Titin Post-Translational Modifications (PTMs)
Definition
Post-translational modifications (PTMs) of titin — primarily acetylation, oxidation, and phosphorylation — are the principal mechanisms by which the heart fine-tunes titin spring stiffness on a near-beat-to-beat basis. Each PTM type acts at specific regions of the elastic I-band and has opposing or complementary effects on titin extensibility. Dysregulation of these PTMs is a consistent feature of heart failure, making them attractive but so far clinically elusive therapeutic targets.
Key Concepts
Acetylation
- Acetylation sites detected along the entire titin molecule by mass spectrometry (rat heart tissue). (sources/TTN-CVResearch-2022, rating: high)
- Proposed mechanism: In HFpEF, reduced SIRT1 (sirtuin-1) deacetylase activity → increased lysine acetylation of elastic titin → addition of negative charges to a positively charged environment → intramolecular interactions within the spring region → increased titin stiffness and diastolic dysfunction. (sources/TTN-CVResearch-2022, rating: high)
- NAD⁺/NAM treatment: Nicotinamide (NAM), a precursor of NAD⁺, restores SIRT1 activity → deacetylation of titin → reduced cardiomyocyte stiffness in HFpEF rat models. Direct SIRT1 application reduced stiffness of skinned rat cardiomyocytes. (sources/TTN-CVResearch-2022, rating: high)
- Conflicting data: HDAC inhibitors (which increase acetylation) also ameliorate diastolic dysfunction in some animal models — suggesting acetylation effects are complex and not yet fully understood. No clinical trials targeting titin acetylation. (sources/TTN-CVResearch-2022, rating: high)
- Titin acetylation may compete with titin ubiquitination at lysine residues, implicating PTM crosstalk in protein quality control. (sources/TTN-CVResearch-2022, rating: high)
Oxidation (UnDOx — Unfolded Domain Oxidation)
- Titin is a primary target of oxidative stress; mass spectrometry detects oxidation sites along the entire titin molecule. Highest relative increase under oxidative stress: I-band hotspots in N2Bus and distal Ig domains. (sources/TTN-CVResearch-2022, rating: high)
- UnDOx mechanism: Cryptic cysteines buried within Ig domains are exposed when the domain unfolds under stretch. Two competing oxidative outcomes determine the stiffness consequence:
- S-glutathionylation → prevents Ig domain from refolding → longer contour length → decreased titin stiffness (increased compliance). Also enables controlled homotypic in-register aggregation of distal I-band titin, potentially aiding force propagation. (sources/TTN-CVResearch-2022, rating: high)
- Disulphide bonding (in Ig domains or within N2Bus) → prevents full domain extension → increased titin stiffness. Disulphide bonds in N2Bus create additional scaffolding to the normally disordered region. (sources/TTN-CVResearch-2022, rating: high)
- Under normal physiological conditions (cardiac stress with increased preload/afterload): low-level oxidation acts as a regulatory mechanism. In ischaemia or HFpEF with metabolic syndrome: oxidation becomes extensive and unregulated → pathological stiffness. (sources/TTN-CVResearch-2022, rating: high)
- Both S-glutathionylation and disulphide bonding are reversible by reductants (dithiothreitol, glutathione) in vitro. Therapeutic oxidation-targeting strategies for HF remain speculative. (sources/TTN-CVResearch-2022, rating: high)
Phosphorylation
- Over 300 titin phosphorylation sites identified. Most clinically studied regions: N2Bus and constitutively expressed PEVK segment — with opposing effects on stiffness. (sources/TTN-CVResearch-2022, rating: high)
- N2Bus phosphorylation → decreased stiffness:
- N2Bus has net negative charge. Additional phosphate groups → electrostatic repulsion → increased persistence length → improved extensibility.
- Key phosphosites: S4010, S4062, S4099, S4185 (human UniProtKB Q8WZ42-1).
- Kinases: PKA, PKG, PKD, ERK2, CaMKIIδ. Dephosphorylation by serine/threonine protein phosphatase 5 (PP5). (sources/TTN-CVResearch-2022, rating: high)
- PEVK phosphorylation → increased stiffness:
- Constitutively expressed PEVK has net positive charge. Phosphorylation → increased intramolecular interactions → reduced extensibility.
- Key phosphosites: S11878, S12022.
- Kinases: PKCα, CaMKIIδ, PKD. (sources/TTN-CVResearch-2022, rating: high)
- In HF: N2Bus hypo-phosphorylation + PEVK hyper-phosphorylation → combined increase in cardiomyocyte passive stiffness. This is a consistent finding across HFpEF, HFrEF, and DCM, though site-specific data is more informative than global titin phosphorylation. (sources/TTN-CVResearch-2022, rating: high)
- cGMP-PKG pathway as therapeutic target:
- PDE5A inhibition (sildenafil) and BNP boosted cGMP → increased total titin phosphorylation and reduced LV stiffness in dog diastolic dysfunction models. RELAX trial in HFpEF: failed to improve diastolic function. (sources/TTN-CVResearch-2022, rating: high)
- sGC stimulators raised cGMP, increased N2Bus phosphorylation and reduced CM stiffness in animal models. VITALITY and SOCRATES trials in HFpEF: no improvement. (sources/TTN-CVResearch-2022, rating: high)
- PDE9A inhibition reduced LV diastolic stiffness in mouse models (titin phosphorylation not directly measured). Clinical trials underway. (sources/TTN-CVResearch-2022, rating: high)
- Proposed reasons for translational failure: dosage/metabolic differences between animals and humans; pathway compartmentalization (compartmentalized cGMP microdomains around titin not yet mapped); complexity of cGMP-PKG intermediate regulatory steps. (sources/TTN-CVResearch-2022, rating: high)
- Metformin and insulin (via ERK1/2, PKCα, PKA) and neuregulin-1 (via ERK1/2/PKG, suppressing PKCα) improved titin N2Bus phosphorylation and diastolic function in diabetic HFpEF animal models — pre-clinical only. (sources/TTN-CVResearch-2022, rating: high)
Contradictions / Open Questions
- Animal-to-human translation failure: Despite consistent pre-clinical evidence that increasing cGMP-PKG signalling reduces titin stiffness, three large clinical trials (RELAX, VITALITY, SOCRATES) in HFpEF showed no benefit. Whether this reflects dosing, patient selection, disease heterogeneity, or fundamental biological differences between species remains unresolved. (sources/TTN-CVResearch-2022, rating: high)
- HDAC inhibitor paradox: HDAC inhibitors increase acetylation (same direction as HFpEF pathology) yet improve diastolic dysfunction in some animal models, contradicting the SIRT1/NAD⁺ deacetylation hypothesis. The net effect of acetylation on titin stiffness may depend on the specific lysine residues modified and regional charge context. (sources/TTN-CVResearch-2022, rating: high)
- Causal vs. compensatory PTM changes in HF: It is not yet established whether titin PTM changes in HFpEF/HFrEF cause increased stiffness or are adaptive responses to other stiffness changes. This distinction matters greatly for therapeutic targeting. (sources/TTN-CVResearch-2022, rating: high)
Connections
- Related to entities/TTN
- Related to concepts/Titin-Isoform-Switch
- Related to concepts/HFpEF
- Related to entities/Heart-Failure
- Related to entities/DCM