Positive AMPA receptor modulation rapidly stimulates BDNF release and increases dendritic mRNA translation

Brain-derived neurotrophic factor Val66Met allele impairs basal and ketamine-stimulated synaptogenesis in prefrontal cortex

BACKGROUND

Knock-in mice with the common human brain-derived neurotrophic factor (BDNF) Val66Met polymorphism have impaired trafficking of BDNF messenger RNA to dendrites. It was hypothesized, given evidence that local synapse formation is dependent on dendritic translation of BDNF messenger RNA, that loss-of-function Met allele mice would show synaptic deficits both at baseline and in response to ketamine, an N-methyl-D-aspartate antagonist that stimulates synaptogenesis in prefrontal cortex (PFC).

METHODS

Whole-cell recordings from layer V medial PFC pyramidal cells in brain slices were combined with two-photon laser scanning for analysis of wildtype, Val/Met, and Met/Met mice both at baseline and in response to a low dose of ketamine.

RESULTS

Val/Met and Met/Met mice were found to have constitutive atrophy of distal apical dendrites and decrements in apically targeted excitatory postsynaptic currents in layer V pyramidal cells of PFC. In addition, spine density and diameter were decreased, indicative of impaired synaptic formation/maturation (synaptogenesis). In Met/Met mice the synaptogenic effect of ketamine was markedly impaired, consistent with the idea that synaptogenesis is dependent on dendritic translation/release of BDNF. In parallel behavioral studies, we found that the antidepressant response to ketamine in the forced swim test was blocked in Met/Met mice.

CONCLUSIONS

The results demonstrate that expression of the BDNF Met allele in mice results in basal synaptic deficits and blocks synaptogenic and antidepressant actions of ketamine in PFC, suggesting that the therapeutic response to this drug might be attenuated or blocked in depressed patients who carry the loss of function Met allele.

Involvement of brain-derived neurotrophic factor and sonic hedgehog in the spinal cord plasticity after neurotoxic partial removal of lumbar motoneurons

Adult mammals could spontaneously achieve a partial sensory-motor recovery after spinal cord injury, by mechanisms including synaptic plasticity. We previously showed that this recovery is associated to the expression of synapsin-I, and that sonic hedgehog and Notch-1 could be also involved in plasticity. The role of brain-derived neurotrophic factor and glutamate receptors in regulating synaptic efficacy has been explored in the last decade but, although these mechanisms are now well-defined in the brain, the molecular mechanisms underlying the so called "spinal learning" are still less clear. Here, we measured the expression levels of choline acetyltransferase, synapsin-I, sonic hedgehog, Notch-1, glutamate receptor subunits (GluR1, GluR2, GluR4, NMDAR1) and brain-derived neurotrophic factor, in a motoneuron-depleted mouse spinal lesion model obtained by intramuscular injection of cholera toxin-B saporin. The lesion caused the down-regulation of the majority of analysed proteins. Moreover, we found that in lesioned but not in control spinal tissue, synapsin-I expression is associated to that of both brain-derived neurotrophic factor and sonic hedgehog, whereas GluR2 expression is linked to that of Shh. These results suggest that brain-derived neurotrophic factor and sonic hedgehog could collaborate in modulating synaptic plasticity after the removal of motoneurons, by a mechanism involving both pre- and post-synaptic processes. Interestingly, the involvement of sonic hedgehog showed here is novel, and offers new routes to address spinal cord plasticity and repair.

Brain-derived neurotrophic factor as a regulator of systemic and brain energy metabolism and cardiovascular health

Overweight sedentary individuals are at increased risk for cardiovascular disease, diabetes, and some neurological disorders. Beneficial effects of dietary energy restriction (DER) and exercise on brain structural plasticity and behaviors have been demonstrated in animal models of aging and acute (stroke and trauma) and chronic (Alzheimer's and Parkinson's diseases) neurological disorders. The findings described later, and evolutionary considerations, suggest brain-derived neurotrophic factor (BDNF) plays a critical role in the integration and optimization of behavioral and metabolic responses to environments with limited energy resources and intense competition. In particular, BDNF signaling mediates adaptive responses of the central, autonomic, and peripheral nervous systems from exercise and DER. In the hypothalamus, BDNF inhibits food intake and increases energy expenditure. By promoting synaptic plasticity and neurogenesis in the hippocampus, BDNF mediates exercise- and DER-induced improvements in cognitive function and neuroprotection. DER improves cardiovascular stress adaptation by a mechanism involving enhancement of brainstem cholinergic activity. Collectively, findings reviewed in this paper provide a rationale for targeting BDNF signaling for novel therapeutic interventions in a range of metabolic and neurological disorders.

Signaling pathways underlying the pathophysiology and treatment of depression: novel mechanisms for rapid-acting agents

Basic and clinical studies demonstrate that stress and depression are associated with atrophy and loss of neurons and glia, which contribute to the decreased size and function of limbic brain regions that control mood and depression, including the prefrontal cortex and hippocampus. Here, we review findings that suggest that opposing effects of stress and/or depression and antidepressants on neurotrophic factor expression and signaling partly explain these effects. We also discuss recent reports that suggest a possible role for glycogen synthase kinase 3 and upstream wingless (Wnt)-frizzled (Fz) signaling pathways in mood disorders. New studies also demonstrate that the rapid antidepressant actions of NMDA receptor antagonists are associated with activation of glutamate transmission and induction of synaptogenesis, providing novel targets for a new generation of fast-acting, more efficacious therapeutic agents.

Signaling pathways underlying the rapid antidepressant actions of ketamine

Currently available medications have significant limitations, most notably low response rate and time lag for treatment response. Recent clinical studies have demonstrated that ketamine, an NMDA receptor antagonist produces a rapid antidepressant response (within hours) and is effective in treatment resistant depressed patients. Molecular and cellular studies in rodent models demonstrate that ketamine rapidly increases synaptogenesis, including increased density and function of spine synapses, in the prefrontal cortex (PFC). Ketamine also produces rapid antidepressant actions in behavioral models of depression, and reverses the deficits in synapse number and behavior resulting from chronic stress exposure. These effects of ketamine are accompanied by stimulation of the mammalian target of rapamycin (mTOR), and increased levels of synaptic proteins. Together these studies indicate that ketamine rapidly reverses the atrophy of spines in the PFC and thereby causes a functional reconnection of neurons that underlies the rapid behavioral responses. These findings identify new targets for rapid acting antidepressants that are safer than ketamine. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Fragile X syndrome and targeted treatment trials

Work in recent years has revealed an abundance of possible new treatment targets for fragile X syndrome (FXS). The use of animal models, including the fragile X knockout mouse which manifests a phenotype very similar to FXS in humans, has resulted in great strides in this direction of research. The lack of Fragile X Mental Retardation Protein (FMRP) in FXS causes dysregulation and usually overexpression of a number of its target genes, which can cause imbalances of neurotransmission and deficits in synaptic plasticity. The use of metabotropic glutamate receptor (mGluR) blockers and gamma amino-butyric acid (GABA) agonists have been shown to be efficacious in reversing cellular and behavioral phenotypes, and restoring proper brain connectivity in the mouse and fly models. Proposed new pharmacological treatments and educational interventions are discussed in this chapter. In combination, these various targeted treatments show promising preliminary results in mitigating or even reversing the neurobiological abnormalities caused by loss of FMRP, with possible translational applications to other neurodevelopmental disorders including autism.

Therapeutic strategies in fragile X syndrome: dysregulated mGluR signaling and beyond

Fragile X syndrome (FXS) is an inherited neurodevelopmental disease caused by loss of function of the fragile X mental retardation protein (FMRP). In the absence of FMRP, signaling through group 1 metabotropic glutamate receptors is elevated and insensitive to stimulation, which may underlie many of the neurological and neuropsychiatric features of FXS. Treatment of FXS animal models with negative allosteric modulators of these receptors and preliminary clinical trials in human patients support the hypothesis that metabotropic glutamate receptor signaling is a valuable therapeutic target in FXS. However, recent research has also shown that FMRP may regulate diverse aspects of neuronal signaling downstream of several cell surface receptors, suggesting a possible new route to more direct disease-targeted therapies. Here, we summarize promising recent advances in basic research identifying and testing novel therapeutic strategies in FXS models, and evaluate their potential therapeutic benefits. We provide an overview of recent and ongoing clinical trials motivated by some of these findings, and discuss the challenges for both basic science and clinical applications in the continued development of effective disease mechanism-targeted therapies for FXS.

The AMPA receptor potentiator Org 26576 modulates stress-induced transcription of BDNF isoforms in rat hippocampus

Brain derived neurotrophic factor (BDNF) is a key mediator of brain plasticity. The modulation of its expression and function is important for cognition and represents a key strategy to enhance neuronal resilience. Within this context, there exists a close interaction between glutamatergic neurotransmission and BDNF activity towards regulating cellular homeostasis and plasticity. The aim of the current study was to investigate the ability of the AMPA receptor potentiator Org 26576 to modulate BDNF expression in selected brain regions under basal conditions or in response to an acute swim stress. Rats subjected to a single intraperitoneal injection with Org 26576 (10mg/kg) or saline were exposed to a swim stress session (5 min) and sacrificed 15 min after the end of stress. Real-time PCR assay was used to determine changes in BDNF transcription in different brain regions. Total BDNF mRNA levels were significantly increased in the hippocampus of animals exposed to the combination of Org 26576 and stress whereas, in prefrontal and frontal cortices, BDNF mRNA levels were modulated by the acute stress, independently from drug treatment. The analysis of BDNF transcripts in the hippocampus revealed a major contribution of exons I and IV. Our results suggest that AMPA receptor potentiation by Org 26576 exerts a positive modulatory influence on BDNF expression during ongoing neuronal activity. Given that these mechanisms are critical for neuronal plasticity, we hypothesized that such changes may facilitate learning/coping mechanisms associated with a mild stressful experience.

Cell atrophy and loss in depression: reversal by antidepressant treatment

Depression is associated with structural alterations in limbic brain regions that control emotion and mood. Studies of chronic stress in animal models and postmortem tissue from depressed subjects demonstrate that these structural alterations result from atrophy and loss of neurons and glial cells. These findings indicate that depression and stress-related mood disorders can be considered mild neurodegenerative disorders. Importantly, there is evidence that these structural alterations can be blocked or even reversed by elimination of stress and by antidepressant treatments. A major focus of current investigations is to characterize the molecular signaling pathways and factors that underlie these effects of stress, depression, and antidepressant treatment. Recent advances in this research area are discussed and potential novel targets for antidepressant development are highlighted.

Brain excitability in stroke: the yin and yang of stroke progression

There is no current medical therapy for stroke recovery. Principles of physiological plasticity have been identified during recovery in both animal models and human stroke. Stroke produces a loss of physiological brain maps in adjacent peri-infarct cortex and then a remapping of motor and sensory functions in this region. This remapping of function in peri-infarct cortex correlates closely with recovery. Recent studies have shown that the stroke produces abnormal conditions of excitability in neuronal circuits adjacent to the infarct that may be the substrate for this process of brain remapping and recovery. Stroke causes a hypoexcitability in peri-infarct motor cortex that stems from increased tonic γ-aminobutyric acid activity onto neurons. Drugs that reverse this γ-aminobutyric acid signaling promote recovery after stroke. Stroke also increases the sensitivity of glutamate receptor signaling in peri-infarct cortex well after the stroke event, and stimulating α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate glutamate receptors in peri-infarct cortex promotes recovery after stroke. Both blocking tonic γ-aminobutyric acid currents and stimulating α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors promote recovery after stroke when initiated at quite a delay, more than 3 to 5 days after the infarct. These changes in the excitability of neuronal circuits in peri-infarct cortex after stroke may underlie the process of remapping motor and sensory function after stroke and may identify new therapeutic targets to promote stroke recovery.

Targeted treatments for fragile X syndrome

Fragile X syndrome (FXS) is the most common identifiable genetic cause of intellectual disability and autistic spectrum disorders (ASD), with up to 50% of males and some females with FXS meeting criteria for ASD. Autistic features are present in a very high percent of individuals with FXS, even those who do not meet full criteria for ASD. Recent major advances have been made in the understanding of the neurobiology and functions of FMRP, the FMR1 (fragile X mental retardation 1) gene product, which is absent or reduced in FXS, largely based on work in the fmr1 knockout mouse model. FXS has emerged as a disorder of synaptic plasticity associated with abnormalities of long-term depression and long-term potentiation and immature dendritic spine architecture, related to the dysregulation of dendritic translation typically activated by group I mGluR and other receptors. This work has led to efforts to develop treatments for FXS with neuroactive molecules targeted to the dysregulated translational pathway. These agents have been shown to rescue molecular, spine, and behavioral phenotypes in the FXS mouse model at multiple stages of development. Clinical trials are underway to translate findings in animal models of FXS to humans, raising complex issues about trial design and outcome measures to assess cognitive change that might be associated with treatment. Genes known to be causes of ASD interact with the translational pathway defective in FXS, and it has been hypothesized that there will be substantial overlap in molecular pathways and mechanisms of synaptic dysfunction between FXS and ASD. Therefore, targeted treatments developed for FXS may also target subgroups of ASD, and clinical trials in FXS may serve as a model for the development of clinical trial strategies for ASD and other cognitive disorders.

Postsynaptic ProSAP/Shank scaffolds in the cross-hair of synaptopathies

Intact synaptic homeostasis is a fundamental prerequisite for a healthy brain. Thus, it is not surprising that altered synaptic morphology and function are involved in the molecular pathogenesis of so-called synaptopathies including autism, schizophrenia (SCZ) and Alzheimer's disease (AD). Intriguingly, various recent studies revealed a crucial role of postsynaptic ProSAP/Shank scaffold proteins in all of the aforementioned disorders. Considering these findings, we follow the hypothesis that ProSAP/Shank proteins are key regulators of synaptic development and plasticity with clear-cut isoform-specific roles. We thus propose a model where ProSAP/Shank proteins are in the center of a postsynaptic signaling pathway that is disrupted in several neuropsychiatric disorders.

Long-term effects of repetitive transcranial magnetic stimulation on markers for neuroplasticity: differential outcomes in anesthetized and awake animals

Long-term effects of repetitive transcranial magnetic stimulation (rTMS) have been associated with neuroplasticity, but most physiological studies have evaluated only the immediate effects of the stimulation on neurochemical markers. Furthermore, although it is known that baseline excitability state plays a major role in rTMS outcomes, the role of spontaneous neural activity in metaplasticity has not been investigated. The first aim of this study was to evaluate and compare the long-term effects of high- and low-frequency rTMS on the markers of neuroplasticity such as BDNF and GluR1 subunit of AMPA receptor. The second aim was to assess whether these effects depend on spontaneous neural activity, by comparing the neurochemical alterations induced by rTMS in anesthetized and awake rats. Ten daily sessions of high- or low-frequency rTMS were applied over the rat brain, and 3 d later, levels of BDNF, GluR1, and phosphorylated GluR1 were assessed in the hippocampus, prelimbic cortex, and striatum. We found that high-frequency stimulation induced a profound effect on neuroplasticity markers; increasing them in awake animals while decreasing them in anesthetized animals. In contrast, low-frequency stimulation did not induce significant long-term effects on these markers in either state. This study highlights the importance of spontaneous neural activity during rTMS and demonstrates that high-frequency rTMS can induce long-lasting effects on BDNF and GluR1 which may underlie the clinical benefits of this treatment in neuroplasticity-related disorders.

AMPA receptor-induced local brain-derived neurotrophic factor signaling mediates motor recovery after stroke

Stroke is the leading cause of adult disability. Recovery after stroke shares similar molecular and cellular properties with learning and memory. A main component of learning-induced plasticity involves signaling through AMPA receptors (AMPARs). We systematically tested the role of AMPAR function in motor recovery in a mouse model of focal stroke. AMPAR function controls functional recovery beginning 5 d after the stroke. Positive allosteric modulators of AMPARs enhance recovery of limb control when administered after a delay from the stroke. Conversely, AMPAR antagonists impair motor recovery. The contributions of AMPARs to recovery are mediated by release of brain-derived neurotrophic factor (BDNF) in periinfarct cortex, as blocking local BDNF function in periinfarct cortex blocks AMPAR-mediated recovery and prevents the normal pattern of motor recovery. In contrast to a delayed AMPAR role in motor recovery, early administration of AMPAR agonists after stroke increases stroke damage. These findings indicate that the role of glutamate signaling through the AMPAR changes over time in stroke: early potentiation of AMPAR signaling worsens stroke damage, whereas later potentiation of the same signaling system improves functional recovery.

AMPA receptor synaptic plasticity induced by psychostimulants: the past, present, and therapeutic future

Experience-dependent plasticity at excitatory synapses of the mesocorticolimbic system is a fundamental brain mechanism that enables adaptation to an ever-changing environment. These synaptic responses are critical for the planning and execution of adaptive behaviors that maximize survival. The mesocorticolimbic system mediates procurement of positive reinforcers such as food and sex; however, drugs of abuse resculpt this crucial circuitry to promote compulsive drug-seeking behavior. This review will discuss the long-term changes in glutamatergic neurotransmission that occur within the mesolimbic system following cocaine exposure. In addition, we will examine how these long-lasting neuroadaptations may drive the pathology of psychostimulant addiction. Finally, we review clinical trials that highlight antagonists at excitatory AMPA receptors as promising targets against cocaine abuse.

Brain-derived neurotrophic factor and epidermal growth factor activate neuronal m-calpain via mitogen-activated protein kinase-dependent phosphorylation

Calpain is a calcium-dependent protease that plays a significant role in synaptic plasticity, cell motility, and neurodegeneration. Two major calpain isoforms are present in brain, with mu-calpain (calpain1) requiring micromolar calcium concentrations for activation and m-calpain (calpain2) needing millimolar concentrations. Recent studies in fibroblasts indicate that epidermal growth factor (EGF) can activate m-calpain independently of calcium via mitogen-activated protein kinase (MAPK)-mediated phosphorylation. In neurons, MAPK is activated by both brain-derived neurotrophic factor (BDNF) and EGF. We therefore examined whether these growth factors could activate m-calpain by MAPK-dependent phosphorylation using cultured primary neurons and HEK-TrkB cells, both of which express BDNF and EGF receptors. Calpain activation was monitored by quantitative analysis of spectrin degradation and by a fluorescence resonance energy transfer (FRET)-based assay, which assessed the truncation of a calpain-specific peptide flanked by the FRET fluorophore pair DABCYL and EDANS. In both cell types, BDNF and EGF rapidly elicited calpain activation, which was completely blocked by MAPK and calpain inhibitors. BDNF stimulated m-calpain but not mu-calpain serine phosphorylation, an effect also blocked by MAPK inhibitors. Remarkably, BDNF- and EGF-induced calpain activation was preferentially localized in dendrites and dendritic spines of hippocampal neurons and was associated with actin polymerization, which was prevented by calpain inhibition. Our results indicate that, in cultured neurons, both BDNF and EGF activate m-calpain by MAPK-mediated phosphorylation. These results strongly support a role for calpain in synaptic plasticity and may explain why m-calpain, although widely expressed in CNS, requires nonphysiological calcium levels for activation.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

Biogenic amines in Rett syndrome: the usual suspects

Rett syndrome (RTT) is a severe postnatal neurological disorder caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. In affected children, most biological parameters, including brain structure, are normal (although acquired microcephaly is usually present). However, in recent years, a deficit in bioaminergic metabolism has been identified at the cellular and molecular levels, in more than 200 patients. Recently available transgenic mouse strains with a defective Mecp2 gene also show abnormalities, strongly suggesting that there is a direct link between the function of the MECP2 protein and the metabolism of biogenic amines. Biogenic amines appear to have an important role in the pathophysiology of Rett syndrome, for several reasons. Firstly, biogenic amines modulate a large number of autonomic and cognitive functions. Secondly, many of these functions are affected in RTT patients. Thirdly, biogenic amines are the only neurotransmitters that have repeatedly been found to be altered in RTT patients. Importantly, pharmacological interventions can be envisaged to try to counteract the deficits observed. Here, we review the available human and mouse data and present how they have been and could be used in the development of pharmacological treatments for children affected by the syndrome. Given our current knowledge and the tools available, modulating biogenic amine metabolism may prove to be the most promising strategy for improving the life quality of Rett syndrome patients in the short term.

Multi-level control of ionotropic glutamate receptor function

Because ionotropic glutamate receptors play critical roles in numerous CNS functions, there has been considerable interest in understanding molecular mechanisms regulating their properties. In particular, the search for ligands and corresponding binding sites providing allosteric regulation of agonist binding and channel opening and closing has been intensely pursued in the hope of developing new approaches for the treatment of a variety of CNS diseases associated with abnormal functioning of glutamatergic systems. Several recent publications have reported detailed structures of the N-terminal domains of NMDA and AMPA receptors and have generated interesting predictions regarding the possibility of finding new ways to control glutamate receptor function. Together with the recently reported control of the receptors by transmembrane proteins, there is now a whole set of potential regulators of these important families of receptors.

Effect of antidepressant drugs on veratridine-evoked glutamate and aspartate release in rat prefrontal cortex

In vivo microdialysis in conscious rats was used to evaluate the effect of local application, through a microdialysis probe, of desipramine (DMI), imipramine and citalopram (CIT), on veratridine-evoked glutamate and aspartate release in rat prefrontal cortex (PFCx). All antidepressant drugs (ADs), given at a concentration of 0.1 mM, significantly inhibited glutamate release, while aspartate release was affected only by DMI and CIT. In contrast, local administration of ADs markedly potentiated veratridine-evoked dopamine and noradrenaline release. Perfusion of clonidine, quinpirole and 1-[3-(trifluoro-methyl)phenyl]-piperazine (TFMPP) at 0.1 mM concentration also diminished, evoked release of glutamate and aspartate. The regulation of amino acid release in rat PFCx may be achieved by direct effect of ADs on Na+ channels or indirectly, by involvement of D2/D3, alpha 2 or 5-HT1B heteroceptors activated by the increased level of monoamines in response to the blockade of respective transporters.