Survival of newborn placental mammals depends on closure of the ductus arteriosus (DA), an arterial connection in the fetus which directs blood away from the pulmonary circulation and towards the placenta where oxygenation occurs. Here we show that morphological changes resulting in closure of the DA in mice are virtually identical to those observed in larger mammals, including humans, and that maintenance of the DA in the open, or patent, state in fetal mice is dependent on prostaglandin synthesis. This requirement is absent in mice lacking the prostaglandin E2 EP4 receptor (EP4(-/-) mice). In EP4(-/-) mice of the 129 strain, remodelling of the DA fails to occur after birth, resulting in a left-to-right shunt of blood and subsequently in death. This suggests that the neonatal drop in prostaglandin E2 that triggers ductal closure is sensed through the EP4 receptor. In contrast, 5% of EP4(-/-) mice of mixed genetic background survive, and selective breeding of these mice leads to a 21% survival rate, suggesting that alleles at other loci can provide an alternative mechanism for ductal closure.

Ephrin (Eph) receptor tyrosine kinases fall into two subclasses (A and B) according to preferences for their ephrin ligands. All published structural studies of Eph receptor/ephrin complexes involve B-class receptors. Here, we present the crystal structures of an A-class complex between EphA2 and ephrin-A1 and of unbound EphA2. Although these structures are similar overall to their B-class counterparts, they reveal important differences that define subclass specificity. The structures suggest that the A-class Eph receptor/ephrin interactions involve smaller rearrangements in the interacting partners, better described by a 'lock-and-key'-type binding mechanism, in contrast to the 'induced fit' mechanism defining the B-class molecules. This model is supported by structure-based mutagenesis and by differential requirements for ligand oligomerization by the two subclasses in cell-based Eph receptor activation assays. Finally, the structure of the unligated receptor reveals a homodimer assembly that might represent EphA2-specific homotypic cell adhesion interactions.

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Mast cells are implicated in the pathogenesis of a broad spectrum of immunological disorders. These cells release inflammatory mediators in response to a number of stimuli, including IgE-Ag complexes. The degranulation of mast cells is modified by PGs. To begin to delineate the pathway(s) used by PGs to regulate mast cell function, we examined bone marrow-derived mast cells (BMMC) cultured from mice deficient in the EP1, EP2, EP3, and EP4 receptors for PGE2. Although BMMCs express all four of these PGE2 receptors, potentiation of Ag-stimulated degranulation and IL-6 cytokine production by PGE2 is dependent on the EP3 receptor. Consistent with the coupling of this receptor to Gαi, PGE2 activation of the EP3 receptor leads to both inhibition of adenylate cyclase and increased intracellular Ca2+. The magnitude of increase in intracellular Ca2+ induced by EP3 activation is similar to that observed after activation of cells with IgE and Ag. Although PGE alone is not sufficient to initiate BMMC degranulation, stimulation of cells with PGE along with PMA induces degranulation. These actions are mediated by the EP3 receptor through signals involving Ca2+ mobilization and/or decreased cAMP levels. Accordingly, these studies identify PGE2/EP3 as a proinflammatory signaling pathway that promotes mast cell activation.

The transition to pulmonary respiration following birth requires rapid alterations in the structure of the mammalian cardiovascular system. One dramatic change that occurs is the closure and remodeling of the ductus arteriosus (DA), an arterial connection in the fetus that directs blood flow away from the pulmonary circulation. A role for prostaglandins in regulating the closure of this vessel has been supported by pharmacological and genetic studies. The production of prostaglandins is dependent on two cyclooxygenases (COX-1 and COX-2), which are encoded by separate genes. We report here that the absence of either or both COX isoforms in mice does not result in premature closure of the DA in utero. However, 35% of COX-2(؊͞؊) mice die with a patent DA within 48 h of birth. In contrast, the absence of only the COX-1 isoform does not affect closure of the DA. The mortality (35%) and patent DA incidence due to absence of COX-2 is, however, significantly increased (79%) when one copy of the gene encoding COX-1 is also inactivated. Furthermore, 100% of the mice deficient in both isoforms die with a patent DA within 12 h of birth, indicating that in COX-2-deficient mice, the contribution of COX-1 to DA closure is gene dosage-dependent. Together, these data establish roles for COX-1, and especially for COX-2, in the transition of the cardiopulmonary circulation at birth. T he ductus arteriosus (DA) is an arterial connection in the fetus between the pulmonary artery and the aorta. The DA directs deoxygenated blood away from the pulmonary circulation toward the descending aorta and to the umbilicoplacental circulation where oxygenation occurs. The DA plays a critical role in the cardiovascular physiology of the fetus and newborn (for review see ref. 1). In utero patency of the DA is essential for proper fetal health, and premature DA closure causes pulmonary hypertension, congestive heart failure, and edema. In contrast, failure of the DA to close after birth, designated patent DA, compromises postnatal health by contributing to respiratory complications, including pulmonary hypertension and edema (2).A family of lipid mediators known as prostaglandins (PGs) are among the factors that have been shown to influence the tone of the DA. The initial reaction in the synthesis of all PGs is catalyzed by prostaglandin G͞H synthase, also known as cyclooxygenase (COX), two isoforms of which have been identified. Both COX-1 and COX-2 catalyze the synthesis of PGH 2 , a product required for the formation of the various biologically active PGs (3). Individual PGs act through specific receptors to mediate their biological effects. The role for PGs in regulation of DA tone was initially determined from the observation that nonsteroidal anti-inflammatory drugs (NSAIDs), which act by inhibiting COX (4), modulate DA tone in utero and following birth (1).The dilation of the DA in utero is an active process maintained primarily by PGE 2 . The PG receptors that may have a role in dilation of the DA include the PGE 2 receptors, EP2 (5) and EP...