Vincent Degos

Characterization of phenotype markers and neuronotoxic potential of polarised primary microglia in vitro

Highlight ► A unique catalogue of phenotype markers and neuronotoxic effects of polarised primary microglia, as a comparative tool to screen neurotherapies.

Activation of microglial N-methyl-D-aspartate receptors triggers inflammation and neuronal cell death in the developing and mature brain

Activated microglia play a central role in the inflammatory and excitotoxic component of various acute and chronic neurological disorders. However, the mechanisms leading to their activation in the latter context are poorly understood, particularly the involvement of N‐methyl‐D‐aspartate receptors (NMDARs), which are critical for excitotoxicity in neurons. We hypothesized that microglia express functional NMDARs and that their activation would trigger neuronal cell death in the brain by modulating inflammation.

Perioperative Cognitive Decline in the Aging Population

Elderly patients who have an acute illness or who undergo surgery often experience cognitive decline. The pathophysiologic mechanisms that cause neurodegeneration resulting in cognitive decline, including protein deposition and neuroinflammation, also play a role in animal models of surgery-induced cognitive decline. With the aging of the population, surgical candidates of advanced age with underlying neurodegeneration are encountered more often, raising concerns that, in patients with this combination, cognitive function will precipitously decline postoperatively. This special article is based on a symposium that the University of California, San Francisco, convened to explore the contributions of surgery and anesthesia to the development of cognitive decline in the aged patient. A road map to further elucidate the mechanisms, diagnosis, risk factors, mitigation, and treatment of postoperative cognitive decline in the elderly is provided.

The neuroinflammatory response of postoperative cognitive decline

BACKGROUND Aseptic surgical trauma provokes a homeostatic neuroinflammatory response to promote healing and protect the organism from further injury. When this response is dysregulated, harmful consequences can follow, including postoperative cognitive decline. SOURCES OF DATA We performed a comprehensive search on PubMed related to postoperative cognitive dysfunction (POCD). AREAS OF AGREEMENT Although the precise pathogenic mechanisms for POCD remain unclear, certain risk factors are known. AREAS OF CONTROVERSY The mechanisms that lead to exaggerated and persistent neuroinflammation and the best way to counteract it are still unknown. AREAS FOR DEVELOPING RESEARCH: It is imperative that we identify the underlying processes that increase the risk of cognitive decline in elderly surgical patients. In this review we explore non-resolution of inflammation as an underlying cause of developing exaggerated and persistent POCD. If interventions can be developed to promote resolution of neuroinflammation, the patients postoperative recovery will be enhanced and long-term consequences can be prevented.

Neuroprotective Effects of Dexmedetomidine against Glutamate Agonist-induced Neuronal Cell Death Are Related to Increased Astrocyte Brain-derived Neurotrophic Factor Expression

Background:Brain-derived neurotrophic factor (BDNF) plays a prominent role in neuroprotection against perinatal brain injury. Dexmedetomidine, a selective agonist of &agr;2-adrenergic receptors, also provides neuroprotection against glutamate-induced damage. Because adrenergic receptor agonists can modulate BDNF expression, our goal was to examine whether dexmedetomidine’s neuroprotective effects are mediated by BDNF modulation in mouse perinatal brain injury. Methods:The protective effects against glutamate-induced injury of BDNF and dexmedetomidine alone or in combination with either a neutralizing BDNF antibody or an inhibitor of the extracellular signal-regulated kinase pathway (PD098059) were compared in perinatal ibotenate-induced cortical lesions (n = 10–20 pups/groups) and in mouse neuronal cultures (300 &mgr;M of ibotenate for 6 h). The effect of dexmedetomidine on BDNF expression was examined in vivo and in vitro with cortical neuronal and astrocyte isolated cultures. Results:Both BDNF and dexmedetomidine produced a significant neuroprotective effect in vivo and in vitro. Dexmedetomidine enhanced Bdnf4 and Bdnf5 transcription and BDNF protein cortical expression in vivo. Dexmedetomidine also enhanced Bdnf4 and Bdnf5 transcription and increased BDNF media concentration in isolated astrocyte cultures but not in neuronal cultures. Dexmedetomidine’s protective effect was inhibited with BDNF antibody (mean lesion size ± SD: 577 ± 148 &mgr;m vs. 1028 ± 213 &mgr;m, n = 14–20, P < 0.001) and PD098059 in vivo but not in isolated neuron cultures. Finally, PD098059 inhibited the increased release of BDNF induced by dexmedetomidine in astrocyte cultures. Conclusion:These results suggest that dexmedetomidine increased astrocyte expression of BDNF through an extracellular signal-regulated kinase-dependent pathway, inducing subsequent neuroprotective effects.

Opposed effects of hypertonic saline on contusions and noncontused brain tissue in patients with severe traumatic brain injury.

Objective:The aim of this study was to quantify the effect of hypertonic saline solution on contused and noncontused brain tissue in patients with traumatic brain injury. We hypothesize that hypertonic saline would increase the volume of brain contusion while decreasing the volume of noncontused hemispheric areas. Design:Prospective observational study. Setting:Neurosciences critical care unit of a university hospital. Patients:Fourteen traumatic brain injury patients with increased intracranial pressure. Interventions:A computed tomography scan was performed before and after a 20-min infusion of 40 mL of 20% saline. Measurements and Main Results:The volume, weight, and specific gravity of contused and noncontused hemispheric areas were assessed from computed tomography DICOM images by using a custom-designed software (BrainView). Physiologic variables and natremia were measured before and after infusion. Hypertonic saline significantly increased natremia from 143 ± 5 to 146 ± 5 mmol/L and decreased intracranial pressure from 23 ± 3 to 17 ± 5 mm Hg. The volume of the noncontused hemispheric areas decreased by 13 ± 8 mL whereas the specific gravity increased by 0.029 ± 0.027%. The volume of contused hemispheric tissue increased by 5 ± 5 mL without any con-comitant change in density. There was a wide interindividual variability in the response of the noncontused hemispheric tissue with changes in specific gravity varying between −0.0124% and 0.0998%. Conclusions:Three days after traumatic brain injury, the blood- brain barrier remains semipermeable in noncontused areas but not in contusions. Further studies are needed to tailor the use of hypertonic saline in patients with traumatic brain injury according to the volume of contusions assessed on computed tomography.

High-mobility Group Box 1 Protein Initiates Postoperative Cognitive Decline by Engaging Bone Marrow-derived Macrophages

Background: Aseptic trauma engages the innate immune response to trigger a neuroinflammatory reaction that results in postoperative cognitive decline. The authors sought to determine whether high-mobility group box 1 protein (HMGB1), an ubiquitous nucleosomal protein, initiates this process through activation and trafficking of circulating bone marrow–derived macrophages to the brain. Methods: The effects of HMGB1 on memory (using trace fear conditioning) were tested in adult C57BL/6J male mice; separate cohorts were tested after bone marrow–derived macrophages were depleted by clodrolip. The effect of anti-HMGB1 neutralizing antibody on the inflammatory and behavioral responses to tibial surgery were investigated. Results: A single injection of HMGB1 caused memory decline, as evidenced by a decrease in freezing time (52 ± 11% vs. 39 ± 5%; n = 16–17); memory decline was prevented when bone marrow–derived macrophages were depleted (39 ± 5% vs. 50 ± 9%; n = 17). Disabling HMGB1 with a blocking monoclonal antibody, before surgery, reduced postoperative memory decline (52 ± 11% vs. 29 ± 5%; n = 15–16); also, hippocampal expression of monocyte chemotactic protein-1 was prevented by the neutralizing antibody (n = 6). Neither the systemic nor the hippocampal inflammatory responses to surgery occurred in mice pretreated with anti-HMGB1 neutralizing antibody (n = 6). Conclusion: Postoperative neuroinflammation and cognitive decline can be prevented by abrogating the effects of HMGB1. Following the earlier characterization of the resolution of surgery-induced memory decline, the mechanisms of its initiation are now described. Together, these data may be used to preoperatively test the risk to surgical patients for the development of exaggerated and prolonged postoperative memory decline that is reflected in delirium and postoperative cognitive dysfunction, respectively.

Depletion of Bone Marrow-derived Macrophages Perturbs the Innate Immune Response to Surgery and Reduces Postoperative Memory Dysfunction

Background:According to rodent models of postoperative cognitive decline, activation of the innate immune response following aseptic surgical trauma results in the elaboration of hippocampal proinflammatory cytokines, which are capable of disrupting long-term potentiation, the neurobiologic correlate of memory. The authors hypothesize that hippocampal recruitment of bone marrow–derived macrophages plays a causal role in these processes, resulting in memory dysfunction. Methods:Clodrolip injection (liposomal formulation of clodronate) before stabilized tibial fracture under general anesthesia was used to deplete bone marrow–derived macrophages. Systemic inflammation and neuroinflammation were studied on postoperative day 1, and memory in a fear-trace conditioning paradigm was assessed on postoperative day 3. CX3CR1GFP/+ CCR2RFP/+ mice were used to identify bone marrow–derived macrophages. Results:Clodrolip effectively depleted splenic CCR2+ bone marrow–derived macrophages. It also attenuated the surgery-induced increase of interleukin-6 in the serum and the hippocampus, and prevented hippocampal infiltration of CCR2+ cells without affecting the number of CX3CR1+ microglia. It did not alter the surgery-induced increase in hippocampal monocyte chemoattractant protein-1, the recruitment signal for CCR2+ cells. Clodrolip prevented surgery-induced memory dysfunction, as evidenced by a significant increase in freezing time (29% [95% CI, 21–38%] vs. 48% [95% CI, 38–58%], n = 20, P = 0.004), but did not affect memory in nonsurgical mice. Conclusion:Depletion of bone marrow–derived macrophages prevents hippocampal neuroinflammation and memory dysfunction after experimental tibial fracture. These data suggest that the hippocampal recruitment of bone marrow–derived macrophages is a necessary mechanism in murine postoperative cognitive dysfunction. Interventions designed to prevent its activation and/or migration into the brain may represent a feasible preemptive strategy.

Neuroprotective Strategies for the Neonatal Brain

Injury to the perinatal brain is a leading cause of childhood mortality and lifelong disability. Cerebral palsy and cognitive impairment are usually related to periventricular white matter damage, which is seen chiefly in babies born before 32 wk gestational age, and to corticosubcortical lesions, which occur mainly in full-term infants. Despite recent improvements in neonatal care, no effective treatment for perinatal brain lesions is available. Several interventions, such as magnesium sulfate in preterm newborns and hypothermia in term newborns, are the focus of completed or continuing clinical trials. Improved understanding of the pathophysiological mechanisms involved in perinatal brain lesions helps to identify potential targets for neuroprotective interventions, as discussed in this review.

Bevacizumab Attenuates VEGF-Induced Angiogenesis and Vascular Malformations in the Adult Mouse Brain

Background and Purpose— Vascular endothelial growth factor (VEGF) expression is elevated in human brain arteriovenous malformations (bAVM). We have developed a bAVM model in the adult mouse by focal Alk1 gene deletion and human VEGF stimulation. We hypothesized that once the abnormal vasculature has been established, tonic VEGF stimulation is necessary to maintain the abnormal phenotype, and VEGF antagonism by bevacizumab (Avastin) would reduce vessel density and attenuate the dysplastic vascular phenotype. Methods— Angiogenesis and bAVM were induced by injection of adeno-associated viral vector expressing human VEGF alone into the brain of wild-type mice or with adenoviral vector expressing Cre recombinase (Ad-Cre) into Alk12f/2f mice. Six weeks later, bevacizumab or trastuzumab (Herceptin, bevacizumab control) was administered. Vessel density, dysplasia index, vascular cell proliferation and apoptosis, and human IgG were assessed (n=6/group). Results— Compared with trastuzumab (15 mg/kg), administration of 5, 10, and 15 mg/kg of bevacizumab to adeno-associated viral vector expressing human VEGF treated wild-type mice reduced focal vessel density (P<0.05); administration of 5 mg/kg bevacizumab decreased proliferating vascular cells (P=0.04) and increased TUNEL-positive vascular cells (P=0.03). More importantly, bevacizumab (5 mg/kg) treatment reduced both vessel density (P=0.01) and dysplasia index (P=0.02) in our bAVM model. Human IgG was detected in the vessel wall and in the parenchyma in the angiogenic foci of bevacizumab-treated mice. Conclusions— We provide proof-of-principle that, once abnormal AVM vessels have formed, VEGF antagonism may reduce the number of dysplastic vessels and should be evaluated further as a therapeutic strategy for the human disease.