Chromatin and transcriptional response to loss of TBX1 in early differentiation of mouse cells

ABSTRACTThe T-box transcription factor TBX1 has critical roles in the cardiopharyngeal lineage and the gene is haploinsufficient in DiGeorge syndrome, a typical developmental anomaly of the pharyngeal apparatus. Despite almost two decades of research, if and how TBX1 function triggers chromatin remodeling is not known.Here, we explored genome-wide gene expression and chromatin remodeling in two independent cellular models of Tbx1 loss of function, mouse embryonic carcinoma cells P19Cl6, and mouse embryonic stem cells (mESCs). The results of our study revealed that the loss or knockdown of TBX1 caused extensive transcriptional changes, some of which were cell type-specific, some were in common between the two models. However, unexpectedly we observed only limited chromatin changes in both systems. In P19Cl6 cells, differentially accessible regions (DARs) were not enriched in T-BOX binding motifs; in contrast, in mESCs, 34% (n=47) of all DARs included a T-BOX binding motif and almost all of them gained accessibility in Tbx1-/- cells.In conclusion, despite a clear transcriptional response of our cell models to loss of TBX1 in early cell differentiation, chromatin changes were relatively modest.

Inhibition of Cardiomyogenesis in Embryocarcinoma Cells Induced by Long-Term High Level of Glucose

Background/Aims: Cardiac myocytes constitute the first differentiated cell type during mammalian heart formation with the ability to beat spontaneously and rhythmically. Hyperglycemia is a primary risk factor for cardiovascular disease in pre-gestational diabetes mellitus (PGDM). However, the impact that hyperglycemia has on cardiac progenitors or on precursors differentiation remains poorly understood. The aim of the present study is to investigate whether hyperglycemia affects cardiomyogenesis of embryocarcinoma cells. Methods: P19CL6 cells differentiation induced by 1% DMSO was evaluated under either normal glucose (5.6 mmol/L) or high level of glucose concentrations (20 mmol/L or 40 mmol/L). To investigate the effect of long-term high level of glucose on cardiomyocytes differentiation, sarcomeric α-actinin, peroxisome proliferator-activated receptor coactivator-1 (PGC-1α), transcription factor GATA4 and Nkx2.5 were assessed by qRT-PCR analysis, western blot and immunofluorescence. Results: We observed that long-term high level of glucose markedly reduced P19CL6 cells differentiation into cardiomyocytes. The change in PGC-1α expression was consistent with changes in cardiac muscle myosin expression after exposure to 20 mmol/L or 40 mmol/L of glucose. On the other hand, the high level of glucose concentration profoundly decreased both GATA4 and Nkx2-5 expressions from day 6 to day 12 after differentiation, which was induced by 1% DMSO. Conclusion: Our results elucidate that the effect resulting from the long-term exposure of cardiac progenitors to high level of glucose is associated with decreased expression of GATA4 and Nkx2.5, providing a novel mechanism by which high glucose is able to affect cell differentiation.

Abstract 46: Temporal Ubiquitin-Proteasomal Degradation of Smad9 Mediated by its Specific E3 Ligase Asb2 is Required for Cardiac Development through the Regulation of Tbx2 Expression

Background: The bone morphogenetic protein (BMP) pathway plays crucial roles in cardiac development. Recent studies have reported that mutations in Smad9, one of the regulatory Smad specific for the BMP pathway, might result in cardiovascular diseases. However, both regulation and function of Smad9 in the cardiovascular system have not been elucidated. Methods and Results: We conducted DNA microarray using P19CL6 cells with forced expression of Smad9. Microarray analysis using Ingenuity Pathway Analysis elucidated that 19 genes including Tbx2 were related to BMP pathway and showed significantly altered expression levels by transient expression of Smad9. We confirmed by qRT-PCR that only Tbx2, but not other Tbx families, were induced by Smad9. Importantly, the expression of Tbx2 was more up-regulated by Smad9 than by Smad1. Moreover, we identified Asb2 as a specific E3 ligase that targets Smad9, but not Smad1/5, for proteasomal degradation. The in situ hybridization using murine embryo revealed that Asb2 is expressed predominantly in the heart during embryonic development, suggesting that Asb2 quantitatively regulates Smad9 in the developing heart. Biochemical analysis demonstrated that Tbx2 expression induced by Smad9 was attenuated by Asb2, which was restored by the treatment with proteasome inhibitor, lactacystin. Developmental studies using both P19CL6 cells and zebrafish showed that the ablation of Asb2 leads accumulation of Smad9 resulting in the up-regulation of Tbx2, which attenuates myocardial development while induces non-myocardial tissue including cardiac cushion. Indeed, alcian blue staining of morpholino-mediated knockdown of zebrafish Asb2 showed significantly dilated ventricle and thinned ventricular wall, accompanied with decreased myocardium and increased cardiac jelly. Conclusions: Smad9 induces the expression of Tbx2 during cardiac development and is temporally and quantitatively regulated by its specific E3 ligase Asb2. This is the first study to show both the target gene and specific E3 ligase of Smad9.