Molecular cloning of cDNAs from androgen-independent mRNA species of DBA/2 mouse sub-maxillary glands

Self-organization and branching morphogenesis of primary salivary epithelial cells

Embryonic tissues may provide clues about mechanisms required for tissue reassembly and regeneration, but few studies have utilized primary embryonic tissue to study tissue assembly. To test the capacity of tissue fragments to regenerate, we cultured fragments of embryonic day 13 (E13) mouse submandibular gland (SMG) epithelium and found that fragments as small as a quarter-bud retain the ability to branch. Further, we found that completely dissociated SMG epithelial cells self-organize into structures that undergo significant branching. Investigation into the mechanisms involved in tissue self-assembly demonstrated that inhibition of beta(1) integrin prevents cell aggregation, while inhibition of E-cadherin hinders aggregate compaction. Immunostaining showed that the cellular architecture and expression patterns of E-cadherin, beta-catenin, and actin in the reassembled aggregates mirror those seen in intact glands. Adding SMG mesenchymal cells to the epithelial cell cultures facilitates branching and morphological differentiation. Quantitative real-time RT-PCR indicated that the aggregates express the differentiation markers aquaporin-5 (AQP5), prolactin-inducible protein (PIP), and SMG protein C (SMGC). Together, these data show that dissociated SMG epithelial cells self-organize and undergo branching morphogenesis to form tissues with structural features and differentiation markers characteristic of the intact gland. These findings provide insights into self-assembly and branching that will facilitate future regeneration strategies in the salivary gland and other organs.

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.

Expression of the mouse homologue for the human GCDFP-15/PIP gene during pre- and early post-natal development

The function of the mouse submaxillary gland/prolactin inducible protein (mSMGP/mPIP), the homologue of the human gross cystic disease fluid protein 15 (GCDFP-15)/prolactin inducible protein (hPIP) remains unknown. The human gene, normally expressed in apocrine glands of healthy individuals, is aberrantly expressed in human breast cancers where it is regulated by hormones including androgens, and in prostate cancers. We have previously reported that in the adult mouse and rat, gene expression is tissue-specific for the salivary and lacrimal glands, and is hormonally regulated. In this study, we examine the endogenous pattern of mouse SMGP/PIP (mSMGP/mPIP) gene expression in mid- and late-embryonic, and in early postnatal development. Gene expression was analyzed by RT-PCR followed by Southern blot analysis, and by in situ hybridization. Gene expression was detected in the submandibular gland as early as embryonic day 14 (E14), a period that coincides with the initiation of submandibular gland development in the embryo, suggesting that mSMGP/mPIP may have a functional role in the developing gland. Nearing the end of gestation, E18, mSMGP/mPIP transcripts were localized in the proacinar cells of the gland, and gene expression continued to be maintained following birth. In addition, during early postnatal development, mSMGP/mPIP gene expression was detected in the other two major salivary glands, the sublingual and parotid, as well as in the lacrimal gland and in reproductive tissues. In the prostate, gene expression was turned off by 10 weeks of age. The spatial and temporal pattern of the mSMGP/mPIP gene expression, in addition to our recent demonstration that mSMGP/mPIP is found in mouse saliva and can bind bacteria, suggest that this protein may have a protective role in the mouse.

Identification of mouse submaxillary gland protein in mouse saliva and its binding to mouse oral bacteria

The mouse submaxillary gland protein (mSMGP) is highly expressed in the submandibular gland of the adult mouse and rat. It shares 51% identity at the amino-acid level with a human protein, the prolactin-inducible protein (PIP)/gross cystic disease fluid protein 15 (GCDFP-15), which has been found in saliva, tears, sweat, seminal plasma, submucosal glands of the lung and amniotic fluid. More recently, the human PIP has been reported to bind to bacterial strains normally found in the mouth, ear canal and human skin. Sequence analysis of mSMGP/PIP earlier identified the presence of a signal peptide, suggesting that it is a secreted protein. Here, by Western blotting, mSMGP/PIP has been identified in mouse saliva. To investigate further the role of this secreted protein, its ability to bind specifically to oral bacteria was examined; the hypothesis was that mSMGP/PIP is involved in non-immune host defence by binding to bacteria. Several bacterial strains, found to belong to the genera Streptococcus, Aerococcus, Pseudomonas, Staphylococcus, Sphingomonas, Vibrio and Aeromonas, were isolated from the mouse oral cavity. Following incubation of these bacteria with (35)S-labeled, in vitro-translated mSMGP/PIP, the protein was found to bind specifically and selectively to several but not all strains tested, showing the highest affinity for the streptococci. The protein also bound specifically to an Aerococcus sp., and a low binding interaction with the Pseudomonas and Staphylococcus spp. was observed. The conservation of SMGP sequences among several animal species suggests that this protein may play an important part in the biology of the submandibular gland. As the function of the mSMGP/PIP is still undetermined, these findings provide insight into a possible involvement of this protein in host defence.

Expression of gross cystic disease fluid protein-15/Prolactin-inducible protein in rat salivary glands

Gross cystic disease fluid protein-15 (GCDFP-15)/prolactin-inducible protein (PIP) is present at moderate levels in human submandibular and sublingual glands and is barely detectable in human parotid gland. The rodent homologue, PIP, has previously been identified in adult submandibular and lacrimal glands. Here we present the molecular characterization of rat PIP and show that this protein is a product of neonatal and adult rat submandibular, sublingual, and parotid glands. cDNA clones encoding rat PIP were isolated and sequenced. The deduced amino acid sequence of rat PIP shows 56% overall identity and 80% similarity with mouse PIP. By SDS-PAGE, secreted rat PIP has an apparent Mr of 17,000, with a minor proportion present as Mr 20-22,000 N-glycosylated forms. PIP was localized in rat salivary glands by immunogold silver staining. PIP was identified in acinar cells of developing and mature submandibular and parotid glands and at very low levels in sublingual gland serous demilunes. Typically, rat submandibular gland secretory proteins are produced by either acinar cell progenitors (Type III cells) or mature acinar cells. The expression pattern observed for PIP is similar to that previously reported for salivary peroxidase, an important component of nonimmune mucosal defense.

A 20-kDa protein in human seminal plasma that is identical to gross cystic disease fluid protein 15 and prolactin-inducible protein

A new method is described for isolating the 20-kDa protein, which used to be purified using an actin-immobilized column from human seminal plasma. This method employed ion-exchange column chromatography and isoelectric focusing separation. The level of seminal protein was determined by sandwich ELISA to be 1.06 +/- 0.27 mg/mL (mean +/- SD, n = 8). Its cDNA was cloned from a human salivary gland cDNA library by immunoscreening. The 553-nucleotide sequence included the 5' untranslated region and extended to the poly(A) tail. It encoded a protein of 118 amino acid residues in addition to a signal sequence of 28 residues. This sequence was identical to those of gross cystic disease fluid protein 15 and prolactin-inducible protein cDNAs. Northern blotting revealed the common expression in the submandibular gland and seminal vesicle. An immunohistochemical study in paraffin-embedded tissues from human male sex organs also indicated that the 20-kDa protein is mainly produced in the seminal vesicle.

EP-GP and the lipocalin VEGh, two different human salivary 20-kDa proteins

Two salivary 20-kDa proteins [the human lipocalin Von Ebner's gland protein (VEGh) and extraparotid glycoprotein (EP-GP)] show several remarkable similarities and differences. The latter is identical to secretory actin-binding protein (SABP), gross cystic disease fluid protein-15 (GCDFP-15), prolactin-induced protein (PIP), and 17-kDA CD4-binding glycoprotein (gp17). Much is known about the distribution, localization, biochemical characteristics, and molecular biology of these two proteins, yet there are only few clues about their functions.

Identity of human extra parotid glycoprotein (EP-GP) with secretory actin binding protein (SABP) and its biological properties

In this paper the identity of the salivary protein EP-GP (extra-parotid glycoprotein) is reported, also apparent in other human secretions. Immunochemical and biochemical analysis demonstrated that EP-GP is similar to the secretory actin-binding protein (SABP), also known as gross cystic disease fluid protein-15 (GCDFP-15) and prolactin-inducible protein (PIP). The molecular mass and charge microheterogeneity of EP-GP, also observed for SABP, was shown to be predominantly caused by the carbohydrate moiety. In addition, evidence was given that EP-GP is not related to the lipocalin Von Ebner's gland protein (human; VEGh). The biological significance of EP-GP and its homologues is not clear. EP-GP bound to actin and fibrinogen as described for SABP and GCDFP-15. However, the affinity for these proteins does not appear to have any direct physiological role in the mucosal secretions. On the other hand, EP-GP binds to several bacteria. By electron microscopy the ultrastructural localization is demonstrated of EP-GP to the cell wall of both Streptococcus salivarius HB and its cell appendage-lacking mutant Streptococcus salivarius HB-C12. Concerning this finding we hypothesize on the possible functional aspects of this enigmatic protein EP-GP.

Three novel SMR1-related cDNAs characterized in the submaxillary gland of mice show extensive evolutionary divergence in the protein coding region

We have previously characterized an abundant male-specific mRNA from the submaxillary gland (SMG) of rats, encoding the SMR1 (androgen-regulated) protein, which has the structure of a prohormone and is processed by maturation enzymes to release a small peptide in the blood and saliva. We have now characterized three SMR1-related cDNAs in the SMG of Balb/c mice. These cDNAs encode three novel proteins, designated MSG1, MSG2 and MSG3. They are 639, 662 and 471 nucleotides (nt) long, respectively, and the corresponding mRNAs appear to be expressed only in the SMG. The putative polypeptides they encode carry an N-terminal secretory peptide sequence and are, therefore, presumably secreted into saliva. Although closely related, the three mRNAs show striking differences: a particularly different expression pattern and an extremely high degree of variability observed in the central part of the molecules. The MSG1 and MSG3 cDNAs are identical, except for a 173-bp insert found only in MSG1. This insert contains three Pro-rich repeats (GPGIGRPPPPPP), reminiscent of the most abundant multigenic family of the SMG, the Pro-rich proteins (PRP). Although MSG1 shares several common features with PRP, it is structurally related to SMR1. The unusually high ratio of replacement/silent nt changes provides a basis to address complex aspects concerning the molecular events leading to the emergence of new proteins in the SMG.

Primary structure of a new actin-binding protein from human seminal plasma

Secretory actin-binding protein (SABP), a glycoprotein from human seminal plasma, was isolated according to Akiyama and Kimura [Akiyama, K. & Kimura, H. (1990) Biochim. Biophys. Acta 1040, 206-210]. The complete amino acid sequence of SABP was determined with the aid of fragments generated by trypsin, Staphylococcus aureus V8 protease and pepsin. The single polypeptide chain of SABP contains 118 amino acids with a calculated Mr of 13,506 and pyroglutamic acid as the N-terminal residue. A single N-glycosidic carbohydrate moiety is located at Asn77. The carbohydrate composition shows an unusually high amount of fucose. The arrangement of the two disulfide bonds is Cys37-Cys63 and Cys61-Cys95. Sequence comparison revealed a high degree of similarity with a 14-kDa submandibular gland protein from mouse (45% identity and 64% similarity). SABP is identical with a prolactin-inducible protein and a protein termed gross cystic disease fluid protein 15 (sequences translated from cDNA clones), both from human breast tissues. Although SABP was also detected in saliva, in extracts of the submandibular gland and seminal vesicles, little is known of its function.

Genetic and tissue-specific variation in the expression of a closely linked murine multigene family on chromosome 15 that encodes salivary and lacrimal proteins

The murine submandibular gland (SMG) produces a novel class of highly acidic salivary proteins encoded by one or more highly abundant mRNA transcripts. In inbred mice, these transcripts are encoded by members of a multigene family comprising approximately 8-12 homologues. Most, and probably all, of these homologues are clustered at a new locus near belted (bt) on chromosome 15, which we designate Spt (salivary protein). Although physically closely linked, Spt genes differ in their patterns of expression both in strains of mice and in their tissues. One gene, Spt-1, is expressed at high levels in the SMG of all inbred strains examined. This gene is also expressed at significant levels in the lacrimal gland. A second gene, Spt-2, appears to be present as a single copy in some strains and as two copies in others. This gene is expressed at high levels only in the SMG of those strains carrying two copies, and Spt-2 mRNA is not detectable in the SMG of strains carrying only one copy. In contrast to Spt-1, the Spt-2 gene is not expressed at detectable levels in the lacrimal gland.

High level of accumulation of a mRNA coding for a precursor-like protein in the submaxillary gland of male rats

NaDodSO4/PAGE analysis of in vitro translation products of rat submaxillary gland (SMG) mRNAs has revealed an important sexual dimorphism. Moreover, most of the rat male-specific major translation products differ in size from those translated from male mouse SMG mRNAs. To characterize proteins accumulated in the rat SMG under androgen control, a cDNA library was constructed. Here we report the nucleotide sequence of a 0.7-kilobase mRNA that is 1000-3000 times more abundant in male rats than in female rats. The predicted corresponding protein, SMR1, has a molecular weight of 16,000 and contains a signal peptide for secretion and potential signals for glycosylation. An interesting feature of SMR1 is the presence, in a hydrophilic region, of the tetrapeptide Gln-His-Asn-Pro surrounded by two pairs of basic residues that represent potential cleavage sites for maturation enzymes. In rats, the tissue distribution of the SMR1 mRNA is restricted to the SMG and the prostate. Only very low amounts of SMR1 mRNA can be detected in the SMG of male or female mice. Southern blot analysis indicates the presence of three genes in rats but only one in mice. Hypotheses on the physiological role of SMR1-derived peptides in male rats are discussed.

A mouse gene family associated with a major submaxillary-gland glycoprotein

A gene family encoding msp36 (mouse submaxillary protein 36), a major product of the mouse submaxillary gland, was shown by Southern analysis and genomic cloning to contain approx. 10 related genes. Heteroduplex mapping has elucidated the structure of one of the genes. Close physical linkage of four of the genes in this family has been demonstrated by the use of specific oligonucleotide probes.