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    Molecular Pathophysiology of Cloned KATP Channels and Implications for Cardiovascular Disease

    Smith, Keith

    [Thesis]. Manchester, UK: The University of Manchester; 2014.

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    Abstract

    ATP-sensitive potassium (KATP) channels are a class of ion channels involved in a multitude of cellular roles. The hallmark feature of these ion channels is that they are inhibited by a rise in intracellular ATP; thus, KATP channels serve as a link between cell metabolism and cell electrical activity. KATP channels are octameric complexes of two subunits: four Kir6.x subunits coassemble to form the channel pore, which is surrounded by four auxiliary SURx subunits that modulate ATP-dependent gating of the channel. Different combinations of Kir6.x and SURx subunits give rise to KATP channels in different tissues.In the heart, KATP channel activation under ischemic conditions results in shortening of the cardiac myocyte action potential and reduces contractility. In this way, KATP channels reduce calcium influx and ATP consumption at times of metabolic stress. In the vascular smooth muscle, KATP channels modulate the membrane resting potential and control arterial tone. Genetic deletion of vascular KATP channels in mice gives rise to a phenotype resembling Prinzmetal angina in humans. Here, we hypothesize that a genetic mutation in the cardiovascular KATP channel in man may result in disease such as angina or cardiomyopathy. In collaboration with the University of Pavia, 1123 patients diagnosed with acute myocardial infarction (AMI) were examined and genetically screened for mutations in the SUR2 gene (ABCC9). It was found that 11 patients presented AMI of vasospastic origin and each patient in this subgroup presented the same single-point mutation in ABCC9. The mutation results in the substitution of valine 734 into isoleucine (V734I) in the SUR2 protein. The main aim of this study was to define the pathophysiological mechanism underscoring the effects of the mutation, and the cause of acute myocardial infarction. The main results and findings are:1) Wild-type channels composed of Kir6.2/SUR2A (cardiac), Kir6.2/SUR2B (endothelial) and Kir6.1/SUR2B (vascular smooth muscle) were reconstituted in a heterologous expression system and the sensitivity of these channels to intracellular MgATP and physiological nucleotide diphosphates (MgNDP) (MgADP, MgGDP and MgUDP) examined with the patch-clamp technique.2) Electrophysiological analysis revealed that the mutation reduced the sensitivity to MgATP inhibition of Kir6.2/SUR2B channels but not of Kir6.2/SUR2A and Kir6.1/SUR2B channels. Furthermore, the stimulatory effects of MgNDP were unaltered in mutant Kir6.2/SUR2A and Kir6.1/SUR2B channels. In contrast, mutant channels composed of Kir6.2 and SUR2B subunits were less sensitive to MgNDP activation. 3) Generation of a structural molecular homology model indicated that V734I may be situated in a vicinity of the MgATP binding site at SUR2.4) The antianginal drug nicorandil activated Kir6.2/SUR2B channels containing the mutation, thus substituting for the loss of MgNDP stimulation, suggesting that this drug could be efficacious in the treatment of AMI associated with V734I.In conclusion, these findings indicate that coronary spasm and AMI in patients presenting V734I are precipitated by an altered KATP channel response to intracellular nucleotides. These findings suggest that vascular KATP channels are a possible pharmacological target for the treatment AMI of vasospastic origin. Furthermore, the results of this research indicate that the region where V734I is located is a domain of the channel involved in ATP-dependent gating.

    Bibliographic metadata

    Type of resource:
    Content type:
    Form of thesis:
    Type of submission:
    Degree type:
    Master of Philosophy
    Degree programme:
    MPhil Physiology (PT)
    Publication date:
    Location:
    Manchester, UK
    Total pages:
    126
    Abstract:
    ATP-sensitive potassium (KATP) channels are a class of ion channels involved in a multitude of cellular roles. The hallmark feature of these ion channels is that they are inhibited by a rise in intracellular ATP; thus, KATP channels serve as a link between cell metabolism and cell electrical activity. KATP channels are octameric complexes of two subunits: four Kir6.x subunits coassemble to form the channel pore, which is surrounded by four auxiliary SURx subunits that modulate ATP-dependent gating of the channel. Different combinations of Kir6.x and SURx subunits give rise to KATP channels in different tissues.In the heart, KATP channel activation under ischemic conditions results in shortening of the cardiac myocyte action potential and reduces contractility. In this way, KATP channels reduce calcium influx and ATP consumption at times of metabolic stress. In the vascular smooth muscle, KATP channels modulate the membrane resting potential and control arterial tone. Genetic deletion of vascular KATP channels in mice gives rise to a phenotype resembling Prinzmetal angina in humans. Here, we hypothesize that a genetic mutation in the cardiovascular KATP channel in man may result in disease such as angina or cardiomyopathy. In collaboration with the University of Pavia, 1123 patients diagnosed with acute myocardial infarction (AMI) were examined and genetically screened for mutations in the SUR2 gene (ABCC9). It was found that 11 patients presented AMI of vasospastic origin and each patient in this subgroup presented the same single-point mutation in ABCC9. The mutation results in the substitution of valine 734 into isoleucine (V734I) in the SUR2 protein. The main aim of this study was to define the pathophysiological mechanism underscoring the effects of the mutation, and the cause of acute myocardial infarction. The main results and findings are:1) Wild-type channels composed of Kir6.2/SUR2A (cardiac), Kir6.2/SUR2B (endothelial) and Kir6.1/SUR2B (vascular smooth muscle) were reconstituted in a heterologous expression system and the sensitivity of these channels to intracellular MgATP and physiological nucleotide diphosphates (MgNDP) (MgADP, MgGDP and MgUDP) examined with the patch-clamp technique.2) Electrophysiological analysis revealed that the mutation reduced the sensitivity to MgATP inhibition of Kir6.2/SUR2B channels but not of Kir6.2/SUR2A and Kir6.1/SUR2B channels. Furthermore, the stimulatory effects of MgNDP were unaltered in mutant Kir6.2/SUR2A and Kir6.1/SUR2B channels. In contrast, mutant channels composed of Kir6.2 and SUR2B subunits were less sensitive to MgNDP activation. 3) Generation of a structural molecular homology model indicated that V734I may be situated in a vicinity of the MgATP binding site at SUR2.4) The antianginal drug nicorandil activated Kir6.2/SUR2B channels containing the mutation, thus substituting for the loss of MgNDP stimulation, suggesting that this drug could be efficacious in the treatment of AMI associated with V734I.In conclusion, these findings indicate that coronary spasm and AMI in patients presenting V734I are precipitated by an altered KATP channel response to intracellular nucleotides. These findings suggest that vascular KATP channels are a possible pharmacological target for the treatment AMI of vasospastic origin. Furthermore, the results of this research indicate that the region where V734I is located is a domain of the channel involved in ATP-dependent gating.
    Thesis main supervisor(s):
    Thesis co-supervisor(s):
    Thesis advisor(s):
    Language:
    en

    Institutional metadata

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    Record metadata

    Manchester eScholar ID:
    uk-ac-man-scw:227797
    Created by:
    Smith, Keith
    Created:
    24th June, 2014, 17:05:22
    Last modified by:
    Smith, Keith
    Last modified:
    1st July, 2019, 14:07:37

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