Mechanical modulation of inflammation in atrial fibrillation: Piezo1 mediates paracrine communication between fibroblasts and endothelial cells

Atrial fibrillation (AF), one of the most prevalent cardiac arrhythmias in humans, is accompanied by atrial dilatation – tissue stretch – fibrotic remodelling and inflammation. Recently, exciting findings shed light on mechanical modulation of the inflammatory response, but the interplay between mechanical stimuli and inflammation is poorly understood in the context of AF. The stretch-activated ion channel Piezo1 is expressed at higher levels in atrial fibroblasts isolated from patients with AF, compared to patients in sinus rhythm.1 It is a functionally relevant mechano-sensor in fibroblasts and contributes to the control of interleukin-6 (IL-6) secretion.2 This suggests that Piezo1 may be a mediator of mechanically-induced inflammatory responses in AF. The aims of this study were i) to assess the effects of differential Piezo1 expression on the inflammatory profile of fibroblasts in an unbiased manner and ii) to investigate the functional relevance of Piezo1-dependent cytokines from fibroblasts for endothelial cells – the central gatekeepers of immune cell extravasation. 

To address these aims, we combined the use of siRNA to knock-down Piezo1 expression in immortalised human atrial fibroblasts, bulk RNA sequencing as well as co-culture assays to assess the interplay between fibroblasts, endothelial cells and THP-1 monocytes.

Using bulk RNA sequencing of fibroblasts with Piezo1 knock-down (as means to identify the role of Piezo1), we found reduced expression of a large number of cytokines compared to control fibroblasts. Many of these are well-known to modulate endothelial cell adhesion properties relevant for immune cell recruitment from the circulation. 

To assess the functional relevance of this finding, we exposed human umbilical vein endothelial cells (HUVEC) for 4 h to culture media primed by control fibroblasts, fibroblasts with knock-down, or fibroblasts with overexpression of Piezo1. After removing the fibroblast-conditioned media, we added untreated THP-1 monocytes to the pre-treated HUVEC and removed non-adherent cells after 2 h. We found that pre-exposure to medium from fibroblasts overexpressing Piezo1 correlated with a ≈25% higher number of THP-1 cells adherent to endothelial cells, compared to pre-exposure to medium from fibroblasts with control or reduced Piezo1 levels (Piezo1 down-regulation did not significantly alter THP-1 cell attachment to endothelial cells), suggesting higher adhesion properties of endothelial cells transiently exposed to medium from Piezo1 overexpressing fibroblasts. Interestingly, Piezo1-dependent secretion of cytokines by human atrial fibroblasts led to elevated mRNA expression of intercellular adhesion molecule-1 (ICAM-1) in HUVEC. 

Taken together, these data indicate that AF-related Piezo1 upregulation in human atrial fibroblasts may aid immune cell recruitment from the circulation by acting on endothelial cell adhesion properties for circulating monocytes. This might be a novel mechanism, linking altered mechano-sensory activity to AF. 

1 Jakob D et al. Journal of Molecular and Cellular Cardiology 2021/158: 49-62.

2 Emig R et al, Cells, 2021/10(3): 663