Cell Molecular Biomechanics: Dynamics of Ca2 Regulation and Muscle Performance

1. Cell & Molecular Biomechanics:Dynamics of Ca2+ Regulation and Muscle Performance P. Bryant Chase Florida State University Dept. of Biological Science and Program in Molecular Biophysics OSU MBI Workshop 4, Signal Transduction II: Muscle and Synapse March 11, 2004

2. From: Berchtold MW, Brinkmeier H, Muntener M. 2000. Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease. Physiol Rev 80(3):1215-65. Fig. 7. Troponin (Tn) C as a myofibrillar Ca2+ switch molecule. Model of the troponin-tropomyosin-actin organization is according to Gagn� et al. (145); TnC is shown in blue for the NH2 domain and pink for the COOH domain. TnI is shown in red (NH2-terminal domain), brown (COOH-terminal domain), and yellow (inhibitory region). TnT is shown in green. Myosin is shown in green (myosin-S1), red (essential light chain), and yellow (regulatory light chain) in stick representation. Tropomyosin is shown in light blue and darker blue stick representation. Note that only TnC, myosin, and tropomyosin are represented by known structure. TnT and TnI structures are modeled. Actin monomers are represented by white spheres. a: Organization in the relaxed state of muscle. The COOH domain of TnC is bound to Mg2+. The NH2-terminal domain of TnI is anchored on the COOH domain of TnC, whereas the inhibitory region and COOH-terminal domain of TnI make contact with actin and tropomyosin. This organization keeps the thin filament in a conformation that prevents myosin from properly interacting with actin. b: Organization after two Ca2+ bind to the NH2 domain of TnC, which in turn interacts with TnI. The inhibitory region and COOH domain of TnI are then released from actin. This leads to a conformation of the thin filament that allows the proper formation of the actomyosin complex. The power stroke can then occur (not shown here) sliding the thin filament to the right. (Figure kindly provided by Drs. S. Gagn� and B. Sykes, Edmonton, Canada.)  From: Berchtold MW, Brinkmeier H, Muntener M. 2000. Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease. Physiol Rev 80(3):1215-65. Fig. 7. Troponin (Tn) C as a myofibrillar Ca2+ switch molecule. Model of the troponin-tropomyosin-actin organization is according to Gagn� et al. (145); TnC is shown in blue for the NH2 domain and pink for the COOH domain. TnI is shown in red (NH2-terminal domain), brown (COOH-terminal domain), and yellow (inhibitory region). TnT is shown in green. Myosin is shown in green (myosin-S1), red (essential light chain), and yellow (regulatory light chain) in stick representation. Tropomyosin is shown in light blue and darker blue stick representation. Note that only TnC, myosin, and tropomyosin are represented by known structure. TnT and TnI structures are modeled. Actin monomers are represented by white spheres. a: Organization in the relaxed state of muscle. The COOH domain of TnC is bound to Mg2+. The NH2-terminal domain of TnI is anchored on the COOH domain of TnC, whereas the inhibitory region and COOH-terminal domain of TnI make contact with actin and tropomyosin. This organization keeps the thin filament in a conformation that prevents myosin from properly interacting with actin. b: Organization after two Ca2+ bind to the NH2 domain of TnC, which in turn interacts with TnI. The inhibitory region and COOH domain of TnI are then released from actin. This leads to a conformation of the thin filament that allows the proper formation of the actomyosin complex. The power stroke can then occur (not shown here) sliding the thin filament to the right. (Figure kindly provided by Drs. S. Gagn� and B. Sykes, Edmonton, Canada.)

3. 1A. Ca2+ modulates isometric force and rate of tension (re)development (kTR)

4. 1B. At sub-saturating [Ca2+], kTR reflects:(i) properties of TnC, and (ii) dynamics of individual regulatory units

5. 1C. A simple, dynamic view ofcalcium regulation of tension development SLIDE 11 - In the time remaining I would like to talk about our recent efforts to model the results with altered substrate conditions and with CDZ. - The idea that Ca2+ controls a process that is separate from the cross-bridge power stroke has been suggested in previous models of the regulation of tension generation. - The model shown here is adapted from one proposed by Landsberg and Sideman and incorporates a mechanism for Ca2+ control of tension generation kinetics that is independent of the rate constants that govern the actomyosin ATPase cycle. - A key feature of this model is that no rate constants are dependent on the [Ca2+]. Instead, Ca2+ exerts it�s regulatory influence on tension via control of thin filament activation and has no direct influence on the kinetic interactions of the cross-bridges. - Briefly, the processes of Ca2+ binding and tension generation are modelled as two-state reactions with forward and reverse rate constants. Since Ca2+ can come off the TF when cross-bridges are in tension generating states, there are 2 possible CB states but four possible TF states. - To simulate dATP the forward and reverse rate constants for tension generation were increased to account for the faster CB cycling that occurs with this nucleotide. Direct evidence for this comes from caged Pi experiments where the forward rate of the tension generating step was found to be increased with dATP. The reverse rate is most likely also elevated since the ss-level of tension in fibers is not altered by dATP. - To simulate 0.5 mM ATP the reverse rate constants of the CB cycle (g, g�) were reduced. - CDZ was modelled by reducing koff and koff� to account for a slower Ca2+ dissociation from TnC. - Ca2+ activation level then simulated by varying kon and kon� SLIDE 11 - In the time remaining I would like to talk about our recent efforts to model the results with altered substrate conditions and with CDZ. - The idea that Ca2+ controls a process that is separate from the cross-bridge power stroke has been suggested in previous models of the regulation of tension generation. - The model shown here is adapted from one proposed by Landsberg and Sideman and incorporates a mechanism for Ca2+ control of tension generation kinetics that is independent of the rate constants that govern the actomyosin ATPase cycle. - A key feature of this model is that no rate constants are dependent on the [Ca2+]. Instead, Ca2+ exerts it�s regulatory influence on tension via control of thin filament activation and has no direct influence on the kinetic interactions of the cross-bridges. - Briefly, the processes of Ca2+ binding and tension generation are modelled as two-state reactions with forward and reverse rate constants. Since Ca2+ can come off the TF when cross-bridges are in tension generating states, there are 2 possible CB states but four possible TF states. - To simulate dATP the forward and reverse rate constants for tension generation were increased to account for the faster CB cycling that occurs with this nucleotide. Direct evidence for this comes from caged Pi experiments where the forward rate of the tension generating step was found to be increased with dATP. The reverse rate is most likely also elevated since the ss-level of tension in fibers is not altered by dATP. - To simulate 0.5 mM ATP the reverse rate constants of the CB cycle (g, g�) were reduced. - CDZ was modelled by reducing koff and koff� to account for a slower Ca2+ dissociation from TnC. - Ca2+ activation level then simulated by varying kon and kon�

6. 2A. Ca2+ control and cardiovascular disease: Regulated in vitro motility assays

7. Mutations in cTnI (a-Tm, cTnT) increase Ca2+ sensitivity, cause hypertrophic cardiomyopathy

8. 3A. Mechanical Tuning Emerged from a 2-D, Two Filament Model with Filament Compliance This is just introductory material.This is just introductory material.

9. UPDATING OUR MODEL OF THE MUSCLE HALF-SARCOMERE WITH FILAMENT COMPLIANCE

10. Model output fromChase, Macpherson & Daniel, submittedQuestion: Is there any experimental evidence that myofilament compliance affects muscle function?

11. Myofilament compliance can explain activation-dependent crossbridge kinetics (rapid tension transients) in muscle fibers

12. Ca2+-activation dependence of rapid tension transient kinetics:model predicts 69% of sarcomere compliance is in the myofilaments

13. CONCLUSIONS

14. Acknowledgements University of Washington Michael Regnier Don Martyn Al Gordon Tony Rivera Mike Macpherson (Stanford) Alan Trimble Tom Daniel