mGluR3 Activation Recruits Cytoplasmic TWIK-1 Channels to Membrane that Enhances Ammonium Uptake in Hippocampal Astrocytes

Abstract

TWIK-1 two-pore domain K ⁺ channels are highly expressed in mature hippocampal astrocytes. While the TWIK-1 activity is readily detectable on astrocyte membrane, the majority of channels are retained in the intracellular compartments, which raises an intriguing question of whether the membrane TWIK-1 channels could be dynamically regulated for functions yet unknown. Here, the regulation of TWIK-1 membrane expression by G _i/G _o-coupled metabotropic glutamate receptor 3 (mGluR3) and its functional impact on ammonium uptake has been studied. Activation of mGluR3 induced a marked translocation of TWIK-1 channels from the cytoplasm to the membrane surface. Consistent with our early observation that membrane TWIK-1 behaves as nonselective monovalent cation channel, mGluR3-mediated TWIK-1 membrane expression was associated with a depolarizing membrane potential ( V _M). As TWIK-1 exhibits a discernibly high permeability to ammonium (NH ₄ ⁺), a critical substrate in glutamate-glutamine cycle for neurotransmitter replenishment, regulation of NH ₄ ⁺ uptake capacity by TWIK-1 membrane expression was determined by response of astrocyte V _M to bath application of 5 mM NH ₄Cl. Stimulation of mGluR3 potentiated NH ₄ ⁺-induced V _M depolarization by ~30 % in wild type, but not in TWIK-1 knockout astrocytes. Furthermore, activation of mGluR3 mediated a coordinated translocation of TWIK-1 channels with recycling endosomes toward astrocyte membrane and the mGluR3-mediated potentiation of NH ₄ ⁺ uptake required a functional Rab-mediated trafficking pathway. Altogether, we demonstrate that the activation of mGluR3 up-regulates the membrane expression of TWIK-1 that in turn enhances NH ₄ ⁺ uptake in astrocytes, a mechanism potentially important for functional coupling of astrocyte glutamate-glutamine cycle with the replenishment of neurotransmitters in neurons.