Co-Expression of SERCA Isoforms, Phospholamban and Sarcolipin in Human Skeletal Muscle FibersReport as inadecuate

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Sarcolipin SLN and phospholamban PLN inhibit the activity of sarcoendoplasmic reticulum Ca2+-ATPases SERCAs by reducing their apparent affinity for Ca2+. A ternary complex between SLN, PLN, and SERCAs results in super-inhibition of SERCA activity. Analysis of skeletal muscle homogenate has limited our current understanding of whether SLN and PLN regulate SERCA1a, SERCA2a, or both in skeletal muscle and whether SLN and PLN are co-expressed in skeletal muscle fibers. Biopsies from human vastus lateralis were analyzed through single fiber Western blotting and immunohisto-fluorescence staining to circumvent this limitation. With a newly generated SLN antibody, we report for the first time that SLN protein is present in human skeletal muscle. Addition of the SLN antibody 50 µg to vastus lateralis homogenates increased the apparent Ca2+ affinity of SERCA KCa, pCa units -Ab, 5.85 ± 0.02 vs. +Ab, 5.95 ± 0.02 and maximal SERCA activity μmol-g protein-min -Ab, 122 ± 6.4 vs. +Ab, 159 ± 11 demonstrating a functional interaction between SLN and SERCAs in human vastus lateralis. Specifically, our results suggest that although SLN and PLN may preferentially regulate SERCA1a, and SERCA2a, respectively, physiologically they both may regulate either SERCA isoform. Furthermore, we show that SLN and PLN co-immunoprecipitate in human vastus lateralis homogenate and are simultaneously expressed in 81% of the fibers analyzed with Western blotting which implies that super-inhibition of SERCA may exist in human skeletal muscle. Finally, we demonstrate unequivocally that mouse soleus contains PLN protein suggesting that super-inhibition of SERCA may also be important physiologically in rodent skeletal muscle.

Author: Val A. Fajardo, Eric Bombardier, Chris Vigna, Tahira Devji, Darin Bloemberg, Daniel Gamu, Anthony O. Gramolini, Joe Quadrilatero,



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