Monocilia on chicken embryonic endocardium in low shear stress areas

Abstract

During cardiovascular development, fluid shear stress patterns change dramatically due to extensive remodeling. This biomechanical force has been shown to drive gene expression in endothelial cells and, consequently, is considered to play a role in cardiovascular development. The mechanism by which endothelial cells sense shear stress is still unidentified. In this study, we postulate that primary cilia function as fluid shear stress sensors of endothelial cells. Such a function already has been attributed to primary cilia on epithelial cells of the adult kidney and of Hensen's node in the embryo where they transduce mechanical signals into an intracellular Ca 2ϩ signaling response. Recently, primary cilia were observed on human umbilical vein endothelial cells. These primary cilia disassembled when subjected to high shear stress levels. Whereas endocardial-endothelial cells have been reported to be more shear responsive than endothelial cells, cilia are not detected, thus far, on endocardial cells. In the present study, we use field emission scanning electron microscopy to show shear stress-related regional differences in cell protrusions within the cardiovasculature of the developing chicken. Furthermore, we identify one of these cell protrusions as a monocilium with monoclonal antibodies against acetylated and detyrosinated alphatubulin. The distribution pattern of the monocilia was compared to the chicken embryonic expression pattern of the high shear stress marker Krü ppel-like factor-2. We demonstrate the presence of monocilia on endocardial-endothelial cells in areas of low shear stress and postulate that they are immotile primary cilia, which function as fluid shear stress sensors.

Primary cilia sensitize endothelial cells for fluid shear stress

“…Previously, we have shown a flow-dependent distribution pattern of ciliated EC in the chicken heart and vasculature that coincided with shear stress-related gene expression (Van der Heiden et al, 2006). In the heart, 30 -70% of the endocardial endothelial cells present a cilium in areas of low and oscillatory shear stress, whereas the arterial endothelial cells are devoid of luminal cilia.…”

“…EC have also been described to possess a primary cilium (Collin and Collin, 2004) that was "shed off" by high levels of fluid flow (Iomini et al, 2004). We have shown that embryonic EC of the heart and blood vessels indeed present a primary cilium in a shear stress-dependent fashion (Van der Heiden et al, 2006). In addition, disturbed flow exposed "athero-prone" areas in the adult mouse vasculature, i.e., the carotid sinus, the branch points of the aortic arch, and its inner curvature, show ciliated EC before and after the onset of atherogenesis .…”

“…Experimental changes in embryonic blood flow lead to changes in gene expression patterning of KLF2, ET1, and eNOS (Groenendijk et al, 2005), and lead to loss of proper cardiovascular function (Stekelenburg-de Vos et al, 2005Ursem et al, 2004) and the development of structural anomalies (Hogers et al, 1997(Hogers et al, , 1999. We have recently identified the inverse correlation between the expression of KLF2 and the presence of endothelial primary cilia in the embryonic heart (Van der Heiden et al, 2006). These sensory membrane extensions are exclusively present on EC in areas of low and disturbed blood flow, in the embryo as well as in the adult , which suggests a role in signal amplification of the fluid shear forces into the cell.…”

Cilia and coordination of signaling networks during heart development

“…The re-absorption of cilia in the endothelium of the endocardial cushions is in agreement with other studies showing that primary cilia are resorbed in response to sheer stress. 135,[137][138][139] Fluid shear stress is known to play important roles in trabeculation, cardiomyocyte proliferation, and establishment of the conduction system, and changes in the fluid flow leads to CHD. 140 Interestingly, mouse endothelial cells that form stunted primary cilia (e.g derived from the Tg737 orpk mutant) or are mutated in Pkd1 show impaired response to shear stress, 141 and mice with mutations in Kif3a, Lrd, Pkd2, 136 and Ift88/Tg737 21,142 have defects in chamber maturation.…”

The primary cilium as a gravitational force transducer and a regulator of transcriptional noise

“…Primary cilia are found on endocardial cells in the developing chick heart, predominantly on cells in areas of low shear forces (Van der Heiden et al, 2006). This localization is probably due to the active remodeling of the heart that takes place in regions of high shear forces.…”