Salivary Gland Dysplasia in Fgf10 Heterozygous Mice: A New Mouse Model of Xerostomia

Fibroblast growth factor 10

Fibroblast growth factor 10 (FGF10) is a major morphoregulatory factor that plays essential signaling roles during vertebrate multiorgan development and homeostasis. FGF10 is predominantly expressed in mesenchymal cells and signals though FGFR2b in adjacent epithelia to regulate branching morphogenesis, stem cell fate, tissue differentiation and proliferation, in addition to autocrine roles. Genetic loss of function analyses have revealed critical requirements for FGF10 signaling during limb, lung, digestive system, ectodermal, nervous system, craniofacial and cardiac development. Heterozygous FGF10 mutations have been identified in human genetic syndromes associated with craniofacial anomalies, including lacrimal and salivary gland aplasia. Elevated Fgf10 expression is associated with poor prognosis in a range of cancers. In addition to developmental and disease roles, FGF10 regulates homeostasis and repair of diverse adult tissues and has been identified as a target for regenerative medicine.

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

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

Id4 modulates salivary gland homeostasis and its expression is downregulated in IgG4-related disease via miR-486-5p

Salivary glands are physiologically orchestrated by the coordinated balance between cell differentiation, proliferation, apoptosis, and interactions between epithelial, mesenchymal endothelial, and neuronal cells, and they are frequent sites of manifestations of Sjögren's syndrome (SS) or IgG4-related disease (IgG4-RD). However, little is known about salivary gland homeostasis and its involvement in those diseases. Inhibitor of DNA binding/differentiation 4 (Id4) is an Id protein involved in the transcriptional control of many biological events, including differentiation. Studies of Id4-deficient mice revealed that Id4-deficient submandibular glands were smaller and exhibited accelerated differentiation, compared with those from wild-type littermates. In addition, dry mouth symptoms and Th17 expansion in splenocytes were also observed in the absence of Id4. Furthermore, Id4 levels in the salivary glands of patients with IgG4-RD, but not SS, were significantly decreased compared with those of healthy controls. miRNA-mRNA integrated analysis demonstrated that miR-486-5p was upregulated in IgG4-RD patients and that it might regulate Id4 in the lesion sites. Together, these results provide evidence for the inhibitory role of Id4 in salivary differentiation, and a critical association between Id4 downregulation and IgG4-RD.

Sci-Seq of Human Fetal Salivary Tissue Introduces Human Transcriptional Paradigms and a Novel Cell Population

Multiple pathologies and non-pathological factors can disrupt the function of the non-regenerative human salivary gland including cancer and cancer therapeutics, autoimmune diseases, infections, pharmaceutical side effects, and traumatic injury. Despite the wide range of pathologies, no therapeutic or regenerative approaches exist to address salivary gland loss, likely due to significant gaps in our understanding of salivary gland development. Moreover, identifying the tissue of origin when diagnosing salivary carcinomas requires an understanding of human fetal development. Using computational tools, we identify developmental branchpoints, a novel stem cell-like population, and key signaling pathways in the human developing salivary glands by analyzing our human fetal single-cell sequencing data. Trajectory and transcriptional analysis suggest that the earliest progenitors yield excretory duct and myoepithelial cells and a transitional population that will yield later ductal cell types. Importantly, this single-cell analysis revealed a previously undescribed population of stem cell-like cells that are derived from SD and expresses high levels of genes associated with stem cell-like function. We have observed these rare cells, not in a single niche location but dispersed within the developing duct at later developmental stages. Our studies introduce new human-specific developmental paradigms for the salivary gland and lay the groundwork for the development of translational human therapeutics.

Deletion of the last two exons of FGF10 in a family with LADD syndrome and pulmonary acinar hypoplasia

Pulmonary acinar hypoplasia (PAH) and lacrimo-auriculo-dento-digital (LADD) syndrome have both been associated with loss-of-function variants in, or deletions of FGF10. Here we report a multi-generational family with seven members manifesting varying features of LADD syndrome, with one individual dying in early infancy of PAH. Whole genome sequencing in one family member identified a 12,158 bp deletion on chromosome 5p12 that removes two of the three exons of FGF10. Allele-specific PCR demonstrated that all affected family members, including the individual with PAH, carried the 12 kb deletion. We conclude the deletion is pathogenic and expands the mutational spectrum of FGF10 variants in LADD syndrome. The common mechanism underlying the variable clinical features of LADD syndrome is defective terminal branching of salivary and lacrimal glands and pulmonary acini, regulated by the TBX4-FGF10-FGFR2 pathway. The variable phenotypic expressivity of FGF10 haploinsufficiency from relatively benign to lethal is likely due to variation at other genetic loci.

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.

Magnetic bioassembly platforms towards the generation of extracellular vesicles from human salivary gland functional organoids for epithelial repair

Salivary glands (SG) are exocrine organs with secretory units commonly injured by radiotherapy. Bio-engineered organoids and extracellular vesicles (EV) are currently under investigation as potential strategies for SG repair. Herein, three-dimensional (3D) cultures of SG functional organoids (SGo) and human dental pulp stem cells (hDPSC) were generated by magnetic 3D bioassembly (M3DB) platforms. Fibroblast growth factor 10 (FGF10) was used to enrich the SGo in secretory epithelial units. After 11 culture days via M3DB, SGo displayed SG-specific acinar epithelial units with functional properties upon neurostimulation. To consistently develop 3D hDPSC in vitro, 3 culture days were sufficient to maintain hDPSC undifferentiated genotype and phenotype for EV generation. EV isolation was performed via sequential centrifugation of the conditioned media of hDPSC and SGo cultures. EV were characterized by nanoparticle tracking analysis, electron microscopy and immunoblotting. EV were in the exosome range for hDPSC (diameter: 88.03 ± 15.60 nm) and for SGo (123.15 ± 63.06 nm). Upon ex vivo administration, exosomes derived from SGo significantly stimulated epithelial growth (up to 60%), mitosis, epithelial progenitors and neuronal growth in injured SG; however, such biological effects were less distinctive with the ones derived from hDPSC. Next, these exosome biological effects were investigated by proteomic arrays. Mass spectrometry profiling of SGo exosomes predicted that cellular growth, development and signaling was due to known and undocumented molecular targets downstream of FGF10. Semaphorins were identified as one of the novel targets requiring further investigations. Thus, M3DB platforms can generate exosomes with potential to ameliorate SG epithelial damage.

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Magnetic bioassembly platforms scale-up the production of salivary gland organoids.

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Exosomes from organoids rescued up to 60% of gland epithelial growth.

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Transplanted gland organoids from magnetic bioassembly platforms rescued 25% of gland epithelial growth.

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Exosomes from dental pulp stem cells in magnetic bioassembly platforms marginally alter epithelial growth.

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99 proteins were differentially expressed in exosomes from organoids.

Loss of Hs3st3a1 or Hs3st3b1 enzymes alters heparan sulfate to reduce epithelial morphogenesis and adult salivary gland function

Heparan sulfate 3-O-sulfotransferases generate highly sulfated but rare 3-O-sulfated heparan sulfate (HS) epitopes on cell surfaces and in the extracellular matrix. Previous ex vivo experiments suggested functional redundancy exists among the family of seven enzymes but that Hs3st3a1 and Hs3st3b1 sulfated HS increases epithelial FGFR signaling and morphogenesis. Single-cell RNAseq analysis of control SMGs identifies increased expression of Hs3st3a1 and Hs3st3b1 in endbud and myoepithelial cells, both of which are progenitor cells during development and regeneration. To analyze their in vivo functions, we generated both Hs3st3a1^-/- and Hs3st3b1^-/- single knockout mice, which are viable and fertile. Salivary glands from both mice have impaired fetal epithelial morphogenesis when cultured with FGF10. Hs3st3b1^-/- mice have reduced intact SMG branching morphogenesis and reduced 3-O-sulfated HS in the basement membrane. Analysis of HS biosynthetic enzyme transcription highlighted some compensatory changes in sulfotransferases expression early in development. The overall glycosaminoglycan composition of adult control and KO mice were similar, although HS disaccharide analysis showed increased N- and non-sulfated disaccharides in Hs3st3a1^-/- HS. Analysis of adult KO gland function revealed normal secretory innervation, but without stimulation there was an increase in frequency of drinking behavior in both KO mice, suggesting basal salivary hypofunction, possibly due to myoepithelial dysfunction. Understanding how 3-O-sulfation regulates myoepithelial progenitor function will be important to manipulate HS-binding growth factors to enhance tissue function and regeneration.

Comparing development and regeneration in the submandibular gland highlights distinct mechanisms

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

Cell signaling regulation in salivary gland development

The mammalian salivary gland develops as a highly branched structure designed to produce and secrete saliva. This review focuses on research conducted on mammalian salivary gland development, particularly on the differentiation of acinar, ductal, and myoepithelial cells. We discuss recent studies that provide conceptual advances in the understanding of the molecular mechanisms of salivary gland development. In addition, we describe the organogenesis of submandibular glands (SMGs), model systems used for the study of SMG development, and the key signaling pathways as well as cellular processes involved in salivary gland development. The findings from the recent studies elucidating the identity of stem/progenitor cells in the SMGs, and the process by which they are directed along a series of cell fate decisions to form functional glands, are also discussed. Advances in genetic tools and tissue engineering strategies will significantly increase our knowledge about the mechanisms by which signaling pathways and cells establish tissue architecture and function during salivary gland development, which may also be conserved in the growth and development of other organ systems. An increased knowledge of organ development mechanisms will have profound implications in the design of therapies for the regrowth or repair of injured tissues. In addition, understanding how the processes of cell survival, expansion, specification, movement, and communication with neighboring cells are regulated under physiological and pathological conditions is critical to the development of future treatments.

Influence of Age and Individual Differences on Mouthfeel Perception of Whey Protein-Fortified Products: A Review

Protein needs are considered to increase with age, with protein consumption being associated with many positive outcomes. Protein-fortified products are often used to improve nutritional status and prevent age-related muscle mass loss in older adults. Accordingly, older adults are commonly provided with products fortified with whey protein; however, such products can cause mouthdrying, limiting consumption and product enjoyment. Currently, the extent to which age and individual differences (e.g., saliva, oral health, food oral processing) influence the perception of whey protein-derived mouthdrying is relatively unclear. Previous research in this area has mainly focused on investigating mouthdrying, without taking into account individual differences that could influence this perception within the target population. Therefore, the main focus of this review is to provide an overview of the relevant individual differences likely to influence mouthfeel perception (specifically mouthdrying) from whey protein-fortified products, thereby enabling the future design of such products to incorporate better the needs of older adults and improve their nutritional status. This review concludes that age and individual differences are likely to influence mouthdrying sensations from whey protein-fortified products. Future research should focus more on the target population and individual differences to maximise the benefits from whey protein fortification.

An Essential Requirement for Fgf10 in Pinna Extension Sheds Light on Auricle Defects in LADD Syndrome

The pinna (or auricle) is part of the external ear, acting to capture and funnel sound toward the middle ear. The pinna is defective in a number of craniofacial syndromes, including Lacrimo-auriculo-dento-digital (LADD) syndrome, which is caused by mutations in FGF10 or its receptor FGFR2b. Here we study pinna defects in the Fgf10 knockout mouse. We show that Fgf10 is expressed in both the muscles and forming cartilage of the developing external ear, with loss of signaling leading to a failure in the normal extension of the pinna over the ear canal. Conditional knockout of Fgf10 in the neural crest fails to recapitulate this phenotype, suggesting that the defect is due to loss of Fgf10 from the muscles, or that this source of Fgf10 can compensate for loss in the forming cartilage. The defect in the Fgf10 null mouse is driven by a reduction in proliferation, rather than an increase in cell death, which can be partially phenocopied by inhibiting cell proliferation in explant culture. Overall, we highlight the mechanisms that could lead to the phenotype observed in LADD syndrome patients and potentially explain the formation of similar low-set and cup shaped ears observed in other syndromes.

Bones, Glands, Ears and More: The Multiple Roles of FGF10 in Craniofacial Development

Members of the fibroblast growth factor (FGF) family have myriad functions during development of both non-vertebrate and vertebrate organisms. One of these family members, FGF10, is largely expressed in mesenchymal tissues and is essential for postnatal life because of its critical role in development of the craniofacial complex, as well as in lung branching. Here, we review the function of FGF10 in morphogenesis of craniofacial organs. Genetic mouse models have demonstrated that the dysregulation or absence of FGF10 function affects the process of palate closure, and FGF10 is also required for development of salivary and lacrimal glands, the inner ear, eye lids, tongue taste papillae, teeth, and skull bones. Importantly, mutations within the FGF10 locus have been described in connection with craniofacial malformations in humans. A detailed understanding of craniofacial defects caused by dysregulation of FGF10 and the precise mechanisms that underlie them offers new opportunities for development of medical treatments for patients with birth defects and for regenerative approaches for cancer patients with damaged gland tissues.

Hypoxia-Activated Adipose Mesenchymal Stem Cells Prevents Irradiation-Induced Salivary Hypofunction by Enhanced Paracrine Effect Through Fibroblast Growth Factor 10

To explore the effects and mechanisms of paracrine factors secreted from human adipose mesenchymal stem cell (hAdMSCs) that are activated by hypoxia on radioprotection against irradiation-induced salivary hypofunction in subjects undergoing radiotherapy for head and neck cancers. An organotypic spheroid coculture model to mimic irradiation (IR)-induced salivary hypofunction was set up for in vitro experiments. Human parotid gland epithelial cells were organized to form three-dimensional (3D) acinus-like spheroids on growth factor reduced -Matrigel. Cellular, structural, and functional damage following IR were examined after cells were cocultured with hAdMSCs preconditioned with either normoxia (hAdMSC^NMX ) or hypoxia (hAdMSC^HPX ). A key paracrine factor secreted by hAdMSCs^HPX was identified by high-throughput microarray-based enzyme-linked immunosorbent assay. Molecular mechanisms and signaling pathways on radioprotection were explored. Therapeutic effects of hAdMSCs^HPX were evaluated after in vivo transplant into mice with IR-induced salivary hypofunction. In our 3D coculture experiment, hAdMSCs^HPX significantly enhanced radioresistance of spheroidal human parotid epithelial cells, and led to greater preservation of salivary epithelial integrity and acinar secretory function relative to hAdMSCs^NMX . Coculture with hAdMSCs^HPX promoted FGFR expression and suppressed FGFR diminished antiapoptotic activity of hAdMSCs^HPX . Among FGFR-binding secreted factors, we found that fibroblast growth factor 10 (FGF10) contributed to therapeutic effects of hAdMSCs^HPX by enhancing antiapoptotic effect, which was dependent on FGFR-PI3K signaling. An in vivo transplant of hAdMSCs^HPX into irradiated salivary glands of mice reversed IR-induced salivary hypofunction where hAdMSC-released FGF10 contributed to tissue remodeling. Our results suggest that hAdMSCs^HPX protect salivary glands from IR-induced apoptosis and preserve acinar structure and functions by activation of FGFR-PI3K signaling via actions of hAdMSC-secreted factors, including FGF10. Stem Cells 2018;36:1020-1032.

Fgf10 and Sox9 are essential for the establishment of distal progenitor cells during mouse salivary gland development

Salivary glands are formed by branching morphogenesis with epithelial progenitors forming a network of ducts and acini (secretory cells). During this process, epithelial progenitors specialise into distal (tips of the gland) and proximal (the stalk region) identities that produce the acini and higher order ducts, respectively. Little is known about the factors that regulate progenitor expansion and specialisation in the different parts of the gland. Here, we show that Sox9 is involved in establishing the identity of the distal compartment before the initiation of branching morphogenesis. Sox9 is expressed throughout the gland at the initiation stage before becoming restricted to the distal epithelium from the bud stage and throughout branching morphogenesis. Deletion of Sox9 in the epithelium results in loss of the distal epithelial progenitors, a reduction in proliferation and a subsequent failure in branching. We demonstrate that Sox9 is positively regulated by mesenchymal Fgf10, a process that requires active Erk signalling. These results provide new insights into the factors required for the expansion of salivary gland epithelial progenitors, which can be useful for organ regeneration therapy.

FGF and EDA pathways control initiation and branching of distinct subsets of developing nasal glands

Hypertrophy, hyperplasia and altered mucus secretion from the respiratory submucosal glands (SMG) are characteristics of airway diseases such as cystic fibrosis, asthma and chronic bronchitis. More commonly, hyper-secretion of the nasal SMGs contributes to allergic rhinitis and upper airway infection. Considering the role of these glands in disease states, there is a significant dearth in understanding the molecular signals that regulate SMG development and patterning. Due to the imperative role of FGF signalling during the development of other branched structures, we investigated the role of Fgf10 during initiation and branching morphogenesis of murine nasal SMGs. Fgf10 is expressed in the mesenchyme around developing SMGs while expression of its receptor Fgfr2 is seen within glandular epithelial cells. In the Fgf10 null embryo, Steno's gland and the maxillary sinus gland were completely absent while other neighbouring nasal glands showed normal duct elongation but defective branching. Interestingly, the medial nasal glands were present in Fgf10 homozygotes but missing in Fgfr2b mutants, with expression of Fgf7 specifically expressed around these developing glands, indicating that Fgf7 might compensate for loss of Fgf10 in this group of glands. Intriguingly the lateral nasal glands were only mildly affected by loss of FGF signalling, while these glands were missing in Eda mutant mice, where the Steno's and maxillary sinus gland developed as normal. This analysis reveals that regulation of nasal gland development is complex with different subsets of glands being regulated by different signalling pathways. This analysis helps shed light on the nasal gland defects observed in patients with hypohidrotic ectodermal dysplasia (HED) (defect EDA pathway) and LADD syndrome (defect FGFR2b pathway).

Multiple Cranial Organ Defects after Conditionally Knocking Out Fgf10 in the Neural Crest

Fgf10 is necessary for the development of a number of organs that fail to develop or are reduced in size in the null mutant. Here we have knocked out Fgf10 specifically in the neural crest driven by Wnt1cre. The Wnt1creFgf10fl/fl mouse phenocopies many of the null mutant defects, including cleft palate, loss of salivary glands, and ocular glands, highlighting the neural crest origin of the Fgf10 expressing mesenchyme surrounding these organs. In contrast tissues such as the limbs and lungs, where Fgf10 is expressed by the surrounding mesoderm, were unaffected, as was the pituitary gland where Fgf10 is expressed by the neuroepithelium. The circumvallate papilla of the tongue formed but was hypoplastic in the conditional and Fgf10 null embryos, suggesting that other sources of FGF can compensate in development of this structure. The tracheal cartilage rings showed normal patterning in the conditional knockout, indicating that the source of Fgf10 for this tissue is mesodermal, which was confirmed using Wnt1cre-dtTom to lineage trace the boundary of the neural crest in this region. The thyroid, thymus, and parathyroid glands surrounding the trachea were present but hypoplastic in the conditional mutant, indicating that a neighboring source of mesodermal Fgf10 might be able to partially compensate for loss of neural crest derived Fgf10.