Functions of glutamate transporters in cerebellar Purkinje cell synapses

Glutamate transporters play a critical role in the maintenance of low extracellular concentrations of glutamate, which prevents the overactivation of post-synaptic glutamate receptors. Four distinct glutamate transporters, GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3 and EAAT4, are distributed in the molecular layer of the cerebellum, especially near glutamatergic synapses in Purkinje cells (PCs). This review summarizes the current knowledge about the differential roles of these transporters at excitatory synapses of PCs. Data come predominantly from electrophysiological experiments in mutant mice that are deficient in each of these transporter genes. GLAST expressed in Bergmann glia contributes to the clearing of the majority of glutamate that floods out of the synaptic cleft immediately after transmitter release from the climbing fibre (CF) and parallel fibre (PF) terminals. It is indispensable to maintain a one-to-one relationship in synaptic transmission at the CF synapses by preventing transcellular glutamate spillover. GLT-1 plays a similar but minor role in the uptake of glutamate as GLAST. Although the loss of neither GLAST nor GLT-1 affects cerebellar morphology, the deletion of both GLAST and GLT-1 genes causes the death of the mutant animal and hinders the folium formation of the cerebellum. EAAT4 removes the low concentrations of glutamate that escape from uptake by glial transporters, preventing the transmitter from spilling over into neighbouring synapses. It also regulates the activation of metabotropic glutamate receptor 1 (mGluR1) in perisynaptic regions at PF synapses, which in turn affects mGluR1-mediated events including slow EPSCs and long-term depression. No change in synaptic function is detected in mice that are deficient in EAAC1.

Functional crosstalk between cell-surface and intracellular channels mediated by junctophilins essential for neuronal functions

Junctophilins (JPs) contribute to the formation of junctional membrane complexes between the plasma membrane and the endoplasmic/sarcoplasmic reticulum, and provide a structural platform for channel communication during excitation-contraction coupling in muscle cells. In the brain, two neuronal JP subtypes are widely expressed in neurons. Recent studies have defined the essential role of neural JPs in the communication between cell-surface and intracellular channels, which modulates the excitability and synaptic plasticity of neurons in the cerebellum and hippocampus.

Unilateral expansion of a narrow mandibular dental arch combined with bimaxillary osteotomies in a patient with hypoglossia

A 23-year-old female with hypoglossia, who had a narrow mandibular dental arch, was treated using the gradual expansion technique. Three lower incisors were missing and the right molar occlusion showed a scissor bite. Her speech was acceptable. Gradual unilateral expansion of the mandibular alveolar bone was performed. Orthodontic tooth alignment was performed prior to surgical treatment. A tooth-borne expander was devised using a hyrax-type screw to move the inclined right alveolar bone into an upright position. Alveolar bone osteotomies were performed under general anesthesia and the expander was placed in the mandibular dental arch. After a 5-day latency period, the screw was activated for 21 days. After expansion, the width of the mandibular dental arch increased by 10mm at the first molar region and the right molars were moved to an upright position. After a consolidation period of 7 days, simultaneous two-jaw surgery that combined Le Fort I osteotomy and intraoral vertical ramus osteotomies was performed to obtain a stable occlusion. After post-surgical orthodontic and prosthodontic treatment, her occlusion improved without deterioration of her speech. The results indicate that this technique is useful for unilateral expansion of distorted mandibular alveolar process.

Temporal switching and cell-to-cell variability in Ca2+ release activity in mammalian cells

Genetically identical cells in a uniform external environment can exhibit different phenotypes, which are often masked by conventional measurements that average over cell populations. Although most studies on this topic have used microorganisms, differentiated mammalian cells have rarely been explored. Here, we report that only approximately 40% of clonal human embryonic kidney 293 cells respond with an intracellular Ca²⁺ increase when ryanodine receptor Ca²⁺ release channels in the endoplasmic reticulum are maximally activated by caffeine. On the other hand, the expression levels of ryanodine receptor showed a unimodal distribution. We showed that the difference in the caffeine sensitivity depends on a critical balance between Ca²⁺ release and Ca²⁺ uptake activities, which is amplified by the regenerative nature of the Ca²⁺ release mechanism. Furthermore, individual cells switched between the caffeine-sensitive and caffeine-insensitive states with an average transition time of approximately 65 h, suggestive of temporal fluctuation in endogenous protein expression levels associated with caffeine response. These results suggest the significance of regenerative mechanisms that amplify protein expression noise and induce cell-to-cell phenotypic variation in mammalian cells.

Involvement of protein-tyrosine phosphatase PTPMEG in motor learning and cerebellar long-term depression

Although protein-tyrosine phosphorylation is important for hippocampus-dependent learning, its role in cerebellum-dependent learning remains unclear. We previously found that PTPMEG, a cytoplasmic protein-tyrosine phosphatase expressed in Purkinje cells (PCs), bound to the carboxyl-terminus of the glutamate receptor delta2 via the postsynaptic density-95/discs-large/ZO-1 domain of PTPMEG. In the present study, we generated PTPMEG-knockout (KO) mice, and addressed whether PTPMEG is involved in cerebellar plasticity and cerebellum-dependent learning. The structure of the cerebellum in PTPMEG-KO mice appeared grossly normal. However, we found that PTPMEG-KO mice showed severe impairment in the accelerated rotarod test. These mice also exhibited impairment in rapid acquisition of the cerebellum-dependent delay eyeblink conditioning, in which conditioned stimulus (450-ms tone) and unconditioned stimulus (100-ms periorbital electrical shock) were co-terminated. Moreover, long-term depression at parallel fiber-PC synapses was significantly attenuated in these mice. Developmental elimination of surplus climbing fibers and the physiological properties of excitatory synaptic inputs to PCs appeared normal in PTPMEG-KO mice. These results suggest that tyrosine dephosphorylation events regulated by PTPMEG are important for both motor learning and cerebellar synaptic plasticity.

Abnormal features in mutant cerebellar Purkinje cells lacking junctophilins

Junctional membrane complexes (JMCs) generated by junctophilins are required for Ca(2+)-mediated communication between cell-surface and intracellular channels in excitable cells. Knockout mice lacking neural junctophilins (JP-DKO) show severe motor defects and irregular cerebellar plasticity due to abolished channel crosstalk in Purkinje cells (PCs). To precisely understand aberrations in JP-DKO mice, we further analyzed the mutant PCs. During the induction of cerebellar plasticity via electrical stimuli, JP-DKO PCs showed insufficient depolarizing responses. Immunochemistry detected mild impairment in synaptic maturation and hyperphosphorylation of protein kinase Cgamma in JP-DKO PCs. Moreover, gene expression was slightly altered in the JP-DKO cerebellum. Therefore, the mutant PCs bear marginal but widespread abnormalities, all of which likely cause cerebellar motor defects in JP-DKO mice.

Postsynaptic GABAB receptor signalling enhances LTD in mouse cerebellar Purkinje cells

Long-term depression (LTD) of excitatory transmission at cerebellar parallel fibre-Purkinje cell synapses is a form of synaptic plasticity crucial for cerebellar motor learning. Around the postsynaptic membrane of these synapses, B-type gamma-aminobutyric acid receptor (GABABR), a Gi/o protein-coupled receptor for the inhibitory transmitter GABA is concentrated and closely associated with type-1 metabotropic glutamate receptors (mGluR1) whose signalling is a key factor for inducing LTD. We found that in cultured Purkinje cells, GABABR activation enhanced LTD of a glutamate-evoked current (LTDglu), increasing the magnitude of depression. It has been reported that parallel fibre-Purkinje cell synapses receive a micromolar level of GABA spilled over from the synaptic terminals of the neighbouring GABAergic interneurons. This level of GABA was able to enhance LTDglu. Our pharmacological analyses revealed that the betagamma subunits but not the alpha subunit of Gi/o protein mediated GABABR-mediated LTDglu enhancement. Gi/o protein activation was sufficient to enhance LTDglu. In this respect, LTDglu enhancement is clearly distinguished from the previously reported GABABR-mediated augmentation of an mGluR1-coupled slow excitatory postsynaptic potential. Baclofen application for only the induction period of LTDglu was sufficient to enhance LTDglu, suggesting that GABABR signalling may modulate mechanisms underlying LTDglu induction. Baclofen augmented mGluR1-coupled Ca2+ release from the intracellular stores in a Gi/o protein-dependent manner. Therefore, GABABR-mediated LTDglu enhancement is likely to result from augmentation of mGluR1 signalling. Furthermore, pharmacological inhibition of GABABR reduced the magnitude of LTD at parallel fibre-Purkinje cell synapses in cerebellar slices. These findings demonstrate a novel mechanism that would facilitate cerebellar motor learning.

Regulation of neurite growth by spontaneous Ca2+ oscillations in astrocytes

Astrocytes play a pivotal role in the regulation of neurite growth, but the intracellular signaling mechanism in astrocytes that mediates this regulation remains unclarified. We studied the relationship between spontaneous Ca(2+) oscillations in astrocytes and the astrocyte-mediated neurite growth. We generated Ca(2+) signal-deficient astrocytes in which spontaneous Ca(2+) oscillations were abolished by a chronic inhibition of IP(3) signaling. When hippocampal neurons were cultured on a monolayer of Ca(2+) signal-deficient astrocytes, the growth of dendrites and axons was inhibited. Time-lapse imaging of the advancement of axonal growth cones indicated the involvement of membrane-bound molecules for this inhibition. Among six candidate membrane-bound molecules that may modulate neuronal growth, N-cadherin was downregulated in Ca(2+) signal-deficient astrocytes. Although a blocking antibody to N-cadherin suppressed the axonal growth on control astrocytes, extrinsic N-cadherin expression rescued the suppressed axonal growth on Ca(2+) signal-deficient astrocytes. These findings suggest that spontaneous Ca(2+) oscillations regulate the astrocytic function to promote neurite growth by maintaining the expression of specific growth-enhancing proteins on their surface, and that N-cadherin is one of such molecules.

Optical glutamate sensor for spatiotemporal analysis of synaptic transmission

Imaging neurotransmission is expected to greatly improve our understanding of the mechanisms and regulations of synaptic transmission. Aiming at imaging glutamate, a major excitatory neurotransmitter in the CNS, we developed a novel optical glutamate probe, which consists of a ligand-binding domain of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor glutamate receptor GluR2 subunit and a small molecule fluorescent dye. We expected that such fluorescent conjugates might report the microenvironmental changes upon protein conformational changes elicited by glutamate binding. After more than 100 conjugates were tested, we finally obtained a conjugate named E (glutamate) optical sensor (EOS), which showed maximally 37% change in fluorescence intensity upon binding of glutamate with a dissociation constant of 148 nm. By immobilizing EOS on the cell surface of hippocampal neuronal culture preparations, we pursued in situ spatial mapping of synaptically released glutamate following presynaptic firing. Results showed that a single firing was sufficient to obtain high-resolution images of glutamate release, indicating the remarkable sensitivity of this technique. Furthermore, we monitored the time course of changes in presynaptic activity induced by phorbol ester and found heterogeneity in presynaptic modulation. These results indicate that EOS can be generally applicable to evaluation of presynaptic modulation and plasticity. This EOS-based glutamate imaging method is useful to address numerous fundamental issues about glutamatergic neurotransmission in the CNS.

Induction of cerebellar long-term depression requires activation of calcineurin in Purkinje cells

Cerebellar long-term depression (LTD) is an activity-dependent depression of synaptic transmission from parallel fibers to Purkinje cells underlying certain forms of motor learning. LTD is induced by the conjunctive stimulation of parallel fibers and climbing fibers, both of which supply excitatory inputs to Purkinje cells. The conjunctive stimulation induces a large increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in Purkinje cells. Although the increase in [Ca(2+)](i) is essential for LTD induction, the downstream signal transduction mechanism remains elusive. In this study, we show that LTD induction requires the activation of the Ca(2+)/calmodulin-dependent protein phosphatase 2B calcineurin. In acute cerebellar slices of mice, the LTD amplitude was significantly reduced in the presence of calcineurin inhibitors (cyclosporin A or FK506), whereas the basic electrophysiological properties of the parallel fiber-Purkinje cell synaptic transmission remained constant. Furthermore, a calcineurin autoinhibitory peptide perfused into Purkinje cells completely blocked LTD induction. On the other hand, microcystin LR, an inhibitor of protein phosphatase 1 and 2A, did not affect the induction of LTD. These results indicate that calcineurin activation is essential for LTD induction downstream of the conjunctive-stimulation-induced Ca(2+) signal in Purkinje cells.

Junctophilin-mediated channel crosstalk essential for cerebellar synaptic plasticity

Functional crosstalk between cell-surface and intracellular ion channels plays important roles in excitable cells and is structurally supported by junctophilins (JPs) in muscle cells. Here, we report a novel form of channel crosstalk in cerebellar Purkinje cells (PCs). The generation of slow afterhyperpolarization (sAHP) following complex spikes in PCs required ryanodine receptor (RyR)-mediated Ca(2+)-induced Ca(2+) release and the subsequent opening of small-conductance Ca(2+)-activated K(+) (SK) channels in somatodendritic regions. Despite the normal expression levels of these channels, sAHP was abolished in PCs from mutant mice lacking neural JP subtypes (JP-DKO), and this defect was restored by exogenously expressing JPs or enhancing SK channel activation. The stimulation paradigm for inducing long-term depression (LTD) at parallel fiber-PC synapses adversely established long-term potentiation in the JP-DKO cerebellum, primarily due to the sAHP deficiency. Furthermore, JP-DKO mice exhibited impairments of motor coordination and learning, although normal cerebellar histology was retained. Therefore, JPs support the Ca(2+)-mediated communication between voltage-gated Ca(2+) channels, RyRs and SK channels, which modulates the excitability of PCs and is fundamental to cerebellar LTD and motor functions.

Coupling of STIM1 to store-operated Ca2+ entry through its constitutive and inducible movement in the endoplasmic reticulum

Depletion of intracellular calcium (Ca(2+)) stores induces store-operated Ca(2+) (SOC) entry across the plasma membrane (PM). STIM1, a putative Ca(2+) sensor in the endoplasmic reticulum (ER), has been recently shown to be necessary for SOC channel activation. Here we show that STIM1 dynamically moves in tubulovesicular shape on the ER and its subcompartment in resting living cells, whereas, upon Ca(2+) store depletion, it is rapidly redistributed into discrete puncta that are located underneath, but not inserted into the PM. Normal constitutive movement of STIM1 is mediated through the coiled-coil and Ser/Thr-rich C-terminal domains in the cytoplasmic region of STIM1, whereas subsequent inducible puncta formation further requires the sterile alpha motif domain protruding into the ER lumen. Each of these three domains (coiled-coil, Ser/Thr-rich, and sterile alpha motif) was essential for activating SOC channels. Hence, our findings based on structure-function experiments suggest that constitutive dynamic movement of STIM1 in the ER and its subcompartment is obligatory for subsequent depletion-dependent redistribution of STIM1 into puncta underneath the PM and activation of SOC channels.

Ca2+ lightning conveys cell-cell contact information inside the cells

Cells communicate with each other to form organized structures by cell-cell adhesion and cell-cell repulsion, but it remains to be clarified how cell-cell contact information is converted into intracellular signals. Here, we show that cells in contact with neighbouring cells generate local transient intracellular Ca(2+) signals (Ca(2+) lightning). Ca(2+) lightning was observed near cell-cell contact regions and was not observed in the central regions of cells or in solitary cells that were not in contact with other cells. We also show that Ca(2+) lightning is able to regulate cell-cell repulsion by means of PYK2, a Ca(2+)-activated protein tyrosine kinase, which induces focal adhesion disassembly in a Ca(2+)-dependent manner. These results show that cell-cell contact information might be transmitted by Ca(2+) lightning to regulate intracellular events.

Postsynaptic inositol 1,4,5-trisphosphate signaling maintains presynaptic function of parallel fiber-Purkinje cell synapses via BDNF

The maintenance of synaptic functions is essential for neuronal information processing, but cellular mechanisms that maintain synapses in the adult brain are not well understood. Here, we report an activity-dependent maintenance mechanism of parallel fiber (PF)-Purkinje cell (PC) synapses in the cerebellum. When postsynaptic metabotropic glutamate receptor (mGluR) or inositol 1,4,5-trisphosphate (IP(3)) signaling was chronically inhibited in vivo, PF-PC synaptic strength decreased because of a decreased transmitter release probability. The same effects were observed when PF activity was inhibited in vivo by the suppression of NMDA receptor-mediated inputs to granule cells. PF-PC synaptic strength similarly decreased after the in vivo application of an antibody against brain-derived neurotrophic factor (BDNF). Furthermore, the weakening of synaptic connection caused by the blockade of mGluR-IP(3) signaling was reversed by the in vivo application of BDNF. These results indicate that a signaling cascade comprising PF activity, postsynaptic mGluR-IP(3) signaling and subsequent BDNF signaling maintains presynaptic functions in the mature cerebellum.

Ca2+ shuttling between endoplasmic reticulum and mitochondria underlying Ca2+ oscillations

Although many cell functions are regulated by Ca(2+) oscillations induced by a cyclic release of Ca(2+) from intracellular Ca(2+) stores, the pacemaker mechanism of Ca(2+) oscillations remains to be explained. Using green fluorescent protein-based Ca(2+) indicators that are targeted to intracellular Ca(2+) stores, the endoplasmic reticulum (ER) and mitochondria, we found that Ca(2+) shuttles between the ER and mitochondria in phase with Ca(2+) oscillations. Following agonist stimulation, Ca(2+) release from the ER generated the first Ca(2+) oscillation and loaded mitochondria with Ca(2+). Before the second Ca(2+) oscillation, Ca(2+) release from the mitochondria by means of the Na(+)/Ca(2+) exchanger caused a gradual increase in cytoplasmic Ca(2+) concentration, inducing a regenerative ER Ca(2+) release, which generated the peak of Ca(2+) oscillation and partially reloaded the mitochondria. This sequence of events was repeated until mitochondrial Ca(2+) was depleted. Thus, Ca(2+) shuttling between the ER and mitochondria may have a pacemaker role in the generation of Ca(2+) oscillations.

Ca2+-Dependent Inositol 1,4,5-Trisphosphate and Nitric Oxide Signaling in Cerebellar Neurons

Intracellular Ca(2+) signals are important for the regulation of synaptic functions in the central nervous system. In this review, I summarize findings of our recent studies on upstream and downstream Ca(2+) signaling mechanisms in cerebellar synapses using novel molecular imaging methods. Inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release plays a pivotal role in central synapses. The visualization of IP(3) at fine dendrites of Purkinje cells (PCs) using a fluorescent IP(3) indicator showed that intracellular Ca(2+) concentration has a stimulatory effect on phospholipase C activity, which catalyzes IP(3) production. This indicates that metabotropic and ionotropic glutamate receptors collaborate to generate IP(3) signals. Using a novel nitric oxide (NO) indicator, the spatial distribution of NO signals originating from parallel fiber (PF) terminals was visualized. Our results show that the NO signal decays steeply with distance from the site of production in the cerebellum and is dependent on PF stimulation frequency in a biphasic manner. NO released from PF terminals generated a synapse-specific long-term potentiation of PF-PC synapse when PF was stimulated at certain frequencies. These imaging studies clarified new aspects of the regulatory mechanisms of synaptic functions.