Reciprocal expression of c-jun, proline-rich protein and amylase genes during rat parotid salivary gland 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.

Development of the rat sublingual gland: a light and electron microscopic immunocytochemical study

Cell differentiation in the rat sublingual gland occurs rapidly and is largely complete by birth. To study differentiation of the serous and mucous cells of the sublingual gland, we used antibodies to the secretory proteins CSP-1, SMGB, PSP, and SMGD, and sublingual mucin as specific cell markers. Glands from rats at ages 18, 19, and 20 days in utero, and postnatal days 0, 1, 5, 9, 14, 18, 25, 40, and 60 were fixed and prepared for morphological analysis and immunocytochemical labeling. At age 18 days in utero, a few cells in the developing terminal bulbs contained mucous-like apical granules that labeled with anti-mucin. Other cells had mixed granules with a peripheral lucent region and a dense core of variable size that occasionally labeled with anti-SMGD. Additionally, presumptive serous cells with small dense granules that contained CSP-1 and SMGB were present. At age 19 days in utero, the dense granules of these cells also labeled with anti-SMGD. By age 20 days in utero, mucous cells were filled with large, pale granules that labeled with anti-mucin, and serous cells had numerous dense granules containing CSP-1, SMGB, PSP, and SMGD. Fewer cells with mixed granules were seen, but dense regions present in some mucous granules (MGs) labeled with anti-SMGD. After birth, fewer MGs had dense regions, and serous cells were organized into well-formed demilunes. Except for PSP, which was undetectable after the fifth postnatal day, the pattern of immunoreactivity observed in glands of neonatal and adult animals was similar to that seen by age 20 days in utero. These results suggest that mucous and serous cells have separate developmental origins, mucous cells differentiate earlier than serous cells, and cells with mixed granules may become mucous cells.

Secretory protein expression patterns during rat parotid gland development

The rat parotid gland produces a number of well-characterized secretory proteins. Relatively little is known, however, about the onset of their synthesis and cellular localization during gland development. Secretory protein expression was studied in parotid glands of fetal and postnatal rats using light and electron microscopic immunocytochemistry and Northern blotting. Amylase, parotid secretory protein (PSP), common salivary protein-1 (CSP-1), and SMGB were first detected by immunofluorescence in parotid glands of 18 day fetuses. By 5 days after birth, light and electron microscopic immunolabeling localized all of these proteins to the secretory granules of developing acinar cells. Labeling of acinar cells for DNAse I, however, was not observed until 18 days after birth. Between 9 and 25 days, CSP-1 and SMGB reactivity of acinar cells declined, but increased in intercalated duct cells. After 25 days, CSP-1 and SMGB were found only in intercalated ducts, and amylase, PSP, and DNAse I were restricted to acinar cells. Levels of CSP-1 and SMGB mRNA were relatively constant through 21 postnatal days, but declined significantly after that. Amylase and PSP mRNA increased rapidly and continuously from five days after birth to the adult stage. In contrast, DNAse I mRNA was not detectable until 18 days after birth. The immunocytochemical and molecular analyses define three basic patterns of protein expression in the rat parotid gland: proteins whose synthesis is initiated early in development and is maintained in the acinar cells, such as amylase and PSP; proteins that are initially synthesized by immature acinar cells but are restricted to intercalated ducts in the adult gland, such as CSP-1 and SMGB; and proteins that are synthesized only by mature acinar cells and first appear during the third postnatal week, such as DNAse I. The parotid gland exhibits four distinct developmental stages: prenatal, from initiation of the gland rudiment until birth; neonatal, from 1 day up to about 9 days postnatal; transitional, from 9 days to 25 days of age; and adult, from 25 days on. Although differences exist in timing and in the specific proteins expressed, these developmental stages are similar to those seen in the rat submandibular gland. Additionally, the results support the suggestion that intercalated ducts may differentiate from the neonatal acini.

Transcriptional regulation of salivary proline-rich protein gene expression

Mechanisms governing gene expression and regulation in eukaryotes are remarkably complex. The results from in vivo transgenic and in vitro transfection studies designed to identify cis-element(s) and trans-factor(s) associated with the salivary proline-rich proteins (PRPs) gene expression are utilized as a paradigm to discuss the regulation of salivary-specific gene expression. Particular attention is given to the molecular mechanism(s) underlying the salivary PRP R15 gene regulation. In rodents, the PRPs are selectively expressed in the acinar cells of salivary glands, and are inducible by the beta-agonist isoproterenol as well as by dietary tannins. The results from a series of experiments using chimeric reporter constructs containing different lengths of the R15 distal enhancer region, their mutations, and various expressing constructs are analyzed and discussed. These data suggest that the inducible nuclear orphan receptor NGFI-B may participate in the regulation of salivary acinar cell-specific and inducible expression of the rat R15 gene via three distinct distal NGFI-B sites. Taken together, a model for the induction of R15 gene expression by isoproterenol is proposed. However, the exact molecular basis of this NGFI-B-mediated transactivation of cAMP-regulated R15 expression remains to be established.

Regulation of salivary-gland-specific gene expression

The results from in vivo transgenic and in vitro transfection studies designed to identify cis-element(s) and transfactor(s) governing the salivary proline-rich proteins (PRPs), amylase, and parotid secretory protein (PSP) gene expression are utilized as a paradigm to discuss the regulation of salivary-specific gene expression. Particular attention is given to the molecular mechanism(s) underlying the salivary PRP R15 gene regulation. In rodents, the PRPs are selectively expressed in the acinar cells of salivary glands, and are inducible by the beta-agonist isoproterenol and by dietary tannins. The results from a series of experiments using chimeric reporter constructs containing different lengths of the R15 distal enhancer region, their mutations, and various expressing constructs are analyzed and discussed. These data suggest that the inducible nuclear orphan receptor NGFI-B may participate in the regulation of salivary acinar-cell-specific and inducible expression of the rat R15 gene via three distinct distal NGFI-B sites. Taken together, a model for the induction of R15 gene expression by Ipr is proposed. However, the exact molecular basis of this NGFI-B-mediated transactivation of cAMP-regulated R15 expression remains to be established.

Proline-rich-protein promoters direct LacZ expression to the granular convoluted tubular cells of the submandibular gland in adult transgenic mice

The ability of two mouse PRP gene promoters to direct the expression of the bacterial lacZ reporter gene was tested in transgenic mice. Transgenes A1-lacZ and C1-lacZ consisted of 8.2 kb A1 and 7.8 kb C1 PRP promoters respectively fused to the lacZ coding sequence. A1 and C1 are two A-type PRP genes isolated from the inbred SWR mice, which show the same gene structure and similar sequence to the closely related MP2 and M14 PRP genes previously cloned from outbred CD-1 mice. We here show that both A1-lacZ and C1-lacZ transgenes have very similar expression patterns: (1) they expressed the lacZ gene in all 14 established transgenic lines under normal (non-stimulated) conditions; (2) the expression was restricted to the granular convoluted tubular cells of the submandibular glands; (3) the expression was developmentally regulated beginning at sexual maturation and lasting to at least 1.5 years of age; and (4) expression in some lines was probably influenced by sex hormones, since higher expression was found in males than in females. A1-lacZ and C1-lacZ are the first transgenes derived from the PRP/GRP (glutamine/glutamic acid-rich protein) gene superfamily to be expressed in the granular convoluted tubular cells (with known endocrine functions), rather than in the acinar cells (with mainly exocrine functions) of the submandibular glands.

Involvement of nuclear orphan receptor NGFI-B in transcriptional activation of salivary-specific R15 gene by cAMP

Proline-rich proteins (PRPs) are selectively expressed in the acinar cells of the salivary glands and are inducible by beta-agonist isoproterenol and dietary tannins. In the previous studies of rat PRP gene, R15, the 5'-flanking region up to -1.7 kilobase pairs (kb), which was thought to contain the necessary proximal regulatory elements, failed to confer the catecholamine isoproterenol and dietary tannin inducibility to the transgene expression in the salivary glands of transgenic mice. Here we analyzed distal 5'-flanking region of R15 in order to understand the mechanisms of tissue-specific and inducible gene regulation. An upstream regulatory region located between -2.4 and -1.7 kb of the R15 5'-flanking region is demonstrated to be indispensable for the salivary-specific and inducible reporter gene expression in vivo, by transgenic approach. Element(s) within the 0.7-kb (-2.4 to -1.7) region that is able to cis-activate the expression of a heterologous reporter gene expression is further elucidated by transient transfection assays in vitro. Three distinct nuclear orphan receptor NGFI-B regulatory sequences are identified within a 184-base pair (bp) minimal control region extended from -1995 to -1812 nucleotides relative to the transcription start site. When reporter gene containing this 184-bp control region and heterologous promoter was cotransfected with the NGFI-B expression construct, a transactivation that mimics the effect of cAMP is observed in the parotid cells. Finally, mutations on all three identified NGFI-B binding sites and coexpression of a dominant negative mutant construct, pCMV-NGFI-B(Delta25-195), abolish this transactivation mediated by NGFI-B. In summary, these data suggest that the inducible nuclear orphan receptor NGFI-B may participate in the regulation of salivary acinar cell-specific and inducible expression of the rat R15 gene via three distinct distal NGFI-B sites.

Temporal and spatial expression of laminin, collagen types IV and I and alpha 6/beta 1 integrin receptor in the developing rat parotid gland

We have examined the temporal expression and cellular localization of the genes and proteins for the extracellular matrix (ECM) proteins laminin (B1, B2 and A chain), collagen types alpha 1 (IV) and alpha 1 (I) and the integrin receptor complex alpha 6/beta 1, during parotid gland postnatal development. Laminin B1 and B2 isoforms and collagens alpha 1 (IV) and alpha 1 (I) mRNA steady-state levels were highest at ages 0, 7 and 14 days after birth and declined to the adult (90 days) level at 21 days and older. Laminin A chain transcripts were not detected at any age. Collagen alpha 1 (IV) and laminin were localized in the basal membrane of the developing acinar and ductal cells, while collagen alpha 1 (I) was localized in the stroma surrounding the cells. The amounts of these ECM components were high at the early stages of development and lower at later times. The pattern of expression of the alpha 6/beta 1 integrin genes during development was similar to those of laminin and collagens alpha 1 (IV) and alpha 1 (I). Accumulations of mRNA were high at 0, 7 and 14 days after birth and lower at 21 days and older. High levels of beta 1 integrin were localized in the developing acinar and ductal cell membranes at early ages (7 days); lower amounts were present in the same distribution pattern at later stages of gland development.(ABSTRACT TRUNCATED AT 250 WORDS)