Alfonso Represa

Objective: Neonatal developmental and epileptic encephalopathy with movement disorder and arthrogryposis (NDEEMA) represents the most severe end of the gain‐of‐function (GOF) SCN1A disorder spectrum. Sporadic cases of congenital arthrogryposis have also been reported in individuals with SCN2A‐ , SCN3A‐, and SCN8A ‐related developmental and epileptic encephalopathy. Here, we investigated whether NDEEMA occurs in other brain‐expressed sodium channelopathies and characterized its features. Methods: Individuals with the clinical phenotype of NDEEMA were identified through internal databases, an international network of epileptologists and geneticists, and the literature. Their clinical and genetic information was analyzed. A literature survey was conducted to review studies describing the functional effects of the pathogenic variants. Results: Of 46 NDEEMA individuals, 25 harbored variants in SCN1A , 13 in SCN2A , one in SCN3A, and seven in SCN8A . Thirty‐five different pathogenic/likely pathogenic missense variants were identified, all of which clustered in evolutionary conserved paralogous Na _V positions. Five individuals died in utero. Thirty‐nine of 41 (95%) liveborn individuals developed neonatal epilepsy with tonic seizures and/or apnea. Thirty‐one individuals tried sodium channel blockers, of whom 21 (68%) experienced seizure reduction. All individuals for whom information was available developed movement disorders, with myoclonus, dystonia, and tremor being the most common features. Literature review of functional studies revealed that nine NDEEMA variants, and the corresponding paralogues of 16 additional NDEEMA variants, have been biophysically characterized as GOF. Significance: This study expands the phenotype of NDEEMA from SCN1A to its paralogue sodium channel genes expressed in the brain: SCN2A, SCN3A , and SCN8A .

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.

The cerebral cortex hosts a diverse array of excitatory glutamatergic and inhibitory GABAergic neuron types, each characterized by distinct positional and synaptic connectivity patterns. However, the molecular mechanisms orchestrating this precise organization remain largely unknown. To identify ligand-receptor (LR) pairs regulating interactions and connectivity among cortical neurons during embryonic and postnatal development, we analyzed the transcriptional dynamics of all genes across major cortical neuron subtypes at 17 developmental time points using single-cell transcriptomics. From these data, we constructed a comprehensive bioinformatic atlas that inferred significant LR-mediated interactions between glutamatergic and GABAergic neurons throughout cortical maturation. This atlas not only corroborated known interactions but also enabled the discovery of novel regulators, identifying two cadherin superfamily members as key mediators of perisomatic inhibition in deep and superficial layer excitatory neurons by parvalbumin-expressing basket cells. These findings underscore the power of large-scale transcriptional profiling to unravel fundamental molecular mechanisms driving cortical circuit assembly.

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.

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. Abstract Figure:

Editorial: What does human pathology bring to the understanding of the fundamental mechanisms of development?

Tuberous sclerosis (TSC) is a multisystem autosomal dominant genetic disorder due to loss of function of TSC1/TSC2 resulting in increased mTOR (mammalian target of rapamycin) signaling. In the brain, TSC is characterized by the formation of specific lesions that include subependymal and white matter nodules and cortical tubers. Cells that constitute TSC lesions are mainly balloon cells and dysmorphic neurons and astrocytes, but normal cells also populate the tubers. Although considered as a developmental disorder, the histopathological features of brain lesions have been described in only a limited number of fetal cases, providing little information on how these lesions develop. In this report we characterized the development of TSC lesions in fetal brains ranging from 19 gestational weeks (GW) to 8th postnatal month. Our data indicate that subcortical lesions, forming within and at the vicinity of germinative zones, are the first alterations (already detected in 19GW brains), characterized by the presence of numerous dysmorphic astrocytes and balloon cells. Our data show that cortical tuber formation is a long process that initiates with the presence of dysmorphic astrocytes (by 19-21GW), followed by the apparition of balloon cells (by 24GW) and acquiring dysmorphic neurons only by the end of gestation (by 36GW). Furthermore, the typical tuberal aspect of cortical lesions is only reached when bundles of neurofilament positive extensions delineate the bottom of the cortical lesion by 36GW. In addition, our study reveals the presence of balloon cells and dysmorphic neurons immunopositive for interneuron markers such as calbindin and parvalbumin, suggesting that TSC lesions would be mosaic lesions generated from different classes of progenitors.