Charged residue changes in the carboxy-terminus of α-tropomyosin alter mouse cardiac muscle contractility

摘要

Striated muscle tropomyosin (TM) is an essential thin filament protein that is sterically and allosterically involved in calcium-mediated cardiac contraction. We have previously shown that overexpressing the β-TM isoform in mouse hearts leads to physiological changes in myocardial relaxation and Ca²⁺ handling of myofilaments. Two important charge differences in β-TM compared to α-TM are the exchange of serine and histidine at positions 229 and 276 with glutamic acid and asparagine, respectively, imparting a more negative charge to β-TM relative to α-TM. Our hypothesis is that the net charge at specific sites on TM might be a major determinant of its role in modulating cardiac muscle performance and in regulating Ca²⁺ sensitivity of the myofilaments. To address this, we generated transgenic (TG) double mutation mouse lines (α-TM DM) expressing mutated α-TM at the two residues that differ between α- and β-TM (Ser229Glu + His276Asn). Molecular analyses show 60–88% of the native TM is replaced with α-TM DM in the different TG lines. Work-performing heart analyses show that α-TM DM mouse hearts exhibit decreased rates of pressure development and relaxation (+dP/dt and –dP/dt). Skinned myofibre preparations from the TG hearts indicate a decrease in calcium sensitivity of steady state force. Protein modelling studies show that these two charge alterations in α-TM cause a change in the surface charges of the molecule. Our results provide the first evidence that charge changes at the carboxy-terminal of α-TM alter the functional characteristics of the heart at both the whole organ and myofilament levels.