Lack of Primary Cilia Primes Shear-Induced Endothelial-to-Mesenchymal Transition

Abstract

Rationale Primary cilia are cellular protrusions which serve as mechanosensors for fluid flow. In endothelial cells (EC) they function in transducing local blood flow information into functional responses, like nitric oxide production and initiation of gene expression. Cilia are present on EC in areas of low or disturbed flow and absent in areas of high flow. In the embryonic heart high flow regime applies to the endocardial cushion area, and the absence of cilia here coincides with the process of endothelial-to-mesenchymal transition (EndoMT). Objective In this study we investigate the role of the primary cilium in defining the responses of EC to fluid shear stress and in EndoMT. Methods and Results Non-ciliated mouse embryonic EC with a mutation in Tg737/Ift88 were used to compare the response to fluid shear stress to that of ciliated EC. In vitro, non-ciliated EC undergo shear-induced EndoMT which is accompanied by downregulation of Klf4. This Tgfβ/Alk5 dependent transformation is prevented by blocking Tgfβ signaling, overexpression of Klf4, or rescue of the primary cilium. In the hearts of Tg737orpk/orpk embryos Tgfβ/Alk5 signaling was activated in areas in which EC would normally be ciliated, but now lack cilia due to the mutation. In these areas EC show increased Smad2 phosphorylation and expression of αSMA. Conclusions This study demonstrates the central role of primary cilia in rendering EC prone to shear-induced activation of Tgfβ/Alk5 signaling and EndoMT, and thereby provides a functional link between primary cilia and flow related endothelial performance.

Cilia and coordination of signaling networks during heart development

“…Furthermore, endocardial cells derived from Ift88/Tg737 −/− mice show disturbances in EndoMT. 136,142,151 Taken together, this Figure 5. examples of signaling networks involved in development of specific anatomical structures of the heart.…”

“…Further, genomic deletion and duplication of the human gene encoding TGFβ Receptor II, TGFBR2, causes heterotaxy, 17 whereas endothelial cells derived from Ift88/Tg737 −/− mouse embryos show defective TGFβ signaling upon sheer induced EndoMT. 151 These results indicate that TGFβ signaling is important at multiple stages during heart development, i.e., both up-and downstream of L-R determination.…”

“…Development of the endocardial cushions depends on EndoMT, a cellular process whereby endothelial cells lose the epithelial phenotype and differentiate into mesenchymal cells that ingress into the underlying cardiac jelly. 151 The endocardial cushions in Kif3a −/− and Pkd2 −/− mouse embryos are acellular and embryos with mutation in the Ift88/Tg737 gene have a decreased amount of mesenchymal cells in the endocardial cushions. Furthermore, endocardial cells derived from Ift88/Tg737 −/− mice show disturbances in EndoMT.…”

Tgfβ/Alk5 signaling is required for shear stress induced klf2 expression in embryonic endothelial cells

“…Primary cilia have been described to play a critical role in the process of mechanosensing by modulating endothelial responses to shear stress (Nauli et al, 2008;Poelmann et al, 2008;Hierck et al, 2008). We have recently demonstrated the central role of primary cilia in rendering EC prone to enhanced shearinduced activation of Tgfb/Alk5 signaling and EndoMT in vitro and in vivo (Egorova et al, 2011). The importance of shear-related signaling is exemplified by the fact that altered intracardiac blood flow patterns, e.g., as seen in the venous clip model, result in the development of congenital heart malformations (Hogers et al, 1997(Hogers et al, , 1999.…”

Telocytes in Human Fallopian Tube and Uterus Express Estrogen and Progesterone Receptors

“…The presumed functions of such a nonmotile cilium could be:-organizer of the mitotic spindle (Alieva et al, 2004), sensory organelle involved in signal transduction -hedgehog pathway (Singla et al, 2006), mechanical sensing and mechano-chemical conversion in endothelial cells (Egorova et al, 2011;Nauli et al, 2008). By analogy, we can presume that TCs could be involved in the signaling process if located near the stromal colony-forming cells/units in human endometrium or might act as stretch sensors if located near smooth muscle structures in both Fallopian tubes and uterus.…”