9.2. Atherosclerosis

between these three elements is related to urinary flow sensor, where a flow-stimulated increase of iCa2+ has been characterized. Initial investigation suggested that deflecting the cilium releases a paracrine factor, such as ATP that can activate a G-protein-coupled receptor and generate inositol triphosphate (IP3) leading to iCa2+ increase throughout the cytosol due to the release of Ca2+ from the intracellular stores. Also, increasing the tubular flow triggered an increase in iCa2+. The same experiments were also performed in renal tubules from mice lacking P2Y2 receptors or cells lacking the primary cilium. In those experiments, the response to tubular flow was markedly reduced only in those cells lacking the P2Y2 [76]. These results strongly suggested that tubular flow triggers ATP release, followed by auto- and paracrine activation of epithelial P2 receptors. However, a direct link to the primary cilium could not be established in these experiments. Despite that, information regarding how purinergic signal-

9. Ciliopathies: an insight into some clinical consequences of impaired

As indicated above, the relevance of the primary cilium function has been well established in the kidney, as evidenced in polycystic kidney disease [37, 77, 78]. Also, previous reports suggest that the outcome of I/R in kidneys is associated with the change of primary cilia length [17]. Physiologically, urinary flow through the nephron is a highly variable process. In the short term, flow changes can be caused by variations in glomerular filtration rate [79], tubuloglomerular feedback [80] and fluid absorption along the nephron [79]. In the long term, urinary flow fluctuations can be caused by a high salt [81] or high protein diet [82], as well as due to hypertension [83] or early stages of diabetes [84]. Variations in luminal urinary flow alter the mechanical forces (shear

Polycystic kidney disease is a genetic disease characterized by bilateral enlarged cystic kidneys. It is caused by mutations of genes encoding for PKD1 and PKD2 linked to polycystic kidney disease type 1 (pkd1) and polycystic kidney disease type 2, respectively [37, 77, 78]. The frequencies of cardiovascular complications are very high in polycystic kidney disease patients. Hypertension occurs in 50–70% of patients even before any substantial kidney disorder [85]. Polycystic kidney disease has been associated with abnormalities in FSS sensing due to primary cilia dysfunction [36]. Mechanistically, polycystic kidney disease patients exhibit impairment of endotheliumdependent relaxation and a decrease of eNOS activity, impaired release of NO and, therefore, endothelial dysfunction [86]. Furthermore, polycystic kidney disease has been associated to the inability of renal epithelia [87] or vascular endothelia [9] to induce Ca2+ influx in response to FSS. Endothelial cells isolated from mice and humans with polycystic kidney disease lack PKD1 and/

or PKD2 in the primary cilium and fail to produce NO in response to FSS [2, 9, 29].

Abnormal PKD2 function or expression has been associated with hypertension [88]. Mutations of pkd2 gene abolish Ca2+ and NO increases in endothelial cells showing that PKD2 mediates FSS sensing in endothelial cells [29]. In addition, PKD2 sensory function as a Ca2+ channel

ing can be associated or not to function of primary cilia is missing.

stress, stretch and pressure) that affect epithelial cells in the nephron.

ciliary function

9.1. Polycystic kidney disease

100 Endothelial Dysfunction - Old Concepts and New Challenges

Initial evidence showed that primary cilia were present in the vascular beds where flow is disturbed, and related to atherosclerosis [89]. Particularly, the primary cilia were found in the endothelial cells of human aortic fatty dots and streaks, but not in those of the normal intima [89]. Moreover, recent evidences also found the primary cilium in cells exposed to laminar blood flow [27]. Regarding atherosclerotic plaque, primary cilia have been shown to be located at the atherosclerotic predilection sites, where flow is disturbed and around atherosclerotic lesions in the aortic arch in wild-type mice and apolipoprotein E-deficient mice, respectively [23]. In addition, experimentally induced pathologic turbulent flow in mice leads to induction of primary cilia, and subsequently to atherogenesis, suggesting a role of primary cilia in endothelial activation and dysfunction [23].

Contrary, another evidence found an inverse correlation between the presence of endothelial primary cilia and vascular calcified areas, although the signaling mechanisms involved remain unknown [22]. In order to analyze this phenomenon, Sanchez-Duffhues et al. [22] used the Tg737 cilium-defective mouse model and they found that non-ciliated aortic endothelial cells acquire the ability to trans-differentiate into mineralizing osteogenic cells. The mechanism for this trans-differentiation requires the presence of bone morphogenetic proteins (BMP). Therefore, these data emphasize the role of the endothelial cells in vascular calcification and generation of atherosclerosis. Whether these findings are associated or not to iCa2+, eNOS activation and NO synthesis remains unclear.

Apparently, differences in blood flow patterns along the endothelium trigger abnormal vascular responses that have been associated with pathophysiological consequences, such as atherosclerosis. While endothelial cells exposed to laminar blood flow are protected from atherosclerosis formation, turbulent blood flow, which occurs at bends and bifurcations of blood vessels, facilitates the process of atherosclerosis. Primary cilia presence and function have barely been studied in both endothelial activation and dysfunction. Hence, more studies are required to better understand these issues.
