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.

Maspin expression inversely correlates with breast tumor progression in MMTV/TGF-alpha transgenic mouse model

Maspin is a novel serine protease inhibitor (serpin) with tumor suppressive activity. To date, despite the mounting evidence implicating the potential diagnostic/prognostic and therapeutic value of maspin in breast and prostate carcinoma, the lack of a suitable animal model hampers the in vivo investigation on the role of maspin at different stages of tumor progression. In this study, we used MMTV/TGF-alpha transgenic mouse model to study the expression profile of maspin in mammary tumor progression. Histopathological examinations of MMTV/TGF-alpha transgenic mice revealed TGF-alpha expression leading to hyperproliferation, hyperplasia, and occasional carcinoma in mammary gland. Interestingly, when MMTV/TGF-alpha transgenic mice were breed to homozygocity, they also developed characteristic skin papillomas. Immunohistochemistry analysis of maspin expression in the breast tissues of TGF-alpha transgenic mice showed a direct correlation between down-regulation of maspin expression and tumor progression. The loss of maspin expression was concomitant with the critical transition from carcinoma in situ to invasive carcinoma. Subsequent in-situ hybridization analyses suggest that the down-regulation of maspin expression is primarily a transcriptional event. This data is consistent with the tumor suppressive role of maspin. Furthermore, our data suggests that MMTV/TGF-alpha transgenic mouse model is advantageous for in vivo evaluation of both the expression and the biological function of maspin during the slow multi-stage carcinogenesis of mammary gland.

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.

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.

Anti-DNA antibodies cross-reacting with laminin inhibit trophoblast attachment and migration: implications for recurrent pregnancy loss in SLE patients

PROBLEM

Systemic lupus erythematosus (SLE), an autoimmune disease, is associated with reduced fetal survival, recurrent abortions, and other pregnancy complications. Some of the autoantibodies found in SLE bind to laminins (LNs), which play an important role in the implantation of the fertilized ovum in humans.

METHOD OF STUDY

To elucidate the role of these specific autoantibodies, chorionic villous explants from 6 7-week-old human placentas were established as organ cultures on laminin-1 (LN-1), collagen IV (CN-IV) or uncoated culture dishes. The cultures were then exposed to a mouse monoclonal anti-DNA/anti-LN-1 antibody, to human polyclonal lupus antibodies cross-reacting with LN-1, a function-blocking polyclonal antibody to LN-1, polyclonal antibodies to CN-IV, or IgG control.

RESULTS

The explants attached to LN-1 and CN-IV, but not to uncoated culture dishes. LN-1 promoted migration of trophoblast, whereas CN-IV promoted migration of fibroblast-like cells. Trophoblast attachment and migration were abolished in a dose-dependent manner by all three antibodies to LN-1, but not by antibodies to CN-IV or IgG control. Furthermore, the effect of anti-LN antibodies was abolished by preincubating them with LN-1.

CONCLUSIONS

These studies suggest that anti-DNA antibodies cross-reacting with LNs may play a role in early pregnancy failure in SLE patients by interfering with placental implantation.

Cell elongation induces laminin alpha2 chain expression in mouse embryonic mesenchymal cells: role in visceral myogenesis

Bronchial smooth muscle (SM) mesenchymal cell precursors change their shape from round to spread/elongated while undergoing differentiation. Here we show that this change in cell shape induces the expression of laminin (LM) alpha2 chain not present in round mesenchymal cells. LM alpha2 expression is reversible and switched on and off by altering the cell's shape in culture. In comparison, the expression of LM beta1 and gamma1 remains unchanged. Functional studies showed that mesenchymal cell spreading and further differentiation into SM are inhibited by an antibody against LM alpha2. Dy/dy mice express very low levels of LM alpha2 and exhibit congenital muscular dystrophy. Lung SM cells isolated from adult dy/dy mice spread defectively and synthesized less SM alpha-actin, desmin, and SM-myosin than controls. These deficiencies were completely corrected by exogenous LM-2. On histological examination, dy/dy mouse airways and gastrointestinal tract had shorter SM cells, and lungs from dy/dy mice contained less SM-specific protein. The intestine, however, showed compensatory hyperplasia, perhaps related to its higher contractile activity. This study therefore demonstrated a novel role for the LM alpha2 chain in SM myogenesis and showed that its decrease in dy/dy mice results in abnormal SM.

Bronchial smooth muscle hypoplasia in mouse embryonic lungs exposed to a laminin beta1 chain antisense oligonucleotide

We used an antisense oligonucleotide (ODN) to inhibit laminin (LM) beta1 chain synthesis in mouse embryonic lung explants and cell cultures. The ODN spanned 17 bases located 13 bases downstream the initiation codon and contained phosphorothioate and C-5 propynyl pyrimidine modifications. Penetration of the ODN into the lung explants was confirmed by fluorescein isothiocyanate (FITC) tagging. 50 microM of antisense ODN decreased LM beta1 chain synthesis by 82+/-6.9% with no significant changes in the synthesis of other LM chains. The same antisense probe but without C-5 propynyl pyrimidine modification, another 17-mer ODN complementary to the LM beta1 initiation codon, and a 17-mer ODN complementary to the LM alpha1 initiation codon had no antisense activity. Lung explants exposed to the active LM beta1 antisense ODN showed decreased LM-1 and collagen type IV deposition at the epithelial-mesenchymal interface and an arrest in bronchial smooth muscle (SM) development. Histological examination and cell motility assays suggested that this arrest was due to impaired spreading and migration of SM cell precursors over the defective basement membrane (BM). Our studies indicate that beta1-chain containing LMs play a role in bronchial myogenesis.

Embryonic mesenchymal cells share the potential for smooth muscle differentiation: myogenesis is controlled by the cell's shape

Undifferentiated embryonic mesenchymal cells are round/cuboidal in shape. During development, visceral myogenesis is shortly preceded by mesenchymal cell elongation. To determine the role of the cell's shape on smooth muscle development, undifferentiated embryonic mesenchymal cells from intestine (abundant visceral muscle), lung (some visceral muscle) or kidney (no visceral muscle) were plated under conditions that maintained cell rounding or promoted elongation. Regardless of their fate in vivo, all the cells differentiated into smooth muscle upon elongation as indicated by the expression of smooth muscle-specific proteins and the development of membrane potentials of −60 mV and voltage-dependent Ca2+ currents, characteristic of excitable cells. Smooth muscle differentiation occurred within 24 hours and was independent of cell proliferation. Regardless of their fate in vivo, all the round cells remained negative for smooth muscle markers, had membrane potentials of −30 mV and showed no voltage-activated current. These cells, however, differentiated into smooth muscle upon elongation. The role of the cell's shape in controlling smooth muscle differentiation was not overcome by treatment with retinoic acid, TGF-beta1, PDGF BB or epithelial-conditioned medium (all modulators of smooth muscle differentiation). These studies suggest that the mesenchymal cell shape plays a main role in visceral myogenesis.

Characteristics of nonmalignant and malignant human prostate in organ culture

Prostate tissue was obtained from 52 radical prostatectomies immediately upon surgery. From each specimen, a small piece of tissue was fixed in 10% buffered formalin and used for histology, cytokeratin staining, staining with the antibodies to the proliferation-associated antigen (Ki-67), and histochemical evaluation of the epithelial-stromal basement membrane. A second piece was used for the isolation of epithelial cells and stromal cells in monolayer culture. The remainder of each specimen was cut into cubes (approximately 1 mm on a side) and incubated in organ culture for up to 20 days. At the end of the incubation period, tissue was fixed in 10% buffered formalin and examined as described above with zero-time tissue. These studies showed that normal epithelial and stromal elements survived in organ culture in the presence of a serum-free medium containing a mixture of growth factors (epidermal growth factor, insulin, pituitary extract, and dihydrotestosterone). In many of the tissues examined at 4 days, individual glands resembled those seen immediately after surgery, with a single layer of basal epithelial cells and a layer of secretory cells above. By Day 8, the secretory epithelium was lost in many places and basal cells proliferated to fill in the lumens of the glands. All of the nonmalignant glands were reactive with the anti-cytokeratin antibody (K903), and there was a large increase in the number of cells staining for Ki-67 as compared with zero-time tissue. Staining with the Periodic Acid Schiff (PAS) and PAS-methenamine silver (PASME) reagents revealed an intact basement membrane around virtually all of the epithelial structures. The basement membrane appeared to be thickened in some areas. In places where a gland was cut during the processing of the tissue, epithelial cells migrated out of the gland and covered the cut surface of the tissue piece. There was no detectable basement membrane separating the epithelium from the stroma at these sites. Whereas nonmalignant epithelial cells were preserved in the growth factor- and dihydrotestosterone-supplemented culture medium, most of the malignant cells rapidly lysed under the same conditions. However, when phorbol myristate acetate was included in the culture medium, many of the tumor cells remained viable. This was seen with the more well-differentiated tumors as well as with tumors that were highly anaplastic. All of the tumor cells were nonreactive with anti-cytokeratin antibody, and only a few cells stained for Ki-67. The basement membrane surrounding malignant cells was thin and, in places, appeared to be discontinuous. Where malignant glands were cut in the processing of the tissue, cells did not migrate out over the cut surface. In summary, this study identifies culture conditions for the successful maintenance of human prostate tissue for several days in organ culture. Histological/histochemical features that distinguish nonmalignant and malignant tissue are present in this model.

Basement membranes in development

The aim of this review is to introduce the reader to the main ECM constituents and to some of their roles in development. The main functions of the ECM during embryogenesis are the production, promotion, and regulation of normal tissue structure. Among the ECM components, LMs have been the most extensively studied in relation to embryo-genesis. Skin and skeletal muscle disorders have been shown to be caused by LM alterations. Additional experiments, e.g., with knockout mice, will help enormously to elucidate the functional significance of many ECM constituents and their involvement in development and disease.

Role of laminin polymerization at the epithelial mesenchymal interface in bronchial myogenesis

Undifferentiated mesenchymal cells were isolated from mouse embryonic lungs and plated at subconfluent and confluent densities. During the first 5 hours in culture, all the cells were negative for smooth muscle markers. After 24 hours in culture, the mesenchymal cells that spread synthesized smooth muscle alpha-actin, muscle myosin, desmin and SM22 in levels comparable to those of mature smooth muscle. The cells that did not spread remained negative for smooth muscle markers. SM differentiation was independent of cell-cell contact or proliferation. In additional studies, undifferentiated lung mesenchymal cells were cocultured with lung embryonic epithelial cells at high density. The epithelial cells aggregated into cysts surrounded by mesenchymal cells and a basement membrane was formed between the two cell types. In these cocultures, the mesenchymal cells in contact with the basement membrane spread and differentiated into smooth muscle. The rest of the mesenchymal cells remained round and negative for smooth muscle markers. Inhibition of laminin polymerization by an antibody to the globular regions of laminin beta1/gamma1 chains blocked basement membrane assembly, mesenchymal cell spreading and smooth muscle differentiation. These studies indicated that lung embryonic mesenchymal cells have the potential to differentiate into smooth muscle and the process is triggered by their spreading along the airway basement membrane.

Laminin fragment E4 inhibition studies: basement membrane assembly and embryonic lung epithelial cell polarization requires laminin polymerization

Laminins (LMs), the main constituents of basement membranes (BMs), are heterotrimeric glycoproteins composed of alpha, beta, and gamma chains held together by disulfide bonds. In the presence of Ca2+, some laminins, such as laminin-1 self-assemble into a polymer through the interaction of their three NH2 termini. Here we exposed lung organotypic cultures to a proteolytic fragment of laminin-1 that blocks laminin polymerization. This fragment, referred as E4, comprises the outer globular region of laminin beta1 chain. Inhibition of laminin polymerization in lung organotypic cultures resulted in impaired basement membrane assembly and failure of epithelial cells to polarize. In addition, we found that in control organotypic cultures, the bronchial smooth muscle cells were arranged in concentric layers around the newly formed epithelium. However, in E4-treated cultures, the smooth muscle cells were in disarray. Exposure of organotypic cultures to laminin-1 proteolytic fragment P1', that comprises part of alpha1, beta1, and gamma1 chains, but does not overlap with fragment E4, had no effect in basement membrane assembly. Exposure to fragment E4 also caused an increased release of laminin-1 into the culture medium, suggesting a failure to retain laminin at the epithelial-mesenchymal interface. These studies provide the first direct evidence linking epithelial cell polarization to laminin polymerization at the epithelial-mesenchymal interface and assign a key role to the outer globular region of laminin beta1 chain.

Glycerol-induced acute renal failure attenuates subsequent HgCl2-associated nephrotoxicity: correlation of renal function and morphology

Glycerol induced acute renal failure (ARF) is known to attenuate subsequent mercuric chloride nephrotoxicity. This protection was evaluated in rats. Glycerol induced varying degrees of renal insufficiency. After 14 days, when serum creatinine (SCr) creatinine clearance (CCr) and fractional excretion of sodium (FENa) had returned to baseline, injection of mercuric chloride caused significantly milder renal insufficiency in recovered rats than in controls (SCr 356 +/- 46 vs. 475 +/- 19 mumol/L; CCr 0.12 +/- 0.02 vs. 0.02 +/- 0.02 mL/min, p < .05; and mortality 0 vs. 45%, respectively, p < .01). A striking finding was that the degree of renal insufficiency induced by mercuric chloride correlated inversely with the degree of renal insufficiency previously induced by glycerol (r = -0.496, p < .05 for SCr and CCr), but there was no correlation with other measures of previous renal function such as urine volume, sodium excretion, or FENa. Glycerol induced ARF also attenuated the renal toxicity of mercuric chloride injected 4 days after glycerol, before full recovery of renal function. The decrements in renal function after the two insults were also inversely related (r = -0.76, p < .01). A third renal insult with a second mercuric chloride injection after three weeks was still attenuated. However, after the third insult, there was no longer an inverse or any statistical relationship with previous measurements of renal function. Histopathology revealed a good correlation between peak Scr after glycerol, and percentage of tubules undergoing re-generation 14 days later (r = 0.97, p < .01). There was an inverse correlation between Scr after mercuric chloride (administered 14 days after glycerol) and percentage of tubular regeneration seen two days later (r = -0.79, p < .05). The correlations of SCr and CCr with regeneration was greater than the correlations with tubular necrosis, suggesting that the regenerative process is involved in the protection from repeated renal insults. In conclusion, glycerol-induced ARF attenuates subsequent mercuric chloride renal insult. The attenuation correlates directly with the initial glycerol-induced damage, so that the more severe the initial renal insufficiency, the milder the renal insufficiency following subsequent mercuric chloride. This protection can be seen as early as 4 days and also 14 days after previous renal insult. The degree of renal tubular regeneration correlates well with the protection seen, and probably plays a role in acquired renal resistance to repeated insults.

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

Laminins, the main components of basement membranes, are heterotrimers consisting of alpha, beta, and gamma polypeptide chains linked together by disulfide bonds. Laminins-1 and -2 are both composed of beta1 and gamma1 chains and differ from each other on their alpha chain, which is alpha1 and alpha2 for laminin-1 and -2, respectively. The present study shows that whereas laminins-1 and -2 are synthesized in the mouse developing lung and in epithelial-mesenchymal cocultures derived from it, epithelial and mesenchymal monocultures lose their ability to synthesize the laminin alpha1 chain. Synthesis of laminin alpha1 chain however returns upon re-establishment of epithelial-mesenchymal contact. Cell-cell contact is critical, since laminin alpha1 chain is not detected in monocultures exposed to coculture-conditioned medium or in epithelial-mesenchymal cocultures in which heterotypic cell-cell contact is prevented by an interposing filter. Immunohistochemical studies on cocultures treated with brefeldin A, an inhibitor of protein secretion, indicated both epithelial and mesenchymal cells synthesize laminin alpha1 chain upon heterotypic cell- cell contact. In a set of functional studies, embryonic lung explants were cultured in the presence of monoclonal antibodies to laminin alpha1, alpha2, and beta/gamma chains. Lung explants exposed to monoclonal antibodies to laminin alpha1 chain exhibited alterations in peribronchial cell shape and decreased smooth muscle development, as indicated by low levels of smooth muscle alpha actin and desmin. Taken together, our studies suggest that laminin alpha1 chain synthesis is regulated by epithelial-mesenchymal interaction and may play a role in airway smooth muscle development.

Laminins in lung development

Laminins are essential components of basement membranes, playing important roles in cell adhesion, proliferation, and differentiation. These heterotrimeric glycoproteins are composed of an alpha, beta, and gamma chains held together by disulfide bonds. The first laminin identified, from the mouse Engelbreth-Holm-Swarm (EHS) tumor, is now referred to as laminin-1. Laminin-1 is expressed in the mouse developing lung by epithelial and mesenchymal cells and plays a role in branching morphogenesis. Since laminins are multidomain proteins, different laminin sites are engaged in promoting lung organogenesis by serving different functions at different stages of development. This study shows that the cross region of the molecule selectively promotes epithelial cell proliferation. The outer globular region of alpha 1 and beta 1 chains mediates laminin polymerization and thereby basement membrane formation and epithelial cell polarization. The inner globular region of laminin beta 1 chain binds to heparan sulfate proteoglycan and both stimulate lumen formation. While the combined effect of these laminin active sites results in normal lung tissue structure and branching morphogenesis, different developmental abnormalities of the lung may result from alterations in each of them.

Laminin and heparan sulfate proteoglycan mediate epithelial cell polarization in organotypic cultures of embryonic lung cells: evidence implicating involvement of the inner globular region of laminin beta 1 chain and the heparan sulfate groups of heparan sulfate proteoglycan

The extracellular matrix and in particular the basement membrane (BM) play an important role in the induction of organotypic rearrangement of cells in culture. This process involves cell aggregation, sorting into epithelial and mesenchymal components, epithelial cell polarization, and lumen formation. In this study, a combination of laminin (LM) and heparan sulfate proteoglycan (HSPG), two major BM constituents, induced organotypic rearrangement of embryonic mouse lung cells. In the absence of LM/HSPG supplementation, the cells sorted into epithelial and mesenchymal compartments but epithelial cell polarization and lumen formation did not occur. Neither LM nor HSPG alone could trigger this process. Synthetic peptide F-9, representing an amino acid sequence from the inner globular region of the laminin beta1 chain (RYVVLPRPVCFEKGMNYTVR) induced organotypic cell rearrangement when substituted for LM. Exogenous LM as well as peptide F-9 were localized at the epithelial-mesenchymal interface of organotypic cultures, where a BM-like structure is formed de novo. Organotypic cell rearrangement was blocked by heparin, heparan sulfate, or antibodies against peptide F-9. Binding assays indicated that peptide F-9 interacts with HSPG but not with LM or type IV collagen. Preincubation of embryonic lung cells with peptide F-9 resulted in a significant increase in cell attachment to HSPG but not to other major BM constituents. These findings suggest that the interaction between LM and BM HSPG is critical for the development of epithelial cell polarization and lumen formation. This interaction occurs at the epithelial-mesenchymal interface and is mediated by a site in the LM molecule represented by peptide F-9 and the heparan sulfate groups of HSPG.

Amphiregulin in lung branching morphogenesis: interaction with heparan sulfate proteoglycan modulates cell proliferation

Epithelial and mesenchymal cells isolated from mouse embryonic lungs synthesized and responded to amphiregulin (AR) in a different fashion. Mesenchymal cells produced and deposited 3- to 4-fold more AR than epithelial cells, proliferated in the presence of exogenous AR, and their spontaneous growth was blocked by up to 85% by anti-AR antibodies. In contrast, epithelial cells exhibited a broad response to this growth regulator factor depending on whether they were supplemented with extracellular matrix (ECM) and whether this ECM was of epithelial or mesenchymal origin. AR-treated epithelial cells proliferated by up to 3-fold in the presence of mesenchymal-deposited ECM, remained unchanged in the presence of epithelial-deposited ECM, and decreased in their proliferation rate below controls in the absence of ECM supplementation. This effect was abolished by treatment with the glycosaminoglycan-degrading enzymes heparinase and heparitinase suggesting the specific involvement of heparan sulfate proteoglycan (HSPG) in AR-mediated cell proliferation. In whole lung explants, branching morphogenesis was inhibited by antibodies against the AR heparan sulfate binding site and stimulated by exogenous AR. Since during development, epithelial cells are in contact with mesenchymal ECM at the tips of the growing buds and alongside the basement membrane, focal variations in the proportion of epithelial and mesenchymal HSPG will focally affect epithelial proliferation rates. Therefore, AR-HSPG interaction may underlie the process of branching morphogenesis by inducing differential cell proliferation.

Two separate domains of laminin promote lung organogenesis by different mechanisms of action

Laminin is a major component of basement membranes. We previously reported that the globular region of laminin B chain(s) and the cross region of the A chain play an active role in mouse lung branching morphogenesis. In this study, basic morphogenic cell behaviors modulated by laminin were analyzed in order to elucidate how this glycoprotein promotes lung development. Cocultures of epithelial and mesenchymal cells from mouse fetal lungs were used to determine the effect of site-specific monoclonal antibodies to laminin (AL-1, AL-2, AL-3, AL-4, and AL-5) on epithelial and mesenchymal cell adhesion, proliferation, and organotypic rearrangement. We found that monoclonal antibody AL-1, directed against the cross region of the laminin A chain, inhibited epithelial and mesenchymal cell attachment and had a selective antiproliferative effect on epithelial cells. In contrast, monoclonal antibody AL-5, directed against the globular region of the B chain(s), blocked epithelial cell polarity. Immunohistochemical studies on epithelial-mesenchymal cocultures exposed to monoclonal antibody AL-5 revealed the absence of laminin deposition at the epithelial-mesenchymal interface, whereas type collagen IV was present at this site. These findings suggest that each of the two laminin domains involved in lung development promotes morphogenesis by a different mechanism of action. The cross-region of the A chain mediates cell adhesion and epithelial cell proliferation, whereas the globular region of the laminin B chain(s) is critical for the process of basement membrane assembly and cell polarization. The combined effect of both laminin domains on epithelial and mesenchymal cells and on the interaction between them seems to be essential for normal lung branching morphogenesis.

Retinoic acid stimulates mouse lung development by a mechanism involving epithelial-mesenchymal interaction and regulation of epidermal growth factor receptors

Retinoic acid (RA) stimulated proliferation of both epithelial and mesenchymal cells in cocultures isolated from developing mouse lungs. There was a corresponding increase in epithelial branching activity in organ culture of embryonic lungs exposed to similar doses of RA. Stimulation was maximal with concentrations of 1 microM and progressively decreased with either lower or higher concentrations. However, when lung cell monocultures of isolated epithelial and mesenchymal cells were exposed to RA, the mitogenic effect was observed only in the mesenchymal population. This suggests that RA may not have a direct mitogenic effect on epithelial cells but rather functions indirectly through the mesenchyme. The cellular response to RA was correlated with an increase in the expression of epidermal growth factor receptor (EGFR). Epidermal growth factor (EGF) also stimulated terminal branch formation in the developing lung. Unlike RA, EGF stimulated proliferation in both epithelial cells and mesenchymal cells in monoculture. In comparison, transforming growth factor-alpha, which also binds to the EGFR, elicited no response. We conclude that RA stimulates cell proliferation and branching activity in the developing mouse lung by a mechanism involving epithelial-mesenchymal interactions. The effect is, in part, produced by stimulation of EGFR expression, with the resulting amplification of the cellular response to EGF or other EGFR ligands. In this process the mesenchyme provides a paracrine support to the epithelium, otherwise unresponsive to RA. Further studies identified the mesenchyme as a major source of EGF in the embryonic lung, suggesting that mesenchymal EGF may represent a paracrine factor involved in the epithelial response to RA.

Effects of all-trans retinoic acid and Ca++ on human skin in organ culture

In this study, we have established an organ culture model of human skin and examined the effects of both all-trans retinoic acid (RA) and extracellular Ca++ on the epidermal and dermal components of the organ-cultured skin. Our data show that while organ cultures maintained in serum-free, growth factor-free culture medium containing 0.15 mM Ca++ degenerated rapidly, those treated with concentrations of RA that have been shown previously to stimulate fibroblast and keratinocyte proliferation in monolayer culture (J Invest Dermatol 1989, 93:449; 1990, 94:717; Am J Pathol 1990, 136:1275) demonstrated a healthy appearance for up to 12 days. Degeneration of the control cultures was characterized by separation of the epidermis from the underlying dermis, progressive cell necrosis leading to a complete absence of viable cells from both the dermal and epidermal compartments, disintegration and fibrillation of the dermal connective tissue, and a cessation of protein synthesis. RA-treated organ cultures contained large numbers of healthy-appearing cells in both the epidermal and dermal compartments. One or several layers of viable basal cells in the epidermis could be seen at least through day 12. However, the upper layers of the epidermis frequently separated from the cells in the basal layer. The dermal connective tissue was histologically well-preserved. Furthermore, the level of protein synthesis was higher in the RA-treated cultures than in the control cultures. In addition to treating organ cultures with RA, other cultures were exposed to serum-free, growth factor-free culture medium containing 1.4 mM Ca++. The presence of the elevated Ca++ concentration also preserved cellular and connective tissue structures in the dermal and epidermal compartments. In comparison to RA there was better preservation of the overall epidermal structure. The upper layers of epidermal cells did not separate from the basal cells, and the various stages of epithelial differentiation could be seen. Histologically, the dermis was well-preserved in the presence of elevated extracellular Ca++. Specimens treated with a combination of Ca++ and RA demonstrated features consistent with the features induced by each treatment separately. This included an expanded basal layer of epithelial cells and a prominent keratotic layer with a fairly orderly pattern of differentiation. The tendency of the upper epidermis to separate from the basal cells was partially mitigated. Taken together, these data indicate that both RA and extracellular Ca++ act to prevent the degeneration of human skin in organ culture but probably do so through different mechanisms.

Laminin expression in the mouse lung increases with development and stimulates spontaneous organotypic rearrangement of mixed lung cells

The recent establishment of a role for laminin in mouse lung organogenesis (Schuger et al. 1990a,b, 1991) prompted us to study its expression in the developing lung. Laminin A and B chains were detected in the murine lung from the first hours of development onward. In situ hybridization of mRNA as well as SDS-PAGE studies of lung cells in monoculture indicated that both epithelium and mesenchyme produce complete laminin molecules. Quantitative analysis of the in situ hybridization studies showed a gradual increase in laminin expression during development which was further supported by immunohistochemistry and ELISA. The overall pattern of expression suggested that the effects of laminin in morphogenesis were not restricted to a particular stage of development. Furthermore, the increase in expression during late development supported a role for the molecule in the fetal lung, which was not previously established. We next determined whether the increase in laminin production modulated the behavior of fetal lung cells as compared with their embryonic counterparts. We previously showed that organotypic pattern formation does not occur in cultures of mixed embryonic lung cells unless exogenous laminin is added (Schuger et al., 1990b). Organotypic pattern formation is the result of cell sorting into epithelial and mesenchymal compartments and further rearrangement in a pattern resembling the tissue of origin. In the present study, we demonstrated that organotypic pattern formation occurs spontaneously in cultures of mixed fetal lung cells, which express high laminin levels. Pattern formation was abolished by antibodies to laminin. These studies suggest a correlation between laminin expression and the ability of lung cells in culture to reproduce normal tissue patterns. We conclude that laminin is critical for epithelial-mesenchymal recognition and further morphogenic interaction during both the embryonic and fetal stages of lung development.

Identification of laminin domains involved in branching morphogenesis: effects of anti-laminin monoclonal antibodies on mouse embryonic lung development

We recently found that polyclonal antibodies to laminin, a basement membrane-related glycoprotein, inhibited murine lung morphogenesis when added to organ cultures of mouse embryonic lung. Using a series of monoclonal anti-laminin antibodies with previously characterized subunit specificity (termed AL-1, AL-2, AL-3, AL-4, and AL-5), the deposition and functional involvement of different laminin domains in the developing lung were investigated. By immunohistochemistry the antibodies' reactivity was largely localized to the basement membrane, but was also present diffusely in the extracellular matrix throughout the mesenchyme. Organ cultures of lung explants from Day 12 embryos were cultured for 3 days in the presence of 50-100 micrograms/ml of each antibody or in the presence of the same concentration of immunoglobulins G and M, laminin-neutralized antibody, or medium alone. Cultures were monitored by phase-contrast microscopy, light microscopy, and immunofluorescence. Although all antibodies penetrated the tissues in culture, only two of them inhibited branching activity. These two antibodies were AL-1, which binds on or near the cross region of laminin, and AL-5, which binds to the lateral short arms at the globular end regions of the B chain of laminin. Inhibition of branching with these two antibodies was dose-dependent and statistically significant for the two concentrations used. AL-2, AL-3, AL-4, laminin-neutralized antibodies and control immunoglobulins did not alter lung morphogenesis. The two domains of laminin that promote lung branching morphogenesis have been reported by others to promote the attachment of a variety of cells and/or bind heparin. These domains of laminin may promote branching morphogenesis by facilitating cell attachment and, consequently, cell proliferation.

Thrombospondin production and thrombospondin-mediated adhesion in U937 cells

U937 cells have low levels of surface thrombospondin (TSP) under control conditions but express higher levels after treatment for 1 day with 100 nM phorbol myristate acetate (PMA). Increased surface expression is due, in part, to increased biosynthesis. Untreated U937 cells do not adhere to TSP-coated plastic culture dishes but adhere strongly to TSP after stimulation with PMA. Untreated U937 cells also adhere weakly to endothelial cell monolayers while PMA-treated U937 cells attach strongly to monolayers of rat pulmonary artery endothelial cells. Endothelial cell adhesion appears to be mediated, in part, by TSP since antibodies to TSP partially inhibit.

Production of fibronectin by human tumor cells and interaction with exogenous fibronectin: comparison of cell lines obtained from colon adenocarcinomas and squamous carcinomas of the upper aerodigestive tract

Cell lines derived from 13 different human colon adenocarcinomas were examined for production of fibronectin by ELISA and for cell-surface expression of fibronectin by indirect immunofluorescence. Two squamous epithelial cell lines obtained from tumors of the upper aerodigestive tract were used as controls. None of the 13 colon carcinoma lines produced detectable amounts of fibronectin or showed detectable cell-surface staining with anti-fibronectin. The 2 squamous epithelial cell lines, in contrast, produced large amounts of fibronectin which could be detected in the culture medium and bound to the substratum. The squamous carcinoma cells also stained brightly when examined in the viable state by immunofluorescence with anti-fibronectin. In addition to being studied for fibronectin production, each cell line was also examined for the ability to interact with exogenous fibronectin in an adhesion assay. None of the colon carcinoma cells were adherent to fibronectin-coated culture dishes while the 2 squamous carcinoma cells rapidly attached and spread on this substratum. These data suggest that cell lines derived from adenocarcinomas of the colon are deficient in production of fibronectin and in their ability to interact with exogenous fibronectin. In their degree of deficiency, the colon carcinoma cells are significantly different from several different types of human tumor cell. The failure of the colon carcinoma cells to synthesize detectable amounts of fibronectin endogenously or to interact with exogenous fibronectin may explain, in part, the low degree of adhesive interaction which these cells have for their substratum. This, in turn, may influence the in vitro and in vivo properties of colon carcinoma cells.

Organotypic arrangement of mouse embryonic lung cells on a basement membrane extract: involvement of laminin

The behavior of embryonic murine lung cells on a basement membrane extract (Matrigel) was investigated. Single cell suspensions generated by trypsinization of lungs removed from day 12 embryos were plated on Matrigel and cultured for up to one week. The basement membrane extract was used as a gel, and as a wet or dried film. In all of these instances, organotypic arrangement of the embryonic lung cells was observed. This process consisted of cell aggregation, sorting, polarization and formation of a tridimensional organization resembling embryonic lung. The maximal degree of organotypic development was obtained by using a thick gel; minimal reorganization was observed using a dried film. A rabbit polyclonal serum to laminin inhibited organotypic pattern formation while normal rabbit serum did not. Culture of lung cells on laminin gels promoted epithelial cyst formation but poor mesenchymal organization. By studying the behavior of epithelial and/or mesenchymal enriched cell populations on Matrigel, it was concluded that organotypic pattern formation on Matrigel required the presence of both cell populations. Cultivation of dissociated lung cells on a gel consisting of a mixture of collagens type I and III (Vitrogen-100) produced only cell aggregation. Cultivation of lung cells on a thin film of Vitrogen-100 or on uncoated tissue culture plastic produced monolayers of mesenchymal cells alone. Cultivation of lung cells in suspension also failed to induce organotypic arrangement even at maximal cell densities. The present study strongly supports a role for the basement membrane in the organotypic rearrangement of embryonic lung cells and subsequent in vitro cyst formation and budding of the reestablished epithelium. This, in turn, reinforces the concept of the basement membrane as a major regulator of organogenesis.