Presynaptically released Cbln1 induces dynamic axonal structural changes by interacting with GluD2 during cerebellar synapse formation

Cbln1 regulates axon growth and guidance in multiple neural regions

The accurate construction of neural circuits requires the precise control of axon growth and guidance, which is regulated by multiple growth and guidance cues during early nervous system development. It is generally thought that the growth and guidance cues that control the major steps of axon development have been defined. Here, we describe cerebellin-1 (Cbln1) as a novel cue that controls diverse aspects of axon growth and guidance throughout the central nervous system (CNS) by experiments using mouse and chick embryos. Cbln1 has previously been shown to function in late neural development to influence synapse organization. Here, we find that Cbln1 has an essential role in early neural development. Cbln1 is expressed on the axons and growth cones of developing commissural neurons and functions in an autocrine manner to promote axon growth. Cbln1 is also expressed in intermediate target tissues and functions as an attractive guidance cue. We find that these functions of Cbln1 are mediated by neurexin-2 (Nrxn2), which functions as the Cbln1 receptor for axon growth and guidance. In addition to the developing spinal cord, we further show that Cbln1 functions in diverse parts of the CNS with major roles in cerebellar parallel fiber growth and retinal ganglion cell axon guidance. Despite the prevailing role of Cbln1 as a synaptic organizer, our study discovers a new and unexpected function for Cbln1 as a general axon growth and guidance cue throughout the nervous system.

Single cell transcriptomics reveals dysregulated cellular and molecular networks in a fragile X syndrome model

Despite advances in understanding the pathophysiology of Fragile X syndrome (FXS), its molecular basis is still poorly understood. Whole brain tissue expression profiles have proved surprisingly uninformative, therefore we applied single cell RNA sequencing to profile an FMRP deficient mouse model with higher resolution. We found that the absence of FMRP results in highly cell type specific gene expression changes that are strongest among specific neuronal types, where FMRP-bound mRNAs were prominently downregulated. Metabolic pathways including translation and respiration are significantly upregulated across most cell types with the notable exception of excitatory neurons. These effects point to a potential difference in the activity of mTOR pathways, and together with other dysregulated pathways, suggest an excitatory-inhibitory imbalance in the Fmr1-knock out cortex that is exacerbated by astrocytes. Our data demonstrate that FMRP loss affects abundance of key cellular communication genes that potentially affect neuronal synapses and provide a resource for interrogating the biological basis of this disorder.

Fragile X syndrome is a leading genetic cause of inherited intellectual disability and autism spectrum disorder. It results from the inactivation of a single gene, FMR1 and hence the loss of its encoded protein FMRP. Despite decades of intensive research, we still lack an overview of the molecular and biological consequences of the disease. Using single cell RNA sequencing, we profiled cells from the brain of healthy mice and of knock-out mice lacking the FMRP protein, a common model for this disease, to identify molecular changes that happen across different cell types. We find neurons are the most impacted cell type, where genes in multiple pathways are similarly impacted. This includes transcripts known to be bound by FMRP, which are collectively decreased only in neurons but not in other cell types. Our results show how the loss of FMRP affects the intricate interactions between different brain cell types, which could provide new perspectives to the development of therapeutic interventions.

Transsynaptic cerebellin 4-neogenin 1 signaling mediates LTP in the mouse dentate gyrus

SignificanceSynapses are controlled by transsynaptic adhesion complexes that mediate bidirectional signaling between pre- and postsynaptic compartments. Long-term potentiation (LTP) of synaptic transmission is thought to enable synaptic modifications during memory formation, but the signaling mechanisms involved remain poorly understood. We show that binding of cerebellin-4 (Cbln4), a secreted ligand of presynaptic neurexin adhesion molecules, to neogenin-1, a postsynaptic surface protein known as a developmental netrin receptor, is essential for normal LTP at entorhinal cortex→dentate gyrus synapses in mice. Cbln4 and neogenin-1 are dispensable for basal synaptic transmission and not involved in establishing synaptic connections as such. Our data identify a netrin receptor as a postsynaptic organizer of synaptic plasticity that collaborates specifically with the presynaptic neurexin-ligand Cbln4.

Neurexins play a crucial role in cerebellar granule cell survival by organizing autocrine machinery for neurotrophins

Neurexins (NRXNs) are key presynaptic cell adhesion molecules that regulate synapse formation and function via trans-synaptic interaction with postsynaptic ligands. Here, we generate cerebellar granule cell (CGC)-specific Nrxn triple-knockout (TKO) mice for complete deletion of all NRXNs. Unexpectedly, most CGCs die in these mice, and this requirement for NRXNs for cell survival is reproduced in cultured CGCs. The axons of cultured Nrxn TKO CGCs that are not in contact with a postsynaptic structure show defects in the formation of presynaptic protein clusters and in action-potential-induced Ca²⁺ influxes. These cells also show impaired secretion of depolarization-induced, fluorescence-tagged brain-derived neurotrophic factor (BDNF) from their axons, and the cell-survival defect is rescued by the application of BDNF. These results suggest that CGC survival is maintained by autocrine neurotrophic factors and that NRXNs organize the presynaptic protein clusters and the autocrine neurotrophic-factor secretory machinery independent of contact with postsynaptic ligands.

Molecular self-avoidance in synaptic neurexin complexes

Synapses are thought to be organized by interactions of presynaptic neurexins with postsynaptic ligands, particularly with neuroligins and cerebellins. However, when a neuron forms adjacent pre- and postsynaptic specializations, as in dendrodendritic or axo-axonic synapses, nonfunctional cis neurexin/ligand interactions would be energetically favored. Here, we reveal an organizational principle for preventing synaptic cis interactions (“self-avoidance”). Using dendrodendritic synapses between mitral and granule cells in the olfactory bulb as a paradigm, we show that, owing to its higher binding affinity, cerebellin-1 blocks the cis interaction of neurexins with neuroligins, thereby enabling trans neurexin/neuroligin interaction. In mitral cells, ablating either cerebellin-1 or neuroligins severely impaired granule cell➔mitral cell synapses, as did overexpression of wild-type neurexins but not of mutant neurexins unable to bind to neuroligins. Our data uncover a molecular interaction network that organizes the self-avoidance of nonfunctional neurexin/ligand cis interactions, thus allowing assembly of physiological trans interactions.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Microglia, Cytokines, and Neural Activity: Unexpected Interactions in Brain Development and Function

Intercellular signaling molecules such as cytokines and their receptors enable immune cells to communicate with one another and their surrounding microenvironments. Emerging evidence suggests that the same signaling pathways that regulate inflammatory responses to injury and disease outside of the brain also play powerful roles in brain development, plasticity, and function. These observations raise the question of how the same signaling molecules can play such distinct roles in peripheral tissues compared to the central nervous system, a system previously thought to be largely protected from inflammatory signaling. Here, we review evidence that the specialized roles of immune signaling molecules such as cytokines in the brain are to a large extent shaped by neural activity, a key feature of the brain that reflects active communication between neurons at synapses. We discuss the known mechanisms through which microglia, the resident immune cells of the brain, respond to increases and decreases in activity by engaging classical inflammatory signaling cascades to assemble, remodel, and eliminate synapses across the lifespan. We integrate evidence from (1) in vivo imaging studies of microglia-neuron interactions, (2) developmental studies across multiple neural circuits, and (3) molecular studies of activity-dependent gene expression in microglia and neurons to highlight the specific roles of activity in defining immune pathway function in the brain. Given that the repurposing of signaling pathways across different tissues may be an important evolutionary strategy to overcome the limited size of the genome, understanding how cytokine function is established and maintained in the brain could lead to key insights into neurological health and disease.

Aerobic Exercise Induces Alternative Splicing of Neurexins in Frontal Cortex

Aerobic exercise (AE) is known to produce beneficial effects on brain health by improving plasticity, connectivity, and cognitive functions, but the underlying molecular mechanisms are still limited. Neurexins (Nrxns) are a family of presynaptic cell adhesion molecules that are important in synapsis formation and maturation. In vertebrates, three-neurexin genes (NRXN1, NRXN2, and NRXN3) have been identified, each encoding for α and β neurexins, from two independent promoters. Moreover, each Nrxns gene (1-3) has several alternative exons and produces many splice variants that bind to a large variety of postsynaptic ligands, playing a role in trans-synaptic specification, strength, and plasticity. In this study, we investigated the impact of a continuous progressive (CP) AE program on alternative splicing (AS) of Nrxns on two brain regions: frontal cortex (FC) and hippocampus. We showed that exercise promoted Nrxns1-3 AS at splice site 4 (SS4) both in α and β isoforms, inducing a switch from exon-excluded isoforms (SS4-) to exon-included isoforms (SS4+) in FC but not in hippocampus. Additionally, we showed that the same AE program enhanced the expression level of other genes correlated with synaptic function and plasticity only in FC. Altogether, our findings demonstrated the positive effect of CP AE on FC in inducing molecular changes underlying synaptic plasticity and suggested that FC is possibly a more sensitive structure than hippocampus to show molecular changes.

Destroy the old to build the new: Activity-dependent lysosomal exocytosis in neurons

Lysosomes are organelles that support diverse cellular functions such as terminal degradation of macromolecules and nutrient recycling. Additionally, lysosomes can fuse with the plasma membrane, a phenomenon referred to as lysosomal exocytosis, to release their contents, including hydrolytic enzymes and cargo proteins. Recently, neuronal activity has been shown to induce lysosomal exocytosis in dendrites and axons. Secreted lysosomal enzyme cathepsin B induces and stabilizes synaptic structural changes by degrading the local extracellular matrix. Extracellular matrix reorganization could also enhance the lateral diffusion of the co-released synaptic organizer Cbln1 along the surface of axons to facilitate new synapse formation. Similarly, lateral diffusion of dendritic AMPA-type glutamate receptors could be facilitated to enhance functional synaptic plasticity. Therefore, lysosomal exocytosis is a powerful way of building new cellular structures through the coordinated destruction of the old environment. Understanding the mechanisms by which lysosomal exocytosis is regulated in neurons is expected to lead to the development of new therapeutics for neuronal plasticity following spinal cord injury or neurodegenerative disease.

Synaptic recognition molecules in development and disease

Synaptic connectivity patterns underlie brain functions. How recognition molecules control where and when neurons form synapses with each other, therefore, is a fundamental question of cellular neuroscience. This chapter delineates adhesion and signaling complexes as well as secreted factors that contribute to synaptic partner recognition in the vertebrate brain. The sections follow a developmental perspective and discuss how recognition molecules (1) guide initial synaptic wiring, (2) provide for the rejection of incorrect partner choices, (3) contribute to synapse specification, and (4) support the removal of inappropriate synapses once formed. These processes involve a rich repertoire of molecular players and key protein families are described, notably the Cadherin and immunoglobulin superfamilies, Semaphorins/Plexins, Leucine-rich repeat containing proteins, and Neurexins and their binding partners. Molecular themes that diversify these recognition systems are defined and highlighted throughout the text, including the neuron-type specific expression and combinatorial action of recognition factors, alternative splicing, and post-translational modifications. Methodological innovations advancing the field such as proteomic approaches and single cell expression studies are additionally described. Further, the chapter highlights the importance of choosing an appropriate brain region to analyze synaptic recognition factors and the advantages offered by laminated structures like the hippocampus or retina. In a concluding section, the profound disease relevance of aberrant synaptic recognition for neurodevelopmental and psychiatric disorders is discussed. Based on the current progress, an outlook is presented on research goals that can further advance insights into how recognition molecules provide for the astounding precision and diversity of synaptic connections.

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.

Paracrine Role for Somatostatin Interneurons in the Assembly of Perisomatic Inhibitory Synapses

GABAergic interneurons represent a heterogenous group of cell types in neocortex that can be clustered based on developmental origin, morphology, physiology, and connectivity. Two abundant populations of cortical GABAergic interneurons include the low-threshold, somatostatin (SST)-expressing cells and the fast-spiking, parvalbumin (PV)-expressing cells. While SST⁺ and PV⁺ interneurons are both early born and migrate into the developing neocortex at similar times, SST⁺ cells are incorporated into functional circuits prior to PV⁺ cells. During this early period of neural development, SST⁺ cells play critical roles in the assembly and maturation of other cortical circuits; however, the mechanisms underlying this process remain poorly understood. Here, using both sexes of conditional mutant mice, we discovered that SST⁺ interneuron-derived Collagen XIX, a synaptogenic extracellular matrix protein, is required for the formation of GABAergic, perisomatic synapses by PV⁺ cells. These results, therefore, identify a paracrine mechanism by which early-born SST⁺ cells orchestrate inhibitory circuit formation in the developing neocortex.SIGNIFICANCE STATEMENT Inhibitory interneurons in the cerebral cortex represent a heterogenous group of cells that generate the inhibitory neurotransmitter GABA. One such interneuron type is the low-threshold, somatostatin (SST)-expressing cell, which is one of the first types of interneurons to migrate into the cerebral cortex and become incorporated into functional circuits. In addition, to contributing important roles in controlling the flow of information in the adult cerebral cortex, SST⁺ cells play important roles in the development of other neural circuits in the developing brain. Here, we identified an extracellular matrix protein that is released by these early-born SST⁺ neurons to orchestrate inhibitory circuit formation in the developing cerebral cortex.

Refinement of Cerebellar Network Organization by Extracellular Signaling During Development

The cerebellum forms regular neural network structures consisting of a few major types of neurons, such as Purkinje cells, granule cells, and molecular layer interneurons, and receives two major inputs from climbing fibers and mossy fibers. Its regular structures consist of three well-defined layers, with each type of neuron designated to a specific location and forming specific synaptic connections. During the first few weeks of postnatal development in rodents, the cerebellum goes through dynamic changes via proliferation, migration, differentiation, synaptogenesis, and maturation, to create such a network structure. The development of this organized network structure presumably relies on the communication between developing elements in the network, including not only individual neurons, but also their dendrites, axons, and synapses. Therefore, it is reasonable that extracellular signaling via synaptic transmission, secreted molecules, and cell adhesion molecules, plays important roles in cerebellar network development. Although it is not yet clear as to how overall cerebellar development is orchestrated, there is indeed accumulating lines of evidence that extracellular signaling acts toward the development of individual elements in the cerebellar networks. In this article, we introduce what we have learned from many studies regarding the extracellular signaling required for cerebellar network development, including our recent study suggesting the importance of unbiased synaptic inputs from parallel fibers.

Genetic insights and neurobiological implications from NRXN1 in neuropsychiatric disorders

Many neuropsychiatric and neurodevelopmental disorders commonly share genetic risk factors. To date, the mechanisms driving the pathogenesis of these disorders, particularly how genetic variations affect the function of risk genes and contribute to disease symptoms, remain largely unknown. Neurexins are a family of synaptic adhesion molecules, which play important roles in the formation and establishment of synaptic structure, as well as maintenance of synaptic function. Accumulating genomic findings reveal that genetic variations within genes encoding neurexins are associated with a variety of psychiatric conditions such as schizophrenia, autism spectrum disorder, and some developmental abnormalities. In this review, we focus on NRXN1, one of the most compelling psychiatric risk genes of the neurexin family. We performed a comprehensive survey and analysis of current genetic and molecular data including both common and rare alleles within NRXN1 associated with psychiatric illnesses, thus providing insights into the genetic risk conferred by NRXN1. We also summarized the neurobiological evidences, supporting the function of NRXN1 and its protein products in synaptic formation, organization, transmission and plasticity, as well as disease-relevant behaviors, and assessed the mechanistic link between the mutations of NRXN1 and synaptic and behavioral pathology in neuropsychiatric disorders.

Transcriptomic analysis reveals new hippocampal gene networks induced by prolactin

Prolactin (Prl) is a pleiotropic hormone with multiple functions in several tissues and organs, including the brain. In the hippocampus, Prl has been implicated in several functions, including neuroprotection against excitotoxicity in lactating rats and in Prl-treated ovariectomized animals. However, the molecular mechanisms involved in Prl actions in the hippocampus have not been completely elucidated. The aim of this study was to analyse the hippocampal transcriptome of female Prl-treated ovariectomized rats. Transcriptomic analysis by RNASeq revealed 162 differentially expressed genes throughout 24 h of Prl treatment. Gene Ontology analysis of those genes showed that 37.65% were involved in brain processes that are regulated by the hippocampus, such as learning, memory and behaviour, as well as new processes that we did not foresee, such as glial differentiation, axogenesis, synaptic transmission, postsynaptic potential, and neuronal and glial migration. Immunodetection analysis demonstrated that Prl significantly modified microglial morphology, reduced the expression of Cd11b/c protein, and altered the content and location of the neuronal proteins Tau, Map2 and Syp, which are involved in axogenic and synaptic functions. This novel delineation of Prl activity in the hippocampus highlights its importance as a neuroactive hormone, opens a new avenue for understanding its actions and supports its participation in neuronal plasticity of this brain area.

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.

The evolving role of neuro-immune interaction in brain repair after cerebral ischemic stroke

Stroke is the world's leading cause of disability with limited brain repair treatments which effectively improve long-term neurological deficits. The neuroinflammatory responses persist into the late repair phase of stroke and participate in all brain repair elements, including neurogenesis, angiogenesis, synaptogenesis, remyelination and axonal sprouting, shedding new light on post-stroke brain recovery. Resident brain glial cells, such as astrocytes not only contribute to neuroinflammation after stroke, but also secrete a wide range of trophic factors that can promote post-stroke brain repair. Alternatively, activated microglia, monocytes, and neutrophils in the innate immune system, traditionally considered as major damaging factors after stroke, have been suggested to be extensively involved in brain repair after stroke. The adaptive immune system may also have its bright side during the late regenerative phase, affecting the immune suppressive regulatory T cells and B cells. This review summarizes the recent findings in the evolving role of neuroinflammation in multiple post-stroke brain repair mechanisms and poses unanswered questions that may generate new directions for future research and give rise to novel therapeutic targets to improve stroke recovery.

Molecular Mechanisms of Synaptic Specificity: Spotlight on Hippocampal and Cerebellar Synapse Organizers

Synapses and neural circuits form with exquisite specificity during brain development to allow the precise and appropriate flow of neural information. Although this property of synapses and neural circuits has been extensively investigated for more than a century, molecular mechanisms underlying this property are only recently being unveiled. Recent studies highlight several classes of cell-surface proteins as organizing hubs in building structural and functional architectures of specific synapses and neural circuits. In the present mini-review, we discuss recent findings on various synapse organizers that confer the distinct properties of specific synapse types and neural circuit architectures in mammalian brains, with a particular focus on the hippocampus and cerebellum.

Improvement of cerebellar ataxic gait by injecting Cbln1 into the cerebellum of cbln1-null mice

Patients and rodents with cerebellar damage display ataxic gaits characterized by impaired coordination of limb movements. Here, gait ataxia in mice with a null mutation of the gene for the cerebellin 1 precursor protein (cbln1-null mice) was investigated by kinematic analysis of hindlimb movements during locomotion. The Cbln1 protein is predominately produced and secreted from cerebellar granule cells. The cerebellum of cbln1-null mice is characterized by an 80% reduction in the number of parallel fiber-Purkinje cell synapses compared with wild-type mice. Our analyses identified prominent differences in the temporal parameters of locomotion between cbln1-null and wild-type mice. The cbln1-null mice displayed abnormal hindlimb movements that were characterized by excessive toe elevation during the swing phase, and by severe hyperflexion of the ankles and knees. When recombinant Cbln1 protein was injected into the cerebellum of cbln1-null mice, the step cycle and stance phase durations increased toward those of wild-type mice, and the angular excursions of the knee during a cycle period showed a much closer agreement with those of wild-type mice. These findings suggest that dysfunction of the parallel fiber-Purkinje cell synapses might underlie the impairment of hindlimb movements during locomotion in cbln1-null mice.

Deletion of the GluRδ2 Receptor in the Hotfoot Mouse Mutant Causes Granule Cell Loss, Delayed Purkinje Cell Death, and Reductions in Purkinje Cell Dendritic Tree Area

Recent studies have found that in the cerebellum, the δ2 glutamate receptor (GluRδ2) plays a key role in regulating the differentiation of parallel fiber-Purkinje synapses and mediating key physiological functions in the granule cell-Purkinje cell circuit. In the hotfoot mutant or GluRδ2 knockout mice, the absence of GluRδ2 expression results in impaired motor-related tasks, ataxia, and disruption of long-term depression at parallel fiber-Purkinje cell synapses. The goal of this study was to determine the long-term consequences of deletion of GluRδ2 expression in the hotfoot mutant (GluRδ2 ^ho/ho ) on Purkinje and granule cell survival and Purkinje cell dendritic differentiation. Quantitative estimates of Purkinje and granule cell numbers in 3-, 12-, and 20-month-old hotfoot mutants and wild-type controls showed that Purkinje cell numbers are within control values at 3 and 12 months in the hotfoot mutant but reduced by 20 % at 20 months compared with controls. In contrast, the number of granule cells is significantly reduced from 3 months onwards in GluRδ2 ^ho/ho mutant mice compared to wild-type controls. Although the overall structure of Purkinje cell dendrites does not appear to be altered, there is a significant 27 % reduction in the cross-sectional area of Purkinje cell dendritic trees in the 20-month-old GluRδ2 ^ho/ho mutants. The interpretation of the results is that the GluRδ2 receptor plays an important role in the long-term organization of the granule-Purkinje cell circuit through its involvement in the regulation of parallel fiber-Purkinje cell synaptogenesis and in the normal functioning of this critical cerebellar circuit.

Non-canonical Signaling, the Hidden Life of Ligand-Gated Ion Channels

Neurotransmitter receptors are responsible for the transfer of information across the synapse. While ionotropic receptors form ion channels and mediate rapid membrane depolarization, so-called metabotropic receptors exert their action though slower, less direct intracellular signaling pathways. Glutamate, GABA, and acetylcholine can activate both ionotropic and metabotropic receptors, yet the distinction between these "canonical" signaling systems has become less clear since ionotropic receptors were proposed to also activate second messenger systems, defining a "non-canonical" signaling pathway. How these alternative pathways affect neuronal circuit activity is not well understood, and their influence could be more significant than previously anticipated. In this review, we examine the evidence available that supports the existence of parallel and unsuspected signaling pathways used by ionotropic neurotransmitter receptors.

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.

The low binding affinity of D-serine at the ionotropic glutamate receptor GluD2 can be attributed to the hinge region

Ionotropic glutamate receptors (iGluRs) are responsible for most of the fast excitatory communication between neurons in our brain. The GluD2 receptor is a puzzling member of the iGluR family: It is involved in synaptic plasticity, plays a role in human diseases, e.g. ataxia, binds glycine and D-serine with low affinity, yet no ligand has been discovered so far that can activate its ion channel. In this study, we show that the hinge region connecting the two subdomains of the GluD2 ligand-binding domain is responsible for the low affinity of D-serine, by analysing GluD2 mutants with electrophysiology, isothermal titration calorimetry and molecular dynamics calculations. The hinge region is highly variable among iGluRs and fine-tunes gating activity, suggesting that in GluD2 this region has evolved to only respond to micromolar concentrations of D-serine.

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.

Shedding of neurexin 3β ectodomain by ADAM10 releases a soluble fragment that affects the development of newborn neurons

Neurexins are transmembrane synaptic cell adhesion molecules involved in the development and maturation of neuronal synapses. In the present study, we report that Nrxn3β is processed by the metalloproteases ADAM10, ADAM17, and by the intramembrane-cleaving protease γ-secretase, producing secreted neurexin3β (sNrxn3β) and a single intracellular domain (Nrxn3β-ICD). We further completed the full characterization of the sites at which Nrxn3β is processed by these proteases. Supporting the physiological relevance of the Nrxn3β processing, we demonstrate in vivo a significant effect of the secreted shedding product sNrxn3β on the morphological development of adult newborn neurons in the mouse hippocampus. We show that sNrxn3β produced by the cells of the dentate gyrus increases the spine density of newborn neurons whereas sNrxn3β produced by the newborn neuron itself affects the number of its mossy fiber terminal extensions. These results support a pivotal role of sNrxn3β in plasticity and network remodeling during neuronal development.

An alternative splicing switch shapes neurexin repertoires in principal neurons versus interneurons in the mouse hippocampus

The unique anatomical and functional features of principal and interneuron populations are critical for the appropriate function of neuronal circuits. Cell type-specific properties are encoded by selective gene expression programs that shape molecular repertoires and synaptic protein complexes. However, the nature of such programs, particularly for post-transcriptional regulation at the level of alternative splicing is only beginning to emerge. We here demonstrate that transcripts encoding the synaptic adhesion molecules neurexin-1,2,3 are commonly expressed in principal cells and interneurons of the mouse hippocampus but undergo highly differential, cell type-specific alternative splicing. Principal cell-specific neurexin splice isoforms depend on the RNA-binding protein Slm2. By contrast, most parvalbumin-positive (PV⁺) interneurons lack Slm2, express a different neurexin splice isoform and co-express the corresponding splice isoform-specific neurexin ligand Cbln4. Conditional ablation of Nrxn alternative splice insertions selectively in PV⁺ cells results in elevated hippocampal network activity and impairment in a learning task. Thus, PV-cell-specific alternative splicing of neurexins is critical for neuronal circuit function

DOI:http://dx.doi.org/10.7554/eLife.22757.001

The Purkinje cell as a model of synaptogenesis and synaptic specificity

Since the groundbreaking work of Ramon y Cajal, the cerebellar Purkinje cell has always represented an ideal model for studying the organization, development and function of synaptic circuits. Purkinje cells receive distinct types of glutamatergic and GABAergic synapses, each characterized by exquisite sub-cellular and molecular specificity. The formation and refinement of these connections results from a temporally-regulated sequence of events that involves molecular interactions between distinct sets of secreted and surface proteins, as well as activity-dependent competition between converging inputs. Insights into the mechanisms controlling synaptic specificity in Purkinje cells may help understand synapse development also in other brain regions and disclose circuit abnormalities that underlie neurodevelopmental disorders.

Functions of synapse adhesion molecules neurexin/neuroligins and neurodevelopmental disorders

Neurexins and neuroligins are two distinct families of single-pass transmembrane proteins localized at pre- and postsynapses, respectively. They trans-synaptically interact with each other and induce synapse formation and maturation. Common variants and rare mutations, including copy number variations, short deletions, and single or small nucleotide changes in neurexin and neuroligin genes have been linked to the neurodevelopmental disorders, such as autism spectrum disorders (ASDs). In this review, we summarize the structure and basic synaptic function of neurexins and neuroligins, followed by behaviors and synaptic phenotypes of knock-in and knock-out mouse of these family genes. From the studies of these mice, it turns out that the effects of neurexins and neuroligins are amazingly neural circuit dependent, even within the same brain region. In addition, neurexins and neuroligins are commonly involved in the endocannabinoid signaling. These finding may provide not only insight into understanding the pathophysiology, but also the concept for strategy of therapeutic intervention for ASDs.

Microglia contact induces synapse formation in developing somatosensory cortex

Microglia are the immune cells of the central nervous system that play important roles in brain pathologies. Microglia also help shape neuronal circuits during development, via phagocytosing weak synapses and regulating neurogenesis. Using in vivo multiphoton imaging of layer 2/3 pyramidal neurons in the developing somatosensory cortex, we demonstrate here that microglial contact with dendrites directly induces filopodia formation. This filopodia formation occurs only around postnatal day 8-10, a period of intense synaptogenesis and when microglia have an activated phenotype. Filopodia formation is preceded by contact-induced Ca(2+) transients and actin accumulation. Inhibition of microglia by genetic ablation decreases subsequent spine density, functional excitatory synapses and reduces the relative connectivity from layer 4 neurons. Our data provide the direct demonstration of microglial-induced spine formation and provide further insights into immune system regulation of neuronal circuit development, with potential implications for developmental disorders of immune and brain dysfunction.

Kinesin-1 sorting in axons controls the differential retraction of arbor terminals

The ability of neurons to generate multiple arbor terminals from a single axon is crucial for establishing proper neuronal wiring. Although growth and retraction of arbor terminals are differentially regulated within the axon, the mechanisms by which neurons locally control their structure remain largely unknown. In the present study, we found that the kinesin-1 (Kif5 proteins) head domain (K5H) preferentially marks a subset of arbor terminals. Time-lapse imaging clarified that these arbor terminals were more stable than others, because of a low retraction rate. Local inhibition of kinesin-1 in the arbor terminal by chromophore-assisted light inactivation (CALI) enhanced the retraction rate. The microtubule turnover was locally regulated depending on the length from the branching point to the terminal end, but did not directly correlate with the presence of K5H. By contrast, F-actin signal values in arbor terminals correlated spatiotemporally with K5H, and inhibition of actin turnover prevented retraction. Results from the present study reveal a new system mediated by kinesin-1 sorting in axons that differentially controls stability of arbor terminals.

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 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.

Spatio-temporal regulations and functions of neuronal alternative RNA splicing in developing and adult brains

Alternative pre-mRNA splicing is a fundamental mechanism that generates molecular diversity from a single gene. In the central nervous system (CNS), key neural developmental steps are thought to be controlled by alternative splicing decisions, including the molecular diversity underlying synaptic wiring, plasticity, and remodeling. Significant progress has been made in understanding the molecular mechanisms and functions of alternative pre-mRNA splicing in neurons through studies in invertebrate systems; however, recent studies have begun to uncover the potential role of neuronal alternative splicing in the mammalian CNS. This article provides an overview of recent findings regarding the regulation and function of neuronal alternative splicing. In particular, we focus on the spatio-temporal regulation of neurexin, a synaptic adhesion molecule, by neuronal cell type-specific factors and neuronal activity, which are thought to be especially important for characterizing neural development and function within the mammalian CNS. Notably, there is increasing evidence that implicates the dysregulation of neuronal splicing events in several neurological disorders. Therefore, understanding the detailed mechanisms of neuronal alternative splicing in the mammalian CNS may provide plausible treatment strategies for these diseases.

The Stress-Induced Atf3-Gelsolin Cascade Underlies Dendritic Spine Deficits in Neuronal Models of Tuberous Sclerosis Complex

Hyperactivation of the mechanistic target of rapamycin (mTOR) kinase, as a result of loss-of-function mutations in tuberous sclerosis complex 1 (TSC1) or TSC2 genes, causes protein synthesis dysregulation, increased cell size, and aberrant neuronal connectivity. Dysregulated synthesis of synaptic proteins has been implicated in the pathophysiology of autism spectrum disorder (ASD) associated with TSC and fragile X syndrome. However, cell type-specific translational profiles in these disease models remain to be investigated. Here, we used high-fidelity and unbiased Translating Ribosome Affinity Purification (TRAP) methodology to purify ribosome-associated mRNAs and identified translational alterations in a rat neuronal culture model of TSC. We find that expression of many stress and/or activity-dependent proteins is highly induced while some synaptic proteins are repressed. Importantly, transcripts for the activating transcription factor-3 (Atf3) and mitochondrial uncoupling protein-2 (Ucp2) are highly induced in Tsc2-deficient neurons, as well as in a neuron-specific Tsc1 conditional knock-out mouse model, and show differential responses to the mTOR inhibitor rapamycin. Gelsolin, a known target of Atf3 transcriptional activity, is also upregulated. shRNA-mediated block of Atf3 induction suppresses expression of gelsolin, an actin-severing protein, and rescues spine deficits found in Tsc2-deficient neurons. Together, our data demonstrate that a cell-autonomous program consisting of a stress-induced Atf3-gelsolin cascade affects the change in dendritic spine morphology following mTOR hyperactivation. This previously unidentified molecular cascade could be a therapeutic target for treating mTORopathies.

SIGNIFICANCE STATEMENT

Tuberous sclerosis complex (TSC) is a genetic disease associated with epilepsy and autism. Dysregulated protein synthesis has been implicated as a cause of this disease. However, cell type-specific translational profiles that are aberrant in this disease are unknown. Here we show that expression of many stress and/or activity-dependent proteins is highly induced while some synaptic proteins are repressed in neurons missing the Tsc2 gene expression. Identification of genes whose translation is abnormal in TSC may provide insights to previously unidentified therapeutic targets.

Parcellation of cerebellins 1, 2, and 4 among different subpopulations of dorsal horn neurons in mouse spinal cord

The cerebellins (Cblns) are a family of secreted proteins that are widely expressed throughout the nervous system, but whose functions have been studied only in the cerebellum and striatum. Two members of the family, Cbln1 and Cbln2, bind to neurexins on presynaptic terminals and to GluRδs postsynaptically, forming trans-synaptic triads that promote synapse formation. Cbln1 has a higher binding affinity for GluRδs and exhibits greater synaptogenic activity than Cbln2. In contrast, Cbln4 does not form such triads and its function is unknown. The different properties of the three Cblns suggest that each plays a distinct role in synapse formation. To begin to elucidate Cbln function in other neuronal systems, we used in situ hybridization to examine Cbln expression in the mouse spinal cord. We find that neurons expressing Cblns 1, 2, and 4 tend to occupy different laminar positions within the dorsal spinal cord, and that Cbln expression is limited almost exclusively to excitatory neurons. Combined in situ hybridization and immunofluorescent staining shows that Cblns 1, 2, and 4 are expressed by largely distinct neuronal subpopulations, defined in part by sensory input, although there is some overlap and some individual neurons coexpress two Cblns. Our results suggest that differences in connectivity between subpopulations of dorsal spinal cord neurons may be influenced by which Cbln each subpopulation contains. Competitive interactions between axon terminals may determine the number of synapses each forms in any given region, and thereby contribute to the development of precise patterns of connectivity in the dorsal gray matter.

A transcription factor network specifying inhibitory versus excitatory neurons in the dorsal spinal cord

The proper balance of excitatory and inhibitory neurons is crucial for normal processing of somatosensory information in the dorsal spinal cord. Two neural basic helix-loop-helix transcription factors (TFs), Ascl1 and Ptf1a, have contrasting functions in specifying these neurons. To understand how Ascl1 and Ptf1a function in this process, we identified their direct transcriptional targets genome-wide in the embryonic mouse neural tube using ChIP-Seq and RNA-Seq. We show that Ascl1 and Ptf1a directly regulate distinct homeodomain TFs that specify excitatory or inhibitory neuronal fates. In addition, Ascl1 directly regulates genes with roles in several steps of the neurogenic program, including Notch signaling, neuronal differentiation, axon guidance and synapse formation. By contrast, Ptf1a directly regulates genes encoding components of the neurotransmitter machinery in inhibitory neurons, and other later aspects of neural development distinct from those regulated by Ascl1. Moreover, Ptf1a represses the excitatory neuronal fate by directly repressing several targets of Ascl1. Ascl1 and Ptf1a bind sequences primarily enriched for a specific E-Box motif (CAGCTG) and for secondary motifs used by Sox, Rfx, Pou and homeodomain factors. Ptf1a also binds sequences uniquely enriched in the CAGATG E-box and in the binding motif for its co-factor Rbpj, providing two factors that influence the specificity of Ptf1a binding. The direct transcriptional targets identified for Ascl1 and Ptf1a provide a molecular understanding of how these DNA-binding proteins function in neuronal development, particularly as key regulators of homeodomain TFs required for neuronal subtype specification.

The role of Cbln1 on Purkinje cell synapse formation

Cbln1 is a glycoprotein which belongs to the C1q family. In the cerebellum, Cbln1 is produced and secreted from granule cells and works as a strong synapse organizer between Purkinje cells and parallel fibers, the axons of the granule cells. In this update article, we will describe the molecular mechanisms by which Cbln1 induces synapse formation and will review our findings on the axonal structural changes which occur specifically during this process. We will also describe our recent finding that Cbln1 has a suppressive role in inhibitory synapse formation between Purkinje cells and molecular layer interneurons. Our results have revealed that Cbln1 plays an essential role to establish parallel fiber-Purkinje cell synapses and to regulate balance between excitatory and inhibitory input on Purkinje cells.

Neural ECM and synaptogenesis

Chemical synapses allow neurons to perform complex computations and regulate other systems of the body. At a chemical synapse, pre- and postsynaptic sites are separated by a small space (the synaptic cleft) and surrounded by astrocytes. The basement membrane (BM), a sheetlike, specialized extracellular matrix (ECM), is found ubiquitously in the PNS. It has become clear that the ECMs not only play a structural role but also serve as barriers and filters in the PNS and CNS. Moreover, proteoglycans and tenascin family proteins in the ECM regulate synapse formation and synaptic plasticity. Although CNS synapses lack the BMs, recent results indicate that the BM-associated collagens are also present in the CNS synaptic cleft and affect synaptogenesis in both the CNS and the PNS. The C1q domain-containing family proteins are important components of the CNS synaptic cleft in regulating synapse formation, maintenance, and the pruning process. The ECM is regarded as a crucial component of the tetrapartite synapse, consisting of pre- and postsynaptic neurons, astrocyte, and ECM.

AMPARs and synaptic plasticity: the last 25 years

The study of synaptic plasticity and specifically LTP and LTD is one of the most active areas of research in neuroscience. In the last 25 years we have come a long way in our understanding of the mechanisms underlying synaptic plasticity. In 1988, AMPA and NMDA receptors were not even molecularly identified and we only had a simple model of the minimal requirements for the induction of plasticity. It is now clear that the modulation of the AMPA receptor function and membrane trafficking is critical for many forms of synaptic plasticity and a large number of proteins have been identified that regulate this complex process. Here we review the progress over the last two and a half decades and discuss the future challenges in the field.

The specific α-neurexin interactor calsyntenin-3 promotes excitatory and inhibitory synapse development

Perturbations of cell surface synapse-organizing proteins, particularly α-neurexins, contribute to neurodevelopmental and psychiatric disorders. From an unbiased screen, we identify calsyntenin-3 (alcadein-β) as a synapse-organizing protein unique in binding and recruiting α-neurexins, but not β-neurexins. Calsyntenin-3 is present in many pyramidal neurons throughout cortex and hippocampus but is most highly expressed in interneurons. The transmembrane form of calsyntenin-3 can trigger excitatory and inhibitory presynapse differentiation in contacting axons. However, calsyntenin-3-shed ectodomain, which represents about half the calsyntenin-3 pool in brain, suppresses the ability of multiple α-neurexin partners including neuroligin 2 and LRRTM2 to induce presynapse differentiation. Clstn3⁻/⁻ mice show reductions in excitatory and inhibitory synapse density by confocal and electron microscopy and corresponding deficits in synaptic transmission. These results identify calsyntenin-3 as an α-neurexin-specific binding partner required for normal functional GABAergic and glutamatergic synapse development.

Neuroimmune regulation of homeostatic synaptic plasticity

Homeostatic synaptic plasticity refers to a set of negative-feedback mechanisms that are used by neurons to maintain activity within a functional range. While it is becoming increasingly clear that homeostatic regulation of synapse function is a key principle in the nervous system, the molecular details of this regulation are only beginning to be uncovered. Recent evidence implicates molecules classically associated with the peripheral immune system in the modulation of homeostatic synaptic plasticity. In particular, the pro-inflammatory cytokine TNFα, class I major histocompatibility complex, and neuronal pentraxin 2 are essential in the regulation of the compensatory synaptic response that occurs in response to prolonged neuronal inactivity. This review will present and discuss current evidence implicating neuroimmune molecules in the homeostatic regulation of synapse function. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.