Abstract
Small-conductance Ca2+-activated K+ (SK) channels mediate medium afterhyperpolarization in the neurons and play a key role in the regulation of neuronal excitability. SK channels are potential drug targets for ataxia and Amyotrophic Lateral Sclerosis (ALS). SK channels are activated exclusively by the Ca2+-bound calmodulin. Previously, we identified an intrinsically disordered fragment that is essential for the mechanical coupling between Ca2+/calmodulin binding and channel opening. Here, we report that substitution of a valine to phenylalanine (V407F) in the intrinsically disordered fragment caused a ~6 fold increase in the Ca2+ sensitivity of SK2-a channels. This substitution resulted in a novel interaction between the ectopic phenylalanine and M411, which stabilized PIP2-interacting residue K405, and subsequently enhanced Ca2+ sensitivity. Also, equivalent valine to phenylalanine substitutions in SK1 or SK3 channels conferred Ca2+ hypersensitivity. An equivalent phenylalanine substitution in the Caenorhabditis elegans (C. elegans) SK2 ortholog kcnl-2 partially rescued locomotion defects in an existing C. elegans ALS model, in which human SOD1G85R is expressed at high levels in neurons, confirming that this phenylalanine substitution impacts channel function in vivo. This work for the first time provides a critical reagent for future studies: an SK channel that is hypersensitive to Ca2+ with increased activity in vivo.
Highlights
Calcium (Ca2+) mediates a variety of cellular signaling processes, including regulation of enzymatic activities, gene expression, synaptic transmission and ion channel activities[1,2]
We considered the possibility that the SK2-a intrinsically disordered fragments (IDFs) might play a critical role in regulation of Small-conductance Ca2+-activated K+ (SK) channel Ca2+ sensitivity
The SK channels are a unique group of potassium channels with an important role in regulating membrane excitability
Summary
Calcium (Ca2+) mediates a variety of cellular signaling processes, including regulation of enzymatic activities, gene expression, synaptic transmission and ion channel activities[1,2]. We reported that SK channels are genetic modifiers in vertebrate and invertebrate models of SMA13 and that SK channels are likely a critical target for the neuroprotective effects of riluzole in these models[14] Because of their critical roles in neuronal excitability, SK channels have been proposed as a drug target for motor neuron diseases and movement disorders[13,14,15,16,17,18]. A simplified gating scheme for Ca2+-dependent SK channel activation includes two steps: (1) binding of Ca2+ to CaM associated with the SK channel and (2) mechanical coupling between Ca2+ binding to CaM, CaMBD conformation change, and consequent channel opening (Fig. 1a).
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