Loss of NDST1 N-sulfotransferase activity is associated with autosomal recessive intellectual disability.

Intellectual Disability (ID) is the major cause of handicap, affecting nearly 3% of the general population, and is highly genetically heterogenous with more than a thousand genes involved. Exome sequencing performed in two independent families identified the same missense variant, p.(Gly611Ser), in the NDST1 (N-deacetylase/N-sulfotransferase member 1) gene. This variant had been previously found in ID patients of two other families but has never been functionally characterized. The NDST1 gene encodes a bifunctional enzyme that catalyzes both N-deacetylation and N-sulfation of N-acetyl-glucosamine residues during heparan sulfate (HS) biosynthesis. This step is essential because it influences the downstream enzymatic modifications and thereby determines the overall structure and sulfation degree of the HS polysaccharide chain. To discriminate between a rare polymorphism and a pathogenic variant, we compared the enzymatic properties of wild-type and mutant NDST1 proteins. We found that the p.(Gly611Ser) variant results in a complete loss of N-sulfotransferase activity while the N-deacetylase activity is retained. NDST1 shows the highest and the most homogeneous expression in the human cerebral structures compared to the other members of the NDST gene family. These results indicate that a loss of NDST1 N-sulfation activity is associated with impaired cognitive functions.

Clinical features and genotype-phenotype correlations in epilepsy patients with de novo DYNC1H1 variants.

OBJECTIVE: DYNC1H1 variants are involved on a disease spectrum from neuromuscular disorders to neurodevelopmental disorders. DYNC1H1-related epilepsy has been reported in small cohorts. We dissect the electroclinical features of 34 patients harboring de novo DYNC1H1 pathogenic variants, identify subphenotypes on the DYNC1H1-related epilepsy spectrum, and compare the genotype-phenotype correlations observed in our cohort with the literature. METHODS: Patients harboring de novo DYNC1H1 pathogenic variants were recruited through international collaborations. Clinical data were retrospectively collected. Latent class analysis was performed to identify subphenotypes. Multivariable binary logistic regression analysis was applied to investigate the association with DYNC1H1 protein domains. RESULTS: DYNC1H1-related epilepsy presented with infantile epileptic spasms syndrome (IESS) in 17 subjects (50%), and in 25% of these individuals the epileptic phenotype evolved into Lennox-Gastaut syndrome (LGS). In 12 patients (35%), focal onset epilepsy was defined. In two patients, the epileptic phenotype consisted of generalized myoclonic epilepsy, with a progressive phenotype in one individual harboring a frameshift variant. In approximately 60% of our cohort, seizures were drug-resistant. Malformations of cortical development were noticed in 79% of our patients, mostly on the lissencephaly-pachygyria spectrum, particularly with posterior predominance in a half of them. Midline and infratentorial abnormalities were additionally reported in 45% and 27% of subjects. We have identified three main classes of subphenotypes on the DYNC1H1-related epilepsy spectrum. SIGNIFICANCE: We propose a classification in which pathogenic de novo DYNC1H1 variants feature drug-resistant IESS in half of cases with potential evolution to LGS (Class 1), developmental and epileptic encephalopathy other than IESS and LGS (Class 2), or less severe focal or genetic generalized epilepsy including a progressive phenotype (Class 3). We observed an association between stalk domain variants and Class 1 phenotypes. The variants p.Arg309His and p.Arg1962His were common and associated with Class 1 subphenotype in our cohort. These findings may aid genetic counseling of patients with DYNC1H1-related epilepsy.

Bumetanide induces post-traumatic microglia-interneuron contact to promote neurogenesis and recovery.

Although the Na-K-Cl cotransporter (NKCC1) inhibitor bumetanide has prominent positive effects on the pathophysiology of many neurological disorders, the mechanism of action is obscure. Attention paid to elucidating the role of Nkcc1 has mainly been focused on neurons, but recent single cell mRNA sequencing analysis has demonstrated that the major cellular populations expressing NKCC1 in the cortex are non-neuronal. We used a combination of conditional transgenic animals, in vivo electrophysiology, two-photon imaging, cognitive behavioural tests and flow cytometry to investigate the role of Nkcc1 inhibition by bumetanide in a mouse model of controlled cortical impact (CCI). Here, we found that bumetanide rescues parvalbumin-positive interneurons by increasing interneuron-microglia contacts shortly after injury. The longitudinal phenotypic changes in microglia were significantly modified by bumetanide, including an increase in the expression of microglial-derived BDNF. These effects were accompanied by the prevention of CCI-induced decrease in hippocampal neurogenesis. Treatment with bumetanide during the first week post-CCI resulted in significant recovery of working and episodic memory as well as changes in theta band oscillations 1 month later. These results disclose a novel mechanism for the neuroprotective action of bumetanide mediated by an acceleration of microglial activation dynamics that leads to an increase in parvalbumin interneuron survival following CCI, possibly resulting from increased microglial BDNF expression and contact with interneurons. Salvage of interneurons may normalize ambient GABA, resulting in the preservation of adult neurogenesis processes as well as contributing to bumetanide-mediated improvement of cognitive performance.

TRAPPC2L-related disorder: first homozygous protein-truncating variant and further delineation of the phenotype.

The TRAPP (TRAfficking Protein Particle) complexes are evolutionarily conserved tethering factors involved in the intracellular transport of vesicles for secretion and autophagy processes. Pathogenic variants in 8 genes (of 14) encoding TRAPP proteins are involved in ultra-rare human diseases, called TRAPPopathies. Seven of them are autosomal recessive neurodevelopmental disorders with overlapping phenotypes. Since 2018, two homozygous missense variants in TRAPPC2L have been reported in five individuals from three unrelated families with early-onset and progressive encephalopathy, with episodic rhabdomyolysis. We now describe the first pathogenic protein-truncating variant in the TRAPPC2L gene found at a homozygous state in two affected siblings. This report provides key genetic evidence invaluable to establishing the gene-disease relationship for this gene and important insights into the TRAPPC2L phenotype. Regression, seizures and postnatal microcephaly initially described are not constant features. Acute episodes of infection do not contribute to the neurological course. HyperCKaemia is part of the clinical picture. Thus, TRAPPC2L syndrome is mainly characterised by a severe neurodevelopmental disorder and a variable degree of muscle involvement, suggesting that it belongs to the clinical entity of rare congenital muscular dystrophies.

NAPB and developmental and epileptic encephalopathy: Description of the electroclinical profile associated with a novel pathogenic variant.

Developmental and epileptic encephalopathies (DEE) are a group of neurodevelopmental disorders characterized by epileptic seizures associated with developmental delay or regression. DEE are genetically heterogeneous, and the proteins involved play roles in multiple pathways such as synaptic transmission, metabolism, neuronal development or maturation, transcriptional regulation, and intracellular trafficking. We performed whole exome sequencing on a consanguineous family with three children presenting an early onset (<6 months) with clusters of seizures characterized by oculomotor and vegetative manifestations, with an occipital origin. Before 1 year of age, interictal electroencephalographic recordings were well organized and neurodevelopment was unremarkable. Then, a severe regression occurred. We identified a novel homozygous protein-truncating variant in the NAPB (N-ethylmaleimide-sensitive fusion [NSF] attachment protein beta) gene that encodes the ßSNAP protein, a key regulator of NSF-adenosine triphosphatase. This enzyme is essential for synaptic transmission by disassembling and recycling proteins of the SNARE complex. Here, we describe the electroclinical profile of each patient during the disease course. Our findings strengthen the association between biallelic variants in NAPB and DEE and refine the associated phenotype. We suggest including this gene in the targeted epilepsy gene panels used for routine diagnosis of unexplained epilepsy.

Biallelic variants in PCDHGC4 cause a novel neurodevelopmental syndrome with progressive microcephaly, seizures, and joint anomalies.

PURPOSE: We aimed to define a novel autosomal recessive neurodevelopmental disorder, characterize its clinical features, and identify the underlying genetic cause for this condition. METHODS: We performed a detailed clinical characterization of 19 individuals from nine unrelated, consanguineous families with a neurodevelopmental disorder. We used genome/exome sequencing approaches, linkage and cosegregation analyses to identify disease-causing variants, and we performed three-dimensional molecular in silico analysis to predict causality of variants where applicable. RESULTS: In all affected individuals who presented with a neurodevelopmental syndrome with progressive microcephaly, seizures, and intellectual disability we identified biallelic disease-causing variants in Protocadherin-gamma-C4 (PCDHGC4). Five variants were predicted to induce premature protein truncation leading to a loss of PCDHGC4 function. The three detected missense variants were located in extracellular cadherin (EC) domains EC5 and EC6 of PCDHGC4, and in silico analysis of the affected residues showed that two of these substitutions were predicted to influence the Ca2+-binding affinity, which is essential for multimerization of the protein, whereas the third missense variant directly influenced the cis-dimerization interface of PCDHGC4. CONCLUSION: We show that biallelic variants in PCDHGC4 are causing a novel autosomal recessive neurodevelopmental disorder and link PCDHGC4 as a member of the clustered PCDH family to a Mendelian disorder in humans.

A knock-in mouse model for KCNQ2-related epileptic encephalopathy displays spontaneous generalized seizures and cognitive impairment.

OBJECTIVE: Early onset epileptic encephalopathy with suppression-burst is one of the most severe epilepsy phenotypes in human patients. A significant proportion of cases have a genetic origin, and the most frequently mutated gene is KCNQ2, encoding Kv7.2, a voltage-dependent potassium channel subunit, leading to so-called KCNQ2-related epileptic encephalopathy (KCNQ2-REE). To study the pathophysiology of KCNQ2-REE in detail and to provide a relevant preclinical model, we generated and described a knock-in mouse model carrying the recurrent p.(Thr274Met) variant. METHODS: We introduced the p.(Thr274Met) variant by homologous recombination in embryonic stem cells, injected into C57Bl/6N blastocysts and implanted in pseudopregnant mice. Mice were then bred with 129Sv Cre-deleter to generate heterozygous mice carrying the p.(Thr274Met), and animals were maintained on the 129Sv genetic background. We studied the development of this new model and performed in vivo electroencephalographic (EEG) recordings, neuroanatomical studies at different time points, and multiple behavioral tests. RESULTS: The Kcnq2Thr274Met/+ mice are viable and display generalized spontaneous seizures first observed between postnatal day 20 (P20) and P30. In vivo EEG recordings show that the paroxysmal events observed macroscopically are epileptic seizures. The brain of the Kcnq2Thr274Met/+ animals does not display major structural defects, similar to humans, and their body weight is normal. Kcnq2Thr274Met/+ mice have a reduced life span, with a peak of unexpected death occurring for 25% of the animals by 3 months of age. Epileptic seizures were generally not observed when animals grew older. Behavioral characterization reveals important deficits in spatial learning and memory in adults but no gross abnormality during early neurosensory development. SIGNIFICANCE: Taken together, our results indicate that we have generated a relevant model to study the pathophysiology of KCNQ2-related epileptic encephalopathy and perform preclinical research for that devastating and currently intractable disease.

The conversion of glutamate by glutamine synthase in neocortical astrocytes from juvenile rat is important to limit glutamate spillover and peri/extrasynaptic activation of NMDA receptors.

Glutamate transporters (EAATs) are important to maintain spatial and temporal specificity of synaptic transmission. Their efficiency to uptake and transport glutamate into the intracellular space depends on several parameters including the intracellular concentrations of Na+ and glutamate, the elevations of which may slow down the cycling rate of EAATs. In astrocytes, glutamate is maintained at low concentration due to the presence of specific enzymes such as glutamine synthase (GS). GS inhibition results in cytosolic accumulation of glutamate suggesting that the conversion of glutamate by GS is important for EAATs operation. Here we recorded astrocytes from juvenile rat neocortical slices and analyzed the consequences of elevated intracellular glutamate concentrations and of GS inhibition on the time course of synaptically evoked transporter current (STC). In slices from rats treated with methionine sulfoximine (MSO), a GS inhibitor, STC evoked by short burst of high frequency stimulation (HFS; 100 Hz for 100 ms) but not by low frequency stimulation (LFS; 0.1 Hz) was twice slower than STC evoked from saline injected rats. Same results were obtained for astrocytes recorded with pipette containing 3-10 mM glutamate and compared with cells recorded with 0 or1 mM glutamate in the patch pipette. We also showed that HFS elicited significantly larger NMDAR-excitatory postsynaptic currents (EPSCs) with a stronger peri/extrasynaptic component in pyramidal cells from MSO-treated compared with saline treated rats. Taken together our data demonstrate that the conversion of glutamate by GS is fundamental to ensure an efficient clearance of glutamate by EAATs and to prevent glutamate spillover. GLIA 2017;65:401-415.

A possible link between KCNQ2- and STXBP1-related encephalopathies: STXBP1 reduces the inhibitory impact of syntaxin-1A on M current.

OBJECTIVE: Kv7 channels mediate the voltage-gated M-type potassium current. Reduction of M current due to KCNQ2 mutations causes early onset epileptic encephalopathies (EOEEs). Mutations in STXBP1 encoding the syntaxin binding protein 1 can produce a phenotype similar to that of KCNQ2 mutations, suggesting a possible link between STXBP1 and Kv7 channels. These channels are known to be modulated by syntaxin-1A (Syn-1A) that binds to the C-terminal domain of the Kv7.2 subunit and strongly inhibits M current. Here, we investigated whether STXBP1could prevent this inhibitory effect of Syn-1A and analyzed the consequences of two mutations in STXBP1 associated with EOEEs. METHODS: Electrophysiologic analysis of M currents mediated by homomeric Kv7.2 or heteromeric Kv7.2/Kv7.3 channels in Chinese hamster ovary (CHO) cells coexpressing Syn-1A and/or STXBP1 or mutants STXBP1 p.W28* and p.P480L. Expression and interaction of these different proteins have been investigated using biochemical and co-immunoprecipitation experiments. RESULTS: Syn-1A decreased M currents mediated by Kv7.2 or Kv7.2/Kv7.3 channels. STXBP1 had no direct effects on M current but dampened the inhibition produced by Syn-1A by abrogating Syn-1A binding to Kv7 channels. The mutation p.W28*, but not p.P480L, failed to rescue M current from Syn-1A inhibition. Biochemical analysis showed that unlike the mutation p.W28*, the mutation p.P480L did not affect STXBP1 expression and reduced the interaction of Syn-1A with Kv7 channels. SIGNIFICANCE: These data indicate that there is a functional link between STXBP1 and Kv7 channels via Syn-1A, which may be important for regulating M-channel activity and neuronal excitability. They suggest also that a defect in Kv7 channel activity or regulation could be one of the consequences of some STXBP1 mutations associated with EOEEs. Furthermore, our data reveal that STXBP1 mutations associated with the Ohtahara syndrome do not necessarily result in protein haploinsufficiency.

A Kv7.2 mutation associated with early onset epileptic encephalopathy with suppression-burst enhances Kv7/M channel activity.

Mutations in the KCNQ2 gene encoding the voltage-gated potassium channel subunit Kv7.2 cause early onset epileptic encephalopathy (EOEE). Most mutations have been shown to induce a loss of function or to affect the subcellular distribution of Kv7 channels in neurons. Herein, we investigated functional consequences and subcellular distribution of the p.V175L mutation of Kv7.2 (Kv7.2(V175L) ) found in a patient presenting EOEE. We observed that the mutation produced a 25-40 mV hyperpolarizing shift of the conductance-voltage relationship of both the homomeric Kv7.2(V175L) and heteromeric Kv7.2(V175L) /Kv7.3 channels compared to wild-type channels and a 10 mV hyperpolarizing shift of Kv7.2(V175L) /Kv7.2/Kv7.3 channels in a 1:1:2 ratio mimicking the patient situation. Mutant channels also displayed faster activation kinetics and an increased current density that was prevented by 1 µm linopirdine. The p.V175L mutation did not affect the protein expression of Kv7 channels and its localization at the axon initial segment. We conclude that p.V175L is a gain of function mutation. This confirms previous observations showing that mutations having opposite consequences on M channels can produce EOEE. These findings alert us that drugs aiming to increase Kv7 channel activity might have adverse effects in EOEE in the case of gain-of-function variants.

A recurrent KCNQ2 pore mutation causing early onset epileptic encephalopathy has a moderate effect on M current but alters subcellular localization of Kv7 channels.

Mutations in the KCNQ2 gene encoding the voltage-dependent potassium M channel Kv7.2 subunit cause either benign epilepsy or early onset epileptic encephalopathy (EOEE). It has been proposed that the disease severity rests on the inhibitory impact of mutations on M current density. Here, we have analyzed the phenotype of 7 patients carrying the p.A294V mutation located on the S6 segment of the Kv7.2 pore domain (Kv7.2(A294V)). We investigated the functional and subcellular consequences of this mutation and compared it to another mutation (Kv7.2(A294G)) associated with a benign epilepsy and affecting the same residue. We report that all the patients carrying the p.A294V mutation presented the clinical and EEG characteristics of EOEE. In CHO cells, the total expression of Kv7.2(A294V) alone, assessed by western blotting, was only 20% compared to wild-type. No measurable current was recorded in CHO cells expressing Kv7.2(A294V) channel alone. Although the total Kv7.2(A294V) expression was rescued to wild-type levels in cells co-expressing the Kv7.3 subunit, the global current density was still reduced by 83% compared to wild-type heteromeric channel. In a configuration mimicking the patients' heterozygous genotype i.e., Kv7.2(A294V)/Kv7.2/Kv7.3, the global current density was reduced by 30%. In contrast to Kv7.2(A294V), the current density of homomeric Kv7.2(A294G) was not significantly changed compared to wild-type Kv7.2. However, the current density of Kv7.2(A294G)/Kv7.2/Kv7.3 and Kv7.2(A294G)/Kv7.3 channels were reduced by 30% and 50% respectively, compared to wild-type Kv7.2/Kv7.3. In neurons, the p.A294V mutation induced a mislocalization of heteromeric mutant channels to the somato-dendritic compartment, while the p.A294G mutation did not affect the localization of the heteromeric channels to the axon initial segment. We conclude that this position is a hotspot of mutation that can give rise to a severe or a benign epilepsy. The p.A294V mutation does not exert a dominant-negative effect on wild-type subunits but alters the preferential axonal targeting of heteromeric Kv7 channels. Our data suggest that the disease severity is not necessarily a consequence of a strong inhibition of M current and that additional mechanisms such as abnormal subcellular distribution of Kv7 channels could be determinant.

Combination of linkage mapping and microarray-expression analysis identifies NF-kappaB signaling defect as a cause of autosomal-recessive mental retardation.

Autosomal-recessive inheritance accounts for nearly 25% of nonsyndromic mental retardation (MR), but the extreme heterogeneity of such conditions markedly hampers gene identification. Combining autozygosity mapping and RNA expression profiling in a consanguineous Tunisian family of three MR children with mild microcephaly and white-matter abnormalities identified the TRAPPC9 gene, which encodes a NF-kappaB-inducing kinase (NIK) and IkappaB kinase complex beta (IKK-beta) binding protein, as a likely candidate. Sequencing analysis revealed a nonsense variant (c.1708C>T [p.R570X]) within exon 9 of this gene that is responsible for an undetectable level of TRAPPC9 protein in patient skin fibroblasts. Moreover, TNF-alpha stimulation assays showed a defect in IkBalpha degradation, suggesting impaired NF-kappaB signaling in patient cells. This study provides evidence of an NF-kappaB signaling defect in isolated MR.

Loss-of-function mutation in the dioxygenase-encoding FTO gene causes severe growth retardation and multiple malformations.

FTO is a nuclear protein belonging to the AlkB-related non-haem iron- and 2-oxoglutarate-dependent dioxygenase family. Although polymorphisms within the first intron of the FTO gene have been associated with obesity, the physiological role of FTO remains unknown. Here we show that a R316Q mutation, inactivating FTO enzymatic activity, is responsible for an autosomal-recessive lethal syndrome. Cultured skin fibroblasts from affected subjects showed impaired proliferation and accelerated senescence. These findings indicate that FTO is essential for normal development of the central nervous and cardiovascular systems in human and establish that a mutation in a human member of the AlkB-related dioxygenase family results in a severe polymalformation syndrome.

Oligosaccharyltransferase-subunit mutations in nonsyndromic mental retardation.

Mental retardation (MR) is the most frequent handicap among children and young adults. Although a large proportion of X-linked MR genes have been identified, only four genes responsible for autosomal-recessive nonsyndromic MR (AR-NSMR) have been described so far. Here, we report on two genes involved in autosomal-recessive and X-linked NSMR. First, autozygosity mapping in two sibs born to first-cousin French parents led to the identification of a region on 8p22-p23.1. This interval encompasses the gene N33/TUSC3 encoding one subunit of the oligosaccharyltransferase (OTase) complex, which catalyzes the transfer of an oligosaccharide chain on nascent proteins, the key step of N-glycosylation. Sequencing N33/TUSC3 identified a 1 bp insertion, c.787_788insC, resulting in a premature stop codon, p.N263fsX300, and leading to mRNA decay. Surprisingly, glycosylation analyses of patient fibroblasts showed normal N-glycan synthesis and transfer, suggesting that normal N-glycosylation observed in patient fibroblasts may be due to functional compensation. Subsequently, screening of the X-linked N33/TUSC3 paralog, the IAP gene, identified a missense mutation (c.932T-->G, p.V311G) in a family with X-linked NSMR. Recent studies of fucosylation and polysialic-acid modification of neuronal cell-adhesion glycoproteins have shown the critical role of glycosylation in synaptic plasticity. However, our data provide the first demonstration that a defect in N-glycosylation can result in NSMR. Together, our results demonstrate that fine regulation of OTase activity is essential for normal cognitive-function development, providing therefore further insights to understand the pathophysiological bases of MR.

High-intensity interval training is superior to moderate intensity training on aerobic capacity in rats: Impact on hippocampal plasticity markers.

The use of endurance regimens could be improved by defining their respective effectiveness on aerobic fitness and brain health that remains controversial. We aimed at comparing work-matched high-intensity interval training (HIIT) with moderate-intensity continuous training (MICT) on aerobic performance and muscular plasticity markers in healthy rats. Cognitive functions and brain plasticity markers were also investigated following the 8-week training. Rats performed the incremental exercise test and behavioural tests before and after training at day 1 (D1), D15, D29 and D57. Key cerebral markers were assessed by Western blot and quantitative polymerase chain reaction to provide information on brain function related to angiogenesis, aerobic metabolism and neurotrophin activity at D59. Muscular protein levels involved in angiogenesis and aerobic metabolism were measured in both triceps brachii and soleus muscles. HIIT induced superior improvement of aerobic fitness compared to MICT, as indicated by enhancement of speed associated with lactate threshold (SLT) and maximal speed (Smax). In the triceps brachii muscle, markers of angiogenesis and aerobic activity were upregulated as well as myokines involved in neuroplasticity. Moreover, levels of key brain plasticity markers increased in the hippocampus after 8 weeks of HIIT, without improving cognitive functions. These findings might contribute to define physical exercise guidelines for maintaining brain health by highlighting the promising role of HIIT when using SLT for distinguishing low running speed from high running speed. Further studies are required to confirm these brain effects by exploring synaptic plasticity and neurogenesis mechanisms when exercise intensity is standardized and individualized.

Protective Role of Low Ethanol Administration Following Ischemic Stroke via Recovery of KCC2 and p75NTR Expression.

A striking result from epidemiological studies show a correlation between low alcohol intake and lower incidence for ischemic stroke and severity of derived brain injury. Although reduced apoptosis and inflammation has been suggested to be involved, little is known about the mechanism mediating this effect in vivo. Increase in intracellular chloride concentration and derived depolarizing GABAAR-mediated transmission are common consequences following various brain injuries and are caused by the abnormal expression levels of the chloride cotransporters NKCC1 and KCC2. Downstream pro-apoptotic signaling through p75NTR may link GABAA depolarization with post-injury neuronal apoptosis. Here, we show that changes in GABAergic signaling, Cl- homeostasis, and expression of chloride cotransporters in the post-traumatic mouse brain can be significantly reduced by administration of 3% ethanol to the drinking water. Ethanol-induced upregulation of KCC2 has a positive impact on neuronal survival, preserving a large part of the cortical peri-infarct zone, as well as preventing the massive post-ischemic upregulation of the pro-apoptotic protein p75NTR. Importantly, intracortical multisite in vivo recordings showed that ethanol treatment could significantly ameliorate stroke-induced reduction in cortical activity. This surprising finding discloses a pathway triggered by low concentration of ethanol as a novel therapeutically relevant target.

Mitochondria and neonatal epileptic encephalopathies with suppression burst.

The mitochondrion is a key cellular structure involved in many metabolic functions such as ATP synthesis by oxidative phosphorylation, tricarboxylic acid cycle or fatty acid oxidation. These pathways are fundamental for biological processes such as cell proliferation or death. In the central nervous system, mitochondria dysfunctions have been involved in many neurological diseases and age-related neurodegenerative disorders, including epilepsy, Alzheimer's and Parkinson's diseases. Mitochondrial diseases are frequently caused by a disruption of the respiratory chain. Nevertheless, other mitochondrial functions, including organellar dynamics or metabolite transport, could also be involved in such pathologies. Here we described mitochondrial dysfunctions in a very severe, intractable and relatively rare neonatal epileptic encephalopathy, the Ohtahara syndrome. This condition is characterized by neonatal onset of seizures, interictal electroencephalogram with suppression burst pattern and a very poor outcome with very severe psychomotor retardation or death. The etiology of this disease remains elusive but seems to be very heterogeneous including brain malformations, metabolic errors, transcription factor and synaptic vesicle release defects. In this review, we discuss first the Ohtahara syndrome caused by mitochondrial respiratory chain damages, suggesting that these defects could be more common than previously thought. Then, we will adress the importance of the mitochondrial glutamate carrier SLC25A22 in these pathologies, since mutations of this gene were described in two distinct families. These findings suggest that glutamate metabolism should also be considered as an important cause of the Ohtahara syndrome.

Novel carbon film induces precocious calcium oscillation to promote neuronal cell maturation.

Different types of carbon materials are biocompatible with neural cells and can promote maturation. The mechanism of this effect is not clear. Here we have tested the capacity of a carbon material composed of amorphous sp3 carbon backbone, embedded with a percolating network of sp2 carbon domains to sustain neuronal cultures. We found that cortical neurons survive and develop faster on this novel carbon material. After 3 days in culture, there is a precocious increase in the frequency of neuronal activity and in the expression of maturation marker KCC2 on carbon films as compared to a commonly used glass surface. Accelerated development is accompanied by a dramatic increase in neuronal dendrite arborization. The mechanism for the precocious maturation involves the activation of intracellular calcium oscillations by the carbon material already after 1 day in culture. Carbon-induced oscillations are independent of network activity and reflect intrinsic spontaneous activation of developing neurons. Thus, these results reveal a novel mechanism for carbon material-induced neuronal survival and maturation.

Report on three additional patients and genotype-phenotype correlation in SLC25A22-related disorders group.

Early infantile epileptic encephalopathy (EIEE) is a heterogeneous group of severe forms of age-related developmental and epileptic encephalopathies with onset during the first weeks or months of life. The interictal electroencephalogram (EEG) shows a "suppression burst" (SB) pattern. The prognosis is usually poor and most children die within the first two years or survive with very severe intellectual disabilities. EIEE type 3 is caused by variants affecting function, in SLC25A22, which is also responsible for epilepsy of infancy with migrating focal seizures (EIMFS). We report a family with a less severe phenotype of EIEE type 3. We performed exome sequencing and identified two unreported variants in SLC25A22 in the compound heterozygous state: NM_024698.4: c.[813_814delTG];[818 G>A] (p.[Ala272Glnfs*144];[Arg273Lys]). Functional studies in cultured skin fibroblasts from a patient showed that glutamate oxidation was strongly defective, based on a literature review. We clustered the 18 published patients (including those from this family) into three groups according to the severity of the SLC25A22-related disorders. In an attempt to identify genotype-phenotype correlations, we compared the variants according to the location depending on the protein domains. We observed that patients with two variants located in helical transmembrane domains presented a severe phenotype, whereas patients with at least one variant outside helical transmembrane domains presented a milder phenotype. These data are suggestive of a continuum of disorders related to SLC25A22 that could be called SLC25A22-related disorders. This might be a first clue to enable geneticists to outline a prognosis based on genetic molecular data regarding the SLC25A22 gene.

Inhibition of the Mitochondrial Glutamate Carrier SLC25A22 in Astrocytes Leads to Intracellular Glutamate Accumulation.

The solute carrier family 25 (SLC25) drives the import of a large diversity of metabolites into mitochondria, a key cellular structure involved in many metabolic functions. Mutations of the mitochondrial glutamate carrier SLC25A22 (also named GC1) have been identified in early epileptic encephalopathy (EEE) and migrating partial seizures in infancy (MPSI) but the pathophysiological mechanism of GC1 deficiency is still unknown, hampered by the absence of an in vivo model. This carrier is mainly expressed in astrocytes and is the principal gate for glutamate entry into mitochondria. A sufficient supply of energy is essential for the proper function of the brain and mitochondria have a pivotal role in maintaining energy homeostasis. In this work, we wanted to study the consequences of GC1 absence in an in vitro model in order to understand if glutamate catabolism and/or mitochondrial function could be affected. First, short hairpin RNA (shRNA) designed to specifically silence GC1 were validated in rat C6 glioma cells. Silencing GC1 in C6 resulted in a reduction of the GC1 mRNA combined with a decrease of the mitochondrial glutamate carrier activity. Then, primary astrocyte cultures were prepared and transfected with shRNA-GC1 or mismatch-RNA (mmRNA) constructs using the Neon® Transfection System in order to target a high number of primary astrocytes, more than 64%. Silencing GC1 in primary astrocytes resulted in a reduced nicotinamide adenine dinucleotide (Phosphate) (NAD(P)H) formation upon glutamate stimulation. We also observed that the mitochondrial respiratory chain (MRC) was functional after glucose stimulation but not activated by glutamate, resulting in a lower level of cellular adenosine triphosphate (ATP) in silenced astrocytes compared to control cells. Moreover, GC1 inactivation resulted in an intracellular glutamate accumulation. Our results show that mitochondrial glutamate transport via GC1 is important in sustaining glutamate homeostasis in astrocytes. Main Points: The mitochondrial respiratory chain is functional in absence of GC1Lack of glutamate oxidation results in a lower global ATP levelLack of mitochondrial glutamate transport results in intracellular glutamate accumulation.