Rat anterior pituitary cells in vitro can partly reconstruct in vivo topographic affinities

Distinct Expression Patterns of Osteopontin and Dentin Matrix Protein 1 Genes in Pituitary Gonadotrophs

Cell-matrix interactions play important roles in pituitary development, physiology, and pathogenesis. In other tissues, a family of non-collagenous proteins, termed SIBLINGs, are known to contribute to cell-matrix interactions. Anterior pituitary gland expresses two SIBLING genes, Dmp1 (dentin matrix protein-1) and Spp1 (secreted phosphoprotein-1) encoding DMP1 and osteopontin proteins, respectively, but their expression pattern and roles in pituitary functions have not been clarified. Here we provide novel evidence supporting the conclusion that Spp1/osteopontin, like Dmp1/DMP1, are expressed in gonadotrophs in a sex- and age-specific manner. Other anterior pituitary cell types do not express these genes. In contrast to Dmp1, Spp1 expression is higher in males; in females, the expression reaches the peak during the diestrus phase of estrous cycle. In further contrast to Dmp1 and marker genes for gonadotrophs, the expression of Spp1 is not regulated by gonadotropin-releasing hormone in vivo and in vitro. However, Spp1 expression increases progressively after pituitary cell dispersion in both female and male cultures. We may speculate that gonadotrophs signal to other pituitary cell types about changes in the structure of pituitary cell-matrix network by osteopontin, a function consistent with the role of this secretory protein in postnatal tissue remodeling, extracellular matrix reorganization after injury, and tumorigenesis.

Reassembly of anterior pituitary organization by hanging drop three-dimensional cell culture

The anterior pituitary gland comprises 5 types of hormone-producing cells and non-endocrine cells, such as folliculostellate (FS) cells. The cells form a lobular structure surrounded by extracellular matrix (ECM) but are not randomly distributed in each lobule; hormone-producing cells have affinities for specific cell types (topographic affinity), and FS cells form a homotypic meshwork. To determine whether this cell and ECM organization can be reproduced in vitro, we developed a 3-dimensional (3D) model that utilizes hanging drop cell culture. We found that the topographic affinities of hormone-producing cells were indeed maintained (ie, GH to ACTH cells, GH to TSH cells, PRL to LH/FSH cells). Fine structures in hormone-producing cells retained their normal appearance. In addition, FS cells displayed well-developed cytoplasmic protrusions, which interconnected with adjacent FS cells to form a 3D meshwork. In addition, reassembly of gap junctions and pseudofollicles among FS cells was observed in cell aggregates. Major ECM components—collagens and laminin—were deposited and distributed around the cells. In sum, the dissociated anterior pituitary cells largely maintained their in vivo anterior pituitary architectures. This culture system appears to be a powerful experimental tool for detailed analysis of anterior pituitary cell organization.

Anterior pituitary cell networks

Both endocrine and non-endocrine cells of the pituitary gland are organized into structural and functional networks which are formed during embryonic development but which may be modified throughout life. Structural mapping of the various endocrine cell types has highlighted the existence of distinct network motifs and relationships with the vasculature which may relate to temporal differences in their output. Functional characterization of the network activity of growth hormone and prolactin cells has revealed a role for cell organization in gene regulation, the plasticity of pituitary hormone output and remarkably the ability to memorize altered demand. As such, the description of these endocrine cell networks alters the concept of the pituitary from a gland which simply responds to external regulation to that of an oscillator which may memorize information and constantly adapt its coordinated networks' responses to the flow of hypothalamic inputs.

Immunohistochemistry of connexin 43 throughout anterior pituitary gland in a transgenic rat with green fluorescent protein-expressing folliculo-stellate cells

Folliculo-stellate (FS) cells in the anterior pituitary gland have been speculated to possess multifunctional properties. Because gap junctions (GJ) have been identified between FS cells, FS cells may be interconnected electrophysiologically by GJ and serve as signal transmission networks to modulate hormone release in the anterior pituitary gland. But whether GJ are localized among FS cells from the pars tuberalis through the pars distalis is unclear. The S100b-GFP transgenic rat has recently been generated, which expresses green fluorescent protein (GFP) specifically in FS cells in the anterior pituitary. This model is expected to be a powerful tool for studies of FS cells. The purpose of the present paper was therefore to examine the localization of GJ on connexin 43 immunohistochemistry throughout the anterior pituitary gland of S100b-GFP rats under confocal laser microscopy. The localization patterns of FS cells was also observed in primary culture of anterior pituitary cells and the question of whether GJ between FS cells are reconstructed in vitro was investigated. In vivo studies showed that GJ were present specifically between FS cells from the pars tuberalis to the pars distalis in the anterior pituitary gland. The appearance of FS cells was distinguished into two types, with localization of GJ differing between types. In vitro, it was observed for the first time that FS cells in primary culture could be categorized into two types. In vivo localization of GJ between FS cells was reconstructed in vitro. These morphological observations are consistent with the hypothesis that FS cells form an electrophysiological network throughout the anterior pituitary for signal transmission.

Anterior pituitary pyroglutamyl peptidase II activity controls TRH-induced prolactin release

Ecto-peptidases modulate the action of peptides in the extracellular space. The relationship between peptide receptor and ecto-peptidase localization, and the physiological role of peptidases is poorly understood. Current evidence suggests that pyroglutamyl peptidase II (PPII) inactivates neuronally released thyrotropin-releasing hormone (TRH). The impact of PPII localization in the anterior pituitary on the endocrine activities of TRH is unknown. We have studied whether PPII influences TRH signaling in anterior pituitary cells in primary culture. In situ hybridization (ISH) experiments showed that PPII mRNA was expressed only in 5-6% of cells. ISH for PPII mRNA combined with immunocytochemistry for prolactin, beta-thyrotropin, or growth hormone, showed that 66% of PPII mRNA expressing cells are lactotrophs, 34% somatotrophs while none are thyrotrophs. PPII activity was reduced using a specific phosphorothioate antisense oligodeoxynucleotide or inhibitors. Compared with mock or scrambled oligodeoxynucleotide-treated controls, knock-down of PPII expression by antisense targeting increased TRH-induced release of prolactin, but not of thyrotropin. Similar data were obtained with either a transition-state or a tight binding inhibitor. These results demonstrate that PPII expression in lactotrophs coincides with its ability to control prolactin release. It may play a specialized role in TRH signaling in the anterior pituitary. Anterior pituitary ecto-peptidases may fulfill unique functions associated with their restricted cell-specific expression.

Gonadotropin-releasing hormone stimulates prolactin release from lactotrophs in photoperiodic species through a gonadotropin-independent mechanism

Previous studies have provided evidence for a paracrine interaction between pituitary gonadotrophs and lactotrophs. Here, we show that GnRH is able to stimulate prolactin (PRL) release in ovine primary pituitary cultures. This effect was observed during the breeding season (BS), but not during the nonbreeding season (NBS), and was abolished by the application of bromocriptine, a specific dopamine agonist. Interestingly, GnRH gained the ability to stimulate PRL release in NBS cultures following treatment with bromocriptine. In contrast, thyrotropin-releasing hormone, a potent secretagogue of PRL, stimulated PRL release during both the BS and NBS and significantly enhanced the PRL response to GnRH during the BS. These results provide evidence for a photoperiodically modulated functional interaction between the GnRH/gonadotropic and prolactin axes in the pituitary gland of a short day breeder. Moreover, the stimulation of PRL release by GnRH was shown not to be mediated by the gonadotropins, since immunocytochemical, Western blotting, and PCR studies failed to detect pituitary LH or FSH receptor protein and mRNA expressions. Similarly, no gonadotropin receptor expression was observed in the pituitary gland of the horse, a long day breeder. In contrast, S100 protein, a marker of folliculostellate cells, which are known to participate in paracrine mechanisms within this tissue, was detected throughout the pituitaries of both these seasonal breeders. Therefore, an alternative gonadotroph secretory product, a direct effect of GnRH on the lactotroph, or another cell type, such as the folliculostellate cell, may be involved in the PRL response to GnRH in these species.

Distinctive localization of N- and E-cadherins in rat anterior pituitary gland

In the rat anterior pituitary gland, folliculo-stellate cells aggregate preferably to form pseudofollicles, and each type of hormone-producing cell shows adhesive affinity with particular types of heterologous hormone-producing cells. Distribution of cadherin types in the rat anterior pituitary was examined immunohistochemically to clarify the unique cell arrangements caused by homologous and heterologous affinities among cells. N- and E-cadherins were detected continuously along cell membranes, while P-cadherin was not. N- and E-cadherins showed distinct isolation in localization, with N-cadherins localized in hormone-producing cells of distal and intermediate lobes in various amounts, and E-cadherins limited to folliculo-stellate cells and marginal layer cells facing the residual lumen of Rathke's pouch. A similar distribution of cadherins was observed in cell clusters of primary cultured anterior pituitary cells. These findings suggest that differential expression of cell adhesion molecules may be partially responsible for localization of hormone-producing cells and folliculo-stellate cells.