Salivary gland development: a template for regeneration

Exploring the Cocktail Factor Approach to Generate Salivary Gland Progenitors through Co-Culture Techniques

BACKGROUND

The derivation of salivary gland (SG) progenitors from pluripotent stem cells (PSCs) presents significant potential for developmental biology and regenerative medicine. However, the existing protocols for inducing SG include limited factors, making it challenging to mimic the in vivo microenvironment of embryonic SGs.

METHODS

We reported a cocktail factor approach to promote the differentiation of mouse embryonic stem cell (mESC)-derived oral epithelium (OE) into SG progenitors through a three-dimensional co-culture method. Upon confirming that the embryonic SG can promote the differentiation of mESC-derived OE, we performed RNA sequence analysis to identify factors involved in the differentiation of SG progenitors.

RESULTS

Our findings highlight several efficient pathways related to SG development, with frequent appearances of four factors: IFN-γ, TGF-β2, EGF, and IGF-1. The combined treatment using these cocktail factors increased the expression of key SG progenitor markers, including Sox9, Sox10, Krt5, and Krt14. However, absence of any one of these cocktail factors did not facilitate differentiation. Notably, aggregates treated with the cocktail factor formed SG epithelial-like structures and pre-bud-like structures on the surface.

CONCLUSION

In conclusion, this study offers a novel approach to developing a differentiation protocol that closely mimics the in vivo microenvironment of embryonic SGs. This provides a foundation for generating PSC-derived organoids with near-physiological cell behaviors and structures.

Salivary gland developmental mechanics

The salivary gland undergoes branching morphogenesis to elaborate into a tree-like structure with numerous saliva-secreting acinar units, all joined by a hierarchical ductal system. The expansive epithelial surface generated by branching morphogenesis serves as the structural basis for the efficient production and delivery of saliva. Here, we elucidate the process of salivary gland morphogenesis, emphasizing the role of mechanics. Structurally, the developing salivary gland is characterized by a stratified epithelium tightly encased by the basement membrane, which is in turn surrounded by a mesenchyme consisting of a dense network of interstitial matrix and mesenchymal cells. Diverse cell types and extracellular matrices bestow this developing organ with organized, yet spatially varied mechanical properties. For instance, the surface epithelial sheet of the bud is highly fluidic due to its high cell motility and weak cell-cell adhesion, rendering it highly pliable. In contrast, the inner core of the bud is more rigid, characterized by reduced cell motility and strong cell-cell adhesion, which likely provide structural support for the tissue. The interactions between the surface epithelial sheet and the inner core give rise to budding morphogenesis. Furthermore, the basement membrane and the mesenchyme offer mechanical constraints that could play a pivotal role in determining the higher-order architecture of a fully mature salivary gland.

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.

Nkx2.3 transcription factor is a key regulator of mucous cell identity in salivary glands

Saliva is vital to oral health, fulfilling multiple functions in the oral cavity. Three pairs of major salivary glands and hundreds of minor salivary glands contribute to saliva production. The secretory acinar cells within these glands include two distinct populations. Serous acinar cells secrete a watery saliva containing enzymes, while mucous acinar cells secrete a more viscous fluid containing highly glycosylated mucins. Despite their shared developmental origins, the parotid gland (PG) is comprised of only serous acinar cells, while the sublingual gland (SLG) contains predominantly mucous acinar cells. The instructive signals that govern the identity of serous versus mucous acinar cell phenotypes are not yet known. The homeobox transcription factor Nkx2.3 is uniquely expressed in the SLG. Disruption of the Nkx2.3 gene was reported to delay the maturation of SLG mucous acinar cells. To examine whether Nkx2.3 plays a role in directing the mucous cell phenotype, we analyzed SLG from Nkx2.3-/- mice using RNAseq, immunostaining and proteomic analysis of saliva. Our results indicate that Nkx2.3, most likely in concert with other transcription factors uniquely expressed in the SLG, is a key regulator of the molecular program that specifies the identity of mucous acinar cells.

Immunolocalization patterns of histone-deacetylases in salivary glands of mice during postnatal development

OBJECTIVE

Histone-deacetylases (HDACs) are epigenetic modulators involved in the control of gene expression. No data are available on the expression or subcellular localization of HDACs in salivary glands. The present study aims to examine the subcellular distribution of HDACs in salivary glands during postnatal development.

DESIGN

The major salivary glands of C57/BL6 mice were separately removed at 10, 25, 30,60 and 90 days after birth. Hematoxylin-eosin (H&E) and immunohistochemical staining were performed for HDACs. Gene Expression of HDACs in C57BL/6. NOD-Aec1Aec2 mice salivary glands during the development of Sjögren's syndrome-like illness were also analyzed by using the gene expression datasets (GSE 15640).

RESULTS

In the mice salivary gland, HDACs were found to have different localization patterns at various stages of development (10, 25, 30, 60, and 90 days). Apart from HDAC6, ductal cells of salivary glands were the primary sites for HDAC localization. HDAC2, 8, 5, 10 and 11 were expressed at high levels in the salivary gland after birth while HDAC6 showed no expression during postnatal development. This suggests that these HDAC subtypes may have different roles in salivary gland function. In the context of Sjögren's syndrome-like illness, HDAC 2, 8 and 10 showed low expression while HDAC1, 6,5,3 and 11 had relatively high expression in the salivary gland.

CONCLUSIONS

This study has provided an important reference for understanding the spatiotemporal-specific expression of HDACs in the salivary gland. These results offer new clues for the experimenters and hold promise for developing innovative therapeutic strategies for salivary gland-related diseases.

Current developments and opportunities of pluripotent stem cells-based therapies for salivary gland hypofunction

Salivary gland hypofunction (SGH) caused by systemic disease, drugs, aging, and radiotherapy for head and neck cancer can cause dry mouth, which increases the risk of disorders such as periodontitis, taste disorders, pain and burning sensations in the mouth, dental caries, and dramatically reduces the quality of life of patients. To date, the treatment of SGH is still aimed at relieving patients' clinical symptoms and improving their quality of life, and is not able to repair and regenerate the damaged salivary glands. Pluripotent stem cells (PSCs), including embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and extended pluripotent stem cells (EPSCs), are an emerging source of cellular therapies that are capable of unlimited proliferation and differentiation into cells of all three germ layers. In recent years, the immunomodulatory and tissue regenerative effects of PSCs, their derived cells, and paracrine products of these cells have received increasing attention and have demonstrated promising therapeutic effects in some preclinical studies targeting SGH. This review outlined the etiologies and available treatments for SGH. The existing efficacy and potential role of PSCs, their derived cells and paracrine products of these cells for SGH are summarized, with a focus on PSC-derived salivary gland stem/progenitor cells (SGS/PCs) and PSC-derived mesenchymal stem cells (MSCs). In this Review, we provide a conceptual outline of our current understanding of PSCs-based therapy and its importance in SGH treatment, which may inform and serve the design of future studies.

Recent progress in regenerative therapy for damaged salivary glands: From bench to bedside

OBJECTIVE

For functional restoration of salivary glands (SGs) injured by radiation therapy or Sjögren's syndrome (SS), various experimental approaches, such as gene therapy, tissue engineering, and cell-based therapy, have been proposed. This narrative review summarized recent progresses in research using cell-based therapies, including promising trials that could lead to bench-to-clinic applications.

METHODS

A literature review based on PubMed publications in the last two decades was performed to summarize progresses in cell-based therapies for SG dysfunction.

RESULTS

Over 100 experimental studies have shown the therapeutic potential of several types of cells, such as SG stem cells and mesenchymal stem cells, as well as effectively conditioned mononuclear cells, in both radiation injury and SS animal models. These therapies affect to slow fibrosis progression and stimulate tissue regeneration in atrophic glands. However, to date, only a total of seven studies have been developed to the stage of clinical study, showing the safety and preliminary efficacy.

CONCLUSION

To lead the radical effectiveness expected in cell-based therapy, advances in reverse translational research and in innovative experimental research, based on the findings of recent clinical studies, will be critical in the next decade.

Salivary Gland Bioengineering

Salivary gland dysfunction affects millions globally, and tissue engineering may provide a promising therapeutic avenue. This review delves into the current state of salivary gland tissue engineering research, starting with a study of normal salivary gland development and function. It discusses the impact of fibrosis and cellular senescence on salivary gland pathologies. A diverse range of cells suitable for tissue engineering including cell lines, primary salivary gland cells, and stem cells are examined. Moreover, the paper explores various supportive biomaterials and scaffold fabrication methodologies that enhance salivary gland cell survival, differentiation, and engraftment. Innovative engineering strategies for the improvement of vascularization, innervation, and engraftment of engineered salivary gland tissue, including bioprinting, microfluidic hydrogels, mesh electronics, and nanoparticles, are also evaluated. This review underscores the promising potential of this research field for the treatment of salivary gland dysfunction and suggests directions for future exploration.

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.

Extracellular matrix turnover in salivary gland disorders and regenerative therapies: Obstacles and opportunities

Salivary gland (SG) extracellular matrix (ECM) has a major influence on tissue development, homeostasis, and tissue regeneration after injury. During aging, disease, and physical insult, normal remodeling of the SG microenvironment (i.e. ECM) becomes dysregulated, leading to alterations in matrix composition which disrupt tissue architecture/structure, alter cell activity, and negatively impact gland function. Matrix metalloproteinases (MMPs) are a large and diverse family of metalloendopeptidases which play a major role in matrix degradation and are intimately involved in regulating development and cell function; dysregulation of these enzymes leads to the production of a fibrotic matrix. In the SG this altered fibrotic ECM (or cell microenvironment) negatively impacts normal cell function and the effectiveness of gene and stem cell therapies which serve as a foundation for many SG regenerative therapies. For this reason, prospective regenerative strategies should prioritize the maintenance and/or restoration of a healthy SG ECM. Mesenchymal stem cells (MSCs) have great potential for mitigating damage to the SG microenvironment by ameliorating inflammation, reducing fibrosis, and repairing the damaged milieu of extracellular regulatory cues, including the matrix. This review addresses our current understanding of the impact of aging and disease on the SG microenvironment and suggests critical deficiencies and opportunities in ECM-targeted therapeutic interventions.

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.

FGFR2 is essential for salivary gland duct homeostasis and MAPK-dependent seromucous acinar cell differentiation

Exocrine acinar cells in salivary glands (SG) are critical for oral health and loss of functional acinar cells is a major clinical challenge. Fibroblast growth factor receptors (FGFR) are essential for early development of multiple organs, including SG. However, the role of FGFR signaling in specific populations later in development and during acinar differentiation are unknown. Here, we use scRNAseq and conditional deletion of murine FGFRs in vivo to identify essential roles for FGFRs in craniofacial, early SG development and progenitor function during duct homeostasis. Importantly, we also discover that FGFR2 via MAPK signaling is critical for seromucous acinar differentiation and secretory gene expression, while FGFR1 is dispensable. We show that FGF7, expressed by myoepithelial cells (MEC), activates the FGFR2-dependent seromucous transcriptional program. Here, we propose a model where MEC-derived FGF7 drives seromucous acinar differentiation, providing a rationale for targeting FGFR2 signaling in regenerative therapies to restore acinar function.

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.

Heterotopia of salivary gland tissue in the pancreas

Heterotopia of the salivary gland occurs mainly in the head and neck region of the human body, rarely in regions such as the rectum, but has never been demonstrated in the pancreas. Within a screening effort of pancreatic samples for detecting ΔNp63 expression, we discovered two pancreatic samples from a 35-year-old male showing salivary gland heterotopia. Immunohistochemical stainings were done for markers of healthy and neoplastic salivary glands and showed expression of calponin, CD142 and KRT14 but not of S100p, GFAP or CD117. A PAS-staining and Alcian Blue staining showed the presence of acid mucins. These staining patterns were consistent with non-neoplastic submandibular gland tissue comprised of abundant seromucous glands, basal cells and myoepithelial cells, all features typically absent in the pancreas. Also, no pancreatic islets of Langerhans were detected. We show for the first time that salivary gland heterotopia can occur at the location of the pancreas.

Salivary gland regeneration: from salivary gland stem cells to three-dimensional bioprinting

Hyposalivation and severe dry mouth syndrome are the most common complications in patients with head and neck cancer (HNC) after receiving radiation therapy. Conventional treatment for hyposalivation relies on the use of sialogogues such as pilocarpine; however, their efficacy is constrained by the limited number of remnant acinar cells after radiation. After radiotherapy, the salivary gland (SG) secretory parenchyma is largely destroyed, and due to the reduced stem cell niche, this gland has poor regenerative potential. To tackle this, researchers must be able to generate highly complex cellularized 3D constructs for clinical transplantation via technologies, including those that involve bioprinting of cells and biomaterials. A potential stem cell source with promising clinical outcomes to reserve dry mouth is adipose mesenchymal stem cells (AdMSC). MSC-like cells like human dental pulp stem cells (hDPSC) have been tested in novel magnetic bioprinting platforms using nanoparticles that can bind cell membranes by electrostatic interaction, as well as their paracrine signals arising from extracellular vesicles. Both magnetized cells and their secretome cues were found to increase epithelial and neuronal growth of in vitro and ex vivo irradiated SG models. Interestingly, these magnetic bioprinting platforms can be applied as a high-throughput drug screening system due to the consistency in structure and functions of their organoids. Recently, exogenous decellularized porcine ECM was added to this magnetic platform to stimulate an ideal environment for cell tethering, proliferation, and/or differentiation. The combination of these SG tissue biofabrication strategies will promptly allow for in vitro organoid formation and establishment of cellular senescent organoids for aging models, but challenges remain in terms of epithelial polarization and lumen formation for unidirectional fluid flow. Current magnetic bioprinting nanotechnologies can provide promising functional and aging features to in vitro craniofacial exocrine gland organoids, which can be utilized for novel drug discovery and/or clinical transplantation.

Dental Pulp Stem Cells for Salivary Gland Regeneration-Where Are We Today?

Xerostomia is the phenomenon of dry mouth and is mostly caused by hypofunction of the salivary glands. This hypofunction can be caused by tumors, head and neck irradiation, hormonal changes, inflammation or autoimmune disease such as Sjögren's syndrome. It is associated with a tremendous decrease in health-related quality of life due to impairment of articulation, ingestion and oral immune defenses. Current treatment concepts mainly consist of saliva substitutes and parasympathomimetic drugs, but the outcome of these therapies is deficient. Regenerative medicine is a promising approach for the treatment of compromised tissue. For this purpose, stem cells can be utilized due to their ability to differentiate into various cell types. Dental pulp stem cells are adult stem cells that can be easily harvested from extracted teeth. They can form tissues of all three germ layers and are therefore becoming more and more popular for tissue engineering. Another potential benefit of these cells is their immunomodulatory effect. They suppress proinflammatory pathways of lymphocytes and could therefore probably be used for the treatment of chronic inflammation and autoimmune disease. These attributes make dental pulp stem cells an interesting tool for the regeneration of salivary glands and the treatment of xerostomia. Nevertheless, clinical studies are still missing. This review will highlight the current strategies for using dental pulp stem cells in the regeneration of salivary gland tissue.

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.

Physical Exercise Mitigates Salivary Gland and Saliva Damages in Rats Exposed to Binge-like Ethanol Pattern

Heavy episodic ethanol (EtOH) consumption is a typical pattern, especially among younger people. The therapeutic effect of exercise on EtOH damage has not yet been fully elucidated. Therefore, this study aims to investigate whether moderate exercise can reduce the damage generated by ethanol consumption in salivary glands and saliva. Thus, 32 male Wistar rats were divided into four groups: control (sedentary animals treated with water); training (trained animals treated with EtOH); EtOH (sedentary animals treated with EtOH); and EtOH + training (trained animals treated with ethanol). EtOH was administered to the animals at a dose of 3 g/kg/day at a concentration of 20% w/v for three consecutive days per week via intragastric gavage. The training was performed on a treadmill for five successive days. At the end of the 4-week experimental protocol, the animals were euthanized, and salivary glands and saliva were collected for oxidative biochemistry analysis. Our results showed that EtOH consumption generated changes in the oxidative biochemistry of the salivary glands and saliva. Thus, it was possible to conclude that moderate physical exercise can significantly recover antioxidant activity, reducing the damage generated by EtOH.

p130Cas is required for androgen-dependent postnatal development regulation of submandibular glands

Salivary glands develop through epithelial-mesenchymal interactions and are formed through repeated branching. The Crk-associated substrate protein (p130Cas) serves as an adapter that forms a complex with various proteins via integrin and growth factor signaling, with important regulatory roles in several essential cellular processes. We found that p130Cas is expressed in ductal epithelial cells of the submandibular gland (SMG). We generated epithelial tissue-specific p130Cas-deficient (p130Cas^Δepi-) mice and aimed to investigate the physiological role of p130Cas in the postnatal development of salivary glands. Histological analysis showed immature development of granular convoluted tubules (GCT) of the SMG in male p130Cas^Δepi- mice. Immunofluorescence staining showed that nuclear-localized androgen receptors (AR) were specifically decreased in GCT cells in p130Cas^Δepi- mice. Furthermore, epidermal growth factor-positive secretory granules contained in GCT cells were significantly reduced in p130Cas^Δepi- mice with downregulated AR signaling. GCTs lacking p130Cas showed reduced numbers and size of secretory granules, disrupted subcellular localization of the cis-Golgi matrix protein GM130, and sparse endoplasmic reticulum membranes in GCT cells. These results suggest that p130Cas plays a crucial role in androgen-dependent GCT development accompanied with ER-Golgi network formation in SMG by regulating the AR signaling.

Genetic Study of Elf5 and Ehf in the Mouse Salivary Gland

Salivary gland (SG) development, maturation, and homeostasis require coordinated roles of transcription factors (TFs) that dictate specific cell identities and fate. The ETS family of proteins are important transcriptional drivers of diverse cell lineages, tissue development, and differentiation programs and hence are also likely to play an important role in the SG. Here we have leveraged genomic and epigenomic data of the SG to examine the expression profile of ETS genes and identified 2 closely related paralogs, Elf5 and Ehf, that are highly expressed in distinct epithelial subpopulations. By using a well-defined mouse knockout model of Elf5, we show that Elf5, despite its enriched expression in the acinar cells, is functionally dispensable for maintaining the homeostatic state of the adult SG epithelium. The lack of a discernible phenotype of the Elf5-null SG might be due to possible functional redundancy with Ehf or other ETS factors. To probe this possibility and to examine the specific consequences of Ehf loss in the SG, we used CRISPR-Cas9 to generate mice in which the DNA-binding ETS domain of Ehf is disrupted due to an insertion mutation. We demonstrate that the Ehf mutant (EhfMut) mice exhibit a distinct cellular phenotype with decreased granular convoluted tubules that are accompanied by an increased accumulation of the intercalated Sox9-positive ductal cell population. Interestingly, the ductal phenotype of the EhfMut animals is highly pronounced in males, reaffirming the established sexual dimorphism of the SG that exists in rodents. Our results show that unlike Elf5, Ehf plays a nonredundant role in directing ductal cell differentiation of the SG and highlights the phenotypic subtlety in mutant mice of closely related TFs and the importance of careful consideration of cell type-specific studies.

Derivation of Human Salivary Epithelial Progenitors from Pluripotent Stem Cells via Activation of RA and Wnt Signaling

Derivation of salivary gland epithelial progenitors (SGEPs) from human pluripotent stem cells (hPSCs) has great potential in developmental biology and regenerative medicine. At present, no efficient method is available to generate salivary gland cells from hPSCs. Here, we described for the first time a robust protocol for direct differentiation of hPSCs into SGEPs by mimicking retinoic acid and Wnt signaling. These hPSC-derived SGEPs expressed SOX9, KRT5, and KRT19, important progenitor markers of developing salivary glands. CD24 and α-SMA positive cells, capable of restoring the functions of injured salivary glands, were also present in SGEP cultures. Importantly, RNA-sequencing revealed that the SGEPs resembled the transcript profiles of human fetal submandibular glands. Therefore, we provided an efficient protocol to induce hPSCs differentiation into SGEPs. Our study provides a foundation for generating functional hPSCs derived salivary gland acinar cells and three-dimensional organoids, potentially serving as new models for basic study and future translational research.

A cellular hierarchy of Notch and Kras signaling controls cell fate specification in the developing mouse salivary gland

The development of the mouse salivary gland involves a tip-driven process of branching morphogenesis that takes place in concert with differentiation into acinar, myoepithelial, and ductal (basal and luminal) sub-lineages. By combining clonal lineage tracing with a three-dimensional (3D) reconstruction of the branched epithelial network and single-cell RNA-seq analysis, we show that in tips, a heterogeneous population of renewing progenitors transition from a Krt14+ multipotent state to unipotent states via two transcriptionally distinct bipotent states, one restricted to the Krt14+ basal and myoepithelial lineage and the other to the Krt8+ acinar and luminal lineage. Using genetic perturbations, we show how the differential expression of Notch signaling correlates with spatial segregation, exits from multipotency, and promotes the Krt8+ lineage, whereas Kras activation promotes proacinar fate. These findings provide a mechanistic basis for how positional cues within growing tips regulate the process of lineage segregation and ductal patterning.

Loss of ATG5 in KRT14+ cells leads to accumulated functional impairments of salivary glands via pyroptosis

Macroautophagy/autophagy is critically involved in the process of salivary gland (SG) diseases such as xerostomia, which has a serious impact on quality of life. KRT14⁺ progenitor cells are found to be the main progenitors for maintaining the ductal homeostasis of the submandibular SGs. In this study, we investigated the role of ATG5 in SG KRT14⁺ cells in mice and humans. Human labial salivary glands (LSG) from primary Sjogren's syndrome (pSS) and non-pSS patients (normal), and submandibular glands (SMG) from Atg5^flox/flox ; Krt14-Cre (cKO) mice were used. ATG5⁺ KRT14⁺ and p62⁺ KRT14⁺ cells were detected by immunofluorescence staining in LSG. TUNEL, immunofluorescence, immunohistochemistry, and western blot were performed to detect cell death in SMG. Saliva was collected in 12-week-old (12 W) and 32-week-old (32 W) mice, then the concentration of calcium and buffering capacity were detected to analyze the function of SG. We found that LSG from pSS patients showed increased p62 and decreased ATG5 in KRT14⁺ cells. We further revealed that in 32 W, (1) the function of salivary glands was significantly impaired in cKO mice, (2) cell death increased in cKO mice, but cl-Caspase 3 was not significantly changed, and (3) cleaved gasdermin D increased and was highly expressed in KRT14⁺ cells of cKO mice. After applying a pyroptosis inhibitor to 32 W mice, the reduced saliva flow rate was rescued. In addition, pyroptosis was also found in KRT14⁺ cells of pSS patients. Collectively, our results indicate that Atg5 deficiency would induce pyroptosis in mice SG, which could lead to functional impairments of SG.

Alterations in DNA Methylation in Orofacial Clefts

Orofacial clefts are among the most common craniofacial anomalies with multifactorial etiologies, including genetics and environments. DNA methylation, one of the most acknowledged mechanisms of epigenetics, is involved in the development of orofacial clefts. DNA methylation has been examined in patients with non-syndromic cleft lip with cleft palate (nsCL/P) from multiple specimens, including blood, saliva, lip, and palate, as well as experimental studies in mice. The results can be reported in two different trends: hypomethylation and hypermethylation. Both hypomethylation and hypermethylation can potentially increase the risk of nsCL/P depending on the types of specimens and the specific regions on each gene and chromosome. This is the most up-to-date review, intending to summarize evidence of the alterations of DNA methylation in association with the occurrence of orofacial clefts. To make things straightforward to understand, we have systematically categorized the data into four main groups: human blood, human tissues, animal models, and the factors associated with DNA methylation. With this review, we are moving closer to the core of DNA methylation associated with nsCL/P development; we hope this is the initial step to find a genetic tool for early detection and prevention of the occurrence of nsCL/P.

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.

Retinoic acid and FGF10 promote the differentiation of pluripotent stem cells into salivary gland placodes

Background

Salivary glands produce saliva that play essential roles in digestion and oral health. Derivation of salivary gland organoids from pluripotent stem cells (PSCs) provides a powerful platform to model the organogenesis processes during development. A few studies attempted to differentiate PSCs into salivary gland organoids. However, none of them could recapitulate the morphogenesis of the embryonic salivary glands, and most of the protocols involved complicated manufacturing processes.

Methods

To generate PSC-derived salivary gland placodes, the mouse embryonic stem cells were first differentiated into oral ectoderm by treatment with BMP4 on day 3. Retinoic acid and bFGF were then applied to the cultures from day 4 to day 6, followed by a 4-day treatment of FGF10. The PSC-derived salivary gland placodes on day 10 were transplanted to kidney capsules to determine the regenerative potential. Quantitative reverse transcriptase–polymerase chain reaction, immunofluorescence, and RNA-sequencing were performed to identify the PSC-derived SG placodes.

Results

We showed that step-wise treatment of retinoic acid and FGF10 promoted the differentiation of PSCs into salivary gland placodes, which can recapitulate the early morphogenetic events of their fetal counterparts, including the thickening, invagination, and then formed initial buds. The PSC-derived salivary gland placodes also differentiated into developing duct structures and could develop to striated and excretory ducts when transplanted in vivo.

Conclusions

The present study provided an easy and safe method to generate salivary gland placodes from PSCs, which offered possibilities for studying salivary gland development in vitro and developing new cell therapies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03033-5.

Parotid Gland Stem Cell Preservation during Intensity-Modulated Radiotherapy for Nasopharyngeal Carcinoma: Dosimetric Analysis and Feasibility

Objective. Parotid gland (PG) is a radiosensitive organ, and xerostomia (XS) is a key factor affecting patients’ life quality after conventional radiotherapy for head and neck tumors. In this study, dosimetry analysis was performed on PG stem cell preservation in intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). Methods. All clinical data of 80 NPC patients diagnosed pathologically in the Radiotherapy Department of Taizhou Hospital of Zhejiang Province Affiliated with Wenzhou Medical University from August 2017 to September 2019 were retrospectively analyzed. Patients were assigned to a regular group and a restricted group according to different IMRT plans, in which a dose limitation for the parotid duct was added in the restricted group in addition to the conventional plan used in the regular group to minimize the parotid duct radiation dose. The differences in planning target volume (PTV) dose distribution, organ at risk (OAR) dose, and dose to the PG and its ducts were compared between the two groups. Results. Significantly higher mean irradiation doses of the brainstem, mandible, and oral cavity were determined in the restricted group compared with the regular group ( $P>0.05$ ), but there was no significant difference in the mean dose of other OARs irradiated ( $P>0.05$ ). As compared to the irradiation of bilateral PGs, no statistical differences were found in the mean irradiation dose and V30 between regular and restricted groups ( $P>0.05$ ), but lower V20 and higher V45 were determined in the restricted group ( $P<0.05$ ). The mean irradiation dose, V15, V20, and V26 of bilateral parotid ducts were lower in the restricted group as compared to the regular group ( $P<0.05$ ). Conclusion. IMRT for NPC can effectively reduce the mean irradiation dose and play a PG stem cell preservation role by giving specific dose limitation conditions to the parotid duct area without affecting PTV dose distribution and OAR irradiation dose, which has certain feasibility.

Enteric viruses replicate in salivary glands and infect through saliva

Enteric viruses like norovirus, rotavirus and astrovirus have long been accepted as spreading in the population through fecal-oral transmission: viruses are shed into feces from one host and enter the oral cavity of another, bypassing salivary glands (SGs) and reaching the intestines to replicate, be shed in feces and repeat the transmission cycle1. Yet there are viruses (for example, rabies) that infect the SGs2,3, making the oral cavity one site of replication and saliva one conduit of transmission. Here we report that enteric viruses productively and persistently infect SGs, reaching titres comparable to those in the intestines. We demonstrate that enteric viruses get released into the saliva, identifying a second route of viral transmission. This is particularly significant for infected infants, whose saliva directly transmits enteric viruses to their mothers' mammary glands through backflow during suckling. This sidesteps the conventional gut-mammary axis route4 and leads to a rapid surge in maternal milk secretory IgA antibodies5,6. Lastly, we show that SG-derived spheroids7 and cell lines8 can replicate and propagate enteric viruses, generating a scalable and manageable system of production. Collectively, our research uncovers a new transmission route for enteric viruses with implications for therapeutics, diagnostics and importantly sanitation measures to prevent spread through saliva.

Reversely immortalized mouse salivary gland cells presented a promising metabolic and fibrotic response upon BMP9/Gdf2 stimulation

The submandibular gland (SMG) and the sublingual gland (SLG) are two of the three major salivary glands in mammals. In mice, they are adjacent to each other and open into the oral cavity, producing saliva to lubricate the mouth and aid in food digestion. Though salivary gland dysfunction accompanied with fibrosis and metabolic disturbance is common in clinic, in-depth mechanistic research is lacking. Currently, research on how to rescue salivary function is challenging, as it must resort to using terminally differentiated acinar cells or precursor acinar cells with unknown differentiation. In this study, we established reversely immortalized mouse primary SMG cells (iSMGCs) and SLG cells (iSLGCs) on the first postnatal day (P0). The iSMGCs and iSLGCs grew well, exhibited many salivary gland characteristics, and retained the metabolism-related genes derived from the original tissue as demonstrated using transcriptome sequencing (RNA-seq) analysis. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of these two cell lines, which overlapped with those of the SMG and SLG, were enriched in cysteine and methionine metabolism. Furthermore, we investigated the role of bone morphogenetic protein 9 (BMP9), also known as growth differentiation factor 2(Gdf2), on metabolic and fibrotic functions in the SMG and SLG. We demonstrated that iSMGCs and iSLGCs presented promising adipogenic and fibrotic responses upon BMP9/Gdf2 stimulation. Thus, our findings indicate that iSMGCs and iSLGCs faithfully reproduce characteristics of SMG and SLG cells and present a promising prospect for use in future study of salivary gland metabolism and fibrosis upon BMP9/Gdf2 stimulation.

Salivary gland organoid culture maintains distinct glandular properties of murine and human major salivary glands

Salivary glands that produce and secrete saliva, which is essential for lubrication, digestion, immunity, and oral homeostasis, consist of diverse cells. The long-term maintenance of diverse salivary gland cells in organoids remains problematic. Here, we establish long-term murine and human salivary gland organoid cultures. Murine and human salivary gland organoids express gland-specific genes and proteins of acinar, myoepithelial, and duct cells, and exhibit gland functions when stimulated with neurotransmitters. Furthermore, human salivary gland organoids are established from isolated basal or luminal cells, retaining their characteristics. Single-cell RNA sequencing also indicates that human salivary gland organoids contain heterogeneous cell types and replicate glandular diversity. Our protocol also enables the generation of tumoroid cultures from benign and malignant salivary gland tumor types, in which tumor-specific gene signatures are well-conserved. In this study, we provide an experimental platform for the exploration of precision medicine in the era of tissue regeneration and anticancer treatment.

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.

Salivary Gland Function, Development and Regeneration

Salivary glands produce and secrete saliva, which is essential for maintaining oral health and overall health. Understanding both the unique structure and physiological function of salivary glands, as well as how they are affected by disease and injury will direct the development of therapy to repair and regenerate them. Significant recent advances, particularly in the OMICS field, increase our understanding of how salivary glands develop at the cellular, molecular and genetic levels; the signaling pathways involved, the dynamics of progenitor cell lineages in development, homeostasis and regeneration and the role of the extracellular matrix microenvironment. These provide a template for cell and gene therapies as well as bioengineering approaches to repair or regenerate salivary function.

Induction of Salivary Gland-Like Tissue by Induced Pluripotent Stem Cells In Vitro

BACKGROUND

To investigate the in vitro induction of salivary gland-like tissue by ips cells in an interferon regulatory factor 6 (IRF6) overexpression and parotid conditioned medium environment.

METHODS

Urine-derived ips cells were isolated, identified, transfected with IRF6 and cultured in parotid conditioned medium to induce ips cells into salivary gland differentiation, morphological changes of ips cells were observed, CCK-8 was used to determine the cell proliferation efficiency and transcriptome sequencing was used to detect the expression of genes related to parotid gland formation.

RESULTS

Immunofluorescence staining showed that the isolated ips cells were positive for NANOG, SSEA4 and OCT4 and had embryonic-like stem cell characteristics; CCK-8 showed that there was no statistical difference in the proliferation efficiency between the IRF6⁺ induced group and the simple induced group after induction of ips cells into salivary glands. The results of transcriptome sequencing showed that there were a total of 643 differentially expressed genes, including 365 up-regulated genes and 278 down-regulated genes in the IRF6⁺ induced group compared to the blank control group, and the salivary gland related genes HAPLN1, CCL2, MSX2, ANXA1, CYP11A1, HES1 and LUM were all highly expressed in the IRF6⁺ induced group.

CONCLUSION

IRF6 promotes salivary gland differentiation in urine-derived iPSCs, and its mechanism of promoting differentiation may be that IRF6 upregulates the expression of HAPLN1, CCL2, MSX2, ANXA1, CYP11A1, HES1 and LUM to promote epithelial differentiation.

Differential expression of CD11c defines two types of tissue-resident macrophages with different origins in steady-state salivary glands

Gland macrophages are primed for gland development and functions through interactions within their niche. However, the phenotype, ontogeny, and function of steady-state salivary gland (SG) macrophages remain unclear. We herein identified CD11c⁺ and CD11c^- subsets among CD64⁺ macrophages in steady-state murine SGs. CD11c^- macrophages were predominant in the SGs of embryonic and newborn mice and decreased with advancing age. CD11c⁺ macrophages were rarely detected in the embryonic period, but rapidly expanded after birth. CD11c⁺, but not CD11c^-, macrophage numbers decreased in mice treated with a CCR2 antagonist, suggesting that CD11c⁺ macrophages accumulate from bone marrow-derived progenitors in a CCR2-dependent manner, whereas CD11c^- macrophages were derived from embryonic progenitors in SGs. CD11c⁺ and CD11c^- macrophages strongly expressed colony-stimulating factor (CSF)-1 receptor, the injection of an anti-CSF-1 receptor blocking antibody markedly reduced both subsets, and SGs strongly expressed CSF-1, indicating the dependency of SG resident macrophage development on CSF-1. The phagocytic activity of SG macrophages was extremely weak; however, the gene expression profile of SG macrophages indicated that SG macrophages regulate gland development and functions in SGs. These results suggest that SG CD11c⁺ and CD11c^- macrophages are developed and instructed to perform SG-specific functions in steady-state SGs.

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.

Acupuncture Therapies for Addressing Post Odontectomy Neuropathy

Background: Third-molar extraction is a common oral surgical procedure that can cause complications. Although rare, nerve injuries that result in permanent neuropathy can occur and include a variety of complaints such as sensory disorders, taste-sensation disorders, speech articulation disorders, etc. Acupuncture is one of several nonpharmacologic therapies that has played a role in managing neuropathic lesions and has been proven to produce good results. Case: A 44-year-old woman with postodontectomy neuropathy developed paresthesia, dysarthria, xerostomia, dysgeusia, pain in the gums and lower right jaw, a chewing disorder, and cephalgia. Her numerical rating scale (NRS) results were: lower right gum pain, 3/10; numbness of the tongue, 4/10; and headache, 1/10. A physical examination revealed dysarthria, a decreased sense of sharpness and dullness in the right mandibular nerve branches, decreased right masseter muscle contractions, and tenderness on the right GB 20 point. Electromyography revealed partial functional lesions in the postganglion at the right fifth cranial nerve. She had body acupuncture therapy at GV 20, GB 20, ST 5, ST 6, ST 7, CV 23, LI 4, HT 5, ST 36, LU 7, and KI 6; ear acupuncture at the Parotid and Shenmen points; and treatment with the Tan Balance Method. Results: After 3 consecutive sessions of acupuncture therapy, this patient's symptoms were reduced. Conclusions: Acupuncture was helpful for reducing paresthesia, dysarthria, xerostomia, dysgeusia, gum and lower right jaw pain, a chewing disorder, and cephalgia in this patient with postodontectomy neuropathy. Clinical trials are needed to support the findings in this case.

Organoid Models for Salivary Gland Biology and Regenerative Medicine

The salivary gland is composed of an elegant epithelial network that secrets saliva and maintains oral homeostasis. While cell lines and animal models furthered our understanding of salivary gland biology, they cannot replicate key aspects of the human salivary gland tissue, particularly the complex architecture and microenvironmental features that dictate salivary gland function. Organoid cultures provide an alternative system to recapitulate salivary gland tissue in vitro, and salivary gland organoids have been generated from pluripotent stem cells and adult stem/progenitor cells. In this review, we describe salivary gland organoids, the advances and limitations, and the promising potential for regenerative medicine.

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.

Developmental morphological analyses on the preglottal salivary gland in Japanese quails (Coturnix japonica)

To understand the development of the mucous preglottal salivary gland in Coturnix japonica (Japanese quail), morphological and histochemical studies were performed on 20 healthy Japanese quail embryos (aging from 10th to 17th incubation days) and 25 healthy quail chicks (aging from 0th to 60th days). The primordia of preglottal salivary gland were observed as an epithelial bud at the early embryonic stage, which then elongated and differentiated into secretory units by the end of this stage. In Japanese quails, the preglottal salivary gland was a mucous polystomatic tubuloalveolar unpaired gland composed of two lateral portions and a middle one embedded into the submucosa of the lingual root. The gland openings accompanied taste pore (8.17 μm) of taste buds associated salivary glands type; some skeletal muscle fibers embedded among secretory lobules extended from muscle cricohyoideus at 14th day-old quail chick. Also, both herbts corpuscles and secretory motor plexus could be detected among secretory lobules. Based on our investigations, the development of the preglottal salivary gland could clearly be distinguished in the embryonic stage into pre bud and bud stages at 10th day old, cord and branching stages ended by cavitation at 11th day old, canalization stage at 13th day old, lobulation and secretory stages by the 17th day old. The secretory materials showed different histochemical reactions ended with highly alcinophilic mucous indicated highly sialomucin (acidic) content. Myoepithelial cells could be demonstrated at a 17-day old quail embryo and thereafter surrounded the secretory endpieces of the preglottal salivary gland.

The intact parasympathetic nerve promotes submandibular gland regeneration through ductal cell proliferation

Objectives

Salivary gland regeneration is closely related to the parasympathetic nerve; however, the mechanism behind this relationship is still unclear. The aim of this study was to evaluate the relationship between the parasympathetic nerve and morphological differences during salivary gland regeneration.

Materials and Methods

We used a duct ligation/deligation‐induced submandibular gland regeneration model of Sprague‐Dawley (SD) rats. The regenerated submandibular gland with or without chorda lingual (CL) innervation was detected by haematoxylin–eosin staining, real‐time PCR (RT‐PCR), immunohistochemistry and Western blotting. We counted the number of Ki67‐positive cells to reveal the proliferation process that occurs during gland regeneration. Finally, we examined the expression of the following markers: aquaporin 5, cytokeratin 7, neural cell adhesion molecule (NCAM) and polysialyltransferases.

Results

Intact parasympathetic innervation promoted submandibular gland regeneration. The process of gland regeneration was significantly repressed by cutting off the CL nerve. During gland regeneration, Ki67‐positive cells were mainly found in the ductal structures. Moreover, the expression of NCAM and polysialyltransferases‐1 (PST) expression in the innervation group was significantly increased during early regeneration and decreased in the late stages. In the denervated submandibular glands, the expression of NCAM decreased during regeneration.

Conclusions

Our findings revealed that the regeneration of submandibular glands with intact parasympathetic innervation was associated with duct cell proliferation and the increased expression of PST and NCAM.

Comparing development and regeneration in the submandibular gland highlights distinct mechanisms

A common question in organ regeneration is the extent to which regeneration recapitulates embryonic development. To investigate this concept, we compared the expression of two highly interlinked and essential genes for salivary gland development, Sox9 and Fgf10, during submandibular gland development, homeostasis and regeneration. Salivary gland duct ligation/deligation model was used as a regenerative model. Fgf10 and Sox9 expression changed during regeneration compared to homeostasis, suggesting that these key developmental genes play important roles during regeneration, however, significantly both displayed different patterns of expression in the regenerating gland compared to the developing gland. Regenerating glands, which during homeostasis had very few weakly expressing Sox9-positive cells in the striated/granular ducts, displayed elevated expression of Sox9 within these ducts. This pattern is in contrast to embryonic development, where Sox9 expression was absent in the proximally developing ducts. However, similar to the elevated expression at the distal tip of the epithelium in developing salivary glands, regenerating glands displayed elevated expression in a subpopulation of acinar cells, which during homeostasis expressed Sox9 at lower levels. A shift in expression of Fgf10 was observed from a widespread mesenchymal pattern during organogenesis to a more limited and predominantly epithelial pattern during homeostasis in the adult. This restricted expression in epithelial cells was maintained during regeneration, with no clear upregulation in the surrounding mesenchyme, as might be expected if regeneration recapitulated development. As both Fgf10 and Sox9 were upregulated in proximal ducts during regeneration, this suggests that the positive regulation of Sox9 by Fgf10, essential during development, is partially reawakened during regeneration using this model. Together these data suggest that developmentally important genes play a key role in salivary gland regeneration but do not precisely mimic the roles observed during development.

DNA Methylation Variation Is Identified in Monozygotic Twins Discordant for Non-syndromic Cleft Lip and Palate

Non-syndromic cleft lip with or without cleft palate (NSCLP) is the most common craniofacial birth defect. The etiology of NSCLP is complex with multiple genes and environmental factors playing causal roles. Although studies have identified numerous genetic markers associated with NSCLP, the role of epigenetic variation remains relatively unexplored. Because of their identical DNA sequences, monozygotic (MZ) twins discordant for NSCLP are an ideal model for examining the potential contribution of DNA methylation to non-syndromic orofacial clefting. In this study, we compared the patterns of whole genome DNA methylation in six MZ twin pairs discordant for NSCLP. Differentially methylated positions (DMPs) and regions (DMRs) were identified in NSCLP candidate genes, including differential methylation in MAFB and ZEB2 in two independent MZ twin pairs. In addition to DNA methylation differences in NSCLP candidate genes, we found common differential methylation in genes belonging to the Hippo signaling pathway, implicating this mechanosensory pathway in the etiology of NSCLP. The results of this novel approach using MZ twins discordant for NSCLP suggests that differential methylation is one mechanism contributing to NSCLP, meriting future studies on the role of DNA methylation in familial and sporadic NSCLP.

Inhibition of Aurora Kinase B activity disrupts development and differentiation of salivary glands

Little is known about the key molecules that regulate cell division during organogenesis. Here we determine the role of the cell cycle promoter aurora kinase B (AURKB) during development, using embryonic salivary glands (E-SGs) as a model. AURKB is a serine/threonine kinase that regulates key events in mitosis, which makes it an attractive target for tailored anticancer therapy. Many reports have elaborated on the role of AURKB in neoplasia and cancer; however, no previous study has shown its role during organ development. Our previous experiments have highlighted the essential requirement for AURKB during adult exocrine regeneration. To investigate if AURKB is similarly required for progression during embryonic development, we pharmacologically inhibited AURKB in developing submandibular glands (SMGs) at embryonic day (E)13.5 and E16.5, using the highly potent and selective drug Barasertib. Inhibition of AURKB interfered with the expansion of the embryonic buds. Interestingly, this effect on SMG development was also seen when the mature explants (E16.5) were incubated for 24 h with another cell cycle inhibitor Aphidicolin. Barasertib prompted apoptosis, DNA damage and senescence, the markers of which (cleaved caspase 3, γH2AX, SA-βgal and p21, respectively), were predominantly seen in the developing buds. In addition to a reduction in cell cycling and proliferation of the epithelial cells in response to AURKB inhibition, Barasertib treatment led to an excessive generation of reactive oxygen species (ROS) that resulted in downregulation of the acinar differentiation marker Mist1. Importantly, inhibition of ROS was able to rescue this loss of identity, with Mist1 expression maintained despite loss of AURKB. Together, these data identify AURKB as a key molecule in supporting embryonic development and differentiation, while inhibiting senescence-inducing signals during organogenesis.

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.

Generation of a Single-Cell RNAseq Atlas of Murine Salivary Gland Development

Understanding the dynamic transcriptional landscape throughout organ development will provide a template for regenerative therapies. Here, we generated a single-cell RNA sequencing atlas of murine submandibular glands identifying transcriptional profiles that revealed cellular heterogeneity during landmark developmental events: end bud formation, branching morphogenesis, cytodifferentiation, maturation, and homeostasis. Trajectory inference analysis suggests plasticity among acinar and duct populations. We identify transcription factors correlated with acinar differentiation including Spdef, Etv1, and Xbp1, and loss of Ybx1, Eno1, Sox11, and Atf4. Furthermore, we characterize two intercalated duct populations defined by either Gfra3 and Kit, or Gstt1. This atlas can be used to investigate specific cell functions and comparative studies predicting common mechanisms involved in development of branching organs.

Secretome Analysis of Inductive Signals for BM-MSC Transdifferentiation into Salivary Gland Progenitors

Severe dry mouth in patients with Sjögren's Syndrome, or radiation therapy for patients with head and neck cancer, significantly compromises their oral health and quality of life. The current clinical management of xerostomia is limited to palliative care as there are no clinically-proven treatments available. Previously, our studies demonstrated that mouse bone marrow-derived mesenchymal stem cells (mMSCs) can differentiate into salivary progenitors when co-cultured with primary salivary epithelial cells. Transcription factors that were upregulated in co-cultured mMSCs were identified concomitantly with morphological changes and the expression of acinar cell markers, such as α-amylase (AMY1), muscarinic-type-3-receptor(M3R), aquaporin-5(AQP5), and a ductal cell marker known as cytokeratin 19(CK19). In the present study, we further explored inductive molecules in the conditioned media that led to mMSC reprogramming by high-throughput liquid chromatography with tandem mass spectrometry and systems biology. Our approach identified ten differentially expressed proteins based on their putative roles in salivary gland embryogenesis and development. Additionally, systems biology analysis revealed six candidate proteins, namely insulin-like growth factor binding protein-7 (IGFBP7), cysteine-rich, angiogenetic inducer, 61(CYR61), agrin(AGRN), laminin, beta 2 (LAMB2), follistatin-like 1(FSTL1), and fibronectin 1(FN1), for their potential contribution to mMSC transdifferentiation during co-culture. To our knowledge, our study is the first in the field to identify soluble inductive molecules that drive mMSC into salivary progenitors, which crosses lineage boundaries.

Pancreas morphogenesis: Branching in and then out

The pancreas of adult mammals displays a branched structure which transports digestive enzymes produced in the distal acini through a tree-like network of ducts into the duodenum. In contrast to several other branched organs, its branching patterns are not stereotypic. Moreover, the branches do not grow from dichotomic splitting of an initial stem but rather from the formation of microlumen in a mass of cells. These lumen progressively assemble into a hyperconnected network that refines into a tree by the time of birth. We review the cell remodeling events and the molecular mechanisms governing pancreas branching, as well as the role of the surrounding tissues in this process. Furthermore, we draw parallels with other branched organs such as the salivary and mammary gland.

Ethanol binge drinking during pregnancy and its effects on salivary glands of offspring rats: oxidative stress, morphometric changes and salivary function impairments

OBJECTIVES

To investigate the biochemical and morphological effects of ethanol (EtOH) binge drinking during pregnancy on parotid glands (PG), submandibular glands (SMG), and saliva of offspring rats.

METHODS

Pregnant Wistar rats (n = 8) were exposed to EtOH consumption (3 g/kg/day - 20 % w/v) for three consecutive days. The saliva of 40-day-old offspring rats was collected to determine amylase activity and total protein concentration. PG and SMG were collected to performe oxidative biochemistry, morphometric and immunohistochemistry analyses (Student's t-test, p < .05).

RESULTS

EtOH consumption during pregnancy significantly decreased the total protein concentration and decreased amylase activity. In the PG, the EtOH group showed increased lipid peroxidation and decreased antioxidant capacity against peroxyl. In the SMG, the EtOH group showed increased lipid peroxidation and NOx metabolite levels. PG exposed to EtOH showed a decrease of acini, ducts, and total parenchymal area. SMG exposed to EtOH showed an increase in the total stromal area. The expression of CK-19 and Vimentin were found not different between groups.

CONCLUSIONS

For the first time, a three-day EtOH binge-drinking protocol during pregnancy is associated with oxidative stress and morphometric alterations in the salivary glands of offspring rats and with the functional reduction of the main salivary enzyme (amylase).

CLINICAL RELEVANCE

EtOH consumption during pregnancy altered the morphology and physiology of the salivary glands of offspring rats.