In the present study, we evaluated the phenotype of mice with deletion of the K2P channel TRESK gene. TRESK homozygous knockout mice appeared no different than wildtype littermates in growth, overall appearance and reproduction, suggesting that TRESK is not an essential gene for these aspects of mouse development. Knockout of the TRESK gene produced no noticeable impairment in the gross behaviors and motor ability of the mice but on closer testing knockout mice were found to have mildly increased thermal sensitivity and decreased inactive behavior compared to wildtype mice. Although we found no compensatory changes in expression of other major K2P channels when TRESK expression is lost, the lack of behavioral phenotype in these mice implies there is redundancy behind the functional contribution that TRESK makes to the function of the central nervous system.
We also investigated the role of TRESK in the sensitivity to volatile anesthetics and found that mice with inactivated TRESK were less sensitive to isoflurane, but had unchanged sensitivity to desflurane, halothane and sevoflurane. The difference in MAC for isoflurane was small but statistically significant. This isolated finding is similar to previously reported changes in anesthetic sensitivity in other K2P
knockout mice. TASK-1 knockout mice have partially reduced sensitivity to isoflurane or halothane depending on the assay30
while TASK-3 knockout mice show decreased sensitivity to halothane but not isoflurane in standard MAC assay.31
Furthermore, knockout mice with deletion of other proteins implicated in anesthesia mechanisms such as the β3 subunit of the γ-aminobutyric acid type A receptor45
or the stomatin gene show variable changes in volatile anesthetic sensitivity.46
Only TREK-1 knockout mice manifest a consistent reduction in volatile anesthetic sensitivity, yet even in those mice the changes in MAC varied widely, from as low as 7% for desflurane to as high as 48% for halothane.29
We believe that our finding of decreased anesthetic sensitivity for only one drug in TRESK knockout mice adds to a growing consensus that volatile anesthetics are acting on multiple targets, possibly including K2P
channels, whose sum total effect achieves the pharmcodynamic action of volatile anesthetics.
The behavioral evaluation of mice with inactivated TRESK gene are also congruent with previous studies of other K2P
family knockout mice.47
TREK-1, TWIK-1, TASK-2 and KCNK7 knockout mice all are healthy, fertile and have normal morphology and behaviors.29,32,33,48
TASK-1 knockout mice act normally in several behavioral tests, with only minor changes in paw withdrawal response and motor function.30,49
TASK-3 knockout mice show no gross abnormality apart from a minor increased locomotor activity during the dark phase and slower swimming ability.31
Even the TASK-1/TASK-3 double-knockout mouse shows no obvious neurological abnormalities or health problems.50
These observations reinforce the idea that the expression of individual members of the K2P
gene family is not essential for normal growth, development and reproduction.
We did document a greater mortality rate in TRESK knockout mice after undergoing MAC testing compared to their wildtype littermates and speculate that this impaired survival ability may result from reduced tolerance to the stress of MAC testing. The MAC assay itself, consisting of a prolonged anesthetic with varying agent levels and exposure to noxious stimuli under at times a light plane of anesthesia represents a significantly stressful event. We do not believe that the higher mortality of knockout versus wildtype mice occurred due to the difference in isoflurane MAC because knockout and wildtype animals received very similar concentrations of isoflurane during testing. In addition, greater mortality also occurred in animals exposed only to the other volatile anesthetics, for which the knockout mice did not display any change in MAC. The exact mechanism underlying reduced survival ability in these mice remains to be further investigated and could include alterations in respiratory dynamics, abnormal response to changes in PaCO2 or interference with other cardiorespiratory functions.
As with all global knockout studies it is possible that a compensatory effect by other K2P
channels or other genes responsible for anesthetic action may occur in response to TRESK gene deletion. Quantitative PCR showed that knockout of TRESK gene did not significantly alter mRNA expression of TREK-1, TASK-1 and TASK-3 in the whole brain, spinal cord and DRG. However, these results do not rule out changes in expression of these K2P
family members at a regional or cellular level or of compensation by other ion channel expression. Changes in gene expression of other anesthetic sensitive ion channels could compensate for loss of TRESK, as has been described with μ opioid receptor knockout.51
Another physiologic effect that could be masking the role of TRESK and other K2P channels in knockout mice is the fact that the anesthetics studied in the MAC assays were delivered in 100% oxygen. Given that several K2P channels are activated by molecular oxygen it is possible that the anesthetic response of knockout animals may have blunted by this factor.
In conclusion, inactivation of TRESK gene expression in mice does not cause a significant effect on development, growth, reproduction and gross behavior, but does cause a slightly decrease in the sensitivity of the mice to isoflurane and increased thermal nociception. Decreased sensitivity to isoflurane but not to halothane, sevoflurane and desflurane indicates that TRESK may partially mediate the action of this volatile anesthetic. A significantly higher death rate in TRESK knockout mice after the MAC assays could indicate a role for TRESK in an endogenous survival mechanism in response to stress.