RNA-Binding Proteins Regulate Post-Transcriptional Responses to TGF-β to Coordinate Function and Mesenchymal Activation of Murine Endothelial Cells

BACKGROUND: Endothelial cells (ECs) are primed to respond to various signaling cues. For example, TGF (transforming growth factor)-beta has major effects on EC function and phenotype by driving ECs towards a more mesenchymal state (ie, triggering endothelial to mesenchymal activation), a dynamic process associated with cardiovascular diseases. Although transcriptional regulation triggered by TGF-beta in ECs is well characterized, post-transcriptional regulatory mechanisms induced by TGF-beta remain largely unknown. METHODS: Using RNA interactome capture, we identified global TGF-beta driven changes in RNA-binding proteins in ECs. We investigated specific changes in the RNA-binding patterns of hnRNP H1 (heterogeneous nuclear ribonucleoprotein H1) and Csde1 (cold shock domain containing E1) using RNA immunoprecipitation and overlapped this with RNA-sequencing data after knockdown of either protein for functional insight. Using a modified proximity ligation assay, we visualized the specific interactions between hnRNP H1 and Csde1 and target RNAs in situ both in vitro and in mouse heart sections. RESULTS: Characterization of TGF-beta-regulated RBPs (RNA-binding proteins) revealed hnRNP H1 and Csde1 as key regulators of the cellular response to TGF-beta at the post-transcriptional level, with loss of either protein-promoting mesenchymal activation in ECs. We found that TGF-beta drives an increase in binding of hnRNP H1 to its target RNAs, offsetting mesenchymal activation, but a decrease in Csde1 RNA-binding, facilitating this process. Both, hnRNP H1 and Csde1, dynamically bind and regulate specific subsets of mRNAs related to mesenchymal activation and endothelial function. CONCLUSIONS: Together, we show that RBPs play a key role in the endothelial response to TGF-beta stimulation at the post-transcriptional level and that the RBPs hnRNP H1 and Csde1 serve to maintain EC function and counteract mesenchymal activation. We propose that TGF-beta profoundly modifies RNA-protein interaction entailing feedback and feed-forward control at the post-transcriptional level, to fine-tune mesenchymal activation in ECs.

Automated summary

This study reveals that RNA-binding proteins hnRNP H1 and Csde1 play a crucial role in the post-transcriptional regulation of endothelial cells (ECs) in response to TGF-beta stimulation. They maintain EC function and counteract mesenchymal activation by modulating their binding to target RNAs. TGF-beta modifies RNA-protein interaction to fine-tune mesenchymal activation in ECs.