Various types of hemodynamic forces acting on the blood vessel wall.
Fluid shear stress (FSS) is able to generate phenotypic changes in the cells in direct contact with the strain force. In order to detect and transduce FSS into intracellular pathways, biological systems use a specific set of sensors, called mechanosensors. The process involves the conversion of the mechanical force into a chemical or electrical signal. Primary cilium is a non-motile organelle that emanates from the cell surface of most mammalian cell types that act as a mechanosensor. Increasing evidence suggests that primary cilia are key coordinators of signaling pathways in tissue homeostasis and when defective may cause human diseases and developmental disorders. Here, we will describe the endothelial primary cilium as a mechanotransductory organelle sensing FSS. To fulfill this function, primary cilium requires the localization of mechanoproteins, polycystin-1 and -2, in their membrane and the structural gene product, polaris. Physiologically, deflection of primary cilium increases the intracellular calcium concentration triggering a signaling pathway that leads to nitric oxide (NO) formation and vasodilation. Additionally, ciliopathies, such as polycystic kidney disease and atherosclerosis, will also be discussed. We also analyze available information regarding a trio of membrane receptors involved in FSS sensing and transducing such as vascular endothelial growth factor receptors (VEGFRs) and its coreceptor neuropilin (NRP), as well as purinergic receptors (P2Y2). Whether or not they modulate, the primary cilium role in sensing FSS is poorly understood. This chapter highlights the main relevance of primary cilium in sensing blood flow, although exact mechanisms are not fully known yet.
- shear stress
- endothelial dysfunction
- primary cilium
- nitric oxide
- reactive oxygen species
- purinergic receptors
Blood, urine and air are primary examples of biological fluids. Biophysically, fluids can be classified into four basic types: ideal fluid, real fluid, Newtonian fluid and non-Newtonian fluid. Among them, biological fluids are classified only as Newtonian and non-Newtonian. Blood and urine belong to non-Newtonian biofluids since their viscosity is not a constant with respect to the rate of shearing stress; moreover, the removal of the stress causes them to return to their initial viscosity state .
In order to regulate blood flow, vascular smooth muscle cells (VSMC) induce changes in blood vessel diameter by contraction and relaxation mechanism. Smooth muscle contraction is regulated by central neuronal as well as by local control mechanisms. In particular, the local control, also termed autoregulation, is an important mechanism of vascular tone regulation, maintaining the immediate control of the amount of blood flow within a specific region. Vessel diameter decreases by a sudden increase of transmural pressure and increases by faster flow or high shear stress . Flow shear stress (FSS) is one of the important blood flow-induced hemodynamic forces (Table 1) acting on the blood vessel and is determined by the velocity of blood flow, fluid viscosity and vessel geometry [2, 3, 4, 5]. An important determinant of shear stress is the viscosity of blood; shear stress is the energy transferred to the vessel wall due to interaction with a fluid in motion . Shear stress forces are imposed directly to the endothelium and modulate endothelial structure and function through local mechanotransduction mechanisms [5, 7]. FSS is crucial for vascular homeostasis .
|Hemodynamic forces||Generated by||Force name|
|Distention force||Surrounding muscle||Stretch force|
|Contractile force||Differential pressure along the vascular system||Compression force|
|Pulsatile force||Turbulent flow of blood||Cyclic strain|
|Systolic force on intima surface (endothelial cells)||Blood flow||Pressure force|
|Drag force on intima surface (endothelial cells)||Blood flow||Shear stress|
In a normal homeostatic mechanism and steady laminar shear stress, endothelial cells respond promptly with an increase in the cytosolic calcium (Ca2+), activation of endothelial nitric oxide synthase (eNOS) and nitric oxide (NO) production [4, 8] and with the ultimate gene modulation [3, 5, 8]. However, besides laminar flow, oscillatory and turbulent flow patterns are also imposed to the endothelium, which has then to continuously fine-tune its activities as a response .
Several structures and processes have been implicated in FSS mechanotransduction into specific biochemical signals, intracellular signaling pathways and gene modulation . Among those structures implicated, the primary cilium emerges as a key sensor of FSS under physiological conditions . Nevertheless, in vascular injury occurring as a result of hypertension for example, normal homeostatic mechanisms are disturbed and vessel wall becomes dysfunctional associated with impaired formation and/or function of primary cilium [5, 10]. Moreover, ciliopathies or ciliary dysfunctions can lead to a series of disorders such as PKD, hypertension and atherosclerotic lesions .
Physiologically, the primary cilium, a solitary non-motile microtubule-based organelle, protrudes from the surface of mammalian cells  into the surrounding tube (vessel/tubule) lumen. Primary cilia work as a chemo- and mechanosensors responding to diverse stimuli, including FSS [12, 13].
Since the importance of the primary cilium as a sensor of FSS has been described mainly in the kidney and in the blood vessels, it is worth to describe in brief some aspects of the renal system.
During the process of urine formation, the flow of the ultrafiltrate through the proximal tubule (PT) is pulsatile, with variable oscillations due to the heart rate and to tubuloglomerular feedback mechanism mediated by the
Recently, several reports showed that an alteration of primary cilia length and function is associated with acute and chronic kidney disease [12, 13, 16, 17, 18, 19, 20, 21]. However, the underlying mechanisms behind these associations are still unclear. The main scope of this chapter focuses on the role of primary cilium as one of the multiple mechanotransduction machineries in sensing FSS in the endothelial vascular and epithelial renal system.
In the blood vessels, endothelial cells exhibit cilia that have been involved in blood vessel autoregulation , as well as in the pathogenesis of hypertension  and atherosclerotic lesions [22, 23].
In this chapter, we will present the physiological role of the endothelial primary cilium as a sensor of FSS. We will make a short review about potential implication of reactive oxygen species (ROS), vascular endothelial growth factor (VEGF) and purinergic signaling as modulators of the function of primary cilium. Finally, implication of the primary cilium dysfunction in the kidney and atherosclerotic lesions will be overviewed.
2. Structure of the primary cilium
Primary cilia differ from motile cilia in both structure and function and are usually classified as non-motile organelles, which were first described in 1867 by Alexander Kowalesky in vertebrate cells . Motile cilia contain microtubules (MT) arranged in a (9 + 2) manner consisting of a nine doublets MT ring surrounding a central pair of MT and presenting protein spokes and dynein inner and outer arms necessary for movement. In contrast the primary cilium shows (9 + 0) organization with nine pairs of MT at the periphery lacking the central pair of MTs, as well as the protein spoke and the dynein arms (Figure 1). In both cases, MT extend from a basal body originating from “mother” centriole of the centrosome . The structure and maintenance of the primary cilium are regulated by intraflagellar transport (IFT) particles .
In physiological conditions, nearly all quiescent differentiated mammalian cells exhibit a primary cilium, which emanates from the surface as a single long hair-shaped projection . Therefore, primary cilia are found in a large number of mammalian cells including stem cells, epithelial and endothelial cells . Their presence was demonstrated in adult vascular system (reviewed in ), developing chicken endocardium , embryonic mouse aortic endothelium , cultured human umbilical vein endothelial cells (HUVECs) [28, 29] and epithelial cells including
The ciliary membrane housing many sensory receptors and channels supporting sensory function of cilia.
The soluble compartment or cilioplasm constituting the fluid between the ciliary membrane and the axoneme and where IFT machinery is located to assemble and maintain the cilia.
The axoneme composed of tubulin that supports ciliary transport. It is composed of nine pairs of MTs.
The ciliary tip is the distal part of the axoneme where specialized proteins localize whose function is still unclear.
The basal body, the network foundation from which the primary cilium emanates.
3. Primary cilium sensing fluid-shear depends on mechanoproteins polycystins and structural polaris
3.1. Intraflagellar transport
IFT is required for assembly and maintenance of cilia. Briefly, ciliogenesis is initiated in the apical cytoplasm at the basal body. Proteins involved in cilium formation concentrate and assemble into complexes that migrate along the cilia axonemal microtubules through a process called IFT. The anterograde movement of particles from the cell body to the tip of the flagella/cilia is driven by kinesin II , whereas the retrograde movement from the tip back to the cell body is driven by cytoplasmic dynein . The protein polaris is the gene product of the IFT particle 88 (
3.2. Polycystin-1, polycystin-2 and polaris
Among sensory molecules housing into the primary cilium, both polycystin-1 (PKD1) and polycystin-2 (PKD2) have been described. These are membrane integral proteins. Experimental data show that they are highly expressed in human endothelial and epithelial cells and are required for normal physiological cilia function (reviewed in ). The importance of these proteins has been highlighted due to the finding that mutations in
PKD1 is a 3327 amino acids long transmembrane protein with 11 membrane-spanning domains. Its long extracellular N-terminus has a mechanosensory function, while its short intracellular C-terminus is involved in intracellular signaling and interaction with PKD2 [34, 35]. PKD1 has been shown to mediate fluid-shear sensing in epithelial and endothelial cells [9, 36].
PKD2, a 968 amino acids long protein, is a non-selective Ca2+ permeable transient receptor potential (TRP) channel consisting of six membrane-spanning domains and intracellular C- and N-terminal domains . The sensory function of PKD2 depends on PKD1 and has to be localized to endothelial primary cilia . Accordingly, PKD2 functioning as a Ca2+ channel  allows extracellular Ca2+ influx into the cilioplasm in response to FSS . Thus, mechanistically, PKD1 and PKD2 interact through their C-terminus [29, 34, 35] and localized to the ciliary membrane; they are able to detect extracellular FSS and to increase cytosolic Ca2+. This turns on a signaling cascade leading to the production of NO [9, 38, 40].
A series of mutation and deletion experiments demonstrated that besides PKD1 and PKD2, the protein polaris also orchestrates FSS sensing. The physiological Ca2+ and NO increase in response to FSS is abolished when the
3.3. Molecular cascade involved in shear stress-induced calcium and NO signaling
FSS leads to cilia bending leading to PKD2-mediated increase of intracellular Ca2+ that leads to activation of ryanodine receptors (RyR) and inositol 1,4,5-triphosphate receptor (InsP3R) present in the endoplasmic reticulum, which then releases its stores of Ca2+ enhancing the intracellular levels of Ca2+ [42, 43]. Subsequently, Ca2+ activates several intracellular signaling pathways, including the activation of the eNOS-bound calmodulin, thus increasing the production of NO that diffuses from endothelial cells to neighboring VSMC inducing vasodilatation [2, 29]. This particular pathway is summarized in Figure 2.
The works of AbouAlaiwi et al.  have helped to elucidate this last mechanism. In order to prove that FSS-dependent primary cilia bending induces extracellular Ca2+ influx, they used Ca2+ chelator EGTA (ethylene glycol-bis (β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid). In these experiments, EGTA abolished both Ca2+ and NO increases. In addition, the inhibitor of the eNOS, NG-nitro-L-arginine methyl ester (L-NAME) blocked the FSS-induced NO release without affecting Ca2+ increase. The same effect was shown after blocking calcium-dependent mechanisms of NO production using calphostin C as an inhibitor of protein kinase C (PKC) or W7 as antagonist of calmodulin. Similarly, inhibiting protein kinase B (PKB)/Akt abolished NO release without altering Ca2+ increase. Inhibiting IP3 kinase using LY-294002 did not alter neither Ca2+ nor NO increase. These findings indicate that calmodulin, PKC and Akt/PKB are downstream of the calcium pathway and that they are necessary for NO release during primary cilium-mediated FSS signaling  (Figure 2).
4. The regulation of ciliary function
Changes in fluid patterns generate differential biomechanical forces, which lead to alteration of cilia function or structure . Indeed, almost all blood vessels possess cilia [4, 23]. Particularly, arteries with low FSS or high fluid turbulence have cilia . A prolonged exposure of endothelial cells to high FSS induces the disassembly of cilia  and inactivation of PKD1 by proteolytic cleavage , suggesting that primary cilia may not be required only to sense high shear stress .
The process of disassembly observed here involves the termination of IFT and the inability of the oldest centriole to maintain or initiate the assembly of primary cilia under laminar shear stress . The disassembly of cilia parallels a major rearrangement of the cytoskeleton and an increase of acetylation of MT [18, 44].
In the renal system, tubular flow and ROS act as potent modulators of epithelial kidney cell phenotype also by affecting the organization of the cytoskeleton and the brush border, changing cell polarity and modifying various cellular functions such as solute reabsorption and extracellular matrix remodeling . Under oxidative stress, ROS directly induce the breakdown of the cell cytoskeleton, activate various cell death-associated signals and regulate elongation, shortening and release of cilia . The mechanism and implications of this regulation are still unclear.
5. Reactive oxygen species, shear stress and cilia function
ROS and NO have been implicated in sensing FSS in both vascular homeostasis and diseases . ROS include collective oxygen (O2) radicals such as superoxide, O2˙− and hydroxyl radical, OH, and non-radical derivatives of O2, including hydrogen peroxide (H2O2) and ozone (O3). Several sources of ROS have been extensively described in the literature, in which the nicotidamine adenine dinucleotide phosphate (NADP) oxidase (Nox) has been described as one of the main cellular sources of ROS generation in endothelial cells under FSS .
Flow patterns and the magnitude of shear determine the amount of ROS produced by endothelial cells, usually an irregular flow pattern (disturbed or oscillatory) producing higher levels of ROS than a regular flow pattern (steady laminar or pulsatile) . In addition to flow pattern, endothelial cells exposed to a prolonged laminar shear stress for more than 24 h display a reduced O2˙− formation and ROS levels . ROS production is closely linked to NO generation: elevated levels of ROS lead to low NO bioavailability, as is often observed in endothelial cells exposed to irregular flow patterns . The low NO bioavailability is partially provoked by ROS reaction with NO to form peroxynitrite (ONOO−), a key molecule that is implicated in oxidative and nitrosative damage . NO can also take part in redox signaling by modifying proteins and lipids via cysteine S-nitrosation and fatty acid nitration, respectively , in this respect affecting the regulation of the vascular system .
5.1. Free radical signaling and primary cilia
Information related to primary cilium and free radical signaling emerges mainly from kidney research area. However, how ROS can regulate this mechanosensory organelle is not well described in the literature [17, 30, 53]. It is known that renal primary cilia protrude from the epithelial cell surface into the lumen detecting fluid flow and responding to diverse stimuli . Indeed, several reports show that an alteration of primary cilia length is associated with acute and chronic kidney disease .
Information about primary cilia acting as an upstream regulator of ROS comes primarily from
The opposite mechanism in which free radical species can regulate primary cilia function is showed mainly in renal ischemia/reperfusion (I/R) experiments. I/R setting is characterized by an increase in free radical species production . Acute tubular necrosis induced by I/R on mice model resulted in changes in primary cilium length. Thus, primary cilium was shortened after 4 h and 1 day of ischemia versus non-ischemic control cells, an effect that was blunted after 16 days . The oxidative stress from I/R derived injury is able to break down cell cytoskeleton and activate various cell death-associated signals, like cell autophagy . As presented by Kim et al. , the treatment with the antioxidant molecule Mn(III) tetrakis (1-methyl-4-pyridyl) porphyrin (MnTMPyP) during the reperfusion (i.e., recovery) period of damaged kidneys accelerated the normalization of cilia length in experiments of I/R. Concomitantly, they also showed that MnTMPyP decrease oxidative stress and recover nephron tubule morphology, indicating that the ROS signals are an integral part of cilium length regulation. In addition, cultured kidney cells treated with H2O2 released a ciliary fragment into the extracellular medium. MnTMPyP treatment inhibited this deciliation process [17, 53]. Moreover, the extracellular signal-regulated kinase (ERK) inhibitor U0126 blocked the cilium elongation of normal and H2O2-treated cells . Taken together, these observations show that primary cilia length can be regulated, at least in part, by H2O2 through an ERK-dependent pathway. Similar results were found related in acute kidney injury after hepatic I/R from liver transplantation or resection experiments in the kidney . In particular, transient hepatic ischemia caused functional and histological kidney damage, including brush border loss of tubular epithelial cells and tubule atrophy. This cellular damage also induces a shortening and deciliation of kidney primary cilia via ROS/oxidative stress, suggesting that the presence of ciliary proteins in the urine could be a potential indication of kidney injury . Therefore, remote organ injury model can increase the content of O2−, and H2O2 subsequently shortening the primary cilium length in several nephron sections . These data confirm that free radical species can modulate the primary cilium length, at least in the kidney, but the mechanism and functional implications of such modulation remain unclear.
6. Vascular endothelial growth factor and shear stress
VEGFs are a complex family of glycoproteins that are structurally related to platelet-derived growth factor (PDGF) . Through alternative RNA splicing, VEGF family is constituted by several isoforms, including VEGF165, which has been named VEGF-A or VEGF, the isoform involved in many of the functions attributed to the VEGF family. All members of the VEGF family activate tyrosine kinase receptors known as VEGF receptors (VEGFRs), which include VEGFR-1 (also known as fms-like tyrosine kinase 1 or Flt-1), VEGFR-2 (or kinase insert-domain containing receptor, KDR) and VEGFR-3 [58, 59]. Activation of VEGFRs has been implicated in several vascular functions, including angiogenesis, vascular tone regulation and endothelial cell survival, among others [59, 60, 61].
Importantly, VEGF and VEGFRs have also been associated with sensing FSS. High expression of VEGF  and the activation of VEGFR2 [63, 64] have been linked to the FSS sensing. Moreover, the activation of VEGFRs generally leads to NO synthesis in many kinds of cells, including endothelial cells . Therefore, it is not surprising that VEGFR2 triggers NO-dependent flow regulation. Jin et al.  showed that FSS leads to VEGFR2 activation in a ligand-independent manner and leads to eNOS activation in cultured endothelial cells. Intracellular downstream pathway associated with NO synthesis due to FSS-stimulated VEGFR2 activation included phosphoinositide 3-kinase (PI3K) and PKB/Akt. Interestingly, contrary to PKB/Akt, the PI3K pathway has not been associated to endothelial primary cilium FSS sensory function . Also,
7. Neuropilins and the primary cilium
VEGF can also bind to neuropilins (NRP), a family of transmembrane glycoproteins that play key role in axonal guidance, angiogenesis, tumorigenesis and immunological response . NRPs have been characterized as co-receptors for VEGFRs and plexins, the receptors of the extracellular secreted ligands, belonging to class III semaphorins . In turn, semaphorins are a class of secreted and membrane proteins that were originally identified as axonal growth cone guidance molecules. At least two neuropilin genes, NRP1 and NRP2, have been identified . Genetic studies in mice have confirmed that NRPs are key components of vasculogenesis, angiogenesis and lymphangiogenesis [65, 66]. Nevertheless, NRPs can bind to growth factors such as VEGF, placental growth factor (PlGF), hepatocyte growth factor (HGF) and fibroblast growth factor (FGF), among others. And due to VEGF binding, NRPs can also modulate blood flow .
In endothelial cells, NRPs are thought to increase signaling through the VEGFRs acting as a co-receptor of VEGF and by stabilizing the VEGF/VEGFR complexes and therefore enhancing VEGF activity. Thus, the interaction of VEGF-A165 with NRP1 is required for stable binding of VEGF-A165 to VEGFR-2, full activation of VEGFR-2 and downstream signaling and biological responses [65, 67].
Limited information about the localization of NRP in primary cilium is available. Before presenting those evidences, we should give some information about hedgehog (HH) signaling. Briefly, HH signaling is essential for tissue patterning and organ formation during embryonic and postnatal development, as well as in cancer development and tissue homeostasis renewal and repair in adult animals . The HH pathway acts via activation of transcriptional effectors, such as the glioblastoma (Gli) proteins, a family of transcription factors whose target genes remain enigmatic. The Gli protein family includes Gli1, Gli2 and Gli3 .
Referring to the primary cilium, studies conducted by Pinskey et al.  found that NRP1 and NRP2 promote the activation of Gli transcription factor. Interestingly, the authors found that a conserved 12 amino acid region of the NRP1 cytoplasmic domain between amino acids 890 and 902 is responsible for the HH-signal promotion. Considering that an intact primary cilium is a main component of the HH signaling, they also looked for the localization of NRP1 in this subcellular compartment and showed the unique evidence until now about the localization of NRP1, but not NRP2, in the primary cilium . Despite the fact that the localization of NRP1 in the primary cilium was not required for HH signaling promotion, it is intriguing why NRP1 is present in primary cilium and what would be its physiological relevance there. This observation is important considering that NRP1, as indicated previously, may interact with growth factors, such as VEGF, PlGF, HGF and FGF, among others, regulating their action. Still more questions than answers emerge and more investigation is required to lighten these intriguing possibilities.
8. Purinergic receptors and the primary cilium
Since early 1970s , adenosine triphosphate (ATP) has been recognized as an extracellular signaling molecule activating a pathway defined as “purinergic signaling” where ATP, ADP and adenosine are involved. The signaling pathway starts with the activation of a family of membrane receptors. At this moment, separate families for adenosine purinergic (P1) and ATP and ADP purinergic (P2) receptors have been characterized. Briefly, adenosine receptor or P1 family includes at least four members of G-protein-coupled receptor subtypes identified as A1, A2A, A2B and A3. In contrast, the P2 family encompasses seven members of purinergic receptor type X (P2X), a family of ion channels receptor subtypes (P2X1–7) and at least eight members of P2Y G-protein-coupled receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13 and P2Y14) . P2Y1, P2Y2, P2Y4 and P2Y6 are associated with the intracellular calcium (iCa2+) signaling pathway, whereas P2Y2, P2Y13 and P2Y14 are associated with cyclic adenosine monophosphate (cAMP) signaling. In contrast, P2Y11 has been shown to be associated with both iCa2+ and cAMP signaling .
ATP is released by almost all cell types after gentle mechanical stimulation and acts in an autocrine or paracrine manner . Living cells under stressful conditions (i.e., hypoxia) or dying cells release ATP . Interestingly, purinergic signaling parallel to flow sensor activity of the primary cilium . Purinergic signaling associated with flow sensing was detected in several structures such as kidney tubules , intrahepatic bile ducts [74, 75], endothelial cells , among other lining cells. Therefore, deflection of the primary cilium has been related with ATP release leading to autocrine or paracrine activation of purinergic receptors.
The relationship between ATP, purinergic signaling and primary cilium has been studied in the kidney tubular system . The main physiological area that established a relationship 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 . 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 signaling can be associated or not to function of primary cilia is missing.
9. Ciliopathies: an insight into some clinical consequences of impaired ciliary function
9.1. Polycystic kidney disease
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 . 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 , tubuloglomerular feedback  and fluid absorption along the nephron . In the long term, urinary flow fluctuations can be caused by a high salt  or high protein diet , as well as due to hypertension  or early stages of diabetes . Variations in luminal urinary flow alter the mechanical forces (shear stress, stretch and pressure) that affect epithelial cells in the nephron.
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 (
Abnormal PKD2 function or expression has been associated with hypertension . Mutations of
Initial evidence showed that primary cilia were present in the vascular beds where flow is disturbed, and related to atherosclerosis . 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 . Moreover, recent evidences also found the primary cilium in cells exposed to laminar blood flow . 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 . 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 .
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 . In order to analyze this phenomenon, Sanchez-Duffhues et al.  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.
10. Concluding remarks
Phenotypic cell alterations resulting from flow-induced mechanical strains and their implication in diseases are a growing field of research in many cell types such as vascular endothelial, smooth muscle, kidney epithelial cells and chondrocytes.
In the chapter, we presented the role of the primary cilium as one of the multiple physiological mechanosensors for FSS in endothelial and renal cells, where it regulates vascular homeostasis and epithelial function. To respond to FSS, a functional primary cilium requires the constitutive proteins, PKD1, PKD2 and polaris. The primary cilium is functional under normal FSS and activates the Ca2+ and NO signaling cascade; nevertheless, it becomes dysfunctional after prolonged exposure to high FSS analogous to a hypertensive situation present in any kind of biological fluid. Respectively, growing evidence implicates the primary cilium and the disruption of its function in many diseases such as hypertension, atherosclerotic lesions and acute and chronic kidney disease.
In this regards, we have summarized evidences implicating that polycystic kidney disease, a pathology characterized by lack of PKD1 and/or PKD2 expression, leads to impaired vascular endothelial FSS sensing. Even when the primary organ affected by the disease is the kidney, the endothelial dysfunction is a common extra renal symptom observed in polycystic kidney disease. Those patients exhibit an impairment of endothelium-dependent relaxation and a decrease of primary cilia-dependent NO production leading to hypertension.
Contrary to its physiological role in sensing FSS, it has also been described that primary cilium is related to plaque formation, since this organelle was present in the endothelial cells of human aortic fatty dots and streaks. Indeed, primary cilium has 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 . In addition, primary cilia have been involved in endothelial activation and dysfunction present in atherosclerosis. Despite relevance of these evidences, it is highlighted in this review that more studies are required to better understand the role of endothelial primary cilium in normal and pathological conditions, such as atherosclerosis.
We also presented examples of regulatory signals that control NO bioavailability or might participate as modulators of primary cilium. For instance, ROS can modulate cilia length and deciliation process in tubular kidney cells. Whether these effects could be extrapolated to endothelial cells is worth of more investigation.
Finally, we presented the interconnected coreceptors VEGF and VEGFRs, neuropilins, ATP, adenosine and purinergic receptors. All have been suggested to be involved in FSS sensing and/or colocalization in the primary cilium. To this respect, we can provide more questions than answers. NRP1, a VEGFR2 receptor, localizes to the primary cilium but its physiological relevance is still unknown. On the other hand, ATP and adenosine are involved in sensing FSS, in a primary cilium-independent manner. Moreover, information regarding whether or not purinergic signaling can be associated to the primary cilia function is missing.
In conclusion, these data emphasize the role of the primary cilium present in endothelial cells as a microsensory organelle transducing FSS. Impairment in the ciliogenesis, cilia length and intracellular pathways can be involved in cardiovascular diseases. The participation of ROS, VEGF and purinergic signaling pathways is being described, but more research is required to elucidate their participation in the primary cilium-mediated sensing of FSS in normal and pathological conditions, such as hypertension, atherosclerosis or polycystic kidney disease.
We would like to thank the research staff of the Vascular Physiology Laboratory; the Group of Investigation in Tumor Angiogenesis (GIANT) from the Universidad del Bío-Bío; Group of Research and Innovation in Vascular Health (GRIVAS Health) group for the outstanding discussion of the ideas presented in this manuscript.
Conflict of interest
Source of funding
This study was supported by Fondecyt Regular 1140586; Fondequip EQM140104; DIUBB 166709 3/R and GI 171709/VC.
|eNOS||Endothelial nitric oxide synthase|
|ERK||Extracellular signal-regulated Kinase|
|FGF||Fibroblast growth factor|
|FSS||Fluid shear stress|
|Gli||Glioblastoma transcription factors|
|HH||Hedgehog signaling pathway|
|HGF||Hepatocyte growth factor|
|HUVECs||Human umbilical vein endothelial cells|
|MMDD1||Immortalized macula densa cell line|
|InsP3R||Inositol 1,4,5-triphosphate receptor|
|ift88 or Tg737||Intraflagellar transport particle 88|
|IFT||Intraflagellar transport particles|
|MnTMPyP||Mn(III) tetrakis (1-methyl-4-pyridyl) porphyrin|
|L-NAME||NG-nitro-L-arginine methyl ester|
|Nox||Nicotidamine adenine dinucleotide phosphate oxidase|
|PlGF||Placental growth factor|
|PDGF||Platelet-derived growth factor|
|PKD||Polycystic kidney disease|
|PKB||Protein kinase B|
|PKC||Protein kinase C|
|ROS||Reactive oxygen species|
|siRNA||Small interference RNA|
|VEGF||Vascular endothelial growth factor|
|VSMC||Vascular smooth muscle cells|
|VEGFR-1||Also known as fms-like tyrosine kinase 1 or Flt-1|
|VEGFR-2||Or kinase insert-domain containing receptor, KDR|
Waite L, Fine J. Applied Bio Fluid Mechanics. McGraw Hill Companies, Inc; 2007
Nauli SM, Jin X, Hierck BP. The mechanosensory role of primary cilia in vascular hypertension. International Journal of Vascular Medicine. Hindawi. 2011; 2011:376281. DOI: 10.1155/2011/376281. PubMed PMID: 21748021; PubMed Central PMCID: PMCPMC3124932
Groenendijk BC, Hierck BP, Gittenberger-De Groot AC, Poelmann RE. Development-related changes in the expression of shear stress responsive genes KLF-2, ET-1, and NOS-3 in the developing cardiovascular system of chicken embryos. Developmental Dynamics. 2004; 230(1):57-68. DOI: 10.1002/dvdy.20029. PubMed PMID: 15108309
Van der Heiden K, Groenendijk BC, Hierck BP, Hogers B, Koerten HK, Mommaas AM, et al. Monocilia on chicken embryonic endocardium in low shear stress areas. Developmental Dynamics. 2006; 235(1):19-28. DOI: 10.1002/dvdy.20557. PubMed PMID: 16145662
Cunningham KS, Gotlieb AI. The role of shear stress in the pathogenesis of atherosclerosis. Laboratory Investigation. 2005; 85(1):9-23. DOI: 10.1038/labinvest.3700215. PubMed PMID: 15568038
Baieth HE. Physical parameters of blood as a non-Newtonian fluid. International Journal of Biomedical Science and Engineering. 2008; 4(4):323-329. PubMed PMID: 23675105; PubMed Central PMCID: PMCPMC3614720
Buschmann I, Schaper W. The pathophysiology of the collateral circulation (arteriogenesis). The Journal of Pathology. 2000; 190(3):338-342. DOI: 10.1002/(SICI)1096-9896(200002)190:3<338::AID-PATH594>3.0.CO;2-7. PubMed PMID: 10685067
Resnick N, Yahav H, Shay-Salit A, Shushy M, Schubert S, Zilberman LC, et al. Fluid shear stress and the vascular endothelium: For better and for worse. Progress in Biophysics and Molecular Biology 2003; 81(3):177-199. PubMed PMID: 12732261
Nauli SM, Kawanabe Y, Kaminski JJ, Pearce WJ, Ingber DE, Zhou J. Endothelial cilia are fluid shear sensors that regulate calcium signaling and nitric oxide production through polycystin-1. Circulation. 2008; 117(9):1161-71. DOI: 10.1161/CIRCULATIONAHA.107.710111. PubMed PMID: 18285569; PubMed Central PMCID: PMCPMC3071982
Della-Morte D, Rendek T. The role of shear stress and arteriogenesis in maintaining vascular homeostasis and preventing cerebral atherosclerosis. Brain Circulation. 2015; 1(1):53-62
Wheatley DN, Wang AM, Strugnell GE. Expression of primary cilia in mammalian cells. Cell Biology International. 1996; 20(1):73-81. DOI: 10.1006/cbir.1996.0011. PubMed PMID: 8936410
Praetorius HA, Spring KR. A physiological view of the primary cilium. Annual Review of Physiology. 2005; 67:515-529. DOI: 10.1146/annurev.physiol.67.040403.101353. PubMed PMID: 15709968
Praetorius HA, Spring KR. Removal of the MDCK cell primary cilium abolishes flow sensing. The Journal of Membrane Biology. 2003; 191(1):69-76. DOI: 10.1007/s00232-002-1042-4. PubMed PMID: 12532278
Raghavan V, Weisz OA. Discerning the role of mechanosensors in regulating proximal tubule function. American Journal of Physiology Renal Physiology. 2016; 310(1):F1-F5. DOI: 10.1152/ajprenal.00373.2015. PubMed PMID: 26662200; PubMed Central PMCID: PMC4675802
Carlstrom M, Wilcox CS, Arendshorst WJ. Renal autoregulation in health and disease. Physiological Reviews. 2015; 95(2):405-511. DOI: 10.1152/physrev.00042.2012. PubMed PMID: 25834230; PubMed Central PMCID: PMC4551215
Verghese E, Ricardo SD, Weidenfeld R, Zhuang J, Hill PA, Langham RG, et al. Renal primary cilia lengthen after acute tubular necrosis. Journal of the American Society of Nephrology : JASN. 2009; 20(10):2147-2153. DOI: 10.1681/ASN.2008101105. PubMed PMID: 19608704; PubMed Central PMCID: PMC2754102
Han SJ, Jang HS, Seu SY, Cho HJ, Hwang YJ, Kim JI, et al. Hepatic ischemia/reperfusion injury disrupts the homeostasis of kidney primary cilia via oxidative stress. Biochimica et Biophysica Acta. 2017; 1863(7):1817-1828. DOI: 10.1016/j.bbadis.2017.05.004. PubMed PMID: 28495528
Sanchez I, Dynlacht BD. Cilium assembly and disassembly. Nature Cell Biology. 2016; 18(7):711-717. DOI: 10.1038/ncb3370. PubMed PMID: 27350441; PubMed Central PMCID: PMCPMC5079433
Satir P, Pedersen LB, Christensen ST. The primary cilium at a glance. Journal of Cell Science. 2010; 123(Pt 4):499-503. DOI: 10.1242/jcs.050377. PubMed PMID: 20144997; PubMed Central PMCID: PMCPMC2818190
Rodat-Despoix L, Delmas P. Ciliar functions in the nephron. Pflügers Archiv. 2009; 458(1):179-187. DOI: 10.1007/s00424-008-0632-0. PubMed PMID: 19153764
Wang L, Weidenfeld R, Verghese E, Ricardo SD, Deane JA. Alterations in renal cilium length during transient complete ureteral obstruction in the mouse. Journal of Anatomy. 2008; 213(2):79-85. DOI: 10.1111/j.1469-7580.2008.00918.x. PubMed PMID: 18537851; PubMed Central PMCID: PMCPMC2526103
Sanchez-Duffhues G, de Vinuesa AG, Lindeman JH, Mulder-Stapel A, DeRuiter MC, Van Munsteren C, et al. SLUG is expressed in endothelial cells lacking primary cilia to promote cellular calcification. Arteriosclerosis, Thrombosis, and Vascular Biology. 2015; 35(3):616-627. DOI: 10.1161/ATVBAHA.115.305268. PubMed PMID: 25633317
Van der Heiden K, Hierck BP, Krams R, de Crom R, Cheng C, Baiker M, et al. Endothelial primary cilia in areas of disturbed flow are at the base of atherosclerosis. Atherosclerosis. 2008; 196(2):542-550. DOI: 10.1016/j.atherosclerosis.2007.05.030. PubMed PMID: 17631294
Kowalewsky A. Entwicklungsgeschichte des Amphioxus Lanceolatus. Mém L’Acad Imp Sci St-Pétersboug. 1867; 11:1-17
Adams M. The primary cilium: An orphan organelle finds a home. Nature Education. 2010; 3(9):54
Kozminski KG, Johnson KA, Forscher P, Rosenbaum JL. A motility in the eukaryotic flagellum unrelated to flagellar beating. Proceedings of the National Academy of Sciences of the United States of America. 1993; 90(12):5519-5523. PubMed PMID: 8516294; PubMed Central PMCID: PMCPMC46752
Satir P, Christensen ST. Overview of structure and function of mammalian cilia. Annual Review of Physiology. 2007; 69:377-400. DOI: 10.1146/annurev.physiol.69.040705.141236. PubMed PMID: 17009929
Iomini C, Tejada K, Mo W, Vaananen H, Piperno G. Primary cilia of human endothelial cells disassemble under laminar shear stress. The Journal of Cell Biology. 2004; 164(6):811-817. DOI: 10.1083/jcb.200312133. PubMed PMID: 15024030; PubMed Central PMCID: PMCPMC2172280
AbouAlaiwi WA, Takahashi M, Mell BR, Jones TJ, Ratnam S, Kolb RJ, et al. Ciliary polycystin-2 is a mechanosensitive calcium channel involved in nitric oxide signaling cascades. Circulation Research. 2009; 104(7):860-869. DOI: 10.1161/CIRCRESAHA.108.192765. PubMed PMID: 19265036; PubMed Central PMCID: PMCPMC3085025
Cabral PD, Garvin JL. Luminal flow regulates NO and O2− along the nephron. American Journal of Physiology Renal Physiology. 2011; 300(5):F1047-F1053. DOI: 10.1152/ajprenal.00724.2010. PubMed PMID: 21345976; PubMed Central PMCID: PMC3094045
Dinsmore C, Reiter JF. Endothelial primary cilia inhibit atherosclerosis. EMBO Reports. 2016; 17(2):156-166. DOI: 10.15252/embr.201541019. PubMed PMID: 26769565; PubMed Central PMCID: PMCPMC5290813
Pazour GJ, Dickert BL, Vucica Y, Seeley ES, Rosenbaum JL, Witman GB, et al. Chlamydomonas IFT88 and its mouse homologue, polycystic kidney disease gene tg737, are required for assembly of cilia and flagella. The Journal of Cell Biology. 2000; 151(3):709-718. PubMed PMID: 11062270; PubMed Central PMCID: PMCPMC2185580
Taulman PD, Haycraft CJ, Balkovetz DF, Yoder BK. Polaris, a protein involved in left-right axis patterning, localizes to basal bodies and cilia. Molecular Biology of the Cell. 2001; 12(3):589-599. PubMed PMID: 11251073; PubMed Central PMCID: PMCPMC30966
Hanaoka K, Qian F, Boletta A, Bhunia AK, Piontek K, Tsiokas L, et al. Co-assembly of polycystin-1 and -2 produces unique cation-permeable currents. Nature. 2000; 408(6815):990-994. DOI: 10.1038/35050128. PubMed PMID: 11140688
Tsiokas L, Kim E, Arnould T, Sukhatme VP, Walz G. Homo- and heterodimeric interactions between the gene products of PKD1 and PKD2. Proceedings of the National Academy of Sciences of the United States of America. 1997; 94(13):6965-6970. PubMed PMID: 9192675; PubMed Central PMCID: PMCPMC21268
Nauli SM, Zhou J. Polycystins and mechanosensation in renal and nodal cilia. BioEssays. 2004; 26(8):844-856. DOI: 10.1002/bies.20069. PubMed PMID: 15273987
Mochizuki T, Wu G, Hayashi T, Xenophontos SL, Veldhuisen B, Saris JJ, et al. PKD2, a gene for polycystic kidney disease that encodes an integral membrane protein. Science. 1996; 272(5266):1339-1342. PubMed PMID: 8650545
Abou Alaiwi WA, Lo ST, Nauli SM. Primary cilia: Highly sophisticated biological sensors. Sensors (Basel). 2009; 9(9):7003-7020. DOI: 10.3390/s90907003. PubMed PMID: 22423203; PubMed Central PMCID: PMCPMC3290460
Kathem SH, Mohieldin AM, Nauli SM. The roles of primary cilia in polycystic kidney disease. AIMS Molecular Science. 2014; 1(1):27-46. DOI: 10.3934/molsci.2013.1.27. PubMed PMID: 25599087; PubMed Central PMCID: PMCPMC4296740
Nauli SM, Jin X, AbouAlaiwi WA, El-Jouni W, Su X, Zhou J. Non-motile primary cilia as fluid shear stress mechanosensors. Methods in Enzymology. 2013; 525:1-20. DOI: 10.1016/B978-0-12-397944-5.00001-8. PubMed PMID: 23522462; PubMed Central PMCID: PMCPMC4096622
Yoder BK, Tousson A, Millican L, Wu JH, Bugg CE Jr, Schafer JA, et al. Polaris, a protein disrupted in orpk mutant mice, is required for assembly of renal cilium. American Journal of Physiology Renal Physiology. 2002; 282(3):F541-F552. DOI: 10.1152/ajprenal.00273.2001. PubMed PMID: 11832437
Anyatonwu GI, Ehrlich BE. Calcium signaling and polycystin-2. Biochemical and Biophysical Research Communications. 2004; 322(4):1364-1373. DOI: 10.1016/j.bbrc.2004.08.043. PubMed PMID: 15336985
Anyatonwu GI, Estrada M, Tian X, Somlo S, Ehrlich BE. Regulation of ryanodine receptor-dependent calcium signaling by polycystin-2. Proceedings of the National Academy of Sciences of the United States of America. 2007; 104(15):6454-6459. DOI: 10.1073/pnas.0610324104. PubMed PMID: 17404231; PubMed Central PMCID: PMCPMC1851053
Seeley ES, Nachury MV. The perennial organelle: Assembly and disassembly of the primary cilium. Journal of Cell Science. 2010; 123(Pt 4):511-518. DOI: 10.1242/jcs.061093. PubMed PMID: 20144999; PubMed Central PMCID: PMCPMC2818191
Navarro-Yepes J, Burns M, Anandhan A, Khalimonchuk O, del Razo LM, Quintanilla-Vega B, et al. Oxidative stress, redox signaling, and autophagy: Cell death versus survival. Antioxidants & Redox Signaling. 2014; 21(1):66-85. doi: 10.1089/ars.2014.5837. PubMed PMID: 24483238; PubMed Central PMCID: PMC4048575
Matlung HL, Bakker EN, VanBavel E. Shear stress, reactive oxygen species, and arterial structure and function. Antioxidants & Redox Signaling. 2009; 11(7):1699-1709. DOI: 10.1089/ARS.2008.2408. PubMed PMID: 19186981
Lassegue B, San Martin A, Griendling KK. Biochemistry, physiology, and pathophysiology of NADPH oxidases in the cardiovascular system. Circulation Research. 2012; 110(10):1364-1390. DOI: 10.1161/CIRCRESAHA.111.243972. PubMed PMID: 22581922; PubMed Central PMCID: PMC3365576
De Keulenaer GW, Chappell DC, Ishizaka N, Nerem RM, Alexander RW, Griendling KK. Oscillatory and steady laminar shear stress differentially affect human endothelial redox state: Role of a superoxide-producing NADH oxidase. Circulation Research. 1998; 82(10):1094-1101. PubMed PMID: 9622162
White SJ, Hayes EM, Lehoux S, Jeremy JY, Horrevoets AJ, Newby AC. Characterization of the differential response of endothelial cells exposed to normal and elevated laminar shear stress. Journal of Cellular Physiology. 2011; 226(11):2841-2848. DOI: 10.1002/jcp.22629. PubMed PMID: 21302282; PubMed Central PMCID: PMC3412226
Cai H, Harrison DG. Endothelial dysfunction in cardiovascular diseases: The role of oxidant stress. Circulation Research. 2000; 87(10):840-844. PubMed PMID: 11073878
Thomas DD, Heinecke JL, Ridnour LA, Cheng RY, Kesarwala AH, Switzer CH, et al. Signaling and stress: The redox landscape in NOS2 biology. Free Radical Biology & Medicine. 2015; 87:204-225. DOI: 10.1016/j.freeradbiomed.2015.06.002. PubMed PMID: 26117324; PubMed Central PMCID: PMC4852151
Heo KS, Fujiwara K, Abe J. Disturbed-flow-mediated vascular reactive oxygen species induce endothelial dysfunction. Circulation Journal. 2011; 75(12):2722-2730. PubMed PMID: 22076424; PubMed Central PMCID: PMCPMC3620204
Kim JI, Kim J, Jang HS, Noh MR, Lipschutz JH, Park KM. Reduction of oxidative stress during recovery accelerates normalization of primary cilia length that is altered after ischemic injury in murine kidneys. American Journal of Physiology Renal Physiology. 2013; 304(10):F1283-F1294. DOI: 10.1152/ajprenal.00427.2012. PubMed PMID: 23515720
Wang L, Shen C, Liu H, Wang S, Chen X, Roman RJ, et al. Shear stress blunts tubuloglomerular feedback partially mediated by primary cilia and nitric oxide at the macula densa. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology. 2015; 309(7):R757-R766. DOI: 10.1152/ajpregu.00173.2015. PubMed PMID: 26269519; PubMed Central PMCID: PMCPMC4666931
Plotnikov EY, Kazachenko AV, Vyssokikh MY, Vasileva AK, Tcvirkun DV, Isaev NK, et al. The role of mitochondria in oxidative and nitrosative stress during ischemia/reperfusion in the rat kidney. Kidney International. 2007; 72(12):1493-1502. DOI: 10.1038/sj.ki.5002568. PubMed PMID: 17914353
Nastos C, Kalimeris K, Papoutsidakis N, Tasoulis MK, Lykoudis PM, Theodoraki K, et al. Global consequences of liver ischemia/reperfusion injury. Oxidative medicine and cellular longevity. 2014; 2014:906965. DOI: 10.1155/2014/906965. PubMed PMID: 24799983; PubMed Central PMCID: PMC3995148
Ruiz de Almodovar C, Lambrechts D, Mazzone M, Carmeliet P. Role and therapeutic potential of VEGF in the nervous system. Physiological Reviews. 2009; 89(2):607-648. Epub 2009/04/04. DOI: 10.1152/physrev.00031.2008. PubMed PMID: 19342615
Olsson AK, Dimberg A, Kreuger J, Claesson-Welsh L. VEGF receptor signalling—In control of vascular function. Nature Reviews. Molecular Cell Biology. 2006; 7(5):359-371. Epub 2006/04/25. DOI: 10.1038/nrm1911. PubMed PMID: 16633338
Shibuya M. Differential roles of vascular endothelial growth factor receptor-1 and receptor-2 in angiogenesis. Journal of Biochemistry and Molecular Biology. 2006; 39(5):469-478. Epub 2006/09/28. PubMed PMID: 17002866
Tammela T, Enholm B, Alitalo K, Paavonen K. The biology of vascular endothelial growth factors. Cardiovascular Research. 2005; 65(3):550-563. DOI: 10.1016/j.cardiores.2004.12.002. PubMed PMID: 15664381
Shibuya M. Vascular endothelial growth factor and its receptor system: Physiological functions in angiogenesis and pathological roles in various diseases. Journal of Biochemistry. 2013; 153(1):13-19. Epub 2012/11/23. DOI: 10.1093/jb/mvs136. PubMed PMID: 23172303; PubMed Central PMCID: PMC3528006
Gee E, Milkiewicz M, Haas TL. p38 MAPK activity is stimulated by vascular endothelial growth factor receptor 2 activation and is essential for shear stress-induced angiogenesis. Journal of Cellular Physiology. 2010; 222(1):120-126. DOI: 10.1002/jcp.21924. PubMed PMID: 19774558; PubMed Central PMCID: PMCPMC3947629
Jin ZG, Ueba H, Tanimoto T, Lungu AO, Frame MD, Berk BC. Ligand-independent activation of vascular endothelial growth factor receptor 2 by fluid shear stress regulates activation of endothelial nitric oxide synthase. Circulation Research. 2003; 93(4):354-363. DOI: 10.1161/01.RES.0000089257.94002.96. PubMed PMID: 12893742
dela Paz NG, Melchior B, Frangos JA. Early VEGFR2 activation in response to flow is VEGF-dependent and mediated by MMP activity. Biochemical and Biophysical Research Communications. 2013; 434(3):641-646. DOI: 10.1016/j.bbrc.2013.03.134. PubMed PMID: 23583373; PubMed Central PMCID: PMCPMC4074593
Sulpice E, Plouet J, Berge M, Allanic D, Tobelem G, Merkulova-Rainon T. Neuropilin-1 and neuropilin-2 act as coreceptors, potentiating proangiogenic activity. Blood. 2008; 111(4):2036-2045. DOI: 10.1182/blood-2007-04-084269. PubMed PMID: 18065694
Rossignol M, Gagnon ML, Klagsbrun M. Genomic organization of human neuropilin-1 and neuropilin-2 genes: Identification and distribution of splice variants and soluble isoforms. Genomics. 2000; 70(2):211-222. DOI: 10.1006/geno.2000.6381. PubMed PMID: 11112349
Fantin A, Vieira JM, Plein A, Denti L, Fruttiger M, Pollard JW, et al. NRP1 acts cell autonomously in endothelium to promote tip cell function during sprouting angiogenesis. Blood. 2013; 121(12):2352-2362. DOI: 10.1182/blood-2012-05-424713. PubMed PMID: 23315162; PubMed Central PMCID: PMC3606070
Pinskey JM, Franks NE, McMellen AN, Giger RJ, Allen BL. Neuropilin-1 promotes hedgehog signaling through a novel cytoplasmic motif. The Journal of Biological Chemistry. 2017; 292(37):15192-15204. DOI: 10.1074/jbc.M117.783845. PubMed PMID: 28667171
Hui CC, Angers S. Gli proteins in development and disease. Annual Review of Cell and Developmental Biology. 2011; 27:513-537. DOI: 10.1146/annurev-cellbio-092910-154048. PubMed PMID: 21801010
Burnstock G. Purinergic nerves. Pharmacological Reviews. 1972; 24(3):509-581. PubMed PMID: 4404211
Burnstock G. Purine and pyrimidine receptors. Cellular and Molecular Life Sciences. 2007; 64(12):1471-1483. DOI: 10.1007/s00018-007-6497-0. PubMed PMID: 17375261
Burnstock G. Purinergic Signaling in the cardiovascular system. Circulation Research. 2017; 120(1):207-228. DOI: 10.1161/CIRCRESAHA.116.309726. PubMed PMID: 28057794
Praetorius HA, Leipziger J. Primary cilium-dependent sensing of urinary flow and paracrine purinergic signaling. Seminars in Cell & Developmental Biology. 2013; 24(1):3-10. DOI: 10.1016/j.semcdb.2012.10.003. PubMed PMID: 23085624
Masyuk AI, Gradilone SA, Banales JM, Huang BQ, Masyuk TV, Lee SO, et al. Cholangiocyte primary cilia are chemosensory organelles that detect biliary nucleotides via P2Y12 purinergic receptors. American Journal of Physiology. Gastrointestinal and Liver Physiology. 2008; 295(4):G725-G734. DOI: 10.1152/ajpgi.90265.2008. PubMed PMID: 18687752; PubMed Central PMCID: PMCPMC2575915
Masyuk AI, Masyuk TV, LaRusso NF. Cholangiocyte primary cilia in liver health and disease. Developmental Dynamics. 2008; 237(8):2007-2012. DOI: 10.1002/dvdy.21530. PubMed PMID: 18407555; PubMed Central PMCID: PMCPMC2574848
Jensen ME, Odgaard E, Christensen MH, Praetorius HA, Leipziger J. Flow-induced [Ca2+]i increase depends on nucleotide release and subsequent purinergic signaling in the intact nephron. Journal of the American Society of Nephrology. 2007; 18(7):2062-2070. DOI: 10.1681/ASN.2006070700. PubMed PMID: 17554149
Hayashi T, Mochizuki T, Reynolds DM, Wu G, Cai Y, Somlo S. Characterization of the exon structure of the polycystic kidney disease 2 gene (PKD2). Genomics. 1997; 44(1):131-136. DOI: 10.1006/geno.1997.4851. PubMed PMID: 9286709
The polycystic kidney disease 1 gene encodes a 14 kb transcript and lies within a duplicated region on chromosome 16. The European Polycystic Kidney Disease Consortium. Cell. 1994; 78(4):725. PubMed PMID: 8069919
Leyssac PP, Karlsen FM, Holstein-Rathlou NH, Skott O. On determinants of glomerular filtration rate after inhibition of proximal tubular reabsorption. The American Journal of Physiology. 1994; 266(5 Pt 2):R1544-R1550. PubMed PMID: 8203631
Holstein-Rathlou NH, Marsh DJ. A dynamic model of the tubuloglomerular feedback mechanism. The American Journal of Physiology. 1990; 258(5 Pt 2):F1448-F1459. PubMed PMID: 2337158
Mozaffari MS, Jirakulsomchok S, Shao ZH, Wyss JM. High-NaCl diets increase natriuretic and diuretic responses in salt-resistant but not salt-sensitive SHR. The American Journal of Physiology. 1991; 260(6 Pt 2):F890-F897. PubMed PMID: 2058709
Seney FD Jr, Persson EG, Wright FS. Modification of tubuloglomerular feedback signal by dietary protein. The American Journal of Physiology. 1987; 252(1 Pt 2):F83-F90. PubMed PMID: 3812704
DiBona GF, Rios LL. Mechanism of exaggerated diuresis in spontaneously hypertensive rats. The American Journal of Physiology. 1978; 235(5):409-416. PubMed PMID: 727292
Pollock CA, Lawrence JR, Field MJ. Tubular sodium handling and tubuloglomerular feedback in experimental diabetes mellitus. The American Journal of Physiology. 1991; 260(6 Pt 2):F946-F952. PubMed PMID: 1829330
Ecder T, Schrier RW. Hypertension in autosomal-dominant polycystic kidney disease: Early occurrence and unique aspects. Journal of the American Society of Nephrology: JASN. 2001; 12(1):194-200. PubMed PMID: 11134267
Ecder T, Schrier RW. Cardiovascular abnormalities in autosomal-dominant polycystic kidney disease. Nature Reviews. Nephrology. 2009; 5(4):221-228. DOI: 10.1038/nrneph.2009.13. PubMed PMID: 19322187; PubMed Central PMCID: PMCPMC2720315
Xu C, Rossetti S, Jiang L, Harris PC, Brown-Glaberman U, Wandinger-Ness A, et al. Human ADPKD primary cyst epithelial cells with a novel, single codon deletion in the PKD1 gene exhibit defective ciliary polycystin localization and loss of flow-induced Ca2+ signaling. American Journal of Physiology Renal Physiology. 2007; 292(3):F930-F945. DOI: 10.1152/ajprenal.00285.2006. PubMed PMID: 17090781; PubMed Central PMCID: PMCPMC3586432
Hateboer N, Veldhuisen B, Peters D, Breuning MH, San-Millan JL, Bogdanova N, et al. Location of mutations within the PKD2 gene influences clinical outcome. Kidney International. 2000; 57(4):1444-1451. DOI: 10.1046/j.1523-1755.2000.00989.x. PubMed PMID: 10760080
Haust MD. Endothelial cilia in human aortic atherosclerotic lesions. Virchows Archiv. A, Pathological Anatomy and Histopathology. 1987; 410(4):317-326. PubMed PMID: 3101281
Ainsworth C. Cilia: Tails of the unexpected. Nature. 2007; 448(7154):638-641. DOI: 10.1038/448638a. PubMed PMID: 17687297