More cross-talk between purinergic receptors

Imaging P2X4 receptor subcellular distribution, trafficking, and regulation using P2X4-pHluorin

A P2X4 receptor labeled with the pH-sensitive GFP superecliptic pHluorin represents a useful probe to investigate P2X4 receptor distribution, trafficking, and up-regulation.

P2X4 receptors are adenosine triphosphate (ATP)-gated cation channels present on the plasma membrane (PM) and also within intracellular compartments such as vesicles, vacuoles, lamellar bodies (LBs), and lysosomes. P2X4 receptors in microglia are up-regulated in epilepsy and in neuropathic pain; that is to say, their total and/or PM expression levels increase. However, the mechanisms underlying up-regulation of microglial P2X4 receptors remain unclear, in part because it has not been possible to image P2X4 receptor distribution within, or trafficking between, cellular compartments. Here, we report the generation of pH-sensitive fluorescently tagged P2X4 receptors that permit evaluations of cell surface and total receptor pools. Capitalizing on information gained from zebrafish P2X4.1 crystal structures, we designed a series of mouse P2X4 constructs in which a pH-sensitive green fluorescent protein, superecliptic pHluorin (pHluorin), was inserted into nonconserved regions located within flexible loops of the P2X4 receptor extracellular domain. One of these constructs, in which pHluorin was inserted after lysine 122 (P2X4-pHluorin123), functioned like wild-type P2X4 in terms of its peak ATP-evoked responses, macroscopic kinetics, calcium flux, current–voltage relationship, and sensitivity to ATP. P2X4-pHluorin123 also showed pH-dependent fluorescence changes, and was robustly expressed on the membrane and within intracellular compartments. P2X4-pHluorin123 identified cell surface and intracellular fractions of receptors in HEK-293 cells, hippocampal neurons, C8-B4 microglia, and alveolar type II (ATII) cells. Furthermore, it showed that the subcellular fractions of P2X4-pHluorin123 receptors were cell and compartment specific, for example, being larger in hippocampal neuron somata than in C8-B4 cell somata, and larger in C8-B4 microglial processes than in their somata. In ATII cells, P2X4-pHluorin123 showed that P2X4 receptors were secreted onto the PM when LBs undergo exocytosis. Finally, the use of P2X4-pHluorin123 showed that the modulator ivermectin did not increase the PM fraction of P2X4 receptors and acted allosterically to potentiate P2X4 receptor responses. Collectively, our data suggest that P2X4-pHluorin123 represents a useful optical probe to quantitatively explore P2X4 receptor distribution, trafficking, and up-regulation.

Imaging P2X4 receptor lateral mobility in microglia: regulation by calcium and p38 MAPK

ATP-gated ionotropic P2X4 receptors are up-regulated in activated microglia and are critical for the development of neuropathic pain, a microglia-associated disorder. However, the nature of how plasma membrane P2X4 receptors are regulated in microglia is not fully understood. We used single-molecule imaging to track quantum dot-labeled P2X4 receptors to explore P2X4 receptor mobility in the processes of resting and activated microglia. We find that plasma membrane P2X4 receptor lateral mobility in resting microglial processes is largely random, consisting of mobile and slowly mobile receptors. Moreover, lateral mobility is P2X subunit- and cell-specific, increased in an ATP activation and calcium-dependent manner, and enhanced in activated microglia by the p38 MAPK pathway that selectively regulates slowly mobile receptors. Thus, our data indicate that P2X4 receptors are dynamically regulated mobile ATP sensors, sampling more of the plasma membrane in response to ATP and during the activated state of microglia that is associated with nervous system dysfunction.

P2X4 receptors in activated C8-B4 cells of cerebellar microglial origin

We investigated the properties and regulation of P2X receptors in immortalized C8-B4 cells of cerebellar microglial origin. Resting C8-B4 cells expressed virtually no functional P2X receptors, but largely increased functional expression of P2X4 receptors within 2–6 h of entering the activated state. Using real-time polymerase chain reaction, we found that P2X4 transcripts were increased during the activated state by 2.4-fold, but this increase was not reflected by a parallel increase in total P2X4 proteins. In resting C8-B4 cells, P2X4 subunits were mainly localized within intracellular compartments, including lysosomes. We found that cell surface P2X4 receptor levels increased by ∼3.5-fold during the activated state. This change was accompanied by a decrease in the lysosomal pool of P2X4 proteins. We next exploited our findings with C8-B4 cells to investigate the mechanism by which antidepressants reduce P2X4 responses. We found little evidence to suggest that several antidepressants were antagonists of P2X4 receptors in C8-B4 cells. However, we found that moderate concentrations of the same antidepressants reduced P2X4 responses in activated microglia by affecting lysosomal function, which indirectly reduced cell surface P2X4 levels. In summary, our data suggest that activated C8-B4 cells express P2X4 receptors when the membrane insertion of these proteins by lysosomal secretion exceeds their removal, and that antidepressants indirectly reduce P2X4 responses by interfering with lysosomal trafficking.