BPS2026 – Protein tyrosine phosphatase 1B as a novel regulator of myofilament function

Phosphorylation of myofilament proteins regulates cardiac contractility and kinetics and is known to be dysregulated in heart failure (HF). While it has been previously shown that the activity of protein tyrosine phosphatase 1B (PTP1B) is upregulated under stress, there are few studies on PTP1B in the heart, and there are no known substrates in the myofilament. We started by examining human left ventricular HF tissue and found increased mRNA and protein expression of PTP1B, compared to non-failing rejected donor hearts, suggesting a role for the phosphatase in cardiomyopathy. We performed phospho-proteomics on LV tissue from cardiomyocyte-specific PTP1B-KO mice subjected to high-fat diet (known to upregulate PTP1B). PTP1B-KO hearts had increased phosphorylation of myofilament proteins, including Troponin I (cTnI), Titin, and Myosin. To explore the functional significance of these targets, we treated a mouse model of heart failure with preserved ejection fraction (HFpEF, two-hit model) with a small molecule inhibitor of PTP1B (DPM-1001) or vehicle. We isolated single-skinned myocytes from the LV from these hearts and assessed their function. DPM-1001 rescued both length-dependent activation (LDA) and titin-based passive stiffness in HFpEF. Corroborating our findings in PTP1B KO mice, we found cTnI phosphorylation of S23/S24 was increased with pharmacological PTP1B inhibition, which may explain the rescue of LDA. PTP1B is typically tethered to the cytosolic face of the ER. It has been shown, although not in cardiomyocytes, that calpains can remove the ER tether sequence, resulting in a cytoplasmic 42 kDa PTP1B product with double the activity. We hypothesized this cleavage might be occurring in cardiomyocytes given PTP1B’s sarcomere targets. Indeed, mice subjected to ischemia-reperfusion injury had a ∼2.6-fold increase in PTP1B cleavage. These results indicate increased PTP1B activity leads to dysregulation of myofilament function, representing a potential therapeutic target.