Ductus arteriosus smooth muscle cell migration on collagen: dependence on laminin and its receptors

[A new therapeutic target for patent ductus arteriosus]

The ductus arteriosus (DA) maintains the fetal circulation by connecting the aorta and pulmonary arteries. Patent ductus arteriosus (PDA) occurs in >70% extremely-low-birth-weight infants. Patients with PDA exhibit circulatory failure, which is caused by left-to-right shunt. The DA immediately contracts after birth in response to the elevation of blood oxygen tension and to the decline in circulating prostaglandin E₂ (PGE₂). Cyclooxygenase inhibitors targeting smooth muscle cell (SMC) contraction represent only pharmacological treatment for PDA. However, it is important for DA anatomical closure that intimal thickening (IT) is appropriately formed between SMC layer and endothelial cells (EC). IT begins to form before the second-trimester and becomes prominent toward the end of third-trimester as an increase in placenta-derived PGE₂. Immature DAs frequently fail to be close due to poorly formed IT. IT consists of extracellular matrices (ECM) and migrated DA-SMCs from the tunica media. A glycoprotein fibulin-1 is expressed in developing cardiovascular system and binds to multiple ECMs. We found that PGE₂ increased fibulin-1 via EP4 in DA-SMCs, and Fbln1-deficient mice exhibited PDA with poor IT formation. Although EP4 is a Gs-coupled GPCR, fibulin-1 was secreted from DA-SMCs through the phospholipase C-protein kinase C-non-canonical NFκB signaling pathway. Fibulin-1 bound to DA-EC-derived versican which is a binding partner of hyaluronan, which promoted directional DA-SMC migration toward ECs and contributed to IT formation in the DA. Fibulin-1 upregulation by the activation of specific downstream pathway of EP4 may serve a new pharmacological strategy for PDA.

Pharmacokinetics of Sirolimus-Eluting Stents Implanted in the Neonatal Arterial Duct

Background—

Sirolimus-eluting stents may have clinical advantages over bare-metal stents in the extremely proliferative environment of the neonatal arterial duct. However, sirolimus has immunosuppressive actions and little is known regarding sirolimus pharmacokinetics in the newborn.

Methods and Results—

This is a retrospective review of sirolimus pharmacokinetics in neonates who underwent sirolimus-eluting stent implantation in the arterial duct for pulmonary blood flow augmentation. Pharmacokinetic parameters were obtained by noncompartmental analysis and by a Bayesian one-compartment nonlinear mixed model. Nine neonates received a single sirolimus-eluting stent with a total sirolimus dose of 245 μg (n=1), 194 μg (n=5), or 143 μg (n=3). Peak sirolimus concentrations were 13.6±4.5 μg/L (24.8 μg/L highest) and clearance was 0.042±0.03 L/hour (noncompartmental analysis) and 0.051 L/hour (95% credible intervals 0.037–0.069, nonlinear mixed model). Sirolimus remained >5 μg/L, the trough level used in oral immunosuppressive therapy, for (95% credible interval) 15.9 (11.4, 22.8), 12.9 (7.6, 19.0), and 8.4 (2.3, 14.5) days for the 245, 194, and 143 μg sirolimus dose stents, respectively. Estimates of the duration of systemic immunosuppression are provided for combinations of 2 stents.

Conclusions—

In neonates after sirolimus-eluting stent implantation, peak sirolimus levels were 20× higher and clearance 30× lower than previously reported in older children and adults. Sirolimus levels were within the immunosuppressive range for a prolonged period, but with no observable clinically significant adverse outcomes.

Transcriptional profiling reveals ductus arteriosus-specific genes that regulate vascular tone

Failure of the ductus arteriosus (DA) to close at birth can lead to serious complications. Conversely, certain profound congenital cardiac malformations require the DA to be patent until corrective surgery can be performed. In each instance, clinicians have a very limited repertoire of therapeutic options at their disposal - indomethacin or ibuprofen to close a patent DA (PDA) and prostaglandin E1 to maintain patency of the DA. Neither treatment is specific to the DA and both may have deleterious off-target effects. Therefore, more therapeutic options specifically targeted to the DA should be considered. We hypothesized the DA possesses a unique genetic signature that would set it apart from other vessels. A microarray was used to compare the genetic profiles of the murine DA and ascending aorta (AO). Over 4,000 genes were differentially expressed between these vessels including a subset of ion channel-related genes. Specifically, the alpha and beta subunits of large-conductance calcium-activated potassium (BKCa) channels are enriched in the DA. Gain- and loss-of-function studies showed inhibition of BKCa channels caused the DA to constrict, while activation caused DA relaxation even in the presence of O2. This study identifies subsets of genes that are enriched in the DA that may be used to develop DA-specific drugs. Ion channels that regulate DA tone, including BKCa channels, are promising targets. Specifically, BKCa channel agonists like NS1619 maintain DA patency even in the presence of O2 and may be clinically useful.

Mechanisms for ductus arteriosus closure

Closure of the ductus arteriosus at birth is a complex phenomenon being conditioned by antenatal events and progressing in preprogrammed steps. Functional at first, narrowing of the vessel is determined by 2 overlapping processes--removal of the prostaglandin E(2)-based relaxation sustaining prenatal patency and activation of a constrictor mechanism by the natural rise in blood oxygen tension. Two schemes have been proposed for oxygen action--one involving a cytochrome P450 hemoprotein (sensor)/endothelin-1 (effector) complex and the other a set of voltage-gated K(+) channels. These proposals, however, are not mutually exclusive. Structural closure follows the constriction through a remodeling process initiated antenatally with the development of intimal cushions and completed postnatally by a host of humoral and mechanical stimuli. Research in this area has already provided clinical applications. Nevertheless, management of premature infants with persistent ductus remains troublesome and calls for an alternative approach to the prostaglandin E(2) inhibitors now in use. Studies in progress on the oxygen-sensing system may lead to a definitive solution for this problem.

Rapamycin-eluting stents in the arterial duct: experimental observations in the pig model

BACKGROUND

Maintaining arterial duct patency by stent implantation may be advantageous in congenital heart disease management algorithms. Rapamycin, an immunosuppressant drug that demonstrates antiproliferative properties and inhibits smooth muscle cell migration, may deter the intimal hyperplasia that occurs during spontaneous closure and after-stent implantation of the arterial duct.

METHODS AND RESULTS

Twenty-eight Yorkshire piglets (7 to 11 days old; weight, 2.2 to 4.9 kg) underwent stent implantation of the arterial duct (rapamycin-eluting (n=14) or bare metal (n=14) stents, 3.5-mm diameter) and were euthanized at 2, 4, and 6 weeks. Dissected arterial ducts were analyzed for lumen diameter, smooth muscle cell, and extracellular matrix components. Isolated arterial duct-derived smooth muscle cells were cultured in the presence or absence of rapamycin. Cellular proliferation rates were assessed by Ki-67 detection and [(3)H]-thymidine incorporation. No significant neointimal proliferation was present in either stent type at 2 weeks. At 4 weeks, the median luminal diameters of the bare metal stents were 87% (P=0.009), 54% (P=0.004), and 77% (P=0.004) that of the drug-eluting stents at the middle and aortic and pulmonary artery ends, respectively. At 6 weeks, the median luminal diameters of the bare metal stents were 0% (P=0.18), 5% (P=0.25), and 61% (P=0.13) that of the drug-eluting stents at the same respective levels. Complete histological occlusion was found in at least 1 level of the lumen in 9 pigs: 1 (17%) in the BMS group at 4 weeks, 5 (83%) in the BMS group at 6 weeks, and 3 (50%) in the DES group at 6 weeks. In vitro studies demonstrated 50%-lower proliferation rates in rapamycin-treated cultures of duct-derived smooth muscle cell cultures (P<0.001).

CONCLUSIONS

Rapamycin has antiproliferative actions on the arterial duct. Drug-eluting stents may be a more efficient tool than current palliative options for maintaining patency in critically duct-dependent states, but there may be a finite time-related benefit.

Mechanisms of neointima formation--lessons from experimental models

The lamina intima of an artery is the region between the endothelial cell surface and the internal elastic lamina, which forms the luminal margin of the media. In humans the intima of atherosclerosis-prone arteries becomes thicker due to accumulation of smooth muscle cells, which originate from the media. The introduction of percutaneous transluminal coronary angioplasty (PTCA) boosted scientific interest in intimal thickening, because restenosis remains an unresolved problem of this intervention. In order to unravel the mechanisms of intimal thickening there is a need for appropriate animal models. A brief overview of these models is given together with factors that control proliferation and/or migration. Despite intensive research on neointima formation, an effective therapy for restenosis has not emerged to date. This may be due to the fact that other processes, such as acute elastic recoil and chronic constrictive remodeling may contribute to lumen narrowing as well. Other limitations of neointima models are related to species and anatomical differences. Most studies are performed in arteries that are either lesion-free, or contain relatively mild plaques, in contrast to the complicated, stenotic lesions that are the substrate for human PTCA. Other differences are the severity of the injury and incorporation of a mural fibrin-rich thrombus. Nevertheless, studies based on superficial injury, like the frequently used balloon denudation model, are useful. There are similarities with angioplasty, such as endothelial cell damage and proliferation of medial and intimal smooth muscle cells. The use of techniques such as differential display, gene transfer and application of antisense oligonucleotides may provide new therapeutic approaches to reduce neointima formation.

Cell-extracellular matrix interactions in the ductus arteriosus and perinatal pulmonary circulation

Our studies and those of others have shown that changes in the extracellular matrix have profound effects on vascular remodeling. In the ductus, increased production of endothelial hyaluronan and smooth muscle cell chondroitin sulfate and fibronectin and impaired elastin fiber assembly are features critical to smooth muscle cell migration into the subendothelium and intimal cushion formation. There is a developmentally orchestrated process that involves post-transcriptional mechanisms of gene regulation. Closure of the ductus arteriosus is associated with further changes in matrix expression and programmed cell death. The changes in the extracellular matrix that induce neointimal formation are also observed in pathological conditions in pulmonary and coronary arteries. Plasticity of the pulmonary circulation in the perinatal period also involves matrix regulation, and processes that prevent the normal decrease in pulmonary vascular resistance will result in impaired matrix regulation, and in the development of structural changes in the pulmonary arteries, including abnormal smooth muscle cell differentiation, hypertrophy and proliferation, which sustain the elevation in pulmonary artery pressure.

Changes in endothelial cell and smooth muscle cell integrin expression during closure of the ductus arteriosus: an immunohistochemical comparison of the fetal, preterm newborn, and full-term newborn rhesus monkey ductus

Anatomical closure of the ductus arteriosus requires normally quiescent luminal endothelial cells and medial smooth muscle cells to migrate into the subendothelial space forming intimal mounds that eventually coalesce and occlude the vessel's lumen. The migration of endothelial cells and smooth muscle cells requires the presence of integrin receptors that interact with the surrounding matrix. We used immunohistochemical staining to examine the repertoires of integrins expressed by endothelial cells and smooth muscle cells during postnatal closure of the ductus arteriosus in full-term and preterm rhesus monkeys. In the fetal ductus, luminal endothelial cells have a limited repertoire of integrins. During postnatal ductus closure, luminal endothelial cells, of both term and preterm monkeys, change their phenotype and express the full repertoire of integrins found on growing capillary endothelial cells (alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha v beta 1, alpha 6 beta 4, and alpha v beta 5). Similarly, during ductus closure, smooth muscle cells of both term and preterm monkeys expand their integrin repertoire to include the alpha 5 beta 1 and alpha v beta 3 integrins; these two integrins have been shown to be essential for smooth muscle cell migration in vitro. These changes in integrin profile occur at the same time the endothelial and smooth muscle cells invade their neighboring compartments. In contrast, preterm monkeys with a persistently patent ductus lumen fail to develop these changes in integrin expression and fail to develop neointimal mounds. No evidence of intimal thickening occurs in the absence of changes in integrin expression. Therefore, endothelial cells and smooth muscle cells change phenotypes to produce the intimal thickening required for ductus closure.

Regulation of beta1-integrin function in cultured human vascular smooth muscle cells

Avidity modulation and function of beta1-integrin receptors in cultured human vascular smooth muscle cells (SMCs) were investigated using monoclonal antibody (mAb) 8A2, which binds to the beta1 subunit of integrin heterodimers and induces a high avidity state. The adhesion of SMCs to extracellular matrix proteins, but not to poly-L-lysine, was enhanced by pretreatment with mAb 8A2. A qualitative alteration of beta1 integrin was assessed with mAb 15/7, which binds to an activation-dependent epitope on the beta1 subunit. Binding of mAb 15/7 was enhanced by mAb 8A2 in a dose-dependent manner. Arg-Gly-Asp peptide and soluble fibronectin also enhanced expression of the 15/7 epitope, suggesting that the 15/7 epitope is closely related to the ligand-occupied state of beta1 integrin. Platelet-derived growth factor (PDGF)-AA and -BB increased SMC adhesion to type I collagen but did not augment mAb 15/7 binding, suggesting that PDGFs increase binding avidity by a postreceptor mechanism. In addition, mAb 8A2 inhibited PDGF-BB-induced SMC migration through Matrigel-coated filters. These results suggest that avidity modulation of beta1 integrin may play an important role in the function of SMCs.

Mapping of network-forming, heparin-binding, and alpha 1 beta 1 integrin-recognition sites within the alpha-chain short arm of laminin-1

Cell-interactive and architecture-forming functions are associated with the short arms of basement membrane laminin-1. To map and characterize these functions, we expressed recombinant mouse laminin-1 alpha-chain extending from the N terminus through one third of domain IIIb. This dumbbell-shaped glycoprotein (r alpha 1(VI-IVb)'), secreted by mammalian cells, was found to possess three activities. 1) Laminin polymerization was quantitatively inhibited by recombinant protein, supporting an alpha-chain role for a three-short arm interaction model of laminin self-assembly. 2) r alpha 1(VI-IVb)' bound to heparin, and the activity was localized to a subfragment corresponding to domain VI by 125I-heparin blotting. 3) PC12 rat pheochromocytoma cells adhered to, and rapidly extended branching neurites on, r alpha 1(VI-IVb)', with adhesion inhibited by alpha 1 and beta 1 integrin chain-specific antibodies. The ability of anti-laminin antibody to block PC12 cell adhesion to laminin was selectively prevented by absorption with r alpha 1(VI-IVb)' or alpha-chain domain VI fragment. This active integrin-recognition site could furthermore be distinguished from a second cryptic alpha 1 beta 1-binding site exposed by heat treatment of fragment P1', a short arm fragment lacking globules. Thus, a polymer-forming, a heparin-binding, and the active alpha 1 beta 1 integrin-recognition site are all clustered at the end of the alpha-chain short arm, the latter two resident solely in domain VI.