Liver organogenesis promoted by endothelial cells prior to vascular function

Two populations of Thy1-positive mesenchymal cells regulate in vitro maturation of hepatic progenitor cells

We previously reported that the in vitro maturation of CD49f(+)Thy1(-)CD45(-) (CD49f positive) fetal hepatic progenitor cells (HPCs) is supported by Thy1-positive mesenchymal cells derived from the fetal liver. These mesenchymal cell preparations contain two populations, one of a cuboidal shape and the other spindle shaped in morphology. In this study, we determined that the mucin-type transmembrane glycoprotein gp38 could distinguish cuboidal cells from spindle cells by immunocytochemistry. RT-PCR analysis revealed differences between isolated CD49f(+/-)Thy1(+)gp38(+)CD45(-) (gp38 positive) cells and CD49f(+/-)Thy1(+)gp38(-)CD45(-) (gp38 negative) cells, whereas both cells expressed mesenchymal cell markers. The coculture with gp38-positive cells promoted the maturation of CD49f-positive HPCs, which was estimated by positivity for periodic acid-Schiff (PAS) staining, whereas the coculture with gp38-negative cells maintained CD49f-positive HPCs negative for PAS staining. The expression of mature hepatocyte markers, such as tyrosine aminotransferase, tryptophan-2,3-dioxygenase, and glucose-6-phosphatase, were upregulated on HPCs by coculture with gp38-positive cells. Furthermore, transmission electron microscopy revealed the acquisition of mature hepatocyte features by HPCs cocultured with gp38-positive cells. This effect on maturation of HPCs was inhibited by the addition of conditioned medium derived from gp38-negative cells. By contrast, the upregulation of bromodeoxyuridine incorporation by HPCs demonstrated the proliferative effect of coculture with gp38-negative cells. In conclusion, these results suggest that in vitro maturation of HPCs promoted by gp38-positive cells may be opposed by an inhibitory effect of gp38-negative cells, which likely maintain the immature, proliferative state of HPCs.

Regulation of cardiac growth and coronary angiogenesis by the Akt/PKB signaling pathway

Postnatal growth of the heart is primarily achieved through hypertrophy of individual myocytes. Cardiac growth observed in athletes represents adaptive or physiological hypertrophy, whereas cardiac growth observed in patients with hypertension or valvular heart diseases is called maladaptive or pathological hypertrophy. These two types of hypertrophy are morphologically, functionally, and molecularly distinct from each other. The serine/threonine protein kinase Akt is activated by various extracellular stimuli in a phosphatidylinositol-3 kinase-dependent manner and regulates multiple aspects of cellular functions including survival, growth and metabolism. In this review we will discuss the role of the Akt signaling pathway in the heart, focusing on the regulation of cardiac growth, contractile function, and coronary angiogenesis. How this signaling pathway contributes to the development of physiological/pathological hypertrophy and heart failure will also be discussed.

Hemangioblasts representing a functional endothelio-hematopoietic entity in ontogeny, postnatal life, and CML neovasculogenesis

The life-long interdependencies/interactions between hemato- and endotheliopoiesis suggest that they form a supplementary functional entity. This view is compatible with the concept of stem cell plasticity as a reversible continuum and is substantiated by the common hematopoietic-endothelial stem cell, i.e., hemangioblasts, with bidirectional, reversible gene transcription and persistence in postnatal life. Indeed, embryonal stem cells/hemangioblasts appear to form a reservior in the adult with the possibility of dedifferentiation of more differentiated progenitor cells back to hemangioblasts. The recent detection of BCR/ABL fusion proteins in endothelial cells during vascular neoangiogenesis in CML suggests that endothelial cells are part of the neoplastic clone, and extends the concept of a functional entity to include CML angiogenesis. Thus, hemangioblasts rather than committed hematopoietic stem cells appear to be target cells for the first oncogenic hit in CML, which could occur as early as during the first steps of embryonal stem cell differentiation towards hemato-endotheliopoiesis and/or in hemangioblasts persisting in adults. The relation of the other leukemias to hemangioblasts is not known.

Development of murine hepatic sinusoidal endothelial cells characterized by the expression of hyaluronan receptors

Endothelial cells (ECs) display distinct structural and functional characteristics depending on the tissue and developmental stage; however, the development of tissue-specific ECs remains poorly understood. Here, we describe the development of hepatic sinusoids in mice based on the expression of hyaluronan receptors Stab2 and Lyve-1. Flk-1(+) cells in and around the liver bud begin to express Stab2 at embryonic day (E) 9.5, before the formation of vascular lumen. Hepatic sinusoidal endothelial cells (HSECs) begin to express Lyve-1 at E10.5, and both markers continue to be expressed in HSECs thereafter. Although HSECs and lymphatic ECs (LECs) are known to share functional and phenotypic characteristics, we clearly show that HSECs can be distinguished from LECs by the expression of molecular markers and higher endocytotic activity. Our results provide new insight into the development of tissue-specific ECs and phenotypic criteria to distinguish HSECs from other types of ECs, including LECs.

Molecular Mechanism of Liver Development and Regeneration

The liver is the central organ for metabolism and has strong regenerative capability. Although the liver has been studied mostly biochemically and histopathologically, genetic studies using gene-targeting technology have identified a number of cytokines, intracellular signaling molecules, and transcription factors involved in liver development and regeneration. In addition, various in vitro systems such as fetal liver explant culture and primary culture of fetal liver cells have been established, and the combination of genetic and in vitro studies has accelerated investigation of liver development. Identification of the cell-surface molecules of liver progenitors has made it possible to identify and isolate liver progenitors, making the liver a unique model for stem cell biology. In this review, we summarize progresses in understanding liver development and regeneration.

Essential function of PTP-PEST during mouse embryonic vascularization, mesenchyme formation, neurogenesis and early liver development

PTP (protein-tyrosine phosphatase)-PEST is a ubiquitously expressed cellular regulator of integrin signalling. It has been shown to bind several molecules such as Shc, paxillin and Grb2, that are involved downstream of FAK (focal adhesion kinase) pathway. Through its specific association to p130cas and further dephosphorylation, PTP-PEST plays a critical role in cell-matrix interactions, which are essential during embryogenesis. We report here that ablation of the gene leads to early embryonic lethality, correlating well with the high expression of the protein during embryonic development. We observed an increased level of tyrosine phosphorylation of p130cas protein in E9.5 PTP-PEST(-/-) embryos, a first evidence of biochemical defect leading to abnormal growth and development. Analysis of null mutant embryos revealed that they reach gastrulation, initiate yolk sac formation, but fail to progress through normal subsequent developmental events. E9.5-10.5 PTP-PEST(-/-) embryos had morphological abnormalities such as defective embryo turning, improper somitogenesis and vasculogenesis, impaired liver development, accompanied by degeneration in both neuroepithelium and somatic epithelia. Moreover, in embryos surviving until E10.5, the caudal region was truncated, with severe mesenchyme deficiency and no successful liver formation. Defects in embryonic mesenchyme as well as subsequent failure of proper vascularization, liver development and somatogenesis, seemed likely to induce lethality at this stage of development, and these results confirm that PTP-PEST plays an essential function in early embryogenesis.

The vascular niche and its basement membrane

Over the past few years, scientists have realized that many cellular and developmental processes, including pancreatic beta-cell growth and differentiation, stem cell and progenitor cell proliferation and cancer cell metastasis, occur in what are known as 'vascular niches'. Despite increasing numbers of reports on these niches, few common mechanisms have been identified to explain their various effects. Here, we define the term 'vascular niche' and suggest that a common and conserved feature of this niche is to provide a basement membrane to cells that are unable to form their own. We further propose that these cells require a vascular niche when they retain a high degree of plasticity.

Microarray analysis of gene expression in Men1 knockout embryoid body reveals genetic events involved in early mouse embryonic development

The Men1 gene has been identified as the gene responsible for MEN1, a hereditary syndrome transmitted with an autosomal dominant trait. Disruption of the Men1 gene results in defects of multiple organs development, including the nervous system, heart, liver, cranium, and face. In this study, we used embryoid bodies (EBs) formed from wild-type and Men1-/- ES cells as a model system to investigate effect of Men1 gene on the embryo development. We characterized in detail gene expression profile of these Men1-/- EBs by microarray techniques and identified a series of putative menin targeted genes, including genes involved in development of bone (e.g., Postn, Runx2, and Msx2), liver (e.g., KDR), blood (e.g., Hox9 and Kitl), and pancreatic islet (e.g., Sox4, Foxa1, Btc, Igf2, and Nfatc1). Further studies may shed light onto the underlying mechanisms of the interplay between menin and these genes.

BMP-4 is required for hepatic specification of mouse embryonic stem cell-derived definitive endoderm

When differentiated in the presence of activin A in serum-free conditions, mouse embryonic stem cells efficiently generate an endoderm progenitor population defined by the coexpression of either Brachyury, Foxa2 and c-Kit, or c-Kit and Cxcr4. Specification of these progenitors with bone morphogenetic protein-4 in combination with basic fibroblast growth factor and activin A results in the development of hepatic populations highly enriched (45-70%) for cells that express the alpha-fetoprotein and albumin proteins. These cells also express transcripts of Afp, Alb1, Tat, Cps1, Cyp7a1 and Cyp3a11; they secrete albumin, store glycogen, show ultrastructural characteristics of mature hepatocytes, and are able to integrate into and proliferate in injured livers in vivo and mature into hepatocytes expressing dipeptidyl peptidase IV or fumarylacetoacetate hydrolase. Together, these findings establish a developmental pathway in embryonic stem cell differentiation cultures that leads to efficient generation of cells with an immature hepatocytic phenotype.

Islet microvasculature in islet hyperplasia and failure in a model of type 2 diabetes

Gene expression profiling of islets from pre-diabetic male Zucker diabetic fatty (ZDF) rats showed increased expression of hypoxia-related genes, prompting investigation of the vascular integrity of the islets. The islet microvasculature was increased approximately twofold in young male ZDF rats by both morphometric analysis and quantifying mRNA levels of endothelial markers. ZDF rats at 12 weeks of age showed a significant reduction in the number of endothelial cells, which was prevented by pretreatment with pioglitazone. Light and electron microscopy of normoglycemic 7-week-old ZDF rats showed thickened endothelial cells with loss of endothelial fenestrations. By 12 weeks of age, there was disruption of the endothelium and intra-islet hemorrhage. Islets from 7- and 12-week-old ZDF rats showed an approximate three- and twofold increase in vascular endothelial growth factor (VEGF)-A mRNA and VEGF protein secretion, respectively, compared with lean controls. Thrombospondin-1 mRNA increased in 7- and 12-week-old rats by 2- and 10-fold, respectively, and was reduced by 50% in 12-week-old rats pretreated with pioglitazone. Islets from young male control rats induced migration of endothelial cells in a collagen matrix only after pretreatment with matrix metalloproteinase (MMP)-9. Islets from 7-week-old ZDF rats showed a fivefold increase in migration score compared with wild-type controls, even without MMP-9 treatment. Islets from 15-week-old ZDF rats did not induce migration; rather, they caused a significant rounding up of the duct-derived cells, suggesting a toxic effect. These data suggest that in the ZDF rat model of type 2 diabetes, an inability of the islet to maintain vascular integrity may contribute to beta-cell failure.

An FGF response pathway that mediates hepatic gene induction in embryonic endoderm cells

While particular combinations of mesodermal signals are known to induce distinct tissue-specific programs in the endoderm, there is little information about the response pathways within endoderm cells that control their specification. We have used signaling inhibitors on embryo tissue explants and whole-embryo cultures as well as genetic approaches to reveal part of an intracellular network by which FGF signaling helps induce hepatic genes and stabilize nascent hepatic cells within the endodermal epithelium. Specifically, we found that hepatic gene induction is elicited by an FGF/MAPK pathway. Although the PI3K pathway is activated in foregut endoderm cells, its inhibition does not block hepatic gene induction in explants; however, it does block tissue growth. We also found that at the onset of hepatogenesis, the FGF/MAPK and PI3K pathways do not crossregulate in the endoderm. The finding of separate pathways for endoderm tissue specification and growth provides insights for guiding cellular regeneration and stem cell differentiation.

Integration of technologies for hepatic tissue engineering

The liver is the largest internal organ in the body, responsible for over 500 metabolic, regulatory, and immune functions. Loss of liver function leads to liver failure which causes over 25,000 deaths/year in the United States. Efforts in the field of hepatic tissue engineering include the design of bioartificial liver systems to prolong patient's lives during liver failure, for drug toxicity screening and for the study of liver regeneration, ischemia/reperfusion injury, fibrosis, viral infection, and inflammation. This chapter will overview the current state-of-the-art in hepatology including isolated perfused liver, culture of liver slices and tissue explants, hepatocyte culture on collagen "sandwich" and spheroids, coculture of hepatocytes with non-parenchymal cells, and the integration of these culture techniques with microfluidics and reactor design. This work will discuss the role of oxygen and medium composition in hepatocyte culture and present promising new technologies for hepatocyte proliferation and function. We will also discuss liver development, architecture, and function as they relate to these culture techniques. Finally, we will review current opportunities and major challenges in integrating cell culture, bioreactor design, and microtechnology to develop new systems for novel applications.

Hematopoietic stem cells and their niche

Recent evidence indicates that osteoblasts are crucial components of the particular microenvironments, or niches, for hematopoietic stem cells (HSCs) in adult bone marrow (BM). Stem cells persist in an immature state within the BM. The quiescence of HSCs is controlled dynamically by the signaling of receptors-ligands and cell-adhesion molecules. In this review, the characteristics of HSCs in the niche are discussed. The understanding of the relationship between normal and cancer stem cells and their niches should lead to the development of new strategies directed toward regeneration medicine and cancer therapeutics.

Disruption of the Arnt gene in endothelial cells causes hepatic vascular defects and partial embryonic lethality in mice

Vascular endothelial cells (ECs) play a critical role in angiogenesis and organogenesis, especially in embryonic liver development. Hypoxia-inducible transcription factors (Hifs) are a key trigger of hypoxic signals, a primary stimulus of angiogenesis. The aryl hydrocarbon receptor nuclear translocator (Arnt), also called Hif-1beta, serves as an obligate heterodimerization partner of Hif-1alpha and Hif-2alpha. Using Cre-Lox technology, the mouse Arnt gene was specifically disrupted in endothelial cells. The resulting mice, designated ArntDeltaEC, developed impaired hepatic vasculature, liver necrosis, and degenerative lesions in cardiac myocytes at the late embryonic stage (E16.5-E18.5), leading to approximately 90% neonatal lethality. Low serum glucose, downregulation of glucose transporter-1 and glucose-6-phosphatase mRNA, and hepatocyte proliferation were observed in ArntDeltaEC embryos. Magnetic resonance imaging on E16.5 embryonic livers revealed that ArntDeltaEC mice had a significant volume of avascular region. ArntDeltaEC mice that survived to the adult stage were fertile, showed normal behavioral activity, but had smaller livers with mild portal fibrosis, dilated blood vessels, abnormal collagen accumulation, and remarkable iron deposition. ArntDeltaEC mice had reduced adiposity, impaired serum lipid homeostasis, and a higher respiratory exchange ratio, indicating they utilized relatively more carbohydrates than their ArntF/F counterparts. In conclusion, endothelial Arnt plays a pivotal role in embryonic liver development. Adult ArntDeltaEC mice carrying embryonic hepatic defects developed what was possibly an early stage of cirrhosis with consequences of limited oxygen availability and altered lipid metabolism.

Endothelium is required for the promotion of interrenal morphogenetic movement during early zebrafish development

The adrenal cortex has a complex vasculature that is essential for growth, tissue maintenance, and access of secreted steroids to the bloodstream. However, the interaction between vasculature and adrenal cortex during early organogenesis remains largely unclear. In this study, we focused on the zebrafish counterpart of adrenal cortex, interrenal tissue, to explore the possible role of endothelium in the development of steroidogenic tissues. The ontogeny of interrenal tissue was found to be tightly associated with the endothelial cells (ECs) that constitute the axial vessels. The early interrenal primordia emerge as two clusters of cells that migrate centrally and converge at the midline, whereas the central convergence was abrogated in the avascular cloche (clo) mutant. Neither loss of blood circulation nor perturbations of vessel assembly could account for the interrenal convergence defect, implying a role of endothelial signaling prior to the formation of axial blood vessels. Moreover, as the absence of trunk endothelium in clo mutant was rescued by the forced expression of SCL, the interrenal fusion defect could be alleviated. We thus conclude that endothelial signaling is involved in the morphogenetic movement of early interrenal tissue.

Puromycin insensitive leucyl-specific aminopeptidase (PILSAP) is required for the development of vascular as well as hematopoietic system in embryoid bodies

We have shown that puromycin insensitive leucyl-specific aminopeptidase (PILSAP) is required for regulation of angiogenesis. However, it remains unclear whether PILSAP plays a role in endothelial cell (EC) differentiation. We examined the role of PILSAP by using an embryoid bodies (EBs) culture system. Fms-like tyrosine kinase-1 (Flk-1) showed two expression peaks on days 4 and 10 of culture. These two peaks represent populations of mesodermal precursors and mature ECs, respectively. Endothelial markers such as VE-cadherin, CD34, CD31 and Tie2 followed the first peak of Flk-1. Interestingly, the expression of PILSAP showed a pattern similar to that of Flk-1. ES cells transfected with mutant PILSAP (mtPILSAP) cDNA of a dominant negative activity organized less vascular structure and showed decreased levels of vascular lineage markers. The similar results were obtained in EBs treated with leucinethiol, a specific inhibitor of leucine aminopeptidase or siRNA for PILSAP. However, Flk-1 expression was unaffected on day 4. The expression of markers for hematopoietic lineage and muscle cells in mtPILSAP-EBs was also reduced. These results suggest that although PILSAP may not function in the initial generation of Flk-1 positive mesodermal precursors, it dose play a role in growth of vascular, hematopoietic, and muscular lineage population from those precursors.

Study of hepatocyte differentiation using embryonic stem cells

The liver has many crucial functions including metabolizing dietary molecules, detoxifying compounds, and storing glycogen. The hepatocytes, comprising most of the liver organ, progressively modify their gene expression profile during the fetal development according to their roles in the different phases of development. Embryonic stem (ES) cells serve as a major tool in understanding liver development. These cells may also serve as a source of hepatic cells for cellular therapy. In this review, we aim to summarize the research that has been performed in the field of hepatocyte differentiation from mouse and human ES cells. We discuss the various methodologies for the differentiation of ES cells towards hepatic cells using either spontaneous or directed differentiation protocols. Although many protocols for differentiating ES cells to hepatic cells have been developed, the analysis of their status is not trivial and can lead to various conclusions. Hence, we discuss the issues of analyzing hepatocytes by means of the specificity of the markers for hepatocytes and the status of the cells as fetal or adult hepatocytes.

Hepatic Sinusoidal Endothelial Cells Promote Hepatocyte Proliferation Early after Partial Hepatectomy in Rats

BACKGROUND

We undertook this study in rats to investigate the role of hepatic sinusoidal endothelial cells (SECs) in hepatocyte proliferation early after partial hepatectomy and the regulatory mechanisms involved.

METHODS

The animal model of 70% hepatectomy was made. Hepatic SECs and hepatocytes were isolated and cultured according to the method of Braet et al. with some modifications. Levels of nitric oxide (NO), interleukin-6 (IL-6), and hepatic growth factor (HGF) in the supernatants of hepatic SEC cultures were measured, and the expression of HGF mRNA by hepatic SECs was analyzed. The relationship between the supernatants of hepatic SEC cultures and hepatocyte proliferation was probed. (3)H-thymidine incorporation and the proliferating cell nuclear antigen (PCNA) labeling index of hepatocytes were used as signs of hepatocyte proliferation.

RESULTS

Levels of NO, IL-6, and HGF in the supernatants of hepatic SECs cultures were increased markedly 6 and 24 h after hepatectomy and then were decreased gradually. The expression of HGF mRNA by cultured SECs was increased markedly 6 and 24 h after hepatectomy, with a peak 6 h after hepatectomy. The PCNA labeling index and (3)H-thymidine incorporation of hepatocytes started to increase 6 h after hepatectomy, with a peak at 24 h. Hepatic SECs were isolated from rats 24 h after partial hepatectomy and cultured for 24 h, and the culture supernatants were obtained. The supernatants not only significantly enhanced the PCNA labeling index and (3)H-thymidine incorporation of proliferating hepatocytes isolated from rats after partial hepatectomy but also obviously increased the DNA synthesis of quiescent hepatocytes from the control rats. The extent of hepatocyte proliferation was closely related to the amount of the SEC culture supernatants added in both rats after partial hepatectomy and control rats.

CONCLUSIONS

These results suggest that cytokines (such as IL-6, HGF and NO) secreted by SECs play important roles in liver regeneration early after partial hepatectomy. We speculate that activated hepatic SECs secrete some substances that induce or trigger liver regeneration after partial hepatectomy.

Epithelial stem cells of the lung: privileged few or opportunities for many?

Most reviews of adult stem cells focus on the relatively undifferentiated cells dedicated to the renewal of rapidly proliferating tissues, such as the skin, gut and blood. By contrast, there is mounting evidence that organs and tissues such as the liver and pancreatic islets, which turn over more slowly,use alternative strategies, including the self-renewal of differentiated cells. The response of these organs to injury may also reveal the potential of differentiated cells to act as stem cells. The lung shows both slow turnover and rapid repair. New experimental approaches, including those based on studies of embryonic development, are needed to identify putative lung stem cells and strategies of lung homeostasis and repair.

VEGF modulates erythropoiesis through regulation of adult hepatic erythropoietin synthesis

Vascular endothelial growth factor (VEGF) exerts crucial functions during pathological angiogenesis and normal physiology. We observed increased hematocrit (60-75%) after high-grade inhibition of VEGF by diverse methods, including adenoviral expression of soluble VEGF receptor (VEGFR) ectodomains, recombinant VEGF Trap protein and the VEGFR2-selective antibody DC101. Increased production of red blood cells (erythrocytosis) occurred in both mouse and primate models, and was associated with near-complete neutralization of VEGF corneal micropocket angiogenesis. High-grade inhibition of VEGF induced hepatic synthesis of erythropoietin (Epo, encoded by Epo) >40-fold through a HIF-1alpha-independent mechanism, in parallel with suppression of renal Epo mRNA. Studies using hepatocyte-specific deletion of the Vegfa gene and hepatocyte-endothelial cell cocultures indicated that blockade of VEGF induced hepatic Epo by interfering with homeostatic VEGFR2-dependent paracrine signaling involving interactions between hepatocytes and endothelial cells. These data indicate that VEGF is a previously unsuspected negative regulator of hepatic Epo synthesis and erythropoiesis and suggest that levels of Epo and erythrocytosis could represent noninvasive surrogate markers for stringent blockade of VEGF in vivo.

Regulation of early lung morphogenesis: questions, facts and controversies

During early respiratory system development, the foregut endoderm gives rise to the tracheal and lung cell progenitors. Through branching morphogenesis, and in coordination with vascular development, a tree-like structure of epithelial tubules forms and differentiates to produce the airways and alveoli. Recent studies have implicated the fibroblast growth factor, sonic hedgehog, bone morphogenetic protein, retinoic acid and Wnt signaling pathways, and various transcription factors in regulating the initial stages of lung development. However, the precise roles of these molecules and how they interact in the developing lung is subject to debate. Here, we review early stages in lung development and highlight questions and controversies regarding their molecular regulation.

Mesodermal Wnt2b signalling positively regulates liver specification

Endodermal organs such as the lung, liver and pancreas emerge at precise locations along the primitive gut tube. Although several signalling pathways have been implicated in liver formation, so far no single gene has been identified that exclusively regulates liver specification. In zebrafish, the onset of liver specification is marked by the localized endodermal expression of hhex and prox1 at 22 hours post fertilization. Here we used a screen for mutations affecting endodermal organ morphogenesis to identify a unique phenotype: prometheus (prt) mutants exhibit profound, though transient, defects in liver specification. Positional cloning reveals that prt encodes a previously unidentified Wnt2b homologue. prt/wnt2bb is expressed in restricted bilateral domains in the lateral plate mesoderm directly adjacent to the liver-forming endoderm. Mosaic analyses show the requirement for Prt/Wnt2bb in the lateral plate mesoderm, in agreement with the inductive properties of Wnt signalling. Taken together, these data reveal an unexpected positive role for Wnt signalling in liver specification, and indicate a possible common theme for the localized formation of endodermal organs along the gut tube.

Preferential expression of connexin37 and connexin40 in the endothelium of the portal veins during mouse liver development

Hepatic blood vessels consist of the hepatic artery and three types of venous channels (the portal veins, the sinusoids, and the hepatic veins). This study was undertaken to analyze, by immunohistochemistry, connexin expression throughout the vascular development of the fetal mouse liver with special attention being given to portal vein development. In the adult liver, connexin37 and connexin40 were expressed in the endothelium of the portal vein and hepatic artery, but not in those of the hepatic vein and sinusoids. Connexin43 was expressed in mesothelial cells and smooth muscle cells of the portal veins. The preferential expression of connexin37 and connexin40 in portal veins was seen throughout liver development, including its primordium formation stage (10.5-day or 11.5-day stage), although connexin37 expression was transiently seen in free nonparenchymal cells in fetal stages. The differentiation of each blood vessel in the hepatic vascular system may occur in early developmental stages, soon after hepatic primordium formation.

Endothelium-Mediated Hepatocyte Recruitment in the Establishment of Liver-like TissueIn Vitro

A major goal of liver tissue engineering is to understand how the constituent cell types interact to achieve liver-specific structure and function. Here we show that hepatocytes migrate toward and adhere to endothelial vascular structures formed on Matrigel in vitro, and that hepatocyte recruitment is dependent on endothelium-derived hepatocyte growth factor. The hepatocyte-decorated endothelial vascular structures resemble In vivo sinusoids containing plate-like structures, bile canaliculi, and a lumen. The sinusoid-like structures retained cytochrome P450 expression and activity, in addition to stable albumin expression and secretion rate for over 2 months in vitro. The stability of the sinusoid-like structures was dependent on the presence of vimentin-positive fibroblasts in culture. The sinusoid-like structures formed by hepatocytes and pure populations of endothelial cells collapsed after 10 days in culture. In contrast, culture of hepatocytes with fibroblast-contaminated human dermal microvascular endothelial cells or a combination of human umbilical vein endothelial cells and normal human dermal fibroblasts resulted in stable sinusoid-like structures surrounded by a fibroblastic capsule that maintained liver specific functions for several months in vitro. These results demonstrate that specification of endothelial cell position ultimately determines hepatocyte position in vitro, suggesting that similar interactions might occur In vivo. The novelty of the culture's sinusoid-like organization and long-term function suggest a new model for the study of liver toxicity, ischemia/reperfusion injury, and fibrosis.

P19 Progenitor Cells Progress to Organized Contracting Myocytes After Chemical and Electrical Stimulation:Implications for Vascular Tissue Engineering

PURPOSE

To test the hypothesis that a level of chemical and electrical stimulation exists that allows differentiation of progenitor cells into organized contracting myocytes.

METHODS

A custom-made bioreactor with the capability of delivering electrical pulses of varying field strengths, widths, and frequencies was constructed. Individual chambers of the bioreactor allowed continuous electrical stimulation of cultured cells under microscopic observation. On day 0, 1% dimethylsulfoxide (DMSO), known to differentiate cells into myocytes, was added to P19 progenitor cells. Additionally, for the next 22 days, electrical pulses of varying field strengths (0-3 V/cm), widths (2-40 ms), and frequencies (10-25 Hz) were continuously applied. On day 5, the medium containing DMSO was exchanged with regular medium, and the electrical stimulation was continued. From days 6-22, the cells were visually assessed for signs of viability, contractility, and organization.

RESULTS

P19 cells remained viable with pulsed electrical fields <3 V/cm, pulse widths <40 ms, and pulse frequencies from 10 to 25 Hz. On day 12, the first spontaneous contractions were observed. For individual colonies, local synchronization and organization occurred; multiple colonies were synchronized with externally applied electrical fields.

CONCLUSION

P19 progenitor cells progress to organized contracting myocytes after chemical and electrical stimulation. Incorporation of such cells into existing methods of producing endothelial cells, fibroblasts, and scaffolds may allow production of improved tissue-engineered vascular grafts.

Capturing complex 3D tissue physiology in vitro

The emergence of tissue engineering raises new possibilities for the study of complex physiological and pathophysiological processes in vitro. Many tools are now available to create 3D tissue models in vitro, but the blueprints for what to make have been slower to arrive. We discuss here some of the 'design principles' for recreating the interwoven set of biochemical and mechanical cues in the cellular microenvironment, and the methods for implementing them. We emphasize applications that involve epithelial tissues for which 3D models could explain mechanisms of disease or aid in drug development.

Vascular endothelial growth factor co-ordinates proper development of lung epithelium and vasculature

The vasculature forms an intrinsic functional component of the lung and its development must be tightly regulated and coordinated with lung epithelial morphogenesis. Vascular endothelial growth factor (VEGF) and its receptors are highly expressed in a complementary pattern in the lungs during embryonic development. VEGF is expressed by epithelium and the receptors in the surrounding mesenchyme. To determine the function of VEGF in lung formation, we inhibited its activity using a soluble receptor in lung renal capsule grafts. Inhibition of VEGF results in inhibition of vascular development and significant alteration in epithelial development. Epithelial proliferation is inhibited, sacculation is impaired, and the epithelium undergoes apoptosis. Interestingly, when VEGF is attenuated, epithelial differentiation still proceeds, as shown by acquisition of both proximal and distal markers. These data show that VEGF co-ordinates epithelial and vascular development. It is required for the development of the lung vasculature and the vasculature is necessary for epithelial proliferation and morphogenesis, but not for cell differentiation.

Vascular endothelial growth factor blockade promotes the transition from compensatory cardiac hypertrophy to failure in response to pressure overload

Cardiac hypertrophy is associated with upregulation of vascular endothelial growth factor (VEGF) in the myocardium. Here, we evaluated the effects of a decoy VEGF receptor on heart morphology and function to a murine model of pressure overload hypertrophy. Mice were administered adenoviral vector encoding a decoy VEGF receptor (Ad-Flk), and their hearts were subjected to pressure overload by transverse aortic constriction (TAC). Treatment with Ad-Flk led to a net reduction in capillary density in hearts subjected to TAC. Ad-Flk also led to a reduction in TAC-induced cardiac hypertrophy and promoted left ventricle dilatation and a loss in contractile function. Treatment with Ad-Flk markedly increased myocardial fibrosis and collagen gene upregulation. In contrast, Ad-Flk had no effect on any of these parameters in sham-treated mice. Administration of a VEGF trap reagent diminished pressure overload cardiac hypertrophy and promoted the progression to heart failure but had no effect on sham-treated animals. These findings suggest that VEGF is required to maintain myocardial capillary density and that reductions in the vascular bed are associated with the transition from compensatory hypertrophy to failure.

A Novel Culture System Shows that Stem Cells Can be Grown in 3D and Under Physiologic Pulsatile Conditions for Tissue Engineering of Vascular Grafts

BACKGROUND

Currently available vascular grafts have been limited by variable patency rates, material availability, and immunological rejection. The creation of a tissue-engineered vascular graft (TEVG) from autologous stem cells would potentially overcome these limitations. As a first step in creating a completely autologous TEVG, our objective was to develop a novel system for culturing undifferentiated mouse embryonic stem cells (mESC) in a three-dimensional (3D) configuration and under physiological pulsatile flow and pressure conditions.

MATERIALS AND METHODS

A bioreactor was created to provide pulsatile conditions to a specially modified four-well Labtek Chamber-Slide culture system. Undifferentiated mESC were either suspended in a 3D Matrigel matrix or suspended only in cell-culture media within the culture system. Pulsatile conditions were applied to the suspended cells and visualized by video microscopy.

RESULTS

Undifferentiated mESC were successfully embedded in a 3D Matrigel matrix and could withstand physiological pulsatile conditions. Video microscopy demonstrated that the mESC in the 3D matrix were constrained to the wells of the culture system, moved in unison with the applied flows, and were not washed downstream; this was in contrast to the mESC suspended in media alone.

CONCLUSIONS

Undifferentiated mESC can be grown in 3D and under pulsatile conditions. We will use these results to study the effects of long-term pulsatile conditions on the differentiation of mESC into endothelial cells, smooth muscle cells, and fibroblast cells with the long-term goal of creating a completely autologous TEVG.

The vascular basement membrane: a niche for insulin gene expression and Beta cell proliferation

Endocrine pancreatic beta cells require endothelial signals for their differentiation and function. However, the molecular basis for such signals remains unknown. Here, we show that beta cells, in contrast to the exocrine pancreatic cells, do not form a basement membrane. Instead, by using VEGF-A, they attract endothelial cells, which form capillaries with a vascular basement membrane next to the beta cells. We have identified laminins, among other vascular basement membrane proteins, as endothelial signals, which promote insulin gene expression and proliferation in beta cells. We further demonstrate that beta1-integrin is required for the beta cell response to the laminins. The proposed mechanism explains why beta cells must interact with endothelial cells, and it may apply to other cellular processes in which endothelial signals are required.

The phosphoinositide 3-kinase/Akt-signal pathway mediates proliferation and secretory function of hepatic sinusoidal endothelial cells in rats after partial hepatectomy

OBJECTIVE

To investigate the role of AKT signaling pathway in hepatic sinusoidal endothelial cells (SECs) early after partial hepatectomy in rats and the regulatory mechanisms involved.

METHODS

The animal model of 70% hepatectomy was made. Hepatic SECs were isolated and cultured according to Braet et al.'s method with some modifications. The cultured hepatic SECs were divided into two groups: 70% partial hepatectomy groups and LY294002 group (LY). We observed the expressions of AKT and NF-kappaB in cultured hepatic SECs by Western blot, measured the levels of NO, NOs, IL-6, and HGF in the supernatants of hepatic SEC cultures and [3H]thymidine incorporation, and analyzed cell cycle of cultured hepatic SECs by flow cytometer. The relationship of the Akt pathway with secretions and proliferation of hepatic SECs after partial hepatectomy was probed.

RESULTS

The levels of Akt protein expression increased significantly after partial hepatectomy in OG group and with a peak at 24 h post operation. Meanwhile, there was a markedly increase in phosphorylated Akt protein during 2-72 h after operation. But the expression and activity of Akt protein did not change significantly after partial hepatectomy in the LY group. So, partial hepatectomy can marked induce Akt expression and result in rapid and marked phosphorylation of Akt from 2 to 72 h thereafter. The changes of NF-kappaB expression in cultured hepatic SECs were similar to those of Akt expression after operation. The concentrations of HGF and IL-6 in the supernatants of cultured hepatic SECs were relatively low in the LY group, and were markedly increased after partial hepatectomy, with a peak at 24 h in the OG group. There were significant differences between the OG and LY groups at 6 and 24 h (P < 0.05). Both NO and NOS secretion was increased in the OG group compared to the LY group within 24 h after partial hepatectomy. But the secretion of NO and NOS was increased more markedly in the LY group than that in the OG beyond 24 h. These findings suggest that the secretion of the cytokines by hepatic SECs is mediated by Akt signaling. Akt signaling pathway in relationship with proliferation of hepatic SECs and suppression of apoptosis. In OG group, the hepatic SECs in S and G2/M obviously increased. The proliferative index of hepatic SECs in OG group had significant differences with that in LY group at 6, 24, and 72 h, P < 0.05. Meanwhile, the cells of apoptosis in OG group were very low, and the cells in LY group gradually increased.

CONCLUSIONS

These results suggest that AKT signaling pathway plays a crucial role in mediating proliferating and secreted signals in hepatic SECs. AKT has been suggested to play a pivotal role in early liver regeneration involved in the induction of secreted cytokines and proliferation of hepatic SECs.

Antiangiogenic therapy decreases integrin expression in normalized tumor blood vessels

Tumor blood vessels normalized by antiangiogenic therapy may provide improved delivery of chemotherapeutic agents during a window of time but it is unknown how protein expression in tumor vascular endothelial cells changes. We evaluated the distribution of RGD-4C phage, which binds alpha(v)beta(3), alpha(v)beta(5), and alpha(5)beta(1) integrins on tumor blood vessels before and after antiangiogenic therapy. Unlike the control phage, fd-tet, RGD-4C phage homed to vascular endothelial cells in spontaneous tumors in RIP-Tag2 transgenic mice in a dose-dependent fashion. The distribution of phage was similar to alpha(v)beta(3) and alpha(5)beta(1) integrin expression. Blood vessels that survived treatment with AG-013736, a small molecule inhibitor of vascular endothelial growth factor and platelet-derived growth factor receptors, had only 4% as much binding of RGD-4C phage compared with vessels in untreated tumors. Cellular distribution of RGD-4C phage in surviving tumor vessels matched the alpha(5)beta(1) integrin expression. The reduction in integrin expression on tumor vessels after antiangiogenic therapy raises the possibility that integrin-targeted delivery of diagnostics or therapeutics may be compromised. Efficacious delivery of drugs may benefit from identification by in vivo phage display of targeting peptides that bind to tumor blood vessels normalized by antiangiogenic agents.

Homing to hypoxia: HIF-1 as a mediator of progenitor cell recruitment to injured tissue

The identification of bone marrow-derived endothelial progenitor cells has altered our understanding of new blood vessel growth and tissue regeneration. Previously, new blood vessel growth in the adult was thought to only occur through angiogenesis, the sprouting of new vessels from existing structures. However, it has become clear that circulating bone marrow-derived cells can form new blood vessels through a process of postnatal vasculogenesis, with endothelial progenitor cells selectively recruited to injured or ischemic tissue. How this process occurs has remained unclear. One common element in the different environments where vasculogenesis is believed to occur is the presence of a hypoxic stimulus. We have identified the chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR4 as critical mediators for the ischemia-specific recruitment of circulating progenitor cells. We have found that the endothelial expression of SDF-1 acts as a signal indicating the presence of tissue ischemia, and that its expression is directly regulated by hypoxia-inducible factor-1. Stromal cell-derived factor 1 is the only chemokine family member known to be regulated in this manner. Later events, including proliferation, patterning, and assembly of recruited progenitors into functional blood vessels, are also influenced by tissue oxygen tension and hypoxia. Interestingly, both SDF-1 and hypoxia are present in the bone marrow niche, suggesting that hypoxia may be a fundamental requirement for progenitor cell trafficking and function. As such, ischemic tissue may represent a conditional stem cell niche, with recruitment and retention of circulating progenitors regulated by hypoxia through differential expression of SDF-1.

COMP-angiopoietin-1 promotes wound healing through enhanced angiogenesis, lymphangiogenesis, and blood flow in a diabetic mouse model

Microvascular dysfunction is a major cause of impaired wound healing seen in diabetic patients. Therefore, reestablishment of structural and functional microvasculature could be beneficial to promote wound healing in these patients. Angiopoietin-1 (Ang1) is a specific growth factor functioning to generate a stable and functional vasculature through the Tie2 and Tie1 receptors. Here we determined the effectiveness of cartilage oligomeric matrix protein (COMP)-Ang1, a soluble, stable, and potent form of Ang1, on promotion of healing in cutaneous wounds of diabetic mice. An excisional full-thickness wound was made in the dorsal side of the tail of diabetic (db/db) mice, and mice were then treated systemically with adenovirus (Ade) encoding COMP-Ang1 or with control virus encoding beta-gal (Ade-beta-gal) or treated topically with recombinant COMP-Ang1 protein or BSA. Time course observations revealed that mice treated with Ade-COMP-Ang1 or COMP-Ang1 protein showed accelerated wound closure and epidermal and dermal regeneration, enhanced angiogenesis and lymphangiogenesis, and higher blood flow in the wound region compared with mice treated with control virus or BSA. COMP-Ang1 promotion of wound closure and angiogenesis was not dependent on endothelial nitric oxide synthase or inducible nitric oxide synthase alone. Taken together, these findings indicate that COMP-Ang1 can promote wound healing in diabetes through enhanced angiogenesis, lymphangiogenesis, and blood flow.

Transforming growth factor-beta3 promotes mesenchymal cell proliferation and angiogenesis mediated by the enhancement of cyclin D1, Flk-1, and CD31 gene expression during CL/Fr mouse lip fusion

BACKGROUND

Cleft lip with or without cleft palate is the most common congenital anomaly in the craniofacial region. Knowledge of the molecular mechanisms behind normal lip fusion can contribute to better intervention and improved functional clinical outcome. Transforming growth factor-beta3 (TGF-beta3) has been implicated in lip morphogenesis. Therefore, we hypothesized that TGF-beta3 functions during lip fusion through regulation of angiogenesis and mesenchymal cell cycle progression during early developmental stages.

METHODS

To test this hypothesis we used the CL/Fraser mouse model, which has a high incidence of cleft lip. Lips isolated from embryonic day (ED) 11.5 mouse embryos were allowed to develop in serum-free organ cultures in the presence or absence of TGF-beta3. The lips that developed in these cultures fused in 2 days.

RESULTS

During normal development, we detected positive immunoreactions for TGF-beta3 at the site of fusion. We also detected mesenchymal cells that were immunopositive for Flk-1 and CD31, which are markers for endothelial cell precursors. Exogenous TGF-beta3 accelerated lip fusion in culture. This enhancement was associated with an increase in the number of capillary blood vessels in the lips cultured in the presence of TGF-beta3, in comparison with controls. In tandem, TGF-beta3 increased the level of expression of both Flk-1 and CD31. Our data suggest that an elevated level of TGF-beta3 may promote angiogenesis in developing lips that is mediated by increased Flk-1 and CD31 expression. We also detected increased cyclin D1 expression (a marker for cell proliferation) in the presence of TGF-beta3, which suggests that TGF-beta3 promoted cell proliferation.

CONCLUSIONS

TGF-beta3 promoted cell proliferation and angiogenesis in lip mesenchymal tissues. These events led to enhanced lip fusion in the presence of TGF-beta3.

Molecular control of liver development

Recent studies using animal models have elucidated a growing number of evolutionarily conserved genes and pathways that control liver development from the embryonic endoderm. It is increasingly clear that the genetic programs active in embryogenesis are often deregulated or reactivated in disease, cancer, and tissue repair. Understanding the molecular control of liver development should impact diagnosis and treatment of pediatric and adult liver diseases and aid in efforts to differentiate liver tissue in vitro for stem cell-based therapies.

Endothelial progenitor cell transplantation improves the survival following liver injury in mice

BACKGROUND & AIMS

Neovascularization, which is vital to the healing of injured tissues, recently has been found to include both angiogenesis, which involves in mature endothelial cells, and vasculogenesis, involving endothelial progenitor cells. The aim of this study was to clarify the possible roles of endothelial progenitor cells during postnatal liver regeneration.

METHODS

To determine how endothelial progenitor cells participate in liver regeneration, human or mouse endothelial progenitor cells were transplanted into the mice with carbon tetrachloride-induced acute liver injury. Survival rate of the mice in endothelial progenitor cell-transplanted and control groups was calculated. Separately, livers removed temporally from both groups were examined.

RESULTS

At an early stage, transplanted human endothelial progenitor cells were seen mainly surrounding hepatic central veins where hepatocytes showed extensive necrosis; later, the transplanted cells formed tubular structures. More of these cells were observed along hepatic sinusoids. Transplantation of human or mouse endothelial progenitor cells improved survival of the mice following liver injury (from 28.6% to 85.7%, P < .0005 and from 33.3% to 80.0%, P < .001, respectively), accompanied by greater proliferation of hepatocytes. Human endothelial progenitor cells produced several growth factors, such as hepatocyte growth factor, transforming growth factor-alpha, heparin-binding epidermal growth factor-like growth factor, and vascular endothelial growth factor, and also elicited endogenous growth factors.

CONCLUSIONS

Endogenous and exogenous growth factors and direct neovascularization after endothelial progenitor cell transplantation promoted liver regeneration, thus improving survival after liver injury. Transplantation of endothelial progenitor cells could represent a new therapeutic strategy for promoting liver regeneration.

Hex homeobox gene controls the transition of the endoderm to a pseudostratified, cell emergent epithelium for liver bud development

Little is known about the mechanism by which embryonic liver, lung, and pancreas progenitor cells emerge from the endodermal epithelium to initiate organogenesis. Understanding this process and its genetic control provides insight into ontogeny, developmental abnormalities, and tissue regeneration. We find that shortly after hepatic endoderm cells are specified, they undergo a transition from a columnar, gut morphology to a pseudostratified morphology, with concomitant "interkinetic nuclear migration" (INM) during cell division. INM is a hallmark of pseudostratified epithelia and the process used by neural progenitors to emerge from the neural epithelium. We find that the transition of the hepatic endoderm, but not the neural epithelium, to a pseudostratified epithelium is dependent upon the cell-autonomous activity of the homeobox gene Hex. In the absence of Hex, hepatic endoderm cells survive but maintain a columnar, simple epithelial phenotype and ectopically express Shh and other genes characteristic of the midgut epithelium. Thus, Hex promotes endoderm organogenesis by promoting the transition to a pseudostratified epithelium, which in turn allows hepatoblasts to emerge into the stromal environment and continue differentiating.

Endothelial cells and VEGF in vascular development

The intricate patterning processes that establish the complex vascular system during development depend on a combination of intrinsic pre-patterning and extrinsic responses to environmental parameters. Mutational studies in mice and fish have shown that the vascular system is highly sensitive to genetic disruption and have identified potential targets for therapeutic interventions. New insights into non-vascular roles of vascular endothelial growth factor and the requirement for endothelial cells in adult organs and stem-cell niches highlight possible side effects of anti-angiogenic therapy and the need for new targets.

An endothelial-mesenchymal relay pathway regulates early phases of pancreas development

Understanding the tissue interactions that induce pancreatic progenitor cells from the embryonic endoderm provides insights into congenital malformations, tissue repair, and differentiating stem cells to a pancreatic fate. The specification of pancreatic progenitors within the dorsal endodermal epithelium has been thought to involve two phases of mesodermal interactions; first with the lateral plate mesoderm and notochord and then with aortic endothelial cells. Afterwards, branching morphogenesis of the pancreatic bud is induced by Isl-1-positive dorsal mesenchyme cells, whose growth is stimulated by factors in the circulation. Using mouse genetic models and embryo tissue explants, we show that the aortic endothelium and dorsal mesenchyme each possess an additional role in pancreatic induction, prior to the branching morphogenesis step. Specifically, we find that aortic endothelial cells promote the survival of nearby, Isl-1-positive dorsal mesenchyme, independently of factors from the circulation. Furthermore, we find that FGF10 signaling from the mesenchyme cells maintains Ptf1a expression in the dorsal pancreatic bud and appears genetically redundant with a role for the transcription factor gene HNF6 in promoting the induction of Pdx-1-positive dorsal endoderm. Together, these studies reveal a relay pathway from aortic endothelium to dorsal mesenchyme and then to the endoderm, along with functions of the dorsal mesenchyme that promote the initial differentiation of the dorsal pancreatic endoderm, prior to organ morphogenesis.

The developing mouse thyroid: Embryonic vessel contacts and parenchymal growth pattern during specification, budding, migration, and lobulation

Normal mouse thyroid development has been revised to identify critical morphogenetic events. The early thyroid primordium associates with the aortic sac endothelium at the time of specification and budding. The vascular contact is lost after the thyroid buds from the pharyngeal endoderm, but is resumed before the gland divides to form two lobes. Lateral expansion of parenchyma takes place along the course of the third pharyngeal arch arteries. Thyroid precursor cells expressing Titf1/Nkx2.1 do not proliferate until the migration stage, implicating that progenitors likely are recruited from outside the thyroid placode. Early lobulation involves engulfment of the entire ultimobranchial bodies by the growing midline thyroid. At the same time, proliferation of the ultimobranchial body epithelium is silenced preceding the differentiation of C cells. Before folliculogenesis, thyroid lobe enlargement is reminiscent of a budding-branching-like growth pattern. It is suggested that thyroid inductive signals arise from embryonic vessels, and that this provides ideas to conceptually new pathogenetic mechanisms of thyroid dysgenesis.

Transplantation of umbilical cord blood-derived endothelial progenitor cells: a promising method of therapeutic revascularisation

Therapeutic neovascularisation by endothelial progenitor cells (EPCs) mediated vascular regeneration is becoming a novel option for the treatment of ischaemic diseases. Recently, human umbilical cord blood (CB) has been found to contain a large number of EPCs and transplantation of CB EPCs led to a successful salvage of the ischaemic limbs through improvement in blood perfusion, indicating the feasibility of using CB cells for therapeutic revascularisation. This review will summarise recent studies in therapeutic revascularisation using CB cells and discuss the potential clinical utilisation of CB cells in ischaemic diseases.

Vascular update: Morphogenesis, tumors, malformations, and molecular dimensions

This vascular review is organized under the following headings: vasculogenesis and angiogenesis; vascular endothelial growth factors, their receptors, TIE receptors, and angiopoietins; other factors in blood vessel formation; parallel patterning in blood vessels and nerves; physiological and pathological neovascularization; the role of VEGF receptors in metastasis; anti-angiogenic therapy for tumors; association of blood vessels with fat; vascular malformations and vascular tumors; infantile hemangiomas; congenital hemangiomas; lymphatic malformations; molecular characteristics of some disorders with vascular malformations; Kasabach-Merritt phenomenon; Sturge-Weber syndrome, Klippel-Trenaunay syndrome, and Parkes Weber syndrome; diagnostic and laboratory studies; and future perspectives.

Loss of PECAM-1 function impairs alveolarization

The final stage of lung development in humans and rodents occurs principally after birth and involves the partitioning of the large primary saccules into smaller air spaces by the inward protrusion of septae derived from the walls of the saccules. Several observations in animal models implicate angiogenesis as critical to this process of alveolarization, but all anti-angiogenic treatments examined to date have resulted in endothelial cell (EC) death. We therefore targeted the function of platelet endothelial cell adhesion molecule, (PECAM-1), an EC surface molecule that promotes EC migration and has been implicated in in vivo angiogenesis. Administration of an anti-PECAM-1 antibody that inhibits EC migration, but not proliferation or survival in vitro, disrupted normal alveolar septation in neonatal rat pups without reducing EC content. Three-dimensional reconstruction of lungs showed that pups treated with a blocking PECAM-1 antibody had remodeling of more proximal branches resulting in large tubular airways. Subsequent studies in PECAM-1-null mice confirmed that the absence of PECAM-1 impaired murine alveolarization, without affecting EC content, proliferation, or survival. Further, cell migration was reduced in lung endothelial cells isolated from these mice. These data suggest that the loss of PECAM-1 function compromises postnatal lung development and provide evidence that inhibition of EC function, in contrast to a loss of viable EC, inhibits alveolarization.

VEGF-A signaling through Flk-1 is a critical facilitator of early embryonic lung epithelial to endothelial crosstalk and branching morphogenesis

Vascular endothelial growth factor-A (VEGF-A) signaling directs both vasculogenesis and angiogenesis. However, the role of VEGF-A ligand signaling in the regulation of epithelial-mesenchymal interactions during early mouse lung morphogenesis remains incompletely characterized. Fetal liver kinase-1 (Flk-1) is a VEGF cognate receptor (VEGF-R2) expressed in the embryonic lung mesenchyme. VEGF-A, expressed in the epithelium, is a high affinity ligand for Flk-1. We have used both gain and loss of function approaches to investigate the role of this VEGF-A signaling pathway during lung morphogenesis. Herein, we demonstrate that exogenous VEGF 164, one of the 3 isoforms generated by alternative splicing of the Vegf-A gene, stimulates mouse embryonic lung branching morphogenesis in culture and increases the index of proliferation in both epithelium and mesenchyme. In addition, it induces differential gene and protein expression among several key lung morphogenetic genes, including up-regulation of BMP-4 and Sp-c expression as well as an increase in Flk-1-positive mesenchymal cells. Conversely, embryonic lung culture with an antisense oligodeoxynucleotide (ODN) to the Flk-1 receptor led to reduced epithelial branching, decreased epithelial and mesenchymal proliferation index as well as downregulating BMP-4 expression. These results demonstrate that the VEGF pathway is involved in driving epithelial to endothelial crosstalk in embryonic mouse lung morphogenesis.

Angioblast-mesenchyme induction of early kidney development is mediated by Wt1 and Vegfa

Most studies on kidney development have considered the interaction of the metanephric mesenchyme and the ureteric bud to be the major inductive event that maintains tubular differentiation and branching morphogenesis. The mesenchyme produces Gdnf, which stimulates branching, and the ureteric bud stimulates continued growth of the mesenchyme and differentiation of nephrons from the induced mesenchyme. Null mutation of the Wt1 gene eliminates outgrowth of the ureteric bud, but Gdnf has been identified as a target of Pax2, but not of Wt1. Using a novel system for microinjecting and electroporating plasmid expression constructs into murine organ cultures, it has been demonstrated that Vegfa expression in the mesenchyme is regulated by Wt1. Previous studies had identified a population of Flk1-expressing cells in the periphery of the induced mesenchyme, and adjacent to the stalk of the ureteric bud, and that Vegfa was able to stimulate growth of kidneys in organ culture. Here it is demonstrated that signaling through Flk1 is required to maintain expression of Pax2 in the mesenchyme of the early kidney, and for Pax2 to stimulate expression of Gdnf. However, once Gdnf stimulates branching of the ureteric bud, the Flk1-dependent angioblast signal is no longer required to maintain branching morphogenesis and induction of nephrons. Thus,this work demonstrates the presence of a second set of inductive events,involving the mesenchymal and angioblast populations, whereby Wt1-stimulated expression of Vegfa elicits an as-yet-unidentified signal from the angioblasts, which is required to stimulate the expression of Pax2 and Gdnf,which in turn elicits an inductive signal from the ureteric bud.

Thinking outside the cell: proteases regulate hepatocyte division

The liver has the unique ability to regenerate after loss of mass and function such as following surgical resection or toxic liver injury. Gene targeting has identified factors crucial to liver development and regeneration. Regeneration occurs through growth-factor- and cytokine-mediated proliferation of differentiated hepatocytes, and extracellular proteases are now recognized to process these molecules. Proteases release cytokines and growth factors that are anchored to the hepatic extracellular matrix or require processing for their bioactivity. Crucial 'start and stop' signals for liver regeneration are regulated by serine proteases and metalloproteases that provide an interface between proteolytic cascades and intracellular signaling during hepatocyte division.

Crucial roles of mesodermal cell lineages in a murine embryonic stem cell-derived in vitro liver organogenesis system

Recent studies in the field of regenerative medicine have exploited the pluripotency of embryonic stem (ES) cells to generate a variety of cell lineages. However, the target has always been only a single lineage, which was isolated from other differentiated cell populations. In the present study, we selected sublines with a high capability for differentiation to contracting cardiomyocytes and also produced germ-line chimeric mice from a parent ES line. We also succeed in establishing embryoid bodies prepared from the ES cells that differentiated into not only hepatocytes but also at least two mesodermal lineages: cardiomyocytes that supported liver development and endothelial cells corresponding to sinusoids. This allowed the development of an in vitro system using murine ES cells that approximated the events of liver development in vivo. The expression of albumin was significantly higher in cardiomyocytes that had arisen in differentiated ES cells than in those that had not. Our in vitro system for liver organogenesis consists of a blood/sinusoid vascular-like network and hepatocyte layers and shows higher levels of hepatic function, such as albumin production and ammonia degradation, than hepatic cell lines and primary cultures of murine adult hepatocytes. This innovative system will lead to the development of second-generation regenerative medicine techniques using ES cells and is expected to be useful for the development of bioartificial liver systems and drug-metabolism assays.

Regulation of Stem Cells in the Niche

Stem cells have a self-renewal capacity as well as the capacity to differentiate into single or multiple lineages. In this review, we discuss the mechanism of maintenance of "stemness" in hematopoietic systems and refer to our studies of corneal epithelial stem cells. The quiescent state is believed to be indispensable for the maintenance of hematopoietic stem cells (HSC). Interaction of HSC with their particular microenvironments, known as stem cell niches, is critical for the cell cycle regulation of HSC. Monitoring of the quiescence of HSC by a side population (SP) revealed that the cell cycle status of HSC is dynamically controlled by microenvironments. We recently discovered a molecular mechanism by which the cell cycle of HSC is regulated by the niche. HSC expressing tyrosine kinase Tie2 receptors adhere to osteoblasts in the bone marrow (BM) niche. The interaction of Tie2 and its ligand angiopoietin-1 (Ang-1) leads to tight adhesion of HSC to stromal cells, resulting in the maintenance of long-term repopulating activity of HSC. Thus, the Tie2/Ang-1 signaling pathway plays a critical role in maintaining HSC in the quiescent state in the BM niche. Understanding of the relationships of stem cells to their niches should lead to development of new strategies directed toward regeneration.