Differential control of thrombospondin over synaptic glycine and AMPA receptors in spinal cord neurons

Astrocyte Regulation of Synapse Formation, Maturation, and Elimination

Astrocytes play an integral role in the development, maturation, and refinement of neuronal circuits. Astrocytes secrete proteins and lipids that instruct the formation of new synapses and induce the maturation of existing synapses. Through contact-mediated signaling, astrocytes can regulate the formation and state of synapses within their domain. Through phagocytosis, astrocytes participate in the elimination of excess synaptic connections. In this work, we will review key findings on the molecular mechanisms of astrocyte-synapse interaction with a focus on astrocyte-secreted factors, contact-mediated mechanisms, and synapse elimination. We will discuss this in the context of typical brain development and maintenance, as well as consider the consequences of dysfunction in these pathways in neurological disorders, highlighting a role for astrocytes in health and disease.

The Contributions of Thrombospondin-1 to Epilepsy Formation

Epilepsy is a neural network disorder caused by uncontrolled neuronal hyperexcitability induced by an imbalance between excitatory and inhibitory networks. Abnormal synaptogenesis plays a vital role in the formation of overexcited networks. Recent evidence has confirmed that thrombospondin-1 (TSP-1), mainly secreted by astrocytes, is a critical cytokine that regulates synaptogenesis during epileptogenesis. Furthermore, numerous studies have reported that TSP-1 is also involved in other processes, such as angiogenesis, neuroinflammation, and regulation of Ca2+ homeostasis, which are closely associated with the occurrence and development of epilepsy. In this review, we summarize the potential contributions of TSP-1 to epilepsy development.

Extracellular matrix and synapse formation

The extracellular matrix (ECM) is a complex molecular network distributed throughout the extracellular space of different tissues as well as the neuronal system. Previous studies have identified various ECM components that play important roles in neuronal maturation and signal transduction. ECM components are reported to be involved in neurogenesis, neuronal migration, and axonal growth by interacting or binding to specific receptors. In addition, the ECM is found to regulate synapse formation, the stability of the synaptic structure, and synaptic plasticity. Here, we mainly reviewed the effects of various ECM components on synapse formation and briefly described the related diseases caused by the abnormality of several ECM components.

How Are Synapses Born? A Functional and Molecular View of the Role of the Wnt Signaling Pathway

Synaptic transmission is a dynamic process that requires precise regulation. Early in life, we must be able to forge appropriate connections (add and remove) to control our behavior. Neurons must recognize appropriate targets, and external soluble factors that activate specific signaling cascades provide the regulation needed to achieve this goal. Wnt signaling has been implicated in several forms of synaptic plasticity, including functional and structural changes associated with brain development. The analysis of synapses from an electrophysiological perspective allows us to characterize the functional role of cellular signaling pathways involved in brain development. The application of quantal theory to principles of developmental plasticity offers the possibility of dissecting the function of structural changes associated with the birth of new synapses as well as the maturation of immature silent synapses. Here, we focus on electrophysiological and molecular evidence that the Wnt signaling pathway regulates glutamatergic synaptic transmission, specifically N-methyl-d-aspartate receptors (NMDARs), to control the birth of new synapses. We also focus on the role of Wnts in the conversion of silent synapses into functional synapses.

CRISPR-mediated activation of autism gene Itgb3 restores cortical network excitability via mGluR5 signaling

Many mutations in autism spectrum disorder (ASD) affect a single allele, indicating a key role for gene dosage in ASD susceptibility. Recently, haplo-insufficiency of ITGB3, the gene encoding the extracellular matrix receptor β3 integrin, was associated with ASD. Accordingly, Itgb3 knockout (KO) mice exhibit autism-like phenotypes. The pathophysiological mechanisms of Itgb3 remain, however, unknown, and the potential of targeting this gene for developing ASD therapies uninvestigated. By combining molecular, biochemical, imaging, and pharmacological analyses, we establish that Itgb3 haplo-insufficiency impairs cortical network excitability by promoting extra-synaptic over synaptic signaling of the metabotropic glutamate receptor mGluR5, which is similarly dysregulated in fragile X syndrome, the most frequent monogenic form of ASD. To assess the therapeutic potential of regulating Itgb3 gene dosage, we implemented CRISPR activation and compared its efficacy with that of a pharmacological rescue strategy for fragile X syndrome. Correction of neuronal Itgb3 haplo-insufficiency by CRISPR activation rebalanced network excitability as effectively as blockade of mGluR5 with the selective antagonist MPEP. Our findings reveal an unexpected functional interaction between two ASD genes, thereby validating the pathogenicity of ITGB3 haplo-insufficiency. Further, they pave the way for exploiting CRISPR activation as gene therapy for normalizing gene dosage and network excitability in ASD.

Synapse Formation and Function Across Species: Ancient Roles for CCP, CUB, and TSP-1 Structural Domains

The appearance of synapses was a crucial step in the creation of the variety of nervous systems that are found in the animal kingdom. With increased complexity of the organisms came a greater number of synaptic proteins. In this review we describe synaptic proteins that contain the structural domains CUB, CCP, or TSP-1. These domains are found in invertebrates and vertebrates, and CUB and CCP domains were initially described in proteins belonging to the complement system of innate immunity. Interestingly, they are found in synapses of the nematode C. elegans, which does not have a complement system, suggesting an ancient function. Comparison of the roles of CUB-, CCP-, and TSP-1 containing synaptic proteins in various species shows that in more complex nervous systems, these structural domains are combined with other domains and that there is partial conservation of their function. These three domains are thus basic building blocks of the synaptic architecture. Further studies of structural domains characteristic of synaptic proteins in invertebrates such as C. elegans and comparison of their role in mammals will help identify other conserved synaptic molecular building blocks. Furthermore, this type of functional comparison across species will also identify structural domains added during evolution in correlation with increased complexity, shedding light on mechanisms underlying cognition and brain diseases.

Astrocyte-Neuron Signaling in Synaptogenesis

Astrocytes are the key component of the central nervous system (CNS), serving as pivotal regulators of neuronal synapse formation and maturation through their ability to dynamically and bidirectionally communicate with synapses throughout life. In the past 20 years, numerous astrocyte-derived molecules promoting synaptogenesis have been discovered. However, our understanding of the cell biological basis underlying intra-neuron processes and astrocyte-mediated synaptogenesis is still in its infancy. Here, we provide a comprehensive overview of the various ways astrocytes talk to neurons, and highlight astrocytes’ heterogeneity that allow them to displays regional-specific capabilities in boosting synaptogenesis. Finally, we conclude with promises and future directions on how organoids generated from human induced pluripotent stem cells (hiPSCs) effectively address the signaling pathways astrocytes employ in synaptic development.

Differential regulation of glycinergic and GABAergic nanocolumns at mixed inhibitory synapses

Super‐resolution imaging has revealed that key synaptic proteins are dynamically organized within sub‐synaptic domains (SSDs). To examine how different inhibitory receptors are regulated, we carried out dual‐color direct stochastic optical reconstruction microscopy (dSTORM) of GlyRs and GABA_ARs at mixed inhibitory synapses in spinal cord neurons. We show that endogenous GlyRs and GABA_ARs as well as their common scaffold protein gephyrin form SSDs that align with pre‐synaptic RIM1/2, thus creating trans‐synaptic nanocolumns. Strikingly, GlyRs and GABA_ARs occupy different sub‐synaptic spaces, exhibiting only a partial overlap at mixed inhibitory synapses. When network activity is increased by 4‐aminopyridine treatment, the GABA_AR copy numbers and the number of GABA_AR SSDs are reduced, while GlyRs remain largely unchanged. This differential regulation is likely the result of changes in gephyrin phosphorylation that preferentially occurs outside of SSDs. The activity‐dependent regulation of GABA_ARs versus GlyRs suggests that different signaling pathways control the receptors' sub‐synaptic clustering. Taken together, our data reinforce the notion that the precise sub‐synaptic organization of GlyRs, GABA_ARs, and gephyrin has functional consequences for the plasticity of mixed inhibitory synapses.

Astrocyte-induced synapse formation and ischemic stroke

Astrocytes are closely associated with the regulation of synapse formation and function. In addition, astrocytes have been shown to block certain brain impairments, including synaptic damage from stroke and other diseases of the central nervous system (CNS). Although astrocytes do not completely prevent synaptic damage, they appear to be protective and to restore synaptic function following damage. The purpose of this study is to discuss the role of astrocytes in synaptogenesis and synaptic damage in ischemic stroke. I detail the mechanism of action of the multiple factors secreted by astrocytes that are involved in synapse formation. In particular, I describe the characteristics and role in synapse formation of each secreted molecule related to synaptic structure and function. Furthermore, I discuss the effect of astrocytes on synaptogenesis and repair in ischemic stroke and in other CNS diseases. Astrocytes release molecules such as thrombospondin, hevin, secreted protein acidic rich in cysteine, etc., due to activation by ischemia to induce synaptic structure and function, an effect associated with protection of the brain from synaptic damage in ischemic stroke. In conclusion, I show that astrocytes may regulate synaptic transmission while having the potential to block and repair synaptic dysfunction in stroke-associated brain damage.

Time-Course Changes of Extracellular Matrix Encoding Genes Expression Level in the Spinal Cord Following Contusion Injury-A Data-Driven Approach

The involvement of the extracellular matrix (ECM) in lesion evolution and functional outcome is well recognized in spinal cord injury. Most attention has been dedicated to the “core” area of the lesion and scar formation, while only scattered reports consider ECM modification based on the temporal evolution and the segments adjacent to the lesion. In this study, we investigated the expression profile of 100 genes encoding for ECM proteins at 1, 8 and 45 days post-injury, in the spinal cord segments rostral and caudal to the lesion and in the scar segment, in a rat model. During both the active lesion phases and the lesion stabilization, we observed an asymmetric gene expression induced by the injury, with a higher regulation in the rostral segment of genes involved in ECM remodeling, adhesion and cell migration. Using bioinformatic approaches, the metalloproteases inhibitor Timp1 and the hyaluronan receptor Cd44 emerged as the hub genes at all post-lesion times. Results from the bioinformatic gene expression analysis were then confirmed at protein level by tissue analysis and by cell culture using primary astrocytes. These results indicated that ECM regulation also takes place outside of the lesion area in spinal cord injury.

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.

Integrin adhesion in brain assembly: From molecular structure to neuropsychiatric disorders

Integrins are extracellular matrix receptors that mediate biochemical and mechanical bi-directional signals between the extracellular and intracellular environment of a cell thanks to allosteric conformational changes. In the brain, they are found in both neurons and glial cells, where they play essential roles in several aspects of brain development and function, such as cell migration, axon guidance, synaptogenesis, synaptic plasticity and neuro-inflammation. Although there are many successful examples of how regulating integrin adhesion and signaling can be used for therapeutic purposes, for example for halting tumor progression, this is not the case for the brain, where the growing evidence of the importance of integrins for brain pathophysiology has not translated yet into medical applications. Here, we review recent literature showing how alterations in integrin structure, expression and signaling may be involved in the etiology of autism spectrum disorder, epilepsy, schizophrenia, addiction, depression and Alzheimer's disease. We focus on common mechanisms and recurrent signaling pathways, trying to bridge studies on the genetics and molecular structure of integrins with those on synaptic physiology and brain pathology. Further, we discuss integrin-targeting strategies and their potential benefits for therapeutic purposes in neuropsychiatric disorders.

Osteopontin and Integrin Mediated Modulation of Post-Synapses in HIV Envelope Glycoprotein Exposed Hippocampal Neurons

The advent of Human Immunodeficiency Virus (HIV) antiretrovirals have reduced the severity of HIV related neurological comorbidities but they nevertheless remain prevalent. Synaptic degeneration due to the action of several viral factors released from infected brain myeloid and glia cells and inflammatory cytokines has been attributed to the manifestation of a range of cognitive and behavioral deficits. The contributions of specific pro-inflammatory factors and their interplay with viral factors in the setting of treatment and persistence are incompletely understood. Exposure of neurons to chemokine receptor-4(CXCR4)-tropic HIV-1 envelope glycoprotein (Env) can lead to post-synaptic degradation of dendritic spines. The contribution of members of the extracellular matrix (ECM) and specifically, of perineuronal nets (PNN) toward synaptic degeneration, is not fully known, even though these structures are found to be disrupted in post-mortem HIV-infected brains. Osteopontin (Opn, gene name SPP1), a cytokine-like protein, is found in abundance in the HIV-infected brain. In this study, we investigated the role of Opn and its ECM integrin receptors, β1- and β3 integrin in modifying neuronal synaptic sculpting. We found that in hippocampal neurons incubated with HIV-1 Env protein and recombinant Opn, post-synaptic-95 (PSD-95) puncta were significantly increased and distributed to dendritic spines when compared to Env-only treated neurons. This effect was mediated through β3 integrin, as silencing of this receptor abrogated the increase in post-synaptic spines. Silencing of β1 integrin, however, did not block the increase of post-synaptic spines in hippocampal cultures treated with Opn. However, a decrease in the PNN to βIII-tubulin ratio was found, indicating an increased capacity to support spine growth. From these results, we conclude that one of the mechanisms by which Opn counters the damaging impact of the HIV Env protein on hippocampal post-synaptic plasticity is through complex interactions between Opn and components of the ECM which activate downstream protective signaling pathways that help maintain the potential for effective post-synaptic plasticity.

Inhibitory Receptor Diffusion Dynamics

The dynamic modulation of receptor diffusion-trapping at inhibitory synapses is crucial to synaptic transmission, stability, and plasticity. In this review article, we will outline the progression of understanding of receptor diffusion dynamics at the plasma membrane. We will discuss how regulation of reversible trapping of receptor-scaffold interactions in combination with theoretical modeling approaches can be used to quantify these chemical interactions at the postsynapse of living cells.

Microglia-neuron crosstalk: Signaling mechanism and control of synaptic transmission

The continuous crosstalk between microglia and neurons is required for microglia housekeeping functions and contributes to brain homeostasis. Through these exchanges, microglia take part in crucial brain functions, including development and plasticity. The alteration of neuron-microglia communication contributes to brain disease states with consequences, ranging from synaptic function to neuronal survival. This review focuses on the signaling pathways responsible for neuron-microglia crosstalk, highlighting their physiological roles and their alteration or specific involvement in disease. In particular, we discuss studies, establishing how these signaling allow microglial cells to control relevant physiological functions during brain development, including synaptic formation and circuit refinement. In addition, we highlight how microglia and neurons interact functionally to regulate highly dynamical synaptic functions. Microglia are able to release several signaling molecules involved in the regulation of synaptic activity and plasticity. On the other side, molecules of neuronal origin control microglial processes motility in an activity-dependent manner. Indeed, the continuous crosstalk between microglia and neurons is required for the sensing and housekeeping functions of microglia and contributes to the maintenance of brain homeostasis and, particularly, to the sculpting of neuronal connections during development. These interactions lay on the delicate edge between physiological processes and homeostasis alteration in pathology and are themselves altered during neuroinflammation. The full description of these processes could be fundamental for understanding brain functioning in health and disease.

Pentraxin 3 regulates synaptic function by inducing AMPA receptor clustering via ECM remodeling and β1-integrin

Control of synapse number and function in the developing central nervous system is critical to the formation of neural circuits. Astrocytes play a key role in this process by releasing factors that promote the formation of excitatory synapses. Astrocyte‐secreted thrombospondins (TSPs) induce the formation of structural synapses, which however remain post‐synaptically silent, suggesting that completion of early synaptogenesis may require a two‐step mechanism. Here, we show that the humoral innate immune molecule Pentraxin 3 (PTX3) is expressed in the developing rodent brain. PTX3 plays a key role in promoting functionally‐active CNS synapses, by increasing the surface levels and synaptic clustering of AMPA glutamate receptors. This process involves tumor necrosis factor‐induced protein 6 (TSG6), remodeling of the perineuronal network, and a β1‐integrin/ERK pathway. Furthermore, PTX3 activity is regulated by TSP1, which directly interacts with the N‐terminal region of PTX3. These data unveil a fundamental role of PTX3 in promoting the first wave of synaptogenesis, and show that interplay of TSP1 and PTX3 sets the proper balance between synaptic growth and synapse function in the developing brain.

Cell adhesion and matricellular support by astrocytes of the tripartite synapse

Astrocytes contribute to the formation, function, and plasticity of synapses. Their processes enwrap the neuronal components of the tripartite synapse, and due to this close interaction they are perfectly positioned to modulate neuronal communication. The interaction between astrocytes and synapses is facilitated by cell adhesion molecules and matricellular proteins, which have been implicated in the formation and functioning of tripartite synapses. The importance of such neuron-astrocyte integration at the synapse is underscored by the emerging role of astrocyte dysfunction in synaptic pathologies such as autism and schizophrenia. Here we review astrocyte-expressed cell adhesion molecules and matricellular molecules that play a role in integration of neurons and astrocytes within the tripartite synapse.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

Cell Biology of Astrocyte-Synapse Interactions

Astrocytes, the most abundant glial cells in the mammalian brain, are critical regulators of brain development and physiology through dynamic and often bidirectional interactions with neuronal synapses. Despite the clear importance of astrocytes for the establishment and maintenance of proper synaptic connectivity, our understanding of their role in brain function is still in its infancy. We propose that this is at least in part due to large gaps in our knowledge of the cell biology of astrocytes and the mechanisms they use to interact with synapses. In this review, we summarize some of the seminal findings that yield important insight into the cellular and molecular basis of astrocyte-neuron communication, focusing on the role of astrocytes in the development and remodeling of synapses. Furthermore, we pose some pressing questions that need to be addressed to advance our mechanistic understanding of the role of astrocytes in regulating synaptic development.

Astrocytes with previous chronic exposure to amyloid β-peptide fragment 1-40 suppress excitatory synaptic transmission

Synaptic dysfunction and neuronal death are responsible for cognitive and behavioral deficits in Alzheimer's disease (AD). It is well known that such neurological abnormalities are preceded by long-term exposure of amyloid β-peptide (Aβ) and/or hyperphosphorylated tau prior. In addition to the neurological deficit, astrocytes as a major glial cell type in the brain, significantly participate in the neuropathogenic mechanisms underlying synaptic modulation. Although astrocytes play a significant key role in modulating synaptic transmission, little is known on whether astrocyte dysfunction caused by such long-term Aβ exposure affects synapse formation and function. Here, we show that synapse formation and synaptic transmission are attenuated in hippocampal-naïve neurons co-cultured with astrocytes that have previously experienced chronic Aβ_1-40 exposure. In this abnormal astrocytic condition, hippocampal neurons exhibit decrements of evoked excitatory post-synaptic currents (EPSCs) and miniature EPSC frequency. Furthermore, size of readily releasable synaptic pools and number of excitatory synapses were also significantly decreased. Contrary to these negative effects, release probability at individual synapses was significantly increased in the same astrocytic condition. Taken together, our data indicate that lower synaptic transmission caused by astrocytes previously, and chronically, exposed to Aβ1-40 is attributable to a small number of synapses with higher release probability.

Estradiol modulates the efficacy of synaptic inhibition by decreasing the dwell time of GABAA receptors at inhibitory synapses

Estrogen plays a critical role in many physiological processes and exerts profound effects on behavior by regulating neuronal excitability. While estrogen has been established to exert effects on dendritic morphology and excitatory neurotransmission its role in regulating neuronal inhibition is poorly understood. Fast synaptic inhibition in the adult brain is mediated by specialized populations of γ-c a_A receptors (GABA_ARs) that are selectively enriched at synapses, a process dependent upon their interaction with the inhibitory scaffold protein gephyrin. Here we have assessed the role that estradiol (E2) plays in regulating the dynamics of GABA_ARs and stability of inhibitory synapses. Treatment of cultured cortical neurons with E2 reduced the accumulation of GABA_ARs and gephyrin at inhibitory synapses. However, E2 exposure did not modify the expression of either the total or the plasma membrane GABA_ARs or gephyrin. Mechanistically, single-particle tracking revealed that E2 treatment selectively reduced the dwell time and thereby decreased the confinement of GABA_ARs at inhibitory synapses. Consistent with our cell biology measurements, we observed a significant reduction in amplitude of inhibitory synaptic currents in both cultured neurons and hippocampal slices exposed to E2, while their frequency was unaffected. Collectively, our results suggest that acute exposure of neurons to E2 leads to destabilization of GABA_ARs and gephyrin at inhibitory synapses, leading to reductions in the efficacy of GABAergic inhibition via a postsynaptic mechanism.

The diverse actions of astrocytes during synaptic development

In the developing brain, cortical circuits are established through a complex process of synaptogenesis, maturation, and synaptic pruning. Astrocytes carry out diverse functions during each of these stages to facilitate the formation of complex networks. Recent work has begun to demonstrate that these heterogeneous roles during excitatory synaptic development are determined by the astrocyte population, brain region, and neuron type. This review will focus on current findings which highlight cell type specific mechanisms of excitatory synaptogenesis, as well as multiple mechanisms engaged by astrocytes to facilitate synaptic maturation and pruning.

Microglia control the glycinergic but not the GABAergic synapses via prostaglandin E2 in the spinal cord

Microglia can influence the excitatory responses of neurons, but less is known about how these immune cells in the brain may influence inhibitory neurotransmitters. Cantaut-Belarif et al. report that prostaglandin production by Toll-like receptor–stimulated microglia can influence the glycinergic but not GABAergic responses of neurons by altering the lateral diffusion of glycine receptors specifically within the synaptic membrane.

Microglia control excitatory synapses, but their role in inhibitory neurotransmission has been less well characterized. Herein, we show that microglia control the strength of glycinergic but not GABAergic synapses via modulation of the diffusion dynamics and synaptic trapping of glycine (GlyR) but not GABA_A receptors. We further demonstrate that microglia regulate the activity-dependent plasticity of glycinergic synapses by tuning the GlyR diffusion trap. This microglia–synapse cross talk requires production of prostaglandin E2 by microglia, leading to the activation of neuronal EP2 receptors and cyclic adenosine monophosphate–dependent protein kinase. Thus, we now provide a link between microglial activation and synaptic dysfunctions, which are common early features of many brain diseases.

Thrombospondin-1 might be a therapeutic target to suppress RB cells by regulating the DNA double-strand breaks repair

Retinoblastoma (RB) arises from the retina, and its growth usually occurs under the retina and toward the vitreous. Ideal therapy should aim to inhibit the tumor and protect neural cells, increasing the patient's life span and quality of life. Previous studies have demonstrated that Thrombospondin-1 (TSP-1) is associated with neurogenesis, neovascularization and tumorigenesis. However, at present, the bioactivity of TSP-1 in retinoblastoma has not been defined. Herein, we demonstrated that TSP-1 was silenced in RB cell lines and clinical tumor samples. HDAC inhibitor, Trichostatin A (TSA), could notably transcriptionally up-regulate TSP-1 in RB cells, WERI-Rb1 cells and Y79 cells. Moreover, we found human recombinant TSP-1 (hTSP-1) could significantly inhibit the cell viability of RB cells both in vitro and in vivo. Interestingly, hTSP-1 could significantly induce the expression of γ-H2AX, a well-characterized in situ marker of DNA double-strand breaks (DSBs) in RB cells. The DNA NHEJ pathway in WERI-Rb1 cells could be significantly inhibited by hTSP-1. A mutation in Rb1 might be involved in the hTSP-1-medicated γ-H2AX increasing in WERI-Rb1 cells. Furthermore, hTSP-1 could inhibit RB cells while promoting retinal neurocyte survival in the neuronal and retinoblastoma cell co-culture system. As such, TSP-1 may become a therapeutic target for treatment of retinoblastoma.

Aberrant LncRNA Expression Profile in a Contusion Spinal Cord Injury Mouse Model

Long noncoding RNAs (LncRNAs) play a crucial role in cell growth, development, and various diseases related to the central nervous system. However, LncRNA differential expression profiles in spinal cord injury are yet to be reported. In this study, we profiled the expression pattern of LncRNAs using a microarray method in a contusion spinal cord injury (SCI) mouse model. Compared with a spinal cord without injury, few changes in LncRNA expression levels were noted 1 day after injury. The differential changes in LncRNA expression peaked 1 week after SCI and subsequently declined until 3 weeks after injury. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to validate the reliability of the microarray, demonstrating that the results were reliable. Gene ontology (GO) analysis indicated that differentially expressed mRNAs were involved in transport, cell adhesion, ion transport, and metabolic processes, among others. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the neuroactive ligand-receptor interaction, the PI3K-Akt signaling pathway, and focal adhesions were potentially implicated in SCI pathology. We constructed a dynamic LncRNA-mRNA network containing 264 LncRNAs and 949 mRNAs to elucidate the interactions between the LncRNAs and mRNAs. Overall, the results from this study indicate for the first time that LncRNAs are differentially expressed in a contusion SCI mouse model.

Thrombospondin-4 reduces binding affinity of [(3)H]-gabapentin to calcium-channel α2δ-1-subunit but does not interact with α2δ-1 on the cell-surface when co-expressed

The α2δ proteins are auxiliary subunits of voltage-gated calcium channels, and influence their trafficking and biophysical properties. The α2δ ligand gabapentin interacts with α2δ-1, and inhibits calcium channel trafficking. However, α2-1 has also been proposed to play a synaptogenic role, independent of calcium channel function. In this regard, α2δ-1 was identified as a ligand of thrombospondins, with the interaction involving the thrombospondin synaptogenic domain and the α2δ-1 von-Willebrand-factor domain. Co-immunoprecipitation between α2δ-1 and the synaptogenic domain of thrombospondin-2 was prevented by gabapentin. We therefore examined whether interaction of thrombospondin with α2δ-1 might reciprocally influence (3)H-gabapentin binding. We concentrated on thrombospondin-4, because, like α2δ-1, it is upregulated in neuropathic pain models. We found that in membranes from cells co-transfected with α2δ-1 and thrombospondin-4, there was a Mg(2+) -dependent reduction in affinity of (3)H-gabapentin binding to α2δ-1. This effect was lost for α2δ-1 with mutations in the von-Willebrand-factor-A domain. However, the effect on (3)H-gabapentin binding was not reproduced by the synaptogenic EGF-domain of thrombospondin-4. Partial co-immunoprecipitation could be demonstrated between thrombospondin-4 and α2δ-1 when co-transfected, but there was no co-immunoprecipitation with thrombospondin-4-EGF domain. Furthermore, we could not detect any association between these two proteins on the cell-surface, indicating the demonstrated interaction occurs intracellularly.

The Role of Synaptopodin in Membrane Protein Diffusion in the Dendritic Spine Neck

The dynamic exchange of neurotransmitter receptors at synapses relies on their lateral diffusion in the plasma membrane. At synapses located on dendritic spines this process is limited by the geometry of the spine neck that restricts the passage of membrane proteins. Biochemical compartmentalisation of the spine is believed to underlie the input-specificity of excitatory synapses and to set the scale on which functional changes can occur. Synaptopodin is located predominantly in the neck of dendritic spines, and is thus ideally placed to regulate the exchange of synaptic membrane proteins. The central aim of our study was to assess whether the presence of synaptopodin influences the mobility of membrane proteins in the spine neck and to characterise whether this was due to direct molecular interactions or to spatial constraints that are related to the structural organisation of the neck. Using single particle tracking we have identified a specific effect of synaptopodin on the diffusion of metabotropic mGluR5 receptors in the spine neck. However, super-resolution STORM/PALM imaging showed that this was not due to direct interactions between the two proteins, but that the presence of synaptopodin is associated with an altered local organisation of the F-actin cytoskeleton, that in turn could restrict the diffusion of membrane proteins with large intracellular domains through the spine neck. This study contributes new data on the way in which the spine neck compartmentalises excitatory synapses. Our data complement models that consider the impact of the spine neck as a function of its shape, by showing that the internal organisation of the neck imposes additional physical barriers to membrane protein diffusion.

Postsynaptic Neurotransmitter Receptor Reserve Pools for Synaptic Potentiation

At excitatory and inhibitory synapses, an immediate transfer of additional neurotransmitter receptors from non-synaptic positions to the synapse mediates synaptic long-term potentiation (LTP). Different types of non-synaptic reserve pools permit the rapid supply of transmembrane neurotransmitter receptors. Recycling endosomes (REs) serve as an intracellular reservoir of receptors that is delivered to the plasma membrane on LTP induction. Furthermore, AMPA receptors at the non-synaptic plasma membrane provide an extrasynaptic reserve pool that is also important to potentiate synapse function. Finally, bidirectional synaptic versus extrasynaptic trapping of freely diffusing plasma membrane GABAA receptors (GABAARs) by scaffolding proteins modulates synaptic transmission. Here we discuss novel findings regarding neurotransmitter receptor reservoirs and potential reserve pool mechanisms for synaptic potentiation.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Increased sensitivity to kindling in mice lacking TSP1

The development of a hyperexcitable neuronal network is thought to be a critical event in epilepsy. Thrombospondins (TSPs) regulate synaptogenesis by binding the neuronal α2δ subunit of the voltage-gated calcium channel. TSPs regulate synapse formation during development and in the mature brain following injury. It is unclear if TSPs are involved in hyperexcitability that contributes to the development of epilepsy. Here we explore the development of epilepsy using a pentylenetetrazole (PTZ) kindling model in mice lacking TSP1 and TSP2. Unexpectedly, we found increased sensitivity to PTZ kindling in mice lacking TSP1, while mice lacking TSP2 kindled similar to wild-type. We found that the increased seizure susceptibility in the TSP1 knockout (KO) mice was not due to a compensatory increase in TSP2 mRNA as TSP1/2 KO mice were sensitive to PTZ, similar to the TSP1 KO mice. Furthermore, there were similar levels of TGF-B signal activation during kindling in the TSP1 KO mice compared to wild-type. We observed decreased expression of voltage-dependent calcium channel subunit CACNA2D1 mRNA in TSP1, TSP2, and TSP1/2 KO mice. Decreased CACNA2D2 mRNA was only detected in mice that lacked TSP1 and α2δ-1/2 protein levels in the cortex were lower in the TSP 1/2 KO mice. CACNA2D2 knockout mice have spontaneous seizures and increased PTZ seizure susceptibility. Here we report similar findings, TSP1, and TSP1/2 KO mice have low levels of CACNA2D2 mRNA expression and α2δ-1/2 receptor level in the cortex, and are more susceptible to seizures. CACNA2D2 mutations in mice and humans can cause epilepsy. Our data suggest TSP1 in particular may control CACNA2D2 levels and could be a modifier of seizure susceptibility.

Involvement of PrP(C) in kainate-induced excitotoxicity in several mouse strains

The cellular prion protein (PrP(C)) has been associated with a plethora of cellular functions ranging from cell cycle to neuroprotection. Mice lacking PrP(C) show an increased susceptibility to epileptic seizures; the protein, then, is neuroprotective. However, lack of experimental reproducibility has led to considering the possibility that other factors besides PrP(C) deletion, such as the genetic background of mice or the presence of so-called "Prnp flanking genes", might contribute to the reported susceptibility. Here, we performed a comparative analysis of seizure-susceptibility using characterized Prnp(+/+) and Prnp(0/0) mice of B6129, B6.129, 129/Ola or FVB/N genetic backgrounds. Our study indicates that PrP(C) plays a role in neuroprotection in KA-treated cells and mice. For this function, PrP(C) should contain the aa32-93 region and needs to be linked to the membrane. In addition, some unidentified "Prnp-flanking genes" play a role parallel to PrP(C) in the KA-mediated responses in B6129 and B6.129 Prnp(0/0) mice.

Astrocyte regulation of synaptic behavior

Astrocytes regulate multiple aspects of neuronal and synaptic function from development through to adulthood. Instead of addressing each function independently, this review provides a comprehensive overview of the different ways astrocytes modulate neuronal synaptic function throughout life, with a particular focus on recent findings in each area. It includes the emerging functions of astrocytes, such as a role in synapse formation, as well as more established roles, including the uptake and recycling of neurotransmitters. This broad approach covers the many ways astrocytes and neurons constantly interact to maintain the correct functioning of the brain. It is important to consider all of these diverse functions of astrocytes when investigating how astrocyte-neuron interactions regulate synaptic behavior to appreciate the complexity of these ongoing interactions.

The Susd2 protein regulates neurite growth and excitatory synaptic density in hippocampal cultures

Complement control protein (CCP) domains have adhesion properties and are commonly found in proteins that control the complement immune system. However, an increasing number of proteins containing CCP domains have been reported to display neuronal functions. Susd2 is a transmembrane protein containing one CCP domain. It was previously identified as a tumor-reversing protein, but has no characterized function in the CNS. The present study investigates the expression and function of Susd2 in the rat hippocampus. Characterization of Susd2 during development showed a peak in mRNA expression two weeks after birth. In hippocampal neuronal cultures, the same expression profile was observed at 15days in vitro for both mRNA and protein, a time consistent with synaptogenesis in our model. At the subcellular level, Susd2 was located on the soma, axons and dendrites, and appeared to associate preferentially with excitatory synapses. Inhibition of Susd2 by shRNAs led to decreased numbers of excitatory synaptic profiles, exclusively. Also, morphological parameters were studied on young (5DIV) developing neurons. After Susd2 inhibition, an increase in dendritic tree length but a decrease in axon elongation were observed, suggesting changes in adhesion properties. Our results demonstrate a dual role for Susd2 at different developmental stages, and raise the question whether Susd2 and other CCP-containing proteins expressed in the CNS could be function-related.

Astrocytes Control Synapse Formation, Function, and Elimination

Astrocytes, through their close associations with synapses, can monitor and alter synaptic function, thus actively controlling synaptic transmission in the adult brain. Besides their important role at adult synapses, in the last three decades a number of critical findings have highlighted the importance of astrocytes in the establishment of synaptic connectivity in the developing brain. In this article, we will review the key findings on astrocytic control of synapse formation, function, and elimination. First, we will summarize our current structural and functional understanding of astrocytes at the synapse. Then, we will discuss the cellular and molecular mechanisms through which developing and mature astrocytes instruct the formation, maturation, and refinement of synapses. Our aim is to provide an overview of astrocytes as important players in the establishment of a functional nervous system.

Astrocytic control of neural circuit formation: highlights on TGF-beta signaling

Brain function depends critically on the coordinated activity of presynaptic and postsynaptic signals derived from both neurons and non-neuronal elements such as glial cells. A key role for astrocytes in neuronal differentiation and circuitry formation has emerged within the last decade. Although the function of glial cells in synapse formation, elimination and efficacy has greatly increased, we are still very far from deeply understanding the molecular and cellular mechanism underlying these events. The present review discusses the mechanisms driving astrocytic control of excitatory and inhibitory synapse formation in the central nervous system, especially the mechanisms mediated by soluble molecules, particularly those from the TGF-β family. Further, we discuss whether and how human astrocytes might contribute to the acquisition of human cognition. We argue that understanding how astrocytic signals regulate synaptic development might offer new insights into human perception, learning, memory, and cognition and, ultimately, provide new targets for the treatment of neurological diseases.

Astrocyte transforming growth factor beta 1 promotes inhibitory synapse formation via CaM kinase II signaling

The balance between excitatory and inhibitory synaptic inputs is critical for the control of brain function. Astrocytes play important role in the development and maintenance of neuronal circuitry. Whereas astrocytes-derived molecules involved in excitatory synapses are recognized, molecules and molecular mechanisms underlying astrocyte-induced inhibitory synapses remain unknown. Here, we identified transforming growth factor beta 1 (TGF-β1), derived from human and murine astrocytes, as regulator of inhibitory synapse in vitro and in vivo. Conditioned media derived from human and murine astrocytes induce inhibitory synapse formation in cerebral cortex neurons, an event inhibited by pharmacologic and genetic manipulation of the TGF-β pathway. TGF-β1-induction of inhibitory synapse depends on glutamatergic activity and activation of CaM kinase II, which thus induces localization and cluster formation of the synaptic adhesion protein, Neuroligin 2, in inhibitory postsynaptic terminals. Additionally, intraventricular injection of TGF-β1 enhanced inhibitory synapse number in the cerebral cortex. Our results identify TGF-β1/CaMKII pathway as a novel molecular mechanism underlying astrocyte control of inhibitory synapse formation. We propose here that the balance between excitatory and inhibitory inputs might be provided by astrocyte signals, at least partly achieved via TGF-β1 downstream pathways. Our work contributes to the understanding of the GABAergic synapse formation and may be of relevance to further the current knowledge on the mechanisms underlying the development of various neurological disorders, which commonly involve impairment of inhibitory synapse transmission.

β-amyloid and ATP-induced diffusional trapping of astrocyte and neuronal metabotropic glutamate type-5 receptors

β-Amyloid (Aβ) oligomers initiate synaptotoxicity following their interaction with the plasma membrane. Several proteins including metabotropic glutamate type 5 receptors (mGluR5s) contribute to this process. We observed an overexpression of mGluR5s in reactive astrocytes surrounding Aβ plaques in brain sections from an Alzheimer's disease mouse model. In a simplified cell culture system, using immunocytochemistry and single molecule imaging, we demonstrated a rapid binding of Aβ oligomers on the plasma membrane of astrocytes. The resulting aggregates of Aβ oligomers led to the diffusional trapping and clustering of mGluR5s. Further, Aβ oligomers induced an increase in ATP release following activation of astroglial mGluR5s by its agonist. ATP slowed mGluR5s diffusion in astrocytes as well as in neurons co-cultured with astrocytes. This effect, which is purinergic receptor-dependent, was not observed in pure neuronal cultures. Thus, Aβ oligomer- and mGluR5-dependent ATP release by astrocytes may contribute to the overall deleterious effect of mGluR5s in Alzheimer's disease. GLIA 2013;61:1673-1686.