Glutamate receptor δ2 is essential for input pathway-dependent regulation of synaptic AMPAR contents in cerebellar Purkinje cells

Cross-species single-cell spatial transcriptomic atlases of the cerebellar cortex

The molecular and cellular organization of the primate cerebellum remains poorly characterized. We obtained single-cell spatial transcriptomic atlases of macaque, marmoset, and mouse cerebella and identified primate-specific cell subtypes, including Purkinje cells and molecular-layer interneurons, that show different expression of the glutamate ionotropic receptor Delta type subunit 2 (GRID2) gene. Distinct gene expression profiles were found in anterior, posterior, and vestibular regions in all species, whereas region-selective gene expression was predominantly observed in the granular layer of primates and in the Purkinje layer of mice. Gene expression gradients in the cerebellar cortex matched well with functional connectivity gradients revealed with awake functional magnetic resonance imaging, with more lobule-specific differences between primates and mice than between two primate species. These comprehensive atlases and comparative analyses provide the basis for understanding cerebellar evolution and function.

Photoproximity labeling of endogenous receptors in the live mouse brain in minutes

Understanding how protein-protein interaction networks in the brain give rise to cognitive functions necessitates their characterization in live animals. However, tools available for this purpose require potentially disruptive genetic modifications and lack the temporal resolution necessary to track rapid changes in vivo. Here we leverage affinity-based targeting and photocatalyzed singlet oxygen generation to identify neurotransmitter receptor-proximal proteins in the live mouse brain using only small-molecule reagents and minutes of photoirradiation. Our photooxidation-driven proximity labeling for proteome identification (named PhoxID) method not only recapitulated the known interactomes of three endogenous neurotransmitter receptors (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), inhibitory γ-aminobutyric acid type A receptor and ionotropic glutamate receptor delta-2) but also uncovered age-dependent shifts, identifying NECTIN3 and IGSF3 as developmentally regulated AMPAR-proximal proteins in the cerebellum. Overall, this work establishes a flexible and generalizable platform to study receptor microenvironments in genetically intact specimens with an unprecedented temporal resolution.

Direct and indirect pathways for heterosynaptic interaction underlying developmental synapse elimination in the mouse cerebellum

Developmental synapse elimination is crucial for shaping mature neural circuits. In the neonatal mouse cerebellum, Purkinje cells (PCs) receive excitatory synaptic inputs from multiple climbing fibers (CFs) and synapses from all but one CF are eliminated by around postnatal day 20. Heterosynaptic interaction between CFs and parallel fibers (PFs), the axons of cerebellar granule cells (GCs) forming excitatory synapses onto PCs and molecular layer interneurons (MLIs), is crucial for CF synapse elimination. However, mechanisms for this heterosynaptic interaction are largely unknown. Here we show that deletion of AMPA-type glutamate receptor functions in GCs impairs CF synapse elimination mediated by metabotropic glutamate receptor 1 (mGlu1) signaling in PCs. Furthermore, CF synapse elimination is impaired by deleting NMDA-type glutamate receptors from MLIs. We propose that PF activity is crucial for CF synapse elimination by directly activating mGlu1 in PCs and indirectly enhancing the inhibition of PCs through activating NMDA receptors in MLIs.

Mapping proteomic composition of excitatory postsynaptic sites in the cerebellar cortex

Functions of the cerebellar cortex, from motor learning to emotion and cognition, depend on the appropriate molecular composition at diverse synapse types. Glutamate receptor distributions have been partially mapped using immunogold electron microscopy. However, information is lacking on the distribution of many other components, such as Shank2, a postsynaptic scaffolding protein whose cerebellar dysfunction is associated with autism spectrum disorders. Here, we used an adapted Magnified Analysis of the Proteome, an expansion microscopy approach, to map multiple glutamate receptors, scaffolding and signaling proteins at single synapse resolution in the cerebellar cortex. Multiple distinct synapse-selective distribution patterns were observed. For example, AMPA receptors were most concentrated at synapses on molecular layer interneurons and at climbing fiber synapses, Shank1 was most concentrated at parallel fiber synapses on Purkinje cells, and Shank2 at both climbing fiber and parallel fiber synapses on Purkinje cells but little on molecular layer interneurons. Our results are consistent with gene expression data but also reveal input-selective targeting within Purkinje cells. In specialized glomerular structures of the granule cell layer, AMPA receptors as well as most other synaptic components preferentially targeted to synapses. However, NMDA receptors and the synaptic GTPase activating protein SynGAP preferentially targeted to extrasynaptic sites. Thus, glomeruli may be considered integrative signaling units through which mossy fibers differentially activate synaptic AMPA and extrasynaptic NMDA receptor complexes. Furthermore, we observed NMDA receptors and SynGAP at adherens junctions, suggesting a role in structural plasticity of glomeruli. Altogether, these data contribute to mapping the cerebellar 'synaptome'.

Bioorthogonal chemical labeling of endogenous neurotransmitter receptors in living mouse brains

Neurotransmitter receptors are essential components of synapses for communication between neurons in the brain. Because the spatiotemporal expression profiles and dynamics of neurotransmitter receptors involved in many functions are delicately governed in the brain, in vivo research tools with high spatiotemporal resolution for receptors in intact brains are highly desirable. Covalent labeling by chemical reaction (chemical labeling) of proteins without genetic manipulation is now a powerful method for analyzing receptors in vitro. However, selective target receptor labeling in the brain has not yet been achieved. This study shows that ligand-directed alkoxyacylimidazole (LDAI) chemistry can be used to selectively tether synthetic probes to target endogenous receptors in living mouse brains. The reactive LDAI reagents with negative charges were found to diffuse well over the whole brain and could selectively label target endogenous receptors, including AMPAR, NMDAR, mGlu1, and GABAAR. This simple and robust labeling protocol was then used for various applications: three-dimensional spatial mapping of endogenous receptors in the brains of healthy and disease-model mice; multi-color receptor imaging; and pulse-chase analysis of the receptor dynamics in postnatal mouse brains. Here, results demonstrated that bioorthogonal receptor modification in living animal brains may provide innovative molecular tools that contribute to the in-depth understanding of complicated brain functions.

Cerebellin-neurexin complexes instructing synapse properties

Cerebellins (Cbln1-4) are secreted adaptor proteins that connect presynaptic neurexins (Nrxn1-3) to postsynaptic ligands (GluD1/2 for Cbln1-3 vs. DCC and Neogenin-1 for Cbln4). Classical studies demonstrated that neurexin-Cbln1-GluD2 complexes organize cerebellar parallel-fiber synapses, but the role of cerebellins outside of the cerebellum has only recently been clarified. In synapses of the hippocampal subiculum and prefrontal cortex, Nrxn1-Cbln2-GluD1 complexes strikingly upregulate postsynaptic NMDA-receptors, whereas Nrxn3-Cbln2-GluD1 complexes conversely downregulate postsynaptic AMPA-receptors. At perforant-path synapses in the dentate gyrus, in contrast, neurexin/Cbln4/Neogenin-1 complexes are essential for LTP without affecting basal synaptic transmission or NMDA- or AMPA-receptors. None of these signaling pathways are required for synapse formation. Thus, outside of the cerebellum neurexin/cerebellin complexes regulate synapse properties by activating specific downstream receptors.

Glyoxal fixation: An approach to solve immunohistochemical problem in neuroscience research

The gold-standard fixative for immunohistochemistry is 4% formaldehyde; however, it limits antibody access to target molecules that are buried within specialized neuronal components, such as ionotropic receptors at the postsynapse and voltage-gated ion channels at the axon initial segment, often requiring additional antigen-exposing techniques to detect their authentic signals. To solve this problem, we used glyoxal, a two-carbon atom di-aldehyde. We found that glyoxal fixation greatly improved antibody penetration and immunoreactivity, uncovering signals for buried molecules by conventional immunohistochemical procedures at light and electron microscopic levels. It also enhanced immunosignals of most other molecules, which are known to be detectable in formaldehyde-fixed sections. Furthermore, we unearthed several specific primary antibodies that were once judged to be unusable in formaldehyde-fixed tissues, allowing us to successfully localize so far controversial synaptic adhesion molecule Neuroligin 1. Thus, glyoxal is a highly effective fixative for immunostaining, and a side-by-side comparison of glyoxal and formaldehyde fixation is recommended for routine immunostaining in neuroscience research.

Enhanced functional detection of synaptic calcium-permeable AMPA receptors using intracellular NASPM

Calcium-permeable AMPA-type glutamate receptors (CP-AMPARs) contribute to many forms of synaptic plasticity and pathology. They can be distinguished from GluA2-containing calcium-impermeable AMPARs by the inward rectification of their currents, which reflects voltage-dependent channel block by intracellular spermine. However, the efficacy of this weakly permeant blocker is differentially altered by the presence of AMPAR auxiliary subunits - including transmembrane AMPAR regulatory proteins, cornichons, and GSG1L - which are widely expressed in neurons and glia. This complicates the interpretation of rectification as a measure of CP-AMPAR expression. Here, we show that the inclusion of the spider toxin analog 1-naphthylacetyl spermine (NASPM) in the intracellular solution results in a complete block of GluA1-mediated outward currents irrespective of the type of associated auxiliary subunit. In neurons from GluA2-knockout mice expressing only CP-AMPARs, intracellular NASPM, unlike spermine, completely blocks outward synaptic currents. Thus, our results identify a functional measure of CP-AMPARs, that is unaffected by their auxiliary subunit content.

Quantitative analysis of NMDA receptor subunits proteins in mouse brain

NMDA-type glutamate receptors (NMDARs) are tetrameric channel complex composed of two subunits of GluN1, which is encoded by a single gene and diversified by alternative splicing, and two subunits from four subtypes of GluN2, leading to various combinations of subunits and channel specificities. However, there is no comprehensive quantitative analysis of GluN subunit proteins for relative comparison, and their compositional ratios at various regions and developmental stages have not been clarified. Here we prepared six chimeric subunits, by fusing an N-terminal side of the GluA1 subunit with a C-terminal side of each of two splicing isoforms of GluN1 subunit and four GluN2 subunits, with which titers of respective NMDAR subunit antibodies could be standardized using common GluA1 antibody, thus enabling quantification of relative protein levels of each NMDAR subunit by western blotting. We determined relative protein amounts of NMDAR subunits in crude, membrane (P2) and microsomal fractions prepared from the cerebral cortex, hippocampus and cerebellum in adult mice. We also examined amount changes in the three brain regions during developmental stages. Their relative amounts in the cortical crude fraction were almost parallel to those of mRNA expression, except for some subunits. Interestingly, a considerable amount of GluN2D protein existed in adult brains, although its transcription level declines after early postnatal stages. GluN1 was larger in quantity than GluN2 in the crude fraction, whereas GluN2 increased in the membrane component-enriched P2 fraction, except in the cerebellum. These data will provide the basic spatio-temporal information on the amount and composition of NMDARs.

Regulation of NETosis and Inflammation by Cyclophilin D in Myeloperoxidase-Positive Antineutrophil Cytoplasmic Antibody-Associated Vasculitis

OBJECTIVE

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is pathologically characterized by focal fibrinoid necrosis, in which ANCA-mediated neutrophil extracellular trap (NET) formation and subsequent endothelial cell necrosis occur. Cyclophilin D (CypD) plays an important role in mediation of cell necrosis and inflammation via the opening of mitochondrial permeability transition pores. This study was undertaken to examine the role of CypD in AAV pathogenesis.

METHODS

We assessed the role and mechanism of CypD in ANCA-stimulated neutrophils in vitro by immunostaining and electron microscopy observation. We performed a comprehensive RNA-sequencing analysis on ANCA-treated murine neutrophils. To investigate the role of CypD in vivo, we assessed disease features in CypD-knockout mice and wild-type mice using 2 different murine AAV models: anti-myeloperoxidase IgG transfer-induced AAV and spontaneous AAV.

RESULTS

In vitro experiments showed that pharmacologic and genetic inhibition of CypD suppressed ANCA-induced NET formation via the suppression of reactive oxygen species and cytochrome c release from the mitochondria. RNA-sequencing analyses in ANCA-treated murine neutrophils revealed the involvement of inflammatory responses, with CypD deficiency reducing ANCA-induced alterations in gene expression. Furthermore, analyses of upstream regulators revealed the relevance of intracellular calcium (CypD activator) and cyclosporin (CypD inhibitor) in ANCA stimulation, indicating that the CypD-dependent opening of mitochondrial permeability transition pores is associated with ANCA-induced neutrophil activation and NETosis. In both AAV mouse models, the genetic deletion of CypD ameliorated crescentic glomerulonephritis via the inhibition of CypD-dependent neutrophil and endothelial necrosis.

CONCLUSION

CypD targeting is a novel and specific therapeutic strategy for AAV via the resolution of necrotizing vasculitis.

CTCF loss induces giant lamellar bodies in Purkinje cell dendrites

CCCTC-binding factor (CTCF) has a key role in higher-order chromatin architecture that is important for establishing and maintaining cell identity by controlling gene expression. In the mature cerebellum, CTCF is highly expressed in Purkinje cells (PCs) as compared with other cerebellar neurons. The cerebellum plays an important role in motor function by regulating PCs, which are the sole output neurons, and defects in PCs cause motor dysfunction. However, the role of CTCF in PCs has not yet been explored. Here we found that the absence of CTCF in mouse PCs led to progressive motor dysfunction and abnormal dendritic morphology in those cells, which included dendritic self-avoidance defects and a proximal shift in the climbing fibre innervation territory on PC dendrites. Furthermore, we found the peculiar lamellar structures known as "giant lamellar bodies" (GLBs), which have been reported in PCs of patients with Werdnig-Hoffman disease, 13q deletion syndrome, and Krabbe disease. GLBs are localized to PC dendrites and are assumed to be associated with neurodegeneration. They have been noted, however, only in case reports following autopsy, and reports of their existence have been very limited. Here we show that GLBs were reproducibly formed in PC dendrites of a mouse model in which CTCF was deleted. GLBs were not noted in PC dendrites at infancy but instead developed over time. In conjunction with GLB development in PC dendrites, the endoplasmic reticulum was almost absent around the nuclei, the mitochondria were markedly swollen and their cristae had decreased drastically, and almost all PCs eventually disappeared as severe motor deficits manifested. Our results revealed the important role of CTCF during normal development and in maintaining PCs and provide new insights into the molecular mechanism of GLB formation during neurodegenerative disease.

Distinct neurexin-cerebellin complexes control AMPA- and NMDA-receptor responses in a circuit-dependent manner

At CA1→subiculum synapses, alternatively spliced neurexin-1 (Nrxn1SS4+) and neurexin-3 (Nrxn3SS4+) enhance NMDA-receptors and suppress AMPA-receptors, respectively, without affecting synapse formation. Nrxn1SS4+ and Nrxn3SS4+ act by binding to secreted cerebellin-2 (Cbln2) that in turn activates postsynaptic GluD1 receptors. Whether neurexin-Cbln2-GluD1 signaling has additional functions besides regulating NMDA- and AMPA-receptors, and whether such signaling performs similar roles at other synapses, however, remains unknown. Here, we demonstrate using constitutive Cbln2 deletions in mice that at CA1→subiculum synapses, Cbln2 performs no additional developmental roles besides regulating AMPA- and NMDA-receptors. Moreover, low-level expression of functionally redundant Cbln1 did not compensate for a possible synapse-formation function of Cbln2 at CA1→subiculum synapses. In exploring the generality of these findings, we examined the prefrontal cortex where Cbln2 was recently implicated in spinogenesis, and the cerebellum where Cbln1 is known to regulate parallel-fiber synapses. In the prefrontal cortex, Nrxn1SS4+-Cbln2 signaling selectively controlled NMDA-receptors without affecting spine or synapse numbers, whereas Nrxn3SS4+-Cbln2 signaling had no apparent role. In the cerebellum, conversely, Nrxn3SS4+-Cbln1 signaling regulated AMPA-receptors, whereas now Nrxn1SS4+-Cbln1 signaling had no manifest effect. Thus, Nrxn1SS4+- and Nrxn3SS4+-Cbln1/2 signaling complexes differentially control NMDA- and AMPA-receptors in different synapses in diverse neural circuits without regulating synapse or spine formation.

Glutamate delta 1 receptor regulates autophagy mechanisms and affects excitatory synapse maturation in the somatosensory cortex

The glutamate delta family of receptors is composed of GluD1 and GluD2 and serve as synaptic organizers. We have previously demonstrated several autism-like molecular and behavioral phenotypes including an increase in dendritic spines in GluD1 knockout mice. Based on previous reports we evaluated whether disruption of autophagy mechanisms may account for these phenotypes. Mouse model with conditional deletion of GluD1 from excitatory neurons in the corticolimbic regions was utilized. GluD1 loss led to overactive Akt-mTOR pathway, higher p62 and a lower LC3-II/LC3-I ratio in the somatosensory cortex suggesting reduced autophagy. Excitatory elements were increased in number but had immature phenotype based on puncta size, lower AMPA subunit GluA1 expression and impaired development switch from predominantly GluN2B to mixed GluN2A/GluN2B subunit expression. Overactive Akt-mTOR signaling and impaired autophagy was also observed in dorsal striatum upon conditional ablation of GluD1 and in the prefrontal cortex and hippocampus in constitutive knockout. Finally, cognitive deficits in novel object recognition test and fear conditioning were observed in mice with conditional ablation of GluD1 from the corticolimbic regions. Together, these results demonstrate a novel function of GluD1 in the regulation of autophagy pathway which may underlie autism phenotypes and is relevant to the genetic association of GluD1 coding, GRID1 gene with autism and other developmental disorders.

Evaluating Large Spontaneous Deletions in a Bovine Cell Line Selected for Bovine Viral Diarrhea Virus Resistance

Bovine viral diarrhea virus’s (BVDV) entry into bovine cells involves attachment of virions to cellular receptors, internalization, and pH-dependent fusion with endosomal membranes. The primary host receptor for BVDV is CD46; however, the complete set of host factors required for virus entry is unknown. The Madin-Darby bovine kidney (MDBK) cell line is susceptible to BVDV infection, while a derivative cell line (CRIB) is resistant at the level of virus entry. We performed complete genome sequencing of each to identify genomic variation underlying the resistant phenotype with the aim of identifying host factors essential for BVDV entry. Three large compound deletions in the BVDV-resistant CRIB cell line were identified and predicted to disrupt the function or expression of the genes PTPN12, GRID2, and RABGAP1L. However, CRISPR/Cas9 mediated knockout of these genes, individually or in combination, in the parental MDBK cell line did not impact virus entry or replication. Therefore, resistance to BVDV in the CRIB cell line is not due to the apparent spontaneous loss of PTPN12, GRID2, or RABGAP1L gene function. Identifying the functional cause of BVDV resistance in the CRIB cell line may require more detailed comparisons of the genomes and epigenomes.

Optical manipulation of local cerebral blood flow in the deep brain of freely moving mice

An artificial tool for manipulating local cerebral blood flow (CBF) is necessary for understanding how CBF controls brain function. Here, we generate vascular optogenetic tools whereby smooth muscle cells and endothelial cells express optical actuators in the brain. The illumination of channelrhodopsin-2 (ChR2)-expressing mice induces a local reduction in CBF. Photoactivated adenylyl cyclase (PAC) is an optical protein that increases intracellular cyclic adenosine monophosphate (cAMP), and the illumination of PAC-expressing mice induces a local increase in CBF. We target the ventral striatum, determine the temporal kinetics of CBF change, and optimize the illumination intensity to confine the effects to the ventral striatum. We demonstrate the utility of this vascular optogenetic manipulation in freely and adaptively behaving mice and validate the task- and actuator-dependent behavioral readouts. The development of vascular optogenetic animal models will help accelerate research linking vasculature, circuits, and behavior to health and disease.

GluA4 facilitates cerebellar expansion coding and enables associative memory formation

AMPA receptors (AMPARs) mediate excitatory neurotransmission in the central nervous system (CNS) and their subunit composition determines synaptic efficacy. Whereas AMPAR subunits GluA1–GluA3 have been linked to particular forms of synaptic plasticity and learning, the functional role of GluA4 remains elusive. Here, we demonstrate a crucial function of GluA4 for synaptic excitation and associative memory formation in the cerebellum. Notably, GluA4-knockout mice had ~80% reduced mossy fiber to granule cell synaptic transmission. The fidelity of granule cell spike output was markedly decreased despite attenuated tonic inhibition and increased NMDA receptor-mediated transmission. Computational network modeling incorporating these changes revealed that deletion of GluA4 impairs granule cell expansion coding, which is important for pattern separation and associative learning. On a behavioral level, while locomotor coordination was generally spared, GluA4-knockout mice failed to form associative memories during delay eyeblink conditioning. These results demonstrate an essential role for GluA4-containing AMPARs in cerebellar information processing and associative learning.

mGluR1 signaling in cerebellar Purkinje cells: Subcellular organization and involvement in cerebellar function and disease

The cerebellum is essential for the control, coordination, and learning of movements, and for certain aspects of cognitive function. Purkinje cells are the sole output neurons in the cerebellar cortex and therefore play crucial roles in the diverse functions of the cerebellum. The type 1 metabotropic glutamate receptor (mGluR1) is prominently enriched in Purkinje cells and triggers downstream signaling pathways that are required for functional and structural plasticity, and for synaptic responses. To understand how mGluR1 contributes to cerebellar functions, it is important to consider not only the operational properties of this receptor, but also its spatial organization and the molecular interactions that enable its proper functioning. In this review, we highlight how mGluR1 and its related signaling molecules are organized into tightly coupled microdomains to fulfill physiological functions. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunction in ataxias of human patients and mouse models.

Absence of GluD2 Antibodies in Patients With Opsoclonus-Myoclonus Syndrome

OBJECTIVE

A recent study showed glutamate receptor delta 2 antibodies (GluD2-ab) in sera of patients with opsoclonus-myoclonus syndrome (OMS). Inconsistencies between cerebellar immunoreactivity and expression of GluD2 led us to hypothesize that these antibodies are not biomarkers of OMS.

METHODS

Serum of 45 children with OMS (10 [22%] with neuroblastoma), 158 adults with OMS (53 [34%] with tumors), and 172 controls including 134 patients with several types of neurologic disorders, 18 with neuroblastoma without OMS, and 20 healthy participants were investigated. Antibodies were determined with 3 different techniques: (1) rat brain immunohistochemistry, (2) a live cell-based assay using a standard secondary antibody (2-step CBA), and (3) a similar CBA with a secondary and tertiary antibodies (3-step CBA). Two plasmids were used in the CBA studies. Three commercial GluD2-ab and 2 human sera with GluD2-ab served as controls for expression of GluD2.

RESULTS

The 3 commercial GluD2-ab showed predominant reactivity with the molecular and Purkinje cell layers (where GluD2 is highly enriched), and were also positive with the indicated CBAs. Substantially milder reactivity with brain tissue and CBA was obtained with the 2 control human sera containing GluD2-ab. None of the 203 patients with OMS and 172 controls showed immunoreactivities consistent with GluD2-abs. Compared with a standard 2-step CBA, the 3-step assay did not improve antibody detection and showed more frequent nonspecific reactivity that was not immunoabsorbed with GluD2.

CONCLUSION

We did not find GluD2-ab in a large cohort of patients with OMS. GluD2-ab should not be considered diagnostic biomarkers of OMS.

AUTS2 Governs Cerebellar Development, Purkinje Cell Maturation, Motor Function and Social Communication

Autism susceptibility candidate 2 (AUTS2), a risk gene for autism spectrum disorders (ASDs), is implicated in telencephalon development. Because AUTS2 is also expressed in the cerebellum where defects have been linked to ASDs, we investigated AUTS2 functions in the cerebellum. AUTS2 is specifically localized in Purkinje cells (PCs) and Golgi cells during postnatal development. Auts2 conditional knockout (cKO) mice exhibited smaller and deformed cerebella containing immature-shaped PCs with reduced expression of Cacna1a. Auts2 cKO and knock-down experiments implicated AUTS2 participation in elimination and translocation of climbing fiber synapses and restriction of parallel fiber synapse numbers. Auts2 cKO mice exhibited behavioral impairments in motor learning and vocal communications. Because Cacna1a is known to regulate synapse development in PCs, it suggests that AUTS2 is required for PC maturation to elicit normal development of PC synapses and thus the impairment of AUTS2 may cause cerebellar dysfunction related to psychiatric illnesses such as ASDs.

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Loss of Auts2 leads to the reduction of cerebellar size

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AUTS2 promotes the dendritic maturation of Purkinje cells

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AUTS2 participates in PF and CF synapse development of Purkinje cells

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Auts2 cKO mice exhibit the impaired motor learning and vocal communications

Trans-synaptic interactions of ionotropic glutamate receptors

Trans-synaptic interactions organize the multiple steps of synaptic development and are critical to generate fully functional neuronal circuits. While trans-synaptic interactions are primarily mediated by cell adhesion molecules (CAMs), some directly involve ionotropic glutamate receptors (iGluRs). Here, we review the expanding extracellular and trans-synaptic proteome of iGluRs. We discuss the role of these molecular networks in specifying the formation of excitatory and inhibitory circuits and in the input-specific recruitment of iGluRs at synapses in various cell types and brain regions. We also shed light on human-specific mutations affecting iGluR-mediated trans-synaptic interactions that may provide unique features to the human brain and contribute to its susceptibility to neurodevelopmental disorders. Together, these data support a view in which iGluR function goes far beyond fast excitatory synaptic transmission by shaping the wiring and the functional properties of neural circuits.

Myosin XVI Regulates Actin Cytoskeleton Dynamics in Dendritic Spines of Purkinje Cells and Affects Presynaptic Organization

The actin cytoskeleton is crucial for function and morphology of neuronal synapses. Moreover, altered regulation of the neuronal actin cytoskeleton has been implicated in neuropsychiatric diseases such as autism spectrum disorder (ASD). Myosin XVI is a neuronally expressed unconventional myosin known to bind the WAVE regulatory complex (WRC), a regulator of filamentous actin (F-actin) polymerization. Notably, the gene encoding the myosin’s heavy chain (MYO16) shows genetic association with neuropsychiatric disorders including ASD. Here, we investigated whether myosin XVI plays a role for actin cytoskeleton regulation in the dendritic spines of cerebellar Purkinje cells (PCs), a neuronal cell type crucial for motor learning, social cognition and vocalization. We provide evidence that both myosin XVI and the WRC component WAVE1 localize to PC spines. Fluorescence recovery after photobleaching (FRAP) analysis of GFP-actin in cultured PCs shows that Myo16 knockout as well as PC-specific Myo16 knockdown, lead to faster F-actin turnover in the dendritic spines of PCs. We also detect accelerated F-actin turnover upon interference with the WRC, and upon inhibition of Arp2/3 that drives formation of branched F-actin downstream of the WRC. In contrast, inhibition of formins that are responsible for polymerization of linear actin filaments does not cause faster F-actin turnover. Together, our data establish myosin XVI as a regulator of the postsynaptic actin cytoskeleton and suggest that it is an upstream activator of the WRC-Arp2/3 pathway in PC spines. Furthermore, ultra-structural and electrophysiological analyses of Myo16 knockout cerebellum reveals the presence of reduced numbers of synaptic vesicles at presynaptic terminals in the absence of the myosin. Therefore, we here define myosin XVI as an F-actin regulator important for presynaptic organization in the cerebellum.

Cerebellar Neurodegeneration and Neuronal Circuit Remodeling in Golgi pH Regulator-Deficient Mice

The Golgi apparatus plays an indispensable role in posttranslational modification and transport of proteins to their target destinations. Although it is well established that the Golgi apparatus requires an acidic luminal pH for optimal activity, morphological and functional abnormalities at the neuronal circuit level because of perturbations in Golgi pH are not fully understood. In addition, morphological alteration of the Golgi apparatus is associated with several neurodegenerative diseases, including Parkinson’s disease, Alzheimer’s disease, and amyotrophic lateral sclerosis. Here, we used anatomical and electrophysiological approaches to characterize morphological and functional abnormalities of neuronal circuits in Golgi pH regulator (GPHR) conditional knock-out mice. Purkinje cells (PCs) from the mutant mice exhibited vesiculation and fragmentation of the Golgi apparatus, followed by axonal degeneration and progressive cell loss. Morphological analysis provided evidence for the disruption of basket cell (BC) terminals around PC soma, and electrophysiological recordings showed selective loss of large amplitude responses, suggesting BC terminal disassembly. In addition, the innervation of mutant PCs was altered such that climbing fiber (CF) terminals abnormally synapsed on the somatic spines of mutant PCs in the mature cerebellum. The combined results describe an essential role for luminal acidification of the Golgi apparatus in maintaining proper neuronal morphology and neuronal circuitry.

Glutamate delta-1 receptor regulates cocaine-induced plasticity in the nucleus accumbens

Cocaine exposure induces plasticity of glutamatergic synapses of medium spiny neurons (MSNs) in the nucleus accumbens (NAc), which has been proposed to contribute to its addictive behavior. The mechanisms underlying cocaine-induced plasticity are not fully understood. The orphan glutamate delta-1 (GluD1) receptor is a member of the ionotropic glutamate receptor family but does not function as a typical ligand-gated ion channel. Instead it serves a synaptogenic function by interacting with presynaptic Neurexin protein. Recent neuroanatomical studies have demonstrated enriched expression of GluD1 in the NAc but its role in reward behavior, MSN function, and drug-induced plasticity remains unknown. Using a combination of constitutive and conditional GluD1 KO models, we evaluated the effect of GluD1 ablation on cocaine-conditioned place preference (CPP) and cocaine-induced structural and functional plasticity. GluD1 KO mice showed higher cocaine CPP. Selective ablation of GluD1 from striatal neurons but not cortico-limbic excitatory neurons reproduced higher CPP. Higher cocaine preference in GluD1 KO correlated with an increase in spine density, greater maturation of dendritic spines, and basally upregulated spine-regulating active cofilin. GluD1 loss did not affect basal excitatory neurotransmission or plasticity but masked the generation of cocaine-induced silent synapses. Finally, loss of GluD1 increased the GluN2B subunit contribution to NMDA receptor currents in MSNs and a partial agonist of GluN2B-containing NMDA receptors normalized the higher active cofilin and cocaine preference in GluD1 KO mice. Together, these findings demonstrate a critical role of GluD1 in controlling susceptibility to cocaine preference and cocaine-induced plasticity by modulating NMDA receptor subunit contribution.

Genetic Ablation of All Cerebellins Reveals Synapse Organizer Functions in Multiple Regions Throughout the Brain

Cerebellins are synaptic organizer molecules that bind to presynaptic neurexins and postsynaptic receptors. They are well studied in the cerebellum, but three of the four cerebellins (Cbln1, Cbln2, and Cbln4) are also broadly expressed outside of the cerebellum, suggesting that they perform general functions throughout the brain. Here, we generated male and female constitutive single (KO), double KO (dKO), and triple KO (tKO) mice of Cbln1, Cbln2, and Cbln4. We found that all constitutive cerebellin-deficient mice were viable and fertile, suggesting that cerebellins are not essential for survival. Cbln1/2 dKO mice exhibited salience-induced seizures that were aggravated in Cbln1/2/4 tKO mice, suggesting that all cerebellins contribute to brain function. As described previously, Cbln1 KO mice displayed major motor impairments that were aggravated by additional KO of Cbln2. Strikingly, the Cbln1/2 dKO did not cause alterations in synapse density in the hippocampus of young adult (1- and 2-month-old) mice, but produced a selective ∼50% decrease in hippocampal synapse density in the stratum lacunosum moleculare of the CA1 region and in the dentate gyrus of aging, 6-month-old mice. A similar decrease in excitatory synapse density was observed in the striatum and retrosplenial cortex. Behaviorally, the Cbln1 KO produced dramatic changes in motor behaviors that were partly aggravated by additional deletion of Cbln2 and/or Cbln4. Our results show that cerebellins are not essential for survival and do not contribute to initial synapse formation, but perform multiple functions throughout the brain; as a consequence, their ablation results in a delayed loss of synapses and in behavioral impairments.SIGNIFICANCE STATEMENT Cerebellins (Cbln1-4) are trans-synaptic cell adhesion molecules. In the cerebellum, Cbln1 functions as a bidirectional organizer of parallel fiber-Purkinje cell synapses by binding to presynaptic neurexins and postsynaptic GluRδ2. Little is known about the function of cerebellins outside of the cerebellum; therefore, the present study used single, double, and triple constitutive KO mice of Cbln1, Cbln2, and Cbln4 to analyze the overall function of cerebellins. We show that cerebellins act as important synaptic organizers in specific subsets of neurons and likely contribute to many different brain functions. We also show that cerebellins are not initially required for synapse formation, but rather for specification and long-term synapse maintenance and demonstrate that all cerebellins, not just Cbln1, contribute to brain function.

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

Synapses are specialized junctions between neurons in brain that transmit and compute information, thereby connecting neurons into millions of overlapping and interdigitated neural circuits. Here, we posit that the establishment, properties, and dynamics of synapses are governed by a molecular logic that is controlled by diverse trans-synaptic signaling molecules. Neurexins, expressed in thousands of alternatively spliced isoforms, are central components of this dynamic code. Presynaptic neurexins regulate synapse properties via differential binding to multifarious postsynaptic ligands, such as neuroligins, cerebellin/GluD complexes, and latrophilins, thereby shaping the input/output relations of their resident neural circuits. Mutations in genes encoding neurexins and their ligands are associated with diverse neuropsychiatric disorders, especially schizophrenia, autism, and Tourette syndrome. Thus, neurexins nucleate an overall trans-synaptic signaling network that controls synapse properties, which thereby determines the precise responses of synapses to spike patterns in a neuron and circuit and which is vulnerable to impairments in neuropsychiatric disorders.

Cerebellar Shank2 Regulates Excitatory Synapse Density, Motor Coordination, and Specific Repetitive and Anxiety-Like Behaviors

Shank2 is a multidomain scaffolding protein implicated in the structural and functional coordination of multiprotein complexes at excitatory postsynaptic sites as well as in psychiatric disorders, including autism spectrum disorders. While Shank2 is strongly expressed in the cerebellum, whether Shank2 regulates cerebellar excitatory synapses, or contributes to the behavioral abnormalities observed in Shank2^-/- mice, remains unexplored. Here we show that Shank2^-/- mice show reduced excitatory synapse density in cerebellar Purkinje cells in association with reduced levels of excitatory postsynaptic proteins, including GluD2 and PSD-93, and impaired motor coordination in the Erasmus test. Shank2 deletion restricted to Purkinje cells (Pcp2-Cre;Shank2^fl/fl mice) leads to similar reductions in excitatory synapse density, synaptic protein levels, and motor coordination. Pcp2-Cre;Shank2^fl/fl mice do not recapitulate autistic-like behaviors observed in Shank2^-/- mice, such as social interaction deficits, altered ultrasonic vocalizations, repetitive behaviors, and hyperactivity. However, Pcp2-Cre;Shank2^fl/fl mice display enhanced repetitive behavior in the hole-board test and anxiety-like behavior in the light-dark test, which are not observed in Shank2^-/- mice. These results implicate Shank2 in the regulation of cerebellar excitatory synapse density, motor coordination, and specific repetitive and anxiety-like behaviors.

SIGNIFICANCE STATEMENT

The postsynaptic side of excitatory synapses contains multiprotein complexes, termed the postsynaptic density, which contains receptors, scaffolding/adaptor proteins, and signaling molecules. Shank2 is an excitatory postsynaptic scaffolding protein implicated in the formation and functional coordination of the postsynaptic density and has been linked to autism spectrum disorders. Using Shank2-null mice and Shank2-conditional knock-out mice with a gene deletion restricted to cerebellar Purkinje cells, we explored functions of Shank2 in the cerebellum. We found that Shank2 regulates excitatory synapse density, motor coordination, and specific repetitive and anxiety-like behaviors, but is not associated with autistic-like social deficits or repetitive behaviors.

Retrograde BDNF to TrkB signaling promotes synapse elimination in the developing cerebellum

Elimination of early-formed redundant synapses during postnatal development is essential for functional neural circuit formation. Purkinje cells (PCs) in the neonatal cerebellum are innervated by multiple climbing fibers (CFs). A single CF is strengthened whereas the other CFs are eliminated in each PC dependent on postsynaptic activity in PC, but the underlying mechanisms are largely unknown. Here, we report that brain-derived neurotrophic factor (BDNF) from PC facilitates CF synapse elimination. By PC-specific deletion of BDNF combined with knockdown of BDNF receptors in CF, we show that BDNF acts retrogradely on TrkB in CFs, and facilitates elimination of CF synapses from PC somata during the third postnatal week. We also show that BDNF shares signaling pathway with metabotropic glutamate receptor 1, a key molecule that triggers a canonical pathway for CF synapse elimination. These results indicate that unlike other synapses, BDNF mediates punishment signal for synapse elimination in the developing cerebellum.

During development, synapses are selectively strengthened or eliminated by activity-dependent competition. Here, the authors show that BDNF-TrkB retrograde signaling is a “punishment” signal that leads to elimination of climbing fiber-onto-Purkinje cell synapses in the developing cerebellum.

Glutamate transporter GLAST controls synaptic wrapping by Bergmann glia and ensures proper wiring of Purkinje cells

Astrocytes regulate synaptic transmission through controlling neurotransmitter concentrations around synapses. Little is known, however, about their roles in neural circuit development. Here we report that Bergmann glia (BG), specialized cerebellar astrocytes that thoroughly enwrap Purkinje cells (PCs), are essential for synaptic organization in PCs through the action of the l-glutamate/l-aspartate transporter (GLAST). In GLAST-knockout mice, dendritic innervation by the main ascending climbing fiber (CF) branch was significantly weakened, whereas the transverse branch, which is thin and nonsynaptogenic in control mice, was transformed into thick and synaptogenic branches. Both types of CF branches frequently produced aberrant wiring to proximal and distal dendrites, causing multiple CF-PC innervation. Our electrophysiological analysis revealed that slow and small CF-evoked excitatory postsynaptic currents (EPSCs) were recorded from almost all PCs in GLAST-knockout mice. These atypical CF-EPSCs were far more numerous and had significantly faster 10-90% rise time than those elicited by glutamate spillover under pharmacological blockade of glial glutamate transporters. Innervation by parallel fibers (PFs) was also affected. PF synapses were robustly increased in the entire dendritic trees, leading to impaired segregation of CF and PF territories. Furthermore, lamellate BG processes were retracted from PC dendrites and synapses, leading to the exposure of these neuronal elements to the extracellular milieus. These synaptic and glial phenotypes were reproduced in wild-type mice after functional blockade of glial glutamate transporters. These findings highlight that glutamate transporter function by GLAST on BG plays important roles in development and maintenance of proper synaptic wiring and wrapping in PCs.

GluD2 Endows Parallel Fiber-Purkinje Cell Synapses with a High Regenerative Capacity

Although injured axons usually do not regenerate in the adult CNS, parallel fibers (PFs) regenerate synaptic connections onto cerebellar Purkinje cells (PCs). In this study, we investigated the role of GluD2 in this regenerative process after PF transection using GluD2-knock-out (KO) mice. All dendritic spines on distal dendrites were innervated by PFs in sham-operated wild-type controls, whereas one-third were devoid of innervation in GluD2-KO mice. In both genotypes, a steep drop in the number of PF synapses occurred with a reciprocal surge in the number of free spines on postlesion day 1, when the PF territory aberrantly expanded toward the proximal dendrites. In wild-type mice, the territory and number of PF synapses were nearly fully restored to normal on postlesion day 7, although PF density remained low. Moreover, presynaptic and postsynaptic elements were markedly enlarged, and the PF terminal-to-PC spine contact ratio increased from 1:1 to 1:2 at most synapses. On postlesion day 30, the size and contact ratio of PF synapses returned to sham-operated control values and PF density recovered through the sprouting and elongation of PF collaterals. However, GluD2-KO mice showed neither a hypertrophic response nor territorial restoration 7 d postlesion, nor the recovery of PF axons or synapses on postlesion day 30. This suggests that PF wiring regenerates initially by inducing hypertrophic responses in surviving synaptic elements (hypertrophic phase), followed by collateral formation by PF axons and retraction of PF synapses (remodeling phase). Without GluD2, no transition to these regenerative phases occurs.

SIGNIFICANCE STATEMENT

The glutamate receptor GluD2 expressed at parallel fiber (PF)-Purkinje cell (PC) synapses regulates the formation and maintenance of the synapses. To investigate the role of GluD2 in their extraordinarily high regenerative capacity, the process after surgical transection of PFs was compared between wild-type and GluD2-knock-out mice. We discovered that, in wild-type mice, PF synapses regenerate initially by inducing hypertrophic responses in surviving synaptic elements, and then by sprouting and elongation of PF collaterals. Subsequently, hypertrophied PF synapses remodel into compact synapses. In GluD2-knock-out mice, PF wiring remains in the degenerative phase, showing neither a hypertrophic response nor recovery of PF axons or synapses. Our finding thus highlights that synaptic connection in the adult brain can regenerate with aid of GluD2.

Purkinje Cells Are More Vulnerable to the Specific Depletion of Cathepsin D Than to That of Atg7

Neurologic phenotypes of cathepsin D (CTSD)-deficient mice, a murine model of neuronal ceroid lipofuscinoses, indicate the importance of CTSD for the maintenance of metabolism in central nervous system neurons. To further understand the role of CTSD in central nervous system neurons, we generated mice with a CTSD deficiency specifically in the Purkinje cells (PCs) (CTSD^Flox/Flox;GRID2-Cre) and compared their phenotypes with those of PC-selective Atg7-deficient (Atg7^Flox/Flox;GRID2-Cre) mice. In both strains of mice, PCs underwent degeneration, but the CTSD-deficient PCs disappeared more rapidly than their Atg7-deficient counterparts. When CTSD-deficient PCs died, the neuronal cell bodies became shrunken, filled with autophagosomes and autolysosomes, and had nuclei with dispersed small chromatin fragments. The dying Atg7-deficient PCs also showed similar ultrastructures, indicating that the neuronal cell death of CTSD- and Atg7-deficient PCs was distinct from apoptosis. Immunohistochemical observations showed the formation of calbindin-positive axonal spheroids and the swelling of vesicular GABA transporter-positive presynaptic terminals that were more pronounced in Atg7-deficient PCs than in CTSD-deficient PCs. An accumulation of tubular vesicles may have derived from the smooth endoplasmic reticulum; nascent autophagosome-like structures with double membranes was a common feature in the swollen axons of these PCs. These results suggested that PCs were more vulnerable to CTSD deficiency in lysosomes than to autophagy impairment, and this vulnerability does not depend on the severity of axonal swelling.

A GluD Coming-Of-Age Story

The GluD1 and GluD2 receptors form the GluD ionotropic glutamate receptor (iGluR) subfamily. Without known endogenous ligands, they have long been referred to as 'orphan' and remained enigmatic functionally. Recent progress has, however, radically changed this view. Both GluD receptors are expressed in wider brain regions than originally thought. Human genetic studies and analyses of knockout mice have revealed their involvement in multiple neurodevelopmental and psychiatric disorders. The discovery of endogenous ligands, together with structural investigations, has opened the way towards a mechanistic understanding of GluD signaling at central nervous system synapses. These studies have also prompted the hypothesis that all iGluRs, and potentially other neurotransmitter receptors, rely on the cooperative binding of extracellular small-molecule and protein ligands for physiological signaling.

TARP γ-2 and γ-8 Differentially Control AMPAR Density Across Schaffer Collateral/Commissural Synapses in the Hippocampal CA1 Area

The number of AMPA-type glutamate receptors (AMPARs) at synapses is the major determinant of synaptic strength and varies from synapse to synapse. To clarify the underlying molecular mechanisms, the density of AMPARs, PSD-95, and transmembrane AMPAR regulatory proteins (TARPs) were compared at Schaffer collateral/commissural (SCC) synapses in the adult mouse hippocampal CA1 by quantitative immunogold electron microscopy using serial sections. We examined four types of SCC synapses: perforated and nonperforated synapses on pyramidal cells and axodendritic synapses on parvalbumin-positive (PV synapse) and pravalbumin-negative interneurons (non-PV synapse). SCC synapses were categorized into those expressing high-density (perforated and PV synapses) or low-density (nonperforated and non-PV synapses) AMPARs. Although the density of PSD-95 labeling was fairly constant, the density and composition of TARP isoforms was highly variable depending on the synapse type. Of the three TARPs expressed in hippocampal neurons, the disparity in TARP γ-2 labeling was closely related to that of AMPAR labeling. Importantly, AMPAR density was significantly reduced at perforated and PV synapses in TARP γ-2-knock-out (KO) mice, resulting in a virtual loss of AMPAR disparity among SCC synapses. In comparison, TARP γ-8 was the only TARP expressed at nonperforated synapses, where AMPAR labeling further decreased to a background level in TARP γ-8-KO mice. These results show that synaptic inclusion of TARP γ-2 potently increases AMPAR expression and transforms low-density synapses into high-density ones, whereas TARP γ-8 is essential for low-density or basal expression of AMPARs at nonperforated synapses. Therefore, these TARPs are critically involved in AMPAR density control at SCC synapses.

SIGNIFICANCE STATEMENT

Although converging evidence implicates the importance of transmembrane AMPA-type glutamate receptor (AMPAR) regulatory proteins (TARPs) in AMPAR stabilization during basal transmission and synaptic plasticity, how they control large disparities in AMPAR numbers or densities across central synapses remains largely unknown. We compared the density of AMPARs with that of TARPs among four types of Schaffer collateral/commissural (SCC) hippocampal synapses in wild-type and TARP-knock-out mice. We show that the density of AMPARs correlates with that of TARP γ-2 across SCC synapses and its high expression is linked to high-density AMPAR expression at perforated type of pyramidal cell synapses and synapses on parvalbumin-positive interneurons. In comparison, TARP γ-8 is the only TARP expressed at nonperforated type of pyramidal cell synapses, playing an essential role in low-density or basal AMPAR expression.

Neuroligins Are Selectively Essential for NMDAR Signaling in Cerebellar Stellate Interneurons

Neuroligins are postsynaptic cell-adhesion molecules that contribute to synapse specification. However, many other postsynaptic cell-adhesion molecules are known and the relative contributions of neuroligins versus other such molecules in different types of synapses and neurons remains largely unknown. Here, we have studied the role of neuroligins in cerebellar stellate interneurons that participate in a well defined circuit that converges on Purkinje cells as the major output neurons of cerebellar cortex. By crossing triple conditional knock-out (cKO) mice targeting all three major neuroligins [neuroligin-1 to neuroligin-3 (NL123)] with parvalbumin-Cre (PV-Cre) transgenic mice, we deleted neuroligins from inhibitory cerebellar interneurons and Purkinje cells, allowing us to study the effects of neuroligin deletions on cerebellar stellate cell synapses by electrophysiology in acute slices. PV-Cre/NL123 cKO mice did not exhibit gross alterations of cerebellar structure or cerebellar interneuron morphology. Strikingly, electrophysiological recordings in stellate cells from these PV-Cre/NL123 cKO mice revealed a large decrease in NMDAR-mediated excitatory synaptic responses, which, in stellate cells, are largely extrasynaptic, without a change in AMPA-receptor-mediated responses. Parallel analyses in PV-Cre/NL1 mice that are single NL1 cKO mice uncovered the same phenotype, demonstrating that NL1 is responsible for recruiting extrasynaptic NMDARs. Moreover, we observed only a modest impairment in inhibitory synaptic responses in stellate cells lacking NL123 despite a nearly complete suppression of inhibitory synaptic transmission in Purkinje cells by the same genetic manipulation. Our results suggest that, unlike other types of neurons investigated, neuroligins are selectively essential in cerebellar stellate interneurons for enabling the function of extrasynaptic NMDARs.

SIGNIFICANCE STATEMENT

Neuroligins are postsynaptic cell-adhesion molecules genetically linked to autism. However, the contributions of neuroligins to interneuron functions remain largely unknown. Here, we analyzed the role of neuroligins in cerebellar stellate interneurons. We deleted neuroligin-1, neuroligin-2, and neuroligin-3, the major cerebellar neuroligin isoforms, from stellate cells in triple NL123 conditional knock-out mice and analyzed synaptic responses by acute slice electrophysiology. We find that neuroligins are selectively essential for extrasynaptic NMDAR-mediated signaling, but dispensable for both AMPAR-mediated and inhibitory synaptic transmission. Our results reveal a critical and selective role for neuroligins in the regulation of NMDAR responses in cerebellar stellate interneurons.

Train stimulation of parallel fibre to Purkinje cell inputs reveals two populations of synaptic responses with different receptor signatures

KEY POINTS

Purkinje cells of the cerebellum receive ∼180,000 parallel fibre synapses, which have often been viewed as a homogeneous synaptic population and studied using single action potentials. Many parallel fibre synapses might be silent, however, and granule cells in vivo fire in bursts. Here, we used trains of stimuli to study parallel fibre inputs to Purkinje cells in rat cerebellar slices. Analysis of train EPSCs revealed two synaptic components, phase 1 and 2. Phase 1 is initially large and saturates rapidly, whereas phase 2 is initially small and facilitates throughout the train. The two components have a heterogeneous distribution at dendritic sites and different pharmacological profiles. The differential sensitivity of phase 1 and phase 2 to inhibition by pentobarbital and NBQX mirrors the differential sensitivity of AMPA receptors associated with the transmembrane AMPA receptor regulatory protein, γ-2, gating in the low- and high-open probability modes, respectively.

ABSTRACT

Cerebellar granule cells fire in bursts, and their parallel fibre axons (PFs) form ∼180,000 excitatory synapses onto the dendritic tree of a Purkinje cell. As many as 85% of these synapses have been proposed to be silent, but most are labelled for AMPA receptors. Here, we studied PF to Purkinje cell synapses using trains of 100 Hz stimulation in rat cerebellar slices. The PF train EPSC consisted of two components that were present in variable proportions at different dendritic sites: one, with large initial EPSC amplitude, saturated after three stimuli and dominated the early phase of the train EPSC; and the other, with small initial amplitude, increased steadily throughout the train of 10 stimuli and dominated the late phase of the train EPSC. The two phases also displayed different pharmacological profiles. Phase 2 was less sensitive to inhibition by NBQX but more sensitive to block by pentobarbital than phase 1. Comparison of synaptic results with fast glutamate applications to recombinant receptors suggests that the high-open-probability gating mode of AMPA receptors containing the auxiliary subunit transmembrane AMPA receptor regulatory protein γ-2 makes a substantial contribution to phase 2. We argue that the two synaptic components arise from AMPA receptors with different functional signatures and synaptic distributions. Comparisons of voltage- and current-clamp responses obtained from the same Purkinje cells indicate that phase 1 of the EPSC arises from synapses ideally suited to transmit short bursts of action potentials, whereas phase 2 is likely to arise from low-release-probability or 'silent' synapses that are recruited during longer bursts.

Antipsychotic pharmacogenomics in first episode psychosis: a role for glutamate genes

Genetic factors may underlie beneficial and adverse responses to antipsychotic treatment. These relationships may be easier to identify among patients early in the course of disease who have limited exposure to antipsychotic drugs. We examined 86 first episode patients (schizophrenia, psychotic bipolar disorder and major depressive disorder with psychotic features) who had minimal to no prior antipsychotic exposure in a 6-week pharmacogenomic study of antipsychotic treatment response. Response was measured by change in Brief Psychiatric Rating Scale total score. Risperidone monotherapy was the primary antipsychotic treatment. Pharmacogenomic association studies were completed to (1) examine candidate single-nucleotide polymorphisms (SNPs) in genes known to be involved with glutamate signaling, and (2) conduct an exploratory genome-wide association study of symptom response to identify potential novel associations for future investigation. Two SNPs in GRM7 (rs2069062 and rs2014195) were significantly associated with antipsychotic response in candidate gene analysis, as were two SNPs in the human glutamate receptor delta 2 (GRID2) gene (rs9307122 and rs1875705) in genome-wide association analysis. Further examination of these findings with those from a separate risperidone-treated study sample demonstrated that top SNPs in both studies were overrepresented in glutamate genes and that there were similarities in neurodevelopmental gene categories associated with drug response from both study samples. These associations indicate a role for gene variants related to glutamate signaling and antipsychotic response with more broad association patterns indicating the potential importance of genes involved in neuronal development.

Synapse-specific expression of calcium-permeable AMPA receptors in neocortical layer 5

Key points

In the hippocampus, calcium‐permeable AMPA receptors have been found in a restricted subset of neuronal types that inhibit other neurons, although their localization in the neocortex is less well understood.

In the present study, we looked for calcium‐permeable AMPA receptors in two distinct populations of neocortical inhibitory neurons: basket cells and Martinotti cells. We found them in the former but not in the latter. Furthermore, in basket cells, these receptors were associated with particularly fast responses.

Computer modelling predicted (and experiments verified) that fast calcium‐permeable AMPA receptors enable basket cells to respond rapidly, such that they promptly inhibit neighbouring cells and shut down activity.

The results obtained in the present study help our understanding of pathologies such as stroke and epilepsy that have been associated with disordered regulation of calcium‐permeable AMPA receptors.

Abstract

AMPA‐type glutamate receptors (AMPARs) lacking an edited GluA2 subunit are calcium‐permeable (CP) and contribute to synaptic plasticity in several hippocampal interneuron types, although their precise role in the neocortex is not well described. We explored the presence of CP‐AMPARs at pyramidal cell (PC) inputs to Martinotti cells (MCs) and basket cells (BCs) in layer 5 of the developing mouse visual cortex (postnatal days 12–21). GluA2 immunolabelling was stronger in MCs than in BCs. A differential presence of CP‐AMPARs at PC‐BC and PC‐MC synapses was confirmed electrophysiologically, based on measures of spermine‐dependent rectification and CP‐AMPAR blockade by 1‐naphtyl acetyl spermine using recordings from synaptically connected cell pairs, NPEC‐AMPA uncaging and miniature current recordings. In addition, CP‐AMPAR expression in BCs was correlated with rapidly decaying synaptic currents. Computer modelling predicted that this reduces spike latencies and sharpens suprathreshold responses in BCs, which we verified experimentally using the dynamic clamp technique. Thus, the synapse‐specific expression of CP‐AMPARs may critically influence both plasticity and information processing in neocortical microcircuits.

In the hippocampus, calcium‐permeable AMPA receptors have been found in a restricted subset of neuronal types that inhibit other neurons, although their localization in the neocortex is less well understood.

In the present study, we looked for calcium‐permeable AMPA receptors in two distinct populations of neocortical inhibitory neurons: basket cells and Martinotti cells. We found them in the former but not in the latter. Furthermore, in basket cells, these receptors were associated with particularly fast responses.

Computer modelling predicted (and experiments verified) that fast calcium‐permeable AMPA receptors enable basket cells to respond rapidly, such that they promptly inhibit neighbouring cells and shut down activity.

The results obtained in the present study help our understanding of pathologies such as stroke and epilepsy that have been associated with disordered regulation of calcium‐permeable AMPA receptors.

Altered Actions of Memantine and NMDA-Induced Currents in a New Grid2-Deleted Mouse Line

Memantine is a non-competitive antagonist of the N-methyl-d-aspartate (NMDA) receptor, and is an approved drug for the treatment of moderate-to-severe Alzheimer’s disease. We identified a mouse strain with a naturally occurring mutation and an ataxic phenotype that presents with severe leg cramps. To investigate the phenotypes of these mutant mice, we screened several phenotype-modulating drugs and found that memantine (10 mg/kg) disrupted the sense of balance in the mutants. Moreover, the mutant mice showed an attenuated optokinetic response (OKR) and impaired OKR learning, which was also observed in wild-type mice treated with memantine. Microsatellite analyses indicated that the Grid2 gene-deletion is responsible for these phenotypes. Patch-clamp analysis showed a relatively small change in NMDA-dependent current in cultured granule cells from Grid2 gene-deleted mice, suggesting that GRID2 is important for correct NMDA receptor function. In general, NMDA receptors are activated after the activation of non-NMDA receptors, such as AMPA receptors, and AMPA receptor dysregulation also occurs in Grid2 mutant mice. Indeed, the AMPA treatment enhanced memantine susceptibility in wild-type mice, which was indicated by balance sense and OKR impairments. The present study explores a new role for GRID2 and highlights the adverse effects of memantine in different genetic backgrounds.

The human δ2 glutamate receptor gene is not mutated in patients with spinocerebellar ataxia

The human glutamate receptor delta 2 gene (GRID2) shares 90% homology with the orthologous mouse gene. The mouse Grid2 gene is involved with functions of the cerebellum and spontaneous mutation of Grid2 leads to a spinocerebellar ataxia-like phenotype. To investigate whether such mutations occur in humans, we screened for mutations in the coding sequence of GRID2 in 24 patients with familial or sporadic spinocerebellar ataxia and in 52 normal controls. We detected no point mutations or insertion/deletion mutations in the 16 exons of GRID2. However, a polymorphic 4 nucleotide deletion (IVS5-121_-118 GAGT) and two single nucleotide polymorphisms (c.1251G>T and IVS14-63C>G) were identified. The frequency of these polymorphisms was similar between spinocerebellar ataxia patients and normal controls. These data indicate that spontaneous mutations do not occur in GRID2 and that the incidence of spinocerebellar ataxia in humans is not associated with GRID2 mutation or polymorphisms.

Enriched expression of GluD1 in higher brain regions and its involvement in parallel fiber-interneuron synapse formation in the cerebellum

Of the two members of the δ subfamily of ionotropic glutamate receptors, GluD2 is exclusively expressed at parallel fiber-Purkinje cell (PF-PC) synapses in the cerebellum and regulates their structural and functional connectivity. However, little is known to date regarding cellular and synaptic expression of GluD1 and its role in synaptic circuit formation. In the present study, we investigated this issue by producing specific and sensitive histochemical probes for GluD1 and analyzing cerebellar synaptic circuits in GluD1-knock-out mice. GluD1 was widely expressed in the adult mouse brain, with high levels in higher brain regions, including the cerebral cortex, striatum, limbic regions (hippocampus, nucleus accumbens, lateral septum, bed nucleus stria terminalis, lateral habenula, and central nucleus of the amygdala), and cerebellar cortex. In the cerebellar cortex, GluD1 mRNA was expressed at the highest level in molecular layer interneurons and its immunoreactivity was concentrated at PF synapses on interneuron somata. In GluD1-knock-out mice, the density of PF synapses on interneuron somata was significantly reduced and the size and number of interneurons were significantly diminished. Therefore, GluD1 is common to GluD2 in expression at PF synapses, but distinct from GluD2 in neuronal expression in the cerebellar cortex; that is, GluD1 in interneurons and GluD2 in PCs. Furthermore, GluD1 regulates the connectivity of PF-interneuron synapses and promotes the differentiation and/or survival of molecular layer interneurons. These results suggest that GluD1 works in concert with GluD2 for the construction of cerebellar synaptic wiring through distinct neuronal and synaptic expressions and also their shared synapse-connecting function.

STIM1 controls neuronal Ca²⁺ signaling, mGluR1-dependent synaptic transmission, and cerebellar motor behavior

In central mammalian neurons, activation of metabotropic glutamate receptor type1 (mGluR1) evokes a complex synaptic response consisting of IP3 receptor-dependent Ca(2+) release from internal Ca(2+) stores and a slow depolarizing potential involving TRPC3 channels. It is largely unclear how mGluR1 is linked to its downstream effectors. Here, we explored the role of stromal interaction molecule 1 (STIM1) in regulating neuronal Ca(2+) signaling and mGluR1-dependent synaptic transmission. By analyzing mouse cerebellar Purkinje neurons, we demonstrate that STIM1 is an essential regulator of the Ca(2+) level in neuronal endoplasmic reticulum Ca(2+) stores. Both mGluR1-dependent synaptic potentials and IP3 receptor-dependent Ca(2+) signals are strongly attenuated in the absence of STIM1. Furthermore, the Purkinje neuron-specific deletion of Stim1 causes impairments in cerebellar motor behavior. Together, our results demonstrate that in the mammalian nervous system STIM1 is a key regulator of intracellular Ca(2+) signaling, metabotropic glutamate receptor-dependent synaptic transmission, and motor coordination.

Cbln1 downregulates the formation and function of inhibitory synapses in mouse cerebellar Purkinje cells

The formation of excitatory and inhibitory synapses must be tightly coordinated to establish functional neuronal circuitry during development. In the cerebellum, the formation of excitatory synapses between parallel fibers and Purkinje cells is strongly induced by Cbln1, which is released from parallel fibers and binds to the postsynaptic δ2 glutamate receptor (GluD2). However, Cbln1's role, if any, in inhibitory synapse formation has been unknown. Here, we show that Cbln1 downregulates the formation and function of inhibitory synapses between Purkinje cells and interneurons. Immunohistochemical analyses with an anti-vesicular GABA transporter antibody revealed an increased density of interneuron-Purkinje cell synapses in the cbln1-null cerebellum. Whole-cell patch-clamp recordings from Purkinje cells showed that both the amplitude and frequency of miniature inhibitory postsynaptic currents were increased in cbln1-null cerebellar slices. A 3-h incubation with recombinant Cbln1 reversed the increased amplitude of inhibitory currents in Purkinje cells in acutely prepared cbln1-null slices. Furthermore, an 8-day incubation with recombinant Cbln1 reversed the increased interneuron-Purkinje cell synapse density in cultured cbln1-null slices. In contrast, recombinant Cbln1 did not affect cerebellar slices from mice lacking both Cbln1 and GluD2. Finally, we found that tyrosine phosphorylation was upregulated in the cbln1-null cerebellum, and acute inhibition of Src-family kinases suppressed the increased inhibitory postsynaptic currents in cbln1-null Purkinje cells. These findings indicate that Cbln1-GluD2 signaling inhibits the number and function of inhibitory synapses, and shifts the excitatory-inhibitory balance towards excitation in Purkinje cells. Cbln1's effect on inhibitory synaptic transmission is probably mediated by a tyrosine kinase pathway.

Reevaluation of the role of parallel fiber synapses in delay eyeblink conditioning in mice using Cbln1 as a tool

The delay eyeblink conditioning (EBC) is a cerebellum-dependent type of associative motor learning. However, the exact roles played by the various cerebellar synapses, as well as the underlying molecular mechanisms, remain to be determined. It is also unclear whether long-term potentiation (LTP) or long-term depression (LTD) at parallel fiber (PF)–Purkinje cell (PC) synapses is involved in EBC. In this study, to clarify the role of PF synapses in the delay EBC, we used mice in which a gene encoding Cbln1 was disrupted (cbln1^-/- mice), which display severe reduction of PF–PC synapses. We showed that delay EBC was impaired in cbln1^-/- mice. Although PF-LTD was impaired, PF-LTP was normally induced in cbln1^-/- mice. A single recombinant Cbln1 injection to the cerebellar cortex in vivo completely, though transiently, restored the morphology and function of PF–PC synapses and delay EBC in cbln1^-/- mice. Interestingly, the cbln1^-/- mice retained the memory for at least 30 days, after the Cbln1 injection’s effect on PF synapses had abated. Furthermore, delay EBC memory could be extinguished even after the Cbln1 injection’s effect were lost. These results indicate that intact PF–PC synapses and PF-LTD, not PF-LTP, are necessary to acquire delay EBC in mice. In contrast, extracerebellar structures or remaining PF–PC synapses in cbln1^-/- mice may be sufficient for the expression, maintenance, and extinction of its memory trace.

Autoantibodies to epilepsy-related LGI1 in limbic encephalitis neutralize LGI1-ADAM22 interaction and reduce synaptic AMPA receptors

More than 30 mutations in LGI1, a secreted neuronal protein, have been reported with autosomal dominant lateral temporal lobe epilepsy (ADLTE). Although LGI1 haploinsufficiency is thought to cause ADLTE, the underlying molecular mechanism that results in abnormal brain excitability remains mysterious. Here, we focused on a mode of action of LGI1 autoantibodies associated with limbic encephalitis (LE), which is one of acquired epileptic disorders characterized by subacute onset of amnesia and seizures. We comprehensively screened human sera from patients with immune-mediated neurological disorders for LGI1 autoantibodies, which also uncovered novel autoantibodies against six cell surface antigens including DCC, DPP10, and ADAM23. Our developed ELISA arrays revealed a specific role for LGI1 antibodies in LE and concomitant involvement of multiple antibodies, including LGI1 antibodies in neuromyotonia, a peripheral nerve disorder. LGI1 antibodies associated with LE specifically inhibited the ligand-receptor interaction between LGI1 and ADAM22/23 by targeting the EPTP repeat domain of LGI1 and reversibly reduced synaptic AMPA receptor clusters in rat hippocampal neurons. Furthermore, we found that disruption of LGI1-ADAM22 interaction by soluble extracellular domain of ADAM22 was sufficient to reduce synaptic AMPA receptors in rat hippocampal neurons and that levels of AMPA receptor were greatly reduced in the hippocampal dentate gyrus in the epileptic LGI1 knock-out mouse. Therefore, either genetic or acquired loss of the LGI1-ADAM22 interaction reduces the AMPA receptor function, causing epileptic disorders. These results suggest that by finely regulating the synaptic AMPA receptors, the LGI1-ADAM22 interaction maintains physiological brain excitability throughout life.

Deletions in GRID2 lead to a recessive syndrome of cerebellar ataxia and tonic upgaze in humans

OBJECTIVE

To identify the genetic cause of a syndrome causing cerebellar ataxia and eye movement abnormalities.

METHODS

We identified 2 families with cerebellar ataxia, eye movement abnormalities, and global developmental delay. We performed genetic analyses including single nucleotide polymorphism genotyping, linkage analysis, array comparative genomic hybridization, quantitative PCR, and Sanger sequencing. We obtained eye movement recordings of mutant mice deficient for the ortholog of the identified candidate gene, and performed immunohistochemistry using human and mouse brain specimens.

RESULTS

All affected individuals had ataxia, eye movement abnormalities, most notably tonic upgaze, and delayed speech and cognitive development. Homozygosity mapping identified the disease locus on chromosome 4q. Within this region, a homozygous deletion of GRID2 exon 4 in the index family and compound heterozygous deletions involving GRID2 exon 2 in the second family were identified. Grid2-deficient mice showed larger spontaneous and random eye movements compared to wild-type mice. In developing mouse and human cerebella, GRID2 localized to the Purkinje cell dendritic spines. Brain MRI in 2 affected children showed progressive cerebellar atrophy, which was more severe than that of Grid2-deficient mice.

CONCLUSIONS

Biallelic deletions of GRID2 lead to a syndrome of cerebellar ataxia and tonic upgaze in humans. The phenotypic resemblance and similarity in protein expression pattern between humans and mice suggest a conserved role for GRID2 in the synapse organization between parallel fibers and Purkinje cells. However, the progressive and severe cerebellar atrophy seen in the affected individuals could indicate an evolutionarily unique role for GRID2 in the human cerebellum.

Disruption of cerebellar microzonal organization in GluD2 (GluRδ2) knockout mouse

Cerebellar cortex has an elaborate rostrocaudal organization comprised of numerous microzones. Purkinje cells (PCs) in the same microzone show synchronous activity of complex spikes (CSs) evoked by excitatory inputs from climbing fibers (CFs) that arise from neurons in the inferior olive (IO). The synchronous CS activity is considered to depend on electrical coupling among IO neurons and anatomical organization of the olivo-cerebellar projection. To determine how the CF–PC wiring contributes to the formation of microzone, we examined the synchronous CS activities between neighboring PCs in the glutamate receptor δ2 knockout (GluD2 KO) mouse in which exuberant surplus CFs make ectopic innervations onto distal dendrites of PCs. We performed in vivo two-photon calcium imaging for PC populations to detect CF inputs. Neighboring PCs in GluD2 KO mice showed higher synchrony of calcium transients than those in wild-type (control) mice. Moreover, the synchrony in GluD2 KO mice hardly declined with mediolateral separation between PCs up to ~200 μm, which was in marked contrast to the falloff of the synchrony in control mice. The enhanced synchrony was only partially affected by the blockade of gap junctional coupling. On the other hand, transverse CF collaterals in GluD2 KO mice extended beyond the border of microzone and formed locally clustered ectopic synapses onto dendrites of neighboring PCs. Furthermore, PCs in GluD2 KO mice exhibited clustered firing (Cf), the characteristic CF response that was not found in PCs of wild-type mice. Importantly, Cf was often associated with localized calcium transients in distal dendrites of PCs, which are likely to contribute to the enhanced synchrony of calcium signals in GluD2 KO mice. Thus, our results indicate that CF signals in GluD2 KO mice propagate across multiple microzones, and that proper formation of longitudinal olivo-cerebellar projection is essential for the spatiotemporal organization of CS activity in the cerebellum.