Jonathan Brouillette

Tau phosphorylation and sevoflurane anesthesia: an association to postoperative cognitive impairment.

Background: There is a growing interest in the involvement of anesthetic agents in the etiology of postoperative cognitive dysfunction. Recent animal studies suggest that acute anesthesia induces transient hyperphosphorylation of tau, an effect essentially ascribed to hypothermia. The main aim of the present study was to investigate effects, in normothermic conditions, of acute or repeated exposure to sevoflurane, a halogenated anesthetic agent, on hippocampal tau phosphorylation and spatial memory in adult mice. Methods: 5 to 6-month-old C57Bl6/J mice were submitted to acute (1 h) or repeated (five exposures of 1h every month) anesthesia using 1.5 or 2.5% sevoflurane, in normothermic conditions. In the acute protocol, animals were sacrificed 1 and 24 h after exposure. In the chronic protocol, spatial memory was evaluated using the Morris water maze following the fourth exposure, and tau phosphorylation evaluated 1 month following the last exposure using bi- and mono-dimensional electrophoresis. Results: Acute sevoflurane anesthesia in normothermic conditions led to a significant dose-dependent and reversible hippocampal tau phosphorylation, 1 h following the end of exposure (P < 0.001). Conversely, repeated anesthesia led to persistent tau hyperphosphorylation and significant memory impairments, as seen in the retention phase of the Morris water maze in sevoflurane-anesthesized animals. These pathologic features may be related to the activation of both Akt and Erk pathways. Conclusions: The present study demonstrates, in mice, that sevoflurane exposure is associated with increased tau phosphorylation through specific kinases activation and spatial memory deficits. These data support a correlation between exposures to this anesthetic agent and cognitive decline.

Neurotoxicity and memory deficits induced by soluble low-molecular-weight amyloid-β1-42 oligomers are revealed in vivo by using a novel animal model.

Neuronal and synaptic degeneration are the best pathological correlates for memory decline in Alzheimers disease (AD). Although the accumulation of soluble low-molecular-weight amyloid-β (Aβ) oligomers has been suggested to trigger neurodegeneration in AD, animal models overexpressing or infused with Aβ lack neuronal loss at the onset of memory deficits. Using a novel in vivo approach, we found that repeated hippocampal injections of small soluble Aβ1–42 oligomers in awake, freely moving mice were able to induce marked neuronal loss, tau hyperphosphorylation, and deficits in hippocampus-dependent memory. The neurotoxicity of small Aβ1–42 species was observed in vivo as well as in vitro in association with increased caspase-3 activity and reduced levels of the NMDA receptor subunit NR2B. We found that the sequestering agent transthyretin is able to bind the toxic Aβ1–42 species and attenuated the loss of neurons and memory deficits. Our novel mouse model provides evidence that small, soluble Aβ1–42 oligomers are able to induce extensive neuronal loss in vivo and initiate a cascade of events that mimic the key neuropathological hallmarks of AD.

Simvastatin improves cerebrovascular function and counters soluble amyloid-beta, inflammation and oxidative stress in aged APP mice.

Cerebrovascular dysfunctions appear to contribute to Alzheimers disease (AD) pathogenesis and the associated cognitive decline. Recently, it has been suggested that statins could be beneficial to AD patients independently from their cholesterol-lowering effects. Using 10 month-old amyloid precursor protein transgenic mice (APP mice), we sought to reverse cerebrovascular, neuronal and memory impairments with simvastatin (20 mg/kg/day, 8 weeks). Simvastatin improved reactivity of cerebral arteries, rescued the blood flow response to neuronal activation, attenuated oxidative stress and inflammation, and reduced cortical soluble amyloid-beta (Abeta) levels and the number of Abeta plaque-related dystrophic neurites. However, at such an advanced stage of the pathology, it failed to reduce Abeta plaque load and normalize cholinergic and memory deficits. These findings demonstrate that low-dose simvastatin treatment in aged APP mice largely salvages cerebrovascular function and has benefits on several AD landmarks, which could explain some of the positive effects of statins reported in AD patients.

Transthyretin: A key gene involved in the maintenance of memory capacities during aging

Aging is often associated with decline of memory function. Aged animals, like humans, can naturally develop memory impairments and thus represent a useful model to investigate genes involved in long-term memory formation that are differentially expressed between aged memory-impaired (AI) and aged memory-unimpaired (AU) animals following stimulation in a spatial memory task. We found that alterations in hippocampal gene expression of transthyretin (TTR), calcineurin, and NAD(P)H dehydrogenase quinone 2 (NQO2) were associated with memory deficits in aged animals. Decreased TTR gene expression could be attributed at least partially to diminish activity of C/EBP immediate-early gene cascade initiated by CREB since protein levels of C/EBP, a transcription factor regulating both TTR and NQO2 expression, was decreased in AI animals. Memory deficits were also found during aging in mice lacking TTR, a retinol transporter known to prevent amyloid-beta aggregation and plaque formation as seen in Alzheimers disease. Treatment with retinoic acid reversed cognitive deficits in these knock-out mice as well as in aged rats. Our study provides genetic, behavioural and molecular evidence that TTR is involved in the maintenance of normal cognitive processes during aging by acting on the retinoid signalling pathway.

Inhibitor of the Tyrosine Phosphatase STEP Reverses Cognitive Deficits in a Mouse Model of Alzheimer's Disease

This study identifies an unusual sulfur-based chemical as a novel and specific inhibitor of the tyrosine phosphatase STEP and shows that it can improve the cognitive function of a mouse model of Alzheimers disease.

The tyrosine phosphatase STEP: implications in schizophrenia and the molecular mechanism underlying antipsychotic medications

Glutamatergic signaling through N-methyl-D-aspartate receptors (NMDARs) is required for synaptic plasticity. Disruptions in glutamatergic signaling are proposed to contribute to the behavioral and cognitive deficits observed in schizophrenia (SZ). One possible source of compromised glutamatergic function in SZ is decreased surface expression of GluN2B-containing NMDARs. STEP61 is a brain-enriched protein tyrosine phosphatase that dephosphorylates a regulatory tyrosine on GluN2B, thereby promoting its internalization. Here, we report that STEP61 levels are significantly higher in the postmortem anterior cingulate cortex and dorsolateral prefrontal cortex of SZ patients, as well as in mice treated with the psychotomimetics MK-801 and phencyclidine (PCP). Accumulation of STEP61 after MK-801 treatment is due to a disruption in the ubiquitin proteasome system that normally degrades STEP61. STEP knockout mice are less sensitive to both the locomotor and cognitive effects of acute and chronic administration of PCP, supporting the functional relevance of increased STEP61 levels in SZ. In addition, chronic treatment of mice with both typical and atypical antipsychotic medications results in a protein kinase A-mediated phosphorylation and inactivation of STEP61 and, consequently, increased surface expression of GluN1/GluN2B receptors. Taken together, our findings suggest that STEP61 accumulation may contribute to the pathophysiology of SZ. Moreover, we show a mechanistic link between neuroleptic treatment, STEP61 inactivation and increased surface expression of NMDARs, consistent with the glutamate hypothesis of SZ.

Neuroprotective Effects of Natural Products: Interaction with Intracellular Kinases, Amyloid Peptides and a Possible Role for Transthyretin

Various studies reported on the neuroprotective effects of natural products, particularly polyphenols, widely present in food and beverages. For example, we have shown that resveratrol, a polyphenol contained present in red wine and other foods, activates the phosphorylation of protein kinase C (PKC), this effect being involved in its neuroprotective action against Aß-induced toxicity. Moreover, tea-derived catechin gallate esters inhibit the formation Aß oligomers/fibrils, suggesting that this action likely contributes to their neuroprotective effects. Interestingly, the effects of polyphenols may be attributable, at least in part, to the presence of specific binding sites. Autoradiographic studies revealed that these binding sites are particularly enriched in choroids plexus in the rat brain. Interestingly, the choroid plexus secretes transthyretin, a protein that has been shown to prevent Aβ aggregation and that may be critical to the maintenance of normal learning capacities in aging. Taken together, these data suggest that polyphenols target multiple enzymes/proteins leading to their neuroprotective actions.

Hippocampal gene expression profiling reveals the possible involvement of Homer1 and GABAB receptors in scopolamine‐induced amnesia

Scopolamine‐treated rats are commonly used as a psychopharmacological model of memory dysfunction and have been extensively studied to establish the effectiveness of acetylcholinesterase inhibitors in the treatment of Alzheimer’s disease. Scopolamine is a muscarinic acetylcholine receptor antagonist that induces memory deficits in young subjects similar to those occurring during aging. The amnesic effect of scopolamine is well established but the molecular and cellular mechanisms that sustain its neuropharmacological action are still unclear. The present genome wide study investigates hippocampal gene expression profiling in scopolamine‐treated adult rats following stimulation in a spatial memory task. Using microarray and quantitative real‐time RT‐PCR approaches, we identified several genes previously known to be associated with memory processes (Homer1, GABAB receptor, early growth response 1, prodynorphin, VGF nerve growth factor inducible) and multiple novel candidate genes possibly involved in cognition (including calcium/calmodulin‐dependent protein kinase kinase 2, dual specificity phosphatase 5 and 6, glycophorin C) that were altered following scopolamine treatment. Moreover, we found that stable over‐expression of glutamatergic components Homer1a and 1c in the hippocampus of adult rats induced by recombinant adeno‐associated virus vector abolished memory improvement produced by the GABAB receptor antagonist SGS742 in scopolamine‐treated rats. Taken together, these results reveal novel genes and mechanisms involved in scopolamine‐induced amnesia, and demonstrate the involvement of both GABA and glutamate neurotransmission in this animal model of cognitive dysfunctions.

Loss of Quinone Reductase 2 Function Selectively Facilitates Learning Behaviors

High levels of reactive oxygen species (ROS) are associated with deficits in learning and memory with age as well as in Alzheimers disease. Using DNA microarray, we demonstrated the overexpression of quinone reductase 2 (QR2) in the hippocampus in two models of learning deficits, namely the aged memory impaired rats and the scopolamine-induced amnesia model. QR2 is a cytosolic flavoprotein that catalyzes the reduction of its substrate and enhances the production of damaging activated quinone and ROS. QR2-like immunostaining is enriched in cerebral structures associated with learning behaviors, such as the hippocampal formation and the temporofrontal cortex of rat, mouse, and human brains. In cultured rat embryonic hippocampal neurons, selective inhibitors of QR2, namely S26695 and S29434, protected against menadione-induced cell death by reversing its proapoptotic action. S26695 (8 mg/kg) also significantly inhibited scopolamine-induced amnesia. Interestingly, adult QR2 knock-out mice demonstrated enhanced learning abilities in various tasks, including Morris water maze, object recognition, and rotarod performance test. Other behaviors related to anxiety (elevated plus maze), depression (forced swim), and schizophrenia (prepulse inhibition) were not affected in QR2-deficient mice. Together, these data suggest a role for QR2 in cognitive behaviors with QR2 inhibitors possibly representing a novel therapeutic strategy toward the treatment of learning deficits especially observed in the aged brain.

Possible Involvement of Transthyretin in Hippocampal β-Amyloid Burden and Learning Behaviors in a Mouse Model of Alzheimer’s Disease (TgCRND8)

Background: Alzheimer’s disease (AD) is a neurodegenerative disease characterized by progressive memory loss, possibly triggered by the accumulation of β-amyloid (Aβ) peptides and the hyperphosphorylation of Tau neurofilament protein. Recent findings have shown that transthyretin (TTR) is a potent scavenger of Aβ peptide deposits, suggesting a possible neuroprotective role for TTR in neurodegenerative processes associated with amyloidogenesis, such as AD. Methods: To investigate the relationship between TTR and Aβ deposition, we crossed mouse carrying a deletion of TTR (TTR–/–) with a transgenic mouse model of AD (TgCRND8), and Aβ burden and spatial learning capacities were evaluated at 4 and 6 months of age (exclusion of the 6 month-old TgCRND8/TTR–/– group due to low survival rate). Results: Rather surprisingly, Aβ plaque burden was significantly reduced in the hippocampus of 4-month-old TgCRND8/TTR+/–, and to a lesser extent in TgCRND8/TTR–/–, as compared to age-matched TgCRND8/TTR+/+. No difference in plaque burden was found between any groups in 6-month-old animals. At 4 and 6 months of age, all populations of these hybrid transgenic mice displayed similar magnitude of spatial memory deficits in the Morris water maze task. Conclusion: Since TgCRND8 mice represent an aggressive model of Aβ deposition with plaques developing as early as 3 months of age, along with spatial learning deficits, it may be already too late at 4 and 6 months of age to observe significant changes due to the deletion of the TTR gene.