2. Structure of the primary cilium

1. Introduction

to their initial viscosity state [1].

90 Endothelial Dysfunction - Old Concepts and New Challenges

vascular homeostasis [5].

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

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 [2]. 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 [2–5]. 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 [6]. 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

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 [5].

Several structures and processes have been implicated in FSS mechanotransduction into specific biochemical signals, intracellular signaling pathways and gene modulation [5]. Among those structures implicated, the primary cilium emerges as a key sensor of FSS under physiological conditions [9]. 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,

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

Table 1. Various types of hemodynamic forces acting on the blood vessel wall.

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 [24]. 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 [25]. The structure and maintenance of the primary cilium are regulated by intraflagellar transport (IFT) particles [26].

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 [27]. Therefore, primary cilia are found in a large number of mammalian cells including stem cells, epithelial and endothelial cells [19]. Their presence was demonstrated in adult vascular system (reviewed in [2]), developing chicken endocardium [4], embryonic mouse aortic endothelium [9], cultured human umbilical vein endothelial cells (HUVECs) [28, 29] and epithelial cells including macula densa [30] or tubular epithelial cells [20]. Nevertheless, alteration in the number, length and structural features has been implicated in pathological conditions such as polycystic kidney disease, atherosclerosis and hypertension, among others [18, 23, 31].

Depending on structural and functional features, five distinct domains were described in the primary cilium [2] (Figure 1):


kinesin II [26], whereas the retrograde movement from the tip back to the cell body is driven by cytoplasmic dynein [32]. The protein polaris is the gene product of the IFT particle 88 (ift88) that in mammalis is homologous to the gene Tg737. This protein is localized to the basal body [26, 33]

Figure 1. Scheme of the primary cilium. Longitudinal section showing the axoneme with the nine doublets of microtubules originating from the basal body. The right part of the figure shows transversal sections of motile and non-motile primary cilia. Note the absence of the central pair of microtubules and dynein arms in the primary cilium. Figure adapted

Sensing Fluid-Shear Stress in the Endothelial System with a Special Emphasis on the Primary Cilium

http://dx.doi.org/10.5772/intechopen.73134

93

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 [2]). The importance of these proteins has been highlighted due to the finding that mutations in pkd1 or pkd2 genes result 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

and is required for ciliogenesis.

with permission of [90].

3.2. Polycystin-1, polycystin-2 and polaris

polycystic kidney disease, hence their name [9].

5. The basal body, the network foundation from which the primary cilium emanates.
