Therapeutic role of mammalian target of rapamycin (mTOR) inhibition in preventing epileptogenesis

Spatiotemporal characterization of mTOR kinase activity following kainic acid induced status epilepticus and analysis of rat brain response to chronic rapamycin treatment

Mammalian target of rapamycin (mTOR) is a protein kinase that senses nutrient availability, trophic factors support, cellular energy level, cellular stress, and neurotransmitters and adjusts cellular metabolism accordingly. Adequate mTOR activity is needed for development as well as proper physiology of mature neurons. Consequently, changes in mTOR activity are often observed in neuropathology. Recently, several groups reported that seizures increase mammalian target of rapamycin (mTOR) kinase activity, and such increased activity in genetic models can contribute to spontaneous seizures. However, the current knowledge about the spatiotemporal pattern of mTOR activation induced by proconvulsive agents is rather rudimentary. Also consequences of insufficient mTOR activity on a status epilepticus are poorly understood. Here, we systematically investigated these two issues. We showed that mTOR signaling was activated by kainic acid (KA)-induced status epilepticus through several brain areas, including the hippocampus and cortex as well as revealed two waves of mTOR activation: an early wave (2 h) that occurs in neurons and a late wave that predominantly occurs in astrocytes. Unexpectedly, we found that pretreatment with rapamycin, a potent mTOR inhibitor, gradually (i) sensitized animals to KA treatment and (ii) induced gross anatomical changes in the brain.

Mechanisms of epileptogenesis: a convergence on neural circuit dysfunction

Epilepsy is a prevalent neurological disorder associated with significant morbidity and mortality, but the only available drug therapies target its symptoms rather than the underlying cause. The process that links brain injury or other predisposing factors to the subsequent emergence of epilepsy is termed epileptogenesis. Substantial research has focused on elucidating the mechanisms of epileptogenesis so as to identify more specific targets for intervention, with the hope of preventing epilepsy before seizures emerge. Recent work has yielded important conceptual advances in this field. We suggest that such insights into the mechanisms of epileptogenesis converge at the level of cortical circuit dysfunction.

Understanding relationships between autism, intelligence, and epilepsy: a cross-disorder approach

AIM

As relationships between autistic traits, epilepsy, and cognitive functioning remain poorly understood, these associations were explored in the biologically related disorders tuberous sclerosis complex (TSC), neurofibromatosis type 1 (NF1), and epilepsy.

METHOD

The Social Responsiveness Scale (SRS), a quantitative measure of autistic traits, was distributed to caregivers or companions of patients with TSC, NF1, and childhood-onset epilepsy of unknown cause (EUC), and these results were compared with SRS data from individuals with idiopathic autism spectrum disorders (ASDs) and their unaffected siblings. Scores and trait profiles of autistic features were compared with cognitive outcomes, epilepsy variables, and genotype.

RESULTS

A total of 180 SRS questionnaires were completed in the TSC, NF1, and EUC outpatient clinics at the Massachusetts General Hospital (90 females, 90 males; mean age 21 y, range 4-63 y), and SRS data from 210 patients with ASD recruited from an autism research collaboration (167 males, 43 females; mean age 9 y, range 4-22 y) and 130 unaffected siblings were available. Regression models showed a significant association between SRS scores and intelligence outcomes (p<0.001) and various seizure variables (p<0.02), but not with a specific underlying disorder or genotype. The level of autistic features was strongly associated with intelligence outcomes in patients with TSC and epilepsy (p<0.01); in patients with NF1 these relationships were weaker (p=0.25). For all study groups, autistic trait subdomains covaried with neurocognitive comorbidity, with endophenotypes similar to that of idiopathic autism.

INTERPRETATION

Our data show that in TSC and childhood-onset epilepsy, the severity and phenotype of autistic features are inextricably linked with intelligence and epilepsy outcomes. Such relationships were weaker for individuals with NF1. Findings suggest that ASDs are not specific in these conditions.

Mammalian target of rapamycin (mTOR) inhibition: potential for antiseizure, antiepileptogenic, and epileptostatic therapy

New epilepsy treatments are needed that not only inhibit seizures symptomatically (antiseizure) but also prevent the development of epilepsy (antiepileptogenic). The mammalian target of rapamycin (mTOR) pathway may mediate mechanisms of epileptogenesis and serve as a rational therapeutic target. mTOR inhibitors have antiepileptogenic and antiseizure effects in animal models of the genetic disease, tuberous sclerosis complex. The mTOR pathway is also implicated in epileptogenesis in animal models of acquired epilepsy and infantile spasms, although the effects of mTOR inhibitors are variable depending on the specific conditions and model. Furthermore, beneficial effects on seizures are lost when treatment is withdrawn, suggesting that mTOR inhibitors are "epileptostatic" in only stalling epilepsy progression during treatment. Clinical studies of rapamycin in human epilepsy are limited, but suggest that mTOR inhibitors at least have antiseizure effects in tuberous sclerosis patients. Further studies are needed to assess the full potential of mTOR inhibitors for epilepsy treatment.

Rapamycin has age-, treatment paradigm-, and model-specific anticonvulsant effects and modulates neuropeptide Y expression in rats

PURPOSE

  Rapamycin (RAP) has certain antiepileptogenic features. However, it is unclear whether these effects can be explained by the anticonvulsant action of RAP, which has not been studied. To address this question, we tested potential anticonvulsant effects of RAP in immature and adult rats using different seizure models and treatment paradigms. In addition, we studied changes in the expression of neuropeptide Y (NPY) induced by RAP, which may serve as an indirect target of the RAP action.

METHODS

  A complex approach was adopted to evaluate the anticonvulsant potential of RAP: We used flurothyl-, pentylenetetrazole (PTZ)-, N-methyl-D-aspartate (NMDA)-, and kainic acid (KA)-induced seizures to test the effects of RAP using different pretreatment protocols in immature and adult rats. We also evaluated expression of NPY within the primary motor cortex, hippocampal CA1, and dentate gyrus (DG) after different pretreatments with RAP in immature rats.

KEY FINDINGS

  We found the following: (1) RAP administered with short-term pretreatment paradigms has a weak anticonvulsant potential in the seizure models with compromised inhibition. (2) Lack of RAP efficacy correlates with decreased NPY expression in the cortex, CA1, and DG. Specifically in immature rats, a single dose of RAP (3 mg/kg) 4 or 24 h before seizure testing had anticonvulsant effects against PTZ-induced seizures. In the flurothyl seizure model only the 4-h pretreatment with RAP was anticonvulsant in the both age groups. Short-term pretreatments with RAP had no effects against NMDA- and KA-induced seizures tested in immature rats. Long-term pretreatments with RAP over 8 days did not show beneficial effect in all tested seizure models in developing rats. Moreover, the long-term pretreatment with RAP had a slight proconvulsant effect on KA-induced seizures. In immature rats, any lack of anticonvulsant effect (including proconvulsant effect of multiple doses of RAP) was associated with downregulation of NPY expression in the cortex and DG. In immature animals, after a single dose of RAP with 24 h delay, we found a decrease of NPY expression in DG, and CA1 as well.

SIGNIFICANCE

  Our data show weak age-, treatment paradigm-, and model-specific anticonvulsant effects of RAP as well as loss of those effects after long-term RAP pretreatment associated with downregulation of NPY expression. These findings suggest that RAP is a poor anticonvulsant and may have beneficial effects only against epileptogenesis. In addition, our data present new insights into mechanisms of RAP action on seizures indicating a possible connection between mammalian target of rapamycin (mTOR) signaling and NPY system.

In vivo imaging of glia activation using 1H-magnetic resonance spectroscopy to detect putative biomarkers of tissue epileptogenicity

PURPOSE

  Long-lasting activation of glia occurs in brain during epileptogenesis, which develops after various central nervous system (CNS) injuries. Glia is the cell source of the biosynthesis and release of molecules that play a role in seizure recurrence and may contribute to epileptogenesis, thus representing a putative biomarker of epilepsy development and severity. In this study, we set up an in vivo longitudinal study using (1) H-magnetic resonance spectroscopy (MRS) to measure metabolite content in the rat hippocampus that could reflect the extent and the duration of glia activation. Our aim was to explore if glia activation during epileptogenesis, or in the chronic epileptic phase, can be used as a biomarker of tissue epileptogenicity (i.e., a measure of epilepsy severity).

METHODS

  (1) H-MRS measurements were done in the adult rat hippocampus every 24 h for 7 days after status epilepticus (SE) and in chronic epileptic rats, using a 7 T Bruker Biospec MRI (magnetic resonance imaging)/MRS scanner. We studied changes in metabolite levels that reflect astrocytes (myo-inositol, mIns; glutathione, GSH), microglia/macrophage activation and the associated neuronal cell injury/dysfunction (lactate, Lac; N-acetyl-aspartate, NAA). (1) H-MRS results were validated by post hoc immunohistochemistry using cell-specific markers. Data analysis was done to determine whether correlations exist between the metabolite changes and spontaneous seizure frequency or the extent of neuronal cell loss.

KEY FINDINGS

  The analysis of (1) H-MRS spectra showed a progressive increase in mIns and GSH levels after SE, which was maintained in epileptic rats. Lac signal transiently increased during epileptogenesis being undetectable in chronic epileptic tissue. NAA levels were chronically reduced from day 2 post-SE. Immunohistochemistry confirmed the activation of microglia and astrocytes and the progressive neuronal cell loss. GSH levels during epileptogenesis showed a negative correlation with the frequency of spontaneous seizures, whereas S100β levels in epileptic tissue were positively correlated with this outcome measure. A negative correlation was also found between GSH or mIns levels during epileptogenesis and the extent of neurodegeneration in hippocampus of epileptic rats.

SIGNIFICANCE

  (1) H-MRS is a valuable in vivo technique for determining the extent and temporal profile of glia activation after an epileptogenic injury. S100β levels measured in the epileptic tissue may represent a biomarker of seizure frequency, whereas GSH levels during epileptogenesis could serve as a predictive marker of seizure frequency. Both mIns and GSH levels measured before the onset of spontaneous seizures predict the extent of neuronal cell loss in epileptic tissue. These findings highlight the potential of serial (1) H-MRS analysis for searching epilepsy biomarkers for prognostic, diagnostic, or therapeutic purposes.

Rapamycin down-regulates KCC2 expression and increases seizure susceptibility to convulsants in immature rats

Seizure susceptibility to neurological insults, including chemical convulsants, is age-dependent and most likely reflective of overall differences in brain excitability. The molecular and cellular mechanisms underlying development-dependent seizure susceptibility remain to be fully understood. Because the mammalian target of rapamycin (mTOR) pathway regulates neurite outgrowth, synaptic plasticity and cell survival, thereby influencing brain development, we tested if exposure of the immature brain to the mTOR inhibitor rapamycin changes seizure susceptibility to neurological insults. We found that inhibition of mTOR by rapamycin in immature rats (3-4 weeks old) increases the severity of seizures induced by pilocarpine, including lengthening the total seizure duration and reducing the latency to the onset of seizures. Rapamycin also reduces the minimal dose of pentylenetetrazol (PTZ) necessary to induce clonic seizures. However, in mature rats, rapamycin does not significantly change the seizure sensitivity to pilocarpine and PTZ. Likewise, kainate sensitivity was not significantly affected by rapamycin treatment in either mature or immature rats. Additionally, rapamycin treatment down-regulates the expression of potassium-chloride cotransporter 2 (KCC2) in the thalamus and to a lesser degree in the hippocampus. Pharmacological inhibition of thalamic mTOR or KCC2 increases susceptibility to pilocarpine-induced seizure in immature rats. Thus, our study suggests a role for the mTOR pathway in age-dependent seizure susceptibility.

mTOR as a potential treatment target for epilepsy

Current treatments for epilepsy suffer from significant limitations, including medical intractability and lack of disease-modifying or anti-epileptogenic actions. As most current seizure medications modulate ion channels and neurotransmitter receptors, more effective therapies likely need to target completely different mechanisms of action. The mammalian target of rapamycin (mTOR) pathway represents a potential novel therapeutic target for epilepsy. mTOR inhibitors can suppress seizures and prevent epilepsy in animal models of certain genetic epilepsies, such as tuberous sclerosis complex. mTOR inhibitors may also be effective in some models of acquired epilepsy related to brain injury, but these effects are more variable and dependent on a number of factors. Some clinical data suggest that mTOR inhibitors decrease seizures in tuberous sclerosis complex patients, but controlled trials are lacking and no clinical data on potential anti-epileptogenic actions exist. Future basic and clinical research will help to determine the full potential of mTOR inhibitors for epilepsy.

Prion-like mechanisms in epileptogenesis

Epilepsy often follows a focal insult, and develops with a time delay so to reveal a complex cascade of events. Both clinical and experimental findings suggest that the initial insult triggers a self-promoted pathological process, currently named epileptogenesis. An early phase reflects the complex response of the nervous system to the insult, which includes pro-injury and pro-repair mechanisms. Successively, the sprouting and probably neurogenesis and gliosis set up the stage for the onset of spontaneous seizures. Thus, local changes in excitability would cause a functional change within a network, and the altered circuitry would favor the seizures. A latent or clinically silent period, as long as years, may precede epilepsy. In spite of the substantial knowledge on the biochemical and morphological changes associated with epileptogenesis, the mechanisms supposedly underlying the process are still uncertain. The uncertainty refers mostly to the silent period, a stage in which most, if not all, the receptor and ion changes are supposedly settled. It is tempting to explore the nature of the factors promoting the epileptogenesis within the notional field of neurodegeneration. Specifically, several observations converge to support the hypothesis that a prion-like mechanism promotes the "maturation" process underlying epileptogenesis. The mechanism, consistently with data from different neurodegenerative diseases, is predictably associated with deposition of self-aggregating misfolded proteins and changes of the ubiquitin proteasome and autophagy-lysosome pathways.

Low-carbohydrate ketogenic diets, glucose homeostasis, and nonalcoholic fatty liver disease

PURPOSE OF REVIEW

Obesity-associated nonalcoholic fatty liver disease (NAFLD) is highly prevalent, for which weight loss is the generally recommended clinical management. Low-carbohydrate ketogenic diets have been successful in promoting weight loss, but variations in the range of metabolic responses to these diets indicate that the effects of altering macronutrient content are not completely understood. This review focuses on the most recent findings that reveal the relationship between low-carbohydrate diets and NAFLD in rodent models and humans.

RECENT FINDINGS

Low-carbohydrate diets have been shown to promote weight loss, decrease intrahepatic triglyceride content, and improve metabolic parameters of patients with obesity. These ketogenic diets also provoke weight loss in rodents. However, long-term maintenance on a ketogenic diet stimulates the development of NAFLD and systemic glucose intolerance in mice. The relationship between ketogenic diets and systemic insulin resistance in both humans and rodents remains to be elucidated.

SUMMARY

Because low-carbohydrate ketogenic diets are increasingly employed for treatment of obesity, NAFLD, and neurological diseases such as epilepsy, understanding the long-term systemic effects of low-carbohydrate diets is crucial to the development of efficacious and safe dietary interventions.

Mapping the spatio-temporal pattern of the mammalian target of rapamycin (mTOR) activation in temporal lobe epilepsy

Growing evidence from rodent models of temporal lobe epilepsy (TLE) indicates that dysregulation of the mammalian target of rapamycin (mTOR) pathway is involved in seizures and epileptogenesis. However, the role of the mTOR pathway in the epileptogenic process remains poorly understood. Here, we used an animal model of TLE and sclerotic hippocampus from patients with refractory TLE to determine whether cell-type specific activation of mTOR signaling occurs during each stage of epileptogenesis. In the TLE mouse model, we found that hyperactivation of the mTOR pathway is present in distinct hippocampal subfields at three different stages after kainate-induced seizures, and occurs in neurons of the granular and pyramidal cell layers, in reactive astrocytes, and in dispersed granule cells, respectively. In agreement with the findings in TLE mice, upregulated mTOR was observed in the sclerotic hippocampus of TLE patients. All sclerotic hippocampus (n = 13) exhibited widespread reactive astrocytes with overactivated mTOR, some of which invaded the dispersed granular layer. Moreover, two sclerotic hippocampus exhibited mTOR activation in some of the granule cells, which was accompanied by cell body hypertrophy. Taken together, our results indicate that mTOR activation is most prominent in reactive astrocytes in both an animal model of TLE and the sclerotic hippocampus from patients with drug resistant TLE.

Posttraumatic Epilepsy: What's Contusion Got to Do With It?

Text of Abstract Liability to develop posttraumatic epilepsy (PTE) correlates in a general way with trauma dose. While contusion of the brain produces an admixture of extravasated blood, edema fluid and necrotic tissue at the site of skull trauma and in regions remote from the direct force, an unpredictable cascade of shearing injury, torsion and rotation and a myriad of physiological changes occur in structures subject to the mechanical pressure wave. Animal models mimic components of injury, some more thoroughly than others. Designing a treatment that is a prophylaxis for the development of PTE awaits understanding the mechanisms of epileptogenesis initiated by trauma.

The ketogenic diet as a treatment paradigm for diverse neurological disorders

Dietary and metabolic therapies have been attempted in a wide variety of neurological diseases, including epilepsy, headache, neurotrauma, Alzheimer disease, Parkinson disease, sleep disorders, brain cancer, autism, pain, and multiple sclerosis. The impetus for using various diets to treat - or at least ameliorate symptoms of - these disorders stems from both a lack of effectiveness of pharmacological therapies, and also the intrinsic appeal of implementing a more "natural" treatment. The enormous spectrum of pathophysiological mechanisms underlying the aforementioned diseases would suggest a degree of complexity that cannot be impacted universally by any single dietary treatment. Yet, it is conceivable that alterations in certain dietary constituents could affect the course and impact the outcome of these brain disorders. Further, it is possible that a final common neurometabolic pathway might be influenced by a variety of dietary interventions. The most notable example of a dietary treatment with proven efficacy against a neurological condition is the high-fat, low-carbohydrate ketogenic diet (KD) used in patients with medically intractable epilepsy. While the mechanisms through which the KD works remain unclear, there is now compelling evidence that its efficacy is likely related to the normalization of aberrant energy metabolism. The concept that many neurological conditions are linked pathophysiologically to energy dysregulation could well provide a common research and experimental therapeutics platform, from which the course of several neurological diseases could be favorably influenced by dietary means. Here we provide an overview of studies using the KD in a wide panoply of neurologic disorders in which neuroprotection is an essential component.

Is there any place for macrolides in mood disorders?

Macrolides are protein synthesis inhibitors exerting an action on the bacterial ribosome. The ribosomes coded for by the human mitochondrial deoxyribonucleic acid (DNA) are similar to those from bacteria in size and structure. In addition, mitochondria are thought to have originated from a symbiotic relationship between an anaerobic proto-eukaryotic cell that engulfed an aerobic bacterium. Morphological changes of mitochondria have been observed in bipolar disorder and schizophrenia. Manic episodes associated with the use of antimicrobial agents have been described since the discovery of isoniazid. The oxidative stress induced in the neuronal mitochondria is thought to underlie this effect. The inhibition of GSK-3β in the intra-mitochondrial Akt signaling pathway is thought to convey mood stabilizing properties. Rapamycin is a macrolide that, besides its antiepileptic effect, restores the Akt function and inhibits the mTOR pathway which may have an antidepressant effect. Accordingly, it is hypothesized that rapamycin may have mood stabilizing properties.

New strategies for preventing epileptogenesis: perspective and overview

Epilepsy is a common disorder with a major negative impact on patient quality of life, yet treatment so far is directed mainly at blocking the symptoms-epileptic seizures, not the underlying cause. In recent years, investigation of epilepsy development or epileptogenesis has yielded new insights into potential therapies that may ultimately prevent epilepsy before it starts. In this special issue of Neuroscience Letters the latest advances in the field are brought together, summarizing: (1) important animal models in both primary and secondary epilepsies, (2) promising biomarkers for monitoring epileptogenesis, (3) cellular and molecular mechanisms which may serve as viable targets for therapy, and (4) translational approaches to human clinical trials. Bringing together these intriguing new approaches to treating epilepsy as a preventable disorder will hopefully soon make symptomatic treatment of epilepsy unnecessary in most patients.