Inhibition of the mammalian target of rapamycin signaling pathway suppresses dentate granule cell axon sprouting in a rodent model of temporal lobe epilepsy

Axon regrowth during development and regeneration following injury share molecular mechanisms

BACKGROUND

The molecular mechanisms that determine axonal growth potential are poorly understood. Intrinsic growth potential decreases with age, and thus one strategy to identify molecular pathways controlling intrinsic growth potential is by studying developing young neurons. The programmed and stereotypic remodeling of Drosophila mushroom body (MB) neurons during metamorphosis offers a unique opportunity to uncover such mechanisms. Despite emerging insights into MB γ-neuron axon pruning, nothing is known about the ensuing axon re-extension.

RESULTS

Using mosaic loss of function, we found that the nuclear receptor UNF (Nr2e3) is cell autonomously required for the re-extension of MB γ-axons following pruning, but not for the initial growth or guidance of any MB neuron type. We found that UNF promotes this process of developmental axon regrowth via the TOR pathway as well as a late axon guidance program via an unknown mechanism. We have thus uncovered a novel developmental program of axon regrowth that is cell autonomously regulated by the UNF nuclear receptor and the TOR pathway.

CONCLUSIONS

Our results suggest that UNF activates neuronal re-extension during development. Taken together, we show that axon growth during developmental remodeling is mechanistically distinct from initial axon outgrowth. Due to the involvement of the TOR pathway in axon regeneration following injury, our results also suggests that developmental regrowth shares common molecular mechanisms with regeneration following injury.

Spatio-temporal expression study of phosphorylated 70-kDa ribosomal S6 kinase (p70S6k) in mesial temporal lobe epilepsy

OBJECTIVE

To determine the spatio-temporal expression of p70S6k activation in hippocampus in mesial temporal lobe epilepsy.

METHODS

Temporal lobe epilepsy model was established by stereotaxically unilateral and intrahippocampal injection of kainite acid (KA) in adult male C57BL/6 mice. Latent and chronic epileptogenesis were represented by mice 5 days after KA injection (n = 5) and mice 5 weeks after KA injection (n = 8), respectively. Control mice (n = 5) were injected with saline. Immunohistochemical assays were performed on brain sections of the mice.

RESULTS

Hippocampus both ipsilateral and contralateral to the KA injection displayed significantly up-regulated pS6 immunoreactivity in dispersed granule cells in 5-day and 5-week model mice.

CONCLUSION

The activation of p70S6k is mainly located in the dentate gyrus in KA-induced mouse model of temporal lobe epilepsy, indicating that the activation may be related with the disperse degree and hypertrophy of granule cells.

The mTOR pathway is activated in glial cells in mesial temporal sclerosis

Mammalian target of rapamycin (mTOR) is a key protein kinase that regulates basic cellular processes, including development and growth. Mutations in mTOR cause tuberous sclerosis complex (TSC), a condition that is characterized by developmental brain malformations (cortical tubers) and epilepsy. Although considerable insight has been gained recently into the pathologic dysfunction of mTOR in tubers in TSC-related epilepsy, data on the mTOR cascade in mesial temporal lobe epilepsy (MTLE) are lacking. Immunohistochemical investigation with confocal microscopy was performed to evaluate mTOR cascade and to correlate its activity with cellular alterations observed in surgically resected samples of human neocortex and hippocampus in MTLE. We compared results in human tissue to findings in the rat pilocarpine model of sclerotic MTLE. In nonsclerotic and control hippocampus, many neurons in the CA1 subfield expressed high levels of phospho-S6 (p-S6), a reliable marker of mTOR activation. In nonsclerotic and control hippocampus, as well as in magnetic resonance imaging (MRI) normal human neocortex, protoplasmic astrocytes did not express p-S6. In contrast, in sclerotic hippocampus, prominent p-S6 immunostaining was observed mainly in astrocytes and microglia located in the areas of neuronal loss and astrogliosis, whereas neurons in preserved areas of CA1 expressed significantly lower levels of p-S6 immunopositivity than neurons in nonsclerotic or control CA1 subfields. In surgically resected neocortex with chronic astroglial scar tissue, only microglia revealed moderate p-S6 immunoreactivity. Different from human sclerotic epileptic hippocampus, astrogliosis in the chronic rat pilocarpine model of epilepsy was not characterized by glial cells with mTOR activation. The mTOR cascade is activated in astroglial cells in sclerotic MTLE, but not in astrocytes in chronic neocortical scarring or in the pilocarpine model of MTLE. These findings suggest that the astroglial "scar" in sclerotic MTLE has active, ongoing cellular changes. Targeting mTOR in MTLE may provide new pathways for the medical therapy of epilepsy.

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.

Lovastatin modulates glycogen synthase kinase-3β pathway and inhibits mossy fiber sprouting after pilocarpine-induced status epilepticus

This study was undertaken to assay the effect of lovastatin on the glycogen synthase kinase-3 beta (GSK-3β) and collapsin responsive mediator protein-2 (CRMP-2) signaling pathway and mossy fiber sprouting (MFS) in epileptic rats. MFS in the dentate gyrus (DG) is an important feature of temporal lobe epilepsy (TLE) and is highly related to the severity and the frequency of spontaneous recurrent seizures. However, the molecular mechanism of MFS is mostly unknown. GSK-3β and CRMP-2 are the genes responsible for axonal growth and neuronal polarity in the hippocampus, therefore this pathway is a potential target to investigate MFS. Pilocarpine-induced status epilepticus animal model was taken as our researching material. Western blot, histological and electrophysiological techniques were used as the studying tools. The results showed that the expression level of GSK-3β and CRMP-2 were elevated after seizure induction, and the administration of lovastatin reversed this effect and significantly reduced the extent of MFS in both DG and CA3 region in the hippocampus. The alteration of expression level of GSK-3β and CRMP-2 after seizure induction proposes that GSK-3β and CRMP-2 are crucial for MFS and epiletogenesis. The fact that lovastatin reversed the expression level of GSK-3β and CRMP-2 indicated that GSK-3β and CRMP-2 are possible to be a novel mechanism of lovatstain to suppress MFS and revealed a new therapeutic target and researching direction for studying the mechanism of MFS and epileptogenesis.

TOR is required for the retrograde regulation of synaptic homeostasis at the Drosophila neuromuscular junction

Homeostatic mechanisms operate to stabilize synaptic function; however, we know little about how they are regulated. Exploiting Drosophila genetics, we have uncovered a critical role for the target of rapamycin (TOR) in the regulation of synaptic homeostasis at the Drosophila larval neuromuscular junction. Loss of postsynaptic TOR disrupts a retrograde compensatory enhancement in neurotransmitter release that is normally triggered by a reduction in postsynaptic glutamate receptor activity. Moreover, postsynaptic overexpression of TOR or a phosphomimetic form of S6 ribosomal protein kinase, a common target of TOR, can trigger a strong retrograde increase in neurotransmitter release. Interestingly, heterozygosity for eIF4E, a critical component of the cap-binding protein complex, blocks the retrograde signal in all these cases. Our findings suggest that cap-dependent translation under the control of TOR plays a critical role in establishing the activity dependent homeostatic response at the NMJ.

Finding a better drug for epilepsy: the mTOR pathway as an antiepileptogenic target

The mammalian target of rapamycin (mTOR) signaling pathway regulates cell growth, differentiation, proliferation, and metabolism. Loss-of-function mutations in upstream regulators of mTOR have been highly associated with dysplasias, epilepsy, and neurodevelopmental disorders. These include tuberous sclerosis, which is due to mutations in TSC1 or TSC2 genes; mutations in phosphatase and tensin homolog (PTEN) as in Cowden syndrome, polyhydramnios, megalencephaly, symptomatic epilepsy syndrome (PMSE) due to mutations in the STE20-related kinase adaptor alpha (STRADalpha); and neurofibromatosis type 1 attributed to neurofibromin 1 mutations. Inhibition of the mTOR pathway with rapamycin may prevent epilepsy and improve the underlying pathology in mouse models with disrupted mTOR signaling, due to PTEN or TSC mutations. However the timing and duration of its administration appear critical in defining the seizure and pathology-related outcomes. Rapamycin application in human cortical slices from patients with cortical dysplasias reduces the 4-aminopyridine-induced oscillations. In the multiple-hit model of infantile spasms, pulse high-dose rapamycin administration can reduce the cortical overactivation of the mTOR pathway, suppresses spasms, and has disease-modifying effects by partially improving cognitive deficits. In post-status epilepticus models of temporal lobe epilepsy, rapamycin may ameliorate the development of epilepsy-related pathology and reduce the expression of spontaneous seizures, but its effects depend on the timing and duration of administration, and possibly the model used. The observed recurrence of seizures and epilepsy-related pathology after rapamycin discontinuation suggests the need for continuous administration to maintain the benefit. However, the use of pulse administration protocols may be useful in certain age-specific epilepsy syndromes, like infantile spasms, whereas repetitive-pulse rapamycin protocols may suffice to sustain a long-term benefit in genetic disorders of the mTOR pathway. In summary, mTOR dysregulation has been implicated in several genetic and acquired forms of epileptogenesis. The use of mTOR inhibitors can reverse some of these epileptogenic processes, although their effects depend upon the timing and dose of administration as well as the model used.

The interaction between early life epilepsy and autistic-like behavioral consequences: a role for the mammalian target of rapamycin (mTOR) pathway

Early life seizures can result in chronic epilepsy, cognitive deficits and behavioral changes such as autism, and conversely epilepsy is common in autistic children. We hypothesized that during early brain development, seizures could alter regulators of synaptic development and underlie the interaction between epilepsy and autism. The mammalian Target of Rapamycin (mTOR) modulates protein translation and is dysregulated in Tuberous Sclerosis Complex, a disorder characterized by epilepsy and autism. We used a rodent model of acute hypoxia-induced neonatal seizures that results in long term increases in neuronal excitability, seizure susceptibility, and spontaneous seizures, to determine how seizures alter mTOR Complex 1 (mTORC1) signaling. We hypothesized that seizures occurring at a developmental stage coinciding with a critical period of synaptogenesis will activate mTORC1, contributing to epileptic networks and autistic-like behavior in later life. Here we show that in the rat, baseline mTORC1 activation peaks during the first three postnatal weeks, and induction of seizures at postnatal day 10 results in further transient activation of its downstream targets phospho-4E-BP1 (Thr37/46), phospho-p70S6K (Thr389) and phospho-S6 (Ser235/236), as well as rapid induction of activity-dependent upstream signaling molecules, including BDNF, phospho-Akt (Thr308) and phospho-ERK (Thr202/Tyr204). Furthermore, treatment with the mTORC1 inhibitor rapamycin immediately before and after seizures reversed early increases in glutamatergic neurotransmission and seizure susceptibility and attenuated later life epilepsy and autistic-like behavior. Together, these findings suggest that in the developing brain the mTORC1 signaling pathway is involved in epileptogenesis and altered social behavior, and that it may be a target for development of novel therapies that eliminate the progressive effects of neonatal seizures.

Rapamycin attenuates aggressive behavior in a rat model of pilocarpine-induced epilepsy

Psychiatric disorders are fairly common comorbidities of epilepsy in humans. Following pilocarpine-induced status epilepticus (SE), experimental animals not only developed spontaneous recurrent seizures, but also exhibited significantly elevated levels of aggressive behavior. The cellular and molecular mechanism triggering these behavioral alterations remains unclear. In the present study, we found that aggression is positively correlated with development of spontaneous seizures. Treatment with rapamycin, a potent mTOR (mammalian target of rapamycin pathway)-pathway inhibitor, markedly diminished aggressive behavior. Therefore, the mTOR pathway may have significance in the underlying molecular mechanism leading to aggression associated with epilepsy.

Differentiation and functional incorporation of embryonic stem cell-derived GABAergic interneurons in the dentate gyrus of mice with temporal lobe epilepsy

Cell therapies for neurological disorders require an extensive knowledge of disease-associated neuropathology and procedures for generating neurons for transplantation. In many patients with severe acquired temporal lobe epilepsy (TLE), the dentate gyrus exhibits sclerosis and GABAergic interneuron degeneration. Mounting evidence suggests that therapeutic benefits can be obtained by transplanting fetal GABAergic progenitors into the dentate gyrus in rodents with TLE, but the scarcity of human fetal cells limits applicability in patient populations. In contrast, virtually limitless quantities of neural progenitors can be obtained from embryonic stem (ES) cells. ES cell-based therapies for neurological repair in TLE require evidence that the transplanted neurons integrate functionally and replace cell types that degenerate. To address these issues, we transplanted mouse ES cell-derived neural progenitors (ESNPs) with ventral forebrain identities into the hilus of the dentate gyrus of mice with TLE and evaluated graft differentiation, mossy fiber sprouting, cellular morphology, and electrophysiological properties of the transplanted neurons. In addition, we compared electrophysiological properties of the transplanted neurons with endogenous hilar interneurons in mice without TLE. The majority of transplanted ESNPs differentiated into GABAergic interneuron subtypes expressing calcium-binding proteins parvalbumin, calbindin, or calretinin. Global suppression of mossy fiber sprouting was not observed; however, ESNP-derived neurons formed dense axonal arborizations in the inner molecular layer and throughout the hilus. Whole-cell hippocampal slice electrophysiological recordings and morphological analyses of the transplanted neurons identified five basic types; most with strong after-hyperpolarizations and smooth or sparsely spiny dendritic morphologies resembling endogenous hippocampal interneurons. Moreover, intracellular recordings of spontaneous EPSCs indicated that the new cells functionally integrate into epileptic hippocampal circuitry.

Identification of new epilepsy treatments: issues in preclinical methodology

Preclinical research has facilitated the discovery of valuable drugs for the symptomatic treatment of epilepsy. Yet, despite these therapies, seizures are not adequately controlled in a third of all affected individuals, and comorbidities still impose a major burden on quality of life. The introduction of multiple new therapies into clinical use over the past two decades has done little to change this. There is an urgent demand to address the unmet clinical needs for: (1) new symptomatic antiseizure treatments for drug-resistant seizures with improved efficacy/tolerability profiles, (2) disease-modifying treatments that prevent or ameliorate the process of epileptogenesis, and (3) treatments for the common comorbidities that contribute to disability in people with epilepsy. New therapies also need to address the special needs of certain subpopulations, that is, age- or gender-specific treatments. Preclinical development in these treatment areas is complex due to heterogeneity in presentation and etiology, and may need to be formulated with a specific seizure, epilepsy syndrome, or comorbidity in mind. The aim of this report is to provide a framework that will help define future guidelines that improve and standardize the design, reporting, and validation of data across preclinical antiepilepsy therapy development studies targeting drug-resistant seizures, epileptogenesis, and comorbidities.

Deconvoluting mTOR biology

In metazoans, TOR is an essential protein that functions as a master regulator of cellular growth and proliferation. Over the past decade, there has been an explosion of information about this critical master kinase, ranging from the composition of the TOR protein complex to its ability to act as an integrator of numerous extracellular signals. Unfortunately, this plethora of information has also raised numerous questions regarding TOR function. Currently, the prevailing view is that mammalian TOR (mTOR) exists in at least two molecular complexes, mTORC1 and mTORC2, which are largely defined by the presence of either RAPTOR or RICTOR. However, additional co-factors have been identified for each complex, and their importance in mediating mTOR signals has been incompletely elucidated. Similarly, there are differences in mTOR function that reflect the tissue of origin. In this review, we present an alternative view to mTOR complex formation and function, which envisions mTOR regulation and signal propagation as a reflection of cell type- and basal state-dependent conditions. The re-interpretation of mTOR biology in this framework may facilitate the design of therapies most likely to effectively inhibit this central regulator of cell behavior.

Pentylenetetrazole-induced seizures cause acute, but not chronic, mTOR pathway activation in rat

PURPOSE

The mammalian target of rapamycin (mTOR) pathway has been implicated in contributing to progressive epileptogenesis in models of chronic epilepsy. Conversely, seizures themselves may directly cause acute activation of the mTOR pathway. To isolate the direct effects of seizures on the mTOR pathway, the time course and mechanisms of mTOR activation were investigated with acute seizures induced by pentylenetetrazole (PTZ), which does not lead to chronic epilepsy.

METHODS

Western blot analysis was used to assay the phosphorylation of Akt and S6, as measures of activation of the phosphoinositide 3-kinase (PI3K)/Akt and mTOR pathways, respectively, at various time points after PTZ-induced seizures in rats. The ability of wortmannin, a PI3K inhibitor, to inhibit PTZ seizure-induced activation of the mTOR pathway was tested.

KEY FINDINGS

PTZ-induced seizures produced an immediate, transient mTOR activation lasting several hours, but no later, more chronic activation over days to weeks. This acute stimulation of the mTOR pathway by PTZ-induced seizures was mediated by upstream PI3K/Akt pathway activation and was blocked by a PI3K inhibitor.

SIGNIFICANCE

Compared with models of chronic epilepsy that exhibit biphasic (acute and chronic) mTOR pathway activation, PTZ-induced seizures produce only acute, but not chronic, mTOR activation. These results in the PTZ seizure model highlight potential differences in the involvement of the mTOR pathway between self-limited seizures and progressive epileptogenesis. These findings also suggest a potential therapeutic role of PI3K inhibitors in epilepsy.

Post-treatment with rapamycin does not prevent epileptogenesis in the amygdala stimulation model of temporal lobe epilepsy

Approximately 30% of all epilepsy cases are acquired. At present there is no effective strategy to stop epilepsy development after the precipitating insult. Recent data from experimental models pointed to the mTOR pathway, which can be potently inhibited by rapamycin. However, data on the antiepileptic and antiepileptogenic properties of rapamycin are conflicting. Therefore, we tested whether rapamycin post-treatment influences epileptogenesis in the amygdala stimulation model of temporal lobe epilepsy in rats. Animals were treated with rapamycin (6mg/kg) or vehicle daily for 2 wks, beginning 24h after stimulation. Sham-operated animals were treated with rapamycin or vehicle but were not stimulated. Animals were video-EEG monitored to detect spontaneous seizures. Animals were sacrificed 4 wks later and brains were collected for Timm staining. There were no significant differences in the number of stimulated rats developing epilepsy; latency to first spontaneous seizure; number of seizures, or seizure frequency in epileptic animals. The area occupied by mossy fibers was significantly increased in stimulated vs. sham-operated animals but was not different in animals treated with rapamycin vs. vehicle. Collectively, our data suggest that the antiepileptic or antiepileptogenic action of rapamycin is not a universal phenomenon and might be limited to certain experimental models or experimental conditions.

S6 kinase localizes to the presynaptic active zone and functions with PDK1 to control synapse development

S6 kinase localizes to the active zone in a Brp-dependent manner and collaborates with presynaptic PDK1 to modulate neuronal cell size, bouton size, active zone number, and neurotransmitter release.

The dimensions of neuronal dendrites, axons, and synaptic terminals are reproducibly specified for each neuron type, yet it remains unknown how these structures acquire their precise dimensions of length and diameter. Similarly, it remains unknown how active zone number and synaptic strength are specified relative the precise dimensions of presynaptic boutons. In this paper, we demonstrate that S6 kinase (S6K) localizes to the presynaptic active zone. Specifically, S6K colocalizes with the presynaptic protein Bruchpilot (Brp) and requires Brp for active zone localization. We then provide evidence that S6K functions downstream of presynaptic PDK1 to control synaptic bouton size, active zone number, and synaptic function without influencing presynaptic bouton number. We further demonstrate that PDK1 is also a presynaptic protein, though it is distributed more broadly. We present a model in which synaptic S6K responds to local extracellular nutrient and growth factor signaling at the synapse to modulate developmental size specification, including cell size, bouton size, active zone number, and neurotransmitter release.

CB2 cannabinoid receptors promote neural progenitor cell proliferation via mTORC1 signaling

The endocannabinoid system is known to regulate neural progenitor (NP) cell proliferation and neurogenesis. In particular, CB(2) cannabinoid receptors have been shown to promote NP proliferation. As CB(2) receptors are not expressed in differentiated neurons, CB(2)-selective agonists are promising candidates to manipulate NP proliferation and indirectly neurogenesis by overcoming the undesired psychoactive effects of neuronal CB(1) cannabinoid receptor activation. Here, by using NP cells, brain organotypic cultures, and in vivo animal models, we investigated the signal transduction mechanism involved in CB(2) receptor-induced NP cell proliferation and neurogenesis. Exposure of hippocampal HiB5 NP cells to the CB(2) receptor-selective agonist HU-308 led to the activation of the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin complex 1 (mTORC1) pathway, which, by inhibiting its downstream target p27Kip1, induced NP proliferation. Experiments conducted with the CB(2) receptor-selective antagonist SR144528, inhibitors of the PI3K/Akt/mTORC1 axis, and CB(2) receptor transient-transfection vector further supported that CB(2) receptors control NP cell proliferation via activation of mTORC1 signaling. Likewise, CB(2) receptor engagement induced cell proliferation in an mTORC1-dependent manner both in embryonic cortical slices and in adult hippocampal NPs. Thus, HU-308 increased ribosomal protein S6 phosphorylation and 5-bromo-2'-deoxyuridine incorporation in wild-type but not CB(2) receptor-deficient NPs of the mouse subgranular zone. Moreover, adult hippocampal NP proliferation induced by HU-308 and excitotoxicity was blocked by the mTORC1 inhibitor rapamycin. Altogether, these findings provide a mechanism of action and a rationale for the use of nonpsychotomimetic CB(2) receptor-selective ligands as a novel strategy for the control of NP cell proliferation and neurogenesis.

Is there a critical period for mossy fiber sprouting in a mouse model of temporal lobe epilepsy?

PURPOSE

Dentate granule cell axon (mossy fiber) sprouting creates an aberrant positive-feedback circuit that might be epileptogenic. Presumably, mossy fiber sprouting is initiated by molecular signals, but it is unclear whether they are expressed transiently or persistently. If transient, there might be a critical period when short preventative treatments could permanently block mossy fiber sprouting. Alternatively, if signals persist, continuous treatment would be necessary. The present study tested whether temporary treatment with rapamycin has long-term effects on mossy fiber sprouting.

METHODS

Mice were treated daily with 1.5 mg/kg rapamycin or vehicle (i.p.) beginning 24 h after pilocarpine-induced status epilepticus. Mice were perfused for anatomic evaluation immediately after 2 months of treatment ("0 delay") or after an additional 6 months without treatment ("6-month delay"). One series of sections was Timm-stained, and an adjacent series was Nissl-stained. Stereologic methods were used to measure the volume of the granule cell layer plus molecular layer and the Timm-positive fraction. Numbers of Nissl-stained hilar neurons were estimated using the optical fractionator method.

KEY FINDINGS

At 0 delay, rapamycin-treated mice had significantly less black Timm staining in the granule cell layer plus molecular layer than vehicle-treated animals. However, by 6-month delay, Timm staining had increased significantly in mice that had been treated with rapamycin. Percentages of the granule cell layer plus molecular layer that were Timm-positive were high and similar in 0 delay vehicle-treated, 6-month delay vehicle-treated, and 6-month delay rapamycin-treated mice. Extent of hilar neuron loss was similar among all groups that experienced status epilepticus and, therefore, was not a confounding factor. Compared to naive controls, average volume of the granule cell layer plus molecular layer was larger in 0 delay vehicle-treated mice. The hypertrophy was partially suppressed in 0 delay rapamycin-treated mice. However, 6-month delay vehicle- and 6-month delay rapamycin-treated animals had similar average volumes of the granule cell layer plus molecular layer that were significantly larger than those of all other groups.

SIGNIFICANCE

Status epilepticus-induced mossy fiber sprouting and dentate gyrus hypertrophy were suppressed by systemic treatment with rapamycin but resumed after treatment ceased. These findings suggest that molecular signals that drive mossy fiber sprouting and dentate gyrus hypertrophy might persist for >2 months after status epilepticus in mice. Therefore, prolonged or continuous treatment might be required to permanently suppress mossy fiber sprouting.

Inhibition of the mammalian target of rapamycin blocks epilepsy progression in NS-Pten conditional knockout mice

PURPOSE

Increased activity of mTOR Complex 1 (mTORC1) has been demonstrated in cortical dysplasia and tuberous sclerosis complex, as well as in animal models of epilepsy. Recent studies in such models revealed that inhibiting mTORC1 with rapamycin effectively suppressed seizure activity. However, seizures can recur after treatment cessation, and continuous rapamycin exposure can adversely affect animal growth and health. Here, we evaluated the efficacy of an intermittent rapamycin treatment protocol on epilepsy progression using neuron subset-specific-Pten (NS-Pten) conditional knockout mice.

METHODS

NS-Pten knockouts were treated with a single course of rapamycin during postnatal weeks 4 and 5, or intermittently over a period of 5 months. Epileptiform activity was monitored using video-electroencephalography (EEG) recordings, and mossy fiber sprouting was evaluated using Timm staining. Survival and body weight were assessed in parallel.

KEY FINDINGS

NS-Pten knockouts treated with a single course of rapamycin had recurrence of epilepsy 4-7 weeks after treatment ended. In contrast, epileptiform activity remained suppressed, and survival increased if knockout mice received additional rapamycin during weeks 10-11 and 16-17. Aberrant mossy fiber sprouting, present by 4 weeks of age and progressing in parallel with epileptiform activity, was also blocked by rapamycin.

SIGNIFICANCE

These findings demonstrate that a single course of rapamycin treatment suppresses epileptiform activity and mossy fiber sprouting for several weeks before epilepsy recurs. However, additional intermittent treatments with rapamycin prevented this recurrence and enhanced survival without compromising growth. Therefore, these studies add to the growing body of evidence implicating an important role for mTORC1 signaling in epilepsy.

Rapamycin suppresses axon sprouting by somatostatin interneurons in a mouse model of temporal lobe epilepsy

PURPOSE

In temporal lobe epilepsy many somatostatin interneurons in the dentate gyrus die. However, some survive and sprout axon collaterals that form new synapses with granule cells. The functional consequences of γ-aminobutyric acid (GABA)ergic synaptic reorganization are unclear. Development of new methods to suppress epilepsy-related interneuron axon sprouting might be useful experimentally.

METHODS

Status epilepticus was induced by systemic pilocarpine treatment in green fluorescent protein (GFP)-expressing inhibitory nerurons (GIN) mice in which a subset of somatostatin interneurons expresses GFP. Beginning 24 h later, mice were treated with vehicle or rapamycin (3 mg/kg intraperitoneally) every day for 2 months. Stereologic methods were then used to estimate numbers of GFP-positive hilar neurons per dentate gyrus and total length of GFP-positive axon in the molecular layer plus granule cell layer. GFP-positive axon density was calculated. The number of GFP-positive axon crossings of the granule cell layer was measured. Regression analyses were performed to test for correlations between GFP-positive axon length versus number of granule cells and dentate gyrus volume.

KEY FINDINGS

After pilocarpine-induced status epilepticus, rapamycin- and vehicle-treated mice had approximately half as many GFP-positive hilar neurons as did control animals. Despite neuron loss, vehicle-treated mice had over twice the GFP-positive axon length per dentate gyrus as controls, consistent with GABAergic axon sprouting. In contrast, total GFP-positive axon length was similar in rapamycin-treated mice and controls. GFP-positive axon length correlated most closely with dentate gyrus volume.

SIGNIFICANCE

These findings suggest that rapamycin suppressed axon sprouting by surviving somatostatin/GFP-positive interneurons after pilocarpine-induced status epilepticus in GIN mice. It is unclear whether the effect of rapamycin on axon length was on interneurons directly or secondary, for example, by suppressing growth of granule cell dendrites, which are synaptic targets of interneuron axons. The mammalian target of rapamycin (mTOR) signaling pathway might be a useful drug target for influencing GABAergic synaptic reorganization after epileptogenic treatments, but additional side effects of rapamycin treatment must be considered carefully.

Frontier of epilepsy research - mTOR signaling pathway

Studies of epilepsy have mainly focused on the membrane proteins that control neuronal excitability. Recently, attention has been shifting to intracellular proteins and their interactions, signaling cascades and feedback regulation as they relate to epilepsy. The mTOR (mammalian target of rapamycin) signal transduction pathway, especially, has been suggested to play an important role in this regard. These pathways are involved in major physiological processes as well as in numerous pathological conditions. Here, involvement of the mTOR pathway in epilepsy will be reviewed by presenting; an overview of the pathway, a brief description of key signaling molecules, a summary of independent reports and possible implications of abnormalities of those molecules in epilepsy, a discussion of the lack of experimental data, and questions raised for the understanding its epileptogenic mechanism.

A pulse rapamycin therapy for infantile spasms and associated cognitive decline

Infantile spasms are seizures manifesting within a spectrum of epileptic encephalopathies of infancy that often lead to cognitive impairment. Their current therapies, including adrenocorticotropic hormone (ACTH), high dose steroids, or vigabatrin, are not always effective and may be associated with serious side effects. Overactivation of the TORC1 complex of the mTOR pathway is implicated in the pathogenesis of certain genetic and acquired disorders that are linked with infantile spasms, like tuberous sclerosis. Here, we tested the therapeutic potential of rapamycin, a TORC1 inhibitor, as a potential treatment for infantile spasms in the multiple-hit rat model of ACTH-refractory symptomatic infantile spasms, which is not linked to tuberous sclerosis. Rapamycin or vehicle was given after spasms appeared. Their effects on spasms, other seizures, performance in Barnes maze, and expression of the phosphorylated S6 ribosomal protein (pS6: a TORC1 target) in the cortex, using immunofluorescence, were compared. Rapamycin suppressed spasms dose-dependently and improved visuospatial learning, although it did not reduce the frequency of other emerging seizures. High-dose pulse rapamycin effected acute and sustained suppression of spasms and improved cognitive outcome, without significant side effects. Therapeutically effective rapamycin doses normalized the pS6 expression, which was increased in perilesional cortical regions of pups with spasms. These findings support that pathological overactivation of TORC1 may be implicated in the pathogenesis of infantile spasms, including those that are not linked to tuberous sclerosis. Furthermore, a high-dose, pulse rapamycin treatment is a promising, well tolerated and disease-modifying new therapy for infantile spasms, including those refractory to ACTH.

Rapamycin suppresses mossy fiber sprouting but not seizure frequency in a mouse model of temporal lobe epilepsy

Temporal lobe epilepsy is prevalent and can be difficult to treat effectively. Granule cell axon (mossy fiber) sprouting is a common neuropathological finding in patients with mesial temporal lobe epilepsy, but its role in epileptogenesis is unclear and controversial. Focally infused or systemic rapamycin inhibits the mammalian target of rapamycin (mTOR) signaling pathway and suppresses mossy fiber sprouting in rats. We tested whether long-term systemic treatment with rapamycin, beginning 1 d after pilocarpine-induced status epilepticus in mice, would suppress mossy fiber sprouting and affect the development of spontaneous seizures. Mice that had experienced status epilepticus and were treated for 2 months with rapamycin displayed significantly less mossy fiber sprouting (42% of vehicle-treated animals), and the effect was dose dependent. However, behavioral and video/EEG monitoring revealed that rapamycin- and vehicle-treated mice displayed spontaneous seizures at similar frequencies. These findings suggest mossy fiber sprouting is neither pro- nor anti-convulsant; however, there are caveats. Rapamycin treatment also reduced epilepsy-related hypertrophy of the dentate gyrus but did not significantly affect granule cell proliferation, hilar neuron loss, or generation of ectopic granule cells. These findings are consistent with the hypotheses that hilar neuron loss and ectopic granule cells might contribute to temporal lobe epileptogenesis.

Circadian regulation of mammalian target of rapamycin signaling in the mouse suprachiasmatic nucleus

Circadian (24-h) rhythms influence virtually every aspect of mammalian physiology. The main rhythm generation center is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, and work over the past several years has revealed that rhythmic gene transcription and post-translational processes are central to clock timing. In addition, rhythmic translation control has also been implicated in clock timing; however the precise cell signaling pathways that drive this process are not well known. Here we report that a key translation activation cascade, the mammalian target of rapamycin (mTOR) pathway, is under control of the circadian clock in the SCN. Using phosphorylated S6 ribosomal protein (pS6) as a marker of mTOR activity, we show that the mTOR cascade exhibits maximal activity during the subjective day, and minimal activity during the late subjective night. Importantly, expression of S6 was not altered as a function of circadian time. Rhythmic S6 phosphorylation was detected throughout the dorsoventral axis of the SCN, thus suggesting that rhythmic mTOR activity was not restricted to a subset of SCN neurons. Rather, rhythmic pS6 expression appeared to parallel the expression pattern of the clock gene period1 (per1). Using a transgenic per1 reporter gene mouse strain, we found a statistically significant cellular level correlation between pS6 and per1 gene expression over the circadian cycle. Further, photic stimulation triggered a coordinate upregulation of per1 and mTOR activation in a subset of SCN cells. Interestingly, this cellular level correlation between mTOR activity and per1 expression appears to be specific, since a similar expression profile for pS6 and per2 or c-FOS was not detected. Finally, we show that mTOR activity is downstream of the ERK/MAPK signal transduction pathway. Together these data reveal that mTOR pathway activity is under the control of the SCN clock, and suggests that mTOR signaling may contribute to distinct aspects of the molecular clock timing process.

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

Traditionally, medical therapy for epilepsy has aimed to suppress seizure activity, but has been unable to alter the progression of the underlying disease. Recent advances in our understanding of mechanisms of epileptogenesis open the door for the development of new therapies which prevent the pathogenic changes in the brain that predispose to spontaneous seizures. In particular, the mammalian target of rapamycin (mTOR) signaling pathway has recently garnered interest as an important regulator of cellular changes involved in epileptogenesis, and mTOR inhibitors have generated excitement as potential antiepileptogenic agents. mTOR hyperactivation occurs in tuberous sclerosis complex (TSC), a common genetic cause of epilepsy, as a result of genetic mutations in upstream regulatory molecules. mTOR inhibition prevents epilepsy and brain pathology in animal models of TSC. mTOR dysregulation has also been demonstrated in a variety of other genetic and acquired epilepsies, including brain tumors, focal cortical dysplasias, and animal models of brain injury due to status epilepticus or trauma. Indeed, mTOR inhibitors appear to possess antiepileptogenic properties in animal models of acquired epilepsy as well. Thus, mTOR dysregulation may represent a final common pathway in epilepsies of various causes. Therefore, mTOR inhibition is an exciting potential antiepileptogenic strategy with broad applications for epilepsy and could be involved in a number of treatment modalities, including the ketogenic diet. Further research is necessary to determine the clinical utility of rapamycin and other mTOR inhibitors for antiepileptogenesis, and to devise new therapeutic targets by further elucidating the signaling molecules involved in epileptogenesis.

RIP2-mediated LKB1 deletion causes axon degeneration in the spinal cord and hind-limb paralysis

Axon degeneration is observed in neurodegenerative diseases and neuroinflammatory disorders, such as Alzheimer's disease, Parkinson's disease and multiple sclerosis. The molecular basis of this process remains largely unknown. Here, we show that mice deleted for the tumour suppressor LKB1 (also called STK11) in the spinal cord, some parts of the brain and in the endocrine pancreas (βLKB1KO mice) develop hind-limb dysfunction and axon degeneration at about 7 weeks. Demyelination and macrophage infiltration are observed in the white matter of these mice, predominantly in the bilateral and anterior funiculi of the thoracic segment of the spinal cord, suggesting damage to the ascending sensory signalling pathway owing to LKB1 deletion in the brain. Microtubule structures were also affected in the degenerated foci, with diminished neurofilament and tubulin expression. Deletion of both PRKAA1 genes, whose products AMPKα1 and AMPKα2 are also downstream targets of LKB1, with the same strategy was without effect. We thus define LKB1 as an intrinsic suppressor of axon degeneration and a possible target for strategies that can reverse this process.

Prevention or modification of epileptogenesis after brain insults: experimental approaches and translational research

Diverse brain insults, including traumatic brain injury, stroke, infections, tumors, neurodegenerative diseases, and prolonged acute symptomatic seizures, such as complex febrile seizures or status epilepticus (SE), can induce "epileptogenesis," a process by which normal brain tissue is transformed into tissue capable of generating spontaneous recurrent seizures. Furthermore, epileptogenesis operates in cryptogenic causes of epilepsy. In view of the accumulating information about cellular and molecular mechanisms of epileptogenesis, it should be possible to intervene in this process before the onset of seizures and thereby either prevent the development of epilepsy in patients at risk or increase the potential for better long-term outcome, which constitutes a major clinical need. For identifying pharmacological interventions that prevent, interrupt or reverse the epileptogenic process in people at risk, two groups of animal models, kindling and SE-induced recurrent seizures, have been recommended as potentially useful tools. Furthermore, genetic rodent models of epileptogenesis are increasingly used in assessing antiepileptogenic treatments. Two approaches have been used in these different model categories: screening of clinically established antiepileptic drugs (AEDs) for antiepileptogenic or disease-modifying potential, and targeting the key causal mechanisms that underlie epileptogenesis. The first approach indicated that among various AEDs, topiramate, levetiracetam, carisbamate, and valproate may be the most promising. On the basis of these experimental findings, two ongoing clinical trials will address the antiepileptogenic potential of topiramate and levetiracetam in patients with traumatic brain injury, hopefully translating laboratory discoveries into successful therapies. The second approach has highlighted neurodegeneration, inflammation and up-regulation of immune responses, and neuronal hyperexcitability as potential targets for antiepileptogenesis or disease modification. This article reviews these areas of progress and discusses the challenges associated with discovery of antiepileptogenic therapies.

Mammalian target of rapamycin: hitting the bull's-eye for neurological disorders

The mammalian target of rapamycin (mTOR) and its associated cell signaling pathways have garnered significant attention for their roles in cell biology and oncology. Interestingly,the explosion of information in this field has linked mTOR to neurological diseases with promising initial studies. mTOR, a 289 kDa serine/threonine protein kinase, plays an important role in cell growth and proliferation and is activated through phosphorylation in response to growth factors, mitogens and hormones. Growth factors, amino acids, cellular nutrients and oxygen deficiency can downregulate mTOR activity. The function of mTOR signaling is mediated primarily through two mTOR complexes: mTORC1 and mTORC2. mTORC1 initiates cap-dependent protein translation, a rate-limiting step of protein synthesis, through the phosphorylation of the targets eukaryotic initiation factor 4E-binding protein 1 (4EBP1) and p70 ribosomal S6 kinase (p70S6K). In contrast, mTORC2 regulates development of the cytoskeleton and also controls cell survival. Although closely tied to tumorigenesis, mTOR and the downstream signaling pathways are significantly involved in the central nervous system (CNS) with synaptic plasticity, memory retention, neuroendocrine regulation associated with food intake and puberty and modulation of neuronal repair following injury. The signaling pathways of mTOR also are believed to be a significant component in a number of neurological diseases, such as Alzheimer disease, Parkinson disease and Huntington disease, tuberous sclerosis, neurofibromatosis, fragile X syndrome, epilepsy, traumatic brain injury and ischemic stroke. Here we describe the role of mTOR in the CNS and illustrate the potential for new strategies directed against neurological disorders.

Therapeutic approaches to epileptogenesis--hope on the horizon

Prevention of epileptogenesis is an unmet need in medicine. During the last 3 years, however, several preclinical studies have demonstrated remarkable favorable effects of novel treatments on genetic and acquired epileptogenesis. These include the use of immunosuppressants and treatments that modify cellular adhesion, proliferation, and/or plasticity. In addition, the use of antiepileptic drugs in rats with genetic epilepsy or proconvulsants in acquired epilepsy models has provided somewhat unexpected favorable effects. This review summarizes these studies, and introduces some caveats when interpreting the data. In particular, the effect of genetic background, the severity of epileptogenic insult, the method and duration of seizure monitoring, and size of animal population are discussed. Furthermore, a novel scheme for defining epileptogenesis-related terms is presented.

Regulation of cell death and epileptogenesis by the mammalian target of rapamycin (mTOR): a double-edged sword?

Identification of cell signaling mechanisms mediating seizure-related neuronal death and epileptogenesis is important for developing more effective therapies for epilepsy. The mammalian target of rapamycin (mTOR) pathway has recently been implicated in regulating neuronal death and epileptogenesis in rodent models of epilepsy. In particular, kainate-induced status epilepticus causes abnormal activation of the mTOR pathway, and the mTOR inhibitor, rapamycin, can decrease the development of neuronal death and chronic seizures in the kainate model. Here, we discuss the significance of these findings and extend them further by identifying upstream signaling pathways through which kainate status epilepticus activates the mTOR pathway and by demonstrating limited situations where rapamycin may paradoxically increase mTOR activation and worsen neuronal death in the kainate model. Thus, the regulation of seizure-induced neuronal death and epileptogenesis by mTOR is complex and may have dual, opposing effects depending on the physiological and pathological context. Overall, these findings have important implications for designing potential neuroprotective and antiepileptogenic therapies that modulate the mTOR pathway.

Downregulation of dendritic HCN channel gating in epilepsy is mediated by altered phosphorylation signaling

The onset of spontaneous seizures in the pilocarpine model of epilepsy causes a hyperpolarized shift in the voltage-dependent activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated current (Ih) in CA1 hippocampal pyramidal neuron dendrites, contributing to neuronal hyperexcitability and possibly to epileptogenesis. However, the specific mechanisms by which spontaneous seizures cause downregulation of HCN channel gating are yet unknown. We asked whether the seizure-dependent downregulation of HCN channel gating was due to altered phosphorylation signaling mediated by the phosphatase calcineurin (CaN) or the kinase p38 mitogen-activated protein kinase (p38 MAPK). We first found that CaN inhibition upregulated HCN channel gating and reduced neuronal excitability under normal conditions, showing that CaN is a strong modulator of HCN channels. We then found that an in vitro model of seizures (1 h in 0 Mg2+ and 50 microM bicuculline at 35-37 degrees C) reproduced the HCN channel gating change seen in vivo. Pharmacological inhibition of CaN or activation of p38 MAPK partially reversed the in vitro seizure-induced hyperpolarized shift in HCN channel gating, and the shift was fully reversed by the combination of CaN inhibition and p38 MAPK activation. We then demonstrated enhanced CaN activity as well as reduced p38 MAPK activity in vivo in the CA1 hippocampal area of chronically epileptic animals. Pharmacological reversal of these phosphorylation changes restored HCN channel gating downregulation and neuronal hyperexcitability in epileptic tissue to control levels. Together, these results suggest that alteration of two different phosphorylation pathways in epilepsy contributes to the downregulation of HCN channel gating, which consequently produces neuronal hyperexcitability and thus may be a target for novel antiepileptic therapies.

Pharmacological inhibition of the mammalian target of rapamycin pathway suppresses acquired epilepsy

Inhibition of mTOR by rapamycin has been shown to suppress seizures in TSC/PTEN genetic models. Rapamycin, when applied immediately before or after a neurological insult, also prevents the development of spontaneous recurrent seizures (epileptogenesis) in an acquired model. In the present study, we examined the mTOR pathway in rats that had already developed chronic spontaneous seizures in a pilocarpine model. We found that mTOR is aberrantly activated in brain tissues from rats with chronic seizures. Furthermore, inhibition of mTOR by rapamycin treatment significantly reduces seizure activity. Finally, mTOR inhibition also significantly suppresses mossy fiber sprouting. Our findings suggest the possibility for a much broader window for intervention for some acquired epilepsies by targeting the mTOR pathway.

Caveat mTOR: aberrant signaling disrupts corticogenesis

The mammalian target of rapamycin (mTOR) signaling pathway is activated in several disorders associated with benign tumors and malformations of the cerebral cortex. In this issue of the JCI, Orlova et al. have now definitively added another disorder to this group by demonstrating that activation of mTOR signaling is associated with polyhydramnios, megalencephaly, and symptomatic epilepsy syndrome (PMSE), which is characterized by severe intractable epilepsy and megalencephaly. PMSE is caused by lack of the pseudokinase STE20-related kinase adaptor alpha (STRADalpha), and Orlova et al. show that reduction of STRADalpha levels during corticogenesis in the mouse results in a cellular phenotype and neuronal migration defects similar to those observed in patients with PMSE, clearly demonstrating a pivotal role for STRADalpha in cell polarity and growth. This study helps pave the way for possible therapeutic intervention with rapamycin to control the epilepsy and learning disabilities associated with this disorder.

Mammalian target of rapamycin (mTOR) inhibition as a potential antiepileptogenic therapy: From tuberous sclerosis to common acquired epilepsies

Most current treatments for epilepsy are symptomatic therapies that suppress seizures but do not affect the underlying course or prognosis of epilepsy. The need for disease-modifying or "antiepileptogenic" treatments for epilepsy is widely recognized, but no such preventive therapies have yet been established for clinical use. A rational strategy for preventing epilepsy is to target primary signaling pathways that initially trigger the numerous downstream mechanisms mediating epileptogenesis. The mammalian target of rapamycin (mTOR) pathway represents a logical candidate, because mTOR regulates multiple cellular functions that may contribute to epileptogenesis, including protein synthesis, cell growth and proliferation, and synaptic plasticity. The importance of the mTOR pathway in epileptogenesis is best illustrated by tuberous sclerosis complex (TSC), one of the most common genetic causes of epilepsy. In mouse models of TSC, mTOR inhibitors prevent the development of epilepsy and underlying brain abnormalities associated with epileptogenesis. Accumulating evidence suggests that mTOR also participates in epileptogenesis due to a variety of other causes, including focal cortical dysplasia and acquired brain injuries, such as in animal models following status epilepticus or traumatic brain injury. Therefore, mTOR inhibition may represent a potential antiepileptogenic therapy for diverse types of epilepsy, including both genetic and acquired epilepsies.

Developing Antiepileptogenic Drugs for Acquired Epilepsy: Targeting the Mammalian Target of Rapamycin (mTOR) Pathway

While current medications for epilepsy are primarily symptomatic treatments that suppress seizures, one of the main goals of future drug development in epilepsy is the identification of antiepileptogenic or disease-modifying therapies that can completely prevent epilepsy or slow its progression. A rational antiepileptogenic strategy is to target primary cell signaling pathways that initially trigger the downstream mechanisms causing epileptogenesis. Recent work implicates the mammalian target of rapamycin (mTOR) pathway as mediating epileptogenesis in a genetic epilepsy, Tuberous Sclerosis Complex (TSC), and suggests that mTOR inhibitors, such as rapamycin, may have antiepileptogenic properties for epilepsy in TSC. As mTOR regulates multiple cellular functions that may contribute to epileptogenesis in general, including ion channel expression, synaptic plasticity, and programmed cell death, mTOR inhibitors might also represent an effective antiepileptogenic therapy for other, more common types of epilepsy, such as acquired epilepsies due to brain injuries. Here, we describe evidence from a recently-published study that mTOR mediates epileptogenesis in a popular animal model of acquired limbic epilepsy due to brain injury following kainate-induced status epilepticus, and that rapamycin has antiepileptogenic effects in this model. Furthermore, putative pathways and mechanisms upstream and downstream from mTOR involved in epileptogenesis in the kainite model are considered, identifying possible additional therapeutic targets. Finally, the potential translational applications of this and other animal model data for developing antiepileptogenic therapies for people with acquired epilepsy due to brain injury are discussed.