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Abstract

With every heartbeat, our cardiomyocytes are exposed to variable degrees of stress and strain of both internal and external origin. The unique mechanical properties of these myocytes are determined by the sarcomeric cytoskeleton. The actin (thin) and myosin (thick) filaments of the sarcomere are mainly relevant for active force generation, whereas the titin filaments determine sarcomeric viscoelasticity. The giant protein titin, which is the focus of this review, has various roles beyond viscoelastic force generation 1-3 : (1) it keeps the thick filaments centered in the sarcomere, allowing optimum active force development; (2) it is important for the assembly of the sarcomeres; (3) it participates in mechano-chemical signaling events via some of its binding partners; and (4) it may be crucial for the length-dependent activation of the contractile apparatus, which underlies the Frank-Starling law. During the past decade, we have learned that titin elasticity is highly variable in the developing and the adult healthy heart and that it can be pathologically altered in heart disease. These alterations greatly affect the extensibility and the diastolic passive stiffness of myocardium and presumably also the mechanosensitivity. Moreover, TTN, which encodes the titin protein, has been recognized as a major human disease gene. In this context, the properties and functions of cardiac titin have become of interest in the continuing search for the molecular origins of human heart disease and the identification of novel potential targets for therapeutic intervention. In our review, we begin with a short clinical perspective establishing the link between diastolic stiffness and titin and briefly compare the importance of titin versus collagen for myocardial passive stiffness. We then cover some details of titin protein structure and elasticity, before summarizing how titin-binding partners affect titin properties and broaden the range of titin functions, with a special focus on the standing of titin in pathways of protein quality control. We continue with a current overview of titin mutations in human skeletal muscle and heart disease. The remainder of the review deals with the contribution of titin to diastolic stiffness and how it is modulated in normal and failing hearts by mechanisms such as titin-isoform switching, titin phosphorylation (including heart failure [HF]-related alterations of it), and oxidative modifications. Clinical Context: Diastolic Function and Diastolic AbnormalitiesDiastolic function has moved into the focus of HF research, because an increasing number of patients with HF (≥50%) present with diastolic rather than systolic dysfunction. 4 Common abnormalities of diastolic function include impaired left ventricular (LV) relaxation, decreased LV distensibility, increased LV end-diastolic stiffness, and pericardial and right ventricular constraint. These abnormalities have been suggested to be contributing factors in diastolic HF or HF with a preserved ejection fraction (HFpEF).5 HFpEF is accompanied by ...

Renin overexpression leads to increased titin-based stiffness contributing to diastolic dysfunction in hypertensive mRen2 rats

“…The main intracellular determinant of passive tension of the contractile machinery is the titin molecule, spanning the half sarcomere (30). This protein has two isoforms in adult mammalian heart, a larger and more compliant N2BA isoform (ϳ3.3 MDa) and a smaller and stiffer N2B isoform (ϳ 3.0 MDa).…”

“…The predominant titin isoform in adult rats is the N2B (11). Stiffness is modulated by titin isoform composition, degradation, oxidative modifications, and phosphorylation by various protein kinases (30) especially within the elastic I-band region at N2-Bus and PEVK domains (1, 17-22, 26, 45).…”

“…The N2-Bus domain (a unique spring segment of titin) has various phosphorylation sites for protein kinase A (PKA), protein kinase G (PKG), extracellular signal-regulated kinase-2 (ERK2), and Ca 2ϩ /calmodulin-dependent protein kinase-II␦ (CaMKII␦) (30). Hypophosphorylation of this region was observed in HFpEF, leading to increased titin-based stiffness (4, 18 -19, 45).…”

“…Myocyte-sized preparations were mounted with silicone adhesive between a high-speed length controller (Aurora Scientific, Aurora, Canada) and a precise force transducer (SensoNor, Horten, Norway) in ISO at 15°C. Cardiomyocyte Ca 2ϩ -activated force generation was evoked by transferring the preparation from relaxing (10.0 mM BES, 37.11 mM KCl, 6.41 mM MgCl 2, 7.0 mM EGTA, 6.94 mM ATP, and 15.0 mM creatine phosphate, pH 7.2) to activating solution (containing Ca 2ϩ -EGTA instead of EGTA, otherwise same composition as relaxing solution) (30). Ca 2ϩ concentrations were expressed as Ϫlog 10[Ca 2ϩ ] (pCa) units, accordingly the pCa of relaxing solution was 9.0, whereas the pCa of maximal activating solution was 4.75.…”

“…Then, gels were washed and handled according to the manufacturer's protocol. Beyond that, Western immunoblotting was applied to assess titin phosphorylation at particular serine (Ser) residues of PEVK segment (rich in proline, glutamate, valine and lysine amino acids) (30). The phospho-specific antibodies were validated by Granzier et al (20,22).…”