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) [57]. 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 insertdomain 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 [59–61].

and postnatal development, as well as in cancer development and tissue homeostasis renewal and repair in adult animals [68]. The HH pathway acts via activation of transcriptional effectors, such as the glioblastoma (Gli) proteins, a family of transcription factors whose target

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

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Referring to the primary cilium, studies conducted by Pinskey et al. [68] 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 [68]. 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.

Since early 1970s [70], 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) [71]. 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

ATP is released by almost all cell types after gentle mechanical stimulation and acts in an autocrine or paracrine manner [72]. Living cells under stressful conditions (i.e., hypoxia) or dying cells release ATP [72]. Interestingly, purinergic signaling parallel to flow sensor activity of the primary cilium [73]. Purinergic signaling associated with flow sensing was detected in several structures such as kidney tubules [20], intrahepatic bile ducts [74, 75], endothelial cells [31], among other lining cells. Therefore, deflection of the primary cilium has been related with

The relationship between ATP, purinergic signaling and primary cilium has been studied in the kidney tubular system [73]. The main physiological area that established a relationship

ATP release leading to autocrine or paracrine activation of purinergic receptors.

genes remain enigmatic. The Gli protein family includes Gli1, Gli2 and Gli3 [69].

8. Purinergic receptors and the primary cilium

both iCa2+ and cAMP signaling [71].

Importantly, VEGF and VEGFRs have also been associated with sensing FSS. High expression of VEGF [62] 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 [58]. Therefore, it is not surprising that VEGFR2 triggers NOdependent flow regulation. Jin et al. [63] 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 [29]. Also, in vivo experiments confirmed that VEGFR2 is a key mechanotransducer that activates eNOS in response to blood flow [63]. Despite these evidences, as far as we known there is no information related to VEGFRs present in the primary cilium as potential regulator of FSS sensing.
