Soluble Phosphoinositide Physical Chemistry and the Interpretation of TRPV1 Potentiation
Many cellular signaling processes depend on phosphoinositide lipids (PIPs), so that several techniques to manipulate plasma membrane PIP levels have been developed. Among the most common is the application of “short-chain” PIPs, usually dioctanoyl PIPs, to excised, inside out plasma membrane patches or to whole cells delivered by dialysis through the patch pipette. Our goal has been to better connect studies using these diC8-PIPs to physiological situations.We have applied isothermal titration calorimetry to determine the partition of these lipids into models of both the intracellular and extracellular leaflets of neuronal plasma membranes. To accurately model the measured heats of transfer of the PIPs from solution to the membrane and account for ion binding as the membrane becomes increasingly charged, applied a variable-charge model of the PIP headgroups. We found, not surprisingly, that for equal solution concentrations, the mole fraction of diC8-PI(4)P in the membrane is about twice as high as the mole fraction of diC8-PI(4,5)P2. More intriguing is the implication that very low concentrations give a number of diC8-PI(4,5)P2 are required to activate TRPV1. We will discuss the conditions under which our observations can be extended to physiological PIPs, as well as implications for other ion channel studies applying these short-chain lipids. Our results suggest novel experiments to examine the role of the membrane in regulating membrane protein function.