Salivary glands: a paradigm for diversity of gland development

Quantitative single cell analysis of cell population dynamics during submandibular salivary gland development and differentiation

Epithelial organ morphogenesis involves reciprocal interactions between epithelial and mesenchymal cell types to balance progenitor cell retention and expansion with cell differentiation for evolution of tissue architecture. Underlying submandibular salivary gland branching morphogenesis is the regulated proliferation and differentiation of perhaps several progenitor cell populations, which have not been characterized throughout development, and yet are critical for understanding organ development, regeneration, and disease. Here we applied a serial multiplexed fluorescent immunohistochemistry technology to map the progressive refinement of the epithelial and mesenchymal cell populations throughout development from embryonic day 14 through postnatal day 20. Using computational single cell analysis methods, we simultaneously mapped the evolving temporal and spatial location of epithelial cells expressing subsets of differentiation and progenitor markers throughout salivary gland development. We mapped epithelial cell differentiation markers, including aquaporin 5, PSP, SABPA, and mucin 10 (acinar cells); cytokeratin 7 (ductal cells); and smooth muscle α-actin (myoepithelial cells) and epithelial progenitor cell markers, cytokeratin 5 and c-kit. We used pairwise correlation and visual mapping of the cells in multiplexed images to quantify the number of single- and double-positive cells expressing these differentiation and progenitor markers at each developmental stage. We identified smooth muscle α-actin as a putative early myoepithelial progenitor marker that is expressed in cytokeratin 5-negative cells. Additionally, our results reveal dynamic expansion and redistributions of c-kit- and K5-positive progenitor cell populations throughout development and in postnatal glands. The data suggest that there are temporally and spatially discreet progenitor populations that contribute to salivary gland development and homeostasis.

Anatomy and histology of rodent and human major salivary glands: -overview of the Japan salivary gland society-sponsored workshop-

MAJOR SALIVARY GLANDS OF BOTH HUMANS AND RODENTS CONSIST OF THREE PAIRS OF MACROSCOPIC GLANDS: parotid, submandibular, and sublingual. These glands secrete serous, mucous or mixed saliva via the proper main excretory ducts connecting the glandular bodies with the oral cavity. A series of discoveries about the salivary ducts in the 17th century by Niels Stensen (1638-1686), Thomas Wharton (1614-1673), and Caspar Bartholin (1655-1738) established the concept of exocrine secretion as well as salivary glands. Recent investigations have revealed the endocrine functions of parotin and a variety of cell growth factors produced by salivary glands.The present review aims to describe macroscopic findings on the major salivary glands of rodents and the microscopic differences between those of humans and rodents, which review should be of interest to those researchers studying salivary glands.

Clock genes show circadian rhythms in salivary glands

Circadian rhythms are endogenous self-sustained oscillations with 24-hour periods that regulate diverse physiological and metabolic processes through complex gene regulation by "clock" transcription factors. The oral cavity is bathed by saliva, and its amount and content are modified within regular daily intervals. The clock mechanisms that control salivary production remain unclear. Our objective was to evaluate the expression and periodicity of clock genes in salivary glands. Real-time quantitative RT-PCR, in situ hybridization, and immunohistochemistry were performed to show circadian mRNA and protein expression and localization of key clock genes (Bmal1, Clock, Per1, and Per2), ion and aqua channel genes (Ae2a, Car2, and Aqp5), and salivary gland markers. Clock gene mRNAs and clock proteins were found differentially expressed in the serous acini and duct cells of all major salivary glands. The expression levels of clock genes and Aqp5 showed regular oscillatory patterns under both light/dark and complete-dark conditions. Bmla1 overexpression resulted in increased Aqp5 expression levels. Analysis of our data suggests that salivary glands have a peripheral clock mechanism that functions both in normal light/dark conditions and in the absence of light. This finding may increase our understanding of the control mechanisms of salivary content and flow.

Human salivary gland stem cells: isolation, propagation, and characterization

Stem cells are of outstanding interest for a variety of applications in regenerative medicine. The identification and characterization of novel tissue sources in order to reduce donor site morbidity and to provide specific cells in clinically applicable numbers have led to the detection of stem cells in almost all adult tissues. Salivary glands are of specific interest to our lab, as these tissues are easily accessible for the head and neck surgeon with low donor site morbidity. On the other hand, they possess an endocrine and exocrine function and thus play a very specific role in the human body. Stem cell identity however can only be demonstrated using a combination of different methods in vitro, as there is not a single marker or feature allowing for definite identification of such cells. In this chapter, we provide a comprehensive summary of our experimental methods for the isolation and characterization of human salivary gland stem cells in vitro.

Induction of Sca-1 in the duct cells of the mouse submandibular gland by obstruction of the main excretory duct

The effect of ligation of the main excretory duct (MED) of the mouse submandibular gland (SMG) on the expression of Sca-1, a stem cell antigen, was examined by Western blotting and immunohistochemistry. By Western blotting, the expression of Sca-1 with a molecular weight of 18 kDa was identified in the normal gland. At 1 day post-ligation, the expression level of Sca-1 was strongly increased in the experimental gland and weakly in the contralateral gland, and such expression in both glands decreased at 6 days. By immunohistochemistry, Sca-1 was detected weakly in the apical membrane of excretory duct (ED) cells of the SMG under the normal condition. By duct ligation, Sca-1 became expressed strongly in most cells of the two major duct systems, i.e., the striated duct (SD) and granular convoluted tubules (GCT), but was not detected in the acinar (Ac) cells. By fluorescence-activated cell sorter (FACS) analysis, the number of side population (SP) cells in this gland was found to be increased by ligation. These results imply that Sca-1-positive cells may have a role in the duct cell proliferation in the regeneration step elicited by MED ligation-induced injury.

Human mesenchymal stem cells cultured with salivary gland biopsies adopt an epithelial phenotype

Sjogren's syndrome and radiotherapy for head and neck cancer result in severe xerostomia and irreversible salivary gland damage for which no effective treatment is currently available. Cell culture methods of primary human salivary gland epithelial cells (huSGs) are slow and cannot provide a sufficient number of cells. In addition, the majority of cultured huSGs are of a ductal phenotype and thus not fluid/saliva secretory cells. Some reports indicated that mesenchymal stem cells (MSCs) possessed the potential to differentiate into epithelial cells. To test this hypothesis with huSGs, a coculture system containing 2 chambers separated by a polyester membrane was used to study the capacity of human MSCs to adopt an epithelial phenotype when cocultured with human salivary gland biopsies. Results were that 20%-40% of cocultured MSCs expressed tight junction proteins [claudin-1 (CLDN-1), -2, -3, and -4; occludin; junctional adhesion molecule-A; and zonula occludens-1] as well as other epithelial markers [aquaporin-5, α-amylase (α-AMY), and E-cadherin], and generated a higher transepithelial electrical resistance. Electron microscopy demonstrated that these MSCs had comparable cellular structures to huSGs, such as tight junction structures and numerous secretory granules. Quantitative real time (RT)-polymerase chain reaction revealed an upregulation of several salivary genes (aquaporin-5, AMY, and CLDN-2). Moreover, the amounts of α-AMY detected in cocultured MSCs were comparable to those detected in huSGs control cultures. These data suggest that cocultured MSCs can demonstrate a temporary change into a salivary gland acinar phenotype.

The effects of antidepressants and pilocarpine on rat parotid glands: an immunohistochemical study

OBJECTIVES

To evaluate the effects of antidepressants and pilocarpine on the quantity of myoepithelial cells and on the proliferation index of the epithelial cells of rat parotid glands.

INTRODUCTION

Hyposalivation, xerostomia, and alterations in saliva composition are important clinical side effects related to the use of antidepressants.

METHODS

Ninety male Wistar rats were allocated to nine groups. The control groups received saline for 30 (group C30) or 60 days (group C60) or pilocarpine for 60 days (group Pilo). The experimental groups were administered fluoxetine (group F30) or venlafaxine for 30 days (group V30); fluoxetine (group FS60) or venlafaxine (group VS60) with saline for 60 days; or fluoxetine (group FP60) or venlafaxine (group VP60) with pilocarpine for 60 days. Parotid gland specimens were processed, and the immunohistochemical expression of calponin and proliferating cell nuclear anti-antigen on the myoepithelial and parenchymal cells, respectively, was evaluated. Analysis of variance (ANOVA), Tukey HSD and Games-Howell tests were applied to detect differences among groups (p<0.05).

RESULTS

Compared with the controls, chronic exposure to antidepressants was associated with an increase in the number of positively stained cells for calponin. In addition, venlafaxine administration for 30 days was associated with an increase in the number of positively stained cells for proliferating cell nuclear anti-antigen. Fluoxetine and pilocarpine (group FP60) induced a significant decrease in the number of positively stained cells for calponin compared with all other groups.

CONCLUSIONS

The number of positively stained cells for calponin increased after chronic administration of antidepressants. The proliferation index of the epithelial cells of rat parotid glands was not altered by the use of antidepressants for 60 days.

Capillary endothelial Na(+), K(+), ATPase transporter homeostasis and a new theory for migraine pathophysiology

BACKGROUND

Cerebrospinal fluid sodium concentration ([Na(+)](csf)) increases during migraine, but the cause of the increase is not known.

OBJECTIVE

Analyze biochemical pathways that influence [Na(+)](csf) to identify mechanisms that are consistent with migraine.

METHOD

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

RESULTS

Increased capillary endothelial cell (CEC) Na(+), K(+), -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na(+)](csf). Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na(+)](e)) and potassium ([K(+)](e)).

CONCLUSIONS

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K(+)](e), facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na(+)](e), increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

Regeneration of acinar cells following ligation of rat submandibular gland retraces the embryonic-perinatal pathway of cytodifferentiation

Rat submandibular gland can regenerate following ligation-induced atrophy, eventually recovering its normal morphology and function. Previous studies have suggested that the regeneration process implies both self-proliferation of existing acini and formation of new acinar cells. One hypothesis is that new acinar cells may differentiate from the ductal cells in a similar fashion to the process of cytodifferentiation occurring during submandibular glandular development. In this study atrophy was induced, under recovery anaesthesia, by applying a metal clip on the main duct of the submandibular gland without including the chorda lingual nerve. After 2 weeks the duct was deligated for 3, 5 or 7 days or 8 weeks and the glands collected. Tissue was prepared for immunohistochemistry, biochemical analysis and RNA extraction. The histology of the regenerated glands shows several normal-looking acini, which have regained their glycoprotein content (AB/PAS positive), data also confirmed by biochemical analysis (SDS-PAGE/PAS). Regenerating tissue was characterized by the presence of embryonic-like branched structures ending with AB/PAS positive acinar cells. The proteins SMG-B and PSP are normally expressed in acinar cell precursors during development but only by intercalated ductal cells in the adult stage. In the adult regenerating gland mRNA levels of both SMG-B and PSP were found to be up-regulated compared to ligated glands and SMG-B expression localized to acinar cells whilst the ductal cells were negative. This study of rat submandibular gland regeneration suggests new acinar cells have differentiated from ducts and express markers of acinar cell precursors in a similar manner to the cytodifferentiation process occurring during glandular development.

On approaches to the functional restoration of salivary glands damaged by radiation therapy for head and neck cancer, with a review of related aspects of salivary gland morphology and development

Radiation therapy for cancer of the head and neck can devastate the salivary glands and partially devitalize the mandible and maxilla. As a result, saliva production is drastically reduced and its quality adversely altered. Without diligent home and professional care, the teeth are subject to rapid destruction by caries, necessitating extractions with attendant high risk of necrosis of the supporting bone. Innovative techniques in delivery of radiation therapy and administration of drugs that selectively protect normal tissues can reduce significantly the radiation effects on salivary glands. Nonetheless, many patients still suffer severe oral dryness. I review here the functional morphology and development of salivary glands as these relate to approaches to preventing and restoring radiation-induced loss of salivary function. The acinar cells are responsible for most of the fluid and organic material in saliva, while the larger ducts influence the inorganic content. A central theme of this review is the extent to which the several types of epithelial cells in salivary glands may be pluripotential and the circumstances that may influence their ability to replace cells that have been lost or functionally inactivated due to the effects of radiation. The evidence suggests that the highly differentiated cells of the acini and large ducts of mature glands can replace themselves except when the respective pools of available cells are greatly diminished via apoptosis or necrosis owing to severely stressful events. Under the latter circumstances, relatively undifferentiated cells in the intercalated ducts proliferate and redifferentiate as may be required to replenish the depleted pools. It is likely that some, if not many, acinar cells may de-differentiate into intercalated duct-like cells and thus add to the pool of progenitor cells in such situations. If the stress is heavy doses of radiation, however, the result is not only the death of acinar cells, but also a marked decline in functional differentiation and proliferative capacity of all of the surviving cells, including those with progenitor capability. Restoration of gland function, therefore, seems to require increasing the secretory capacity of the surviving cells, or replacing the acinar cells and their progenitors either in the existing gland remnants or with artificial glands.

Long-term transduction of miniature pig parotid glands using serotype 2 adeno-associated viral vectors

BACKGROUND

Previously, using an adenoviral vector, we showed that miniature pigs could provide a valuable and affordable large animal model for pre-clinical gene therapy studies to correct parotid gland radiation damage. However, adenoviral vectors lead to short-term transgene expression and, ideally, a more stable correction is required. In the present study, we examined the suitability of using a serotype 2 adeno-associated viral (AAV2) vector to mediate more stable gene transfer in the parotid glands of these animals.

METHODS

Heparan sulfate proteoglycan was detected by immunohistochemistry. beta-galactosidase expression was determined histochemically. An AAV2 vector encoding human erythropoietin (hEpo) was administered via Stensen's duct. Salivary and serum hEpo levels were measured using an enzyme-linked immunosorbent assay. Serum chemistry and hematological analyses were performed and serum antibodies to hEpo were measured throughout the study. Vector distribution was determined by a quantitative polymerase chain reaction.

RESULTS

Transgene expression was vector dose-dependent, with high levels of hEpo being detected for up to 32 weeks (i.e. the longest time studied). hEpo reached maximal levels during weeks 4-8, but declined to approximately 25% of these values by week 32. Haematocrits were elevated from week 2. Transduced animals exhibited low serum anti-hEpo antibodies (1 : 8-1 : 16). Vector biodistribution at animal sacrifice revealed that most copies were in the targeted parotid gland, with few being detected elsewhere. No consistent adverse changes in serum chemistry or hematology parameters were seen.

CONCLUSIONS

AAV2 vectors mediate extended gene transfer to miniature pig parotid glands and should be useful for testing pre-clinical gene therapy strategies aiming to correct salivary gland radiation damage.

Stem cell therapy to reduce radiation-induced normal tissue damage

Normal tissue damage after radiotherapy is still a major problem in cancer treatment. Stem cell therapy may provide a means to reduce radiation-induced side effects and improve the quality of life of patients. This review discusses the current status in stem cell research with respect to their potential to reduce radiation toxicity. A number of different types of stem cells are being investigated for their potential to treat a variety of disorders. Their current status, localization, characterization, isolation, and potential in stem cell-based therapies are addressed. Although clinical adult stem cell research is still at an early stage, preclinical experiments show the potential these therapies may have. Based on the major advances made in this field, stem cell-based therapy has great potential to allow prevention or treatment of normal tissue damage after radiotherapy.

Role for Notch signaling in salivary acinar cell growth and differentiation

The Notch pathway is crucial for stem/progenitor cell maintenance, growth and differentiation in a variety of tissues. The Notch signaling is essential for Drosophila salivary gland development but its role in mammalian salivary gland remains unclear. The human salivary epithelial cell line, HSG, was studied to determine the role of Notch signaling in salivary epithelial cell differentiation. HSG expressed Notch 1 to 4, and the Notch ligands Jagged 1 and 2 and Delta 1. Treatment of HSG cells with inhibitors of gamma-secretase, which is required for Notch cleavage and activation, blocked vimentin and cystatin S expression, an indicator of HSG differentiation. HSG differentiation was also associated with Notch downstream signal Hes-1 expression, and Hes-1 expression was inhibited by gamma-secretase inhibitors. siRNA corresponding to Notch 1 to 4 was used to show that silencing of all four Notch receptors was required to inhibit HSG differentiation. Normal human submandibular gland expressed Notch 1 to 4, Jagged 1 and 2, and Delta 1, with nuclear localization indicating Notch signaling in vivo. Hes-1 was also expressed in the human tissue, with staining predominantly in the ductal cells. In salivary tissue from rats undergoing and recovering from ductal obstruction, we found that Notch receptors and ligands were expressed in the nucleus of the regenerating epithelial cells. Taken together, these data suggest that Notch signaling is critical for normal salivary gland cell growth and differentiation.

Expression of Muc19/Smgc gene products during murine sublingual gland development: cytodifferentiation and maturation of salivary mucous cells

Muc19/Smgc expresses two splice variants, Smgc (submandibular gland protein C) and Muc19 (mucin 19), the latter a major exocrine product of differentiated murine sublingual mucous cells. Transcripts for Smgc were detected recently in neonatal sublingual glands, suggesting that SMGC proteins are expressed during initial salivary mucous cell cytodifferentiation. We therefore compared developmental expression of transcripts and translation products of Smgc and Muc19 in sublingual glands. We find abundant expression of SMGC within the initial terminal bulbs, with a subsequent decrease as Muc19 expression increases. During postnatal gland expansion, SMGC is found in presumptive newly formed acinar cells and then persists in putative acinar stem cells. Mucin levels increase 7-fold during the first 3 weeks of life, with little change in transcript levels, whereas between postnatal days 21 and 28, there is a 3-fold increase in Muc19 mRNA and heteronuclear RNA. Our collective results demonstrate the direct transition from SMGC to Muc19 expression during early mucous cell cytodifferentiation and further indicate developmentally regulated changes in Muc19/Smgc transcription, alternative splicing, and translation. These changes in Muc19/Smgc gene expression delineate multiple stages of salivary mucous cell cytodifferentiation and subsequent maturation during embryonic gland development through the first 4 weeks of postnatal life.

Drainage post-thyroglossal duct remnant surgery: Possible source and analysis

Objective: To establish the distance between tongue base salivary tissue and hyoid. Also, to identify protein differences between thyroglossal duct (TGD) remnants and salivary tissue in order to distinguish drainage source post Sistrunk surgery.

Methods/Setting: The anterior neck block was obtained from 10 adult cadavers with no known neck pathology. The distance between the normal salivary tissue and hyoid was measured histologically. Immunohistochemistry (IHC) was then performed on 20 archived tissue blocks from pediatric patients post Sistrunk surgery to identify the presence of amylase, MUC5AC (tracheobronchial mucin), and MUC7 (salivary mucin) within the excised specimen.

Results: Average distance between salivary tissue and the hyoid within adult human cadavers was 3.3 mm (range, 1.0-4.2 mm). IHC revealed all excised TGD remnants contained amylase and MUC5AC but none contained MUC7. Both amylase and MUC7 were present within adjacent salivary tissues.

Conclusions: Salivary tissue of the tongue base normally resides an average of 3.3 mm from the hyoid within the adult population. Biochemical analysis showed MUC5AC was specific for TGD remnants while MUC7 was specific for salivary tissue. Amylase does not distinguish between the two tissues.

Diverse roles of E-cadherin in the morphogenesis of the submandibular gland: insights into the formation of acinar and ductal structures

The formation of acinar and ductal structures during epithelial tissue branching morphogenesis is not well understood. We report that in the mouse submandibular gland (SMG), acinar and ductal cell fates are determined early in embryonic morphogenesis with E-cadherin playing pivotal roles in development. We identified two morphologically distinct cell populations at the single bud stage, destined for different functions. The outer layer of columnar cells with organized E-cadherin junctions expressed the neonatal acinar marker B1 by E13.5, demonstrating their acinar fate. The interior cells initially lacked distinct E-cadherin junctions, but with morphogenesis formed cytokeratin 7 (K7) -positive ductal structures with organized E-cadherin junctions and F-actin filaments. Inhibition of E-cadherin function with either siRNA or function blocking antibody caused extensive apoptosis of ductal cells and aberrantly dilated lumens, providing the first evidence that E-cadherin regulates ductal lumen formation during branching morphogenesis of the salivary gland.

Aquaporins in secretory glands and their role in Sjögren's syndrome

Salivary, lacrimal and pancreatic secretions are known to account for multiple physiological functions. These exocrine secretions are watery fluids containing electrolytes, and a mixture of proteins, and can be stimulated by a number of agonists. Since water movement is involved in exocrine secretion, aquaporins (AQPs) have been hypothesised to contribute to fluid production in exocrine glands. This chapter will focus on the expression, localisation and function of AQPs in salivary and lacrimal glands and pancreas. The role of multiple water and ion transporters and channels in exocrine fluid secretion will also be reviewed. Finally, this chapter will address the potential role of AQPs in Sjögren's syndrome.

Altered plasticity of the parasympathetic innervation in the recovering rat submandibular gland following extensive atrophy

Adult rat submandibular glands have a rich autonomic innervation, with parasympathetic and sympathetic nerves working in synergy rather than antagonistically. Ligation of the secretory duct rapidly causes atrophy and the loss of most acini, which are the main target cell for parasympathetic nerves. Following deligation, there is a recovery of gland structure and function, as assessed by autonomimetic stimulation. This study examines whether the parasympathetic nerves reattach to new target cells to form functional neuro-effector junctions. Under recovery anaesthesia, the submandibular duct of adult male rats was ligated via an intra-oral approach to avoid damaging the chorda-lingual nerve. Four weeks later, rats were either killed or anaesthetized and the ligation clip removed. Following a further 8 weeks, both submandibular ducts were cannulated under terminal anaesthesia. Salivary flows were then stimulated electrically (chorda-lingual nerve at 2, 5 and 10 Hz) and subsequently by methacholine (whole-body infusion at two doses). Glands were excised, weighed and divided for further in vitro studies or fixed for histological examination. Ligation of ducts caused 75% loss of gland weight, with the loss of most acinar cells. Of the remaining acini, only 50% were innervated despite unchanged choline acetyltransferase activity, suggesting few parasympathetic nerves had died. Following deligation, submandibular glands recovered half their weight and had normal morphology. Salivary flows from both glands (per unit of gland tissue) were similar when evoked by methacholine but greater from the deligated glands when evoked by nerve stimulation. This suggests that parasympathetic nerves had reattached to new target cells in the recovered glands at a greater ratio than normal, confirming reinnervation of the regenerating gland.

Transient TWEAK overexpression leads to a general salivary epithelial cell proliferation

OBJECTIVES

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a multifunctional cytokine that has pro-apoptotic, pro-angiogenic and pro-inflammatory effects. In liver, TWEAK leads to proliferation of progenitor oval cells, but not of mature hepatocytes. This study evaluated the hypothesis that TWEAK overexpression in salivary glands would lead to the proliferation of a salivary progenitor cell.

METHODS

A recombinant, serotype 5 adenoviral vector encoding human TWEAK, AdhTWEAK, was constructed, initially tested in vitro, and then administered to male Balb/c mice via cannulation of Wharton's duct. TWEAK expression in vivo was monitored as protein secreted into saliva and serum by enzyme-linked immunosorbent assays. Salivary cell proliferation was monitored by proliferating cell nuclear antigen staining and apoptosis was monitored using TUNEL staining.

RESULTS

AdhTWEAK administration led to a dose-dependent, transient TWEAK protein expression, detected primarily in saliva. Salivary epithelial cell proliferation was generalized, peaking on approximately days 2 and 3. TWEAK expression had no detectable effect on apoptosis of salivary epithelial cells.

CONCLUSION

Transient overexpression of TWEAK in murine salivary glands leads to a general proliferation of epithelial cells vs a selective stimulation of a salivary progenitor cell.

Visualizing form and function in organotypic slices of the adult mouse parotid gland

An organotypic slice preparation of the adult mouse parotid salivary gland amenable to a variety of optical assessments of fluid and protein secretion dynamics is described. The semi-intact preparation rendered without the use of enzymatic treatment permitted live-cell imaging and multiphoton analysis of cellular and supracellular signals. Toward this end we demonstrated that the parotid slice is a significant addition to the repertoire of tools available to investigators to probe exocrine structure and function since there is currently no cell culture system that fully recapitulates parotid acinar cell biology. Importantly, we show that a subpopulation of the acinar cells of parotid slices can be maintained in short-term culture and retain their morphology and function for up to 2 days. This in vitro model system is a significant step forward compared with enzymatically dispersed acini that rapidly lose their morphological and functional characteristics over several hours, and it was shown to be long enough for the expression and trafficking of exogenous protein following adenoviral infection. This system is compatible with a variety of genetic and physiological approaches used to study secretory function.

Rescue of salivary gland function after stem cell transplantation in irradiated glands

Head and neck cancer is the fifth most common malignancy and accounts for 3% of all new cancer cases each year. Despite relatively high survival rates, the quality of life of these patients is severely compromised because of radiation-induced impairment of salivary gland function and consequential xerostomia (dry mouth syndrome). In this study, a clinically applicable method for the restoration of radiation-impaired salivary gland function using salivary gland stem cell transplantation was developed. Salivary gland cells were isolated from murine submandibular glands and cultured in vitro as salispheres, which contained cells expressing the stem cell markers Sca-1, c-Kit and Musashi-1. In vitro, the cells differentiated into salivary gland duct cells and mucin and amylase producing acinar cells. Stem cell enrichment was performed by flow cytrometric selection using c-Kit as a marker. In vitro, the cells differentiated into amylase producing acinar cells. In vivo, intra-glandular transplantation of a small number of c-Kit⁺ cells resulted in long-term restoration of salivary gland morphology and function. Moreover, donor-derived stem cells could be isolated from primary recipients, cultured as secondary spheres and after re-transplantation ameliorate radiation damage. Our approach is the first proof for the potential use of stem cell transplantation to functionally rescue salivary gland deficiency.

Gender differences in serotype 2 adeno-associated virus biodistribution after administration to rodent salivary glands

Salivary glands (SGs) have proven useful targets for clinical applications of gene therapeutics. In this toxicology and biodistribution study, which conforms to U.S. Food and Drug Administration Good Laboratory Practice regulations, four doses (10(7)-10(10) particles) of a serotype 2 adeno-associated viral (AAV2) vector encoding human erythropoietin were directly administered to the right submandibular gland of male and female BALB/c mice (n = 21 per gender dose group). Control-treated (saline administered; n = 66) and vector-treated (n = 168) animals did not differ in clinical appearance, morbidity and mortality rates, food and water consumption, weight gain ratios, and final weight. Clinical hematology values also were unaffected by AAV2 administration except for parameters influenced by the expression of the recombinant protein (e.g., hematocrit). Mice were killed on days 3, 30, 55, and 92. No major vector-related toxicity was uncovered after complete pathology and histopathology review. However, a significant gender-related difference in vector biodistribution was revealed by quantitative polymerase chain reaction. In male mice vector (group receiving 10(10) particles/animal) effectively transduced, and was primarily confined within, the SGs (i.e., approximately 800 times more copies in SGs than in liver; day 3) and long lived. In contrast, in female mice, SG transduction was less efficient (260-fold less than in males; day 3) and short lived, and vector was disseminated widely via both the bloodstream (SG:liver copy ratio, approximately 1) and saliva (30-fold greater than in males). The observed vector biodistribution is likely due to differences in AAV2 receptor targets and structural differences affecting SG integrity. Sexual dimorphism is a factor of major significance that could potentially affect gene therapy clinical applications in SGs.

Label-retaining cells in the rat submandibular gland

To identify stem cells in salivary glands, label-retaining cells (LRCs) were established in rat submandibular glands. Developing and regenerating glands were labeled with bromodeoxyuridine (BrdU). To cause gland regeneration, the glands were injured by duct obstruction. BrdU LRCs were observed in all the parenchymal structures except for the acinus of the glands labeled during regeneration. Among these LRCs, a few, but not many, expressed neither keratin18 (K18; an acinar/duct cell marker) nor alpha-smooth muscle actin (alphaSMA; a myoepithelial cell marker), and thus were putative stem cells. These (K18 and alphaSMA)(neg) LRCs were invariably observed in the intercalated duct and the excretory duct. In the intercalated duct, they were at the proximal end bordering the acinus (the neck of the intercalated duct). Next, to test the above identification, gland extirpation experiments were performed. LRCs were established by labeling developing glands with iododeoxyuridine (IdU) in place of BrdU. Removal of one submandibular gland forced the IdU-LRCs in the remaining gland to divide. They were labeled with chlorodeoxyuridine (CldU). The (K18 and alphaSMA)(neg) LRCs in the neck of the intercalated duct and in the excretory duct did not change in number or in IdU label. The CldU label appeared in these cells and then disappeared. These results indicate that the (K18 and alphaSMA)(neg) LRCs have divided asymmetrically and are thus considered salivary gland stem cells.

Differential expression of Prominin-1 (CD133) and Prominin-2 in major cephalic exocrine glands of adult mice

The major cephalic exocrine glands share many morphological and functional features and so can be simultaneously affected in certain autoimmune- and inherited disorders leading to glandular hypofunction. Phenotypic characterization of these exocrine glands is not only an interesting biological issue, but might also be of considerable clinical relevance. The major salivary and lacrimal glands might therefore be potential subjects of future cell-based regenerative/tissue engineering therapeutic approaches. In the present study, we described the expression of the stem and progenitor cell marker Prominin-1 and those of its paralogue, Prominin-2, in the three pairs of major salivary glands, i.e., submandibular-, major sublingual-, and parotid glands in adult mice. We have also documented their expression in the extraorbital lacrimal and meibomian glands (Glandulae tarsales) of the eyelid (Palpebra). Our analysis revealed that murine Prominin-1 and Prominin-2 were differentially expressed in these major cephalic exocrine organs. Expression of Prominin-1 was found to be associated with the duct system, while Prominin-2 expression was mostly, but not exclusively, found in the acinar compartment of these organs with marked differences among the various glands. Finally, we report that Prominin-2, like Prominin-1, is released into the human saliva associated with small membrane particles holding the potential for future diagnostic applications.

Temporospatially regulated expression of subtilisin-like proprotein convertase PACE4 (SPC4) during development of the rat submandibular gland

The temporospatial expression of PACE4, a member of the mammalian subtilisin-like proprotein convertase family involved in the activation of growth/differentiation factors, was investigated by in situ hybridization during the development of the rat submandibular gland (SMG). At the initiation stage (day 15.5 of gestation; E15), PACE4 was intensely expressed in the submandibular epithelium, but weakly expressed in the mesenchymal cells. At E16 when the branching morphogenesis becomes obvious, the expression of PACE4 in the mesenchyme was further decreased, although its level in the submandibular epithelium had not changed remarkably from that at E15. During the next stage of embryonic development (E17-E20), PACE4 was expressed in the cells derived from the submandibular epithelium, which include the proacinar, terminal tubular, and presumptive ductal cells. In the perinatal SMG, PACE4 was still expressed intensely in the terminal portion of the SMG containing the proacinar and terminal tubular cells, whereas its expression in the ductal cells was obviously decreased at the second postnatal day (P2) and at P6. Acinar cells expressing no PACE4 appeared, and their numbers increased following their development (P9-P20). At P30 when the PACE4 expression in the acinar cells was completely suppressed, its expression in the ductal cells became intense again. This temporospatially regulated expression of PACE4 suggests its apparent association with the proliferation, differentiation, and establishment of functional acinar and ductal cells of the SMG.

Salivary gland development

Salivary glands provide an excellent model for the study of epithelial-mesenchymal interactions and branching morphogenesis. This review will discuss the anatomy of different types of glands, in a range of different organisms. Then, concentrating on the mouse submandibular gland, the stages of salivary gland development will be reviewed and the relative role of the mesenchyme and the epithelium will be discussed. Finally, the genes thought to play a role in development of the glands from initiation to differentiation will be reviewed.

Regulation of salivary gland function by autonomic nerves

Oral homeostasis is dependent upon saliva and its content of proteins. Reflex salivary flow occurs at a low 'resting' rate and for short periods of the day more intense taste or chewing stimuli evoke up to ten fold increases in salivation. The secretion of salivary fluid and proteins is controlled by autonomic nerves. All salivary glands are supplied by cholinergic parasympathetic nerves which release acetylcholine that binds to M3 and (to a lesser extent) M1 muscarinic receptors, evoking the secretion of saliva by acinar cells in the endpieces of the salivary gland ductal tree. Most salivary glands also receive a variable innervation from sympathetic nerves which released noradrenaline from which tends to evoke greater release of stored proteins, mostly from acinar cells but also ductal cells. There is some 'cross-talk' between the calcium and cyclic AMP intracellular pathways coupling autonomic stimulation to secretion and salivary protein secretion is augmented during combined stimulation. Other non-adrenergic, non-cholinergic neuropeptides released from autonomic nerves evoke salivary gland secretion and parasympathetically derived vasointestinal peptide, acting through endothelial cell derived nitric oxide, plays a role in the reflex vasodilatation that accompanies secretion. Neuronal type, calcium-activated, soluble nitric oxide within salivary cells appears to play a role in mediating salivary protein secretion in response to autonomimetics. Fluid secretion by salivary glands involves aquaporin 5 and the extent to which the expression of aquaporin 5 on apical acinar cell membranes is upregulated by cholinomimetics remains uncertain. Extended periods of autonomic denervation, liquid diet feeding (reduced reflex stimulation) or duct ligation cause salivary gland atrophy. The latter two are reversible, demonstrating that glands can regenerate provided that the autonomic innervation remains intact. The mechanisms by which nerves integrate with salivary cells during regeneration or during salivary gland development remain to be elucidated.

Identification of a human ortholog of the mouseDcppgene locus, encoding a novel member of the CSP-1/Dcpp salivary protein family

Salivary fluid, the collective product of numerous major and minor salivary glands, contains a range of secretory proteins that play key defensive, digestive, and gustatory roles in the oral cavity. To understand the distinct protein “signature” contributed by individual salivary glands to salivary secretions, we studied a family of proteins shown by in vitro mRNA translation to be abundantly expressed in mouse sublingual glands. Molecular cloning, Southern blotting, and restriction fragment length polymorphism analyses showed these to represent one known and two novel members of the common salivary protein (CSP-1)/Demilune cell and parotid protein (Dcpp) salivary protein family, the genes for which are closely linked in the T-complex region of mouse chromosome 17. Bioinformatic analysis identified a putative human CSP-1/Dcpp ortholog, HRPE773, expressed predominantly in human salivary tissue, that shows 31% amino acid identity and 45% amino acid similarity to the mouse Dcpp query sequence. The corresponding human gene displays a similar structure to the mouse Dcpp genes and is located on human chromosome 16 in a region known to be syntenic with the T-complex region of mouse chromosome 17. The predicted mouse and human proteins both display classical NH2-terminal signal sequences, putative jacalin-related lectin domains, and potential N-linked glycosylation sites, suggesting secretion via sublingual saliva into the oral cavity where they may display antimicrobial activity or provide a defensive coating to enamel. Identification of a human CSP-1/Dcpp ortholog therefore provides a key tool for investigation of salivary protein function in human oral health and disease.

Salivary gland branching morphogenesis

Salivary gland branching morphogenesis involves coordinated cell growth, proliferation, differentiation, migration, apoptosis, and interaction of epithelial, mesenchymal, endothelial, and neuronal cells. The ex vivo analysis of embryonic mouse submandibular glands, which branch so reproducibly and beautifully in culture, is a powerful tool to investigate the molecular mechanisms regulating epithelium-mesenchyme interactions during development. The more recent analysis of genetically modified mice provides insight into the genetic regulation of branching morphogenesis. The review begins, as did the field historically, focusing on the role of the extracellular matrix (ECM), and its components such as glycosaminoglycans, collagens, and laminins. Following sections describe the modification of the ECM by proteases and the role of cell-matrix and cell-cell receptors. The review then focuses on two major families of growth factors implicated in salivary gland development, the fibroblast growth factors (FGFs) and the epidermal growth factors (EGFs). The salivary gland phenotypes in mice with genetic modification of FGFs and their receptors highlight the central role of FGFs during salivary gland branching morphogenesis. A broader section mentions other molecules implicated from analysis of the phenotypes of genetically modified mice or organ culture experiments. The review concludes with speculation on some future areas of research.

Subcellular localization of epidermal growth factor receptor in human submandibular gland

The subcellular distribution of the epidermal growth factor receptor (EGFr) was demonstrated in the normal human submandibular gland by means of immunogold cytochemistry. EGFr labelling appeared in both acinar and ductal cells, where strong immunoreactivity was associated with a tubulovesicular system near the basolateral surfaces. In addition, groups of reactive vesicles were highlighted among secretory granules of both serous and mucous cells and at the apex of ductal cells. Basolateral vesicles were interpreted as being a result of EGFr internalization after activation by an exogenous ligand, although the functional meaning of those located apically remains unclear.

An Overview of Autonomic Regulation of Parotid Gland Activity: Influence of Orchiectomy

The parotid gland participates in the digestive process by providing fluid, electrolytes and enzymes that facilitate the onset of digestion. Neurotransmitters, hormones and biologically active peptides regulate its activity. The autonomic system is the main regulatory mechanism of the gland. Sympathetic stimulation induces amylase release through beta(1)-receptor activation and few fluid secretion by alpha(1)-receptor activation. The parasympathetic system controls basal activity of the gland acting on M(1) and M(3) muscarinic acetylcholine receptors and induces the secretion of fluid saliva rich in electrolytes through the modulation of ion channels and the Na(+)-K(+)-ATPase activity. In addition, its activation induces amylase release. The mechanisms involved in amylase secretion by isoproterenol and carbachol, as well as the mechanism of the cholinergic regulation of Na(+)-K(+)-ATPase activity and the changes observed after orchiectomy, are the scope of this review.

The effect of β-bungarotoxin, or geniculate ganglion lesion on taste bud development in the chick embryo

Chick taste bud (gemmal) primordia normally appear on embryonic day (E) 16 and incipient immature, spherical-shaped buds at E17. In ovo injection of beta-bungarotoxin at E12 resulted in a complete absence of taste buds in lower beak and palatal epithelium at developmental ages E17 and E21. However, putative gemmal primordia (solitary clear cells; small, cell groupings) remained, lying adjacent to salivary gland duct openings as seen in normal chick gemmal development. Oral epithelium was immunonegative to neural cell adhesion molecule (NCAM) suggesting gemmal primordia are nerve-independent. Some NCAM immunoreactivity was evident in autonomic ganglion-like cells and nerve fibers in connective tissue. After unilateral geniculate ganglion/otocyst excision on E2.5, at developmental ages E18 and posthatching day 1, approximately 12% of surviving ipsilateral geniculate ganglion cells sustained approximately 54% of the unoperated gemmal counts. After E18, proportional stages of differentiation in surviving developing buds probably reflect their degree of innervation, as well as rate of differentiation. Irrespective of the degree of geniculate ganglion damage, the proportion of surviving buds can be sustained at the same differentiated bud stage as on the unoperated side, or may differentiate to a later bud stage, consistent with the thesis that bud maturation, maintenance, and survival are nerve-dependent.

Clonal proliferation of multipotent stem/progenitor cells in the neonatal and adult salivary glands

Salivary gland stem/progenitor cells are thought to be present in intercalated ductal cells, but the fact is unclear. In this study, we sought to clarify if stem/progenitor cells are present in submandibular glands using colony assay, which is one of the stem cell assay methods. Using a low-density culture of submandibular gland cells of neonatal rats, we developed a novel culture system that promotes single cell colony formation. Average doubling time for the colony-forming cells was 24.7 (SD=+/-7.02)h, indicating high proliferative potency. When epidermal growth factor (EGF) and hepatocyte growth factor (HGF) were added to the medium, the number of clonal colonies increased greater than those cultured without growth factors (13.2+/-4.18 vs. 4.5+/-1.73). The RT-PCR and immunostaining demonstrated expressing acinar, ductal, and myoepithelial cell lineage markers. This study demonstrated the presence of the salivary gland stem/progenitor cells that are highly proliferative and multipotent in salivary glands.

Salivary heterotopia of the parathyroid gland: A report of two cases and review of the literature

Two cases of periparathyroid salivary gland heterotopia are described. A review of the records of the Department of Pathology, Long Island Jewish Medical Center, over a 4-year period, identified 759 surgical specimens containing parathyroid gland tissue. Of these, 2 (0.26%) contained foci of ectopic salivary gland tissue. Both cases were associated with cyst formation. To date, 9 additional cases of heterotopic salivary gland tissue associated with the parathyroid gland have been described in the literature.

Transglutaminase 2 expression in the salivary myoepithelial cells of mouse embryo

Earlier a strong transient expression of transglutaminase 2 (TGase 2) localized at the anchoring sites of muscle bundles in human embryo was observed. In this study, we report a similar transient expression of the TGase 2 in the salivary myoepithelial cells of mouse embryo by immunohistochemistry, RNA in situ hybridisation, and RT-PCR. From 35 submandibular glands of mouse embryos and postnatal mice, a consistent expression of TGase 2 in the myoepithelial cells via a stage-specific manner was identified by mono-clonal antibody to TGase 2 immunostaining. A similar expression pattern of TGase 2 in the myoepithelial cells was also observed by RNA in situ hybridisation analysis. The expression of TGase 2 in the salivary epithelium and mesenchyme during the prenatal 14.5-15.5 days was found minimally diffusely spread and became intensely focalised in the myoepithelial cells of salivary acini and ducts during the prenatal 16.5-18.5 days but thereafter gradually decreased until postnatal 7 days and remained weak in postnatal 3 weeks. Such transient rise and fall expressions of TGase 2 were also found with the sequential amount of RT-PCR products during the same period. The alpha-smooth muscle actin (alpha-SMA) as a positive control in the myoepithelial cells of mouse submandibular glands was consistently expressed during the prenatal and postnatal period. These results of transient expression of TGase 2 in the myoepithelial cells coincided with the formation of the dendritic basket structure in the periphery of acini and ducts, suggest a possible catalytic role of transglutaminase in a newly formed cellular matrixes during the cytodifferentiating stage of mouse prenatal and neonatal submandibular glands.

Morphogenesis of the juxtaoral organ in humans

The juxtaoral organ was studied using light microscopy in 55 human embryos and 90 fetuses at different stages of development. The juxtaoral organ arises from the epithelium at the bottom of the transverse opening of the primitive mouth during O'Rahilly stage 16 and becomes detached from the epithelium after O'Rahilly stage 18. The juxtaoral organ is innervated by the buccal nerve from O'Rahilly stage 20 onward, and its connective tissue capsule is clearly visible after week 11 of development. This study enabled us to describe three main periods of juxtaoral organ development: (1) the period of epithelial condensation and invagination, at O'Rahilly stages 16-17; (2) the period during which the juxtaoral organ becomes detached from the oral epithelium and is innervated, at O'Rahilly stages 18-23; and (3) the period during which the connective tissue capsule is formed, after week 11 of development. We also analysed the juxtaoral organ of five additional fetuses by immunohistochemistry with anti-NF-200 to verify their innervation. The results show that the juxtaoral organ may have a function in the mechanical activity of the region.

Mist1 is necessary for the establishment of granule organization in serous exocrine cells of the gastrointestinal tract

Establishing a pool of granules at the luminal border is a key step during exocrine cell development in the pancreas and is necessary for efficient release of digestive enzymes through regulated exocytosis. Several proteins have been linked to maintaining granule organization, but it is unclear which regulatory mechanisms are necessary to establish organization. Based on temporal and spatial expression, the transcription factor Mist1 is an excellent candidate, and analysis of mice that do not express Mist1 (Mist1KO) reveal disrupted cell morphology in adult pancreatic acini. To address Mist1's role in establishing granule location, we have characterized the organization of pancreatic acini throughout development in Mist1KO mice. Using various histological approaches, we have determined that correct granule organization is never established in pancreatic acini of Mist1KO mice. Further examination indicates that this disruption in granule targeting may be the primary defect in Mist1KO mice as granule organization is affected in other serous exocrine cells that normally express Mist1. To identify a mechanistic link between granule targeting and the loss of Mist1 function, intercellular junctions and the expression of Rab3D were assessed. While both of these factors are affected in Mist1KO mice, these changes alone do not account for the disorganization observed in Mist1KO tissues. Therefore, we conclude that Mist1 is necessary for complete differentiation and maturation of serous exocrine cells through the combined regulation of several exocrine specific genes.

Molecular cloning and characterization of the neonatal rat and mouse submandibular gland protein SMGC

We report the molecular cloning and characterization of SMGC, a major secretory product and a marker of the type I (terminal tubule) cells of the neonatal rat and mouse submandibular gland. SMGC is expressed in the submandibular gland at high levels through postnatal day 20, but in the adult is present only in some intercalated duct cells. Rat and mouse SMGC have deduced molecular weights of 67.8 and 74.4 kDa, respectively, are 37% Ser+Gly+Thr, and contain tandem repeats of between 8 and 60 amino acids. Secreted SMGC visualized by SDS-PAGE and silver staining is 89 kDa in rat and 105 kDa in mouse, although Western blot analyses with anti-SMGC antisera demonstrate multiple additional lower molecular weight forms. Contributions to the heterogeneity of SMGC include alternate splicing, proteolysis and N-glycosylation. Smgc is localized on rat chromosome 7q34-35 and on mouse chromosome 15E3, both immediately upstream of the high molecular weight salivary mucin, Muc19. Amino acid sequence identity between the signal peptides of SMGC, human MUC19 and pig submaxillary mucin suggest that rat and mouse Smgc and Muc19 arose from a single ancestral mucin gene.

Reengineered salivary glands are stable endogenous bioreactors for systemic gene therapeutics

The use of critical-for-life organs (e.g., liver or lung) for systemic gene therapeutics can lead to serious safety concerns. To circumvent such issues, we have considered salivary glands (SGs) as an alternative gene therapeutics target tissue. Given the high secretory abilities of SGs, we hypothesized that administration of low doses of recombinant adeno-associated virus (AAV) vectors would allow for therapeutic levels of transgene-encoded secretory proteins in the bloodstream. We administered 10(9) particles of an AAV vector encoding human erythropoietin (hEPO) directly to individual mouse submandibular SGs. Serum hEPO reached maximum levels 8-12 weeks after gene delivery and remained relatively stable for 54 weeks (longest time studied). Hematocrit levels were similarly increased. Moreover, these effects proved to be vector dose-dependent, and even a dosage as low as 10(8) particles per animal led to significant increases in hEPO and hematocrit levels. Vector DNA was detected only within the targeted SGs, and levels of AAV copies within SGs were highly correlated with serum hEPO levels (r = 0.98). These results show that SGs appear to be promising targets with potential clinical applicability for systemic gene therapeutics.

Morphogenesis of the human lacrimal gland

The aim of this study was to determine the main stages of the lacrimal gland's developmental process in humans and to establish its precise morphogenetic timetable. Its onset is generally assumed to take place at O'Rahilly's stage 21, arising from an epithelial thickening of the superior extreme of the temporary conjunctival fornix. However, the present study points to a prior stage in the process: the presence of epithelial-mesenchymal changes in embryos at O'Rahilly's stage 19. The study was performed using light microscopy on serial sections of 37 human specimens: 23 embryos and 14 fetuses ranging from 15 to 137 mm crown-rump length (7-116 weeks of development). Three stages in lacrimal gland morphogenesis were identified: (1) the presumptive glandular stage, O'Rahilly's stages 19-20, characterized by a thickening of the superior fornix epithelium together with surrounding mesenchymal condensation; (2) the bud stage, generally assumed to be the first manifestation of glandular origin, characterized initially by the appearance of nodular formations in the region of the superior conjunctival fornix and concluding with the appearance of lumina within the epithelial buds; and (3) the glandular maturity stage, weeks 9-16, the period in which the gland begins to take on the morphology of adulthood.

Androgen regulation of SMR2 gene expression in rat submandibular gland: evidence for a graded but not a binary response

Expression of SMR2, a member of the gene family encoding salivary glutamine/glutamic acid-rich proteins, is regulated by androgens in rat submandibular gland acinar cells. To further characterize SMR2 regulation, we analyzed SMR2 expression during submandibular gland postnatal development and rat puberty at both a global and a single-cell level. Using in situ detection of mature and primary SMR2 transcripts, we show that SMR2 expression is heterogeneous among acinar cells. However, only one cell population with various amounts of mRNAs can be defined. The number of high-expressing cells increases in males during puberty and in females up to 6 weeks of age, suggesting that some factor in addition to acinar differentiation might be important for SMR2 expression in female rats. Involvement of the beta-adrenergic system in regulating SMR2 expression was tested in rats exposed daily to isoproterenol for 4 days. Under these conditions we found an increase in SMR2 expression in female rats, associated with an increase in SMR2 mRNA levels in most acinar cells. This suggests that a signaling cascade, elicited by beta-adrenergic stimuli, might act in concert with androgens to regulate SMR2 expression.

Salivary (SD-type) cystatins: over one billion years in the making--but to what purpose?

Human saliva contains relatively abundant proteins that are related ancestrally in sequence to the cystatin superfamily. Most, although not all, members of this superfamily are potent inhibitors of cysteine peptidases. Four related genes have been identified, CST1, 2, 4 and 5, encoding cystatins SN, SA, S, and D, respectively. CST1, 4, and probably CST5 are now known to be expressed in a limited number of other tissues in the body, primarily in exocrine epithelia, and the term SD-type cystatin is more appropriate than 'salivary cystatin'. These genes are co-ordinately regulated in the submandibular gland during post-natal development. The organization of these tissue-specifically-expressed genes in the genome, and their phylogeny, indicate that they evolved from an ancestral housekeeping gene encoding the ubiquitously expressed cystatin C, and are members of a larger protein family. Their relationship to rat cystatin S, a developmentally regulated rodent submandibular gland protein, remains to be established. In this review, the evolution of the SD-type cystatins in the cystatin superfamily, their genomics, expression, and structure-function relationships are examined and compared with known cystatin functions, with the goal of providing clues to their biological roles.