Epigenetic marking and neuronal plasticity

Epigenetic Regulation of Circadian Clocks and Its Involvement in Drug Addiction

Based on studies describing an increased prevalence of addictive behaviours in several rare sleep disorders and shift workers, a relationship between circadian rhythms and addiction has been hinted for more than a decade. Although circadian rhythm alterations and molecular mechanisms associated with neuropsychiatric conditions are an area of active investigation, success is limited so far, and further investigations are required. Thus, even though compelling evidence connects the circadian clock to addictive behaviour and vice-versa, yet the functional mechanism behind this interaction remains largely unknown. At the molecular level, multiple mechanisms have been proposed to link the circadian timing system to addiction. The molecular mechanism of the circadian clock consists of a transcriptional/translational feedback system, with several regulatory loops, that are also intricately regulated at the epigenetic level. Interestingly, the epigenetic landscape shows profound changes in the addictive brain, with significant alterations in histone modification, DNA methylation, and small regulatory RNAs. The combination of these two observations raises the possibility that epigenetic regulation is a common plot linking the circadian clocks with addiction, though very little evidence has been reported to date. This review provides an elaborate overview of the circadian system and its involvement in addiction, and we hypothesise a possible connection at the epigenetic level that could further link them. Therefore, we think this review may further improve our understanding of the etiology or/and pathology of psychiatric disorders related to drug addiction.

Normalizing JMJD6 Expression in Rat Spinal Dorsal Horn Alleviates Hyperalgesia Following Chronic Constriction Injury

Jumonji domain-containing protein 6 (JMJD6) is a homolog of hypoxia-inducible factor (HIF) asparaginyl hydroxylase, an inhibitor of HIF. HIF-1α is known to participate in neuropathic pain (NPP) during chronic constriction injury (CCI); however, the roles of JMJD6 in NPP have not been systematically investigated. In this study, we examined the temporal distribution and cellular location of JMJD6 in the spinal cord during CCI. In addition, we assessed behavioral changes representative of NPP in rats. Following CCI, lentiviral vectors (LV-JMJD6) were intrathecally administered to observe the changes in the expression of JMJD6, HIF-1α, and its downstream factor caspase-3. Co-immunoprecipitation was used to detect potential interactions between JMJD6 and HIF-1α. We found that JMJD6 was decreased in rats following CCI, which was accompanied by significant NPP–associated behavioral changes. JMJD6 was mainly expressed in neurons. Intrathecal injection of LV-JMJD6 following CCI alleviated the thermal and mechanical hyperalgesia, normalized JMJD6 protein expression, and decreased HIF-1α protein expression with a corresponding reduction in caspase-3 protein expression. Furthermore, the co-immunoprecipitation analyses showed that JMJD6 and HIF-1α protein immunoprecipitated with each other, indicating an interaction between these two proteins. Taken together, the results suggest that JMJD6 may serve as a sensor in neurons of the adult rat spinal cord during the CCI state. Furthermore, JMJD6 may exert its function in NPP by regulating HIF-1α in rats exposed to CCI.

DNA methyl transferase (DNMT) gene polymorphisms could be a primary event in epigenetic susceptibility to schizophrenia

DNA methylation has been implicated in the etiopathology of various complex disorders. DNA methyltransferases are involved in maintaining and establishing new methylation patterns. The aim of the present study was to investigate the inherent genetic variations within DNA methyltransferase genes in predisposing to susceptibility to schizophrenia. We screened for polymorphisms in DNA methyltransferases, DNMT1, DNMT3A, DNMT3B and DNMT3L in 330 schizophrenia patients and 302 healthy controls for association with Schizophrenia in south Indian population. These polymorphisms were also tested for subgroup analysis with patient's gender, age of onset and family history. DNMT1 rs2114724 (genotype P = .004, allele P = 0.022) and rs2228611 (genotype P = 0.004, allele P = 0.022) were found to be significantly associated at genotypic and allelic level with Schizophrenia in South Indian population. DNMT3B rs2424932 genotype (P = 0.023) and allele (P = 0.0063) increased the risk of developing schizophrenia in males but not in females. DNMT3B rs1569686 (genotype P = 0.027, allele P = 0.033) was found to be associated with early onset of schizophrenia and also with family history and early onset (genotype P = 0.009). DNMT3L rs2070565 (genotype P = 0.007, allele P = 0.0026) confers an increased risk of developing schizophrenia at an early age in individuals with family history. In-silico prediction indicated functional relevance of these SNPs in regulating the gene. These observations might be crucial in addressing and understanding the genetic control of methylation level differences from ethnic viewpoint. Functional significance of genotype variations within the DNMTs indeed suggest that the genetic nature of methyltransferases should be considered while addressing epigenetic events mediated by methylation in Schizophrenia.

Total levels of hippocampal histone acetylation predict normal variability in mouse behavior

Background

Genetic, pharmacological, and environmental interventions that alter total levels of histone acetylation in specific brain regions can modulate behaviors and treatment responses. Efforts have been made to identify specific genes that are affected by alterations in total histone acetylation and to propose that such gene specific modulation could explain the effects of total histone acetylation levels on behavior — the implication being that under naturalistic conditions variability in histone acetylation occurs primarily around the promoters of specific genes.

Methods/Results

Here we challenge this hypothesis by demonstrating with a novel flow cytometry based technique that normal variability in open field exploration, a hippocampus-related behavior, was associated with total levels of histone acetylation in the hippocampus but not in other brain regions.

Conclusions

Results suggest that modulation of total levels of histone acetylation may play a role in regulating biological processes. We speculate in the discussion that endogenous regulation of total levels of histone acetylation may be a mechanism through which organisms regulate cellular plasticity. Flow cytometry provides a useful approach to measure total levels of histone acetylation at the single cell level. Relating such information to behavioral measures and treatment responses could inform drug delivery strategies to target histone deacetylase inhibitors and other chromatin modulators to places where they may be of benefit while avoiding areas where correction is not needed and could be harmful.

Pharmacogenetics of mood disorders: what clinicians need to know

Pharmacogenetics brought the promise of matching individuals with treatments that would be efficacious while minimizing adverse events. This has been long needed in psychiatry, where treatment options have been empirical and treatment choices have been made largely based on clinical judgment. The efficacy and tolerability of antidepressants, the most common drugs used in mood disorders, have been widely studied in pharmacogenetics. Genetic association studies have been reported for pharmacokinetic genes such as the CYP450 isoenzymes or MDR1, and pharmacodynamic genes such as the serotonin transporter (SLC6A4) or the serotonin 2A receptor (HTR2A). However, despite the large number of reports, clinically useful predictors are still scarce for antidepressant monotherapy. Pharmacogenetic predictors of efficacy for mood stabilizers such as lithium and anticonvulsants have not had a dissimilar fate, and clinically meaningful markers are yet to emerge. The lack of consistent results may be in part due to small samples of heterogeneous populations and lack of consensus on phenotype definitions. Current pharmacogenetic recommendations include testing for HLA-B*1502 when using carbamazepine in Asian ancestry populations to prevent Stevens–Johnson syndrome, CYP2D6 genotypes when using pimozide, and CYP2D6 in polypharmacy to minimize drug interactions. This review, which is aimed at clinicians, lays the basis for understanding strengths and weaknesses of pharmacogenetic studies and outlines current clinical uses of these biomarkers.

Epigenetic mechanisms and endocannabinoid signalling

The endocannabinoid system, composed of endogenous lipids, their target receptors and metabolic enzymes, has been implicated in multiple biological functions in health and disease, both in the central nervous system and in peripheral organs. Despite the exponential growth of experimental evidence on the key role of endocannabinoid signalling in basic cellular processes, and on its potential exploitation for therapeutic interventions, much remains to be clarified about the respective regulatory mechanisms. Epigenetics refers to a set of post-translational modifications that regulate gene expression without causing variation in DNA sequence, endowed with a major impact on signal transduction pathways. The epigenetic machinery includes DNA methylation, histone modifications, nucleosome positioning and non-coding RNAs. Due to the reversibility of epigenetic changes, an emerging field of interest is the possibility of an 'epigenetic therapy' that could possibly be applied also to endocannabinoids. Here, we review current knowledge of epigenetic regulation of endocannabinoid system components under both physiological and pathological conditions, as well as the epigenetic changes induced by endocannabinoid signalling.

Neuronal plasticity and antidepressant actions

Antidepressant treatments enhance plasticity and increase neurogenesis in the adult brain, but it has been unclear how these effects influence mood. We propose that, like environmental enrichment and exercise, antidepressant treatments enhance adaptability by increasing structural variability within the nervous system at many levels, from proliferating precursors to immature synaptic contacts. Conversely, sensory deprivation and chronic stress reduce this structural variability. Activity-dependent competition within the mood-related circuits, guided by rehabilitation, then selects for the survival and stabilization of those structures that best represent the internal or external milieu. Increased variability together with competition-mediated selection facilitates normal function, such as pattern separation within the dentate gyrus and other mood-related circuits, thereby enhancing adaptability toward novel experiences.

Antidepressants elevate GDNF expression and release from C₆ glioma cells in a β-arrestin1-dependent, CREB interactive pathway

Glial cell line-derived neurotrophic factor (GDNF), essential for neuronal survival, plasticity and development, has been implicated in the mechanism of action of antidepressant drugs (ADs). β-arrestin1, a member of the arrestin protein family, was found to play a role in AD mechanism of action. The present study aimed at evaluating whether the effect of ADs on GDNF in C6 rat glioma cells is exerted through a β-arrestin1-dependent, CREB-interactive pathway. For chronic treatment, C6 rat glioma cells were treated for 3 d with different classes of ADs: imipramine - a non-selective monoamine reuptake inhibitor, citalopram - a serotonin selective reuptake inhibitor (SSRI) or desipramine - a norepinephrine selective reuptake inhibitor (NSRI) and compared to mood stabilizers (lithium and valproic acid) or to the antipsychotic haloperidol. Only ADs significantly elevated β-arrestin1 levels in the cytosol, while reducing phospho-β-arrestin1 levels in the cell nuclear fraction. ADs significantly increased both GDNF expression and release from the cells, but were unable to induce such effects in β-arrestin1 knock-down cells. Chronic AD treatment significantly increased CREB phosphorylation without altering the level of total CREB in the nuclear fraction of the cells. Moreover, treatment with ADs significantly increased β-arrestin1/CREB interaction. These findings support the involvement of β-arrestin1 in the mechanism of action of ADs. We suggest that following AD treatment, β-arrestin1 generates a transcription complex involving CREB essential for GDNF expression and release, thus enhancing GDNF's neuroprotective action that promotes cellular survival and plasticity when the survival and function of neurons is compromised as occurs in major depression.

Genome-wide association studies of antidepressant outcome: a brief review

Genome-wide association studies (GWAS) of antidepressant treatment outcome have been at the forefront of psychiatric pharmacogenetics. Such studies may ultimately help match medications with patients, maximizing efficacy while minimizing adverse effects. The hypothesis-free approach of the GWAS has the advantage of interrogating genes that otherwise would have not been considered as candidates due to our limited understanding of their function, and may also uncover important regulatory variation within the large regions of the genome that do not contain protein-coding genes. Three independent samples have so far been studied using a genome-wide approach: The Sequenced Treatment Alternatives to Relieve Depression sample (STAR*D) (n=1953), the Munich Antidepressant Response Signature (MARS) sample (n=339) and the Genome-based Therapeutic Drugs for Depression (GENDEP) sample (n=706). None of the studies reported results that achieved genome-wide significance, suggesting that larger samples and better outcome measures will be needed. This review discusses the published GWAS studies, their strengths, limitations, and possible future directions.

Transcriptional and epigenetic regulation of opioid receptor genes: present and future

Three opioid receptors (ORs) are known: μ opioid receptors (MORs), δ opioid receptors (DORs), and κ opioid receptors (KORs). Each is encoded by a distinct gene, and the three OR genes share a highly conserved genomic structure and promoter features, including an absence of TATA boxes and sensitivity to extracellular stimuli and epigenetic regulation. However, each of the genes is differentially expressed. Transcriptional regulation engages both basal and regulated transcriptional machineries and employs activating and silencing mechanisms. In retinoic acid-induced neuronal differentiation, the opioid receptor genes undergo drastically different chromatin remodeling processes and display varied patterns of epigenetic marks. Regulation of KOR expression is distinctly complex, and KOR exerts a unique function in neurite extension, indicating that KOR is not simply a pharmacological cousin of MOR and DOR. As the expression of OR proteins is ultimately controlled by extensive posttranscriptional processing, the pharmacological implication of OR gene regulation at the transcriptional level remains to be determined.

Subregion-specific p300 conditional knock-out mice exhibit long-term memory impairments

Histone acetylation plays a critical role during long-term memory formation. Several studies have demonstrated that the histone acetyltransferase (HAT) CBP is required during long-term memory formation, but the involvement of other HAT proteins has not been extensively investigated. The HATs CBP and p300 have at least 400 described interacting proteins including transcription factors known to play a role in long-term memory formation. Thus, CBP and p300 constitute likely candidates for transcriptional coactivators in memory formation. In this study, we took a loss-of-function approach to evaluate the role of p300 in long-term memory formation. We used conditional knock-out mice in which the deletion of p300 is restricted to the postnatal phase and to subregions of the forebrain. We found that p300 is required for the formation of long-term recognition memory and long-term contextual fear memory in the CA1 area of the hippocampus and cortical areas.

Eating disorders, gene-environment interactions and epigenetics

This review describes the various subtypes of eating disorders and examines factors associated with the risk of illness. It considers evidence that the development and maintenance of eating disorders is due to gene-environment interactions (GxE) that alter genetic expression via epigenetic processes. It describes how environmental factors such as those associated with nutrition and/or stress may cause epigenetic changes which have transcriptional and phenotypic effects, which, in turn, alter the long term risk of developing an eating disorder. It reviews theoretical and practical issues associated with epigenetic studies in psychiatry and how these are relevant to eating disorders. It examines the limited number of epigenetic studies which have been conducted in eating disorders and suggests directions for further research. Understanding the relationship between epigenetic processes and the risk of an eating disorder opens possibilities for preventive and/or therapeutic interventions. For example, epigenetic changes associated with diet and weight may be reversible and those associated with cognitive processes may be accessible to pharmacological interventions.

Setdb1 histone methyltransferase regulates mood-related behaviors and expression of the NMDA receptor subunit NR2B

Histone methyltransferases specific for the histone H3-lysine 9 residue, including Setdb1 (Set domain, bifurcated 1)/Eset/Kmt1e are associated with repressive chromatin remodeling and expressed in adult brain, but potential effects on neuronal function and behavior remain unexplored. Here, we report that transgenic mice with increased Setdb1 expression in adult forebrain neurons show antidepressant-like phenotypes in behavioral paradigms for anhedonia, despair, and learned helplessness. Chromatin immunoprecipitation in conjunction with DNA tiling arrays (ChIP-chip) revealed that genomic occupancies of neuronal Setdb1 are limited to <1% of annotated genes, which include the NMDA receptor subunit NR2B/Grin2B and other ionotropic glutamate receptor genes. Chromatin conformation capture and Setdb1-ChIP revealed a loop formation tethering the NR2B/Grin2b promoter to the Setdb1 target site positioned 30 kb downstream of the transcription start site. In hippocampus and ventral striatum, two key structures in the neuronal circuitry regulating mood-related behaviors, Setdb1-mediated repressive histone methylation at NR2B/Grin2b was associated with decreased NR2B expression and EPSP insensitivity to pharmacological blockade of NR2B, and accelerated NMDA receptor desensitization consistent with a shift in NR2A/B subunit ratios. In wild-type mice, systemic treatment with the NR2B antagonist, Ro25-6981 [R-(R,S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol], and hippocampal small interfering RNA-mediated NR2B/Grin2b knockdown resulted in behavioral changes similar to those elicited by the Setdb1 transgene. Together, these findings point to a role for neuronal Setdb1 in the regulation of affective and motivational behaviors through repressive chromatin remodeling at a select set of target genes, resulting in altered NMDA receptor subunit composition and other molecular adaptations.

CDKL5 is a brain MeCP2 target gene regulated by DNA methylation

Rett syndrome and its "early-onset seizure" variant are severe neurodevelopmental disorders associated with mutations within the MECP2 and the CDKL5 genes. Antidepressants and drugs of abuse induce the expression of the epigenetic factor MeCP2, thereby influencing chromatin remodeling. We show that increased MeCP2 levels resulted in the repression of Cdkl5 in rat brain structures in response to cocaine, as well as in cells exposed to serotonin, or overexpressing MeCP2. In contrast, Cdkl5 was induced by siRNA-mediated knockdown of Mecp2 and by DNA-methyltransferase inhibitors, demonstrating its regulation by MeCP2 and by DNA methylation. Cdkl5 gene methylation and its methylation-dependent binding to MeCP2 were increased in the striatum of cocaine-treated rats. Our data demonstrate that Cdkl5 is a MeCP2-repressed target gene providing a link between genes the mutation of which generates overlapping symptoms. They highlight DNA methylation changes as a potential mechanism participating in the long-term plasticity triggered by pharmacological agents.

Protein phosphatase 1 regulates the histone code for long-term memory

Chromatin remodeling through histone posttranslational modifications (PTMs) and DNA methylation has recently been implicated in cognitive functions, but the mechanisms involved in such epigenetic regulation remain poorly understood. Here, we show that protein phosphatase 1 (PP1) is a critical regulator of chromatin remodeling in the mammalian brain that controls histone PTMs and gene transcription associated with long-term memory. Our data show that PP1 is present at the chromatin in brain cells and interacts with enzymes of the epigenetic machinery including HDAC1 (histone deacetylase 1) and histone demethylase JMJD2A (jumonji domain-containing protein 2A). The selective inhibition of the nuclear pool of PP1 in forebrain neurons in transgenic mice is shown to induce several histone PTMs that include not only phosphorylation but also acetylation and methylation. These PTMs are residue-specific and occur at the promoter of genes important for memory formation like CREB (cAMP response element-binding protein) and NF-kappaB (nuclear factor-kappaB). These histone PTMs further co-occur with selective binding of RNA polymerase II and altered gene transcription, and are associated with improved long-term memory for objects and space. Together, these findings reveal a novel mechanism for the epigenetic control of gene transcription and long-term memory in the adult brain that depends on PP1.

[What is migraine?]

Migraine is a multifactorial and heterogeneous disorder. Diagnostic criteria have been established by the International Headache Society, however these are only supportive in terms of definition. The pathophysiology involves neuronal and vascular phenomena. The former is supported by the cortical spreading depression being the aura correlate and by brainstem and hypothalamic activation during the pain phase; the latter is suggested by the association between migraine and cardiovascular disease and findings of pathological vasoreactivity and endothelial dysfunction. Triptans and calcitonin gene-related peptide receptor antagonists show only a relative migraine-specific action; up to 30% of patients are nonresponders. Despite a clear genetic component, the discovery of specific genes for common forms of migraine remains elusive. Electrophysiological studies consistently indicate a characteristic "dyshabituation" concurring with clinical features of altered sensory perception. The age- and sex-specific pattern along with the effect of external factors on the course of migraine argue in favor of the involvement of epigenetic mechanisms. Knowledge about migraine is still limited, which hampers a definition.

Plasticity of defensive behavior and fear in early development

Animals have the ability to respond to threatening situations with sets of defensive behaviors. This review demonstrates that defensive behaviors change during early life in mammals. First, unlearned responses are reorganized during early ontogeny and expressed in an age-specific way. Second, the expression of defensive responses is influenced by early experience prior to the first encounter with a threat. Third, once animals have been exposed to a threatening stimulus they subsequently modify their behavior. The neural bases of defensive behavior and the processes that alter them during development are discussed. Maturation of components and connections of the fear circuit seem to contribute to changes in unlearned fear responses. Early experience and learning modify these developmental processes and shape the expression of defensive behavior. Continuous reorganization of the neural substrate and defensive behavior during ontogeny seems to allow the animal to adjust to the conditions it encounters at a given age in a given environment. It is proposed that the developmental changes in defensive behavior can be conceptualized as phenotypic plasticity.

Repeated risperidone treatment increases the expression of NCAM and PSA-NCAM protein in the rat medial prefrontal cortex

The present study investigates whether the anti-schizophrenic drug risperidone may evoke changes in the expression of NCAM/PSA-NCAM proteins, an indispensable element in the remodeling of synaptic arrangements, in the medial prefrontal cortex (mPFC). Rats were treated with risperidone (0.2 mg/kg, i.p.) either once or repeatedly (once a day, for 21 days). The expression of NCAM and PSA-NCAM proteins was analyzed via western blot and immunohistochemistry at intervals of 3 h and 3, 6, and 9 days after the single or the last risperidone dose. Repeated (but not acute) administration of risperidone was found to increase the expression of NCAM-180, NCAM-140 and PSA-NCAM proteins at 3 or 6 days after treatment. PSA-NCAM immunoreactivity was found in cell bodies, perisomatic-like sites, and in the neuropil of the mPFC. Neither single nor repeated risperidone administration changed the number of PSA-NCAM neurons in the mPFC. In contrast, the repeated risperidone treatment increased the number of PSA-NCAM perisomatic-like sites and the length density of PSA-NCAM positive neuropil at 3 days after the last injection. The data obtained indicate that risperidone, given repeatedly, may promote the remodeling of the structure of presumably GABA-ergic interneurons and that it may evoke the rearrangement of the synaptic contact in the mPFC.

Epigenetic regulation in human brain-focus on histone lysine methylation

Alterations in RNA levels are frequently reported in brain of subjects diagnosed with autism, schizophrenia, depression, and other psychiatric diseases, but it remains unclear whether the underlying molecular pathology involves changes in gene expression, as opposed to alterations in messenger RNA processing. Pre-clinical studies have revealed that stress, drugs, and a variety of other environmental factors lead to changes in RNA levels in brain via epigenetic mechanisms, including modification of histone proteins. A number of site-specific modifications of the nucleosome core histones-including the trimethylated forms of histone H3 lysines K4, K9, and K27-are of particular interest for postmortem research, because these marks differentiate between active and inactive chromatin and seem to remain relatively stable during tissue autolysis. Therefore, histone methylation profiling at promoter regions could provide important clues about mechanisms of gene expression in human brain during development and in disease. Intriguingly, mutations within the genes encoding the H3K9-specific methyltransferase, EHMT1, and the H3K4-specific histone demethylase, JARID1C/SMCX, have been linked to mental retardation and autism, respectively. In addition, the H3K4-specific methyltransferase, MLL1, is essential for hippocampal synaptic plasticity and might be involved in cortical dysfunction of some cases of schizophrenia. Together, these findings emphasize the potential significance of histone lysine methylation for orderly brain development and also as a molecular toolbox to study chromatin function in postmortem tissue.

Ocular HSV-1 latency, reactivation and recurrent disease

Ocular infection with HSV-1 continues to be a serious clinical problem despite the availability of effective antivirals. Primary infection with HSV-1 can involve ocular and adenaxial sites and can manifest as blepharitis, conjunctivitis, or corneal epithelial keratitis. After initial ocular infection, HSV-1 can establish latent infection in the trigeminal ganglia for the lifetime of the host. During latency, the viral genome is retained in the neuron without producing viral proteins. However, abundant transcription occurs at the region encoding the latency-associated transcript, which may play significant roles in the maintenance of latency as well as neuronal reactivation. Many host and viral factors are involved in HSV-1 reactivation from latency. HSV-1 DNA is shed into tears and saliva of most adults, but in most cases this does not result in lesions. Recurrent disease occurs as HSV-1 is carried by anterograde transport to the original site of infection, or any other site innervated by the latently infected ganglia, and can reinfect the ocular tissues. Recurrent corneal disease can lead to corneal scarring, thinning, stromal opacity and neovascularization and, eventually, blindness. In spite of intensive antiviral and anti-inflammatory therapy, a significant percentage of patients do not respond to chemotherapy for herpetic necrotizing stromal keratitis. Therefore, the development of therapies that would reduce asymptomatic viral shedding and lower the risks of recurrent disease and transmission of the virus is key to decreasing the morbidity of ocular herpetic disease. This review will highlight basic HSV-1 virology, and will compare the animal models of latency, reactivation, and recurrent ocular disease to the current clinical data.

Epigenetic targets of HDAC inhibition in neurodegenerative and psychiatric disorders

Epigenetic chromatin remodeling and modifications of DNA represent central mechanisms for regulation of gene expression during brain development and in memory formation. Emerging evidence implicates epigenetic modifications in disorders of synaptic plasticity and cognition. This review focuses on recent findings that HDAC inhibitors can ameliorate deficits in synaptic plasticity, cognition, and stress-related behaviors in a wide range of neurologic and psychiatric disorders including Huntington's disease, Parkinson's disease, anxiety and mood disorders, Rubinstein-Taybi syndrome, and Rett syndrome. These agents may prove useful in the clinic for the treatment of the cognitive impairments that are central elements of many neurodevelopmental, neurological, and psychiatric disorders.

Drugs and their molecular targets: an updated overview

About 330 targets bind approved drugs, 270 encoded by the human genome and 60 belonging to pathogenic organisms. A large number of druggable targets have been recently proposed from preclinical and first clinical data, but a huge reservoir of putative drug targets, possibly several thousands, remains to be explored. This overview considers the different types of ligands and their selectivity in the main superfamilies of drug targets, enzymes, membrane transporters and ion channels, and the various classes of membrane and nuclear receptors with their signalling pathway. Recently approved drugs such as monoclonal antibodies, tyrosine kinase and proteasome inhibitors, and major drugs under clinical studies are reviewed with their molecular target and therapeutic interest. The druggability of emerging targets is discussed, such as multidrug resistance transporters and cystic fibrosis transmembrane conductance regulator (CFTR), hyperpolarization-activated cyclic nucleotides-gated (HCN), cyclic nucleotide-gated (CNG) and transient receptor potential (TRP) ion channels, tumour necrosis factor (TNF) and receptor activator of NFkappaB (RANK) receptors, integrins, and orphan or recently deorphanized G-protein-coupled and nuclear receptors. Large advances have been made in the therapeutical use of recombinant cytokines and growth factors (i.e. tasonermin, TNFalpha-1a; becaplermin, platelet-derived growth factor (PDGF); dibotermin-alpha, bone morphogenetic proteins (BMP)2; anakinra, interleukin-1 receptor antagonist protein (IRAP), and in enzyme replacement therapy, i.e. algasidase (alpha-galactosidase) and laronidase (alpha-l-iduronidase). New receptor classes are emerging, e.g. membrane aminopeptidases, and novel concepts are stimulating drug research, e.g. epigenetic therapy, but the molecular target of some approved drugs, such as paracetamol and imidazolines, still need to be identified.