Intracellular Ca2+ stores regulate muscarinic receptor stimulation of phospholipase C in cerebellar granule cells

Costimulation of AMPA and metabotropic glutamate receptors underlies phospholipase C activation by glutamate in hippocampus

Glutamate, a major neurotransmitter in the brain, activates ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs, respectively). The two types of glutamate receptors interact with each other, as exemplified by the modulation of iGluRs by mGluRs. However, the other way of interaction (i.e., modulation of mGluRs by iGluRs) has not received much attention. In this study, we found that group I mGluR-specific agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) alone is not sufficient to activate phospholipase C (PLC) in rat hippocampus, while glutamate robustly activates PLC. These results suggested that additional mechanisms provided by iGluRs are involved in group I mGluR-mediated PLC activation. A series of experiments demonstrated that glutamate-induced PLC activation is mediated by mGluR5 and is facilitated by local Ca(2+) signals that are induced by AMPA-mediated depolarization and L-type Ca(2+) channel activation. Finally, we found that PLC and L-type Ca(2+) channels are involved in hippocampal mGluR-dependent long-term depression (mGluR-LTD) induced by paired-pulse low-frequency stimulation, but not in DHPG-induced chemical LTD. Together, we propose that AMPA receptors initiate Ca(2+) influx via the L-type Ca(2+) channels that facilitate mGluR5-PLC signaling cascades, which underlie mGluR-LTD in rat hippocampus.

Nicotine-like effects of the neonicotinoid insecticides acetamiprid and imidacloprid on cerebellar neurons from neonatal rats

Background

Acetamiprid (ACE) and imidacloprid (IMI) belong to a new, widely used class of pesticide, the neonicotinoids. With similar chemical structures to nicotine, neonicotinoids also share agonist activity at nicotinic acetylcholine receptors (nAChRs). Although their toxicities against insects are well established, their precise effects on mammalian nAChRs remain to be elucidated. Because of the importance of nAChRs for mammalian brain function, especially brain development, detailed investigation of the neonicotinoids is needed to protect the health of human children. We aimed to determine the effects of neonicotinoids on the nAChRs of developing mammalian neurons and compare their effects with nicotine, a neurotoxin of brain development.

Methodology/Principal Findings

Primary cultures of cerebellar neurons from neonatal rats allow for examinations of the developmental neurotoxicity of chemicals because the various stages of neurodevelopment—including proliferation, migration, differentiation, and morphological and functional maturation—can be observed in vitro. Using these cultures, an excitatory Ca²⁺-influx assay was employed as an indicator of neural physiological activity. Significant excitatory Ca²⁺ influxes were evoked by ACE, IMI, and nicotine at concentrations greater than 1 µM in small neurons in cerebellar cultures that expressed the mRNA of the α3, α4, and α7 nAChR subunits. The firing patterns, proportion of excited neurons, and peak excitatory Ca²⁺ influxes induced by ACE and IMI showed differences from those induced by nicotine. However, ACE and IMI had greater effects on mammalian neurons than those previously reported in binding assay studies. Furthermore, the effects of the neonicotinoids were significantly inhibited by the nAChR antagonists mecamylamine, α-bungarotoxin, and dihydro-β-erythroidine.

Conclusions/Significance

This study is the first to show that ACE, IMI, and nicotine exert similar excitatory effects on mammalian nAChRs at concentrations greater than 1 µM. Therefore, the neonicotinoids may adversely affect human health, especially the developing brain.

Phospholipase Cbeta serves as a coincidence detector through its Ca2+ dependency for triggering retrograde endocannabinoid signal

Endocannabinoids mediate retrograde signal and modulate transmission efficacy at various central synapses. Although endocannabinoid release is induced by either depolarization or activation of G(q/11)-coupled receptors, it is markedly enhanced by the coincidence of depolarization and receptor activation. Here we report that this coincidence is detected by phospholipase Cbeta1 (PLCbeta1) in hippocampal neurons. By measuring cannabinoid-sensitive synaptic currents, we found that the receptor-driven endocannabinoid release was dependent on physiological levels of intracellular Ca(2+) concentration ([Ca(2+)](i)), and markedly enhanced by depolarization-induced [Ca(2+)](i) elevation. Furthermore, we measured PLC activity in intact neurons by using exogenous TRPC6 channel as a biosensor for the PLC product diacylglycerol and found that the receptor-driven PLC activation exhibited similar [Ca(2+)](i) dependence to that of endocannabinoid release. Neither endocannabinoid release nor PLC activation was induced by receptor activation in PLCbeta1 knockout mice. We therefore conclude that PLCbeta1 serves as a coincidence detector through its Ca(2+) dependency for endocannabinoid release in hippocampal neurons.

Disruption of Intraneuronal Divalent Cation Regulation by Methylmercury: Are Specific Targets Involved in Altered Neuronal Development and Cytotoxicity in Methylmercury Poisoning?

Methylmercury is an environmental contaminant which causes relatively specific degeneration of the granular layer of the cerebellum, despite its ability to bind thiol groups in proteins of all cell types. The mechanisms underlying the specific targeting of cells during MeHg poisoning may depend on specific receptors and other targets related to divalent cation homeostasis, particularly intracellular calcium (Ca(2+)(i) signaling. MeHg disrupts Ca(2+)(i) homeostasis in a number of neuronal models, including cerebellar granule cells in primary culture, and contributes to MeHg-induced cell death, impaired synaptic function and disruption of neuronal development. Interestingly, the disruption of [Ca(2+)](i) regulation occurs through specific pathways which affect Ca(2+) regulation by organelles, particularly mitochondria and the smooth endoplasmic reticulum (SER). Cholinergic pathways which affect [Ca(2+)](i) signaling also appear to be critical targets, particularly muscarinic acetylcholine (ACh) receptors which are linked to Ca(2+) release through inositol-1,4,5-triphosphate (IP(3)) receptors. [Ca(2+)](i) dysregulation may also underlie observed alterations in cerebellar neuron development through interaction with specific target(s) in the developing axon. In this review, we examine the hypothesis that MeHg affects specific targets to cause disruption of neuronal development and cell death.

The endoplasmic reticulum-related events in S-nitrosoglutathione-induced neurotoxicity in cerebellar granule cells

Nitric oxide (NO)-induced neurotoxicities are involved in the pathogenesis of several neurodegenerative disorders featured by misfolded proteins. However, the details remain to be investigated. In the present work, we focus on the study of some endoplasmic reticulum-related events in S-nitrosoglutathione (GSNO)-induced neurotoxicity in cerebellar granule cells (CGCs) and we demonstrated that: (1) GSNO caused sustained elevation of intracellular calcium; (2) This calcium elevation resulted partially from the depletion of endoplasmic reticulum (ER) calcium stores; (3) There was ER stress which was indicated by the incomplete splicing of X-box binding protein (XBP-1) mRNA by 8-polysialyltransferase (Pst1); (4) GSNO upregulated the expression of the proapoptotic growth arrest and DNA damage-inducible gene (Gadd153) and caused the depletion of intracellular glutathione (GSH) pools. At the same time, GSNO downregulated the expression of the antiapoptotic gene Sarco/endoplasmic reticulum calcium-ATPase (SERCA2b) in parallel with the downregulation of the antiapoptotic ER chaperones-glucose-regulated protein genes (Grp78 and Grp94). These effects indicate that ER is one of the NO targets in GSNO-induced neurotoxicity in cerebellar granule cells besides mitochondria.

The multiple actions of black widow spider toxins and their selective use in neurosecretion studies

The black widow spider venom contains several large protein toxins--latrotoxins--that are selectively targeted against different classes of animals: vertebrates, insects, and crustaceans. These toxins are synthesised as large precursors that undergo proteolytic processing and activation in the lumen of the venom gland. The mature latrotoxins demonstrate strong functional structure conservation and contain multiple ankyrin repeats, which mediate toxin oligomerisation. The three-dimensional structure has been determined for alpha-latrotoxin (alphaLTX), a representative venom component toxic to vertebrates. This reconstruction explains the mechanism of alphaLTX pore formation by showing that it forms tetrameric complexes, harbouring a central channel, and that it is able to insert into lipid membranes. All latrotoxins cause massive release of neurotransmitters from nerve terminals of respective animals after binding to specific neuronal receptors. A G protein-coupled receptor latrophilin and a single-transmembrane receptor neurexin have been identified as major high-affinity receptors for alphaLTX. Latrotoxins act by several Ca(2+)-dependent and -independent mechanisms based on pore formation and activation of receptors. Mutant recombinant alphaLTX that does not form pores has been used to dissect the multiple actions of this toxin. As a result, important insights have been gained into the receptor signalling and the role of intracellular Ca(2+) stores in the effect of alphaLTX.

NMDA‐receptor regulation of muscarinic‐receptor stimulated inositol 1,4,5‐trisphosphate production and protein kinase C activation in single cerebellar granule neurons

Inositol 1,4,5-trisphosphate (InsP(3)) production in single cerebellar granule neurons (CGNs) grown in culture was measured using the PH domain of phospholipase C delta1 tagged with enhanced green fluorescent protein (eGFP-PH(PLCdelta1)). These measurements were correlated with changes in intracellular free Ca2+ determined by single cell imaging. In control CGNs, intracellular Ca2+ stores appeared replete. However, the refilling state of these stores appeared dependent on the fluorophore used to measure Ca2+-release. Thus, methacholine (MCH), acting via muscarinic acetylcholine-receptors (mAchRs), mobilised intracellular Ca2+ in cells loaded with fluo-3 and fura-4f, but not fura-2. Confocal measurements of single CGNs expressing eGFP-PH(PLCdelta1) demonstrated that MCH stimulated a robust peak increase in InsP(3), which was followed by a sustained plateau phase of InsP(3) production. In contrast, glutamate-induced InsP(3) signals were weak or not detectable. MCH-stimulated InsP(3) production was reduced by chelation of intracellular Ca2+ with BAPTA, and emptying of intracellular stores with thapsigargin, indicated a positive feedback effect of Ca2+ mobilisation onto PLC activity. In CGNs, NMDA- and KCl-mediated Ca2+-entry significantly enhanced MCH-induced InsP(3) production. Furthermore, mAchR-mediated PLC activation appeared sensitive to the full dynamic range of intracellular Ca2+ increases stimulated by 100 microm NMDA. This dynamic regulation was also observed at the level of PKC activation indicated by an enhanced translocation of eGFP-tagged myristoylated alanine-rich C kinase substrate (MARCKS) protein in cells stimulated with MCH. Thus, NMDA-mediated Ca2+ influx and PLC activation may represent a coincident-detection system whereby ionotropic and metabotropic signals combine to stimulate InsP(3) production and PKC-mediated phosphorylation events in CGNs.

Opposed effects of lithium on the MEK-ERK pathway in neural cells: inhibition in astrocytes and stimulation in neurons by GSK3 independent mechanisms

Lithium is widely used in the treatment of bipolar disorder, but despite its proven therapeutic efficacy, the molecular mechanisms of action are not fully understood. The present study was undertaken to explore lithium effects of the MEK/ERK cascade of protein kinases in astrocytes and neurons. In asynchronously proliferating rat cortical astrocytes, lithium decreased time- and dose-dependently the phosphorylation of MEK and ERK, with 1 mM concentrations achieving 60 and 50% inhibition of ERK and MEK, respectively, after a 7-day exposure. Lithium also inhibited [3H]thymidine incorporation into DNA and induced a G2/M cell cycle arrest. In serum-deprived, quiescent astrocytes, pre-exposure to lithium resulted in the inhibition of cell cycle re-entry as stimulated by the mitogen endothelin-1: under this experimental setting, lithium did not affect the rapid, peak phosphorylation of MEK taking place after 3-5 min, but was effective in inhibiting the long-term, sustained phosphorylation of MEK. Lithium inhibition of the astrocyte MEK/ERK pathway was independent of inositol depletion. Further, compound SB216763 inhibited Tau phosphorylation at Ser396 and stabilized cytosolic beta-catenin, consistent with the inhibition of glycogen synthase kinase-3 beta (GSK-3 beta), but failed to reproduce lithium effects on MEK and ERK phosphorylation and cell cycle arrest. In cerebellar granule neurons, millimolar concentrations of lithium enhanced MEK and ERK phosphorylation in a concentration-dependent manner, again through an inositol and GSK-3 beta independent mechanism. These opposing effects in astrocytes and neurons make lithium treatment a promising strategy to favour neural repair and reduce reactive gliosis after traumatic injury.

Characterization of the metabotropic glutamate receptors mediating phospholipase C activation and calcium release in cerebellar granule cells: calcium-dependence of the phospholipase C response

In this study we have determined the metabotropic glutamate receptors (mGluRs) involved in the glutamate activation of phospholipase C (PLC) and Ca(2+) mobilization in cerebellar granule cells at 9 days in vitro; and studied the Ca(2+) modulation of the PLC response. Both PLC activation and Ca(2+) signalling were found to be mediated exclusively by the mGluR1 subtype, although both group I mGluRs, mGluR1 alpha and mGluR5, could be detected in cell extracts. Exposure of cells to medium devoid of Ca(2+) for various times before agonist stimulation reduced the PLC response, which was quickly recovered following the re-exposure of cells to Ca(2+)-containing medium. The extent of the glutamate response correlated well with changes in the cytosolic Ca(2+) concentration. On the other hand, loading of the intracellular Ca(2+) stores by a transient depolarization followed by washing in nondepolarizing buffer, allowed glutamate to release stored Ca(2+) in the majority of cells and enhanced glutamate activation of PLC. Under such conditions, the absence of extracellular Ca(2+) during stimulation and the chelation of cytosolic Ca(2+) with BAPTA/AM inhibited both glutamate-elicited Ca(2+) response and PLC activation. Overall, these results indicate that the mGluR-mediated activation of PLC depends on the presence of extracellular Ca(2+) and can be modulated by moderate changes of cytosolic Ca(2+). Furthermore, ryanodine reduced PLC stimulation by glutamate in predepolarized cells but not in control cells, suggesting that ryanodine receptors could play a role in the potentiation of the mGluR-mediated activation of PLC by Ca(2+) release in predepolarized cells.

The role of Ca2+ stores in the muscarinic inhibition of the K+ current IK(SO) in neonatal rat cerebellar granule cells

Cerebellar granule neurons (CGNs) possess a standing outward potassium current (IK(SO)) which shares many similarities with current through the two-pore domain potassium channel TASK-1 and which is inhibited following activation of muscarinic acetylcholine receptors. The action of muscarine on IK(SO) was unaffected by the M2 receptor antagonist methoctramine (100 nM) but was blocked by the M3 antagonist zamifenacin, which, at a concentration of 100 nM, shifted the muscarine concentration-response curve to the right by around 50-fold. Surprisingly, M3 receptor activation rarely produced a detectable increase in [Ca2+]i unless preceded by depolarization of the cells with 25 mM K+. Experiments with thapsigargin and ionomycin suggested that the endoplasmic reticulum Ca2+ stores in CGNs were depleted at rest. In contrast, cerebellar glial cells in the same fields of cells possessed substantial endoplasmic reticulum Ca2+ stores at rest. Pretreatment of the cells with BAPTA AM, thapsigargin or the phospholipase C (PLC) inhibitor U-73122 all blocked the muscarine-induced Ca2+ signal but had little or no effect on muscarinic inhibition of IK(SO). Raising [Ca2+]i directly with ionomycin caused a small but significant inhibition of IK(SO). It is concluded that muscarine acts on M3 muscarinic acetylcholine receptors both to inhibit IK(SO) and to mobilize Ca2+ from intracellular stores in CGNs. While the mobilization of Ca2+ occurs through activation of PLC, this does not seem to be the primary mechanism underlying muscarinic inhibition of IK(SO).