Tuning the molecular giant titin through phosphorylation: role in health and disease

Abstract

Titin is a giant multi-functional filament that spans half of the sarcomere. Titin's extensible I-band region functions as a molecular spring that provides passive stiffness to cardiac myocytes. Elevated diastolic stiffness is found in a large fraction of heart failure patients and thus understanding the normal mechanisms and pathophysiology of passive stiffness modulation is clinically important. Here we provide first a brief general background on titin including what is known about titin isoforms and then focus on recently discovered post-translational modifications of titin that alter passive stiffness. We discuss the various kinases that have been shown to phosphorylate titin and address the possible roles of titin phosphorylation in cardiac disease, including heart failure with preserved ejection fraction (HFpEF).

Figure 1

A) Schematic of titin in the cardiac sarcomere. Single titin molecules (shown in blue and yellow) span from Z-disk (N-terminus) to M-band (C-terminus). B) Composition of extensible I-band region of the N2B and N2BA titin isoforms (found in adults). Red blocks denote Ig-like domains, blue is unique sequence and yellow is PEVK sequence. Also indicated are known phosphorylation sites for PKA/PKG (blue) and PKCα (yellow). C) Schematic of force-extension curves of titin isoforms and the effects of phosphorylation on passive tension.

Figure 2

A) Schematic of titin in the cardiac sarcomere with at bottom the domain structure of regions within titin that are phosphorylated. There are two unique sequences in the Z-disk (Zis1 (exon 6) and Zis5(exon 25)) that are phosphorylated by ERK1/2 and cdc2 kinase(Sebestyen et al., 1995; Gautel et al., 1996); the large unique sequence in N2B element (exon 49) is phosphorylated on 3 sites by ERK2(Raskin et al., 2012) and on one by PKA and PKG(Kruger et al., 2009); the PEVK is phosphorylated by PKCα and CaMKIIδ (exons 219 and 225)(Hidalgo et al., 2009; Hidalgo et al., 2012) and a unique sequence in the M-band region (Mis4(exon 358)) is phosphorylated by cdc2(Sebestyen et al., 1995). B) Conservation of titin sequence that can be phosphorylated. Bottom shows the human titin sequence with in red the residue that is phosphorylated (number in superscript is based on Q8WZ42-1). (Rhesus monkey F7FBM0, bovine F1N757, pig F1RZC8, dog F1PV45, rabbit G1U9S3, rat F1M7S9, and mouse E9Q8N1 all in UnitProKB Protein Knowledgebase). Each row indicates a different species and each column a different phospho site in the human titin sequence. Sites that are absent are indicated by a cross. Except for the sites in the N2B unique sequence where conservation is limited all sites are highly conserved. C) Protein kinases that have been shown to phosphorylate the indicated sites.