Titin-based tension in the cardiac sarcomere: molecular origin and physiological adaptations

Abstract

The passive stiffness of cardiac muscle plays a critical role in ventricular filling during diastole and is determined by the extracellular matrix and the sarcomeric protein titin. Titin spans from the Z-disk to the M-band of the sarcomere and also contains a large extensible region that acts as a molecular spring and develops passive force during sarcomere stretch. This extensible segment is titin's I-band region, and its force-generating mechanical properties determine titin-based passive tension. The properties of titin's I-band region can be modulated by isoform splicing and post-translational modification and are intimately linked to diastolic function. This review discusses the physical origin of titin-based passive tension, the mechanisms that alter titin stiffness, and titin's role in stress-sensing signaling pathways.

Figure 1

A) A schematic of the sarcomere. A single titin molecule spans from the Z-disk to the M-band and contains a spring-like region that develops force upon sarcomere stretch. B) The I-band region composition of the three cardiac titin isoforms. The adult N2BA isoform and fetal cardiac titin isoform contain a middle tandem Ig segment, the N2A element, and a longer PEVK sequence than the short N2B isoform. C) Titin-based passive tension levels are determined by titin isoform composition and can also be modulated by phosphorylation of I-band elements.

Figure 2

Simple models of random coil proteins. A) The freely-jointed chain is a 3D random walk of rigid monomers of length L, and B) the wormlike chain model represents a continuous, homogenous polymer. Both chains can occupy numerous conformations at low fractional extensions (end-to-end distance z ≪ molecular contour length) but only one conformation when z = L_c. C) External force is required to extend a flexible polymer. The molecular restoring force than opposes the external force is the entropic force generated by the polymer. This entropic force is powered by diffusion and is the apparent force that recoils the protein when external force is removed.

Figure 3

The exon structure of the human titin gene (based on (Bang et al. 2001)). Titin contains 363 exons, ∼220 of which are found in the I-band region. Titin's I-band region comprises immunoglobulin(Ig)-like domains, the PEVK element, and unique sequences. The thick filament bound A-band region of titin contains Ig and fibronectin-III (Fn-III) domains exclusively. Domains and exons alluded to in the text are labeled.

Figure 4

A) A ribbon schematic of five serially-linked Ig domains. β-strands are colored blue and alpha helices are red. The atomic coordinates were downloaded from the Worldwide Protein Data Bank, PDB ID: 3B43 (Von Castelmur et al. 2008). Each folded Ig domain is 4-5 nm in diameter and connected by short linker sequences. B) Simplified AFM schematic. The cantilever tip probes a protein-coated surface until a molecule is tethered. The molecule is then stretched and the force that develops is determined by measuring the deflection of the laser off the cantilever. C) A tethered polyprotein is stretched to induce domain unfolding. Large structural transitions, such as complete unfolding of an Ig domain, result in a large contour length increase, a release of tension in the molecule, and a sharp unfolding force peak. In the example shown, the molecule is still attached after all five domains unfold; the last force peak is due to stretching the completely unfolded peptide that behaves as an entropic spring and is well-described by the WLC equation (fitted dashed line).

Figure 5

A) The relationship between stress and strain in cardiac muscle. The difference between peak stress and steady-state stress is due to viscosity. B) Stress leads strain when sinusoidal length changes are imposed on myocardial tissue and is due to the presence of viscosity. The elastic modulus (EM) and viscous modulus (VM) are determined from the amplitudes of stress and strain and the phase difference between them.

Figure 6

Signaling hotspots in titin. At the Z-disk, the titin-binding protein T-cap interacts with muscle LIM protein (MLP), a nuclear regulator of myogenesis. In the I-band, the N2B and N2A elements interact with four-and-a-half-LIM protein (FHL) and muscle ankyrin-repeat proteins (MARPs), respectively. The N2B element and FHL are thought to form a force-dependent stretch-sensing complex that also involves components of MAPK signaling, which regulates hypertrophy. The interaction of the N2A element and MARPs is hypothesized to influence transcription as a function of mechanical strain. The M-band region of titin interacts with MURFs and Nbr1, and these interactions may play an important role in calcium signaling and hypertrophy.