Functional Regulation of KATP Channels and Mutant Insight Into Clinical Therapeutic Strategies in Cardiovascular Diseases

ATP-sensitive potassium channels (KATP channels) play pivotal roles in excitable cells and link cellular metabolism with membrane excitability. The action potential converts electricity into dynamics by ion channel-mediated ion exchange to generate systole, involved in every heartbeat. Activation of the KATP channel repolarizes the membrane potential and decreases early afterdepolarization (EAD)-mediated arrhythmias. KATP channels in cardiomyocytes have less function under physiological conditions but they open during severe and prolonged anoxia due to a reduced ATP/ADP ratio, lessening cellular excitability and thus preventing action potential generation and cell contraction. Small active molecules activate and enhance the opening of the KATP channel, which induces the repolarization of the membrane and decreases the occurrence of malignant arrhythmia. Accumulated evidence indicates that mutation of KATP channels deteriorates the regulatory roles in mutation-related diseases. However, patients with mutations in KATP channels still have no efficient treatment. Hence, in this study, we describe the role of KATP channels and subunits in angiocardiopathy, summarize the mutations of the KATP channels and the functional regulation of small active molecules in KATP channels, elucidate the potential mechanisms of mutant KATP channels and provide insight into clinical therapeutic strategies.


INTRODUCTION
The aging of the population and improved survival after acute myocardial infarction have resulted in high morbidity and mortality, a poor clinical prognosis and high expenses due to heart failure (HF) (Savarese et al., 2022). The prevalence of HF is predicted to increase by 46% from 2012 to 2030. After several years of therapeutic exploration, the prognosis of HF remains poor, with a 5-years mortality of ≈40%-50%, and the projections suggest that the total costs for HF in 2030 will be close to $38.99 billion in the United States (Kaneko et al., 2021;Yu et al., 2021;Savarese et al., 2022). Sudden cardiac death (SCD) is the leading cause of death in HF, and malignant arrhythmia is regarded as the overriding risk within SCD (Akhtar et al., 2021;Grune et al., 2021;Mulder et al., 2021). HF involves numerous physiological and pathological processes, among which calcium (Ca 2+ ) overload is a typical representative. Ca 2+ overload destroys membranes, organelles and DNA, leading to structural and functional disruption of cells and tissues, eventually promoting cardiomyopathy, ventricular fibrillation and sudden death (Yang et al., 2021).
ATP-sensitive potassium channels (K ATP channels) were first discovered in cardiac muscle in 1983 by Noma (Noma, 1983) and were successively found in skeletal muscle, the digestive system, urinary system, integumentary system, reproductive system, and central nervous system (Huang et al., 2019;Zhao et al., 2020) ( Figure 1). Activating K ATP channels shorten the action potential duration, reduce intracellular Ca 2+ entry to suppress calcium overload, inhibit contractility, and prevent arrhythmias and cardiac insufficiency caused by calcium overload; however, completely opening K ATP channels in the heart may result in complete cessation of cardiac electrical activity and contractile failure (Huang et al., 2019). Hence, K ATP channels play an irreplaceable role in HF, whether from myocardial ischemia or arrhythmia. K ATP channels are widely distributed in various organs, but the assembly of their subunits varies depending upon the tissue and they may confer different functional and pharmacological properties depending on which subunits are present Stewart and Turner, 2021) (Table 1). K ATP channels are comprised of four sulfonylurea receptors (SURx) and four K + inward rectifiers (Kir6. x) that assemble to form hetero-octameric protein complexes. The pore-forming subunit Kir6. x (Kir6.1 and Kir6.2) has intracellular N-and C-termini and two transmembrane segments M1 and M2, encoded by KCNJ8 and KCNJ11, respectively. The modulatory subunit SURx (SUR1, SUR2A, SUR2B) consists of three groups of transmembrane domains (TMD0, TMD1 and TMD2) and extracellular N-and intracellular C-termini, encoded by ABCC8 and ABCC9. There are two intracellular nucleotide binding folds (NBD1 and NBD2) within the SUR subunit. ABCC8 and KCNJ11 are adjacent to each FIGURE 1 | The Structure of the K ATP Channel. (A). K ATP channels are comprised of four sulfonylurea receptors (SURx) and four K + inward rectifiers (Kir6. x) that assemble to form hetero-octameric protein complexes. The green and pink sections on the left represent the side view of the Kir6. x subunit, and the gold and purple sections on the right represent the side view of the SURx subunit. FASTA comes from NCBI, designed by www.swissmodel.com, Image designed by Swiss-Pdbviewer software. (B). The pore-forming subunit Kir6. x (Kir6.1 and Kir6.2) has intracellular N-and C-termini and two transmembrane segments M1 and M2, encoded by KCNJ8 and KCNJ11, respectively. The modulatory subunit SURX (SUR1, SUR2A, SUR2B) consists of three groups of transmembrane domains (TMD0, TMD1 and TMD2) and extracellular N-and intracellular C-termini, encoded by ABCC8 and ABCC9. There are two intracellular nucleotide binding folds (NBD1 and NBD2) within the SUR subunit. ABCC8 and KCNJ11 are adjacent to each other on chromosome 11p15.1, with ABCC9 and KCNJ8 on chromosome 12P12.1.

System
Organ/Tissue/Cell Subunit Types Disease Contributory other on chromosome 11p15.1, with ABCC9 and KCNJ8 on chromosome 12P12.1. Cardiac K ATP channels provide cardioprotection against ischemia/reperfusion injury; in contrast, overexpressed cardiac K ATP channels have proarrhythmic effects, which associates them with profound value for clinical applications and exploration. There are three subtypes of K ATP channels found within the cardiovascular system: two widely accepted channels, mitoK ATP and sarcolemma K ATP , and a controversial K ATP channel, plasma membrane K ATP (Iguchi et al., 2019;Pertiwi et al., 2019;Aziz et al., 2020;Jiang et al., 2021) . Different cardiac K ATP channels play different roles in the cardiovascular system, which will be explained here. Recent evidence has shown that several refractory diseases are closely related to mutations in K ATP channel subunits. Disease-related clinical symptoms and high medical costs will burden the patient, the family, and society. Most encouraging, some K ATP channel activators and antagonists have shown good results for treating K ATP channel subunit mutation-related diseases, such as Cantú syndrome, congenital hyperinsulinism (CHI), neonatal diabetes mellitus (NDM), developmental delay epilepsy and neonatal diabetes (DEND) and ABCC9-related intellectual disability myopathy Syndrome (AIMS) (Demirbilek et al., 2019;Martin et al., 2020;McClenaghan et al., 2020).
In this review, we focus on the regulatory mechanism of K ATP channels during angiocardiopathy and provide insights into how mutations in K ATP channelopathies lead to some incurable diseases. Furthermore, we will explore the therapeutic strategy of targeting K ATP channel drugs in clinical practice.

K ATP CHANNELS IN CARDIOVASCULAR DISEASES
Mitochondrial ATP-Sensitive Potassium Channels (mitoK ATP Channels) As an independent factor associated with high mortality, acute myocardial infarction is an irreversible process characterized by glycogen depletion, margination of nuclear chromatin, mitochondrial swelling and sarcolemmal breaks. Myocardial infarct size and the duration of ischemia are the main determinants of the prognosis (Heusch, 2020). Rapidly restoring blood flow is the key to successful salvage of ischemic myocardium; however, reperfusion not only salvages ischemic myocardium from infarction but also induces an increased risk of additional complications and further cardiomyocyte death, a process called myocardial ischemia reperfusion injury (MIRI) (Griffiths et al., 2021). In myocardial ischemia, the mitochondrial matrix is damaged and extensively broken, and it dissolves the mitochondrial crest, ruptures and vacuolates the mitochondrial membrane, significantly decreases glycogen granules, increases intracellular Ca 2+ , diminishes ATP production, and induces myocardial cell apoptosis (Paggio et al., 2019;Basalay et al., 2020;Wang et al., 2020;Bai et al., 2021;Wang et al., 2021). A novel autologous mitochondrial transplantation therapy, in which respirationcompetent mitochondria are isolated from autologous nonischemic tissue and transplanted into ischemic myocardium, improves the contractile function and tissue viability of the injured myocardium, proving that mitochondrial injury is the main pathogenesis of MIRI (Shin et al., 2019). mitoK ATP channels are involved in a series of physiological and pathophysiological changes to mitigate cardiomyocyte injury and apoptosis. mitoK ATP channels have been described as being located in the inner mitochondrial membrane and they have protective properties for ischemic myocardium; moreover, their existence has been the subject of heated debate (Bezerra Palácio et al., 2021). Recently, the molecular composition of mitoK ATP was shown by Paggio et al., and they are comprised of poreforming (MITOK, encoded by the CCDC51 gene (NCBI ID 79714)) and regulatory (MITOSUR, tissue expression correlates with ABCB8) subunits (Paggio et al., 2019) ( Figure 2). The opening of mitoK ATP channels promotes mitochondrial K + inward flow into the deeply negative polarized matrix (mitochondrial membrane potential (Ψ m )), decreases the transmembrane potential discrepancy, depolarizes the Ψ m , reduces Ca 2+ inward flow dynamics, inhibits Ca 2+ inward flow, and prevents mitochondrial calcium overload, leading to mitochondrial relaxation, enhanced fatty acid oxidation, oxidative phosphorylation, respiratory function, and ATP production, thus improving myocardial cell survival (Sakamoto and Kurokawa, 2019;Jiang et al., 2021).
During the process of MIRI, the activation of mitochondrial K ATP channels depolarizes the mitochondrial membrane, reduces the driving force of mitochondrial calcium uptake, and prevents Ca 2+ accumulation in the mitochondrial matrix, thus preventing the formation of the mitochondrial permeability transition pore (MPTP) (Testai et al., 2021). Currently, the main components of MPTP are as follows: adenine nucleotide translocator (ANT) and mitochondrial phosphate carrier (PiC) on the inner mitochondrial membrane (IMM), mammalian F1Fo(F)-ATP synthase (the Fo subunit consists of a, e, f, g and A6 L; the F1 subunit α, β, and the ε and γ subunits needed for ATP synthase dimer formation, the peripheral stem consists of b, d, and the F6 subunit and oligomycin sensitivity conferring protein (OSCP)) (Kent et al., 2021) (Figure 2). MPTP is the point at which reactive oxygen species (ROS), Ca 2+ , cytochrome C (Cyt C), and other small molecule modulators escape from the mitochondrial matrix, and its opening leads to the loss of oxidative phosphorylation capacity as well as the release of pro-death mitochondrial proteins (mitochondrial swelling and membrane rupture), increased Bax expression and decreased Bcl 2 expression, ultimately activating the apoptotic cascade in the mitochondria in ischemia-reperfusion heart tissues (Jiang et al., 2021;Kent et al., 2021;Pereira and Kowaltowski, 2021). Mitochondrial ROS spreading from the electron transport chain damages the mitochondrial DNA, which can cause mitochondrial dysfunction and affect nuclear gene expression, ion handling, and mitochondrial metabolism, finally causing the activation of an inflammatory response, apoptotic signaling, and endoplasmic reticulum stress in the cardiovascular system (Bou-Teen et al., 2021). Indeed, MIRI triggers a cascade involving increased NO production (NO function; anti-inflammatory and antioxidant effects) and leads to the superfluous formation of peroxynitrite (ONOO − ) with increased production of ROS, which mediates the pernicious impact of NO (Liu et al., 2021a). The activation of mitoK ATP during ischemia plays a role in the cardioprotective function by inhibiting the overproduction of ROS and stimulating the NO effects on anti-inflammatory and antioxidant activities in the ischemic myocardium (Liu et al., 2021a;Rameshrad et al., 2021). Ischemic postconditioning activates protein kinase C and reperfusion injury salvage kinase pathways through modulating the intracellular concentrations of adenosine and NO, ultimately acting on the mitoK ATP pathway to protect the myocardium (Li et al., 2020a). mitoK ATP channels also affect other cardiac components. Within rat cardiac fibroblasts (CFs), mitoK ATP channels prevent the transdifferentiation of CFs to myofibroblasts (MFs) to reduce MF maturation and antagonize cardiac pathological remodeling following simulated ischemia-reperfusion injury (Stewart and Turner, 2021). ROMK (a kidney mRNA detectable in the thick ascending limb and the distal nephron) participates in K + reabsorption and secretion. An experiment performed by Irina B. Krylova et al. implicated ROMK in Ca 2+ -induced MPTP opening but did not play a role in mitoK ATP activity in the mouse heart (Papanicolaou et al., 2020). Electrophysiological analysis revealed that 10 μM testosterone increased the open probability of mitoK ATP channels, which offered cytoprotection against MIRI (Sakamoto and Kurokawa, 2019).
The recently discovered signal-regulated pathways involved in mitoK ATP channels are as follows. Penehyclidine hydrochloride (PHC) preconditioning plays a cardioprotective role by regulating the mitochondrial K ATP channel and Akt/GSK-3β and Akt/mTOR signaling pathways (Zi et al., 2020). Uridine attenuates myocardial injury and oxidative stress in MIRI, which may be mediated by activation of the mitoK ATP channel, achieved by reducing excessive ROS production and preventing the appearance of calcium overload (Krylova et al., 2021). GST (genistein, a phytoestrogen) provides a cardioprotective function that depends on protein kinase C and activates mitoK ATP channels via a typical pathway, such as PI3K/Akt and NO synthases (Colareda et al., 2020).

Sarcolemma ATP-Sensitive Potassium Channels
Early evidence indicated that sarcolemma ATP-sensitive potassium (sarcK ATP ) channels play a crucial role in ischemic preconditioning and myocardial resistance to ischemia, which close during general conditions and open in response to increased [ADP]/[ATP], linking membrane excitability to the balance of ATP production and shortening action potential (AP) duration (APD) via the efflux of K + (Garrott et al., 2017;Sudhir et al., 2020). SarcK ATP channels improve adaptation to physical stress and profoundly alter membrane excitability and other membrane FIGURE 2 | The role of mitoK ATP in myocardial ischemia and the regulation of the signaling pathways involved. (A). Mitochondrial injury and mitoK ATP play a role in cardiac protection during the process of MIRI; MIRI increased Bax expression and decreased BCL 2 expression, triggering a cascade involving increased NO production and leading to the superficial formation of ONOO − with increased production of ROS. Activation of mitoK ATP stimulates the anti-inflammatory and antioxidant effects of NO on ischemic myocardium by inhibiting the overproduction of ROS. Ischemic postadaptation activates protein kinase C and the reperfusion injury salvage kinase pathway through the intracellular concentration of adenosine and NO, and it acts on the mitoK ATP channel to protect the myocardium; activation of the mitoK ATP channel leads to cell hyperpolarization, resulting in reduced Ca 2+ entry, and a reduced driving force of mitochondrial calcium uptake, prevent Ca 2+ accumulation in the matrix and MPTP formation. The MPTP is the escape pore of ROS, Cyt C, Ca 2+ and other signaling molecules. CF indicates cardiac fibroblasts; MF, myofibroblasts; MIRI, myocardial ischaemia reperfusion injury; and ONOO − , peroxynitrite. (B). The structure of MPTP; F1Fo ATP synthase: The Fo subunit consists of a, e, f, g and A6 L, the F1 component consists of α and ß subunits, labeled in bottle green and yellow, respectively, and the γ, ε subunits. The F1 peripheral stalk is comprised of subunits b, d, F6, and OSCP. The C cyclic subunit, ANT and PiC are overlaid by IMM; the dotted line represents the MPTP signal molecule outflow position. OSCP indicates oligomycin sensitivity conferring protein; IMM, inner mitochondrial membrane; ANT, adenine nucleotide translocase; and PiC, phosphate carrier. (C). mitoK ATP channels prevent transdifferentiation of CF to MF to reduce MF maturation. Testosterone (10 μM) increases the opening probability of mitoK ATP channels, PHC regulates the mitochondrial K ATP channel, Akt/GSK-3β, and Akt/mTOR signaling pathways, GST depends on protein-kinase C and mitoK ATP channel pathway activation by some typical pathways such as PI3K/Akt and NO synthases, and plays roles in cardioprotection. Uridine attenuates myocardial injury and oxidative stress by activating the mitoK ATP channel to reduce excessive ROS production and prevent calcium overload. PHC indicates penehyclidine hydrochloride; GST, genistein.
Frontiers in Pharmacology | www.frontiersin.org June 2022 | Volume 13 | Article 868401 potential-related functions, such as Ca 2+ overload, thus helping to maintain cellular homeostasis during cardiac challenge (i.v. adenosine) (Zhang et al., 2016). The partial opening of sarcK ATP channels plays a crucial role in the regional depolarization of Ψm, which can transform cellular electrical excitability and increase the propensity for reentry arrhythmogenesis (Solhjoo and O'Rourke, 2015). An unstable or oscillating Ψm can expose cardiomyocytes to ROS or result in glutathione depletion, activate sarcK ATP channels and abate the cellular ATP/ADP ratio, which has been deemed to be a dominant factor in arrhythmogenesis during MIRI (Solhjoo and O'Rourke, 2015). Increased activation of the sarcK ATP channel (a role in cardioprotection) does not participate in the protection provided by ordinary cardioprotective stimulation . sarcK ATP opening actually occurs later during metabolic inhibition (after cardioprotection), cardioprotective stimuli prolong normal mitochondrial function during ischemia, and the delay in the opening of sarcK ATP channels is a consequence of the continuation of ATP production, so sarcK ATP channel opening is the last defense of cardiomyocytes to preserve ATP and limit the Ca 2+ overload during ischemia (Brennan et al., 2015). The density of sarcK ATP channels under physiological conditions plays a significant role in cardioprotection; however, certain pathophysiologic circumstances give rise to a declining density of sarcK ATP channels, including hyperinsulinemia and cardiac ischemia (Yang et al., 2018). The lower basal expression level of sarcK ATP channels in hESCs (human embryonic stem cells)-VCMs (ventricular cardiomyocytes) (~1/8 of adults) means they were partially activated and sufficient to cause APD shortening and accelerate AP firing; when fully activated, sarcK ATP channels silenced automaticity without compromising intrinsic cellular excitability (Keung et al., 2016).
Studies on the cardiac sarcK ATP channel regulatory subunit SUR2A/SUR2B are ongoing. The activation of ß 1 -adrenoceptors upregulates SUR2B/Kir6.2, in which SUR2B physically associates with Kir6.2 to act as a regulatory subunit in sarcK ATP channels to offer cardioprotection (Jovanovic et al., 2016). With an increasing number of sarcK ATP channels, increased expression of SUR2A regulates cardiac physiology and improves the adaptation to physical stress by shortening the action potential and improving cardiac Ca 2+ homeostasis (Zhang et al., 2016).
The recently discovered signal-regulated pathways and regulatory proteins involved in sarcK ATP channels are as follows. Eps15 homology domain-containing protein (EHD)-2 affects the sarcK ATP channel by stabilizing sarcK ATP channelcontaining caveolar structures to increase its surface density, which results in a reduced rate of endocytosis. Pathophysiologically, EHD-2 mutant-activated cardiomyocytes may be cardioprotective against ischemic damage (Yang et al., 2018). In rat cardiomyocytes, the sarcK ATP channel exerts a cardioprotective effect against lipopolysaccharide (LPS)induced apoptosis and it is mediated by mitochondrial Ca 2+ (Zhang et al., 2016). The cardioprotective effect of BNP is related to sarcK ATP channel opening. Additionally, the cardioprotective effects of ANP and cANP4-23 are mediated via sarcK ATP channel opening (Krylatov et al., 2021). ANP (atrial natriuretic peptide) positively regulates the function of the sarcK ATP channel in adult rabbit ventricular cardiomyocytes by activating NPR-A (natriuretic peptide receptor type A), an effect mediated by intracellular signaling mechanisms that cover PKG (cGMP-dependent protein kinase), ROS, ERK (extracellular signal-regulated protein kinase)1/2, CaMK II (calcium/ calmodulin-dependent protein kinase II), and RyR (ryanodine receptor)-2; meanwhile, RyR2 (activation) is feasibly situated downstream of ROS/H 2 O 2 , which process enhances the opening frequency whereas it labilizes the long closures of the channel, thereby heightening channel activity (Zhang and Lin, 2020).

MUTATION OF K ATP CHANNELS
Kir 6.1 Endothelium-expressed Kir6.1 is located on human chromosome 12p, and via elevated endothelin-1 release it controls vascular tone. Smooth muscle Kir6.1 gain-of-function mutation causes overt hypertension and hypotension; notably, autosomal dominant hypertension is related to chromosome 12p recombination, and postural hypotension is related to chromosome 12 (Li et al., 2013) (Table 2). In gain-of-function mutation Kir6.1 [GD-QR] (point mutations in two C-terminal residues of Kir6.1; Gly343Asp and Gln53Arg), lymphatic smooth muscle and vascular dysfunction are present, and lymphatic smooth muscle-specific expression subunit mutations result in profound lymphatic contractile dysfunction and lymphatic smooth muscle hyperpolarization rather than lymphatic endothelial cells (Davis et al., 2020). In a CS animal model, the Kir6.1 wt/VM mutation directly and/or indirectly affects the skeletal muscle through vascular dysfunction, resulting in reduced limb strength, skeletal muscle atrophy, autophagy, and myofiber connective tissue replacement (Scala et al., 2020). The S422 L mutation, a missense mutation in the KCNJ8 gene, leads to a gain-of-function Kir6.1 channel, which leads to shortened repolarization in ventricular tissue; nevertheless, it could shorten repolarization in the atrium to increase atrial fibrillation susceptibility (Delaney et al., 2012).

Kir 6.2
Approximately 38.5% of mutations in the KCNJ11 gene, which encodes Kir 6.2 and consists of a single exon containing 390 amino acids, have been identified, which is associated with clinical diseases including but not limited to neonatal diabetes mellitus, maturity-onset diabetes of the young, type 2 diabetes mellitus, and even persistent hyperinsulinemic hypoglycemia of infancy (He et al., 2021). Patients with the E227K mutation in the KCNJ11 gene typically manifest with transient neonatal diabetes, which remits spontaneously, usually within 4-60 weeks of onset; however, more than half of these patients relapse into permanent diabetes in adolescence or early adulthood (Devaraja et al., 2020). rs5215 G/G (nucleotide change; G-A, amino acid change; Val337Ile) of the KCNJ11 gene, located at 11p15.1 and encoding the Kir6.2 subunit, causes valine-isoleucine substitution in exon 1,009 (ATC-GTC), and it is associated Frontiers in Pharmacology | www.frontiersin.org June 2022 | Volume 13 | Article 868401 with a gain of function of the K ATP channel, leading to vasodilation augmentation and shear stress reduction, which protects humans from lower coronary microvascular dysfunction, reducing the risk of ischemic heart disease in women (Severino et al., 2020). In a hypertension mouse model, the Kir6.2 mutation led to heart failure and death, involving knockout mutation-induced myocardial incommensurate remodeling (Liu et al., 2021b). In neurons, Kir6.2 has critical roles in glucose sensing and neuronal excitability in response to metabolic demands, and the KCNJ11 p. V59 M mutation was strongly associated with intellectual disability (Moriguchi et al., 2018;Svalastoga et al., 2020).

SUR1
SUR1 is mainly expressed in the pancreas, and its mutations may lead to neonatal diabetes by disrupting inhibitory binding/gating or enhancing nucleotide stimulation. Some SUR1 mutant models in mice did not recapitulate the human phenotype (Sachse et al., 2020;Usher et al., 2020). SUR1-mutant (a homozygous c.560T > A (V187D) mutation in exon four of the ABCC8 gene encoding the SUR1 protein) stem cell-derived islet-like clusters (SC islets) leads to increased beta-cell proliferation and mass, higher insulin secretion in hypoglycemia and makes K ATP channels-acting pharmaceuticals ineffective (Lithovius et al., 2021). The homozygous p. H1401Tfs ABCC8 mutation could cause significant clinical heterogeneity congenital hyperinsulinemia, ranging from a late-onset and diazoxide-responsive mild form to an extremely early-onset severe form requiring multimodality treatment with a full-course assessment of neurodevelopment and glycometabolism (Takasawa et al., 2021). Some SUR1 mutations resulted in increased channel activity in MgATP/MgADP and drastically reduced K ATP channel surface expression, which suggests that the overactive defects due to altered nucleotide sensitivities outweigh their biogenesis and surface expression defects and lead to an overall gain-of-channel-function effect and the neonatal diabetes mellitus disease phenotype (Balamurugan et al., 2019).

SUR2A
Due to the strong difficulties and inferior feasibility of single subunit mutation research, we mainly noted several common cases herein. In individuals with idiopathic dilated cardiomyopathy, two heterozygous mutations in exon 38 of ABCC9 encode at the C-terminal domain of SUR2A, Fs1524 (a frameshift at Leu1524, which introduces four anomalous terminal residues followed by a premature stop codon) and A1513T (a missense mutation (4537G→A) causing the amino acid substitution), substantially diminishing the maximal rate of the NBD2 ATPase reaction without altering the Michaelis-Menten constant of catalysis, resulting in abnormal hydrolytic dynamics of the regulatory channel subunits, disrupting catalysisdependent gating and impairing metabolic decoding, resulting in severely dilated hearts with impaired systolic function and arrhythmia (Bienengraeber et al., 2004).

SUR2B
The SUR2B mutation R659C located in the secondary structure region in the L1 linker (it has the greatest α-helical propensity) most stably interacts with NBD1, which could cause heart disease and even lead to early repolarization syndrome, a life-threatening condition (Sooklal et al., 2018). During colonic inflammation, two specific mutations within SUR2B (C24S and C1455S) prevent the detrimental effects of sulfhydration and NaHS-induced tyrosine nitration from reducing the pore-forming subunit (Kir6.1) (Kang et al., 2015).

Multiple Subunits Mutations of K ATP Channels
Cantú syndrome (CS) is an ultrarare autosomal dominant inherited disorder caused by dominant gain-of-function mutations in both the SUR2A and Kir6.1 subunits of the K ATP channel, which is also characterized by multiple cardiovascular abnormalities, including edema, pericardial effusion, pulmonary hypertension, dilated and tortuous blood vessels with decreased  Table 3). CHI is a rare genetically heterogeneous disorder caused by inactivating mutations in the SUR1 and Kir6.2 subunits of the K ATP channel and it is characterized by persistent hypoglycemia in infants and children, which may increase the risk of permanent brain damage (Boodhansingh et al., 2019;Rosenfeld et al., 2019;Männistö et al., 2020;Rosenfeld et al., 2021). NDM is characterized by the development of hyperglycemia within the first 6 months of life, beta-cell destruction, pancreatic hypoplasia or aplasia, impaired beta-cell function or severe insulin resistance resulting from impaired insulin secretion caused by gain-offunction mutations in KCNJ11 and/or ABCC8 subunits of the K ATP channel, which can be divided into two transient diabetes mellitus (TNDM) and perma-nent diabetes mellitus (PNDM) clinical subtypes, depending on the length of the disease course (Cao et al., 2020;Dahl and Kumar, 2020;Pipatpolkai et al., 2020;Horita et al., 2021). DEND syndrome is a severe pathological condition of neonatal diabetes with developmental delay, muscle weakness, and epilepsy caused by gain-of-function mutations in Kir 6.2 and SUR1 (Dahl and Kumar, 2020;Pipatpolkai et al., 2020;Gopi et al., 2021). AIMS is characterized by delayed psychomotor development with intellectual disability, anxiety, muscle weakness and fatigability and some shared dysmorphic features caused by loss-of-function mutations in ABCC9 (SUR2A and/or SUR2B) (Smeland et al., 2019).

REGULATION OF K ATP CHANNELS BY SMALL ACTIVE MOLECULES Hydrogen Sulfide
Hydrogen sulfide (H 2 S), as a gaseous signaling molecule, has a wide range of biological functions, including vasodilatation, antiendoplasmic reticulum stress, anti-apoptotic and antiinflammatory functions, and it contributes to ameliorating ventricular structural remodeling and cardiac function . In cardiac tissue, the most important enzyme for the synthesis of H 2 S is cystathionine γ-lyase (CSE), which has reduced activity in atherosclerotic patients connected with angina and atrial fibrillation (Bibli et al., 2021) (Figure 3). H 2 S has many significant bioactivities, including cytoprotective, antioxidant, anti-inflammatory, antiapoptotic, and smooth muscle relaxing effects, in part because it acts as a K ATP channel opener (Fouad et al., 2020). H 2 S partially inhibits phosphodiesterase-5 through the activation of K ATP channels and increases intracellular cGMP to evoke direct vasorelaxing responses (Citi et al., 2020). H 2 S activates the K ATP channel and inhibits insulin secretion in INS-1E cells (a pancreatic ßcell line), but the function of hyperpolarizing the plasma membrane and closing voltage-gated Ca 2+ channels is not mediated by the K ATP channel (Lu et al., 2019;Shoji et al., 2019). H 2 S modulates K ATP channel activity, promotes protective effects against pulmonary hypertension and increases uterine blood flow by antagonizing vasoconstriction (Guerra and Hurt, 2019;Roubenne et al., 2021). H 2 S protects the embryonic heart from I/R injury by opening the K ATP channel rather than increasing coronary artery flow, demonstrating that H 2 S treatment of the embryonic heart is independent of the mother and the underdeveloped placenta (Hess et al., 2020). NaHS, a rapid-releasing H 2 S donor, stimulates ANP secretion via the K ATP channel under hypoxic conditions, resulting in decreased blood pressure, ECF volume and antiproliferation of vascular smooth muscle cells in the cardiovascular system (Yu et al., 2019). Briefly, the interaction between H 2 S and K ATP plays an irreplaceable role in cardiovascular disease.

Nitric Oxide (NO)
The NO-cyclic guanosine monophosphate (cGMP) signaling pathway is a potential therapeutic target for heart failure, and a reduction in NO bioavailability may result in the decreased production of cGMP, which could lead to decreased protection against myocardial injury, vascular and ventricular sclerosis, fibrosis, hypertrophy, and cardiorenal syndrome (Udelson et al., 2020). K ATP channels can activate the L-arginine/NO/ cGMP cascade pathway to induce membrane hyperpolarization, which results in shortening the action potential and restricting Ca 2+ entry through Ca 2+ channels, thus contributing to cardioprotection and vasodilatation (Wang et al., 2019a;Iguchi et al., 2019). Reductions in the arterial blood pressure effect of white mulberry fruit polysaccharides, the vascular relaxation effect of tetrahydropalmatine on rat aortae, and the vasodilatory effect of formaldehyde, either partially or completely, are all mediated by the NO/cGMP/K ATP pathway (Wang et al., 2019a;  The first 6 months of life, beta-cell destruction, pancreatic hypoplasia or aplasia, impaired beta-cell function or severe insulin resistance Kir 6.2 and SUR1 DEND syndrome Neonatal diabetes with developmental delay, muscle weakness, and epilepsy SUR2A and/or SUR2B ABCC9-related intellectual disability myopathy syndrome Intellectual disability, anxiety, muscle weakness and fatigability, and some shared dysmorphic features Frontiers in Pharmacology | www.frontiersin.org June 2022 | Volume 13 | Article 868401 2019a; Zhao et al., 2019). Lipofundin MCT/LCT is involved in attenuating K ATP channel-induced vasodilation by inhibiting basally released endothelial NO and/or cGMP (Lee et al., 2020). The antinociceptive effects of methanol extracts of B. spectabilis, cardamonin and Lonchocarpus araripensis lectin ether are partially or completely mediated by the NO/cGMP/ K ATP pathways (Assreuy et al., 2020;Ferdous et al., 2020;Pui Ping et al., 2020). The K ATP channel, a gastroprotective factor, is involved in the gastroprotective effects of N-acylarylhydrazone derivatives on ethanol-induced gastric lesions in mice via the NO/ cGMP pathway (da Silva Monteiro et al., 2019). Therefore, the NO/cGMP/K ATP pathway is involved in a variety of organoprotective and vasodilative pharmacological processes.

Oxygen(O 2 )
The heart operates exclusively under aerobic metabolism and three factors, heart rate, contractility, and ventricular wall tension, require myocardial mitochondria for maintaining sufficient O 2 to sustain oxidative phosphorylation. Hypoxia causes the opening of the K ATP channel due to a decline in the ATP:ADP ratio, which couples cellular metabolism to excitability to prevent action potential generation and cell contraction, ultimately leading to coronary artery smooth muscle cell hyperpolarization and the closure of voltagedependent Ca 2+ channels and relaxation (Yang et al., 2020a). Vascular K ATP channels supporting skeletal muscle convective and diffusive O 2 transport and oxidative phosphorylation sustain submaximal exercise tolerance; conversely, K ATP channel inhibitors may exacerbate exercise intolerance in healthy rats (Colburn et al., 2020). Additionally, K ATP channel activation modulates the anterior circulation and total cerebral perfusion, contributing to cerebral blood flow and oxygen delivery responses to hypoxia, maintaining a constant cerebral blood supply, avoiding disturbances in the precise regulation of cerebral perfusion and oxygen delivery, and preventing severe tissue damage and even death (Smith et al., 2020).

Regulated by Other Factors
CORM-3, a water-soluble CO-releasing molecule that can mimic the HO-1/CO (heme oxygenase-1/carbon monoxide) pathway by FIGURE 3 | The functional regulation of active small molecules on K ATP channels. (A) H 2 S participates in K ATP channel regulation. H 2 S acts on the corresponding organ/tissues/cytochemical small molecules in the oval through K ATP channels to produce physiological effects in a rectangular box. The black up and down arrows inside the ellipse represent increasing and decreasing, respectively. (B). NO participates in K ATP channel regulation. The relative drugs in the ellipses produce physiological effects in the rectangular boxes through the K ATP channels and NO/CGMP signaling pathways, which the reaction goes in the direction of the arrows in the same color. (C). O 2 participates in K ATP channel regulation. Hypoxia results in a decrease in ATP/ADP acting on K ATP channels to prevent action potential generation and cell contraction; The K ATP channel acts on skeletal muscle and affects O 2 transport to sustain submaximal exercise tolerance; The K ATP channel affects anterior cerebral circulation and total cerebral perfusion through O 2 transport to prevent action potential generation and cell contraction. The black up and down arrows inside the ellipse represent increasing and decreasing, respectively. (D). Other small active molecules. CORM-3 can mimic the HO-1/CO pathway, activate mitoK ATP channels and elicit cardioprotection against hypoxia-reoxygenation injury by inhibiting the Na + /HCO 3 − transporter; CORM-2 alleviates gastric lesions at the systemic level via K ATP channels, reducing gastric DNA oxidation and inflammatory responses; SO 2 plays a vasodilatory role through K ATP channel activation in the peripheral cardiovascular system at high concentrations (>500 μmol/L).
Frontiers in Pharmacology | www.frontiersin.org June 2022 | Volume 13 | Article 868401 liberating CO under appropriate conditions in biological systems, activates mitoK ATP channels and elicits cardioprotection against hypoxia-reoxygenation injury by inhibiting a bicarbonate transporter (most likely Na + /HCO3 − ) during reoxygenation (Portal et al., 2019). CORM-2 increases the gastric mucosal CO content and blood carboxyhemoglobin concentration, resulting in gastroprotection, alleviation of gastric lesions, decreased gastric DNA oxidation and the inflammatory response at the systemic level, which is partly mediated by K ATP channels . Sulfur dioxide (SO 2 ), a major toxic gas and environmental pollutant, plays a vasodilatory role through K ATP channel activation in the peripheral cardiovascular system at high concentrations (>500 μmol/L) (Magierowska et al., 2019). The search for signaling molecular regulatory pathways related to K ATP channels is still in progress.
Highly specific HCN 2 (hyperpolarization-activated, cyclicnucleotide gated channels 2) in ventricular myocytes, an integral part of ventricular electric remodeling, and the reduced expression of its mRNA, leads to the downregulation of the K ATP channel current, which is one of the partial causes of arrhythmia in diabetic rats (Hadova et al., 2021;White et al., 2021). Exogenous cholesterol eliminated the increase in SUR2, suggesting that cholesterol may regulate K ATP channel expression and explain why patients with hypercholesterolemia were also able to cope with ischemic events (Geiger et al., 2021). Low-density lipoprotein (LDL)5, the most negatively charged subfraction of circulating LDL, which has been considered a novel factor for predicting coronary vascular disease and stroke, prolongs the APD and increases the current density of K ATP channels, and may induce arrhythmias . Under 5-Hz pacing conditions, the ATP-sensitive potassium current of ZFHX3 knockdown (zinc finger homeobox three gene) cells was increased compared with that under other conditions, confirming that ZFHX3 knockdown and tachypacing are related to increased stress (Lkhagva et al., 2021). Statins increased the ADP/ATP ratio and activated K ATP channels to dedifferentiate myofibroblasts, while inhibition of K ATP channels weakened the role of statin-induced myofibroblast dedifferentiation (Emelyanova et al., 2019). K ATP channels participate in the cardiomyocyte-specific expression of photoinduced proton pump inhibitors, hyperpolarizing the intact heart to terminate ventricular arrhythmias (Funken et al., 2019). Resistin, secreted by PVAT (the fat reserve surrounding blood vessels consisting of fat cells, immune cells, fibroblasts, and endothelial cells), did not alter K ATP channel-mediated relaxation in males, while K ATP channel-mediated relaxation was significantly reduced in females (Small et al., 2019).

CLINICAL THERAPY OF TARGETED K ATP CHANNELS
The functional regulation of small active molecules on K ATP channels and the potential mechanisms of mutant K ATP channels have been introduced in the previous content, and the relevant clinical effects and pharmacological mechanisms of some irre-placeable K ATP channel openers and inhibitors will be introduced.
Nicorandil is a renowned cardioprotective drug that is characterized by opening K ATP channels. It participates in the regulation of multiple signaling pathways and can be used to treat arrhythmias, chronic heart failure, stable angina, and acute coronary syndromes, including post-PCI (percutaneous coronary intervention). Nicorandil regulates coronary blood flow, protects cardiomyocytes from ischemia-reperfusion injury, alleviates endothelial dysfunction and reduces myocardial necrosis due to its K ATP channel-opening effects, thereby relieving angina symptoms and limiting infarct size and subsequent severe ischemic insult (Jiang et al., 2021). A systematic review and meta-analysis demonstrated that nicorandil could effectively improve microvascular perfusion, alleviate microvascular spasms, reduce platelet aggregation, open K ATP channels, reduce the excessive production of oxygen free radicals and myocardial ischemia, improve myocardial antioxidant capacity, and inhibit myocardial apoptosis and inflammatory reactions after ischemia to treat unstable angina pectoris and related microvascular complications (Zhang et al., 2021b).
Levosimendan is a calcium sensitization agent and K ATP channel opener that is clinically used for the treatment of decompensated heart failure, which is characterized by inducing vasodilation of the pulmonary, coronary, and peripheral arteries and venous circulation, anti-inflammatory and antioxidant effects, and then it exerts a cardioprotective effect in various settings (Herpain et al., 2019;Efentakis et al., 2020). Levosimendan may be considered for the prevention of overt acute heart failure and cardiogenic shock due to its hemodynamic and anti-ischemia effects and its pharmacodynamic properties (Cosentino et al., 2020).
Sulfonamides, as an antibacterial drug, Marcel Janbon discovered its hypoglycemic side effects, and A. Loubatières proved that its hypoglycemic mechanism is to promote insulin secretion, so they were widely used in clinic as hypoglycemic drugs . In pancreatic ß cells, when blood glucose concentration increases, intracellular ATP concentration increases with active glucose uptake and metabolism and inhibits K ATP channels, leading to cell plasma membrane depolarization, activation of voltage-gated calcium channels, and calcium influx triggering insulin release (Yang et al., 2020b). Sulfonamides bind to K ATP channel sulfonylurea receptors, inhibit the opening of K ATP channel, promote the release of insulin, and reduce blood glucose and the risk of microvascular complications associated with diabetes (Wang et al., 2019b). Sulfonylureas might increase the risk of adverse cardiovascular events, due to K ATP channel closure in the heart, in Neil Dhopeshwarkar et al. cohort included 268,094 glipizide users and 124,354 glimepiride users in Medicaid, they found that glimepiride (as opposed to glipizide) was associated with an elevated risk of sudden cardiac death/fatal ventricular arrhythmia, in Abdelmoneim et al. cohort included 7,441 gliclazide and 13 884 glyburide users, and they observed that statistically significant 14% higher risk of acute coronary syndrome was observed in patients taking glyburide compared with those taking gliclazide (Leroy et al., 2006;Rieg et al., 2020). In conclusion, the exploration of pancreatic specific K ATP channel inhibitors can Frontiers in Pharmacology | www.frontiersin.org June 2022 | Volume 13 | Article 868401 help control patients' blood glucose, reduce microvascular complications. At present, there is no specific pharmacotherapeutic treatment options are currently suitable for the diseases of K ATP channels mutation (Rosenfeld et al., 2019). Glibenclamide inhibited K ATP and slightly improved sensorimotor performance in DEND patients, but did not improve cognitive deficits caused by neuronal K ATP gain-offunction expression (Jin et al., 2020). Glibenclamide directly act on SUR1, leading to the closure of K ATP channel and the normal release of insulin, improving the growth imbalance, nervous system disorders and muscle strength of some PNDM children, but may cause hypoglycemia, temporary diarrhea, tooth staining, long Q-T syndrome and other adverse reactions (Cao et al., 2020). Chen et al. verified that Cantú Mutations C166S (Kir6.2) and S1020P (SUR2A) are inhibited by travoprost, betaxolol, and ritodrine, meanwhile, these compounds are not known to cause cardiac side effects or hypoglycemia (Chen et al., 2019). Scala et al. 's animal experiments demonstrated that glibenclamide treatment may help to reverse or avoid muscle weakness and atrophy in CS (Li et al., 2020b). In addition, in partially effective treatment regimens for patients with CHI, diazoxide opens the sarcK ATP channel and inhibits insulin secretion, octreopeptide and long-acting somatostatin analogues act downstream of the K ATP channel, inhibition of insulin secretion, subtotal pancreatectomy is used to reduce insulin production in focal and medically responsive non-focal cases (Rosenfeld et al., 2019). K ATP openers may indeed prove beneficial in some AIMS patients (Bibli et al., 2021). K ATP channel activator tifenazoxide, VU0071063 can be unlocked by opening the K ATP channel, providing CHI patients with a new pharmacological option for CHI therapy to maintain normal blood glucose and reduce drug side effects and postoperative complications (Sim et al., 2002). Interestingly, CRISPR-based genome editing techniques were found to detect changes in ABCC8 and SUR1 expression levels in type 2 diabetes, suggesting that gene editing could be useful in diagnosing and treating K ATP channel mutations in the future (Zhou et al., 2019b).

CONCLUSION
At present, no specific pharmacotherapeutic treatment options are currently suitable for CS, but glibenclamide can partially reverse the vascular symptoms of CS by inhibiting the overactivity of the K ATP channel (McClenaghan et al., 2020;Laimon et al., 2021). Sulfonylureas can inhibit the effect of ABCC8 and KCNJ11 activation mutations that prevent the closure of K ATP channels leading to insulin deficiency, reversing a condition that has historically been treated only with insulin (Laimon et al., 2021). The early ascertainment of a genetic diagnosis help us find the underlying cause which is the optimal treatment of the diseases of mutations in K ATP channels. These facts support the hypothesis that the study of K ATP channels may improve the prognosis, alleviate pain, and reduce the economic burden on patients. In the short run, Patients with cardiovascular disease and refractory K ATP channel subunit mutations will have more treatment options, more promising outcomes, and acceptable medical costs.

AUTHOR CONTRIBUTIONS
ZW contributed to literature search, and drafted the manuscript; WB, YY contributed to literature search; D-MZ contributed to review design, wrote and revised the manuscript. All authors reviewed the manuscript.