Anesthetic isoflurane increases phosphorylated tau levels mediated by caspase activation and Aβ generation

The role of hippocampal tau protein phosphorylation in isoflurane-induced cognitive dysfunction in transgenic APP695 mice

BACKGROUND

Previous studies have shown that exposure to inhaled anesthetics can cause cognitive dysfunction, suggesting that general anesthesia might be a risk factor for the development of Alzheimer disease. However, the underlying mechanisms remain to be elucidated. In the present study, we tested our hypothesis that enhanced tau protein phosphorylation in hippocampus contributes to isoflurane-induced cognitive dysfunction in a mouse model of Alzheimer disease.

METHODS

Fifty-four male wild-type (WT) mice (12 months old) and 54 male amyloid precursor protein 695 (APP695) mice (12 months old) were either anesthetized for 4 hours with 1.0 minimum alveolar concentration isoflurane or sham-anesthetized (control). Learning and memory behaviors were measured using the Morris Water Maze test for mice. Phosphorylation of hippocampal tau protein at Ser262 site was analyzed with quantitative Western blotting.

RESULTS

In the Morris Water Maze test, both WT and transgenic APP695 mice showed decreased latency times during a 4-day training period. Isoflurane exposure significantly increased the latency times on days 2 and 3 in WT mice as well as on days 3 and 4 in APP695 mice (WT: P = 0.005 for day 2 and P = 0.002 for day 3; APP695: P = 0.001 for day 3 and P < 0.0001 for day 4) and reduced platform quadrant times (WT: P < 0.0001; APP695: P < 0.0001) in both types of mice. Compared with WT mice, transgenic APP695 mice displayed worse learning and memory behaviors after isoflurane exposure (P = 0.0005 for escape latency testing on day 4 training; P = 0.009 for platform probe testing). Western blot analysis showed that the levels of phosphorylation of hippocampal tau protein at Ser262 site (tau[pS262]) in the transgenic APP695 mice were higher than those in WT mice (P < 0.0001) and that isoflurane exposure time dependently enhanced the hippocampal tau[pS262] levels in both types of mice, but this effect was much more significant in the transgenic APP695 mice (P < 0.0001). Our data also showed that isoflurane exposure had no effect on the expression of total tau protein in the hippocampi of all mice (P ≥ 0.54).

CONCLUSIONS

Isoflurane may induce cognitive dysfunction by enhancing phosphorylation of hippocampal tau protein at Ser262 site, and this effect is more significant in transgenic APP695 mice.

Sevoflurane induces tau phosphorylation and glycogen synthase kinase 3β activation in young mice

BACKGROUND

Children with multiple exposures to anesthesia and surgery may have an increased risk of developing cognitive impairment. Sevoflurane is a commonly used anesthetic in children. Tau phosphorylation contributes to cognitive dysfunction. The authors therefore assessed the effects of sevoflurane on Tau phosphorylation and the underlying mechanisms in young mice.

METHODS

Six-day-old wild-type and Tau knockout mice were exposed to sevoflurane. The authors determined the effects of sevoflurane anesthesia on Tau phosphorylation, levels of the kinases and phosphatase related to Tau phosphorylation, interleukin-6 and postsynaptic density protein-95 in hippocampus, and cognitive function in both young wild-type and Tau knockout mice.

RESULTS

Anesthesia with 3% sevoflurane 2 h daily for 3 days induced Tau phosphorylation (257 vs. 100%, P = 0.0025, n = 6) and enhanced activation of glycogen synthase kinase 3β, which is the kinase related to Tau phosphorylation in the hippocampus of postnatal day-8 wild-type mice. The sevoflurane anesthesia decreased hippocampus postsynaptic density protein-95 levels and induced cognitive impairment in the postnatal day-31 mice. Glycogen synthase kinase 3β inhibitor lithium inhibited the sevoflurane-induced glycogen synthase kinase 3β activation, Tau phosphorylation, increased levels of interleukin-6, and cognitive impairment in the wild-type young mice. Finally, the sevoflurane anesthesia did not induce an increase in interleukin-6 levels, reduction in postsynaptic density protein-95 levels in hippocampus, or cognitive impairment in Tau knockout young mice.

CONCLUSIONS

These data suggested that sevoflurane induced Tau phosphorylation, glycogen synthase kinase 3β activation, increase in interleukin-6 and reduction in postsynaptic density protein-95 levels in hippocampus of young mice, and cognitive impairment in the mice. Future studies will dissect the cascade relation of these effects.

The spectrum of neurobehavioral sequelae after repetitive mild traumatic brain injury: a novel mouse model of chronic traumatic encephalopathy

There has been an increased focus on the neurological sequelae of repetitive mild traumatic brain injury (TBI), particularly neurodegenerative syndromes, such as chronic traumatic encephalopathy (CTE); however, no animal model exists that captures the behavioral spectrum of this phenomenon. We sought to develop an animal model of CTE. Our novel model is a modification and fusion of two of the most popular models of TBI and allows for controlled closed-head impacts to unanesthetized mice. Two-hundred and eighty 12-week-old mice were divided into control, single mild TBI (mTBI), and repetitive mTBI groups. Repetitive mTBI mice received six concussive impacts daily for 7 days. Behavior was assessed at various time points. Neurological Severity Score (NSS) was computed and vestibulomotor function tested with the wire grip test (WGT). Cognitive function was assessed with the Morris water maze (MWM), anxiety/risk-taking behavior with the elevated plus maze, and depression-like behavior with the forced swim/tail suspension tests. Sleep electroencephalogram/electromyography studies were performed at 1 month. NSS was elevated, compared to controls, in both TBI groups and improved over time. Repetitive mTBI mice demonstrated transient vestibulomotor deficits on WGT. Repetitive mTBI mice also demonstrated deficits in MWM testing. Both mTBI groups demonstrated increased anxiety at 2 weeks, but repetitive mTBI mice developed increased risk-taking behaviors at 1 month that persist at 6 months. Repetitive mTBI mice exhibit depression-like behavior at 1 month. Both groups demonstrate sleep disturbances. We describe the neurological sequelae of repetitive mTBI in a novel mouse model, which resemble several of the neuropsychiatric behaviors observed clinically in patients sustaining repetitive mild head injury.

Intranasal insulin prevents anesthesia-induced hyperphosphorylation of tau in 3xTg-AD mice

Background: It is well documented that elderly individuals are at increased risk of cognitive decline after anesthesia. General anesthesia is believed to be a risk factor for Alzheimer’s disease (AD). Recent studies suggest that anesthesia may increase the risk for cognitive decline and AD through promoting abnormal hyperphosphorylation of tau, which is crucial to neurodegeneration seen in AD.

Methods: We treated 3xTg-AD mice, a commonly used transgenic mouse model of AD, with daily intranasal administration of insulin (1.75 U/day) for one week. The insulin- and control-treated mice were then anesthetized with single intraperitoneal injection of propofol (250 mg/kg body weight). Tau phosphorylation and tau protein kinases and phosphatases in the brains of mice 30 min and 2 h after propofol injection were then investigated by using Western blots and immunohistochemistry.

Results: Propofol strongly promoted hyperphosphorylation of tau at several AD-related phosphorylation sites. Intranasal administration of insulin attenuated propofol-induced hyperphosphorylation of tau, promoted brain insulin signaling, and led to up-regulation of protein phosphatase 2A, a major tau phosphatase in the brain. Intranasal insulin also resulted in down-regulation of several tau protein kinases, including cyclin-dependent protein kinase 5, calcium/calmodulin-dependent protein kinase II, and c-Jun N-terminal kinase.

Conclusion: Our results demonstrate that pretreatment with intranasal insulin prevents AD-like tau hyperphosphorylation. These findings provide the first evidence supporting that intranasal insulin administration might be used for the prevention of anesthesia-induced cognitive decline and increased risk for AD and dementia.

Sleep deprivation impairs memory, tau metabolism, and synaptic integrity of a mouse model of Alzheimer's disease with plaques and tangles

Several studies have highlighted the frequency of sleep disturbances in Alzheimer's disease (AD). However, whether they are secondary to the disease or per se increase its risk remains to be fully investigated. The aim of the current investigation was to study the effect of sleep deprivation (SD) on the development of AD phenotype in a transgenic mouse model with plaques and tangles, the 3xTg mice. We evaluated the functional and biological consequences on 3xTg mice that underwent 4 hours sleep restrain per day for 8 weeks. Compared with controls, behavioral assessment showed that SD-treated mice had a significant decline in their learning and memory. Although no differences were detected in the levels of soluble amyloid-β peptides, the same animals displayed a decrease in tau phosphorylation, which associated with a significant increase in its insoluble fraction. In addition, we observed that SD resulted in lower levels of postsynaptic density protein 95 and increased glial fibrillary acidic protein levels. Finally, although total levels of the transcription factor cellular response element binding protein were unchanged, its phosphorylated form was significantly diminished in brains of sleep-deprived mice when compared with controls. Our study underlines the importance of SD as a chronic stressor, which by modulating biochemical processes influences the development of memory impairments and AD neuropathologies. Correction of SD could be a viable therapeutic strategy to prevent the onset or slow the progression of AD in individuals bearing this risk factor.

Anesthesia and tau pathology

Alzheimer's disease (AD) is the most common form of dementia and remains a growing worldwide health problem. As life expectancy continues to increase, the number of AD patients presenting for surgery and anesthesia will steadily rise. The etiology of sporadic AD is thought to be multifactorial, with environmental, biological and genetic factors interacting together to influence AD pathogenesis. Recent reports suggest that general anesthetics may be such a factor and may contribute to the development and exacerbation of this neurodegenerative disorder. Intra-neuronal neurofibrillary tangles (NFT), composed of hyperphosphorylated and aggregated tau protein are one of the main neuropathological hallmarks of AD. Tau pathology is important in AD as it correlates very well with cognitive dysfunction. Lately, several studies have begun to elucidate the mechanisms by which anesthetic exposure might affect the phosphorylation, aggregation and function of this microtubule-associated protein. Here, we specifically review the literature detailing the impact of anesthetic administration on aberrant tau hyperphosphorylation as well as the subsequent development of neurofibrillary pathology and degeneration.

Tau hyperphosphorylation: a downstream effector of isoflurane-induced neuroinflammation in aged rodents

Postoperative cognitive dysfunction (POCD) is a severe neurological sequela after anesthesia and surgery. Multiple risk factors, such as advanced age and anesthesia duration, relevant to POCD have been made out, although the pathophysiological mechanisms of this complication need to be further elucidated. To date, there is a substantial body of evidence implicating that neuroinflammatory cytokines and the subsequent neuroinflammatory response contribute to the cognitive impairment in aged rodents exposed to isoflurane, a commonly used general anesthetic. Interestingly, this cognitive disorder is mitigated by anti-inflammatory agents even 14 days after isoflurane exposure. In addition, isoflurane-induced upregulation of neuroinflammatory cytokines is only limited within 48 h. So a first possibility to consider is a downstream effector of isoflurane-induced neuroinflammatory cytokines which contributes to the long-lasting cognitive dysfunction. In Alzheimer's disease (AD) models, proinflammatory cytokines can induce tau hyperphosphorylation which is associated with synaptic abnormality and further cognitive impairment. It is unknown whether isoflurane-induced neuroinflammatory cytokines can trigger tau hyperphosphorylation. Taken together, we hypothesize that tau hyperphosphorylation is a downstream target of isoflurane-induced neuroinflammatory response and thus bridges the isoflurane-induced relatively transient neuroinflammatory process to the long-term cognitive impairment.

Effects of anesthetic isoflurane and desflurane on human cerebrospinal fluid Aβ and τ level

BACKGROUND

Accumulation of β-amyloid protein (Aβ) and tau protein is the main feature of Alzheimer disease neuropathogenesis. Anesthetic isoflurane, but not desflurane, may increase Aβ levels in vitro and in animals. Therefore, we set out to determine the effects of isoflurane and desflurane on cerebrospinal fluid (CSF) levels of Aβ and tau in humans.

METHODS

The participants were assigned into spinal anesthesia (N=35), spinal plus desflurane anesthesia (N=33), or spinal plus isoflurane anesthesia (N=38) group by randomization using computer-generated lists. Pre- and postoperative human CSF samples were obtained through an inserted spinal catheter. The levels of Aβ (Aβ40 and Aβ42) and total tau in the CSF were determined.

RESULTS

Here, we show that isoflurane, but not desflurane, was associated with an increase in human CSF Aβ40 levels (from 10.90 to 12.41 ng/ml) 24 h after the surgery under anesthesia compared to spinal anesthesia (from 11.59 to 11.08 ng/ml), P=0.022. Desflurane, but not isoflurane, was associated with a decrease in Aβ42 levels 2 h after the surgery under anesthesia (from 0.39 to 0.35 ng/ml) compared to spinal anesthesia (from 0.43 to 0.44 ng/ml), P=0.006. Isoflurane and desflurane did not significantly affect the tau levels in human CSF.

CONCLUSIONS

These studies have established a system to study the effects of anesthetics on human biomarkers associated with Alzheimer disease and cognitive dysfunction. These findings have suggested that isoflurane and desflurane may have different effects on human CSF Aβ levels.

Isoflurane increases neuronal cell death vulnerability by downregulating miR-214

Since accumulating evidence suggests the application of anesthetics may increase the risk of Alzheimer’s disease (AD), we investigated the cytotoxicity of inhaled general anesthesia in neurons and its underlying mechanism. Using primary cultured rat hippocampal neurons as the study model, here we show that isoflurane increases vulnerability to intracellular or extracellular amyloid β with or without serum deprivation. This isoflurane-induced effect is mediated by the downregulation of miR-214 level that lead to an elevated expression of Bax, a prominent target for miR-214. We conclude that isoflurane increases cell death in the presence of amyloid β by increasing Bax level through downregulating miR-214. Our data provide a new insight for inhaled anesthetics toxicity and indicate a possible mechanistic link between anesthetic application and neurodegenration in AD.

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 patient's postoperative recovery will be enhanced and long-term consequences can be prevented.

Gender-specific differences in the central nervous system's response to anesthesia

Males and females are physiologically distinct in their responses to various anesthetic agents. The brain and central nervous system (CNS), the main target of anesthesia, are sexually dimorphic from birth and continue to differentiate throughout life. Accordingly, gender has a substantial impact on the influence of various anesthetic agents in the brain and CNS. Given the vast differences in the male and female CNS, it is surprising to find that females are often excluded from basic and clinical research studies of anesthesia. In animal research, males are typically studied to avoid the complication of breeding, pregnancy, and hormonal changes in females. In clinical studies, females are also excluded for the variations that occur in the reproductive cycle. Being that approximately half of the surgical population is female, the exclusion of females in anesthesia-related research studies leaves a huge knowledge gap in the literature. In this review, we examine the reported sex-specific differences in the central nervous system's response to anesthesia. Furthermore, we suggest that anesthesia researchers perform experiments on both sexes to further evaluate such differences. We believe a key goal of research studying the interaction of the brain and anesthesia should include the search for knowledge of sex-specific mechanisms that will improve anesthetic care and management in both sexes.