Activation of group III metabotropic glutamate receptors attenuates rotenone toxicity on dopaminergic neurons through a microtubule-dependent mechanism

Targeting metabotropic glutamate receptor 4 for cancer immunotherapy

In this study, we report a novel role of metabotropic glutamate receptor 4 (GRM4) in suppressing antitumor immunity. We revealed in three murine syngeneic tumor models (B16, MC38, and 3LL) that either genetic knockout (Grm4−/−) or pharmacological inhibition led to significant delay in tumor growth. Mechanistically, perturbation of GRM4 resulted in a strong antitumor immunity by promoting natural killer (NK), CD4+, and CD8+ T cells toward an activated, proliferative, and functional phenotype. Single-cell RNA sequencing and T cell receptor profiling further defined the clonal expansion and immune landscape changes in CD8+ T cells. We further showed that Grm4−/− intrinsically activated interferon-γ production in CD8+ T cells through cyclic adenosine 3′,5′-monophosphate (cAMP)/cAMP response element binding protein–mediated pathway. Our study appears to be of clinical significance as a signature of NKhigh-GRM4low and CD8high-GRM4low correlated with improved survival in patients with melanoma. Targeting GRM4 represents a new approach for cancer immunotherapy.

Metabotropic glutamate receptors and nitric oxide in dopaminergic neurotoxicity

Dopaminergic neurotoxicity is characterized by damage and death of dopaminergic neurons. Parkinson's disease (PD) is a neurodegenerative disorder that primarily involves the loss of dopaminergic neurons in the substantia nigra. Therefore, the study of the mechanisms, as well as the search for new targets for the prevention and treatment of neurodegenerative diseases, is an important focus of modern neuroscience. PD is primarily caused by dysfunction of dopaminergic neurons; however, other neurotransmitter systems are also involved. Research reports have indicated that the glutamatergic system is involved in different pathological conditions, including dopaminergic neurotoxicity. Over the last two decades, the important functional interplay between dopaminergic and glutamatergic systems has stimulated interest in the possible role of metabotropic glutamate receptors (mGluRs) in the development of extrapyramidal disorders. However, the specific mechanisms driving these processes are presently unclear. The participation of the universal neuronal messenger nitric oxide (NO) in the mechanisms of dopaminergic neurotoxicity has attracted increased attention. The current paper aims to review the involvement of mGluRs and the contribution of NO to dopaminergic neurotoxicity. More precisely, we focused on studies conducted on the rotenone-induced PD model. This review is also an outline of our own results obtained using the method of electron paramagnetic resonance, which allows quantitation of NO radicals in brain structures.

Neddylation is required for presynaptic clustering of mGlu7 and maturation of presynaptic terminals

Neddylation is a posttranslational modification in which NEDD8 is conjugated to a target substrate by cellular processes similar to those involved in ubiquitination. Recent studies have identified PSD-95 and cofilin as substrates for neddylation in the brain and have shown that neddylation modulates the maturation and stability of dendritic spines in developing neurons. However, the precise substrates and functional consequences of neddylation at presynaptic terminals remain elusive. Here, we provide evidence that the mGlu7 receptor is a target of neddylation in heterologous cells and rat primary cultured neurons. We found that mGlu7 neddylation is reduced by agonist treatment and is required for the clustering of mGlu7 in the presynaptic active zone. In addition, we observed that neddylation is not required for the endocytosis of mGlu7, but it facilitates the ubiquitination of mGlu7 and stabilizes mGlu7 protein expression. Finally, we demonstrate that neddylation is necessary for the maturation of excitatory presynaptic terminals, providing a key role for neddylation in synaptic function.

Pathogenic GRM7 Mutations Associated with Neurodevelopmental Disorders Impair Axon Outgrowth and Presynaptic Terminal Development

Metabotropic glutamate receptor 7 (mGlu7) is an inhibitory heterotrimeric G-protein-coupled receptor that modulates neurotransmitter release and synaptic plasticity at presynaptic terminals in the mammalian central nervous system. Recent studies have shown that rare mutations in glutamate receptors and synaptic scaffold proteins are associated with neurodevelopmental disorders (NDDs). However, the role of presynaptic mGlu7 in the pathogenesis of NDDs remains largely unknown. Recent whole-exome sequencing (WES) studies in families with NDDs have revealed that several missense mutations (c.1865G>A:p.R622Q; c.461T>C:p.I154T; c.1972C>T:p.R658W and c.2024C>A:p.T675K) or a nonsense mutation (c.1757G>A:p.W586X) in the GRM7 gene may be linked to NDDs. In the present study, we investigated the mechanistic links between GRM7 point mutations and NDD pathology. We find that the pathogenic GRM7 I154T and R658W/T675K mutations lead to the degradation of the mGlu7 protein. In particular, the GRM7 R658W/T675K mutation results in a lack of surface mGlu7 expression in heterologous cells and cultured neurons isolated from male and female rat embryos. We demonstrate that the expression of mGlu7 variants or exposure to mGlu7 antagonists impairs axon outgrowth through the mitogen-activated protein kinase (MAPK)-cAMP-protein kinase A (PKA) signaling pathway during early neuronal development, which subsequently leads to a decrease in the number of presynaptic terminals in mature neurons. Treatment with an mGlu7 agonist restores the pathologic phenotypes caused by mGlu7 I154T but not by mGlu7 R658W/T675K because of its lack of neuronal surface expression. These findings provide evidence that stable neuronal surface expression of mGlu7 is essential for neural development and that mGlu7 is a promising therapeutic target for NDDs.SIGNIFICANCE STATEMENT Neurodevelopmental disorders (NDDs) affect brain development and function by multiple etiologies. Metabotropic glutamate receptor 7 (mGlu7) is a receptor that controls excitatory neurotransmission and synaptic plasticity. Since accumulating evidence indicates that the GRM7 gene locus is associated with NDD risk, we analyzed the functional effects of human GRM7 variants identified in patients with NDDs. We demonstrate that stable neuronal surface expression of mGlu7 is essential for axon outgrowth and presynaptic terminal development in neurons. We found that mitogen-activated protein kinase (MAPK)-cAMP-protein kinase A (PKA) signaling and subsequent cytoskeletal dynamics are defective because of the degradation of mGlu7 variants. Finally, we show that the defects caused by mGlu7 I154T can be reversed by agonists, providing the rationale for proposing mGlu7 as a potential therapeutic target for NDDs.

Internal Clocks, mGluR7 and Microtubules: A Primer for the Molecular Encoding of Target Durations in Cerebellar Purkinje Cells and Striatal Medium Spiny Neurons

The majority of studies in the field of timing and time perception have generally focused on sub- and supra-second time scales, specific behavioral processes, and/or discrete neuronal circuits. In an attempt to find common elements of interval timing from a broader perspective, we review the literature and highlight the need for cell and molecular studies that can delineate the neural mechanisms underlying temporal processing. Moreover, given the recent attention to the function of microtubule proteins and their potential contributions to learning and memory consolidation/re-consolidation, we propose that these proteins play key roles in coding temporal information in cerebellar Purkinje cells (PCs) and striatal medium spiny neurons (MSNs). The presence of microtubules at relevant neuronal sites, as well as their adaptability, dynamic structure, and longevity, makes them a suitable candidate for neural plasticity at both intra- and inter-cellular levels. As a consequence, microtubules appear capable of maintaining a temporal code or engram and thereby regulate the firing patterns of PCs and MSNs known to be involved in interval timing. This proposed mechanism would control the storage of temporal information triggered by postsynaptic activation of mGluR7. This, in turn, leads to alterations in microtubule dynamics through a "read-write" memory process involving alterations in microtubule dynamics and their hexagonal lattice structures involved in the molecular basis of temporal memory.

Nedd4 E3 ligase and beta-arrestins regulate ubiquitination, trafficking, and stability of the mGlu7 receptor

The metabotropic glutamate receptor 7 (mGlu7) is a class C G protein-coupled receptor that modulates excitatory neurotransmitter release at the presynaptic active zone. Although post-translational modification of cellular proteins with ubiquitin is a key molecular mechanism governing protein degradation and function, mGlu7 ubiquitination and its functional consequences have not been elucidated yet. Here, we report that Nedd4 ubiquitin E3 ligase and β-arrestins regulate ubiquitination of mGlu7 in heterologous cells and rat neurons. Upon agonist stimulation, β-arrestins recruit Nedd4 to mGlu7 and facilitate Nedd4-mediated ubiquitination of mGlu7. Nedd4 and β-arrestins regulate constitutive and agonist-induced endocytosis of mGlu7 and are required for mGlu7-dependent MAPK signaling in neurons. In addition, Nedd4-mediated ubiquitination results in the degradation of mGlu7 by both the ubiquitin-proteasome system and the lysosomal degradation pathway. These findings provide a model in which Nedd4 and β-arrestin act together as a complex to regulate mGlu7 surface expression and function at presynaptic terminals.

From the Cover: Manganese and Rotenone-Induced Oxidative Stress Signatures Differ in iPSC-Derived Human Dopamine Neurons

Parkinson's disease (PD) is the result of complex interactions between genetic and environmental factors. Two chemically distinct environmental stressors relevant to PD are the metal manganese and the pesticide rotenone. Both are thought to exert neurotoxicity at least in part via oxidative stress resulting from impaired mitochondrial activity. Identifying shared mechanism of action may reveal clues towards an understanding of the mechanisms underlying PD pathogenesis. Here we compare the effects of manganese and rotenone in human-induced pluripotent stem cells-derived postmitotic mesencephalic dopamine neurons by assessing several different oxidative stress endpoints. Manganese, but not rotenone caused a concentration and time-dependent increase in intracellular reactive oxygen/nitrogen species measured by quantifying the fluorescence of oxidized chloromethyl 2',7'-dichlorodihydrofluorescein diacetate (DCF) assay. In contrast, rotenone but not manganese caused an increase in cellular isoprostane levels, an indicator of lipid peroxidation. Manganese and rotenone both caused an initial decrease in cellular reduced glutathione; however, glutathione levels remained low in neurons treated with rotenone for 24 h but recovered in manganese-exposed cells. Neurite length, a sensitive indicator of overall neuronal health was adversely affected by rotenone, but not manganese. Thus, our observations suggest that the cellular oxidative stress evoked by these 2 agents is distinct yielding unique oxidative stress signatures across outcome measures. The protective effect of rasagiline, a compound used in the clinic for PD, had negligible impact on any of oxidative stress outcome measures except a subtle significant decrease in manganese-dependent production of reactive oxygen/nitrogen species detected by the DCF assay.

The neuroprotective effects of orthosteric agonists of group II and III mGluRs in primary neuronal cell cultures are dependent on developmental stage

Activation of metabotropic glutamate receptors (mGluRs) modulates neuronal excitability. Here, we evaluated the neuroprotective potential of four structurally diverse activators of group II and III mGluRs: an orthosteric agonist of group II (LY354740), an orthosteric agonist of group III (ACPT-I), an allosteric agonist of mGluR7 (AMN082) and a positive allosteric modulator (PAM) of mGluR4 (VU0361737). Neurotoxicity was induced by the pro-apoptotic agents: staurosporine (St) and doxorubicin (Dox) or the excitotoxic factor glutamate (Glu). The effects were analyzed in primary hippocampal (HIP) and cerebellar granule cell (CGC) cultures at two developmental stages, at 7 and 12 days in vitro (DIV). The data reveal a general neuroprotective effect of group II and III mGluR activators against the St- and Glu- but not Dox-induced cell damage. We found that neuroprotective effects of group II and III mGluR orthosteric agonists (LY354740 and ACPT-I) were higher at 12 DIV when compared to 7 DIV cells. In contrast, the efficiency of allosteric mGluR agents (AMN082 and VU0361737) did not differ between 7 and 12 DIV in both, St and Glu models of neuronal cell damage. Interestingly, the protective effects of activators of group II and III mGluRs were blocked by relevant antagonists only against Glu-induced neurotoxicity. Moreover, the observed neuroprotective action of group II and III mGluR activators in the St model was associated with a decreased number of PI-positive cells and no alterations in the caspase-3 activity. Finally, we showed that MAPK/ERK pathway activation was potentially involved in the mechanism of ACPT-I- and AMN082-induced neuroprotection against the St-evoked cellular damage. Our comparative study demonstrated the developmental stage-dependent neuroprotective effect of orthosteric group II and III mGluR agonists. In comparison to allosteric modulators, orthosteric compounds may provide more specific tools for suppression of neuronal cell loss associated with various chronic neurodegenerative conditions. Our results also suggest that the inhibition of intracellular pathways mediating necrotic, rather than apoptotic cascades, may be involved in neuroprotective effects of activators of group II and III mGluRs.

Microtubule: A Common Target for Parkin and Parkinson’s Disease Toxins

Parkinson’s disease (PD) is characterized by the selective loss of nigral dopaminergic (DA) neurons, which have long axons enriched with microtubules. Depolymerization of microtubules by PD toxins such as rotenone disrupts vesicular transport. The ensuing accumulation of vesicles in the cell body leads to increased cytosolic concentration of dopamine due to leakage of the vesicles. Elevated oxidative stress induced by dopamine oxidation may thus trigger the selective demise of DA neurons. Many strategies have been developed to protect DA neurons by stabilizing microtubules either directly or through intracellular signaling cascades. On the other hand, parkin, one of the most frequently mutated genes in PD, encodes for a protein-ubiquitin E3 ligase that strongly binds to microtubules. Parkin stabilizes microtubules through three domains that provide strong and independent interactions with tubulin and microtubules. These interactions anchor parkin on microtubules and may facilitate its E3 ligase activity on misfolded proteins transported along microtubules. Thus, parkin and rotenone, two prominent genetic and environmental factors linked to PD, act in an opposing manner on the same molecular target in the cell, microtubules, whose destruction underlies the selective vulnerability of dopaminergic neurons.

Allosteric modulation of metabotropic glutamate receptor 4 activates IDO1-dependent, immunoregulatory signaling in dendritic cells

Metabotropic glutamate receptor 4 (mGluR4) possesses immune modulatory properties in vivo, such that a positive allosteric modulator (PAM) of the receptor confers protection on mice with relapsing-remitting experimental autoimmune encephalomyelitis (RR-EAE). ADX88178 is a newly-developed, one such mGluR4 modulator with high selectivity, potency, and optimized pharmacokinetics. Here we found that application of ADX88178 in the RR-EAE model system converted disease into a form of mild—yet chronic—neuroinflammation that remained stable for over two months after discontinuing drug treatment. In vitro, ADX88178 modulated the cytokine secretion profile of dendritic cells (DCs), increasing production of tolerogenic IL-10 and TGF-β. The in vitro effects required activation of a G_i-independent, alternative signaling pathway that involved phosphatidylinositol-3-kinase (PI3K), Src kinase, and the signaling activity of indoleamine 2,3-dioxygenase 1 (IDO1). A PI3K inhibitor as well as small interfering RNA targeting Ido1—but not pertussis toxin, which affects G_i protein-dependent responses—abrogated the tolerogenic effects of ADX88178-conditioned DCs in vivo. Thus our data indicate that, in DCs, highly selective and potent mGluR4 PAMs such as ADX88178 may activate a G_i-independent, long-lived regulatory pathway that could be therapeutically exploited in chronic autoimmune diseases such as multiple sclerosis.

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ADX88178, a selective mGluR4 PAM, exerts long-term therapeutic effects in RR-EAE.

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ADX88178 activates a noncanonical mGluR4 signaling in DCs.

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ADX88178 induces a tolerogenic functional phenotype in DCs via immunoregulatory IDO1.

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Highly selective mGluR4 PAMs may represent novel drugs in chronic neuroinflammation.

Neuroprotective effects of metabotropic glutamate receptor group II and III activators against MPP(+)-induced cell death in human neuroblastoma SH-SY5Y cells: the impact of cell differentiation state

Recent studies have documented that metabotropic glutamate receptors from group II and III (mGluR II/III) are a potential target in the symptomatic treatment of Parkinson's disease (PD), however, the neuroprotective effects of particular mGluR II/III subtypes in relation to PD pathology are recognized only partially. In the present study, we investigated the effect of various mGluR II/III activators in the in vitro model of PD using human neuroblastoma SH-SY5Y cell line and mitochondrial neurotoxin MPP(+). We demonstrated that all tested mGluR ligands: mGluR II agonist - LY354740, mGluR III agonist - ACPT-I, mGluR4 PAM - VU0361737, mGluR8 agonist - (S)-3,4-DCPG, mGluR8 PAM - AZ12216052 and mGluR7 allosteric agonist - AMN082 were protective against MPP(+)-evoked cell damage in undifferentiated (UN-) SH-SY5Y cells with the highest neuroprotection mediated by mGluR8-specific agents. However, in retinoic acid- differentiated (RA-) SH-SY5Y cells we found protection mediated only by mGluR8 activators. We also demonstrated the cell proliferation stimulating effect for mGluR4 and mGluR8 PAMs. Next, we showed that the protection mediated by mGluR II/III activators in UN-SH-SY5Y was not accompanied by the modulation of caspase-3 activity, however, a decrease in the number of apoptotic nuclei was found. Finally, we showed that the inhibitor of necroptosis, necrostatin-1 blocked the mGluR III-mediated protection. Altogether our comparative in vitro data add a further proof to neuroprotective effects of mGluR agonists or PAMs and point to mGluR8 as a promising target for neuroprotective interventions in PD. The results also suggest the participation of necroptosis-related molecular pathways in neuroprotective effects of mGluR III activation.

Neuroprotective and symptomatic effects of targeting group III mGlu receptors in neurodegenerative disease

Neurodegenerative disorders possess common pathological mechanisms, such as protein aggregation, inflammation, oxidative stress (OS) and excitotoxicity, raising the possibility of shared therapeutic targets. As a result of the selective cellular and regional expression of group III metabotropic glutamate (mGlu) receptors, drugs targeting such receptors have demonstrated both neuroprotective properties and symptomatic improvements in several models of neurodegeneration. In recent years, the discovery and development of subtype-selective ligands for the group III mGlu receptors has gained pace, allowing further research into the functions of these receptors and revealing their roles in health and disease. Activation of this class of receptors results in neuroprotection, with a variety of underlying mechanisms implicated. Group III mGlu receptor stimulation prevents excitotoxicity by inhibiting glutamate release from neurons and microglia and increasing glutamate uptake by astrocytes. It also attenuates the neuroinflammatory response by reducing glial reactivity and encourages neurotrophic phenotypes. This article will review the current literature with regard to the neuroprotective and symptomatic effects of group III mGlu receptor activation and discuss their promise as therapeutic targets in neurodegenerative disease. We review the neuroprotective and symptomatic effects of targeting group III mGlu receptors in neurodegenerative disease: Excess extracellular glutamate causes overactivation of NMDA receptors resulting in excitotoxicity. Externalization of phosphatidylserine stimulates phagocytosis of neurons by activated microglia, which contribute to damage through glutamate and pro-inflammatory factor release. Reactive astrocytes produce cytotoxic factors enhancing neuronal cell death. Activation of group III mGlu receptors by glutamate and/or mGlu receptor ligands results in inhibition of glutamate release from presynaptic terminals and microglia, reducing excitotoxicity. Astrocytic glutamate uptake is increased and microglia produce neurotrophic factors.

Automated imaging system for fast quantitation of neurons, cell morphology and neurite morphometry in vivo and in vitro

Quantitation of neurons using stereologic approaches reduces bias and systematic error, but is time-consuming and labor-intensive. Accurate methods for quantifying neurons in vitro are lacking; conventional methodologies are limited in reliability and application. The morphological properties of the soma and neurites are a key aspect of neuronal phenotype and function, but the assays commonly used in such evaluations are beset with several methodological drawbacks. Herein we describe automated techniques to quantify the number and morphology of neurons (or any cell type, e.g., astrocytes) and their processes with high speed and accuracy. Neuronal quantification from brain tissue using a motorized stage system yielded results that were statistically comparable to those generated by stereology. The approach was then adapted for in vitro neuron and neurite outgrowth quantification. To determine the utility of our methods, rotenone was used as a neurotoxicant leading to morphological changes in neurons and cell death, astrocytic activation, and loss of neurites. Importantly, our technique counted about 8 times as many neurons in less than 5-10% of the time taken by manual stereological analysis.

Therapeutic potential of metabotropic glutamate receptor modulators

Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and is a major player in complex brain functions. Glutamatergic transmission is primarily mediated by ionotropic glutamate receptors, which include NMDA, AMPA and kainate receptors. However, glutamate exerts modulatory actions through a family of metabotropic G-protein-coupled glutamate receptors (mGluRs). Dysfunctions of glutamatergic neurotransmission have been implicated in the etiology of several diseases. Therefore, pharmacological modulation of ionotropic glutamate receptors has been widely investigated as a potential therapeutic strategy for the treatment of several disorders associated with glutamatergic dysfunction. However, blockade of ionotropic glutamate receptors might be accompanied by severe side effects due to their vital role in many important physiological functions. A different strategy aimed at pharmacologically interfering with mGluR function has recently gained interest. Many subtype selective agonists and antagonists have been identified and widely used in preclinical studies as an attempt to elucidate the role of specific mGluRs subtypes in glutamatergic transmission. These studies have allowed linkage between specific subtypes and various physiological functions and more importantly to pathological states. This article reviews the currently available knowledge regarding the therapeutic potential of targeting mGluRs in the treatment of several CNS disorders, including schizophrenia, addiction, major depressive disorder and anxiety, Fragile X Syndrome, Parkinson’s disease, Alzheimer’s disease and pain.

Regulation of N-methyl-D-aspartic acid (NMDA) receptors by metabotropic glutamate receptor 7

Emerging evidence suggests that metabotropic glutamate receptors (mGluRs) are potential novel targets for brain disorders associated with the dysfunction of prefrontal cortex (PFC), a region critical for cognitive and emotional processes. Because N-methyl-D-aspartic acid receptor (NMDAR) dysregulation has been strongly associated with the pathophysiology of mental illnesses, we examined the possibility that mGluRs might be involved in modulating PFC functions by targeting postsynaptic NMDARs. We found that application of prototypical group III mGluR agonists significantly reduced NMDAR-mediated synaptic and ionic currents in PFC pyramidal neurons, which was mediated by mGluR7 localized at postsynaptic neurons and involved the β-arrestin/ERK signaling pathway. The mGluR7 modulation of NMDAR currents was prevented by agents perturbing actin dynamics and by the inhibitor of cofilin, a major actin-depolymerizing factor. Consistently, biochemical and immunocytochemical results demonstrated that mGluR7 activation increased cofilin activity and F-actin depolymerization via an ERK-dependent mechanism. Furthermore, mGluR7 reduced the association of NMDARs with the scaffolding protein PSD-95 and the surface level of NMDARs in an actin-dependent manner. These data suggest that mGluR7, by affecting the cofilin/actin signaling, regulates NMDAR trafficking and function. Because ablation of mGluR7 leads to a variety of behavioral symptoms related to PFC dysfunction, such as impaired working memory and reduced anxiety and depression, our results provide a potential mechanism for understanding the role of mGluR7 in mental health and disorders.

The microtubule cytoskeleton acts as a key downstream effector of neurotransmitter signaling

Microtubules are well known to play a key role in the trafficking of neurotransmitters to the synapse. However, less attention has been paid to their role as downstream effectors of neurotransmitter signaling in the target neuron. Here, we show that neurotransmitter-based signaling to the microtubule cytoskeleton regulates downstream microtubule function through several mechanisms. These include tubulin posttranslational modification, binding of microtubule-associated proteins, release of microtubule-interacting second messenger molecules, and regulation of tubulin expression levels. We review the evidence for neurotransmitter regulation of the microtubule cytoskeleton, focusing on the neurotransmitters serotonin, melatonin, dopamine, glutamate, glycine, and acetylcholine. Some evidence suggests that microtubules may even play a more direct role in propagating action potentials through conductance of electric current. In turn, there is evidence for the regulation of neurotransmission by the microtubule cytoskeleton.

Therapeutic potential of targeting group III metabotropic glutamate receptors in the treatment of Parkinson's disease

Current drugs used in the treatment of Parkinson's disease (PD), for example, L-DOPA and dopamine agonists, are very effective at reversing the motor symptoms of the disease. However, they do little to combat the underlying degeneration of dopaminergic neurones in the substantia nigra pars compacta (SNc) and their long-term use is associated with the appearance of adverse effects such as L-DOPA-induced dyskinesia. Much emphasis has therefore been placed on finding alternative non-dopaminergic drugs that may circumvent some or all of these problems. Group III metabotropic glutamate (mGlu) receptors were first identified in the basal ganglia a decade ago. One or more of these receptors (mGlu4, mGlu7 or mGlu8) is found on pre-synaptic terminals of basal ganglia pathways whose overactivity is implicated not only in the generation of motor symptoms in PD, but also in driving the progressive SNc degeneration. The finding that drugs which activate group III mGlu receptors can inhibit transmission across these overactive synapses has lead to the proposal that group III mGlu receptors are promising targets for drug discovery in PD. This paper provides a comprehensive review of the role and target potential of group III mGlu receptors in the basal ganglia. Overwhelming evidence obtained from in vitro studies and animal models of PD supports group III mGlu receptors as potentially important drug targets for providing both symptom relief and neuroprotection in PD.

A molecular basis for the increased vulnerability of substantia nigra dopamine neurons in aging and Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder of unknown etiology. There is no cure or proven strategy for slowing the progression of the disease. Although there are signs of pathology in many brain regions, the core symptoms of PD are attributable to the selective degeneration of dopaminergic neurons in the substantia nigra pars compacta. A potential clue to the vulnerability of these neurons is an increasing reliance with age upon L-type Ca(2+) channels with a pore-forming Cav1.3 subunit to support autonomous activity. This reliance could pose a sustained stress on mitochondrial ATP generating oxidative phosphorylation, accelerating cellular aging and death. Systemic administration of isradipine, a dihydropyridine blocker of these channels, forces dopaminergic neurons in rodents to revert to a juvenile, L-type Ca(2+) channel independent mechanism to generate autonomous activity. This "rejuvenation" confers protection against toxins that produce experimental Parkinsonism, pointing to a potential neuroprotective strategy for PD. Their decades-long track record of safe use in the treatment of hypertension makes dihydropyridines particularly attractive as a therapeutic tool in PD.

Mitochondrial reactive oxygen species control T cell activation by regulating IL-2 and IL-4 expression: mechanism of ciprofloxacin-mediated immunosuppression

This article shows that T cell activation-induced expression of the cytokines IL-2 and -4 is determined by an oxidative signal originating from mitochondrial respiratory complex I. We also report that ciprofloxacin, a fluoroquinolone antibiotic, exerts immunosuppressive effects on human T cells suppressing this novel mechanism. Sustained treatment of preactivated primary human T cells with ciprofloxacin results in a dose-dependent inhibition of TCR-induced generation of reactive oxygen species (ROS) and IL-2 and -4 expression. This is accompanied by the loss of mitochondrial DNA and a resulting decrease in activity of the complex I. Consequently, using a complex I inhibitor or small interfering RNA-mediated downregulation of the complex I chaperone NDUFAF1, we demonstrate that TCR-triggered ROS generation by complex I is indispensable for activation-induced IL-2 and -4 expression and secretion in resting and preactivated human T cells. This oxidative signal (H(2)O(2)) synergizes with Ca(2+) influx for IL-2/IL-4 expression and facilitates induction of the transcription factors NF-kappaB and AP-1. Moreover, using T cells isolated from patients with atopic dermatitis, we show that inhibition of complex I-mediated ROS generation blocks disease-associated spontaneous hyperexpression and TCR-induced expression of IL-4. Prolonged ciprofloxacin treatment of T cells from patients with atopic dermatitis also blocks activation-induced expression and secretion of IL-4. Thus, our work shows that the activation phenotype of T cells is controlled by a mitochondrial complex I-originated oxidative signal.

Arrestin development: emerging roles for beta-arrestins in developmental signaling pathways

Arrestins were identified as mediators of G protein-coupled receptor (GPCR) desensitization and endocytosis. However, it is now clear that they scaffold many intracellular signaling networks to modulate the strength and duration of signaling by diverse types of receptors--including those relevant to the Hedgehog, Wnt, Notch, and TGFbeta pathways--and downstream kinases such as the MAPK and Akt/PI3K cascades. The involvement of arrestins in many discrete developmental signaling events suggests an indispensable role for these multifaceted molecular scaffolds.

Vascular Pathology in Male Lewis Rats following Short-Term, Low-Dose Rotenone Administration

The long-term administration of low doses of rotenone has been used to produce a model of Parkinson disease (PD) in rats. However, only about 50% of similarly treated rats develop the PD-like syndrome, with many dying during the first few days of treatment. The lesions in male Lewis rats that became moribund or died after short-term, low-dose rotenone administration are described. Dosed rats had fibrinoid change and acute hemorrhage involving small arteries and arterioles of the brain and lungs. The thalamus, hypothalamus, and medulla oblongata were most frequently and severely affected. Blood vessels in the brain of some male Lewis rats appeared acutely susceptible to the effects of rotenone. Understanding the selective nature of the fibrinoid change and hemorrhage might explain how rotenone produces PD-like signs and lesions in rats, and it might also provide the basis for a model of intraparenchymal hemorrhagic cerebrovascular disease (i.e., hemorrhagic strokes) in humans.

Activation of 5-HT2A/C receptors counteracts 5-HT1A regulation of n-methyl-D-aspartate receptor channels in pyramidal neurons of prefrontal cortex

Abnormal serotonin-glutamate interaction in prefrontal cortex (PFC) is implicated in the pathophysiology of many mental disorders, including schizophrenia and depression. However, the mechanisms by which this interaction occurs remain unclear. Our previous study has shown that activation of 5-HT_1A receptors inhibits N-methyl-d-aspartate (NMDA) receptor (NMDAR) currents in PFC pyramidal neurons by disrupting microtubule-based transport of NMDARs. Here we found that activation of 5-HT_2A/C receptors significantly attenuated the effect of 5-HT_1A on NMDAR currents and microtubule depolymerization. The counteractive effect of 5-HT_2A/C on 5-HT_1A regulation of synaptic NMDAR response was also observed in PFC pyramidal neurons from intact animals treated with various 5-HT-related drugs. Moreover, 5-HT_2A/C stimulation triggered the activation of extracellular signal-regulated kinase (ERK) in dendritic processes. Inhibition of the β-arrestin/Src/dynamin signaling blocked 5-HT_2A/C activation of ERK and the counteractive effect of 5-HT_2A/C on 5-HT_1A regulation of NMDAR currents. Immunocytochemical studies showed that 5-HT_2A/C treatment blocked the inhibitory effect of 5-HT_1A on surface NR2B clusters on dendrites, which was prevented by cellular knockdown of β-arrestins. Taken together, our study suggests that serotonin, via 5-HT_1A and 5-HT_2A/C receptor activation, regulates NMDAR functions in PFC neurons in a counteractive manner. 5-HT_2A/C, by activating ERK via the β-arrestin-dependent pathway, opposes the 5-HT_1A disruption of microtubule stability and NMDAR transport. These findings provide a framework for understanding the complex interactions between serotonin and NMDARs in PFC, which could be important for cognitive and emotional control in which both systems are highly involved.

‘Rejuvenation’ protects neurons in mouse models of Parkinson’s disease

Why dopamine-containing neurons of the brain's substantia nigra pars compacta die in Parkinson's disease has been an enduring mystery. Our studies suggest that the unusual reliance of these neurons on L-type Ca(v)1.3 Ca2+ channels to drive their maintained, rhythmic pacemaking renders them vulnerable to stressors thought to contribute to disease progression. The reliance on these channels increases with age, as juvenile dopamine-containing neurons in the substantia nigra pars compacta use pacemaking mechanisms common to neurons not affected in Parkinson's disease. These mechanisms remain latent in adulthood, and blocking Ca(v)1.3 Ca2+ channels in adult neurons induces a reversion to the juvenile form of pacemaking. Such blocking ('rejuvenation') protects these neurons in both in vitro and in vivo models of Parkinson's disease, pointing to a new strategy that could slow or stop the progression of the disease.

Beta-arrestins and cell signaling

Upon their discovery, beta-arrestins 1 and 2 were named for their capacity to sterically hinder the G protein coupling of agonist-activated seven-transmembrane receptors, ultimately resulting in receptor desensitization. Surprisingly, recent evidence shows that beta-arrestins can also function to activate signaling cascades independently of G protein activation. By serving as multiprotein scaffolds, the beta-arrestins bring elements of specific signaling pathways into close proximity. beta-Arrestin regulation has been demonstrated for an ever-increasing number of signaling molecules, including the mitogen-activated protein kinases ERK, JNK, and p38 as well as Akt, PI3 kinase, and RhoA. In addition, investigators are discovering new roles for beta-arrestins in nuclear functions. Here, we review the signaling capacities of these versatile adapter molecules and discuss the possible implications for cellular processes such as chemotaxis and apoptosis.

Novel role for mitochondria: protein kinase Ctheta-dependent oxidative signaling organelles in activation-induced T-cell death

Reactive oxygen species (ROS) play a key role in regulation of activation-induced T-cell death (AICD) by induction of CD95L expression. However, the molecular source and the signaling steps necessary for ROS production are largely unknown. Here, we show that the proximal T-cell receptor-signaling machinery, including ZAP70 (zeta chain-associated protein kinase 70), LAT (linker of activated T cells), SLP76 (SH2 domain-containing leukocyte protein of 76 kDa), PLCgamma1 (phospholipase Cgamma1), and PKCtheta (protein kinase Ctheta), are crucial for ROS production. PKCtheta is translocated to the mitochondria. By using cells depleted of mitochondrial DNA, we identified the mitochondria as the source of activation-induced ROS. Inhibition of mitochondrial electron transport complex I assembly by small interfering RNA (siRNA)-mediated knockdown of the chaperone NDUFAF1 resulted in a block of ROS production. Complex I-derived ROS are converted into a hydrogen peroxide signal by the mitochondrial superoxide dismutase. This signal is essential for CD95L expression, as inhibition of complex I assembly by NDUFAF1-specific siRNA prevents AICD. Similar results were obtained when metformin, an antidiabetic drug and mild complex I inhibitor, was used. Thus, we demonstrate for the first time that PKCtheta-dependent ROS generation by mitochondrial complex I is essential for AICD.

mGluR7 undergoes rapid internalization in response to activation by the allosteric agonist AMN082

The G-protein coupled receptor (GPCR) metabotropic glutamate receptor 7 (mGluR7) is widely expressed throughout the nervous system and is implicated in diverse physiological processes ranging from synaptic plasticity to neuroprotection. To date, unequivocally assigning specific functions to mGluR7 has been hampered by a lack of specific pharmacological tools, however, an mGluR7 specific allosteric agonist, AMN082, was recently discovered. Accumulating evidence indicates that in addition to G-protein activation, GPCRs trigger critical intracellular signalling cascades during agonist-induced internalization. Thus, to determine if AMN082 will be useful for evaluating signalling events related to mGluR7 internalization as well as receptor activation we have examined whether AMN082 induces mGluR7 endocytosis. Using an immunofluorescence assay we demonstrate that AMN082 induces robust internalization of mGluR7 overexpressed in dissociated hippocampal neurons. AMN082-induced mGluR7 internalization was resistant to inhibition by a competitive antagonist consistent with the distinct binding site of the allosteric agonist from the glutamate-binding pocket utilized by conventional orthosteric ligands. Finally, as an independent assay of receptor internalization we overexpressed N-terminal pHluorin-tagged mGluR7 in neurons, allowing live imaging of surface receptors in real time. AMN082 treatment produced a rapid loss of surface mGluR7 as indicated by decreased fluorescence confirming the ability of allosteric receptor activation to trigger mGluR7 endocytosis. Thus, AMN082 will be effective for investigating physiological processes related to both mGluR7 activation and internalization such as control of bidirectional plasticity at mossy fiber-st. lucidum interneuron synapses.

Selective Activation of Group III Metabotropic Glutamate Receptors by l-(+)-2-Amino-4-phosphonobutryic Acid Protects the Nigrostriatal System against 6-Hydroxydopamine Toxicity in Vivo

Evidence from several studies suggests that the progressive degeneration of dopaminergic (DA) neurones of the substantia nigra pars compacta (SNc) in Parkinson's disease (PD) may in part be due to excessive release of glutamate from subthalamic projections onto nigral DA neurones. Previous in vitro studies have demonstrated that selective activation of Group III metabotropic glutamate receptors (mGluR) negatively modulates excitatory transmission in the SNc and is neuroprotective against glutamate-mediated toxicity. Consistent with this, we have reported preliminary data indicating that the selective group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) can also protect the nigrostriatal system against 6-hydroxydopamine (6-OHDA) toxicity in vivo. We have now extended these preliminary studies in this model and report here that both acute and subchronic intranigral injections of l-AP4 provide significant protection of the nigrostriatal system against 6-OHDA toxicity. This neuroprotection displays a bell-shaped profile with a clear concentration-dependent relationship. In contrast, when administered to animals 7 days post-6-OHDA lesioning, l-AP4 significantly protects the functionality but not the integrity of the nigrostriatal system. We further demonstrate that neuroprotection by l-AP4 in vivo is reversed by coadministration of the selective Group III mGluR antagonist (RS)-alpha-methylserine-O-phosphate, confirming a receptor-mediated mechanism of action. These data provide further compelling evidence that selective activation of Group III mGluR is neuroprotective in an in vivo experimental model of PD, a finding that may have important implications for the future treatment of this disease.

Interaction of alpha-synuclein and dopamine metabolites in the pathogenesis of Parkinson's disease: a case for the selective vulnerability of the substantia nigra

Parkinson's disease (PD) is the most common movement disorder. Major disease symptoms are due to the loss of dopaminergic (DA) neurons in substantia nigra (SN). The pathologic hallmark of PD is Lewy bodies (LBs) in the SN and the major protein in LBs is alpha-synuclein (AS). A plethora of evidence points towards the culpability of AS in the pathogenesis of PD including: (1) linkage of AS mutations to familial forms of PD, (2) triplication of the AS locus causing PD, and (3) overexpression of AS in transgenic mice and Drosophila leads to PD-like phenotypes. Studies of purified AS have revealed its ability to interact with diverse molecules including monoamines. Monoamine metabolism is associated with oxidative stress conditions that may contribute to DA-AS interactions promoting aggregation and neuronal damage. However, in order to explain the selective vulnerability of DA neurons there needs to be a link between DA metabolism and AS aggregation. Since only the DA neurons contain significant amounts of DA, this has been hypothesized to account for the selective vulnerability of SN neurons. However, DA itself may not be toxic at physiologic relevant doses, so it is probable that other DA metabolites may play a major role in AS aggregation. In this review, we discuss the role of the DA metabolite 3,4-dihydroxyphenylacetaldehyde to provide a plausible link between DA production and metabolism, AS aggregation and the pathogenesis of PD.