Genetic program of neuronal differentiation and growth induced by specific activation of NMDA receptors

The role of glutamate dehydrogenase in the ageing brain

The homeostasis of glutamate, the primary excitatory neurotransmitter in the brain and is crucial for normal brain function. The mitochondrial enzyme glutamate dehydrogenase (GDH) connects the multifunctional amino acid glutamate, which is intimately related to glutamate metabolism, to the Krebs cycle. As a result, GDH reglutes the synthesis and uptake of the chemical messenger glutamate in neuroendocrine cells, playing a crucial role in the metabolism of proteins and carbohydrates. Nonetheless, brain ageing and numerous neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease, have been linked to GDH malfunction or dysregulation. In this review, we summarize the dynamics of GDH levels in the ageing brain and provide additional details about the role of GDH in the ageing brain. Understanding the metabolic mechanisms underlying glutamate homeostasis in the aging brain and how GDH regulates glutamate-dependent metabolic processes at synapses may lead to novel therapeutic approaches for neurodegenerative and psychiatric disorders, potentially slowing the aging process and promoting brain regeneration.

A genetically encoded secreted toxin potentiates synaptic NMDA receptors in hippocampal neurons and confers neuroprotection

NMDA receptors (NMDARs) play essential roles in neuronal development, survival, and synaptic plasticity, to name a few. However, dysregulation in receptors' activity can lead to neuronal and synaptic damage, contributing to the development of various brain pathologies. Current pharmacological treatments targeting NMDARs remain limited, for instance due to insufficient receptor selectivity and poor spatial targeting. Genetic approaches hold promise to overcome some of these issues; however, require genetically encodable NMDAR-modulating peptides, which are scarce. Here, we explored NMDAR-selective peptide toxins from marine cone snails, which resulted in the necessary engineering of a posttranslational modification-free variant of Conantokin-P (naked Con-P). The naked form is essential for expression in mammalian cells. We systematically explored the naked variant and discovered that naked Con-P maintains its ability to inhibit GluN2B-containing receptors, but uniquely acquired the ability to potentiate GluN2A-containing synaptic receptors. We then engineered a secreted naked Con-P that readily enhances NMDAR-mediated synaptic events in primary hippocampal neurons, and mitigates neuronal damage induced by staurosporine. We therefore provide a genetically encodable, subtype selective, and secreted bimodulator of NMDARs. This new variant and approach should pave the way for the development of additional genetic tools, specifically tailored to target NMDARs within distinct cellular populations in the brain.

Multi-faceted regulation of CREB family transcription factors

cAMP response element-binding protein (CREB) is a ubiquitously expressed nuclear transcription factor, which can be constitutively activated regardless of external stimuli or be inducibly activated by external factors such as stressors, hormones, neurotransmitters, and growth factors. However, CREB controls diverse biological processes including cell growth, differentiation, proliferation, survival, apoptosis in a cell-type-specific manner. The diverse functions of CREB appear to be due to CREB-mediated differential gene expression that depends on cAMP response elements and multi-faceted regulation of CREB activity. Indeed, the transcriptional activity of CREB is controlled at several levels including alternative splicing, post-translational modification, dimerization, specific transcriptional co-activators, non-coding small RNAs, and epigenetic regulation. In this review, we present versatile regulatory modes of CREB family transcription factors and discuss their functional consequences.

Maternal upbringing and selective breeding for voluntary exercise behavior modify patterns of DNA methylation and expression of genes in the mouse brain

Selective breeding has been utilized to study the genetic basis of exercise behavior, but research suggests that epigenetic mechanisms, such as DNA methylation, also contribute to this behavior. In a previous study, we demonstrated that the brains of mice from a genetically selected high runner (HR) line have sex-specific changes in DNA methylation patterns in genes known to be genomically imprinted compared to those from a non-selected control (C) line. Through cross-fostering, we also found that maternal upbringing can modify the DNA methylation patterns of additional genes. Here, we identify an additional set of genes in which DNA methylation patterns and gene expression may be altered by selection for increased wheel-running activity and maternal upbringing. We performed bisulfite sequencing and gene expression assays of 14 genes in the brain and found alterations in DNA methylation and gene expression for Bdnf, Pde4d and Grin2b. Decreases in Bdnf methylation correlated with significant increases in Bdnf gene expression in the hippocampus of HR compared to C mice. Cross-fostering also influenced the DNA methylation patterns for Pde4d in the cortex and Grin2b in the hippocampus, with associated changes in gene expression. We also found that the DNA methylation patterns for Atrx and Oxtr in the cortex and Atrx and Bdnf in the hippocampus were further modified by sex. Together with our previous study, these results suggest that DNA methylation and the resulting change in gene expression may interact with early-life influences to shape adult exercise behavior.

Regulation of CREB Phosphorylation in Nucleus Accumbens after Relief Conditioning

Relief learning is the association of environmental cues with the cessation of aversive events. While there is increasing knowledge about the neural circuitry mediating relief learning, the respective molecular pathways are not known. Therefore, the aim of the present study was to examine different putative molecular pathways underlying relief learning. To this purpose, male rats were subjected either to relief conditioning or to a pseudo conditioning procedure. Forty-five minutes or 6 h after conditioning, samples of five different brain regions, namely the prefrontal cortex, nucleus accumbens (NAC), dorsal striatum, dorsal hippocampus, and amygdala, were collected. Using quantitative Western blots, the expression level of CREB, pCREB, ERK1/2, pERK1/2, CaMKIIα, MAP2K, PKA, pPKA, Akt, pAkt, DARPP-32, pDARPP-32, 14-3-3, and neuroligin2 were studied. Our analyses revealed that relief conditioned rats had higher CREB phosphorylation in NAC 6 h after conditioning than pseudo conditioned rats. The data further revealed that this CREB phosphorylation was mainly induced by dopamine D1 receptor-mediated activation of PKA, however, other kinases, downstream of the NMDA receptor, may also contribute. Taken together, the present study suggests that CREB phosphorylation, induced by a combination of different molecular pathways downstream of dopamine D1 and NMDA receptors, is essential for the acquisition and consolidation of relief learning.

Comparison of induced neurons reveals slower structural and functional maturation in humans than in apes

We generated induced excitatory neurons (iNeurons, iNs) from chimpanzee, bonobo, and human stem cells by expressing the transcription factor neurogenin-2 (NGN2). Single-cell RNA sequencing showed that genes involved in dendrite and synapse development are expressed earlier during iNs maturation in the chimpanzee and bonobo than the human cells. In accordance, during the first 2 weeks of differentiation, chimpanzee and bonobo iNs showed repetitive action potentials and more spontaneous excitatory activity than human iNs, and extended neurites of higher total length. However, the axons of human iNs were slightly longer at 5 weeks of differentiation. The timing of the establishment of neuronal polarity did not differ between the species. Chimpanzee, bonobo, and human neurites eventually reached the same level of structural complexity. Thus, human iNs develop slower than chimpanzee and bonobo iNs, and this difference in timing likely depends on functions downstream of NGN2.

Development of the Adverse Outcome Pathway (AOP): Chronic binding of antagonist to N-methyl-d-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities of children

The Adverse Outcome Pathways (AOPs) are designed to provide mechanistic understanding of complex biological systems and pathways of toxicity that result in adverse outcomes (AOs) relevant to regulatory endpoints. AOP concept captures in a structured way the causal relationships resulting from initial chemical interaction with biological target(s) (molecular initiating event) to an AO manifested in individual organisms and/or populations through a sequential series of key events (KEs), which are cellular, anatomical and/or functional changes in biological processes. An AOP provides the mechanistic detail required to support chemical safety assessment, the development of alternative methods and the implementation of an integrated testing strategy. An example of the AOP relevant to developmental neurotoxicity (DNT) is described here following the requirements of information defined by the OECD Users' Handbook Supplement to the Guidance Document for developing and assessing AOPs. In this AOP, the binding of an antagonist to glutamate receptor N-methyl-d-aspartate (NMDAR) receptor is defined as MIE. This MIE triggers a cascade of cellular KEs including reduction of intracellular calcium levels, reduction of brain derived neurotrophic factor release, neuronal cell death, decreased glutamate presynaptic release and aberrant dendritic morphology. At organ level, the above mentioned KEs lead to decreased synaptogenesis and decreased neuronal network formation and function causing learning and memory deficit at organism level, which is defined as the AO. There are in vitro, in vivo and epidemiological data that support the described KEs and their causative relationships rendering this AOP relevant to DNT evaluation in the context of regulatory purposes.

Functional network stability and average minimal distance - A framework to rapidly assess dynamics of functional network representations

BACKGROUND

Recent advances in neurophysiological recording techniques have increased both the spatial and temporal resolution of data. New methodologies are required that can handle large data sets in an efficient manner as well as to make quantifiable, and realistic, predictions about the global modality of the brain from under-sampled recordings.

NEW METHOD

To rectify both problems, we first propose an analytical modification to an existing functional connectivity algorithm, Average Minimal Distance (AMD), to rapidly capture functional network connectivity. We then complement this algorithm by introducing Functional Network Stability (FuNS), a metric that can be used to quickly assess the global network dynamic changes over time, without being constrained by the activities of a specific set of neurons.

RESULTS

We systematically test the performance of AMD and FuNS (1) on artificial spiking data with different statistical characteristics, (2) from spiking data generated using a neural network model, and (3) using in vivo data recorded from mouse hippocampus during fear learning. Our results show that AMD and FuNS are able to monitor the change in network dynamics during memory consolidation.

COMPARISON WITH OTHER METHODS

AMD outperforms traditional bootstrapping and cross-correlation (CC) methods in both significance and computation time. Simultaneously, FuNS provides a reliable way to establish a link between local structural network changes, global dynamics of network-wide representations activity, and behavior.

CONCLUSIONS

The AMD-FuNS framework should be universally useful in linking long time-scale, global network dynamics and cognitive behavior.

NMDA receptor dependent changes in c-fos and p-CREB signaling following extinction and reinstatement of morphine place preference

Neural circuitry comprising the ventral tegmental area, nucleus accumbens (NAc), prefrontal cortex (PFC) and hippocampus (HIP) has a main role in reward phenomena. Previous behavioral studies indicated that intracerebroventricular administration of AP5 (NMDA glutamate receptor antagonist) and CNQX (AMPA/kainate glutamate receptor antagonist) during the extinction and before reinstatement of morphine-induced conditioned place preference (CPP) reduced the extinction period and reinstatement of morphine-CPP. Therefore, in the present study, we tried to evaluate the effect of antagonism of NMDA glutamate receptors on the p-CREB/CREB ratio and c-fos expression in the NAc, PFC and HIP during these two phases of morphine-CPP in male adult albino Wistar rats. The p-CREB/CREB ratio and c-fos levels were estimated by Western blot analysis. The results revealed that these two factors decreased by antagonism of NMDA glutamate receptors (different doses of AP5) compared to saline-control group in aforementioned regions. The reduction of molecular markers, especially the p-CREB/CREB ratio, after AP5 administration was more during the extinction period. Therefore, it can be assumed that consolidation and reconsolidation of morphine memory via intra-PFC, -NAc and -HIP NMDA glutamate receptors are in accordance with changes in p-CREB/CREB ratio and c-fos levels.

A new perspective on the role of the CREB family of transcription factors in memory consolidation via adult hippocampal neurogenesis

Adult neurogenesis is the process by which new neurons are generated in the brains of adults. Since its discovery 50 years ago, adult neurogenesis has been widely studied in the mammalian brain and has provided a new perspective on the pathophysiology of many psychiatric and neurodegenerative disorders, some of which affect memory. In this regard, adult hippocampal neurogenesis (AHN), which occurs in the subgranular zone (SGZ) of the dentate gyrus (DG), has been suggested to play a role in the formation and consolidation of new memories. This process involves many transcription factors, of which cyclic AMP (cAMP)-responsive element-binding protein (CREB) is a well-documented one. In the developing brain, CREB regulates crucial cell stages (e.g., proliferation, differentiation, and survival), and in the adult brain, it participates in neuronal plasticity, learning, and memory. In addition, new evidence supports the hypothesis that CREB may also participate in learning and memory through its involvement in AHN. This review examines the CREB family of transcription factors, including the different members and known signaling pathways. It highlights the role of CREB as a modulator of AHN, which could underlie its function in memory consolidation mechanisms.

Effects of treadmill exercise combined with MK 801 treatment on neuroblast differentiation in the dentate gyrus in rats

N-methyl-D-aspartate receptor (NR) is involved in activity-dependent synaptic plasticity, such as associative long-term potentiation, and in related central functions, such as learning and memory. In this study, we observed effects of treadmill exercise on NR1 and doublecortin (DCX, a marker for neuroblast differentiation) in the subgranular zone of the dentate gyrus (DG). At 6 weeks of age, rats were put on a treadmill with or without running for 1 h/day for 5 consecutive days at 22 m/min for 5 weeks. Exercise increased NR1 immunoreactivity and protein level in the hippocampus. To identify the correlations between NR and neuroblasts, we intraperitoneally administered a NR antagonist, MK-801, to the exercised rats. MK-801 treatment reduced NR1 protein level in the hippocampus of the exercised rats. In addition, in the MK-801-treated group, the number of DCX cells was significantly decreased in the subgranular zone of the DG. These results suggest that NR may be one of the important factors that modulate neuroblast differentiation during exercise in rats.

Transcriptomic responses in mouse brain exposed to chronic excess of the neurotransmitter glutamate

Background

Increases during aging in extracellular levels of glutamate (Glu), the major excitatory neurotransmitter in the brain, may be linked to chronic neurodegenerative diseases. Little is known about the molecular responses of neurons to chronic, moderate increases in Glu levels. Genome-wide gene expression in brain hippocampus was examined in a unique transgenic (Tg) mouse model that exhibits moderate Glu hyperactivity throughout the lifespan, the neuronal Glutamate dehydrogenase (Glud1) mouse, and littermate 9 month-old wild type mice.

Results

Integrated bioinformatic analyses on transcriptomic data were used to identify bio-functions, pathways and gene networks underlying neuronal responses to increased Glu synaptic release. Bio-functions and pathways up-regulated in Tg mice were those associated with oxidative stress, cell injury, inflammation, nervous system development, neuronal growth, and synaptic transmission. Increased gene expression in these functions and pathways indicated apparent compensatory responses offering protection against stress, promoting growth of neuronal processes (neurites) and re-establishment of synapses. The transcription of a key gene in the neurite growth network, the kinase Ptk2b, was significantly up-regulated in Tg mice as was the activated (phosphorylated) form of the protein. In addition to genes related to neurite growth and synaptic development, those associated with neuronal vesicle trafficking in the Huntington's disease signalling pathway, were also up-regulated.

Conclusions

This is the first study attempting to define neuronal gene expression patterns in response to chronic, endogenous Glu hyperactivity at brain synapses. The patterns observed were characterized by a combination of responses to stress and stimulation of nerve growth, intracellular transport and recovery.

A risk PRODH haplotype affects sensorimotor gating, memory, schizotypy, and anxiety in healthy male subjects

BACKGROUND

Significant associations have been shown for haplotypes comprising three PRODH single nucleotide polymorphisms (SNPs; 1945T/C, 1766A/G, 1852G/A) located in the 3' region of the gene, suggesting a role of these variants in the etiopathogenesis of schizophrenia. We assessed the relationship between these high-risk PRODH polymorphisms and schizophrenia-related endophenotypes in a large and highly homogeneous cohort of healthy males.

METHODS

Participants (n = 217) were tested in prepulse inhibition (PPI), verbal and working memory, trait anxiety and schizotypy. The QTPHASE from the UNPHASED package was used for the association analysis of each SNP or haplotype data. This procedure revealed significant phenotypic impact of the risk CGA haplotype. Subjects were then divided in two groups; levels of PPI, anxiety, and schizotypy, verbal and working memory were compared with analysis of variance.

RESULTS

CGA carriers (n = 32) exhibited attenuated PPI (p < .001) and verbal memory (p < .001) and higher anxiety (p < .004) and schizotypy (p < .008) compared with the noncarriers (n = 185). There were no differences in baseline startle, demographics, and working memory. The main significant correlations were schizotypy x PPI [85-dB, 120-msec trials] in the carriers and schizotypy x anxiety in the entire group and the noncarriers but not the carriers group.

CONCLUSIONS

Our results strongly support PPI as a valid schizophrenia endophenotype and highlight the importance of examining the role of risk haplotypes on multiple endophenotypes and have implications for understanding the continuum from normality to psychosis, transitional states, and the genetics of schizophrenia-related traits.

A critical importance of polyamine site in NMDA receptors for neurite outgrowth and fasciculation at early stages of P19 neuronal differentiation

We have investigated the role of N-methyl-d-aspartate receptors (NMDARs) and gamma-aminobutyric acid receptors type A (GABA(A)Rs) at an early stage of P19 neuronal differentiation. The subunit expression was profiled in 24-hour intervals with RT-PCR and functionality of the receptors was verified via fluo-3 imaging of Ca(2+) dynamics in the immature P19 neurons showing that both NMDA and GABA excite neuronal bodies, but only polyamine-site sensitive NMDAR stimulation leads to enhanced Ca(2+) signaling in the growth cones. Inhibition of NR1/NR2B NMDARs by 1 muM ifenprodil severely impaired P19 neurite extension and fasciculation, and this negative effect was fully reversible by polyamine addition. In contrast, GABA(A)R antagonism by a high dose of 200 microM bicuculline had no observable effect on P19 neuronal differentiation and fasciculation. Except for the differential NMDAR and GABA(A)R profiles of Ca(2+) signaling within the immature P19 neurons, we have also shown that inhibition of NR1/NR2B NMDARs strongly decreased mRNA level of NCAM-180, which has been previously implicated as a regulator of neuronal growth cone protrusion and neurite extension. Our data thus suggest a critical role of NR1/NR2B NMDARs during the process of neuritogenesis and fasciculation of P19 neurons via differential control of local growth cone Ca(2+) surges and NCAM-180 signaling.

Rare structural variants disrupt multiple genes in neurodevelopmental pathways in schizophrenia

Schizophrenia is a devastating neurodevelopmental disorder whose genetic influences remain elusive. We hypothesize that individually rare structural variants contribute to the illness. Microdeletions and microduplications >100 kilobases were identified by microarray comparative genomic hybridization of genomic DNA from 150 individuals with schizophrenia and 268 ancestry-matched controls. All variants were validated by high-resolution platforms. Novel deletions and duplications of genes were present in 5% of controls versus 15% of cases and 20% of young-onset cases, both highly significant differences. The association was independently replicated in patients with childhood-onset schizophrenia as compared with their parents. Mutations in cases disrupted genes disproportionately from signaling networks controlling neurodevelopment, including neuregulin and glutamate pathways. These results suggest that multiple, individually rare mutations altering genes in neurodevelopmental pathways contribute to schizophrenia.