Ischemia-induced translocation of Ca2+/calmodulin-dependent protein kinase II: potential role in neuronal damage

Calcium/calmodulin-dependent protein kinase II activity in focal ischemia with reperfusion in rats

BACKGROUND AND PURPOSE

Evidence linking changes in calcium/calmodulin-dependent protein kinase II activity with ischemic cell death has been reported in animal models of global ischemia. The purpose of this study was to delineate the course of these changes after focal ischemia and to clarify the relation of changes in activity of calcium/calmodulin-dependent protein kinase II to the process of ischemic cell death.

METHODS

Change in calcium/calmodulin-dependent protein kinase II activity was evaluated in a rat model of focal ischemia after 5 minutes, 30 minutes, and 1 hour of tandem middle cerebral artery and common carotid artery occlusion both with and without reperfusion.

RESULTS

Calcium/calmodulin-dependent protein kinase II activity was significantly decreased after all three durations of ischemia followed by immediate decapitation compared with sham-operated animals, in both ischemic core and border-zone regions (P < .05 for all groups). Depression of activity occurred in a regionally graded fashion, with the most severe decrease in infarct core and progressively smaller decreases in samples moving out from the center, corresponding to the severity of histological injury later detected in infarct core and border-zone regions. There were only minor differences between the three durations of ischemia in the degree of enzyme depression noted in the more peripheral regions, indicating that the initial decrease in calcium/calmodulin-dependent protein kinase II activity is an early, sensitive marker for an ischemic insult. After reperfusion, the differences between the 5-minute group and longer periods of ischemia widened because of an increase toward baseline in the 5-minute group and a trend toward further decrease in the 30- and 60-minute groups.

CONCLUSIONS

The extreme sensitivity of calcium/calmodulin-dependent protein kinase II to focal ischemia and the parallel temporal and regional changes in its activity to those of more delayed cell injury point to a potential role for this enzyme in the process of excitotoxic injury.

mRNA levels of Ca(2+)-independent forms of protein kinase C in postischemic gerbil brain by northern blot analysis

To investigate the role of Ca(2+)-independent forms of protein kinase C (PKC) in ischemic neuronal injury, mRNA expression of PKC was studied by Northern blot analysis. Ischemia was produced in gerbils by 10-min bilateral carotid artery occlusion and was followed by recirculation for 15 min, 6 h, and 24 h. Brains of postischemic and sham-operated animals were removed, forebrains fresh frozen, and processed for Northern blot analysis. Three synthetic oligonucleotide probes based on published cDNA sequences of rat brain PKC for the isozymes delta, epsilon, and zeta were utilized for hybridization. Northern blot analysis showed increased hybridization signal for all three PKC isozymes examined in the 6- and 24-h postischemic groups. Of these, the twofold increases in the expression of PKC delta and zeta were statistically significant in comparison to the control. These results suggest that the mRNA levels of Ca(2+)-independent forms of PKC, in particular, delta and zeta, are temporally stimulated by ischemic injury in the brain and may imply an important role of the enzyme in postischemic neuronal damage. However, since the protein itself was not examined in this study, the significance of the increased expression cannot be ascertained. However, it may reflect a compensatory response to the loss of PKC reported to occur in the reperfusion phase.

Threshold of calcium disturbances after focal cerebral ischemia in rats. Implications of the window of therapeutic opportunity

BACKGROUND AND PURPOSE

One explanation for inconclusive results with calcium channel blockers in human acute stroke trials may be incomplete information about the time course of calcium-mediated ischemic neuronal injury. This study explores the temporal relation between duration of focal ischemia and the functional activity of increased intracellular calcium as measured by calcium-calmodulin binding.

METHODS

Calcium-calmodulin binding, determined by immunohistochemical assay of free calmodulin, was measured in 60 male spontaneously hypertensive rats after 2 minutes and after 1, 2, 4, and 24 hours of permanent tandem common carotid and middle cerebral artery occlusion, and after 1 and 2 hours of reversible middle cerebral artery occlusion followed by 1 and 22 hours of reperfusion, respectively. Light microscopic histological damage was measured after 1 hour of occlusion with 23 hours of reperfusion and after 24 hours of occlusion.

RESULTS

Significant loss of calmodulin staining in the core of the infarction was noted by 1 hour and became maximal after 4 hours of ischemia. No reversal of calmodulin staining loss was noted after reperfusion following 1 and 2 hours of ischemia. Cortical necrosis seen by light microscopy correlated well with the area of maximal calcium-calmodulin binding. The border zone area, represented by a mild loss of calmodulin staining surrounding the central core of maximal binding, gradually decreased in size and became incorporated into the central core after 4 hours of ischemia; it may represent an area of reversible ischemia.

CONCLUSIONS

Calcium-calmodulin binding correlates with duration of focal ischemia, and histological neuronal necrosis corresponds to the cortical areas displaying a significant loss of calmodulin staining. Inasmuch as loss of calmodulin staining represents a marker for calcium-mediated activity after ischemia, it suggests a window of opportunity within 4 hours after acute stroke for therapeutic intervention with calcium antagonists.

Inactivation and subcellular redistribution of Ca2+/calmodulin-dependent protein kinase II following spinal cord ischemia

Reversible spinal cord ischemia in rabbits induced a rapid loss of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) activity measured as incorporation of phosphate into exogenous substrates. About 70% of the activity was lost from the cytosolic fraction of spinal cord homogenates after 15 min of ischemia preceding irreversible paraplegia, which takes 25 min in this model. The loss of enzyme activity correlated with a loss of in situ renaturable autophosphorylation activity and a loss of CaM kinase II alpha and beta subunits in the cytosol detected by immunoblotting. CaM kinase II activity in the particulate fraction also decreased but the protein levels of the alpha and beta subunits increased. Thus ischemia resulted in an inactivation of CaM kinase II and a sequential or concurrent subcellular redistribution of the enzyme. However, denaturation and renaturation in situ of the CaM kinase subunits immobilized on membranes partly reversed the apparent inactivation of the enzyme in the particulate fraction. CaM kinase II activity was restored after reperfusion following short (< or = 25 min) durations of ischemia but not after longer durations (60 min) that result in irreversible paraplegia. The ischemia-induced inactivation of CaM kinase II, which phosphorylates proteins regulating many cellular processes, may be important in the cascade of events leading to delayed neuronal cell death.

Neuronal protection and preservation of calcium/calmodulin-dependent protein kinase II and protein kinase C activity by dextrorphan treatment in global ischemia

This study analyzed the ability of the N-methyl-D-aspartate receptor antagonist dextrorphan (DX) to prevent neuronal degeneration (analyzed by light microscopy), calmodulin (CaM) redistribution (analyzed by immunocytochemistry) and changes in activity of two major Ca(2+)-dependent protein kinases--calcium/calmodulin-dependent protein kinase II (CaM-KII) and protein kinase C (PKC) (analyzed by specific substrate phosphorylation) after 20 min of global ischemia (four-vessel occlusion model) in rats. DX treatment before and after ischemia significantly protected hippocampal and cortical neurons from neurodegeneration whereas DX posttreatment alone did not have any effect on preservation of neuronal morphology as compared with placebo treatment analyzed 72 h after 20 min of ischemia. Similarly to histological changes, DX exhibited protection against redistribution of CaM observed after ischemia. These changes were detected both in hippocampus as well as in cerebral cortex. Finally, DX administered before ligation of the carotid arteries reduced loss in both CaM-KII and PKC activity evoked by ischemia.

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.

Autophosphorylation of neuronal calcium/calmodulin-stimulated protein kinase II

A unique feature of neuronal calcium/calmodulin-stimulated protein kinase II (CaM-PK II) is its autophosphorylation. A number of sites are involved and, depending on the in vitro conditions used, three serine and six threonine residues have been tentatively identified as autophosphorylation sites in the alpha subunit. These sites fall into three categories. Primary sites are phosphorylated in the presence of calcium and calmodulin, but under limiting conditions of temperature, ATP, Mg2+, or time. Secondary sites are phosphorylated in the presence of calcium and calmodulin under nonlimiting conditions. Autonomous sites are phosphorylated in the absence of calcium and calmodulin after initial phosphorylation of Thr-286. Mechanisms that lead to a decrease in CaM-PK II autophosphorylation include the thermolability of the enzyme and the activity of protein phosphatases. A range of in vitro inhibitors of CaM-PK II autophosphorylation have recently been identified. Autophosphorylation of CaM-PK II leads to a number of consequences in vitro, including generation of autonomous activity and subcellular redistribution, as well as alterations in conformation, activity, calmodulin binding, substrate specificity, and susceptibility to proteolysis. It is established that CaM-PK II is autophos-phorylated in neuronal cells under basal conditions. Depolarization and/or activation of receptors that lead to an increase in intracellular calcium induces a marked rise in the autophosphorylation of CaM-PK II in situ. The incorporation of phosphate is mainly found on Thr-286, but other sites are also phosphorylated at a slower rate. One consequence of the increase in CaM-PK II autophosphorylation in situ is an increase in the level of autonomous kinase activity. It is proposed that the formation of an autonomous enzyme is only one of the consequences of CaM-PK II autophosphorylation in situ and that some of the other consequences observed in vitro will also be seen. CaM-PK II is involved in the control of neuronal plasticity, including neurotransmitter release and long-term modulation of postreceptor events. In order to understand the function of CaM-PK II, it will be essential to ascertain more fully the mechanisms of its autophosphorylation in situ, including especially the sites involved, the consequences of this autophosphorylation for the kinase activity, and the relationships between the state of CaM-PK II autophosphorylation and the physiological events within neurons.