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

Salivary Gland Tissue Recombination Can Modify Cell Fate

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

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.

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.

Salivary Gland Bioengineering

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

BMP-2-mediated signaling suppresses salivary gland development

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

Craniofacial developmental biology in the single-cell era

The evolution of a unique craniofacial complex in vertebrates made possible new ways of breathing, eating, communicating and sensing the environment. The head and face develop through interactions of all three germ layers, the endoderm, ectoderm and mesoderm, as well as the so-called fourth germ layer, the cranial neural crest. Over a century of experimental embryology and genetics have revealed an incredible diversity of cell types derived from each germ layer, signaling pathways and genes that coordinate craniofacial development, and how changes to these underlie human disease and vertebrate evolution. Yet for many diseases and congenital anomalies, we have an incomplete picture of the causative genomic changes, in particular how alterations to the non-coding genome might affect craniofacial gene expression. Emerging genomics and single-cell technologies provide an opportunity to obtain a more holistic view of the genes and gene regulatory elements orchestrating craniofacial development across vertebrates. These single-cell studies generate novel hypotheses that can be experimentally validated in vivo. In this Review, we highlight recent advances in single-cell studies of diverse craniofacial structures, as well as potential pitfalls and the need for extensive in vivo validation. We discuss how these studies inform the developmental sources and regulation of head structures, bringing new insights into the etiology of structural birth anomalies that affect the vertebrate head.

Slow hydrogel matrix degradation enhances salivary gland mimetic phenotype

We recently developed a salivary gland tissue mimetic (SGm), comprised of salivary gland cells encapsulated in matrix metalloproteinase (MMP)-degradable poly(ethylene glycol) hydrogels within arrays of ∼320 µm diameter spherical cavities molded in PDMS. The SGm provides a functional and physiologically relevant platform well-suited to high-throughput drug screening for radioprotective compounds. However, the utility of the SGm would benefit from improved retention of acinar cell phenotype and function. We hypothesized that tuning biochemical cues presented within the PEG hydrogel matrix would improve maintenance of acinar cell phenotype and function by mimicking the natural extracellular matrix microenvironment of the intact gland. Hydrogels formed using slower-degrading MMP-sensitive peptide crosslinkers showed >2-fold increase in sphere number formed at 48 h, increased expression of acinar cell markers, and more robust response to calcium stimulation by the secretory agonist, carbachol, with reduced SGm tissue cluster disruption and outgrowth during prolonged culture. The incorporation of adhesive peptides containing RGD or IKVAV improved calcium flux response to secretory agonists at 14 days of culture. Tuning the hydrogel matrix improved cell aggregation, and promoted acinar cell phenotype, and stability of the SGm over 14 days of culture. Furthermore, combining this matrix with optimized media conditions synergistically prolonged the retention of the acinar cell phenotype in SGm. STATEMENT OF SIGNIFICANCE: Salivary gland (SG) dysfunction occurs due to off-target radiation due to head and neck cancer treatments. Progress in understanding gland dysfunction and developing therapeutic strategies for the SG are hampered by the lack of in vitro models, as salivary gland cells rapidly lose critical secretory function within 24 hours in vitro. Herein, we identify properties of poly(ethylene glycol) hydrogel matrices that enhance the secretory phenotype of SG tissue mimetics within the previously-described SG-microbubble tissue chip environment. Combining slow-degrading hydrogels with media conditions optimized for secretory marker expression further enhanced functional secretory response and secretory marker expression.

Identifying fibrogenic cells following salivary gland obstructive injury

Fibrosis results from excess extracellular matrix accumulation, which alters normal tissue architecture and impedes function. In the salivary gland, fibrosis can be induced by irradiation treatment for cancer therapy, Sjögren's Disease, and other causes; however, it is unclear which stromal cells and signals participate in injury responses and disease progression. As hedgehog signaling has been implicated in fibrosis of the salivary gland and other organs, we examined contributions of the hedgehog effector, Gli1, to fibrotic responses in salivary glands. To experimentally induce a fibrotic response in female murine submandibular salivary glands, we performed ductal ligation surgery. We detected a progressive fibrotic response where both extracellular matrix accumulation and actively remodeled collagen significantly increased at 14 days post-ligation. Macrophages, which participate in extracellular matrix remodeling, and Gli1+ and PDGFRα+ stromal cells, which may deposit extracellular matrix, both increased with injury. Using single-cell RNA-sequencing, Gli1 + cells were not found in discrete clusters at embryonic day 16 but were found in clusters expressing the stromal genes Pdgfra and/or Pdgfrb. In adult mice, Gli1+ cells were similarly heterogeneous but more cells co-expressed PDGFRα and PDGFRβ. Using Gli1-CreERT2; ROSA26tdTomato lineage-tracing mice, we found that Gli1-derived cells expand with ductal ligation injury. Although some of the Gli1 lineage-traced tdTomato+ cells expressed vimentin and PDGFRβ following injury, there was no increase in the classic myofibroblast marker, smooth muscle alpha-actin. Additionally, there was little change in extracellular matrix area, remodeled collagen area, PDGFRα, PDGFRβ, endothelial cells, neurons, or macrophages in Gli1 null salivary glands following injury when compared with controls, suggesting that Gli1 signaling and Gli1+ cells have only a minor contribution to mechanical injury-induced fibrotic changes in the salivary gland. We used scRNA-seq to examine cell populations that expand with ligation and/or showed increased expression of matrisome genes. Some Pdgfra + /Pdgfrb + stromal cell subpopulations expanded in response to ligation, with two stromal cell subpopulations showing increased expression of Col1a1 and a greater diversity of matrisome genes, consistent with these cells being fibrogenic. However, only a few cells in these subpopulations expressed Gli1, consistent with a minor contribution of these cells to extracellular matrix production. Defining the signaling pathways driving fibrotic responses in stromal cell sub-types could reveal future therapeutic targets.

Gene expression in organoids: an expanding horizon

Recent development of human three-dimensional organoid cultures has opened new doors and opportunities ranging from modelling human development in vitro to personalised cancer therapies. These new in vitro systems are opening new horizons to the classic understanding of human development and disease. However, the complexity and heterogeneity of these models requires cutting-edge techniques to capture and trace global changes in gene expression to enable identification of key players and uncover the underlying molecular mechanisms. Rapid development of sequencing approaches made possible global transcriptome analyses and epigenetic profiling. Despite challenges in organoid culture and handling, these techniques are now being adapted to embrace organoids derived from a wide range of human tissues. Here, we review current state-of-the-art multi-omics technologies, such as single-cell transcriptomics and chromatin accessibility assays, employed to study organoids as a model for development and a platform for precision medicine.

FGF signaling regulates salivary gland branching morphogenesis by modulating cell adhesion

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

Identifying Fibrogenic Cells Following Salivary Gland Obstructive Injury

Fibrosis results from excess extracellular matrix accumulation, which alters normal tissue architecture and impedes function. In the salivary gland, fibrosis can be induced by irradiation treatment for cancer therapy, Sjögren's Disease, and other causes; however, it is unclear which stromal cells and signals participate in injury responses and disease progression. As hedgehog signaling has been implicated in fibrosis of the salivary gland and other organs, we examined contributions of the hedgehog effector, Gli1, to fibrotic responses in salivary glands. To experimentally induce a fibrotic response in female murine submandibular salivary glands, we performed ductal ligation surgery. We detected a progressive fibrotic response where both extracellular matrix accumulation and actively remodeled collagen trended upwards at 7 days and significantly increased at 14 days post- ligation. Macrophages, which participate in extracellular matrix remodeling, Gli1 + and PDGFRα + stromal cells, which may deposit extracellular matrix, both increased with injury. Using single-cell RNA-sequencing, we found that a majority of Gli1 + cells at embryonic day 16 also express Pdgfra and/or Pdgfrb. However, in adult mice, only a small subset of Gli1 + cells express PDGFRα and/or PDGFRβ at the protein level. Using lineage-tracing mice, we found that Gli1-derived cells expand with ductal ligation injury. Although some of the Gli1 lineage-traced tdTomato + cells expressed vimentin and PDGFRβ following injury, there was no increase in the classic myofibroblast marker, smooth muscle alpha-actin. Additionally, there was little change in extracellular matrix area, remodeled collagen area, PDGFRα, PDGFRβ, endothelial cells, neurons, or macrophages in Gli1 null salivary glands following injury when compared with controls, suggesting that Gli1 signaling and Gli1 + cells have only a minor contribution to mechanical injury-induced fibrotic changes in the salivary gland. We used scRNA-seq to examine cell populations that expand with ligation and/or showed increased expression of matrisome genes. Pdgfra + /Pdgfrb + stromal cell subpopulations both expanded in response to ligation, showed increased expression and a greater diversity of matrisome genes expressed, consistent with these cells being fibrogenic. Defining the signaling pathways driving fibrotic responses in stromal cell sub-types could reveal future therapeutic targets.

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.

Raman microspectroscopy fingerprinting of organoid differentiation state

Background

Organoids, which are organs grown in a dish from stem or progenitor cells, model the structure and function of organs and can be used to define molecular events during organ formation, model human disease, assess drug responses, and perform grafting in vivo for regenerative medicine approaches. For therapeutic applications, there is a need for nondestructive methods to identify the differentiation state of unlabeled organoids in response to treatment with growth factors or pharmacologicals.

Methods

Using complex 3D submandibular salivary gland organoids developed from embryonic progenitor cells, which respond to EGF by proliferating and FGF2 by undergoing branching morphogenesis and proacinar differentiation, we developed Raman confocal microspectroscopy methods to define Raman signatures for each of these organoid states using both fixed and live organoids.

Results

Three separate quantitative comparisons, Raman spectral features, multivariate analysis, and machine learning, classified distinct organoid differentiation signatures and revealed that the Raman spectral signatures were predictive of organoid phenotype.

Conclusions

As the organoids were unlabeled, intact, and hydrated at the time of imaging, Raman spectral fingerprints can be used to noninvasively distinguish between different organoid phenotypes for future applications in disease modeling, drug screening, and regenerative medicine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11658-022-00347-3.

Salivary gland organoids have unique Raman signatures detectable with a confocal-based Raman imaging approach.

Raman signatures can be detected in unlabeled fixed or live organoids.

Raman spectral signatures effectively predict organoid phenotypes.

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.