Molecular organization of cerebellar glutamate synapses

Circadian Clock Proteins and Melatonin Receptors in Neurons and Glia of the Sapajus apella Cerebellum

Oscillations of brain proteins in circadian rhythms are important for determining several cellular and physiological processes in anticipation of daily and seasonal environmental rhythms. In addition to the suprachiasmatic nucleus, the primary central oscillator, the cerebellum shows oscillations in gene and protein expression. The variety of local circuit rhythms that the cerebellar cortex contains influences functions such as motivational processes, regulation of feeding, food anticipation, language, and working memory. The molecular basis of the cerebellar oscillator has been demonstrated by "clock gene" expression within cells of the cerebellar layers. Genetic and epidemiological evidence suggests that disruption of circadian rhythms in humans can lead to many pathological conditions. Despite this importance, data about clock gene and protein expression in the cerebellum of diurnal (day-active) species, specifically primates, is currently poorly explored, mainly in regard to cellular identity, as well as the relationship with other molecules also involved in cerebellar functions. These studies could contribute to clarification of the possible mechanisms behind cerebellar rhythmicity. Considering that calcium binding proteins (CaBPs) play crucial roles in preserving and modulating cerebellar functions and that clock gene expression can be controlled by afferent projections or paracrine circadian signals such as the hormone melatonin, the present study aimed to describe cellular identities, distribution patterns and day/night expression changes in PER1, PER2, CaBPs, and MT₁ and MT₂ melatonin receptors in the cerebellar cortex of a diurnal primate using conventional fluorescence and peroxidase-antiperoxidase immunocytochemical techniques. PER1 and PER2 immunoreactive (IR) cells were observed in the Purkinje cells of the cerebellum, and MT₁ and MT₂ receptors were localized around Purkinje cells in the Pj layer in Bergmann cells. This identity was confirmed by the S100β-IR of these cells. The highest expression of PER seen in the daytime analysis coincided with the highest expression of melatonin receptors. CaBPs showed day/night morphological and density changes in the cerebellar cortex. The presence of the same temporal variations in the expression of PER in the Pj neurons and in MT₁ and MT₂ receptors in Bergmann cells indicates a possible relation between these cells during the rhythmic processing of the cerebellum, in addition to the CaBP temporal morphological and density changes.

Microglia: a new frontier for synaptic plasticity, learning and memory, and neurodegenerative disease research

We focus on emerging roles for microglia in synaptic plasticity, cognition and disease. We outline evidence that ramified microglia, traditionally thought to be functionally "resting" (i.e. quiescent) in the normal brain, in fact are highly dynamic and plastic. Ramified microglia continually and rapidly extend processes, contact synapses in an activity and experience dependent manner, and play a functionally dynamic role in synaptic plasticity, possibly through release of cytokines and growth factors. Ramified microglial also contribute to structural plasticity through the elimination of synapses via phagocytic mechanisms, which is necessary for normal cognition. Microglia have numerous mechanisms to monitor neuronal activity and numerous mechanisms also exist to prevent them transitioning to an activated state, which involves retraction of their surveying processes. Based on the evidence, we suggest that maintaining the ramified state of microglia is essential for normal synaptic and structural plasticity that supports cognition. Further, we propose that change of their ramified morphology and function, as occurs in inflammation associated with numerous neurological disorders such as Alzheimer's and Parkinson's disease, disrupts their intricate and essential synaptic functions. In turn altered microglia function could cause synaptic dysfunction and excess synapse loss early in disease, initiating a range of pathologies that follow. We conclude that the future of learning and memory research depends on an understanding of the role of non-neuronal cells and that this should include using sophisticated molecular, cellular, physiological and behavioural approaches combined with imaging to causally link the role of microglia to brain function and disease including Alzheimer's and Parkinson's disease and other neuropsychiatric disorders.

GLAST/EAAT1-induced glutamine release via SNAT3 in Bergmann glial cells: evidence of a functional and physical coupling

Glutamate, the major excitatory transmitter in the vertebrate brain, is removed from the synaptic cleft by a family of sodium-dependent glutamate transporters profusely expressed in glial cells. Once internalized, it is metabolized by glutamine synthetase to glutamine and released to the synaptic space through sodium-dependent neutral amino acid carriers of the N System (SNAT3/slc38a3/SN1, SNAT5/slc38a5/SN2). Glutamine is then taken up by neurons completing the so-called glutamate/glutamine shuttle. Despite of the fact that this coupling was described decades ago, it is only recently that the biochemical framework of this shuttle has begun to be elucidated. Using the established model of cultured cerebellar Bergmann glia cells, we sought to characterize the functional and physical coupling of glutamate uptake and glutamine release. A time-dependent Na⁺-dependent glutamate/aspartate transporter/EAAT1-induced System N-mediated glutamine release could be demonstrated. Furthermore, D-aspartate, a specific glutamate transporter ligand, was capable of enhancing the co-immunoprecipitation of Na⁺-dependent glutamate/aspartate transporter and Na⁺-dependent neutral amino acid transporter 3, whereas glutamine tended to reduce this association. Our results suggest that glial cells surrounding glutamatergic synapses may act as sensors of neuron-derived glutamate through their contribution to the neurotransmitter turnover.

Identification of novel glial genes by single-cell transcriptional profiling of Bergmann glial cells from mouse cerebellum

Bergmann glial cells play critical roles in the structure and function of the cerebellum. During development, their radial processes serve as guides for migrating granule neurons and their terminal endfeet tile to form the glia limitans. As the cerebellum matures, Bergmann glia perform important roles in synaptic transmission and synapse maintenance, while continuing to serve as essential structural elements. Despite growing evidence of the diverse functions of Bergmann glia, the molecular mechanisms that mediate these functions have remained largely unknown. As a step toward identifying the molecular repertoire underlying Bergmann glial function, here we examine global gene expression in individual Bergmann glia from developing (P6) and mature (P30) mouse cerebellum. When we select for developmentally regulated genes, we find that transcription factors and ribosomal genes are particularly enriched at P6 relative to P30; whereas synapse associated molecules are enriched at P30 relative to P6. We also analyze genes expressed at high levels at both ages. In all these categories, we find genes that were not previously known to be expressed in glial cells, and discuss novel functions some of these genes may potentially play in Bergmann glia. We also show that Bergmann glia, even in the adult, express a large set of genes thought to be specific to stem cells, suggesting that Bergmann glia may retain neural precursor potential as has been proposed. Finally, we highlight several genes that in the cerebellum are expressed in Bergmann glia but not astrocytes, and may therefore serve as new, specific markers for Bergmann glia.

Brief bursts of parallel fiber activity trigger calcium signals in bergmann glia

Changes in synaptic strength during ongoing activity are often mediated by neuromodulators. At the synapse between cerebellar granule cell parallel fibers (PFs) and Purkinje cells (PCs), brief bursts of stimuli can evoke endocannabinoid release from PCs and GABA release from interneurons that both inhibit transmission by activating presynaptic G-protein-coupled receptors. Studies in several brain regions suggest that synaptic activity can also evoke calcium signals in astrocytes, thereby causing them to release a transmitter, which acts presynaptically to regulate neurotransmitter release. In the cerebellum, Bergmann glia cells (BGs) are intimately associated with PF synapses. However, the mechanisms leading to calcium signals in BGs under physiological conditions and the role of BGs in regulating ongoing synaptic transmission are poorly understood. We found that brief bursts of PF activity evoke calcium signals in BGs that are triggered by the activation of metabotropic glutamate receptor 1 and purinergic receptors and mediated by calcium release from IP3-sensitive internal stores. We found no evidence for modulation of release from PFs mediated by BGs, even when endocannabinoid- and GABA-mediated presynaptic modulation was prominent. Thus, despite the fact that PF activation can reliably evoke calcium transients within BGs, it appears that BGs do not regulate synaptic transmission on the time scale of seconds to tens of seconds. Instead, endocannabinoid release from PCs and GABA release from molecular layer interneurons provide the primary means of feedback that dynamically regulate release from PF synapses.

Glutamate transporters GLAST and EAAT4 regulate postischemic Purkinje cell death: An in vivo study using a cardiac arrest model in mice lacking GLAST or EAAT4

Cerebellar Purkinje cells represent a group of neurons highly vulnerable to ischemia. Excitotoxicity is thought to be an important pathophysiological mechanism in Purkinje cell death following ischemia. The glutamate transporter is the only mechanism for the removal of glutamate from the extracellular fluid in the brain. Therefore, glutamate transporters are believed to play a critical role in protecting Purkinje cells from ischemia-induced damage. Two distinct glutamate transporters, GLAST and EAAT4, are expressed most abundantly in the cerebellar cortex. GLAST is expressed in Bergmann glia, whereas EAAT4 is concentrated in the perisynaptic regions of Purkinje cell spines. However, the in vivo functional significance of these glial and neuronal glutamate transporters in postischemic Purkinje cell death is largely unknown. To clarify the role of these glutamate transporters in the protection of Purkinje cells after global brain ischemia, we evaluated Purkinje cell loss after cardiac arrest in mice lacking GLAST or EAAT4. We found that Purkinje cells with low EAAT4 expression were selectively lost after cardiac arrest in GLAST mutant mice. This result demonstrates that GLAST plays a role in preventing excitotoxic cerebellar damage after ischemia in concert with EAAT4.

Astrocytic exocytosis vesicles and glutamate: A high-resolution immunofluorescence study

Physiological evidence has demonstrated that cultured astrocytes can release glutamate via Ca2+-dependent mechanisms. Also, glutamate released from astrocytes in the hippocampal slice interferes with synaptic neurotransmission. Since these observations suggest vesicular glutamate release from astrocytes, the presence of glutamate-containing exocytosis vesicles was investigated. We applied immunofluorescence techniques combined with high-performance deconvolution microscopy, which yields a resolution of <200 nm and permits evaluation of double labeling in individual vesicles. Using a well-characterized anti-glutamate antiserum and parameters minimizing fixative-induced autofluorescence, glutamate-immunoreactive (ir) puncta were found all over the astrocyte but were conspicuously dense at the cell boundary and in filopodia. Images were very similar with antibodies against vesicular glutamate transporters (vGluT1 and vGluT2). Labeling for the exocytosis markers rab3, synaptophysin, or synaptobrevin was also punctate, particularly dense at the cell boundary, but disappearing toward the perinuclear region. Sections of the cell boundary were delineated by rab3 immunoreactivity. In double-labeled cells, vesicular colocalization of glutamate and any of the exocytosis markers was frequent in filopodia and at the cell boundary. Within the cell, single-labeled glutamate-ir vesicles prevailed; double-labeled vesicles were infrequently present. By resolving single vesicles, in cultured astrocytes we visualize glutamate-containing vesicles, vesicles displaying vGluT1 or vGluT2, and exocytosis vesicles displaying glutamate-ir. This may provide the morphological correlate of Ca2+-dependent glutamate release from astrocytes, possibly occurring at defined sections of the cell membrane and at filopodia. However, since vGluTs and exocytosis markers are classically restricted to nerve terminals in the CNS, glutamate release from astrocytes in the CNS remains to be studied.

Increased glutamate, GABA and glutamine in lateral geniculate nucleus but not in medial geniculate nucleus caused by visual attention to novelty

This study is concerned with cortico-thalamic neural mechanisms underlying attentional phenomena. Previous results from this laboratory demonstrated that the visual sector of the GABAergic thalamic reticular nucleus is selectively c-fos activated in rats that are naturally paying attention to features of a novel-complex environment, and that this activation is dependent on top-down glutamatergic inputs from the primary visual cortex. By contrast, the acoustic sector of the thalamic reticular nucleus is not activated despite noise generated by exploration and c-fos activation of brainstem acoustic centers (e.g. dorsal cochlear nucleus, inferior colliculus). A prediction of these results is that the levels of the neurotransmitters glutamate and GABA, and the glutamate-related amino acid glutamine, will be increased in the lateral geniculate nucleus (LGN), but not in the medial geniculate nucleus (MGN) of rats that explore a novel-complex environment in comparison to levels of these amino acids in control rats. By means of neurochemical analysis of these amino acids (HPLC) the results of this study confirmed this prediction. The results are consistent with the previously proposed 'focal attention' hypothesis postulating that a focus of attention in the primary visual cortex generates top-down center-surround facilitatory-inhibitory effects on geniculocortical transmission via corticoreticulogeniculate pathways. The results also supports the notion that a main function of corticothalamic pathways to relay thalamic nuclei is attention-dependent modulation of thalamocortical transmission.

TRPC1 and TRPC5 form a novel cation channel in mammalian brain

TRP proteins are cation channels responding to receptor-dependent activation of phospholipase C. Mammalian (TRPC) channels can form hetero-oligomeric channels in vitro, but native TRPC channel complexes have not been identified to date. We demonstrate here that TRPC1 and TRPC5 are subunits of a heteromeric neuronal channel. Both TRPC proteins have overlapping distributions in the hippocampus. Coexpression of TRPC1 and TRPC5 in HEK293 cells resulted in a novel nonselective cation channel with a voltage dependence similar to NMDA receptor channels, but unlike that of any reported TRPC channel. TRPC1/TRPC5 heteromers were activated by G(q)-coupled receptors but not by depletion of intracellular Ca(2+) stores. In contrast to the more common view of the TRP family as comprising store-operated channels, we propose that many TRPC heteromers form diverse receptor-regulated nonselective cation channels in the mammalian brain.

Glutamate stimulates ascorbate transport by astrocytes

The concentrations of glutamate and ascorbate in brain extracellular fluid increase following seizure activity, trauma and ischemia. Extracellular ascorbate concentration also rises following intracerebral glutamate injection. We hypothesized that glutamate triggers the release of ascorbate from astrocytes. We observed in primary cultures of rat cerebral astrocytes that glutamate increased ascorbate efflux significantly within 30 min. The half-maximal effective concentration of glutamate was 180+/-30 microM. Glutamate-stimulated efflux of ascorbate was attenuated by hypertonic media. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid inhibited both Na(+)-dependent glutamate uptake and ascorbate efflux. Two other inhibitors of volume-sensitive organic anion channels (1, 9-dideoxyforskolin and 5-nitro-2-(3-phenylpropylamino) benzoic acid) did not slow glutamate uptake but prevented stimulation of ascorbate efflux. Glutamate also stimulated the uptake of ascorbate by ascorbate-depleted astrocytes. In contrast, glutamate uptake was not affected by intracellular ascorbate, thus ruling out a putative glutamate-ascorbate heteroexchange mechanism. These results are consistent with activation by glutamate of ascorbate-permeant channels in astrocytes.

Ion channel genes and human neurological disease: recent progress, prospects, and challenges

What do epilepsy, migraine headache, deafness, episodic ataxia, periodic paralysis, malignant hyperthermia, and generalized myotonia have in common? These human neurological disorders can be caused by mutations in genes for ion channels. Many of the channel diseases are "paroxysmal disorders" whose principal symptoms occur intermittently in individuals who otherwise may be healthy and active. Some of the ion channels that cause human neurological disease are old acquaintances previously cloned and extensively studied by channel specialists. In other cases, however, disease-gene hunts have led the way to the identification of new channel genes. Progress in the study of ion channels has made it possible to analyze the effects of human neurological disease-causing channel mutations at the level of the single channel, the subcellular domain, the neuronal network, and the behaving organism.