The endogenous inhibitor of Akt, CTMP, is critical to ischemia-induced neuronal death

Estradiol pretreatment ameliorates impaired synaptic plasticity at synapses of insulted CA1 neurons after transient global ischemia

Global ischemia in humans or induced experimentally in animals causes selective and delayed neuronal death in pyramidal neurons of the hippocampal CA1. The ovarian hormone estradiol administered before or immediately after insult affords histological protection in experimental models of focal and global ischemia and ameliorates the cognitive deficits associated with ischemic cell death. However, the impact of estradiol on the functional integrity of Schaffer collateral to CA1 (Sch-CA1) pyramidal cell synapses following global ischemia is not clear. Here we show that long term estradiol treatment initiated 14 days prior to global ischemia in ovariectomized female rats acts via the IGF-1 receptor to protect the functional integrity of CA1 neurons. Global ischemia impairs basal synaptic transmission, assessed by the input/output relation at Sch-CA1 synapses, and NMDA receptor (NMDAR)-dependent long term potentiation (LTP), assessed at 3 days after surgery. Presynaptic function, assessed by fiber volley and paired pulse facilitation, is unchanged. To our knowledge, our results are the first to demonstrate that estradiol at near physiological concentrations enhances basal excitatory synaptic transmission and ameliorates deficits in LTP at synapses onto CA1 neurons in a clinically-relevant model of global ischemia. Estradiol-induced rescue of LTP requires the IGF-1 receptor, but not the classical estrogen receptors (ER)-α or β. These findings support a model whereby estradiol acts via the IGF-1 receptor to maintain the functional integrity of hippocampal CA1 synapses in the face of global ischemia. This article is part of a Special Issue entitled SI: Brain and Memory.

High-fat diet reduces neuroprotection of isoflurane post-treatment: Role of carboxyl-terminal modulator protein-Akt signaling

OBJECTIVE

High-fat diet (HFD) contributes to the increased prevalence of obesity and hyperlipidemia in young adults, a possible cause for their recent increase in stroke. Isoflurane post-treatment provides neuroprotection. Isoflurane post-treatment induced neuroprotection in HFD-fed mice was determined.

METHODS

Six-week old CD-1 male mice were fed HFD or regular diet (RD) for 5 or 10 weeks. Their hippocampal slices (400 µm) were subjected to oxygen-glucose deprivation (OGD). Some slices were exposed to isoflurane for 30 min immediately after OGD. Some mice had a 90-min middle cerebral arterial occlusion and were post-treated with 2% isoflurane for 30 min.

RESULTS

OGD time-dependently induced cell injury. This injury was dose-dependently reduced by isoflurane. The effect was apparent at 1% or 2% isoflurane in RD-fed mice but required 3% isoflurane in HFD-fed mice. HFD influenced the isoflurane effects in DG. OGD increased carboxyl-terminal modulator protein (CTMP), an Akt inhibitor, and decreased Akt signaling. Isoflurane reduced these effects. LY294002, an Akt activation inhibitor, attenuated the isoflurane effects. HFD increased CTMP and reduced Akt signaling. Isoflurane improved neurological outcome in the RD-fed mice but not in the HFD-fed mice.

CONCLUSIONS

HFD attenuated isoflurane post-treatment-induced neuroprotection possibly because of decreased prosurvival Akt signaling.

Dual role of Src kinase in governing neuronal survival

BACKGROUND

Src-family kinases (SFKs) are involved in neuronal survival and their aberrant regulation contributes to neuronal death. However, how they control neuronal survival and death remains unclear.

OBJECTIVE

To define the effect of inhibition of Src activity and expression on neuronal survival.

RESULTS

In agreement with our previous findings, we demonstrated that Src was cleaved by calpain to form a 52-kDa truncated fragment in neurons undergoing excitotoxic cell death, and expression of the recombinant truncated Src fragment induced neuronal death. The data confirm that the neurotoxic signaling pathways are intact in the neurons we used for our study. To define the functional role of neuronal SFKs, we treated these neurons with SFK inhibitors and discovered that the treatment induced cell death, suggesting that the catalytic activity of one or more of the neuronal SFKs is critical to neuronal survival. Using small hairpin RNAs that suppress Src expression, we demonstrated that Src is indispensable to neuronal survival. Additionally, we found that neuronal death induced by expression of the neurotoxic truncated Src mutant, treatment of SFK inhibitors or knock-down of Src expression caused inhibition of the neuroprotective protein kinases Erk1/2, or Akt.

CONCLUSIONS

Src is critical to both neuronal survival and death. Intact Src sustains neuronal survival. However, in the excitotoxic condition, calpain cleavage of Src generates a neurotoxic truncated Src fragment. Both intact Src and the neurotoxic truncated Src fragment exert their biological actions by controlling the activities of neuroprotective protein kinases.

TRAF1 is a critical regulator of cerebral ischaemia-reperfusion injury and neuronal death

Stroke is a leading global cause of mortality and disability. Less than 5% of patients are able to receive tissue plasminogen activator thrombolysis within the necessary timeframe. Focusing on the process of neuronal apoptosis in the penumbra, which lasts from hours to days after ischaemia, appears to be promising. Here we report that tumour necrosis factor receptor-associated factor 1 (TRAF1) expression is markedly induced in wild-type mice 6 h after stroke onset. Using genetic approaches, we demonstrate that increased neuronal TRAF1 leads to elevated neuronal death and enlarged ischaemic lesions, whereas TRAF1 deficiency is neuroprotective. In addition, TRAF1-mediated neuroapoptosis correlates with the activation of the JNK pro-death pathway and inhibition of the Akt cell survival pathway. Finally, TRAF1 is found to exert pro-apoptotic effects via direct interaction with ASK1. Thus, ASK1 positively and negatively regulates the JNK and Akt signalling pathways, respectively. Targeting the TRAF1/ASK1 pathway may provide feasible therapies for stroke long after onset.

Sevoflurane preconditioning-induced neuroprotection is associated with Akt activation via carboxy-terminal modulator protein inhibition

BACKGROUND

Sevoflurane preconditioning has a neuroprotective effect, but the underlying mechanism is not fully understood. The aim of the present investigation was to evaluate whether sevoflurane-induced cerebral preconditioning involves inhibition of carboxy-terminal modulator protein (CTMP), an endogenous inhibitor of Akt, in a rat model of focal cerebral ischaemia.

METHODS

Male Sprague-Dawley rats were exposed to 2.7% sevoflurane for 45 min. One hour later, rats were subjected to 60 min of focal cerebral ischaemia. The phosphoinositide 3-kinase inhibitors wortmannin and LY294002 were administered 10 min before preconditioning. Rats in the lentiviral transduction group received an intracerebroventricular injection of lentiviral vector Ubi-MCS-CTMP 3 days before ischaemia. Neurological deficits and infarct volumes were evaluated 24 h and 7 days after reperfusion. Phosphorylation of Akt, glycogen synthase kinase-3β (GSK3β), and expression of CTMP were determined at 1, 3, 12, and 24 h after reperfusion. Akt activity was measured at 3 h after reperfusion.

RESULTS

Sevoflurane preconditioning improved neurological score and reduced infarct size at 24 h of reperfusion. Pretreatment with wortmannin or LY294002 attenuated these neuroprotective effects. Expression of CTMP correlated with reduced Akt activity after ischaemia, while sevoflurane preconditioning preserved Akt activity and increased phosphorylation of GSK3β. CTMP over-expression diminished the beneficial effects of sevoflurane preconditioning.

CONCLUSIONS

Activation of Akt signalling via inhibition of CTMP is involved in the mechanism of neuroprotection provided by sevoflurane preconditioning.

The gene silencing transcription factor REST represses miR-132 expression in hippocampal neurons destined to die

The gene silencing transcription factor REST [repressor element 1 silencing transcription factor]/NRSF (neuron-restrictive silencer factor) actively represses a large array of coding and noncoding neuron-specific genes important to synaptic plasticity including miR-132. miR-132 is a neuron-specific microRNA and plays a pivotal role in synaptogenesis, synaptic plasticity and structural remodeling. However, a role for miR-132 in neuronal death is not, as yet, well-delineated. Here we show that ischemic insults promote REST binding and epigenetic remodeling at the miR-132 promoter and silencing of miR-132 expression in selectively vulnerable hippocampal CA1 neurons. REST occupancy was not altered at the miR-9 or miR-124a promoters despite the presence of repressor element 1 sites, indicating REST target specificity. Ischemia induced a substantial decrease in two marks of active gene transcription, dimethylation of lysine 4 on core histone 3 (H3K4me2) and acetylation of lysine 9 on H3 (H3K9ac) at the miR-132 promoter. RNAi-mediated depletion of REST in vivo blocked ischemia-induced loss of miR-132 in insulted hippocampal neurons, consistent with a causal relation between activation of REST and silencing of miR-132. Overexpression of miR-132 in primary cultures of hippocampal neurons or delivered directly into the CA1 of living rats by means of the lentiviral expression system prior to induction of ischemia afforded robust protection against ischemia-induced neuronal death. These findings document a previously unappreciated role for REST-dependent repression of miR-132 in the neuronal death associated with global ischemia and identify a novel therapeutic target for amelioration of the neurodegeneration and cognitive deficits associated with ischemic stroke.

Casein kinase 1 suppresses activation of REST in insulted hippocampal neurons and halts ischemia-induced neuronal death

Repressor Element-1 (RE1) Silencing Transcription Factor/Neuron-Restrictive Silencer Factor (REST/NRSF) is a gene-silencing factor that is widely expressed during embryogenesis and plays a strategic role in neuronal differentiation. Recent studies indicate that REST can be activated in differentiated neurons during a critical window of time in postnatal development and in adult neurons in response to neuronal insults such as seizures and ischemia. However, the mechanism by which REST is regulated in neurons is as yet unknown. Here, we show that REST is controlled at the level of protein stability via β-TrCP-dependent, ubiquitin-based proteasomal degradation in differentiated neurons under physiological conditions and identify Casein Kinase 1 (CK1) as an upstream effector that bidirectionally regulates REST cellular abundance. CK1 associates with and phosphorylates REST at two neighboring, but distinct, motifs within the C terminus of REST critical for binding of β-TrCP and targeting of REST for proteasomal degradation. We further show that global ischemia in rats in vivo triggers a decrease in CK1 and an increase in REST in selectively vulnerable hippocampal CA1 neurons. Administration of the CK1 activator pyrvinium pamoate by in vivo injection immediately after ischemia restores CK1 activity, suppresses REST expression, and rescues neurons destined to die. Our results identify a novel and previously unappreciated role for CK1 as a brake on REST stability and abundance in adult neurons and reveal that loss of CK1 is causally related to ischemia-induced neuronal death. These findings point to CK1 as a potential therapeutic target for the amelioration of hippocampal injury and cognitive deficits associated with global ischemia.

Novel link of anti-apoptotic ATF3 with pro-apoptotic CTMP in the ischemic brain

Activating transcription factor 3 (ATF3) is a stress-induced transcription factor with diverse functions under disease states in multiple cell types. ATF3 has neuroprotective action against cerebral ischemia, which may involve caspase 3. However, the molecular mechanisms underlying ATF3 regulation of apoptosis are largely unknown. Here, we used gain- and loss-of-function and rescue approaches to demonstrate ATF3 attenuating hypoxic neuronal apoptosis. As well, the protective effect of ATF3 was mediated by downregulation of carboxyl-terminal modulator protein (CTMP), a pro-apoptotic factor that inhibits the anti-apoptotic Akt/PKB cascade. ATF3 (1) downregulated the mRNA and protein levels of CTMP; (2) its temporal expression pattern was reciprocal to that of CTMP; and (3) nuclear localization suggested that ATF3 may regulate CTMP transcription following hypoxic insult. Reporter assays demonstrated that ATF3 suppressed CTMP transcription, whereas ATF3 fusion with VP16, converting ATF3 to transcriptional activator, boosted CTMP transcription. By contrast, NF-κB increased CTMP transcription, and degradation-resistant IκBα decreased CTMP transcription. ChIP assays further confirmed that binding of ATF3 to the ATF/CREB site hindered NF-κB binding to the CTMP promoter, which repressed CTMP expression. Furthermore, CTMP siRNA treatment reduced hypoxic neuronal apoptosis by increasing p-Akt (Ser473) levels and leaving the upstream ATF3 level unchanged. We have identified an endogenous neuroprotective ATF3→CTMP signal cascade that may be a therapeutic target for reducing ischemic brain injury.

Small interfering RNA directed against CTMP reduces acute traumatic brain injury in a mouse model by activating Akt

OBJECTIVE

Protein kinase B (PKB/Akt), which is phosphorylated and activated by upstream activators, exerts critical neuroprotective effects by phosphorylating downstream targets after traumatic brain injury (TBI). Studies on the regulation of Akt will be crucial for our understanding of neuronal survival. The goal of this study is to investigate the effects of carboxyl-terminal modulator protein (CTMP) on phosphorylation of Akt and neurological function in a mouse model of TBI.

METHODS

Traumatic brain injury in mice was performed by a controlled cortical impact device. The expression of Akt, phospho-Akt, and CTMP was examined in the injured cortices by immunohistochemistry and Western blot analysis. To determine the effects of CTMP, small interfering RNAs (siRNAs) directed against CTMP were injected in mice with TBI, and the expression of phosphorylated Akt and neurological function were evaluated.

RESULTS

Phospho-Akt significantly increased at 4 hours post-TBI in the nucleus (P < 0.01) and remained at high levels until 72 hours after TBI, as shown by Western blot analysis. In the cytosol, the expression of phospho-Akt reached its peak at 4 hours post-TBI, but decreased markedly at 24 hours and maintained below pre-TBI levels until 72 hours post-TBI. Interestingly, the expression of CTMP significantly increased 4 hours after TBI (P < 0.01) and sustained those levels until 72 hours without dramatic changes. Treatment with CTMP siRNA effectively augmented the phosphorylation of Akt and significantly improved the neurological functional recovery up to 28 days post-TBI.

CONCLUSION

We conclude that Akt is phosphorylated and translocated to nucleus after TBI to exert neuroprotective effects. However, CTMP is simultaneously triggered to inhibit the phosphorylation of Akt. Inhibition of CTMP by siRNA improves the recovery of neurological functions after TBI.

New players in high fat diet-induced obesity: LETM1 and CTMP

OBJECTIVE

Obesity contributes to insulin resistance and is a risk factor for diabetes. C-terminal modulator protein (CTMP) and leucine zipper/EF-hand-containing transmembrane protein 1 (LETM1) have been reported to influence the phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB) signaling pathway via the modulation of PKB activity, a key player for insulin signaling. However, it remains unclear whether CTMP and LETM1 are associated with PI3K/PKB signaling in mouse models of obesity.

MATERIALS/METHODS

To address this question, we used two different mouse models of obesity, including high-fat diet (HFD)-induced diabetic mice and genetically modified obese mice (ob/ob mice). The levels of insulin-signaling molecules in these mice were determined by immunohistochemical and Western blot analyses. The involvement of CTMP and LETM1 in PI3K/PKB signaling was investigated in HEK293 cells by transient transfection and adenovirus-mediated infection.

RESULTS

We found that the levels of insulin receptor, phosphorylated PKB, and LETM1 were lower and the level of CTMP was higher in the adipose tissue of obese mice on an HFD compared to lean mice on a chow diet. Similar results were obtained in ob/ob mice. In HEK293 cells, the activation of PKB increased the LETM1 level, and inhibition of PKB increased the CTMP level. The overexpression of CTMP suppressed the insulin-induced increase in PKB phosphorylation, which was abrogated by co-overexpression with LETM1.

CONCLUSION

These results suggest that CTMP and LETM1 may participate in impaired insulin signaling in the adipose tissue of obese mice, raising the possibility that these parameters may serve as new candidate biomarkers or targets in the development of new therapeutic approaches for diabetes.

Akt signaling in platelets and thrombosis

Akt is a Ser-Thr kinase with pleiotropic effects on cell survival, growth and metabolism. Recent evidence from gene-deletion studies in mice, and analysis of human platelets treated with Akt inhibitors, suggest that Akt regulates platelet activation, with potential consequences for thrombosis. Akt activation is regulated by the level of phosphoinositide 3-phosphates, and proteins that regulate concentrations of this lipid also regulate Akt activation and platelet function. Although the effectors through which Akt contributes to platelet activation are not definitively known, several candidates are discussed, including endothelial nitric oxide synthase, glycogen synthase kinase 3β, phosphodiesterase 3A and the integrin β(3) tail. Selective inhibitors of Akt isoforms or of proteins that contribute to its activation, such as individual PI3K isoforms, may make attractive targets for antithrombotic therapy. This review summarizes the current literature describing Akt activity and its regulation in platelets, including speculation regarding the future of Akt or its regulatory pathways as targets for the development of antithrombotic therapies.

Survivin Is a transcriptional target of STAT3 critical to estradiol neuroprotection in global ischemia

Transient global ischemia causes selective, delayed death of hippocampal CA1 pyramidal neurons in humans and animals. It is well established that estrogens ameliorate neuronal death in animal models of focal and global ischemia. However, the role of signal transducer and activator of transcription-3 (STAT3) and its target genes in estradiol neuroprotection in global ischemia remains unclear. Here we show that a single intracerebral injection of 17β-estradiol to ovariectomized female rats immediately after ischemia rescues CA1 neurons destined to die. Ischemia promotes activation of STAT3 signaling, association of STAT3 with the promoters of target genes, and STAT3-dependent mRNA and protein expression of prosurvival proteins in the selectively vulnerable CA1. In animals subjected to ischemia, acute postischemic estradiol further enhances activation and nuclear translocation of STAT3 and STAT3-dependent transcription of target genes. Importantly, we show that STAT3 is critical to estradiol neuroprotection, as evidenced by the ability of STAT3 inhibitor peptide and STAT3 shRNA delivered directly into the CA1 of living animals to abolish neuroprotection. In addition, we identify survivin, a member of the inhibitor-of-apoptosis family of proteins and known gene target of STAT3, as essential to estradiol neuroprotection, as evidenced by the ability of shRNA to survivin to reverse neuroprotection. These findings indicate that ischemia and estradiol act synergistically to promote activation of STAT3 and STAT3-dependent transcription of survivin in insulted CA1 neurons and identify STAT3 and survivin as potentially important therapeutic targets in an in vivo model of global ischemia.

Pharmacological inhibition of pleckstrin homology domain leucine-rich repeat protein phosphatase is neuroprotective: differential effects on astrocytes

Pleckstrin homology domain and leucine-rich repeat protein phosphatase 1 (PHLPP1) inhibits protein kinase B (AKT) survival signaling in neurons. Small molecule pan-PHLPP inhibitors (selective for PHLPP1 and PHLPP2) may offer a translatable method to induce AKT neuroprotection. We tested several recently discovered PHLPP inhibitors (NSC117079 and NSC45586; benzoic acid, 5-[2-[4-[2-(2,4-diamino-5-methylphenyl)diazenyl]phenyl]diazenyl]-2-hydroxy-,sodium salt.) in rat cortical neurons and astrocytes and compared the biochemical response of these agents with short hairpin RNA (shRNA)-mediated PHLPP1 knockdown (KD). In neurons, both PHLPP1 KD and experimental PHLPP inhibitors activated AKT and ameliorated staurosporine (STS)-induced cell death. Unexpectedly, in astrocytes, both inhibitors blocked AKT activation, and NSC117079 reduced viability. Only PHLPP2 KD mimicked PHLPP inhibitors on astrocyte biochemistry. This suggests that these inhibitors could have possible detrimental effects on astrocytes by blocking novel PHLPP2-mediated prosurvival signaling mechanisms. Finally, because PHLPP1 levels are reportedly high in the hippocampus (a region prone to ischemic death), we characterized hippocampal changes in PHLPP and several AKT targeting prodeath phosphatases after cardiac arrest (CA)-induced brain injury. PHLPP1 levels increased in rat brains subjected to CA. None of the other AKT inhibitory phosphatases increased after global ischemia (i.e., PHLPP2, PTEN, PP2A, and PP1). Selective PHLPP1 inhibition (such as by shRNA KD) activates AKT survival signaling in neurons and astrocytes. Nonspecific PHLPP inhibition (by NSC117079 and NSC45586) only activates AKT in neurons. Taken together, these results suggest that selective PHLPP1 inhibitors should be developed and may yield optimal strategies to protect injured hippocampal neurons and astrocytes-namely from global brain ischemia.

Induction of apoptosis by c9, t11-CLA in human endometrial cancer RL 95-2 cells via ERα-mediated pathway

Numerous studies have shown that conjugated linoleic acid (CLA) can inhibit cancer cells growth and induce apoptosis in vitro and in vivo. The aim of the present study was to investigate the effects of CLA, including cis9, trans11-conjugated linoleic acid (c9, t11-CLA) and trans10, cis12-conjugated linoleic acid (t10, c12-CLA), on apoptosis of human endometrial cancer RL 95-2 cells and its related mechanisms. The MTT analysis was used to evaluate the effect of CLA isomers on the viability of endometrial cancer RL 95-2 cells. We then estimated the apoptosis by Morphological observation and Annexin V-FITC/PI staining and flow cytometry. We also used Western blot analysis to assess the expression of caspase-3, Bax, Bcl-2 proteins and the activation of Akt/p-Akt and ERα/p-ERα. Propylpyrazole-triol (PPT), a selective ERα agonist was used to confirm the induction of apoptosis by c9, t11 CLA may relate to ERα-mediated pathway. In CLA-treated RL 95-2 cells, we found that c9, t11-CLA inhibited viability and trigged apoptosis, as judged from nuclear morphology and flow cytometric analysis. The expression of caspase-3 and the ratio of Bax/Bcl-2 were significant increased, but no obvious change was observed about Akt and p-Akt in c9, t11-CLA-treated cells. However, the expression of total ERα level in RL 95-2 cells-treated with c9, t11-CLA was unchanged, while in the concentration of 80 mM, c9, t11-CLA down-regulated the protein expression level of p-ERα. Then PPT has the antagonistic action on growth inhibitory effect in RL 95-2 cells incubated with c9, t11-CLA. This study demonstrated that c9, t11- CLA could induce apoptosis in RL 95-2 cells, and may involve in ERα-mediated pathway. These results indicated that c9, t11- CLA could induce apoptosis of endometrial cancer cells and may be potential agents for the treatment of endometrial cancer.

Carboxyl-terminal modulator protein positively regulates Akt phosphorylation and acts as an oncogenic driver in breast cancer

Akt activation has been implicated broadly in tumorigenesis, but the basis for its dysregulation in cancer cells is incompletely understood. In this study, we sought to clarify a regulatory role for the Akt-binding carboxy-terminal modulator protein (CTMP), which has been controversial. In evaluating CTMP expression in paired normal-tumor specimens of 198 patients with breast cancer, we found that CTMP was upregulated in breast tumors, where it was associated with poor patient survival. Notably, CTMP expression also correlated positively with Akt phosphorylation in breast cancer clinical specimens and cell lines. Furthermore, ectopic expression of CTMP promoted cell proliferation and enhanced the tumorigenic properties of estrogen-dependent breast cancer cells. This effect was correlated with increased sensitivity to insulin-induced Akt phosphorylation, which is mediated primarily by the phosphoinositide 3-kinase-Akt pathway. In contrast, short hairpin RNA-mediated silencing of endogenous CTMP decreased the proliferation of estrogen-dependent or estrogen-independent breast cancer cells. Mechanistic investigations defined the N-terminal domain of CTMP at amino acids 1 to 64 as responsible for Akt binding. Taken together, our results firmly corroborate the concept that CTMP promotes Akt phosphorylation and functions as an oncogenic molecule in breast cancer.

Synergy of homocysteine, microRNA, and epigenetics: a novel therapeutic approach for stroke

Homocysteine (Hcy) is a thiol-containing amino acid formed during methionine metabolism. Elevated level of Hcy is known as hyperhomocysteinemia (HHcy). HHcy is an independent risk factor for cerebrovascular diseases such as stroke, dementia, Alzheimer's disease, etc. Stroke, which is caused by interruption of blood supply to the brain, is one of the leading causes of death and disability in a number of people worldwide. The HHcy causes an increased carotid artery plaque that may lead to ischemic stroke but the mechanism is currently not well understood. Though mutations or polymorphisms in the key genes of Hcy metabolism pathway have been well elucidated in stroke, emerging evidences suggested epigenetic mechanisms equally play an important role in stroke development such as DNA methylation, chromatin remodeling, RNA editing, noncoding RNAs (ncRNAs), and microRNAs (miRNAs). However, there is no review available yet that describes the role of genetics and epigenetics during HHcy in stroke. The current review highlights the role of genetics and epigenetics in stroke during HHcy and the role of epigenetics in its therapeutics. The review also highlights possible epigenetic mechanisms, potential therapeutic molecules, putative challenges, and approaches to deal with stroke during HHcy.

A truncated fragment of Src protein kinase generated by calpain-mediated cleavage is a mediator of neuronal death in excitotoxicity

Excitotoxicity resulting from overstimulation of glutamate receptors is a major cause of neuronal death in cerebral ischemic stroke. The overstimulated ionotropic glutamate receptors exert their neurotoxic effects in part by overactivation of calpains, which induce neuronal death by catalyzing limited proteolysis of specific cellular proteins. Here, we report that in cultured cortical neurons and in vivo in a rat model of focal ischemic stroke, the tyrosine kinase Src is cleaved by calpains at a site in the N-terminal unique domain. This generates a truncated Src fragment of ~52 kDa, which we localized predominantly to the cytosol. A cell membrane-permeable fusion peptide derived from the unique domain of Src prevents calpain from cleaving Src in neurons and protects against excitotoxic neuronal death. To explore the role of the truncated Src fragment in neuronal death, we expressed a recombinant truncated Src fragment in cultured neurons and examined how it affects neuronal survival. Expression of this fragment, which lacks the myristoylation motif and unique domain, was sufficient to induce neuronal death. Furthermore, inactivation of the prosurvival kinase Akt is a key step in its neurotoxic signaling pathway. Because Src maintains neuronal survival, our results implicate calpain cleavage as a molecular switch converting Src from a promoter of cell survival to a mediator of neuronal death in excitotoxicity. Besides unveiling a new pathological action of Src, our discovery of the neurotoxic action of the truncated Src fragment suggests new therapeutic strategies with the potential to minimize brain damage in ischemic stroke.

PHLPP1 gene deletion protects the brain from ischemic injury

A recently discovered protein phosphatase PHLPP (PH domain Leucine-rich repeat Protein Phosphatase) has been shown to dephosphorylate Akt on its hydrophobic motif (Ser473) thereby decreasing Akt kinase activity. We generated PHLPP1 knockout (KO) mice and used them to explore the ability of enhanced in vivo Akt signaling to protect the brain against ischemic insult. Brains from KO mice subjected to middle cerebral artery occlusion (MCAO) for 2 hours showed significantly greater increases in Akt activity and less neurovascular damage after reperfusion than wild-type (WT) mice. Remarkably, infarct volume in the PHLPP1 KO was significantly reduced compared with WT (12.7±2.7% versus 22.9±3.1%) and this was prevented by Akt inhibition. Astrocytes from KO mice and neurons in which PHLPP1 was downregulated showed enhanced Akt activation and diminished cell death in response to oxygen-glucose deprivation. Thus, deletion of PHLPP1 can enhance Akt activation in neurons and astrocytes, and can significantly increase cell survival and diminish infarct size after MCAO. Inhibition of PHLPP could be a therapeutic approach to minimize damage after focal ischemia.

Emerging molecular targets for brain repair after stroke

The field of neuroprotection generated consistent preclinical findings of mechanisms of cell death but these failed to be translated into clinics. The approaches that combine the modulation of the inhibitory environment together with the promotion of intrinsic axonal outgrowth needs further work before combined therapeutic strategies will be transferable to clinic trials. It is likely that only when some answers have been found to these issues will our therapeutic efforts meet our expectations. Stroke is a clinically heterogeneous disease and combinatorial treatments require much greater work in pharmacological and toxicological testing. Advances in genetics and results of the Whole Human Genome Project (HGP) provided new unknown information in relation to stroke. Genetic factors are not the only determinants of responses to some diseases. It was recognized early on that "epigenetic" factors were major players in the aetiology and progression of many diseases like stroke. The major players are microRNAs that represent the best-characterized subclass of noncoding RNAs. Epigenetic mechanisms convert environmental conditions and physiological stresses into long-term changes in gene expression and translation. Epigenetics in stroke are in their infancy but offer great promise for better understanding of stroke pathology and the potential viability of new strategies for its treatment.

New players on the metabolic stage: How do you like Them Acots?

Members of the acyl-CoA thioesterase (Acot) gene family catalyze the hydrolysis of fatty acyl-CoA thioesters. Thioesterase superfamily member (Them) 1 (synonym: Acot11) is enriched in brown adipose tissue and is markedly upregulated when mice are exposed to cold ambient temperatures. In a recent study, we demonstrated that Them1^−/− mice exhibit increased energy expenditure and are resistant to diet-induced obesity and its metabolic consequences. This mini-review places these findings in the context of an emerging understanding of Them/Acot genes.

Neural stem cell grafting counteracts hippocampal injury-mediated impairments in mood, memory, and neurogenesis

The hippocampus is vital for functions such as mood and memory. Hippocampal injury typically leads to mood and memory impairments associated with reduced and aberrant neurogenesis in the dentate gyrus. We examined whether neural stem cell (NSC) grafting after hippocampal injury would counteract impairments in mood, memory, and neurogenesis. We expanded NSCs from the anterior subventricular zone (SVZ) of postnatal F344 rat pups expressing the human placental alkaline phosphatase and grafted them into the hippocampus of young adult F344 rats at 5 days after an injury inflicted through a unilateral intracerebroventricular administration of kainic acid. Analyses through forced swim, water maze, and novel object recognition tests revealed significant impairments in mood and memory function in animals that underwent injury and sham-grafting surgery. In contrast, animals that received SVZ-NSC grafts after injury exhibited mood and memory function comparable to those of naïve control animals. Graft-derived cells exhibited excellent survival and pervasive migration, and they differentiated into neurons, subtypes of inhibitory GABAergic interneurons, astrocytes, oligodendrocytes, and oligodendrocyte progenitors. Significant fractions of graft-derived cells also expressed beneficial neurotrophic factors such as the glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, fibroblast growth factor, and vascular endothelial growth factor. Furthermore, SVZ-NSC grafting counteracted the injury-induced reductions and abnormalities in neurogenesis by both maintaining a normal level of NSC activity in the subgranular zone and providing protection to reelin+ interneurons in the dentate gyrus. These results underscore that early SVZ-NSC grafting intervention after hippocampal injury is efficacious for thwarting mood and memory dysfunction and abnormal neurogenesis.

Effects of global ischemia and estradiol pretreatment on phosphorylation of Akt, CREB and STAT3 in hippocampal CA1 of young and middle-aged female rats

Transient global ischemia induces selective, delayed neuronal death of pyramidal neurons in the hippocampal CA1. Whereas long term treatment of middle-aged female rats with estradiol at physiological doses ameliorates neuronal death, the signaling pathways that mediate the neuroprotection are, as yet, unknown. Protein kinase B (Akt) and downstream transcription factors, the cAMP response element binding protein (CREB) and signal transducer and activator of transcription (STAT3) are critical players in cellular survival following injury. The present study was undertaken to determine whether long term estradiol alters the phosphorylation status and activity of Akt, STAT3 and CREB in ovariohysterectomized, middle-aged and young female rats subjected to global ischemia. Irrespective of either hormone or ischemic condition, middle-aged females exhibited lower levels of p-CREB and higher levels of Akt and STAT3 in CA1 than young females, as assessed by Western blot. In middle-aged animals, ischemia increased the phosphorylation status/activity of Akt and STAT3, and decreased the phosphorylation status/activity of CREB in the hippocampal CA1. Whereas estradiol did not detectably alter the phosphorylation status/activity of Akt or STAT3, it prevented the ischemia-induced decrease in nuclear p-CREB. Similar results were observed for the young females. Collectively, these data demonstrate that CREB, STAT3, and Akt are involved in the molecular response to global ischemia and that age influences the status of CREB, STAT3 and Akt activity in CA1 under physiological as well as pathological conditions, further emphasizing the importance of including older rodents in neuroprotection studies.

Bidirectional control of mRNA translation and synaptic plasticity by the cytoplasmic polyadenylation complex

Translational control of mRNAs in dendrites is essential for certain forms of synaptic plasticity and learning and memory. CPEB is an RNA-binding protein that regulates local translation in dendrites. Here, we identify poly(A) polymerase Gld2, deadenylase PARN, and translation inhibitory factor neuroguidin (Ngd) as components of a dendritic CPEB-associated polyadenylation apparatus. Synaptic stimulation induces phosphorylation of CPEB, PARN expulsion from the ribonucleoprotein complex, and polyadenylation in dendrites. A screen for mRNAs whose polyadenylation is altered by Gld2 depletion identified >100 transcripts including one encoding NR2A, an NMDA receptor subunit. shRNA depletion studies demonstrate that Gld2 promotes and Ngd inhibits dendritic NR2A expression. Finally, shRNA-mediated depletion of Gld2 in vivo attenuates protein synthesis-dependent long-term potentiation (LTP) at hippocampal dentate gyrus synapses; conversely, Ngd depletion enhances LTP. These results identify a pivotal role for polyadenylation and the opposing effects of Gld2 and Ngd in hippocampal synaptic plasticity.

Small molecule-induced cytosolic activation of protein kinase Akt rescues ischemia-elicited neuronal death

Elevating Akt activation is an obvious clinical strategy to prevent progressive neuronal death in neurological diseases. However, this endeavor has been hindered because of the lack of specific Akt activators. Here, from a cell-based high-throughput chemical genetic screening, we identified a small molecule SC79 that inhibits Akt membrane translocation, but paradoxically activates Akt in the cytosol. SC79 specifically binds to the PH domain of Akt. SC79-bound Akt adopts a conformation favorable for phosphorylation by upstream protein kinases. In a hippocampal neuronal culture system and a mouse model for ischemic stroke, the cytosolic activation of Akt by SC79 is sufficient to recapitulate the primary cellular function of Akt signaling, resulting in augmented neuronal survival. Thus, SC79 is a unique specific Akt activator that may be used to enhance Akt activity in various physiological and pathological conditions.

Lithium treatment reduces brain injury induced by focal ischemia with partial reperfusion and the protective mechanisms dispute the importance of akt activity

Lithium is a mood stabilizer shown to have neuroprotective effects against several chronic and acute neuronal injuries, including stroke. However, it is unknown whether lithium treatment protects against brain injury post-stroke in a rat model of permanent distal middle cerebral artery occlusion (MCAo) combined with transient bilateral common carotid artery occlusion (CCAo), a model that mimics human stroke with partial reperfusion. In addition, whether lithium treatment alters Akt activity as measured by the kinase activity assay has not been reported, although it is known to inhibit GSK3β activity. After stroke, Akt activity contributes to neuronal survival while GSK3β activity causes neuronal death. We report that a bolus of lithium injection at stroke onset robustly reduced infarct size measured by 2,3,5-triphenyltetrazolium chloride (TTC) staining at 48 h post-stroke and inhibited cell death in the ischemic penumbra, but not in the ischemic core, as shown by TUNEL staining performed 24 h post-stroke. However, lithium treatment did not alter the reduction in Akt activity as measured by Akt kinase assay. We further showed that lithium did not alter phosphorylated GSK3β protein levels, or the degradation of β-catenin, a substrate of GSK3β, which is consistent with previous findings that long-term treatment is required for lithium to alter GSK3β phosphorylation. In summary, we show innovative data that lithium protects against stroke in a focal ischemia model with partial reperfusion, however, our results dispute the importance of Akt activity in the protective effects of lithium.

Ras family small GTPase-mediated neuroprotective signaling in stroke

Selective neuronal cell death is one of the major causes of neuronal damage following stroke, and cerebral cells naturally mobilize diverse survival signaling pathways to protect against ischemia. Importantly, therapeutic strategies designed to improve endogenous anti-apoptotic signaling appear to hold great promise in stroke treatment. While a variety of complex mechanisms have been implicated in the pathogenesis of stroke, the overall mechanisms governing the balance between cell survival and death are not well-defined. Ras family small GTPases are activated following ischemic insults, and in turn, serve as intrinsic switches to regulate neuronal survival and regeneration. Their ability to integrate diverse intracellular signal transduction pathways makes them critical regulators and potential therapeutic targets for neuronal recovery after stroke. This article highlights the contribution of Ras family GTPases to neuroprotective signaling cascades, including mitogen-activated protein kinase (MAPK) family protein kinase- and AKT/PKB-dependent signaling pathways as well as the regulation of cAMP response element binding (CREB), Forkhead box O (FoxO) and hypoxiainducible factor 1(HIF1) transcription factors, in stroke.

Repressor element-1 silencing transcription factor (REST)-dependent epigenetic remodeling is critical to ischemia-induced neuronal death

Dysregulation of the transcriptional repressor element-1 silencing transcription factor (REST)/neuron-restrictive silencer factor is important in a broad range of diseases, including cancer, diabetes, and heart disease. The role of REST-dependent epigenetic modifications in neurodegeneration is less clear. Here, we show that neuronal insults trigger activation of REST and CoREST in a clinically relevant model of ischemic stroke and that REST binds a subset of "transcriptionally responsive" genes (gria2, grin1, chrnb2, nefh, nfκb2, trpv1, chrm4, and syt6), of which the AMPA receptor subunit GluA2 is a top hit. Genes with enriched REST exhibited decreased mRNA and protein. We further show that REST assembles with CoREST, mSin3A, histone deacetylases 1 and 2, histone methyl-transferase G9a, and methyl CpG binding protein 2 at the promoters of target genes, where it orchestrates epigenetic remodeling and gene silencing. RNAi-mediated depletion of REST or administration of dominant-negative REST delivered directly into the hippocampus in vivo prevents epigenetic modifications, restores gene expression, and rescues hippocampal neurons. These findings document a causal role for REST-dependent epigenetic remodeling in the neurodegeneration associated with ischemic stroke and identify unique therapeutic targets for the amelioration of hippocampal injury and cognitive deficits.

N-terminally cleaved Bcl-xL mediates ischemia-induced neuronal death

Transient global ischemia in rats induces delayed death of hippocampal CA1 neurons. Early events include caspase activation, cleavage of anti-death Bcl-2 family proteins and large mitochondrial channel activity. However, whether these events have a causal role in ischemia-induced neuronal death is unclear. We found that the Bcl-2 and Bcl-x(L) inhibitor ABT-737, which enhances death of tumor cells, protected rats against neuronal death in a clinically relevant model of brain ischemia. Bcl-x(L) is prominently expressed in adult neurons and can be cleaved by caspases to generate a pro-death fragment, ΔN-Bcl-x(L). We found that ABT-737 administered before or after ischemia inhibited ΔN-Bcl-x(L)-induced mitochondrial channel activity and neuronal death. To establish a causal role for ΔN-Bcl-x(L), we generated knock-in mice expressing a caspase-resistant form of Bcl-x(L). The knock-in mice exhibited markedly reduced mitochondrial channel activity and reduced vulnerability to ischemia-induced neuronal death. These findings suggest that truncated Bcl-x(L) could be a potentially important therapeutic target in ischemic brain injury.

Ischemic tolerance in the brain: endogenous adaptive machinery against ischemic stress

Although more than 100 drugs have been examined clinically, tissue plasminogen activator remains the only drug approved for the treatment of acute ischemic stroke. Since the discovery of ischemic tolerance, it has been widely recognized that the brain possesses an endogenous protective machinery to protect against ischemic stress. Recent studies have clarified that both the upregulation of neuroprotective signaling and the downregulation of inflammatory or apoptotic pathways are involved equally in the acquisition of ischemic tolerance. The triggering stimuli for ischemic stresses are divided into hypoxic, oxidant/inflammatory, and glutamate stress. Glutamate stress, particularly the synaptic stimulation of the N-methyl-D-aspartate receptor, leads to activation of the cAMP response element-binding protein, which could subsequently induce gene expression of several neuroprotective molecules. Gene reprogramming and metabolic downregulation are intimately involved in ischemic tolerance as well as in hibernation and hypothermia. Micro-RNAs may be a key player for tuning the level of gene expression in ischemic tolerance. Future research should be performed to investigate the most effective combination for brain protection, enhancement of cell survival signaling, and inhibition of the inflammatory or apoptotic pathways.

Protracted Tyrosine Phosphorylation of the Glutamate Receptor Subunit NR2 in the Rat Hippocampus Following Transient Cerebral Ischemia is Prevented by Intra-Ischemic Hypothermia

Changes in the dynamic interactions of macromolecules in cell membranes appear to underlie the robust neuroprotective effect of hypothermia against selective neuronal degeneration in the CA1 region of the rat hippocampus after transient cerebral ischemia, but the detailed mechanisms are still elusive. Using the two-vessel occlusion model of transient normothermic cerebral ischemia of 15 min duration, we investigated the tyrosine phosphorylation of synaptic proteins in general and that of the NMDA receptor subunits in particular, at different times of recirculation. Specifically, the effect of intra-ischemic hypothermia (33°C), which provides neuroprotection to the CA1 region of the hippocampus, was studied. Phosphorylation of tyrosine residues on the NMDA receptor (NR) 2, but not of the NR1 or the AMPA receptor subunit 1 (GluR1) proteins, was markedly enhanced following cerebral ischemia. Protein tyrosine phosphorylation was persistently increased in the postsynaptic densities of the vulnerable CA1 region, but was transient in the CA3/dentate gyrus (DG) neurons where cell death was not evident. The phospho-tyrosine phosphatase activity decreased during reperfusion in the CA1 region but not in CA3/DG. Importantly, decreasing body temperature to 33°C during ischemia modified the dynamics of the protein tyrosine phosphorylation of NR2 in the CA1 region, which was transient and similar in time course to that seen in the CA3/DG region after normothermic ischemia. We conclude that the protracted tyrosine phosphorylation of the NR2 subunit in the hippocampus CA1 region following normothermic ischemia is attenuated by hypothermia and therefore constitutes an important target for hypothermic neuroprotection.

D1 and D2 dopamine receptors differentially mediate the activation of phosphoproteins in the striatum of amphetamine-sensitized rats

RATIONALE

Extracellular signal-regulated kinase (ERK), cAMP response element binding protein (CREB), and protein kinase B (PKB or Akt) in the striatum are differentially activated by acute and repeated amphetamine (AMPH) administration. However, the dopamine receptor subtypes that mediate transient vs. prolonged phosphorylation changes in these proteins induced by AMPH challenge in AMPH-sensitized rats are unknown.

OBJECTIVES

The role of the D1 and D2 class of dopamine receptors in the differential phosphorylation of striatal ERK, CREB, Thr308-Akt and Ser473-Akt and the expression of behavioral sensitization induced by AMPH challenge in AMPH-pretreated rats were determined.

METHODS

D1 or D2 dopamine receptor antagonists were injected before an AMPH challenge in AMPH-sensitized rats. After behavioral activity was recorded, rats were euthanized either 15 min or 2 h after AMPH challenge and striatal phosphoprotein status was analyzed by Western blotting.

RESULTS

The D1 receptor antagonist (SCH23390) decreased stereotypical behavior whereas the D2 receptor antagonist (eticlopride) decreased all behavioral activity induced by an AMPH challenge in AMPH-sensitized rats. SCH23390, but not eticlopride, significantly decreased ERK, CREB, and Thr308-Akt phosphorylation in the striatum 15 min, and ERK and CREB phosphorylation 2 h, after AMPH challenge in AMPH-sensitized rats. In contrast, eticlopride, but not SCH23390, prevented a decrease in Akt phosphorylation 2 h after AMPH challenge.

CONCLUSIONS

These data indicate that the time course of phosphoprotein signaling is differentially regulated by D1 and D2 receptors in the striatum of AMPH-sensitized rats, suggesting that complex regulatory interactions are activated by repeated AMPH exposure.

PTEN gene silencing prevents HIV-1 gp120(IIIB)-induced degeneration of striatal neurons

To assess the role of the phosphatase and tensin homologue on chromosome 10 (PTEN) in mediating envelope glycoprotein 120 (gp120)-induced neurotoxicity in the striatum, PTEN was silenced using short interfering RNA (siRNA) vectors. PTEN activity directs multiple downstream pathways implicated in gp120-induced neuronal injury and death. PTEN is a negative regulator of Akt (protein kinase B) phosphorylation, but has also been shown to directly activate extrasynaptic NMDA receptors and dephosphorylate focal adhesion kinase. Rodent striatal neurons were nucleofected with green fluorescent protein (GFP)-expressing siRNA constructs to silence PTEN (PTENsi-GFP) or with negative-control (NCsi-GFP) vectors, and exposed to HIV-1 gp120(IIIB) using rigorously controlled, cell culture conditions including computerized time-lapse microscopy to track the fate of individual neurons following gp120 exposure. Immunofluorescence labeling showed that subpopulations of striatal neurons possess CXCR4 and CCR5 co-receptor immunoreactivity and that gp120(IIIB) was intrinsically neurotoxic to isolated striatal neurons. Importantly, PTENsi-GFP, but not control NCsi-GFP, constructs markedly decreased PTEN mRNA and protein levels and significantly attenuated gp120-induced death. These findings implicate PTEN as a critical factor in mediating the direct neurotoxic effects of HIV-1 gp120, and suggest that effectors downstream of PTEN such as Akt or other targets are potentially affected. The selective abatement of PTEN activity in neurons may represent a potential therapeutic strategy for the CNS complications of HIV-1.

The emerging role of epigenetics in stroke: II. RNA regulatory circuitry

Recent scientific advances have demonstrated the existence of extensive RNA-based regulatory networks involved in orchestrating nearly every cellular process in health and various disease states. This previously hidden layer of functional RNAs is derived largely from non-protein-coding DNA sequences that constitute more than 98% of the genome in humans. These non-protein-coding RNAs (ncRNAs) include subclasses that are well known, such as transfer RNAs and ribosomal RNAs, as well as those that have more recently been characterized, such as microRNAs, small nucleolar RNAs, and long ncRNAs. In this review, we examine the role of these novel ncRNAs in the nervous system and highlight emerging evidence that implicates RNA-based networks in the molecular pathogenesis of stroke. We also describe RNA editing, a related epigenetic mechanism that is partly responsible for generating the exquisite degrees of environmental responsiveness and molecular diversity that characterize ncRNAs. In addition, we discuss the development of future therapeutic strategies for locus-specific and genome-wide regulation of genes and functional gene networks through the modulation of RNA transcription, posttranscriptional RNA processing (eg, RNA modifications, quality control, intracellular trafficking, and local and long-distance intercellular transport), and RNA translation. These novel approaches for neural cell- and tissue-selective reprogramming of epigenetic regulatory mechanisms are likely to promote more effective neuroprotective and neural regenerative responses for safeguarding and even restoring central nervous system function.

Role of salt-induced kinase 1 in androgen neuroprotection against cerebral ischemia

Androgens within physiological ranges protect castrated male mice from cerebral ischemic injury. Yet, underlying mechanisms are unclear. Here, we report that, after middle cerebral artery occlusion (MCAO), salt-induced kinase 1 (SIK1) was induced by a potent androgen-dihydrotestosterone (DHT) at protective doses. To investigate whether SIK1 contributes to DHT neuroprotection after cerebral ischemia, we constructed lentivirus-expressing small interference RNA (siRNA) against SIK1. The SIK1 knockdown by siRNA exacerbated oxygen-glucose deprivation (OGD)-induced cell death in primary cortical neurons, suggesting that SIK1 is an endogenous neuroprotective gene against cerebral ischemia. Furthermore, lentivirus-mediated SIK1 knockdown increased both cortical and striatal infarct sizes in castrated mice treated with a protective dose of DHT. Earlier studies show that SIK1 inhibits histone deacetylase (HDAC) activities by acting as a class IIa HDAC kinase. We observed that SIK1 knockdown decreased histone H3 acetylation in primary neurons. The SIK1 siRNA also exacerbated OGD-induced neuronal death in the presence of trichostatin A (TSA), an HDAC inhibitor, and decreased histone H3 acetylation at 4 hours reoxygenation in TSA-treated neurons. Finally, we showed that DHT at protective doses prevented ischemia-induced histone deacetylation after MCAO. Our finding suggests that SIK1 contributes to neuroprotection by androgens within physiological ranges by inhibiting histone deacetylation.

PTEN deletion prevents ischemic brain injury by activating the mTOR signaling pathway

It is increasingly clear that the tumor suppressor PTEN (phosphatase and tensin homolog deleted on chromosome 10) is a negative regulator of neuronal cell survival. However, its molecular mechanisms remain poorly understood. Here we found that PTEN/mTOR is critical for controlling neuronal cell death after ischemic brain injury. Male rats were subjected to MCAO (middle cerebral artery occlusion) followed by pretreating with bpv (pic), a potent inhibitor for PTEN, or by intra-cerebroventricular infusion of PTEN siRNA. bpv (pic) significantly decreased infarct volume and reduced the number of TUNEL-positive cells. We further demonstrated that although bpv (pic) did not affect brain injury-induced mTOR protein expression, bpv (pic) prevented decrease in phosphorylation of mTOR, and the subsequent decrease in S6. Similarly, down-regulation of PTEN expression also reduced the number of TUNEL-positive cells, and increased phospho-mTOR. These data suggest that PTEN deletion prevents neuronal cell death resulting from ischemic brain injury and that its neuroprotective effects are mediated by increasing the injury-induced mTOR phosphorylation.

Acute estradiol protects CA1 neurons from ischemia-induced apoptotic cell death via the PI3K/Akt pathway

Global ischemia arising during cardiac arrest or cardiac surgery causes highly selective, delayed death of hippocampal CA1 neurons. Exogenous estradiol ameliorates global ischemia-induced neuronal death and cognitive impairment in male and female rodents. However, the molecular mechanisms by which a single acute injection of estradiol administered after the ischemic event intervenes in global ischemia-induced apoptotic cell death are unclear. Here we show that acute estradiol acts via the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) signaling cascade to protect CA1 neurons in ovariectomized female rats. We demonstrate that global ischemia promotes early activation of glycogen synthase kinase-3beta (GSK3beta) and forkhead transcription factor of the O class (FOXO)3A, known Akt targets that are related to cell survival, and activation of caspase-3. Estradiol prevents ischemia-induced dephosphorylation and activation of GSK3beta and FOXO3A, and the caspase death cascade. These findings support a model whereby estradiol acts by activation of PI3K/Akt signaling to promote neuronal survival in the face of global ischemia.

Circulating insulin-like growth factor I and cognitive function: neuromodulation throughout the lifespan

Insulin-like growth factor I (IGF-I) is central to the somatotropic (growth hormone) axis. It promotes tissue growth and continues to have anabolic effects in adulthood. Accumulating evidence from the last decade, however, reveals that circulating levels of IGF-I also significantly affects cognitive brain function. Specifically, the decline of serum IGF-I might be associated with the age-related cognitive decline in elderly people. Moreover, psychiatric and neurological conditions characterized by cognitive impairment may be characterized by altered levels of IGF-I. Some evidence is emerging that interventions that target the GH/IGF-I axis may improve cognitive functioning, at least in deficient states. As there is evidence linking high serum IGF-I levels with cancer risk, these interventions should be carefully evaluated. On a cellular and molecular level, IGF-I may be a crucial component of neural homeostasis since disturbed IGF-I input is inevitably linked to perturbed function. Consistent with this, all nerve cells are potential targets of IGF-I actions, including neurons, glia, endothelial, epithelial, and perivascular cells. Indeed, many key cellular processes in the brain are affected by IGF-I's neurotrophic and modulatory actions. We review the regulation by IGF-I of neurotransmission and neuronal plasticity and conclude that serum IGF-I is an important mediator of neuronal growth, survival and function throughout the lifespan. The role of IGF-I in synaptic plasticity render its neurotrophic potential a key target for remediating the cognitive impairment associated with a range of neurological conditions.

Heat shock protein 70-mediated sensitization of cells to apoptosis by Carboxyl-Terminal Modulator Protein

BACKGROUND

The serine/threonine protein kinase B (PKB/Akt) is involved in insulin signaling, cellular survival, and transformation. Carboxyl-terminal modulator protein (CTMP) has been identified as a novel PKB binding partner in a yeast two-hybrid screen, and appears to be a negative PKB regulator with tumor suppressor-like properties. In the present study we investigate novel mechanisms by which CTMP plays a role in apoptosis process.

RESULTS

CTMP is localized to mitochondria. Furthermore, CTMP becomes phosphorylated following the treatment of cells with pervanadate, an insulin-mimetic. Two serine residues (Ser37 and Ser38) were identified as novel in vivo phosphorylation sites of CTMP. Association of CTMP and heat shock protein 70 (Hsp70) inhibits the formation of complexes containing apoptotic protease activating factor 1 and Hsp70. Overexpression of CTMP increased the sensitivity of cells to apoptosis, most likely due to the inhibition of Hsp70 function.

CONCLUSION

Our data suggest that phosphorylation on Ser37/Ser38 of CTMP is important for the prevention of mitochondrial localization of CTMP, eventually leading to cell death by binding to Hsp70. In addition to its role in PKB inhibition, CTMP may therefore play a key role in mitochondria-mediated apoptosis by localizing to mitochondria.