Anesthesia dulls the senses by design, but for elderly people the after-effects might be more than they bargained for. Months of confusion and even permanent cognitive decline can occur after surgery under general anesthesia. Some epidemiological studies have suggested that anesthesia in elderly patients is a risk factor for developing AD. Other studies have suggested that some commonly used inhaled anesthetics might exacerbate amyloid pathology by promoting the oligomerization of amyloid-β (Aβ) peptides (Eckenhoff et al., 2004; Bianchi et al., 2007), or their production and toxicity to cells (see ARF related news story).
Now, tau is getting dragged into the act, too, with new work showing that, in mice, several anesthetics cause rapid and dramatic increases in tau phosphorylation. Emmanuel Planel and Karen Duff at Columbia University Medical Center in New York, demonstrate that the spike in phospho-tau is not caused by the anesthetics themselves, but instead occurs in response to a rapid and prolonged anesthetic-induced lowering of body temperature, which decreases the activity of the protein phosphatase 2A (PP2A), a major tau phosphatase in brain. Joining Planel as coauthors on the March 21 publication in the Journal of Neuroscience are Lit-Fui Lau and colleagues at Pfizer Global Research and Development in Groton, Connecticut.
In the study, the researchers treated mice with any of three anesthetics—intraperitoneal chloral hydrate, pentobarbital, or inhaled isoflurane—and looked at tau by Western blot 1 hour later. In each case, they saw massively increased phosphorylation at every one of the eight phosphoepitopes they checked, including pathological epitopes associated with paired helical filament formation. Anesthesia was accompanied by a dramatic drop in core body temperature to as low as 26-27 degrees centigrade, and phosphorylation at the AT8 epitope went up as body temperature dropped. When the scientists kept anesthetized animals in heated chambers to prevent the drop, the hyperphosphorylation disappeared.
Under these acute treatment conditions, the researchers did not see changes in amyloid precursor protein metabolism or Aβ levels. This is in contrast to experiments in cell culture, where isoflurane enhanced Aβ accumulation and apoptosis in human neuroglioma cells (Xie et al., 2006).
It is not news that lowering body temperature jacks up tau phosphorylation. Planel and colleagues previously showed a reversible hyperphosphorylation of tau epitopes by hypothermia in mice with impaired glucose metabolism (Planel et al., 2004), and studies from Thomas Arendt and colleagues at Leipzig University, Germany, found the same in hibernating ground squirrels (Arendt et al., 2003). In both cases, the phosphorylation rapidly reverses when the animals warm up to normal body temperature, and indeed the current work shows the same result.
However, no one knows what effect anesthesia and hypothermia might have when the brain is already afflicted with a tauopathy or AD. “We must redo these experiments in tau transgenic mice, or in a mouse model of AD. If we see more accumulation of tangles, it will be further evidence that anesthesia can have pathological effects,” Planel told ARF.
The results also carry a warning for researchers studying mouse models of AD that fluctuations in body temperature during experiments may produce artifacts of tau phosphorylation. The authors cite one study that showed APP transgenic mice are more prone to hypothermia than normal mice (Huitron-Resendiz et al., 2002), and another that found the standard procedure for the Morris water maze leaves mice hypothermic (Iivonen et al., 2003). “The main implication of this work is that people should be very careful when they analyze their mice,” Planel said. “These are not subtle changes. We found that a 3-4-degree drop in temperature can increase AT8 phosphorylation by 400-500 percent.”
How does hypothermia affect tau? Planel’s previous work showed that lowering body temperature decreases the activity of the protein phosphatase 2A (PP2A). In agreement with this, the scientists now found that when they reduced the assay temperature from 37 to 26 degrees, PP2A in brain extracts from anesthetized mice were inhibited by 40 percent. PP2A is the main enzyme responsible for dephosphorylating tau in the brain, and low PP2A activity has been implicated in AD pathology (reviewed in Tian and Wang, 2002).
Finally, the authors call for a re-examination of epidemiological studies looking at the association between anesthesia and AD. Where data are available about how changes in body temperature were managed, looking at the outcome in cases where anesthesia resulted in documented hypothermia may give a different picture of the anesthesia-AD link.—Pat McCaffrey
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