PROJECT SUMMARY
Heart Failure with Preserved Ejection (HFpEF) is one of the largest unmet needs of all cardiovascular disease.
Although it now is the most common form of heart failure, to date, it has little to no specific effective therapy. An
obesity pandemic has now changed its phenotype, with obesity and metabolic syndrome now significant drivers
of the disease. We recently reported that an obese-HFpEF phenotype exhibits striking depression of right
ventricular myocyte tension generation at higher (contraction-related) levels of calcium. Critically, the mechanism
by which this occurs is unknown. Myocyte tension is regulated by both the thick filament, consisting of myosin,
and the thin filament, consisting of actin, tropomyosin, and cardiac troponins. In the thick filament, approximately
half of all myosin heads are in a conformation known as the super-relaxed (SRX) state, and the proportion of
myosin in this state is an important regulator of tension. The thin filament regulates tension by altering calcium
sensitivity, and one regulator is phosphorylation of cardiac troponin I (cTnI). In exciting new preliminary data, I
find that thick filament structure and phosphorylation of myofilament proteins are altered in obese-HFpEF. This
proposal derives from these data and aims to elucidate how obesity alters the thick and thin filament in human
HFpEF. In Aim 1, I will test the hypothesis that structural inactivation of the thick filament in obese-HFpEF results
from an excess of SRX myosin. To assess thick filament structure, I use small angle x-ray diffraction, a technique
that leverages the ordered structure of cardiac muscle to quantify distances between sarcomere proteins. This
technique is performed at the synchrotron at Argonne National Laboratory, one of few locations globally that can
perform the assay, and this proposal describes the first application of this technique to endomyocardial biopsies
from human HFpEF patients. While informative, X-ray diffraction on its own cannot prove the presence of excess
SRX myosin. For this, I will measure the myosin ATP turnover from single cardiomyocytes from HFpEF patients.
I will then explore whether obesity is a driver of excess SRX myosin by measuring both assays in HFpEF patients
with both obesity and hypertension/hypertrophy. In Aim 2, I explore the mechanism underlying how
hyperphosphorylation alters calcium activated tension. My preliminary data finds that the exposure to
enzymatically active protein phosphatase 2A (PP2A) partially reverses the deficit observed in calcium activated
tension in obese HFpEF, but the mechanism is unknown. I will test if this is from thick filament activation by
measuring x-ray diffraction patterns and myosin ATP turnover after PP2A exposure. I also test if this results from
thin filament hyperphosphorylation, specifically at cTnI, in HFpEF. We have identified a novel threonine 181
residue of cTnI to be hyperphosphorylated in HFpEF, but its function is unknown. Phospho-null/mimetic
transgenic cTnI Thr181 will be swapped into skinned myocytes from HFpEF patients, and myocyte tension
measured. These studies will advance our understanding of the thick and thin filament in HFpEF and could pave
the way for new therapies with small molecule sarcomere enhancers that target these mechanisms.