Studies over the past decade have demonstrated that SR Ca2+
release is abnormal in patients with chronic AF.2
Whereas the amplitude of the L-type Ca2+
current is generally decreased in AF, Ca2+
leak through RyR2 is typically elevated despite similar or decreased SR Ca2+
It has been proposed that triggered activity due to DADs is caused by an inward depolarizing INCX
current, which occurs in response to the removal of excess Ca2+
release from the cytosol.9,10
Here we provide direct experimental evidence for this mechanism in the FKBP12.6−/− mouse model of AF. Our data revealed that FKBP12.6−/− mice exhibit atrial focal activity and AF caused by SR Ca2+
leak, NCX activation and DADs generation. Because recent studies revealed that CaMKII phosphorylation of RyR2 at S2814 is elevated in patients with chronic AF,4,5
we investigated whether inhibition of S2814 phosphorylation of RyR2 affected susceptibility of FKBP12.6−/− to AF. Our results demonstrate that inhibition of S2814 but not S2808 phosphorylation suppressed pacing-induced AF in FKBP12.6−/− mice by preventing spontaneous SCaWs and related DADs. Therefore, our studies suggest that elevated CaMKII phosphorylation on RyR2 might be the primary phosphorylation event associated with triggered activity and AF induction,2
at least in the particular mutant mice examined in this study. This data are consistent with evidence showing that expression levels and activity of cytosolic CaMKII are upregulated in patients with chronic AF.4,5
Elevated CaMKII activity not only leads to increased RyR2 phosphorylation at S2814 but also causes increased PLN phosphorylation at T17, which might help preserve SR Ca2+
content in AF,11
by increasing SR Ca2+
uptake through SERCA2a disinhibition. We demonstrated that (partial) inhibition of S2814 phosphorylation on RyR2 is sufficient to suppress inducibility of AF following atrial-burst pacing in R176Q/+:S2814A/+ mice. Whereas this suggests that CaMKII phosphorylation of RyR2 contributes to SR Ca2+
leak associated with arrhythmogenesis, this did not exclude the possibility that CaMKII phosphorylation of PLN or other ion channels/transporters also contribute under some circumstances.
In conclusion, our data demonstrate that an increase in CaMKII phosphorylation of RyR2 at S2814 contributes to AF initiation in FKBP12.6−/− mice by amplifying SR Ca2+ leak and inducing DADs. Conversely, inhibition of CaMKII phosphorylation of RyR2 prevents AF initiation by decreasing aberrant SR Ca2+ release, NCX activation and DADs generation, whereas inhibition of S2808 phosphorylation of RyR2 failed to prevent AF induction. Together, our findings imply that CaMKII phosphorylation of S2814 on RyR2 might play an important role in enhancing RyR2-mediated SR Ca2+ leak that promotes DADs and atrial triggered activity associated with AF.
Novelty and Significance
What Is Known?
- Atrial fibrillation (AF) is the most prevalent sustained cardiac arrhythmia.
- Increased open probability of ryanodine receptors (RyR2) contributes to defective intracellular Ca2+ handling in AF.
- Ca2+/calmodulin-dependent kinase II (CaMKII) is upregulated in patients with chronic AF.
What New Information Does This Article Contribute?
- Genetic inhibition of CaMKII phosphorylation of RyR2 prevents induction of AF in FKBP12.6 deficient mice.
- CaMKII phosphorylation of RyR2 promotes spontaneous Ca2+ waves, activation of inward Na+/Ca2+ exchange current, and delayed after-depolarizations in atrial myocytes from FKBP12.6 deficient mice.
Previous studies demonstrated higher open probability of RyR2 in patients with chronic atrial fibrillation. Biochemical studies revealed increased phosphorylation levels of serine 2808 and serine 2814 on RyR2, as well as increased CaMKII activity. Here, we present evidence that inhibition of serine 2814 but not serine 2808 prevents induction of atrial fibrillation in the FKBP12.6-deficient mouse model of AF. Atrial myocytes from FKBP12.6-deficient mice exhibited spontaneous Ca2+ waves (SCaWs) leading to Na+/Ca2+-exchange current activation, and delayed afterdepolarizations (DADs). We therefore conclude that serine 2814 is an important downstream target of CaMKII in atrial fibrillation. We propose that defective sarcoplasmic reticulum calcium release via hyperphosphorylated RyR2 may cause triggered activity in atria, and contribute to the initiation of AF. Our findings suggest that inhibition of CaMKII phosphorylation of RyR2 could be a potential target for AF treatment.