High laminar flow shear stress activates signaling by PIEZO1 and PECAM1:CDH5:KDR in endothelial cells

Laminar shear stress on endothelial cells is detected by the glycocalyx, caveolae, cilia, the mechanosensitive ion channel PIEZO1 located on the apex of the cell, and the PECAM1:CDH5:KDR (PECAM1:VE-cadherin:VEGFR2) complex located on the lateral surfaces between adjacent cells (reviewed in Tanaka et al. 2021). The active molecular components, mechanisms of activation, and downstream events related to the glycocalyx, caveolae, and cilia are incompletely characterized so the annotation here focuses more on PIEZO1 and the PECAM1:CDH5:KDR complex.

The force of the flow on the membrane of the endothelial cell activates the mechanosensitive ion channel PIEZO1 and, indirectly, the ion channel TRPV4 to transport cations, notably calcium, from the extracellular region to the cytosol (reviewed in Li et al. 2014, Ranade et al. 2014, Fang et al. 2021, Xiao et al. 2023). Cytosolic calcium activates the protease complex Calpain2 to cleave the cytoskeletal proteins TALIN1 and VINCULIN, resulting in changes to the cytoskeleton that alter the shape of the endothelial cell.

Flow-sensitive potassium channels (which may include Kir2.1 and TREK1) and chloride channels (which may include LRRC8A) are also observed to open, however their mechanisms of activation and downstream events are incompletely characterized (reviewed in Tanaka et al. 2021).

Cytosolic calcium activates Pannexin channels to release ATP, which binds the P2RY2 (P2Y2) receptor on the cell surface in an autocrine and paracrine manner and thereby activates Galpha(q/11)-PI3K-AKT1 signaling. Both signaling by P2RY2 and signaling by a mechanosensitive complex containing PECAM1 and KDR (VEGFR2) (inferred from mouse homologs in Tzima et al. 2005) produce phosphatidylinositol 3,4,5-trisphosphate (PIP3), which binds AKT1 and enhances the phosphorylation of AKT1 on serine-475 by the mTORC2 complex.

Through a PI3K-independent mechanism, P2RY2 signaling and cytosolic calcium activate the kinase PDPK1, which phosphorylates the kinase PKN2 (PRK2). Phospho-PKN2 then phosphorylates AKT1 on threonine-308. Phospho-T308,S475-AKT1 phosphorylates serine-1177 of NOS3 (eNOS) while phospho-PKN2 also phosphorylates serine-1179 of NOS3, causing increased nitric oxide production (reviewed in Cabou and Martinez 2022).

Laminar shear stress increases secretion of Adrenomedullin (ADM), a vasodilator, by endothelial cells through an uncharacterized mechanism. ADM binds the AM1 receptor and signals through G-alpha(s), adenylate cyclase, and resultant cAMP to activate protein kinase A (PKA) to phosphorylate serine-633 of NOS3, further increasing nitric oxide production.

The sphingosine 1-phosphate receptor S1PR1, which couples to Galpha(i1) and Galpha(i3), contributes in a ligand-independent manner to activation of AKT and NOS3, however the intermediate steps are incompletely characterized (reviewed in Tanaka et al. 2021). Other GPCRs such as GPR68 also become activated, possibly through flow-induced deformation of the extracellular domain (reviewed in Tanaka et al. 2021).