Two-Stage Translational Control of Dentate Gyrus LTP Consolidation Is Mediated by Sustained BDNF-TrkB Signaling to MNK

Repurposing of niclosamide, an anthelmintic, by targeting ERK/MAPK signaling pathway in the experimental paradigm of autism spectrum disorders

AIM

The current study explores niclosamide's neuroprotective potential in an animal model of autism spectrum disorder (ASD) and goes further to understand how the ERK/MAPK signaling pathway is thought to contribute to this activity.

METHODS

In order to create an autism-like phenotype in rats, 4 μl of 1 M PPA was infused intracerebroventricularly. The oral treatment with niclosamide (50 and 100 mg/kg) and risperidone (1 mg/kg) (used as standard) was given from 3rd to 30th day. Between the 14th and 28th day, behavioral assessments were made for sociability, stereotypy, anxiety, depression, novelty preference, repetitive behavior, and perseverative behavior. The animals were euthanized on the 29th day, and oxidative stress markers were assessed in the brain homogenate. The levels of neuroinflammatory cytokines such as TNF-α, IL-6, NF-κB, IFN-γ and glutamate were estimated using ELISA kits. To assess the involvement of the ERK/MAPK signaling pathway, levels of Nrf2 and ERK2 were also measured.

KEY FINDINGS

Niclosamide therapy significantly restored behavioral, biochemical, neurological, and molecular impairments. Hence, niclosamide could be a potential neurotherapeutic candidate for further studies for use in ASD.

BDNF-dependent nano-organization of Neogenin and the WAVE regulatory complex promotes actin remodeling in dendritic spines

Synaptic structural plasticity, the expansion of dendritic spines in response to synaptic stimulation, is essential for experience-dependent plasticity and is driven by branched actin polymerization. The WAVE regulatory complex (WRC) is confined to nanodomains at the postsynaptic membrane where it catalyzes actin polymerization. As the netrin/RGM receptor Neogenin is a critical regulator of the WRC, its nanoscale organization may be an important determinant of WRC nanoarchitecture and function. Using super-resolution microscopy, we reveal that Neogenin is highly organized on the spine membrane at the nanoscale level. We show that Neogenin binding to the WRC promotes co-clustering into nanodomains in response to brain-derived neurotrophic factor (BDNF), indicating that nanoclustering occurs in response to synaptic stimulation. Disruption of Neogenin/WRC binding not only prevents BDNF-mediated actin remodeling but also inhibits BDNF-induced calcium signaling. We conclude that the assembly of Neogenin/WRC nanodomains is a prerequisite for BDNF-mediated structural and synaptic plasticity.

Role of brain renin-angiotensin system in depression: A new perspective

Depression is a mood disorder characterized by abnormal thoughts. The pathophysiology of depression is related to the deficiency of serotonin (5HT), which is derived from tryptophan (Trp). Mitochondrial dysfunction, oxidative stress, and neuroinflammation are involved in the pathogenesis of depression. Notably, the renin-angiotensin system (RAS) is involved in the pathogenesis of depression, and different findings revealed that angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs) may be effective in depression. However, the underlying mechanism for the role of dysregulated brain RAS-induced depression remains speculative. Therefore, this review aimed to revise the conceivable role of ACEIs and ARBs and how these agents ameliorate the pathophysiology of depression. Dysregulation of brain RAS triggers the development and progression of depression through the reduction of brain 5HT and expression of brain-derived neurotrophic factor (BDNF) and the induction of mitochondrial dysfunction, oxidative stress, and neuroinflammation. Therefore, inhibition of central classical RAS by ARBS and ACEIs and activation of non-classical RAS prevent the development of depression by regulating 5HT, BDNF, mitochondrial dysfunction, oxidative stress, and neuroinflammation.

Different dysregulations of CYFIP1 and CYFIP2 in distinct types of dementia

In humans, the cytoplasmic FMR1 interacting protein (CYFIP) family consists of two members, namely CYFIP1 and CYFIP2. Both CYFIP1 and CYFIP2 function in the WAVE regulatory complex (WRC), which regulates actin polymerization. Additionally, these two proteins form a posttranscriptional regulatory complex with the fragile X mental retardation protein (FMRP), which suppresses mRNA translation. Thus, CYFIP1 and CYFIP2 are important signalling regulators at synapses, and mutations in their genes are associated with neurodevelopmental and neuropsychiatric disorders, including intellectual disabilities. Moreover, dysregulation of the CYFIP protein family is involved in Alzheimer's disease (AD). However, the relevance of the CYFIP family in other dementias is largely unknown. Here, we compared CYFIP1/2 protein levels in the post-mortem hippocampus from patients with AD, dementia with Lewy bodies (DLB), vascular dementia (VaD) and frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP). Consistent with previous findings, CYFIP2 was reduced in AD hippocampus. In DLB and VaD hippocampus, the protein level of CYFIP2 and CYFIP1 was unaltered. Finally, an increase in the protein level of both CYFIP1 and CYFIP2 was noted in FTLD-TDP hippocampus. These findings reveal that the protein levels of the CYFIP family is distinct in different types of dementia, suggesting that the pathogenesis of these neurodegenerative disorders has divergent impacts on hippocampal synaptic function.

Therapeutic Strategies Aimed at Improving Neuroplasticity in Alzheimer Disease

Alzheimer disease (AD) is the most prevalent form of dementia among elderly people. Owing to its varied and multicausal etiopathology, intervention strategies have been highly diverse. Despite ongoing advances in the field, efficient therapies to mitigate AD symptoms or delay their progression are still of limited scope. Neuroplasticity, in broad terms the ability of the brain to modify its structure in response to external stimulation or damage, has received growing attention as a possible therapeutic target, since the disruption of plastic mechanisms in the brain appear to correlate with various forms of cognitive impairment present in AD patients. Several pre-clinical and clinical studies have attempted to enhance neuroplasticity via different mechanisms, for example, regulating glucose or lipid metabolism, targeting the activity of neurotransmitter systems, or addressing neuroinflammation. In this review, we first describe several structural and functional aspects of neuroplasticity. We then focus on the current status of pharmacological approaches to AD stemming from clinical trials targeting neuroplastic mechanisms in AD patients. This is followed by an analysis of analogous pharmacological interventions in animal models, according to their mechanisms of action.

Maintenance of a short-lived protein required for long-term memory involves cycles of transcription and local translation

Activity-dependent expression of immediate early genes (IEGs) is critical for long-term synaptic remodeling and memory. It remains unknown how IEGs are maintained for memory despite rapid transcript and protein turnover. To address this conundrum, we monitored Arc, an IEG essential for memory consolidation. Using a knockin mouse where endogenous Arc alleles were fluorescently tagged, we performed real-time imaging of Arc mRNA dynamics in individual neurons in cultures and brain tissue. Unexpectedly, a single burst stimulation was sufficient to induce cycles of transcriptional reactivation in the same neuron. Subsequent transcription cycles required translation, whereby new Arc proteins engaged in autoregulatory positive feedback to reinduce transcription. The ensuing Arc mRNAs preferentially localized at sites marked by previous Arc protein, assembling a "hotspot" of translation, and consolidating "hubs" of dendritic Arc. These cycles of transcription-translation coupling sustain protein expression and provide a mechanism by which a short-lived event may support long-term memory.

Visualizing Arc protein dynamics and localization in the mammalian brain using AAV-mediated in situ gene labeling

The activity-regulated cytoskeleton-associated (Arc) protein is essential for synaptic plasticity and memory formation. The Arc gene, which contains remnants of a structural GAG retrotransposon sequence, produces a protein that self-assembles into capsid-like structures harboring Arc mRNA. Arc capsids, released from neurons, have been proposed as a novel intercellular mechanism for mRNA transmission. Nevertheless, evidence for intercellular transport of Arc in the mammalian brain is still lacking. To enable the tracking of Arc molecules from individual neurons in vivo, we devised an adeno-associated virus (AAV) mediated approach to tag the N-terminal of the mouse Arc protein with a fluorescent reporter using CRISPR/Cas9 homologous independent targeted integration (HITI). We show that a sequence coding for mCherry can successfully be knocked in at the 5' end of the Arc open reading frame. While nine spCas9 gene editing sites surround the Arc start codon, the accuracy of the editing was highly sequence-dependent, with only a single target resulting in an in-frame reporter integration. When inducing long-term potentiation (LTP) in the hippocampus, we observed an increase of Arc protein highly correlated with an increase in fluorescent intensity and the number of mCherry-positive cells. By proximity ligation assay (PLA), we demonstrated that the mCherry-Arc fusion protein retains the Arc function by interacting with the transmembrane protein stargazin in postsynaptic spines. Finally, we recorded mCherry-Arc interaction with presynaptic protein Bassoon in mCherry-negative surrounding neurons at close proximity to mCherry-positive spines of edited neurons. This is the first study to provide support for inter-neuronal in vivo transfer of Arc in the mammalian brain.

Detection of Arc/Arg3.1 oligomers in rat brain: constitutive and synaptic activity-evoked dimer expression in vivo

The immediate early gene product activity-regulated cytoskeleton-associated protein (Arc or Arg3.1) is a major regulator of long-term synaptic plasticity with critical roles in postnatal cortical development and memory formation. However, the molecular basis of Arc function is undefined. Arc is a hub protein with interaction partners in the postsynaptic neuronal compartment and nucleus. Previous in vitro biochemical and biophysical analysis of purified recombinant Arc showed formation of low-order oligomers and larger particles including retrovirus-like capsids. Here, we provide evidence for naturally occurring Arc oligomers in the mammalian brain. Using in situ protein crosslinking to trap weak Arc-Arc interactions, we identified in various preparations a prominent Arc immunoreactive band on SDS-PAGE of molecular mass corresponding to a dimer. While putative trimers, tetramers and heavier Arc species were detected, they were of lower abundance. Stimulus-evoked induction of Arc expression and dimer formation was first demonstrated in SH-SY5Y neuroblastoma cells treated with the muscarinic cholinergic agonist, carbachol, and in primary cortical neuronal cultures treated with brain-derived neurotrophic factor (BDNF). In the dentate gyrus (DG) of adult anesthetized rats, induction of long-term potentiation (LTP) by high-frequency stimulation (HFS) of medial perforant synapses or by brief intrahippocampal infusion of BDNF led to a massive increase in Arc dimer expression. Arc immunoprecipitation of crosslinked DG tissue showed enhanced dimer expression during 4 h of LTP maintenance. Mass spectrometric proteomic analysis of immunoprecipitated, gel-excised bands corroborated detection of Arc dimer. Furthermore, Arc dimer was constitutively expressed in naïve cortical, hippocampal and DG tissue, with the lowest levels in the DG. Taken together the results implicate Arc dimer as the predominant low-oligomeric form in mammalian brain, exhibiting regional differences in its constitutive expression and enhanced synaptic activity-evoked expression in LTP.

eIF4E phosphorylation recruits β-catenin to mRNA cap and promotes Wnt pathway translation in dentate gyrus LTP maintenance

The mRNA cap-binding protein, eukaryotic initiation factor 4E (eIF4E), is crucial for translation and regulated by Ser209 phosphorylation. However, the biochemical and physiological role of eIF4E phosphorylation in translational control of long-term synaptic plasticity is unknown. We demonstrate that phospho-ablated Eif4e^S209A Knockin mice are profoundly impaired in dentate gyrus LTP maintenance in vivo, whereas basal perforant path-evoked transmission and LTP induction are intact. mRNA cap-pulldown assays show that phosphorylation is required for synaptic activity-induced removal of translational repressors from eIF4E, allowing initiation complex formation. Using ribosome profiling, we identified selective, phospho-eIF4E-dependent translation of the Wnt signaling pathway in LTP. Surprisingly, the canonical Wnt effector, β-catenin, was massively recruited to the eIF4E cap complex following LTP induction in wild-type, but not Eif4e^S209A, mice. These results demonstrate a critical role for activity-evoked eIF4E phosphorylation in dentate gyrus LTP maintenance, remodeling of the mRNA cap-binding complex, and specific translation of the Wnt pathway.

Mnk1/2 kinases regulate memory and autism-related behaviours via Syngap1

MAPK interacting protein kinases 1 and 2 (Mnk1/2) regulate a plethora of functions, presumably via phosphorylation of their best characterized substrate, eukaryotic translation initiation factor 4E (eIF4E) on Ser209. Here, we show that, whereas deletion of Mnk1/2 (Mnk double knockout) impairs synaptic plasticity and memory in mice, ablation of phospho-eIF4E (Ser209) does not affect these processes, suggesting that Mnk1/2 possess additional downstream effectors in the brain. Translational profiling revealed only a small overlap between the Mnk1/2- and phospho-eIF4E(Ser209)-regulated translatome. We identified the synaptic Ras GTPase activating protein 1 (Syngap1), encoded by a syndromic autism gene, as a downstream target of Mnk1 because Syngap1 immunoprecipitated with Mnk1 and showed reduced phosphorylation (S788) in Mnk double knockout mice. Knockdown of Syngap1 reversed memory deficits in Mnk double knockout mice and pharmacological inhibition of Mnks rescued autism-related phenotypes in Syngap1+/- mice. Thus, Syngap1 is a downstream effector of Mnk1, and the Mnks-Syngap1 axis regulates memory formation and autism-related behaviours.

CYFIP1 Dosages Exhibit Divergent Behavioral Impact via Diametric Regulation of NMDA Receptor Complex Translation in Mouse Models of Psychiatric Disorders

BACKGROUND

Gene dosage imbalance caused by copy number variations (CNVs) is a prominent contributor to brain disorders. In particular, 15q11.2 CNV duplications and deletions have been associated with autism spectrum disorder and schizophrenia, respectively. The mechanism underlying these diametric contributions remains unclear.

METHODS

We established both loss-of-function and gain-of-function mouse models of Cyfip1, one of four genes within 15q11.2 CNVs. To assess the functional consequences of altered CYFIP1 levels, we performed systematic investigations on behavioral, electrophysiological, and biochemical phenotypes in both mouse models. In addition, we utilized RNA immunoprecipitation sequencing (RIP-seq) analysis to reveal molecular targets of CYFIP1 in vivo.

RESULTS

Cyfip1 loss-of-function and gain-of function mouse models exhibited distinct and shared behavioral abnormalities related to autism spectrum disorder and schizophrenia. RIP-seq analysis identified messenger RNA targets of CYFIP1 in vivo, including postsynaptic NMDA receptor (NMDAR) complex components. In addition, these mouse models showed diametric changes in levels of postsynaptic NMDAR complex components at synapses because of dysregulated protein translation, resulting in bidirectional alteration of NMDAR-mediated signaling. Importantly, pharmacological balancing of NMDAR signaling in these mouse models with diametric Cyfip1 dosages rescues behavioral abnormalities.

CONCLUSIONS

CYFIP1 regulates protein translation of NMDAR and associated complex components at synapses to maintain normal synaptic functions and behaviors. Our integrated analyses provide insight into how gene dosage imbalance caused by CNVs may contribute to divergent neuropsychiatric disorders.

Molecular physiology of Arc/Arg3.1: The oligomeric state hypothesis of synaptic plasticity

The immediate early gene, Arc, is a pivotal regulator of synaptic plasticity, memory, and cognitive flexibility. But what is Arc protein? How does it work? Inside the neuron, Arc is a protein interaction hub and dynamic regulator of intra-cellular signaling in synaptic plasticity. In remarkable contrast, Arc can also self-assemble into retrovirus-like capsids that are released in extracellular vesicles and capable of intercellular transfer of RNA. Elucidation of the molecular basis of Arc hub and capsid functions, and the relationship between them, is vital for progress. Here, we discuss recent findings on Arc structure-function and regulation of oligomerization that are giving insight into the molecular physiology of Arc. The unique features of mammalian Arc are emphasized, while drawing comparisons with Drosophila Arc and retroviral Gag. The Arc N-terminal domain, found only in mammals, is proposed to play a key role in regulating Arc hub signaling, oligomerization, and formation of capsids. Bringing together several lines of evidence, we hypothesize that Arc function in synaptic plasticity-long-term potentiation (LTP) and long-term depression (LTD)-are dictated by different oligomeric forms of Arc. Specifically, monomer/dimer function in LTP, tetramer function in basic LTD, and 32-unit oligomer function in enhanced LTD. The role of mammalian Arc capsids is unclear but likely depends on the cross-section of captured neuronal activity-induced RNAs. As the functional states of Arc are revealed, it may be possible to selectively manipulate specific forms of Arc-dependent plasticity and intercellular communication involved in brain function and dysfunction.

BDNF and its Role in the Alcohol Abuse Initiated During Early Adolescence: Evidence from Preclinical and Clinical Studies

Brain-derived neurotrophic factor (BDNF) is a crucial brain signaling protein that is integral to many signaling pathways. This neurotrophin has shown to be highly involved in brain plastic processes such as neurogenesis, synaptic plasticity, axonal growth, and neurotransmission, among others. In the first part of this review, we revise the role of BDNF in different neuroplastic processes within the central nervous system. On the other hand, its deficiency in key neural circuits is associated with the development of psychiatric disorders, including alcohol abuse disorder. Many people begin to drink alcohol during adolescence, and it seems that changes in BDNF are evident after the adolescent regularly consumes alcohol. Therefore, the second part of this manuscript addresses the involvement of BDNF during adolescent brain maturation and how this process can be negatively affected by alcohol abuse. Finally, we propose different BNDF enhancers, both behavioral and pharmacological, which should be considered in the treatment of problematic alcohol consumption initiated during the adolescence.

Progression in Time of Dentate Gyrus Granule Cell Layer Widening due to Excitotoxicity Occurs along In Vivo LTP Reinstatement and Contextual Fear Memory Recovery

The dentate gyrus (DG) is the gateway of sensory information arriving from the perforant pathway (PP) to the hippocampus. The adequate integration of incoming information into the DG is paramount in the execution of hippocampal-dependent cognitive functions. An abnormal DG granule cell layer (GCL) widening due to granule cell dispersion has been reported under hyperexcitation conditions in animal models as well as in patients with mesial temporal lobe epilepsy, but also in patients with no apparent relation to epilepsy. Strikingly, it is unclear whether the presence and severity of GCL widening along time affect synaptic processing arising from the PP and alter the performance in hippocampal-mediated behaviors. To evaluate the above, we injected excitotoxic kainic acid (KA) unilaterally into the DG of mice and analyzed the evolution of GCL widening at 10 and 30 days post injection (dpi), while analyzing if KA-induced GCL widening affected in vivo long-term potentiation (LTP) in the PP-DG pathway, as well as the performance in learning and memory through contextual fear conditioning. Our results show that at 10 dpi, when a subtle GCL widening was observed, LTP induction, as well as contextual fear memory, were impaired. However, at 30 dpi when a pronounced increase in GCL widening was found, LTP induction and contextual fear memory were already reestablished. These results highlight the plastic potential of the DG to recover some of its functions despite a major structural alteration such as abnormal GCL widening.

Brain-derived neurotrophic factor-induced regulation of RNA metabolism in neuronal development and synaptic plasticity

The neurotrophin brain-derived neurotrophic factor (BDNF) plays multiple roles in the nervous system, including in neuronal development, in long-term synaptic potentiation in different brain regions, and in neuronal survival. Alterations in these regulatory mechanisms account for several diseases of the nervous system. The synaptic effects of BDNF mediated by activation of tropomyosin receptor kinase B (TrkB) receptors are partly mediated by stimulation of local protein synthesis which is now considered a ubiquitous feature in both presynaptic and postsynaptic compartments of the neuron. The capacity to locally synthesize proteins is of great relevance at several neuronal developmental stages, including during neurite development, synapse formation, and stabilization. The available evidence shows that the effects of BDNF-TrkB signaling on local protein synthesis regulate the structure and function of the developing and mature synapses. While a large number of studies have illustrated a wide range of effects of BDNF on the postsynaptic proteome, a growing number of studies also point to presynaptic effects of the neurotrophin in the local regulation of the protein composition at the presynaptic level. Here, we will review the latest evidence on the role of BDNF in local protein synthesis, comparing the effects on the presynaptic and postsynaptic compartments. Additionally, we overview the relevance of BDNF-associated local protein synthesis in neuronal development and synaptic plasticity, at the presynaptic and postsynaptic compartments, and their relevance in terms of disease. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Export and Localization > RNA Localization.

BDNF Therapeutic Mechanisms in Neuropsychiatric Disorders

Brain-derived neurotrophic factor (BDNF) is the most abundant neurotrophin in the adult brain and functions as both a primary neurotrophic signal and a neuromodulator. It serves essential roles in neuronal development, maintenance, transmission, and plasticity, thereby influencing aging, cognition, and behavior. Accumulating evidence associates reduced central and peripheral BDNF levels with various neuropsychiatric disorders, supporting its potential utilization as a biomarker of central pathologies. Subsequently, extensive research has been conducted to evaluate restoring, or otherwise augmenting, BDNF transmission as a potential therapeutic approach. Promising results were indeed observed for genetic BDNF upregulation or exogenous administration using a multitude of murine models of neurological and psychiatric diseases. However, varying mechanisms have been proposed to underlie the observed therapeutic effects, and many findings indicate the engagement of disease-specific and other non-specific mechanisms. This is because BDNF essentially affects all aspects of neuronal cellular function through tropomyosin receptor kinase B (TrkB) receptor signaling, the disruptions of which vary between brain regions across different pathologies leading to diversified consequences on cognition and behavior. Herein, we review the neurophysiology of BDNF transmission and signaling and classify the converging and diverging molecular mechanisms underlying its therapeutic potentials in neuropsychiatric disorders. These include neuroprotection, synaptic maintenance, immunomodulation, plasticity facilitation, secondary neuromodulation, and preservation of neurovascular unit integrity and cellular viability. Lastly, we discuss several findings suggesting BDNF as a common mediator of the therapeutic actions of centrally acting pharmacological agents used in the treatment of neurological and psychiatric illness.

RNA and neuronal function: the importance of post-transcriptional regulation

The brain represents an organ with a particularly high diversity of genes that undergo post-transcriptional gene regulation through multiple mechanisms that affect RNA metabolism and, consequently, brain function. This vast regulatory process in the brain allows for a tight spatiotemporal control over protein expression, a necessary factor due to the unique morphologies of neurons. The numerous mechanisms of post-transcriptional regulation or translational control of gene expression in the brain include alternative splicing, RNA editing, mRNA stability and transport. A large number of trans-elements such as RNA-binding proteins and micro RNAs bind to specific cis-elements on transcripts to dictate the fate of mRNAs including its stability, localization, activation and degradation. Several trans-elements are exemplary regulators of translation, employing multiple cofactors and regulatory machinery so as to influence mRNA fate. Networks of regulatory trans-elements exert control over key neuronal processes such as neurogenesis, synaptic transmission and plasticity. Perturbations in these networks may directly or indirectly cause neuropsychiatric and neurodegenerative disorders. We will be reviewing multiple mechanisms of gene regulation by trans-elements occurring specifically in neurons.

Recent Advances on the Role of Brain-Derived Neurotrophic Factor (BDNF) in Neurodegenerative Diseases

Neurotrophins, such as brain-derived neurotrophic factor (BDNF), are essential for neuronal survival and growth. The signaling cascades initiated by BDNF and its receptor are the key regulators of synaptic plasticity, which plays important role in learning and memory formation. Changes in BDNF levels and signaling pathways have been identified in several neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Huntington's disease, and have been linked with the symptoms and course of these diseases. This review summarizes the current understanding of the role of BDNF in several neurodegenerative diseases, as well as the underlying molecular mechanism. The therapeutic potential of BDNF treatment is also discussed, in the hope of discovering new avenues for the treatment of neurodegenerative diseases.

The role of altered translation in intellectual disability and epilepsy

A very high proportion of cases of intellectual disability are genetic in origin and are associated with the occurrence of epileptic seizures during childhood. These two disorders together effect more than 5% of the world's population. One feature linking the two diseases is that learning and memory require the synthesis of new synaptic components and ion channels, while maintenance of overall excitability also requires synthesis of similar proteins in response to altered neuronal stimulation. Many of these disorders result from mutations in proteins that regulate mRNA processing, translation initiation, translation elongation, mRNA stability or upstream translation modulators. One theme that emerges on reviewing this field is that mutations in proteins that regulate changes in translation following neuronal stimulation are more likely to result in epilepsy with intellectual disability than general translation regulators with no known role in activity-dependent changes. This is consistent with the notion that activity-dependent translation in neurons differs from that in other cells types in that the changes in local cellular composition, morphology and connectivity that occur generally in response to stimuli are directly coupled to local synaptic activity and persist for months or years after the original stimulus.

Neuroligins in neurodevelopmental conditions: how mouse models of de novo mutations can help us link synaptic function to social behavior

Neurodevelopmental conditions (or neurodevelopmental disorders, NDDs) are highly heterogeneous with overlapping characteristics and shared genetic etiology. The large symptom variability and etiological heterogeneity have made it challenging to understand the biological mechanisms underpinning NDDs. To accommodate this individual variability, one approach is to move away from diagnostic criteria and focus on distinct dimensions with relevance to multiple NDDs. This domain approach is well suited to preclinical research, where genetically modified animal models can be used to link genetic variability to neurobiological mechanisms and behavioral traits. Genetic factors associated with NDDs can be grouped functionally into common biological pathways, with one prominent functional group being genes associated with the synapse. These include the neuroligins (Nlgns), a family of postsynaptic transmembrane proteins that are key modulators of synaptic function. Here, we review how research using Nlgn mouse models has provided insight into how synaptic proteins contribute to behavioral traits associated with NDDs. We focus on how mutations in different Nlgns affect social behaviors, as differences in social interaction and communication are a common feature of most NDDs. Importantly, mice carrying distinct mutations in Nlgns share some neurobiological and behavioral phenotypes with other synaptic gene mutations. Comparing the functional implications of mutations in multiple synaptic proteins is a first step towards identifying convergent neurobiological pathways in multiple brain regions and circuits.

Environmental Enrichment Protects Against Sepsis-Associated Encephalopathy-Induced Learning and Memory Deficits by Enhancing the Synthesis and Release of Vasopressin in the Supraoptic Nucleus

Background

As a severe complication of sepsis, sepsis-associated encephalopathy (SAE) usually manifests as impaired learning and memory ability in survivors. Previous studies have reported that environmental enrichment (EE) can increase the learning and memory ability in different brain injury models. However, there has been no research on the possible positive effect of EE on SAE.

Aim

The present study aimed to test the effect of EE on SAE-induced impairment of learning and memory and its related mechanisms.

Methods

A Morris water maze test (MWM) was used to evaluate the learning and memory ability of SAE rats that received EE housing or not. The expression of vasopressin (VP) was assessed using immunofluorescence microscopy and enzyme-linked immunosorbent assays (ELISAs). The synthesis of VP in the supraoptic nucleus (SON) was determined using quantitative real-time reverse transcription-PCR analysis. Moreover, inflammatory markers and brain-derived neurotrophic factor (BDNF) were detected using ELISA.

Results

The results showed that SAE induced a decreased learning and memory ability, while EE reversed this impairment. EE also enhanced the synthesis and secretion of VP in the SON. Blocking the action of VP in the hippocampus interrupted the EE-induced amelioration of learning and memory impairment. Moreover, EE induced changes to the levels of BDNF and cytokines in the hippocampus and these effects were mediated by VP binding to the VP receptor 1a.

Conclusion

Our findings demonstrated that the enhanced synthesis and secretion of VP in the SON are a key determinant responsible for EE-induced alleviation of learning and memory deficits caused by SAE.

The GSK-3β/β-Catenin Signaling-Mediated Brain-Derived Neurotrophic Factor Pathway Is Involved in Aluminum-Induced Impairment of Hippocampal LTP In Vivo

The neurotoxic effects of aluminum (Al) are associated with the impairment of synaptic plasticity, the biological basis of learning and memory, the major form of which is long-term potentiation (LTP). The canonical glycogen synthase kinase-3β (GSK-3β)/β-catenin signaling-mediated brain-derived neurotrophic factor (BDNF) pathway has been suggested to play important roles in memory. Thus, Al may affect LTP through this pathway. In this study, a Sprague-Dawley rat model of neurotoxicity was established through intracerebroventricular (i.c.v.) injection of aluminum maltol (Al(mal)₃), which was achieved by preimplantation of a cannula into the lateral ventricle. The rats in the control and Al-treated groups received a daily injection of SB216763, an inhibitor of GSK-3β. Electrophysiology and western blot analysis were used to investigate the regulatory effect of the GSK-3β/β-catenin signaling-mediated BDNF pathway on LTP impairment induced by Al(mal)₃. The results confirmed that i.c.v. injection of Al(mal)₃ significantly suppressed the field excitatory postsynaptic potential (fEPSP) amplitude, as indicated by a decrease in BDNF protein expression, which was accompanied by dose-dependent decreases in β-catenin protein expression and the phosphorylation of GSK-3β at Ser9. Rats that received SB216763, a GSK-3β inhibitor, exhibited higher fEPSP amplitudes than control rats. Furthermore, SB216763 treatment upregulated the hippocampal protein expression of BDNF and β-catenin while increasing the ratio of p-GSK-3β/GSK-3β. From the perspective of the identified β-catenin-BDNF axis, Al impairs hippocampal LTP, possibly through the GSK-3β/β-catenin signaling-mediated BDNF pathway.

Translational control in neurovascular brain development

The human brain carries out complex tasks and higher functions and is crucial for organismal survival, as it senses both intrinsic and extrinsic environments. Proper brain development relies on the orchestrated development of different precursor cells, which will give rise to the plethora of mature brain cell-types. Within this process, neuronal cells develop closely to and in coordination with vascular cells (endothelial cells (ECs), pericytes) in a bilateral communication process that relies on neuronal activity, attractive or repulsive guidance cues for both cell types and on tight-regulation of gene expression. Translational control is a master regulator of the gene-expression pathway and in particular for neuronal and ECs, it can be localized in developmentally relevant (axon growth cone, endothelial tip cell) and mature compartments (synapses, axons). Herein, we will review mechanisms of translational control relevant to brain development in neurons and ECs in health and disease.

Astrocytes in Neural Circuits: Key Factors in Synaptic Regulation and Potential Targets for Neurodevelopmental Disorders

Astrocytes are the major glial cells in the brain, which play a supporting role in the energy and nutritional supply of neurons. They were initially regarded as passive space-filling cells, but the latest progress in the study of the development and function of astrocytes highlights their active roles in regulating synaptic transmission, formation, and plasticity. In the concept of "tripartite synapse," the bidirectional influence between astrocytes and neurons, in addition to their steady-state and supporting function, suggests that any negative changes in the structure or function of astrocytes will affect the activity of neurons, leading to neurodevelopmental disorders. The role of astrocytes in the pathophysiology of various neurological and psychiatric disorders caused by synaptic defects is increasingly appreciated. Understanding the roles of astrocytes in regulating synaptic development and the plasticity of neural circuits could help provide new treatments for these diseases.

Optimization of 4,6-Disubstituted Pyrido[3,2-d]pyrimidines as Dual MNK/PIM Inhibitors to Inhibit Leukemia Cell Growth

Mitogen-activated protein kinase-interacting kinases (MNKs) and provirus integration in maloney murine leukemia virus kinases (PIMs) are downstream enzymes of cell proliferation signaling pathways associated with the resistance of tyrosine kinase inhibitors. MNKs and PIMs have complementary effects to regulate cap-dependent translation of oncoproteins. Dual inhibitors of MNKs and PIMs have not been developed. We developed a novel 4,6-disubstituted pyrido[3,2-d]pyrimidine compound 21o with selective inhibition of MNKs and PIMs. The IC50's of 21o to inhibit MNK1 and MNK2 are 1 and 7 nM and those to inhibit PIM1, PIM2, and PIM3 are 43, 232, and 774 nM, respectively. 21o inhibits the growth of myeloid leukemia K562 and MOLM-13 cells with GI50's of 2.1 and 1.2 μM, respectively. 21o decreases the levels of p-eIF4E and p-4EBP1, the downstream products of MNKs and PIMs, as well as cap-dependent proteins c-myc, cyclin D1, and Mcl-1. 21o inhibits the growth of MOLM-13 cell xenografts without causing evident toxicity. 21o represents an innovative dual MNK/PIM inhibitor with a good pharmacokinetic profile.

Neuronal activity-induced SUMOylation of Akt1 by PIAS3 is required for long-term potentiation of synaptic transmission

Neuronal activity regulates spatial distribution of the SUMOylation system in cytosolic and dendritic sites, which has been implicated in learning, memory, and underlying synaptic structural and functional remodeling in the hippocampus. However, the functional target proteins for activated small ubiquitin-like modifiers (SUMOs) and downstream molecular consequences behind long-term potentiation (LTP) of synaptic plasticity remain to be elucidated. In this study, we showed that N-methyl-D-aspartate receptor-mediated neuronal activity induced the covalent modification of cytosolic Akt1 by small ubiquitin-like modifier 1 (SUMO1) in rat cortical and hippocampal CA1 neurons. Protein inhibitor of activated STAT3 (PIAS3) was involved in the activity-induced Akt1 SUMO1-ylation, and K64 and K276 residues were major SUMOylated sites. Importantly, Akt1 SUMOylation at K64 and K276 enhanced its enzymatic activity and facilitated T308 phosphorylation. Furthermore, the N-terminal SAP domain of PIAS3 bound Akt1 directly. The disruption of Akt1-PIAS3 interaction by Tat-SAP, a synthetic Tat-fused cell-permeable peptide containing PIAS3 SAP domain, inhibited neuronal activity-induced Akt1 SUMOylation and impaired LTP expression and late phase LTP maintenance in the hippocampus. Correlatedly, Tat-SAP not only blocked the LTP-related extracellular signal-regulated kinase (ERK)1/2-Elk-1-brain-derived neurotrophic factor (BDNF)/Arc signaling, but also disrupted mammalian target of rapamycin (mTOR)-eIF4E-binding protein 1 (4E-BP1) pathway. These findings reveal an activity-induced Akt1 SUMOylation by PIAS3 that contributes to ERK1/2-BDNF/Arc and mTOR-4E-BP1 cascades, and in turn, long-lasting excitatory synaptic responses.

A white paper on a neurodevelopmental framework for drug discovery in autism and other neurodevelopmental disorders

In the last decade there has been a revolution in terms of genetic findings in neurodevelopmental disorders (NDDs), with many discoveries critical for understanding their aetiology and pathophysiology. Clinical trials in single-gene disorders such as fragile X syndrome highlight the challenges of investigating new drug targets in NDDs. Incorporating a developmental perspective into the process of drug development for NDDs could help to overcome some of the current difficulties in identifying and testing new treatments. This paper provides a summary of the proceedings of the 'New Frontiers Meeting' on neurodevelopmental disorders organised by the European College of Neuropsychopharmacology in conjunction with the Innovative Medicines Initiative-sponsored AIMS-2-TRIALS consortium. It brought together experts in developmental genetics, autism, NDDs, and clinical trials from academia and industry, regulators, patient and family associations, and other stakeholders. The meeting sought to provide a platform for focused communication on scientific insights, challenges, and methodologies that might be applicable to the development of CNS treatments from a neurodevelopmental perspective. Multidisciplinary translational consortia to develop basic and clinical research in parallel could be pivotal to advance knowledge in the field. Although implementation of clinical trials for NDDs in paediatric populations is widely acknowledged as essential, safety concerns should guide each aspect of their design. Industry and academia should join forces to improve knowledge of the biology of brain development, identify the optimal timing of interventions, and translate these findings into new drugs, allowing for the needs of users and families, with support from regulatory agencies.

Rapid Aging in the Perforant Path Projections to the Rodent Dentate Gyrus

Why layers II/III of entorhinal cortex (EC) deteriorate in advance of other regions during the earliest stages of Alzheimer's disease is poorly understood. Failure of retrograde trophic support from synapses to cell bodies is a common cause of neuronal atrophy, and we accordingly tested for early-life deterioration in projections of rodent layer II EC neurons. Using electrophysiology and quantitative imaging, changes in EC terminals during young adulthood were evaluated in male rats and mice. Field excitatory postsynaptic potentials, input/output curves, and frequency following capacity by lateral perforant path (LPP) projections from lateral EC to dentate gyrus were unchanged from 3 to 8-10 months of age. In contrast, the unusual presynaptic form of long-term potentiation (LTP) expressed by the LPP was profoundly impaired by 8 months in rats and mice. This impairment was accompanied by a reduction in the spine to terminal endocannabinoid signaling needed for LPP-LTP induction and was offset by an agent that enhances signaling. There was a pronounced age-related increase in synaptophysin within LPP terminals, an effect suggestive of incipient pathology. Relatedly, presynaptic levels of TrkB-receptors mediating retrograde trophic signaling-were reduced in the LPP terminal field. LTP and TrkB content were also reduced in the medial perforant path of 8- to 10-month-old rats. As predicted, performance on an LPP-dependent episodic memory task declined by late adulthood. We propose that memory-related synaptic plasticity in EC projections is unusually sensitive to aging, which predisposes EC neurons to pathogenesis later in life.SIGNIFICANCE STATEMENT Neurons within human superficial entorhinal cortex are particularly vulnerable to effects of aging and Alzheimer's disease, although why this is the case is not understood. Here we report that perforant path projections from layer II entorhinal cortex to the dentate gyrus exhibit rapid aging in rodents, including reduced synaptic plasticity and abnormal protein content by 8-10 months of age. Moreover, there was a substantial decline in the performance of an episodic memory task that depends on entorhinal cortical projections at the same ages. Overall, the results suggest that the loss of plasticity and related trophic signaling predispose the entorhinal neurons to functional decline in relatively young adulthood.

Brain-Derived Neurotrophic Factor/Tropomyosin Receptor Kinase B Signaling Controls Excitability and Long-Term Depression in Oval Nucleus of the BNST

Dysregulation of proteins involved in synaptic plasticity is associated with pathologies in the CNS, including psychiatric disorders. The bed nucleus of the stria terminalis (BNST), a brain region of the extended amygdala circuit, has been identified as the critical hub responsible for fear responses related to stress coping and pathologic systems states. Here, we report that one particular nucleus, the oval nucleus of the BNST (ovBNST), is rich in brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) receptor. Whole-cell patch-clamp recordings of neurons from male mouse ovBNST in vitro showed that the BDNF/TrkB interaction causes a hyperpolarizing shift of the membrane potential from resting value, mediated by an inwardly rectifying potassium current, resulting in reduced neuronal excitability in all major types of ovBNST neurons. Furthermore, BDNF/TrkB signaling mediated long-term depression (LTD) at postsynaptic sites in ovBNST neurons. LTD of ovBNST neurons was prevented by a BDNF scavenger or in the presence of TrkB inhibitors, indicating the contribution to LTD induction. Our data identify BDNF/TrkB signaling as a critical regulator of synaptic activity in ovBNST, which acts at postsynaptic sites to dampen excitability at short and long time scales. Given the central role of ovBNST in mediating maladaptive behaviors associated with stress exposure, our findings suggest a synaptic entry point of the BDNF/TrkB system for adaptation to stressful environmental encounters.

Translational control of coronaviruses

Coronaviruses represent a large family of enveloped RNA viruses that infect a large spectrum of animals. In humans, the severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) is responsible for the current COVID-19 pandemic and is genetically related to SARS-CoV and Middle East respiratory syndrome-related coronavirus (MERS-CoV), which caused outbreaks in 2002 and 2012, respectively. All viruses described to date entirely rely on the protein synthesis machinery of the host cells to produce proteins required for their replication and spread. As such, virus often need to control the cellular translational apparatus to avoid the first line of the cellular defense intended to limit the viral propagation. Thus, coronaviruses have developed remarkable strategies to hijack the host translational machinery in order to favor viral protein production. In this review, we will describe some of these strategies and will highlight the role of viral proteins and RNAs in this process.

Arc self-association and formation of virus-like capsids are mediated by an N-terminal helical coil motif

Activity-regulated cytoskeleton-associated protein (Arc) is a protein interaction hub with diverse roles in intracellular neuronal signaling, and important functions in neuronal synaptic plasticity, memory, and postnatal cortical development. Arc has homology to retroviral Gag protein and is capable of self-assembly into virus-like capsids implicated in the intercellular transfer of RNA. However, the molecular basis of Arc self-association and capsid formation is largely unknown. Here, we identified a 28-amino-acid stretch in the mammalian Arc N-terminal (NT) domain that is necessary and sufficient for self-association. Within this region, we identified a 7-residue oligomerization motif, critical for the formation of virus-like capsids. Purified wild-type Arc formed capsids as shown by transmission and cryo-electron microscopy, whereas mutant Arc with disruption of the oligomerization motif formed homogenous dimers. An atomic-resolution crystal structure of the oligomerization region peptide demonstrated an antiparallel coiled-coil interface, strongly supporting NT-NT domain interactions in Arc oligomerization. The NT coil-coil interaction was also validated in live neurons using fluorescence lifetime FRET imaging, and mutation of the oligomerization motif disrupted Arc-facilitated endocytosis. Furthermore, using single-molecule photobleaching, we show that Arc mRNA greatly enhances higher-order oligomerization in a manner dependent on the oligomerization motif. In conclusion, a helical coil in the Arc NT domain supports self-association above the dimer stage, mRNA-induced oligomerization, and formation of virus-like capsids. DATABASE: The coordinates and structure factors for crystallographic analysis of the oligomerization region were deposited at the Protein Data Bank with the entry code 6YTU.

FMRP and CYFIP1 at the Synapse and Their Role in Psychiatric Vulnerability

There is increasing awareness of the role genetic risk variants have in mediating vulnerability to psychiatric disorders such as schizophrenia and autism. Many of these risk variants encode synaptic proteins, influencing biological pathways of the postsynaptic density and, ultimately, synaptic plasticity. Fragile-X mental retardation 1 (FMR1) and cytoplasmic fragile-X mental retardation protein (FMRP)-interacting protein 1 (CYFIP1) contain 2 such examples of highly penetrant risk variants and encode synaptic proteins with shared functional significance. In this review, we discuss the biological actions of FMRP and CYFIP1, including their regulation of (i) protein synthesis and specifically FMRP targets, (ii) dendritic and spine morphology, and (iii) forms of synaptic plasticity such as long-term depression. We draw upon a range of preclinical studies that have used genetic dosage models of FMR1 and CYFIP1 to determine their biological function. In parallel, we discuss how clinical studies of fragile X syndrome or 15q11.2 deletion patients have informed our understanding of FMRP and CYFIP1, and highlight the latest psychiatric genomic findings that continue to implicate FMRP and CYFIP1. Lastly, we assess the current limitations in our understanding of FMRP and CYFIP1 biology and how they must be addressed before mechanism-led therapeutic strategies can be developed for psychiatric disorders.

Arc/Arg3.1 function in long-term synaptic plasticity: Emerging mechanisms and unresolved issues

Arc (activity-regulated cytoskeleton-associated protein) is posited as a critical regulator of long-term synaptic plasticity at excitatory synapses, including long-term potentiation, long-term depression, inverse synaptic tagging and homoeostatic scaling, with pivotal roles in memory and postnatal cortical development. However, the mechanisms underlying the bidirectional regulation of synaptic strength are poorly understood. Here we review evidence from different plasticity paradigms, highlight outstanding issues and discuss stimulus-specific mechanisms that dictate Arc function. We propose a model in which Arc bidirectionally controls synaptic strength by coordinate regulation of AMPA-type glutamate receptor (AMPAR) trafficking and actin cytoskeletal dynamics in dendritic spines. Key to this model, Arc is proposed to function as an activity-dependent regulator of AMPAR lateral membrane diffusion and trapping at synapses.

BDNF and nicotine dependence: associations and potential mechanisms

Smoking is the leading preventable cause of death worldwide and tobacco addiction has become a serious public health problem. Nicotine is the main addictive component of tobacco, and the majority of people that smoke regularly develop nicotine dependence. Nicotine addiction is deemed to be a chronic mental disorder. Although it is well known that nicotine binds to the nicotinic acetylcholine receptors (nAChRs) and activates the mesolimbic dopaminergic system (MDS) to generate the pleasant and rewarding effects, the molecular mechanisms of nicotine addiction are not fully understood. Brain-derived neurotrophic factor (BDNF) is the most prevalent growth factor in the brain, which regulates neuron survival, differentiation, and synaptic plasticity, mainly through binding to the high affinity receptor tyrosine kinase receptor B (TrkB). BDNF gene polymorphisms are associated with nicotine dependence and blood BDNF levels are altered in smokers. In this review, we discussed the effects of nicotine on BDNF expression in the brain and summarized the underlying signaling pathways, which further indicated BDNF as a key regulator in nicotine dependence. Further studies that aim to understand the neurobiological mechanism of BDNF in nicotine addcition would provide a valuable reference for quitting smoking and developing the treatment of other addictive substances.

Dysregulation of translational control signaling in autism spectrum disorders

Deviations from the optimal level of mRNA translation are linked to disorders with high rates of autism. Loss of function mutations in genes encoding translational repressors such as PTEN, TSC1, TSC2, and FMRP are associated with autism spectrum disorders (ASDs) in humans and their deletion in animals recapitulates many ASD-like phenotypes. Importantly, the activity of key translational control signaling pathways such as PI3K-mTORC1 and ERK is frequently dysregulated in autistic patients and animal models and their normalization rescues many abnormal phenotypes, suggesting a causal relationship. Mutations in several genes encoding proteins not directly involved in translational control have also been shown to mediate ASD phenotypes via altered signaling upstream of translation. This raises the possibility that the dysregulation of translational control signaling is a converging mechanism not only in familiar but also in sporadic forms of autism. Here, we overview the current knowledge on translational signaling in ASD and highlight how correcting the activity of key pathways upstream of translation reverses distinct ASD-like phenotypes.

Cognitive function and brain plasticity in a rat model of shift work: role of daily rhythms, sleep and glucocorticoids

Many occupations require operations during the night-time when the internal circadian clock promotes sleep, in many cases resulting in impairments in cognitive performance and brain functioning. Here, we use a rat model to attempt to identify the biological mechanisms underlying such impaired performance. Rats were exposed to forced activity, either in their rest-phase (simulating night-shift work; rest work) or in their active-phase (simulating day-shift work; active work). Sleep, wakefulness and body temperature rhythm were monitored throughout. Following three work shifts, spatial memory performance was tested on the Morris Water Maze task. After 4 weeks washout, the work protocol was repeated, and blood and brain tissue collected. Simulated night-shift work impaired spatial memory and altered biochemical markers of cerebral cortical protein synthesis. Measures of daily rhythm strength were blunted, and sleep drive increased. Individual variation in the data suggested differences in shift work tolerance. Hierarchical regression analyses revealed that type of work, changes in daily rhythmicity and changes in sleep drive predict spatial memory performance and expression of brain protein synthesis regulators. Moreover, serum corticosterone levels predicted expression of brain protein synthesis regulators. These findings open new research avenues into the biological mechanisms that underlie individual variation in shift work tolerance.

eEF2/eEF2K Pathway in the Mature Dentate Gyrus Determines Neurogenesis Level and Cognition

Levels of adult neurogenesis in the dentate gyrus (DG) of the hippocampus are correlated with unique cognitive functions. However, the molecular pathways controlling it are poorly understood. Here, we found that the known physiological ways to enhance neurogenesis converged on the eEF2/eEF2K pathway via AMPK in the DG. Enhancing the elongation phase of mRNA translation in eEF2K-knockout (eEF2K-KO) mice induced the expression of neurogenesis-related proteins in the hippocampus. We thus tested the hypothesis that inducing eEF2K-KO in mature neurons of the DG controls neurogenesis. Indeed, both general eEF2K-KO and targeted KO in DG excitatory mature neurons resulted in enhanced neurogenesis levels and upregulation of neurogenesis-related proteins. Increased neurogenesis was correlated with enhanced performance in DG-dependent learning. Moreover, general and local eEF2K-KO in old mice rejuvenated the DG, paving the way for better mechanistic understanding of how neurogenesis is controlled in the mature DG and possible treatments for incurable aging-associated diseases.

ERK signalling: a master regulator of cell behaviour, life and fate

The proteins extracellular signal-regulated kinase 1 (ERK1) and ERK2 are the downstream components of a phosphorelay pathway that conveys growth and mitogenic signals largely channelled by the small RAS GTPases. By phosphorylating widely diverse substrates, ERK proteins govern a variety of evolutionarily conserved cellular processes in metazoans, the dysregulation of which contributes to the cause of distinct human diseases. The mechanisms underlying the regulation of ERK1 and ERK2, their mode of action and their impact on the development and homeostasis of various organisms have been the focus of much attention for nearly three decades. In this Review, we discuss the current understanding of this important class of kinases. We begin with a brief overview of the structure, regulation, substrate recognition and subcellular localization of ERK1 and ERK2. We then systematically discuss how ERK signalling regulates six fundamental cellular processes in response to extracellular cues. These processes are cell proliferation, cell survival, cell growth, cell metabolism, cell migration and cell differentiation.

The Role of BDNF on Neural Plasticity in Depression

Using behavioral, pharmacological, and molecular methods, lots of studies reveal that depression is closely related to the abnormal neural plasticity processes occurring in the prefrontal cortex and limbic system such as the hippocampus and amygdala. Meanwhile, functions of the brain-derived neurotrophic factor (BDNF) and the other neurotrophins in the pathogenesis of depression are well known. The maladaptive neuroplastic in depression may be related to alterations in the levels of neurotrophic factors, which play a central role in plasticity. Enhancement of neurotrophic factors signaling has great potential in therapy for depression. This review highlights the relevance of neurotrophic factors mediated neural plasticity and pathophysiology of depression. These studies reviewed here may suggest new possible targets for antidepressant drugs such as neurotrophins, their receptors, and relevant signaling pathways, and agents facilitating the activation of gene expression and increasing the transcription of neurotrophic factors in the brain.

Monosomes actively translate synaptic mRNAs in neuronal processes

To accommodate their complex morphology, neurons localize messenger RNAs (mRNAs) and ribosomes near synapses to produce proteins locally. However, a relative paucity of polysomes (considered the active sites of translation) detected in electron micrographs of neuronal processes has suggested a limited capacity for local protein synthesis. In this study, we used polysome profiling together with ribosome footprinting of microdissected rodent synaptic regions to reveal a surprisingly high number of dendritic and/or axonal transcripts preferentially associated with monosomes (single ribosomes). Furthermore, the neuronal monosomes were in the process of active protein synthesis. Most mRNAs showed a similar translational status in the cell bodies and neurites, but some transcripts exhibited differential ribosome occupancy in the compartments. Monosome-preferring transcripts often encoded high-abundance synaptic proteins. Thus, monosome translation contributes to the local neuronal proteome.

CREB Family Transcription Factors Are Major Mediators of BDNF Transcriptional Autoregulation in Cortical Neurons

BDNF signaling via its transmembrane receptor TrkB has an important role in neuronal survival, differentiation, and synaptic plasticity. Remarkably, BDNF is capable of modulating its own expression levels in neurons, forming a transcriptional positive feedback loop. In the current study, we have investigated this phenomenon in primary cultures of rat cortical neurons using overexpression of dominant-negative forms of several transcription factors, including CREB, ATF2, C/EBP, USF, and NFAT. We show that CREB family transcription factors, together with the coactivator CBP/p300, but not the CRTC family, are the main regulators of rat BDNF gene expression after TrkB signaling. CREB family transcription factors are required for the early induction of all the major BDNF transcripts, whereas CREB itself directly binds only to BDNF promoter IV, is phosphorylated in response to BDNF-TrkB signaling, and activates transcription from BDNF promoter IV by recruiting CBP. Our complementary reporter assays with BDNF promoter constructs indicate that the regulation of BDNF by CREB family after BDNF-TrkB signaling is generally conserved between rat and human. However, we demonstrate that a nonconserved functional cAMP-responsive element in BDNF promoter IXa in humans renders the human promoter responsive to BDNF-TrkB-CREB signaling, whereas the rat ortholog is unresponsive. Finally, we show that extensive BDNF transcriptional autoregulation, encompassing all major BDNF transcripts, occurs also in vivo in the adult rat hippocampus during BDNF-induced LTP. Collectively, these results improve the understanding of the intricate mechanism of BDNF transcriptional autoregulation.SIGNIFICANCE STATEMENT Deeper understanding of stimulus-specific regulation of BDNF gene expression is essential to precisely adjust BDNF levels that are dysregulated in various neurological disorders. Here, we have elucidated the molecular mechanisms behind TrkB signaling-dependent BDNF mRNA induction and show that CREB family transcription factors are the main regulators of BDNF gene expression after TrkB signaling. Our results suggest that BDNF-TrkB signaling may induce BDNF gene expression in a distinct manner compared with neuronal activity. Moreover, our data suggest the existence of a stimulus-specific distal enhancer modulating BDNF gene expression.

Optogenetic Modulation of TrkB Signaling in the Mouse Brain

Optogenetic activation of receptors has advantages compared with chemical or ligand treatment because of its high spatial and temporal precision. Especially in the brain, the use of a genetically encoded light-tunable receptor is superior to direct infusion or systemic drug treatment. We applied light-activatable TrkB receptors in the mouse brain with reduced basal activity by incorporating Cry2PHR mutant, Opto-cytTrkB(E281A). Upon AAV mediated gene delivery, this form was expressed at sufficient levels in the mouse hippocampus (HPC) and medial entorhinal cortex (MEC) retaining normal canonical signal transduction by the blue light stimulus, even by delivery of noninvasive LED light on the mouse head. Within target cells, where its expression was driven by a cell type-specific promoter, Opto-cytTrkB(E281A)-mediated TrkB signaling could be controlled by adjusting light-stimulating conditions. We further demonstrated that Opto-cytTrkB(E281A) could locally induce TrkB signaling in axon terminals in the MEC-HPC. In summary, Opto-cytTrkB(E281A) will be useful for elucidating time- and region-specific roles of TrkB signaling ranging from cellular function to neural circuit mechanisms.

MicroRNA-34a Acutely Regulates Synaptic Efficacy in the Adult Dentate Gyrus In Vivo

Activity-dependent synaptic plasticity involves rapid regulation of neuronal protein synthesis on a time-scale of minutes. miRNA function in synaptic plasticity and memory formation has been elucidated by stable experimental manipulation of miRNA expression and activity using transgenic approaches and viral vectors. However, the impact of rapid miRNA modulation on synaptic efficacy is unknown. Here, we examined the effect of acute (12 min), intrahippocampal infusion of a miR-34a antagonist (antimiR) on medial perforant path-evoked synaptic transmission in the dentate gyrus of adult anesthetised rats. AntimiR-34a infusion acutely depressed medial perforant path-evoked field excitatory post-synaptic potentials (fEPSPs). The fEPSP decrease was detected within 9 min of infusion, lasted for hours, and was associated with knockdown of antimiR-34a levels. AntimiR-34a-induced synaptic depression was sequence-specific; no changes were elicited by infusion of scrambled or mismatch control. The rapid modulation suggests that a target, or set of targets, is regulated by miR-34a. Western blot analysis of dentate gyrus lysates revealed enhanced expression of Arc, a known miR-34a target, and four novel predicted targets (Ctip2, PKI-1α, TCF4 and Ube2g1). Remarkably, antimiR-34a had no effect when infused during the maintenance phase of long-term potentiation. We conclude that miR-34a regulates basal synaptic efficacy in the adult dentate gyrus in vivo. To our knowledge, these in vivo findings are the first to demonstrate acute (< 9 min) regulation of synaptic efficacy in the adult brain by a miRNA.

Oleanolic Acid Ameliorates Aβ25-35 Injection-induced Memory Deficit in Alzheimer's Disease Model Rats by Maintaining Synaptic Plasticity

Background:

Abnormal amyloid β (Aβ) accumulation and deposition in the hippocampus is an essential process in Alzheimer’s disease (AD).

Objective:

To investigate whether Oleanolic acid (OA) could improve memory deficit in AD model and its possible mechanism.

Methods:

Forty-five SD rats were randomly divided into sham operation group, model group, and OA group. AD models by injection of Aβ25-35 were built. Morris water maze (MWM) was applied to inves-tigate learning and memory, transmission electron microscope (TEM) to observe the ultrastructure of synapse, western blot to the proteins, electrophysiology for long-term potentiation (LTP), and Ca2+ con-centration in synapse was also measured.

Results:

The latency time in model group was significantly longer than that in sham operation group (P=0.0001); while it was significantly shorter in the OA group than that in model group (P=0.0001); compared with model group, the times of cross-platform in OA group significantly increased (P=0.0001). TEM results showed OA could alleviate neuron damage and synapses changes induced by Aβ25-35. The expressions of CaMKII, PKC, NMDAR2B, BDNF, TrkB, and CREB protein were signif-icantly improved by OA (P=0.0001, 0.036, 0.041, 0.0001, 0.0001, 0.026, respectively) compared with that in model group; the concentration of Ca2+ was significantly lower in OA group (1.11±0.42) than that in model group (1.68±0.18); and the slope rate (P=0.0001) and amplitude (P=0.0001) of f-EPSP significantly increased in OA group.

Conclusion:

The present results support that OA could ameliorate Aβ-induced memory loss of AD rats by maintaining synaptic plasticity of the hippocampus

Identification of protein targets for the antidepressant effects of Kai-Xin-San in Chinese medicine using isobaric tags for relative and absolute quantitation

Kai-Xin-San consists of Ginseng Radix, Polygalae Radix, Acori Tatarinowii Rhizoma, and Poria at a ratio of 3:3:2:2. Kai-Xin-San has been widely used for the treatment of emotional disorders in China. However, no studies have identified the key proteins implicated in response to Kai-Xin-San treatment. In this study, rat models of chronic mild stress were established using different stress methods over 28 days. After 14 days of stress stimulation, rats received daily intragastric administrations of 600 mg/kg Kai-Xin-San. The sucrose preference test was used to determine depression-like behavior in rats, while isobaric tags were used for relative and absolute quantitation-based proteomics to identify altered proteins following Kai-Xin-San treatment. Kai-Xin-San treatment for 2 weeks noticeably improved depression-like behaviors in rats with chronic mild stress. We identified 33 differentially expressed proteins: 7 were upregulated and 26 were downregulated. Functional analysis showed that these differentially expressed proteins participate in synaptic plasticity, neurodevelopment, and neurogenesis. Our results indicate that Kai-Xin-San has an important role in regulating the key node proteins in the synaptic signaling network, and are helpful to better understand the mechanism of the antidepressive effects of Kai-Xin-San and to provide objective theoretical support for its clinical application. The study was approved by the Ethics Committee for Animal Research from the Chinese PLA General Hospital (approval No. X5-2016-07) on March 5, 2016.

Rbfox1 up-regulation impairs BDNF-dependent hippocampal LTP by dysregulating TrkB isoform expression levels

Brain-derived neurotrophic factor (BDNF) is a potent modulator of brain synaptic plasticity. Signaling defects caused by dysregulation of its Ntrk2 (TrkB) kinase (TrkB.FL) and truncated receptors (TrkB.T1) have been linked to the pathophysiology of several neurological and neurodegenerative disorders. We found that upregulation of Rbfox1, an RNA binding protein associated with intellectual disability, epilepsy and autism, increases selectively hippocampal TrkB.T1 isoform expression. Physiologically, increased Rbfox1 impairs BDNF-dependent LTP which can be rescued by genetically restoring TrkB.T1 levels. RNA-seq analysis of hippocampi with upregulation of Rbfox1 in conjunction with the specific increase of TrkB.T1 isoform expression also shows that the genes affected by Rbfox1 gain of function are surprisingly different from those influenced by Rbfox1 deletion. These findings not only identify TrkB as a major target of Rbfox1 pathophysiology but also suggest that gain or loss of function of Rbfox1 regulate different genetic landscapes.