Laminin alpha1 chain synthesis in the mouse developing lung: requirement for epithelial-mesenchymal contact and possible role in bronchial smooth muscle development

The control of lung branching morphogenesis

Branching morphogenesis generates epithelial trees which facilitate gas exchange, filtering, as well as secretion processes with their large surface to volume ratio. In this review, we focus on the developmental mechanisms that control the early stages of lung branching morphogenesis. Lung branching morphogenesis involves the stereotypic, recurrent definition of new branch points, subsequent epithelial budding, and lung tube elongation. We discuss current models and experimental evidence for each of these steps. Finally, we discuss the role of the mesenchyme in determining the organ-specific shape.

Three-dimensional culture models of mammary gland

The mammary gland is a complex tissue comprised of a branching network of ducts embedded within an adipocyte-rich stroma. The ductal epithelium is a bi-layer of luminal and myoepithelial cells, the latter being in contact with a basement membrane. During pregnancy, tertiary branching occurs and lobuloalveolar structures, which produce milk during lactation, form in response to hormonal and cytokine signals. Postlactational regression is characterized by extensive cell death and tissue remodeling. These complex developmental events have been difficult to mimic in cell culture although many useful culture models exist. Recently, considerable advances in three-dimensional modelling of the mammary gland have been made with the use of collagen and other biomaterials for the study of branching morphogenesis and tumorigenesis, techniques which may enable rapid advances in our understanding of both basic biology and the study of cancer therapeutics.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

The laminin β1-competing peptide YIGSR induces a hypercontractile, hypoproliferative airway smooth muscle phenotype in an animal model of allergic asthma

Background

Fibroproliferative airway remodelling, including increased airway smooth muscle (ASM) mass and contractility, contributes to airway hyperresponsiveness in asthma. In vitro studies have shown that maturation of ASM cells to a (hyper)contractile phenotype is dependent on laminin, which can be inhibited by the laminin-competing peptide Tyr-Ile-Gly-Ser-Arg (YIGSR). The role of laminins in ASM remodelling in chronic asthma in vivo, however, has not yet been established.

Methods

Using an established guinea pig model of allergic asthma, we investigated the effects of topical treatment of the airways with YIGSR on features of airway remodelling induced by repeated allergen challenge, including ASM hyperplasia and hypercontractility, inflammation and fibrosis. Human ASM cells were used to investigate the direct effects of YIGSR on ASM proliferation in vitro.

Results

Topical administration of YIGSR attenuated allergen-induced ASM hyperplasia and pulmonary expression of the proliferative marker proliferating cell nuclear antigen (PCNA). Treatment with YIGSR also increased both the expression of sm-MHC and ASM contractility in saline- and allergen-challenged animals; this suggests that treatment with the laminin-competing peptide YIGSR mimics rather than inhibits laminin function in vivo. In addition, treatment with YIGSR increased allergen-induced fibrosis and submucosal eosinophilia. Immobilized YIGSR concentration-dependently reduced PDGF-induced proliferation of cultured ASM to a similar extent as laminin-coated culture plates. Notably, the effects of both immobilized YIGSR and laminin were antagonized by soluble YIGSR.

Conclusion

These results indicate that the laminin-competing peptide YIGSR promotes a contractile, hypoproliferative ASM phenotype in vivo, an effect that appears to be linked to the microenvironment in which the cells are exposed to the peptide.

Lung self-assembly is modulated by tissue surface tensions

To identify cell-intrinsic properties that facilitate interaction between epithelial endodermal and mesenchymal mesodermal cells during lung morphogenesis, we developed a model of lung self-assembly that mimics fetal lung formation in structure, polarity, vasculature, and extracellular matrix expression. Three-dimensional pulmonary bodies (PBs) spontaneously self-assemble from single-cell suspensions and exhibit liquid-like properties that allow measurements of compaction rate and cohesion, and that may help to specify cellular self-organization. We hypothesized that changes in one or more of these parameters could potentially explain the lung hypoplasia associated with abnormal lung development. We examined the impact of endothelial/monocyte-activating polypeptide (EMAP) II in PBs, because EMAPII is highly expressed in lung hypoplasia. EMAPII significantly increased compaction rate and decreased overall cohesion of PBs composed of both epithelial and mesenchymal cells. Moreover, the effects of EMAPII on compaction and cohesion act exclusively through the mesenchymal cell population by interfering with fibronectin matrix assembly. We also show that EMAPII alters epithelial cell polarity and surfactant protein C expression. Our findings demonstrate, for the first time, that PBs possess liquid-like properties that can help to guide the self-assembly of fetal lungs, and that EMAPII expression can influence both mesenchymal and epithelial cells but through different molecular mechanisms.

Development, repair and fibrosis: what is common and why it matters

The complex structure of the lung is developed sequentially, initially by epithelial tube branching and later by septation of terminal air sacs with accompanying coordinated growth of a variety of lung epithelial and mesenchymal cells. Groups of transcriptional factors, peptide growth factors and their intracellular signaling regulators, as well as extracellular matrix proteins are programmed to be expressed at appropriate levels in the right place at the right time to control normal lung formation. Studies of lung development and lung repair/fibrosis to date have discovered that many of the same factors that control normal development are also key players in lung injury repair and fibrosis. Transforming growth factor-beta (TGF-beta) family peptide signaling is a prime example. Lack of TGF-beta signaling results in abnormal lung branching morphogenesis and alveolarization during development, whereas excessive amounts of TGF-beta signaling cause severe hypoplasia in the immature lung and fibrosis in mature lung. This leads us to propose the 'Goldilocks' hypothesis of regulatory signaling in lung development and injury repair that everything must be done just right!

Insulin-induced laminin expression promotes a hypercontractile airway smooth muscle phenotype

Airway smooth muscle (ASM) plays a key role in the development of airway hyperresponsiveness and remodeling in asthma, which may involve maturation of ASM cells to a hypercontractile phenotype. In vitro studies have indicated that long-term exposure of bovine tracheal smooth muscle (BTSM) to insulin induces a functional hypercontractile, hypoproliferative phenotype. Similarly, the extracellular matrix protein laminin has been found to be involved in both the induction and maintenance of a contractile ASM phenotype. Using BTSM, we now investigated the role of laminins in the insulin-induced hypercontractile, hypoproliferative ASM phenotype. The results demonstrate that insulin-induced hypercontractility after 8 days of tissue culture was fully prevented by combined treatment of BTSM-strips with the laminin competing peptides Tyr-Ile-Gly-Ser-Arg (YIGSR) and Arg-Gly-Asp-Ser (RGDS). YIGSR also prevented insulin-induced increases in sm-myosin expression and abrogated the suppressive effects of prolonged insulin treatment on platelet-derived growth factor-induced DNA synthesis in cultured cells. In addition, insulin time-dependently increased laminin alpha2, beta1, and gamma1 chain protein, but not mRNA abundance in BTSM strips. Moreover, as previously found for contractile protein accumulation, signaling through PI3-kinase- and Rho kinase-dependent pathways was required for the insulin-induced increase in laminin abundance and contractility. Collectively, our results indicate a critical role for beta1-containing laminins, likely laminin-211, in the induction of a hypercontractile, hypoproliferative ASM phenotype by prolonged insulin exposure. Increased laminin production by ASM could be involved in the increased ASM contractility and contractile protein expression in asthma. Moreover, the results may be of interest for the use of inhaled insulin administrations by diabetics.

CD117-positive cells in adult human heart are localized in the subepicardium, and their activation is associated with laminin-1 and alpha6 integrin expression

CD117-positive cells contributing to cardiac cell turnover in normal and pathological conditions have recently been described in adult human heart. Since the precise spatial and temporal expression of extracellular matrix proteins and their receptors is critical for organ formation, we compared the distribution of cardiac primitive CD117-positive cells in the human adult normal and pathological hearts with ischemic cardiomyopathy, with respect to localization and expression of laminin and integrin isoforms. In the pathological hearts, CD117-positive cells were significantly more numerous than in the normal hearts. They were localized mainly in the atria and were up to 38-fold more numerous in the subepicardium than in the myocardium. Compared with normal hearts, most CD117-positive cells in the subepicardium of pathological hearts were alpha(6) integrin-positive. Laminin-1, typical of developing heart, was found predominantly in the subepicardium of adult heart. Immunoblotting revealed its highest expression in the normal atrium and pathological left ventricle. Both laminin isoforms reduced apoptosis and increased proliferation and migration of CD117-positive cells in vitro with respect to control, but the effects of laminin-1 significantly outweighed those of laminin-2. Signaling mediated by alpha(6) integrin was implicated in the migration and protection from apoptosis, as documented by transfection with specific small interfering RNA. These data reveal that the increase in the number of cardiac CD117-positive cells and the expression of laminin-1 are observed in ischemic cardiomyopathy. Subepicardial localization of CD117-positive cells and expression of laminin-1 and alpha(6) integrin subunits may all correspond to the activation of regeneration involving an epithelial-mesenchymal transition recently described in adult heart.

Embryological origin of airway smooth muscle

Airway smooth muscle (SM) develops from local mesenchymal cells located around the tips of growing epithelial buds. These cells gradually displace from distal to proximal position alongside the bronchial tree, elongate, and begin to synthesize SM-specific proteins. Mechanical tension (either generated by cell spreading/elongation or stretch), as well as epithelial paracrine factors, regulates the process of bronchial myogenesis. The specific roles of many of these paracrine factors during normal lung development are currently unknown. It is also unknown how and if mechanical and paracrine signals integrate into a common myogenic pathway. Furthermore, as with vascular SM and other types of visceral SM, we are just beginning to elucidate the intracellular signaling pathways and the genetic program that controls lung myogenesis. Here we present what we have learned so far about the embryogenesis of bronchial muscle.

Laminin-binding integrin alpha7 is required for contractile phenotype expression by human airway myocytes

Contractile airway smooth muscle (ASM) cells retain the ability for phenotype plasticity in response to multiple stimuli, which equips them with capacity to direct modeling and remodeling during development, and in disease states such as asthma. We have shown that endogenously expressed laminin is required for maturation of human ASM cells to a contractile phenotype, as occurs during ASM thickening in asthma. In this study, we profiled the expression of laminin-binding integrins alpha3beta1, alpha6beta1, and alpha7beta1, and tested whether they are required for laminin-induced myocyte maturation. Immunoblotting revealed that myocyte maturation induced by prolonged serum withdrawal, which was marked by the accumulation of contractile phenotype marker protein desmin, was also associated with the accumulation of alpha3A, alpha6A, and alpha7B. Flow cytometry revealed that alpha7B expression was a distinct feature of individual myocytes that acquired a contractile phenotype. siRNA knockdown of alpha7, but not alpha3 or alpha6, suppressed myocyte maturation. Thus, alpha7B is a novel marker of the contractile phenotype, and alpha7 expression is essential for human ASM cell maturation, which is a laminin-dependent process. These observations provide new insight into mechanisms that likely underpin normal development and remodeling associated with airways disease.

Endogenous laminin is required for human airway smooth muscle cell maturation

Background

Airway smooth muscle (ASM) contraction underlies acute bronchospasm in asthma. ASM cells can switch between a synthetic-proliferative phenotype and a contractile phenotype. While the effects of extracellular matrix (ECM) components on modulation of ASM cells to a synthetic phenotype have been reported, the role of ECM components on maturation of ASM cells to a contractile phenotype in adult lung is unclear. As both changes in ECM components and accumulation of contractile ASM are features of airway wall remodelling in asthma, we examined the role of the ECM protein, laminin, in the maturation of contractile phenotype in human ASM cells.

Methods

Human ASM cells were made senescence-resistant by stable expression of human telomerase reverse transcriptase. Maturation to a contractile phenotype was induced by 7-day serum deprivation, as assessed by immunoblotting for desmin and calponin. The role of laminin on ASM maturation was investigated by comparing the effects of exogenous laminin coated on culture plates, and of soluble laminin peptide competitors. Endogenous expression of laminin chains during ASM maturation was also measured.

Results

Myocyte binding to endogenously expressed laminin was required for ASM phenotype maturation, as laminin competing peptides (YIGSR or GRGDSP) significantly reduced desmin and calponin protein accumulation that otherwise occurs with prolonged serum deprivation. Coating of plastic cell culture dishes with different purified laminin preparations was not sufficient to further promote accumulation of desmin or calponin during 7-day serum deprivation. Expression of α2, β1 and γ1 laminin chains by ASM cells was specifically up-regulated during myocyte maturation, suggesting a key role for laminin-2 in the development of the contractile phenotype.

Conclusion

While earlier reports suggest exogenously applied laminin slows the spontaneous modulation of ASM to a synthetic phenotype, we show for the first time that endogenously expressed laminin is required for ASM maturation to the contractile phenotype. As endogenously expressed laminin chains α2, β1 and γ1 are uniquely increased during myocyte maturation, these laminin chains may be key in this process. Thus, human ASM maturation appears to involve regulated endogenous expression of a select set of laminin chains that are essential for accumulation of contractile phenotype myocytes.

Laminin isoforms and lung development: All isoforms are not equal

Laminins are a major component of basement membranes. Each laminin molecule is a heterotrimeric glycoprotein composed of one alpha, one beta, and one gamma chain. Fifteen laminin isoforms exist, assembled from various combinations of 5alpha, 3beta, and 3gamma chains. The embryonic lung has abundant laminin isoforms. Increasing evidence suggests that different laminin isoforms have unique functions in lung development. Studies of embryonic lung explants and organotypic co-cultures show that laminin alpha1 and laminin 111 are important for epithelial branching morphogenesis and that laminin alpha2 and laminin 211 have a role in smooth muscle cell differentiation. In vivo studies of laminin alpha5-deficient mice indicate that this laminin chain, found in laminins 511 and 521, is essential for normal lobar septation in early lung development and normal alveolization and distal epithelial cell differentiation and maturation in late lung development. However, not all of the laminin chains present in the developing lung appear to be necessary for normal lung development since laminin alpha4 null mice do not have obvious lung abnormalities and laminin gamma2 null mice have only minimal changes in lung development. The mechanisms responsible for the lung phenotypes in mice with laminin mutations are unknown, but it is clear that multiple laminin isoforms are crucial for lung development and that different laminin isoforms exhibit specific, non-overlapping functions.

Parabronchial smooth muscle cells and alveolar myofibroblasts in lung development

Epithelial-mesenchymal interactions and extracellular matrix remodeling are key processes of embryonic lung development. Lung smooth muscle cells, which are derived from the mesenchyme, form a sheath around bronchi and blood vessels. During lung organogenesis, smooth muscle differentiation coincides with epithelial branching morphogenesis and closely follows developing airways spatially and temporally. The precise function of parabronchial smooth muscle (PBSM) cells in healthy adult lung remains unclear. However, PBSM may regulate epithelial branching morphogenesis during lung development by the induction of mechanical stress or through regulation of paracrine signaling pathways. Alveolar myofibroblasts are interstitial contractile cells that share features and may share an origin with smooth muscle cells. Alveolar myofibroblasts are essential for secondary septation, a process critical for the development of the gas-exchange region of the lung. Dysregulation of PBSM or alveolar myofibroblast development is thought to underlie the pathogenesis of many lung diseases, including bronchopulmonary dysplasia, asthma, and interstitial fibrosis. We review the current understanding of the regulation of PBSM and alveolar myofibroblast development, and discuss the role of PBSM in lung development. We specifically focus on the role of these cells in the context of fibroblast growth factor-10, sonic hedgehog, bone morphogenetic protein-4, retinoic acid, and Wnt signaling pathways in the regulation of lung branching morphogenesis.

Monoclonal anti-mouse laminin antibodies: AL-1 reacts with laminin alpha1 chain, AL-2 with laminin beta1 chain, and AL-4 with the coiled-coil domain of laminin beta1 chain

We analyzed the reactivity of three different commercially available rat monoclonal antibodies raised against mouse laminin-alpha1beta1gamma1 (laminin-111), AL-1, AL-2, and AL-4. Using ELISA assays, Western blot analysis and immunostainings we present refined epitope maps for these three laminin monoclonals. AL-1 reacted, as predicted with laminin alpha1 chain. AL-4 has also been marketed as an alpha1 chain specific probe, but we show here that AL-4 detects mouse laminin beta1 chain, in the distal part of the coiled-coil region. AL-2 was predicted to react with all three chains near the cross-region, but seems to primarily react with laminin beta1 chain.

The association of caveolae, actin, and the dystrophin-glycoprotein complex: a role in smooth muscle phenotype and function?

Smooth muscle cells exhibit phenotypic and mechanical plasticity. During maturation, signalling pathways controlling actin dynamics modulate contractile apparatus-associated gene transcription and contractile apparatus remodelling resulting from length change. Differentiated myocytes accumulate abundant caveolae that evolve from the structural association of lipid rafts with caveolin-1, a protein with domains that confer unique functional properties. Caveolae and caveolin-1 modulate and participate in receptor-mediated signalling, and thus contribute to functional diversity of phenotypically similar myocytes. In mature smooth muscle, caveolae are partitioned into discrete linear domains aligned with structural proteins that tether actin to the extracellular matrix. Caveolin-1 binds with beta-dystroglycan, a subunit of the dystrophin glycoprotein complex (DGC), and with filamin, an actin binding protein that organizes cortical actin, to which integrins and focal adhesion complexes are anchored. The DGC is linked to the actin cytoskeleton by a dystrophin subunit and is a receptor for extracellular laminin. Thus, caveolae and caveolin-associated signalling proteins and receptors are linked via structural proteins to a dynamic filamentous actin network. Despite development of transgenic models to investigate caveolins and membrane-associated actin-linking proteins in skeletal and cardiac muscle function, only superficial understanding of this association in smooth muscle phenotype and function has emerged.

Fetal oxygen tension promotes tenascin-C-dependent lung branching morphogenesis

Tenascin-C (TN-C) is a mesenchyme-derived extracellular matrix (ECM) glycoprotein required for fetal lung branching morphogenesis. Given that the low oxygen (O(2)) environment of the fetus is also essential for normal lung branching morphogenesis, we determined whether fetal O(2) tension supports this process by promoting TN-C expression. Initial studies showed that 15-day fetal rat lung explants cultured for 2 days at 3% O(2) not only branched well, but they also expressed higher levels of TN-C when compared to lungs maintained at 21% O(2), which branched poorly. Antisense oligonucleotide studies demonstrated that TN-C produced in response to 3% O(2) was essential for lung branching morphogenesis. As well, exogenous TN-C protein was shown to promote branching of lung epithelial rudiments cultured at 21% O(2). Because ECM-degrading proteinases are capable of catabolizing TN-C protein, we reasoned that 3% O(2) might promote TN-C deposition by limiting the activity of these enzymes within the fetal lung. Consistent with this idea, gelatin zymography showed that the activity of a 72-kDa gelatinase, identified as matrix metalloproteinase-2 (MMP-2), was lower at 3% O(2) vs. 21% O(2). Furthermore, pharmacologic inhibition of MMP-2 activity in fetal lung explants cultured at 21% O(2) resulted in increased TN-C deposition within the mesenchyme, as well as enhanced branching morphogenesis. Collectively, these studies indicate that fetal O(2) tension promotes TN-C-dependent lung epithelial branching morphogenesis by limiting the proteolytic turnover of this ECM component within the adjacent mesenchyme.

Cell Biology of Bladder Development and the Role of the Extracellular Matrix

The bladder is unique. It is the most distensible organ in the body. If the normal pattern of development is disturbed, the bladder can become dysfunctional and poorly compliant. Renal outcome is intimately linked to bladder function and hence bladder development. In this review we describe our current knowledge of the molecular processes that govern bladder development, with particular emphasis on the role of the extracellular matrix. Finally, we suggest a hypothetical model for bladder development.

Epithelial–Mesenchymal Interactions Are Linked to Neovascularization

Lung morphogenesis is dependent on interactions between mesenchymal and epithelial cells. We have previously demonstrated that inhibition of neovascularization by endothelial monocyte-activating polypeptide (EMAP) II also attenuates fetal lung morphogenesis in vivo, and hypothesized that epithelial-mesenchymal interactions are regulated by vascular signals. To address this postulate, we evaluated the formation of epithelial cysts in vitro and assessed this complex interaction through: (i) identification of vascular formation in vitro; (ii) assessment of the effect of selective vascular inhibition on cell viability, proliferation, and cellular interactions as measured by epithelial cyst formation; and (iii) examination of whether there is an interdependent relationship between epithelial and mesenchymal cells and a vascular mediator's protein expression. Vascular networks in vitro formed in direct relationship to the presence of epithelial cysts. Disruption of the vasculature by delivery of a selective antiangiogenic protein EMAP II was associated with disruption of epithelial cyst formation. Lastly, control of the vascular formation regulatory protein EMAP II is a direct result of epithelial-mesenchymal cell interactions. These findings suggest that vascular formation modulates and is modulated by the normal cellular communication and interactions that direct lung morphology.

Role of stretch in activation of smooth muscle cell lineage

Mechanical force is a critical modulator of smooth muscle (SM) function and gene expression. Very little is known, however, about its contribution to SM myogenesis. This review presents and discusses what has been learned about the role of mechanical force in inducing SM myogenesis and some of the signaling mechanisms involved in this process.

Mechanisms of inflammation-mediated airway smooth muscle plasticity and airways remodeling in asthma

Recent evidence points to progressive structural change in the airway wall, driven by chronic local inflammation, as a fundamental component for development of irreversible airway hyperresponsiveness. Acute and chronic inflammation is orchestrated by cytokines from recruited inflammatory cells, airway myofibroblasts and myocytes. Airway myocytes exhibit functional plasticity in their capacity for contraction, proliferation, and synthesis of matrix protein and cytokines. This confers a principal role in driving different components of the airway remodeling process, and mediating constrictor hyperresponsiveness. Functional plasticity of airway smooth muscle (ASM) is regulated by an array of environmental cues, including cytokines, which mediate their effects through receptors and a number of intracellular signaling pathways. Despite numerous studies of the cellular effects of cytokines on cultured airway myocytes, few have identified how intracellular signaling pathways modulate or induce these cellular responses. This review summarizes current understanding of these concepts and presents a model for the effects of inflammatory mediators on functional plasticity of ASM in asthma.

Involvement of serum response factor isoforms in myofibroblast differentiation during bleomycin-induced lung injury

Serum response factor (SRF) is a transcription factor essential for smooth muscle (SM) myogenesis. Its role in myofibroblast differentiation is, however, unknown. We studied the expression and the localization of SRF in bleomycin-induced pulmonary fibrosis, where myofibroblasts are abundant. We found that SRF levels were upregulated in bleomycin-exposed mouse lungs mainly due to de novo synthesis of SRFDelta5, a less myogenic SRF isoform. Before myofibroblast differentiation, SRF/SRFDelta5 was immunolocalized mostly in the cytoplasm of scattered fibroblasts at lesion sites. With the development of myofibroblasts, however, SRF/SRFDelta5 was found in myofibroblast nuclei. cDNA array analysis showed that SRFDelta5 and SRF induced expression of transforming growth factor-beta1, a critical factor in myofibroblast differentiation. This was accompanied by de novo expression of several inflammatory cell-specific mRNAs. The latter was confirmed by reverse transcriptase-polymerase chain reaction. Treatment of lung fibroblasts with tumor necrosis factor-alpha, which is produced early in the bleomycin model, induced SRFDelta5 expression and SRF/SRFDelta5 cytoplasmic accumulation, whereas addition of transforming growth factor-beta1 caused SRF/SRFDelta5 nuclear translocation followed by SM alpha-actin synthesis. Interleukin-4, another cytokine involved in myofibroblast differentiation, did not affect SRF or induce SRFDelta5 expression. Our studies therefore suggested a new mechanism whereby SRF and SRFDelta5 contribute to the emergence of myofibroblasts in lung injury and fibrosis.

Evidence that bone morphogenetic protein 4 has multiple biological functions during kidney and urinary tract development

BACKGROUND

We have suggested that bone morphogenetic protein 4 (BMP4), acting on the Wolffian duct and ureter epithelium, determines the budding site of the ureter by locally antagonizing ubiquitous inductive signal(s) from the metanephric mesenchyme. In the present study, we examine the effect of BMP4 on the development of metanephric and periureteral mesenchymal cells, which express the BMP type I receptor gene, Bmpr1a (Alk3).

METHODS

Urogenital tissues obtained from Bmp4 heterozygous null mutant (Bmp4+/-) embryos at different stages, and metanephric and ureteral tissue explants cultured in the presence of recombinant BMP4 were subjected to morphologic, immunohistochemical and in situ hybridization analyses. To examine the chemotactic activity of BMP4 for periureteral mesenchymal cells, a modified Boyden chamber assay was performed.

RESULTS

Many of the kidneys of newborn Bmp4+/- mice contained multicystic dysplastic regions. This morphology was preceded by abnormally high apoptotic activity in the metanephric mesenchyme of mutant embryos at E14.5. In whole metanephric explants, BMP4 uniformly promoted the expansion of the Pax2-negative and weakly Foxd1 (previously Bf2) -positive peripheral stromal compartment of metanephric mesenchyme in the presence of fibroblast growth factor 2 (FGF2). In addition, in isolated metanephric mesenchyme, BMP4-loaded beads prevented apoptosis locally. Thus, BMP4 prevents cell death and promotes the growth of the metanephric mesenchyme. The effect of BMP4 on periureteral mesenchyme is different from its effect on metanephric mesenchyme. In utero, periureteral mesenchymal cells condense around the ureter epithelium, followed by differentiation into smooth muscle cells at a site where Bmp4 is intensely expressed. Analysis of Bmp4+/- ureters at E15.5 reveals that the alpha-smooth muscle actin (alpha-SMA)-positive cells are low in number. In vitro, BMP4-loaded beads promote the accumulation of periureteral mesenchymal cells to form several cell layers surrounding the beads. In addition, in a Boyden chamber assay, BMP4 increases the migration of periureteral mesenchymal cells through the filter. Thus, BMP4 can serve as a chemoattractant for periureteral mesenchymal cells and induce locally the smooth muscle layer of the ureter at Bmp4-expressing sites.

CONCLUSION

Depending on local context, BMP4 has several biological actions on the morphogenesis of different portions of the excretory system, namely, the development of the ureterovesical junction, the ureter, and the kidney.

P311 induces a TGF-beta1-independent, nonfibrogenic myofibroblast phenotype

P311, also called PTZ17, was identified by suppressive subtraction hybridization as potentially involved in smooth muscle (SM) myogenesis. P311 is an 8-kDa protein with several PEST-like motifs found in neurons and muscle. P311 transfection into two fibroblast cell lines, NIH 3T3 and C3H10 T1/2, induced phenotypic changes consistent with myofibroblast transformation, including upregulation of SM alpha-actin and SM22, induction of FGF-2, VEGF, PDGF, and PDGF receptors, upregulation of integrins alpha3 and alpha5, and increased proliferation rate. The P311-mediated changes differed, however, from the well-characterized myofibroblast in that P311 inhibited TGF-beta1, TGF-beta receptor 2, and TGF-beta1-activating MMP-2 and MMP-9, with the resultant decrease in collagen 1 and 3 expression. The effect of P311 on collagen was overcome by exogenous TGF-beta1, indicating that the cells were responsive to TGF-beta1 paracrine stimulus. In support of a role for P311 in vivo, immunohistochemical examination of human wounds showed P311 only in myofibroblasts and their activated precursors. To our knowledge, these studies are the first to implicate P311 in myofibroblast transformation, to demonstrate that transformation may occur independently of TGF-beta1, and to suggest that P311 may prevent fibrosis.

Mesenchymal-epithelial interactions in lung development and repair: are modeling and remodeling the same process?

We propose that lung morphogenesis and repair are characterized by complex cell-cell interactions of endodermal and mesodermal origin, leading to (or returning back to) an alveolar structure that can effectively exchange gases between the circulation and the alveolar space. We provide the developmental basis for cell/molecular control of lung development and disease, what is known about growth and transcription factors in normal and abnormal lung development, and how endodermal and mesodermal cell origins interact during lung development and disease. The global mechanisms that mediate mesenchymal-epithelial interactions and the plasticity of mesenchymal cells in normal lung development and remodeling provide a functional genomic model that may bring these concepts closer together. We present a synopsis followed by a vertical integration of the developmental and injury/repair mechanisms.

PDGF-BB enhances expression of, and reduces adhesion to, laminin-5 in vascular smooth muscle cells

The laminin family of extracellular matrix (ECM) proteins plays crucial roles in regulating cellular growth, migration, and differentiation. We report here that laminin-5 is expressed in the tunica media of the rat aorta and pulmonary arteries. Using indirect immunofluorescence microscopy, Western blots, and RT-PCR analysis, we found that primary cultures of rat arterial smooth muscle cells express laminin-5 and deposit it into their insoluble ECM. These cells also attach strongly to laminin-5 via beta1 integrin receptors in 30 min adhesion assays. Laminin-5 expression in these cells is upregulated by growth factors in vitro and platelet-derived growth factor (PDGF-BB) stimulation reduces adhesion to laminin-5. As laminin-5 promotes enhanced migration of other cell types, the production of and adhesion to laminin-5 by vascular smooth muscle cells may play a role in the pathological growth and migration of these cells associated with restenosis following vascular injury.

Laminin alpha 5 is required for lobar septation and visceral pleural basement membrane formation in the developing mouse lung

Laminin alpha/beta/gamma heterotrimers are the major noncollagenous components of all basement membranes. To date, five alpha, three beta, and three gamma chains have been identified. Laminin alpha 5 is expressed early in lung development and colocalizes with laminin alpha1. While laminin alpha1 expression in the lung is restricted to the embryonic period, laminin alpha 5 expression persists throughout embryogenesis and adulthood. Targeted mutation of the mouse laminin alpha 5 gene Lama5 causes embryonic lethality at E14-E17 associated with exencephaly, syndactyly, placentopathy, and kidney defects, all attributable to abnormal basement membranes. In this investigation, lung development in Lama5(-/-) mice up to E16.5 was examined. We observed normal lung branching morphogenesis and vasculogenesis, but incomplete lobar septation and absence of the visceral pleura basement membrane. Preservation of branching morphogenesis was associated with ectopic deposition of laminin alpha 4 in the airway basement membrane. Perturbation of pleural basement membrane formation and right lung septation correlated with absence of laminin alpha 5, which was found to be the only laminin alpha chain present in the normal visceral pleura basement membrane. Our finding of normal lung branching morphogenesis with abnormal lobar septation demonstrates that these processes are not obligatorily linked.

Pro-EMAP II is not primarily cleaved by caspase-3 and -7

Endothelial monocyte-activating polypeptide (EMAP) II is a unique cytokine, also known as p43, the active mature form of which exhibits antiangiogenic properties in vivo and in vitro. The proteolytic enzymes associated with the cleavage and release of the active mature form, however, remain unclear. Here we show that, in contrast to prior observations, purified pro-EMAP II is not cleaved by either caspase-3 or -7 in vivo or in vitro. Thus other proteolytic processes, which allow it to induce apoptosis via caspase-3 activation in migrating and dividing endothelium, may be involved in the release of the active mature EMAP II.

Expression and distribution of laminin alpha1 and alpha2 chains in embryonic and adult mouse tissues: an immunochemical approach

Protein levels, mRNA expression, and localization of laminin alpha1 and alpha2 chains in development and in adult mice were examined. Recombinant fragments were used to obtain high-titer-specific polyclonal antibodies for establishing quantitative radioimmuno-inhibition assays. This often demonstrated an abundance of alpha2 chain, but also distinct amounts of alpha1 chain for adult tissues. The highest amounts of alpha1 were found in placenta, kidney, testis, and liver and exceeded those of alpha2. All other tissue extracts showed a higher content of alpha2, which was particularly high in heart and muscle when compared to alpha1. Content of gamma1 chain, shared by most laminins, was also analyzed. This demonstrated gamma1 chain levels being equal to or moderately exceeding the sum of alpha1 and alpha2 chains, indicating that these isoforms represent the major known laminin isoforms in most adult mouse tissues so far examined. Moreover, we found good correlation between radioimmuno-inhibition data and mRNA levels of adult tissues as measured by quantitative real-time reverse transcriptase-PCR. Embryonic tissues were also analyzed by radioimmuno-inhibition assays. This demonstrated for day 11 embryos comparable amounts of alpha1 and gamma1 and a more than 25-fold lower content of alpha2. This content increased to about 10% of alpha1 in day 13 embryos. The day 18 embryo showed in heart, kidney, and liver, but not yet in brain and lung, alpha1/alpha2 chain ratios comparable to those in adult tissues. Immunostaining demonstrated alpha1 in Reichert's membrane (day 7.5), while alpha2 could not be detected before day 11.5. These data were compared with immunohistochemical localization results on several more embryonic and adult tissue sections. Our results regarding localization are consistent with those of earlier work with some notable exceptions. This was in part due to epitope masking for monoclonal antibodies commonly used in previous studies in esophagus, intestine, stomach, liver, kidney, and spleen.

Development of neural tissue and airway smooth muscle in fetal mouse lung explants: a role for glial-derived neurotrophic factor in lung innervation

We have characterized the distribution of neural tissue and its primary target tissue, airway smooth muscle (ASM), in an in vitro mouse model of early lung development comprising left lung lobes at embryonic Day 12, cultured for 2 or 5 d. Neural tissue was detected with antibodies to protein gene product 9.5 (PGP 9.5), synapsin, and p75NTR (the low-affinity neurotrophin receptor), and smooth muscle with an antibody to alpha-actin. Imaging by confocal microscopy revealed few PGP 9.5-positive neurons at the start of culture; after 2 d clusters of neurons and nerve fibers had appeared along the lobar bronchus and after 5 d along the secondary and tertiary branches. Neural tissue did not just follow the smooth muscle-covered tubules, as seen in vivo, but also grew outside the lobes onto a wide layer of alpha-actin-positive cells, suggesting that smooth muscle may express a trophic factor that attracts nerves. Explants cultured with glial-derived neurotrophic factor (GDNF) exhibited a striking increase in the amount of p75NTR- and PGP 9.5-positive tissue outside the lobes, whereas GDNF-impregnated beads attracted neuronal precursors and influenced the direction of neurite extension. We show that the mouse lung explant is suitable for investigating trophic signals involved in pulmonary innervation and that GDNF may have a role in the early innervation of the developing airways.

High RhoA activity maintains the undifferentiated mesenchymal cell phenotype, whereas RhoA down-regulation by laminin-2 induces smooth muscle myogenesis

Round embryonic mesenchymal cells have the potential to differentiate into smooth muscle (SM) cells upon spreading/elongation (Yang, Y., K.C. Palmer, N. Relan, C. Diglio, and L. Schuger. 1998. Development. 125:2621-2629; Yang, Y., N.K. Relan, D.A. Przywara, and L. Schuger. 1999. Development. 126:3027-3033; Yang, Y., S. Beqaj, P. Kemp, I. Ariel, and L. Schuger. 2000. J. Clin. Invest. 106:1321-1330). In the developing lung, this process is stimulated by peribronchial accumulation of laminin (LN)-2 (Relan, N.K., Y. Yang, S. Beqaj, J.H. Miner, and L. Schuger. 1999. J. Cell Biol. 147:1341-1350). Here we show that LN-2 stimulates bronchial myogenesis by down-regulating RhoA activity. Immunohistochemistry, immunoblotting, and reverse transcriptase-PCR indicated that RhoA, a small GTPase signaling protein, is abundant in undifferentiated embryonic mesenchymal cells and that its levels decrease along with SM myogenesis. Functional studies using agonists and antagonists of RhoA activation and dominant positive and negative plasmid constructs demonstrated that high RhoA activity was required to maintain the round undifferentiated mesenchymal cell phenotype. This was in part achieved by restricting the localization of the myogenic transcription factor serum response factor (SRF) mostly to the mesenchymal cell cytoplasm. Upon spreading on LN-2 but not on other main components of the extracellular matrix, the activity and level of RhoA decreased rapidly, resulting in translocation of SRF to the nucleus. Both cell elongation and SRF translocation were prevented by overexpression of dominant positive RhoA. Once the cells underwent SM differentiation, up-regulation of RhoA activity induced rather than inhibited SM gene expression. Therefore, our studies suggest a novel mechanism whereby LN-2 and RhoA modulate SM myogenesis.

An organotypic in vitro model that mimics the dento-epithelial junction

BACKGROUND

The dento-epithelial junction forms the primary periodontal defense structure against oral microbes. The cells of the junctional epithelium (JE) attach both to a basement membrane (BM) facing the connective tissue and to a hard dental tissue by structurally similar but molecularly distinct mechanisms. Here we describe a new organotypic cell culture model for the dento-epithelial junction comprising not only epithelial and mesenchymally derived components, but also a tooth surface equivalent.

METHODS

Rat palatal keratinocytes were seeded on fibroblast-collagen gels. A tooth slice was placed on top of the epithelial cells and the multilayer cultures were grown at the air-liquid interface. Formation of the epithelial structures, BM components, and the epithelial attachment to the tooth surface were studied by immunofluorescence and light and electron microscopy. The findings were compared to the structure of the dento-epithelial junction in vivo.

RESULTS

A well-differentiated stratified epithelium was formed. Under the tooth slice the epithelium remained thin and non-differentiated. Attachment of the epithelial cells to the tooth surface was mediated by hemidesmosomes (HDs) as in vivo. Laminin-5 (Ln-5) was present in the extracellular matrix (ECM) between the tooth and the epithelium as well as in the BM structure between the epithelium and the fibroblast-collagen matrix. Instead, Ln-10/11 was present only at the mesenchymal tissue side as is known to be the case in vivo.

CONCLUSIONS

The organotypic model presented expresses the characteristic structural and molecular features of the dento-epithelial junction and may be applied for studying physiological and pathological processes in the epithelial attachment.

Perturbation of extracellular matrix prevents association of the otic primordium with the posterior rhombencephalon and inhibits subsequent invagination

In the avian embryo, the otic primordia become visible by Hamburger and Hamilton stage 10 as a pair of thickened regions of head ectoderm. In contrast to other epithelial primordia, invagination occurs by means of formation of a series of folds in distinct areas of the primordium, giving the otic vesicle a box-like appearance. Because previous work has shown that otic invagination is ATP and calcium independent, it is unlikely that cytoskeletal changes are the primary mechanism responsible for invagination as in other epithelial primordia. Interaction of the primordium with surrounding tissues may provide the force for otic invagination. These extracellular forces may be transduced through extracellular matrix macromolecules and their cell surface receptors. This investigation tests the hypothesis that fusion of the otic and hindbrain basal laminae between stages 11 and 13 is necessary for normal invagination. Perturbation of binding of the otic primordium to the neural tube was accomplished by means of microinjection of antibodies to various extracellular matrix components and integrin subunits into the head mesenchyme in the otic region at stage 10. Only antibodies to laminin and integrins caused detachment of the otic primordium from the hindbrain. These experiments suggest that fusion of the otic and hindbrain basal laminae is required for subsequent invagination and, furthermore, that this event is mediated by components of the extracellular matrix.

Dystroglycan binding to laminin alpha1LG4 module influences epithelial morphogenesis of salivary gland and lung in vitro

Dystroglycan is a receptor for the basement membrane components laminin-1, -2, perlecan, and agrin. Genetic studies have revealed a role for dystroglycan in basement membrane formation of the early embryo. Dystroglycan binding to the E3 fragment of laminin-1 is involved in kidney epithelial cell development, as revealed by antibody perturbation experiments. E3 is the most distal part of the carboxyterminus of laminin alpha1 chain, and is composed of two laminin globular (LG) domains (LG4 and LG5). Dystroglycan-E3 interactions are mediated solely by discrete domains within LG4. Here we examined the role of this interaction for the development of mouse embryonic salivary gland and lung. Dystroglycan mRNA was expressed in epithelium of developing salivary gland and lung. Immunofluorescence demonstrated dystroglycan on the basal side of epithelial cells in these tissues. Antibodies against dystroglycan that block binding of alpha-dystroglycan to laminin-1 perturbed epithelial branching morphogenesis in salivary gland and lung organ cultures. Inhibition of branching morphogenesis was also seen in cultures treated with polyclonal anti-E3 antibodies. One monoclonal antibody (mAb 200) against LG4 blocked interactions between a-dystroglycan and recombinant laminin alpha1LG4-5, and also inhibited salivary gland and lung branching morphogenesis. Three other mAbs, also specific for the alpha1 carboxyterminus and known not to block branching morphogenesis, failed to block binding of alpha-dystroglycan to recombinant laminin alpha1LG4-5. These findings clarify why mAbs against the carboxyterminus of laminin alpha1 differ in their capacity to block epithelial morphogenesis and suggest that dystroglycan binding to alpha1LG4 is important for epithelial morphogenesis of several organs.

The laminins in the murine inner ear: developmental transitions and expression in cochlear basement membranes

The laminins are a family of heterotrimeric extracellular matrix molecules that form suprastructural networks in basement membranes and elsewhere. They interact with integrin receptors, playing key roles in modulating programs of cytodifferentiation and maintaining tissue homeostasis in animals. Earlier studies have demonstrated an extensive laminin network in both the developing and adult cochlea, primarily associated with the basement membranes. These studies, however, did not address the laminin chain composition of these networks. In this study, we used antibodies specific for the known laminin chains to examine the composition of laminins in both the developing and adult murine cochlea. The results illustrate a complex and dynamic postnatal developmental regulation pattern for most of these chains, and suggest that an unusually large number of laminin heterotrimers are present in both the developing and adult cochlea. The laminin composition at postnatal day 2 is relatively simple. By postnatal day 7, however, activation of several laminin chains results in a very complex laminin composition. In the basement membrane underlying the region of the basilar membrane under the developing organ of Corti, eight of the 11 known basement membrane laminins are possible by co-localization inference. Dynamic changes in expression continue through day 14, but simplify by adulthood. Thus, the most dynamic period for laminin expression in the mouse cochlea coincides with terminal cytodifferentiation of the cochlear epithelial structures. Considering the well established role of laminins in regulating both embryonic and organ development in other systems, these data suggest a closer look at the role of the laminins in cochlear development and function may be warranted.

Heterogeneous nuclear ribonucleoprotein-H plays a suppressive role in visceral myogenesis

Mouse embryonic mesenchymal cells undergo spontaneous smooth muscle (SM) differentiation upon spreading/elongation in culture (Relan et al., J. Cell Biol. 147 (1999) 1341; Yang et al., Development 125 (1998) 2621; Yang et al., Development 126 (1999) 3027). Using these cells we generated a subtracted cDNA library to identify potential suppressors of SM myogenesis. One of the differentially expressed genes was heterogeneous nuclear ribonucleoprotein-H (hnRNP-H), which is involved in pre-mRNA alternative splicing. hnRNP-H was highly expressed in mesenchymal cells prior to the onset of SM differentiation, but its expression rapidly decreased in mesenchymal cells undergoing SM myogenesis. In vivo, the drop in hnRNP-H expression was restricted to visceral SM cells. Antisense oligodeoxynucleotide and antisense RNA were used to inhibit hnRNP-H synthesis in SM-differentiating mesenchymal cells and in embryonic lung explants. A decrease in hnRNP-H levels resulted in upregulation of SM-specific gene expression and increased bronchial SM development in lung explants. hnRNP-H overexpression in cell cultures had the opposite effect. These studies, therefore, indicate a novel role for hnRNP-H in the control of visceral myogenesis.

Molecular mechanisms of phenotypic plasticity in smooth muscle cells

Morphological, functional, molecular and cell biology studies have revealed a striking multifunctional nature of individual smooth muscle cells (SMC). SMCs manifest phenotypic plasticity in response to changes in environment and functional requirements, acquiring a range of structural and functional properties bounded by two extremes, called "synthetic" and "contractile." Each phenotypic state is characterized by expression of a unique set of structural, contractile, and receptor proteins and isoforms that correlate with differing patterns of gene expression. Recent studies have identified signaling pathways and transcription factors (e.g., RhoA GTPase/ROCK, also known as Rho kinase, and serum response factor) that regulate the transcriptional activities of genes encoding proteins associated with the contractile apparatus. Mechanical plasticity of contractile-state smooth muscle further extends SMC functional diversity. This may also be regulated, in part, by the RhoA GTPase/ROCK pathway, via reorganization of cytoskeletal and contractile proteins. Future studies that define transcriptional and posttranscriptional mechanisms of SMC plasticity are necessary to fully understand the role of SMC in the pathogenesis and morbidity of human diseases of the airways, vasculature, and gastrointestinal tract.

Expression patterns of laminin alpha1 and alpha5 in human lung during development

Laminins are trimeric glycoprotein components of basement membranes. Each laminin has three structurally similar chains, designated alpha, beta, and gamma. Five laminin alpha chains are now known. In previous studies using monoclonal antibody 4C7, laminin alpha1 was thought to be present in basement membranes of human lung throughout development and in the adult, but recent expression studies have demonstrated that 4C7 identifies laminin alpha5 rather than alpha1. To determine the temporal and spatial patterns of laminin alpha1 and laminin alpha5 in developing human lung, we prepared complementary DNA probes specific for laminin alpha1 and alpha5 messenger RNAs (mRNAs). By Northern analysis, laminin alpha1 mRNA was prominent in first-trimester fetal lung, but was not detectable at 23 wk or at later times. In contrast, laminin alpha5 mRNA was readily detected in early fetal lung and remained present thereafter. Immunohistochemical staining demonstrated laminin alpha1 only in early fetal lung, whereas laminin alpha5 was persistent from the early fetal period. In situ hybridization localized laminin alpha1 expression to distal epithelium in the first-trimester lung, and laminin alpha5 to all epithelium and developing pulmonary arteries from the first trimester through the perinatal period. These studies indicate that laminin alpha1 expression is restricted to early human lung morphogenesis, whereas the expression of laminin alpha5 in human lung is continuous from early lung development through adult life. It is evident that laminin alpha1 and laminin alpha5 have different roles in the development of the human lung.

Differential expression of laminin alpha chains during proliferative and differentiation stages in a model for skin morphogenesis

The purpose of this study was to determine the mRNA and protein expression of laminin alpha chains at various stages of in vitro skin morphogenesis. Fibroblasts in mono-cultures express low levels of the mRNA of laminin alpha1,alpha2, alpha3 and alpha4 chains. When co-cultured with keratinocytes for 28 days, they expressed the mRNA for all these chains. Keratinocytes in monolayer expressed the laminin alpha3 chain mRNA and very low levels of the mRNA of the alpha1 and alpha2 chains, although, when recombined with fibroblasts they also expressed laminin alpha1and alpha2 mRNA, but not the laminin alpha4 mRNA. Immunocytochemistry of cells in co-culture showed that laminin alpha1, alpha3 and alpha5 chains were expressed in the epidermis, while the laminin alpha2, beta1, and gamma1 chains were noted in the dermis and at the epidermo-dermal interface. The laminin alpha1chain was first expressed during the proliferative stage (14-21 days) and the laminin alpha2 and alpha5 chains appeared later, during the differentiation stage (28-42 days). The above results suggest that epithelial-mesenchymal interactions are involved in the expression of laminin alpha chain mRNA during in vitro skin morphogenesis. In addition, there is distinct temporal and spatial expression of these chains during proliferative and differentiation stages, possibly reflecting different functions.

Stretch-induced alternative splicing of serum response factor promotes bronchial myogenesis and is defective in lung hypoplasia

Smooth muscle (SM) develops only in organs and sites that sustain mechanical tensions. Therefore, we determined the role of stretch in mouse and human bronchial myogenesis. Sustained stretch induced expression of SM proteins in undifferentiated mesenchymal cells and accelerated the differentiation of cells undergoing myogenesis. Moreover, bronchial myogenesis was entirely controlled in lung organ cultures by the airway intraluminal pressure. Serum response factor (SRF) is a transcription factor critical for the induction of muscle-specific gene expression. Recently, a SRF-truncated isoform produced by alternative splicing of exon 5 has been identified (SRFDelta5). Here we show that undifferentiated mesenchymal cells synthesize both SRF and SRFDelta5 but that SRFDelta5 synthesis is suppressed during bronchial myogenesis in favor of increased SRF production. Stretch induces the same change in SRF alternative splicing, and its myogenic effect is abrogated by overexpressing SRFDelta5. Furthermore, human hypoplastic lungs related to conditions that hinder cell stretching continue to synthesize SRFDelta5 and show a marked decrease in bronchial and interstitial SM cells and their ECM product, tropoelastin. Taken together, our findings indicate that stretch plays a critical role in SM myogenesis and suggest that its decrease precludes normal bronchial muscle development.

Differential effects of extracellular matrix proteins on human airway smooth muscle cell proliferation and phenotype

Mature airway smooth muscle cells are characterized by a low proliferative index and expression of contractile marker proteins such as smooth muscle alpha-actin (sm-alpha-actin), calponin, and smooth muscle myosin heavy chain (sm-MHC). In the present study, defined extracellular matrix (ECM) components were examined on the proliferative and phenotypic status of mitogen-stimulated, cultured human airway smooth muscle cells. The results demonstrate that although cells adhered and spread on plates precoated with (1 to 100 microg/ml) of fibronectin (FN), collagen I (Col I), laminin (LN), or Matrigel, their subsequent proliferative response varied qualitatively. FN and Col I enhanced proliferation in response to either platelet-derived growth factor (PDGF)-BB or alpha-thrombin, compared with cells on plastic. LN, however, reduced mitogen-stimulated proliferation. A similar reduction was found in cells cultured on Matrigel. The effect of ECM substrates on contractile phenotype was determined by examining cellular expression of sm-alpha-actin, sm-MHC, and calponin using immunocytochemical and flow cytometric methods. Approximately 75% of PDGF-BB-stimulated cells, cultured on LN or Matrigel, expressed sm-alpha-actin, calponin, and sm-MHC, but only 8 to 10% stained for the Ki67 nuclear antigen proliferation marker. In contrast, more than 75% of cells cultured on FN or Col I were positive for Ki67 antigen, but only 20% were positive for contractile proteins. Flow cytometric analysis of sm-alpha-actin and DNA content confirmed the immunocytochemical findings and showed that the observed reduction in sm-alpha-actin content after culture on FN or Col I, compared with LN and Matrigel, occurred in the majority of the cell population, supporting bidirectional phenotype modulation. Overall, the data suggest that ECM substrates modulate both proliferation and phenotype of human airway smooth muscle cells in culture.

Laminin alpha1-chain shows a restricted distribution in epithelial basement membranes of fetal and adult human tissues

Two novel monoclonal antibodies were raised and used to study the expression of laminin (Ln) alpha1-chain in developing and adult human tissues. In both fetal and adult kidney, a distinct immunoreactivity was seen in basement membranes (BM) of most proximal tubules but not in the distal tubular or glomerular BM or in the basal laminae of blood vessels. Immunoprecipitation of metabolically labeled cultured human renal proximal tubular cells showed an abundant production and deposition of Ln alpha1-chain to the extracellular matrix, suggestive of an epithelial origin of kidney Ln-1. Quantitative cell adhesion experiments with JAR choriocarcinoma cells showed that purified human Ln-1 is a good substrate for cell adhesion that it is differently recognized by integrin receptors when compared to mouse Ln-1. In fetal and adult testes immunoreactivity was solely confined to BM of the seminiferous epithelium. In the airways BM-confined reaction was only seen in fetal budding bronchial tubules (16-19 weeks) at the pseudoglandular stage of development. In the skin a distinct immunoreactivity was confined to BM of developing hair buds but not in epithelial BMs of adult epidermis or of epidermal appendages. In other adult tissues, immunoreactivity was found in BMs of thyroid, salivary, and mammary glands as well as in BMs of endometrium and endocervix, but not of ectocervix or vagina. No immunoreactivity was found in BMs of most of the digestive tract, including the liver and pancreas, except for BMs of esophageal submucosal glands and duodenal Brunner's glands. In fetal specimens, BMs of the bottoms of the intestinal and gastric glands were positive. Basal laminae of blood vessels were generally negative for Ln alpha1 chain with the exception of specimens of both fetal and adult central nervous system in which immunoreactivity for Ln alpha1 chain was prominently confined to capillary walls. The results suggest that outside the central nervous system, Ln alpha1 chain shows a restricted and developmentally regulated expression in BMs of distinct epithelial tissues.

Form and function: the laminin family of heterotrimers

The laminins are a family of glycoproteins that provide an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Each laminin is a heterotrimer assembled from alpha, beta, and gamma chain subunits, secreted and incorporated into cell-associated extracellular matrices. The laminins can self-assemble, bind to other matrix macromolecules, and have unique and shared cell interactions mediated by integrins, dystroglycan, and other receptors. Through these interactions, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue phenotypes, and promotion of tissue survival. Recent advances in the characterization of genetic disruptions in humans, mice, nematodes and flies have revealed developmental roles for the different laminin subunits in diverse cell types, affecting differentiation from blastocyst formation to the post-natal period. These genetic defects have challenged some of the previous concepts about basement membranes and have shed new light on the diversity and complexity of laminin functions as well as established the molecular basis of several human diseases.

The molecular basis of lung morphogenesis

To form a diffusible interface large enough to conduct respiratory gas exchange with the circulation, the lung endoderm undergoes extensive branching morphogenesis and alveolization, coupled with angiogenesis and vasculogenesis. It is becoming clear that many of the key factors determining the process of branching morphogenesis, particularly of the respiratory organs, are highly conserved through evolution. Synthesis of information from null mutations in Drosophila and mouse indicates that members of the sonic hedgehog/patched/smoothened/Gli/FGF/FGFR/sprouty pathway are functionally conserved and extremely important in determining respiratory organogenesis through mesenchymal-epithelial inductive signaling, which induces epithelial proliferation, chemotaxis and organ-specific gene expression. Transcriptional factors including Nkx2.1, HNF family forkhead homologues, GATA family zinc finger factors, pou and hox, helix-loop-helix (HLH) factors, Id factors, glucocorticoid and retinoic acid receptors mediate and integrate the developmental genetic instruction of lung morphogenesis and cell lineage determination. Signaling by the IGF, EGF and TGF-beta/BMP pathways, extracellular matrix components and integrin signaling pathways also directs lung morphogenesis as well as proximo-distal lung epithelial cell lineage differentiation. Soluble factors secreted by lung mesenchyme comprise a 'compleat' inducer of lung morphogenesis. In general, peptide growth factors signaling through cognate receptors with tyrosine kinase intracellular signaling domains such as FGFR, EGFR, IGFR, PDGFR and c-met stimulate lung morphogenesis. On the other hand, cognate receptors with serine/threonine kinase intracellular signaling domains, such as the TGF-beta receptor family are inhibitory, although BMP4 and BMPR also play key inductive roles. Pulmonary neuroendocrine cells differentiate earliest in gestation from among multipotential lung epithelial cells. MASH1 null mutant mice do not develop PNE cells. Proximal and distal airway epithelial phenotypes differentiate under distinct transcriptional control mechanisms. It is becoming clear that angiogenesis and vasculogenesis of the pulmonary circulation and capillary network are closely linked with and may be necessary for lung epithelial morphogenesis. Like epithelial morphogenesis, pulmonary vascularization is subject to a fine balance between positive and negative factors. Angiogenic and vasculogenic factors include VEGF, which signals through cognate receptors flk and flt, while novel anti-angiogenic factors include EMAP II.

Assembly of the exogenous extracellular matrix during basement membrane formation by alveolar epithelial cells in vitro

We found that immortalized alveolar type II epithelial cells (SV40-T2 cells) that were cultured on dense fibrillar collagen supplemented with Matrigel gel formed a thin and continuous lamina densa beneath them. Immunohistochemical analysis of laminin-1, type IV collagen, entactin (nidogen) and perlecan in the culture indicated that all these components were integrated into a sheet structure of basement membrane beneath the cells. Analysis of the temporal and spatial distribution of the basement membrane macromolecules revealed that the initial deposits of laminin-1 and entactin were significantly greater in area in the presence of Matrigel. These globular deposits and the coarse mesh of basement membrane macromolecules developed into a flat membranous basement membrane. In the absence of Matrigel, the SV40-T2 cells failed to form a continuous lamina densa, and the deposits stayed in the coarse mesh. The major biotinylated Matrigel components that were integrated into the basement membrane were laminin-1 and entactin. Furthermore, SV40-T2 cells supplemented with exogenous laminin-1 alone as well as laminin-1 contaminated with entactin formed a continuous lamina densa. These results indicate that the laminin-1 and entactin supplied from the Matrigel were incorporated into a basement membrane beneath the SV40-T2 cells, and contributed to the formation of basement membrane. Therefore, we concluded that the alveolar epithelial cells synthesize laminin-1, entactin, type IV collagen, and perlecan, but that they also needed to assemble exogenous laminin-1 into the basement membrane to complete its formation in vitro.

Nidogen-1 regulates laminin-1-dependent mammary-specific gene expression

Nidogen-1 (entactin) acts as a bridge between the extracellular matrix molecules laminin-1 and type IV collagen, and thus participates in the assembly of basement membranes. To investigate the role of nidogen-1 in regulating cell-type-specific gene expression in mammary epithelium, we designed a culture microecosystem in which each component, including epithelial cells, mesenchymal cells, lactogenic hormones and extracellular matrix, could be controlled. We found that primary and established mesenchymal and myoepithelial cells synthesized and secreted nidogen-1, whereas expression was absent in primary and established epithelial cells. In an epithelial cell line containing mesenchymal cells, nidogen-1 was produced by the mesenchymal cells but deposited between the epithelial cells. In this mixed culture, mammary epithelial cells express (beta)-casein in the presence of lactogenic hormones. Addition of either laminin-1 plus nidogen-1, or laminin-1 alone, to mammary epithelial cells induced (beta)-casein production. We asked whether recombinant nidogen-1 alone could signal directly for (beta)-casein. Nidogen-1 did not induce (beta)-casein synthesis in epithelial cells, but it augmented the inductive capacity of laminin-1. These data suggest that nidogen-1 can cooperate with laminin-1 to regulate (beta)-casein expression. Addition of full-length nidogen-1 to the mixed cultures had no effect on (beta)-casein gene expression; however, a nidogen-1 fragment containing the laminin-1 binding domain, but lacking the type IV collagen-binding domain, had a dominant negative effect on (beta)-casein expression. These data point to a physiological role for nidogen-1 in the basement membrane-induced gene expression by epithelial cells.