The Gs-Linked Receptor GPR3 Inhibits the Proliferation of Cerebellar Granule Cells during Postnatal Development

Background

During postnatal murine and rodent cerebellar development, cerebellar granule precursors (CGP) gradually stop proliferating as they differentiate after migration to the internal granule layer (IGL). Molecular events that govern this program remain to be fully elucidated. GPR3 belongs to a family of Gs-linked receptors that activate cyclic AMP and are abundantly expressed in the adult brain.

Methodology/Principal Findings

To investigate the role of this orphan receptor in CGP differentiation, we determined that exogenous GPR3 expression in rat cerebellar granule neurons partially antagonized the proliferative effect of Sonic hedgehog (Shh), while endogenous GPR3 inhibition by siRNA stimulated Shh-induced CGP proliferation. In addition, exogenous GPR3 expression in CGPs correlated with increased p27/kip expression, while GPR3 knock-down led to a decrease in p27/kip expression. In wild-type mice, GPR3 expression increased postnatally and its expression was concentrated in the internal granular layer (IGL). In GPR3 −/− mice, the IGL was widened with increased proliferation of CGPs, as measured by bromodeoxyuridine incorporation. Cell cycle kinetics of GPR3-transfected medulloblastoma cells revealed a G0/G1 block, consistent with cell cycle exit.

Conclusions/Significance

These results thus indicate that GPR3 is a novel antiproliferative mediator of CGPs in the postnatal development of murine cerebellum.     

Induction of Ornithine Decarboxylase by N-Methyl-D-Aspartate Receptor Activation Is Unrelated to Potentiation of Glutamate Excitotoxicity by Polyamines in Cerebellar Granule Neurons

Abstract: Polyamines positively modulate the activity of the N -methyl- d -aspartate (NMDA)-sensitive glutamate receptors. The concentration of polyamines in the brain increases in certain pathological conditions, such as ischemia and brain trauma, and these compounds have been postulated to play a role in excitotoxic neuronal death. In primary cultures of rat cerebellar granule neurons, exogenous application of the polyamines spermidine and spermine (but not putrescine) potentiated the delayed neurotoxicity elicited by NMDA receptor stimulation with glutamate. Furthermore, both toxic and nontoxic concentrations of glutamate stimulated the activity of ornithine decarboxylase (ODC)—the key regulatory enzyme in polyamine synthesis—and increased the concentration of ODC mRNA in cerebellar granule neurons but not in glial cells. Glutamate-induced ODC activation but not neurotoxicity was blocked by the ODC inhibitor difluoromethylornithine. Thus, high extracellular polyamine concentrations potentiate glutamate-triggered neuronal death, but the glutamate-induced increase in neuronal ODC activity may not play a determinant role in the cascade of intracellular events responsible for delayed excitotoxicity.     

Crucial Positively Charged Residues for Ligand Activation of the GPR35 Receptor

GPR35 is a G protein-coupled receptor expressed in the immune, gastrointestinal, and nervous systems in gastric carcinomas and is implicated in heart failure and pain perception. We investigated residues in GPR35 responsible for ligand activation and the receptor structure in the active state. GPR35 contains numerous positively charged amino acids that face into the binding pocket that cluster in two distinct receptor regions, TMH3-4-5-6 and TMH1-2-7. Computer modeling implicated TMH3-4-5-6 for activation by the GPR35 agonists zaprinast and pamoic acid. Mutation results for the TMH1-2-7 region of GPR35 showed no change in ligand efficacies at the K1.32A, R2.65A, R7.33A, and K7.40A mutants. However, mutation of arginine residues in the TMH3-4-5-6 region (R4.60, R6.58, R3.36, R(164), and R(167) in the EC2 loop) had effects on signaling for one or both agonists tested. R4.60A resulted in a total ablation of agonist-induced activation in both the β-arrestin trafficking and ERK1/2 activation assays. R6.58A increased the potency of zaprinast 30-fold in the pERK assay. The R(167)A mutant decreased the potency of pamoic acid in the β-arrestin trafficking assay. The R(164)A and R(164)L mutants decreased potencies of both agonists. Similar trends for R6.58A and R(167)A were observed in calcium responses. Computer modeling showed that the R6.58A mutant has additional interactions with zaprinast. R3.36A did not express on the cell surface but was trapped in the cytoplasm. The lack of surface expression of R3.36A was rescued by a GPR35 antagonist, CID2745687. These results clearly show that R4.60, R(164), R(167), and R6.58 play crucial roles in the agonist initiated activation of GPR35.     

Ras Protein Activation Is a Key Event in Activity-dependent Survival of Cerebellar Granule Neurons

Neuronal activity promotes the survival of cerebellar granule neurons (CGNs) during the postnatal development of cerebellum. CGNs that fail to receive excitatory inputs will die by apoptosis. This process could be mimicked in culture by exposing CGNs to either a physiological concentration of KCl (5 mm or K5) plus N-methyl-d-aspartate (NMDA) or to 25 mm KCl (K25). We have previously described that a 24-h exposure to NMDA (100 μm) or K25 at 2 days in vitro induced long term survival of CGNs in K5 conditions. Here we have studied the molecular mechanisms activated at 2 days in vitro in these conditions. First we showed that NMDA or K25 addition promoted a rapid stimulation of PI3K and a biphasic phosphorylation on Ser-473 of Akt, a PI3K substrate. Interestingly, we demonstrated that only the first wave of Akt phosphorylation is necessary for the NMDA- and K25-mediated survival. Additionally, we detected that both NMDA and K25 increased ERK activity with a similar time-course. Moreover, our results showed that NMDA-mediated activation of the small G-protein Ras is necessary for PI3K/Akt pathway activation, whereas Rap1 was involved in NMDA phosphorylation of ERK. On the other hand, Ras, but not Rap1, mediates K25 activation of PI3K/Akt and MEK/ERK pathways. Because neuroprotection by NMDA or K25 is mediated by Ras (and not by Rap1) activation, we propose that Ras stimulation is a crucial event in NMDA- and K25-mediated survival of CGNs through the activation of PI3K/Akt and MEK/ERK pathways.     

非基因型雌激素膜性受体GPR30在生后雌性大鼠海马内的发育学表达与亚细胞水平定位研究

为了研究非基因型雌激素膜性受体GPR30对海马的结构和功能的调节作用,应用硫酸镍铵增强显色的免疫组化技术以及酶标免疫电镜技术,观察了生后雌性大鼠海马内GPR30表达的变化及其免疫阳性产物在神经元亚细胞水平的定位情况．结果显示,GPR30免疫阳性产物主要位于海马CA区的锥体层神经元与齿状回颗粒层的神经元内,其表达水平随发育呈增加趋势．P0时在雌性大鼠海马未发现明显GPR30免疫阳性反应,P7后免疫阳性物质开始在CA2出现,P14时见于 CA1、CA2和齿状回,P30和P60主要见于CA1、CA2、CA3和齿状回．在光镜下,GPR30免疫阳性产物位于细胞核外的胞浆中,细胞核未见免疫阳性反应．在透射电镜下可见其位于神经元的胞浆内,可能主要是粗面内质网,也可见于线粒体和细胞膜．以上结果证实,GPR30是一种位于细胞核外的、非基因型作用的雌激素受体,可能参与了雌激素对海马锥体神经元突触可塑性和学习记忆等功能的调节,还可能参与了对齿状回成年神经干细胞某些活动的调节．     

Long-Term GABA Treatment Elicits Supersensitivity of Quisqualate-Preferring Metabotropic Glutamate Receptor in Cultured Rat Cerebellar Neurons

Abstract: In primary cultures of rat cerebellar granule neurons, GABA treatment (50 μ M , 7 days) caused a withdrawal supersensitivity selective for the metabotropic glutamate receptors that mainly prefer l -glutamate, quisqua- late and, to a lesser extent, kainate. The withdrawal supersensitivity was absent when 10 μ M SR-95531 was coadministered with GABA during the treatment period, an event that suggests the GABA_A receptors primarily produced the GABA treatment effect. This was supported further by the inability of baclofen treatment to mimic completely the treatment effect of GABA. Withdrawal from 7 days of baclofen treatment only produced a slight increase in the metabotropic effect of l -glutamate and carbachol. In addition, in untreated neurons, baclofen had no acute effect, whereas GABA inhibited the effect of l -glutamate and carbachol. The inhibitory effect of GABA was reversed by SR-95531 and was absent in neurons treated with GABA. These observations suggest the involvement of GABA_A receptors and the apparent development of tolerance to GABA, respectively. Also, dependence on GABA may have occurred; the metabotropic effects of glutamate, kainate, and quisqualate were not altered in neurons maintained with GABA treatment.     

Recognition and Activation Domains Contribute to Allele-Specific Responses of an Arabidopsis NLR Receptor to an Oomycete Effector Protein

In plants, specific recognition of pathogen effector proteins by nucleotide-binding leucine-rich repeat (NLR) receptors leads to activation of immune responses. RPP1, an NLR from Arabidopsis thaliana, recognizes the effector ATR1, from the oomycete pathogen Hyaloperonospora arabidopsidis, by direct association via C-terminal leucine-rich repeats (LRRs). Two RPP1 alleles, RPP1-NdA and RPP1-WsB, have narrow and broad recognition spectra, respectively, with RPP1-NdA recognizing a subset of the ATR1 variants recognized by RPP1-WsB. In this work, we further characterized direct effector recognition through random mutagenesis of an unrecognized ATR1 allele, ATR1-Cala2, screening for gain-of-recognition phenotypes in a tobacco hypersensitive response assay. We identified ATR1 mutants that a) confirm surface-exposed residues contribute to recognition by RPP1, and b) are recognized by and activate the narrow-spectrum allele RPP1-NdA, but not RPP1-WsB, in co-immunoprecipitation and bacterial growth inhibition assays. Thus, RPP1 alleles have distinct recognition specificities, rather than simply different sensitivity to activation. Using chimeric RPP1 constructs, we showed that RPP1-NdA LRRs were sufficient for allele-specific recognition (association with ATR1), but insufficient for receptor activation in the form of HR. Additional inclusion of the RPP1-NdA ARC2 subdomain, from the central NB-ARC domain, was required for a full range of activation specificity. Thus, cooperation between recognition and activation domains seems to be essential for NLR function.     

Activation of Nuclear Calcium Dynamics by Synaptic Stimulation in Cultured Cortical Neurons

L-type voltage-sensitive Ca2+ channels (VSCCs) are enriched on the neuronal soma and trigger gene expression during synaptic activity. To understand better how these channels regulate somatic and nuclear Ca2+ dynamics, we have investigated Ca2+ influx through L-type VSCCs following synaptic stimulation, using the long-wavelength Ca2+ indicator fluo-3 combined with laser scanning confocal microscopy. Single synaptic stimuli resulted in rapid Ca2+ transients in somatic cytoplasmic compartments (<5 ms rise time). Nuclear Ca2+ elevations lagged behind cytoplasmic levels by approximately 60 ms, consistent with a dependence on diffusion from a cytoplasmic source. Pharmacological experiments indicated that L-type VSCCs mediated approximately 50% of the nuclear and somatic (cytoplasmic) Ca2+ elevation in response to strong synaptic stimulation. In contrast, relatively weak excitatory postsynaptic potentials (EPSPs; approximately 15 mV) or single action potentials were much less effective at activating L-type VSCCs. Antagonist experiments indicated that activation of the NMDA-type glutamate receptor leads to a long-lasting somatic depolarization necessary to activate L-type VSCCs effectively during synaptic stimuli. Simulation of action potential and somatic EPSP depolarization using voltage-clamp pulses indicated that nuclear Ca2+ transients mediated by L-type VSCCs were produced by sustained depolarization positive to -25 mV. In the absence of synaptic stimulation, action potential stimulation alone led to elevations in nuclear Ca2+ mediated by predominantly non-L-type VSCCs. Our results suggest that action potentials, in combination with long-lived synaptic depolarizations, facilitate the activation of L-type VSCCs. This activity elevates somatic Ca2+ levels that spread to the nucleus.     

Dynamics of the Ligand Binding Domain Layer during AMPA Receptor Activation

Ionotropic glutamate receptors are postsynaptic tetrameric ligand-gated channels whose activity mediates fast excitatory transmission. Glutamate binding to clamshell-shaped ligand binding domains (LBDs) triggers opening of the integral ion channel, but how the four LBDs orchestrate receptor activation is unknown. Here, we present a high-resolution x-ray crystal structure displaying two tetrameric LBD arrangements fully bound to glutamate. Using a series of engineered metal ion trapping mutants, we showed that the more compact of the two assemblies corresponds to an arrangement populated during activation of full-length receptors. State-dependent cross-linking of the mutants identified zinc bridges between the canonical active LBD dimers that formed when the tetramer was either fully or partially bound by glutamate. These bridges also stabilized the resting state, consistent with the recently published full-length apo structure. Our results provide insight into the activation mechanism of glutamate receptors and the complex conformational space that the LBD layer can sample.     

Non-Equilibrium Dynamics Contribute to Ion Selectivity in the KcsA Channel

The ability of biological ion channels to conduct selected ions across cell membranes is critical for the survival of both animal and bacterial cells. Numerous investigations of ion selectivity have been conducted over more than 50 years, yet the mechanisms whereby the channels select certain ions and reject others are not well understood. Here we report a new application of Jarzynski’s Equality to investigate the mechanism of ion selectivity using non-equilibrium molecular dynamics simulations of Na⁺ and K⁺ ions moving through the KcsA channel. The simulations show that the selectivity filter of KcsA adapts and responds to the presence of the ions with structural rearrangements that are different for Na⁺ and K⁺. These structural rearrangements facilitate entry of K⁺ ions into the selectivity filter and permeation through the channel, and rejection of Na⁺ ions. A mechanistic model of ion selectivity by this channel based on the results of the simulations relates the structural rearrangement of the selectivity filter to the differential dehydration of ions and multiple-ion occupancy and describes a mechanism to efficiently select and conduct K⁺. Estimates of the K⁺/Na⁺ selectivity ratio and steady state ion conductance for KcsA from the simulations are in good quantitative agreement with experimental measurements. This model also accurately describes experimental observations of channel block by cytoplasmic Na⁺ ions, the “punch through” relief of channel block by cytoplasmic positive voltages, and is consistent with the knock-on mechanism of ion permeation.     

Cardiac Fibroblasts Contribute to Myocardial Dysfunction in Mice with Sepsis: The Role of NLRP3 Inflammasome Activation

Myocardial contractile dysfunction in sepsis is associated with the increased morbidity and mortality. Although the underlying mechanisms of the cardiac depression have not been fully elucidated, an exaggerated inflammatory response is believed to be responsible. Nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome is an intracellular platform that is involved in the maturation and release of interleukin (IL)-1β. The aim of the present study is to evaluate whether sepsis activates NLRP3 inflammasome/caspase-1/IL-1β pathway in cardiac fibroblasts (CFs) and whether this cytokine can subsequently impact the function of cardiomyocytes (cardiac fibroblast-myocyte cross-talk). We show that treatment of CFs with lipopolysaccharide (LPS) induces upregulation of NLRP3, activation of caspase-1, as well as the maturation (activation) and release of IL-1β. In addition, the genetic (small interfering ribonucleic acid [siRNA]) and pharmacological (glyburide) inhibition of the NLRP3 inflammasome in CFs can block this signaling pathway. Furthermore, the inhibition of the NLRP3 inflammasome in cardiac fibroblasts ameliorated the ability of LPS-chalenged CFs to impact cardiomyocyte function as assessed by intracellular cyclic adenosine monophosphate (cAMP) responses in cardiomyocytes. Salient features of this the NLP3 inflammasome/ caspase-1 pathway were confirmed in in vivo models of endotoxemia/sepsis. We found that inhibition of the NLRP3 inflammasome attenuated myocardial dysfunction in mice with LPS and increased the survival rate in mice with feces-induced peritonitis. Our results indicate that the activation of the NLRP3 inflammasome in cardiac fibroblasts is pivotal in the induction of myocardial dysfunction in sepsis.     

Neuritin Activates Insulin Receptor Pathway to Up-regulate Kv4.2-mediated Transient Outward K+ Current in Rat Cerebellar Granule Neurons

Neuritin is a new neurotrophic factor discovered in a screen to identify genes involved in activity-dependent synaptic plasticity. Neuritin also plays multiple roles in the process of neural development and synaptic plasticity. The receptors for binding neuritin and its downstream signaling effectors, however, remain unclear. Here, we report that neuritin specifically increases the densities of transient outward K⁺ currents (I_A) in rat cerebellar granule neurons (CGNs) in a time- and concentration-dependent manner. Neuritin-induced amplification of I_A is mediated by increased mRNA and protein expression of Kv4.2, the main α-subunit of I_A. Exposure of CGNs to neuritin markedly induces phosphorylation of ERK (pERK), Akt (pAkt), and mammalian target of rapamycin (pmTOR). Neuritin-induced I_A and increased expression of Kv4.2 are attenuated by ERK, Akt, or mTOR inhibitors. Unexpectedly, pharmacological blockade of insulin receptor, but not the insulin-like growth factor 1 receptor, abrogates the effect of neuritin on I_A amplification and Kv4.2 induction. Indeed, neuritin activates downstream signaling effectors of the insulin receptor in CGNs and HeLa. Our data reveal, for the first time, an unanticipated role of the insulin receptor in previously unrecognized neuritin-mediated signaling.     

At the Edge of Chaos: How Cerebellar Granular Layer Network Dynamics Can Provide the Basis for Temporal Filters

Models of the cerebellar microcircuit often assume that input signals from the mossy-fibers are expanded and recoded to provide a foundation from which the Purkinje cells can synthesize output filters to implement specific input-signal transformations. Details of this process are however unclear. While previous work has shown that recurrent granule cell inhibition could in principle generate a wide variety of random outputs suitable for coding signal onsets, the more general application for temporally varying signals has yet to be demonstrated. Here we show for the first time that using a mechanism very similar to reservoir computing enables random neuronal networks in the granule cell layer to provide the necessary signal separation and extension from which Purkinje cells could construct basis filters of various time-constants. The main requirement for this is that the network operates in a state of criticality close to the edge of random chaotic behavior. We further show that the lack of recurrent excitation in the granular layer as commonly required in traditional reservoir networks can be circumvented by considering other inherent granular layer features such as inverted input signals or mGluR2 inhibition of Golgi cells. Other properties that facilitate filter construction are direct mossy fiber excitation of Golgi cells, variability of synaptic weights or input signals and output-feedback via the nucleocortical pathway. Our findings are well supported by previous experimental and theoretical work and will help to bridge the gap between system-level models and detailed models of the granular layer network.     

Glutamate Receptor Subtypes in Cultured Cerebellar Neurons: Modulation of Glutamate and γ-Aminobutyric Acid Release

Using cerebellar, neuron-enriched primary cultures, we have studied the glutamate receptor subtypes coupled to neurotransmitter amino acid release. Acute exposure of the cultures to micromolar concentrations of kainate and quisqualate stimulated D-[3H]aspartate release, whereas N-methyl-D-aspartate, as well as dihydrokainic acid, were ineffective. The effect of kainic acid was concentration dependent in the concentration range of 20-100 microM. Quisqualic acid was effective at lower concentrations, with maximal releasing activity at about 50 microM. Kainate and dihydrokainate (20-100 microM) inhibited the initial rate of D-[3H]aspartate uptake into cultured granule cells, whereas quisqualate and N-methyl-DL-aspartate were ineffective. D-[3H]Aspartate uptake into confluent cerebellar astrocyte cultures was not affected by kainic acid. The stimulatory effect of kainic acid on D-[3H]aspartate release was Na+ independent, and partly Ca2+ dependent; the effect of quisqualate was Na+ and Ca2+ independent. Kynurenic acid (50-200 microM) and, to a lesser extent, 2,3-cis-piperidine dicarboxylic acid (100-200 microM) antagonized the stimulatory effect of kainate but not that of quisqualate. Kainic and quisqualic acid (20-100 microM) also stimulated gamma-[3H]-aminobutyric acid release from cerebellar cultures, and kynurenic acid antagonized the effect of kainate but not that of quisqualate. In conclusion, kainic acid and quisqualic acid appear to activate two different excitatory amino acid receptor subtypes, both coupled to neurotransmitter amino acid release. Moreover, kainate inhibits D-[3H]aspartate neuronal uptake by interfering with the acidic amino acid high-affinity transport system.     

Effects of Oxidants and Glutamate Receptor Activation on Mitochondrial Membrane Potential in Rat Forebrain Neurons

Abstract: Both glutamate and reactive oxygen species have been implicated in excitotoxic neuronal injury, and mitochondria may play a key role in the mediation of this process. In this study, we examined whether glutamate-receptor stimulation and oxidative stress interact to affect the mitochondrial membrane potential (ΔΨ). We measured ΔΨ in rat forebrain neurons with the ratiometric fluorescent dye JC-1 by using laser scanning confocal imaging. Intracellular oxidant levels were measured by using the oxidation-sensitive dyes 2',7'-dichlorodihydrofluorescein (DCFH₂) and dihydroethidium (DHE). Application of hydrogen peroxide (0.3–3 m M ) or 1 m M xanthine/0.06 U/ml xanthine oxidase decreased ΔΨ in a way that was independent of the presence of extracellular Ca²⁺ and was not affected by the addition of cyclosporin A, suggesting the presence of either a cyclosporin A-insensitive form of permeability transition, or a separate mechanism. tert -Butylhydroperoxide (730 µ M ) had less of an effect on ΔΨ than hydrogen peroxide despite similar effects on intracellular DCFH₂ or DHE oxidation. Hydrogen peroxide-, tert -butylhydroperoxide-, and superoxide-enhanced glutamate, but not kainate, induced decreases in ΔΨ. The combined effect of peroxide or superoxide plus glutamate was Ca²⁺ dependent and was partially inhibited by cyclosporin A. These results suggest that oxidants and glutamate depolarize mitochondria by different mechanisms, and that oxidative stress may enhance glutamate-mediated mitochondrial depolarization.     

Spatio-Temporal Cellular Dynamics of the Arabidopsis Flagellin Receptor Reveal Activation Status-Dependent Endosomal Sorting

The activity of surface receptors is location specific, dependent upon the dynamic membrane trafficking network and receptor-mediated endocytosis (RME). Therefore, the spatio-temporal dynamics of RME are critical to receptor function. The plasma membrane receptor FLAGELLIN SENSING2 (FLS2) confers immunity against bacterial infection through perception of flagellin (flg22). Following elicitation, FLS2 is internalized into vesicles. To resolve FLS2 trafficking, we exploited quantitative confocal imaging for colocalization studies and chemical interference. FLS2 localizes to bona fide endosomes via two distinct endocytic trafficking routes depending on its activation status. FLS2 receptors constitutively recycle in a Brefeldin A (BFA)–sensitive manner, while flg22-activated receptors traffic via ARA7/Rab F2b– and ARA6/Rab F1–positive endosomes insensitive to BFA. FLS2 endocytosis required a functional Rab5 GTPase pathway as revealed by dominant-negative ARA7/Rab F2b. Flg22-induced FLS2 endosomal numbers were increased by Concanamycin A treatment but reduced by Wortmannin, indicating that activated FLS2 receptors are targeted to late endosomes. RME inhibitors Tyrphostin A23 and Endosidin 1 altered but did not block induced FLS2 endocytosis. Additional inhibitor studies imply the involvement of the actin-myosin system in FLS2 internalization and trafficking. Altogether, we report a dynamic pattern of subcellular trafficking for FLS2 and reveal a defined framework for ligand-dependent endocytosis of this receptor.