Biomaterials-based strategies for salivary gland tissue regeneration

Bioengineering from the laboratory to clinical translation in oral and maxillofacial reconstruction

Background

Conventional techniques used in oral and maxillofacial reconstruction focus mainly on utilizing autologous tissues that have unquestionably improved function and esthetics for many patients, worldwide. However, the success depends on countless factors such as: donor and recipient sites conditions, patient's medical history, surgeon's experience, restricted availability of high-quality autogenous tissues or stem cells, and increased surgical cost and time.

Materials and Methods

Lately, teaming researchers, scientists, surgeons, and engineers, to address these limitations, have allowed tremendous progress in recombinant protein therapy, cell-based therapy, and gene therapy.

Results

Over the past few years, biomedical engineering has been evolving from the laboratory to clinical applications, for replacement of damaged body tissues due to trauma, cancer, congenital or acquired disorders.

Conclusions

This review provides an outlook on the content, benefits, recent advances, limitations, and future expectations of biomedical engineering for salivary glands, oral mucosa, dental structures, and maxillofacial reconstruction.

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.

Bioengineered Salivary Gland Microtissues─A Review of 3D Cellular Models and their Applications

Salivary glands (SGs) play a vital role in maintaining oral health through the production and release of saliva. Injury to SGs can lead to gland hypofunction and a decrease in saliva secretion manifesting as xerostomia. While symptomatic treatments for xerostomia exist, effective permanent solutions are still lacking, emphasizing the need for innovative approaches. Significant progress has been made in the field of three-dimensional (3D) SG bioengineering for applications in gland regeneration. This has been achieved through a major focus on cell culture techniques, including soluble cues and biomaterial components of the 3D niche. Cells derived from both adult and embryonic SGs have highlighted key in vitro characteristics of SG 3D models. While still in its first decade of exploration, SG spheroids and organoids have so far served as crucial tools to study SG pathophysiology. This review, based on a literature search over the past decade, covers the importance of SG cell types in the realm of their isolation, sourcing, and culture conditions that modulate the 3D microenvironment. We discuss different biomaterials employed for SG culture and the current advances made in bioengineering SG models using them. The success of these 3D cellular models are further evaluated in the context of their applications in organ transplantation and in vitro disease modeling.

Mesenchymal Stromal/Stem Cell Therapy Improves Salivary Flow Rate in Radiation-Induced Salivary Gland Hypofunction in Preclinical in vivo Models: A Systematic Review and Meta-Analysis

BACKGROUND

Mesenchymal stromal/stem cells (MSCs) have been suggested for salivary gland (SG) restoration following radio-induced salivary gland damage. This study aimed to determine the safety and effectiveness of MSC therapy on radio-induced SG damage and hypofunction in preclinical in vivo studies.

METHODS

PubMed and EMBASE were systematically searched for preclinical in vivo interventional studies evaluating efficacy and safety of MSC treatment following radio-induced salivary gland damage published before 10th of January 2022. The primary endpoint was salivary flow rate (SFR) evaluated in a meta-analysis. The study protocol was published and registered on PROSPERO ( www.crd.ac.uk/prospero ), registration number CRD42021227336.

RESULTS

A total of 16 preclinical in vivo studies were included for qualitative analysis (858 experimental animals) and 13 in the meta-analysis (404 experimental animals). MSCs originated from bone marrow (four studies), adipose tissue (10 studies) and salivary gland tissue (two studies) and were administered intravenously (three studies), intra-glandularly (11 studies) or subcutaneously (one study). No serious adverse events were reported. The overall effect on SFR was significantly increased with a standardized mean difference (SMD) of 6.99 (95% CI: 2.55-11.42). Studies reported improvements in acinar tissue, vascular areas and paracrine factors.

CONCLUSION

In conclusion, this systematic review and meta-analysis showed a significant effect of MSC therapy for restoring SG functioning and regenerating SG tissue following radiotherapy in preclinical in vivo studies without serious adverse events. MSC therapy holds significant therapeutic potential in the treatment of radio-induced xerostomia, but comprehensive, randomized, clinical trials in humans are required to ascertain their efficacy in a clinical setting.

Cell type-specific transforming growth factor-β (TGF-β) signaling in the regulation of salivary gland fibrosis and regeneration

Salivary gland damage and hypofunction result from various disorders, including autoimmune Sjögren's disease (SjD) and IgG4-related disease (IgG4-RD), as well as a side effect of radiotherapy for treating head and neck cancers. There are no therapeutic strategies to prevent the loss of salivary gland function in these disorders nor facilitate functional salivary gland regeneration. However, ongoing aquaporin-1 gene therapy trials to restore saliva flow show promise. To identify and develop novel therapeutic targets, we must better understand the cell-specific signaling processes involved in salivary gland regeneration. Transforming growth factor-β (TGF-β) signaling is essential to tissue fibrosis, a major endpoint in salivary gland degeneration, which develops in the salivary glands of patients with SjD, IgG4-RD, and radiation-induced damage. Though the deposition and remodeling of extracellular matrix proteins are essential to repair salivary gland damage, pathological fibrosis results in tissue hardening and chronic salivary gland dysfunction orchestrated by multiple cell types, including fibroblasts, myofibroblasts, endothelial cells, stromal cells, and lymphocytes, macrophages, and other immune cell populations. This review is focused on the role of TGF-β signaling in the development of salivary gland fibrosis and the potential for targeting TGF-β as a novel therapeutic approach to regenerate functional salivary glands. The studies presented highlight the divergent roles of TGF-β signaling in salivary gland development and dysfunction and illuminate specific cell populations in damaged or diseased salivary glands that mediate the effects of TGF-β. Overall, these studies strongly support the premise that blocking TGF-β signaling holds promise for the regeneration of functional salivary glands.

Evaluation of Alginate Hydrogel Microstrands for Stromal Cell Encapsulation and Maintenance

Mesenchymal stromal cells (MSCs) have displayed potential in regenerating organ function due to their anti-fibrotic, anti-inflammatory, and regenerative properties. However, there is a need for delivery systems to enhance MSC retention while maintaining their anti-fibrotic characteristics. This study investigates the feasibility of using alginate hydrogel microstrands as a cell delivery vehicle to maintain MSC viability and phenotype. To accommodate cell implantation needs, we invented a Syringe-in-Syringe approach to reproducibly fabricate microstrands in small numbers with a diameter of around 200 µm and a porous structure, which would allow for transporting nutrients to cells by diffusion. Using murine NIH 3T3 fibroblasts and primary embryonic 16 (E16) salivary mesenchyme cells as primary stromal cell models, we assessed cell viability, growth, and expression of mesenchymal and fibrotic markers in microstrands. Cell viability remained higher than 90% for both cell types. To determine cell number within the microstrands prior to in vivo implantation, we have further optimized the alamarBlue assay to measure viable cell growth in microstrands. We have shown the effect of initial cell seeding density and culture period on cell viability and growth to accommodate future stromal cell delivery and implantation. Additionally, we confirmed homeostatic phenotype maintenance for E16 mesenchyme cells in microstrands.

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.

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.

Matrix Degradability Contributes to the Development of Salivary Gland Progenitor Cells with Secretory Functions

Synthetic matrices that are cytocompatible, cell adhesive, and cell responsive are needed for the engineering of implantable, secretory salivary gland constructs to treat radiation induced xerostomia or dry mouth. Here, taking advantage of the bioorthogonality of the Michael-type addition reaction, hydrogels with comparable stiffness but varying degrees of degradability (100% degradable, 100DEG; 50% degradable, 50DEG; and nondegradable, 0DEG) by cell-secreted matrix metalloproteases (MMPs) were synthesized using thiolated HA (HA-SH), maleimide (MI)-conjugated integrin-binding peptide (RGD-MI), and MI-functionalized peptide cross-linkers that are protease degradable (GIW-bisMI) or nondegradable (GIQ-bisMI). Organized multicellular structures developed readily in all hydrogels from dispersed primary human salivary gland stem cells (hS/PCs). As the matrix became progressively degradable, cells proliferated more readily, and the multicellular structures became larger, less spherical, and more lobular. Immunocytochemical analysis showed positive staining for stem/progenitor cell markers CD44 and keratin 5 (K5) in all three types of cultures and positive staining for the acinar marker α-amylase under 50DEG and 100DEG conditions. Quantitatively at the mRNA level, the expression levels of key stem/progenitor markers KIT, KRT5, and ETV4/5 were significantly increased in the degradable gels as compared to the nondegradable counterparts. Western blot analyses revealed that imparting matrix degradation led to >3.8-fold increase in KIT expression by day 15. The MMP-degradable hydrogels also promoted the development of a secretary phenotype, as evidenced by the upregulation of acinar markers α-amylase (AMY), aquaporin-5 (AQP5), and sodium-potassium chloride cotransporter 1 (SLC12A2). Collectively, we show that cell-mediated matrix remodeling is necessary for the development of regenerative pro-acinar progenitor cells from hS/PCs.

Optimizing phenol-modified hyaluronic acid for designing shape-maintaining biofabricated hydrogel scaffolds in soft tissue engineering

In this study, we developed a well-printable biomaterial ink for 3D printing of shape-maintaining hydrogel scaffolds. The hydrogel base comprised tyramine-modified hyaluronic acid (HA-Tyr) and gelatin methacrylate (GelMA) and was dually cross-linked. Using the Box-Behnken design, we explored how varying the ink composition affected fiber formation and shape preservation. By adjusting the polymer ratios, we produced a stable hydrogel with varying responses, from a viscous liquid to a thick gel, and optimized 3D scaffolds that were structurally stable both during and after printing, offering precision and flexibility. Our ink exhibited shear-thinning behavior and high swelling capacity, as well as ECM-like characteristics and biocompatibility, making it an ideal candidate for soft tissues matrices with storage modulus of around 300 Pa. Animal trials and CAM assays confirmed its biocompatibility and integration with host tissue.

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.

Organ-specific extracellular matrix directs trans-differentiation of mesenchymal stem cells and formation of salivary gland-like organoids in vivo

BACKGROUND

Current treatments for salivary gland (SG) hypofunction are palliative and do not address the underlying cause or progression of the disease. SG-derived stem cells have the potential to treat SG hypofunction, but their isolation is challenging, especially when the tissue has been damaged by disease or irradiation for head and neck cancer. In the current study, we test the hypothesis that multipotent bone marrow-derived mesenchymal stem cells (BM-MSCs) in a rat model are capable of trans-differentiating to the SG epithelial cell lineage when induced by a native SG-specific extracellular matrix (SG-ECM) and thus may be a viable substitute for repairing damaged SGs.

METHODS

Rat BM-MSCs were treated with homogenates of decellularized rat SG-ECM for one hour in cell suspension and then cultured in tissue culture plates for 7 days in growth media. By day 7, the cultures contained cell aggregates and a cell monolayer. The cell aggregates were hand-selected under a dissecting microscope, transferred to a new tissue culture dish, and cultured for an additional 7 days in epithelial cell differentiation media. Cell aggregates and cells isolated from the monolayer were evaluated for expression of SG progenitor and epithelial cell specific markers, cell morphology and ultrastructure, and ability to form SG-like organoids in vivo.

RESULTS

The results showed that this approach was very effective and guided the trans-differentiation of a subpopulation of CD133-positive BM-MSCs to the SG epithelial cell lineage. These cells expressed amylase, tight junction proteins (Cldn 3 and 10), and markers for SG acinar (Aqp5 and Mist 1) and ductal (Krt 14) cells at both the transcript and protein levels, produced intracellular secretory granules which were morphologically identical to those found in submandibular gland, and formed SG-like organoids when implanted in the renal capsule in vivo.

CONCLUSIONS

The results of this study suggest the feasibility of using autologous BM-MSCs as an abundant source of stem cells for treating SG hypofunction and restoring the production of saliva in these patients.

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.

Mesenchymal stromal/stem cell therapy for radiation-induced salivary gland hypofunction in animal models: a protocol for a systematic review and meta-analysis

BACKGROUND

Salivary gland (SG) hypofunction (objectively reduced saliva flow rate) and xerostomia (subjective sensation of dry mouth) are common and burdensome side effects of radiotherapy to the head and neck region. Currently, only sparse symptomatic treatment is available to ease the discomfort of xerostomia. The objective of this study is to assess the effect of mesenchymal stem cell (MSC) therapy on SG function after radiation-induced injury.

METHODS

This systematic review will include animal intervention studies assessing efficacy and safety of MSCs in treating radiation-induced SG hypofunction. The primary outcome is the effect of MSC administration on salivary flow rates (SFR), by comparing treated groups to control groups when available. Secondary outcomes are morphological and immunohistochemical effects as well as safety of MSC treatment. Electronic searches in MEDLINE (PubMed) and Embase databases will be constructed and validated according to the peer review of electronic search strategies (PRESS) and assessed by two independent researchers. Data from eligible studies will be extracted, pooled, and analyzed using random-effects models. Risk of bias will be evaluated with the Systematic Review Centre for Laboratory animal Experimentation (SYRCLE) risk of bias tool.

DISCUSSION

Thus far, critical appraisal of MSC therapy as an effective treatment for SG hypofunction caused solely by radiation injury has not been conducted. A summary of the existing literature on preclinical studies concerning this issue can provide valuable information about effectiveness, mode of action, and safety, allowing further optimization of preclinical and clinical trials.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42021227336.

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.

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.

RGDSP-Decorated Hyaluronate Hydrogels Facilitate Rapid 3D Expansion of Amylase-Expressing Salivary Gland Progenitor Cells

In vitro engineering of salivary glands relies on the availability of synthetic matrices presenting essential cell-instructive signals to guide tissue growth. Here, we describe a biomimetic, hyaluronic acid (HA)-based hydrogel platform containing covalently immobilized bioactive peptides derived from perlecan domain IV (TWSKV), laminin-111 (YIGSR, IKVAV), and fibronectin (RGDSP). The HA network was established by the thiol/acrylate reaction, and bioactive peptides were conjugated to the network with high efficiency without significantly altering the mechanical property of the matrix. When encapsulated as single cells in peptide-modified HA hydrogels, human salivary gland stem/progenitor cells (hS/PCs) spontaneously organized into multicellular spheroids with close cell-cell contacts. Conjugation of RGDSP and TWSKV signals in HA gels significantly accelerated cell proliferation, with the largest spheroids observed in RGDSP-tagged gels. Peptide conjugation did not significantly alter the expression of acinar (AMY1), ductal (TFCP2L1), and progenitor (KRT14) markers at the mRNA level. Characterization of three-dimensional (3D) cultures by immunocytochemistry showed positive staining for keratin-5 (K5), keratin-14 (K14), integrin-β1, and α-amylase under all culture conditions, confirming the maintenance of the secretory progenitor cell population. Two-dimensional (2D) adhesion studies revealed that integrin-β1 played a key role in facilitating cell-matrix interaction in gels with RGDSP, IKVAV, and TWSKV signals. Overall, conjugation of the RGDSP peptide to HA gels improved cell viability, accelerated the formation of epithelial spheroids, and promoted the expansion of the progenitor cell population in 3D. This work represents an essential first step toward the development of an engineered salivary gland.

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.

Scaffold-based developmental tissue engineering strategies for ectodermal organ regeneration

Tissue engineering (TE) is a multidisciplinary research field aiming at the regeneration, restoration, or replacement of damaged tissues and organs. Classical TE approaches combine scaffolds, cells and soluble factors to fabricate constructs mimicking the native tissue to be regenerated. However, to date, limited success in clinical translations has been achieved by classical TE approaches, because of the lack of satisfactory biomorphological and biofunctional features of the obtained constructs. Developmental TE has emerged as a novel TE paradigm to obtain tissues and organs with correct biomorphology and biofunctionality by mimicking the morphogenetic processes leading to the tissue/organ generation in the embryo. Ectodermal appendages, for instance, develop in vivo by sequential interactions between epithelium and mesenchyme, in a process known as secondary induction. A fine artificial replication of these complex interactions can potentially lead to the fabrication of the tissues/organs to be regenerated. Successful developmental TE applications have been reported, in vitro and in vivo, for ectodermal appendages such as teeth, hair follicles and glands. Developmental TE strategies require an accurate selection of cell sources, scaffolds and cell culture configurations to allow for the correct replication of the in vivo morphogenetic cues. Herein, we describe and discuss the emergence of this TE paradigm by reviewing the achievements obtained so far in developmental TE 3D scaffolds for teeth, hair follicles, and salivary and lacrimal glands, with particular focus on the selection of biomaterials and cell culture configurations.

Oral Organoids: Progress and Challenges

Oral organoids are complex 3-dimensional structures that develop from stem cells or organ-specific progenitors through a process of self-organization and re-create architectures and functionalities similar to in vivo organs and tissues in the oral and maxillofacial region. Recently, striking advancements have been made in the construction and application of oral organoids of the tooth, salivary gland, and tongue. Dental epithelial and mesenchymal cells isolated from tooth germs or derived from pluripotent stem cells could generate tooth germ-like organoids by self-organization in a specific culture system. Tooth organoids can also be constructed based on tissue engineering principles by seeding stem cells on a scaffold with the bioregulatory functions of odontogenic differentiation. Two main approaches have been used to construct salivary gland organoids: 1) incubation of salivary gland-derived stem/progenitor cells in a 3-dimensional culture system to form the structure of the gland through mimicking regenerative processes and 2) inducing of pluripotent stem cells to generate embryonic salivary glands by replicating the development process. Taste bud organoids can be generated by embedding isolated circumvallate papilla tissue in Matrigel with a mixture of growth factors, while lingual epithelial organoids have been constructed using lingual stem cells in a suitable culture system containing specific signaling molecules. These oral organoids usually maintain the main functions and characteristic structures of the corresponding organ to a certain extent. Furthermore, using cells isolated from patients, oral organoids could replicate specific diseases such as maxillofacial tumors and tooth dysplasia. Until now, oral organoids have been applied in the study of mechanisms of tooth development, pathology and regeneration of the salivary gland, and precision therapeutics for tongue cancer. These findings strongly demonstrate that the organoid technique is a novel paradigm for the study of the development, pathology, and regeneration of oral and maxillofacial tissue.

Radiation-Induced Salivary Gland Dysfunction: Mechanisms, Therapeutics and Future Directions

Salivary glands sustain collateral damage following radiotherapy (RT) to treat cancers of the head and neck, leading to complications, including mucositis, xerostomia and hyposalivation. Despite salivary gland-sparing techniques and modified dosing strategies, long-term hypofunction remains a significant problem. Current therapeutic interventions provide temporary symptom relief, but do not address irreversible glandular damage. In this review, we summarize the current understanding of mechanisms involved in RT-induced hyposalivation and provide a framework for future mechanistic studies. One glaring gap in published studies investigating RT-induced mechanisms of salivary gland dysfunction concerns the effect of irradiation on adjacent non-irradiated tissue via paracrine, autocrine and direct cell-cell interactions, coined the bystander effect in other models of RT-induced damage. We hypothesize that purinergic receptor signaling involving P2 nucleotide receptors may play a key role in mediating the bystander effect. We also discuss promising new therapeutic approaches to prevent salivary gland damage due to RT.

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.

Building a Functional Salivary Gland for Cell-Based Therapy: More than Secretory Epithelial Acini

A few treatment options exist for patients experiencing xerostomia due to hyposalivation that occurs as a result of disease or injury to the gland. An opportunity for a permanent solution lies in the field of salivary gland replacement through tissue engineering. Recent success emboldens in the vision of producing a tissue-engineered salivary gland composed of differentiated salivary epithelial cells that are able to differentiate to form functional units that produce and deliver saliva to the oral cavity. This vision is augmented by advances in understanding cellular mechanisms that guide branching morphogenesis and salivary epithelial cell polarization in both acinar and ductal structures. Growth factors and other guidance cues introduced into engineered constructs help to develop a more complex glandular structure that seeks to mimic native salivary gland tissue. This review describes the separate epithelial phenotypes that make up the gland, and it describes their relationship with the other cell types such as nerve and vasculature that surround them. The review is organized around the links between the native components that form and contribute to various aspects of salivary gland development, structure, and function and how this information can drive the design of functional tissue-engineered constructs. In addition, we discuss the attributes of various biomaterials commonly used to drive function and form in engineered constructs. The review also contains a current description of the state-of-the-art of the field, including successes and challenges in creating materials for preclinical testing in animal models. The ability to integrate biomolecular cues in combination with a range of materials opens the door to the design of increasingly complex salivary gland structures that, once accomplished, can lead to breakthroughs in other fields of tissue engineering of epithelial-based exocrine glands or oral tissues.

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.

Harnessing biomolecules for bioinspired dental biomaterials

Dental clinicians have relied for centuries on traditional dental materials (polymers, ceramics, metals, and composites) to restore oral health and function to patients. Clinical outcomes for many crucial dental therapies remain poor despite many decades of intense research on these materials. Recent attention has been paid to biomolecules as a chassis for engineered preventive, restorative, and regenerative approaches in dentistry. Indeed, biomolecules represent a uniquely versatile and precise tool to enable the design and development of bioinspired multifunctional dental materials to spur advancements in dentistry. In this review, we survey the range of biomolecules that have been used across dental biomaterials. Our particular focus is on the key biological activity imparted by each biomolecule toward prevention of dental and oral diseases as well as restoration of oral health. Additional emphasis is placed on the structure-function relationships between biomolecules and their biological activity, the unique challenges of each clinical condition, limitations of conventional therapies, and the advantages of each class of biomolecule for said challenge. Biomaterials for bone regeneration are not reviewed as numerous existing reviews on the topic have been recently published. We conclude our narrative review with an outlook on the future of biomolecules in dental biomaterials and potential avenues of innovation for biomaterial-based patient oral care.

Trimers Conjugated to Fibrin Hydrogels Promote Salivary Gland Function

New strategies for tissue engineering have great potential for restoring and revitalizing impaired tissues and organs, including the use of smart hydrogels that can be modified to enhance organization and functionality of the salivary glands. For instance, monomers of laminin-111 peptides chemically conjugated to fibrin hydrogel (L_1pM-FH) promote cell cluster formation in vitro and salivary gland regeneration in vivo when compared with fibrin hydrogel (FH) alone; however, L_1pM-FH produce only weak expression of acinar differentiation markers in vivo (e.g., aquaporin-5 and transmembrane protein 16). Since previous studies demonstrated that a greater impact can be achieved when trimeric forms were used as compared with monomeric or dimeric forms, we investigated the extent to which trimers of laminin-111 chemically conjugated to FH (L_1pT-FH) can increase the expression of acinar differentiation markers and elevate saliva secretion. In vitro studies using Par-C10 acinar cells demonstrated that when compared with L_1pM-FH, L_1pT-FH induced similar levels of acinar-like cell clustering, polarization, lumen formation, and calcium signaling. To assess the performance of the trimeric complex in vivo, we compared the ability of L_1pM-FH and L_1pT-FH to increase acinar differentiation markers and restore saliva flow rate in a salivary gland wound model of C57BL/6 mice. Our results show that L_1pT-FH applied to wounded mice significantly improved the expression of the acinar differentiation markers and saliva secretion when compared with the monomeric form. Together, these positive effects of L_1pT-FH warrant its future testing in additional models of hyposalivation with the ultimate goal of applying this technology in humans.

CERE-120 Prevents Irradiation-Induced Hypofunction and Restores Immune Homeostasis in Porcine Salivary Glands

Salivary gland hypofunction causes significant morbidity and loss of quality of life for head and neck cancer patients treated with radiotherapy. Preventing hypofunction is an unmet therapeutic need. We used an adeno-associated virus serotype 2 (AAV2) vector expressing the human neurotrophic factor neurturin (CERE-120) to treat murine submandibular glands either pre- or post-irradiation (IR). Treatment with CERE-120 pre-IR, not post-IR, prevented hypofunction. RNA sequencing (RNA-seq) analysis showed reduced gene expression associated with fibrosis and the innate and humoral immune responses. We then used a minipig model with CERE-120 treatment pre-IR and also compared outcomes of the contralateral non-IR gland. Analysis of gene expression, morphology, and immunostaining showed reduced IR-related immune responses and improved secretory mechanisms. CERE-120 prevented IR-induced hypofunction and restored immune homeostasis, and there was a coordinated contralateral gland response to either damage or treatment. CERE-120 gene therapy is a potential treatment for head and neck cancer patients to influence communication among neuronal, immune, and epithelial cells to prevent IR-induced salivary hypofunction and restore immune homeostasis.

Mouth-Watering Results: Clinical Need, Current Approaches, and Future Directions for Salivary Gland Regeneration

Permanent damage to the salivary glands and resulting hyposalivation and xerostomia have a substantial impact on patient health, quality of life, and healthcare costs. Currently, patients rely on lifelong treatments that alleviate the symptoms, but no long-term restorative solutions exist. Recent advances in adult stem cell enrichment and transplantation, bioengineering, and gene transfer have proved successful in rescuing salivary gland function in a number of animal models that reflect human diseases and that result in hyposalivation and xerostomia. By overcoming the limitations of stem cell transplants and better understanding the mechanisms of cellular plasticity in the adult salivary gland, such studies provide encouraging evidence that a regenerative strategy for patients will be available in the near future.

Salivary gland tissue engineering to attain clinical benefits: a special report

The salivary glands produce saliva, which helps in mediating the oral colonization of microbes, the repair of mucosa, the remineralization of teeth, lubrication and gustation. However, certain medications, therapeutic radiation and certain autoimmune diseases can cause a reduction in the salivary flow. The aim of this report was to review and highlight the indications and techniques of salivary gland engineering to counter hyposalivation. This report concludes that in the literature, numerous strategies have been suggested and discussed pertaining to the engineering of salivary gland, however, challenges remain in terms of its production and accurate function. Dedicated efforts are required from researchers all over the world to obtain the maximum benefits from salivary gland engineering techniques.

NiCHE Platform: Nature-Inspired Catechol-Conjugated Hyaluronic Acid Environment Platform for Salivary Gland Tissue Engineering

Recently, there has been growing interest in replacing severely damaged salivary glands with artificial salivary gland functional units created in vitro by tissue engineering approaches. Although various materials such as poly(lactic-co-glycolic acid), polylactic acid, poly(glycolic acid), and polyethylene glycol hydrogels have been used as scaffolds for salivary gland tissue engineering, none of them is effective enough to closely recapitulate the branched structural complexity and heterogeneous cell population of native salivary glands. Instead of discovering new biomaterial candidates, we synthesized hyaluronic acid-catechol (HACA) conjugates to establish a versatile hyaluronic acid coating platform named "NiCHE (nature-inspired catechol-conjugated hyaluronic acid environment)" for boosting the salivary gland tissue engineering efficacy of the previously reported biomaterials. By mimicking hyaluronic acid-rich niche in the mesenchyme of embryonic submandibular glands (eSMGs) with NiCHE coating on substrates including polycarbonate membrane, stiff agarose hydrogel, and polycaprolactone scaffold, we observed significantly enhanced cell adhesion, vascular endothelial and progenitor cell proliferation, and branching of in vitro-cultured eSMGs. High mechanical stiffness of the substrate is known to inhibit eSMG growth, but the NiCHE coating significantly reduced such stiffness-induced negative effects, leading to successful differentiation of progenitor cells to functional acinar and myoepithelial cells. These enhancement effects of the NiCHE coating were due to the increased proliferation of vascular endothelial cells via interaction between CD44 and surface-immobilized HAs. As such, our NiCHE coating platform renders any kind of material highly effective for salivary gland tissue culture by mimicking in vivo embryonic mesenchymal HA. Based on our results, we expect the NiCHE coating to expand the range of biomaterial candidates for salivary glands and other branching epithelial organs.

Active agents, biomaterials, and technologies to improve biolubrication and strengthen soft tissues

Normal functioning of articulating tissues is required for many physiological processes occurring across length scales from the molecular to whole organism. Lubricating biopolymers are present natively on tissue surfaces at various sites of biological articulation, including eyelid, mouth, and synovial joints. The range of operating conditions at these disparate interfaces yields a variety of tribological mechanisms through which compressive and shear forces are dissipated to protect tissues from material wear and fatigue. This review focuses on recent advances in active agents and biomaterials for therapeutic augmentation of friction, lubrication, and wear in disease and injured states. Various small-molecule, biological, and gene delivery therapies are described, as are tribosupplementation with naturally-occurring and synthetic biolubricants and polymer reinforcements. While reintroduction of a diseased tissue's native lubricant received significant attention in the past, recent discoveries and pre-clinical research are capitalizing on concurrent advances in the molecular sciences and bioengineering fields, with an understanding of the underlying tissue structure and physiology, to afford a desired, and potentially patient-specific, tissue mechanical response for restoration of normal function. Small and large molecule drugs targeting recently elucidated pathways as well as synthetic and hybrid natural/synthetic biomaterials for restoring a desired tissue mechanical response are being investigated for treatment of, for example, keratoconjunctivitis sicca, xeroderma, and osteoarthritis.

Core-shell patterning of synthetic hydrogels via interfacial bioorthogonal chemistry for spatial control of stem cell behavior

A new technique is described for the patterning of cell-guidance cues in synthetic extracellular matrices (ECM) for tissue engineering applications. Using s-tetrazine modified hyaluronic acid (HA), bis-trans-cyclooctene (TCO) crosslinkers and monofunctional TCO conjugates, interfacial bioorthogonal crosslinking was used to covalently functionalize hydrogels as they were synthesized at the liquid-gel interface. Through temporally controlled introduction of TCO conjugates during the crosslinking process, the enzymatic degradability, cell adhesivity, and mechanical properties of the synthetic microenvironment can be tuned with spatial precision. Using human mesenchymal stem cells (hMSCs) and hydrogels with a core-shell structure, we demonstrated the ability of the synthetic ECM with spatially defined guidance cues to modulate cell morphology in a biomimetic fashion. This new method for the spatially resolved introduction of cell-guidance cues for the establishment of functional tissue constructs complements existing methods that require UV-light or specialized equipment.

Bottom-up assembly of salivary gland microtissues for assessing myoepithelial cell function

Myoepithelial cells are flat, stellate cells present in exocrine tissues including the salivary glands. While myoepithelial cells have been studied extensively in mammary and lacrimal gland tissues, less is known of the function of myoepithelial cells derived from human salivary glands. Several groups have isolated tumorigenic myoepithelial cells from cancer specimens, however, only one report has demonstrated isolation of normal human salivary myoepithelial cells needed for use in salivary gland tissue engineering applications. Establishing a functional organoid model consisting of myoepithelial and secretory acinar cells is therefore necessary for understanding the coordinated action of these two cell types in unidirectional fluid secretion. Here, we developed a bottom-up approach for generating salivary gland microtissues using primary human salivary myoepithelial cells (hSMECs) and stem/progenitor cells (hS/PCs) isolated from normal salivary gland tissues. Phenotypic characterization of isolated hSMECs confirmed that a myoepithelial cell phenotype consistent with that from other exocrine tissues was maintained over multiple passages of culture. Additionally, hSMECs secreted basement membrane proteins, expressed adrenergic and cholinergic neurotransmitter receptors, and released intracellular calcium [Ca²⁺_i] in response to parasympathetic agonists. In a collagen I contractility assay, activation of contractile machinery was observed in isolated hSMECs treated with parasympathetic agonists. Recombination of hSMECs with assembled hS/PC spheroids in a microwell system was used to create microtissues resembling secretory complexes of the salivary gland. We conclude that the engineered salivary gland microtissue complexes provide a physiologically relevant model for both mechanistic studies and as a building block for the successful engineering of the salivary gland for restoration of salivary function in patients suffering from hyposalivation.

Interaction of chitin/chitosan with salivary and other epithelial cells-An overview

Chitin and its deacetylated form, chitosan, have been widely used for tissue engineering of both epithelial and mesenchymal tissues. Epithelial cells characterised by their sheet-like tight cellular arrangement and polarised nature, constitute a major component in various organs and play a variety of roles including protection, secretion and maintenance of tissue homeostasis. Regeneration of damaged epithelial tissues has been studied using biomaterials such as chitin, chitosan, hyaluronan, gelatin and alginate. Chitin and chitosan are known to promote proliferation of various embryonic and adult epithelial cells. However it is not clearly understood how this activity is achieved or what are the mechanisms involved in the chitin/chitosan driven proliferation of epithelial cells. Mechanistic understanding of influence of chitin/chitosan on epithelial cells will guide us to develop more targeted regenerative scaffold/hydrogel systems. Therefore, current review attempts to elicit a mechanistic insight into how chitin and chitosan interact with salivary, mammary, skin, nasal, lung, intestinal and bladder epithelial cells.

Tuning Hydrogel Properties to Promote the Assembly of Salivary Gland Spheroids in 3D

Current treatments for chronic xerostomia, or "dry mouth", do not offer long-term therapeutic benefits for head and neck cancer survivors previously treated with curative radiation. Towards the goal of creating tissue-engineered constructs for the restoration of salivary gland functions, we developed new hyaluronic acid (HA)-based hydrogels using thiolated HA (HA-SH) and acrylated HA (HA-AES) with a significant molecular weight mismatch. Four hydrogel formulations with varying HA concentration, (1-2.4 wt%) and thiol/acrylate ratios (2/1 to 36/1) and elastic moduli (G': 35 to 1897 Pa, 2 h post-mixing) were investigated. In our system, thiol/acrylate reaction was initiated rapidly upon mixing of HA-SH/HA-AES to establish thioether crosslinks with neighboring ester groups, and spontaneous sulfhydryl oxidation occurred slowly over several days to install a secondary network. The concurrent reactions cooperatively create a cell-permissive network to allow for cell expansion and aggregation. Multicellular spheroids formed readily from a robust ductal epithelial cell line (Madin-Darby Canine Kidney, MDCK cells) in all hydrogel formulations investigated. Primary salivary human stem/progenitor cells (hS/PCs), on the other hand, are sensitive to the synthetic extracellular environment, and organized acini-like structures with an average diameter of 50 µm were obtained only in gels with G' ≤ 216 Pa and a thiol/acrylate ratio ≥18. The spheroid size and size distribution were dependent on the HA content in the hydrogel. Cells in hS/PC spheroids formed tight junctions (occludin), remained viable and proliferative, secreted structural proteins (collagen IV and laminin) found in the basement membrane and maintained key stem/progenitor markers. We conclude that incorporation of time-dependent, dynamic features into a covalently crosslinked HA network produces an adaptable hydrogel framework that promotes hS/PC assembly and supports early aspects of salivary morphogenesis, key to reconstitution of a fully functional implantable salivary gland.

Graphene oxide and sulfonated polyanion co-doped hydrogel films for dual-layered membranes with superior hemocompatibility and antibacterial activity

GO based dual-layered membranes with superior hemocompatibility and antibacterial activity have potential application for clinical hemodialysis and many other biomedical therapies.