Extracellular matrix involvement in epithelial branching morphogenesis

Cryopreservation of Biologically Functional Submandibular Gland Rudiments from Fetal Mice

BACKGROUND/AIM

Cryopreservation of cell lines has been widely used in the laboratory; however, cryopreservation of organs is still considered to be difficult. The submandibular gland (SMG) of fetal mice is one of the best-characterized organs. We investigated the conditions for cryopreserving SMG rudiments.

MATERIALS AND METHODS

Embryonic day 13 SMG rudiments were cryopreserved with or without a cryoprotectant. They were thawed and incubated in DMEM/F12 medium. Moreover, the influence of EGF stimulation on the signaling cascade after frozen-thawing the rudiments was analyzed by Western blotting.

RESULTS

When SMG rudiments were cryopreserved without a cryoprotectant, all cells in the rudiments died. However, the SMG rudiments that had been preserved in a cryoprotectant showed branching morphogenesis. Additionally, the responsiveness of signaling cascades to EGF did not differ between frozen with a cryoprotectant and non-frozen rudiments.

CONCLUSION

Cryopreservation might be a useful technology for preserving tissues from small organs, such as fetal SMG rudiments.

CD29 is highly expressed on epithelial, myoepithelial, and mesenchymal stromal cells of human salivary glands

OBJECTIVE

The phenotype of the cells present in the ductal region of salivary glands has been well characterized. However, it is imperative to identify novel biomarkers that can identify different cell types present in other glandular components for the development of therapeutic strategies and diagnostics of salivary gland disorders and malignancies. Our study aimed at the characterization of the expression and distribution of various cell surface markers, especially with a focus on CD29 in human fetal as well as adult glands.

MATERIALS AND METHODS

Paired human midgestation fetal and adult parotid, sublingual, and submandibular glands were collected. Phenotypic expression of various lineage-specific cell surface markers including CD29 was investigated in freshly collected glands. The findings were further corroborated by immunohistochemistry.

RESULTS

Enriched expression of CD29 was found on acinar and ductal epithelial, mesenchymal stromal, and myoepithelial cells; CD29⁺ cells co-expressed epithelial (CD324, CD326, NKCC1, and CD44), mesenchymal (CD73, CD90, vimentin, and CD34), and myoepithelial (α-SMA) cell-specific progenitor markers in both fetal as well as adult salivary glands.

CONCLUSION

CD29 is widely expressed in human salivary glands, and it could serve as a potential biomarker for devising novel cellular therapeutic and diagnostic strategies for salivary gland disorders and malignancies.

TGFβ signaling promotes matrix assembly during mechanosensitive embryonic salivary gland restoration

Mechanical properties of the microenvironment regulate cell morphology and differentiation within complex organs. However, methods to restore morphogenesis and differentiation in organs in which compliance is suboptimal are poorly understood. We used mechanosensitive mouse salivary gland organ explants grown at different compliance levels together with deoxycholate extraction and immunocytochemistry of the intact, assembled matrices to examine the compliance-dependent assembly and distribution of the extracellular matrix and basement membrane in explants grown at permissive or non-permissive compliance. Extracellular matrix and basement membrane assembly were disrupted in the glands grown at low compliance compared to those grown at high compliance, correlating with defective morphogenesis and decreased myoepithelial cell differentiation. Extracellular matrix and basement membrane assembly as well as myoepithelial differentiation were restored by addition of TGFβ1 and by mechanical rescue, and mechanical rescue was prevented by inhibition of TGFβ signaling during the rescue. We detected a basal accumulation of active integrin β1 in the differentiating myoepithelial cells that formed a continuous peripheral localization around the proacini and in clefts within active sites of morphogenesis in explants that were grown at high compliance. The pattern and levels of integrin β1 activation together with myoepithelial differentiation were interrupted in explants grown at low compliance but were restored upon mechanical rescue or with application of exogenous TGFβ1. These data suggest that therapeutic application of TGFβ1 to tissues disrupted by mechanical signaling should be examined as a method to promote organ remodeling and regeneration.

ECM-modulated cellular dynamics as a driving force for tissue morphogenesis

The extracellular matrix (ECM) plays diverse regulatory roles throughout development. Coordinate interactions between cells within a tissue and the ECM result in the dynamic remodeling of ECM structure. Both chemical signals and physical forces that result from such microenvironmental remodeling regulate cell behavior that sculpts tissue structure. Here, we review recent discoveries illustrating different ways in which ECM remodeling promotes dynamic cell behavior during tissue morphogenesis. We focus first on new insights that identify localized ECM signaling as a regulator of cell migration, shape, and adhesion during branching morphogenesis. We also review mechanisms by which the ECM and basement membrane can both sculpt and stabilize epithelial tissue structure, using as examples Drosophila egg chamber development and cleft formation in epithelial organs. Finally, we end with an overview of the dynamic mechanisms by which the ECM can regulate stem cell differentiation to contribute to proper tissue morphogenesis.

Btbd7 regulates epithelial cell dynamics and branching morphogenesis

During embryonic development, many organs form by extensive branching of epithelia through the formation of clefts and buds. In cleft formation, buds are delineated by the conversion of epithelial cell-cell adhesions to cell-matrix adhesions, but the mechanisms of cleft formation are not clear. We have identified Btbd7 as a dynamic regulator of branching morphogenesis. Btbd7 provides a mechanistic link between the extracellular matrix and cleft propagation through its highly focal expression leading to local regulation of Snail2 (Slug), E-cadherin, and epithelial cell motility. Inhibition experiments show that Btbd7 is required for branching of embryonic mammalian salivary glands and lungs. Hence, Btbd7 is a regulatory gene that promotes epithelial tissue remodeling and formation of branched organs.

Salivary gland branching morphogenesis

Salivary gland branching morphogenesis involves coordinated cell growth, proliferation, differentiation, migration, apoptosis, and interaction of epithelial, mesenchymal, endothelial, and neuronal cells. The ex vivo analysis of embryonic mouse submandibular glands, which branch so reproducibly and beautifully in culture, is a powerful tool to investigate the molecular mechanisms regulating epithelium-mesenchyme interactions during development. The more recent analysis of genetically modified mice provides insight into the genetic regulation of branching morphogenesis. The review begins, as did the field historically, focusing on the role of the extracellular matrix (ECM), and its components such as glycosaminoglycans, collagens, and laminins. Following sections describe the modification of the ECM by proteases and the role of cell-matrix and cell-cell receptors. The review then focuses on two major families of growth factors implicated in salivary gland development, the fibroblast growth factors (FGFs) and the epidermal growth factors (EGFs). The salivary gland phenotypes in mice with genetic modification of FGFs and their receptors highlight the central role of FGFs during salivary gland branching morphogenesis. A broader section mentions other molecules implicated from analysis of the phenotypes of genetically modified mice or organ culture experiments. The review concludes with speculation on some future areas of research.

Lgl1, a mesenchymal modulator of early lung branching morphogenesis, is a secreted glycoprotein imported by late gestation lung epithelial cells

Secreted glycoproteins serve a variety of functions related to cell-cell communication in developmental systems. We cloned LGL1, a novel glucocorticoid-inducible gene in foetal lung, and described its temporal and spatial localization in the rat. Disruption of foetal mesenchyme-specific LGL1 expression using antisense oligodeoxynucleotides, which was associated with a 50% decrease in lgl1 protein levels, inhibited airway epithelial branching in foetal rat gestational day 13 lung buds in explant culture. These findings suggested that lgl1 functions as a secreted signalling molecule. We now provide evidence supporting a role for lgl1 in mesenchymal-epithelial interactions that govern lung organogenesis. Lgl1 is a secreted glycoprotein with a conserved N-terminal secretory signal peptide. Using dual immunofluorescence, intracellular lgl1 was found to co-localize with markers of the Golgi apparatus and endoplasmic reticulum, consistent with its association with secretory vesicles. Using pulse-chase studies, we show that lgl1 is a stable protein with a half-life of 11.5 h. Furthermore, at gestational days 20 and 21 (term=22), foetal distal lung epithelial cells import lgl1 protein. Taken together, our findings support distinct roles for lgl1 as a mediator of glucocorticoid-induced mesenchymal-epithelial interactions in early and late foetal lung organogenesis.

EGF-stimulated signaling by means of PI3K, PLCgamma1, and PKC isozymes regulates branching morphogenesis of the fetal mouse submandibular gland

Branching morphogenesis of fetal mouse submandibular glands (SMGs) partly depends on the epidermal growth factor (EGF) receptor that triggers at least three intracellular signaling pathways involving (1) the mitogen-activated protein kinases ERK-1/2, (2) phospholipase Cgamma1 (PLCgamma1), and (3) phosphatidylinositol-3-kinase (PI3K). PLCgamma1 directly activates protein kinase C (PKC) isozymes; PI3K stimulates protein kinase B (PKB, also known as Akt), which ultimately activates PKCs and other proteins. We reported that the pattern of phosphorylation of ERK-1/2 in response to EGF in SMGs varies with fetal age and that blockade of EGF-stimulated ERK-1/2 signaling partially inhibits branching (Kashimata et al. [2000] Dev. Biol. 220:183-196). Here, we report on components of the PLCgamma1, PI3K, and PKC families of signaling molecules in fetal SMGs from the 13th day of gestation to postnatal ages. Western blotting revealed that (1) PLCgamma1 is present from E13 to E18 but drops off precipitously to negligible levels on the day of birth and thereafter, and (2) PI3K, PKB(Akt), and several PKC isozymes are expressed from E13 onward through adult life. Both PLCgamma1 and PI3K are phosphorylated in response to EGF. Inhibition of PI3K by LY294002 inhibited EGF-stimulated branching, but inhibition of PLCgamma1 by U73122 had no effect. Western blotting showed that the concentrations of 8 PKC isozymes vary with age in the fetal and postnatal SMG. However, general inhibition of PKCs by Calphostin C or specific inhibition of PKCalpha or of PKCepsilon by Gö6976 or Ro-32-0432, respectively, increased EGF-stimulated branching. Calphostin C also increased EGF-stimulated phosphorylation of ERK-1/2. These findings indicate that signaling from the EGF receptor in the fetal mouse SMG varies with development and triggers stimulatory effects by means of ERK-1/2 and PI3K but inhibitory effects by means of PKC isozymes.

Transforming growth factor beta2, but not beta1 and beta3, is critical for early rat lung branching

Mesenchymal-epithelial tissue interactions are critical for lung branching morphogenesis, and polypeptide growth factors are likely involved in these tissue interactions. Transforming growth factorbetas (TGFbetas) have been implicated in lung development, but their involvement in early lung branching morphogenesis is unclear. In the present study, we investigated the role of the three mammalian TGFbeta subtypes (beta1, beta2, and beta3) and their receptors (type III (TbetaR-III), type II (TbetaR-II), and two types I (TbetaR-I), ALK-1 and ALK-5) in early rat lung organogenesis by using an embryonic rat lung explant culture. Transcripts and proteins for all three TGFbetas and their receptors were detected during the embryonic period of fetal rat lung development. Inhibition of TGFbeta2, but not beta1 and beta3, with antisense oligonucleotides and neutralizing antibodies resulted in significant inhibition of early lung branching in culture. Addition of minute amounts (</=1 ng/ml) of exogenous TGFbeta2, but not beta1 and beta3, restored the branching of TGFbeta2 antisense-treated explants. Higher concentrations of TGFbeta2 were inhibitory. BrdU labeling of lung explants was not altered by antisense TGFbeta2 treatment, but low concentrations of TGFbeta2 increased thymidine uptake by isolated epithelial cells. Fibronectin and metallogelatinase activities of embryonic lung cells were not affected by any TGFbeta isoform but TGFbeta2 specifically decreased mesenchymal hyaluronan synthesis. Antisense inhibition of ALK-5 and TbetaR-II showed a similar reduction in early lung branching as observed with antisense TGFbeta2. Incubation of lung explants with soluble TbetaR-II receptors also abrogated lung branching. ALK-1 antisense treatment did not affect early branching. Administration of neither activin A, which can act via ALK-1, nor follistatin, the natural inhibitor of activin, to the explants cultures had any significant effect on lung branching. Antisense inhibition of the activin receptor-II (Act-RII) also did not affect lung branching. These results are consistent with TGFbeta2, but not beta1 and beta3, regulating pattern formation during early rat lung organogenesis. This TGFbeta signaling in rat lung branching in vitro appears to be predominantly mediated via the TbetaR-I(ALK-5)/TbetaR-II heteromeric complex.

Epidermal growth factor system is a physiological regulator of development of the mouse fetal submandibular gland and regulates expression of the alpha6-integrin subunit

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) regulate branching morphogenesis of fetal mouse submandibular gland (SMG) rudiments in vitro. The EGF system (EGF, TGF-alpha, and their shared receptor, EGFR) also regulates expression of integrins and their ligands in the extracellular matrix. We show here that inhibition of EGFR tyrosine-kinase activity by a tyrphostin retards in vitro development of SMGs. Using total RNA isolated from pooled SMGs taken from intact mouse fetuses, mRNA transcripts for EGF, TGF-alpha, and EGFR were detected by reverse transcription-polymerase chain reaction (RT-PCR), and age-dependent variations in the levels of these mRNA were quantitatively determined by nuclease protection assays. These findings suggest that the EGF system is operative in the in vivo development of this gland. alpha6-Integrin subunit was localized by immunofluorescence at the basal surface of epithelial cells. Branching morphogenesis of cultured SMG rudiments was inhibited by anti-alpha6 antibodies. Synthesis of alpha6-subunit in cultured SMGs, detected by metabolic labeling and immunoprecipitation, was increased by EGF and drastically reduced by tyrphostin. RT-PCR revealed that mRNAs for alpha6- and beta1- and beta4-integrin subunits are expressed at all ages between embryonic day 13 and postnatal day 7. These findings suggest that 1) the EGF system is a physiologic regulator of development of fetal mouse SMG, and 2) one mechanism by which it acts may be by regulating expression of integrins, which in turn control interaction of epithelial cells with the extracellular matrix.

Phosphorylation of a membrane-intercalated proteoglycan, syndecan-2, expressed in a stroma-inducing clone from a mouse Lewis lung carcinoma

We previously reported that a mouse Lewis lung carcinoma-derived stroma-inducing clone, P29, highly expresses a syndecan-like proteoglycan exhibiting specific binding to fibronectin, a major constituent of the interstitial matrix formed by the induced stromal cells, via its heparan sulphate chains [Itano, Oguri, Nakanishi and Okayama (1993) J. Biochem. (Tokyo) 114, 862-873]. On metabolic labelling of the proteoglycan with [32P]Pi, followed by identification of the radiolabelled material using glycanases, almost all the isotope was found to have been incorporated into a core portion of molecular mass 48 kDa, which was generated by digestion with heparan sulphate lyase I plus chondroitin ABC lyase. Immunoblotting of the core protein with a monoclonal antibody, F58-6G12, demonstrated that the proteoglycan was mouse syndecan-2. CsCl-density-gradient centrifugation after mild treatment of liposome-intercalated 32P-labelled syndecan-2 with trypsin resulted in clear separation of the radioactivity into a bottom fraction containing all the glycosaminoglycans (accounting for 40% of the total radioactivity) and a top fraction containing liposome-associated peptides (60%). The former isotope was shown to be linked covalently to both heparan sulphate and chondroitin sulphate chains, probably at their bridge regions. The latter was mostly attributed to phosphoserine, the one and only phosphorylated amino acid released on acid hydrolysis of this proteoglycan, strongly suggesting that the phosphorylation occurs at a specific serine residue(s) in the cytoplasmic domain of the core protein.

Gravity in mammalian organ development: differentiation of cultured lung and pancreas rudiments during spaceflight

Organ culture of embryonic mouse lung and pancreas rudiments has been used to investigate development and differentiation, and to assess the effects of microgravity on culture differentiation, during orbital spaceflight of the shuttle Endeavour (mission STS-54). Lung rudiments continue to grow and branch during spaceflight, an initial result that should allow future detailed study of lung morphogenesis in microgravity. Cultured embryonic pancreas undergoes characteristic exocrine acinar tissue and endocrine islet tissue differentiation during spaceflight, and in ground controls. The rudiments developing in the microgravity environment of spaceflight appear to grow larger than their ground counterparts, and they may have differentiated more rapidly than controls, as judged by exocrine zymogen granule presence.

Localization of extracellular matrix components in developing mouse salivary glands by confocal microscopy

The importance of the extracellular matrix (ECM) in epithelial-mesenchymal interactions in developing organisms is well established. Proteoglycans and interstitial collagens are required for the growth, morphogenesis, and differentiation of epithelial organs and the distribution of these molecules has been described. However, much less is known about other ECM macromolecules in developing epithelial organs. We used confocal microscopy to examine the distribution of laminin, heparan sulfate (BM-1) proteoglycan, fibronectin, and collagen types I, IV, and V, in mouse embryonic salivary glands. Organ rudiments were isolated from gestational day 13 mouse embryos and cultured for 24, 48, or 72 hours. Whole mounts were stained by indirect immunofluorescence and then examined using a Zeiss Laser Scan Microscope. We found that each ECM component examined had a distinct distribution and that the distribution of some molecules varied with culture time. Laminin was mainly restricted to the basement membrane. BM-1 proteoglycan was concentrated in the basement membrane and also formed a fine network throughout the mesenchyme. Type IV collagen was mainly located in the basement membrane of the epithelium, but it was also present throughout the mesenchyme. Type V collagen was distributed throughout the mesenchyme at 24 hours, but at 48 hours was principally located in the basement membrane. Type I collagen was distributed throughout the mesenchyme at all culture times, and accumulated in the clefts and particularly at the epithelial-mesenchymal interface as time in culture increased. Fibronectin was observed throughout the mesenchyme at all times.

Development of the discoidal hemochorial placenta in the black mastiff bat, Molossus ater: evidence for a role of maternal endothelial cells in the control of trophoblastic growth

In an effort to define further the factors that can influence trophoblastic growth, development of the discoidal chorioallantoic placenta was examined histologically in laboratory-bred black mastiff bats, Molossus ater. This placenta normally forms only at the cranial end of the right uterine horn. Its positioning was found to be related to the development of an unusual vascular tuft in the endometrium at this site following ovulation. When a decidual reaction occurred, the endothelial cells of the tuft vessels hypertrophied and their basal laminae became more prominent in sections stained for glycoproteins. Cytotrophoblast then proliferated preferentially around the tuft in the right horn and its vessels became surrounded by trophoblastic cuffs. A functional placenta was formed when trophoblastic tubules grew out from these cuffs, became interconnected, developed lumina, and began to carry maternal blood. Maternal endothelial cells generally persisted only in portions of the larger vascular tubules. These observations suggest that the endothelial cells of the tuft vessels may be secreting factors that influence early trophoblastic growth and are at least partially incorporated into the basal laminae of the endothelial cells. Evidence is reviewed which raises the possibility that endometrial endothelial cells might have a similar morphogenetic role in many other mammals with invasive trophoblast. Mitotic activity in the trophoblastic tubules of M. ater ceased when the tubules became patent and began to carry blood. Further growth was then accomplished by hypertrophy of the existing cytotrophoblast cells. Electron microscopic examination of near-term placentas confirmed that the interhemal barrier was hemomonochorial and lacked a continuous layer of syncytiotrophoblast.

Extracellular matrix of cultivated, poorly differentiated human gastric adenocarcinoma cells promotes attachment and spreading of mesenchymal cells

To clarify interactions between carcinoma and mesenchymal cells, we examined the extracellular matrix-substance remaining on culture dishes after confluent growths of gastric carcinoma cells were removed with EDTA. The matrix synthesized by poorly differentiated adenocarcinoma cells (cell lines KATO-III and MKN-45) cultivated in serum-free medium has a fibroblast (cell line WI38)-attachment activity, which is not detected in the matrix synthesized by a well differentiated adenocarcinoma (cell line MKN-28). This activity was not observed in KATO-III-matrix extracted with 6 M urea, but could be detected in a 1% SDS extract from the remaining matrix on the culture dishes after 6 M urea extraction. The activity was abolished by treatment with pronase (16 micrograms/ml), trypsin (0.005%) or alkali, but was unaffected by collagenase (80 micrograms/ml, 4 h) or chondroitinase ABC (1 U ml, 1 h). It is conceivable that the fibroblast-attachment activity of the matrix produced by poorly differentiated adenocarcinoma cells is related to the proliferation of interstitial connective tissue in vivo.

An autoradiographic analysis of N-linked glycoconjugates in embryonic salivary gland morphogenesis

The synthesis, deposition, and loss of mannose-bearing glycoconjugates during branching morphogenesis of embryonic mouse salivary glands has been evaluated. Day 13 embryonic mouse salivary glands were cultured for 44 hr, pulse labeled 4 hr with [3H]mannose, then fixed after 0, 2, 4, 8, or 24 hr of chase in nonradioactive medium, and processed for autoradiography. Light microscopic autoradiograms of sectioned rudiments reveal extensive label within the epithelium, little label over the mesenchyme, and a concentration of radioactivity at the basal surface of the epithelium. Autoradiograms of "chased" rudiments reveal a) no detectable loss of label from the epithelium or the basal epithelial surface over the first 8 hr, and b) significant label loss by 24 hr of chase at the basal epithelial surface, while moderate amounts of radioactivity remain throughout the rest of the epithelium. The [3H]bound material is insensitive to chondroitinase ABC, a glycosaminoglycan degradative enzyme, but is sensitive to tunicamycin presence in the culture medium. Earlier studies showed that embryonic mouse salivary glands cultured in medium containing tunicamycin (25 ng/ml) continued normal epithelial branching while epithelial growth was inhibited. The present autoradiographic studies of [3H]mannose-labeled rudiments demonstrate that tunicamycin causes a significant decrease in radioactivity, relative to controls. Thus, our results suggest that epithelial branching activity is independent of control levels of mannose-containing/tunicamycin-sensitive, glycoconjugate deposition.