Salivary gland morphogenesis and basement membranes

Basement Membrane Mimetic Hydrogel Cooperates with Rho-Associated Protein Kinase Inhibitor to Promote the Development of Acini-Like Salivary Gland Spheroids

Successful engineering of functional salivary glands necessitates the creation of cell-instructive environments for ex vivo expansion and lineage specification of primary human salivary gland stem cells (hS/PCs). Herein, basement membrane mimetic hydrogels were prepared using hyaluronic acid, cell adhesive peptides, and hyperbranched polyglycerol (HPG), with or without sulfate groups, to produce "hyperGel+" or "hyperGel", respectively. Differential scanning fluorescence experiments confirmed the ability of the sulphated HPG precursor to stabilize fibroblast growth factor 10. The hydrogels were nanoporous, cytocompatibile and cell-permissive, enabling the development of multicellular hS/PC spheroids in 14 days. Incorporation of sulfated HPG species in the hydrogel enhanced cell proliferation. Culture of hS/PCs in hyperGel+ in the presence of a Rho kinase inhibitor, Y-27632 (Y-27), led to the development of spheroids with a central lumen, increased the expression of acinar marker aquaporin-3 at the transcript level (AQP3), and decreased the expression of ductal marker keratin 7 at both the transcript (KRT7) and the protein levels (K7). Reduced expression of transforming growth factor beta (TGF-β) targets SMAD2/3 was also observed in Y27-treated cultures, suggesting attenuation of TGF-β signaling. Thus, hyperGel+ cooperates with the ROCK inhibitor to promote the development of lumened spheroids with enhanced expression of acinar markers.

Slow hydrogel matrix degradation enhances salivary gland mimetic phenotype

We recently developed a salivary gland tissue mimetic (SGm), comprised of salivary gland cells encapsulated in matrix metalloproteinase (MMP)-degradable poly(ethylene glycol) hydrogels within arrays of ∼320 µm diameter spherical cavities molded in PDMS. The SGm provides a functional and physiologically relevant platform well-suited to high-throughput drug screening for radioprotective compounds. However, the utility of the SGm would benefit from improved retention of acinar cell phenotype and function. We hypothesized that tuning biochemical cues presented within the PEG hydrogel matrix would improve maintenance of acinar cell phenotype and function by mimicking the natural extracellular matrix microenvironment of the intact gland. Hydrogels formed using slower-degrading MMP-sensitive peptide crosslinkers showed >2-fold increase in sphere number formed at 48 h, increased expression of acinar cell markers, and more robust response to calcium stimulation by the secretory agonist, carbachol, with reduced SGm tissue cluster disruption and outgrowth during prolonged culture. The incorporation of adhesive peptides containing RGD or IKVAV improved calcium flux response to secretory agonists at 14 days of culture. Tuning the hydrogel matrix improved cell aggregation, and promoted acinar cell phenotype, and stability of the SGm over 14 days of culture. Furthermore, combining this matrix with optimized media conditions synergistically prolonged the retention of the acinar cell phenotype in SGm. STATEMENT OF SIGNIFICANCE: Salivary gland (SG) dysfunction occurs due to off-target radiation due to head and neck cancer treatments. Progress in understanding gland dysfunction and developing therapeutic strategies for the SG are hampered by the lack of in vitro models, as salivary gland cells rapidly lose critical secretory function within 24 hours in vitro. Herein, we identify properties of poly(ethylene glycol) hydrogel matrices that enhance the secretory phenotype of SG tissue mimetics within the previously-described SG-microbubble tissue chip environment. Combining slow-degrading hydrogels with media conditions optimized for secretory marker expression further enhanced functional secretory response and secretory marker expression.

Dynamics, structure and assembly of the basement membrane in developing salivary glands revealed by an exogenous EGFP-tagged nidogen probe

Most epithelial tissues rapidly become complex during embryonic development while being surrounded by the basement membrane (BM). Thus, the BM shape is thought to change dramatically as the epithelium grows, but the underlying mechanism is not yet clear. Nidogen-1 is ubiquitous in the BM and binds to various other BM components, including laminin and type IV collagen. To elucidate the behavior of the BM during epithelial morphogenesis, we attempted to live-label the developing BM with recombinant human nidogen-1 fused to an enhanced green fluorescent protein (hNid1-EGFP). Submandibular glands of mouse embryos were cultured in glass-bottomed dishes and incubated in media containing hNid1-EGFP. Subsequent confocal microscopy clearly visualized the BMs surrounding the epithelial end buds. On three-dimensional reconstruction from Z-series confocal sections, the epithelial BM was observed as a thin sheet that expanded continuously around the entire epithelial basal surface. Because the explants continued to grow well in the presence of hNid1-EGFP, time-lapse confocal microscopy was performed to follow the dynamics of the BM. We found that the epithelial BM is an adaptive structure that deforms in accordance with the rapid shape changes of the developing epithelium. Furthermore, hNid1-EGFP was found to be incorporated differently into the epithelial BM compared with that reported for fibronectin or type IV collagen, suggesting that individual BM components assemble in different ways to form the BM.

Fibrin with Laminin-Nidogen Reduces Fibrosis and Improves Soft Palate Regeneration Following Palatal Injury

This study aimed to analyze the effects of fibrin constructs enhanced with laminin-nidogen, implanted in the wounded rat soft palate. Fibrin constructs with and without laminin-nidogen were implanted in 1 mm excisional wounds in the soft palate of 9-week-old rats and compared with the wounded soft palate without implantation. Collagen deposition and myofiber formation were analyzed at days 3, 7, 28 and 56 after wounding by histochemistry. In addition, immune staining was performed for a-smooth muscle actin (a-SMA), myosin heavy chain (MyHC) and paired homeobox protein 7 (Pax7). At day 56, collagen areas were smaller in both implant groups (31.25 ± 7.73% fibrin only and 21.11 ± 6.06% fibrin with laminin-nidogen)) compared to the empty wounds (38.25 ± 8.89%, p < 0.05). Moreover, the collagen area in the fibrin with laminin-nidogen group was smaller than in the fibrin only group (p ˂ 0.05). The areas of myofiber formation in the fibrin only group (31.77 ± 10.81%) and fibrin with laminin-nidogen group (43.13 ± 10.39%) were larger than in the empty wounds (28.10 ± 11.68%, p ˂ 0.05). Fibrin-based constructs with laminin-nidogen reduce fibrosis and improve muscle regeneration in the wounded soft palate. This is a promising strategy to enhance cleft soft palate repair and other severe muscle injuries.

Single-Cell RNA-seq Identifies Cell Diversity in Embryonic Salivary Glands

Branching organs, including the salivary and mammary glands, lung, and kidney, arise as epithelial buds that are morphologically very similar. However, the mesenchyme is known to guide epithelial morphogenesis and to help govern cell fate and eventual organ specificity. We performed single-cell transcriptome analyses of 14,441 cells from embryonic day 12 submandibular and parotid salivary glands to characterize their molecular identities during bud initiation. The mesenchymal cells were considerably more heterogeneous by clustering analysis than the epithelial cells. Nonetheless, distinct clusters were evident among even the epithelial cells, where unique molecular markers separated presumptive bud and duct cells. Mesenchymal cells formed separate, well-defined clusters specific to each gland. Neuronal and muscle cells of the 2 glands in particular showed different markers and localization patterns. Several gland-specific genes were characteristic of different rhombomeres. A muscle cluster was prominent in the parotid, which was not myoepithelial or vascular smooth muscle. Instead, the muscle cluster expressed genes that mediate skeletal muscle differentiation and function. Striated muscle was indeed found later in development surrounding the parotid gland. Distinct spatial localization patterns of neuronal and muscle cells in embryonic stages appear to foreshadow later differences in adult organ function. These findings demonstrate that the establishment of transcriptional identities emerges early in development, primarily in the mesenchyme of developing salivary glands. We present the first comprehensive description of molecular signatures that define specific cellular landmarks for the bud initiation stage, when the neural crest-derived ectomesenchyme predominates in the salivary mesenchyme that immediately surrounds the budding epithelium. We also provide the first transcriptome data for the largely understudied embryonic parotid gland as compared with the submandibular gland, focusing on the mesenchymal cell populations.

3D Culture Histology Cryosectioned Well Insert Technology Preserves the Structural Relationship between Cells and Biomaterials for Time-Lapse Analysis of 3D Cultures

When performing histology of softer biomaterials, aspiration disrupts the cellular and molecular location information. This study aims to develop a cryosectionable well insert able to preserve the biomaterial and cell's original 3D conformation from the well to histology analysis. The well insert is composed of a paraffin-coated gelatine pill. Within the coated capsule, the human epithelial cell line (NS-SV-AC) is cultured in Matrigel, GrowDex, Myogel, Myogel + GrowDex, or cell culture media for 14 days. At 0 and 14 days, the samples are frozen in liquid nitrogen and cryotome is used to create sections. The slides are stained by Sirius Red and immunohistochemistry using antibodies human collagens I-V and human Ki-67. Sirius Red shows pink shades of biomaterials and the best cellular vertical distribution throughout the sagittal section of the well is achieved with Matrigel, GrowDex, and Myogel + GrowDex; in Myogel and media, the cells sink. For collagen protein expression, only Matrigel induces a notable difference while in the other materials, collagen staining is weak or difficult to distinguish from endogenous collagens. Ki-67 expression is maintained over time. The 3D-cryo well insert provides a new time-lapse histology perspective of analysis for liquid or gel cultures that maintains cells and macromolecules in their unaltered in-well configuration.

Basement Membranes in Development and Disease

The basement membrane is a thin but dense, sheet-like specialized type of extracellular matrix that has remarkably diverse functions tailored to individual tissues and organs. Tightly controlled spatial and temporal changes in its composition and structure contribute to the diversity of basement membrane functions. These different basement membranes undergo dynamic transformations throughout animal life, most notably during development. Numerous developmental mechanisms are regulated or mediated by basement membranes, often by a combination of molecular and mechanical processes. A particularly important process involves cell transmigration through a basement membrane because of its link to cell invasion in disease. While developmental and disease processes share some similarities, what clearly distinguishes the two is dysregulation of cells and extracellular matrices in disease. With its relevance to many developmental and disease processes, the basement membrane is a vitally important area of research that may provide novel insights into biological mechanisms and development of innovative therapeutic approaches. Here we present a review of developmental and disease dynamics of basement membranes in Caenorhabditis elegans, Drosophila, and vertebrates.

Human saliva as a diagnostic material

Today blood biochemical laboratory tests are essential elements to the diagnosis and monitoring of the treatment of diseases. However, many researchers have suggested saliva as an preferable diagnostic material. The collection of saliva is simple, painless, cheap and safe, both for patients and medical staff. An additional advantage of saliva is the fact that it may be retrieved several times a day, which makes repeat analysis much easier. Furthermore, saliva has very high durability. Although 94-99% of salivary content is water, saliva also contains numerous cellular elements and many organic and inorganic substances, including most biological markers present in the blood and urine that may be used in the early detection and monitoring of many dental and general diseases.

Shh/Ptch and EGF/ErbB cooperatively regulate branching morphogenesis of fetal mouse submandibular glands

The hedgehog family includes Sonic hedgehog (Shh), Desert hedgehog, and Indian hedgehog, which are well known as a morphogens that play many important roles during development of numerous organs such as the tongue, pancreas, kidney, cartilage, teeth and salivary glands (SMG). In Shh null mice, abnormal development of the salivary gland is seen after embryonic day 14 (E14). Shh also induced lobule formation and lumen formation in acini-like structures in cultured E14 SMG. In this study, we investigated the relationship between Shh and epidermal growth factor (EGF)/ErbB signaling in developing fetal mouse SMG. Administration of Shh to cultured E13 SMG stimulated branching morphogenesis (BrM) and induced synthesis of mRNAs for EGF ligands and receptors of the ErbB family. Shh also stimulated activation of ErbB signaling system such as ERK1/2. AG1478, a specific inhibitor of ErbB receptors, completely suppressed BrM and activation of EGF/ErbB/ERK1/2 cascade in E13 SMGs cultured with Shh. The expressions of mRNA for Egf in mesenchyme and mRNA for Erbb1, Erbb2 and Erbb3 in epithelium of E13 SMG were specifically induced by administration of Shh. These results show that Shh stimulates BrM of fetal mouse SMG, at least in part, through activation of the EGF/ErbB/ERK1/2 signaling system.

Chitosan facilitates structure formation of the salivary gland by regulating the basement membrane components

Tissue structure is important for inherent physiological function and should be recapitulated during tissue engineering for regenerative purposes. The salivary gland is a branched organ that is responsible for saliva secretion and regulation. The salivary glands develop from epithelial-mesenchymal interactions, and depend on the support of the basement membrane (BM). Chitosan-based biomaterials have been demonstrated to be competent in facilitating the formation of salivary gland tissue structure. However, the underlying mechanisms have remained elusive. In the developing submandibular gland (SMG), the chitosan effect was found to diminish when collagen and laminin were removed from cultured SMG explants. Chitosan increased the expression of BM components including collagen, laminin, and heparan sulfate proteoglycan, and also facilitated BM components and the corresponding receptors to be expressed in tissue-specific patterns beneficial for SMG branching. The chitosan effect decreased when either laminin components or receptors were inhibited, as well when the downstream signaling was blocked. Our results revealed that chitosan promotes salivary glands branching through the BM. By regulating BM components and receptors, chitosan efficiently stimulated downstream signaling to facilitate salivary gland branching. The present study revealed the underlying mechanism of the chitosan effect in engineering SMG structure formation.

Silk fibroin scaffolds promote formation of the ex vivo niche for salivary gland epithelial cell growth, matrix formation, and retention of differentiated function

Salivary gland hypofunction often results from a number of causes, including the use of various medications, radiation for head and neck tumors, autoimmune diseases, diabetes, and aging. Since treatments for this condition are lacking and adult salivary glands have little regenerative capacity, there is a need for cell-based therapies to restore salivary gland function. Development of these treatment strategies requires the establishment of a system that is capable of replicating the salivary gland cell "niche" to support the proliferation and differentiation of salivary gland progenitor cells. In this study, a culture system using three-dimensional silk fibroin scaffolds (SFS) and primary salivary gland epithelial cells (pSGECs) from rat submandibular (SM) gland and parotid gland (PG) was established and characterized. pSGECs grown on SFS, but not tissue culture plastic (TCP), formed aggregates of cells with morphological features resembling secretory acini. High levels of amylase were released into the media by both cell types after extended periods in culture on SFS. Remarkably, cultures of PG-derived cells on SFS, but not SM cells, responded to isoproterenol, a β-adrenergic receptor agonist, with increased enzyme release. This behavior mimics that of the salivary glands in vivo. Decellularized extracellular matrix (ECM) formed by pSGECs in culture on SFS contained type IV collagen, a major component of the basement membrane. These results demonstrate that pSGECs grown on SFS, but not TCP, retain important functional and structural features of differentiated salivary glands and produce an ECM that mimics the native salivary gland cell niche. These results demonstrate that SFS has potential as a scaffold for creating the salivary gland cell niche in vitro and may provide an approach for inducing multipotent stem cells to provide therapeutically meaningful numbers of salivary gland progenitor cells for regenerating these tissues in patients.

Expression and localization of laminin 5, laminin 10, type IV collagen, and amelotin in adult murine gingiva

The biochemical composition of the internal and external basal laminae in the junctional epithelium differs significantly, and the precise cellular origin of their respective molecules remains to be determined. In the present study, the expression and localization of three basement membrane-specific molecules-laminin 5 (γ2 chain), type IV collagen (α1 chain), and laminin 10 (α5 chain)-and one tooth-specific molecule, amelotin, was analyzed in adult murine gingiva by using in situ hybridization and immunohistochemistry. The results showed that the outermost cells in junctional epithelium facing the tooth enamel strongly expressed laminin 5 mRNA, supporting the immunohistochemical staining data. This suggests that laminin 5 is actively synthesized in junctional epithelial cells and that the products are incorporated into the internal basal lamina to maintain firm epithelial adhesion to the tooth enamel throughout life. Conversely, no amelotin mRNA signals were detected in the junctional epithelial cells, suggesting that the molecules localized on the internal basal lamina are mainly derived from maturation-stage ameloblasts. Weak and sporadic expression of type IV collagen in addition to laminin 10 in the gingiva indicates that these molecules undergo turnover less frequently in adult animals.

Region-specific epithelial cell dynamics during branching morphogenesis

BACKGROUND

Epithelial cells of developing embryonic organs, such as salivary glands, can display substantial motility during branching morphogenesis. Their dynamic movements and molecules involved in their migration are not fully characterized.

RESULTS

We generated transgenic mice expressing photo-convertible KikGR and tracked the movements of individual cells highlighted by red fluorescence in different regions of developing salivary glands. Motility was highest for outer bud epithelial cells adjacent to the basement membrane, lower in inner bud cells, and lowest in duct cells. The highly motile outer cells contacting the basement membrane were pleomorphic, whereas inner cells were rounded. Peripheral cell motility was disrupted by antibodies inhibiting α6+β1 integrins and the nonmuscle myosin II inhibitor blebbistatin. Inner bud cell migration was unaffected by these inhibitors, but their rate of migration was stimulated by inhibiting E-cadherin.

CONCLUSIONS

Cell motility in developing salivary glands was highest in cells in contact with the basement membrane. The basement membrane-associated motility of these outer bud cells depended on integrins and myosin II, but not E-cadherin. In contrast, motility of inner bud cells was restrained by E-cadherin. These findings identify the importance of integrin-dependent basement membrane association for the morphology, tissue organization, and lateral motility of morphogenetic epithelial cells.

Chronic alcohol consumption promotes alterations on salivary gland regeneration process

The aim of this study is to investigate the histological effect of alcohol ingestion on the regeneration of the submandibular gland (SMG) in rats. Twelve 60-day-old male Wistar rats were randomized into two experimental groups. Test group (TG) animals ingested 40° GL of alcohol for 45 days before surgery, being its concentration gradually increased 10° GL/week for 4 weeks to achieve the final concentration of 40° GL. The control group (CG) received water during the whole experimental period. One-third of the left SMG lobe was removed. Three and seven days after, the whole gland was excised and analyzed. In the TG, the inflammatory process was pronounced when comparing the CG on day 3. The inverse aspect was observed on day 7, associated with an advanced parenchyma development. Changes in laminin expression and glycoproteins production were observed in the TG, causing advanced morphogenesis and delay in cytodifferentiation during the salivary gland regeneration, probably due to alcohol effects. Animals who received ethanol showed alterations in the pattern of glandular regeneration.

O-glycosylation modulates integrin and FGF signalling by influencing the secretion of basement membrane components

Extracellular microenvironments have crucial roles in modulating cell interactions during development. Here we discover that a conserved protein modification (O-glycosylation) influences extracellular matrix composition during mammalian organogenesis, affecting integrin signalling and fibroblast growth factor-mediated cell proliferation. Specifically, mice deficient for an enzyme (Galnt1) that adds sugars to proteins during early stages of organogenesis resulted in intracellular accumulation of major basement membrane proteins and endoplasmic reticulum stress, with resultant effects on fibroblast growth factor signalling, epithelial cell proliferation and organ growth. Exogenous addition of basement membrane components rescued fibroblast growth factor signalling and the growth defects in a β1-integrin-dependent manner. Our work demonstrates for the first time that O-glycosylation influences the composition of the extracellular matrix during mammalian organ development, influencing specific aspects of the endoplasmic reticulum stress response, cell signalling, cell proliferation and organ growth. Our work provides insight into the role of this conserved protein modification in both development and disease.

COMPARISON OF PLGA, PCL, AND CHITOSAN IN SALIVARY GLAND BRANCHING MORPHOGENESIS

Branching morphogenesis is a fundamental morphogenetic process in generating glandular tissues. Although the mechanism of branching morphogenesis has been well-explored in the salivary gland development, its interaction with different biodegradable materials has never been investigated. For the purpose of salivary gland regeneration, recapitulation of morphogenetic processes on biodegradable materials might be requisite. Toward this aim, biodegradable biomaterials including poly-lactic-co-glycolic acid (PLGA), poly-epsilon-caprolactone (PCL), and chitosan were examined in the submandibular gland (SMG) culture systems to elucidate their possible impact on salivary morphogenesis. It was found that when SMG explants were cultured on PLGA and PCL, the explants failed to form well-developed branching phenotypes with limited cell migration (5.6 ± 8.8 μm; 10.0 ± 14.1 μm) and decreasing cell viability (56.9% ± 12.5%; 50.3% ± 8.1%). On the contrary, explants cultured on chitosan showed well-developed branches, which were superior in number to those on the control substrata, without any alteration of the morphogenetic phenotypes. Furthermore, the increased cell migration (267.8 ± 45.2 μm) and explants viability (146.8% ± 18.4%) along with the greater deposition of type III collagen, altogether account for better SMG morphogenesis on chitosan. According to the results, it was found that branching morphogenesis of SMG was affected by different biodegradable materials. Chitosan might be an appropriate biodegradable material for salivary morphogenesis, and has applicable potential in the regeneration of salivary tissue.

Dynamics of salivary gland morphogenesis

Salivary glands form during embryonic development by a complex process that creates compact, highly organized secretory organs with functions essential for oral health. The architecture of these glands is generated by branching morphogenesis, revealed by recent research to involve unexpectedly dynamic cell motility and novel regulatory pathways. Numerous growth factors, extracellular matrix molecules, gene regulatory pathways, and mechanical forces contribute to salivary gland morphogenesis, but local gene regulation and morphological changes appear to play particularly notable roles. Here we review these recent advances and their potential application to salivary gland tissue engineering.

Cellular dynamics of epithelial clefting during branching morphogenesis of the mouse submandibular gland

We cultured the rudimental submandibular gland (SMG) of mice with a non-cell-permeable fluorescent tracer, and observed cell behavior during epithelial branching morphogenesis using confocal time-lapse microscopy. We traced movements of individual cells as shadowgraph movies. Individual epithelial cells migrated dynamically but erratically. The epithelial cleft extended by wiggling and separated a cluster of cells into two buds during branching. We examined the ultrastructure of the clefts in SMG rudiments treated with the laminin peptide A5G77f, which induces epithelial clefting. A short cytoplasmic shelf with a core of microfilaments was found at the deep end of the cleft. We propose that epithelial clefting involves a dynamic movement of cells at the base of the cleft, and the formation of a shelf within a cleft cell. The shelf might form a matrix attachment point at the base of the cleft with a core of microfilaments driving cleft elongation.

Branching morphogenesis in the fetal mouse submandibular gland is codependent on growth factors and extracellular matrix

Branching morphogenesis (BrM) is a basic developmental process for the formation of the lung, kidney, and all exocrine glands, including the salivary glands. This process proceeds as follows. An epithelial downgrowth invaginates into underlying mesenchyme, and forms a cleft at its distal end, which is the site of dichotomous branching and elongation; this process of clefting and elongation is repeated many times at the distal ends of the invading epithelium until the desired final extent of branching is reached. The distal ends of the epithelium differentiate into the secretory endpieces, and the elongated segments become the ducts. This presentation is a brief historical review of studies on BrM during the development of the submandibular gland (SMG).

Identification of alpha-dystroglycan binding sequences in the laminin alpha2 chain LG4-5 module

The biological activities of the laminin alpha2 chain LG4-5 module result from interactions with cell surface receptors, such as heparan sulfate proteoglycans and alpha-dystroglycan. In this study, heparin and alpha-dystroglycan binding sequences were identified using 42 overlapping synthetic peptides from the LG4-5 module and using recombinant LG4-5 protein (rec-alpha2LG4-5). Physiological activities of the active peptides were also examined in explants of submandibular glands. Heparin binding screens showed that the A2G78 peptide (GLLFYMARINHA) bound to heparin and prevented its binding to rec-alpha2LG4-5. Furthermore, alanine substitution of the arginine residue in the A2G78 site on rec-alpha2LG4-5 decreased heparin binding activity. When alpha-dystroglycan binding of the peptides was screened, two peptides, A2G78 and A2G80 (VQLRNGFPYFSY), bound alpha-dystroglycan. A2G78 and A2G80 also inhibited alpha-dystroglycan binding of rec-alpha2LG4-5. A2G78 and A2G80 specifically inhibited end bud formation of submandibular glands in culture. These results suggest that the A2G78 and A2G80 sites play functional roles as heparan sulfate- and alpha-dystroglycan-binding sites in the module. These peptides are useful for elucidating molecular mechanisms of heparan sulfate- and/or alpha-dystroglycan-mediated biological functions of the laminin alpha2 chain.

Chitosan cooperates with mesenchyme-derived factors in regulating salivary gland epithelial morphogenesis

Chitosan is a widely used biocompatible biomaterial in the tissue regeneration, but its utility and application in the tissue morphogenesis of salivary gland remains unclear. The study aimed to explore the effects of chitosan on the epithelial morphogenesis of submandibular gland (SMG). With chitosan, the branching morphogenesis of the whole SMG explant was facilitated, and the morphogenetic-promoting effects of mesenchymal tissue on SMG were further enhanced. Furthermore, chitosan was competent to induce recombined SMG epithelium to form branches in the serum-free condition independently. In the presence of chitosan, the morphogenetic efficacy of mesenchyme-derived growth factors responsible for epithelial morphogenesis including fibroblast growth factors 7, fibroblast growth factor 10 and hepatocyte growth factor increased. The specific epithelial phenotype induced by individual growth factor, which was required for the accomplishment of salivary epithelial morphogenesis, was promoted by chitosan. Moreover, the proliferative and the chemotactic properties of these growth factors towards the SMG epithelia were also reinforced by chitosan. Therefore, in orchestrating and intensifying the essential mesenchyme-derived growth factors, chitosan is versatile in mediating SMG epithelium to form a predetermined phenotype more efficiently and comprehensively. This study suggested that chitosan is a morphogenetic-regulating biomaterial for salivary tissue, which might be useful for the future salivary gland investigation and regeneration.

FGF alters epithelial competence for EGF at the initiation of branching morphogenesis of mouse submandibular gland

Embryonic day 13 mouse submandibular gland (E13-SMG) rudiments with two to four clefts have been commonly used in culture experiments to show that growth factors, such as epidermal growth factor (EGF) -family and fibroblast growth factor (FGF) -family ligands, are involved in branching morphogenesis. In the present study, we focused on E12 rudiments and attempted to elucidate the roles of EGF- and FGF-family ligands in SMG development from E12 to E13. In mesenchyme-free, Matrigel-embedded cultures, EGF + lysophosphatidic acid (LPA) induced branching in E13 epithelium, whereas E12 epithelium remained spherical and no branching occurred under the same culture conditions; however, both E12 and E13 epithelia elongated in response to FGF10. Reverse transcriptase-polymerase chain reaction studies showed that the expression of ErbB1 among four EGF receptors and Lpa3 among three LPA receptors was lower in E12 than in E13 epithelia. Fgf10, Fgf7, and their major receptor Fgfr2b were highly and equally expressed in E12 and E13 rudiments. After 24 hr of mesenchyme-free culture with FGF10 or FGF7, E12 epithelium was primed to initiate branching morphogenesis in response to EGF + LPA coincident with ErbB1 and Lpa3 up-regulation. These results suggest that the EGF-family ligand-receptor system is undeveloped at E12 and that it becomes primed on E13 by the FGF ligand-receptor system to play an important role in the induction of branching morphogenesis.

Signaling pathways activated by epidermal growth factor receptor or fibroblast growth factor receptor differentially regulate branching morphogenesis in fetal mouse submandibular glands

Although growth factor signaling is required for embryonic development of organs, individual signaling mechanisms regulating these organotypic processes are just beginning to be defined. We compared signaling activated in fetal mouse submandibular glands (SMGs) by three growth factors, epidermal growth factor (EGF), fibroblast growth factor (FGF) 7, or FGF10, and correlated it with specific events of branching morphogenesis. Immunoblotting showed that EGF strongly stimulated phosphorylation of extracellular signal-regulated kinase-1/2 (ERK-1/2) and weakly stimulated phosphorylation of phospholipase Cgamma1 (PLCgamma1) and phosphatidylinositol-3 kinase (PI3K) in cultured E14 SMG. However, FGF7 and FGF10 stimulated phosphorylation of both PLCgamma1 and PI3K, but elicited only minimal phosphorylation of ERK-1/2. Morphological study of mesenchyme-free SMG epithelium cultured in Matrigel revealed that EGF induced cleft formation of endpieces, that FGF7 stimulated both cleft formation and stalk elongation, but that FGF10 induced only stalk elongation. In mesenchyme-free SMG epithelium cultured with EGF, FGF7 and FGF10, U0126 (MEK inhibitor) completely blocked cleft formation, whereas U73122 (PLCgamma1 inhibitor) suppressed stalk elongation. These finding suggest that EGF stimulates cleft formation and drives branch formation via ERK-1/2, and that FGF7 stimulates both cleft formation and stalk elongation via PLCgamma1 and partly via ERK-1/2, but that FGF10 stimulates stalk elongation mainly via PLCgamma1.

Diverse roles of E-cadherin in the morphogenesis of the submandibular gland: insights into the formation of acinar and ductal structures

The formation of acinar and ductal structures during epithelial tissue branching morphogenesis is not well understood. We report that in the mouse submandibular gland (SMG), acinar and ductal cell fates are determined early in embryonic morphogenesis with E-cadherin playing pivotal roles in development. We identified two morphologically distinct cell populations at the single bud stage, destined for different functions. The outer layer of columnar cells with organized E-cadherin junctions expressed the neonatal acinar marker B1 by E13.5, demonstrating their acinar fate. The interior cells initially lacked distinct E-cadherin junctions, but with morphogenesis formed cytokeratin 7 (K7) -positive ductal structures with organized E-cadherin junctions and F-actin filaments. Inhibition of E-cadherin function with either siRNA or function blocking antibody caused extensive apoptosis of ductal cells and aberrantly dilated lumens, providing the first evidence that E-cadherin regulates ductal lumen formation during branching morphogenesis of the salivary gland.

EGF-receptor regulates salivary gland branching morphogenesis by supporting proliferation and maturation of epithelial cells and survival of mesenchymal cells

Epidermal growth factor receptor (EGF-R) regulates epithelial morphogenesis during development and is important for the proper branching of the lung, mammary gland, and pancreas. We analyzed the salivary gland phenotype of EGF-R-deficient mice and showed impaired growth, branching, and maturation of the epithelium. Furthermore, treatment of wild-type E13 salivary glands with gefitinib, a small molecular inhibitor of EGF-R, led to apoptosis of the mesenchyme. Interestingly, MMP2 and plasminogen activators were upregulated upon inhibition of EGF-R signaling. To summarize, we show that EGF-R is a physiological regulator of salivary gland development and its main function is to support the proliferation and maturation of the epithelium and the survival of the mesenchyme.

Involvement of REG Ialpha protein in the regeneration of ductal epithelial cells in the minor salivary glands of patients with Sjögren's syndrome

The regenerating gene (Reg) was originally isolated from regenerating rat pancreatic islets and revealed recently to constitute a multi-gene family in humans. REG Ialpha protein is known to be overexpressed not only in various human inflammatory diseases but also in various experimental models of inflammation in animal tissues. However, its involvement in pathophysiology of the minor salivary gland (MSG) is not clear. We investigated REG Ialpha expression in the MSG of patients with primary Sjögren's syndrome (SS) and assessed its role in ductal epithelial cell proliferation in such tissues. Lip biopsy specimens were obtained from 40 patients with primary SS and examined using immunohistochemistry for REG Ialpha protein, Ki67 and single-strand DNA (ssDNA). The relationships among clinicopathological factors and expression of REG Ialpha protein, Ki67 and ssDNA in the MSG were then analysed. REG Ialpha protein was expressed rarely in ductal epithelial cells of the normal MSG but was apparently overexpressed in those of patients with SS. The labelling indices for both Ki67 and ssDNA in the ductal cells of the MSGs were significantly higher in SS patients than in controls. Moreover, these labelling indices were significantly higher in REG Ialpha-positive than in negative SS patients. REG Ialpha protein may play a role in the regeneration of ductal epithelial cells in the MSGs of patients with SS.

Adhesion and protease activity in cell lines from human salivary gland tumors are regulated by the laminin-derived peptide AG73, syndecan-1 and beta1 integrin

We studied the induction of protease activity by the laminin alpha1-derived peptide AG73 in cells from adenoid cystic carcinoma (CAC2) and myoepithelioma (M1), respectively a malignant and a benign salivary gland tumors. Laminin alpha1 chain and MMP9 were immunolocalized in adenoid cystic carcinoma and myoepithelioma in vivo and in vitro. Cells grown inside AG73-enriched laminin-111 exhibited large spaces in the extracellular matrix, suggestive of remodeling. The broad spectrum MMP inhibitor GM6001 decreased spaces induced by AG73 in CAC2 and M1 cells. This result strongly suggests that AG73-mediated matrix remodeling involves matrix metalloproteinases. CAC2 and M1 cells cultured on AG73 showed a dose-dependent increase of MMP9 secretion, as detected by zymography. Furthermore, siRNA silencing of MMP9 decreased remodeling in 3D cultures. We searched for AG73 receptors regulating MMP9 activity in our cell lines. CAC2 and M1 cells grown on AG73 exhibited colocalization of syndecan-1 and beta1 integrin. siRNA knockdown of syndecan-1 expression in these cells resulted in decreased adhesion to AG73 and reduced protease and remodeling activity. We investigated syndecan-1 co-receptors in both cell lines. Silencing beta1 integrin inhibited adhesion to AG73, matrix remodeling and protease activity. Double-knockdown experiments were carried out to further explore syndecan-1 and beta1 integrin cooperation. CAC2 cells transfected with both syndecan-1 and beta1 integrin siRNA oligos showed significant decrease in adhesion to AG73. Simultaneous silencing of receptors also induced a decrease in protease activity. Our results suggest that syndecan-1 and beta1 integrin signaling downstream of AG73 regulate adhesion and MMP production by CAC2 and M1 cells.

Temporary accumulation of glycogen in the epithelial cells of the developing mouse submandibular gland

Temporary accumulation of glycogen in the epithelial cells of the developing mouse submandibular gland was examined under light microscopic histochemistry and electron microscopy. To avoid loss of water-soluble glycogen during histological tissue preparation, fixation with ethanol and embedding in hydrophilic glycol methacrylate resin was used for light microscopy, and high-pressure freezing/freeze substitution for electron microscopy. Glycogen was detected on periodic acid-Schiff stain, periodic acid-thiosemicarbazide-silver proteinate reaction, and the digestion test with alpha-amylase. On embryonic day 14, glycogen began to accumulate in the proximal portions of the developing epithelial cords. On embryonic day 17, marked glycogen particles were seen at the basal portion of the ductal epithelial cells and an abrupt increase of glycogen accumulation occurred in the secretory cells in the terminal bulbs. Ultrastructural observation indicated large clumps of glycogen particles localized in the basal portion of the terminal bulb cells. The initiation of glycogen accumulation preceded the formation of lumens in the ducts and terminal bulbs. Furthermore, proliferation analysis by bromodeoxyuridine labeling showed that this glycogen accumulation followed the cessation of the epithelial cell proliferation. Postnatally, glycogen accumulation in the terminal bulbs became gradually inconspicuous and completely disappeared by postnatal day 3, but that in the ducts was retained until around postnatal day 12. Temporary glycogen accumulation after the cell proliferation and before/during the lumen formation and secretory granule formation suggests significant involvement of the carbohydrate metabolism in the organogenesis of the submandibular gland.

SIKVAV, a laminin alpha1-derived peptide, interacts with integrins and increases protease activity of a human salivary gland adenoid cystic carcinoma cell line through the ERK 1/2 signaling pathway

Adenoid cystic carcinoma is a frequently occurring malignant salivary gland neoplasm. We studied the induction of protease activity by the laminin-derived peptide, SIKVAV, in cells (CAC2) derived from this neoplasm. Laminin alpha1 and matrix metalloproteinases (MMPs) 2 and 9 were immunolocalized in adenoid cystic carcinoma cells in vivo and in vitro. CAC2 cells cultured on SIKVAV showed a dose-dependent increase of MMP9 as detected by zymography and colocalization of alpha3 and alpha6 integrins. Small interfering RNA (siRNA) knockdown of integrin expression in CAC2 cells resulted in decreased adhesion to the peptide. SIKVAV affinity chromatography and immunoblot analysis showed that alpha3, alpha6, and beta1 integrins were eluted from the SIKVAV column, which was confirmed by mass spectrometry and a solid-phase binding assay. Small interfering RNA experiments also showed that these integrins, through extracellular signal-regulated kinase (ERK) 1/2 signaling, regulate MMP secretion induced by SIKVAV in CAC2 cells. We propose that SIKVAV increases protease activity of a human salivary gland adenoid cystic carcinoma cell line through alpha3beta1 and alpha6beta1 integrins and the ERK 1/2 signaling pathway.

Semaphorin signaling facilitates cleft formation in the developing salivary gland

Semaphorin signaling plays integral roles in multiple developmental processes. Branching morphogenesis is one such role that has not been thoroughly explored. Here, we show in mice that functional blockage of neuropilin 1 (Npn1) inhibits cleft formation in the developing submandibular gland (SMG) cultured ex vivo. This Npn1-dependent morphogenesis is mediated by Sema3A and Sema3C in an additive manner, and can be abolished by decreasing the expression of plexin A2 or plexin D1. VEGF, another known Npn1 ligand, has no apparent effects on SMG development. FGF signaling, which also mediates SMG branching morphogenesis, acts in parallel with semaphorin signaling. Finally, in contrast to the effect of FGF signaling, we find that semaphorins do not stimulate the proliferation of SMG epithelial cells. Instead, the semaphorin signals act locally on the epithelial cells to facilitate SMG cleft formation.

Laminin alpha5 is necessary for submandibular gland epithelial morphogenesis and influences FGFR expression through beta1 integrin signaling

Laminin alpha chains have unique spatiotemporal expression patterns during development and defining their function is necessary to understand the regulation of epithelial morphogenesis. We investigated the function of laminin alpha5 in mouse submandibular glands (SMGs). Lama5(-/-) SMGs have a striking phenotype: epithelial clefting is delayed, although proliferation occurs; there is decreased FGFR1b and FGFR2b, but no difference in Lama1 expression; later in development, epithelial cell organization and lumen formation are disrupted. In wild-type SMGs alpha5 and alpha1 are present in epithelial clefts but as branching begins alpha5 expression increases while alpha1 decreases. Lama5 siRNA decreased branching, p42 MAPK phosphorylation, and FGFR expression, and branching was rescued by FGF10. FGFR siRNA decreased Lama5 suggesting that FGFR signaling provides positive feedback for Lama5 expression. Anti-beta1 integrin antibodies decreased FGFR and Lama5 expression, suggesting that beta1 integrin signaling provides positive feedback for Lama5 and FGFR expression. Interestingly, the Itga3(-/-):Itga6(-/-) SMGs have a similar phenotype to Lama5(-/-). Our findings suggest that laminin alpha5 controls SMG epithelial morphogenesis through beta1 integrin signaling by regulating FGFR expression, which also reciprocally regulates the expression of Lama5. These data link changes in basement membrane composition during branching morphogenesis with FGFR expression and signaling.

Demystifying the effects of a three-dimensional microenvironment in tissue morphogenesis

Tissue morphogenesis and homeostasis are dependent on a complex dialogue between multiple cell types and chemical and physical cues in the surrounding microenvironment. The emergence of engineered three-dimensional (3D) tissue constructs and the development of tractable methods to recapitulate the native tissue microenvironment ex vivo has led to a deeper understanding of tissue-specific behavior. However, much remains unclear about how the microenvironment and aberrations therein directly affect tissue morphogenesis and behavior. Elucidating the role of the microenvironment in directing tissue-specific behavior will aid in the development of surrogate tissues and tractable approaches to diagnose and treat chronic-debilitating diseases such as cancer and atherosclerosis. Toward this goal, 3D organotypic models have been developed to clarify the mechanisms of epithelial morphogenesis and the subsequent maintenance of tissue homeostasis. Here we describe the application of these 3D culture models to illustrate how the microenvironment plays a critical role in regulating mammary tissue function and signaling, and discuss the rationale for applying precisely defined organotypic culture assays to study epithelial cell behavior. Experimental methods are provided to generate and manipulate 3D organotypic cultures to study the effect of matrix stiffness and matrix dimensionality on epithelial tissue morphology and signaling. We end by discussing technical limitations of currently available systems and by presenting opportunities for improvement.

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.

Lysophosphatidic acid cooperates with EGF in inducing branching morphogenesis of embryonic mouse salivary epithelium

Epithelial morphogenesis is supported by diffusible growth factors and by nondiffusible cell substrata, such as laminin and fibronectin. When embedded in a laminin-rich basement-membrane substratum, embryonic mouse submandibular epithelium undergoes cell proliferation and branching morphogenesis in response to epidermal growth factor (EGF) in mesenchyme-free culture but not in serum-free medium. In this study, we sought to identify the biologically active factor in serum. As this factor was heat-stable and trypsin-resistant, the lipid fraction was analyzed. Horse serum was fractionated by ethanol extraction, Folch partition with chloroform-methanol-water, and high-performance liquid chromatography, and we tested the branch-inducing activity of each fraction. We also analyzed the partially purified fraction with a mass spectrometer, indicating that the active fraction largely consisted of lysophosphatidyl-hexose. Finally we identified the molecule as lysophosphatidic acid (LPA), because, whereas lysophosphatidyl-inositol had only a slight branch-inducing activity, its relevant LPA fully substituted for serum and induced branching morphogenesis in cooperation with EGF. LPA receptor genes were expressed in submandibular epithelial cells. DNA-synthesizing cells were abundant only when cultured in the presence of both EGF and LPA, but not either singly.