Matrigel improves functional properties of primary human salivary gland cells

Bioprinting salivary gland models and their regenerative applications

OBJECTIVE

Salivary gland (SG) hypofunction is a common clinical condition arising from radiotherapy to suppress head and neck cancers. The radiation often destroys the SG secretory acini, and glands are left with limited regenerative potential. Due to the complex architecture of SG acini and ducts, three-dimensional (3D) bioprinting platforms have emerged to spatially define these in vitro epithelial units and develop mini-organs or organoids for regeneration. Due to the limited body of evidence, this comprehensive review highlights the advantages and challenges of bioprinting platforms for SG regeneration.

METHODS

SG microtissue engineering strategies such as magnetic 3D bioassembly of cells and microfluidic coaxial 3D bioprinting of cell-laden microfibers and microtubes have been proposed to replace the damaged acinar units, avoid the use of xenogeneic matrices (like Matrigel), and restore salivary flow.

RESULTS

Replacing the SG damaged organ is challenging due to its complex architecture, which combines a ductal network with acinar epithelial units to facilitate a unidirectional flow of saliva. Our research group was the first to develop 3D bioassembly SG epithelial functional organoids with innervation to respond to both cholinergic and adrenergic stimulation. More recently, microtissue engineering using coaxial 3D bioprinting of hydrogel microfibers and microtubes could also supported the formation of viable epithelial units. Both bioprinting approaches could overcome the need for Matrigel by facilitating the assembly of adult stem cells, such as human dental pulp stem cells, and primary SG cells into micro-sized 3D constructs able to produce their own matrix and self-organize into micro-modular tissue clusters with lumenized areas. Furthermore, extracellular vesicle (EV) therapies from organoid-derived secretome were also designed and validated ex vivo for SG regeneration after radiation damage.

CONCLUSION

Magnetic 3D bioassembly and microfluidic coaxial bioprinting platforms have the potential to create SG mini-organs for regenerative applications via organoid transplantation or organoid-derived EV therapies.

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.

Involvement of aquaporin 5 in Sjögren's syndrome

Sjögren's syndrome (SS) is a chronic autoimmune disease with the pathological hallmark of lymphoplasmacytic infiltration of exocrine glands - more specifically salivary and lacrimal glands - resulting in a diminished production of tears and saliva (sicca syndrome). The pathophysiology underscoring the mechanisms of the sicca symptoms in SS has still yet to be unraveled but recent advances have identified a cardinal role of aquaporin-5 (AQP5) as a key player in saliva secretion as well as salivary gland epithelial cell dysregulation. AQP5 expression and localization are significantly altered in salivary glands from patients and mice models of the disease, shedding light on a putative mechanism accounting for diminished salivary flow. Furthermore, aberrant expression and localization of AQP5 protein partners, such as prolactin-inducible protein and ezrin, may account for altered AQP5 localization in salivary glands from patients suffering from SS and are considered as new players in SS development. This review provides an overview of the role of AQP5 in SS salivary gland epithelial cell dysregulation, focusing on its trafficking and protein-protein interactions.

Engineered organoids in oral and maxillofacial regeneration

Oral and maxillofacial organoids, as three-dimensional study models of organs, have attracted increasing attention in tissue regeneration and disease modeling. However, traditional strategies for organoid construction still fail to precisely recapitulate the key characteristics of real organs, due to the difficulty in controlling the self-organization of cells in vitro. This review aims to summarize the recent progress of novel approaches to engineering oral and maxillofacial organoids. First, we introduced the necessary components and their roles in forming oral and maxillofacial organoids. Besides, we discussed cutting-edge technology in advancing the architecture and function of organoids, especially focusing on oral and maxillofacial tissue regeneration via novel strategy with designed cell-signal scaffold compounds. Finally, current limitations and future prospects of oral and maxillofacial organoids were represented to provide guidance for further disciplinary progression and clinical application to achieve organ regeneration.

Hydrogels for Salivary Gland Tissue Engineering

Mimicking the complex architecture of salivary glands (SGs) outside their native niche is challenging due their multicellular and highly branched organization. However, significant progress has been made to recapitulate the gland structure and function using several in vitro and ex vivo models. Hydrogels are polymers with the potential to retain a large volume of water inside their three-dimensional structure, thus simulating extracellular matrix properties that are essential for the cell and tissue integrity. Hydrogel-based culture of SG cells has seen a tremendous success in terms of developing platforms for cell expansion, building an artificial gland, and for use in transplantation to rescue loss of SG function. Both natural and synthetic hydrogels have been used widely in SG tissue engineering applications owing to their properties that support the proliferation, reorganization, and polarization of SG epithelial cells. While recent improvements in hydrogel properties are essential to establish more sophisticated models, the emphasis should still be made towards supporting factors such as mechanotransduction and associated signaling cues. In this concise review, we discuss considerations of an ideal hydrogel-based biomaterial for SG engineering and their associated signaling pathways. We also discuss the current advances made in natural and synthetic hydrogels for SG tissue engineering applications.

Current approaches for the regeneration and reconstruction of ocular surface in dry eye

Significant research revealed the preocular tear film composition and regulations that remain vital for maintaining Ocular surface functional integrity. Inflammation triggered by many factors is the hallmark of Ocular surface disorders or dry eyes syndrome (DES). The tear deficiencies may lead to ocular surface desiccation, corneal ulceration and/or perforation, higher rates of infectious disease, and the risk of severe visual impairment and blindness. Clinical management remains largely supportive, palliative, and frequent, lifelong use of different lubricating agents. However, few advancements such as punctal plugs, non-steroidal anti-inflammatory drugs, and salivary gland autografts are of limited use. Cell-based therapies, tissue engineering, and regenerative medicine, have recently evolved as long-term cures for many diseases, including ophthalmic diseases. The present article focuses on the different regenerative medicine and reconstruction/bioengineered lacrimal gland formation strategies reported so far, along with their limiting factors and feasibility as an effective cure in future.

Isolation, Culture, and Characterization of Primary Salivary Gland Cells

Primary cells are an essential tool for in vitro studies and are obtained directly from living tissues or organs. They closely mimic the physiological state and maintain in vivo functions for short periods of time under optimal conditions. Isolation and culture of salivary gland (SG) cells are useful to decipher the various mechanisms involved in salivary gland dysfunction. However, unlike some other primary cell cultures, SG cell cultures from patient-derived tissues present several challenges. They are difficult to obtain, culture, expand, and characterize due to their sensitive heterogenous cell population and limited expansion potential. In addition, the majority of saliva-secreting acinar cells fail to maintain a differentiated state ex vivo for long periods, and eventually succumb to an acinar-to-ductal metaplasia, losing their secretory phenotype and functions. Herein, we describe two detailed protocols for primary SG cell isolation, culture, and expansion from human (or mouse) salivary tissues using serum-free culture media. We also describe the growth kinetics of these primary cells along with their immunocytochemical characterization. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Preparation of SG single-cell culture from freshly obtained human or mouse SG tissues. Basic Protocol 2: Preparation of SG explant culture from freshly obtained human or mouse SG tissues.

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.

Encapsulation of Primary Salivary Gland Acinar Cell Clusters and Intercalated Ducts (AIDUCs) within Matrix Metalloproteinase (MMP)-Degradable Hydrogels to Maintain Tissue Structure and Function

Progress in the development of salivary gland regenerative strategies is limited by poor maintenance of the secretory function of salivary gland cells (SGCs) in vitro. To reduce the precipitous loss of secretory function, a modified approach to isolate intact acinar cell clusters and intercalated ducts (AIDUCs), rather than commonly used single cell suspension, is investigated. This isolation approach yields AIDUCs that maintain many of the cell-cell and cell-matrix interactions of intact glands. Encapsulation of AIDUCs in matrix metalloproteinase (MMP)-degradable PEG hydrogels promotes self-assembly into salivary gland mimetics (SGm) with acinar-like structure. Expression of Mist1, a transcription factor associated with secretory function, is detectable throughout the in vitro culture period up to 14 days. Immunohistochemistry also confirms expression of acinar cell markers (NKCC1, PIP and AQP5), duct cell markers (K7 and K5), and myoepithelial cell markers (SMA). Robust carbachol and ATP-stimulated calcium flux is observed within the SGm for up to 14 days after encapsulation, indicating that secretory function is maintained. Though some acinar-to-ductal metaplasia is observed within SGm, it is reduced compared to previous reports. In conclusion, cell-cell interactions maintained within AIDUCs together with the hydrogel microenvironment may be a promising platform for salivary gland regenerative strategies.

Alginate Hydrogel Microtubes for Salivary Gland Cell Organization and Cavitation

Understanding the different regulatory functions of epithelial and mesenchymal cell types in salivary gland development and cellular organization is essential for proper organoid formation and salivary gland tissue regeneration. Here, we demonstrate a biocompatible platform using pre-formed alginate hydrogel microtubes to facilitate direct epithelial–mesenchymal cell interaction for 3D salivary gland cell organization, which allows for monitoring cellular organization while providing a protective barrier from cell-cluster loss during medium changes. Using mouse salivary gland ductal epithelial SIMS cells as the epithelial model cell type and NIH 3T3 fibroblasts or primary E16 salivary mesenchyme cells as the stromal model cell types, self-organization from epithelial–mesenchymal interaction was examined. We observed that epithelial and mesenchymal cells undergo aggregation on day 1, cavitation by day 4, and generation of an EpCAM-expressing epithelial cell layer as early as day 7 of the co-culture in hydrogel microtubes, demonstrating the utility of hydrogel microtubes to facilitate heterotypic cell–cell interactions to form cavitated organoids. Thus, pre-formed alginate microtubes are a promising co-culture method for further understanding epithelial and mesenchymal interaction during tissue morphogenesis and for future practical applications in regenerative medicine.

Egg White Alginate as a Novel Scaffold Biomaterial for 3D Salivary Cell Culturing

Saliva production by salivary glands play a crucial role in oral health. The loss of salivary gland function could lead to xerostomia, a condition also known as dry mouth. Significant reduction in saliva production could lead to further complications such as difficulty in speech, mastication, and increased susceptibility to dental caries and oral infections and diseases. While some palliative treatments are available for xerostomia, there are no curative treatments to date. This study explores the use of Egg White Alginate (EWA), as an alternative scaffold to Matrigel^® for culturing 3D salivary gland cells. A protocol for an optimized EWA was established by comparing cell viability using 1%, 2%, and 3% alginate solution. The normal salivary simian virus 40-immortalized acinar cell (NS-SV-AC) and the submandibular gland-human-1 (SMG-hu-1) cell lines were also used to compare the spheroid formation and cell viability properties of both scaffold biomaterials; cell viability was observed over 10 days using a Live–Dead Cell Assay. Cell viability and spheroid size in 2% EWA was significantly greater than 1% and 3%. It is evident that EWA can support salivary cell survivability as well as form larger spheroids when compared to cells grown in Matrigel^®. However, further investigations are necessary as it is unclear if cultured cells were proliferating or aggregating.

Salivary Gland Tissue Engineering Approaches: State of the Art and Future Directions

Salivary gland regeneration is important for developing treatments for radiation-induced xerostomia, Sjögren's syndrome, and other conditions that cause dry mouth. Culture conditions adopted from tissue engineering strategies have been used to recapitulate gland structure and function to study and regenerate the salivary glands. The purpose of this review is to highlight current trends in the field, with an emphasis on soluble factors that have been shown to improve secretory function in vitro. A PubMed search was conducted to identify articles published in the last 10 years and articles were evaluated to identify the most promising approaches and areas for further research. Results showed increasing use of extracellular matrix mimetics, such as Matrigel®, collagen, and a variety of functionalized polymers. Soluble factors that provide supportive cues, including fibroblast growth factors (FGFs) and neurotrophic factors, as well as chemical inhibitors of Rho-associated kinase (ROCK), epidermal growth factor receptor (EGFR), and transforming growth factor β receptor (TGFβR) have shown increases in important markers including aquaporin 5 (Aqp5); muscle, intestine, and stomach expression 1 (Mist1); and keratin (K5). However, recapitulation of tissue function at in vivo levels is still elusive. A focus on identification of soluble factors, cells, and/or matrix cues tested in combination may further increase the maintenance of salivary gland secretory function in vitro. These approaches may also be amenable for translation in vivo to support successful regeneration of dysfunctional glands.

Three-Dimensional Culture of Ameloblast-Originated HAT-7 Cells for Functional Modeling of Defective Tooth Enamel Formation

Background: Amelogenesis, the formation of dental enamel, is well understood at the histomorphological level but the underlying molecular mechanisms are poorly characterized. Ameloblasts secrete enamel matrix proteins and Ca²⁺, and also regulate extracellular pH as the formation of hydroxyapatite crystals generates large quantities of protons. Genetic or environmental impairment of transport and regulatory processes (e.g. dental fluorosis) leads to the development of enamel defects such as hypomineralization.

Aims: Our aims were to optimize the culture conditions for the three-dimensional growth of ameloblast-derived HAT-7 cells and to test the effects of fluoride exposure on HAT-7 spheroid formation.

Methods: To generate 3D HAT-7 structures, cells were dispersed and plated within a Matrigel extracellular matrix scaffold and incubated in three different culture media. Spheroid formation was then monitored over a two-week period. Ion transporter and tight-junction protein expression was investigated by RT-qPCR. Intracellular Ca²⁺ and pH changes were measured by microfluorometry using the fluorescent dyes fura-2 and BCECF.

Results: A combination of Hepato-STIM epithelial cell differentiation medium and Matrigel induced the expansion and formation of 3D HAT-7 spheroids. The cells retained their epithelial cell morphology and continued to express both ameloblast-specific and ion transport-specific marker genes. Furthermore, like two-dimensional HAT-7 monolayers, the HAT-7 spheroids were able to regulate their intracellular pH and to show intracellular calcium responses to extracellular stimulation. Finally, we demonstrated that HAT-7 spheroids may serve as a disease model for studying the effects of fluoride exposure during amelogenesis.

Conclusion: In conclusion, HAT-7 cells cultivated within a Matrigel extracellular matrix form three-dimensional, multi-cellular, spheroidal structures that retain their functional capacity for pH regulation and intracellular Ca²⁺ signaling. This new 3D model will allow us to gain a better understanding of the molecular mechanisms involved in amelogenesis, not only in health but also in disorders of enamel formation, such as those resulting from fluoride exposure.

Development of a functional salivary gland tissue chip with potential for high-content drug screening

Radiation therapy for head and neck cancers causes salivary gland dysfunction leading to permanent xerostomia. Limited progress in the discovery of new therapeutic strategies is attributed to the lack of in vitro models that mimic salivary gland function and allow high-throughput drug screening. We address this limitation by combining engineered extracellular matrices with microbubble (MB) array technology to develop functional tissue mimetics for mouse and human salivary glands. We demonstrate that mouse and human salivary tissues encapsulated within matrix metalloproteinase-degradable poly(ethylene glycol) hydrogels formed in MB arrays are viable, express key salivary gland markers, and exhibit polarized localization of functional proteins. The salivary gland mimetics (SGm) respond to calcium signaling agonists and secrete salivary proteins. SGm were then used to evaluate radiosensitivity and mitigation of radiation damage using a radioprotective compound. Altogether, SGm exhibit phenotypic and functional parameters of salivary glands, and provide an enabling technology for high-content/throughput drug testing.

The Optimization of a Novel Hydrogel-Egg White-Alginate for 2.5D Tissue Engineering of Salivary Spheroid-Like Structure

Hydrogels have been used for a variety of biomedical applications; in tissue engineering, they are commonly used as scaffolds to cultivate cells in a three-dimensional (3D) environment allowing the formation of organoids or cellular spheroids. Egg white-alginate (EWA) is a novel hydrogel which combines the advantages of both egg white and alginate; the egg white material provides extracellular matrix (ECM)-like proteins that can mimic the ECM microenvironment, while alginate can be tuned mechanically through its ionic crosslinking property to modify the scaffold's porosity, strength, and stiffness. In this study, a frozen calcium chloride (CaCl2) disk technique to homogenously crosslink alginate and egg white hydrogel is presented for 2.5D culture of human salivary cells. Different EWA formulations were prepared and biologically evaluated as a spheroid-like structure platform. Although all five EWA hydrogels showed biocompatibility, the EWA with 1.5% alginate presented the highest cell viability, while EWA with 3% alginate promoted the formation of larger size salivary spheroid-like structures. Our EWA hydrogel has the potential to be an alternative 3D culture scaffold that can be used for studies on drug-screening, cell migration, or as an in vitro disease model. In addition, EWA can be used as a potential source for cell transplantation (i.e., using this platform as an ex vivo environment for cell expansion). The low cost of producing EWA is an added advantage.

Tissue Engineering of Oral Mucosa and Salivary Gland: Disease Modeling and Clinical Applications

Oral mucosa and salivary gland are composed of complex and dynamic networks of extracellular matrix, multiple cell types, vasculature, and various biochemical agents. Two-dimensional (2D) cell culture is commonly used in testing new drugs and experimental therapies. However, 2D cell culture cannot fully replicate the architecture, physiological, and pathological microenvironment of living human oral mucosa and salivary glands. Recent microengineering techniques offer state of the science cell culture models that can recapitulate human organ structures and functions. This narrative review describes emerging in vitro models of oral and salivary gland tissue such as 3D cell culture models, spheroid and organoid models, tissue-on-a-chip, and functional decellularized scaffolds. Clinical applications of these models are also discussed in this review.

Immortalization of Salivary Gland Epithelial Cells of Xerostomic Patients: Establishment and Characterization of Novel Cell Lines

Primary Sjögren’s Syndrome (pSS) is an autoimmune disease mainly affecting salivary and lacrimal glands. Previous pSS studies have relied on primary cell culture models or cancer cell lines with limited relevance to the disease. Our objective was to generate and characterize immortalized salivary gland epithelial cells (iSGECs) derived from labial salivary gland (LSG) biopsies of pSS patients (focus score > 1) and non-Sjögren’s Syndrome (nSS) xerostomic (i.e., sicca) female patients. To characterize iSGECs (n = 3), mRNA expression of specific epithelial and acinar cell markers was quantified by qRT-PCR. Protein expression of characterization markers was determined by immunocytochemistry and Western blot. Secretion of α-amylase by iSGECs was confirmed through colorimetric activity assay. Spheroid formation and associated alterations in expression markers were determined using matrigel-coated cell culture plates. Consistent mRNA and protein expressions of both epithelial and pro-acinar cell markers were observed in all three iSGEC lines. When cultured on matrigel medium, iSGECs formed spheroids, secreted α-amylase after β-adrenergic stimulation, and expressed multiple acinar cell markers at late passages. One iSGEC line retained adequate cell morphology without a loss of SV40Lt expression and proliferation potential after over 100 passages. In conclusion, our established iSGEC lines represent a viable model for salivary research due to their passaging capacity and maintenance of pro-acinar cell characteristics.

Self-organization of organoids from endoderm-derived cells

Organoids constitute biological systems which are used to model organ development, homeostasis, regeneration, and disease in vitro and hold promise for use in therapy. Reflecting in vivo development, organoids form from tissue cells or pluripotent stem cells. Cues provided from the media and individual cells promote self-organization of these uniform starting cells into a structure, with emergent differentiated cells, morphology, and often functionality that resemble the tissue of origin. Therefore, organoids provide a complement to two-dimensional in vitro culture and in vivo animal models of development, providing the experimental control and flexibility of in vitro methods with the three-dimensional context of in vivo models, with fewer ethical restraints than human or animal work. However, using organoids, we are only just beginning to understand on the cellular level how the external conditions and signaling between individual cells promote the emergence of cells and structures. In this review, we focus specifically on organoids derived from endodermal tissues: the starting conditions of the cells, signaling mechanisms, and external media that allow the emergence of higher order self-organization.

3D Cell Culture of Human Salivary Glands Using Nature-Inspired Functional Biomaterials: The Egg Yolk Plasma and Egg White

The egg yolk plasma (EYP)-a translucent fraction of the egg yolk (EY) obtained by centrifugation-was tested as a developmentally encouraging, cost-effective, biomaterial for salivary gland (SG) tissue engineering. To find optimal incubating conditions for both the human NS-SV-AC SG acinar cell line and SG fibroblasts, cells were stained with Live/Dead®. The cellular contents of 96-well plates were analyzed by high content screening image analysis. Characteristically, the EYP biomaterial had lipid and protein content resembling the EY. On its own, the EYP was non-conducive to cell survival. EYP's pH of 6 mainly contributed to cell death. This was demonstrated by titrating EYP's pH with different concentrations of either commercial cell culture media, NaOH, or egg white (EW). These additives improved SG mesenchymal and epithelial cell survival. The best combinations were EYP diluted with (1) 70% commercial medium, (2) 0.02 M NaOH, or (3) 50% EW. Importantly, commercial medium-free growth was obtained with EYP + NaOH or EYP + EW. Furthermore, 3D cultures were obtained as a result of EW's gelatinous properties. Here, the isolation, characterization, and optimization of three EYP-based biomaterial combinations are shown; two were free of commercial medium or supplements and supported both SG cells' survival.

Insight into Salivary Gland Aquaporins

The main role of salivary glands (SG) is the production and secretion of saliva, in which aquaporins (AQPs) play a key role by ensuring water flow. The AQPs are transmembrane channel proteins permeable to water to allow water transport across cell membranes according to osmotic gradient. This review gives an insight into SG AQPs. Indeed, it gives a summary of the expression and localization of AQPs in adult human, rat and mouse SG, as well as of their physiological role in SG function. Furthermore, the review provides a comprehensive view of the involvement of AQPs in pathological conditions affecting SG, including Sjögren's syndrome, diabetes, agedness, head and neck cancer radiotherapy and SG cancer. These conditions are characterized by salivary hypofunction resulting in xerostomia. A specific focus is given on current and future therapeutic strategies aiming at AQPs to treat xerostomia. A deeper understanding of the AQPs involvement in molecular mechanisms of saliva secretion and diseases offered new avenues for therapeutic approaches, including drugs, gene therapy and tissue engineering. As such, AQP5 represents a potential therapeutic target in different strategies for the treatment of xerostomia.

In vitro three-dimensional culture systems of salivary glands

Dry mouth can be caused by salivary gland hypofunction due to Sjögren's syndrome (SS) or radiation therapy for head and neck cancer, and it can also be a side effect of medications. The use of sialagogues effectively increases saliva secretion in patients with dry mouth. However, the application of sialagogues is not always satisfactory because of their side effects, such as sweating, nausea, runny nose and diarrhea. Two-dimensional (2D) cell cultures have been used not only for drug screening and discovery but also to clarify disease mechanisms. However, three-dimensional (3D) cell cultures are expected to be even more advantageous than 2D cell cultures. Therefore, we have tried to develop an in vitro cell culture system that can reconstitute 3D salivary glands. Sox9 and Foxc1 were identified as important genes that differentiate mouse embryonic stem cell-derived oral ectoderm into salivary gland placode. Using these genes and organoid culture systems, we succeeded in generating salivary gland organoids that exhibited a morphology and gene expression profile that were similar to those of the embryonic rudiment from which salivary glands arise in normal mice. These organoids are expected to be a promising tool for disease modeling, drug discovery and regenerative medicine in salivary glands.

P2 Receptors as Therapeutic Targets in the Salivary Gland: From Physiology to Dysfunction

Although often overlooked in our daily lives, saliva performs a host of necessary physiological functions, including lubricating and protecting the oral cavity, facilitating taste sensation and digestion and maintaining tooth enamel. Therefore, salivary gland dysfunction and hyposalivation, often resulting from pathogenesis of the autoimmune disease Sjögren's syndrome or from radiotherapy of the head and neck region during cancer treatment, severely reduce the quality of life of afflicted patients and can lead to dental caries, periodontitis, digestive disorders, loss of taste and difficulty speaking. Since their initial discovery in the 1970s, P2 purinergic receptors for extracellular nucleotides, including ATP-gated ion channel P2X and G protein-coupled P2Y receptors, have been shown to mediate physiological processes in numerous tissues, including the salivary glands where P2 receptors represent a link between canonical and non-canonical saliva secretion. Additionally, extracellular nucleotides released during periods of cellular stress and inflammation act as a tissue alarmin to coordinate immunological and tissue repair responses through P2 receptor activation. Accordingly, P2 receptors have gained widespread clinical interest with agonists and antagonists either currently undergoing clinical trials or already approved for human use. Here, we review the contributions of P2 receptors to salivary gland function and describe their role in salivary gland dysfunction. We further consider their potential as therapeutic targets to promote physiological saliva flow, prevent salivary gland inflammation and enhance tissue regeneration.

Investigation of Cellular Function and DNA Integrity during 2D in vitro Culture of Human Salivary Gland Epithelial Cells

In vitro culture of human salivary gland epithelial cells (SGEC) is still a challenge. A high quantity and quality of cells are needed for the cultivation of 3D matrices. Furthermore, it is known that DNA damage is supposed to be an important factor involved in carcinogenesis. This study investigates cellular function and DNA integrity of human SGEC during 3 passage steps in 2 groups (group 1: n = 10; group 2: n = 9). Cellular function was analyzed by immunofluorescence, transmission electron microscopy (TEM), and quantitative real-time polymerase chain reaction (qPCR). DNA integrity was tested via the comet assay. Immunohistochemistry and qPCR results showed stable α-amylase and pan-cytokeratin levels; TEM revealed functional cells; and no significant DNA damage could be detected in the comet assay during 3 culture steps. The study shows that not only at cellular but also at DNA level human SGEC can be safely quantified over 3 passages for preclinical tissue engineering without loss of differentiation and function.

3D Cultures of Salivary Gland Cells in Native or Gelled Egg Yolk Plasma, Combined with Egg White and 3D-Printing of Gelled Egg Yolk Plasma

For salivary gland (SG) tissue engineering, we cultured acinar NS-SV-AC cell line or primary SG fibroblasts for 14 days in avian egg yolk plasma (EYP). Media or egg white (EW) supplemented the cultures as they grew in 3D-Cryo histology well inserts. In the second half of this manuscript, we measured EYP’s freeze-thaw gelation and freeze-thaw induced gelled EYP (_GEYP), and designed and tested further _GEYP tissue engineering applications. With a 3D-Cryo well insert, we tested _GEYP as a structural support for 3D cell culture or as a bio-ink for 3D-Bioprinting fluorescent cells. In non-printed EYP + EW or _GEYP + EW cultures, sagittal sections of the cultures showed cells remaining above the well’s base. Ki-67 expression was lacking for fibroblasts, contrasting NS-SV-AC’s constant expression. Rheological viscoelastic measurements of _GEYP at 37 °C on seven different freezing periods showed constant increase from 0 in mean storage and loss moduli, to 320 Pa and 120 Pa, respectively, after 30 days. We successfully 3D-printed _GEYP with controlled geometries. We manually extruded _GEYP bio-ink with fluorescence cells into a 3D-Cryo well insert and showed cell positioning. The 3D-Cryo well inserts reveal information on cells in EYP and we demonstrated _GEYP cell culture and 3D-printing applications.

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

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

Cell culture of differentiated human salivary epithelial cells in a serum-free and scalable suspension system: The salivary functional units model

Saliva aids in digestion, lubrication, and protection of the oral cavity against dental caries and oropharyngeal infections. Reduced salivary secretion, below an adequate level to sustain normal oral functions, is unfortunately experienced by head and neck cancer patients treated with radiotherapy and by patients with Sjögren's syndrome. No disease-modifying therapies exist to date to address salivary gland hypofunction (xerostomia, dry mouth) because pharmacotherapies are limited by the need for residual secretory acinar cells, which are lost at the time of diagnosis, whereas novel platforms such as cell therapies are yet immature for clinical applications. Autologous salivary gland primary cells have clinical utility as personalized cell therapies, if they could be cultured to a therapeutically useful mass while maintaining their in vivo phenotype. Here, we devised a serum-free scalable suspension culture system that grows partially digested human salivary tissue filtrates composing of acinar and ductal cells attached to their native extracellular matrix components while retaining their 3D in vivo spatial organization; we have coined these salivary spheroids as salivary functional units (SFU). The proposed SFU culture system was sub-optimal, but we have found that the cells could still survive and grow into larger salivary spheroids through cell proliferation and aggregation for 5 to 10 days within the oxygen diffusion rates in vitro. In summary, by using a less disruptive cell isolation procedure as the starting point for primary cell culture of human salivary epithelial cells, we demonstrated that aggregates of cells remained proliferative and continued to express acinar and ductal cell-specific markers.

Three-Dimensional Culture of Salivary Gland Stem Cell in Orthotropic Decellularized Extracellular Matrix Hydrogels

Radiotherapy in patients with cancer can kill cancer cells but also damage normal cells or tissues. During the treatment of patients with head and neck cancer or thyroid cancer, hyposalivation is a representative chronic side effect of radio-damaged salivary glands (SGs). The major symptom of hyposalivation is mouth dryness, resulting in several subsequent long-term complications. No effective therapeutic approaches have been developed to manage this symptom. In this study, we developed the first rat SG tissue-derived decellularized extracellular matrix hydrogel (DSGM-hydrogel) as a functional orthotropic bioscaffold for future efficient SG stem cell therapy. DSGM-hydrogels were characterized by rheological or biochemical analyses, and rat SG stem/progenitor cells (rSGSCs) were then subjected to three-dimensional culture in the DSGM-hydrogels. Interestingly, DSGM-hydrogel-embedded rSGSCs survived and expressed SG functional differentiation marker of amylase IA and increased enzyme activity of α-amylase in protein level, whereas they showed reduced levels of adult ductal stem/progenitor markers, including c-Kit, c-Met, and CD44. Furthermore, the expression levels of basic epithelial tight junction markers were recovered to levels similar to those naked SG tissues after culture in DSGM-hydrogels in transcription level. Therefore, our findings suggested that the DSGM-hydrogels could provide an appropriate microenvironment for stem/progenitor cell survival and a source of SG cytodifferentiation. This approach could be an applicable method to SG stem cell research as a potential source for an organoid and for clinical regenerative reagents to manage radio-damaged SGs in vivo. Impact Statement In this study, we established the first rat salivary gland (SG) tissue-derived decellularized extracellular matrix hydrogel (DSGM-hydrogel) and assessed the role of this hydrogel as a functional orthotropic bioscaffold. Our findings provide important insights into the applications of the DSGM-hydrogel as a biocompatible matrix for regenerative therapy of radio-damaged SGs.

A magnetic three-dimensional levitated primary cell culture system for the development of secretory salivary gland-like organoids

Salivary gland (SG) hypofunction and oral dryness can be induced by radiotherapy for head and neck cancers or autoimmune disorders. These are common clinical conditions that involve loss of saliva-secreting epithelial cells. Several oral complications arise with SG hypofunction that interfere with routine daily activities such as chewing, swallowing, and speaking. Hence, there is a need for replacing these saliva-secreting cells. Recently, researchers have proposed to repair SG hypofunction via various cell-based approaches in three-dimensional (3D) scaffold-based systems. However, majority of the scaffolds used cannot be translated clinically due to the presence of non-human-based substrates. Herein, saliva-secreting organoids/mini-glands were developed using a new scaffold/substrate-free culture system named magnetic 3D levitation (M3DL), which assembles and levitates magnetized primary SG-derived cells (SGDCs), allowing them to produce their own extracellular matrices. Primary SGDCs were assembled in M3DL to generate SG-like organoids in well-established SG epithelial differentiation conditions for 7 days. After such culture time, these organoids consistently presented uniform spheres with greater cell viability and pro-mitotic cells, when compared with conventional salisphere cultures. Additionally, organoids formed by M3DL expressed SG-specific markers from different cellular compartments: acinar epithelial including adherens junctions (NKCC1, cholinergic muscarinic receptor type 3, E-cadherin, and EpCAM); ductal epithelial and myoepithelial (cytokeratin 14 and α-smooth muscle actin); and neuronal (β3-tubulin and vesicular acetylcholine transferase). Lastly, intracellular calcium and α-amylase activity assays showed functional organoids with SG-specific secretory activity upon cholinergic stimulation. Thus, the functional organoid produced herein indicate that this M3DL system can be a promising tool to generate SG-like mini-glands for SG secretory repair.

Unveiling Stem Cell Heterogeneity Toward the Development of Salivary Gland Regenerative Strategies

Epithelial damage in the salivary gland (SG) resulting in irreversible dry mouth can be commonly induced by gamma radiation therapy. This radiation depletes the SG stem/progenitor cell niche slowing healing and natural gland regeneration. Biologists have been focused in understanding the development and differentiation of epithelial stem and progenitor cell niches during SG organogenesis. These organogenesis studies gave insights into novel cell-based therapies to recreate the three-dimensional (3D) salivary gland (SG) organ, recapitulate the SG native physiology, and restore saliva secretion. Such therapeutical strategies apply techniques that assemble, in a 3D organotypic culture, progenitor and stem cell lines to develop SG organ-like organoids or mini-transplants. Future studies will employ a combination of organoids, decellularized matrices, and smart biomaterials to create viable and functional SG transplants to repair the site of SG injury and reestablish saliva production.

Cell culture models of oral mucosal barriers: A review with a focus on applications, culture conditions and barrier properties

Understanding the function of oral mucosal epithelial barriers is essential for a plethora of research fields such as tumor biology, inflammation and infection diseases, microbiomics, pharmacology, drug delivery, dental and biomarker research. The barrier properties are comprised by a physical, a transport and a metabolic barrier, and all these barrier components play pivotal roles in the communication between saliva and blood. The sum of all epithelia of the oral cavity and salivary glands is defined as the blood-saliva barrier. The functionality of the barrier is regulated by its microenvironment and often altered during diseases. A huge array of cell culture models have been developed to mimic specific parts of the blood-saliva barrier, but no ultimate standard in vitro models have been established. This review provides a comprehensive overview about developed in vitro models of oral mucosal barriers, their applications, various cultivation protocols and corresponding barrier properties.

Engineering innervated secretory epithelial organoids by magnetic three-dimensional bioprinting for stimulating epithelial growth in salivary glands

Current saliva-based stimulation therapies for radiotherapy-induced xerostomia are not fully effective due to the presence of damaged secretory epithelia and nerves in the salivary gland (SG). Hence, three-dimensional bio-engineered organoids are essential to regenerate the damaged SG. Herein, a recently validated three-dimensional (3D) biofabrication system, the magnetic 3D bioprinting (M3DB), is tested to generate innervated secretory epithelial organoids from a neural crest-derived mesenchymal stem cell, the human dental pulp stem cell (hDPSC). Cells are tagged with magnetic nanoparticles (MNP) and spatially arranged with magnet dots to generate 3D spheroids. Next, a SG epithelial differentiation stage was completed with fibroblast growth factor 10 (4-400 ng/ml) to recapitulate SG epithelial morphogenesis and neurogenesis. The SG organoids were then transplanted into ex vivo model to evaluate their epithelial growth and innervation. M3DB-formed spheroids exhibited both high cell viability rate (>90%) and stable ATP intracellular activity compared to MNP-free spheroids. After differentiation, spheroids expressed SG epithelial compartments including secretory epithelial, ductal, myoepithelial, and neuronal. Fabricated organoids also produced salivary α-amylase upon FGF10 stimulation, and intracellular calcium mobilization and trans-epithelial resistance was elicited upon neurostimulation with different neurotransmitters. After transplantation, the SG-like organoids significantly stimulated epithelial and neuronal growth in damaged SG. It is the first time bio-functional innervated SG-like organoids are bioprinted. Thus, this is an important step towards SG regeneration and the treatment of radiotherapy-induced xerostomia.

Comparing human and mouse salivary glands: A practice guide for salivary researchers

Mice are a widely utilized in vivo model for translational salivary gland research but must be used with caution. Specifically, mouse salivary glands are similar in many ways to human salivary glands (i.e., in terms of their anatomy, histology, and physiology) and are both readily available and relatively easy and affordable to maintain. However, there are some significant differences between the two organisms, and by extension, the salivary glands derived from them must be taken into account for translational studies. The current review details pertinent similarities and differences between human and mouse salivary glands and offers practical guidelines for using both for research purposes.

Stem cell properties of human clonal salivary gland stem cells are enhanced by three-dimensional priming culture in nanofibrous microwells

BACKGROUND

Three-dimensional (3D) cultures recapitulate the microenvironment of tissue-resident stem cells and enable them to modulate their properties. We determined whether salivary gland-resident stem cells (SGSCs) are primed by a 3D spheroid culture prior to treating irradiation-induced salivary hypofunction using in-vitro coculture and in-vivo transplant models.

METHODS

3D spheroid-derived SGSCs (SGSCs^3D) were obtained from 3D culture in microwells consisting of a nanofiber bottom and cell-repellent hydrogel walls, and were examined for salivary stem or epithelial gene/protein expression, differentiation potential, and paracrine secretory function compared with monolayer-cultured SGSCs (SGSCs^2D) in vitro and in vivo.

RESULTS

SGSCs^3D expressed increased salivary stem cell markers (LGR5 and THY1) and pluripotency markers (POU5F1 and NANOG) compared with SGSCs^2D. Also, SGSCs^3D exhibited enhanced potential to differentiate into salivary epithelial cells upon differentiation induction and increased paracrine secretion as compared to SGSCs^2D. Wnt signaling was activated by 3D spheroid formation in the microwells and suppression of the Wnt/β-catenin pathway led to reduced stemness of SGSCs^3D. Enhanced radioprotective properties of SGSCs^3D against radiation-induced salivary hypofunction was confirmed by an organotypic 3D coculture and in-vivo transplantation experiments.

CONCLUSION

The 3D spheroid culture of SGSCs in nanofibrous microwells promotes stem cell properties via activation of Wnt signaling. This may contribute to SGSC priming prior to regenerative therapy to restore salivary hypofunction after radiotherapy.

Development of Human Salivary Gland-Like Tissue In Vitro

The loss of salivary gland function caused by radiation therapy of the head and neck is a serious condition and it affects a patient's quality of life. The current lack of effective therapies demands new options to be explored. This study tested whether human salivary gland epithelial cells (SGECs) could be successfully cultured on a decellularized porcine gut matrix (SIS-muc) in both mono- and coculture with microvascular endothelial cells (mvECs). By performing immunofluorescence imaging, transmission as well as scanning electron microscopy (SEM), quantitative polymerase chain reaction (qPCR), and an amylase enzyme assay, it was investigated as to what extent the three-dimensional (3D)-cultured cells could maintain their molecular differentiation and the production of working α-amylase (α-AMY) compared with two-dimensional (2D) culture. In both 3D mono- and coculture, SGECs were successfully cultured and formed acinar-like structures. Those findings were confirmed by SEM imaging. Immunofluorescence imaging revealed that 3D-cultured cells expressed α-AMY, Claudin-1 (CL-1), and water channel protein aquaporin-5 (AQP-5). Two-dimensional-cultured cells only were positive for α-AMY. Real time (RT)-qPCR analysis showed that α-AMY relative gene expression was higher in both 3D mono- and coculture than in 2D culture. In α-AMY enzyme assay, cocultured SGECs showed about 25 times increased enzyme activity compared with 2D-cultured cells. In conclusion, the SIS-muc combined with endothelial coculture seems a suitable culture setting for the tissue engineering of functional human salivary gland tissue.

Three-dimensional cell culture models for anticancer drug screening: Worth the effort?

High attrition of new oncology drug candidates in clinical trials is partially caused by the poor predictive capacity of artificial monolayer cell culture assays early in drug discovery. Monolayer assays do not take the natural three-dimensional (3D) microenvironment of cells into account. As a result, false positive compounds often enter clinical trials, leading to high dropout rates and a waste of time and money. Over the past 2 decades, tissue engineers and cell biologists have developed a broad range of 3D in vitro culturing tools that better represent in vivo cell biology. These tools preserve the 3D architecture of cells and can be used to predict toxicity of and resistance against antitumor agents. Recent progress in tissue engineering further improves 3D models by taking into account the tumor microenvironment, which is important for metastatic progression and vascularization. However, the widespread implementation of 3D cell cultures into cell-based research programs has been limited by various factors, including their cost and reproducibility. In addition, different 3D cell culture techniques often produce spheroids of different size and shape, which can strongly influence drug efficacy and toxicity. Hence, it is imperative to morphometrically characterize multicellular spheroids to avoid generalizations among different spheroid types. Standardized 3D culturing procedures could further reduce data variability and enhance biological relevance. Here, we critically evaluate the benefits and challenges inherent to growing cells in 3D, along with an overview of the techniques used to form spheroids. This is done with a specific focus on antitumor drug screening.

Organotypic Spheroid Culture to Mimic Radiation-Induced Salivary Hypofunction

Radiation treatment often leads to irreversible damage to normal salivary glands (SGs) because of their proximity to head and neck cancers. Optimization of the in vitro model of irradiation (IR)-induced SG damage is warranted to investigate pathophysiology and monitor treatment outcome. Here, we present an organotypic spheroid culture model to investigate the impact of IR on SGs and the mechanisms underlying IR-induced structural and functional changes. Human parotid epithelial cells were obtained from human parotid glands and plated on either plastic plates or Matrigel. A number of 3-dimensional (3D) spheroids were assembled on Matrigel. After IR at 10 and 20 Gy, morphologic changes in cells in 2D monolayers and 3D spheroids were observed. As the structural integrity of the 3D spheroids was destroyed by IR, the expression levels of salivary epithelial and structural proteins and genes decreased proportionally with radiation dosage. Furthermore, the spheroid culture allowed better measurement of functional alterations following IR relative to the monolayer culture, in which IR-inflicted spheroids exhibited a loss of acinar-specific cellular functions that enable Ca²⁺ influx or secretion of α-amylase in response to cholinergic or β-adrenergic agonists. p53-mediated apoptotic cell death was observed under both culture conditions, and its downstream signals increased, such as p53 upregulated modulator of apoptosis (PUMA), Bax, cytochrome c, caspase 9, and caspase 3. These results suggest that the organotypic spheroid culture could provide a useful alternative model for exploration of radiobiology and mode of action of new therapies for prevention of radiation-induced salivary hypofunction.

Functional spheroid organization of human salivary gland cells cultured on hydrogel-micropatterned nanofibrous microwells

Development of a tissue-engineered, salivary bio-gland will benefit patients suffering from xerostomia due to loss of fluid-secreting acinar cells. This study was conducted to develop a bioengineering system to induce self-assembly of human parotid epithelial cells (hPECs) cultured on poly ethylene glycol (PEG) hydrogel-micropatterned polycaprolactone (PCL) nanofibrous microwells. Microwells were fabricated by photopatterning of PEG hydrogel in the presence of an electrospun PCL nanofibrous scaffold. hPECs were plated on plastic dishes, Matrigel, PCL nanofibers, or PCL nanofibrous microwells. When the cells were plated onto plastic, they did not form spheres, but aggregated to form 3D acinar-like spheroids when cultured on Matrigel, PCL, and PCL microwells, with the greatest aggregating potency being observed on the PCL microwells. The 3D-assembled spheroids in the PCL microwells expressed higher levels of salivary epithelial markers (α-amylase and AQP5), tight junction proteins (ZO-1 and occludin), adherence protein (E-cadherin), and cytoskeletal protein (F-actin) than those on the Matrigel and PCL. Furthermore, the 3D-assembled spheroids in the PCL microwells showed higher levels of α-amylase secretion and intracellular calcium concentration ([Ca²⁺]_i) than those on the Matrigel and PCL nanofibers, suggesting more functional organization of hPECs. We established a bioengineering 3D culture system to promote robust and functional acinar-like organoids from hPECs. PCL nanofibrous microwells can be applied in the future for bioengineering of an artificial bio-salivary gland for restoration of salivary function.

STATEMENT OF SIGNIFICANCE

Three dimensional (3D) cultures of salivary glandular epithelial cells using nanofibrous bottom facilitate the formation of acinar-like organoids. In this study, we adapted a PEG hydrogel-micropatterned PCL nanofibrous microwell for the efficient bioengineering of human salivary gland organoids, in which we could easily produce uniform size of 3D organoids. This 3D culture system supports spherical organization, gene and protein expression of acinar markers, TJ proteins, adherence, and cytoskeletal proteins, as well as to promote epithelial structural integrity and acinar secretory functions, and results showed superior efficiency relative to Matrigel and nanofibrous scaffold culture. This 3D culture system has benefits in terms of inert, non-animal and serum-free culture conditions, as well as controllable spheroid size and scalable production of functional SG organoids and is applicable to bioengineering approaches for an artificial bio-gland, as well as to investigations of salivary gland physiology and regeneration.

Concise Review: Salivary Gland Regeneration: Therapeutic Approaches from Stem Cells to Tissue Organoids

The human salivary gland (SG) has an elegant architecture of epithelial acini, connecting ductal branching structures, vascular and neuronal networks that together function to produce and secrete saliva. This review focuses on the translation of cell- and tissue-based research toward therapies for patients suffering from SG hypofunction and related dry mouth syndrome (xerostomia), as a consequence of radiation therapy or systemic disease. We will broadly review the recent literature and discuss the clinical prospects of stem/progenitor cell and tissue-based therapies for SG repair and/or regeneration. Thus far, several strategies have been proposed for the purpose of restoring SG function: (1) transplanting autologous SG-derived epithelial stem/progenitor cells; (2) exploiting nonepithelial cells and/or their bioactive lysates; and (3) tissue engineering approaches using 3D (three-dimensional) biomaterials loaded with SG cells and/or bioactive cues to mimic in vivo SGs. We predict that further scientific improvement in each of these areas will translate to effective therapies toward the repair of damaged glands and the development of miniature SG organoids for the fundamental restoration of saliva secretion.

Three-dimensional organotypic culture of human salivary glands: the slice culture model

OBJECTIVE

A challenge in studying human salivary glands is to maintain the cells ex vivo in their three-dimensional (3D) morphology with an intact native extracellular matrix (ECM) environment. This paper established a human salivary 3D organotypic slice culture model that could maintain its physiological functions as well as allowing a direct visualization of the cells.

METHODS

Human salivary biopsies from six patients were embedded in agarose and submerged in cold buffer for thin (50 μm) sectioning using a vibratome. 'Salivary slices' were mechanically supported by a porous membrane insert that allowed an air-liquid interface and cultured in serum-free culture media. Cell viability, proliferation, apoptosis, physiological functions, and gene expression were assessed during 14 days of culture.

RESULTS

Human salivary slices maintained cell survival (70-40%) and proliferation (6-17%) for 14 days ex vivo. The protein secretory (amylase) function decreased, but fluid (intracellular calcium mobilization) function was maintained. Acinar, ductal, and myoepithelial cell populations survived and maintained their 3D organization within the slice culture model.

CONCLUSION

The human salivary slice culture model kept cells alive ex vivo for 14 days as well as maintaining their 3D morphology and physiological functions.

The role of human fibronectin- or placenta basement membrane extract-based gels in favouring the formation of polarized salivary acinar-like structures

Head and neck cancer patients treated with radiotherapy commonly experience hyposalivation and oral/tooth infections, leading to a reduced quality of life. Clinical management is currently unsatisfactory for dry mouth. Thus, there is a need for growing salivary fluid-secreting (acinar) cells for these patients. However, functionally-grown salivary acinar cells are cultured in Matrigel, a product that cannot be used clinically, owing to its source from a mouse sarcoma. Therefore, finding a gel suitable for clinical use and possessing properties similar to that of Matrigel would allow biopsied salivary cells to be expanded in vitro and transplanted into the mouths of xerostomic patients. This study tested gels made with human placenta basement membrane extract (BME) or fibronectin for the growth and differentiation of human salivary biopsies into acinar cells. We report here that, following expansion of primary human salivary gland epithelial cells (huSGs) in serum-free medium, using these gels (made from human proteins) allowed morphological and functional differentiation of salivary ductal cells into acinar-like cells. These (human) gels gave comparable results to Matrigel, such as differentiation into polarized acinar 3D units or monolayers with tight junction proteins (claudin-1, -2, -3) and exhibiting adequate transepithelial electrical resistance, acinar proteins (AQP5, α-amylase, mucin-1, NKCC1) and acinar adhesion-related cell markers (CD44, CD166). Ultrastructural, mRNA and protein analyses confirmed the formation of differentiated acinar polarized cells. The mitotic activity was highest with human placenta BME gel. This human culture model provided a reproducible approach to studying human salivary cell expansion and differentiation for tissue engineering. Copyright © 2016 John Wiley & Sons, Ltd.

Current trends in salivary gland tight junctions

Tight junctions form a continuous intercellular barrier between epithelial cells that is required to separate tissue spaces and regulate selective movement of solutes across the epithelium. They are composed of strands containing integral membrane proteins (e.g., claudins, occludin and tricellulin, junctional adhesion molecules and the coxsackie adenovirus receptor). These proteins are anchored to the cytoskeleton via scaffolding proteins such as ZO-1 and ZO-2. In salivary glands, tight junctions are involved in polarized saliva secretion and barrier maintenance between the extracellular environment and the glandular lumen. This review seeks to provide an overview of what is currently known, as well as the major questions and future research directions, regarding tight junction expression, organization and function within salivary glands.

Biomaterials-based strategies for salivary gland tissue regeneration

The salivary gland is a complex, secretory tissue that produces saliva and maintains oral homeostasis. Radiation induced salivary gland atrophy, manifested as "dry mouth" or xerostomia, poses a significant clinical challenge. Tissue engineering recently has emerged as an alternative, long-term treatment strategy for xerostomia. In this review, we summarize recent efforts towards the development of functional and implantable salivary glands utilizing designed polymeric substrates or synthetic matrices/scaffolds. Although the in vitro engineering of a complex implantable salivary gland is technically challenging, opportunities exist for multidisciplinary teams to assemble implantable and secretory tissue modules by combining stem/progenitor cells found in the adult glands with biomimetic and cell-instructive materials.

Natural extracellular matrix scaffolds recycled from human salivary digests: a morphometric study

OBJECTIVE

A challenge in engineering tissues is to supply parenchymal cells with suitable scaffolds which ideally reproduce the extracellular matrix (ECM). This study tested the hypothesis of preserving the 'residual connective tissue' remaining after mechanical and enzymatic release of cells from human submandibular gland biopsies (that we named 'natural ExtraCellular Matrix scaffolds', nECMsc) to be used as recycled natural scaffolds. The objective was to test whether nECMsc and native salivary tissue were comparable morphologically, in ECM proteins composition, and in cell seeding efficiency.

METHODS

Following cell isolation procedures, nECMsc were kept, either fresh or frozen (sectioned into 12-μm-thick slices), and examined with high-resolution electron microscopy (HRSEM) for its three-dimensional structure, and with picrosirius red staining and immunogold staining for ECM protein composition and distribution, respectively. nECMsc were seeded with human epithelial cells and fibroblasts to assess cell attachment and proliferation in short-term experiments.

RESULTS

Under HRSEM, nECMsc had comparable fiber arrangement to original glands. Histochemical and immunogold-labeling examinations revealed the presence of collagen types I, III, and IV. Seeded epithelial cells and fibroblasts attached, proliferated (14-55%), and were alive (86-99%) after 4-8 days of culture.

CONCLUSIONS

nECMsc retained native ECM proteins and maintained their distribution. Seeded cells remained viable on nECMsc.

Evidence for Active Electrolyte Transport by Two-Dimensional Monolayers of Human Salivary Epithelial Cells

Functional reconstruction of lost tissue by regenerative therapy of salivary glands would be of immense benefit following radiotherapy or in the treatment of Sjogren's syndrome. The purpose of this study was to develop primary cultures of human salivary gland cells as potential regenerative resources and to characterize their acinar/ductal phenotype using electrophysiological measurements of ion transport. Human salivary gland cultures were prepared either from adherent submandibular gland cells (huSMG) or from mixed adherent and nonadherent cells (PTHSG) and were cultivated in Hepato-STIM or minimum essential medium (MEM). Expression of key epithelial marker proteins was determined by quantitative reverse transcription polymerase chain reaction (RT-PCR). Transepithelial electrical resistance (TER) was monitored following seeding the cells on Transwell membranes. Transepithelial ion transport was estimated by short-circuit current (Isc) measurements in an Ussing chamber. Both huSMG and PTHSG cells showed epithelial characteristics when cultivated in Hepato-STIM, while fibroblast-like elements dominated in MEM. Compared to intact tissue, cultivation of the cells resulted in substantial decreases in AQP5 and NKCC1 expression and moderate increases in claudin-1 and ENaC expression. Both cultures achieved high TER and transepithelial electrolyte movement in Hepato-STIM, but not in MEM. The Isc was substantially reduced by basolateral Cl(-) and bicarbonate withdrawal, indicating the involvement of basolateral-to-apical anion transport, and by the blockade of apical ENaC by amiloride, indicating the involvement of apical-to-basolateral Na(+) transport. An almost complete inhibition was observed following simultaneous ENaC block and withdrawal of the two anions. Isc was enhanced by either apical adenosine triphosphate (ATP) or basolateral carbachol application, but not by forskolin, confirming the expected role of Ca(2+)-activated regulatory pathways in electrolyte secretion. Inhibition of basolateral NKCC1 by bumetanide reduced the response to ATP, indicating the active involvement of this transporter in Cl(-) secretion. In conclusion, we have demonstrated that both PTHSG and huSMG primary cultures cultivated in Hepato-STIM form two-dimensional monolayers in vitro on permeable supports and achieve active vectorial transepithelial electrolyte transport. The presence of both basolateral-to-apical anion fluxes and an apical-to-basolateral Na(+) flux indicates both acinar and ductal characteristics. With further refinement, this model should provide a firm basis for new interventions to correct salivary gland dysfunction.

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

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

Regulation of proximal tubular cell differentiation and proliferation in primary culture by matrix stiffness and ECM components

To explore whether matrix stiffness affects cell differentiation, proliferation, and transforming growth factor (TGF)-β1-induced epithelial-mesenchymal transition (EMT) in primary cultures of mouse proximal tubular epithelial cells (mPTECs), we used a soft matrix made from monomeric collagen type I-coated polyacrylamide gel or matrigel (MG). Both kinds of soft matrix benefited primary mPTECs to retain tubular-like morphology with differentiation and growth arrest and to evade TGF-β1-induced EMT. However, the potent effect of MG on mPTEC differentiation was suppressed by glutaraldehyde-induced cross-linking and subsequently stiffening MG or by an increasing ratio of collagen in the soft mixed gel. Culture media supplemented with MG also helped mPTECs to retain tubular-like morphology and a differentiated phenotype on stiff culture dishes as soft MG did. We further found that the protein level and activity of ERK were scaled with the matrix stiffness. U-0126, a MEK inhibitor, abolished the stiff matrix-induced dedifferentiation and proliferation. These data suggest that the ERK signaling pathway plays a vital role in matrix stiffness-regulated cell growth and differentiation. Taken together, both compliant property and specific MG signals from the matrix are required for the regulation of epithelial differentiation and proliferation. This study provides a basic understanding of how physical and chemical cues derived from the extracellular matrix regulate the physiological function of proximal tubules and the pathological development of renal fibrosis.

An in vitro culture system for long-term expansion of epithelial and mesenchymal salivary gland cells: role of TGF-β1 in salivary gland epithelial and mesenchymal differentiation

Despite a pivotal role in salivary gland development, homeostasis, and disease, the role of salivary gland mesenchyme is not well understood. In this study, we used the Col1a1-GFP mouse model to characterize the salivary gland mesenchyme in vitro and in vivo. The Col1a1-GFP transgene was exclusively expressed in the salivary gland mesenchyme. Ex vivo culture of mixed salivary gland cells in DMEM plus serum medium allowed long-term expansion of salivary gland epithelial and mesenchymal cells. The role of TGF-β1 in salivary gland development and disease is complex. Therefore, we used this in vitro culture system to study the effects of TGF-β1 on salivary gland cell differentiation. TGF-β1 induced the expression of collagen, and inhibited the formation of acini-like structures in close proximity to mesenchymal cells, which adapted a fibroblastic phenotype. In contrast, TGF-βR1 inhibition increased acini genes and fibroblast growth factors (Fgf-7 and Fgf-10), decreased collagen and induced formation of larger, mature acini-like structures. Thus, inhibition of TGF-β signaling may be beneficial for salivary gland differentiation; however, due to differential effects of TGF-β1 in salivary gland epithelial versus mesenchymal cells, selective inhibition is desirable. In conclusion, this mixed salivary gland cell culture system can be used to study epithelial-mesenchymal interactions and the effects of differentiating inducers and inhibitors.

Growth factors polymerized within fibrin hydrogel promote amylase production in parotid cells

Salivary gland cell differentiation has been a recurring challenge for researchers as primary salivary cells show a loss of phenotype in culture. Particularly, parotid cells show a marked decrease in amylase expression, the loss of tight junction organization and proper cell function. Previously, Matrigel has been used successfully as an extracellular matrix; however, it is not practical for in vivo applications as it is tumorigenic. An alternative method could rely on the use of fibrin hydrogel (FH), which has been used extensively in biomedical engineering applications ranging from cardiovascular tissue engineering to wound-healing experiments. Although several groups have examined the effects of a three-dimensional (3D) environment on salivary cell cultures, little is known about the effects of FH on salivary cell cultures. The current study developed a 3D cell culture model to support parotid gland cell differentiation using a combination of FH and growth factor-reduced Matrigel (GFR-MG). Furthermore, FH polymerized with a combination of EGF and IGF-1 induced formation of 3D spheroids capable of amylase expression and an agonist-induced increase in the intracellular Ca(2+) concentration ([Ca(2+)]i) in salivary cells. These studies represent an initial step toward the construction of an artificial salivary gland to restore salivary gland dysfunction. This is necessary to reduce xerostomia in patients with compromised salivary function.