**4.4 Cell proliferation**

*Basic and Clinical Understanding of Microcirculation*

part to play in the wound healing process.

**4.3 Dopamine signaling**

effects in peripheral arteries [77].

activation [68]. Moreover, data from recent studies on endothelial progenitor cells indicate that platelet-derived growth factor AA (PDGF-AA) might contribute a vital role in wound healing, possibly by its effects on angiogenesis through the PI3K/ Akt/eNOS signaling pathway [69]. Bone morphological protein (BMP) receptor II (BMPRII), which is highly expressed on endothelial cells in lung vasculature, as well as moderately expressed in smooth muscle, is also involved in cell wound migration. In this pathway, migration is trigged by the ligands BMP2 and BMP4, which result in eNOS being phosphorylated [70]. While not conclusive, this evidence, when taken all together, suggests that primary cilia may have a significant

When tissues begin to repair themselves after a wound, clots must be dissolved to maintain proper blood flow. This is known as clot retraction and platelet inhibition. NO is known to inhibit platelet aggregation, secretion, adhesion, and fibrinogen binding; all through activation of guanylyl cyclase and cGMP, alongside the inhibition of thromboxane A2. By this mechanism, platelet aggregation and accumulation are reduced, enabling the clot to dissolve, and the wound to heal fully [71–73]. Given the evidence, it is possible that an interaction between primary cilia

Hypertension present in polycystic kidney disease (PKD) patients in the later stages of the disease is made worse by increased kidney volume. However, hypertension can also be seen in children, as well as the early stages of PKD, long before renal function starts to deteriorate. Some evidence suggests that an increase in sympathetic activation occurs in these patients, independently of their kidney function. Dopamine, an endogenous neuronal hormone that acts within the sympathetic nervous system, is confirmed to be involved in the regulation of blood pressure. Abnormal dopamine signaling can lead to hypertensive states in humans. Dopamine receptor 1 (D1) and dopamine receptor 5 (D5) receptors have been found to localize to primary cilia [39, 74–76]. While there are no current therapies that target D1 or D5, some studies using dopamine 1-like receptor subtypes showed vasodilatory

The D5 receptor is thought to have both a chemosensory and mechanosensory role within primary cilia. Subjecting endothelial ciliary knockout cells *pkd1−/−* (lacking PC-1), and *Tg737orpk/orpk* cells that have no cilia, to dopamine under static conditions revealed a significantly subdued calcium influx when compared to the control cells. The researchers contributed this to the presence of underdeveloped cilia in the knockout cells, which would have less D5 receptors on them due to their smaller size. Under flow conditions with added dopamine, the mechanosensory function of the cilia knockout cells was restored, in comparison to the untreated knockout cells. Because calcium influx in these cell lines is associated with eNOS activation, the results of this study suggest a potential restoration of lost vasodilatory responses caused by a failed ciliary induction of NO biosynthesis [74]. There is additional evidence that suggests dopamine receptor 2 (D2) may also localize, or possibly get transported to, the primary cilia [78]. In one study, cerebral vasospasms were reversed with dopamine treatment; but when haloperidol, a D2 selective antagonist drug, was administered, the vasorelaxation failed to occur. It was also reported that, after administration of dopamine, a large increase in eNOS and iNOS expression

was seen, and administration of haloperidol also blocked this effect [79].

D2 is also possibly transported to the primary cilia under specific conditions to mediate NOS activity within cells. Evidence supporting the role of ciliary dopamine receptors in the mediation of NO can be found in Autosomal Dominant Polycystic

and NO could be important in the wound healing and repair processes.

**56**

Primary cilia also help regulate cell proliferation. As stated in the ciliogenesis section, the cilia extends from the basal body, which is composed of mother and daughter centrioles, and cilia are reabsorbed after cell cycle re-entry [12]. In cancerous cell clusters, cilia are missing from the more prolific dividers, which suggests that despite not playing a major role in cell division, primary cilia are important for starting and stopping cell mitosis [81–84].

NO possibly plays a role in cell proliferation as well, in conjunction with primary cilia. NO has been proven to halt the cell cycle by preventing the transition from G1 to S phase, in a dose dependent manner. The spike in NO is caused by an increase in free l-arginine, which is mediated by various cytokines. PC-1 is a known mediator of the JAK/STAT pathway by activating STAT3; when the cytosolic tail of PC-1 is cleaved upon once luminal flow halts, it can coactivate STAT-1, −3, and −6, along with JAK2. The PC-1 tail causes the cells to sensitize to cytokines and growth factor signaling, which then causes an exaggerated cellular response, which could potentially lead to an increase in l-arginine [85, 86]. Through this mechanism, overly prolific cell division would be arrested.

The superfamily of TGF-β signaling provides a fascinating system of cellular crosstalk, in which the effects of the same ligand can be unique depending on the cell type and the physiological conditions. This family is composed of more than 30 different ligand types of the TGF-β-activin-Nodal BMP subfamilies that can activate receptor serine/threonine kinases of types I and II (TGFβRI/II and BMP-RI/II, respectively). Ciliopathies widely overlap with phenotypes associated with aberrant TGF-β/BMP signaling. Prominent examples include structural heart defects associated with congenital heart disease (CHD) [87], suggesting that cardiac primary cilia may contribute to cellular events regulated by TGF-β/BMP signaling events during heart development. Moreover, different components of the TGF-β signalosome, including TGF-βRI, TGF-βRII, SMAD2/3, SMAD4, and SMAD7 are present at the cilia-centrosome axis. In a recent study, Feng et al. concluded that high salt (HS)-induced endothelial dysfunction and the development of salt-dependent increases in blood pressure (BP) were related to endothelial TGF-β signaling. Specifically, TGF-β-dependent ALK5 signaling increases endothelial NADPH oxidase-4 (NOX4), an enzyme that produces hydrogen peroxide, which limits NO bioavailability and ultimately promotes increased BP [88]. BMPRII contributes to cell proliferation through its interactions with primary cilia, eNOS, and NO. Using pulmonary artery endothelial cells, studies have shown that stimulation of BMPRII results in eNOS activation. BMPRII ligands BMP2 and BMP4 stimulate eNOS phosphorylation at a regulatory site via activation by protein kinase A. This eNOS

stimulation results in increased NO bioavailability; loss of BMPRII function, therefore, is proposed to contribute to endothelial dysfunction [70].
