Potassium channels participate in many biological functions, from ion homeostasis to generation and modulation of the electrical membrane potential. They are involved in a large variety of diseases. ...In the human genome, 15 genes code for K+ channels with two pore domains (K2P). These channels form dimers of pore‐forming subunits that produce background conductances finely regulated by a range of natural and chemical effectors, including signalling lipids, temperature, pressure, pH, antidepressants and volatile anaesthetics. Since the cloning of TWIK1, the prototypical member of this family, a lot of work has been carried out on their structure and biology. These studies are still in progress, but data gathered so far show that K2P channels are central players in many processes, including ion homeostasis, hormone secretion, cell development and excitability. A growing number of studies underline their implication in physiopathological mechanisms, such as vascular and pulmonary hypertension, cardiac arrhythmias, nociception, neuroprotection and depression. This review gives a synthetic view of the most noticeable features of these channels.
Potassium channels form the largest family of ion channels with more than 80 members involved in cell excitability and signalling. Most of them exist as homomeric channels, whereas specific ...conditions are required to obtain heteromeric channels. It is well established that heteromerization of voltage‐gated and inward rectifier potassium channels affects their function, increasing the diversity of the native potassium currents. For potassium channels with two pore domains (K2P), homomerization has long been considered the rule, their polymodal regulation by a wide diversity of physical and chemical stimuli being responsible for the adaptation of the leak potassium currents to cellular needs. This view has recently evolved with the accumulation of evidence of heteromerization between different K2P subunits. Several functional intragroup and intergroup heteromers have recently been identified, which contribute to the functional heterogeneity of this family. K2P heteromerization is involved in the modulation of channel expression and trafficking, promoting functional and signalling diversity. As illustrated in the Figure, heteromerization of TREK1 and TRAAK provides the cell with more possibilities of regulation. It is becoming increasingly evident that K2P heteromers contribute to important physiological functions including neuronal and cardiac excitability. Since heteromerization also affects the pharmacology of K2P channels, this understanding helps to establish K2P heteromers as new therapeutic targets for physiopathological conditions.
figure legend TREK1 and TRAAK, two K2P channel subunits, assemble to form functional homodimers and heterodimers. Characterization of their regulations by protein kinase A and C, phospholipase D, intracellular and extracellular protons and chemical compounds such as ML67 and fluoxetine has shown that the TREK1/TRAAK heteromers have unique properties compared to TREK1 and TRAAK homomers.
Highly selective for K
at neutral pH, the TWIK1 channel becomes permeable to Na
upon acidification. Using molecular dynamics simulations, we identify a network of residues involved in this unique ...property. Between the open and closed states previously observed by electron microscopy, molecular dynamics simulations show that the channel undergoes conformational changes between pH 7.5-6 involving residues His122, Glu235, Lys246 and Phe109. A complex network of interactions surrounding the selectivity filter at high pH transforms into a simple set of stronger interactions at low pH. In particular, His122 protonated by acidification moves away from Lys246 and engages in a salt bridge with Glu235. In addition, stacking interactions between Phe109 and His122, which stabilize the selectivity filter in its K
-selective state at high pH, disappear upon acidification. This leads to dissociation of the Phe109 aromatic side chain from this network, resulting in the Na
-permeable conformation of the channel.
Mutations that modulate the activity of ion channels are essential tools to understand the biophysical determinants that control their gating. Here, we reveal the conserved role played by a single ...amino acid position (TM2.6) located in the second transmembrane domain of two-pore domain potassium (K2P) channels. Mutations of TM2.6 to aspartate or asparagine increase channel activity for all vertebrate K2P channels. Using two-electrode voltage-clamp and single-channel recording techniques, we find that mutation of TM2.6 promotes channel gating via the selectivity filter gate and increases single channel open probability. Furthermore, channel gating can be progressively tuned by using different amino acid substitutions. Finally, we show that the role of TM2.6 was conserved during evolution by rationally designing gain-of-function mutations in four Caenorhabditis elegans K2P channels using CRISPR/Cas9 gene editing. This study thus describes a simple and powerful strategy to systematically manipulate the activity of an entire family of potassium channels.
The tandem of pore domain in a weak inwardly rectifying K⁺ channel (Twik)-related acid-arachidonic activated K⁺ channel (TRAAK) and Twik-related K⁺ channels (TREK) 1 and TREK2 are active as ...homodimers gated by stretch, fatty acids, pH, and G protein-coupled receptors. These two-pore domain potassium (K2P) channels are broadly expressed in the nervous system where they control excitability. TREK/TRAAK KO mice display altered phenotypes related to nociception, neuroprotection afforded by polyunsaturated fatty acids, learning and memory, mood control, and sensitivity to general anesthetics. These channels have emerged as promising targets for the development of new classes of anesthetics, analgesics, antidepressants, neuroprotective agents, and drugs against addiction. Here, we show that the TREK1, TREK2, and TRAAK subunits assemble and form active heterodimeric channels with electrophysiological, regulatory, and pharmacological properties different from those of homodimeric channels. Heteromerization occurs between all TREK variants produced by alternative splicing and alternative translation initiation. These results unveil a previously unexpected diversity of K2P channels that will be challenging to analyze in vivo, but which opens new perspectives for the development of clinically relevant drugs.
Autosomal-dominant polycystic kidney disease, the most frequent monogenic cause of kidney failure, is induced by mutations in the
PKD1 or
PKD2 genes, encoding polycystins TRPP1 and TRPP2, ...respectively. Polycystins are proposed to form a flow-sensitive ion channel complex in the primary cilium of both epithelial and endothelial cells. However, how polycystins contribute to cellular mechanosensitivity remains obscure. Here, we show that TRPP2 inhibits stretch-activated ion channels (SACs). This specific effect is reversed by coexpression with TRPP1, indicating that the TRPP1/TRPP2 ratio regulates pressure sensing. Moreover, deletion of TRPP1 in smooth muscle cells reduces SAC activity and the arterial myogenic tone. Inversely, depletion of TRPP2 in TRPP1-deficient arteries rescues both SAC opening and the myogenic response. Finally, we show that TRPP2 interacts with filamin A and demonstrate that this actin crosslinking protein is critical for SAC regulation. This work uncovers a role for polycystins in regulating pressure sensing.
TWIK1 belongs to the family of background K⁺ channels with two pore domains. In native and transfected cells, TWIK1 is detected mainly in recycling endosomes. In principal cells in the kidney, TWIK1 ...gene inactivation leads to the loss of a nonselective cationic conductance, an unexpected effect that was attributed to adaptive regulation of other channels. Here, we show that TWIK1 ion selectivity is modulated by extracellular pH. Although TWIK1 is K⁺ selective at neutral pH, it becomes permeable to Na⁺ at the acidic pH found in endosomes. Selectivity recovery is slow after restoration of a neutral pH. Such hysteresis makes plausible a role of TWIK1 as a background channel in which selectivity and resulting inhibitory or excitatory influences on cell excitability rely on its recycling rate between internal acidic stores and the plasma membrane. TWIK1⁻/⁻ pancreatic β cells are more polarized than control cells, confirming a depolarizing role of TWIK1 in kidney and pancreatic cells.
Despite a high level of sequence homology, tandem pore domain halothane-inhibited K+ channel 1 (THIK1) produces background K+ currents, whereas THIK2 is silent. This lack of activity is due to a ...unique combination of intracellular retention and weak basal activity in the plasma membrane. Here, we designed THIK subunits containing dominant negative mutations (THIK1DN and THIK2DN). THIK2DN mutant inhibits THIK1 currents, whereas THIK1DN inhibits an activated form of THIK2 (THIK2-A155P-I158D). In situ proximity ligation assays and Förster/fluorescence resonance energy transfer (FRET) experiments support a physical association between THIK1 and THIK2. Next, we expressed covalent tandems of THIK proteins to obtain expression of pure heterodimers. Td-THIK1-THIK2 (where Td indicates tandem) produces K+ currents of amplitude similar to Td-THIK1-THIK1 but with a noticeable difference in the current kinetics. Unlike Td-THIK2-THIK2 that is mainly detected in the endoplasmic reticulum, Td-THIK1-THIK2 distributes at the plasma membrane, indicating that THIK1 can mask the endoplasmic reticulum retention/retrieval motif of THIK2. Kinetics and unitary conductance of Td-THIK1-THIK2 are intermediate between THIK1 and THIK2. Altogether, these results show that THIK1 and THIK2 form active heteromeric channels, further expanding the known repertoire of K+ channels.
Inhibitory potassium channels of the TREK1/TRAAK family are integrators of multiple stimuli, including temperature, membrane stretch, polyunsaturated fatty acids and pH. How these signals affect the ...gating of these channels is the subject of intense research. We have previously identified a cytoplasmic domain, pCt, which plays a major role in controlling channel activity. Here, we use pharmacology to show that the effects of pCt, arachidonic acid, and extracellular pH converge to the same gate within the channel. Using a state-dependent inhibitor, fluoxetine, as well as natural and synthetic openers, we provide further evidence that the “up” and “down” conformations identified by crystallography do not correspond to open and closed states of these channels.
Mechano-gated ion channels play a key physiological role in cardiac, arterial, and skeletal myocytes. For instance, opening of the non-selective stretch-activated cation channels in smooth muscle ...cells is involved in the pressure-dependent myogenic constriction of resistance arteries. These channels are also implicated in major pathologies, including cardiac hypertrophy or Duchenne muscular dystrophy. Seminal work in prokaryotes and invertebrates highlighted the role of transient receptor potential (TRP) channels in mechanosensory transduction. In mammals, recent findings have shown that the canonical TRPC1 and TRPC6 channels are key players in muscle mechanotransduction. In the present review, we will focus on the functional properties of TRPC1 and TRPC6 channels, on their mechano-gating, regulation by interacting cytoskeletal and scaffolding proteins, physiological role and implication in associated diseases.
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