NMDA receptor surface trafficking and synaptic subunit composition are developmentally regulated by the extracellular matrix protein Reelin

Reelin Deficiency and Synaptic Impairment in the Adolescent Prefrontal Cortex Following Initial Synthetic Cannabinoid Exposure

Background

Adolescent cannabinoid exposure can have long-lasting effects on the brain, particularly in the prefrontal cortex, where the reelin protein plays a crucial role in neural organization. Chronic cannabinoid exposure leads to reelin deficiency and behavioral abnormalities, but the underlying mechanisms remain unclear. With the increasing use of synthetic cannabinoids (SCs) among young people, understanding these effects is crucial.

Methods

We examined the cellular and synaptic consequences of initial SC exposure in adolescent male mice 1 day after a single in vivo exposure to WIN 55,212-2. Our approach combined immunohistochemistry, Western blots, conditional CB1 receptor (CB1R) knockout mouse lines, quantitative polymerase chain reaction, and ex vivo electrophysiology to investigate the effects of SC on reelin expression and synaptic plasticity. Additionally, single-molecule fluorescent in situ hybridization profiling was used to identify cellular coexpression patterns of reelin and CB1Rs.

Results

Our findings indicate that a single exposure to SC decreased reelin expression in specific prefrontal cortex layers accompanied by disrupted proteolytic fragmentation but not changes in messenger RNA expression. Single-molecule fluorescent in situ hybridization profiling revealed a strong coexpression of CB1R and reelin. Furthermore, our pharmacological and genetic approaches demonstrated that CB1Rs in GABAergic (gamma-aminobutyric acidergic) neurons mediate the SC-induced decrease in reelin. This decrease in reelin results in a reduction in long-term potentiation, phenocopying reelin haploinsufficient mice. Notably, we restored long-term potentiation by infusing reelin bilaterally, establishing a functional link between reelin depletion and synaptic deficits.

Conclusions

These findings provide new insights into the neural consequences of adolescent cannabinoid consumption and highlight the critical role of reelin in the cellular mechanisms associated with SC initiation during adolescence.

Regulatory mechanism of Reelin activity: a platform for exploiting Reelin as a therapeutic agent

Reelin is a secreted glycoprotein that was initially investigated in the field of neuronal development. However, in recent decades, its role in the adult brain has become increasingly important, and it is now clear that diminished Reelin function is involved in the pathogenesis and progression of neuropsychiatric and neurodegenerative disorders, including schizophrenia and Alzheimer's disease (AD). Reelin activity is regulated at multiple steps, including synthesis, posttranslational modification, secretion, oligomerization, proteolytic processing, and interactions with extracellular molecules. Moreover, the differential use of two canonical receptors and the presence of non-canonical receptors and co-receptors add to the functional diversity of Reelin. In this review, I summarize recent findings on the molecular mechanisms of Reelin activity. I also discuss possible strategies to enhance Reelin's function. A complete understanding of Reelin function and its regulatory mechanisms in the adult central nervous system could help ameliorate neuropsychiatric and neurodegenerative disorders.

Glutamate signaling and neuroligin/neurexin adhesion play opposing roles that are mediated by major histocompatibility complex I molecules in cortical synapse formation

Although neurons release neurotransmitter before contact, the role for this release in synapse formation remains unclear. Cortical synapses do not require synaptic vesicle release for formation 1-4 , yet glutamate clearly regulates glutamate receptor trafficking 5,6 and induces spine formation 7-11 . Using a culture system to dissect molecular mechanisms, we found that glutamate rapidly decreases synapse density specifically in young cortical neurons in a local and calcium-dependent manner through decreasing NMDAR transport and surface expression as well as co-transport with neuroligin (NL1). Adhesion between NL1 and neurexin 1 protects against this glutamate-induced synapse loss. Major histocompatibility I (MHCI) molecules are required for the effects of glutamate in causing synapse loss through negatively regulating NL1 levels. Thus, like acetylcholine at the NMJ, glutamate acts as a dispersal signal for NMDARs and causes rapid synapse loss unless opposed by NL1-mediated trans-synaptic adhesion. Together, glutamate, MHCI and NL1 mediate a novel form of homeostatic plasticity in young neurons that induces rapid changes in NMDARs to regulate when and where nascent glutamatergic synapses are formed.

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.

Molecular Mechanisms of Reelin in the Enteric Nervous System and the Microbiota-Gut-Brain Axis: Implications for Depression and Antidepressant Therapy

Current pharmacological treatments for depression fail to produce adequate remission in a significant proportion of patients. Increasingly, other systems, such as the microbiome-gut-brain axis, are being looked at as putative novel avenues for depression treatment. Dysbiosis and dysregulation along this axis are highly comorbid with the severity of depression symptoms. The endogenous extracellular matrix protein reelin is present in all intestinal layers as well as in myenteric and submucosal ganglia, and its receptors are also present in the gut. Reelin secretion from subepithelial myofibroblasts regulates cellular migration along the crypt-villus axis in the small intestine and colon. Reelin brain expression is downregulated in mood and psychotic disorders, and reelin injections have fast antidepressant-like effects in animal models of depression. This review seeks to discuss the roles of reelin in the gastrointestinal system and propose a putative role for reelin actions in the microbiota-gut-brain axis in the pathogenesis and treatment of depression, primarily reflecting on alterations in gut epithelial cell renewal and in the clustering of serotonin transporters.

The Inflammation-Induced Dysregulation of Reelin Homeostasis Hypothesis of Alzheimer's Disease

Alzheimer's disease (AD) accounts for most dementia cases, but we lack a complete understanding of the mechanisms responsible for the core pathology associated with the disease (e.g., amyloid plaque and neurofibrillary tangles). Inflammation has been identified as a key contributor of AD pathology, with recent evidence pointing towards Reelin dysregulation as being associated with inflammation. Here we describe Reelin signaling and outline existing research involving Reelin signaling in AD and inflammation. Research is described pertaining to the inflammatory and immunological functions of Reelin before we propose a mechanism through which inflammation renders Reelin susceptible to dysregulation resulting in the induction and exacerbation of AD pathology. Based on this hypothesis, it is predicted that disorders of both inflammation (including peripheral inflammation and neuroinflammation) and Reelin dysregulation (including disorders associated with upregulated Reelin expression and disorders of Reelin downregulation) have elevated risk of developing AD. We conclude with a description of AD risk in various disorders involving Reelin dysregulation and inflammation.

Reelin Signaling and Synaptic Plasticity in Schizophrenia

Recent research emphasizes the significance of studying the quality of life of schizophrenia patients, considering the complex nature of the illness. Identifying neuronal markers for early diagnosis and treatment is crucial. Reelin (RELN) stands out among these markers, with genetic studies highlighting its role in mental health. Suppression of RELN expression may contribute to cognitive deficits by limiting dendritic proliferation, affecting neurogenesis, and leading to improper neuronal circuits. Although the physiological function of reelin is not fully understood, it plays a vital role in hippocampal cell stratification and neuroglia formation. This analysis explores reelin's importance in the nervous system, shedding light on its impact on mental disorders such as schizophrenia, paving the way for innovative therapeutic approaches, and at the same time, raises the following conclusions: increased methylation levels of the RELN gene in patients with a diagnosis of schizophrenia results in a multiple decrease in the expression of reelin, and monitoring of this indicator, i.e., methylation levels, can be used to monitor the severity of symptoms in the course of schizophrenia.

The role of integrin beta in schizophrenia: a preliminary exploration

Integrins are transmembrane heterodimeric (αβ) receptors that transduce mechanical signals between the extracellular milieu and the cell in a bidirectional manner. Extensive research has shown that the integrin beta (β) family is widely expressed in the brain and that they control various aspects of brain development and function. Schizophrenia is a relatively common neurological disorder of unknown etiology and has been found to be closely related to neurodevelopment and neurochemicals in neuropathological studies of schizophrenia. Here, we review literature from recent years that shows that schizophrenia involves multiple signaling pathways related to neuronal migration, axon guidance, cell adhesion, and actin cytoskeleton dynamics, and that dysregulation of these processes affects the normal function of neurons and synapses. In fact, alterations in integrin β structure, expression and signaling for neural circuits, cortex, and synapses are likely to be associated with schizophrenia. We explored several aspects of the possible association between integrin β and schizophrenia in an attempt to demonstrate the role of integrin β in schizophrenia, which may help to provide new insights into the study of the pathogenesis and treatment of schizophrenia.

Natural antioxidant formula ameliorates lipopolysaccharide-induced impairment of hippocampal neurogenesis and contextual fear memory through suppression of neuroinflammation in rats

This study investigated the ameliorating effects of a natural antioxidant formula (NAF) consisting of Ginkgo biloba leaf extract, docosahexaenoic acid/eicosapentaenoic acid, ferulic acid, flaxseed oil, vitamin E, and vitamin B12 on a lipopolysaccharide (LPS)-induced cognitive dysfunction model in rats. Six-week-old rats received a diet containing 0.5% (w/w) NAF for 38 days from Day 1, and LPS (1 mg/kg body weight) was administered intraperitoneally once daily on Days 8 and 10. On Day 11, LPS alone increased interleukin-1β and tumor necrosis factor-α in the hippocampus and cerebral cortex and the numbers of M1-type microglia/macrophages and GFAP+ reactive astrocytes in the hilus of the hippocampal dentate gyrus. NAF treatment decreased brain proinflammatory cytokine levels and increased the number of M2-type microglia/macrophages. During Days 34-38, LPS alone impaired fear memory acquisition and the extinction learning process, and NAF facilitated fear extinction learning. On Day 38, LPS alone decreased the number of type-3 neural progenitor cells in the hippocampal neurogenic niche, and NAF restored the number of type-3 neural progenitor cells and increased the numbers of both immature granule cells in the neurogenic niche and reelin+ hilar interneurons. Thus, NAF exhibited anti-inflammatory effects and ameliorated LPS-induced adverse effects on hippocampal neurogenesis and fear memory learning, possibly through amplification of reelin signaling by hilar interneurons. These results suggest that neuroinflammation is a key factor in the development of LPS-induced impairment of fear memory learning, and supplementation with NAF in the present study helped to prevent hippocampal neurogenesis and disruptive neurobehaviors caused by neuroinflammation.

GluN3A subunit tunes NMDA receptor synaptic trafficking and content during postnatal brain development

Signaling via N-methyl-d-aspartate receptors (NMDARs) is critical for the maturation of glutamatergic synapses, partly through a developmental switch from immature synapses expressing primarily GluN2B- and GluN3A-containing subtypes to GluN2A-rich mature ones. This subunit switch is thought to underlie the synaptic stabilization of NMDARs necessary for neural network consolidation. However, the cellular mechanisms controlling the NMDAR exchange remain unclear. Using a combination of single-molecule and confocal imaging and biochemical and electrophysiological approaches, we show that surface GluN3A-NMDARs form a highly diffusive receptor pool that is loosely anchored to synapses. Remarkably, changes in GluN3A subunit expression selectively alter the surface diffusion and synaptic anchoring of GluN2A- but not GluN2B-NMDARs, possibly through altered interactions with cell surface receptors. The effects of GluN3A on NMDAR surface diffusion are restricted to an early time window of postnatal development in rodents, allowing GluN3A subunits to control the timing of NMDAR signaling maturation and neuronal network refinements.

NMDA receptor functions in health and disease: Old actor, new dimensions

N-Methyl-D-aspartate ionotropic glutamate receptors (NMDARs) play key roles in synaptogenesis, synaptic maturation, long-term plasticity, neuronal network activity, and cognition. Mirroring this wide range of instrumental functions, abnormalities in NMDAR-mediated signaling have been associated with numerous neurological and psychiatric disorders. Thus, identifying the molecular mechanisms underpinning the physiological and pathological contributions of NMDAR has been a major area of investigation. Over the past decades, a large body of literature has flourished, revealing that the physiology of ionotropic glutamate receptors cannot be restricted to fluxing ions, and involves additional facets controlling synaptic transmissions in health and disease. Here, we review newly discovered dimensions of postsynaptic NMDAR signaling supporting neural plasticity and cognition, such as the nanoscale organization of NMDAR complexes, their activity-dependent redistributions, and non-ionotropic signaling capacities. We also discuss how dysregulations of these processes may directly contribute to NMDAR-dysfunction-related brain diseases.

Quantifying postsynaptic receptor dynamics: insights into synaptic function

The molecular composition of presynaptic and postsynaptic neuronal terminals is dynamic, and yet long-term stabilizations in postsynaptic responses are necessary for synaptic development and long-term plasticity. The need to reconcile these concepts is further complicated by learning- and memory-related plastic changes in the molecular make-up of synapses. Advances in single-particle tracking mean that we can now quantify the number and diffusive properties of specific synaptic molecules, while statistical thermodynamics provides a framework to analyse these molecular fluctuations. In this Review, we discuss the use of these approaches to gain quantitative descriptions of the processes underlying the turnover, long-term stability and plasticity of postsynaptic receptors and show how these can help us to understand the balance between local molecular turnover and synaptic structural identity and integrity.

Adult-specific Reelin expression alters striatal neuronal organization: implications for neuropsychiatric disorders

In addition to neuronal migration, brain development, and adult plasticity, the extracellular matrix protein Reelin has been extensively implicated in human psychiatric disorders such as schizophrenia, bipolar disorder, and autism spectrum disorder. Moreover, heterozygous reeler mice exhibit features reminiscent of these disorders, while overexpression of Reelin protects against its manifestation. However, how Reelin influences the structure and circuits of the striatal complex, a key region for the above-mentioned disorders, is far from being understood, especially when altered Reelin expression levels are found at adult stages. In the present study, we took advantage of complementary conditional gain- and loss-of-function mouse models to investigate how Reelin levels may modify adult brain striatal structure and neuronal composition. Using immunohistochemical techniques, we determined that Reelin does not seem to influence the striatal patch and matrix organization (studied by μ-opioid receptor immunohistochemistry) nor the density of medium spiny neurons (MSNs, studied with DARPP-32). We show that overexpression of Reelin leads to increased numbers of striatal parvalbumin- and cholinergic-interneurons, and to a slight increase in tyrosine hydroxylase-positive projections. We conclude that increased Reelin levels might modulate the numbers of striatal interneurons and the density of the nigrostriatal dopaminergic projections, suggesting that these changes may be involved in the protection of Reelin against neuropsychiatric disorders.

Specific contribution of Reelin expressed by Cajal-Retzius cells or GABAergic interneurons to cortical lamination

The extracellular protein Reelin, expressed by Cajal-Retzius (CR) cells at early stages of cortical development and at late stages by GABAergic interneurons, regulates radial migration and the "inside-out" pattern of positioning. Current models of Reelin functions in corticogenesis focus on early CR cell-derived Reelin in layer I. However, developmental disorders linked to Reelin deficits, such as schizophrenia and autism, are related to GABAergic interneuron-derived Reelin, although its role in migration has not been established. Here we selectively inactivated the Reln gene in CR cells or GABAergic interneurons. We show that CR cells have a major role in the inside-out order of migration, while CR and GABAergic cells sequentially cooperate to prevent invasion of cortical neurons into layer I. Furthermore, GABAergic cell-derived Reelin compensates some features of the reeler phenotype and is needed for the fine tuning of the layer-specific distribution of cortical neurons. In the hippocampus, the inactivation of Reelin in CR cells causes dramatic alterations in the dentate gyrus and mild defects in the hippocampus proper. These findings lead to a model in which both CR and GABAergic cell-derived Reelin cooperate to build the inside-out order of corticogenesis, which might provide a better understanding of the mechanisms involved in the pathogenesis of neuropsychiatric disorders linked to abnormal migration and Reelin deficits.

Amyloid β / PKC-dependent alterations in NMDA receptor composition are detected in early stages of Alzheimer´s disease

Amyloid beta (Aβ)-mediated synapse dysfunction is an early event in Alzheimer’s disease (AD) pathogenesis and previous studies suggest that NMDA receptor (NMDAR) dysregulation may contribute to these pathological effects. Although Aβ peptides impair NMDAR expression and activity, the mechanisms mediating these alterations in the early stages of AD are unclear. Here, we observed that NMDAR subunit NR2B and PSD-95 levels were aberrantly upregulated and correlated with Aβ₄₂ load in human postsynaptic fractions of the prefrontal cortex in early stages of AD patients, as well as in the hippocampus of 3xTg-AD mice. Importantly, NR2B and PSD95 dysregulation was revealed by an increased expression of both proteins in Aβ-injected mouse hippocampi. In cultured neurons, Aβ oligomers increased the NR2B-containing NMDAR density in neuronal membranes and the NMDA-induced intracellular Ca²⁺ increase, in addition to colocalization in dendrites of NR2B subunit and PSD95. Mechanistically, Aβ oligomers required integrin β1 to promote synaptic location and function of NR2B-containing NMDARs and PSD95 by phosphorylation through classic PKCs. These results provide evidence that Aβ oligomers modify the contribution of NR2B to NMDAR composition and function in the early stages of AD through an integrin β1 and PKC-dependent pathway. These data reveal a novel role of Aβ oligomers in synaptic dysfunction that may be relevant to early-stage AD pathogenesis.

New Strategies for the Treatment of Neuropsychiatric Disorders Based on Reelin Dysfunction

Reelin is an extracellular matrix protein that is mainly produced in Cajal-Retzius cells and controls neuronal migration, which is important for the proper formation of cortical layers in the developmental stage of the brain. In the adult brain, Reelin plays a crucial role in the regulation of N-methyl-D-aspartate receptor-dependent synaptic function, and its expression decreases postnatally. Clinical studies showed reductions in Reelin protein and mRNA expression levels in patients with psychiatric disorders; however, the causal relationship remains unclear. Reelin-deficient mice exhibit an abnormal neuronal morphology and behavior, while Reelin supplementation ameliorates learning deficits, synaptic dysfunctions, and spine loss in animal models with Reelin deficiency. These findings suggest that the neuronal deficits and brain dysfunctions associated with the down-regulated expression of Reelin are attenuated by enhancements in its expression and functions in the brain. In this review, we summarize findings on the role of Reelin in neuropsychiatric disorders and discuss potential therapeutic approaches for neuropsychiatric disorders associated with Reelin dysfunctions.

Microglia as hackers of the matrix: sculpting synapses and the extracellular space

Microglia shape the synaptic environment in health and disease, but synapses do not exist in a vacuum. Instead, pre- and postsynaptic terminals are surrounded by extracellular matrix (ECM), which together with glia comprise the four elements of the contemporary tetrapartite synapse model. While research in this area is still just beginning, accumulating evidence points toward a novel role for microglia in regulating the ECM during normal brain homeostasis, and such processes may, in turn, become dysfunctional in disease. As it relates to synapses, microglia are reported to modify the perisynaptic matrix, which is the diffuse matrix that surrounds dendritic and axonal terminals, as well as perineuronal nets (PNNs), specialized reticular formations of compact ECM that enwrap neuronal subsets and stabilize proximal synapses. The interconnected relationship between synapses and the ECM in which they are embedded suggests that alterations in one structure necessarily affect the dynamics of the other, and microglia may need to sculpt the matrix to modify the synapses within. Here, we provide an overview of the microglial regulation of synapses, perisynaptic matrix, and PNNs, propose candidate mechanisms by which these structures may be modified, and present the implications of such modifications in normal brain homeostasis and in disease.

Microinjection of Reelin into the mPFC prevents MK-801-induced recognition memory impairment in mice

Reelin, a large extracellular matrix protein, helps to regulate neuronal plasticity and cognitive function. Several studies have shown that Reelin dysfunction, resulting from factors such as mutations in gene RELN or low Reelin expression, is associated with schizophrenia (SCZ). We previously reported that microinjection of Reelin into cerebral ventricle prevents phencyclidine-induced cognitive and sensory-motor gating deficits. However, it remains unclear whether and how Reelin ameliorates behavioral abnormalities in the animal model of SCZ. In the present study, we evaluated the effect of recombinant Reelin microinjection into the medial prefrontal cortex (mPFC) on abnormal behaviors induced by MK-801, an N-methyl-D-aspartate receptor antagonist. Microinjection of Reelin into the mPFC prevented impairment of recognition memory of MK-801-treated mice in the novel object recognition test (NORT). On the other hand, the same treatment had no effect on deficits in sensory-motor gating and short-term memory in the pre-pulse inhibition and Y-maze tests, respectively. To establish the neural substrates that respond to Reelin, the number of c-Fos-positive cells in the mPFC was determined. A significant increase in c-Fos-positive cells in the mPFC of MK-801-treated mice was observed when compared with saline-treated mice, and this change was suppressed by microinjection of Reelin into the mPFC. A K2360/2467A Reelin that cannot bind to its receptor failed to ameliorate MK-801-induced cognitive deficits in NORT. These results suggest that Reelin prevents MK-801-induced recognition memory impairment by acting on its receptors to suppress neural activity in the mPFC of mice.

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

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

Reelin Regulates Neuronal Excitability through Striatal-Enriched Protein Tyrosine Phosphatase (STEP61) and Calcium Permeable AMPARs in an NMDAR-Dependent Manner

Alzheimer's disease (AD) is a progressive neurodegenerative disease marked by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles. Aβ oligomers cause synaptic dysfunction early in AD by enhancing long-term depression (LTD; a paradigm for forgetfulness) via metabotropic glutamate receptor (mGluR)-dependent regulation of striatal-enriched tyrosine phosphatase (STEP61). Reelin is a neuromodulator that signals through ApoE (apolipoprotein E) receptors to protect the synapse against Aβ toxicity (Durakoglugil et al., 2009) Reelin signaling is impaired by ApoE4, the most important genetic risk factor for AD, and Aβ-oligomers activate metabotropic glutamate receptors (Renner et al., 2010). We therefore asked whether Reelin might also affect mGluR-LTD. To this end, we induced chemical mGluR-LTD using DHPG (Dihydroxyphenylglycine), a selective mGluR5 agonist. We found that exogenous Reelin reduces the DHPG-induced increase in STEP61, prevents the dephosphorylation of GluA2, and concomitantly blocks mGluR-mediated LTD. By contrast, Reelin deficiency increased expression of Ca2+-permeable GluA2-lacking AMPA receptors along with higher STEP61 levels, resulting in occlusion of DHPG-induced LTD in hippocampal CA1 neurons. We propose a model in which Reelin modulates local protein synthesis as well as AMPA receptor subunit composition through modulation of mGluR-mediated signaling with implications for memory consolidation or neurodegeneration.SIGNIFICANCE STATEMENT Reelin is an important neuromodulator, which in the adult brain controls synaptic plasticity and protects against neurodegeneration. Amyloid-β has been shown to use mGluRs to induce synaptic depression through endocytosis of NMDA and AMPA receptors, a mechanism referred to as LTD, a paradigm of forgetfulness. Our results show that Reelin regulates the phosphatase STEP, which plays an important role in neurodegeneration, as well as the expression of calcium-permeable AMPA receptors, which play a role in memory formation. These data suggest that Reelin uses mGluR LTD pathways to regulate memory formation as well as neurodegeneration.

Synaptic and Genetic Bases of Impaired Motor Learning Associated with Modified Experience-Dependent Cortical Plasticity in Heterozygous Reeler Mutants

Patients with neurodevelopmental disorders show impaired motor skill learning. It is unclear how the effect of genetic variation on synaptic function and transcriptome profile may underlie experience-dependent cortical plasticity, which supports the development of fine motor skills. RELN (reelin) is one of the genes implicated in neurodevelopmental psychiatric vulnerability. Heterozygous reeler mutant (HRM) mice displayed impairments in reach-to-grasp learning, accompanied by less extensive cortical map reorganization compared with wild-type mice, examined after 10 days of training by intracortical microstimulation. Assessed by patch-clamp recordings after 3 days of training, the training induced synaptic potentiation and increased glutamatergic-transmission of cortical layer III pyramidal neurons in wild-type mice. In contrast, the basal excitatory and inhibitory synaptic functions were depressed, affected both by presynaptic and postsynaptic impairments in HRM mice; and thus, no further training-induced synaptic plasticity occurred. HRM exhibited downregulations of cortical synaptophysin, immediate-early gene expressions, and gene enrichment, in response to 3 days of training compared with trained wild-type mice, shown using quantitative reverse transcription polymerase chain reaction, immunohistochemisty, and RNA-sequencing. We demonstrated that motor learning impairments associated with modified experience-dependent cortical plasticity are at least partially attributed by the basal synaptic alternation as well as the aberrant early experience-induced gene enrichment in HRM.

Nanoscale synapse organization and dysfunction in neurodevelopmental disorders

Neurodevelopmental disorders such as those linked to intellectual disabilities or autism spectrum disorder are thought to originate in part from genetic defects in synaptic proteins. Single gene mutations linked to synapse dysfunction can broadly be separated in three categories: disorders of transcriptional regulation, disorders of synaptic signaling and disorders of synaptic scaffolding and structures. The recent developments in super-resolution imaging technologies and their application to synapses have unraveled a complex nanoscale organization of synaptic components. On the one hand, part of receptors, adhesion proteins, ion channels, scaffold elements and the pre-synaptic release machinery are partitioned in subsynaptic nanodomains, and the respective organization of these nanodomains has tremendous impact on synaptic function. For example, pre-synaptic neurotransmitter release sites are partly aligned with nanometer precision to postsynaptic receptor clusters. On the other hand, a large fraction of synaptic components is extremely dynamic and constantly exchanges between synaptic domains and extrasynaptic or intracellular compartments. It is largely the combination of the exquisitely precise nanoscale synaptic organization of synaptic components and their high dynamic that allows the rapid and profound regulation of synaptic function during synaptic plasticity processes that underlie adaptability of brain function, learning and memory. It is very tempting to speculate that genetic defects that lead to neurodevelopmental disorders and target synaptic scaffolds and structures mediate their deleterious impact on brain function through perturbing synapse nanoscale dynamic organization. We discuss here how applying super-resolution imaging methods in models of neurodevelopmental disorders could help in addressing this question.

Selective Inactivation of Reelin in Inhibitory Interneurons Leads to Subtle Changes in the Dentate Gyrus But Leaves Cortical Layering and Behavior Unaffected

Reelin is an extracellular matrix protein, known for its dual role in neuronal migration during brain development and in synaptic plasticity at adult stages. During the perinatal phase, Reelin expression switches from Cajal-Retzius (CR) cells, its main source before birth, to inhibitory interneurons (IN), the main source of Reelin in the adult forebrain. IN-derived Reelin has been associated with schizophrenia and temporal lobe epilepsy; however, the functional role of Reelin from INs is presently unclear. In this study, we used conditional knockout mice, which lack Reelin expression specifically in inhibitory INs, leading to a substantial reduction in total Reelin expression in the neocortex and dentate gyrus. Our results show that IN-specific Reelin knockout mice exhibit normal neuronal layering and normal behavior, including spatial reference memory. Although INs are the major source of Reelin within the adult stem cell niche, Reelin from INs does not contribute substantially to normal adult neurogenesis. While a closer look at the dentate gyrus revealed some unexpected alterations at the cellular level, including an increase in the number of Reelin expressing CR cells, overall our data suggest that Reelin derived from INs is less critical for cortex development and function than Reelin expressed by CR cells.

Regulation of membrane NMDA receptors by dynamics and protein interactions

Understanding neurotransmitter system crosstalk in the brain is a major challenge in neurobiology. Several intracellular and genomic cascades have been identified in this crosstalk. However, the discovery that neurotransmitter receptors are highly diffusive in the plasma membrane of neurons, where they form heterocomplexes with other proteins, has profoundly changed our view of neurotransmitter signaling. Here, we review new insights into neurotransmitter crosstalk at the plasma membrane. We focus on the membrane organization and interactome of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR) that plays a central role in excitatory synaptic and network physiology and is involved in the etiology of several major neuropsychiatric disorders. The nanoscale organization and dynamics of NMDAR is a key regulatory process for glutamate synapse transmission, plasticity, and crosstalk with other neurotransmitter systems, such as the monoaminergic ones. The plasma membrane appears to be a prime regulatory compartment for spatial and temporal crosstalk between neurotransmitter systems in the healthy and diseased brain. Understanding the molecular mechanisms regulating membrane neurotransmitter receptor crosstalk will likely open research avenues for innovative therapeutical strategies.

Increased Callosal Connectivity in Reeler Mice Revealed by Brain-Wide Input Mapping of VIP Neurons in Barrel Cortex

The neocortex is composed of layers. Whether layers constitute an essential framework for the formation of functional circuits is not well understood. We investigated the brain-wide input connectivity of vasoactive intestinal polypeptide (VIP) expressing neurons in the reeler mouse. This mutant is characterized by a migration deficit of cortical neurons so that no layers are formed. Still, neurons retain their properties and reeler mice show little cognitive impairment. We focused on VIP neurons because they are known to receive strong long-range inputs and have a typical laminar bias toward upper layers. In reeler, these neurons are more dispersed across the cortex. We mapped the brain-wide inputs of VIP neurons in barrel cortex of wild-type and reeler mice with rabies virus tracing. Innervation by subcortical inputs was not altered in reeler, in contrast to the cortical circuitry. Numbers of long-range ipsilateral cortical inputs were reduced in reeler, while contralateral inputs were strongly increased. Reeler mice had more callosal projection neurons. Hence, the corpus callosum was larger in reeler as shown by structural imaging. We argue that, in the absence of cortical layers, circuits with subcortical structures are maintained but cortical neurons establish a different network that largely preserves cognitive functions.

N-Methyl-D-Aspartate (NMDA) receptor modulators: a patent review (2015-present)

INTRODUCTION

- The NMDA receptor is implicated in various diseases including neurodegenerative, neurodevelopmental and mood disorders. However, only a limited number of clinically approved NMDA receptor modulators are available. Today, apparent NMDA receptor drug development strategies entail 1) exploring the unknown chemical space to identify novel scaffolds; 2) using the clinically available NMDA receptor modulators to expand the therapeutic indication space; 3) and to trace physiological functions of the NMDA receptor.

AREAS COVERED

- The current review reflects on the functional and pharmacological facets of NMDA receptors and the current clinical status quo of NMDA receptor modulators. Patent literature covering 2015 till April 2020 is discussed with emphasis on new indications.

EXPERT OPINION

- Supporting evidence shows that subtype-selective NMDA receptor antagonists show an improved safety profile compared to broad-spectrum channel blockers. Although GluN2B-selective antagonists are by far the most extensively investigated subtype-selective modulators, they have shown only modest clinical efficacy so far. To overcome the limitations that have hampered the clinical development of previous subtype-selective NMDA receptor antagonists, future studies with improved animal models that better reflect human NMDA receptor pathophysiology are warranted. The increased availability of subtype-selective probes will allow target engagement studies and proper dose finding in future clinical trials.

Spinal Inhibition of GABAB Receptors by the Extracellular Matrix Protein Fibulin-2 in Neuropathic Rats

In the central nervous system, the inhibitory GABAB receptor is the archetype of heterodimeric G protein-coupled receptors (GPCRs). Receptor interaction with partner proteins has emerged as a novel mechanism to alter GPCR signaling in pathophysiological conditions. We propose here that GABAB activity is inhibited through the specific binding of fibulin-2, an extracellular matrix protein, to the B1a subunit in a rat model of neuropathic pain. We demonstrate that fibulin-2 hampers GABAB activation, presumably through decreasing agonist-induced conformational changes. Fibulin-2 regulates the GABAB-mediated presynaptic inhibition of neurotransmitter release and weakens the GABAB-mediated inhibitory effect in neuronal cell culture. In the dorsal spinal cord of neuropathic rats, fibulin-2 is overexpressed and colocalized with B1a. Fibulin-2 may thus interact with presynaptic GABAB receptors, including those on nociceptive afferents. By applying anti-fibulin-2 siRNA in vivo, we enhanced the antinociceptive effect of intrathecal baclofen in neuropathic rats, thus demonstrating that fibulin-2 limits the action of GABAB agonists in vivo. Taken together, our data provide an example of an endogenous regulation of GABAB receptor by extracellular matrix proteins and demonstrate its functional impact on pathophysiological processes of pain sensitization.

Integrin adhesion in brain assembly: From molecular structure to neuropsychiatric disorders

Integrins are extracellular matrix receptors that mediate biochemical and mechanical bi-directional signals between the extracellular and intracellular environment of a cell thanks to allosteric conformational changes. In the brain, they are found in both neurons and glial cells, where they play essential roles in several aspects of brain development and function, such as cell migration, axon guidance, synaptogenesis, synaptic plasticity and neuro-inflammation. Although there are many successful examples of how regulating integrin adhesion and signaling can be used for therapeutic purposes, for example for halting tumor progression, this is not the case for the brain, where the growing evidence of the importance of integrins for brain pathophysiology has not translated yet into medical applications. Here, we review recent literature showing how alterations in integrin structure, expression and signaling may be involved in the etiology of autism spectrum disorder, epilepsy, schizophrenia, addiction, depression and Alzheimer's disease. We focus on common mechanisms and recurrent signaling pathways, trying to bridge studies on the genetics and molecular structure of integrins with those on synaptic physiology and brain pathology. Further, we discuss integrin-targeting strategies and their potential benefits for therapeutic purposes in neuropsychiatric disorders.

Fyn Tyrosine Kinase as Harmonizing Factor in Neuronal Functions and Dysfunctions

Fyn is a non-receptor or cytoplasmatic tyrosine kinase (TK) belonging to the Src family kinases (SFKs) involved in multiple transduction pathways in the central nervous system (CNS) including synaptic transmission, myelination, axon guidance, and oligodendrocyte formation. Almost one hundred years after the original description of Fyn, this protein continues to attract extreme interest because of its multiplicity of actions in the molecular signaling pathways underlying neurodevelopmental as well as neuropathologic events. This review highlights and summarizes the most relevant recent findings pertinent to the role that Fyn exerts in the brain, emphasizing aspects related to neurodevelopment and synaptic plasticity. Fyn is a common factor in healthy and diseased brains that targets different proteins and shapes different transduction signals according to the neurological conditions. We will primarily focus on Fyn-mediated signaling pathways involved in neuronal differentiation and plasticity that have been subjected to considerable attention lately, opening the fascinating scenario to target Fyn TK for the development of potential therapeutic interventions for the treatment of CNS injuries and certain neurodegenerative disorders like Alzheimer’s disease.

Linking glutamate receptor movements and synapse function

Regulation of neurotransmitter receptor content at synapses is achieved through a dynamic equilibrium between biogenesis and degradation pathways, receptor stabilization at synaptic sites, and receptor trafficking in and out synapses. In the past 20 years, the movements of receptors to and from synapses have emerged as a series of highly regulated processes that mediate postsynaptic plasticity. Our understanding of the properties and roles of receptor movements has benefited from technological advances in receptor labeling and tracking capacities, as well as from new methods to interfere with their movements. Focusing on two key glutamatergic receptors, we review here our latest understanding of the characteristics of receptor movements and their role in tuning the efficacy of synaptic transmission in health and brain disease.

NMDA receptor C-terminal signaling in development, plasticity, and disease

The NMDA subtype of ionotropic glutamate receptor is a sophisticated integrator and transducer of information. NMDAR-mediated signals control diverse processes across the life course, including synaptogenesis and synaptic plasticity, as well as contribute to excitotoxic processes in neurological disorders. At the basic biophysical level, the NMDAR is a coincidence detector, requiring the co-presence of agonist, co-agonist, and membrane depolarization in order to open. However, the NMDAR is not merely a conduit for ions to flow through; it is linked on the cytoplasmic side to a large network of signaling and scaffolding proteins, primarily via the C-terminal domain of NMDAR GluN2 subunits. These physical interactions help to organize the signaling cascades downstream of NMDAR activation. Notably, the NMDAR does not come in a single form: the subunit composition of the NMDAR, particularly the GluN2 subunit subtype (GluN2A-D), influences the biophysical properties of the channel. Moreover, a growing number of studies have illuminated the extent to which GluN2 C-terminal interactions vary according to GluN2 subtype and how this impacts on the processes that NMDAR activity controls. We will review recent advances, controversies, and outstanding questions in this active area of research.

Quantitative Three-Dimensional Reconstructions of Excitatory Synaptic Boutons in the "Barrel Field" of the Adult "Reeler" Mouse Somatosensory Neocortex: A Comparative Fine-Scale Electron Microscopic Analysis with the Wild Type Mouse

Synapses are key structural determinants for information processing and computations in the normal and pathologically altered brain. Here, the quantitative morphology of excitatory synaptic boutons in the "reeler" mutant, a model system for various neurological disorders, was investigated and compared with wild-type (WT) mice using high-resolution, fine-scale electron microscopy (EM) and quantitative three-dimensional (3D) models of synaptic boutons. Beside their overall geometry, the shape and size of presynaptic active zones (PreAZs) and postsynaptic densities (PSDs) forming the active zones and the three pools of synaptic vesicles (SVs), namely the readily releasable pool (RRP), the recycling pool (RP), and the resting pool, were quantified. Although the reeler mouse neocortex is severely disturbed, no significant differences were found in most of the structural parameters investigated: the size of boutons (~3 μm2), size of the PreAZs and PSDs (~0.17 μm2), total number of SVs, and SVs within a perimeter (p) of 10 nm and p20 nm RRP; the p60 nm, p100 nm, and p60-p200 nm RP; and the resting pool, except the synaptic cleft width. Taken together, the synaptic organization and structural composition of synaptic boutons in the reeler neocortex remain comparably "normal" and may thus contribute to a "correct" wiring of neurons within the reeler cortical network.

Co-expression of glutamatergic and autism-related genes in the hippocampus of male mice with disturbances of social behavior

В настоящее время существует представление о вовлеченности глутаматергической системы (ГГ) в механизмы развития аутизма. В предыдущих исследованиях нами было показано, что негативный социальный опыт, приобретенный в ежедневных межсамцовых конфронтациях, приводит к нарушениям в социальном поведении: снижению коммуникативности, нарушению социализации, появлению стереотипных форм поведения, которые могут рассматриваться как симптомы аутистического спектра. В связи с этим целью нашей работы было изучение с помощью транскриптомного анализа изменений экспрессии генов, кодирующих белки, вовлеченные в функционирование глутаматергической системы, и генов, связанных с патологией аутизма (ГА), в гиппокампе. В эксперименте использовали животных с нарушениями социального поведения, вызванными повторным опытом социальных побед или поражений в ежедневных агонистических взаимодействиях. Для формирования групп животных с контрастными типами поведения использовали модель сенсорного контакта (хронического социального стресса). Полученные образцы мозга были секвенированы в ЗАО «Геноаналитика» (http://genoanalytica.ru/, Россия, Москва). Транскриптомный анализ показал, что у агрессивных животных снижается экспрессия генов Shank3, Auts2, Ctnnd2, Nrxn2, для которых показано участие в развитии аутизма, а также глутаматергического гена Grm4. В то же время у животных с негативным социальным опытом экспрессия ГА Shank2, Nlgn2, Ptcdh10, Reln, Arx возрастает. При этом ГГ (Grik3, Grm2, Grm4, Slc17a7, Slc1a4, Slc25a22), за исключением гена Grin2a, повышают свою экспрессию. Корреляционный анализ выявил статистически значимую взаимосвязь из- мененной экспрессии ГГ и ГА. Полученные результаты, с одной стороны, могут служить подтверждением участия ГГ в патофизиологии развития симптомов аутистического спектра, с другой – свидетельствовать о коэкспрессии ГГ и ГА в гиппокампе, развивающейся под влиянием социальной среды. Так как большинство ГА, изменивших экспрессию в настоящем исследовании, являются генами, связанными с клеточным скелетом и внеклеточным матриксом, в частности участвующими в формировании синапсов, а ГГ, изменившие свою экспрессию, – генами, кодирующими субъединицы рецепторов, то можно предположить, что вовлечение ГГ в патофизиологию аутизма происходит на уровне рецепторов.

Modulation of Hippocampal Gamma Oscillations by Dopamine in Heterozygous Reeler Mice in vitro

The reelin haploinsufficient heterozygous reeler mice (HRM), an animal model of schizophrenia, have altered mesolimbic dopaminergic pathways and share similar neurochemical and behavioral properties with patients with schizophrenia. Dysfunctional neural circuitry with impaired gamma (γ) oscillation (30–80 Hz) has been implicated in abnormal cognition in patients with schizophrenia. However, the function of neural circuitry in terms of γ oscillation and its modulation by dopamine (DA) has not been reported in HRM. In this study, first, we recorded γ oscillations in CA3 from wild-type mice (WTM) and HRM hippocampal slices, and we studied the effects of DA on γ oscillations. We found that there was no difference in γ power between WTM and HRM and that DA increased γ power of WTM but not HRM, suggesting that DA modulations of network oscillations in HRM are impaired. Second, we found that N-methyl-D-aspartate receptor (NMDAR) antagonist MK-801 itself increased γ power and occluded DA-mediated enhancement of γ power in WTM but partially restored DA modulation of γ oscillations in HRM. Third, inhibition of phosphatidylinositol 3-kinase (PI3K), a downstream molecule of NMDAR, increased γ power and blocked the effects of DA on γ oscillation in WTM and had no significant effect on γ power but largely restored DA modulation of γ oscillations in HRM. Our results reveal that impaired DA function in HRM is associated with dysregulated NMDAR–PI3K signaling, a mechanism that may be relevant in the pathology of schizophrenia.

A novel method using ambient glutamate for the electrophysiological quantification of extrasynaptic NMDA receptor function in acute brain slices

KEY POINTS

We present a novel protocol to quantify extrasynaptic NMDA receptor function utilizing the semi-selective activation of extrasynaptic receptors by ambient extracellular glutamate in acute brain slices from adult rats. We use whole cell patch clamp to measure the effect of the NMDA receptor antagonist MK-801 on both synaptic and brief, local agonist application-evoked responses. The level of ambient glutamate was estimated from tonic NMDA receptor activity to be ∼77 nM and an equivalent concentration of NMDA was used to estimate the degree of extrasynaptic blockade (>82%) by our MK-801 protocol. The extrasynaptic component of the total NMDA receptor pool can be mathematically derived from these data and was estimated to be 29-39% in the stratum radiatum of the CA1 region of the rat hippocampus. This technique could be used to quantify extrasynaptic NMDA receptor function in rodent models of diseases where extrasynaptic NMDA receptors are implicated in neuron death.

ABSTRACT

Synaptic NMDA receptors (NMDARs) play a central role in pro-survival signalling and synaptic plasticity in the majority of excitatory synapses in the central nervous system whereas extrasynaptic NMDARs (ES-NMDARs) activate pro-death pathways and have been implicated in many neurodegenerative diseases. ES-NMDARs have been characterized in acute brain slice preparations using the largely irreversible, activity-dependent NMDAR antagonist MK-801 to block synaptic NMDARs. This approach is limited by the concomitant MK-801 blockade of ES-NMDARs activated by ambient extracellular glutamate, which is largely absent from the synaptic cleft due to the high density of nearby glutamate transporters. In acute hippocampal slices from rats aged 35-42 postnatal days, we estimated ambient glutamate to be 72-83 nM resulting in a block of more than 82% of ES-NMDARs during a 5 min MK-801 application. This paper describes a novel electrophysiological and mathematical method to quantify the proportion of NMDARs located at extrasynaptic locations in a confined region of an acute brain slice preparation using MK-801 to preferentially block ES-NMDARs. The protocol uses whole cell patch clamp measurement of NMDAR responses to synaptic stimulation and brief local pressure application of NMDA before and after MK-801 application. After mathematically correcting for the relative block of both synaptic and extrasynaptic receptors, ES-NMDARs were estimated to comprise 29-39% of the total NMDAR pool in the apical dendrites of hippocampal CA1 pyramidal neurons. This new method may prove useful for accurate quantification of NMDAR distributions in neurodegenerative diseases that are associated with increased toxic ES-NMDAR signalling.

Interaction between leptin and glutamatergic system on food intake regulation in neonatal chicken: role of NMDA and AMPA receptors

Objective: The aim of the current study was to determine the possible interaction of the central leptin and Glutamatergic systems on feeding behavior in neonatal 3-hours food deprived (FD₃) broilers chickens.Methods: In experiment 1, FD₃ chicken received intracerebroventricular (ICV) injection of control solution (group i) and 2.5, 5 and 10 µg of Leptin (groups ii-iv). In experiment 2, FD₃ chicken were ICV injected with (group i) control solution and groups ii-iv with 2.5, 5 and 10 nmol of AG-490 (JAK₂ antagonist). In experiment 3, injections were (i) control solution, (ii) Leptin (10 µg), (iii) AG-490 (2.5 nmol) and (iv) Leptin + AG-490. In experiment 4, broiler chickens were ICV injected with (i) control solution, (ii) Leptin (10 µg), (iii) MK-801 (NMDA glutamate receptors antagonist; 15 nmol) and (iv) Leptin + MK-801. Experiments 5-9 were similar to experiment 1, except chicken were ICV injected with CNQX (AMPA receptor antagonist, 390 nmol), UBP-302 (Kainate receptor antagonist, 390 nmol), AIDA (mGluR1 antagonist, 2 nmol), LY341495 (mGluR2 antagonist, 150 nmol) and UBP1112 (mGluR3 antagonist, 2 nmol) instead of MK-801. Then, food intake was measured until 120 min after injection.Results: ICV injection of leptin (2.5, 5 and 10 µg) significantly decreased food intake in a dose dependent manner (p < 0.05). Also, ICV injection of the JAK₂ antagonist (2.5, 5 and 10 nmol) had hyperphagic effect in chicken (p < 0.05). Co-administration of leptin + AG-490, partially decreased leptin-induced hypophagia in broiler chicken (p < 0.05). In addition, co-injection of leptin + MK-801 significalty inhibited leptin-induced hypophagia in neonatal chicken (p < 0.05). Also, co-administration of leptin + CNQX partially attenuated hypophagic effect of leptin in chicken (p < 0.05).Conclusion: The results of present study suggest that leptin has hypophagic effect in neonatal chicken and this effect is probably mediated via NMDA and AMPA glutamatergic receptors.

Reelin reverts biochemical, physiological and cognitive alterations in mouse models of Tauopathy

Reelin is an extracellular protein crucial for adult brain plasticity. Moreover, Reelin is protective against amyloid-β (Aβ) pathology in Alzheimer's Disease (AD), reducing plaque deposition, synaptic loss and cognitive decline. Given that Tau protein plays a key role in AD pathogenesis, and that the Reelin pathway modulates Tau phosphorylation, here we explored the involvement of Reelin in AD-related Tau pathology. We found that Reelin overexpression modulates the levels of Tau phosphorylation in AD-related epitopes in VLW mice expressing human mutant Tau. in vitro, Reelin reduced the Aβ-induced missorting of axonal Tau and neurofilament proteins to dendrites. Reelin also reverted in vivo the toxic somatodendritic localization of phosphorylated Tau. Finally, overexpression of Reelin in VLW mice improved long-term potentiation and long-term memory cognitive performance thus masking the cognitive and physiological deficits in VLW mice. These data suggest that the Reelin pathway, which is also protective against Aβ pathology, modulates fundamental traits of Tau pathology, strengthening the potential of Reelin as a therapeutic target in AD.

Inhibitory Receptor Diffusion Dynamics

The dynamic modulation of receptor diffusion-trapping at inhibitory synapses is crucial to synaptic transmission, stability, and plasticity. In this review article, we will outline the progression of understanding of receptor diffusion dynamics at the plasma membrane. We will discuss how regulation of reversible trapping of receptor-scaffold interactions in combination with theoretical modeling approaches can be used to quantify these chemical interactions at the postsynapse of living cells.

NMDA receptor C-terminal signaling in development, plasticity, and disease

The NMDA subtype of ionotropic glutamate receptor is a sophisticated integrator and transducer of information. NMDAR-mediated signals control diverse processes across the life course, including synaptogenesis and synaptic plasticity, as well as contribute to excitotoxic processes in neurological disorders. At the basic biophysical level, the NMDAR is a coincidence detector, requiring the co-presence of agonist, co-agonist, and membrane depolarization in order to open. However, the NMDAR is not merely a conduit for ions to flow through; it is linked on the cytoplasmic side to a large network of signaling and scaffolding proteins, primarily via the C-terminal domain of NMDAR GluN2 subunits. These physical interactions help to organize the signaling cascades downstream of NMDAR activation. Notably, the NMDAR does not come in a single form: the subunit composition of the NMDAR, particularly the GluN2 subunit subtype (GluN2A–D), influences the biophysical properties of the channel. Moreover, a growing number of studies have illuminated the extent to which GluN2 C-terminal interactions vary according to GluN2 subtype and how this impacts on the processes that NMDAR activity controls. We will review recent advances, controversies, and outstanding questions in this active area of research.

Functional Role of SIL1 in Neurodevelopment and Learning

Background

Sil1 is the causative gene of Marinesco-Sjӧgren Syndrome (MSS). The mutated Sil1 generates shortened SIL1 protein which will form aggregation and be degraded rapidly. Mental retardation is a major symptom of MSS which suggests a role of SIL1 in the development of the central nervous system, but how SIL1 functions remains unclear.

Objectives

The aim of this study is to explore the role of SIL1 in regulating cerebral development and its underlying molecular mechanism.

Methods

The basic expression pattern of SIL1 in tissues and cultured cortical neurons is measured by immunostaining and Western blot. The expression of SIL1 is reduced in vitro and in vivo through RNA interference delivered by a lentivirus. The expression of NMDA receptor subunits and the function of the Reelin signaling pathway are then examined by surface biotinylation and Western blot subsequently. Finally, the spatial learning of young mice was assessed by the Barnes maze task.

Results

SIL1 deficiency caused a diminished expression of both Reelin receptors and therefore impaired the Reelin signaling pathway. It then inhibited the developmental expression of GluN2A and impaired the spatial learning of 5-week-old mice.

Conclusions

These results suggested that SIL1 is required for the development of the central nervous system which is associated with its role in Reelin signaling.

Surface trafficking of neurotransmitter receptors: From cultured neurons to intact brain preparations

Over the last decade, developments in single molecule imaging have changed our vision of synaptic physiology. By providing high spatio-temporal resolution maps of the molecular actors of neurotransmissions, these techniques have revealed that pre- and post-synaptic proteins are not randomly distributed but precisely organized at the nanoscale, and that this specific organization is dynamically regulated. At the centre of synaptic transmissions, neurotransmitter receptors have been shown to form nanodomains at synapses and to dynamically move in and out of these confinement areas through lateral diffusion within the membrane plane on millisecond timescales, thereby directly contributing to the regulation of synaptic transmission and plasticity. Since the vast majority of these discoveries originated from observations made on dissociated neurons lacking several features of brain tissue (e.g. three-dimensional organization, tissue density), they were initially considered with caution. However, the recent implementation of single-particle tracking (SPT) approaches in cultured and acute brain preparations confirmed that early findings on the dynamic properties of receptors at the surface of neurons can be extended to more physiological conditions. Taking example of dopamine D1 and NMDA glutamate receptors we here review our current knowledge of the features of neurotransmitter receptor surface diffusion in intact brain tissue. Through detailed comparison with cultured neurons, we also discuss how these biophysical properties are influenced by the complexity of the extracellular environment. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Developmental NMDA receptor dysregulation in the infantile neuronal ceroid lipofuscinosis mouse model

Protein palmitoylation and depalmitoylation alter protein function. This post-translational modification is critical for synaptic transmission and plasticity. Mutation of the depalmitoylating enzyme palmitoyl-protein thioesterase 1 (PPT1) causes infantile neuronal ceroid lipofuscinosis (CLN1), a pediatric neurodegenerative disease. However, the role of protein depalmitoylation in synaptic maturation is unknown. Therefore, we studied synapse development in Ppt1^-/- mouse visual cortex. We demonstrate that the developmental N-methyl-D-aspartate receptor (NMDAR) subunit switch from GluN2B to GluN2A is stagnated in Ppt1^-/- mice. Correspondingly, Ppt1^-/- neurons exhibit immature evoked NMDAR currents and dendritic spine morphology in vivo. Further, dissociated Ppt1^-/- cultured neurons show extrasynaptic, diffuse calcium influxes and enhanced vulnerability to NMDA-induced excitotoxicity, reflecting the predominance of GluN2B-containing receptors. Remarkably, Ppt1^-/- neurons demonstrate hyperpalmitoylation of GluN2B as well as Fyn kinase, which regulates surface retention of GluN2B. Thus, PPT1 plays a critical role in postsynapse maturation by facilitating the GluN2 subunit switch and proteostasis of palmitoylated proteins.

The Role of NMDA Receptors in Alzheimer's Disease

In Alzheimer’s disease (AD), early synaptic dysfunction is associated with the increased oligomeric amyloid-beta peptide, which causes NMDAR-dependent synaptic depression and spine elimination. Memantine, low-affinity NMDAR channel blocker, has been used in the treatment of moderate to severe AD. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between NMDARs dysfunction and AD. This review focuses on not only changes in expression of different NMDAR subunits, but also some unconventional modes of NMDAR action.

Distal Dendritic Enrichment of HCN1 Channels in Hippocampal CA1 Is Promoted by Estrogen, but Does Not Require Reelin

HCN1 compartmentalization in CA1 pyramidal cells, essential for hippocampal information processing, is believed to be controlled by the extracellular matrix protein Reelin. Expression of Reelin, in turn, is stimulated by 17β-estradiol (E2). In this study, we therefore tested whether E2 regulates the compartmentalization of HCN1 in CA1 via Reelin. In organotypic entorhino-hippocampal cultures, we found that E2 promotes HCN1 distal dendritic enrichment via the G protein-coupled estrogen receptor GPER1, but apparently independent of Reelin, because GST-RAP, known to reduce Reelin signaling, did not prevent E2-induced HCN1 enrichment in distal CA1. We therefore re-examined the role of Reelin for the regulation of HCN1 compartmentalization and could not detect effects of reduced Reelin signaling on HCN1 distribution in CA1, either in the (developmental) slice culture model or in tamoxifen-inducible conditional reelin knockout mice during adulthood. We conclude that for HCN1 channel compartmentalization in CA1 pyramidal cells, Reelin is not as essential as previously proposed, and E2 effects on HCN1 distribution in CA1 are mediated by mechanisms that do not involve Reelin. Because HCN1 localization was not altered at different phases of the estrous cycle, gonadally derived estradiol is unlikely to regulate HCN1 channel compartmentalization, while the pattern of immunoreactivity of aromatase, the final enzyme of estradiol synthesis, argues for a role of local hippocampal E2 synthesis.

Involvement of extrasynaptic glutamate in physiological and pathophysiological changes of neuronal excitability

Glutamate is the most abundant neurotransmitter of the central nervous system, as the majority of neurons use glutamate as neurotransmitter. It is also well known that this neurotransmitter is not restricted to synaptic clefts, but found in the extrasynaptic regions as ambient glutamate. Extrasynaptic glutamate originates from spillover of synaptic release, as well as from astrocytes and microglia. Its concentration is magnitudes lower than in the synaptic cleft, but receptors responding to it have higher affinity for it. Extrasynaptic glutamate receptors can be found in neuronal somatodendritic location, on astroglia, oligodendrocytes or microglia. Activation of them leads to changes of neuronal excitability with different amplitude and kinetics. Extrasynaptic glutamate is taken up by neurons and astrocytes mostly via EAAT transporters, and astrocytes, in turn metabolize it to glutamine. Extrasynaptic glutamate is involved in several physiological phenomena of the central nervous system. It regulates neuronal excitability and synaptic strength by involving astroglia; contributing to learning and memory formation, neurosecretory and neuromodulatory mechanisms, as well as sleep homeostasis.The extrasynaptic glutamatergic system is affected in several brain pathologies related to excitotoxicity, neurodegeneration or neuroinflammation. Being present in dementias, neurodegenerative and neuropsychiatric diseases or tumor invasion in a seemingly uniform way, the system possibly provides a common component of their pathogenesis. Although parts of the system are extensively discussed by several recent reviews, in this review I attempt to summarize physiological actions of the extrasynaptic glutamate on neuronal excitability and provide a brief insight to its pathology for basic understanding of the topic.

Tracking single membrane targets of human autoantibodies using single nanoparticle imaging

BACKGROUND

Over the past decade, an increasing number of neurological and neuropsychiatric diseases have been associated with the expression of autoantibodies directed against neuronal targets, including neurotransmitter receptors. Although cell-based assays are routinely used in clinics to detect the presence of immunoglobulins, such tests often provide heterogeneous outcomes due to their limited sensitivity, especially at low titers. Thus, there is an urging need for new methods allowing the detection of autoantibodies in seropositive patients that cannot always be clinically distinguished from seronegative ones.

NEW METHOD

Here we make a case for single nanoparticle imaging approaches as a highly sensitive antibody detection assay. Through high-affinity interactions between functionalized nanoparticles and autoantibodies that recognize extracellular domains of membrane neuronal targets, single nanoparticle imaging allows a live surface staining of transmembrane proteins and gives access to their surface dynamics.

RESULTS AND COMPARISON WITH EXISTING METHOD(S)

We show here that this method is well-suited to detect low titers of purified immunoglobulin G (IgG) from first-episode psychotic patients and demonstrate that these IgG target glutamatergic N-Methyl-d-Aspartate receptors (NMDAR) in live hippocampal neurons. The molecular behaviors of targeted membrane receptors were indistinguishable from those of endogenous GluN1 NMDAR subunit and were virtually independent of the IgG concentration present in the sample contrary to classical cell-based assays.

CONCLUSIONS

Single nanoparticle imaging emerges as a real-time sensitive method to detect IgG directed against neuronal surface proteins, which could be used as an additional step to rule out ambiguous seropositivity diagnoses.

Pathogenicity of Antibodies against NMDA Receptor: Molecular Insights into Autoimmune Psychosis

Recent years have seen a flourishing literature on detection of circulating autoantibodies against neurotransmitter receptors in patients with neuropsychiatric disorders. These studies have generated hope for a better understanding of the underlying molecular dysfunctions and for appropriate therapeutic strategies. However, the detection of these autoantibodies in healthy subjects, and the lack of mechanistic insights have fostered debate about the pathogenic role of such autoantibodies. Here, we specifically discuss the biological evidence linking autoantibodies directed against the glutamatergic N-methyl-d-aspartate (NMDA) receptor (NMDAR-Abs) and psychosis, emphasising recent single-molecule imaging investigations that unveiled the impaired surface trafficking of NMDAR in the presence of NMDAR-Abs from psychotic patients. Although still in its infancy, the hypothesis that NMDAR-Abs from patients with psychosis play a pathogenic role is thus gaining support, opening avenues of fundamental and translational investigations.

TRPC1 Channels Are Expressed in Pyramidal Neurons and in a Subset of Somatostatin Interneurons in the Rat Neocortex

Disturbances in calcium homeostasis due to canonical transient receptor potential (TRPC) and/or store-operated calcium (SOC) channels can play a key role in a large number of brain disorders. TRPC channels are plasma membrane cation channels included in the transient receptor potential (TRP) superfamily. The most widely distributed member of the TRPC subfamily in the brain is TRPC1, which is frequently linked to group I metabotropic glutamate receptors (mGluRs) and to the components of SOC channels. Proposing TRPC/SOC channels as a therapeutic target in neurological diseases previously requires a detailed knowledge of the distribution of such molecules in the brain. The aim of our study was to analyze the neuroanatomical distribution of TRPC1 in the rat neocortex. By double- and triple-labeling and confocal microscopy, we tested the presence of TRPC1 by using a series of specific neurochemical markers. TRPC1 was abundant in SMI 32-positive pyramidal neurons, and in some glutamic acid decarboxylase 67 (GAD67) interneurons, but was lacking in glial fibrillary acidic protein (GFAP)-positive glial cells. In neurons it colocalized with postsynaptic marker MAP2 in cell bodies and apical dendritic trunks and it was virtually absent in synaptophysin-immunoreactive terminals. By using a panel of antibodies to classify interneurons, we identified the GABAergic interneurons that contained TRPC1. TRPC1 was lacking in basket and chandelier parvalbumin (PVALB) cells, and a very low percentage of calretinin (CALR) or calbindin (CALB) interneurons expressed TRPC1. Moreover, 63% of somatostatin (SST) expressing-cells and 37% of reelin-positive cells expressed TRPC1. All the SST/TRPC1 double-labeled cells, many of which were presumptive Martinotti cells (MC), were positive for reelin. The presence of TRPC1 in the somata and apical dendritic trunks of neocortical pyramidal cells suggests a role for this channel in sensory processing and synaptic plasticity. Conversely in SST/reelin interneurons, TRPC1 could modulate GABAergic transmission, which is responsible for shaping the coordinated activity of the pyramidal cells in the cortical network. In future studies, it would be relevant to investigate whether TRPC1 could be involved in the expression or processing of reelin in SST inhibitory interneurons.