Inhibition of mammalian target of rapamycin reduces epileptogenesis and blood–brain barrier leakage but not microglia activation

Abstract

SUMMARYPurpose: Previous studies have shown that inhibition of the mammalian target of rapamycin (mTOR) pathway with rapamycin prevents epileptogenesis after pharmacologically induced status epilepticus (SE) in rat models of temporal lobe epilepsy. Because rapamycin is also known for its immunosuppressant properties we hypothesized that one of the mechanisms by which it exerts this effect could be via suppression of brain inflammation, a process that has been suggested to play a major role in the development and progression of epilepsy. Methods: Rats were treated with rapamycin or vehicle once daily for 7 days (6 mg/kg/day, i.p.) starting 4 h after the induction of SE, which was evoked by electrical stimulation of the angular bundle. Hereafter rapamycin was administered every other day until rats were sacrificed, 6 weeks after SE. Video-electroencephalography was used to monitor the occurrence of seizures. Neuronal death, synaptic reorganization, and microglia and astrocyte activation were assessed by immunohistologic staining. Fluorescein was administered to quantify blood-brain barrier leakage. Key Findings: Rapamycin treatment did not alter SE severity and duration compared to vehicle treatment rats. Rapamycin-treated rats developed hardly (n = 9) or no (n = 3) seizures during the 6-week treatment, whereas vehicle-treated rats showed a progressive increase of seizures starting 1 week after SE (mean 8 ± 2 seizures per day during the sixth week). Cell loss and sprouting that normally occur after SE were prominent but on average significantly less in rapamycin-treated rats versus vehicletreated rats. Nevertheless, various inflammation markers (CD11b/c and CD68) were dramatically upregulated and not significantly different between post-SE groups. Of interest, blood-brain barrier leakage was barely detected in the rapamycin-treated group, whereas it was prominent in the vehicle-treated group. Significance: mTOR inhibition led to strong reduction of seizure development despite the presence of microglia activation, suggesting that effects of rapamycin on seizure development are not due to a control of inflammation. Whether the effects on blood-brain barrier leakage in rapamycin-treated rats are a consequence of seizure suppressing properties of the drug, or contribute to a real antiepileptogenic effect still needs to be determined.

mTOR Signaling in Epilepsy: Insights from Malformations of Cortical Development

“…Despite the known association between focal MCD and intractable seizures, distinguishing the differential contributions of altered brain structure, the effects of mutations on downstream gene, and protein expression, mTOR hyperactivation to epileptogenesis has been a challenge (see Aronica and Crino 2013). Although enhanced mTOR signaling is detected in knockout mouse models of mTOR regulatory genes associated with spontaneous seizures (e.g., Tsc1, Tsc2, Pten), hyperactive mTOR signaling is also found in animal models of seizures such as kainate treatment (Zeng et al 2009;Sha et al 2012) or electrical brain stimulation (van Vliet et al 2012) in the absence of structural changes in the neocortex. Enhanced mTOR activation has been linked to mouse models of infantile spasms (see Raffo et al 2011) and seizures induced in a hypoxia model lead to enhanced expression of genesencoding components of the mTOR pathway (Theilhaber et al 2013).…”

m-TOR Inhibitors in the Current Practice

“…Furthermore, there is increasing interest as to whether the mTOR pathway may be involved in other types of epilepsy (other than genetic epilepsies), such as epilepsy following acquired brain injury. There is some evidence that mTOR inhibitors can inhibit existing seizures or can prevent epilepsy in some animal models of acquired epilepsy, though other studies have found negative results [65][66][67][68][69].…”

Effects of rapamycin and curcumin treatment on the development of epilepsy after electrically induced status epilepticus in rats

“…4,5 In TLE patients with hippocampal sclerosis the mTOR pathway is activated, 6 and accordingly, mTOR hyperactivation has been demonstrated in various animal models for TLE. [6][7][8][9][10][11][12] Although the exact mechanisms through which mTOR hyperactivation could contribute to epileptogenesis are not yet fully understood, mTOR inhibition has been studied as a potential antiepileptogenic strategy in genetic as well as in acquired rodent models of epilepsy. 13 Antiepileptogenic effects of the mTOR inhibitor rapamycin have been reported in rodent post-SE models of TLE.…”

“…13 Antiepileptogenic effects of the mTOR inhibitor rapamycin have been reported in rodent post-SE models of TLE. 7,11,12 After kainic acid (KA)-induced SE in rats, a reduction in seizure frequency and mossy fiber sprouting was observed after rapamycin treatment. 12 In the same study, it was shown that rapamycin treatment prior to SE prevented the progressive development of spontaneous seizures.…”

“…9 In the post-SE model, where SE is induced by electrical stimulation of the angular bundle, rapamycin reduced seizure frequency, mossy fiber sprouting, and cell loss when treatment was started 4 h after SE and continued thereafter. 11 Thus although several antiepileptogenic effects of rapamycin have been suggested it is still debated whether mTOR inhibition is truly antiepileptogenic.…”