Abstract 4348658: Titin hyperphosphorylation of skeletal muscle from heart failure with preserved ejection fraction patients is correlated with their peakVO ₂

Background: Heart failure with preserved ejection fraction (HFpEF) goes along with a variety of molecular alterations. Amongst others, dysregulation of titin in different heart failure forms has been reported. Titin, a giant protein that is conserved over different species, expresses different tissue-dependent isoforms. In the myocardium 2 isoforms (N2B and N2BA) can be detected, while in the skeletal muscle (SKM) only N2A gets expressed. Modulation of myocardial titin e.g. by phosphorylation, is often described to account for cardiomyocyte and myocardial stiffness. While for HFpEF patients a hypophosphorylation of myocardial titin has been shown, driven by a decreased N2B- and an increased PEVK (conserved among all isoforms) phosphorylation, recent data from our group showed that SKM titin is hyperphosphorylated in a HFpEF rat model, partially driven by phosphorylation at the PEVK-region. The aim of the present study was to determine the phosphorylation status of SKM titin in HFpEF patients, compared to healthy controls and to analyze proteins, regulating muscle structure and function. Methods: SKM biopsies were obtained from patients with HFpEF (OptimEx trial, ClinicalTrials.gov Identifier: NCT02078947) and healthy controls (LEICA study, ClinicalTrials.gov Identifier: NCT00176319). A vertical agarose gel electrophoresis, of the homogenized biopsies, was performed, followed by a gel stain, to assess titin expression and phosphorylation. Protein expression (α-actinin and myostatin) was determined by western blot. Results: With respect to SKM titin, we detected not only disturbed titin expression (-9.8 %, p=0.03, but we could also confirm titin hyperphosphorylation (+15.5 %, p<0.0001) in HFpEF-patients compared to healthy controls. Additionally, a negative correlation between titin phosphorylation and peakVO 2 was evident (r=0.506, p=0.003). Moreover, HFpEF patients revealed a decreased α-actinin (important for sarcomere stability) and increased myostatin (regulator of muscle mass) expression. Conclusion: While in HFpEF the myocardium reveals a titin hypophosphorylation, a hyperphosphorylation of titin in the skeletal muscle is evident, which may be associated with decreased exercise capacity. Furthermore, signs for impaired sarcomere structure and atrophy were evident in the samples. Further trials are necessary to evaluate the direct effect of titin phosphorylation on muscle function and its role in HFpEF, to estimate its potential as a therapeutic target.