Transgenic Mice Lacking NMDAR-Dependent LTD Exhibit Deficits in Behavioral Flexibility

Working memory forgetting: Bridging gaps between human and animal studies

The causes of forgetting in working memory (WM) remain a source of debate in cognitive psychology, partly because it has always been challenging to probe the complex neural mechanisms that govern rapid cognitive processes in humans. In this review, we argue that neural, and more precisely animal models, provide valuable tools for exploring the precise mechanisms of WM forgetting. First, we discuss theoretical perspectives concerning WM forgetting in humans. Then, we present neuronal correlates of WM in animals, starting from the initial evidence of delay activity observed in the prefrontal cortex to the later synaptic theory of WM. In the third part, specific theories of WM are discussed, including the notion that silent versus non-silent activity is more consistent with the processes of refreshing and decay proposed in human cognitive models. The review concludes with an exploration of the relationship between long-term memory and WM, revealing connections between these two forms of memory through the long-term synaptic hypothesis, which suggests that long-term storage of interference can potentially disrupt WM.

Apoptosis signal-regulating kinase 1 (Ask1) deficiency alleviates MPP+-induced impairment of evoked dopamine release in the mouse hippocampus

The dopaminergic system is susceptible to dysfunction in numerous neurological diseases, including Parkinson's disease (PD). In addition to motor symptoms, some PD patients may experience non-motor symptoms, including cognitive and memory deficits. A possible explanation for their manifestation is a disturbed pattern of dopamine release in brain regions involved in learning and memory, such as the hippocampus. Therefore, investigating neuropathological alterations in dopamine release prior to neurodegeneration is imperative. This study aimed to characterize evoked hippocampal dopamine release and assess the impact of the neurotoxin MPP+ using a genetically encoded dopamine sensor and gene expression analysis. Additionally, considering the potential neuroprotective attributes demonstrated by apoptosis signal-regulating kinase 1 (Ask1) in various animal-disease-like models, the study also aimed to determine whether Ask1 knockdown restores MPP+-altered dopamine release in acute hippocampal slices. We applied variations of low- and high-frequency stimulation to evoke dopamine release within different hippocampal regions and discovered that acute application of MPP+ reduced the amount of dopamine released and hindered the recovery of dopamine release after repeated stimulation. In addition, we observed that Ask1 deficiency attenuated the detrimental effects of MPP+ on the recovery of dopamine release after repeated stimulation. RNA sequencing analysis indicated that genes associated with the synaptic pathways are involved in response to MPP+ exposure. Notably, Ask1 deficiency was found to downregulate the expression of Slc5a7, a gene encoding a sodium-dependent high-affinity choline transporter that regulates acetylcholine levels. Respective follow-up experiments indicated that Slc5a7 plays a role in Ask1 deficiency-mediated protection against MPP+ neurotoxicity. In addition, increasing acetylcholine levels using an acetylcholinesterase inhibitor could exacerbate the toxicity of MPP+. In conclusion, our data imply that the modulation of the dopamine-acetylcholine balance may be a crucial mechanism of action underlying the neuroprotective effects of Ask1 deficiency in PD.

BK channels sustain neuronal Ca2+ oscillations to support hippocampal long-term potentiation and memory formation

Mutations of large conductance Ca2+- and voltage-activated K+ channels (BK) are associated with cognitive impairment. Here we report that CA1 pyramidal neuron-specific conditional BK knock-out (cKO) mice display normal locomotor and anxiety behavior. They do, however, exhibit impaired memory acquisition and retrieval in the Morris Water Maze (MWM) when compared to littermate controls (CTRL). In line with cognitive impairment in vivo, electrical and chemical long-term potentiation (LTP) in cKO brain slices were impaired in vitro. We further used a genetically encoded fluorescent K+ biosensor and a Ca2+-sensitive probe to observe cultured hippocampal neurons during chemical LTP (cLTP) induction. cLTP massively reduced intracellular K+ concentration ([K+]i) while elevating L-Type Ca2+ channel- and NMDA receptor-dependent Ca2+ oscillation frequencies. Both, [K+]i decrease and Ca2+ oscillation frequency increase were absent after pharmacological BK inhibition or in cells lacking BK. Our data suggest that L-Type- and NMDAR-dependent BK-mediated K+ outflow significantly contributes to hippocampal LTP, as well as learning and memory.

RNA-binding deficient TDP-43 drives cognitive decline in a mouse model of TDP-43 proteinopathy

TDP-43 proteinopathies including frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative disorders characterized by aggregation and mislocalization of the nucleic acid-binding protein TDP-43 and subsequent neuronal dysfunction. Here, we developed endogenous models of sporadic TDP-43 proteinopathy based on the principle that disease-associated TDP-43 acetylation at lysine 145 (K145) alters TDP-43 conformation, impairs RNA-binding capacity, and induces downstream mis-regulation of target genes. Expression of acetylation-mimic TDP-43K145Q resulted in stress-induced nuclear TDP-43 foci and loss of TDP-43 function in primary mouse and human-induced pluripotent stem cell (hiPSC)-derived cortical neurons. Mice harboring the TDP-43K145Q mutation recapitulated key hallmarks of FTLD, including progressive TDP-43 phosphorylation and insolubility, TDP-43 mis-localization, transcriptomic and splicing alterations, and cognitive dysfunction. Our study supports a model in which TDP-43 acetylation drives neuronal dysfunction and cognitive decline through aberrant splicing and transcription of critical genes that regulate synaptic plasticity and stress response signaling. The neurodegenerative cascade initiated by TDP-43 acetylation recapitulates many aspects of human FTLD and provides a new paradigm to further interrogate TDP-43 proteinopathies.

A centronuclear myopathy-causing mutation in dynamin-2 disrupts neuronal morphology and excitatory synaptic transmission in a murine model of the disease

AIMS

Dynamin-2 is a large GTPase, a member of the dynamin superfamily that regulates membrane remodelling and cytoskeleton dynamics. Mutations in the dynamin-2 gene (DNM2) cause autosomal dominant centronuclear myopathy (CNM), a congenital neuromuscular disorder characterised by progressive weakness and atrophy of the skeletal muscles. Cognitive defects have been reported in some DNM2-linked CNM patients suggesting that these mutations can also affect the central nervous system (CNS). Here we studied how a dynamin-2 CNM-causing mutation influences the CNS function.

METHODS

Heterozygous mice harbouring the p.R465W mutation in the dynamin-2 gene (HTZ), the most common causing autosomal dominant CNM, were used as disease model. We evaluated dendritic arborisation and spine density in hippocampal cultured neurons, analysed excitatory synaptic transmission by electrophysiological field recordings in hippocampal slices, and evaluated cognitive function by performing behavioural tests.

RESULTS

HTZ hippocampal neurons exhibited reduced dendritic arborisation and lower spine density than WT neurons, which was reversed by transfecting an interference RNA against the dynamin-2 mutant allele. Additionally, HTZ mice showed defective hippocampal excitatory synaptic transmission and reduced recognition memory compared to the WT condition.

CONCLUSION

Our findings suggest that the dynamin-2 p.R465W mutation perturbs the synaptic and cognitive function in a CNM mouse model and support the idea that this GTPase plays a key role in regulating neuronal morphology and excitatory synaptic transmission in the hippocampus.

Adnp-mutant mice with cognitive inflexibility, CaMKIIα hyperactivity, and synaptic plasticity deficits

ADNP syndrome, involving the ADNP transcription factor of the SWI/SNF chromatin-remodeling complex, is characterized by developmental delay, intellectual disability, and autism spectrum disorders (ASD). Although Adnp-haploinsufficient (Adnp-HT) mice display various phenotypic deficits, whether these mice display abnormal synaptic functions remain poorly understood. Here, we report synaptic plasticity deficits associated with cognitive inflexibility and CaMKIIα hyperactivity in Adnp-HT mice. These mice show impaired and inflexible contextual learning and memory, additional to social deficits, long after the juvenile-stage decrease of ADNP protein levels to ~10% of the newborn level. The adult Adnp-HT hippocampus shows hyperphosphorylated CaMKIIα and its substrates, including SynGAP1, and excessive long-term potentiation that is normalized by CaMKIIα inhibition. Therefore, Adnp haploinsufficiency in mice leads to cognitive inflexibility involving CaMKIIα hyperphosphorylation and excessive LTP in adults long after its marked expressional decrease in juveniles.

Regulation of the Ca2+ Channel CaV1.2 Supports Spatial Memory and Its Flexibility and LTD

Widespread release of norepinephrine (NE) throughout the forebrain fosters learning and memory via adrenergic receptor (AR) signaling, but the molecular mechanisms are largely unknown. The β2 AR and its downstream effectors, the trimeric stimulatory Gs-protein, adenylyl cyclase (AC), and the cAMP-dependent protein kinase A (PKA), form a unique signaling complex with the L-type Ca2+ channel (LTCC) CaV1.2. Phosphorylation of CaV1.2 by PKA on Ser1928 is required for the upregulation of Ca2+ influx on β2 AR stimulation and long-term potentiation induced by prolonged theta-tetanus (PTT-LTP) but not LTP induced by two 1-s-long 100-Hz tetani. However, the function of Ser1928 phosphorylation in vivo is unknown. Here, we show that S1928A knock-in (KI) mice of both sexes, which lack PTT-LTP, express deficiencies during initial consolidation of spatial memory. Especially striking is the effect of this mutation on cognitive flexibility as tested by reversal learning. Mechanistically, long-term depression (LTD) has been implicated in reversal learning. It is abrogated in male and female S1928A knock-in mice and by β2 AR antagonists and peptides that displace β2 AR from CaV1.2. This work identifies CaV1.2 as a critical molecular locus that regulates synaptic plasticity, spatial memory and its reversal, and LTD.SIGNIFICANCE STATEMENT We show that phosphorylation of the Ca2+ channel CaV1.2 on Ser1928 is important for consolidation of spatial memory and especially its reversal, and long-term depression (LTD). Identification of Ser1928 as critical for LTD and reversal learning supports the model that LTD underlies flexibility of reference memory.

Ubiquitination of the GluA1 Subunit of AMPA Receptors Is Required for Synaptic Plasticity, Memory, and Cognitive Flexibility

Activity-dependent changes in the number of AMPA-type glutamate receptors (AMPARs) at the synapse underpin the expression of LTP and LTD, cellular correlates of learning and memory. Post-translational ubiquitination has emerged as a key regulator of the trafficking and surface expression of AMPARs, with ubiquitination of the GluA1 subunit at Lys-868 controlling the post-endocytic sorting of the receptors into the late endosome for degradation, thereby regulating their stability at synapses. However, the physiological significance of GluA1 ubiquitination remains unknown. In this study, we generated mice with a knock-in mutation in the major GluA1 ubiquitination site (K868R) to investigate the role of GluA1 ubiquitination in synaptic plasticity, learning, and memory. Our results reveal that these male mice have normal basal synaptic transmission but exhibit enhanced LTP and deficits in LTD. They also display deficits in short-term spatial memory and cognitive flexibility. These findings underscore the critical roles of GluA1 ubiquitination in bidirectional synaptic plasticity and cognition in male mice.SIGNIFICANCE STATEMENT Subcellular targeting and membrane trafficking determine the precise number of AMPA-type glutamate receptors at synapses, processes that are essential for synaptic plasticity, learning, and memory. Post-translational ubiquitination of the GluA1 subunit marks AMPARs for degradation, but its functional role in vivo remains unknown. Here we demonstrate that the GluA1 ubiquitin-deficient mice exhibit an altered threshold for synaptic plasticity accompanied by deficits in short-term memory and cognitive flexibility. Our findings suggest that activity-dependent ubiquitination of GluA1 fine-tunes the optimal number of synaptic AMPARs required for bidirectional synaptic plasticity and cognition in male mice. Given that increases in amyloid-β cause excessive ubiquitination of GluA1, inhibiting that GluA1 ubiquitination may have the potential to ameliorate amyloid-β-induced synaptic depression in Alzheimer's disease.

Long-Term-But Not Short-Term-Plasticity at the Mossy Fiber-CA3 Pyramidal Cell Synapse in Hippocampus Is Altered in M1/M3 Muscarinic Acetylcholine Receptor Double Knockout Mice

Muscarinic acetylcholine receptors are well-known for their crucial involvement in hippocampus-dependent learning and memory, but the exact roles of the various receptor subtypes (M1-M5) are still not fully understood. Here, we studied how M1 and M3 receptors affect plasticity at the mossy fiber (MF)-CA3 pyramidal cell synapse. In hippocampal slices from M1/M3 receptor double knockout (M1/M3-dKO) mice, the signature short-term plasticity of the MF-CA3 synapse was not significantly affected. However, the rather unique NMDA receptor-independent and presynaptic form of long-term potentiation (LTP) of this synapse was much larger in M1/M3-deficient slices compared to wild-type slices in both field potential and whole-cell recordings. Consistent with its presynaptic origin, induction of MF-LTP strongly enhanced the excitatory drive onto single CA3 pyramidal cells, with the effect being more pronounced in M1/M3-dKO cells. In an earlier study, we found that the deletion of M2 receptors in mice disinhibits MF-LTP in a similar fashion, suggesting that endogenous acetylcholine employs both M1/M3 and M2 receptors to constrain MF-LTP. Importantly, such synergism was not observed for MF long-term depression (LTD). Low-frequency stimulation, which reliably induced LTD of MF synapses in control slices, failed to do so in M1/M3-dKO slices and gave rise to LTP instead. In striking contrast, loss of M2 receptors augmented LTD when compared to control slices. Taken together, our data demonstrate convergence of M1/M3 and M2 receptors on MF-LTP, but functional divergence on MF-LTD, with the net effect resulting in a well-balanced bidirectional plasticity of the MF-CA3 pyramidal cell synapse.

Ca2+-stimulated ADCY1 and ADCY8 regulate distinct aspects of synaptic and cognitive flexibility

The type 1 and 8 adenylyl cyclase (ADCY1 and ADCY8) exclusively account for Ca2+-stimulated cyclic AMP (cAMP) production and regulate activity-dependent synaptic modification. In this study, we examined distinct forms of synaptic plasticity in the hippocampus of Adcy1-/- and Adcy8-/- mice. We found that, at the Schaffer collateral-CA1 synapses, while the Adcy8-/- mice displayed normal long-term potentiation (LTP) following various induction protocols with high-frequency stimulation (HFS), the Adcy1-/- mice showed protocol-dependent deficits in LTP. We also found that long-term depression (LTD) requires ADCY1 but not ADCY8. Interestingly, both Adcy1-/- and Adcy8-/- mice showed defective synaptic depotentiation (i.e., activity-dependent reversal of LTP); the deficits in Adcy8-/- mice were dependent on the induction protocol. Examination of spatial memory found that ADCY1 is required for the formation of both initial and reversal memory. ADCY8 is only required for reversal memory formation. These data demonstrate that ADCY1 and ADCY8 play distinct roles in regulating synaptic and cognitive flexibility that involves bidirectional modification of synaptic function.

Arc ubiquitination regulates endoplasmic reticulum-mediated Ca2+ release and CaMKII signaling

Synaptic plasticity relies on rapid, yet spatially precise signaling to alter synaptic strength. Arc is a brain enriched protein that is rapidly expressed during learning-related behaviors and is essential for regulating metabotropic glutamate receptor-mediated long-term depression (mGluR-LTD). We previously showed that disrupting the ubiquitination capacity of Arc enhances mGluR-LTD; however, the consequences of Arc ubiquitination on other mGluR-mediated signaling events is poorly characterized. Here we find that pharmacological activation of Group I mGluRs with S-3,5-dihydroxyphenylglycine (DHPG) increases Ca2+ release from the endoplasmic reticulum (ER). Disrupting Arc ubiquitination on key amino acid residues enhances DHPG-induced ER-mediated Ca2+ release. These alterations were observed in all neuronal subregions except secondary branchpoints. Deficits in Arc ubiquitination altered Arc self-assembly and enhanced its interaction with calcium/calmodulin-dependent protein kinase IIb (CaMKIIb) and constitutively active forms of CaMKII in HEK293 cells. Colocalization of Arc and CaMKII was altered in cultured hippocampal neurons, with the notable exception of secondary branchpoints. Finally, disruptions in Arc ubiquitination were found to increase Arc interaction with the integral ER protein Calnexin. These results suggest a previously unknown role for Arc ubiquitination in the fine tuning of ER-mediated Ca2+ signaling that may support mGluR-LTD, which in turn, may regulate CaMKII and its interactions with Arc.

Cyclin Y regulates spatial learning and memory flexibility through distinct control of the actin pathway

Spatial learning and memory flexibility are known to require long-term potentiation (LTP) and long-term depression (LTD), respectively, on a cellular basis. We previously showed that cyclin Y (CCNY), a synapse-remodeling cyclin, is a novel actin-binding protein and an inhibitory regulator of functional and structural LTP in vitro. In this study, we report that Ccny knockout (KO) mice exhibit enhanced LTP and weak LTD at Schaffer collateral-CA1 synapses in the hippocampus. In accordance with enhanced LTP, Ccny KO mice showed improved spatial learning and memory. However, although previous studies reported that normal LTD is necessary for memory flexibility, Ccny KO mice intriguingly showed improved memory flexibility, suggesting that weak LTD could exert memory flexibility when combined with enhanced LTP. At the molecular level, CCNY modulated spatial learning and memory flexibility by distinctively affecting the cofilin-actin signaling pathway in the hippocampus. Specifically, CCNY inhibited cofilin activation by original learning, but reversed such inhibition by reversal learning. Furthermore, viral-mediated overexpression of a phosphomimetic cofilin-S3E in hippocampal CA1 regions enhanced LTP, weakened LTD, and improved spatial learning and memory flexibility, thus mirroring the phenotype of Ccny KO mice. In contrast, the overexpression of a non-phosphorylatable cofilin-S3A in hippocampal CA1 regions of Ccny KO mice reversed the synaptic plasticity, spatial learning, and memory flexibility phenotypes observed in Ccny KO mice. Altogether, our findings demonstrate that LTP and LTD cooperatively regulate memory flexibility. Moreover, CCNY suppresses LTP while facilitating LTD in the hippocampus and negatively regulates spatial learning and memory flexibility through the control of cofilin-actin signaling, proposing CCNY as a learning regulator modulating both memorizing and forgetting processes.

Long-term potentiation and depression regulatory microRNAs were highlighted in Bisphenol A induced learning and memory impairment by microRNA sequencing and bioinformatics analysis

The mechanisms of Bisphenol A (BPA) induced learning and memory impairment have still not been fully elucidated. MicroRNAs (miRNAs) are endogenous non-coding small RNA molecules involved in the process of toxicant-induced neurotoxicity. To investigate the role of miRNAs in BPA-induced learning and memory impairment, we analyzed the impacts of BPA on miRNA expression profile by high-throughput sequencing in mice hippocampus. Results showed that mice treated with BPA displayed impairments of spatial learning and memory and changes in the expression of miRNAs in the hippocampus. Seventeen miRNAs were significantly differentially expressed after BPA exposure, of these, 13 and 4 miRNAs were up- and downregulated, respectively. Bioinformatic analysis of Gene Ontology (GO) and pathway suggests that BPA exposure significantly triggered transcriptional changes of miRNAs associated with learning and memory; the top five affected pathways involved in impairment of learning and memory are: 1) Long-term depression (LTD); 2) Thyroid hormone synthesis; 3) GnRH signaling pathway; 4) Long-term potentiation (LTP); 5) Serotonergic synapse. Eight BPA-responsive differentially expressed miRNAs regulating LTP and LTD were further screened to validate the miRNA sequencing data using Real-Time PCR. The deregulation expression levels of proteins of five target genes (CaMKII, MEK1/2, IP3R, AMPAR1 and PLCβ4) were investigated via western blot, for further verifying the results of gene target analysis. Our results showed that LTP and LTD related miRNAs and their targets could contribute to BPA-induced impairment of learning and memory. This study provides valuable information for novel miRNA biomarkers to detect changes in impairment of learning and memory induced by BPA exposure.

The Long-Term Pannexin 1 Ablation Produces Structural and Functional Modifications in Hippocampal Neurons

Enhanced activity and overexpression of Pannexin 1 (Panx1) channels contribute to neuronal pathologies such as epilepsy and Alzheimer's disease (AD). The Panx1 channel ablation alters the hippocampus's glutamatergic neurotransmission, synaptic plasticity, and memory flexibility. Nevertheless, Panx1-knockout (Panx1-KO) mice still retain the ability to learn, suggesting that compensatory mechanisms stabilize their neuronal activity. Here, we show that the absence of Panx1 in the adult brain promotes a series of structural and functional modifications in the Panx1-KO hippocampal synapses, preserving spontaneous activity. Compared to the wild-type (WT) condition, the adult hippocampal neurons of Panx1-KO mice exhibit enhanced excitability, a more complex dendritic branching, enhanced spine maturation, and an increased proportion of multiple synaptic contacts. These modifications seem to rely on the actin-cytoskeleton dynamics as an increase in the actin polymerization and an imbalance between the Rac1 and the RhoA GTPase activities were observed in Panx1-KO brain tissues. Our findings highlight a novel interaction between Panx1 channels, actin, and Rho GTPases, which appear to be relevant for synapse stability.

NMDA and AMPA Receptors at Synapses: Novel Targets for Tau and α-Synuclein Proteinopathies

A prominent feature of neurodegenerative diseases is synaptic dysfunction and spine loss as early signs of neurodegeneration. In this context, accumulation of misfolded proteins has been identified as one of the most common causes driving synaptic toxicity at excitatory glutamatergic synapses. In particular, a great effort has been placed on dissecting the interplay between the toxic deposition of misfolded proteins and synaptic defects, looking for a possible causal relationship between them. Several studies have demonstrated that misfolded proteins could directly exert negative effects on synaptic compartments, altering either the function or the composition of pre- and post-synaptic receptors. In this review, we focused on the physiopathological role of tau and α-synuclein at the level of postsynaptic glutamate receptors. Tau is a microtubule-associated protein mainly expressed by central nervous system neurons where it exerts several physiological functions. In some cases, it undergoes aberrant post-translational modifications, including hyperphosphorylation, leading to loss of function and toxic aggregate formation. Similarly, aggregated species of the presynaptic protein α-synuclein play a key role in synucleinopathies, a group of neurological conditions that includes Parkinson’s disease. Here, we discussed how tau and α-synuclein target the postsynaptic compartment of excitatory synapses and, specifically, AMPA- and NMDA-type glutamate receptors. Notably, recent studies have reported their direct functional interactions with these receptors, which in turn could contribute to the impaired glutamatergic transmission observed in many neurodegenerative diseases.

The Intriguing Contribution of Hippocampal Long-Term Depression to Spatial Learning and Long-Term Memory

Long-term potentiation (LTP) and long-term depression (LTD) comprise the principal cellular mechanisms that fulfill established criteria for the physiological correlates of learning and memory. Traditionally LTP, that increases synaptic weights, has been ascribed a prominent role in learning and memory whereas LTD, that decreases them, has often been relegated to the category of “counterpart to LTP” that serves to prevent saturation of synapses. In contradiction of these assumptions, studies over the last several years have provided functional evidence for distinct roles of LTD in specific aspects of hippocampus-dependent associative learning and information encoding. Furthermore, evidence of the experience-dependent “pruning” of excitatory synapses, the majority of which are located on dendritic spines, by means of LTD has been provided. In addition, reports exist of the temporal and physical restriction of LTP in dendritic compartments by means of LTD. Here, we discuss the role of LTD and LTP in experience-dependent information encoding based on empirical evidence derived from conjoint behavioral and electrophysiological studies conducted in behaving rodents. We pinpoint the close interrelation between structural modifications of dendritic spines and the occurrence of LTP and LTD. We report on findings that support that whereas LTP serves to acquire the general scheme of a spatial representation, LTD enables retention of content details. We argue that LTD contributes to learning by engaging in a functional interplay with LTP, rather than serving as its simple counterpart, or negator. We propose that similar spatial experiences that share elements of neuronal representations can be modified by means of LTD to enable pattern separation. Therewith, LTD plays a crucial role in the disambiguation of similar spatial representations and the prevention of generalization.

Astrocytes Render Memory Flexible by Releasing D-Serine and Regulating NMDA Receptor Tone in the Hippocampus

BACKGROUND

NMDA receptor (NMDAR) hypofunction has been implicated in several psychiatric disorders with impairment of cognitive flexibility. However, the molecular mechanism of how NMDAR hypofunction with decreased NMDAR tone causes the impairment of cognitive flexibility has been minimally understood. Furthermore, it has been unclear whether hippocampal astrocytes regulate NMDAR tone and cognitive flexibility.

METHODS

We employed cell type-specific genetic manipulations, ex vivo electrophysiological recordings, sniffer patch recordings, cutting-edge biosensor for norepinephrine, and behavioral assays to investigate whether astrocytes can regulate NMDAR tone by releasing D-serine and glutamate. Subsequently, we further investigated the role of NMDAR tone in heterosynaptic long-term depression, metaplasticity, and cognitive flexibility.

RESULTS

We found that hippocampal astrocytes regulate NMDAR tone via BEST1-mediated corelease of D-serine and glutamate. Best1 knockout mice exhibited reduced NMDAR tone and impairments of homosynaptic and α₁ adrenergic receptor-dependent heterosynaptic long-term depression, which leads to defects in metaplasticity and cognitive flexibility. These impairments in Best1 knockout mice can be rescued by hippocampal astrocyte-specific BEST1 expression or enhanced NMDAR tone through D-serine supplement. D-serine injection in Best1 knockout mice during initial learning rescues subsequent reversal learning.

CONCLUSIONS

These findings indicate that NMDAR tone during initial learning is important for subsequent learning, and hippocampal NMDAR tone regulated by astrocytic BEST1 is critical for heterosynaptic long-term depression, metaplasticity, and cognitive flexibility.

Inhibition of the ISR abrogates mGluR5-dependent long-term depression and spatial memory deficits in a rat model of Alzheimer's disease

Soluble amyloid-β-protein (Aβ) oligomers, a major hallmark of AD, trigger the integrated stress response (ISR) via multiple pathologies including neuronal hyperactivation, microvascular hypoxia, and neuroinflammation. Increasing eIF2α phosphorylation, the core event of ISR, facilitates metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), and suppressing its phosphorylation has the opposite effect. Having found the facilitation of mGluR5-LTD by Aβ in live rats, we wondered if suppressing eIF2α phosphorylation cascade would protect against the synaptic plasticity and cognitive disrupting effects of Aβ. We demonstrate here that the facilitation of mGluR5-LTD in a delayed rat model by single i.c.v. injection of synthetic Aβ_1-42. Systemic administration of the small-molecule inhibitor of the ISR called ISRIB (trans-isomer) prevents Aβ-facilitated LTD and abrogates spatial learning and memory deficits in the hippocampus in exogenous synthetic Aβ-injected rats. Moreover, ex vivo evidence indicates that ISRIB normalizes protein synthesis in the hippocampus. Targeting the ISR by suppressing the eIF2α phosphorylation cascade with the eIF2B activator ISRIB may provide protective effects against the synaptic and cognitive disruptive effects of Aβ which likely mediate the early stage of sporadic AD.

Dendritic autophagy degrades postsynaptic proteins and is required for long-term synaptic depression in mice

The pruning of dendritic spines during development requires autophagy. This process is facilitated by long-term depression (LTD)-like mechanisms, which has led to speculation that LTD, a fundamental form of synaptic plasticity, also requires autophagy. Here, we show that the induction of LTD via activation of NMDA receptors or metabotropic glutamate receptors initiates autophagy in the postsynaptic dendrites in mice. Dendritic autophagic vesicles (AVs) act in parallel with the endocytic machinery to remove AMPA receptor subunits from the membrane for degradation. During NMDAR-LTD, key postsynaptic proteins are sequestered for autophagic degradation, as revealed by quantitative proteomic profiling of purified AVs. Pharmacological inhibition of AV biogenesis, or conditional ablation of atg5 in pyramidal neurons abolishes LTD and triggers sustained potentiation in the hippocampus. These deficits in synaptic plasticity are recapitulated by knockdown of atg5 specifically in postsynaptic pyramidal neurons in the CA1 area. Conducive to the role of synaptic plasticity in behavioral flexibility, mice with autophagy deficiency in excitatory neurons exhibit altered response in reversal learning. Therefore, local assembly of the autophagic machinery in dendrites ensures the degradation of postsynaptic components and facilitates LTD expression.

The GSK-3 Inhibitor CT99021 Enhances the Acquisition of Spatial Learning and the Accuracy of Spatial Memory

Glycogen synthase kinase 3 (GSK-3) is a Ser/Thr protein kinase that regulates many cellular processes, including synaptic plasticity. Previously, we reported that inhibition of GSK-3 prevents the induction of one of the major forms of synaptic plasticity, N-methyl-D-aspartate receptor (NMDAR)-dependent long-term depression (LTD), in hippocampal slices. In the present study, we have investigated the effects of inhibiting GSK-3 on learning and memory in healthy naïve animals. Systemic administration of a highly selective GSK-3 inhibitor, CT99021, reversibly blocked NMDAR-dependent LTD in the CA1 region of the hippocampus in anesthetized adult mice. In behavioral tasks, CT99021 had no effect on locomotor activity, anxiety, hippocampus-dependent contextual fear memory, and hippocampus-dependent reversal learning. However, CT99021 facilitated the rate of learning in the Morris water maze (MWM) and T-maze and enhanced the accuracy of long-term spatial memory in the MWM. These findings suggest that GSK-3 regulates the accuracy of spatial memory acquisition and recall.

Propofol improves learning and memory in post-traumatic stress disorder (PTSD) mice via recovering hippocampus synaptic plasticity

AIMS

Propofol, the most commonly used intravenous anesthetic, is known for its protective effect in various human and animal disease models such as post-traumatic stress disease (PTSD). However, it still needs efforts to clarify the effect of propofol on fear memory extinction and the relevant mechanisms.

METHODS

Fear memory extinction was examined in PTSD mice model. Thirty-six mice were randomly divided into three groups: a shock + propofol group (sh + Pro), shock + normal saline group (sh + NS), and sham group. The mice were treated with propofol (150 mg/kg) or normal saline (of the same volume) intraperitoneally 30 min after the conditioning. These mice's behavior was analysed with contextual test, sucrose preference test (SPT) and Morris water maze (MWM). Additionally, the synaptic plasticity of the hippocampus was examined by long-term potentiation (LTP) and long-term depression (LTD).

KEY FINDINGS

Compared with the sham group, the sh + NS group showed increased freezing time and depressive behavior, meanwhile the sh + Pro group showed minor behavioral changes. What's more, we found that propofol rescued the impaired long-term potentiation (LTP) and long-term depression (LTD) in hippocampus of PTSD mice. All these suggest that propofol can accelerate fear memory extinction and change synaptic plasticity of PTSD mice.

SIGNIFICANCE

The study proved that propofol can protect the mice from PTSD by reserving synaptic plasticity and brought a new insight into PTSD treatment indicating that propofol maybe a potential cure for PTSD.

Age-Dependent Contributions of NMDA Receptors and L-Type Calcium Channels to Long-Term Depression in the Piriform Cortex

In the hippocampus, the contributions of N-methyl-D-aspartate receptors (NMDARs) and L-type calcium channels (LTCCs) to neuronal transmission and synaptic plasticity change with aging, underlying calcium dysregulation and cognitive dysfunction. However, the relative contributions of NMDARs and LTCCs in other learning encoding structures during aging are not known. The piriform cortex (PC) plays a significant role in odor associative memories, and like the hippocampus, exhibits forms of long-term synaptic plasticity. Here, we investigated the expression and contribution of NMDARs and LTCCs in long-term depression (LTD) of the PC associational fiber pathway in three cohorts of Sprague Dawley rats: neonatal (1-2 weeks), young adult (2-3 months) and aged (20-25 months). Using a combination of slice electrophysiology, Western blotting, fluorescent immunohistochemistry and confocal imaging, we observed a shift from an NMDAR to LTCC mediation of LTD in aged rats, despite no difference in the amount of LTD expression. These changes in plasticity are related to age-dependent differential receptor expression in the PC. LTCC Cav1.2 expression relative to postsynaptic density protein 95 is increased in the associational pathway of the aged PC layer Ib. Enhanced LTCC contribution in synaptic depression in the PC may contribute to altered olfactory function and learning with aging.

Neonatal NMDA blockade alters the LTP, LTD and cognitive functions in male and female Wistar rats

There is compelling evidence that neonatal blockade of NMDA receptors by phencyclidine (PCP) is associated with cognitive impairment in adulthood but little is known about the effects of early life PCP treatment on synaptic function later in life. Here, we sought to determine whether early life exposure to PCP alters the electrophysiologic function of hippocampal CA1 neurons in adult rats. To this end, male and female Wistar rats received either saline or PCP (10 mg/kg) on postnatal days (PND) 7, 9, and 11, and then underwent separate behavioral and electrophysiology tests in adulthood. Neonatal PCP treatment did not alter basic synaptic transmission and had only a modest effect on frequency following (FF) capacity but significantly decreased the paired-pulse facilitation (PPF) in the Schaffer collateral (SC)-CA1 pathway. We found that PCP treatment significantly attenuated the long-term potentiation (LTP) and long-term depression (LTD) in CA1 neurons accompanied by pronounced alteration in complex response profile in adult rats. The electrophysiology data were comparable in male and female rats and reliably associated with impaired spatial reference and working memories in these animals. Overall, this study suggests that blockade of NMDA receptors during early life deteriorates the short-term and long-term synaptic plasticity and complex response profile of CA1 neurons in adulthood.

Selective expression of the neurexin substrate for presenilin in the adult forebrain causes deficits in associative memory and presynaptic plasticity

Presenilins (PS) form the active subunit of the gamma-secretase complex, which mediates the proteolytic clearance of a broad variety of type-I plasma membrane proteins. Loss-of-function mutations in PSEN1/2 genes are the leading cause of familial Alzheimer's disease (fAD). However, the PS/gamma-secretase substrates relevant for the neuronal deficits associated with a loss of PS function are not completely known. The members of the neurexin (Nrxn) family of presynaptic plasma membrane proteins are candidates to mediate aspects of the synaptic and memory deficits associated with a loss of PS function. Previous work has shown that fAD-linked PS mutants or inactivation of PS by genetic and pharmacological approaches failed to clear Nrxn C-terminal fragments (NrxnCTF), leading to its abnormal accumulation at presynaptic terminals. Here, we generated transgenic mice that selectively recreate the presynaptic accumulation of NrxnCTF in adult forebrain neurons, leaving unaltered the function of PS/gamma-secretase complex towards other substrates. Behavioral characterization identified selective impairments in NrxnCTF mice, including decreased fear-conditioning memory. Electrophysiological recordings in medial prefrontal cortex-basolateral amygdala (mPFC-BLA) of behaving mice showed normal synaptic transmission and uncovered specific defects in synaptic facilitation. These data functionally link the accumulation of NrxnCTF with defects in associative memory and short-term synaptic plasticity, pointing at impaired clearance of NrxnCTF as a new mediator in AD.

Role of hippocampal NF-κB and GluN2B in the memory acquisition impairment of experiences gathered prior to cocaine administration in rats

Cocaine can induce severe neurobehavioral changes, among others, the ones involved in learning and memory processes. It is known that during drug consumption, cocaine-associated memory and learning processes take place. However, much less is known about the effects of this drug upon the mechanisms involved in forgetting.The present report focuses on the mechanisms by which cocaine affects memory consolidation of experiences acquired prior to drug administration. We also study the involvement of hippocampus in these processes, with special interest on the role of Nuclear factor kappa B (NF-κB), N-methyl-D-aspartate glutamate receptor 2B (GluN2B), and their relationship with other proteins, such as cyclic AMP response element binding protein (CREB). For this purpose, we developed a rat experimental model of chronic cocaine administration in which spatial memory and the expression or activity of several proteins in the hippocampus were assessed after 36 days of drug administration. We report an impairment in memory acquisition of experiences gathered prior to cocaine administration, associated to an increase in GluN2B expression in the hippocampus. We also demonstrate a decrease in NF-κB activity, as well as in the expression of the active form of CREB, confirming the role of these transcription factors in the cocaine-induced memory impairment.

Lack of Sez6 Family Proteins Impairs Motor Functions, Short-Term Memory, and Cognitive Flexibility and Alters Dendritic Spine Properties

Seizure-related gene 6 (Sez6), Sez6-Like (Sez6L), and Sez6-Like 2 (Sez6L2) comprise a family of homologous proteins widely expressed throughout the brain that have been linked to neurodevelopmental and psychiatric disorders. Here, we use Sez6 triple knockout (TKO) mice, which lack all three Sez6 family proteins, to demonstrate that Sez6 family proteins regulate dendritic spine structure and cognitive functions, motor learning, and maintenance of motor functions across the lifespan. Compared to WT controls, we found that Sez6 TKO mice had impaired motor learning and their motor coordination was negatively affected from 6 weeks old and declined more rapidly as they aged. Sez6 TKO mice had reduced spine density in the hippocampus and dendritic spines were shifted to more immature morphologies in the somatosensory cortex. Cognitive testing revealed that they had enhanced stress responsiveness, impaired working, and spatial short-term memory but intact spatial long-term memory in the Morris water maze albeit accompanied by a reversal deficit. Our study demonstrates that the lack of Sez6 family proteins results in phenotypes commonly associated with neuropsychiatric disorders making it likely that Sez6 family proteins contribute to the complex etiologies of these disorders.

Disruption of Long-Term Depression Potentiates Latent Inhibition: Key Role for Central Nucleus of the Amygdala

BACKGROUND

Latent inhibition (LI) reflects an adaptive form of learning impaired in certain forms of mental illness. Glutamate receptor activity is linked to LI, but the potential role of synaptic plasticity remains unspecified.

METHODS

Accordingly, the present study examined the possible role of long-term depression (LTD) in LI induced by prior exposure of rats to an auditory stimulus used subsequently as a conditional stimulus to signal a pending footshock. We employed 2 mechanistically distinct LTD inhibitors, the Tat-GluA23Y peptide that blocks endocytosis of the GluA2-containing glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor, or the selective glutamate n-methyl-d-aspartate receptor 2B antagonist, Ro25-6981, administered prior to the acquisition of 2-way conditioned avoidance with or without tone pre-exposure.

RESULTS

Systemic LTD blockade with the Tat-GluA23Y peptide strengthened the LI effect by further impairing acquisition of conditioned avoidance in conditional stimulus-preexposed rats compared with normal conditioning in non-preexposed controls. Systemic Ro25-6981 had no significant effects. Brain region-specific microinjections of the Tat-GluA23Y peptide into the nucleus accumbens, medial prefrontal cortex, or central or basolateral amygdala demonstrated that disruption of glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor endocytosis in the central amygdala also potentiated the LI effect.

CONCLUSIONS

These data revealed a previously unknown role for central amygdala LTD in LI as a key mediator of cognitive flexibility required to respond to previously irrelevant stimuli that acquire significance through reinforcement. The findings may have relevance both for our mechanistic understanding of LI and its alteration in disease states such as schizophrenia, while further elucidating the role of LTD in learning and memory.

Calcineurin Participation in Hebbian and Homeostatic Plasticity Associated With Extinction

In nature, animals need to adapt to constant changes in their environment. Learning and memory are cognitive capabilities that allow this to happen. Extinction, the reduction of a certain behavior or learning previously established, refers to a very particular and interesting type of learning that has been the basis of a series of therapies to diminish non-adaptive behaviors. In recent years, the exploration of the cellular and molecular mechanisms underlying this type of learning has received increasing attention. Hebbian plasticity (the activity-dependent modification of the strength or efficacy of synaptic transmission), and homeostatic plasticity (the homeostatic regulation of plasticity) constitute processes intimately associated with memory formation and maintenance. Particularly, long-term depression (LTD) has been proposed as the underlying mechanism of extinction, while the protein phosphatase calcineurin (CaN) has been widely related to both the extinction process and LTD. In this review, we focus on the available evidence that sustains CaN modulation of LTD and its association with extinction. Beyond the classic view, we also examine the interconnection among extinction, Hebbian and homeostatic plasticity, as well as emergent evidence of the participation of kinases and long-term potentiation (LTP) on extinction learning, highlighting the importance of the balance between kinases and phosphatases in the expression of extinction. Finally, we also integrate data that shows the association between extinction and less-studied phenomena, such as synaptic silencing and engram formation that open new perspectives in the field.

Long-term plasticity in the hippocampus: maintaining within and 'tagging' between synapses

Synapses between neurons are malleable biochemical structures, strengthening and diminishing over time dependent on the type of information they receive. This phenomenon known as synaptic plasticity underlies learning and memory, and its different forms, long-term potentiation (LTP) and long-term depression (LTD), perform varied cognitive roles in reinforcement, relearning and associating memories. Moreover, both LTP and LTD can exist in an early transient form (early-LTP/LTD) or a late persistent form (late-LTP/LTD), which are triggered by different induction protocols, and also differ in their dependence on protein synthesis and the involvement of key molecular players. Beyond homosynaptic modifications, synapses can also interact with one another. This is encapsulated in the synaptic tagging and capture hypothesis (STC), where synapses expressing early-LTP/LTD present a 'tag' that can capture the protein synthesis products generated during a temporally proximal late-LTP/LTD induction. This 'tagging' phenomenon forms the framework of synaptic interactions in various conditions and accounts for the cellular basis of the time-dependent associativity of short-lasting and long-lasting memories. All these synaptic modifications take place under controlled neuronal conditions, regulated by subcellular elements such as epigenetic regulation, proteasomal degradation and neuromodulatory signals. Here, we review current understanding of the different forms of synaptic plasticity and its regulatory mechanisms in the hippocampus, a brain region critical for memory formation. We also discuss expression of plasticity in hippocampal CA2 area, a long-overlooked narrow hippocampal subfield and the behavioural correlate of STC. Lastly, we put forth perspectives for an integrated view of memory representation in synapses.

Working and Reference Memory Tasks Trigger Opposed Long-Term Synaptic Changes in the Rat Dentate Gyrus

Long-term storage of information into memory is supposed to rely on long-term synaptic plasticity processes. The detection of such synaptic changes after training in long-term/reference memory (RM) tasks has yet been scarce, variable and only studied on a short time scale. Short-term or working memory (WM) is largely known to depend on persistent neuronal activity or short-term plasticity. However, processing information into WM could also involve long-term synaptic changes that could be responsible for the erasure/forgetting of items previously stored in WM and acting as proactive interference. In order to study long-term synaptic changes associated with RM or WM, we trained chronically implanted rats in 3 different radial maze tasks: a classical RM task and 2 WM tasks involving different levels of proactive interference. Synaptic responses in the dentate gyrus were recorded during 2 × 24 h in freely moving rats after training. We found that consolidation of long-term information leads first to a delayed synaptic potentiation, occurring 9 h after RM training that is replaced by a synaptic depression once the RM rule is fully acquired. In contrast, optimal information processing into WM triggers a synaptic depression immediately after training and lasting 3 h that could act as a mechanism for interference erasure/forgetting.

Defects in syntabulin-mediated synaptic cargo transport associate with autism-like synaptic dysfunction and social behavioral traits

The formation and maintenance of synapses require long-distance delivery of newly synthesized synaptic proteins from the soma to distal synapses, raising the fundamental question of whether impaired transport is associated with neurodevelopmental disorders such as autism. We previously revealed that syntabulin acts as a motor adapter linking kinesin-1 motor and presynaptic cargos. Here, we report that defects in syntabulin-mediated transport and thus reduced formation and maturation of synapses are one of core synaptic mechanisms underlying autism-like synaptic dysfunction and social behavioral abnormalities. Syntabulin expression in the mouse brain peaks during the first 2 weeks of postnatal development and progressively declines during brain maturation. Neurons from conditional syntabulin-/- mice (stb cKO) display impaired transport of presynaptic cargos, reduced synapse density and active zones, and altered synaptic transmission and long-term plasticity. Intriguingly, stb cKO mice exhibit core autism-like traits, including defective social recognition and communication, increased stereotypic behavior, and impaired spatial learning and memory. These phenotypes establish a new mechanistic link between reduced transport of synaptic cargos and impaired maintenance of synaptic transmission and plasticity, contributing to autism-associated behavioral abnormalities. This notion is further confirmed by the human missense variant STB-R178Q, which is found in an autism patient and loses its adapter capacity for binding kinesin-1 motors. Expressing STB-R178Q fails to rescue reduced synapse formation and impaired synaptic transmission and plasticity in stb cKO neurons. Altogether, our study suggests that defects in syntabulin-mediated transport mechanisms underlie the synaptic dysfunction and behavioral abnormalities that bear similarities to autism.

Molecular Basis of Neuronal Autophagy in Ageing: Insights from Caenorhabditis elegans

Autophagy is an evolutionarily conserved degradation process maintaining cell homeostasis. Induction of autophagy is triggered as a response to a broad range of cellular stress conditions, such as nutrient deprivation, protein aggregation, organelle damage and pathogen invasion. Macroautophagy involves the sequestration of cytoplasmic contents in a double-membrane organelle referred to as the autophagosome with subsequent degradation of its contents upon delivery to lysosomes. Autophagy plays critical roles in development, maintenance and survival of distinct cell populations including neurons. Consequently, age-dependent decline in autophagy predisposes animals for age-related diseases including neurodegeneration and compromises healthspan and longevity. In this review, we summarize recent advances in our understanding of the role of neuronal autophagy in ageing, focusing on studies in the nematode Caenorhabditis elegans.

Mutation in PHACTR1 associated with multifocal epilepsy with infantile spasms and hypsarrhythmia

A young boy with multifocal epilepsy with infantile spasms and hypsarrhythmia with minimal organic lesions of brain structures underwent DNA diagnosis using whole‐exome sequencing. A heterozygous amino‐acid substitution p.L519R in a PHACTR1 gene was identified. PHACTR1 belongs to a protein family of G‐actin binding protein phosphatase 1 (PP1) cofactors and was not previously associated with a human disease. The missense single nucleotide variant in the proband was shown to occur de novo in the paternal allele. The mutation was shown in vitro to reduce the affinity of PHACTR1 for G‐actin, and to increase its propensity to form complexes with the catalytic subunit of PP1. These properties are associated with altered subcellular localization of PHACTR1 and increased ability to induce cytoskeletal rearrangements. Although the molecular role of the PHACTR1 in neuronal excitability and differentiation remains to be defined, PHACTR1 has been previously shown to be involved in Slack channelopathy pathogenesis, consistent with our findings. We conclude that this activating mutation in PHACTR1 causes a severe type of sporadic multifocal epilepsy in the patient.

The Na+-activated K+ channel Slack contributes to synaptic development and plasticity

Human mutations of the Na⁺-activated K⁺ channel Slack (KCNT1) are associated with epilepsy and intellectual disability. Accordingly, Slack knockout mice (Slack^-/-) exhibit cognitive flexibility deficits in distinct behavioral tasks. So far, however, the underlying causes as well as the role of Slack in hippocampus-dependent memory functions remain enigmatic. We now report that infant (P6-P14) Slack^-/- lack both hippocampal LTD and LTP, likely due to impaired NMDA receptor (NMDAR) signaling. Postsynaptic GluN2B levels are reduced in infant Slack^-/-, evidenced by lower amplitudes of NMDAR-meditated excitatory postsynaptic potentials. Low GluN2B affected NMDAR-mediated Ca²⁺-influx, rendering cultured hippocampal Slack^-/-neurons highly insensitive to the GluN2B-specific inhibitor Ro 25-6981. Furthermore, dephosphorylation of the AMPA receptor (AMPAR) subunit GluA1 at S845, which is involved in AMPAR endocytosis during homeostatic and neuromodulator-regulated plasticity, is reduced after chemical LTD (cLTD) in infant Slack^-/-. We additionally detect a lack of mGluR-induced LTD in infant Slack^-/-, possibly caused by upregulation of the recycling endosome-associated small GTPase Rab4 which might accelerate AMPAR recycling from early endosomes. Interestingly, LTP and mGluR LTD, but not LTD and S845 dephosphorylation after cLTD are restored in adult Slack^-/-. This together with normalized expression levels of GluN2B and Rab4 hints to developmental "restoration" of LTP expression despite Slack ablation, whereas in infant and adult brain, NMDAR-dependent LTD induction depends on this channel. Based on the present findings, NMDAR and vesicular transport might represent novel targets for the therapy of intellectual disability associated with Slack mutations. Consequently, careful modulation of hippocampal Slack activity should also improve learning abilities.

Date seeds alleviate behavioural and neuronal complications of metabolic syndrome in rats

Unhealthy dietary habits can play a crucial role in metabolic damages, promoting alteration of neural functions through the lifespan. Recently, dietary change has been perceived as the first line intervention in prevention and/or treatment of metabolic damages and related diseases. In this context, our study was designed to assess the eventual therapeutic effect of date seeds administration on memory and learning and on neuronal markers in a rat Metabolic Syndrome model. For this purpose, 32 adult male Wistar rats were fed with standard diet or high-fat high-sugar diet during ten weeks. After this, 16 rats were sacrified and the remaining rats received an oral administration of 300 mg of date seeds/kg of body weight during four supplementary weeks. Before sacrifice, we evaluate cognitive performances by the Barnes maze test. Afterwards, neuronal, astrocytic, microtubular and oxidative markers were investigated by immunoblotting methods. In Metabolic syndrome rats, results showed impairment of spatial memory and histological alterations. We identified neuronal damages in hippocampus, marked by a decrease of NeuN and an increase of GFAP and pTau396. Finally, we recorded an increase in protein oxidation and lipid peroxidation, respectively identified by an up-regulation of protein carbonyls and 4-HNe. Interestingly, date seeds administration improved these behavioural, histological, neuronal and oxidative damages highlighting the neuroprotective effect of this natural compound. Liquid Chromatography-Mass Spectrometry (LC-MS) identified, in date seeds, protocatechuic acid, caffeoylshikimic acid and vanillic acid, that could potentially prevent the progression of neurodegenerative diseases, acting through their antioxidant properties.

Noncanonical cytoplasmic poly(A) polymerases regulate RNA levels, alternative RNA processing, and synaptic plasticity but not hippocampal-dependent behaviours

Noncanonical poly(A) polymerases are frequently tethered to mRNA 3' untranslated regions and regulate poly(A) tail length and resulting translation. In the brain, one such poly(A) polymerase is Gld2, which is anchored to mRNA by the RNA-binding protein CPEB1 to control local translation at postsynaptic regions. Depletion of CPEB1 or Gld2 from the mouse hippocampus results in a deficit in long-term potentiation (LTP), but only depletion of CPEB1 alters animal behaviour. To test whether a related enzyme, Gld4, compensates for the lack of Gld2, we separately or simultaneously depleted both proteins from hippocampal area CA1 and again found little change in animal behaviour, but observed a deficit in LTP as well as an increase in long-term depression (LTD), two forms of protein synthesis-dependent synaptic plasticity. RNA-seq data from Gld2, Gld4, and Gld2/Gld4-depleted hippocampus show widespread changes in steady state RNA levels, alternative splicing, and alternative poly(A) site selection. Many of the RNAs subject to these alterations encode proteins that mediate synaptic function, suggesting a molecular foundation for impaired synaptic plasticity.

Chronic Sodium Selenate Treatment Restores Deficits in Cognition and Synaptic Plasticity in a Murine Model of Tauopathy

A major goal in diseases is identifying a potential therapeutic agent that is cost-effective and can remedy some, if not all, disease symptoms. In Alzheimer’s disease (AD), aggregation of hyperphosphorylated tau protein is one of the neuropathological hallmarks, and Tau pathology correlates better with cognitive impairments in AD patients than amyloid-β load, supporting a key role of tau-related mechanisms. Selenium is a non-metallic trace element that is incorporated in the brain into selenoproteins. Chronic treatment with sodium selenate, a non-toxic selenium compound, was recently reported to rescue behavioral phenotypes in tau mouse models. Here, we focused on the effects of chronic selenate application on synaptic transmission and synaptic plasticity in THY-Tau22 mice, a transgenic animal model of tauopathies. Three months with a supplement of sodium selenate in the drinking water (12 μg/ml) restored not only impaired neurocognitive functions but also rescued long-term depression (LTD), a major form of synaptic plasticity. Furthermore, selenate reduced the inactive demethylated catalytic subunit of protein phosphatase 2A (PP2A) in THY-Tau22 without affecting total PP2A.Our study provides evidence that chronic dietary selenate rescues functional synaptic deficits of tauopathy and identifies activation of PP2A as the putative mechanism.

Age-related memory decline, dysfunction of the hippocampus and therapeutic opportunities

While the aging of the population is a sign of progress for societies, it also carries its load of negative aspects. Among them, cognitive decline and in particular memory loss is a common feature of non-pathological aging. Autobiographical memories, which rely on the hippocampus, are a primary target of age-related cognitive decline. Here, focusing on the neurobiological mechanisms of memory formation and storage, we describe how hippocampal functions are altered across time in non-pathological mammalian brains. Several hallmarks of aging have been well described over the last decades; among them, we consider altered synaptic communication and plasticity, reduction of adult neurogenesis and epigenetic alterations. Building on the neurobiological processes of cognitive aging that have been identified to date, we review some of the strategies based on lifestyle manupulation allowing to address age-related cognitive deficits.

Effects of exercise on proactive interference in memory: potential neuroplasticity and neurochemical mechanisms

Proactive interference occurs when consolidated memory traces inhibit new learning. This kind of interference decreases the efficiency of new learning and also causes memory errors. Exercise has been shown to facilitate some types of cognitive function; however, whether exercise reduces proactive interference to enhance learning efficiency is not well understood. Thus, this review discusses the effects of exercise on proactive memory interference and explores potential mechanisms, such as neurogenesis and neurochemical changes, mediating any effect.

Autophagy controls the induction and developmental decline of NMDAR-LTD through endocytic recycling

NMDA receptor-dependent long-term depression (NMDAR-LTD) is a long-lasting form of synaptic plasticity. Its expression is mediated by the removal of AMPA receptors from postsynaptic membranes. Under basal conditions, endocytosed AMPA receptors are rapidly recycled back to the plasma membrane. In NMDAR-LTD, however, they are diverted to late endosomes for degradation. The mechanism for this switch is largely unclear. Additionally, the inducibility of NMDAR-LTD is greatly reduced in adulthood. The underlying mechanism and physiological significance of this phenomenon are elusive. Here, we report that autophagy inhibition is essential for the induction and developmental dampening of NMDAR-LTD. Autophagy is inhibited during NMDAR-LTD to decrease endocytic recycling. Autophagy inhibition is both necessary and sufficient for LTD induction. In adulthood, autophagy is up-regulated to make LTD induction harder, thereby preventing the adverse effect of excessive LTD on memory consolidation. These findings reveal the unrecognized functions of autophagy in synaptic plasticity, endocytic recycling, and memory.

NMDA receptor-dependent long-term depression (NMDAR-LTD) is a form of synaptic plasticity mediated by reduced recycling of AMPA receptors to the plasma membrane. Here the authors show that autophagy is a regulator of this endocytic recycling and autophagy upregulation dampens NMDAR-LTD in adulthood.

The Role of Tau in the Post-synapse

It is well documented that tauopathy is involved in various forms of neurodegenerative disease. However, there is a huge gap in terms of our understanding of the neurophysiological roles of tau, and how these can be aberrantly regulated by pathological processes. Tau is enriched in the axon but is also localized to synapses. The finding of synaptically localised tau has undoubtedly created more questions than it has answered. What is the physiological role of tau at the synapse? Whether and how does tau interact with and effect other synaptic proteins to mediate this function? Are these effects regulated by post-translational modifications of tau, such as phosphorylation? Such questions require significant attention from the scientific community if we are to resolve this critical aspect of tau biology. This chapter will describe our current understanding of synaptic tau and its functions and illuminate the numerous remaining challenges in this evolving research area.

Caspase-2 promotes AMPA receptor internalization and cognitive flexibility via mTORC2-AKT-GSK3β signaling

Caspase-2 is the most evolutionarily conserved member in the caspase family of proteases and is constitutively expressed in most cell types including neurons; however, its physiological function remains largely unknown. Here we report that caspase-2 plays a critical role in synaptic plasticity and cognitive flexibility. We found that caspase-2 deficiency led to deficits in dendritic spine pruning, internalization of AMPA receptors and long-term depression. Our results indicate that caspase-2 degrades Rictor, a key mTOR complex 2 (mTORC2) component, to inhibit Akt activation, which leads to enhancement of the GSK3β activity and thereby long-term depression. Furthermore, we found that mice lacking caspase-2 displayed elevated levels of anxiety, impairment in reversal water maze learning, and little memory loss over time. These results not only uncover a caspase-2-mTORC2-Akt-GSK3β signaling pathway, but also suggest that caspase-2 is important for memory erasing and normal behaviors by regulating synaptic number and transmission.

Astrocytic p38α MAPK drives NMDA receptor-dependent long-term depression and modulates long-term memory

NMDA receptor-dependent long-term depression (LTD) in the hippocampus is a well-known form of synaptic plasticity that has been linked to different cognitive functions. The core mechanism for this form of plasticity is thought to be entirely neuronal. However, we now demonstrate that astrocytic activity drives LTD at CA3-CA1 synapses. We have found that LTD induction enhances astrocyte-to-neuron communication mediated by glutamate, and that Ca²⁺ signaling and SNARE-dependent vesicular release from the astrocyte are required for LTD expression. In addition, using optogenetic techniques, we show that low-frequency astrocytic activation, in the absence of presynaptic activity, is sufficient to induce postsynaptic AMPA receptor removal and LTD expression. Using cell-type-specific gene deletion, we show that astrocytic p38α MAPK is required for the increased astrocytic glutamate release and astrocyte-to-neuron communication during low-frequency stimulation. Accordingly, removal of astrocytic (but not neuronal) p38α abolishes LTD expression. Finally, this mechanism modulates long-term memory in vivo.

How astrocytes influence neuronal plasticity remains unclear, as they are typically considered as modulators of core mechanisms driven by neuronal components. Here, authors show that Long-term depression (LTD) induction in the hippocampus triggers calcium signaling in the astrocyte and enhances SNARE-dependent astrocytic glutamate release, which is then responsible for the activation of postsynaptic NMDA receptors and synaptic depression.

VGF has Roles in the Pathogenesis of Major Depressive Disorder and Schizophrenia: Evidence from Transgenic Mouse Models

Mental disorders, such as major depressive disorder and schizophrenia, are complex multigenetic conditions, but focused studies of single genes might reveal genes involved in the pathogenesis of mental disorders, including major depressive disorder and schizophrenia. Several candidate genes have been identified using transgenic mice. VGF nerve growth factor inducible (VGF) is a neuropeptide expression of which is induced by nerve growth factor (NGF). VGF is robustly and exclusively synthesized in neuronal and neuroendocrine cells. In central nervous system (CNS), VGF is extensively expressed especially in the cerebral cortex, hippocampus, and hypothalamus. VGF has many roles in the CNS, such as promotion of synaptic plasticity, neurogenesis, and neurite outgrowth. In clinical studies, altered expression and genetic mutations of VGF have been reported in patients with major depressive disorder and schizophrenia. On this basis, studies using transgenic mice to overexpress or knockout VGF have been performed to investigate the roles of upregulation or downregulation of VGF. In this review, we will discuss studies of the roles of VGF using transgenic mice and its relevance to pathologies in major depressive disorder and schizophrenia.

Perineuronal Nets Restrict the Induction of Long-Term Depression in the Mouse Hippocampal CA1 Region

Long-term depression (LTD) of synaptic efficacy is widely regarded as a cellular basis of learning and memory. The magnitude of hippocampal CA1 LTD induced by low-frequency stimulation (LFS) declines with age, but the mechanisms involved remain poorly understood. Perineuronal nets (PNNs) are specialized extracellular matrix structures that function in dampening synaptic plasticity during postnatal development, suggesting that PNN formation may restrict LTD induction in the adult hippocampus. Here, we show that PNNs tightly enwrap a subpopulation of parvalbumin (PV) interneurons in the hippocampal CA1 region and enzymatic removal of PNNs with the chondroitinase ABC alters the excitatory/inhibitory synaptic balance toward more excitation and restores the ability of LFS to induce an N-methyl-D-aspartate receptor-dependent LTD at Schaffer collateral-CA1 synapses in slices from male adult mice. Early interference with depolarizing GABA with Na⁺-K⁺-2Cl^- cotransporter inhibitor bumetanide impairs the maturation of PNNs and enhances LTD induction. These results provide novel insights into a previously unrecognized role for PNNs around PV interneurons in restricting long-term synaptic plasticity at excitatory synapses on hippocampal CA1 neurons in adulthood.

Synaptic retinoic acid receptor signaling mediates mTOR-dependent metaplasticity that controls hippocampal learning

Homeostatic synaptic plasticity is a stabilizing mechanism engaged by neural circuits in response to prolonged perturbation of network activity. The non-Hebbian nature of homeostatic synaptic plasticity is thought to contribute to network stability by preventing "runaway" Hebbian plasticity at individual synapses. However, whether blocking homeostatic synaptic plasticity indeed induces runaway Hebbian plasticity in an intact neural circuit has not been explored. Furthermore, how compromised homeostatic synaptic plasticity impacts animal learning remains unclear. Here, we show in mice that the experience of an enriched environment (EE) engaged homeostatic synaptic plasticity in hippocampal circuits, thereby reducing excitatory synaptic transmission. This process required RARα, a nuclear retinoic acid receptor that doubles as a cytoplasmic retinoic acid-induced postsynaptic regulator of protein synthesis. Blocking RARα-dependent homeostatic synaptic plasticity during an EE experience by ablating RARα signaling induced runaway Hebbian plasticity, as evidenced by greatly enhanced long-term potentiation (LTP). As a consequence, RARα deletion in hippocampal circuits during an EE experience resulted in enhanced spatial learning but suppressed learning flexibility. In the absence of RARα, moreover, EE experience superactivated mammalian target of rapamycin (mTOR) signaling, causing a shift in protein translation that enhanced the expression levels of AMPA-type glutamate receptors. Treatment of mice with the mTOR inhibitor rapamycin during an EE experience not only restored normal AMPA-receptor expression levels but also reversed the increases in runaway Hebbian plasticity and learning after hippocampal RARα deletion. Thus, our findings reveal an RARα- and mTOR-dependent mechanism by which homeostatic plasticity controls Hebbian plasticity and learning.

Ouabain increases neuronal branching in hippocampus and improves spatial memory

Previous research shows Ouabain (OUA) to bind Na, K-ATPase, thereby triggering a number of signaling pathways, including the transcription factors NFᴋB and CREB. These transcription factors play a key role in the regulation of BDNF and WNT-β-catenin signaling cascades, which are involved in neuroprotection and memory regulation. This study investigated the effects of OUA (10 nM) in the modulation of the principal signaling pathways involved in morphological plasticity and memory formation in the hippocampus of adult rats. The results show intrahippocampal injection of OUA 10 nM to activate the Wnt/β-Catenin signaling pathway and to increase CREB/BDNF and NFᴋB levels. These effects contribute to important changes in the cellular microenvironment, resulting in enhanced levels of dendritic branching in hippocampal neurons, in association with an improvement in spatial reference memory and the inhibition of long-term memory extinction.