Interaction of transcription factors Islet2 and Nr2f1b to control vascular patterning during zebrafish development.

Many regulators controlling arterial identity are well described; however, transcription factors that promote vein identity and vascular patterning have remained largely unknown. We previously identified the transcription factors Islet2 (Isl2) and Nr2f1b required for specification of the vein and tip cell identity mediated by notch pathway in zebrafish. However, the interaction between Isl2 and Nr2f1b is not known. In this study, we report that Nr2f2 plays minor roles on vein and intersegmental vessels (ISV) growth and dissect the genetic interactions among the three transcription factors Isl2, Nr2f1b, and Nr2f2 using a combinatorial knockdown strategy. The double knockdown of isl2/nr2f1b, isl2/nr2f2, and nr2f1b/nr2f2 showed the enhanced defects in vasculature including less completed ISV, reduced veins, and ISV cells. We further tested the genetic relationship among these three transcription factors. We found isl2 can regulate the expression of nr2f1b and nr2f2, suggesting a model where Isl2 functions upstream of Nr2f1b and Nr2f2. We hypothsized that Isl2 and Nr2f1b can function together through cis-regulatory binding motifs. In-vitro luciferase assay results, we showed that Isl2 and Nr2f1b can cooperatively enhance gene expression. Moreover, co-immunoprecipitation results indicated that Isl2 and Nr2f1b interact physically. Together, we showed that the interaction of the Nr2f1b and Nr2f2 transcription factors in combination with the Islet2 play coordinated roles in the vascular development of zebrafish.

WASp controls oriented migration of endothelial cells to achieve functional vascular patterning.

Endothelial cell migration and proliferation are essential for the establishment of a hierarchical organization of blood vessels and optimal distribution of blood. However, how these cellular processes are quantitatively coordinated to drive vascular network morphogenesis remains unknown. Here, using the zebrafish vasculature as a model system, we demonstrate that the balanced distribution of endothelial cells, as well as the resulting regularity of vessel calibre, is a result of cell migration from veins towards arteries and cell proliferation in veins. We identify the Wiskott-Aldrich Syndrome protein (WASp) as an important molecular regulator of this process and show that loss of coordinated migration from veins to arteries upon wasb depletion results in aberrant vessel morphology and the formation of persistent arteriovenous shunts. We demonstrate that WASp achieves its function through the coordination of junctional actin assembly and PECAM1 recruitment and provide evidence that this is conserved in humans. Overall, we demonstrate that functional vascular patterning in the zebrafish trunk is established through differential cell migration regulated by junctional actin, and that interruption of differential migration may represent a pathomechanism in vascular malformations.

MAPKs Are Highly Abundant but Do Not Contribute to α1-Adrenergic Contraction of Rat Saphenous Arteries in the Early Postnatal Period.

Previously, the abundance of p42/44 and p38 MAPK proteins had been shown to be higher in arteries of 1- to 2-week-old compared to 2- to 3-month-old rats. However, the role of MAPKs in vascular tone regulation in early ontogenesis remains largely unexplored. We tested the hypothesis that the contribution of p42/44 and p38 MAPKs to the contraction of peripheral arteries is higher in the early postnatal period compared to adulthood. Saphenous arteries of 1- to 2-week-old and 2- to 3-month-old rats were studied using wire myography and western blotting. The α1-adrenoceptor agonist methoxamine did not increase the phosphorylation level of p38 MAPK in either 1- to 2-week-old or 2- to 3-month-old rats. Accordingly, inhibition of p38 MAPK did not affect arterial contraction to methoxamine in either age group. Methoxamine increased the phosphorylation level of p42/44 MAPKs in arteries of 2- to 3-month-old and of p44 MAPK in 1- to 2-week-old rats. Inhibition of p42/44 MAPKs reduced methoxamine-induced contractions in arteries of 2- to 3-month-old, but not 1- to 2-week-old rats. Thus, despite a high abundance in arterial tissue, p38 and p42/44 MAPKs do not regulate contraction of the saphenous artery in the early postnatal period. However, p42/44 MAPK activity contributes to arterial contractions in adult rats.

Elastic Laminar Reorganization Occurs with Outward Diameter Expansion during Collateral Artery Growth and Requires Lysyl Oxidase for Stabilization.

When a large artery becomes occluded, hemodynamic changes stimulate remodeling of arterial networks to form collateral arteries in a process termed arteriogenesis. However, the structural changes necessary for collateral remodeling have not been defined. We hypothesize that deconstruction of the extracellular matrix is essential to remodel smaller arteries into effective collaterals. Using multiphoton microscopy, we analyzed collagen and elastin structure in maturing collateral arteries isolated from ischemic rat hindlimbs. Collateral arteries harvested at different timepoints showed progressive diameter expansion associated with striking rearrangement of internal elastic lamina (IEL) into a loose fibrous mesh, a pattern persisting at 8 weeks. Despite a 2.5-fold increase in luminal diameter, total elastin content remained unchanged in collaterals compared with control arteries. Among the collateral midzones, baseline elastic fiber content was low. Outward remodeling of these vessels with a 10-20 fold diameter increase was associated with fractures of the elastic fibers and evidence of increased wall tension, as demonstrated by the straightening of the adventitial collagen. Inhibition of lysyl oxidase (LOX) function with ß-aminopropionitrile resulted in severe fragmentation or complete loss of continuity of the IEL in developing collaterals. Collateral artery development is associated with permanent redistribution of existing elastic fibers to accommodate diameter growth. We found no evidence of new elastic fiber formation. Stabilization of the arterial wall during outward remodeling is necessary and dependent on LOX activity.

Arterialization requires the timely suppression of cell growth.

The formation of arteries is thought to occur by the induction of a highly conserved arterial genetic programme in a subset of vessels that will later experience an increase in oxygenated blood flow1,2. The initial steps of arterial specification require both the VEGF and Notch signalling pathways3-5. Here, we combine inducible genetic mosaics and transcriptomics to modulate and define the function of these signalling pathways in cell proliferation, arteriovenous differentiation and mobilization. We show that endothelial cells with high levels of VEGF or Notch signalling are intrinsically biased to mobilize and form arteries; however, they are not genetically pre-determined, and can also form veins. Mechanistically, we found that increased levels of VEGF and Notch signalling in pre-arterial capillaries suppresses MYC-dependent metabolic and cell-cycle activities, and promotes the incorporation of endothelial cells into arteries. Mosaic lineage-tracing studies showed that endothelial cells that lack the Notch-RBPJ transcriptional activator complex rarely form arteries; however, these cells regained the ability to form arteries when the function of MYC was suppressed. Thus, the development of arteries does not require the direct induction of a Notch-dependent arterial differentiation programme, but instead depends on the timely suppression of endothelial cell-cycle progression and metabolism, a process that precedes arterial mobilization and complete differentiation.

Conduit arterial wave reflection promotes pressure transmission but impedes hydraulic energy transmission to the microvasculature.

Current thinking suggests that wave reflection in arteries limits pulse pressure and hydraulic energy (HE) transmission to the microvasculature and that this protective effect reduces with advancing age. However, according to transmission line theory, pressure transmission (Tp) and reflection (R) coefficients are proportional (Tp = 1 + R), implying that wave reflection would promote rather than limit pressure transmission. We hypothesized that increasing distal pulse pressure (PPd) with age is instead related to increased proximal pulse pressure (PPp) and its forward component and that these are modulated by arterial compliance. A one-dimensional model of a fractal arterial tree containing 21 generations was constructed. Wave speed in each vessel was prescribed to achieve a uniform R at every junction, with changes in R achieved by progressively stiffening proximal or distal vessels. For both stiffening scenarios, decreasing reflection led to a decrease or no change in PPd when forward pressure or compliance were held constant, respectively, suggesting that wave reflection per se does not limit pressure transmission. Proximal pulse pressure, its forward component, and PPd increased with decreasing compliance; furthermore, proximal and distal pulse pressures were approximately proportional. With fixed compliance but decreasing reflection, HE transmission increased, whereas pressure transmission decreased, consistent with transmission line theory. In conclusion, wave reflection does not protect the microvasculature from high PPd; rather, PPp and PPd are modulated by arterial compliance, which reduces with age. Wave reflection has opposing effects on pressure and HE transmission; hence, the relative importance of pressure versus HE in contributing to microvascular damage warrants investigation.NEW & NOTEWORTHY With aging, a reduction in the stiffness gradient between elastic and muscular arteries is thought to reduce wave reflection in conduit arteries, leading to increased pulsatile pressure transmission into the microvasculature. This assumes that wave reflection limits pressure transmission in arteries. However, using a computational model, we showed that wave reflection promotes pulsatile pressure transmission, although it does limit hydraulic energy transmission. Increased microvascular pulse pressure with aging is instead related to decreasing arterial compliance.

TIM­3 inhibits PDGF­BB­induced atherogenic responses in human artery vascular smooth muscle cells.

Increasing evidence suggests that T­cell immunoglobulin and mucin domain 3 (TIM­3) displays anti­atherosclerotic effects, but its role in vascular smooth muscle cells (VSMCs) has not been reported. The present study aimed to investigate the function of TIM­3 and its roles in human artery VSMCs (HASMCs). A protein array was used to investigate the TIM­3 protein expression profile, which indicated that TIM­3 expression was increased in the serum of patients with lower extremity arteriosclerosis obliterans disease (LEAOD) compared with healthy individuals. Immunohistochemistry and western blotting of arterial tissue further revealed that TIM­3 expression was increased in LEAOD artery tissue compared with normal artery tissue. Additionally, platelet­derived growth factor­BB (PDGF­BB) displayed a positive correlation with TIM­3 expression in HASMCs. TIM­3 decreased the migration and proliferation of PDGF­BB­induced HASMCs, and anti­TIM­3 blocked the effects of TIM­3. The effect of TIM­3 on the proliferation and migration of HASMCs was further investigated using LV­TIM­3­transduced cells. The results revealed that TIM­3 also inhibited PDGF­BB­induced expression of the inflammatory factors interleukin­6 and tumor necrosis factor­α by suppressing NF­κB activation. In summary, the present study revealed that TIM­3 displayed a regulatory role during the PDGF­BB­induced inflammatory reaction in HASMCs, which indicated that TIM­3 may display anti­atherosclerotic effects.

Why Should Growth Hormone (GH) Be Considered a Promising Therapeutic Agent for Arteriogenesis? Insights from the GHAS Trial.

Despite the important role that the growth hormone (GH)/IGF-I axis plays in vascular homeostasis, these kind of growth factors barely appear in articles addressing the neovascularization process. Currently, the vascular endothelium is considered as an authentic gland of internal secretion due to the wide variety of released factors and functions with local effects, including the paracrine/autocrine production of GH or IGF-I, for which the endothelium has specific receptors. In this comprehensive review, the evidence involving these proangiogenic hormones in arteriogenesis dealing with the arterial occlusion and making of them a potential therapy is described. All the elements that trigger the local and systemic production of GH/IGF-I, as well as their possible roles both in physiological and pathological conditions are analyzed. All of the evidence is combined with important data from the GHAS trial, in which GH or a placebo were administrated to patients suffering from critical limb ischemia with no option for revascularization. We postulate that GH, alone or in combination, should be considered as a promising therapeutic agent for helping in the approach of ischemic disease.

NKX2-6 related congenital heart disease: Biallelic homeodomain-disrupting variants and truncus arteriosus.

Congenital heart defects (CHD) are the most common birth defect and are both clinically and genetically heterogeneous. Truncus arteriosus (TA), characterized by a single arterial vessel arising from both ventricles giving rise to the coronary, pulmonary and systemic arteries, is rare and only responsible for 1% of all CHD. Two consanguineous families with TA were previously identified to have homozygous nonsense variants within the gene NKX2-6. NKX2-6 is a known downstream target of TBX1, an important transcriptional regulator implicated in the cardiac phenotype of 22q11.2 microdeletion syndrome. Herein, we report two siblings with TA presumably caused by compound heterozygous NKX2-6 variants without a history of consanguinity. Two in-house cohorts with conotruncal defects (CTD) were sequenced for variants in NKX2-6 and no additional cases of biallelic NKX2-6 variants were identified. The similar phenotype of these cases, and the clustering of variants that likely result in a truncated protein that disrupts the homeobox domain, suggest that biallelic loss of function for NKX2-6 is a rare genetic etiology for TA in particular, and possibly other types of CHD.

Exercise-Induced Vascular Adaptations under Artificially Versus Pathologically Reduced Blood Flow: A Focus Review with Special Emphasis on Arteriogenesis.

BACKGROUND: The vascular effects of training under blood flow restriction (BFR) in healthy persons can serve as a model for the exercise mechanism in lower extremity arterial disease (LEAD) patients. Both mechanisms are, inter alia, characterized by lower blood flow in the lower limbs. We aimed to describe and compare the underlying mechanism of exercise-induced effects of disease- and external application-BFR methods. METHODS: We completed a narrative focus review after systematic literature research. We included only studies on healthy participants or those with LEAD. Both male and female adults were considered eligible. The target intervention was exercise with a reduced blood flow due to disease or external application. RESULTS: We identified 416 publications. After the application of inclusion and exclusion criteria, 39 manuscripts were included in the vascular adaption part. Major mechanisms involving exercise-mediated benefits in treating LEAD included: inflammatory processes suppression, proinflammatory immune cells, improvement of endothelial function, remodeling of skeletal muscle, and additional vascularization (arteriogenesis). Mechanisms resulting from external BFR application included: increased release of anabolic growth factors, stimulated muscle protein synthesis, higher concentrations of heat shock proteins and nitric oxide synthase, lower levels in myostatin, and stimulation of S6K1. CONCLUSIONS: A main difference between the two comparators is the venous blood return, which is restricted in BFR but not in LEAD. Major similarities include the overall ischemic situation, the changes in microRNA (miRNA) expression, and the increased production of NOS with their associated arteriogenesis after training with BFR.

Importance of endothelial Hey1 expression for thoracic great vessel development and its distal enhancer for Notch-dependent endothelial transcription.

Thoracic great vessels such as the aorta and subclavian arteries are formed through dynamic remodeling of embryonic pharyngeal arch arteries (PAAs). Previous work has shown that loss of a basic helix-loop-helix transcription factor Hey1 in mice causes abnormal fourth PAA development and lethal great vessel anomalies resembling congenital malformations in humans. However, how Hey1 mediates vascular formation remains unclear. In this study, we revealed that Hey1 in vascular endothelial cells, but not in smooth muscle cells, played essential roles for PAA development and great vessel morphogenesis in mouse embryos. Tek-Cre-mediated Hey1 deletion in endothelial cells affected endothelial tube formation and smooth muscle differentiation in embryonic fourth PAAs and resulted in interruption of the aortic arch and other great vessel malformations. Cell specificity and signal responsiveness of Hey1 expression were controlled through multiple cis-regulatory regions. We found two distal genomic regions that had enhancer activity in endothelial cells and in the pharyngeal epithelium and somites, respectively. The novel endothelial enhancer was conserved across species and was specific to large-caliber arteries. Its transcriptional activity was regulated by Notch signaling in vitro and in vivo, but not by ALK1 signaling and other transcription factors implicated in endothelial cell specificity. The distal endothelial enhancer was not essential for basal Hey1 expression in mouse embryos but may likely serve for Notch-dependent transcriptional control in endothelial cells together with the proximal regulatory region. These findings help in understanding the significance and regulation of endothelial Hey1 as a mediator of multiple signaling pathways in embryonic vascular formation.

A new finite-element shell model for arterial growth and remodeling after stent implantation.

The goal of this paper is to study computationally how blood vessels adapt when they are exposed to a mechanobiological insult, namely, a sudden change of their biomechanical conditions such as proteolytic injuries or implantation. Adaptation occurs through growth and remodeling (G&R), consisting of mass production or removal of structural proteins, such as collagen, until restoring the initial homeostatic biomechanical conditions. In some circumstances, the initial conditions can never be recovered, and arteries evolve towards unstable pathological conditions, such as aneurysms, which are responsible for significant morbidity and mortality. Therefore, computational predictions of G&R under different circumstances can be helpful in understanding fundamentally how arterial pathologies progress. For that, we have developed a low-cost open-source finite-element 2D axisymmetric shell model (FEM) of the arterial wall. The constitutive equations for static equilibrium used to model the stress-strain behavior and the G&R response are expressed within the homogenized constrained mixture theory. The originality is to integrate the layer-specific behavior of both arterial layers (media and adventitia) into the model. Considering different mechanobiological insults, our results show that the resulting arterial dilatation is strongly correlated with the media thickness. The adaptation to stent implantation is particularly interesting. For large stent oversizing ratios, the artery cannot recover from the mechanobiological insult and dilates forever, whereas dilatation stabilizes after a transient period for more moderate oversizing ratios. We also show that stent implantation induces a different response in an aneurysm or in a healthy artery, the latter yielding more unstable G&R. Finally, our G&R model can efficiently predict, with very low computational cost, fundamental aspects of arterial adaptation induced by clinical procedures.

The Extraordinary Role of Extracellular RNA in Arteriogenesis, the Growth of Collateral Arteries.

Arteriogenesis is an intricate process in which increased shear stress in pre-existing arteriolar collaterals induces blood vessel expansion, mediated via endothelial cell activation, leukocyte recruitment and subsequent endothelial and smooth muscle cell proliferation. Extracellular RNA (eRNA), released from stressed cells or damaged tissue under pathological conditions, has recently been discovered to be liberated from endothelial cells in response to increased shear stress and to promote collateral growth. Until now, eRNA has been shown to enhance coagulation and inflammation by inducing cytokine release, leukocyte recruitment, and endothelial permeability, the latter being mediated by vascular endothelial growth factor (VEGF) signaling. In the context of arteriogenesis, however, eRNA has emerged as a transmitter of shear stress into endothelial activation, mediating the sterile inflammatory process essential for collateral remodeling, whereby the stimulatory effects of eRNA on the VEGF signaling axis seem to be pivotal. In addition, eRNA might influence subsequent steps of the arteriogenesis cascade as well. This article provides a comprehensive overview of the beneficial effects of eRNA during arteriogenesis, laying the foundation for further exploration of the connection between the damaging and non-damaging effects of eRNA in the context of cardiovascular occlusive diseases and of sterile inflammation.

Isoform-Specific Roles of ERK1 and ERK2 in Arteriogenesis.

Despite the clinical importance of arteriogenesis, this biological process is poorly understood. ERK1 and ERK2 are key components of a major intracellular signaling pathway activated by vascular endothelial growth (VEGF) and FGF2, growth factors critical to arteriogenesis. To investigate the specific role of each ERK isoform in arteriogenesis, we used mice with a global Erk1 knockout as well as Erk1 and Erk2 floxed mice to delete Erk1 or Erk2 in endothelial cells, macrophages, and smooth muscle cells. We found that ERK1 controls macrophage infiltration following an ischemic event. Loss of ERK1 in endothelial cells and macrophages induced an excessive macrophage infiltration leading to an increased but poorly functional arteriogenesis. Loss of ERK2 in endothelial cells leads to a decreased arteriogenesis due to decreased endothelial cell proliferation and a reduced eNOS expression. These findings show for the first time that isoform-specific roles of ERK1 and ERK2 in the control of arteriogenesis.

Heterogeneous Cellular Contributions to Elastic Laminae Formation in Arterial Wall Development.

RATIONALE: Elastin is an important ECM (extracellular matrix) protein in large and small arteries. Vascular smooth muscle cells (SMCs) produce the layered elastic laminae found in elastic arteries but synthesize little elastin in muscular arteries. However, muscular arteries have a well-defined internal elastic lamina (IEL) that separates endothelial cells (ECs) from SMCs. The extent to which ECs contribute elastin to the IEL is unknown. OBJECTIVE: To use targeted elastin (Eln) deletion in mice to explore the relative contributions of SMCs and ECs to elastic laminae formation in different arteries. METHODS AND RESULTS: We used SMC- and EC-specific Cre recombinase transgenes with a novel floxed Eln allele to focus gene inactivation in mice. Inactivation of Eln in SMCs using Sm22aCre resulted in depletion of elastic laminae in the arterial wall with the exception of the IEL and SMC clusters in the outer media near the adventitia. Inactivation of elastin in ECs using Tie2Cre or Cdh5Cre resulted in normal medial elastin and a typical IEL in elastic arteries. In contrast, the IEL was absent or severely disrupted in muscular arteries. Interruptions in the IEL resulted in neointimal formation in the ascending aorta but not in muscular arteries. CONCLUSIONS: Combined with lineage-specific fate mapping systems, our knockout results document an unexpected heterogeneity in vascular cells that produce the elastic laminae. SMCs and ECs can independently form an IEL in most elastic arteries, whereas ECs are the major source of elastin for the IEL in muscular and resistance arteries. Neointimal formation at IEL disruptions in the ascending aorta confirms that the IEL is a critical physical barrier between SMCs and ECs in the large elastic arteries. Our studies provide new information about how SMCs and ECs contribute elastin to the arterial wall and how local elastic laminae defects may contribute to cardiovascular disease.

[The functions of arterial sympathetic innervation: from development to pathology]. / Les fonctions de l'innervation sympathique artérielle - Du développement à la pathologie.

Arterial sympathetic innervation (ASI) is a complex biological process requiring a fine axonal guidance by arteries. Its physiological impact has remained unknown for decades but recently started to be better understood and recognized. ASI is a key element of the adaptive response of the cardiovascular system to challenging situations (exposure to cold, exercise…) as ASI controls the diameter of resistance arteries, thus blood supply to organs and systemic arterial blood pressure via arterial tone modulation. Defaults in ASI can lead to diseases, acting as a main cause or as an aggravating factor. Its impact is actively studied in cardiovascular diseases representing major public health issues, like hypertension, but ASI could also play a role in aging and many more pathological processes including cancer.

TITLE: Les fonctions de l'innervation sympathique artérielle - Du développement à la pathologie. ABSTRACT: L'innervation sympathique artérielle (ISA) est un processus biologique complexe nécessitant un guidage fin des axones des neurones sympathiques par les artères. L'ISA est un élément clé de l'adaptation du système cardiovasculaire aux différentes contraintes (exposition au froid, exercice, etc.) : elle contrôle le diamètre des artères de résistance, donc le flux sanguin parvenant aux organes et la pression artérielle systémique via la modulation du tonus artériel. Son importance lors du vieillissement et dans de nombreux contextes pathologiques est de mieux en mieux reconnue et comprise. Son intégration à la prise en charge de nombreuses maladies (hypertension, cancer, etc.) permettrait d'en améliorer traitements et pronostic.

In vivo efficacy of endothelial growth medium stimulated mesenchymal stem cells derived from patients with critical limb ischemia.

BACKGROUND: Cell therapy has been proposed for patients with critical limb ischemia (CLI). Autologous bone marrow derived cells (BMCs) have been mostly used, mesenchymal stem cells (MSCs) being an alternative. The aim of this study was to characterize two types of MSCs and evaluate their efficacy. METHODS: MSCs were obtained from CLI-patients BMCs. Stimulated- (S-) MSCs were cultured in endothelial growth medium. Cells were characterized by the expression of cell surface markers, the relative expression of 6 genes, the secretion of 10 cytokines and the ability to form vessel-like structures. The cell proangiogenic properties was analysed in vivo, in a hindlimb ischemia model. Perfusion of lower limbs and functional tests were assessed for 28 days after cell infusion. Muscle histological analysis (neoangiogenesis, arteriogenesis and muscle repair) was performed. RESULTS: S-MSCs can be obtained from CLI-patients BMCs. They do not express endothelial specific markers but can be distinguished from MSCs by their secretome. S-MSCs have the ability to form tube-like structures and, in vivo, to induce blood flow recovery. No amputation was observed in S-MSCs treated mice. Functional tests showed improvement in treated groups with a superiority of MSCs and S-MSCs. In muscles, CD31+ and αSMA+ labelling were the highest in S-MSCs treated mice. S-MSCs induced the highest muscle repair. CONCLUSIONS: S-MSCs exert angiogenic potential probably mediated by a paracrine mechanism. Their administration is associated with flow recovery, limb salvage and muscle repair. The secretome from S-MSCs or secretome-derived products may have a strong potential in vessel regeneration and muscle repair. Trial registration NCT00533104.

Angiogenesis and tissue formation driven by an arteriovenous loop in the mouse.

The rapid vascularisation of biomaterials and artificial tissues is a key determinant for their in vivo viability and ultimately for their integration in a host; therefore promoting angiogenesis and maintaining the newly formed vascular beds has become a major goal of tissue engineering. The arteriovenous loop (AVL) has been an extensively studied platform which integrates microsurgery with cells scaffolds and growth factors to form neotissues. Most AVL studies to date are limited to larger animal models, which are surgically easier to perform, but have inherent limits for the understanding and interrogation of the underlying in vivo mechanisms due the paucity of transgenic models. Here, we demonstrate for the first time in a mouse model the utility of the AVL in the de novo production of vascularized tissue. We also present the combined use of the model with 3D printed chambers, which allow us to dictate size and shape of the tissues formed. This novel platform will allow for an understanding of the fundamental mechanisms involved in tissue generation de novo.

Isl-1 positive pharyngeal mesenchyme subpopulation and its role in the separation and remodeling of the aortic sac in embryonic mouse heart.

BACKGROUND: Second heart field cells and neural crest cells have been reported to participate in the morphogenesis of the pharyngeal arch arteries (PAAs); however, how the PAAs grow out and are separated from the aortic sac into left and right sections is unknown. RESULTS: An Isl-1 positive pharyngeal mesenchyme protrusion in the aortic sac ventrally extends and fuses with the aortic sac wall to form a midsagittal septum that divides the aortic sac. The aortic sac division separates the left and right PAAs to form independent arteries. The midsagittal septum dividing the aortic sac has a different expression pattern from the aortic-pulmonary (AP) septum in which Isl-1 positive cells are absent. At 11 days post-conception (dpc) in a mouse embryo, the Isl-1 positive mesenchyme protrusion appears as a heart-shaped structure, in which subpopulations with Isl-1+ Tbx3+ and Isl-1+ Nkx2.5+ cells are included. CONCLUSIONS: The aortic sac is a dynamic structure that is continuously divided during the migration from the pharyngeal mesenchyme to the pericardial cavity. The separation of the aortic sac is not complete until the AP septum divides the aortic sac into the ascending aorta and pulmonary trunk. Moreover, the midsagittal septum and the AP septum are distinct structures.

Chronic administration of minoxidil protects elastic fibers and stimulates their neosynthesis with improvement of the aorta mechanics in mice.

Arterial wall elastic fibers, made of 90% elastin, are arranged into elastic lamellae which are responsible for the resilience and elastic properties of the large arteries (aorta and its proximal branches). Elastin is synthesized only in early life and adolescence mainly by the vascular smooth muscles cells (VSMC) through the cross-linking of its soluble precursor, tropoelastin. In normal aging, the elastic fibers become fragmented and the mechanical load is transferred to collagen fibers, which are 100-1000 times stiffer than elastic fibers. Minoxidil, an ATP-dependent K+ channel opener, has been shown to stimulate elastin expression in vitro, and in vivo in the aorta of male aged mice and young adult hypertensive rats. Here, we have studied the effect of a 3-month chronic oral treatment with minoxidil (120 mg/L in drinking water) on the abdominal aorta structure and function in adult (6-month-old) and aged (24-month-old) male and female mice. Our results show that minoxidil treatment preserves elastic lamellae integrity at both ages, which is accompanied by the formation of newly synthesized elastic fibers in aged mice. This leads to a generally decreased pulse pressure and a significant improvement of the arterial biomechanical properties in female mice, which present an increased distensibility and a decreased rigidity of the aorta. Our studies show that minoxidil treatment reversed some of the major adverse effects of arterial aging in mice and could be an interesting anti-arterial aging agent, also potentially usable for female-targeted therapies.