Label-retaining cells in the adult murine salivary glands possess characteristics of adult progenitor cells

Duct ligation/de-ligation model: exploring mechanisms for salivary gland injury and regeneration

Sialadenitis and sialadenitis-induced sialopathy are typically caused by obstruction of the salivary gland ducts. Atrophy of the salivary glands in experimental animals caused by duct ligation exhibits a histopathology similar to that of salivary gland sialadenitis. Therefore, a variety of duct ligation/de-ligation models have been commonly employed to study salivary gland injury and regeneration. Duct ligation is mainly characterised by apoptosis and activation of different signaling pathways in parenchymal cells, which eventually leads to gland atrophy and progressive dysfunction. By contrast, duct de-ligation can initiate the recovery of gland structure and function by regenerating the secretory tissue. This review summarizes the animal duct ligation/de-ligation models that have been used for the examination of pathological fundamentals in salivary disorders, in order to unravel the pathological changes and underlying mechanisms involved in salivary gland injury and regeneration. These experimental models have contributed to developing effective and curative strategies for gland dysfunction and providing plausible solutions for overcoming salivary disorders.

Epithelial plasticity enhances regeneration of committed taste receptor cells following nerve injury

Taste receptor cells are taste bud epithelial cells that are dependent upon the innervating nerve for continuous renewal and are maintained by resident tissue stem/progenitor cells. Transection of the innervating nerve causes degeneration of taste buds and taste receptor cells. However, a subset of the taste receptor cells is maintained without nerve contact after glossopharyngeal nerve transection in the circumvallate papilla in adult mice. Here, we revealed that injury caused by glossopharyngeal nerve transection triggers the remaining differentiated K8-positive taste receptor cells to dedifferentiate and acquire transient progenitor cell-like states during regeneration. Dedifferentiated taste receptor cells proliferate, express progenitor cell markers (K14, Sox2, PCNA) and form organoids in vitro. These data indicate that differentiated taste receptor cells can enter the cell cycle, acquire stemness, and participate in taste bud regeneration. We propose that dedifferentiated taste receptor cells in combination with stem/progenitor cells enhance the regeneration of taste buds following nerve injury.

Role of quiescent cells in the homeostatic maintenance of the adult submandibular salivary gland

Stem/progenitor cells are required for maintenance of salivary gland (SG) function and serve as untapped reservoirs to create functional cells. Despite recent advancements in the identification of stem/progenitor pools, in the submandibular gland (SMG), a knowledge gap remains. Furthermore, the contribution to adult SMG homeostasis of stem/progenitor cells originating from embryonic development is unclear. Here, we employ an H2B-GFP embryonic and adult pulse-and-chase system to characterize potential SMG stem/progenitor cells (SGSCs) based on quiescence at different stages. Phenotypical profiling of quiescent cells in the SMG revealed that label-retaining cells (LRCs) of embryonic or adult origin co-localized with CK8+ ductal or vimentin + mesenchymal, but not with CK5+ or CK14 + stem/progenitor cells. These SMG LRCs failed to self-renew in vitro while non-label retaining cells displayed differentiation and long-term expansion potential as organoids. Collectively, our data suggest that an active cycling population of cells is responsible for SMG homeostasis with organoid forming potential.

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Embryonic quiescent cells do not retain stemness in the adult submandibular gland (SMG)

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Postnatal quiescent cells do not exhibit stem/progenitor cell potency in the adult SMG

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Quiescent cells do not contribute to the homeostatic maintenance of the murine SMG

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Adult murine SMG stem/progenitor cells are likely to be an actively cycling population

Aldehyde dehydrogenase 3A1 deficiency leads to mitochondrial dysfunction and impacts salivary gland stem cell phenotype

Adult salivary stem/progenitor cells (SSPC) have an intrinsic property to self-renew in order to maintain tissue architecture and homeostasis. Adult salivary glands have been documented to harbor SSPC, which have been shown to play a vital role in the regeneration of the glandular structures postradiation damage. We have previously demonstrated that activation of aldehyde dehydrogenase 3A1 (ALDH3A1) after radiation reduced aldehyde accumulation in SSPC, leading to less apoptosis and improved salivary function. We subsequently found that sustained pharmacological ALDH3A1 activation is critical to enhance regeneration of murine submandibular gland after radiation damage. Further investigation shows that ALDH3A1 function is crucial for SSPC self-renewal and survival even in the absence of radiation stress. Salivary glands from Aldh3a1 -/- mice have fewer acinar structures than wildtype mice. ALDH3A1 deletion or pharmacological inhibition in SSPC leads to a decrease in mitochondrial DNA copy number, lower expression of mitochondrial specific genes and proteins, structural abnormalities, lower membrane potential, and reduced cellular respiration. Loss or inhibition of ALDH3A1 also elevates ROS levels, depletes glutathione pool, and accumulates ALDH3A1 substrate 4-hydroxynonenal (4-HNE, a lipid peroxidation product), leading to decreased survival of murine SSPC that can be rescued by treatment with 4-HNE specific carbonyl scavengers. Our data indicate that ALDH3A1 activity protects mitochondrial function and is important for the regeneration activity of SSPC. This knowledge will help to guide our translational strategy of applying ALDH3A1 activators in the clinic to prevent radiation-related hyposalivation in head and neck cancer patients.

Thermoresponsive fiber-based microwells capable of formation and retrieval of salivary gland stem cell spheroids for the regeneration of irradiation-damaged salivary glands

Three-dimensional spheroid culture enhances cell-to-cell interactions among stem cells and promotes the expression of stem cell properties; however, subsequent retrieval and delivery of these cells remain a challenge. We fabricated a thermoresponsive fiber-based microwell scaffold by combining electrospinning and hydrogel micropatterning. The resultant scaffold appeared to facilitate the formation of cellular spheroids of uniform size and enabled the expression of more stem cell-secreting growth factor genes ( EGF, IGF-1, FGF1, FGF2, and HGF), pluripotent stem cell-related genes ( SOX2 and NANOG), and adult epithelial stem cell-related genes ( LGR4, LGR5, and LGR6) than salivary gland stem cells in a monolayer culture (SGSCmonolayer). The spheroids could be retrieved efficiently by decreasing temperature. SGSC-derived spheroid (SGSCspheroid) cells were then implanted into the submandibular glands of mice at 2 weeks after fractionated X-ray irradiation at a dose of 7.5 Gy/day. At 16 weeks post-irradiation, restoration of salivary function was detected only in SGSCspheroid-implanted mice. The production of submandibular acini specific mucin increased in SGSCspheroid-implanted mice, compared with PBS control. More MIST1+ mature acinar cells were preserved in the SGSCspheroid-implanted group than in the PBS control group. Intriguingly, SGSCspheroid-implanted mice exhibited greater amelioration of tissue damage and preservation of KRT7+ terminally differentiated luminal ductal cells than SGSCmonolayer-implanted mice. The SGSCspheroid-implanted mice also showed less DNA damage and apoptotic cell death than the SGSCmonolayer-implanted mice at 2 weeks post-implantation. Additionally, a significant increase in Ki67+AQP5+ proliferative acinar cells was noted only in SGSCspheroid-implanted mice. Our results suggest that a thermoresponsive fiber-based scaffold could be of use to facilitate the production of function-enhanced SGSCspheroid cells and their subsequent retrieval and delivery to damaged salivary glands to alleviate radiation-induced apoptotic cell death and promote salivary gland regeneration.

The Hippo signaling pathway effector YAP promotes salivary gland regeneration after injury

[Figure: see text].

Aging-associated stem/progenitor cell dysfunction in the salivary glands of mice

Although stem cell aging leads to a decline in tissue homeostasis and regenerative capacity, it remains unclear whether salivary gland stem cell function changes during this process. However, the salivary glands are gradually replaced by connective tissue during aging. Here, we show a decline in the stem cell ability of CD133-positive stem/progenitor cells in the salivary glands of aged mice. The CD133-positive cells were isolated from young, adult, and aged mice. The number of CD133-positive cells was significantly decreased in aged mice. They also showed a lower sphere formation capacity compared to young and adult mice. RNA sequencing revealed that CD133-positive cells in aged mice exhibited lower gene expression of several aging-related genes, including FoxO3a, than those in young and adult mice. Salivary gland cells infected with a recombinant lentivirus encoding the FoxO3a gene showed a reduction in oxidative stress induced by hydrogen peroxide compared with those infected with a control virus. Thus, FoxO3a may inhibit stem cell aging via oxidative stress.

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.

The evolving definition of salivary gland stem cells

Dysfunction of the salivary gland and irreversible hyposalivation are the main side effects of radiotherapy treatment for head and neck cancer leading to a drastic decrease of the quality of life of the patients. Approaches aimed at regenerating damaged salivary glands have been proposed as means to provide long-term restoration of tissue function in the affected patients. In studies to elucidate salivary gland regenerative mechanisms, more and more evidence suggests that salivary gland stem/progenitor cell behavior, like many other adult tissues, does not follow that of the hard-wired professional stem cells of the hematopoietic system. In this review, we provide evidence showing that several cell types within the salivary gland epithelium can serve as stem/progenitor-like cells. While these cell populations seem to function mostly as lineage-restricted progenitors during homeostasis, we indicate that upon damage specific plasticity mechanisms might be activated to take part in regeneration of the tissue. In light of these insights, we provide an overview of how recent developments in the adult stem cell research field are changing our thinking of the definition of salivary gland stem cells and their potential plasticity upon damage. These new perspectives may have important implications on the development of new therapeutic approaches to rescue radiation-induced hyposalivation.

Yap activation in irradiated parotid salivary glands is regulated by ROCK activity

Radiotherapy plays a major role in the curative treatment of head and neck cancer, either as a single modality therapy, or in combination with surgery or chemotherapy, or both. Despite advances to limit radiation-induced side-effects, the major salivary glands are often affected. This frequently leads to hyposalivation which causes an increased risk for xerostomia, dental caries, mucositis, and malnutrition culminating in a significant impact on patients' quality of life. Previous research demonstrated that loss of salivary function is associated with a decrease in polarity regulators and an increase in nuclear Yap localization in a putative stem and progenitor cell (SPC) population. Yap activation has been shown to be essential for regeneration in intestinal injury models; however, the highest levels of nuclear Yap are observed in irradiated salivary SPCs that do not regenerate the gland. Thus, elucidating the inputs that regulate nuclear Yap localization and determining the role that Yap plays within the entire tissue following radiation damage and during regeneration is critical. In this study, we demonstrate that radiation treatment increases nuclear Yap localization in acinar cells and Yap-regulated genes in parotid salivary tissues. Conversely, administration of insulin-like growth factor 1 (IGF1), known to restore salivary function in mouse models, reduces nuclear Yap localization and Yap transcriptional targets to levels similar to untreated tissues. Activation of Rho-associated protein kinase (ROCK) using calpeptin results in increased Yap-regulated genes in primary acinar cells while inhibition of ROCK activity (Y-27632) leads to decreased Yap transcriptional targets. These results suggest that Yap activity is dependent on ROCK activity and provides new mechanistic insights into the regulation of radiation-induced hyposalivation.

Origin and Lineage Plasticity of Endogenous Lacrimal Gland Epithelial Stem/Progenitor Cells

The lacrimal gland (LG) is an exocrine organ responsible for the secretion of aqueous tear film. Regenerative and stem cell therapies that target LG repair are coming to the fore, although our understanding of LG cell lineage hierarchy is still incomplete. We utilize the analysis of label-retaining cells (LRCs) and genetic lineage tracing to define LG cell lineage hierarchy. Our study suggests that embryonic LG contains unique long-lived multipotent stem cells that give rise to all postnatal epithelial cell types. Following birth, lineages become established and the fate of progenitor cell descendants becomes restricted. However, some cell lineages retain plasticity after maturation and can trans-differentiate into other cell types upon injury. The demonstration that the LG contains progenitor cells with different levels of plasticity has profound implications for our understanding of LG gland function in homeostasis and disease and will be helpful for developing stem cell-based therapies in the future.

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Multipotent stem cells differentiate into distal Sox10+ and proximal Sox10− lineages

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Lineage-restricted progenitor cells sustain the long-term lacrimal gland maintenance

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Label-retaining cells are localized in the intercalated ducts and excretory ducts

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Some cell lineages in the adult lacrimal gland retain plasticity

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.

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

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

Establishment of a Murine Pro-acinar Cell Line to Characterize Roles for FGF2 and α3β1 Integrins in Regulating Pro-acinar Characteristics

Radiation therapy for head and neck cancers results in permanent damage to the saliva producing acinar compartment of the salivary gland. To date, a pure pro-acinar cell line to study underlying mechanisms of acinar cell differentiation in culture has not been described. Here, we report the establishment of a pro-acinar (mSG-PAC1) and ductal (mSG-DUC1) cell line, from the murine submandibular salivary gland (SMG), which recapitulate developmental milestones in differentiation. mSG-DUC1 cells express the ductal markers, keratin-7 and keratin-19, and form lumenized spheroids. mSG-PAC1 cells express the pro-acinar markers SOX10 and aquaporin-5. Using the mSG-PAC1 cell line, we demonstrate that FGF2 regulates specific steps during acinar cell maturation. FGF2 up-regulates aquaporin-5 and the expression of the α3 and α6 subunits of the α3β1 and α6β1 integrins that are known to promote SMG morphogenesis and differentiation. mSG-DUC1 and mSG-PAC1 cells were derived from genetically modified mice, homozygous for floxed alleles of the integrin α3 subunit. Similar to SMGs from α3-null mice, deletion of α3 alleles in mSG-PAC1 cells results in the up-regulation of E-cadherin and the down-regulation of CDC42. Our data indicate that mSG-DUC1 and mSG-PAC1 cells will serve as important tools to gain mechanistic insight into salivary gland morphogenesis and differentiation.

PKCζ and JNK signaling regulate radiation-induced compensatory proliferation in parotid salivary glands

Radiotherapy is a common treatment option for head and neck cancer patients; however, the surrounding healthy salivary glands are often incidentally irradiated during the process. As a result, patients often experience persistent xerostomia and hyposalivation, which deceases their quality of life. Clinically, there is currently no standard of care available to restore salivary function. Repair of epithelial wounds involves cellular proliferation and establishment of polarity in order to regenerate the tissue. This process is partially mediated by protein kinase C zeta (PKCζ), an apical polarity regulator; however, its role following radiation damage is not completely understood. Using an in vivo radiation model, we show a significant decrease in active PKCζ in irradiated murine parotid glands, which correlates with increased proliferation that is sustained through 30 days post-irradiation. Additionally, salivary glands in PKCζ null mice show increased basal proliferation which radiation treatment did not further potentiate. Radiation damage also activates Jun N-terminal kinase (JNK), a proliferation-inducing mitogen-activated protein kinase normally inhibited by PKCζ. In both a PKCζ null mouse model and in primary salivary gland cell cultures treated with a PKCζ inhibitor, there was increased JNK activity and production of downstream proliferative transcripts. Collectively, these findings provide a potential molecular link by which PKCζ suppression following radiation damage promotes JNK activation and radiation-induced compensatory proliferation in the salivary gland.

Concise Review: A Critical Evaluation of Criteria Used to Define Salivary Gland Stem Cells

In the effort to develop cell-based therapies to treat salivary gland dysfunction, many different populations of cells in the adult salivary glands have been proposed as stem cells. These cell populations vary, depending on the assay used, and are often nonoverlapping, leading to the conclusion that salivary glands harbor multiple stem cells. The goal of this review is to critically appraise the assays and properties used to identify stem cells in the adult salivary gland, and to consider the caveats of each. Re-evaluation of the defining criteria may help to reconcile the many potential stem cell populations described in the salivary gland, in order to increase comparability between studies and build consensus in the field. Stem Cells 2019;37:1144-1150.

Neurturin-containing laminin matrices support innervated branching epithelium from adult epithelial salispheres

Cell transplantation of autologous adult biopsies, grown ex vivo as epithelial organoids or expanded as spheroids, are proposed treatments to regenerate damaged branching organs. However, it is not clear whether transplantation of adult organoids or spheroids alone is sufficient to initiate a fetal-like program of branching morphogenesis in which coordinated branching of multiple cell types including nerves, mesenchyme and blood vessels occurs. Yet this is an essential concept for the regeneration of branching organs such as lung, pancreas, and lacrimal and salivary glands. Here, we used factors identified from fetal organogenesis to maintain and expand adult murine and human epithelial salivary gland progenitors in non-adherent spheroid cultures, called salispheres. These factors stimulated critical developmental pathways, and increased expression of epithelial progenitor markers such as Keratin5, Keratin14, FGFR2b and KIT. Moreover, physical recombination of adult salispheres in a laminin-111 extracellular matrix with fetal salivary mesenchyme, containing endothelial and neuronal cells, only induced branching morphogenesis when neurturin, a neurotrophic factor, was added to the matrix. Neurturin was essential to improve neuronal survival, axon outgrowth, innervation of the salispheres, and resulted in the formation of branching structures with a proximal-distal axis that mimicked fetal branching morphogenesis, thus recapitulating organogenesis. Epithelial progenitors were also maintained, and developmental differentiation programs were initiated, showing that the fetal microenvironment provides a template for adult epithelial progenitors to initiate branching and differentiation. Further delineation of secreted and physical cues from the fetal niche will be useful to develop novel regenerative therapies that instruct adult salispheres to resume a developmental-like program in vitro and to regenerate branching organs in vivo.

Sox9 regulates the luminal stem/progenitor cell properties of salivary glands

Exocrine glands share a common morphology consisting of ductal, acinar, and basal/myoepithelial cells, but their functions and mechanisms of homeostasis differ among tissues. Salivary glands are an example of exocrine glands, and they have been reported to contain multipotent stem cells that differentiate into other tissues. In this study, we purified the salivary gland stem/progenitor cells of adult mouse salivary glands using the cell surface marker CD133 by flow cytometry. CD133⁺ cells possessed stem cell capacity, and the transplantation of CD133⁺ cells into the submandibular gland reconstituted gland structures, including functional acinar. CD133⁺ cells were sparsely distributed in the intercalated and exocrine ducts and expressed Sox9 at higher levels than CD133^- cells. Moreover, we demonstrated that Sox9 was required for the stem cell properties CD133⁺ cells, including colony and sphere formation. Thus, the Sox9-related signaling may control the regeneration salivary glands.

Diverse progenitor cells preserve salivary gland ductal architecture after radiation-induced damage

The ductal system of the salivary gland has long been postulated to be resistant to radiation-induced damage, a common side effect incurred by head and neck cancer patients receiving radiotherapy. Yet, whether the ducts are capable of regenerating after genotoxic injury, or whether damage to ductal cells induces lineage plasticity, as has been reported in other organ systems, remains unknown. Here, using the murine salivary gland, we show that two ductal progenitor populations, marked exclusively by KRT14 and KIT, maintain non-overlapping ductal compartments after radiation exposure but do so through distinct cellular mechanisms. KRT14⁺ progenitor cells are fast-cycling cells that proliferate in response to radiation-induced damage in a sustained manner and divide asymmetrically to produce differentiated cells of the larger granulated ducts. Conversely, KIT⁺ intercalated duct cells are long-lived progenitors for the intercalated ducts that undergo few cell divisions either during homeostasis or after gamma radiation, thus maintaining ductal architecture with slow rates of cell turnover. Together, these data illustrate the regenerative capacity of the salivary ducts and highlight the heterogeneity in the damage responses used by salivary progenitor cells to maintain tissue architecture.

Salivary gland stem cells: A review of development, regeneration and cancer

Salivary glands are responsible for maintaining the health of the oral cavity and are routinely damaged by therapeutic radiation for head and neck cancer as well as by autoimmune diseases such as Sjögren's syndrome. Regenerative approaches based on the reactivation of endogenous stem cells or the transplant of exogenous stem cells hold substantial promise in restoring the structure and function of these organs to improve patient quality of life. However, these approaches have been hampered by a lack of knowledge on the identity of salivary stem cell populations and their regulators. In this review we discuss our current knowledge on salivary stem cells and their regulators during organ development, homeostasis and regeneration. As increasing evidence in other systems suggests that progenitor cells may be a source of cancer, we also review whether these same salivary stem cells may also be cancer initiating cells.

aPKCζ-dependent Repression of Yap is Necessary for Functional Restoration of Irradiated Salivary Glands with IGF-1

Xerostomia and salivary hypofunction often result as a consequence of radiation therapy for head and neck cancers, which are diagnosed in roughly 60,000 individuals every year in the U.S. Due to the lack of effective treatments for radiation-induced salivary hypofunction, stem cell-based therapies have been suggested to regenerate the irradiated salivary glands. Pharmacologically, restoration of salivary gland function has been accomplished in mice by administering IGF-1 shortly after radiation treatment, but it is not known if salivary stem and progenitor cells play a role. We show that radiation inactivates aPKCζ and promotes nuclear redistribution of Yap in a population of label-retaining cells in the acinar compartment of the parotid gland (PG)- which comprises a heterogeneous pool of salivary progenitors. Administration of IGF-1 post-radiation maintains activation of aPKCζ and partially rescues Yap's cellular localization in label retaining cells, while restoring salivary function. Finally, IGF-1 fails to restore saliva production in mice lacking aPKCζ, demonstrating the importance of the kinase as a potential therapeutic target.

Administration of growth factors promotes salisphere formation from irradiated parotid salivary glands

Worldwide, 500,000 cases of head and neck cancer (HNC) are reported each year and the primary treatment for HNC is radiotherapy. Although the goal of radiotherapy is to target the tumor, secondary exposure occurs in surrounding normal tissues, such as the salivary glands. As a result, despite successful treatment of the cancer, patients are left with long-term side effects due to direct damage to the salivary glands. The effect is chronic and currently there is no treatment. Stem cells are an attractive therapeutic option for treatment of radiation-induced glandular dysfunction because of the potential to regenerate damaged cell populations and restore salivary gland function. However, limited knowledge about the endogenous stem cell population post irradiation hinders the development for stem cell-based therapies. In this study, an ex vivo sphere formation cell culture system was utilized to assess the self-renewal capacity of cells derived from parotid salivary glands at a chronic time point following radiation. Salivary glands from irradiated mice generate significantly fewer salispheres, but can be stimulated with fetal bovine serum (FBS) to generate an equivalent number of salispheres as unirradiated salivary glands. Interestingly, the number and size of salispheres formed is dependent on the concentration of FBS supplemented into the media. Salispheres derived from irradiated glands and cultured in FBS media were found to contain cells that proliferate and express progenitor and acinar cell markers such as Keratin 5, Keratin 14, Aquaporin 5, and NKCC1. Utilization of insulin-like growth factor (IGF1) injections following radiation treatment restores salivary gland function and improves salisphere generation. These findings indicate that stimulation of these cellular populations may provide a promising avenue for the development of cell-based therapies for radiation-induced salivary gland damage.

Immunolocalization patterns of cytokeratins during salivary acinar cell development in mice

Embryonic development of the mouse salivary glands begins with epithelial thickening and continues with sequential changes from the pre-bud to terminal bud stages. After birth, morphogenesis proceeds, and the glands develop into a highly branched epithelial structure that terminates with saliva-producing acinar cells at the adult stage. Acinar cells derived from the epithelium are differentiated into serous, mucous, and seromucous types. During differentiation, cytokeratins, intermediate filaments found in most epithelial cells, play vital roles. Although the localization patterns and developmental roles of cytokeratins in different epithelial organs, including the mammary glands, circumvallate papilla, and sweat glands, have been well studied, their stage-specific localization and morphogenetic roles during salivary gland development have yet to be elucidated. Therefore, the aim of this study was to determine the stage and acinar cell type-specific localization pattern of cytokeratins 4, 5, 7, 8, 13, 14, 18, and 19 in the major salivary glands (submandibular, sublingual, and parotid glands) of the mouse at the E15.5, PN0, PN10, and adult stages. In addition, cell physiology, including cell proliferation, was examined during development via immunostaining for Ki67 to understand the cellular mechanisms that govern acinar cell differentiation during salivary gland morphogenesis. The distinct localization patterns of cytokeratins in conjunction with cell physiology will reveal the roles of epithelial cells in salivary gland formation during the differentiation of serous, mucous or seromucous salivary glands.

Insulin-Like Growth Factor-1-Mediated DNA Repair in Irradiated Salivary Glands Is Sirtuin-1 Dependent

Ionizing radiation is one of the most common cancer treatments; however, the treatment leads to a wide range of debilitating side effects. In patients with head and neck cancer (HNC), the surrounding normal salivary gland is extremely sensitive to therapeutic radiation, and damage to this tissue results in various oral complications and decreased quality of life (QOL). In the current study, mice treated with targeted head and neck radiation showed a significant increase in double-stranded breaks (DSB) in the DNA of parotid salivary gland cells immediately after treatment, and this remained elevated 3 h posttreatment. In contrast, mice pretreated with insulin-like growth factor-1 (IGF-1) showed resolution of the same amount of initial DNA damage by 3 h posttreatment. At acute time points (30 min to 2 h), irradiated parotid glands had significantly decreased levels of the histone deactylase Sirtuin-1 (SirT-1) which has been previously shown to function in DNA repair. Pretreatment with IGF-1 increased SirT-1 protein levels and increased deacetylation of SirT-1 targets involved in DNA repair. Pharmacological inhibition of SirT-1 activity decreased the IGF-1-mediated resolution of DSB. These data suggest that IGF-1 promotes DNA repair in irradiated parotid glands through the maintenance and activation of SirT-1.

Single Cell Clones Purified from Human Parotid Glands Display Features of Multipotent Epitheliomesenchymal Stem Cells

A better understanding of the biology of tissue-resident stem cell populations is essential to development of therapeutic strategies for regeneration of damaged tissue. Here, we describe the isolation of glandular stem cells (GSCs) from a small biopsy specimen from human parotid glands. Single colony-forming unit-derived clonal cells were isolated through a modified subfractionation culture method, and their stem cell properties were examined. The isolated clonal cells exhibited both epithelial and mesenchymal stem cell (MSC)-like features, including differentiation potential and marker expression. The cells transiently displayed salivary progenitor phenotypes during salivary epithelial differentiation, suggesting that they may be putative multipotent GSCs rather than progenitor cells. Both epithelial and mesenchymal-expressing putative GSCs, LGR5⁺CD90⁺ cells, were found in vivo, mostly in inter-secretory units of human salivary glands. Following in vivo transplantation into irradiated salivary glands of mice, these cells were found to be engrafted around the secretory complexes, where they contributed to restoration of radiation-induced salivary hypofunction. These results showed that multipotent epitheliomesenchymal GSCs are present in glandular mesenchyme, and that isolation of homogenous GSC clones from human salivary glands may promote the precise understanding of biological function of bona fide GSCs, enabling their therapeutic application for salivary gland regeneration.

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

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

STATEMENT OF SIGNIFICANCE

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

Primary Salivary Human Stem/Progenitor Cells Undergo Microenvironment-Driven Acinar-Like Differentiation in Hyaluronate Hydrogel Culture

Radiotherapy for head and neck cancer often has undesirable effects on salivary glands that lead to xerostomia or severe dry mouth, which can increase oral infections. Our goal is to engineer functional, three‐dimensional (3D) salivary gland neotissue for autologous implantation to provide permanent relief. An immediate need exists to obtain autologous adult progenitor cells as the use of embryonic and induced pluripotent stem cells potentially pose serious risks such as teratogenicity and immunogenic rejection. Here, we report an expandable population of primary salivary human stem/progenitor cells (hS/PCs) that can be reproducibly and scalably isolated and propagated from tissue biopsies. These cells have increased expression of progenitor markers (K5, K14, MYC, ETV4, ETV5) compared with differentiation markers of the parotid gland (acinar: MIST1/BHLHA15 and AMY1A; ductal: K19 and TFCP2L1). Isolated hS/PCs grown in suspension formed primary and secondary spheres and could be maintained in long‐term 3D hydrogel culture. When grown in a customized 3D modular hyaluronate‐based hydrogel system modified with bioactive basement membrane‐derived peptides, levels of progenitor markers, indices of proliferation, and viability of hS/PCs were enhanced. When appropriate microenvironmental cues were provided in a controlled manner in 3D, such as stimulation with β‐adrenergic and cholinergic agonists, hS/PCs differentiated into an acinar‐like lineage, needed for saliva production. We conclude that the stem/progenitor potential of adult hS/PCs isolated without antigenic sorting or clonal expansion in suspension, combined with their ability to differentiate into specialized salivary cell lineages in a human‐compatible culture system, makes them ideal for use in 3D bioengineered salivary gland applications. Stem Cells Translational Medicine2017;6:110–120

Immunohistochemical localization of keratin 5 in the submandibular gland in adult and postnatal developing mice

Keratin 5 (K5) is a marker of basal progenitor cells in the epithelia of a number of organs. During prenatal development of the submandibular gland (SMG) in mice, K5(+) progenitor cells in the developing epithelia play important roles in its organogenesis. Although K5(+) cells are also present in the adult mouse SMG and may function in tissue regeneration, their histological localization has not yet investigated in detail. In the present study, we examined the immunohistochemical localization of K5 in the SMG in adult and postnatal developing mice. At birth, K5 immunoreactivity was detected in the entire duct system, in which it was localized in the basal cells of a double-layered epithelium, but was not detected in the terminal tubule or myoepithelial cells. At postnatal weeks 1-3, with the development of intercalated ducts (ID), striated ducts (SD), and excretory ducts (ED), K5-immunoreactive basal cells were gradually restricted to the ED and the proximal double-layered portions of the ID connecting to the SD. At the same time, K5 immunoreactivity appeared in myoepithelial cells, in which its positive ratio gradually increased. In adults, K5 immunoreactivity was localized to most myoepithelial cells, most basal cells in the ED, and a small number of ID cells at the boundary between the ID and SD in the female SMG or between the ID and granular convoluted tubules in the male SMG. These results suggest that K5 is a marker of differentiated myoepithelial cells and duct progenitor cells in the mouse SMG.

Regulatory Mechanisms Driving Salivary Gland Organogenesis

Salivary glands develop as highly branched structures designed to produce and secrete saliva. Advances in mouse genetics, stem cell biology, and regenerative medicine are having a tremendous impact on our understanding of salivary gland organogenesis. Understanding how submandibular gland (SMG) initiation, branching morphogenesis, and cell differentiation occur, as well as defining the progenitor/stem cells and cell and tissue interactions that drive SMG development will help guide regenerative approaches for patients suffering from loss of salivary gland function. This review focuses on recent literature from the past 5 years investigating the regulatory mechanisms driving SMG organogenesis.

Salivary Glands: Stem Cells, Self-duplication, or Both?

Understanding the intrinsic potential for renewal and regeneration within a tissue is critical for the rational design of reparative strategies. Maintenance of the salivary glands is widely thought to depend on the differentiation of stem cells. However, there is also new evidence that homeostasis of the salivary glands, like that of the liver and pancreas, relies on self-renewal of differentiated cells rather than a stem cell pool. Here, we review the evidence for both modes of turnover and consider the implications for the process of regeneration. We propose that the view of salivary glands as postmitotic and dependent on stem cells for renewal be revised to reflect the proliferative activity of acinar cells and their role in salivary gland homeostasis.

The contribution of specific cell subpopulations to submandibular salivary gland branching morphogenesis

Branching morphogenesis is the developmental program responsible for generating a large surface to volume ratio in many secretory and absorptive organs. To accomplish branching morphogenesis, spatiotemporal regulation of specific cell subpopulations is required. Here, we review recent studies that define the contributions of distinct cell subpopulations to specific cellular processes during branching morphogenesis in the mammalian submandibular salivary gland, including the initiation of the gland, the coordination of cleft formation, and the contribution of stem/progenitor cells to morphogenesis. In conclusion, we provide an overview of technological advances that have opened opportunities to further probe the contributions of specific cell subpopulations and to define the integration of events required for branching morphogenesis.