Sense and stretchability: the role of titin and titin-associated proteins in myocardial stress-sensing and mechanical dysfunction

Mechanical stress signals transmitted through the heart walls during hemodynamic loading are sensed by the myocytes, which respond with changes in contractile performance and gene expression. External forces play an important role in physiological heart development and hypertrophy, but disruption of the well-balanced stress-sensing machinery causes mechanical dysregulation, cardiac remodelling, and heart failure. Nodal points of mechanosensing in the cardiomyocytes may reside in the Z-disk, I-band, and M-band regions of the sarcomeres. Longitudinal linkage of these regions is provided by the titin filament, and several 'hot spots' along this giant protein, in complex with some of its >20 ligands, may be pivotal to the myofibrillar stress or stretch response. This review outlines the known interaction partners of titin, highlights the putative stress/stretch-sensor complexes at titin's NH(2) and COOH termini and their role in myopathies, and summarizes the known disease-associated mutations in those titin regions. Another focus is the elastic I-band titin section, which interacts with a diverse number of proteins and whose main function is as a determinant of diastolic distensibility and passive stiffness. The discussion centers on recent insights into the plasticity, mechanical role, and regulation of the elastic titin springs during cardiac development and in human heart disease. Titin and titin-based protein complexes are now recognized as integral parts of the mechanosensitive protein network and as critical components in cardiomyocyte stress/stretch signalling.

Titin isoforms, extracellular matrix, and global chamber remodeling in experimental dilated cardiomyopathy: functional implications and mechanistic insight

BACKGROUND

Altered titin isoforms may modify cardiac function in heart failure (HF), but the nature of isoform switches and associated functional implications are not well defined. Limited studies have reported an increased compliant isoform (N2BA) expression in human systolic HF. Titin may also modulate stretch-regulated responses such as myocardial natriuretic peptide production.

METHODS AND RESULTS

We characterized titin isoform expression and extracellular matrix in all 4 cardiac chambers and the left ventricular (LV) epicardium and endocardium in normal dogs (n=6) and those with HF (n=6) due to tachypacing and characterized functional implications at the LV myofiber and chamber level. Recognizing the potential for uncoupling of the extracellular matrix and cardiomyocyte in tachypacing, myocardial natriuretic peptide production, a molecular marker of stretch-regulated responses, was also assessed. All chambers were dilated in HF, but the extracellular matrix was not increased. HF dogs had markedly lower N2BA in the atria and right ventricle. In failing LVs, N2BA was decreased only in the epicardium, where myofiber passive stiffness was increased. However, LV chamber mechanics were driven by the marked LV dilatation, with no increase in LV diastolic stiffness. Natriuretic peptide concentrations increased markedly in the endocardium in relation to increases in LV wall stress.

CONCLUSIONS

Tachypacing HF is characterized by decreases in compliant titin isoform expression in the atria, right ventricle, and LV epicardium. However, LV chamber mechanics are principally determined by geometric and extracellular matrix changes rather than titin-based myofiber stiffness in this model. Stretch-regulated myocardial responses (natriuretic peptide production) appeared intact, suggesting that the mechanotransduction role of titin was not impaired in HF.

Thyroid Hormone Regulates Developmental Titin Isoform Transitions via the Phosphatidylinositol-3-Kinase/ AKT Pathway

Titins, giant sarcomere proteins with major mechanical/signaling functions, are expressed in 2 main isoform classes in the mammalian heart: N2B (3000 kDa) and N2BA (>3200 kDa). A dramatic isoform switch occurs during cardiac development, from fetal N2BA titin (3700 kDa) expressed before birth to a mix of smaller N2BA/N2B isoforms found postnatally; adult rat hearts almost exclusively have N2B titin. The isoform switch, which can be reversed in chronic human heart failure, alters myocardial distensibility and mechanosignaling. Here we determined factors regulating this switch using, as a model system, primary cardiomyocyte cultures prepared from embryonic rats. In standard culture, the mean N2B percentage initially was 14% and increased by ≈60% within 1 week, resembling the in vivo switching. The titin isoform transition was independent of endothelin-1–induced myocyte hypertrophy and was not altered by pacing, contractile arrest, or cell stretch; however, it was modestly impaired by decreasing substrate rigidity and strongly dependent on serum components. Angiotensin II significantly promoted the transition. The mean N2B proportion in 1-week-old cultures dropped 20% to 25% in hormone-reduced medium, but addition of 3,5,3′-triiodo- l -thyronine (T3) nearly restored the proportion to that found in standard culture. This T3 effect was not prevented by bisphenol A, a specific inhibitor of the classic genomic pathway of T3 action. In contrast, the titin switch could be stalled by the phosphatidylinositol 3-kinase inhibitor LY294002, which decreased the proportion of N2B mRNA transcripts within hours and suppressed a rapid T3-induced increase in Akt phosphorylation. Also, angiotensin II, but not endothelin-1 or cell stretch, enhanced Akt phosphorylation. Thus, although matrix stiffness modulates developmental titin isoform transitions, these transitions are mainly regulated through phosphatidylinositol 3-kinase/Akt-dependent signaling triggered particularly by T3 via a rapid action pathway.

Mineralocorticoid Signaling in Transition to Heart Failure With Normal Ejection Fraction

Heart failure with normal ejection fraction occurs in elderly patients with hypertensive heart disease. We hypothesized that, in such patients, mineralocorticoid receptor activation accelerates the types of ventricular and vascular remodeling and dysfunction believed important in the transition to heart failure. We tested this hypothesis by administering deoxycorticosterone acetate (DOCA) without salt loading or nephrectomy to elderly dogs with experimental hypertension. Elderly dogs were made hypertensive by renal wrapping. After 5 weeks, dogs were randomly assigned to DOCA (1 mg/kg per day IM; old hypertensive [OH]+DOCA; n=11) or not (OH; n=11) for 3 weeks. At week 8, conscious echocardiography and hemodynamic assessment under anesthesia were performed. DOCA resulted in further increases in conscious blood pressure ( P <0.05) without increases in cardiac output or diastolic volume. In the conscious state, effective arterial elastance ( P <0.05) and systemic vascular resistance ( P =0.06) were increased, and systemic arterial compliance ( P <0.05) was decreased in OH+DOCA animals. After anesthesia, instrumentation, and autonomic blockade, blood pressure was lower, whereas left ventricular (LV) systolic elastance, LV diastolic stiffness, and ex vivo myofiber diastolic stiffness were increased in OH+DOCA animals. LV collagen was increased in OH+DOCA animals ( P <0.05 for all), but LV mass, LV brain natriuretic peptide, and titin isoform profiles were not. Neither aortic stiffness nor aortic structure was altered in OH+DOCA animals. These findings suggest that age and hypertensive heart disease enhance sensitivity to exogenous mineralocorticoid administration and that mineralocorticoid receptor activation could contribute to the transition to heart failure in elderly persons by promoting increases in LV diastolic and systolic stiffness.

Acute and Chronic Ventricular-Arterial Coupling in Systole and Diastole

Aging and hypertension lead to arterial remodeling and tandem increases in arterial (Ea) and ventricular (LV) systolic stiffness (ventricular-arterial [VA] coupling). Age and hypertension also predispose to heart failure with normal ejection fraction (HFnlEF), where symptoms during hypertensive urgencies or exercise are common. We hypothesized that: (1) chronic VA coupling also occurs in diastole, (2) acute changes in Ea are coupled with shifts in the diastolic and systolic pressure-volume relationships (PVR), and (3) diastolic VA coupling reflects changes in LV diastolic stiffness rather than external forces or relaxation. Old chronically hypertensive (OHT, n=8) and young normal (YNL, n=7) dogs underwent assessment of PVR (caval occlusion) and of aortic pressure, dimension, and flow, at baseline and during changes in afterload and preload. Concomitant changes in the slope/position of PVR were accounted for by calculating systolic (ESV 200 ) and diastolic (EDV 20 ) volumes at common pressures (capacitance). OHT displayed marked vascular remodeling. Indices reflecting the pulsatile component of Ea (aortic stiffness and systemic arterial compliance) were more impaired in OHT at any distending pressure. In both groups, acute increases in Ea were associated with decreases in ESV 200 and EDV 20 . However, at any load, OHT had lower ESV 200 and EDV 20 , associated with LV remodeling and myocardial endothelin activation. Acute changes in EDV 20 were not mediated by changes in relaxation or external forces. These observations provide insight into the mechanisms whereby arterial remodeling and acute and chronic VA coupling in both systole and diastole may predispose to and interact with increases in load to cause HFnlEF.

Plasticity of cardiac titin/connectin in heart development

Many sarcomeric proteins in the myocardium alter their isoform pattern during perinatal development to adjust to the intensified pump function of the postnatal heart. These changes also involve the giant protein titin/connectin. Here we show by low-percentage polyacrylamide-gel electrophoresis that developmentally regulated switching of cardiac titin/connectin size occurs in the hearts of mouse, rat, pig, and chicken. Mammalian hearts express, well before birth, large foetal (approximately 3.7 MDa) N2BA-titin/connectin isoform but no N2B-isoform (3.0 MDa). During perinatal heart development the 3.7-MDa N2BA-isoform is replaced by a mix of smaller isoforms. At birth a plethora of intermediate-size N2BA-isoforms appears together with the N2B-isoform. In postnatal heart development the larger-size N2BA-isoforms disappear and smaller-size N2BA-isoforms are upregulated, whereas the proportion of N2B-titin/connectin increases to species-specific adult levels. The time courses of isoform switching are faster in small than in large mammals. Titin/connectin isoform switching also takes place in developing chicken hearts, but the largest embryonic isoform found here was less than 3.4 MDa. At hatching, various smaller-size isoforms appeared and within a week the adult expression pattern was established representing a major 3.0-MDa isoform and a minor 3.15-MDa isoform. The ratio between the two adult isoforms differed between the left ventricle and the right atrium. The perinatal changes toward smaller cardiac titin/connectin isoforms in mammals and chicken greatly increase the myofibrillar passive tension of postnatal hearts. Plasticity of titin/connectin at approximately the time of birth thus affects myocardial mechanics but could also be an important factor in developmentally regulated assembly and signalling processes.

Fibre type-specific increase in passive muscle tension in spinal cord-injured subjects with spasticity

Patients with spasticity typically present with an increased muscle tone that is at least partly caused by an exaggerated stretch reflex. However, intrinsic changes in the skeletal muscles, such as altered mechanical properties of the extracellular matrix or the cytoskeleton, have been reported in response to spasticity and could contribute to hypertonia, although the underlying mechanisms are poorly understood. Here we examined the vastus lateralis muscles from spinal cord-injured patients with spasticity (n = 7) for their passive mechanical properties at three different levels of structural organization, in comparison to healthy controls (n = 7). We also assessed spasticity-related alterations in muscle protein expression and muscle ultrastructure. At the whole-muscle level in vivo, we observed increased passive tension (PT) in some spasticity patients particularly at long muscle lengths, unrelated to stretch reflex activation. At the single-fibre level, elevated PT was found in cells expressing fast myosin heavy chain (MyHC) isoforms, especially MyHC-IIx, but not in those expressing slow MyHC. Type IIx fibres were present in higher than normal proportions in spastic muscles, whereas type I fibres were proportionately reduced. At the level of the isolated myofibril, however, there were no differences in PT between patients and controls. The molecular size of the giant protein titin, a main contributor to PT, was unchanged in spasticity, as was the titin : MyHC ratio and the relative desmin content. Electron microscopy revealed extensive ultrastructural changes in spastic muscles, especially expanded connective tissue, but also decreased mitochondrial volume fraction and appearance of intracellular amorphous material. Results strongly suggest that the global passive muscle stiffening in spasticity patients is caused to some degree by elevated PT of the skeletal muscles themselves. We conclude that this increased PT component arises not only from extracellular matrix remodelling, but also from structural and functional adaptations inside the muscle cells, which alter their passive mechanical properties in response to spasticity in a fibre type-dependent manner.

Protein kinase-A phosphorylates titin in human heart muscle and reduces myofibrillar passive tension

Protein kinase-A (PKA) is activated during beta-adrenergic stimulation of the heart and is known to phosphorylate several sarcomeric proteins including the giant polypeptide titin. A PKA phosphorylation site on titin is located within the N2B-unique sequence, which is present in the elastic segment of the two major isoforms of cardiac titin, N2B and N2BA, but not in the skeletal-muscle isoforms of the N2A-type. In bovine and rat cardiomyocytes, PKA-mediated phosphorylation decreases passive tension (PT), an effect ascribed to titin phosphorylation. Whether titin is phosphorylated by PKA upon beta-adrenergic stimulation in human heart has not been shown to date. Here we report that PKA induces phosphorylation of N2B and N2BA titin isoforms, as well as a characteristic proteolytic fragment of titin, T2, in human donor hearts. The PKA-induced phosphorylation signals were stronger when myofilaments were first de-phosphorylated by protein phosphatase-1, suggesting inherent phosphorylation of titin in human heart. Titin phosphorylation was associated with a reduction in PT of skinned human cardiac strips; the relative decrease was higher at shorter than at longer physiological sarcomere lengths. The PKA-dependent PT drop was substantially larger when fibers were pre-treated with protein phosphatase-1, indicating that inherent phosphorylation of titin is important for the basal myocardial PT level. Mechanical measurements on isolated myofibrils from rat heart confirmed the PKA effect on passive stiffness and also showed a more pronounced effect in the presence of reducing agent, DTT. In contrast, PKA did not alter the PT of single skinned rat diaphragm muscle fibers; however, the kinase was still able to phosphorylate the skeletal N2A-titin isoform, which lacks the N2B-unique sequence. Thus, an additional phosphorylation site in titin may exist outside the cardiac N2B-unique sequence. We conclude that PKA mediates phosphorylation of titin in normal human myocardium. Titin phosphorylation lowers titin-based passive stiffness in heart but not in skeletal muscle.

Mechanical properties of cardiac titin’s N2B-region by single-molecule atomic force spectroscopy

Titin is a giant protein responsible for passive-tension generation in muscle sarcomeres. Here, we used single-molecule AFM force spectroscopy to investigate the mechanical characteristics of a recombinant construct from the human cardiac-specific N2B-region, which harbors a 572-residue unique sequence flanked by two immunoglobulin (Ig) domains on either side. Force-extension curves of the N2B-construct revealed mean unfolding forces for the Ig-domains similar to those of a recombinant fragment from the distal Ig-region in titin (I91-98). The mean contour length of the N2B-unique sequence was 120 nm, but there was a bimodal distribution centered at approximately 95 nm (major peak) and 180 nm (minor peak). These values are lower than expected if the N2B-unique sequence were a permanently unfolded entropic spring, but are consistent with the approximately 100 nm maximum extension of that segment measured in isolated stretched cardiomyofibrils. A contour-length below 200 nm would be reasonable, however, if the N2B-unique sequence were stabilized by a disulphide bridge, as suggested by several disulphide connectivity prediction algorithms. Since the N2B-unique sequence can be phosphorylated by protein kinase A (PKA), which lowers titin-based stiffness, we studied whether addition of PKA (+ATP) affects the mechanical properties of the N2B-construct, but found no changes. The softening effect of PKA on N2B-titin may require specific conditions/factors present inside the cardiomyocytes.

Developmental changes in passive stiffness and myofilament Ca2+sensitivity due to titin and troponin-I isoform switching are not critically triggered by birth

The giant protein titin, a major contributor to myocardial mechanics, is expressed in two main cardiac isoforms: stiff N2B (3.0 MDa) and more compliant N2BA (>3.2 MDa). Fetal hearts of mice, rats, and pigs express a unique N2BA isoform (∼3.7 MDa) but no N2B. Around birth the fetal N2BA titin is replaced by smaller-size N2BA isoforms and N2B, which predominates in adult hearts, stiffening their sarcomeres. Here we show that perinatal titin-isoform switching and corresponding passive stiffness (STp) changes do not occur in the hearts of guinea pig and sheep. In these species the shift toward “adult” proportions of N2B isoform is almost completed by midgestation. The relative contributions of titin and collagen to STpwere estimated in force measurements on skinned cardiac muscle strips by selective titin proteolysis, leaving the collagen matrix unaffected. Titin-based STpcontributed between 42% and 58% to total STpin late-fetal and adult sheep/guinea pigs and adult rats. However, only ∼20% of total STpwas titin based in late-fetal rat. Titin-borne passive tension and the proportion of titin-based STpgenerally scaled with the N2B isoform percentage. The titin isoform transitions were correlated to a switch in troponin-I (TnI) isoform expression. In rats, fetal slow skeletal TnI (ssTnI) was replaced by adult carciac TnI (cTnI) shortly after birth, thereby reducing the Ca2+sensitivity of force development. In contrast, guinea pig and sheep coexpressed ssTnI and cTnI in fetal hearts, and skinned fibers from guinea pig showed almost no perinatal shift in Ca2+sensitivity. We conclude that TnI-isoform and titin-isoform switching and corresponding functional changes during heart development are not initiated by birth but are genetically programmed, species-specific regulated events.

Myocardial Structure and Function Differ in Systolic and Diastolic Heart Failure

Background— To support the clinical distinction between systolic heart failure (SHF) and diastolic heart failure (DHF), left ventricular (LV) myocardial structure and function were compared in LV endomyocardial biopsy samples of patients with systolic and diastolic heart failure.

Methods and Results— Patients hospitalized for worsening heart failure were classified as having SHF (n=22; LV ejection fraction (EF) 34±2%) or DHF (n=22; LVEF 62±2%). No patient had coronary artery disease or biopsy evidence of infiltrative or inflammatory myocardial disease. More DHF patients had a history of arterial hypertension and were obese. Biopsy samples were analyzed with histomorphometry and electron microscopy. Single cardiomyocytes were isolated from the samples, stretched to a sarcomere length of 2.2 μm to measure passive force (F passive ), and activated with calcium-containing solutions to measure total force. Cardiomyocyte diameter was higher in DHF (20.3±0.6 versus 15.1±0.4 μm, P <0.001), but collagen volume fraction was equally elevated. Myofibrillar density was lower in SHF (36±2% versus 46±2%, P <0.001). Cardiomyocytes of DHF patients had higher F passive (7.1±0.6 versus 5.3±0.3 kN/m 2 ; P <0.01), but their total force was comparable. After administration of protein kinase A to the cardiomyocytes, the drop in F passive was larger ( P <0.01) in DHF than in SHF.

Conclusions— LV myocardial structure and function differ in SHF and DHF because of distinct cardiomyocyte abnormalities. These findings support the clinical separation of heart failure patients into SHF and DHF phenotypes.

The molecular elasticity of the insect flight muscle proteins projectin and kettin

Projectin and kettin are titin-like proteins mainly responsible for the high passive stiffness of insect indirect flight muscles, which is needed to generate oscillatory work during flight. Here we report the mechanical properties of kettin and projectin by single-molecule force spectroscopy. Force-extension and force-clamp curves obtained from Lethocerus projectin and Drosophila recombinant projectin or kettin fragments revealed that fibronectin type III domains in projectin are mechanically weaker (unfolding force, F(u) approximately 50-150 pN) than Ig-domains (F(u) approximately 150-250 pN). Among Ig domains in Sls/kettin, the domains near the N terminus are less stable than those near the C terminus. Projectin domains refolded very fast [85% at 15 s(-1) (25 degrees C)] and even under high forces (15-30 pN). Temperature affected the unfolding forces with a Q(10) of 1.3, whereas the refolding speed had a Q(10) of 2-3, probably reflecting the cooperative nature of the folding mechanism. High bending rigidities of projectin and kettin indicated that straightening the proteins requires low forces. Our results suggest that titin-like proteins in indirect flight muscles could function according to a folding-based-spring mechanism.

Isoform diversity of giant proteins in relation to passive and active contractile properties of rabbit skeletal muscles

The active and passive contractile performance of skeletal muscle fibers largely depends on the myosin heavy chain (MHC) isoform and the stiffness of the titin spring, respectively. Open questions concern the relationship between titin-based stiffness and active contractile parameters, and titin's importance for total passive muscle stiffness. Here, a large set of adult rabbit muscles (n = 37) was studied for titin size diversity, passive mechanical properties, and possible correlations with the fiber/MHC composition. Titin isoform analyses showed sizes between ∼3300 and 3700 kD; 31 muscles contained a single isoform, six muscles coexpressed two isoforms, including the psoas, where individual fibers expressed similar isoform ratios of 30:70 (3.4:3.3 MD). Gel electrophoresis and Western blotting of two other giant muscle proteins, nebulin and obscurin, demonstrated muscle type–dependent size differences of ≤70 kD. Single fiber and single myofibril mechanics performed on a subset of muscles showed inverse relationships between titin size and titin-borne tension. Force measurements on muscle strips suggested that titin-based stiffness is not correlated with total passive stiffness, which is largely determined also by extramyofibrillar structures, particularly collagen. Some muscles have low titin-based stiffness but high total passive stiffness, whereas the opposite is true for other muscles. Plots of titin size versus percentage of fiber type or MHC isoform (I-IIB-IIA-IID) determined by myofibrillar ATPase staining and gel electrophoresis revealed modest correlations with the type I fiber and MHC-I proportions. No relationships were found with the proportions of the different type II fiber/MHC-II subtypes. Titin-based stiffness decreased with the slow fiber/MHC percentage, whereas neither extramyofibrillar nor total passive stiffness depended on the fiber/MHC composition. In conclusion, a low correlation exists between the active and passive mechanical properties of skeletal muscle fibers. Slow muscles usually express long titin(s), predominantly fast muscles can express either short or long titin(s), giving rise to low titin-based stiffness in slow muscles and highly variable stiffness in fast muscles. Titin contributes substantially to total passive stiffness, but this contribution varies greatly among muscles.

Multiple sources of passive stress relaxation in muscle fibres

The forces developed during stretch of nonactivated muscle consist of velocity-sensitive (viscous/viscoelastic) and velocity-insensitive (elastic) components. At the myofibrillar level, the elastic-force component has been described in terms of the entropic-spring properties of the giant protein titin, but entropic elasticity cannot account for viscoelastic properties, such as stress relaxation. Here we examine the contribution of titin to passive stress relaxation of isolated rat-cardiac myofibrils depleted of actin by gelsolin treatment. Monte Carlo simulations show that, up to approximately 5 s after a stretch, the time course of stress relaxation can be described assuming unfolding of 1-2 immunoglobulin domains per titin molecule. For extended periods of stress relaxation, the simulations failed to correctly describe the myofibril data, suggesting that in situ, titin-Ig domains may be more stable than predicted in earlier single-molecule atomic-force-microscopy studies. The reasons behind this finding remain unknown; simply assuming a reduced unfolding probability of domains--an effect found here by AFM force spectroscopy on titin-Ig domains in the presence of a chaperone, alpha-B-crystallin--did not help correctly simulate the time course of stress relaxation. We conclude that myofibrillar stress relaxation likely has multiple sources. Evidence is provided that in intact myofibrils, an initial, rapid phase of stress relaxation results from viscous resistance due to the presence of actin filaments.

Passive Stiffness Changes Caused by Upregulation of Compliant Titin Isoforms in Human Dilated Cardiomyopathy Hearts

In the pathogenesis of dilated cardiomyopathy, cytoskeletal proteins play an important role. In this study, we analyzed titin expression in left ventricles of 19 control human donors and 9 severely diseased (nonischemic) dilated cardiomyopathy (DCM) transplant-patients, using gel-electrophoresis, immunoblotting, and quantitative RT-PCR. Both human-heart groups coexpressed smaller (≈3 MDa) N2B-isoform and longer (3.20 to 3.35 MDa) N2BA-isoforms, but the average N2BA:N2B-protein ratio was shifted from ≈30:70 in controls to 42:58 in DCM hearts, due mainly to increased expression of N2BA-isoforms >3.30 MDa. Titin per unit tissue was decreased in some DCM hearts. The titin-binding protein obscurin also underwent isoform-shifting in DCM. Quantitative RT-PCR revealed a 47% reduction in total-titin mRNA levels in DCM compared with control hearts, but no differences in N2B, all-N2BA, and individual-N2BA transcripts. The reduction in total-titin transcripts followed from a decreased area occupied by myocytes and increased connective tissue in DCM hearts, as detected by histological analysis. Force measurements on isolated cardiomyofibrils showed that sarcomeric passive tension was reduced on average by 25% to 30% in DCM, a reduction readily predictable with a model of wormlike-chain titin elasticity. Passive-tension measurements on human-heart fiber bundles, before and after titin proteolysis, revealed a much-reduced relative contribution of titin to total passive stiffness in DCM. Results suggested that the titin-isoform shift in DCM depresses the proportion of titin-based stiffness by ≈10%. We conclude that a lower-than-normal proportion of titin-based stiffness in end-stage failing hearts results partly from loss of titin and increased fibrosis, partly from titin-isoform shift. The titin-isoform shift may be beneficial for myocardial diastolic function, but could impair the contractile performance in systole.

Gigantic variety: expression patterns of titin isoforms in striated muscles and consequences for myofibrillar passive stiffness

The giant muscle protein titin has become a focus of research interests in the field of muscle mechanics due to its importance for passive muscle stiffness. Here we summarize research activities leading to current understanding of titin's mechanical role in the sarcomere. We then show how low-porosity polyacrylamide-gel electrophoresis, optimised for resolving megadalton proteins, can identify differences in titin-isoform expression in the hearts of 10 different vertebrate species and in several skeletal muscles of the rabbit. A large variety of titin-expression patterns is apparent, which is analysed in terms of its effect on the passive tension of isolated myofibrils obtained from selected muscle types. We show and discuss evidence indicating that vertebrate striated muscle cells are capable of adjusting their passive stiffness in the following ways: (1) Cardiomyocytes co-express long (N2BA) and short (N2B) titin isoform in the same half-sarcomeres and vary the N2BA:N2B ratio to adjust stiffness. Hearts from different mammalian species vary widely in their N2BA:N2B ratio; right ventricles show higher ratios than left ventricles. There is also a significant gradient of N2BA:N2B ratio in a given heart, from basal to apical; transmural ratio differences are less distinct. (2) Skeletal muscles can express longer or shorter I-band-titin (N2A-isoform) to achieve lower or higher titin-derived stiffness, respectively. (3) Some skeletal muscles co-express longer (N2A(L)) and shorter (N2A(S)) titin isoforms, also at the single-fibre level (e.g., rabbit psoas); variations in overall N2A(L):N2A(S) ratio may add to the fine-tuning of titin-based stiffness in the whole muscle. Whereas it is established that titin, together with extracellular collagen, determines the passive tension at physiological sarcomere lengths in cardiac muscle, it remains to be seen to which degree titin and/or extracellular structures are important for the physiological passive-tension generation of whole skeletal muscle.

Developmentally regulated switching of titin size alters myofibrillar stiffness in the perinatal heart

Before birth, the compliance of the heart is limited predominantly by extracardiac constraint. Reduction of this constraint at birth requires that myocardial compliance be determined mainly by the heart's own constituents. Because titin is a principal contributor to ventricular passive tension (PT), we studied the expression and mechanics of cardiac-titin isoforms during perinatal rat heart development. Gel electrophoresis and immunoblotting revealed a single, 3.7-MDa, N2BA isoform present 6 days before birth and an additional, also previously unknown, N2BA isoform of 3.5 to 3.6 MDa expressed in the near-term fetus. These large isoforms rapidly disappear after birth and are replaced by a small N2B isoform (3.0 MDa) predominating in 1-week-old and adult rats. In addition, neonatal pig hearts showed large N2BA-titin isoforms distinct from those present in the adult porcine myocardium. By quantitative reverse transcriptase-polymerase chain reaction, developmentally expressed titin-mRNA species were detected in rat heart. Titin-based PT was much lower (approximately 15 times) in fetal than adult rat cardiomyocytes, and measured PT levels were readily predictable with a model of worm-like chain titin elasticity. Immunofluorescence microscopy showed the extensibility of the differentially spliced molecular spring regions of fetal/neonatal titin isoforms in isolated rat cardiomyofibrils. Whereas the titin-isoform shift by 700 kDa ensures high passive stiffness of the postnatal cardiac myofibrils, the expression of specific fetal/neonatal cardiac-titin isoforms may also have important functions for contractile properties, myofibril assembly or turnover, and myocardial signaling during perinatal heart development.

A troponin switch that regulates muscle contraction by stretch instead of calcium

The flight muscles of many insects have a form of regulation enabling them to contract at high frequencies. The muscles are activated by periodic stretches at low Ca2+ levels. The same muscles also give isometric contractions in response to higher Ca2+. We show that the two activities are controlled by different isoforms of TnC (F1 and F2) within single myofibrils. F1 binds one Ca2+ with high affinity in the C-terminal domain and F2 binds one Ca2+ in the C-terminal domain and one exchangeable Ca2+ in the N-terminal domain. We have characterised the isoforms and determined their effect on the development of stretch-activated and Ca2+-activated tension by replacing endogenous TnC in Lethocerus flight muscle fibres with recombinant isoforms. Fibres with F1 gave stretch-activated tension and minimal isometric tension; those with F2 gave Ca2+-dependent isometric tension and minimal stretch-activated tension. Regulation by a TnC responding to stretch rather than Ca2+ is unprecedented and has resulted in the ability of insect flight muscle to perform oscillatory work at low Ca2+ concentrations, a property to which a large number of flying insects owe their evolutionary success.

Association of the Chaperone αB-crystallin with Titin in Heart Muscle

alphaB-crystallin, a major component of the vertebrate lens, is a chaperone belonging to the family of small heat shock proteins. These proteins form oligomers that bind to partially unfolded substrates and prevent denaturation. alphaB-crystallin in cardiac muscle binds to myofibrils under conditions of ischemia, and previous work has shown that the protein binds to titin in the I-band of cardiac fibers (Golenhofen, N., Arbeiter, A., Koob, R., and Drenckhahn, D. (2002) J. Mol. Cell. Cardiol. 34, 309-319). This part of titin extends as muscles are stretched and is made up of immunoglobulin-like modules and two extensible regions (N2B and PEVK) that have no well defined secondary structure. We have followed the position of alphaB-crystallin in stretched cardiac fibers relative to a known part of the titin sequence. alphaB-crystallin bound to a discrete region of the I-band that moved away from the Z-disc as sarcomeres were extended. In the physiological range of sarcomere lengths, alphaB-crystallin bound in the position of the N2B region of titin, but not to PEVK. In overstretched myofibrils, it was also in the Ig region between N2B and the Z-disc. Binding between alphaB-crystallin and N2B was confirmed using recombinant titin fragments. The Ig domains in an eight-domain fragment were stabilized by alphaB-crystallin; atomic force microscopy showed that higher stretching forces were needed to unfold the domains in the presence of the chaperone. Reversible association with alphaB-crystallin would protect I-band titin from stress liable to cause domain unfolding until conditions are favorable for refolding to the native state.

Damped elastic recoil of the titin spring in myofibrils of human myocardium

The giant protein titin functions as a molecular spring in muscle and is responsible for most of the passive tension of myocardium. Because the titin spring is extended during diastolic stretch, it will recoil elastically during systole and potentially may influence the overall shortening behavior of cardiac muscle. Here, titin elastic recoil was quantified in single human heart myofibrils by using a high-speed charge-coupled device-line camera and a nanonewtonrange force sensor. Application of a slack-test protocol revealed that the passive shortening velocity (Vp) of nonactivated cardiomyofibrils depends on: (i) initial sarcomere length, (ii) release-step amplitude, and (iii) temperature. Selective digestion of titin, with low doses of trypsin, decelerated myofibrillar passive recoil and eventually stopped it. Selective extraction of actin filaments with a Ca2+-independent gelsolin fragment greatly reduced the dependency of Vp on release-step size and temperature. These results are explained by the presence of viscous forces opposing myofibrillar passive recoil that are caused mainly by weak actin-titin interactions. Thus, Vp is determined by two distinct factors: titin elastic recoil and internal viscous drag forces. The recoil could be modeled as that of a damped entropic spring consisting of independent worm-like chains. The functional importance of myofibrillar elastic recoil was addressed by comparing instantaneous Vp to unloaded shortening velocity, which was measured in demembranated, fully Ca2+-activated, human cardiac fibers. Titin-driven passive recoil was much faster than active unloaded shortening velocity in early phases of isotonic contraction. Damped myofibrillar elastic recoil could help accelerate active contraction speed of human myocardium during early systolic shortening.

Varieties of elastic protein in invertebrate muscles

Elastic proteins in the muscles of a nematode (Caenorhabditis elegans), three insects (Drosophila melanogaster, Anopheles gambiae, Bombyx mori) and a crustacean (Procambus clarkii) were compared. The sequences of thick filament proteins, twitchin in the worm and projectin in the insects, have repeating modules with fibronectin-like (Fn) and immunoglobulin-like (Ig) domains conserved between species. Projectin has additional tandem Igs and an elastic PEVK domain near the N-terminus. All the species have a second elastic protein we have called SLS protein after the Drosophila gene, sallimus. SLS protein is in the I-band. The N-terminal region has the sequence of kettin which is a spliced product of the gene composed of Ig-linker modules binding to actin. Downstream of kettin, SLS protein has two PEVK domains, unique sequence, tandem Igs, and Fn domains at the end. PEVK domains have repeating sequences: some are long and highly conserved and would have varying elasticity appropriate to different muscles. Insect indirect flight muscle (IFM) has short I-bands and electron micrographs of Lethocerus IFM show fine filaments branching from the end of thick filaments to join thin filaments before they enter the Z-disc. Projectin and kettin are in this region and the contribution of these to the high passive stiffness of Drosophila IFM myofibrils was measured from the force response to length oscillations. Kettin is attached both to actin near the Z-disc and to the end of thick filaments, and extraction of actin or digestion of kettin leads to rapid decrease in stiffness; residual tension is attributable to projectin. The wormlike chain model for polymer elasticity fitted the force-extension curve of IFM myofibrils and the number of predicted Igs in the chain is consistent with the tandem Igs in Drosophila SLS protein. We conclude that passive tension is due to kettin and projectin, either separate or linked in series.

Cardiac titin: molecular basis of elasticity and cellular contribution to elastic and viscous stiffness components in myocardium

Myocardium resists the inflow of blood during diastole through stretch-dependent generation of passive tension. Earlier we proposed that this tension is mainly due to collagen stiffness at degrees of stretch corresponding to sarcomere lengths (SLS) > or = 2.2 microns, but at shorter lengths, is principally determined by the giant sarcomere protein titin. Myocardial passive force consists of stretch-velocity-sensitive (viscous/viscoelastic) and velocity-insensitive (elastic) components; these force components are seen also in isolated cardiac myofibrils or skinned cells devoid of collagen. Here we examine the cellular/myofibrillar origins of passive force and describe the contribution of titin, or interactions involving titin, to individual passive-force components. We construct force-extension relationships for the four distinct elastic regions of cardiac titin, using results of in situ titin segment-extension studies and force measurements on isolated cardiac myofibrils. Then, we compare these relationships with those calculated for each region with the wormlike-chain (WLC) model of entropic polymer elasticity. Parameters used in the WLC calculations were determined experimentally by single-molecule atomic force-microscopy measurements on engineered titin domains. The WLC modelling faithfully predicts the steady-state-force vs. extension behavior of all cardiac-titin segments over much of the physiological SL range. Thus, the elastic-force component of cardiac myofibrils can be described in terms of the entropic-spring properties of titin segments. In contrast, entropic elasticity cannot account for the passive-force decay of cardiac myofibrils following quick stretch (stress relaxation). Instead, slower (viscoelastic) components of stress relaxation could be simulated by using a Monte-Carlo approach, in which unfolding of a few immunoglobulin domains per titin molecule explains the force decay. Fast components of stress relaxation (viscous drag) result mainly from interaction between actin and titin filaments; actin extraction of cardiac sarcomeres by gelsolin immediately suppressed the quickly decaying force transients. The combined results reveal the sources of velocity sensitive and insensitive force components of cardiomyofibrils stretched in diastole.

Alcohol affects the skeletal muscle proteins, titin and nebulin in male and female rats

Alcoholic myopathy is characterized by decreased protein synthesis and contents resulting in atrophy of muscle fibers. We investigated the effect of alcohol on the cytoskeletal muscle proteins, nebulin and titin. Because women are more susceptible than men to the toxic effects of alcohol, male and female rats were included. Four groups were investigated: alcoholic males, pair-fed males, alcoholic females, pair-fed females. Alcohol consumption per unit body weight was 12.9 g/kg.d, with no difference between males and females. After 10 wk, male and female rats fed alcohol had lower gastrocnemius and plantaris protein and RNA contents (P < 0.001), with no effect on soleus, indicating myopathy of type II fibers. The gastrocnemius was fractionated to measure myofibrillary protein contents. Low percentage SDS-gel electrophoresis was performed to determine myosin heavy chain (MHC), nebulin and titin contents. Alcohol reduced gastrocnemius myofibrillary protein and MHC contents, and the plantaris RNA/protein ratio (P < 0.01). The titin/MHC and nebulin/MHC ratios were unaffected, suggesting a concomitant reduction in titin and nebulin. The decreases in titin and nebulin contents may affect muscle function. An interaction between gender and alcohol was noted for the plantaris RNA/protein ratio (P < 0.025), suggesting a reduced capacity for muscle protein synthesis in females.