A Review of Current Treatments for Drug-Resistant Epilepsy via the mTOR Signaling Pathway

Rafi Matin^1*, Tristen Nimojan^2*, Arhum Razi^1*
1Honours Neuroscience, Class of 2021, McMaster University
²Honours Biochemistry, Class of 2021, McMaster University
matinr1@mcmaster.ca
nimojant@mcmaster.ca
razia1@mcmaster.ca
*authors contributed equally to this work

BACKGROUND 

Epilepsy is a neurological condition that affects 50-300 million people globally and is characterized by abnormal brain activity resulting in the presence and recurrence of seizures.¹ Of the affected individuals, 30% exhibit drug-resistant epilepsy (DRE), a form of epilepsy in which current anti-epileptic drugs are ineffective in treating their condition.² As a result, individuals with DRE experience several health issues and are at higher risk of experiencing deficits in cognitive and psychosocial function with increasing age.² 

In epilepsy, alterations in neurotransmitter and ion channel function results in hyperexcitability of neurons and epileptogenesis.³ For 70% of patients that experience conventional epilepsy, antiepileptic drugs are used to target and modify ion channels or neurotransmitter receptors that directly alter neuronal excitability. However, in cases involving DRE, it may be seen that there are secondary mutations or a loss of sensitivity which may impact the target of the pharmaceutical intervention.² Therefore, it is important to consider treatments that target alternate pathways, as a direct approach may not be as effective. 

One specific pathway that is thought to be strongly associated with epileptic activity and is a potential target for alternative DRE treatment is the mechanistic target of rapamycin (mTOR) pathway as it is involved in several cellular processes.⁴ Gene mutations that alter the mTOR pathway can result in hyperexcitability of certain neuronal populations.⁴ Through this review, we will discuss and critique the current literature on treatment methods for DRE that utilize the mTOR pathway such as mTOR inhibitors, the ketogenic diet, and neurostimulation. Currently, as there are few feasible treatment methods available for DRE, we identify areas of study that are potential future treatments. 

THE PI3K-MTOR SIGNALING PATHWAY 

The PI3K/AKT/mTOR pathway is a common intracellular signaling pathway that may be altered in epilepsy and can be targeted for potential treatments; this pathway is involved with regulating protein synthesis and mRNA translation of cell cycle genes. Key components of the pathway include the kinase proteins PI3K, AKT, and mTOR.⁴ The tuberous sclerosis complex (TSC) genes encode a complex of proteins that are involved in regulating the mTOR pathway by inhibiting mTOR (4). Through the phosphorylation of AKT by PI3K, the PI3K/AKT pathway is involved in the inhibition of the aforementioned TSC protein complex.^4,5 This increases mTOR activity, which is then indicated by greater phosphorylation of the downstream protein, S6.^4,5 Therefore, increased expression of phosphorylated AKT (pAKT) and phosphorylated S6 (pS6) indicate hyperactivity of the mTOR pathway. 

Findings from previous studies suggest neuronal hyperactivity of the mTOR pathway causes changes in protein expression and structural changes that result in enhanced excitatory synaptic transmission and epileptogenesis.^6,7 Alternative treatments for DRE may target the mTOR pathway to counter the enhanced neuronal excitation which is present in epilepsy. 

ALTERNATIVE TREATMENTS 

mTOR Inhibitors 

The anti-seizure effects of mTOR inhibitors have been widely researched as it is hypothesized that mTOR inhibitors directly reduce the activity of the mTOR pathway and counter the effects on neuronal excitability.⁵ From current research, it is thought to be unlikely that mTOR inhibitors alter ion channels or receptors directly, but are instead responsible for the eventual changes in expression of certain ion channels and receptor proteins through the activity of the mTOR pathway.^8,9

The effects of mTOR inhibitors on epileptic activity have been well studied in the TSC knockout (KO) mice model. In TSC KO mice, hyperactivity of the mTOR pathway and increased epileptic activity was observed. Zeng et al. studied the effect of rapamycin, an mTOR inhibitor, on TSC KO mice and found that treating these mice with rapamycin reduced seizures and epileptic spike frequencies.¹⁰ The researchers further found that TSC KO mice had reduced levels of specific astrocyte glutamate transporters, indicating that there is less excitatory activity. Rapamycin treatment was able to reverse this effect and increase the expression of glutamate transporters.¹⁰ These results suggest that rapamycin can be used to inhibit the TSC proteins, which in turn decreases epileptic activity. However, further research would be required into how the inhibition of mTOR would lead to the reduction of astrocyte glutamate transporters and allow for a decrease in epileptic activity. 

These findings were supported by studies that used the pilocarpine/kainate rat model, a separate epilepsy model. Pilocarpine and kainate are convulsive agents that cause several neurobiological changes similar to changes seen in temporal lobe epilepsy. Pilocarpine/kainate show increased mTOR activity which is indicated by increased S6 phosphorylation in the hippocampus.¹¹ 

Ketogenic Diet 

A common lifestyle change recommended for epilepsy patients is to follow a ketogenic diet which involves increasing fat consumption and decreasing carbohydrate and protein consumption.¹² This has been shown to alleviate epilepsy symptoms by inhibiting the hyperactivity of the mTOR pathway. Using the kainic acid (KA) induced status epilepticus (SE) mouse model which promotes epileptogenesis and recurrent seizures in the mouse, McDaniel et al. studied this effect.¹² McDaniel looked at whether the mTOR pathway activation markers pS6 and pAKT would be reduced in these mice when fed using the ketogenic diet.¹² After mice were fed with a normal diet, epileptogenesis was induced with KA leading to increased pS6 and pAKT expression in the hippocampus; this expression remained at baseline. When the mice were fed the ketogenic diet right after the epileptic episode pS6 and pAKT expression decreased below baseline, indicating less mTOR activity.¹² Another study done by Singh et al. used the Kcnal-null mouse model to create KO mice that were spontaneously epileptic and fed them either a normal diet or the ketogenic diet.¹³ The mice that were fed a ketogenic diet were found to have less pS6 markers, similar to the study by McDaniel.¹² 

Neurostimulation 

Neurostimulation utilizes focal stimulation towards epileptogenic regions of the brain with abnormal electrographic discharges in an attempt to stop a seizure. Transcranial Focused Ultrasound (tFUS) is newer, promising alternative neurostimulation technique with several improvements. tFUS is more appealing than current neurostimulation techniques as it is non-invasive and provides greater spatial resolution. A recent study by Chen et al. examined the effect of FUS treatment on rats that had induced-epileptogenic activity via alterations in the PI3K-mTOR signaling pathway. FUS exposure resulted in decreased epileptic spikes and decreased AKT and S6 indicating the normalization of the hyperactive PI3K-mTOR signaling pathway. It was also found that FUS treatment resulted in decreased c-Fos expression and increased GAD65 expression indicating a decrease in excitatory neural activity.¹⁴ Another study by Zou et al. explored FUS treatment in non human-primate models and found that FUS can be administered safely and effectively to deep brain regions.¹⁵ 

CONCLUSION 

Epilepsy is a condition that affects millions of people around the world, impacting both their current health and their future outlooks on life. Focusing specifically on DRE, we examined the current literature on treatment methods that regulate the mTOR pathway. In these cases, interventions for conventional epilepsy cannot be implemented and therefore other methods were reviewed. Through the use of mTOR inhibitors, the ketogenic diet, and tFUS, reductive effects on hyperexcitability were seen by targeting the mTOR pathway. 

Surveying the current literature, there seem to be consistent limitations that arise. In studying the mTOR pathway and its treatments, researchers often focus on the clinical aspect rather than focusing on the direct mechanisms that alter ion channels or receptors. Future research efforts can focus on identifying specific components of the mTOR pathway that directly contribute to epileptogenesis. Through the study of DRE, the pathways which it encompasses, and possible treatment methods, the lives of millions of people can be altered for the better. 

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