Role of the mTOR signalling pathway in salivary gland development

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.

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.

Localization of phospholipid-related signal molecules in salivary glands of rodents: A review

BACKGROUND

In the 1950s, Hokin conducted initial studies on phosphoinositide turnover/cycle in salivary glandular cells. From these studies, the idea emerged that receptor-mediated changes in intramembranous levels of phosphoinositides represent an early step in the stimulus-response pathway. Based on this idea and the general view that knowledge of the exact localization of a given endogenous molecule in cells in situ is important for understanding its functional significance, we have reviewed available information about the localization of several representative phosphoinositide-signaling molecules in the salivary glands in situ in mice.

HIGHLIGHT

We focused on phosphatidylinositol 4-kinase, phosphatidylinositol 4 phosphate 5-kinase α, β, γ, phospholipase Cβ, muscarinic cholinoceptors 1 and 3, diacylglycerol kinase ζ, phospholipase D1 and 2, ADP-ribosylation factor 6 and its exchange factors for Arf6, and cannabinoid receptors. These molecules individually exhibit differential localization in a spatiotemporal manner in the exocrine glands, making it possible to deduce their functional significance, such as their involvement in secretion and cell differentiation.

CONCLUSION

Although phosphoinositide-signaling molecules whose in situ localization in glandular cells has been clarified are still limited, the obtained information on their localization suggests that their functional significance is more valuable in glandular ducts than in acini. It thus suggests the necessity of greater attention to the ducts in their physio-pharmacological analyses. The purpose of this review is to encourage more in situ localization studies of phosphoinositide-signaling molecules with an aim to further understand their possible involvement in the pathogenesis of salivary gland diseases.

Simultaneous Expression of PD-1 and PD-L1 in Peripheral and Central Immune Cells and Tumor Cells in the Benign and Malignant Salivary Gland Tumors Microenvironment

BACKGROUND

To investigate the differential expression of PD-1 and PD-L1 in salivary gland tumors (SGTs, malignant and benign subtypes) and determine their association with the clinicopathological characterization of the patients.

METHODS

The immunohistochemistry was used to examine PD-1 and PD-L1 expression in specimens from 83 patients with primary SGTs including salivary ductal carcinoma (SDC), adenoid cystic carcinoma (AdCC), acinic cell carcinoma (ACC), mucoepidermoid carcinoma (MEC), warthin's tumors (WT), poleomorphic adenoma (PA) and other subtypes.

RESULTS

The expression of PD-1 in peripheral and central immune cells (ICs) of MEC, and peripheral ICs of ACC was significantly higher than those with AdCC (P = 0.02, P = 0.02, P = 0.03, respectively). Interestingly, the expression of PD-1 was also observed in peripheral and central malignant tumor cells (TCs), particularly in SDC and ACC. Despite no significant difference in PD-L1 expression of TCs among malignant subtypes, the peripheral and central ICs of ACC and MEC were revealed to express PDL-1 significantly more than those with AdCC (P < 0.05). WTs were rich in PD-1/PD-L1 expressing ICs. However, the tumor microenvironment of PA generally had low levels of PD-1/PD-L1 expression. In general, the expression of PD-1 in peripheral and central TCs was found to be significantly higher in malignant tumors than in benign ones (P = 0.002 and P = 0.003, respectively).

CONCLUSION

The simultaneous presentation of PD-1 and PD-L1 in TCs and ICs of SGTs, their significant association with disease severity as well as the positive correlation between these immune checkpoints may suggest the therapeutic potential of anti-PD-1 and anti-PDL-1 combinational immunotherapy for SGTs.

Immunological and biological dissection of normal and tumoral salivary glands

Salivary glands naturally play central roles in oral immunity. The salivary glands microenvironment inevitable may be exposed to exogenous factors consequently triggering the initiation and formation of various malignant and benign tumors. Mesenchymal stem cells are recruited into salivary gland microenvironment, interact with tumor cells, and induce inhibitory cytokines as well as cells with immunosuppressive phenotypes such as myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs). The immune components and tumor immune responses in malignant and benign SGTs are still under investigation. Immune responses may directly play a limiting role in tumor growth and expansion, or may participate in formation of a rich milieu for tumor growth in cooperation with other cellular and regulatory molecules. Immune checkpoint molecules (e.g. PDLs, HLA-G and LAG3) are frequently expressed on tumor cells and/or tumor-infiltrating lymphocytes (TILs) in salivary gland microenvironment, and an increase in their expression is associated with T cell exhaustion, immune tolerance and tumor immune escape. Chemokines and chemokine receptors have influential roles on aggressive behaviors of SGTs, and thereby they could be candidate targets for cancer immunotherapy. To present a broad knowledge on salivary glands, this review first provides a brief description on immunological functions of normal salivary glands, and then describe the SGT's tumor microenvironment, by focusing on mesenchymal stem cells, immune cell subsets, immune checkpoint molecules, chemokines and chemokine receptors, and finally introduces immune checkpoint inhibitors as well as potential targets for cancer therapy.

The Role of mTOR and Injury in Developing Salispheres

Salispheres are the representative primitive cells of salivary glands grown in vitro in a nonadherent system. In this study, we used the ligation model for salisphere isolation after seven days of obstruction of the main excretory duct of the submandibular gland. The mammalian target of rapamycin (mTOR) is a critical signalling pathway involved in many cellular functions and is suggested to play a role in atrophy. We determined the role of mTOR and injury in the formation and development of salispheres. Morphological assessments and Western blot analysis illustrated how mTOR inhibition by rapamycin impaired the assembly of salispheres and how indirect stimulation of mTOR by lithium chloride (LiCl) assisted in the expansion of the salispheres. The use of rapamycin highlighted the necessity of mTOR for the development of salispheres as it affected the morphology and inhibited the phosphorylation of the eukaryotic translation initiation factor 4E-binding protein (4e-bp1). mTOR activity also appeared to be a crucial regulator for growing salispheres, even from the ligated gland. However, atrophy induced by ductal ligation resulted in a morphological alteration. The phosphorylation of 4e-bp1 and S6 ribosomal protein in cultured salispheres from ligated glands suggests that mTOR was not responsible for the morphological modification, but other unexplored factors were involved. This exploratory study indicates that active mTOR is essential for growing healthy salispheres. In addition, mTOR stimulation by LiCl could effectively play a role in the expansion of salispheres. The impact of atrophy on salispheres suggests a complex mechanism behind the morphological alteration, which requires further investigation.

Establishment of a Mouse Submandibular Salivary Gland Organ Culture

The salivary glands produce saliva and are important in maintaining oral health. Saliva keeps the mouth moist, cleanses the oral cavity, aids digestion, and has antibacterial properties. Saliva also helps in swallowing and speech. Investigating the development of the salivary glands is thus relevant in the context of both health and disease. Various cell culture methods have been used to study salivary gland development, including culturing cells in two dimensions (2D). Under physiological conditions, cells constantly interact with other cells and the extracellular matrix, which controls complex biological functions such as cell migration and apoptosis, and can modulate gene expression. Since many of these functions are not accurately represented or reproduced in 2D culture, the results of in vitro experiments using such culture methods are often not reflected in vivo. The use of 3D cultures, such as organ cultures, has helped address this issue and has emerged as a model that better reflects the in vivo physiological environment. Here, we describe a protocol for establishing submandibular salivary gland organ culture that is more concise and simpler than previous methods and includes the separation and dissection of the salivary glands. We also describe the use of environmental stress (hypoxic stimulation) and inhibitors (U0126, LY294002, and rapamycin) to elucidate signaling pathways involved in salivary gland development. This protocol can provide researchers with a simpler and more robust method of salivary gland organ culture, enabling analysis of organ-based signaling pathways to advance developmental biology research. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Submandibular salivary gland organ culture Basic Protocol 2: Analysis of salivary gland development in the presence of hypoxia and signaling pathway inhibitors Basic Protocol 3: Western blotting using submandibular salivary gland organ culture.

Oxidative Stress and Metabolism: A Mechanistic Insight for Glyphosate Toxicology

Glyphosate (GLYP) is a widely used pesticide; it is considered to be a safe herbicide for animals and humans because it targets 5-enolpyruvylshikimate-3-phosphate synthase. However, there has been increasing evidence that GLYP causes varying degrees of toxicity. Moreover, oxidative stress and metabolism are highly correlated with toxicity. This review provides a comprehensive introduction to the toxicity of GLYP and, for the first time, systematically summarizes the toxicity mechanism of GLYP from the perspective of oxidative stress, including GLYP-mediated oxidative damage, changes in antioxidant status, altered signaling pathways, and the regulation of oxidative stress by exogenous substances. In addition, the metabolism of GLYP is discussed, including metabolites,metabolic pathways, metabolic enzymes, and the toxicity of metabolites. This review provides new ideas for the toxicity mechanism of GLYP and proposes effective strategies for reducing its toxicity.

The HIF-1α pathway plays a critical role in salivary gland development in ex vivo organ cultures

The transcription factor, hypoxia‐inducible factor‐1α (HIF‐1α), has previously been shown to upregulate the expression of hypoxia‐related genes, including erythropoietin (EPO). However, the role of hypoxia‐inducible factor‐1α in morphogenesis during salivary gland development is unclear. We investigated the function of HIF‐1α in submandibular gland (SMG) organ cultures obtained from embryonic day 13.5 embryos from ICR female mice. Expression of HIF‐1α, glucose transporter 1, and vascular endothelial growth factor was induced under hypoxia (5% O₂). We further showed that BAY 87‐2243‐mediated inhibition of HIF‐1α suppressed salivary gland development. Under severe hypoxia (1% O₂), HIF‐1α did not promote salivary gland development; this was due to suppression of cell proliferation and inhibition of the cell cycle and not because of autophagy and apoptosis. Additionally, using the inhibitor U0126, we verified that the ERK1/2 pathway is upstream of HIF‐1α. Overall, we found that the HIF‐1α signaling pathway plays a critical role in salivary gland development in ex vivo SMG organ cultures.