Glial cells as active partners in synaptic functions

Glial Cell Modulation of Dendritic Spine Structure and Synaptic Function

Glia comprise a heterogeneous group of cells involved in the structure and function of the central and peripheral nervous system. Glial cells are found from invertebrates to humans with morphological specializations related to the neural circuits in which they are embedded. Glial cells modulate neuronal functions, brain wiring and myelination, and information processing. For example, astrocytes send processes to the synaptic cleft, actively participate in the metabolism of neurotransmitters, and release gliotransmitters, whose multiple effects depend on the targeting cells. Human astrocytes are larger and more complex than their mice and rats counterparts. Astrocytes and microglia participate in the development and plasticity of neural circuits by modulating dendritic spines. Spines enhance neuronal connectivity, integrate most postsynaptic excitatory potentials, and balance the strength of each input. Not all central synapses are engulfed by astrocytic processes. When that relationship occurs, a different pattern for thin and large spines reflects an activity-dependent remodeling of motile astrocytic processes around presynaptic and postsynaptic elements. Microglia are equally relevant for synaptic processing, and both glial cells modulate the switch of neuroendocrine secretion and behavioral display needed for reproduction. In this chapter, we provide an overview of the structure, function, and plasticity of glial cells and relate them to synaptic maturation and modulation, also involving neurotrophic factors. Together, neurons and glia coordinate synaptic transmission in both normal and abnormal conditions. Neglected over decades, this exciting research field can unravel the complexity of species-specific neural cytoarchitecture as well as the dynamic region-specific functional interactions between diverse neurons and glial subtypes.

The Time Course of MHC-I Expression in C57BL/6J and A/J Mice Correlates with the Degree of Retrograde Gliosis in the Spinal Cord following Sciatic Nerve Crush

The pleiotropic role of the major histocompatibility complex class I (MHC-I) reflects the close association between the nervous and immune systems. In turn, MHC-I upregulation postinjury is associated with a better regenerative outcome in isogenic mice following peripheral nerve damage. In the present work, we compared the time course of neuronal, glial, and sensorimotor recovery (1, 3, 5, 7, and 28 days after lesion—dal) following unilateral sciatic nerve crush in A/J and C57BL/6J mice. The A/J strain showed higher expression of MHC-I (7 dal, ** p < 0.01), Iba-1 (microglial reaction, 7 dal, *** p < 0.001), and GFAP (astrogliosis, 5 dal, * p < 0.05) than the C57BL/6J counterpart. Synaptic coverage (synaptophysin) was equivalent in both strains over time. In addition, mRNA expression of microdissected spinal motoneurons revealed an increase in cytoskeleton-associated molecules (cofilin, shp2, and crmp2, * p < 0.05), but not trkB, in C57BL/6J mice. Gait recovery, studied by the sciatic functional index, was faster in the A/J strain, despite the equivalent results of C57BL/6J at 28 days after injury. A similar recovery was also seen for the nociceptive threshold (von Frey test). Interestingly, when evaluating proprioceptive recovery, C57BL/6J animals showed an enlarged base of support, indicating abnormal ambulation postinjury. Overall, the present results reinforce the role of MHC-I expression in the plasticity of the nervous system following axotomy, which in turn correlates with the variable recovery capacity among strains of mice.

Peripheral glia diversity

Recent years have seen an evolving appreciation for the role of glial cells in the nervous system. As we move away from the typical neurocentric view of neuroscience, the complexity and variability of central nervous system glia is emerging, far beyond the three main subtypes: astrocytes, oligodendrocytes, and microglia. Yet the diversity of the glia found in the peripheral nervous system remains rarely discussed. In this review, we discuss the developmental origin, morphology, and function of the different populations of glia found in the peripheral nervous system, including: myelinating Schwann cells, Remak Schwann cells, repair Schwann cells, satellite glia, boundary cap-derived glia, perineurial glia, terminal Schwann cells, glia found in the skin, olfactory ensheathing cells, and enteric glia. The morphological and functional heterogeneity of glia found in the periphery reflects the diverse roles the nervous system performs throughout the body. Further, it highlights a complexity that should be appreciated and considered when it comes to a complete understanding of the peripheral nervous system in health and disease.

Terminal Schwann Cell Aging: Implications for Age-Associated Neuromuscular Dysfunction

Action potential is transmitted to muscle fibers through specialized synaptic interfaces called neuromuscular junctions (NMJs). These structures are capped by terminal Schwann cells (tSCs), which play essential roles during formation and maintenance of the NMJ. tSCs are implicated in the correct communication between nerves and muscles, and in reinnervation upon injury. During aging, loss of muscle mass and strength (sarcopenia and dynapenia) are due, at least in part, to the progressive loss of contacts between muscle fibers and nerves. Despite the important role of tSCs in NMJ function, very little is known on their implication in the NMJ-aging process and in age-associated denervation. This review summarizes the current knowledge about the implication of tSCs in the age-associated degeneration of NMJs. We also speculate on the possible mechanisms underlying the observed phenotypes.

Morphological peculiarities of neuromuscular junctions among different fiber types: Effect of exercise

The aim of our research was to examine whether there are differences in the morphology of neuromuscular junctions of different types of muscle fibers in rodents, and after their adaptation to six weeks endurance exercise training. After 5-day acclimation, Wistar rats were subjected to run with the speed 35 m/min during 6 week, 5 days per week and the training volume reached 60 min per day. Muscle samples for ultrastructural studies were fixed, dehydrated and embedded in Epon-812. Ultra-thin sections were cut from longitudinally and transversely oriented blocs, using 4 blocks from each animal. The area of axon terminals on fast- twitch fibers is 1.5 time large (p<0.001) and the perimeter of terminals is 1.7 time large in comparison with slow- twitch oxidative fibers (p<0.001) in control group. There are correlation between cross-sectional area of different muscle fibers and length of axon terminals (r=0.72), between cross-sectional area and with of axon terminal (r=-0.62), and between turnover rate of contractile proteins and length of axon terminal (r=0.75). Fast remodeling of synapse on oxidative and oxidative-glycolytic muscle fibers during endurance training seems to guarantees the intensive renewal of the structures of muscle fibers with higher oxidative capacity.

Neuromuscular Junction Impairment in Amyotrophic Lateral Sclerosis: Reassessing the Role of Acetylcholinesterase

Amyotrophic Lateral Sclerosis (ALS) is a highly debilitating disease caused by progressive degeneration of motorneurons (MNs). Due to the wide variety of genes and mutations identified in ALS, a highly varied etiology could ultimately converge to produce similar clinical symptoms. A major hypothesis in ALS research is the “distal axonopathy” with pathological changes occurring at the neuromuscular junction (NMJ), at very early stages of the disease, prior to MNs degeneration and onset of clinical symptoms. The NMJ is a highly specialized cholinergic synapse, allowing signaling between muscle and nerve necessary for skeletal muscle function. This nerve-muscle contact is characterized by the clustering of the collagen-tailed form of acetylcholinesterase (ColQ-AChE), together with other components of the extracellular matrix (ECM) and specific key molecules in the NMJ formation. Interestingly, in addition to their cholinergic role AChE is thought to play several “non-classical” roles that do not require catalytic function, most prominent among these is the facilitation of neurite growth, NMJ formation and survival. In all this context, abnormalities of AChE content have been found in plasma of ALS patients, in which AChE changes may reflect the neuromuscular disruption. We review these findings and particularly the evidences of changes of AChE at neuromuscular synapse in the pre-symptomatic stages of ALS.

Mitochondrial dynamics in astrocytes

Astrocytes exhibit cellular excitability through variations in their intracellular calcium (Ca²⁺) levels in response to synaptic activity. Astrocyte Ca²⁺ elevations can trigger the release of neuroactive substances that can modulate synaptic transmission and plasticity, hence promoting bidirectional communication with neurons. Intracellular Ca²⁺ dynamics can be regulated by several proteins located in the plasma membrane, within the cytosol and by intracellular organelles such as mitochondria. Spatial dynamics and strategic positioning of mitochondria are important for matching local energy provision and Ca²⁺ buffering requirements to the demands of neuronal signalling. Although relatively unresolved in astrocytes, further understanding the role of mitochondria in astrocytes may reveal more about the complex bidirectional relationship between astrocytes and neurons in health and disease. In the present review, we discuss some recent insights regarding mitochondrial function, transport and turnover in astrocytes and highlight some important questions that remain to be answered.

Gliotransmitters travel in time and space

The identification of the presence of active signaling between astrocytes and neurons in a process termed gliotransmission has caused a paradigm shift in our thinking about brain function. However, we are still in the early days of the conceptualization of how astrocytes influence synapses, neurons, networks, and ultimately behavior. In this Perspective, our goal is to identify emerging principles governing gliotransmission and consider the specific properties of this process that endow the astrocyte with unique functions in brain signal integration. We develop and present hypotheses aimed at reconciling confounding reports and define open questions to provide a conceptual framework for future studies. We propose that astrocytes mainly signal through high-affinity slowly desensitizing receptors to modulate neurons and perform integration in spatiotemporal domains complementary to those of neurons.

Mps one binder 2 gene upregulation in the stellation of astrocytes induced by cAMP-dependent pathway

Astrocytes, the major glial population in the central nervous system (CNS), play an important role in neuronal homeostasis, neurogenesis, and synaptogenesis. The cells have a stellate shape with elaborated processes in the developing CNS. Cultured astrocytes become stellate when the cells undergo differentiation in response to stimuli. Nevertheless, the molecular mechanism for astrocytic stellation is poorly understood. Here, we showed that the addition of serum induced a flat polygonal shape in cultured astrocytes with a reduced level of Mps one binder 2 (Mob2) that is involved in neurite growth by forming stable complex with a nuclear Ser/Thr kinase Dbf2-related protein kinase 1 (NDR1). Furthermore, exposure to a membrane permeable cAMP analogue, dbcAMP, not only induced astrocytic stellation, but also caused an increase in Mob2 expression. Similarly, the upregulation of Mob2 mRNA expression was induced by exposure of astrocytes to pituitary adenylyl cyclase-activating polypeptide (PACAP). Pretreatment with a cAMP/protein kinase A (PKA) inhibitor, KT-5720, significantly blocked the effect of dbcAMP and PACAP on induced upregulation of Mob2 mRNA expression in astrocytes. In addition, the process withdrawal of dbcAMP-treated astrocytes was caused by the inhibition of Mob2 expression using lentivirus-mediated Mob2 shRNA delivery system. Based on our findings, we suggest that Mob2 is involved in PKA signaling-mediated astrocytic stellation.

Using comparative anatomy in the axotomy model to identify distinct roles for microglia and astrocytes in synaptic stripping

The synaptic terminals' withdrawal from the somata and proximal dendrites of injured motoneuron by the processes of glial cells following facial nerve axotomy has been the subject of research for many years. This phenomenon is referred to as synaptic stripping, which is assumed to help survival and regeneration of neurons via reduction of synaptic inputs. Because there is no disruption of the blood-brain barrier or infiltration of macrophages, the axotomy paradigm has the advantage of being able to selectively investigate the roles of resident glial cells in the brain. Although there have been numerous studies of synaptic stripping, the detailed mechanisms are still under debate. Here we suggest that the species and strain differences that are often present in previous work might be related to the current controversies of axotomy studies. For instance, the survival ratios of axotomized neurons were generally found to be higher in rats than in mice. However, some studies have used the axotomy paradigm to follow the glial reactions and did not assess variations in neuronal viability. In the first part of this article, we summarize and discuss the current knowledge on species and strain differences in neuronal survival, glial augmentation and synaptic stripping. In the second part, we focus on our recent findings, which show the differential involvement of microglia and astrocytes in synaptic stripping and neuronal survival. This article suggests that the comparative study of the axotomy paradigm across various species and strains may provide many important and unexpected discoveries on the multifaceted roles of microglia and astrocytes in injury and repair.

Synapse-glia interactions are governed by synaptic and intrinsic glial properties

It is believed that glial cell activation and their interactions with synapses are predominantly dependent upon the characteristics of synaptic activity and the level of transmitter release. Because synaptic properties vary from one type of synapse to another, synapse-glia interactions should differ accordingly. The goal of this work was to examine how glial cell activation is dependent upon the properties of their respective synapses as well as the level of synaptic activity. We contrasted Ca(2+) responses of perisynaptic Schwann cells (PSCs) at neuromuscular junctions (NMJs) with different synaptic properties; the slow-twitch soleus (SOL) and the fast-twitch levator auris longus (LAL) muscles. Amplitude of PSC Ca(2+) responses elicited by repeated motor nerve stimulation at 40, 50 and 100 Hz were larger and their kinetics faster at LAL NMJs and this, at all frequencies examined. In addition, a greater number of PSCs per NMJ was activated by sustained synaptic transmission at NMJs of LAL in comparison to SOL. Differences in PSC activation could not be explained solely by differences in levels of transmitter release but also by intrinsic PSC properties since increasing transmitter release with tetraethylammonium chloride (TEA) did not increase their responsiveness. As a whole, these results indicate that PSC responsiveness at NMJs of slow- and fast-twitch muscles differ not only according to the level of activity of their synaptic partner but also in accordance with inherent glial properties.

The similarity of astrocytes number in dentate gyrus and CA3 subfield of rats hippocampus

The dentate gyrus is a part of hippocampal formation that it contains granule cells, which project to the pyramidal cells and interneurons of the CA3 subfield of the hippocampus. Astrocytes play a more active role in neuronal activity, including regulating ion flux currents, energy production, neurotransmitter release and synaptogenesis. Astrocytes are the only cells in the brain that contain the energy molecule glycogen. The close relationship between dentate gyrus and CA3 area can cause the similarity of the number of astrocytes in these areas. In this study 5 male albino wistar rats were used. Rats were housed in large plastic cage in animal house and were maintained under standard conditions, after histological processing, The 7 microm slides of the brains were stained with PTAH staining for showing the astrocytes. This staining is specialized for astrocytes. We showed that the number of astrocytes in different (ant., mid., post) parts of dentate gyrus and CA3 of hippocampus is the same. For example, the anterior parts of two area have the most number of astrocytes and the middle parts of two area have the least number of astrocytes. We concluded that dentate gyrus and CA3 area of hippocampus have the same group of astrocytes.

Cooperative astrocyte and dendritic spine dynamics at hippocampal excitatory synapses

Accumulating evidence is redefining the importance of neuron-glial interactions at synapses in the CNS. Astrocytes form "tripartite" complexes with presynaptic and postsynaptic structures and regulate synaptic transmission and plasticity. Despite our understanding of the importance of neuron-glial relationships in physiological contexts, little is known about the structural interplay between astrocytes and synapses. In the past, this has been difficult to explore because studies have been hampered by the lack of a system that preserves complex neuron-glial relationships observed in the brain. Here we present a system that can be used to characterize the intricate relationship between astrocytic processes and synaptic structures in situ using organotypic hippocampal slices, a preparation that retains the three-dimensional architecture of astrocyte-synapse interactions. Using time-lapse confocal imaging, we demonstrate that astrocytes can rapidly extend and retract fine processes to engage and disengage from motile postsynaptic dendritic spines. Surprisingly, astrocytic motility is, on average, higher than its dendritic spine counterparts and likely relies on actin-based cytoskeletal reorganization. Changes in astrocytic processes are typically coordinated with changes in spines, and astrocyte-spine interactions are stabilized at larger spines. Our results suggest that dynamic structural changes in astrocytes help control the degree of neuron-glial communication at hippocampal synapses.

Astrocyte reactivity influences the number of presynaptic terminals apposed to spinal motoneurons after axotomy

Although synaptic plasticity is a widespread phenomenon, the underlying mechanisms leading to its occurrence are virtually unknown. In this sense, glial cells, especially astrocytes, may have a role in network changes of the nervous system, influencing the retraction of boutons as well as providing a proper perisynaptic environment, thereby affecting the replacement of inputs. Interestingly, the glial reaction does vary between strains of rats and mice. In this sense, we present evidence that C57BL/6J and A/J isogenic mice present different astrocyte reactivity after a peripheral lesion in vivo as well as in vitro, by analyzing primary cell cultures. Such a difference in the glial reaction has a direct influence on in vivo number of pre-synaptic terminals and on in vitro synaptogenesis.

Synaptic plasticity, astrocytes and morphological homeostasis

Recent discoveries suggest that astrocytes are an integral part of synaptic connections, as they sense and modulate synaptic activity. Moreover, there is evidence that astrocytes change the number of synaptic connections directly via synaptogenic signals or indirectly, by modifying the morphology of axons and dendrites. Here, we formulate the hypothesis that astrocytes mediate the morphological homeostasis of nerve cells, which is any adaptation of the morphology of a neuron to preserve its ability to respond to and generate synaptic activity during learning and memory-induced changes. We argue that astrocytes control neuronal morphology locally and across long-ranging assemblies of neurons and that on the other hand, astrocytes are part of the engram with plasticity-related changes affecting their morphology.

Synaptic information processing by astrocytes

Glial cells were classically considered as supportive cells that do not contribute to information processing in the nervous system. However, considerable amount of evidence obtained by several groups during the last few years has demonstrated the existence of a bidirectional communication between astrocytes and neurons, which prompted a re-examination of the role of glial cells in the physiology of the nervous system. This review will discuss recent advances in the neuron-to-astrocyte communication, focusing on the recently reported properties of the synaptically evoked astrocyte Ca2+ signal that indicate that astrocytes show integrative properties for synaptic information processing. Indeed, we have recently shown that hippocampal astrocytes discriminate between the activity of different synapses, and respond selectively to different axon pathways. Furthermore, the astrocyte Ca2+ signal is modulated by the simultaneous activity of different synaptic inputs. This Ca2+ signal modulation depends on cellular intrinsic properties of the astrocytes, is bidirectionally regulated by the level of synaptic activity, and controls the spatial extension of the intracellular Ca2+ signal. Consequently, we propose that astrocytes can be considered as cellular elements involved in information processing by the nervous system.

Anti-disialosyl antibodies mediate selective neuronal or Schwann cell injury at mouse neuromuscular junctions

The human paralytic neuropathy, Miller Fisher syndrome (MFS) is associated with autoantibodies specific for disialosyl epitopes on gangliosides GQ1b, GT1a, and GD3. Since these gangliosides are enriched in synaptic membranes, anti-ganglioside antibodies may target neuromuscular junctions (NMJs), thereby contributing to disease symptoms. We have shown previously that at murine NMJs, anti-disialosyl antibodies induce an alpha-latrotoxin-like effect, electrophysiologically characterized by transient massive increase of spontaneous neurotransmitter release followed by block of evoked release, resulting in paralysis of the muscle preparation. Morphologically, motor nerve terminal damage, as well as perisynaptic Schwann cell (pSC) death is observed. The relative contributions of neuronal and pSC injury to the paralytic effect and subsequent repair are unknown. In this study, we have examined the ability of subsets of anti-disialosyl antibodies to discriminate between the neuronal and glial elements of the NMJ and thereby induce either neuronal injury or pSC death. Most antibodies reactive with GD3 induced pSC death, whereas antibody reactivity with GT1a correlated with the extent of nerve terminal injury. Motor nerve terminal injury resulted in massive uncontrolled exocytosis with paralysis. However, pSC ablation induced no acute (within 1 h) electrophysiological or morphological changes to the underlying nerve terminal. These data suggest that at mammalian NMJs, acute pSC injury or ablation has no major deleterious influence on synapse function. Our studies provide evidence for highly selective targeting of mammalian NMJ membranes, based on ganglioside composition, that can be exploited for examining axonal-glial interactions both in disease states and in normal NMJ homeostasis.

Nitric oxide, cAMP and the biphasic muscarinic modulation of ACh release at the lizard neuromuscular junction

In this study, we characterized the pharmacology and physiology of the automodulation of ACh release at the lizard neuromuscular junction (NMJ). The activation of muscarinic ACh receptors generated a biphasic modulation of synaptic transmission. Muscarine-induced activation of M3 receptors (0-12 min) decreased release, whereas M1 activation (> 12 min) enhanced release. Both phases of the biphasic effect are dependent on nitric oxide. However, cAMP acting via protein kinase A is also necessary for the M1 effect. In summary, we present a novel biphasic role for muscarine and implicate M3 receptors in the inhibition and M1 receptors in the enhancement of transmitter releaseat the cholinergic lizard NMJ.

Glial cells maintain synaptic structure and function and promote development of the neuromuscular junction in vivo

To investigate the in vivo role of glial cells in synaptic function, maintenance, and development, we have developed an approach to selectively ablate perisynaptic Schwann cells (PSCs), the glial cells at the neuromuscular junction (NMJ), en masse from live frog muscles. In adults, following acute PSC ablation, synaptic structure and function were not altered. However, 1 week after PSC ablation, presynaptic function decreased by approximately half, while postsynaptic function was unchanged. Retraction of nerve terminals increased over 10-fold at PSC-ablated NMJs. Furthermore, nerve-evoked muscle twitch tension was reduced. In tadpoles, repeated in vivo observations revealed that PSC processes lead nerve terminal growth. In the absence of PSCs, growth and addition of synapses was dramatically reduced, and existing synapses underwent widespread retraction. Our findings provide in vivo evidence that glial cells maintain presynaptic structure and function at adult synapses and are vital for the growth and stability of developing synapses.

Functional implications of seizure-induced neurogenesis

The neurobiological doctrine governing the concept of neurogenesis has undergone a revolution in the past few years. What was once considered dubious is now well accepted: new neurons are born in the adult brain. Science fiction is quickly becoming a reality as scientists discover ways to convert skin, bone, or blood cells into neurons. In the epilepsy arena, widespread interest has developed because of the evidence that neurogenesis increases after seizures, trauma, and other insults or injuries that alter seizure susceptibility. This review discusses some of the initial studies in this field, and their often surprising functional implications. The emphasis will be on the granule cells of hippocampus, because they are perhaps more relevant to epilepsy than other areas in which neurogenesis occurs throughout life, the olfactory bulb and subventricular zone. In particular, the following questions will be addressed: 1. Do granule cells that are born in the adult brain become functional, and what are the limits of their function? Do they behave homogeneously? Results from our own laboratory have focused on cells that become established outside the normal boundaries of the granule cell layer, forming a group of "ectopic" granule cells in the hilar region. 2. Is increased neurogenesis beneficial, or might it actually exacerbate seizures? Evidence is presented that supports the hypothesis that new granule cells may not necessarily act to ameliorate seizures, and might even contribute to them. Furthermore, cognitive deficits following seizures might in part be due to new circuits that develop between new cells and the host brain. 3. How do the new cells interact with the host brain? Several changes occur in the dentate gyrus after seizures, and increased neurogenesis is only one of many. What is the interdependence of this multitude of changes, if any? 4. Is neurogenesis increased after seizures in man? Research suggests that the data from human epileptics are actually inconsistent with the studies in animal models of epilepsy, because there is little evidence of increased neurogenesis in epileptic tissue resected from intractable epileptics. Yet neurogenesis has been shown to occur in humans throughout adult life. What might be the reasons for these seemingly disparate results?

Glial modulation of synaptic transmission in culture

Accumulating evidence has demonstrated the existence of bidirectional communication between glial cells and neurons, indicating an important active role of glia in the physiology of the nervous system. Neurotransmitters released by presynaptic terminals during synaptic activity increase intracellular Ca(2+) concentration in adjacent glial cells. In turn, activated glia may release different transmitters that can feed back to neuronal synaptic elements, regulating the postsynaptic neuronal excitability and modulating neurotransmitter release from presynaptic terminals. As a consequence of this evidence, a new concept of the synaptic physiology, the tripartite synapse, has been proposed, in which glial cells play an active role as dynamic regulatory elements in neurotransmission. In the present article we review evidence showing the ability of astrocytes to modulate synaptic transmission directly, with the focus on studies performed on cell culture preparations, which have been proved extremely useful in the characterization of molecular and cellular processes involved in astrocyte-mediated neuromodulation.

New roles for astrocytes: regulation of CNS synaptogenesis

The notion that astrocytes have a profound influence on the function of synapses between CNS neurons implies that the development of synaptic connections and their glial neighbors are controlled by reciprocally acting signals. Currently, however, synaptogenesis is considered a purely neuronal affair. This article summarizes recent experimental evidence suggesting that this may not be the case. Astrocytes may indeed regulate the formation, maturation and maintenance of synapses. The recent advances caution that synapses cannot develop correctly without astrocytes. Further progress on this issue requires new experimental models to identify signaling pathways and to scrutinize the relevance of glia-synapse interactions in vivo.

Dual action of hydrogen peroxide on synaptic transmission at the frog neuromuscular junction

There is evidence that reactive oxygen species (ROS) are produced and released during neuromuscular activity, but their role in synaptic transmission is not known. Using a two-electrode voltage-clamp technique, at frog neuromuscular junctions, the action H2O2 on end-plate currents (EPC) was studied to determine the targets for this membrane-permeable ROS. In curarized or cut muscles, micromolar concentrations of H2O2 increased the amplitude of EPCs. Higher (> 30 microM) doses inhibited EPCs and prolonged current decay. These effects were presynaptic since H2O2 did not change the amplitude or duration of miniature EPCs (although it reduced the rate of spontaneous release at high concentrations). Quantal analysis and deconvolution methods showed that facilitation of EPCs was due to increased quantal release, while depression was accompanied by temporal dispersion of evoked release. Extracellular recordings revealed prolonged presynaptic Ca2+ entry in the presence of high H2O2. Both low and high H2O2 increased presynaptic potentiation during high-frequency stimulation. Pro-oxidant Fe2+ did not affect facilitation by low doses of H2O2 but augmented the inhibition of EPCs by high H2O2, indicating involvement of hydroxyl radicals. High Mg2+ and the ROS scavenger N-acetylcysteine eliminated both the facilitatory and depressant effects of H2O2. The facilitatory effect of H2O2 was prevented by protein kinase C (PKC) inhibitors and 4beta-phorbol 12-myristate, 13-acetate (PMA), an activator of PKC. PKC inhibitors but not PMA also abolished the depressant effect of H2O2. Our data suggest complex presynaptic actions of H2O2, which could serve as a fast feedback modulator of intense neuromuscular transmission.

Proteomic analysis of astrocytic secretion in the mouse. Comparison with the cerebrospinal fluid proteome

Astrocytes, the most abundant cell type in the central nervous system, are intimately associated with synapses. They play a pivotal role in neuronal survival and the brain inflammatory response. Some astrocytic functions are mediated by the secretion of polypeptides. Using a proteomic approach, we have identified more than 30 proteins released by cultured astrocytes. These include proteases and protease inhibitors, carrier proteins, and antioxidant proteins. Exposing astrocytes to brefeldin A, which selectively blocks secretory vesicle assembly, suppressed the release of some of these proteins. This indicates that astrocytes secrete these proteins by a classic vesicular mechanism and others by an alternative pathway. Astrocytes isolated from different brain regions secreted a similar pattern of proteins. However, the secretion of some of them, including metalloproteinase inhibitors and apolipoprotein E, was region-specific. In addition, pro-inflammatory treatments modified the profile of astrocytic protein secretion. Finally, more than two thirds of the proteins identified in the astrocyte-conditioned medium were detectable in the mouse cerebrospinal fluid, suggesting that astrocytes contribute to the cerebrospinal fluid protein content. In conclusion, this study provides the first unbiased characterization of the major proteins released by astrocytes, which may play a crucial role in the modulation of neuronal survival and function.

Role of glia in synapse development

Recent studies suggest that glial cells regulate certain aspects of synapse development. Neurons can form synapses without glia, but may require glia-derived cholesterol to form numerous and efficient synapses. During synapse maturation, soluble and contact-dependent factors from glia may influence the composition of the postsynaptic density. Finally, synaptic connections appear to require glia to support their structural stability. Given the new evidence, it may be time now to acknowledge glia as a source for synaptogenesis-promoting signals. Scrutinizing the molecular mechanisms underlying this new function of glia and testing its relevance in vivo may help to understand how synapses develop and why they degenerate under pathological conditions.

Microglia and astrocytes may participate in the shaping of visual callosal projections during postnatal development

In the adult cat, axons running through the corpus callosum interconnect the border between the visual cortical areas 17 and 18 (A17 and A18) of both hemispheres. This specific pattern emerges during postnatal development, under normal viewing conditions (NR), from the elimination of initially exuberant callosal projections. In contrast, if the postnatal visual experience is monocular from birth (MD), juvenile callosal projections are stabilised throughout A17 and A18. The present study aimed at using such a model in vivo to find indications of a contribution of glial cells in the shaping of projections in the developing CNS through interactions with neurones, both in normal and pathological conditions. As a first stage, the distribution and the morphology of microglial cells and astrocytes were investigated from 2 weeks to adulthood. Microglial cells, stained with isolectin-B4, were clustered in the white matter below A17 and A18. Until one month, these clustered cells displayed an ameboid morphology in NR group, while they were more ramified in MD animals. Their phenotype thus depends on the postnatal visual experience, which indicates that microglial cells may interact with axons of visual neurones. It also suggests that they may differentially contribute to the elimination and the stabilisation of juvenile exuberant callosal fibres in NR and MD animals respectively. Beyond one month, microglial cells were very ramified in both experimental groups. Astrocytes were labelled with a GFAP-antibody. The distributions of connexins 43 (Cx43) and 30 (Cx30), the main proteic components of gap junction channels in astrocytes, were also investigated using specific antibodies. Both in NR and MD groups, until 1 month, GFAP-positive astrocytes and Cx43 were mainly localised within the subcortical white matter. Then GFAP, Cx43 and Cx30 stainings progressively appeared within the cortex, throughout A17 and A18 but with a differential laminar expression according to the age. Thus, the distributions of both astrocytes and connexins changed with age; however, the monocular occlusion had no visible effect. This suggests that astrocytes may contribute to the postnatal development of neuronal projections to the primary visual cortex, including visual callosal projections.

Synapse-glia interactions at the mammalian neuromuscular junction

Perisynaptic Schwann cells (PSCs) play critical roles in regulating and stabilizing nerve terminals at the mammalian neuromuscular junction (NMJ). However, although these functions are likely regulated by the synaptic properties, the interactions of PSCs with the synaptic elements are not known. Therefore, our goal was to study the interactions between mammalian PSCs in situ and the presynaptic terminals using changes in intracellular Ca(2+) as an indicator of cell activity. Motor nerve stimulation induced an increase in intracellular Ca(2+) in PSCs, and this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of acetylcholine induced Ca(2+) responses that were blocked by the muscarinic antagonist atropine and mimicked by the muscarinic agonist muscarine. The nicotinic antagonist alpha-bungarotoxin had no effect on Ca(2+) responses induced by acetylcholine. Local application of the cotransmitter ATP induced Ca(2+) responses that were unaffected by the P2 antagonist suramin, whereas local application of adenosine induced Ca(2+) responses that were greatly reduced by the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). However, the presence of the A1 antagonist in the perfusate did not block responses induced by ATP. Ca(2+) responses evoked by stimulation of the motor nerve were reduced in the presence of CPT, whereas atropine almost completely abolished them. Ca(2+) responses were further reduced when both antagonists were present simultaneously. Hence, PSCs at the mammalian NMJ respond to the release of neurotransmitter induced by stimulation of the motor nerve through the activation of muscarinic and adenosine A1 receptors.

Differential regulation of transmitter release by presynaptic and glial Ca2+ internal stores at the neuromuscular synapse

The differential regulation of synaptic transmission by internal Ca(2+) stores of presynaptic terminals and perisynaptic Schwann cells (PSCs) was studied at the frog neuromuscular junction. Thapsigargin (tg), an inhibitor of Ca(2+)-ATPase pumps of internal stores, caused a transient Ca(2+) elevation in PSCs, whereas it had no effect on Ca(2+) stores of presynaptic terminals at rest. Tg prolonged presynaptic Ca(2+) responses evoked by single action potentials with no detectable increase in the resting Ca(2+) level in nerve terminals. However, Ca(2+) accumulation was observed during high frequency stimulation. Tg induced a rapid rise in endplate potential (EPP) amplitude, accompanied by a delayed and transient increase. The effects appeared presynaptic, as suggested by the lack of effects of tg on the amplitude and time course of miniature EPPs (MEPPs). However, MEPP frequency was increased when preparations were stimulated tonically (0.2 Hz). The delayed and transient increase in EPP amplitude was occluded by injections of the Ca(2+) chelator BAPTA into PSCs before tg application, whereas a rise in intracellular Ca(2+) in PSCs induced by inositol 1,4,5-triphosphate (IP(3)) injections potentiated transmitter release. Furthermore, increased Ca(2+) buffering capacity after BAPTA injection in PSCs resulted in a more pronounced synaptic depression induced by high frequency stimulation of the motor nerve (10 Hz/80 sec). It is concluded that presynaptic Ca(2+) stores act as a Ca(2+) clearance mechanism to limit the duration of transmitter release, whereas Ca(2+) release from glial stores initiates Ca(2+)-dependent potentiation of synaptic transmission.