A mechanosensitive transcriptional mechanism that controls angiogenesis. Nature. 2009;457:1103-1108. [PMID: 19242469 DOI: 10.1038/nature07765]

Fibronectin conformation regulates the proangiogenic capability of tumor-associated adipogenic stromal cells

BACKGROUND

Changes in fibronectin (Fn) matrix remodeling contribute to mammary tumor angiogenesis and are related to altered behavior of adipogenic stromal cells; yet, the underlying mechanisms remain unclear due in part to a lack of reductionist model systems that allow the inherent complexity of cell-derived extracellular matrices (ECMs) to be deciphered. In particular, breast cancer-associated adipogenic stromal cells not only enhance the composition, quantity, and rigidity of deposited Fn, but also partially unfold these matrices. However, the specific effect of Fn conformation on tumor angiogenesis is undefined.

METHODS

Decellularized matrices and a conducting polymer device consisting of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) were used to examine the effect of Fn conformation on the behavior of 3T3-L1 preadipocytes. Changes in cell adhesion and proangiogenic capability were tested via cell counting and by quantification of vascular endothelial growth factor (VEGF) secretion, respectively. Integrin-blocking antibodies were utilized to examine varied integrin specificity as a potential mechanism.

RESULTS

Our findings suggest that tumor-associated partial unfolding of Fn decreases adhesion while enhancing VEGF secretion by breast cancer-associated adipogenic precursor cells, and that altered integrin specificity may underlie these changes.

CONCLUSIONS AND GENERAL SIGNIFICANCE

These results not only have important implications for our understanding of tumorigenesis, but also enhance knowledge of cell-ECM interactions that may be harnessed for other applications including advanced tissue engineering approaches. This article is part of a Special Issue entitled Organic Bioelectronics - Novel Applications in Biomedicine.

Physicochemical regulation of endothelial sprouting in a 3D microfluidic angiogenesis model

Both physiological and pathological tissue remodeling (e.g., during wound healing and cancer, respectively) require new blood vessel formation via angiogenesis, but the underlying microenvironmental mechanisms remain poorly defined due in part to the lack of biologically relevant in vitro models. Here, we present a biomaterials-based microfluidic 3D platform for analysis of endothelial sprouting in response to morphogen gradients. This system consists of three lithographically defined channels embedded in type I collagen hydrogels. A central channel is coated with endothelial cells, and two parallel side channels serve as a source and a sink for the steady-state generation of biochemical gradients. Gradients of vascular endothelial growth factor (VEGF) promoted sprouting, whereby endothelial cell responsiveness was markedly dependent on cell density and vessel geometry regardless of treatment conditions. These results point toward mechanical and/or autocrine mechanisms that may overwhelm pro-angiogenic paracrine signaling under certain conditions. To date, neither geometrical effects nor cell density have been considered critical determinants of angiogenesis in health and disease. This biomimetic vessel platform demonstrated utility for delineating hitherto underappreciated contributors of angiogenesis, and future studies may enable important new mechanistic insights that will inform anti-angiogenic cancer therapy.

Negative pressure therapy stimulates healing of critical-size calvarial defects in rabbits

Negative pressure therapy (NPT) is the controlled application of subatmospheric pressure to wounds. It has been shown to stimulate healing across a broad spectrum of soft-tissue wounds, at least in part from the application of mechanical stress on cells and tissues in the wound environment. This study tests the hypothesis that application of NPT to cranial critical-size defects (CSD) in skeletally mature rabbits leads to osseous healing. NPT was delivered 1, 4, 6 or 10 days over CSD-containing calcium phosphate scaffolds placed in contact with intact dura. At 12 weeks after defect creation, NPT groups exhibited significantly greater defect bridging and bone within the scaffolds (P<0.01). Increasing duration of NPT did not result in a greater amount of bone within the scaffolds, but did increase the amount of bone distributed in the upper half of the scaffolds. Appearance of tissue within defects immediately following the removal of NPT at day 6 suggests alternating regions of dural compression and distention indicative of cell stretching. Dura and adjacent tissue were composed of multiple cell layers that extended up into the scaffolds, lining struts and populating pore spaces. An extracellular matrix densely populated with cells and capillaries, as well as larger vessels, infiltrated pores of NPT-treated scaffolds, while scattered spindle-shaped cells and sparse stroma are present within pores of control scaffolds. This rabbit model data suggest that NPT activates within mature dura a natural healing cascade that results in osseous tissue formation without the addition of exogenous factors or progenitor cells.

A cell-based model of extracellular-matrix-guided endothelial cell migration during angiogenesis

Angiogenesis, the formation of new blood vessels sprouting from existing ones, occurs in several situations like wound healing, tissue remodeling, and near growing tumors. Under hypoxic conditions, tumor cells secrete growth factors, including VEGF. VEGF activates endothelial cells (ECs) in nearby vessels, leading to the migration of ECs out of the vessel and the formation of growing sprouts. A key process in angiogenesis is cellular self-organization, and previous modeling studies have identified mechanisms for producing networks and sprouts. Most theoretical studies of cellular self-organization during angiogenesis have ignored the interactions of ECs with the extra-cellular matrix (ECM), the jelly or hard materials that cells live in. Apart from providing structural support to cells, the ECM may play a key role in the coordination of cellular motility during angiogenesis. For example, by modifying the ECM, ECs can affect the motility of other ECs, long after they have left. Here, we present an explorative study of the cellular self-organization resulting from such ECM-coordinated cell migration. We show that a set of biologically-motivated, cell behavioral rules, including chemotaxis, haptotaxis, haptokinesis, and ECM-guided proliferation suffice for forming sprouts and branching vascular trees.

Tissue mechanics and fibrosis

Mechanical forces are essential to the development and progression of fibrosis, and are likely to be as important as soluble factors. These forces regulate the phenotype and proliferation of myofibroblasts and other cells in damaged tissues, the activation of growth factors, the structure and mechanics of the matrix, and, potentially, tissue patterning. Better understanding of the variety and magnitude of forces, the characteristics of those forces in biological tissues, and their impact on fibrosis in multiple tissues is needed and may lead to identification of important new therapeutic targets. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Cell friction

Cells sense and respond to their environment. Mechanotransduction is the process by which mechanical forces, stress, and strains are converted into biochemical signals that control cell behavior. In recent decades it has been shown that appropriate mechanical signals are essential to tissue health, but the role of friction and direct contact shearing across cell surfaces has been essentially unexplored. This, despite the obvious existence of numerous biological tissues whose express function depends on sliding contacts. In our studies on frictional interactions of corneal cells we find that the friction coefficients are on the order of mu = 0.03-0.06 for in vitro and in vivo experiments. Additionally, we observe cell death after single cycles of sliding at contact pressures estimated to be approximately 12 kPa. These experimental results suggest that frictional contact forces produce mechanical stresses and strains that are in the cellular mechanosensing ranges.

Stem-cell niche based comparative analysis of chemical and nano-mechanical material properties impacting ex vivo expansion and differentiation of hematopoietic and mesenchymal stem cells

The ability of stem cells to self-renew with minimal or no differentiation and, when appropriately cued, to give rise to many types of progenitor and mature cells, is the basis for applications in regenerative and transfusion medicine, but also in drug discovery and in vitro toxicology. Inspired by the complex interactions between stem cells and their microenvironment, the so-called stem-cell niche, the properties of supporting biomaterials, including surface biochemistry, topography (type, size, organization, and geometry of nanostructures), and mechanical properties, have been identified as important determinants of stem-cell fate in vitro. 3D culture environments that could recapitulate the complexity of the in vivo stem-cell microenvironment could further expand the complexity and repertoire of engineered environments with exciting translational applications. Herein, the material aspects that affect the expansion and differentiation fate of adult hematopoietic stem/progenitor cells (HSPCs) and mesenchymal stem cells (MSCs), two powerful cell types that co-reside in the bone-marrow niche, but with distinct, sometime complementary, differentiation fates, properties, and translational applications, are examined. Although MSCs are adherent cells and, in contrast, HSPCs are non- or weakly adherent cells, both can sense and respond to material properties, including surface (bio)chemistry, ECM composition, topography, and matrix elasticity, possibly through similar molecular mechanisms.

Engineering an in situ crosslinkable hydrogel for enhanced remyelination

Remyelination has to occur to fully regenerate injured spinal cords or brain tissues. A growing body of evidence has suggested that exogenous cell transplantation is one promising strategy to promote remyelination. However, direct injection of neural stem cells or oligodendrocyte progenitor cells (OPCs) to the lesion site may not be an optimal therapeutic strategy due to poor viability and functionality of transplanted cells resulted from the local hostile tissue environment. The overall objective of this study was to engineer an injectable biocompatible hydrogel system as a supportive niche to provide a regeneration permissive microenvironment for transplanted OPCs to survive, functionally differentiate, and remyelinate central nervous system (CNS) lesions. A highly biocompatible hydrogel, based on thiol-functionalized hyaluronic acid and thiol-functionalized gelatin, which can be crosslinked by poly-(ethylene glycol) diacrylate (PEGDA), was used. These hydrogels were optimized first regarding cell adhesive properties and mechanical properties to best support the growth properties of OPCs in culture. Transplanted OPCs with the hydrogels optimized in vitro exhibited enhanced survival and oligodendrogenic differentiation and were able to remyelinate demyelinated axons inside ethidium bromide (EB) demyelination lesion in adult spinal cord. This study provides a new possible therapeutic approach to treat CNS injuries in which cell therapies may be essential.

Evolution of general transcription factors

Three genes GTF2IRD1, GTF2I, and GTF2IRD2, which encode members of the GTF2I (or TFII-I) family of so-called general transcription factors, were discovered and studied during the last two decades. Chromosome location and similarity of exon-intron structures suggest that the family evolved by duplications. The initial duplication of ancestral proto-GTF2IRD1 gene likely occurred in early vertebrates prior to origin of cartilaginous fish and led to formation of GTF2I (>450 MYA), which was later lost in bony fish but successfully evolved in the land vertebrates. The second duplication event, which created GTF2IRD2, occurred prior to major radiation events of eutherian mammalian evolution (>100 MYA). During recent steps of primate evolution there was another duplication which led to formation of GTF2IRD2B (<4 MYA). Two latest duplications were coupled with inversions. Genes belonging to the family have several highly conservative repeats which are implicated in DNA binding. Phylogenetic analysis of the repeats revealed a pattern of intragenic duplications, deletions and substitutions which led to diversification of the genes and proteins. Distribution of statistically rare atypical substitutions (p ≤ 0.01) sheds some light on structural differentiation of repeats and hence evolution of the genes. The atypical substitutions are often located on secondary structures joining α-helices and affect 3D arrangement of the protein globule. Such substitutions are commonly traced at the early stages of evolution in Tetrapoda, Amniota, and Mammalia.

Netrin-1 promotes glioblastoma cell invasiveness and angiogenesis by multiple pathways including activation of RhoA, cathepsin B, and cAMP-response element-binding protein

Glioblastomas are very difficult tumors to treat because they are highly invasive and disseminate within the normal brain, resulting in newly growing tumors. We have identified netrin-1 as a molecule that promotes glioblastoma invasiveness. As evidence, netrin-1 stimulates glioblastoma cell invasion directly through Matrigel-coated transwells, promotes tumor cell sprouting and enhances metastasis to lymph nodes in vivo. Furthermore, netrin-1 regulates angiogenesis as shown in specific angiogenesis assays such as enhanced capillary endothelial cells (EC) sprouting and by increased EC infiltration into Matrigel plugs in vivo, as does VEGF-A. This netrin-1 signaling pathway in glioblastoma cells includes activation of RhoA and cyclic AMP response element-binding protein (CREB). A novel finding is that netrin-1-induced glioblastoma invasiveness and angiogenesis are mediated by activated cathepsin B (CatB), a cysteine protease that translocates to the cell surface as an active enzyme and co-localizes with cell surface annexin A2 (ANXA2). The specific CatB inhibitor CA-074Me inhibits netrin-1-induced cell invasion, sprouting, and Matrigel plug angiogenesis. Silencing of CREB suppresses netrin-1-induced glioblastoma cell invasion, sprouting, and CatB expression. It is concluded that netrin-1 plays an important dual role in glioblastoma progression by promoting both glioblastoma cell invasiveness and angiogenesis in a RhoA-, CREB-, and CatB-dependent manner. Targeting netrin-1 pathways may be a promising strategy for brain cancer therapy.

Engineering approaches for investigating tumor angiogenesis: exploiting the role of the extracellular matrix

A major paradigm shift in cancer research is the emergence of multidisciplinary approaches to investigate complex cell behaviors, to elucidate regulatory mechanisms and to identify therapeutic targets. Recently, efforts are focused on the engineering of complex in vitro models, which more accurately recapitulate the growth and progression of cancer. These strategies have proven vital for investigating and targeting the events that control tumor angiogenesis. In this review, we explore how the emerging engineering approaches are being used to unlock the complex mechanisms regulating tumor angiogenesis. Emphasis is placed on models using natural and synthetic biomaterials to generate scaffolds mimicking the extracellular matrix, which is known to play a critical role in angiogenesis. While the models presented in this review are revolutionary, improvements are still necessary and concepts for advancing and perfecting engineering approaches for modeling tumor angiogenesis are proposed. Overall, the marriage between disparate scientific fields is expected to yield significant improvements in our understanding and treatment of cancer.

Differential roles of angiogenic chemokines in endothelial progenitor cell-induced angiogenesis

This study aimed to analyze the role of endothelial progenitor cell (EPC)-derived angiogenic factors and chemokines in the multistep process driving angiogenesis with a focus on the recently discovered macrophage migration inhibitory factor (MIF)/chemokine receptor axis. Primary murine and murine embryonic EPCs (eEPCs) were analyzed for the expression of angiogenic/chemokines and components of the MIF/CXC chemokine receptor axis, focusing on the influence of hypoxic versus normoxic stimulation. Hypoxia induced an upregulation of CXCR2 and CXCR4 but not CD74 on EPCs and triggered the secretion of CXCL12, CXCL1, MIF, and vascular endothelial growth factor (VEGF). These factors stimulated the transmigration activity and adhesive capacity of EPCs, with MIF and VEGF exhibiting the strongest effects under hypoxia. MIF-, VEGF-, CXCL12-, and CXCL1-stimulated EPCs enhanced tube formation, with MIF and VEGF exhibiting again the strongest effect following hypoxia. Tube formation following in vivo implantation utilizing angiogenic factor-loaded Matrigel plugs was only promoted by VEGF. Coloading of plugs with eEPCs led to enhanced tube formation only by CXCL12, whereas MIF was the only factor which induced differentiation towards an endothelial and smooth muscle cell (SMC) phenotype, indicating an angiogenic and differentiation capacity in vivo. Surprisingly, CXCL12, a chemoattractant for smooth muscle progenitor cells, inhibited SMC differentiation. We have identified a role for EPC-derived proangiogenic MIF, VEGF and MIF receptors in EPC recruitment following hypoxia, EPC differentiation and subsequent tube and vessel formation, whereas CXCL12, a mediator of early EPC recruitment, does not contribute to the remodeling process. By discerning the contributions of key angiogenic chemokines and EPCs, these findings offer valuable mechanistic insight into mouse models of angiogenesis and help to define the intricate interplay between EPC-derived angiogenic cargo factors, EPCs, and the angiogenic target tissue.

The role of extracellular matrix in vascular branching morphogenesis

Angiogenesis requires the development of a hierarchically branched network of vessels, which undergoes radial expansion and anastomosis to form a close circuit. Branching is achieved by coordinated behavior of endothelial cells that organize into leading “tip” cells and trailing “stalk” cells. Such organization is under control of the Dll4-Notch signaling pathway, which sets a hierarchy in receptiveness of cells to VEGF-A. Recent studies have shed light on a control of the Notch pathway by basement membrane proteins and integrin signaling, disclosing that extracellular matrix exerts active control on vascular branching morphogenesis. We will survey in the present review how extracellular matrix is a multifaceted substrate, which behind a classical structural role hides a powerful conductor function to shape the branching pattern of vessels.

Stiffness-controlled three-dimensional extracellular matrices for high-resolution imaging of cell behavior

Regulation of cell functions by the physical properties of the extracellular matrix (ECM) has emerged as a crucial contributor to development and disease. Two specific physical properties of the ECM, stiffness and dimensionality, each influence cell signaling and function. As these ECM physical properties are linked to other properties that also regulate cell behavior, e.g., integrin ligand density, parsing the specific contributions of ECM stiffness and dimensionality has proven difficult. Here we detail a simple protocol, which can be completed in 1-2 d, for combining three-dimensional (3D) ECM engagement with controlled underlying ECM stiffness. In these 'sandwich gels', cells are sandwiched between a 3D fibrillar ECM and an ECM-coupled polyacrylamide gel of defined compliance, allowing the study of the specific effects of ECM compliance on cell function in physiologically relevant 3D ECMs. This type of system enables high-resolution time-lapse imaging and is suitable for a wide range of cell types and molecular perturbations.

Chemical and physical properties of regenerative medicine materials controlling stem cell fate

Regenerative medicine is a multidisciplinary field utilizing the potential of stem cells and the regenerative capability of the body to restore, maintain, or enhance tissue and organ functions. Stem cells are unspecialized cells that can self-renew but also differentiate into several somatic cells when subjected the appropriate environmental cues. The ability to reliably direct stem cell fate would provide tremendous potential for basic research and clinical therapies. Proper tissue function and regeneration rely on the spatial and temporal control of biophysical and biochemical cues, including soluble molecules, cell-cell contacts, cell-extracellular matrix contacts, and physical forces. The mechanisms involved remain poorly understood. This review focuses on the stem cell-extracellular matrix interactions by summarizing the observations of the effects of material variables (such as overall architecture, surface topography, charge, ζ-potential, surface energy, and elastic modulus) on the stem cell fate. It also deals with the mechanisms underlying the effects of these extrinsic, material variables. Insight in the environmental interactions of the stem cells is crucial for the development of new material-based approaches for cell culture experiments and future experimental and clinical regenerative medicine applications.

Bioeffects of ultrasound-stimulated microbubbles on endothelial cells: gene expression changes associated with radiation enhancement in vitro

Ultrasound can be used to target endothelial cells in cancer therapy where the destruction of vasculature leads to tumor cell death. Here, we demonstrate ultrasound bioeffects in which the levels of genes in endothelial cells can be significantly altered by ultrasound-stimulated microbubble exposure. These were compared with established effects of radiation on endothelial cells at a gene level. Human-endothelial cells were exposed to ultrasound and microbubbles, radiation or combinations of ultrasound, microbubbles and radiation. Gene expression analyses revealed an up-regulation of genes known to be involved in apoptosis and ceramide-induced apoptotic pathways, including SMPD2, UGT8, COX6B1, Caspase 9 and MAP2K1 with ultrasound-stimulated microbubble exposure but not SMPD1. This was supported by immunohistochemistry and morphologic changes examined with cell microscopy, which showed changes in SMPD1 gene product in cells with microbubble exposure. This supports the hypothesis that ultrasound-stimulated microbubbles can induce significant bioeffect-related changes in gene expression and can affect ceramide signaling pathways in endothelial cells, leading to apoptosis.

Paxillin controls directional cell motility in response to physical cues

Physical cues from the extracellular environment that influence cell shape and directional migration are transduced into changes in cytoskeletal organization and biochemistry through integrin-based cell adhesions to extracellular matrix (ECM). Paxillin is a focal adhesion (FA) scaffold protein that mediates integrin anchorage to the cytoskeleton, and has been implicated in regulation of FA assembly and cell migration. To determine whether paxillin is involved in coupling mechanical distortion with directional movement, cell shape was physically constrained by culturing cells on square-shaped fibronectin-coated adhesive islands surrounded by non-adhesive barrier regions that were created with a microcontact printing technique. Square-shaped cells preferentially formed FAs and extended lamellipodia from their corner regions when stimulated with PDGF, and loss of paxillin resulted in loss of this polarized response. Selective expression of the N- and C-terminal domains of paxillin produced opposite, but complementary, effects on suppressing or promoting lamellipodia formation in different regions of square cells, which corresponded to directional motility defects in vitro. Paxillin loss or mutation was also shown to affect the formation of circular dorsal ruffles, and this corresponded to changes in cell invasive behavior in 3D. This commentary addresses the implications of these findings in terms of how a multifunctional FA scaffold protein can link physical cues to cell adhesion, protrusion and membrane trafficking so as to control directional migration in 2D and 3D. We also discuss how microengineered ECM islands and in vivo model systems can be used to further elucidate the functions of paxillin in directional migration.

Adhesion regulates MAP kinase/ternary complex factor exchange to control a proliferative transcriptional switch

BACKGROUND

The ternary complex factors (TCFs; Elk1, Net, and Sap-1) are growth factor-responsive transcription cofactors of serum response factor (SRF) and are activated by MAP kinase (MAPK) phosphorylation to regulate immediate early gene transcription. Although cell adhesion also can regulate immediate early genes and proliferation, the mechanism for this effect has remained unexplored.

RESULTS

Restricting adhesion and spreading of G(0)-synchronized cells on substrates with decreasing size of micropatterned islands of fibronectin suppressed serum-induced immediate early gene expression and S phase entry. Knockdown of Sap-1 decreased expression of the immediate early genes egr1 and fos and subsequent proliferation normally present with high adhesion, whereas knockdown of Net rescued egr1 and fos expression and proliferation normally suppressed by low adhesion. Chromatin immunoprecipitation studies showed increased occupancy of egr1 and fos promoters by Sap-1 with high adhesion, whereas low adhesion increased Net occupancy. This switch in TCF promoter binding was regulated by an adhesion-mediated switch in MAPK activity. Increasing adhesion enhanced serum-induced JNK activity while suppressing p38 activity, leading to increased Sap-1 phosphorylation and Net dephosphorylation, and switching Net with Sap-1 at egr1 and fos promoters to support proliferation. Microarray studies confirmed this switch in TCF regulation of proliferative genes and uncovered novel gene targets and functions coregulated by Sap-1 and Net.

CONCLUSIONS

These data demonstrate a key role for the TCFs in adhesion-induced transcription and proliferation and reveal a novel MAPK/TCF transcriptional switch that controls this process.

Cis-element mutated in GATA2-dependent immunodeficiency governs hematopoiesis and vascular integrity

Haploinsufficiency for GATA2 causes human immunodeficiency syndromes characterized by mycobacterial infection, myelodysplasia, lymphedema, or aplastic anemia that progress to myeloid leukemia. GATA2 encodes a master regulator of hematopoiesis that is also linked to endothelial biology. Though the disease-causing mutations commonly occur in the GATA-2 DNA binding domain, we identified a patient with mycobacterial infection and myelodysplasia who had an uncharacterized heterozygous deletion in a GATA2 cis-element consisting of an E-box and a GATA motif. Targeted deletion of the equivalent murine element to yield homozygous mutant mice revealed embryonic lethality later than occurred with global Gata2 knockout, hematopoietic stem/progenitor cell depletion, and impaired vascular integrity. Heterozygous mutant mice were viable, but embryos exhibited deficits in definitive, but not primitive, hematopoietic stem/progenitor activity and reduced expression of Gata2 and its target genes. Mechanistic analysis revealed disruption of the endothelial cell transcriptome and loss of vascular integrity. Thus, the composite element disrupted in a human immunodeficiency is essential for establishment of the murine hematopoietic stem/progenitor cell compartment in the fetal liver and for essential vascular processes.

From mechanical force to RhoA activation

Throughout their lives, all cells constantly experience and respond to various mechanical forces. These frequently originate externally but can also arise internally as a result of the contractile actin cytoskeleton. Mechanical forces trigger multiple signaling pathways. Several converge and result in the activation of the GTPase RhoA. In this review, we focus on the pathways by which mechanical force leads to RhoA regulation, especially when force is transmitted via cell adhesion molecules that mediate either cell-matrix or cell-cell interactions. We discuss both the upstream signaling events that lead to activation of RhoA and the downstream consequences of this pathway. These include not only cytoskeletal reorganization and, in a positive feedback loop, increased myosin-generated contraction but also profound effects on gene expression and differentiation.

Control of stem cell fate and function by engineering physical microenvironments

The phenotypic expression and function of stem cells are regulated by their integrated response to variable microenvironmental cues, including growth factors and cytokines, matrix-mediated signals, and cell–cell interactions. Recently, growing evidence suggests that matrix-mediated signals include mechanical stimuli such as strain, shear stress, substrate rigidity and topography, and these stimuli have a more profound impact on stem cell phenotypes than had previously been recognized, e.g. self-renewal and differentiation through the control of gene transcription and signaling pathways. Using a variety of cell culture models enabled by micro and nanoscale technologies, we are beginning to systematically and quantitatively investigate the integrated response of cells to combinations of relevant mechanobiological stimuli. This paper reviews recent advances in engineering physical stimuli for stem cell mechanobiology and discusses how micro- and nanoscale engineered platforms can be used to control stem cell niche environments and regulate stem cell fate and function.

Dynamical systems approach to endothelial heterogeneity

Endothelial cells display remarkable phenotypic heterogeneity. An important goal is to elucidate the scope and mechanisms of endothelial heterogeneity and to use this information to develop vascular bed-specific therapies. We reexamine our current understanding of the molecular basis of endothelial heterogeneity. We introduce multistability as a new explanatory framework in vascular biology. We draw on the field of nonlinear dynamics to propose a dynamical systems framework for modeling multistability and its derivative properties, including robustness, memory, and plasticity. Our perspective allows for both a conceptual and quantitative description of system-level features of endothelial regulation.

Nanopatterned cardiac cell patches promote stem cell niche formation and myocardial regeneration

Stem cell-based methods for myocardial regeneration suffer from considerable cell attrition. Artificial matrices reproducing mechanical and structural properties of the native tissue may facilitate survival, retention and functional integration of adult stem or progenitor cells, by conditioning the cells prior to, and during, transplantation. Here we combined autologous cardiosphere-derived cells (CDCs) with nanotopographically defined hydrogels mimicking the native myocardial matrix, to form in vitro cardiac stem cell niches, and control cell function and fate. These platforms were used to produce cardiac patches that could be transplanted at the site of infarct. In culture, highly anisotropic, but not more randomized nanotopographic, control augmented cell adhesion, migration, and proliferation. It also dramatically enhanced early, and, in the presence of mature cardiomyocytes, late cardiomyogenesis. Nanotopography sensing and transcriptional response was mediated via p190RhoGAP. In a rat infarction model, engraftment of nanofabricated scaffolds with CDCs enhanced retention and growth of transplanted cells, and their integration with the host tissue. The infarcted ventricle wall increased in thickness, with higher cell viability and better collagen organization. These results suggest that nanostructured polymeric materials that closely mimic the extracellular matrix structure on which cardiac cells reside in vivo can be both very effective tools in investigating the mechanisms of cardiac differentiation and the basis for cardiac tissue engineering, thus facilitating stem cell-based therapy in the heart.

LRP5 regulates development of lung microvessels and alveoli through the angiopoietin-Tie2 pathway

Angiogenesis is crucial for lung development. Although there has been considerable exploration, the mechanism by which lung vascular and alveolar formation is controlled is still not completely understood. Here we show that low-density lipoprotein receptor-related protein 5 (LRP5), a component of the Wnt ligand-receptor complex, regulates angiogenesis and alveolar formation in the lung by modulating expression of the angiopoietin (Ang) receptor, Tie2, in vascular endothelial cells (ECs). Vascular development in whole mouse lungs and in cultured ECs is controlled by LRP5 signaling, which is, in turn, governed by a balance between the activities of the antagonistic Tie2 ligands, Ang1 and Ang2. Under physiological conditions when Ang1 is dominant, LRP5 knockdown decreases Tie2 expression and thereby, inhibits vascular and alveolar development in the lung. Conversely, when Ang2 dominates under hyperoxia treatment in neonatal mice, high LRP5 and Tie2 expression suppress angiogenesis and lung development. These findings suggest that the LRP5-Tie2-Ang signaling axis plays a central role in control of both angiogenesis and alveolarization during postnatal lung development, and that deregulation of this signaling mechanism might lead to developmental abnormalities of the lung, such as are observed in bronchopulmonary dysplasia (BPD).

Cardiovascular importance of the microRNA-23/27/24 family

MicroRNAs (miRNAs) are a class of highly conserved, noncoding short RNA molecules that regulate gene expression on the post-transcriptional level. MiRNAs are involved in a variety of processes such as proliferation, differentiation, and apoptosis. Deregulated expression of miRNAs has been linked to the development of diseases including cardiovascular disorders. Recently, the miR-23/27/24 cluster has been shown to be involved in angiogenesis and endothelial apoptosis in cardiac ischemia and retinal vascular development. In the present review, we summarize and discuss the role and importance of the miRNA-23/27/24 cluster during cardiovascular angiogenesis. Moreover, we illustrate a novel therapeutic application of the miRNA-23/27/24 cluster in vascular disorders and ischemic heart disease.

Mechanosensitive mechanisms in transcriptional regulation

Transcriptional regulation contributes to the maintenance of pluripotency, self-renewal and differentiation in embryonic cells and in stem cells. Therefore, control of gene expression at the level of transcription is crucial for embryonic development, as well as for organogenesis, functional adaptation, and regeneration in adult tissues and organs. In the past, most work has focused on how transcriptional regulation results from the complex interplay between chemical cues, adhesion signals, transcription factors and their co-regulators during development. However, chemical signaling alone is not sufficient to explain how three-dimensional (3D) tissues and organs are constructed and maintained through the spatiotemporal control of transcriptional activities. Accumulated evidence indicates that mechanical cues, which include physical forces (e.g. tension, compression or shear stress), alterations in extracellular matrix (ECM) mechanics and changes in cell shape, are transmitted to the nucleus directly or indirectly to orchestrate transcriptional activities that are crucial for embryogenesis and organogenesis. In this Commentary, we review how the mechanical control of gene transcription contributes to the maintenance of pluripotency, determination of cell fate, pattern formation and organogenesis, as well as how it is involved in the control of cell and tissue function throughout embryogenesis and adult life. A deeper understanding of these mechanosensitive transcriptional control mechanisms should lead to new approaches to tissue engineering and regenerative medicine.

The contribution of cellular mechanotransduction to cardiomyocyte form and function

Myocardial development is regulated by an elegantly choreographed ensemble of signaling events mediated by a multitude of intermediates that take a variety of forms. Cellular differentiation and maturation are a subset of vertically integrated processes that extend over several spatial and temporal scales to create a well-defined collective of cells that are able to function cooperatively and reliably at the organ level. Early efforts to understand the molecular mechanisms of cardiomyocyte fate determination focused primarily on genetic and chemical mediators of this process. However, increasing evidence suggests that mechanical interactions between the extracellular matrix (ECM) and cell surface receptors as well as physical interactions between neighboring cells play important roles in regulating the signaling pathways controlling the developmental processes of the heart. Interdisciplinary efforts have made it apparent that the influence of the ECM on cellular behavior occurs through a multitude of physical mechanisms, such as ECM boundary conditions, elasticity, and the propagation of mechanical signals to intracellular compartments, such as the nucleus. In addition to experimental studies, a number of mathematical models have been developed that attempt to capture the interplay between cells and their local microenvironment and the influence these interactions have on cellular self-assembly and functional behavior. Nevertheless, many questions remain unanswered concerning the mechanism through which physical interactions between cardiomyocytes and their environment are translated into biochemical cellular responses and how these signaling modalities can be utilized in vitro to fabricate myocardial tissue constructs from stem cell-derived cardiomyocytes that more faithfully represent their in vivo counterpart. These studies represent a broad effort to characterize biological form as a conduit for information transfer that spans the nanometer length scale of proteins to the meter length scale of the patient and may yield new insights into the contribution of mechanotransduction into heart development and disease.

Implanted adipose progenitor cells as physicochemical regulators of breast cancer

Multipotent adipose-derived stem cells (ASCs) are increasingly used for regenerative purposes such as soft tissue reconstruction following mastectomy; however, the ability of tumors to commandeer ASC functions to advance tumor progression is not well understood. Through the integration of physical sciences and oncology approaches we investigated the capability of tumor-derived chemical and mechanical cues to enhance ASC-mediated contributions to tumor stroma formation. Our results indicate that soluble factors from breast cancer cells inhibit adipogenic differentiation while increasing proliferation, proangiogenic factor secretion, and myofibroblastic differentiation of ASCs. This altered ASC phenotype led to varied extracellular matrix (ECM) deposition and contraction thereby enhancing tissue stiffness, a characteristic feature of breast tumors. Increased stiffness, in turn, facilitated changes in ASC behavior similar to those observed with tumor-derived chemical cues. Orthotopic mouse studies further confirmed the pathological relevance of ASCs in tumor progression and stiffness in vivo. In summary, altered ASC behavior can promote tumorigenesis and, thus, their implementation for regenerative therapy should be carefully considered in patients previously treated for cancer.

Matrix rigidity controls endothelial differentiation and morphogenesis of cardiac precursors

Tissue development and regeneration involve tightly coordinated and integrated processes: selective proliferation of resident stem and precursor cells, differentiation into target somatic cell type, and spatial morphological organization. The role of the mechanical environment in the coordination of these processes is poorly understood. We show that multipotent cells derived from native cardiac tissue continually monitored cell substratum rigidity and showed enhanced proliferation, endothelial differentiation, and morphogenesis when the cell substratum rigidity closely matched that of myocardium. Mechanoregulation of these diverse processes required p190RhoGAP, a guanosine triphosphatase-activating protein for RhoA, acting through RhoA-dependent and -independent mechanisms. Natural or induced decreases in the abundance of p190RhoGAP triggered a series of developmental events by coupling cell-cell and cell-substratum interactions to genetic circuits controlling differentiation.

Spatial control of cell-mediated degradation to regulate vasculogenesis and angiogenesis in hyaluronan hydrogels

Matrix remodeling is crucial for neovascularization, however its utilization to control this process in synthetic biomaterials has been limited. Here, we utilized hyaluronic acid (HA) hydrogels to spatially control cellular remodeling during vascular network formation. Specifically, we exploited a secondary radical polymerization to alter the ability of cells to degrade the hydrogel and utilized it to create spatial patterning using light initiation. We first demonstrated the ability of the hydrogel to either support or inhibit in vitro vasculogenesis of endothelial colony-forming cells (ECFCs) or angiogenesis from ex ovo chorioallantoic membranes. We showed that vascular tube branching and sprouting, which required matrix metalloproteinases (MMPs)-dependent remodeling, could be achieved in hydrogels formed by primary addition-crosslinking only. Although ECFCs expressed higher levels of MMPs in the hydrogels with the secondary radical crosslinking, the generated kinetic chains disabled cell-mediated remodeling and therefore vascular formation was arrested at the vacuole and lumen stage. We then patterned hydrogels to have regions that either permitted or inhibited cell-mediated degradation during in vitro vasculogenesis or angiogenesis. Our ability to control degradation cues that regulate vascular tube formation is important for the study of vascular biology and the application of synthetic biomaterials in tissue regeneration.

Signaling in cell differentiation and morphogenesis

All the information to make a complete, fully functional living organism is encoded within the genome of the fertilized oocyte. How is this genetic code translated into the vast array of cellular behaviors that unfold during the course of embryonic development, as the zygote slowly morphs into a new organism? Studies over the last 30 years or so have shown that many of these cellular processes are driven by secreted or membrane-bound signaling molecules. Elucidating how the genetic code is translated into instructions or signals during embryogenesis, how signals are generated at the correct time and place and at the appropriate level, and finally, how these instructions are interpreted and put into action, are some of the central questions of developmental biology. Our understanding of the causes of congenital malformations and disease has improved substantially with the rapid advances in our knowledge of signaling pathways and their regulation during development. In this article, I review some of the signaling pathways that play essential roles during embryonic development. These examples show some of the mechanisms used by cells to receive and interpret developmental signals. I also discuss how signaling pathways downstream from these signals are regulated and how they induce specific cellular responses that ultimately affect cell fate and morphogenesis.

Rho GTPases mediate the mechanosensitive lineage commitment of neural stem cells

Adult neural stem cells (NSCs) play important roles in learning and memory and are negatively impacted by neurological disease. It is known that biochemical and genetic factors regulate self-renewal and differentiation, and it has recently been suggested that mechanical and solid-state cues, such as extracellular matrix (ECM) stiffness, can also regulate the functions of NSCs and other stem cell types. However, relatively little is known of the molecular mechanisms through which stem cells transduce mechanical inputs into fate decisions, the extent to which mechanical inputs instruct fate decisions versus select for or against lineage-committed blast populations, or the in vivo relevance of mechanotransductive signaling molecules in native stem cell niches. Here we demonstrate that ECM-derived mechanical signals act through Rho GTPases to activate the cellular contractility machinery in a key early window during differentiation to regulate NSC lineage commitment. Furthermore, culturing NSCs on increasingly stiff ECMs enhances RhoA and Cdc42 activation, increases NSC stiffness, and suppresses neurogenesis. Likewise, inhibiting RhoA and Cdc42 or downstream regulators of cellular contractility rescues NSCs from stiff matrix- and Rho GTPase-induced neurosuppression. Importantly, Rho GTPase expression and ECM stiffness do not alter proliferation or apoptosis rates indicating that an instructive rather than selective mechanism modulates lineage distributions. Finally, in the adult brain, RhoA activation in hippocampal progenitors suppresses neurogenesis, analogous to its effect in vitro. These results establish Rho GTPase-based mechanotransduction and cellular stiffness as biophysical regulators of NSC fate in vitro and RhoA as an important regulatory protein in the hippocampal stem cell niche.

Functional Human Vascular Network Generated in Photocrosslinkable Gelatin Methacrylate Hydrogels

The generation of functional, 3D vascular networks is a fundamental prerequisite for the development of many future tissue engineering-based therapies. Current approaches in vascular network bioengineering are largely carried out using natural hydrogels as embedding scaffolds. However, most natural hydrogels present a poor mechanical stability and a suboptimal durability, which are critical limitations that hamper their widespread applicability. The search for improved hydrogels has become a priority in tissue engineering research. Here, the suitability of a photopolymerizable gelatin methacrylate (GelMA) hydrogel to support human progenitor cell-based formation of vascular networks is demonstrated. Using GelMA as the embedding scaffold, it is shown that 3D constructs containing human blood-derived endothelial colony-forming cells (ECFCs) and bone marrow-derived mesenchymal stem cells (MSCs) generate extensive capillary-like networks in vitro. These vascular structures contain distinct lumens that are formed by the fusion of ECFC intracellular vacuoles in a process of vascular morphogenesis. The process of vascular network formation is dependent on the presence of MSCs, which differentiate into perivascular cells occupying abluminal positions within the network. Importantly, it is shown that implantation of cell-laden GelMA hydrogels into immunodeficient mice results in a rapid formation of functional anastomoses between the bioengineered human vascular network and the mouse vasculature. Furthermore, it is shown that the degree of methacrylation of the GelMA can be used to modulate the cellular behavior and the extent of vascular network formation both in vitro and in vivo. These data suggest that GelMA hydrogels can be used for biomedical applications that require the formation of microvascular networks, including the development of complex engineered tissues.

Cellular and physiological mechanisms underlying blood flow regulation in the retina and choroid in health and disease

We review the cellular and physiological mechanisms responsible for the regulation of blood flow in the retina and choroid in health and disease. Due to the intrinsic light sensitivity of the retina and the direct visual accessibility of fundus blood vessels, the eye offers unique opportunities for the non-invasive investigation of mechanisms of blood flow regulation. The ability of the retinal vasculature to regulate its blood flow is contrasted with the far more restricted ability of the choroidal circulation to regulate its blood flow by virtue of the absence of glial cells, the markedly reduced pericyte ensheathment of the choroidal vasculature, and the lack of intermediate filaments in choroidal pericytes. We review the cellular and molecular components of the neurovascular unit in the retina and choroid, techniques for monitoring retinal and choroidal blood flow, responses of the retinal and choroidal circulation to light stimulation, the role of capillaries, astrocytes and pericytes in regulating blood flow, putative signaling mechanisms mediating neurovascular coupling in the retina, and changes that occur in the retinal and choroidal circulation during diabetic retinopathy, age-related macular degeneration, glaucoma, and Alzheimer's disease. We close by discussing issues that remain to be explored.

Differential effects of substrate modulus on human vascular endothelial, smooth muscle, and fibroblastic cells

Regenerative medicine approaches offer attractive alternatives to standard vascular reconstruction; however, the biomaterials to be used must have optimal biochemical and mechanical properties. To evaluate the effects of biomaterial properties on vascular cells, heparinized poly(ethylene glycol) (PEG)-based hydrogels of three different moduli, 13.7, 5.2, and 0.3 kPa, containing fibronectin and growth factor were utilized to support the growth of three human vascular cell types. The cell types exhibited differences in attachment, proliferation, and gene expression profiles associated with the hydrogel modulus. Human vascular smooth muscle cells demonstrated preferential attachment on the highest-modulus hydrogel, adventitial fibroblasts demonstrated preferential growth on the highest-modulus hydrogel, and human umbilical vein endothelial cells demonstrated preferential growth on the lowest-modulus hydrogel investigated. Our studies suggest that the growth of multiple vascular cell types can be supported by PEG hydrogels and that different populations can be controlled by altering the mechanical properties of biomaterials.

Tissue deformation spatially modulates VEGF signaling and angiogenesis

Physical forces play a major role in the organization of developing tissues. During vascular development, physical forces originating from a fluid phase or from cells pulling on their environment can alter cellular signaling and the behavior of cells. Here, we observe how tissue deformation spatially modulates angiogenic signals and angiogenesis. Using soft lithographic templates, we assemble three-dimensional, geometric tissues. The tissues contract autonomously, change shape stereotypically and form patterns of vascular structures in regions of high deformations. We show that this emergence correlates with the formation of a long-range gradient of Vascular Endothelial Growth Factor (VEGF) in interstitial cells, the local overexpression of the corresponding receptor VEGF receptor 2 (VEGFR-2) and local differences in endothelial cells proliferation. We suggest that tissue contractility and deformation can induce the formation of gradients of angiogenic microenvironments which could contribute to the long-range patterning of the vascular system.

Downregulation of endothelial microRNA-200b supports cutaneous wound angiogenesis by desilencing GATA binding protein 2 and vascular endothelial growth factor receptor 2

OBJECTIVE

MicroRNAs (miRs) regulate angiogenesis by posttranscriptional silencing of target genes. The significance of angiostatic miR-200b in switching on skin wound angiogenesis was tested.

METHODS AND RESULTS

Wounding caused imminent and transient downregulation of miR-200b in dermal wound-edge endothelial cells. Derailing this injury response by lentiviral delivery of miR-200b in vivo impaired wound angiogenesis. Computational prediction, target reporter luciferase assay, and Western blot analysis provided first evidence that miR-200b targets globin transcription factor binding protein 2 (GATA2) and vascular endothelial growth factor receptor 2 (VEGFR2). Overexpression of GATA2 or VEGFR2 in endothelial cells rescued the angiostatic effect of miR-200b in vitro. Downregulation of miR-200b derepressed GATA2 and VEGFR2 expression to switch on wound angiogenesis, which was disrupted in diabetic wounds. Treatment of endothelial cells with tumor necrosis factor-α, a proinflammatory cytokine abundant in diabetic wounds, induced miR-200b expression, silenced GATA2 and VEGFR2, and suppressed angiogenesis. These outcomes were attenuated using anti-miR-200b strategy. Neutralization of tumor necrosis factor-α in the diabetic wounds improved wound angiogenesis and closure, which was accompanied by downregulation of miR-200b expression and desilencing of GATA2 and VEGFR2.

CONCLUSIONS

Injury-induced repression of miR-200b turned on wound angiogenesis. In mice with diabetes mellitus,excessive tumor necrosis factor-α induced miR-200b blunting proangiogenic functions of GATA2 and VEGFR2.

New insights into the plasticity of the endothelial phenotype

The mammalian vascular system consists of two distinct, but closely related, networks: the blood vasculature (itself divided into arterial and venous networks) and the lymphatic vasculature. EC (endothelial cell) lineage specification has been proposed to be determined during embryonic development, after which the ECs are committed to their fate. However, increasing evidence suggests that ECs retain various degrees of plasticity, and have the ability to express characteristics of alternative cell lineages. Therapeutic control of endothelial plasticity will allow greater understanding of the genesis and treatment of several vascular diseases.

A heterogeneous in vitro three dimensional model of tumour-stroma interactions regulating sprouting angiogenesis

Angiogenesis, the formation of new blood vessels, is an essential process for tumour progression and is an area of significant therapeutic interest. Different in vitro systems and more complex in vivo systems have been described for the study of tumour angiogenesis. However, there are few human 3D in vitro systems described to date which mimic the cellular heterogeneity and complexity of angiogenesis within the tumour microenvironment. In this study we describe the Minitumour model--a 3 dimensional human spheroid-based system consisting of endothelial cells and fibroblasts in co-culture with the breast cancer cell line MDA-MB-231, for the study of tumour angiogenesis in vitro. After implantation in collagen-I gels, Minitumour spheroids form quantifiable endothelial capillary-like structures. The endothelial cell pre-capillary sprouts are supported by the fibroblasts, which act as mural cells, and their growth is increased by the presence of cancer cells. Characterisation of the Minitumour model using small molecule inhibitors and inhibitory antibodies show that endothelial sprout formation is dependent on growth factors and cytokines known to be important for tumour angiogenesis. The model also shows a response to anti-angiogenic agents similar to previously described in vivo data. We demonstrate that independent manipulation of the different cell types is possible, using common molecular techniques, before incorporation into the model. This aspect of Minitumour spheroid analysis makes this model ideal for high content studies of gene function in individual cell types, allowing for the dissection of their roles in cell-cell interactions. Finally, using this technique, we were able to show the requirement of the metalloproteinase MT1-MMP in endothelial cells and fibroblasts, but not cancer cells, for sprouting angiogenesis.

The impact of adhesion peptides within hydrogels on the phenotype and signaling of normal and cancerous mammary epithelial cells

The microenviroment contributes to directing mammary epithelial cell (MEC) development and the progression of breast cancer. Three-dimensional culture models have been used to support formation of structures that display varying degrees of disorganization that parallel the degree of cancer. Synthetic hydrogels were employed to investigate the mechanisms by which specific adhesion signals in the microenvironment directed development. Polyethylene glycol-based hydrogels supported 3D growth of MECs and directed formation of a range of phenotypes that were functions of genotype, and identity and concentration of adhesion peptides RGD and YIGSR. Non-cancerous and cancerous MECs responded differentially to the same adhesion cues and produced variable structural organizations. An analysis of dynamic signaling pathways revealed differential activities of transcription factors within the MAPK and JAK/STAT pathways in response to genotype and adhesion. These results directly implicate adhesion in cancer development and demonstrate that AP1, CREB, STAT1, and STAT3 all contribute to the genotype dependence of cellular response to adhesion peptides. The tools presented in this work could be applied to other systems and connect extracellular cues with intracellular signaling to molecularly dissect tissue development and further biomaterials development.

Mechanisms of vascular endothelial growth factor-induced pathfinding by endothelial sprouts in biomaterials

A critical property of biomaterials for use in regenerative medicine applications is the ability to promote angiogenesis, the formation of new vascular networks, to support regenerating tissues. Recent studies have demonstrated that a complex interplay exists between biomechanical and biochemical regulators of endothelial cell sprouting, an early step in angiogenesis. Here, we use a microfluidic platform to study the pathfinding behaviors induced by various stable vascular endothelial growth factor (VEGF) gradients during sprouting morphogenesis within biomaterials. Quantitative, time-lapse analysis of endothelial sprouting demonstrated that the ability of VEGF to regulate sprout orientation during several stages of sprouting morphogenesis (initiation, elongation, and turning navigation) was biomaterial dependent. Identical VEGF gradients induced different types of coordinated cell movements depending on the density of the surrounding collagen/fibronectin matrix. In denser matrices, sprouts were more likely to have an initial orientation aligned parallel to the VEGF gradient. In contrast, in less dense matrices, sprouts were more likely to initially misalign with the VEGF gradient; however, these sprouts underwent significant turning and navigation to eventually reorient to be parallel to the VEGF gradient. These less dense matrices required shallower VEGF gradients and demonstrated lower activating VEGF thresholds to induce proper sprout alignment and pathfinding. These results encourage the future use of microfluidic platforms to probe fundamental aspects of matrix effects on angiogenesis, to screen biomaterials for angiogenic potential, and to design ex vivo tissues with aligned vascular networks.

Quantification of local matrix deformations and mechanical properties during capillary morphogenesis in 3D

Reciprocal mechanical interactions between cells and the extracellular matrix (ECM) are thought to play important instructive roles in branching morphogenesis. However, most studies to date have failed to characterize these interactions on a length scale relevant to cells, especially in three-dimensional (3D) matrices. Here we utilized two complementary methods, spatio-temporal image correlation spectroscopy (STICS) and laser optical tweezers-based active microrheology (AMR), to quantify endothelial cell (EC)-mediated deformations of individual ECM elements and the local ECM mechanical properties, respectively, during the process of capillary morphogenesis in a 3D cell culture model. In experiments in which the ECM density was systematically varied, STICS revealed that the rate at which ECs deformed individual ECM fibers on the microscale positively correlated with capillary sprouting on the macroscale. ECs expressing constitutively active V14-RhoA displaced individual matrix fibers at significantly faster rates and displayed enhanced capillary sprouting relative to wild-type cells, while those expressing dominant-negative N19-RhoA behaved in an opposite fashion. In parallel, AMR revealed a local stiffening of the ECM proximal to the tips of sprouting ECs. By quantifying the dynamic physical properties of the cell-ECM interface in both space and time, we identified a correlation linking ECM deformation rates and local ECM stiffening at the microscale with capillary morphogenesis at the macroscale.

Neural crest specification by noncanonical Wnt signaling and PAR-1

Neural crest (NC) cells are multipotent progenitors that form at the neural plate border, undergo epithelial-mesenchymal transition and migrate to diverse locations in vertebrate embryos to give rise to many cell types. Multiple signaling factors, including Wnt proteins, operate during early embryonic development to induce the NC cell fate. Whereas the requirement for the Wnt/β-catenin pathway in NC specification has been well established, a similar role for Wnt proteins that do not stabilize β-catenin has remained unclear. Our gain- and loss-of-function experiments implicate Wnt11-like proteins in NC specification in Xenopus embryos. In support of this conclusion, modulation of β-catenin-independent signaling through Dishevelled and Ror2 causes predictable changes in premigratory NC. Morpholino-mediated depletion experiments suggest that Wnt11R, a Wnt protein that is expressed in neuroectoderm adjacent to the NC territory, is required for NC formation. Wnt11-like signals might specify NC by altering the localization and activity of the serine/threonine polarity kinase PAR-1 (also known as microtubule-associated regulatory kinase or MARK), which itself plays an essential role in NC formation. Consistent with this model, PAR-1 RNA rescues NC markers in embryos in which noncanonical Wnt signaling has been blocked. These experiments identify novel roles for Wnt11R and PAR-1 in NC specification and reveal an unexpected connection between morphogenesis and cell fate.

New insights into the role of angiogenin in actin polymerization

Angiogenin is a potent stimulator of angiogenesis. It interacts with endothelial cells and induces a wide range of cellular responses initiating a process of blood vessel formation. One important target of angiogenin is endothelial cell-surface actin, and their interaction might be one of crucial steps in angiogenin-induced neovascularization. Recently, it was shown that angiogenin inhibits polymerization of G-actin and changes the physical properties of F-actin. These observations suggest that angiogenin may cause changes in the cell cytoskeleton. This chapter reviews the current state of the literature regarding angiogenin structure and function and discusses the relationship between the angiogenin and actin and possible functional roles of their interaction.

On the somatosensation of vision

The interconnection between vision and somatosensation is already well-established and is further supplemented by the evolutionary link between eyes and photoreceptors, and the functional connection between photosensation and thermoreception. However, our analysis shows that the relation between vision and somatosensation is much deeper and suggests that somatosensation may possibly be the basis of vision. Surprisingly, our photoreceptor itself needs somatosensory proteins for its functioning, and our entire visual pathway depends on somatosensory cues for its functioning.