Calpain-induced proteolysis after transient global cerebral ischemia and ischemic tolerance in a rat model

In vitro ischemia triggers a transcriptional response to down-regulate synaptic proteins in hippocampal neurons

Transient global cerebral ischemia induces profound changes in the transcriptome of brain cells, which is partially associated with the induction or repression of genes that influence the ischemic response. However, the mechanisms responsible for the selective vulnerability of hippocampal neurons to global ischemia remain to be clarified. To identify molecular changes elicited by ischemic insults, we subjected hippocampal primary cultures to oxygen-glucose deprivation (OGD), an in vitro model for global ischemia that resulted in delayed neuronal death with an excitotoxic component. To investigate changes in the transcriptome of hippocampal neurons submitted to OGD, total RNA was extracted at early (7 h) and delayed (24 h) time points after OGD and used in a whole-genome RNA microarray. We observed that at 7 h after OGD there was a general repression of genes, whereas at 24 h there was a general induction of gene expression. Genes related with functions such as transcription and RNA biosynthesis were highly regulated at both periods of incubation after OGD, confirming that the response to ischemia is a dynamic and coordinated process. Our analysis showed that genes for synaptic proteins, such as those encoding for PICK1, GRIP1, TARPγ3, calsyntenin-2/3, SAPAP2 and SNAP-25, were down-regulated after OGD. Additionally, OGD decreased the mRNA and protein expression levels of the GluA1 AMPA receptor subunit as well as the GluN2A and GluN2B subunits of NMDA receptors, but increased the mRNA expression of the GluN3A subunit, thus altering the composition of ionotropic glutamate receptors in hippocampal neurons. Together, our results present the expression profile elicited by in vitro ischemia in hippocampal neurons, and indicate that OGD activates a transcriptional program leading to down-regulation in the expression of genes coding for synaptic proteins, suggesting that the synaptic proteome may change after ischemia.

Excitotoxicity and stroke: identifying novel targets for neuroprotection

Excitotoxicity, the specific type of neurotoxicity mediated by glutamate, may be the missing link between ischemia and neuronal death, and intervening the mechanistic steps that lead to excitotoxicity can prevent stroke damage. Interest in excitotoxicity began fifty years ago when monosodium glutamate was found to be neurotoxic. Evidence soon demonstrated that glutamate is not only the primary excitatory neurotransmitter in the adult brain, but also a critical transmitter for signaling neurons to degenerate following stroke. The finding led to a number of clinical trials that tested inhibitors of excitotoxicity in stroke patients. Glutamate exerts its function in large by activating the calcium-permeable ionotropic NMDA receptor (NMDAR), and different subpopulations of the NMDAR may generate different functional outputs, depending on the signaling proteins directly bound or indirectly coupled to its large cytoplasmic tail. Synaptic activity activates the GluN2A subunit-containing NMDAR, leading to activation of the pro-survival signaling proteins Akt, ERK, and CREB. During a brief episode of ischemia, the extracellular glutamate concentration rises abruptly, and stimulation of the GluN2B-containing NMDAR in the extrasynaptic sites triggers excitotoxic neuronal death via PTEN, cdk5, and DAPK1, which are directly bound to the NMDAR, nNOS, which is indirectly coupled to the NMDAR via PSD95, and calpain, p25, STEP, p38, JNK, and SREBP1, which are further downstream. This review aims to provide a comprehensive summary of the literature on excitotoxicity and our perspectives on how the new generation of excitotoxicity inhibitors may succeed despite the failure of the previous generation of drugs.

The translational repressor eIF4E-binding protein 2 (4E-BP2) correlates with selective delayed neuronal death after ischemia

Transient brain ischemia induces an inhibition of translational rates and causes delayed neuronal death in selective regions and cognitive deficits, whereas these effects do not occur in resistant areas. The translational repressor eukaryotic initiation factor (eIF) 4E-binding protein-2 (4E-BP2) specifically binds to eIF4E and is critical in the control of protein synthesis. To link neuronal death to translation inhibition, we study the eIF4E association with 4E-BP2 under ischemia reperfusion in a rat model of transient forebrain ischemia. Upon reperfusion, a selective neuronal apoptosis in the hippocampal cornu ammonis 1 (CA1) region was induced, while it did not occur in the cerebral cortex. Confocal microscopy analysis showed a decrease in 4E-BP2/eIF4E colocalization in resistant cortical neurons after reperfusion. In contrast, in vulnerable CA1 neurons, 4E-BP2 remains associated to eIF4E with a higher degree of 4E-BP2/eIF4E colocalization and translation inhibition. Furthermore, the binding of a 4E-BP2 peptide to eIF4E induced neuronal apoptosis in the CA1 region. Finally, pharmacological-induced protection of CA1 neurons inhibited neuronal apoptosis, decreased 4E-BP2/eIF4E association, and recovered translation. These findings documented specific changes in 4E-BP2/eIF4E association during ischemic reperfusion, linking the translation inhibition to selective neuronal death, and identifying 4E-BP2 as a novel target for protection of vulnerable neurons in ischemic injury.

Deregulation of HDAC1 by p25/Cdk5 in neurotoxicity

Aberrant cell-cycle activity and DNA damage are emerging as important pathological components in various neurodegenerative conditions. However, their underlying mechanisms are poorly understood. Here, we show that deregulation of histone deacetylase 1 (HDAC1) activity by p25/Cdk5 induces aberrant cell-cycle activity and double-strand DNA breaks leading to neurotoxicity. In a transgenic model for neurodegeneration, p25/Cdk5 activity elicited cell-cycle activity and double-strand DNA breaks that preceded neuronal death. Inhibition of HDAC1 activity by p25/Cdk5 was identified as an underlying mechanism for these events, and HDAC1 gain of function provided potent protection against DNA damage and neurotoxicity in cultured neurons and an in vivo model for ischemia. Our findings outline a pathological signaling pathway illustrating the importance of maintaining HDAC1 activity in the adult neuron. This pathway constitutes a molecular link between aberrant cell-cycle activity and DNA damage and is a potential target for therapeutics against diseases and conditions involving neuronal death.

Synthetic calpain activator boosts neuronal excitability without extra Ca2+

Earlier we have shown that an equimolar mixture of calpastatin subdomains A and C (19 amino acids each) strongly activates m-calpain in vitro. In the present work we developed a membrane-permeable activator system, by conjugating an oligo-arginine tail to both peptides. We tested calpain activation as well as synaptic excitability on rat brain slices ex vivo. In hippocampal slices both basic excitability and long-term synaptic efficacy were significantly increased upon treatment with the activator. We propose that the activator peptide conjugates can be used with any mammalian cell, to specifically challenge the calpain system apparently without raising cytoplasmic Ca2+. Such an effector may be a useful tool in dissecting intracellular mechanisms involving the calpain system.

Nitric oxide mediates NMDA-induced persistent inhibition of protein synthesis through dephosphorylation of eukaryotic initiation factor 4E-binding protein 1 and eukaryotic initiation factor 4G proteolysis

Cerebral ischaemia causes long-lasting protein synthesis inhibition that is believed to contribute to brain damage. Energy depletion promotes translation inhibition during ischaemia, and the phosphorylation of eIF (eukaryotic initiation factor) 2α is involved in the translation inhibition induced by early ischaemia/reperfusion. However, the molecular mechanisms underlying prolonged translation down-regulation remain elusive. NMDA (N-methyl-D-aspartate) excitotoxicity is also involved in ischaemic damage, as exposure to NMDA impairs translation and promotes the synthesis of NO (nitric oxide), which can also inhibit translation. In the present study, we investigated whether NO was involved in NMDA-induced protein synthesis inhibition in neurons and studied the underlying molecular mechanisms. NMDA and the NO donor DEA/NO (diethylamine–nitric oxide sodium complex) both inhibited protein synthesis and this effect persisted after a 30 min exposure. Treatments with NMDA or NO promoted calpain-dependent eIF4G cleavage and 4E-BP1 (eIF4E-binding protein 1) dephosphorylation and also abolished the formation of eIF4E–eIF4G complexes; however, they did not induce eIF2α phosphorylation. Although NOS (NO synthase) inhibitors did not prevent protein synthesis inhibition during 30 min of NMDA exposure, they did abrogate the persistent inhibition of translation observed after NMDA removal. NOS inhibitors also prevented NMDA-induced eIF4G degradation, 4E-BP1 dephosphorylation, decreased eIF4E–eIF4G-binding and cell death. Although the calpain inhibitor calpeptin blocked NMDA-induced eIF4G degradation, it did not prevent 4E-BP1 dephosphorylation, which precludes eIF4E availability, and thus translation inhibition was maintained. The present study suggests that eIF4G integrity and hyperphosphorylated 4E-BP1 are needed to ensure appropriate translation in neurons. In conclusion, our data show that NO mediates NMDA-induced persistent translation inhibition and suggest that deficient eIF4F activity contributes to this process.

Molecular mechanisms of apoptosis in cerebral ischemia: multiple neuroprotective opportunities

Cerebral ischemia/reperfusion (I/R) injury triggers multiple and distinct but overlapping cell signaling pathways, which may lead to cell survival or cell damage. There is overwhelming evidence to suggest that besides necrosis, apoptosis do contributes significantly to the cell death subsequent to I/R injury. Both extrinsic and intrinsic apoptotic pathways play a vital role, and upon initiation, these pathways recruit downstream apoptotic molecules to execute cell death. Caspases and Bcl-2 family members appear to be crucial in regulating multiple apoptotic cell death pathways initiated during I/R. Similarly, inhibitor of apoptosis family of proteins (IAPs), mitogen-activated protein kinases, and newly identified apoptogenic molecules, like second mitochondrial-activated factor/direct IAP-binding protein with low pI (Smac/Diablo), omi/high-temperature requirement serine protease A2 (Omi/HtrA2), X-linked mammalian inhibitor of apoptosis protein-associated factor 1, and apoptosis-inducing factor, have emerged as potent regulators of cellular apoptotic/antiapoptotic machinery. All instances of cell survival/death mechanisms triggered during I/R are multifaceted and interlinked, which ultimately decide the fate of brain cells. Moreover, apoptotic cross-talk between major subcellular organelles suggests that therapeutic strategies should be optimally directed at multiple targets/mechanisms for better therapeutic outcome. Based on the current knowledge, this review briefly focuses I/R injury-induced multiple mechanisms of apoptosis, involving key apoptotic regulators and their emerging roles in orchestrating cell death programme. In addition, we have also highlighted the role of autophagy in modulating cell survival/death during cerebral ischemia. Furthermore, an attempt has been made to provide an encouraging outlook on emerging therapeutic approaches for cerebral ischemia.

Regulatory proteins of eukaryotic initiation factor 2-alpha subunit (eIF2 alpha) phosphatase, under ischemic reperfusion and tolerance

Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha), which is one of the substrates of protein phosphatase 1 (PP1), occurs rapidly during the first minutes of post-ischemic reperfusion after an episode of cerebral ischemia. In the present work, two experimental models of transient global ischemia and ischemic tolerance (IT) were used to study PP1 interacting/regulatory proteins following ischemic reperfusion. For that purpose we utilized PP1 purified by microcystin chromatography, as well as 2D DIGE of PP1alpha and PP1gamma immunoprecipitates. The highest levels of phosphorylated eIF2alpha found after 30 min reperfusion in rats without IT, correlated with increased levels in PP1 immunoprecipitates of the inhibitor DARPP32 as well as GRP78 and HSC70 proteins. After 4 h reperfusion, the levels of these proteins in PP1c complexes had returned to control values, in parallel to a significant decrease in eIF2alpha phosphorylated levels. IT that promoted a decrease in eIF2alpha phosphorylated levels after 30 min reperfusion induced the association of GADD34 with PP1c, while prevented that of DARPP32, GRP78, and HSC70. Different levels of HSC70 and DARPP32 associated with PP1alpha and PP1gamma isoforms, whereas GRP78 was only detected in PP1gamma immunoprecipitates. Here we suggest that PP1, through different signaling complexes with their interacting proteins, may modulate the eIF2alpha phosphorylation/dephosphorylation during reperfusion after a transient global ischemia in the rat brain. Of particular interest is the potential role of GADD34/PP1c complexes after tolerance acquisition.

Calpain cleavage of collapsin response mediator proteins in ischemic mouse brain

Collapsin response mediator proteins (CRMPs) are important brain-specific proteins with distinct functions in modulating growth cone collapse and axonal guidance during brain development. Our previous studies have shown that calpain cleaves CRMP3 in the adult mouse brain during cerebral ischemia [S.T. Hou et al. (2006) J. Neurosci., 26, 2241-2249]. Here, the expression of all CRMP family members (1-5) was examined in mouse brains that were subjected to middle cerebral artery occlusion. Among the five CRMPs, the expressions of CRMP1, CRMP3 and CRMP5 were the most abundant in the cerebral cortex and all CRMPs were targeted for cleavage by ischemia-activated calpain. Sub-cellular fractionation analysis showed that cleavage of CRMPs by calpain occurred not only in the cytoplasm but also in the synaptosomes isolated from ischemic brains. Moreover, synaptosomal CRMPs appeared to be at least one-fold more sensitive to cleavage compared with those isolated from the cytosolic fraction in an in-vitro experiment, suggesting that synaptosomal CRMPs are critical targets during cerebral ischemia-induced neuronal injury. Finally, the expression of all CRMPs was colocalized with TUNEL-positive neurons in the ischemic mouse brain, which further supports the notion that CRMPs may play an important role in neuronal death following cerebral ischemia. Collectively, these studies demonstrated that CRMPs are targets of calpains during cerebral ischemia and they also highlighted an important potential role that CRMPs may play in modulating ischemic neuronal death.