SCN1A-deficient excitatory neuronal networks display mutation-specific phenotypes

Engineered modular neuronal networks-on-chip represent structure-function relationship

Brain function is substantially linked to the highly organized modular structure of neuronal networks. However, the structure of in vitro assembled neuronal circuits often exhibits variability, complicating the consistent recording of network functional output and its correlation to network structure. Therefore, engineering neuronal structures with predefined geometry and reproducible functional features is essential to precisely model in vivo neuronal circuits. Here, we engineered microchannel devices to assemble 2D and 3D modular networks. The microchannel devices were coupled with a multi-electrode array (MEA) electrophysiology system to enable recordings from circuits. Each network consisted of 64 modules connected to their adjacent modules by micron-sized channels. Modular circuits within microchannel devices showed enhanced activity and functional connectivity traits. This includes metrics such as connection weights, clustering coefficient, global efficiency, and the number of hub neurons with higher betweenness centrality. In addition, modular networks demonstrated an increased functional modularity score compared to the randomly formed circuits. Neurons within individual modules displayed uniform network characteristics and predominantly participated in their respective functional communities within the same or neighboring physical modules. These observations highlight that the modular network structure promotes the development of segregated functional connectivity traits while simultaneously enhancing the efficiency of overall network connectivity. Our findings emphasize the significant impact of physical constraints on the activity patterns and functional organization within engineered modular networks. These circuits, characterized by stable modular architecture and intricate functional dynamics-key features of the brain networks-offer a robust in vitro model for advancing neuroscience research.

Breaking the burst: Unveiling mechanisms behind fragmented network bursts in patient-derived neurons

Fragmented network bursts (NBs) are observed as a phenotypic driver in many patient-derived neuronal networks on multi-electrode arrays (MEAs), but the pathophysiological mechanisms underlying this phenomenon are unknown. Here, we used our previously developed biophysically detailed in silico model to investigate these mechanisms. Fragmentation of NBs in our model simulations occurred only when the level of short-term synaptic depression (STD) was enhanced, suggesting that STD is a key player. Experimental validation with Dynasore, an STD enhancer, induced fragmented NBs in healthy neuronal networks in vitro. Additionally, we showed that strong asynchronous neurotransmitter release, NMDA currents, or short-term facilitation (STF) can support the emergence of multiple fragments in NBs by producing excitation that persists after high-frequency firing stops. Our results provide important insights into disease mechanisms and potential pharmaceutical targets for neurological disorders modeled using human induced pluripotent stem cell (hiPSC)-derived neurons.

The Biallelic Inheritance of Two Novel SCN1A Variants Results in Developmental and Epileptic Encephalopathy Responsive to Levetiracetam

Loss-, gain-of-function and mixed variants in SCN1A (Nav1.1 voltage-gated sodium channel) have been associated with a spectrum of neurologic disorders with different severity and drug-responsiveness. Most SCN1A variants are heterozygous changes occurring de novo or dominantly inherited; recessive inheritance has been reported in a few cases. Here, we report a family in which the biallelic inheritance of two novel SCN1A variants, N935Y and H1393Q, occurs in two siblings presenting with drug-responsive developmental and epileptic encephalopathy and born to heterozygous asymptomatic parents. To assess the genotype-phenotype correlation and support the treatment choice, HEK 293 cells were transfected with different combinations of the SCN1A WT and mutant cDNAs, and the resulting sodium currents were recorded through whole-cell patch-clamp. Functional studies showed that the N935Y and H1393Q channels and their combinations with the WT (WT + N935Y and WT + H1393Q) had current densities and biophysical properties comparable with those of their respective control conditions. This explains the asymptomatic condition of the probands' parents. The co-expression of the N935Y + H1393Q channels, mimicking the recessive inheritance of the two variants in siblings, showed ~20% reduced current amplitude compared with WT and with parental channels. This mild loss of Nav1.1 function may contribute in part to the disease pathogenesis, although other mechanisms may be involved.

In vitro human cell culture models in a bench-to-bedside approach to epilepsy

Epilepsy is the most common chronic neurological disease, affecting nearly 1%-2% of the world's population. Current pharmacological treatment and regimen adjustments are aimed at controlling seizures; however, they are ineffective in one-third of the patients. Although neuronal hyperexcitability was previously thought to be mainly due to ion channel alterations, current research has revealed other contributing molecular pathways, including processes involved in cellular signaling, energy metabolism, protein synthesis, axon guidance, inflammation, and others. Some forms of drug-resistant epilepsy are caused by genetic defects that constitute potential targets for precision therapy. Although such approaches are increasingly important, they are still in the early stages of development. This review aims to provide a summary of practical aspects of the employment of in vitro human cell culture models in epilepsy diagnosis, treatment, and research. First, we briefly summarize the genetic testing that may result in the detection of candidate pathogenic variants in genes involved in epilepsy pathogenesis. Consequently, we review existing in vitro cell models, including induced pluripotent stem cells and differentiated neuronal cells, providing their specific properties, validity, and employment in research pipelines. We cover two methodological approaches. The first approach involves the utilization of somatic cells directly obtained from individual patients, while the second approach entails the utilization of characterized cell lines. The models are evaluated in terms of their research and clinical benefits, relevance to the in vivo conditions, legal and ethical aspects, time and cost demands, and available published data. Despite the methodological, temporal, and financial demands of the reviewed models they possess high potential to be used as robust systems in routine testing of pathogenicity of detected variants in the near future and provide a solid experimental background for personalized therapy of genetic epilepsies. PLAIN LANGUAGE SUMMARY: Epilepsy affects millions worldwide, but current treatments fail for many patients. Beyond traditional ion channel alterations, various genetic factors contribute to the disorder's complexity. This review explores how in vitro human cell models, either from patients or from cell lines, can aid in understanding epilepsy's genetic roots and developing personalized therapies. While these models require further investigation, they offer hope for improved diagnosis and treatment of genetic forms of epilepsy.

SCN1A-Characterization of the Gene's Variants in the Polish Cohort of Patients with Dravet Syndrome: One Center Experience

The aim of this study was to characterize the genotype and phenotype heterogeneity of patients with SCN1A gene mutations in the Polish population, fulfilling the criteria for the diagnosis of Dravet syndrome (DRVT). Particularly important was the analysis of the clinical course, the type of epileptic seizures and the co-occurrence of additional features such as intellectual disability, autism or neurological symptoms such as ataxia or gait disturbances. Based on their results and the available literature, the authors discuss potential predictors for DRVT. Identifying these early symptoms has important clinical significance, affecting the course and disease prognosis. 50 patients of the Pediatric Neurology Clinic of the Institute of Mother and Child in Warsaw clinically diagnosed with DRVT and carriers of SCN1A pathogenic variants were included. Clinical data were retrospectively collected from caregivers and available medical records. Patients in the study group did not differ significantly in parameters such as type of first seizure and typical epileptic seizures from those described in other studies. The age of onset of the first epileptic seizure was 2-9 months. The co-occurrence of intellectual disability was confirmed in 71% of patients and autism in 18%. The study did not show a correlation between genotype and phenotype, considering the severity of the disease course, clinical symptoms, response to treatment, the presence of intellectual disability, autism symptoms or ataxia. From the clinical course, a significant problem was the differentiation between complex febrile convulsions and symptoms of DRVT. The authors suggest that parameters such as the age of the first seizure, less than one year of age, the onset of a seizure up to 72 h after vaccination and the presence of more than two features of complex febrile seizures are more typical of DRVT, which should translate into adequate diagnostic and clinical management. The substantial decrease in the age of genetic verification of the diagnosis, as well as the decline in the use of sodium channel inhibitors, underscores the growing attention of pediatric neurologists in Poland to the diagnosis of DRVT.

Targeted blockade of aberrant sodium current in a stem cell-derived neuron model of SCN3A encephalopathy

Missense variants in SCN3A encoding the voltage-gated sodium (Na+) channel α subunit Nav1.3 are associated with SCN3A-related neurodevelopmental disorder (SCN3A-NDD), a spectrum of disease that includes epilepsy and malformation of cortical development. How genetic variation in SCN3A leads to pathology remains unclear, as prior electrophysiological work on disease-associated variants has been performed exclusively in heterologous cell systems. To further investigate the mechanisms of SCN3A-NDD pathogenesis, we used CRISPR/Cas9 gene editing to modify a control human induced pluripotent stem cell (iPSC) line to express the recurrent de novo missense variant SCN3A c.2624T>C (p.Ile875Thr). With the established Ngn2 rapid induction protocol, we generated glutamatergic forebrain-like neurons (iNeurons), which we showed to express SCN3A mRNA and Nav1.3-mediated Na+ currents. We performed detailed whole-cell patch clamp recordings to determine the effect of the SCN3A-p.Ile875Thr variant on endogenous Na+ currents in, and intrinsic excitability of, human neurons. Compared to control iNeurons, variant-expressing iNeurons exhibit markedly increased slowly-inactivating/persistent Na+ current, abnormal firing patterns with paroxysmal bursting and plateau-like potentials with action potential failure, and a hyperpolarized voltage threshold for action potential generation. We then validated these findings using a separate iPSC line generated from a patient harbouring the SCN3A-p.Ile875Thr variant compared to a corresponding CRISPR-corrected isogenic control line. Finally, we found that application of the Nav1.3-selective blocker ICA-121431 normalizes action potential threshold and aberrant firing patterns in SCN3A-p.Ile1875Thr iNeurons; in contrast, consistent with action as a Na+ channel blocker, ICA-121431 decreases excitability of control iNeurons. Our findings demonstrate that iNeurons can model the effects of genetic variation in SCN3A yet reveal a complex relationship between gain-of-function at the level of the ion channel versus impact on neuronal excitability. Given the transient expression of SCN3A in the developing human nervous system, selective blockade or suppression of Nav1.3-containing Na+ channels could represent a therapeutic approach towards SCN3A-NDD.

Neural and metabolic dysregulation in PMM2-deficient human in vitro neural models

Phosphomannomutase 2-congenital disorder of glycosylation (PMM2-CDG) is a rare inborn error of metabolism caused by deficiency of the PMM2 enzyme, which leads to impaired protein glycosylation. While the disorder presents with primarily neurological symptoms, there is limited knowledge about the specific brain-related changes caused by PMM2 deficiency. Here, we demonstrate aberrant neural activity in 2D neuronal networks from PMM2-CDG individuals. Utilizing multi-omics datasets from 3D human cortical organoids (hCOs) derived from PMM2-CDG individuals, we identify widespread decreases in protein glycosylation, highlighting impaired glycosylation as a key pathological feature of PMM2-CDG, as well as impaired mitochondrial structure and abnormal glucose metabolism in PMM2-deficient hCOs, indicating disturbances in energy metabolism. Correlation between PMM2 enzymatic activity in hCOs and symptom severity suggests that the level of PMM2 enzyme function directly influences neurological manifestations. These findings enhance our understanding of specific brain-related perturbations associated with PMM2-CDG, offering insights into the underlying mechanisms and potential directions for therapeutic interventions.

Unraveling Dravet Syndrome: Exploring the complex effects of sodium channel mutations on neuronal networks

Dravet Syndrome (DS) is a severe developmental epileptic encephalopathy with frequent intractable seizures accompanied by cognitive impairment, often caused by pathogenic variants in SCN1A encoding sodium channel NaV1.1. Recent research utilizing in vitro patient-derived neuronal networks and accompanying in silico models uncovered that not just sodium-but also potassium-and synaptic currents were impaired in DS networks. Here, we explore the implications of these findings for three questions that remain elusive in DS: How do sodium channel impairments result in epilepsy? How can identical variants lead to varying phenotypes? What mechanisms underlie the developmental delay in DS patients? We speculate that impaired potassium currents might be a secondary effect to NaV1.1 mutations and could result in hyperexcitable neurons and epileptic networks. Moreover, we reason that homeostatic plasticity is actively engaged in DS networks, possibly affecting the phenotype and impairing learning and development when driven to extremes.