ABSTRACT: Background: Embryonic trancription factor TBX18 re-expression has been demonstrated to be capable of converting ventricular myocytes to pacemakers cells. The goals of this study are to investigate the effects of TBX18 on myocytes and non-myocytes at day 3, day 6 and day 14, as well as tracking the key node of reprogramming process and mechanism. Methods: Neonatal rat ventricular myocytes (NRVC) were treated by adeno-TBX18 or GFP for 3 days to investigate the early effects of TBX18 on myocytes and non-myocytes. Except for these 2 treatments, Adeno-TBX18 with one specific Tgfβ inhibitor, A83-01 (0.2uM) was treated for NRVC for 14 days for explore the effects of Tgfβ inhibition on fibroblast activation and myocyte reprogramming process. Attached NRVC monolayers were digested with 0.25% trypsin (Cat# LS003707, Corning, NY) for 5 min at 37 °C, and filtered through a 100-μm cell strainer (Fischer Scientific, Waltham, MA) to generate a single-cell suspension. Single cells were resuspended to a concentration of 700-2,000 cells per microliter. Gel beads-in-emulsion (GEMs) were generated with Chromium X (10X Genomics, Pleasanton, CA) and RNA libraries were prepared using the Chromium Next GEM Single Cell 3' GEM kit, v3.1 (10X Genomics Inc., Pleasanton, CA) with a targeted cell recovery of 4,000-10,000 cells per reaction. Quality control of cDNA and scRNA-seq libraries were performed using the Bioanalyzer High Sensitivity kit (Agilent Technologies Inc., CA). Libraries were sequenced using Novaseq 6000 (Illumina Inc., CA) with a minimum depth of 50,000–65,000 read pairs per cell. Result: Upon QA/QC filtering, 13,437 cells met the quality thresholds for subsequent analysis. We focused on three cell types: cardiomyocytes (CMs, Tnni3-positive), total FBs (Pdgfrb and Tcf21-positive) with 2 subpopulations, including myoFBs (Pdgfrb, Tcf21, Acta2-high) and quiescent FBs (Pdgfrb, Tcf21, Acta2-low). Pathway analysis further revealed that upregulated DEGs by TBX18 are involved in fibroblast activation and ECM remodeling. Among those, Tgfβ signaling, which is one of the major molecular mediators of fibrosis in the heart, figured prominently in TBX18-transduced CMs and FBs. This was evidenced by upregulated expression of Tgfβ ligands and profibrotic genes such as Tgfb1, Tgfb2, and Acta2 in TBX18-NRVCs. Accordingly, higher proportion of activated myofibroblasts (39.3% vs. 25.4%) and lower proportion of quiescent FBs were observed in TBX18-NRVCs compared to control (Fig. 2F). To test the causative role of Tgfβ signaling in triggering fibrosis upon TBX18 re-expression, monolayers of NRVCs were treated with a small molecule inhibitor of type I Tgfβ receptors, A83-01. Treatment with A83-01 significantly decreased the population of myoFBs from 68.0% in TBX18-NRVC to 49.5% of total cell population at d14. Heat map analysis of all genes indicated that TBX18-CMs show upregulation of fibroblast-related genes, which is mitigated with A83-01 treatment. The upregulated DEGs in TBX18-FBs pointed to ECM remodeling as well as inflammatory response involving Tgfβ and other cytokines. Again, treatment with A83-01 abrogated the rise of these pathways. Profibrotic genes such as Tgfb2, Tgfb3, Acta2, and inflammatory genes were upregulated in FBs transduced with Ad-TBX18, which was mitigated by A83-01 treatment. The upregulated gene clusters in TBX18-FBs were also upregulated in TBX18-CMs, particularly, interferon signaling, interleukin-4 regulation of apoptosis, which may have led to the loss of TBX18-CMs over time. The inflammatory and profibrotic pathways were suppressed by treatment with A83-01 and may explain the higher population of TBX18-CMs with A83-01 at d14.We also quantified the number of de novo iPMs based on: Tnni3high, Hcn4+, Nkx2-5low, Gja1low through our scRNA-seq analysis and demonstrated the proportions of Hcn4+ iPMs was similar in TBX18-CMs regardless of Tgfβ inhibition but was higher compared to control. Conclusion: Tgfβ signaling inhibition with a83-01 prevents against myofibroblast activation, cardiomyocytes apoptosis and inflammatory response induced by TBX18, and short term treatment of A8301 does not strongly interfere the reprogramming process and efficiency.
