Gene expression profiles of mouse submandibular gland development: FGFR1 regulates branching morphogenesis in vitro through BMP- and FGF-dependent mechanisms

Salivary Gland Tissue Recombination Can Modify Cell Fate

Although mesenchyme is essential for inducing the epithelium of ectodermal organs, its precise role in organ-specific epithelial fate determination remains poorly understood. To elucidate the roles of tissue interactions in cellular differentiation, we performed single-cell RNA sequencing and imaging analyses on recombined tissues, where mesenchyme and epithelium were switched ex vivo between two types of embryonic mouse salivary glands: the parotid gland (a serous gland) and the submandibular gland (a predominantly mucous gland). We found partial induction of molecules that define gland-specific acinar and myoepithelial cells in recombined salivary epithelium. The parotid epithelium recombined with submandibular mesenchyme began to express mucous acinar genes not intrinsic to the parotid gland. While myoepithelial cells do not normally line parotid acini, newly induced myoepithelial cells densely populated recombined parotid acini. However, mucous acinar and myoepithelial markers continued to be expressed in submandibular epithelial cells recombined with parotid mesenchyme. Consequently, some epithelial cells appeared to be plastic, such that their fate could still be modified in response to mesenchymal signaling, whereas other epithelial cells appeared to be already committed to a specific fate. We also discovered evidence for bidirectional induction: transcriptional changes were observed not only in the epithelium but also in the mesenchyme after heterotypic tissue recombination. For example, parotid epithelium induced the expression of muscle-related genes in submandibular fibroblasts that began to mimic parotid fibroblast gene expression. These studies provide the first comprehensive unbiased molecular characterization of tissue recombination approaches exploring the regulation of cell fate.

Design principles and therapeutic applications of novel synthetic WNT signaling agonists

Wingless-related integration site or Wingless and Int-1 or Wingless-Int (WNT) signaling is crucial for embryonic development, and adult tissue homeostasis and regeneration, through its essential roles in cell fate, patterning, and stem cell regulation. The biophysical characteristics of WNT ligands have hindered efforts to interrogate ligand activity in vivo and prevented their development as therapeutics. Recent breakthroughs have enabled the generation of synthetic WNT signaling molecules that possess characteristics of natural ligands and potently activate the pathway, while also providing distinct advantages for therapeutic development and manufacturing. This review provides a detailed discussion of the protein engineering of these molecular platforms for WNT signaling agonism. We discuss the importance of WNT signaling in several organs and share insights from the initial application of these new classes of molecules in vitro and in vivo. These molecules offer a unique opportunity to enhance our understanding of how WNT signaling agonism promotes tissue repair, enabling targeted development of tailored therapeutics.

Generation of salivary glands derived from pluripotent stem cells via conditional blastocyst complementation

Whole salivary gland generation and transplantation offer potential therapies for salivary gland dysfunction. However, the specific lineage required to engineer complete salivary glands has remained elusive. In this study, we identify the Foxa2 lineage as a critical lineage for salivary gland development through conditional blastocyst complementation (CBC). Foxa2 lineage marking begins at the boundary between the endodermal and ectodermal regions of the oral epithelium before the formation of the primordial salivary gland, thereby labeling the entire gland. Ablation of Fgfr2 within the Foxa2 lineage in mice leads to salivary gland agenesis. We reversed this phenotype by injecting donor pluripotent stem cells into the mouse blastocysts, resulting in mice that survived to adulthood with salivary glands of normal size, comparable to those of their littermate controls. These findings demonstrate that CBC-based salivary gland regeneration serves as a foundational experimental approach for future advanced cell-based therapies.

ΔNp63 regulates Sfrp1 expression to direct salivary gland branching morphogenesis

Branching morphogenesis is a complex process shared by many organs including the lungs, kidney, prostate, as well as several exocrine organs including the salivary, mammary and lacrimal glands. This critical developmental program ensures the expansion of an organ's surface area thereby maximizing processes of cellular secretion or absorption. It is guided by reciprocal signaling from the epithelial and mesenchymal cells. While signaling pathways driving salivary gland branching morphogenesis have been relatively well-studied, our understanding of the underlying transcriptional regulatory mechanisms directing this program, is limited. Here, we performed in vivo and ex vivo studies of the embryonic mouse submandibular gland to determine the function of the transcription factor ΔNp63, in directing branching morphogenesis. Our studies show that loss of ΔNp63 results in alterations in the differentiation program of the ductal cells which is accompanied by a dramatic reduction in branching morphogenesis that is mediated by dysregulation of WNT signaling. We show that ΔNp63 modulates WNT signaling to promote branching morphogenesis by directly regulating Sfrp1 expression. Collectively, our findings have revealed a novel role for ΔNp63 in the regulation of this critical process and offers a better understanding of the transcriptional networks involved in branching morphogenesis.

Ser252Trp mutation in fibroblast growth factor receptor 2 promotes branching morphogenesis in mouse salivary glands

OBJECTIVES

The purpose of this study was to perform morphological and immunohistochemical (IHC) analysis of the submandibular glands (SMGs) in early development in Apert syndrome model mice (Ap mice).

METHODS

ACTB-Cre homozygous mice were mated with fibroblast growth factor receptor 2 (Fgfr2+/Neo-S252W) mice; ACTB-Cre heterozygous mice (ACTB-Cre mice) at embryonic day (E) 13.5 served as the control group, and Fgfr2+/S252W mice (Ap mice) served as the experimental group. Hematoxylin and eosin (H&E) staining was performed on SMGs; Total SMG area and epithelial area were determined, and the epithelial occupancy ratio was calculated. Immunostaining was performed to assess the localization of FGF signaling-related proteins. Next, bromodeoxyuridine (BrdU)-positive cells were evaluated to assess cell proliferation. Finally, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was performed to assess apoptosis in SMGs.

RESULTS

The epithelial occupancy ratio was significantly higher in SMGs of Ap mice compared with that in SMGs of controls. FGF7 and bone morphogenetic protein 4 (BMP4) exhibited different localizations in SMGs of Ap mice compared with SMGs of controls. Cell proliferation was higher in SMGs of Ap mice compared with that of controls; however, apoptosis did not different significantly between the two groups.

CONCLUSION

Our results suggest that enhanced FGF signaling conferred by missense mutations in FGFR2 promotes branching morphogenesis in SMGs of Ap mice.

Fibrin hydrogels fortified with FGF-7/10 and laminin-1 peptides promote regeneration of irradiated salivary glands

Ionizing radiation, commonly used for head and neck cancer treatment, typically damages the salivary glands, resulting in hyposalivation. The development of treatments to restore this lost function is crucial for improving the quality of life for patients suffering from this condition. To address this clinical need, we have developed an innovative hydrogel by chemically conjugating laminin-1 peptides (A99 and YIGSR) and growth factors, FGF-7 and FGF-10, to fibrin hydrogels. Our results demonstrate that FGF-7/10 and laminin-1 peptides fortified fibrin hydrogel [enhanced laminin-1 peptides fibrin hydrogel (Ep-FH)] promotes salivary gland regeneration and functionality by improving epithelial tissue organization, establishing a healthy network of blood vessels and nerves, while reducing fibrosis in a head and neck irradiated mouse model. These results indicate that fibrin hydrogel-based implantable scaffolds containing pro-regenerative signals promote sustained secretory function of irradiated salivary glands, offering a potential alternative treatment for hyposalivation in head and neck cancer patients undergoing radiation treatment. These unique findings emphasize the potential of fibrin hydrogel-based implantable scaffolds enriched with pro-regenerative signals in sustaining the secretory function of irradiated salivary glands and offer a promising alternative treatment for addressing hyposalivation in head and neck cancer patients undergoing radiation therapy. STATEMENT OF SIGNIFICANCE: Radiation therapies used to treat head and neck cancers often result in damaged salivary gland, leading to severe dryness of the oral cavity. In this study, we engineered FGF-7 and FGF-10 and immobilized them into L1p-FH. The resulting hydrogel, Ep-FH, restored irradiated salivary gland functionality by enhancing epithelial tissue organization, promoting the development of a healthy network of blood vessels and nerves as well as reduction of fibrosis.

Generation of salivary glands derived from pluripotent stem cells via conditional blastocyst complementation

Various patients suffer from dry mouth due to salivary gland dysfunction. Whole salivary gland generation and transplantation is a potential therapy to resolve this issue. However, the lineage permissible to design the entire salivary gland generation has been enigmatic. Here, we discovered Foxa2 as a lineage critical for generating a salivary gland via conditional blastocyst complementation (CBC). Foxa2 linage, but not Shh nor Pitx2, initiated to label between the boundary region of the endodermal and the ectodermal oral mucosa before primordial salivary gland formation, resulting in marking the entire salivary gland. The salivary gland was agenesis by depleting Fgfr2 under the Foxa2 lineage in the mice. We rescued this phenotype by injecting donor pluripotent stem cells into the mouse blastocysts. Those mice survived until adulthood with normal salivary glands compatible in size compared with littermate controls. These results indicated that CBC-based salivary gland generation is promising for next-generation cell-based therapy.

BMP-2-mediated signaling suppresses salivary gland development

Research regarding the process of salivary gland development and elucidation of related mechanisms are considered essential for development of effective treatments for conditions associated with salivary disease. Various reports regarding the effects of bone morphogenetic protein (BMP)-2 on hard tissue cells have been presented, though few have examined those related to salivary gland formation. Using an organ culture system, the present study was conducted to investigate the function of BMP-2 in salivary gland formation. Salivary glands obtained from embryonic day 13.5 mice and treated with BMP-2 showed suppression of primordial cell differentiation and also gland formation in a concentration-dependent manner. Furthermore, gland formation inhibition was suppressed by concurrent treatment with dorsomorphin, an inhibitor of the Smad pathway. Expression levels of AQP5, a marker gene for acinar cells, and Prol1, an opiorphin expressed in the lacrimal gland, were decreased in salivary glands treated with BMP-2. The present findings indicate that suppression of salivary gland formation, especially acinar differentiation, is induced by BMP-2, a phenomenon considered to be related to the Smad pathway.

CSF1R-dependent macrophages in the salivary gland are essential for epithelial regeneration after radiation-induced injury

The salivary glands often become damaged in individuals receiving radiotherapy for head and neck cancer, resulting in chronic dry mouth. This leads to detrimental effects on their health and quality of life, for which there is no regenerative therapy. Macrophages are the predominant immune cell in the salivary glands and are attractive therapeutic targets due to their unrivaled capacity to drive tissue repair. Yet, the nature and role of macrophages in salivary gland homeostasis and how they may contribute to tissue repair after injury are not well understood. Here, we show that at least two phenotypically and transcriptionally distinct CX3CR1+ macrophage populations are present in the adult salivary gland, which occupy anatomically distinct niches. CD11c+CD206-CD163- macrophages typically associate with gland epithelium, whereas CD11c-CD206+CD163+ macrophages associate with blood vessels and nerves. Using a suite of complementary fate mapping systems, we show that there are highly dynamic changes in the ontogeny and composition of salivary gland macrophages with age. Using an in vivo model of radiation-induced salivary gland injury combined with genetic or antibody-mediated depletion of macrophages, we demonstrate an essential role for macrophages in clearance of cells with DNA damage. Furthermore, we show that epithelial-associated macrophages are indispensable for effective tissue repair and gland function after radiation-induced injury, with their depletion resulting in reduced saliva production. Our data, therefore, provide a strong case for exploring the therapeutic potential of manipulating macrophages to promote tissue repair and thus minimize salivary gland dysfunction after radiotherapy.

Submandibular gland epithelial development and the importance of junctions

Salivary glands consist of highly specialized epithelial cells that secrete the fluid, saliva, and/or transport saliva into the oral cavity. Saliva is essential to lubricate the oral cavity for food consumption and to maintain the hygiene of the oral cavity. In this review, we will focus on the formation of the epithelial cell lineage and the cell junctions that are essential for formation of saliva and maintenance of the epithelial barrier between the ducts that transport saliva and the extracellular environment.

Primer on FGF3

Though initially discovered as a proto-oncogene in virally induced mouse mammary tumors, FGF3 is primarily active in prenatal stages, where it is found at various sites at specific times. FGF3 is crucial during development, as its roles include tail formation, inner ear development and hindbrain induction and patterning. FGF3 expression and function are highly conserved in vertebrates, while it also interacts with other FGFs in various developmental processes. Intriguingly, while it is classified as a classical paracrine signaling factor, murine FGF3 was uniquely found to also act in an intracrine manner, depending on alternative translation initiation sites. Corresponding with its conserved role in inner ear morphogenesis, mutations in FGF3 in humans are associated with LAMM syndrome, a disorder that include hearing loss and inner ear malformations. While recent studies indicate of some FGF3 presence in post-natal stages, emerging evidences of its upregulation in various human tumors and cariogenic processes in mouse models, highlights the importance of its close regulation in adult tissues. Altogether, the broad and dynamic expression pattern and regulation of FGF3 in embryonic and adult tissues together with its link to congenital malformations and cancer, calls for further discoveries of its diverse roles in health and disease.

Autologous mesenchymal stem cells offer a new paradigm for salivary gland regeneration

Salivary gland (SG) dysfunction, due to radiotherapy, disease, or aging, is a clinical manifestation that has the potential to cause severe oral and/or systemic diseases and compromise quality of life. Currently, the standard-of-care for this condition remains palliative. A variety of approaches have been employed to restore saliva production, but they have largely failed due to damage to both secretory cells and the extracellular matrix (niche). Transplantation of allogeneic cells from healthy donors has been suggested as a potential solution, but no definitive population of SG stem cells, capable of regenerating the gland, has been identified. Alternatively, mesenchymal stem cells (MSCs) are abundant, well characterized, and during SG development/homeostasis engage in signaling crosstalk with the SG epithelium. Further, the trans-differentiation potential of these cells and their ability to regenerate SG tissues have been demonstrated. However, recent findings suggest that the "immuno-privileged" status of allogeneic adult MSCs may not reflect their status post-transplantation. In contrast, autologous MSCs can be recovered from healthy tissues and do not present a challenge to the recipient's immune system. With recent advances in our ability to expand MSCs in vitro on tissue-specific matrices, autologous MSCs may offer a new therapeutic paradigm for restoration of SG function.

FGF signaling regulates salivary gland branching morphogenesis by modulating cell adhesion

Loss of FGF signaling leads to defects in salivary gland branching, but the mechanisms underlying this phenotype remain largely unknown. We disrupted expression of Fgfr1 and Fgfr2 in salivary gland epithelial cells and found that both receptors function coordinately in regulating branching. Strikingly, branching morphogenesis in double knockouts is restored by Fgfr1 and Fgfr2 (Fgfr1/2) knock-in alleles incapable of engaging canonical RTK signaling, suggesting that additional FGF-dependent mechanisms play a role in salivary gland branching. Fgfr1/2 conditional null mutants showed defective cell-cell and cell-matrix adhesion, both of which have been shown to play instructive roles in salivary gland branching. Loss of FGF signaling led to disordered cell-basement membrane interactions in vivo as well as in organ culture. This was partially restored upon introducing Fgfr1/2 wild-type or signaling alleles that are incapable of eliciting canonical intracellular signaling. Together, our results identify non-canonical FGF signaling mechanisms that regulate branching morphogenesis through cell-adhesion processes.

Developmental function of Piezo1 in mouse submandibular gland morphogenesis

Mechanically activated factors are important in organogenesis, especially in the formation of secretory organs, such as salivary glands. Piezo-type mechanosensitive ion channel component 1 (Piezo1), although previously studied as a physical modulator of the mechanotransduction, was firstly evaluated on its developmental function in this study. The detailed localization and expression pattern of Piezo1 during mouse submandibular gland (SMG) development were analyzed using immunohistochemistry and RT-qPCR, respectively. The specific expression pattern of Piezo1 was examined in acinar-forming epithelial cells at embryonic day 14 (E14) and E16, which are important developmental stages for acinar cell differentiation. To understand the precise function of Piezo1 in SMG development, siRNA against Piezo1 (siPiezo1) was employed as a loss-of-function approach, during in vitro organ cultivation of SMG at E14 for the designated period. Alterations in the histomorphology and expression patterns of related signaling molecules, including Bmp2, Fgf4, Fgf10, Gli1, Gli3, Ptch1, Shh, and Tgfβ-3, were examined in acinar-forming cells after 1 and 2 days of cultivation. Particularly, altered localization patterns of differentiation-related signaling molecules including Aquaporin5, E-cadherin, Vimentin, and cytokeratins would suggest that Piezo1 modulates the early differentiation of acinar cells in SMGs by modulating the Shh signaling pathway.

Id4 modulates salivary gland homeostasis and its expression is downregulated in IgG4-related disease via miR-486-5p

Salivary glands are physiologically orchestrated by the coordinated balance between cell differentiation, proliferation, apoptosis, and interactions between epithelial, mesenchymal endothelial, and neuronal cells, and they are frequent sites of manifestations of Sjögren's syndrome (SS) or IgG4-related disease (IgG4-RD). However, little is known about salivary gland homeostasis and its involvement in those diseases. Inhibitor of DNA binding/differentiation 4 (Id4) is an Id protein involved in the transcriptional control of many biological events, including differentiation. Studies of Id4-deficient mice revealed that Id4-deficient submandibular glands were smaller and exhibited accelerated differentiation, compared with those from wild-type littermates. In addition, dry mouth symptoms and Th17 expansion in splenocytes were also observed in the absence of Id4. Furthermore, Id4 levels in the salivary glands of patients with IgG4-RD, but not SS, were significantly decreased compared with those of healthy controls. miRNA-mRNA integrated analysis demonstrated that miR-486-5p was upregulated in IgG4-RD patients and that it might regulate Id4 in the lesion sites. Together, these results provide evidence for the inhibitory role of Id4 in salivary differentiation, and a critical association between Id4 downregulation and IgG4-RD.

Sci-Seq of Human Fetal Salivary Tissue Introduces Human Transcriptional Paradigms and a Novel Cell Population

Multiple pathologies and non-pathological factors can disrupt the function of the non-regenerative human salivary gland including cancer and cancer therapeutics, autoimmune diseases, infections, pharmaceutical side effects, and traumatic injury. Despite the wide range of pathologies, no therapeutic or regenerative approaches exist to address salivary gland loss, likely due to significant gaps in our understanding of salivary gland development. Moreover, identifying the tissue of origin when diagnosing salivary carcinomas requires an understanding of human fetal development. Using computational tools, we identify developmental branchpoints, a novel stem cell-like population, and key signaling pathways in the human developing salivary glands by analyzing our human fetal single-cell sequencing data. Trajectory and transcriptional analysis suggest that the earliest progenitors yield excretory duct and myoepithelial cells and a transitional population that will yield later ductal cell types. Importantly, this single-cell analysis revealed a previously undescribed population of stem cell-like cells that are derived from SD and expresses high levels of genes associated with stem cell-like function. We have observed these rare cells, not in a single niche location but dispersed within the developing duct at later developmental stages. Our studies introduce new human-specific developmental paradigms for the salivary gland and lay the groundwork for the development of translational human therapeutics.

The Semaphorin 3A-AKT axis-mediated cell proliferation in salivary gland morphogenesis and adenoid cystic carcinoma pathogenesis

We recently demonstrated that Semaphorin 3 A (Sema3A), the expression of which is negatively regulated by Wnt/β-catenin signaling, promotes odontogenic epithelial cell proliferation, suggesting the involvement of Sema3A in tooth germ development. Salivary glands have a similar developmental process to tooth germ development, in which reciprocal interactions between the oral epithelium and adjacent mesenchyme proceeds via stimulation with several growth factors; however, the role of Sema3A in the development of salivary glands is unknown. There may thus be a common mechanism between epithelial morphogenesis and pathogenesis; however, the role of Sema3A in salivary gland tumors is also unclear. The current study investigated the involvement of Sema3A in submandibular gland (SMG) development and its expression in adenoid cystic carcinoma (ACC) specimens. Quantitative RT-PCR and immunohistochemical analyses revealed that Sema3A was expressed both in epithelium and in mesenchyme in the initial developmental stages of SMG and their expressions were decreased during the developmental processes. Loss-of-function experiments using an inhibitor revealed that Sema3A was required for AKT activation-mediated cellular growth and formation of cleft and bud in SMG rudiment culture. In addition, Wnt/β-catenin signaling decreased the Sema3A expression in the rudiment culture. ACC arising from salivary glands frequently exhibits malignant potential. Immunohistochemical analyses of tissue specimens obtained from 10 ACC patients showed that Sema3A was hardly observed in non-tumor regions but was strongly expressed in tumor lesions, especially in myoepithelial neoplastic cells, at high frequencies where phosphorylated AKT expression was frequently detected. These results suggest that the Sema3A-AKT axis promotes cell growth, thereby contributing to morphogenesis and pathogenesis, at least in ACC, of salivary glands.

Bioengineering in salivary gland regeneration

Salivary gland (SG) dysfunction impairs the life quality of many patients, such as patients with radiation therapy for head and neck cancer and patients with Sjögren’s syndrome. Multiple SG engineering strategies have been considered for SG regeneration, repair, or whole organ replacement. An in-depth understanding of the development and differentiation of epithelial stem and progenitor cells niche during SG branching morphogenesis and signaling pathways involved in cell–cell communication constitute a prerequisite to the development of suitable bioengineering solutions. This review summarizes the essential bioengineering features to be considered to fabricate an engineered functional SG model using various cell types, biomaterials, active agents, and matrix fabrication methods. Furthermore, recent innovative and promising approaches to engineering SG models are described. Finally, this review discusses the different challenges and future perspectives in SG bioengineering.

Growth factor dependence of the proliferation and survival of cultured lacrimal gland epithelial cells isolated from late-embryonic mice

Epidermal growth factor (EGF) and hepatocyte growth factor (HGF) regulate the growth and morphogenesis of various exocrine glands with branched morphologies. Their roles in lacrimal gland (LG) development remain unknown, but fibroblast growth factor (FGF) 10 is crucial for early LG organogenesis. To clarify the roles of EGF, HGF, and FGF10 in LG development, LG epithelial cells were isolated from late-embryonic and neonatal mice; cultured; and treated with EGF, HGF, or FGF10 and their respective receptor tyrosine kinase (RTK) inhibitors AG1478, PHA665752, or SU5402. EGF and HGF increased the number of viable cells by enhancing DNA synthesis, FGF10 and SU5402 showed no such effect, and RTK inhibitors exhibited the opposite effect. EGF and HGF receptors were immunostained in cultured late-embryonic LG epithelial cells and terminal LG acini from late embryos and adult mice. HGF was detected in neonatal LG epithelial cell culture supernatants by western blotting. In the absence of EGF and HGF RTK inhibitors, growth factor addition increased the number of viable cells and suppressed cell death. However, when one RTK was inhibited and a growth factor targeting an intact RTK was added, the number of dead cells increased as the number of viable cells increased. No cells survived when both RTKs were inhibited. In explant cultures of LGs from embryos, AG1478 or PHA665752 decreased the number of Ki67-positive proliferating epithelial cells in terminal acini. Thus, EGF and HGF may function in a cooperative autocrine manner, supporting cell proliferation and survival during LG development in late-embryonic and neonatal mice.

Single-cell RNA sequencing reveals PDGFRα+ stromal cell subpopulations that promote proacinar cell differentiation in embryonic salivary gland organoids

Stromal cells can direct the differentiation of epithelial progenitor cells during organ development. Fibroblast growth factor (FGF) signaling is essential for submandibular salivary gland development. Through stromal fibroblast cells, FGF2 can indirectly regulate proacinar cell differentiation in organoids, but the mechanisms are not understood. We performed single-cell RNA-sequencing and identified multiple stromal cell subsets, including Pdgfra+ stromal subsets expressing both Fgf2 and Fgf10. When combined with epithelial progenitor cells in organoids, magnetic-activated cell-sorted PDGFRα+ cells promoted proacinar cell differentiation similarly to total stroma. Gene expression analysis revealed that FGF2 increased the expression of multiple stromal genes, including Bmp2 and Bmp7. Both BMP2 and BMP7 synergized with FGF2, stimulating proacinar cell differentiation but not branching. However, stromal cells grown without FGF2 did not support proacinar organoid differentiation and instead differentiated into myofibroblasts. In organoids, TGFβ1 treatment stimulated myofibroblast differentiation and inhibited the proacinar cell differentiation of epithelial progenitor cells. Conversely, FGF2 reversed the effects of TGFβ1. We also demonstrated that adult salivary stromal cells were FGF2 responsive and could promote proacinar cell differentiation. These FGF2 signaling pathways may have applications in future regenerative therapies.

Spatiotemporal Expression Patterns of Critical Genes Involved in FGF Signaling During Morphogenesis and Odontogenesis of Deciduous Molars in Miniature Pigs

The fibroblast growth factor (FGF) pathway plays an important role in epithelial-mesenchymal interactions during tooth development. Nevertheless, how the ligands, receptors, and antagonists of the FGF pathway are involved in epithelial-mesenchymal interactions remains largely unknown. Miniature pigs exhibit tooth anatomy and replacement patterns like those in humans and hence can serve as large animal models. The present study investigated the spatiotemporal expression patterns of critical genes encoding FGF ligands (FGF3, FGF4, FGF7, and FGF9), antagonists (SPRY2 and SPRY4) and receptors (FGFR1, FGFR2, and FGFR3) in the third deciduous molars of miniature pigs at the cap (embryonic day 40, E40), early bell (E50), and late bell (E60) stages. The results of in situ hybridization (ISH) with tyramide signal amplification and of qRT-PCR analysis revealed increased expression of FGF7, FGFR1, FGFR2, and SPRY4 in dental epithelium and of FGF7 and FGFR1 in mesenchyme from E40 to E50. In contrast, the results revealed decreased expression of FGF3, FGF4, FGF9, and FGFR3 in dental epithelium and of FGF4, FGF9, FGFR2, and FGFR3 in the mesenchyme from E40 to E60. Mesenchyme signals of FGF3, FGF4, FGF7, SPRY2, FGFR2, and FGFR3 were concentrated in the odontoblast layer from E50 to E60. The distinct expression patterns of these molecules indicated elaborate regulation during dental morphogenesis. Our results provide a foundation for further investigation into fine-tuning dental morphogenesis and odontogenesis by controlling interactions between dental epithelium and mesenchyme, thus promoting tooth regeneration in large mammals.

The role of salivary gland macrophages in infection, disease and repair

Macrophages are mononuclear innate immune cells which have become of increasing interest in the fields of disease and regeneration, as their non-classical functions have been elucidated in addition to their classical inflammatory functions. Macrophages can regulate tissue remodeling, by both mounting and reducing inflammatory responses; and exhibit direct communication with other cells to drive tissue turnover and cell replacement. Furthermore, macrophages have recently become an attractive therapeutic target to drive tissue regeneration. The major salivary glands are glandular tissues that are exposed to pathogens through their close connection with the oral cavity. Moreover, there are a number of diseases that preferentially destroy the salivary glands, causing irreversible injury, highlighting the need for a regenerative strategy. However, characterization of macrophages in the mouse and human salivary glands is sparse and has been mostly determined from studies in infection or autoimmune pathologies. In this review, we describe the current literature around salivary gland macrophages, and speculate about the niches they inhabit and how their role in development, regeneration and cancer may inform future therapeutic advances.

Loss of Hs3st3a1 or Hs3st3b1 enzymes alters heparan sulfate to reduce epithelial morphogenesis and adult salivary gland function

Heparan sulfate 3-O-sulfotransferases generate highly sulfated but rare 3-O-sulfated heparan sulfate (HS) epitopes on cell surfaces and in the extracellular matrix. Previous ex vivo experiments suggested functional redundancy exists among the family of seven enzymes but that Hs3st3a1 and Hs3st3b1 sulfated HS increases epithelial FGFR signaling and morphogenesis. Single-cell RNAseq analysis of control SMGs identifies increased expression of Hs3st3a1 and Hs3st3b1 in endbud and myoepithelial cells, both of which are progenitor cells during development and regeneration. To analyze their in vivo functions, we generated both Hs3st3a1^-/- and Hs3st3b1^-/- single knockout mice, which are viable and fertile. Salivary glands from both mice have impaired fetal epithelial morphogenesis when cultured with FGF10. Hs3st3b1^-/- mice have reduced intact SMG branching morphogenesis and reduced 3-O-sulfated HS in the basement membrane. Analysis of HS biosynthetic enzyme transcription highlighted some compensatory changes in sulfotransferases expression early in development. The overall glycosaminoglycan composition of adult control and KO mice were similar, although HS disaccharide analysis showed increased N- and non-sulfated disaccharides in Hs3st3a1^-/- HS. Analysis of adult KO gland function revealed normal secretory innervation, but without stimulation there was an increase in frequency of drinking behavior in both KO mice, suggesting basal salivary hypofunction, possibly due to myoepithelial dysfunction. Understanding how 3-O-sulfation regulates myoepithelial progenitor function will be important to manipulate HS-binding growth factors to enhance tissue function and regeneration.

3D Organoid Culture From Adult Salivary Gland Tissues as an ex vivo Modeling of Salivary Gland Morphogenesis

Lumen formation of salivary glands has been investigated using in vivo or ex vivo rudiment culture models. In this study, we used a three-dimensional (3D) salivary gland organoid culture system and demonstrated that lumen formation could be recapitulated in mouse SMG organoids. In our organoid culture system, lumen formation was induced by vasoactive intestinal peptide and accelerated by treatment with RA. Furthermore, lumen formation was observed in branching duct-like structure when cultured in combination of fibroblast growth factors (FGF) in the presence of retinoic acid (RA). We suggest RA signaling-mediated regulation of VIPR1 and KRT7 as the underlying mechanism for lumen formation, rather than apoptosis in the organoid culture system. Collectively, our results support a fundamental role for RA in lumen formation and demonstrate the feasibility of 3D organoid culture as a tool for studying salivary gland 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.

Cell signaling regulation in salivary gland development

The mammalian salivary gland develops as a highly branched structure designed to produce and secrete saliva. This review focuses on research conducted on mammalian salivary gland development, particularly on the differentiation of acinar, ductal, and myoepithelial cells. We discuss recent studies that provide conceptual advances in the understanding of the molecular mechanisms of salivary gland development. In addition, we describe the organogenesis of submandibular glands (SMGs), model systems used for the study of SMG development, and the key signaling pathways as well as cellular processes involved in salivary gland development. The findings from the recent studies elucidating the identity of stem/progenitor cells in the SMGs, and the process by which they are directed along a series of cell fate decisions to form functional glands, are also discussed. Advances in genetic tools and tissue engineering strategies will significantly increase our knowledge about the mechanisms by which signaling pathways and cells establish tissue architecture and function during salivary gland development, which may also be conserved in the growth and development of other organ systems. An increased knowledge of organ development mechanisms will have profound implications in the design of therapies for the regrowth or repair of injured tissues. In addition, understanding how the processes of cell survival, expansion, specification, movement, and communication with neighboring cells are regulated under physiological and pathological conditions is critical to the development of future treatments.

Endothelial Wnts control mammary epithelial patterning via fibroblast signaling

Endothelial and fibroblast niches are crucial for epithelial organs. How these heterotypic cells interact is of great interest. In this study, we reveal an axis of signaling in which fibroblasts relay Wnt signals from the endothelial niche to organize epithelial patterning. We generate an Axin2-membrane GFP (mGFP) reporter mouse and observe robust Wnt/β-catenin signaling activities in fibroblasts surrounding the mammary epithelium. To enable cell-type-specific gene manipulation in vitro, we establish an organoid system via coculture of endothelial cells (ECs), fibroblasts, and mammary epithelial cells. Deletion of β-catenin in fibroblasts impedes epithelium branching, and ECs are responsible for the activation of Wnt/β-catenin signaling in fibroblasts. In vivo, EC deletion of Wntless inhibits Wnt/β-catenin signaling activity in fibroblasts, rendering a reduction in epithelial branches. These findings highlight the significance of the endothelial niche in tissue patterning, shedding light on the interactive mechanisms in which distinct niche components orchestrate epithelial organogenesis and tissue homeostasis.

A Ser252Trp substitution in mouse FGFR2 results in hyperplasia of embryonic salivary gland parenchyma

OBJECTIVES

Mutations in the fibroblast growth factor receptor 2 (FGFR2) gene are responsible for several severe forms of craniosynostotic disorders, such as Apert and Crouzon syndromes. Patients with craniosynostotic disorders caused by a mutation in Fgfr2 present with several clinical symptoms, including hypersalivation. Here we used a transgenic mouse model of Apert syndrome (Fgfr2^+/S252W mice) to evaluate the morphology of the submandibular glands at embryonic day 15.5 (E15.5), the time point reported to mark the start of lumen formation.

METHODS

Fgfr2^+/S252W mice were generated by crossing ACTB-Cre^+/+ and Fgfr2^+/Neo-S252W mice. After measuring body weight, the submandibular glands were collected at E15.5. H&E staining, immunostaining, and RT-qPCR were performed to investigate the development of the submandibular gland.

RESULTS

The number of ducts and acini in Fgfr2^+/S252W mice was significantly higher than in control littermates; however, lumen formation was not affected. The mRNA expression of Fgf1, Fgfr1, Mmp2, Bmp4, Bmp7, Dusp6, and Etv5 in Fgfr2^+/S252W mice was significantly higher compared to control littermates. Immunoreactivity for FGF3, FGF1, BMP4, and F4/80 was detected in the parenchyma of Fgfr2^+/S252W mice. The area of apoptotic cells stained with TUNEL in Fgfr2^+/S252W mice was significantly larger than that of the control littermates.

CONCLUSIONS

These results suggested that increased FGFR1 signaling and apoptosis in the submandibular glands of Fgfr2^+/S252W mice occurred at E15.5, leading to parenchymal hyperplasia. This study demonstrated that a Ser252Trp substitution in mouse FGFR2 resulted in hyperplasia of the submandibular gland parenchyma during development.

Specification of the patterning of a ductal tree during branching morphogenesis of the submandibular gland

The development of ductal structures during branching morphogenesis relies on signals that specify ductal progenitors to set up a pattern for the ductal network. Here, we identify cellular asymmetries defined by the F-actin cytoskeleton and the cell adhesion protein ZO-1 as the earliest determinants of duct specification in the embryonic submandibular gland (SMG). Apical polarity protein aPKCζ is then recruited to the sites of asymmetry in a ZO-1-dependent manner and collaborates with ROCK signaling to set up apical-basal polarity of ductal progenitors and further define the path of duct specification. Moreover, the motor protein myosin IIB, a mediator of mechanical force transmission along actin filaments, becomes localized to vertices linking the apical domains of multiple ductal epithelial cells during the formation of ductal lumens and drives duct maturation. These studies identify cytoskeletal, junctional and polarity proteins as the early determinants of duct specification and the patterning of a ductal tree during branching morphogenesis of the SMG.

Evidence Implicating Non-Dioxin-Like Congeners as the Key Mediators of Polychlorinated Biphenyl (PCB) Developmental Neurotoxicity

Despite being banned from production for decades, polychlorinated biphenyls (PCBs) continue to pose a significant risk to human health. This is due to not only the continued release of legacy PCBs from PCB-containing equipment and materials manufactured prior to the ban on PCB production, but also the inadvertent production of PCBs as byproducts of contemporary pigment and dye production. Evidence from human and animal studies clearly identifies developmental neurotoxicity as a primary endpoint of concern associated with PCB exposures. However, the relative role(s) of specific PCB congeners in mediating the adverse effects of PCBs on the developing nervous system, and the mechanism(s) by which PCBs disrupt typical neurodevelopment remain outstanding questions. New questions are also emerging regarding the potential developmental neurotoxicity of lower chlorinated PCBs that were not present in the legacy commercial PCB mixtures, but constitute a significant proportion of contemporary human PCB exposures. Here, we review behavioral and mechanistic data obtained from experimental models as well as recent epidemiological studies that suggest the non-dioxin-like (NDL) PCBs are primarily responsible for the developmental neurotoxicity associated with PCBs. We also discuss emerging data demonstrating the potential for non-legacy, lower chlorinated PCBs to cause adverse neurodevelopmental outcomes. Molecular targets, the relevance of PCB interactions with these targets to neurodevelopmental disorders, and critical data gaps are addressed as well.

Regulation of bone morphogenetic protein 4 on epithelial tissue

Epithelial tissues provide tissue barriers and specialize in organs and glands. When epithelial homeostasis is physiologically or pathologically stimulated, epithelial cells produce mesenchymal cells through the epithelial-mesenchymal transition, forming new tissues, promoting the cure of diseases or leading to illness. A variety of cytokines are involved in the regulation of epithelial cell differentiation. Bone morphogenetic proteins (BMPs), especially the bone morphogenetic protein 4 (BMP4) has a variety of biological functions and plays a prominent role in the regulation of epithelial cell differentiation. BMP4 is an important regulatory factor of a series of life activities in vertebrates, which is also related to cell proliferation, differentiation and mobility, it also has relation with tumor development. This paper mainly reviews the mechanism of BMP4's regulation on epithelial tissues, as well as its effect on the growth, differentiation, benign lesions and malignant lesions of epithelial tissues, and expounds the function of BMP4 in epithelial tissues, to provide theoretical support for the research on reducing epithelial diseases.

Establishment of a Murine Pro-acinar Cell Line to Characterize Roles for FGF2 and α3β1 Integrins in Regulating Pro-acinar Characteristics

Radiation therapy for head and neck cancers results in permanent damage to the saliva producing acinar compartment of the salivary gland. To date, a pure pro-acinar cell line to study underlying mechanisms of acinar cell differentiation in culture has not been described. Here, we report the establishment of a pro-acinar (mSG-PAC1) and ductal (mSG-DUC1) cell line, from the murine submandibular salivary gland (SMG), which recapitulate developmental milestones in differentiation. mSG-DUC1 cells express the ductal markers, keratin-7 and keratin-19, and form lumenized spheroids. mSG-PAC1 cells express the pro-acinar markers SOX10 and aquaporin-5. Using the mSG-PAC1 cell line, we demonstrate that FGF2 regulates specific steps during acinar cell maturation. FGF2 up-regulates aquaporin-5 and the expression of the α3 and α6 subunits of the α3β1 and α6β1 integrins that are known to promote SMG morphogenesis and differentiation. mSG-DUC1 and mSG-PAC1 cells were derived from genetically modified mice, homozygous for floxed alleles of the integrin α3 subunit. Similar to SMGs from α3-null mice, deletion of α3 alleles in mSG-PAC1 cells results in the up-regulation of E-cadherin and the down-regulation of CDC42. Our data indicate that mSG-DUC1 and mSG-PAC1 cells will serve as important tools to gain mechanistic insight into salivary gland morphogenesis and differentiation.

Morphogenesis and Mucus Production of Epithelial Tissues of Three Major Salivary Glands of Embryonic Mouse in 3D Culture

Embryonic mouse submandibular epithelia initiate branching morphogenesis within two days when embedded in Matrigel and stimulated by members of the epidermal growth factor family. However, it is unknown whether the end buds further branch over longer culture periods, and whether saliva-producing cells differentiate there. In the present study, we cultivated three major (submandibular, sublingual and parotid) salivary epithelia from 13-day embryos for 14 days in mesenchyme-free cultures. All epithelia continued to grow and branch to form numerous acinus-like structures in medium supplemented with neuregulin 1, fibroblast growth factor 1, and insulintransferrin-sodium selenite. Alcian blue staining to detect mucous cells showed that each epithelium differentiated via three distinct modes, as seen in normal development, although the staining intensities were weaker than in normal development. RT-PCR analysis of the amylase gene showed that no epithelia expressed amylase after 14 days of culture, which is inconsistent with the fact that only parotid epithelium does so at postnatal day 7 during normal development. These results suggest that cytodifferentiation progresses to a lesser extent in mesenchyme-free cultures than in vivo.

Expression patterns of genes critical for SHH, BMP, and FGF pathways during the lumen formation of human salivary glands

Sjögren's syndrome or radiotherapy for head and neck cancer leads to the irreversible hypofunction of salivary gland (SG). The stem/progenitor cell-based regenerative strategy has been proven to be the most promising approach to repair the function of SG. The molecular mechanisms that regulate SG morphogenesis, especially during lumen formation, provide valuable hints for establishment of such regenerative strategies. It has been demonstrated that numerous growth factors particularly belonging to SHH, BMP, and FGF signaling pathway are involved in the regulation of lumen formation and have shown protective effects on the SG from irradiation in mouse models. However, it remains elusive whether the expression pattern and function of these signaling molecules are conserved in humans. In this study, we examined the expression patterns of the molecules critical for SHH, BMP, and FGF signaling cascades from the canalicular stage to the terminal bud stage, the key stages for lumen formation, in human SG and compared them with the expression data observed in mice. Our results manifested that genes involved in SHH signaling pathway showed identical expression patterns, while genes involved in BMP as well as FGF pathway exhibited similar but distinct expression patterns in humans to those in the mouse. We concluded that the expression patterns of genes involved in SHH, BMP, and FGF pathways in the development of human SG exhibit high similarity to that in the development of mouse SG during lumen formation, suggesting that the molecular mechanism regulating the morphogenesis of SG during lumen formation may be conserved in mice and humans. Our results will have an implication in the future establishment of stem-cell based approaches for the repair of SG function.

Loss of Fam20c causes defects in the acinar and duct structure of salivary glands in mice

Family with sequence similarity 20-member C (FAM20C), a recently characterized Golgi kinase, performs numerous biological functions by phosphorylating more than 100 secreted proteins. However, the role of FAM20C in the salivary glands remains undefined. The present study demonstrated that FAM20C is mainly located in the cytoplasm of duct epithelial cells in the salivary glands. Fam20c^f/f; Mmtv-Cre mice were created in which Fam20c was inactivated in the salivary gland cells and observed that the number of ducts and the ductal cross-sectional area increased significantly, while the number of acinar cells was reduced. The granular convoluted tubules (GCTs) exhibited an accumulation of aberrant secretory granules, along with a reduced expression and altered distribution patterns of β nerve growth factor, α-amylase and bone morphogenetic protein (BMP) 4. This abnormality suggested that the GCT cells were immature and exhibited defects in developmental and secretory functions. In accordance with the morphological alterations and the reduced number of acinar cells, FAM20C deficiency in the salivary glands significantly decreased the salivary flow rate. The Na⁺, Cl⁻ and K⁺ concentrations in the saliva were all significantly increased due to dysfunction of the ducts. Furthermore, Fam20c deficiency significantly increased BMP2 and BMP7 expression, decreased BMP4 expression, and attenuated the canonical and noncanonical BMP signaling pathways in the salivary glands. Collectively, the results of the present study demonstrate that FAM20C is a key regulator of acinar and duct structure and duct maturation and provide a novel avenue for investigating novel therapeutic targets for oral diseases including xerostomia.

Structural Biology of the FGF7 Subfamily

Mammalian fibroblast growth factor (FGF) signaling is intricately regulated via selective binding interactions between 18 FGF ligands and four FGF receptors (FGFR1–4), three of which (FGFR1–3) are expressed as either epithelial (“b”) or mesenchymal (“c”) splice isoforms. The FGF7 subfamily, consisting of FGF3, FGF7, FGF10, and FGF22, is unique among FGFs in that its members are secreted exclusively by the mesenchyme, and specifically activate the “b” isoforms of FGFR1 (FGFR1b) and FGFR2 (FGFR2b) present in the overlying epithelium. This unidirectional mesenchyme-to-epithelium signaling contributes to the development of essentially all organs, glands, and limbs. Structural analysis has shown that members of the FGF7 subfamily achieve their restricted specificity for FGFR1b/FGFR2b by engaging in specific contacts with two alternatively spliced loop regions in the immunoglobulin-like domain 3 (D3) of these receptors. Weak basal receptor-binding affinity further constrains the FGF7 subfamily’s specificity for FGFR1b/2b. In this review, we elaborate on the structural determinants of FGF7 subfamily receptor-binding specificity, and discuss how affinity differences among the four members for the heparin sulfate (HS) co-receptor contribute to their disparate biological activities.

Regulatory mechanisms of branching morphogenesis in mouse submandibular gland rudiments

Branching morphogenesis is an important developmental process for many organs, including the salivary glands. Whereas epithelial–mesenchymal interactions, which are cell-to-cell communications, are known to drive branching morphogenesis, the molecular mechanisms responsible for those inductive interactions are still largely unknown. Cell growth factors and integrins are known to be regulators of branching morphogenesis of salivary glands. In addition, functional microRNAs (miRNAs) have recently been reported to be present in the developing submandibular gland. In this review, the authors describe the roles of various cell growth factors, integrins and miRNAs in branching morphogenesis of developmental mouse submandibular glands.

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.

Salivary glands regenerate after radiation injury through SOX2-mediated secretory cell replacement

Salivary gland acinar cells are routinely destroyed during radiation treatment for head and neck cancer that results in a lifetime of hyposalivation and co-morbidities. A potential regenerative strategy for replacing injured tissue is the reactivation of endogenous stem cells by targeted therapeutics. However, the identity of these cells, whether they are capable of regenerating the tissue, and the mechanisms by which they are regulated are unknown. Using in vivo and ex vivo models, in combination with genetic lineage tracing and human tissue, we discover a SOX2+ stem cell population essential to acinar cell maintenance that is capable of replenishing acini after radiation. Furthermore, we show that acinar cell replacement is nerve dependent and that addition of a muscarinic mimetic is sufficient to drive regeneration. Moreover, we show that SOX2 is diminished in irradiated human salivary gland, along with parasympathetic nerves, suggesting that tissue degeneration is due to loss of progenitors and their regulators. Thus, we establish a new paradigm that salivary glands can regenerate after genotoxic shock and do so through a SOX2 nerve-dependent mechanism.

FGF2-dependent mesenchyme and laminin-111 are niche factors in salivary gland organoids

Epithelial progenitor cells are dependent upon a complex 3D niche to promote their proliferation and differentiation during development, which can be recapitulated in organoids. The specific requirements of the niche remain unclear for many cell types, including the proacinar cells that give rise to secretory acinar epithelial cells that produce saliva. Here, using ex vivo cultures of E16 primary mouse submandibular salivary gland epithelial cell clusters, we investigated the requirement for mesenchymal cells and other factors in producing salivary organoids in culture. Native E16 salivary mesenchyme, but not NIH3T3 cells or mesenchymal cell conditioned medium, supported robust protein expression of the progenitor marker Kit and the acinar/proacinar marker AQP5, with a requirement for FGF2 expression by the mesenchyme. Enriched salivary epithelial clusters that were grown in laminin-enriched basement membrane extract or laminin-111 together with exogenous FGF2, but not with EGF, underwent morphogenesis to form organoids that displayed robust expression of AQP5 in terminal buds. Knockdown of FGF2 in the mesenchyme or depletion of mesenchyme cells from the organoids significantly reduced AQP5 levels even in the presence of FGF2, suggesting a requirement for autocrine FGF2 signaling in the mesenchyme cells for AQP5 expression. We conclude that basement membrane proteins and mesenchyme cells function as niche factors in salivary organoids.

Endothelial cell regulation of salivary gland epithelial patterning

Perfusion-independent regulation of epithelial pattern formation by the vasculature during organ development and regeneration is of considerable interest for application in restoring organ function. During murine submandibular salivary gland development, the vasculature co-develops with the epithelium during branching morphogenesis; however, it is not known whether the vasculature has instructive effects on the epithelium. Using pharmacological inhibitors and siRNA knockdown in embryonic organ explants, we determined that VEGFR2-dependent signaling is required for salivary gland epithelial patterning. To test directly for a requirement for endothelial cells in instructive epithelial patterning, we developed a novel ex vivo cell fractionation/reconstitution assay. Immuno-depletion of CD31⁺ endothelial cells in this assay confirmed a requirement for endothelial cells in epithelial patterning of the gland. Depletion of endothelial cells or inhibition of VEGFR2 signaling in organ explants caused an aberrant increase in cells expressing the ductal proteins K19 and K7, with a reduction in Kit⁺ progenitor cells in the endbuds of reconstituted glands. Addition of exogenous endothelial cells to reconstituted glands restored epithelial patterning, as did supplementation with the endothelial cell-regulated mesenchymal factors IGFBP2 and IGFBP3. Our results demonstrate that endothelial cells promote expansion of Kit⁺ progenitor cells and suppress premature ductal differentiation in early developing embryonic submandibular salivary gland buds.

Identification of the protective mechanisms of Lactoferrin in the irradiated salivary gland

Radiotherapy is commonly used in patients with head and neck cancer, and usually results in irreversible salivary glands damage and hypofunction. It is therefore important to manage such irradiation to prevent damage to the salivary glands. A previous study showed that Lactoferrin (LF) has a radioprotective effect, but the mechanism was not determined in salivary glands. In the present study, we investigated the detailed radioprotective effect of LF using both ex vivo submandibular salivary gland organ culture and ICR male mice in vivo. We found that LF had effects on both cell proliferation and CyclinD1-mediated cell-cycle progression which were regulated via the ERK1/2 and AKT signal transduction pathways. In addition, LF affected acinar cell structure and function after irradiation. These findings suggest that LF may be a useful agent to prevent irradiation effects in salivary glands.

Functional expression of BMP7 receptors in oral epithelial cells. Interleukin-17F production in response to BMP7

BACKGROUND

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-β (TGF-β) superfamily. Recently, BMP7 has been demonstrated to be produced by salivary glands and contribute to embryonic branching in mice. The BMP7 in saliva is thought to be delivered to the oral cavity and is expected to contact with stratified squamous epithelial cells which line the surface of oral mucosa. In this study, we attempted to investigate the effects of BMP7 on oral epithelial cells.

METHODS

The expression of BMP receptors was examined by reverse transcriptase-polymerase chain reaction (RT-PCR). OSCCs were stimulated with human recombinant BMP7 (hrBMP7) and the phosphorylation status of Smad1/5/8 was examined by western blotting. For microarray analysis, Ca9-22 cells were stimulated with 100 ng/mL of hrBMP7 and total RNA was extracted and subjected to real-time PCR. The 5'-untranslated region (5'-UTR) of IL-17 F gene was cloned to pGL4-basic vector and used for luciferase assay. Ca9-22 cells were pre-incubated with DM3189, a specific inhibitor of Smad1/5/8, for inhibition assay.

RESULTS

All isoforms of type I and type II BMP receptors were expressed in both Ca9-22 and HSC3 cells and BMP7 stimulation resulted in the phosphorylation of Smad1/5/8 in both cell lines. The microarray analysis revealed the induction of interleukin-17 F (IL-17 F), netrin G2 (NTNG2) and hyaluronan synthase 1 (HAS1). Luciferase assay using the 5'-UTR of the IL-17 F gene revealed transcriptional regulation. Induced IL-17 F production was further confirmed at the protein level by ELISA. Smad1/5/8 inhibitor pretreatment decreased IL-17 F expression levels in the cells.

De novo transcriptome analysis shows differential expression of genes in salivary glands of edible bird's nest producing swiftlets

BACKGROUND

Edible bird's nest (EBN), produced from solidified saliva secretions of specific swiftlet species during the breeding season, is one of the most valuable animal by-products in the world. The composition and medicinal benefits of EBN have been extensively studied, however, genomic and transcriptomic studies of the salivary glands of these birds have not been conducted.

RESULTS

The study described the transcriptomes of salivary glands from three swiftlet species (28 samples) generated by RNASeq. A total of 14,835 annotated genes and 428 unmapped genes were cataloged. The current study investigated the genes and pathways that are associated with the development of salivary gland and EBN composition. Differential expression and pathway enrichment analysis indicated that the expression of CREB3L2 and several signaling pathways involved in salivary gland development, namely, the EGFR, BMP, and MAPK signaling pathways, were up-regulated in swiftlets producing white EBN (Aerodramus fuciphagus) and black EBN (Aerodramus maximus) compared with non-EBN-producing swiftlets (Apus affinis). Furthermore, MGAT, an essential gene for the biosynthesis of N-acetylneuraminic acid (sialic acid), was highly expressed in both white- and black-nest swiftlets compared to non-EBN-producing swiftlets. Interspecies comparison between Aerodramus fuciphagus and Aerodramus maximus indicated that the genes involved in N-acetylneuraminic and fatty acid synthesis were up-regulated in Aerodramus fuciphagus, while alanine and aspartate synthesis pathways were up-regulated in Aerodramus maximus. Furthermore, gender-based analysis revealed that N-glycan trimming pathway was significantly up-regulated in male Aerodramus fuciphagus from its natural habitat (cave) compared to their female counterpart.

CONCLUSIONS

Transcriptomic analysis of salivary glands of different swiftlet species reveal differential expressions of candidate genes that are involved in salivary gland development and in the biosynthesis of various bioactive compounds found in EBN.

SOX2 regulates acinar cell development in the salivary gland

Acinar cells play an essential role in the secretory function of exocrine organs. Despite this requirement, how acinar cells are generated during organogenesis is unclear. Using the acini-ductal network of the developing human and murine salivary gland, we demonstrate an unexpected role for SOX2 and parasympathetic nerves in generating the acinar lineage that has broad implications for epithelial morphogenesis. Despite SOX2 being expressed by progenitors that give rise to both acinar and duct cells, genetic ablation of SOX2 results in a failure to establish acini but not ducts. Furthermore, we show that SOX2 targets acinar-specific genes and is essential for the survival of acinar but not ductal cells. Finally, we illustrate an unexpected and novel role for peripheral nerves in the creation of acini throughout development via regulation of SOX2. Thus, SOX2 is a master regulator of the acinar cell lineage essential to the establishment of a functional organ.

DOI:http://dx.doi.org/10.7554/eLife.26620.001

The salivary glands produce fluid that contains enzymes to help us to digest our food. These glands contain a tree-like network of cells – known as acinar cells – that produce the fluid, and cells that form ducts to transport the fluid out of the glands. Both types of cells form from stem cells as animal embryos develop. Like all developing organs, the salivary glands receive many different signals that guide how they grow. However, the identity of the cues that instruct a stem cell to produce a new acinar cell or duct cell are not known.

Emmerson et al. studied how the salivary glands develop in mouse embryos. The experiments show that a protein called SOX2 – which is an essential regulator of stem cells in embryos – is required for acinar cells to form. Loss of SOX2 inhibited the production of acinar but not duct cells. Furthermore, nerves that surround the gland provide support to cells that produce SOX2 and promote the formation of acinar cells.

Further experiments suggest that the nerves also play the same role in humans. Adult organs often use developmental signals to repair or regenerate tissue. As such, understanding how an organ develops may lead to new therapies that can stimulate salivary glands and other organs to regenerate after they have been damaged in adults.

DOI:http://dx.doi.org/10.7554/eLife.26620.002

Identification and characterization of a rich population of CD34+ mesenchymal stem/stromal cells in human parotid, sublingual and submandibular glands

Mesenchymal stem/stromal cells (MSCs) play crucial roles in maintaining tissue homeostasis during physiological turnovers and injuries. Very little is known about the phenotype, distribution and molecular nature of MSCs in freshly isolated human salivary glands (SGs) as most reports have focused on the analysis of cultured MSCs. Our results demonstrate that the cell adhesion molecule CD34 was widely expressed by the MSCs of human major SGs, namely parotid (PAG), sublingual (SLG) and submandibular (SMG) glands. Further, gene expression analysis of CD34⁺ cells derived from fetal SMGs showed significant upregulation of genes involved in cellular adhesion, proliferation, branching, extracellular matrix remodeling and organ development. Moreover, CD34⁺ SMG cells exhibited elevated expression of genes encoding extracellular matrix, basement membrane proteins, and members of ERK, FGF and PDGF signaling pathways, which play key roles in glandular development, branching and homeostasis. In vitro CD34⁺ cell derived SG-MSCs revealed multilineage differentiation potential. Intraglandular transplantation of cultured MSCs in immunodeficient mice led to their engraftment in the injected and uninjected contralateral and ipsilateral glands. Engrafted cells could be localized to the stroma surrounding acini and ducts. In summary, our data show that CD34⁺ derived SG-MSCs could be a promising cell source for adoptive cell-based SG therapies, and bioengineering of artificial SGs.

Defining epithelial cell dynamics and lineage relationships in the developing lacrimal gland

The tear-producing lacrimal gland is a tubular organ that protects and lubricates the ocular surface. The lacrimal gland possesses many features that make it an excellent model in which to investigate tubulogenesis, but the cell types and lineage relationships that drive lacrimal gland formation are unclear. Using single-cell sequencing and other molecular tools, we reveal novel cell identities and epithelial lineage dynamics that underlie lacrimal gland development. We show that the lacrimal gland from its earliest developmental stages is composed of multiple subpopulations of immune, epithelial and mesenchymal cell lineages. The epithelial lineage exhibits the most substantial cellular changes, transitioning through a series of unique transcriptional states to become terminally differentiated acinar, ductal and myoepithelial cells. Furthermore, lineage tracing in postnatal and adult glands provides the first direct evidence of unipotent KRT5⁺ epithelial cells in the lacrimal gland. Finally, we show conservation of developmental markers between the developing mouse and human lacrimal gland, supporting the use of mice to understand human development. Together, our data reveal crucial features of lacrimal gland development that have broad implications for understanding epithelial organogenesis.

Fgf10 and Sox9 are essential for the establishment of distal progenitor cells during mouse salivary gland development

Salivary glands are formed by branching morphogenesis with epithelial progenitors forming a network of ducts and acini (secretory cells). During this process, epithelial progenitors specialise into distal (tips of the gland) and proximal (the stalk region) identities that produce the acini and higher order ducts, respectively. Little is known about the factors that regulate progenitor expansion and specialisation in the different parts of the gland. Here, we show that Sox9 is involved in establishing the identity of the distal compartment before the initiation of branching morphogenesis. Sox9 is expressed throughout the gland at the initiation stage before becoming restricted to the distal epithelium from the bud stage and throughout branching morphogenesis. Deletion of Sox9 in the epithelium results in loss of the distal epithelial progenitors, a reduction in proliferation and a subsequent failure in branching. We demonstrate that Sox9 is positively regulated by mesenchymal Fgf10, a process that requires active Erk signalling. These results provide new insights into the factors required for the expansion of salivary gland epithelial progenitors, which can be useful for organ regeneration therapy.