Molecular Genetics of ATP-Sensitive Potassium Channels in Cardiovascular Diseases

K _ATP Channel Structure

K _ATP channels are hetero-octameric complexes of four Kir6 pore-forming subunit members of the inward rectifier channel (Kir) protein family, in association with four regulatory SUR subunits, which are members of the ATP binding cassette (ABC) family of membrane proteins ( Fig. 11.1 ). There are two Kir6-encoding genes, KCNJ8 (Kir6.1) and KCNJ11 (Kir6.2), ^, and two SUR-encoding genes, ABCC8 (SUR1) and ABCC9 (SUR2) ( Fig. 11.2A ). Alternative RNA splicing can give rise to multiple SUR protein variants; the best characterized are SUR2A and SUR2B (generated by alternate splicing of the C-terminal exon; see Fig. 11.2A ), which confer distinct physiologic and pharmacologic properties on channel complexes. ^, The ABCC8 and KCNJ11 genes are immediately adjacent to each other on human chromosome 11p15.1, whereas ABCC9 and KCNJ8 are also adjacent to one another on chromosome 12p12.1. ^, These arrangements implicate both an evolutionary duplication and suggest potential coregulation at the gene level. Consistent with this, as discussed later, the majority of native KATP channel complexes are generated by either one gene pair or the other, even though all subunit combinations can be functionally engineered in recombinant systems.

Kir6 subunits maintain the conserved architecture of other Kir channels, with two transmembrane helices (M1, M2) bridged by an extracellular loop that generates the narrow portion of the pore and controls ion selectivity (see Fig. 11.1A ), and harboring the inhibitory ATP binding site. As with other ABCC family members, SURs contain two six-helix transmembrane domains, TMD1 and TMD2, and contain an additional N-terminal five-helix TMD0 domain (see Fig. 11.1A ), which is critical for Kir6.x trafficking and gating. SURs contain one nucleotide binding fold (NBF1) between TMD1 and TMD2, and a second (NBF2) after TMD2 in the cytoplasmic loops (see Fig. 11.1A ). These NBFs, which pair up as head-to-tail dimers (see Fig. 11.1C ) to generate two nucleotide binding sites (NBDs) at the interface between them, are responsible for channel activation by Mg-nucleotides. A flurry of structural studies of SUR1/Kir6.2 complexes using Cryo-EM have recently confirmed K _ATP complexes to be formed by intimate packing of 4 SUR and 4 Kir6.x subunits (see Fig. 11.1C ). Very similar structures were independently reported at approximately 6 Å resolution by two groups in 2017, ^, and subsequently refined structures have since been reported at near-atomic 3.6 Å resolution. In these structures, the binding of glibenclamide, repaglinide, and carbamazepine, three distinct K _ATP channel inhibitors, is well resolved at essentially the same location within the SUR core (see Fig. 11.1C ).

Molecular Regulation of K _ATP Channel Activity

K _ATP channel regulation is complex and involves metabolites, hormones, and neurotransmitters and transcriptional mechanisms. Micromolar ATP inhibits channels by direct interaction with Kir6 subunits, and because the cellular ATP concentration is relatively high (i.e., millimolar) in physiologic conditions, ATP inhibition is usually sufficient to maintain cardiac K _ATP channels in predominantly closed states (see Fig. 11.1B ). However, ATP inhibition is overridden by MgATP and MgADP interacting with SUR subunit NBFs, and ATP sensitivity is also reduced by membrane phosphoinositides, such as phosphatidylinositol-4,5-bisphosphate (PIP ₂ ), and by long-chain acyl-CoA esters (LC-CoAs), and metabolic derivatives of free fatty acids, which act on Kir6.2 to antagonize ATP inhibition and increase channel open probability. Negative coupling between PIP ₂ activation and ATP sensitivity of K _ATP channels means that ATP sensitivity is not a fixed parameter, and it may change dynamically with changes in membrane composition. Consistent with an electrostatic interaction between the negatively charged head group and a region of the Kir6.2 protein close to the membrane, a number of positively charged amino acid residues in the slide helix region directly interact with PIP ₂ in the membrane. ^{, ,} PIP ₂ interaction with Kir6.1 has been less extensively studied; Kir6.1 channels are activated by PIP ₂ , suggesting conservation of the fundamental determinants of phospholipid binding and gating. In addition, other metabolic factors, including pH, nitric oxide (NO), eicosanoids, hydrogen sulfide (H ₂ S), and hormones and neurotransmitters, can also affect K _ATP channel activity.