A Titin mutation defines roles for circulation in endothelial morphogenesis

Origins of Aortic Coarctation: A Vascular Smooth Muscle Compartment Boundary Model

Compartment boundaries divide the embryo into segments with distinct fates and functions. In the vascular system, compartment boundaries organize endothelial cells into arteries, capillaries, and veins that are the fundamental units of a circulatory network. For vascular smooth muscle cells (SMCs), such boundaries produce mosaic patterns of investment based on embryonic origins with important implications for the non-uniform distribution of vascular disease later in life. The morphogenesis of blood vessels requires vascular cell movements within compartments as highly-sensitive responses to changes in fluid flow shear stress and wall strain. These movements underline the remodeling of primitive plexuses, expansion of lumen diameters, regression of unused vessels, and building of multilayered artery walls. Although the loss of endothelial compartment boundaries can produce arterial-venous malformations, little is known about the consequences of mislocalization or the failure to form SMC-origin-specific boundaries during vascular development. We propose that the failure to establish a normal compartment boundary between cardiac neural-crest-derived SMCs of the 6th pharyngeal arch artery (future ductus arteriosus) and paraxial-mesoderm-derived SMCs of the dorsal aorta in mid-gestation embryos leads to aortic coarctation observed at birth. This model raises new questions about the effects of fluid flow dynamics on SMC investment and the formation of SMC compartment borders during pharyngeal arch artery remodeling and vascular development.

Cardiac MRI as an Imaging Tool in Titin Variant-Related Dilated Cardiomyopathy

Dilated Cardiomyopathy is a common myocardial disease characterized by dilation and loss of function of one or both ventricles. A variety of etiologies have been implicated including genetic variation. Advancement in genetic sequencing, and diagnostic imaging allows for detection of genetic mutations in sarcomere protein titin (TTN) and high resolution assessment of cardiac function. This review article discusses the role of cardiac MRI in diagnosing dilated cardiomyopathy in patients with TTN variant related cardiomyopathy.

ELTD1 is present in extracellular vesicles derived from endothelial cells as a cleaved extracellular domain which induces in vivo angiogenesis

ELTD1/ADGRL4 is an adhesion GPCR with an important role in angiogenesis. We recently identified a role for ELTD1 in wound repair and inflammation. Activation of ELTD1 in endothelial cells results in a type II EMT to myofibroblast-like cells that have enhanced angiogenic ability. Furthermore, expression of Eltd1 in murine breast cancer cells increases tumour growth by increasing blood vessel size and perfusion and by creating an immunosuppressive microenvironment. As extracellular vesicles (EVs) are known to be involved in vascular development, growth and maturation we investigated the composition and functional effects of the EVs isolated from ELTD1 expressing cells to elucidate their role in these processes. A highly glycosylated form of the extracellular domain (ECD) of ELTD1 is readily incorporated into EVs. Using mass spectrometry-based proteomics we identified proteins that are enriched in ELTD1-EVs and are involved in haemostasis and immune responses. ELTD1 enriched EVs were pro-angiogenic in vivo and in vitro and the presence of the ECD alone induced endothelial sprouting. In endothelial cells experiencing laminar flow, ELTD1 levels were reduced in the EVs when they are quiescent, showing a relationship between ELTD1 and the activation state of the endothelium. Using FACS, we detected a significant increase in vesicular ELTD1 in the plasma of patients with preeclampsia, a condition characterized by endothelial dysfunction. These data confirm a role for ELTD1 in wound repair and inflammation and reveal its potential as a biomarker of vessel dysfunction.

Whole-genome sequencing reveals de-novo mutations associated with nonsyndromic cleft lip/palate

The majority (85%) of nonsyndromic cleft lip with or without cleft palate (nsCL/P) cases occur sporadically, suggesting a role for de novo mutations (DNMs) in the etiology of nsCL/P. To identify high impact protein-altering DNMs that contribute to the risk of nsCL/P, we conducted whole-genome sequencing (WGS) analyses in 130 African case-parent trios (affected probands and unaffected parents). We identified 162 high confidence protein-altering DNMs some of which are based on available evidence, contribute to the risk of nsCL/P. These include novel protein-truncating DNMs in the ACTL6A, ARHGAP10, MINK1, TMEM5 and TTN genes; as well as missense variants in ACAN, DHRS3, DLX6, EPHB2, FKBP10, KMT2D, RECQL4, SEMA3C, SEMA4D, SHH, TP63, and TULP4. Many of these protein-altering DNMs were predicted to be pathogenic. Analysis using mouse transcriptomics data showed that some of these genes are expressed during the development of primary and secondary palate. Gene-set enrichment analysis of the protein-altering DNMs identified palatal development and neural crest migration among the few processes that were significantly enriched. These processes are directly involved in the etiopathogenesis of clefting. The analysis of the coding sequence in the WGS data provides more evidence of the opportunity for novel findings in the African genome.

Modeling Rare Human Disorders in Mice: The Finnish Disease Heritage

The modification of genes in animal models has evidently and comprehensively improved our knowledge on proteins and signaling pathways in human physiology and pathology. In this review, we discuss almost 40 monogenic rare diseases that are enriched in the Finnish population and defined as the Finnish disease heritage (FDH). We will highlight how gene-modified mouse models have greatly facilitated the understanding of the pathological manifestations of these diseases and how some of the diseases still lack proper models. We urge the establishment of subsequent international consortiums to cooperatively plan and carry out future human disease modeling strategies. Detailed information on disease mechanisms brings along broader understanding of the molecular pathways they act along both parallel and transverse to the proteins affected in rare diseases, therefore also aiding understanding of common disease pathologies.

Developmental Perspectives on Arterial Fate Specification

Blood vessel acquisition of arterial or venous fate is an adaptive phenomenon in response to increasing blood circulation during vascular morphogenesis. The past two decades of effort in this field led to development of a widely accepted paradigm of molecular regulators centering on VEGF and Notch signaling. More recent findings focused on shear stress-induced cell cycle arrest as a prerequisite for arterial specification substantially modify this traditional understanding. This review aims to summarize key molecular mechanisms that work in concert to drive the acquisition of arterial fate in two distinct developmental settings of vascular morphogenesis: de novo vasculogenesis of the dorsal aorta and postnatal retinal angiogenesis. We will also discuss the questions and conceptual controversies that potentially point to novel directions of investigation and possible clinical relevance.

Vascular dysfunction and pathology: focus on mechanical forces

The role of mechanical forces is emerging as a new player in the pathophysiologic programming of the cardiovascular system. The ability of the cells to 'sense' mechanical forces does not relate only to perception of movement or flow, as intended traditionally, but also to the biophysical properties of the extracellular matrix, the geometry of the tissues, and the force distribution inside them. This is also supported by the finding that cells can actively translate mechanical cues into discrete gene expression and epigenetic programming. In the present review, we will contextualize these new concepts in the vascular pathologic programming.

Pulmonary Hypoplasia Resulting from Pulmonary Artery Banding in Infancy: A Neonatal Rat Model Study

The aim of this study was to establish a neonatal rat model of decreased pulmonary blood flow (PBF) for studying pulmonary pathophysiological changes in newborn lung development with reduced PBF. Horizontal thoracotomy surgery with banding of the main pulmonary artery (PA) was performed on 30 rats in the PA banding (PAB) group and without banding on another 30 rats in the sham group within 6 h after birth. The body growth and mortality were recorded. Constriction of PA was checked by echocardiography on postnatal day 7 (P7). Lung morphology was assessed with computed tomography scanning and three-dimensional reconstruction. Histological differences of two groups were evaluated using hematoxylin and eosin (H&E) staining, Masson's trichrome staining, TdT-mediated dUTP nick-end labeling assay, and CD31 labeling with microscopic examination. PA ultrasound confirmed the establishment of constriction on P7. Relative to the sham group, the neonates' physical growth, survival fraction, and lung geometry volume were decreased in the PAB group over time (p < 0.05). Histologic appearance with reduced PBF characterized a markedly simplified alveolarization with noted lower radial alveolar count and alveolar septal thickness in the PAB group (p < 0.0001), pulmonary arteries with thinner/uneven membranous layers and smaller lumina. The deficient alveolar capillary bed, enhanced pulmonary collagen deposition, and increased apoptotic alveolar epithelium were significant in the PAB group compared to the sham group (p < 0.0001). A neonatal rat PAB model demonstrated that PBF reduction during early infancy impairs alveolarization and pulmonary microvasculature.

Role of activin receptor-like kinase 1 in vascular development and cerebrovascular diseases

Activin receptor-like kinase 1 (ALK1) is a transmembrane serine/threonine receptor kinase of the transforming growth factor beta (TGFβ) receptor superfamily. ALK1 is specifically expressed in vascular endothelial cells, and its dynamic changes are closely related to the proliferation of endothelial cells, the recruitment of pericytes to blood vessels, and functional differentiation during embryonic vascular development. The pathophysiology of many cerebrovascular diseases is today understood as a disorder of endothelial cell function and an imbalance in the proportion of vascular cells. Indeed, mutations in ALK1 and its co-receptor endoglin are major genetic risk factors for vascular arteriovenous malformation. Many studies have shown that ALK1 is closely related to the development of cerebral aneurysms, arteriovenous malformations, and cerebral atherosclerosis. In this review, we describe the various roles of ALK1 in the regulation of angiogenesis and in the maintenance of cerebral vascular homeostasis, and we discuss its relationship to functional dysregulation in cerebrovascular diseases. This review should provide new perspectives for basic research on cerebrovascular diseases and offer more effective targets and strategies for clinical diagnosis, treatment, and prevention.

Endothelial responses to shear stress in atherosclerosis: a novel role for developmental genes

Flowing blood generates a frictional force called shear stress that has major effects on vascular function. Branches and bends of arteries are exposed to complex blood flow patterns that exert low or low oscillatory shear stress, a mechanical environment that promotes vascular dysfunction and atherosclerosis. Conversely, physiologically high shear stress is protective. Endothelial cells are critical sensors of shear stress but the mechanisms by which they decode complex shear stress environments to regulate physiological and pathophysiological responses remain incompletely understood. Several laboratories have advanced this field by integrating specialized shear-stress models with systems biology approaches, including transcriptome, methylome and proteome profiling and functional screening platforms, for unbiased identification of novel mechanosensitive signalling pathways in arteries. In this Review, we describe these studies, which reveal that shear stress regulates diverse processes and demonstrate that multiple pathways classically known to be involved in embryonic development, such as BMP-TGFβ, WNT, Notch, HIF1α, TWIST1 and HOX family genes, are regulated by shear stress in arteries in adults. We propose that mechanical activation of these pathways evolved to orchestrate vascular development but also drives atherosclerosis in low shear stress regions of adult arteries.

The effects of reduced hemodynamic loading on morphogenesis of the mouse embryonic heart

Development of the embryonic heart involves an intricate network of biochemical and genetic cues to ensure its proper growth and morphogenesis. However, studies from avian and teleost models reveal that biomechanical force, namely hemodynamic loading (blood pressure and shear stress), plays a significant role in regulating heart development. To study how hemodynamic loading impacts development of the mammalian embryonic heart, we utilized mouse embryo culture and manipulation techniques and performed optical projection tomography imaging followed by morphometric analysis to determine how reduced-loading affects heart volume, myocardial thickness, trabeculation and looping. Our results reveal that hemodynamic loading can regulate these features at different thresholds. Intermediate levels of hemodynamic loading are sufficient to promote proper myocardial growth and heart size, but insufficient to promote looping and trabeculation. Whereas, low levels of hemodynamic loading fails to promote proper growth of the myocardium and heart size. These results reveal that the regulation of heart development by biomechanical force is conserved across many vertebrate classes, and this study begins to elucidate how these specific forces regulate development of the mammalian heart.

Abnormal arterial-venous fusions and fate specification in mouse embryos lacking blood flow

The functions of blood flow in the morphogenesis of mammalian arteries and veins are not well understood. We examined the development of the dorsal aorta (DA) and the cardinal vein (CV) in Ncx1 -/- mutants, which lack blood flow due to a deficiency in a sodium calcium ion exchanger expressed specifically in the heart. The mutant DA and CV were abnormally connected. The endothelium of the Ncx1 -/- mutant DA lacked normal expression of the arterial markers ephrin-B2 and Connexin-40. Notch1 activation, known to promote arterial specification, was decreased in mutant DA endothelial cells (ECs), which ectopically expressed the venous marker Coup-TFII. These findings suggest that flow has essential functions in the DA by promoting arterial and suppressing venous marker expression. In contrast, flow plays a lesser role in the CV, because expression of arterial-venous markers in CV ECs was not as dramatically affected in Ncx1 -/- mutants. We propose a molecular mechanism by which blood flow mediates DA and CV morphogenesis, by regulating arterial-venous specification of DA ECs to ensure proper separation of the developing DA and CV.

Genetic epidemiology of titin-truncating variants in the etiology of dilated cardiomyopathy

Heart failure (HF) is a complex clinical syndrome defined by the inability of the heart to pump enough blood to meet the body's metabolic demands. Major causes of HF are cardiomyopathies (diseases of the myocardium associated with mechanical and/or electrical dysfunction), among which the most common form is dilated cardiomyopathy (DCM). DCM is defined by ventricular chamber enlargement and systolic dysfunction with normal left ventricular wall thickness, which leads to progressive HF. Over 60 genes are linked to the etiology of DCM. Titin (TTN) is the largest known protein in biology, spanning half the cardiac sarcomere and, as such, is a basic structural and functional unit of striated muscles. It is essential for heart development as well as mechanical and regulatory functions of the sarcomere. Next-generation sequencing (NGS) in clinical DCM cohorts implicated truncating variants in titin (TTNtv) as major disease alleles, accounting for more than 25% of familial DCM cases, but these variants have also been identified in 2-3% of the general population, where these TTNtv blur diagnostic and clinical utility. Taking into account the published TTNtv and their association to DCM, it becomes clear that TTNtv harm the heart with position-dependent occurrence, being more harmful when present in the A-band TTN, presumably with dominant negative/gain-of-function mechanisms. However, these insights are challenged by the depiction of position-independent toxicity of TTNtv acting via haploinsufficient alleles, which are sufficient to induce cardiac pathology upon stress. In the current review, we provide an overview of TTN and discuss studies investigating various TTN mutations. We also present an overview of different mechanisms postulated or experimentally validated in the pathogenicity of TTNtv. DCM-causing genes are also discussed with respect to non-truncating mutations in the etiology of DCM. One way of understanding pathogenic variants is probably to understand the context in which they may or may not affect protein-protein interactions, changes in cell signaling, and substrate specificity. In this regard, we also provide a brief overview of TTN interactions in situ. Quantitative models in the risk assessment of TTNtv are also discussed. In summary, we highlight the importance of gene-environment interactions in the etiology of DCM and further mechanistic studies used to delineate the pathways which could be targeted in the management of DCM.

A Combined Linkage and Exome Sequencing Analysis for Electrocardiogram Parameters in the Erasmus Rucphen Family Study

Electrocardiogram (ECG) measurements play a key role in the diagnosis and prediction of cardiac arrhythmias and sudden cardiac death. ECG parameters, such as the PR, QRS, and QT intervals, are known to be heritable and genome-wide association studies of these phenotypes have been successful in identifying common variants; however, a large proportion of the genetic variability of these traits remains to be elucidated. The aim of this study was to discover loci potentially harboring rare variants utilizing variance component linkage analysis in 1547 individuals from a large family-based study, the Erasmus Rucphen Family Study (ERF). Linked regions were further explored using exome sequencing. Five suggestive linkage peaks were identified: two for QT interval (1q24, LOD = 2.63; 2q34, LOD = 2.05), one for QRS interval (1p35, LOD = 2.52) and two for PR interval (9p22, LOD = 2.20; 14q11, LOD = 2.29). Fine-mapping using exome sequence data identified a C > G missense variant (c.713C > G, p.Ser238Cys) in the FCRL2 gene associated with QT (rs74608430; P = 2.8 × 10^-4, minor allele frequency = 0.019). Heritability analysis demonstrated that the SNP explained 2.42% of the trait’s genetic variability in ERF (P = 0.02). Pathway analysis suggested that the gene is involved in cytosolic Ca²⁺ levels (P = 3.3 × 10^-3) and AMPK stimulated fatty acid oxidation in muscle (P = 4.1 × 10^-3). Look-ups in bioinformatics resources showed that expression of FCRL2 is associated with ARHGAP24 and SETBP1 expression. This finding was not replicated in the Rotterdam study. Combining the bioinformatics information with the association and linkage analyses, FCRL2 emerges as a strong candidate gene for QT interval.

Neuromuscular Disease Models and Analysis

Neuromuscular diseases can affect the survival of peripheral neurons, their axons extending to peripheral targets, their synaptic connections onto those targets, or the targets themselves. Examples include motor neuron diseases such as Amyotrophic Lateral Sclerosis, peripheral neuropathies such as Charcot-Marie-Tooth diseases, myasthenias, and muscular dystrophies. Characterizing these phenotypes in mouse models requires an integrated approach, examining both the nerve and muscle histologically, anatomically, and functionally by electrophysiology. Defects observed at these levels can be related back to onset, severity, and progression, as assessed by "Quality of life measures" including tests of gross motor performance such as gait or grip strength. This chapter describes methods for assessing neuromuscular disease models in mice, and how interpretation of these tests can be complicated by the inter-relatedness of the phenotypes.

Cardiovascular Patterning as Determined by Hemodynamic Forces and Blood Vessel Genetics

Background

Vascular patterning depends on coordinated timing of arteriovenous specification of endothelial cells and the concomitant hemodynamic forces supplied by the onset of cardiac function. Using a combination of 3D imaging by OPT and embryo registration techniques, we sought to identify structural differences between three different mouse models of cardiovascular perturbation.

Results

Endoglin mutant mice shared a high degree of similarity to Mlc2a mutant mice, which have been shown to have a primary developmental heart defect causing secondary vessel remodeling failures. Dll4 mutant mice, which have well-characterized arterial blood vessel specification defects, showed distinct differences in vascular patterning when compared to the disruptions seen in Mlc2a^-/- and Eng^-/- models. While Mlc2a^-/- and Eng^-/- embryos exhibited significantly larger atria than wild-type, Dll4^-/- embryos had significantly smaller hearts than wild-type, but this quantitative volume decrease was not limited to the developing atrium. Dll4^-/- embryos also had atretic dorsal aortae and smaller trunks, suggesting that the cardiac abnormalities were secondary to primary arterial blood vessel specification defects.

Conclusions

The similarities in Eng^-/- and Mlc2a^-/- embryos suggest that Eng^-/- mice may suffer from a primary heart developmental defect and secondary defects in vessel patterning, while defects in Dll4^-/- embryos are consistent with primary defects in vessel patterning.

Growth and hemodynamics after early embryonic aortic arch occlusion

The majority of severe clinically significant forms of congenital heart disease (CHD) are associated with great artery lesions, including hypoplastic, double, right or interrupted aortic arch morphologies. While fetal and neonatal interventions are advancing, their potential ability to restore cardiac function, optimal timing, location, and intensity required for intervention remain largely unknown. Here, we combine computational fluid dynamics (CFD) simulations with in vivo experiments to test how individual pharyngeal arch artery hemodynamics alter as a result of local interventions obstructing individual arch artery flow. Simulated isolated occlusions within each pharyngeal arch artery were created with image-derived three-dimensional (3D) reconstructions of normal chick pharyngeal arch anatomy at Hamburger-Hamilton (HH) developmental stages HH18 and HH24. Acute flow redistributions were then computed using in vivo measured subject-specific aortic sinus inflow velocity profiles. A kinematic vascular growth-rendering algorithm was then developed and implemented to test the role of changing local wall shear stress patterns in downstream 3D morphogenesis of arch arteries. CFD simulations predicted that altered pressure gradients and flow redistributions were most sensitive to occlusion of the IVth arches. To evaluate these simulations experimentally, a novel in vivo experimental model of pharyngeal arch occlusion was developed and implemented using two-photon microscopy-guided femtosecond laser-based photodisruption surgery. The right IVth arch was occluded at HH18, and resulting diameter changes were followed for up to 24 h. Pharyngeal arch diameter responses to acute hemodynamic changes were predicted qualitatively but poorly quantitatively. Chronic growth and adaptation to hemodynamic changes, however, were predicted in a subset of arches. Our findings suggest that this complex biodynamic process is governed through more complex forms of mechanobiological vascular growth rules. Other factors in addition to wall shear stress or more complex WSS rules are likely important in the long-term arterial growth and patterning. Combination in silico/experimental platforms are essential for accelerating our understanding and prediction of consequences from embryonic/fetal cardiovascular occlusions and lay the foundation for noninvasive methods to guide CHD diagnosis and fetal intervention.

Investigating developmental cardiovascular biomechanics and the origins of congenital heart defects

Innovative research on the interactions between biomechanical load and cardiovascular (CV) morphogenesis by multiple investigators over the past 3 decades, including the application of bioengineering approaches, has shown that the embryonic heart adapts both structure and function in order to maintain cardiac output to the rapidly growing embryo. Acute adaptive hemodynamic mechanisms in the embryo include the redistribution of blood flow within the heart, dynamic adjustments in heart rate and developed pressure, and beat to beat variations in blood flow and vascular resistance. These biomechanically relevant events occur coincident with adaptive changes in gene expression and trigger adaptive mechanisms that include alterations in myocardial cell growth and death, regional and global changes in myocardial architecture, and alterations in central vascular morphogenesis and remodeling. These adaptive mechanisms allow the embryo to survive these biomechanical stresses (environmental, maternal) and to compensate for developmental errors (genetic). Recent work from numerous laboratories shows that a subset of these adaptive mechanisms is present in every developing multicellular organism with a “heart” equivalent structure. This chapter will provide the reader with an overview of some of the approaches used to quantify embryonic CV functional maturation and performance, provide several illustrations of experimental interventions that explore the role of biomechanics in the regulation of CV morphogenesis including the role of computational modeling, and identify several critical areas for future investigation as available experimental models and methods expand.

Vascular development and hemodynamic force in the mouse yolk sac

Vascular remodeling of the mouse embryonic yolk sac is a highly dynamic process dependent on multiple genetic signaling pathways as well as biomechanical factors regulating proliferation, differentiation, migration, cell-cell, and cell-matrix interactions. During this early developmental window, the initial primitive vascular network of the yolk sac undergoes a dynamic remodeling process concurrent with the onset of blood flow, in which endothelial cells establish a branched, hierarchical structure of large vessels and smaller capillary beds. In this review, we will describe the molecular and biomechanical regulators which guide vascular remodeling in the mouse embryonic yolk sac, as well as live imaging methods for characterizing endothelial cell and hemodynamic function in cultured embryos.

Dynamic responses of endothelial cells to changes in blood flow during vascular remodeling of the mouse yolk sac

Despite extensive work showing the importance of blood flow in angiogenesis and vessel remodeling, very little is known about how changes in vessel diameter are orchestrated at the cellular level in response to mechanical forces. To define the cellular changes necessary for remodeling, we performed live confocal imaging of cultured mouse embryos during vessel remodeling. Our data revealed that vessel diameter increase occurs via two distinct processes that are dependent on normal blood flow: vessel fusions and directed endothelial cell migrations. Vessel fusions resulted in a rapid change in vessel diameter and were restricted to regions that experience the highest flow near the vitelline artery and vein. Directed cell migrations induced by blood flow resulted in the recruitment of endothelial cells to larger vessels from smaller capillaries and were observed in larger artery segments as they expanded. The dynamic and specific endothelial cell behaviors captured in this study reveal how sensitive endothelial cells are to changes in blood flow and how such responses drive vascular remodeling.

Mechanotransduction in embryonic vascular development

A plethora of biochemical signals provides spatial and temporal cues that carefully orchestrate the complex process of vertebrate embryonic development. The embryonic vasculature develops not only in the context of these biochemical cues, but also in the context of the biomechanical forces imparted by blood flow. In the mature vasculature, different blood flow regimes induce distinct genetic programs, and significant progress has been made toward understanding how these forces are perceived by endothelial cells and transduced into biochemical signals. However, it cannot be assumed that paradigms that govern the mature vasculature are pertinent to the developing embryonic vasculature. The embryonic vasculature can respond to the mechanical forces of blood flow, and these responses are critical in vascular remodeling, certain aspects of sprouting angiogenesis, and maintenance of arterial-venous identity. Here, we review data regarding mechanistic aspects of endothelial cell mechanotransduction, with a focus on the response to shear stress, and elaborate upon the multifarious effects of shear stress on the embryonic vasculature. In addition, we discuss emerging predictive vascular growth models and highlight the prospect of combining signaling pathway information with computational modeling. We assert that correlation of precise measurements of hemodynamic parameters with effects on endothelial cell gene expression and cell behavior is required for fully understanding how blood flow-induced loading governs normal vascular development and shapes congenital cardiovascular abnormalities.

Extracellular matrix and the mechanics of large artery development

The large, elastic arteries, as their name suggests, provide elastic distention and recoil during the cardiac cycle in vertebrate animals. The arteries are distended from the pressure of ejecting blood during the active contraction of the left ventricle (LV) during systole and recoil to their original dimensions during relaxation of the LV during diastole. The cyclic distension occurs with minimal energy loss, due to the elastic properties of one of the major structural extracellular matrix (ECM) components, elastin. The maximum distension is limited to prevent damage to the artery by another major ECM component, collagen. The mix of ECM components in the wall largely determines the passive mechanical behavior of the arteries and the subsequent load on the heart during systole. While much research has focused on initial artery formation, there has been less attention on the continuing development of the artery to produce the mature composite wall complete with endothelial cells (ECs), smooth muscle cells (SMCs), and the necessary mix of ECM components for proper cardiovascular function. This review focuses on the physiology of large artery development, including SMC differentiation and ECM production. The effects of hemodynamic forces and ECM deposition on the evolving arterial structure and function are discussed. Human diseases and mouse models with genetic mutations in ECM proteins that affect large artery development are summarized. A review of constitutive models and growth and remodeling theories is presented, along with future directions to improve understanding of ECM and the mechanics of large artery development.

Mechanotransduction, PROX1, and FOXC2 cooperate to control connexin37 and calcineurin during lymphatic-valve formation

Lymphatic valves are essential for efficient lymphatic transport, but the mechanisms of early lymphatic-valve morphogenesis and the role of biomechanical forces are not well understood. We found that the transcription factors PROX1 and FOXC2, highly expressed from the onset of valve formation, mediate segregation of lymphatic-valve-forming cells and cell mechanosensory responses to shear stress in vitro. Mechanistically, PROX1, FOXC2, and flow coordinately control expression of the gap junction protein connexin37 and activation of calcineurin/NFAT signaling. Connexin37 and calcineurin are required for the assembly and delimitation of lymphatic valve territory during development and for its postnatal maintenance. We propose a model in which regionally increased levels/activation states of transcription factors cooperate with mechanotransduction to induce a discrete cell-signaling pattern and morphogenetic event, such as formation of lymphatic valves. Our results also provide molecular insights into the role of endothelial cell identity in the regulation of vascular mechanotransduction.

Lack of protein S in mice causes embryonic lethal coagulopathy and vascular dysgenesis

Protein S (ProS) is a blood anticoagulant encoded by the Pros1 gene, and ProS deficiencies are associated with venous thrombosis, stroke, and autoimmunity. These associations notwithstanding, the relative risk that reduced ProS expression confers in different disease settings has been difficult to assess without an animal model. We have now described a mouse model of ProS deficiency and shown that all Pros1-/- mice die in utero,from a fulminant coagulopathy and associated hemorrhages. Although ProS is known to act as a cofactor for activated Protein C (aPC), plasma from Pros1+/- heterozygous mice exhibited accelerated thrombin generation independent of aPC, and Pros1 mutants displayed defects in vessel development and function not seen in mice lacking protein C. Similar vascular defects appeared in mice in which Pros1 was conditionally deleted in vascular smooth muscle cells. Mutants in which Pros1 was deleted specifically in hepatocytes, which are thought to be the major source of ProS in the blood, were viable as adults and displayed less-severe coagulopathy without vascular dysgenesis. Finally, analysis of mutants in which Pros1 was deleted in endothelial cells indicated that these cells make a substantial contribution to circulating ProS. These results demonstrate that ProS is a pleiotropic anticoagulant with aPC-independent activities and highlight new roles for ProS in vascular development and homeostasis.

NME1 at the human maternal-fetal interface downregulates titin expression and invasiveness of trophoblast cells via MAPK pathway in early pregnancy

Nometastatic gene 23-H1 (NME1, also known as nm23-H1) is a wide-spectrum tumor metastasis suppressor gene that plays an important role in suppressing the invasion and metastasis of tumor cells. It has been demonstrated that NME1 is also expressed in human first-trimester placenta, but its function at maternal-fetal interface is not clear. The present study aimed to elucidate the biological function of NME1 at the maternal-fetal interface, especially on invasion of the human extravillous cytotrophoblasts (EVCTs). NME1 has been identified in both human trophoblast cells and decidual stromal cells (DSCs) in early pregnancy. We have proved that NME1 silencing in vitro increases the titin protein translation in the invasive EVCTs. Moreover, NME1 can inactivate the phospho-extracellular signal-regulated kinase 1/2 (P-ERK1/2) in trophoblasts in a time-dependent manner, and U0126, an inhibitor of MAPK/ERK, can inhibit partly the enhanced invasiveness and titin expression in trophoblasts induced by NME1 silencing. Interestingly, the expression of NME1 in either villi or decidua is higher significantly in miscarriage than that of the normal early pregnancy. These findings first reveal that the NME1 expressed in trophoblasts and DSCs controls the inappropriate invasion of human first-trimester trophoblast cells via MAPK/ERK1/2 signal pathway, and the overexpression of NME1 at maternal-fetal interface leads to pregnancy wastage.

Gene expression in chorionic villous samples at 11 weeks of gestation in women who develop preeclampsia later in pregnancy: implications for screening

OBJECTIVES

To determine the gene expression profile in chorionic villous samples (CVS) of women destined to develop preeclampsia.

METHOD

cDNA microarray technology was employed. Ten singleton fetuses of women who subsequently developed preeclampsia where compared with a pool of 50 controls. The mRNA expression of some of the genes previously found to be up- or down-regulated were validated by RT-PCR in peripheral blood from 23 pregnant women at term affected with preeclampsia and 23 controls.

RESULTS

Altered expression was found among several genes including those involved in invasion of human trophoblasts (Titin), in inflammatory stress (Lactotransferrin), endothelial aberration (Claudin 6), angiogenesis (Vasohibin 1), blood pressure control (Adducin 1). Also the peripheral blood from preeclampsia patients showed significant differences for all the genes studied.

CONCLUSION

CVS show an aberrant gene profile prior to preeclampsia onset that may be predictive of the disease.

Neuromuscular disease models and analysis

Neuromuscular diseases can affect the survival of peripheral neurons, their axons extending to peripheral targets, their synaptic connections onto those targets, or the targets themselves. Examples include motor neuron diseases such as amyotrophic lateral sclerosis, peripheral neuropathies, such as Charcot-Marie-Tooth diseases, myasthenias, and muscular dystrophies. Characterizing these phenotypes in mouse models requires an integrated approach, examining both the nerve and the muscle histologically, anatomically, and functionally by electrophysiology. Defects observed at these levels can be related back to onset, severity, and progression, as assessed by "quality-of-life measures" including tests of gross motor performance such as gait or grip strength. This chapter describes methods for assessing neuromuscular disease models in mice, and how interpretation of these tests can be complicated by the inter-relatedness of the phenotypes.

Endoglin is dispensable for angiogenesis, but required for endocardial cushion formation in the midgestation mouse embryo

Vascular patterning depends on precisely coordinated timing of endothelial cell differentiation and onset of cardiac function. Endoglin is a transmembrane receptor for members of the TGF-beta superfamily that is expressed on endothelial cells from early embryonic gestation to adult life. Heterozygous loss of function mutations in human ENDOGLIN cause Hereditary Hemorrhagic Telangiectasia Type 1, a vascular disorder characterized by arteriovenous malformations that lead to hemorrhage and stroke. Endoglin null mice die in embryogenesis with numerous lesions in the cardiovascular tree including incomplete yolk sac vessel branching and remodeling, vessel dilation, hemorrhage and abnormal cardiac morphogenesis. Since defects in multiple cardiovascular tissues confound interpretations of these observations, we performed in vivo chimeric rescue analysis using Endoglin null embryonic stem cells. We demonstrate that Endoglin is required cell autonomously for endocardial to mesenchymal transition during formation of the endocardial cushions. Endoglin null cells contribute widely to endothelium in chimeric embryos rescued from cardiac development defects, indicating that Endoglin is dispensable for angiogenesis and vascular remodeling in the midgestation embryo, but is required for early patterning of the heart.

Global survey of protein expression during gonadal sex determination in mice

The development of an embryo as male or female depends on differentiation of the gonads as either testes or ovaries. A number of genes are known to be important for gonadal differentiation, but our understanding of the regulatory networks underpinning sex determination remains fragmentary. To advance our understanding of sexual development beyond the transcriptome level, we performed the first global survey of the mouse gonad proteome at the time of sex determination by using two-dimensional nanoflow LC-MS/MS. The resulting data set contains a total of 1037 gene products (154 non-redundant and 883 redundant proteins) identified from 620 peptides. Functional classification and biological network construction suggested that the identified proteins primarily serve in RNA post-transcriptional modification and trafficking, protein synthesis and folding, and post-translational modification. The data set contains potential novel regulators of gonad development and sex determination not revealed previously by transcriptomics and proteomics studies and more than 60 proteins with potential links to human disorders of sexual development.

A broken heart: a stretch too far: an overview of mouse models with mutations in stretch-sensor components

With every heartbeat the heart must contract and relax. This seemingly trivial process critically needs tight control of contraction and relaxation phases, and extremely efficient coordination between these two phases to control blood flow and maintain cardiac homeostasis. To achieve this, specialized sensors are required to detect the inherent repeatedly changing environment and needs. One sensor is a stretch-sensor that monitors the filling of the ventricles. Its molecular identity and localization are only partly understood. Here we give a synopsis of the genetic models that leap into our understanding of stretch-sensors. We focus on the widely acknowledged sarcomeric sensor at the Z-disc and the costamere sensor at the sarcolemma. Recently, several novel components of both sensors were discovered. Given that these two sensors seem physically connected, it is likely that these two models are not mutually exclusive and might even communicate. We describe briefly how candidate and known proteins within these sensors receive and transduce mechanical signals in the cardiomyocyte that lead to changes in gene expression underlying homeostasis and its restoration in the heart. Emphasis is placed on the putative link between altered stretch-sensor function and heart failure observed in different genetic mouse models of stretch-sensor components.

Separating genetic and hemodynamic defects in neuropilin 1 knockout embryos

Targeted inactivation of genes involved in murine cardiovascular development frequently leads to abnormalities in blood flow. As blood fluid dynamics play a crucial role in shaping vessel morphology, the presence of flow defects generally prohibits the precise assignment of the role of the mutated gene product in the vasculature. In this study, we show how to distinguish between genetic defects caused by targeted inactivation of the neuropilin 1 (Nrp1) receptor and hemodynamic defects occurring in homozygous knockout embryos. Our analysis of a Nrp1 null allele bred onto a C57BL/6 background shows that vessel remodeling defects occur concomitantly with the onset of blood flow and cause death of homozygous mutants at E10.5. Using mouse embryo culture, we establish that hemodynamic defects are already present at E8.5 and continuous circulation is never established in homozygous mutants. The geometry of yolk sac blood vessels is altered and remodeling into yolk sac arteries and veins does not occur. To separate flow-induced deficiencies from those caused by the Nrp1 mutation, we arrested blood flow in cultured wild-type and mutant embryos and followed their vascular development. We find that loss of Nrp1 function rather than flow induces the altered geometry of the capillary plexus. Endothelial cell migration, but not replication, is altered in Nrp1 mutants. Gene expression analysis of endothelial cells isolated from freshly dissected wild-type and mutants and after culture in no-flow conditions showed down-regulation of the arterial marker genes connexin 40 and ephrin B2 related to the loss of Nrp1 function. This method allows genetic defects caused by loss-of-function of a gene important for cardiovascular development to be isolated even in the presence of hemodynamic defects.

Vascular remodeling of the mouse yolk sac requires hemodynamic force

The embryonic heart and vessels are dynamic and form and remodel while functional. Much has been learned about the genetic mechanisms underlying the development of the cardiovascular system, but we are just beginning to understand how changes in heart and vessel structure are influenced by hemodynamic forces such as shear stress. Recent work has shown that vessel remodeling in the mouse yolk sac is secondarily effected when cardiac function is reduced or absent. These findings indicate that proper circulation is required for vessel remodeling, but have not defined whether the role of circulation is to provide mechanical cues, to deliver oxygen or to circulate signaling molecules. Here, we used time-lapse confocal microscopy to determine the role of fluid-derived forces in vessel remodeling in the developing murine yolk sac. Novel methods were used to characterize flows in normal embryos and in embryos with impaired contractility (Mlc2a(-/-)). We found abnormal plasma and erythroblast circulation in these embryos, which led us to hypothesize that the entry of erythroblasts into circulation is a key event in triggering vessel remodeling. We tested this by sequestering erythroblasts in the blood islands, thereby lowering the hematocrit and reducing shear stress, and found that vessel remodeling and the expression of eNOS (Nos3) depends on erythroblast flow. Further, we rescued remodeling defects and eNOS expression in low-hematocrit embryos by restoring the viscosity of the blood. These data show that hemodynamic force is necessary and sufficient to induce vessel remodeling in the mammalian yolk sac.

What Determines Blood Vessel Structure? Genetic Prespecification vs. Hemodynamics

Vascular network remodeling, angiogenesis, and arteriogenesis play an important role in the pathophysiology of ischemic cardiovascular diseases and cancer. Based on recent studies of vascular network development in the embryo, several novel aspects to angiogenesis have been identified as crucial to generate a functional vascular network. These aspects include specification of arterial and venous identity in vessels and network patterning. In early embryogenesis, vessel identity and positioning are genetically hardwired and involve neural guidance genes expressed in the vascular system. We demonstrated that, during later stages of embryogenesis, blood flow plays a crucial role in regulating vessel identity and network remodeling. The flow-evoked remodeling process is dynamic and involves a high degree of vessel plasticity. The open question in the field is how genetically predetermined processes in vessel identity and patterning balance with the contribution of blood flow in shaping a functional vascular architecture. Although blood flow is essential, it remains unclear to what extent flow is able to act on the developing cardiovascular system. There is significant evidence that mechanical forces created by flowing blood are biologically active within the embryo and that the level of mechanical forces and the type of flow patterns present in the embryo are able to affect gene expression. Here, we highlight the pivotal role for blood flow and physical forces in shaping the cardiovascular system.

Molecular mechanisms controlling the coupled development of myocardium and coronary vasculature

Cardiac failure affects 1.5% of the adult population and is predominantly caused by myocardial dysfunction secondary to coronary vascular insufficiency. Current therapeutic strategies improve prognosis only modestly, as the primary cause -- loss of normally functioning cardiac myocytes -- is not being corrected. Adult cardiac myocytes are unable to divide and regenerate to any significant extent following injury. New cardiac myocytes are, however, created during embryogenesis from progenitor cells and then by cell division from existing cardiac myocytes. This process is intimately linked to the development of coronary vasculature from progenitors originating in the endothelium, the proepicardial organ and neural crest. In this review, we systematically evaluate approx. 90 mouse mutations that impair heart muscle growth during development. These studies provide genetic evidence for interactions between myocytes, endothelium and cells derived from the proepicardial organ and the neural crest that co-ordinate myocardial and coronary vascular development. Conditional knockout and transgenic rescue experiments indicate that Vegfa, Bmpr1a (ALK3), Fgfr1/2, Mapk14 (p38), Hand1, Hand2, Gata4, Zfpm2 (FOG2), Srf and Txnrd2 in cardiac myocytes, Rxra and Wt1 in the proepicardial organ, EfnB2, Tek, Mapk7, Pten, Nf1 and Casp8 in the endothelium, and Bmpr1a and Pax3 in neural crest cells are key molecules controlling myocardial development. Coupling of myocardial and coronary development is mediated by BMP (bone morphogenetic protein), FGF (fibroblast growth factor) and VEGFA (vascular endothelial growth factor A) signalling, and also probably involves hypoxia. Pharmacological targeting of these molecules and pathways could, in principle, be used to recreate the embryonic state and achieve coupled myocardial and coronary vascular regeneration in failing hearts.

M line-deficient titin causes cardiac lethality through impaired maturation of the sarcomere

Titin, the largest protein known to date, has been linked to sarcomere assembly and function through its elastic adaptor and signaling domains. Titin's M-line region contains a unique kinase domain that has been proposed to regulate sarcomere assembly via its substrate titin cap (T-cap). In this study, we use a titin M line-deficient mouse to show that the initial assembly of the sarcomere does not depend on titin's M-line region or the phosphorylation of T-cap by the titin kinase. Rather, titin's M-line region is required to form a continuous titin filament and to provide mechanical stability of the embryonic sarcomere. Even without titin integrating into the M band, sarcomeres show proper spacing and alignment of Z discs and M bands but fail to grow laterally and ultimately disassemble. The comparison of disassembly in the developing and mature knockout sarcomere suggests diverse functions for titin's M line in embryonic development and the adult heart that not only involve the differential expression of titin isoforms but also of titin-binding proteins.

In vivo biofluid dynamic imaging in the developing zebrafish

Flow-structure interactions are ubiquitous in nature, and are important factors in the proper development of form and function in living organisms. In order to uncover the mechanisms by which flow-structure interactions affect vertebrate development, we first need to establish the techniques necessary to quantitatively describe the fluid flow environment within the embryo. To do this, we must bring dynamic, in vivo imaging methods to bear on living systems. Traditional avian and mammalian model systems can be problematic in this regard. The zebrafish (Danio rerio) is widely accepted as an excellent model organism for the study of vertebrate biology, as it shows substantial anatomical and genetic conservation with higher vertebrates, including humans. Their small size, optical transparency, and external development make zebrafish the ideal model system for dynamic imaging. This article reviews the current state of research in imaging biofluid flow within and around developing zebrafish embryos, with an emphasis on dynamic imaging modalities.