A sensitive and selective assay of neuronal degeneration in cell culture

Novel Positive Allosteric Modulators of Glutamate Transport Have Neuroprotective Properties in an in Vitro Excitotoxic Model

Dysfunction of excitatory amino acid transporters (EAATs) has been implicated in the pathogenesis of various neurological disorders, such as stroke, brain trauma, epilepsy, and several neurodegenerative disorders. EAAT2 is the main transporter subtype responsible for glutamate clearance in the brain, and plays a key role in regulating neurotransmission and preventing excitotoxicity. Therefore, compounds that increase the activity of EAAT2 have therapeutic potential for neuroprotection. In previous studies, we used virtual screening approaches to identify novel positive allosteric modulators (PAMs) of EAAT2. These compounds were shown to selectively increase the activity of EAAT2 and increase V_max of transport, without changing substrate affinity. In this work, our major effort was to investigate whether increasing the activity of EAAT2 by allosteric modulation would translate to neuroprotection in in vitro primary culture models of excitotoxicity. To investigate potential neuroprotective effects of one EAAT2 PAM, GT949, we subjected cultures to acute and prolonged excitotoxic insults by exogenous application of glutamate, or oxidative stress by application of hydrogen peroxide. GT949 administration did not result in neuroprotection in the oxidative stress model, likely due to damage of the glutamate transporters. However, GT949 displayed neuroprotective properties after acute and prolonged glutamate-mediated excitotoxicity. We propose that this compound prevents excess glutamate signaling by increasing the rate of glutamate clearance by EAAT2, thereby preventing excitotoxic damage and cell death. This novel class of compounds is therefore an innovative approach for neuroprotection with potential for translation in in vivo animal models of excitotoxicity.

Mitogen- and Stress-Activated Protein Kinase 1 Regulates Status Epilepticus-Evoked Cell Death in the Hippocampus

Mitogen-activated protein kinase (MAPK) signaling has been implicated in a wide range of neuronal processes, including development, plasticity, and viability. One of the principal downstream targets of both the extracellular signal-regulated kinase/MAPK pathway and the p38 MAPK pathway is Mitogen- and Stress-activated protein Kinase 1 (MSK1). Here, we sought to understand the role that MSK1 plays in neuroprotection against excitotoxic stimulation in the hippocampus. To this end, we utilized immunohistochemical labeling, a MSK1 null mouse line, cell viability assays, and array-based profiling approaches. Initially, we show that MSK1 is broadly expressed within the major neuronal cell layers of the hippocampus and that status epilepticus drives acute induction of MSK1 activation. In response to the status epilepticus paradigm, MSK1 KO mice exhibited a striking increase in vulnerability to pilocarpine-evoked cell death within the CA1 and CA3 cell layers. Further, cultured MSK1 null neurons exhibited a heighted level of N-methyl-D-aspartate-evoked excitotoxicity relative to wild-type neurons, as assessed using the lactate dehydrogenase assay. Given these findings, we examined the hippocampal transcriptional profile of MSK1 null mice. Affymetrix array profiling revealed that MSK1 deletion led to the significant (>1.25-fold) downregulation of 130 genes and an upregulation of 145 genes. Notably, functional analysis indicated that a subset of these genes contribute to neuroprotective signaling networks. Together, these data provide important new insights into the mechanism by which the MAPK/MSK1 signaling cassette confers neuroprotection against excitotoxic insults. Approaches designed to upregulate or mimic the functional effects of MSK1 may prove beneficial against an array of degenerative processes resulting from excitotoxic insults.

Inhibition of soluble tumor necrosis factor is therapeutic in Huntington's disease

Neuroinflammation is a common feature of many neurodegenerative diseases, including Huntington's disease (HD). HD is an autosomal dominant genetic disease caused by an expanded CAG repeat in exon 1 of the huntingtin (HTT) gene. Previous studies demonstrated that levels of several proinflammatory cytokines, including tumor necrosis factor (TNF)-α, were higher in the plasma and brain tissues of mice and patients with HD, suggesting that inflammation may contribute to HD progression. To evaluate the pathological role of TNF-α in HD pathogenesis, we blocked TNF-α signaling using a dominant negative inhibitor of soluble TNF-α (XPro1595). XPro1595 effectively suppressed the inflammatory responses of primary astrocytes-enriched culture isolated from a transgenic mouse model (R6/2) and human astrocytes-enriched culture derived from induced pluripotent stem cells (iPSCs) of HD patients evoked by lipopolysaccharide and cytokines, respectively. Moreover, XPro1595 protected the cytokine-induced toxicity of primary R6/2 neurons and human neurons derived from iPSCs of HD patients. To assess the beneficial effect of XPro1595 in vivo, an intracerebroventricular (i.c.v.) infusion was provided with an osmotic minipump. ELISA analyses showed that i.c.v. infusion of XPro1595 decreased elevated levels of TNFα in the cortex and striatum, improved motor function, reduced caspase activation, diminished the amount of mutant HTT aggregates, increased neuronal density and decreased gliosis in brains of R6/2 mice. Moreover, reducing the peripheral inflammatory response by a systemic injection of XPro1595 improved the impaired motor function of R6/2 mice but did not affect caspase activation. Collectively, our findings suggest that an effective and selective anti-inflammatory treatment targeting the abnormal brain inflammatory response is a potential therapeutic strategy for HD.

5-Hydroxytryptamine type 7 receptor neuroprotection against NMDA-induced excitotoxicity is PDGFβ receptor dependent

The serotonin (5-HT) type 7 receptor is expressed throughout the CNS including the hippocampus. Long-term (2-24 h) activation of 5-HT7 receptors regulates growth factor receptor expression, including the expression of platelet-derived growth factor (PDGF) β receptors. Direct activation of PDGFβ receptors in primary hippocampal and cortical neurons inhibits NMDA receptor activity and attenuates NMDA receptor-induced neurotoxicity. Our objective was to investigate whether the 5-HT7 receptor-induced increase in PDGFβ receptor expression would be similarly neuroprotective. We demonstrate that 5-HT7 receptor agonist treatment in primary hippocampal neurons also increases the expression of phospholipase C (PLC) γ, a downstream effector of PDGFβ receptors associated with the inhibition of NMDA receptor activity. To determine if the up-regulation of PDGFβ receptors is neuroprotective, primary hippocampal neurons were incubated with the 5-HT7 receptor agonist, LP 12, for 24 h. Indeed, LP 12 treatment prevented NMDA-induced neurotoxicity and this effect was dependent on PDGFβ receptor kinase activity. Treatment of primary neurons with LP 12 also differentially altered NMDA receptor subunit expression, reducing the expression of NR1 and NR2B, but not NR2A. These findings demonstrate the potential for providing growth factor receptor-dependent neuroprotective effects using small-molecule ligands of G protein-coupled receptors.

A critical role of astrocyte-mediated nuclear factor-κB-dependent inflammation in Huntington's disease

Huntington's disease (HD) is an autosomal disease caused by a CAG repeat expansion in the huntingtin (HTT) gene. The resultant mutant HTT protein (mHTT) forms aggregates in various types of cells, including neurons and glial cells and preferentially affects brain function. We found that two HD mouse models (Hdh(150Q) and R6/2) were more susceptible than wild-type (WT) mice to lipopolysaccharide-evoked systemic inflammation and produced more proinflammatory cytokines in the brain. Such an enhanced inflammatory response in the brain was not observed in N171- 82Q mice that express mHTT only in neurons, but not in glial cells. Thus, HD glia might play an important role in chronic inflammation that accelerates disease progression in HD mice. Intriguingly, enhanced activation of nuclear factor (NF)-κB-p65 (p65), a transcriptional mediator of inflammatory responses, was observed in astrocytes of patients and mice with HD. Results obtained using primary R6/2 astrocytes suggest that these cells exhibited higher IκB kinase (IKK) activity that caused prolongation of NF-κB activation, thus upregulating proinflammatory factors during inflammation. R6/2 astrocytes also produced a more-damaging effect on primary R6/2 neurons than did WT astrocytes during inflammation. Blockage of IKK reduced the neuronal toxicity caused by R6/2 astrocytes and ameliorated several HD symptoms of R6/2 mice (e.g. decreased neuronal density, impaired motor coordination and poor cognitive function). Collectively, our results indicate that enhancement of the p65-mediated inflammatory response in astrocytes contributes to HD pathogenesis. Therapeutic interventions aimed at preventing neuronal inflammation may be an important strategy for treating HD.

Development of Neh2-luciferase reporter and its application for high throughput screening and real-time monitoring of Nrf2 activators

The NF-E2-related factor 2 (Nrf2) is a key transcriptional regulator of antioxidant defense and detoxification. To directly monitor stabilization of Nrf2, we fused its Neh2 domain, responsible for the interaction with its nucleocytoplasmic regulator, Keap1, to firefly luciferase (Neh2-luciferase). We show that Neh2 domain is sufficient for recognition, ubiquitination, and proteasomal degradation of Neh2-luciferase fusion protein. The Neh2-luc reporter system allows direct monitoring of the adaptive response to redox stress and classification of drugs based on the time course of reporter activation. The reporter was used to screen the Spectrum library of 2000 biologically active compounds to identify activators of Nrf2. The most robust and yet nontoxic Nrf2 activators found--nordihydroguaiaretic acid, fisetin, and gedunin--induced astrocyte-dependent neuroprotection from oxidative stress via an Nrf2-dependent mechanism.

Physiological oxygen level is critical for modeling neuronal metabolism in vitro

In vitro models are important tools for studying the mechanisms that govern neuronal responses to injury. Most neuronal culture methods employ nonphysiological conditions with regard to metabolic parameters. Standard neuronal cell culture is performed at ambient (21%) oxygen levels, whereas actual tissue oxygen levels in the mammalian brain range from 1% to 5%. In this study, we examined the consequences of oxygen level on the viability and metabolism of primary cultures of cortical neurons. Our results indicate that physiological oxygen level (5% O(2)) has a beneficial effect on cortical neuronal survival and mitochondrial function in vitro. Moreover, oxygen level affects metabolic fluxes: glucose uptake and glycolysis was enhanced at physiological oxygen level, whereas glucose oxidation and fatty acid oxidation were reduced. Adenosine monophosphate-activated protein kinase (AMPK) was more activated in 5% O(2) and appears to play a role in these metabolic effects. Inhibiting AMPK activity with compound C decreased glucose uptake, intracellular ATP level, and viability in neurons cultured in 5% O(2). These data indicate that oxygen level is an important parameter to consider when modeling neuronal responses to stress in vitro.

Assessment of general anaesthetic cytotoxicity in murine cortical neurones in dissociated culture

General anaesthetics are proposed to cause unconsciousness by modulating neuronal excitability in the mammalian brain through mechanisms that include enhancement of inhibitory GABA(A) receptor currents and suppression of excitatory glutamate receptor responses. Both intravenous and volatile agents may produce neurotoxic effects during early postnatal rodent brain development through similar mechanisms. In the following study, we investigated anaesthetic cytotoxicity in primary cortical neurones and glia from postnatal day 2-8 mice. Cultures at 4-20 days in vitro were exposed to combinations of ketamine (100 μM to 3 mM), nitrous oxide (75%, v/v) and/or isoflurane (1.5-5%, v/v) for 6-12 h. Neuronal survival and cell death were measured via microtubule associated protein 2 immunoassay and lactate dehydrogenase release assays, respectively. Clinically relevant anaesthetic concentrations of ketamine, nitrous oxide and isoflurane had no significant neurotoxic effects individually or when given as anaesthetic cocktails, even with up to 12 h exposure. This lack of neurotoxicity was observed regardless of whether cultures were prepared from postnatal day 0-2 or day 8 mice, and was also unaffected by number of days in vitro (DIV 4-20). Significant neurotoxic effects were only observed at supraclinical concentrations (e.g. 1-3 mM ketamine). Our study suggests that neurotoxicity previously reported in vivo is not due to direct cytotoxicity of anaesthetic agents, but results from other impacts of the anaesthetised state during early brain development.

Effects of mutant huntingtin on mGluR5-mediated dual signaling pathways: implications for therapeutic interventions

Glutamate excitotoxicity is thought to play an important role in Huntington's disease (HD), which is caused by a polyglutamine expansion in the HD protein huntingtin (htt). Overactivation of group I metabotropic glutamate receptors (mGluRs), which include mGluR1 as well as mGluR5 and are coupled via phospholipase C to the inositol phosphate pathway, is found to be involved in mutant htt-mediated neurotoxicity. However, activation of mGluR5 also leads to neuronal protection. Here, we report that mutant htt can activate both mGluR5-mediated ERK and JNK signaling pathways. While increased JNK signaling causes cell death, activation of ERK signaling pathway is protective against cell death. Expression of mutant htt in cultured cells causes greater activation of JNK than ERK. These findings suggest that selective inhibition of the JNK signaling pathway may offer an effective therapeutic approach for reducing htt-mediated excitotoxicity.

Single-cell ELISA and flow cytometry as methods for highlighting potential neuronal and astrocytic toxicant specificity

The timeline imposed by recent worldwide chemical legislation is not amenable to conventional in vivo toxicity testing, requiring the development of rapid, economical in vitro screening strategies which have acceptable predictive capacities. When acquiring regulatory neurotoxicity data, distinction on whether a toxic agent affects neurons and/or astrocytes is essential. This study evaluated neurofilament (NF) and glial fibrillary acidic protein (GFAP) directed single-cell (S-C) ELISA and flow cytometry as methods for distinguishing cell-specific cytoskeletal responses, using the established human NT2 neuronal/astrocytic (NT2.N/A) co-culture model and a range of neurotoxic (acrylamide, atropine, caffeine, chloroquine, nicotine) and non-neurotoxic (chloramphenicol, rifampicin, verapamil) test chemicals. NF and GFAP directed flow cytometry was able to identify several of the test chemicals as being specifically neurotoxic (chloroquine, nicotine) or astrocytoxic (atropine, chloramphenicol) via quantification of cell death in the NT2.N/A model at cytotoxic concentrations using the resazurin cytotoxicity assay. Those neurotoxicants with low associated cytotoxicity are the most significant in terms of potential hazard to the human nervous system. The NF and GFAP directed S-C ELISA data predominantly demonstrated the known neurotoxicants only to affect the neuronal and/or astrocytic cytoskeleton in the NT2.N/A cell model at concentrations below those affecting cell viability. This report concluded that NF and GFAP directed S-C ELISA and flow cytometric methods may prove to be valuable additions to an in vitro screening strategy for differentiating cytotoxicity from specific neuronal and/or astrocytic toxicity. Further work using the NT2.N/A model and a broader array of toxicants is appropriate in order to confirm the applicability of these methods.

Twenty-First Century Challenges for In Vitro Neurotoxicity

During the last 40 years, studies incorporating in vitro methodologies have greatly advanced our understanding of human nerve cell biology. Attempts have been made to apply these to investigations of neurotoxicity. Due to the complexity of the nervous system, underpinned by an array of integrated interactions between a host of cell types, it is concluded that, at present, alternative neural models are most successful in determining the underlying mechanisms which can cause perturbation of normal functioning of the nervous system, both in adults and during the embryonic period. The use of tiered batteries of test models has been proposed in screening programmes for neurotoxicity, with the generation of much encouraging data in laboratories across the globe. This review aims to discuss the development of neural alternatives, considers the various model systems available, and highlights specific neuronal endpoints which can be tested, in addition to the cytotoxic evaluation of neuronal viability. Developments in molecular and stem cell biology, which are appropriate to neural tissue, and which offer the prospect of exciting advances for the next decade, are cited.

Corticospinal neurons respond differentially to neurotrophins and myelin-associated glycoprotein in vitro

Elucidating the mechanisms that regulate the survival and outgrowth of corticospinal tract (CST) neurons and other CNS tracts will be a key component in developing novel approaches for the treatment of central nervous system (CNS) disorders, including stroke, spinal cord injury (SCI), and motor neuron disease (MND). However, the in vivo complexities of these diseases make a systematic evaluation of potential therapeutics that directly affect corticospinal regeneration or survival very challenging. Here, we use Thy1.2 transgenic mice expressing yellow fluorescent protein (YFP) in postnatal day 8 (P8) corticospinal neurons, as a source of CST neurons that have already established synapses in the spinal cord, to assess factors that influence neurite outgrowth and survival of axotomized CST neurons. After culture, YFP-positive corticospinal neurons represent an enriched neuronal population over other glia and interneurons, survive, and extend processes over time. YFP-positive CST neurons also continue to express the corticospinal markers CTIP2 and Otx1. CST neurons display different degrees of axon extension, dendritic branch length and elaboration, and neurite elongation in response to neurotrophin-3 and ciliary neurotrophic factor, and an inhibitory outgrowth response when cultured on myelin-associated glycoprotein. Some CST neurons are lost with extended culture, which provides a baseline from which we can also assess factors that enhance CST neuron survival. This assay thus allows us to assess independent aspects of CST axonal and dendritic outgrowth kinetics, which allows for the rapid and sensitive investigation of new therapies to address corticospinal neuron outgrowth in the context of CNS injury and neurodegenerative disorders.

Extrasynaptic NMDA receptors couple preferentially to excitotoxicity via calpain-mediated cleavage of STEP

NMDA receptor (NMDAR)-mediated excitotoxicity plays an important role in several CNS disorders, including epilepsy, stroke, and ischemia. Here we demonstrate the involvement of striatal-enriched protein tyrosine phosphatase (STEP) in this critical process. STEP(61) is an alternatively spliced member of the family that is present in postsynaptic terminals. In an apparent paradox, STEP(61) regulates extracellular signal-regulated kinase 1/2 (ERK1/2) and p38, two proteins with opposing functions; activated p38 promotes cell death, whereas activated ERK1/2 promotes cell survival. We found that synaptic stimulation of NMDARs promoted STEP(61) ubiquitination and degradation, concomitant with ERK1/2 activation. In contrast, extrasynaptic stimulation of NMDARs invoked calpain-mediated proteolysis of STEP(61), producing the truncated cleavage product STEP(33) and activation of p38. The calpain cleavage site on STEP was mapped to the kinase interacting motif, a domain required for substrate binding. As a result, STEP(33) neither interacts with nor dephosphorylates STEP substrates. A synthetic peptide spanning the calpain cleavage site efficiently reduced STEP(61) degradation and attenuated p38 activation and cell death in slice models. Furthermore, this peptide was neuroprotective when neurons were subjected to excitotoxicity or cortical slices were exposed to ischemic conditions. These findings suggest a novel mechanism by which differential NMDAR stimulation regulates STEP(61) to promote either ERK1/2 or p38 activation and identifies calpain cleavage of STEP(61) as a valid target for the development of neuroprotective therapy.

Platelet-derived growth factor selectively inhibits NR2B-containing N-methyl-D-aspartate receptors in CA1 hippocampal neurons

Platelet-derived growth factor (PDGF) beta receptor activation inhibits N-methyl-d-aspartate (NMDA)-evoked currents in hippocampal and cortical neurons via the activation of phospholipase Cgamma, PKC, the release of intracellular calcium, and a rearrangement of the actin cytoskeleton. In the hippocampus, the majority of NMDA receptors are heteromeric; most are composed of 2 NR1 subunits and 2 NR2A or 2 NR2B subunits. Using NR2B- and NR2A-specific antagonists, we demonstrate that PDGF-BB treatment preferentially inhibits NR2B-containing NMDA receptor currents in CA1 hippocampal neurons and enhances long-term depression in an NR2B subunit-dependent manner. Furthermore, treatment of hippocampal slices or cultures with PDGF-BB decreases the surface localization of NR2B but not of NR2A subunits. PDGFbeta receptors colocalize to a higher degree with NR2B subunits than with NR2A subunits. After neuronal injury, PDGFbeta receptors and PDGF-BB are up-regulated and PDGFbeta receptor activation is neuroprotective against glutamate-induced neuronal damage in cultured neurons. We demonstrate that the neuroprotective effects of PDGF-BB are occluded by the NR2B antagonist, Ro25-6981, and that PDGF-BB promotes NMDA signaling to CREB and ERK1/2. We conclude that PDGFbetaR signaling, by preferentially targeting NR2B receptors, provides an important mechanism for neuroprotection by growth factors in the central nervous system.

Specific AAV serotypes stably transduce primary hippocampal and cortical cultures with high efficiency and low toxicity

Most current methods of gene delivery for primary cultured hippocampal neurons are limited by toxicity, transient expression, the use of immature neurons and/or low efficiency. We performed a direct comparison of seven serotypes of adeno-associated virus (AAV) vectors for genetic manipulation of primary cultured neurons in vitro. Serotypes 1, 2, 7, 8 and 9 mediated highly efficient, nontoxic, stable long-term gene expression in cultured cortical and hippocampal neurons aged 0-4 weeks in vitro; serotypes 5 and 6 were associated with toxicity at high doses. AAV1 transduced over 90% of all cells with approximately 80% of the transduced cells being neurons. The method was readily adapted to a high-throughput format to demonstrate neurotrophin-mediated neuroprotection from glutamate toxicity in cultured neurons at 2 weeks in vitro. These vectors should prove highly useful for efficient overexpression or downregulation of genes in primary neuronal cultures at any developmental stage.

Bioenergetic and oxidative effects of free 3-nitrotyrosine in culture: selective vulnerability of dopaminergic neurons and increased sensitivity of non-dopaminergic neurons to dopamine oxidation

Protein bound and free 3-nitrotyrosine (3NT) levels are elevated in neurodegenerative diseases and have been used as evidence for peroxynitrite generation. Intrastriatal injection of free 3NT causes dopaminergic neuron injury and represents a new mouse model of Parkinson's disease (PD). We are investigating the nature of free 3NT neurotoxicity. In primary ventral midbrain cultures, free 3NT damaged dopaminergic neurons, while adjacent non-dopaminergic neurons were unaffected. Combined treatment with free 3NT and subtoxic amounts of dopamine caused extensive death of non-dopaminergic forebrain neurons in culture. Free 3NT alone directly inhibited mitochondrial complex I, decreased ATP, sensitized neurons to mitochondrial depolarization, and increased superoxide production. Subtoxic concentrations of rotenone (instead of free 3NT) caused similar results. Additionally, free 3NT and dopamine combined increased extraneuronal hydrogen peroxide and decreased intraneuronal glutathione levels more than dopamine alone. Oxidative and bioenergetic processes have been proposed to contribute to neurodegeneration in PD. As free 3NT is a compound that is increased in PD, damages dopamine neurons in vivo and in vitro and has detrimental effects on neuronal bioenergetics, it is possible that free 3NT is an endogenous contributing factor to neuronal loss, in addition to being a marker of oxidative and nitrative processes.

PLA(2) signaling is involved in calpain-mediated degradation of synaptic dihydropyrimidinase-like 3 protein in response to NMDA excitotoxicity

Dihydropyrimidinase-like 3 (DPYSL3) is believed to play a role in neuronal differentiation, axonal outgrowth and neuronal regeneration, as well as cytoskeleton organization. Recently we have shown that glutamate excitotoxicity and oxidative stress result in calpain-dependent cleavage of DPYSL3, and that NOS plays a role in this process [R. Kowara, Q. Chen, M. Milliken, B. Chakravarthy, Calpain-mediated truncation of dihydropyrimidinase-like 3 protein (DPYSL3) in response to NMDA and H2O2 toxicity, J. Neurochem. 95 (2005) 466-474; R. Kowara, K.L. Moraleja, B. Chakravarthy, Involvement of nitric oxide synthase and ROS-mediated activation of L-type voltage-gated Ca(2+) channels in NMDA-induced DPYSL3 degradation, Brain Res. 1119 (2006) 40-49]. The present study investigates the involvement of PLA(2) signaling in NMDA-induced DPYSL3 degradation. Exposure of rat primary cortical neurons (PCN) to PLA(2) and COX-2 inhibitors significantly prevented NMDA-induced DPYSL3 degradation. Since the metabolic product of PLA(2) signaling, PGE(2), which augments toxic effect of NMDA, is known to stimulate cAMP, the effect of adenyl cyclase activator (forskolin plus IBMX) and inhibitor (MDL12,300) on NMDA-induced DPYSL3 degradation was tested. Our data indicate that the activation of adenyl cyclase contributes to NMDA-induced DPYSL3 degradation. Furthermore, cAMP-dependent protein kinase (PKA) inhibitor PKI (14-22) provided additional evidence of PKA involvement in NMDA-induced DPYSL3 degradation. In summary, the obtained data show the contribution of PLA(2) signaling to NMDA-induced calpain activation and subsequent degradation of synaptic protein DPYSL3.