Seizures can give rise to enduring changes that reflect alterations in gene expression patterns, intra and inter cellular signaling and ultimately network alterations that are a hallmark of epilepsy. A growing body of literature suggests that long-term changes in gene transcription associated with epilepsy are mediated via modulation of chromatin structure. One transcription factor in particular, REST (repressor element 1-silencing transcription factor), has received a lot of attention due to the possibility that it may control fundamental transcription patterns that drive circuit excitability, seizures and epilepsy. REST represses a suite of genes in the nervous system by utilizing nuclear protein complexes that were originally identified as mediators of epigenetic inheritance. Epigenetics has traditionally referred to mechanisms that allow a heritable change in gene expression in the absence of DNA mutation. However a more contemporaneous definition acknowledges that many of the mechanisms used to perpetuate epigenetic traits in dividing cells are utilized by neurons to control activity dependent gene expression.
This review will survey what is currently understood about the role of epigenetic mechanisms in epilepsy. We discuss how REST controls gene expression to effect circuit excitability and neurogenesis in epilepsy. We also discuss how the repressor MeCP2 and activator CREB regulate neuronal activity and are themselves controlled by activity. Finally we highlight possible future directions in the field of epigenetics and epilepsy.
REST; RE1; NRSF; epilepsy; epigenetics; chromatin; histone; neurogenesis; MeCP2; CREB; seizure; excitability; G9a; CoREST; SIN3; HDAC
Pilocarpine induces prolonged status epilepticus (SE) in rodents that results in neurodegeneration and cognitive deficits, both commonly observed to be associated with human temporal lobe epilepsy. The multifunctional neuronal modulator, cyclooxygenase-2 (PTGS2 or COX-2), is rapidly induced after SE, mainly in principal neurons of the hippocampal formation and cortex. We used mice in which COX-2 is conditionally ablated in principal forebrain neurons to investigate the involvement of neuron-derived COX-2 in delayed mortality and performance in the Barnes maze.
Using the COX-2 conditional knockout mouse (nCOX-2 cKO) and their littermate wildtype controls, we compared motor behavior and performance in the Barnes maze before and three weeks after the induction of SE by pilocarpine. Mortality rate was also measured during SE and in the week following SE.
nCOX-2 cKO mice showed less delayed mortality than wildtype mice in the week after SE. Although motor behavior and most cognitive measures were not different in the nCOX-2 cKO, upon re-exposure to the maze three weeks after pilocarpine the latency to find the previously-learned target hole was significantly shorter in the nCOX-2 cKO than their wildtype littermate controls. By this measure pilocarpine-treated nCOX-2 cKO mice were identical to mice that had not experienced SE.
Results point to a role for neuronal COX-2 in delayed mortality in mice during the week following SE and suggest that neuronal COX-2 contributes to selected cognitive deficits observed after SE. Taking into consideration our previous findings that neurodegeneration and neuroinflammation after SE are reduced in the nCOX-2 cKO, and opening of the blood-brain barrier after pilocarpine is prevented, we conclude that neuronal COX-2 induction is an early step in many of the deleterious consequences of SE.
Seizure-induced neuronal COX-2; pilocarpine; retrograde memory; anterograde memory; Barnes maze
Functional MRI (fMRI) indirectly measures neural activity by detecting the signal change associated with the hemodynamic response following brain activation. In order to alleviate the temporal and spatial specificity problems associated with fMRI, a number of attempts have been made to detect neural magnetic fields (NMFs) with MRI directly, but have thus far provided conflicting results. In the present study, we used magnetic resonance to detect axonal NMFs in the median giant fiber of the earthworm, Lumbricus terrestris, by examining the free-induction decay (FID) with a sampling interval of 0.32 ms. The earthworm nerve cords were isolated from the vasculature and stimulated at the threshold of action potential generation. FIDs were acquired shortly after the stimulation and simultaneous field potential recordings identified the presence or absence of single evoked action potentials. FIDs acquired when the stimulus did not evoke an action potential were summed as background. The phase of the background-subtracted FID exhibited a systematic change, with a peak phase difference of [-1.2 ± 0.3] ×10-5 radians occurring at a time corresponding to the timing of the action potential. In addition, we calculated the possible changes in the FID magnitude and phase due to a simulated action potential using a volume conductor model. The measured phase difference matched the theoretical prediction well in both amplitude and temporal characteristics. This study provides the first evidence for the direct detection of a magnetic field from an evoked action potential using magnetic resonance.
Förster (fluorescence) resonance energy transfer (FRET) and fluorescence polarization (FP) are widely used technologies for monitoring bimolecular interactions and have been extensively used in high-throughput screening (HTS) for probe and drug discovery. Despite their popularity in HTS, it has been recognized that different assay technologies may generate different hit lists for the same biochemical interaction. Due to the high cost of large-scale HTS campaigns, one has to make a critical choice to employee one assay platform for a particular HTS. Here we report the design and development of a dual-readout HTS assay that combines two assay technologies into one system using the Mcl-1 and Noxa BH3 peptide interaction as a model system. In this system, both FP and FRET signals were simultaneously monitored from one reaction, which is termed “Dual-Readout F2 assay” with F2 for FP and FRET. This dual-readout technology has been optimized in a 1,536-well ultra-HTS format for the discovery of Mcl-1 protein inhibitors and achieved a robust performance. This F2 assay was further validated by screening a library of 102,255 compounds. As two assay platforms are utilized for the same target simultaneously, hit information is enriched without increasing the screening cost. This strategy can be generally extended to other FP-based assays and is expected to enrich primary HTS information and enhance the hit quality of HTS campaigns.
Coronaviruses are a family of enveloped single-stranded positive-sense RNA viruses causing respiratory, enteric, and neurologic diseases in mammals and fowl. Human coronaviruses are recognized to cause up to a third of common colds and are suspected to be involved in enteric and neurologic diseases. Coronavirus replication involves the generation of nested subgenomic mRNAs (sgmRNAs) with a common capped 5′ leader sequence. The translation of most of the sgmRNAs is thought to be cap dependent and displays a requirement for eukaryotic initiation factor 4F (eIF4F), a heterotrimeric complex needed for the recruitment of 40S ribosomes. We recently reported on an ultrahigh-throughput screen to discover compounds that inhibit eIF4F activity by blocking the interaction of two of its subunits (R. Cencic et al., Proc. Natl. Acad. Sci. U. S. A. 108:1046–1051, 2011). Herein we describe a molecule from this screen that prevents the interaction between eIF4E (the cap-binding protein) and eIF4G (a large scaffolding protein), inhibiting cap-dependent translation. This inhibitor significantly decreased human coronavirus 229E (HCoV-229E) replication, reducing the percentage of infected cells and intra- and extracellular infectious virus titers. Our results support the strategy of targeting the eIF4F complex to block coronavirus infection.
Interactions between cholinergic and glutamatergic neurotransmitter systems influence synaptic transmission and plasticity. While previous studies have examined cross-talk between acetylcholine (ACh) and NMDA or AMPA receptors, little is known about the effect of ACh on kainate receptors (KARs). We show that stimulation of m1 or m3 muscarinic ACh receptors (mAChRs) for 2 min potentiates recombinant KAR currents in a long lasting fashion. Muscarinic AChR activation potentiates heteromeric GluK2/GluK4 and GluK2/GluK5 receptors, but not homomeric GluK2 receptors. In hippocampal slices kainate potentiates mossy fiber axon excitability. Transient mAChR activation enhances this action of kainate, suggesting a novel mechanism through which acetylcholine could modulate synaptic transmission in the hippocampus. KAR over-activation has been implicated in excitotoxic cell death. To establish the functional significance of the interaction between mAChRs and KARs we examined the effect of mAChR activation on KAR-mediated excitotoxicity. We find that during pharmacological blockade of NMDA and AMPA receptors, KAR activation with AMPA produces significant cell death in primary cortical culture. Concanavalin A (Con A), which selectively blocks KAR desensitization, markedly increases this KAR-mediated neurotoxicity. Brief activation of mAChRs with pilocarpine significantly enhances KAR-mediated excitotoxicity both in the presence and absence of Con A. We conclude that KARs are modulated in a subunit dependent manner by mAChRs. We suggest that ACh may induce long lasting alterations in neuronal excitability and enhance excitotoxicity in part by potentiating KAR function.
Excitotoxicity; GluK2; GluK5; Hippocampus; Glutamate; Acetylcholine
Enantiomeric propanolamines have been identified as a new class of NR2B-selective NMDA receptor antagonists. The most effective agents are biaryl structures, synthesized in six steps with overall yields ranging from 11–64%. The compounds are potent and selective inhibitors of NR2B-containing recombinant NMDA receptors with IC50 values between 30–100 nM. Potency is strongly controlled by substitution on both rings and the centrally located amine nitrogen. SAR analysis suggests that well-balanced polarity and chain-length factors provide the greatest inhibitory potency. Structural comparisons based on 3D shape analysis and electrostatic complementarity support this conclusion. The antagonists are neuroprotective in both in vitro and in vivo models of ischemic cell death. In addition, some compounds exhibit anticonvulsant properties. Unlike earlier generation NMDA receptor antagonists and some NR2B-selective antagonists, the present series of propanolamines does not cause increased locomotion in rodents. Thus, the NR2B-selective antagonists exhibit a range of therapeutically interesting properties.
Dopamine (DA) neurons in the substantia nigra (SNDA neurons) are among the most severely affected in Parkinson’s disease (PD). Mitochondrial complex I inhibition by rotenone or MPTP can induce SNDA neurodegeneration and recapitulate motor disability in rodents. We performed a transcriptional analysis of the midbrain response to complex I inhibition focused on selected metabolic transcripts using quantitative real-time RT-PCR in conjunction with laser-capture microdissection (LCM) of immunofluorescently-targeted SNDA and ventral tegmental area (VTA) DA neurons. There were DA neuron-selective alterations in metabolic transcripts in response to generalized complex I inhibition dependent on the behavioral response of the animal, and vulnerable SNDA neurons were more dynamic in their metabolic transcriptional response than less vulnerable VTADA neurons. The metabolic transcriptional response of DA neurons may contribute significantly to the ultimate toxicity associated with mitochondrial inhibition, and better understanding of this response may provide insight into potential targets for neuroprotection in PD.
Parkinson’s disease; mitochondria; gene expression; rotenone; MPTP; dopamine; substantia nigra; ventral tegmental area
We have used laser-capture microdissection and microarray hybridization to characterize gene expression in the three principal neuron layers of rat hippocampus. Correlative and clustering analyses revealed all three layers to be easily differentiated from one another based on gene expression profile alone. A greater disparity in gene expression exists between dentate granule and pyramidal cell layers, reflecting phenotypic and ontological differences between those cell populations. Remarkably, the level of more than 45% of expressed transcripts was significantly different among the three neuron populations, with more than a third of those (>1,000 transcripts) being at least 2-fold different between layers. Even CA1 and CA3 pyramidal cell layers were dramatically different on a transcriptional level with a separate analysis indicating that nearly 20% of transcripts are differentially expressed between them. Only a small number of transcripts were specific for a given hippocampal cell layer, suggesting that functional differences are more likely secondary to wide-ranging expression differences of modest magnitude rather than very large disparities in a few genes. Categorical analysis of transcript abundance revealed concerted differences in gene expression among the three cell layers referable to specific cellular pathways. For instance, transcripts encoding proteins involved in glucose metabolism are most highly expressed in the CA3 pyramidal layer, which may reflect an underlying greater metabolic rate of these neurons and partially explain their exquisite vulnerability to seizure-induced damage. Conversely, transcripts related to MAP kinase signaling pathways and transcriptional regulator activity are prominent in the dentate granule cell layer, which could contribute to its resistance to damage following seizure activity by positioning these neurons to respond to external stimuli by altering transcription. Taken together, these data suggest that unique physiological characteristics of major cell layers, such as neuronal activity, neuronal plasticity, and vulnerability to neurodegeneration, are reflected in substantial transcriptional heterogeneity within the hippocampus.
laser capture microdissection; pyramidal neuron; dentate gyrus; granule cell; microarray
Therapeutic block of estrogen action is typically achieved with conventional antagonists (CAs), compounds that displace estradiol from the estrogen receptor (ER) and induce formation of an ER conformation that cannot bind to coactivator proteins, such as the steroid receptor coactivators (SRCs). As an alternative mode for blocking estrogen action, we are seeking small molecules that act as coactivator binding inhibitors (CBIs), i.e., they compete directly with SRC3 for interaction with estradiol-bound ER. CBIs would be interesting mechanistic probes of estrogen action and might also provide an alternative, more durable endocrine therapy for hormone-responsive breast cancer, where cellular adaptations lead to resistance to CAs. We have designed and optimized a set of time-resolved fluorescence resonance energy transfer (TR-FRET) assays to monitor the interaction of ER with SRC3 and ligands, and we have used them in high-throughput screens to discover small molecule CBIs that are able to disrupt this interaction. These assays also distinguish CBIs from CAs. These robust and sensitive “mix and measure” assays use low concentrations of ER labeled with a europium chelate as FRET donor and a Cy5-labeled SRC as acceptor. This multiplexed protocol produces excellent signal to noise ratios (> 100) and Z' values (> 0.8).
The GluR2 subunit determines many of the functional properties of the AMPA subtype of glutamate receptor. The roles of untranslated regions (UTRs) in mRNA stability, transport or translation are increasingly recognized. The 3′end of the GluR2 transcripts are alternatively processed to form a short and long 3′UTR, giving rise to two pools of GluR2 mRNA of 4 and 6 kb in length respectively in the mammalian brain. However, the role of these alternative 3′UTRs in GluR2 expression has not been reported. We demonstrate that in the cytoplasm of rat hippocampus, native GluR2 mRNAs bearing the long 3′UTR are mostly retained in translationally dormant complexes of ribosome-free messenger ribonucleoprotein (mRNP), whereas GluR2 transcripts bearing the short 3′UTR are predominantly associated with actively translating ribosomes. One day after pilocarpine-induced status epilepticus (SE), the levels of both long and short GluR2 transcripts were markedly decreased in rat hippocampus. However, after SE GluR2 mRNAs bearing long 3′UTRs were shifted from untranslating mRNP complexes to ribosome-containing complexes, pointing to a selective translational derepression of GluR2 mRNA mediated by the long 3′UTR. In Xenopus oocytes, expression of firefly luciferase reporters bearing alternative GluR2 3′UTRs confirmed that the long 3′UTR is sufficient to suppress translation. The stability of reporter mRNAs in oocytes was not significantly influenced by alternative 5′ or 3′UTRs of GluR2 over the time period examined. Overall our findings that the long 3′UTR of GluR2 mRNA alone is sufficient to suppress translation, and the evidence for seizure-induced derepression of translation of GluR2 via the long 3′UTR, strongly suggests that a regulatory signaling mechanism exists that differentially targets GluR2 transcripts with alternative 3′UTRs.
The molecular chaperone Hsp90 plays important roles in maintaining the malignant phenotypes. Recent studies suggest that Hsp90 exerts high affinity interactions with multiple oncoproteins, which are essential for the growth of tumor cells. As a result, research has been focused on finding Hsp90 probes as potential and selective anticancer agents. In a high-throughput screening exercise, we identified quinoline 7 as a moderate inhibitor of Hsp90. Further hit identification, SAR studies and biological investigation revealed several synthetic analogs in this series with micromolar activities in both fluorescent polarization (FP) assay and a cell-based western-blot (WB) assay. These compounds represent a new class of Hsp90 inhibitors with simple chemical structures.
Heat Shock Protein 90 (Hsp90); high-throughput screening (HTS); aminoquinoline
Many mechanisms of neurodegeneration have been implicated in Parkinson’s disease, but which ones are most important and potential interactions among them are unclear. To provide a broader perspective on the parkinsonian neurodegenerative process, we have performed a global analysis of gene expression changes caused by chronic, low-level exposure of neuroblastoma cells to the mitochondrial complex I inhibitor and parkinsonian neurotoxin rotenone. Undifferentiated SK-N-MC neuroblastoma cells were grown in the presence of rotenone (5 nM), and RNA was extracted at three different time points (baseline, 1 week, and 4 weeks) for labeling and hybridization to Affymetrix Human U133 Plus 2.0 GeneChips. Our results show that rotenone induces concerted alterations in gene expression that change over time. Particularly, alterations in transcripts related to DNA damage, energy metabolism, and protein metabolism are prominent during chronic complex I inhibition. These data suggest that early augmentation of capacity for energy production in response to mitochondrial inhibition might be deleterious to cellular function and survival. These experiments provide the first transcriptional analysis of a rotenone model of Parkinson’s disease and insight into which mechanisms of neurodegeneration may be targeted for therapeutic intervention.
Parkinson’s disease; rotenone; mitochondria; microarray; oxidative stress; neurodegeneration
Rodents experience hippocampal damage after status epilepticus (SE) mainly in pyramidal cells while sparing the dentate granule cell layer (DGCL). Hippocampal damage was prevented in rats that had been preconditioned by brief seizures on two consecutive days before SE. To identify neuroprotective genes and biochemical pathways changed after preconditioning we compared the effect of preconditioning on gene expression in the CA1 and CA3 pyramidal and DGCLs, harvested by laser capture microscopy. In the DGCL the expression of 632 genes was altered, compared to only 151 and 58 genes in CA1 and CA3 pyramidal cell layers. Most of the differentially expressed genes regulate tissue structure and intra- and extracellular signaling, including neurotransmission. A selective upregulation of energy metabolism transcripts occurred in CA1 pyramidal cells relative to the DGCL. These results reveal a broad transcriptional response of the DGCL to preconditioning, and suggest several mechanisms underlying the neuroprotective effect of preconditioning seizures.
kainate; status epilepticus; microarray; hippocampus; neuroprotection; epilepsy; rat; pilocarpine; CA1; CA3