Project description:Direct conversion of fibroblasts to induced cardiomyocytes (iCMs) has great potential for regenerative medicine. Recent publications have reported significant progress, but the evaluation of reprogramming has relied upon non-functional measures such as flow cytometry for cardiomyocyte markers or GFP expression driven by a cardiomyocyte-specific promoter. The issue is one of practicality: the most stringent measures - electrophysiology to detect cell excitation and the presence of spontaneously contracting myocytes - are not readily quantifiable in the large numbers of cells screened in reprogramming experiments. However, excitation and contraction are linked by a third functional characteristic of cardiomyocytes: the rhythmic oscillation of intracellular calcium levels. We set out to optimize direct conversion of fibroblasts to iCMs with a quantifiable calcium reporter to rapidly assess functional transdifferentiation. We constructed a reporter system in which the calcium indicator GCaMP is driven by the cardiomyocyte-specific Troponin T promoter. Using calcium activity as our primary outcome measure, we compared several published combinations of transcription factors along with novel combinations in mouse embryonic fibroblasts. The most effective combination consisted of Hand2, Nkx2.5, Gata4, Mef2c, and Tbx5 (HNGMT). This combination is >50-fold more efficient than GMT alone and produces iCMs with cardiomyocyte marker expression, robust calcium oscillation, and spontaneous beating that persists for weeks following inactivation of reprogramming factors. HNGMT is also significantly more effective than previously published factor combinations for the transdifferentiation of adult mouse cardiac fibroblasts to iCMs. Quantification of calcium function is a convenient and effective means for the identification and evaluation of cardiomyocytes generated by direct reprogramming. Using this stringent outcome measure, we conclude that HNGMT produces iCMs more efficiently than previously published methods. Mouse embryonic fibroblasts were treated with different combinations of transcription factors to drive transdifferentiation to induced cardiomyocytes (iCMs). Putative iCMs were enriched by zeocin selection. Zeocin resistance was conferred to iCMs via the TroponinT-GCaMP5-Zeo lentiviral reporter.

Project description:Acute occlusion of a coronary artery results in swift tissue necrosis. Bordering areas of the infarcted myocardium may also experience impaired blood supply and reduced oxygen delivery leading to altered metabolic and mechanical processes. While transcriptional changes in hypoxic cardiomyocytes are well-studied, little is known about the proteins that are actively secreted from these cells. We established a novel secretome analysis of cardiomyocytes by combining stable isotope labeling and click chemistry with subsequent mass spectrometry analysis. Further functional validation experiments included ELISA measurement of human samples, murine LAD ligation and adeno-associated virus (AAV) 9-mediated in vivo overexpression in mice. The presented approach is feasible for the analysis of the secretome of primary cardiomyocytes without serum starvation. 1026 proteins were identified to be secreted within 24 hours, indicating a 5-fold increase in detection compared to former approaches. Among them, a variety of proteins have so far not been explored in the context of cardiovascular pathologies. One of the most strongly upregulated secreted factors upon hypoxia was proprotein convertase subtilisin/kexin type 6 (PCSK6). Validation experiments revealed an increase of PCSK6 on mRNA and protein level in hypoxic cardiomyocytes. PCSK6 expression was elevated in hearts of mice following 3 days of ligation of the left anterior descending artery, a finding confirmed by immunohistochemistry. ELISA measurements in human serum also indicate distinct kinetics for PCSK6 in patients suffering from acute myocardial infarction, with a peak on day 3 post-infarction. Transfer of PCSK6-depleted cardiomyocyte secretome resulted in decreased expression of collagen I and III in fibroblasts compared to control treated cells, and siRNA mediated knockdown of PCSK6 in cardiomyocytes impacted transforming growth factor-β activation and mothers against decapentaplegic homolog 3 (SMAD3) translocation in fibroblasts. An Adeno-associated virus (AAV) 9-mediated, cardiomyocyte-specific overexpression of PCSK6 in mice resulted in increased collagen expression and cardiac fibrosis as well as decreased left ventricular function after myocardial infarction. In conclusion, a novel mass spectrometry-based approach allows the investigation of the secretome of primary cardiomyocytes. Analysis of hypoxia-induced secretion led to the identification of PCSK6 to be crucially involved in cardiac remodeling after acute myocardial infarction. Secretome analysis was performed on neonatal rat ventricular cardiomyocytes (NRVCMs) which were incubated under hypoxic conditions (1.5% O2, 5% CO2, 93.5% N2) for 12 (Hypoxia 0-12h), 24 (Hypoxia 0-24h) and 30 (Hypoxia 24-30h) hours. Furthermore, knockdown (KD) of PCSK6 in vitro mediated by small interfering RNA (siRNA) was performed to investigate changes in the secretome of cardiomyocytes with PCSK6 KD vs. control (control siRNA) during 24 hours of hypoxia (PCSK6 KD 0-24h). Cells were pulse-labeled with AHA (L-azidohomoalanine) and SILAC (stable isotope labeling with amino acids in cell culture) for 12 (Hypoxia 0-12h), 24 (Hypoxia 0-24h/PCSK6 KD 0-24h) and 6 hours (Hypoxia 24-30h). For Hypoxia 0-12h 3 replicates, Hypoxia 0-24h 6 replicates, PCSK6 KD 0-24h 3 Replicates and Hypoxia 24-30h 2 replicates were performed with label-swap.

Project description:Background: Direct cardiac reprogramming of fibroblasts into cardiomyocytes has emerged as one of the promising strategies to remuscularize the injured myocardium. Yet, it is still insufficient to generate functional induced cardiomyocytes (iCMs) from human fibroblasts using conventional reprogramming cocktails, such as our previously published combination consisting of MEF2C, GATA4, TBX5 and microRNA miR-133 (MGT133). Results: To discover potential missing factors for human direct reprogramming, we performed transcriptomic comparison between human iCMs and functional cardiomyocytes (CMs). We identified T-box transcription factor TBX20 as the top CM gene that is unable to be activated by MGT133. TBX20 is required for normal heart development and cardiac function in adult CMs but its role on cardiac reprogramming remains undefined. Here, we found that transduction of MGT133+TBX20 in human cardiac fibroblasts resulted in enhanced reprogramming featured with significantly activated contractility gene programs and signatures more similar to ventricular CMs. Human iCMs produced with MGT133+TBX20 more frequently demonstrated beating and calcium oscillation in co-culture with pluripotent stem cell derived CMs. More mitochondria and higher mitochondrial respiration were also detected in iCMs after TBX20 overexpression. Mechanistically, comprehensive transcriptomic, chromatin occupancy and epigenomic integration revealed that TBX20 localized to the cis-regulatory enhancers of under-expressed cardiac genes, such as MYBPC3, MYH7 and MYL4, to activate gene expression via strengthening the occupancy and co-occupancy of transcription factors. Furthermore, we identified TBX20-regulated enhancers and confirmed the synergistic effect of MGT and TBX20 on enhancer activation. Conclusions: TBX20 promotes cardiac cell fate conversion via direct activating cardiac enhancers. Human iCMs generated with TBX20 showed enhanced cardiac function in terms of contractility and mitochondrial respiration.

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FAM13B knockdown in iCMs

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