Gene expression of vascular endothelial growth factor-A in the pituitary during formation of the vascular system in the hypothalamic-pituitary axis of the rat

Primary Cilia are Required for Cell-Type Determination and Angiogenesis in Pituitary Development

The functional maturation of the pituitary gland requires adequate cell differentiation and vascular network formation. Although spatiotemporal signaling and transcription factors are known to govern pituitary development, the involvement of primary cilia, nonmoving hair-like organelles, remains unclear. In this study, we uncovered the contribution of primary cilia to cell-type determination and vascular network formation during pituitary development. Homozygous knockout mice lacking a ciliary kinase, Dyrk2-/-, exhibit abnormalities in ciliary structure and pituitary hypoplasia, accompanied by varying degrees of failure in differentiation among all types of hormone-producing cells in the anterior lobe. Aberrations in cell differentiation in Dyrk2-/- mice arise from a decrease in the expression of crucial transcription factors, Lhx4, Lhx3, and Prop1, resulting from the inactivity of Hedgehog (Hh) signaling during the early stages of development. Furthermore, the loss of Dyrk2 results in vascular system abnormalities during the middle to late stages of development. Mechanistically, transcriptome analyses revealed the downregulation of vitronectin-integrin αvβ3-VEGFR2 signaling, essential for orchestrating vascular development. Collectively, our findings demonstrate that primary cilia play a pivotal role as critical regulators of cell survival, cell determination, and angiogenesis during pituitary gland development through the activation of Hh signaling. These findings expand our understanding of the potential link between pituitary dysfunction in human disorders and ciliopathies.

History of the Development of Knowledge about the Neuroendocrine Control of Ovulation-Recent Knowledge on the Molecular Background

The physiology of reproduction has been of interest to researchers for centuries. The purpose of this work is to review the development of our knowledge on the neuroendocrine background of the regulation of ovulation. We first describe the development of the pituitary gland, the structure of the median eminence (ME), the connection between the hypothalamus and the pituitary gland, the ovarian and pituitary hormones involved in ovulation, and the pituitary cell composition. We recall the pioneer physiological and morphological investigations that drove development forward. The description of the supraoptic-paraventricular magnocellular and tuberoinfundibular parvocellular systems and recognizing the role of the hypophysiotropic area were major milestones in understanding the anatomical and physiological basis of reproduction. The discovery of releasing and inhibiting hormones, the significance of pulse and surge generators, the pulsatile secretion of the gonadotropin-releasing hormone (GnRH), and the subsequent pulsatility of luteinizing (LH) and follicle-stimulating hormones (FSH) in the human reproductive physiology were truly transformative. The roles of three critical neuropeptides, kisspeptin (KP), neurokinin B (NKB), and dynorphin (Dy), were also identified. This review also touches on the endocrine background of human infertility and assisted fertilization.

Collagen XIII Is the Key Molecule of Neurovascular Junctions in the Neuroendocrine System

INTRODUCTION

Axons of magnocellular neurosecretory cells project from the hypothalamus to the posterior lobe (PL) of the pituitary. In the PL, a wide perivascular space exists between the outer basement membrane (BM), where nerve axons terminate, and the inner BM lining the fenestrated capillaries. Hypothalamic axon terminals and outer BMs in the PL form neurovascular junctions. We previously had found that collagen XIII is strongly localized in the outer BMs. In this study, we investigated the role of collagen XIII in the PL of rat pituitaries.

METHODS

We first studied the expression of Col13a1, the gene encoding the α1 chains of collagen XIII, in rat pituitaries via quantitative real-time polymerase chain reaction and in situ hybridization. We observed the distribution of COL13A1 in the rat pituitary using immunohistochemistry and immunoelectron microscopy. We examined the expression of Col13a1 and the distribution of COL13A1 during the development of the pituitary. In addition, we examined the effects of water deprivation and arginine vasopressin (AVP) signaling on the expression of Col13a1 in the PL.

RESULTS

Col13a1 was expressed in NG2-positive pericytes, and COL13A1 signals were localized in the outer BM of the PL. The expression of Col13a1 was increased by water deprivation and was regulated via the AVP/AVPR1A/Gαq/11 cascade in pericytes of the PL.

CONCLUSION

These results suggest that pericytes surrounding fenestrated capillaries in the PL secrete COL13A1 and are involved in the construction of neurovascular junctions. COL13A1 is localized in the outer BM surrounding capillaries in the PL and may be involved in the connection between capillaries and axon terminals.

Caveolae-mediated endocytosis pathway regulates endothelial fenestra homeostasis in the rat pituitary

Endothelial fenestrae are transcellular pores separated by diaphragms formed by plasmalemma vesicle-associated proteins (PLVAP) and function as channels for peptide hormones and other substances. Caveola, a key regulator of clathrin-independent endocytosis, may be involved in the invagination and fusion of plasma membranes, which are essential for fenestra formation. In this study, we first found that caveolin-1 and -2, the major components of caveolae, was localized in fenestrated endothelial cells in the anterior lobe of the rat pituitary by immunohistochemistry. As we also observed caveolae in the endothelial cells of the anterior lobe of the rat pituitary by transmission electron microscopy, we studied the relationship between the caveolae-mediated endocytosis pathway and fenestrae structure in cultured endothelial cells isolated from the anterior lobe of the rat pituitary (CECAL) by immunofluorescence staining and scanning electron microscopy. The inhibition of caveolae-mediated endocytosis by genistein enlarged the PLVAP-positive oval-shaped structure that represented the sieve plate and induced the formation of a doughnut-shaped bulge around the fenestra in CECAL. In contrast, the acceleration of caveolae-mediated endocytosis by okadaic acid induced the diffusion of PLVAP-positive signals in the cytoplasm and reduced the number of fenestrae in CECAL. These results indicate that the caveolae-mediated endocytosis pathway is involved in the fenestra homeostasis in the fenestrated endothelial cells of the rat pituitary.

Regulation of fenestra formation via actin-dynamin2 interaction in rat pituitary endothelial cells

Endothelial fenestrae are transcellular pores divided by a diaphragm consisting of plasmalemma vesicle-associated protein (PLVAP). They function as a channel for peptide hormones and other substances. Invagination of the plasma membrane is necessary for the fenestra formation. The actin cytoskeleton is essential for scission of endocytic vesicles from the invaginated plasma membrane. Therefore, we examined the involvement of the actin cytoskeleton in fenestra formation in cultured endothelial cells isolated from the anterior lobe (AL) of the rat pituitary, using immunofluorescence and scanning electron microscopy. Inhibition of polymerization and depolymerization of the actin cytoskeleton by latrunculin A and jasplakinolide, respectively, remarkably increased the PLVAP-positive sieve plate area and number of fenestrae. Jasplakinolide significantly affected the arrangement of the fenestra on the cell surface, resulting in parallel serpentine furrows of the fenestra. These results suggest that the actin cytoskeleton not only induces fenestra formation but also regulates cell arrangement. Dynamin is a scission protein of the invaginated plasma membrane and interacts with the actin cytoskeleton. We found that dynamin2 is mainly expressed in the endothelial cells of the rat AL. We then investigated the function of dynamin2 by the treatment with dyngo-4a, a potent inhibitor of dynamin1 and dynamin2, on the fenestra formation. As a result, the PLVAP-positive area is significantly increased by the treatment. These results show that the actin-dynamin2 interaction is essential for the control of the fenestra formation in endothelial cells of rat AL. In conclusion, the actin cytoskeleton and dynamin2 function as regulators of endothelial fenestra formation.

Pituitary Apoplexy: Risk Factors and Underlying Molecular Mechanisms

Pituitary apoplexy is a rare syndrome, graded from asymptomatic subclinical apoplexy to a life-threatening condition due to pituitary ischemia or haemorrhage of an enlarged pituitary gland. The risk factors and the molecular underlying mechanisms are yet to be elucidated. We provide an overview of the general concepts, the potential factors associated with pituitary adenoma susceptibility for apoplectic events and the molecular mechanisms that could be involved such as HIF-1α/VEGF pathways and metalloproteinases activation, among others. The knowledge of the molecular mechanisms that could participate in the pathogenesis of pituitary apoplexy is crucial to advancement in the identification of future diagnostic tools and therapeutic targets in this rare but sometimes fatal condition.

Fibronectin is essential for formation of fenestrae in endothelial cells of the fenestrated capillary

Endothelial fenestrae are transcellular pores that pierce the capillary walls in endocrine glands such as the pituitary. The fenestrae are covered with a thin fibrous diaphragm consisting of the plasmalemma vesicle-associated protein (PLVAP) that clusters to form sieve plates. The basal surface of the vascular wall is lined by basement membrane (BM) composed of various extracellular matrices (ECMs). However, the relationship between the ECMs and the endothelial fenestrae is still unknown. In this study, we isolated fenestrated endothelial cells from the anterior lobe of the rat pituitary, using a dynabeads-labeled antibody against platelet endothelial cell adhesion molecule 1 (PECAM1). We then analyzed the gene expression levels of several endothelial marker genes and genes for integrin α subunits, which function as the receptors for ECMs, by real-time polymerase chain reaction (PCR). The results showed that the genes for the integrin α subunit, which binds to collagen IV, fibronectin, laminin-411, or laminin-511, were highly expressed. When the PECAM1-positive cells were cultured for 7 days on collagen IV-, fibronectin-, laminins-411-, or laminins-511-coated coverslips, the sieve plate structures equipped with probably functional fenestrae were maintained only when the cells were cultured on fibronectin. Additionally, real-time PCR analysis showed that the fibronectin coating was effective in maintaining the expression pattern of several endothelial marker genes that were preferentially expressed in the endothelial cells of the fenestrated capillaries. These results indicate that fibronectin functions as the principal factor in the maintenance of the sieve plate structures in the endothelial cells of the fenestrated capillary.

Fibronectin-integrin signaling regulates PLVAP localization at endothelial fenestrae by microtubule stabilization

Endothelial fenestrae are the transcellular pores existing on the capillary walls which are organized in clusters referred to as sieve plates. They are also divided by a diaphragm consisting of plasmalemma vesicle-associated protein (PLVAP). In this study, we examined the involvement of fibronectin signaling in the formation of fenestra and diaphragm in endothelial cells. Results showed that Itga5 and Itgb1 were expressed in PECAM1-positive endothelial cells isolated from the anterior lobe (AL) of the rat pituitary, and integrin α5 was localized at the fenestrated capillaries of the rat pituitary and cultured PECAM1-positive endothelial cells isolated from AL (CECAL). Inhibition of both integrin α5β1 and FAK, a key molecule for integrin-microtubule signaling, respectively, by ATN-161 and FAK inhibitor 14, caused the delocalization of PLVAP at the sieve plates and depolymerization of microtubules in CECAL. Paclitaxel prevented the delocalization of PLVAP by the inhibition of integrin α5β1. Microtubule depolymerization induced by colcemid also caused the delocalization of PLVAP. Treatment of CECAL with ATN-161 and colcemid caused PLVAP localization at the Golgi apparatus. The localization of PLVAP at the sieve plates was inhibited by BFA treatment in a time-dependent manner and spread diffusely to the cytoplasm. These results indicate that a constant supply of PLVAP proteins by the endomembrane system via the Golgi apparatus is essential for the localization of PLVAP at sieve plates. In conclusion, the endomembrane transport pathway from the Golgi apparatus to sieve plates requires microtubule cytoskeletons, which are regulated by fibronectin-integrin α5β1 signaling.

Easy detection of hormone secretion from LβT2 cells by using Gaussia luciferase

Reproduction is regulated by gonadotropins secreted from gonadotrophs. The production and secretion of gonadotropins are mainly regulated by gonadotropin-releasing hormone (GnRH). Agonists or antagonists that influence GnRH action on gonadotrophs are important to regulate reproduction; however, these factors have not been fully characterized due to the lack of simple and easy-to-use techniques to detect gonadotropin secretion from gonadotropin-producing cells. In the present study, we found that Gaussia luciferase (Gluc), which was expressed in LβT2 cells, can be secreted like a luteinizing-hormone (LH) upon stimulation with GnRH. The Gluc secreted into the medium was easily monitored as luminescence signals. The detection range of the GnRH-induced Gluc activity was comparable to that of the enzyme-linked immunosorbent assay for LH. In addition, when the Gluc was expressed in AtT20 cells, which produce adrenocorticotropic hormone (ACTH), the Gluc activity in the medium increased in parallel with the ACTH secretion upon stimulation with corticotropin-releasing hormone. Thus, the Gluc assay in the present study can be easily used for high-throughput screening of factors that influence LH or ACTH secretion from LβT2 or AtT20 cells, respectively.

Vascular endothelial growth factor secretion from pituitary folliculostellate cells: role of KATP channels

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen responsible for physiological and pathological angiogenesis. Abnormal regulation of VEGF expression in anterior pituitary folliculostellate (FS) cells has been implicated in pituitary tumour progression. FS and endocrine cells express VEGF, which is considered to be secreted by the constitutive pathway. The present study investigated the mechanism of VEGF secretion in TtT/GF cells, a mouse FS cell line. TtT/GF cells were shown to express VEGF(164), the most potent and bioavailable isoform of VEGF. Immunofluorescence and immunogold electron microscopy localised VEGF to the cytoplasm and small electron-lucent vesicles. Pituitary adenylate cyclase-activating polypeptide (PACAP), a well-documented stimulant of VEGF secretion, caused a robust increase in VEGF secretion over 24 h. Glyburide, an ABCA1 and K(ATP) channel blocker, also caused an increase in VEGF secretion when applied alone, and amplified the response to PACAP. Other ABCA1 transport blockers did not affect VEGF secretion. Exposure of TtT/GF cells to cycloheximide with PACAP or glyburide inhibited the increased secretion of VEGF, consistent with control of secretion at the transcription level. The SUR2B/Kir6.1 form of K(ATP) channels was shown to be expressed by TtT/GF cells. Diazoxide, a K(ATP) activator, inhibited PACAP- and PACAP + glyburide-stimulated VEGF secretion but not that of glyburide alone. These data suggest that K(ATP) channels are expressed by FS cells and play a significant role in the control of VEGF secretion, and also that activation of K(ATP) channels inhibits the secretion of VEGF at the level of transcription.

Angiogenesis in the intermediate lobe of the pituitary gland alters its structure and function

The pars distalis (PD) and the pars intermedia (PI) have the same embryonic origin, but their morphological and functional characteristics diverge during development. The PD is highly vascularized, whereas the highly innervated PI is essentially non-vascularized. Based on our previous finding that vascular endothelial growth factor-A (VEGF-A) is involved in vascularization of the rat PD, attempt was made to generate transgenic Xenopus expressing VEGF-A specifically in the melanotrope cells of the PI as a model system for studying the significance of vascularization or avascularization for the functional differentiation of the pituitary. The PI of the transgenic frogs, examined after metamorphosis, were distinctly vascularized but poorly innervated. The experimentally induced vascularization in the PI resulted in a marked increase in tissue volume and a decrease in the expression of both alpha-melanophore-stimulating hormone (α-MSH) and prohormone convertase 2, a cleavage enzyme essential for generating α-MSH. The transgenic animals had low plasma α-MSH concentrations and displayed incomplete adaptation to a black background. To our knowledge, this is the first report indicating that experimentally induced angiogenesis in the PI may bring about functional as well as structural alterations in this tissue.

Pituitary gland development and disease: from stem cell to hormone production

Many aspects of pituitary development have become better understood in the past two decades. The signaling pathways regulating pituitary growth and shape have emerged, and the balancing interactions between the pathways are now appreciated. Markers for multipotent progenitor cells are being identified, and signature transcription factors have been discovered for most hormone-producing cell types. We now realize that pulsatile hormone secretion involves a 3D integration of cellular networks. About a dozen genes are known to cause pituitary hypoplasia when mutated due to their essential roles in pituitary development. Similarly, a few genes are known that predispose to familial endocrine neoplasia, and several genes mutated in sporadic pituitary adenomas are documented. In the next decade, we anticipate gleaning a deeper appreciation of these processes at the molecular level, insight into the development of the hypophyseal portal blood system, and evolution of better therapeutics for congenital and acquired hormone deficiencies and for common craniopharyngiomas and pituitary adenomas.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Expression of a mammalian aquaporin 3 homolog in the anterior pituitary gonadotrophs of the tree frog, Hyla japonica

Aquaporins (AQPs) are a family of water channel proteins that play a major role in maintaining water homeostasis in various organisms. Several AQPs have been identified in the tree frog, Hyla japonica. Of these, AQP-h3BL, which is expressed in the basolateral membrane of the epithelial cells, is a homolog of mammalian AQP3. Using immunohistochemistry and in situ RT-PCR, we have demonstrated that AQP-h3BL is expressed in the anterior pituitary gonadotrophs of the tree frog but not in the other hormone-producing cells of the anterior pituitary. In gonadotrophs labeled for luteinizing hormone subunit-β (LHβ), AQP-h3BL protein was found to reside in the plasma membrane, the nuclear membrane and the cytoplasm. Double-labeling of AQP-h3BL mRNA and LHβ protein revealed that AQP-h3BL mRNA is expressed in the gonadotrophs. Following stimulation by gonadotropin-releasing hormone (GnRH), the label for AQP-h3BL localized in the plasma membrane became more intense, concomitant with the transport of LHβ-positive materials to the plasma membrane. These developments coincided with a decrease in the labeling density in the cytoplasm and near the nuclear membrane, suggesting that the latter localizations may function as "storage area" for AQP-h3BL. Immunoelectron microscopy also confirmed these localizations of AQP-h3BL protein. Based on these results, we suggest that AQP-h3BL protein in the frog gonadotrophs is involved in the formation of secretory granules, the swelling and increase in the volume of the granules and exocytosis.

Expression of IGFBP7 mRNA in corticotrophs in the anterior pituitary of adrenalectomized rats

The number of corticotrophs increases in the anterior pituitary (AP) gland in adrenalectomized (AdX) rats. In this study, aimed at identifying the growth factor implicated in this proliferation, we analyzed proteins secreted from a cDNA library of the AP of AdX rats, using the signal sequence trap method. A PCR analysis of several cDNAs that coded for insulin-like growth factor binding protein (IGFBP) 5, IGFBP7, and vacuolar H+-ATPase accessory subunit Ac45 revealed an increased and decreased expression level of IGFBP7 mRNA in the AP of AdX rats and AdX rats injected with dexamethasone, respectively. IGFBP7 mRNA was predominately expressed in the corticotrophs of the APs of both sham-operated and AdX rats. The AP of AdX rats contained an increased number of IGFBP7 mRNA-expressing cells and corticotrophs compared with that of sham-operated rats, but the ratio of IGFBP7 mRNA-positive corticotrophs per total number of corticotrophs did not significantly change in either group. Histochemical analysis of labeled proliferating cell nuclear antigen (PCNA) and sex-determining region Y box-2 (SOX2) revealed the presence of several PCNA-positive signals and the absence of SOX2 cells among the corticotrophs, suggesting that IGFBP7 mRNA-expressing corticotrophs are derived from in situ corticotrophs and that they increase in number as corticotrophs increase. The possible roles of IGFBP7 in the corticotrophs are also discussed.

Sinusoid development and morphogenesis may be stimulated by VEGF-Flk-1 signaling during fetal mouse liver development

Early morphogenesis of hepatic sinusoids was histochemically and experimentally analyzed, and the importance of VEGF-Flk-1 signaling in the vascular development was examined during murine liver organogenesis. FITC-gelatin injection experiments into young murine fetuses demonstrated that all primitive sinusoidal structures were confluent with portal and central veins, suggesting that hepatic vessel development may occur via angiogenesis. At 12.5-14.5 days of gestation, VEGF receptors designated Flk-1, especially their mature form, were highly expressed in endothelial cells of primitive sinusoidal structures and highly phosphorylated on their tyrosine residues. At the same time, VEGF was also detected in hepatoblasts/hepatocytes, hemopoietic cells, and megakaryocytes of the whole liver parenchyma. Furthermore, the addition of VEGF to E12.5 liver cell cultures significantly induced the growth and branching morphogenesis of sinusoidal endothelial cells. Therefore, VEGF-Flk-1 signaling may play an important role in the growth and morphogenesis of primitive sinusoids during fetal liver development.

The spatial and temporal expression of delta-like protein 1 in the rat pituitary gland during development

An analysis of secreted proteins by the signal sequence trap method using a cDNA library of the rat pituitary anlage at embryonic days (E) 13.5 revealed the abundant expression of delta-like protein 1 (Dlk1) in the pituitary gland. Dlk1, an epidermal growth factor-like repeat protein in preadipocytes, functions in maintaining the preadipose state. Expression of Dlk1 mRNA in the pituitary at E13.5 and in the adult pituitary was confirmed by in situ hybridization. The expression pattern of Dlk1 during pituitary development was also studied by immunohistochemistry. Dlk1 protein first appeared in Rathke's pouch and the infundibulum at E11.5; as development proceeded, expression became restricted to the pars distalis and pars tuberalis (PT). Dlk1 was expressed in most ACTH cells during the embryonic stages, but its expression was limited to only a few ACTH cells in the adult pituitary. It was also expressed in a small population of TSH, GTH, and PRL cells throughout development, whereas it was present in the cytoplasm of most GH cells at all developmental stages. Similarly, Dlk1 was localized in the cytoplasm of PT cells during development. These findings provide new insights into the mechanism of Dlk1 regarding its regulation of pituitary hormone-secreting cells during development.