OBJECTIVE:Mutations in genes of the mTOR pathway have been identified as a major cause of hemimegalencephaly (HMG), focal cortical dysplasia type II, and tuberous sclerosis, cortical malformations associated with epilepsy. These conditions are characterized at the cellular level by increased size of pyramidal neurons that grow with dysmorphic features and in some cases by the presence of giant balloon cells. Our previous research in tuberous sclerosis has shown that parvalbumin (Pvalb) and calbindin immunoreactive cells in cortical and subcortical tuberal lesions show cytomegalic features, suggesting the involvement of GABAergic cells in mTOR-related pathologies. In the present report, we propose to deepen our understanding of the role of interneurons in mTOR-related cortical malformations by analyzing the maturation of Pvalb neurons in fetal samples of HMG. METHODS:We performed immunohistochemical staining of cortical samples from individuals with HMG from 21 gestational weeks to 10 postnatal months. The study focused on Pvalb cells, and pS6 counterstaining was performed to assess the activation of the mTOR pathway. To investigate the pathomechanisms behind the cytomegalic features, we examined mTOR pathway gene expression in Pvalb interneurons and cortical projection neurons using a single-cell transcriptomic atlas of the human neocortex. RESULTS:Our results revealed cytomegalic features in Pvalb interneurons, indicating abnormal development in HMG patients compared to controls. This phenotype progressively worsened over time, suggesting ongoing developmental abnormalities associated with mTOR dysregulation, which may underlie the pathology of cortical malformations in HMG. Our transcriptomic data revealed similar expression patterns of mTOR and its upstream regulators in both Pvalb and glutamatergic neurons during development, suggesting that mTOR pathway disorders may induce similar phenotypes in both cell types. SIGNIFICANCE:The present data suggest that Pvalb interneurons are involved in the development of mTOR-related cortical dysplasia and that they may be a contributor to the clinical phenotype of these patients.

Brain somatic variants in the SLC35A2 gene, encoding for a Golgi galactose transporter, represent the major cause of mild malformation of cortical development with oligodendroglial hyperplasia in epilepsy (MOGHE). Clinical features associated with MOGHE include early-onset epileptic encephalopathy, drug-resistant focal epilepsy with developmental delay, and intellectual disability. Half of somatic SLC35A2 variants identified in MOGHE patients are predicted to encode full-length SLC35A2 protein or stable protein products. We investigated the pathophysiological basis of MOGHE by analyzing the functional consequences of SLC35A2 pathogenetic variants in vitro and in vivo models. We assessed how different SLC35A2 variants impact protein stability and expression in transfected cellular models. We used in utero electroporation in the rat brain to model mosaic expression of SLC35A2 pathogenetic variants in the cerebral cortex and assessed their effect on neurons migration and morphology. We found that SLC35A2 variants identified in MOGHE patients variably impact on SLC35A2 protein expression. In utero expression of a SLC35A2 missense (p.G282A) or frameshift (p.F280Tfs*10) variants resulted in neuronal heterotopia in the white matter and impaired dendritogenesis at postnatal stages, suggesting a cell autonomous role for SLC35A2 in neuronal development. These phenotypes were recapitulated by in utero silencing of rat Slc35a2 gene. We successfully developed an in vivo mosaic model for the characterization of SLC35A2 variants identified in MOGHE patients and demonstrated that the expression of single SLC35A2 variants triggers the pathophysiological cascade associated with SLC35A2 dysfunction in neurons.

Brain architectures vary widely across species, yet how neuronal positioning constrains the type of circuits that can be made, and their function, remains poorly understood. Here we examine how neuronal position affects molecular identity, connectivity and function by studying Eml1 knockout mice, which exhibit abnormally located (heterotopic) neurons beneath the cortex. Heterotopic neurons maintained their molecular signatures, formed appropriate long-range connections and exhibited coherent electrophysiological properties. They organized into functional sensory-processing centers that mirrored their cortical counterparts, with preserved somatotopic mapping and responsiveness to sensory stimuli. Remarkably, cortical silencing did not impair sensory discrimination, revealing that heterotopic neurons were the main drivers of this function. Hence, equivalent circuits can emerge in different spatial configurations, allowing diverse brain architectures to converge on similar functional outcomes.

Microglia (MG), the brain-resident macrophages, play major roles in health and disease via a diversity of cellular states. While embryonic MG display a large heterogeneity of cellular distribution and transcriptomic states, their functions remain poorly characterized. Here, we uncovered a role for MG in the maintenance of structural integrity at two fetal cortical boundaries. At these boundaries between structures that grow in distinct directions, embryonic MG accumulate, display a state resembling post-natal axon-tract-associated microglia (ATM) and prevent the progression of microcavities into large cavitary lesions, in part via a mechanism involving the ATM-factor Spp1. MG and Spp1 furthermore contribute to the rapid repair of lesions, collectively highlighting protective functions that preserve the fetal brain from physiological morphogenetic stress and injury. Our study thus highlights key major roles for embryonic MG and Spp1 in maintaining structural integrity during morphogenesis, with major implications for our understanding of MG functions and brain development.

Grey matter heterotopia (GMH) are neurodevelopmental disorders associated with abnormal cortical function and epilepsy. Subcortical band heterotopia (SBH) and periventricular nodular heterotopia (PVNH) are two well-recognized GMH subtypes in which neurons are misplaced, either forming nodules lining the ventricles in PVNH, or forming bands in the white matter in SBH. Although both PVNH and SBH are commonly associated with epilepsy, it is unclear whether these two GMH subtypes differ in terms of pathological consequences or, on the contrary, share common altered mechanisms. Here, we studied two robust preclinical models of SBH and PVNH, and performed a systematic comparative assessment of the physiological and morphological diversity of heterotopia neurons, as well as the dynamics of epileptiform activity and input connectivity. We uncovered a complex set of altered properties, including both common and distinct physiological and morphological features across heterotopia subtypes, and associated with specific dynamics of epileptiform activity. Taken together, these results suggest that pro-epileptic circuits in GMH are, at least in part, composed of neurons with distinct, subtype-specific, physiological and morphological properties depending on the heterotopia subtype. Our work supports the notion that GMH represent a complex set of disorders, associating both shared and diverging pathological consequences, and contributing to forming epileptogenic networks with specific properties. A deeper understanding of these properties may help to refine current GMH classification schemes by identifying morpho-electric signatures of GMH subtypes, to potentially inform new treatment strategies.

Periventricular nodular heterotopia (PNH), the most common brain malformation diagnosed in adulthood, is characterized by the presence of neuronal nodules along the ventricular walls. PNH is mainly associated with mutations in the FLNA gene - encoding an actin-binding protein - and patients often develop epilepsy. However, the molecular mechanisms underlying the neuronal failure still remain elusive. It has been hypothesized that dysfunctional cortical circuitry, rather than ectopic neurons, may explain the clinical manifestations. To address this issue, we depleted FLNA from cortical pyramidal neurons of a conditional Flnaflox/flox mice by timed in utero electroporation of Cre recombinase. We found that FLNA regulates dendritogenesis and spinogenesis thus promoting an appropriate excitatory/inhibitory inputs balance. We demonstrated that FLNA modulates RAC1 and cofilin activity through its interaction with the Rho-GTPase Activating Protein 24 (ARHGAP24). Collectively, we disclose an uncharacterized role of FLNA and provide strong support for neural circuit dysfunction being a consequence of FLNA mutations.

Brains come in various sizes and shapes, yet how neuronal position constrains the type of circuits that they can form remains largely unknown. The spatial layout of anatomical structures with corresponding functions varies widely across species ( J - 4 ). Also, during evolution, anatomical structures have duplicated and then diverged to generate new circuits and functions ( [5][1] , [6][2] ). Thus, it is critical to understand how the position of neurons constrains their integration into circuits and, ultimately, their function. To address this question, we studied EmlJ knockout mice in which subsets of neocortical neurons form a new structure below the neocortex termed heterotopia (Ht). We examined how this new location affects the molecular identity, topography, input-output circuit connectivity, electrophysiology, and functional properties of these neurons. Our results reveal a striking conservation of the cellular features and circuit properties of Ht neurons, despite their abnormal location and misorientation. Supporting this observation, these neurons were able to functionally substitute for overlying neocortical neurons in a behaviorally relevant task when the latter were optogenetically silenced. Hence, specific neuronal identities and associated function can be reproduced in altered anatomical settings, revealing a remarkable level of self-organization and adaptability of neocortical circuits. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-5 [2]: #ref-6

Grey matter heterotopia (GMH) are neurodevelopmental disorders associated with abnormal cortical function and epilepsy. Subcortical band heterotopia (SBH) and periventricular nodular heterotopia (PVNH) are two well-recognized GMH subtypes in which neurons are misplaced, either forming nodules lining the ventricles in PVNH, or forming bands in the white matter in SBH. Although both PVNH and SBH are commonly associated with epilepsy, it is unclear whether these two GMH subtypes differ in terms of pathological consequences or, on the contrary, share common altered mechanisms. Here, we studied two robust preclinical models of SBH and PVNH, and performed a systematic comparative assessment of the physiological and morphological diversity of heterotopia neurons, as well as the dynamics of epileptiform activity and input connectivity. We uncovered a complex set of altered properties, including both common and distinct physiological and morphological features across heterotopia subtypes, and associated with specific dynamics of epileptiform activity. Taken together, these results suggest that pro-epileptic circuits in GMH are, at least in part, composed of neurons with distinct, subtype-specific, physiological and morphological properties depending on the heterotopia subtype. Our work supports the notion that GMH represent a complex set of disorders, associating both shared and diverging pathological consequences, and contributing to forming epileptogenic networks with specific properties. A deeper understanding of these properties may help to refine current GMH classification schemes by identifying morpho-electric signatures of GMH subtypes, to potentially inform new treatment strategies.

Malformations of cortical development represent a major cause of epilepsy in childhood. However, the pathological substrate and dynamic changes leading to the development and progression of epilepsy remain unclear. Here, we characterized an etiology-relevant rat model of subcortical band heterotopia (SBH), a diffuse type of cortical malformation associated with drug-resistant seizures in humans. We used longitudinal electrographic recordings to monitor the age-dependent evolution of epileptiform discharges during the course of epileptogenesis in this model. We found both quantitative and qualitative age-related changes in seizures properties and patterns, accompanying a gradual progression towards a fully developed seizure pattern seen in adulthood. We also dissected the relative contribution of the band heterotopia and the overlying cortex to the development and age-dependent progression of epilepsy using timed and spatially targeted manipulation of neuronal excitability. We found that an early suppression of neuronal excitability in SBH slows down epileptogenesis in juvenile rats, whereas epileptogenesis is paradoxically exacerbated when excitability is suppressed in the overlying cortex. However, in rats with active epilepsy, similar manipulations of excitability have no effect on chronic spontaneous seizures. Together, our data support the notion that complex developmental alterations occurring in both the SBH and the overlying cortex concur to creating pathogenic circuits prone to generate seizures. Our study also suggests that early and targeted interventions could potentially influence the course of these altered developmental trajectories, and favorably modify epileptogenesis in malformations of cortical development.

Studies conducted in human and rodent models have suggested that preexisting neurodevelopmental defects could predispose immature brains to febrile seizures (FS). However, the impact of the anatomical extent of preexisting cortical malformations on FS susceptibility was never assessed. Here, we induced hyperthermic seizures (HS) in rats with bilateral subcortical band heterotopia (SBH) and found variable degrees of HS susceptibility depending on inter-individual anatomical differences in size and extent of SBH. This indicates that an association exists between the overall extent or location of a cortical malformation, and the predisposition to FS. This also suggests that various predisposing factors and underlying causes may contribute to the etiology of complex FS.

Subcortical band heterotopia (SBH), also known as doublecortex syndrome, is a malformation of cortical development resulting from mutations in the doublecortin gene (DCX). It is characterized by a lack of migration of cortical neurons that accumulate in the white matter forming a heterotopic band. Patients with SBH may present mild to moderate intellectual disability as well as epilepsy. The SBH condition can be modeled in rats by in utero knockdown (KD) of Dcx. The affected cells form an SBH reminiscent of that observed in human patients and the animals develop a chronic epileptic condition in adulthood. Here, we investigated if the presence of a SBH is sufficient to induce cognitive impairment in juvenile Dcx-KD rats, before the onset of epilepsy. Using a wide range of behavioral tests, we found that the presence of SBH did not appear to affect motor control or somatosensory processing. In addition, cognitive abilities such as learning, short-term and long-term memory, were normal in pre-epileptic Dcx-KD rats. We suggest that the SBH presence is not sufficient to impair these behavioral functions.

Single germline or somatic activating mutations of mammalian target of rapamycin (mTOR) pathway genes are emerging as a major cause of type II focal cortical dysplasia (FCD), hemimegalencephaly (HME) and tuberous sclerosis complex (TSC). A double-hit mechanism, based on a primary germline mutation in one allele and a secondary somatic hit affecting the other allele of the same gene in a small number of cells, has been documented in some patients with TSC or FCD. In a patient with HME, severe intellectual disability, intractable seizures and hypochromic skin patches, we identified the ribosomal protein S6 (RPS6) p.R232H variant, present as somatic mosaicism at ~15.1% in dysplastic brain tissue and ~11% in blood, and the MTOR p.S2215F variant, detected as ~8.8% mosaicism in brain tissue, but not in blood. Overexpressing the two variants independently in animal models, we demonstrated that MTOR p.S2215F caused neuronal migration delay and cytomegaly, while RPS6 p.R232H prompted increased cell proliferation. Double mutants exhibited a more severe phenotype, with increased proliferation and migration defects at embryonic stage and, at postnatal stage, cytomegalic cells exhibiting eccentric nuclei and binucleation, which are typical features of balloon cells. These findings suggest a synergistic effect of the two variants. This study indicates that, in addition to single activating mutations and double-hit inactivating mutations in mTOR pathway genes, severe forms of cortical dysplasia can also result from activating mutations affecting different genes in this pathway. RPS6 is a potential novel disease-related gene.

The neocortex is a six-layered laminated structure with a precise anatomical and functional organization ensuring proper function. Laminar positioning of cortical neurons, as determined by termination of neuronal migration, is a key determinant of their ability to assemble into functional circuits. However, the exact contribution of laminar placement to dendrite morphogenesis and synapse formation remains unclear. Here we manipulated the laminar position of cortical neurons by knocking down Dcx, a crucial effector of migration, and show that misplaced neurons fail to properly form dendrites, spines and functional glutamatergic synapses. We further show that knocking down Dcx in properly positioned neurons induces similar but milder defects, suggesting that the laminar misplacement is the primary cause of altered neuronal development. Thus, the specific laminar environment of their fated layers is crucial for the maturation of cortical neurons, and influences their functional integration into developing cortical circuits.

SummaryObjectiveMalformations of cortical development are common causes of intellectual disability and epilepsy, yet there is a crucial lack of relevant preclinical models associating seizures and cortical malformations. Here, we describe a novel rat model with bilateral subcortical band heterotopia (SBH) and examine whether this model develops spontaneous epileptic seizures.MethodsTo generate bilateral SBH in rats, we combined RNAi‐mediated knockdown of Dcx and in utero electroporation with a tripolar electrode configuration enabling simultaneous transfection of the two brain hemispheres. To determine whether bilateral SBH leads to epileptiform activity, rats of various ages were implanted for telemetric electrocorticographic recordings and histopathological examination was carried out at the end of the recording sessions.ResultsBy 2 months, rats with bilateral SBH showed nonconvulsive spontaneous seizures consisting of spike‐and‐wave discharges (SWDs) with dominant frequencies in the alpha and theta bands and secondarily in higher‐frequency bands. SWDs occurred during both the dark and the light period, but were more frequent during quiet awake state than during sleep. Also, SWDs were more frequent and lasted longer at older ages. No sex differences were found. Although frequencies and durations of SWDs were found to be uncorrelated with the size of SBH, SWDs were initiated in some occasions from brain hemispheres comprising a larger SBH. Lastly, SWDs exhibited absence‐like pharmacological properties, being temporarily alleviated by ethosuximide administration.SignificanceThis novel model of bilateral SBH with spontaneous epilepsy may potentially provide valuable new insights into causality between cortical malformations and seizures, and help translational research aiming at designing novel treatment strategies for epilepsy.

Subcortical band heterotopia (SBH), also known as double-cortex syndrome, is a neuronal migration disorder characterized by an accumulation of neurons in a heterotopic band below the normotopic cortex. The majority of patients with SBH have mild to moderate intellectual disability and intractable epilepsy. However, it is still not clear how cortical networks are organized in SBH patients and how this abnormal organization contributes to improper brain function. In this study, cortical networks were investigated in the barrel cortex in an animal model of SBH induced by in utero knockdown of Dcx, main causative gene of this condition in human patients. When the SBH was localized below the Barrel Field (BF), layer (L) four projection to correctly positioned L2/3 pyramidal cells was weakened due to lower connectivity. Conversely, when the SBH was below an adjacent cortical region, the excitatory L4 to L2/3 projection was stronger due to increased L4 neuron excitability, synaptic strength and excitation/inhibition ratio of L4 to L2/3 connection. We propose that these developmental alterations contribute to the spectrum of clinical dysfunctions reported in patients with SBH.

The series of atlases of the developing rat brain in stereotaxic coordinates that we propose here1,2 has been conceived as a tool for the exploration of the rat brain in vivo during the postnatal period, offering a choice of bregma and lambda as the reference points. We suggest to use this atlas, which provides stereotaxic coordinates, in combination with other histological atlases, where brain structures and boundaries between them were assessed using histochemistry or quantative magnetic resonance techniques, but where stereotaxic coordinates are not available (Ramachandra and Subramanian, 2011; Calabrese et al., 2013; Ashwell and Paxinos, 2015) or provided (at P10, P21, and P39) but not within the bregma-lambda metrics (Sherwood and Timiras, 1970). The neonatal rat is a powerful model for the study of the early stages of central nervous system development. Comparative developmental studies suggest that the first two postnatal weeks in the rat roughly correspond to the second half of gestation and the early postnatal period in humans, with the electrical brain activity patterns expressed during this period and their developmental trajectories matching the patterns observed in premature human neonates (Clancy et al., 2001, 2007; Khazipov and Luhmann, 2006; Colonnese et al., 2010; Workman et al., 2013) (see also http://www.translatingtime.net/). Recent advances in the understanding of the developing brain functions largely came from the use of techniques to record brain activity from neonatal and adolescent rats in vivo, notably in head restrained animals (Leinekugel et al., 2002; Khazipov et al., 2004; Minlebaev et al., 2011; Tiriac et al., 2012; Yang et al., 2013). However, the rat brain grows rapidly and non-proportionally during the postnatal period and brain structures change their position in relation to the conventional skull marks such as bregma and lambda, making difficult to perform targeted recordings, stimulations, lesions, or local drug injections in precise anatomical locations. Hence the importance of a developmental atlas of the rat brain in stereotaxic coordinates. Several examples of histology atlases series of the developing rat brain have been published (Table ​(Table1).1). The Atlas by Sherwood and Timiras “A Stereotaxic Atlas of the Developing Rat Brain” atlas includes three postnatal time points: postnatal days P10, P21, and P39 (Sherwood and Timiras, 1970). Ashwell and Paxinos' “Atlas of the Developing Rat Nervous System” provides photographs and accompanying diagrams of coronal and sagittal sections of rats aged E12, E13, E14, E16, E17, E19, and P0 (Ashwell and Paxinos, 2015). “Atlas of the Neonatal Rat Brain” by Ramachandra and Subramanian describes rat brains at P1, P7, and P14 (Ramachandra and Subramanian, 2011). More recently, Calabrese, Badea, Watson, and Johnson published their “Quantitative magnetic resonance histology atlas of postnatal rat brain development with regional estimates of growth and variability” based on the study of P0, P2, P4, P8, P12, P18, P24, P40, and P80 rats (Calabrese et al., 2013). Among these atlases, only the atlas by Sherwood and Timiras (1970) provides stereotaxic coordinates and yet for only three postnatal days P10, 21, and P39. Stereotaxic atlases for the early postnatal period are not available, however. Table 1 Atlases of the developing rat brain. The series of atlases of the developing rat brain in stereotaxic coordinates that we propose here has been conceived as a tool for the exploration of the rat brain in vivo during the postnatal period, offering a choice of bregma and lambda as the reference points. Each atlas contains a series of microphotographs of wet, non-stained 200 micron coronal brain sections in oblique light, obtained from postnatal days P0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 14, and 21 rats, with an indication of the main brain structures. Wet non-stained sections were used to avoid deformation, shrinkage and lessening associated with staining and mounting procedures, particularly when manipulating immature tissue. Because we did not stain sections, the images do not provide cellular resolution and not all brain structures and boundaries between them can be easily identified. Therefore, only certain brain structures are labeled without boundaries. Used in combination with the existing high image resolution atlases of the developing rat brain (Sherwood and Timiras, 1970; Ramachandra and Subramanian, 2011; Calabrese et al., 2013; Ashwell and Paxinos, 2015), this atlas may be useful for targeted recordings, stimulations, and drug injections in different brain structures at various postnatal ages. This atlas has been successfully used in several previous studies (Minlebaev et al., 2011; Petit et al., 2014; Tyzio et al., 2014).