**Recent Advances Concerning the Molecular Mechanism of Patent Ductus Arteriosus**

Susumu Minamisawa1 and Utako Yokoyama2 *1Waseda University 2Yokohama City University Japan* 

#### **1. Introduction**

84 Congenital Heart Disease – Selected Aspects

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LH.; Ansink, TJ.; Cozijnsen, L.; Pieper, PG.; Meijboom, FJ.; Grobbee, DE. & Mulder, BJ. (2010). Circumstances of death in adult congenital heart disease. *International*  The ductus arteriosus (DA), a fetal arterial shunt between the main pulmonary artery and the descending aorta, is a normal and essential fetal structure. Normally, the DA begins to close immediately after birth, but in some cases it remains patent after birth. Postnatal patent DA (PDA) is a major cause of morbidity and mortality in premature infants, leading to severe complications including pulmonary hypertension, right ventricular dysfunction, postnatal infections, and respiratory failure (Hermes-DeSantis & Clyman, 2006). The incidence of PDA among full-term newborns has been estimated at one in 500, and in preterm newborns it accounts for the majority of all congenital heart disease cases (Mitchell, 1971). The incidence of PDA exceeds 30% in preterm babies with birth weights <1,500 g (Van Overmeire, 2004). Curiously, patent DA can be essential for patients with complex congenital heart diseases in which the systemic or pulmonary circulation is dependent on the passage of blood through the DA. Therefore, a thorough understanding of the precise molecular mechanism underlying DA closure is very important in pediatric cardiovascular medicine.

Closure of the human DA occurs in two phases: functional closure of the lumen within the first hours after birth by smooth muscle constriction, and anatomic occlusion of the lumen over the next several days due to extensive neointimal thickening and vascular remodelling. Although this overall process is similar among all mammals, the time course of the two phases varies among species.

DA constriction after birth is induced by an increase in arterial oxygen tension, a dramatic decline in circulating prostaglandinE2 (PGE2), and a decrease in blood pressure within the DA lumen (Smith 1998; Clyman 2006). Anatomical closure of the DA is associated with a unique system of differentiation of the vessel wall. The most prominent phenotypic change is intimal thickening, a process characterized by (a) an area of subendothelial deposition of extracellular matrix, (b) the disassembly of the internal elastic lamina and loss of elastic fiber in the medial layer, and (c) the migration of undifferentiated medial smooth muscle cells (SMCs) into the subendothelial space. The DA later undergoes permanent closure through structural remodelling and fibrosis. The resulting fibrous band with no lumen persists in the adult as the ligamentum arteriosum (Fay & Cooke 1972). The cascade of events is thought to orchestrate the activation of subsequent signalling pathways, leading finally to the complete obliteration of the DA. In this chapter, we focus on reviewing the current state of knowledge regarding the mechanisms by which vascular remodelling of the DA is regulated.

Recent Advances Concerning the Molecular Mechanism of Patent Ductus Arteriosus 87

literature why COX deletion causes PDA in mice, we assume that the same mechanism should work as described below in mice harboring deletion of the EP4 gene, a predominant PGE2 receptor in the DA. Trivedi et al. have demonstrated that COX2 expression is attenuated in EP4-deleted mice (Trivedi,. 2006), suggesting the existence of a positive

Dogs have been studied as an animal model of inherited PDA because their histological features of normal DA and PDA closely resemble those of humans. Through such studies, De Reeder et al. have demonstrated that the expression of PGI2 synthase is high in the endothelium and low in vascular SMCs in PDA and other patent neighboring arteries. In normally closing DA, in contrast, high amounts of PGI2 are found in the vascular SMCs of the intimal cushions, suggesting that PGI2 plays a role in the onset of intimal thickening (de

PGE2, the most potent vasodilator affecting the DA, is produced in the placenta (Smith 1998) and in the DA itself (Clyman, 1978; Coceani, 1978). During gestation, PGE2 contributes to DA patency *in utero*. Stimulation of PGE2 receptors activates adenylyl cyclases (ACs). The resulting increased intracellular concentrations of cyclic AMP (cAMP) inhibit myosin light chain kinase, inducing DA relaxation (Smith 1998). The dilator effect of PGE2 on the mammalian DA is mediated mainly by the PGE2 receptor, EP4. After birth, the concentration of circulating PGE2 declines dramatically as the placenta is removed and PGE2 is rapidly catabolised through lung circulation. Furthermore, the expression levels of PGE2 receptors

Although PGE2 plays a primary role in maintaining the patency of the DA, previous studies have demonstrated that genetic disruption of EP4 paradoxically results in fatal PDA in mice (Nguyen, 1997; Segi, 1998). We have found that intimal thickening was completely absent in DA from EP4-disrupted neonatal mice (Yokoyama, 2006a). Moreover, a marked reduction in hyaluronan production was found in EP4-disrupted DA, whereas a thick layer of hyaluronan deposit was present in wild-type DA. PGE2-EP4-cAMP-protein kinase A (PKA) signalling up-regulates hyaluronan synthase type 2 mRNA, which increases hyaluronan production in the DA. Accumulation of hyaluronan then promotes SMCs migration into the subendothelial layer to induce intimal thickening (Yokoyama, 2006a). Therefore, signalling through PGE2-EP4 plays two essential roles in DA development, namely, vascular dilation

Chronic PGE2-EP4-AC-cAMP-PKA signalling during gestation induces vascular remodelling of the DA and thereby promotes hyaluronan-mediated intimal thickening and structural closure of the vascular lumen. Both PGE1 and PGE2 also induce vasodilation in the DA. Since intracellular cAMP is synthesized by ACs, which are transmembrane enzymes activated by G protein-coupled receptors, including PGE receptors, ACs must play an important role in regulating vasodilation and remodelling in the DA. To date, nine different isoforms of membrane-bound forms of ACs (AC1 through AC9) have been identified in vertebrate tissues. Most tissues express several AC isoforms, which exhibit remarkable diversity in their biochemical properties (Sunahara & Taussig 2002). Since SMCs in the DA exhibit biological

**2.2.3 PGE2 – EP4: A critical player in regulating intimal thickening** 

**2.2.4 Specific adenylyl cyclases regulate intimal thickening of the DA** 

feedback loop in COX-PGE2 cascades.

are decreased in the DA wall (Smith 1998).

and intimal thickening.

**2.2.2 Prostacyclin (PGI2)** 

Reeder, 1989).
