Global forebrain ischemia induces a posttranslational modification of multifunctional calcium- and calmodulin-dependent kinase II

Calcium/calmodulin-dependent protein kinase II (CaMKII) inhibition induces neurotoxicity via dysregulation of glutamate/calcium signaling and hyperexcitability

Aberrant glutamate and calcium signalings are neurotoxic to specific neuronal populations. Calcium/calmodulin-dependent kinase II (CaMKII), a multifunctional serine/threonine protein kinase in neurons, is believed to regulate neurotransmission and synaptic plasticity in response to calcium signaling produced by neuronal activity. Importantly, several CaMKII substrates control neuronal structure, excitability, and plasticity. Here, we demonstrate that CaMKII inhibition for >4 h using small molecule and peptide inhibitors induces apoptosis in cultured cortical neurons. The neuronal death produced by prolonged CaMKII inhibition is associated with an increase in TUNEL staining and caspase-3 cleavage and is blocked with the translation inhibitor cycloheximide. Thus, this neurotoxicity is consistent with apoptotic mechanisms, a conclusion that is further supported by dysregulated calcium signaling with CaMKII inhibition. CaMKII inhibitory peptides also enhance the number of action potentials generated by a ramp depolarization, suggesting increased neuronal excitability with a loss of CaMKII activity. Extracellular glutamate concentrations are augmented with prolonged inhibition of CaMKII. Enzymatic buffering of extracellular glutamate and antagonism of the NMDA subtype of glutamate receptors prevent the calcium dysregulation and neurotoxicity associated with prolonged CaMKII inhibition. However, in the absence of CaMKII inhibition, elevated glutamate levels do not induce neurotoxicity, suggesting that a combination of CaMKII inhibition and elevated extracellular glutamate levels results in neuronal death. In sum, the loss of CaMKII observed with multiple pathological states in the central nervous system, including epilepsy, brain trauma, and ischemia, likely exacerbates programmed cell death by sensitizing vulnerable neuronal populations to excitotoxic glutamate signaling and inducing an excitotoxic insult itself.

Mechanisms of dendritic spine remodeling in a rat model of traumatic brain injury

Traumatic brain injury (TBI), a leading cause of death and disability in the United States, causes potentially preventable damage in part through the dysregulation of neural calcium levels. Calcium dysregulation could affect the activity of the calcium-sensitive phosphatase calcineurin (CaN), with serious implications for neural function. The present study used both an in vitro enzymatic assay and Western blot analyses to characterize the effects of lateral fluid percussion injury on CaN activity and CaN-dependent signaling in the rat forebrain. TBI resulted in an acute alteration of CaN phosphatase activity and long-lasting alterations of its downstream effector, cofilin, an actin-depolymerizing protein. These changes occurred bilaterally in the neocortex and hippocampus, appeared to persist for hours after injury, and coincided with synapse degeneration, as suggested by a loss of the excitatory post-synaptic protein PSD-95. Interestingly, the effect of TBI on cofilin in some brain regions was blocked by a single bolus of the CaN inhibitor FK506, given 1 h post-TBI. Overall, these findings suggest a loss of synapse stability in both hemispheres of the laterally-injured brain, and offer evidence for region-specific, CaN-dependent mechanisms.

Auditory impairment in infants at risk for bilirubin-induced neurologic dysfunction

Classical and subtypes of kernicterus associated with bilirubin toxicity can be differentiated in part with physiological auditory measures that include auditory-evoked potentials and measures of cochlear integrity. The combination of these auditory measures suggests that bilirubin exposure results in auditory system damage initially at the level of the brainstem, progressing to the level of the VIII cranial nerve and then to greater neural centers. There is no evidence of neural damage at the level of the cochlea. Auditory neural damage from bilirubin toxicity ranges from neural timing deficits, including neural firing delays and dyssynchrony, to neural response reduction and even elimination of auditory neural responses. This condition is comprehensively described as auditory neuropathy spectrum disorder. Independent measures of cochlear function and auditory neural function up to the level of the brainstem can effectively diagnose auditory neural damage resulting from bilirubin neurotoxicity. Intervention, including cochlear implants can be effective.

Calcium/calmodulin-dependent protein kinases as potential targets of nitric oxide

Nitric oxide (NO) synthesis is controlled by Ca(2+)/calmodulin (CaM) binding with and kinase-dependent phosphorylation of constitutive NO synthases, which catalyze the formation of NO and L-citrulline from L-arginine. NO operates as a mediator of important cell signaling pathways, such as cGMP signaling cascade. Another mechanism by which NO exerts biological effects is mediated via post-translational modification of redox-sensitive cysteine thiols of proteins. The Ca(2+)/CaM-dependent protein kinases (CaM kinases) such as CaM kinase I, CaM kinase II, and CaM kinase IV, are a family of protein kinases which requires binding of Ca(2+)/CaM to and subsequent phosphorylation of the enzymes to initiate its activation process. We report other regulation mechanisms of CaM kinases, such as S-glutathionylation of CaM kinase I at Cys(179) and S-nitrosylation of CaM kinase II at Cys(6/30). Such unique post-translational modification of CaMKs by NO shed light on a new area of mutual regulation of NO- and CaM kinases-signals. Based on the novel direct regulation of these kinases, we propose that CaM kinases/NO signaling would be good targets for understanding how they can participate in neuronal physiology and disease.

A cellular mechanism for dendritic spine loss in the pilocarpine model of status epilepticus

PURPOSE

Previous studies have documented a synaptic translocation of calcineurin (CaN) and increased CaN activity following status epilepticus (SE); however, the cellular effect of these changes in CaN in the pathology of SE remains to be elucidated. This study examined a CaN-dependent modification of the dendritic cytoskeleton. CaN has been shown to induce dephosphorylation of cofilin, an actin depolymerization factor. The ensuing actin depolymerization can lead to a number of physiological changes that are of interest in SE.

METHODS

SE was induced by pilocarpine injection, and seizure activity was monitored by video-EEG. Subcellular fractions were isolated by differential centrifugation. CaN activity was assayed using a paranitrophenol phosphate (pNPP) assay protocol. Cofilin phosphorylation was assessed using phosphocofilin-specific antibodies. Cofilin-actin binding was determined by coimmunoprecipitation, and actin polymerization was measured using a triton-solubilization protocol. Spines were visualized using a single-section rapid Golgi impregnation procedure.

RESULTS

The immunoreactivity of phosphocofilin decreased significantly in hippocampal and cortical synaptosomal samples after SE. SE-induced cofilin dephosphorylation could be partially blocked by the preinjection of CaN inhibitors. Cofilin activation could be further demonstrated by increased actin-cofilin binding and a significant depolymerization of neuronal actin, both of which were also blocked by CaN inhibitors. Finally, we demonstrated a CaN-dependent loss of dendritic spines histologically.

DISCUSSION

The data demonstrate a CaN-dependent, cellular mechanism through which prolonged seizure activity results in loss of dendritic spines via cofilin activation. Further research into this area may provide useful insights into the pathology of SE and epileptogenic mechanisms.

Nitric oxide-mediated modulation of calcium/calmodulin-dependent protein kinase II

The mechanisms of NO inhibition of CaMK [Ca2+/CaM (calmodulin)-dependent protein kinase] II activity were studied. In rat pituitary tumour GH3 cells, TRH [thyrotrophin (TSH)-releasing hormone]-stimulated phosphorylation of nNOS [neuronal NOS (NO synthase)] at Ser847 was sensitive to an inhibitor of CaMKs, KN-93, and was enhanced by inhibition of nNOS with 7NI (7-nitroindazole). Enzyme activity of CaMKII following in situ treatment with 7NI was also increased. The in vitro activity of CaMKII was inhibited by co-incubation either with nNOS and L-arginine or with NO donors SNAP (S-nitroso-N-acetyl-DL-penicillamine) and DEA-NONOate [diethylamine-NONOate (diazeniumdiolate)]. Once inhibited by these treatments, CaMKII was observed to undergo full reactivation on the addition of a reducing reagent, DTT (dithiothreitol). In transfected cells expressing CaMKII and nNOS, treatment with the calcium ionophore A23187 further revealed nNOS phosphorylation at Ser847, which was enhanced by 7NI and CaMKII S-nitrosylation. Mutated CaMKII (C6A), in which Cys6 was substituted with an alanine residue, was refractory to 7NI-induced enhancement of nNOS phosphorylation or to CaMKII S-nitrosylation. Furthermore, we could identify Cys6 as a direct target for S-nitrosylation of CaMKII using MS. In addition, treatment with glutamate caused an increase in CaMKII S-nitrosylation in rat hippocampal slices. This glutamate-induced S-nitrosylation was blocked by 7NI. These results suggest that inactivation of CaMKII mediated by S-nitrosylation at Cys6 may contribute to NO-induced neurotoxicity in the brain.

Protein phosphatase-2A is activated in pig brain following cardiac arrest and resuscitation

Protein phosphatase-2A (PP-2A) interacts with several regulators of cell death pathways and is therefore a potential component of signaling pathways linking global cerebral ischemia to cell death. Using a novel procedure to quantify PP-2A activity, we find that cardiac arrest with resuscitation and reperfusion leads to activation of PP-2A by 1.6-fold in pig brain extract and by 3.4-fold after partial purification of PP-2A. This is the first demonstration of PP-2A activation in a clinically relevant model of transient global cerebral ischemia. These results suggest that inhibition of PP-2A activity may be neuroprotective in global cerebral ischemia.

Molecular targets in cerebral ischemia for developing novel therapeutics

Cerebral ischemia (stroke) triggers a complex series of biochemical and molecular mechanisms that impairs the neurologic functions through breakdown of cellular integrity mediated by excitotoxic glutamatergic signalling, ionic imbalance, free-radical reactions, etc. These intricate processes lead to activation of signalling mechanisms involving calcium/calmodulin-dependent kinases (CaMKs) and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK). The distribution of these transducers bring them in contact with appropriate molecular targets leading to altered gene expression, e.g. ERK and JNK mediated early gene induction, responsible for activation of cell survival/damaging mechanisms. Moreover, inflammatory reactions initiated at the neurovascular interface and alterations in the dynamic communication between the endothelial cells, astrocytes and neurons are thought to substantially contribute to the pathogenesis of the disease. The damaging mechanisms may proceed through rapid nonspecific cell lysis (necrosis) or by active form of cell demise (apoptosis or necroptosis), depending upon the severity and duration of the ischemic insult. A systematic understanding of these molecular mechanisms with prospect of modulating the chain of events leading to cellular survival/damage may help to generate the potential strategies for neuroprotection. This review briefly covers the current status on the molecular mechanisms of stroke pathophysiology with an endeavour to identify potential molecular targets such as targeting postsynaptic density-95 (PSD-95)/N-methyl-d-aspartate (NMDA) receptor interaction, certain key proteins involved in oxidative stress, CaMKs and MAPKs (ERK, p38 and JNK) signalling, inflammation (cytokines, adhesion molecules, etc.) and cell death pathways (caspases, Bcl-2 family proteins, poly (ADP-ribose) polymerase-1 (PARP-1), apoptosis-inducing factor (AIF), inhibitors of apoptosis proteins (IAPs), heat shock protein 70 (HSP70), receptor interacting protein (RIP), etc., besides targeting directly the genes itself. However, selecting promising targets from various signalling cascades, for drug discovery and development is very challenging, nevertheless such novel approaches may lead to the emergence of new avenues for therapeutic intervention in cerebral ischemia.

A role for calcium/calmodulin-dependent protein kinase II in cardiac disease and arrhythmia

More than 20 years have passed since the discovery that a collection of specific calcium/calmodulin-dependent phosphorylation events is the result of a single multifunctional kinase. Since that time, we have learned a great deal about this multifunctional and ubiquitous kinase, known today as calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII is interesting not only for its widespread distribution and broad specificity but also for its biophysical properties, most notably its activation by the critical second messenger complex calcium/calmodulin and its autophosphorylating capability. A central role for CaMKII has been identified in regulating a diverse array of fundamental cellular activities. Furthermore, altered CaMKII activity profoundly impacts function in the brain and heart. Recent findings that CaMKII expression in the heart changes during hypertrophy, heart failure, myocardial ischemia, and infarction suggest that CaMKII may be a viable therapeutic target for patients suffering from common forms of heart disease.

Coupling between NMDA Receptor and Acid-Sensing Ion Channel Contributes to Ischemic Neuronal Death

Acid-sensing ion channels (ASICs) composed of ASIC1a subunit exhibit a high Ca(2+) permeability and play important roles in synaptic plasticity and acid-induced cell death. Here, we show that ischemia enhances ASIC currents through the phosphorylation at Ser478 and Ser479 of ASIC1a, leading to exacerbated ischemic cell death. The phosphorylation is catalyzed by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity, as a result of activation of NR2B-containing N-methyl-D-aspartate subtype of glutamate receptors (NMDARs) during ischemia. Furthermore, NR2B-specific antagonist, CaMKII inhibitor, or overexpression of mutated form of ASIC1a with Ser478 or Ser479 replaced by alanine (ASIC1a-S478A, ASIC1a-S479A) in cultured hippocampal neurons prevented ischemia-induced enhancement of ASIC currents, cytoplasmic Ca(2+) elevation, as well as neuronal death. Thus, NMDAR-CaMKII cascade is functionally coupled to ASICs and contributes to acidotoxicity during ischemia. Specific blockade of NMDAR/CaMKII-ASIC coupling may reduce neuronal death after ischemia and other pathological conditions involving excessive glutamate release and acidosis.

Modulation of CaM Kinase II Activity Is Coincident with Induction of Status Epilepticus in the Rat Pilocarpine Model

PURPOSE

This study was conducted to characterize the early cellular changes in CaM kinase II activity that occur during the induction of status epilepticus (SE).

METHODS

The pilocarpine model of SE was characterized both behaviorally and electrographically. At specific time points after the first discrete seizure, specific brain regions were isolated for biochemical study. Phosphate incorporation into a CaM kinase II-specific substrate, autocamtide III, was used to determine kinase activity.

RESULTS

After the development of SE, the data show an immediate inhibition of both cortical and hippocampal CaM kinase II activity in homogenate, but a delayed inhibition in synaptic kinase activity. The maintenance of synaptic kinase activity was due to a translocation of CaM kinase II protein to the synapse. However, despite the translocation of functional kinase, CaM kinase II activity was not maintained, membrane potential was not restored, and the newly translocated CaM kinase II did not terminate the SE event. Unlike the homogenate samples, in the crude synaptoplasmic membrane (SPM) subcellular fractions, a positive correlation is found between the duration of SE and the inhibition of CaM kinase II activity in both the cortex and hippocampus.

CONCLUSIONS

The data support the hypothesis that alterations of CaM kinase II activity are involved in the early events of SE pathology.

Age dependence of pilocarpine-induced status epilepticus and inhibition of CaM kinase II activity in the rat

This study was conducted to characterize the post-pubertal developmental aspects on seizure susceptibility and severity as well as calcium/calmodulin protein kinase type II (CaM kinase II) activity in status epilepticus (SE). Thirty- to ninety-day-old rats, in 10-day increments, were studied. This corresponds to a developmental age group that has not received thorough attention. The pilocarpine model of SE was characterized both behaviorally and electrographically. Seven criteria were analyzed for electrographical characterization: seizure severity, SE susceptibility, the average number of discrete seizures, average time until first seizure, average time to SE, average time from first discrete seizure to SE, and death. After 1 h of SE, specific brain regions were isolated for biochemical study. Phosphate incorporation into a CaM kinase II-specific substrate, autocamtide III, was used to determine kinase activity. There was no developmental effect on the average number of discrete seizures, average time until first seizure, average time to SE, average time from first discrete seizure to SE, and death; however, there was a significant effect on SE probability and seizure severity. Once SE was expressed, all animals showed a decrease in both cortical and hippocampal CaM kinase II activities. Conversely, seizure activity in the absence of SE did not result in a decrease in CaM kinase II activity. The data suggest that there is a gradual age-dependent modulation of SE susceptibility and seizure severity within the developmental stages studied. Additionally, once status epilepticus is observed at any age, there is a corresponding SE-induced inhibition of CaM kinase II.

Calcium/calmodulin-dependent kinase II phosphorylation of the GABAA receptor alpha1 subunit modulates benzodiazepine binding

gamma-Aminobutyric acid (GABA) is the primary neurotransmitter that is responsible for the fast inhibitory synaptic transmission in the central nervous system. A major post-translational mechanism that can rapidly regulate GABAAR function is receptor phosphorylation. This study was designed to test the effect of endogenous calcium and calmodulin-dependent kinase II (CaM kinase II) activation on both allosteric modulator binding and GABAA receptor subunit phosphorylation. Endogenous CaM kinase II activity was stimulated, and GABAA receptors were subsequently analyzed for bothallosteric modulator binding properties and immunoprecipitated and analyzed for subunit phosphorylation levels. A significant increase in allosteric-modulator binding of the GABAAR was observed under conditions maximal for CaM kinase II activation. In addition, CaM kinase II activation resulted in a direct increase in phosphorylation of the GABAA receptor alpha1 subunit. The data suggest that the CaM kinase II-dependent phosphorylation of the GABAA receptor alpha1 subunit modulated allosteric modulator binding to the GABAA receptor.

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.

Calcium/calmodulin-dependent protein kinase II inhibitor protein: localization of isoforms in rat brain

A second isoform of Ca2+/calmodulin-dependent-kinase II inhibitor protein (CaM-KIIN) has been identified using the yeast two-hybrid screen. The 1.8kb message encodes a 78 residue CaM-KIINalpha that is 65% identical in its putative open-reading frame and 95% identical in its inhibitory domain to the previously characterized CaM-KIINbeta. CaM-KIINalpha exhibits inhibitory properties towards recombinant mouse CaM-kinase IIalpha indistinguishable from CaM-KIINbeta. The 27 amino acid inhibitory peptide (CaM-KIINtide) derived from CaM-KIIN has the ability to inhibit brain CaM-kinase II activity from multiple organisms including rat, Drosophila and goldfish. Northern analysis of various rat tissues indicates that CaM-KIINalpha is specific to brain whereas CaM-KIINbeta message is also present in testis. In situ hybridization shows a general distribution of both isoforms in rat brain with stronger localization of CaM-KIINbeta in cerebellum and hindbrain and CaM-KIINalpha in frontal cortex, hippocampus and inferior colliculus. An antibody that recognizes both isoforms shows a distribution of CaM-KIIN in rat brain that correlates with immunoreactivity of CaM-kinase II. In cultured mature hippocampal neurons, CaM-KIIN is present in cell bodies and dendrites but, unlike CaM-kinase II, does not display punctate staining at synapses. These results suggest a localized function for CaM-KIIN in inhibiting specialized pools of CaM-kinase II.

Chronic inhibition of Ca(2+)/calmodulin kinase II activity in the pilocarpine model of epilepsy

The development of symptomatic epilepsy is a model of long-term plasticity changes in the central nervous system. The rat pilocarpine model of epilepsy was utilized to study persistent alterations in calcium/calmodulin-dependent kinase II (CaM kinase II) activity associated with epileptogenesis. CaM kinase II-dependent substrate phosphorylation and autophosphorylation were significantly inhibited for up to 6 weeks following epileptogenesis in both the cortex and hippocampus, but not in the cerebellum. The net decrease in CaM kinase II autophosphorylation and substrate phosphorylation was shown to be due to decreased kinase activity and not due to increased phosphatase activity. The inhibition in CaM kinase II activity and the development of epilepsy were blocked by pretreating seizure rats with MK-801 indicating that the long-lasting decrease in CaM kinase II activity was dependent on N-methyl-D-aspartate receptor activation. In addition, the inhibition of CaM kinase II activity was associated in time and regional localization with the development of spontaneous recurrent seizure activity. The decrease in enzyme activity was not attributed to a decrease in the alpha or beta kinase subunit protein expression level. Thus, the significant inhibition of the enzyme occurred without changes in kinase protein expression, suggesting a long-lasting, post-translational modification of the enzyme. This is the first published report of a persistent, post-translational alteration of CaM kinase II activity in a model of epilepsy characterized by spontaneous recurrent seizure activity.

The role of complement anaphylatoxin C5a in neurodegeneration: implications in Alzheimer's disease

There is evidence that the complement system, a major component of inflammatory responses, may play an important role in neurodegenerative conditions such as Alzheimer's disease (AD). Work from our lab demonstrated that mice genetically deficient in the complement component C5 are more susceptible to hippocampal excitotoxic lesions (Pasinetti et al., 1996) and that the C5-derived ana;hylatoxin C5a may protect against excitotoxicity in vitro and in vivo (Osaka et al., 1999). Potential mechanisms identified in C5a-mediated neuroprotection include activation of mitogen activated protein (MAP)-kinase (Osaka et al., 1998; Osaka et al., 1999). This novel neuroprotective role of C5a complicates current theories that complement proteins augment beta-amyloid (Abeta) toxicity in AD. In view of the fact that the complement system represents a target for therapeutic interventions in AD, further characterization of the complex role of complement proteins is essential. Towards this aim, we have characterized a transgenic C5a receptor (C5aR) knockout (KO) mouse. Recent studies in the lab using C5aR-KO mice show that disruption of C5aR alters calcium calmodulin kinase (CaM-KII) signal transduction in brain cells. We are presently using C5aR-KO mice to study the role of C5a in caspase mediated apoptotic neuronal death. In this review we will attempt to delineate possible neuroprotective roles for C5a in mechanisms of neurotoxicity pertaining to AD.

Subcellular distribution and autophosphorylation of calcium/calmodulin-dependent protein kinase II-alpha in rat hippocampus in a model of ischemic tolerance

A brief period of sublethal ischemia induces resistance to a subsequent, otherwise lethal, ischemic insult, a process named ischemic tolerance or preconditioning. A persistently disturbed cell signaling during reperfusion after cerebral ischemia has been proposed to contribute to ischemic cell death. Here, we report on the effect of ischemic preconditioning on the levels of the regulatory alpha-subunit of calcium/calmodulin protein kinase II and its phosphorylation in the hippocampal CA1 region. We found that during and following lethal cerebral ischemia, calcium/calmodulin protein kinase II-alpha is persistently translocated to cell membranes, where it becomes phosphorylated at threonine 286. In contrast, in the preconditioned brains the translocation and phosphorylation are transient and return to preischemic values after one day of reperfusion. At this time of reperfusion, the total level of calcium/calmodulin protein kinase II-alpha is significantly lower in preconditioned animals compared to the sham and non-conditioned animals. After one day of reperfusion, the level of calcium/calmodulin protein kinase II-alpha messenger RNA decreases in the non-conditioned brains, whereas it is unchanged in preconditioned brains. We conclude that, during and after ischemia, calcium/calmodulin protein kinase II-alpha is translocated to cell membranes and becomes phosphorylated at threonine 286. This could detrimentally influence cell survival by changing receptor function and ion channel conductance. Ischemic preconditioning prevents the persistent presence of calcium/calmodulin protein kinase II-alpha at cell membranes, presumably by enhancing its degradation, which could be part of a neuroprotective mechanism of ischemic tolerance.

Status epilepticus results in an N-methyl-D-aspartate receptor-dependent inhibition of Ca2+/calmodulin-dependent kinase II activity in the rat

Status epilepticus is a major medical emergency that results in significant alteration of neuronal function. Status epilepticus involves seizure activity recurring frequently enough to induce a sustained alteration in brain function. This study was initiated to investigate how status epilepticus affects the activity of calcium and calmodulin-dependent kinase II in the brain. Calcium and calmodulin-dependent kinase II is a neuronally enriched signal transducing system involved in the regulation of neurotransmitter synthesis and release, cytoskeletal function, gene transcription, neurotransmitter receptor function and neuronal excitability. Therefore, alteration of this signal transduction system would have significant physiological effects. Status epilepticus was induced in rats by pilocarpine injection, allowed to progress for 60 min and terminated by repeated diazepam injections. Animals were killed at specific time-points and examined for calcium and calmodulin-dependent kinase II activity. Calcium and calmodulin-dependent kinase II activity was significantly reduced in cerebral cortex and hippocampal homogenates obtained from status epilepticus rats when compared with control animals. Once established, the status epilepticus-induced inhibition of calcium and calmodulin-dependent kinase II activity was observed at all time-points tested following the termination of seizure activity. However, calcium and calmodulin-dependent kinase II activity was not significantly decreased in thalamus and cerebellar homogenates. In addition, status epilepticus-induced inhibition of calcium and calmodulin-dependent kinase II activity was dependent upon activation of N-methyl-D-aspartate subtype of glutamatergic receptors. Thus, status epilepticus induced a significant inhibition of calcium and calmodulin-dependent kinase II activity that involves N-methyl-D-aspartate receptor activation. The data support the hypothesis that inhibition of calcium and calmodulin-dependent kinase II activity may be involved in the alteration of neuronal function following status epilepticus.

Cumulative effects of glutamate microstimulation on Ca(2+) responses of CA1 hippocampal pyramidal neurons in slice

Glutamate stimulation of hippocampal CA1 neurons in slice was delivered via iontophoresis from a microelectrode. Five pulses (approximately 5 muA, 10 s duration, repeated at 1 min intervals) were applied with the electrode tip positioned in the stratum radiatum near the dendrites of a neuron filled with the Ca(2+) indicator fura-2. A single stimulus set produced Ca(2+) elevations that ranged from several hundred nM to several microM and that, in all but a few neurons, recovered within 1 min of stimulus termination. Subsequent identical stimulation produced Ca(2+) elevations that outlasted the local glutamate elevations by several minutes as judged by response recoveries in neighboring cells or in other parts of the same neuron. These long responses ultimately recovered but persisted for up to 10 min and were most prominent in the mid and distal dendrites. Recovery was not observed for responses that spread to the soma. The elevated Ca(2+) levels were accompanied by membrane depolarization but did not appear to depend on the depolarization. High-resolution images demonstrated responsive areas that involved only a few mu(m) of dendrite. Our results confirm the previous general findings from isolated and cell culture neurons that glutamate stimulation, if carried beyond a certain range, results in long-lasting Ca(2+) elevation. The response characterized here in mature in situ neurons was significantly different in terms of time course and reversibility. We suggest that the extended Ca(2+) elevations might serve not only as a trigger for delayed neuron death but, where more spatially restricted, as a signal for local remodeling in dendrites.

Long-lasting decrease in neuronal Ca2+/calmodulin-dependent protein kinase II activity in a hippocampal neuronal culture model of spontaneous recurrent seizures

Ca2+/calmodulin-dependent protein kinase II (CaM Kinase II) activity was evaluated in a well-characterized in vitro model of epileptiform activity. Long-lasting spontaneous recurrent seizure (SRS) activity was induced in hippocampal neuronal cultures by exposure to low Mg2+ media for 3 h. Analysis of endogenous Ca2+/calmodulin-dependent phosphorylation revealed a significant long-lasting decrease in 32P incorporation into the alpha (50 kDa) and beta (60 kDa) subunits of CaM kinase II in association with the induction of SRS activity in this preparation. Ca2+/calmodulin-dependent substrate phosphorylation of the synthetic peptides, Autocamtide-2 and Syntide II, was also significantly reduced following the induction of SRSs and persisted for the life of the neurons in culture. The decrement in CaM kinase II activity associated with low Mg2+ treatment remained significantly decreased when values were corrected for changes in levels of alpha subunit immunoreactivity and neuronal cell loss. Addition of the protein phosphatase inhibitors, okadaic acid and cyclosporin A, to the phosphorylation reaction did not block the SRS-associated decrease in substrate phosphorylation, indicating that enhanced phosphatase activity was not a contributing factor to the observed decrease in phosphate incorporation. The findings of this study demonstrate that CaM kinase II activity is decreased in association with epileptogenesis observed in these hippocampal cultures and may contribute to the production and maintenance of SRSs in this model.

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.

Global ischemia-induced inhibition of the coupling ratio of calcium uptake and ATP hydrolysis by rat whole brain microsomal Mg(2+)/Ca(2+) ATPase

Ischemia is associated with a loss of cytosolic calcium homeostasis. Intracellular stores, particularly in endoplasmic reticulum, are critical for the maintenance of calcium homeostasis. Recent studies have shown that ischemia significantly inhibited microsomal calcium uptake mediated by Mg(2+)/Ca(2+) ATPase, the major mechanism of endoplasmic reticulum calcium sequestration. This study was initiated to determine whether the decreased calcium uptake caused by ischemia was the result of inhibition of Mg(2+)/Ca(2+) ATPase activity or an uncoupling of calcium uptake from ATP hydrolysis. The microsomal Mg(2+)/Ca(2+) ATPase specific inhibitor thapsigargin partially inhibited ATPase activity and completely inhibited calcium uptake. ATPase inhibited by thapsigargin was considered microsomal Mg(2+)/Ca(2+) ATPase. Ischemia from 5 to 60 min had no significant effect on thapsigargin sensitive ATPase activity. However, under identical conditions, increasing ischemia from 5 to 60 min significantly inhibited microsomal calcium uptake. Comparing calcium uptake to ATP hydrolysis as ischemia increased from 5 to 60 min revealed that the coupling ratio of calcium molecules sequestered to ATP molecules hydrolyzed became significantly decreased. The results demonstrated that the effect of ischemia on microsomal calcium uptake was mediated by an uncoupling of calcium transport from Mg(2+)/Ca(2+) ATPase activity.

Potential role of calcineurin for brain ischemia and traumatic injury

Calcineurin belongs to the family of Ca2+/calmodulin-dependent protein phosphatase, protein phosphatase 2B. Calcineurin is the only protein phosphatase which is regulated by a second messenger, Ca2+. Furthermore, calcineurin is highly localized in the central nervous system, especially in those neurons vulnerable to ischemic and traumatic insults. For these reasons, calcineurin is considered to play important roles in neuron-specific functions. Recently, on the basis of the finding that FK506 and cyclosporin A serve as calcineurin-specific inhibitors, this enzyme has become the subject of much study. It is clear that calcineurin is involved in many neuronal (or non-neuronal) functions such as neurotransmitter release, regulation of receptor functions, signal transduction systems, neurite outgrowth, gene expression and neuronal cell death. In this review, we describe the calcineurin functions, functions of the substrates, and the pathogenesis of traumatic and ischemic insults, and we discuss the potential role of calcineurin. There are many similarities in traumatic and ischemic pathogenesis of the brain in which the release of excessive glutamate is followed by an intracellular Ca2+ increase. However, the intracellular cascade which leads to neuronal cell death after the release of excess Ca2+ is unclear. Although calcineurin is thought to be a key toxic enzyme on the basis of studies using immunosuppressants (FK506 or cyclosporin A), many of the functions of the substrates for calcineurin protect against neuronal cell death. We concluded that calcineurin is a bi-directional enzyme for neuronal cell death, having protective and toxic actions, and the balance of the bi-directional effects may be important in ischemic and traumatic pathogenesis.

In vivo glutamate neurotoxicity is associated with reductions in calcium/calmodulin-dependent protein kinase II immunoreactivity

Calcium/calmodulin-dependent protein kinase II (CaM kinase) activity is inhibited in cultured hippocampal cells following direct application of glutamate. The goal of the present study was to determine if hippocampal regions that undergo delayed cell death following glutamate microinfusion would exhibit changes in CaM kinase immunoreactivity. Gerbils received bilateral intra-hippocampal infusions of L-glutamate (34 microg/microl), or control treatments of D-glutamate or saline. Animals were sacrificed at 12 or 24 hr to assess cell loss and determine changes in CaM kinase-like immunoreactivity. Hippocampi of gerbils euthanized 12 hr following L-glutamate, or 24 hr following D-glutamate, did not exhibit cell death in the hippocampal CA1 region. Animals injected with L-glutamate and sacrificed 24 hr after infusion had extensive cell damage that was restricted to the hippocampal CA1 region. CaM kinase-like immunoreactivity was absent in the hippocampal CA1 region of all L-glutamate treated animals sacrificed at 12 hr. In these same sections, CaM kinase immunoreactivity was evident in the subiculum, CA2 and CA3 regions. Reduction in CaM kinase immunoreactivity following L-glutamate were also observed using Western analysis. The results confirm and extend the findings of earlier cell culture studies by demonstrating a reduction in CaM kinase immunoreactivity that occurred prior to cell death.

CaMKII-dependent phosphorylation of NR2A and NR2B is decreased in animals characterized by hippocampal damage and impaired LTP

The calcium-calmodulin-dependent protein kinase II (CaMKII) subserves activity-dependent plasticity in central neurons. To examine in vivo the implication of CaMKII activity in synaptic plasticity, we used an animal model characterized by developmentally induced targeted neuronal ablation within the cortex and the hippocampus, and showing, at presynaptic level, molecular alterations leading to facilitation of glutamate release in hippocampal synapses (methylazoxymethanol-treated rats, MAM-rats). We report here that at the postsynaptic side, the activity of CaMKII is markedly decreased in MAM-rats when compared to controls, although the concentration of the enzyme in Post Synaptic Density (PSD) is not altered. This effect is confined to PSD-associated CaMKII, as enzyme activity tested in the soluble fraction is unchanged in MAM-rats. In addition, the decreased activity is not due to inhibition by autophosphorylation in specific sites within the calmodulin-binding domain, as preincubation with purified phosphatases 1 and 2A failed to restore CaMKII activity in PSD of MAM-rats. The CaMKII-dependent phosphorylation of NR2A/B subunits of NMDA receptor is lower in MAM-rats when compared to controls (51.77 +/- 7.39% of controls level), as revealed in back-phosphorylation experiments. In addition, a treatment able to restore long-term potentiation (LTP) in hippocampal slices from MAM-rats, e.g. exposure to D-serine, is able to restore CaMKII activity to the control value.

Modulation of GABAergic receptor binding by activation of calcium and calmodulin-dependent kinase II membrane phosphorylation

gamma-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Because of the important role that GABA plays in the CNS, alteration of GABAA receptor function would significantly affect neuronal excitability. Protein phosphorylation is a major mechanism for regulating receptor function in the brain and has been implicated in modulating GABAA receptor function. Therefore, this study was initiated to determine the role of calmodulin-dependent kinase II (CaM kinase II) membrane phosphorylation on GABAA receptor binding. Synaptosomal membrane fractions were tested for CaM kinase II activity towards endogenous substrates. In addition, muscimol binding was evaluated under equilibrium conditions in synaptosomal membrane fractions subjected to either basal (Mg2+ alone) or maximal CaM kinase II-dependent phosphorylation. Activation of endogenous CaM kinase II-dependent phosphorylation resulted in a significant enhancement of the apparent Bmax for muscimol binding without significantly altering the apparent binding affinity. The enhanced muscimol binding could be increased further by the addition of exogenous CaM kinase II to synaptosomal membrane fractions. Co-incubation with inhibitors of kinase activity during the phosphorylation reactions blocked the CaM kinase II-dependent increase in muscimol binding. The data support the hypothesis that activation of CaM kinase II-dependent phosphorylation caused an increased GABAA receptor binding and may play an important role in modulating the function of this inhibitory receptor/chloride ion channel complex.

Modifications in brain cAMP- and calcium/calmodulin-dependent protein kinases induced by treatment with S-adenosylmethionine

Several lines of evidence suggest that the mechanism of action of antidepressant drugs (AD) involves adaptive changes occurring in intraneuronal post-receptor signal transduction cascades. Protein phosphorylation has a key role in signal transduction and was previously found to be a target in the action of AD (5-HT and/or NA reuptake blockers). Several studies showed that cAMP- and type II Ca2+/calmodulin-dependent protein kinases (PKA and CaMKII) are markedly affected by typical AD in two different and complementary cellular districts, respectively microtubules (a somatodendritic compartment) and synaptic vesicles (a presynaptic terminal compartment). In order to investigate whether the effect on protein kinases may be involved in the therapeutic action of drugs it is interesting to compare the effect of atypical AD with that of typical drugs. In this study the effect of the atypical AD S-adenosylmethionine (SAMe) was tested. Repeated (12 days) SAMe treatment induced in cerebrocortical microtubules an increase in the binding of cAMP to the RII PKA regulatory subunit and an increase in the endogenous phosphorylation of microtubule-associated protein 2, an effect resembling that of typical AD. In synaptic terminals the treatment induced an increase in the activity of CaMKII and in the endogenous phosphorylation of vesicular substrates. However, this modification was found in the cerebral cortex rather than in the hippocampus, where typical AD affect CaMKII. In addition the synapsin I level was decreased in the hippocampus and increased in the cerebral cortex, an effect not detected with typical AD.

Metabolic inhibition potentiates AMPA-induced Ca2+ fluxes and neurotoxicity in rat cerebellar granule cells

The effects of partial metabolic inhibition (induced by 2 h exposure to low concentrations of cyanide (NaCN)) on the glutamate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-induced excitotoxicity and elevation of free cytoplasmic Ca2+ levels ([Ca2+]i) were studied in glucose-deprived primary cultures of cerebellar granule cells. Co-application of AMPA plus NaCN caused a marked increase of cell death, with morphological features of both necrotic and apoptotic cell death as estimated by the capacity of cultured cerebellar granule cells to metabolize 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide into formazan (MTT method), and by measuring the amount of DNA fragmentation in neurons using an ELISA test for histone-bound DNA fragments, respectively. Cell morphology was assessed by confocal microscopy of propidium iodide-stained cultures. No toxic effects were observed when AMPA or a low concentration of NaCN (0.1-0.3 mM; in the presence of NMDA receptor antagonist MK-801; 10 microM) were applied alone. The neurotoxic actions induced by AMPA plus NaCN were preceded and accompanied by a significant elevation of [Ca2+]i, as well as by depletion of neuronal ATP stores. The marked enhancement in the functional responsiveness of AMPA receptors in energetically compromised neurons suggests that at least under certain conditions AMPA receptors may play an important role in excitotoxic processes which might be of relevance for the slowly developing neuronal death seen in several neurodegenerative diseases.

Postischemic hypothermia. A critical appraisal with implications for clinical treatment

The use of hypothermia to mitigate cerebral ischemic injury is not new. From early studies, it has been clear that cooling is remarkably neuroprotective when applied during global or focal ischemia. In contrast, the value of postischemic cooling is typically viewed with skepticism because of early clinical difficulties and conflicting animal data. However, more recent rodent experiments have shown that a protracted reduction in temperature of only a few degrees Celsius can provide sustained behavioral and histological neuroprotection. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute clinical stroke. A thorough mechanistic understanding of postischemic hypothermia would lead to a more selective and effective therapy. Unfortunately, few studies have investigated the mechanisms by which postischemic cooling conveys its beneficial effect. The purpose of this article is to evaluate critically the effects of postischemic temperature changes with a comparison to some current drug therapies. This article will stimulate new research into the mechanisms of lengthy postischemic hypothermia and its potential as a therapy for stroke patients.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Ca2+/calmodulin-dependent protein kinase II in postsynaptic densities after reversible cerebral ischemia in rats

Compartmentalization of protein kinases and association of the enzyme with strategic cellular substrates may be important for regulating signal transduction in neurons. Cerebral ischemia produced by transient 20 min occlusion of common carotid and vertebral arteries in rats caused a dramatic (3-fold) increase in Ca2+/Calmodulin-dependent protein kinase II (CaM-KII) in the fraction enriched in postsynaptic density (PSDf), the compartment of the neuron that is involved in signal transduction. This change in compartmentalization was not reversible for up to 24 h after termination of the occlusion and was followed by reduction of CaM-KII to 50% of control content one week after the insult. The observed changes in CaM-KII content did not represent general protein redistribution in PSDf after ischemia since there were no parallel changes in PSDf actin concentration. The redistribution of CaM-KII coincided with gradual (up to 80%) reduction of its activity in PSDf as tested using specific peptide substrate and endogenous CaM-KII substrates. This work provides evidence that ischemia disturbs CaM-KII distribution and activity in PSDf and this may lead to long lasting disruption of signal transduction at the synaptic level.

Transient cerebral ischemia decreases calcium/calmodulin-dependent protein kinase II immunoreactivity, but not mRNA levels in the gerbil hippocampus

During transient cerebral ischemia, intracellular calcium increases initiating a cascade of events which leads to the delayed death of neurons located in the hippocampus. Coupled to this calcium disturbance is the rapid decrease of calcium/calmodulin kinase II (CaM kinase) activity, a protein kinase critical to neuronal functioning. The present study correlated the increased locomotor activity following ischemic insult with alterations in CaM kinase mRNA levels and immunocytochemical labeling of alpha and beta CaM kinase subunits in the hippocampus. The protective effect of hypothermia was also compared with CaM kinase mRNA levels and immunoreactivity. Levels of CaM kinase message for either alpha or beta subunits was not altered in ischemic gerbils compared to sham or hypothermic ischemic conditions. Immunoreactivity for both the alpha and beta subunits was markedly reduced in the vulnerable CA1 region of ischemic animals compared to sham controls. Gerbils that underwent the ischemic insult while hypothermic showed no decrement in staining. CaM kinase-like immunoreactivity in the ischemia-resistant CA3 sector was not altered following ischemia. These data suggest that the loss of hippocampal CaM kinase immunoreactivity observed at 24 h following ischemia is not associated with a reduction in CaM kinase mRNA levels and support the notion that the rapid decline in CaM kinase activity following ischemic insult is a result of a posttranslational modification and/or translocation of the enzyme.

Alterations of Ca2+/calmodulin-dependent protein kinase II and its messenger RNA in the rat hippocampus following normo- and hypothermic ischemia

The change in the subcellular distribution of Ca2+/calmodulin-dependent protein kinase II was studied in the rat hippocampus following normothermic and hypothermic transient cerebral ischemia of 15 min duration. A decrease in immunostaining of Ca2+/calmodulin-dependent protein kinase II was observed at 1 h of reperfusion which persisted until cell death in the CA1 region. In the CA3 and dentate gyrus areas immunostaining recovered at one to three days of reperfusion. The CA2+/calmodulin-dependent protein kinase II was translocated to synaptic junctions during ischemia and reperfusion which could be due to a persistent change in the intracellular calcium ion homeostasis. The expression of the messenger RNA of the alpha-subunit of Ca2+/calmodulin-dependent protein kinase II decreased in the entire hippocampus during reperfusion, and was most marked in the dentate gyrus at 12 h of reperfusion. This decrease could be a feedback downregulation of the mRNA due to increased Ca2+/calmodulin-dependent protein kinase II activation. Intraischemic hypothermia protected against ischemic neuronal damage and attenuated the ischemia-induced decrease of Ca2+/calmodulin-dependent protein kinase II immunostaining in all hippocampal regions. Hypothermia also reduced the translocation of Ca2+/calmodulin-dependent protein kinase II and restored Ca2+/calmodulin-dependent protein kinase II alpha messenger RNA after ischemia. The data suggest that ischemia leads to an aberrant Ca2+/calmodulin-dependent protein kinase II mediated signal transduction in the CA1 region, which is important for the development of delayed neuronal damage. Hypothermia enhances the restoration of the Ca2+/calmodulin-dependent protein kinase II mediated cell signalling.

Inability to restore resting intracellular calcium levels as an early indicator of delayed neuronal cell death

The hippocampus is especially vulnerable to excitotoxicity and delayed neuronal cell death. Chronic elevations in free intracellular calcium concentration ([Ca2+]i) following glutamate-induced excitotoxicity have been implicated in contributing to delayed neuronal cell death. However, no direct correlation between delayed cell death and prolonged increases in [Ca2+]i has been determined in mature hippocampal neurons in culture. This investigation was initiated to determine the statistical relationship between delayed neuronal cell death and prolonged increases in [Ca2+]i in mature hippocampal neurons in culture. Using indo-1 confocal fluorescence microscopy, we observed that glutamate induced a rapid increase in [Ca2+]i that persisted after the removal of glutamate. Following excitotoxic glutamate exposure, neurons exhibited prolonged increases in [Ca2+]i, and significant delayed neuronal cell death was observed. The N-methyl-D-aspartate (NMDA) channel antagonist MK-801 blocked the prolonged increases in [Ca2+]i and cell death. Depolarization of neurons with potassium chloride (KCl) resulted in increases in [Ca2+]i, but these increases were buffered immediately upon removal of the KCl, and no cell death occurred. Linear regression analysis revealed a strong correlation (R = 0.973) between glutamate-induced prolonged increases in [Ca2+]i and delayed cell death. These data suggest that excitotoxic glutamate exposure results in an NMDA-induced inability to restore resting [Ca2+]i (IRRC) that is a statistically significant indicator of delayed neuronal cell death.

N-methyl-D-aspartate and nitric oxide regulate the expression of calcium/calmodulin-dependent kinase II in the hippocampal dentate gyrus

Injection of small volumes of N-methyl-D-aspartate (NMDA) or Sin-1 molsidomine (a nitric oxide releasing agent) onto the dendrites of granule cells in the hippocampal dentate gyrus leads to changes in the level of expression of a number of genes. There is a fall in prodynorphin mRNA levels with a corresponding increase in proenkephalin mRNA levels. Similar changes in opioid gene expression occur following the induction of long-term potentiation (LTP). We report here that at short time periods (1-6 h) after injections of NMDA or sin-1 molsidomine, there is an increase in the levels of the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II (CaMKII alpha), consistent with a report of elevated CaMKII alpha mRNA in postsynaptic neurons in the CA1 region of the hippocampus following LTP induction [54]. However, we also report that 24 h after injection of NMDA or sin-1, there is a dramatic decrease in CaMKII alpha mRNA levels in the vicinity of the injection. This effect is specific for CaMKII alpha mRNA, in that many other mRNA species are not affected, and occurs in the dendritic population of CaMKII alpha mRNA as well as in the pool of mRNA in the granule cell bodies. The effect is blocked by an inhibitor of cGMP-dependent protein kinase. The biphasic regulation of CaMKII alpha mRNA may be of considerable functional importance for the long-term response of granule cells to local stimulation of NMDA receptors or NO release.

Effect of cerebral ischemia on calcium/calmodulin-dependent protein kinase II activity and phosphorylation

The effects of cerebral ischemia on calcium/calmodulin-dependent kinase II (CaM kinase II) were investigated using the rat four-vessel occlusion model. In agreement with previous results using rat or gerbil models of cerebral ischemia or a rabbit model of spinal cord ischemia, this report demonstrates that transient forebrain ischemia leads to a reduction in CaM kinase II activity within 5 min of occlusion onset. Loss of activity from the cytosol fractions of homogenates from the neocortex, striatum, and hippocampus correlated with a decrease in the amount of CaM kinase alpha and beta isoforms detected by immunoblotting. In contrast, there was an apparent increase in the amount of CaM kinase alpha and beta in the particulate fractions. The decrease in the amount of CaM kinase isoforms from the cytosol but not the particulate fractions was confirmed by autophosphorylation of CaM kinase II after denaturation and renaturation in situ of the blotted proteins. These results indicate that ischemia causes a rapid inhibition of CaM kinase II activity and a change in the partitioning of the enzyme between the cytosol and particulate fractions. CaM kinase II is a multifunctional protein kinase, and the loss of activity may play a critical role in initiating the changes leading to ischemia-induced cell death. To identify a structural basis for the decrease in enzyme activity, tryptic peptide maps of CaM kinase II phosphorylated in vitro were compared. Phosphopeptide maps of CaM kinase alpha from particulate fractions of control and ischemic samples revealed not only reduced incorporation of phosphate into the protein but also the absence of a limited number of peptides in the ischemic samples. This suggested that certain sites are inaccessible, possibly due to a conformational change, a covalent modification of CaM kinase II, or steric hindrance by an associated molecule. Verifying one of these possibilities should help to elucidate the mechanism of ischemia-induced modulation of CaM kinase II.

Specific distribution of Ca2+/calmodulin-dependent protein kinase II alpha and beta isoforms in some structures of the rat forebrain

The immunohistochemical distribution of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) alpha and beta isoforms in the rat forebrain was examined by using monoclonal antibodies specific to each isoform. The present study confirmed that alpha and beta immunoreactivities are localized only in neuronal elements. At the light microscopic level, specific distribution patterns of these isoforms and staining characteristics were recognized in some regions of the forebrain as follows. Firstly, alpha-immunoreactive neurons were more homogeneously distributed throughout the cellular layers of the cerebral cortex (i.e., layers II-VI) than beta-immunoreactive ones. Secondly, neurons in the globus pallidus were immunostained by the anti-beta antibody, but not by the anti-alpha antibody. Thirdly, neurons in the medial habenular nucleus, the subthalamic nucleus and the reticular thalamic nucleus were more densely stained with the anti-beta antibody than with the anti-alpha antibody. However, marked differences were not observed in the hippocampal formation at the light microscopic level. The electron microscopic analysis of the cerebral cortex demonstrated that subcellular localizations of alpha- and beta-immunoreactive products within the cortical neurons were quite dissimilar: (i) the nucleus was stained only with the anti-alpha antibody, but not with the anti-beta antibody, and (ii) beta-immunoreactive products were more sporadically localized in the cytoplasms of the perikarya and dendrites than the alpha-immunoreactive ones. These regional and subcellular differences between the distribution patterns of alpha and beta immunoreactivities suggest the functional diversity of CaM kinase II alpha and beta isoforms in the central nervous system.

Endogenous phosphorylation of a 61,000 dalton hippocampal protein increases following traumatic brain injury

Acute biochemical consequences of moderate traumatic brain injury (TBI) include activation of kinases, including protein kinase C (PKC). To determine the possible consequences of PKC activation at the substrate level, we have examined protein phosphorylation patterns 1 h following injury. Although the phosphorylation of most proteins remained unchanged following injury, we observed a significant increase in the phosphorylation of a 61,000 dalton protein (TBI61) in injured rat hippocampus (121% higher than sham control) in vitro. TBI61 phosphorylation could be enhanced by phosphatidyl serine and diacylglycerol or by addition of exogenous PKC. In addition, TBI61 phosphorylation was inhibited by the PKC inhibitor, staurosporine, suggesting further that this protein may be a PKC substrate. These data suggest that TBI increases the phosphorylation of a 61 kD hippocampal protein in vitro. Increases in the protein level and activity of PKC could contribute to this increased phosphorylation.

Decreased protein phosphorylation induced by anoxia in proximal renal tubules

Anoxia-induced depletion of cellular ATP may affect the degree of protein phosphorylation due to kinase inhibition. In this study, protein phosphorylation was measured in rabbit kidney proximal tubules under normoxic or anoxic conditions in a medium containing 32P. During the first 20 min of normoxia, phosphate incorporation was linear, averaging 17 +/- 5 pmol.mg protein-1.min-1 and was 70% inhibited by the protein kinase C inhibitor chelerythrine chloride. Phosphorylation measurements initiated simultaneously with anoxic conditions (95% N2-5% CO2) significantly reduced the initial rate to 58% of control, saturating after 15 min, and reaching 28 +/- 5% of the normoxic value after 60 min of incubation. The phosphatase inhibitor calyculin A did not affect the initial rate of phosphate incorporation by anoxic tubules but increased phosphate incorporation at 60 min to 43 +/- 4% of normoxia. Addition of 32P after 15 min of anoxia abolished phosphate incorporation, demonstrating that kinase activity was completely inhibited. Cellular phosphate uptake was measured and found not to be rate limiting for phosphorylation. Chelerythrine chloride increased lactate dehydrogenase (LDH) release during normoxia, and calyculin A decreased anoxia-induced LDH release, suggesting that protein phosphorylation events may control plasma membrane permeability.

Time course of the translocation and inhibition of protein kinase C during complete cerebral ischemia in the rat

The time course for the ischemia-induced changes in the subcellular distribution of protein kinase C (PKC) (alpha), (beta II), and (gamma) and the activity of PKC were studied in the neocortex of rats subjected to 1, 2, 3, 5, 10, and 15 min of global cerebral ischemia. In the particulate fraction, a 14-fold increase in PKC (gamma) levels was seen at 3 min of ischemia, which further increased at 5-15 min of ischemia. At 15 min of ischemia, PKC (alpha) and (beta II) levels had increased two- and six-fold, respectively. In the cytosolic fraction, a transient early 1.4-fold increase in PKC (beta II) and PKC (gamma) levels was seen, whereas no change in the levels PKC (alpha) was noted. PKC (gamma) levels then progressively declined, reaching 50% at 15 min of ischemia. At 5 min of ischemia, a 43% decrease in PKC activity was seen in the particulate fraction, reaching 50% at 15 min of ischemia concomitant with a 27% decrease in the cytosolic fraction. There was no change in the activator-independent PKC activity. Pretreatment with the ganglioside AGF2 prevented the redistribution of PKC (gamma) in the particulate fraction at 5 min, but not at 10 min of ischemia. The observed time course for the translocation of PKC (gamma) parallels the ischemia-induced release of neurotransmitters and increased levels of diacylglycerols, arachidonate, and increased levels of diacylglycerols, arachidonate, and intracellular calcium and delineates this subspecies as especially ischemia-sensitive. Ganglioside pretreatment delayed the translocation of PKC (gamma), possibly by counter-acting the effects of ischemia-induced factors that favor PKC binding to cell membranes.

Excitotoxicity affects membrane potential and calmodulin kinase II activity in cultured rat cortical neurons

BACKGROUND AND PURPOSE

Glutamate-induced excitotoxicity has been implicated as a causative factor for selective neuronal loss in ischemia and hypoxia. Toxic exposure of neurons to glutamate results in an extended neuronal depolarization that precedes delayed neuronal death. Because both delayed neuronal death and extended neuronal depolarization are dependent on calcium, we examined the effect of glutamate exposure on extended neuronal depolarization and calcium/calmodulin-dependent protein kinase II (CaM kinase II) activity.

METHODS

Three-week-old cortical cell cultures from embryonic rats were exposed to 500 microM glutamate and 10 microM glycine or to control medium for 10 minutes. Cells were examined for neuronal toxicity, electrophysiology, and biochemical alterations. In one set of experiments, whole-cell current clamp recording was performed throughout the experiment. In a parallel experiment, cortical cultures were allowed to recover from glutamate exposure for 1 hour, at which time the cells were homogenized and CaM kinase II activity was assayed using standard techniques.

RESULTS

Excitotoxic exposure to glutamate resulted in extended neuronal depolarization, which remained after removal of the glutamate. Glutamate exposure also resulted in delayed neuronal death, which was preceded by significant inhibition of CaM kinase II activity. The excitotoxic inhibition of CaM kinase II correlated with neuronal loss, was N-methyl-D-aspartate receptor-mediated, and was not due to autophosphorylation of the enzyme.

CONCLUSIONS

Glutamate-induced delayed neuronal toxicity correlates with extended neuronal depolarization and inhibition of CaM kinase II activity. Because inhibition of CaM kinase II activity significantly preceded the histological loss of neurons, the data suggest that modulation of CaM kinase II activity may be involved in the cascade of events resulting in loss of calcium homeostasis and delayed neuronal death.

Global forebrain ischemia results in decreased immunoreactivity of calcium/calmodulin-dependent protein kinase II

Previous studies utilizing crude brain homogenates have shown that forebrain ischemia results in inhibition of calcium/calmodulin-dependent protein kinase II (CaM kinase II) activity without large-scale proteolysis of the enzyme. In this report, a monoclonal antibody (1C3-3D6) directed against the beta- (60-kDa) subunit of CaM kinase II that does not recognize ischemically altered enzyme was utilized to further investigate the ischemia-induced inhibition of CaM kinase II. Immunohistochemical investigations showed that the ischemia-induced decreased immunoreactivity of CaM kinase II occurred immediately following ischemic insult in ischemia-sensitive cells such as pyramidal cells of the hippocampus. No decrease in CaM kinase II immunoreactivity was observed in ischemia-resistant cells such as granule cells of the dentate gyrus. The decreased immunoreactivity was observed for CaM kinase II balanced for protein staining and calmodulin binding in vitro. In addition, autophosphorylation of CaM kinase II in the presence of low (7 microM) or high (500 microM) ATP did not alter immunoreactivity of the enzyme with 1C3-3D6. The data demonstrate the production of a monoclonal antibody that recognizes the beta-subunit of CaM kinase II in a highly specific manner, but does not recognize ischemic enzyme. Together with previous studies, the data support the hypothesis that rapid, ischemia-induced inhibition of CaM kinase II activity may be involved in the cascade of events that lead to selective neuronal cell loss in stroke.