An unusual sexually dimorphic mosaic distribution of a subset of kallikreins in the granular convoluted tubule of the mouse submandibular gland detected by an antibody with restricted immunoreactivity

Sex-dependent Regeneration Patterns in Mouse Submandibular Glands

Our previous studies indicated that YIGSR-A99 peptides chemically conjugated to fibrin hydrogel (FH) and applied to wounded submandibular gland (SMG) in vivo, formed new organized salivary tissue, whereas wounded SMG treated with FH alone or in the absence of a scaffold showed disorganized collagen formation and poor tissue healing. While these studies indicated that damaged SMG grow and differentiate when treated with FH containing L1 peptide, they were performed only in female mice. However, there is a well-established sexual dimorphism present in mouse SMG (e.g., males develop well-differentiated granular convoluted tubules, but these structures are poorly developed in females) and little is known about how these sex differences influence wound healing events. Therefore, the goal of this study was to conduct comparative analyses of regeneration patterns in male and female mice using L_1p-FH in a wounded SMG mouse model. Particularly, we focused on sex-dependent wound healing events such as macrophage polarization, vascularization, tissue organization, and collagen deposition, and how these events affect salivary gland functioning.

Transcriptome analysis reveals potential immune function-related regulatory genes/pathways of female Lubo goat submandibular glands at different developmental stages

Background

The submandibular glands, as major salivary glands, participate in rumen digestion in goats. Sialic acid, lysozyme, immunoglobulin A (IgA), lactoferrin and other biologically active substances secreted in the submandibular glands were reported in succession, which suggests that the submandibular gland may have immune functions in addition to participating in digestion. The aim of this study was to map the expression profile of differentially expressed genes (DEGs) at three different stages by transcriptome sequencing, screen immune-related genes and pathways by bioinformatics methods, and predict the immune function of submandibular glands at different developmental stages.

Methods

Nine submandibular gland tissue samples were collected from groups of 1-month-old kids, 12-month-old adolescent goats and 24-month-old adult goats (3 samples from each group), and high-throughput transcriptome sequencing was conducted on these samples. The DEGs among the three stages were screened and analysed. Key genes and signalling pathways were selected via protein-protein interaction (PPI) network analysis.

Results

The results revealed 2,706, 2,525 and 52 DEGs between 1-month-old and 12-month-old goats, between 1-month-old and 24-month-old goats, and between 12-month-old and 24-month-old goats, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses indicated that most of the DEGs were enriched in immune- related GO terms and pathways. Based on functional enrichment analysis and network analysis, 10 genes (PTPRC, CD28, SELL, LCP2, MYC, LCK, ZAP70, ITGB2, SYK and CCR7), two signalling pathways (the T cell receptor signalling pathway and the NF-κβ signalling pathway) and eight GO terms (T cell receptor signalling pathway, neutrophil mediated immunity, B cell mediated immunity, regulation of alpha-beta T cell activation, positive regulation of T cell proliferation, regulation of leukocyte differentiation, positive regulation of antigen receptor-mediated signalling pathway, positive regulation of lymphocyte proliferation) that may play key roles in the immune functions of the goat submandibular glands at different developmental stages were identified. Moreover, we found that eight antibacterial peptide-encoding genes were downregulated in the tuberculosis and salivary secretion pathways, while all immunoglobulins were upregulated in 10 immune system pathways. These findings indicate that the submandibular glands may be important immunological organs during the growth process of goats and that the immune function of these glands gradually weakens with age up to 12 months but remains relatively stable after 12 months of age. Overall, this study will improve our understanding of transcriptional regulation related to goat submandibular gland immune function.

Reversible Conversion among Subtypes of Salivary Gland Duct Cells as Identified by Production of a Variety of Bioactive Polypeptides

Four major kallikreins (mK1, mK22, mK9, and mK13) were identified in the mouse submandibular gland (SMG). mK1, a true tissue kallikrein, was used as a protein marker to identify different types of SMG granular convoluted tubule (GCT) cells along with epidermal growth factor (EGF), nerve growth factor (NGF), and renin. Kallikrein mK1 was localized in a very small number (~5%) of GCT cells, which were scattered throughout the GCT, indicating that the majority of GCT cells are mK1-negative. Among mK1-positive cells, particularly strong signals were observed in a small number of narrow cells, recognized as slender granular cells (SG cells, Type IV), in the GCT. After postnatal development of the SMG, GCT cells are no longer uniform based on the bioactive substances (mK1, EGF, NGF, and renin) that they produce and secrete. GCT cells were classified into four subtypes, Types I–IV, and it became clear that these subtypes are complicatedly and reversibly converted by the endocrine hormones 5α-dihydrotestosterone (DHT) and triiodothyronine (T₃). Duct segments with similar morphology or hormone dependency were recognized in the sublingual and parotid glands. The presence of duct cells with such characteristics is therefore a common feature of the three major salivary glands of rodents.

Androgen Regulation of the Cellular Distribution of the True Tissue Kallikrein mK1 in the Submandibular Gland of the Mouse

The kallikrein gene family encodes for at least four different proteases in the mouse submandibular gland (SMG): mK1 (true tissue kallikrein), mK9, mK13, and mK22. These enzymes and many other biologically active proteins are synthesized by the granular convoluted tubule (GCT), a specialized segment of the SMG duct system. The GCT is under multihormonal regulation by androgens, thyroid hormones, and adrenocortical hormones. Androgens suppress synthesis of mK1 in the SMG but enhance expression of the other three kallikreins. We prepared an antibody with limited immunoreactivity for mK1 and used it to examine the effects of androgen status on the distribution of this isozyme in the SMGs of developing and mature mice by immunoperoxidase staining for the light microscope and immunogold labeling for the electron microscope. In prepubertal mice, every immature GCT cell contains mK1, confined to an accumulation of small granules in the subluminal cytoplasm. In mature mice, not every GCT cell contains mK1, and in those cells that do there is considerable intergranular variation in the intensity of staining for mK1. GCT cells containing mK1 are much more abundant in the glands of females than of males, resulting in a peculiar sexually dimorphic mosaic distribution of this isozyme in the mature SMG. Castration of adult males increases the number of GCT cells expressing mK1. Administration of androgen to intact or castrated males or to intact females reduces the number of cells staining for mK1. In all cases, immunogold labeling for mK1 is confined to secretory granules. No fine structural differences were noted between cells that were positively or negatively stained for mK1. Therefore, although GCT cells appear to be composed of a uniform population of cells on the basis of morphology alone, they are not homogeneous in their content of secretory proteins. These results indicate that androgen regulation of GCT cells is more complex than has been appreciated to date.

Immunocytochemical study of granular duct cells with a hormonally enhanced granular cell phenotype in the mouse parotid gland

In the parotid glands (PGs) of intact male mice (12 weeks of age, ICR strain), immunofluorescence labels for a true tissue kallikrein, mK1, and for nerve growth factor (NGF) were recognized through the subluminal edges of the striated duct (SD) segments and interlobular duct segments. Because of their small size, secretory granules were not detectable by light microscopy in any of the duct cells. Full-fledged granular cells, containing large secretory granules that were visible by light microscopy, were induced in the SD segments of male mice after the injection of 5alpha-dehydrotestosterone (DHT) and triiodothyronine (T(3)), given either alone or in combination every other day for 2 weeks. A stronger effect was detected in the mice that were concomitantly injected with DHT and T(3), and more abundant, fully developed granular cells appeared in the SD segments of these mice. These full-fledged granular cells were immunoreactive for mK1, NGF, and epidermal growth factor, but not for renin. The present results indicate that some of the SD cells with small granules in the mouse PG can develop a granular cell phenotype, producing more kinds of growth factors, as a result of the actions of androgen and thyroid hormone.

Hypophysectomy and hormonal therapy modulate mK1-immunoreactive duct cells in the mice sublingual glands

The immunocytochemical localization of a true tissue kallikrein, mK1, in mouse sublingual glands (SLGs) was examined following hypophysectomy and hormonal replacement therapy. In the glands of intact mice (14 weeks of age), mK1 was detected in the striated ducts (SDs). Full-fledged granular cells were scattered in the SDs of male mice (but not in those of female mice), showing a cellular mosaic distribution of mK1 with some being positive and others being negative. mK1 was also detected in transitional-type granular cells, though the secretory granules were too small and scarce to be visible by a light microscopy. Hypophysectomy in male mice resulted in the atrophy and loss of secretory granules in many SD cells. Granulation recovered after the repeated injection of 5alpha-dihydrotestosterone (DHT), 3,5,3'-triiodo-L: thyronine (T3), and dexamethasone (Dex), given either alone or in combination to the hypophysectomized mice. The concomitant injection of DHT and T3, with or without Dex, resulted in the reappearance of the full-fledged granular cells, only some of which were mK1-positive. Electron microscopy revealed mK1 to be present exclusively in the secretory granules of these mK1-positive cells, and no ultrastructural differences were observed between mK1-positive and mK1-negative full-fledged granular cells. These results show that the differentiation of the granular cell phenotype in the mouse SLG duct system requires the concomitant action of androgen and thyroid hormone and retards mK1 synthesis.

Repeated androgen and thyroid hormone injection modulates the morphology of hormone-responsive duct cells in the mouse parotid gland

When the parotid glands of normal male and female ICR mice (12 weeks of age) were examined under a light microscope, no granular cells were seen in the duct system. However, transmission electron microscopy revealed that, in both sexes, many striated duct cells contained a few electron-dense secretory granules in their subluminal cytoplasm and had formed so-called granular striated tubules (GSTs) in some of the striated duct segments. These secretory granules were not large enough to be visible with a light microscope. Fully fledged granular cells, containing large secretory granules visible with a light microscope, could be induced in the GST segments of the glands of males by injection with 5alpha-dihydrotestosterone (DHT), triiodothyronine (T(3)), and dexamethasone (Dex), given alone or in combination every other day for 2 weeks. Dex alone showed no effect on the GSTs in this study. Both DHT and T(3), either individually or with Dex, were moderately effective, inducing a few scattered fully fledged granular cells. A stronger effect was detected after concomitant injection of DHT and T(3), with or without Dex, with more abundant fully developed granular cells appearing in the GST segments. Electron microscopy revealed that these granular cells had abundant large secretory granules in their apical two-thirds, a basal nucleus, and modest basal infoldings. By contrast, the effect of the same hormones was very weak in the glands of females, and even the concomitant injection of DHT and T(3), with or without Dex, rarely induced fully fledged granular cells. These results indicate a close similarity between the ductal systems of the major salivary glands of the mouse, in terms of some of the striated duct segments containing secretory granules, being under the same multihormonal regulation, and being sexually dimorphic.

Tissue kallikrein mK13 is a candidate processing enzyme for the precursor of interleukin-1beta in the submandibular gland of mice

By using Western blot analysis, high levels of 17.5- and 20-kDa interleukin-1beta (IL-1beta) proteins were detected in the submandibular gland (SMG) of mice. Despite this fact, the amount of pro-IL-1beta protein, a precursor of IL-1beta, with a molecular size of 35 kDa in this tissue was below the detectable level, although strong expression of pro-IL-1beta mRNA was observed. A large amount of 17.5-kDa IL-1beta also appeared in the saliva of mice injected with lipopolysaccharide, suggesting that this IL-1beta is a secretory form produced by the SMG. The protein for IL-1beta-converting enzyme, a processing enzyme for pro-IL-1beta, was expressed only at a low level in the SMG as compared with its level in various epithelial tissues or lipopolysaccharide-stimulated macrophages. On the other hand, mK1, mK9, mK13, and mK22, members of the kallikrein family, were detected strongly in the SMG but not in other tissues. By incubation with mK13, but not with mK1, mK9, or mK22, the 35-kDa pro-IL-1beta was cleaved into two major products with molecular masses of 17.5 and 22 kDa, and production was inhibited by phenylmethylsulfonyl fluoride, a serine protease inhibitor, but not by IL-1beta-converting enzyme inhibitors. A peptide segment corresponding to amino acid residues 107-121 of mouse pro-IL-1beta (107WDDDDNLLVCDVPIR) was cleaved by incubation with mK13, generating two peptides, 107WDDDDNL and 114LVCDVPIR. Therefore, kallikrein mK13 would appear to hydrolyze pro-IL-1beta between its Leu113 and Leu114 residues. The results of immunohistochemistry and an autonomic therapy experiment showed that IL-1beta and kallikrein mK13 were co-localized in the secretory granules of granular convoluted tubular cells. Our present results thus suggest kallikrein mK13 is a plausible candidate for the processing enzyme for pro-IL-1beta in the SMG of mice.

Dynamic rearrangement of telomeres during spermatogenesis in mice

Chromosomal structure within the nucleus influences various biological processes such as transcription and replication. Telomeres are located at the end of eukaryotic chromosomes and they can be a decisive factor for correct chromosomal positioning. To gain new insight into telomere dynamics, we examined telomere length and positional changes during spermatogenesis using improved fluorescence in situ hybridization (FISH) and in situ telomeric repeat amplification protocols (TRAP) on histological sections. FISH revealed telomere length and chromosome position within nuclei change dynamically. Telomere extension occurred during spermiogenesis. In situ TRAP analysis verified elevated telomerase activity in elongating spermatids. Together, these data show that elongated spermatids have longer telomeres than precursor spermatogenic cells. This observation indicates that telomere elongation in haploid cells occurs after meiosis and in the absence of genomic replication. Analyses of testes from telomerase null mice further support the significance of telomere dynamics during spermatogenesis and the existence of an alternative telomere extension pathway.

Role of thyroid hormone in the initiation of EGF (epidermal growth factor) expression in the sublingual gland of the postnatal mouse

The effect of triiodo-L-thyronine (T3) and propylthiouracil (PTU) on the initiation of epidermal growth factor (EGF) expression in the sublingual glands (SLGs) of postnatal mice was investigated by indirect enzyme-labeled and immunogold antibody methods for light and electron microscopy, respectively. In normal males, EGF immunoreactivity first appeared in a few scattered granular cells of striated ducts (SDs) at 5 weeks of age, and the immunoreactive cells had increased in number at 6 weeks of age. No EGF expression was observed in the glands of females at any ages examined. When T3 (1 mg/kg body weight) was given to males every other day for 2 weeks before examination, EGF expression began earlier; the immunoreactive granular cells were first detected at 4 weeks of age, and at later ages they were markedly increased in number compared to those of normal males. Moreover, T3 was capable of inducing EGF in the female glands. After T3 was administered to females in the same manner as in males, a few immunoreactive cells were first detected at 5 weeks of age, and increased numbers were detected at later ages. By contrast, when PTU (1 mg/kg body weight) was given to male mice every other day for 2 weeks before examination, the EGF-immunoreactive cells were markedly decreased in number compared to those of normal males of the same age. Electron microscopy revealed that many SD cells contained secretory granules, and that these cells constituted the granular striated tubule (GST) in a portion of SDs, but they were undetectable by light microscopy, because their secretory granules were minimal in size and few in number. Gold-labeling of EGF was confined to the secretory granules of scattered granular cells, whose secretory granules were far larger in size and more abundant than those of the GST cells. These results suggest that thyroid hormone is essential to differentiation of the cellular phenotype of GST precursor cells into typical granular cells (detectable by light microscopy) that express EGF in the mouse SLG, showing a close resemblance to the submandibular granular convoluted tubule cells.

Additive and/or synergistic action (downregulation) of androgens and thyroid hormones on the cellular distribution and localization of a true tissue kallikrein, mK1, in the mouse submandibular gland

We investigated the effects of 5alpha-dihydrotestosterone (DHT), 3,5,3'-triiodo-l-thyronine (T(3)), and dexamethasone (Dex) on the expression of mK1 in the granular convoluted tubule (GCT) cells of the submandibular gland (SMG) of hypophysectomized (Hypox) male mice by indirect enzyme-labeled antibody and immunogold antibody methods for light and electron microscopy. Hypox resulted in considerable atrophy of the GCT cells, which were always immunoreactive for mK1, and the cells were characterized by apical small dense secretory granules labeled with gold particles suggesting the presence of mK1, small Golgi apparatus, sparse rough endoplasmic reticulum (RER), and developed basal infoldings. Each of the hormones, DHT, T(3), and Dex, enhanced the GCT phenotype to various degrees in Hypox male mice. Both DHT alone and T(3) alone moderately inhibited mK1 synthesis by increasing the number of mK1-immunonegative GCT cells in Hypox males, but Dex alone had no inhibitory effect on mK1 synthesis. A significant trophic effect on GCT cells was induced by combined injection of DHT and T(3) or of all three hormones, and was reflected in the appearance of abundant large secretory granules, well-developed Golgi apparatus and RER, and reduced basal infoldings. Only a few such GCT cells were immunopositive for mK1, and the pattern of immunopositive and immunonegative cells very closely resembled the mosaic pattern seen in normal male GCTs. These findings suggested that the sexual dimorphism of mK1 expression and the morphological appearance of GCT cells can be induced by treatment with two hormones, DHT and T(3), but not by either of them alone. T(3) appears to have a permissive effect on committed GCT cells that results in downregulation of mK1 expression in these cells.

Developmental and androgenic regulation of the immunocytochemical distribution of mK1, a true tissue kallikrein, in the granular convoluted tubule of the mouse submandibular gland

The action of androgens on the immunocytochemical distribution of mK1, a true tissue kallikrein, was examined in the submandibular gland (SMG) of developing and adult mice by indirect enzyme-labeled and immunogold-labeled antibody methods for light and electron microscopy, respectively. In both sexes at 3 weeks of age, essentially all of the immature granular convoluted tubule (GCT) cells were uniformly immunostained. At 4 weeks of age (the onset of puberty), morphological differences between the two sexes appeared in the GCTs, in which some cells became immunonegative. Thereafter, the immunonegative GCT cells became more abundant in the SMG of males than of females and considerable intercellular variation in staining intensity for mK1 was seen, especially in males. A few slender GCT cells with strong immunoreactivity appeared in GCT segments only in males. Castration of males resulted in an increase in the number of immunopositive GCT cells, whereas administration of dihydrotestosterone (DHT) decreased the number of immunopositive GCT cells in the SMGs of both sexes. Slender GCT cells immunoreactive for mK1 were seen in females treated with DHT for 6 days. However, there were no immunostained slender GCT cells in female SMGs after injection of DHT for 2 weeks. Immunoelectron microscopy disclosed this type of cell in male SMGs, which closely resembles immature GCT cells of prepubertal mice, with a few small secretory granules uniformly labeled with gold particles, a sparse Golgi apparatus and RER, and basal infoldings. In mature male SMGs and in SMGs of DHT-treated females and castrated males, typical GCT cells had a well-developed Golgi apparatus and a net-like RER but few to no basal infoldings, whereas in the female gland equivalent cells had moderately developed RER and some basal infoldings. These results suggest that mK1 is one of the enzymes characteristically present in immature GCT cells and that its synthesis is inhibited in part by androgens, resulting in decreased numbers of immunopositive cells.

Secretion by striated ducts of mammalian major salivary glands: review from an ultrastructural, functional, and evolutionary perspective

In addition to their role in electrolyte homeostasis, striated ducts (SDs) in the major salivary glands of many mammalian species engage in secretion of organic products. This phenomenon usually is manifested as the presence of small serous-like secretory granules in the apical cytoplasm of SD cells. The composition of these granules is largely unknown, except in the case of the cat and rat submandibular gland, where the granules have unequivocally been shown to contain kallikrein. In some species, the apical cytoplasm of SD cells contains variable numbers of vesicles, both spherical and elongated, that vary in appearance from 'empty' to moderately dense. In the rat parotid gland, lucent vesicles transport glycoproteins to the luminal surface where they are incorporated into the apical plasmalemma and the glycocalyx. There is a strong possibility that in various species some of these vesicles are involved in transcytosis of antibodies to the saliva from their source (plasma cells) in the surrounding connective tissue. In addition, vesicles may engage in transfer of growth factors from the saliva to the interstitium. In a few species, conventional SDs have been replaced by ducts that are wholly given over to secretion, i.e., they entirely lack basal striations; although such ducts occupy the histological position of conventional SDs, it is not clear whether they represent a new type of duct or merely are modifications of SDs. Broad-based comparisons of ultrastructural and other data about SDs offer some insight into evolutionary history of salivary glands and their role in the adaptive radiation of mammals. Evolutionary patterns emerged when we made interspecific comparisons across mammalian orders. Among the bats, there is a clear relationship between SD secretion and general categories of diet.