Feature Review

Molecular Psychiatry (2012) 17, 584–596; doi:10.1038/mp.2011.107; published online 6 September 2011

Epigenetic regulation of the BDNF gene: implications for psychiatric disorders

F Boulle1,2, D L A van den Hove1,3, S B Jakob3, B P Rutten1, M Hamon2, J van Os4, K-P Lesch1,3, L Lanfumey2, H W Steinbusch1 and G Kenis1

  1. 1Department of Psychiatry and Neuropsychology, Maastricht University, European Graduate School for Neuroscience (EURON), Maastricht, The Netherlands
  2. 2Center for Psychiatry and Neuroscience, INSERM U894, University Pierre and Marie Curie, Paris, France
  3. 3Department of Psychiatry, Psychosomatics and Psychotherapy, University of Wurzburg, Wurzburg, Germany
  4. 4Department of Psychosis Studies, King's College London, King's Health Partner, Institute of Psychiatry, London, UK

Correspondence: Dr DLA van den Hove, Department of Psychiatry and Neuropsychology, Maastricht University, School for Mental Health and Neuroscience (MHENS), Universiteitsingel 50, 6200 MD Maastricht, The Netherlands. E-mail: d.vandenhove@maastrichtuniversity.nl

Received 20 January 2011; Revised 18 July 2011; Accepted 1 August 2011
Advance online publication 6 September 2011

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Abstract

Abnormal brain-derived neurotrophic factor (BDNF) signaling seems to have a central role in the course and development of various neurological and psychiatric disorders. In addition, positive effects of psychotropic drugs are known to activate BDNF-mediated signaling. Although the BDNF gene has been associated with several diseases, molecular mechanisms other than functional genetic variations can impact on the regulation of BDNF gene expression and lead to disturbed BDNF signaling and associated pathology. Thus, epigenetic modifications, representing key mechanisms by which environmental factors induce enduring changes in gene expression, are suspected to participate in the onset of various psychiatric disorders. More specifically, various environmental factors, particularly when occurring during development, have been claimed to produce long-lasting epigenetic changes at the BDNF gene, thereby affecting availability and function of the BDNF protein. Such stabile imprints on the BDNF gene might explain, at least in part, the delayed efficacy of treatments as well as the high degree of relapses observed in psychiatric disorders. Moreover, BDNF gene has a complex structure displaying differential exon regulation and usage, suggesting a subcellular- and brain region-specific distribution. As such, developing drugs that modify epigenetic regulation at specific BDNF exons represents a promising strategy for the treatment of psychiatric disorders. Here, we present an overview of the current literature on epigenetic modifications at the BDNF locus in psychiatric disorders and related animal models.

Keywords:

BDNF gene; environmental factors; epigenetic; psychiatric disorders; treatments

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Introduction

Brain-derived neurotrophic factor (BDNF) is a small secreted protein that is a member of the neurotrophin family of growth factors.1 Its action depends on two distinct receptors differing in their respective downstream signaling pathways. The p75 neurotrophin receptor is activated by all neurotrophins and, on activation, has various effects, which are dependent on, for example, the ligand bound to it and the type of cell it is expressed in Refs. 2 and 3. Further, its action depends on the presence of the tropomyosin-related kinase receptor B (TrkB), the other receptor for BDNF, and neurotrophin-4/5.4 BDNF-dependent activation of TrkB is essential for the proper development of the vertebrate nervous system and impairment of its associated signaling pathways is likely to be at the origin of many neurological and psychiatric disorders.5, 6, 7 Activation and subsequent phosphorylation of the different tyrosine residues in the catalytic domain of the TrkB receptor has been associated with at least three intracellular signaling cascades, which are widely interconnected.8 Briefly, the mitogen-activated protein kinase pathway is primarily implicated in neuronal differentiation and neurite outgrowth, the phosphoinositide-3 kinase pathway mainly enables cell survival, and the phospholipase-gamma pathway is particularly involved in synaptic plasticity and facilitates neurotransmission.9, 10, 11, 12 As such, BDNF has an important role in normal neural development.13 BDNF is highly expressed in limbic structures and cerebral cortex, and is important for long-term potentiation and neurogenesis, making this trophic factor a key player in learning and memory as well as in reward-related processes.14, 15, 16 BDNF is meticulously regulated in the brain and modifications of neuronal BDNF expression or release are thought to induce abnormal functioning of some brain areas.17, 18 Recent interest has been directed to the epigenetic regulation of BDNF that mediates the effects of environmental factors on the BDNF gene resulting in enduring changes of its expression.19 Epigenetics refers to processes, such as DNA methylation, histone acetylation or nucleosome sliding, which are dynamic events controlling the expression of genes without affecting the DNA sequence (for reviews, see (refs. 20 and 21).

This review gives an overview of recent findings on epigenetic mechanisms associated with BDNF gene regulation in psychiatric disorders and animal models. The origin of such modifications, that is, risk factors such as neurodevelopmental stress exposure, in these pathologies is not under focus here because it has already been the matter of an extensive recent review elsewhere.19 Our objective here is to provide an overview of new insights into the potential role of the epigenetic regulation of BDNF in the pathophysiology of these disorders. Further, we will discuss some pharmacological perspectives, focusing on drugs that can modify BDNF gene expression by affecting epigenetic regulation.

Regulation of BDNF gene expression

The BDNF gene
 

BDNF has a complex gene structure, which has been documented and revisited extensively.22, 23, 24, 25, 26, 27, 28 A number of studies have presented substantial similarities in rodents and humans (Figure 1). Briefly, the human BDNF gene consists of several untranslated 5′ exons with independent promoters. These can be connected to a 3′ coding exon to form a bipartite or tripartite transcript providing different splice variants of BDNF mRNA.24 The 3′ coding exon (exon IX) contains the sequence that codes for the pro-BDNF protein. In addition, the 3′ untranslated region of exon IX is composed of two alternative polyadenylation (polyA) sites, so as to generate one short splice variant and one long splice variant.22 The use of distinct BDNF mRNA splice variants differing either by the 5′ or 3′ extremity allows for temporal and spatial regulation of BDNF expression, which appears to be critical in the modulation of synaptic plasticity and spine development in dendrites.29, 30, 31 This temporo-spatial regulation depends largely on the various promoters of the BDNF gene, which are differentially targeted in response to diverse stimuli and signaling events.32, 33, 34 In addition to regulation at the promoter level, BDNF gene expression is also controlled at the post-transcriptional level. In particular, all Bdnf mRNAs are translated into pro-BDNF and are further cleaved into mature BDNF by several mechanisms.35 The balance between pro-BDNF and mature BDNF levels in the synaptic cleft is controlled by tissue plasminogen activator and is essential for neuronal plasticity.36 Although pro-BDNF binds specifically to p75 and primarily promotes cell death and long-term depression, mature BDNF binds more readily to TrkB, particularly enabling long-term potentiation and cell survival.37, 38 Therefore, the balance between pro- and mature BDNF on the one hand, and between the p75 and TrkB receptors on the other, is of critical importance in determining the functional characteristics of the BDNF signal.39 Moreover, a common single-nucleotide polymorphism within the BDNF gene causes a valine (Val) to methionine (Met) substitution at codon 66 of the prodomain. This single-nucleotide polymorphism is thought to alter BDNF mRNA and protein trafficking, and has been widely implicated in psychiatric disorders.40, 41, 42, 43

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Structure of the human and rodent BDNF gene. Exons are represented as boxes and the introns as lines. Numbers of the exons are indicated in roman numerals and the size of exons and introns is indicated in Arabic numerals. The 3′ coding exon (exon IX) contains two polyadenylation sites (poly A). The red boxes represent the start codon ATG that marks the initiation of transcription. The green box shows the region of exon IX coding for the pro-BDNF protein, including the rs6265 genetic variant implicated in the Val66Met polymorphism. Some exons, like exon II and IX, contain different transcript variants with alternative splice-donor sites (A, B, C, D). CpG islands were predicted with Methprimer software and determined as sequences of at least 200 pairs of bases with a GC percentage greater than 50%. Adapted from Aid et al.23 and Pruunsild et al.24

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Epigenetic control of BDNF gene expression
 

Various epigenetic mechanisms have been associated with repression or activation of the rodent Bdnf gene (Figure 2). For example, the methyl-CpG binding protein 2 (MeCP2) is known for its repression of Bdnf gene transcription.47, 48 MeCP2 binds selectively to methylated DNA at the rat's Bdnf promoter IV, where it is associated with the co-repressor molecules Sin3a and histone deacetylase 1 (HDAC1) to form a complex that maintains the repressed state of the Bdnf gene.49 It has been shown that increased Bdnf transcription after membrane depolarization in cultured neurons correlates with the phosphorylation and dissociation of MeCP2 from Bdnf promoter IV.44, 49 Other proteins such as the growth arrest and DNA-damage-inducible protein b (Gadd45b) have been shown to be required for activity-induced DNA demethylation at Bdnf promoter IX, which is associated with increased hippocampal neurogenesis in mice.45 In addition to changes in DNA-methylation, post-translational modifications of histones at distinct amino acid residues on their amino-terminal tails have been reported at the Bdnf gene. Some of these covalent modifications have been shown to modulate Bdnf gene expression. For example, methylation at lysine (K) 27 on histone H3 (H3K27) is usually associated with transcriptional repression, whereas acetylation on histone H3 and H4 is associated with transcriptional activation.46 Rodent studies have revealed that some promoters are preferentially targeted by epigenetic regulation. This is notably the case for promoter IV, that contains a specific binding site for the cyclic-AMP-responsive element-binding protein (CREB).32, 50, 51 CREB is known to have a specific binding domain for CREB-binding protein, which has a central role in the regulation of gene activity and influences chromatin remodeling because of its histone acetyl-transferase properties.52

Figure 2.
Figure 2 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Epigenetic mechanism associated with repression and activation of Bdnf exon IV transcription. The BDNF exon IV displays 12 distinct CpG sites, which can be methylated and interact selectively with MeCP2 to form complexes that repress gene transcription. Histone methyltransferases (HMTs) are responsible for adding methyl groups at histone tails (H3K27), whereas histone deacetylases (HDACs) remove acetylation at histone tails, both processes that repress gene transcription. Moreover, low levels of nicotinamine adenine dinucleotide (NAD) promote DNA methylation at the BDNF locus.53 Conversely, Bdnf gene activation is associated with increased histone H3 and H4 acetylation, which is mediated by histone acetyl transferase (HAT) activity. Derepression of BDNF exon IV can be observed after membrane depolarization or NMDA receptor activation, both inducing the dissociation of MeCP2 from methylated DNA.33, 44 In addition, NMDA receptor stimulation has also been shown to reduce the HDAC1 occupancy of BDNF promoter IV.33 DNA demethylation might be facilitated by growth arrest and DNA damage proteins such as Gadd45b.45 An increased binding of CREB to its specific binding protein, CREB binding protein (CBP), is also associated with an increase in BDNF gene transcription. Membrane depolarization has been shown to increase this CBP-mediated regulation via calcium/calmodulin-dependent protein kinase (CaMK) signaling.46 Hypothetically, TrkB, whose signaling is known to activate the CaMK pathway, might induce CREB phosphorylation and subsequent binding to CPB, thereby increasing BDNF expression. See text for more details.

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BDNF, epigenetics and psychiatric disorders

A prevailing current theory posits that multiple genetic and environmental factors contribute to the development of most neurological and psychiatric disorders.53, 54 In particular, diverse environmental stressors have been shown to modulate BDNF availability and function in rodents.55, 56, 57 Importantly, stress-induced changes in Bdnf mRNA expression have been shown to depend on the timing, type, duration and frequency of the stressor.58 Indeed, some risk factors may cause persisting changes in BDNF gene regulation, underlying a lasting epigenetic imprint on the genome, thereby increasing susceptibility to psychopathology. In this section, the epigenetic regulation of BDNF in psychiatric disorders and related phenotypes will be discussed.

Epigenetic regulation of BDNF in learning and memory
 

BDNF has been shown to mediate synaptic plasticity, known to be critically important in learning and memory processes,59 in particular for the consolidation and extinction of fear memory.60, 61, 62 It has been shown that the BDNF val66met polymorphism affects memory and extinction learning in rodents and humans, which may have its importance in anxiety-related behavior.41, 42, 63 Recent data about BDNF protein levels led to the suggestion that this trophic factor is required for consolidation and extinction of fear memory in the prelimbic cortex as well as in the amygdala in rodents.64, 65 Expression of Bdnf exons I and III was significantly upregulated in the amygdala of fear-conditioned rats, whereas extinction of conditioned fear in the prefrontal cortex (PFC) was accompanied by a significant increase in Bdnf exons I and IV.66, 67 Interestingly, Bdnf exon IV upregulation was associated with hyperacetylation of histone H4 near its concurrent promoter, suggesting a relationship between long-lasting extinction learning and histone modifications at Bdnf gene promoters in the PFC.67 Another study has shown that consolidation of fear learning was associated with a transcriptional upregulation of Bdnf exon IV, whereas context exposure alone leads to increased levels of Bdnf exons I and VI.68 These data suggest differential exon usage in the hippocampus in response to distinct cognitive tasks such as learning a novel environment versus associating an emotion with that same environment. Interestingly, increased levels of Bdnf exon IV-containing transcripts correlated with the decrease in DNA methylation at the corresponding promoter, which was directly linked to an overall increase in total Bdnf mRNA (exon IX) in the hippocampus during fear memory consolidation.68 Moreover, there is increasing evidence that DNA methylation at the Bdnf gene represents a crucial mechanism that regulates associated changes in hippocampal synaptic plasticity.69 Regarding recognition memory, it has been shown that performance in the novel object recognition task correlates with increased Bdnf expression and methylation state at the Bdnf promoter I in the hippocampus of mice.70 Nevertheless, although epigenetic regulation of the Bdnf gene seems to have a crucial role in the dynamic process of learning, memory and related pathological disorders, compelling evidence from human studies to support findings observed in animal models is lacking.

Epigenetic regulation of BDNF in depression
 

Some studies on human patients suffering from major depression have shown reduced hippocampal volumes71 and decreased circulating levels of BDNF, which normalize after chronic antidepressant treatment.72, 73 Post-mortem analyses have shown decreased BDNF levels in the brains of suicide victims and depressed patients.74, 75 Regarding rodent models of depression, mounting evidence indicates that BDNF is implicated in stress-induced hippocampal alterations.76, 77 Reduced levels of BDNF have been shown to contribute to impaired neuronal differentiation in the dentate gyrus and depression-related behavior in rodents.78 In contrast, infusion of BDNF into the hippocampus has antidepressant properties in rodent models of depression.79 In addition, antidepressants such as selective serotonin reuptake inhibitors are known to activate BDNF-mediated signaling and reverse neuronal atrophy and cell loss induced by chronic stress in rodents, whereas Bdnf +/− mice showed an impaired response to antidepressants.80, 81 Altogether, these findings have spurred researchers to further investigate the involvement of chromatin remodeling at the BDNF gene in depression and antidepressant action. Indeed, in chronic social defeat stress, a rodent model of depression, levels of Bdnf mRNA IV and V were decreased in the hippocampus of stressed mice.82 Interestingly, this robust decrease in expression was associated with long-lasting H3K27 hypermethylation at the corresponding promoters. A similar study, using perinatal exposure to methylmercury in mice as a model for depressive-like behavior has demonstrated that this developmental exposure was associated with long-lasting changes at Bdnf promoter IV in the hippocampus.83 Onishchenko et al.83 showed that the observed decrease in hippocampal Bdnf mRNA levels was mediated by H3K27 hypermethylation and H3 hypoacetylation at this specific promoter.

Taken together, these studies underscore the importance of BDNF gene expression in the pathophysiology of depression and in the mechanisms of antidepressant treatments. Further, animal models of stress and depression induce specific epigenetic changes at the Bdnf gene within the hippocampus, possibly contributing to the onset of associated behavioral disorders. However, the role of epigenetic modifications of the BDNF gene in depressed patients has yet to be investigated.

Epigenetic regulation of BDNF in addiction
 

Both BDNF and the mesolimbic dopamine system are known to have a central role in addiction.84, 85, 86 In the mesolimbic system, dopaminergic neurons in the ventral tegmental area project to the nucleus accumbens (NAc). In this respect, BDNF has been shown to mediate long-term neuronal adaptation in pathological conditions by controlling dopamine receptor expression, thereby inducing behavioral sensitization.87 Changes in Bdnf mRNA and protein have been examined in multiple brain regions such as the ventral tegmental area and NAc following administration and/or withdrawal of many addictive compounds.88, 89, 90 A recent study found that acute cocaine administration induces a transient increase in BDNF protein levels and activates TrkB-mediated signaling in the NAc of rats.91 Furthermore, BDNF infusions in the NAc are known to increase cocaine self-administration as well as relapse to cocaine seeking in withdrawal.91 These investigators suggest that BDNF release during cocaine use is important for the development and persistence of addictive behavior. In the ventral tegmental area, an increase in BDNF induces a transition to an opiate-dependent motivational state in naïve rats.92 Previous findings showed that changes in BDNF levels following cocaine administration are persistent,93 suggesting that epigenetic modifications could be implicated in drug-induced modifications in gene expression. However, only few studies have focused on epigenetic regulation at the BDNF gene in relation to addictive disorders. A recent study has revealed that chronic administration of cocaine increases H3 acetylation at Bdnf promoter II in the rat striatum, whereas no significant changes have been observed in H4 acetylation at the same promoter.94 Interestingly, this hyperacetylation significantly increased after 1 week of withdrawal from cocaine. However, the increase in H3 acetylation did not immediately correlate with an increase in Bdnf mRNA and protein levels, which progressively increased during cocaine withdrawal.93 Similar results have shown that chronic administration of cocaine in rats caused a robust induction of H3, but not H4, acetylation at Bdnf promoters II and III in the NAc shell, which was reverted by HDAC4 overexpression in this same region.99 Moreover, cocaine self-administration increased acetylation of histone H3 and reduced MeCP2 occupancy at Bdnf promoter IV in the PFC of rats, leading to decreased levels of Bdnf exon IV mRNA.100 Another study has focused on the effects of cocaine abstinence on the endocrine and molecular response to stress, reporting that stress exposure resulted in an increase in Bdnf mRNA in the NAc only in mice subjected to prolonged cocaine abstinence.101 The increase in Bdnf mRNA was associated with H3 hyperacetylation at BDNF promoter I. Regarding human findings, it has been shown that prenatal exposure to maternal smoking is associated with higher rates of DNA methylation at BDNF promoter VI in white blood cells of adolescents, but only in those that are homozygous (Table 1) for the Val allele of the Val66Met polymorphism.98 Interestingly, adolescents whose mothers smoked during pregnancy also showed an increase in drug experimentation.98 Altogether, these studies support the notion that drug-induced plasticity may be partially caused by persistent epigenetically regulated changes in BDNF expression.


Epigenetic regulation of BDNF in schizophrenia.
 

Human studies have revealed that the val66met polymorphism within the BDNF gene may be associated with schizophrenia and may be one of the major factors affecting brain volume reductions associated with this disorder.102 Furthermore, analysis of human post-mortem brains has shown that BDNF mRNA as well as BDNF protein synthesis and availability are significantly reduced in layer III, V and VI of the dorsolateral PFC of schizophrenic patients.103 In addition, it has been shown that levels of BDNF were significantly reduced in the serum of schizophrenic patients.104, 105 However, these findings should be interpreted with caution because of the heterogeneity across the different studies and the complex etiology of the schizophrenic spectrum. Further, the TrkB receptor is widely expressed in γ-aminobutyric acid (GABA) neurons, and impairment of BDNF/TrkB signaling might contribute to a change in GABAergic interneuron functioning observed in schizophrenic patients.106 GABAergic neurons can be divided in different sub-populations including the parvalbumin-positive cells, which exert a powerful negative control on pyramidal cells, by high-frequency firing and fast-spiking action potentials.107 A recent study has demonstrated that mice mutant for Bdnf promoter IV show a significant deficit in GABAergic signaling by interneurons in the PFC, particularly those expressing parvalbumin.108 These data suggest that decreased levels of BDNF in the brain may relate to alterations in the GABAergic system within the PFC of schizophrenic patients. Although that BDNF has a crucial role in many neurodevelopmental processes and that schizophrenia is strongly linked to aberrant neurodevelopment,109 only few studies have addressed epigenetic changes at BDNF gene promoters induced by developmental perturbations.110, 111 Whether epigenetic mechanisms are responsible of the abnormal levels of BDNF observed in post-mortem brains of schizophrenic patients remains unclear. A recent observation has suggested a link between the BDNF Val66Met polymorphism and abnormal DNA methylation levels at CpG islands at the BDNF locus in the frontal cortex of schizophrenic patients.97 More specifically, it was shown that there was a decrease in DNA methylation at exonic CpG islands of the BDNF gene in the frontal cortex of individuals carrying the Met allele. All in all, these data support the notion that genetic variations in BDNF are associated with a distinct epigenetic regulation of BDNF gene expression, which may be important in the pathophysiology of schizophrenia.

Pharmaceutical drugs targeting BDNF gene expression

Since aberrant BDNF signaling might have a critical role in the pathophysiology and/or treatment of various psychiatric disorders, directly targeting epigenetic modifications at the BDNF gene promoters might represent a promising strategy to enduringly reverse abnormal BDNF expression and engender long-lasting clinical benefits. In the next section, we will present recent findings demonstrating that common psychotropic drugs (such as antidepressants and mood stabilizers) interfere with epigenetic processes at the BDNF gene, and discuss the potential use of other emerging drugs.

Antidepressants
 

Chronic defeat stress is commonly used to model depressive-like behavior in rodents. Whereas chronic treatment with antidepressants is able to restore a normal phenotype in socially defeated animals,82, 112 chronic imipramine administration could not reverse the enduring H3K27 hypermethylation at Bdnf promoters IV and V induced by chronic social defeat in mice. Interestingly, it did reverse the repression of Bdnf expression at promoter IV and V by inducing H3-K9 and H3-K14 acetylation resulting in an activation of Bdnf gene transcription.82 Additional experiments further supported the notion that imipramine exerts its therapeutic effects by increasing BDNF levels, at least partly through downregulation of HDAC5.82 Another study showed that chronic treatment with fluoxetine did not alter the methylation state of H3K27 induced by perinatal exposure to methylmercury in mice, but significantly upregulated H3 acetylation at Bdnf promoter IV83 (Table 2). Moreover, electroconvulsive shock therapy, still the most effective treatment for refractory depressed patients,120 is thought to be mediated by enduring changes in gene expression, including that of neurotrophic factors like BDNF. Acute electroconvulsive shock therapy results in increased H4 acetylation at Bdnf promoter II and correlates with increases in total Bdnf mRNA in rats.121 On the other hand, chronic electroconvulsive shock therapy in healthy rats has been shown to increase H3 acetylation at Bdnf promoters IV and V and to result in an upregulation of the corresponding Bdnf mRNA.121 These data suggest that antidepressant drugs as well as other antidepressant therapies reverse environmentally induced reductions in BDNF expression through changes in covalent modifications at histones tails.


HDAC inhibitors
 

HDACs, which are classified in four different classes and generally associated with transcriptional repression,122 represent interesting therapeutic targets for a wide range of human disorders.123, 124 Many HDAC inhibitors are currently being tested in clinical trials and the Food and Drug Administration has already approved some of them for the treatment of specific types of cancer.125 A possible role for HDAC inhibitors in treating certain mental disorders has emerged from the observation that valproic acid, a commonly used antiepileptic and mood stabilizing drug, is a nonspecific inhibitor of class I and II HDACs.126 Interestingly, valproic acid has been shown to modulate the expression of BDNF protein and mRNA.127 Bdnf exon IV mRNA expression was increased within the PFC of mice treated with valproic acid during a fear extinction paradigm, the effect of which was mediated by H4 hyperacetylation at the corresponding gene promoter.67 Another interesting study has demonstrated that the valproic acid-induced increase in Bdnf expression depends on a novel responsive region in the neighborhood of promoter IV.127 In addition, other HDAC inhibitors such as sodium butyrate and trichostatin A have been shown to induce an upregulation of Bdnf gene transcription in both cortical neuronal and astrocyte cultures, although the exact mechanisms involved have not been elucidated yet.127, 128

DNA methylation modulators
 

Several proteins belonging to the DNA methyltransferase (DNMT) family are implicated in regulating the maintenance methylation and de novo methylation at CpG dinucleotides.21 Many clinical trials with DNMT inhibitors such as the nucleoside analogue 5-aza-cytidine and its deoxy analogue 5-aza-deoxycytidine have been investigated in the treatment of hematological malignancies.129 Further, 5-aza-deoxycytidine has been shown to reduce the degree of methylation at Bdnf promoter I in mouse neuroblastoma cells.130 Additionally, other DNMT inhibitors such as zebularine have been shown to induce a decrease in DNA methylation at the Bdnf gene.68, 110 Infusion of zebularine in the hippocampus of mice significantly increased the levels of Bdnf exons I, IV and VI mRNA. In that study, demethylation at Bdnf promoters I, IV and VI was associated with enhanced consolidation of conditioned fear memories.68 Zebularine has been shown to decrease DNA methylation at Bdnf promoter IV in the PFC of rats subjected to early-life adversity.110 Apart from targeting DNMTs, inhibition of MeCP2, which selectively binds to BDNF promoter IV to enhance DNA methylation,44 could represent an interesting target to increase Bdnf expression (Table 3).


Preclinical and clinical perspectives

There is growing evidence that BDNF has a central role in psychiatric disorders owing to its action on neuronal and synaptic plasticity.7 Various environmental factors, particularly when exposure occurs during early development, have been shown to produce disturbances in BDNF availability and function, thus increasing vulnerability to mental illness. As outlined in this review, long-lasting changes in epigenetic processes such as DNA methylation and histone modifications, thereby regulating BDNF gene expression, seem to have a crucial role in this respect.

Relevance to psychiatric neuroscience
 

The proper regulation of BDNF expression is critical for key regulatory processes related to, for example, learning, memory and reward. In addition, animal studies have revealed a major role for abnormal BDNF signaling in the pathophysiology of many psychiatric phenotypes. While memory formation and drugs of abuse are able to increase levels of Bdnf mRNA and protein in various brain areas, chronic stress exposure and associated negative emotional responses are generally linked to decreased levels of BDNF mainly in the hippocampus and PFC. In various animal models, different long-lasting epigenetic changes at the Bdnf gene are associated with abnormal Bdnf mRNA and protein levels.

While most studies have focused on chromatin remodeling, particularly involving histone tail modifications, relatively few studies have assessed DNA methylation changes. Interestingly, aberrant DNA methylation seems to be critically involved in developmental programming of adult health and disease. In fact, DNA methylation footprints are relatively stable and may even be transmitted to the next generation, suggesting an important role for BDNF in the transgenerational inheritance of emotional traits.110 Regarding human studies, it has been shown that DNA methylation at the BDNF gene was indeed increased in the Wernicke area of suicide patients as well as in the white blood cells of healthy adolescents prenatally exposed to maternal smoking.96, 98 On the other hand, decreased DNA methylation levels at the BDNF gene were observed in the PFC of schizophrenic patients carrying the Met allele as compared with patients homozygous for the Val allele.97

Whereas both aberrant DNA methylation and histone modifications at the BDNF gene have been implicated in the pathophysiology of psychiatric disorders and related phenotypes, it is yet to be determined whether these epigenetic changes are the cause or the consequence of the associated pathology. The fact that epidemiologically relevant environmental risk factors for psychiatric disorders (for example, chronic stress in major depression) modulate BDNF levels in animal models through epigenetic modifications, and that pharmacological interventions (for example, antidepressants) are able to restore BDNF levels via changes in the epigenome, suggest that epigenetic changes at the BDNF gene are rather causal in the pathology than merely being an epiphenomenon. Along similar lines, it is tempting to speculate that the effects of repetitive administration of drugs of abuse that modulate BDNF levels, thereby enduringly modifying the neuronal network and changing local plasticity in reward circuits, are mediated by epigenetic modifications.

Differential BDNF transcript regulation
 

BDNF expression is tightly regulated at the level of transcription with differential exon usage, resulting in the specific involvement of the various transcripts in different functions and related processes. Among other things, it has been reported that long and short Bdnf transcripts are differentially engaged in protein synthesis.29 Furthermore, the use of distinct Bdnf mRNA splice variants differing in either their 5′ or 3′ extremity allows a temporal and spatial regulation of BDNF expression. For example, the various BDNF transcripts display a tissue-specific expression pattern within the human body, as well as a region-specific distribution within the central nervous system.24 The expression of BDNF protein at local sites of action depends on restricted regulation of Bdnf mRNA trafficking, which is controlled by the 5′ non-coding exon, defining the distinct subcellular target areas of the different transcripts.134 In rodent primary neuronal cultures, it has been shown that Bdnf exon IV was restricted to the soma, whereas Bdnf exon II and VII were predominantly observed in dendrites, and Bdnf exon I was distributed in both soma and dendrites.40, 135 It is suggested that transcripts localized in the dendrites are most likely to participate in local synthesis of BDNF protein and modulate morphological and biochemical changes related to synaptic plasticity.136, 137 Similarly, Bdnf transcripts primarily expressed in the soma would be particularly involved in processes related to the synthesis of neurotransmitters or hormones.138 Bdnf exon II seems to be particularly involved in mediating synaptic plasticity within the reward circuitry, as well as in the molecular and cellular changes underlying drug abuse in the striatum.94, 99 Furthermore, Bdnf exon VI is likely to be involved in cognitive processes related to learning and memory primarily in the hippocampus and PFC of rodents.139 Bdnf exon IV, the promoter of which contains specific binding sites for CREB and MeCP2, making it a preferential epigenetic target, displays a ubiquitous repartition in the brain and seems to have a key role in many processes related to mood, emotion, reward, learning and memory. Evidently, the exact role of the various BDNF transcripts and the complex regulation of BDNF mRNA and protein processing await further research.

In relation to the differential functions of the various BDNF transcripts, modulation of the expression of specific BDNF exons in a targeted fashion may represent a promising strategy to restore enduring changes in gene expression induced by, for example, repeated environmental insults. In particular, HDAC inhibitors and DNMT inhibitors represent potentially powerful agents for the treatment of psychiatric disorders through regulation of, for example, BDNF gene expression. Conversely, these molecular agents are expected to exert their effects on a substantial proportion of the genome, thereby most likely inducing numerous negative side effects. More specifically, inhibitors of DNA methylation have been suggested to promote cancer metastasis, lupus or autoimmune diseases.125 In view of the region-specific distribution of BDNF transcripts in the brain and their functionally distinct roles, developing compounds that modulate the transcription of specific BDNF exons will provide considerable advantages and possibly avoid undesirable side effects.

Perspectives in psychiatric research
 

Another promising approach may be to directly target TrkB signaling, whose abnormal regulation may underlie lasting epigenetic changes influencing the transcription of many genes important in brain functioning. Directly targeting TrkB has the capacity to modulate the functional balance between p75 and TrkB signaling so as to avoid the possible negative effects mediated by the pro-apoptotic p75 receptor. Recent findings have described new specific TrkB agonists and antagonists.140, 141, 142, 143, 144 Chronic administration of such agents might induce, through epigenetic modifications, lasting changes in the expression of TrkB target genes. Finding molecules that target specific brain regions and/or genetic loci, thus limiting their impact on nonspecific adverse epigenetic processes in these areas, is a major challenge in psychiatric drug discovery.

In order to gain more insight into the exact role of epigenetic regulation at the BDNF locus in psychiatric disorders, future studies should focus on human post-mortem brain material to assess whether epigenetic modifications in the human brain are comparable to those observed in rodent models. A recent study has suggested that DNA methylation patterns at gene promoters are preserved up to a post-mortem delay of at least 48h, enabling researchers to reliably study DNA methylation in human post-mortem brains.145 Moreover, further studies should address the regulation of BDNF gene expression in blood cells and assess whether and, if so, to what extent, changes observed within peripheral blood mononuclear cells reflect those as observed within the brains of patients suffering from psychiatric disorders. This would indicate whether or not peripheral blood mononuclear cells represent a suitable surrogate tissue to examine epigenetic processes relevant to brain disorders.146, 147 Adverse environmental exposures occurring during the various stages of brain development are known risk factors for developing psychopathology, including affective disorders and schizophrenia.109, 148 To further elucidate the role of BDNF in this respect, the impact of such environmental factors on epigenetic changes at the Bdnf locus should be examined in animal models, both in relevant brain regions and peripheral blood cells, while simultaneously assessing possible associations between epigenetic states and intermediate behavioral phenotypes. Monitoring such changes in humans might be helpful to identify individuals at risk for pathology and/or predict transition from subclinical symptoms to disease. Finally, it is also important to understand the impact of the BDNF val66met polymorphism on DNA methylation and/or histone modifications at the BDNF gene. If it is confirmed that this genetic variation indeed can influence DNA methylation at CpG islands within the human BDNF promoter region,63 this would add to the evidence of the BDNF gene being a vulnerability substrate for psychiatric disorders, thus implicating a novel–epigenetic–mechanism in the effect of the polymorphism.

In conclusion, enduring epigenetic changes at the BDNF gene seem to underlie various neurobiological and behavioral phenotypes in animal models of psychiatric disorders. Recent evidence suggests that such epigenetic modifications are also present in human patients. A better understanding of the exon-specific regulation of the BDNF gene and associated epigenetic remodeling in psychiatric disorders and related phenotypes might open new therapeutic perspectives for these disorders.

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Conflict of interest

The authors declare no conflict of interest.

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References

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