Single nuclei transcriptomics in human and non-human primate striatum in opioid use disorder.

In brain, the striatum is a heterogenous region involved in reward and goal-directed behaviors. Striatal dysfunction is linked to psychiatric disorders, including opioid use disorder (OUD). Striatal subregions are divided based on neuroanatomy, each with unique roles in OUD. In OUD, the dorsal striatum is involved in altered reward processing, formation of habits, and development of negative affect during withdrawal. Using single nuclei RNA-sequencing, we identified both canonical (e.g., dopamine receptor subtype) and less abundant cell populations (e.g., interneurons) in human dorsal striatum. Pathways related to neurodegeneration, interferon response, and DNA damage were significantly enriched in striatal neurons of individuals with OUD. DNA damage markers were also elevated in striatal neurons of opioid-exposed rhesus macaques. Sex-specific molecular differences in glial cell subtypes associated with chronic stress were found in OUD, particularly female individuals. Together, we describe different cell types in human dorsal striatum and identify cell type-specific alterations in OUD.

Selective Neuronal Knockout of STAT3 Function Inhibits Epilepsy Progression, Improves Cognition, and Restores Dysregulated Gene Networks in a Temporal Lobe Epilepsy Model.

OBJECTIVE: Temporal lobe epilepsy (TLE) is a progressive disorder mediated by pathological changes in molecular cascades and hippocampal neural circuit remodeling that results in spontaneous seizures and cognitive dysfunction. Targeting these cascades may provide disease-modifying treatments for TLE patients. Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) inhibitors have emerged as potential disease-modifying therapies; a more detailed understanding of JAK/STAT participation in epileptogenic responses is required, however, to increase the therapeutic efficacy and reduce adverse effects associated with global inhibition. METHODS: We developed a mouse line in which tamoxifen treatment conditionally abolishes STAT3 signaling from forebrain excitatory neurons (nSTAT3KO). Seizure frequency (continuous in vivo electroencephalography) and memory (contextual fear conditioning and motor learning) were analyzed in wild-type and nSTAT3KO mice after intrahippocampal kainate (IHKA) injection as a model of TLE. Hippocampal RNA was obtained 24 h after IHKA and subjected to deep sequencing. RESULTS: Selective STAT3 knock-out in excitatory neurons reduced seizure progression and hippocampal memory deficits without reducing the extent of cell death or mossy fiber sprouting induced by IHKA injection. Gene expression was rescued in major networks associated with response to brain injury, neuronal plasticity, and learning and memory. We also provide the first evidence that neuronal STAT3 may directly influence brain inflammation. INTERPRETATION: Inhibiting neuronal STAT3 signaling improved outcomes in an animal model of TLE, prevented progression of seizures and cognitive co-morbidities while rescuing pathogenic changes in gene expression of major networks associated with epileptogenesis. Specifically targeting neuronal STAT3 may be an effective disease-modifying strategy for TLE. ANN NEUROL 2023;94:106-122.

Regulation of Inhibitory Signaling at the Receptor and Cellular Level; Advances in Our Understanding of GABAergic Neurotransmission and the Mechanisms by Which It Is Disrupted in Epilepsy.

Inhibitory signaling in the brain organizes the neural circuits that orchestrate how living creatures interact with the world around them and how they build representations of objects and ideas. Without tight control at multiple points of cellular engagement, the brain's inhibitory systems would run down and the ability to extract meaningful information from excitatory events would be lost leaving behind a system vulnerable to seizures and to cognitive decline. In this review, we will cover many of the salient features that have emerged regarding the dynamic regulation of inhibitory signaling seen through the lens of cell biology with an emphasis on the major building blocks, the ligand-gated ion channel receptors that are the first transduction point when the neurotransmitter GABA is released into the synapse. Epilepsy association will be used to indicate importance of key proteins and their pathways to brain function and to introduce novel areas for therapeutic intervention.

Evidence for a non-canonical JAK/STAT signaling pathway in the synthesis of the brain's major ion channels and neurotransmitter receptors.

BACKGROUND: Brain-derived neurotrophic factor (BDNF) is a major signaling molecule that the brain uses to control a vast network of intracellular cascades fundamental to properties of learning and memory, and cognition. While much is known about BDNF signaling in the healthy nervous system where it controls the mitogen activated protein kinase (MAPK) and cyclic-AMP pathways, less is known about its role in multiple brain disorders where it contributes to the dysregulated neuroplasticity seen in epilepsy and traumatic brain injury (TBI). We previously found that neurons respond to prolonged BDNF exposure (both in vivo (in models of epilepsy and TBI) and in vitro (in BDNF treated primary neuronal cultures)) by activating the Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) signaling pathway. This pathway is best known for its association with inflammatory cytokines in non-neuronal cells. RESULTS: Here, using deep RNA-sequencing of neurons exposed to BDNF in the presence and absence of well characterized JAK/STAT inhibitors, and without non-neuronal cells, we determine the BDNF transcriptome that is specifically regulated by agents that inhibit JAK/STAT signaling. Surprisingly, the BDNF-induced JAK/STAT transcriptome contains ion channels and neurotransmitter receptors coming from all the major classes expressed in the brain, along with key modulators of synaptic plasticity, neurogenesis, and axonal remodeling. Analysis of this dataset has revealed a unique non-canonical mechanism of JAK/STATs in neurons as differential gene expression mediated by STAT3 is not solely dependent upon phosphorylation at residue 705 and may involve a BDNF-induced interaction of STAT3 with Heterochromatin Protein 1 alpha (HP1α). CONCLUSIONS: These findings suggest that the neuronal BDNF-induced JAK/STAT pathway involves more than STAT3 phosphorylation at 705, providing the first evidence for a non-canonical mechanism that may involve HP1α. Our analysis reveals that JAK/STAT signaling regulates many of the genes associated with epilepsy syndromes where BDNF levels are markedly elevated. Uncovering the mechanism of this novel form of BDNF signaling in the brain may provide a new direction for epilepsy therapeutics and open a window into the complex mechanisms of STAT3 transcriptional regulation in neurological disease.

Nuclear Respiratory Factor 1 (NRF-1) Controls the Activity Dependent Transcription of the GABA-A Receptor Beta 1 Subunit Gene in Neurons.

While the exact role of ß1 subunit-containing GABA-A receptors (GABARs) in brain function is not well understood, altered expression of the ß1 subunit gene (GABRB1) is associated with neurological and neuropsychiatric disorders. In particular, down-regulation of ß1 subunit levels is observed in brains of patients with epilepsy, autism, bipolar disorder and schizophrenia. A pathophysiological feature of these disease states is imbalance in energy metabolism and mitochondrial dysfunction. The transcription factor, nuclear respiratory factor 1 (NRF-1), has been shown to be a key mediator of genes involved in oxidative phosphorylation and mitochondrial biogenesis. Using a variety of molecular approaches (including mobility shift, promoter/reporter assays, and overexpression of dominant negative NRF-1), we now report that NRF-1 regulates transcription of GABRB1 and that its core promoter contains a conserved canonical NRF-1 element responsible for sequence specific binding and transcriptional activation. Our identification of GABRB1 as a new target for NRF-1 in neurons suggests that genes coding for inhibitory neurotransmission may be coupled to cellular metabolism. This is especially meaningful as binding of NRF-1 to its element is sensitive to the kind of epigenetic changes that occur in multiple disorders associated with altered brain inhibition.

Changes in neuronal immunofluorescence in the C- versus N-terminal domains of hnRNP H following D1 dopamine receptor activation.

RNA binding proteins are a diverse class of proteins that regulate all aspects of RNA metabolism. Accumulating studies indicate that heterogeneous nuclear ribonucleoproteins are associated with cellular adaptations in response to drugs of abuse. We recently mapped and validated heterogeneous nuclear ribonucleoprotein H1 (Hnrnph1) as a quantitative trait gene underlying differential behavioral sensitivity to methamphetamine. The molecular mechanisms by which hnRNP H1 alters methamphetamine behaviors are unknown but could involve pre- and/or post-synaptic changes in protein localization and function. Methamphetamine initiates post-synaptic D1 dopamine receptor signaling indirectly by binding to pre-synaptic dopamine transporters and vesicular monoamine transporters of midbrain dopaminergic neurons which triggers reverse transport and accumulation of dopamine at the synapse. Here, we examined changes in neuronal localization of hnRNP H in primary rat cortical neurons that express dopamine receptors that can be modulated by the D1 or D2 dopamine receptor agonists SKF38393 and (-)-Quinpirole HCl, respectively. Basal immunostaining of hnRNP H was localized primarily to the nucleus. D1 dopamine receptor activation induced an increase in hnRNP H nuclear immunostaining as detected by immunocytochemistry with a C-domain directed antibody containing epitope near the glycine-rich domain but not with an N-domain specific antibody. Although there was no change in hnRNP H protein in the nucleus or cytoplasm, there was a decrease in Hnrnph1 transcript following D1 receptor stimulation. Taken together, these results suggest that D1 receptor activation increases availability of the hnRNP H C-terminal epitope, which could potentially reflect changes in protein-protein interactions. Thus, D1 receptor signaling could represent a key molecular post-synaptic event linking Hnrnph1 polymorphisms to drug-induced behavior.

Rapid Increases in proBDNF after Pilocarpine-Induced Status Epilepticus in Mice Are Associated with Reduced proBDNF Cleavage Machinery.

Brain-derived neurotrophic factor (BDNF) levels are elevated after status epilepticus (SE), leading to activation of multiple signaling pathways, including the janus kinase/signal transducer and activator of transcription pathway that mediates a decrease in GABAA receptor α1 subunits in the hippocampus (Lund et al., 2008). While BDNF can signal via its pro or mature form, the relative contribution of these forms to signaling after SE is not fully known. In the current study, we investigate changes in proBDNF levels acutely after SE in C57BL/6J mice. In contrast to previous reports (Unsain et al., 2008; Volosin et al., 2008; VonDran et al., 2014), our studies found that levels of proBDNF in the hippocampus are markedly elevated as early as 3 h after SE onset and remain elevated for 7 d. Immunohistochemistry studies indicate that seizure-induced BDNF localizes to all hippocampal subfields, predominantly in principal neurons and also in astrocytes. Analysis of the proteolytic machinery that cleaves proBDNF to produce mature BDNF demonstrates that acutely after SE there is a decrease in tissue plasminogen activator and an increase in plasminogen activator inhibitor-1 (PAI-1), an inhibitor of extracellular and intracellular cleavage, which normalizes over the first week after SE. In vitro treatment of hippocampal slices from animals 24 h after SE with a PAI-1 inhibitor reduces proBDNF levels. These findings suggest that rapid proBDNF increases following SE are due in part to reduced cleavage, and that proBDNF may be part of the initial neurotrophin response driving intracellular signaling during the acute phase of epileptogenesis.

Brief Dark Exposure Reduces Tonic Inhibition in Visual Cortex.

Tonic inhibition mediated by extrasynaptic GABA(A) receptors (GABARs) sensing ambient levels of GABA can profoundly alter the membrane input resistance to affect cellular excitability. Therefore, regulation of tonic inhibition is an attractive mechanism to control the levels of cortical firing. In cortical pyramidal cells, tonic inhibition is regulated by age and several neurotransmitters and is affected by stroke and epilepsy. However, the possible role of sensory experience has not been examined. Here, we report that a brief 2-day exposure to dark reduces by 1/3 the inhibitory tonic conductance recorded in layer II/III pyramidal cells of the mouse juvenile (postnatal day 12-27) visual cortex. In these cells, tonic inhibition is carried primarily by GABARs containing the δ subunit. Consistently, the dark exposure reduction in conductance was associated with a reduction in δ subunit levels, which were not affected in control frontal cortex. We propose that a deprivation-induced reduction in tonic inhibition might serve a homeostatic function by increasing the firing levels of cells in deprived cortical circuits.

JAK/STAT pathway regulation of GABAA receptor expression after differing severities of experimental TBI.

Synaptic inhibition in the adult brain is primarily mediated by the γ-aminobutyric acid (GABA) type A receptor (GABA(A)R). The distribution, properties, and dynamics of these receptors are largely determined by their subunit composition. Alteration of subunit composition after a traumatic brain injury (TBI) may result in abnormal increased synaptic firing and possibly contribute to injury-related pathology. Several studies have shown that the Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) signaling pathway can alter GABA(A)R subunit expression. The present study investigated changes in JAK/STAT pathway activation after two different severities of experimental TBI in the mouse using the controlled cortical impact (CCI) model. It also investigated whether modulating the activation of the JAK/STAT pathway after severe controlled cortical impact (CCI-S) with a JAK/STAT inhibitor (WP1066) alters post-traumatic epilepsy development and/or neurological recovery after injury. Our results demonstrated differential changes in both the activation of STAT3 and the expression of the GABA(A)R α1 and γ2 subunit levels that were dependent on the severity of the injury. The change in the GABA(A)R α1 subunit levels appeared to be at least partly transcriptionally mediated. We were able to selectively reverse the decrease in GABA(A)R α1 protein levels with WP1066 treatment after CCI injury. WP1066 treatment also improved the degree of recovery of vestibular motor function after injury. These findings suggest that the magnitude of JAK/STAT pathway activation and GABA(A)R α1 subunit level decrease is dependent on injury severity in this mouse model of TBI. In addition, reducing JAK/STAT pathway activation after severe experimental TBI reverses the decrease in the GABA(A)R α1 protein levels and improves vestibular motor recovery.

Effect of spontaneous seizures on GABAA receptor α4 subunit expression in an animal model of temporal lobe epilepsy.

OBJECTIVE: Temporal lobe epilepsy (TLE) is frequently medically intractable and often progressive. Compromised inhibitory neurotransmission due to altered γ-aminobutyric acid (GABA)A receptor α4 subunit (GABAA Rα4) expression has been emphasized as a potential contributor to the initial development of epilepsy following a brain insult (primary epileptogenesis), but the regulation of GABAA Rα4 during chronic epilepsy, specifically, how expression is altered following spontaneous seizures, is less well understood. METHODS: Continuous video-electroencephalography (EEG) recordings from rats with pilocarpine-induced TLE were used to capture epileptic animals within 3 h of a spontaneous seizure (SS), or >24 h after the last SS, to determine whether recent occurrence of a seizure was associated with altered levels of GABAA Rα4 expression. We further evaluated whether this GABAA Rα4 plasticity is regulated by signaling mechanisms active in primary epileptogenesis, specifically, increases in brain-derived neurotrophic factor (BDNF) and early growth response factor 3 (Egr3). RESULTS: Elevated levels of GABAA Rα4 messenger RNA (mRNA) and protein were observed following spontaneous seizures, and were associated with higher levels of BDNF and Egr3 mRNA. SIGNIFICANCE: These data suggest that spontaneous, recurrent seizures that define chronic epilepsy may influence changes in GABAA Rα4 expression, and that signaling pathways known to regulate GABAA Rα4 expression after status epilepticus may also be activated after spontaneous seizures in chronically epileptic animals.

A role for picomolar concentrations of pregnenolone sulfate in synaptic activity-dependent Ca2+ signaling and CREB activation.

Fast excitatory synaptic transmission that is contingent upon N-methyl d-aspartate receptor (NMDAR) function contributes to core information flow in the central nervous system and to the plasticity of neural circuits that underlie cognition. Hypoactivity of excitatory NMDAR-mediated neurotransmission is hypothesized to underlie the pathophysiology of schizophrenia, including the associated cognitive deficits. The neurosteroid pregnenolone (PREG) and its metabolites pregnenolone sulfate (PregS) and allopregnanolone in serum are inversely associated with cognitive improvements after oral PREG therapy, raising the possibility that brain neurosteroid levels may be modulated therapeutically. PregS is derived from PREG, the precursor of all neurosteroids, via a single sulfation step and is present at low nanomolar concentrations in the central nervous system. PregS, but not PREG, augments long-term potentiation and cognitive performance in animal models of learning and memory. In this report, we communicate the first observation that PregS, but not PREG, is a potent (EC50 ∼2 pM) enhancer of intracellular Ca(2+) that is contingent upon neuronal activity, NMDAR-mediated synaptic activity, and L-type Ca(2+) channel activity. Low picomolar PregS similarly activates cAMP response element-binding protein (CREB) phosphorylation (within 10 minutes), an essential memory molecule, via an extracellular-signal-regulated kinase/mitogen-activated protein kinase signal transduction pathway. Taken together, the results are consistent with a novel biologic role for the neurosteroid PregS that acts at picomolar concentrations to intensify the intracellular response to glutamatergic signaling at synaptic but not extrasynaptic, NMDARs by differentially augmenting CREB activation. This provides a genomic signal transduction mechanism by which PregS could participate in memory consolidation of relevance to cognitive function.

Polycomblike protein PHF1b: a transcriptional sensor for GABA receptor activity.

BACKGROUND: The γ-aminobutyric acid (GABA) type A receptor (GABA(A)R) contains the recognition sites for a variety of agents used in the treatment of brain disorders, including anxiety and epilepsy. A better understanding of how receptor expression is regulated in individual neurons may provide novel opportunities for therapeutic intervention. Towards this goal we have studied transcription of a GABA(A)R subunit gene (GABRB1) whose activity is autologously regulated by GABA via a 10 base pair initiator-like element (ß(1)-INR). METHODS: By screening a human cDNA brain library with a yeast one-hybrid assay, the Polycomblike (PCL) gene product PHD finger protein transcript b (PHF1b) was identified as a ß(1)-INR associated protein. Promoter/reporter assays in primary rat cortical cells demonstrate that PHF1b is an activator at GABRB1, and chromatin immunoprecipitation assays reveal that presence of PHF1 at endogenous Gabrb1 is regulated by GABA(A)R activation. RESULTS: PCL is a member of the Polycomb group required for correct spatial expression of homeotic genes in Drosophila. We now show that PHF1b recognition of ß(1)-INR is dependent on a plant homeodomain, an adjacent helix-loop-helix, and short glycine rich motif. In neurons, it co-immunoprecipitates with SUZ12, a key component of the Polycomb Repressive Complex 2 (PRC2) that regulates a number of important cellular processes, including gene silencing via histone H3 lysine 27 trimethylation (H3K27me3). CONCLUSIONS: The observation that chronic exposure to GABA reduces PHF1 binding and H3K27 monomethylation, which is associated with transcriptional activation, strongly suggests that PHF1b may be a molecular transducer of GABA(A)R function and thus GABA-mediated neurotransmission in the central nervous system.

The neuroactive steroid pregnenolone sulfate stimulates trafficking of functional N-methyl D-aspartate receptors to the cell surface via a noncanonical, G protein, and Ca2+-dependent mechanism.

N-methyl D-aspartate (NMDA) receptors (NMDARs) mediate fast excitatory synaptic transmission and play a critical role in synaptic plasticity associated with learning and memory. NMDAR hypoactivity has been implicated in the pathophysiology of schizophrenia, and clinical studies have revealed reduced negative symptoms of schizophrenia with a dose of pregnenolone that elevates serum levels of the neuroactive steroid pregnenolone sulfate (PregS). This report describes a novel process of delayed-onset potentiation whereby PregS approximately doubles the cell's response to NMDA via a mechanism that is pharmacologically and kinetically distinct from rapid positive allosteric modulation by PregS. The number of functional cell-surface NMDARs in cortical neurons increases 60-100% within 10 minutes of exposure to PregS, as shown by surface biotinylation and affinity purification. Delayed-onset potentiation is reversible and selective for expressed receptors containing the NMDAR subunit subtype 2A (NR2A) or NR2B, but not the NR2C or NR2D, subunits. Moreover, substitution of NR2B J/K helices and M4 domain with the corresponding region of NR2D ablates rapid allosteric potentiation of the NMDA response by PregS but not delayed-onset potentiation. This demonstrates that the initial phase of rapid positive allosteric modulation is not a first step in NMDAR upregulation. Delayed-onset potentiation by PregS occurs via a noncanonical, pertussis toxin-sensitive, G protein-coupled, and Ca(2+)-dependent mechanism that is independent of NMDAR ion channel activation. Further investigation into the sequelae for PregS-stimulated trafficking of NMDARs to the neuronal cell surface may uncover a new target for the pharmacological treatment of disorders in which NMDAR hypofunction has been implicated.

Molecular pathways controlling inhibitory receptor expression.

Epilepsy is a disease of complex etiology, and multiple molecular mechanisms contribute to its development. Temporal lobe epilepsy (TLE) may result from an initial precipitating event such as hypoxia, head injury, or prolonged seizure (i.e., status epilepticus [SE]), that is followed by a latent period of months to years before spontaneous seizures occur. γ-Aminobutyric acid (GABA)(A) receptor (GABA(A) R) subunit changes occur during this latent period and may persist following the onset of spontaneous seizures. Research into the molecular mechanisms regulating these changes and potential targets for intervention to reverse GABA(A) R subunit alterations have uncovered seizure-induced pathways that contribute to epileptogenesis. Several growth or transcription factors are known to be activated by SE, including (but not limited to): brain-derived neurotrophic factor (BDNF), cAMP response element binding protein (CREB), inducible cAMP early repressor (ICER), and early growth response factors (Egrs). Results of multiple studies suggest that these factors transcriptionally regulate GABA(A) R subunit gene expression in a way that is pertinent to the development of epilepsy. This article focuses on these signaling elements and describes their possible roles in gene regulatory pathways that may be critical in the development of chronic epilepsy.

GABA(A) receptor regulation after experimental traumatic brain injury.

The gamma-aminobutyric acid (GABA) type A receptor (GABA(A)R) is responsible for most fast synaptic inhibition in the adult brain. The GABA(A)R protein is composed of multiple subunits that determine the distribution, properties, and dynamics of the receptor. Several studies have shown that the Janus kinase/signal transducer and activator of transcription (JaK/STAT) and early growth response 3 (Egr3) signaling pathways can alter GABA(A)R subunit expression after status epilepticus (SE). In this study we investigated changes in these pathways after experimental TBI in the rat using a lateral fluid percussion injury (FPI) model. Our results demonstrated changes in the expression of several GABA(A)R subunit levels after injury, including GABA(A)R α1 and α4 subunits. This change appears to be transcriptional, and there is an associated increase in the phosphorylation of STAT3, and an increase in the expression of Egr3 and inducible cAMP element repressor (ICER) after FPI. These findings suggest that the activation of the JaK/STAT and Egr3 pathways after TBI may regulate injury-related changes in GABA(A)R subunit expression.

Brain-derived neurotrophic factor uses CREB and Egr3 to regulate NMDA receptor levels in cortical neurons.

Regulation of gene expression via brain-derived neurotrophic factor (BDNF) is critical to the development of the nervous system and may well underlie cognitive performance throughout life. We now describe a mechanism by which BDNF can exert its effects on postsynaptic receptor populations that may have relevance to both the normal and diseased brain where BDNF levels either rise or fall in association with changes in excitatory neurotransmission. Increased levels of NMDA receptors (NMDARs) occur in rat cortical neurons via synthesis of new NMDA receptor 1 (NR1) subunits. The majority of synthesis is controlled by binding of cAMP response element binding protein (CREB) and early growth response factor 3 (Egr3) to the core NR1 promoter (NR1-p) region. BDNF-mediated NR1 transcription depends upon induction of the mitogen-activated protein kinase (MAPK) pathway through activation of the TrK-B receptor. Taken together with the fact that NMDAR activation stimulates BDNF synthesis, our results uncover a feed-forward gene regulatory network that may enhance excitatory neurotransmission to change neuronal behavior over time.

A steroid modulatory domain in NR2A collaborates with NR1 exon-5 to control NMDAR modulation by pregnenolone sulfate and protons.

NMDA receptor (NMDAR)-mediated excitatory synaptic transmission plays a critical role in synaptic plasticity and memory formation, whereas its dysfunction may underlie neuropsychiatric and neurodegenerative diseases. The neuroactive steroid pregnenolone sulfate (PS) acts as a cognitive enhancer in impaired animals, augments LTP in hippocampal slices by enhancing NMDAR activity, and may participate in the reduction of schizophrenia's negative symptoms by systemic pregnenolone. We report that the effects of PS on NMDAR function are diverse, varying with subunit composition and NR1 splice variant. While PS potentiates NR1-1a/NR2B receptors through a critical steroid modulatory domain in NR2B that also modulates tonic proton inhibition, potentiation of the NMDA response is not dependent upon relief of such inhibition, a finding that distinguishes it from spermine. In contrast, the presence of an NR2A subunit confers enhanced PS-potentiation at reduced pH, suggesting that it may indeed act like spermine does at NR2B-containing receptors. Additional tuning of the NMDAR response by PS comes via the N-terminal exon-5 splicing insert of NR1-1b, which regulates the magnitude of proton-dependent PS potentiation. For NR2C- and NR2D-containing receptors, negative modulation at NR2C receptors is pH-independent (like NR2B) while negative modulation at NR2D receptors is pH-dependent (like NR2A). Taken together, PS displays a rich modulatory repertoire that takes advantage of the structural diversity of NMDARs in the CNS. The differential pH sensitivity of NMDAR isoforms to PS modulation may be especially important given the emerging role of proton sensors to both learning and memory, as well as brain injury.

Alteration of epileptogenesis genes.

Retrospective studies suggest that precipitating events such as prolonged seizures, stroke, or head trauma increase the risk of developing epilepsy later in life. The process of epilepsy development, known as epileptogenesis, is associated with changes in the expression of a myriad of genes. One of the major challenges for the epilepsy research community has been to determine which of these changes contributes to epileptogenesis, which may be compensatory, and which may be noncontributory. Establishing this for any given gene is essential if it is to be considered a therapeutic target for the prevention or treatment of epilepsy. Our laboratories have examined alterations in gene expression related to inhibitory neurotransmission that have been proposed as contributing factors in epileptogenesis. The GABA(A) receptor mediates most fast synaptic inhibition, and changes in GABA(A) receptor subunit expression and function have been reported in adult animals beginning immediately after prolonged seizures (status epilepticus [SE]) and continue as animals become chronically epileptic. Prevention of GABA(A) receptor subunit changes after SE using viral gene transfer inhibits development of epilepsy in an animal model, suggesting that these changes directly contribute to epileptogenesis. The mechanisms that regulate differential expression of GABA(A) receptor subunits in hippocampus after SE have recently been identified, and include the CREB-ICER, JAK-STAT, BDNF, and Egr3 signaling pathways. Targeting signaling pathways that alter the expression of genes involved in epileptogenesis may provide novel therapeutic approaches for preventing or inhibiting the development of epilepsy after a precipitating insult.

BDNF selectively regulates GABAA receptor transcription by activation of the JAK/STAT pathway.

The gamma-aminobutyric acid (GABA) type A receptor (GABA(A)R) is the major inhibitory neurotransmitter receptor in the brain. Its multiple subunits show regional, developmental, and disease-related plasticity of expression; however, the regulatory networks controlling GABA(A)R subunit expression remain poorly understood. We report that the seizure-induced decrease in GABA(A)R alpha1 subunit expression associated with epilepsy is mediated by the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway regulated by brain-derived neurotrophic factor (BDNF). BDNF- and seizure-dependent phosphorylation of STAT3 cause the adenosine 3',5'-monophosphate (cAMP) response element-binding protein (CREB) family member ICER (inducible cAMP early repressor) to bind with phosphorylated CREB at the Gabra1:CRE site. JAK/STAT pathway inhibition prevents the seizure-induced decrease in GABA(A)R alpha1 abundance in vivo and, given that BDNF is known to increase the abundance of GABA(A)R alpha4 in a JAK/STAT-independent manner, indicates that BDNF acts through at least two distinct pathways to influence GABA(A)R-dependent synaptic inhibition.

BDNF and the diseased nervous system: a delicate balance between adaptive and pathological processes of gene regulation.

It is clear that brain-derived neurotrophic factor (BDNF) plays a crucial role in organizing the response of the genome to dynamic changes in the extracellular environment that enable brain plasticity. BDNF has emerged as one of the most important signaling molecules for the developing nervous system as well as the impaired nervous system, and multiple diseases, such as Alzheimer's, Parkinson's, Huntington's, epilepsy, Rett's syndrome, and psychiatric depression, are linked by their association with potential dysregulation of BDNF-driven signal transduction programs. These programs are responsible for controlling the amount of activated transcription factors, such as cAMP response element binding protein, that coordinate the expression of multiple brain proteins, like ion channels and early growth response factors, whose job is to maintain the balance of excitation and inhibition in the nervous system. In this review, we will explore the evidence for BDNF's role in gene regulation side by side with its potential role in the etiology of neurological diseases. It is hoped that by bringing the datasets together in these diverse fields we can help develop the foundation for future studies aimed at understanding basic principles of gene regulation in the nervous system and how they can be harnessed to develop new therapeutic opportunities.