Novel Cardiac Troponin I S43/45N Substitution does not Cause Aberrant Cardiac Remodeling or Dysfunction in Mice

Abstract

Elevated protein kinase C (PKC) expression and activity, along with downstream phosphorylation of cardiac troponin I (cTnI) are linked to cardiac dysfunction. Specifically, elevated cTnI S43/45 phosphorylation is reported in human and rodent models of heart failure, but the exact role of phosphorylation at this site is still debated. Transgenic (tg) mice expressing cardiac-specific phospho-mimetic S43/45D develop contractile dysfunction, followed by cardiac hypertrophy, fibrosis, and progressive heart failure. Thus, S43/45 phosphorylation is postulated to be a sarcomeric node for modulating contractile function in the heart. To further test this idea, it is critical to show that a phospho-null substitution at this site does not alter wild-type function. Multiple groups have used the conventional nonpolar alanine (A) phospho-null substitution at S43/45 and report diminished myocyte and contractile function, which resembles the outcome after phosphorylation. We postulate the nonpolar properties of A potentially alter interactions between cTnI, troponin T (TnT) and C (TnC). As a result, the amide R side chain in asparagine (N), with a polarity somewhat similar to the hydroxyl group in S, could make N a reasonable non-phosphorylatable residue without overt effects on wild-type function. Previously, adult rat myocytes virally transduced with S43/45N (SN)in our laboratory did not display overt changes in contractile function compared to wild-type myocytes. In ongoing experiments, generation of tg mice expressing a cardiac-specific cTnISN also show no changes in in vivo cardiac contractile function or evidence of remodeling in mice up to at least four months of age. These findings suggest the novel N substitution acts as a “functionally conservative” phospho-null substitution at cTnIS43/45 in myocardium.