A plethora of presynaptic proteins associated with ATP-storing organelles in cultured astrocytes

The Key Role of Astrocytes in Amyotrophic Lateral Sclerosis and Their Commitment to Glutamate Excitotoxicity

In the last two decades, there has been increasing evidence supporting non-neuronal cells as active contributors to neurodegenerative disorders. Among glial cells, astrocytes play a pivotal role in driving amyotrophic lateral sclerosis (ALS) progression, leading the scientific community to focus on the "astrocytic signature" in ALS. Here, we summarized the main pathological mechanisms characterizing astrocyte contribution to MN damage and ALS progression, such as neuroinflammation, mitochondrial dysfunction, oxidative stress, energy metabolism impairment, miRNAs and extracellular vesicles contribution, autophagy dysfunction, protein misfolding, and altered neurotrophic factor release. Since glutamate excitotoxicity is one of the most relevant ALS features, we focused on the specific contribution of ALS astrocytes in this aspect, highlighting the known or potential molecular mechanisms by which astrocytes participate in increasing the extracellular glutamate level in ALS and, conversely, undergo the toxic effect of the excessive glutamate. In this scenario, astrocytes can behave as "producers" and "targets" of the high extracellular glutamate levels, going through changes that can affect themselves and, in turn, the neuronal and non-neuronal surrounding cells, thus actively impacting the ALS course. Moreover, this review aims to point out knowledge gaps that deserve further investigation.

EPA stronger than DHA increases the mitochondrial membrane potential and cardiolipin levels but does not change the ATP level in astrocytes

Astrocytes are highly energy-consuming glial cells critical for metabolic support to neurons. A growing body of evidence suggests that mitochondrial dysfunction in astrocytes is involved in age-related neurodegenerative disorders and that fish oil, rich in docosahexaenoic (DHA) and eicosapentaenoic (EPA) fatty acids, may alleviate cognition impairment in Parkinson's and Alzheimer's diseases. The present study examines the effect of DHA and EPA on mitochondrial membrane potential (MMP), apoptosis activation and ATP levels in astrocytes cultured in medium containing glucose or galactose, which limits oxidative phosphorylation (OXPHOS). MMP, expressed as the ratio of red to green JC-10 and MitoTracker fluorescence, increased in EPA-incubated cells in a dose dependent manner and was higher than in DHA-incubated astrocytes, also after uncoupling of OXPHOS by carbonyl cyanide 3-chlorophenylhydrazone (CCCP). In cells cultured in glucose and galactose medium mitochondrial hyperpolarization had no impact on intracellular ATP level. Furthermore, both EPA and DHA elevated mitochondrial cardiolipin content, however only EPA did so in a dose-dependent manner and reduced apoptosis which was analyzed by flow cytometry.

SNARE protein VAMP-2, but not syntaxin-1, SNAP-25 and synaptotagmin 1, expressed in perisynaptic astrocytic processes in the CA1 area of the rat hippocampus

OBJECTIVE

Perisynaptic astrocytic processes have been suggested as sites for the regulated release of neuroactive substances. However, very little is known about the molecular properties of regulated exocytosis in these processes. Soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) proteins mediate synaptic vesicle exocytosis from neuronal cells and might be candidates for regulated exocytosis also from astrocytic processes. The expression of SNARE proteins in astrocytes, however, is not clarified. Thus, we aimed to investigate the localization and relative concentrations of neuronal SNARE proteins syntaxin-1, synaptosomal nerve-associated protein 25 (SNAP-25), vesicle-associated membrane protein 2 (VAMP-2) (synaptobrevin-2) and calcium sensor synaptotagmin 1 in perisynaptic astrocytic processes compared to nerve terminals and dendrites.

METHODS

We used quantitative immunogold electron microscopy of the rat hippocampus to investigate the localization and concentration of neuronal SNARE proteins.

RESULTS

As expected, analysis of the immunogold data revealed a lower labeling density of SNARE proteins in the perisynaptic astrocytic processes than in presynaptic terminals. The same was also true when compared to dendrites. Contrary to VAMP-2, labeling intensities for syntaxin-1, SNAP-25 and synaptotagmin 1 were not distinguishable from background labeling in the processes. The relative concentration of VAMP-2 stands out, as the mean perisynaptic astrocytic process concentration of the protein was only 68 % lower than in presynaptic terminals and still 32 % higher than in dendrites. VAMP-2 was associated with small vesicles in the processes. Some gold particles were located over the astrocytic plasma membrane.

CONCLUSION

VAMP-2 is expressed in perisynaptic astrocytic processes, with a concentration higher than in the dendrites. Our results are compatible with the role of VAMP-2 in exocytosis from perisynaptic astrocytic processes.

Synapsin III Regulates Dopaminergic Neuron Development in Vertebrates

Attention deficit and hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by alterations in the mesocorticolimbic and nigrostriatal dopaminergic pathways. Polymorphisms in the Synapsin III (Syn III) gene can associate with ADHD onset and even affect the therapeutic response to the gold standard ADHD medication, methylphenidate (MPH), a monoamine transporter inhibitor whose efficacy appears related with the stimulation of brain-derived neurotrophic factor (BDNF). Interestingly, we previously showed that MPH can bind Syn III, which can regulate neuronal development. These observations suggest that Syn III polymorphism may impinge on ADHD onset and response to therapy by affecting BDNF-dependent dopaminergic neuron development. Here, by studying zebrafish embryos exposed to Syn III gene knock-down (KD), Syn III knock-out (ko) mice and human induced pluripotent stem cells (iPSCs)-derived neurons subjected to Syn III RNA interference, we found that Syn III governs the earliest stages of dopaminergic neurons development and that this function is conserved in vertebrates. We also observed that in mammals Syn III exerts this function acting upstream of brain-derived neurotrophic factor (BDNF)- and cAMP-dependent protein kinase 5 (Cdk5)-stimulated dendrite development. Collectively, these findings own significant implications for deciphering the biological basis of ADHD.

Regulation of glutamate homeostasis in the nucleus accumbens by astrocytic CB1 receptors and its role in cocaine-motivated behaviors

Cannabinoid type 1 receptors (CB1Rs) orchestrate brain reward circuitry and are prevalent neurobiological targets for endocannabinoids and cannabis in the mammalian brain. Decades of histological and electrophysiological studies have established CB1R as presynaptic G-protein coupled receptors (GPCRs) that inhibit neurotransmitter release through retrograde signaling mechanisms. Recent seminal work demonstrates CB1R expression on astrocytes and the pivotal function of glial cells in endocannabinoid-mediated modulation of neuron-astrocyte signaling. Here, we review key facets of CB1R-mediated astroglia regulation of synaptic glutamate transmission in the nucleus accumbens with a specific emphasis on cocaine-directed behaviors.

Imaging Synaptic Density: The Next Holy Grail of Neuroscience?

The brain is the central and most complex organ in the nervous system, comprising billions of neurons that constantly communicate through trillions of connections called synapses. Despite being formed mainly during prenatal and early postnatal development, synapses are continually refined and eliminated throughout life via complicated and hitherto incompletely understood mechanisms. Failure to correctly regulate the numbers and distribution of synapses has been associated with many neurological and psychiatric disorders, including autism, epilepsy, Alzheimer’s disease, and schizophrenia. Therefore, measurements of brain synaptic density, as well as early detection of synaptic dysfunction, are essential for understanding normal and abnormal brain development. To date, multiple synaptic density markers have been proposed and investigated in experimental models of brain disorders. The majority of the gold standard methodologies (e.g., electron microscopy or immunohistochemistry) visualize synapses or measure changes in pre- and postsynaptic proteins ex vivo. However, the invasive nature of these classic methodologies precludes their use in living organisms. The recent development of positron emission tomography (PET) tracers [such as (¹⁸F)UCB-H or (¹¹C)UCB-J] that bind to a putative synaptic density marker, the synaptic vesicle 2A (SV2A) protein, is heralding a likely paradigm shift in detecting synaptic alterations in patients. Despite their limited specificity, novel, non-invasive magnetic resonance (MR)-based methods also show promise in inferring synaptic information by linking to glutamate neurotransmission. Although promising, all these methods entail various advantages and limitations that must be addressed before becoming part of routine clinical practice. In this review, we summarize and discuss current ex vivo and in vivo methods of quantifying synaptic density, including an evaluation of their reliability and experimental utility. We conclude with a critical assessment of challenges that need to be overcome before successfully employing synaptic density biomarkers as diagnostic and/or prognostic tools in the study of neurological and neuropsychiatric disorders.

Exocytosis in Astrocytes

Until recently, astrocytes were thought to be a part of a simple "brain glue" providing only a supporting role for neurons. However, the discoveries of the last two decades have proven astrocytes to be dynamic partners participating in brain metabolism and actively influencing communication between neurons. The means of astrocyte-neuron communication are diverse, although regulated exocytosis has received the most attention but also caused the most debate. Similar to most of eukaryotic cells, astrocytes have a complex range of vesicular organelles which can undergo exocytosis as well as intricate molecular mechanisms that regulate this process. In this review, we focus on the components needed for regulated exocytosis to occur and summarise the knowledge about experimental evidence showing its presence in astrocytes.

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

Glial Synaptobrevin mediates peripheral nerve insulation, neural metabolic supply, and is required for motor function

Peripheral nerves contain sensory and motor neuron axons coated by glial cells whose interplay ensures function, but molecular details are lacking. SNARE-proteins mediate the exchange and secretion of cargo by fusing vesicles with target organelles, but how glial SNAREs contribute to peripheral nerve function is largely unknown. We, here, identify non-neuronal Synaptobrevin (Syb) as the essential vesicular SNARE in Drosophila peripheral glia to insulate and metabolically supply neurons. We show that tetanus neurotoxin light chain (TeNT-LC), which potently inhibits SNARE-mediated exocytosis from neurons, also impairs peripheral nerve function when selectively expressed in glia, causing nerve disintegration, defective axonal transport, tetanic muscle hyperactivity, impaired locomotion, and lethality. While TeNT-LC disrupts neural function by cleaving neuronal Synaptobrevin (nSyb), it targets non-neuronal Synaptobrevin (Syb) in glia, which it cleaves at low rates: Glial knockdown of Syb (but not nSyb) phenocopied glial TeNT-LC expression whose effects were reverted by a TeNT-LC-insensitive Syb mutant. We link Syb-necessity to two distinct glial subtypes: Impairing Syb function in subperineurial glia disrupted nerve morphology, axonal transport, and locomotion, likely, because nerve-isolating septate junctions (SJs) could not form as essential SJ components (like the cell adhesion protein Neurexin-IV) were mistargeted. Interference with Syb in axon-encircling wrapping glia left nerve morphology and locomotion intact but impaired axonal transport, likely because neural metabolic supply was disrupted due to the mistargeting of metabolite shuffling monocarboxylate transporters. Our study identifies crucial roles of Syb in various glial subtypes to ensure glial-glial and glial-neural interplay needed for proper nerve function, animal motility, and survival.

New frontiers in probing the dynamics of purinergic transmitters in vivo

Purinergic transmitters such as adenosine, ADP, ATP, UTP, and UDP-glucose play important roles in a wide range of physiological processes, including the sleep-wake cycle, learning and memory, cardiovascular function, and the immune response. Moreover, impaired purinergic signaling has been implicated in various pathological conditions such as pain, migraine, epilepsy, and drug addiction. Examining the function of purinergic transmission in both health and disease requires direct, sensitive, non-invasive tools for monitoring structurally similar purinergic transmitters; ideally, these tools should have high spatial and temporal resolution in in vivo applications. Here, we review the recent progress with respect to the development and application of new methods for detecting purinergic transmitters, focusing on optical tools; in addition, we provide discussion regarding future perspectives.

Gliocrine System: Astroglia as Secretory Cells of the CNS

Astrocytes are secretory cells, actively participating in cell-to-cell communication in the central nervous system (CNS). They sense signaling molecules in the extracellular space, around the nearby synapses and also those released at much farther locations in the CNS, by their cell surface receptors, get excited to then release their own signaling molecules. This contributes to the brain information processing, based on diffusion within the extracellular space around the synapses and on convection when locales relatively far away from the release sites are involved. These functions resemble secretion from endocrine cells, therefore astrocytes were termed to be a part of the gliocrine system in 2015. An important mechanism, by which astrocytes release signaling molecules is the merger of the vesicle membrane with the plasmalemma, i.e., exocytosis. Signaling molecules stored in astroglial secretory vesicles can be discharged into the extracellular space after the vesicle membrane fuses with the plasma membrane. This leads to a fusion pore formation, a channel that must widen to allow the exit of the Vesiclal cargo. Upon complete vesicle membrane fusion, this process also integrates other proteins, such as receptors, transporters and channels into the plasma membrane, determining astroglial surface signaling landscape. Vesiclal cargo, together with the whole vesicle can also exit astrocytes by the fusion of multivesicular bodies with the plasma membrane (exosomes) or by budding of vesicles (ectosomes) from the plasma membrane into the extracellular space. These astroglia-derived extracellular vesicles can later interact with various target cells. Here, the characteristics of four types of astroglial secretory vesicles: synaptic-like microvesicles, dense-core vesicles, secretory lysosomes, and extracellular vesicles, are discussed. Then machinery for vesicle-based exocytosis, second messenger regulation and the kinetics of exocytotic vesicle content discharge or release of extracellular vesicles are considered. In comparison to rapidly responsive, electrically excitable neurons, the receptor-mediated cytosolic excitability-mediated astroglial exocytotic vesicle-based transmitter release is a relatively slow process.

Astrocytes Maintain Glutamate Homeostasis in the CNS by Controlling the Balance between Glutamate Uptake and Release

Glutamate is one of the most prevalent neurotransmitters released by excitatory neurons in the central nervous system (CNS); however, residual glutamate in the extracellular space is, potentially, neurotoxic. It is now well-established that one of the fundamental functions of astrocytes is to uptake most of the synaptically-released glutamate, which optimizes neuronal functions and prevents glutamate excitotoxicity. In the CNS, glutamate clearance is mediated by glutamate uptake transporters expressed, principally, by astrocytes. Interestingly, recent studies demonstrate that extracellular glutamate stimulates Ca²⁺ release from the astrocytes’ intracellular stores, which triggers glutamate release from astrocytes to the adjacent neurons, mostly by an exocytotic mechanism. This released glutamate is believed to coordinate neuronal firing and mediate their excitatory or inhibitory activity. Therefore, astrocytes contribute to glutamate homeostasis in the CNS, by maintaining the balance between their opposing functions of glutamate uptake and release. This dual function of astrocytes represents a potential therapeutic target for CNS diseases associated with glutamate excitotoxicity. In this regard, we summarize the molecular mechanisms of glutamate uptake and release, their regulation, and the significance of both processes in the CNS. Also, we review the main features of glutamate metabolism and glutamate excitotoxicity and its implication in CNS diseases.

Botulinum toxin: A review of the mode of action in migraine

Botulinum toxin serotype A (BoNT/A) was originally used in neurology for the treatment of dystonia and blepharospasms, but is now clinically used worldwide for the treatment of chronic migraine. Still, the possible mode of action of BoNT/A in migraine is not fully known. However, the mode of action of BoNT/A has been investigated in experimental pain as well as migraine models, which may elucidate the underlying mechanisms in migraine. The aim of this study was to review studies on the possible mode of action of BoNT/A in relation to chronic migraine treatment. Observations suggest that the mode of action of BoNT/A may not be limited to the injection site, but also includes anatomically connected sites due to axonal transport. The mechanisms behind the effect of BoNT/A in chronic migraine may also include modulation of neurotransmitter release, changes in surface expression of receptors and cytokines as well as enhancement of opioidergic transmission. Clinical and experimental studies with botulinum toxin in the last decade have advanced our understanding of headache and other pain states. More research into botulinum toxin as treatment for headache is warranted as it can be an attractive alternative for patients who do not respond positively to other drugs.

Selective Blockade of the Metabotropic Glutamate Receptor mGluR5 Protects Mouse Livers in In Vitro and Ex Vivo Models of Ischemia Reperfusion Injury

2-Methyl-6-(phenylethynyl)pyridine (MPEP), a negative allosteric modulator of the metabotropic glutamate receptor (mGluR) 5, protects hepatocytes from ischemic injury. In astrocytes and microglia, MPEP depletes ATP. These findings seem to be self-contradictory, since ATP depletion is a fundamental stressor in ischemia. This study attempted to reconstruct the mechanism of MPEP-mediated ATP depletion and the consequences of ATP depletion on protection against ischemic injury. We compared the effects of MPEP and other mGluR5 negative modulators on ATP concentration when measured in rat hepatocytes and acellular solutions. We also evaluated the effects of mGluR5 blockade on viability in rat hepatocytes exposed to hypoxia. Furthermore, we studied the effects of MPEP treatment on mouse livers subjected to cold ischemia and warm ischemia reperfusion. We found that MPEP and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) deplete ATP in hepatocytes and acellular solutions, unlike fenobam. This finding suggests that mGluR5s may not be involved, contrary to previous reports. MPEP, as well as MTEP and fenobam, improved hypoxic hepatocyte viability, suggesting that protection against ischemic injury is independent of ATP depletion. Significantly, MPEP protected mouse livers in two different ex vivo models of ischemia reperfusion injury, suggesting its possible protective deployment in the treatment of hepatic inflammatory conditions.

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.

Neuronal P2X7 Receptor: Involvement in Neuronal Physiology and Pathology

The proposed presence of P2X7 receptor (P2X7R) in neurons has been the source of some contention. Initial studies suggested an absence of P2X7R mRNA in neurons, and the apparent nonspecificity of the antibodies used to identify P2X7R raised further doubts. However, subsequent studies using new pharmacological and biomolecular tools provided conclusive evidence supporting the existence of functional P2X7Rs in neurons. The P2X7 receptor has since been shown to play a leading role in multiple aspects of neuronal physiology, including axonal elongation and branching and neurotransmitter release. P2X7R has also been implicated in neuronal pathologies, in which it may influence neuronal survival. Together, this body of research suggests that P2X7R may constitute an important therapeutic target for a variety of neurological disorders.

Mutant Huntingtin Impairs BDNF Release from Astrocytes by Disrupting Conversion of Rab3a-GTP into Rab3a-GDP

Brain-derived neurotrophic factor (BDNF) is essential for neuronal differentiation and survival. We know that BDNF levels decline in the brains of patients with Huntington's disease (HD), a neurodegenerative disease caused by the expression of mutant huntingtin protein (mHtt), and furthermore that administration of BDNF in HD mice is protective against HD neuropathology. BDNF is produced in neurons, but astrocytes are also an important source of BDNF in the brain. Nonetheless, whether mHtt affects astrocytic BDNF in the HD brain remains unknown. Here we investigated astrocytes from HD140Q knock-in mice and uncovered evidence that mHtt decreases BDNF secretion from astrocytes, which is mediated by exocytosis in astrocytes. Our results demonstrate that mHtt associates with Rab3a, a small GTPase localized on membranes of dense-core vesicles, and prevents GTP-Rab3a from binding to Rab3-GAP1, disrupting the conversion of GTP-Rab3a into GDP-Rab3a and thus impairing the docking of BDNF vesicles on plasma membranes of astrocytes. Importantly, overexpression of Rab3a rescues impaired BDNF vesicle docking and secretion from HD astrocytes. Moreover, ATP release and the number of ATP-containing dense-core vesicles docking are decreased in HD astrocytes, suggesting that the exocytosis of dense-core vesicles is impaired by mHtt in HD astrocytes. Further, Rab3a overexpression reduces reactive astrocytes in the striatum of HD140Q knock-in mice. Our results indicate that compromised exocytosis of BDNF in HD astrocytes contributes to the decreased BDNF levels in HD brains and underscores the importance of improving glial function in the treatment of HD.

SIGNIFICANCE STATEMENT

Huntington's disease (HD) is an inherited neurodegenerative disorder that affects one in every 10,000 Americans. To date, there is no effective treatment for HD, in part because the pathogenic mechanism driving the disease is not fully understood. The dysfunction of astrocytes is known to contribute to the pathogenesis of HD. One important role of astrocytes is to synthesize and release brain-derived neurotrophic factor (BDNF), which is vital for neuronal survival, development, and function. We found that mutant huntingtin protein (mHtt) at the endogenous level decreases BDNF secretion from astrocytes by disrupting the conversion of GTP-Rab3a into GDP-Rab3a and that overexpressing Rab3a can rescue this deficient BDNF release and early neuropathology in HD knock-in mouse brain. Our study suggests that astrocytic Rab3a is a potential therapeutic target for HD treatment.

Dynamin regulates the fusion pore of endo- and exocytotic vesicles as revealed by membrane capacitance measurements

BACKGROUND

Dynamin is a multidomain GTPase exhibiting mechanochemical and catalytic properties involved in vesicle scission from the plasmalemma during endocytosis. New evidence indicates that dynamin is also involved in exocytotic release of catecholamines, suggesting the existence of a dynamin-regulated structure that couples endo- to exocytosis.

METHODS

Thus we here employed high-resolution cell-attached capacitance measurements and super-resolution structured illumination microscopy to directly examine single vesicle interactions with the plasmalemma in cultured rat astrocytes treated with distinct pharmacological modulators of dynamin activity. Fluorescent dextrans and the lipophilic plasmalemmal marker DiD were utilized to monitor uptake and distribution of vesicles in the peri-plasmalemmal space and in the cell cytosol.

RESULTS

Dynamin inhibition with Dynole™-34-2 and Dyngo™-4a prevented vesicle internalization into the cytosol and decreased fusion pore conductance of vesicles that remained attached to the plasmalemma via a narrow fusion pore that lapsed into a state of repetitive opening and closing - flickering. In contrast, the dynamin activator Ryngo™-1-23 promoted vesicle internalization and favored fusion pore closure by prolonging closed and shortening open fusion pore dwell times. Immunocytochemical staining revealed dextran uptake into dynamin-positive vesicles and increased dextran uptake into Syt4- and VAMP2-positive vesicles after dynamin inhibition, indicating prolonged retention of these vesicles at the plasmalemma.

CONCLUSIONS

Our results have provided direct evidence for a role of dynamin in regulation of fusion pore geometry and kinetics of endo- and exocytotic vesicles, indicating that both share a common dynamin-regulated structural intermediate, the fusion pore.

The structural and functional evidence for vesicular release from astrocytes in situ

The concept of the tripartite synapse states that bi-directional signalling between perisynaptic astrocyte processes, presynaptic axonal boutons and postsynaptic neuronal structures defines the properties of synaptic information processing. Ca²⁺-dependent vesicular release from astrocytes, as one of the mechanisms of astrocyte-neuron communication, has attracted particular attention but has also been the subject of intense debate. In neurons, regulated vesicular release is a strongly coordinated process. It requires a complex release machinery comprised of many individual components ranging from vesicular neurotransmitter transporters and soluble NSF attachment protein receptors (SNARE) proteins to Ca²⁺-sensors and the proteins that spatially and temporally control exocytosis of synaptic vesicles. If astrocytes employ similar mechanisms to release neurotransmitters is less well understood. The aim of this review is therefore to discuss recent experimental evidence that sheds light on the central structural components responsible for vesicular release from astrocytes in situ.

Astrocytes as secretory cells of the central nervous system: idiosyncrasies of vesicular secretion

Astrocytes are housekeepers of the central nervous system (CNS) and are important for CNS development, homeostasis and defence. They communicate with neurones and other glial cells through the release of signalling molecules. Astrocytes secrete a wide array of classic neurotransmitters, neuromodulators and hormones, as well as metabolic, trophic and plastic factors, all of which contribute to the gliocrine system. The release of neuroactive substances from astrocytes occurs through several distinct pathways that include diffusion through plasmalemmal channels, translocation by multiple transporters and regulated exocytosis. As in other eukaryotic cells, exocytotic secretion from astrocytes involves divergent secretory organelles (synaptic-like microvesicles, dense-core vesicles, lysosomes, exosomes and ectosomes), which differ in size, origin, cargo, membrane composition, dynamics and functions. In this review, we summarize the features and functions of secretory organelles in astrocytes. We focus on the biogenesis and trafficking of secretory organelles and on the regulation of the exocytotic secretory system in the context of healthy and diseased astrocytes.

Current status and future directions of botulinum neurotoxins for targeting pain processing

Current evidence suggests that botulinum neurotoxins (BoNTs) A1 and B1, given locally into peripheral tissues such as skin, muscles, and joints, alter nociceptive processing otherwise initiated by inflammation or nerve injury in animal models and humans. Recent data indicate that such locally delivered BoNTs exert not only local action on sensory afferent terminals but undergo transport to central afferent cell bodies (dorsal root ganglia) and spinal dorsal horn terminals, where they cleave SNAREs and block transmitter release. Increasing evidence supports the possibility of a trans-synaptic movement to alter postsynaptic function in neuronal and possibly non-neuronal (glial) cells. The vast majority of these studies have been conducted on BoNT/A1 and BoNT/B1, the only two pharmaceutically developed variants. However, now over 40 different subtypes of botulinum neurotoxins (BoNTs) have been identified. By combining our existing and rapidly growing understanding of BoNT/A1 and /B1 in altering nociceptive processing with explorations of the specific characteristics of the various toxins from this family, we may be able to discover or design novel, effective, and long-lasting pain therapeutics. This review will focus on our current understanding of the molecular mechanisms whereby BoNTs alter pain processing, and future directions in the development of these agents as pain therapeutics.

Functional distribution of synapsin I in human sperm

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Synapsin I was localized in the human sperm equatorial segment.

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Presence of synapsin I was confirmed by dot and Western blotting techniques.

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Treatment of sperm with anti-synapsin antibodies significantly decreased motility.

Proteins known to function during cell–cell communication and exocytosis in neurons and other secretory cells have recently been reported in human sperm. Synapsins are a group of proteins that have been very well characterized in neurons, but little is known about synapsin function in other cell types. Based upon previous findings and the known function of synapsin, we tested the hypothesis that synapsin I was present in human sperm. Washed, capacitated, and acrosome induced sperm preparations were used to evaluate the functional distribution of synapsin I using immunocytochemistry. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were used for protein blotting techniques. Immunolocalization revealed synapsin I was enriched in the sperm equatorial segment. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were positive for synapsin I using several different antibodies, and dot blot results were confirmed by Western blot analyses. Finally, treatment of capacitated and acrosome reaction induced samples with anti-synapsin antibodies significantly reduced sperm motility. Localization of synapsin I in human sperm is a novel finding. The association of synapsin I with the sperm equatorial segment and effects on motility are suggestive of a role associated with capacitation and/or acrosome reaction, processes that render sperm capable of fertilizing an oocyte.

Quantitative Proteomic Analysis Reveals Molecular Adaptations in the Hippocampal Synaptic Active Zone of Chronic Mild Stress-Unsusceptible Rats

BACKGROUND

While stressful events are recognized as an important cause of major depressive disorder, some individuals exposed to life stressors maintain normal psychological functioning. The molecular mechanism(s) underlying this phenomenon remain unclear. Abnormal transmission and plasticity of hippocampal synapses have been implied to play a key role in the pathoetiology of major depressive disorder.

METHODS

A chronic mild stress protocol was applied to separate susceptible and unsusceptible rat subpopulations. Proteomic analysis using an isobaric tag for relative and absolute quantitation coupled with tandem mass spectrometry was performed to identify differential proteins in enriched hippocampal synaptic junction preparations.

RESULTS

A total of 4318 proteins were quantified, and 89 membrane proteins were present in differential amounts. Of these, SynaptomeDB identified 81 (91%) having a synapse-specific localization. The unbiased profiles identified several candidate proteins within the synaptic junction that may be associated with stress vulnerability or insusceptibility. Subsequent functional categorization revealed that protein systems particularly involved in membrane trafficking at the synaptic active zone exhibited a positive strain as potential molecular adaptations in the unsusceptible rats. Moreover, through STRING and immunoblotting analysis, membrane-associated GTP-bound Rab3a and Munc18-1 appear to coregulate syntaxin-1/SNAP25/VAMP2 assembly at the hippocampal presynaptic active zone of unsusceptible rats, facilitating SNARE-mediated membrane fusion and neurotransmitter release, and may be part of a stress-protection mechanism in actively maintaining an emotional homeostasis.

CONCLUSIONS

The present results support the concept that there is a range of potential protein adaptations in the hippocampal synaptic active zone of unsusceptible rats, revealing new investigative targets that may contribute to a better understanding of stress insusceptibility.

Loose excitation-secretion coupling in astrocytes

Astrocytes play an important housekeeping role in the central nervous system. Additionally, as secretory cells, they actively participate in cell-to-cell communication, which can be mediated by membrane-bound vesicles. The gliosignaling molecules stored in these vesicles are discharged into the extracellular space after the vesicle membrane fuses with the plasma membrane. This process is termed exocytosis, regulated by SNARE proteins, and triggered by elevations in cytosolic calcium levels, which are necessary and sufficient for exocytosis in astrocytes. For astrocytic exocytosis, calcium is sourced from the intracellular endoplasmic reticulum store, although its entry from the extracellular space contributes to cytosolic calcium dynamics in astrocytes. Here, we discuss calcium management in astrocytic exocytosis and the properties of the membrane-bound vesicles that store gliosignaling molecules, including the vesicle fusion machinery and kinetics of vesicle content discharge. In astrocytes, the delay between the increase in cytosolic calcium activity and the discharge of secretions from the vesicular lumen is orders of magnitude longer than that in neurons. This relatively loose excitation-secretion coupling is likely tailored to the participation of astrocytes in modulating neural network processing.

Botulinum neurotoxin type A modulates vesicular release of glutamate from satellite glial cells

This study investigated the presence of cell membrane docking proteins synaptosomal-associated protein, 25 and 23 kD (SNAP-25 and SNAP-23) in satellite glial cells (SGCs) of rat trigeminal ganglion; whether cultured SGCs would release glutamate in a time- and calcium-dependent manner following calcium-ionophore ionomycin stimulation; and if botulinum neurotoxin type A (BoNTA), in a dose-dependent manner, could block or decrease vesicular release of glutamate. SGCs were isolated from the trigeminal ganglia (TG) of adult Wistar rats and cultured for 7 days. The presence of SNAPs in TG sections and isolated SGCs were investigated using immunohistochemistry and immunocytochemistry, respectively. SGCs were stimulated with ionomycin (5 μM for 4, 8, 12 and 30 min.) to release glutamate. SGCs were then pre-incubated with BoNTA (24 hrs with 0.1, 1, 10 and 100 pM) to investigate if BoNTA could potentially block ionomycin-stimulated glutamate release. Glutamate concentrations were measured by ELISA. SNAP-25 and SNAP-23 were present in SGCs in TG sections and in cultured SGCs. Ionomycin significantly increased glutamate release from cultured SGCs 30 min. following the treatment (P < 0.001). BoNTA (100 pM) significantly decreased glutamate release (P < 0.01). Results from this study demonstrated that SGCs, when stimulated with ionomycin, released glutamate that was inhibited by BoNTA, possibly through cleavage of SNAP-25 and/or SNAP-23. These novel findings demonstrate the existence of vesicular glutamate release from SGCs, which could potentially play a role in the trigeminal sensory transmission. In addition, interaction of BoNTA with non-neuronal cells at the level of TG suggests a potential analgesic mechanism of action of BoNTA.

Excitable Astrocytes: Ca(2+)- and cAMP-Regulated Exocytosis

During neural activity, neurotransmitters released at synapses reach neighbouring cells, such as astrocytes. These get excited via numerous mechanisms, including the G protein coupled receptors that regulate the cytosolic concentration of second messengers, such as Ca(2+) and cAMP. The stimulation of these pathways leads to feedback modulation of neuronal activity and the activity of other cells by the release of diverse substances, gliosignals that include classical neurotransmitters such as glutamate, ATP, or neuropeptides. Gliosignal molecules are released from astrocytes through several distinct molecular mechanisms, for example, by diffusion through membrane channels, by translocation via plasmalemmal transporters, or by vesicular exocytosis. Vesicular release regulated by a stimulus-mediated increase in cytosolic second messengers involves a SNARE-dependent merger of the vesicle membrane with the plasmalemma. The coupling between the stimulus and vesicular secretion of gliosignals in astrocytes is not as tight as in neurones. This is considered an adaptation to regulate homeostatic processes in a slow time domain as is the case in the endocrine system (slower than the nervous system), hence glial functions constitute the gliocrine system. This article provides an overview of the mechanisms of excitability, involving Ca(2+) and cAMP, where the former mediates phasic signalling and the latter tonic signalling. The molecular, anatomic, and physiologic properties of the vesicular apparatus mediating the release of gliosignals is presented.

Astrocytic vesicles and gliotransmitters: Slowness of vesicular release and synaptobrevin2-laden vesicle nanoarchitecture

Neurotransmitters released at synapses activate neighboring astrocytes, which in turn, modulate neuronal activity by the release of diverse neuroactive substances that include classical neurotransmitters such as glutamate, GABA or ATP. Neuroactive substances are released from astrocytes through several distinct molecular mechanisms, for example, by diffusion through membrane channels, by translocation via plasmalemmal transporters or by vesicular exocytosis. Vesicular release regulated by a stimulus-mediated increase in cytosolic calcium involves soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptor (SNARE)-dependent merger of the vesicle membrane with the plasmalemma. Up to 25 molecules of synaptobrevin 2 (Sb2), a SNARE complex protein, reside at a single astroglial vesicle; an individual neuronal, i.e. synaptic, vesicle contains ∼70 Sb2 molecules. It is proposed that this paucity of Sb2 molecules in astrocytic vesicles may determine the slow secretion. In the present essay we shall overview multiple aspects of vesicular architecture and types of vesicles based on their cargo and dynamics in astroglial cells.

Protein kinase A mediates adenosine A2a receptor modulation of neurotransmitter release via synapsin I phosphorylation in cultured cells from medulla oblongata

Synaptic transmission is an essential process for neuron physiology. Such process is enabled in part due to modulation of neurotransmitter release. Adenosine is a synaptic modulator of neurotransmitter release in the Central Nervous System, including neurons of medulla oblongata, where several nuclei are involved with neurovegetative reflexes. Adenosine modulates different neurotransmitter systems in medulla oblongata, specially glutamate and noradrenaline in the nucleus tractussolitarii, which are involved in hypotensive responses. However, the intracellular mechanisms involved in this modulation remain unknown. The adenosine A2a receptor modulates neurotransmitter release by activating two cAMP protein effectors, the protein kinase A and the exchange protein activated by cAMP. Therefore, an in vitro approach (cultured cells) was carried out to evaluate modulation of neurotransmission by adenosine A2a receptor and the signaling intracellular pathway involved. Results show that the adenosine A2a receptor agonist, CGS 21680, increases neurotransmitter release, in particular, glutamate and noradrenaline and such response is mediated by protein kinase A activation, which in turn increased synapsin I phosphorylation. This suggests a mechanism of A2aR modulation of neurotransmitter release in cultured cells from medulla oblongata of Wistar rats and suggest that protein kinase A mediates this modulation of neurotransmitter release via synapsin I phosphorylation.

Role of glycogenolysis in stimulation of ATP release from cultured mouse astrocytes by transmitters and high K+ concentrations

This study investigates the role of glycogenolysis in stimulated release of ATP as a transmitter from astrocytes. Within the last 20 years our understanding of brain glycogenolysis has changed from it being a relatively uninteresting process to being a driving force for essential brain functions like production of transmitter glutamate and homoeostasis of potassium ions (K⁺) after their release from excited neurons. Simultaneously, the importance of astrocytic handling of adenosine, its phosphorylation to ATP and release of some astrocytic ATP, located in vesicles, as an important transmitter has also become to be realized. Among the procedures stimulating Ca²⁺-dependent release of vesicular ATP are exposure to such transmitters as glutamate and adenosine, which raise intra-astrocytic Ca²⁺ concentration, or increase of extracellular K⁺ to a depolarizing level that opens astrocytic L-channels for Ca²⁺ and thereby also increase intra-astrocytic Ca²⁺ concentration, a prerequisite for glycogenolysis. The present study has confirmed and quantitated stimulated ATP release from well differentiated astrocyte cultures by glutamate, adenosine or elevated extracellular K⁺ concentrations, measured by a luciferin/luciferase reaction. It has also shown that this release is virtually abolished by an inhibitor of glycogenolysis as well as by inhibitors of transmitter-mediated signaling or of L-channel opening by elevated K⁺ concentrations.

Stimulation of Ca²⁺-dependent astrocytic ATP release by glutamate, adenosine or high K⁺ concentration is abolished by inhibition of glycogenolysis, probably due to signaling inhibition. This is consistent with the K⁺-mediated pathway, but glycogenolysis-sensitive steps in the transmitter pathways are unknown.

Botulinum toxin B in the sensory afferent: transmitter release, spinal activation, and pain behavior

We addressed the hypothesis that intraplantar botulinum toxin B (rimabotulinumtoxin B: BoNT-B) has an early local effect upon peripheral afferent terminal releasing function and, over time, will be transported to the central terminals of the primary afferent. Once in the terminals it will cleave synaptic protein, block spinal afferent transmitter release, and thereby prevent spinal nociceptive excitation and behavior. In mice, C57Bl/6 males, intraplantar BoNT-B (1 U) given unilaterally into the hind paw had no effect upon survival or motor function, but ipsilaterally decreased: (1) intraplantar formalin-evoked flinching; (2) intraplantar capsaicin-evoked plasma extravasation in the hind paw measured by Evans blue in the paw; (3) intraplantar formalin-evoked dorsal horn substance P (SP) release (neurokinin 1 [NK1] receptor internalization); (4) intraplantar formalin-evoked dorsal horn neuronal activation (c-fos); (5) ipsilateral dorsal root ganglion (DRG) vesicle-associated membrane protein (VAMP); (6) ipsilateral SP release otherwise evoked bilaterally by intrathecal capsaicin; (7) ipsilateral activation of c-fos otherwise evoked bilaterally by intrathecal SP. These results indicate that BoNT-B, after unilateral intraplantar delivery, is taken up by the peripheral terminal, is locally active (blocking plasma extravasation), is transported to the ipsilateral DRG to cleave VAMP, and is acting presynaptically to block release from the spinal peptidergic terminal. The observations following intrathecal SP offer evidence for a possible transsynaptic effect of intraplantar BoNT. These results provide robust evidence that peripheral BoNT-B can alter peripheral and central terminal release from a nociceptor and attenuate downstream nociceptive processing via a presynaptic effect, with further evidence suggesting a possible postsynaptic effect.

The role of non-synaptic extracellular glutamate

Although there are some mechanisms which allow the direct crossing of substances between the cytoplasm of adjacent cells (gap junctions), most substances use the extracellular space to diffuse between brain cells. The present work reviews the behavior and functions of extracellular glutamate (GLU). There are two extracellular pools of glutamate (GLU) in the brain, a synaptic pool whose functions in the excitatory neurotransmission has been widely studied and an extrasynaptic GLU pool although less known nonetheless is gaining attention among a growing number of researchers. Evidence accumulated over the last years shows a number of mechanisms capable of releasing glial GLU to the extracellular medium, thus modulating neurons, microglia and oligodendrocytes, and regulating the immune response, cerebral blood flow, neuronal synchronization and other brain functions. This new scenario is expanding present knowledge regarding the role of GLU in the brain under different physiological and pathological conditions. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

The analgesic effect on neuropathic pain of retrogradely transported botulinum neurotoxin A involves Schwann cells and astrocytes

In recent years a growing debate is about whether botulinum neurotoxins are retrogradely transported from the site of injection. Immunodetection of cleaved SNAP-25 (cl-SNAP-25), the protein of the SNARE complex targeted by botulinum neurotoxin serotype A (BoNT/A), could represent an excellent approach to investigate the mechanism of action on the nociceptive pathways at peripheral and/or central level. After peripheral administration of BoNT/A, we analyzed the expression of cl-SNAP-25, from the hindpaw's nerve endings to the spinal cord, together with the behavioral effects on neuropathic pain. We used the chronic constriction injury of the sciatic nerve in CD1 mice as animal model of neuropathic pain. We evaluated immunostaining of cl-SNAP-25 in the peripheral nerve endings, along the sciatic nerve, in dorsal root ganglia and in spinal dorsal horns after intraplantar injection of saline or BoNT/A, alone or colocalized with either glial fibrillar acidic protein, GFAP, or complement receptor 3/cluster of differentiation 11b, CD11b, or neuronal nuclei, NeuN, depending on the area investigated. Immunofluorescence analysis shows the presence of the cl-SNAP-25 in all tissues examined, from the peripheral endings to the spinal cord, suggesting a retrograde transport of BoNT/A. Moreover, we performed in vitro experiments to ascertain if BoNT/A was able to interact with the proliferative state of Schwann cells (SC). We found that BoNT/A modulates the proliferation of SC and inhibits the acetylcholine release from SC, evidencing a new biological effect of the toxin and further supporting the retrograde transport of the toxin along the nerve and its ability to influence regenerative processes. The present results strongly sustain a combinatorial action at peripheral and central neural levels and encourage the use of BoNT/A for the pathological pain conditions difficult to treat in clinical practice and dramatically impairing patients' quality of life.

The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation

Following the discovery of the vasorelaxant properties of nitric oxide (NO) by Furchgott and Ignarro, the finding by Bredt and coll. of a constitutively expressed NO synthase in neurons (nNOS) led to the presumption that neuronal NO may control cerebrovascular functions. Consequently, numerous studies have sought to determine whether neuraly-derived NO is involved in the regulation of cerebral blood flow (CBF). Anatomically, axons, dendrites, or somata of NO neurons have been found to contact the basement membrane of blood vessels or perivascular astrocytes in all segments of the cortical microcirculation. Functionally, various experimental approaches support a role of neuronal NO in the maintenance of resting CBF as well as in the vascular response to neuronal activity. Since decades, it has been assumed that neuronal NO simply diffuses to the local blood vessels and produce vasodilation through a cGMP-PKG dependent mechanism. However, NO is not the sole mediator of vasodilation in the cerebral microcirculation and is known to interact with a myriad of signaling pathways also involved in vascular control. In addition, cerebrovascular regulation is the result of a complex orchestration between all components of the neurovascular unit (i.e., neuronal, glial, and vascular cells) also known to produce NO. In this review article, the role of NO interneuron in the regulation of cortical microcirculation will be discussed in the context of the neurovascular unit.

Synaptotagmin-1 promotes the formation of axonal filopodia and branches along the developing axons of forebrain neurons

Synaptotagmin-1 (syt1) is a Ca(2+)-binding protein that functions in regulation of synaptic vesicle exocytosis at the synapse. Syt1 is expressed in many types of neurons well before synaptogenesis begins both in vivo and in vitro. To determine if expression of syt1 has a functional role in neuronal development before synapse formation, we examined the effects of syt1 overexpression and knockdown on the growth and branching of the axons of cultured primary embryonic day 8 chicken forebrain neurons. In vivo these neurons express syt1, and most have not yet extended axons. We present evidence that syt1 plays a role in regulating axon branching, while not regulating overall axon length. To study the effects of overexpression of syt1, we used adenovirus-mediated infection to introduce a syt1-YFP construct, or control GFP construct, into neurons. Syt1 levels were reduced using RNA interference. Overexpression of syt1 increased the formation of axonal filopodia and branches. Conversely, knockdown of syt1 decreased the number of axonal filopodia and branches. Time-lapse analysis of filopodial dynamics in syt1-overexpressing cells demonstrated that elevation of syt1 levels increased both the frequency of filopodial initiation and their lifespan. Taken together these data indicate that syt1 regulates the formation of axonal filopodia and branches before engaging in its conventional functions at the synapse.

Exocytosis in astrocytes: transmitter release and membrane signal regulation

Astrocytes, a type of glial cells in the brain, are eukaryotic cells, and a hallmark of these are subcellular organelles, such as secretory vesicles. In neurons vesicles play a key role in signaling. Upon a stimulus-an increase in cytosolic concentration of free Ca(2+) ([Ca(2+)](i))-the membrane of vesicle fuses with the presynaptic plasma membrane, allowing the exit of neurotransmitters into the extracellular space and their diffusion to the postsynaptic receptors. For decades it was thought that such vesicle-based mechanisms of gliotransmitter release were not present in astrocytes. However, in the last 30 years experimental evidence showed that astrocytes are endowed with mechanisms for vesicle- and non-vesicle-based gliotransmitter release mechanisms. The aim of this review is to focus on exocytosis, which may play a role in gliotransmission and also in other forms of cell-to-cell communication, such as the delivery of transporters, ion channels and antigen presenting molecules to the cell surface.

Astroglial excitability and gliotransmission: an appraisal of Ca2+ as a signalling route

Astroglial cells, due to their passive electrical properties, were long considered subservient to neurons and to merely provide the framework and metabolic support of the brain. Although astrocytes do play such structural and housekeeping roles in the brain, these glial cells also contribute to the brain's computational power and behavioural output. These more active functions are endowed by the Ca²⁺-based excitability displayed by astrocytes. An increase in cytosolic Ca²⁺ levels in astrocytes can lead to the release of signalling molecules, a process termed gliotransmission, via the process of regulated exocytosis. Dynamic components of astrocytic exocytosis include the vesicular-plasma membrane secretory machinery, as well as the vesicular traffic, which is governed not only by general cytoskeletal elements but also by astrocyte-specific IFs (intermediate filaments). Gliotransmitters released into the ECS (extracellular space) can exert their actions on neighbouring neurons, to modulate synaptic transmission and plasticity, and to affect behaviour by modulating the sleep homoeostat. Besides these novel physiological roles, astrocytic Ca²⁺ dynamics, Ca²⁺-dependent gliotransmission and astrocyte–neuron signalling have been also implicated in brain disorders, such as epilepsy. The aim of this review is to highlight the newer findings concerning Ca²⁺ signalling in astrocytes and exocytotic gliotransmission. For this we report on Ca²⁺ sources and sinks that are necessary and sufficient for regulating the exocytotic release of gliotransmitters and discuss secretory machinery, secretory vesicles and vesicle mobility regulation. Finally, we consider the exocytotic gliotransmission in the modulation of synaptic transmission and plasticity, as well as the astrocytic contribution to sleep behaviour and epilepsy.

Synapsin II is involved in the molecular pathway of lithium treatment in bipolar disorder

Bipolar disorder (BD) is a debilitating psychiatric condition with a prevalence of 1–2% in the general population that is characterized by severe episodic shifts in mood ranging from depressive to manic episodes. One of the most common treatments is lithium (Li), with successful response in 30–60% of patients. Synapsin II (SYN2) is a neuronal phosphoprotein that we have previously identified as a possible candidate gene for the etiology of BD and/or response to Li treatment in a genome-wide linkage study focusing on BD patients characterized for excellent response to Li prophylaxis. In the present study we investigated the role of this gene in BD, particularly as it pertains to Li treatment. We investigated the effect of lithium treatment on the expression of SYN2 in lymphoblastoid cell lines from patients characterized as excellent Li-responders, non-responders, as well as non-psychiatric controls. Finally, we sought to determine if Li has a cell-type-specific effect on gene expression in neuronal-derived cell lines. In both in vitro models, we found SYN2 to be modulated by the presence of Li. By focusing on Li-responsive BD we have identified a potential mechanism for Li response in some patients.

Adenosine A2A receptors modulate glutamate uptake in cultured astrocytes and gliosomes

Glutamate is the primary excitatory neurotransmitter in the central nervous system, where its toxic build-up leads to synaptic dysfunction and excitotoxic cell death that underlies many neurodegenerative diseases. Therefore, efforts have been made to understand the regulation of glutamate transporters, which are responsible for the clearance of extracellular glutamate. We now report that adenosine A(2A) receptors (A(2A) R) control the uptake of D-aspartate in primary cultured astrocytes as well as in an ex vivo preparation enriched in glial plasmalemmal vesicles (gliosomes) from adult rats, whereas A(1) R and A(3) R were devoid of effects. Thus, the acute exposure to the A(2A) R agonist, CGS 21680, inhibited glutamate uptake, an effect prevented by the A(2A) R antagonist, SCH 58261, and abbrogated in cultured astrocytes from A(2A) R knockout mice. Furthermore, the prolonged activation of A(2A) R lead to a cAMP/protein kinase A-dependent reduction of GLT-I and GLAST mRNA and protein levels, which leads to a sustained decrease of glutamate uptake. This dual mechanism of inhibition of glutamate transporters by astrocytic A(2A) R provides a novel candidate mechanism to understand the ability of A(2) (A) R to control synaptic plasticity and neurodegeneration, two conditions tightly associated with the control of extracellular glutamate levels by glutamate transporters.

Surface-associated astrocytes, not endfeet, form the glia limitans in posterior piriform cortex and have a spatially distributed, not a domain, organization

"Surface-associated astrocytes" (SAAs) in posterior piriform cortex (PPC) are unique by virtue of a direct apposition to the cortical surface and large-caliber processes that descend into layer I. In this study additional unique and functionally relevant features of SAAs in PPC of the rat were identified by light and electron microscopy. Examination of sections cut parallel to the surface of PPC and stained for glial fibrillar acidic protein revealed that, in addition to descending processes, SAAs give rise to an extensive matrix of "superficial processes." Electron microscopy revealed that these superficial processes, together with cell bodies, form a continuous sheet at the surface of PPC with features in common with the glia limitans that is formed by endfeet in other cortical areas. These include a glia limiting membrane with basal lamina and similar associated organelles, including a striking array of mitochondria. Of particular interest, SAAs lack the domain organization observed in neocortex and hippocampus. Rather, superficial processes overlap extensively with gap junctions between their proximal regions as well as between cell bodies. Study of the descending processes revealed thin extensions, many of which appose synaptic profiles. We conclude that SAAs provide a potential substrate for bidirectional signaling and transport between brain and the pial arteries and cerebrospinal fluid in the subarachnoid space. We postulate that the spatially distributed character of SAAs in PPC reflects and supports the spatially distributed circuitry and sensory representation that are also unique features of this area.

Synapsin I is an oligomannose-carrying glycoprotein, acts as an oligomannose-binding lectin, and promotes neurite outgrowth and neuronal survival when released via glia-derived exosomes

Oligomannosidic glycans play important roles in nervous system development and function. By performing a phage display screening with oligomannose-specific antibodies, we identified an oligomannose-mimicking peptide that was functionally active in modulating neurite outgrowth and neuron-astrocyte adhesion. Using the oligomannose-mimicking peptide in crosslinking experiments, synapsin I was identified as a novel oligomannose-binding protein in mouse brain. Further analyses not only verified that synapsin I is an oligomannose-binding lectin, but also indicated that it is a glycoprotein carrying oligomannose and Lewis(x). We also found that synapsin I is expressed in glia-enriched cultures and is released from glial cells via exosomes. Incubation of glial-derived exosomes in the presence of high KCl concentrations or subjecting glial cell cultures to either oxygen/glucose deprivation or hydrogen peroxide resulted in release of synapsin I from exosomes. Application of synapsin I promoted neurite outgrowth from hippocampal neurons and increased survival of cortical neurons upon hydrogen peroxide treatment or oxygen/glucose deprivation. Coculture experiments using wild-type hippocampal neurons and wild-type or synapsin-deficient glial cells showed enhanced neurite outgrowth when synapsin was expressed by glial cells. Synapsin-induced neurite outgrowth was dependent on oligomannose on synapsin I and the neural cell adhesion molecule NCAM at the neuronal cell surface. The data indicate that, under conditions of high neuronal activity and/or oxidative stress, synapsin can be released from glial-derived exosomes and promotes neurite outgrowth and neuronal survival by modulating the interactions between glia and neurons.

Temporal characteristics of vesicular fusion in astrocytes: examination of synaptobrevin 2-laden vesicles at single vesicle resolution

Astrocytes can release various gliotransmitters in response to stimuli that cause increases in intracellular Ca(2+) levels; this secretion occurs via a regulated exocytosis pathway. Indeed, astrocytes express protein components of the vesicular secretory apparatus. However, the detailed temporal characteristics of vesicular fusions in astrocytes are not well understood. In order to start addressing this issue, we used total internal reflection fluorescence microscopy (TIRFM) to visualize vesicular fusion events in astrocytes expressing the fluorescent synaptobrevin 2 derivative, synapto-pHluorin. Although our cultured astrocytes from visual cortex express synaptosome-associated protein of 23 kDa (SNAP23), but not of 25 kDa (SNAP25), these glial cells exhibited a slow burst of exocytosis under mechanical stimulation; the expression of SNAP25B did not affect bursting behaviour. The relative amount of two distinct types of events observed, transient and full fusions, depended on the applied stimulus. Expression of exogenous synaptotagmin 1 (Syt1) in astrocytes endogenously expressing Syt4, led to a greater proportion of transient fusions when astrocytes were stimulated with bradykinin, a stimulus otherwise resulting in more full fusions. Additionally, we studied the stability of the transient fusion pore by measuring its dwell time, relation to vesicular size, flickering and decay slope; all of these characteristics were secretagogue dependent. The expression of SNAP25B or Syt1 had complex effects on transient fusion pore stability in a stimulus-specific manner. SNAP25B obliterated the appearance of flickers and reduced the dwell time when astrocytes were mechanically stimulated, while astrocytes expressing SNAP25B and stimulated with bradykinin had a reduction in decay slope. Syt1 reduced the dwell time when astrocytes were stimulated either mechanically or with bradykinin. Our detailed study of temporal characteristics of astrocytic exocytosis will not only aid the general understanding of this process, but also the interpretation of the events at the tripartite synapse, both in health and disease.

SNARE protein expression in synaptic terminals and astrocytes in the adult hippocampus: a comparative analysis

Several evidences suggest that astrocytes release small transmitter molecules, peptides, and protein factors via regulated exocytosis, implying that they function as specialized neurosecretory cells. However, very little is known about the molecular and functional properties of regulated secretion in astrocytes in the adult brain. Establishing these properties is central to the understanding of the communication mode(s) of these cells and their role(s) in the control of synaptic functions and of cerebral blood flow. In this study, we have set-up a high-resolution confocal microscopy approach to distinguish protein expression in astrocytic structures and neighboring synaptic terminals in adult brain tissue. This approach was applied to investigate the expression pattern of core SNARE proteins for vesicle fusion in the dentate gyrus and CA1 regions of the mouse hippocampus. Our comparative analysis shows that astrocytes abundantly express, in their cell body and main processes, all three protein partners necessary to form an operational SNARE complex but not in the same isoforms expressed in neighbouring synaptic terminals. Thus, SNAP25 and VAMP2 are absent from astrocytic processes and typically concentrated in terminals, while SNAP23 and VAMP3 have the opposite expression pattern. Syntaxin 1 is present in both synaptic terminals and astrocytes. These data support the view that astrocytes in the adult hippocampus can communicate via regulated exocytosis and also indicates that astrocytic exocytosis may differ in its properties from action potential-dependent exocytosis at neuronal synapses, as it relies on a distinctive set of SNARE proteins.

Ionotropic ATP receptors in neuronal-glial communication

In the central nervous system ATP is released from both neurones and astroglial cells acting as a homo- and heterocellular neurotransmitter. Glial cells express numerous purinoceptors of both ionotropic (P2X) and metabotropic (P2Y) varieties. Astroglial P2X receptors can be activated by ongoing synaptic transmission and can mediate fast local signalling through elevation in cytoplasmic Ca(2+) and Na(+) concentrations. These ionic signals can be translated into various physiological messages by numerous pathways, including release of gliotransmitters, metabolic support of neurones and regulation of activity of postsynaptic glutamate and GABA receptors. Ionotropic purinoceptors represent a novel pathway of glia-driven modulation of synaptic signalling that involves the release of ATP from neurones and astrocytes followed by activation of P2X receptors which can regulate synaptic activity by variety of mechanisms expressed in both neuronal and glial compartments.

Is astrocyte calcium signaling relevant for synaptic plasticity?

Astrocytes constitute a major group of glial cells which were long regarded as passive elements, fulfilling nutritive and structural functions for neurons. Calcium rise in astrocytes propagating to neurons was the first demonstration of direct interaction between the two cell types. Since then, calcium has been widely used, not only as an indicator of astrocytic activity but also as a stimulator switch to control astrocyte physiology. As a result, astrocytes have been elevated from auxiliaries to neurons, to cells involved in processing synaptic information. Curiously, while there is evidence that astrocytes play an important role in synaptic plasticity, the data relating to calcium's pivotal role are inconsistent. In this review, we will detail the various mechanisms of calcium flux in astrocytes, then briefly present the calcium-dependent mechanisms of gliotransmitter release. Finally, we will discuss the role of calcium in plasticity and present alternative explanations that could reconcile the conflicting results published recently.