Activin type II receptor (ActRII) ligands have been implicated in muscle wasting in aging and disease. However, the role of these ligands and ActRII signaling in the heart remains unclear. Here, we investigated this catabolic pathway in human aging and heart failure (HF) using circulating follistatin-like 3 (FSTL3) as a potential indicator of systemic ActRII activity. FSTL3 is a downstream regulator of ActRII signaling, whose expression is up-regulated by the major ActRII ligands, activin A, circulating growth differentiation factor-8 (GDF8), and GDF11. In humans, we found that circulating FSTL3 increased with aging, frailty, and HF severity, correlating with an increase in circulating activins. In mice, increasing circulating activin A increased cardiac ActRII signaling and FSTL3 expression, as well as impaired cardiac function. Conversely, ActRII blockade with either clinical-stage inhibitors or genetic ablation reduced cardiac ActRII signaling while restoring or preserving cardiac function in multiple models of HF induced by aging, sarcomere mutation, or pressure overload. Using unbiased RNA sequencing, we show that activin A, GDF8, and GDF11 all induce a similar pathologic profile associated with up-regulation of the proteasome pathway in mammalian cardiomyocytes. The E3 ubiquitin ligase, Smurf1, was identified as a key downstream effector of activin-mediated ActRII signaling, which increased proteasome-dependent degradation of sarcoplasmic reticulum Ca2+ ATPase (SERCA2a), a critical determinant of cardiomyocyte function. Together, our findings suggest that increased activin/ActRII signaling links aging and HF pathobiology and that targeted inhibition of this catabolic pathway holds promise as a therapeutic strategy for multiple forms of HF.

Heart failure with preserved ejection fraction (HFpEF) is the most common type of HF in older adults. Although no pharmacological therapy has yet improved survival in HFpEF, exercise training (ExT) has emerged as the most effective intervention to improving functional outcomes in this age‐related disease. The molecular mechanisms by which ExT induces its beneficial effects in HFpEF, however, remain largely unknown. Given the strong association between aging and HFpEF, we hypothesized that ExT might reverse cardiac aging phenotypes that contribute to HFpEF pathophysiology and additionally provide a platform for novel mechanistic and therapeutic discovery. Here, we show that aged (24–30 months) C57BL/6 male mice recapitulate many of the hallmark features of HFpEF, including preserved left ventricular ejection fraction, subclinical systolic dysfunction, diastolic dysfunction, impaired cardiac reserves, exercise intolerance, and pathologic cardiac hypertrophy. Similar to older humans, ExT in old mice improved exercise capacity, diastolic function, and contractile reserves, while reducing pulmonary congestion. Interestingly, RNAseq of explanted hearts showed that ExT did not significantly modulate biological pathways targeted by conventional HF medications. However, it reversed multiple age‐related pathways, including the global downregulation of cell cycle pathways seen in aged hearts, which was associated with increased capillary density, but no effects on cardiac mass or fibrosis. Taken together, these data demonstrate that the aged C57BL/6 male mouse is a valuable model for studying the role of aging biology in HFpEF pathophysiology, and provide a molecular framework for how ExT potentially reverses cardiac aging phenotypes in HFpEF.

Introduction: Peripartum cardiomyopathy (PPCM) is a rare form of pregnancy-related heart failure associated with preeclampsia. Our understanding of their shared pathophysiology is limited as are treatment options. Hypothesis: We hypothesized that plasma proteomic profiling would identify deleterious circulating proteins and provide insights into shared mechanisms driving preeclampsia and PPCM. Methods: We performed serum proteomic case-control studies in women with preeclampsia or PPCM to identify biological processes common to both conditions. The top candidate was validated in independent preeclampsia (n=58) and PPCM (n=114) cohorts. Preeclamptic placenta was examined as a potential source of circulating proteins. Therapeutic targeting the top candidate was examined in an animal model of PPCM. Results: Paradoxically, we found the senescence-associated secretory phenotype (SASP), a marker of biological aging, to be the most upregulated process in relatively young women with preeclampsia (normalized enrichment score [NES]=3.3;p adj =4.0x10 -6 ) or PPCM (NES=3.2;p adj =1.9x10 -5 ). 28 circulating proteins contributed to this common signal, and were strongly enriched in preeclamptic placenta, which expressed markers of increased senescence. The TGFb family member, Activin-A, was the most highly upregulated SASP gene in preeclamptic placentas (5-fold;p<0.001). Circulating Activin-A was elevated in preeclampsia (2.9-fold) and PPCM (2.5-fold) validation cohorts (p<0.001) and correlated with impaired left ventricular global longitudinal strain (r=0.3-0.5;p<0.001). In PPCM, Activin-A increased with worsening heart failure, reflected by NYHA class and BNP levels (p<0.05), and cardiac FSTL3 expression, an indicator of Activin-A signaling, was increased (2.2-fold;p=0.005). Inhibition of Activin-A receptors with a monoclonal antibody improved systolic function and adverse cardiac remodeling in a murine PPCM model. Conclusions: These data implicate placental senescence in the increased deleterious circulating proteins seen in preeclampsia and PPCM. We identify multiple new circulating factors associated with these diseases and demonstrate that targeting SASP proteins, such as Activin-A, can mitigate PPCM in mice.

Sarco/endoplasmic reticulum Ca 2+ -ATPase 2a (SERCA2a) is a critical regulator of cardiac function whose expression is decreased in aging and heart failure (HF). Activin-A, a member of the TGFβ superfamily, has been shown to be a potent paracrine inhibitor of SERCA2a expression in cardiomyocytes (CM). However, the mechanism by which this occurs remains unclear. Here, we confirm that Activin-A decreases SERCA2a protein expression in isolated CMs (reductions in SERCA2a expression at 25ng/ml: 35±5%, p=0.02; 50ng/ml: 55±14%, p= 0.003; 100ng/ml: 56±8%, p.= 0.001; n=2-3/group, three replicates). Moreover, we show that increasing circulating Activin-A levels in vivo also decreases cardiac SERCA2a protein (reductions in SERCA2a expression at 1-25ng/ml: 22±12%, p=0.27; 25-50ng/ml: 63±14%, p=0.0015; >50ng/ml: 79±2%, p= 0.0002; n=4 per group). Using systematic gene set enrichment analysis of RNAseq data from hearts and CMs exposed to increased Activin-A, we identify the ubiquitin-proteasome system (UPS) as one of the most highly up-regulated pathways induced by Activin-A. With ubiquitination prediction modeling revealing multiple ubiquitin binding sites on SERCA2a, we hypothesized that Activin-A regulates SERCA2a expression through modulation of its UPS-mediated degradation. Indeed, inhibiting the proteasome with MG132 attenuated the inhibitory effects of Activin-A on CM SERCA2a expression. Although Activin-A did not directly affect proteasome activity in CMs, we found that it increased ubiquitination of SERCA2a, resulting in increased proteasome-mediated degradation. Additionally, targeted inhibition of the Activin type II receptors (ActRII) with a monoclonal ActRII blocking antibody reversed the effects of Activin-A on SERCA2a protein expression in isolated CMs in a dose-dependent fashion. Importantly, these effects of Activin-A/ActRII blockade on SERCA2a expression were recapitulated in vivo in a transverse aortic constriction model of HF, and further led to improvements in cardiac function. Collectively, these data identify a novel UPS-mediated mechanism by which Activin-A regulates CM SERCA2a expression, which warrants further investigation as a potential therapeutic approach to augmenting SERCA2a in HF.

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Aptamers—artificially generated three dimensional strands of RNA—might revolutionise therapeutics, says Ryan Hobson

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