GRIP: a synaptic PDZ domain-containing protein that interacts with AMPA receptors

WWC2 modulates GABAA-receptor-mediated synaptic transmission, revealing class-specific mechanisms of synapse regulation by WWC family proteins

The WW and C2 domain-containing protein (WWC2) is implicated in several neurological disorders. Here, we demonstrate that WWC2 interacts with inhibitory, but not excitatory, postsynaptic scaffolds, consistent with prior proteomic identification of WWC2 as a putative component of the inhibitory postsynaptic density. Using mice lacking WWC2 expression in excitatory forebrain neurons, we show that WWC2 suppresses γ-aminobutyric acid type-A receptor (GABAAR) incorporation into the plasma membrane and regulates HAP1 and GRIP1, which form a complex promoting GABAAR recycling to the membrane. Inhibitory synaptic transmission is increased in CA1 pyramidal cells lacking WWC2. Furthermore, unlike the WWC2 homolog KIBRA (kidney/brain protein; WWC1), a key regulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking at excitatory synapses, the deletion of WWC2 does not affect synaptic AMPAR expression. In contrast, loss of KIBRA does not affect GABAAR membrane expression. These data reveal synapse class-selective functions for WWC proteins as regulators of ionotropic neurotransmitter receptors and provide insight into mechanisms regulating GABAAR membrane expression.

Deciphering the role of siRNA in anxiety and depression

Anxiety and depression are central nervous system illnesses that are among the most prevalent medical concerns of the twenty-first century. Patients with this condition and their families bear psychological, financial, and societal hardship. There are currently restrictions when utilizing the conventional course of treatment. RNA interference is expected to become an essential approach in anxiety and depression due to its potent and targeted gene silencing. Silencing of genes by post-transcriptional modification is the mechanism of action of small interfering RNA (siRNA). The suppression of genes linked to disease is typically accomplished by siRNA molecules in an efficient and targeted manner. Unfavourable immune responses, off-target effects, naked siRNA instability, nuclease vulnerability, and the requirement to create an appropriate delivery method are some of the challenges facing the clinical application of siRNA. This review focuses on the use of siRNA in the treatment of anxiety and depression.

Exploring the role of AMPA receptor auxiliary proteins in synaptic functions and diseases

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) ionotropic glutamate receptors (AMPARs) mediate rapid excitatory synaptic transmission in the mammalian brain, primarily driven by the neurotransmitter glutamate. The modulation of AMPAR activity, particularly calcium-permeable AMPARs (CP-AMPARs), is crucially influenced by various auxiliary subunits. These subunits are integral membrane proteins that bind to the receptor's core and modify its functional properties, including ion channel kinetics and receptor trafficking. This review comprehensively catalogs all known AMPAR auxiliary proteins, providing vital insights into the biochemical mechanisms governing synaptic modulation and the specific impact of CP-AMPARs compared to their calcium-impermeable AMPA receptor (CI-AMPARs). Understanding the complex interplay between AMPARs and their auxiliary subunits in different brain regions is essential for elucidating their roles in cognitive functions such as learning and memory. Importantly, alterations in these auxiliary proteins' expression, function or interactions have been implicated in various neurological disorders. Aberrant signaling through CP-AMPARs, in particular, is associated with severe synaptic dysfunctions across neurodevelopmental, neurodegenerative and psychiatric conditions. Targeting the distinct properties of AMPAR-auxiliary subunit complexes, especially those involving CP-AMPARs, could disclose new therapeutic strategies, potentially allowing for more precise interventions in treating complex neuronal disorders.

Palmitoylation of synaptic proteins: roles in functional regulation and pathogenesis of neurodegenerative diseases

Palmitoylation is a type of lipid modification that plays an important role in various aspects of neuronal function. Over the past few decades, several studies have shown that the palmitoylation of synaptic proteins is involved in neurotransmission and synaptic functions. Palmitoyl acyltransferases (PATs), which belong to the DHHC family, are major players in the regulation of palmitoylation. Dysregulated palmitoylation of synaptic proteins and mutated/dysregulated DHHC proteins are associated with several neurodegenerative diseases, such as Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD). In this review, we summarize the recent discoveries on the subcellular distribution of DHHC proteins and analyze their expression patterns in different brain cells. In particular, this review discusses how palmitoylation of synaptic proteins regulates synaptic vesicle exocytotic fusion and the localization, clustering, and transport of several postsynaptic receptors, as well as the role of palmitoylation of other proteins in regulating synaptic proteins. Additionally, some of the specific known associations of these factors with neurodegenerative disorders are explored, with a few suggestions for the development of therapeutic strategies. Finally, this review provides possible directions for future research to reveal detailed and specific mechanisms underlying the roles of synaptic protein palmitoylation.

Trans-synaptic Association of Vesicular Zinc Transporter 3 and Shank3 Supports Synapse-Specific Dendritic Spine Structure and Function in the Mouse Auditory Cortex

Shank3 is a synaptic scaffolding protein that assists in tethering and organizing structural proteins and glutamatergic receptors in the postsynaptic density of excitatory synapses. The localization of Shank3 at excitatory synapses and the formation of stable Shank3 complexes is regulated by the binding of zinc to the C-terminal sterile-alpha-motif (SAM) domain of Shank3. Mutations in the SAM domain of Shank3 result in altered synaptic function and morphology, and disruption of zinc in synapses that express Shank3 leads to a reduction of postsynaptic proteins important for synaptic structure and function. This suggests that zinc supports the localization of postsynaptic proteins via Shank3. Many regions of the brain are highly enriched with free zinc inside glutamatergic vesicles at presynaptic terminals. At these synapses, zinc transporter 3 (ZnT3) moves zinc into vesicles where it is co-released with glutamate. Alterations in ZnT3 are implicated in multiple neurodevelopmental disorders, and ZnT3 knock-out (KO) mice-which lack synaptic zinc-show behavioral deficits associated with autism spectrum disorder and schizophrenia. Here we show that male and female ZnT3 KO mice have smaller dendritic spines and miniature excitatory postsynaptic current amplitudes than wildtype (WT) mice in the auditory cortex. Additionally, spine size deficits in ZnT3 KO mice are restricted to synapses that express Shank3. In WT mice, synapses that express both Shank3 and ZnT3 have larger spines compared to synapses that express Shank3 but not ZnT3. Together these findings suggest a mechanism whereby presynaptic ZnT3-dependent zinc supports postsynaptic structure and function via Shank3 in a synapse-specific manner.

Inhibiting Hippo pathway kinases releases WWC1 to promote AMPAR-dependent synaptic plasticity and long-term memory in mice

The localization, number, and function of postsynaptic AMPA-type glutamate receptors (AMPARs) are crucial for synaptic plasticity, a cellular correlate for learning and memory. The Hippo pathway member WWC1 is an important component of AMPAR-containing protein complexes. However, the availability of WWC1 is constrained by its interaction with the Hippo pathway kinases LATS1 and LATS2 (LATS1/2). Here, we explored the biochemical regulation of this interaction and found that it is pharmacologically targetable in vivo. In primary hippocampal neurons, phosphorylation of LATS1/2 by the upstream kinases MST1 and MST2 (MST1/2) enhanced the interaction between WWC1 and LATS1/2, which sequestered WWC1. Pharmacologically inhibiting MST1/2 in male mice and in human brain-derived organoids promoted the dissociation of WWC1 from LATS1/2, leading to an increase in WWC1 in AMPAR-containing complexes. MST1/2 inhibition enhanced synaptic transmission in mouse hippocampal brain slices and improved cognition in healthy male mice and in male mouse models of Alzheimer's disease and aging. Thus, compounds that disrupt the interaction between WWC1 and LATS1/2 might be explored for development as cognitive enhancers.

Modulation of GABA A receptor trafficking by WWC2 reveals class-specific mechanisms of synapse regulation by WWC family proteins

WWC2 (WW and C2 domain-containing protein) is implicated in several neurological disorders, however its function in the brain has yet to be determined. Here, we demonstrate that WWC2 interacts with inhibitory but not excitatory postsynaptic scaffolds, consistent with prior proteomic identification of WWC2 as a putative component of the inhibitory postsynaptic density. Using mice lacking WWC2 expression in excitatory forebrain neurons, we show that WWC2 suppresses GABA A R incorporation into the plasma membrane and regulates HAP1 and GRIP1, which form a complex promoting GABA A R recycling to the membrane. Inhibitory synaptic transmission is dysregulated in CA1 pyramidal cells lacking WWC2. Furthermore, unlike the WWC2 homolog KIBRA (WWC1), a key regulator of AMPA receptor trafficking at excitatory synapses, deletion of WWC2 does not affect synaptic AMPAR expression. In contrast, loss of KIBRA does not affect GABA A R membrane expression. These data reveal unique, synapse class-selective functions for WWC proteins as regulators of ionotropic neurotransmitter receptors and provide insight into mechanisms regulating GABA A R membrane expression.

Circulating ovarian hormones interact with protein interacting with C kinase (PICK1) within the medial prefrontal cortex to influence cocaine seeking in female mice

Protein interacting with C kinase 1 (PICK1) is an AMPA receptor binding protein that works in conjunction with glutamate receptor interacting protein (GRIP) to balance the number of GluA2-containing AMPARs in the synapse. In male mice, disrupting PICK1 in the medial prefrontal cortex (mPFC) leads to a decrease in cue-induced cocaine seeking and disrupting GRIP in the mPFC has the opposing effect, consistent with other evidence that removal of GluA2-containing AMPARs potentiates reinstatement. However, PICK1 does not seem to play the same role in female mice, as knockdown of either PICK1 or GRIP in the mPFC leads to similar increases in cue-induced cocaine seeking. These previous findings indicate that the role of PICK1 in the prefrontal cortex is sex specific. The goal of the current study was to examine whether ovarian hormones contribute to the effect of prefrontal PICK1 knockdown on reinstatement of cocaine seeking. While we replicated the increased cue-induced cocaine seeking in prefrontal PICK1 knockdown sham mice, we did not see any difference between the GFP control mice and PICK1 knockdowns following ovariectomy. However, this effect was driven primarily by an increase in cocaine seeking in ovariectomized GFP control mice while there was no effect ovariectomy in PICK1 knockdown mice. Taken together, these findings suggest that circulating ovarian hormones interact with the effects of PICK1 on cue-induced reinstatement.

Kaixin-San improves Aβ-induced synaptic plasticity inhibition by affecting the expression of regulation proteins associated with postsynaptic AMPAR expression

Objective: To explore the mechanism underlying Kaixin-San (KXS) regulation of postsynaptic AMPA receptor (AMPAR) expression to mitigate toxic effects of the amyloid-β protein (Aβ). Methods: An animal model was established via intracerebroventricular injection of Aβ1-42. The Morris water maze test was conducted to evaluate learning and memory, while electrophysiological recording was conducted to assess the hippocampal long-term potentiation (LTP). Western blotting was used to detect expression levels of the hippocampal postsynaptic AMPAR and its accessory proteins. Results: The time spent to find the platform was significantly prolonged, the number of mice crossing the target site was significantly reduced, and the maintenance of LTP was inhibited in the Aβ group than in the control group. In the Aβ/KXS group, the time taken to find the platform was significantly shortened and the number of mice crossing the target site was significantly increased than in the Aβ group; furthermore, the inhibition of LTP induced by Aβ was reversed. The expression of GluR1, GluR2, ABP, GRIP1, NSF, and pGluR1-Ser845 was upregulated, while that of pGluR2-Ser880 and PKC δ was downregulated in the Aβ/KXS group. Conclusion: The increased expression of ABP, GRIP1, NSF, and pGluR1-Ser845 and the decreased expression of pGluR2-Ser880 and PKC δ under the influence of KXS, followed by the upregulation of postsynaptic GluR1 and GluR2, alleviated the inhibition of LTP induced by Aβ. Ultimately, the memory function of model animals was improved by KXS. Our study provides novel insights into the mechanism underlying KXS mitigation of Aβ-induced synaptic plasticity inhibition and memory impairment by altering the levels of accessory proteins associated with AMPAR expression.

GluA3 subunits are required for appropriate assembly of AMPAR GluA2 and GluA4 subunits on cochlear afferent synapses and for presynaptic ribbon modiolar-pillar morphology

Cochlear sound encoding depends on α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs), but reliance on specific pore-forming subunits is unknown. With 5-week-old male C57BL/6J Gria3-knockout mice (i.e., subunit GluA3KO) we determined cochlear function, synapse ultrastructure, and AMPAR molecular anatomy at ribbon synapses between inner hair cells (IHCs) and spiral ganglion neurons. GluA3KO and wild-type (GluA3WT) mice reared in ambient sound pressure level (SPL) of 55-75 dB had similar auditory brainstem response (ABR) thresholds, wave-1 amplitudes, and latencies. Postsynaptic densities (PSDs), presynaptic ribbons, and synaptic vesicle sizes were all larger on the modiolar side of the IHCs from GluA3WT, but not GluA3KO, demonstrating GluA3 is required for modiolar-pillar synapse differentiation. Presynaptic ribbons juxtaposed with postsynaptic GluA2/4 subunits were similar in quantity, however, lone ribbons were more frequent in GluA3KO and GluA2-lacking synapses were observed only in GluA3KO. GluA2 and GluA4 immunofluorescence volumes were smaller on the pillar side than the modiolar side in GluA3KO, despite increased pillar-side PSD size. Overall, the fluorescent puncta volumes of GluA2 and GluA4 were smaller in GluA3KO than GluA3WT. However, GluA3KO contained less GluA2 and greater GluA4 immunofluorescence intensity relative to GluA3WT (threefold greater mean GluA4:GluA2 ratio). Thus, GluA3 is essential in development, as germline disruption of Gria3 caused anatomical synapse pathology before cochlear output became symptomatic by ABR. We propose the hearing loss in older male GluA3KO mice results from progressive synaptopathy evident in 5-week-old mice as decreased abundance of GluA2 subunits and an increase in GluA2-lacking, GluA4-monomeric Ca2+-permeable AMPARs.

The Role of Zinc and NMDA Receptors in Autism Spectrum Disorders

NMDA-type glutamate receptors are critical for synaptic plasticity in the central nervous system. Their unique properties and age-dependent arrangement of subunit types underpin their role as a coincidence detector of pre- and postsynaptic activity during brain development and maturation. NMDAR function is highly modulated by zinc, which is co-released with glutamate and concentrates in postsynaptic spines. Both NMDARs and zinc have been strongly linked to autism spectrum disorders (ASDs), suggesting that NMDARs are an important player in the beneficial effects observed with zinc in both animal models and children with ASDs. Significant evidence is emerging that these beneficial effects occur via zinc-dependent regulation of SHANK proteins, which form the backbone of the postsynaptic density. For example, dietary zinc supplementation enhances SHANK2 or SHANK3 synaptic recruitment and rescues NMDAR deficits and hypofunction in Shank3^ex13-16-/- and Tbr1^+/- ASD mice. Across multiple studies, synaptic changes occur in parallel with a reversal of ASD-associated behaviours, highlighting the zinc-dependent regulation of NMDARs and glutamatergic synapses as therapeutic targets for severe forms of ASDs, either pre- or postnatally. The data from rodent models set a strong foundation for future translational studies in human cells and people affected by ASDs.

Hippo-released WWC1 facilitates AMPA receptor regulatory complexes for hippocampal learning

Learning and memory rely on changes in postsynaptic glutamergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type receptor (AMPAR) number, spatial organization, and function. The Hippo pathway component WW and C2 domain-containing protein 1 (WWC1) regulates AMPAR surface expression and impacts on memory performance. However, synaptic binding partners of WWC1 and its hierarchical position in AMPAR complexes are largely unclear. Using cell-surface proteomics in hippocampal tissue of Wwc1-deficient mice and by generating a hippocampus-specific interactome, we show that WWC1 is a major regulatory platform in AMPAR signaling networks. Under basal conditions, the Hippo pathway members WWC1 and large tumor-suppressor kinase (LATS) are associated, which might prevent WWC1 effects on synaptic proteins. Reduction of WWC1/LATS binding through a point mutation at WWC1 elevates the abundance of WWC1 in AMPAR complexes and improves hippocampal-dependent learning and memory. Thus, uncoupling of WWC1 from the Hippo pathway to AMPAR-regulatory complexes provides an innovative strategy to enhance synaptic transmission.

Expression and Interaction Proteomics of GluA1- and GluA3-Subunit-Containing AMPARs Reveal Distinct Protein Composition

The AMPA glutamate receptor (AMPAR) is the major type of synaptic excitatory ionotropic receptor in the brain. AMPARs have four different subunits, GluA1-4 (each encoded by different genes, Gria1, Gria2, Gria3 and Gria4), that can form distinct tetrameric assemblies. The most abundant AMPAR subtypes in the hippocampus are GluA1/2 and GluA2/3 heterotetramers. Each subtype contributes differentially to mechanisms of synaptic plasticity, which may be in part caused by how these receptors are regulated by specific associated proteins. A broad range of AMPAR interacting proteins have been identified, including the well-studied transmembrane AMPA receptor regulatory proteins TARP-γ2 (also known as Stargazin) and TARP-γ8, Cornichon homolog 2 (CNIH-2) and many others. Several interactors were shown to affect biogenesis, AMPAR trafficking, and channel properties, alone or in distinct assemblies, and several revealed preferred binding to specific AMPAR subunits. To date, a systematic specific interactome analysis of the major GluA1/2 and GluA2/3 AMPAR subtypes separately is lacking. To reveal interactors belonging to specific AMPAR subcomplexes, we performed both expression and interaction proteomics on hippocampi of wildtype and Gria1- or Gria3 knock-out mice. Whereas GluA1/2 receptors co-purified TARP-γ8, synapse differentiation-induced protein 4 (SynDIG4, also known as Prrt1) and CNIH-2 with highest abundances, GluA2/3 receptors revealed strongest co-purification of CNIH-2, TARP-γ2, and Noelin1 (or Olfactomedin-1). Further analysis revealed that TARP-γ8-SynDIG4 interact directly and co-assemble into an AMPAR subcomplex especially at synaptic sites. Together, these data provide a framework for further functional analysis into AMPAR subtype specific pathways in health and disease.

Alteration in the Synaptic and Extrasynaptic Organization of AMPA Receptors in the Hippocampus of P301S Tau Transgenic Mice

Tau pathology is a hallmark of Alzheimer's disease (AD) and other tauopathies, but how pathological tau accumulation alters the glutamate receptor dynamics driving synaptic dysfunction is unclear. Here, we determined the impact of tau pathology on AMPAR expression, density, and subcellular distribution in the hippocampus of P301S mice using immunoblot, histoblot, and quantitative SDS-digested freeze-fracture replica labeling (SDS-FRL). Histoblot and immunoblot showed differential regulation of GluA1 and GluA2 in the hippocampus of P301S mice. The GluA2 subunit was downregulated in the hippocampus at 3 months while both GluA1 and GluA2 subunits were downregulated at 10 months. However, the total amount of GluA1-4 was similar in P301S mice and in age-matched wild-type mice. Using quantitative SDS-FRL, we unraveled the molecular organization of GluA1-4 in various synaptic connections at a high spatial resolution on pyramidal cell spines and interneuron dendrites in the CA1 field of the hippocampus in 10-month-old P301S mice. The labeling density for GluA1-4 in the excitatory synapses established on spines was significantly reduced in P301S mice, compared to age-matched wild-type mice, in the strata radiatum and lacunosum-moleculare but unaltered in the stratum oriens. The density of synaptic GluA1-4 established on interneuron dendrites was significantly reduced in P301S mice in the three strata. The labeling density for GluA1-4 at extrasynaptic sites was significantly reduced in several postsynaptic compartments of CA1 pyramidal cells and interneurons in the three dendritic layers in P301S mice. Our data demonstrate that the progressive accumulation of phospho-tau is associated with alteration of AMPARs on the surface of different neuron types, including synaptic and extrasynaptic membranes, leading to a decline in the trafficking and synaptic transmission, thereby likely contributing to the pathological events taking place in AD.

Glutamate receptor-interacting protein 1 in D1- and D2-dopamine receptor-expressing medium spiny neurons differentially regulates cocaine acquisition, reinstatement, and associated spine plasticity

BACKGROUND

The nucleus accumbens (NAc) is involved in the expression of cocaine addictive phenotypes, including acquisition, extinction, and reinstatement. In the NAc, D1-medium spiny neurons (MSNs) encode cocaine reward, whereas D2-MSNs encode aversive responses in drug addiction. Glutamate receptor-interacting protein 1 (GRIP1) is known to be associated with cocaine addiction, but the role of GRIP1 in D1-MSNs and D2-MSNs of the NAc in cocaine acquisition and reinstatement remains unknown.

METHODS

A conditioned place preference apparatus was used to establish cocaine acquisition, extinction, and reinstatement in mouse models. GRIP1 expression was evaluated using Western blotting. Furthermore, GRIP1-siRNA and GRIP1 overexpression lentivirus were used to interfere with GRIP1 in the NAc. After the behavioral test, green fluorescent protein immunostaining of brain slices was used to detect spine density.

RESULTS

GRIP1 expression decreased during cocaine acquisition and reinstatement. GRIP1-siRNA enhanced cocaine-induced CPP behavior in acquisition and reinstatement and regulated associated spine plasticity. Importantly, the decreased GRIP1 expression that mediated cocaine acquisition and reinstatement was mainly driven by the interference of the GRIP1-GluA2 interaction in D1-MSNs and could be blocked by the interference of the GRIP1-GluA2 interaction in D2-MSNs. Interference with the GRIP1-GluA2 interaction in D1- and D2-MSNs decreased spine density in D1- and D2-MSNs, respectively.

CONCLUSION

GRIP1 in D1- and D2-MSNs of the NAc differentially modulates cocaine acquisition and reinstatement. GRIP1 downregulation in D1-MSNs has a positive effect on cocaine acquisition and reinstatement, while GRIP1 downregulation in D2-MSNs has a negative effect. Additionally, GRIP1 downregulation in D1-MSNs plays a leading role in cocaine acquisition and reinstatement.

Glutamate Receptor Interacting Protein 1 in the Dorsal CA1 Drives Alpha-amino-3-hydroxy-5-methyl-4-Isoxazolepropionic Acid Receptor Endocytosis and Exocytosis Bidirectionally and Mediates Forgetting, Exploratory, and Anxiety-like Behavior

Endocytosis of GluA2-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) in CA1 of the hippocampus regulates forgetting; deficits in forgetting nociceptive memory can induce serious stress disorders. As a transporter of GluA2-containing AMPAR, the functions of glutamate receptor interacting protein 1 (GRIP1) in forgetting and possible stress responses remain unclear. Lentivirus-mediated interference of GRIP1 expression or function in the dorsal CA1 of the hippocampus in mice indicated that GRIP1 overexpression enhanced spatial memory, impaired forgetting in a Barnes maze, and induced anxiety-like behavior in the open field and elevated plus-maze test. In contrast, GRIP1 knockdown impaired learning capacity. Furthermore, inhibition of the PDZ2 and PDZ4/5 domains of GRIP1 and GluA2-dn enhanced learning capacity, whereas GluA2-dn impaired spatial memory; inhibition of the PDZ2 and PDZ4/5 domains of GRIP1 also decreased forgetting capacity to some extent. Importantly, inhibition of both the PDZ2 and PDZ4/5 domains of GRIP1 induced anxiety-like behavior but not GluA2-dn. Furthermore, optogenetic control of both GluA1 and GluA2 insertion into the postsynaptic membrane impaired memory and induced anxiety-like behavior. In vitro experiments showed that GRIP1-ov and -dn, inhibition of PDZ2 and PDZ4/5 domains of GRIP1, and GluA2-dn decreased glycine-induced GluA1 and GluA2 exocytosis; meanwhile, GRIP1-ov and -dn, and interference of PDZ2 and PDZ4/5 domains of GRIP1 blocked AMPA- and NMDA-induced GluA1 and GluA2 endocytosis. Overall, these results suggest that GRIP1 drives AMPA receptor endocytosis and exocytosis bidirectionally; furthermore, GRIP1-induced stabilization of anchoring postsynaptic GluA1 and GluA2 impairs forgetting and induces anxiety-like behavior. GRIP1 may provide a potential therapeutic target in posttraumatic syndromes and anxiety disorders.

Serine Racemase Expression Differentiates Aging from Alzheimer's Brain

Aging is an inevitable process characterized by progressive loss of physiological integrity and increased susceptibility to cancer, diabetes, cardiovascular, and neurodegenerative diseases; aging is the primary risk factor for Alzheimer's disease (AD), the most common cause of dementia. AD is characterized by brain pathology, including extracellular deposition of amyloid aggregation and intracellular accumulation of neurofibrillary tangles composed of hyperphosphorylated tau protein. In addition, losses of synapses and a wide range of neurons are pivotal pathologies in the AD brain. Accumulating evidence demonstrates hypoactivation of hippocampal neural networks in the aging brain, whereas AD-related mild cognitive impairment (AD-MCI) begins with hyperactivation, followed by a diminution of hippocampal activity as AD develops. The biphasic trends of the activity of the hippocampal neural network are consistent with the alteration of N-methyl-D-aspartate receptor (NMDA-R) activity from aging to prodromal (AD-MCI) to mid-/late stage AD. D-serine, a product of racemization catalyzed by serine racemase (SR), is an important co-agonist of the NMDA-R which is involved in synaptic events including neurotransmission, synaptogenesis, long-term potentiation (LTP), development, and excitotoxicity. SR and D-serine are decreased in the hippocampus of the aging brain, correlating with impairment of cognitive function. By contrast, SR is increased in AD brain, which is associated with a greater degree of cognitive dysfunction. Emerging studies suggest that D-serine levels in the brain or in cerebral spinal fluid from AD patients are higher than in age-matched controls, but the results are inconsistent. Very recently, serum D-serine levels in AD were reported to correlate with sex and clinical dementia rating (CDR) stage. This review will discuss alterations of NMDA-R and SR in aging and AD brain, and the mechanisms underlying the differential regulation of SR will be probed. Collectively, we propose that SR may be a molecular switch that distinguishes the effects of aging from those of AD on the brain.

The Yin and Yang of GABAergic and Glutamatergic Synaptic Plasticity: Opposites in Balance by Crosstalking Mechanisms

Synaptic plasticity is a critical process that regulates neuronal activity by allowing neurons to adjust their synaptic strength in response to changes in activity. Despite the high proximity of excitatory glutamatergic and inhibitory GABAergic postsynaptic zones and their functional integration within dendritic regions, concurrent plasticity has historically been underassessed. Growing evidence for pathological disruptions in the excitation and inhibition (E/I) balance in neurological and neurodevelopmental disorders indicates the need for an improved, more "holistic" understanding of synaptic interplay. There continues to be a long-standing focus on the persistent strengthening of excitation (excitatory long-term potentiation; eLTP) and its role in learning and memory, although the importance of inhibitory long-term potentiation (iLTP) and depression (iLTD) has become increasingly apparent. Emerging evidence further points to a dynamic dialogue between excitatory and inhibitory synapses, but much remains to be understood regarding the mechanisms and extent of this exchange. In this mini-review, we explore the role calcium signaling and synaptic crosstalk play in regulating postsynaptic plasticity and neuronal excitability. We examine current knowledge on GABAergic and glutamatergic synapse responses to perturbances in activity, with a focus on postsynaptic plasticity induced by short-term pharmacological treatments which act to either enhance or reduce neuronal excitability via ionotropic receptor regulation in neuronal culture. To delve deeper into potential mechanisms of synaptic crosstalk, we discuss the influence of synaptic activity on key regulatory proteins, including kinases, phosphatases, and synaptic structural/scaffolding proteins. Finally, we briefly suggest avenues for future research to better understand the crosstalk between glutamatergic and GABAergic synapses.

The Shaping of AMPA Receptor Surface Distribution by Neuronal Activity

Neurotransmission is critically dependent on the number, position, and composition of receptor proteins on the postsynaptic neuron. Of these, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are responsible for the majority of postsynaptic depolarization at excitatory mammalian synapses following glutamate release. AMPARs are continually trafficked to and from the cell surface, and once at the surface, AMPARs laterally diffuse in and out of synaptic domains. Moreover, the subcellular distribution of AMPARs is shaped by patterns of activity, as classically demonstrated by the synaptic insertion or removal of AMPARs following the induction of long-term potentiation (LTP) and long-term depression (LTD), respectively. Crucially, there are many subtleties in the regulation of AMPARs, and exactly how local and global synaptic activity drives the trafficking and retention of synaptic AMPARs of different subtypes continues to attract attention. Here we will review how activity can have differential effects on AMPAR distribution and trafficking along with its subunit composition and phosphorylation state, and we highlight some of the controversies and remaining questions. As the AMPAR field is extensive, to say the least, this review will focus primarily on cellular and molecular studies in the hippocampus. We apologise to authors whose work could not be cited directly owing to space limitations.

Dysfunction of Glutamatergic Synaptic Transmission in Depression: Focus on AMPA Receptor Trafficking

Pharmacological and anatomical evidence suggests that abnormal glutamatergic neurotransmission may be associated with the pathophysiology of depression. Compounds that act as NMDA receptor antagonists may be a potential treatment for depression, notably the rapid-acting agent ketamine. The rapid-acting and sustained antidepressant effects of ketamine rely on the activation of AMPA receptors (AMPARs). As the key elements of fast excitatory neurotransmission in the brain, AMPARs are crucially involved in synaptic plasticity and memory. Recent efforts have been directed toward investigating the bidirectional dysregulation of AMPAR-mediated synaptic transmission in depression. Here, we summarize the published evidence relevant to the dysfunction of AMPAR in stress conditions and review the recent progress toward the understanding of the involvement of AMPAR trafficking in the pathophysiology of depression, focusing on the roles of AMPAR auxiliary subunits, key AMPAR-interacting proteins, and posttranslational regulation of AMPARs. We also discuss new prospects for the development of improved therapeutics for depression.

Molecular and cellular mechanisms of spastin in neural development and disease (Review)

Spastin is a microtubule (MT)‑severing enzyme identified from mutations of hereditary spastic paraplegia in 1999 and extensive studies indicate its vital role in various cellular activities. In the past two decades, efforts have been made to understand the underlying molecular mechanisms of how spastin is linked to neural development and disease. Recent studies on spastin have unraveled the mechanistic processes of its MT‑severing activity and revealed that spastin acts as an MT amplifier to mediate its remodeling, thus providing valuable insight into the molecular roles of spastin under physiological conditions. In addition, recent research has revealed multiple novel molecular mechanisms of spastin in cellular biological pathways, including endoplasmic reticulum shaping, calcium trafficking, fatty acid trafficking, as well as endosomal fission and trafficking. These processes are closely involved in axonal and dendritic development and maintenance. The current review presents recent biological advances regarding the molecular mechanisms of spastin at the cellular level and provides insight into how it affects neural development and disease.

AMPA Receptors: A Key Piece in the Puzzle of Memory Retrieval

Retrieval constitutes a highly regulated and dynamic phase in memory processing. Its rapid temporal scales require a coordinated molecular chain of events at the synaptic level that support transient memory trace reactivation. AMPA receptors (AMPAR) drive the majority of excitatory transmission in the brain and its dynamic features match the singular fast timescales of memory retrieval. Here we provide a review on AMPAR contribution to memory retrieval regarding its dynamic movements along the synaptic compartments, its changes in receptor number and subunit composition that take place in activity dependent processes associated with retrieval. We highlight on the differential regulations exerted by AMPAR subunits in plasticity processes and its impact on memory recall.

Kainate and AMPA receptors in epilepsy: Cell biology, signalling pathways and possible crosstalk

Epilepsy is caused when rhythmic neuronal network activity escapes normal control mechanisms, resulting in seizures. There is an extensive and growing body of evidence that the onset and maintenance of epilepsy involves alterations in the trafficking, synaptic surface expression and signalling of kainate and AMPA receptors (KARs and AMPARs). The KAR subunit GluK2 and AMPAR subunit GluA2 are key determinants of the properties of their respective assembled receptors. Both subunits are subject to extensive protein interactions, RNA editing and post-translational modifications. In this review we focus on the cell biology of GluK2-containing KARs and GluA2-containing AMPARs and outline how their regulation and dysregulation is implicated in, and affected by, seizure activity. Further, we discuss role of KARs in regulating AMPAR surface expression and plasticity, and the relevance of this to epilepsy. This article is part of the special issue on 'Glutamate Receptors - Kainate receptors'.

Interaction and Subcellular Association of PRRT1/SynDIG4 With AMPA Receptors

AMPA receptors (AMPAR) are organized into supramolecular complexes in association with other membrane proteins that provide exquisite regulation of their biophysical properties and subcellular trafficking. Proline-rich transmembrane protein 1 (PRRT1), also named as SynDIG4, is a component of native AMPAR complexes in multiple brain regions. Deletion of PRRT1 leads to altered surface levels and phosphorylation status of AMPARs, as well as impaired forms of synaptic plasticity. Here, we have investigated the mechanisms underlying the observed regulation of AMPARs by investigating the interaction properties and subcellular localization of PRRT1. Our results show that PRRT1 can interact physically with all AMPAR subunits GluA1-GluA4. We decipher the membrane topology of PRRT1 to find that contrary to the predicted dual membrane pass, only the second hydrophobic segment spans the membrane completely, and is involved in mediating the interaction with AMPARs. We also report a physical interaction of PRRT1 with phosphatase PP2B that dephosphorylates AMPARs during synaptic plasticity. Our co-localization analysis in primary neuronal cultures identifies that PRRT1 associates with AMPARs extrasynaptically where it localizes to early and recycling endosomes as well as to the plasma membrane. These findings advance the understanding of the mechanisms by which PRRT1 regulates AMPARs under basal conditions and during synaptic plasticity.

Genetic manipulations of AMPA glutamate receptors in hippocampal synaptic plasticity

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are the principal mediators of fast excitatory synaptic transmission and they are required for various forms of synaptic plasticity, including long-term potentiation (LTP) and depression (LTD), which are key mechanisms of learning and memory. AMPARs are tetrameric complexes assembled from four subunits (GluA1-4), however, the lack of subunit-specific pharmacological tools has made the assessment of individual subunits difficult. The application of genetic techniques, particularly gene targeting, allows for precise manipulation and dissection of each subunit in the regulation of neuronal function and behaviour. In this review, we summarize studies using various mouse models with genetically altered AMPARs and focus on their roles in basal synaptic transmission, LTP, and LTD at the hippocampal CA1 synapse. These studies provide strong evidence that there are multiple forms of LTP and LTD at this synapse which can be induced by various induction protocols, and they are differentially regulated by different AMPAR subunits and domains. We conclude that it is necessary to delineate the mechanism of each of these forms of plasticity and their contribution to memory and brain disorders.

Comparative Analysis of Multiple Neurodegenerative Diseases Based on Advanced Epigenetic Aging Brain

Background: Neurodegenerative Diseases (NDs) are age-dependent and include Alzheimer’s disease (AD), Parkinson’s disease (PD), progressive supranuclear palsy (PSP), frontotemporal dementia (FTD), and so on. There have been numerous studies showing that accelerated aging is closely related (even the driver of) ND, thus promoting imbalances in cellular homeostasis. However, the mechanisms of how different ND types are related/triggered by advanced aging are still unclear. Therefore, there is an urgent need to explore the potential markers/mechanisms of different ND types based on aging acceleration at a system level.

Methods: AD, PD, PSP, FTD, and aging markers were identified by supervised machine learning methods. The aging acceleration differential networks were constructed based on the aging score. Both the enrichment analysis and sensitivity analysis were carried out to investigate both common and specific mechanisms among different ND types in the context of aging acceleration.

Results: The extracellular fluid, cellular metabolisms, and inflammatory response were identified as the common driving factors of cellular homeostasis imbalances during the accelerated aging process. In addition, Ca ion imbalance, abnormal protein depositions, DNA damage, and cytoplasmic DNA in macrophages were also revealed to be special mechanisms that further promote AD, PD, PSP, and FTD, respectively.

Conclusion: The accelerated epigenetic aging mechanisms of different ND types were integrated and compared through our computational pipeline.

Rabies virus glycoprotein enhances spatial memory via the PDZ binding motif

Rabies is a life-threatening viral infection of the brain. Rabies virus (RABV) merely infects excitable cells including neurons provoking drastic behaviors including negative emotional memories. RABV glycoprotein (RVG) plays a critical role in RABV pathogenesis. RVG interacts with various cytoplasmic PDZ (PSD-95/Dlg/ZO-1) containing proteins through its PDZ binding motif (PBM). PTZ domains have crucial role in formation and function of signal transduction. Hippocampus is one of the cerebral regions that contain high load of viral antigens. We examined impact of RVG expression in the dorsal hippocampus on aversive as well as spatial learning and memory performance in rats. Two microliter of the lentiviral vector (~10⁸ T.U./ml) encoding RVG or ∆RVG (deleted PBM) genomes was microinjected into the hippocampal CA1. After 1 week, rat's brain was cross-sectioned and RVG/∆RVG-expressing neuronal cells were confirmed by fluorescent microscopy. Passive avoidance and spatial learning and memory were assessed in rats by Shuttle box and Morris water maze (MWM). In the shuttle box, both RVG and ∆RVG decreased the time spent in the dark compartment compared to control (p < 0.05). In MWM, RVG and ∆RVG did not affect the acquisition of spatial task. In the probe test, RVG-expressing rats spent more time in the target quadrant, and also reached the platform position sooner than control group (p < 0.05). Rats expressing ∆RVG significantly swam farther from the hidden platform than RVG group (p < 0.05). Our data indicate RVG expression in the hippocampus strengthens aversive and spatial learning and memory performance. The boosting effect on spatial but not avoidance memory is mediated through PBM.

EphrinB2 and GRIP1 stabilize mushroom spines during denervation-induced homeostatic plasticity

Despite decades of work, much remains elusive about molecular events at the interplay between physiological and structural changes underlying neuronal plasticity. Here, we combined repetitive live imaging and expansion microscopy in organotypic brain slice cultures to quantitatively characterize the dynamic changes of the intracellular versus surface pools of GluA2-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) across the different dendritic spine types and the shaft during hippocampal homeostatic plasticity. Mechanistically, we identify ephrinB2 and glutamate receptor interacting protein (GRIP) 1 as mediating AMPAR relocation to the mushroom spine surface following lesion-induced denervation. Moreover, stimulation with the ephrinB2 specific receptor EphB4 not only prevents the lesion-induced disappearance of mushroom spines but is also sufficient to shift AMPARs to the surface and rescue spine recovery in a GRIP1 dominant-negative background. Thus, our results unravel a crucial role for ephrinB2 during homeostatic plasticity and identify a potential pharmacological target to improve dendritic spine plasticity upon injury.

Roles of palmitoylation in structural long-term synaptic plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are important cellular mechanisms underlying learning and memory processes. N-Methyl-d-aspartate receptor (NMDAR)-dependent LTP and LTD play especially crucial roles in these functions, and their expression depends on changes in the number and single channel conductance of the major ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) located on the postsynaptic membrane. Structural changes in dendritic spines comprise the morphological platform and support for molecular changes in the execution of synaptic plasticity and memory storage. At the molecular level, spine morphology is directly determined by actin cytoskeleton organization within the spine and indirectly stabilized and consolidated by scaffold proteins at the spine head. Palmitoylation, as a uniquely reversible lipid modification with the ability to regulate protein membrane localization and trafficking, plays significant roles in the structural and functional regulation of LTP and LTD. Altered structural plasticity of dendritic spines is also considered a hallmark of neurodevelopmental disorders, while genetic evidence strongly links abnormal brain function to impaired palmitoylation. Numerous studies have indicated that palmitoylation contributes to morphological spine modifications. In this review, we have gathered data showing that the regulatory proteins that modulate the actin network and scaffold proteins related to AMPAR-mediated neurotransmission also undergo palmitoylation and play roles in modifying spine architecture during structural plasticity.

The palmitoyl acyltransferase ZDHHC14 controls Kv1-family potassium channel clustering at the axon initial segment

The palmitoyl acyltransferase (PAT) ZDHHC14 is highly expressed in the hippocampus and is the only PAT predicted to bind Type-I PDZ domain-containing proteins. However, ZDHHC14’s neuronal roles are unknown. Here, we identify the PDZ domain-containing Membrane-associated Guanylate Kinase (MaGUK) PSD93 as a direct ZDHHC14 interactor and substrate. PSD93, but not other MaGUKs, localizes to the axon initial segment (AIS). Using lentiviral-mediated shRNA knockdown in rat hippocampal neurons, we find that ZDHHC14 controls palmitoylation and AIS clustering of PSD93 and also of Kv1 potassium channels, which directly bind PSD93. Neurodevelopmental expression of ZDHHC14 mirrors that of PSD93 and Kv1 channels and, consistent with ZDHHC14’s importance for Kv1 channel clustering, loss of ZDHHC14 decreases outward currents and increases action potential firing in hippocampal neurons. To our knowledge, these findings identify the first neuronal roles and substrates for ZDHHC14 and reveal a previously unappreciated role for palmitoylation in control of neuronal excitability.

Glutamatergic Receptor Trafficking and Delivery: Role of the Exocyst Complex

Cells comprise several intracellular membrane compartments that allow them to function properly. One of these functions is cargo movement, typically proteins and membranes within cells. These cargoes ride microtubules through vesicles from Golgi and recycling endosomes to the plasma membrane in order to be delivered and exocytosed. In neurons, synaptic functions employ this cargo trafficking to maintain inter-neuronal communication optimally. One of the complexes that oversee vesicle trafficking and tethering is the exocyst. The exocyst is a protein complex containing eight subunits first identified in yeast and then characterized in multicellular organisms. This complex is related to several cellular processes, including cellular growth, division, migration, and morphogenesis, among others. It has been associated with glutamatergic receptor trafficking and tethering into the synapse, providing the molecular machinery to deliver receptor-containing vesicles into the plasma membrane in a constitutive manner. In this review, we discuss the evidence so far published regarding receptor trafficking and the exocyst complex in both basal and stimulated levels, comparing constitutive trafficking and long-term potentiation-related trafficking.

AMPA Receptor Surface Expression Is Regulated by S-Nitrosylation of Thorase and Transnitrosylation of NSF

The regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking affects multiple brain functions, such as learning and memory. We have previously shown that Thorase plays an important role in the internalization of AMPARs from the synaptic membrane. Here, we show that N-methyl-d-aspartate receptor (NMDAR) activation leads to increased S-nitrosylation of Thorase and N-ethylmaleimide-sensitive factor (NSF). S-nitrosylation of Thorase stabilizes Thorase-AMPAR complexes and enhances the internalization of AMPAR and interaction with protein-interacting C kinase 1 (PICK1). S-nitrosylated NSF is dependent on the S-nitrosylation of Thorase via trans-nitrosylation, which modulates the surface insertion of AMPARs. In the presence of the S-nitrosylation-deficient C137L Thorase mutant, AMPAR trafficking, long-term potentiation, and long-term depression are impaired. Overall, our data suggest that both S-nitrosylation and interactions of Thorase and NSF/PICK1 are required to modulate AMPAR-mediated synaptic plasticity. This study provides critical information that elucidates the mechanism underlying Thorase and NSF-mediated trafficking of AMPAR complexes.

GRIP1 regulates synaptic plasticity and learning and memory

Hebbian plasticity is a key mechanism for higher brain functions, such as learning and memory. This form of synaptic plasticity primarily involves the regulation of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) abundance and properties, whereby AMPARs are inserted into synapses during long-term potentiation (LTP) or removed during long-term depression (LTD). The molecular mechanisms underlying AMPAR trafficking remain elusive, however. Here we show that glutamate receptor interacting protein 1 (GRIP1), an AMPAR-binding protein shown to regulate the trafficking and synaptic targeting of AMPARs, is required for LTP and learning and memory. GRIP1 is recruited into synapses during LTP, and deletion of Grip1 in neurons blocks synaptic AMPAR accumulation induced by glycine-mediated depolarization. In addition, Grip1 knockout mice exhibit impaired hippocampal LTP, as well as deficits in learning and memory. Mechanistically, we find that phosphorylation of serine-880 of the GluA2 AMPAR subunit (GluA2-S880) is decreased while phosphorylation of tyrosine-876 on GluA2 (GluA2-Y876) is elevated during chemically induced LTP. This enhances the strength of the GRIP1-AMPAR association and, subsequently, the insertion of AMPARs into the postsynaptic membrane. Together, these results demonstrate an essential role of GRIP1 in regulating AMPAR trafficking during synaptic plasticity and learning and memory.

Subunit-Specific Augmentation of AMPA Receptor Ubiquitination by Phorbol Ester

Excitatory neurotransmission relies on the precise targeting of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors to the neuronal plasma membrane. Activity-dependent ubiquitination of AMPA receptor (AMPAR) subunits sorts internalised receptors to late endosomes for degradation, which ultimately determines the number of AMPARs on neuronal membrane. Our recent study has demonstrated a functional cross-talk between the phosphorylation and ubiquitination of the GluA1 subunit in mammalian central neurons. However, the existence of such a cross modulation for the GluA2 subunit remains unknown. Here, we have shown that bicuculline induced GluA2 ubiquitination on the same lysine residues (Lys-870 and Lys-882) in the C-terminal as those elicited by the AMPA treatment. Interestingly, bicuculline-induced ubiquitination was markedly enhanced by the phospho-mimetic GluA2 S880E mutant. Pharmacological activation of protein kinase C (PKC) by phorbol ester, which mediates the phosphorylation of GluA2 at Ser-880, augmented bicuculline-induced ubiquitination of GluA2 in cultured neurons. This effect was specific for the GluA2 subunit because phorbol ester did not alter the level of GluA1 ubiquitination. However, phorbol ester-induced enhancement of GluA2 ubiquitination did not require Ser-880 phosphorylation. This suggests that pseudo-phosphorylation of Ser-880 is sufficient but is not necessary for the augmentation of bicuculline-induced GluA2 ubiquitination. Collectively, these data provide the first demonstration of subunit-specific modulation of AMPAR ubiquitination by the PKC-dependent signalling pathway in mammalian central neurons.

The biophysical, molecular, and anatomical landscape of pigeon CRY4: A candidate light-based quantal magnetosensor

The biophysical and molecular mechanisms that enable animals to detect magnetic fields are unknown. It has been proposed that birds have a light-dependent magnetic compass that relies on the formation of radical pairs within cryptochrome molecules. Using spectroscopic methods, we show that pigeon cryptochrome clCRY4 is photoreduced efficiently and forms long-lived spin-correlated radical pairs via a tetrad of tryptophan residues. We report that clCRY4 is broadly and stably expressed within the retina but enriched at synapses in the outer plexiform layer in a repetitive manner. A proteomic survey for retinal-specific clCRY4 interactors identified molecules that are involved in receptor signaling, including glutamate receptor-interacting protein 2, which colocalizes with clCRY4. Our data support a model whereby clCRY4 acts as an ultraviolet-blue photoreceptor and/or a light-dependent magnetosensor by modulating glutamatergic synapses between horizontal cells and cones.

Phosphorylation-Dependent Regulation of Ca2+-Permeable AMPA Receptors During Hippocampal Synaptic Plasticity

Experience-dependent learning and memory require multiple forms of plasticity at hippocampal and cortical synapses that are regulated by N-methyl-D-aspartate receptors (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptors (NMDAR, AMPAR). These plasticity mechanisms include long-term potentiation (LTP) and depression (LTD), which are Hebbian input-specific mechanisms that rapidly increase or decrease AMPAR synaptic strength at specific inputs, and homeostatic plasticity that globally scales-up or -down AMPAR synaptic strength across many or even all inputs. Frequently, these changes in synaptic strength are also accompanied by a change in the subunit composition of AMPARs at the synapse due to the trafficking to and from the synapse of receptors lacking GluA2 subunits. These GluA2-lacking receptors are most often GluA1 homomeric receptors that exhibit higher single-channel conductance and are Ca2+-permeable (CP-AMPAR). This review article will focus on the role of protein phosphorylation in regulation of GluA1 CP-AMPAR recruitment and removal from hippocampal synapses during synaptic plasticity with an emphasis on the crucial role of local signaling by the cAMP-dependent protein kinase (PKA) and the Ca2+calmodulin-dependent protein phosphatase 2B/calcineurin (CaN) that is coordinated by the postsynaptic scaffold protein A-kinase anchoring protein 79/150 (AKAP79/150).

Lamina-specific AMPA receptor dynamics following visual deprivation in vivo

Regulation of AMPA receptor (AMPAR) expression is central to synaptic plasticity and brain function, but how these changes occur in vivo remains elusive. Here, we developed a method to longitudinally monitor the expression of synaptic AMPARs across multiple cortical layers in awake mice using two-photon imaging. We observed that baseline AMPAR expression in individual spines is highly dynamic with more dynamics in primary visual cortex (V1) layer 2/3 (L2/3) neurons than V1 L5 neurons. Visual deprivation through binocular enucleation induces a synapse-specific and depth-dependent change of synaptic AMPARs in V1 L2/3 neurons, wherein deep synapses are potentiated more than superficial synapses. The increase is specific to L2/3 neurons and absent on apical dendrites of L5 neurons, and is dependent on expression of the AMPAR-binding protein GRIP1. Our study demonstrates that specific neuronal connections, across cortical layers and even within individual neurons, respond uniquely to changes in sensory experience.

Smart motors and cargo steering drive kinesin-mediated selective transport

Neurons are polarized cells, with dendrites and axons that require different complements of membrane proteins to fulfill their specialized functions. Membrane proteins are synthesized in the somatodendritic domain and delivered to their target membranes via long-range vesicle transport. Most anterograde vesicle transport is mediated by kinesin motors, but it is unclear how kinesins are targeted to axons or dendrites. Two main models have been proposed to explain kinesin selectivity. In the smart motor model, kinesin selectivity is conferred by a preference of the kinesin motor domain for specific subsets of microtubules. In the cargo steering model, kinesin selectivity is modulated by the vesicular cargo to which the motor is bound. We evaluate the evidence for both models and conclude that while the smart motor model may explain axonal selectivity, cargo steering is required for dendritic selectivity. Future work will determine the relative contributions of these models to polarized transport in living neurons.

Modified Glutamatergic Postsynapse in Neurodegenerative Disorders

The postsynaptic density (PSD) is a complex subcellular domain important for postsynaptic signaling, function, and plasticity. The PSD is present at excitatory synapses and specialized to allow for precise neuron-to-neuron transmission of information. The PSD is localized immediately underneath the postsynaptic membrane forming a major protein network that regulates postsynaptic signaling and synaptic plasticity. Glutamatergic synaptic dysfunction affecting PSD morphology and signaling events have been described in many neurodegenerative disorders, either sporadic or familial forms. Thus, in this review we describe the main protein players forming the PSD and their activity, as well as relevant modifications in key components of the postsynaptic architecture occurring in Huntington's, Parkinson's and Alzheimer's diseases.

Prediction of Protein-Protein Interactions Based on Domain

Protein-protein interactions (PPIs) play a crucial role in various biological processes. To better comprehend the pathogenesis and treatments of various diseases, it is necessary to learn the detail of these interactions. However, the current experimental method still has many false-positive and false-negative problems. Computational prediction of protein-protein interaction has become a more important prediction method which can overcome the obstacles of the experimental method. In this work, we proposed a novel computational domain-based method for PPI prediction, and an SVM model for the prediction was built based on the physicochemical property of the domain. The outcomes of SVM and the domain-domain score were used to construct the prediction model for protein-protein interaction. The predicted results demonstrated the domain-based research can enhance the ability to predict protein interactions.

Glutamate receptor interacting protein acts within the prefrontal cortex to blunt cocaine seeking

Glutamate receptor interacting protein (GRIP) is a neuronal scaffolding protein that anchors GluA2-containing AMPA receptors to the cell membrane. GRIP plays a critical role in activity-dependent synaptic plasticity, including that which occurs after drug exposure. Given that cocaine administration alters glutamate receptor trafficking within the prefrontal cortex (PFC), a better understanding of the role of receptor trafficking proteins could lead to a more complete understanding of addictive phenotypes. AMPA receptor trafficking in general, and GRIP specifically, is known to play a role in cocaine seeking and conditioned reward in the nucleus accumbens, but its role in the PFC has not been characterized. The current study demonstrates that conditional deletion of GRIP1 in the medial prefrontal cortex increases the motivation for cocaine and potentiates cue-induced reinstatement of cocaine seeking in male and female mice. As no effects of PFC GRIP1 deletion were seen in reinstatement of food seeking, strategy set-shifting, or reversal learning the effects on cocaine seeking are not related to generalized alterations in cognitive function. While disrupting GRIP1 might be expected to lead to decreased AMPA transmission, our electrophysiological data indicate an increase in sEPSC amplitude in the prefrontal cortex and a corresponding decrease in paired pulse facilitation in the nucleus accumbens. Taken together this suggests a strengthening of the PFC to NAc input following prefrontal GRIP1 deletion that may mediate the enhanced drug seeking behavior.

AMPA receptors and their minions: auxiliary proteins in AMPA receptor trafficking

To correctly transfer information, neuronal networks need to continuously adjust their synaptic strength to extrinsic stimuli. This ability, termed synaptic plasticity, is at the heart of their function and is, thus, tightly regulated. In glutamatergic neurons, synaptic strength is controlled by the number and function of AMPA receptors at the postsynapse, which mediate most of the fast excitatory transmission in the central nervous system. Their trafficking to, at, and from the synapse, is, therefore, a key mechanism underlying synaptic plasticity. Intensive research over the last 20 years has revealed the increasing importance of interacting proteins, which accompany AMPA receptors throughout their lifetime and help to refine the temporal and spatial modulation of their trafficking and function. In this review, we discuss the current knowledge about the roles of key partners in regulating AMPA receptor trafficking and focus especially on the movement between the intracellular, extrasynaptic, and synaptic pools. We examine their involvement not only in basal synaptic function, but also in Hebbian and homeostatic plasticity. Included in our review are well-established AMPA receptor interactants such as GRIP1 and PICK1, the classical auxiliary subunits TARP and CNIH, and the newest additions to AMPA receptor native complexes.

Palmitoylation-mediated synaptic regulation of AMPA receptor trafficking and function

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a major glutamate-gated ion channel in the brain and is important for synaptic transmission, synaptic plasticity, and learning. Palmitoylation, a post-translational modification, is a critical process regulating AMPAR trafficking, synaptic function and plasticity, and learning and memory in health and diseases. In this review, we discuss current knowledge on the palmitoylation-dependent regulation of AMPAR trafficking and functions. We focus on the palmitoylation of AMPARs and other synaptic proteins that directly or indirectly interact with AMPARs, including postsynaptic density 95, glutamate receptor-interacting protein/AMPAR-binding protein, A-kinase anchoring protein 79/150, and protein interacting with C kinase 1. Finally, we discuss what future studies should address in the field of palmitoylation-dependent AMPAR trafficking and function with regard to physiology and neurodegenerative diseases.

Protein Kinase C in the Cerebellum: Its Significance and Remaining Conundrums

Protein kinase C (PKC), a family of serine/threonine protein kinases, mediates a myriad of patho-physiological cellular events in various tissues. The originally discovered PKC (conventional) requires the binding of diacylglycerol and Ca²⁺ for full activation. The conventional PKC consists of four isoforms, PKCα, PKCβI/βII, and PKCγ. PKCα and PKCβI/βII are expressed in the cells of various tissues including the brain, while PKCγ is present specifically in neurons of the brain and spinal cord. The cerebellum expresses the largest amount of PKC with all its four isoforms. Purkinje cells express PKCα and PKCγ. Previous studies have shown that PKCα is involved in the induction of long-term depression (LTD) at parallel fiber-Purkinje cell synapses. On the other hand, analysis of PKCγ-deficient mice has revealed that PKCγ plays a critical role in eliminating supernumerary climbing fiber synapses from developing Purkinje cells. Although why PKCα has no compensatory action in climbing fiber pruning in PKCγ-deficient Purkinje cells had so far remained unclear, we have recently demonstrated that PKCα is also capable of pruning supernumerary climbing fiber synapses, but the expression levels of PKCα are too low to achieve pruning in PKCγ-null Purkinje cells. Notably, although PKCγ is most abundant in Purkinje cells, its physiological role in mature Purkinje cells remained totally unknown. In addition to a concise review of the physiological and pathological roles of conventional PKCs in Purkinje cells, this report postulates a contribution of PKCα in developing Purkinje cells and a possible involvement of PKCγ in motor coordination in the mature cerebellum.

Mechanisms of postsynaptic localization of AMPA-type glutamate receptors and their regulation during long-term potentiation

l-Glutamate is the main excitatory neurotransmitter in the brain, with postsynaptic responses to its release predominantly mediated by AMPA-type glutamate receptors (AMPARs). A critical component of synaptic plasticity involves changes in the number of responding postsynaptic receptors, which are dynamically recruited to and anchored at postsynaptic sites. Emerging findings continue to shed new light on molecular mechanisms that mediate AMPAR postsynaptic trafficking and localization. Accordingly, unconventional secretory trafficking of AMPARs occurs in dendrites, from the endoplasmic reticulum (ER) through the ER-Golgi intermediary compartment directly to recycling endosomes, independent of the Golgi apparatus. Upon exocytosis, AMPARs diffuse in the plasma membrane to reach the postsynaptic site, where they are trapped to contribute to transmission. This trapping occurs through a combination of both intracellular interactions, such as TARP (transmembrane AMPAR regulatory protein) binding to α-actinin-stabilized PSD-95, and extracellular interactions through the receptor amino-terminal domain. These anchoring mechanisms may facilitate precise receptor positioning with respect to glutamate release sites to enable efficient synaptic transmission.

Phosphorylation of the AMPAR-TARP Complex in Synaptic Plasticity

Synaptic plasticity has been considered a key mechanism underlying many brain functions including learning, memory, and drug addiction. An increase or decrease in synaptic activity of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) complex mediates the phenomena as shown in the cellular models of synaptic plasticity, long-term potentiation (LTP), and depression (LTD). In particular, protein phosphorylation shares the spotlight in expressing the synaptic plasticity. This review summarizes the studies on phosphorylation of the AMPAR pore-forming subunits and auxiliary proteins including transmembrane AMPA receptor regulatory proteins (TARPs) and discusses its role in synaptic plasticity.

Recent Findings on AMPA Receptor Recycling

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPA-Rs) are tetrameric protein complexes that mediate most of the fast-excitatory transmission in response to the neurotransmitter glutamate in neurons. The abundance of AMPA-Rs at the surface of excitatory synapses establishes the strength of the response to glutamate. It is thus evident that neurons need to tightly regulate this feature, particularly in the context of all synaptic plasticity events, which are considered the biological correlates of higher cognitive functions such as learning and memory. AMPA-R levels at the synapse are regulated by insertion of newly synthesized receptors, lateral diffusion on the plasma membrane and endosomal cycling. The latter is likely the most important especially for synaptic plasticity. This process starts with the endocytosis of the receptor from the cell surface and is followed by either degradation, if the receptor is directed to the lysosomal compartment, or reinsertion at the cell surface through a specialized endosomal compartment called recycling endosomes. Although the basic steps of this process have been discovered, the details and participation of additional regulatory proteins are still being discovered. In this review article, we describe the most recent findings shedding light on this crucial mechanism of synaptic regulation.