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Previously, we reported that heterologous expression of an embryonic transcription factor, Tbx18, reprograms ventricular cardiomyocytes into induced pacemaker cells (Tbx18-iPMs), though the key pathways are unknown. Here, we have used a tandem mass tag proteomic approach to characterize the impact of Tbx18 on neonatal rat ventricular myocytes. Tbx18 expression triggered vast proteome remodeling. Tbx18-iPMs exhibited increased expression of known pacemaker ion channels, including Hcn4 and Cx45 as well as upregulation of the mechanosensitive ion channels Piezo1, Trpp2 (PKD2), and TrpM7. Metabolic pathways were broadly downregulated, as were ion channels associated with ventricular excitation-contraction coupling. Tbx18-iPMs also exhibited extensive intracellular cytoskeletal and extracellular matrix remodeling, including 96 differentially expressed proteins associated with the epithelial-to-mesenchymal transition (EMT). RNAseq extended coverage of low abundance transcription factors, revealing upregulation of EMT-inducing Snai1, Snai2, Twist1, Twist2, and Zeb2. Finally, network diffusion mapping ofgt;200 transcriptional regulators indicates EMT and heart development factors occupy adjacent network neighborhoods downstream of Tbx18 but upstream of metabolic control factors. In conclusion, transdifferentiation of cardiac myocytes into pacemaker cells entails massive electrogenic, metabolic, and cytostructural remodeling. Structural changes exhibit hallmarks of the EMT. The results aid ongoing efforts to maximize the yield and phenotypic stability of engineered biological pacemakers.

Implantable cardiac pacemaker devices are generally effective for patients with symptomatic bradyarrhythmia. However, device-dependent cardiac pacing is far from ideal and often inadequate, particularly for pediatric patients who need to go through invasive revision of the indwelling hardware. Biological pacemakers have been proposed as device-free alternatives to the current treatment, but sustained, unwavering biological pacing beyond days after the biologic delivery has not been demonstrated. We have previously demonstrated that re-expression of an embryonic transcription factor, TBX18, could reprogram ventricular cardiomyocytes into induced pacemaker myocytes (iPMs). Here, we report that exogenous expression of TBX18 per se leads to severe fibrosis in situ, impairing the iPMs’ ability to pace together. Acute fibrosis is accompanied with proliferation and activation of cardiac fibroblasts via Tgfβ-Smad2/3 pathway. Small molecule inhibition of Tgfβ signaling mitigated the interstitial remodeling, independent from TBX18-induced iPM reprogramming at the single-cell level. Direct and focal gene transfer of TBX18 into the left ventricular myocardium created ventricular pacing in a rat model of chronic atrioventricular block, but such activity began to wane in a week. In contrast, a combination therapy consisting of TBX18 gene transfer and Tgfβ inhibition enabled sustained biological pacing beyond the four-week study period. Our data demonstrate that inhibition of Tgfβ signaling suffices to achieve durable cardiac pacing by TBX18-induced biological pacemakers.

Cardiac muscle cells (CMCs) are the unit cells that comprise the heart. CMCs go through different stages of differentiation and maturation pathways to fully mature into beating cells. These cells can sense and respond to mechanical cues through receptors such as integrins which influence maturation pathways. For example, cell traction forces are important for the differentiation and development of functional CMCs, as CMCs cultured on varying substrate stiffness function differently. Most work in this area has focused on understanding the role of bulk extracellular matrix stiffness in mediating the functional fate of CMCs. Given that stiffness sensing mechanisms are mediated by individual integrin receptors, an important question in this area pertains to the specific magnitude of integrin piconewton (pN) forces that can trigger CMC functional maturation. To address this knowledge gap, we used DNA adhesion tethers that rupture at specific thresholds of force (∼12, ∼56, and ∼160 pN) to test whether capping peak integrin tension to specific magnitudes affects CMC function. We show that adhesion tethers with greater force tolerance lead to functionally mature CMCs as determined by morphology, twitching frequency, transient calcium flux measurements, and protein expression (F-actin, vinculin, α-actinin, YAP, and SERCA2a). Additionally, sarcomeric actinin alignment and multinucleation were significantly enhanced as the mechanical tolerance of integrin tethers was increased. Taken together, the results show that CMCs harness defined pN integrin forces to influence early stage development. This study represents an important step toward biophysical characterization of the contribution of pN forces in early stage cardiac differentiation.

Remarkable progress has been made in the direct differentiation of human pluripotent stem cells (hPSCs) into the major sources of cell types found in the heart. The understanding of cardiovascular development in vivo can provide the guidelines and help us to facilitate the process of generation of cells. In this chapter, we review the state of methods in generating cardiovascular cells from hPSCs according to our understanding of heart development. We also stress the important role of developmental biology during the differentiation from hPSCs. We then highlight the application in disease modeling, drug screening, and cell therapy based on cell transplantation. With the advances of their application, the promise of hPSCs-related products will provide new treatments for degenerative diseases of the heart in the future.