# Shear Stress Initiates Endothelial-to-Mesenchymal Transition in Endocardial Endothelial Cells

## TL;DR

This study shows that high shear stress in heart cells can trigger a transformation linked to fibrosis, a key process in a congenital heart disease called DSS.

## Contribution

The study demonstrates that elevated shear stress induces endothelial-to-mesenchymal transition in endocardial endothelial cells, potentially explaining fibrotic membrane formation in DSS.

## Key findings

- Elevated shear stress triggers EEC alignment and EndMT signaling via SNAI1 upregulation.
- Snail1 inhibition with CYD19 transiently reduced EndMT markers but was less effective than shear stress.
- Shear stress had a stronger impact on EEC gene and protein expression than Snail1 inhibition.

## Abstract

Discrete subaortic stenosis (DSS) is a congenital heart disease in which a fibrotic membrane forms below the aortic valve; the underlying cellular mechanisms are currently unknown. Since an elevated pressure gradient in the left ventricular outflow tract (LVOT) is a distinguishing feature of DSS, it is hypothesized that the membrane formation is caused by elevated wall shear stress applied to the endocardial endothelial cells (EECs) that line the LVOT, triggering fibrosis. To correlate shear stress to an EEC fibrotic phenotype, we applied fluid shear stress to EECs at physiological and pathological shear rates using a cone-and-plate device, designed to recapitulate physiological wall shear stress in a controlled in vitro environment. Controlled shear stress regimes were applied to EECs to replicate the conditions observed in DSS patients. We found that elevated shear stress triggered EEC alignment as well as endothelial-to-mesenchymal transformation (EndMT) signaling pathways driven by upregulation of SNAI1 gene expression. The EECs were then treated with a small molecule inhibitor of Snail1 protein, CYD19, to attempt to attenuate EndMT signaling, and subsequently subjected to pathological shear stress. The Snail1 inhibitor did downregulate selected markers of EndMT signaling, although only transiently. Interestingly, the application of shear stress had a greater effect on the EEC gene and protein expression than did the Snail1 inhibition. This investigation of EEC response to shear stress reveals the pronounced and complex effect of this mechanical stimulation on the EEC phenotype. Further study should reveal the mechanisms that drive fibrosis and the formation of the DSS membrane.