Sleep and wake cycles dynamically modulate hippocampal inhibitory synaptic plasticity.

Sleep is an essential process that consolidates memories by modulating synapses through poorly understood mechanisms. Here, we report that GABAergic synapses in hippocampal CA1 pyramidal neurons undergo daily rhythmic alterations. Specifically, wake inhibits phasic inhibition, whereas it promotes tonic inhibition compared to sleep. We further utilize a model of chemically induced inhibitory long-term potentiation (iLTP) to examine inhibitory plasticity. Intriguingly, while CA1 pyramidal neurons in both wake and sleep mice undergo iLTP, wake mice have a much higher magnitude. We also employ optogenetics and observe that inhibitory inputs from parvalbumin-, but not somatostatin-, expressing interneurons contribute to dynamic iLTP during sleep and wake. Finally, we demonstrate that synaptic insertion of α5-GABAA receptors underlies the wake-specific enhancement of iLTP at parvalbumin-synapses, which is independent of time of the day. These data reveal a previously unappreciated daily oscillation of inhibitory LTP in hippocampal neurons and uncover a dynamic contribution of inhibitory synapses in memory mechanisms across sleep and wake.

Shisa7-Dependent Regulation of GABAA Receptor Single-Channel Gating Kinetics.

GABAA receptors (GABAARs) mediate the majority of fast inhibitory transmission throughout the brain. Although it is widely known that pore-forming subunits critically determine receptor function, it is unclear whether their single-channel properties are modulated by GABAAR-associated transmembrane proteins. We previously identified Shisa7 as a GABAAR auxiliary subunit that modulates the trafficking, pharmacology, and deactivation properties of these receptors. However, whether Shisa7 also regulates GABAAR single-channel properties has yet to be determined. Here, we performed single-channel recordings of α2ß3γ2L GABAARs cotransfected with Shisa7 in HEK293T cells and found that while Shisa7 does not change channel slope conductance, it reduced the frequency of receptor openings. Importantly, Shisa7 modulates GABAAR gating by decreasing the duration and open probability within bursts. Through kinetic analysis of individual dwell time components, activation modeling, and macroscopic simulations, we demonstrate that Shisa7 accelerates GABAAR deactivation by governing the time spent between close and open states during gating. Together, our data provide a mechanistic basis for how Shisa7 controls GABAAR gating and reveal for the first time that GABAAR single-channel properties can be modulated by an auxiliary subunit. These findings shed light on processes that shape the temporal dynamics of GABAergic transmission.SIGNIFICANCE STATEMENT Although GABAA receptor (GABAAR) single-channel properties are largely determined by pore-forming subunits, it remains unknown whether they are also controlled by GABAAR-associated transmembrane proteins. Here, we show that Shisa7, a recently identified GABAAR auxiliary subunit, modulates GABAAR activation by altering single-channel burst kinetics. These results reveal that Shisa7 primarily decreases the duration and open probability of receptor burst activity during gating, leading to accelerated GABAAR deactivation. These experiments are the first to assess the gating properties of GABAARs in the presence of an auxiliary subunit and provides a kinetic basis for how Shisa7 modifies temporal attributes of GABAergic transmission at the single-channel level.

Vascular endothelial growth factor increases the function of calcium-impermeable AMPA receptor GluA2 subunit in astrocytes via activation of protein kinase C signaling pathway.

Astrocytic calcium signaling plays pivotal roles in the maintenance of neural functions and neurovascular coupling in the brain. Vascular endothelial growth factor (VEGF), an original biological substance of vessels, regulates the movement of calcium and potassium ions across neuronal membrane. In this study, we investigated whether and how VEGF regulates glutamate-induced calcium influx in astrocytes. We used cultured astrocytes combined with living cell imaging to detect the calcium influx induced by glutamate. We found that VEGF quickly inhibited the glutamate/hypoxia-induced calcium influx, which was blocked by an AMPA receptor antagonist CNQX, but not D-AP5 or UBP310, NMDA and kainate receptor antagonist, respectively. VEGF increased phosphorylation of PKCα and AMPA receptor subunit GluA2 in astrocytes, and these effects were diminished by SU1498 or calphostin C, a PKC inhibitor. With the pHluorin assay, we observed that VEGF significantly increased membrane insertion and expression of GluA2, but not GluA1, in astrocytes. Moreover, siRNA-produced knockdown of GluA2 expression in astrocytes reversed the inhibitory effect of VEGF on glutamate-induced calcium influx. Together, our results suggest that VEGF reduces glutamate-induced calcium influx in astrocytes via enhancing PKCα-mediated GluA2 phosphorylation, which in turn promotes the membrane insertion and expression of GluA2 and causes AMPA receptors to switch from calcium-permeable to calcium-impermeable receptors, thereby inhibiting astrocytic calcium influx. The present study reveals that excitatory neurotransmitter glutamate-mediated astrocytic calcium influx can be regulated by vascular biological factor via activation of AMPA receptor GluA2 subunit and uncovers a novel coupling mechanism between astrocytes and endothelial cells within the neurovascular unit.

Endothelial cells promote excitatory synaptogenesis and improve ischemia-induced motor deficits in neonatal mice.

Brain microvascular endothelial cells (BMEC) are highly complex regulatory cells that communicate with other cells in the neurovascular unit. Cerebral ischemic injury is known to produce detectable synaptic dysfunction. This study aims to investigate whether endothelial cells in the brain regulate postnatal synaptic development and to elucidate their role in functional recovery after ischemia. Here, we found that in vivo engraftment of endothelial cells increased synaptic puncta and excitatory postsynaptic currents in layers 2/3 of the motor cortex. This pro-synaptogenic effect was blocked by the depletion of VEGF in the grafted BMEC. The in vitro results showed that BMEC conditioned medium enhanced spine and synapse formation but conditioned medium without VEGF had no such effects. Moreover, under pathological conditions, transplanted endothelial cells were capable of enhancing angiogenesis and synaptogenesis and improved motor function in the ischemic injury model. Collectively, our findings suggest that endothelial cells promote excitatory synaptogenesis via the paracrine factor VEGF during postnatal development and exert repair functions in hypoxia-ischemic neonatal mice. This study highlights the importance of the endothelium-neuron interaction not only in regulating neuronal development but also in maintaining healthy brain function.

MicroRNA-365 modulates astrocyte conversion into neuron in adult rat brain after stroke by targeting Pax6.

Reactive astrocytes induced by ischemia can transdifferentiate into mature neurons. This neurogenic potential of astrocytes may have therapeutic value for brain injury. Epigenetic modifications are widely known to involve in developmental and adult neurogenesis. PAX6, a neurogenic fate determinant, contributes to the astrocyte-to-neuron conversion. However, it is unclear whether microRNAs (miRs) modulate PAX6-mediated astrocyte-to-neuron conversion. In the present study we used bioinformatic approaches to predict miRs potentially targeting Pax6, and transient middle cerebral artery occlusion (MCAO) to model cerebral ischemic injury in adult rats. These rats were given striatal injection of glial fibrillary acidic protein targeted enhanced green fluorescence protein lentiviral vectors (Lv-GFAP-EGFP) to permit cell fate mapping for tracing astrocytes-derived neurons. We verified that miR-365 directly targets to the 3'-UTR of Pax6 by luciferase assay. We found that miR-365 expression was significantly increased in the ischemic brain. Intraventricular injection of miR-365 antagomir effectively increased astrocytic PAX6 expression and the number of new mature neurons derived from astrocytes in the ischemic striatum, and reduced neurological deficits as well as cerebral infarct volume. Conversely, miR-365 agomir reduced PAX6 expression and neurogenesis, and worsened brain injury. Moreover, exogenous overexpression of PAX6 enhanced the astrocyte-to-neuron conversion and abolished the effects of miR-365. Our results demonstrate that increase of miR-365 in the ischemic brain inhibits astrocyte-to-neuron conversion by targeting Pax6, whereas knockdown of miR-365 enhances PAX6-mediated neurogenesis from astrocytes and attenuates neuronal injury in the brain after ischemic stroke. Our findings provide a foundation for developing novel therapeutic strategies for brain injury.

Shisa7 phosphorylation regulates GABAergic transmission and neurodevelopmental behaviors.

GABA-A receptors (GABAARs) are crucial for development and function of the brain. Altered GABAergic transmission is hypothesized to be involved in neurodevelopmental disorders. Recently, we identified Shisa7 as a GABAAR auxiliary subunit that modulates GABAAR trafficking and GABAergic transmission. However, the underlying molecular mechanisms remain elusive. Here we generated a knock-in (KI) mouse line that is phospho-deficient at a phosphorylation site in Shisa7 (S405) and combined with electrophysiology, imaging and behavioral assays to illustrate the role of this site in GABAergic transmission and plasticity as well as behaviors. We found that expression of phospho-deficient mutants diminished α2-GABAAR trafficking in heterologous cells. Additionally, α1/α2/α5-GABAAR surface expression and GABAergic inhibition were decreased in hippocampal neurons in KI mice. Moreover, chemically induced inhibitory long-term potentiation was abolished in KI mice. Lastly, KI mice exhibited hyperactivity, increased grooming and impaired sleep homeostasis. Collectively, our study reveals a phosphorylation site critical for Shisa7-dependent GABAARs trafficking which contributes to behavioral endophenotypes displayed in neurodevelopmental disorders.

Distinct regulation of tonic GABAergic inhibition by NMDA receptor subtypes.

Tonic inhibition mediated by extrasynaptic GABAARs regulates various brain functions. However, the mechanisms that regulate tonic inhibition remain largely unclear. Here, we report distinct actions of GluN2A- and GluN2B-NMDA receptors (NMDARs) on tonic inhibition in hippocampal neurons under basal and high activity conditions. Specifically, overexpression of GluN2B, but not GluN2A, reduces α5-GABAAR surface expression and tonic currents. Additionally, knockout of GluN2A and GluN2B decreases and increases tonic currents, respectively. Mechanistically, GluN2A-NMDARs inhibit and GluN2B-NMDARs promote α5-GABAAR internalization, resulting in increased and decreased surface α5-GABAAR expression, respectively. Furthermore, GluN2A-NMDARs, but not GluN2B-NMDARs, are required for homeostatic potentiation of tonic inhibition induced by prolonged increase of neuronal activity. Last, tonic inhibition decreases during acute seizures, whereas it increases 24 h later, involving GluN2-NMDAR-dependent signaling. Collectively, these data reveal an NMDAR subunit-specific regulation of tonic inhibition in physiological and pathological conditions and provide mechanistic insight into activity-dependent modulation of tonic inhibition.

Activity- and sleep-dependent regulation of tonic inhibition by Shisa7.

Tonic inhibition mediated by extrasynaptic γ-aminobutyric acid type A receptors (GABAARs) critically regulates neuronal excitability and brain function. However, the mechanisms regulating tonic inhibition remain poorly understood. Here, we report that Shisa7 is critical for tonic inhibition regulation in hippocampal neurons. In juvenile Shisa7 knockout (KO) mice, α5-GABAAR-mediated tonic currents are significantly reduced. Mechanistically, Shisa7 is crucial for α5-GABAAR exocytosis. Additionally, Shisa7 regulation of tonic inhibition requires protein kinase A (PKA) that phosphorylates Shisa7 serine 405 (S405). Importantly, tonic inhibition undergoes activity-dependent regulation, and Shisa7 is required for homeostatic potentiation of tonic inhibition. Interestingly, in young adult Shisa7 KOs, basal tonic inhibition in hippocampal neurons is unaltered, largely due to the diminished α5-GABAAR component of tonic inhibition. However, at this stage, tonic inhibition oscillates during the daily sleep/wake cycle, a process requiring Shisa7. Together, these data demonstrate that intricate signaling mechanisms regulate tonic inhibition at different developmental stages and reveal a molecular link between sleep and tonic inhibition.

An Epilepsy-Associated GRIN2A Rare Variant Disrupts CaMKIIα Phosphorylation of GluN2A and NMDA Receptor Trafficking.

Rare variants in GRIN genes, which encode NMDAR subunits, are strongly associated with neurodevelopmental disorders. Among these, GRIN2A, which encodes the GluN2A subunit of NMDARs, is widely accepted as an epilepsy-causative gene. Here, we functionally characterize the de novo GluN2A-S1459G mutation identified in an epilepsy patient. We show that S1459 is a CaMKIIα phosphorylation site, and that endogenous phosphorylation is regulated during development and in response to synaptic activity in a dark rearing model. GluN2A-S1459 phosphorylation results in preferential binding of NMDARs to SNX27 and a corresponding decrease in PSD-95 binding, which consequently regulates NMDAR trafficking. Furthermore, the epilepsy-associated GluN2A-S1459G variant displays defects in interactions with both SNX27 and PSD-95, resulting in trafficking deficits, reduced spine density, and decreased excitatory synaptic transmission. These data demonstrate a role for CaMKIIα phosphorylation of GluN2A in receptor targeting and implicate NMDAR trafficking defects as a link to epilepsy.

A Cluster of Autism-Associated Variants on X-Linked NLGN4X Functionally Resemble NLGN4Y.

Autism spectrum disorder (ASD) is more prevalent in males; however, the etiology for this sex bias is not well understood. Many mutations on X-linked cell adhesion molecule NLGN4X result in ASD or intellectual disability. NLGN4X is part of an X-Y pair, with NLGN4Y sharing ∼97% sequence homology. Using biochemistry, electrophysiology, and imaging, we show that NLGN4Y displays severe deficits in maturation, surface expression, and synaptogenesis regulated by one amino acid difference with NLGN4X. Furthermore, we identify a cluster of ASD-associated mutations surrounding the critical amino acid in NLGN4X, and these mutations phenocopy NLGN4Y. We show that NLGN4Y cannot compensate for the functional deficits observed in ASD-associated NLGN4X mutations. Altogether, our data reveal a potential pathogenic mechanism for male bias in NLGN4X-associated ASD.

A Conserved Tyrosine Residue in Slitrk3 Carboxyl-Terminus Is Critical for GABAergic Synapse Development.

Single-passing transmembrane protein, Slitrk3 (Slit and Trk-like family member 3, ST3), is a synaptic cell adhesion molecule highly expressed at inhibitory synapses. Recent studies have shown that ST3, through its extracellular domain, selectively regulates inhibitory synapse development via the trans-synaptic interaction with presynaptic cell adhesion molecule, receptor protein tyrosine phosphatase δ (PTPδ) and the cis-interaction with postsynaptic cell adhesion molecule, Neuroligin 2 (NL2). However, little is known about the physiological function of ST3 intracellular, carboxyl (C)-terminal region. Here we report that in heterologous cells, ST3 C-terminus is not required for ST3 homo-dimerization and trafficking to the cell surface. In contrast, in hippocampal neurons, ST3 C-terminus, more specifically, the conserved tyrosine Y969 (in mice), is critical for GABAergic synapse development. Indeed, overexpression of ST3 Y969A mutant markedly reduced the gephyrin puncta density and GABAergic transmission in hippocampal neurons. In addition, single-cell genetic deletion of ST3 strongly impaired GABAergic transmission. Importantly, wild-type (WT) ST3, but not the ST3 Y969A mutant, could fully rescue GABAergic transmission deficits in neurons lacking endogenous ST3, confirming a critical role of Y969 in the regulation of inhibitory synapses. Taken together, our data identify a single critical residue in ST3 C-terminus that is important for GABAergic synapse development and function.

Shisa7 is a GABAA receptor auxiliary subunit controlling benzodiazepine actions.

The function and pharmacology of γ-aminobutyric acid type A receptors (GABAARs) are of great physiological and clinical importance and have long been thought to be determined by the channel pore-forming subunits. We discovered that Shisa7, a single-passing transmembrane protein, localizes at GABAergic inhibitory synapses and interacts with GABAARs. Shisa7 controls receptor abundance at synapses and speeds up the channel deactivation kinetics. Shisa7 also potently enhances the action of diazepam, a classic benzodiazepine, on GABAARs. Genetic deletion of Shisa7 selectively impairs GABAergic transmission and diminishes the effects of diazepam in mice. Our data indicate that Shisa7 regulates GABAAR trafficking, function, and pharmacology and reveal a previously unknown molecular interaction that modulates benzodiazepine action in the brain.

VEGF Axonal Transport Dependent on Kinesin-1B and Microtubules Dynamics.

Axon-transport plays an important role in neuronal activity and survival. Reduced endogenous VEGF can cause neuronal damage and axon degeneration. It is unknown at this time if VEGF can be transported within the axon or whether it can be released by axonal depolarization. We transfected VEGF-eGFP plasmids in cultured hippocampal neurons and tracked their movement in the axons by live-cell confocal imaging. Then, we co-transfected phVEGF-eGFP and kinesin-1B-DsRed vectors into neurons and combined with immunoprecipitation and two-color imaging to study the mechanism of VEGF axon-trafficking. We found that VEGF vesicles morphologically co-localized and biochemically bounded with kinesin-1B, as well as co-trafficked with it in the axons. Moreover, the capacity for axonal trafficking of VEGF was reduced by administration of nocodazole, an inhibitor of microtubules, or kinesin-1B shRNA. In addition, we found that VEGF could release from the cultured neurons under acute depolarizing stimulation with potassium chloride. Therefore, present findings suggest that neuronal VEGF is stored in the vesicles, actively released, and transported in the axons, which depends on the presence of kinesin-1B and functional microtubules. These results further help us to understand the importance of neuronal VEGF in the maintenance of neuronal activity and survival throughout life.

Neurovascular Interaction Promotes the Morphological and Functional Maturation of Cortical Neurons.

Brain microvascular endothelial cells (BMEC) have been found to guide the migration, promote the survival and regulate the differentiation of neural cells. However, whether BMEC promote development and maturation of immature neurons is still unknown. Therefore, in this study, we used a direct endothelium-neuron co-culture system combined with patch clamp recordings and confocal imaging analysis, to investigate the effects of endothelial cells on neuronal morphology and function during development. We found that endothelial cells co-culture or BMEC-conditioned medium (B-CM) promoted neurite outgrowth and spine formation, accelerated electrophysiological development and enhanced synapse function. Moreover, B-CM treatment induced vascular endothelial growth factor (VEGF) expression and p38 phosphorylation in the cortical neurons. Through pharmacological analysis, we found that incubation with SU1498, an inhibitor of VEGF receptor, abolished B-CM-induced p-p38 upregulation and suppressed the enhancement of synapse formation and transmission. SB203580, an inhibitor of p38 MAPK also blocked B-CM-mediated synaptic regulation. Together these results clearly reveal that the endothelium-neuron interactions promote morphological and functional maturation of neurons. In addition, neurovascular interaction-mediated promotion of neural network maturation relies on activation of VEGF/Flk-1/p38 MAPK signaling. This study provides novel aspects of endothelium-neuron interactions and novel mechanism of neurovascular crosstalk.

Neurovascular coupling protects neurons against hypoxic injury via inhibition of potassium currents by generation of nitric oxide in direct neuron and endothelium cocultures.

This study examined the effect of neuron-endothelial coupling on the survival of neurons after ischemia and the possible mechanism underlying that effect. Whole-cell patch-clamp experiments were performed on cortical neurons cultured alone or directly cocultured with brain microvascular endothelial cells (BMEC). Propidium iodide (PI) and NeuN staining were performed to examine neuronal death following oxygen and glucose deprivation (OGD). We found that the neuronal transient outward potassium currents (IA) decreased in the coculture system, whereas the outward delayed-rectifier potassium currents (IK) did not. Sodium nitroprusside, a NO donor, enhanced BMEC-induced IA inhibition and nitro-l-arginine methylester, a NOS inhibitor, partially prevented this inhibition. Moreover, the neurons directly cocultured with BMEC showed more resistance to OGD-induced injury compared with the neurons cultured alone, and that neuroprotective effect was abolished by treatment with NS5806, an activator of the IA. These results indicate that vascular endothelial cells assist neurons to prevent hypoxic injury via inhibiting neuronal IA by production of NO in the direct neuron-BMEC coculture system. These results further provide direct evidence of functional coupling between neurons and vascular endothelial cells. This study clearly demonstrates that vascular endothelial cells play beneficial roles in the pathophysiological processes of neurons after hypoxic injury, suggesting that the improvement of neurovascular coupling or functional remodeling may become an important therapeutic target for preventing brain injury.

Aminotriazole alleviates acetaminophen poisoning via downregulating P450 2E1 and suppressing inflammation.

Aminotriazole (ATZ) is commonly used as a catalase (CAT) inhibitor. We previously found ATZ attenuated oxidative liver injury, but the underlying mechanisms remain unknown. Acetaminophen (APAP) overdose frequently induces life-threatening oxidative hepatitis. In the present study, the potential hepatoprotective effects of ATZ on oxidative liver injury and the underlying mechanisms were further investigated in a mouse model with APAP poisoning. The experimental data indicated that pretreatment with ATZ dose- and time-dependently suppressed the elevation of plasma aminotransferases in APAP exposed mice, these effects were accompanied with alleviated histological abnormality and improved survival rate of APAP-challenged mice. In mice exposed to APAP, ATZ pretreatment decreased the CAT activities, hydrogen peroxide (H2O2) levels, malondialdehyde (MDA) contents, myeloperoxidase (MPO) levels in liver and reduced TNF-α levels in plasma. Pretreatment with ATZ also downregulated APAP-induced cytochrome P450 2E1 (CYP2E1) expression and JNK phosphorylation. In addition, posttreatment with ATZ after APAP challenge decreased the levels of plasma aminotransferases and increased the survival rate of experimental animals. Posttreatment with ATZ had no effects on CYP2E1 expression or JNK phosphorylation, but it significantly decreased the levels of plasma TNF-α. Our data indicated that the LD50 of ATZ in mice was 5367.4 mg/kg body weight, which is much higher than the therapeutic dose of ATZ in the present study. These data suggested that ATZ might be effective and safe in protect mice against APAP-induced hepatotoxicity, the beneficial effects might resulted from downregulation of CYP2E1 and inhibiton of inflammation.

Involvement of catalase in the protective benefits of metformin in mice with oxidative liver injury.

Metformin is a commonly used anti-diabetic drug with AMP-activated protein kinase (AMPK)-dependent hypoglycemic activities. Recent studies have revealed its anti-inflammatory and anti-oxidative properties. In the present study, the anti-oxidative potential of metformin and its potential mechanisms were investigated in a mouse model with carbon tetrachloride (CCl2)-induced severe oxidative liver injury. Our results showed that treatment with metformin significantly attenuated CCl2-induced elevation of serum aminotransferases and hepatic histological abnormalities. The alleviated liver injury was associated with decreased hepatic contents of oxidized glutathione (GSSG) and malondialdehyde (MDA). In addition, metformin treatment dose-dependently enhanced the activities of catalase (CAT) and decreased CCl4-induced elevation of hepatic H2O2 levels, but it had no obvious effects on the protein level of CAT. We also found that metformin increased the level of phosphorylated AMP-activated protein kinase (AMPK), but treatment with AMPK activator AICAR had no obvious effects on CAT activity. A molecular docking analysis indicated that metformin might interact with CAT via hydrogen bonds. These data suggested that metformin effectively alleviated CCl4-induced oxidative liver injury in mice and these hepatoprotective effects might be associated with CAT.

Heterologous expression of a Nelumbo nucifera phytochelatin synthase gene enhances cadmium tolerance in Arabidopsis thaliana.

Phytochelatin synthase (PCS) is a key enzyme involved in the synthesis of phytochelatins, which are thought to play important roles in heavy metal detoxification. The sacred lotus (Nelumbo nucifera), one of the most popular ornamental species, has been shown to be a potential phytoremediator of heavy metal polluted water. However, the phytochelatin synthase gene in N. nucifera has not been identified yet. Here, we report the isolation and function characterization of a N. nucifera homologue of phytochelatin synthase. The sequence obtained shares a high degree of similarity with PCSs from other plant species and was named as Nelumbo nucifera phytochelatin synthase1 (NnPCS1). By using quantitative RT-PCR, we found that the expression of NnPCS1 in leaves of N. nucifera was dramatically increased in response to Cadmium (Cd) treatment. We further showed that, when exposed to Cd stress, Arabidopsis transgenic plants heterologous expressing NnPCS1 accumulated more Cd when compared with wild type. These results suggest that NnPCS1 involved in the response of N. nucifera to Cd stress and may represent a useful target gene for the phytoremediation of Cd-polluted water.