Group II mGlu receptor agonists fail to protect against various neurotoxic insults induced in murine cortical, striatal and cerebellar granular pure neuronal cultures

Inhibition of bioenergetics provides novel insights into recruitment of PINK1-dependent neuronal mitophagy

Contributions of damaged mitochondria to neuropathologies have stimulated interest in mitophagy. We investigated triggers of neuronal mitophagy by disruption of mitochondrial energy metabolism in primary neurons. Mitophagy was examined in cultured murine cerebellar granule cells after inhibition of mitochondrial respiratory chain by drugs rotenone, 3-nitropropionic acid, antimycin A, and potassium cyanide, targeting complexes I, II, III, and IV, respectively. Inhibitor concentrations producing slow cellular demise were determined from analyses of cellular viability, morphology of neuritic damage, plasma membrane permeability, and oxidative phosphorylation. Live cell imaging of dissipation of mitochondrial membrane potential (ΔΨ_m ) by drugs targeting mitochondrial complexes was referenced to complete depolarization by carbonyl cyanide m-chlorophenyl hydrazone. While inhibition of complexes I, III and IV effected rapid dissipation of ΔΨ_m , inhibition of complex II using 3-nitropropionic acid led to minimal depolarization of mitochondria. Nonetheless, all respiratory chain inhibitors triggered mitophagy as indicated by increased aggregation of mitochondrially localized PINK1. Mitophagy was further analyzed using a dual fluorescent protein biosensor reporting mitochondrial relocation to acidic lysosomal environment. Significant acidification of mitochondria was observed in neurons treated with rotenone or 3-nitropropionic acid, revealing mitophagy at distal processes. Neurons treated with antimycin A or cyanide failed to show mitochondrial acidification. Minor dissipation of ΔΨ_m by 3-nitropropionic acid coupled with vigorous triggering of mitophagy suggested depolarization of mitochondria is not a necessary condition to trigger mitophagy. Moreover, weak elicitation of mitophagy by antimycin A, subsequent to loss of ΔΨ_m , suggested that mitochondrial depolarization is not a sufficient condition for triggering robust neuronal mitophagy. Our findings provide new insight into complexities of mitophagic clearance of neuronal mitochondria.

Impaired expression and function of group II metabotropic glutamate receptors in pilocarpine-treated chronically epileptic rats

Group II metabotropic (mGlu II) receptor subtypes mGlu2 and mGlu3 are important modulators of synaptic plasticity and glutamate release in the brain. Accordingly, several pharmacological ligands have been designed to target these receptors for the treatment of neurological disorders characterized by anomalous glutamate regulation including epilepsy. In this study, we examine whether the expression level and function of mGlu2 and mGlu3 are altered in experimental epilepsy by using immunohistochemistry, Western blot analysis, RT-PCR and extracellular recordings. A down-regulation of mGlu2/3 protein expression at the mossy fiber pathway was associated with a significant reduction in mGlu2/3 protein expression in the hippocampus and cortex of chronically epileptic rats. Moreover, a reduction in mGlu2 and mGlu3 transcripts levels was noticed as early as 24 h after pilocarpine-induced status epilepticus (SE) and persisted during subsequent "latent" and chronic periods. In addition, a significant impairment of mGlu II-mediated depression of field excitatory postsynaptic potentials at mossy fiber-CA3 synapses was detected in chronically epileptic rats. Application of mGlu II agonists (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) induced a significant reduction of the fEPSP amplitude in control rats, but not in chronic epileptic rats. These data indicate a long-lasting impairment of mGlu2/3 expression that may contribute to abnormal presynaptic plasticity, exaggerate glutamate release and hyperexcitability in temporal lobe epilepsy.

Reduced cortical inhibition in a mouse model of familial childhood absence epilepsy

Mutations in the GABA(A) receptor gamma2 subunit are associated with childhood absence epilepsy and febrile seizures. To understand better the molecular basis of absence epilepsy in man, we developed a mouse model harboring a gamma2 subunit point mutation (R43Q) found in a large Australian family. Mice heterozygous for the mutation demonstrated behavioral arrest associated with 6-to 7-Hz spike-and-wave discharges, which are blocked by ethosuximide, a first-line treatment for absence epilepsy in man. Seizures in the mouse showed an abrupt onset at around age 20 days corresponding to the childhood nature of this disease. Reduced cell surface expression of gamma2(R43Q) was seen in heterozygous mice in the absence of any change in alpha1 subunit surface expression, ruling out a dominant-negative effect. GABA(A)-mediated synaptic currents recorded from cortical pyramidal neurons revealed a small but significant reduction that was not seen in the reticular or ventrobasal thalamic nuclei. We hypothesize that a subtle reduction in cortical inhibition underlies childhood absence epilepsy seen in humans harboring the R43Q mutation.

Abnormal mGluR2/3 expression in the perforant path termination zones and mossy fibers of chronically epileptic rats

Epilepsy is characterized by hyperexcitability of hippocampal networks, excessive release of glutamate, and progressive neurodegeneration. Presynaptic group II metabotropic receptors (mGluR2 and mGluR3) are among different mechanisms that modulate presynaptic release of glutamate, especially at the mossy fibers in the hippocampus. Here, we explore whether mGluR2/3 expression is affected in a rat model of temporal lobe epilepsy obtained via pilocarpine-induced status epilepticus (SE). Immunohistochemical assays were performed in age-matched controls and two groups of epileptic rats sacrificed at 25-35 days (1 month post-SE) and at 55-65 days (2 months post-SE) following SE onset. A dramatic lessening of mGluR2/3 immunofluorescence was observed at CA1 and CA3 stratum lacunosum/molecular (SLM) declining to 60% and 68% of control values in 1-month and 2-month post-SE, respectively. Additionally, thickness of mGluR2/3-stained SLM layer narrowed up to 70% of controls indicating atrophy at this branch of the perforant path. Epileptic rats exhibited a marked and progressive down-regulation of mGluR2/3 expression in mossy fiber at hilus and CA3 stratum lucidum in contrast with an enhanced expression of vesicular glutamate transporter type 1 (VGluT1) at the mossy fibers. Intense VGluT1 punctated staining was detected at the inner third molecular layer indicating glutamatergic sprouting. In the molecular layer, mGluR2/3 labeling slightly declined in the 1-month post-SE group but then increased in the 2-month post-SE group although it was diffusely distributed. Down-regulation of mGluR2/3 at the mossy fibers and the SLM may render epileptic hippocampal networks hyperexcitable and susceptible to glutamate-mediated excitotoxicity and neurodegeneration.

Establishment of primary cultures of rat olfactory bulb under serum-free conditions for studies of cellular injury

Olfactory dysfunction has been implicated in various neurodegenerative diseases including Parkinson's and Alzheimer's disease but, despite intense interest in the neurobiology of the olfactory bulb (OB), studies of neurodegenerative mechanisms have not been attempted in primary OB cultures. This study was aimed at developing a primary OB culture under serum-free conditions in order to investigate injury and excitotoxicity in vitro. Olfactory bulbs from rat pups were rapidly trypsinised and mechanically dissociated and the resultant single cell suspension was centrifuged through a high bovine serum albumin concentration gradient to reduce cellular debris before being seeded in multi-well culture plates. Cells were plated in neurobasal medium containing 0.5 mM glutamine, 25 mM K(+), 2% B27 and 10% fetal calf serum (FCS) for 24 h and, after 1 day in vitro (div1), were maintained without FCS. At div8, neurones exhibited extensive neuritic networks, were present as a monolayer and were mainly bipolar and immunopositive for gamma-aminobutyric acid indicating that they were intrinsic OB neurones. At div8, neurones (positive for microtubule-associated protein-2, 73%) predominated over astrocytes (positive for glial fibrillary acidic protein, 27%). Cellular injury produced by staurosporine, hydrogen peroxide and kainate, when assessed by morphological and biochemical procedures, was shown to be concentration-dependent and significantly reduced the numbers of neurones and astrocytes. Further analyses of kainate-induced injury revealed the presence of TUNEL-positive cells (indicative of apoptosis) and increases in intracellular free calcium, both of which were antagonised by CNQX. Thus, the serum-free culture developed here is amenable to morphological and high throughput neurochemical analyses of mechanisms contributing to the injury of OB neurones in vitro.

Neuroprotective activity of the mGluR5 antagonists MPEP and MTEP against acute excitotoxicity differs and does not reflect actions at mGluR5 receptors

1 Neuroprotection has been reported after either activation or blockade of the group I metabotropic glutamate receptor subtype 5 (mGluR5). However, some recent evidence suggests that protection provided by mGluR5 antagonists may reflect their ability to inhibit N-methyl-D-aspartate (NMDA) receptor activity. 2 Here, in both rat and mouse cortical neurons, we compare the neuroprotective actions of two mGluR5 antagonists: 2-methyl-6-(phenylethynyl)-pyridine (MPEP), which has been commonly used and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), a more recently developed compound believed to have greater mGluR5 selectivity. We have previously shown that MPEP directly reduces single-channel NMDA receptor open time at the same concentrations (20 microM or greater) that show neuroprotection, whereas MPEP antagonizes mGluR5 agonist ((RS)-2-chloro-5-hydroxyphenylglycine (CHPG))-induced changes in inositol phosphates (IP) at concentrations as low as 0.2 microM. 3 In the present studies, MTEP significantly inhibited CHPG-mediated IP hydrolysis at concentrations as low as 0.02 microM. In contrast to MPEP, which significantly reduced glutamate- or NMDA-mediated cell death in primary rat neuronal cultures at a concentration of 20 microM, small neuroprotective effects were observed with MTEP only at a concentration of 200 microM. Neither MPEP- nor MTEP-mediated mGluR5 inhibition had any effect on etoposide-induced apoptotic cell death. In rat cortical neurons, the neuroprotective effects of MTEP at very high concentrations, like those of MPEP, reflect ability to directly reduce NMDA receptor peak and steady-state currents. 4 We also compared the effects of MPEP and MTEP in primary cortical neuronal cultures from parental and mGluR5 knockout mice. Both agents were neuroprotective, at high concentrations in normal as well as in the knockout cultures. In contrast to rat cortical neurons, neither MPEP nor MTEP appears to directly alter NMDA receptor activity. 5 Combined, these studies support the conclusion that MTEP has greater mGluR5 selectivity than MPEP, and that neuroprotection provided by either antagonist in neuronal cultures does not reflect inhibition of mGluR5 receptors.

The role of metabotropic glutamate receptors for the generation of calcium oscillations in rat hippocampal astrocytes in situ

Ca2+ oscillations are part of the intra- and intercellular signalling in many cell types. We have studied Ca2+ oscillations in astrocytes in acute brain slices of the hippocampus of juvenile rats (postnatal 8-14 days old), using confocal laser scanning microscopy and bulk-loading of the Ca2+ -sensitive dye Fluo-4. Astrocytes were identified morphologically in the stratum radiatum, and by their Ca2+ response in the absence of external K+. Thirty-five per cent of astrocytes (43 slices) showed spontaneous Ca2+ oscillations, with a frequency of 1.26 +/- 0.11 transients/min (n = 366). These Ca2+ signals were unaffected by tetrodotoxin (0.5 microM) and Ni2+ (2 mM), but were sensitive to interference with the phospholipase C-mediated Ca2+ release from intracellular stores. Spontaneous Ca2+ oscillations were reduced or suppressed by antagonists of metabotropic glutamate receptors (mGluRs) of groups I and II, but not affected by antagonists of group III. Glutamate (1-100 microM) and specific agonists of mGluR groups I and II evoked concentration-dependent Ca2+ signals, which were oscillatory at intermediate concentrations (e.g. at 10 microM glutamate). Our results indicate that mGluRs of both groups I and II are involved in mediating Ca2+ oscillations in astrocytes, which might be glial responses to micromolar changes of glutamate in the extracellular spaces.

Group I and II metabotropic glutamate receptors alter brain cortical metabolic and glutamate/glutamine cycle activity: a 13C NMR spectroscopy and metabolomic study

Metabotropic glutamate receptors (mGluR) modulate neuronal function. Here, we tested the effect on metabolism of a range of Group I and II mGluR ligands in Guinea pig brain cortical tissue slices, applying 13C NMR spectroscopy and metabolomic analysis using multivariate statistics. The effects of Group I agonists (S)-3,5-dihydroxyphenylglycine (DHPG) and (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) depended upon concentration and were mostly stimulatory, increasing both net metabolic flux through the Krebs cycle and glutamate/glutamine cycle activity. Only the higher (50 microm) concentrations of CHPG had the opposite effect. The Group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), consistent with its neuroprotective role, caused significant decreases in metabolism. With principal components analysis of the metabolic profiles generated by these ligands, the effects could be separated by two principal components. Agonists at Group II mGluR [(2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC)] generally stimulated metabolism, including glutamate/glutamine cycling, although this varied with concentration. The antagonist (2S)-alpha-ethylglutamic acid (EGLU) stimulated astrocyte metabolism with minimal impact on glutamate/glutamine cycling. (RS)-1-Aminophosphoindan-1-carboxylic acid (APICA) decreased metabolism at 5 microm but had a stimulatory effect at 50 microm. All ligand effects were separated from control and from each other using two principal components. The ramifications of these findings are discussed.

Protection against glucose-induced neuronal death by NAAG and GCP II inhibition is regulated by mGluR3

Glutamate carboxypeptidase II (GCP II) inhibition has previously been shown to be protective against long-term neuropathy in diabetic animals. In the current study, we have determined that the GCP II inhibitor 2-(phosphonomethyl) pentanedioic acid (2-PMPA) is protective against glucose-induced programmed cell death (PCD) and neurite degeneration in dorsal root ganglion (DRG) neurons in a cell culture model of diabetic neuropathy. In this model, inhibition of caspase activation is mediated through the group II metabotropic glutamate receptor, mGluR3. 2-PMPA neuroprotection is completely reversed by the mGluR3 antagonist (S)-alpha-ethylglutamic acid (EGLU). In contrast, group I and III mGluR inhibitors have no effect on 2-PMPA neuroprotection. Furthermore, we show that two mGluR3 agonists, the direct agonist (2R,4R)-4-aminopyrrolidine-2, 4-dicarboxylate (APDC) and N-acetyl-aspartyl-glutamate (NAAG) provide protection to neurons exposed to high glucose conditions, consistent with the concept that 2-PMPA neuroprotection is mediated by increased NAAG activity. Inhibition of GCP II or mGluR3 may represent a novel mechanism to treat neuronal degeneration under high-glucose conditions.

Modulation of intracellular Ca2+ levels by Scorpaenidae venoms

The crude venoms of the soldierfish (Gymnapistes marmoratus), the lionfish (Pterois volitans) and the stonefish (Synanceia trachynis) display pronounced neuromuscular activity. Since [Ca(2+)](i) is a key regulator in many aspects of neuromuscular function we sought to determine its involvement in the neuromuscular actions of the venoms. In the chick biventer cervicis muscle, all three venoms produced a sustained contraction (approx 20-30% of 1mM acetylcholine). Blockade of nicotinic receptors with tubocurarine (10 micro M) failed to attenuate the contractile response to either G. marmoratus venom or P. volitans venom, but produced slight inhibition of the response to S. trachynis venom. All three venoms produced a rise in intracellular Ca(2+) (approx. 200-300% of basal) in cultured murine cortical neurons. The Ca(2+)-channel blockers omega-conotoxin MVIIC, omega-conotoxin GVIA, omega-agatoxin IVa and nifedipine (each at 1 micro M) potentiated the increase in [Ca(2+)](i) in response to G. marmoratus venom and P. volitans venom, while attenuating the response to S. trachynis venom. Removal of extracellular Ca(2+), replacement of Ca(2+) with La(3+) (0.5mM), or addition of stonefish antivenom (3units/ml) inhibited both the venom-induced increase in [Ca(2+)](i) in cultured neurones and contraction in chick biventer cervicis muscle. Venom-induced increases in [Ca(2+)](i) correlated with an increased cell death of cultured neurones as measured using propidium iodide (1 micro g/ml). Morphological analysis revealed cellular swelling and neurite loss consistent with necrosis. These data indicate that the effects of all three venoms are due in part to an increase in intracellular Ca(2+), possibly via the formation of pores in the cellular membrane which, under certain conditions, can lead to necrosis.

DCG-IV but not other group-II metabotropic receptor agonists induces microglial BDNF mRNA expression in the rat striatum. Correlation with neuronal injury

We have previously described a neuroprotective action of (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV), an agonist for group-II metabotropic receptors, on dopaminergic nerve terminals against the degeneration induced by 1-methyl-4-phenylpyridinium (MPP+). This effect was accompanied by an up-regulation of brain-derived neurotrophic factor (BDNF) mRNA expression in the rat striatum. We have now analyzed the phenotypic nature of the BDNF mRNA-expressing cells in response to intrastriatal injection of DCG-IV. Dual in situ hybridization and immunohistochemistry revealed that microglial cells but not astrocytes were responsible for this induction. Subsequent analysis demonstrated that this effect was accompanied by striking loss of striatal glutamic acid decarboxylase (GAD) mRNA and massive appearance of internucleosomal DNA fragmentation, a hallmark of apoptosis. A dose-response study demonstrated that doses of DCG-IV as low as 5 nmol was very toxic in terms GAD mRNA and apoptosis. 0.5 nmol of DCG-IV did not induce toxicity at all in terms of GAD mRNA and apoptosis. Activation of group-II metabotropic receptors in striatum with N-Acetyl-Asp-Glu (NAAG; a mGlu3 agonist) and (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (a mGlu2 and mGlu3 agonist) did not induce neither loss of GAD mRNA nor appearance of apoptosis (doses up to 20 nmol). In additional experiments, NAAG, in contrast to DCG-IV, failed to protect the striatal dopaminergic system against the degeneration induced by MPP+ as studied by microdialysis. Finally, we studied the mechanism by which DCG-IV is highly toxic. For that, selective antagonists of either metabotropic--(R,S)-alpha-methyl-4-carboxyphenylglycine and LY 341495--or ionotropic (N-methyl-D-aspartate, NMDA)--DL-2-amino-5-phosphonovaleric acid (AP-5) glutamate receptors --were co-administered with DCG-IV. Only AP-5 highly protected the striatum against the degeneration induced by DCG-IV. Since DCG-IV also activates the NMDA receptor at concentrations higher than 3 microM, it is conceivable that a intrastriatal concentration equal or higher than 3 microM after a single striatal injection of 5-20 nmol of DCG-IV. Our findings suggest that much caution must be exerted when testing the numerous neuroprotective effects ascribed to group-II metabotropic receptor activation, in particular when using DCG-IV. We conclude that the neuroprotectant capability of a given compound on a specific system does not exclude the possibility of inducing toxicity on a different one.

Astrocyte mGlu(2/3)-mediated cAMP potentiation is calcium sensitive: studies in murine neuronal and astrocyte cultures

Signal transduction mechanisms of group II metabotropic glutamate receptors (mGlu(2/3)) remains a matter of some controversy, therefore we sought to gain new insights into its regulation by studying cAMP production in cultured neurons and astrocytes, and by examining inter-relationships of mGlu(2/3)-induced signalling with cellular calcium and various signalling cascades. mGlu(2/3) agonists 2R,4R-4-aminopyrrolidine-2,4-dicarboxylic acid (2R,4R-APDC) and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268) inhibited 10 microM forskolin-stimulated production of cAMP in murine cortical neurons, striatal neurons and forebrain astrocytes in the absence of extracellular Ca(2+). These agonists potentiated cAMP production in the presence of 1.8 mM Ca(2+) in astrocytes only. This potentiation was dependent on the extracellular Ca(2+) concentration (0.001-10 mM) and inhibited by the mGlu(2/3) antagonist LY341495 (1 microM), adenosine deaminase (1 U/ml) and the adenosine A(2A) receptor antagonist ZM241385 (1 microM). Pre-incubation with the phospholipase C (PLC) inhibitor U73122 (10 microM), L-type Ca(2+)-channel blockers nifedipine (1 microM) and nimodipine (1 microM), the calmodulin kinase II (CaMKII) inhibitor KN-62 (10 microM) or pertussis toxin (100 ng/ml) inhibited this potentiation. In the absence of 1.8 mM Ca(2+), thapsigargin (1 microM) facilitated the potentiation of cAMP production. Measurement of the Ca(2+)-binding dye Fluo-3/AM showed that, compared to Ca(2+)-free conditions, thapsigargin and 1.8 mM Ca(2+) elevated [Ca(2+)](i) in astrocytes; the latter effect being prevented by L-type Ca(2+)-channel blockers. Potentiation of cAMP production was also demonstrated when astrocytes were stimulated with the beta-adrenoceptor agonist isoprenaline (10 microM) in the presence of 1.8 mM Ca(2+), but not with the adenosine agonist NECA (10 microM) or the group I mGlu receptor agonist DHPG (100 microM). BaCl(2) (1.8 mM) in place of Ca(2+) did not facilitate forskolin-stimulated mGlu(2/3)-potentiation of cAMP. In short, this study in astrocytes demonstrates that under physiological Ca(2+) and adenylate cyclase stimulation an elevation of cAMP production is achieved that is mediated by PLC/IP(3)- and CaMKII-dependent pathways and results in the release of endogenous adenosine which acts at G(s) protein-coupled A(2A) receptors. These findings provide new insights into mGlu(2/3) signalling in astrocytes versus neurons, and which could determine the functional phenotypy of astrocytes under physiological and pathological conditions.