Correct Laminar Positioning in the Neocortex Influences Proper Dendritic and Synaptic Development.

The neocortex is a 6-layered laminated structure with a precise anatomical and functional organization ensuring proper function. Laminar positioning of cortical neurons, as determined by termination of neuronal migration, is a key determinant of their ability to assemble into functional circuits. However, the exact contribution of laminar placement to dendrite morphogenesis and synapse formation remains unclear. Here we manipulated the laminar position of cortical neurons by knocking down doublecortin (Dcx), a crucial effector of migration, and show that misplaced neurons fail to properly form dendrites, spines, and functional glutamatergic and GABAergic synapses. We further show that knocking down Dcx in properly positioned neurons induces similar but milder defects, suggesting that the laminar misplacement is the primary cause of altered neuronal development. Thus, the specific laminar environment of their fated layers is crucial for the maturation of cortical neurons, and influences their functional integration into developing cortical circuits.

Depolarizing γ-aminobutyric acid contributes to glutamatergic network rewiring in epilepsy.

OBJECTIVE: Rewiring of excitatory glutamatergic neuronal circuits is a major abnormality in epilepsy. Besides the rewiring of excitatory circuits, an abnormal depolarizing γ-aminobutyric acidergic (GABAergic) drive has been hypothesized to participate in the epileptogenic processes. However, a remaining clinically relevant question is whether early post-status epilepticus (SE) evoked chloride dysregulation is important for the remodeling of aberrant glutamatergic neuronal circuits. METHODS: Osmotic minipumps were used to infuse intracerebrally a specific inhibitor of depolarizing GABAergic transmission as well as a functionally blocking antibody toward the pan-neurotrophin receptor p75 (p75NTR ). The compounds were infused between 2 and 5 days after pilocarpine-induced SE. Immunohistochemistry for NKCC1, KCC2, and ectopic recurrent mossy fiber (rMF) sprouting as well as telemetric electroencephalographic and electrophysiological recordings were performed at day 5 and 2 months post-SE. RESULTS: Blockade of NKCC1 after SE with the specific inhibitor bumetanide restored NKCC1 and KCC2 expression, normalized chloride homeostasis, and significantly reduced the glutamatergic rMF sprouting within the dentate gyrus. This mechanism partially involves p75NTR signaling, as bumetanide application reduced SE-induced p75NTR expression and functional blockade of p75NTR decreased rMF sprouting. The early transient (3 days) post-SE infusion of bumetanide reduced rMF sprouting and recurrent seizures in the chronic epileptic phase. INTERPRETATION: Our findings show that early post-SE abnormal depolarizing GABA and p75NTR signaling fosters a long-lasting rearrangement of glutamatergic network that contributes to the epileptogenic process. This finding defines promising and novel targets to constrain reactive glutamatergic network rewiring in adult epilepsy. Ann Neurol 2017;81:251-265.

Neuronal chloride accumulation and excitatory GABA underlie aggravation of neonatal epileptiform activities by phenobarbital.

Phenobarbital produces its anti-epileptic actions by increasing the inhibitory drive of γ-aminobutyric acid. However, following recurrent seizures, γ-aminobutyric acid excites neurons because of a persistent increase of chloride raising the important issue of whether phenobarbital could aggravate persistent seizures. Here we compared the actions of phenobarbital on initial and established ictal-like events in an in vitro model of mirror focus. Using the in vitro three-compartment chamber preparation with the two hippocampi and their commissural fibres placed in three different chambers, kainate was applied to one hippocampus and phenobarbital contralaterally, either after one ictal-like event or after many recurrent ictal-like events that produce an epileptogenic mirror focus. Field, perforated patch and single-channel recordings were used to determine the effects of γ-aminobutyric acid and their modulation by phenobarbital, and alterations of the chloride cotransporters were investigated using sodium-potassium-chloride cotransporter 1 and potassium chloride cotransporter 2 antagonists, potassium chloride cotransporter 2 immunocytochemistry and sodium-potassium-chloride cotransporter 1 knockouts. Phenobarbital reduced initial ictal-like events and prevented the formation of a mirror focus when applied from the start. In contrast, phenobarbital aggravated epileptiform activities when applied after many ictal-like events by enhancing the excitatory actions of γ-aminobutyric acid due to increased chloride. The accumulation of chloride and the excitatory actions of γ-aminobutyric acid in mirror foci neurons are mediated by the sodium-potassium-chloride cotransporter 1 chloride importer and by downregulation and internalization of the chloride-exporter potassium-chloride cotransporter 2. Finally, concomitant applications of the sodium-potassium-chloride cotransporter 1 antagonist bumetanide and phenobarbital decreased excitatory actions of γ-aminobutyric acid and prevented its paradoxical actions on mirror focus. Therefore, the history of seizures prior to phenobarbital applications determines its effects and rapid treatment of severe potentially epileptogenic-neonatal seizures is recommended to prevent secondary epileptogenesis associated with potassium chloride cotransporter 2 downregulation and acquisition of the excitatory γ-aminobutyric acid phenotype.

Long term osmotic stress exposure outcomes on rat dopaminergic innervations and the associated motor behavior.

The osmotic stress is a powerful stimulus that elicits profound peripheral and central disturbances. In the mammalian brain, osmotic stress has been associated to several glial and neuronal changes. The lack of data regarding the impact on the dopaminergic system and locomotion led us to investigate the effect of prolonged water deprivation in rat on the midbrain dopaminergic system and locomotor performance by dehydrating rats for one and two weeks. Locomotor activity and tyrosine hydroxylase (TH) expression were assessed using the open field test and immunohistochemistry respectively. Water deprivation was accompanied with a significant increment of TH expression within substantia nigra compacta (SNc) and ventral tegmental area (VTA) gradually as the duration of dehydration increases. While locomotor activity showed the inverse tendency manifested by a drop of crossed boxes number following one and two weeks of water deprivation. Our data suggest a substantial implication of midbrain dopaminergic system in the central response to the osmotic stimuli accompanied with locomotor deficiencies.

GABA action in immature neocortical neurons directly depends on the availability of ketone bodies.

In the early postnatal period, energy metabolism in the suckling rodent brain relies to a large extent on metabolic pathways alternate to glucose such as the utilization of ketone bodies (KBs). However, how KBs affect neuronal excitability is not known. Using recordings of single NMDA and GABA-activated channels in neocortical pyramidal cells we studied the effects of KBs on the resting membrane potential (E(m)) and reversal potential of GABA-induced anionic currents (E(GABA)), respectively. We show that during postnatal development (P3-P19) if neocortical brain slices are adequately supplied with KBs, E(m) and E(GABA) are both maintained at negative levels of about -83 and -80 mV, respectively. Conversely, a KB deficiency causes a significant depolarization of both E(m) (>5 mV) and E(GABA) (>15 mV). The KB-mediated shift in E(GABA) is largely determined by the interaction of the NKCC1 cotransporter and Cl(-)/HCO3 transporter(s). Therefore, by inducing a hyperpolarizing shift in E(m) and modulating GABA signaling mode, KBs can efficiently control the excitability of neonatal cortical neurons.

Increased neurogenesis in adult mCD24-deficient mice.

mCD24, a glycosylphosphatidylinositol-anchored highly glycosylated molecule, is expressed on differentiating neurons during development. In the adult CNS, its expression is restricted to immature neurons located in two regions showing ongoing neurogenesis: the subventricular zone (SVZ) of the lateral ventricle pathway and the dentate gyrus (DG) of the hippocampal formation. Here, combining bromodeoxyuridine (BrdU) and proliferating cell nuclear antigen labelings we confirmed that mCD24 is expressed on proliferating cells. To determine whether the inactivation of the molecule may affect adult neurogenesis, we analyzed the phenotype of mCD24-deficient mice (mCD24-/-). We labeled cells in S-phase with a pulse, a long, or a cumulative administration of BrdU and analyzed cells in different zones according to their dividing rate (rapid and slow) both in the control and mCD24-/-. We found a significant increase in the number of rapid (in the SVZ and the DG) and slow (in the SVZ) proliferating cells. Cumulative assays revealed a global reduction of the total cell cycle duration of rapidly proliferating precursors of SVZ. We investigated the fate of supernumerary cells and observed an increased number of apoptotic cells (terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling) in the mutant SVZ. Furthermore, we found no difference in the size of the olfactory bulb between wild-type (WT) and mutant mice. In support, mCD24 deletion did not appear to affect migration in the migratory stream. A comparison of the organization of migrating precursors between WT and mCD24 -/-, both in vivo at the optic and electron microscopic levels and in SVZ cultured explants, did not show any changes in the arrangement of neuroblasts in chain-like structures. Altogether, our data suggest that mCD24 regulates negatively cell proliferation in zones of secondary neurogenesis.

Detrimental effect of post Status Epilepticus treatment with ROCK inhibitor Y-27632 in a pilocarpine model of temporal lobe epilepsy.

Temporal lobe epilepsy (TLE) is the most common type of epilepsy in adults where 20-30% of the patients are refractory to currently available anti-epileptic drugs. The RhoA/Rho-kinase signaling pathway activation has been involved in inflammatory responses, neurite outgrowth and neuronal death under pathological conditions such as epileptic insults. Acute preventive administration of ROCK inhibitor has been reported to have beneficial outcomes in Status Epilepticus (SE) epilepsy. In the present study, we evaluate the effect of chronic post SE treatment with the ROCK inhibitor Y-27632 in a rat pilocarpine model of TLE. We used chronic i.p. injections of Y-27632 for 5 days in 6 week old control rats or rats subjected to pilocarpine treatment as a model of TLE. Surprisingly, our findings demonstrate that a systemic administration of Y-27632 in pilocarpine-treated rats increases neuronal death in the CA3 region and ectopic recurrent mossy fiber sprouting (rMFS) in the dentate gyrus of the hippocampal formation. Interestingly, we found that chronic treatment with Y-27632 exacerbates the down-regulation and pathological distribution of the K(+)-Cl(-) cotransporter KCC2, thus providing a putative mechanism for post SE induced neuronal death. The involvement of astrogliosis in this mechanism appears to be intricate as ROCK inhibition reduces reactive astrogliosis in pilocarpine rats. Conversely, in control rats, chronic Y-27632 treatment increases astrogliosis. Together, our findings suggest that Y-27632 has a detrimental effect when chronically used post SE in a rat pilocarpine model of TLE.

MIM-Induced Membrane Bending Promotes Dendritic Spine Initiation.

Proper morphogenesis of neuronal dendritic spines is essential for the formation of functional synaptic networks. However, it is not known how spines are initiated. Here, we identify the inverse-BAR (I-BAR) protein MIM/MTSS1 as a nucleator of dendritic spines. MIM accumulated to future spine initiation sites in a PIP2-dependent manner and deformed the plasma membrane outward into a proto-protrusion via its I-BAR domain. Unexpectedly, the initial protrusion formation did not involve actin polymerization. However, PIP2-dependent activation of Arp2/3-mediated actin assembly was required for protrusion elongation. Overexpression of MIM increased the density of dendritic protrusions and suppressed spine maturation. In contrast, MIM deficiency led to decreased density of dendritic protrusions and larger spine heads. Moreover, MIM-deficient mice displayed altered glutamatergic synaptic transmission and compatible behavioral defects. Collectively, our data identify an important morphogenetic pathway, which initiates spine protrusions by coupling phosphoinositide signaling, direct membrane bending, and actin assembly to ensure proper synaptogenesis.

Cortical GABAergic excitation contributes to epileptic activities around human glioma.

Brain gliomas are highly epileptogenic. Excitatory glutamatergic mechanisms are involved in the generation of epileptic activities in the neocortex surrounding gliomas. However, chloride homeostasis is known to be perturbed in glioma cells. Thus, the contribution of γ-aminobutyric acidergic (GABAergic) mechanisms that depend on intracellular chloride merits closer study. We studied the occurrence, networks, cells, and signaling basis of epileptic activities in neocortical slices from the peritumoral surgical margin resected around human brain gliomas. Postoperative glioma tissue from 69% of patients spontaneously generated interictal-like discharges, synchronized, with a high-frequency oscillation signature, in superficial layers of neocortex around areas of glioma infiltration. Interictal-like events depended both on glutamatergic AMPA receptor-mediated transmission and on depolarizing GABAergic signaling. GABA released by interneurons depolarized 65% of pyramidal cells, in which chloride homeostasis was perturbed because of changes in expression of neuronal chloride cotransporters: KCC2 (K-Cl cotransporter 2) was reduced by 42% and expression of NKCC1 (Na-K-2Cl cotransporter 1) increased by 144%. Ictal-like activities were initiated by convulsant stimuli exclusively in these epileptogenic areas. This study shows that epileptic activities are sustained by excitatory effects of GABA in human peritumoral neocortex, as reported in temporal lobe epilepsies, suggesting that both glutamate and GABA signaling and cellular chloride regulation processes, all also involved in oncogenesis as already shown, induce an imbalance between synaptic excitation and inhibition underlying epileptic discharges in glioma patients. Thus, the control of chloride in neurons and glioma cells may provide a therapeutic target for patients with epileptogenic gliomas.

Oxytocin-mediated GABA inhibition during delivery attenuates autism pathogenesis in rodent offspring.

We report that the oxytocin-mediated neuroprotective γ-aminobutyric acid (GABA) excitatory-inhibitory shift during delivery is abolished in the valproate and fragile X rodent models of autism. During delivery and subsequently, hippocampal neurons in these models have elevated intracellular chloride levels, increased excitatory GABA, enhanced glutamatergic activity, and elevated gamma oscillations. Maternal pretreatment with bumetanide restored in offspring control electrophysiological and behavioral phenotypes. Conversely, blocking oxytocin signaling in naïve mothers produced offspring having electrophysiological and behavioral autistic-like features. Our results suggest a chronic deficient chloride regulation in these rodent models of autism and stress the importance of oxytocin-mediated GABAergic inhibition during the delivery process. Our data validate the amelioration observed with bumetanide and oxytocin and point to common pathways in a drug-induced and a genetic rodent model of autism.

Enhanced Synaptic Activity and Epileptiform Events in the Embryonic KCC2 Deficient Hippocampus.

The neuronal potassium-chloride co-transporter 2 [indicated thereafter as KCC2 (for protein) and Kcc2 (for gene)] is thought to play an important role in the post natal excitatory to inhibitory switch of GABA actions in the rodent hippocampus. Here, by studying hippocampi of wild-type (Kcc2(+/+)) and Kcc2 deficient (Kcc2(-/-)) mouse embryos, we unexpectedly found increased spontaneous neuronal network activity at E18.5, a developmental stage when KCC2 is thought not to be functional in the hippocampus. Embryonic Kcc2(-/-) hippocampi have also an augmented synapse density and a higher frequency of spontaneous glutamatergic and GABA-ergic postsynaptic currents than naïve age matched neurons. However, intracellular chloride concentration ([Cl(-)](i)) and the reversal potential of GABA-mediated currents (E(GABA)) were similar in embryonic Kcc2(+/+) and Kcc2(-/-) CA3 neurons. In addition, KCC2 immunolabeling was cytoplasmic in the majority of neurons suggesting that the molecule is not functional as a plasma membrane chloride co-transporter. Collectively, our results show that already at an embryonic stage, KCC2 controls the formation of synapses and, when deleted, the hippocampus has a higher density of GABA-ergic and glutamatergic synapses and generates spontaneous and evoked epileptiform activities. These results may be explained either by a small population of orchestrating neurons in which KCC2 operates early as a chloride exporter or by transporter independent actions of KCC2 that are instrumental in synapse formation and networks construction.

Coronin-1A links cytoskeleton dynamics to TCR alpha beta-induced cell signaling.

Actin polymerization plays a critical role in activated T lymphocytes both in regulating T cell receptor (TCR)-induced immunological synapse (IS) formation and signaling. Using gene targeting, we demonstrate that the hematopoietic specific, actin- and Arp2/3 complex-binding protein coronin-1A contributes to both processes. Coronin-1A-deficient mice specifically showed alterations in terminal development and the survival of alpha beta T cells, together with defects in cell activation and cytokine production following TCR triggering. The mutant T cells further displayed excessive accumulation yet reduced dynamics of F-actin and the WASP-Arp2/3 machinery at the IS, correlating with extended cell-cell contact. Cell signaling was also affected with the basal activation of the stress kinases sAPK/JNK1/2; and deficits in TCR-induced Ca2+ influx and phosphorylation and degradation of the inhibitor of NF-kappaB (I kappa B). Coronin-1A therefore links cytoskeleton plasticity with the functioning of discrete TCR signaling components. This function may be required to adjust TCR responses to selecting ligands accounting in part for the homeostasis defect that impacts alpha beta T cells in coronin-1A deficient mice, with the exclusion of other lympho/hematopoietic lineages.

Mouse CD24 is required for homeostatic cell renewal.

Under physiological conditions, some adult tissues retain a capacity for self-renewal. This property is attributable to the proliferation and differentiation of stem, transit-amplifying, and differentiating cells, which are regulated by cell-cell or cell-matrix interactions or by secreted factors. By gain and loss of function experiments, we demonstrate the involvement of mouse CD24 (mouse cluster of differentiation 24), which is a glycosyl phosphatidylinositol (GPI)-anchored cell-surface glycoprotein, in the regulation of homeostatic cell renewal. BrdU incorporation observations, at optical and electron-microscopic levels, have revealed increased cell proliferation in the developing brain and in the epithelia of mCD24-deleted mice. We have observed ectopic proliferative cells in the suprabasal layers of the mutant skin leading to a general disruption of basal and suprabasal layers. By contrast, ectopic mCD24 expression mediated by retroviral infection of the embryonic brain leads to a decreased number of clusters of cells generated in the progeny. Together, these results and our previous published data indicate that mCD24 contributes to the regulation of the production of differentiated cells by controlling the proliferation/differentiation balance between transit-amplifying and committed differentiated cells.

Neuronal distribution of spatial in the developing cerebellum and hippocampus and its somatodendritic association with the kinesin motor KIF17.

We identified the Spatial (Stromal Protein Associated with Thymii and Lymph-node) gene from an adult thymus mouse library of cDNA clones. By RT-PCR, we reported that Spatial was highly expressed in restricted areas of the central nervous system. Here, we characterize the precise cellular localization of Spatial during mouse brain development in the cerebellum, hippocampus and cortex. Five different transcript isoforms have been described for Spatial and among those, only Spatial-epsilon and -beta present a tightly controlled expression. In the cerebellum, Spatial expression is detected in the external precursor granular layer and persists as these cells migrate and differentiate to form the internal granular layer. It is also expressed in differentiating Purkinje cells with a specific somatodendritic distribution. Spatial expression in the hippocampus is spatially and temporally regulated: it is first expressed in the CA3 field, then in CA1 and later in the dentate gyrus. Interestingly, Spatial-beta expression tightly overlaps with the beginning of neuronal differentiation in both structures. Using cultured hippocampal neurons, we show that Spatial also exhibits a somatodendritic distribution and it is concentrated in some synaptic regions. Moreover, the vesicle-like cellular distribution of Spatial protein in dendrites is similar to that described for the kinesin motor protein KIF17. Immunofluorescence analyses show that Spatial colocalizes with KIF17 in dendrites of hippocampal neurons in primary culture. Additionally, coimmunoprecipitation experiments of endogenous proteins from hippocampus confirmed that Spatial and KIF17 physically interact. These findings suggest that Spatial may play a role in neuronal morphogenesis and synaptic plasticity through its interaction with the kinesin motor KIF17 in dendrites.

ABCA2 is a marker of neural progenitors and neuronal subsets in the adult rodent brain.

The notion that the ATP-binding cassette transporter-A2 (ABCA2) may be involved in brain sterol homeostasis and is associated with early onset Alzheimer's disease led us to explore its neural expression. Our data support and extend the previous reports on ABCA2 expression by oligodendrocytes. They evidence that ABCA2 (i) is located in intracellular vesicles, identified in transfected cells as lysosome-related organelles only partially overlapping with classical endolysosomes; (ii) is a marker of neural progenitors as it is expressed in the subventricular zone of the lateral ventricle and the dentate gyrus of the hippocampal formation, sites of continual neurogenesis in the adult brain, and in nestin(+) cells differentiated in vitro from embryonic stem cells; (iii) persists, in the adult rodent brain, in a subset of GABAergic and glutamatergic neurons. Considering that the latter are targets of Alzheimer's lesions, these data provide a new rationale to explore the neuropathological consequences of ABCA2 functional dysregulations.

mCD24 regulates proliferation of neuronal committed precursors in the subventricular zone.

We previously showed that deletion of the cell surface molecule mCD24 resulted in an increased proliferation in adult subventricular zone (SVZ). Here, we report an increased PSA-NCAM+/TuJ1- population in the mCD24-/- in vivo SVZ as well as in vitro neurospheres. Isolated in vitro, these cells were able to generate neurospheres. Proliferation studies, using BrdU incorporation, showed an increased proliferation in P7 mCD24-/- SVZ and neurospheres. Using electron microscopy, the same cell types were identified in the in vivo SVZ as well as in vitro neurospheres from the WT and mCD24-/- mice. In mixed neurospheres, formed with WT and EGFP/KO cells (enhanced green fluorescent protein mCD24-/-), the WT environment was able to control the proliferation rate of the mCD24-/- cells, but was unable to regulate their differentiation. We concluded that mCD24 acts cell nonautonomously to regulate transit-amplifying cells proliferation and/or differentiation.