Ovine middle cerebral artery characterization and quantification of ultrastructure and other features: changes with development

Vascular Stem Cells and the Role of B-Raf Kinase in Survival, Proliferation, and Apoptosis

Neovascularization is an essential process in organismal development and aging. With aging, from fetal to adult life, there is a significant reduction in neovascularization potential. However, the pathways which play a role in increased neovascularization potential during fetal life are unknown. Although several studies proposed the idea of vascular stem cells (VSCs), the identification and essential survival mechanism are still not clear. In the present study, we isolated fetal VSCs from the ovine carotid artery and identified the pathways involved in their survival. We tested the hypothesis that fetal vessels contain a population of VSCs, and that B-Raf kinase is required for their survival. We conducted viability, apoptotic, and cell cycle stage assays on fetal and adult carotid arteries and isolated cells. To determine molecular mechanisms, we conducted RNAseq, PCR, and western blot experiments to characterize them and identify pathways essential for their survival. Results: A stem cell-like population was isolated from fetal carotid arteries grown in serum-free media. The isolated fetal VSCs contained markers for endothelial, smooth muscle, and adventitial cells, and formed a de novo blood vessel ex vivo. A transcriptomic analysis that compared fetal and adult arteries identified pathway enrichment for several kinases, including B-Raf kinase in fetal arteries. Furthermore, we demonstrated that B-Raf- Signal Transducer and Activator of Transcription 3 (STAT3)-Bcl2 is critical for the survival of these cells. Fetal arteries, but not adult arteries, contain VSCs, and B-Raf-STAT3-Bcl2 plays an important role in their survival and proliferation.

Fetal acute cerebral vasoreactivity to maternal hyperoxia in low-risk pregnancies: a cross-sectional study

OBJECTIVE

To establish whether fetal cerebral vasoreactivity (CVR_O2 ), following maternal hyperoxia, is predicted by fetal cerebral and uteroplacental Doppler pulsatility indices (PI) at baseline, fetal pulmonary vasoreactivity to oxygen (PVR_O2 ), gestational age (GA), or sex.

METHODS

Pulsatility index of middle (MCA), anterior (ACA), posterior cerebral (PCA), umbilical (UA), uterine (UtA), and branch of the pulmonary arteries (PA) were obtained, by ultrasound, before (baseline), during (hyperoxia) and after 15 minutes of maternal administration of 8 L/min of 100% oxygen, through a non-rebreathing face mask, in normal singleton pregnancies within 20 to 38 weeks' gestation. CVR_O2 was defined as changes greater than zero in z score of PI of the cerebral arteries from baseline to hyperoxia. Logistic modeling was applied to identify CVR_O2 predictors.

RESULTS

A total of 97 pregnancies were eligible. In the overall population, median z scores of PI of MCA, ACA, and PCA did not differ between study phases. Based on the logistic model, baseline z scores for cerebral PI and GA were the best predictors of CVR_O2 .

CONCLUSIONS

In low-risk pregnancies, fetal CVR_O2 to hyperoxia does not occur uniformly but depends on cerebral PI and GA at baseline. These findings may provide useful reference points when oxygen is administered in high-risk pregnancies.

Fetal Cerebral Circulation as Target of Maternal Alcohol Consumption

Alcohol (ethanol [EtOH]) is one of the most widely used psychoactive substances worldwide. Alcohol consumption during pregnancy may result in a wide range of morphological and neurodevelopmental abnormalities termed fetal alcohol spectrum disorders (FASD), with the most severe cases diagnosed as fetal alcohol syndrome (FAS). FAS and FASD are not readily curable and currently represent the leading preventable causes of birth defect and neurodevelopmental delay in the United States. The etiology of FAS/FASD remains poorly understood. This review focuses on the effects of prenatal alcohol exposure (PAE) on fetal cerebrovascular function. A brief introduction to the epidemiology of alcohol consumption and the developmental characteristics of fetal cerebral circulation is followed by several sections that discuss current evidence documenting alcohol-driven alterations of fetal cerebral blood flow, artery function, and microvessel networks. The material offers mechanistic insights at the vascular level itself into the pathophysiology of PAE.

Developmental Maturation and Alpha-1 Adrenergic Receptors-Mediated Gene Expression Changes in Ovine Middle Cerebral Arteries

The Alpha Adrenergic Signaling Pathway is one of the chief regulators of cerebrovascular tone and cerebral blood flow (CBF), mediating its effects in the arteries through alpha1-adrenergic receptors (Alpha1AR). In the ovine middle cerebral artery (MCA), with development from a fetus to an adult, others and we have shown that Alpha1AR play a key role in contractile responses, vascular development, remodeling, and angiogenesis. Importantly, Alpha1AR play a significant role in CBF autoregulation, which is incompletely developed in a premature fetus as compared to a near-term fetus. However, the mechanistic pathways are not completely known. Thus, we tested the hypothesis that as a function of maturation and in response to Alpha1AR stimulation there is a differential gene expression in the ovine MCA. We conducted microarray analysis on transcripts from MCAs of premature fetuses (96-day), near-term fetuses (145-day), newborn lambs, and non-pregnant adult sheep (2-year) following stimulation of Alpha1AR with phenylephrine (a specific agonist). We observed several genes which belonged to pro-inflammatory and vascular development/angiogenesis pathway significantly altered in all of the four age groups. We also observed age-specific changes in gene expression–mediated by Alpha1AR stimulation in the different developmental age groups. These findings imply complex regulatory mechanisms of cerebrovascular development.

Fetal Sheep Mesenteric Resistance Arteries: Functional and Structural Maturation

BACKGROUND

Fetal blood pressure increases during late gestation; however, the underlying vascular mechanisms are unclear. Knowledge of the maturation of resistance arteries is important to identify the mechanisms and vulnerable periods for the development of vascular dysfunction in adulthood.

METHODS

We determined the functional and structural development of fetal sheep mesenteric resistance arteries using wire myography and immunohistochemistry.

RESULTS

Media mass and distribution of myosin heavy-chain isoforms showed no changes between 0.7 (100 ± 3 days) and 0.9 (130 ± 3 days) gestation. However, from 0.7 to 0.9 gestation, the resting wall tension increased accompanied by non-receptor-dependent (potassium) and receptor-dependent (noradrenaline; endothelin-1) increases in vasocontraction. Angiotensin II had no contractile effect at both ages. Endothelium-dependent relaxation to acetylcholine and prostaglandin E2 was absent at 0.7 but present at 0.9 gestation. Augmented vascular responsiveness was paralleled by the maturation of sympathetic and sensory vascular innervation. Non-endothelium-dependent relaxation to nitric oxide showed no maturational changes. The expression of vasoregulator receptors/enzymes did not increase between 0.7 and 0.9 gestation.

CONCLUSION

Vascular maturation during late ovine gestation involves an increase in resting wall tension and the vasoconstrictor and vasodilator capacity of the mesenteric resistance arteries. Absence of structural changes in the tunica media and the lack of an increase in vasoregulator receptor/enzyme expression suggest that vasoactive responses are due to the maturation of intracellular pathways at this gestational age.

Mechanical and vasomotor properties of piglet isolated middle cerebral artery

Piglets are often used as experimental models for studying cerebrovascular responses in newborn infants. However, the mechanical characteristics of piglets’ middle cerebral arteries (MCA) are not well characterized. Additionally, the vessels’ response to dopamine, the most commonly used vasopressor in newborns, is not characterized in piglets’ MCA. Finally, the influence of preterm birth on the dopamine response is not known. The aim of this current was to compare by wire myography the active and passive mechanical characteristics and dopamine concentration–response relations of MCAs isolated from preterm and term newborn piglets. Second‐order branches of the MCA with a diameter <400 μm were chosen for study. The active and passive mechanical properties were comparable between vessels from six preterm (90% gestation, n_segments = 11) and nine term (n_segments = 22) newborn piglets. The response to increasing concentrations of dopamine was biphasic, starting with vasodilation in the 1 nmol/L–0.3 μmol/L concentration range followed by vasoconstriction at higher concentrations. The response was very similar between the two groups. In conclusion, the mechanical properties of the MCA as well as the response to dopamine were comparable between term and 90% gestation preterm piglets.

Development of Mechanical and Failure Properties in Sheep Cerebral Arteries

Traumatic brain injury (TBI) is a devastating problem for people of all ages, but the nature of the response to such injury is often different in children than in adults. Cerebral vessel damage and dysfunction are common following TBI, but age-dependent, large-deformation vessel response has not been characterized. Our objective was to investigate the mechanical properties of cerebral arteries as a function of development. Sheep middle cerebral arteries from four age groups (fetal, newborn, juvenile, and adult) were subjected to biaxial loading around physiological conditions and then to failure in the axial direction. Results show little difference among age groups under physiological loading conditions, but response varied significantly with age in response to large axial deformation. Vessels from all age groups reached the same ultimate stretch level, but the amount of stress carried at a given level of stretch increased significantly with age through the developmental period (fetal to juvenile). Our results are the first to identify changes in cerebral vessel response to large deformations with age and may lead to new insights regarding differences in response to TBI with age.

The fetal cerebral circulation: three decades of exploration by the LLU Center for Perinatal Biology

For more than three decades, research programs in the Center of Perinatal Biology have focused on the vascular biology of the fetal cerebral circulation. In the 1980s, research in the Center demonstrated that cerebral autoregulation operated over a narrower pressure range, and was more vulnerable to insults, in fetuses than in adults. Other studies were among the first to establish that compared to adult cerebral arteries, fetal cerebral arteries were more hydrated, contained smaller smooth muscle cells and less connective tissue, and had endothelium less capable of producing NO. Work in the 1990s revealed that pregnancy depressed reactivity to NO in extra-cerebral arteries, but elevated it in cerebral arteries through effects involving changes in cGMP metabolism. Comparative studies verified that fetal lamb cerebral arteries were an excellent model for cerebral arteries from human infants. Biochemical studies demonstrated that cGMP metabolism was dramatically upregulated, but that contraction was far more dependent on calcium influx, in fetal compared to adult cerebral arteries. Further studies established that chronic hypoxia accelerates functional maturation of fetal cerebral arteries, as indicated by increased contractile responses to adrenergic agonists and perivascular adrenergic nerves. In the 2000s, studies of signal transduction established age-dependent roles for PKG, PKC, PKA, ERK, ODC, IP3, myofilament calcium sensitivity, and many other mechanisms. These diverse studies clearly demonstrated that fetal cerebral arteries were functionally quite distinct compared to adult cerebral arteries. In the current decade, research in the Center has expanded to a more molecular focus on epigenetic mechanisms and their role in fetal vascular adaptation to chronic hypoxia, maternal drug abuse, and nutrient deprivation. Overall, the past three decades have transformed thinking about, and understanding of, the fetal cerebral circulation due in no small part to the sustained research efforts by faculty and staff in the Center for Perinatal Biology.

Antenatal maternal long-term hypoxia: acclimatization responses with altered gene expression in ovine fetal carotid arteries

In humans and other species, long-term hypoxia (LTH) during pregnancy can lead to intrauterine growth restriction with reduced body/brain weight, dysregulation of cerebral blood flow (CBF), and other problems. To identify the signal transduction pathways and critical molecules, which may be involved in acclimatization to high altitude LTH, we conducted microarray with advanced bioinformatic analysis on carotid arteries (CA) from the normoxic near-term ovine fetus at sea-level and those acclimatized to high altitude for 110+ days during gestation. In response to LTH acclimatization, in fetal CA we identified mRNA from 38 genes upregulated >2 fold (P<0.05) and 9 genes downregulated >2-fold (P<0.05). The major genes with upregulated mRNA were SLC1A3, Insulin-like growth factor (IGF) binding protein 3, IGF type 2 receptor, transforming growth factor (TGF) Beta-3, and genes involved in the AKT and BCL2 signal transduction networks. Most genes with upregulated mRNA have a common motif for Pbx/Knotted homeobox in the promoter region, and Sox family binding sites in the 3′ un translated region (UTR). Genes with downregulated mRNA included those involved in the P53 pathway and 5-lipoxygenase activating proteins. The promoter region of all genes with downregulated mRNA, had a common 49 bp region with a binding site for DOT6 and TOD6, components of the RPD3 histone deacetylase complex RPD3C(L). We also identified miRNA complementary to a number of the altered genes. Thus, the present study identified molecules in the ovine fetus, which may play a role in the acclimatization response to high-altitude associated LTH.

Cerebral artery signal transduction mechanisms: developmental changes in dynamics and Ca2+ sensitivity

As compared to the adult, the developing fetus and newborn infant are at much greater risk for dysregulation of cerebral blood flow (CBF), with complications such as intraventricular and germinal matrix hemorrhage with resultant neurologic sequelae. To minimize this dysregulation and its consequences presents a major challenge. Although in many respects the fundamental signal transduction mechanisms that regulate relaxation and contraction pathways, and thus cerebrovascular tone and CBF in the immature organism are similar to those of the adult, the individual elements, pathways, and roles differ greatly. Here, we review aspects of these maturational changes of relaxation/contraction mechanisms in terms of both electro-mechanical and pharmaco-mechanical coupling, their biochemical pathways and signaling networks. In contrast to the adult cerebrovasculature, in addition to attenuated structure with differences in multiple cytoskeletal elements, developing cerebrovasculature of fetus and newborn differs in many respects, such as a strikingly increased sensitivity to [Ca(2+)]i and requirement for extracellular Ca(2+) for contraction. In essence, the immature cerebrovasculature demonstrates both "hyper-relaxation" and "hypo-contraction". A challenge is to unravel the manner in which these mechanisms are integrated, particularly in terms of both Ca(2+)-dependent and Ca(2+)-independent pathways to increase Ca(2+) sensitivity. Gaining an appreciation of these significant age-related differences in signal mechanisms also will be critical to understanding more completely the vulnerability of the developing cerebral vasculature to hypoxia and other stresses. Of vital importance, a more complete understanding of these mechanisms promises hope for improved strategies for therapeutic intervention and clinical management of intensive care of the premature newborn.

Depolarization-dependent contraction increase after birth and preservation following long-term hypoxia in sheep pulmonary arteries

Membrane depolarization is critical to pulmonary arterial (PA) contraction. Both L-type Ca²⁺ channels (Ca_L) and Rho-kinase are important signaling components of this process and mitochondrial and non-mitochondrial generated superoxides can be part of the signaling process. Maturation and long-term hypoxia (LTH) each can modify depolarization-dependent contraction and the role of superoxides. By the use of wire myography, we tested the hypothesis that maturation and LTH increase pulmonary arterial reactivity to high-K⁺-induced membrane depolarization through enhancements in the importance of Ca_Land Rho-kinase-dependent pathways. The data show that maturation, but not LTH, increases contraction to 125 mM KCl (high-K⁺) without altering the EC₅₀. High-K⁺-dependent contraction was inhibited to a similar extent in fetal and adult PA by multiple Ca_L blockers, including 10 μM diltiazem, 10 μM verapamil, and 10 μM nifedipine. Postnatal maturation increased the role for 10 μM nifedipine-sensitive Ca_L, and decreased that for 10 μM Y-27632-sensitive Rho-kinase. In all groups, the combination of nifedipine and Y-27632 effectively inhibited high-K⁺ contraction. Tempol (3 mM) but not 100 μM apocynin slightly reduced contraction in arteries from fetal hypoxic and adult normoxic and hypoxic sheep, indicating a limited role for non-mitochondrial derived superoxide to high-K⁺-induced contraction. Western immunoblot for alpha smooth muscle actin indicated small increases in relative abundance in the adult. The data suggest that while Ca_L therapies more effectively vasodilate PA in adults and rho-kinase therapies are more effective in newborns, combination therapies would provide greater efficacy in both young and mature patients regardless of normoxic or hypoxic conditions.

Gene expression in sheep carotid arteries: major changes with maturational development

BACKGROUND

With development from immature fetus to near-term fetus, newborn, and adult, the cerebral vasculature undergoes a number of fundamental changes. Although the near-term fetus is prepared for a transition from an intra- to extra-uterine existence, this is not necessarily the case with the premature fetus, which is more susceptible to cerebrovascular dysregulation. In this study, we tested the hypothesis that the profound developmental and age-related differences in cerebral blood flow are associated with significant underlying changes in gene expression.

METHODS

With the use of oligonucleotide microarray and pathway analysis, we elucidated significant changes in the transcriptome with development in sheep carotid arteries.

RESULTS

As compared with adult, we demonstrate a U-shaped relationship of gene expression in major cerebrovascular network/pathways during early life, e.g., the level of gene expression in the premature fetus and newborn is considerably greater than that of the near-term fetus. Specifically, cell proliferation, growth, and assembly pathway genes were upregulated during early life. In turn, as compared with adult, mitogen-activated protein kinase-extracellular regulated kinase, actin cytoskeleton, and integrin-signaling pathways were downregulated during early life.

CONCLUSION

In cranial vascular smooth muscle, highly significant changes occur in important cellular and signaling pathways with maturational development.