The versatile regulation of K2P channels by polyanionic lipids of the phosphoinositide and fatty acid metabolism

Work over the past three decades has greatly advanced our understanding of the regulation of Kir K+ channels by polyanionic lipids of the phosphoinositide (e.g., PIP2) and fatty acid metabolism (e.g., oleoyl-CoA). However, comparatively little is known regarding the regulation of the K2P channel family by phosphoinositides and by long-chain fatty acid–CoA esters, such as oleoyl-CoA. We screened 12 mammalian K2P channels and report effects of polyanionic lipids on all tested channels. We observed activation of members of the TREK, TALK, and THIK subfamilies, with the strongest activation by PIP2 for TRAAK and the strongest activation by oleoyl-CoA for TALK-2. By contrast, we observed inhibition for members of the TASK and TRESK subfamilies. Our results reveal that TASK-2 channels have both activatory and inhibitory PIP2 sites with different affinities. Finally, we provided evidence that PIP2 inhibition of TASK-1 and TASK-3 channels is mediated by closure of the recently identified lower X-gate as critical mutations within the gate (i.e., L244A, R245A) prevent PIP2-induced inhibition. Our findings establish that K+ channels of the K2P family are highly sensitive to polyanionic lipids, extending our knowledge of the mechanisms of lipid regulation and implicating the metabolism of these lipids as possible effector pathways to regulate K2P channel activity.

Heteromerization of alkaline-sensitive two-pore domain potassium channels

Two-pore domain (K2P) potassium channels are active as dimers. They produce inhibitory currents regulated by a variety of stimuli. Among them, TALK1, TALK2 and TASK2 form a subfamily of structurally related K2P channels stimulated by extracellular alkalosis. The human genes encoding them are clustered on chromosomal region 6p21. They are expressed in different tissues including the pancreas. By analyzing single cell transcriptomic data, we show that these channels are co-expressed in insulin-secreting pancreatic β cells. By different approaches we show that they form functional heterodimers. Heteromerization of TALK2 with TALK1 or with TASK2 endorses TALK2 with sensitivity to extracellular alkalosis in the physiological range. The association of TASK2 with TALK1 and TALK2 increases their unitary conductance. These results provide a new example of heteromerization in the K2P channel family expanding the range of their potential physiological and pathophysiological roles.

Two-Pore-Domain Potassium (K2P-) Channels: Cardiac Expression Patterns and Disease-Specific Remodelling Processes

Two-pore-domain potassium (K2P-) channels conduct outward K+ currents that maintain the resting membrane potential and modulate action potential repolarization. Members of the K2P channel family are widely expressed among different human cell types and organs where they were shown to regulate important physiological processes. Their functional activity is controlled by a broad variety of different stimuli, like pH level, temperature, and mechanical stress but also by the presence of lipids or pharmacological agents. In patients suffering from cardiovascular diseases, alterations in K2P-channel expression and function have been observed, suggesting functional significance and a potential therapeutic role of these ion channels. For example, upregulation of atrial specific K2P3.1 (TASK-1) currents in atrial fibrillation (AF) patients was shown to contribute to atrial action potential duration shortening, a key feature of AF-associated atrial electrical remodelling. Therefore, targeting K2P3.1 (TASK-1) channels might constitute an intriguing strategy for AF treatment. Further, mechanoactive K2P2.1 (TREK-1) currents have been implicated in the development of cardiac hypertrophy, cardiac fibrosis and heart failure. Cardiovascular expression of other K2P channels has been described, functional evidence in cardiac tissue however remains sparse. In the present review, expression, function, and regulation of cardiovascular K2P channels are summarized and compared among different species. Remodelling patterns, observed in disease models are discussed and compared to findings from clinical patients to assess the therapeutic potential of K2P channels.

Two-pore domain potassium channels (K2P) in GtoPdb v.2021.3

The 4TM family of K channels mediate many of the background potassium currents observed in native cells. They are open across the physiological voltage-range and are regulated by a wide array of neurotransmitters and biochemical mediators. The pore-forming α-subunit contains two pore loop (P) domains and two subunits assemble to form one ion conduction pathway lined by four P domains. It is important to note that single channels do not have two pores but that each subunit has two P domains in its primary sequence; hence the name two-pore domain, or K2P channels (and not two-pore channels). Some of the K2P subunits can form heterodimers across subfamilies (e.g. K2P3.1 with K2P9.1). The nomenclature of 4TM K channels in the literature is still a mixture of IUPHAR and common names. The suggested division into subfamilies, described in the More detailed introduction, is based on similarities in both structural and functional properties within subfamilies and this explains the "common abbreviation" nomenclature in the tables below.

Negative Influence by the Force: Mechanically Induced Hyperpolarization via K2P Background Potassium Channels

The two-pore domain K2P subunits form background (leak) potassium channels, which are characterized by constitutive, although not necessarily constant activity, at all membrane potential values. Among the fifteen pore-forming K2P subunits encoded by the KCNK genes, the three members of the TREK subfamily, TREK-1, TREK-2, and TRAAK are mechanosensitive ion channels. Mechanically induced opening of these channels generally results in outward K+ current under physiological conditions, with consequent hyperpolarization and inhibition of membrane potential-dependent cellular functions. In the past decade, great advances have been made in the investigation of the molecular determinants of mechanosensation, and members of the TREK subfamily have emerged among the best-understood examples of mammalian ion channels directly influenced by the tension of the phospholipid bilayer. In parallel, the crucial contribution of mechano-gated TREK channels to the regulation of membrane potential in several cell types has been reported. In this review, we summarize the general principles underlying the mechanical activation of K2P channels, and focus on the physiological roles of mechanically induced hyperpolarization.

Contribution of Neuronal and Glial Two-Pore-Domain Potassium Channels in Health and Neurological Disorders

Two-pore-domain potassium (K2P) channels are widespread in the nervous system and play a critical role in maintaining membrane potential in neurons and glia. They have been implicated in many stress-relevant neurological disorders, including pain, sleep disorder, epilepsy, ischemia, and depression. K2P channels give rise to leaky K+ currents, which stabilize cellular membrane potential and regulate cellular excitability. A range of natural and chemical effectors, including temperature, pressure, pH, phospholipids, and intracellular signaling molecules, substantially modulate the activity of K2P channels. In this review, we summarize the contribution of K2P channels to neuronal excitability and to potassium homeostasis in glia. We describe recently discovered functions of K2P channels in glia, such as astrocytic passive conductance and glutamate release, microglial surveillance, and myelin generation by oligodendrocytes. We also discuss the potential role of glial K2P channels in neurological disorders. In the end, we discuss current limitations in K2P channel researches and suggest directions for future studies.

The versatile regulation of K2P channels by polyanionic lipids of the phosphoinositide (PIP2) and fatty acid metabolism (LC-CoA)

Work of the past three decades provided tremendous insight into the regulation of K+ channels - in particular Kir channels - by polyanionic lipids of the phosphoinositide (e.g. PIP2) and fatty acid metabolism (e.g. oleoyl-CoA). However, comparatively little is known regarding the phosphoinositide regulation in the K2P channel family and the effects of long-chain fatty acid CoA esters (LC-CoA, e.g. oleoyl-CoA) are so far unexplored. By screening most mammalian K2P channels (12 in total), we report strong effects of polyanionic lipids (activation and inhibition) for all tested K2P channels. In most cases the effects of PIP2 and oleoyl-CoA were similar causing either activation or inhibition depending on the respective subgroup. Activation was observed for members of the TREK, TALK and THIK subfamily with the strongest activation by PIP2 seen for TRAAK (~110-fold) and by oleoyl-CoA for TALK-2 (~90-fold). In contrast, inhibition was observed for members of the TASK and TRESK subfamilies up to ~85 %. In TASK-2 channels our results indicated an activatory as well as an inhibitory PIP2 site with different affinities. Finally, we provided evidence that PIP2 inhibition in TASK-1 and TASK-3 channels is mediated by closure of the recently identified lower X-gate as critical mutations within the gate (i.e. L244A, R245A) prevent PIP2 induced inhibition. Our results disclosed K2P channels as a family of ion channels highly sensitive to polyanionic lipids (PIP2 and LC-CoA), extended our knowledge on the mechanisms of lipid regulation and implicate the metabolisms of these lipids as possible effector pathways to regulate K2P channel activity.