Molecular mechanisms regulating cardiac contractility: subfunctionalization of fish‐specific paralogs of troponin C in Danio rerio

Abstract

Myofilament Ca2+ sensitivity is a critical factor in regulating cardiac contractility. The key regulator, cardiac troponin (cTn), is made up of three proteins (cTnC, cTnI, cTnT). Though Ca2+‐activated component of cTn,TnC is highly conserved across phylogenetically diverse groups, subtle variations are seen as interspecies orthologs and intraspecies paralogs between tissues. In mammals, two forms of TnC exist with distinct localization patterns (fast skeletal muscle and slow skeletal/cardiac muscle) and Ca2+ affinities, but in fish a third paralog exists annotated ssTnC in zebrafish. Tissue‐specific mRNA profiles in adult zebrafish revealed a distinct localization of gene expression, with ssTnC preferentially expressed in the atrium and cTnC is almost exclusively expressed in the zebrafish ventricle, which may reflect the differing contractile properties between the cardiac chambers. The functionality of these differences is being compared biochemically through measurement of Ca2+ binding kinetics. This subfunctionalization of TnC isoforms lends insight the consequences of variation to the structure‐function of cTn and variation in cardiac muscle contraction.