Farnesoid X receptor knockout protects brain against ischemic injury through reducing neuronal apoptosis in mice

Background Farnesoid X receptor (FXR) is a nuclear receptor that plays a critical role in controlling cell apoptosis in diverse diseases. Previous studies have shown that knocking out FXR improved cardiac function by reducing cardiomyocyte apoptosis in myocardial ischemic mice. However, the role of FXR after cerebral ischemia remains unknown. In this study, we explored the effects and mechanisms of FXR knockout (KO) on the functional recovery of mice post cerebral ischemia-reperfusion. Methods Adult male C57BL/6 wild type and FXR KO mice were subjected to 90-min transient middle cerebral artery occlusion (tMCAO). The mice were divided into five groups: sham, wild-type tMCAO, FXR KO tMCAO, wild-type tMCAO treated with calcium agonist Bayk8644, and FXR KO tMCAO treated with Bayk8644. FXR expression was examined using immunohistochemistry and Western blot. Brain infarct and brain atrophy volume were examined at 3 and 14 days after stroke respectively. Neurobehavioral tests were conducted up to 14 days after stroke. The protein levels of apoptotic factors (Bcl-2, Bax, and Cleaved caspase-3) and mRNA levels of pro-inflammatory factors (TNF-alpha, IL-6, IL-1 beta, IL-17, and IL-18) were examined using Western blot and RT-PCR. TUNEL staining and calcium imaging were obtained using confocal and two-photon microscopy. Results The expression of FXR was upregulated after ischemic stroke, which is located in the nucleus of the neurons. FXR KO was found to reduce infarct volume and promote neurobehavioral recovery following tMCAO compared to the vehicle. The expression of apoptotic and pro-inflammatory factors decreased in FXR KO mice compared to the control. The number of NeuN(+)/TUNEL+ cells declined in the peri-infarct area of FXR KO mice compared to the vehicle. We further demonstrated that inhibition of FXR reduced calcium overload and addition of ionomycin could reverse this neuroprotective effect in vitro. What is more, in vivo results showed that enhancement of intracellular calcium concentrations could aggravate ischemic injury and reverse the neuroprotective effect of FXR KO in mice. Conclusions FXR KO can promote neurobehavioral recovery and attenuate ischemic brain injury, inflammatory release, and neuronal apoptosis via reducing calcium influx, suggesting its role as a therapeutic target for stroke treatments.