Characterization of P2X7 isoform B sheds light on the role of pore versus channel activity in P2X7 mediated life and death.

P2X7 isoform B (Gene bank AY847299, P2X7B) is a naturally occurring splice variant of P2X7 full-length receptor (Gene Bank accession n NT-009775, P2X7A). P2X7B mRNA retains introns between exons 10 and 11 thus causing the loss of the C terminal tail and incorporation of 18 amminoacids. If compared to P2X7A, P2X7B variant shows a reduced activity as ion channel and a complete loss of the pore forming ability. We engineered and stably expressed P2X7B in HEK293 cells were we could confirm that its activity is restricted to that of small anion channel. P2X7B also loose P2X7A ability of ATP release in the extracellular milieu. Nevertheless, P2X7B expression confers to HEK293 cells a longer survival in the absence of serum. P2X7B transfectants also gain an increased competence in growth and migration in serum free soft agar, suggesting a possible role of the receptor in facilitating tissue infiltration. This increased resistance to unfavorable colture conditions is dependent on extracellular ATP, as treatment with apyrase significantly reduces it. P2X7B transfectants show an higher content of intracellular ATP if compared to mock cells, suggesting an increased metabolic activity. This hypothesis is also supported by a greater content of reticular calcium in P2X7B cells and by an increased nuclear translocation of the calcineurin activated nuclear factor NFATc1. Moreover, in the same cells there is and loss of Bz-ATP evoked cell blebbing. It is tempting to speculate that while P2X7B expression will represent a proliferative advantage for the cell its counterpart, P2X7A, will also expose it to death. These different phenotypes will be dependent on channel versus pore forming activity.