Calpain I activation in rat hippocampal neurons in culture is NMDA receptor selective and not essential for excitotoxic cell death

Lipopolysaccharide From E. coli Increases Glutamate-Induced Disturbances of Calcium Homeostasis, the Functional State of Mitochondria, and the Death of Cultured Cortical Neurons

Lipopolysaccharide (LPS), a fragment of the bacterial cell wall, specifically interacting with protein complexes on the cell surface, can induce the production of pro-inflammatory and apoptotic signaling molecules, leading to the damage and death of brain cells. Similar effects have been noted in stroke and traumatic brain injury, when the leading factor of death is glutamate (Glu) excitotoxicity too. But being an amphiphilic molecule with a significant hydrophobic moiety and a large hydrophilic region, LPS can also non-specifically bind to the plasma membrane, altering its properties. In the present work, we studied the effect of LPS from Escherichia coli alone and in combination with the hyperstimulation of Glu-receptors on the functional state of mitochondria and Ca2+ homeostasis, oxygen consumption and the cell survival in primary cultures from the rats brain cerebellum and cortex. In both types of cultures, LPS (0.1–10 μg/ml) did not change the intracellular free Ca2+ concentration ([Ca2+]i) in resting neurons but slowed down the median of the decrease in [Ca2+]i on 14% and recovery of the mitochondrial potential (ΔΨm) after Glu removal. LPS did not affect the basal oxygen consumption rate (OCR) of cortical neurons; however, it did decrease the acute OCR during Glu and LPS coapplication. Evaluation of the cell culture survival using vital dyes and the MTT assay showed that LPS (10 μg/ml) and Glu (33 μM) reduced jointly and separately the proportion of live cortical neurons, but there was no synergism or additive action. LPS-effects was dependent on the type of culture, that may be related to both the properties of neurons and the different ratio between neurons and glial cells in cultures. The rapid manifestation of these effects may be the consequence of the direct effect of LPS on the rheological properties of the cell membrane.

Activity-dependent cleavage of dyskinesia-related proline-rich transmembrane protein 2 (PRRT2) by calpain in mouse primary cortical neurons

Mutations of PRRT2 (proline-rich transmembrane protein 2) cause several neurological disorders, represented by paroxysmal kinesigenic dyskinesia (PKD), which is characterized by attacks of involuntary movements triggered by sudden voluntary movements. PRRT2 is reported to suppress neuronal excitation, but it is unclear how the function of PRRT2 is modulated during neuronal excitation. We found that PRRT2 is processed to a 12 kDa carboxy-terminal fragment (12K-CTF) by calpain, a calcium-activated cysteine protease, in a neuronal activity-dependent manner, predominantly via NMDA receptors or voltage-gated calcium channels. Furthermore, we clarified that 12K-CTF is generated by sequential cleavages at Q220 and S244. The amino-terminal fragment (NTF) of PRRT2, which corresponds to PKD-related truncated mutants, is not detected, probably due to rapid cleavage at multiple positions. Given that 12K-CTF lacks most of the proline-rich domain, this cleavage might be involved in the activity-dependent enhancement of neuronal excitation perhaps through transient retraction of PRRT2's function. Therefore, PRRT2 might serve as a buffer for neuronal excitation, and lack of this function in PKD patients might cause neuronal hyperexcitability in their motor circuits.

Ankyrin-G induces nucleoporin Nup358 to associate with the axon initial segment of neurons

Nup358 (also known as RanBP2) is a member of the large nucleoporin family that constitutes the nuclear pore complex. Depending on the cell type and the physiological state, Nup358 interacts with specific partner proteins and influences distinct mechanisms independent of its role in nucleocytoplasmic transport. Here, we provide evidence that Nup358 associates selectively with the axon initial segment (AIS) of mature neurons, mediated by the AIS scaffold protein ankyrin-G (AnkG, also known as Ank3). The N-terminus of Nup358 is demonstrated to be sufficient for its localization at the AIS. Further, we show that Nup358 is expressed as two isoforms, one full-length and another shorter form of Nup358. These isoforms differ in their subcellular distribution in neurons and expression level during neuronal development. Overall, the present study highlights an unprecedented localization of Nup358 within the AIS and suggests its involvement in neuronal function.This article has an associated First Person interview with the first author of the paper.

Calpain Inhibition Increases SMN Protein in Spinal Cord Motoneurons and Ameliorates the Spinal Muscular Atrophy Phenotype in Mice

Spinal muscular atrophy (SMA), a leading genetic cause of infant death, is caused by the loss of survival motor neuron 1 (SMN1) gene. SMA is characterized by the degeneration and loss of spinal cord motoneurons (MNs), muscular atrophy, and weakness. SMN2 is the centromeric duplication of the SMN gene, whose numbers of copies determine the intracellular levels of SMN protein and define the disease onset and severity. It has been demonstrated that elevating SMN levels can be an important strategy in treating SMA and can be achieved by several mechanisms, including promotion of protein stability. SMN protein is a direct target of the calcium-dependent protease calpain and induces its proteolytic cleavage in muscle cells. In this study, we examined the involvement of calpain in SMN regulation on MNs. In vitro experiments showed that calpain activation induces SMN cleavage in CD1 and SMA mouse spinal cord MNs. Additionally, calpain 1 knockdown or inhibition increased SMN level and prevent neurite degeneration in these cells. We examined the effects of calpain inhibition on the phenotype of two severe SMA mouse models. Treatment with the calpain inhibitor, calpeptin, significantly improved the lifespan and motor function of these mice. Our observations show that calpain regulates SMN level in MNs and calpeptin administration improves SMA phenotype demonstrating the potential utility of calpain inhibitors in SMA therapy.

The neuroprotective effect of hesperidin in NMDA-induced retinal injury acts by suppressing oxidative stress and excessive calpain activation

We found that hesperidin, a plant-derived bioflavonoid, may be a candidate agent for neuroprotective treatment in the retina, after screening 41 materials for anti-oxidative properties in a primary retinal cell culture under oxidative stress. We found that the intravitreal injection of hesperidin in mice prevented reductions in markers of the retinal ganglion cells (RGCs) and RGC death after N-methyl-D-aspartate (NMDA)-induced excitotoxicity. Hesperidin treatment also reduced calpain activation, reactive oxygen species generation and TNF-α gene expression. Finally, hesperidin treatment improved electrophysiological function, measured with visual evoked potential, and visual function, measured with optomotry. Thus, we found that hesperidin suppressed a number of cytotoxic factors associated with NMDA-induced cell death signaling, such as oxidative stress, over-activation of calpain, and inflammation, thereby protecting the RGCs in mice. Therefore, hesperidin may have potential as a therapeutic supplement for protecting the retina against the damage associated with excitotoxic injury, such as occurs in glaucoma and diabetic retinopathy.

Activation of NMDA receptors thickens the postsynaptic density via proteolysis

The postsynaptic density (PSD) is a protein complex that is critical for synaptic transmission. Ultrastructural changes in the PSD are therefore likely to modify synaptic functions. In this study, we investigated the ultrastructural changes in the PSD in the hippocampal CA1 stratum radiatum following neuronal excitation. Oxygen-glucose deprivation-induced PSD thickening in hippocampal slice cultures was blocked by the N-methyl-d-aspartate (NMDA) receptor antagonist MK801. To gain more insight into the mechanisms underlying NMDA receptor-mediated PSD thickening, we assessed the area, length, and thickness of the PSD after NMDA treatment. The PSDs thickened with just 2 min of NMDA receptor stimulation, and this treatment was considered sublethal. When N-acetyl-leucyl-leucyl-norleucinal, an inhibitor of calpain, cathepsins, and the proteasome, was applied, NMDA-induced PSD thickening was abolished. Furthermore, the calcium-induced calcium release inhibitor, ryanodine, reduced NMDA receptor-mediated PSD thickening. These results suggest that NMDA receptor activation induces PSD thickening by proteolysis through intracellular calcium increase, including that induced by calcium.

Phosphorylation of synaptic GTPase-activating protein (synGAP) by Ca2+/calmodulin-dependent protein kinase II (CaMKII) and cyclin-dependent kinase 5 (CDK5) alters the ratio of its GAP activity toward Ras and Rap GTPases

synGAP is a neuron-specific Ras and Rap GTPase-activating protein (GAP) found in high concentrations in the postsynaptic density (PSD) fraction from the mammalian forebrain. We have previously shown that, in situ in the PSD fraction or in recombinant form in Sf9 cell membranes, synGAP is phosphorylated by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), another prominent component of the PSD. Here, we show that recombinant synGAP (r-synGAP), lacking 102 residues at the N terminus, can be purified in soluble form and is phosphorylated by cyclin-dependent kinase 5 (CDK5) as well as by CaMKII. Phosphorylation of r-synGAP by CaMKII increases its HRas GAP activity by 25% and its Rap1 GAP activity by 76%. Conversely, phosphorylation by CDK5 increases r-synGAP's HRas GAP activity by 98% and its Rap1 GAP activity by 20%. Thus, phosphorylation by both kinases increases synGAP activity; CaMKII shifts the relative GAP activity toward inactivation of Rap1, and CDK5 shifts the relative activity toward inactivation of HRas. GAP activity toward Rap2 is not altered by phosphorylation by either kinase. CDK5 phosphorylates synGAP primarily at two sites, Ser-773 and Ser-802. Phosphorylation at Ser-773 inhibits r-synGAP activity, and phosphorylation at Ser-802 increases it. However, the net effect of concurrent phosphorylation of both sites, Ser-773 and Ser-802, is an increase in GAP activity. synGAP is phosphorylated at Ser-773 and Ser-802 in the PSD fraction, and its phosphorylation by CDK5 and CaMKII is differentially regulated by activation of NMDA-type glutamate receptors in cultured neurons.

NMDA receptor activation and calpain contribute to disruption of dendritic spines by the stress neuropeptide CRH

The complex effects of stress on learning and memory are mediated, in part, by stress-induced changes in the composition and structure of excitatory synapses. In the hippocampus, the effects of stress involve several factors including glucocorticoids and the stress-released neuropeptide corticotropin-releasing hormone (CRH), which influence the integrity of dendritic spines and the structure and function of the excitatory synapses they carry. CRH, at nanomolar, presumed-stress levels, rapidly abolishes short-term synaptic plasticity and destroys dendritic spines, yet the mechanisms for these effects are not fully understood. Here we tested the hypothesis that glutamate receptor-mediated processes, which shape synaptic structure and function, are engaged by CRH and contribute to spine destabilization. In cultured rat hippocampal neurons, CRH application reduced dendritic spine density in a time- and dose-dependent manner, and this action depended on the CRH receptor type 1. CRH-mediated spine loss required network activity and the activation of NMDA, but not of AMPA receptors; indeed GluR1-containing dendritic spines were resistant to CRH. Downstream of NMDA receptors, the calcium-dependent enzyme, calpain, was recruited, resulting in the breakdown of spine actin-interacting proteins including spectrin. Pharmacological approaches demonstrated that calpain recruitment contributed critically to CRH-induced spine loss. In conclusion, the stress hormone CRH co-opts mechanisms that contribute to the plasticity and integrity of excitatory synapses, leading to selective loss of dendritic spines. This spine loss might function as an adaptive mechanism preventing the consequences of adverse memories associated with severe stress.

Nerve injury increases GluA2-lacking AMPA receptor prevalence in spinal cords: functional significance and signaling mechanisms

The glutamate α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are critically involved in the excitatory synaptic transmission, and blocking AMPARs at the spinal level reverses neuropathic pain. However, little is known about changes in the composition of synaptic AMPARs in the spinal dorsal horn after peripheral nerve injury. AMPARs lacking the GluA2 subunit are permeable to Ca(2+), and their currents show unique inward rectification. We found that AMPAR-mediated excitatory postsynaptic currents (AMPAR-EPSCs) of spinal dorsal horn neurons exhibited a linear current-voltage relationship in control rats, whereas AMPAR-EPSCs of dorsal horn neurons displayed inward rectification in rats with spinal nerve injury. In nerve-injured rats, compared with control rats, the GluA2 protein level was significantly less in the plasma membrane but was greater in the cytosolic vesicle fraction in the dorsal spinal cord. However, the GluA1 protein levels in these fractions did not differ significantly between nerve-injured and control rats. Blocking N-methyl-d-aspartate receptors (NMDARs) abolished inward rectification of AMPAR-EPSCs of dorsal horn neurons in nerve-injured rats. Furthermore, inhibition of calpain or calcineurin, but not protein kinase C, completely blocked nerve injury-induced inward rectification of AMPAR-EPSCs of dorsal horn neurons. In addition, blocking GluA2-lacking AMPARs at the spinal cord level reduced nerve injury-induced pain hypersensitivity. Our study suggests that nerve injury increases GluA2 internalization and the prevalence of GluA2-lacking AMPARs in the spinal dorsal horn to maintain chronic neuropathic pain. Increased prevalence of spinal GluA2-lacking AMPARs in neuropathic pain is mediated by NMDARs and subsequent stimulation of calpain and calcineurin signaling.

Presenilin-1/γ-secretase controls glutamate release, tyrosine phosphorylation, and surface expression of N-methyl-D-aspartate receptor (NMDAR) subunit GluN2B

Abnormally high concentrations of extracellular glutamate in the brain may cause neuronal damage via excitotoxicity. Thus, tight regulation of glutamate release is critical to neuronal function and survival. Excitotoxicity is caused mainly by overactivation of the extrasynaptic NMDA receptor (NMDAR) and results in specific cellular changes, including calcium-induced activation of calpain proteases. Here, we report that presenilin-1 (PS1) null mouse cortical neuronal cultures have increased amounts of calpain-dependent spectrin breakdown products (SBDPs) compared with WT cultures. NMDAR antagonists blocked accumulation of SBDPs, suggesting abnormal activation of this receptor in PS1 null cultures. Importantly, an increase in SBDPs was detected in cultures of at least 7 days in vitro but not in younger cultures. Conditioned medium from PS1 null neuronal cultures at 8 days in vitro contained higher levels of glutamate than medium from WT cultures and stimulated production of SBDPs when added to WT cultures. Use of glutamate reuptake inhibitors indicated that accumulation of this neurotransmitter in the media of PS1 null cultures was due to increased rates of release. PS1 null neurons showed decreased cell surface expression and phosphorylation of the GluN2B subunit of NMDAR, indicating decreased amounts of extrasynaptic NMDAR in the absence of PS1. Inhibition of γ-secretase activity in WT neurons caused changes similar to those observed in PS1 null neurons. Together, these data indicate that the PS1/γ-secretase system regulates release of glutamate, tyrosine phosphorylation, and surface expression of GluN2B-containing NMDARs.

Beneficial effects of antecedent exercise training on limb motor function and calpain expression in a rat model of stroke

[Purpose] In the present study, we investigated the effects of antecedent exercise on functional recovery and calpain protein expression following focal cerebral ischemia injury. [Subjects and Methods] The rat middle cerebral artery occlusion model was employed. Adult male Sprague-Dawley rats were randomly divided into 4 groups. Group I comprised untreated normal rats (n=10); Group II comprised untreated rats with focal cerebral ischemia (n=10); Group III comprised rats that performed treadmill exercise (20 m/min) training after focal cerebral ischemia (n=10); and Group IV comprised rats that performed antecedent treadmill exercise (20 m/min) training before focal cerebral ischemia (n=10). At different time points (1, 7, 14, and 21 days), limb placement test score and the levels of calpain protein in the hippocampus were examined. [Results] In the antecedent exercise group, improvements in the motor behavior index (limb placement test) were observed and hippocampal calpain protein levels were decreased. [Conclusion] These results indicated that antecedent treadmill exercise prior to focal cerebral ischemia exerted neuroprotective effects against ischemic brain injury by improving motor performance and decreasing the levels of calpain expression. Furthermore, these results suggest that antecedent treadmill exercise of an appropriate intensity is critical for post-stroke rehabilitation.

Calpain 2 activated through N-methyl-D-aspartic acid receptor signaling cleaves CPEB3 and abrogates CPEB3-repressed translation in neurons

Long-term memory requires the activity-dependent reorganization of the synaptic proteome to modulate synaptic efficacy and consequently consolidate memory. Activity-regulated RNA translation can change the protein composition at the stimulated synapse. Cytoplasmic polyadenylation element-binding protein 3 (CPEB3) is a sequence-specific RNA-binding protein that represses translation of its target mRNAs in neurons, while activation of N-methyl-d-aspartic acid (NMDA) receptors alleviates this repression. Although recent research has revealed the mechanism of CPEB3-inhibited translation, how NMDA receptor signaling modulates the translational activity of CPEB3 remains unclear. This study shows that the repressor CPEB3 is degraded in NMDA-stimulated neurons and that the degradation of CPEB3 is accompanied by the elevated expression of CPEB3's target, epidermal growth factor receptor (EGFR), mostly at the translational level. Using pharmacological and knockdown approaches, we have identified that calpain 2, activated by the influx of calcium through NMDA receptors, proteolyzes the N-terminal repression motif but not the C-terminal RNA-binding domain of CPEB3. As a result, the calpain 2-cleaved CPEB3 fragment binds to RNA but fails to repress translation. Therefore, the cleavage of CPEB3 by NMDA-activated calpain 2 accounts for the activity-related translation of CPEB3-targeted RNAs.

Phosphorylation and cleavage of the family of collapsin response mediator proteins may play a central role in neurodegeneration after CNS trauma

The family of the collapsin response mediator proteins (CRMPs) plays a significant physiological role in neuronal cell bodies and axons within the integrated mammalian central nervous system (CNS). Trauma-induced damage to the CNS results in variable degrees of axonal degeneration, and this may lead to neuronal cell death in key grey matter regions. Site-specific phosphorylation of certain CRMPs has been associated with trauma-induced axonal degeneration. Moreover, recent data implicate the pro-apoptotic, calcium-dependent protease calpain as a key initiator of CRMP cleavage. The primary cleavage product of injury-induced neuronal calpain activation is a C-terminus truncated 55- to 58-kDa form of CRMP, which may exert its effects within the cytoplasm and axonal core, or alternatively through its translocation into the nucleus, initiating neuronal cell death. The precise structure of cleaved CRMP has yet to be elucidated, as is the reason for nuclear translocation. Once the crystal structure of the cytoplasmic and nuclear-translocated forms of CRMPs is determined, a greater molecular understanding of why these forms can initiate neurodegeneration following CNS injury will be established. Such information will be particularly informative in the design of inhibitors of specific protein-protein interaction sites between cleaved CRMP and vital cytosolic or nuclear molecules.

Switch to glutamate receptor 2-lacking AMPA receptors increases neuronal excitability in hypothalamus and sympathetic drive in hypertension

Glutamatergic synaptic input in the hypothalamic paraventricular nucleus (PVN) plays a critical role in regulating sympathetic outflow in hypertension. GluR2-lacking AMPA receptors (AMPARs) are permeable to Ca(2+), and their currents show unique inward rectification. However, little is known about changes in the AMPAR composition and its functional significance in hypertension. In this study, we found that AMPAR-mediated EPSCs (AMPAR-EPSCs) of retrogradely labeled spinally projecting PVN neurons exhibited a linear current-voltage relationship in Wistar-Kyoto (WKY) rats. However, AMPAR-EPSCs of labeled PVN neurons in spontaneously hypertensive rats (SHR) displayed inward rectification at positive holding potentials, which were not altered by lowering blood pressure with celiac ganglionectomy. Blocking GluR2-lacking AMPARs with 1-naphthyl acetyl spermine (NAS) caused a greater reduction in the AMPAR-EPSC amplitude and firing activity of PVN neurons in SHR than in WKY rats. Furthermore, blocking NMDA receptors and inhibition of calpain or calcineurin abolished inward rectification of AMPAR-EPSCs of PVN neurons in SHR. The GluR2 protein level was significantly less in the plasma membrane but greater in the cytosolic vesicle fraction in SHR than in WKY rats. In addition, microinjection of NAS into the PVN decreased blood pressure and lumbar sympathetic nerve activity in SHR but not in WKY rats. Our study reveals that increased GluR2-lacking AMPAR activity of PVN neurons results from GluR2 internalization through NMDA receptor-calpain-calcineurin signaling in hypertension. This phenotype switch in synaptic AMPARs contributes to increased excitability of PVN presympathetic neurons and sympathetic vasomotor tone in hypertension.

Neuroprotection against traumatic brain injury by a peptide derived from the collapsin response mediator protein 2 (CRMP2)

Neurological disabilities following traumatic brain injury (TBI) may be due to excitotoxic neuronal loss. The excitotoxic loss of neurons following TBI occurs largely due to hyperactivation of N-methyl-d-aspartate receptors (NMDARs), leading to toxic levels of intracellular Ca(2+). The axon guidance and outgrowth protein collapsin response mediator protein 2 (CRMP2) has been linked to NMDAR trafficking and may be involved in neuronal survival following excitotoxicity. Lentivirus-mediated CRMP2 knockdown or treatment with a CRMP2 peptide fused to HIV TAT protein (TAT-CBD3) blocked neuronal death following glutamate exposure probably via blunting toxicity from delayed calcium deregulation. Application of TAT-CBD3 attenuated postsynaptic NMDAR-mediated currents in cortical slices. In exploring modulation of NMDARs by TAT-CBD3, we found that TAT-CBD3 induced NR2B internalization in dendritic spines without altering somal NR2B surface expression. Furthermore, TAT-CBD3 reduced NMDA-mediated Ca(2+) influx and currents in cultured neurons. Systemic administration of TAT-CBD3 following a controlled cortical impact model of TBI decreased hippocampal neuronal death. These findings support TAT-CBD3 as a novel neuroprotective agent that may increase neuronal survival following injury by reducing surface expression of dendritic NR2B receptors.

Calpain activation and Na+/Ca2+ exchanger degradation occur downstream of calcium deregulation in hippocampal neurons exposed to excitotoxic glutamate

Delayed calcium deregulation (DCD) plays an essential role in glutamate excitotoxicity, a major detrimental factor in stroke, traumatic brain injury, and various neurodegenerations. In the present study, we examined the role of calpain activation and Na(+)/Ca(2+) exchanger (NCX) degradation in DCD and excitotoxic cell death in cultured hippocampal neurons. Exposure of neurons to glutamate caused DCD accompanied by secondary mitochondrial depolarization. Activation of calpain was evidenced by detecting NCX isoform 3 (NCX3) degradation products. Degradation of NCX isoform 1 (NCX1) was below the detection limit of Western blotting. Degradation of NCX3 was detected only after 1 hr of incubation with glutamate, whereas DCD occurred on average within 15 min after glutamate application. Calpeptin, an inhibitor of calpain, significantly attenuated NCX3 degradation but failed to inhibit DCD and excitotoxic neuronal death. Calpain inhibitors I, III, and VI also failed to influence DCD and glutamate-induced neuronal death. On the other hand, MK801, an inhibitor of the NMDA subtype of glutamate receptors, added shortly after the initial glutamate-induced jump in cytosolic Ca(2+), completely prevented DCD and activation of calpain and strongly protected neurons against excitotoxicity. Taken together, our results suggest that, in glutamate-treated hippocampal neurons, the initial increase in cytosolic Ca(2+) that precedes DCD is insufficient for sustained calpain activation, which most likely occurs downstream of DCD.

Excitotoxicity effects of glutamate on human neuroblastoma SH-SY5Y cells via oxidative damage

OBJECTIVE

To investigate the mechanisms of excitotoxic effects of glutamate on human neuroblastoma SH-SY5Y cells.

METHODS

SH-SY5Y cell viability was measured by MTT assay. Other damaged profile was detected by lactate dehydrogenase (LDH) release and by 4', 6-diamidino-2-phenylindole (DAPI) staining. The cytosolic calcium concentration was tested by calcium influx assay. The glutamate-induced oxidative stress was analyzed by cytosolic glutathione assay, superoxide dismutase (SOD) assay and extracellular malondialdehyde (MDA) assay.

RESULTS

Glutamate treatment caused damage in SH-SY5Y cells, including the decrease of cell viability, the increase of LDH release and the alterations of morphological structures. Furthermore, the concentration of cytoplasmic calcium in SH-SY5Y cells was not changed within 20 min following glutamate treatment, while cytosolic calcium concentration significantly increased within 24 h after glutamate treatment, which could not be inhibited by MK801, an antagonist of NMDA receptors, or by LY341495, an antagonist of metabotropic glutamate receptors. On the other hand, oxidative damage was observed in SH-SY5Y cells treated with glutamate, including decreases in glutathione content and SOD activity, and elevation of MDA level, all of which could be alleviated by an antioxidant Tanshinone IIA (Tan IIA, a major active ingredient from a Chinese plant Salvia Miltiorrhiza Bge).

CONCLUSION

Glutamate exerts toxicity in human neuroblastoma SH-SY5Y cells possibly through oxidative damage, not through calcium homeostasis destruction mediated by NMDA receptors.

Transfection of nerve cells

Transfection is a method of transforming cells based on the introduction into living cells of plasmids encoding a particular protein or RNA. This review describes the main methods of transfection and considers their advantages and disadvantages. Most attention is paid to lentivirus transduction as one of the most efficient methods for transforming nerve cells. The development of current transfection systems based on lentivirus vectors is described and a brief review of studies performed using in vivo and in vitro lentivirus transfection of nerve cells is presented.

Specific vulnerability of mouse spinal cord motoneurons to membrane depolarization

Intracellular calcium (Ca(2+)) concentration determines neuronal dependence on neurotrophic factors (NTFs) and susceptibility to cell death. Ca(2+) overload induces neuronal death and the consequences are thought to be a probable cause of motoneuron (MN) degeneration in neurodegenerative diseases. In the present study, we show that membrane depolarization with elevated extracellular potassium (K(+)) was toxic to cultured embryonic mouse spinal cord MNs even in the presence of NTFs. Membrane depolarization induced an intracellular Ca(2+) increase. Depolarization-induced toxicity and increased intracellular Ca(2+) were blocked by treatment with antagonists to some of the voltage-gated Ca(2+) channels (VGCCs), indicating that Ca(2+) influx through these channels contributed to the toxic effect of depolarization. Ca(2+) activates the calpains, cysteine proteases that degrade a variety of substrates, causing cell death. We investigated the functional involvement of calpain using a calpain inhibitor and calpain gene silencing. Pre-treatment of MNs with calpeptin (a cell-permeable calpain inhibitor) rescued MNs survival; calpain RNA interference had the same protective effect, indicating that endogenous calpain contributes to the cell death caused by membrane depolarization. These findings suggest that MNs are especially vulnerable to extracellular K(+) concentration, which induces cell death by causing both intracellular Ca(2+) increase and calpain activation.

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.

Polyglutamine-modulated striatal calpain activity in YAC transgenic huntington disease mouse model: impact on NMDA receptor function and toxicity

Huntington disease (HD), caused by CAG expansion in the ubiquitously expressed huntingtin gene, is characterized by early dysfunction and death of striatal medium-sized spiny neurons (MSNs). Previous work has shown MSN-specific alterations in NMDA receptor (NMDAR) expression and cell death signaling. Furthermore, studies in HD human brain tissue and a knock-in mouse model demonstrate increases in calpain activity, which can be stimulated by NMDARs and contribute to excitotoxicity. Here, we report increased calpain activity in MSNs from the yeast artificial chromosome (YAC) transgenic mouse model of HD, expressing human full-length huntingtin with 128 polyglutamine repeats (YAC128), compared with wild type. Moreover, the calpain-cleaved product of NMDAR subunit NR2B is increased early, and NR2B expression levels are reduced, in YAC128 striatum. Although steady-state NMDAR surface expression is similar in wild-type and YAC128 MSNs, the rate of loss of NR2B-containing surface receptors is enhanced in YAC128 MSNs, suggesting that NMDAR forward trafficking to the surface is also faster, as previously reported for YAC72 MSNs. Calpain inhibitor-1 treatment normalized the loss rate of surface NMDARs in YAC128 MSNs to that of wild type, and significantly increased surface NMDAR expression in YAC128, but not in wild type or YAC72. With acute NMDAR overstimulation, the increase in calpain activity correlated with polyglutamine length, and calpain inhibitor treatment reduced NMDA-induced apoptosis in YAC72 and YAC128 MSNs to wild-type levels. Thus, the cumulative effect of increasing huntingtin polyglutamine length is to enhance MSN sensitivity to excitotoxicity at least in part by calpain-mediated cell death signaling.

Growth-compromised HSV-2 vector Delta RR protects from N-methyl-D-aspartate-induced neuronal degeneration through redundant activation of the MEK/ERK and PI3-K/Akt survival pathways, either one of which overrides apoptotic cascades

We have previously shown that intrastriatal injection of Delta RR, the growth-compromised herpes simplex virus type 2 (HSV-2) vector for the antiapoptotic protein ICP10PK, prevents apoptosis caused by the excitotoxin N-methyl-D-aspartate (NMDA) in a mouse model of glutamatergic neuronal cell death (Golembewski et al. [2007] Exp. Neurol. 203:381-393). Because apoptosis regulation is stimulus and cell type specific, our studies were designed to examine the mechanism of Delta RR-mediated neuroprotection in striatal neurons. Organotypic striatal cultures (OSC) that retain much of the synaptic circuitry of the intact striatum were infected with Delta RR or a growth-compromised HSV-2 vector that lacks ICP10PK (Delta PK) and examined for neuroprotection-associated signaling. The mutated ICP10 proteins (p175 and p95) were expressed in 70-80% of neurons from Delta RR- and Delta PK-infected cultures, respectively, as determined by double-immunofluorescent staining with antibodies to ICP10 and NeuN or GAD65. Delta RR- but not Delta PK-treated OSC were protected from NMDA-induced apoptosis, as verified by ethidium homodimer staining, TUNEL, caspase-3 activation, and poly(AD-ribose) polymerase (PARP) cleavage. Neuroprotection was through ICP10PK-mediated activation of the survival pathways MEK/ERK and PI3-K/Akt, up-regulation of the antiapoptotic proteins Bag-1 and Bcl-2, and phosphorylation (inactivation) of the proapoptotic protein Bad. It was blocked by the MEK inhibitor U0126 or the PI3-K inhibitor LY294002, suggesting that either pathway can prevent NMDA-induced apoptosis. The data indicate that Delta RR-delivered ICP10PK stimulates redundant survival pathways that override proapoptotic cascades. Delta RR is a promising gene therapy platform against glutamatergic cell death.

Postsynaptic density-95 (PSD-95) and calcineurin control the sensitivity of N-methyl-D-aspartate receptors to calpain cleavage in cortical neurons

The N-methyl-D-aspartate receptor (NMDAR) is a Ca(2+)-permeable glutamate receptor mediating many neuronal functions under normal and pathological conditions. Ca(2+) influx via NMDARs activates diverse intracellular targets, including Ca(2+)-dependent protease calpain. Biochemical studies suggest that NR2A and NR2B subunits of NMDARs are substrates of calpain. Our physiological data showed that calpain, activated by prolonged NMDA treatment (100 microM, 5 min) of cultured cortical neurons, irreversibly decreased the whole-cell currents mediated by extrasynaptic NMDARs. Animals exposed to transient forebrain ischemia, a condition that activates calpain, exhibited the reduced NMDAR current density and the lower full-length NR2A/B level in a calpain-dependent manner. Disruption of the association between NMDARs and the scaffolding protein postsynaptic density (PSD)-95 facilitated the calpain regulation of synaptic NMDAR responses and NR2 cleavage in cortical slices, whereas inhibition of calcineurin activity blocked the calpain effect on NMDAR currents and NR2 cleavage. Calpain-cleaved NR2B subunits were removed from the cell surface. Moreover, cell viability assays showed that calpain, by targeting NMDARs, provided a negative feedback to dampen neuronal excitability in excitotoxic conditions. These data suggest that calpain activation suppresses NMDAR function via proteolytic cleavage of NR2 subunits in vitro and in vivo, and the susceptibility of NMDARs to calpain cleavage is controlled by PSD-95 and calcineurin.

Changes in calcium dynamics following the reversal of the sodium-calcium exchanger have a key role in AMPA receptor-mediated neurodegeneration via calpain activation in hippocampal neurons

Proteolytic cleavage of the Na(+)/Ca(2+) exchanger (NCX) by calpains impairs calcium homeostasis, leading to a delayed calcium overload and excitotoxic cell death. However, it is not known whether reversal of the exchanger contributes to activate calpains and trigger neuronal death. We investigated the role of the reversal of the NCX in Ca(2+) dynamics, calpain activation and cell viability, in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-stimulated hippocampal neurons. Selective overactivation of AMPA receptors caused the reversal of the NCX, which accounted for approximately 30% of the rise in intracellular free calcium concentration ([Ca(2+)](i)). The NCX reverse-mode inhibitor, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea (KB-R7943), partially inhibited the initial increase in [Ca(2+)](i), and prevented a delayed increase in [Ca(2+)](i). In parallel, overactivation of AMPA receptors strongly activated calpains and led to the proteolysis of NCX3. KB-R7943 prevented calpain activation, cleavage of NCX3 and was neuroprotective. Silencing of NCX3 reduced Ca(2+) uptake, calpain activation and was neuroprotective. Our data show for the first time that NCX reversal is an early event following AMPA receptor stimulation and is linked to the activation of calpains. Since calpain activation subsequently inactivates NCX, causing a secondary Ca(2+) entry, NCX may be viewed as a new suicide substrate operating in a Ca(2+)-dependent loop that triggers cell death and as a target for neuroprotection.

Calpain-mediated collapsin response mediator protein-1, -2, and -4 proteolysis after neurotoxic and traumatic brain injury

Collapsin response mediator proteins (CRMPs) are important molecules in neurite outgrowth and axonal guidance. Within the CRMP family, CRMP-2 has been implicated in several neurological diseases (Alzheimer's, epilepsy, and ischemia). Here, we investigated the integrity of CRMPs (CRMP-1, -2, -4, -5) after in vitro neurotoxin treatment and in vivo traumatic brain injury (TBI). After maitotoxin (MTX) and NMDA treatment of primary cortical neurons, a dramatic decrease of intact CRMP-1, -2 and -4 proteins were observed, accompanied by the appearance of distinct 55-kDa and 58-kDa breakdown products (BDP) for CRMP-2 and -4, respectively. Inhibition of calpain activation prevented NMDA-induced CRMP-2 proteolysis and redistribution of CRMP-2 from the neurites to the cell body, while attenuating neurite damage and neuronal cell injury. Similarly, CRMP-1, -2, and -4 were also found degraded in rat cortex and hippocampus following controlled cortical impact (CCI), an in vivo model of TBI. The appearance of the 55-kDa CRMP-2 BDP was observed to increase, in a time-dependent manner, between 24 and 48 h in the ipsilateral cortex, and by 48 hours in the hippocampus. The observed 55-kDa CRMP-2 BDP following TBI was reproduced by in vitro incubation of naive brain lysate with activated calpain-2, but not activated caspase-3. Sequence analysis revealed several possible cleavage sites near the C-terminus of CRMP-2. Collectively, this study demonstrated that CRMP-1, -2, and -4 are degraded following both acute traumatic and neurotoxic injury. Furthermore, calpain-2 was identified as the possible proteolytic mediator of CRMP-2 following excitotoxic injury and TBI, which appears to correlate well with neuronal cell injury and neurite damage. It is possible that the calpain-mediated truncation of CRMPs following TBI may be an inhibiting factor for post-injury neurite regeneration.

Sequential degradation of alphaII and betaII spectrin by calpain in glutamate or maitotoxin-stimulated cells

Calpain-catalyzed proteolysis of II-spectrin is a regulated event associated with neuronal long-term potentiation, platelet and leukocyte activation, and other processes. Calpain proteolysis is also linked to apoptotic and nonapoptotic cell death following excessive glutamate exposure, hypoxia, HIV-gp120/160 exposure, or toxic injury. The molecular basis for these divergent consequences of calpain action, and their relationship to spectrin proteolysis, is unclear. Calpain preferentially cleaves II spectrin in vitro in repeat 11 between residues Y1176 and G1177. Unless stimulated by Ca++ and calmodulin (CaM), betaII spectrin proteolysis in vitro is much slower. We identify additional unrecognized sites in spectrin targeted by calpain in vitro and in vivo. Bound CaM induces a second II spectrin cleavage at G1230*S1231. BetaII spectrin is cleaved at four sites. One cleavage only occurs in the absence of CaM at high enzyme-to-substrate ratios near the betaII spectrin COOH-terminus. CaM promotes II spectrin cleavages at Q1440*S1441, S1447*Q1448, and L1482*A1483. These sites are also cleaved in the absence of CaM in recombinant II spectrin fusion peptides, indicating that they are probably shielded in the spectrin heterotetramer and become exposed only after CaM binds alphaII spectrin. Using epitope-specific antibodies prepared to the calpain cleavage sites in both alphaII and betaII spectrin, we find in cultured rat cortical neurons that brief glutamate exposure (a physiologic ligand) rapidly stimulates alphaII spectrin cleavage only at Y1176*G1177, while II spectrin remains intact. In cultured SH-SY5Y cells that lack an NMDA receptor, glutamate is without effect. Conversely, when stimulated by calcium influx (via maitotoxin), there is rapid and sequential cleavage of alphaII and then betaII spectrin, coinciding with the onset of nonapoptotic cell death. These results identify (i) novel calpain target sites in both alphaII and betaII spectrin; (ii) trans-regulation of proteolytic susceptibility between the spectrin subunits in vivo; and (iii) the preferential cleavage of alphaII spectrin vs betaII spectrin when responsive cells are stimulated by engagement of the NMDA receptor. We postulate that calpain proteolysis of spectrin can activate two physiologically distinct responses: one that enhances skeletal plasticity without destroying the spectrin-actin skeleton, characterized by preservation of betaII spectrin; or an alternative response closely correlated with nonapoptotic cell death and characterized by proteolysis of betaII spectrin and complete dissolution of the spectrin skeleton.

Calpain regulation of AMPA receptor channels in cortical pyramidal neurons

AMPA receptors (AMPARs) are the principal glutamate receptors mediating fast excitatory synaptic transmission in neurons. Aberrant extracellular glutamate has long been recognized as a hallmark phenomenon during neuronal excitotoxicity. Excessive glutamate triggers massive Ca(2+) influx through NMDA receptors (NMDARs), which in turn can activate Ca(2+)-dependent protease, calpain. In the present study, we found that prolonged NMDA treatment (100 microM, 10 min) caused a sustained and irreversible suppression of AMPAR-mediated currents in cortical pyramidal neurons, which was largely blocked by selective calpain inhibitors. Biochemical and immunocytochemical studies demonstrated that in cortical cultures, prolonged glutamate or NMDA treatment reduced the level of surface and total GluR1, but not GluR2, subunits in a calpain-dependent manner. Consistent with the in vitro data, in animals exposed to transient ischaemic insults, calpain was strongly activated, and the AMPAR current density and GluR1 expression level were substantially reduced. Moreover, calpain inhibitors blocked the ischaemia-induced depression of AMPAR currents, and the NMDAR-induced, calpain-mediated depression of AMPA responses was occluded in ischaemic animals. Taken together, our studies show that overstimulation of NMDARs reduces AMPAR functions in cortical pyramidal neurons through activation of endogenous calpain, and calpain mediates the ischaemia-induced synaptic depression. The down-regulation of AMPARs by calpain provides a negative feedback to dampen neuronal excitability in excitotoxic conditions like ischaemia and epilepsy.

Involvement of calpain in AMPA-induced toxicity to rat cerebellar Purkinje neurons

AMPA receptor-elicited excitotoxicity is manifested as both a type of programmed cell death termed dark cell degeneration and edematous necrosis, both of which are linked to increased intracellular Ca2+ concentration. The appearance of marked cytoskeletal changes in response to abusive AMPA receptor activation, coupled with increased intracellular Ca2+ concentration suggests activation of various destructive enzymes such as calpains, a family of Ca2+-dependent cysteine proteases. Since calpains and AMPA have been linked to both necrotic cell death and programmed cell death, we sought to determine the role of calpains in mediating both types of AMPA-mediated toxicity in Purkinje neurons of the cerebellum. These studies employed immunohistochemistry for cytoskeletal breakdown products of calpain activity coupled with confocal microscopy and pharmacological interventions with calpain and AMPA receptor antagonists. The present study identifies an early involvement of calpains in mediating AMPA-induced dark cell degeneration, but not edematous necrosis, based upon the effectiveness of AMPA to generate calpain-derived alpha-spectrin cleavage products in cerebellar Purkinje neurons that express dark cell degeneration, and the effectiveness of calpain antagonists, PD150606 and MDL28170, to attenuate AMPA-induced dark cell degeneration. Moreover, the AMPA receptor antagonist CNQX, a proven inhibitor of AMPA-elicited dark cell degeneration, also blocked AMPA-induced increases in alpha-spectrin, further suggesting interplay between abusive AMPA receptor activation, calpain activation and dark cell degeneration. Since AMPA-induced dark cell degeneration possesses morphological changes that resemble those that occur following brain ischemia in vivo, hypoglycemia, or extended seizure episodes, the involvement of calpains as mediators of cell death is potentially far reaching and has widespread therapeutic implications in numerous CNS disorders.

Calpain product of WT-CRMP2 reduces the amount of surface NR2B NMDA receptor subunit

The brain is particularly vulnerable to ischaemia; however, neurons can become tolerant to ischaemic insult. This tolerance has been shown to involve activation of NMDA receptors, but its mechanisms have not yet been fully elucidated. Using a preconditioning protocol, we show that neurons surviving to a transient NMDA exposure become resistant to the glutamatergic agonist. Using a proteomic approach, we found that alterations of the protein pattern of NMDA-resistant neurons are restricted mainly to the five collapsin response mediator proteins (CRMPs). A sustained increase in calpain activity following NMDA treatment is responsible for the production of cleaved CRMPs. Finally, we provide evidence for the involvement of the cleaved form of WT-CRMP2 in the down-regulation of NR2B. Our data suggests that, beside their role in neuronal morphogenesis, CRMPs may contribute to neuronal plasticity.

Glutamate-induced apoptosis in neuronal cells is mediated via caspase-dependent and independent mechanisms involving calpain and caspase-3 proteases as well as apoptosis inducing factor (AIF) and this process is inhibited by equine estrogens

BACKGROUND

Glutamate, a major excitatory amino acid neurotransmitter, causes apoptotic neuronal cell death at high concentrations. Our previous studies have shown that depending on the neuronal cell type, glutamate-induced apoptotic cell death was associated with regulation of genes such as Bcl-2, Bax, and/or caspase-3 and mitochondrial cytochrome c. To further delineate the intracellular mechanisms, we have investigated the role of calpain, an important calcium-dependent protease thought to be involved in apoptosis along with mitochondrial apoptosis inducing factor (AIF) and caspase-3 in primary cortical cells and a mouse hippocampal cell line HT22.

RESULTS

Glutamate-induced apoptotic cell death in neuronal cells was associated with characteristic DNA fragmentation, morphological changes, activation of calpain and caspase-3 as well as the upregulation and/or translocation of AIF from mitochondria into cytosol and nuclei. Our results reveal that primary cortical cells and HT22 cells display different patterns of regulation of these genes/proteins. In primary cortical cells, glutamate induces activation of calpain, caspase-3 and translocation of AIF from mitochondria to cytosol and nuclei. In contrast, in HT22 cells, only the activation of calpain and upregulation and translocation of AIF occurred. In both cell types, these processes were inhibited/reversed by 17beta-estradiol and Delta8,17beta-estradiol with the latter being more potent.

CONCLUSION

Depending upon the neuronal cell type, at least two mechanisms are involved in glutamate-induced apoptosis: a caspase-3-dependent pathway and a caspase-independent pathway involving calpain and AIF. Since HT22 cells lack caspase-3, glutamate-induced apoptosis is mediated via the caspase-independent pathway in this cell line. Kinetics of this apoptotic pathway further indicate that calpain rather than caspase-3, plays a critical role in the glutamate-induced apoptosis. Our studies further indicate that glutamate- induced changes of these proteins can be inhibited by estrogens, with Delta8,17beta-estradiol, a novel equine estrogen being more potent than 17beta-estradiol. To our knowledge, this is the first demonstration that glutamate-induced apoptosis involves regulation of multiple apoptotic effectors that can be inhibited by estrogens. Whether these observations can help in the development of novel therapeutic approaches for the prevention of neurodegenerative diseases with estrogens and calpain inhibitors remains to be investigated.

Ester Derivatives of Tournefolic Acid B Attenuate N-Methyl-d-aspartate-Mediated Excitotoxicity in Rat Cortical Neurons

The effects of tournefolic acid B (TAB) and two ester derivatives, TAB methyl ester (TABM) and TAB ethyl ester (TABE), on N-methyl-D-aspartate (NMDA)-mediated excitotoxicity and the underlying mechanisms were investigated. Treatment with 50 microM NMDA elicited neuronal death by 48.7 +/- 5.1%, coinciding with the appearance of injured morphology. TABM (50 microM) attenuated the NMDA-induced cell death by 60.9 +/- 19.7%, and to a lesser extent by TABE. The NMDA-mediated activation of calpain was not affected by TABM and TABE, as determined by the cleavage of alpha-spectrin. NMDA increased the activity of caspases 2, 3, 6, 8, and 9 and reached the maximum after 8-h treatment. TABM and TABE abrogated NMDA-induced activation of caspases 2, 3, 6, and 8 by approximately 80 to 90% and 50 to 60%, respectively, and to a higher extent for caspase 9. TABM and TABE also blocked the NMDA-mediated activation of caspase 12. Furthermore, TABM and TABE eliminated the NMDA-induced accumulation of superoxide anion (O2-*). NMDA evoked significant depolarization of mitochondria, whereas TABM elicited a mild decrease of mitochondrial membrane potential as determined by tetramethylrhodamine methyl ester perchlorate. NMDA treatment induced elevation of Ca2+ levels in cytosol, endoplasmic reticulum (ER), and mitochondria. TABM (50 microM) significantly diminished the NMDA-induced elevation of Ca2+ levels in mitochondria and ER but not cytosol. Therefore, TABM decreased mitochondrial membrane potential and attenuated the NMDA-mediated Ca2+-loading in ER and mitochondria. These events subsequently eliminated the accumulation of O2-* and blocked the activation of caspase cascade, thereby conferring their neuroprotective effects on NMDA-mediated excitotoxicity.

Calpain activation in apoptosis of ventral spinal cord 4.1 (VSC4.1) motoneurons exposed to glutamate: calpain inhibition provides functional neuroprotection

Glutamate toxicity has been implicated in cell death in neurodegenerative diseases and injuries. Glutamate-induced Ca2+ influx may mediate activation of calpain, a Ca2+-dependent cysteine protease, which in turn may degrade key cytoskeletal proteins. We investigated glutamate-mediated apoptosis of VSC4.1 motoneurons and functional neuroprotection by calpain inhibition. Exposure of VSC4.1 cells to 10 microM glutamate for 24 hr caused significant increases in intracellular free [Ca2+], as determined by fura-2 assay. Pretreatment of cells with 10 or 25 microM calpeptin (a cell-permeable calpain-specific inhibitor) for 1 hr prevented glutamate-induced Ca2+ influx. Western blot analyses showed an increase in Bax:Bcl-2 ratio, release of cytochrome c from mitochondria, and calpain and caspase-3 activities during apoptosis. Cell morphology, as evaluated by Wright staining, indicated predominantly apoptotic features following glutamate exposure. ApopTag assay further substantiated apoptotic features morphologically as well as biochemically. Our data showed that calpeptin mainly prevented calpain-mediated proteolysis and apoptosis and maintained whole-cell membrane potential, indicating functional neuroprotection. The results imply that calpeptin may serve as a therapeutic agent for preventing motoneuron degeneration, which occurs in amyotrophic lateral sclerosis and spinal cord injury. In this investigation, we also examined glutamate receptor subtypes involved in the initiation of apoptosis in VSC4.1 cells following exposure to glutamate. Our results indicated that the N-methyl-D-aspartate (NMDA) receptors contributed more than alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptors to glutamate-mediated Ca2+ influx and cell death mechanism. Inhibition of the activities of both NMDA and AMPA receptors protected VSC4.1 cells from glutamate toxicity and preserved whole-cell membrane potential.

Transcription of the NR1 Subunit of the N-Methyl-d-aspartate Receptor Is Down-regulated by Excitotoxic Stimulation and Cerebral Ischemia

The N-methyl-D-aspartate (NMDA) type of glutamate receptor (NMDAR) plays central roles in normal and pathological neuronal functioning. We have examined the regulation of the NR1 subunit of the NMDAR in response to excessive activation of this receptor in in vitro and in vivo models of excitotoxicity. NR1 protein expression in cultured cortical neurons was specifically reduced by stimulation with 100 microM NMDA or glutamate. NMDA decreased NR1 protein amounts by 71% after 8 h. Low NMDA concentrations (< or = 10 microM) had no effect. NR1 down-regulation was inhibited by the general NMDAR antagonist DL-AP5 and also by ifenprodil, which specifically antagonizes NMDARs containing NR2B subunits. Arrest of NMDAR signaling with DL-AP5 after brief exposure to NMDA did not prevent subsequent NR1 decrease. Down-regulation of NR1 did not involve calpain cleavage but resulted from a decrease in de novo synthesis consequence of reduced mRNA amounts. In contrast, NMDA did not alter the expression of NR2A mRNA or newly synthesized protein. In neurons transiently transfected with an NR1 promoter/luciferase reporter construct, promoter activity was reduced by 68% after 2 h of stimulation with NMDA, and its inhibition required extracellular calcium. A similar mechanism of autoregulation of the receptor probably operates during cerebral ischemia, because NR1 mRNA and protein were strongly decreased at early stages of blood reperfusion in the infarcted brains of rats subjected to occlusion of the middle cerebral artery. Because NR1 is the obligatory subunit of NMDARs, this regulatory mechanism will be fundamental to NMDAR functioning.

Calpain Sensitive Regions in the N-terminal Cytoplasmic Domains of Glycine Transporters GlyT1A and GlyT1B

Glycine transporters are members of the Na+/Cl- dependent transporter gene family and play crucial roles in regulating inhibitory as well as excitatory neurotransmission. In this report we show that calcium elevation in spinal cord synaptosomes decreases the levels of glycine transporter, GlyT1, N-terminal immunoreactivity, and that this decrease can be blocked by calpain inhibitor. Sequencing of GST fusion proteins containing the N-terminal domains of GlyT1A and B splice variants cleaved with rat recombinant calpain identified calpain cleavage sites after glycine 17 in GlyT1B and N-terminally of the first conserved arginine residue in both GlyT1A and GlyT1B. Expression in HEK293 cells revealed that truncation of the N-terminus of GlyT1 results in significant inhibition of glycine uptake. A syntaxin1A GST fusion protein was able to pull-down N-terminally deleted GlyT1, indicating that calpain cleavage does not eliminate syntaxin1A binding. These results suggest that calpain cleavage may regulate the transport activity/turnover of GlyT1 in vivo by cleaving its N-terminal domain.

Negative regulation of neurotransmitter release by calpain: a possible involvement of specific SNAP-25 cleavage

Synaptic transmission is conducted by neurotransmitters released from presynaptic nerve terminals by means of Ca2+-dependent exocytosis of synaptic vesicles. Formation of a complex of soluble N-ethylmaleimide-sensitive fusion protein receptor (SNARE) proteins, including vesicle-associated membrane protein-2 (VAMP-2) in the synaptic vesicle membrane, and syntaxin 1 and synaptosomal-associated protein of 25 kDa (SNAP-25) in the plasma membrane, is essential for exocytosis. Ionomycin treatment of cultured rat cerebellar granule cells led to cleavage of SNAP-25, but not syntaxin 1 and VAMP-2, that was dependent on extracellular Ca2+. Cleavage was also induced by N-methyl-D-aspartate (NMDA) treatment, but not by depolarization. The use of various site-specific antibodies to SNAP-25, suggested that the cleavage site was in the N-terminal domain of SNAP-25. Calpain inhibitors abolished the Ca2+-dependent cleavage of SNAP-25 and markedly facilitated Ca2+-dependent glutamate (Glu) release from cerebellar granule cells. These results suggest that calpain may play an important role in the long-lasting regulation of synaptic transmission by suppressing neurotransmitter release, possibly through the proteolytic cleavage of SNAP-25.

Calpain inhibitor inhibits p35-p25-Cdk5 activation, decreases tau hyperphosphorylation, and improves neurological function after spinal cord hemisection in rats

Aberrant calpain activation is a key mediator of neuron death. We examined the cell-permeable calpain inhibitor MDL28170 in the pathophysiological processes after spinal cord injury (SCI) including p35-p25- cyclin-dependent kinase-5 (Cdk5) activation, tau hyperphosphorylation, neuron cell death, calpain I activation, astrogliosis, and microglia activation. Our study showed that intrathecal administration of MDL28170 improved neurologic dysfunction, prevented neuron loss, decreased the number of apoptotic cells, and abated astrogliosis and microglia activation 7 days after spinal cord hemisection in rats. Reverse transcription polymerase chain reaction demonstrated calpain inhibition significantly attenuated the ratio of proapoptotic Bax/anti-apoptotic Bcl-2 mRNA in the lesion and penumbra after SCI. Calpain, the calcium-activated proteolytic enzyme, was found to digest p35 to its truncated product, p25. Moreover, abnormal Cdk5 activation by p25 and subsequent tau hyperphosphorylation triggers pathologic events leading to neurodegeneration and neurofibrillary tangles. We found p35-p25-Cdk5 activation and tau hyperphosphorylation in SCI, and then we showed that intrathecal MDL28170 treatment could diminish p35 truncation, and abrogate aberrant tau phosphorylation. Double labeling of calpain I and phosphorylated tau (AT8) in the same cells of spinal cord lesion further implicated pathogenesis of SCI. In conclusion, MDL28170 abated calpain I activation, inhibited apoptosis and neuron loss, quenched microglia and astrocyte activation, and significantly improved neurologic deficit one week after spinal cord hemisection. The neuroprotective mechanisms of calpain inhibitor in SCI could be attenuating upregulation of Bax/Bcl-2 ratio, preventing p35 truncation in the lesion and penumbra, and abrogating tau hyperphosphorylation.

Selective activation induced cleavage of the NR2B subunit by calpain

Although activation of calcium-activated neutral protease (calpain) by the NMDA receptor has been suggested to play critical roles in synaptic modulation and neurologic disease, the nature of its substrates has not been completely defined. In this study, we examined the ability of calpain to cleave the NMDA receptor in cultured hippocampal neurons. Activation of the NMDA receptor by agonist application led to rapid calpain-specific proteolysis of spectrin and decreased levels of NR2A/2B subunits. Cleavage of the NR2A/2B subunit created a 115 kDa product that retained the ability to bind 125I-MK-801 and is predicted to be active. Increases in levels of this product appeared within 5 min of NMDA receptor activation and were stable for periods of >30 min. Subtype-specific antibodies demonstrated that the NR2B subunit was cleaved in these primary cultures, but the NR2A subunit was not. An inhibitor of calpain blocked both the decrease of intact NR2B and the increase of the low molecular weight form, whereas neither caspase nor cathepsin inhibitors had an effect on these events. Cell surface biotinylation experiments demonstrated that the 115 kDa fragment remained on the cell surface. This NR2B fragment was also found in the rat hippocampus after transient forebrain ischemia, showing that this process also occurs in vivo. This suggests that calpain-mediated cleavage of the NR2B subunit occurs in neurons and gives rise to active NMDA receptor forms present on the cell surface after excitotoxic glutamatergic stimulation. Such forms could contribute to excitotoxicity and synaptic remodeling.

Influence of cytosolic and mitochondrial Ca2+, ATP, mitochondrial membrane potential, and calpain activity on the mechanism of neuron death induced by 3-nitropropionic acid

3-Nitropropionic acid (3NP), an irreversible inhibitor of succinate dehydrogenase, induces both rapid necrotic and slow apoptotic death in rat hippocampal neurons. Low levels of extracellular glutamate (10 microM) shift the 3NP-induced cell death mechanism to necrosis, while NMDA receptor blockade results in predominantly apoptotic death. In this study, we examined the 3NP-induced alterations in free cytosolic and mitochondrial calcium levels, ATP levels, mitochondrial membrane potential, and calpain and caspase activity, under conditions resulting in the activation of apoptotic and necrotic pathways. In the presence of 10 microM glutamate, 3NP administration resulted in a massive elevation in [Ca(2+)](c) and [Ca(2+)](m), decreased ATP, rapid mitochondrial membrane depolarization, and a rapid activation of calpain but not caspase activity. In the presence of the NMDA receptor antagonist MK-801, 3NP did not induce a significant elevation of [Ca(2+)](c) within the 24h time period examined, nor increase [Ca(2+)](m) within 1h. ATP was maintained at control levels during the first hour of treatment, but declined 64% by 16h. Calpain and caspase activity were first evident at 24h following 3NP administration. 3NP treatment alone resulted in a more rapid decline in ATP, more rapid calpain activation (within 8h), and elevated [Ca(2+)](m) as compared to the results obtained with added MK-801. Together, the results demonstrate that 3NP-induced necrotic neuron death is associated with a massive calcium influx through NMDA receptors, resulting in mitochondrial depolarization and calpain activation; while 3NP-induced apoptotic neuron death is not associated with significant elevations in [Ca(2+)](c), nor with early changes in [Ca(2+)](m), mitochondrial membrane potential, ATP levels, or calpain activity.

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.

To die or not to die for neurons in ischemia, traumatic brain injury and epilepsy: a review on the stress-activated signaling pathways and apoptotic pathways

After a severe episode of ischemia, traumatic brain injury (TBI) or epilepsy, it is typical to find necrotic cell death within the injury core. In addition, a substantial number of neurons in regions surrounding the injury core have been observed to die via the programmed cell death (PCD) pathways due to secondary effects derived from the various types of insults. Apart from the cell loss in the injury core, cell death in regions surrounding the injury core may also contribute to significant losses in neurological functions. In fact, it is the injured neurons in these regions around the injury core that treatments are targeting to preserve. In this review, we present our cumulated understanding of stress-activated signaling pathways and apoptotic pathways in the research areas of ischemic injury, TBI and epilepsy and that gathered from concerted research efforts in oncology and other diseases. However, it is obvious that our understanding of these pathways in the context of acute brain injury is at its infancy stage and merits further investigation. Hopefully, this added research effort will provide a more detailed knowledge from which better therapeutic strategies can be developed to treat these acute brain injuries.

Proteolysis of the N-methyl-d-aspartate receptor by calpain in situ

N-Methyl-D-aspartate (NMDA) receptors are calcium-permeable glutamate receptors that play putative roles in learning, memory, and excitotoxicity. NMDA receptor-mediated calcium entry can activate the calcium-dependent protease calpain, leading to substrate degradation. The major NMDA receptor 2 (NR2) subunits of the receptor are in vitro substrates for calpain at selected sites in the C-terminal region. In the present study, we assessed the ability of calpain-mediated proteolysis to modulate the NR1a/2A subtype in a heterologous expression system. Human embryonic kidney (HEK293t) cells, which endogenously express calpain, were cotransfected with NR1a/2A in addition to the calpain inhibitor calpastatin or empty vector as control. Receptor activation by glutamate and glycine as co-agonists led to calpain activation as measured by succinyl-L-leucyl-L-leucyl-L-valyl-L-tyrosyl-aminomethyl coumarin (Suc-LLVY-AMC). Calpain activation also resulted in the degradation of NR2A and decreased binding of (125)I-MK-801 ((125)I-dizocilpine) to NR1a/2A receptors. No stable N-terminal fragment of the NMDA receptor was formed after calpain activation, suggesting calpain regulation of NMDA receptor levels in ways distinct from that previously observed with in vitro cleavage. NR2 subunit constructs lacking the final 420 amino acids were not degraded by calpain. Agonist-stimulated NR1a/2A-transfected cells also had decreased calcium uptake and produced lower changes in agonist-stimulated intracellular calcium compared with cells cotransfected with calpastatin. Calpastatin had no effect on either calcium uptake or intracellular calcium levels when the NR2A subunit lacked the final 420 amino acids. These studies demonstrate that NR2A is a substrate for calpain in situ and that this proteolytic event can modulate NMDA receptor levels.

Calpain activates caspase-3 during UV-induced neuronal death but only calpain is necessary for death

While caspases have been strongly implicated in delayed neuronal death in a variety of experimental paradigms, other proteases such as calpain can also contribute to neuronal death. To evaluate the relative roles of caspase and calpain, we used a model system wherein UV treatment induced moderate or severe delayed cortical neuronal death, as quantified by propidium iodide and calcein AM. UV treatment led to increases in both caspase and calpain activation. Calpain inhibitor III (MDL-28170) reduced caspase activation, suggesting that caspase activation was mediated by calpain. Calpain contributed to neuronal death, as indicated by strong neuroprotection provided by calpain inhibitor III, calpeptin, or Ca2+-free medium. In contrast, caspase inhibitors were not neuroprotective. These results suggest that UV neurotoxicity is mediated by a loss of Ca2+ homeostasis which leads to a calpain-dependent, caspase-independent cell death. That calpain, but not caspase, may mediate death in instances involving the activation of both proteases may have relevance to other neuronal death models.

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.