Ischemic depolarization monitoring: evaluation of protein synthesis in the hippocampal CA1 after brief unilateral ischemia in a gerbil model

Prostacyclin receptor deletion aggravates hippocampal neuronal loss after bilateral common carotid artery occlusion in mouse

Transient global cerebral ischemia causes delayed neuronal death in the hippocampal CA1 region. It also induces an increase in cyclooxygenase 2 (COX-2), which generates several metabolites of arachidonic acid, known as prostanoids, including prostacyclin (PGI(2)). To determine the role of the PGI(2) receptor (IP) in post-ischemic delayed cell death, wild-type and IP knockout (IP(-/-)) C57Bl/6 mice were subjected to 12-min bilateral common carotid artery occlusion or sham surgery, followed by 7 days of reperfusion. In the sham-operated mice, no statistical difference in CA1 hippocampal neuronal density was observed between the wild-type (2836+/-18/mm(2)) and IP(-/-) (2793+/-43/mm(2)) mice. Interestingly, in animals subjected to ischemia, surviving neuronal density in wild-type mice decreased to 50.5+/-7.9% and that of IP(-/-) mice decreased to 23.0+/-4.5% of their respective sham-operated controls (P<0.05). The results establish a role for the IP receptor in protecting pyramidal hippocampal neurons after this global ischemic model and suggest that IP receptor agonists could be developed to prevent delayed pyramidal neuronal cell death.

Optimized protocol to reduce variable outcomes for the bilateral common carotid artery occlusion model in mice

The pre-clinical global ischemia model transient bilateral common carotid artery occlusion addresses the unique cascade of events leading to delayed neuronal cell death. However, the inconsistent occurrence of posterior communicating arteries (PcomA) in mice might cause high outcome variability. To determine a means for reducing variability, CD1 mice were subjected to bilateral common carotid artery occlusion for 12-40 min. Occlusion duration> or =18 min was applied to mice with bilateral regional cerebral blood flow (rCBF)> or =10% of baseline at 2.5 min of ischemia. However, only groups with ischemic duration< or =18 min were used for statistical analysis because of the high mortality in the other groups. After 7 days, patency of PcomA and hippocampal neuronal loss in the CA1 subfield were evaluated. Outcome variability was reduced when hemispheres containing PcomA were excluded from analysis; ischemic outcome was not affected by the presence of a contralateral PcomA. Extending ischemic duration based on rCBF did not reduce outcome variability because the initial rCBF could not reliably predict PcomA. Therefore, after an optimal ischemic duration, evaluating hippocampal injury in each hemisphere independently according to the existence of PcomA is an effective and reliable method to obtain consistent results in this pre-clinical mouse model.

Blood pressure in the artery distal to an intraarterial embolus during thrombolytic therapy for occlusion of a major artery: a predictor of cerebral infarction following good recanalization

Object. The aim in this study was the investigation of back pressure in arteries distal to the occlusion site during intraarterial thrombolysis as well as the usefulness of back pressure measurement in combination with diffusion-weighted (DW) magnetic resonance (MR) imaging to predict the occurrence of ischemic lesions following good recanalization.

Methods. Twenty-five consecutive patients with severe hemiparesis caused by embolism of the internal carotid artery (10 patients) and the proximal middle cerebral artery (15 patients) were treated using intraarterial thrombolysis. Systolic back pressure, measured through a microcatheter in the artery just distal to the emboli, ranged from 22 to 78 mm Hg. According to an angiographic inclusion criterion for good recanalization—that is, recanalization of the M2 or more distal arteries at the end of thrombolysis—21 of 25 patients underwent evaluation in this study. In 14 patients volumes of low-density areas on computerized tomography (CT) scans obtained 2 months postthrombolysis were smaller in comparison with volumes of hyperintense areas on DW MR images acquired before treatment, whereas these low-density areas were larger in seven patients. Compared with those on initial DW MR images, the volume of abnormalities on CT scans obtained 2 months posttreatment were significantly reduced in patients with a systolic back pressure greater than 30 mm Hg (16 patients) than in those with a back pressure of 30 mm Hg or less (five patients) (p < 0.05). Systolic back pressures greater than 30 mm Hg were associated with significantly better modified Rankin Scale scores than those 30 mm Hg or less (p < 0.05).

Conclusions. Back pressure measurement in combination with DW MR imaging can be used to predict the occurrence of infarction as demonstrated on CT scans following thrombolysis.

Time course and cellular distribution of hsp27 and hsp72 stress protein expression in a quantitative gerbil model of ischemic injury and tolerance: thresholds for hsp72 induction and hilar lesioning in the context of ischemic preconditioning

The distribution and time course of expression of the heat shock/stress proteins, hsp27 and hsp72, were evaluated in a highly controlled gerbil model of ischemic injury and tolerance induction, in which the duration of ischemic depolarization in each hippocampus provides a precise quantitative index of insult severity. Gerbils were subjected to brief priming insults (2- to 3.5-minute depolarization) that produce optimal preconditioning, to severe test insults (6- to 8.5-minute depolarization) that produce complete CA1 neuron loss in naive animals, or to combined insults administered 1 week apart, after which almost complete tolerance to CA1 neuron injury is observed. Immunoreactivities of hsp27, hsp72, glial fibrillary acidic protein and microtubule-associated protein 2 (MAP2) were evaluated in animals perfused at defined intervals after the final insult in each treatment group, using a variation of established antigen-retrieval procedures that significantly improves detection of many proteins in vibratome brain sections. Hsp72 was detected in CA1 neurons of some hippocampi 2 to 4 days after preconditioning, but this was only seen after the longest priming depolarizations, whereas shorter insults that still induced optimal tolerance failed to induce hsp72. Hsp72 was induced after test insults in preconditioned hippocampi, but at a higher depolarization threshold than observed for naive animals. An astrocytic localization of hsp27 was observed in regions of neuron injury, as indicated by reduced MAP2 immunoreactivity, and was primarily restricted to dentate hilus after preconditioning insults. These results establish that limited hilar lesions are characteristic of optimal preconditioning, whereas prior neuronal expression of either hsp72 or hsp27 is not required for ischemic tolerance.

Induced hippocampal neuron protection in an optimized gerbil ischemia model: insult thresholds for tolerance induction and altered gene expression defined by ischemic depolarization

Preconditioning of hippocampal CA1 neurons was evaluated in a gerbil model of transient global ischemia using extracellular recording of DC potential shifts characteristic of ischemic depolarization to precisely define the duration of both priming and test insults. Brief ischemia resulting in depolarizations of 2.5 to 3.5 minutes consistently induced maximal tolerance (95% protection) against subsequent challenges 2 days later with an approximate doubling of the insult duration required for complete CA1 neuron loss from 6 to 12 minutes depolarization when evaluated 1 week after the test insult. Significant protection persisted at 2 months survival, although the apparent injury threshold regressed to approximately 8 minutes, indicating delayed progression of injury after longer test insults. In situ hybridization was used to evaluate depolarization thresholds for induction of mRNAs encoding the 70 kDa heat shock/stress protein, hsp72, as well as several immediate-early genes (c-fos, c-jun, junB, and junD). Immediate-early genes were prominently expressed after short insults inducing tolerance, whereas appreciable hsp72 induction only occurred after insults approaching the threshold for neuron injury. These results establish an ischemic preconditioning model with the predictability needed for mechanistic studies and demonstrate that prior transcriptional activation of the postischemic heat shock response is not required for expression of delayed tolerance.

Alternative programs of cell death in developing retinal tissue

We examined cell death in developing retinal tissue, following inhibition of protein synthesis, which kills undifferentiated post-mitotic cells. Ultrastructural features were found of both apoptosis and autophagy. Only approximately half of the degenerating cells were either terminal dUTP nick-end labeling (TUNEL)-positive or reacted with antibodies specific for activated caspases-3 or -9. Bongkrekic acid completely inhibited any appearance of cell death, whereas inhibitors of autophagy, caspases-9 or -3, prevented only TUNEL-positive cell death. Interestingly, inhibition of caspase-6 blocked TUNEL-negative cell death. Simultaneous inhibition of caspases-9 and -6 prevented cell death almost completely, but degeneration dependent on autophagy/caspase-9 still occurred under inhibition of both caspases-3 and -6. Thus, inhibition of protein synthesis induces in the developing retina various post-translational, mitochondria-dependent pathways of cell death. Autophagy precedes sequential activation of caspases-9 and -3, and DNA fragmentation, whereas, in parallel, caspase-6 leads to a TUNEL-negative form of cell death. Additional mechanisms of cell death may be engaged upon selective caspase inhibition.

Endoplasmic reticulum: a primary target in various acute disorders and degenerative diseases of the brain

Changes in neuronal calcium activity in the various subcellular compartments have divergent effects on affected cells. In the cytoplasm and mitochondria, where calcium activity is normally low, a prolonged excessive rise in free calcium levels is believed to be toxic, in the endoplasmic reticulum (ER), in contrast, calcium activity is relatively high and severe stress is caused by a depletion of ER calcium stores. Besides its role in cellular calcium signaling, the ER is the site where membrane and secretory proteins are folded and processed. These calcium-dependent processes are fundamental to normal cell functioning. Under conditions of ER dysfunction unfolded proteins accumulate in the ER lumen, a signal responsible for activation of the unfolded protein response (UPR) and the ER-associated degradation (ERAD). UPR is characterized by activation of two ER-resident kinases, PKR-like ER kinase (PERK) and IRE1. PERK induces phosphorylation of the eukaryotic initiation factor (eIF2alpha), resulting in a shut-down of translation at the initiation step. This stress response is needed to block new synthesis of proteins that cannot be correctly folded, and thus to protect cells from the effect of unfolded proteins which tend to form toxic aggregates. IRE1, on the other hand, is turned after activation into an endonuclease that cuts out a sequence of 26 bases from the coding region of xbp1 mRNA. Processed xbp1 mRNA is translated into the respective protein, an active transcription factor specific for ER stress genes such as grp78. In acute disorders and degenerative diseases, the ER calcium pool is a primary target of toxic metabolites or intermediates, such as oxygen free radicals, produced during the pathological process. Affected neurons need to activate the entire UPR to cope with the severe form of stress induced by ER dysfunction. This stress response is however hindered under conditions where protein synthesis is suppressed to such an extent that processed xbp1 mRNA is not translated into the processed XBP1 protein (XBP1(proc)). Furthermore, activation of ERAD is important for the degradation of unfolded proteins through the ubiquitin/proteasomal pathway, which is impaired in acute disorders and degenerative diseases, resulting in further ER stress. ER functioning is thus impaired in two different ways: first by the direct action of toxic intermediates, produced in the course of the pathological process, hindering vital ER reactions, and second by the inability of cells to fully activate UPR and ERAD, leaving them unable to withstand the severe form of stress induced by ER dysfunction.