Silent but not dumb: how cellular trafficking and pore gating modulate expression of TWIK1 and THIK2

Abstract

Among K2P channels, a few of them turned out to be difficult to express in heterologous systems and were coined "silent subunits". Recent studies have shed light on the mechanisms behind this apparent lack of channel activity at the plasma membrane. For TWIK1 and THIK2 channels, silence is related to a combination of intracellular retention and low intrinsic activity. TWIK1 is constitutively endocytosed from the plasma membrane before being transported to recycling endosomes, whereas THIK2 is restricted to endoplasmic reticulum. These intracellular localizations are related to trafficking signals located in the cytoplasmic parts of the channels. When these motifs are mutated or masked, channels are redistributed at the plasma membrane and produce measurable currents. However, these currents are of modest amplitude. This weak basal activity is due to a hydrophobic barrier in the deep pore that limits water and ions in the conduction pathway. Other silent channels KCNK7, TWIK2, and TASK5 are still under study. Expression and characterization of these K2P channels pave the way for a better understanding of the mechanisms controlling intracellular trafficking of membrane proteins, ion conduction, and channel gating.

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

“…A diisoleucine repeat located in the cytoplasmic carboxyl-terminus of TWIK-1 has been identified as a critical retrieval motif in both cultured kidney cells and transfected oocytes [4, 9, 11]; currently, our results could not fully exclude the possibility that mGluR3 activation may also inhibit TWIK-1 retrieval through endocytosis as a second mechanism for the observed TWIK-1 accumulation in astrocyte membrane.…”

“…The newly inserted TWIK-1 channels stay on membrane only transiently; then, the channels are retrieved back to the recycling endosomes [11]. Thus, it is possible that a short-term inhibition of constitutive vesicle exocytosis may prevent mGluR3-mediated membrane accumulation of TWIK-1 channels and abolish the potentiation of NH 4 + uptake, while leaving the function of the conventional K + channels, such as K ir 4.1, relatively intact in astrocytes.…”

“…Association of TWIK-1 with recycling endosomes has been further identified as a responsible mechanism for a lack of functional membrane currents in cultured kidney cells [9]. In the event of a fall in extracellular K + (hypokalemia) or pH, TWIK-1 switches to conduct Na + [4, 10]; thus, the leak Na + behavior of TWIK-1 in astrocyte and other native cells is likely attributable to a transient presence and rapid retrieval of membrane channels to the acidic environment inside recycling endosomes [11]. …”

TASK, TREK & Co.: a mutable potassium channel family for diverse tasks in the brain

“…Among them also so-called 'silent' members of the K 2P -channel family, THIK-2 and TASK-5, that, until now, is not shown to mediate current in heterologous expression systems. Recent studies in heterologous expression systems reveal that the apparent lack of THIK-2 channel activity on the cell surface is induced by the channels' retention in membranes of the endoplasmic reticulum as well as by their weak intrinsic activity [40]. It remains to be seen which physiological functions K 2P -channels and especially the 'silent' family members fulfil in local interneurons of the thalamus.…”

Recombinant tandem of pore-domains in a Weakly Inward rectifying K+ channel 2 (TWIK2) forms active lysosomal channels

“…The levels of currents produced by these channels are not only controlled by regulation of their activity at the plasma membrane, but also indirectly via controlled biogenesis, intracellular sorting and trafficking 8 . Among the K 2P channel family, 4 of them were considered as “silent” because of their inability to produce measurable currents in heterologous expression systems: TWIK1 (KCNK1, K 2P 1.1) 9 , THIK2 (KCNK12, K 2P 12.1) 10, 11 , KCNK7 (K 2P 7.1) 12 and TASK5 (KCNK15, K 2P 15.1) 13, 14 .…”