In vivo nanoscopic landscape of neurexin ligands underlying anterograde synapse specification.

Excitatory synapses are formed and matured by the cooperative actions of synaptic organizers, such as neurexins (Nrxns), neuroligins (Nlgns), LRRTMs, and Cbln1. Recent super-resolution nanoscopy developments have revealed that many synaptic organizers, as well as glutamate receptors and glutamate release machinery, exist as nanoclusters within synapses. However, it is unclear how such nanodomains interact with each other to organize excitatory synapses in vivo. By applying X10 expansion microscopy to epitope tag knockin mice, we found that Cbln1, Nlgn1, and LRRTM1, which share Nrxn as a common presynaptic receptor, form overlapping or separate nanodomains depending on Nrxn with or without a sequence encoded by splice site 4. The size and position of glutamate receptor nanodomains of GluD1, NMDA, and AMPA receptors were regulated by Cbln1, Nlgn1, and LRRTM1 nanodomains, respectively. These findings indicate that Nrxns anterogradely regulate the postsynaptic nanoscopic architecture of glutamate receptors through competition and coordination of Nrxn ligands.

Optimizing Nervous System-Specific Gene Targeting with Cre Driver Lines: Prevalence of Germline Recombination and Influencing Factors.

The Cre-loxP system is invaluable for spatial and temporal control of gene knockout, knockin, and reporter expression in the mouse nervous system. However, we report varying probabilities of unexpected germline recombination in distinct Cre driver lines designed for nervous system-specific recombination. Selective maternal or paternal germline recombination is showcased with sample Cre lines. Collated data reveal germline recombination in over half of 64 commonly used Cre driver lines, in most cases with a parental sex bias related to Cre expression in sperm or oocytes. Slight differences among Cre driver lines utilizing common transcriptional control elements affect germline recombination rates. Specific target loci demonstrated differential recombination; thus, reporters are not reliable proxies for another locus of interest. Similar principles apply to other recombinase systems and other genetically targeted organisms. We hereby draw attention to the prevalence of germline recombination and provide guidelines to inform future research for the neuroscience and broader molecular genetics communities.

Activity-Dependent Secretion of Synaptic Organizer Cbln1 from Lysosomes in Granule Cell Axons.

Synapse formation is achieved by various synaptic organizers. Although this process is highly regulated by neuronal activity, the underlying molecular mechanisms remain largely unclear. Here we show that Cbln1, a synaptic organizer of the C1q family, is released from lysosomes in axons but not dendrites of cerebellar granule cells in an activity- and Ca2+-dependent manner. Exocytosed Cbln1 was retained on axonal surfaces by binding to its presynaptic receptor neurexin. Cbln1 further diffused laterally along the axonal surface and accumulated at boutons by binding postsynaptic δ2 glutamate receptors. Cbln1 exocytosis was insensitive to tetanus neurotoxin, accompanied by cathepsin B release, and decreased by disrupting lysosomes. Furthermore, overexpression of lysosomal sialidase Neu1 not only inhibited Cbln1 and cathepsin B exocytosis in vitro but also reduced axonal bouton formation in vivo. Our findings imply that co-release of Cbln1 and cathepsin B from lysosomes serves as a new mechanism of activity-dependent coordinated synapse modification.

Optogenetic Control of Synaptic AMPA Receptor Endocytosis Reveals Roles of LTD in Motor Learning.

Long-term depression (LTD) of AMPA-type glutamate receptor (AMPA receptor)-mediated synaptic transmission has been proposed as a cellular substrate for learning and memory. Although activity-induced AMPA receptor endocytosis is believed to underlie LTD, it remains largely unclear whether LTD and AMPA receptor endocytosis at specific synapses are causally linked to learning and memory in vivo. Here we developed a new optogenetic tool, termed PhotonSABER, which enabled the temporal, spatial, and cell-type-specific control of AMPA receptor endocytosis at active synapses, while the basal synaptic properties and other forms of synaptic plasticity were unaffected. We found that fiberoptic illumination to Purkinje cells expressing PhotonSABER in vivo inhibited cerebellar motor learning during adaptation of the horizontal optokinetic response and vestibulo-ocular reflex, as well as synaptic AMPA receptor decrease in the flocculus. Our results demonstrate that LTD and AMPA receptor endocytosis at specific neuronal circuits were directly responsible for motor learning in vivo. VIDEO ABSTRACT.

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.

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.

Structural basis for integration of GluD receptors within synaptic organizer complexes.

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 ß-neurexin 1 (ß-NRX1) via Cbln1, a C1q-like synaptic organizer. We show how Cbln1 hexamers "anchor" GluD2 amino-terminal domain dimers to monomeric ß-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.

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.

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.

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 delta2 glutamate receptors (GluD2s), which are predominantly expressed in cerebellar Purkinje cells, how the mutant receptor (GluD2(Lc)) 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 GluD2(Lc) in heterologous cells and cultured neurons, they were blocked by the introduction of mutations in the channel pore domain of GluD2(Lc) or by removal of extracellular Na(+). In addition, although GluD2(Lc) 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 GluD2(Lc) 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 GluD2(Lc)-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.

The N-terminal domain of GluD2 (GluRdelta2) recruits presynaptic terminals and regulates synaptogenesis in the cerebellum in vivo.

The delta2 glutamate receptor (GluRdelta2; 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 (GluD2(wt)) or mutant GluD2 into the subarachnoid supracerebellar space of GluD2-null mice. Remarkably, the expression of GluD2(wt), but not of a mutant GluD2 lacking the NTD (GluD2(DeltaNTD)), 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 (GluD2(NTD)-GluK2) did. Similarly, GluD2(wt) and GluD2(NTD)-GluK2, but not GluD2(DeltaNTD), 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.

Differential regulation of synaptic plasticity and cerebellar motor learning by the C-terminal PDZ-binding motif of GluRdelta2.

The delta2 glutamate receptor (GluRdelta2) is predominantly expressed in Purkinje cells and plays crucial roles in cerebellar functions: GluRdelta2-/- 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 GluRdelta2-/- Purkinje cells. Recently, we demonstrated that abrogated LTD was restored in GluRdelta2-/- Purkinje cells by the virus-mediated expression of the wild-type GluRdelta2 transgene (Tg(wt)) but not by that of mutant GluRdelta2 lacking the C-terminal seven residues to which several PDZ proteins bind (Tg(DeltaCT7)). These results indicated that the C terminus of GluRdelta2 conveys the signal(s) necessary for LTD. In contrast, other phenotypes of GluRdelta2-/- 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 GluRdelta2 in vivo, we generated transgenic mice that expressed Tg(DeltaCT7) on a GluRdelta2-/- background. Interestingly, although Tg(DeltaCT7) restored abnormal PF and CF synapse formation almost completely, it could not rescue abrogated LTD in GluRdelta2-/- Purkinje cells. Furthermore, although the gross motor discoordination of GluRdelta2-/- 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 GluRdelta2, whereas other functions may be differentially regulated by other regions of GluRdelta2.

Ca2+ permeability of the channel pore is not essential for the delta2 glutamate receptor to regulate synaptic plasticity and motor coordination.

The delta2 glutamate receptor (GluRdelta2) plays a crucial role in cerebellar functions; mice with a disrupted GluRdelta2 gene (GluRdelta2-/-) display impaired synapse formation and abrogated long-term depression (LTD). However, the mechanisms by which GluRdelta2 functions have remained unclear. Because a GluRdelta2 mutation in lurcher mice causes channel activities characterized by Ca2+ permeability, GluRdelta2 was previously suggested to serve as a Ca2+-permeable channel in Purkinje cells. To test this hypothesis, we introduced a GluRdelta2 transgene, which had a mutation (Gln618Arg) in the putative channel pore, into GluRdelta2-/- mice. Interestingly, the mutant transgene rescued the major functional and morphological abnormalities of GluRdelta2-/- 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 GluRdelta2.

Ho15J: a new hotfoot allele in a hot spot in the gene encoding the delta2 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.

MR study of lateral retropharyngeal lymph nodes at different ages.

OBJECTIVE: The purpose of this study was to investigate the age-related morphologic changes of the lateral retropharyngeal lymph nodes using MR imaging. STUDY DESIGN: The maximal axial diameter of the nodes was measured on the MR image in 120 healthy subjects (younger group 6-19 years (n = 40), middle group 20-38 years (n = 48), older group 42-74 years (n = 32)). Between-group differences in the diameter of the nodes were analyzed. RESULTS: Mean values of the maximal axial diameter of the nodes were 9.0 +/- 1.6 mm, 6.6 +/- 1.7 mm, and 5.3 +/- 1.6 mm, corresponding to younger group, middle group, and older group, respectively ( P <.01). CONCLUSION: The information obtained in the current study is useful for the differential diagnosis of normal and abnormal lateral retropharyngeal lymph nodes according to age.

A BMAL1 mutant with arginine 91 substituted with alanine acts as a dominant negative inhibitor.

Basic-Helix-Loop-Helix-Per-Arnt-Sim (bHLH-PAS) transcription factor, Brain-Muscle-Arnt-Like-protein 1 (BMAL1), forms a heterodimer with the CLOCK protein. The BMAL1/CLOCK complex binds to a specific DNA sequence and plays an essential role in the generation of the circadian rhythm. The basic region of BMAL1 contains an E-R-X-R motif that is highly conserved among basic-helix-loop-helix (bHLH) transcription factors that bind to the E-box transcription element, and is thus thought to constitute a structure required for recognition of this DNA sequence. To understand the role of arginine 91 (E-K-R-R) within the basic region of BMAL1, we examined the effect of mutation of this residue on BMAL1 function. Co-immunoprecipitation and electrophoretic mobility shift assays (EMSA) showed that a BMAL1 R91A mutant forms a heterodimer with CLOCK, but is unable to support DNA binding in vitro. Consistent with these observations, plasmids encoding the R91A and R91H mutants of BMAL1 were unable to stimulate transcription from an E-box containing reporter construct in transient co-transfection analyses in NIH 3T3 cells. More importantly, these mutants suppressed the activation of transcription from the reporter construct mediated by wild-type BMAL1, indicating that they possess dominant negative activity in this assay. These results document further the importance of the basic region of BMAL1 in binding to DNA and suggest that this new mutant might be a useful tool for further analysis of BMAL1 function.

T2-weighted MRI for the assessment of joint effusion: comparative study of conventional spin-echo and fast spin-echo sequences.

OBJECTIVE: This study aimed to evaluate the usefulness of suitable conventional spin-echo (CSE) and fast spin-echo (FSE) T2-weighted imaging parameters for the assessment of joint effusion in a phantom study and in a comparative study of CSE and FSE using clinical cases. STUDY DESIGN: In the phantom study, the signal ratios of water and oil signal fields were determined and studied comparatively. The shape and size of signals were evaluated separately. In the study of joint effusion images, 318 joints were evaluated. CSE T2-weighted imaging and FSE T2-weighted imaging were carried out, and a comparative assessment was performed. RESULTS: In both CSE and FSE imaging, the ratios of mean MRI signal values showed divergence as TR/TE values increased. The evaluation of joint effusion with FSE TR/TE 8000/120 msec was significantly better than that in all other groups (P <.01). CONCLUSION: The use of FSE requires investigation of TR/TE values. When a 0.5 T static field strength MRI apparatus is employed, TR/TE 8000/120 msec is recommended.

Magnetic resonance imaging of normal and osteomyelitis in the mandible: assessment of short inversion time inversion recovery sequence.

OBJECTIVE: We sought to determine the suitable magnetic resonance imaging conditions for the short inversion time inversion recovery (STIR) sequence through the use of phantoms; to describe the signal characteristics of normal structures in the mandible; and to evaluate the usefulness of STIR images in enabling the identification of mandibular osteomyelitis on conventional T1- and T2-weighted spin-echo images. STUDY DESIGN: Suitable mandibular STIR imaging conditions were determined by varying inversion time and repetition time in each sequence. STIR magnetic resonance images of 162 healthy subjects and STIR and T1- and T2-weighted spin-echo images of 21 subjects with mandibular osteomyelitis were evaluated. RESULTS: In STIR imaging, the signal of oil was suppressed at an inversion time equaling 100 milliseconds and a repetition time equaling 1500 to 3000 milliseconds. In healthy subjects, the mandibular marrow was revealed to have high signal intensities (100%) and cortical bone had no signal intensities (100%) on STIR images. In surrounding soft tissue in these healthy subjects, the submandibular glands were shown to have high signal intensities (100%); the parotid glands had intermediate to high signal intensities (100%); the sublingual glands had high (88.9%) and intermediate to high (11.1%) signal intensities; lymph nodes had high signal intensities (100%); and the masseter muscles had intermediate signal intensities (100%) on STIR images. The lesions in bone marrow had low (75%) and low to intermediate (25%) signal intensities on T1-weighted images and high (54%), intermediate to high (29%), and intermediate (17%) signal intensities on T2-weighted images. On STIR images, the signal intensities resulted in high (75%), intermediate to high (21%), and intermediate (4%) signal intensities. CONCLUSIONS: STIR imaging is highly effective for the evaluation of bone marrow and surrounding soft tissue in terms of the detection of osteomyelitis in the mandible and the identification of inflammation spreading to soft tissue.