Abstract Body (Do not enter title and authors here): Phosphorylation of myofilament proteins is a critical regulatory mechanism for cardiac contractility and kinetics and is known to be dysregulated in heart failure (HF). Most studies focus on kinase dysregulation, however, and phosphatases are relatively less understood. Protein Tyrosine Phosphatase 1B (PTP1B) has recently been found to have upregulated activity under stress, and, indeed, we found that human HF samples have increased mRNA and protein expression of PTP1B. Recent work has focused on its role in cardiac metabolism since its known target is the insulin receptor, however, no studies have probed PTP1B’s role in direct regulation of myofilament proteins.

We performed phospho-proteomics on LV tissue from cardiomyocyte-specific PTP1B KO mice subjected to a high-fat diet (known to upregulate PTP1B). PTP1B KO hearts had upregulated phosphorylation on myofilament contractile proteins, including Troponin I, 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 either a small molecule inhibitor of PTP1B (DPM-1001) or vehicle. Single skinned myocytes were isolated from the LV from these hearts and assessed their function. We found that DPM-1001 rescued both length-dependent activation (LDA) and titin-based passive stiffness in HFpEF. To corroborate our findings in PTP1B KO mice, we found cardiac troponin I (cTnI) phosphorylation of S23/S24 was increased with pharmacological PTP1B inhibition. S23/S24 cTnI plays a critical role in Ca²⁺ sensitivity and may explain the rescue of LDA. Similarly, we also found PTP1B upregulation in HF may modulate glycogen synthase kinase 3β localization, driving it away from the z-disc and resulting in loss of LDA.

PTP1B is typically tethered to the cytosolic face of the ER, but in other cell types, calpain can remove the ER tether sequence, resulting in a 42 kDa PTP1B product with double the activity. This mechanism has not been studied in cardiomyocytes, but we hypothesized it might be occurring given PTP1B’s sarcomere targets. Indeed, we found mice subjected to ischemia-reperfusion injury had significantly upregulated cleaved PTP1B expression. This evidence suggests that PTP1B upregulation and cleavage under stress may lead to direct modulation of myofilament proteins in disease.