Junko Motohashi

D -Serine regulates cerebellar LTD and motor coordination through the δ2 glutamate receptor

D-Serine (D-Ser) is an endogenous co-agonist for NMDA receptors and regulates neurotransmission and synaptic plasticity in the forebrain. D-Ser is also found in the cerebellum during the early postnatal period. Although D-Ser binds to the δ2 glutamate receptor (GluD2, Grid2) in vitro, its physiological significance has remained unclear. Here we show that D-Ser serves as an endogenous ligand for GluD2 to regulate long-term depression (LTD) at synapses between parallel fibers and Purkinje cells in the immature cerebellum. D-Ser was released mainly from Bergmann glia after the burst stimulation of parallel fibers in immature, but not mature, cerebellum. D-Ser rapidly induced endocytosis of AMPA receptors and mutually occluded LTD in wild-type, but not Grid2-null, Purkinje cells. Moreover, mice expressing mutant GluD2 in which the binding site for D-Ser was disrupted showed impaired LTD and motor dyscoordination during development. These results indicate that glial D-Ser regulates synaptic plasticity and cerebellar functions by interacting with GluD2.

Differential Regulation of Synaptic Plasticity and Cerebellar Motor Learning by the C-Terminal PDZ-Binding Motif of GluRδ2

The δ2 glutamate receptor (GluRδ2) is predominantly expressed in Purkinje cells and plays crucial roles in cerebellar functions: GluRδ2−/− mice display ataxia and impaired motor learning. In addition, long-term depression (LTD) at parallel fiber (PF)–Purkinje cell synapses is abrogated, and synapse formation with PFs and climbing fibers (CFs) is severely disturbed in GluRδ2−/− Purkinje cells. Recently, we demonstrated that abrogated LTD was restored in GluRδ2−/− Purkinje cells by the virus-mediated expression of the wild-type GluRδ2 transgene (Tgwt) but not by that of mutant GluRδ2 lacking the C-terminal seven residues to which several PDZ proteins bind (TgΔCT7). These results indicated that the C terminus of GluRδ2 conveys the signal(s) necessary for LTD. In contrast, other phenotypes of GluRδ2−/− cerebellum, especially morphological abnormalities at PF and CF synapses, could not be rescued by virus-mediated transient expression. Thus, whether these phenotypes are mediated by the same signaling pathway remains unclear. To address these issues and to further delineate the function of GluRδ2 in vivo, we generated transgenic mice that expressed TgΔCT7 on a GluRδ2−/− background. Interestingly, although TgΔCT7 restored abnormal PF and CF synapse formation almost completely, it could not rescue abrogated LTD in GluRδ2−/− Purkinje cells. Furthermore, although the gross motor discoordination of GluRδ2−/− mice was restored, the cerebellar motor learning underlying delayed eyeblink conditioning remained impaired. These results indicate that LTD induction and motor learning are regulated by signaling via the C-terminal end of GluRδ2, whereas other functions may be differentially regulated by other regions of GluRδ2.

Anterograde C1ql1 Signaling Is Required in Order to Determine and Maintain a Single-Winner Climbing Fiber in the Mouse Cerebellum

Neuronal networks are dynamically modified by selective synapse pruning during development and adulthood. However, how certain connections win the competition with others and are subsequently maintained is not fully understood. Here, we show that C1ql1, a member of the C1q family of proteins, is provided by climbing fibers (CFs) and serves as a crucial anterograde signal to determine and maintain the single-winner CF in the mouse cerebellum throughout development and adulthood. C1ql1 specifically binds to the brain-specific angiogenesis inhibitor 3 (Bai3), which is a member of the cell-adhesion G-protein-coupled receptor family and expressed on postsynaptic Purkinje cells. C1ql1-Bai3 signaling is required for motor learning but not for gross motor performance or coordination. Because related family members of C1ql1 and Bai3 are expressed in various brain regions, the mechanism described here likely applies to synapse formation, maintenance, and function in multiple neuronal circuits essential for important brain functions.

The N-Terminal Domain of GluD2 (GluRδ2) Recruits Presynaptic Terminals and Regulates Synaptogenesis in the Cerebellum In Vivo

The δ2 glutamate receptor (GluRδ2; GluD2), which is predominantly expressed on postsynaptic sites at parallel fiber (PF)–Purkinje cell synapses in the cerebellum, plays two crucial roles in the cerebellum: the formation of PF synapses and the regulation of long-term depression (LTD), a form of synaptic plasticity underlying motor learning. Although the induction of LTD and motor learning absolutely require signaling via the cytoplasmic C-terminal domain of GluD2, the mechanisms by which GluD2 regulates PF synaptogenesis have remained unclear. Here, we examined the role of the extracellular N-terminal domain (NTD) of GluD2 on PF synaptogenesis by injecting Sindbis virus carrying wild-type (GluD2wt) or mutant GluD2 into the subarachnoid supracerebellar space of GluD2-null mice. Remarkably, the expression of GluD2wt, but not of a mutant GluD2 lacking the NTD (GluD2ΔNTD), rapidly induced PF synapse formation and rescued gross motor dyscoordination in adult GluD2-null mice just 1 d after injection. In addition, although the kainate receptor GluR6 (GluK2) did not induce PF synaptogenesis, a chimeric GluK2 that contained the NTD of GluD2 (GluD2NTD–GluK2) did. Similarly, GluD2wt and GluD2NTD–GluK2, but not GluD2ΔNTD, induced synaptogenesis in heterologous cells in vitro. In contrast, LTD was restored in GluD2-null Purkinje cells expressing a mutant GluD2 lacking the NTD. These results indicate that the NTD of GluD2 is necessary and sufficient for the function of GluD2 in the regulation of PF–Purkinje cell synaptogenesis. Furthermore, our results suggest that GluD2 differently regulates PF synaptogenesis and cerebellar LTD through the extracellular NTD and the cytoplasmic C-terminal end, respectively.

Ca2+ permeability of the channel pore is not essential for the δ2 glutamate receptor to regulate synaptic plasticity and motor coordination

The δ2 glutamate receptor (GluRδ2) plays a crucial role in cerebellar functions; mice with a disrupted GluRδ2 gene (GluRδ2−/−) display impaired synapse formation and abrogated long‐term depression (LTD). However, the mechanisms by which GluRδ2 functions have remained unclear. Because a GluRδ2 mutation in lurcher mice causes channel activities characterized by Ca2+ permeability, GluRδ2 was previously suggested to serve as a Ca2+‐permeable channel in Purkinje cells. To test this hypothesis, we introduced a GluRδ2 transgene, which had a mutation (Gln618Arg) in the putative channel pore, into GluRδ2−/− mice. Interestingly, the mutant transgene rescued the major functional and morphological abnormalities of GluRδ2−/− Purkinje cells, such as enhanced paired‐pulse facilitation, impaired LTD at parallel fibre synapses, and sustained innervation by multiple climbing fibres. These results indicate that the conserved glutamine residue in the channel pore, which is crucial for all Ca2+‐permeable glutamate receptors, is not essential for the function of GluRδ2.

Structural basis for integration of GluD receptors within synaptic organizer complexes

Transmitting signals across the synapse Glutamate receptors located on neuronal cells play a role in mediating electrical signals at excitatory synapses. These glutamatergic synapses are extremely important for nearly all cognitive functions. Elegheert et al. analyzed a complex that bridges the synapse, comprising β-neurexin 1, a cell adhesion molecule on the surface of presynaptic axons; cerebellin 1, a synaptic organizer; and the postsynaptic glutamate receptor GluD2. The structural and functional analysis provides insight into the mechanism of synaptic signaling. Science, this issue p. 295 A molecular bridge across excitatory synapses provides a structural framework that facilitates signaling in the cerebellum. Ionotropic glutamate receptor (iGluR) family members are integrated into supramolecular complexes that modulate their location and function at excitatory synapses. However, a lack of structural information beyond isolated receptors or fragments thereof currently limits the mechanistic understanding of physiological iGluR signaling. Here, we report structural and functional analyses of the prototypical molecular bridge linking postsynaptic iGluR δ2 (GluD2) and presynaptic β-neurexin 1 (β-NRX1) via Cbln1, a C1q-like synaptic organizer. We show how Cbln1 hexamers “anchor” GluD2 amino-terminal domain dimers to monomeric β-NRX1. This arrangement promotes synaptogenesis and is essential for d-serine–dependent GluD2 signaling in vivo, which underlies long-term depression of cerebellar parallel fiber–Purkinje cell (PF-PC) synapses and motor coordination in developing mice. These results lead to a model where protein and small-molecule ligands synergistically control synaptic iGluR function.

Reevaluation of Neurodegeneration in lurcher Mice: Constitutive Ion Fluxes Cause Cell Death with, Not by, Autophagy

The lurcher (Lc) mice have served as a valuable model for neurodegeneration for decades. Although the responsible mutation was identified in genes encoding δ2 glutamate receptors (GluD2s), which are predominantly expressed in cerebellar Purkinje cells, how the mutant receptor (GluD2Lc) triggers cell death has remained elusive. Here, taking advantage of recent knowledge about the domain structure of GluD2, we reinvestigated Lc-mediated cell death, focusing on the “autophagic cell death” hypothesis. Although autophagy and cell death were induced by the expression of GluD2Lc in heterologous cells and cultured neurons, they were blocked by the introduction of mutations in the channel pore domain of GluD2Lc or by removal of extracellular Na+. In addition, although GluD2Lc is reported to directly activate autophagy, mutant channels that are not associated with n-PIST (neuronal isoform of protein-interacting specifically with TC10)–Beclin1 still caused autophagy and cell death. Furthermore, cells expressing GluD2Lc showed decreased ATP levels and increased AMP-activated protein kinase (AMPK) activities in a manner dependent on extracellular Na+. Thus, constitutive currents were likely necessary and sufficient to induce autophagy via AMPK activation, regardless of the n-PIST–Beclin1 pathway in vitro. Interestingly, the expression of dominant-negative AMPK suppressed GluD2Lc-induced autophagy but did not prevent cell death in heterologous cells. Similarly, the disruption of Atg5, a gene crucial for autophagy, did not prevent but rather aggravated Purkinje-cell death in Lc mice. Furthermore, calpains were specifically activated in Lc Purkinje cells. Together, these results suggest that Lc-mediated cell death was not caused by autophagy but necrosis with autophagic features both in vivo and in vitro.

Ho15J- : A new hotfoot allele in a hot spot in the gene encoding the δ2 glutamate receptor

Hotfoot, a recessive mouse mutation characterized by ataxia and jerky movements of the hindlimbs, is caused by various mutations in the gene (Grid2) encoding the delta2 glutamate receptor (GluRdelta2). So far, at least 20 alleles, arising either spontaneously or through the random insertion of transgenes, have been described. Interestingly, most hotfoot mutants have deletions of one or more exons coding for portions of the most amino-terminal domain of GluRdelta2. However, because live mice colonies are no longer available for most hotfoot mutants, the possibility that the loss of a part of an intron might affect the splicing of other exons or the general efficiency of transcription could not be ruled out. Here, we report that a newly identified hotfoot mutant, ho15J, was caused by an intragenic deletion of the Grid2 gene, which indeed resulted in a new type of 52-amino-acid deletion in the most amino-terminal domain of GluRdelta2. Like GluRdelta2 proteins in ho4J mutants, GluRdelta2 proteins in ho15J mice were retained in the soma of Purkinje cells, where they were degraded. Long-term depression, a form of synaptic plasticity underlying information storage in the cerebellum, was abrogated, and ho15J mice showed severe motor discoordination on rotarod tests. The agreement between the PCR results for genomic DNA and the RT-PCR results for the ho15J allele supports the view that PCR analyses of grid2 genomic DNA can predict alterations in mRNA and protein. In addition, the present findings underscore the importance of the most amino-terminal domain in GluRdelta2 signaling and cerebellar functions.

Roles of Cbln1 in non-motor functions of mice

The cerebellum is thought to be involved in cognitive functions in addition to its well established role in motor coordination and motor learning in humans. Cerebellin 1 (Cbln1) is predominantly expressed in cerebellar granule cells and plays a crucial role in the formation and function of parallel fiber–Purkinje cell synapses. Although genes encoding Cbln1 and its postsynaptic receptor, the delta2 glutamate receptor (GluD2), are suggested to be associated with autistic-like traits and many psychiatric disorders, whether such cognitive impairments are caused by cerebellar dysfunction remains unclear. In the present study, we investigated whether and how Cbln1 signaling is involved in non-motor functions in adult mice. We show that acquisition and retention/retrieval of cued and contextual fear memory were impaired in Cbln1-null mice. In situ hybridization and immunohistochemical analyses revealed that Cbln1 is expressed in various extracerebellar regions, including the retrosplenial granular cortex and the hippocampus. In the hippocampus, Cbln1 immunoreactivity was present at the molecular layer of the dentate gyrus and the stratum lacunosum-moleculare without overt mRNA expression, suggesting that Cbln1 is provided by perforant path fibers. Retention/retrieval, but not acquisition, of cued and contextual fear memory was impaired in forebrain-predominant Cbln1-null mice. Spatial learning in the radial arm water maze was also abrogated. In contrast, acquisition of fear memory was affected in cerebellum-predominant Cbln1-null mice. These results indicate that Cbln1 in the forebrain and cerebellum mediates specific aspects of fear conditioning and spatial memory differentially and that Cbln1 signaling likely regulates motor and non-motor functions in multiple brain regions. SIGNIFICANCE STATEMENT Despites its well known role in motor coordination and motor learning, whether and how the cerebellum is involved in cognitive functions remains less clear. Cerebellin 1 (Cbln1) is highly expressed in the cerebellum and serves as an essential synaptic organizer. Although genes encoding Cbln1 and its receptor are associated with many psychiatric disorders, it remains unknown whether such cognitive impairments are caused by cerebellar dysfunction. Here, we show that Cbln1 is also expressed in the forebrain, including the hippocampus and retrosplenial granular cortex. Using forebrain- and cerebellum-predominant conditional Cbln1-null mice, we show that Cbln1 in the forebrain and cerebellum mediates specific aspects of fear conditioning and spatial memory differentially, indicating that Cbln1 signaling regulates both motor and non-motor functions in multiple brain regions.

Cellular and Subcellular Localization of Endogenous Neuroligin-1 in the Cerebellum

Synapses are precisely established, maintained, and modified throughout life by molecules called synaptic organizers, which include neurexins and neuroligins (Nlgn). Despite the importance of synaptic organizers in defining functions of neuronal circuits, the cellular and subcellular localization of many synaptic organizers has remained largely elusive because of the paucity of specific antibodies for immunohistochemical studies. In the present study, rather than raising specific antibodies, we generated knock-in mice in which a hemagglutinin (HA) epitope was inserted in the Nlgn1 gene. We have achieved high-throughput and precise gene editing by delivering the CRISPR/Cas9 system into zygotes. Using HA-Nlgn1 mice, we found that HA-Nlgn1 was enriched at synapses between parallel fibers and molecular layer interneurons (MLIs) and the glomeruli, in which mossy fiber terminals synapse onto granule cell dendrites. HA immunoreactivity was colocalized with postsynaptic density 95 at these synapses, indicating that endogenous Nlgn1 is localized at excitatory postsynaptic sites. In contrast, HA-Nlgn1 signals were very weak in dendrites and somata of Purkinje cells. Interestingly, HA-immunoreactivities were also observed in the pinceau, a specialized structure formed by MLI axons and astrocytes. HA-immunoreactivities in the pinceau were significantly reduced by knockdown of Nlgn1 in MLIs, indicating that in addition to postsynaptic sites, Nlgn1 is also localized at MLI axons. Our results indicate that epitope-tagging by electroporation-based gene editing with CRISPR/Cas9 is a viable and powerful method for mapping endogenous synaptic organizers with subcellular resolution, without the need for specific antibodies for each protein.