A P2X7 receptor stimulates plasma membrane trafficking in the FRTL rat thyrocyte cell line

Abstract

Thyroid cells express a variety of P2Y and P2X purinergic receptor subtypes. G protein-coupled P2Y receptors influence a wide variety of thyrocyte-specific functions; however, functional P2X receptor-gated channels have not been observed. In this study, we used whole cell patch-clamp recording and fluorescence imaging of the plasma membrane marker FM1-43 to examine the effects of extracellular ATP on membrane permeability and trafficking in the Fisher rat thyroid cell line FRTL. We found a cation-selective current that was gated by ATP and 2′,3′- O-(4-benzoylbenzoyl)-ATP but not by UTP. The ATP-evoked currents were inhibited by pyridoxal phosphate 6-azophenyl-2′,4′-disulfonic acid, adenosine 5′-triphosphate-2′,3′-dialdehyde, 100 μM Zn2+, and 50 μM Cu2+. Fluorescence imaging revealed pronounced, temperature-sensitive stimulation of exocytosis and membrane internalization by ATP with the same pharmacological profile as observed for activation of current. The EC50 for ATP stimulation of internalization was 440 μM in saline containing 2 mM Ca2+ and 2 mM Mg2+, and 33 μM in low-Mg2+, nominally Ca2+-free saline. Overall, the results are most consistent with activation of a P2X7 receptor by ATP4−. However, low permeability to N-methyl-d-glucamine+ and the propidium cation YO-PRO-1 indicates absence of the cytolytic pore that often accompanies P2X7 receptor activation. ATP stimulation of internalization occurs in Na+-free, Ca2+-free, or low-Mg2+ saline and therefore does not depend on cation influx through the ATP-gated channel. We conclude that ATP activation of a P2X7 receptor stimulates membrane internalization in FRTL cells via a transduction pathway that does not depend on cation influx.