route (left side broken arrow). The inhibition of PARP in cells entering

necrotic route preserves cellular energy stores and thus enables apoptotic machinery to operate

(dashed arrow on the right).

Fig. 1 Schematic presentation of the mechanisms underlying the protective effects of

PARP1inhibition.

1. PARP1 inhibition could produce protective effects by inhibiting inflammation through its

effects on NFκB and high-mobility group protein box 1 (HMGB1). 2. PARP1 inhibition can

reduce NAD+ depletion, which has been shown to prevent inhibition of glycolysis and

mitochondrial alterations including mitochondrial permeability transition (MPT) and

mitochondrial depolarization. Reducing NAD+ deletion may lead to prevention of SIRT1

inhibition, thus blocking p53 activation and inflammatory responses. 3. PARP1 inhibition could

produce its protective effects by affecting Akt activity. 4. PARP1 inhibition could prevent ADP-

ribose generation and subsequent activation of transient receptor potential melastatin2 (TRPM2)

receptors, leading to prevention of cell death.

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Highlights of the review:

� Short introduction on poly(ADP-ribose)polymerase (PARP1) and its structural

features

� Overview of PARP1 activity

� Succinct outline of various implications of PARP1

� Short account on the development of PARP1 inhibitors

� A colossal narration on the effects various studies of PARP1inhibition in multiple

central nervous system ailments

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