Cardiac Muscle and Skeletal Muscle Physiology

Cardiac Titin-Isoform Changes in Heart Development and in Chronic Heart Disease

Recent investigations using quantitative RT-PCR and high-resolution SDS-polyacrylamide gel electrophoresis showed that the titin isoform expression pattern is modified dramatically during perinatal heart development (Opitz et al., 2004). To a lesser degree, a titin-isoform shift occurs also in diseased myocardium, including human heart (Neagoe et al., 2002). Titin-stiffness is tuned by changing both the length of titin in the I-band and the expression ratio of longer, more compliant N2BA-isoforms relative to short, stiff N2B-isoform. In the perinatal heart a shift occurs from very long embryonic/fetal N2BA-titin isoforms (3.6-3.7 MDa) towards shorter N2BA-isoforms and a predominant N2B-isoform (3.0 MDa); these changes lead to increased titin-based stiffness in the postnatal heart (Opitz et al., 2004). In contrast, iIn end-stage failing human myocardium, a shift towards higher proportions of long N2BA-isoforms causes decreased myofibrillar passive stiffness (Neagoe et al., 2002; Makarenko et al., 2004).

Human-heart sarcomeres coexpress N2BA and N2B titin isoforms (A). The isoform ratio is shifted towards large N2BA in human heart disease. In contrast, a shift towards shorter isoforms occurs during postnatal heart development (e.g., pig; Opitz et al., 2004). (B) The N2BA:N2B ratio is about 30:70 in normal human heart, but is increased in failing hearts from coronary artery disease (CAD) or dilated cardiomyopathy (DCM) patients. (C) The titin-isoform shift led to decreased titin-derived stiffness of the cardiomyofibrils, although whole hearts were globally stiffened due to increased fibrosis (Neagoe et al., 2002). Thus, titin expression and stiffness can be modified in response to chronic human heart disease (Makarenko et al., 2004).

Makarenko, I., C.A. Opitz, M.C. Leake, C. Neagoe, M. Kulke, J.K. Gwathmey, F. del Monte, R.J. Hajjar & W.A. Linke (2004) Passive stiffness changes caused by upregulation of compliant titin isoforms in human dilated cardiomyopathy hearts. Circ. Res. 95:708-716. free pdf from publisher and free online data supplement

Neagoe C., M. Kulke, F. del Monte, J.K. Gwathmey, P.P. de Tombe, R.J. Hajjar & W.A. Linke (2002) Titin isoform switch in ischemic human heart disease. Circulation. 106:1333-1341. free pdf from publisher

Opitz, C.A., M.C. Leake, I. Makarenko, V. Benes & W.A. Linke (2004) Developmentally regulated switching of titin size alters myofibrillar stiffness in the perinatal heart. Circ. Res. 94:967-975. free pdf from publisher , online data supplement , and Editorial

Altered Cardiac Contractility in "Knock-in" Mice with Truncated Cardiac Myosin-Binding Protein-C

We generated a "knock-in" mouse model replacing native cardiac myosin-binding protein-C with N-terminally truncated cMyBP-C and measured the calcium-sensitivity of active force in skinned cardiac fibers (Witt et al., 2001). An increased calcium sensitivity was found, likely due to higher mobility of myosin heads after truncation of cMyBP-C (see Figure).

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Proposed effect of myosin-binding protein-C (red) on the mobility of myosin heads (green) in cardiac muscle (Witt et al., 2001). Normal cMyBP-C (left), which binds to the myosin neck region but can be unbound by the action of cAMP-dependent protein kinase (PKA), restricts the myosin-head mobility. Genetically truncated cMyBP-C in cardiac fibers from "knock-in" mice (right) allows higher mobility of myosin heads. The truncation led to increased calcium-sensitivity of active force in skinned fibers of the mouse hearts (Witt et al., 2001).

Witt, C.C., B. Gerull, M.J. Davies, T. Centner, W.A. Linke & L. Thierfelder (2001) Hypercontractile properties of cardiac muscle fibers in a knock-in mouse model of cardiac myosin-binding protein-C. J. Biol. Chem. 276:5353-5359. pdf available free from publisher.

Determinants of Shortening Velocity in Muscle Fibers

A novel "slack test" method developed by us has allowed the quantitation of the passive recoil speed of titin in rabbit psoas myofibrils (Minajeva et al., 2002) and human heart myofibrils (Opitz et al., 2003). In the presence of titin-based passive force, titin elastic recoil importantly contributes to the shortening velocity of muscle fibers.

Minajeva, A., C. Neagoe, M. Kulke & W.A. Linke (2002) Titin-based contribution to shortening velocity of rabbit skeletal myofibrils. J. Physiol. 540:177-188. pdf available free from publisher.

Opitz, C.A., M. Kulke, M. C. Leake, C. Neagoe, H. Hinssen, R.J. Hajjar & W.A. Linke (2003) Damped elastic recoil of the titin spring in myofibrils of human myocardium. Proc. Natl. Acad. Sci. USA. 100:12688-12693.  free pdf available from Pubmed Central plus online supplement.

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