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There is broad evidence that association of transmembrane receptors and signalling molecules with lipid rafts/caveolae provides an enriched environment for protein-protein interactions necessary for signal transduction, and a mechanism for the modulation of neurotransmitter and/or growth factor receptor function. Several receptors translocate into submembrane compartments after ligand binding, while others move in the opposite direction. The role of such a dynamic localization and functional facilitation is signalling modulation and receptor desensitization or internalization. Purine and pyrimidine nucleotides have been viewed as primordial precursors in the evolution of all forms of intercellular communication, and they are now regarded as fundamental extracellular signalling molecules. They propagate the purinergic signalling by binding to ionotropic and metabotropic receptors expressed on the plasma membrane of almost all cell types, tissues and organs. Here, we have illustrated the localization in lipid rafts/caveolae of G protein-coupled P1 receptors for adenosine and P2Y receptors for nucleoside tri- and di-phosphates. We have highlighted that microdomain partitioning of these purinergic GPCRs is cell-specific, as is the overall expression levels of these same receptors. Moreover, we have described that disruption of submembrane compartments can shift the purinergic receptors from raft/caveolar to non-raft/non-caveolar fractions, and then abolish their ability to activate lipid signalling pathways and to integrate with additional lipid-controlled signalling events. This modulates the biological response to purinergic ligands and most of all indicates that the topology of the various purinergic components at the cell surface not only organizes the signal transduction machinery, but also controls the final cellular response.

In a previous study we used P2 receptor antagonists to inhibit diverse responses that nerve growth factor (NGF) promotes and coordinates in PC12 cells and we suggested that P2 receptors partake in the NGF signalling cascade. In this paper, we examine the direct role of extracellular P2 receptor agonists as neurotrophic factors. ATP and 2-Cl-ATP promote neurite regeneration after priming PC12 cells with NGF and the effect is dose-dependent, with an EC50of about 5 and 3 μM, respectively. The number of cell clumps bearing neurites was maximally induced in day 1 and it was maintained up to about one week by ATP, or up to at least 2 weeks by 2-Cl-ATP. The involvement of P1 receptors or intracellular inosine in these actions was excluded, whereas various antagonists of P2 receptors were inhibitory. Moreover, NGF and ATP caused a direct up-regulation of P2X2, P2X3, P2X4and P2Y2, but not P2Y4receptor proteins under neurite-regenerating conditions, as well as extracellular signal-regulated kinase (Erk)1-2 tyrosine/threonine phosphorylation and activation. Finally, ATP, 2-Cl-ATP and ATPγS enhanced neurite initiation evoked by sub-optimal NGF concentrations and ATP and 2-Cl-ATP fully sustained survival of PC12 cells after serum deprivation.