doi: 10.1161/01.RES.0000247031.56868.10
© 2006 American Heart Association, Inc.
Editorials |
New Insights in the Role of Cardiac Myosin Binding Protein C As a Regulator of Cardiac Contractility
From the Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology (H.L.G., K.B.C.), Washington State University, Wash.
Correspondence to Henk L. Granzier, Department VCAPP, Washington State University, Pullman, WA 99164-6520. E-mail granzier{at}wsu.edu
See related article, pages 884–890
Key Words: stretch activation • ß–adrenergic stimulation • crossbridge kinetics • cooperativity
Myosin Binding Protein C is a large multidomain protein that is found in the A-band region of the sarcomere where it associates with the thick filament.1 Mutations in cardiac myosin binding protein C (cMyBP-C) are a major cause of familial hypertrophic cardiomyopathy2 but the functions of cMyBP-C are not well resolved. In this issue Moss and colleagues (Stelzer et al3) provide important new insights in the role of cMyBP-C in the regulation of cardiac muscle contraction.
A binding site for the light meromyosin (LMM) domain of myosin is found near the C-terminus of MyBP-C and a second site that binds the S2 domain of myosin is present near the N-terminus of MyBP-C.4 The cardiac MyBP-C isoform contains protein kinase A (PKA) phosphorylation sites within the S2 binding site that are absent in skeletal muscle isoforms. Although phosphorylating these residues abolishes binding to myosin S2,4 the functional role of this phosphorylation is not well understood. Stelzer et al3 examined the effect of PKA on stretch activation in skinned cardiac myocytes of wildtype and cMyBP-C KO mice. Their findings provide an important piece of the puzzle—cMyBP-C slows crossbridge cycling kinetics (confirming earlier results5) and, importantly, this effect can be relieved by PKA-based phosphorylation of cMyBP-C.
It is well known from work initiated by Professor Pringle mid last century that stretch activation is most pronounced in asynchronous flight muscle, where it gives rise to wing beat frequencies that far exceed the capacity of the sarcoplasmic reticulum to activate and relax myofilaments.6 Although cardiac muscle largely relies on calcium cycling for activation and relaxation, several decades ago Steiger and colleagues7 and more recently Moss and colleagues8 have shown that stretch activation also exists in skinned myocardium. Concomitant with a stretch, force increases because of strain of attached crossbridges. This is followed by a rapid decrease in force (with a rate constant Krel) because of crossbridge detachment. Finally, a delayed force increase occurs (with a rate constant Kdf) to a maximal level that exceeds the prestretch force level. The delayed force rise is thought to involve both stretch-induced cooperative effects that result from crossbridge-based activation of the thin filament and direct stretch-induced recruitment of cross-bridges.3 Thus, stretch activation positively affects muscle function by increasing the force generating capability beyond the level determined by cytosolic calcium.
Noninvasive phase labeled MRI has provided convincing evidence for stretch of the papillary muscles during systole9 and stretch activation is likely to be a prominent feature of papillary muscle function. Because of the limited resolution of MRI it is less clear whether other parts of the chamber walls are stretched during systole. However, considering the counterhelical fiber orientation of epicardial and endocardial left ventricular fibers, the differences in timing of activation (endocardium activates first), and differences in the strength of contraction (the epicardial fibers dominate), local stretch of activated fibers is likely to occur.9 Indeed microsonometry has provided evidence that endocardial fibers are stretched during early systole.10 Thus, stretch activation is likely to have relevance for cardiac function.
Stelzer et al3 showed that PKA treatment accelerates crossbridge detachment (reflected in the significantly increased Krel) as well as rates of force generating crossbridge transitions (increased Kdf). A powerful aspect of their work is the comparison between wildtype and cMyBP-C KO mice. Stretch activation is accelerated in cardiac myocytes from the KO mice and, importantly, is insensitive to PKA treatment. Considering that no differences in TnI phosphorylation between wildtype and KO mice were noted, the authors conclude that the accelerated stretch activation that is present following PKA treatment of wildtype cardiac myocytes is because of cMyBP-C phosphorylation.3 Thus, phosphorylation of cMyBP-C is likely to be a factor in the accelerated contractility because of ß-adrenergic stimulation.
Considering that phosphorylation of cMyBP-C and ablation of cMyBP-C both result in accelerated stretch activation, it is worthwhile reviewing relevant physiological findings with mice that do not contain cMyBP-C, either because they express truncated cMyBP-C or because they are cMyBP-C null.11) A unique feature of these models is that their early systolic function is relatively normal but their late systolic function is abnormal, with relaxation occurring prematurely.11 This indicates that cMyBP-C is not critical for initiating contraction but that it is important for sustaining force throughout ejection. It is noteworthy that it recently has been proposed that the mechanisms that govern stretch activation might also exert effects when muscle shortens. As explained in detail elsewhere,12 crossbridge-based activation slows the attainment of a lower force level following shortening and in effect "remembers" the higher force produced at the longer length. Thus, mechanisms of stretch activation are important for sustaining force during cardiac ejection and when they are accelerated (or absent) the ejection period is abbreviated.
Stelzer et al speculate that accelerated stretch activation by cMyBP-C phosphorylation is because of a disrupted binding of cMyBP-C to the S2 domain of myosin that allows myosin heads to move away from the thick filament backbone (schematically depicted in Figure 1). This proposal is consistent with the findings of Levine et al who studied the effect of PKA-based phosphorylation on myosin head disposition of isolated thick flaments.13 The reduced distance between S1 and actin that results from cMyBP-C phosphorylation might increase the probability of cross-bridge formation and accelerate the transition to force generation. This potential mechanism warrants testing in intact myocardium using low angle X-ray diffraction.
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Alternative mechanisms might involve binding of cMyBP-C to actin, via the proline-rich region found near cMyBP-C’s N-terminus.14 (This region shares homology to the PEVK region of titin, which is known to bind to actin.15,16) This proline-rich region has been suggested to bind to the thin filament in a manner that is controlled by the phosphorylation state of cMyBP-C.14 Further work is required to establish the physiological significance of cMyBP-C–actin interaction and whether it has relevance to the accelerated stretch activation following cMyBP-C phosphorylation.
Outstanding issues also include establishing how cMyBP-C exerts a controlling influence over myosin despite the low 
1:8 molar ratio of cMyBP-C to myosin. It is thought that cMyBP-C molecules form collars around the thick filament backbone, spaced 43 nm apart with each collar consisting of 3 cMyBP-C molecules17 and this arrangement potentially allows a limited number of cMyBP-C molecules to affect a much larger number of myosins. Another issue is that cMyBP-C is located in only 2 regions adjacent to the M-line region (the so-called C-zones) and, thus, that the peripheral 1/3 of the half thick filament (D-zone) is devoid of MyBP-C17. It is important to understand whether the effect of cMyBP-C propagates into the D-zone of the sarcomere or whether the effect is restricted to the C-zone. Finally, experiments performed using ssTnI transgenic mice have provided supportive evidence that phosphorylation of cTnI does play a role in accelerating crossbridge kinetics during ß-adrenergic stimulation of isolated heart preparations.18 It is possible that the cTnI effect is brought about indirectly and involves the changes in the calcium transients and cTnC calcium binding affinity that take place during ß-adrenergic stimulation, whereas cMyBP-C has a more direct effect on crossbridge kinetics. We consider it important to dissect the role of cMyBP-C phosphorylation, relative to that of cTnI, in the intact heart with phosphorylation brought about by physiological signaling pathways. Excellent mouse models and experimental tools are presently available for this work.
In summary, mechanical-perturbation protocols that elucidated stretch activation led to a major advance in understanding how contractility is regulated in insect flight muscle.6 More than half a century later the technique has been revived and this has resulted in novel insights in the regulation of heart muscle contraction. Professor Pringle anticipated an important role for stretch activation in cardiac muscle and would be pleased with these new results. The present work on stretch-activation in skinned cardiac myocytes by Stelzer et al3 shows that the PKA sites of cMyBP-C allow myofilament kinetics to be slowed (dephosphorylation) or sped up (phosporylation). Thus, cMyBP-C phosphorylation is likely to play a significant role in achieving the hyperdynamic state of the heart that results from ß–adrenergic stimulation.
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Acknowledgments |
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Sources of Funding
Work of the authors is supported by a grant from the American Heart Association (K.B.C.) and the National Institutes of Health (HL61497 and HL62881 to H.L.G.).
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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 Top References |
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1. Starr R, Offer G. The interaction of C-protein with heavy meromyosin and subfragment-2. Biochem J. 1978; 171: 813–816.[Medline] [Order article via Infotrieve]
2. Richard P, Charron P, Carrier L, Ledeuil C, Cheav T, Pichereau C, Benaiche A, Isnard R, Dubourg O, Burban M, Gueffet JP, Millaire A, Desnos M, Schwartz K, Hainque B, Komajda M. Hypertrophic cardiomyopathy: distribution of disease genes, spectrum of mutations, and implications for a molecular diagnosis strategy. Circulation. 2003; 107: 2227–2232.
3. Stelzer JE, Patel JR, Moss RL. PKA-mediated acceleration of the stretch activation response in murine skinned myocardium is eliminated by ablation of cMyBP-C. Circ Res. 2006; 99: 884–890.
4. Gruen M, Prinz H, Gautel M. cAPK-phosphorylation controls the interaction of the regulatory domain of cardiac myosin binding protein C with myosin-S2 in an on-off fashion. FEBS Lett. 1999; 453: 254–259.[CrossRef][Medline] [Order article via Infotrieve]
5. Stelzer JE, Dunning SB, Moss RL. Ablation of cardiac myosin-binding protein-C accelerates stretch activation in murine skinned myocardium. Circ Res. 2006; 98: 1212–1218.
6. Pringle JW. The Croonian Lecture, 1977. Stretch activation of muscle: function and mechanism. Proc R Soc Lond B Biol Sci. 1978; 201: 107–130.[Medline] [Order article via Infotrieve]
7. Steiger GJ. Tension transients in extracted rabbit heart muscle preparations. J Mol Cell Cardiol. 1977; 9: 671–685.[Medline] [Order article via Infotrieve]
8. Stelzer JE, Larsson L, Fitzsimons DP, Moss RL. Activation dependence of stretch activation in mouse skinned myocardium: implications for ventricular function. J Gen Physiol. 2006; 127: 95–107.
9. Davis JS, Hassanzadeh S, Winitsky S, Lin H, Satorius C, Vemuri R, Aletras AH, Wen H, Epstein ND. The overall pattern of cardiac contraction depends on a spatial gradient of myosin regulatory light chain phosphorylation. Cell. 2001; 107: 631–641.[CrossRef][Medline] [Order article via Infotrieve]
10. Sengupta PP, Khandheria BK, Korinek J, Wang J, Belohlavek M. Biphasic tissue Doppler waveforms during isovolumic phases are associated with asynchronous deformation of subendocardial and subepicardial layers. J Appl Physiol. 2005; 99: 1104–1111.
11. Palmer BM, Georgakopoulos D, Janssen PM, Wang Y, Alpert NR, Belardi DF, Harris SP, Moss RL, Burgon PG, Seidman CE, Seidman JG, Maughan DW, Kass DA. Role of cardiac myosin binding protein C in sustaining left ventricular systolic stiffening. Circ Res. 2004; 94: 1249–1255.
12. Campbell KB, Chandra M. Functions of stretch activation in heart muscle. J Gen Physiol. 2006; 127: 89–94.
13. Levine R, Weisberg A, Kulikovskaya I, McClellan G, Winegrad S. Multiple structures of thick filaments in resting cardiac muscle and their influence on cross-bridge interactions. Biophys J. 2001; 81: 1070–1082.[Medline] [Order article via Infotrieve]
14. Kulikovskaya I, McClellan G, Flavigny J, Carrier L, Winegrad S. Effect of MyBP-C binding to actin on contractility in heart muscle. J Gen Physiol. 2003; 122: 761–774.
15. Yamasaki R, Berri M, Wu Y, Trombitas K, McNabb M, Kellermayer MS, Witt C, Labeit D, Labeit S, Greaser M, Granzier H. Titin-actin interaction in mouse myocardium: passive tension modulation and its regulation by calcium/S100A1. Biophys J. 2001; 81: 2297–2313.[Medline] [Order article via Infotrieve]
16. Granzier HL, Labeit S. The giant protein titin: a major player in myocardial mechanics, signaling, and disease. Circ Res. 2004; 94: 284–295.
17. Flashman E, Redwood C, Moolman-Smook J, Watkins H. Cardiac myosin binding protein C: its role in physiology and disease. Circ Res. 2004; 94: 1279–1289.
18. Layland J, Solaro RJ, Shah AM. Regulation of cardiac contractile function by troponin I phosphorylation. Cardiovasc Res. 2005; 66: 12–21.
19. de Tombe PP. Myosin binding protein C in the heart. Circ Res. 2006; 98: 1234–1236.
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