Guojun Chen

Regulation of synaptic inhibition by phospho-dependent binding of the AP2 complex to a YECL motif in the GABAA receptor γ2 subunit

The regulation of the number of γ2-subunit-containing GABAA receptors (GABAARs) present at synapses is critical for correct synaptic inhibition and animal behavior. This regulation occurs, in part, by the controlled removal of receptors from the membrane in clathrin-coated vesicles, but it remains unclear how clathrin recruitment to surface γ2-subunit-containing GABAARs is regulated. Here, we identify a γ2-subunit-specific Yxxφ-type-binding motif for the clathrin adaptor protein, AP2, which is located within a site for γ2-subunit tyrosine phosphorylation. Blocking GABAAR-AP2 interactions via this motif increases synaptic responses within minutes. Crystallographic and biochemical studies reveal that phosphorylation of the Yxxφ motif inhibits AP2 binding, leading to increased surface receptor number. In addition, the crystal structure provides an explanation for the high affinity of this motif for AP2 and suggests that γ2-subunit-containing heteromeric GABAARs may be internalized as dimers or multimers. These data define a mechanism for tyrosine kinase regulation of GABAAR surface levels and synaptic inhibition.

Dopamine D3 Receptors Regulate GABAA Receptor Function through a Phospho-Dependent Endocytosis Mechanism in Nucleus Accumbens

The dopamine D3 receptor, which is highly enriched in nucleus accumbens (NAc), has been suggested to play an important role in reinforcement and reward. To understand the potential cellular mechanism underlying D3 receptor functions, we examined the effect of D3 receptor activation on GABAA receptor (GABAAR)-mediated current and inhibitory synaptic transmission in medium spiny neurons of NAc. Application of PD128907 [(4aR,10bR)-3,4a,4,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride], a specific D3 receptor agonist, caused a significant reduction of GABAAR current in acutely dissociated NAc neurons and miniature IPSC amplitude in NAc slices. This effect was blocked by dialysis with a dynamin inhibitory peptide, which prevents the clathrin/activator protein 2 (AP2)-mediated GABAA receptor endocytosis. In addition, the D3 effect on GABAAR current was prevented by agents that manipulate protein kinase A (PKA) activity. Infusion of a peptide derived from GABAAR β subunits, which contains an atypical binding motif for the clathrin AP2 adaptor complex and the major PKA phosphorylation sites and binds with high affinity to AP2 only when dephosphorylated, diminished the D3 regulation of IPSC amplitude. The phosphorylated equivalent of the peptide was without effect. Moreover, PD128907 increased GABAAR internalization and reduced the surface expression of GABAA receptor β subunits in NAc slices, which was prevented by dynamin inhibitory peptide or cAMP treatment. Together, our results suggest that D3 receptor activation suppresses the efficacy of inhibitory synaptic transmission in NAc by increasing the phospho-dependent endocytosis of GABAA receptors.

Molecular determinants for the interaction between AMPA receptors and the clathrin adaptor complex AP-2

α-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors undergo constitutive and ligand-induced internalization that requires dynamin and the clathrin adaptor complex AP-2. We report here that an atypical basic motif within the cytoplasmic tails of AMPA-type glutamate receptors directly associates with μ2-adaptin by a mechanism similar to the recognition of the presynaptic vesicle protein synaptotagmin 1 by AP-2. A synaptotagmin 1-derived AP-2 binding peptide competes the interaction of the AMPA receptor subunit GluR2 with AP-2μ and increases the number of surface active glutamate receptors in living neurons. Moreover, fusion of the GluR2-derived tail peptide with a synaptotagmin 1 truncation mutant restores clathrin/AP-2-dependent internalization of the chimeric reporter protein. These data suggest that common mechanisms regulate AP-2-dependent internalization of pre- and postsynaptic membrane proteins.

Increased Expression of DNA methyltransferase 1 and 3a in Human Temporal Lobe Epilepsy

DNA methylation is a key epigenetic modification of DNA that is catalyzed by DNA methyltransferase (DNMT). Increasing evidence suggests that DNA methylation in neurons regulates synaptic plasticity as well as neuronal network activity. Here, we evaluated DNA methyltransferase 1 (Dnmt1) and Dnmt3a expression in brain tissues of epileptic patients to explore their possible role in epileptogenesis. Tissue samples from temporal neocortices of 25 patients with intractable temporal lobe epilepsy (TLE) and ten histologically normal temporal lobes from control patients were used to detect Dnmt1 and Dnmt3a expression through immunohistochemistry, immunofluorescence, and Western blotting analysis. We found that both Dnmt1 and Dnmt3a expression were principally expressed in the nucleus and the cytoplasm of NeuN-positive neurons, but not in GFAP-positive astrocytes. Levels of the two DNMT proteins were significantly increased in patients with TLE. Our study suggests that DNMT1 and DNMT3a may play a role in the pathogenesis of TLE.

Oestrogen upregulates L‐type Ca2+ channels via oestrogen‐receptor‐α by a regional genomic mechanism in female rabbit hearts

Non‐technical summary  Women during their child‐bearing years have longer QT intervals in their electrocardiograms than men and are more susceptible to lethal arrhythmias elicited by drugs that delay repolarization. Current theories posit that women have a reduced ‘repolarization reserve’ due to reduced potassium currents resulting in longer QT and greater repolarization delays. We proposed an alternative mechanism of  higher calcium currents in women which would likewise prolong QT intervals, delay repolarization while increasing the force of contractions and intracellular calcium load. Here, we show that physiological concentrations of oestrogen increase the calcium current only in cells from the base of the heart, by increasing messenger RNA and proteins levels that encode for the calcium current. Moreover, oestrogen acts by interacting with oestrogen receptors (ER)α but not ERβ which may explain why hormone replacement therapy increases the risk of arrhythmia and offers a possible protective solution of using an oestrogen mimetic that selectively binds to ERβ.

Altered Expression of CX3CL1 in Patients with Epilepsy and in a Rat Model

Chemokine C-X3-C motif ligand 1 (CX3CL1, alias fractalkine), is highly expressed in the central nervous system and participates in inflammatory responses. Recent studies indicated that inflammatory processes within the brain constitute a common and crucial mechanism in the pathophysiological characteristics of epilepsy. This study investigated the expression pattern of CX3CL1 in epilepsy and its relationship with neuronal loss. Double immunolabeling, IHC, and immunoblotting results showed that CX3CL1 expression was up-regulated in the temporal neocortex of patients with temporal lobe epilepsy. In a rat model of epilepsy, CX3CL1 up-regulation began 6 hours after epilepsy, with relatively high expression for 60 days. In addition, ELISA revealed that the concentrations of CX3CL1 in cerebrospinal fluid and serum were higher in epileptic patients than in patients with neurosis but lower than in patients with inflammatory neurological diseases. Moreover, H&E staining demonstrated significant neuronal loss in the brains of epileptic patients and in the rat model. Finally, the expression of tumor necrosis factor-related apoptosis-inducing ligand was significantly increased in both patients and the animal model, suggesting that tumor necrosis factor-related apoptosis-inducing ligand may play a role in CX3CL1-induced cell death. Thus, our results indicate that CX3CL1 may serve as a possible biomarker of brain inflammation in epileptic patients.

Neuronal thread protein regulation and interaction with microtubule-associated proteins in SH-Sy5y neuronal cells

In Alzheimer’s disease (AD), neuronal thread protein (NTP) accumulates in cortical neurons and colocalizes with phospho-tau-immunoreactive cytoskeletal lesions that correlate with dementia. To generate additional information about the potential role of NTP in AD, we characterized its expression and regulation in human SH-Sy5y neuronal cells. Quantitative real-time reverse transcription-polymerase chain reactin and Western blot analysis demonstrated prominent insulin, moderate insulin-like growth factor, type 1 (IGF-1) and minimal nerve growth factor stimulation of NTP expression. In addition, NTP protein was more stable and it progressively accumulated in cells that were stimulated with insulin for 24 or 48 h. Metabolic labeling and phospho-amino acid analysis demonstrated phosphorylation of NTP on Serine residues, 30–60 min after insulin or IGF-1 stimulation, when glycogen synthase kinase 3β (GSK-3β) activity would no longer have been suppressed. Kinase inhibitor and in vitro phosphorylation studies demonstrated a role for GSK-3β in the positive regulation of NTP expression and phosphorylation. Coimmunoprecipitation studies demonstrated physical interactions between NTP and tau or microtubule-associated protein 1b (MAP-1b), and ubiquitin immunoreactivity in NTP immunoprecipitates. In summary, these studies showed that (i) NTP expression is regulated at the level of transcription by insulin and IGF-1 stimulation; (ii) NTP is phosphorylated by GSK-3β; (iii) NTP can physically interact with tau and MAP-1b and (iv) NTP-MAP complexes are ubiquitinated. The results suggest a functional role for NTP in relation to the turnover or processing of neuronal cytoskeletal proteins, attributes that may be modulated by insulin/IGF-1-mediated signaling.

Association of Mitochondrial Letm1 with Epileptic Seizures

Leucine zipper-EF-hand containing transmembrane protein 1 (Letm1) is a mitochondrial protein that is associated with seizure attacks in Wolf-Hirschhorn syndrome. This study aimed to investigate the expression pattern of Letm1 in patients with temporal lobe epilepsy (TLE) and pilocarpine-induced rat model of epilepsy, and to determine if altered Letm1 leads to mitochondrial dysfunction and increased susceptibility to seizures. Using immunohistochemical, immunofluorescent, western blotting, and transmission electron microscopic methods, we have found that Letm1 was significantly decreased in TLE patients, and gradually decreased in experimental rats from 1 to 7 days after onset of seizures. Letm1 knock-down by a lentivirus bearing LV-Letm1-sh resulted in mitochondrial swelling and decreased expression of Letm1 target protein mitochondrially encoded cytochrome B (MT-CYB). Behavioral study revealed that inhibition of Letm1 caused early onset of the first seizure, increased seizure frequency, and duration. However, administration of Letm1 homolog nigericin failed to prevent epilepsy. These results indicate that inhibition of Letm1 and mitochondrial dysfunctions contributes to the development of epileptic seizures. Appropriate Letm1 level may be critical for maintaining normal neuronal functions.

Regional genomic regulation of cardiac sodium–calcium exchanger by oestrogen

Non‐technical summary  Women of child bearing age are known to have longer QT intervals on their electrocardiogram recordings due to sex‐differences in the heart that are mediated by the sex hormone oestrogen. This results in a higher risk of lethal arrhythmias in women when their QT interval is prolonged further through drugs or inherited congenital diseases (i.e. long QT syndrome). Using rabbit hearts, which exhibit the same sex‐differences as in humans, it is here shown for the first time that oestrogen up‐regulates the sodium–calcium exchanger (NCX), a critical protein that regulates calcium ions in the heart and thereby alters the force of contraction, but only at the base of the ventricles (where the heart has its widest cross section). This effect accounts for the greater risk of long QT‐related arrhythmias in women.

Methylphenidate Exerts Dose-Dependent Effects on Glutamate Receptors and Behaviors

BACKGROUND Methylphenidate (MPH), a psychostimulant drug used to treat attention-deficit/hyperactivity disorder, produces the effects of increasing alertness and improving attention. However, misuse of MPH has been associated with an increased risk of aggression and psychosis. We sought to determine the molecular mechanism underlying the complex actions of MPH. METHODS Adolescent (4-week-old) rats were given one injection of MPH at different doses. The impact of MPH on glutamatergic signaling in pyramidal neurons of prefrontal cortex was measured. Behavioral changes induced by MPH were also examined in parallel. RESULTS Administration of low-dose (.5 mg/kg) MPH selectively potentiated N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory postsynaptic currents (EPSCs) via adrenergic receptor activation, whereas high-dose (10 mg/kg) MPH suppressed both NMDAR-mediated and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor-mediated EPSCs. The dual effects of MPH on EPSCs were associated with bidirectional changes in the surface level of glutamate receptor subunits. Behavioral tests also indicated that low-dose MPH facilitated prefrontal cortex-mediated temporal order recognition memory and attention. Animals injected with high-dose MPH exhibited significantly elevated locomotive activity. Inhibiting the function of synaptosomal-associated protein 25, a key SNARE protein involved in NMDAR exocytosis, blocked the increase of NMDAR-mediated EPSCs by low-dose MPH. In animals exposed to repeated stress, administration of low-dose MPH effectively restored NMDAR function and temporal order recognition memory via a mechanism dependent on synaptosomal-associated protein 25. CONCLUSIONS These results provide a potential mechanism underlying the cognitive-enhancing effects of low-dose MPH as well as the psychosis-inducing effects of high-dose MPH.