Sarcoplasmic reticulum calcium uptake and speed of relaxation are depressed in nebulin-free skeletal muscle.
Previous work suggested that altered Ca(2+) homeostasis might contribute to dysfunction of nebulin-free muscle, as gene expression analysis revealed that the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-inhibitor sarcolipin (SLN) is up-regulated >70-fold in nebulin knockout mice, and here we tested this proposal. We investigated SLN protein expression in nebulin-free and wild-type skeletal muscle, as well as expression of other Ca(2+)-handling proteins. Ca(2+) uptake capacity was determined in isolated sarcoplasmic reticulum vesicles and in intact myofibers by measuring Ca(2+) transients. Muscle contractile performance was determined in skinned muscle activated with exogenous Ca(2+), as well as in electrically stimulated intact muscle. We found profound up-regulation of SLN protein in nebulin-free skeletal muscle, whereas expression of other Ca(2+)-handling proteins was not (calsequestrin and phospholamban) or was minimally (SERCA) affected. Speed of Ca(2+) uptake was >3-fold decreased in sarcoplasmic reticulum vesicles isolated from nebulin-free muscle as well as in nebulin-free intact myofibers. Ca(2+)-activated stress in skinned muscle and stress produced by intact nebulin-free muscle were reduced to a similar extent compared with wild type. Half-relaxation time was significantly longer in nebulin-free compared with wild-type muscle. Thus, the present study demonstrates for the first time that nebulin might also be involved in physiological Ca(2+) handling of the SR-myofibrillar system.
Ottenheijm
Coen A C
CA
Dept. of Molecular and Cellular Biology, University of Arizona, PO Box 245217, Tucson, AZ 85724, USA.
Fong
Chi
C
Vangheluwe
Peter
P
Wuytack
Frank
F
Babu
Gopal J
GJ
Periasamy
Muthu
M
Witt
Christian C
CC
Labeit
Siegfried
S
Granzier
Henk
H
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation; D066298:In Vitro Techniques; D017136:Ion Transport; D007700:Kinetics; D051379:Mice; D018345:Mice, Knockout; D008861:Microsomes / Q000378:metabolism; D009124:Muscle Proteins / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism; D009126:Muscle Relaxation / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D009210:Myofibrils / Q000378:metabolism; D011510:Proteolipids / Q000235:genetics / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D015854:Up-Regulation
D009124:Muscle Proteins; D011510:Proteolipids; C035620:nebulin; C077258:sarcolipin; D002118:Calcium