High-level salivary gland expression in transgenic mice

cDNA cloning, genomic structure, chromosomal mapping, and expression analysis of parotid secretory protein in pig

A novel cDNA has been isolated from pig parotid glands by 3' and 5' rapid amplification of cDNA ends and designated parotid secretory protein (PSP). The open reading frame of this cDNA covers 714 bases, encoding 238 amino acids, which show 56% identity with human PSP at the level of the primary protein structure. The PSP genomic sequence comprises eight exons and seven introns, is approximately 22 kb in size, determined by sequencing, and maps to pig chromosome 17q21-q23. RT-PCR, dot blot, and Northern blot analyses demonstrated that PSP is strongly expressed in parotid glands, but is not present in heart, liver, lung, kidney, muscle, or stomach. A search for functionally significant protein motifs revealed consensus sequences for casein kinase II phosphorylation and N-myristoylation. We observed a unique amino acid sequence pattern consisting of the residues Leu-X(6)-Leu-X(6)-Leu-X(7)-Leu-X(6)-Leu-X(6)-Leu near the amino-terminal portion of the protein, which is similar to the leucine zipper.

Protein-binding elements in the proximal parotid secretory protein gene enhancer essential for salivary-gland-specific expression

The murine parotid secretory protein (PSP) gene is expressed at high levels in the parotid gland and at lower levels in the sublingual gland. A proximal enhancer core necessary for tissue-specific expression was identified previously, and it was demonstrated that one element, parotid gland element I (PGE I), exhibited specific binding of parotid gland nuclear proteins. In the present study, we demonstrate that a related adjacent element, PGE II, which binds nuclear proteins in a much less tissue-restricted manner, is able to compete with PGE I for binding of parotid-gland-specific factors. The functional significance of PGE I and PGE II was examined in transgenic mice. Deletion of PGE II reduced transgene expression only in the parotid gland, whereas deletion of PGE I appeared to reduce expression in both of the PSP-expressing salivary glands. Combined deletion of PGE I and PGE II reduced expression below the limit of detection. Thus PGE I and PGE II are functionally important salivary-gland-specific binding elements that are necessary for the salivary-gland-specific expression of a PSP minigene in transgenic mice.

Low levels of GRP-Ca expression in transgenic mice

The GRP-Ca gene of the rat encodes a member of the glutamine/glutamic-acid-rich protein (GRP) family. This gene is expressed in a highly tissue-specific fashion, with transcription being found only in the acinar cells of the submandibular gland (SMG). To begin to define the cis-acting elements governing GRP-Ca expression, we constructed transgenic mice containing the rat GRP-Ca gene plus 9.5 kb of 5' genomic sequence and 1 kb of 3 genomic sequence. Expression of GRP-Ca was detectable in progeny from only 1 of 3 independent founders. Expression levels of the transgenic GRP-Ca were much lower than the native GRP-Ca found in the rat SMG. Furthermore, GRP-Ca in transgenic mice was not tissue-specifically expressed, being found in both the SMG/SLG complex and the liver. These results indicate that the genomic region of GRP-Ca included in these transgenic mice is not sufficient to confer the high levels of tissue-specific expression seen in the rat.

Structure and chromosomal localization of the rat salivary Psp and Smgb genes

SMGB and PSP are among the most abundant products of the immature acinar cells in developing rat parotid and submandibular glands and are also products of the sublingual gland serous demilunes. Previous analysis of Smgb and Psp cDNA clones demonstrated a high degree of sequence similarity between the signal peptide-encoding and 3' untranslated regions of these transcripts, although the secreted proteins themselves are more divergent. The current study reports the upstream sequences, genomic organization and localization of the Psp and Smgb genes. Both structural genes contain nine exons and are present at 3q41-3q42, where they are arranged in tandem and separated by 21kb. In addition to the previously observed sequence similarity, Psp and Smgb are highly homologous throughout exon 1 and at 365 of 600bp immediately upstream of the transcription start site. These findings indicate that the Psp and Smgb genes arose by tandem duplication and divergence. The similar neonatal submandibular and parotid gland expression patterns observed for these genes are likely to be due to closely conserved or shared enhancer(s).

The main regulatory region in the murine PSP gene is a parotid gland enhancer

The murine PSP gene is expressed at a high-level in the parotid glands. To extend the knowledge of parotid gland expression and develop tools for expression of heterologous proteins in this tissue, the regulation of the PSP gene was studied using transgenic mice. High-level parotid gland expression of the PSP gene was indicated to depend on a novel regulatory region situated between -8.0 and -6.5 kb. Together with previous results this indicates that the main regulatory elements in the PSP gene are situated between -8.0 to -3.1 kb. This region was shown to activate a heterologus SV40 early promoter in the parotid glands of transgenic mice, suggesting that the PSP gene is controlled by enhancer sequences. A novel Psp derived 9.7 kb parotid gland expression cassette, Lama IV, carrying all known regulatory regions in the PSP gene was expressed at high-levels in the parotid glands and should prove highly useful for expression of heterologous proteins in the saliva of transgenic mice.

Novel salivary gland specific binding elements located in the PSP proximal enhancer core

The murine parotid secretory protein (PSP) gene is expressed selectively at high levels in parotid and sublingual salivary glands. Previously, the transcriptional activity of a PSP mini-gene, called Lama, was shown to be dependent on a 1.5 kb region located 3 kb upstream of the transcription start site. Here, functional studies in transgenic mice demonstrate that this proximal regulatory region has properties of a parotid and sublingual gland specific enhancer. Protein-binding experiments identify multiple sequence-specific binding complexes spanning the entire 1.5 kb enhancer region. Several sequence elements bound specifically by parotid and/or sublingual gland nuclear extracts, including consensus binding elements for previously described transcription factors as well as novel binding elements are located in the proximal enhancer region. A deletion analysis of the enhancer region in transgenic mice identified a core sequence of 700 bp. This region contains five elements bound specifically by nuclear proteins isolated from the PSP-expressing parotid and sublingual glands. Two of these elements, denoted parotid gland element I (PGE I) and sublingual gland element I (SLE I), are novel salivary gland specific binding elements, bound uniquely by parotid and sublingual gland nuclear extracts, respectively.

Tissue-specific expression of human salivary mucin gene, MUC7, in transgenic mice

The MUC7 gene encodes the protein core of the low molecular weight human salivary mucin (MG2, mucin glycoprotein 2) and is expressed in a tissue-specific manner in salivary glands. The purpose of this study was to examine MUC7 expression by transgenic mouse technology. A 16 kb DNA fragment, containing the MUC7 gene (10 kb) and 3 kb of the upstream and 3 kb of the downstream sequences, was used to generate transgenic mice. We have identified five transgenic founder mice which were propagated as individual transgenic lines and analysed. Tissues of transgenic offspring from each line were analysed by RT-PCR to determine the sites of the MUC7 expression. The results indicated that only line 3 and line 5 expressed the MUC7 gene in salivary glands. The level of MUC7 expression in selected tissues was then determined by northern blot analyses. The results showed that line 3 mice contained high levels of MUC7 transcripts in the sublingual glands of both males and females and indicated low levels of MUC7 transcripts in the submandibular glands of females. No MUC7 expression was detected in this line by northern blot analysis in any other tissue tested. On the other hand, no expression of MUC7 was detected in any tissues of line 5 mice examined by northern blot analysis. A Southern blot analysis of human and mouse genomic DNA demonstrated multiple copies of the MUC7 transgene in line 3 and a single copy in line 5. Collectively, these results indicate that the regulatory sequences required for the tissue-specific expression of MUC7 are within the MUC7 transgene. However, the sequences necessary for expression comparable to that of MUC7 in human salivary glands may be missing from this construct. Western blot analysis of protein extracts from different tissues of transgenic mice line 3 showed that MUC7 gene product was produced in the submandibular-sublingual gland complex of both male and female mice and not in the other tissues examined.

Characterization of the rat salivary-gland B1-immunoreactive proteins

The B1-immunoreactive proteins (B1-IPs) are major secretory products of rat submandibular gland acinar-cell progenitors, and are also produced by neonatal and adult rat sublingual and parotid glands. In order to characterize the B1-IPs, we have previously isolated cDNA clones encoding rat parotid secretory protein (PSP; the predominant parotid B1-IP) and the related clone ZZ3, which is developmentally regulated in the neonatal submandibular gland. The remainder of the B1-IPs were uncharacterized. This report demonstrates that all of the B1-IPs are derived from the PSP and ZZ3 transcripts. Molecular cloning and Western-blot analyses using PSP- and ZZ3-specific antisera show that, of the B1-IPs, only PSP and neonatal submandibular gland protein A (SMGA) are products of the Psp gene. This finding corrects our previous assertion that SMGA is derived from ZZ3. Neonatal submandibular gland proteins B1 and B2, as well as apparent Mr 26000-28000 and Mr 18000-20000 forms in submandibular, sublingual and parotid glands, are derived from the gene encoding ZZ3 by differential N-glycosylation and by proteolytic cleavage. The apparent Mr 18000-20000 proteolytic products are significant in secretion product collected in vitro, but rare in gland homogenate and submandibular/sublingual saliva. The gene encoding ZZ3 has been named Smgb. Psp and Smgb are regulated similarly in the developing submandibular gland, but differently in the sublingual and parotid glands. The expression pattern of Psp is conserved between rat and mouse. However, no evidence for proteins derived from an Smgb-like gene was observed in neonatal mouse submandibular or sublingual glands.

A cassette for high-level expression in the mouse salivary glands

Expression in the mouse salivary glands may be used as a model system for studies involving oral cavity delivery of gene products. Previously, sequences from the mouse Psp gene were used to build a minigene construct denoted 'Lama'. This construct was used as a cassette for expression of human factor VIII light chain in mouse saliva. However, whereas the endogenous Psp mRNA is the most abundant protein-coding transcript in the parotid glands, the Lama mRNA was expressed below 1% of the level of Psp mRNA in these glands. Here, we show that a 25-kb cosmid-derived DNA fragment (PspX25) carrying the structural gene and large flanking areas of Psp is expressed in all 14 analysed lines in the parotid glands. The average level of transgene expression was estimated to be 45% of that of the endogenous Psp gene. More importantly, it was possible to transfer PspX25's ability for high-level parotid gland expression to the Lama construct.

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.

Molecular cloning and characterization of the gene encoding rat submandibular gland apomucin, Mucsmg

Mucin glycoproteins are a major constituent of salivary secretions and play a primary role in the protection of the oral cavity. Rat submandibular glands (RSMG) synthesize and secrete a low molecular weight (114 kDa) mucin glycoprotein. We have isolated, partially sequenced, and characterized the gene which encodes the RSMG apomucin. The gene is encoded by three exons of 106 nt, 69 nt, and 991 nt, separated by introns of 921 nt and 12.5 kb. CAAT and TATA elements are present, at -68 and -26, respectively, in the 5' flanking sequence of the RSMG apomucin gene. The tandem repeat domain present in exon III consists of ten tandem repeats of 39 nt encoding the consensus sequence PTTDSTTPAPTTK. Sequence comparison and organization of the nucleic acid sequence encoding the tandem repeats of two alleles for this gene suggests that the apomucin gene has undergone recombinational events during its evolution. No significant sequence similarity was found with other mucin genes, or with other known salivary gland-specific genes. The gene was localized to rat chromosome 14 using somatic cell hybrids that segregate rat chromosomes. Since this, to our knowledge, represents the first RSMG mucin gene cloned, we have designated this gene Mucsmg.