Borrowing from the Platypus: Proline Substitution in Cardiac Troponin I

Abstract

166 million years ago, egg-laying mammals branched off the evolutionary tree from their viviparous counterparts to give rise to the modern order Monotremata. There are only five extant monotremes, the platypus and four species of echidna; consequently the platypus represents a unique divergence in mammalian evolution. We investigated whether unique adaptations in cardiac sarcomere arising in these species confer physiological performance advantages. The heterotrimeric cardiac troponin complex (cTn) is a key regulator of contractile events, and confers calcium-sensitivity to the sarcomere. During ischemic-injury, protons accumulate in the myoplasm and acidify the sarcomere, resulting in markedly reduced calcium-sensitivity. Unlike its adult counterpart, force generation in fetal cardiac tissue is uniquely pH-insensitive. Replacement of the adult cardiac troponin I isoform (cTnI) with the fetal isoform (ssTnI) renders adult cardiac tissue relatively insensitive to acidification. Alignment and function studies have revealed that this insensitivity is derived almost exclusively from the histidine at position 132 in ssTnI. Substitution of histidine at the cognate position 164 in cTnI confers the same pH insensitivity to adult tissue. TNNI3, the gene encoding for cTnI, encodes for an alanine at position 164 in all known mammals, except the Platypus whose TNNI3 encodes for a proline. Prolines are biophysically intriguing due their helix-breaking nature; position 164 occurs in helix 4 of TnI, an important motif belonging to the critical switch region of cTnI. MD simulations of the cTnI switch region interaction with the N-terminal domain of troponin C provide evidence of a conformational change in this region that may play a role in how the switch region interacts with troponin C. The functional and biophysical consequences of a cTnI-A164P replacement in the context of isolated myocytes by acute gene transfer as well as molecular dynamics simulations will be discussed.