The Hippo pathway effector YAP is an essential regulator of ductal progenitor patterning in the mouse submandibular gland

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.

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.

Generation of salivary glands derived from pluripotent stem cells via conditional blastocyst complementation

Whole salivary gland generation and transplantation offer potential therapies for salivary gland dysfunction. However, the specific lineage required to engineer complete salivary glands has remained elusive. In this study, we identify the Foxa2 lineage as a critical lineage for salivary gland development through conditional blastocyst complementation (CBC). Foxa2 lineage marking begins at the boundary between the endodermal and ectodermal regions of the oral epithelium before the formation of the primordial salivary gland, thereby labeling the entire gland. Ablation of Fgfr2 within the Foxa2 lineage in mice leads to salivary gland agenesis. We reversed this phenotype by injecting donor pluripotent stem cells into the mouse blastocysts, resulting in mice that survived to adulthood with salivary glands of normal size, comparable to those of their littermate controls. These findings demonstrate that CBC-based salivary gland regeneration serves as a foundational experimental approach for future advanced cell-based therapies.

Salivary gland developmental mechanics

The salivary gland undergoes branching morphogenesis to elaborate into a tree-like structure with numerous saliva-secreting acinar units, all joined by a hierarchical ductal system. The expansive epithelial surface generated by branching morphogenesis serves as the structural basis for the efficient production and delivery of saliva. Here, we elucidate the process of salivary gland morphogenesis, emphasizing the role of mechanics. Structurally, the developing salivary gland is characterized by a stratified epithelium tightly encased by the basement membrane, which is in turn surrounded by a mesenchyme consisting of a dense network of interstitial matrix and mesenchymal cells. Diverse cell types and extracellular matrices bestow this developing organ with organized, yet spatially varied mechanical properties. For instance, the surface epithelial sheet of the bud is highly fluidic due to its high cell motility and weak cell-cell adhesion, rendering it highly pliable. In contrast, the inner core of the bud is more rigid, characterized by reduced cell motility and strong cell-cell adhesion, which likely provide structural support for the tissue. The interactions between the surface epithelial sheet and the inner core give rise to budding morphogenesis. Furthermore, the basement membrane and the mesenchyme offer mechanical constraints that could play a pivotal role in determining the higher-order architecture of a fully mature salivary gland.

Tlx1 regulates acinar and duct development in mouse salivary glands

Tlx1 encodes a transcription factor expressed in several craniofacial structures of developing mice. The role of Tlx1 in salivary gland development was examined using morphological and immunohistochemical analyses of Tlx1 null mice. Tlx1 is expressed in submandibular and sublingual glands but not parotid glands of neonatal and adult male and female C57Bl/6J (Tlx1+/+ ) mice. TLX1 protein was localized to the nuclei of terminal tubule cells, developing duct cells and mesenchymal cells in neonatal submandibular and sublingual glands, and to nuclei of duct cells and connective tissue cells in adult glands. Occasionally, TLX1 was observed in nuclei of epithelial cells in or adjacent to the acini. Submandibular glands were smaller and sublingual glands were larger in size in mutant mice (Tlx1-/- ) compared to wild-type mice. Differentiation of terminal tubule and proacinar cells of neonatal Tlx1-/- submandibular glands was abnormal; expression of their characteristic products, submandibular gland protein C and parotid secretory protein, respectively, was reduced. At 3 weeks postnatally, terminal tubule cells at the acinar-intercalated duct junction were poorly developed or absent in Tlx1-/- mice. Granular convoluted ducts in adult mutant mice were decreased, and epidermal growth factor and nerve growth factor expression were reduced. Along with normal acinar cell proteins, adult acinar cells of Tlx1-/- mice continued to express neonatal proteins and expressed parotid proteins not normally present in submandibular glands. Sublingual gland mucous acinar and serous demilune cell differentiation were altered. Tlx1 is necessary for proper differentiation of submandibular and sublingual gland acinar cells, and granular convoluted ducts. The mechanism(s) underlying Tlx1 regulation of salivary gland development and differentiation remains unknown.

Generation of salivary glands derived from pluripotent stem cells via conditional blastocyst complementation

Various patients suffer from dry mouth due to salivary gland dysfunction. Whole salivary gland generation and transplantation is a potential therapy to resolve this issue. However, the lineage permissible to design the entire salivary gland generation has been enigmatic. Here, we discovered Foxa2 as a lineage critical for generating a salivary gland via conditional blastocyst complementation (CBC). Foxa2 linage, but not Shh nor Pitx2, initiated to label between the boundary region of the endodermal and the ectodermal oral mucosa before primordial salivary gland formation, resulting in marking the entire salivary gland. The salivary gland was agenesis by depleting Fgfr2 under the Foxa2 lineage in the mice. We rescued this phenotype by injecting donor pluripotent stem cells into the mouse blastocysts. Those mice survived until adulthood with normal salivary glands compatible in size compared with littermate controls. These results indicated that CBC-based salivary gland generation is promising for next-generation cell-based therapy.

Efficient Gene Knockout in Salivary Gland Epithelial Explant Cultures

We have developed methods to achieve efficient CRISPR-Cas9-mediated gene knockout in ex vivo mouse embryonic salivary epithelial explants. Salivary epithelial explants provide a valuable model for characterizing cell signaling, differentiation, and epithelial morphogenesis, but research has been limited by a paucity of efficient gene perturbation methods. Here, we demonstrate highly efficient gene perturbation by transient transduction of guide RNA-expressing lentiviruses into Cas9-expressing salivary epithelial buds isolated from Cas9 transgenic mice. We first show that salivary epithelial explants can be cultured in low-concentration, nonsolidified Matrigel suspensions in 96-well plates, which greatly increases sample throughput compared to conventional cultures embedded in solidified Matrigel. We further show that salivary epithelial explants can grow and branch with FGF7 alone, while supplementing with insulin, transferrin, and selenium (ITS) enhances growth and branching. We then describe an efficient workflow to produce experiment-ready, high-titer lentiviruses within 1 wk after molecular cloning. To track transduced cells, we designed the lentiviral vector to coexpress a nuclear fluorescent reporter with the guide RNA. We routinely achieved 80% transduction efficiency when antibiotic selection was used. Importantly, we detected robust loss of targeted protein products when testing 9 guide RNAs for 3 different genes. Moreover, targeting the β1 integrin gene (Itgb1) inhibited branching morphogenesis, which supports the importance of cell-matrix adhesion in driving branching morphogenesis. In summary, we have established a lentivirus-based method that can efficiently perturb genes of interest in salivary epithelial explants, which will greatly facilitate studies of specific gene functions using this system.

A cellular hierarchy of Notch and Kras signaling controls cell fate specification in the developing mouse salivary gland

The development of the mouse salivary gland involves a tip-driven process of branching morphogenesis that takes place in concert with differentiation into acinar, myoepithelial, and ductal (basal and luminal) sub-lineages. By combining clonal lineage tracing with a three-dimensional (3D) reconstruction of the branched epithelial network and single-cell RNA-seq analysis, we show that in tips, a heterogeneous population of renewing progenitors transition from a Krt14+ multipotent state to unipotent states via two transcriptionally distinct bipotent states, one restricted to the Krt14+ basal and myoepithelial lineage and the other to the Krt8+ acinar and luminal lineage. Using genetic perturbations, we show how the differential expression of Notch signaling correlates with spatial segregation, exits from multipotency, and promotes the Krt8+ lineage, whereas Kras activation promotes proacinar fate. These findings provide a mechanistic basis for how positional cues within growing tips regulate the process of lineage segregation and ductal patterning.

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.

Adipose‑derived stem cells postpone the progression of Sjögren's syndrome by upregulating the Hippo signaling pathway

The aim of the present study was to explore the effect and mechanism of action of adipose-derived stem cells (ADSCs) on Sjögren syndrome (SS) to develop novel and more effective methods for SS treatment. ADSCs, dexamethasone or normal saline was injected into the submandibular gland (SMG) of three 12-week-old non-obese diabetic (NOD) mice. The degree of lymphocyte infiltration was considered as a criterion for judging disease progression, hematoxylin and eosin staining was performed to observe the pathological state, and the expression levels of TAZ, E-cadherin and α-catenin were assessed by western blotting. ADSC transplantation triggered an inhibitory effect on the progression of SS, which was slightly stronger compared with that of dexamethasone treatment. This was found to be related to the Hippo signaling pathway. In addition, TAZ protein expression levels decreased gradually with the progression of the disease; immunofluorescence staining showed that the expression of E-cadherin and TAZ followed similar trends. Notably, the expression of TAZ, p-TAZ, E-cadherin and α-catenin in NOD mice were lower compared with that in Control mice. Similarly, the ratio of p-TAZ/TAZ also decreased, which means that the activation level of Hippo signal pathway decreased. The results suggest that ADSCs may exert a therapeutic effect against SS and may postpone its progression by upregulating the Hippo signaling pathway.

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.

The Semaphorin 3A-AKT axis-mediated cell proliferation in salivary gland morphogenesis and adenoid cystic carcinoma pathogenesis

We recently demonstrated that Semaphorin 3 A (Sema3A), the expression of which is negatively regulated by Wnt/β-catenin signaling, promotes odontogenic epithelial cell proliferation, suggesting the involvement of Sema3A in tooth germ development. Salivary glands have a similar developmental process to tooth germ development, in which reciprocal interactions between the oral epithelium and adjacent mesenchyme proceeds via stimulation with several growth factors; however, the role of Sema3A in the development of salivary glands is unknown. There may thus be a common mechanism between epithelial morphogenesis and pathogenesis; however, the role of Sema3A in salivary gland tumors is also unclear. The current study investigated the involvement of Sema3A in submandibular gland (SMG) development and its expression in adenoid cystic carcinoma (ACC) specimens. Quantitative RT-PCR and immunohistochemical analyses revealed that Sema3A was expressed both in epithelium and in mesenchyme in the initial developmental stages of SMG and their expressions were decreased during the developmental processes. Loss-of-function experiments using an inhibitor revealed that Sema3A was required for AKT activation-mediated cellular growth and formation of cleft and bud in SMG rudiment culture. In addition, Wnt/β-catenin signaling decreased the Sema3A expression in the rudiment culture. ACC arising from salivary glands frequently exhibits malignant potential. Immunohistochemical analyses of tissue specimens obtained from 10 ACC patients showed that Sema3A was hardly observed in non-tumor regions but was strongly expressed in tumor lesions, especially in myoepithelial neoplastic cells, at high frequencies where phosphorylated AKT expression was frequently detected. These results suggest that the Sema3A-AKT axis promotes cell growth, thereby contributing to morphogenesis and pathogenesis, at least in ACC, of salivary glands.

Regulation of myoepithelial differentiation

The salivary gland can be permanently impaired by radiation treatment for head and neck cancers. Efforts at tissue regeneration have focused on saliva-producing acinar cells. However, myoepithelial cells are also critical to gland function, but mechanisms that regulate their differentiation are poorly defined. To study myoepithelial differentiation, we employed mSG-PAC1 murine salivary gland epithelial cells. We demonstrate that mSG-PAC1 spheroids exhibit phenotypic plasticity between pro-acinar and myoepithelial cell fates. Increased expression of pro-acinar/acinar or myoepithelial RNAs was identified from spheroids cultured under different media conditions by microarray followed by gene-set enrichment analysis. Spheroids cultured with different medium components expressed proteins typical of either acinar or myoepithelial cells, as detected by immunocytochemistry. We demonstrate that the pattern of TAZ expression in the epithelial compartment of the differentiating murine salivary gland correlates with the expression of the myoepithelial marker alpha-SMA, as is the case for TAZ expression in mSG-PAC1 spheroids. Our analysis also indicates that YAP/TAZ target genes are upregulated together with myoepithelial markers. Importantly, siRNA targeting of TAZ expression in mSG-PAC1 spheroids diminished the expression of myoepithelial markers. Our results in this in vitro cell model implicate TAZ signaling in myoepithelial differentiation.

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

[Figure: see text].

The Role of EREG/EGFR Pathway in Tumor Progression

Aberrant activation of the epidermal growth factor receptor (EGFR/ERBB1) by erythroblastic leukemia viral oncogene homolog (ERBB) ligands contributes to various tumor malignancies, including lung cancer and colorectal cancer (CRC). Epiregulin (EREG) is one of the EGFR ligands and is low expressed in most normal tissues. Elevated EREG in various cancers mainly activates EGFR signaling pathways and promotes cancer progression. Notably, a higher EREG expression level in CRC with wild-type Kirsten rat sarcoma viral oncogene homolog (KRAS) is related to better efficacy of therapeutic treatment. By contrast, the resistance of anti-EGFR therapy in CRC was driven by low EREG expression, aberrant genetic mutation and signal pathway alterations. Additionally, EREG overexpression in non-small cell lung cancer (NSCLC) is anticipated to be a therapeutic target for EGFR-tyrosine kinase inhibitor (EGFR-TKI). However, recent findings indicate that EREG derived from macrophages promotes NSCLC cell resistance to EGFR-TKI treatment. The emerging events of EREG-mediated tumor promotion signals are generated by autocrine and paracrine loops that arise from tumor epithelial cells, fibroblasts, and macrophages in the tumor microenvironment (TME). The TME is a crucial element for the development of various cancer types and drug resistance. The regulation of EREG/EGFR pathways depends on distinct oncogenic driver mutations and cell contexts that allows specific pharmacological targeting alone or combinational treatment for tailored therapy. Novel strategies targeting EREG/EGFR, tumor-associated macrophages, and alternative activation oncoproteins are under development or undergoing clinical trials. In this review, we summarize the clinical outcomes of EREG expression and the interaction of this ligand in the TME. The EREG/EGFR pathway may be a potential target and may be combined with other driver mutation targets to combat specific cancers.

The Hippo pathway: a renewed insight in the craniofacial diseases and hard tissue remodeling

The Hippo pathway plays an important role in many pathophysiological processes, including cell proliferation and differentiation, cell death, cell migration and invasion. Because of its extensive functions, Hippo pathway is closely related to not only growth and development, but also many diseases, including inflammation and cancer. In this study, the role of Hippo pathway in craniofacial diseases and hard tissue remodeling was reviewed, in attempting to find new research directions.

Budding epithelial morphogenesis driven by cell-matrix versus cell-cell adhesion

Many embryonic organs undergo epithelial morphogenesis to form tree-like hierarchical structures. However, it remains unclear what drives the budding and branching of stratified epithelia, such as in the embryonic salivary gland and pancreas. Here, we performed live-organ imaging of mouse embryonic salivary glands at single-cell resolution to reveal that budding morphogenesis is driven by expansion and folding of a distinct epithelial surface cell sheet characterized by strong cell-matrix adhesions and weak cell-cell adhesions. Profiling of single-cell transcriptomes of this epithelium revealed spatial patterns of transcription underlying these cell adhesion differences. We then synthetically reconstituted budding morphogenesis by experimentally suppressing E-cadherin expression and inducing basement membrane formation in 3D spheroid cultures of engineered cells, which required β1-integrin-mediated cell-matrix adhesion for successful budding. Thus, stratified epithelial budding, the key first step of branching morphogenesis, is driven by an overall combination of strong cell-matrix adhesion and weak cell-cell adhesion by peripheral epithelial cells.

TAZ inhibits acinar cell differentiation but promotes immature ductal cell proliferation in adult mouse salivary glands

There are currently no treatments for salivary gland diseases, making it vital to understand signaling mechanisms operating in acinar and ductal cells so as to develop regenerative therapies. To date, little work has focused on elucidating the signaling cascades controlling the differentiation of these cell types in adult mammals. To analyze the function of the Hippo-TAZ/YAP1 pathway in adult mouse salivary glands, we generated adMOB1DKO mice in which both MOB1A and MOB1B were TAM-inducibly deleted when the animals were adults. Three weeks after TAM treatment, adMOB1DKO mice exhibited smaller submandibular glands (SMGs) than controls with a decreased number of acinar cells and an increased number of immature dysplastic ductal cells. The mutants suffered from reduced saliva production accompanied by mild inflammatory cell infiltration and fibrosis in SMGs, similar to the Sjogren's syndrome. MOB1-deficient acinar cells showed normal proliferation and apoptosis but decreased differentiation, leading to an increase in acinar/ductal bilineage progenitor cells. These changes were TAZ-dependent but YAP1-independent. Biochemically, MOB1-deficient salivary epithelial cells showed activation of the TAZ/YAP1 and β-catenin in ductal cells, but reduced SOX2 and SOX10 expression in acinar cells. Thus, Hippo-TAZ signaling is critical for proper ductal and acinar cell differentiation and function in adult mice.

A mechanogenetic role for the actomyosin complex in branching morphogenesis of epithelial organs

The actomyosin complex plays crucial roles in various life processes by balancing the forces generated by cellular components. In addition to its physical function, the actomyosin complex participates in mechanotransduction. However, the exact role of actomyosin contractility in force transmission and the related transcriptional changes during morphogenesis are not fully understood. Here, we report a mechanogenetic role of the actomyosin complex in branching morphogenesis using an organotypic culture system of mouse embryonic submandibular glands. We dissected the physical factors arranged by characteristic actin structures in developing epithelial buds and identified the spatial distribution of forces that is essential for buckling mechanism to promote the branching process. Moreover, the crucial genes required for the distribution of epithelial progenitor cells were regulated by YAP and TAZ through a mechanotransduction process in epithelial organs. These findings are important for our understanding of the physical processes involved in the development of epithelial organs and provide a theoretical background for developing new approaches for organ regeneration.

Inactivation of Lats1 and Lats2 highlights the role of hippo pathway effector YAP in larynx and vocal fold epithelium morphogenesis

Proliferation and differentiation of vocal fold epithelial cells during embryonic development is poorly understood. We examined the role of Hippo signaling, a vital pathway known for regulating organ size, in murine laryngeal development. Conditional inactivation of the Hippo kinase genes Lats1 and Lats2, specifically in vocal fold epithelial cells, resulted in severe morphogenetic defects. Deletion of Lats1 and Lats2 caused abnormalities in epithelial differentiation, epithelial lamina separation, cellular adhesion, basement membrane organization with secondary failed cartilage, and laryngeal muscle development. Further, Lats1 and Lats2 inactivation led to failure in differentiation of p63+ basal progenitors. Our results reveal novel roles of Hippo-Lats-YAP signaling in proper regulation of VF epithelial fate and larynx morphogenesis.

Specification of the patterning of a ductal tree during branching morphogenesis of the submandibular gland

The development of ductal structures during branching morphogenesis relies on signals that specify ductal progenitors to set up a pattern for the ductal network. Here, we identify cellular asymmetries defined by the F-actin cytoskeleton and the cell adhesion protein ZO-1 as the earliest determinants of duct specification in the embryonic submandibular gland (SMG). Apical polarity protein aPKCζ is then recruited to the sites of asymmetry in a ZO-1-dependent manner and collaborates with ROCK signaling to set up apical-basal polarity of ductal progenitors and further define the path of duct specification. Moreover, the motor protein myosin IIB, a mediator of mechanical force transmission along actin filaments, becomes localized to vertices linking the apical domains of multiple ductal epithelial cells during the formation of ductal lumens and drives duct maturation. These studies identify cytoskeletal, junctional and polarity proteins as the early determinants of duct specification and the patterning of a ductal tree during branching morphogenesis of the SMG.

Immuno-hippo: Research progress of the hippo pathway in autoimmune disease

Extensive research in Drosophila and mammals has identified the core components of Hippo signaling, which controls gene expression. Studies of Drosophila have demonstrated the highly conserved Hippo pathway controls tissue homeostasis and organ size by regulating the balance between cell proliferation and apoptosis. Recent work has indicated a potential role of the Hippo pathway in regulating the immune system, which is the key player in autoimmune disease (AID). Therefore, the Hippo pathway may become a novel target for curing AID. Although the pivotal role of both the Hippo pathway in tumorigenesis has been thoroughly investigated, the role of it in AID is still poorly understood. Elucidating the role of Hippo signaling pathways in the activation and expression of specific molecules involved in immune regulation is important for understanding the pathogenesis of AID and exploring novel therapeutic targets. To aid in further research, this review describes the relationship between the Hippo pathway and inflammatory signals such as NF-κB and JAK-STAT, the function of the Hippo pathway in the formation and differentiation of immune cells, and the regulatory role of the Hippo pathway in AID.

Functional Specialization of Human Salivary Glands and Origins of Proteins Intrinsic to Human Saliva

Salivary proteins are essential for maintaining health in the oral cavity and proximal digestive tract, and they serve as potential diagnostic markers for monitoring human health and disease. However, their precise organ origins remain unclear. Through transcriptomic analysis of major adult and fetal salivary glands and integration with the saliva proteome, the blood plasma proteome, and transcriptomes of 28+ organs, we link human saliva proteins to their source, identify salivary-gland-specific genes, and uncover fetal- and adult-specific gene repertoires. Our results also provide insights into the degree of gene retention during gland maturation and suggest that functional diversity among adult gland types is driven by specific dosage combinations of hundreds of transcriptional regulators rather than by a few gland-specific factors. Finally, we demonstrate the heterogeneity of the human acinar cell lineage. Our results pave the way for future investigations into glandular biology and pathology, as well as saliva's use as a diagnostic fluid.

The Hippo Pathway Blocks Mammalian Retinal Müller Glial Cell Reprogramming

In response to retinal damage, the Müller glial cells (MGs) of the zebrafish retina have the ability to undergo a cellular reprogramming event in which they enter the cell cycle and divide asymmetrically, thereby producing multipotent retinal progenitors capable of regenerating lost retinal neurons. However, mammalian MGs do not exhibit such a proliferative and regenerative ability. Here, we identify Hippo pathway-mediated repression of the transcription cofactor YAP as a core regulatory mechanism that normally blocks mammalian MG proliferation and cellular reprogramming. MG-specific deletion of Hippo pathway components Lats1 and Lats2, as well as transgenic expression of a Hippo non-responsive form of YAP (YAP5SA), resulted in dramatic Cyclin D1 upregulation, loss of adult MG identity, and attainment of a highly proliferative, progenitor-like cellular state. Our results reveal that mammalian MGs may have latent regenerative capacity that can be stimulated by repressing Hippo signaling.

Rueda et al. identify the Hippo pathway as an endogenous molecular mechanism normally preventing mammalian Müller glial reprogramming to a proliferative, progenitor-like state.

The HIPPO pathway in gynecological malignancies

The Hippo pathway has been initially discovered by screening genes that regulate organ size in Drosophila. Recent studies have highlighted the role of the Hippo pathway in controlling organ size, tissue homeostasis and regeneration, and signaling dysregulation, especially the overactivation of the transcriptional coactivator YAP/TAZ, which leads to uncontrolled cell growth and malignant transformation. The core components of the Hippo pathway may initiate tumorigenesis by inducing tumor stem cells and proliferation, ultimately leading to metastasis and drug resistance, which occurs extensively in gynecological malignancies, including cervical cancer, ovarian cancer, and endometrial cancer. In this review, we attempt to systematically summarize recent progress in our understanding of the mechanism of Hippo pathway regulation in tumorigenesis and the mechanisms that underlie alterations during gynecological malignancies, as well as new therapeutic strategies.

Effect of antenatal tetramethylpyrazine on lung development and YAP expression in rat model of experimental congenital diaphragmatic hernia

The aim of this study was to investigate the therapeutic effects and underlying mechanism of tetramethylpyrazine (TMP) on lung development using a rat model of congenital diaphragmatic hernia (CDH). Nitrofen was used to induce CDH. Pregnant rats were divided into three groups: control, CDH, and CDH+TMP. In the CDH and CDH+TMP, fetuses only with left diaphragmatic hernias were chosen for analysis. Lung and body weight were recorded and lung histologic evaluations, image analysis, and western blot analysis of YAP, p-YAP and LATS1 were performed after lung processing. A markedly abnormal structure was observed, as evidenced by pulmonary hypoplasia and vascular remodeling, in the CHD. These abnormalities were improved in the CDH+TMP. There were significant differences between the CHD and CHD+TMP in percentage of medial wall thickness, arteriole muscularization, radial alveolar counts, AA%, and alveolar septal thickness. YAP expression was markedly increased in the CHD compared to the controls, which was not affected by antenatal TMP administration. However, prenatal TMP intervention significantly increased expression of LATS1 and phosphorylation of YAP in the CDH fetuses. Our results demonstrate that antenatal TMP administration improved vascular remodeling and promoted lung development in a rat model of CHD, potentially through increasing expression of LATS1 and phosphorylation of YAP.

Specific complexes derived from extracellular matrix facilitate generation of structural and drug-responsive human salivary gland microtissues through maintenance stem cell homeostasis

Three-dimensional cultured salivary glands (SGs) microtissues hold great potentials for clinical research. However, most SGs microtissues still lack convincing structure and function due to poor supplementation of factors to maintain stem cell homeostasis. Extracellular matrix (ECM) plays a crucial role in regulating stem cell behavior. Thus, it is necessary to model stem cell microenvironment in vitro by supplementing culture medium with proteins derived from ECM. We prepared specific complexes from human SG ECM (s-Ecx) and analyzed the components of the s-Ecx. Human SG epithelial and mesenchymal cells were used to generate microtissues, and the optimum seeding cell number and ratio of two cell types were determined. Then, the s-Ecx was introduced to the culture medium to assess its effect on stem cell behavior. Multiple specific factors were presented in s-Ecx. s-Ecx promoted maintenance of the stem cell and formation of specific structures resembling that of salivary glands and containing mucins, which suggested stem cell differentiation potential. Moreover, treatment of the microtissues with s-Ecx increased their sensitivity to neurotransmitters. On the basis of the analysis of components, we believed that the presented growth factors are able to interact with stem cell they encountered in vivo, which promote the capacity to maintain stem cell homeostasis. This work provided foundations to study molecular mechanism of stem cell homeostasis in SGs and develop novel therapies for dry mouth through new drug discovery and disease modeling.

Hippo-Yap/Taz signaling: Complex network interactions and impact in epithelial cell behavior

The Hippo pathway has emerged as a crucial integrator of signals in biological events from development to adulthood and in diseases. Although extensively studied in Drosophila and in cell cultures, major gaps of knowledge still remain on how this pathway functions in mammalian systems. The pathway consists of a growing number of components, including core kinases and adaptor proteins, which control the subcellular localization of the transcriptional co-activators Yap and Taz through phosphorylation of serines at key sites. When localized to the nucleus, Yap/Taz interact with TEAD transcription factors to induce transcriptional programs of proliferation, stemness, and growth. In the cytoplasm, Yap/Taz interact with multiple pathways to regulate a variety of cellular functions or are targeted for degradation. The Hippo pathway receives cues from diverse intracellular and extracellular inputs, including growth factor and integrin signaling, polarity complexes, and cell-cell junctions. This review highlights the mechanisms of regulation of Yap/Taz nucleocytoplasmic shuttling and their implications for epithelial cell behavior using the lung as an intriguing example of this paradigm. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Signaling Pathways > Cell Fate Signaling Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

Epithelial Cell Lineage and Signaling in Murine Salivary Glands

Maintaining salivary gland function is critical for oral health. Loss of saliva is a common side effect of therapeutic irradiation for head and neck cancer or autoimmune diseases such as Sjögren's syndrome. There is no curative treatment, and current strategies proposed for functional regeneration include gene therapy to reengineer surviving salivary gland tissue, cell-based transplant therapy, use of bioengineered glands, and development of drugs/biologics to stimulate in vivo regeneration or increase secretion. Understanding the genetic and cellular mechanisms required for development and homeostasis of adult glands is essential to the success of these proposed treatments. Recent advances in genetic lineage tracing provide insight into epithelial lineage relationships during murine salivary gland development. During early fetal gland development, epithelial cells expressing keratin 14 (K14) Sox2, Sox9, Sox10, and Trp63 give rise to all adult epithelium, but as development proceeds, lineage restriction occurs, resulting in separate lineages of myoepithelial, ductal, and acinar cells in postnatal glands. Several niche signals have been identified that regulate epithelial development and lineage restriction. Fibroblast growth factor signaling is essential for gland development, and other important factors that influence epithelial patterning and maturation include the Wnt, Hedgehog, retinoic acid, and Hippo signaling pathways. In addition, other cell types in the local microenvironment, such as endothelial and neuronal cells, can influence epithelial development. Emerging evidence also suggests that specific epithelial cells will respond to different types of salivary gland damage, depending on the cause and severity of damage and the resulting damaged microenvironment. Understanding how regeneration occurs and which cell types are affected, as well as which signaling factors drive cell lineage decisions, provides specific targets to manipulate cell fate and improve regeneration. Taken together, these recent advances in understanding cell lineages and the signaling factors that drive cell fate changes provide a guide to develop novel regenerative treatments.

The Hippo Signaling Pathway in Development and Disease

The Hippo signaling pathway regulates diverse physiological processes, and its dysfunction has been implicated in an increasing number of human diseases, including cancer. Here, we provide an updated review of the Hippo pathway; discuss its roles in development, homeostasis, regeneration, and diseases; and highlight outstanding questions for future investigation and opportunities for Hippo-targeted therapies.

Inductive signals in branching morphogenesis - lessons from mammary and salivary glands

Branching morphogenesis is a fundamental developmental program that generates large epithelial surfaces in a limited three-dimensional space. It is regulated by inductive tissue interactions whose effects are mediated by soluble signaling molecules, and cell-cell and cell-extracellular matrix interactions. Here, we will review recent studies on inductive signaling interactions governing branching morphogenesis in light of phenotypes of mouse mutants and ex vivo organ culture studies with emphasis on developing mammary and salivary glands. We will highlight advances in understanding how cell fate decisions are intimately linked with branching morphogenesis. We will also discuss novel insights into the molecular control of cellular mechanisms driving the formation of these arborized ductal structures and reflect upon how distinct spatial patterns are generated.

Characterization of Krt19 CreERT allele for targeting the nucleus pulposus cells in the postnatal mouse intervertebral disc

Intervertebral disc degeneration and associated back pain are relatively common but sparsely understood conditions, affecting over 70% of the population during some point of life. Disc degeneration is often associated with a loss of nucleus pulposus (NP) cells. Genetic mouse models offer convenient avenues to understand the cellular and molecular regulation of the disc during its formation, growth, maintenance, and aging. However, due to the lack of inducible driver lines to precisely target NP cells in the postnatal mouse disc, progress in this area of research has been moderate. NP cells are known to express cytokeratin 19 (Krt19), and tamoxifen (Tam)‐inducible Krt19^CreERT allele is available. The current study describes the characterization of Krt19^CreERT allele to specifically and efficiently target NP cells in neonatal, skeletally mature, middle‐aged, and aged mice using two independent fluorescent reporter lines. The efficiency of recombination at all ages was validated by immunostaining for KRT19. Results show that following Tam induction, Krt19^CreERT specifically drives recombination of NP cells in the spine of neonatal and aged mice, while no recombination was detected in the surrounding tissues. Knee joints from skeletally mature Tam‐treated Krt19^CreERT/+; R26^tdTOM mouse show the absence of recombination in all tissues and cells of the knee joint. Thus, this study provides evidence for the use of Krt19^CreERT allele for genetic characterization of NP cells at different stages of the mouse life.

Lats1/2 inactivation reveals Hippo function in alveolar type I cell differentiation during lung transition to air breathing

Lung growth to its optimal size at birth is driven by reiterative airway branching followed by differentiation and expansion of alveolar cell types. How this elaborate growth is coordinated with the constraint of the chest is poorly understood. Here, we investigate the role of Hippo signaling, a cardinal pathway in organ size control, in mouse lung development. Unexpectedly, we found that epithelial loss of the Hippo kinase genes Lats1 and Lats2 (Lats1/2) leads to a striking reduction of lung size owing to an early arrest of branching morphogenesis. This growth defect is accompanied by abnormalities in epithelial cell polarity, cell division plane and extracellular matrix deposition, as well as precocious and increased expression of markers for type 1 alveolar epithelial cells (AEC1s), an indicator of terminal differentiation. Increased AEC1s were also observed in transgenic mice with overexpression of a constitutive nuclear form of downstream transcriptional effector YAP. Conversely, loss of Yap and Taz led to decreased AEC1s, demonstrating that the canonical Hippo signaling pathway is both sufficient and necessary to drive AEC1 fate. These findings together reveal unique roles of Hippo-LATS-YAP signaling in the developing mouse lung.

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.

Integrin-FAK-CDC42-PP1A signaling gnaws at YAP/TAZ activity to control incisor stem cells

How epithelial tissues are able to self-renew to maintain homeostasis and regenerate in response to injury remains a persistent question. The transcriptional effectors YAP and TAZ are increasingly being recognized as central mediators of epithelial stem cell biology, and a wealth of recent studies have been directed at understanding the control and activity of these factors. Recent work by Hu et al. has added to this knowledge, as they identify an Integrin-FAK-CDC42-PP1A signaling cascade that directs nuclear YAP/TAZ activity in stem cell populations of the mouse incisor, and define convergence on mTORC1 signaling as an important mediator of the proliferation of these cells. Here, we review recent studies on YAP/TAZ function and regulation in epithelial tissue-specific stem cells, merging the Hu et al. study together with our current knowledge of YAP/TAZ.