Wnt7b regulates mesenchymal proliferation and vascular development in the lung

Wnt5a regulates Shh and Fgf10 signaling during lung development

The role of WNT signaling and its interactions with other morphogenetic pathways were investigated during lung development. Previously, we showed that targeted disruption of Wnt5a results in over-branching of the epithelium and thickening of the interstitium in embryonic lungs. In this study, we generated and characterized transgenic mice with lung-specific over-expression of Wnt5a from the SpC promoter. Over-expression of Wnt5a interfered with normal epithelial-mesenchymal interactions resulting in reduced epithelial branching and dilated distal airways. During early lung development, over-expression of Wnt5a in the epithelium resulted in increased Fgf10 in the mesenchyme and decreased Shh in the epithelium. Both levels and distribution of SHH receptor, Ptc were reduced in SpC-Wnt5a transgenic lungs and were reciprocally correlated to changes of Fgf10 in the mesenchyme, suggesting that SHH signaling is decreased by over-expression of Wnt5a. Cultured mesenchyme-free epithelial explants from SpC-Wnt5a transgenic lungs responded abnormally to recombinant FGF10 supplied uniformly in the Matrigel with dilated branch tips that mimic the in vivo phenotype. In contrast, chemotaxis of transgenic epithelial explants towards a directional FGF10 source was inhibited. These suggest that over-expression of Wnt5a disrupts epithelial-response to FGF10. In conclusion, Wnt5a regulates SHH and FGF10 signaling during lung development.

Gli3 is required for the specification and differentiation of preplate neurons

During corticogenesis, the cerebral cortex develops a laminated structure which is essential for its function. Early born neurons of the preplate and its derivatives, the marginal zone (MZ) and the subplate (SP), serve as a framework during the cortical lamination process. Here, I report on defects in the generation and specification of these early born cortical neurons in extra-toes (Xt(J)) mice which are defective for the Gli3 zinc finger transcription factor. The Gli3 mutation dramatically disrupts early steps in the cortical lamination process. The MZ, SP and the cortical plate (CP) do not form layers but cortical neurons are arranged in clusters. These defects start to become evident at E12.5 when the cortex forms several protrusions and the ventricular zone becomes undulated. At this stage, cortical progenitor cells start to loose their apical/basal cell polarity correlating with an ectopic expression of Wnt7b in the ventricular zone. In addition, the cellular composition of the preplate is severely altered. Cajal-Retzius cells are reduced in numbers while early born Calretinin(+) neurons are overproduced. These results show that multiple aspects of corticogenesis including the organization of the venticular zone, the apical/basal cell polarity of cortical progenitors and the differentiation of early born cortical neurons are affected in the Gli3 mutant.

WNT7b mediates macrophage-induced programmed cell death in patterning of the vasculature

Macrophages have a critical role in inflammatory and immune responses through their ability to recognize and engulf apoptotic cells. Here we show that macrophages initiate a cell-death programme in target cells by activating the canonical WNT pathway. We show in mice that macrophage WNT7b is a short-range paracrine signal required for WNT-pathway responses and programmed cell death in the vascular endothelial cells of the temporary hyaloid vessels of the developing eye. These findings indicate that macrophages can use WNT ligands to influence cell-fate decisions--including cell death--in adjacent cells, and raise the possibility that they do so in many different cellular contexts.

Wnt/beta-catenin signaling acts upstream of N-myc, BMP4, and FGF signaling to regulate proximal-distal patterning in the lung

Branching morphogenesis in the lung serves as a model for the complex patterning that is reiterated in multiple organs throughout development. Beta-catenin and Wnt signaling mediate critical functions in cell fate specification and differentiation, but specific functions during branching morphogenesis have remained unclear. Here, we show that Wnt/beta-catenin signaling regulates proximal-distal differentiation of airway epithelium. Inhibition of Wnt/beta-catenin signaling, either by expression of Dkk1 or by tissue-specific deletion of beta-catenin, results in disruption of distal airway development and expansion of proximal airways. Wnt/beta-catenin functions upstream of BMP4, FGF signaling, and N-myc. Moreover, we show that beta-catenin and LEF/TCF activate the promoters of BMP4 and N-myc. Thus, Wnt/beta-catenin signaling is a critical upstream regulator of proximal-distal patterning in the lung, in part, through regulation of N-myc, BMP4, and FGF signaling.

Similarities and dissimilarities of branching and septation during lung development

The lungs of small premature babies are at a developmental stage of finalizing their airway tree by a process called branching morphogenesis, and of creating terminal gas exchange units by a mechanism called septation. If the branching process is disturbed, the lung has a propensity to be hypoplastic. If septation is impaired, the terminal gas exchange units, the alveoli, tend to be enlarged and reduced in number, an entity known as bronchopulmonary dysplasia. Here, we review current knowledge of key molecules influencing branching and septation. In particular, we discuss the molecular similarities and dissimilarities between the two processes of airspace enlargement. Understanding of the molecular mechanisms regulating branching and septation may provide perinatologists with targets for improving lung growth and maturation.

Wnt/frizzled signaling during vertebrate retinal development

Multiple signaling pathways are known to be involved in regulating development of the vertebrate neural retina. Recent publications have demonstrated that Wnt/Frizzled (Fz) signaling components are expressed in the developing retina and may play a fundamental role in retinogenesis. In this review, we summarize Wnt/Fz expression patterns in the developing vertebrate retina, mainly from chick and mouse, and compare them with Wnt/beta-catenin reporter activity. Consistent with the dynamic expression patterns of Wnt pathway components, evidence suggests that Wnt/Fz signaling has multiple roles during retinal development.

Wnt7b activates canonical signaling in epithelial and vascular smooth muscle cells through interactions with Fzd1, Fzd10, and LRP5

Wnt7b is a Wnt ligand that has been demonstrated to play critical roles in several developmental processes, including lung airway and vascular development and chorion-allantois fusion during placental development. Wnt signaling involves the binding of Wnt ligands to cell surface receptors of the frizzled family and coreceptors of the LRP5/6 family. However, little is known of the ligand-receptor specificity exhibited by different Wnts, Fzds, and LRPs in Wnt signaling. Expression analysis of Fzds and LRP5/6 in the developing lung and vasculature showed that Fzd1, -4, -7, and -10 and LRP5/6 are expressed in tissue-specific patterns during lung development. Fzd1, -4, and -7 are expressed primarily in the developing lung mesenchyme, and Fzd10 is expressed in airway epithelium. LRP5 and LRP6 are expressed in airway epithelium during lung development, whereas LRP5 but not LRP6 expression is observed in the muscular component of large blood vessels, including the aorta. Cell transfection studies demonstrate that Wnt7b can activate the canonical Wnt pathway but not the noncanonical Wnt pathway in a cell-specific manner. Biochemical analysis demonstrates that Wnt7b can bind to Fzd1 and -10 on the cell surface and cooperatively activate canonical Wnt signaling with these receptors in the presence of LRP5. Together, these data demonstrate that Wnt7b signals through Fzd1 and -10 and LRP5 and implicate these Wnt coreceptors in the regulation of lung airway and vascular development.

The molecular basis of transdifferentiation

There is now excellent experimental evidence demonstrating the remarkable ability of some differentiated cells to convert to a completely different phenotype. The conversion of one cellular phenotype to another is referred to as 'transdifferentiation' and belongs to a wider class of cell-type switches termed 'metaplasias'. Defining the molecular steps in transdifferentiation will help us to understand the developmental biology of the cells that interconvert, as well as help identify key regulatory transcription factors that may be important for the reprogramming of stem cells. Ultimately, being able to produce cells at will offers a compelling new approach to therapeutic transplantation and therefore the treatment and cure of diseases such as diabetes.

Wnt Signaling in Stem Cells and Non–Small-Cell Lung Cancer

Evidence suggests that stem cells may be the source of mutant cells that cause cancers to develop and proliferate. Wnt signaling has been shown to promote self-renewal in gut epithelial and hematopoietic stem cells and to trigger critical pathways in carcinogenesis. In this review, we highlight the progress in understanding how the Wnt pathway contributes to stem cell maintenance and its role in lung carcinogenesis. Although the function of stem cells in solid tumor development is unclear, the Wnt pathway's role in determining the fate and self-renewal potential of cancer stem cells suggests a critical role in carcinogenesis and that developing drugs to inhibit this pathway may be of therapeutic interest.

Wnt signaling in lung cancer

Wnt signaling has recently emerged as a critical pathway in lung carcinogenesis as already demonstrated in many cancers and particularly in colorectal cancer. We critically discuss in this review the individual components of the Wnt pathway and their role in lung cancer development. We propose that activation of the Wnt-mediated signal occurs in a different manner in lung cancer than in colorectal cancer. In lung cancer, mutations of APC or beta-catenin are rare and the Wnt pathway appears to be activated upstream of beta-catenin. We identified at least three mechanisms of activation: overexpression of Wnt effectors such as Dvl, activation of a non-canonical pathway involving JNK and repression of Wnt antagonists such as WIF-1. The respective relevance of each event and their likely relationship remain unclear. Nevertheless, we propose that many of the studied components of the Wnt pathway may serve as potential targets in the search for therapeutic agents and we can reasonably argue that blockade of Wnt pathway may lead to new treatment strategies in lung cancer.

The Forkhead Box M1 Transcription Factor Is Essential for Embryonic Development of Pulmonary Vasculature

Transgenic and gene knock-out studies demonstrated that the mouse Forkhead Box m1 (Foxm1 or Foxm1b) transcription factor (previously called HFH-11B, Trident, Win, or MPP2) is essential for hepatocyte entry into mitosis during liver development, regeneration, and liver cancer. Targeted deletion of Foxm1 gene in mice produces an embryonic lethal phenotype due to severe abnormalities in the development of liver and heart. In this study, we show for the first time that Foxm1(-/-) lungs exhibit severe hypertrophy of arteriolar smooth muscle cells and defects in the formation of peripheral pulmonary capillaries as evidenced by significant reduction in platelet endothelial cell adhesion molecule 1 staining of the distal lung. Consistent with these findings, significant reduction in proliferation of the embryonic Foxm1(-/-) lung mesenchyme was found, yet proliferation levels were normal in the Foxm1-deficient epithelial cells. Severe abnormalities of the lung vasculature in Foxm1(-/-) embryos were associated with diminished expression of the transforming growth factor beta receptor II, a disintegrin and metalloprotease domain 17 (ADAM-17), vascular endothelial growth factor receptors, Polo-like kinase 1, Aurora B kinase, laminin alpha4 (Lama4), and the Forkhead Box f1 transcription factor. Cotransfection studies demonstrated that Foxm1 stimulates transcription of the Lama4 promoter, and this stimulation requires the Foxm1 binding sites located between -1174 and -1145 bp of the mouse Lama4 promoter. In summary, development of mouse lungs depends on the Foxm1 transcription factor, which regulates expression of genes essential for mesenchyme proliferation, extracellular matrix remodeling, and vasculogenesis.

Angiotensin II modulates frizzled-2 receptor expression in rat vascular smooth muscle cells

Ang II (angiotensin II) has multiple effects on vascular smooth muscle cells through the modulation of different classes of genes. Using the mRNA differential-display method to investigate gene expression in rat aortic smooth muscle cells in culture in response to 3 h of Ang II stimulation, we observed that Ang II down-regulated the expression of a member of the family of transmembrane receptors for Wnt proteins that was identified as Fzd2 [Fzd (frizzled)-2 receptor]. Fzds are a class of highly conserved genes playing a fundamental role in the developmental processes. In vitro, time course experiments demonstrated that Ang II induced a significant increase (P<0.05) in Fzd2 expression after 30 min, whereas it caused a significant decrease (P<0.05) in Fzd2 expression at 3 h. A similar rapid up-regulation after Ang II stimulation for 30 min was evident for TGFβ1 (transforming growth factor β1; P<0.05). To investigate whether Ang II also modulated Fzd2 expression in vivo, exogenous Ang II was administered to Sprague–Dawley rats (200 ng·kg−1 of body weight·min−1; subcutaneously) for 1 and 4 weeks. Control rats received normal saline. After treatment, systolic blood pressure was significantly higher (P<0.01), whereas plasma renin activity was suppressed (P<0.01) in Ang II- compared with the saline-treated rats. Ang II administration for 1 week did not modify Fzd2 expression in aorta of Ang II-treated rats, whereas Ang II administration for 4 weeks increased Fzd2 mRNA expression (P<0.05) in the tunica media of the aorta, resulting in a positive immunostaining for fibronectin at this time point. In conclusion, our data demonstrate that Ang II modulates Fzd2 expression in aortic smooth muscle cells both in vitro and in vivo.

Lung vascular development: implications for the pathogenesis of bronchopulmonary dysplasia

Past studies have primarily focused on how altered lung vascular growth and development contribute to pulmonary hypertension. Recently, basic studies of vascular growth have led to novel insights into mechanisms underlying development of the normal pulmonary circulation and the essential relationship of vascular growth to lung alveolar development. These observations have led to new concepts underlying the pathobiology of developmental lung disease, especially the inhibition of lung growth that characterizes bronchopulmonary dysplasia (BPD). We speculate that understanding basic mechanisms that regulate and determine vascular growth will lead to new clinical strategies to improve the long-term outcome of premature babies with BPD.

Effects of canonical Wnt signaling on dorso-ventral specification of the mouse telencephalon

Wnt signaling is involved in numerous processes during vertebrate CNS development. In this study, we used conditional Cre/loxP system in mouse to ablate or activate beta-catenin in the telencephalon in two time windows: before and after the onset of neurogenesis. We show that beta-catenin mediated Wnt signals are required to maintain the molecular identity of the pallium. Inactivation of beta-catenin in the telencephalon before neurogenesis results in downregulated expression of dorsal markers Emx1, Emx2 and Ngn2, and in ectopic up-regulation of ventral markers Gsh2, Mash1 and Dlx2 in the pallium. In contrast, ablation of ss-catenin after the onset of cortical neurogenesis (E11.5) does not result in a dorso-ventral fate shift. In addition, activation of canonical Wnt signaling in the subpallium leads to a repression of ventral telencephalic cell identities as shown by the down-regulation of subpallial markers Dlx2, Nkx2.1, Gsh2, Olig2 and Mash1. This was accompanied with an expansion of dorsal identities ventrally as shown by the expanded expression domains of pallial markers Pax6 and Ngn2. Thus, our data suggest that canonical Wnt signals are involved in maintaining the identity of the pallium by controlling expression of dorsal markers and by suppressing ventral programs from being activated in pallial progenitor cells.

Can we differentiate between airway and vascular smooth muscle?

1. Airway smooth muscle (ASM) has recently been termed the 'frustrated' cell of the lung given that contraction of ASM has no proven useful physiological function in adults and yet is indelibly associated with pathological conditions by virtue of its unwanted airflow-limiting actions in asthma. In contrast, pulmonary vascular smooth muscle contraction plays an essential role in the control of blood flow through the lung. 2. Little is known of the differences in phenotype between human ASM and pulmonary vascular smooth muscle (VSM) tissues, but differences in contractile protein and transcription factor expression and regulation of contractile protein promoter activity have been documented. Similarly, the embryological signals in mice required for differentiation of ASM versus pulmonary VSM are distinct. 3. Bronchoconstriction in asthma is currently treated with beta2-adrenoceptor agonists, which relax contracted ASM cells. An additional approach may be to use gene therapy to render ASM unable to contract (via disruption of their contractile apparatus organization). 4. Application of ASM-specific gene therapies would rely on minimal actions on other lung smooth muscle tissues, including pulmonary and bronchial vascular smooth muscle. The combination of mRNA analysis of laser-captured microdissected tissue with in situ immunohistochemical staining for protein should be very useful in terms of being able to characterize definitively the differences in mRNA and protein expression between the smooth muscle species of the lung. Any discovery of an ASM-selective target could provide a novel lead for ASM-directed anti-asthma therapy.

Frzb modulates Wnt-9a-mediated beta-catenin signaling during avian atrioventricular cardiac cushion development

Normal development of the cardiac atrioventricular (AV) endocardial cushions is essential for proper ventricular septation and morphogenesis of the mature mitral and tricuspid valves. In this study, we demonstrate spatially restricted expression of both Wnt-9a (formerly Wnt-14) and the secreted Wnt antagonist Frzb in AV endocardial cushions of the developing chicken heart. Wnt-9a expression is detected only in AV canal endocardial cells, while Frzb expression is detected in both endocardial and transformed mesenchymal cells of the developing AV cardiac cushions. We present evidence that Wnt-9a promotes cell proliferation in the AV canal and overexpression of Wnt-9a in ovo results in enlarged endocardial cushions and AV inlet obstruction. Wnt-9a stimulates beta-catenin-responsive transcription in AV canal cells, duplicates the embryonic axis upon ventral injections in Xenopus embryos and appears to regulate cell proliferation by activating a Wnt/beta-catenin signaling pathway. Additional functional studies reveal that Frzb inhibits Wnt-9a-mediated cell proliferation in cardiac cushions. Together, these data argue that Wnt-9a and Frzb regulate mesenchymal cell proliferation leading to proper AV canal cushion outgrowth and remodeling in the developing avian heart.

The Wnt signaling pathway in development and disease

Tight control of cell-cell communication is essential for the generation of a normally patterned embryo. A critical mediator of key cell-cell signaling events during embryogenesis is the highly conserved Wnt family of secreted proteins. Recent biochemical and genetic analyses have greatly enriched our understanding of how Wnts signal, and the list of canonical Wnt signaling components has exploded. The data reveal that multiple extracellular, cytoplasmic, and nuclear regulators intricately modulate Wnt signaling levels. In addition, receptor-ligand specificity and feedback loops help to determine Wnt signaling outputs. Wnts are required for adult tissue maintenance, and perturbations in Wnt signaling promote both human degenerative diseases and cancer. The next few years are likely to see novel therapeutic reagents aimed at controlling Wnt signaling in order to alleviate these conditions.

Compensatory roles of Foxa1 and Foxa2 during lung morphogenesis

Foxa1 and Foxa2 are closely related family members of the Foxa group of transcription factors that are coexpressed in subsets of respiratory epithelial cells throughout lung morphogenesis. Shared patterns of expression, conservation of DNA binding, and transcriptional activation domains indicate that they may serve complementary functions in the regulation of gene expression during lung morphogenesis. Whereas branching morphogenesis of the fetal lung occurs normally in the Foxa2Delta/Delta and Foxa1-/- mice, deletion of both Foxa1 and Foxa2 (in Foxa2Delta/Delta, Foxa1-/- mice) inhibited cell proliferation, epithelial cell differentiation, and branching. Dilation of terminal lung tubules and decreased branching were observed as early as embryonic day 12.5. Foxa1 and Foxa2 regulated Shh (sonic hedgehog) and Shh-dependent genes in the respiratory epithelial cells that influenced the expression of genes in the pulmonary mesenchyme that are required for branching morphogenesis. Epithelial cell differentiation, as indicated by lack of expression of surfactant protein B, surfactant protein C, the Clara cell secretory protein, and Foxj1, was inhibited. Foxa family members regulate signaling and transcriptional programs required for morphogenesis and cell differentiation during formation of the lung.

Dickkopf-1 (DKK1) reveals that fibronectin is a major target of Wnt signaling in branching morphogenesis of the mouse embryonic lung

Members of the Dickkopf (Dkk) family of secreted proteins are potent inhibitors of Wnt/beta-catenin signaling. In this study we show that Dkk1, -2, and -3 are expressed distally in the epithelium, while Kremen1, the needed co-receptor, is expressed throughout the epithelium of the developing lung. Using TOPGAL mice [DasGupta, R., Fuchs, E., 1999. Multiple roles for activated LEF/TCF transcription complexes during hair follicle development and differentiation. Development 126, 4557-4568] to monitor the Wnt pathway, we show that canonical Wnt signaling is dynamic in the developing lung and is active throughout the epithelium and in the proximal smooth muscle cells (SMC) until E12.5. However, from E13.5 onwards, TOPGAL activity is absent in the SMC and is markedly reduced in the distal epithelium coinciding with the onset of Dkk-1 expression in the distal epithelium. To determine the role of Wnt signaling in early lung development, E11.5 organ cultures were treated with recombinant DKK1. Treated lungs display impaired branching, characterized by failed cleft formation and enlarged terminal buds, and show decreased alpha-smooth muscle actin (alpha-SMA) expression as well as defects in the formation of the pulmonary vasculature. These defects coincide with a pattern of decreased fibronectin (FN) deposition. DKK1-induced morphogenetic defects can be mimicked by inhibition of FN and overcome by addition of exogenous FN, suggesting an involvement of FN in Wnt-regulated morphogenetic processes.

Fgf18 is required for embryonic lung alveolar development

Fgf18 is abundantly expressed in mouse embryonic lungs. To elucidate the roles of Fgf18 in mouse embryonic lung development, we examined the Fgf18-/- embryonic lungs. Although the sizes of the Fgf18-/- lungs were a little smaller in appearance than those of wild-type lungs, neither proximal nor distal airway branching in the Fgf18-/- lungs was impaired. However, the Fgf18-/- lungs at E18.5 had reduced alveolar space, thicker interstitial mesenchymal compartments, and many embedded capillaries. Cell proliferation in the Fgf18-/- lungs was also transiently reduced around E17.5, although the expression of marker genes for lung epithelial cells in the Fgf18-/- lungs was not impaired. The present findings indicate that the Fgf18 plays roles in lung alveolar development during late embryonic lung development stages. The cell proliferation during the terminal saccular stage stimulated by Fgf18 might play roles in the remodeling of the distal lung.

Global analysis of genes differentially expressed in branching and non-branching regions of the mouse embryonic lung

During development, the proximal and distal regions of respiratory tract undergo distinct processes that ultimately give rise to conducting airways and alveoli. To gain insights into the genetic pathways differentially activated in these regions when branching morphogenesis is initiating, we characterized their transcriptional profiles in murine rudiments isolated at embryonic (E) day 11.5. By using oligonucleotide microarrays, we identified 83 and 128 genes preferentially expressed in branching and non-branching regions, respectively. The majority of these genes (85%) had not been previously described in the lung, or in other organs. We report restricted expression patterns of 22 of these genes were by in situ hybridization. Among them in the lung potential components of the Wnt, TGF beta, FGF and retinoid pathways identified in other systems, and uncharacterized genes, such as translocases, small GTPases and splicing factors. In addition, we provide a more detailed analysis of the expression pattern and regulation of a representative gene from the distal (transforming growth factor, beta induced) and proximal (WW domain-containing protein 2) regions. Our data suggest that these genes may regulate focal developmental events specific of each of these regions during respiratory tract formation.

Segmental cystic malformation of the lung with chondrolipomatous component

A peculiar cystic and chondrolipomatous malformation affecting the right upper lobe of the lung as presented in a 10-year-old girl with a history of recurrent pneumonia and pneumothorax is reported. The lesion combined bronchial-type cavities with intervening stroma containing adipose tissue, with cartilage nodules of different sizes. The findings do not fit with any already-known malformations of the lung.

Sonic hedgehog signaling regulates Gli3 processing, mesenchymal proliferation, and differentiation during mouse lung organogenesis

Lack of Sonic hedgehog (Shh) signaling, mediated by the Gli proteins, leads to severe pulmonary hypoplasia. However, the precise role of Gli genes in lung development is not well established. We show Shh signaling prevents Gli3 proteolysis to generate its repressor forms (Gli3R) in the developing murine lung. In Shh(-/-) or cyclopamine-treated wild-type (WT) lung, we found that Gli3R level is elevated, and this upregulation appears to contribute to defects in proliferation and differentiation observed in the Shh(-/-) mesenchyme, where Gli3 is normally expressed. In agreement, we found Shh(-/-);Gli3(-/-) lungs exhibit enhanced growth potential. Vasculogenesis is also enhanced; in contrast, bronchial myogenesis remains absent in Shh(-/-);Gli3(-/-) compared with Shh(-/-) lungs. Genes upregulated in Shh(-/-);Gli3(-/-) relative to Shh(-/-) lung include Wnt2 and, surprisingly, Foxf1 whose expression has been reported to be Shh-dependent. Cyclins D1, D2, and D3 antibody labelings also reveal distinct expression patterns in the normal and mutant lungs. We found significant repression of Tbx2 and Tbx3, both linked to inhibition of cellular senescence, in Shh(-/-) and partial derepression in Shh(-/-); Gli3(-/-) lungs, while Tbx4 and Tbx5 expressions are less affected in the mutants. Our findings shed light on the role of Shh signaling on Gli3 processing in lung growth and differentiation by regulating several critical genes.

Epithelial-mesenchymal interactions in the developing lung

Classical experiments in embryology have shown that normal growth, morphogenetic patterning, and cellular differentiation in the developing lung depend on interactive signaling between the endodermal epithelium and mesenchyme derived from splanchnic mesoderm. These interactions are mediated by a myriad of diffusible factors that are precisely regulated in their temporal and spatial expression. In this review we first describe factors regulating formation of the embryonic foregut. We then discuss the experiments demonstrating the importance of tissue interactions in lung patterning and differentiation. Finally, we detail the roles that a few key signaling systems-fibroblast growth factors and their receptors, sonic hedgehog and Gli genes, Wnt genes and beta-catenin, and BMP4-play as mediators of epithelial-mesenchymal interactions in the developing lung.

Renal development: perspectives on a Wnt-dependent process

Specification of embryonic progenitors to generate the branched collecting duct system and tubular epithelia of the nephron in the metanephros is mediated by families of soluble factors that cooperate to regulate morphogenesis. These include multiple members of the FGF, TGF-beta, and Wnt families; however, the complexity of interactions through cell-cell and extracellular matrix-mediated contacts, the redundancy of factors involved, and multiplicity of cooperative signaling mechanisms limit our understanding of events responsible for this development. With available in vitro and targeted mutagenesis models, we are now beginning to comprehend how the secreted inductive proteins and associated transcription factors direct competent cells to produce a functional filtering tubular epithelium and its tightly integrated vascular network.

Inhibition of alveolarization and altered pulmonary mechanics in mice expressing GATA-6

GATA-6, a member of a family of zinc finger transcription factors, is expressed in epithelial cells of the developing lung. To further assess the role of GATA-6 in lung morphogenesis, GATA-6 was expressed in respiratory epithelial cells of the developing mouse lung under control of the surfactant protein C promoter (hSP-CGATA-6 mice). Although GATA-6 did not alter lung morphology at embryonic day 18.5, defects in alveolar septation were observed early in the neonatal period, and air space enlargement persisted to adulthood. Airway resistance, airway elastance, tissue damping, and tissue elastance were significantly decreased, and lung volumes were significantly increased at 12 wk of age. Normal postnatal morphogenesis of the lung depends upon precise temporal-spatial regulation of GATA-6.

beta-Catenin is required for specification of proximal/distal cell fate during lung morphogenesis

The lungs are divided, both structurally and functionally, into two distinct components, the proximal airways, which conduct air, and the peripheral airways, which mediate gas exchange. The mechanisms that control the specification of these two structures during lung development are currently unknown. Here we show that beta-catenin signaling is required for the formation of the distal, but not the proximal, airways. When the gene for beta-catenin was conditionally excised in epithelial cells of the developing mouse lung prior to embryonic day 14.5, the proximal lung tubules grew and differentiated appropriately. The mice, however, died at birth because of respiratory failure. Analysis of the lungs by in situ hybridization and immunohistochemistry, using molecular markers of the epithelial and mesenchymal components of both proximal and peripheral airways, showed that the lungs were composed primarily of proximal airways. These observations establish, for the first time, both the sites and timing of specification of the proximal and peripheral airways in the developing lung, and that beta-catenin is one of the essential components of this specification.

Characterization of Wnt signaling components and activation of the Wnt canonical pathway in the murine retina

The neuroepithelial layer of the developing eyecup contains multipotential precursor cells that give rise to all of the neurons and the one glial cell type present in the adult retina. Patterning within the retinal neuroepithelium is regulated by cell intrinsic as well as cell extrinsic mechanisms. Although the identity of some of the signaling molecules that regulate retinal development is known, the function of many others, especially members of the Wnt family, has yet to be characterized in the context of retinal development. We undertook a comprehensive in situ hybridization analysis to examine the expression of Wnt pathway components in the developing and adult mouse neural retina. Our findings confirm and extend previous expression studies in mice and other vertebrates, as we show that Wnt-3, -5a, -5b, and -7b are expressed in the neural retina and that there is a dynamic pattern of Wnt receptor (Mouse frizzled [Mfz]) and Wnt antagonist (Secreted-frizzled-related protein [Sfrp]) gene expression in the embryonic and perinatal neural retina. Moreover, we show that Wnt-13 is expressed in the pigment epithelium overlying the distal part of the eyecup and the ciliary margin and that Mfz-4, -6, and -7 are expressed in different regions within the ciliary margin. To determine where activation of canonical Wnt signaling is occurring in the retina, we examined reporter gene expression in TCF/Lef-LacZ mice and we demonstrate that the highest levels of beta-gal activity are found in the ciliary margin, adjacent to and within the Wnt-13 expression domain, implicating Wnt-13 signaling in the development of the ciliary margin and its derivatives.

Branching morphogenesis of the lung: new molecular insights into an old problem

It has been known for decades that branching morphogenesis of the lung is mediated through reciprocal interactions between the epithelium and its underlying mesenchyme. In recent years, several key players, in particular members of the major signaling pathways that mediate this interaction, have been identified. Here, we review the genetic and molecular studies of these key components, which have provided a conceptual framework for understanding the interactions of these major signaling pathways in branching morphogenesis. The future challenge is to translate understanding of the signaling cascade into knowledge of the cellular responses, including cell proliferation, migration and differentiation, that lead to the stereotyped branching.*