Tsc2 gene inactivation causes a more severe epilepsy phenotype than Tsc1 inactivation in a mouse model of tuberous sclerosis complex

Deletion of mTOR in Reactive Astrocytes Suppresses Chronic Seizures in a Mouse Model of Temporal Lobe Epilepsy

Germline and somatic mutations in key genes of the mammalian target of rapamycin (mTOR) pathway have been identified in seizure-associated disorders. mTOR mutations lead to aberrant activation of mTOR signaling, and, although affected neurons are critical for epileptogenesis, the role of mTOR activation in glial cells remains poorly understood. We previously reported a consistent activation of the mTOR pathway in astrocytes in the epileptic foci of temporal lobe epilepsy. In this study, it was demonstrated that mTOR deletion from reactive astrocytes prevents increases in seizure frequency over the disease course. By using a tamoxifen-inducible mTOR conditional knockout system and kainic acid, a model was developed that allowed astrocyte-specific mTOR gene deletion in mice with chronic epilepsy. Animals in which mTOR was deleted from 44 % of the astrocyte population exhibited a lower seizure frequency compared with controls. Down-regulation of mTOR significantly ameliorated astrogliosis in the sclerotic hippocampus but did not rescue mossy fiber sprouting. In cultured astrocytes, the mTOR pathway modulated the stability of the astroglial glutamate transporter 1 (Glt1) and influenced the ability of astrocytes to remove extracellular glutamate. Taken together, these data indicate that astrocytes with activated mTOR signaling may provide conditions that are favorable for spontaneous recurrent seizures.

mTOR, a Potential Target to Treat Autism Spectrum Disorder

Mammalian target of rapamycin (mTOR) is a key regulator in various cellular processes, including cell growth, gene expression, and synaptic functions. Autism spectrum disorder (ASD) is frequently accompanied by monogenic disorders, such as tuberous sclerosis complex, phosphatase and tensin homolog tumor hamartoma syndrome, neurofibromatosis 1, and fragile X syndrome, in which mTOR is hyperactive. Mutations in the genes involved in the mTOR-mediated signaling pathway have been identified in some cases of syndromic ASD. Evidences indicate a pathogenic role for hyperactive mTOR-mediated signaling in ASD associated with these monogenic disorders, and mTOR inhibitors are a potential pharmacotherapy for ASD. Abnormal synaptic transmission through metabotropic glutamate receptor 5 may underlie in a part of ASD associated with hyperactive mTOR-mediated signaling. In this review, the relationship between mTOR and ASD is discussed.

Translational Mouse Models of Autism: Advancing Toward Pharmacological Therapeutics

Animal models provide preclinical tools to investigate the causal role of genetic mutations and environmental factors in the etiology of autism spectrum disorder (ASD). Knockout and humanized knock-in mice, and more recently knockout rats, have been generated for many of the de novo single gene mutations and copy number variants (CNVs) detected in ASD and comorbid neurodevelopmental disorders. Mouse models incorporating genetic and environmental manipulations have been employed for preclinical testing of hypothesis-driven pharmacological targets, to begin to develop treatments for the diagnostic and associated symptoms of autism. In this review, we summarize rodent behavioral assays relevant to the core features of autism, preclinical and clinical evaluations of pharmacological interventions, and strategies to improve the translational value of rodent models of autism.

Ribosomal Protein S6 Phosphorylation in the Nervous System: From Regulation to Function

Since the discovery of the phosphorylation of the 40S ribosomal protein S6 (rpS6) about four decades ago, much effort has been made to uncover the molecular mechanisms underlying the regulation of this post-translational modification. In the field of neuroscience, rpS6 phosphorylation is commonly used as a readout of the mammalian target of rapamycin complex 1 signaling activation or as a marker for neuronal activity. Nevertheless, its biological role in neurons still remains puzzling. Here we review the pharmacological and physiological stimuli regulating this modification in the nervous system as well as the pathways that transduce these signals into rpS6 phosphorylation. Altered rpS6 phosphorylation observed in various genetic and pathophysiological mouse models is also discussed. Finally, we examine the current state of knowledge on the physiological role of this post-translational modification and highlight the questions that remain to be addressed.

Neonatal and Infantile Epilepsy: Acquired and Genetic Models

The incidence of seizures and epilepsies is particularly high during the neonatal and infantile periods. We will review selected animal models of early-life epileptic encephalopathies that have addressed the dyscognitive features of frequent interictal spikes, the pathogenesis and treatments of infantile spasms (IS) or Dravet syndrome, disorders with mammalian target of rapamycin (mTOR) dysregulation, and selected early-life epilepsies with genetic defects. Potentially pathogenic mechanisms in these conditions include interneuronopathies in IS or Dravet syndrome and mTOR dysregulation in brain malformations, tuberous sclerosis, and related genetic disorders, or IS of acquired etiology. These models start to generate the first therapeutic drugs, which have been specifically developed in immature animals. However, there are challenges in translating preclinical discoveries into clinically relevant findings. The advances made so far hold promise that the new insights may potentially have curative or disease-modifying potential for many of these devastating conditions.

Complex Neurological Phenotype in Mutant Mice Lacking Tsc2 in Excitatory Neurons of the Developing Forebrain(123)

Mutations in the TSC1 and TSC2 genes cause tuberous sclerosis complex (TSC), a genetic disease often associated with epilepsy, intellectual disability, and autism, and characterized by the presence of anatomical malformations in the brain as well as tumors in other organs. The TSC1 and TSC2 proteins form a complex that inhibits mammalian target of rapamycin complex 1 (mTORC1) signaling. Previous animal studies demonstrated that Tsc1 or Tsc2 loss of function in the developing brain affects the intrinsic development of neural progenitor cells, neurons, or glia. However, the interplay between different cellular elements during brain development was not previously investigated. In this study, we generated a novel mutant mouse line (NEX-Tsc2) in which the Tsc2 gene is deleted specifically in postmitotic excitatory neurons of the developing forebrain. Homozygous mutant mice failed to thrive and died prematurely, whereas heterozygous mice appeared normal. Mutant mice exhibited distinct neuroanatomical abnormalities, including malpositioning of selected neuronal populations, neuronal hypertrophy, and cortical astrogliosis. Intrinsic neuronal defects correlated with increased mTORC1 signaling, whereas astrogliosis did not result from altered intrinsic signaling, since these cells were not directly affected by the gene knockout strategy. All neuronal and non-neuronal abnormalities were suppressed by continuous postnatal treatment with the mTORC1 inhibitor RAD001. The data suggest that the loss of Tsc2 and mTORC1 signaling activation in excitatory neurons not only disrupts their intrinsic development, but also disrupts the development of cortical astrocytes, likely through the mTORC1-dependent expression of abnormal signaling proteins. This work thus provides new insights into cell-autonomous and non-cell-autonomous functions of Tsc2 in brain development.

A novel mouse model of tuberous sclerosis complex (TSC): eye-specific Tsc1-ablation disrupts visual-pathway development

Tuberous sclerosis complex (TSC) is an autosomal dominant syndrome that is best characterised by neurodevelopmental deficits and the presence of benign tumours (called hamartomas) in affected organs. This multi-organ disorder results from inactivating point mutations in either the TSC1 or the TSC2 genes and consequent activation of the canonical mammalian target of rapamycin complex 1 signalling (mTORC1) pathway. Because lesions to the eye are central to TSC diagnosis, we report here the generation and characterisation of the first eye-specific TSC mouse model. We demonstrate that conditional ablation of Tsc1 in eye-committed progenitor cells leads to the accelerated differentiation and subsequent ectopic radial migration of retinal ganglion cells. This results in an increase in retinal ganglion cell apoptosis and consequent regionalised axonal loss within the optic nerve and topographical changes to the contra- and ipsilateral input within the dorsal lateral geniculate nucleus. Eyes from adult mice exhibit aberrant retinal architecture and display all the classic neuropathological hallmarks of TSC, including an increase in organ and cell size, ring heterotopias, hamartomas with retinal detachment, and lamination defects. Our results provide the first major insight into the molecular etiology of TSC within the developing eye and demonstrate a pivotal role for Tsc1 in regulating various aspects of visual-pathway development. Our novel mouse model therefore provides a valuable resource for future studies concerning the molecular mechanisms underlying TSC and also as a platform to evaluate new therapeutic approaches for the treatment of this multi-organ disorder.

Editors' choice: Conditional deletion of Tsc1 in the eye results in hamartoma formation and defects in retinal ganglion cell development – a novel mouse model providing insights into visual pathway involvement in TSC.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.

Catastrophic childhood epilepsy: a recent convergence of basic and clinical neuroscience

Advances in understanding the genetics and underlying pathology of the catastrophic childhood epilepsies are pointing toward treatments.

Tuberous Sclerosis: A New Frontier in Targeted Treatment of Autism

Tuberous sclerosis complex (TSC) is a genetic disorder with a high prevalence of autism spectrum disorder (ASD). Tremendous progress in understanding the pathogenesis of TSC has been made in recent years, along with initial trials of medical treatment aimed specifically at the underlying mechanism of the disorder. At the cellular level, loss of TSC1 or TSC2 results in upregulation of the mechanistic target of rapamycin (mTOR) pathway. At the circuitry level, TSC and mTOR play crucial roles in axonal, dendritic, and synaptic development and function. In this review, we discuss the molecular mechanism underlying TSC, and how this disease results in aberrant neural connectivity at multiple levels in the central nervous system, leading to ASD symptoms. We then review recent advances in mechanism-based treatments of TSC, and the promise that these treatments provide for future mechanism-based treatment of ASD. Because of these recent advances, TSC represents an ideal model for how to make progress in understanding and treating the mechanisms that underlie ASD in general.

Intermittent dosing of rapamycin maintains antiepileptogenic effects in a mouse model of tuberous sclerosis complex

OBJECTIVE

Inhibitors of the mechanistic target of rapamycin (mTOR) pathway have antiepileptogenic effects in preventing epilepsy and pathologic and molecular mechanisms of epileptogenesis in mouse models of tuberous sclerosis complex (TSC). However, long-term treatment with mTOR inhibitors may be required to maintain efficacy and potentially has chronic side effects, such as immunosuppression. Attempts to minimize drug exposure will facilitate translational efforts to develop mTOR inhibitors as antiepileptogenic agents for patients with TSC. In this study, we tested intermittent dosing paradigms of mTOR inhibitors for antiepileptogenic properties in a TSC mouse model.

METHODS

Western blot analysis of phosphorylation of S6 protein was used to assess the dose- and time-dependence of mTOR inhibition by rapamycin in control mice and conditional knockout mice with inactivation of the Tsc1 gene in glial fibrillary acidic protein (GFAP)-expressing cells (Tsc1(GFAP)CKO mice). Based on the Western blot studies, different dosing paradigms of rapamycin starting at postnatal day 21 were tested for their ability to prevent epilepsy or pathologic abnormalities in Tsc1(GFAP)CKO mice: 4 days of rapamycin only (4-∞), 4 days on-24 days off (4-24), and 4 days on-10 days off (4-10).

RESULTS

mTOR activity was inhibited by rapamycin in a dose-dependent fashion and recovered to baseline by about 10 days after the last rapamycin dose. The 4-10 and 4-24 dosing paradigms almost completely prevented epilepsy and the 4-10 paradigm inhibited glial proliferation and megalencephaly in Tsc1(GFAP)CKO mice.

SIGNIFICANCE

Intermittent dosing of rapamycin, with drug holidays of more than 3 weeks, maintains significant antiepileptogenic properties in mouse models of TSC. These findings have important translational applications in developing mTOR inhibitors as antiepileptogenic agents in TSC patients by minimizing drug exposure and potential side effects.

Animal models of tumour-associated epilepsy

Brain tumours cause a sizeable proportion of epilepsies in adulthood, and actually can be etiologically responsible also for childhood epilepsies. Conversely, seizures are often first clinical signs of a brain tumour. Nevertheless, several issues of brain-tumour associated seizures and epilepsies are far from understood, or clarified regarding clinical consensus. These include both the specific mechanisms of epileptogenesis related to different tumour types, the possible relationship between malignancy and seizure emergence, the interaction between tumour mass and surrounding neuronal networks, and - not least - the best treatment options depending on different tumour types. To investigate these issues, experimental models of tumour-induced epilepsies are necessary. This review concentrates on the description of currently used models, focusing on methodological aspects. It highlights advantages and shortcomings of these models, and identifies future experimental challenges.

Inflammatory mechanisms contribute to the neurological manifestations of tuberous sclerosis complex

Epilepsy and other neurological deficits are common, disabling manifestations of the genetic disorder, tuberous sclerosis complex (TSC). Brain inflammation has been implicated in contributing to epileptogenesis in acquired epilepsy due to brain injury, but the potential role of inflammatory mechanisms in genetic epilepsies is relatively unexplored. In this study, we investigated activation of inflammatory mediators and tested the effects of anti-inflammatory treatment on epilepsy in the Tsc1-GFAP conditional knock-out mouse model of TSC (Tsc1(GFAP)CKO mice). Real-time quantitative RT-PCR, immunohistochemistry, and Western blotting demonstrated increased expression of specific cytokines and chemokines, particularly IL-1β and CXCL10, in the neocortex and hippocampus of Tsc1(GFAP)CKO mice, which was reversed by treatment with a mammalian target of rapamycin complex 1 (mTORC1) inhibitor. Double-labeling immunohistochemical studies indicated that the increased IL-1β was localized primarily to astrocytes. Importantly, the increase in inflammatory markers was also observed in astrocyte culture in vitro and at 2 weeks of age in Tsc1(GFAP)CKO mice before the onset of epilepsy in vivo, indicating that the inflammatory changes were not secondary to seizures. Epicatechin-3-gallate, an inhibitor of IL-1β and CXCL10, at least partially reversed the elevated cytokine and chemokine levels, reduced seizure frequency, and prolonged survival of Tsc1(GFAP)CKO mice. These findings suggest that mTOR-mediated inflammatory mechanisms may be involved in epileptogenesis in the genetic epilepsy, TSC.

Genetic animal models of malformations of cortical development and epilepsy

Malformations of cortical development constitute a variety of pathological brain abnormalities that commonly cause severe, medically-refractory epilepsy, including focal lesions, such as focal cortical dysplasia, heterotopias, and tubers of tuberous sclerosis complex, and diffuse malformations, such as lissencephaly. Although some cortical malformations result from environmental insults during cortical development in utero, genetic factors are increasingly recognized as primary pathogenic factors across the entire spectrum of malformations. Genes implicated in causing different cortical malformations are involved in a variety of physiological functions, but many are focused on regulation of cell proliferation, differentiation, and neuronal migration. Advances in molecular genetic methods have allowed the engineering of increasingly sophisticated animal models of cortical malformations and associated epilepsy. These animal models have identified some common mechanistic themes shared by a number of different cortical malformations, but also revealed the diversity and complexity of cellular and molecular mechanisms that lead to the development of the pathological lesions and resulting epileptogenesis.

mTOR signaling in epilepsy: insights from malformations of cortical development

Over the past decade enhanced activation of the mammalian target of rapamycin (mTOR)-signaling cascade has been identified in focal malformations of cortical development (MCD) subtypes, which have been collectively referred to as "mTORopathies." Mutations in mTOR regulatory genes (e.g., TSC1, TSC2, AKT3, DEPDC5) have been associated with several focal MCD highly associated with epilepsy such as tuberous sclerosis complex (TSC), hemimegalencephaly (HME; brain malformation associated with dramatic enlargement of one brain hemisphere), and cortical dysplasia. mTOR plays important roles in the regulation of cell division, growth, and survival, and, thus, aberrant activation of the cascade during cortical development can cause dramatic alterations in cell size, cortical lamination, and axon and dendrite outgrowth often observed in focal MCD. Although it is widely believed that structural alterations induced by hyperactivated mTOR signaling are critical for epileptogenesis, newer evidence suggests that mTOR activation on its own may enhance neuronal excitability. Clinical trials with mTOR inhibitors have shown efficacy in the treatment of seizures associated with focal MCD.

Mechanistic target of rapamycin (mTOR) in tuberous sclerosis complex-associated epilepsy

BACKGROUND

Tuberous sclerosis complex is a multiorgan disease resulting from a mutation of one of two TSC genes. The two gene products form a functional complex that regulates the mTOR signaling pathway (mTOR initially represented mammalian target of rapamycin, but increasingly the term mechanistic target of rapamycin is used to reflect the ubiquitous occurrence of mTOR). Epilepsy is the most common neurological symptom of tuberous sclerosis complex, occurring in 80% to 90% of affected individuals over the course of their lifetimes and causing significant morbidity and mortality. The mechanistic target of rapamycin (mTOR) signaling pathway is intricately involved in multiple cellular functions--including protein synthesis, cell growth and proliferation, and synaptic plasticity--which may influence neuronal excitability and precipitate epileptogenesis. Recent preclinical and clinical studies have increased interest in the potential role of mTOR inhibitors for the treatment of tuberous sclerosis complex-related epilepsy.

METHODS

Medline and PubMed database searches were used to identify relevant studies and other information on tuberous sclerosis complex-related epilepsies, the mTOR pathway, and current advances in treatment approaches.

RESULTS

Although current management strategies that provide symptomatic relief are effective at reducing the frequency of seizures in individuals with tuberous sclerosis complex, there is further room for the exploration of therapies that directly address hyperactive mTOR signaling--the underlying etiology of the disease. The role of the antiepileptic effect of mTOR inhibition was first demonstrated in knockout TSC1 mouse models. Additionally, several case studies demonstrated a positive effect on seizure frequency and severity in patients with pharmacoresistant epilepsy. In a phase 1/2 clinical trial with 28 patients, clinically relevant reduction in overall seizure frequency was documented in individuals treated with the mTOR inhibitor everolimus. In a phase 3 trial evaluating the role of everolimus in subependymal giant cell astrocytoma, seizures were a secondary end point. Because the median seizure frequency was zero in this study, the analysis was inconclusive.

CONCLUSION

Various preclinical models provide substantial evidence for the role of mTOR inhibition in the treatment of epilepsy in individuals with tuberous sclerosis complex. Preliminary clinical studies provide supportive evidence for a role of mTOR inhibition in the management of tuberous sclerosis complex-associated epilepsy and pave the way for new randomized placebo-controlled studies. This article reviews current treatment recommendations for the management of tuberous sclerosis complex-associated epilepsy as well as the rationale and evidence to support the use of mTOR inhibitors.

Importance of EEG in validating the chronic effects of drugs: suggestions from animal models of epilepsy treated with rapamycin

PURPOSE

The development of new drugs for the treatment of epilepsy is a major challenge for modern neurology and its first steps demand basic research. Preclinical studies on animal models of epilepsy are mainly based on the analysis of brain electrical activity to detect seizures, when they are not just limited to behavioral tests like the Racine scale.

METHODS

In the present review, we discuss the importance of using time-locked video and EEG recordings (Video-EEG) coupled with behavioral tests as tools to monitor and analyze the effects of anti-epileptic drugs in pre-clinical research. Particularly, we focus on the utility of a multimodal approach based on EEG/behavioral analysis to study the beneficial effects of chronic rapamycin treatment as a potential anti-epileptogenic therapy for a broad spectrum of epilepsy, including both genetic (as in tuberous sclerosis complex) and acquired diseases.

RESULTS

Changes and synchronization of neuronal activity of different areas have been correlated with specific behavior in both physiological and pathological conditions. In the epileptic brain, during a seizure there is an abnormal activation of many cells all at once, altering different networks.

CONCLUSION

A multimodal approach based on video, EEG analysis and behavioral tests would be the best option in preclinical studies of epilepsy.

Prolonging the survival of Tsc2 conditional knockout mice by glutamine supplementation

The genetic disease tuberous sclerosis complex (TSC) is an autosomal dominant disorder caused by loss of function mutations in either TSC1 (hamartin) or TSC2 (tuberin), which serve as negative regulators of mechanistic target of rapamycin complex 1 (mTORC1) activity. TSC patients exhibit developmental brain abnormalities and tuber formations that are associated with neuropsychological and neurocognitive impairments, seizures and premature death. Mechanistically, TSC1 and TSC2 loss of function mutations result in abnormally high mTORC1 activity. Thus, the development of a strategy to inhibit abnormally high mTORC1 activity may have therapeutic value in the treatment of TSC. mTORC1 is a master regulator of growth processes, and its activity can be reduced by withdrawal of growth factors, decreased energy availability, and by the immunosuppressant rapamycin. Recently, glutamine has been shown to alter mTORC1 activity in a TSC1-TSC2 independent manner in cells cultured under amino acid- and serum-deprived conditions. Since starvation culture conditions are not physiologically relevant, we examined if glutamine can regulate mTORC1 in non-deprived cells and in a murine model of TSC. Our results show that glutamine can reduce phosphorylation of S6 and S6 kinase, surrogate indicators of mTORC1 activity, in both deprived and non-deprived cells, although higher concentrations were required for non-deprived cultures. When administered orally to TSC2 knockout mice, glutamine reduced S6 phosphorylation in the brain and significantly prolonged their lifespan. Taken together, these results suggest that glutamine supplementation can be used as a potential treatment for TSC.

mTOR inhibition in epilepsy: rationale and clinical perspectives

Despite a large number of available medical options, many individuals with epilepsy are refractory to existing therapies that mainly target neurotransmitter or ion channel activity. A growing body of preclinical data has uncovered a molecular pathway that appears crucial in many genetic and acquired epilepsy syndromes. The mammalian target of rapamycin (mTOR) pathway regulates a number of cellular processes required in the growth, metabolism, structure, and cell-cell interactions of neurons and glia. Rapamycin and similar compounds inhibit mTOR complex 1 and decrease seizures, delay seizure development, or prevent epileptogenesis in many animal models of mTOR hyperactivation. However, the exact mechanisms by which mTOR inhibition drives decreased seizure activity have not been completely determined. Nonetheless, these preclinical data have led to limited use in humans with epilepsy due to tuberous sclerosis complex and polyhydramnios, megalencephaly, and symptomatic epilepsy with promising results. Currently, larger controlled studies are underway using mTOR inhibitors in individuals with tuberous sclerosis complex and intractable epilepsy.

The neural crest lineage as a driver of disease heterogeneity in Tuberous Sclerosis Complex and Lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) is a rare neoplastic disease, best characterized by the formation of proliferative nodules that express smooth muscle and melanocytic antigens within the lung parenchyma, leading to progressive destruction of lung tissue and function. The pathological basis of LAM is associated with Tuberous Sclerosis Complex (TSC), a multi-system disorder marked by low-grade tumors in the brain, kidneys, heart, eyes, lung and skin, arising from inherited or spontaneous germ-line mutations in either of the TSC1 or TSC2 genes. LAM can develop either in a patient with TSC (TSC-LAM) or spontaneously (S-LAM), and it is clear that the majority of LAM lesions of both forms are characterized by an inactivating mutation in either TSC1 or TSC2, as in TSC. Despite this genetic commonality, there is considerable heterogeneity in the tumor spectrum of TSC and LAM patients, the basis for which is currently unknown. There is extensive clinical evidence to suggest that the cell of origin for LAM, as well as many of the TSC-associated tumors, is a neural crest cell, a highly migratory cell type with extensive multi-lineage potential. Here we explore the hypothesis that the types of tumors that develop and the tissues that are affected in TSC and LAM are dictated by the developmental timing of TSC gene mutations, which determines the identities of the affected cell types and the size of downstream populations that acquire a mutation. We further discuss the evidence to support a neural crest origin for LAM and TSC tumors, and propose approaches for generating humanized models of TSC and LAM that will allow cell of origin theories to be experimentally tested. Identifying the cell of origin and developing appropriate humanized models is necessary to truly understand LAM and TSC pathology and to establish effective and long-lasting therapeutic approaches for these patients.

Epilepsy in newborns with tuberous sclerosis complex

BACKGROUND

Epilepsy affects up to 90% of TSC patients and majority of them have seizure at the age of 3-5 months, after a period of latent epileptogenesis, but some develop epilepsy earlier.

AIMS

The aim of this work was to identify incidence, clinical characteristics, and risk factors for neonatal onset of epilepsy in a large cohort of TSC patients.

METHODS

A retrospective review of medical data of 421 TSC patients was performed. Patients who developed epilepsy within first 4 weeks of life were included in the study. Clinical and treatment data, EEG, MRI, and genetic analyses were assessed.

RESULTS

Epilepsy was present in 366 (86.9%) patients. Twenty-one (5.7%) developed epilepsy as newborns. Mean follow-up was 44.86 (6-170) months. Six patients were seizure free and 15 had drug-resistant seizures at the end of follow-up. Mental retardation was found in 81% of patients. In 11 (52.4%) patients brain MRI revealed large malformations of cerebral cortex, meeting the criteria for focal cortical dysplasia (FCD). FCD was revealed in both TSC1 and TSC2 mutation cases. Other risk factors for neonatal epilepsy included: perinatal complications and congenital SEGAs. Presence of FCD was associated with more severe epilepsy and worse neuropsychological outcome. Epilepsy surgery resulted in improvement in seizure control.

CONCLUSIONS

Neonatal onset of epilepsy in TSC is frequently associated with large malformations of cerebral cortex. Patients with FCD are at high risk of severe drug-resistant epilepsy and poor neuropsychological outcome. Early epilepsy surgery may be beneficial and should be considered in such cases.

The neurology of mTOR

The mechanistic target of rapamycin (mTOR) signaling pathway is a crucial cellular signaling hub that, like the nervous system itself, integrates internal and external cues to elicit critical outputs including growth control, protein synthesis, gene expression, and metabolic balance. The importance of mTOR signaling to brain function is underscored by the myriad disorders in which mTOR pathway dysfunction is implicated, such as autism, epilepsy, and neurodegenerative disorders. Pharmacological manipulation of mTOR signaling holds therapeutic promise and has entered clinical trials for several disorders. Here, we review the functions of mTOR signaling in the normal and pathological brain, highlighting ongoing efforts to translate our understanding of cellular physiology into direct medical benefit for neurological disorders.

Recent advances in animal model experimentation in autism research

OBJECTIVE

Autism, a lifelong neuro-developmental disorder is a uniquely human condition. Animal models are not the perfect tools for the full understanding of human development and behavior, but they can be an important place to start. This review focused on the recent updates of animal model research in autism.

METHODS

We have reviewed the publications over the last three decades, which are related to animal model study in autism.

RESULTS

Animal models are important because they allow researchers to study the underlying neurobiology in a way that is not possible in humans. Improving the availability of better animal models will help the field to increase the development of medicines that can relieve disabling symptoms. Results from the therapeutic approaches are encouraging remarkably, since some behavioral alterations could be reversed even when treatment was performed on adult mice. Finding an animal model system with similar behavioral tendencies as humans is thus vital for understanding the brain mechanisms, supporting social motivation and attention, and the manner in which these mechanisms break down in autism. The ongoing studies should therefore increase the understanding of the biological alterations associated with autism as well as the development of knowledge-based treatments therapy for those struggling with autism.

CONCLUSION

In this review, we have presented recent advances in research based on animal models of autism, raising hope for understanding the disease biology for potential therapeutic intervention to improve the quality of life of autism individuals.

Rapamycin reveals an mTOR-independent repression of Kv1.1 expression during epileptogenesis

Changes in ion channel expression are implicated in the etiology of epilepsy. However, the molecular leading to long-term aberrant expression of ion channels are not well understood. The mechanistic/mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that mediates activity-dependent protein synthesis in neurons. mTOR is overactive in epilepsy, suggesting that excessive protein synthesis may contribute to the neuronal pathology. In contrast, we found that mTOR activity and the microRNA miR-129-5p reduce the expression of the voltage-gated potassium channel Kv1.1 in an animal model of temporal lobe epilepsy (TLE). When mTOR activity is low, Kv1.1 expression is high and the frequency of behavioral seizures is low. However, as behavioral seizure activity rises, mTOR activity increases and Kv1.1 protein levels drop. In CA1 pyramidal neurons, the reduction in Kv1.1 lowers the threshold for action potential firing. Interestingly, blocking mTOR activity with rapamycin reduces behavioral seizures and temporarily keeps Kv1.1 levels elevated. Overtime, seizure activity increases and Kv1.1 protein decreases in all animals, even those treated with rapamycin. Notably, the concentration of miR-129-5p, the negative regulator of Kv1.1 mRNA translation, increases by 21days post-status epilepticus (SE), sustaining Kv1.1 mRNA translational repression. Our results suggest that following kainic-acid induced status epilepticus there are two phases of Kv1.1 repression: (1) an initial mTOR-dependent repression of Kv1.1 that is followed by (2) a miR-129-5p persistent reduction of Kv1.1.

Hippocampal granule cell pathology in epilepsy - a possible structural basis for comorbidities of epilepsy?

Temporal lobe epilepsy in both animals and humans is characterized by abnormally integrated hippocampal dentate granule cells. Among other abnormalities, these cells make axonal connections with inappropriate targets, grow dendrites in the wrong direction, and migrate to ectopic locations. These changes promote the formation of recurrent excitatory circuits, leading to the appealing hypothesis that these abnormal cells may by epileptogenic. While this hypothesis has been the subject of intense study, less attention has been paid to the possibility that abnormal granule cells in the epileptic brain may also contribute to comorbidities associated with the disease. Epilepsy is associated with a variety of general findings, such as memory disturbances and cognitive dysfunction, and is often comorbid with a number of other conditions, including schizophrenia and autism. Interestingly, recent studies implicate disruption of common genes and gene pathways in all three diseases. Moreover, while neuropsychiatric conditions are associated with changes in a variety of brain regions, granule cell abnormalities in temporal lobe epilepsy appear to be phenocopies of granule cell deficits produced by genetic mouse models of autism and schizophrenia, suggesting that granule cell dysmorphogenesis may be a common factor uniting these seemingly diverse diseases. Disruption of common signaling pathways regulating granule cell neurogenesis may begin to provide mechanistic insight into the cooccurrence of temporal lobe epilepsy and cognitive and behavioral disorders.

Comparative analysis of Tsc1 and Tsc2 single and double radial glial cell mutants

Tuberous sclerosis complex (TSC) is a neurodevelopmental disorder with variable expressivity. Heterozygous mutations in either of two genes, TSC1 (hamartin) or TSC2 (tuberin), are responsible for most cases. Hamartin and tuberin form a heterodimer that functions as a major cellular inhibitor of the mammalian target of rapamycin complex 1 (mTORC1) kinase. Genotype-phenotype studies suggest that TSC2 mutations are associated with a more severe neurologic phenotype, although the biologic basis for the difference between TSC1- and TSC2-based disease is unclear. Here we performed a study to compare and contrast the brain phenotypes of Tsc1 and Tsc2 single and double mutants. Using Tsc1 and Tsc2 floxed alleles and a radial glial transgenic Cre driver (FVB-Tg(GFAP-cre)25Mes/J), we deleted Tsc1 and/or Tsc2 in radial glial progenitor cells. Single and double mutants had remarkably similar phenotypes: early postnatal mortality, brain overgrowth, laminar disruption, astrogliosis, a paucity of oligodendroglia, and myelination defects. Double Tsc1/Tsc2 mutants died earlier than single mutants, and single mutants showed differences in the location of heterotopias and the organization of the hippocampal stratum pyramidale. The differences were not due to differential mTORC1 activation or feedback inhibition on Akt. These data provide further genetic evidence for individual hamartin and tuberin functions that may explain some of the genotype-phenotype differences seen in the human disease.

Monoallelic germline TSC1 mutations are permissive for T lymphocyte development and homeostasis in tuberous sclerosis complex individuals

Germline and somatic biallelic mutations of the Tuberous sclerosis complex (TSC) 1 and TSC2 gene products cause TSC, an autosomal dominant multifocal hamartomatosis with variable neurological manifestations. The consequences of TSC1 or TSC2 loss in cells of hematopoietic origin have recently started to be unveiled in mice and showed to hinder the development of proper T cell immunity. To date, the consequences of germline TSC1 mutations and/or its loss in mature human T cells remain to be determined. To address these issues, we analyzed subset representation, phenotype and responsiveness to mitogens in T cells from patients with inherited monoallelic TSC1 mutations, and induced shRNA-mediated TSC1 down-regulation in primary and transformed human T cells. We report that, the distribution of peripheral CD4 and CD8 T cell subsets, their cytokine-secretion profile, and responsiveness to in vitro stimulation were largely preserved in TSC subjects with monoallelic TSC1 germline mutations when compared to healthy controls. Sufficient levels of hamartin and tuberin and proper control of mTOR-dependent signaling in primary T cells from TSC subjects best explained this. In contrast, shRNA-induced down-regulation of TSC1, likely mimicking biallelic inactivation of TSC1, compromised hamartin and tuberin expression and mTORC2/AKT/FoxO1/3 signaling causing both primary and transformed T cells to die by apoptosis. Thus, our results indicate that, while one functional TSC1 allele preserves human T lymphocytes development and homeostasis, TSC1 acute down-regulation is detrimental to the survival of both primary and transformed T cells.

Mechanisms regulating neuronal excitability and seizure development following mTOR pathway hyperactivation

The phosphatidylinositol-3-kinase/phosphatase and tensin homolog (PTEN)-mammalian target of rapamycin (mTOR) pathway regulates a variety of neuronal functions, including cell proliferation, survival, growth, and plasticity. Dysregulation of the pathway is implicated in the development of both genetic and acquired epilepsies. Indeed, several causal mutations have been identified in patients with epilepsy, the most prominent of these being mutations in PTEN and tuberous sclerosis complexes 1 and 2 (TSC1, TSC2). These genes act as negative regulators of mTOR signaling, and mutations lead to hyperactivation of the pathway. Animal models deleting PTEN, TSC1, and TSC2 consistently produce epilepsy phenotypes, demonstrating that increased mTOR signaling can provoke neuronal hyperexcitability. Given the broad range of changes induced by altered mTOR signaling, however, the mechanisms underlying seizure development in these animals remain uncertain. In transgenic mice, cell populations with hyperactive mTOR have many structural abnormalities that support recurrent circuit formation, including somatic and dendritic hypertrophy, aberrant basal dendrites, and enlargement of axon tracts. At the functional level, mTOR hyperactivation is commonly, but not always, associated with enhanced synaptic transmission and plasticity. Moreover, these populations of abnormal neurons can affect the larger network, inducing secondary changes that may explain paradoxical findings reported between cell and network functioning in different models or at different developmental time points. Here, we review the animal literature examining the link between mTOR hyperactivation and epileptogenesis, emphasizing the impact of enhanced mTOR signaling on neuronal form and function.

A critical review of mTOR inhibitors and epilepsy: from basic science to clinical trials

Present medications for epilepsy have substantial limitations, such as medical intractability in many patients and lack of antiepileptogenic properties to prevent epilepsy. Drugs with novel mechanisms of action are needed to overcome these limitations. The mTOR signaling pathway has emerged as a possible therapeutic target for epilepsy. Preliminary clinical trials suggest that mTOR inhibitors reduce seizures in tuberous sclerosis complex (TSC) patients with intractable epilepsy. Furthermore, mTOR inhibitors have antiepileptogenic properties in preventing epilepsy in animal models of TSC. Besides TSC, accumulating preclinical data suggest that mTOR inhibitors may have antiseizure or antiepileptogenic actions in other types of epilepsy, including infantile spasms, neonatal hypoxic seizures, absence epilepsy and acquired temporal lobe epilepsy following brain injury, but these effects depend on a number of conditions. Future clinical and basic research is needed to establish whether mTOR inhibitors are an effective treatment for epilepsy.

Mammalian target of rapamycin (mTOR) pathways in neurological diseases

The mammalian target of rapamycin (mTOR) pathway is an essential cellular signaling pathway involved in a number of important physiological functions, including cell growth, proliferation, metabolism, protein synthesis, and autophagy. Dysregulation of the mTOR pathway has been implicated in the pathophysiology of a number of neurological diseases. Hyperactivation of the mTOR pathway, leading to increased cell growth and proliferation, has been most convincingly shown to stimulate tumor growth in the brain and other organs in the genetic disorder, tuberous sclerosis complex (TSC). In addition, mTOR may also play a role in promoting epileptogenesis or maintaining seizures in TSC, as well as in acquired epilepsies following brain injury. Finally, the mTOR pathway may also be involved in the pathogenesis of cognitive dysfunction and other neurological deficits in developmental disorders and neurodegenerative diseases. mTOR inhibitors, such as rapamycin and its analogs, may represent novel, rational therapies for a variety of neurological disorders.

Early susceptibility for epileptiform activity in malformed cortex

Despite early disruption of developmental processes, hyperexcitability is often delayed after the induction of cortical malformations. In the freeze-lesion model of microgyria, interictal activity cannot be evoked in vitro until postnatal day (P)12, despite the increased excitatory afferent input to the epileptogenic region by P10. In order to determine the most critical time period for assessment of epileptogenic mechanisms, here we have used low-Mg(2+) aCSF as a second hit after the neonatal freeze lesion to examine whether there is an increased susceptibility prior to the overt expression of epileptiform activity. This two-hit model produced increased interictal activity in freeze-lesioned relative to control cortex. We quantified this with measures of incidence by sweep, time to first epileptiform event, and magnitude of late activity. The increase was present even in the P7-9 survival group, before increased excitatory afferents invade, as well as in the P10-11 and P12-15 groups. In our young adult group (P28-36), the amount of interictal activity did not differ, but only the lesioned cortices produced ictal activity. We conclude that epileptogenic processes begin early and continue beyond the expression of interictal activity, with different time courses for susceptibility for interictal and ictal activity.

Rapamycin prevents seizures after depletion of STRADA in a rare neurodevelopmental disorder

A rare neurodevelopmental disorder in the Old Order Mennonite population called PMSE (polyhydramnios, megalencephaly, and symptomatic epilepsy syndrome; also called Pretzel syndrome) is characterized by infantile-onset epilepsy, neurocognitive delay, craniofacial dysmorphism, and histopathological evidence of heterotopic neurons in subcortical white matter and subependymal regions. PMSE is caused by a homozygous deletion of exons 9 to 13 of the LYK5/STRADA gene, which encodes the pseudokinase STRADA, an upstream inhibitor of mammalian target of rapamycin complex 1 (mTORC1). We show that disrupted pathfinding in migrating mouse neural progenitor cells in vitro caused by STRADA depletion is prevented by mTORC1 inhibition with rapamycin or inhibition of its downstream effector p70 S6 kinase (p70S6K) with the drug PF-4708671 (p70S6Ki). We demonstrate that rapamycin can rescue aberrant cortical lamination and heterotopia associated with STRADA depletion in the mouse cerebral cortex. Constitutive mTORC1 signaling and a migration defect observed in fibroblasts from patients with PMSE were also prevented by mTORC1 inhibition. On the basis of these preclinical findings, we treated five PMSE patients with sirolimus (rapamycin) without complication and observed a reduction in seizure frequency and an improvement in receptive language. Our findings demonstrate a mechanistic link between STRADA loss and mTORC1 hyperactivity in PMSE, and suggest that mTORC1 inhibition may be a potential treatment for PMSE as well as other mTOR-associated neurodevelopmental disorders.

Role of the PTEN signaling pathway in autism spectrum disorder

Autism is an etiologically heterogeneous group of neurodevelopmental disorders, diagnosed mostly by the clinical behavioral phenotypes. The concept that the tumor-related gene PTEN plays a critical role in autism spectrum disorder has emerged over the last decade. In this review, we focus on the essential role of the PTEN signaling pathway in neuronal differentiation and the formation of neural circuitry, as well as genetic mouse models with Pten manipulations. Particularly, accumulated data suggest that the effect of PTEN on neural stem-cell development contributes significantly to the pathophysiology of autism spectrum disorders.

The role of dopamine signaling in epileptogenesis

Clinical and experimental studies implicate most neuromodulatory systems in epileptogenesis. The dopaminergic system has a seizure-modulating effect that crucially depends on the different subtypes of dopamine (DA) receptors involved and the brain regions in which they are activated. Specifically, DA plays a major role in the control of seizures arising in the limbic system. Studies performed in a wide variety of animal models contributed to illustrate the opposite actions of D1-like and D2-like receptor signaling in limbic epileptogenesis. Indeed, signaling from D1-like receptors is generally pro-epileptogenic, whereas D2-like receptor signaling exerts an anti-epileptogenic effect. However, this view might appear quite simplistic as the complex neuromodulatory action of DA in the control of epileptogenesis likely requires a physiological balance in the activation of circuits modulated by these two major DA receptor subtypes, which determines the response to seizure-promoting stimuli. Here we will review recent evidences on the identification of molecules activated by DA transduction pathways in the generation and spread of seizures in the limbic system. We will discuss the intracellular signaling pathways triggered by activation of different DA receptors in relation to their role in limbic epileptogenesis, which lead to the activation of neuronal death/survival cascades. A deep understanding of the signaling pathways involved in epileptogenesis is crucial for the identification of novel targets for the treatment of epilepsy.

TORC1-dependent epilepsy caused by acute biallelic Tsc1 deletion in adult mice

OBJECTIVE

Seizure development in tuberous sclerosis complex (TSC) correlates with the presence of specific lesions called cortical tubers. Moreover, heterozygous TSC animal models do not show gross brain pathology and are seizure-free, suggesting that such pathology is a prerequisite for the development of epilepsy. However, cells within TSC lesions show increased activity of the target of rapamycin complex 1 (TORC1) pathway, and recent studies have implicated this pathway in non-TSC-related animal models of epilepsy and neuronal excitability. These findings imply a direct role for TORC1 in epilepsy. Here, we investigate the effect of increased TORC1 signaling induced by acute biallelic deletion of Tsc1 in healthy adult mice.

METHODS

Biallelic Tsc1 gene deletion was induced in adult Tsc1 heterozygous and wild-type mice. Seizures were monitored by electroencephalographic and video recordings. Molecular and cellular changes were investigated by Western blot analysis, immunohistochemistry, and electrophysiology.

RESULTS

Mice developed epilepsy a few days after biallelic Tsc1 deletion. Acute gene deletion was not accompanied by any obvious histological changes, but resulted in activation of the TORC1 pathway, enhanced neuronal excitability, and a decreased threshold for protein-synthesis-dependent long-term potentiation preceding the onset of seizures. Rapamycin treatment after seizure onset reduced TORC1 activity and fully abolished the seizures.

INTERPRETATION

Our data indicate a direct role for TORC1 signaling in epilepsy development, even in the absence of major brain pathology. This suggests that TORC1 is a promising target for treating seizures not only in TSC but also in other forms of epilepsy that result from increased TORC1 activation.

Potential for treatment of severe autism in tuberous sclerosis complex

The Food and Drug Administration (FDA) has approved two mechanism-based treatments for tuberous sclerosis complex (TSC)-everolimus and vigabatrin. However, these treatments have not been systematically studied in individuals with TSC and severe autism. The aim of this review is to identify the clinical features of severe autism in TSC, applicable preclinical models, and potential barriers that may warrant strategic planning in the design phase of clinical trial development. A comprehensive search strategy was formed and searched across PubMed, Embase and SCOPUS from their inception to 2/21/12, 3/16/12, and 3/12/12 respectively. After the final search date, relevant, updated articles were selected from PubMed abstracts generated electronically and emailed daily from PubMed. The references of selected articles were searched, and relevant articles were selected. A search of clinicaltrials.gov was completed using the search term “TSC” and “tuberous sclerosis complex”. Autism has been reported in as many as 60% of individuals with TSC; however, review of the literature revealed few data to support clear classification of the severity of autism in TSC. Variability was identified in the diagnostic approach, assessment of cognition, and functional outcome among the reviewed studies and case reports. Objective outcome measures were not used in many early studies; however, diffusion tensor imaging of white matter, neurophysiologic variability in infantile spasms, and cortical tuber subcategories were examined in recent studies and may be useful for objective classification of TSC in future studies. Mechanism-based treatments for TSC are currently available. However, this literature review revealed two potential barriers to successful design and implementation of clinical trials in individuals with severe autism-an unclear definition of the population and lack of validated outcome measures. Recent studies of objective outcome measures in TSC and further study of applicable preclinical models present an opportunity to overcome these barriers.

How does the ketogenic diet work? Four potential mechanisms

The ketogenic diet and its newer variants are clinically useful in treating epilepsy. They can also have antiepileptogenic properties and can eventually have a role in treating other neurologic and nonneurologic conditions. Despite being nearly a century old, identifying the molecular underpinnings of the ketogenic diet has been challenging. However, recent studies provide experimental evidence for 4 distinct mechanisms that could contribute to the antiseizure and other beneficial effects of these diets. These mechanisms include carbohydrate reduction, activation of adenosine triphosphate (ATP)-sensitive potassium channels by mitochondrial metabolism, inhibition of the mammalian target of rapamycin pathway, and inhibition of glutamatergic excitatory synaptic transmission.

mTOR signaling in neural stem cells: from basic biology to disease

The mammalian target of rapamycin (mTOR) pathway is a central controller of growth and homeostasis, and, as such, is implicated in disease states where growth is deregulated, namely cancer, metabolic diseases, and hamartoma syndromes like tuberous sclerosis complex (TSC). Accordingly, mTOR is also a pivotal regulator of the homeostasis of several distinct stem cell pools in which it finely tunes the balance between stem cell self-renewal and differentiation. mTOR hyperactivation in neural stem cells (NSCs) has been etiologically linked to the development of TSC-associated neurological lesions, such as brain hamartomas and benign tumors. Animal models generated by deletion of mTOR upstream regulators in different types of NSCs reproduce faithfully some of the TSC neurological alterations. Thus, mTOR dysregulation in NSCs seems to be responsible for the derangement of their homeostasis, thus leading to TSC development. Here we review recent advances in the molecular dissection of the mTOR cascade, its involvement in the maintenance of stem cell compartments, and in particular the implications of mTOR hyperactivation in NSCs in vivo and in vitro.

Focal malformations of cortical development: new vistas for molecular pathogenesis

Focal malformations of cortical development (FMCD) are highly associated with several neurological disorders including intractable epilepsy and neurocognitive disabilities. Over the past decade, several FMCD subtypes have been linked to hyperactivation of the mammalian target of rapamycin (mTOR) signaling cascade. In view of the roles that mTOR plays in cell proliferation, size, motility, and stem cell phenotype, many of the features of FMCD such as cytomegaly, disorganized lamination, and expression of stem cell markers can be explained by enhanced mTOR signaling. FMCD result from several distinct and fascinating molecular mechanisms including biallelic gene inactivation, somatic mutation, and potentially, viral infection. These mechanisms have been directly linked to mTOR activation. Perhaps most compelling, pharmacological inhibition of mTOR has been implemented successfully in clinical trials for select FMCD and provides a new vista for treatment.

Role of the mTOR signaling pathway in epilepsy

Epilepsy, a common neurological disorder and cause of significant morbidity and mortality, places an enormous burden on the individual and society. Presently, most drugs for epilepsy primarily suppress seizures as symptomatic therapies but do not possess actual antiepileptogenic or disease-modifying properties. The mTOR (mammalian target of rapamycin) signaling pathway is involved in major multiple cellular functions, including protein synthesis, cell growth and proliferation and synaptic plasticity, which may influence neuronal excitability and be responsible for epileptogenesis. Intriguing findings of the frequent hyperactivation of mTOR signaling in epilepsy make it a potential mechanism in the pathogenesis as well as an attractive target for the therapeutic intervention, and have driven the significant ongoing efforts to pharmacologically target this pathway. This review explores the relevance of the mTOR pathway to epileptogenesis and its potential as a therapeutic target in epilepsy treatment by presenting the current results on mTOR inhibitors, in particular, rapamycin, in animal models of diverse types of epilepsy. Limited clinical studies in human epilepsy, some paradoxical experimental data and outstanding questions have also been discussed.

Rapamycin reverses impaired social interaction in mouse models of tuberous sclerosis complex

Impairment of reciprocal social interaction is a core symptom of autism spectrum disorder. Genetic disorders frequently accompany autism spectrum disorder, such as tuberous sclerosis complex caused by haploinsufficiency of the TSC1 and TSC2 genes. Accumulating evidence implicates a relationship between autism spectrum disorder and signal transduction that involves tuberous sclerosis complex 1, tuberous sclerosis complex 2 and mammalian target of rapamycin. Here we show behavioural abnormalities relevant to autism spectrum disorder and their recovery by the mammalian target of rapamycin inhibitor rapamycin in mouse models of tuberous sclerosis complex. In Tsc2(+/-) mice, we find enhanced transcription of multiple genes involved in mammalian target of rapamycin signalling, which is dependent on activated mammalian target of rapamycin signalling with a minimal influence of Akt. The findings indicate a crucial role of mammalian target of rapamycin signalling in deficient social behaviour in mouse models of tuberous sclerosis complex, supporting the notion that mammalian target of rapamycin inhibitors may be useful for the pharmacological treatment of autism spectrum disorder associated with tuberous sclerosis complex and other conditions that result from dysregulated mammalian target of rapamycin signalling.

Timing of mTOR activation affects tuberous sclerosis complex neuropathology in mouse models

Tuberous sclerosis complex (TSC) is a dominantly inherited disease with high penetrance and morbidity, and is caused by mutations in either of two genes, TSC1 or TSC2. Most affected individuals display severe neurological manifestations – such as intractable epilepsy, mental retardation and autism – that are intimately associated with peculiar CNS lesions known as cortical tubers (CTs). The existence of a significant genotype-phenotype correlation in individuals bearing mutations in either TSC1 or TSC2 is highly controversial. Similar to observations in humans, mouse modeling has suggested that a more severe phenotype is associated with mutation in Tsc2 rather than in Tsc1. However, in these mutant mice, deletion of either gene was achieved in differentiated astrocytes. Here, we report that loss of Tsc1 expression in undifferentiated radial glia cells (RGCs) early during development yields the same phenotype detected upon deletion of Tsc2 in the same cells. Indeed, the same aberrations in cortical cytoarchitecture, hippocampal disturbances and spontaneous epilepsy that have been detected in RGC-targeted Tsc2 mutants were observed in RGC-targeted Tsc1 mutant mice. Remarkably, thorough characterization of RGC-targeted Tsc1 mutants also highlighted subventricular zone (SVZ) disturbances as well as STAT3-dependent and -independent developmental-stage-specific defects in the differentiation potential of ex-vivo-derived embryonic and postnatal neural stem cells (NSCs). As such, deletion of either Tsc1 or Tsc2 induces mostly overlapping phenotypic neuropathological features when performed early during neurogenesis, thus suggesting that the timing of mTOR activation is a key event in proper neural development.

Conditional and domain-specific inactivation of the Tsc2 gene in neural progenitor cells

Tuberous sclerosis complex (TSC) is a genetic disease characterized by multiorgan benign tumors as well as neurological manifestations. Epilepsy and autism are two of the more prevalent neurological complications and are usually severe. TSC is caused by mutations in either the TSC1 (encodes hamartin) or the TSC2 (encodes tuberin) genes with TSC2 mutations being associated with worse outcomes. Tuberin contains a highly conserved GTPase-activating protein (GAP) domain that indirectly inhibits mammalian target of rapamycin complex 1 (mTORC1). mTORC1 dysregulation is currently thought to cause much of the pathogenesis in TSC but mTORC1-independent mechanisms may also contribute. We generated a novel conditional allele of Tsc2 by flanking exons 36 and 37 with loxP sites. Mice homozygous for this knock-in Tsc2 allele are viable and fertile with normal appearing growth and development. Exposure to Cre recombinase then creates an in-frame deletion involving critical residues of the GAP domain. Homozygous conditional mutant mice generated using Emx1(Cre) have increased cortical mTORC1 signaling, severe developmental brain anomalies, seizures, and die within 3 weeks. We found that the normal levels of the mutant Tsc2 mRNA, though GAP-deficient tuberin protein, appear unstable and rapidly degraded. This novel animal model will allow further study of tuberin function including the requirement of the GAP domain for protein stability.

Rheb activation in subventricular zone progenitors leads to heterotopia, ectopic neuronal differentiation, and rapamycin-sensitive olfactory micronodules and dendrite hypertrophy of newborn neurons

Mammalian target of rapamycin (mTOR) hyperactivity in perinatal neural progenitor cells (NPCs) of tuberous sclerosis complex 1 (Tsc1) heterozygote mice leads to heterotopia and abnormal neuronal morphogenesis as seen in patients with tuberous sclerosis. Considering that pathological hyperactive mTOR also occurs in individuals carrying no genetic mutations, we examined whether increasing mTOR activity in neonatal NPCs of wild-type mice would recapitulate the above phenotypes. Electroporation of a plasmid encoding constitutively active Ras-homolog enriched in brain (Rheb(CA)) into subventricular zone NPCs increased mTOR activity in newborn cells. At 19 d post-electroporation (dpe), heterotopia and ectopic cells with a neuronal morphology were observed along the migratory path [rostral migratory stream (RMS)] and in the olfactory bulb (OB). These ectopic cells displayed action potentials and received synaptic inputs identifying them as synaptically integrated neurons. RMS heterotopias contained astrocytes, neurons, and entrapped neuroblasts. Immunostaining at 3 dpe revealed the presence of Mash1(+) Olig2(-) cells in the migratory route accompanied by ectopic neuronal differentiation and altered direction and speed of neuroblast migration at 7 dpe, suggesting a non-cell-autonomous disruption of migration. At >19 dpe, newborn Rheb(CA)-expressing neurons displayed altered distribution and formed micronodules in the OB. In addition, they displayed increased dendritic complexity along with altered membrane biophysics and increased frequency of GABAergic synaptic inputs. OB heterotopia, micronodules, and dendrite hypertrophy were notably prevented by rapamycin treatment, suggesting their mTOR dependence. Collectively, these data show that increasing mTOR activity in neonatal NPCs of wild-type mice recapitulate the pathologies observed in Tsc1 mutant mice. In addition, increased mTOR activity in individuals without known mutations could significantly impact neurogenesis and circuit formation.

Differential response of skeletal muscles to mTORC1 signaling during atrophy and hypertrophy

BACKGROUND

Skeletal muscle mass is determined by the balance between protein synthesis and degradation. Mammalian target of rapamycin complex 1 (mTORC1) is a master regulator of protein translation and has been implicated in the control of muscle mass. Inactivation of mTORC1 by skeletal muscle-specific deletion of its obligatory component raptor results in smaller muscles and a lethal dystrophy. Moreover, raptor-deficient muscles are less oxidative through changes in the expression PGC-1α, a critical determinant of mitochondrial biogenesis. These results suggest that activation of mTORC1 might be beneficial to skeletal muscle by providing resistance to muscle atrophy and increasing oxidative function. Here, we tested this hypothesis by deletion of the mTORC1 inhibitor tuberous sclerosis complex (TSC) in muscle fibers.

METHOD

Skeletal muscles of mice with an acute or a permanent deletion of raptor or TSC1 were examined using histological, biochemical and molecular biological methods. Response of the muscles to changes in mechanical load and nerve input was investigated by ablation of synergistic muscles or by denervation .

RESULTS

Genetic deletion or knockdown of raptor, causing inactivation of mTORC1, was sufficient to prevent muscle growth and enhance muscle atrophy. Conversely, short-term activation of mTORC1 by knockdown of TSC induced muscle fiber hypertrophy and atrophy-resistance upon denervation, in both fast tibialis anterior (TA) and slow soleus muscles. Surprisingly, however, sustained activation of mTORC1 by genetic deletion of Tsc1 caused muscle atrophy in all but soleus muscles. In contrast, oxidative capacity was increased in all muscles examined. Consistently, TSC1-deficient soleus muscle was atrophy-resistant whereas TA underwent normal atrophy upon denervation. Moreover, upon overloading, plantaris muscle did not display enhanced hypertrophy compared to controls. Biochemical analysis indicated that the atrophy response of muscles was based on the suppressed phosphorylation of PKB/Akt via feedback inhibition by mTORC1 and subsequent increased expression of the E3 ubiquitin ligases MuRF1 and atrogin-1/MAFbx. In contrast, expression of both E3 ligases was not increased in soleus muscle suggesting the presence of compensatory mechanisms in this muscle.

CONCLUSIONS

Our study shows that the mTORC1- and the PKB/Akt-FoxO pathways are tightly interconnected and differentially regulated depending on the muscle type. These results indicate that long-term activation of the mTORC1 signaling axis is not a therapeutic option to promote muscle growth because of its strong feedback induction of the E3 ubiquitin ligases involved in protein degradation.

The ability of high field strength 7-T magnetic resonance imaging to reveal previously uncharacterized brain lesions in patients with tuberous sclerosis complex

OBJECT

Tuberous sclerosis complex (TSC) brain pathology is characterized on MRI by cortical tubers, subependymal nodules, and subependymal giant cell astrocytomas. Seizures, the prominent feature of TSC, are frequently intractable to medical therapy and, in many patients, resection of tubers results in seizure control. However, in approximately 40% of patients, resection of tubers does not control seizures. This fact, as well as evidence from invasive electrophysiological recordings and experimental animal models, suggests that in patients with TSC, there may be extratuberal epileptogenic brain that does not display any apparent abnormality on conventional MRI. The authors hypothesized that high field strength MRI might uncover lesions not seen on conventional MRI in these patients.

METHODS

Institutional review board approval was obtained to scan 4 patients with TSC (ages 18-26 years) in a 7-T MR unit. Optimized 7-T sequences, including T1- and T2-weighted, FLAIR, SPACE FLAIR, T2*, and MPRAGE studies, were performed. Imaging studies were compared with identical sequences performed using a conventional 1.5-T MR scanner.

RESULTS

In all 4 patients, there was improved visualization of the findings demonstrated on conventional imaging. Importantly, new lesions were detected in all 4 patients, which were not well visualized with conventional MRI. Newly detected lesions included microtubers, radial glial signal abnormalities, subependymal nodules arising from the caudate nucleus, and caudate nucleus lesions.

CONCLUSIONS

High field strength MRI detects previously uncharacterized lesions in patients with TSC and allows better detection and delineation of subtle abnormalities. In addition, the data demonstrate a compelling relationship between intraventricular lesions and the caudate nucleus. These data support previous electrophysiological and animal-model findings that demonstrate neurological pathology beyond the conventionally detected lesions in TSC.

Evolving neurobiology of tuberous sclerosis complex

Over the past decade, there have been numerous advances in our understanding of the molecular pathogenesis of tuberous sclerosis complex (TSC). Following the identification of the TSC1 and TSC2 genes, a link to regulatory control of the mammalian target of rapamycin (mTOR) signaling pathway has paved the way for new therapeutic interventions, and now even approved therapies for TSC. Gene identification has permitted establishment of cell lines and conditional knockout mouse strains to assay how abnormalities in brain structure lead to enhanced excitability, seizures, cognitive disabilities, and other neuropsychological disorders in TSC. Furthermore, work in in vitro systems and analysis of rodent models and human tissue has allowed investigators to study how brain lesions form in TSC. Evolving questions over the next decade include understanding the high clinical variability of TSC, defining why there is a lack of clear genotype-phenotype correlations, and identifying biomarkers for prognosis and stratification. The study of TSC has in many ways reflected a paradigm "bench-to-bedside" success story that serves as a model of many other neurological disorders.

A magnetic resonance imaging study of cerebellar volume in tuberous sclerosis complex

The cerebellum plays an important role in motor learning and cognition, and structural cerebellar abnormalities have been associated with cognitive impairment. In tuberous sclerosis complex, neurologic outcome is highly variable, and no consistent imaging or pathologic determinant of cognition has been firmly established. The cerebellum calls for specific attention because mouse models of tuberous sclerosis complex have demonstrated a loss of cerebellar Purkinje cells, and cases of human histologic data have demonstrated a similar loss in patients. We hypothesized that there might be a common cerebellar finding in tuberous sclerosis complex that could be measured as morphometric changes with magnetic resonance imaging. Using a robust, automated image analysis procedure, we studied 36 patients with tuberous sclerosis complex and age-matched control subjects and observed significant volume loss among patients in the cerebellar cortices and vermis. Furthermore, this effect was strongest in a subgroup of 19 patients with a known, pathogenic mutation of the tuberous sclerosis 2 gene and impacted all cerebellar structures. We conclude that patients with tuberous sclerosis complex exhibit volume loss in the cerebellum, and this loss is larger and more widespread in patients with a tuberous sclerosis 2 mutation.