Focal cortical dysplasia (FCD), a local malformation of cortical development, is the most common cause of pharmacoresistant epilepsy associated with life-long neurocognitive impairments. It remains unclear whether neuronal misplacement is required for seizure activity. Here we show that dyslamination and white matter heterotopia are not necessary for seizure generation in a murine model of type II FCDs. These experimental FCDs generated by increasing mTOR activity in layer 2/3 neurons of the medial prefrontal cortex are associated with tonic-clonic seizures and a normal survival rate. Preventing all FCD-related defects, including neuronal misplacement and dysmorphogenesis, with rapamycin treatments from birth eliminates seizures, but seizures recur after rapamycin withdrawal. In addition, bypassing neuronal misplacement and heterotopia using inducible vectors do not prevent seizure occurrence. Collectively, data obtained using our new experimental FCD-associated epilepsy suggest that life-long treatment to reduce neuronal dysmorphogenesis is required to suppress seizures in individuals with FCD.

Hyperactive mammalian target of rapamycin complex 1 (mTORC1) is a shared molecular hallmark in several neurodevelopmental disorders characterized by abnormal brain cytoarchitecture. The mechanisms downstream of mTORC1 that are responsible for these defects remain unclear. We show that focally increasing mTORC1 activity during late corticogenesis leads to ectopic placement of upper-layer cortical neurons that does not require altered signaling in radial glia and is accompanied by changes in layer-specific molecular identity. Importantly, we found that decreasing cap-dependent translation by expressing a constitutively active mutant of the translational repressor eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) prevents neuronal misplacement and soma enlargement, while partially rescuing dendritic hypertrophy induced by hyperactive mTORC1. Furthermore, overactivation of translation alone through knockdown of 4E-BP2 was sufficient to induce neuronal misplacement. These data show that many aspects of abnormal brain cytoarchitecture can be prevented by manipulating a single intracellular process downstream of mTORC1, cap-dependent translation.veractive mammalian target of rapamycin complex 1 (mTORC1) signaling is a signature of many disorders with cortical malformations (1), ranging from tuberous sclerosis complex with focal dysplasias to hemimegalencephaly with more diffuse, hemispheric aberrations. The high incidence of negative outcomes in individuals with such malformations (2), which are often associated with intractable childhood seizures, underscores the need to better understand the molecular etiology of these developmental lesions. Animal models have demonstrated the causative effect of increased mTORC1 signaling on mislamination (3-7). The pharmacological mTORC1 blocker rapamycin has also been shown to reverse some of the developmental abnormalities and associated seizure activity in several of these mouse models (5, 6, 8-10), further emphasizing the importance of mTORC1 in the disease pathogenesis.Despite the demonstrated relevance of mTORC1 signaling, there is less known about the molecular mechanisms by which mTORC1 alters cortical development. Addressing this question is complicated by the wide range of cellular processes regulated by mTORC1 through independent downstream targets. Among these regulated processes are autophagy, lysosomal function, lipid synthesis, and, one of the best-studied functions, cap-dependent translation (11). Because current drugs that suppress mTORC1 activity can have serious side effects (12, 13) and do not fully block some of mTORC1's functions (14), a more specific understanding of how mTORC1 contributes to cortical mislamination could yield better targets for treatment.This study therefore aimed to more closely characterize the cytoarchitectural aberrations generated by hyperactive mTORC1 and to examine the contribution of translational regulation to these cortical malformations. Using in utero electroporation, we generated and characterized focal mislamination an...

Developmental nicotine exposure causes persistent changes in cortical neuron morphology and in behavior. We used microarray screening to identify master transcriptional or epigenetic regulators mediating these effects of nicotine and discovered increases in Ash2l, a component of a histone methyltransferase complex. We therefore examined genome-wide changes in H3K4 tri-methylation, a mark induced by the Ash2l complex associated with increased gene transcription. A significant number of regulated promoter sites were involved in synapse maintenance. We found that Mef2c interacts with Ash2l and mediates changes in H3K4 tri-methylation. Knockdown of Ash2l or Mef2c abolishes nicotine-mediated alterations of dendritic complexity in vitro and in vivo, and attenuates nicotine-dependent changes in passive avoidance behavior. In contrast, overexpression mimics nicotine-mediated alterations of neuronal structure and passive avoidance behavior. These studies identify Ash2l as a novel target induced by nicotinic stimulation that couples developmental nicotine exposure to changes in brain epigenetic marks, neuronal structure and behavior.

Tuberous sclerosis complex (TSC) is an autosomal dominant monogenetic disorder that is characterized by the formation of benign tumors in several organs as well as brain malformations and neuronal defects. TSC is caused by inactivating mutations in one of two genes, TSC1 and TSC2, resulting in increased activity of the mammalian Target of Rapamycin (mTOR). Here, we explore the cytoarchitectural and functional CNS aberrations that may account for the neurological presentations of TSC, notably seizures, hydrocephalus, and cognitive and psychological impairments. In particular, recent mouse models of brain lesions are presented with an emphasis on using electroporation to allow the generation of discrete lesions resulting from loss of heterozygosity during perinatal development. Cortical lesions are thought to contribute to epileptogenesis and worsening of cognitive defects. However, it has recently been suggested that being born with a mutant allele without loss of heterozygosity and associated cortical lesions is sufficient to generate cognitive and neuropsychiatric problems. We will thus discuss the function of mTOR hyperactivity on neuronal circuit formation and the potential consequences of being born heterozygote on neuronal function and the biochemistry of synaptic plasticity, the cellular substrate of learning and memory. Ultimately, a major goal of TSC research is to identify the cellular and molecular mechanisms downstream of mTOR underlying the neurological manifestations observed in TSC patients and identify novel therapeutic targets to prevent the formation of brain lesions and restore neuronal function.

Summary Abnormal dendritic complexity is a shared feature of many neurodevelopmental disorders associated with neurological defects. Here, we found that the actin-crosslinking protein filamin A (FLNA) is overexpressed in tuberous sclerosis complex (TSC) mice, a PI3K-mTOR model of neurodevelopmental disease that is associated with abnormal dendritic complexity. Both under-and overexpression of FLNA in wild-type neurons led to more complex dendritic arbors in vivo, suggesting that an optimal level of FLNA expression is required for normal dendritogenesis. In Tsc1null neurons, knocking down FLNA in vivo prevented dendritic abnormalities. Surprisingly, FLNA overexpression in Tsc1null neurons was dependent on MEK1/2 but not mTOR activity despite both pathways being hyperactive. In addition, increasing MEK-ERK1/2 activity led to dendritic abnormalities via FLNA and decreasing MEK-ERK1/2 signaling in Tsc1null neurons rescued dendritic defects. These data demonstrate that altered FLNA expression increases dendritic complexity and contributes to pathologic dendritic patterning in TSC in an mTOR-independent, ERK1/2-dependent manner.