Alternative splice isoforms of small conductance calcium-activated SK2 channels differ in molecular interactions and surface levels

Abstract

Small conductance Ca2+-sensitive potassium (SK2) channels are voltage-independent, Ca2+-activated ion channels that conduct potassium cations and thereby modulate the intrinsic excitability and synaptic transmission of neurons and sensory hair cells. In the cochlea, SK2 channels are functionally coupled to the highly Ca2+ permeant α9/10-nicotinic acetylcholine receptors (nAChRs) at olivocochlear postsynaptic sites. SK2 activation leads to outer hair cell hyperpolarization and frequency-selective suppression of afferent sound transmission. These inhibitory responses are essential for normal regulation of sound sensitivity, frequency selectivity, and suppression of background noise. However, little is known about the molecular interactions of these key functional channels. Here we show that SK2 channels co-precipitate with α9/10-nAChRs and with the actin-binding protein α-actinin-1. SK2 alternative splicing, resulting in a 3 amino acid insertion in the intracellular 3′ terminus, modulates these interactions. Further, relative abundance of the SK2 splice variants changes during developmental stages of synapse maturation in both the avian cochlea and the mammalian forebrain. Using heterologous cell expression to separately study the 2 distinct isoforms, we show that the variants differ in protein interactions and surface expression levels, and that Ca2+ and Ca2+-bound calmodulin differentially regulate their protein interactions. Our findings suggest that the SK2 isoforms may be distinctly modulated by activity-induced Ca2+ influx. Alternative splicing of SK2 may serve as a novel mechanism to differentially regulate the maturation and function of olivocochlear and neuronal synapses.

Mesenchymal stem cell differentiation: Control by calcium‐activated potassium channels

“…SK channels are encoded by three genes: SK1/KCNN1 (4 splice variants), SK2/KCNN2 (7 splice variants), and SK3/KCNN3 (5 splice variants) (Cunningham et al, ). The SK2‐ARK transcript was shown (i) to have an insertion of three amino acids (Alanine–Arginine–Lysine) that reduced its cell‐surface expression and (ii) to be developmentally regulated in the cochlea (Scholl, Pirone, Cox, Duncan, & Jacob, ). Thus, the alternate transcripts generate considerable functional diversity by determining sensitivity to Ca 2+ and V m as well as domain interactions and subcellular localization (Johnson et al, ; Scholl et al, ).…”

Physiological Roles and Therapeutic Potential of Ca2+ Activated Potassium Channels in the Nervous System

“…They are ubiquitously expressed and sensitive to iberiotoxin, charybdotoxin, and paxilline ( Latorre et al, 2017 ). The diversity of K Ca physiological roles is widened by the existence of numerous splicing variants (specially for BK and SK channels) in different tissues ( Navaratnam et al, 1997 ; Shipston, 2001 ; Chen et al, 2005 ; Scholl et al, 2014 ), as well as their association with auxiliary subunits. In vertebrates, BK channels co-assemble with modulatory β (β 1-4 ) and γ (γ 1-4 ) subunits to modify the channel functional properties and pharmacology ( Latorre et al, 2017 ).…”

A V-to-F substitution in SK2 channels causes Ca2+ hypersensitivity and improves locomotion in a C. elegans ALS model

“…For example, phosphorylation of CaM threonine79 (T79) inhibits SK channels; this led to creation of the phosphomimetic CaM-T79D substitution that negatively modulates SK2-a channel Ca 2+ sensitivity 8 . Or, in another example, an alternative RNA splice variants of rat or chicken SK2 channels (SK2-b) reduces channel Ca 2+ sensitivity 35 , 41 . Also, disturbing positively charged residues in the putative SK2 PIP 2 binding site decreases Ca 2+ sensitivity 31 , 32 .…”