Postsynaptic scaffolds of excitatory and inhibitory synapses in hippocampal neurons: maintenance of core components independent of actin filaments and microtubules

Mitigation of synaptic and memory impairments via F-actin stabilization in Alzheimer's disease

BACKGROUND

Synaptic dysfunction, characterized by synapse loss and structural alterations, emerges as a prominent correlate of cognitive decline in Alzheimer's disease (AD). Actin cytoskeleton, which serves as the structural backbone of synaptic architecture, is observed to be lost from synapses in AD. Actin cytoskeleton loss compromises synaptic integrity, affecting glutamatergic receptor levels, neurotransmission, and synaptic strength. Understanding these molecular changes is crucial for developing interventions targeting synaptic dysfunction, potentially mitigating cognitive decline in AD.

METHODS

In this study, we investigated the synaptic actin interactome using mass spectrometry in a mouse model of AD, APP/PS1. Our objective was to explore how alterations in synaptic actin dynamics, particularly the interaction between PSD-95 and actin, contribute to synaptic and cognitive impairment in AD. To assess the impact of restoring F-actin levels on synaptic and cognitive functions in APP/PS1 mice, we administered F-actin stabilizing agent, jasplakinolide. Behavioral deficits in the mice were evaluated using the contextual fear conditioning paradigm. We utilized primary neuronal cultures to study the synaptic levels of AMPA and NMDA receptors and the dynamics of PSD-95 actin association. Furthermore, we analyzed postmortem brain tissue samples from subjects with no cognitive impairment (NCI), mild cognitive impairment (MCI), and Alzheimer's dementia (AD) to determine the association between PSD-95 and actin.

RESULTS

We found a significant reduction in PSD-95-actin association in synaptosomes from middle-aged APP/PS1 mice compared to wild-type (WT) mice. Treatment with jasplakinolide, an actin stabilizer, reversed deficits in memory recall, restored PSD-95-actin association, and increased synaptic F-actin levels in APP/PS1 mice. Additionally, actin stabilization led to elevated synaptic levels of AMPA and NMDA receptors, enhanced dendritic spine density, suggesting improved neurotransmission and synaptic strength in primary cortical neurons from APP/PS1 mice. Furthermore, analysis of postmortem human tissue with NCI, MCI and AD subjects revealed disrupted PSD-95-actin interactions, underscoring the clinical relevance of our preclinical studies.

CONCLUSION

Our study elucidates disrupted PSD-95 actin interactions across different models, highlighting potential therapeutic targets for AD. Stabilizing F-actin restores synaptic integrity and ameliorates cognitive deficits in APP/PS1 mice, suggesting that targeting synaptic actin regulation could be a promising therapeutic strategy to mitigate cognitive decline in AD.

Imbalance of synaptic and extrasynaptic NMDA receptors induced by the deletion of CRMP1 accelerates age-related cognitive decline in mice

Collapsin response mediator protein 1 (CRMP1) is involved in semaphorin 3A signaling pathway, promoting neurite extension and growth cone collapse. It is highly expressed in the nervous system, especially the hippocampus. The crmp1 knockout (KO) mice display impaired spatial learning and memory, and this phenomenon seemingly tends to deteriorate with age. Here we investigated whether CRMP1 is involved in age-related cognitive decline in WT and crmp1 KO mice at adult, middle-aged and older stages. The results revealed that cognitive dysfunction in the Morris water maze task became more severe and decreased glutamate and glutamine level in middle-aged crmp1 KO mice. Additionally, increasing levels of extrasynaptic NMDA receptors and phosphorylation of Tau were observed in middle-aged crmp1 KO mice, leading to synaptic and neuronal loss in the CA3 regions of hippocampus. These findings suggest that deletion of CRMP1 accelerates age-related cognitive decline by disrupting the balance between synaptic and extrasynaptic NMDA receptors, resulting in the loss of synapses and neurons.

Acute reorganization of postsynaptic GABAA receptors reveals the functional impact of molecular nanoarchitecture at inhibitory synapses

Neurotransmitter receptors partition into nanometer-scale subdomains within the postsynaptic membrane that are precisely aligned with presynaptic neurotransmitter release sites. While spatial coordination between pre- and postsynaptic elements is observed at both excitatory and inhibitory synapses, the functional significance of this molecular architecture has been challenging to evaluate experimentally. Here we utilized an optogenetic clustering approach to acutely alter the nanoscale organization of the postsynaptic inhibitory scaffold gephyrin while monitoring synaptic function. Gephyrin clustering rapidly enlarged postsynaptic area, laterally displacing GABAA receptors from their normally precise apposition with presynaptic active zones. Receptor displacement was accompanied by decreased synaptic GABAA receptor currents even though presynaptic release probability and the overall abundance and function of synaptic GABAA receptors remained unperturbed. Thus, acutely repositioning neurotransmitter receptors within the postsynaptic membrane profoundly influences synaptic efficacy, establishing the functional importance of precision pre-/postsynaptic molecular coordination at inhibitory synapses.

Neurons: The Interplay between Cytoskeleton, Ion Channels/Transporters and Mitochondria

Neurons are permanent cells whose key feature is information transmission via chemical and electrical signals. Therefore, a finely tuned homeostasis is necessary to maintain function and preserve neuronal lifelong survival. The cytoskeleton, and in particular microtubules, are far from being inert actors in the maintenance of this complex cellular equilibrium, and they participate in the mobilization of molecular cargos and organelles, thus influencing neuronal migration, neuritis growth and synaptic transmission. Notably, alterations of cytoskeletal dynamics have been linked to alterations of neuronal excitability. In this review, we discuss the characteristics of the neuronal cytoskeleton and provide insights into alterations of this component leading to human diseases, addressing how these might affect excitability/synaptic activity, as well as neuronal functioning. We also provide an overview of the microscopic approaches to visualize and assess the cytoskeleton, with a specific focus on mitochondrial trafficking.

Single-cell reconstruction reveals input patterns and pathways into corticotropin-releasing factor neurons in the central amygdala in mice

Corticotropin-releasing factor (CRF) neurons are one of the most densely distributed cell types in the central amygdala (CeA), and are involved in a wide range of behaviors including anxiety and learning. However, the fundamental input circuits and patterns of CeA-CRF neurons are still unclear. Here, we generate a monosynaptic-input map onto CeA-CRF neurons at single-cell resolution via a retrograde rabies-virus system. We find all inputs are located in 44 nested subregions that directly innervate CeA-CRF neurons; most of them are top-down convergent inputs expressing Ca²⁺/calmodulin-dependent protein kinase II, and are centralized in cortex, especially in the layer 4 of the somatosensory cortex, which may directly relay information from the thalamus. While the bottom-up divergent inputs have the highest proportion of glutamate decarboxylase expression. Finally, en passant structures of single input neuron are revealed by in-situ reconstruction in a modified 3D-reference atlas, represented by a Periaqueductal gray-Subparafascicular nucleus-Subthalamic nucleus-Globus pallidus-Caudoputamen-CeA pathway. Taken together, our findings provide morphological and connectivity properties of inputs onto CeA-CRF neurons, which may provide insights for future studies interrogating circuit mechanisms of CeA-CRF neurons in mediating various functions.

Viral retrograde tracing identifies input regions and patterns into the corticotropin releasing factor-expressing neurons in central amygdala, providing an important resource to disentangle the role of these cells in fear and anxiety.

Control of Synapse Structure and Function by Actin and Its Regulators

Neurons transmit and receive information at specialized junctions called synapses. Excitatory synapses form at the junction between a presynaptic axon terminal and a postsynaptic dendritic spine. Supporting the shape and function of these junctions is a complex network of actin filaments and its regulators. Advances in microscopic techniques have enabled studies of the organization of actin at synapses and its dynamic regulation. In addition to highlighting recent advances in the field, we will provide a brief historical perspective of the understanding of synaptic actin at the synapse. We will also highlight key neuronal functions regulated by actin, including organization of proteins in the pre- and post- synaptic compartments and endocytosis of ion channels. We review the evidence that synapses contain distinct actin pools that differ in their localization and dynamic behaviors and discuss key functions for these actin pools. Finally, whole exome sequencing of humans with neurodevelopmental and psychiatric disorders has identified synaptic actin regulators as key disease risk genes. We briefly summarize how genetic variants in these genes impact neurotransmission via their impact on synaptic actin.

Intramyocardial injection of human adipose-derived stem cells ameliorates cognitive deficit by regulating oxidative stress-mediated hippocampal damage after myocardial infarction

Cognitive impairment is a serious side effect of post-myocardial infarction (MI) course. We have recently demonstrated that human adipose-derived stem cells (hADSCs) ameliorated myocardial injury after MI by attenuating reactive oxygen species (ROS) levels. Here, we studied whether the beneficial effects of intramyocardial hADSC transplantation can extend to the brain and how they may attenuate cognitive dysfunction via modulating ROS after MI. After coronary ligation, male Wistar rats were randomized via an intramyocardial route to receive either vehicle, hADSC transplantation (1 × 106 cells), or the combination of hADSCs and 3-Morpholinosydnonimine (SIN-1, a peroxynitrite donor). Whether hADSCs migrated into the hippocampus was assessed by using human-specific primers in qPCR reactions. Passive avoidance test was used to assess cognitive performance. Postinfarction was associated with increased oxidative stress in the myocardium, circulation, and hippocampus. This was coupled with decreased numbers of dendritic spines as well as a significant downregulation of synaptic plasticity consisting of synaptophysin and PSD95. Step-through latency during passive avoidance test was impaired in vehicle-treated rats after MI. Intramyocardial hADSC injection exerted therapeutic benefits in improving cardiac function and cognitive impairment. None of hADSCs was detected in rat's hippocampus at the 3rd day after intramyocardial injection. The beneficial effects of hADSCs on MI-induced histological and cognitive changes were abolished after adding SIN-1. MI-induced ROS attacked the hippocampus to induce neurodegeneration, resulting in cognitive deficit. The remotely intramyocardial administration of hADSCs has the capacity of improved synaptic neuroplasticity in the hippocampus mediated by ROS, not the cell engraftment, after MI. KEY MESSAGES: Human adipose-derived stem cells (hADSCs) ameliorated injury after myocardial infarction by attenuating reactive oxygen species (ROS) levels. Intramyocardial administration of hADSCs remotely exerted therapeutic benefits in improving cognitive impairment after myocardial infarction. The improved synaptic neuroplasticity in the hippocampus was mediated by hADSC-inhibiting ROS, not by the stem cell engraftment.

Exercise intensities modulate cognitive function in spontaneously hypertensive rats through oxidative mediated synaptic plasticity in hippocampus

Oxidative damage in the brain may lead to cognitive impairments. There was considerable debate regarding the beneficial effects of physical exercise on cognitive functions because exercise protocols have varied widely across studies. We investigated whether different exercise intensities alter performance on cognitive tasks. The experiment was performed on spontaneously hypertensive rats (6 months at the established phase of hypertension) distributed into 3 groups: sedentary, low-intensity exercise and high-intensity exercise. Systolic blood pressure measurements confirmed hypertension in spontaneously hypertensive rats. In comparison to normotensive Wistar-Kyoto rats, sedentary spontaneously hypertensive rats had similar escape latencies and a similar preference for the correct quadrant in the probe trial. Compared to the sedentary group, the low-intensity exercise group had significantly better improvements in spatial memory assessed by Morris water maze. Low-intensity exercise was associated with attenuated reactive oxygen species, as measured by dihydroethidine fluorescence and nitrotyrosine staining in the dentate gyrus of the hippocampus. This was coupled with increased numbers of neurons and dendritic spines as well as a significant upregulation of synaptic density. In contrast, the beneficial effects of low-intensity exercise are abolished in high-intensity exercise as shown by increased free radical levels and an impairment in spatial memory. We concluded that exercise is an effective strategy to improve spatial memory in spontaneously hypertensive rats even at an established phase of hypertension. Low-intensity exercise exhibited better improvement on cognitive deficits than high-intensity exercise by attenuating free radical levels and improving downstream synaptic plasticity.

Binding of gephyrin to microtubules is regulated by its phosphorylation at Ser270

Gephyrin is a multifunctional scaffolding protein anchoring glycine- and subtypes of GABA type A- receptors at inhibitory postsynaptic membrane specializations by binding to the microtubule (MT) and/or the actin cytoskeleton. However, the conditions under which gephyrin can bind to MTs and its regulation are currently unknown. Here, we demonstrate that during the purification of MTs from rat brain by sedimentation of polymerized tubulin using high-speed centrifugation a fraction of gephyrin was bound to MTs, whereas gephyrin phosphorylated at the CDK5-dependent site Ser270 was detached from MTs and remained in the soluble protein fraction. Moreover, after collybistin fostered phosphorylation at Ser270 the binding of a recombinant gephyrin to MTs was strongly reduced in co-sedimentation assays. Correspondingly, upon substitution of wild-type gephyrin with recombinant gephyrin carrying alanine mutations at putative CDK5 phosphorylation sites the binding of gephyrin to MTs was increased. Furthermore, the analysis of cultured HEK293T and U2OS cells by immunofluorescence-microscopy disclosed a dispersed and punctuated endogenous gephyrin immunoreactivity co-localizing with MTs which was evidently not phosphorylated at Ser270. Thus, our study provides additional evidence for the binding of gephyrin to MTs in brain tissue and in in vitro cell systems. More importantly, our findings indicate that gephyrin-MT binding is restricted to a specific gephyrin fraction and depicts phosphorylation of gephyrin as a regulatory mechanism of this process by showing that soluble gephyrin detached from MTs can be detected specifically with the mAb7a antibody, which recognizes the Ser270 phosphorylated- version of gephyrin.

The microtubule cytoskeleton at the synapse

In neurons, microtubules (MTs) provide routes for transport throughout the cell and structural support for dendrites and axons. Both stable and dynamic MTs are necessary for normal neuronal functions. Research in the last two decades has demonstrated that MTs play additional roles in synaptic structure and function in both pre- and postsynaptic elements. Here, we review current knowledge of the functions that MTs perform in excitatory and inhibitory synapses, as well as in the neuromuscular junction and other specialized synapses, and discuss the implications that this knowledge may have in neurological disease.

PLPP/CIN-mediated NF2-serine 10 dephosphorylation regulates F-actin stability and Mdm2 degradation in an activity-dependent manner

Neurofibromin 2 (NF2, also known as merlin) is a tumor suppressor protein encoded by the neurofibromatosis type 2 gene NF2. NF2 is also an actin-binding protein that functions in an intrinsic signaling network critical for actin dynamics. Although protein kinase A (PKA)-mediated NF2-serin (S) 10 phosphorylation stabilizes filamentous actin (F-actin), the underlying mechanisms of NF2-S10 dephosphorylation and the role of NF2 in seizures have been elusive. Here, we demonstrate that pyridoxal-5′-phosphate phosphatase/chronophin (PLPP/CIN) dephosphorylated NF2-S10 site as well as cofilin-S3 site. In addition, NF2-S10 dephosphorylation reversely regulated murine double minute-2 (Mdm2) and postsynaptic density 95 (PSD95) degradations in an activity-dependent manner, which increased seizure intensity and its progression in response to kainic acid (KA). In addition, NF2 knockdown facilitated seizure intensity and its progress through F-actin instability independent of cofilin-mediated actin dynamics. Therefore, we suggest that PLPP/CIN may be a potential therapeutic target for epileptogenesis and NF2-associated diseases.

The actin binding protein α-actinin-2 expression is associated with dendritic spine plasticity and migrating granule cells in the rat dentate gyrus following pilocarpine-induced seizures

α-actinin-2 (α-actn-2) is an F-actin-crosslinking protein, localized in dendritic spines. In vitro studies suggested that it is involved in spinogenesis, morphogenesis, actin organization, cell migration and anchoring of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptors in dendritic spines. However, little is known regarding its function in vivo. We examined the levels of α-actn-2 expression within the dentate gyrus (DG) during the development of chronic limbic seizures (epileptogenesis) induced by pilocarpine in rats. In this model, plasticity of the DG glutamatergic granule cells including spine loss, spinogenesis, morphogenesis, neo-synaptogenesis, aberrant migration, and alterations of NMDA receptors have been well characterized. We showed that α-actn-2 immunolabeling was reduced in the inner molecular layer at 1-2 weeks post-status epilepticus (SE), when granule cell spinogenesis and morphogenesis occur. This low level persisted at the chronic stage when new functional synapses are established. This decreased of α-actn-2 protein is concomitant with the recovery of drebrin A (DA), another actin-binding protein, at the chronic stage. Indeed, we demonstrated in cultured cells that in contrast to DA, α-actn-2 did not protect F-actin destabilization and DA inhibited α-actn-2 binding to F-actin. Such alteration could affect the anchoring of NR1 in dendritic spines. Furthermore, we showed that the expression of α-actn-2 and NR1 are co-down-regulated in membrane fractions of pilocarpine animals at chronic stage. Last, we showed that α-actn-2 is expressed in migrating newly born granule cells observed within the hilus of pilocarpine-treated rats. Altogether, our results suggest that α-actn-2 is not critical for the structural integrity and stabilization of granule cell dendritic spines. Instead, its expression is regulated when spinogenesis and morphogenesis occur and within migrating granule cells. Our data also suggest that the balance between α-actn-2 and DA expression levels may modulate NR1 anchoring within dendritic spines.

White matter disturbances in phenylketonuria: Possible underlying mechanisms

White matter pathologies, as well as intellectual disability, microcephaly, and other central nervous system injuries, are clinical traits commonly ascribed to classic phenylketonuria (PKU). PKU is an inherited metabolic disease elicited by the deficiency of phenylalanine hydroxylase. Accumulation of l-phenylalanine (Phe) and its metabolites is found in tissues and body fluids in phenylketonuric patients. In order to mitigate the clinical findings, rigorous dietary Phe restriction constitutes the core of therapeutic management in PKU. Myelination is the process whereby the oligodendrocytes wrap myelin sheaths around the axons, supporting the conduction of action potentials. White matter injuries are implicated in the brain damage related to PKU, especially in untreated or poorly treated patients. The present review summarizes evidence toward putative mechanisms driving the white matter pathology in PKU patients.

Cytoskeletal makeup of the synapse: Shaft versus spine

The ability of neurons to communicate and store information depends on the activity of synapses which can be located on small protrusions (dendritic spines) or directly on the dendritic shaft. The formation, plasticity, and stability of synapses are regulated by the neuronal cytoskeleton. Actin filaments together with microtubules, neurofilaments, septins, and scaffolding proteins orchestrate the structural organization of both shaft and spine synapses, enabling their efficacy in response to synaptic activation. Synapses critically depend on several factors, which are also mediated by the cytoskeleton, including transport and delivery of proteins from the soma, protein synthesis, as well as surface diffusion of membrane proteins. In this minireview, we focus on recent progress made in the field of cytoskeletal elements of the postsynapse and discuss the differences and similarities between synapses located in the spines versus dendritic shaft.

Oligodendrocytes express synaptic proteins that modulate myelin sheath formation

Vesicular release from neurons promotes myelin sheath growth on axons. Oligodendrocytes express proteins that allow dendrites to respond to vesicular release at synapses, suggesting that axon-myelin contacts use similar communication mechanisms as synapses to form myelin sheaths. To test this, we used fusion proteins to track synaptic vesicle localization and membrane fusion in zebrafish during developmental myelination and investigated expression and localization of PSD95, a dendritic post-synaptic protein, within oligodendrocytes. Synaptic vesicles accumulate and exocytose at ensheathment sites with variable patterning and most sheaths localize PSD95 with patterning similar to exocytosis site location. Disruption of candidate PDZ-binding transsynaptic adhesion proteins in oligodendrocytes cause variable effects on sheath length and number. One candidate, Cadm1b, localizes to myelin sheaths where both PDZ binding and extracellular adhesion to axons mediate sheath growth. Our work raises the possibility that axon-glial communication contributes to myelin plasticity, providing new targets for mechanistic unraveling of developmental myelination.

Role of microtubules in late-associative plasticity of hippocampal Schaffer collateral-CA1 synapses in mice

The microtubule network represents a key scaffolding structure that forms part of the neuronal cytoskeleton and contributes to biomolecule exchange within neurons. However, researchers have not determined whether an intact microtubule network is required for late associative plasticity. Therefore, the late associative plasticity of field excitatory postsynaptic potentials from two synaptic inputs was analyzed. Synaptic potentiation was induced through alternating tetanization of hippocampal Schaffer-collateral CA1 synaptic populations in acute slices prepared from young-adult C57BL/6 mice. Vincristine was applied to depolymerize microtubules. Vincristine did not alter the phosphorylation levels of plasticity-related pre- or postsynaptic proteins but reduced the level of a protein marker of the ER-Golgi intermediate compartment (ERGIC-53/p58). Vincristine did not alter the magnitude or maintenance of the synaptic potentiation evoked by repeated tetanization (3 × 100 stimuli at 100 Hz) of one synaptic population. However, this synaptic potentiation was sensitive to the coapplication of a protein synthesis inhibitor, such as rapamycin, anisomycin or cycloheximide, indicating that protein synthesis has become essential in depolymerized microtubules during the first hour of the synaptic potentiation. The application of vincristine up to a 70 stimuli, 100 Hz tetanization of a second synaptic input prevented the transformation of short-term potentiation into long-term potentiation (LTP), further indicating that intact microtubules are required for the late associative properties of synaptic plasticity. Therefore, activity-dependent synaptic plasticity does not rely on microtubules within the first two hours after tetanization; however, the associative interaction of independent synaptic inputs relies on their proper function. In addition, either new protein synthesis or microtubule-based processes are sufficient to stabilize LTP within the first 3 h after tetanization, and a deficit in synaptic plasticity is only observable when both processes are blocked.

Gephyrin Palmitoylation in Basolateral Amygdala Mediates the Anxiolytic Action of Benzodiazepine

BACKGROUND

Benzodiazepines (BZDs) have been used to treat anxiety disorders for more than five decades as the allosteric modulator of the gamma-aminobutyric acid A receptor (GABA_AR). Little is known about other mechanisms of BZDs. Here, we describe how the rapid stabilization of postsynaptic GABA_AR is essential and sufficient for the anxiolytic effect of BZDs via a palmitoylation-dependent mechanism.

METHODS

Palmitoylated proteins in the basolateral amygdala (BLA) of rats with different anxious states were assessed by a biotin exchange protocol. Both pharmacological and genetic approaches were used to investigate the role of palmitoylation in anxiety behavior. Electrophysiological recording, reverse transcription polymerase chain reaction, Western blotting, and coimmunoprecipitation were used to investigate the mechanisms.

RESULTS

Highly anxious rats were accompanied by the deficiency of gephyrin palmitoylation and decreased the synaptic function of GABA_AR in the BLA. We then identified that the dysfunction of DHHC12, a palmitoyl acyltransferase that specifically palmitoylates gephyrin, contributed to the high-anxious state. Furthermore, diazepam, as an anxiolytic drug targeting GABA_ARs, was found to increase gephyrin palmitoylation in the BLA via a GABA_AR-dependent manner to activate DHHC12. The anxiolytic effect of diazepam was nearly abolished by the DHHC12 knockdown. Specifically, similar to the effect of BZD, the overexpression of DHHC12 in the BLA exerted a significant anxiolytic action, which was prevented by flumazenil.

CONCLUSIONS

Our results support the view that the strength of inhibitory synapse was controlled by gephyrin palmitoylation in vivo and proposes a previously unknown palmitoylation-centered mode of BZD's action.

α-Actinin Anchors PSD-95 at Postsynaptic Sites

Despite the central role PSD-95 plays in anchoring postsynaptic AMPARs, how PSD-95 itself is tethered to postsynaptic sites is not well understood. Here we show that the F-actin binding protein α-actinin binds to the very N terminus of PSD-95. Knockdown (KD) of α-actinin phenocopies KD of PSD-95. Mutating lysine at position 10 or lysine at position 11 of PSD-95 to glutamate, or glutamate at position 53 or glutamate and aspartate at positions 213 and 217 of α-actinin, respectively, to lysine impairs, in parallel, PSD-95 binding to α-actinin and postsynaptic localization of PSD-95 and AMPARs. These experiments identify α-actinin as a critical PSD-95 anchor tethering the AMPAR-PSD-95 complex to postsynaptic sites.

Fast interaction between AMPA and NMDA receptors by intracellular calcium

Suppression of NMDA receptor (NMDAR)-mediated currents by intracellular Ca²⁺ has been described as a negative feedback loop in NMDAR modulation. In the time scale of tenths of milliseconds the depth of the suppression does not depend on the Ca²⁺ source. It may be caused by Ca²⁺ influx through voltage-gated calcium channels, NMDAR channels or release from intracellular stores. However, NMDARs are often co-expressed in synapses with Ca²⁺-permeable AMPA receptors (AMPARs). Due to significant differences in activation kinetics between these two types of glutamate receptors (GluRs), Ca²⁺ entry through AMPARs precedes full activation of NMDARs, and therefore, might have an impact on the amplitude of NMDAR-mediated currents. The study of Ca²⁺-mediated crosstalk between AMPAR and NMDAR in native synapses is challenging due to high NMDAR Ca²⁺ permeability. Therefore, recombinant Ca²⁺-permeable AMPAR and Ca²⁺-impermeable NMDAR mutant channels were co-expressed in HEK 293 cells to examine their interaction. An AMPAR-mediated increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) reversibly reduced the size of NMDAR-mediated whole-cell currents. The time course of the NMDAR channel inactivation and recovery from inactivation followed the time course of the [Ca²⁺]_i transient. When brief (1ms) pulses of glutamate were applied to outside-out patches, the degree of NMDAR inactivation increased with the increase in charge carried by the currents through co-activated AMPARs. However, AMPAR-mediated NMDAR inactivation was abolished in the presence of intracellular fast Ca²⁺ buffer BAPTA or in Ca²⁺-free extracellular solution. We conclude that Ca²⁺ entering through AMPARs inactivates co-localized NMDARs in the time range of excitatory postsynaptic currents.

PLPP/CIN regulates bidirectional synaptic plasticity via GluN2A interaction with postsynaptic proteins

Dendritic spines are dynamic structures whose efficacies and morphologies are modulated by activity-dependent synaptic plasticity. The actin cytoskeleton plays an important role in stabilization and structural modification of spines. However, the regulatory mechanism by which it alters the plasticity threshold remains elusive. Here, we demonstrate the role of pyridoxal-5′-phosphate phosphatase/chronophin (PLPP/CIN), one of the cofilin-mediated F-actin regulators, in modulating synaptic plasticity in vivo. PLPP/CIN transgenic (Tg) mice had immature spines with small heads, while PLPP/CIN knockout (KO) mice had gigantic spines. Furthermore, PLPP/CIN Tg mice exhibited enhanced synaptic plasticity, but KO mice showed abnormal synaptic plasticity. The PLPP/CIN-induced alterations in synaptic plasticity were consistent with the acquisition and the recall capacity of spatial learning. PLPP/CIN also enhanced N-methyl-D-aspartate receptor (GluN) functionality by regulating the coupling of GluN2A with interacting proteins, particularly postsynaptic density-95 (PSD95). Therefore, these results suggest that PLPP/CIN may be an important factor for regulating the plasticity threshold.

Shank-cortactin interactions control actin dynamics to maintain flexibility of neuronal spines and synapses

The family of Shank scaffolding molecules (comprising Shank1, 2 and 3) are core components of the postsynaptic density (PSD) in neuronal synapses. Shanks link surface receptors to other scaffolding molecules within the PSD, as well as to the actin cytoskeleton. However, determining the function of Shank proteins in neurons has been complicated because the different Shank isoforms share a very high degree of sequence and domain homology. Therefore, to control Shank content while minimizing potential compensatory effects, a miRNA-based knockdown strategy was developed to reduce the expression of all synaptically targeted Shank isoforms simultaneously in rat hippocampal neurons. Using this approach, a strong (>75%) reduction in total Shank protein levels was achieved at individual dendritic spines, prompting an approximately 40% decrease in mushroom spine density. Furthermore, Shank knockdown reduced spine actin levels and increased sensitivity to the actin depolymerizing agent Latrunculin A. A SHANK2 mutant lacking the proline-rich cortactin-binding motif (SHANK2-ΔPRO) was unable to rescue these defects. Furthermore, Shank knockdown reduced cortactin levels in spines and increased the mobility of spine cortactin as measured by single-molecule tracking photoactivated localization microscopy, suggesting that Shank proteins recruit and stabilize cortactin at the synapse. Furthermore, it was found that Shank knockdown significantly reduced spontaneous remodelling of synapse morphology that could not be rescued by the SHANK2-ΔPRO mutant. It was concluded that Shank proteins are key intermediates between the synapse and the spine interior that, via cortactin, permit the actin cytoskeleton to dynamically regulate synapse morphology and function.

Neural plasticity and memory: molecular mechanism

Deciphering the cellular and molecular mechanisms of memory has been an important topic encompassing the learning and memory domain besides the neurodegenerative disorders. Synapses accumulate cognitive information from life-lasting alterations of their molecular and structural composition. Current memory storage models identify posttranslational modification imperative for short-term information storage and mRNA translation for long-term information storage. However, the precise account of these modifications has not been summarized at the individual synapse level. Therefore, herein we describe the spatiotemporal reorganization of synaptic plasticity at the dendritic spine level to elucidate the mechanism through which synaptic substructures are remodeled; though at the molecular level, such mechanisms are still quite unclear. It has thus been concluded that the existing mechanisms do not entirely elaborate memory storage processes. Further efforts are therefore encouraged to delineate the mechanism of neuronal connectivity at the chemical level as well, including inter- or intramolecular bonding patterns at the synaptic level, which may be a permissive and vital step of memory storage.

Gephyrin: a central GABAergic synapse organizer

Gephyrin is a central element that anchors, clusters and stabilizes glycine and γ-aminobutyric acid type A receptors at inhibitory synapses of the mammalian brain. It self-assembles into a hexagonal lattice and interacts with various inhibitory synaptic proteins. Intriguingly, the clustering of gephyrin, which is regulated by multiple posttranslational modifications, is critical for inhibitory synapse formation and function. In this review, we summarize the basic properties of gephyrin and describe recent findings regarding its roles in inhibitory synapse formation, function and plasticity. We will also discuss the implications for the pathophysiology of brain disorders and raise the remaining open questions in this field.

PTBP-dependent PSD-95 and CamKIIα alternative splicing in the lens

PURPOSE

Parallels described between neurons and lens fiber cells include detailed similarities in sub-cellular structures that increasingly show shared expression of genes involved in the construction and function of these structures in neurons. Intriguingly, associated modes of molecular regulation of these genes that had been thought to distinguish neurons have been identified in the lens as well. Both elongated cell types form membrane protrusions with similar size, shape, and spacing that exclude microtubules, contain F-actin, and are coated with the clathrin/AP-2 adaptor. Lenses express glutamate and gamma-aminobutyric acid (GABA) receptors with signaling and channel proteins shown to act together at neuronal membranes. Postsynaptic density protein 95 (PSD-95) and Ca(2+)/calmodulin-dependent protein kinase (CaMKIIα) expression and functions illustrate the integration of aspects of neuronal molecular and cell biology and were investigated here in the lens.

METHODS

Immunofluorescence, immunoblot, and RT-PCR methods were used to assess protein expression and alternative transcript splicing.

RESULTS

We showed the essential dendritic spine scaffold protein PSD-95 is expressed in lenses and demonstrated lens PSD-95 transcripts undergo polypyrimidine tract binding protein (PTBP)-dependent alternative splicing of its pivotal exon 18 required to avoid nonsense-mediated decay, and showed PTBP-dependent alternative splicing of CaMKIIα transcripts in the lens. The PSD-95 protein was observed at fiber cell membranes overlapping with N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate and GABA receptor proteins, tyrosine phosphatase STEP, CaMKIIα, the Ca(V)1.3 calcium channel, and clathrin, which were previously identified at lens fiber cell membranes. During neurogenesis, miR-124 is expressed that suppresses PTBP1 and promotes these splicing events. miR-124 is also expressed in mammalian lenses and upregulated during lens regeneration in amphibians, consistent with previous demonstrations of PTBP1,2 and PTBP-dependent PTBP2 exon 10 splicing in rodent lenses.

CONCLUSIONS

Findings of this dendritic spine scaffold protein and conservation of its key mode of molecular regulation in the lens provides further evidence that key aspects of the neuron morphogenetic program are shared with the lens.

Barriers in the brain: resolving dendritic spine morphology and compartmentalization

Dendritic spines are micron-sized protrusions that harbor the majority of excitatory synapses in the central nervous system. The head of the spine is connected to the dendritic shaft by a 50-400 nm thin membrane tube, called the spine neck, which has been hypothesized to confine biochemical and electric signals within the spine compartment. Such compartmentalization could minimize interspinal crosstalk and thereby support spine-specific synapse plasticity. However, to what extent compartmentalization is governed by spine morphology, and in particular the diameter of the spine neck, has remained unresolved. Here, we review recent advances in tool development - both experimental and theoretical - that facilitate studying the role of the spine neck in compartmentalization. Special emphasis is given to recent advances in microscopy methods and quantitative modeling applications as we discuss compartmentalization of biochemical signals, membrane receptors and electrical signals in spines. Multidisciplinary approaches should help to answer how dendritic spine architecture affects the cellular and molecular processes required for synapse maintenance and modulation.

Modulation of the NMDA Receptor Through Secreted Soluble Factors

Synaptic activity is a critical determinant in the formation and development of excitatory synapses in the central nervous system (CNS). The excitatory current is produced and regulated by several ionotropic receptors, including those that respond to glutamate. These channels are in turn regulated through several secreted factors that function as synaptic organizers. Specifically, Wnt, brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF), and transforming growth factor (TGF) particularly regulate the N-methyl-D-aspartate receptor (NMDAR) glutamatergic channel. These factors likely regulate early embryonic development and directly control key proteins in the function of important glutamatergic channels. Here, we review the secreted molecules that participate in synaptic organization and discuss the cell signaling behind of this fine regulation. Additionally, we discuss how these factors are dysregulated in some neuropathologies associated with glutamatergic synaptic transmission in the CNS.

GIT1 and βPIX are essential for GABA(A) receptor synaptic stability and inhibitory neurotransmission

Effective inhibitory synaptic transmission requires efficient stabilization of GABA_A receptors (GABA_ARs) at synapses, which is essential for maintaining the correct excitatory-inhibitory balance in the brain. However, the signaling mechanisms that locally regulate synaptic GABA_AR membrane dynamics remain poorly understood. Using a combination of molecular, imaging, and electrophysiological approaches, we delineate a GIT1/βPIX/Rac1/PAK signaling pathway that modulates F-actin and is important for maintaining surface GABA_AR levels, inhibitory synapse integrity, and synapse strength. We show that GIT1 and βPIX are required for synaptic GABA_AR surface stability through the activity of the GTPase Rac1 and downstream effector PAK. Manipulating this pathway using RNAi, dominant-negative and pharmacological approaches leads to a disruption of GABA_AR clustering and decrease in the strength of synaptic inhibition. Thus, the GIT1/βPIX/Rac1/PAK pathway plays a crucial role in regulating GABA_AR synaptic stability and hence inhibitory synaptic transmission with important implications for inhibitory plasticity and information processing in the brain.

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GIT1 and βPIX are present at inhibitory synapses and complex with GABA_ARs

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GIT1 and βPIX are important for GABA_AR clustering and inhibitory transmission

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Rac1 and PAK activity is required for stabilization of GABA_ARs at synapses

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A GIT1/βPIX/Rac1/PAK pathway is required for inhibitory synaptic transmission

α-Actinin-2 mediates spine morphology and assembly of the post-synaptic density in hippocampal neurons

Dendritic spines are micron-sized protrusions that constitute the primary post-synaptic sites of excitatory neurotransmission in the brain. Spines mature from a filopodia-like protrusion into a mushroom-shaped morphology with a post-synaptic density (PSD) at its tip. Modulation of the actin cytoskeleton drives these morphological changes as well as the spine dynamics that underlie learning and memory. Several PSD molecules respond to glutamate receptor activation and relay signals to the underlying actin cytoskeleton to regulate the structural changes in spine and PSD morphology. α-Actinin-2 is an actin filament cross-linker, which localizes to dendritic spines, enriched within the post-synaptic density, and implicated in actin organization. We show that loss of α-actinin-2 in rat hippocampal neurons creates an increased density of immature, filopodia-like protrusions that fail to mature into a mushroom-shaped spine during development. α-Actinin-2 knockdown also prevents the recruitment and stabilization of the PSD in the spine, resulting in failure of synapse formation, and an inability to structurally respond to chemical stimulation of the N-methyl-D-aspartate (NMDA)-type glutamate receptor. The Ca2+-insensitive EF-hand motif in α-actinin-2 is necessary for the molecule's function in regulating spine morphology and PSD assembly, since exchanging it for the similar but Ca2+-sensitive domain from α-actinin-4, another α-actinin isoform, inhibits its function. Furthermore, when the Ca2+-insensitive domain from α-actinin-2 is inserted into α-actinin-4 and expressed in neurons, it creates mature spines. These observations support a model whereby α-actinin-2, partially through its Ca2+-insensitive EF-hand motif, nucleates PSD formation via F-actin organization and modulates spine maturation to mediate synaptogenesis.

Activity-induced Nr4a1 regulates spine density and distribution pattern of excitatory synapses in pyramidal neurons

Excitatory synapses occur mainly on dendritic spines, and spine density is usually correlated with the strength of excitatory synaptic transmission. We report that Nr4a1, an activity-inducible gene encoding a nuclear receptor, regulates the density and distribution of dendritic spines in CA1 pyramidal neurons. Nr4a1 overexpression resulted in elimination of the majority of spines; however, postsynaptic densities were preserved on dendritic shafts, and the strength of excitatory synaptic transmission was unaffected, showing that excitatory synapses can be dissociated from spines. mRNA expression profiling studies suggest that Nr4a1-mediated transcriptional regulation of the actin cytoskeleton contributes to this effect. Under conditions of chronically elevated activity, when Nr4a1 was induced, Nr4a1 knockdown increased the density of spines and PSDs specifically at the distal ends of dendrites. Thus, Nr4a1 is a key component of an activity-induced transcriptional program that regulates the density and distribution of spines and synapses.

Gephyrin: a master regulator of neuronal function?

The neurotransmitters GABA and glycine mediate fast synaptic inhibition by activating ligand-gated chloride channels--namely, type A GABA (GABA(A)) and glycine receptors. Both types of receptors are anchored postsynaptically by gephyrin, which self-assembles into a scaffold and interacts with the cytoskeleton. Current research indicates that postsynaptic gephyrin clusters are dynamic assemblies that are held together and regulated by multiple protein-protein interactions. Moreover, post-translational modifications of gephyrin regulate the formation and plasticity of GABAergic synapses by altering the clustering properties of postsynaptic scaffolds and thereby the availability and function of receptors and other signalling molecules. Here, we discuss the formation and regulation of the gephyrin scaffold, its role in GABAergic and glycinergic synaptic function and the implications for the pathophysiology of brain disorders caused by abnormal inhibitory neurotransmission.

Mobility of acetylcholine receptors in command Helix lucorum neurons in a cellular analog of habituation

We investigated the role of the mobility of acetylcholine receptors in the depression of an acetylcholine-induced inward current (ACh-current) of Helix lucorum (a land snail) command neurons of defensive behavior in a cellular analog of habituation. The inhibitors of endocytosis and exocytosis, actin microfilaments and cytoskeleton microtubules, serine/threonine protein kinases (PKA, PKG, calcium calmodulin-dependent PK II, p38 mitogen-activated PK), tyrosine kinases (including Src-family kinases), serine/threonine phosphatases (PP1, PP2A, PP2B, PPM1D), and tyrosine protein phosphatases altered the depression of the ACh-current. A comparison of experimentally calculated curves of the ACh-current of these neurons and those obtained by mathematical modeling revealed the following: (a) ACh-current depression is caused by the reduction in the number of membranous ACh-receptors, which results from the shift in the balance of multidirectional transport processes of receptors toward the predominance of ACh-receptor internalization over their recycling; (b) depression of ACh-current depends on the activity of serine/threonine and tyrosine protein kinases and protein phosphatases, whose one of the main targets is the neuron transport system-actin microfilaments and microtubules of cytoskeleton, as well as motor proteins.

Extracellular chaperones prevent Aβ42-induced toxicity in rat brains

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by cognitive decline, formation of the extracellular amyloid β (Aβ42) plaques, neuronal and synapse loss, and activated microglia and astrocytes. Extracellular chaperones, which are known to inhibit amyloid fibril formation and promote clearance of misfolded aggregates, have recently been shown to reduce efficiently the toxicity of HypF-N misfolded oligomers to immortalised cell lines, by binding and clustering them into large species. However, the role of extracellular chaperones on Aβ oligomer toxicity remains unclear, with reports often appearing contradictory. In this study we microinjected into the hippocampus of rat brains Aβ42 oligomers pre-incubated for 1h with two extracellular chaperones, namely clusterin and α2-macroglobulin. The chaperones were found to prevent Aβ42-induced learning and memory impairments, as assessed by the Morris Water Maze test, and reduce Aβ42-induced glia inflammation and neuronal degeneration in rat brains, as probed by fluorescent immunohistochemical analyses. Moreover, the chaperones were able to prevent Aβ42 colocalisation with PSD-95 at post-synaptic terminals of rat primary neurons, suppressing oligomer cytotoxicity. All such effects were not effective by adding pre-formed oligomers and chaperones without preincubation. Molecular chaperones have therefore the potential to prevent the early symptoms of AD, not just by inhibiting Aβ42 aggregation, as previously demonstrated, but also by suppressing the toxicity of Aβ42 oligomers after they are formed. These findings elect them as novel neuroprotectors against amyloid-induced injury and excellent candidates for the design of therapeutic strategies against AD.

Septin 6 localizes to microtubules in neuronal dendrites

In neuronal dendrites, septins localize to the base of the spine, a unique position which is sandwiched between the microtubule (MT)-rich dendritic shaft and the actin filament-rich spine. Here, we provide evidence for the association of SEPT6 with MTs in cultured rat hippocampal neurons. In normal cultures, SEPT6 clusters localized to MTs, but not to actin clusters. Only MT-disrupting agents (vincristine and nocodazole), but not microfilament-disrupting one (latrunculin A), induced the redistribution of SEPT6 to the disrupted MTs. The nascent MT fibers that were recovered from vincristine or nocodazole treatments also accompanied SEPT6. Blocking MT disruption by Taxol prevented such phenomena, proving that the redistribution of SEPT6 was due to the MT disruption. Our results indicate that SEPT6 complexes at the base of the dendritic spine are associated with MTs.

Substrate-selective and calcium-independent activation of CaMKII by α-actinin

Protein-protein interactions are thought to modulate the efficiency and specificity of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) signaling in specific subcellular compartments. Here we show that the F-actin-binding protein α-actinin targets CaMKIIα to F-actin in cells by binding to the CaMKII regulatory domain, mimicking CaM. The interaction with α-actinin is blocked by CaMKII autophosphorylation at Thr-306, but not by autophosphorylation at Thr-305, whereas autophosphorylation at either site blocks Ca(2+)/CaM binding. The binding of α-actinin to CaMKII is Ca(2+)-independent and activates the phosphorylation of a subset of substrates in vitro. In intact cells, α-actinin selectively stabilizes CaMKII association with GluN2B-containing glutamate receptors and enhances phosphorylation of Ser-1303 in GluN2B, but inhibits CaMKII phosphorylation of Ser-831 in glutamate receptor GluA1 subunits by competing for activation by Ca(2+)/CaM. These data show that Ca(2+)-independent binding of α-actinin to CaMKII differentially modulates the phosphorylation of physiological targets that play key roles in long-term synaptic plasticity.

LIS1-dependent retrograde translocation of excitatory synapses in developing interneuron dendrites

Synaptic remodelling coordinated with dendritic growth is essential for proper development of neural connections. After establishment of synaptic contacts, synaptic junctions are thought to become stationary and provide fixed anchoring points for further dendritic growth. However, the possibility of active translocation of synapses along dendritic protrusions, to guide the proper arrangement of synaptic distribution, has not yet been fully investigated. Here we show that immature dendrites of γ-aminobutyric acid-positive interneurons form long protrusions and that these protrusions serve as conduits for retrograde translocation of synaptic contacts to the parental dendrites. This translocation process is dependent on microtubules and the activity of LIS1, an essential regulator of dynein-mediated motility. Suppression of this retrograde translocation results in disorganized synaptic patterns on interneuron dendrites. Taken together, these findings suggest the existence of an active microtubule-dependent mechanism for synaptic translocation that helps in the establishment of proper synaptic distribution on dendrites.

Maturation of synaptic junctions is important for proper neuronal connections. Using live cell imaging, Okabe et al. show that interneuron dendrites extend filopodia-like projections and use microtubule-dependent retrograde transport to guide proper synaptic distribution on dendrites.

Regulation of Synapse Composition by Protein Acetylation: The Role of Acetylated Cortactin

Protein acetylation affects synaptic plasticity and memory, but its effects on synapse composition have not been addressed. We found that protein acetylation promotes the dendritic clustering of the excitatory postsynaptic scaffold protein PSD95 in hippocampal neurons, without affecting the total levels of this protein. Cortactin, an F-actin-binding protein enriched in dendritic spines, is a substrate for acetylation and has a role in spine morphogenesis. Recent studies showed that cortactin acetylation changes its ability to bind F-actin and regulates cellular motility, but the function of cortactin acetylation in neuronal cells is so far unknown. We tested whether acetylation of cortactin influences its morphogenic function by overexpressing wild-type cortactin, or the mimetic mutants for acetylated or deacetylated cortactin, in hippocampal neurons, and found that cortactin acetylation has an impact on PSD95 clustering, independent from its function as actin dynamics regulator. Moreover, acetylated cortactin can rescue the reduction in PSD95 clustering mediated by knockdown of cortactin. We also found that acetylation of cortactin is correlated with decreased cortactin interaction with p140Cap and Shank1, and with lower cortactin phosphorylation at tyrosine 421. The neurotrophin BDNF promoted the acetylation of cortactin in hippocampal neurons, suggesting that BDNF may regulate excitatory synapses and PSD95 dendritic clustering at least in part by changing the acetylation level of cortactin. Our findings unravel an unsuspected role for cortactin acetylation in the regulation of PSD95 dendritic clustering, which may work in concert with cortactin's role in spine development.

A network of networks: cytoskeletal control of compartmentalized function within dendritic spines

Almost 30 years ago, actin was identified as the major cytoskeletal component of dendritic spines. Since then, its role in the remarkable dynamics of spine morphology have been detailed with live-cell views establishing that spine shape dynamics are an important requirement for synaptogenesis and synaptic plasticity. However, the actin cytoskeleton is critical to numerous and varied processes within the spine which contribute to the maintenance and plasticity of synaptic function. Here, we argue that the spatial and temporal distribution of actin-dependent processes within spines suggests that the spine cytoskeleton should not be considered a single entity, but an interacting network of nodes or hubs that are independently regulated and balanced to maintain synapse function. Disruptions of this balance within the spine are likely to lead to psychiatric and neurological dysfunction.

Immobilized pool of NCAM180 in the postsynaptic membrane is homeostatically replenished by the flux of NCAM180 from extrasynaptic regions

Homeostatic mechanisms maintaining high levels of adhesion molecules in synapses over prolonged periods of time remain incompletely understood. We used fluorescence recovery after photobleaching experiments to analyze the steady state turnover of the immobile pool of green fluorescent protein-labeled NCAM180, the largest postsynaptically accumulating isoform of the neural cell adhesion molecule (NCAM). We show that there is a continuous flux of NCAM180 to the postsynaptic membrane from nonsynaptic regions of dendrites by diffusion. In the postsynaptic membrane, the newly delivered NCAM180 slowly intermixes with the immobilized pool of NCAM180. Preferential immobilization and accumulation of NCAM180 in the postsynaptic membrane is reduced after disruption of the association of NCAM180 with the spectrin cytoskeleton and in the absence of the homophilic interactions of NCAM180 in synapses. Our observations indicate that the homophilic interactions and binding to the cytoskeleton promote immobilization of NCAM180 and its accumulation in the postsynaptic membrane. Flux of NCAM180 from extrasynaptic regions and its slow intermixture with the immobile pool of NCAM180 in the postsynaptic membrane may be important for the continuous homeostatic replenishment of NCAM180 protein at synaptic contacts without compromising the long term synaptic contact stability.

Repeated carbenoxolone injections during late pregnancy alter Snk-SPAR and PSD-95 expression in the hippocampus of rat pups

Homeostasis of circulating cortisol is maintained by the 11β-HSD2 enzyme which inactivates cortisol into cortisone. It is abundantly expressed in the placenta where it protects the fetus from high levels of maternal glucocorticoids (GCs). Maternal administration of Carbenoxolone (Cbx), a powerful 11β-HSD2 inhibitor, leads to an increase in fetal cortisol. Previous data showed that intrauterine environment plays a crucial role in determining hippocampal structure and function. Exposure of pregnant rats to high levels of GC leads to low birth weight in offspring and an increased risk of age related memory and cognitive deficits later in life. Glutamate receptors are localized in the postsynaptic density (PSD), where many signaling proteins, cytoskeleton proteins, and ion channels are found. Any change in the number of these molecules can influence the morphology and function of the dendritic spine. We proposed that repeated Cbx injections during late pregnancy may alter the scaffolding proteins of the NMDA receptor in the pup's brain. We investigated the effects of repeated maternal Cbx injections on the scaffolding proteins of NMDA receptor in the hippocampus of rat pups. We showed that injecting pregnant rats with Cbx injections (30mg/kg) during GD 14-21 leads to a significant decrease in SPAR (Spine Associated Rap Guanylate kinase activating protein) (p<0.001) and PSD-95 (p<0.05) but a significant increase in Snk (Serum inducible kinase) (p<0.001) in the pup's hippocampus at P40. In general, Snk is induced by neuronal activity and plays an important role in phosphorylating SPAR. The phosphorylated SPAR is then recognized and degraded by ubiquitin proteasome system (UPS), causing the depletion of SPAR and PSD-95 from the spines. The results suggest that fetal exposure to excessive GC levels may activate the Snk/SPAR pathway and lead to the depletion of SPAR and PSD-95. Since GCs drugs are commonly used in various obstetric and pediatric conditions, it is important to consider the risks and benefits of prenatal GCs exposure in order to prevent neurodevelopmental delay in the offspring.

"Memory bytes" - molecular match for CaMKII phosphorylation encoding of microtubule lattices

Learning, memory and long-term potentiation (LTP) are supported by factors including post-synaptic calcium ion flux activating and transforming the hexagonal calcium-calmodulin kinase II (CaMKII) holoenzyme. Upon calcium-induced activation, up to six kinase domains extend upward, and up to six kinase domains extend downward from the CaMKII association domain, the fully activated holoenzyme resembling a robotic insect 20 nanometers in length. Each extended kinase domain can be phosphorylated, and able to phosphorylate other proteins, thus potentially further encoding synaptic information at intraneuronal molecular sites for memory storage, processing and distribution. Candidate sites for phosphorylation-encoded molecular memory include microtubules, cylindrical lattice polymers of the protein tubulin. Using molecular modeling, we find spatial dimensions and geometry of the six extended CaMKII kinase domains can precisely match those of microtubule hexagonal lattice neighborhoods (both A- and B-lattices), and show two feasible phosphorylation mechanisms. In one, phosphorylation sites (e.g., valine 208) on a CaMKII extended kinase domain interact with serine 444 on a C-terminal "tail" of tubulin. In the second, the CaMKII kinase domain unfurls, enabling phosphorylation sites to contact threonine and serine sites on the tubulin surface. We suggest sets of six CaMKII kinase domains phosphorylate hexagonal microtubule lattice neighborhoods collectively, e.g., conveying synaptic information as ordered arrays of six "bits", and thus a "byte", with (minimally) 2⁶ (64) possible bit states per CaMKII-microtubule interaction. We model two levels of interaction between CaMKII and microtubules, suggesting a testable framework for molecular memory encoding.

NCAM/spectrin complex disassembly results in PSD perforation and postsynaptic endocytic zone formation

Mechanisms inducing perforation of the postsynaptic density (PSD) are poorly understood. We show that neural cell adhesion molecule- deficient (NCAM-/-) hippocampal neurons have an abnormally high percentage of synapses with perforated PSDs. The percentage of synapses with perforated PSDs is also increased in wild-type (NCAM+/+) neurons after the disruption of the NCAM/spectrin complex indicating that the NCAM-assembled spectrin cytoskeleton maintains the structural integrity of PSDs. We demonstrate that PSD perforations contain endocytic zones involved in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) internalization. Induction of long-term potentiation in NCAM+/+ neurons accompanied by insertion of AMPAR into the neuronal cell surface is subsequently followed by formation of perforated synapses and AMPAR endocytosis suggesting that perforation of PSDs is important for membrane homeostasis in activated synapses. In NCAM-/- or NCAM+/+ neurons with dissociated spectrin meshwork, AMPAR endocytosis is enhanced under conditions of basal activity. An abnormally high rate of postsynaptic membrane endocytosis may thus contribute to brain pathologies associated with mutations in NCAM or spectrin.

Experience-induced plasticity of thalamocortical axons in both juveniles and adults

We examined the effect of sensory deprivation on thalamocortical (TC) projections to the rat primary somatosensory cortex at different postnatal ages ranging from P0 to P96. Rats had their whiskers clipped off with one or two vibrissae spared. TC axons innervating barrel cortex were specifically labeled by injecting virus expressing fluorescent proteins into the corresponding primary (VPM) and/or secondary (POm) thalamic nuclei. The density of VPM axons in deprived columns was ≈34% lower relative to spared columns with a concomitant decrease in bouton density, suggesting a deprivation-induced retraction of VPM axons. Axonal changes were reversible upon regrowth of the clipped whiskers and independent of age at deprivation, indicating the absence of a critical period for anatomical plasticity. The POm projection was not obviously altered by sensory deprivation. We suggest that retraction and regrowth of TC axons substantially contribute to long-term deprivation-dependent functional plasticity.

A novel approach reveals temporal patterns of synaptogenesis between the isolated growth cones of Lymnaea neurons

All brain functions, ranging from motor behaviour to cognition, depend on precise developmental patterns of synapse formation between the growth cones of both pre- and postsynaptic neurons. While the molecular evidence for the presence of 'pre-assembled' elements of synaptic machinery prior to physical contact is beginning to emerge, the precise timing of functional synaptogenesis between the growth cones has not yet been defined. Moreover, it is unclear whether an initial assembly of various synaptic molecules located at the extrasomal regions (e.g. growth cones) can indeed result in fully mature and consolidated synapses in the absence of somata signalling. Such evidence is difficult to obtain both in vivo and in vitro because the extrasomal sites are often challenging, if not impossible, to access for electrophysiological analysis. Here we demonstrate a novel approach to precisely define various steps underlying synapse formation between the isolated growth cones of individually identifiable pre- and postsynaptic neurons from the mollusc Lymnaea stagnalis. We show for the first time that isolated growth cones transformed into 'growth balls' have an innate propensity to develop specific and multiple synapses within minutes of physical contact. We also demonstrate that a prior 'synaptic history' primes the presynaptic growth ball to form synapses quicker with subsequent partners. This is the first demonstration that isolated Lymnaea growth cones have the necessary machinery to form functional synapses.