Integrin-driven actin polymerization consolidates long-term potentiation

Metabotropic NMDAR Signaling Contributes to Sex Differences in Synaptic Plasticity and Episodic Memory

NMDA receptor (NMDAR)-mediated calcium influx triggers the induction and initial expression of long-term potentiation (LTP). Here we report that in male rodents, ion flux-independent (metabotropic) NMDAR signaling is critical for a third step in the production of enduring LTP, i.e., cytoskeletal changes that stabilize the activity-induced synaptic modifications. Surprisingly, females rely upon estrogen receptor alpha (ERα) for the metabotropic NMDAR operations used by males. Blocking NMDAR channels with MK-801 eliminated LTP expression in hippocampal field CA1 of both sexes but left intact theta burst stimulation (TBS)-induced actin polymerization within dendritic spines. A selective antagonist (Ro25-6981) of the NMDAR GluN2B subunit had minimal effects on synaptic responses but blocked actin polymerization and LTP consolidation in males only. Conversely, an ERα antagonist thoroughly disrupted TBS-induced actin polymerization and LTP in females while having no evident effect in males. In an episodic memory paradigm, Ro25-6981 prevented acquisition of spatial locations by males but not females, whereas an ERα antagonist blocked acquisition in females but not males. Sex differences in LTP consolidation were accompanied by pronounced differences in episodic memory in tasks involving minimal (for learning) cue sampling. Males did better on acquisition of spatial information whereas females had much higher scores than males on tests for acquisition of the identity of cues (episodic "what") and the order in which the cues were sampled (episodic "when"). We propose that sex differences in synaptic processes used to stabilize LTP result in differential encoding of the basic elements of episodic memory.

Unlocking mechanosensitivity: integrins in neural adaptation

Mechanosensitivity extends beyond sensory cells to encompass most neurons in the brain. Here, we explore recent research on the role of integrins, a diverse family of adhesion molecules, as crucial biomechanical sensors translating mechanical forces into biochemical and electrical signals in the brain. The varied biomechanical properties of neuronal integrins, including their force-dependent conformational states and ligand interactions, dictate their specific functions. We discuss new findings on how integrins regulate filopodia and dendritic spines, shedding light on their contributions to synaptic plasticity, and explore recent discoveries on how they engage with metabotropic receptors and ion channels, highlighting their direct participation in electromechanical transduction. Finally, to facilitate a deeper understanding of these developments, we present molecular and biophysical models of mechanotransduction.

Alterations of synaptic plasticity in Angelman syndrome model mice are rescued by 5-HT7R stimulation

Angelman syndrome (AS) is a severe neurodevelopmental disorder characterized by motor disfunction, seizures, intellectual disability, speech deficits, and autism-like behavior, showing high comorbidity with Autism Spectrum Disorders (ASD). It is known that stimulation of the serotonin receptor 7 (5-HT7R) can rescue some of the behavioral and neuroplasticity dysfunctions in animal models of Fragile X and Rett syndrome, two pathologies associated with ASD. In view of these observations, we hypothesised that alterations of 5-HT7R signalling might also be involved in AS. To test this hypothesis, we stimulated 5-HT7R with the selective agonist LP-211 to investigate its possible beneficial effects on synaptic dysfunctions and altered behavior in the AS mice model. In mutant mice, we observed impairment of the synaptic machinery of protein synthesis, which was reversed by 5-HT7R activation. Moreover, stimulation of 5-HT7R was able to: i) enhance dendritic spine density in hippocampal neurons, which was reduced in AS mice; ii) restore impaired long-term potentiation (LTP) in hippocampal slices of the AS mice; iii) improve cognitive performance of the mutant animals subjected to the fear conditioning paradigm. Altogether, our results, showing beneficial effects of 5-HT7R stimulation in restoring molecular and cognitive deficits associated with AS, suggest that targeting 5-HT7R could be a promising therapeutic approach for the pathology.

Contributions of site- and sex-specific LTPs to everyday memory

Commentaries about long-term potentiation (LTP) generally proceed with an implicit assumption that largely the same physiological effect is sampled across different experiments. However, this is clearly not the case. We illustrate the point by comparing LTP in the CA3 projections to CA1 with the different forms of potentiation in the dentate gyrus. These studies lead to the hypothesis that specialized properties of CA1-LTP are adaptations for encoding unsupervised learning and episodic memory, whereas the dentate gyrus variants subserve learning that requires multiple trials and separation of overlapping bodies of information. Recent work has added sex as a second and somewhat surprising dimension along which LTP is also differentiated. Triggering events for CA1-LTP differ between the sexes and the adult induction threshold is significantly higher in females; these findings help explain why males have an advantage in spatial learning. Remarkably, the converse is true before puberty: Females have the lower LTP threshold and are better at spatial memory problems. A mechanism has been identified for the loss-of-function in females but not for the gain-of-function in males. We propose that the many and disparate demands of natural environments, with different processing requirements across ages and between sexes, led to the emergence of multiple LTPs. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Anesthetic Sevoflurane Induces Enlargement of Dendritic Spine Heads in Mouse Neurons via Tau-Dependent Mechanisms

BACKGROUND

Sevoflurane induces neuronal dysfunction and cognitive impairment. However, the underlying mechanism remains largely to be determined. Tau, cyclophilin D, and dendritic spine contribute to cognitive function. But whether changes in dendritic spines are involved in the effects of sevoflurane and the potential association with tau and cyclophilin D is not clear.

METHODS

We harvested hippocampal neurons from wild-type mice, tau knockout mice, and cyclophilin D knockout mice. We treated these neurons with sevoflurane at day in vitro 7 and measured the diameter of dendritic spine head and the number of dendritic spines. Moreover, we determined the effects of sevoflurane on the expression of excitatory amino acid transporter 3 (EAAT3), extracellular glutamate levels, and miniature excitatory postsynaptic currents (mEPSCs). Finally, we used lithium, cyclosporine A, and overexpression of EAAT3 in the interaction studies.

RESULTS

Sevoflurane-induced tau phosphgorylation increased the diameter of dendritic spine head and decreased the number of dendritic spines in neurons harvested from wild-type and cyclophilin D knockout mice, but not tau knockout mice. Sevoflurane decreased the expression of EAAT3, increased extracellular glutamate levels, and decreased the frequency of mEPSCs in the neurons. Overexpression of EAAT3 mitigated the effects of sevoflurane on dendritic spines. Lithium, but not cyclosporine A, attenuated the effects of sevoflurane on dendritic spines. Lithium also inhibited the effects of sevoflurane on EAAT3 expression and mEPSCs.

CONCLUSIONS

These data suggest that sevoflurane induces a tau phosphorylation-dependent demtrimental effect on dendritic spine via decreasing EAAT3 expression and increasing extracellular glutamate levels, leading to neuronal dysfunction.

Pre- versus Post-synaptic Forms of LTP in Two Branches of the Same Hippocampal Afferent

There has been considerable controversy about pre- versus postsynaptic expression of memory-related long-term potentiation (LTP), with corresponding disputes about underlying mechanisms. We report here an instance in male mice, in which both types of potentiation are expressed but in separate branches of the same hippocampal afferent. Induction of LTP in the dentate gyrus (DG) branch of the lateral perforant path (LPP) reduces paired-pulse facilitation, is blocked by antagonism of cannabinoid receptor type 1, and is not affected by suppression of postsynaptic actin polymerization. These observations are consistent with presynaptic expression. The opposite pattern of results was obtained in the LPP branch that innervates the distal dendrites of CA3: LTP did not reduce paired-pulse facilitation, was unaffected by the cannabinoid receptor blocker, and required postsynaptic actin filament assembly. Differences in the two LPP termination sites were also noted for frequency facilitation of synaptic responses, an effect that was reproduced in a two-step simulation by small adjustments to vesicle release dynamics. These results indicate that different types of glutamatergic neurons impose different forms of filtering and synaptic plasticity on their afferents. They also suggest that inputs are routed to, and encoded by, different sites within the hippocampus depending upon the pattern of activity arriving over the parent axon.

Metabotropic NMDA Receptor Signaling Contributes to Sex Differences in Synaptic Plasticity and Episodic Memory

Men generally outperform women on encoding spatial components of episodic memory whereas the reverse holds for semantic elements. Here we show that female mice outperform males on tests for non-spatial aspects of episodic memory ("what", "when"), suggesting that the human findings are influenced by neurobiological factors common to mammals. Analysis of hippocampal synaptic plasticity mechanisms and encoding revealed unprecedented, sex-specific contributions of non-classical metabotropic NMDA receptor (NMDAR) functions. While both sexes used non-ionic NMDAR signaling to trigger actin polymerization needed to consolidate long-term potentiation (LTP), NMDAR GluN2B subunit antagonism blocked these effects in males only and had the corresponding sex-specific effect on episodic memory. Conversely, blocking estrogen receptor alpha eliminated metabotropic stabilization of LTP and episodic memory in females only. The results show that sex differences in metabotropic signaling critical for enduring synaptic plasticity in hippocampus have significant consequences for encoding episodic memories.

An antagonism between Spinophilin and Syd-1 operates upstream of memory-promoting presynaptic long-term plasticity

We still face fundamental gaps in understanding how molecular plastic changes of synapses intersect with circuit operation to define behavioral states. Here, we show that an antagonism between two conserved regulatory proteins, Spinophilin (Spn) and Syd-1, controls presynaptic long-term plasticity and the maintenance of olfactory memories in Drosophila. While Spn mutants could not trigger nanoscopic active zone remodeling under homeostatic challenge and failed to stably potentiate neurotransmitter release, concomitant reduction of Syd-1 rescued all these deficits. The Spn/Syd-1 antagonism converged on active zone close F-actin, and genetic or acute pharmacological depolymerization of F-actin rescued the Spn deficits by allowing access to synaptic vesicle release sites. Within the intrinsic mushroom body neurons, the Spn/Syd-1 antagonism specifically controlled olfactory memory stabilization but not initial learning. Thus, this evolutionarily conserved protein complex controls behaviorally relevant presynaptic long-term plasticity, also observed in the mammalian brain but still enigmatic concerning its molecular mechanisms and behavioral relevance.

Recruitment of the SNX17-Retriever recycling pathway regulates synaptic function and plasticity

Trafficking of cell-surface proteins from endosomes to the plasma membrane is a key mechanism to regulate synaptic function. In non-neuronal cells, proteins recycle to the plasma membrane either via the SNX27-Retromer-WASH pathway or via the recently discovered SNX17-Retriever-CCC-WASH pathway. While SNX27 is responsible for the recycling of key neuronal receptors, the roles of SNX17 in neurons are less understood. Here, using cultured hippocampal neurons, we demonstrate that the SNX17 pathway regulates synaptic function and plasticity. Disruption of this pathway results in a loss of excitatory synapses and prevents structural plasticity during chemical long-term potentiation (cLTP). cLTP drives SNX17 recruitment to synapses, where its roles are in part mediated by regulating the surface expression of β1-integrin. SNX17 recruitment relies on NMDAR activation, CaMKII signaling, and requires binding to the Retriever and PI(3)P. Together, these findings provide molecular insights into the regulation of SNX17 at synapses and define key roles for SNX17 in synaptic maintenance and in regulating enduring forms of synaptic plasticity.

Sex differences in synaptic plasticity underlying learning

Although sex differences in learning behaviors are well documented, sexual dimorphism in the synaptic processes of encoding is only recently appreciated. Studies in male rodents have built upon the discovery of long-term potentiation (LTP), and acceptance of this activity-dependent increase in synaptic strength as a mechanism of encoding, to identify synaptic receptors and signaling activities that coordinate the activity-dependent remodeling of the subsynaptic actin cytoskeleton that is critical for enduring potentiation and memory. These molecular substrates together with other features of LTP, as characterized in males, have provided an explanation for a range of memory phenomena including multiple stages of consolidation, the efficacy of spaced training, and the location of engrams at the level of individual synapses. In the present report, we summarize these findings and describe more recent results from our laboratories showing that in females the same actin regulatory mechanisms are required for hippocampal LTP and memory but, in females only, the engagement of both modulatory receptors such as TrkB and synaptic signaling intermediaries including Src and ERK1/2 requires neuron-derived estrogen and signaling through membrane-associated estrogen receptor α (ERα). Moreover, in association with the additional ERα involvement, females exhibit a higher threshold for hippocampal LTP and spatial learning. We propose that the distinct LTP threshold in females contributes to as yet unappreciated sex differences in information processing and features of learning and memory.

Presynaptic Plasticity Is Associated with Actin Polymerization

Modulation of presynaptic short-term plasticity induced by actin polymerization was studied in rat hippocampal slices using the paired-pulse paradigm. Schaffer collaterals were stimulated with paired pulses with a 70-ms interstimulus interval every 30 s before and during perfusion with jasplakinolide, an activator of actin polymerization. Jasplakinolide application resulted in the increase in the amplitudes of CA3-CA1 responses (potentiation) accompanied by a decrease in the paired-pulse facilitation, suggesting induction of presynaptic modifications. Jasplakinolide-induced potentiation depended on the initial paired-pulse rate. These data indicate that the jasplakinolide-mediated changes in actin polymerization increased the probability of neurotransmitter release. Less typical for CA3-CA1 synapses responses, such as a very low paired-pulse ratio (close to 1 or even lower) or even paired-pulse depression, were affected differently. Thus, jasplakinolide caused potentiation of the second, but not the first response to the paired stimulus, which increased the paired-pulse ratio from 0.8 to 1.0 on average, suggesting a negative impact of jasplakinolide on the mechanisms promoting paired-pulse depression. In general, actin polymerization facilitated potentiation, although the patterns of potentiation differed depending on the initial synapse characteristics. We conclude that in addition to the increase in the neurotransmitter release probability, jasplakinolide induced other actin polymerization-dependent mechanisms, including those involved in the paired-pulse depression.

The Potential Role of Integrin Signaling in Memory and Cognitive Impairment

Dementia currently has no cure and, due to the increased prevalence and associated economic and personal burden of this condition, current research efforts for the development of potential therapies have intensified. Recently, targeting integrins as a strategy to ameliorate dementia and other forms of cognitive impairment has begun to gain traction. Integrins are major bidirectional signaling receptors in mammalian cells, mediating various physiological processes such as cell-cell interaction and cell adhesion, and are also known to bind to the extracellular matrix. In particular, integrins play a critical role in the synaptic transmission of signals, hence their potential contribution to memory formation and significance in cognitive impairment. In this review, we describe the physiological roles that integrins play in the blood-brain barrier (BBB) and in the formation of memories. We also provide a clear overview of how integrins are implicated in BBB disruption following cerebral pathology. Given that vascular contributions to cognitive impairment and dementia and Alzheimer's' disease are prominent forms of dementia that involve BBB disruption, as well as chronic inflammation, we present current approaches shown to improve dementia-like conditions with integrins as a central focus. We conclude that integrins are vital in memory formation and that their disruption could lead to various forms of cognitive impairment. While further research to understand the relationships between integrins and memory is needed, we propose that the translational relevance of research efforts in this area could be improved through the use of appropriately aged, comorbid, male and female animals.

The MeshCODE to scale-visualising synaptic binary information

The Mercator projection map of the world provides a useful, but distorted, view of the relative scale of countries. Current cellular models suffer from a similar distortion. Here, we undertook an in-depth structural analysis of the molecular dimensions in the cell's computational machinery, the MeshCODE, that is assembled from a meshwork of binary switches in the scaffolding proteins talin and vinculin. Talin contains a series of force-dependent binary switches and each domain switching state introduces quantised step-changes in talin length on a micrometre scale. The average dendritic spine is 1 μm in diameter so this analysis identifies a plausible Gearbox-like mechanism for dynamic regulation of synaptic function, whereby the positioning of enzymes and substrates relative to each other, mechanically-encoded by the MeshCODE switch patterns, might control synaptic transmission. Based on biophysical rules and experimentally derived distances, this analysis yields a novel perspective on biological digital information.

Dysregulation of integrin αvβ3 and α5β1 impedes migration of placental endothelial cells in fetal growth restriction

Placentas from pregnancies complicated by severe early-onset fetal growth restriction (FGR) exhibit diminished vascular development mediated by impaired angiogenesis, but underlying mechanisms remain unknown. In this study, we show that FGR endothelial cells demonstrate inherently reduced migratory capacity despite the presence of fibronectin, a matrix protein abundant in placental stroma that displays abnormal organization in FGR placentas. Thus, we hypothesized that aberrant endothelial-fibronectin interactions in FGR are a key mechanism underlying impaired FGR endothelial migration. Using human fetoplacental endothelial cells isolated from uncomplicated term control and FGR pregnancies, we assessed integrin α5β1 and αvβ3 regulation during cell migration. We show that endothelial integrin α5β1 and αvβ3 interactions with fibronectin are required for migration and that FGR endothelial cells responded differentially to integrin inhibition, indicating integrin dysregulation in FGR. Whole-cell expression was not different between groups. However, there were significantly more integrins in focal adhesions and reduced intracellular trafficking in FGR. These newly identified changes in FGR endothelial cellular processes represent previously unidentified mechanisms contributing to persistent angiogenic deficiencies in FGR.

NX210c Peptide Promotes Glutamatergic Receptor-Mediated Synaptic Transmission and Signaling in the Mouse Central Nervous System

NX210c is a disease-modifying dodecapeptide derived from the subcommissural organ-spondin that is under preclinical and clinical development for the treatment of neurological disorders. Here, using whole-cell patch-clamp recordings, we demonstrate that NX210c increased α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)- and GluN2A-containing N-methyl-D-aspartate receptor (GluN2A-NMDAR)-mediated excitatory postsynaptic currents in the brain. Accordingly, using extracellular field excitatory postsynaptic potential recordings, an enhancement of synaptic transmission was shown in the presence of NX210c in two different neuronal circuits. Furthermore, the modulation of synaptic transmission and GluN2A-NMDAR-driven signaling by NX210c restored memory in mice chronically treated with the NMDAR antagonist phencyclidine. Overall, by promoting glutamatergic receptor-related neurotransmission and signaling, NX210c represents an innovative therapeutic opportunity for patients suffering from CNS disorders, injuries, and states with crippling synaptic dysfunctions.

Integrins Bidirectionally Regulate the Efficacy of Inhibitory Synaptic Transmission and Control GABAergic Plasticity

For many decades, synaptic plasticity was believed to be restricted to excitatory transmission. However, in recent years, this view started to change, and now it is recognized that GABAergic synapses show distinct forms of activity-dependent long-term plasticity, but the underlying mechanisms remain obscure. Herein, we asked whether signaling mediated by β1 or β3 subunit-containing integrins might be involved in regulating the efficacy of GABAergic synapses, including the NMDA receptor-dependent inhibitory long-term potentiation (iLTP) in the hippocampus. We found that activation of β3 integrin with fibrinogen induced a stable depression, whereas inhibition of β1 integrin potentiated GABAergic synapses at CA1 pyramidal neurons in male mice. Additionally, compounds that interfere with the interaction of β1 or β3 integrins with extracellular matrix blocked the induction of NMDA-iLTP. In conclusion, we provide the first evidence that integrins are key players in regulating the endogenous modulatory mechanisms of GABAergic inhibition and plasticity in the hippocampus.SIGNIFICANCE STATEMENT Epilepsy, schizophrenia, and anxiety are just a few medical conditions associated with dysfunctional inhibitory synaptic transmission. GABAergic synapses are known for their extraordinary susceptibility to modulation by endogenous factors and exogenous pharmacological agents. We describe here that integrins, adhesion proteins, play a key role in the modulation of inhibitory synaptic transmission. Specifically, we show that interference with integrin-dependent adhesion results in a variety of effects on the amplitude and frequency of GABAergic mIPSCs. Activation of β3 subunit-containing integrins induces inhibitory long-term depression, whereas the inhibition of β1 subunit-containing integrins induces iLTP. Our results unveil an important mechanism controlling synaptic inhibition, which opens new avenues into the usage of integrin-aimed pharmaceuticals as modulators of GABAergic synapses.

Kallikrein 8: A key sheddase to strengthen and stabilize neural plasticity

Neural networks are modified and reorganized throughout life, even in the matured brain. Synapses in the networks form, change, or disappear dynamically in the plasticity state. The pre- and postsynaptic signaling, transmission, and structural dynamics have been studied considerably well. However, not many studies have shed light on the events in the synaptic cleft and intercellular space. Neural activity-dependent protein shedding is a phenomenon in which (1) presynaptic excitation evokes secretion or activation of sheddases, (2) sheddases are involved not only in cleavage of membrane- or matrix-bound proteins but also in mechanical modulation of cell-to-cell connectivity, and (3) freed activity domains of protein factors play a role in receptor-mediated or non-mediated biological actions. Kallikrein 8/neuropsin (KLK8) is a kallikrein family serine protease rich in the mammalian limbic brain. Accumulated evidence has suggested that KLK8 is an important modulator of neural plasticity and consequently, cognition. Insufficiency, as well as excess of KLK8 may have detrimental effects on limbic functions.

Biliverdin reductase bridges focal adhesion kinase to Src to modulate synaptic signaling

Synapses connect discrete neurons into vast networks that send, receive, and encode diverse forms of information. Synaptic function and plasticity, the neuronal process of adapting to diverse and variable inputs, depend on the dynamic nature of synaptic molecular components, which is mediated in part by cell adhesion signaling pathways. Here, we found that the enzyme biliverdin reductase (BVR) physically links together key focal adhesion signaling molecules at the synapse. BVR-null (BVR-/-) mice exhibited substantial deficits in learning and memory on neurocognitive tests, and hippocampal slices in which BVR was postsynaptically depleted showed deficits in electrophysiological responses to stimuli. RNA sequencing, biochemistry, and pathway analyses suggested that these deficits were mediated through the loss of focal adhesion signaling at both the transcriptional and biochemical level in the hippocampus. Independently of its catalytic function, BVR acted as a bridge between the primary focal adhesion signaling kinases FAK and Pyk2 and the effector kinase Src. Without BVR, FAK and Pyk2 did not bind to and stimulate Src, which then did not phosphorylate the N-methyl-d-aspartate (NMDA) receptor, a critical posttranslational modification for synaptic plasticity. Src itself is a molecular hub on which many signaling pathways converge to stimulate NMDAR-mediated neurotransmission, thus positioning BVR at a prominent intersection of synaptic signaling.

Cell adhesion presence during adolescence controls the architecture of projection-defined prefrontal cortical neurons and reward-related action strategies later in life

Adolescent brain development is characterized by neuronal remodeling in the prefrontal cortex; relationships with behavior are largely undefined. Integrins are cell adhesion factors that link the extracellular matrix with intracellular actin cytoskeleton. We find that β1-integrin presence in the prelimbic prefrontal cortex (PL) during adolescence, but not adulthood, is necessary for mice to select actions based on reward likelihood and value. As such, adult mice that lacked β1-integrin during adolescence failed to modify response strategies when rewards lost value or failed to be delivered. This pattern suggests that β1-integrin-mediated neuronal development is necessary for PL function in adulthood. We next visualized adolescent PL neurons, including those receiving input from the basolateral amygdala (BLA) - thought to signal salience - and projecting to the dorsomedial striatum (DMS) - the striatal output by which the PL controls goal-seeking behavior. Firstly, we found that these projection-defined neurons had a distinct morphology relative to general layer V PL neurons. Secondly, β1-integrin loss triggered the overexpression of stubby-type dendritic spines at the expense of mature spines, including on projection-defined neurons. This phenotype was not observed when β1-integrins were silenced before or after adolescence. Altogether, our experiments localize β1-integrin-mediated cell adhesion within a developing di-synaptic circuit coordinating adaptive action.

The Synaptic Extracellular Matrix: Long-Lived, Stable, and Still Remarkably Dynamic

In the adult brain, synapses are tightly enwrapped by lattices of the extracellular matrix that consist of extremely long-lived molecules. These lattices are deemed to stabilize synapses, restrict the reorganization of their transmission machinery, and prevent them from undergoing structural or morphological changes. At the same time, they are expected to retain some degree of flexibility to permit occasional events of synaptic plasticity. The recent understanding that structural changes to synapses are significantly more frequent than previously assumed (occurring even on a timescale of minutes) has called for a mechanism that allows continual and energy-efficient remodeling of the extracellular matrix (ECM) at synapses. Here, we review recent evidence for such a process based on the constitutive recycling of synaptic ECM molecules. We discuss the key characteristics of this mechanism, focusing on its roles in mediating synaptic transmission and plasticity, and speculate on additional potential functions in neuronal signaling.

Single-Molecule Imaging Reveals Rapid Estradiol Action on the Surface Movement of AMPA Receptors in Live Neurons

Gonadal steroid 17β-estradiol (E2) exerts rapid, non-genomic effects on neurons and strictly regulates learning and memory through altering glutamatergic neurotransmission and synaptic plasticity. However, its non-genomic effects on AMPARs are not well understood. Here, we analyzed the rapid effect of E2 on AMPARs using single-molecule tracking and super-resolution imaging techniques. We found that E2 rapidly decreased the surface movement of AMPAR via membrane G protein-coupled estrogen receptor 1 (GPER1) in neurites in a dose-dependent manner. The cortical actin network played a pivotal role in the GPER1 mediated effects of E2 on the surface mobility of AMPAR. E2 also decreased the surface movement of AMPAR both in synaptic and extrasynaptic regions on neurites and increased the synaptic dwell time of AMPARs. Our results provide evidence for understanding E2 action on neuronal plasticity and glutamatergic neurotransmission at the molecular level.

Extracellular Metalloproteinases in the Plasticity of Excitatory and Inhibitory Synapses

Long-term synaptic plasticity is shaped by the controlled reorganization of the synaptic proteome. A key component of this process is local proteolysis performed by the family of extracellular matrix metalloproteinases (MMPs). In recent years, considerable progress was achieved in identifying extracellular proteases involved in neuroplasticity phenomena and their protein substrates. Perisynaptic metalloproteinases regulate plastic changes at synapses through the processing of extracellular and membrane proteins. MMP9 was found to play a crucial role in excitatory synapses by controlling the NMDA-dependent LTP component. In addition, MMP3 regulates the L-type calcium channel-dependent form of LTP as well as the plasticity of neuronal excitability. Both MMP9 and MMP3 were implicated in memory and learning. Moreover, altered expression or mutations of different MMPs are associated with learning deficits and psychiatric disorders, including schizophrenia, addiction, or stress response. Contrary to excitatory drive, the investigation into the role of extracellular proteolysis in inhibitory synapses is only just beginning. Herein, we review the principal mechanisms of MMP involvement in the plasticity of excitatory transmission and the recently discovered role of proteolysis in inhibitory synapses. We discuss how different matrix metalloproteinases shape dynamics and turnover of synaptic adhesome and signal transduction pathways in neurons. Finally, we discuss future challenges in exploring synapse- and plasticity-specific functions of different metalloproteinases.

The Mechanical Basis of Memory - the MeshCODE Theory

One of the major unsolved mysteries of biological science concerns the question of where and in what form information is stored in the brain. I propose that memory is stored in the brain in a mechanically encoded binary format written into the conformations of proteins found in the cell-extracellular matrix (ECM) adhesions that organise each and every synapse. The MeshCODE framework outlined here represents a unifying theory of data storage in animals, providing read-write storage of both dynamic and persistent information in a binary format. Mechanosensitive proteins that contain force-dependent switches can store information persistently, which can be written or updated using small changes in mechanical force. These mechanosensitive proteins, such as talin, scaffold each synapse, creating a meshwork of switches that together form a code, the so-called MeshCODE. Large signalling complexes assemble on these scaffolds as a function of the switch patterns and these complexes would both stabilise the patterns and coordinate synaptic regulators to dynamically tune synaptic activity. Synaptic transmission and action potential spike trains would operate the cytoskeletal machinery to write and update the synaptic MeshCODEs, thereby propagating this coding throughout the organism. Based on established biophysical principles, such a mechanical basis for memory would provide a physical location for data storage in the brain, with the binary patterns, encoded in the information-storing mechanosensitive molecules in the synaptic scaffolds, and the complexes that form on them, representing the physical location of engrams. Furthermore, the conversion and storage of sensory and temporal inputs into a binary format would constitute an addressable read-write memory system, supporting the view of the mind as an organic supercomputer.

Retrograde enhancement of episodic learning by a postlearning stimulus

Evidence suggests encoding of recent episodic experiences may be enhanced by a subsequent salient event. We tested this hypothesis by giving rats a 3-min unsupervised experience with four odors and measuring retention after different delays. Animals recognized that a novel element had been introduced to the odor set at 24 but not 48 h. However, when odor sampling was followed within 5 min by salient light flashes or bedding odor, the memory lasted a full 2 d. These results describe a retroactive influence of salience to promote storage of episodic information and introduce a unique model for studying underlying plasticity mechanisms.

Estradiol induces synaptic rearrangements

17β-Estradiol (E2) is a potent steroid hormone of both gonadal and neuronal origin that exerts profound effects on neuroplasticity in several brain regions. Dendritic spine and synapse formation and rearrangements are modulated and mediated by estrogens. In this chapter, we highlighted the essential background concerning the effects of E2 on synaptic rearrangements accompanied by synaptic plasticity in E2-sensitive brain regions that mediate learning and memory, i.e., cortex and hippocampus. We also address details of the molecular mechanisms underlying E2 regulation of spine dynamics. The proposed models of action of E2 overlaps with that for well-established synaptic modulators, such as adenosine. Thus, the possible synergistic effects of those two molecules in respect to synaptic rearrangement and plasticity were presented.

A TrkB agonist and ampakine rescue synaptic plasticity and multiple forms of memory in a mouse model of intellectual disability

Fragile X syndrome (FXS) is associated with deficits in various types of learning, including those that require the hippocampus. Relatedly, hippocampal long-term potentiation (LTP) is impaired in the Fmr1 knockout (KO) mouse model of FXS. Prior research found that infusion of brain-derived neurotrophic factor (BDNF) rescues LTP in the KOs. Here, we tested if, in Fmr1 KO mice, up-regulating BDNF production or treatment with an agonist for BDNF's TrkB receptor restores synaptic plasticity and improves learning. In hippocampal slices, bath infusion of the TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) completely restored otherwise impaired hippocampal field CA1 LTP of Fmr1 KOs without effect in wild types (WTs). Similarly, acute, semi-chronic, or chronic treatments with 7,8-DHF rescued a simple hippocampus-dependent form of spatial learning (object location memory: OLM) in Fmr1 KOs without effect in WTs. The agonist also restored object recognition memory, which depends on cortical regions. Semi-chronic, but not acute, treatment with the ampakine CX929, which up-regulates BDNF expression, lowered the training threshold for OLM in WT mice and rescued learning in the KOs. Positive results were also obtained in a test for social recognition. An mGluR5 antagonist did not improve learning. Quantification of synaptic immunolabeling demonstrated that 7,8-DHF and CX929 increase levels of activated TrkB at excitatory synapses. Moreover, CX929 induced a robust synaptic activation of the TrkB effector ERK1/2. These results suggest that enhanced synaptic BDNF signaling constitutes a plausible strategy for treating certain aspects of the cognitive disabilities associated with FXS.

Atypical Endocannabinoid Signaling Initiates a New Form of Memory-Related Plasticity at a Cortical Input to Hippocampus

Endocannabinoids (ECBs) depress transmitter release at sites throughout the brain. Here, we describe another form of ECB signaling that triggers a novel form of long-term potentiation (LTP) localized to the lateral perforant path (LPP) which conveys semantic information from cortex to hippocampus. Two cannabinoid CB1 receptor (CB1R) signaling cascades were identified in hippocampus. The first is pregnenolone sensitive, targets vesicular protein Munc18-1 and depresses transmitter release; this cascade is engaged by CB1Rs in Schaffer-Commissural afferents to CA1 but not in the LPP, and it does not contribute to LTP. The second cascade is pregnenolone insensitive and LPP specific; it entails co-operative CB1R/β1-integrin signaling to effect synaptic potentiation via stable enhancement of transmitter release. The latter cascade is engaged during LPP-dependent learning. These results link atypical ECB signaling to the encoding of a fundamental component of episodic memory and suggest a novel route whereby endogenous and exogenous cannabinoids affect cognition.

Extracellular Matrix Signaling Through β3 Integrin Mediates Cocaine Cue-Induced Transient Synaptic Plasticity and Relapse

BACKGROUND

Cue-induced relapse to drug use is a primary symptom of cocaine addiction. Cue-induced transient excitatory synaptic potentiation (t-SP) induced in the nucleus accumbens mediates cued cocaine seeking in rat models of relapse. Cue-induced t-SP depends on extracellular signaling by matrix metalloproteases (MMPs), but it is unknown how this catalytic activity communicates with nucleus accumbens neurons to induce t-SP and cocaine seeking.

METHODS

Male Sprague Dawley rats (N = 125) were trained to self-administer cocaine, after which self-administration was extinguished and then reinstated by cocaine-conditioned cues. We used a morpholino antisense strategy to knock down the β1 or β3 integrin subunits or inhibitors to prevent phosphorylation of the integrin signaling kinases focal adhesion kinase (FAK) or integrin-linked kinase. We quantified protein changes with immunoblotting and t-SP by measuring dendritic spine morphology and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate glutamate currents. Integrin signaling was stimulated by microinjecting an MMP activator or integrin peptide ligand into the accumbens.

RESULTS

Knockdown of β3 integrin or FAK inhibitor, but not β1 integrin or integrin-linked kinase inhibitor, prevented cue-induced cocaine seeking but not sucrose seeking. β3 integrin knockdown prevented t-SP as measured by preventing the cue-induced increases in both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate glutamate ratio and spine head diameter. Activating MMP gelatinases with tissue plasminogen activator potentiated cue-induced reinstatement, which was prevented by β3 integrin knockdown and FAK inhibition. Stimulating integrin receptors with the RGD ligand liberated by MMP gelatinase activity also potentiated cued cocaine seeking.

CONCLUSIONS

Activation of MMP gelatinase in the extracellular space is necessary for and potentiates cued cocaine seeking. This extracellular catalysis stimulates β3 integrins and activates FAK to induce t-SP and promote cue-induced cocaine seeking.

β1-Integrins in the Developing Orbitofrontal Cortex Are Necessary for Expectancy Updating in Mice

Navigating a changing environment requires associating stimuli and actions with their likely outcomes and modifying these associations when they change. These processes involve the orbitofrontal cortex (OFC). Although some molecular mediators have been identified, developmental factors are virtually unknown. We hypothesized that the cell adhesion factor β1-integrin is essential to OFC function, anticipating developmental windows during which β1-integrins might be more influential than others. We discovered that OFC-selective β1-integrin silencing before adolescence, but not later, impaired the ability of mice to extinguish conditioned fear and select actions based on their likely outcomes. Early-life knock-down also reduced the densities of dendritic spines, the primary sites of excitatory plasticity in the brain, and weakened sensitivity to cortical inputs. Notwithstanding these defects in male mice, females were resilient to OFC (but not hippocampal) β1-integrin loss. Existing literature suggests that resilience may be explained by estradiol-mediated transactivation of β1-integrins and tropomyosin receptor kinase B (trkB). Accordingly, we discovered that a trkB agonist administered during adolescence corrected reward-related decision making in β1-integrin-deficient males. In sum, developmental β1-integrins are indispensable for OFC function later in life.SIGNIFICANCE STATEMENT The orbitofrontal cortex (OFC) is a subregion of the frontal cortex that allows organisms to link behaviors and stimuli with anticipated outcomes, and to make predictions about the consequences of one's behavior. Aspects of OFC development are particularly prolonged, extending well into adolescence, likely optimizing organisms' abilities to prospectively calculate the consequences of their actions and select behaviors appropriately; these decision making strategies improve as young individuals mature into adulthood. Molecular factors are not, however, well understood. Our experiments reveal that a cell adhesion protein termed "β1-integrin" is necessary for OFC neuronal maturation and function. Importantly, β1-integrins operate during a critical period equivalent to early adolescence in humans to optimize the ability of organisms to update expectancies later in life.

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

Early Synaptic Alterations and Selective Adhesion Signaling in Hippocampal Dendritic Zones Following Organophosphate Exposure

Organophosphates account for many of the world's deadliest poisons. They inhibit acetylcholinesterase causing cholinergic crises that lead to seizures and death, while survivors commonly experience long-term neurological problems. Here, we treated brain explants with the organophosphate compound paraoxon and uncovered a unique mechanism of neurotoxicity. Paraoxon-exposed hippocampal slice cultures exhibited progressive declines in synaptophysin, synapsin II, and PSD-95, whereas reduction in GluR1 was slower and NeuN and Nissl staining showed no indications of neuronal damage. The distinctive synaptotoxicity was observed in dendritic zones of CA1 and dentate gyrus. Interestingly, declines in synapsin II dendritic labeling correlated with increased staining for β1 integrin, a component of adhesion receptors that regulate synapse maintenance and plasticity. The paraoxon-induced β1 integrin response was targeted to synapses, and the two-fold increase in β1 integrin was selective as other synaptic adhesion molecules were unchanged. Additionally, β1 integrin-cofilin signaling was triggered by the exposure and correlations were found between the extent of synaptic decline and the level of β1 integrin responses. These findings identified organophosphate-mediated early and lasting synaptotoxicity which can explain delayed neurological dysfunction later in life. They also suggest that the interplay between synaptotoxic events and compensatory adhesion responses influences neuronal fate in exposed individuals.

How the epigenome integrates information and reshapes the synapse

In the past few decades, the field of neuroepigenetics has investigated how the brain encodes information to form long-lasting memories that lead to stable changes in behaviour. Activity-dependent molecular mechanisms, including, but not limited to, histone modification, DNA methylation and nucleosome remodelling, dynamically regulate the gene expression required for memory formation. Recently, the field has begun to examine how a learning experience is integrated at the level of both chromatin structure and synaptic physiology. Here, we provide an overview of key established epigenetic mechanisms that are important for memory formation. We explore how epigenetic mechanisms give rise to stable alterations in neuronal function by modifying synaptic structure and function, and highlight studies that demonstrate how manipulating epigenetic mechanisms may push the boundaries of memory.

Osteopontin counters human immunodeficiency virus type 1-induced impairment of neurite growth through mammalian target of rapamycin and beta-integrin signaling pathways

Despite the fact that human immunodeficiency virus type 1 (HIV-1) does not enter or replicate in neurons, its infection of a subset of resident brain glia cells (microglia and astrocytes) induces via disparate mechanisms, dysregulation of glutamate metabolism, neurotoxicity, and inflammation. Antiretroviral therapies suppress viral load, but cellular activation and release of proinflammatory factors, some of which is likely related to viral reservoirs, continue to promote a microenvironment that is injurious to neurons. However, the molecular mechanisms remain to be identified. Osteopontin (OPN) is a proinflammatory cytokine-like, extracellular matrix protein that is elevated within the brain and CSF in several neurodegenerative disorders, including HIV-associated cognitive disorder. However, the impact of elevated OPN on neuronal integrity and function in HIV-infected individuals who exhibit cognitive dysfunction remains unknown. In this study, using a neuronal cell line and primary cultures of cortical rat neurons, we identify the mammalian target of rapamycin pathway involvement in a signaling interaction between OPN-β1-integrins and the HIV-1 envelope glycoprotein, which stimulates neurite growth. These findings link for the first time HIV X4-envelope receptor engagement and osteopontin-mediated signaling through β1-integrin receptors to the mTOR pathway and alterations in the cytoskeleton of cortical neurons.

The role of nonmuscle myosin II in polydrug memories and memory reconsolidation

Using pharmacologic and genetic approaches targeting actin or the actin-driving molecular motor, nonmuscle myosin II (NMII), we previously discovered an immediate, retrieval-independent, and long-lasting disruption of methamphetamine- (METH-) and amphetamine-associated memories. A single intrabasolateral amygdala complex infusion or systemic administration of the NMII inhibitor Blebbistatin (Blebb) is sufficient to produce this disruption, which is selective, having no retrieval-independent effect on memories for fear, food reward, cocaine, or morphine. However, it was unclear if Blebb treatment would disrupt memories of other stimulants and amphetamine class drugs, such as nicotine (NIC) or mephedrone (MEPH; bath salts). Moreover, many individuals abuse multiple drugs, but it was unknown if Blebb could disrupt polydrug memories, or if the inclusion of another substance would render Blebb no longer able to disrupt METH-associated memories. Therefore, the present study had two primary goals: (1) to determine the ability of Blebb to disrupt NIC- or MEPH-associated memories, and (2) to determine the ability of METH to modify other unconditioned stimulus (US) associations’ susceptibility to Blebb. To this end, using the conditional place preference model, mice were conditioned to NIC and MEPH alone or METH in combination with NIC, morphine, or foot shock. We report that, unlike METH, there was no retrieval-independent effect of Blebb on NIC- or MEPH-associated memories. However, similar to cocaine, reconsolidation of the memory for both drugs was disrupted. Further, when combined with METH administration, NIC- and morphine-, but not fear-, associated memories were rendered susceptible to disruption by Blebb. Given the high rate of polydrug use and the resurgence of METH use, these results have important implications for the treatment of substance use disorder.

Memory-Related Synaptic Plasticity Is Sexually Dimorphic in Rodent Hippocampus

Men are generally superior to women in remembering spatial relationships, whereas the reverse holds for semantic information, but the neurobiological bases for these differences are not understood. Here we describe striking sexual dimorphism in synaptic mechanisms of memory encoding in hippocampal field CA1, a region critical for spatial learning. Studies of acute hippocampal slices from adult rats and mice show that for excitatory Schaffer-commissural projections, the memory-related long-term potentiation (LTP) effect depends upon endogenous estrogen and membrane estrogen receptor α (ERα) in females but not in males; there was no evident involvement of nuclear ERα in females, or of ERβ or GPER1 (G-protein-coupled estrogen receptor 1) in either sex. Quantitative immunofluorescence showed that stimulation-induced activation of two LTP-related kinases (Src, ERK1/2), and of postsynaptic TrkB, required ERα in females only, and that postsynaptic ERα levels are higher in females than in males. Several downstream signaling events involved in LTP were comparable between the sexes. In contrast to endogenous estrogen effects, infused estradiol facilitated LTP and synaptic signaling in females via both ERα and ERβ. The estrogen dependence of LTP in females was associated with a higher threshold for both inducing potentiation and acquiring spatial information. These results indicate that the observed sexual dimorphism in hippocampal LTP reflects differences in synaptic kinase activation, including both a weaker association with NMDA receptors and a greater ERα-mediated kinase activation in response to locally produced estrogen in females. We propose that male/female differences in mechanisms and threshold for field CA1 LTP contribute to differences in encoding specific types of memories.SIGNIFICANCE STATEMENT There is good evidence for male/female differences in memory-related cognitive function, but the neurobiological basis for this sexual dimorphism is not understood. Here we describe sex differences in synaptic function in a brain area that is critical for learning spatial cues. Our results show that female rodents have higher synaptic levels of estrogen receptor α (ERα) and, in contrast to males, require membrane ERα for the activation of signaling kinases that support long-term potentiation (LTP), a form of synaptic plasticity thought to underlie learning. The additional requirement of estrogen signaling in females resulted in a higher threshold for both LTP and hippocampal field CA1-dependent spatial learning. These results describe a synaptic basis for sexual dimorphism in encoding spatial information.

Rapid non-classical effects of steroids on the membrane receptor dynamics and downstream signaling in neurons

Contribution to Special Issue on Fast effects of steroids. Although rapid effects of steroid hormones on membrane receptors and intracellular signaling molecules have been extensively studied in neurons, we are only beginning to understand the molecular mechanisms behind these non-classical steroid actions. Single molecule tracking (SMT) studies on live cells demonstrated that surface trafficking of membrane receptors determines their ligand binding properties and downstream signaling events. Recent findings suggest that one of the underlying mechanisms of non-classical steroid actions is the alteration of receptor movements on the membrane surface. In order to highlight this novel aspect of steroid effects, we first address the types of receptor movements in the plasma membrane and the role of cortical actin dynamics in receptor movement. We then discuss how single molecules and the surface movements of receptors can be detected in live cells. Next, we review the fundamental processes, which determine the effect of steroids on the plasma membrane: steroid movement through the lipid bilayer and the role of steroid membrane receptors. Using glutamate and neurotrophin receptors (NTRs) as examples, we demonstrate the features of receptor dynamics in the membrane. In addition, we survey the available data of rapid steroid actions on membrane receptor trafficking: we discuss how glucocorticoids act on the surface diffusion of glutamate receptor molecules and how estradiol acts on NTRs and gamma-aminobutyric acid type A receptors (GABA_ARs) and their related signaling events as well as on cortical actin. Finally, we address the physiological relevance of rapid steroid action on membrane receptors dynamics.

UBE3A-mediated p18/LAMTOR1 ubiquitination and degradation regulate mTORC1 activity and synaptic plasticity

Accumulating evidence indicates that the lysosomal Ragulator complex is essential for full activation of the mechanistic target of rapamycin complex 1 (mTORC1). Abnormal mTORC1 activation has been implicated in several developmental neurological disorders, including Angelman syndrome (AS), which is caused by maternal deficiency of the ubiquitin E3 ligase UBE3A. Here we report that Ube3a regulates mTORC1 signaling by targeting p18, a subunit of the Ragulator. Ube3a ubiquinates p18, resulting in its proteasomal degradation, and Ube3a deficiency in the hippocampus of AS mice induces increased lysosomal localization of p18 and other members of the Ragulator-Rag complex, and increased mTORC1 activity. p18 knockdown in hippocampal CA1 neurons of AS mice reduces elevated mTORC1 activity and improves dendritic spine maturation, long-term potentiation (LTP), as well as learning performance. Our results indicate that Ube3a-mediated regulation of p18 and subsequent mTORC1 signaling is critical for typical synaptic plasticity, dendritic spine development, and learning and memory.

Integrin activity in neuronal connectivity

The formation of correct synaptic structures and neuronal connections is paramount for normal brain development and a functioning adult brain. The integrin family of cell adhesion receptors and their ligands play essential roles in the control of several processes regulating neuronal connectivity - including neurite outgrowth, the formation and maintenance of synapses, and synaptic plasticity - that are affected in neurodevelopmental disorders, such as autism spectrum disorders (ASDs) and schizophrenia. Many ASD- and schizophrenia-associated genes are linked to alterations in the genetic code of integrins and associated signalling pathways. In non-neuronal cells, crosstalk between integrin-mediated adhesions and the actin cytoskeleton, and the regulation of integrin activity (affinity for extracellular ligands) are widely studied in healthy and pathological settings. In contrast, the roles of integrin-linked pathways in the central nervous system remains less well defined. In this Review, we will provide an overview of the known pathways that are regulated by integrin-ECM interaction in developing neurons and in adult brain. We will also describe recent advances in the identification of mechanisms that regulate integrin activity in neurons, and highlight the interesting emerging links between integrins and neurodevelopment.

Nogo-66 Restricts Synaptic Strengthening via Lingo1 and the ROCK2-Cofilin Pathway to Control Actin Dynamics

Nogo-A restricts long-term potentiation (LTP) at the Schaffer collateral-CA1 pathway in the adult hippocampus via 2 extracellular domains: Nogo-A-Δ20 and Nogo-66. Nogo-66 signals via Nogo Receptor 1 (NgR1) to regulate synaptic function. Whether the NgR1 coreceptors Lingo1 and p75NTR are involved in the signaling in this context is still not known. Moreover, the intracellular cascade mediating the activity of Nogo-66 in restricting LTP is unexplored. We combine electrophysiology and biochemistry in acute hippocampal slices and demonstrate that a loss of function for Lingo1 results in a significant increase in LTP levels at the Schaffer collateral-CA1 pathway, and that Lingo1 is the NgR1 coreceptor mediating the role of Nogo-66 in restricting LTP. Our data show that p75NTR is not involved in mediating the Nogo-66 effect on LTP. Moreover, loss of function for p75NTR and NgR1 equally attenuate LTD, suggesting that p75NTR might mediate the NgR1-dependent regulation of LTD, independently of Nogo-66. Finally, our results indicate that Nogo-66 signaling limits LTP via the ROCK2-Cofilin pathway to control the dynamics of the actin cytoskeleton. The present results elucidate the signaling pathway activated by Nogo-66 to control LTP and contribute to the understanding of how Nogo-A stabilizes the neural circuits to limit activity-dependent plasticity events in the mature hippocampus.

Differential proteome profiling in the hippocampus of amnesic mice

Amnesia or memory loss is associated with brain aging and several neurodegenerative pathologies including Alzheimer's disease (AD). This can be induced by a cholinergic antagonist scopolamine but the underlying molecular mechanism is poorly understood. This study of proteome profiling in the hippocampus could provide conceptual insights into the molecular mechanisms involved in amnesia. To reveal this, mice were administered scopolamine to induce amnesia and memory impairment was validated by novel object recognition test. Using two-dimensional gel electrophoresis coupled with MALDI-MS/MS, we have analyzed the hippocampal proteome and identified 18 proteins which were differentially expressed. Out of these proteins, 11 were downregulated and 7 were upregulated in scopolamine-treated mice as compared to control. In silico analysis showed that the majority of identified proteins are involved in metabolism, catalytic activity, and cytoskeleton architectural functions. STRING interaction network analysis revealed that majority of identified proteins exhibit common association with Actg1 cytoskeleton and Vdac1 energy transporter protein. Furthermore, interaction map analysis showed that Fascin1 and Coronin 1b individually interact with Actg1 and regulate the actin filament dynamics. Vdac1 was significantly downregulated in amnesic mice and showed interaction with other proteins in interaction network. Therefore, we silenced Vdac1 in the hippocampus of normal young mice and found similar impairment in recognition memory of Vdac1 silenced and scopolamine-treated mice. Thus, these findings suggest that Vdac1-mediated disruption of energy metabolism and cytoskeleton architecture might be involved in scopolamine-induced amnesia.

Differential expression of cytoskeletal regulatory factors in the adolescent prefrontal cortex: Implications for cortical development

The prevalence of depression, anxiety, schizophrenia, and drug and alcohol use disorders peaks during adolescence. Further, up to 50% of "adult" mental health disorders emerge in adolescence. During adolescence, the prefrontal cortex (PFC) undergoes dramatic structural reorganization, in which dendritic spines and synapses are refined, pruned, and stabilized. Understanding the molecular mechanisms that underlie these processes should help to identify factors that influence the development of psychiatric illness. Here we briefly discuss the anatomical connections of the medial and orbital prefrontal cortex (mPFC and OFC, respectively). We then present original findings suggesting that dendritic spines on deep-layer excitatory neurons in the mouse mPFC and OFC prune at different adolescent ages, with later pruning in the OFC. In parallel, we used Western blotting to define levels of several cytoskeletal regulatory proteins during early, mid-, and late adolescence, focusing on tropomyosin-related kinase receptor B (TrkB) and β1-integrin-containing receptors and select signaling partners. We identified regional differences in the levels of several proteins in early and midadolescence that then converged in early adulthood. We also observed age-related differences in TrkB levels, both full-length and truncated isoforms, Rho-kinase 2, and synaptophysin in both PFC subregions. Finally, we identified changes in protein levels in the dorsal and ventral hippocampus that were distinct from those in the PFC. We conclude with a general review of the manner in which TrkB- and β1-integrin-mediated signaling influences neuronal structure in the postnatal brain. Elucidating the role of cytoskeletal regulatory factors throughout adolescence may identify critical mechanisms of PFC development. © 2016 Wiley Periodicals, Inc.

Cofilin Activation Is Temporally Associated with the Cessation of Growth in the Developing Hippocampus

Dendritic extension and synaptogenesis proceed at high rates in rat hippocampus during early postnatal life but markedly slow during the third week of development. The reasons for the latter, fundamental event are poorly understood. Here, we report that levels of phosphorylated (inactive) cofilin, an actin depolymerizing factor, decrease by 90% from postnatal days (pnds) 10 to 21. During the same period, levels of total and phosphorylated Arp2, which nucleates actin branches, increase. A search for elements that could explain the switch from inactive to active cofilin identified reductions in β1 integrin, TrkB, and LIM domain kinase 2b, upstream proteins that promote cofilin phosphorylation. Moreover, levels of slingshot 3, which dephosphorylates cofilin, increase during the period in which growth slows. Consistent with the cofilin results, in situ phalloidin labeling of F-actin demonstrated that spines and dendrites contained high levels of dynamic actin filaments during Week 2, but these fell dramatically by pnd 21. The results suggest that the change from inactive to constitutively active cofilin leads to a loss of dynamic actin filaments needed for process extension and thus the termination of spine formation and synaptogenesis. The relevance of these events to the emergence of memory-related synaptic plasticity is described.

Activity-Induced Synaptic Structural Modifications by an Activator of Integrin Signaling at the Drosophila Neuromuscular Junction

Activity-induced synaptic structural modification is crucial for neural development and synaptic plasticity, but the molecular players involved in this process are not well defined. Here, we report that a protein named Shriveled (Shv) regulates synaptic growth and activity-dependent synaptic remodeling at the Drosophila neuromuscular junction. Depletion of Shv causes synaptic overgrowth and an accumulation of immature boutons. We find that Shv physically and genetically interacts with βPS integrin. Furthermore, Shv is secreted during intense, but not mild, neuronal activity to acutely activate integrin signaling, induce synaptic bouton enlargement, and increase postsynaptic glutamate receptor abundance. Consequently, loss of Shv prevents activity-induced synapse maturation and abolishes post-tetanic potentiation, a form of synaptic plasticity. Our data identify Shv as a novel trans-synaptic signal secreted upon intense neuronal activity to promote synapse remodeling through integrin receptor signaling.SIGNIFICANCE STATEMENT The ability of neurons to rapidly modify synaptic structure in response to neuronal activity, a process called activity-induced structural remodeling, is crucial for neuronal development and complex brain functions. The molecular players that are important for this fundamental biological process are not well understood. Here we show that the Shriveled (Shv) protein is required during development to maintain normal synaptic growth. We further demonstrate that Shv is selectively released during intense neuronal activity, but not mild neuronal activity, to acutely activate integrin signaling and trigger structural modifications at the Drosophila neuromuscular junction. This work identifies Shv as a key modulator of activity-induced structural remodeling and suggests that neurons use distinct molecular cues to differentially modulate synaptic growth and remodeling to meet synaptic demand.

Memory disrupting effects of nonmuscle myosin II inhibition depend on the class of abused drug and brain region

Depolymerizing actin in the amygdala through nonmuscle myosin II inhibition (NMIIi) produces a selective, lasting, and retrieval-independent disruption of the storage of methamphetamine-associated memories. Here we report a similar disruption of memories associated with amphetamine, but not cocaine or morphine, by NMIIi. Reconsolidation appeared to be disrupted with cocaine. Unlike in the amygdala, methamphetamine-associated memory storage was not disrupted by NMIIi in the hippocampus, nucleus accumbens, or orbitofrontal cortex. NMIIi in the hippocampus did appear to disrupt reconsolidation. Identification of the unique mechanisms responsible for NMII-mediated, amygdala-dependent disruption of memory storage associated with the amphetamine class may enable induction of retrieval-independent vulnerability to other pathological memories.

Role of the adhesion molecule F3/Contactin in synaptic plasticity and memory

Cell adhesion molecules (CAMs) have a pivotal role in building and maintaining synaptic structures during brain development participating in axonal elongation and pathfinding, glial guidance of neuronal migration, as well as myelination. CAMs expression persists in the adult brain particularly in structures undergoing postnatal neurogenesis and involved in synaptic plasticity and memory as the hippocampus. Among the neural CAMs, we have recently focused on F3/Contactin, a glycosylphosphatidyl inositol-anchored glycoprotein belonging to the immunoglobulin superfamily, involved in neuronal development, synaptic maintenance and organization of neuronal networks. Here, we discuss our recent data suggesting that F3/Contactin exerts a role in hippocampal synaptic plasticity and memory in adult and aged mice. In particular, we have studied long-term potentiation (LTP), spatial and object recognition memory, and phosphorylation of the transcription factor cAMP-Responsive-Element Binding protein (CREB) in a transgenic mouse model of F3/Contactin overexpression. We also investigated whether F3/Contactin might influence neuronal apoptosis and the production of amyloid-beta peptide (Aβ), known to be one of the main pathogenetic hallmarks of Alzheimer's disease (AD). In conclusion, a further understanding of F3/Contactin role in synaptic plasticity and memory might have interesting clinical outcomes in cognitive disorders, such as aging and AD, offering innovative therapeutic opportunities.

Integrins in synapse regulation

Integrins are a large family of extracellular matrix (ECM) receptors. In the developing and adult brain, many integrins are present at high levels at synapses. The tetrapartite structure of synapses - which comprises presynaptic and postsynaptic neurons, the ECM and glial processes - places synaptic integrins in an excellent position to sense dynamic changes in the synaptic environment and use this information to coordinate further changes in synapse structure and function that will shape neural circuit properties. Recent developments in our understanding of the cellular and physiological roles of integrins, which range from control of neural process outgrowth and synapse formation to regulation of synaptic plasticity and memory, enable us to attempt a synthesis of synaptic integrin function.

Estrogen's Effects on Excitatory Synaptic Transmission Entail Integrin and TrkB Transactivation and Depend Upon β1-integrin function

Estradiol (E2) perfusion rapidly increases the strength of fast excitatory transmission and facilitates long-term potentiation in the hippocampus, two effects likely related to its memory-enhancing properties. Past studies showed that E2's facilitation of transmission involves activation of RhoA signaling leading to actin polymerization in dendritic spines. Here we report that brief exposure of adult male hippocampal slices to 1 nM E2 increases the percentage of postsynaptic densities associated with high levels of immunoreactivity for activated forms of the BDNF receptor TrkB and β1-integrins, two synaptic receptors that engage actin regulatory RhoA signaling. The effects of E2 on baseline synaptic responses were unaffected by pretreatment with the TrkB-Fc scavenger for extracellular BDNF or TrkB antagonism, but were eliminated by neutralizing antisera for β1-integrins. E2 effects on synaptic responses were also absent in conditional β1-integrin knockouts, and with inhibition of matrix metalloproteinases, extracellular enzymes that generate integrin ligands. We propose that E2, acting through estrogen receptor-β, transactivates synaptic TrkB and β1-integrin, and via mechanisms dependent on integrin activation and signaling, reversibly reorganizes the spine cytoskeleton and thereby enhances synaptic responses in adult hippocampus.