Titin (TTN): from molecule to modifications, mechanics, and medical significance

Figures

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  Figure 1. Known changes in cardiac titin properties under physiological stress and as a cause or consequence of heart disease.

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  Figure 2. Titin isoforms in cardiac muscle, isoform switch, and consequences for titin-based stiffness. (A) Architecture of cardiac titin N2B and N2BA isoforms in the half-sarcomere. Note the abbreviated A-band region (double-line break). (B) Titin isoforms (‘Full length’ is a theoretical variant); only main I-band segments are shown. Dotted lines indicate alternative splicing. (C) Titin species separated on a typical Coomassie-stained titin gel (1.8% SDS–PAGE) of adult human heart tissue. (D) The proportion of N2BA (green) vs. N2B (red) titin isoforms can change under numerous conditions. This isoform switch determines a change in titin spring stiffness. AngII, angiotensin II; HFpEF, heart failure with preserved ejection fraction; HFrEF, heart failure with reduced ejection fraction; Ig, immunoglobulin-like domain; PEVK, segment rich in proline (P), glutamic acid (E), valine (V), and lysine (K) residues; RBM20, RNA-binding motif protein 20; T3, thyroid hormone.

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  Figure 3. Locations and proposed mechanism of titin acetylation. (A) Titin acetylation sites detected in rat heart tissue by mass spectrometry (data obtained from Ref.70). (B) Locally increased acetylation of elastic titin in HFpEF may be due to reduced deacetylase activity of SIRT1; this increases titin-based stiffness. Treatment with NAD+/NAM restores SIRT1 activity to reverse this process. Brown arches depict titin spring stiffness. aa, amino acid.

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  Figure 4. Titin oxidation sites and mechanisms of how S-glutathionylation and disulphide bonding alter titin stiffness in health and disease. (A) Titin oxidation sites detected in mouse heart tissue by mass spectrometry (data obtained from Ref.76). (B) Changes in titin stiffness are dependent on whether more S-glutathionylation or more disulphide bonding occurs in unfolded I-band titin domains. Increased compliance is mediated via the S-glutathionylation mechanism involving Ig domains and increased stiffness via the disulphide bonding mechanism involving Ig domains or the N2Bus. Brown arches depict titin spring stiffness. (C) Observed changes in I-band titin oxidation under cardiac stress (increased preload/afterload) or in diseased states (ischaemia, HFpEF) modulate titin stiffness in a complex manner. S-glutathionylation under cardiac stress can cause controlled in-register aggregation of distal I-band titin, whereas disease states can be associated with a general increase in titin oxidation. aa, amino acid; HFpEF, heart failure with preserved ejection fraction; Ig, immunoglobulin-like; UnDOx, unfolded domain oxidation.

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  Figure 5. Where and how titin becomes phosphorylated, how this affects titin stiffness, and the role titin phosphorylation changes play in heart disease. (A) The number of times a titin phosphorylation site has been referenced (ref.) in the literature from either high-throughput (mass spectrometry; blue) or low-throughput, site-specific (antibody detection, mutagenesis; red) detection in human heart tissue. Data obtained from www.phosphosite.org.83 Note that phosphosite S4062 is currently not detected in this database but has been added based on the publication by Ref.95 (B) Specific phosphorylation sites detected in different regions of titin, known PKs/phosphatases involved, and differential effects of I-band titin phosphorylation on titin stiffness. (C) Hypo-phosphorylation of the N2Bus and hyper-phosphorylation of constitutively expressed PEVK titin are thought to increase titin stiffness in HF. (D) Proposed treatment strategies in pre-clinical tests aimed at correcting titin phosphorylation in animal models of heart disease, to reduce titin-based cardiac stiffness. ‘Trial’ highlights results of large clinical trials on human HF patients using the respective drugs. Brown arches depict…

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  Figure 6. Titin-PQC pathways. (A) Interaction of different regions in cardiac titin (N2BA isoform) with components of the PQC machinery: UPS, ubiquitin-proteasome system (purple), proteases (blue), chaperones (green), and autophagy-lysosomal pathways (yellow). (B) Heart disease-related changes in PQC pathways, effects on titin, and suggestions for therapeutic approaches that could be taken to improve titin properties but are still speculative. CRYAB, alpha-B-crystallin; HSP27, heat shock protein 27; HSP90, heat shock protein 90; Ig, immunoglobulin-like domain; Mdm2, mouse-double-minute 2 homolog; MMP2, matrix metalloproteinase-2; MuRF1/2, muscle RING-finger protein-1/2; Nbr1, neighbor-of-BRCA1-gene-1; PEVK, segment rich in proline (P), glutamic acid (E), valine (V), and lysine (K) residues; Smyd2, SET and MYND domain containing protein 2; SQSTM1/p62, sequestosome 1; T-CAP, telethonin; Ub, ubiquitin.

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  Figure 7. Pathomechanisms observed in human cardiomyopathy due to a TTNtv. (A) Titin domain architecture (according to human titin meta-transcript) and TTN exon usage in adult human hearts.150 PSI above 90% indicates (nearly) constitutive expression, lower PSI values are due to differential splicing. (B) Location of heterozygous TTNtv detected by next-generation sequencing in the general population and in DCM patients, and odds ratio indicating the chance to get DCM if the TTNtv is in a specific titin region (marked by red boxes).150 (C) Recently identified pathomechanisms of human TTNtv-DCM.4 (D) Specific treatment strategies suggested.4