Cav1.2 Ca2+ channel knock‐down alters colonic smooth muscle function

Abstract

Timothy Syndrome (TS) arises from a point mutation in the voltage‐gated Cav1.2 L‐type Ca2+ channel in exon 8 (TS1) or exon 8a (TS2). We used a point mutation knock‐in technique to create a TS2 mouse (TS2 NEO) which contains the TS2 G406R mutation in exon 8a, and a NEO cassette‐induced Cav1.2 knock‐down. In vitro Topo cloning determined 17% of colonies expressed exon 8a (0% mutant) in distal colon of TS negative mice, and 30% of colonies (35% mutant) in TS2 NEO mice. To determine the effects of the mutation on function, we measured isometric force generation in cross sectional rings (circular muscle) or longitudinal strips (longitudinal muscle) of proximal colon using force‐displacement transducers. In circular muscle in Ca2+‐free, high‐K+, bath less tension was generated during a Ca2+ dose‐response in TS2 NEO mice compared to negative littermates. Electric field stimulation (EFS) of longitudinal muscle from TS negative mice produced frequency‐dependent relaxation followed by rebound contraction (EFS off), consistent with release of multiple neurotransmitters; muscle from TS2 NEO littermates produced a strong EFS off contraction, but only a weak relaxation response. TS2 NEO is a Cav1.2 knock‐down mouse, consistent with reduced Ca2+ entry into colonic circular smooth muscle and reduced enteric nerve‐stimulated relaxation of longitudinal smooth muscle. (DOD‐USAMRMC support)