Akt Contributes to Neuroprotection by Hypothermia against Cerebral Ischemia in Rats

171

110

Recent studies have revealed that the phosphatidylinositol 3-kinase (PI3-K) pathway is involved in apoptotic cell death after experimental cerebral ischemia. The serine-threonine kinase, Akt, functions in the PI3-K pathway and prevents apoptosis by phosphorylation at Ser473 after a variety of cell death stimuli. After phosphorylation, activated Akt inactivates other apoptogenic factors, including glycogen synthase kinase-3b (GSK3b), thereby inhibiting cell death. However, the role of Akt/GSK3b signaling in the delayed death of hippocampal neurons in the CA1 subregion after transient global cerebral ischemia (tGCI) has not been clarified. Transient global cerebral ischemia for 5 mins was induced by bilateral common carotid artery occlusion combined with hypotension. Western blot analysis showed a significant increase in phospho-Akt (Ser473) and phospho-GSK3b (Ser9) in the hippocampal CA1 subregion after tGCI. Immunohistochemistry showed that expression of phospho-Akt (Ser473) and phospho-GSK3b (Ser9) was markedly increased in the vulnerable CA1 subregion, but not in the ischemic-tolerant CA3 subregion. Double staining with phospho-GSK3b (Ser9) and terminal deoxynucleotidyl transferase-mediated uridine 5 0 -triphosphate-biotin nick end labeling showed different cellular distributions in the CA1 subregion 3 days after tGCI. Phosphorylation of Akt and GSK3b was prevented by LY294002, a PI3-K inhibitor, which facilitated subsequent DNA fragmentation 3 days after tGCI. Moreover, transgenic rats that overexpress copper/zinc-superoxide dismutase, which is known to be neuroprotective against delayed hippocampal CA1 injury after tGCI, had enhanced and persistent phosphorylation of both Akt and GSK3b after tGCI. These findings suggest that activation of the Akt/GSK3b signaling pathway may mediate survival of vulnerable hippocampal CA1 neurons after tGCI.

Section: Discussionsupporting

confidence: 91%

Activation of the Akt/GSK3β Signaling Pathway Mediates Survival of Vulnerable Hippocampal Neurons after Transient Global Cerebral Ischemia in Rats

Endo

Nito

Kamada

et al. 2006

171

110

“…We therefore explored if hypothermia affects the downstream target of GSK3b, bcatenin, a critical transcription factor. Glycogen synthase kinase 3b phosphorylase b-catenin leading to its degradation and apoptosis (Zhao et al, 2005a). Despite protein levels of phosphorylated b-catenin being unknown, we have found that b-catenin translocates into nuclei in the penumbra after stroke, and that hypothermia blocks such translocation.…”

Section: Caspase and Cytochrome C-mediated Apoptotic Pathwaysmentioning

confidence: 77%

“…Glycogen synthase kinase 3b activity is downregulated when phosphorylated at Ser-9 by Akt. Although hypothermia maintains many steps in the Akt pathway after stroke, it does not elevate GSK3b (Ser-9) phosphorylation (Zhao et al, 2005a). It is a puzzle that the protein level of the active form of nonphosphorylated GSK3b was increased after hypothermic stroke, which is supposed to induce cell death, but no ischemic damage was detected under hypothermia.…”

Section: Caspase and Cytochrome C-mediated Apoptotic Pathwaysmentioning

confidence: 99%

General versus Specific Actions of Mild-Moderate Hypothermia in Attenuating Cerebral Ischemic Damage

Zhao

Steinberg

Sapolsky

2007

152

134

Mild or moderate hypothermia is generally thought to block all changes in signaling events that are detrimental to ischemic brain, including ATP depletion, glutamate release, Ca 2 + mobilization, anoxic depolarization, free radical generation, inflammation, blood-brain barrier permeability, necrotic, and apoptotic pathways. However, the effects and mechanisms of hypothermia are, in fact, variable. We emphasize that, even in the laboratory, hypothermic protection is limited. In certain models of permanent focal ischemia, hypothermia may not protect at all. In cases where hypothermia reduces infarct, some studies have overemphasized its ability to maintain cerebral blood flow and ATP levels, and to prevent anoxic depolarization, glutamate release during ischemia. Instead, hypothermia may protect against ischemia by regulating cascades that occur after reperfusion, including blood-brain barrier permeability and the changes in gene and protein expressions associated with necrotic and apoptotic pathways. Hypothermia not only blocks multiple damaging cascades after stroke, but also selectively upregulates some protective genes. However, most of these mechanisms are addressed in models with intraischemic hypothermia; much less information is available in models with postischemic hypothermia. Moreover, although it has been confirmed that mild hypothermia is clinically feasible for acute focal stroke treatment, no definite beneficial effect has been reported yet. This lack of clinical protection may result from suboptimal criteria for patient entrance into clinical trials. To facilitate clinical translation, future efforts in the laboratory should focus more on the protective mechanisms of postischemic hypothermia, as well as on the effects of sex, age and rewarming during reperfusion on hypothermic protection.

“…Focal cerebral ischemia was generated as described (Zhao et al, 2005b). Male Sprague-Dawley rats (350 to 450 g) were used.…”

Section: Focal Cerebral Ischemiamentioning

confidence: 99%

“…The brains were removed and tissue corresponding to the ischemic core and penumbra was dissected for Western blots as described (Zhao et al, 2005b). The ischemic penumbra, a viable ischemic region that will eventually evolve into infarction, was defined as the tissue saved by hypothermia 48 h after CCA release.…”

Section: Sample Preparations For Western Blotmentioning

confidence: 99%

Suppression of δPKC Activation after Focal Cerebral Ischemia Contributes to the Protective Effect of Hypothermia

Shimohata

Zhao

Sung

et al. 2007

Mild hypothermia is a robust neuroprotective treatment for stroke. Understanding the mechanisms underlying hypothermia's benefits will lead to more effective treatments to prevent stroke damage. Delta protein kinase C (dPKC) is a kinase that has been strongly implicated in executing ischemic damage. We investigated the effects of hypothermia on dPKC activation, as determined by its subcellular translocation, proteolytic cleavage, and phosphorylation in a focal cerebral ischemia model. The amount of constitutively activated C-terminal catalytic fragment of dPKC (CF-dPKC) increased after stroke. Both hypothermia (301C) and the caspase-3-specific inhibitor, Z-DQMD-FMK, blocked the accumulation of activated dPKC in the penumbra. Other hallmarks of dPKC activation, its translocation to the mitochondria, and nucleus were observed in the penumbra as early as 10 mins after reperfusion. These events were blocked by hypothermia. Hypothermia also blocked CF-dPKC increases in the mitochondria and nuclei. Conversely, a specific dPKC activator, wdRACK, decreased the neuroprotective effect of hypothermia. Finally, dPKC activity may lead to mitochondrial injury and cytochrome c release, as the timing of cytochrome c release corresponded to the time course of dPKC translocation. Both cytochrome c release and dPKC translocation were blocked by hypothermia. In conclusion, hypothermia protects against ischemic damage in part by suppressing dPKC activation after stroke.