Glutamate receptor-mediated taurine release from the hippocampus during oxidative stress

Tweety-homolog (Ttyh) Family Encodes the Pore-forming Subunits of the Swelling-dependent Volume-regulated Anion Channel (VRACswell) in the Brain

In the brain, a reduction in extracellular osmolality causes water-influx and swelling, which subsequently triggers Cl^-- and osmolytes-efflux via volume-regulated anion channel (VRAC). Although LRRC8 family has been recently proposed as the pore-forming VRAC which is activated by low cytoplasmic ionic strength but not by swelling, the molecular identity of the pore-forming swelling-dependent VRAC (VRAC_swell) remains unclear. Here we identify and characterize Tweety-homologs (TTYH1, TTYH2, TTYH3) as the major VRAC_swell in astrocytes. Gene-silencing of all Ttyh1/2/3 eliminated hypo-osmotic-solution-induced Cl^- conductance (I_Cl,swell) in cultured and hippocampal astrocytes. When heterologously expressed in HEK293T or CHO-K1 cells, each TTYH isoform showed a significant I_Cl,swell with similar aquaporin-4 dependency, pharmacological properties and glutamate permeability as I_Cl,swell observed in native astrocytes. Mutagenesis-based structure-activity analysis revealed that positively charged arginine residue at 165 in TTYH1 and 164 in TTYH2 is critical for the formation of the channel-pore. Our results demonstrate that TTYH family confers the bona fide VRAC_swell in the brain.

Understanding taurine CNS activity using alternative zebrafish models

Taurine is a highly abundant "amino acid" in the brain. Despite the potential neuroactive role of taurine in vertebrates has long been recognized, the underlying molecular mechanisms related to its pleiotropic effects in the brain remain poorly understood. Due to the genetic tractability, rich behavioral repertoire, neurochemical conservation, and small size, the zebrafish (Danio rerio) has emerged as a powerful candidate for neuropsychopharmacology investigation and in vivo drug screening. Here, we summarize the main physiological roles of taurine in mammals, including neuromodulation, osmoregulation, membrane stabilization, and antioxidant action. In this context, we also highlight how zebrafish models of brain disorders may present interesting approaches to assess molecular mechanisms underlying positive effects of taurine in the brain. Finally, we outline recent advances in zebrafish drug screening that significantly improve neuropsychiatric translational researches and small molecule screens.

Circulating Glutamate and Taurine Levels Are Associated with the Generation of Reactive Oxygen Species in Paroxysmal Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, but its proarrhythmic mechanism remains to be elucidated. Glutamate (Glu) and taurine (Tau) are present in the myocardium at substantially higher concentrations than in the plasma, suggesting their active role in myocardium. Here, we tested the hypothesis that the metabolism of Glu and Tau is altered in association with the generation of reactive oxygen species (ROS) in patients with AF. Fifty patients with paroxysmal AF and 50 control subjects without a history of AF were consecutively enrolled. Circulating Glu and Tau levels were measured and correlations between Glu/Tau and ROS levels were examined. Glu/Tau content was significantly higher in patients with AF versus controls (Glu: 79.2 ± 23.9 versus 60.5 ± 25.2 nmol/L; Tau: 78.8 ± 19.8 versus 68.5 ± 20.8 nmol/L; mean ± standard deviation (SD), p < 0.001 for both). Glu/Tau levels also showed an independent association with AF by multiple logistic regression analysis. Glu and Tau levels both showed significant positive associations with plasma hydroperoxide concentrations. These data suggest a novel pathophysiological role of Glu and Tau in association with ROS production in paroxysmal AF, providing new insights into the elevated amino acid content in cardiac disease.

Activation of the ACE2/Ang-(1-7)/Mas pathway reduces oxygen-glucose deprivation-induced tissue swelling, ROS production, and cell death in mouse brain with angiotensin II overproduction

We previously demonstrated that mice which overexpress human renin and angiotensinogen (R+A+) show enhanced cerebral damage in both in vivo and in vitro experimental ischemia models. Angiotensin-converting enzyme 2 (ACE2) counteracts the effects of angiotensin (Ang-II) by transforming it into Ang-(1-7), thus reducing the ligand for the AT1 receptor and increasing stimulation of the Mas receptor. Triple transgenic mice, SARA, which specifically overexpress ACE2 in neurons of R+A+ mice were used to study the role of ACE2 in ischemic stroke using oxygen and glucose deprivation (OGD) of brain slices as an in vitro model. We examined tissue swelling, the production of reactive oxygen species (ROS), and cell death in the cerebral cortex (CX) and the hippocampal CA1 region during OGD. Expression levels of NADPH oxidase (Nox) isoforms, Nox2 and Nox4 were measured using western blots. Results show that SARA mice and R+A+ mice treated with the Mas receptor agonist Ang-(1-7) had less swelling, cell death, and ROS production in CX and CA1 areas compared to those in R+A+ animals. Treatment of slices from SARA mice with the Mas antagonist A779 eliminated this protection. Finally, western blots revealed less Nox2 and Nox4 expression in SARA mice compared with R+A+ mice both before and after OGD. We suggest that reduced brain swelling and cell death observed in SARA animals exposed to OGD result from diminished ROS production coupled with lower expression of Nox isoforms. Thus, the ACE2/Ang-(1-7)/Mas receptor pathway plays a protective role in brain ischemic damage by counteracting the detrimental effects of Ang-II-induced ROS production.

Repeated exposure of adult rats to Aroclor 1254 induces neuronal injury and impairs the neurochemical manifestations of the NMDA receptor-mediated intracellular signaling in the hippocampus

Aroclor 1254 is a mixture of polychlorinated biphenyls (PCBs), a class of environmental toxins which cause a wide spectrum of neurotoxic effects. Learning and memory deficits are the profound effects of PCBs which may be related to hippocampal dysfunction. To get insight into the underlying neurochemical mechanisms, we employed the microdialysis technique to investigate the effect of repeated exposure of adult male Wistar rats to Aroclor 1254 (10mg/kg b.w., daily, ig., for 14days), on the neurochemical parameters of NMDA receptor-mediated glutamatergic signaling in the hippocampus in vivo assessed using the microdialysis technique. The results demonstrated that exposure to Aroclor 1254, which was associated with substantial neuronal damage and loss in the hippocampus, markedly decreased the NMDA-induced extracellular accumulation of newly loaded (45)CaCl(2), cGMP and glutamate, and reduced the basal content of the NO precursor, arginine, indicating inhibition of the NMDA/NO/cGMP pathway. Aroclor 1254 exposure also decreased the basal microdialysate content of glutamate and glutamine, which may cause inadequate supply of the neurotransmitter glutamate, while the level of two other neuroactive amino acids, aspartate or taurine was not affected by the exposure. The results underscore neuronal lesion and inhibition of NMDA receptor-mediated glutamatergic signaling in hippocampus as a potential major contributor to the cognitive deficits associated with exposure to PCB.

GABA(A) receptor and glycine receptor activation by paracrine/autocrine release of endogenous agonists: more than a simple communication pathway

It is a common and widely accepted assumption that glycine and GABA are the main inhibitory transmitters in the central nervous system (CNS). But, in the past 20 years, several studies have clearly demonstrated that these amino acids can also be excitatory in the immature central nervous system. In addition, it is now established that both GABA receptors (GABARs) and glycine receptors (GlyRs) can be located extrasynaptically and can be activated by paracrine release of endogenous agonists, such as GABA, glycine, and taurine. Recently, non-synaptic release of GABA, glycine, and taurine gained further attention with increasing evidence suggesting a developmental role of these neurotransmitters in neuronal network formation before and during synaptogenesis. This review summarizes recent knowledge about the non-synaptic activation of GABA(A)Rs and GlyRs, both in developing and adult CNS. We first present studies that reveal the functional specialization of both non-synaptic GABA(A)Rs and GlyRs and we discuss the neuronal versus non-neuronal origin of the paracrine release of GABA(A)R and GlyR agonists. We then discuss the proposed non-synaptic release mechanisms and/or pathways for GABA, glycine, and taurine. Finally, we summarize recent data about the various roles of non-synaptic GABAergic and glycinergic systems during the development of neuronal networks and in the adult.