Bergmann glia and the recognition molecule CHL1 organize GABAergic axons and direct innervation of Purkinje cell dendrites

Potential roles for microglia in drug addiction: Adolescent neurodevelopment and beyond

Adolescence is a sensitive period for development of addiction-relevant brain circuits, and it is also when people typically start experimenting with drugs. Unfortunately, such substance use may cause lasting impacts on the brain, and might increase vulnerability to later-life addictions. Microglia are the brain's immune cells, but their roles in shaping neural connectivity and synaptic plasticity, especially in developmental sensitive periods like adolescence, may also contribute to addiction-related phenomena. Here, we overview how drugs of abuse impact microglia, and propose that they may play poorly-understood, but important roles in addiction vulnerability and progression.

The Hippo effector TEAD1 regulates postnatal murine cerebellar development

The Hippo signalling cascade is an evolutionarily conserved pathway critical for the development of numerous organ systems and is required for the development of many parts of the mammalian nervous system, including the cerebellum. The Hippo pathway converges, via the nuclear YAP/TAZ co-transcription factors, on transcription factors of the TEA Domain (TEAD) family (TEAD1-4) and promotes the expression of pro-proliferative genes. Despite the importance of TEAD function, our understanding of spatial and temporal expression of this family is limited, as is our understanding of which TEAD family members regulate Hippo-dependent organ development. Here, we focus on TEAD1 and how this factor contributes to postnatal murine cerebellar development. We find expression of TEAD1 within cerebellar progenitor cells and glial cells, including astrocytes and Bergmann glia, as well as by some interneurons within the granular layer. The importance of TEAD1 expression for cerebellar development was investigated using a conditional ablation approach, which revealed a range of developmental deficits in Tead1 mutants, including an underdeveloped cerebellum, morphological defects in Bergmann Glia and Purkinje Neurons, as well as granule neuron migration defects. Collectively, these findings suggest a major role for TEAD1 as an effector of the Hippo pathway during cerebellar development.

Slit regulates compartment-specific targeting of dendrites and axons in the Drosophila brain

Proper functioning of the nervous system requires precise neuronal connections at subcellular domains, which can be achieved by projection of axons or dendrites to subcellular domains of target neurons. Here we studied subcellular-specific targeting of dendrites and axons in the Drosophila mushroom body (MB), where mushroom body output neurons (MBONs) and local dopaminergic neurons (DAN) project their dendrites and axons, respectively, to specific compartments of MB axons. Through genetic ablation, we demonstrate that compartment-specific targeting of MBON dendrites and DAN axons involves mutual repulsion of MBON dendrites and/or DAN axons between neighboring compartments. We further show that Slit expressed in subset of DANs mediates such repulsion by acting through different Robo receptors in different neurons. Loss of Slit-mediated repulsion leads to projection of MBON dendrites and DAN axons into neighboring compartments, resulting formation of ectopic synaptic contacts between MBONs and DANs and changes in olfactory-associative learning. Together, our findings suggest that Slit-mediated repulsion controls compartment-specific targeting of MBON dendrites and DAN axons, which ensures precise connections between MBON dendrites and DAN axons and proper learning and memory formation.

Cellular and Molecular Mechanisms Underlying Synaptic Subcellular Specificity

Chemical synapses are essential for neuronal information storage and relay. The synaptic signal received or sent from spatially distinct subcellular compartments often generates different outcomes due to the distance or physical property difference. Therefore, the final output of postsynaptic neurons is determined not only by the type and intensity of synaptic inputs but also by the synaptic subcellular location. How synaptic subcellular specificity is determined has long been the focus of study in the neurodevelopment field. Genetic studies from invertebrates such as Caenorhabditis elegans (C. elegans) have uncovered important molecular and cellular mechanisms required for subcellular specificity. Interestingly, similar molecular mechanisms were found in the mammalian cerebellum, hippocampus, and cerebral cortex. This review summarizes the comprehensive advances in the cellular and molecular mechanisms underlying synaptic subcellular specificity, focusing on studies from C. elegans and rodents.

Artificial Neural Network Model with Astrocyte-Driven Short-Term Memory

In this study, we introduce an innovative hybrid artificial neural network model incorporating astrocyte-driven short-term memory. The model combines a convolutional neural network with dynamic models of short-term synaptic plasticity and astrocytic modulation of synaptic transmission. The model's performance was evaluated using simulated data from visual change detection experiments conducted on mice. Comparisons were made between the proposed model, a recurrent neural network simulating short-term memory based on sustained neural activity, and a feedforward neural network with short-term synaptic depression (STPNet) trained to achieve the same performance level as the mice. The results revealed that incorporating astrocytic modulation of synaptic transmission enhanced the model's performance.

Anisotropy and Frequency Dependence of Signal Propagation in the Cerebellar Circuit Revealed by High-Density Multielectrode Array Recordings

The cerebellum is one of the most connected structures of the central nervous system and receives inputs over an extended frequency range. Nevertheless, the frequency dependence of cerebellar cortical processing remains elusive. In this work, we characterized cerebellar cortex responsiveness to mossy fibers activation at different frequencies and reconstructed the spread of activity in the sagittal and coronal planes of acute mouse cerebellar slices using a high-throughput high-density multielectrode array (HD-MEA). The enhanced spatiotemporal resolution of HD-MEA revealed the frequency dependence and spatial anisotropy of cerebellar activation. Mossy fiber inputs reached the Purkinje cell layer even at the lowest frequencies, but the efficiency of transmission increased at higher frequencies. These properties, which are likely to descend from the topographic organization of local inhibition, intrinsic electroresponsiveness, and short-term synaptic plasticity, are critical elements that have to be taken into consideration to define the computational properties of the cerebellar cortex and its pathological alterations.

Model simulations unveil the structure-function-dynamics relationship of the cerebellar cortical microcircuit

The cerebellar network is renowned for its regular architecture that has inspired foundational computational theories. However, the relationship between circuit structure, function and dynamics remains elusive. To tackle the issue, we developed an advanced computational modeling framework that allows us to reconstruct and simulate the structure and function of the mouse cerebellar cortex using morphologically realistic multi-compartmental neuron models. The cerebellar connectome is generated through appropriate connection rules, unifying a collection of scattered experimental data into a coherent construct and providing a new model-based ground-truth about circuit organization. Naturalistic background and sensory-burst stimulation are used for functional validation against recordings in vivo, monitoring the impact of cellular mechanisms on signal propagation, inhibitory control, and long-term synaptic plasticity. Our simulations show how mossy fibers entrain the local neuronal microcircuit, boosting the formation of columns of activity travelling from the granular to the molecular layer providing a new resource for the investigation of local microcircuit computation and of the neural correlates of behavior.

Cross-regional homeostatic and reactive glial signatures in multiple sclerosis

Multiple sclerosis (MS) is a multifocal and progressive inflammatory disease of the central nervous system (CNS). However, the compartmentalized pathology of the disease affecting various anatomical regions including gray and white matter and lack of appropriate disease models impede understanding of the disease. Utilizing single-nucleus RNA-sequencing and multiplex spatial RNA mapping, we generated an integrated transcriptomic map comprising leukocortical, cerebellar and spinal cord areas in normal and MS tissues that captures regional subtype diversity of various cell types with an emphasis on astrocytes and oligodendrocytes. While we found strong cross-regional diversity among glial subtypes in control tissue, regional signatures become more obscure in MS. This suggests that patterns of transcriptomic changes in MS are shared across regions and converge on specific pathways, especially those regulating cellular stress and immune activation. In addition, we found evidence that a subtype of white matter oligodendrocytes appearing across all three CNS regions adopt pro-remyelinating gene signatures in MS. In summary, our data suggest that cross-regional transcriptomic glial signatures overlap in MS, with different reactive glial cell types capable of either exacerbating or ameliorating pathology.

Axon-dendrite and apical-basolateral sorting in a single neuron

Cells are highly organized machines with functionally specialized compartments. For example, membrane proteins are localized to axons or dendrites in neurons and to apical or basolateral surfaces in epithelial cells. Interestingly, many sensory cells-including vertebrate photoreceptors and olfactory neurons-exhibit both neuronal and epithelial features. Here, we show that Caenorhabditis elegans amphid neurons simultaneously exhibit axon-dendrite sorting like a neuron and apical-basolateral sorting like an epithelial cell. The distal ∼5-10 µm of the dendrite is apical, while the remainder of the dendrite, soma, and axon are basolateral. To determine how proteins are sorted among these compartments, we studied the localization of the conserved adhesion molecule SAX-7/L1CAM. Using minimal synthetic transmembrane proteins, we found that the 91-aa cytoplasmic tail of SAX-7 is necessary and sufficient to direct basolateral localization. Basolateral localization can be fully recapitulated using either of 2 short (10-aa or 19-aa) tail sequences that, respectively, resemble dileucine and Tyr-based motifs known to mediate sorting in mammalian epithelia. The Tyr-based motif is conserved in human L1CAM but had not previously been assigned a function. Disrupting key residues in either sequence leads to apical localization, while "improving" them to match epithelial sorting motifs leads to axon-only localization. Indeed, changing only 2 residues in a short motif is sufficient to redirect the protein between apical, basolateral, and axonal localization. Our results demonstrate that axon-dendrite and apical-basolateral sorting pathways can coexist in a single cell, and suggest that subtle changes to short sequence motifs are sufficient to redirect proteins between these pathways.

The expression and function of glutamate aspartate transporters in Bergmann glia are decreased in neuronal nitric oxide synthase-knockout mice during postnatal development

Bergmann glia (BG) predominantly use glutamate/aspartate transporters (GLAST) for glutamate uptake in the cerebellum. Recently, nitric oxide (NO) treatment has been shown to upregulate GLAST function and increase glutamate uptake in vitro. We previously discovered that neuronal nitric oxide synthase knockout (nNOS^−/−) mice displayed structural and functional neuronal abnormalities in the cerebellum during development, in addition to previously reported motor deficits. Although these developmental deficits have been identified in the nNOS^−/− cerebellum, it is unknown whether BG morphology and GLAST expression are also affected in the absence of nNOS in vivo. This study is the first to characterize BG morphology and GLAST expression during development in nNOS^−/− mice using immunohistochemistry and western blotting across postnatal development. Results showed that BG in nNOS^−/− mice exhibited abnormal morphology and decreased GLAST expression compared with wildtype (WT) mice across postnatal development. Treating ex vivo WT cerebellar slices with the NOS inhibitor L‐NAME decreased GLAST expression while treating nNOS^−/− slices with the slow‐release NO‐donor NOC‐18 increased GLAST expression when compared with their respective controls. In addition, treating primary BG isolated from WT mice with the selective nNOS inhibitor 7N decreased the membrane expression of GLAST and influx of Ca²⁺/Na⁺, while treating nNOS^−/− BG with SNAP increased the membrane expression of GLAST and Ca²⁺/Na⁺ influx. Moreover, the effects of SNAP on GLAST expression and Ca²⁺/Na⁺ influx in nNOS^−/− BG were significantly reduced by a PKG inhibitor. Together, these results reveal a novel role for nNOS/NO signaling in BG development, regulated by a PKG‐mediated mechanism.

Interplay in neural functions of cell adhesion molecule close homolog of L1 (CHL1) and Programmed Cell Death 6 (PDCD6)

Close homolog of L1 (CHL1) is a cell adhesion molecule of the immunoglobulin superfamily. It promotes neuritogenesis and survival of neurons in vitro. In vivo, CHL1 promotes nervous system development, regeneration after trauma, and synaptic function and plasticity. We identified programmed cell death 6 (PDCD6) as a novel binding partner of the CHL1 intracellular domain (CHL1-ICD). Co-immunoprecipitation, pull-down assay with CHL1-ICD, and proximity ligation in cerebellum and pons of 3-day-old and 6-month-old mice, as well as in cultured cerebellar granule neurons and cortical astrocytes indicate an association between PDCD6 and CHL1. The Ca2+-chelator BAPTA-AM inhibited the association between CHL1 and PDCD6. The treatment of cerebellar granule neurons with a cell-penetrating peptide comprising the cell surface proximal 30 N-terminal amino acids of CHL1-ICD inhibited the association between CHL1 and PDCD6 and PDCD6- and CHL1-triggered neuronal survival. These results suggest that PDCD6 contributes to CHL1 functions in the nervous system.

Mechanisms Underlying Target Selectivity for Cell Types and Subcellular Domains in Developing Neocortical Circuits

The cerebral cortex contains numerous neuronal cell types, distinguished by their molecular identity as well as their electrophysiological and morphological properties. Cortical function is reliant on stereotyped patterns of synaptic connectivity and synaptic function among these neuron types, but how these patterns are established during development remains poorly understood. Selective targeting not only of different cell types but also of distinct postsynaptic neuronal domains occurs in many brain circuits and is directed by multiple mechanisms. These mechanisms include the regulation of axonal and dendritic guidance and fine-scale morphogenesis of pre- and postsynaptic processes, lineage relationships, activity dependent mechanisms and intercellular molecular determinants such as transmembrane and secreted molecules, many of which have also been implicated in neurodevelopmental disorders. However, many studies of synaptic targeting have focused on circuits in which neuronal processes target different lamina, such that cell-type-biased connectivity may be confounded with mechanisms of laminar specificity. In the cerebral cortex, each cortical layer contains cell bodies and processes from intermingled neuronal cell types, an arrangement that presents a challenge for the development of target-selective synapse formation. Here, we address progress and future directions in the study of cell-type-biased synaptic targeting in the cerebral cortex. We highlight challenges to identifying developmental mechanisms generating stereotyped patterns of intracortical connectivity, recent developments in uncovering the determinants of synaptic target selection during cortical synapse formation, and current gaps in the understanding of cortical synapse specificity.

Cell adhesion molecules regulating astrocyte-neuron interactions

A tripartite synapse comprises a neuronal presynaptic axon and a postsynaptic dendrite, which are closely ensheathed by a perisynaptic astrocyte process. Through their structural and functional association with thousands of neuronal synapses, astrocytes regulate synapse formation and function. Recent work revealed a diverse range of cell adhesion-based mechanisms that mediate astrocyte-synapse interactions at tripartite synapses. Here, we will review some of these findings unveiling a highly dynamic bidirectional signaling between astrocytes and synapses, which orchestrates astrocyte morphological maturation and synapse development. Moreover, we will discuss the roles of these newly discovered molecular pathways in brain physiology and function both in health and disease.

Excitatory granule neuron precursors orchestrate laminar localization and differentiation of cerebellar inhibitory interneuron subtypes

GABAergic interneurons migrate long distances through stereotyped migration programs toward specific laminar positions. During their migration, GABAergic interneurons are morphologically alike but then differentiate into a rich array of interneuron subtypes critical for brain function. How interneuron subtypes acquire their final phenotypic traits remains largely unknown. Here, we show that cerebellar molecular layer GABAergic interneurons, derived from the same progenitor pool, use separate migration paths to reach their laminar position and differentiate into distinct basket cell (BC) and stellate cell (SC) GABAergic interneuron subtypes. Using two-photon live imaging, we find that SC final laminar position requires an extra step of tangential migration supported by a subpopulation of glutamatergic granule cells (GCs). Conditional depletion of GCs affects SC differentiation but does not affect BCs. Our results reveal how timely feedforward control of inhibitory interneuron migration path regulates their terminal differentiation and, thus, establishment of the local inhibitory circuit assembly.

Graphene glial-interfaces: challenges and perspectives

Graphene nanosheets are mechanically strong but flexible, electrically conductive and bio-compatible. Thus, due to these unique properties, they are being intensively studied as materials for the next generation of neural interfaces. Most of the literature focused on optimizing the interface between these materials and neurons. However, one of the most common causes of implant failure is the adverse inflammatory reaction of glial cells. These cells are not, as previously considered, just passive and supportive cells, but play a crucial role in the physiology and pathology of the nervous system, and in the interaction with implanted electrodes. Besides providing structural support to neurons, glia are responsible for the modulation of synaptic transmission and control of central and peripheral homeostasis. Accordingly, knowledge on the interaction between glia and biomaterials is essential to develop new implant-based therapies for the treatment of neurological disorders, such as epilepsy, brain tumours, and Alzheimer's and Parkinson's disease. This work provides an overview of the emerging literature on the interaction of graphene-based materials with glial cells, together with a complete description of the different types of glial cells and problems associated with them. We believe that this description will be important for researchers working in materials science and nanotechnology to develop new active materials to interface, measure and stimulate these cells.

Establishment and Maintenance of Neural Circuit Architecture

The ability to sense the world, process information, and navigate the environment depends on the assembly and continuous function of neural circuits in the brain. Within the past two decades, new technologies have rapidly advanced our understanding of how neural circuits are wired during development and how they are stably maintained, often for years. Electron microscopy reconstructions of model organism connectomes have provided a map of the stereotyped (and variable) connections in the brain; advanced light microscopy techniques have enabled direct observation of the cellular dynamics that underlie circuit construction and maintenance; transcriptomic and proteomic surveys of both developing and mature neurons have provided insights into the molecular and genetic programs governing circuit establishment and maintenance; and advanced genetic techniques have allowed for high-throughput discovery of wiring regulators. These tools have empowered scientists to rapidly generate and test hypotheses about how circuits establish and maintain connectivity. Thus, the set of principles governing circuit formation and maintenance have been expanded. These principles are discussed in this review.

Molecular Mechanisms of L1 and NCAM Adhesion Molecules in Synaptic Pruning, Plasticity, and Stabilization

Mammalian brain circuits are wired by dynamic formation and remodeling during development to produce a balance of excitatory and inhibitory synapses. Synaptic regulation is mediated by a complex network of proteins including immunoglobulin (Ig)- class cell adhesion molecules (CAMs), structural and signal-transducing components at the pre- and post-synaptic membranes, and the extracellular protein matrix. This review explores the current understanding of developmental synapse regulation mediated by L1 and NCAM family CAMs. Excitatory and inhibitory synapses undergo formation and remodeling through neuronal CAMs and receptor-ligand interactions. These responses result in pruning inactive dendritic spines and perisomatic contacts, or synaptic strengthening during critical periods of plasticity. Ankyrins engage neural adhesion molecules of the L1 family (L1-CAMs) to promote synaptic stability. Chondroitin sulfates, hyaluronic acid, tenascin-R, and linker proteins comprising the perineuronal net interact with L1-CAMs and NCAM, stabilizing synaptic contacts and limiting plasticity as critical periods close. Understanding neuronal adhesion signaling and synaptic targeting provides insight into normal development as well as synaptic connectivity disorders including autism, schizophrenia, and intellectual disability.

Role of astrocytes in synapse formation and maturation

Astrocytes are the most abundant glial cells in the mammalian brain and directly participate in the proper functioning of the nervous system by regulating ion homeostasis, controlling glutamate reuptake, and maintaining the blood-brain barrier. In the last two decades, a growing body of work also identified critical roles for astrocytes in regulating synaptic connectivity. Stemming from the observation that functional and morphological development of astrocytes occur concurrently with synapse formation and maturation, these studies revealed that both developmental processes are directly linked. In fact, astrocytes both physically contact numerous synaptic structures and actively instruct many aspects of synaptic development and function via a plethora of secreted and adhesion-based molecular signals. The complex astrocyte-to-neuron signaling modalities control different stages of synaptic development such as regulating the initial formation of structural synapses as well as their functional maturation. Furthermore, the synapse-modulating functions of astrocytes are evolutionarily conserved and contribute to the development and plasticity of diverse classes of synapses and circuits throughout the central nervous system. Importantly, because impaired synapse formation and function is a hallmark of many neurodevelopmental disorders, deficits in astrocytes are likely to be major contributors to disease pathogenesis. In this chapter, we review our current understanding of the cellular and molecular mechanisms by which astrocytes contribute to synapse development and discuss the bidirectional secretion-based and contact-mediated mechanisms responsible for these essential developmental processes.

Cell Adhesion Molecules Involved in Neurodevelopmental Pathways Implicated in 3p-Deletion Syndrome and Autism Spectrum Disorder

Autism spectrum disorder (ASD) is characterized by impaired social interaction, language delay and repetitive or restrictive behaviors. With increasing prevalence, ASD is currently estimated to affect 0.5–2.0% of the global population. However, its etiology remains unclear due to high genetic and phenotypic heterogeneity. Copy number variations (CNVs) are implicated in several forms of syndromic ASD and have been demonstrated to contribute toward ASD development by altering gene dosage and expression. Increasing evidence points toward the p-arm of chromosome 3 (chromosome 3p) as an ASD risk locus. Deletions occurring at chromosome 3p result in 3p-deletion syndrome (Del3p), a rare genetic disorder characterized by developmental delay, intellectual disability, facial dysmorphisms and often, ASD or ASD-associated behaviors. Therefore, we hypothesize that overlapping molecular mechanisms underlie the pathogenesis of Del3p and ASD. To investigate which genes encoded in chromosome 3p could contribute toward Del3p and ASD, we performed a comprehensive literature review and collated reports investigating the phenotypes of individuals with chromosome 3p CNVs. We observe that high frequencies of CNVs occur in the 3p26.3 region, the terminal cytoband of chromosome 3p. This suggests that CNVs disrupting genes encoded within the 3p26.3 region are likely to contribute toward the neurodevelopmental phenotypes observed in individuals affected by Del3p. The 3p26.3 region contains three consecutive genes encoding closely related neuronal immunoglobulin cell adhesion molecules (IgCAMs): Close Homolog of L1 (CHL1), Contactin-6 (CNTN6), and Contactin-4 (CNTN4). CNVs disrupting these neuronal IgCAMs may contribute toward ASD phenotypes as they have been associated with key roles in neurodevelopment. CHL1, CNTN6, and CNTN4 have been observed to promote neurogenesis and neuronal survival, and regulate neuritogenesis and synaptic function. Furthermore, there is evidence that these neuronal IgCAMs possess overlapping interactomes and participate in common signaling pathways regulating axon guidance. Notably, mouse models deficient for these neuronal IgCAMs do not display strong deficits in axonal migration or behavioral phenotypes, which is in contrast to the pronounced defects in neuritogenesis and axon guidance observed in vitro. This suggests that when CHL1, CNTN6, or CNTN4 function is disrupted by CNVs, other neuronal IgCAMs may suppress behavioral phenotypes by compensating for the loss of function.

Single Cell Transcriptome Analysis of Niemann-Pick Disease, Type C1 Cerebella

Niemann-Pick disease, type C1 (NPC1) is a lysosomal disease characterized by endolysosomal storage of unesterified cholesterol and decreased cellular cholesterol bioavailability. A cardinal symptom of NPC1 is cerebellar ataxia due to Purkinje neuron loss. To gain an understanding of the cerebellar neuropathology we obtained single cell transcriptome data from control (Npc1^+/+) and both three-week-old presymptomatic and seven-week-old symptomatic mutant (Npc1^-/-) mice. In seven-week-old Npc1^-/- mice, differential expression data was obtained for neuronal, glial, vascular, and myeloid cells. As anticipated, we observed microglial activation and increased expression of innate immunity genes. We also observed increased expression of innate immunity genes by other cerebellar cell types, including Purkinje neurons. Whereas neuroinflammation mediated by microglia may have both neuroprotective and neurotoxic components, the contribution of increased expression of these genes by non-immune cells to NPC1 pathology is not known. It is possible that dysregulated expression of innate immunity genes by non-immune cells is neurotoxic. We did not anticipate a general lack of transcriptomic changes in cells other than microglia from presymptomatic three-week-old Npc1^-/- mice. This observation suggests that microglia activation precedes neuronal dysfunction. The data presented in this paper will be useful for generating testable hypotheses related to disease progression and Purkinje neurons loss as well as providing insight into potential novel therapeutic interventions.

Function of Cathepsin K in the Central Nervous System of Male Mice is Independent of Its Role in the Thyroid Gland

Cathepsin K deficiency in male mice (Ctsk-/-) results in decreased numbers of hippocampal astrocytes and altered neuronal patterning as well as learning and memory deficits. Additionally, cathepsin K carries essential roles in the thyroid gland where it contributes to the liberation of thyroid hormones (TH). Because TH are essential for brain development, in particular for the cerebellum, we investigated whether cathepsin K's function in the thyroid is directly linked to the brain phenotype of Ctsk-/- mice. Serum levels of thyroid stimulating hormone, brain concentrations of free TH, and deiodinase 2 (Dio2) activity in brain parenchyma as well as cerebellar development were comparable in Ctsk-/- and WT animals, suggesting regular thyroid states and TH metabolism. Despite unaltered transcript levels, protein expression of two TH transporters was enhanced in specific brain regions in Ctsk-/- mice, suggesting altered TH supply to these regions. Thyrotropin releasing hormone (Trh) mRNA levels were enhanced threefold in the hippocampus of Ctsk-/- mice. In the striatum of Ctsk-/- mice the mRNA for Dio2 and hairless were approximately 1.3-fold enhanced, while mRNA levels for monocarboxylate transporter 8 and Trh were reduced to 60% and 40%, respectively, pointing to altered striatal physiology. We conclude that the role of cathepsin K in the thyroid gland is not directly associated with its function in the central nervous system (CNS) of mice. Future studies will show whether the brain region-specific alterations in Trh mRNA may eventually result in altered neuroprotection that could explain the neurobehavioral defects of Ctsk-/- mice.

The Role of Astrocytes in the Development of the Cerebellum

Astrocytes, initially described as merely support cells, are now known as a heterogeneous population of cells actively involved in a variety of biological functions such as: neuronal migration and differentiation; regulation of cerebral blood flow; metabolic control of extracellular potassium concentration; and modulation of synapse formation and elimination; among others. Cerebellar glial cells have been shown to play a significant role in proliferation, differentiation, migration, and synaptogenesis. However, less evidence is available about the role of neuron-astrocyte interactions during cerebellar development and their impact on diseases of the cerebellum. In this review, we will focus on the mechanisms underlying cellular interactions, specifically neuron-astrocyte interactions, during cerebellar development, function, and disease. We will discuss how cerebellar glia, astrocytes, and Bergmann glia play a fundamental role in several steps of cerebellar development, such as granule cell migration, axonal growth, neuronal differentiation, and synapse formation, and in diseases associated with the cerebellum. We will focus on how astrocytes and thyroid hormones impact cerebellar development. Furthermore, we will provide evidence of how growth factors secreted by glial cells, such as epidermal growth factor and transforming growth factors, control cerebellar organogenesis. Finally, we will argue that glia are a key mediator of cerebellar development and that identification of molecules and pathways involved in neuron-glia interactions may contribute to a better understanding of cerebellar development and associated disorders.

A muscle-epidermis-glia signaling axis sustains synaptic specificity during allometric growth in Caenorhabditis elegans

Synaptic positions underlie precise circuit connectivity. Synaptic positions can be established during embryogenesis and sustained during growth. The mechanisms that sustain synaptic specificity during allometric growth are largely unknown. We performed forward genetic screens in C. elegans for regulators of this process and identified mig-17, a conserved ADAMTS metalloprotease. Proteomic mass spectrometry, cell biological and genetic studies demonstrate that MIG-17 is secreted from cells like muscles to regulate basement membrane proteins. In the nematode brain, the basement membrane does not directly contact synapses. Instead, muscle-derived basement membrane coats one side of the glia, while glia contact synapses on their other side. MIG-17 modifies the muscle-derived basement membrane to modulate epidermal-glial crosstalk and sustain glia location and morphology during growth. Glia position in turn sustains the synaptic pattern established during embryogenesis. Our findings uncover a muscle-epidermis-glia signaling axis that sustains synaptic specificity during the organism's allometric growth.

Emotional Stress Induces Structural Plasticity in Bergmann Glial Cells via an AC5-CPEB3-GluA1 Pathway

Stress alters brain function by modifying the structure and function of neurons and astrocytes. The fine processes of astrocytes are critical for the clearance of neurotransmitters during synaptic transmission. Thus, experience-dependent remodeling of glial processes is anticipated to alter the output of neural circuits. However, the molecular mechanisms that underlie glial structural plasticity are not known. Here we show that a single exposure of male and female mice to an acute stress produced a long-lasting retraction of the lateral processes of cerebellar Bergmann glial cells. These cells express the GluA1 subunit of AMPA-type glutamate receptors, and GluA1 knockdown is known to shorten the length of glial processes. We found that stress reduced the level of GluA1 protein and AMPA receptor-mediated currents in Bergmann glial cells, and these effects were absent in mice devoid of CPEB3, a protein that binds to GluA1 mRNA and regulates GluA1 protein synthesis. Administration of a β-adrenergic receptor blocker attenuated the reduction in GluA1, and deletion of adenylate cyclase 5 prevented GluA1 suppression. Therefore, stress suppresses GluA1 protein synthesis via an adrenergic/adenylyl cyclase/CPEB3 pathway, and reduces the length of astrocyte lateral processes. Our results identify a novel mechanism for GluA1 subunit plasticity in non-neuronal cells and suggest a previously unappreciated role for AMPA receptors in stress-induced astrocytic remodeling.SIGNIFICANCE STATEMENT Astrocytes play important roles in synaptic transmission by extending fine processes around synapses. In this study, we showed that a single exposure to an acute stress triggered a retraction of lateral/fine processes in mouse cerebellar astrocytes. These astrocytes express GluA1, a glutamate receptor subunit known to lengthen astrocyte processes. We showed that astrocytic structural changes are associated with a reduction of GluA1 protein levels. This requires activation of β-adrenergic receptors and is triggered by noradrenaline released during stress. We identified adenylyl cyclase 5, an enzyme that elevates cAMP levels, as a downstream effector and found that lowering GluA1 levels depends on CPEB3 proteins that bind to GluA1 mRNA. Therefore, stress regulates GluA1 protein synthesis via an adrenergic/adenylyl cyclase/CPEB3 pathway in astrocytes and remodels their fine processes.

Glial Cell-Axonal Growth Cone Interactions in Neurodevelopment and Regeneration

The developing nervous system is a complex yet organized system of neurons, glial support cells, and extracellular matrix that arranges into an elegant, highly structured network. The extracellular and intracellular events that guide axons to their target locations have been well characterized in many regions of the developing nervous system. However, despite extensive work, we have a poor understanding of how axonal growth cones interact with surrounding glial cells to regulate network assembly. Glia-to-growth cone communication is either direct through cellular contacts or indirect through modulation of the local microenvironment via the secretion of factors or signaling molecules. Microglia, oligodendrocytes, astrocytes, Schwann cells, neural progenitor cells, and olfactory ensheathing cells have all been demonstrated to directly impact axon growth and guidance. Expanding our understanding of how different glial cell types directly interact with growing axons throughout neurodevelopment will inform basic and clinical neuroscientists. For example, identifying the key cellular players beyond the axonal growth cone itself may provide translational clues to develop therapeutic interventions to modulate neuron growth during development or regeneration following injury. This review will provide an overview of the current knowledge about glial involvement in development of the nervous system, specifically focusing on how glia directly interact with growing and maturing axons to influence neuronal connectivity. This focus will be applied to the clinically-relevant field of regeneration following spinal cord injury, highlighting how a better understanding of the roles of glia in neurodevelopment can inform strategies to improve axon regeneration after injury.

Dendrites with specialized glial attachments develop by retrograde extension using SAX-7 and GRDN-1

Dendrites develop elaborate morphologies in concert with surrounding glia, but the molecules that coordinate dendrite and glial morphogenesis are mostly unknown. C. elegans offers a powerful model for identifying such factors. Previous work in this system examined dendrites and glia that develop within epithelia, similar to mammalian sense organs. Here, we focus on the neurons BAG and URX, which are not part of an epithelium but instead form membranous attachments to a single glial cell at the nose, reminiscent of dendrite-glia contacts in the mammalian brain. We show that these dendrites develop by retrograde extension, in which the nascent dendrite endings anchor to the presumptive nose and then extend by stretching during embryo elongation. Using forward genetic screens, we find that dendrite development requires the adhesion protein SAX-7/L1CAM and the cytoplasmic protein GRDN-1/CCDC88C to anchor dendrite endings at the nose. SAX-7 acts in neurons and glia, while GRDN-1 acts in glia to non-autonomously promote dendrite extension. Thus, this work shows how glial factors can help to shape dendrites, and identifies a novel molecular mechanism for dendrite growth by retrograde extension.

Daily changes in GFAP expression in radial glia of the olfactory bulb in rabbit pups entrained to circadian feeding

When food is restricted daily to a fixed time, animals show uncoupled molecular, physiological and behavioral circadian rhythms from those entrained by light and controlled by the suprachiasmatic nucleus. The loci of the food-entrainable oscillator and the mechanisms by which rhythms emerge are unclear. Using animals entrained to the light-dark cycle, recent studies indicate that astrocytes in the suprachiasmatic nucleus play a key role in the regulation of circadian rhythms. However, it is unknown whether astrocytic cells can be synchronized by circadian restricted feeding. Studying the olfactory bulb (OB) of rabbit pups entrained to daily feeding, we hypothesized that the expression of glial fibrillary acidic protein (GFAP) and the morphology of GFAP-immunopositive cells change in synchrony with timing of feeding. By using pups fed at 1000 h or 2200 h, we found that GFAP protein expression in the OB changes with a nadir at feeding time and a peak 16 h after feeding. We also found that length of radial glia processes, the most abundant GFAP⁺ cell in the rabbit pup OB, shows a daily change also coupled to feeding time. These temporal changes of GFAP were expressed in anti-phase to the rhythms of locomotor activity and c-Fos immunoreactivity. The results indicate that GFAP expression and elongation-retraction of radial glia processes are coupled by feeding time and suggest that glia cells may play an important functional role in food entraining of the OB circadian oscillator.

Immunoglobulin-Like Receptors and Their Impact on Wiring of Brain Synapses

Synapse formation is mediated by a surprisingly large number and wide variety of genes encoding many different protein classes. One of the families increasingly implicated in synapse wiring is the immunoglobulin superfamily (IgSF). IgSF molecules are by definition any protein containing at least one Ig-like domain, making this family one of the most common protein classes encoded by the genome. Here, we review the emerging roles for IgSF molecules in synapse formation specifically in the vertebrate brain, focusing on examples from three classes of IgSF members: ( a) cell adhesion molecules, ( b) signaling molecules, and ( c) immune molecules expressed in the brain. The critical roles for IgSF members in regulating synapse formation may explain their extensive involvement in neuropsychiatric and neurodevelopmental disorders. Solving the IgSF code for synapse formation may reveal multiple new targets for rescuing IgSF-mediated deficits in synapse formation and, eventually, new treatments for psychiatric disorders caused by altered IgSF-induced synapse wiring.

Maternal marginal iodine deficiency delays cerebellar Bergmann glial cell development in rat offspring: Involvement of Notch signaling pathway

During early pregnancy, iodine deficiency (ID) is linked to adverse effects on child motor and psychomotor function. Maternal marginal ID has become a common public health problem. It is unclear whether marginal ID influences the development of the cerebellum or its underlying mechanisms. Thus, the purpose of this study was to determine the effects of marginal ID on the development of cerebellar Bergmann glial cells (BGs) and investigate the activation of the Notch signaling pathway, which is crucial for the development and morphology of BGs. We treated Wistar rats with an ID diet (iodine content 60 ± 1.5 ng/g) supplemented with deionized water containing different concentrations of potassium iodide (KI) (183, 117, and 0 μg/L for the control, marginal ID, and severe ID groups, respectively) during pregnancy and lactation. We explored the morphology of the BGs by Golgi-Cox staining and immunofluorescence and investigated the Notch signaling pathway using western blot. Our results showed that the marginal ID and severe ID groups had decreased cerebellar BG fiber lengths (P < 0.05 and 0.01, respectively) and numbers (P < 0.01 for both) on postnatal day (PN) 7, PN14, and PN21 compared to the control group. Moreover, the data showed that severe ID significantly reduced Dll1, Notch1, RBP-Jκ, and BLBP protein levels at all three time points. Marginal ID slightly reduced the expression of Notch1 on PN7 (P < 0.05) and PN21 (P < 0.01), RBP-Jκ on PN14 (P < 0.01) and PN21 (P < 0.05), and BLBP on PN7 (P < 0.05). There was no significant difference in Dll1 protein levels between the marginal ID and control groups at any time point. Our study suggests that marginal ID leads to mild damage to BG morphogenesis in the cerebellum. The abnormal regulation of the Notch signaling pathway may be involved in the damage to BGs.

Assembly and maintenance of GABAergic and Glycinergic circuits in the mammalian nervous system

Inhibition in the central nervous systems (CNS) is mediated by two neurotransmitters: gamma-aminobutyric acid (GABA) and glycine. Inhibitory synapses are generally GABAergic or glycinergic, although there are synapses that co-release both neurotransmitter types. Compared to excitatory circuits, much less is known about the cellular and molecular mechanisms that regulate synaptic partner selection and wiring patterns of inhibitory circuits. Recent work, however, has begun to fill this gap in knowledge, providing deeper insight into whether GABAergic and glycinergic circuit assembly and maintenance rely on common or distinct mechanisms. Here we summarize and contrast the developmental mechanisms that regulate the selection of synaptic partners, and that promote the formation, refinement, maturation and maintenance of GABAergic and glycinergic synapses and their respective wiring patterns. We highlight how some parts of the CNS demonstrate developmental changes in the type of inhibitory transmitter or receptor composition at their inhibitory synapses. We also consider how perturbation of the development or maintenance of one type of inhibitory connection affects other inhibitory synapse types in the same circuit. Mechanistic insight into the development and maintenance of GABAergic and glycinergic inputs, and inputs that co-release both these neurotransmitters could help formulate comprehensive therapeutic strategies for treating disorders of synaptic inhibition.

Inhibitory neuron-specific Cre-dependent red fluorescent labeling using VGAT BAC-based transgenic mouse lines with identified transgene integration sites

Inhibitory neurons are crucial for shaping and regulating the dynamics of the entire network, and disturbances in these neurons contribute to brain disorders. Despite the recent progress in genetic labeling techniques, the heterogeneity of inhibitory neurons requires the development of highly characterized tools that allow accurate, convenient, and versatile visualization of inhibitory neurons in the mouse brain. Here, we report a novel genetic technique to visualize the vast majority and/or sparse subsets of inhibitory neurons in the mouse brain without using techniques that require advanced skills. We developed several lines of Cre-dependent tdTomato reporter mice based on the vesicular GABA transporter (VGAT)-BAC, named VGAT-stop-tdTomato mice. The most useful line (line #54) was selected for further analysis based on two characteristics: the inhibitory neuron-specificity of tdTomato expression and the transgene integration site, which confers efficient breeding and fewer adverse effects resulting from transgene integration-related genomic disruption. Robust and inhibitory neuron-specific expression of tdTomato was observed in a wide range of developmental and cellular contexts. By breeding the VGAT-stop-tdTomato mouse (line #54) with a novel Cre driver mouse line, Galntl4-CreER, sparse labeling of inhibitory neurons was achieved following tamoxifen administration. Furthermore, another interesting line (line #58) was generated through the unexpected integration of the transgene into the X-chromosome and will be used to map X-chromosome inactivation of inhibitory neurons. Taken together, our studies provide new, well-characterized tools with which multiple aspects of inhibitory neurons can be studied in the mouse.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Embryology

With the growing recognition of the extent and prevalence of human cerebellar disorders, an understanding of developmental programs that build the mature cerebellum is necessary. In this chapter we present an overview of the basic epochs and key molecular regulators of the developmental programs of cerebellar development. These include early patterning of the cerebellar territory, the genesis of cerebellar cells from multiple spatially distinct germinal zones, and the extensive migration and coordinated cellular rearrangements that result in the formation of the exquisitely foliated and laminated mature cerebellum. This knowledge base is founded on extensive analysis of animal models, particularly mice, due in large part to the ease of genetic manipulation of this important model organism. Since cerebellar structure and function are largely conserved across species, mouse cerebellar development is highly relevant to humans and has led to important insights into the developmental pathogenesis of human cerebellar disorders. Human fetal cerebellar development remains largely undescribed; however, several human-specific developmental features are known which are relevant to human disease and underline the importance of ongoing human fetal research.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Propofol Exposure in Early Life Induced Developmental Impairments in the Mouse Cerebellum

Propofol is a widely used anesthetic in the clinic while several studies have demonstrated that propofol exposure may cause neurotoxicity in the developing brain. However, the effects of early propofol exposure on cerebellar development are not well understood. Propofol (30 or 60 mg/kg) was administered to mice on postnatal day (P)7; Purkinje cell dendritogenesis and Bergmann glial cell development were evaluated on P8, and granule neuron migration was analyzed on P10. The results indicated that exposure to propofol on P7 resulted in a significant reduction in calbindin-labeled Purkinje cells and their dendrite length. Furthermore, propofol induced impairments in Bergmann glia development, which might be involved in the delay of granule neuron migration from the external granular layer (EGL) to the internal granular layer (IGL) during P8 to P10 at the 60 mg/kg dosage, but not at the 30 mg/kg dosage. Several reports have suggested that the Notch signaling pathway plays instructive roles in the morphogenesis of Bergmann glia. Here, it was revealed that propofol treatment decreased Jagged1 and Notch1 protein levels in the cerebellum on P8. Taken together, exposure to propofol during the neonatal period impairs Bergmann glia development and may therefore lead to cerebellum development defects. Our results may aid in the understanding of the neurotoxic effects of propofol when administrated to infants.

Roles of Glial Cells in Sculpting Inhibitory Synapses and Neural Circuits

Glial cells are essential for every aspect of normal neuronal development, synapse formation, and function in the central nervous system (CNS). Astrocytes secrete a variety of factors that regulate synaptic connectivity and circuit formation. Microglia also modulate synapse development through phagocytic activity. Most of the known actions of CNS glial cells are limited to roles at excitatory synapses. Nevertheless, studies have indicated that both astrocytes and microglia shape inhibitory synaptic connections through various mechanisms, including release of regulatory molecules, direct contact with synaptic terminals, and utilization of mediators in the extracellular matrix. This review summarizes recent investigations into the mechanisms underlying CNS glial cell-mediated inhibitory synapse development.

Stratification of astrocytes in healthy and diseased brain

Astrocytes, a subtype of glial cells, come in variety of forms and functions. However, overarching role of these cell is in the homeostasis of the brain, be that regulation of ions, neurotransmitters, metabolism or neuronal synaptic networks. Loss of homeostasis represents the underlying cause of all brain disorders. Thus, astrocytes are likely involved in most if not all of the brain pathologies. We tabulate astroglial homeostatic functions along with pathological condition that arise from dysfunction of these glial cells. Classification of astrocytes is presented with the emphasis on evolutionary trails, morphological appearance and numerical preponderance. We note that, even though astrocytes from a variety of mammalian species share some common features, human astrocytes appear to be the largest and most complex of all astrocytes studied thus far. It is then an imperative to develop humanized models to study the role of astrocytes in brain pathologies, which is perhaps most abundantly clear in the case of glioblastoma multiforme.

Homophilic binding of the neural cell adhesion molecule CHL1 regulates development of ventral midbrain dopaminergic pathways

Abnormal development of ventral midbrain (VM) dopaminergic (DA) pathways, essential for motor and cognitive function, may underpin a number of neurological disorders and thereby highlight the importance of understanding the birth and connectivity of the associated neurons. While a number of regulators of VM DA neurogenesis are known, processes involved in later developmental events, including terminal differentiation and axon morphogenesis, are less well understood. Recent transcriptional analysis studies of the developing VM identified genes expressed during these stages, including the cell adhesion molecule with homology to L1 (Chl1). Here, we map the temporal and spatial expression of CHL1 and assess functional roles of substrate-bound and soluble-forms of the protein during VM DA development. Results showed early CHL1 in the VM, corresponding with roles in DA progenitor migration and differentiation. Subsequently, we demonstrated roles for CHL1 in both axonal extension and repulsion, selectively of DA neurons, suggestive of a role in guidance towards forebrain targets and away from hindbrain nuclei. In part, CHL1 mediates these roles through homophilic CHL1-CHL1 interactions. Collectively, these findings enhance our knowledge of VM DA pathways development, and may provide new insights into understanding DA developmental conditions such as autism spectrum disorders.

The cell adhesion molecule CHL1 interacts with patched-1 to regulate apoptosis during postnatal cerebellar development

The immunoglobulin superfamily adhesion molecule close homolog of L1 (CHL1) plays important roles during nervous system development. Here, we identified the hedgehog receptor patched-1 (PTCH1) as a novel CHL1-binding protein and showed that CHL1 interacts with the first extracellular loop of PTCH1 via its extracellular domain. Colocalization and co-immunoprecipitation of CHL1 with PTCH1 suggest an association of CHL1 with this major component of the hedgehog signaling pathway. The trans-interaction of CHL1 with PTCH1 promotes neuronal survival in cultures of dissociated cerebellar granule cells and of organotypic cerebellar slices. An inhibitor of the PTCH1-regulated hedgehog signal transducer, smoothened (SMO), and inhibitors of RhoA and Rho-associated kinase (ROCK) 1 and 2 prevent CHL1-dependent survival of cultured cerebellar granule cells and survival of cerebellar granule and Purkinje cells in organotypic cultures. In histological sections from 10- and 14-day-old CHL1-deficient mice, enhanced apoptosis of granule, but not Purkinje, cells was observed. The results of the present study indicate that CHL1 triggers PTCH1-, SMO-, RhoA- and ROCK-dependent signal transduction pathways to promote neuronal survival after cessation of the major morphogenetic events during mouse cerebellar development.

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.

Dual Function of NRP1 in Axon Guidance and Subcellular Target Recognition in Cerebellum

Subcellular target recognition in the CNS is the culmination of a multiple-step program including axon guidance, target recognition, and synaptogenesis. In cerebellum, basket cells (BCs) innervate the soma and axon initial segment (AIS) of Purkinje cells (PCs) to form the pinceau synapse, but the underlying mechanisms remain incompletely understood. Here, we demonstrate that neuropilin-1 (NRP1), a Semaphorin receptor expressed in BCs, controls both axonal guidance and subcellular target recognition. We show that loss of Semaphorin 3A function or specific deletion of NRP1 in BCs alters the stereotyped organization of BC axon and impairs pinceau synapse formation. Further, we identified NRP1 as a trans-synaptic binding partner of the cell adhesion molecule neurofascin-186 (NF186) expressed in the PC AIS during pinceau synapse formation. These findings identify a dual function of NRP1 in both axon guidance and subcellular target recognition in the construction of GABAergic circuitry.

Conversion of Fibroblasts to Parvalbumin Neurons by One Transcription Factor, Ascl1, and the Chemical Compound Forskolin

Abnormalities in parvalbumin (PV)-expressing interneurons cause neurodevelopmental disorders such as epilepsy, autism, and schizophrenia. Unlike other types of neurons that can be efficiently differentiated from pluripotent stem cells, PV neurons were minimally generated using a conventional differentiation strategy. In this study we developed an adenovirus-based transdifferentiation strategy that incorporates an additional chemical compound for the efficient generation of induced PV (iPV) neurons. The chemical compound forskolin combined with Ascl1 induced ∼80% of mouse fibroblasts to iPV neurons. The iPV neurons generated by this procedure matured 5-7 days post infection and were characterized by electrophysiological properties and known neuronal markers, such as PV and GABA. Our studies, therefore, identified an efficient approach for generating PV neurons.

A Two-Immunoglobulin-Domain Transmembrane Protein Mediates an Epidermal-Neuronal Interaction to Maintain Synapse Density

Synaptic maintenance is essential for neural circuit function. In the C. elegans locomotor circuit, motor neurons are in direct contact with the epidermis. Here, we reveal a novel epidermal-neuronal interaction mediated by a two-immunoglobulin domain transmembrane protein, ZIG-10, that is necessary for maintaining cholinergic synapse density. ZIG-10 is localized at the cell surface of epidermis and cholinergic motor neurons, with high levels at areas adjacent to synapses. Loss of zig-10 increases the number of cholinergic excitatory synapses and exacerbates convulsion behavior in a seizure model. Mis-expression of zig-10 in GABAergic inhibitory neurons reduces GABAergic synapse number, dependent on the presence of ZIG-10 in the epidermis. Furthermore, ZIG-10 interacts with the tyrosine kinase SRC-2 to regulate the phagocytic activity of the epidermis to restrict cholinergic synapse number. Our studies demonstrate the highly specific roles of non-neuronal cells in modulating neural circuit function, through neuron-type-specific maintenance of synapse density.

Interaction between CHL1 and serotonin receptor 2c regulates signal transduction and behavior in mice

The serotonergic system plays important roles in multiple functions of the nervous system and its malfunctioning leads to neurological and psychiatric disorders. Here, we show that the cell adhesion molecule close homolog of L1 (CHL1), which has been linked to mental disorders, binds to a peptide stretch in the third intracellular loop of the serotonin 2c (5-HT2c) receptor through its intracellular domain. Moreover, we provide evidence that CHL1 deficiency in mice leads to 5-HT2c-receptor-related reduction in locomotor activity and reactivity to novelty, and that CHL1 regulates signaling pathways triggered by constitutively active isoforms of the 5-HT2c receptor. Furthermore, we found that the 5-HT2c receptor and CHL1 colocalize in striatal and hippocampal GABAergic neurons, and that 5-HT2c receptor phosphorylation and its association with phosphatase and tensin homolog (PTEN) and β-arrestin 2 is regulated by CHL1. Our results demonstrate that CHL1 regulates signal transduction pathways through constitutively active 5-HT2c receptor isoforms, thereby altering 5-HT2c receptor functions and implicating CHL1 as a new modulator of the serotonergic system.

Age-dependent loss of parvalbumin-expressing hippocampal interneurons in mice deficient in CHL1, a mental retardation and schizophrenia susceptibility gene

In humans, deletions/mutations in the CHL1/CALL gene are associated with mental retardation and schizophrenia. Juvenile CHL1-deficient (CHL1(-/-) ) mice have been shown to display abnormally high numbers of parvalbumin-expressing (PV(+) ) hippocampal interneurons and, as adults, display behavioral traits observed in neuropsychiatric disorders. Here, we addressed the question whether inhibitory interneurons and synaptic plasticity in the CHL1(-/-) mouse are affected during brain maturation and in adulthood. We found that hippocampal, but not neocortical, PV(+) interneurons were reduced with age in CHL1(-/-) mice, from a surplus of +27% at 1 month to a deficit of -20% in adulthood compared with wild-type littermates. This loss occurred during brain maturation, correlating with microgliosis and enhanced interleukin-6 expression. In parallel with the loss of PV(+) interneurons, the inhibitory input to adult CA1 pyramidal cells was reduced and a deficit in short- and long-term potentiation developed at CA3-CA1 excitatory synapses between 2 and 9 months of age in CHL1(-/-) mice. This deficit could be abrogated by a GABAA receptor agonist. We propose that region-specific aberrant GABAergic synaptic connectivity resulting from the mutation and a subsequently enhanced synaptic elimination during brain maturation lead to microgliosis, increase in pro-inflammatory cytokine levels, loss of interneurons, and impaired synaptic plasticity. Close homolog of L1-deficient (CHL1(-/-) ) mice have abnormally high numbers of parvalbumin (PV)-expressing hippocampal interneurons in juvenile animals, but in adult animals a loss of these cells is observed. This loss correlates with an increased density of microglia (M), enhanced interleukin-6 (IL6) production and a deficit in short- and long-term potentiation at CA3-CA1 excitatory synapses. Furthermore, adult CHL1(-/-) mice display behavioral traits similar to those observed in neuropsychiatric disorders of humans.

Molecular layer interneurons of the cerebellum: developmental and morphological aspects

During the past 25 years, our knowledge on the development of basket and stellate cells (molecular layer interneurons [MLIs]) has completely changed, not only regarding their origin from the ventricular zone, corresponding to the primitive cerebellar neuroepithelium, instead of the external granular layer, but above all by providing an almost complete account of the genetic regulations (transcription factors and other genes) involved in their differentiation and synaptogenesis. Moreover, it has been shown that MLIs' precursors (dividing neuroblasts) and not young postmitotic neurons, as in other germinal neuroepithelia, leave the germinative zone and migrate all along a complex and lengthy path throughout the presumptive cerebellar white matter, which provides suitable niches exerting epigenetic influences on their ultimate neuronal identities. Recent studies carried out on the anatomical-functional properties of adult MLIs emphasize the importance of these interneurons in regulating PC inhibition, and point out the crucial role played by electrical synaptic transmission between MLIs as well as ephaptic interactions between them and Purkinje cells at the pinceaux level, in the regulation of this inhibition.

Calcium-permeable AMPA receptors trigger vesicular glutamate release from Bergmann gliosomes

The Bergmann glia is equipped with Ca2+-permeable AMPA receptors for glutamate, indispensable for structural and functional relations between the Bergmann glia and parallel/climbing fibers-Purkinje cell synapses. To better understand roles for the Bergmann AMPA receptors, herein we investigate on gliotransmitter release and Ca2+ signals in isolated Bergmann glia processes obtained from adult rat cerebellum. We found that: 1) the rat cerebellar purified astrocyte processes (gliosomes) expressed astrocytic and Bergmann markers and exhibited negligible contamination by nerve terminals, microglia, or oligodendrocytes; 2) activation of Ca2+-permeable AMPA receptors caused Ca2+ signals in the processes, and the release of glutamate from the processes; 3) effectiveness of rose bengal, trypan blue or bafilomycin A1, indicated that activation of the AMPA receptors evoked vesicular glutamate release. Cerebellar purified nerve terminals appeared devoid of glutamate-releasing Ca2+-permeable AMPA receptors, indicating that neuronal contamination may not be the source of the signals detected. Ultrastructural analysis indicated the presence of vesicles in the cytoplasm of the processes; confocal imaging confirmed the presence of vesicular glutamate transporters in Bergmann glia processes. We conclude that: a vesicular mechanism for release of the gliotransmitter glutamate is present in mature Bergmann processes; entry of Ca2+ through the AMPA receptors located on Bergmann processes is coupled with vesicular glutamate release. The findings would add a new role for a well-known Bergmann target for glutamate (the Ca2+-permeable AMPA receptors) and a new actor (the gliotransmitter glutamate) at the cerebellar excitatory synapses onto Purkinje cells.

Adenosine Monophosphate-activated Protein Kinase Regulates Interleukin-1β Expression and Glial Glutamate Transporter Function in Rodents with Neuropathic Pain

BACKGROUND

Neuroinflammation and dysfunctional glial glutamate transporters (GTs) in the spinal dorsal horn are implicated in the genesis of neuropathic pain. The authors determined whether adenosine monophosphate-activated protein kinase (AMPK) in the spinal dorsal horn regulates these processes in rodents with neuropathic pain.

METHODS

Hind paw withdrawal responses to radiant heat and mechanical stimuli were used to assess nociceptive behaviors. Spinal markers related to neuroinflammation and glial GTs were determined by Western blotting. AMPK activities were manipulated pharmacologically and genetically. Regulation of glial GTs was determined by measuring protein expression and activities of glial GTs.

RESULTS

AMPK activities were reduced in the spinal dorsal horn of rats (n = 5) with thermal hyperalgesia induced by nerve injury, which were accompanied with the activation of astrocytes, increased production of interleukin-1β and activities of glycogen synthase kinase 3β, and suppressed protein expression of glial glutamate transporter-1. Thermal hyperalgesia was reversed by spinal activation of AMPK in neuropathic rats (n = 10) and induced by inhibiting spinal AMPK in naive rats (n = 7 to 8). Spinal AMPKα knockdown (n = 6) and AMPKα1 conditional knockout (n = 6) induced thermal hyperalgesia and mechanical allodynia. These genetic alterations mimicked the changes of molecular markers induced by nerve injury. Pharmacological activation of AMPK enhanced glial GT activity in mice with neuropathic pain (n = 8) and attenuated glial glutamate transporter-1 internalization induced by interleukin-1β (n = 4).

CONCLUSIONS

These findings suggest that enhancing spinal AMPK activities could be an effective approach for the treatment of neuropathic pain.