Bao Mei Liu

Vasa vasorum hypoperfusion is responsible for medial hypoxia and anatomic remodeling in the newborn lamb ductus arteriosus

Postnatal constriction of the full-term ductus arteriosus produces hypoxia of the muscle media. This is associated with anatomic remodeling (including smooth muscle death) that prevents subsequent reopening. We used late-gestation fetal and neonatal lambs to determine which factors are responsible for the postnatal hypoxia. Hypoxia [measured by 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide technique] and cell death (measured by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling technique) were observed in regions of the constricted ductus wall within 4 h after delivery. Although there was a decrease in ductus luminal flow during the first 6 h after delivery (measured by Doppler transducer), the amount of oxygen delivered to the ductus lumen (3070 ± 1880 μmol O2 · min−1 · g−1) far exceeded the amount of oxygen consumed by the constricted ductus (0.052 ± 0.021 μmol O2 · min−1 · g−1, measured in vitro). Postnatal constriction increased the effective oxygen diffusion distance across the ductus wall to >3× the limit that can be tolerated for normal tissue homeostasis. This was owing to both an increase in the thickness of the ductus (fetus, 1.12 ± 0.20 mm; newborn, 1.60 ± 0.17 mm;p < 0.01) and a marked reduction in vasa vasorum flow (fetus, 0.99 ± 0.44 mL · min−1 · g−1; newborn, 0.21 ± 0.08 mL · min−1 · g−1;p < 0.01). These findings suggest that hypoxic cell death in the full-term ductus is caused primarily by changes in vasa vasorum flow and muscle media thickness and can occur before luminal flow has been eliminated. We speculate that in contrast with the full-term ductus, the preterm ductus is much less likely to develop the degree of hypoxia needed for vessel remodeling inasmuch as it only is capable of increasing its oxygen diffusion distance to 1.3× the maximally tolerated limit.

The role of monocyte-derived cells and inflammation in baboon ductus arteriosus remodeling

Inflammatory processes play a crucial role in the pathogenesis of atherosclerosis and other vascular disorders. We hypothesized that ischemia of the ductus arteriosus might initiate an active inflammatory response that could play a role in ductus remodeling and permanent closure. To test this hypothesis, we studied effects of postnatal ductus construction on inflammatory processes and remodeling in late-gestation fetal and newborn baboons, and preterm newborn baboons. After postnatal ductus constriction, the expression of several genes known to be essential for atherosclerotic remodeling [vascular cell adhesion molecule (VCAM)-1, E-selectin, IL-8, macrophage colony stimulating factor-1, CD154, interferon-γ, IL-6, and tumor necrosis factor-α] was increased in the ductus wall. We were unable to detect intercellular adhesion molecule (ICAM)-1, ICAM-2, P-selectin, monocyte chemoattractant protein-1, or IL-1 by either real-time PCR or immunohistochemistry. VCAM-1, which is newly expressed by luminal cells of the closed ductus, is an important ligand for the mononuclear cell adhesion receptor VLA4. After postnatal constriction, VLA4+ monocytes/macrophages (CD68+ and CD14+) and, to a lesser extent, T-lymphocytes adhered to the ductus wall. Neutrophils and platelets were not observed. The extent of postnatal neointimal remodeling (both endothelial cell layering and subendothelial space thickening) was associated with the degree of mononuclear cell adhesion. Similarly, the extent of vasa vasorum ingrowth correlated with the invasion of CD68+ cells, from the adventitia into the muscle media. Based on these data, we conclude that the inflammatory response following postnatal ductus constriction may be as necessary for ductus remodeling as it is for atherosclerotic remodeling.

ATP depletion and cell death in the neonatal lamb ductus arteriosus.

Postnatal constriction of the full-term ductus arteriosus produces cell death and remodeling of the ductus wall. Using a bioluminescence imaging technique, we found that after birth, the lamb ductus develops ATP, glucose, and glycogen depletion in addition to hypoxia. In vitro studies showed that cell death correlates best with ATP depletion and is most marked when both glucose and oxygen are severely depleted; in addition, the degree of ATP depletion found in vivo is sufficient to account for the extensive degree of cell death that occurs after birth. Under hypoxic conditions, the immature ductus is more capable of preserving its ATP supply than the mature ductus as a result of increased glucose availability, glycogen stores, and glucose utilization. However, the immature ductus is just as susceptible as the mature ductus to ATP depletion when glucose supplies are restricted. The extensive degree of cell death that occurs in the newborn ductus after birth is associated primarily with ATP depletion. The increased glycolytic capacity of the immature ductus may enable it to tolerate episodes of hypoxia and nutrient shortage, making it more resistant to developing postnatal cell death and permanent closure.

Effects of Hypoxia, Hypoglycemia, and Muscle Shortening on Cell Death in the Sheep Ductus Arteriosus

After birth, constriction of the full-term ductus arteriosus produces ischemic hypoxia, caspase activation, DNA fragmentation (>70% of cell nuclei are positive by the terminal deoxynucleotidyl transferase nick-end labeling [TUNEL] technique), and permanent ductus closure. In contrast, the preterm ductus frequently fails to develop these changes. We used the TUNEL technique to examine rings of fetal ductus arteriosus (incubated for 24 h at different oxygen and glucose concentrations) to determine the roles of 1) constriction and shortening, 2) hypoxia, and 3) hypoglycemia in producing cell death. Under controlled conditions, late-gestation ductus rings had a low rate of TUNEL-positive staining (0.6 ± 0.9%) that did not change during muscle shortening. Although hypoxia (6.9 ± 3.5%) and hypoglycemia (2.4 ± 1.9%) increased the incidence of TUNEL-positive staining, only the combination of hypoxia-plus-hypoglycemia increased the incidence to the range found in vivo (83 ± 9.5%). The combination of hypoxia-plus-hypoglycemia was associated with an oligonucleosomal pattern of DNA fragmentation. Under the same experimental conditions, the preterm ductus was capable of developing a similar degree of TUNEL-positive staining as found at term. Although caspase-3 and caspase-7 were activated in rings exposed to hypoxia-plus-hypoglycemia, a nonselective caspase inhibitor, Z-VAD.FMK (which inhibited caspase-3 and caspase-7 cleavage in the rings), did not diminish the degree of TUNEL-positive staining. We hypothesize that the preterm ductus is capable of developing an extensive degree of cell death, if it can develop the same degree of hypoxia and hypoglycemia found in the full-term newborn ductus. We also hypothesize that cell death in the ductus wall may involve pathways that are not dependent on caspase-3 or -7 activation.

Anatomic Closure of the Premature Patent Ductus Arteriosus: The Role of CD14 + /CD163 + Mononuclear Cells and VEGF in Neointimal Mound Formation

Permanent closure of the newborn ductus arteriosus requires the development of neointimal mounds to completely occlude its lumen. VEGF is required for neointimal mound formation. The size of the neointimal mounds (composed of proliferating endothelial and migrating smooth muscle cells) is directly related to the number of VLA4+ mononuclear cells that adhere to the ductus lumen after birth. We hypothesized that VEGF plays a crucial role in attracting CD14+/CD163+ mononuclear cells (expressing VLA4+) to the ductus lumen and that CD14+/CD163+ cell adhesion to the ductus lumen is important for neointimal growth. We used neutralizing antibodies against VEGF and VLA-4+ to determine their respective roles in remodeling the ductus of premature newborn baboons. Anti-VEGF treatment blocked CD14+/CD163+ cell adhesion to the ductus lumen and prevented neointimal growth. Anti-VLA-4 treatment blocked CD14+/CD163+ cell adhesion to the ductus lumen, decreased the expression of PDGF-B (which promotes smooth muscle migration), and blocked smooth muscle influx into the neointimal subendothelial space (despite the presence of increased VEGF in the ductus wall). We conclude that VEGF is necessary for CD14+/CD163+ cell adhesion to the ductus lumen and that CD14+/CD163+ cell adhesion is essential for VEGF-induced expansion of the neointimal subendothelial zone.

Understanding broad-spectrum durable resistance in rice

A long-standing goal in rice disease control is to identify and incorporate broadspectrum durable resistance (BSDR). Although quantitative resistance can potentially contribute to BSDR, neither the genes responsible for quantitative resistance nor the pathways or mechanisms by which they may function to contribute to BSDR are understood. Using varieties that show durable resistance historically, we have identified rice genes that are candidates for contributing to BSDR through co-localization with disease resistance QTLs in mapping studies. Several of these genes are known as disease defense response genes (e.g., oxalate oxidase, chitinase, PR1, etc.), whereas others are of unknown function. Genome-wide expression analyses at critical stages of host-pathogen interactions are also being used to reveal additional genes that may play a role in quantitative resistance. By combining chromosomal segments associated with five different candidate genes by marker-assisted selection, rice lines were produced that exhibited a high level of resistance to rice blast in multilocation trials. The current challenge is to understand if and how these candidate genes contribute to BSDR as well as the allelic variation that accounts for function in some lines but not in others. Targeted gene expression and functional analyses of candidate gene family members, for example, the oxalate oxidase gene families, are being used to focus on gene members involved in BSDR, and to determine what gene structural features are key to involvement. Sequence comparisons are providing clues as to critical allelic variation in rice germplasm. Finally, analysis of mutants exhibiting inappropriate activation of defense pathways is guiding the selection of candidate genes or genic regions. The integration of expression, mapping, and allelic diversity data is expected to unveil genes or gene interactions with significant phenotypic effects that can be used in breeding programs.