Taurine analog modulation of taurine uptake by two different mechanisms in cultured glial cells

Mechanism for modulation of gating of connexin26-containing channels by taurine

The mechanisms of action of endogenous modulatory ligands of connexin channels are largely unknown. Previous work showed that protonated aminosulfonates (AS), notably taurine, directly and reversibly inhibit homomeric and heteromeric channels that contain Cx26, a widely distributed connexin, but not homomeric Cx32 channels. The present study investigated the molecular mechanisms of connexin channel modulation by taurine, using hemichannels and junctional channels composed of Cx26 (homomeric) and Cx26/Cx32 (heteromeric). The addition of a 28-amino acid "tag" to the carboxyl-terminal domain (CT) of Cx26 (Cx26(T)) eliminated taurine sensitivity of homomeric and heteromeric hemichannels in cells and liposomes. Cleavage of all but four residues of the tag (Cx26(Tc)) resulted in taurine-induced pore narrowing in homomeric hemichannels, and restored taurine inhibition of heteromeric hemichannels (Cx26(Tc)/Cx32). Taurine actions on junctional channels were fully consistent with those on hemichannels. Taurine-induced inhibition of Cx26/Cx32(T) and nontagged Cx26 junctional channels was blocked by extracellular HEPES, a blocker of the taurine transporter, confirming that the taurine-sensitive site of Cx26 is cytoplasmic. Nuclear magnetic resonance of peptides corresponding to Cx26 cytoplasmic domains showed that taurine binds to the cytoplasmic loop (CL) and not the CT, and that the CT and CL directly interact. ELISA showed that taurine disrupts a pH-dependent interaction between the CT and the CT-proximal half of the CL. These studies reveal that AS disrupt a pH-driven cytoplasmic interdomain interaction in Cx26-containing channels, causing closure, and that the Cx26CT has a modulatory role in Cx26 function.

Comparison of the effects of taurine with those of related sulfur-containing compounds on pyridoxal-induced adrenomedullary catecholamine release and glycogenolysis in the rat

Taurine (2-aminoethanesulfonic acid) is known to attenuate the release of adrenomedullary catecholamines and ensuing hepatic glycogenolysis and hyperglycemia induced by pyridoxal in the rat. Using this animal model, the present study was undertaken to assess the impact that simple structural modifications of the taurine molecule might have on its antagonistic actions against PL. Removal of the amino group (ethanesulfonic acid) or shortening the carbon chain by one methylene (2-aminomethanesulfonic acid) raised the protective actions of taurine. While N-alkylation (N-methyltaurine) or replacement of the amino group by hydroxyl (isethionic acid) had a lowering effect, substituting a sulfhydryl group (2-mercaptoethanesulfonic acid) for the 2-amino group abolished all antagonistic properties associated with taurine. The sulfinic acid analog (hypotaurine) was equipotent with taurine, but the carboxylate isostere (beta-alanine) was inactive. Propranolol, a nonspecific beta-adrenoceptor antagonist, enhanced the antiglycogenolytic effect of taurine and of all structurally related compounds capable of attenuating the outflow of adrenal catecholamines elicited by pyridoxal.

Extracellular taurine in the substantia nigra: Taurine-glutamate interaction

Taurine has been proposed as an inhibitory transmitter in the substantia nigra (SN), but the mechanisms involved in its release and uptake remain practically unexplored. We studied the extracellular pool of taurine in the rat's SN by using microdialysis methods, paying particular attention to the taurine-glutamate (GLU) interaction. Extracellular taurine increased after cell depolarization with high-K(+) in a Ca(2+)-dependent manner, being modified by the local perfusion of GLU, GLU receptor agonists, and zinc. Nigral administration of taurine increased the extracellular concentration of gamma-aminobutyric acid (GABA) and GLU, the transmitters of the two main inputs of the SN. The modification of the glial metabolism with fluocitrate and L-methionine sulfoximine also changed the extracellular concentration of taurine. The complex regulation of the extracellular pool of taurine, its interaction with GABA and GLU, and the involvement of glial cells in its regulation suggest a volume transmission role for taurine in the SN.

Carrier-mediated uptake and release of taurine from Bergmann glia in rat cerebellar slices

Taurine uptake is essential for the maintenance of millimolar intracellular concentrations of taurine, which is released during ischaemia and is thought to be neuroprotective. To determine whether Bergmann glia express functional transporters that can mediate both taurine uptake and efflux, whole-cell patch-clamp recordings were obtained from these cells in rat cerebellar slices. Taurine-induced inward currents can be pharmacologically separated into GABA(A) receptor and taurine transporter currents. In the presence of GABA receptor blockers, residual taurine currents averaged -28 pA at -70 mV and were strictly inwardly rectifying between -70 and +50 mV. These residual currents were also abolished by external Na+ removal and diminished by reduction of external Cl-, consistent with transport currents. Taurine transport currents were reduced by a taurine transporter inhibitor, guanidinoethyl sulphonate (GES). Other classical inhibitors reduced taurine transport currents with an order of potency (hypotaurine > beta-alanine > GES > GABA) similar to that reported for cloned rat taurine transporters. Following intracellular taurine perfusion during the recording, a progressively developing outward current could be observed at -50 mV but not at -70 mV. Intracellular perfusion of taurine also decreased taurine-induced inward currents at both holding potentials. Outward currents induced by intracellular taurine increased in amplitude with depolarization, activated near -50 mV, and were affected by GES. For the first time, these results demonstrate that taurine activates both GABA(A) receptors and Na+/Cl--dependent taurine transporters in Bergmann glia in slices. In addition, our data show that taurine transporters can work in reverse and can probably mediate taurine efflux under ischaemic conditions.

In vitro taurine uptake into cell culture influenced by using media with or without CO2

Buffers used to incubate cells for pharmacological or toxicological studies are usually of very simple composition, far from the composition of biological fluids or cell culture media. Comparative studies on taurine uptake levels by cultured cells show that a new CO2-Independent Medium (CIM) is suitable for incubating cells in place of the Krebs-Ringer buffer (KR) usually used. Basal uptake level of taurine was lower for cells incubated in CIM or in other culture media when compared to those incubated whether in KR or in other "physiological buffers." Isoproterenol depressed similarly the taurine uptake in cells incubated in CIM or KR. The same uptake modulation by beta-alanine, GES, GABA, or HEPES was observed for cells incubated in CIM or KR. C6 cells growth in CIM was dependent on the starting cell density when classically vented T-flasks were used, growth being notably reduced at low density. In tightly closed flasks cells grew in CIM similarly to control cultures maintained in M199 medium or DMEM.

Release of taurine and its effects on release of neurotransmitter amino acids in rat cerebral cortex

Taurine has been postulated to function as a neurotransmitter or neuromodulator. The possibility of depolarization-evoked release of taurine from nerve terminals, and the effects of taurine on release of endogenous glutamate (Glu), aspartate (Asp), and gamma-aminobutyrate (GABA) were examined using a superfusion of crude synaptosomes prepared from rat cerebral cortex. Taurine contents in cerebral cortex and its synaptosomes were 31.7 and 25.2 nmol/mg protein, respectively. Although the basal rate of taurine release was 35.3 pmol/min/mg protein of synaptosomes (second highest releasing rate), the 2-min stimulation with KCl (30 mM) evoked only a 1.3-fold increase in release of taurine (47.3 pmol/min/mg). The increase was largely Ca(2+)-dependent. The addition of taurine to the perfusion medium significantly reduced the depolarization-evoked increases in Glu, Asp, and GABA release. The taurine-induced reduction in GABA release was attenuated by phaclofen, a GABAB antagonist, but not by bicuculline, a GABAA antagonist. However, these antagonists did not block the effects on Glu and Asp release. These data suggest that taurine may be only partly released from nerve terminals by depolarization in the cerebral cortex, but that taurine may act upon nerve terminals to regulate the release of excitatory and inhibitory amino acid transmitters.