Sylvain Doré

Ecosystem carbon dioxide fluxes after disturbance in forests of North America

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER.

Haem oxygenase-1 prevents cell death by regulating cellular iron

Haem oxygenase-1 (HO1) is a heat-shock protein that is induced by stressful stimuli. Here we demonstrate a cytoprotective role for HO1: cell death produced by serum deprivation, staurosporine or etoposide is markedly accentuated in cells from mice with a targeted deletion of the HO1 gene, and greatly reduced in cells that overexpress HO1. Iron efflux from cells is augmented by HO1 transfection and reduced in HO1-deficient fibroblasts. Iron accumulation in HO1-deficient cells explains their death: iron chelators protect HO1-deficient fibroblasts from cell death. Thus, cytoprotection by HO1 is attributable to its augmentation of iron efflux, reflecting a role for HO1 in modulating intracellular iron levels and regulating cell viability.

Inflammation after Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a devastating clinical event without effective therapies. Increasing evidence suggests that inflammatory mechanisms are involved in the progression of ICH-induced brain injury. Inflammation is mediated by cellular components, such as leukocytes and microglia, and molecular components, including prostaglandins, chemokines, cytokines, extracellular proteases, and reactive oxygen species. Better understanding of the role of the ICH-induced inflammatory response and its potential for modulation might have profound implications for patient treatment. In this review, a summary of the available literature on the inflammatory responses after ICH is presented along with discussion of some of the emerging opportunities for potential therapeutic strategies. In the near future, additional strategies that target inflammation could offer exciting new promise in the therapeutic approach to ICH.

The ginkgo biloba extract (EGb 761) protects hippocampal neurons against cell death induced by β-amyloid

Substantial evidence suggests that the accumulation of β‐amyloid (Aβ)‐derived peptides, and to a lesser extent free radicals, may contribute to the aetiology and/or progression of Alzheimers disease (AD). Ginkgo biloba extract (EGb 761) is a well‐defined plant extract containing two major groups of constituents, i.e. flavonoids and terpenoids. It is viewed as a polyvalent agent with a possible therapeutic use in the treatment of neurodegenerative diseases of multifactorial origin, e.g. AD. We have investigated here the potential effectiveness of EGb 761 against toxicity induced by (Aβ)‐derived peptides (Aβ25−35, Aβ1−40 and Aβ1−42) on hippocampal primary cultured cells, this area being severely affected in AD. A co‐treatment with EGb 761 concentration‐dependently (10–100 μg/mL) protected hippocampal neurons against toxicity induced by Aβ fragments, with a maximal and complete protection at the highest concentration tested. Similar, albeit less potent protective effects were seen with the flavonoid fraction of the extract (CP 205), while the terpenes were ineffective. Most interestingly, EGb 761 (100 μg/mL) was even able to protect (up to 8 h) hippocampal cells from a pre‐exposure to Aβ25−35 and Aβ1−40. EGb 761 was also able to both protect and rescue hippocampal cells from toxicity induced by H2O2 (50–150 μm), a major peroxide possibly involved in mediating Aβ toxicity. Moreover, EGb 761 (10–100 μg/mL), and to a lesser extent CP 205 (10–50 μg/mL), completely blocked Aβ‐induced events, e.g. reactive oxygen species accumulation and apoptosis. These results suggest that the neuroprotective effects of EGb 761 are partly associated with its antioxidant properties and highlight its possible effectiveness in neurodegenerative diseases, e.g. AD via the inhibition of Aβ‐induced toxicity and cell death.

Amyloid Precursor Proteins Inhibit Heme Oxygenase Activity and Augment Neurotoxicity in Alzheimer's Disease

Amyloid precursor protein (APP) generates the beta-amyloid peptide, postulated to participate in the neurotoxicity of Alzheimers disease. We report that APP and APLP bind to heme oxygenase (HO), an enzyme whose product, bilirubin, is antioxidant and neuroprotective. The binding of APP inhibits HO activity, and APP with mutations linked to the familial Alzheimers disease (FAD) provides substantially greater inhibition of HO activity than wild-type APP. Cortical cultures from transgenic mice expressing Swedish mutant APP have greatly reduced bilirubin levels, establishing that mutant APP inhibits HO activity in vivo. Oxidative neurotoxicity is markedly greater in cerebral cortical cultures from APP Swedish mutant transgenic mice than wild-type cultures. These findings indicate that augmented neurotoxicity caused by APP-HO interactions may contribute to neuronal cell death in Alzheimers disease.

Potential mechanism by which resveratrol, a red wine constituent, protects neurons

Abstract: Polyphenolic compounds, such as resveratrol, are naturally present at high concentration in grape skin, seeds, and red wine. Resveratrol is present in cis and trans isoforms and the major trans isomer is the biologically active one. Epidemiologic studies have revealed a reduced incidence of cardiovascular risk associated with consumers of red wine; this has been popularized as the French paradox. Resveratrol has been shown to have significant antioxidant properties in a variety of in vitro and in vivo models. It can reduce ischemic damage in heart ischemia reperfusion injury and also in brain ischemia/reperfusion in rodent models. Due to the high rate of oxygen consumption in the brain, and especially low levels of antioxidant defense enzymes, this organ is particularly susceptible of free radical damage. Most of the protective biological actions associated with resveratrol have been associated with its intrinsic radical scavenger properties. We have investigated the possibility of other indirect pathways by which resveratrol can exert its neuroprotective abilities. We have specifically tested whether heme oxygenase neuroprotective enzyme could be stimulated after resveratrol treatment. Using primary neuronal cultures, resveratrol was able to significantly induce heme oxygenase 1, whereas vehicle control showed no effect. No detectable toxicity was quantified. It is well established that after stroke significant levels of intracellular heme levels increase. The source of free heme comes mainly from several heme‐containing enzymes. Heme (iron‐protoporphyrin IX) is a pro‐oxidant and its rapid degradation by heme oxygenase is believed to be protective. Moreover, the generation of heme metabolites can also have their own intrinsic cellular properties. All together, increased heme oxygenase activity by resveratrol is a unique pathway by which this compound can exert its neuroprotective actions.

Neuronal overexpression of cyclooxygenase-2 increases cerebral infarction

Increases in COX‐2 enzymatic activity and prostaglandin production have been associated with neuronal injury in both acute and age‐related degenerative neurological diseases. In this study, we tested the effects of increased COX‐2 activity in a model of transient focal ischemia using a transgenic mouse model in which human COX‐2 is constitutively expressed selectively in neurons of the striatum, cerebral cortex, and hippocampus. These COX‐2 transgenic mice harbor elevated levels of PGE2 that are 10‐fold higher than nontransgenic levels. A significant increase in infarct volume was observed after middle cerebral artery occlusion with 4 days of reperfusion in COX‐2 transgenic mice as compared with nontransgenic littermates. Pretreatment of nontransgenic mice with the selective COX‐2 inhibitor SC58236 resulted in a significant reduction of infarct volume in nontransgenic mice, consistent with previous pharmacological studies. However, transgenic COX‐2 mice treated with SC58236 did not show a significant reduction. This suggests that chronic increases in COX‐2 expression and enzymatic activity, which can occur in aging and in pathological states characterized by oxidative stress and chronic inflammatory processes, can lead to downstream cellular changes that have a negative impact on neuronal survival in cerebrovascular disease. Ann Neurol 2003

The Flavanol (−)-Epicatechin Prevents Stroke Damage through the Nrf2/HO1 Pathway

Epidemiologic studies have shown that foods rich in polyphenols, such as flavanols, can lower the risk of ischemic heart disease; however, the mechanism of protection has not been clearly established. In this study, we investigated whether epicatechin (EC), a flavanol in cocoa and tea, is protective against brain ischemic damage in mice. Wild-type mice pretreated orally with 5, 15, or 30 mg/kg EC before middle cerebral artery occlusion (MCAO) had significantly smaller brain infarcts and decreased neurologic deficit scores (NDS) than did the vehicle-treated group. Mice that were posttreated with 30 mg/kg of EC at 3.5 hours after MCAO also had significantly smaller brain infarcts and decreased NDS. Similarly, WT mice pretreated with 30 mg/kg of EC and subjected to N-methyl-D-aspartate (NMDA)-induced excitotoxicity had significantly smaller lesion volumes. Cell viability assays with neuronal cultures further confirmed that EC could protect neurons against oxidative insults. Interestingly, the EC-associated neuroprotection was mostly abolished in mice lacking the enzyme heme oxygenase 1 (HO1) or the transcriptional factor Nrf2, and in neurons derived from these knockout mice. These results suggest that EC exerts part of its beneficial effect through activation of Nrf2 and an increase in the neuroprotective HO1 enzyme.

Heme oxygenase-2 is neuroprotective in cerebral ischemia.

Heme oxygenase (HO) is believed to be a potent antioxidant enzyme in the nervous system; it degrades heme from heme-containing proteins, giving rise to carbon monoxide, iron, and biliverdin, which is rapidly reduced to bilirubin. The first identified isoform of the enzyme, HO1, is an inducible heat-shock protein expressed in high levels in peripheral organs and barely detectable under normal conditions in the brain, whereas HO2 is constitutive and most highly concentrated in the brain. Interestingly, although HO2 is constitutively expressed, its activity can be modulated by phosphorylation. We demonstrated that bilirubin, formed from HO2, is neuroprotectant, as neurotoxicity is augmented in neuronal cultures from mice with targeted deletion of HO2 (HO2−/−) and reversed by low concentrations of bilirubin. We now show that neural damage following middle cerebral artery occlusion (MCAO) and reperfusion, a model of focal ischemia of vascular stroke, is substantially worsened in HO2−/− animals. By contrast, stroke damage is not significantly altered in HO1−/− mice, despite their greater debility. Neural damage following intracranial injections of N-methyl-d-aspartate (NMDA) is also accentuated in HO2−/− animals. These findings establish HO2 as an endogenous neuroprotective system in the brain whose pharmacologic manipulation may have therapeutic relevance.

Heme oxygenase-2 acts to prevent neuronal death in brain cultures and following transient cerebral ischemia

Heme oxygenase (HO) cleaves the heme ring to form biliverdin, which is rapidly reduced to bilirubin, carbon monoxide, and iron. HO1, the first form of the enzyme discovered, is an inducible protein, concentrated in tissues that are exposed to degrading red blood cells and stimulated by hemolysis and numerous other toxic perturbations to eliminate potentially toxic heme. By contrast, HO2 is constitutive and most highly concentrated in neural tissues. Carbon monoxide, formed from HO2, is a putative neurotransmitter in the brain and peripheral autonomic nervous system. HO1 regulates the efflux of potentially toxic iron from cells, as iron efflux is deficient in mice with targeted deletion of HO1 (HO1(-/-)), and transfection of HO1 facilitates iron efflux. Bilirubin appears to be a physiologic neuroprotectant. Activation of HO2 by phorbol esters, that stimulate protein kinase C to phosphorylate HO2, augments production of bilirubin which protects brain cultures from oxidative stress. Bilirubin itself in nanomolar concentrations is neuroprotective, while HO2 deletion (HO2(-/-)) leads to increased neurotoxicity in brain cultures and increased neural damage following transient cerebral ischemia in intact mice. Mechanisms whereby HO2 provides neuroprotection have not been clarified including whether protection is primarily associated with apoptotic or necrotic cell death. Moreover, the generality of neurotoxic stimuli influenced by HO2 has been unclear. We now demonstrate increased neuronal death in cerebellar granule cultures of HO2(-/-) mice with a selective augmentation of apoptotic death. We also demonstrate that HO2 transfection rescues apoptotic death. In intact mice, we show an increased incidence of apoptotic morphology in the penumbra area surrounding the infarct core in HO2(-/-) mice undergoing transient focal ischemia.