Developmental and epileptic encephalopathies: from genetic heterogeneity to phenotypic continuum.

Developmental and epileptic encephalopathies (DEEs) are a heterogeneous group of disorders characterized by early-onset, often severe epileptic seizures and EEG abnormalities on a background of developmental impairment that tends to worsen as a consequence of epilepsy. DEEs may result from both nongenetic and genetic etiologies. Genetic DEEs have been associated with mutations in many genes involved in different functions including cell migration, proliferation, and organization, neuronal excitability, and synapse transmission and plasticity. Functional studies performed in different animal models and clinical trials on patients have contributed to elucidate pathophysiological mechanisms underlying many DEEs and have explored the efficacy of different treatments. Here, we provide an extensive review of the phenotypic spectrum included in the DEEs and of the genetic determinants and pathophysiological mechanisms underlying these conditions. We also provide a brief overview of the most effective treatment now available and of the emerging therapeutic approaches.

Apache is a neuronal player in autophagy required for retrograde axonal transport of autophagosomes.

Neurons are dependent on efficient quality control mechanisms to maintain cellular homeostasis and function due to their polarization and long-life span. Autophagy is a lysosomal degradative pathway that provides nutrients during starvation and recycles damaged and/or aged proteins and organelles. In neurons, autophagosomes constitutively form in distal axons and at synapses and are trafficked retrogradely to the cell soma to fuse with lysosomes for cargo degradation. How the neuronal autophagy pathway is organized and controlled remains poorly understood. Several presynaptic endocytic proteins have been shown to regulate both synaptic vesicle recycling and autophagy. Here, by combining electron, fluorescence, and live imaging microscopy with biochemical analysis, we show that the neuron-specific protein APache, a presynaptic AP-2 interactor, functions in neurons as an important player in the autophagy process, regulating the retrograde transport of autophagosomes. We found that APache colocalizes and co-traffics with autophagosomes in primary cortical neurons and that induction of autophagy by mTOR inhibition increases LC3 and APache protein levels at synaptic boutons. APache silencing causes a blockade of autophagic flux preventing the clearance of p62/SQSTM1, leading to a severe accumulation of autophagosomes and amphisomes at synaptic terminals and along neurites due to defective retrograde transport of TrkB-containing signaling amphisomes along the axons. Together, our data identify APache as a regulator of the autophagic cycle, potentially in cooperation with AP-2, and hypothesize that its dysfunctions contribute to the early synaptic impairments in neurodegenerative conditions associated with impaired autophagy.

Early endocannabinoid-mediated depolarization-induced suppression of excitation delays the appearance of the epileptic phenotype in synapsin II knockout mice.

The mechanism underlying the transition from the pre-symptomatic to the symptomatic state is a crucial aspect of epileptogenesis. SYN2 is a member of a multigene family of synaptic vesicle phosphoproteins playing a fundamental role in controlling neurotransmitter release. Human SYN2 gene mutations are associated with epilepsy and autism spectrum disorder. Mice knocked out for synapsin II (SynII KO) are prone to epileptic seizures that appear after 2 months of age. However, the involvement of the endocannabinoid system, known to regulate seizure development and propagation, in the modulation of the excitatory/inhibitory balance in the epileptic hippocampal network of SynII KO mice has not been explored. In this study, we investigated the impact of endocannabinoids on glutamatergic and GABAergic synapses at hippocampal dentate gyrus granule cells in young pre-symptomatic (1-2 months old) and adult symptomatic (5-8 months old) SynII KO mice. We observed an increase in endocannabinoid-mediated depolarization-induced suppression of excitation in young SynII KO mice, compared to age-matched wild-type controls. In contrast, the endocannabinoid-mediated depolarization-induced suppression of inhibition remained unchanged in SynII KO mice at both ages. This selective alteration of excitatory synaptic transmission was accompanied by changes in hippocampal endocannabinoid levels and cannabinoid receptor type 1 distribution among glutamatergic and GABAergic synaptic terminals contacting the granule cells of the dentate gyrus. Finally, inhibition of type-1 cannabinoid receptors in young pre-symptomatic SynII KO mice induced seizures during a tail suspension test. Our results suggest that endocannabinoids contribute to maintaining network stability in a genetic mouse model of human epilepsy.

Homeostatic regulation of brain activity: from endogenous mechanisms to homeostatic nanomachines.

After the initial concepts of the constancy of the internal milieu or homeostasis, put forward by Claude Bernard and Walter Cannon, homeostasis emerged as a mechanism to control oscillations of biologically meaningful variables within narrow physiological ranges. This is a primary need in the central nervous system that is continuously subjected to a multitude of stimuli from the internal and external environments that affect its function and structure, allowing to adapt the individual to the ever-changing daily conditions. Preserving physiological levels of activity despite disturbances that could either depress neural computation or excessively stimulate neural activity is fundamental, and failure of these homeostatic mechanisms can lead to brain diseases. In this review, we cover the role and main mechanisms of homeostatic plasticity involving the regulation of excitability and synaptic strength from the single neuron to the network level. We analyze the relationships between homeostatic and Hebbian plasticity and the conditions under which the preservation of the excitatory/inhibitory balance fails, triggering epileptogenesis and eventually epilepsy. Several therapeutic strategies to cure epilepsy have been designed to strengthen homeostasis when endogenous homeostatic plasticity mechanisms have become insufficient or ineffective to contrast hyperactivity. We describe "on demand" gene therapy strategies including optogenetics, chemogenetics, and chemo-optogenetics, and particularly focus on new closed loop sensor-actuator strategies mimicking homeostatic plasticity that can be endogenously expressed to strengthen the homeostatic defenses against brain diseases.

The intramembrane COOH-terminal domain of PRRT2 regulates voltage-dependent Na+ channels.

Proline-rich transmembrane protein 2 (PRRT2) is the single causative gene for pleiotropic paroxysmal syndromes, including epilepsy, kinesigenic dyskinesia, episodic ataxia, and migraine. PRRT2 is a neuron-specific type-2 membrane protein with a COOH-terminal intramembrane domain and a long proline-rich NH2-terminal cytoplasmic region. A large array of experimental data indicates that PRRT2 is a neuron stability gene that negatively controls intrinsic excitability by regulating surface membrane localization and biophysical properties of voltage-dependent Na+ channels Nav1.2 and Nav1.6, but not Nav1.1. To further investigate the regulatory role of PRRT2, we studied the structural features of this membrane protein with molecular dynamics simulations, and its structure-function relationships with Nav1.2 channels by biochemical and electrophysiological techniques. We found that the intramembrane COOH-terminal region maintains a stable conformation over time, with the first transmembrane domain forming a helix-loop-helix motif within the bilayer. The unstructured NH2-terminal cytoplasmic region bound to the Nav1.2 better than the isolated COOH-terminal intramembrane domain, mimicking full-length PRRT2, while the COOH-terminal intramembrane domain was able to modulate Na+ current and channel biophysical properties, still maintaining the striking specificity for Nav1.2 versus Nav1.1. channels. The results identify PRRT2 as a dual-domain protein in which the NH2-terminal cytoplasmic region acts as a binding antenna for Na+ channels, while the COOH-terminal membrane domain regulates channel exposure on the membrane and its biophysical properties.

Biomolecular Corona of Gold Nanoparticles: The Urgent Need for Strong Roots to Grow Strong Branches.

Gold nanoparticles (GNPs) are largely employed in diagnostics/biosensors and are among the most investigated nanomaterials in biology/medicine. However, few GNP-based nanoformulations have received FDA approval to date, and promising in vitro studies have failed to translate to in vivo efficacy. One key factor is that biological fluids contain high concentrations of proteins, lipids, sugars, and metabolites, which can adsorb/interact with the GNP's surface, forming a layer called biomolecular corona (BMC). The BMC can mask prepared functionalities and target moieties, creating new surface chemistry and determining GNPs' biological fate. Here, the current knowledge is summarized on GNP-BMCs, analyzing the factors driving these interactions and the biological consequences. A partial fingerprint of GNP-BMC analyzing common patterns of composition in the literature is extrapolated. However, a red flag is also risen concerning the current lack of data availability and regulated form of knowledge on BMC. Nanomedicine is still in its infancy, and relying on recently developed analytical and informatic tools offers an unprecedented opportunity to make a leap forward. However, a restart through robust shared protocols and data sharing is necessary to obtain "stronger roots". This will create a path to exploiting BMC for human benefit, promoting the clinical translation of biomedical nanotools.

Low glycemic index diet restrains epileptogenesis in a gender-specific fashion.

Dietary restriction, such as low glycemic index diet (LGID), have been successfully used to treat drug-resistant epilepsy. However, if such diet could also counteract antiepileptogenesis is still unclear. Here, we investigated whether the administration of LGID during the latent pre-epileptic period, prevents or delays the appearance of the overt epileptic phenotype. To this aim, we used the Synapsin II knockout (SynIIKO) mouse, a model of temporal lobe epilepsy in which seizures manifest 2-3 months after birth, offering a temporal window in which LGID may affect epileptogenesis. Pregnant SynIIKO mice were fed with either LGID or standard diet during gestation and lactation. Both diets were maintained in weaned mice up to 5 months of age. LGID delayed the seizure onset and induced a reduction of seizures severity only in female SynIIKO mice. In parallel with the epileptic phenotype, high-density multielectrode array recordings revealed a reduction of frequency, amplitude, duration, velocity of propagation and spread of interictal events by LGID in the hippocampus of SynIIKO females, but not mutant males, confirming the gender-specific effect. ELISA-based analysis revealed that LGID increased cortico-hippocampal allopregnanolone (ALLO) levels only in females, while it was unable to affect ALLO plasma concentrations in either sex. The results indicate that the gender-specific interference of LGID with the epileptogenic process can be ascribed to a gender-specific increase in cortical ALLO, a neurosteroid known to strengthen GABAergic transmission. The study highlights the possibility of developing a personalized gender-based therapy for temporal lobe epilepsy.

Catalytic Bioswitch of Platinum Nanozymes: Mechanistic Insights of Reactive Oxygen Species Scavenging in the Neurovascular Unit.

Oxidative stress is known to be the cause of several neurovascular diseases, including neurodegenerative disorders, since the increase of reactive oxygen species (ROS) levels can lead to cellular damage, blood-brain barrier leaking, and inflammatory pathways. Herein, we demonstrate the therapeutic potential of 5 nm platinum nanoparticles (PtNPs) to effectively scavenge ROS in different cellular models of the neurovascular unit. We investigated the mechanism underlying the PtNP biological activities, analyzing the influence of the evolving biological environment during particle trafficking and disclosing a key role of the protein corona, which elicited an effective switch-off of the PtNP catalytic properties, promoting their selective in situ activity. Upon cellular internalization, the lysosomal environment switches on and boosts the enzyme-like activity of the PtNPs, acting as an intracellular "catalytic microreactor" exerting strong antioxidant functionalities. Significant ROS scavenging was observed in the neurovascular cellular models, with an interesting protective mechanism of the Pt-nanozymes along lysosomal-mitochondrial axes.

Interactions of Graphene Oxide and Few-Layer Graphene with the Blood-Brain Barrier.

Thanks to their biocompatibility and high cargo capability, graphene-based materials (GRMs) might represent an ideal brain delivery system. The capability of GRMs to reach the brain has mainly been investigated in vivo and has highlighted some controversy. Herein, we employed two in vitro BBB models of increasing complexity to investigate the bionano interactions with graphene oxide (GO) and few-layer graphene (FLG): a 2D murine Transwell model, followed by a 3D human multicellular assembloid, to mimic the complexity of the in vivo architecture and intercellular crosstalk. We developed specific methodologies to assess the translocation of GO and FLG in a label-free fashion and a platform applicable to any nanomaterial. Overall, our results show good biocompatibility of the two GRMs, which did not impact the integrity and functionality of the barrier. Sufficiently dispersed subpopulations of GO and FLG were actively uptaken by endothelial cells; however, the translocation was identified as a rare event.

Alterations in KIDINS220/ARMS Expression Impact Sensory Processing and Social Behavior in Adult Mice.

Kinase D-interacting substrate of 220 kDa (Kidins220) is a transmembrane protein that participates in neural cell survival, maturation, and plasticity. Mutations in the human KIDINS220 gene are associated with a neurodevelopmental disorder ('SINO' syndrome) characterized by spastic paraplegia, intellectual disability, and in some cases, autism spectrum disorder. To better understand the pathophysiology of KIDINS220-linked pathologies, in this study, we assessed the sensory processing and social behavior of transgenic mouse lines with reduced Kidins220 expression: the CaMKII-driven conditional knockout (cKO) line, lacking Kidins220 in adult forebrain excitatory neurons, and the Kidins220floxed line, expressing constitutively lower protein levels. We show that alterations in Kidins220 expression levels and its splicing pattern cause impaired response to both auditory and olfactory stimuli. Both transgenic lines show impaired startle response to high intensity sounds, with preserved pre-pulsed inhibition, and strongly reduced social odor recognition. In the Kidins220floxed line, olfactory alterations are associated with deficits in social memory and increased aggressive behavior. Our results broaden our knowledge of the SINO syndrome; understanding sensory information processing and its deviations under neuropathological conditions is crucial for devising future therapeutic strategies to enhance the quality of life of affected individuals.

Neuron-restrictive silencer factor/repressor element 1-silencing transcription factor (NRSF/REST) controls spatial K+ buffering in primary cortical astrocytes.

Neuron-restrictive silencer factor/repressor element 1 (RE1)-silencing transcription factor (NRSF/REST) is a transcriptional repressor of a large cluster of neural genes containing RE1 motifs in their promoter region. NRSF/REST is ubiquitously expressed in non-neuronal cells, including astrocytes, while it is down-regulated during neuronal differentiation. While neuronal NRSF/REST homeostatically regulates intrinsic excitability and synaptic transmission, the role of the high NRSF/REST expression levels in the homeostatic functions of astrocytes is poorly understood. Here, we investigated the functional consequences of NRSF/REST deletion in primary cortical astrocytes derived from NRSF/REST conditional knockout mice (KO). We found that NRSF/REST KO astrocyte displayed a markedly reduced activity of inward rectifying K+ channels subtype 4.1 (Kir4.1) underlying spatial K+ buffering that was associated with a decreased expression and activity of the glutamate transporter-1 (GLT-1) responsible for glutamate uptake by astrocytes. The effects of the impaired astrocyte homeostatic functions on neuronal activity were investigated by co-culturing wild-type hippocampal neurons with NRSF/REST KO astrocytes. Interestingly, neurons experienced increased neuronal excitability at high firing rates associated with decrease after hyperpolarization and increased amplitude of excitatory postsynaptic currents. The data indicate that astrocytic NRSF/REST directly participates in neural circuit homeostasis by regulating intrinsic excitability and excitatory transmission and that dysfunctions of NRSF/REST expression in astrocytes may contribute to the pathogenesis of neurological disorders.

Missense mutations in the membrane domain of PRRT2 affect its interaction with Nav1.2 voltage-gated sodium channels.

PRRT2 is a neuronal protein that controls neuronal excitability and network stability by modulating voltage-gated Na+ channel (Nav). PRRT2 pathogenic variants cause pleiotropic syndromes including epilepsy, paroxysmal kinesigenic dyskinesia and episodic ataxia attributable to loss-of-function pathogenetic mechanism. Based on the evidence that the transmembrane domain of PRRT2 interacts with Nav1.2/1.6, we focused on eight missense mutations located within the domain that show expression and membrane localization similar to the wild-type protein. Molecular dynamics simulations showed that the mutants do not alter the structural stability of the PRRT2 membrane domain and preserve its conformation. Using affinity assays, we found that the A320V and V286M mutants displayed respectively decreased and increased binding to Nav1.2. Accordingly, surface biotinylation showed an increased Nav1.2 surface exposure induced by the A320V mutant. Electrophysiological analysis confirmed the lack of modulation of Nav1.2 biophysical properties by the A320V mutant with a loss-of-function phenotype, while the V286M mutant displayed a gain-of-function with respect to wild-type PRRT2 with a more pronounced left-shift of the inactivation kinetics and delayed recovery from inactivation. The data confirm the key role played by the PRRT2-Nav interaction in the pathogenesis of the PRRT2-linked disorders and suggest an involvement of the A320 and V286 residues in the interaction site. Given the similar clinical phenotype caused by the two mutations, we speculate that circuit instability and paroxysmal manifestations may arise when PRRT2 function is outside the physiological range.

A developmental stage- and Kidins220-dependent switch in astrocyte responsiveness to brain-derived neurotrophic factor.

Astroglial cells are key to maintain nervous system homeostasis. Neurotrophins are known for their pleiotropic effects on neuronal physiology but also exert complex functions to glial cells. Here, we investigated (i) the signaling competence of mouse embryonic and postnatal primary cortical astrocytes exposed to brain-derived neurotrophic factor (BDNF) and, (ii) the role of kinase D-interacting substrate of 220 kDa (Kidins220), a transmembrane scaffold protein that mediates neurotrophin signaling in neurons. We found a shift from a kinase-based response in embryonic cells to a response predominantly relying on intracellular Ca2+ transients [Ca2+]i within postnatal cultures, associated with a decrease in the synthesis of full-length BDNF receptor TrkB, with Kidins220 contributing to the BDNF-activated kinase and [Ca2+]i pathways. Finally, Kidins220 participates in the homeostatic function of astrocytes by controlling the expression of the ATP-sensitive inward rectifier potassium channel 10 (Kir4.1) and the metabolic balance of embryonic astrocytes. Overall, our data contribute to the understanding of the complex role played by astrocytes within the central nervous system, and identify Kidins220 as a novel actor in the increasing number of pathologies characterized by astrocytic dysfunctions. This article has an associated First Person interview with the first authors of the paper.

Dual Thermal- and Oxidation-Responsive Polymers Synthesized by a Sequential ROP-to-RAFT Procedure Inherently Temper Neuroinflammation.

This study is about multiple responsiveness in biomedical materials. This typically implies "orthogonality" (i.e., one response does not affect the other) or synergy (i.e., one increases efficacy or selectivity of the other), but an antagonist effect between responses may also occur. Here, we describe a family of very well-defined amphiphilic and micelle-forming block copolymers, which show both oxidative and temperature responses. They are produced via successive anionic ring-opening polymerization of episulfides and RAFT polymerization of dialkylacrylamides and differ only in the ratio between inert (N,N-dimethylacrylamide, DMA) and temperature-sensitive (N,N-diethylacrylamide, DEA) units. By scavenging Reactive Oxygen Species (ROS), these polymers are anti-inflammatory; through temperature responsiveness, they can macroscopically aggregate, which may allow them to form depots upon injection. The localization of the anti-inflammatory action is an example of synergy. An extensive evaluation of toxicity and anti-inflammatory effects on in vitro models, including BV2 microglia, C8D30 astrocytes and primary neurons, shows a link between capacity of aggregation and detrimental effects on viability which, albeit mild, can hinder the anti-inflammatory potential (antagonist action). Although limited in breadth (e.g., only in vitro models and only DEA as a temperature-responsive unit), this study suggests that single-responsive controls should be used to allow for a precise assessment of the (synergic or antagonist) potential of double-responsive systems.

Phenotypic and genetic spectrum of ATP6V1A encephalopathy: a disorder of lysosomal homeostasis.

Vacuolar-type H+-ATPase (V-ATPase) is a multimeric complex present in a variety of cellular membranes that acts as an ATP-dependent proton pump and plays a key role in pH homeostasis and intracellular signalling pathways. In humans, 22 autosomal genes encode for a redundant set of subunits allowing the composition of diverse V-ATPase complexes with specific properties and expression. Sixteen subunits have been linked to human disease. Here we describe 26 patients harbouring 20 distinct pathogenic de novo missense ATP6V1A variants, mainly clustering within the ATP synthase α/ß family-nucleotide-binding domain. At a mean age of 7 years (extremes: 6 weeks, youngest deceased patient to 22 years, oldest patient) clinical pictures included early lethal encephalopathies with rapidly progressive massive brain atrophy, severe developmental epileptic encephalopathies and static intellectual disability with epilepsy. The first clinical manifestation was early hypotonia, in 70%; 81% developed epilepsy, manifested as developmental epileptic encephalopathies in 58% of the cohort and with infantile spasms in 62%; 63% of developmental epileptic encephalopathies failed to achieve any developmental, communicative or motor skills. Less severe outcomes were observed in 23% of patients who, at a mean age of 10 years and 6 months, exhibited moderate intellectual disability, with independent walking and variable epilepsy. None of the patients developed communicative language. Microcephaly (38%) and amelogenesis imperfecta/enamel dysplasia (42%) were additional clinical features. Brain MRI demonstrated hypomyelination and generalized atrophy in 68%. Atrophy was progressive in all eight individuals undergoing repeated MRIs. Fibroblasts of two patients with developmental epileptic encephalopathies showed decreased LAMP1 expression, Lysotracker staining and increased organelle pH, consistent with lysosomal impairment and loss of V-ATPase function. Fibroblasts of two patients with milder disease, exhibited a different phenotype with increased Lysotracker staining, decreased organelle pH and no significant modification in LAMP1 expression. Quantification of substrates for lysosomal enzymes in cellular extracts from four patients revealed discrete accumulation. Transmission electron microscopy of fibroblasts of four patients with variable severity and of induced pluripotent stem cell-derived neurons from two patients with developmental epileptic encephalopathies showed electron-dense inclusions, lipid droplets, osmiophilic material and lamellated membrane structures resembling phospholipids. Quantitative assessment in induced pluripotent stem cell-derived neurons identified significantly smaller lysosomes. ATP6V1A-related encephalopathy represents a new paradigm among lysosomal disorders. It results from a dysfunctional endo-lysosomal membrane protein causing altered pH homeostasis. Its pathophysiology implies intracellular accumulation of substrates whose composition remains unclear, and a combination of developmental brain abnormalities and neurodegenerative changes established during prenatal and early postanal development, whose severity is variably determined by specific pathogenic variants.

The light-dependent pseudo-capacitive charging of conjugated polymer nanoparticles coupled with the depolarization of the neuronal membrane.

The mechanism underlying visual restoration in blind animal models of retinitis pigmentosa using a liquid retina prosthesis based on semiconductive polymeric nanoparticles is still being debated. Through the application of mathematical models and specific experiments, we developed a coherent understanding of abiotic/biotic coupling, capturing the essential mechanism of photostimulation responsible for nanoparticle-induced retina activation. Our modeling is based on the solution of drift-diffusion and Poisson-Nernst-Planck models in the multi-physics neuron-cleft-nanoparticle-extracellular space domain, accounting for the electro-chemical motion of all the relevant species following photoexcitation. Modeling was coupled with electron microscopy to estimate the size of the neuron-nanoparticle cleft and electrophysiology on retina explants acutely or chronically injected with nanoparticles. Overall, we present a consistent picture of electrostatic depolarization of the bipolar cell driven by the pseudo-capacitive charging of the nanoparticle. We demonstrate that the highly resistive cleft composition, due to filling by adhesion/extracellular matrix proteins, is a crucial ingredient for establishing functional electrostatic coupling. Additionally, we show that the photo-chemical generation of reactive oxygen species (ROS) becomes relevant only at very high light intensities, far exceeding the physiological ones, in agreement with the lack of phototoxicity shown in vivo.

Ca2+ binding to synapsin I regulates resting Ca2+ and recovery from synaptic depression in nerve terminals.

Synapsin I (SynI) is a synaptic vesicle (SV)-associated phosphoprotein that modulates neurotransmission by controlling SV trafficking. The SynI C-domain contains a highly conserved ATP binding site mediating SynI oligomerization and SV clustering and an adjacent main Ca2+ binding site, whose physiological role is unexplored. Molecular dynamics simulations revealed that the E373K point mutation irreversibly deletes Ca2+ binding to SynI, still allowing ATP binding, but inducing a destabilization of the SynI oligomerization interface. Here, we analyzed the effects of this mutation on neurotransmitter release and short-term plasticity in excitatory and inhibitory synapses from primary hippocampal neurons. Patch-clamp recordings showed an increase in the frequency of miniature excitatory postsynaptic currents (EPSCs) that was totally occluded by exogenous Ca2+ chelators and associated with a constitutive increase in resting terminal Ca2+ concentrations. Evoked EPSC amplitude was also reduced, due to a decreased readily releasable pool (RRP) size. Moreover, in both excitatory and inhibitory synapses, we observed a marked impaired recovery from synaptic depression, associated with impaired RRP refilling and depletion of the recycling pool of SVs. Our study identifies SynI as a novel Ca2+ buffer in excitatory terminals. Blocking Ca2+ binding to SynI results in higher constitutive Ca2+ levels that increase the probability of spontaneous release and disperse SVs. This causes a decreased size of the RRP and an impaired recovery from depression due to the failure of SV reclustering after sustained high-frequency stimulation. The results indicate a physiological role of Ca2+ binding to SynI in the regulation of SV clustering and trafficking in nerve terminals.

Synaptic genes and neurodevelopmental disorders: From molecular mechanisms to developmental strategies of behavioral testing.

Synaptopathies are a class of neurodevelopmental disorders caused by modification in genes coding for synaptic proteins. These proteins oversee the process of neurotransmission, mainly controlling the fusion and recycling of synaptic vesicles at the presynaptic terminal, the expression and localization of receptors at the postsynapse and the coupling between the pre- and the postsynaptic compartments. Murine models, with homozygous or heterozygous deletion for several synaptic genes or knock-in for specific pathogenic mutations, have been developed. They have proved to be extremely informative for understanding synaptic physiology, as well as for clarifying the patho-mechanisms leading to developmental delay, epilepsy and motor, cognitive and social impairments that are the most common clinical manifestations of neurodevelopmental disorders. However, the onset of these disorders emerges during infancy and adolescence while the behavioral phenotyping is often conducted in adult mice, missing important information about the impact of synaptic development and maturation on the manifestation of the behavioral phenotype. Here, we review the main achievements obtained by behavioral testing in murine models of synaptopathies and propose a battery of behavioral tests to improve classification, diagnosis and efficacy of potential therapeutic treatments. Our aim is to underlie the importance of studying behavioral development and better focusing on disease onset and phenotypes.

Presynaptic L-Type Ca2+ Channels Increase Glutamate Release Probability and Excitatory Strength in the Hippocampus during Chronic Neuroinflammation.

Neuroinflammation is involved in the pathogenesis of several neurologic disorders, including epilepsy. Both changes in the input/output functions of synaptic circuits and cell Ca2+ dysregulation participate in neuroinflammation, but their impact on neuron function in epilepsy is still poorly understood. Lipopolysaccharide (LPS), a toxic byproduct of bacterial lysis, has been extensively used to stimulate inflammatory responses both in vivo and in vitro LPS stimulates Toll-like receptor 4, an important mediator of the brain innate immune response that contributes to neuroinflammation processes. Although we report that Toll-like receptor 4 is expressed in both excitatory and inhibitory mouse hippocampal neurons (both sexes), its chronic stimulation by LPS induces a selective increase in the excitatory synaptic strength, characterized by enhanced synchronous and asynchronous glutamate release mechanisms. This effect is accompanied by a change in short-term plasticity with decreased facilitation, decreased post-tetanic potentiation, and increased depression. Quantal analysis demonstrated that the effects of LPS on excitatory transmission are attributable to an increase in the probability of release associated with an overall increased expression of L-type voltage-gated Ca2+ channels that, at presynaptic terminals, abnormally contributes to evoked glutamate release. Overall, these changes contribute to the excitatory/inhibitory imbalance that scales up neuronal network activity under inflammatory conditions. These results provide new molecular clues for treating hyperexcitability of hippocampal circuits associated with neuroinflammation in epilepsy and other neurologic disorders.SIGNIFICANCE STATEMENT Neuroinflammation is thought to have a pathogenetic role in epilepsy, a disorder characterized by an imbalance between excitation/inhibition. Fine adjustment of network excitability and regulation of synaptic strength are both implicated in the homeostatic maintenance of physiological levels of neuronal activity. Here, we focused on the effects of chronic neuroinflammation induced by lipopolysaccharides on hippocampal glutamatergic and GABAergic synaptic transmission. Our results show that, on chronic stimulation with lipopolysaccharides, glutamatergic, but not GABAergic, neurons exhibit an enhanced synaptic strength and changes in short-term plasticity because of an increased glutamate release that results from an anomalous contribution of L-type Ca2+ channels to neurotransmitter release.

Increased responsiveness at the cerebellar input stage in the PRRT2 knockout model of paroxysmal kinesigenic dyskinesia.

PRoline-Rich Transmembrane protein-2 (PRRT2) is a recently described neuron-specific type-2 integral membrane protein with a large cytosolic N-terminal domain that distributes in presynaptic and axonal domains where it interacts with several presynaptic proteins and voltage-gated Na+ channels. Several PRRT2 mutations are the main cause of a wide and heterogeneous spectrum of paroxysmal disorders with a loss-of-function pathomechanism. The highest expression levels of PRRT2 in brain occurs in cerebellar granule cells (GCs) and cerebellar dysfunctions participate in the dyskinetic phenotype of PRRT2 knockout (KO) mice. We have investigated the effects of PRRT2 deficiency on the intrinsic excitability of GCs and the input-output relationships at the mossy fiber-GC synapses. We show that PRRT2 KO primary GCs display increased expression of Na+ channels, increased amplitude of Na+ currents and increased length of the axon initial segment, leading to an overall enhancement of intrinsic excitability. In acute PRRT2 KO cerebellar slices, GCs were more prone to action potential discharge in response to mossy fiber activation and exhibited an enhancement of transient and persistent Na+ currents, in the absence of changes at the mossy fiber-GC synapses. The results support a key role of PRRT2 expressed in GCs in the physiological regulation of the excitatory input to the cerebellum and are consistent with a major role of a cerebellar dysfunction in the pathogenesis of the PRRT2-linked paroxysmal pathologies.