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Voltage-Driven Ca2+ Binding at the L‑Type Ca2+ Channel Triggers Cardiac Excitation–Contraction Coupling Prior to Ca2+ Influx
journal contribution
posted on 2012-12-04, 00:00 authored by Liron
S. Gez, Yamit Hagalili, Asher Shainberg, Daphne AtlasThe activation of the ryanodine Ca2+ release channels
(RyR2) by the entry of Ca2+ through the L-type Ca2+ channels (Cav1.2) is believed to be the primary mechanism of excitation–contraction
(EC) coupling in cardiac cells. This proposed mechanism of Ca2+-induced Ca2+ release (CICR) cannot fully account
for the lack of a termination signal for this positive feedback process.
Using Cav1.2 channel mutants, we demonstrate that the Ca2+-impermeable α11.2/L775P/T1066Y mutant introduced
through lentiviral infection into neonate cardiomyocytes triggers
Ca2+ transients in a manner independent of Ca2+ influx. In contrast, the α11.2/L775P/T1066Y/4A
mutant, in which the Ca2+-binding site of the channel was
destroyed, supports neither the spontaneous nor the electrically evoked
contractions. Ca2+ bound at the channel selectivity filter
appears to initiate a signal that is conveyed directly from the channel
pore to RyR2, triggering contraction of cardiomyocytes prior to Ca2+ influx. Thus, RyR2 is activated in response to a conformational
change in the L-type channel during membrane depolarization and not
through interaction with Ca2+ ions diffusing in the junctional
gap space. Accordingly, termination of the RyR2 activity is achieved
when the signal stops upon the return of the L-channel to the resting
state. We propose a new model in which the physical link between Cav1.2
and RyR2 allows propagation of a conformational change induced at
the open pore of the channel to directly activate RyR2. These results
highlight Cav1.2 as a signaling protein and provide a mechanism for
terminating the release of Ca2+ from RyR2 through protein–protein
interactions. In this model, the L-type channel is a master regulator
of both initiation and termination of EC coupling in neonate cardiomyocytes.