Charged residue alterations in the inner-core domain and carboxy-terminus of α-tropomyosin differentially affect mouse cardiac muscle contractility

Two important charge differences between the α- and, β-tropomyosin (TM) isoforms are the exchange of a serine residue in the inner-core region at position 229, and a histidine residue at the carboxy-terminal end at position 276, with glutamic acid and asparagine, respectively. We have recently shown that altering these two residues in α-TM to their β-TM counterparts in transgenic (TG) mouse hearts causes a depression in both +dP/dt and -dP/dt and a decrease in calcium sensitivity. In this study, we address whether independent charge changes at these two residues in α-TM modulate cardiac function differentially. To test this hypothesis we generated two TG lines: α-TMSer229Glu and α-TMHis276Asn. Molecular analyses show that 98% of native α-TM is replaced by mutated protein in α-TM229 hearts whereas α-TM276 hearts show 82% replacement with the mutated protein. Isolated working heart data show that α -TM229 TG hearts exhibit a significant decrease in both +dP/dt (7%) and -dP/dt (8%) compared with nontransgenics (NTGs) and time to peak pressure (TPP) is also reduced in α-TM229 hearts. α-TM276 hearts show a decrease only in -dP/dt (14%) and TPP is increased. pCa²⁺-tension relationships in skinned fibre preparations indicate decreased calcium sensitivity in α-TM229 but no change in α-TM276 preparations. Force-[Ca²⁺]_IC measurements from intact papillary fibres indicate that α-TM276 fibres produce more force per given [Ca²⁺]_IC when compared to NTG fibres, while α-TM229 fibres produce less force per given [Ca²⁺]_IC. These data demonstrate that changing charged residues at either the inner-core domain or the carboxyl end of TM alters sarcomeric performance differently, suggesting that the function of TM is compartmentalized along its length.