P2X7 in normal and cystic kidney development
P2X7 is a unique member of the P2X family of membrane receptors for extracellular ATP. As well as being a non-selective cation channel, this membrane receptor is implicated in several biological functions, including cell death and proliferation. P2X7 expression, initially thought restricted to immune cells, also occurs in epithelial and other cell types. I hypothesised that P2X7 is functionally expressed in the renal tract, particularly in situations in which cell turnover is prominent. I have demonstrated expression of P2X7 in both mouse and human kidney. During mouse nephrogenesis P2X7 was detected in the condensing mesenchyme in early metanephrogenesis and was subsequently restricted to derivatives of the ureteric bud i.e. collecting duct and ureter. P2X7 was immunolocalised in regions of cell turnover, consistent with a role for the nucleotide receptor in nephrogenesis. P2X7 was also detected in cystic collecting ducts of both the cpk/cpk mouse model of autosomal recessive polycystic kidney disease (ARPKD) and human ARPKD. Next I investigated the potential function of P2X7 in cystogenesis using a 3D suspension culture model from the cpk/cpk mouse. Exposure to agonists of the P2X7 receptor caused a significant reduction in numbers of cysts forming in vitro. This was inhibited by P2X7 antagonists, and was greater than the response to other nucleotides, supporting a specific mediation by the P2X7 receptor. My study did not demonstrate a significant effect on markers of cell proliferation, apoptosis or necrosis correlating with P2X7-mediated reduction in cyst numbers, suggesting an alternative function for the receptor in cyst formation. To further understand the molecular mechanisms by which P2X7 mediates its functions, particularly apoptosis, I have developed an in vitro expression system. Stable transfection of a chicken lymphocytes with rat P2X7 enabled characterisation of the receptor's membrane properties, both ion fluxes and pore potential, and established a novel tool with which to further examine the mechanisms by which P2X7 mediates cell death. Further understanding of the molecular mechanisms of this unique nucleotide receptor, and its functional roles in the kidney, particularly in the setting of polycystic kidney disease may in the future lead to a novel therapeutic target for the manipulation of progression of renal injury, via its apoptotic pathways, or other as yet undefined pathways.