Rad as a novel regulator of excitation-contraction coupling and beta-adrenergic signaling in heart.
BACKGROUND
Rad (Ras associated with diabetes) GTPase, a monomeric small G protein, binds to Ca(v)beta subunit of the L-type Ca(2+) channel (LCC) and thereby regulates LCC trafficking and activity. Emerging evidence suggests that Rad is an important player in cardiac arrhythmogenesis and hypertrophic remodeling. However, whether and how Rad involves in the regulation of excitation-contraction (EC) coupling is unknown.


OBJECTIVE
This study aimed to investigate possible role of Rad in cardiac EC coupling and beta-adrenergic receptor (betaAR) inotropic mechanism.


RESULTS
Adenoviral overexpression of Rad by 3-fold in rat cardiomyocytes suppressed LCC current (I(Ca)), [Ca(2+)](i) transients, and contractility by 60%, 42%, and 38%, respectively, whereas the "gain" function of EC coupling was significantly increased, due perhaps to reduced "redundancy" of LCC in triggering sarcoplasmic reticulum release. Conversely, approximately 70% Rad knockdown by RNA interference increased I(Ca) (50%), [Ca(2+)](i) transients (52%) and contractility (58%) without altering EC coupling efficiency; and the dominant negative mutant RadS105N exerted a similar effect on I(Ca). Rad upregulation caused depolarizing shift of LCC activation and hastened time-dependent LCC inactivation; Rad downregulation, however, failed to alter these attributes. The Na(+)/Ca(2+) exchange activity, sarcoplasmic reticulum Ca(2+) content, properties of Ca(2+) sparks and propensity for Ca(2+) waves all remained unperturbed regardless of Rad manipulation. Rad overexpression, but not knockdown, negated betaAR effects on I(Ca) and Ca(2+) transients.


CONCLUSIONS
These results establish Rad as a novel endogenous regulator of cardiac EC coupling and betaAR signaling and support a parsimonious model in which Rad buffers Ca(v)beta to modulate LCC activity, EC coupling, and betaAR responsiveness.
Wang
Gang
G
Institute of Molecular Medicine, College of Life Science, Peking University, Beijing, China.
Zhu
Xiaojun
X
Xie
Wenjun
W
Han
Peidong
P
Li
Kaitao
K
Sun
Zhongcui
Z
Wang
Yanru
Y
Chen
Chunlei
C
Song
Ruisheng
R
Cao
Chunmei
C
Zhang
Jifeng
J
Wu
Caihong
C
Liu
Jie
J
Cheng
Heping
H
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D018398:Homeodomain Proteins / Q000378:metabolism; D007545:Isoproterenol / Q000494:pharmacology; D008564:Membrane Potentials; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D018408:Patch-Clamp Techniques; D034622:RNA Interference; D051381:Rats; D017207:Rats, Sprague-Dawley; D011943:Receptors, Adrenergic, beta / Q000378:metabolism / Q000502:physiology*; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D015398:Signal Transduction; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism*
D000318:Adrenergic beta-Agonists; D018398:Homeodomain Proteins; D011943:Receptors, Adrenergic, beta; C490031:Rrad protein, rat; D019837:Ryanodine Receptor Calcium Release Channel; C107836:Tlx2 protein, mouse; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D018631:ras Proteins; D007545:Isoproterenol; D002118:Calcium