TnI Structural Interface with the N-Terminal Lobe of TnC as a Determinant of Cardiac Contractility

Abstract

The heterotrimeric cardiac troponin complex is a key regulator of contraction and plays an essential role in conferring Ca²⁺ sensitivity to the sarcomere. During ischemic injury, rapidly accumulating protons acidify the myoplasm, resulting in markedly reduced Ca²⁺ sensitivity of the sarcomere. Unlike the adult heart, sarcomeric Ca²⁺ sensitivity in fetal cardiac tissue is comparatively pH insensitive. Replacement of the adult cardiac troponin I (cTnI) isoform with the fetal troponin I (ssTnI) isoform renders adult cardiac contractile machinery relatively insensitive to acidification. Alignment and functional studies have determined histidine 132 of ssTnI to be the predominant source of this pH insensitivity. Substitution of histidine at the cognate position 164 in cTnI confers the same pH insensitivity to adult cardiac myocytes. An alanine at position 164 of cTnI is conserved in all mammals, with the exception of the platypus, which expresses a proline. Prolines are biophysically unique because of their innate conformational rigidity and helix-disrupting function. To provide deeper structure-function insight into the role of the TnC-TnI interface in determining contractility, we employed a live-cell approach alongside molecular dynamics simulations to ascertain the chemo-mechanical implications of the disrupted helix 4 of cTnI where position 164 exists. This important motif belongs to the critical switch region of cTnI. Substitution of a proline at position 164 of cTnI in adult rat cardiac myocytes causes increased contractility independent of alterations in the Ca²⁺ transient. Free-energy perturbation calculations of cTnC-Ca²⁺ binding indicate no difference in cTnC-Ca²⁺ affinity. Rather, we propose the enhanced contractility is derived from new salt bridge interactions between cTnI helix 4 and cTnC helix A, which are critical in determining pH sensitivity and contractility. Molecular dynamics simulations demonstrate that cTnI A164P structurally phenocopies ssTnI under baseline but not acidotic conditions. These findings highlight the evolutionarily directed role of the TnI-cTnC interface in determining cardiac contractility.

Figure 1

Replacement and incorporation of the adenovirus-transduced adult cardiac myocytes. (A) Shown is a representative Western blot of adult rat ventricular myocytes on day 3 after transduction, probed with a TnI antibody. Expected sizes are denoted on the left; all adenovirus-transduced cTnI isoforms are Flag tagged. All protein samples are from the same blot with the intervening lanes removed. (B) Shown is the densitometry of cTnI replacement based on a Western blot comparing exogenous (cTnI Flag band) to total cTnI (n = 3–5 for each group from two to three independent experiments). Mean ± standard error (SE) are presented.

Figure 2

Sarcomere length and cytosolic calcium dynamics at physiologic baseline. (A and B) Shown are representative traces of sarcomere length dynamics, with normalized baseline sarcomere lengths shown as a percent change in length from baseline; a group summary is also given. n = 37–55 myocytes from four independent experiments for each group. ^∗p < 0.05 from a one-way analysis of variance. (C) Shown is a representative Fura-2-acetoxymethyl ester fluorescence trace of a cTnI Flag adenovirus-transduced myocyte (solid line) and a cTnI A164P adenovirus-transduced myocyte (dotted line). (D) Peak amplitude of the calcium transient reveals no changes in the amplitude of the calcium transient; n = 6–12 myocytes from two different experiments for each group. Mean ± SE are presented, and results were not significant by Student’s t-test.

Figure 3

Sarcomere length dynamics at physiologic baseline and in acidosis. (A–C) Shown are representative traces of sarcomere length dynamics, with normalized baseline sarcomere lengths shown as a percent change in length from baseline (solid trace = pH 7.4, dotted track = pH 6.2). (D) pH 6.2 sarcomere length dynamics showing the fractional shortening summary are given. (E) Shown is the change in fractional shortening from pH 6.2 to pH 7.4. n = 37–55 myocytes from four independent experiments for each group. Mean ± SE are presented. ^∗p < 0.05 from a one-way analysis of variance.

Figure 4

Atomistic ribbon structure representations of the protein structures at the end of the 40 ns simulation. Red is cTnC 1–90, blue is cTnI 148–174 or ssTnI 115–140, and yellow spheres represent the calcium ion bound to cTnC site II. (A) Shown are cTnI-cTnC starting structures from PDB: 1J1E with key motifs labeled; amino acid position 164 is highlighted in yellow for reference. (B) Shown are HSP (histidine protonated) and HSD (histidine deprotonated) final 40 ns ribbon structures for each of the cTnI structures. In the lower right of each ribbon structure is a simplified stick figure representation of the orientation between the cTnC A helix (red) and the TnI switch peptide and helix 4 (blue). To see this figure in color, go online.

Figure 5

Atomistic ribbon structures of TnI helix 4 (blue) interactions with cTnC (red) (A) A representative 40 ns frame of ssTnI cTnI HSD showing R139 of ssTnI interaction with cTnC E15 is given. The inset shows distances between E15 and R139. (B) A representative 40 ns frame of cTnI A164P HSD showing R171 of cTnI interaction with cTnC E15 is given. The inset shows distances between E15 and R171. (C) Shown is the calculated distance measurement from TnI R139/171 for ssTnI and cTnI A164P, respectively, to cTnC E15 during the 40 ns simulation. (D) The mean distance between ssTnI R139 or cTnI A164P R171 to cTnC E15 over the entire stable 40 ns of each simulation is given. Error bars are ±SE, and ^∗p < 0.05 from a one-way analysis of variance. To see this figure in color, go online.

Figure 6

Relative Ca²⁺ binding free energy. The change in Ca²⁺ binding free energy is normalized to the average of WT cTnI, plotted as ΔΔ $G_{bind}^{wild\to mut}$. Four representative frames from all simulations for a given isoform were calculated and averaged; error bars are ±SE. Mean ± SE are presented. ^∗p < 0.05 from a Student’s t-test to the WT HSD value.

Figure 7

Summary of isoform-specific substitutions and pH as molecular determinants of TnI function. ↔ indicates no change in contractility compared to cTnI at pH 7.4, ↑ indicates increased contractility compared to cTnI at pH 7.4, and ↓ indicates decreased contractility with acidosis relative to the same isoform at baseline. Atomistic stick figures show the orientation of cTnC helix A (red) in relation to cTnI switch peptide and helix 4 (blue) from MD simulations. + indicates the presence of a stable salt bridge interaction between helix 4 of cTnI and the A helix of cTnC, and − indicates the absence of any observed stable salt bridges. To see this figure in color, go online.