Impaired relaxation in transgenic mice overexpressing junctin.
OBJECTIVE
Junctin is a major transmembrane protein in cardiac junctional sarcoplasmic reticulum, which forms a quaternary complex with the ryanodine receptor (Ca(2+) release channel), triadin, and calsequestrin.


METHODS
To better understand the role of junctin in excitation-contraction coupling in the heart, we generated transgenic mice with targeted overexpression of junctin to mouse heart, using the alpha-MHC promoter to drive protein expression.


RESULTS
The protein was overexpressed 10-fold in mouse ventricles and overexpression was accompanied by cardiac hypertrophy (19%). The levels of two other junctional SR-proteins, the ryanodine receptor and triadin, were reduced by 32% and 23%, respectively. However, [3H]ryanodine binding and the expression levels of calsequestrin, phospholamban and SERCA2a remained unchanged. Cardiomyocytes from junctin-overexpressing mice exhibited impaired relaxation: Ca(2+) transients decayed at a slower rate and cell relengthening was prolonged. Isolated electrically stimulated papillary muscles from junctin-overexpressing hearts exhibited prolonged mechanical relaxation, and echocardiographic parameters of relaxation were prolonged in the living transgenic mice. The amplitude of caffeine-induced Ca(2+) transients was lower in cardiomyocytes from junctin-overexpressing mice. The inactivation kinetics of L-type Ca(2+) channel were prolonged in junctin-overexpressing cardiomyocytes using Ca(2+) or Ba(2+) as charge carriers.


CONCLUSIONS
Our data provide evidence that cardiac-specific overexpression of junctin is accompanied by impaired myocardial relaxation with prolonged Ca(2+) transient kinetics on the cardiomyocyte level.
Kirchhefer
Uwe
U
Institut für Pharmakologie und Toxikologie, Westfälische Wilhelms-Universität, Domagkstrasse 12, 48149 Münster, Germany. kirchef@uni-muenster.de
Neumann
Joachim
J
Bers
Donald M
DM
Buchwalow
Igor B
IB
Fabritz
Larissa
L
Hanske
Gabriela
G
Justus
Isabel
I
Riemann
Burkhard
B
Schmitz
Wilhelm
W
Jones
Larry R
LR
D000818:Animals; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000378:metabolism; D002135:Calcium-Binding Proteins; D006332:Cardiomegaly / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D048429:Cell Size; D015150:Echocardiography, Doppler; D004558:Electric Stimulation; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D051379:Mice; D008822:Mice, Transgenic; D006899:Mixed Function Oxygenases; D009124:Muscle Proteins / Q000235:genetics* / Q000378:metabolism; D009126:Muscle Relaxation; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac / Q000378:metabolism*; D010210:Papillary Muscles; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism
D020746:Calcium Channels, L-Type; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; C498269:TRDN protein, human; C498270:Trdn protein, mouse; C070048:triadin; C498841:Asph protein, mouse; D006899:Mixed Function Oxygenases; D002118:Calcium