Unresolved issues and controversies in purinergic signalling

Abstract

Some areas of current interest in the rapidly expanding purinergic signalling field that are controversial or are unresolved are highlighted in this review. These include the mechanisms underlying: ATP transport across cell and vesicle membranes; the interaction of multiple receptors for purines and pyrimidines on single cells; the blocking effect of antagonists to P2X(4) and P2X(7) receptors expressed by microglial cells in neuropathic and inflammatory pain; and the complex actions mediated by P2X(7) receptors. Some desirable areas for further research are also discussed including: comparative studies of the evolution of purinergic signalling; studies of purinergic signalling in development and regeneration, including the involvement of stem cells; behavioural studies; and therapeutic strategies.

Figure 1

A, the molecular topology of the P2X_1–6 and the P2X₇ receptors. The key feature of the P2X₇ receptor distinguishing it from the other P2X receptors is its long carboxy-terminus tail. B, the 3 different forms of the P2X₇ receptor upon activation by ATP: closed channel, open channel, and pore formation. Brief ATP activation leads to opening of an ion channel permeable to small ions, such as Na⁺, K⁺ and Ca²⁺, whereas prolonged ATP exposure results in the pore formation, permeable to high molecular weight organic cations, e.g. N-methyl-d-glucamine (NMDG) and YO-PRO-1 dye. C, naturally occurring splice variants of the human P2X₇ gene are shown. Exons are numbered and indicated by boxes. The newly identified exon N3 is indicated. Introns are represented by horizontal lines between the exons. Below the genomic structure is depicted the P2X₇ transcript with relevant domains indicated. D, the exon arrangement of the various splice variants isolated is shown. New stop codons are indicated by asterisks and new start codons indicated by the letter M. The approximate positions of PCR primers used for real-time expression analysis are indicated by arrowheads placed below variants A, B and H. The sequences of the seven splice variants have been deposited in GenBank with the following accession nos.: AY847298, AY847299, AY847300, AY847301, AY847302, AY847303 and AY847304. (A and B are reproduced from Gunosewoyo et al. 2007, with permission from Bentham Science Publishers, Ltd; C and D are reproduced from Cheewatrakoolpong et al. 2005, with permission from Elsevier.)