Journal of Biological Chemistry
Volume 284, Issue 8, 20 February 2009, Pages 5097-5106
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Protein Synthesis, Post-Translational Modification, and Degradation
Ablation of Ventricular Myosin Regulatory Light Chain Phosphorylation in Mice Causes Cardiac Dysfunction in Situ and Affects Neighboring Myofilament Protein Phosphorylation*

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There is little direct evidence on the role of myosin regulatory light chain phosphorylation in ejecting hearts. In studies reported here we determined the effects of regulatory light chain (RLC) phosphorylation on in situ cardiac systolic mechanics and in vitro myofibrillar mechanics. We compared data obtained from control nontransgenic mice (NTG) with a transgenic mouse model expressing a cardiac specific nonphosphorylatable RLC (TG-RLC(P-). We also determined whether the depression in RLC phosphorylation affected phosphorylation of other sarcomeric proteins. TG-RLC(P-) demonstrated decreases in base-line load-independent measures of contractility and power and an increase in ejection duration together with a depression in phosphorylation of myosin-binding protein-C (MyBP-C) and troponin I (TnI). Although TG-RLC(P-) displayed a significantly reduced response to β1-adrenergic stimulation, MyBP-C and TnI were phosphorylated to a similar level in TG-RLC(P-) and NTG, suggesting cAMP-dependent protein kinase signaling to these proteins was not disrupted. A major finding was that NTG controls were significantly phosphorylated at RLC serine 15 following β1-adrenergic stimulation, a mechanism prevented in TG-RLC(P-), thus providing a biochemical difference in β1-adrenergic responsiveness at the level of the sarcomere. Our measurements of Ca2+ tension and Ca2+-ATPase rate relations in detergent-extracted fiber bundles from LV trabeculae demonstrated a relative decrease in maximum Ca2+-activated tension and tension cost in TG-RLC(P-) fibers, with no change in Ca2+ sensitivity. Our data indicate that RLC phosphorylation is critical for normal ejection and response to β1-adrenergic stimulation. Our data also indicate that the lack of RLC phosphorylation promotes compensatory changes in MyBP-C and TnI phosphorylation, which when normalized do not restore function.

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*

This work was supported, in whole or in part, by National Institutes of Health Grants PO1 HL62426 (to R. J. S., P. P. D., and P. M. B.), RO1 HL22231 (to R. J. S.), T32 HL07962 (to S. B. S. and A. C. H.), and F32 HL086023 (to A. C. H.). This work was also supported by American Heart Association Grant 0710031Z (to S. B. S.) and funds from the Temple Hoyne Buell Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.