Summary Although studies have shown that continuous positive airway pressure (CPAP) therapy to treat obstructive sleep apnea improves left ventricular diastolic function, modifiers of improvement are unknown. We explored race and pre‐treatment 24‐h non‐dipping blood pressure status as modifiers of improved diastolic function. Participants (N = 220) with obstructive sleep apnea (apnea–hypopnea index ≥15 events/h) and hypertension were recruited to a cohort study that examined effects of 3‐month CPAP therapy on 24‐h blood pressure. Those who completed echocardiogram at baseline and follow‐up were included in this analysis. Diastolic function parameters (E, A, e′, E/A, E/e′) were assessed. Race was categorised to African American versus others. Participants were categorised as nocturnal dippers (night‐time blood pressure decrease by ≥10%) versus non‐dippers. We compared changes in parameters of diastolic function by race and nocturnal dipping status. A total of 92 participants were included. They were men (86%), African American (67.4%), and current smoker (29.5%). Mean apnea–hypopnea index was 32.9 events/h. Mean CPAP usage was 3.15 h/day. After 3 months of CPAP treatment, there were significant improvements in measures of diastolic function: a median (interquartile range [IQR]) increase in E velocity by 4.00 (−5.75 to 13.75) cm/s, an increase in e′ by 2.00 (0–4.00) cm/s, and a decrease in the E/e′ ratio by 1.74 (−4.27 to 0.00) at follow‐up (p < 0.05). These changes did not differ by race or nocturnal dipping status. Improvements in diastolic function after CPAP therapy did not differ by race or nocturnal dipping status. Further studies are needed to understand predictors of CPAP effects on diastolic function.

Introduction: Atrial fibrillation (AF) is the most common sustained arrhythmia, with an estimated prevalence in the U.S. of 6.1 million . AF increases the risk of a thromboembolic stroke in five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function in AF remains unknown. We have recently identified protein phosphatase 1 subunit 12c (PPP1R12C) as a key molecule targeting myosin light chain phosphorylation in AF. Objective We hypothesize that the overexpression of PPP1R12C causes hypophosphorylation of atrial myosin light chain 2 (MLC2a), thereby decreasing atrial contractility in AF. Methods and Results Left and right atrial appendage tissues were isolated from AF patients versus sinus rhythm (SR). To evaluate the role of the PP1c-PPP1R12C interaction in MLC2a de-phosphorylation, we utilized Western blots, co-immunoprecipitation, and phosphorylation assays. In patients with AF, PPP1R12C expression was increased 3.5-fold versus SR controls with an 88% reduction in MLC2a phosphorylation. PPP1R12CPP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF. In vitro studies of either pharmacologic (BDP5290) or genetic (T560A) PPP1R12C activation demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Additionally, to evaluate the role of PPP1R12C expression in cardiac function, mice with lentiviral cardiac-specific overexpression of PPP1R12C (Lenti-12C) were evaluated for atrial contractility using echocardiography, versus wild-type and Lenti-controls. Lenti-12C mice demonstrated a 150% increase in left atrium size versus controls, with reduced atrial strain and atrial ejection fraction. Also, programmed electrical stimulation was performed to evaluate AF inducibility in vivo. Pacing-induced AF in Lenti-12C mice was significantly higher than controls. Conclusion The overexpression of PPP1R12C increases PP1c targeting to MLC2a and provokes dephosphorylation, associated with a reduction in atrial contractility and increase in AF inducibility. All these discoveries suggest that PP1 regulation of sarcomere function at MLC2a is a main regulator of atrial contractility in AF.

Background: Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, increases thromboembolic stroke risk five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function remain unknown. We tested the hypothesis that increased expression of PPP1R12C, the PP1 regulatory subunit targeting atrial myosin light chain 2 (MLC2a), causes hypophosphorylation of MLC2a and results in atrial hypocontractility. Methods: Right atrial appendage tissues were isolated from human AF patients versus sinus rhythm controls. Western blots, co-immunoprecipitation, and phosphorylation studies were performed to examine how the PP1c-PPP1R12C interaction causes MLC2a de-phosphorylation. In vitro studies of pharmacologic MRCK inhibitor (BDP5290) in atrial HL-1 cells were performed to evaluate PP1 holoenzyme activity on MLC2a. Cardiac-specific lentiviral PPP1R12C overexpression was performed in mice to evaluate atrial remodeling with atrial cell shortening assays, echocardiography, and AF inducibility with EP studies. Results: In human patients with AF, PPP1R12C expression was increased two-fold versus SR controls (P=2.0x10-2, n=12,12 in each group) with >40% reduction in MLC2a phosphorylation (P=1.4x10-6, n=12,12 in each group). PPP1R12C-PP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF (P=2.9x10-2 and 6.7x10-3 respectively, n=8,8 in each group). In vitro studies utilizing drug BDP5290, which inhibits T560-PPP1R12C phosphorylation, demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Lenti-12C mice demonstrated a 150% increase in LA size versus controls (P=5.0x10-6, n=12,8,12), with reduced atrial strain and atrial ejection fraction. Pacing-induced AF in Lenti-12C mice was significantly higher than controls (P=1.8x10-2 and 4.1x10-2 respectively, n= 6,6,5). Conclusions: AF patients exhibit increased levels of PPP1R12C protein compared to controls. PPP1R12C overexpression in mice increases PP1c targeting to MLC2a and causes MLC2a dephosphorylation, which reduces atrial contractility and increases AF inducibility. These findings suggest that PP1 regulation of sarcomere function at MLC2a is a key determinant of atrial contractility in AF.

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Introduction: Atrial fibrillation (AF), is the most common sustained arrhythmia, with an estimated prevalence in the U.S. of 2.7 million to 6.1 million and is predictive to increase to 12.1 million in 2030. AF increases the chances of a thromboembolic stroke in five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function in AF remains unknown. Objective: The overexpression of PPP1R12C, causes hypophosphorylation of atrial myosin light chain 2 (MLC2a), decreasing atrial contractility. Methods and Results: Left and right atrial appendage tissues were isolated from AF patients versus sinus rhythm (SR). To evaluated the role of PP1c-PPP1R12C interaction in MLC2a de-phosphorylation we used Western blots, coimmunoprecipitation, and phosphorylation assays. In patients with AF, PPP1R12C expression was increased 3.5-fold versus SR controls with an 88% reduction in MLC2a phosphorylation. PPP1R12C-PP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF. In vitro studies of either pharmacologic (BDP5290) or genetic (T560A) PPP1R12C activation demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Additionally, to evaluate the role of PPP1R12C expression in cardiac function, mice with lentiviral cardiac-specific overexpression of PPP1R12C (Lenti-12C) were evaluated for atrial contractility using echocardiography, versus wild-type and Lenti-controls. Lenti-12C mice demonstrated a 150% increase in left atrium size versus controls, with reduced atrial strain and atrial ejection fraction. Also, programmed electrical stimulation was performed to evaluate AF inducibility in vivo. Pacing-induced AF in Lenti-12C mice was significantly higher than controls. Conclusion: The Overexpression of PPP1R12C increases PP1c targeting to MLC2a and provokes dephosphorylation, that cause a reduction in atrial contractility and increases AF inducibility. All these discoveries advocate that PP1 regulation of sarcomere function at MLC2a is a main regulator of atrial contractility in AF.