parkinson 's disease ( pd ) is a common , progressive neurodegenerative disorder , the incidence of which rises with age , with the life - time risk of developing the disease standing at 1.5% ( lees et al . , 2009 ) .
the two classical hallmarks are a progressive loss of dopaminergic neurons from the substantia nigra pars compacta ( snpc ) , and the presence of aggregates of -synuclein , called lewy bodies , that are present in many regions of the central and peripheral nervous systems ( lees et al . , 2009 ) .
the progressive loss of dopaminergic neurons in the snpc leads to the motor features of the disorder which are characterised by bradykinesia , akinesia and a resting tremor .
there are also many non - motor symptoms such as cognitive dysfunction , orthostatic hypotension and gastrointestinal disturbances ( lees et al . , 2009 ) .
though administration of the dopamine precursor drug , levodopa , is a successful symptomatic treatment , its effectiveness wears off over time and levodopa - induced dyskinesias develop with prolonged use .
there is therefore a critical need to develop new drugs and drugs targets to protect dopaminergic neurons and their axons from degeneration in pd .
it has become increasingly recognized that epigenetic disturbances are found in patients with pd , and that these may play a role in parkinsonian pathology .
one of the most intensively studied modes of epigenetic regulation is dna methylation ( figure 1 ) .
this refers to the covalent methylation of residues in cpg dinucleotides within the dna sequence , by enzymes called dna methyltranferases ( dnmts ) , and most commonly results in gene repression by blocking access to dna by transcription factors ( labbe et al . , 2016 ) .
evidence implicating global changes in the methylome is supported by observations of genome - wide changes in dna methylation in brain and blood samples from pd patients ( masliah et al . , 2013 ) .
a distinctive pattern of methylation ( both increased ( hyper ) and decreased ( hypo ) ) was observed in both brain and peripheral blood leukocytes from these patients , involving many genes previously associated with pd ( masliah et al . ,
such findings highlight the fact that methylation patterns have the potential to be employed as epigenetic biomarkers for pd . in support of these findings , a recent study examining methylation at the cellular level using induced pluripotent stem cell ( ipsc)-derived dopaminergic neurons from patients with genetic ( monogenic lrrk2-associated pd ) and sporadic forms of pd showed extensive differences in dna methylation , and subsequent gene expression , in ipsc - derived dopaminergic neurons from these individuals compared to controls ( fernandez - santiago et al . ,
intriguingly , these differences were not seen in parental skin cells , undifferentiated ipscs or ipscs not enriched in dopaminergic neurons ( fernandez - santiago et al . , 2015 ) , suggesting that there may be something unique about the dopaminergic methylome in pd .
for example , administration of methionine ( which increases global levels of dna methylation ) was found to decrease levodopa - induced dyskinesias , whereas administration of rg-108 ( which reduces global levels of dna methylation ) exacerbated levodopa - induced dyskinesias ( figge et al . , 2016 ) .
this highlights the importance of the dna methylome in pd - associated motor function , and raises the potential for pharmacological manipulation of the methylome as a viable therapeutic strategy for pd .
histone acetylation occurs when enzymes called histone acetyltransferases acetylate lysine residues in the n - terminal of histone proteins .
this neutralizes the positive charge on the histone tail , decreasing their interaction with negatively - charged dna .
this results in loosening of the dna around the histones , rendering the dna more accessible to transcription factor binding and enhanced gene transcription .
these densely - methylated islands can directly prevent transcription factor binding and therefore reduce gene expression .
ac : acetyl group ; cpg : cytosine - phosphoric acid - guanine motif ; dnmt : dna methyltranferase ; hat : histone acetyltransferase(s ) ; hdac : histone deacetylase(s ) ; mbp : myelin basic protein .
a second intensively - studied mechanism of epigenetic regulation is post - translational modification of the n - terminal tails of histone proteins , around which dna is normally coiled ( figure 1 ) .
although there are many types of post - translational histone modifications , including methylation , acetylation , phosphorylation , ubiquitination and sumoylation , histone acetylation at lysine residues is particularly important ( labbe et al . , 2016 ) .
histone acetylation is regulated by a balance between histone acetyltransferases ( hats ) and histone deacetylases ( hdacs ) .
hats add acetyl groups to histones , resulting in a less condensed chromatin structure , thereby facilitating transcriptional activation , whereas hdacs remove acetyl groups from histones , exerting the opposite effect ( labbe et al . , 2016 ) .
as dysregulation in histone acetylation has been implicated in the pathogenesis of pd , drugs which alter levels of histone acetylation may have therapeutic potential ( harrison and dexter , 2013 ) .
a recent study has shown that the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium ( mpp ) increases the levels of acetylated histones in experimental models of pd , and that there are increased levels of acetylated histones in the brains of pd patients ( park et al . , 2016 ) .
this suggests that changes in the levels of histone acetylation may either 1 ) play a causative role in the neurodegenerative process in pd , or 2 ) contribute to an endogenous compensatory response in an attempt to counteract the neurodegeneration .
a number of studies have shown that pan- and class - specific hdac inhibitors have neuroprotective effects in cellular models ( harrison and dexter , 2013 ) .
however , it is important to note that not all hdac inhibitors have been shown to be neuroprotective , with a recent study showing that a hdac1/2 inhibitor exacerbates mpp - induced toxicity in a sh - sy5y cell model of pd ( park et al . , 2016 ) .
conversely , another study showed that an alternative hdac1/2 inhibitor was neuroprotective against mpp - induced dopaminergic toxicity both in vitro and in vivo ( choong et al . , 2016 ) .
moreover , it has also been recently shown that other hdac inhibitors can protect other neuronal cell types affected by pd , as it was shown that hdac inhibition protected both dopaminergic and sympathetic neurons from mpp - induced cytotoxicity ( collins et al . , 2015 ) .
these contrasting findings are not easy to reconcile , and may reflect subtle compositional and functional differences in the hdac inhibitor molecules .
furthermore , the precise phenotypic outcomes of hdac inhibitors may also be concentration dependent , as highlighted by our recent work on the p300/cbp hat described below .
the potential of hdac inhibitors for clinical translation is highlighted by an on - going phase i clinical trial of the fda - approved drug glycerol phenylbutyrate ( an hdac inhibitor ) , which is exploring the potential of this drug to increase the removal of -synuclein from the brain ( nct02046434 ) .
in addition to hdac inhibition , an alternative approach to increase histone acetylation is the induction of hat activity using hat activators .
however , there has been limited research into the potential of hat activators as potential drug therapies for pd . to begin to address this potential
, we employed a selective and potent small molecular activator of p300/cbp known as ctpb ( n-(4-chloro-3-trifluoromethyl - phenyl)-2-ethoxy-6-pentadecyl - benzamide ) ( balasubramanyam et al . , 2003 ) .
ctpb is a benzamide that activates p300/cbp hat activity and induces p300/cbp hat - dependent transcriptional activation , but has no effect on p300/cbp - associated factor ( pcaf ) or histone deacetylase activity ( balasubramanyam et al . , 2003 ) . to investigate the neurotrophic potential of ctpb in pd
, we examined the survival- and growth - promoting effects of ctpb in the sh - sy5y neuronal cell line , a widely used model of human dopaminergic and sympathetic neurons ( hegarty et al . , 2016 ) .
we found that ctpb - induced p300/cbp hat activation dose - dependently promoted the survival and neurite growth of sh - sy5y cells , and that ctpb significantly increased histone acetylation in these cells , most likely through induction of p300/cbp hat activity .
moreover , this study found that ctpb was capable of protecting sh - sy5y cells from the cell death induced by the dopaminergic neurotoxin 6-hydroxydopamine ( 6-ohda ) ( hegarty et al . , 2016 ) .
collectively these data suggest that increasing the levels of histone acetylation either through hdac inhibition or hat activation may be neuroprotective .
however in contrast to this , another recent study found that garcinol - mediated inhibition of p300/cbp and pcaf hats protected sh - sy5y cells against mpp+-induced cell death ( park et al . , 2016 ) .
again , it is difficult to rationalize why both activation and inhibition of p300/cbp hats could be neuroprotective in these cellular models of pd , but it could reflect intrinsic differences between ctbp and garcinol , with garcinol ( but not ctpb ) targeting pcaf hat activity . moreover , unpublished observations from our laboratory have shown that garcinol induces cell death in sh - sy5y cells , and that it exacerbates the toxic effects of 6-ohda ( figure 2 ) .
further research is required to determine the basis of these contrasting findings , but again such discrepancies may reflect differences between the concentrations of garcinol used in these studies . a key challenge for future research is how to optimize the delivery of hat - targetting molecules to the brain .
interestingly , carbon nanosphere - conjugated ctpb has the ability to cross the blood - brain barrier , localize to specific nuclei in the brain and induce hyperacetlyation in vivo ( selvi et al . , 2008 ) .
taken together , these studies demonstrate that small molecule - mediated p300/cbp hat activation may be an avenue to explore for neurotrophic effects in pd .
garcinol dose - dependently induces cell death in sh - sy5y cells , and significantly exacerbates the neurotoxic effects of 6-ohda .
( a ) standardised thiazolyl blue tetrazolium bromide ( mtt ) assay of sh - sy5y cells treated daily for 4div with either control or garcinol ( 0.5100 m ) , as indicated ( * * p < 0.01 , * * * p < 0.001 , vs. control ; anova with post - hoc tukey 's test .
standardized mtt assay ( b ) and lactate dehydrogenase ( ldh ) assay ( c ) on sh - sy5y cells treated with 15 m 6-ohda in the presence or absence of 1 m garcinol for 24 hours ( * * p < 0.01 , * * * p < 0.001 , vs. control ; + p < 0.05 , + + + p < 0.001 , vs. 6-ohda alone ; one - way anova with post - hoc tukey 's test ; n = 6 ) .
in summary , small molecule epigenetic modulators , such as those targeting dnmts , hdacs and hats , hold much promise as pharmacological modifiers of the epigenetic status of the cns , especially considering their ability to cross the blood - brain barrier .
these drugs therefore have the ability to act both centrally and peripherally in the nervous system , and have the potential to protect all neurons affected by pd .
however , further research is required to elucidate the precise mechanisms leading to the chronic epigenetic dysregulation observed in neurodegenerative diseases , such as pd .
in addition to this , much more work is needed in translational animal models of pd for rationalizing the use of small molecule epigenetic modulators as a potential neuroprotective therapy for this disorder , including exploring strategies to deliver these drugs to the brain .
as shown for ctpb , carbon nanosphere - conjugation offers a method to deliver such molecules to the brain .
although the beneficial effects of epigenetic modulators , such as hdac inhibitors , are yet to be reported in clinical trials for pd , there is much evidence to support the continued study of molecules that target the epigenome as novel neuroprotective therapies for pd .
 parkinson 's disease ( pd ) is a common , progressive neurodegenerative disease characterised by degeneration of nigrostriatal dopaminergic neurons , aggregation of -synuclein and motor symptoms . 
 current dopamine - replacement strategies provide symptomatic relief , however their effectiveness wear off over time and their prolonged use leads to disabling side - effects in pd patients . 
 there is therefore a critical need to develop new drugs and drug targets to protect dopaminergic neurons and their axons from degeneration in pd . over recent years 
 , there has been robust evidence generated showing that epigenetic dysregulation occurs in pd patients , and that epigenetic modulation is a promising therapeutic approach for pd . 
 this article first discusses the present evidence implicating global , and dopaminergic neuron - specific , alterations in the methylome in pd , and the therapeutic potential of pharmacologically targeting the methylome . 
 it then focuses on another mechanism of epigenetic regulation , histone acetylation , and describes how the histone acetyltransferase ( hat ) and histone deacetylase ( hdac ) enzymes that mediate this process are attractive therapeutic targets for pd . 
 it discusses the use of activators and/or inhibitors of hdacs and hats in models of pd , and how these approaches for the selective modulation of histone acetylation elicit neuroprotective effects . 
 finally , it outlines the potential of employing small molecule epigenetic modulators as neuroprotective therapies for pd , and the future research that will be required to determine and realise this therapeutic potential . 