Physiological stress improves stem cell modeling of dystrophic cardiomyopathy

Heart failure contributes to Duchenne muscular dystrophy (DMD), which arises from mutations that ablate dystrophin, rendering the plasma membrane prone to disruption. Cardiomyocyte membrane breakdown in patients with DMD yields a serum injury profile similar to other types of myocardial injury with the release of creatine kinase and troponin isoforms. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are highly useful but can be improved. We generated hiPSC-CMs from a patient with DMD and subjected these cells to equibiaxial mechanical strain to mimic in vivo stress. Compared to healthy cells, DMD hiPSC-CMs demonstrated greater susceptibility to equibiaxial strain after 2 h at 10% strain. We generated an aptamer-based profile of proteins released from hiPSC-CMs both at rest and subjected to strain and identified a strong correlation in the mechanical stress-induced proteome from hiPSC-CMs and serum from patients with DMD. We exposed hiPSC-CMs to recombinant annexin A6, a protein resealing agent, and found reduced biomarker release in DMD and control hiPSC-CMs subjected to strain. Thus, the application of mechanical strain to hiPSC-CMs produces a model that reflects an in vivo injury profile, providing a platform to assess pharmacologic intervention.

Transthyretin deposition alters cardiomyocyte sarcomeric architecture, calcium transients, and contractile force

Age-related wild-type transthyretin amyloidosis (wtATTR) is characterized by systemic deposition of amyloidogenic fibrils of misfolded transthyretin (TTR) in the connective tissue of many organs. In the heart, this leads to age-related heart failure with preserved ejection fraction (HFpEF). The hypothesis tested is that TTR deposited in vitro disrupts cardiac myocyte cell-to-cell and cell-to-matrix adhesion complexes, resulting in altered calcium handling, force generation, and sarcomeric disorganization. Human iPSC-derived cardiomyocytes and neonatal rat ventricular myocytes (NRVMs), when grown on TTR-coated polymeric substrata mimicking the stiffness of the healthy human myocardium (10 kPa), had decreased contraction and relaxation velocities as well as decreased force production measured using traction force microscopy. Both NRVMs and adult mouse atrial cardiomyocytes had altered calcium kinetics with prolonged transients when cultured on TTR fibril-coated substrates. Furthermore, NRVMs grown on stiff (~GPa), flat or microgrooved substrates coated with TTR fibrils exhibited significantly decreased intercellular electrical coupling as shown by FRAP dynamics of cells loaded with the gap junction-permeable dye calcein-AM, along with decreased gap junction content as determined by quantitative connexin 43 staining. Significant sarcomeric disorganization and loss of sarcomere content, with increased ubiquitin localization to the sarcomere, were seen in NRVMs on various TTR fibril-coated substrata. TTR presence decreased intercellular mechanical junctions as evidenced by quantitative immunofluorescence staining of N-cadherin and vinculin. Current therapies for wtATTR are cost-prohibitive and only slow the disease progression; therefore, better understanding of cardiomyocyte maladaptation induced by TTR amyloid may identify novel therapeutic targets.

Transthyretin amyloid fibrils alter primary fibroblast structure, function, and inflammatory gene expression

Age-related wild-type transthyretin amyloidosis (wtATTR) is characterized by systemic deposition of amyloidogenic fibrils of misfolded transthyretin (TTR) in the connective tissue of many organs. In the heart, this leads to cardiac dysfunction, which is a significant cause of age-related heart failure. The hypothesis tested is that TTR affects cardiac fibroblasts in ways that may contribute to fibrosis. When primary cardiac fibroblasts were cultured on TTR-deposited substrates, the F-actin cytoskeleton was disorganized, focal adhesion formation was decreased, and nuclear shape was flattened. Fibroblasts had faster collective and single-cell migration velocities on TTR-deposited substrates. In addition, fibroblasts cultured on microposts with TTR deposition had reduced attachment and increased proliferation above untreated. Transcriptomic and proteomic analyses of fibroblasts grown on glass covered with TTR showed significant upregulation of inflammatory genes after 48 h, indicative of progression in TTR-based diseases. Together, results suggest that TTR deposited in tissue extracellular matrix may affect the structure, function, and gene expression of cardiac fibroblasts. As therapies for wtATTR are cost-prohibitive and only slow disease progression, better understanding of cellular maladaptation may elucidate novel therapeutic targets.NEW & NOTEWORTHY Transthyretin (TTR) cardiac amyloidosis involves deposition of fibrils of misfolded TTR in the aging human heart, leading to cardiac dysfunction and heart failure. Our novel in vitro studies show that TTR fibrils alter primary cardiac fibroblast cytoskeletal and nuclear structure and focal adhesion formation. Furthermore, both fibrillar and tetrameric TTR significantly increased cellular migration velocity and caused upregulation of inflammatory genes determined by transcriptomic RNA and protein analysis. These findings may suggest new therapeutic approaches.