Facilitation of AMPA receptor synaptic delivery as a molecular mechanism for cognitive enhancement

Interplay between hippocampal TACR3 and systemic testosterone in regulating anxiety-associated synaptic plasticity

Tachykinin receptor 3 (TACR3) is a member of the tachykinin receptor family and falls within the rhodopsin subfamily. As a G protein-coupled receptor, it responds to neurokinin B (NKB), its high-affinity ligand. Dysfunctional TACR3 has been associated with pubertal failure and anxiety, yet the mechanisms underlying this remain unclear. Hence, we have investigated the relationship between TACR3 expression, anxiety, sex hormones, and synaptic plasticity in a rat model, which indicated that severe anxiety is linked to dampened TACR3 expression in the ventral hippocampus. TACR3 expression in female rats fluctuates during the estrous cycle, reflecting sensitivity to sex hormones. Indeed, in males, sexual development is associated with a substantial increase in hippocampal TACR3 expression, coinciding with elevated serum testosterone and a significant reduction in anxiety. TACR3 is predominantly expressed in the cell membrane, including the presynaptic compartment, and its modulation significantly influences synaptic activity. Inhibition of TACR3 activity provokes hyperactivation of CaMKII and enhanced AMPA receptor phosphorylation, associated with an increase in spine density. Using a multielectrode array, stronger cross-correlation of firing was evident among neurons following TACR3 inhibition, indicating enhanced connectivity. Deficient TACR3 activity in rats led to lower serum testosterone levels, as well as increased spine density and impaired long-term potentiation (LTP) in the dentate gyrus. Remarkably, aberrant expression of functional TACR3 in spines results in spine shrinkage and pruning, while expression of defective TACR3 increases spine density, size, and the magnitude of cross-correlation. The firing pattern in response to LTP induction was inadequate in neurons expressing defective TACR3, which could be rectified by treatment with testosterone. In conclusion, our study provides valuable insights into the intricate interplay between TACR3, sex hormones, anxiety, and synaptic plasticity. These findings highlight potential targets for therapeutic interventions to alleviate anxiety in individuals with TACR3 dysfunction and the implications of TACR3 in anxiety-related neural changes provide an avenue for future research in the field.

PDI augments kainic acid-induced seizure activity and neuronal death by inhibiting PP2A-GluA2-PICK1-mediated AMPA receptor internalization in the mouse hippocampus

Protein disulfide isomerase (PDI) is a redox-active enzyme and also serves as a nitric oxide donor causing S-nitrosylation of cysteine residues in various proteins. Although PDI knockdown reduces α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR)-mediated neuronal activity, the underlying mechanisms are largely unknown. In the present study, we found that under physiological condition PDI knockdown increased CaMKII activity (phosphorylation) in the mouse hippocampus. However, PDI siRNA inhibited protein phosphatase (PP) 2A-mediated GluA2 S880 dephosphorylation by increasing PP2A oxidation, independent of S-nitrosylation. PDI siRNA also enhanced glutamate ionotropic receptor AMPA type subunit 1 (GluA1) S831 and GluA2 S880, but not GluA1 S845 and GluA2 Y869/Y873/Y876 phosphorylations, concomitant with the enhanced protein interacting with C kinase 1 (PICK1)-mediated AMPAR internalization. Furthermore, PDI knockdown attenuated seizure activity and neuronal damage in response to kainic acid (a non-desensitizing agonist of AMPAR). Therefore, these findings suggest that PDI may regulate surface AMPAR expression through PP2A-GluA2-PICK1 signaling pathway, and that PDI may be one of the therapeutic targets for epilepsy via AMPAR internalization without altering basal neurotransmission.

Cell-type specific changes in PKC-delta neurons of the central amygdala during alcohol withdrawal

The central amygdala (CeA) contains a diverse population of cells, including multiple subtypes of GABAergic neurons, along with glia and epithelial cells. Specific CeA cell types have been shown to affect alcohol consumption in animal models of dependence and may be involved in negative affect during alcohol withdrawal. We used single-nuclei RNA sequencing to determine cell-type specificity of differential gene expression in the CeA induced by alcohol withdrawal. Cells within the CeA were classified using unbiased clustering analyses and identified based on the expression of known marker genes. Differential gene expression analysis was performed on each identified CeA cell-type. It revealed differential gene expression in astrocytes and GABAergic neurons associated with alcohol withdrawal. GABAergic neurons were further subclassified into 13 clusters of cells. Analyzing transcriptomic responses in these subclusters revealed that alcohol exposure induced multiple differentially expressed genes in one subtype of CeA GABAergic neurons, the protein kinase C delta (PKCδ) expressing neurons. These results suggest that PKCδ neurons in the CeA may be uniquely sensitive to the effects of alcohol exposure and identify a novel population of cells in CeA associated with alcohol withdrawal.

Prenatal glucocorticoid exposure selectively impairs neuroligin 1-dependent neurogenesis by suppressing astrocytic FGF2-neuronal FGFR1 axis

Exposure to maternal stress irreversibly impairs neurogenesis of offspring by inducing life-long effects on interaction between neurons and glia under raging differentiation process, culminating in cognitive and neuropsychiatric abnormalities in adulthood. We identified that prenatal exposure to stress-responsive hormone glucocorticoid impaired neurogenesis and induced abnormal behaviors in ICR mice. Then, we used human induced pluripotent stem cell (iPSC)-derived neural stem cell (NSC) to investigate how neurogenesis deficits occur. Following glucocorticoid treatment, NSC-derived astrocytes were found to be A1-like neurotoxic astrocytes. Moreover, cortisol-treated astrocytic conditioned media (ACM) then specifically downregulated AMPA receptor-mediated glutamatergic synaptic formation and transmission in differentiating neurons, by inhibiting localization of ionotropic glutamate receptor (GluR)1/2 into synapses. We then revealed that downregulated astrocytic fibroblast growth factor 2 (FGF2) and nuclear fibroblast growth factor receptor 1 (FGFR1) of neurons are key pathogenic factors for reducing glutamatergic synaptogenesis. We further confirmed that cortisol-treated ACM specifically decreased the binding of neuronal FGFR1 to the synaptogenic NLGN1 promoter, but this was reversed by FGFR1 restoration. Upregulation of neuroligin 1, which is important in scaffolding GluR1/2 into the postsynaptic compartment, eventually normalized glutamatergic synaptogenesis and subsequent neurogenesis. Moreover, pretreatment of FGF2 elevated neuroligin 1 expression and trafficking of GluR1/2 into the postsynaptic compartment of mice exposed to prenatal corticosterone, improving spatial memory and depression/anxiety-like behaviors. In conclusion, we identified neuroligin 1 restoration by astrocytic FGF2 and its downstream neuronal nuclear FGFR1 as a critical target for preventing prenatal stress-induced dysfunction in glutamatergic synaptogenesis, which recovered both neurogenesis and hippocampal-related behaviors.

TRPV1 Antagonist Prevents Neonatal Sevoflurane-Induced Synaptic Abnormality and Cognitive Impairment in Mice Through Regulating the Src/Cofilin Signaling Pathway

Long-term neurodevelopmental disorders following neonatal anesthesia have been reported both in young animals and in children. The activation of transient receptor potential vanilloid 1 (TRPV1) channels in hippocampus adversely affects neurodevelopment. The current study explored the underlying mechanism of TRPV1 channels on long-lasting cognitive dysfunction induced by anesthetic exposure to the developing brain. we demonstrated that TRPV1 expression was increased after sevoflurane exposure both in vitro and in vivo. Sevoflurane exposure to hippocampal neurons decreased the synaptic density and the surface GluA1 expression, as well as increased co-localization of internalized AMPAR in early and recycling endosomes. Sevoflurane exposure to newborn mice impaired learning and memory in adulthood, and reduced AMPAR subunit GluA1, 2 and 3 expressions in the crude synaptosomal fractions from mouse hippocampus. The inhibition of TRPV1 reversed the phenotypic changes induced by sevoflurane. Moreover, sevoflurane exposure increased Src phosphorylation at tyrosine 416 site thereby reducing cofilin phosphorylation. TRPV1 blockade reversed these suppressive effects of sevoflurane. Our data suggested that TRPV1 antagonist may protect against synaptic damage and cognitive dysfunction induced by sevoflurane exposure during the brain developing stage.

FORTIS: a live-cell assay to monitor AMPA receptors using pH-sensitive fluorescence tags

The real-time live fluorescent monitoring of surface AMPA receptors (AMPARs) could open new opportunities for drug discovery and phenotypic screening concerning neuropsychiatric disorders. We have developed FORTIS, a tool based on pH sensitivity capable of detecting subtle changes in surface AMPARs at a neuronal population level. The expression of SEP-GluA1 or pHuji-GluA1 recombinant AMPAR subunits in mammalian neurons cultured in 96-well plates enables surface AMPARs to be monitored with a microplate reader. Thus, FORTIS can register rapid changes in surface AMPARs induced by drugs or genetic modifications without having to rely on conventional electrophysiology or imaging. By combining FORTIS with pharmacological manipulations, basal surface AMPARs, and plasticity-like changes can be monitored. We expect that employing FORTIS to screen for changes in surface AMPARs will accelerate both neuroscience research and drug discovery.

Aberrant Synaptic PTEN in Symptomatic Alzheimer's Patients May Link Synaptic Depression to Network Failure

In Alzheimer’s disease (AD), Amyloid β (Aβ) impairs synaptic function by inhibiting long-term potentiation (LTP), and by facilitating long-term depression (LTD). There is now evidence from AD models that Aβ provokes this shift toward synaptic depression by triggering the access to and accumulation of PTEN in the postsynaptic terminal of hippocampal neurons. Here we quantified the PTEN in 196,138 individual excitatory dentate gyrus synapses from AD patients at different stages of the disease and from controls with no neuropathological findings. We detected a gradual increase of synaptic PTEN in AD brains as the disease progresses, in conjunction with a significant decrease in synaptic density. The synapses that remain in symptomatic AD patients are more likely to be smaller and exhibit fewer AMPA receptors (AMPARs). Hence, a high Aβ load appears to strongly compromise human hippocampal synapses, as reflected by an increase in PTEN, inducing a loss of AMPARs that may eventually provoke synaptic failure and loss.

Network-Based Analysis of Cognitive Impairment and Memory Deficits from Transcriptome Data

Aging is an inevitable process that negatively affects all living organisms and their vital functions. The brain is one of the most important organs in living beings and is primarily impacted by aging. The molecular mechanisms of learning, memory and cognition are altered over time, and the impairment in these mechanisms can lead to neurodegenerative diseases. Transcriptomics can be used to study these impairments to acquire more detailed information on the affected molecular mechanisms. Here we analyzed learning- and memory-related transcriptome data by mapping it on the organism-specific protein-protein interactome network. Subnetwork discovery algorithms were applied to discover highly dysregulated subnetworks, which were complemented with co-expression-based interactions. The functional analysis shows that the identified subnetworks are enriched with genes having roles in synaptic plasticity, gliogenesis, neurogenesis and cognition, which are reported to be related to memory and learning. With a detailed analysis, we show that the results from different subnetwork discovery algorithms or from different transcriptomic datasets can be successfully reconciled, leading to a memory-learning network that sheds light on the molecular mechanisms behind aging and memory-related impairments.

Nose-to-brain delivery of hyaluronate - FG loop peptide conjugate for non-invasive hypoxic-ischemic encephalopathy therapy

The intranasal drug administration has attracted great interest as a non-invasive route allowing targeted delivery of drugs directly to the brain. However, one of the main issues in nasal drug administration is mucociliary clearance. Hyaluronate (HA) has been widely used as a mucoadhesive excipient for ocular, rectal, and vaginal delivery. Here, FG loop peptide (FGL) was conjugated to HA for improving enzymatic stability and delivery efficiency from the nose to the brain. The successful conjugation of FGL to aldehyde modified HA was confirmed by gel permeation chromatography (GPC) and ¹H nuclear magnetic resonance (NMR). The outstanding enzymatic stability of HA-FGL conjugate was also corroborated by the GPC. The HA-FGL conjugate showed enhanced binding affinity onto nasal epithelial cells. In addition, in vivo nose-to-brain delivery of HA-FGL conjugate could be visualized by using an IVIS imaging system and fluorescence microscopy. Finally, in vivo therapeutic effect of HA-FGL conjugate was successfully confirmed by histological analysis, transferase-mediated uridine 5-triphosphate-biotin nick end-labeling (TUNEL) assay, immunofluorescent staining, transmission electron microscopy (TEM), and rotarod tests in hypoxic-ischemic encephalopathy model animals.

Systemic administration of a fibroblast growth factor receptor 1 agonist rescues the cognitive deficit in aged socially isolated rats

Social isolation predominantly occurs in elderly people and it is strongly associated with cognitive decline. However, the mechanisms that produce isolation-related cognitive dysfunction during aging remain unclear. Here, we evaluated the cognitive, electrophysiological, and morphological effects of short- (4 weeks) and long-term (12 weeks) social isolation in aged male Wistar rats. Long-term but not short-term social isolation increased the plasma corticosterone levels and impaired spatial memory in the Morris water maze. Moreover, isolated animals displayed dampened hippocampal long-term potentiation in vivo, both in the dentate gyrus (DG) and CA1, as well as a specific reduction in the volume of the stratum oriens and spine density in CA1. Interestingly, social isolation induced a transient increase in hippocampal basic fibroblast growth factor (FGF2), whereas fibroblast growth factor receptor 1 (FGFR1) levels only increased after long-term isolation. Importantly, subchronic systemic administration of FGL, a synthetic peptide that activates FGFR1, rescued spatial memory in long-term isolated rats. These findings provide new insights into the neurobiological mechanisms underlying the detrimental effects on memory of chronic social isolation in the aged.

Influence of pharmacological and epigenetic factors to suppress neurotrophic factors and enhance neural plasticity in stress and mood disorders

Stress-induced major depression and mood disorders are characterized by behavioural abnormalities and psychiatric illness, leading to disability and immature mortality worldwide. Neurobiological mechanisms of stress and mood disorders are discussed considering recent findings, and challenges to enhance pharmacological effects of antidepressant, and mood stabilizers. Pharmacological enhancement of ketamine and scopolamine regulates depression at the molecular level, increasing synaptic plasticity in prefrontal regions. Blood-derived neurotrophic factors facilitate mood-deficit symptoms. Epigenetic factors maintain stress-resilience in hippocampal region. Regulation of neurotrophic factors blockades stress, and enhances neuronal survival though it paralyzes limbic regions. Molecular agents and neurotrophic factors also control behavioral and synaptic plasticity in addiction and stress disorders. Future research on neuronal dynamics and cellular actions can be directed to obtain the etiology of synaptic dysregulation in mood disorder and stress. For the first time, the current review contributes to the literature of synaptic plasticity representing the role of epigenetic mechanisms and glucocorticoid receptors to predict depression and anxiety in clinical conditions.

Neural cell adhesion molecule peptide mimetics modulate emotionality: pharmacokinetic and behavioral studies in rats and non-human primates

Recent evidence highlights the fibroblast growth factor (FGF) family in emotion modulation. Although ligands that activate FGF receptors have antidepressant and anxiolytic effects in animal models, FGF ligands have a broad range of actions both in the brain and the periphery. Therefore, identifying molecular partners that may function as allosteric modulators could offer new avenues for drug development. Since neural cell adhesion molecule (NCAM) activates FGF receptors, we asked whether peripherally administered NCAM peptide mimetics penetrate the brain and alter the behavior of standardized tests that have predictive validity for drug treatments of anxiety or depression. The NCAM peptide mimetic, plannexin, acutely increased and chronically decreased anxiety, but did not have antidepressant effects in rats. Another NCAM peptide mimetic, FGL_L, had acute anxiogenic effects and chronic antidepressant effects in rats. A related NCAM peptide mimetic, FGL_S, had antidepressant effects without modulating anxiety-like behavior, and these antidepressant effects were blocked by an AMPA receptor antagonist. Cisternal cerebrospinal fluid (CSF) levels of FGLs correlated with blood plasma levels in rats and non-human primates, and CSF-to-blood ratios of FGL_S were comparable in both species. Results indicate that NCAM peptide mimetics penetrate the brain and support the suggestion that FGL_S may be a candidate for further development as a novel treatment for major depressive disorder in humans.

Neurotrophin and FGF Signaling Adapter Proteins, FRS2 and FRS3, Regulate Dentate Granule Cell Maturation and Excitatory Synaptogenesis

Dentate granule cells (DGCs) play important roles in cognitive processes. Knowledge about how growth factors such as FGFs and neurotrophins contribute to the maturation and synaptogenesis of DGCs is limited. Here, using brain-specific and germline mouse mutants we show that a module of neurotrophin and FGF signaling, the FGF Receptor Substrate (FRS) family of intracellular adapters, FRS2 and FRS3, are together required for postnatal brain development. In the hippocampus, FRS promotes dentate gyrus morphogenesis and DGC maturation during developmental neurogenesis, similar to previously published functions for both neurotrophins and FGFs. Consistent with a role in DGC maturation, two-photon imaging revealed that Frs2,3-double mutants have reduced numbers of dendritic branches and spines in DGCs. Functional analysis further showed that double-mutant mice exhibit fewer excitatory synaptic inputs onto DGCs. These observations reveal roles for FRS adapters in DGC maturation and synaptogenesis and suggest that FRS proteins may act as an important node for FGF and neurotrophin signaling in postnatal hippocampal development.

Aging Triggers a Repressive Chromatin State at Bdnf Promoters in Hippocampal Neurons

Cognitive capacities decline with age, an event accompanied by the altered transcription of synaptic plasticity genes. Here, we show that the transcriptional induction of Bdnf by a mnemonic stimulus is impaired in aged hippocampal neurons. Mechanistically, this defect is due to reduced NMDA receptor (NMDAR)-mediated activation of CaMKII. Decreased NMDAR signaling prevents changes associated with activation at specific Bdnf promoters, including displacement of histone deacetylase 4, recruitment of the histone acetyltransferase CBP, increased H3K27 acetylation, and reduced H3K27 trimethylation. The decrease in NMDA-CaMKII signaling arises from constitutive reduction of synaptic cholesterol that occurs with normal aging. Increasing the levels of neuronal cholesterol in aged neurons in vitro, ex vivo, and in vivo restored NMDA-induced Bdnf expression and chromatin remodeling. Furthermore, pharmacological prevention of age-associated cholesterol reduction rescued signaling and cognitive deficits of aged mice. Thus, reducing hippocampal cholesterol loss may represent a therapeutic approach to reverse cognitive decline during aging.

Peptides Acting as Cognitive Enhancers

The aim of this paper is to present an overview of three peptides that, by improving synaptic function, enhance learning and memory in laboratory rodents. We summarize their structure, their mechanisms of action, and their effects on synaptic and cognitive function. First we describe FGL, a peptide derived from the neural cell adhesion molecule which improves cognition by the activation of the PKC pathway that triggers an activity-dependent delivery of AMPA receptors to the synapses. Then we describe PTD4-PI3KAc peptide that by activating PI3K signaling pathway it promotes synapse and spine formation and enhances hippocampal dependent memory. Lastly, we describe a new peptide derived from the well-known tumor suppressor PTEN that prevents pathological interactions between PTEN and PDZ proteins at synapses during exposure to Amyloid beta. This action prevents memory deterioration in mouse model of Alzheimer's disease. Together, this review indicates how learning and memory can be improved by manipulating synaptic function and number through pharmacological treatment with peptides, and it establishes synaptic function as a valid target for cognitive enhancement.

Unilateral stimulation of the lateral division of the dorsal telencephalon induces synaptic plasticity in the bilateral medial division of zebrafish

This study was aimed to evaluate the synaptic plasticity in projections from the dorsal lateral region (Dl) to the bilateral dorsal medial region (Dm) of the zebrafish telencephalon. The results showed that unilateral electrical stimulation of the Dl evokes a negative field potential (FP) in both the contralateral and ipsilateral side of the Dm. We tested synaptic plasticity, including high-frequency stimulation-induced LTP (HFS-LTP) and low-frequency stimulation-induced LTD (LFS-LTD). We demonstrated that HFS-induced bilateral LTP is NMDAR-dependent by the application of an NMDAR antagonist, DL-AP5 (30 μM, suprafused for 10 min), which blocked the HFS-induced LTP in both the contralateral and ipsilateral Dm. In addition, LTP was restored after DL-AP5 was washed out by continuous aCSF suprafusion. These results suggested that the potentiation is NMDAR-dependent. Either LFS (1 Hz for 20 min) or applying the mGluR agonist, DHPG (40 μM, suprafused for 10 min) successfully induced bilateral LTD for at least 1 h. Furthermore, both the contralateral fEPSP and LTP vanished after ablation of the anterior commissure. In conclusion, the results of the present study suggested that the projection between the Dl and contralateral Dm in the telencephalon of zebrafish is via the anterior commissure and possesses synaptic plasticity.

Neural Cell Adhesion Molecules of the Immunoglobulin Superfamily Regulate Synapse Formation, Maintenance, and Function

Immunoglobulin superfamily adhesion molecules are among the most abundant proteins in vertebrate and invertebrate nervous systems. Prominent family members are the neural cell adhesion molecules NCAM and L1, which were the first to be shown to be essential not only in development but also in synaptic function and as key regulators of synapse formation, synaptic activity, plasticity, and synaptic vesicle recycling at distinct developmental and activity stages. In addition to interacting with each other, adhesion molecules interact with ion channels and cytokine and neurotransmitter receptors. Mutations in their genes are linked to neurological disorders associated with abnormal development and synaptic functioning. This review presents an overview of recent studies on these molecules and their crucial impact on neurological disorders.

BBB-Permeable, Neuroprotective, and Neurotrophic Polysaccharide, Midi-GAGR

An enormous amount of efforts have been poured to find an effective therapeutic agent for the treatment of neurodegenerative diseases including Alzheimer’s disease (AD). Among those, neurotrophic peptides that regenerate neuronal structures and increase neuron survival show a promise in slowing neurodegeneration. However, the short plasma half-life and poor blood-brain-barrier (BBB)-permeability of neurotrophic peptides limit their in vivo efficacy. Thus, an alternative neurotrophic agent that has longer plasma half-life and better BBB-permeability has been sought for. Based on the recent findings of neuroprotective polysaccharides, we searched for a BBB-permeable neuroprotective polysaccharide among natural polysaccharides that are approved for human use. Then, we discovered midi-GAGR, a BBB-permeable, long plasma half-life, strong neuroprotective and neurotrophic polysaccharide. Midi-GAGR is a 4.7kD cleavage product of low acyl gellan gum that is approved by FDA for human use. Midi-GAGR protected rodent cortical neurons not only from the pathological concentrations of co-/post-treated free reactive radicals and Aβ₄₂ peptide but also from activated microglial cells. Moreover, midi-GAGR showed a good neurotrophic effect; it enhanced neurite outgrowth and increased phosphorylated cAMP-responsive element binding protein (pCREB) in the nuclei of primary cortical neurons. Furthermore, intra-nasally administered midi-GAGR penetrated the BBB and exerted its neurotrophic effect inside the brain for 24 h after one-time administration. Midi-GAGR appears to activate fibroblast growth factor receptor 1 (FGFR1) and its downstream neurotrophic signaling pathway for neuroprotection and CREB activation. Additionally, 14-day intranasal administration of midi-GAGR not only increased neuronal activity markers but also decreased hyperphosphorylated tau, a precursor of neurofibrillary tangle, in the brains of the AD mouse model, 3xTg-AD. Taken together, midi-GAGR with good BBB-permeability, long plasma half-life, and strong neuroprotective and neurotrophic effects has a great therapeutic potential for the treatment of neurodegenerative diseases, especially AD.

Hippocampal Dendritic Spines Are Segregated Depending on Their Actin Polymerization

Dendritic spines are mushroom-shaped protrusions of the postsynaptic membrane. Spines receive the majority of glutamatergic synaptic inputs. Their morphology, dynamics, and density have been related to synaptic plasticity and learning. The main determinant of spine shape is filamentous actin. Using FRAP, we have reexamined the actin dynamics of individual spines from pyramidal hippocampal neurons, both in cultures and in hippocampal organotypic slices. Our results indicate that, in cultures, the actin mobile fraction is independently regulated at the individual spine level, and mobile fraction values do not correlate with either age or distance from the soma. The most significant factor regulating actin mobile fraction was the presence of astrocytes in the culture substrate. Spines from neurons growing in the virtual absence of astrocytes have a more stable actin cytoskeleton, while spines from neurons growing in close contact with astrocytes show a more dynamic cytoskeleton. According to their recovery time, spines were distributed into two populations with slower and faster recovery times, while spines from slice cultures were grouped into one population. Finally, employing fast lineal acquisition protocols, we confirmed the existence of loci with high polymerization rates within the spine.

The synthetic NCAM mimetic peptide FGL mobilizes neural stem cells in vitro and in vivo

The neural cell adhesion molecule (NCAM) plays a role in neurite outgrowth, synaptogenesis, and neuronal differentiation. The NCAM mimetic peptide FG Loop (FGL) promotes neuronal survival in vitro and enhances spatial learning and memory in rats. We here investigated the effects of FGL on neural stem cells (NSC) in vitro and in vivo. In vitro, cell proliferation of primary NSC was assessed after exposure to various concentrations of NCAM or FGL. The differentiation potential of NCAM- or FGL-treated cells was assessed immunocytochemically. To investigate its influence on endogenous NSC in vivo, FGL was injected subcutaneously into adult rats. The effects on NSC mobilization were studied both via non-invasive positron emission tomography (PET) imaging using the tracer [(18)F]-fluoro-L-thymidine ([(18)F]FLT), as well as with immunohistochemistry. Only FGL significantly enhanced NSC proliferation in vitro, with a maximal effect at 10 μg/ml. During differentiation, NCAM promoted neurogenesis, while FGL induced an oligodendroglial phenotype; astrocytic differentiation was neither affected by NCAM or FGL. Those differential effects of NCAM and FGL on differentiation were mediated through different receptors. After FGL-injection in vivo, proliferative activity of NSC in the subventricular zone (SVZ) was increased (compared to placebo-treated animals). Moreover, non-invasive imaging of cell proliferation using [(18)F]FLT-PET supported an FGL-induced mobilization of NSC from both the SVZ and the hippocampus. We conclude that FGL robustly induces NSC mobilization in vitro and in vivo, and supports oligodendroglial differentiation. This capacity renders FGL a promising agent to facilitate remyelinization, which may eventually make FGL a drug candidate for demyelinating neurological disorders.

D-cycloserine prevents relational memory deficits and suppression of long-term potentiation induced by scopolamine in the hippocampus

Previous research has demonstrated that systemic D-cycloserine (DCS), a partial agonist of the N-methyl-D-aspartate receptor (NMDAR), enhances memory processes in different learning paradigms and attenuates mnemonic deficits produced by diverse pharmacological manipulations. In the present study two experiments were conducted in rats to investigate whether DCS administered in the hippocampus may rescue relational memory deficits and improve deficient synaptic plasticity, both induced by an intracerebral injection of the muscarinic receptor antagonist scopolamine (SCOP). In experiment 1, we assessed whether DCS would prevent SCOP-induced amnesia in an olfactory learning paradigm requiring the integrity of the cholinergic system, the social transmission of food preference (STFP). The results showed that DCS (10 μg/site) injected into the ventral hippocampus (vHPC) before STFP acquisition compensated the 24-h retention deficit elicited by post-training intra-vHPC SCOP (40 μg/site), although it did not affect memory expression in non-SCOP treated rats. In experiment 2, we evaluated whether the perfusion of DCS in hippocampal slices may potentiate synaptic plasticity in CA1 synapses and thus recover SCOP-induced deficits in long-term potentiation (LTP). We found that DCS (50 µM and 100 µM) was able to rescue SCOP (100 µM)-induced LTP maintenance impairment, in agreement with the behavioral findings. Additionally, DCS alone (50 µM and 100 µM) enhanced field excitatory postsynaptic potentials prior to high frequency stimulation, although it did not significantly potentiate LTP. Our results suggest that positive modulation of the NMDAR, by activation of the glycine-binding site, may compensate relational memory impairments due to hippocampal muscarinic neurotransmission dysfunction possibly through enhancements in LTP maintenance.

The role of low levels of fullerene C60 nanocrystals on enhanced learning and memory of rats through persistent CaMKII activation

Engineered nanomaterials are known to exhibit diverse and sometimes unexpected biological effects. Fullerene nanoparticles have been reported to specifically bind to and elicit persistent activation of hippocampal Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), a multimeric intracellular serine/threonine kinase central to Ca(2+) signal transduction and critical for synaptic plasticity, but the functional consequence of that modulation is unknown. Here we show that low doses of fullerene C60 nanocrystals (Nano C60), delivered through intrahippocampal infusion and without any obvious cytotoxicity in hippocampal neuronal cells, enhance the long-term potentiation (LTP) of rats. Intraperitoneal injection of 320 μg/kg of Nano C60, once daily for 10 days, also enhanced spatial memory of rats in addition to an increase of LTP. In parallel, both the IH and IP administration of Nano C60 increased the autonomous activity and the level of threonine 286 (T286) autophosphorylation of CaMKII, enhanced post-synaptic AMPA/NMDA ratio, and triggered time-dependent activation of ERK and CREB. Our results reveal a striking and highly unexpected ability of Nano C60 in positively modulating learning and memory, an effect that is most likely manifested through locking CaMKII in an active conformation, and may have significant implications for the potential therapeutic applications of fullerene C60, a classic engineered nanomaterial.

Glutamatergic autoencephalitides: an emerging field

Autoimmune responses targeting synaptic proteins are associated with a wide range of neurologic symptoms. Among these disorders are those associated with antibodies to ionotropic glutamate receptors, including the NMDAR (N-methyl-D-aspartate receptor) and AMPAR (α-amino-3-hydrozy-5-methyl-4-isoxazolepropionic acid receptor). Patients with anti-NMDAR encephalitis present with psychiatric symptoms, seizures, movement disorders, impaired consciousness, and autonomic derangements; half of patients have an associated ovarian teratoma, and most patients respond to immunosuppressive therapies. Patients' antibodies bind to the amino terminal domain of the NMDAR, and result in loss of NMDARs from synapses with subsequent NMDAR hypofunction. Anti-NMDAR antibodies have now been reported in other neuropsychiatric conditions, including psychosis, dementia, and HSV encephalitis. The pathophysiologic relevance of anti-NMDAR antibodies in these disorders is not yet clear, but their presence may indicate a role for immunotherapy in some patients. Although considerable work remains to be done, our understanding of disorders associated with anti-glutamate receptor antibodies has grown exponentially since they were first described just over 7 years ago, revolutionizing neurology. These antibodies, by interfering with synaptic function, readily link basic science and clinical medicine, and have revealed the impact of sudden but sustained loss of specific neurotransmitter receptors in humans. Improved understanding of their pathophysiology will lead to better treatments for these diseases while providing novel insights regarding the roles of glutamate receptors in learning, memory, and neuropsychiatric disease.

Ovarian hormone loss impairs excitatory synaptic transmission at hippocampal CA3-CA1 synapses

Premature and long-term ovarian hormone loss following ovariectomy (OVX) is associated with cognitive impairment. This condition is prevented by estradiol (E2) therapy when initiated shortly following OVX but not after substantial delay. To determine whether these clinical findings are correlated with changes in synaptic functions, we used adult OVX rats to evaluate the consequences of short-term (7-10 d, OVXControl) and long-term (∼5 months, OVXLT) ovarian hormone loss, as well as subsequent in vivo E2 treatment, on excitatory synaptic transmission at the hippocampal CA3-CA1 synapses important for learning and memory. The results show that ovarian hormone loss was associated with a marked decrease in synaptic strength. E2 treatment increased synaptic strength in OVXControl but not OVXLT rats, demonstrating a change in the efficacy for E2 5 months following OVX. E2 also had a more rapid effect: within minutes of bath application, E2 acutely increased synaptic strength in all groups except OVXLT rats that did not receive in vivo E2 treatment. E2's acute effect was mediated postsynaptically, and required Ca(2+) influx through the voltage-gated Ca(2+) channels. Despite E2's acute effect, synaptic strength of OVXLT rats remained significantly lower than that of OVXControl rats. Thus, changes in CA3-CA1 synaptic transmission associated with ovarian hormone loss cannot be fully reversed with delayed E2 treatment. Given that synaptic strength at CA3-CA1 synapses is related to the ability to learn hippocampus-dependent tasks, these findings provide additional insights for understanding cognitive impairment-associated long-term ovarian hormone loss and ineffectiveness for delayed E2 treatment to maintain cognitive functions.

Hippocampal protein kinase C family members in spatial memory retrieval in the mouse

Although a few individual members of the protein kinase C (PKC) family were studied in spatial memory no systematic approach was carried out to concomitantly determine all described PKC family members in spatial memory of the mouse. It was therefore the aim of the current study to link hippocampal PKCs to memory retrieval in the Morris water maze (MWM). CD1 mice were trained (n=9) or untrained (n=9) in the MWM, hippocampi were taken 6h following the test for memory retrieval and PKCs were determined in mouse hippocampi by immunoblotting. The trained animals learned the spatial memory task and kept memory at the probe trial. PKCs alpha and epsilon were comparable between groups while PKCs beta, delta, gamma (two forms, i.e. two bands on Western blotting), zeta (2 forms) were higher in trained mice and theta (2 forms) were lower in trained mice. PKC gamma (1 form) was significantly correlating with the time spent in the target quadrant (r=0.7933; P=0.0188). Changes of hippocampal levels of PKCs beta, delta, gamma, zeta and theta were paralleling memory retrieval of the MWM task but correlations revealed that spatial memory retrieval was only linked to one form of PKC gamma. Results are also in agreement with a recent publication showing that PKM zeta is not required for memory formation. These findings may be relevant for the interpretation of previous work and the design of future work on the protein kinase C family in spatial memory of the mouse.

Fibroblast growth factor-2 signaling in neurogenesis and neurodegeneration

Fibroblast growth factor-2 (FGF2), also known as basic FGF, is a multi-functional growth factor. One of the 22-member FGF family, it signals through receptor tyrosine kinases encoding FGFR1-4. FGF2 activates FGFRs in cooperation with heparin or heparin sulfate proteoglycan to induce its pleiotropic effects in different tissues and organs, which include potent angiogenic effects and important roles in the differentiation and function of the central nervous system (CNS). FGF2 is crucial to development of the CNS, which explains its importance in adult neurogenesis. During development, high levels of FGF2 are detected from neurulation onwards. Moreover, developmental expression of FGF2 and its receptors is temporally and spatially regulated, concurring with development of specific brain regions including the hippocampus and substantia nigra pars compacta. In adult neurogenesis, FGF2 has been implicated based on its expression and regulation of neural stem and progenitor cells in the neurogenic niches, the subventricular zone (SVZ) and the subgranular zone (SGZ) of the hippocampal dentate gyrus. FGFR1 signaling also modulates inflammatory signaling through the surface glycoprotein CD200, which regulates microglial activation. Because of its importance in adult neurogenesis and neuroinflammation, manipulation of FGF2/FGFR1 signaling has been a focus of therapeutic development for neurodegenerative disorders, such as Alzheimer's disease, multiple sclerosis, Parkinson's disease and traumatic brain injury. Novel strategies include intranasal administration of FGF2, administration of an NCAM-derived FGFR1 agonist, and chitosan-based nanoparticles for the delivery of FGF2 in pre-clinical animal models. In this review, we highlight current research towards therapeutic interventions targeting FGF2/FGFR1 in neurodegenerative disorders.

Long-term social isolation in the adulthood results in CA1 shrinkage and cognitive impairment

Social isolation in adulthood is a psychosocial stressor that can result in endocrinological and behavioral alterations in different species. In rodents, controversial results have been obtained in fear conditioning after social isolation at adulthood, while neural substrates underlying these differences are largely unknown. Neural cell adhesion molecule (NCAM) and its polysialylated form (PSA-NCAM) are prominent modulators of synaptic plasticity underlying memory processes in many tasks, including fear conditioning. In this study, we used adult female Octodon degus to investigate the effects of long-term social isolation on contextual and cued fear conditioning, and the possible modulation of the synaptic levels of NCAM and PSA-NCAM in the hippocampus. After 6½ months of social isolation, adult female degus showed a normal auditory-cued fear memory, but a deficit in contextual fear memory, a hippocampal dependent task. Subsequently, we observed reduced hippocampal synaptic levels of PSA-NCAM in isolated compared to grouped-housed female degus. No significant differences were found between experimental groups in hippocampal levels of the three main isoforms of NCAM (NCAM180, NCAM140 and NCAM120). Interestingly, social isolation reduced the volume of the hippocampal CA1 subfield, without affecting the volume of the CA3 subregion or the total hippocampus. Moreover, attenuated body weight gain and reduced number of granulocytes were detected in isolated animals. Our findings indicate for the first time, that long-term social isolation of adult female animals induces a specific shrinkage of CA1 and a decrease in synaptic levels of PSA-NCAM in the hippocampus. These effects may be related to the deficit in contextual fear memory observed in isolated female degus.

Age-related cognitive impairments in mice with a conditional ablation of the neural cell adhesion molecule

Most of the mechanisms involved in neural plasticity support cognition, and aging has a considerable effect on some of these processes. The neural cell adhesion molecule (NCAM) of the immunoglobulin superfamily plays a pivotal role in structural and functional plasticity and is required to modulate cognitive and emotional behaviors. However, whether aging is associated with NCAM alterations that might contribute to age-related cognitive decline is not currently known. In this study, we determined whether conditional NCAM-deficient mice display increased vulnerability to age-related cognitive and emotional alterations. We assessed the NCAM expression levels in the hippocampus and medial prefrontal cortex (mPFC) and characterized the performance of adult and aged conditional NCAM-deficient mice and their age-matched wild-type littermates in a delayed matching-to-place test in the Morris water maze and a delayed reinforced alternation test in the T-maze. Although aging in wild-type mice is associated with an isoform-specific reduction of NCAM expression levels in the hippocampus and mPFC, these mice exhibited only mild impairments in working/episodic-like memory performance. However, aged conditional NCAM-deficient mice displayed pronounced impairments in both the delayed matching-to-place and the delayed reinforced alternation tests. Importantly, the deficits of aged NCAM-deficient mice in these working/episodic-like memory tasks could not be attributed to increased anxiety-like behaviors or to differences in locomotor activity. Taken together, these data indicate that reduced NCAM expression in the forebrain might be a critical factor for the occurrence of cognitive impairments during aging.

NCAM function in the adult brain: lessons from mimetic peptides and therapeutic potential

Neural cell adhesion molecules (NCAMs) are complexes of transmembranal proteins critical for cell-cell interactions. Initially recognized as key players in the orchestration of developmental processes involving cell migration, cell survival, axon guidance, and synaptic targeting, they have been shown to retain these functions in the mature adult brain, in relation to plastic processes and cognitive abilities. NCAMs are able to interact among themselves (homophilic binding) as well as with other molecules (heterophilic binding). Furthermore, they are the sole molecule of the central nervous system undergoing polysialylation. Most interestingly polysialylated and non-polysialylated NCAMs display opposite properties. The precise contributions each of these characteristics brings in the regulations of synaptic and cellular plasticity in relation to cognitive processes in the adult brain are not yet fully understood. With the aim of deciphering the specific involvement of each interaction, recent developments led to the generation of NCAM mimetic peptides that recapitulate identified binding properties of NCAM. The present review focuses on the information such advances have provided in the understanding of NCAM contribution to cognitive function.

Differences between synaptic plasticity thresholds result in new timing rules for maximizing long-term potentiation

The fundamental observation that the temporal spacing of learning episodes plays a critical role in the efficiency of memory encoding has had little effect on either research on long-term potentiation (LTP) or efforts to develop cognitive enhancers. Here we review recent findings describing a spaced trials phenomenon for LTP that appears to be related to recent evidence that plasticity thresholds differ between synapses in the adult hippocampus. Results of tests with one memory enhancing drug suggest that the compound potently facilitates LTP via effects on 'high threshold' synapses and thus alters the temporally extended timing rules. Possible implications of these results for our understanding of LTP substrates, neurobiological contributors to the distributed practice effect, and the consequences of memory enhancement are discussed. This article is part of a Special Issue entitled 'Cognitive Enhancers'.