Adult teleost heart expresses two distinct troponin C paralogs: cardiac TnC and a novel and teleost-specific ssTnC in a chamber- and temperature-dependent manner

Abstract

The teleost-specific whole genome duplication created multiple copies of genes allowing for subfunctionalization of isoforms. In this study, we show that the teleost cardiac Ca2+-binding troponin C (TnC) is the product of two distinct genes: cardiac TnC (cTnC, TnnC1a) and a fish-specific slow skeletal TnC (ssTnC, TnnC1b). The ssTnC gene is novel to teleosts as mammals have a single gene commonly referred as cTnC but which is also expressed in slow skeletal muscle. In teleosts, the data strongly indicate that these are two TnC genes are different paralogs. Because we determined that ssTnC exists across many teleosts but not in basal ray-finned fish (e.g., bichir), we propose that these paralogs are the result of an ancestral tandem gene duplication persisting only in teleosts. Quantification of mRNA levels was used to demonstrate distinct expression localization patterns of the paralogs within the chambers of the heart. In the adult zebrafish acclimated at 28°C, ssTnC mRNA levels are twofold greater than cTnC mRNA levels in the atrium, whereas cTnC mRNA was almost exclusively expressed in the ventricle. Meanwhile, rainbow trout acclimated at 5°C showed cTnC mRNA levels in both chambers significantly greater than ssTnC. Distinct responses to temperature acclimation were also quantified in both adult zebrafish and rainbow trout, with mRNA in both chambers shifting to express higher levels of cTnC in 18°C acclimated zebrafish and 5°C acclimated trout. Possible subfunctionalization of TnC isoforms may provide insight into how teleosts achieve physiological versatility in chamber-specific contractile properties.

Characterization of Zebrafish Cardiac and Slow Skeletal Troponin C Paralogs by MD Simulation and ITC

“…Ectothermic species tolerate a range of acute and seasonal temperatures through plasticity in protein function and changes in gene expression (1,2). Central to this tolerance is the maintenance of cardiac function across a range of temperatures.…”

“…The adaptation to acute temperature changes occurs more quickly than transcriptional changes can account for. Long-term temperature acclimation confers greater tolerance through altered composition of the cardiomyocyte transcriptome, which includes critical proteins of the contractile apparatus such as members of the troponin complex (1,12).…”

Cold acclimation increases cardiac myofilament function and ventricular pressure generation in trout

“…Previous studies have demonstrated that cold acclimation of rainbow trout results in an increase in the maximal rate of cardiac actomyosin ATPase (AM-ATPase) (Yang et al, 2000;Klaiman et al, 2011) and the speed of twitch kinetics of intact fibers (Aho and Vornanen, 1999), indicating changes to the contractile machinery of the heart. Cold acclimation of trout also affects transcript levels of troponin I (Alderman et al, 2012), troponin C (Genge et al, 2013) and sarcoplasmic reticulum Ca 2+ -ATPase (SERCA) (Korajoki and Vornanen, 2012), all of which are key proteins involved in contraction. Korajoki and Vornanen (Korajoki and Vornanen, 2013) have also found that cold acclimation of burbot causes a fourfold increase in the levels of SERCA in the heart, a modification thought to help maintain Ca 2+ cycling at low temperatures.…”

The regulation of troponins I, C and ANP by GATA4 and NKX2-5 in heart of hibernating thirteen-Lined ground squirrels, ictidomys tridecemlineatus

“…5). This theory is further complemented by another study which has shown that coldtolerant teleosts produce more cardiac troponin C when acclimated to colder temperatures [58]. Similarly, the temperature-dependent troponin C expression and increased Ca 2+ affinity in teleost cardiac troponin C has also been proposed to be an adaptive mechanism to allow for proper contraction at lower temperatures [58].…”

“…This theory is further complemented by another study which has shown that coldtolerant teleosts produce more cardiac troponin C when acclimated to colder temperatures [58]. Similarly, the temperature-dependent troponin C expression and increased Ca 2+ affinity in teleost cardiac troponin C has also been proposed to be an adaptive mechanism to allow for proper contraction at lower temperatures [58]. Indeed, our data, along with data observed in teleosts [58], would support the hypothesis that increases in troponin C expression may occur in other cold-adapted animals and, by extension, may represent a generalized adaptive mechanism which preserves cardiomyocyte function at cold temperatures.…”