Showing papers by "Piotr Bregestovski published in 2001"

Activation of human α1 and α2 homomeric glycine receptors by taurine and GABA

Abstract: In the mammalian central nervous system, inhibitory glycine receptors (GlyRs) are mainly expressed in the spinal cord and in the midbrain where they control motor and sensory pathways (Breitinger & Becker, 1998) They form chloride-selective ionic channels which are activated by glycine and, to a lesser extent, by β-alanine, taurine and several other amino acids (Werman, 1972; Schmieden et al 1995, 1999) Four α subunits and one β subunit have been cloned from mammals It is generally believed that in adult, GlyRs are heteromers mainly composed of three α1 and two β subunits, whereas fetal and neonatal receptors are homomeric α2 GlyRs (for reviews, see Rajendra et al 1997; Betz et al 1999), although strong functional evidence of the presence of synaptic homomeric GlyRs is still lacking (see Singer et al 1998; Ali et al 2000) The different GlyR subtypes exhibit different functional properties during ontogenesis (Takahashi et al 1992; Singer et al 1998; Ali et al 2000) We recently cloned an α subunit from zebrafish GlyR (named αZ1) which displays high sequence similarities to mammalian α1 subunits (David-Watine et al 1999) Like all the α subunits identified so far, αZ1 is able to form a functional homomeric GlyR in Xenopus oocytes or in transiently transfected human cell lines The functional properties of this GlyR are, however, surprisingly different from those composed of human subunits (David-Watine et al 1999; Fucile et al 1999) First, αZ1 GlyRs are highly sensitive to taurine despite the presence of a valine at position 111, a residue that is thought to confer a low sensitivity to taurine on human GlyRs (Schmieden et al 1992) Furthermore, αZ1 GlyRs can be activated by GABA in the absence of mutations F159 and Y161 which are apparently necessary to transform GABA-insensitive human α1 GlyRs into GABA-sensitive GlyRs (Schmieden et al 1993) To determine whether these discrepancies are related to species differences, we first re-examined the actions of taurine and GABA on homomeric αH1 and αH2 GlyRs We have also previously demonstrated that for αZ1 GlyR the EC50 for glycine (EC50gly) and the relative maximum response of GABA (defined as the ratio ImaxGABA/Imaxgly) are closely correlated (Fucile et al 1999) This implies that variations in EC50gly alter the response to the other agonists dramatically Although similar properties have never been established for the mammalian GlyRs, various data suggest that the ability of taurine and GABA to activate these GlyRs may also be correlated with the EC50gly Firstly, Taleb & Betz (1994) reported that when the EC50gly of human αH1 GlyRs is lowered at high receptor density in Xenopus oocytes, the sensitivity to taurine and to GABA increased Secondly, the Imaxtau/Imaxgly ratio is always higher if the receptors are expressed in HEK cells, in which they display an EC50gly of approximatively 30 μm (Rajendra et al 1995; Lynch et al 1997; Moorhouse et al 1999), than in Xenopus oocytes, where the EC50gly is usually above 200 μm (Schmieden et al 1992, 1993, 1995, 1999) Thirdly, several mutations in the α1 subunit which increase the relative maximum response of taurine are accompanied by an elevation of the sensitivity of GlyR to glycine (Schmieden et al 1999) Finally, αH1 GlyRs become sensitive to GABA when their EC50gly is decreased by the double mutation F159Y-Y161F (Schmieden et al 1993) Thus, two other aims of our study were (i) to determine the relationships between the maximal responses to agonists (taurine or GABA) and the EC50gly and (ii) to elucidate whether these relations are different for αH1 and αH2 GlyRs Preliminary results of this study have appeared in abstract form (De Saint Jan et al 1999)