Gating mechanisms underlying deactivation slowing by atrial fibrillation mutations and small molecule activators of KCNQ1
Creators: Peng, Gary
Publisher: name: Columbia University
Year: 2017
Subjects: Atrial fibrillation
Potassium channels
Membrane proteins
Arrhythmia--Pathophysiology
Ion channels
Biophysics

description: Ion channels are membrane proteins that facilitate electrical signaling in important physiological processes, such as the rhythmic contraction of the heart.  KCNQ1 is the pore-forming subunit of a voltage-gated potassium channel that assembles with the β-subunit KCNE1 in the heart to generate the IKs current, which is critical to cardiac action potential repolarization and electrical conduction in the heart.  Mutations in IKs subunits can cause potentially lethal arrhythmia, including long QT syndrome, short QT syndrome, and atrial fibrillation.  Each channel consists of four voltage-sensing domains and a central pore through which ions permeate.  Voltage-dependent gating occurs when movement of voltage sensors cause pore opening/closing through coupling mechanisms.  Although KCNQ1 by itself is able to form a voltage-dependent potassium channel, its assembly with KCNE1 is essential to generating the physiologically critical cardiac IKs current, characterized by a delay in the onset of activation, an increase in current amplitude, and a depolarizing shift in the current-voltage relationship.  KCNE1 is thought to have multiple points of contact with KCNQ1 that reside within both the voltage-sensing domain and the pore domain, allowing for extensive modulation of channel function.
      Atrial fibrillation is the most common cardiac arrhythmia and affects more than 3 million adults in the United States.  Much rarer, genetic forms of atrial fibrillation have been associated with gain-of-function mutations in KCNQ1, such as two adjacent mutations, S140G and V141M.  Both mutations drastically slow channel deactivation, which underlies their pathophysiology.  Deactivation slowing causes accumulation of open channels in the context of repeated stimulation, which abnormally increases the repolarizing K+ current, excessively shortens the action potential duration, and predisposes to re-entry arrhythmia such as atrial fibrillation.  Although both mutations are located in the voltage-sensing domain, their mechanisms of action remain unknown.  Understanding the gating mechanisms underlying deactivation slowing may provide key insights for the development of mechanism-based pharmacologic therapies for arrhythmias associated with KCNQ1 mutations.
      In addition to gain-of-function mutations, molecular activators of KCNQ1 can slow deactivation and increase channel activity. An existing problem in the pharmacologic treatment of arrhythmia is that many antiarrhythmic drugs do not have specific targets and cause undesired side effects such as additional arrhythmia.  Thus, developing mechanism-based therapies may optimize clinical treatment for patients with specific forms of channel dysfunction.  Two KCNQ1 activators, ML277 and R-L3, have been previously shown to slow current deactivation, but the underlying gating mechanisms remain known.  Although these modulators are unlikely to serve directly as antiarrhythmic therapy, investigating their mechanisms will likely provide fundamental insights on channel modulation and guide future efforts to develop personalized therapies for arrhythmia, such as congenital long QT syndrome.
      Given the central importance of deactivation slowing in both pathophysiology and pharmacology, we focused on investigating gating mechanisms that underlie deactivation slowing.  To this end, we utilized voltage clamp fluorometry, a technique that simultaneously assays for voltage sensor movement and ionic current through the channel pore.  In Chapter 1, we begin our study by examining the gating mechanisms of KCNQ1 atrial fibrillation mutations in the absence of KCNE1.  We show that S140G slows voltage sensor deactivation, which indirectly slows current deactivation.  On the other hand, V141M neither slows voltage sensor nor current deactivation.  This is followed by Chapter 2, where we examine the gating mechanisms underlying deactivation slowing by atrial fibrillation mutations in the presence of KCNE1.  We show that both S140G and V141M slow IKs deactivation by slowing pore closing and altering voltage sensor-pore coupling.  Based on these findings, we proposed a molecular mechanism in which both mutations disrupt the orientation of KCNE1 relative to KCNQ1 and thus impede pore closing, implying that future efforts to modulate KCNQ1 function can benefit from targeting the β-subunit.  Finally, in Chapter 3, we explore the gating mechanisms underlying deactivation slowing for two small-molecule activators of KCNQ1.  We show that ML277 predominantly slows pore transitions, whereas R-L3 slows voltage sensor deactivation, which indirectly slows current deactivation.  Taken together, these studies guide future efforts to develop mechanism-based therapies for arrhythmia.