Prospects for re-engineering salivary glands

Evidence for Active Electrolyte Transport by Two-Dimensional Monolayers of Human Salivary Epithelial Cells

Functional reconstruction of lost tissue by regenerative therapy of salivary glands would be of immense benefit following radiotherapy or in the treatment of Sjogren's syndrome. The purpose of this study was to develop primary cultures of human salivary gland cells as potential regenerative resources and to characterize their acinar/ductal phenotype using electrophysiological measurements of ion transport. Human salivary gland cultures were prepared either from adherent submandibular gland cells (huSMG) or from mixed adherent and nonadherent cells (PTHSG) and were cultivated in Hepato-STIM or minimum essential medium (MEM). Expression of key epithelial marker proteins was determined by quantitative reverse transcription polymerase chain reaction (RT-PCR). Transepithelial electrical resistance (TER) was monitored following seeding the cells on Transwell membranes. Transepithelial ion transport was estimated by short-circuit current (Isc) measurements in an Ussing chamber. Both huSMG and PTHSG cells showed epithelial characteristics when cultivated in Hepato-STIM, while fibroblast-like elements dominated in MEM. Compared to intact tissue, cultivation of the cells resulted in substantial decreases in AQP5 and NKCC1 expression and moderate increases in claudin-1 and ENaC expression. Both cultures achieved high TER and transepithelial electrolyte movement in Hepato-STIM, but not in MEM. The Isc was substantially reduced by basolateral Cl(-) and bicarbonate withdrawal, indicating the involvement of basolateral-to-apical anion transport, and by the blockade of apical ENaC by amiloride, indicating the involvement of apical-to-basolateral Na(+) transport. An almost complete inhibition was observed following simultaneous ENaC block and withdrawal of the two anions. Isc was enhanced by either apical adenosine triphosphate (ATP) or basolateral carbachol application, but not by forskolin, confirming the expected role of Ca(2+)-activated regulatory pathways in electrolyte secretion. Inhibition of basolateral NKCC1 by bumetanide reduced the response to ATP, indicating the active involvement of this transporter in Cl(-) secretion. In conclusion, we have demonstrated that both PTHSG and huSMG primary cultures cultivated in Hepato-STIM form two-dimensional monolayers in vitro on permeable supports and achieve active vectorial transepithelial electrolyte transport. The presence of both basolateral-to-apical anion fluxes and an apical-to-basolateral Na(+) flux indicates both acinar and ductal characteristics. With further refinement, this model should provide a firm basis for new interventions to correct salivary gland dysfunction.

Silk fibroin scaffolds promote formation of the ex vivo niche for salivary gland epithelial cell growth, matrix formation, and retention of differentiated function

Salivary gland hypofunction often results from a number of causes, including the use of various medications, radiation for head and neck tumors, autoimmune diseases, diabetes, and aging. Since treatments for this condition are lacking and adult salivary glands have little regenerative capacity, there is a need for cell-based therapies to restore salivary gland function. Development of these treatment strategies requires the establishment of a system that is capable of replicating the salivary gland cell "niche" to support the proliferation and differentiation of salivary gland progenitor cells. In this study, a culture system using three-dimensional silk fibroin scaffolds (SFS) and primary salivary gland epithelial cells (pSGECs) from rat submandibular (SM) gland and parotid gland (PG) was established and characterized. pSGECs grown on SFS, but not tissue culture plastic (TCP), formed aggregates of cells with morphological features resembling secretory acini. High levels of amylase were released into the media by both cell types after extended periods in culture on SFS. Remarkably, cultures of PG-derived cells on SFS, but not SM cells, responded to isoproterenol, a β-adrenergic receptor agonist, with increased enzyme release. This behavior mimics that of the salivary glands in vivo. Decellularized extracellular matrix (ECM) formed by pSGECs in culture on SFS contained type IV collagen, a major component of the basement membrane. These results demonstrate that pSGECs grown on SFS, but not TCP, retain important functional and structural features of differentiated salivary glands and produce an ECM that mimics the native salivary gland cell niche. These results demonstrate that SFS has potential as a scaffold for creating the salivary gland cell niche in vitro and may provide an approach for inducing multipotent stem cells to provide therapeutically meaningful numbers of salivary gland progenitor cells for regenerating these tissues in patients.

Elements toward novel therapeutic targeting of the adrenergic system

Adrenergic receptors belong to the family of the G protein coupled receptors that represent important targets in the modern pharmacotherapies. Studies on different physiological and pathophysiological properties of the adrenergic system have led to novel evidences and theories that suggest novel possible targeting of such system in a variety of pathologies and disorders, even beyond the classical known therapeutic possibilities. Herein, those advances have been illustrated with selected concepts and different examples. Furthermore, we illustrated the applications and the therapeutic implications that such findings and advances might have in the contexts of experimental pharmacology, therapeutics and clinic. We hope that the content of this work will guide researches devoted to the adrenergic aspects that combine neurosciences with pharmacology.

Protective MCMV immunity by vaccination of the salivary gland via Wharton's duct: replication-deficient recombinant adenovirus expressing individual MCMV genes elicits protection similar to that of MCMV

Salivary glands, a major component of the mucosal immune system, confer antigen-specific immunity to mucosally acquired pathogens. We investigated whether a physiological route of inoculation and a subunit vaccine approach elicited MCMV-specific and protective immunity. Mice were inoculated by retrograde perfusion of the submandibular salivary glands via Wharton's duct with tcMCMV or MCMV proteins focused to the salivary gland via replication-deficient adenovirus expressing individual MCMV genes (gB, gH, IE1; controls: saline and replication deficient adenovirus without MCMV inserts). Mice were evaluated for MCMV-specific antibodies, T-cell responses, germinal center formation, and protection against a lethal MCMV challenge. Retrograde perfusion with tcMCMV or adenovirus expressed MCMV proteins induced a 2- to 6-fold increase in systemic and mucosal MCMV-specific antibodies, a 3- to 6-fold increase in GC marker expression, and protection against a lethal systemic challenge, as evidenced by up to 80% increased survival, decreased splenic pathology, and decreased viral titers from 10(6) pfu to undetectable levels. Thus, a focused salivary gland immunization via a physiological route with a protein antigen induced systemic and mucosal protective immune responses. Therefore, salivary gland immunization can serve as an alternative mucosal route for administering vaccines, which is directly applicable for use in humans.

The protective efficacy of basic fibroblast growth factor in radiation-induced salivary gland dysfunction in mice

OBJECTIVES/HYPOTHESIS

Radiotherapy is one of the most effective treatments for head and neck cancer. However, in addition to the target tumor, normal salivary glands are also included in the irradiation field. This unavoidably results in dry mouth syndrome as a side effect. In this study, the protective efficacy of basic fibroblast growth factor (bFGF) was investigated in radiation-damaged salivary glands.

STUDY DESIGN

Prospective animal experiment with control.

METHODS

Nine-week-old female C57BL/6 mice were divided into three groups. All mice in two of the three groups were irradiated (10 Gy) at the same time. In the bFGF-treated group, bFGF was administered to the submandibular glands for 3 consecutive days after neck irradiation. Mice in the untreated control group were administered distilled water. Mice in the third group were not irradiated and did not receive any additional treatments. Saliva flow rate and submandibular gland morphology were assessed, and the apoptotic response of irradiated submandibular glands was also evaluated.

RESULTS

Administration of bFGF improved hyposalivation 8 weeks after irradiation, and histologic analysis revealed that bFGF-treated glands contained more acinar cells compared to untreated glands. The apoptotic response to irradiation, examined 1 and 2 days after irradiation, was reduced, and quantitative real-time polymerase chain reaction revealed a paracrine effect for bFGF in the glands that received bFGF treatment.

CONCLUSIONS

Our study indicates that bFGF prevents salivary gland dysfunction after irradiation. The protective benefits of bFGF may be attributed to the inhibition of radiation-induced apoptosis as well as the paracrine effect it has in these tissues.

Regeneration of radiation damaged salivary glands with adipose-derived stromal cells

OBJECTIVES

Radiotherapy is one of the most effective treatments for head and neck cancer. However, the development of dry mouth syndrome is an unavoidable side effect because, in addition to the tumor, the normal salivary glands are included in the irradiation field. Previously, we investigated the protective efficacy of basic fibroblast growth factor (bFGF) in radiation-damaged salivary glands. In this study, we investigated the efficacy of adipose-derived stromal cell (ADSC) transplantation for the regeneration of radiation damaged salivary glands.

STUDY DESIGN

Prospective animal experiment with control.

METHODS

ADSCs (500,000 cells isolated from green fluorescent protein mice) were transplanted into submandibular glands of C57BL/6 mice 10 weeks after 10 Gy irradiation (ADSC group). Phosphate-buffered saline was administered to mice as a control (sham group). Submandibular gland morphology and saliva flow rate were assessed just prior to transplantation and at 5 and 10 weeks posttransplantation. Quantitative RT-PCR was performed to examine the response of irradiated submandibular glands to ADSC transplantation.

RESULTS

Saliva flow rate in the ADSC group was improved compared to the sham group. Although acinar cell damage was detected in both groups, the proliferation of blood vessels and other paracrine effects were observed only in the ADSC group. Additionally, the differentiation of ADSCs into blood endothelial cells and ductal cells was also observed.

CONCLUSIONS

Our study indicates that ADSCs have the potential to restore salivary gland function after irradiation. The restoration of blood flow within submandibular gland tissue may explain the beneficial effects of ADSCs.

Salivary gland progenitor cell biology provides a rationale for therapeutic salivary gland regeneration

An irreversible loss of salivary gland function often occurs in humans after removal of salivary tumors, after therapeutic radiation of head and neck tumors, as a result of Sjögren's syndrome and in genetic syndromes affecting gland development. The permanent loss of gland function impairs the oral health of these patients and broadly affects their quality of life. The regeneration of functional salivary gland tissue is thus an important therapeutic goal for the field of regenerative medicine and will likely involve stem/progenitor cell biology and/or tissue engineering approaches. Recent reports demonstrate how both innervation of the salivary gland epithelium and certain growth factors influence progenitor cell growth during mouse salivary gland development. These advances in our understanding suggest that developmental mechanisms of mouse salivary gland development may provide a paradigm for postnatal regeneration of both mice and human salivary glands. Herein, we will discuss the developmental mechanisms that influence progenitor cell biology and the implications for salivary gland regeneration.

Dynamics of salivary gland morphogenesis

Salivary glands form during embryonic development by a complex process that creates compact, highly organized secretory organs with functions essential for oral health. The architecture of these glands is generated by branching morphogenesis, revealed by recent research to involve unexpectedly dynamic cell motility and novel regulatory pathways. Numerous growth factors, extracellular matrix molecules, gene regulatory pathways, and mechanical forces contribute to salivary gland morphogenesis, but local gene regulation and morphological changes appear to play particularly notable roles. Here we review these recent advances and their potential application to salivary gland tissue engineering.

Inhibition of Src and p38 MAP kinases suppresses the change of claudin expression induced on dedifferentiation of primary cultured parotid acinar cells

Sjögren's syndrome and therapeutic radiation for head and neck cancers result in irreversible changes in the parenchyma of salivary glands, loss of acinar cells, prominence of duct cells, and fibrosis. To clarify mechanisms of salivary gland dysfunction, we identified a signaling pathway involved in the dedifferentiation of primary cultures of parotid acinar cells. We reported previously that the expression pattern of claudins changes during culture, is related to the three-dimensional organization of the cells, and reflects their ability to function as acinar cells. In this study, we found that this change of claudin expression is a process of dedifferentiation, because expression of other differentiation markers also changes during culture. The expression levels of claudins-4 and -6, cytokeratin 14, and vimentin are increased, and those of claudin-10, aquaporin 5, and amylase are decreased. Inhibitors of Src and p38 MAP kinases suppress these changes and increase the expression of acinar marker proteins. Differences in extracellular matrix components have no effect. Activation of p38 MAP kinase occurs during cell isolation from the parotid glands and is retained up to 6 h after the isolation. In contrast, activation of Src kinases does not increase during the cell isolation. The Src inhibitor PP1 suppresses the activation of p38 MAP kinase. Therefore, cellular stresses induced during cell isolation cause dedifferentiation and transition to duct-like cells through activation of p38 MAP kinase and constitutively active Src kinases.

[Cell-based strategies for salivary gland regeneration]

Xerostomia as a side effect of radiotherapy or due to Sjögren's disease leads to considerable impairment of the quality of life of the affected patients. Preventive treatment approaches such as intensity-modulated radiotherapy, surgical transfer of a submandibular gland to a site outside the radiation field or administration of amifostin during radiation treatment are not yet completely established in clinical practice and are not applicable for all patients. Symptomatic treatment with pilocarpin or synthetic saliva leads to an improvement of the symptoms only in some patients, and in the case of pilocarpin significant systemic anticholinergic side-effects might occur. Because large numbers of patients are affected and current treatment options are not satisfactory, it is essential to develop new treatment options. In parallel with the in vitro production of functional salivary gland constructs by means of tissue engineering techniques, attempts are currently under way to experimentally restore salivary gland function by genetic treatment approaches such as transfection of the affected salivary glands with aquaporins or pro-angiogenic factors. In addition, the in vivo application of stem cells is under investigation. In the present paper, we discuss the clinical and radiobiological background of xerostomia and highlight possible innovative future treatment options.

Transplantation of cultured salivary gland cells into an atrophic salivary gland

Patients with dry mouth have been treated with salivary substitutes and/or medications such as pilocarpine or cevimeline hydrochloride. These treatments temporarily relieve their symptoms and induce salivation from residual tissue. However, no treatment is available for the purpose of regenerating an atrophic gland. In this study, the feasibility of a cell transplantation therapy for the atrophic submandibular glands was investigated in rats. Further, the potential of cell differentiation into a useful phenotype was assessed by immunohistochemistry together with cell tracking with the fluorescent dye PKH 26. Rat submandibular glands were excised, and the salivary gland epithelial cells were cultured for 3 weeks with 3T3 cells as a feeder layer. Ductal ligation of the submandibular gland was employed to generate an atrophic gland. One week after the operation, the ligation was removed, and the cultured cells labeled with PKH 26 were injected into the atrophic submandibular glands. As a control, the cultured cells were also injected into normal submandibular glands. Two weeks after cell transplantation, the transplanted cells were detectable in both the experimental and control groups. The cells were clustered in the connective tissue between the lobules. Four weeks after transplantation, the labeled cells were detectable in the experimental group but not in the control group. In the atrophic glands, the scattered transplanted cells were observed over a broad area of the gland but localized mainly around the acini and ductal region. Immunostaining results showed a possible involvement of the transplanted cells in ductal regeneration, while neither myoepithelial nor acinar differentiations were observed within the 4 weeks since transplantation. This study demonstrated that cell transplantation to the salivary gland is feasible, and that the transplanted cells were selectively attracted to and remained in the damaged area without affecting normal tissue.

Regeneration in chronic sialadenitis: an analysis of proliferation and apoptosis based on double immunohistochemical labelling

The understanding of regeneration in salivary glands as a finely tuned balance of cellular proliferation, differentiation and apoptosis has been limited by the difficulty of identifying proliferating cells. This has been overcome in the present investigation by double immunohistochemical labelling for the proliferation-associated antigen Ki67 and for different cell-type-specific antigens applied to 8 specimens of normal parotids and 16 specimens of chronic parotid sialadenitis with particular reference to acini and intercalated ducts. In comparison with low baseline rates of proliferation in normal parotids, proliferative indices were significantly increased in chronic sialadenitis in mature acinar cells, intercalated ductal cells and myoepithelial cells without evidence of proliferation by an additional population of cells. In accordance with findings in glands of experimental animals, the present data do not support the previously postulated concept of regeneration of acini and intercalated ducts by a hypothetical population of uncommitted ductal stem cells. The demonstration of a profound capacity for intrinsic glandular regeneration from differentiated cells represents a biological basis for the good results obtained from conservative therapy of chronic sialadenitis and offers hope for novel therapies designed to reconstitute impaired salivary flow.

The oral component of Sjögren's syndrome: pass the scalpel and check the water

The labial salivary gland biopsy is a diagnostic test for the oral component of Sjögren's syndrome (SS) that has been the subject of controversy and re-examination for many years. Despite multiple recent challenges to the significance of this test, when correctly done, it remains one of the most informative, specific, and technically simple tests available for the oral component of SS. Because of compromised salivary gland function, patients with SS are at risk for dental caries. Within the past decade, a paradigm shift has occurred within the field of caries research. The caries process was previously thought irreversible once initiated. Research has shown that the "early" carious lesion can be remineralized. Thus, the "early" carious lesion may be prevented and even repaired. The process of remineralization requires appropriate conditions to occur, and one of those conditions is pH. Because water is often the preferred wetting agent/beverage for patients with SS, the data show that, with regard to pH, not all waters are created equal.

Age-related changes in IGF-1 expression in submandibular glands of senescence-accelerated mice

Saliva is known to play important roles in such functions as swallowing, mastication, speech, and taste. Furthermore, salivary glands synthesize and secrete a number of growth factors involved in cell/tissue homeostasis. It has been demonstrated that IGF-1, which is structurally analogous to insulin, has been shown to be expressed in mouse submandibular glands, and that IGF-1 stimulates DNA synthesis, amino acid uptake, protein synthesis, and glucose transport in various cells. Diminished function of the salivary glands is thought to lead to increased dental caries and periodontal diseases, which are commonly associated with aging. However, very little is known regarding the effects of age on IGF-1 expression in submandibular glands. The senescence-accelerated mouse (SAM), an experimental murine model of accelerated aging, has been extensively used to examine the mechanisms responsible for aging. In the present study, IGF-1 production and mRNA levels in the submandibular glands of SAM-P1 mice were examined. IGF-1 levels were determined by radioimmunoassay and IGF-1 mRNA levels by semi-quantitative RT-PCR. We found that IGF-1 protein levels in homogenates and IGF-1 mRNA levels decreased with age in SAMP1 mice. These findings suggest that IGF-1 synthesis in submandibular glands decreases with aging, and this may result in lower levels of cellular proliferation, regeneration and wound healing in aged oral tissues.