Protein phosphatase 2A affects myofilament contractility in non-failing but not in failing human myocardium

Abstract

Protein phosphatase (PP) type 2A is a multifunctional serine/threonine phosphatase that is involved in cardiac excitation–contraction coupling. The PP2A core enzyme is a dimer, consisting of a catalytic C and a scaffolding A subunit, which is targeted to several cardiac proteins by a regulatory B subunit. At present, it is controversial whether PP2A and its subunits play a critical role in end-stage human heart failure. Here we report that the application of purified PP2AC significantly increased the Ca2+-sensitivity (ΔpCa50 = 0.05 ± 0.01) of the contractile apparatus in isolated skinned myocytes of non-failing (NF) hearts. A higher phosphorylation of troponin I (cTnI) was found at protein kinase A sites (Ser23/24) in NF compared to failing myocardium. The basal Ca2+-responsiveness of myofilaments was enhanced in myocytes of ischemic (ICM, ΔpCa50 = 0.10 ± 0.03) and dilated (DCM, ΔpCa50 = 0.06 ± 0.04) cardiomyopathy compared to NF. However, in contrast to NF myocytes the treatment with PP2AC did not shift force-pCa relationships in failing myocytes. The higher basal Ca2+-sensitivity in failing myocytes coincided with a reduced protein expression of PP2AC in left ventricular tissue from patients suffering from ICM and DCM (by 50 and 56% compared to NF, respectively). However, PP2A activity was unchanged in failing hearts despite an increase of both total PP and PP1 activity. The expression of PP2AB56α was also decreased by 51 and 62% in ICM and DCM compared to NF, respectively. The phosphorylation of cTnI at Ser23/24 was reduced by 66 and 49% in ICM and DCM compared to NF hearts, respectively. Our results demonstrate that PP2A increases myofilament Ca2+-sensitivity in NF human hearts, most likely via cTnI dephosphorylation. This effect is not present in failing hearts, probably due to the lower baseline cTnI phosphorylation in failing compared to non-failing hearts.

Transmural Heterogeneity of Cellular Level Power Output Is Reduced in Human Heart Failure

“…For example, separate studies using epicardial biopsies [10] and samples from unspecified regions of the heart [11] have shown that isometric force is reduced by ~80% in patients with heart failure. Other works, using samples from unspecified regions of the left ventricle, suggests that there is no significant effect of heart failure on maximum isometric force [12][13][14]. In the present experiments, heart failure reduced the average force generated by the experimental preparations (Figure 2, p=0.042).…”

“…Other groups have shown that cTnI phosphorylation can also influence Ca 2+ sensitivity [12,23]. In this study, site-specific cTnI phosphorylation was reduced in heart failure samples at Ser23/24 ( Figure S5, p=0.012) but there was not a significant relationship between this parameter and pCa 50 (r=0.034, p=0.850, data not shown).…”

Cardiac Function Is Regulated by B56α-mediated Targeting of Protein Phosphatase 2A (PP2A) to Contractile Relevant Substrates

“…PP2A induced uniform dephosphorylation of cTnI in preparations previously treated with both PKA and PKC (38). We have recently demonstrated that the administration of purified PP2A C dephosphorylated cTnI at PKA sites Ser 23/24 , resulting in an increased Ca 2ϩ sensitivity of the contractile apparatus in isolated skinned human cardiomyocytes (39). A decreased cTnI phosphorylation at PKA-dependent phosphosites was also found in a dog heart failure model (40).…”

Molecular Mechanisms Underlying Cardiac Protein Phosphatase 2A Regulation in Heart

“…In myocytes, PP2A activity is linked with multiple targets important in membrane excitability and excitation-contraction coupling including the ryanodine receptor (RyR2), connexin43, Ca v 1.2, troponin, Na ϩ /Ca 2ϩ exchanger (NCX), and phospholamban (16 -22). Although manipulation of PP2A activity or expression in animal or cell models produces defects in myocyte physiology and cardiac phenotypes (19,20,(23)(24)(25)(26), the role and regulation of this critical enzyme family in cardiovascular disease is still largely elusive. In fact, we lack even a basic fundamental understanding of the expression, activity, and regulation of protein phosphatases in heart and/or in cardiovascular disease.…”