A possible role of the junctional face protein JP-45 in modulating Ca2+ release in skeletal muscle.
We investigated the functional role of JP-45, a recently discovered protein of the junctional face membrane (JFM) of skeletal muscle. For this purpose, we expressed JP-45 C-terminally tagged with the fluorescent protein DsRed2 by nuclear microinjection in myotubes derived from the C2C12 skeletal muscle cell line and performed whole-cell voltage-clamp experiments. We recorded in parallel cell membrane currents and Ca(2+) signals using fura-2 during step depolarization. It was found that properties of the voltage-activated Ca(2+) current were not significantly changed in JP-45-DsRed2-expressing C2C12 myotubes whereas the amplitude of depolarization-induced Ca(2+) transient was decreased compared to control myotubes expressing only DsRed2. Converting Ca(2+) transients to Ca(2+) input flux using a model fit approach to quantify Ca(2+) removal, the change could be attributed to an alteration in voltage-activated Ca(2+) permeability rather than to altered removal properties or a lower Ca(2+) content of the sarcoplasmic reticulum (SR). Determining non-linear capacitive currents revealed a reduction of Ca(2+) permeability per voltage-sensor charge. The results may be explained by a modulatory effect of JP-45 related to its reported in vitro interaction with the dihydropyridine receptor and the SR Ca(2+) binding protein calsequestrin (CSQ).
Gouadon
E
E
University of Ulm, Department of Applied Physiology, Albert-Einstein-Allee 11, D-89069 Ulm, Germany.
Schuhmeier
R P
RP
Ursu
D
D
Anderson
A A
AA
Treves
S
S
Zorzato
F
F
Lehmann-Horn
F
F
Melzer
W
W
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D002460:Cell Line; D006801:Humans; D015640:Ion Channel Gating / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D008565:Membrane Proteins / Q000378:metabolism*; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000502:physiology*
C418633:JSRP1 protein, human; D008565:Membrane Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium