Delayed antagonism of calpain reduces excitotoxicity in cultured neurons

Trauma in Neonatal Acute Brain Slices Alters Calcium and Network Dynamics and Causes Calpain-Mediated Cell Death

Preparing acute brain slices produces trauma that mimics severe penetrating brain injury. In neonatal acute brain slices, the spatiotemporal characteristics of trauma-induced calcium dynamics in neurons and its effect on network activity are relatively unknown. Using multiphoton laser scanning microscopy of the somatosensory neocortex in acute neonatal mouse brain slices (P8-12), we simultaneously imaged neuronal Ca2+ dynamics (GCaMP6s) and cytotoxicity (propidium iodide or PI) to determine the relationship between cytotoxic Ca2+ loaded neurons (GCaMP-filled) and cell viability at different depths and incubation times. PI+ cells and GCaMP-filled neurons were abundant at the surface of the slices, with an exponential decrease with depth. Regions with high PI+ cells correlated with elevated neuronal and neuropil Ca2+ The number of PI+ cells and GCaMP-filled neurons increased with prolonged incubation. GCaMP-filled neurons did not participate in stimulus-evoked or seizure-evoked network activity. Significantly, the superficial tissue, with a higher degree of trauma-induced injury, showed attenuated seizure-related neuronal Ca2+ responses. Calpain inhibition prevented the increase in PI+ cells and GCaMP-filled neurons in the deep tissue and during prolonged incubation times. Isoform-specific pharmacological inhibition implicated calpain-2 as a significant contributor to trauma-induced injury in acute slices. Our results show a calpain-mediated spatiotemporal relationship between cell death and aberrant neuronal Ca2+ load in acute neonatal brain slices. Also, we demonstrate that neurons in acute brain slices exhibit altered physiology depending on the degree of trauma-induced injury. Blocking calpains may be a therapeutic option to prevent acute neuronal death during traumatic brain injury in the young brain.

Calpain-mediated cleavage of Fbxw7 during excitotoxicity

Neuronal cell death induced by ischemic injury has been attributed to glutamate receptor-mediated excitotoxicity, which is known to be accompanied by Ca²⁺ overload in the cytoplasm with concomitant activation of calcium-dependent mechanisms. More specifically, the overactivation of calpains, calcium-dependent cysteine proteases, have been associated with neuronal cell death following glutamate treatment. Previously, we observed decreased expression levels of F-box/WD repeat domain-containing protein 7 (Fbxw7) after the hyperactivation of cyclin-dependent kinase 5 (Cdk5) in cortical neurons challenged with glutamate. As determined using in vitro calpain cleavage assays, we demonstrated that the cleavage of Fbxw7 was mediated by activated calpain and attenuated in the presence of the calpain inhibitor, calpeptin. Using the rat middle cerebral artery occlusion model, we confirmed that Fbxw7 was indeed cleaved by activated calpain in the ipsilateral cortex. Based on our data, we hypothesize that the negative regulation of Fbxw7 by calpain may contribute to neuronal cell death and that the preservation of Fbxw7 by the inhibition of calpain, Cdk5, or both composes a novel protective mechanism following excitotoxicity.

Quantitative proteomic analyses of dynamic signalling events in cortical neurons undergoing excitotoxic cell death

Excitotoxicity, caused by overstimulation or dysregulation of ionotropic glutamate receptors (iGluRs), is a pathological process directing neuronal death in many neurological disorders. The aberrantly stimulated iGluRs direct massive influx of calcium ions into the affected neurons, leading to changes in expression and phosphorylation of specific proteins to modulate their functions and direct their participation in the signalling pathways that induce excitotoxic neuronal death. To define these pathways, we used quantitative proteomic approaches to identify these neuronal proteins (referred to as the changed proteins) and determine how their expression and/or phosphorylation dynamically changed in association with excitotoxic cell death. Our data, available in ProteomeXchange with identifier PXD008353, identified over 100 changed proteins exhibiting significant alterations in abundance and/or phosphorylation levels at different time points (5–240 min) in neurons after glutamate overstimulation. Bioinformatic analyses predicted that many of them are components of signalling networks directing defective neuronal morphology and functions. Among them, the well-known neuronal survival regulators including mitogen-activated protein kinases Erk1/2, glycogen synthase kinase 3 (GSK3) and microtubule-associated protein (Tau), were selected for validation by biochemical approaches, which confirmed the findings of the proteomic analysis. Bioinformatic analysis predicted Protein Kinase B (Akt), c-Jun kinase (JNK), cyclin-dependent protein kinase 5 (Cdk5), MAP kinase kinase (MEK), Casein kinase 2 (CK2), Rho-activated protein kinase (Rock) and Serum/glucocorticoid-regulated kinase 1 (SGK1) as the potential upstream kinases phosphorylating some of the changed proteins. Further biochemical investigation confirmed the predictions of sustained changes of the activation states of neuronal Akt and CK2 in excitotoxicity. Thus, future investigation to define the signalling pathways directing the dynamic alterations in abundance and phosphorylation of the identified changed neuronal proteins will help elucidate the molecular mechanism of neuronal death in excitotoxicity.

NFAT3/c4-mediated excitotoxicity in hippocampal apoptosis during radiation-induced brain injury

Whole brain irradiation (WBI) has become an indispensible tool in the treatment of head and neck cancer, and it has greatly improved patient survival rate and total survival time. In addition, prophylactic cranial irradiation (PCI) has dramatically decreased the incidence of brain metastatic carcinoma. However, WBI may induce temporary functional deficits or even progressive, irreversible cognitive dysfunction that compromises the quality of life for survivors. Unfortunately, the exact molecular mechanisms for cognitive damage remain elusive, and no treatment or preventative measures are available for use in the clinic. In the present study, the nuclear factor of activated T cells isoform 4 (NFAT3/c4) was found to play a vital role in excitotoxic hippocampus cell apoptosis induced by radiation. Sprague-Dawley (SD) rats received 20 Gy WBI, after which we detected NFAT3/c4-mediated excitotoxicity. We found that radiation caused hippocampus excitotoxicity, resulting from overactivation of the N-methyl-D-aspartate receptor (NMDAR) and always accompanied by subsequent elevation of the intracellular calcium level and activation of calcineurin (CaN). P-NFAT3/c4 was the principal downstream target of CaN, including regulation of its nuclear translocation as well as transcriptional activities. Radiation recruited NMDAR/NFAT3/c4 activation and subsequent Bax induction in hippocampus cells. Once treated with the NFAT3/c4 inhibitor 11R-VIVIT peptide pre-irradiation, hippocampal proliferation and neuron survival (dentate gyrus cells in particular) were protected from radiation-induced injury, resulting in inhibition of the apoptosis marker Bax. Our principal aim was to illuminate the role of NFAT3/c4-mediated excitotoxicity in hippocampal apoptosis during radiation-induced brain injury. This study is the first time that radiation-induced activation of NFAT3/c4 has been recorded, and our results suggest that NFAT3/c4 may be a novel target for prevention and treatment of radiation-induced brain injury.

Inhibitory effects of imidazoline receptor ligands on basal and kainic acid-induced neurotoxic signalling in mice

This in vivo study assessed the potential of the imidazoline receptor (IR) ligands moxonidine (selective I1-IR), BU224 (selective I2-IR) and LSL61122 (mixed I1/I2-IR) to dampen excitotoxic signalling induced by kainic acid (KA; 45 mg/kg) in the mouse brain (hippocampus and cerebral cortex). KA triggered a strong behavioural syndrome (seizures; maximal at 60-90 minutes) and sustained stimulation (at 72 hours with otherwise normal mouse behaviour) of pro-apoptotic c-Jun-N-terminal kinases (JNK) and calpain with increased cleavage of p35 into neurotoxic p25 (cyclin-dependent kinase 5 [Cdk5] activators) in mouse hippocampus. Pretreatment (five days) with LSL61122 (10 mg/kg), but not moxonidine (1 mg/kg) or BU224 (20 mg/kg), attenuated the KA-induced behavioural syndrome, and all three IR ligands inhibited JNK and calpain activation, as well as p35/p25 cleavage after KA in the hippocampus (effects also observed after acute IR drug treatments). Efaroxan (I1-IR, 10 mg/kg) and idazoxan (I2-IR, 10 mg/kg), postulated IR antagonists, did not antagonise the effects of moxonidine and LSL61122 on KA targets (these IR ligands showed agonistic properties inhibiting pro-apoptotic JNK). Brain subcellular preparations revealed reduced synaptosomal postsynaptic density-95 protein contents (a mediator of JNK activation) and indicated increased p35/Cdk5 complexes (with pro-survival functions) after treatment with moxonidine, BU224 and LSL61122. These results showed that I1- and I2-IR ligands (moxonidine and BU224), and especially the mixed I1/I2-IR ligand LSL61122, are partly neuroprotective against KA-induced excitotoxic signalling. These findings suggest a therapeutic potential of IR drugs in disorders associated with glutamate-mediated neurodegeneration.

Neuroprotective strategies against calpain-mediated neurodegeneration

Calpains are calcium-dependent proteolytic enzymes that have deleterious effects on neurons upon their pathological over-activation. According to the results of numerous studies to date, there is no doubt that abnormal calpain activation triggers activation and progression of apoptotic processes in neurodegeneration, leading to neuronal death. Thus, it is very crucial to unravel all the aspects of calpain-mediated neurodegeneration in order to protect neurons through eliminating or at least minimizing its lethal effects. Protecting neurons against calpain-activated apoptosis basically requires developing effective, reliable, and most importantly, therapeutically applicable approaches to succeed. From this aspect, the most significant studies focusing on preventing calpain-mediated neurodegeneration include blocking the N-methyl-d-aspartate (NMDA)-type glutamate receptor activities, which are closely related to calpain activation; directly inhibiting calpain itself via intrinsic or synthetic calpain inhibitors, or inhibiting its downstream processes; and utilizing the neuroprotectant steroid hormone estrogen and its receptors. In this review, the most remarkable neuroprotective strategies for calpain-mediated neurodegeneration are categorized and summarized with respect to their advantages and disadvantages over one another, in terms of their efficiency and applicability as a therapeutic regimen in the treatment of neurodegenerative diseases.

GABA(A) receptor dephosphorylation followed by internalization is coupled to neuronal death in in vitro ischemia

Cerebral ischemia is characterized by an early disruption of GABAergic neurotransmission contributing to an imbalance of the excitatory/inhibitory equilibrium and neuronal death, but the molecular mechanisms involved are not fully understood. Here we report a downregulation of GABA(A) receptor (GABA(A)R) expression, affecting both mRNA and protein levels of GABA(A)R subunits, in hippocampal neurons subjected to oxygen-glucose deprivation (OGD), an in vitro model of ischemia. Similar alterations in the abundance of GABA(A)R subunits were observed in in vivo brain ischemia. OGD reduced the interaction of surface GABA(A)R with the scaffold protein gephyrin, followed by clathrin-dependent receptor internalization. Internalization of GABA(A)R was dependent on glutamate receptor activation and mediated by dephosphorylation of the β3 subunit at serine 408/409. Expression of phospho-mimetic mutant GABA(A)R β3 subunits prevented receptor internalization and protected hippocampal neurons from ischemic cell death. The results show a key role for β3 GABA(A)R subunit dephosphorylation in the downregulation of GABAergic synaptic transmission in brain ischemia, contributing to neuronal death. GABA(A)R phosphorylation might be a therapeutic target to preserve synaptic inhibition in brain ischemia.

Excitotoxicity and stroke: identifying novel targets for neuroprotection

Excitotoxicity, the specific type of neurotoxicity mediated by glutamate, may be the missing link between ischemia and neuronal death, and intervening the mechanistic steps that lead to excitotoxicity can prevent stroke damage. Interest in excitotoxicity began fifty years ago when monosodium glutamate was found to be neurotoxic. Evidence soon demonstrated that glutamate is not only the primary excitatory neurotransmitter in the adult brain, but also a critical transmitter for signaling neurons to degenerate following stroke. The finding led to a number of clinical trials that tested inhibitors of excitotoxicity in stroke patients. Glutamate exerts its function in large by activating the calcium-permeable ionotropic NMDA receptor (NMDAR), and different subpopulations of the NMDAR may generate different functional outputs, depending on the signaling proteins directly bound or indirectly coupled to its large cytoplasmic tail. Synaptic activity activates the GluN2A subunit-containing NMDAR, leading to activation of the pro-survival signaling proteins Akt, ERK, and CREB. During a brief episode of ischemia, the extracellular glutamate concentration rises abruptly, and stimulation of the GluN2B-containing NMDAR in the extrasynaptic sites triggers excitotoxic neuronal death via PTEN, cdk5, and DAPK1, which are directly bound to the NMDAR, nNOS, which is indirectly coupled to the NMDAR via PSD95, and calpain, p25, STEP, p38, JNK, and SREBP1, which are further downstream. This review aims to provide a comprehensive summary of the literature on excitotoxicity and our perspectives on how the new generation of excitotoxicity inhibitors may succeed despite the failure of the previous generation of drugs.

Calcineurin signaling mediates activity-dependent relocation of the axon initial segment

The axon initial segment (AIS) is a specialized neuronal subcompartment located at the beginning of the axon that is crucially involved in both the generation of action potentials and the regulation of neuronal polarity. We recently showed that prolonged neuronal depolarization produces a distal shift of the entire AIS structure away from the cell body, a change associated with a decrease in neuronal excitability. Here, we used dissociated rat hippocampal cultures, with a major focus on the dentate granule cell (DGC) population, to explore the signaling pathways underlying activity-dependent relocation of the AIS. First, a pharmacological screen of voltage-gated calcium channels (VGCCs) showed that AIS relocation is triggered by activation of L-type Cav1 VGCCs with negligible contribution from any other VGCC subtypes. Additional pharmacological analysis revealed that downstream signaling events are mediated by the calcium-sensitive phosphatase calcineurin; inhibition of calcineurin with either FK506 or cyclosporin A totally abolished both depolarization- and optogenetically-induced activity-dependent AIS relocation. Furthermore, calcineurin activation is sufficient for AIS plasticity, because expression of a constitutively active form of the phosphatase resulted in relocation of the AIS of DGCs without a depolarizing stimulus. Finally, we assessed the role of calcineurin in other forms of depolarization-induced plasticity. Neither membrane resistance changes nor spine density changes were affected by FK506 treatment, suggesting that calcineurin acts via a separate pathway to modulate AIS plasticity. Together, these results emphasize calcineurin as a vital player in the regulation of intrinsic plasticity as governed by the AIS.

Non-conventional apoptotic response to ionising radiation mediated by N-methyl D-aspartate receptors in immature neuronal cells

During cortical development, N-methyl D-aspartate (NMDA) receptors are highly involved in neuronal maturation and synapse establishment. Their implication in the phenomenon of excitotoxicity has been extensively described in several neurodegenerative diseases due to the permissive entry of Ca²⁺ ions and massive accumulation in the intracellular compartment, which is highly toxic to cells. Ionising radiation is also a source of stress to the cells, particularly immature neurons. Their capacity to induce cell death has been described for various cell types either by directly damaging the DNA or indirectly through the generation of reactive oxygen species responsible for the activation of a battery of stress response effectors leading in certain cases, to cell death. In this study, in order to determine whether a link exists between NMDA receptors-mediated excitotoxicity and radiation-induced cell death, we evaluated radiation-induced cell death in vitro and in vivo in maturing neurons during the fetal period. Cell death induction was assessed by TUNEL, caspase-3 activity and DNA ladder assays, with or without the administration of dizocilpine (MK-801), a non-competitive NMDA receptor antagonist which blocks neuronal Ca²⁺ influx. To further investigate the possible involvement of Ca²⁺-dependent enzyme activation, known to occur at high Ca²⁺ concentrations, we examined the protective effect of a calpain inhibitor on cell death induced by radiation. Doses ranging from 0.2 to 0.6 Gy of X-rays elicited a clear apoptotic response that was prevented by the injection of dizocilpine (MK-801) or calpain inhibitor. These data demonstrate the involvement of NMDA receptors in radiation-induced neuronal death by the activation of downstream effectors, including calpain-related pathways. An increased apoptotic process elicited by radiation, occurring independently of the normal developmental scheme, may eliminate post-mitotic but immature neuronal cells and deeply impair the establishment of the neuronal network, which in the case of cortical development is critical for cognitive capacities.

Calpain dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the presence of senile plaques and neurofibrillary tangles in the neocortex and hippocampus of AD patients. In addition, a marked decrease in synaptic contacts has been detected in these affected brain areas. Due to its prevalence in the aging population, this disease has been the focus of numerous studies. The data obtained from those studies suggest that the mechanisms leading to the formation of the hallmark lesions of AD might be linked. One of such mechanisms seems to be the dysregulation of calcium homeostasis that results in the abnormal activation of calpains. Calpains are a family of Ca(2+)-dependent cysteine proteases that play a key role in multiple cell functions including cell development, differentiation and proliferation, axonal guidance, growth cone motility, and cell death, among others. In this paper, we briefly reviewed data on the structure of these proteases and their regulation under normal conditions. We also summarized data underscoring the participation of calpains in the neurodegenerative mechanisms associated with AD.

Calpains are downstream effectors of bax-dependent excitotoxic apoptosis

Excitotoxicity resulting from excessive Ca(2+) influx through glutamate receptors contributes to neuronal injury after stroke, trauma, and seizures. Increased cytosolic Ca(2+) levels activate a family of calcium-dependent proteases with papain-like activity, the calpains. Here we investigated the role of calpain activation during NMDA-induced excitotoxic injury in embryonic (E16-E18) murine cortical neurons that (1) underwent excitotoxic necrosis, characterized by immediate deregulation of Ca(2+) homeostasis, a persistent depolarization of mitochondrial membrane potential (Δψ(m)), and insensitivity to bax-gene deletion, (2) underwent excitotoxic apoptosis, characterized by recovery of NMDA-induced cytosolic Ca(2+) increases, sensitivity to bax gene deletion, and delayed Δψ(m) depolarization and Ca(2+) deregulation, or (3) that were tolerant to excitotoxic injury. Interestingly, treatment with the calpain inhibitor calpeptin, overexpression of the endogenous calpain inhibitor calpastatin, or gene silencing of calpain protected neurons against excitotoxic apoptosis but did not influence excitotoxic necrosis. Calpeptin failed to exert a protective effect in bax-deficient neurons but protected bid-deficient neurons similarly to wild-type cells. To identify when calpains became activated during excitotoxic apoptosis, we monitored calpain activation dynamics by time-lapse fluorescence microscopy using a calpain-sensitive Förster resonance energy transfer probe. We observed a delayed calpain activation that occurred downstream of mitochondrial engagement and directly preceded neuronal death. In contrast, we could not detect significant calpain activity during excitotoxic necrosis or in neurons that were tolerant to excitotoxic injury. Oxygen/glucose deprivation-induced injury in organotypic hippocampal slice cultures confirmed that calpains were specifically activated during bax-dependent apoptosis and in this setting function as downstream cell-death executioners.

Stroke intervention pathways: NMDA receptors and beyond

Despite abundant evidence from basic/preclinical research that excessive NMDAR (N-methyl-d-aspartate receptor) stimulation is a crucial step required for brain damage following a stroke, clinical trials for NMDAR blockers have all ended with disappointments. The past decade of stroke research has revealed distinct NMDAR subpopulations and many specific effectors downstream of these receptors that are differentially responsible for neuronal survival and death. These new advancements provide promising targets for the development of novel NMDAR-based neuroprotective stroke therapies that could have greater therapeutic windows and reduced side effects. In this review, we discuss these advancements with a particular emphasis on the identification of novel signaling effectors downstream of proneuronal death NMDARs and the potential implications of these findings for the development of stroke therapeutics.

Glutamate receptors, neurotoxicity and neurodegeneration

Glutamate excitotoxicity is a hypothesis that states excessive glutamate causes neuronal dysfunction and degeneration. As glutamate is a major excitatory neurotransmitter in the central nervous system (CNS), the implications of glutamate excitotoxicity are many and far-reaching. Acute CNS insults such as ischaemia and traumatic brain injury have traditionally been the focus of excitotoxicity research. However, glutamate excitotoxicity has also been linked to chronic neurodegenerative disorders such as amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease and others. Despite the continued research into the mechanisms of excitotoxicity, there are currently no pharmacological interventions capable of providing significant neuroprotection in the clinical setting of brain ischaemia or injury. This review addresses the current state of excitotoxic research, focusing on the structure and physiology of glutamate receptors; molecular mechanisms underlying excitotoxic cell death pathways and their interactions with each other; the evidence for glutamate excitotoxicity in acute neurologic diseases; laboratory and clinical attempts at modulating excitotoxicity; and emerging targets for excitotoxicity research.

Real-time visualization of cytoplasmic calpain activation and calcium deregulation in acute glutamate excitotoxicity

Although calpain (EC 3.4.22) protease activation was suggested to contribute to excitotoxic delayed calcium deregulation (DCD) via proteolysis of Na+/Ca2+ exchanger 3 (NCX3), cytoplasmic calpain activation in relation to DCD has never been visualized in real-time. We employed a calpain fluorescence resonance energy transfer substrate to simultaneously image calpain activation and calcium deregulation in live cortical neurons. A calpain inhibitor-sensitive decline in fluorescence resonance energy transfer was observed at 39 +/- 5 min after the occurrence of DCD in neurons exposed to continuous glutamate (100 microM). Inhibition of calpain by calpeptin did not delay the onset of DCD, recovery from DCD-like reversible calcium elevations, or cell death despite inhibiting alpha-spectrin processing by > 90%. NCXs reversed during glutamate exposure, the NCX antagonist KB-R7943 prolonged the time to DCD, and significant NCX3 cleavage following 90 min of glutamate exposure was not observed. Our findings suggest that robust calpain activation associated with acute glutamate toxicity occurs only after a sustained loss in calcium homeostasis. Processing of NCX3 or other calpain substrates is unlikely to be the primary cause of acute excitotoxicity in cortical neurons. However, a role for calpain as a contributing factor or in response to milder glutamate insults is not excluded.

Knockdown of m-calpain increases survival of primary hippocampal neurons following NMDA excitotoxicity

The calpain family of cysteine proteases has a well-established causal role in neuronal cell death following acute brain injury. However, the relative contribution of calpain isoforms to the various forms of injury has not been determined as available calpain inhibitors are not isoform-specific. In this study, we evaluated the relative role of m-calpain and mu-calpain in a primary hippocampal neuron model of NMDA-mediated excitotoxicity. Baseline mRNA expression for the catalytic subunit of m-calpain (capn2 ) was found to be 50-fold higher than for the mu-calpain catalytic subunit (capn1) based on quantitative real-time PCR. Adeno-associated viral vectors designed to deliver short hairpin RNAs targeting capn1 or capn2 resulted in 60% and 90% knockdown of message respectively. Knockdown of capn2 but not capn1 increased neuronal survival after NMDA exposure at 21 days in vitro. Nuclear translocation of calpain substrates apoptosis inducing factor, p35/p25 and collapsin response mediator protein (CRMP) 2-4 was not detected after NMDA exposure in this model. However, nuclear translocation of CRMP-1 was observed and was prevented by capn2 knockdown. These findings provide insight into potential mechanisms of calpain-mediated neurodegeneration and have important implications for the development of isoform-specific calpain inhibitor therapy.

Intraspinal MDL28170 microinjection improves functional and pathological outcome following spinal cord injury

Although calpain (calcium-activated cysteine protease) inhibition represents a rational therapeutic target for spinal cord injury (SCI), few studies have reported improved functional outcomes with post-injury administration of calpain inhibitors. This reflects the weak potency and limited aqueous solubility of current calpain inhibitors. Previously, we demonstrated that intraspinal microinjection of the calpain inhibitor MDL28170 resulted in greater inhibition of calpain activity as compared to systemic administration of the same compound. In the present study, we evaluated the ability of intraspinal MDL28170 microinjection to spare spinal tissue and locomotor dysfunction following SCI. Contusion SCI was produced in female Long-Evans rats using the Infinite Horizon impactor at the 200-kdyn force setting. Open-field locomotion was evaluated until 6 weeks post-injury. Histological assessment of tissue sparing was performed at 6 weeks after SCI. The results demonstrate that MDL28170, administered with a single post-injury intraspinal microinjection (50 nmoles), significantly improves both locomotor function and pathological outcome measures following SCI.

Mechanistic role of calpains in postischemic neurodegeneration

The calpain family of proteases is causally linked to postischemic neurodegeneration. However, the precise mechanisms by which calpains contribute to postischemic neuronal death have not been fully elucidated. This review outlines the key features of the calpain system, and the evidence for its causal role in postischemic neuronal pathology. Furthermore, the consequences of specific calpain substrate cleavage at various subcellular locations are explored. Calpain substrates within synapses, plasma membrane, endoplasmic reticulum, lysosomes, mitochondria, and the nucleus, as well as the overall effect of postischemic calpain activity on calcium regulation and cell death signaling are considered. Finally, potential pathways for calpain-mediated neurodegeneration are outlined in an effort to guide future studies aimed at understanding the downstream pathology of postischemic calpain activity and identifying optimal therapeutic strategies.

Calcineurin is involved in the early activation of NMDA-mediated cell death in mutant huntingtin knock-in striatal cells

Excitotoxicity has been proposed as one of the mechanisms involved in the specific loss of striatal neurons that occurs in Huntington's disease. Here, we studied the role of calcineurin in the vulnerability of striatal neurons expressing mutant huntingtin to excitotoxicity. To this end, we induced excitotoxicity by adding NMDA to a striatal precursor cell line expressing full-length wild-type (STHdh(Q7/Q7)) or mutant (STHdh(Q111/Q111)) huntingtin. We observed that cell death appeared earlier in STHdh(Q111/Q111) cells than in STHdh(Q7/Q7) cells. Interestingly, these former cells expressed higher levels of calcineurin A that resulted in a greater increase of its activity after NMDA receptor stimulation. Moreover, transfection of full-length mutant huntingtin in different striatal-derived cells (STHdh(Q7/Q7), M213 and primary cultures) increased calcineurin A protein levels. To determine whether high levels of calcineurin A might account for the earlier activation of cell death in mutant huntingtin knock-in cells, wild-type cells were transfected with calcineurin A. Calcineurin A-transfected STHdh(Q7/Q7) cells displayed a significant increase in cell death compared with that recorded in green fluorescent protein-transfected cells after NMDA treatment. Notably, addition of the calcineurin inhibitor FK-506 produced a more robust reduction in cell death in mutant huntingtin knock-in cells than it did in wild-type cells. These results suggest that high levels of calcineurin A could account for the increased vulnerability of striatal cells expressing mutant huntingtin to excitotoxicity.

Differential regulation of the NMDA receptor by acute and sub-chronic phencyclidine administration in the developing rat

Neurodegeneration induced by the NMDA receptor antagonist, phencyclidine (PCP), has been used to model the pathogenesis of schizophrenia in the developing rat. Acute and sub-chronic administration of PCP in perinatal rats results in different patterns of neurodegeneration. The potential role of an alteration in the membrane expression of NMDA receptors in PCP-induced degeneration is unknown. Acute PCP treatment on postnatal day 7 increased membrane levels of both NMDA receptor subunit 1 (NR1) and NMDA receptor subunit 2B (NR2B) proteins in the frontal cortex; conversely, NR1 and NR2B protein levels in the endoplasmic reticulum fraction were decreased. Acute PCP administration also resulted in increased membrane cortical protein levels of post-synaptic density-95, as well as the activation of calpain, which paralleled the observed increase in membrane expression of NR1 and NR2B. Further, administration of the calpain inhibitor, MDL28170, prevented PCP-induced up-regulation of NR1 and NR2B. On the other hand, sub-chronic PCP treatment on postnatal days 7, 9 and 11 caused an increase in NR1 and NR2A expression, which was accompanied by an increase in both NR1 and NR2A in the endoplasmic reticulum fraction. Sub-chronic PCP administration did not alter levels of post-synaptic density-95 and had no effect on activation of calpain. These data suggest that increased trafficking accounts for up-regulation of cortical NR1/NR2B subunits following acute PCP administration, while increased protein synthesis likely accounts for the increased expression of NR1/NR2A following sub-chronic PCP treatment of the developing rat. These results are discussed in the context of the differential neurodegeneration caused by acute and subchronic PCP administration in the developing rat brain.

Sustained calpain inhibition improves locomotor function and tissue sparing following contusive spinal cord injury

Following contusive spinal cord injury (SCI), calpain activity is dramatically increased and remains elevated for days to weeks. Although calpain inhibition has previously been demonstrated to be neuroprotective following spinal cord injury, most studies administered the calpain inhibitor at a single time point. We hypothesized that sustained calpain inhibition would improve functional and pathological outcomes, as compared to the results obtained with a single postinjury administration of the calpain inhibitor. Contusion SCI was produced in female Long-Evans rats using the Infinite Horizon spinal cord injury impactor at the 200 kdyn force setting. Open-field locomotor function was evaluated until 6 weeks postinjury. Histological assessment of lesion volume and tissue sparing was performed at 6 weeks after SCI. Calpain inhibitor MDL28170 administered as a single postinjury i.v. bolus (20 mg/kg) or as a daily i.p. dose (1 mg/kg) improved locomotor function, but did not increase tissue sparing. Combined i.v. and daily i.p. MDL28170 administration resulted in significant improvement in both functional and pathological outcome measures, supporting the calpain theory of SCI proposed by Dr. Banik and colleagues.

Does treating schizophrenia reduce the chances of developing amyotrophic lateral sclerosis?

The development of amyotrophic lateral sclerosis (ALS) in the relatively common psychiatric disorder schizophrenia is very rare. This observation has been made by us and a number of other neuromuscular specialists at large ALS centers. We propose that the use of neuroleptics and some antidepressants, which are chronically prescribed to schizophrenics and which have neuroprotective properties and some of which promote neurogenesis, may confer protection against this deadly neurodegenerative disease ALS. Such an observation may have important implications towards the therapy and understanding the pathophysiology of this deadly neurodegenerative disease.

Calpain activation is required for glutamate-induced p27 down-regulation in cultured cortical neurons

Recent evidence suggests that cell cycle-related molecules play pivotal roles in multiple forms of cell death in post-mitotic neurons. Nevertheless, it remains unclear what molecular mechanisms are involved in the regulation of expression levels and activities of these molecules. We showed previously that treatment with extracellular glutamate decreases cyclin-dependent kinase inhibitor p27 before neuronal cell death. In this study, we demonstrate that reductions of both p27 and neuronal viability were dependent on activity of calpain, a Ca(2+)-dependent protease, but not on activity of caspase 3. Interestingly, the glutamate-induced reduction of p27 was not dependent on the ubiquitin-proteasome system. In fact, p27 was present only in the neuronal nucleus, whereas calpain 1, a ubiquitous calpain, was observed both in the neuronal nucleus and cytoplasm in control cultures. Glutamate treatment did not change the localization patterns of p27 and calpain 1. It reduced p27 expression level in the nucleus in a calpain-dependent manner. In vitro experiments using neuronal cell lysate and p27 recombinant protein revealed that p27 was degraded as a substrate of activated calpain 1. These results suggest that calpain(s), activated by glutamate treatment, degrade(s) p27 in the nucleus of neurons, which might promote aberrant cell cycle progression.

Developmental changes in NMDA neurotoxicity reflect developmental changes in subunit composition of NMDA receptors

Excitotoxicity is generally studied in dissociated neurons, cultured hippocampal slices, or intact animals. However, the requirements of dissociated neurons or cultured slices to use prenatal or juvenile rats seriously limit the advantages of these systems, whereas the complexity of intact animals prevents detailed molecular investigations. In the present experiments, we studied developmental changes in NMDA neurotoxicity in acute hippocampal slices with lactate dehydrogenase (LDH) release in medium, propidium iodide (PI) uptake, and Nissl staining as markers of cell damage. Calpain-mediated spectrin degradation was used to test calpain involvement in NMDA neurotoxicity. NMDA treatment produced increased LDH release, PI uptake, and spectrin degradation in slices from juvenile rats but not adult rats. NMDA-induced changes in slices from young rats were blocked completely by the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801) and by the antagonists of NR2B receptor ifenprodil and R-(R, S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol and were partly blocked by calpain inhibitor III but were not affected by the NR2A-specific antagonist [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid. NMDA-induced changes in Nissl staining were also different in slices from young and adult rats and blocked by NR2B but not NR2A antagonists. In contrast to NMDA treatment, oxygen/glucose deprivation (OGD) induced neurotoxicity in slices from both young and adult rats, although OGD-induced toxicity was attenuated by MK-801 only in slices from young rats. Our results are consistent with the idea that NMDA-mediated toxicity is caused by activation of NR2B- but not NR2A-containing NMDA receptors leading to calpain activation and that developmental changes in NMDA toxicity reflect developmental changes in NMDA receptor subunit composition.

Calpain and N-methyl-d-aspartate (NMDA)-induced excitotoxicity in rat retinas

Calpain-mediated proteolysis has been implicated as a major process in neuronal cell death in both acute insults and the chronic neurodegenerative disorders in the central nerves system. However, activation of calpain also plays a protective function in the early phase of excitotoxic neuronal death. The exact role of calpains in neuronal death and recovery after exposure to N-methyl-D-aspartate (NMDA) is not clearly known. The purpose of present study was to examine the involvement of mu- and m-calpain in NMDA-induced excitotoxicity in the adult rat retina. Increased immunoreactivity of mu-calpain was noted in RGC layer cells and in the inner nuclear layer with maximal expression at 12 h after NMDA injection. This was further confirmed with Western blotting. TdT-mediated biotin-dUTP nick end labeling (TUNEL) positive cells in the inner retina co-localized with moderate or intense mu-calpain immunoreactivity. In contrast, there was no remarkable change in m-calpain immunoreactivity at any time point after NMDA injection. Simultaneous injection of 2 nmol of a calpain inhibitor (calpain inhibitor II) significantly reduced the number of TUNEL-positive cells in the inner retina at 18 h after NMDA injection and preserved RGC-like cells counted at 7 days after injection. The results of this study showed that mu-calpain may be involved in mediating NMDA-induced excitotoxicity in the rat retina and calpain inhibitors may play a therapeutic role in NMDA related disease.

Calpain inhibitor MDL 28170 protects hypoxic–ischemic brain injury in neonatal rats by inhibition of both apoptosis and necrosis

MDL 28170 is a CNS-penetrating calpain inhibitor, and we examined the effects of MDL 28170 on hypoxic-ischemic brain injury in immature brain using the Rice-Vannucci model. Immediately after hypoxic exposure, 24 mg/kg of MDL 28170 was injected intraperitoneally as an initial dose, followed by 12 mg/kg every 4 h for a total dose of 60 mg/kg over 12 h post-HI. A vehicle control group received peanut oil injection instead. Macroscopic evaluation of brain injury revealed the neuroprotective effect of MDL 28170 after 12 h post-HI. Neuropathological quantitative analysis of cell death showed that MDL 28170 significantly decreased the number of necrotic cells in all the examined regions except for cingular cortex, and the number of apoptotic cells in caudate putamen, parietal cortex, hippocampus CA1, and laterodorsal thalamus. Western blots showed that MDL 28170 suppressed 145/150 kDa subunits of alpha-spectrin breakdown products (SBDP) in cortex, hippocampus, thalamus, and striatum, and also 120-kDa subunit of SBDP in all regions except for striatum. This suggests that MDL 28170 inhibited activation of calpain and caspase-3, respectively. Our results indicate that post-hypoxic MDL 28170 injection is neuroprotective in HI newborn rat brain by decreasing both necrosis and apoptosis. SBDP expression also suggests that MDL 28170 injection inhibits both calpain and caspase-3 activation after HI insult.

Both N-methyl-d-aspartate (NMDA) and non-NMDA receptors mediate glutamate-induced cleavage of the cyclin-dependent kinase 5 (cdk5) activator p35 in cultured rat hippocampal neurons

Cyclin-dependent kinase 5 (cdk5) regulates crucial neurobiological events, and deregulation of cdk5 has been implicated in several neurodegenerative disorders. The deregulation is suggested to occur due to cleavage of the cdk5 activator protein p35 to a smaller p25 fragment by the calcium-activated protease calpain. Here we have elucidated the role of different calcium-permeable ionotropic glutamate receptors in the induction of p35 cleavage in cultured rat hippocampal neurons. The glutamate receptor agonists glutamic acid, N-methyl-D-aspartate (NMDA), kainic acid, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) were all able to induce p35 cleavage, in a manner depending on extracellular calcium. The effect of glutamate was mediated by NMDA receptors, as it was prevented by the NMDA antagonist MK-801. Cyclothiazide (CTZ), an inhibitor of AMPA receptor desensitization, enhanced glutamate-induced p35 cleavage. In immature 6-day-old cultures the non-NMDA agonist kainic acid provoked p35 cleavage, whereas glutamate and NMDA were ineffective. The data suggest that both NMDA and non-NMDA receptors are able to induce p35 cleavage. Different factors, such as maturation state of neurons or desensitization properties of non-NMDA receptors, may determine which receptor predominantly mediates the effect of glutamate on p35 cleavage.

Effects of amyloid-beta on cholinergic and acetylcholinesterase-positive cells in cultured basal forebrain neurons of embryonic rat brain

The neurotoxic effects of amyloid-beta(1-42) and amyloid-beta(25-35) (A beta) on cholinergic and acetylcholinesterase-positive neurons were investigated in primary cultures derived from embryonic 18-day-old rat basal forebrain. After various time intervals, the cultures were treated with 1, 5, 10 or 20 microM A beta for different time periods. The cholinergic neurons and their axon terminals were revealed by vesicular acetylcholine transporter immunohistochemistry and the cholinoceptive cells by acetylcholinesterase histochemical staining. To assess the toxic effects of these A beta peptides on the cholinergic neurons, image analysis was applied for quantitative determination of the numbers of axon varicosities/terminals and cells. The results demonstrate that, following treatment with 1 or 5 microM A beta for 5, 10, 30, 60 or 120 min, no changes in vesicular acetylcholine transporter immunohistochemical staining were observed. However, after treatment for 30 min with 10 or 20 microM A beta, the number of stained axon varicosities was reduced, and treatment for 2 h they had disappeared. In contrast, vesicular acetylcholine transporter-positivity could be seen in some of the neuronal perikarya even after 3 days after treatment. The acetylcholinesterase staining was homogeneously distributed in the control neurons. After A beta treatment, the histochemical reaction end-product was detected in some of the neuronal perikarya or in the dendritic processes near to the soma. It is concluded that the neurotoxic effects of A beta appear more rapidly in the cholinergic axon terminals than in the cholinergic and acetylcholinesterase-positive neuronal perikarya.

Calpain facilitates the neuron death induced by 3-nitropropionic acid and contributes to the necrotic morphology

3-Nitropropionic acid (3NP), an irreversible inhibitor of succinate dehydrogenase, has been used to model features of neurodegenerative disorders including Huntington disease, as well as acute neuronal insults such as cerebral ischemia. 3NP induces rapid necrosis and delayed apoptosis in primary cultures of rat hippocampal neurons. Low levels of extracellular glutamate shift the cell death mechanism to necrosis, whereas antagonism of NMDA receptors results in predominately apoptotic death. In the present study, the involvement of cysteine proteases in the morphologic and biochemical alterations accompanying 3NP-induced neuron death was investigated. Immunoblots of spectrin breakdown products indicated Ca(2+)-dependent cysteine protease (calpain) activation within the 8 hours of 3NP administration, whereas caspase-3 activation was not evident until 16 to 48 hours after treatment. The NMDA receptor antagonist MK-801 (dizocilpine) decreased 3NP-induced calpain activity, but did not alter caspase-3 activity. Similar to MK-801, calpain inhibitors (Z-Val-Phe.H and Z-Leu-Phe-CONHEt) shifted the cell death morphology towards apoptosis and delayed, but did not prevent, the 3NP-induced cell death. Together, the results indicate that following 3NP administration, increased calpain activity precedes caspase-3 activation, contributes to the necrotic morphology, and facilitates and accelerates the cell death.

Dual regulation of NMDA receptor functions by direct protein-protein interactions with the dopamine D1 receptor

Dopamine D1-like receptors, composed of D1 and D5 receptors, have been documented to modulate glutamate-mediated fast excitatory synaptic neurotransmission. Here, we report that dopamine D1 receptors modulate NMDA glutamate receptor-mediated functions through direct protein-protein interactions. Two regions in the D1 receptor carboxyl tail can directly and selectively couple to NMDA glutamate receptor subunits NR1-1a and NR2A. While one interaction is involved in the inhibition of NMDA receptor-gated currents, the other is implicated in the attenuation of NMDA receptor-mediated excitotoxicity through a PI-3 kinase-dependent pathway.

Peptidyl alpha-keto amide inhibitor of calpain blocks excitotoxic damage without affecting signal transduction events

The cysteine protease calpain is activated by calcium and has a wide range of substrates. Calpain-mediated cellular damage is associated with many neuropathologies, and calpain also plays a role in signal transduction events that are essential for cell maintenance, including the activation of important kinases and transcription factors. In the present study, the hippocampal slice culture was used as a model of excitotoxicity to test whether the neuroprotection elicited by selective calpain inhibition is associated with changes in cell signaling. Peptidyl alpha-keto amide and alpha-keto acid inhibitors reduced both calpain-mediated cytoskeletal damage and the concomitant synaptic deterioration resulting from an N-methyl-D-aspartate exposure. The alpha-keto amide CX295 was protective when infused into slice cultures before or after the excitotoxic episode. The slices protected with CX295 exhibited normal activation levels of mitogen-activated protein kinase and the transcription factor nuclear factor-kappaB. Thus, selective inhibition of calpain provides neuroprotection without influencing critical signaling pathways.

Cross-tolerance to otherwise lethal N-methyl-D-aspartate and oxygen-glucose deprivation in preconditioned cortical cultures

In vitro ischemic preconditioning induced by subjecting rat cortical cultures to nonlethal oxygen-glucose deprivation protects against a subsequent exposure to otherwise lethal oxygen-glucose deprivation. We provide evidence that attenuation of the postsynaptic N-methyl-D-aspartate (NMDA) receptor- and Ca(2+)-dependent neurotoxicity underlies oxygen-glucose deprivation tolerance. It is demonstrated that extended tolerance to otherwise lethal NMDA or oxygen-glucose deprivation can be induced by either of their sublethal forms of preconditioning. These four pathways are linked, since NMDA receptor blockade during preconditioning by oxygen-glucose deprivation eliminates tolerance. These results suggest that NMDA tolerance, induced by nonlethal activation of these receptors during oxygen-glucose deprivation preconditioning, underlies oxygen-glucose deprivation tolerance. Several neurotoxic downstream Ca(2+)-dependent signaling events specifically linked to NMDA receptor activation are attenuated during otherwise lethal oxygen-glucose deprivation in preconditioned cultures. Specifically, calpain activation, as well as degradation of microtubule-associated protein-2 and postsynaptic density-95, are attenuated 2 h following otherwise lethal NMDA treatment alone or oxygen-glucose deprivation in preconditioned cultures. Formation of microtubule-associated protein-2-labeled dendritic varicosities is also attenuated in preconditioned cultures within 1 h of lethal oxygen-glucose deprivation or NMDA application. Intracellular Ca(2+) levels, measured using the high- or low-affinity dyes Fluo-4 (K(d) approximately equal 345 nM) or Fluo-4FF (K(d) approximately equal 9.7 microM) respectively, are markedly attenuated during lethal oxygen-glucose deprivation in preconditioned cultures.Collectively, the results suggest the attenuation of the postsynaptic NMDA-mediated component of otherwise lethal oxygen-glucose deprivation through the suppression of Ca(2+)-dependent neurotoxic signaling, a mechanism that is initially induced by transient nonlethal activation of this receptor during ischemic preconditioning.

Excitotoxicity: perspectives based on N-methyl-D-aspartate receptor subtypes

Since excitotoxicity has been implicated in a variety of neuropathological conditions, understanding the pathways involved in this type of cell death is of critical importance to the future clinical treatment of many diseases. The N-methyl-D-aspartate (NMDA) receptor has become a primary focus of excitotoxic research because early studies demonstrated that antagonism of this receptor subtype was neuroprotective. However, initial pharmacological agents were not clinically useful due to the adverse effects of complete NMDA receptor blockade. Understanding the biochemical properties of the multitude of NMDA receptor subtypes offers the possibility of developing more effective and clinically useful drugs. With the discovery of the basis of heterogeneity of NMDA receptors through molecular biological approaches, many new potential therapeutic targets have been uncovered, and several model systems have been developed for the study of NMDA receptor-mediated cell death. This review discusses these models and the current understanding of the relationship between NMDA receptor subtypes and excitotoxicity.

Ca(2+)-induced inhibition of apoptosis in human SH-SY5Y neuroblastoma cells: degradation of apoptotic protease activating factor-1 (APAF-1)

During apoptotic and excitotoxic neuron death, challenged mitochondria release the pro-apoptotic factor cytochrome c. In the cytosol, cytochrome c is capable of binding to the apoptotic protease-activating factor-1 (APAF-1). This complex activates procaspase-9 in the presence of dATP, resulting in caspase-mediated execution of apoptotic neuron death. Many forms of Ca(2+)-mediated neuron death, however, do not lead to prominent activation of the caspase cascade despite significant release of cytochrome c from mitochondria. We demonstrate that elevation of cytosolic Ca(2+) induced prominent degradation of APAF-1 in human SH-SY5Y neuroblastoma cells and in a neuronal cell-free apoptosis system. Loss of APAF-1 correlated with a reduced ability of cytochrome c to activate caspase-3-like proteases. Ca(2+) induced the activation of calpains, monitored by the cleavage of full-length alpha-spectrin into a calpain-specific 150-kDa breakdown product. However, pharmacological inhibition of calpain activity indicated that APAF-1 degradation also occurred via calpain-independent pathways. Our data suggest that Ca(2+) inhibits caspase activation during Ca(2+)-mediated neuron death by triggering the degradation of the cytochrome c-binding protein APAF-1.

Strain-dependent differences in calcium signaling predict excitotoxicity in murine hippocampal neurons

Commonly used inbred murine strains differ substantially in their vulnerability to excitotoxic insults. We investigated whether differences in dendritic Ca(2+) signaling could underlie the differential vulnerability of C57BL/6 (resistant to kainate excitotoxicity) and C57BL/10 strains (vulnerable). A striking difference was found in fine dendrite Ca(2+) responses after kainate exposure. Ca(2+) signals in distal dendrites were large in C57BL/10 neurons, and, if a threshold concentration of approximately 1.5 microm was reached, a region of sustained high Ca(2+) was established in the distal dendritic tree. This region then served as an initiation site for a degenerative cascade, producing high Ca(2+) levels that slowly spread to involve the entire neuron and led to cell death. Dendritic Ca(2+) signals in C57BL/6 neurons were much smaller and did not trigger these propagating secondary responses. Strain differences in dendritic Ca(2+) signaling were also evident after tetanic stimulation of Schaffer collaterals. Ca(2+) responses were much larger and peaked earlier in distal dendrites of C57BL/10 compared with those in C57BL/6. Neurons from both strains had similar membrane properties and responded to kainate with intense action potential firing. Degenerative Ca(2+) responses were seen in both strains if soma Ca(2+) could be sustained above 1.5 microm. The early phases of secondary Ca(2+) responses were attributable to Ca(2+) influx and were abolished rapidly by buffered zero Ca(2+) saline. Taken together, these data indicate that the substantial difference in Ca(2+) signals in fine distal dendrites and in the initiation of spreading secondary responses may underlie the selective vulnerability of these neurons to excitotoxic insults.

Calpain activity in the rat brain after transient forebrain ischemia

Activity of the Ca(2+)-dependent protease calpain is increased in neurons after global and focal brain ischemia, and may contribute to postischemic injury cascades. Understanding the time course and location of calpain activity in the post-ischemic brain is essential to establishing causality and optimizing therapeutic interventions. This study examined the temporal and spatial characteristics of brain calpain activity after transient forebrain ischemia (TFI) in rats. Male Long Evans rats underwent 10 min of normothermic TFI induced by bilateral carotid occlusion with hypovolemic hypotension (MABP 30 mm Hg). Brain calpain activity was examined between 1 and 72 h after reperfusion. Western blot analysis of regional brain homogenates demonstrated a bimodal pattern of calpain-mediated alpha-spectrin degradation in the hippocampus, cortex, and striatum with an initial increase at 1 h followed by a more prominent secondary increase at 36 h after reperfusion. Immunohistochemical analysis revealed that calpain activity was primarily localized to dendritic fields of selectively vulnerable neurons at one hour after reperfusion. Between 24 and 48 h after reperfusion neuronal calpain activity progressed from the dorsal to ventral striatum, medial to lateral CA1 hippocampus, and centripetally expanded from watershed foci in the cerebral cortex. This progression was associated with fragmentation of dendritic processes, calpain activation in the neuronal soma and subsequent neuronal degeneration. These observations demonstrate a clear association between calpain activation and subsequent delayed neuronal death and suggest broad therapeutic window for interventions aimed at preventing delayed intracellular Ca(2+) overload and pathologic calpain activation.

A selective cysteine protease inhibitor is non-toxic and cerebroprotective in rats undergoing transient middle cerebral artery ischemia

Ischemic neuronal injury mediated by cysteine proteases such as calpains and caspases has been demonstrated in various experimental models. Cathepsins B and L are also cysteine proteases which may contribute to neuronal death after ischemia. The authors measured in vitro and in vivo toxicity and post-ischemic cytoprotective effects of a cysteine protease inhibitor which does not block calpain or caspase but, rather, is relatively selective for cathepsins B and L. The compound belongs to the peptidyl-diazomethane family (cysteine protease inhibitor 1, termed CP-1). In vitro toxicity was measured using an assay of cell viability, and in vivo toxicity was measured by histological tissue analysis after infusion of CP-1 in rats. Two hours of middle cerebral artery (MCA) occlusion in rats was performed by the intravascular suture method. Immediately following reperfusion, intravenous infusion of CP-1 or vehicle was performed for 4 h at 0.9 ml/h. After a 7-day survival, the infarct volumes were measured. CP-1 was non-toxic to cultured glial cells to a local concentration of 200 microM, and relatively non-toxic to cultured endothelial cells at concentrations of 100-200 microM. No animal exhibited toxic effects at any of the doses used. Histologic comparisons revealed no signs of tissue toxicity. CP-1 significantly reduced hemispheric infarct volume compared to control (37+/-8.2%) at concentrations of 10, 50, and 250 microM [22+/-15%, P=0.008; 20+/-13%, P=0.002; 23+/-15%, P=0.022, respectively (mean+/-standard deviation; N=7-10 per group)]. CP-1, at the concentration of 50 microM, improved the functional score of the animals, but did not significantly alter cerebral blood flow. This study supports the hypothesis that the lysosomal cathepsins B and/or L contribute to cerebral injury after focal ischemia with reperfusion. Cysteine protease inhibitors which are relatively selective for cathepsins B and L, but not the calpains or caspases, are effective at reducing infarct volume after intravenous post-ischemic administration.

Transient NMDA receptor inactivation provides long-term protection to cultured cortical neurons from a variety of death signals

NMDA receptor antagonists, such as (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), potently block glutamate-induced neuronal death in myriad in vitro cell models and effectively attenuate ischemic damage in vivo. In this report, a novel role for MK-801 and other NMDA receptor antagonists in preconditioning neurons to withstand a wide range of subsequent lethal insults is described. A brief 30 min exposure to 0.1 microM MK-801, applied up to 96 hr before a "lethal" insult, protected primary cortical neurons from a diverse group of neurotoxic agents, including NMDA, beta-amyloid, staurosporine, etoposide, and oxygen-glucose deprivation. This neuroprotective preconditioning by MK-801 arose from transient NMDA receptor inactivation, because the noncompetitive NMDA receptor antagonists memantine and nylindin and the competitive antagonist AP-5 gave similar effects. MK-801 protection was dependent on new protein synthesis during the first 2 hr, but not from 2 to 5 hr, after MK-801 exposure. The MK-801 transient did not alter the ability of NMDA to trigger normally lethal [Ca(2+)](i) influx 48 hr later, but it did block early downstream signaling events coupled to NMDA neurotoxicity, including PKC inactivation and the activation of calpain. Moreover, MK-801 protected neurons from staurosporine-induced apoptosis, although caspase activation in these cells was unimpeded. It is likely that the stress associated with transient inactivation of NMDA receptors triggered a rapid compensatory survival response that provided long-term protection from a spectrum of insults, inducing apoptotic and nonapoptotic death. The possibility that MK-801 preconditioning blocks an event common to seemingly diverse death mechanisms suggests it will be an important tool for obtaining a clearer understanding of the salient molecular events at work in neuronal death and survival pathways.

E-64-d prevents both calpain upregulation and apoptosis in the lesion and penumbra following spinal cord injury in rats

Calpain, a Ca(2+)-dependent cysteine protease, has been implicated in cytoskeletal protein degradation and neurodegeneration in the lesion and adjacent areas following spinal cord injury (SCI). To attenuate apoptosis or programmed cell death (PCD) in SCI, we treated injured rats with E-64-d, a cell permeable and selective inhibitor of calpain. SCI was induced on T12 by the weight-drop (40 g-cm force) method. Within 15 min, E-64-d (1 mg/kg) in 1.5% DMSO was administered i.v. to the SCI rats. Following 24 h treatment, a 5-cm long spinal cord section with the lesion in the center was collected. The spinal cord section was divided equally into five 1-cm segments (S1: distant rostral, S2: near rostral, S3: lesion or injury, S4: near caudal and S5: distant caudal) for analysis. Determination of mRNA levels by reverse transcriptase-polymerase chain reaction (RT-PCR) indicated that ratios of bax/bcl-2 and calpain/calpastatin were increased in spinal cord segments from injured rats compared to controls. Degradation of the 68-kD neurofilament protein and internucleosomal DNA fragmentation were also increased. All of these changes were maximally increased in the lesion and gradually decreased in the adjacent areas of SCI rats, while largely undetectable in E-64-d treated rats and absent in sham controls. The results indicate that apoptosis in rat SCI appears to be associated with calpain activity which can be attenuated by the calpain inhibitor E-64-d.

Calpain inhibitors confer biochemical, but not electrophysiological, protection against anoxia in rat optic nerves

Calpains are ubiquitous Ca(2+)-activated neutral proteases that have been implicated in ischemic and traumatic CNS injury. Ischemia and trauma of central white matter are dependent on Ca2+ accumulation, and calpain overactivation likely plays a significant role in the pathogenesis. Adult rat optic nerves, representative central white matter tracts, were studied in an in vitro anoxic model. Functional recovery following 60 min of anoxia and reoxygenation was measured electrophysiologically. Calpain activation was assessed using western blots with antibodies against calpain-cleaved spectrin breakdown products. Sixty minutes of in vitro anoxia increased the amount of spectrin breakdown approximately 20-fold over control, with a further increase after reoxygenation to >70 times control, almost as much as 2 h of continuous anoxia. Blocking voltage-gated Na+ channels with tetrodotoxin or removing bath Ca2+ was highly neuroprotective electrophysiologically and resulted in a marked reduction of spectrin degradation. The membrane-permeable calpain inhibitors MDL 28,170 and calpain inhibitor-I (10-100 microM) were effective at reducing spectrin breakdown in anoxic and reoxygenated optic nerves, but no electrophysiological improvement was observed. We conclude that calpain activation is an important step in anoxic white matter injury, but inhibition of this Ca(2+)-dependent process in isolation does not improve functional outcome, probably because other deleterious Ca(2+)-activated pathways proceed unchecked.

Synthesis, assembly, and turnover of alpha and beta-erythroid and nonerythroid spectrins in rat hippocampal neurons

The synthesis and turnover of alpha-erythroid, beta-erythroid, alpha-nonerythroid and beta-nonerythroid spectrins was investigated in cultured rat hippocampal neurons. [35S]methionine and subunit specific antibodies were used to label and immunoprecipitate newly synthesized spectrins in 12- to 14-day-old cultures. Synthesis experiments, performed under normal resting conditions, showed that the ratio of newly synthesized alpha-erythroid/beta-erythroid and alpha-nonerythroid/beta-nonerythroid spectrins is 1/1 (mol/mol) both in the soluble and insoluble fractions. Soluble and insoluble alpha and beta erythroid spectrin turn over rapidly (half-life=16-24 min). Soluble nonerythroid alpha-spectrin (half-life=80 min) and beta spectrin (half-life=53 min) turn over more slowly than their insoluble counterparts (30-34 min). The nonerythroid alpha spectrin turnover was significantly different (p<0.05) from the other measurements except for nonerythroid beta spectrin, indicating that these subunits are protected from rapid proteolytic degradation until they are assembled in the membrane skeleton.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Hydroxyoxazolidines as alpha-aminoacetaldehye equivalents: novel inhibitors of calpain

The synthesis of [1-[(5-hydroxy-4-(phenylmethyl)-3-oxazolidinyl)carbonyl]-2-ethylpropy lcarbamic acid phenylmethyl ester (2; MDL 104,903), a potent inhibitor of calpain, is described. Synthesis of related compounds, which offer insights into the mechanism of action for 2, are also described, as is an O-acetyl prodrug derivative of 2.

Inducible nitric oxide production and expression of transforming growth factor-beta1 in serum and CSF after cerebral ischaemic stroke in man

A residual blood supply to the ischaemic brain is a crucial determinant for tissue survival. Early changes in the vascular network and subsequent angiogenesis may be mediated by short-lived molecules like nitric oxide (NO) or growth factors such as transforming growth factor-beta1 (TGF-beta1). Although TGF-beta1 can inhibit NO production, this interaction has not been studied after ischaemia in humans. Serum samples were taken from patients at 24 h and 6 months and cerebrospinal fluid (CSF) samples at 24 h and 1 week later for possible correlation between the two factors. Tissue expression of TGF-beta1 and of the inducible isoform of NO synthase (NOS2) was assessed by immunohistochemistry. CSF levels of NO2-/NO3- as well as total (active + latent) TGF-beta1 were higher in stroke patients as compared to controls 24 h after the stroke. Both NO2-/NO3- and TGF-beta1 were lower 6 months after the stroke compared to 24 h. Levels of NO2-/NO3- correlated with levels of TGF-beta1 within the time points (P = 0.041, Kendall correlation coefficient). There was a strong staining for NOS2 in brain tissue sections in neurones, reactive astrocytes, infiltrating white blood cells, and endothelial cells of larger microvessels. TGF-beta1 expression was mainly limited to neurones and reactive astrocytes. These findings suggest that the interaction between TGF-beta1 and NOS2 might be important for angiogenesis after cerebral ischaemia and may indicate that TGF-beta1 is upregulated as a negative feedback response to elevated levels of NO.

Selective coupling of mu-calpain activation with the NMDA receptor is independent of translocation and autolysis in primary cortical neurons

Excessive mu-calpain activation has been linked to several cellular pathologies including excitotoxicity and ischemia. In erythrocytes and other non-central nervous system (CNS) cells, calpain activation is thought to occur following a Ca2+-induced translocation of inactive cytosolic enzyme to membranes and subsequent autolysis. In the present report, we show that transiently exposing primary rat cortical neurons to lethal (50 microM) N-methyl-D-aspartic acid (NMDA) caused protracted calpain activation, measured as increased spectrin hydrolysis, but this was independent of translocation or autolysis of the protease. An anti-mu-calpain antibody showed that calpain was largely membrane associated in cortical neurons, and, consequently, neither translocation nor autolysis of the protease was observed following ionomycin or lethal NMDA treatment. By contrast, in rat erythrocytes, calpain was largely cytosolic and underwent rapid translocation and autolysis in response to ionomycin. Calpain-mediated spectrin hydrolysis was specifically coupled to Ca2+ entry through the NMDA receptor because nonspecific Ca2+ influx via ionomycin or KCl-mediated depolarization failed to activate the enzyme. Thus, calpain appears selectively linked to glutamate receptors in cortical neurons and regulated by mechanisms distinct from that occurring in many non-CNS cells. The data suggest that intracellular signals coupled to the NMDA receptor are responsible for activating calpain already associated with cellular membranes in cortical cells.