No abstract

Nicotine is a neuroteratogen and is the likely link between maternal cigarette smoking during pregnancy and sudden infant death syndrome (SIDS).

The P2X 7 receptor is a non-selective cationic channel activated by extracellular ATP, belonging to the P2X receptor family. To assess the role of extracellular histidines on the allosteric modulation of the rat P2X 7 receptor by divalent metals (copper, zinc and magnesium) and protons, these amino acid residues were singly substituted for corresponding alanines. Wild-type and mutated receptors were injected to Xenopus laevis oocytes; metal-related effects were evaluated by the two-electrode voltage-clamp technique. Copper inhibited the ATP-gated currents with a median inhibitory concentration of 4.4 ± 1.0 lmol/L. The inhibition was noncompetitive and time-dependent; copper was 60-fold more potent than zinc. The mutant H267A, resulted in a copper resistant receptor; mutants H201A and H130A were less sensitive to copper inhibition (p < 0.05). The rest of the mutants examined, H62A, H85A, and H219A, conserved the copper-induced inhibition. Only mutants H267A and H219Awere less sensitive to the modulator action of zinc. Moreover, the magnesium-induced inhibition was abolished exclusively on the H130A and H201A mutants, suggesting that this metal may act at a novel cationic modulator site. Media acidification inhibited the ATP-gated current 87 ± 3%; out of the six mutants examined, only H130A was significantly less sensitive to the change in pH, suggesting that His-130 could be involved as a pH sensor. In conclusion, while His-267 is critically involved in the copper or zinc allosteric modulation, the magnesium inhibitory effects is related to His-130 and His-201, His-130 is involved in proton sensing, highlighting the role of defined extracellular histidines in rat P2X 7 receptor allosteric modulation.

Seven mammalian purinergic receptor subunits, denoted P2X1 to P2X7, and several spliced forms of these subunits have been cloned. When heterologously expressed, these cDNAs encode ATP-gated non-selective cation channels organized as trimers. All activated receptors produce cell depolarization and promote Ca2+ influx through their pores and indirectly by activating voltage-gated calcium channels. However, the biophysical and pharmacological properties of these receptors differ considerably, and the majority of these subunits are also capable of forming heterotrimers with other members of the P2X receptor family, which confers further different properties. These channels have three ATP binding domains, presumably located between neighboring subunits, and occupancy of at least two binding sites is needed for their activation. In addition to the orthosteric binding sites for ATP, these receptors have additional allosteric sites that modulate the agonist action at receptors, including sites for trace metals, protons, neurosteroids, reactive oxygen species and phosphoinositides. The allosteric regulation of P2X receptors is frequently receptor-specific and could be a useful tool to identify P2X members in native tissues and their roles in signaling. The focus of this review is on common and receptor-specific allosteric modulation of P2X receptors and the molecular base accounting for allosteric binding sites.