Sci. Aging Knowl. Environ., 11 December 2002
Vol. 2002, Issue 49, p. pe20
[DOI: 10.1126/sageke.2002.49.pe20]

PERSPECTIVES

Titin--Springing Back to Youth?

Fawzia Huq, E. Kevin Heist, and Roger J. Hajjar

The authors are at the Cardiovascular Research Center, Massachusetts General Hospital, Boston, MA 02129, USA. E-mail: hajjar{at}cvrc.mgh.harvard.edu (R.J.H.)

http://sageke.sciencemag.org/cgi/content/full/sageke;2002/49/pe20

Key Words: titin • sarcomere • heart failure • cardiomyopathy • cardiomyocyte

Could being young at heart simply be a matter of how big-hearted we are? It might indeed, according to two recent reports (1, 2). These studies have drawn attention to the giant protein titin, a component of muscle fibers (myofibrils) that confers passive elasticity on striated muscle. This type of muscle, which includes cardiac and skeletal muscle, has a highly organized subcellular architecture that produces visible striations. Each titin molecule spans half a sarcomere--the 2-µm contractile unit of the myofibril in striated muscle (Fig. 1). Titin is anchored to the Z band at one end of the sarcomere, where it tethers the myosin molecule, and it extends to the middle of aligned myosin filaments (the M band) at the center of the sarcomere (3). In contrast to myosin, which produces active contractile forces by adenosine triphosphate-dependent ratcheting along the actin filament, titin acts as a molecular spring to generate passive forces that provide elasticity to the sarcomere and permit elastic recoil (3, 4). Titin provides the retracting force, known as passive stiffness, that is generated when a striated muscle is stretched. As the length of a sarcomere increases, the titin molecule first unfolds; once a certain length is reached (2 µm in a cardiac sarcomere), titin stretches further like an elastic band and will cause the sarcomere to retract once the stretching forces are removed (3).



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Fig. 1. Schematic diagram showing the position of titin in the cardiac sarcomere. Titin spans the distance between the M line and the Z disk and has a mass of about 2.9 MD. The portion of the protein near the Z disk is the highly folded, elastic segment of the molecule. [Courtesy of Dr. Wolfgang Linke/University of Heidelberg]

 
Alternative splicing results in the production of multiple titin isoforms with varying degrees of elasticity (5), and different isoforms are coexpressed in different species. In rodents, for example, the N2B isoform (which is smaller and stiffer than other isoforms) is exclusively expressed; whereas in bovine atria, the longer and more elastic N2BA isoform predominates. Humans (and most other species) express a mixture of both isoforms (5).

Given the crucial functions performed by titin in muscle physiology, one might suspect that mutant forms of titin might lie at the heart of certain muscle abnormalities. In this Perspective, we consider the role titin abnormalities might play in cardiac muscle dysfunction. We have only begun to get glimpses of how changes in titin isoforms correlate with disease states. Earlier this year, analysis of a zebrafish mutant by Xu et al. revealed that a T-to-G mutation in the cardiac-specific N2Bb exon causes cardiomyopathy characterized by poor pumping of blood by the ventricle during contraction. These researchers also found that normal sarcomere development fails to occur in this mutant (6). In addition, two other recent reports have described, for the first time, mutations in human titin that are associated with dilated cardiomyopathy, a condition in which pathologically enlarged heart chambers fail to pump blood effectively (7, 8). Despite affecting different regions of the titin protein, all three mutations resulted in heart failure.

The regulation of titin isoform expression in response to myocardial stress is also under investigation, and the results have proved both puzzling and intriguing. Two recent reports published simultaneously in the journal Circulation shed further insight, and generate further controversy, on the role of titin in cardiac dysfunction. In one study, Wu and colleagues increased the heart rates of live dogs for 4 weeks by inserting wires into their hearts that deliver rapid electrical stimulation. This technique (referred to as "pacing") is an established model of experimentally induced heart failure. After pacing the dogs, Wu and colleagues analyzed titin isoform expression in slices of tissue derived from the midventricular region of the hearts (1). Although the total amount of titin expressed was unchanged, there was an increase in the expression of the stiff, smaller N2B isoform and a corresponding decrease in the expression of the long, compliant N2BA isoform in these animals. Skinned muscle strips from the hearts of the paced animals displayed increased passive tension (a marker for cardiac relaxation dysfunction) that appeared to be caused by variations in both collagen and titin expression. This is a somewhat surprising finding, because this same group had reported previously that after 2 weeks of pacing in a similar canine model, no change in titin isoform ratios was observed, although the gradient of isoform expression between the inside (endocardium) and the outside (epicardium) of ventricular muscle was altered (9). In control animals, there was a greater ratio of the compliant N2BA titin isoform versus the stiffer N2B isoform in the endocardium compared to the epicardium. In paced animals, this gradient between the endocardium and epicardium increased, although the functional significance of this finding is not known. Why 2 weeks of rapid pacing produces different effects on titin expression than 4 weeks of pacing is currently the subject of speculation and further investigation.

In a second study, Neagoe and colleagues examined the heart tissue of end-stage heart failure patients at the time of cardiac transplant, in which a donor heart is surgically implanted to replace a failing heart (2). These researchers found that hearts that had failed as a result of coronary artery disease and past heart attacks had similar total titin expression but displayed a relative decrease in the expression of the stiff N2B isoform versus the compliant N2BA isoform, as compared to normal hearts and to hearts that failed for reasons other than previous heart attacks. This finding is in marked contrast to what was observed in the paced dogs described above. A change in the titin expression pattern similar to that observed in the human patients was also demonstrated by this group in a rat model (2); rats subjected to heart attack by ligation of a major coronary artery displayed increased expression of the compliant N2BA titin isoform, which is not normally present in significant quantities in rodents. It was also noted in this study that the overall concentration of titin in heart tissue is reduced in older versus younger rats.

So are the switches in titin isoform expression seen in response to different models of cardiac failure compensatory or maladaptive? It is tempting to speculate that in end-stage heart failure patients, the increased formation of fibrous (nonelastic) tissue in the failing heart might, as a compensatory response, induce increased expression of a more compliant titin isoform. The structural changes in cardiac muscle that occurred in the pacing model remain obscure, with different groups reporting increased, decreased, or unchanged amounts of fibrosis. In the case of myocardial dilation that follows pacing, it is possible that the switch to a less compliant titin isoform might initially be cardioprotective. Finally, it has been shown that both systolic function (contractility) and diastolic function (relaxation) are attenuated in the sensescent myocardium (10). These changes have been attributed to the altered expression and function of calcium-regulated proteins essential for sarcomere function and to increased collagen and fibronectin deposition in the extracellular matrix (10). It would be intriguing to explore the role of titin in the diastolic dysfunction seen in senescent hearts. Molecular characterization of the titin-related alterations that occur with age might eventualy allow us to remain young at heart in the true sense of the word.


December 11, 2002
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Citation: F. Huq, E. K. Heist, R. J. Hajjar, Titin--Springing Back to Youth? Science's SAGE KE (11 December 2002), http://sageke.sciencemag.org/cgi/content/full/sageke;2002/49/pe20








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