Regulation of ryanodine receptors by sphingosylphosphorylcholine: involvement of both calmodulin-dependent and -independent mechanisms.
Sphingosylphosphorylcholine (SPC), a lipid mediator with putative second messenger functions, has been reported to regulate ryanodine receptors (RyRs), Ca2+ channels of the sarco/endoplasmic reticulum. RyRs are also regulated by the ubiquitous Ca2+ sensor calmodulin (CaM), and we have previously shown that SPC disrupts the complex of CaM and the peptide corresponding to the CaM-binding domain of the skeletal muscle Ca2+ release channel (RyR1). Here we report that SPC also displaces Ca2+-bound CaM from the intact RyR1, which we hypothesized might lead to channel activation by relieving the negative feedback Ca2+CaM exerts on the channel. We could not demonstrate such channel activation as we have found that SPC has a direct, CaM-independent inhibitory effect on channel activity, confirmed by both single channel measurements and [3H]ryanodine binding assays. In the presence of Ca2+CaM, however, the addition of SPC did not reduce [3H]ryanodine binding, which we could explain by assuming that the direct inhibitory action of the sphingolipid was negated by the simultaneous displacement of inhibitory Ca2+CaM. Additional experiments revealed that RyRs are unlikely to be responsible for SPC-elicited Ca2+ release from brain microsomes, and that SPC does not exert detergent-like effects on sarcoplasmic reticulum vesicles. We conclude that regulation of RyRs by SPC involves both CaM-dependent and -independent mechanisms, thus, the sphingolipid might play a physiological role in RyR regulation, but channel activation previously attributed to SPC is unlikely.
Kovacs
Erika
E
Institute of Enzymology, Biological Research Center, Hungarian Academy of Sciences, Karolina ut 29, H-1113 Budapest, Hungary. kovacs@enzim.hu
Xu
Le
L
Pasek
Daniel A
DA
Liliom
Karoly
K
Meissner
Gerhard
G
D000818:Animals; D002147:Calmodulin / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D010539:Permeability; D010767:Phosphorylcholine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D011817:Rabbits; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology
D002147:Calmodulin; D019837:Ryanodine Receptor Calcium Release Channel; C005356:sphingosine phosphorylcholine; D010767:Phosphorylcholine; D013110:Sphingosine