Promotion of intragranular co-aggregation with LH by enhancement of secretogranin II storage resulted in increased intracellular granule storage in gonadotrophs of GnRH-deprived male mice

Function of secretoneurin in regulating the expression of reproduction-related genes in ovoviviparous black rockfish (Sebastes schlegelii)

Secretoneurin (SN), a conserved peptide derived from secretogranin-2 (scg2), also known as secretogranin II or chromogranin C, plays an important role in regulating gonadotropin in the pituitary, which affects the reproductive system. This study aimed to clarify the mode of action of scg2 in regulating gonad development and maturation and the expression of mating behavior-related genes. Two scg2 cDNAs were cloned from the ovoviviparity teleost black rockfish (Sebastes schlegelii). In situ hybridization detected positive scg2 mRNA signals in the telencephalon and hypothalamus, where sgnrh and kisspeptin neurons were reported to be located and potentially regulated by scg2. In vivo, intracerebral ventricular injections of synthetic black rockfish SNa affected brain cgnrh, sgnrh, kisspeptin1, pituitary lh and fsh and gonad steroidogenesis-related gene expression levels with sex dimorphism. In vitro, a similar effect was found in primary cultured brain and pituitary cells. Thus, SN could contribute to the regulation of gonadal development, as well as reproductive behaviors, including mating and parturition.

The Hippo Pathway Effectors YAP and TAZ Regulate LH Release by Pituitary Gonadotrope Cells in Mice

The Hippo transcriptional coactivators YAP and TAZ exert critical roles in morphogenesis, organ size determination and tumorigenesis in many tissues. Although Hippo kinase cascade activity was recently reported in the anterior pituitary gland in mice, the role of the Hippo effectors in regulating gonadotropin production remains unknown. The objective of this study was therefore to characterize the roles of YAP and TAZ in gonadotropin synthesis and secretion. Using a conditional gene targeting approach (cKO), we found that gonadotrope-specific inactivation of Yap and Taz resulted in increased circulating levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in adult male mice, along with increased testosterone levels and testis weight. Female cKO mice had increased circulating LH (but not FSH) levels, which were associated with a hyperfertility phenotype characterized by higher ovulation rates and larger litter sizes. Unexpectedly, the loss of YAP/TAZ did not appear to affect the expression of gonadotropin subunit genes, yet both basal and GnRH-induced LH secretion were increased in cultured pituitary cells from cKO mice. Likewise, pharmacologic inhibition of YAP binding to the TEAD family of transcription factors increased both basal and GnRH-induced LH secretion in LβT2 gonadotrope-like cells in vitro without affecting Lhb expression. Conversely, mRNA levels of ChgA and SgII, which encode key secretory granule cargo proteins, were decreased following pharmacologic inhibition of YAP/TAZ, suggesting a mechanism whereby YAP/TAZ regulate the LH secretion machinery in gonadotrope cells. Together, these findings represent the first evidence that Hippo signaling may play a role in regulating pituitary LH secretion.

In early pubertal boys, testosterone and LH are associated with improved anti-oxidation during an aerobic exercise bout

PURPOSE

To investigate the association of the hypothalamic-pituitary-testicular (HPT) axis with pro- and anti- oxidation, in relation to puberty and obesity in boys, before and after an aerobic exercise bout.

METHODS

This is a cross-sectional human observational study of 92 healthy normal-weight, obese pre- and early- pubertal boys that underwent a blood sampling, before, and after an aerobic exercise bout at 70% VO₂max, until exhaustion. LH, FSH, total testosterone (tT) and markers of pro- (TBARS and PCs) and anti- (GSH, GSSG, GPX, catalase, TAC) oxidation were measured.

RESULTS

Baseline LH, FSH, and tT concentrations were greater in early, than in pre- pubertal boys, independently of weight status. Post-exercise, LH concentrations decreased in early pubertal boys while FSH concentrations did not change in any of the studied groups. Baseline and post-exercise tT concentrations were lower in obese than in normal-weight early pubertal boys, while baseline and post-exercise LH and FSH concentrations did not differ between these groups. Post-exercise tT concentrations increased in early pubertal obese boys. Baseline LH, FSH and tT concentrations correlated positively with baseline anti-oxidation markers concentrations in pre-pubertal boys. Baseline tT concentrations correlated positively with the increase of TAC concentrations in early pubertal normal-weight boys. In all boys, baseline LH concentrations were the best positive predictors for the exercise-associated increase of TAC concentrations.

CONCLUSIONS

It appears that the HPT axis maturation during puberty (in particular its LH and testosterone components) is positively associated with the increase of anti-oxidation during a bout of aerobic exercise.

Comparative proteomic analysis of pituitary glands from Huoyan geese between pre-laying and laying periods using an iTRAQ-based approach

In this study, we performed a comprehensive evaluation of the proteomic profile of the pituitary gland of the Huoyan goose during the laying period compared to the pre-laying period using an iTRAQ-based approach. Protein samples were prepared from pituitary gland tissues of nine pre-laying period and nine laying period geese. Then the protein samples from three randomly selected geese within each period were pooled in equal amounts to generate one biological sample pool. We identified 684 differentially expressed proteins, including 418 up-regulated and 266 down-regulated proteins. GO annotation and KEGG pathway analyses of these proteins were conducted. Some of these proteins were found to be associated with hormone and neurotransmitter secretion and transport, neuropeptide signalling and GnRH signalling pathways, among others. Subsequently, the modification of the abundance of three proteins (prolactin, chromogranin-A and ITPR3) was verified using western blotting. Our results will provide a new source for mining genes and gene products related to the egg-laying performance of Huoyan geese, and may provide important information for the conservation and utilization of local goose breeds.

Anatomical and functional gonadotrope networks in the teleost pituitary

Mammalian pituitaries exhibit a high degree of intercellular coordination; this enables them to mount large-scale coordinated responses to various physiological stimuli. This type of communication has not been adequately demonstrated in teleost pituitaries, which exhibit direct hypothalamic innervation and expression of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in distinct cell types. We found that in two fish species, namely tilapia and zebrafish, LH cells exhibit close cell-cell contacts and form a continuous network throughout the gland. FSH cells were more loosely distributed but maintained some degree of cell-cell contact by virtue of cytoplasmic processes. These anatomical differences also manifest themselves at the functional level as evidenced by the effect of gap-junction uncouplers on gonadotropin release. These substances abolished the LH response to gonadotropin-releasing hormone stimulation but did not affect the FSH response to the same stimuli. Dye transfer between neighboring LH cells provides further evidence for functional coupling. The two gonadotropins were also found to be differently packaged within their corresponding cell types. Our findings highlight the evolutionary origin of pituitary cell networks and demonstrate how the different levels of cell-cell coordination within the LH and FSH cell populations are reflected in their distinct secretion patterns.

GnRH pulse frequency-dependent differential regulation of LH and FSH gene expression

The pituitary gonadotropin hormones, FSH and LH, are essential for fertility. Containing an identical α-subunit (CGA), they are comprised of unique β-subunits, FSHβ and LHβ, respectively. These two hormones are regulated by the hypothalamic decapeptide, GnRH, which is released in a pulsatile manner from GnRH neurons located in the hypothalamus. Varying frequencies of pulsatile GnRH stimulate distinct signaling pathways and transcriptional machinery after binding to the receptor, GnRHR, on the cell surface of anterior pituitary gonadotropes. This ligand-receptor binding and activation orchestrates the synthesis and release of FSH and LH, in synergy with other effectors of gonadotropin production, such as activin, inhibin and steroids. Current research efforts aim to discover the mechanisms responsible for the decoding of the GnRH pulse signal by the gonadotrope. Modulating the response to GnRH has the potential to lead to new therapies for patients with altered gonadotropin secretion, such as those with hypothalamic amenorrhea or polycystic ovarian syndrome.

An investigation into pituitary gonadotrophic hormone synthesis, secretion, subunit gene expression and cell structure in normal and mutant male mice

To investigate brain-pituitary-gonadal inter-relationships, we have compared the effects of mutations that perturb the hypothalamic-pituitary-gonadal axis in male mice. Specifically, serum and pituitary gonadotrophin concentrations, gonadotrophin gene expression, and gonadotroph structure and number were measured. Follicle-stimulating hormone (FSH)β knockout (FSHβKO), FSH receptor knockout (FSHRKO), luteinising hormone (LH) receptor knockout (LuRKO), hypogonadal (hpg), testicular feminised (tfm) and gonadectomised mice were compared with control wild-type mice or heterozygotes. Serum levels of LH were similar in FSHβKO, FSHRKO and heterozygote males despite decreased androgen production in KO males. As expected, there was no detectable FSH in the serum or pituitary and an absence of expression of the FSHβ subunit gene in FSHβKO mice. However, there was a significant increase in expression of the common α and LHβ subunit genes in FSHRKO males. The morphology of FSHβKO and FSHRKO gonadotrophs was not significantly different from controls, except that the subpopulation of granules consisting of an electron-dense core and electron-lucent 'halo' was not observed in FSHβKO gonadotrophs and the granules were smaller in diameter. In the gonadotrophin-releasing hormone deficient hpg mouse, gonadotrophin mRNA and hormone levels were significantly lower compared to control mice and gonadotrophs were correspondingly smaller, with less abundant endoplasmic reticulum and reduced secretory granules. In LuRKO, tfm and gonadectomised mice, hyperstimulation of LHβ and FSHβ mRNA and serum protein concentrations was reflected by subcellular changes in gonadotroph morphology, including more dilated rough endoplasmic reticulum and more secretory granules distributed adjacent to the plasma membrane. In summary, major differences in pituitary content and serum concentrations of the gonadotrophins LH and FSH have been found between normal and mutant male mice. These changes are associated with changes in transcriptional activity of the gonadotrophin subunit genes and are reflected by changes in the cellular structure and secretory granule architecture within the gonadotroph cells.

Is secretoneurin a new hormone?

Numerous small potentially bioactive peptides are derived from the selective processing of the ~600 amino acid secretogranin II (SgII) precursor, but only the 31-42 amino acid segment termed secretoneurin (SN) is well-conserved from sharks to mammals. Both SNa and SNb paralogs have been identified in some teleosts, likely arising as a result of the specific genome duplication event in this lineage. Only one copy of the putative lamprey SgII (188 amino acids) could be identified which gives rise to a divergent agnathan SN that contains the signature YTPQ-X-LA-X(7)-EL sequence typical of the central core of all known SN peptides. In rodent models, SN has regulatory effects on neuroinflammation and neurotransmitter release, and possesses therapeutic potential for the induction of angiogenesis. The wide distribution of SN in neuroendocrine neurons and pituitary cells suggests important endocrine roles. The clearest example of the endocrine action of SN is the stimulatory effects on pituitary luteinizing hormone release from goldfish pituitary and mouse LβT2 gonadotroph cells, indicative of an important role in reproduction. Several lines of evidence suggest that the SN receptor is most likely a G-protein coupled protein. Microarray analysis of SN effects on dispersed goldfish pituitary cells in vitro reveals novel SN actions that include effects on genes involved in notch signaling and the guanylate cyclase pathway. Intracerebroventricular injection of SN increases feeding and locomotory behaviors in goldfish. Given that SgII appeared early in vertebrate evolution, SN is an old peptide with emerging implications as a new multifunctional hormone.

Secretoneurin stimulates the production and release of luteinizing hormone in mouse L{beta}T2 gonadotropin cells

Secretoneurin (SN) is a functional secretogranin II (SgII)-derived peptide that stimulates luteinizing hormone (LH) production and its release in the goldfish. However, the effects of SN on the pituitary of mammalian species and the underlying mechanisms remain poorly understood. To study SN in mammals, we adopted the mouse LβT2 gonadotropin cell line that has characteristics consistent with normal pituitary gonadotrophs. Using radioimmunoassay and real-time RT-PCR, we demonstrated that static treatment with SN induced a significant increment of LH release and production in LβT2 cells in vitro. We found that GnRH increased cellular SgII mRNA level and total SN-immunoreactive protein release into the culture medium. We also report that SN activated the extracellular signal-regulated kinases (ERK) in either 10-min acute stimulation or 3-h chronic treatment. The SN-induced ERK activation was significantly blocked by pharmacological inhibition of MAPK kinase (MEK) with PD-98059 and protein kinase C (PKC) with bisindolylmaleimide. SN also increased the total cyclic adenosine monophosphate (cAMP) levels similarly to GnRH. However, SN did not activate the GnRH receptor. These data indicate that SN activates the protein kinase A (PKA) and cAMP-induced ERK signaling pathways in the LH-secreting mouse LβT2 pituitary cell line.

A dileucine determinant in the carboxyl terminal sequence of the LHβ subunit is implicated in the regulated secretion of lutropin from transfected GH3 cells

LH and FSH are essential for control of gonadal function. They are synthesized in the same gonadotrope but differ in their mode of secretion. LH release is regulated, while FSH is secreted constitutively. One unique feature of LHβ is a carboxyl terminal hydrophobic heptapeptide. We demonstrated that deleting the heptapeptide diverted the truncated LH dimer to the constitutive pathway in vitro. To examine if the residues of this heptapeptide play a role in LH sorting, leucines 118-119 were substituted with alanine (L118A and L119A, respectively). The intracellular pool of the L118A mutant protein decreased with a corresponding increase in constitutive secretion. Moreover, immunofluorescence microscopy revealed that the L118A mutant exhibited fewer puncta as compared to wild-type LH. L119A behaved similar to wild-type LH, indicating that a single leucine residue at position 118, rather than a dileucine motif, contributes to the process that sorts LH into the regulated pathway.

Regulation and distribution of squirrel monkey chorionic gonadotropin and secretogranin II in the pituitary

Secretogranin II (SgII) is a member of the granin family of proteins found in neuroendocrine and endocrine cells. The expression and storage of SgII in the pituitary gland of Old World primates and rodents have been linked with those of luteinizing hormone (LH). However, New World primates including squirrel monkeys do not express LH in the pituitary gland, but rather CG is expressed. If CG takes on the luteotropic role of LH in New World primates, SgII may be associated with the expression and storage of CG in the pituitary gland. The goal of this study was to evaluate the regulation and distribution of CG and SgII in the squirrel monkey. A DNA fragment containing approximately 750 bp of squirrel monkey SgII promoter was isolated from genomic DNA and found to contain a cyclic-AMP response element that is also present in the human SgII promoter and important for GnRH responsiveness. The squirrel monkey and human SgII promoters were similarly activated by GnRH in luciferase reporter gene assays in LβT2 cells. Double immunofluorescence microscopy demonstrated close association of SgII and CG in gonadotrophs of squirrel monkey pituitary gland. These results suggest that CG and SgII have a similar intercellular distribution and are coregulated in squirrel monkey pituitary gland.

The hypothalamic-pituitary-ovarian axis and manipulations of the oestrous cycle in the brushtail possum

The main purpose of this review is to provide a comprehensive update on what is known about the regulatory mechanisms of the hypothalamic-pituitary-ovarian axis in the brushtail possum, and to report on the outcomes of attempts made to manipulate by hormonal means, these processes in the possum. Over the last 15 years, several unique features of possum reproductive physiology have been discovered. These include an extended follicular phase despite elevated concentrations of FSH during the luteal phase, and early expression of LH receptors on granulosa cells of small antral follicles, suggesting a different mechanism for the selection of a dominant follicle. The use of routine synchronisation protocols that are effective in eutherians has failed to be effective in possums, and so the ability to reliably synchronise oestrus in this species remains a challenge.

LIM homeodomain transcription factor Isl-1 enhances follicle stimulating hormone-beta and luteinizing hormone-beta gene expression and mediates the activation of leptin on gonadotropin synthesis

The Lin-11, Isl-1, and Mec-3 (LIM) homeodomain transcription factor Isl-1 has been reported to be involved in pituitary development in the early stages of mouse embryogenesis. Our recent studies have shown that Isl-1 is mainly located in the pituitary gonadotropes throughout pituitary development and persists to adulthood. We still do not know the physiological functions of Isl-1 expression and its related mechanisms in the pituitary gland. The aim of the present study was to examine the hypothesis that Isl-1 is involved in regulating pituitary gonadotropin hormone (FSH/LH) production by activating FSHβ and LHβ gene expressions. We have shown that Isl-1 activates FSHβ and LHβ subunit promoters and endogenous gene transcription in LβT2 cells. In addition, Isl-1 overexpression significantly increased FSH synthesis and secretion but not LH. The actions of Isl-1 were not observed when the homeodomain or LIM1 domains are mutated. This demonstrates that Isl-1 induction of FSHβ and LHβ is by both direct and indirect binding of Isl-1 to DNA sequences. Furthermore, Isl-1 expressional level was up-regulated in LβT2 cells after exposure to GnRH, activin, and leptin. However, RNA interference-induced knockdown of Isl-1 significantly reduced the effect of leptin but did not obviously influence the stimulating effects of GnRH and activin on LH and FSH production. In conclusion, the results demonstrate that the LIM-homeodomain transcription factor Isl-1 functions to increase FSHβ/LHβ gene transcription, and mediates the effects of leptin on gonadotropin synthesis.

Rerouting of a follicle-stimulating hormone analog to the regulated secretory pathway

LH and FSH are produced by the same gonadotrope cells of the anterior pituitary but differ in their mode of secretion. This coordinated secretion of LH and FSH is essential for normal follicular development and ovulation in females and for spermatogenesis in males. The structural signals encoded in the LH and FSH subunits that govern the intracellular sorting of LH through the regulated secretory pathway and FSH through the constitutive pathway are largely unknown. Our laboratory recently identified the seven amino acid carboxy tail of LH beta as a sorting signal for LH in GH(3) cells. Here we compared the morphological features of GH(3) cells expressing an FSH analog containing the heptapeptide (FL7AA) with wild-type FSH using confocal microscopy. These experiments were performed to develop a rerouting model for examining structure-function links between secretion pathways of FSH/LH and their biological action. Both FSH- and LH-expressing cells exhibit a fluorescence pattern of randomly dispersed cytoplasmic puncta. FL7AA expressing cells have more intracellular accumulation compared with wild-type FSH and display a unique halo pattern of fluorescence near the plasma membrane. Such a pattern was not observed in cells expressing FSH or LH. Our results demonstrate that this FSH analog containing the carboxy heptapeptide of LH beta is rerouted to the regulated secretory pathway in GH(3) cells. This rerouted gonadotropin provides a unique model to study the trafficking, regulation, and function of LH and FSH.

Sulfation of LH does not affect intracellular trafficking

LH and FSH are produced by the same gonadotrope cells of the anterior pituitary but differ in their mode of secretion. LH secretion is primarily episodic, or regulated, while FSH secretion is primarily basal, or constitutive. The asparagine (N)-linked oligosaccharides of LH and FSH terminate with sulfate and sialic acid, respectively. TSH also contains sulfated N-linked oligosaccharides and is secreted through the regulated pathway. It has been hypothesized that sulfate plays a role in segregating LH to the regulated pathway. Using a mouse pituitary model, we tested this hypothesis by examining the secretory fate of LH from pituitaries treated with sodium chlorate, a known inhibitor of sulfation. Here we show that mouse LH is sulfated and secreted through the regulated pathway, while FSH is secreted constitutively. LH secretion from chlorate-treated pituitaries, which showed complete inhibition of sulfation, was similar to untreated pituitaries. These data suggest that the metabolic role for sulfated N-linked oligosaccharides is not for intracellular trafficking but for the extracellular bioactivity of LH.

The secretogranin II-derived peptide secretoneurin stimulates luteinizing hormone secretion from gonadotrophs

Secretoneurin (SN) is a 33- to 34-amino acid neuropeptide derived from secretogranin-II, a member of the chromogranin family. We previously synthesized a putative goldfish (gf) SN and demonstrated its ability to stimulate LH release in vivo. However, it was not known whether goldfish actually produced the free SN peptide or whether SN directly stimulates LH release from isolated pituitary cells. Using a combination of reverse-phase HPLC and mass spectrometry analysis, we isolated for the first time a 34-amino acid free gfSN peptide from the whole brain. Moreover, Western blot analysis indicated the existence of this peptide in goldfish pituitary. Immunocytochemical localization studies revealed the presence of SN immunoreactivity in prolactin cells of rostral pars distalis of the anterior pituitary. Additionally, we found that magnocellular cells of the goldfish preoptic region are highly immunoreactive for SN. These neurons send heavily labeled projections that pass through the pituitary stalk and innervate the neurointermediate and anterior lobes. In static 12-h incubation of dispersed pituitary cells, application of SN antiserum reduced LH levels, whereas 1 and 10 nM gfSN, respectively, induced 2.5-fold (P < 0.001) and 1.9-fold (P < 0.01) increments of LH release into the medium, increases similar to those elicited by 100 nM concentrations of GnRH. Like GnRH, gfSN elevated intracellular Ca(2+) in identified gonadotrophs. Whereas we do not yet know the relative contribution of neural SN or pituitary SN to LH release, we propose that SN could act as a neuroendocrine and/or paracrine factor to regulate LH release from the anterior pituitary.

Gene expression and secretion of LH and FSH in relation to gene expression of GnRH receptors in the brushtail possum (Trichosurus vulpecula) demonstrates highly conserved mechanisms

In eutherian mammals, the gonadotrophins (LH and FSH) are synthesized and stored in gonadotroph cells under the regulation of multiple mechanisms including GnRH. Very little is known about the regulation of gonadotrophin secretion and storage in pituitary glands of marsupials. This study revealed, using quantitative PCR and heterologous RIA techniques, thatLHBmRNA expression levels remained constant over the oestrous cycle, regardless of the presence of a preovulatory LH surge, which is characteristic of a hormone secreted under regulation. Our sampling regime was unable to detect pulses of LH during the follicular phase, althoughGNRHRmRNA levels had increased at this time. Pulses of LH were, however, detected in the luteal phase of cycling females, in anoestrus females and in males. There was a positive correlation between gene expression ofFSHBand plasma levels of FSH at different stages of the oestrous cycle and no pulses of FSH were detected at any time; all characteristics of a hormone secreted via the constitutive pathway. Usingin situhybridisation and immunohistochemistry methods, we determined that mRNA expression ofLHBandFSHB, and protein storage of gonadotrophins exhibited a similar pattern of localisation within the pituitary gland. Additionally, sexual dimorphism of gonadotroph populations was evident. In summary, these findings are similar to that reported in eutherians and considering that marsupial evolution diverged from eutherians over 100 million years ago suggests that the regulation of gonadotrophins is highly conserved indeed.

3', 5'-cyclic adenosine 5'-monophosphate response element-dependent transcriptional regulation of the secretogranin II gene promoter depends on gonadotropin-releasing hormone-induced mitogen-activated protein kinase activation and the transactivator activating transcription factor 3

Previous studies demonstrated that GnRH-induced secretogranin II (SgII) promoter regulation required a consensus cAMP response element (CRE) and protein kinase A/CRE binding protein. The present studies examined the role of additional components of the GnRH signaling network on SgII promoter activity with particular attention devoted to CRE-dependent gene regulation. Disruption of the SgII CRE by mutagenesis resulted in inhibition of GnRH agonist (GnRHa) induction of this promoter in alphaT3-1 cells. Pharmacological and dominant-negative inhibition of the ERK and c-Jun N-terminal kinase (JNK) signaling pathways revealed that GnRHa-induced SgII promoter activity required functional JNK and ERK modules. Combined inhibition of both pathways nearly abolished GnRHa-induced SgII promoter activity. Specific induction of the ERK cascade alone using overexpression of Raf-CAAX was not sufficient to activate the SgII gene promoter. In contrast, overexpression of the catalytic domain of the more pleiotropic MAPK activator, MAPK/ERK kinase-1, was sufficient to induce SgII promoter activity. The effect(s) of mitogen-activated protein/ERK kinase-1 on SgII promoter activity was CRE dependent and was reversed by the combined pharmacological inhibition of both JNK and ERK modules. CRE DNA binding studies demonstrated the recruitment of activating transcription factor (ATF)-3 and c-Jun to the CRE after administration of GnRHa to alphaT3-1 cells. Specific small interfering RNA knockdown of ATF3 reduced ATF3 DNA binding and the effect of GnRHa on the SgII promoter. These studies support the conclusion that MAPK signaling and ATF3 action are essential for full SgII promoter activation by GnRHa through a canonical CRE. Moreover, we suggest that within the GnRH signaling network, CRE-dependent gene regulation in general may be mediated primarily through the immediate early response gene ATF3.

Effects of a depot formulation of the GnRH agonist leuprorelin on the ultrastructure of male rat pituitary gonadotropes

To clarify the acute and chronic effects of GnRH agonists on pituitary gonadotropes, changes in the ultrastructure of male rat gonadotropes were examined immunocytochemically and morphometrically after the administration of a one-month depot formulation of the GnRH agonist, leuprorelin. Immediately after the depot administration, the relative amounts of secretory granules drastically decreased in gonadotropes concomitantly with a marked increase in the plasma LH level. After the acute hyperstimulated phase, secretory granules in gonadotropes were gradually restored although the newly synthesized granules were less densely immunolabeled for LHbeta; their relative amounts and sizes were still significantly smaller than the controls after depot treatment for 28 days. Eighty-four days after the leuprorelin depot administration, however, the ultrastructural characteristics of pituitary gonadotropes appeared to recover as observed in controls: there were no significant differences in the relative amounts, sizes, and labeling densities for LHbeta of secretory granules, and the amounts of chromogranin A (CgA) and secretogranin II (SgII) were restored in secretory granules to control levels. When the rats were repeatedly treated with the leuprorelin depot at intervals of 4 weeks, the expression and intracellular storage levels of gonadotropins remained highly suppressed, judging from the labeling density for LHbeta. These findings suggest that the depot formulation of the GnRH agonist could suppress both the biosynthesis and release of gonadotropins for a month by synergistically depleting the intracellular storage of secretory granules at the onset of the treatment and by inducing the subsequent desensitization of the GnRH receptor signaling.

Comparative response of rams and bulls to long-term treatment with gonadotropin-releasing hormone analogs

The objective was to compare the relative response between rams and bulls in characteristics of LH, FSH and testosterone (T) secretion, during and after long-term treatment with GnRH analogs. Animals were treated with GnRH agonist, GnRH antagonist, or vehicle (Control) for 28 days. Serial blood samples were collected on day 21 of treatment, and at several intervals after treatment. Injections of natural sequence GnRH were used to evaluate the capacity of the pituitary to release gonadotropins during and after treatment. Treatment with GnRH agonist increased basal LH and T concentrations in both rams and bulls, with a greater relative increase in bulls. Endogenous LH pulses and LH release after administration of GnRH were suppressed during treatment with GnRH agonist. Treatment with GnRH antagonist decreased mean hormone concentrations, LH and T pulse frequency, and the release of LH and T after exogenous GnRH, with greater relative effects in bulls. Rams previously treated with antagonist had a greater release of LH after administration of GnRH compared with control rams, while rams previously treated with agonist showed a reduced LH response. Bulls previously treated with agonist had reduced FSH concentrations and LH pulse amplitudes compared with control bulls while bulls previously treated with antagonist had greater T concentrations and pulse frequency. The present study was the first direct comparison between domestic species of the response in males to treatment with GnRH analogs. The findings demonstrated that differences do occur between rams and bulls in LH, FSH and testosterone secretion during and after treatment. Also, the consequences of treatment with either GnRH analog can persist for a considerable time after discontinuation of treatment.

Fetal gonadotrope cell origin of FSH-secreting pituitary adenoma - insight into human pituitary tumour pathogenesis

OBJECTIVE

The pathogenesis of human pituitary adenomas remains unclear, but we report a case of FSH-secreting pituitary adenoma whose monohormonal phenotype suggests it was of fetal origin.

PATIENTS

A 28-year-old woman presented with abdominal discomfort and irregular menses, enlarged multicystic ovaries and elevated serum oestradiol, with sustained high-normal FSH and low LH levels.

MEASUREMENTS

Endocrine studies were performed before and after curative surgery, with assessment of tumour hormone secretion in vitro, and immunostaining of tumour tissue for a series of gonadotrope proteins.

RESULTS

Immunocytochemistry showed that tumour cells were monohormonal for FSH. Normal components of gonadotrope signalling pathways were expressed, including oestrogen receptor-alpha, activin receptors, secretogranin-II and chromogranin-A. beta-glycan, the putative inhibin coreceptor, was absent. Tumour culture in vitro confirmed secretion of FSH with minimal LH, that was unsuppressed by oestradiol or inhibin-A. Human fetal pituitary tissue contained FSH-only cells at 18 weeks gestation, whereas normal adult pituitary tissue contained only bihormonal gonadotropes.

CONCLUSIONS

We propose that this pituitary adenoma represents an indolent tumour of monohormonal fetal gonadotrope cells that originated early in gestation. Pituitary tumours may therefore arise from abnormal persistence of fetal cell types, with extremely slow growth over many years until reaching a size threshold to generate an endocrine syndrome. Understanding fetal pituitary architecture and function may be more informative for new insights into pituitary tumour pathogenesis than classical theories of cancer biology that invoke unrestrained cell proliferation.

Identification of transcripts related to high egg production in the chicken hypothalamus and pituitary gland

To identify transcripts related to high egg production expressed specifically in the hypothalamus and pituitary gland of the chicken, two subtracted cDNA libraries were constructed. Two divergently selected strains of Taiwan Country Chickens (TCCs), B (sire line) and L2 (dam line) were used; they had originated from a single population and were further subjected (since 1982) to selection for egg production to 40 wk of age and body weight/comb size, respectively. A total of 324 and 370 clones were identified from the L2-B (L2-subtract-B) and the B-L2 subtracted cDNA libraries, respectively. After sequencing and annotation, 175 and 136 transcripts that represented 53 known and 65 unknown non-redundant sequences were characterized in the L2-B subtracted cDNA library. Quantitative reverse-transcription (RT)-PCR was used to screen the mRNA expression levels of 32 randomly selected transcripts in another 78 laying hens from five different strains. These strains included the two original strains (B and L2) used to construct the subtracted cDNA libraries and an additional three commercial strains, i.e., Black- and Red-feather TCCs and Single-Comb White Leghorn (WL) layer. The mRNA expression levels of 16 transcripts were significantly higher in the L2 than in the B strain, whereas the mRNA expression levels of nine transcripts, BDH, NCAM1, PCDHA@, PGDS, PLAG1, PRL, SAR1A, SCG2 and STMN2, were significantly higher in two high egg production strains, L2 and Single-Comb WL; this indicated their usefulness as molecular markers of high egg production.

Proteolytic processing and differential distribution of secretogranin-II in goldfish

Secretoneurin (SN) is a 33-34 amino acid neuropeptide derived by endoproteolysis of secretogranin-II (SgII), a chromogranin. A multi-antigenic strategy was used to generate a rabbit polyclonal goldfish SN antiserum that was characterized for Western blot analysis. In the goldfish pituitary two intermediate proteins containing SN and likely processed from the 69.6-kDa SgII precursor were detected. No immunoreactive proteins were observed in the goldfish interrenal, ovary, cerebellum, and telencephalon whereas SgII mRNA was expressed in all these tissues. Immunoreactive levels of the approximately 57 kDa product were higher in the pars distalis (PD) than in the neurointermediate lobe (NIL). The abundance of the approximately 57 kDa protein indicates that this SgII-product containing the SN sequence is a major stored form in secretory granules of the goldfish pituitary. High expression and processing of SN in the hypothalamus and pituitary suggest important roles for SgII-derived peptides in neuroendocrine tissues.

Differential expression and processing of chromogranin A and secretogranin II in relation to the secretory status of endocrine cells

Chromogranin A (CgA) and secretogranin II (SgII) are neuroendocrine secretory proteins that participate in regulation of the secretory pathway and also serve as precursors of biologically active peptides. To investigate whether there is a relationship between the expression, distribution, and processing of CgA and SgII and the degree of secretory activity, we employed two melanotrope subpopulations of the pituitary intermediate lobe that exhibit opposite secretory phenotypes. Thus, although one of the melanotrope subtypes shows high secretory activity, the other exhibits characteristics of a hormone storage phenotype. Our data show that SgII expression levels were higher in secretory melanotropes, whereas CgA expression showed similar rates in both cell subsets. The use of various antibodies revealed the presence of the unprocessed proteins as well as three CgA-derived peptides (67, 45, and 30 kDa) and six SgII-derived peptides (81, 66, 55, 37, 32, and 30 kDa) in both subpopulations. However, the smallest molecular forms of both granins predominated in secretory melanotropes, whereas the largest SgII- and CgA-immunoreactive peptides were more abundant in storage melanotropes, which is suggestive of a more extensive processing of granins in the secretory subset. Confocal microscopy studies showed that CgA immunoreactivity was higher in storage cells, but SgII immunoreactivity was higher in secretory melanotropes. Taken together, our results indicate that SgII and CgA are differentially regulated in melanotrope subpopulations. Thus, SgII expression is strongly related to the secretory activity of melanotrope cells, whereas CgA expression may not be related to secretory rate, but, rather, to hormone storage in this endocrine cell type.

Interaction between secretogranin III and carboxypeptidase E facilitates prohormone sorting within secretory granules

Secretogranin III (SgIII) and carboxypeptidase E (CPE) bind specifically to cholesterol-rich secretory granule (SG) membranes. We previously showed that SgIII binds chromogranin A (CgA) and targets CgA to the SGs in endocrine cells. We investigated the binding of SgIII and CPE because they frequently localize close to the periphery of SGs, and they bind each other in mouse corticotrope-derived AtT-20 cells. In Cpe fat mouse corticotropes, which have defective CPE, proopiomelanocortin (POMC)-derived adrenocorticotrophin hormone (ACTH)-containing peptides were distributed over the entire surface of the SGs, and displayed a regulated secretion by secretagogues. The Cpe fat pituitary exhibited elevated levels of SgIII and CgA, which suggests that they compensate for a sorting function of CPE for POMC and its intermediates to ACTH. Indeed, both SgIII and CgA were able to bind POMC-derived intermediates. In a competitive pull-down assay, excessive SgIII led to a decrease in CPE-bound POMC-derived intermediate molecules, and SgIII pulled-down by anti-ACTH antibody increased proportionately. We suggest that SgIII and CPE form the separate functional sorting complex by anchoring to cholesterol-rich SG membranes, and POMC-derived peptides are transferred from CPE to SgIII, and subsequently to CgA.

Microarray profiling of gene expression patterns in adult male rat brain following acute progesterone treatment

Progesterone can influence various behaviors in adult male rats, however, little is known about which particular genes are regulated by progesterone in the male rat brain. Using focused microarray technology, we where able to define a subset of genes that are responsive to progesterone. Nylon membrane-based cDNA microarrays were used to profile gene expression patterns in the preoptic area/mediobasal hypothalamus (POA/MBH) of male rat brain 7 h following a single injection of progesterone. RNA was isolated from the brains of 6 male rats injected with progesterone and 6 male rats injected with sesame oil. Next, we hybridized the RNA from each animal to individual cDNA microarrays that contained more than 100 target genes, all of which are involved in cAMP and or calcium signaling pathways. Direct side-by-side comparison of all 12 arrays revealed differences in the expression patterns of 12 different genes. We confirmed the data gathered from the arrays on 4 different genes using Real-Time PCR. These data begin to outline the important role played by progesterone in mediating changes in gene expression within the male brain.

Melanotrope cells as a model to understand the (patho)physiological regulation of hormone secretion

Regulation of hormone secretion is a complex process that comprises the sequential participation of numerous subcellular mechanisms. Hormone secretion is dictated by extracellular stimuli that are transduced intracellularly into activation/deactivation of different mechanisms, such as hormone expression, processing and exocytosis, which will ultimately determine the precise availability of hormone to be secreted. Malfunction in any of these steps may result in deficient or excessive hormone release and the subsequent appearance of endocrine disorders. Given the complexity of this system, it is difficult to find appropriate cellular models wherein to investigate the multiple components of the secretory process in a physiologically relevant, experimentally manipulable setting. In this review, we present recent evidence on the use of the intermediate lobe (IL) of the pituitary as a powerful tool to understand different aspects of the regulated secretory pathway. IL is composed of a single endocrine cell type, alpha-melanocyte stimulating hormone (alpha-MSH)-producing melanotropes, a fact that greatly facilitates its study. Furthermore, melanotropes can be separated using classic cell separation techniques into two cell subtypes showing opposite morphophysiological phenotypes of hypo- and hypersecretory cells. Comparison of their gene expression fingerprints has unveiled the existence of certain genes preferentially expressed in each melanotrope subtype. Because of their direct participation in the secretory pathway, we postulate that characterization of these gene products in an endocrine cell type may represent novel and useful markers for reliably determining the general secretory status in an endocrine gland, as well as a valuable new tool to further investigate this complex process.

The pituitary effects of GnRH

Advances in our understanding of the complexity of GnRH actions at the pituitary and the various mechanisms involved in mediating differential LH and FSH biosynthesis and secretion at the gonadotrope, are continually emerging. In this review, we summarise recent studies pertaining to GnRH and GnRH receptor phylogeny, the divergent signalling and trafficking pathways initiated and utilised by GnRH and its receptor, and the pathways that mediate gonadotropin secretion from the gonadotrope.

Seasonal cyclicity of secretogranin-II expression and its modulation by sex steroids and GnRH in the female goldfish pituitary

The secretory vesicle protein secretogranin-II (SgII), a precursor for the bioactive peptide secretoneurin, is expressed at all levels of the goldfish reproductive axis, including the hypothalamus, pituitary and ovaries. These findings led us to hypothesize that SgII is involved in reproduction and is physiologically regulated. We investigated the effects of different sex steroids on pituitary SgII expression throughout the seasonal reproductive cycle of the female goldfish, as well as the effects of GnRH and testosterone on pituitary LHbeta subunit, GH, and SgII expression in sexually recrudescent female fish using northern blot analysis. We demonstrated that SgII expression levels vary seasonally, with levels being highest in winter and lowest in spring. Sex steroids did not alter SgII expression at any of the time periods studied. In sexually mature goldfish, injection of a GnRH agonist stimulated the expression of LHbeta and SgII specifically in the pars distalis but not the neurointermediate lobe of the pituitary. Although testosterone alone did not alter the expression of either of these genes, it did abolish the stimulatory effects of GnRH on both LHbeta and SgII expression. This represents the first study where testosterone is shown to modulate SgII expression in the pituitary.

Gonadotrophin inhibitory hormone depresses gonadotrophin alpha and follicle-stimulating hormone beta subunit expression in the pituitary of the domestic chicken

Studies performed in vitro suggest that a novel 12 amino acid RF amide peptide, isolated from the quail hypothalamus, is a gonadotrophin inhibitory hormone (GnIH). The aim of the present study was to investigate this hypothesis in the domestic chicken. Injections of GnIH into nest-deprived incubating hens failed to depress the concentration of plasma luteinizing hormone (LH). Addition of GnIH to short-term (120 min) cultures of diced pituitary glands from adult cockerels depressed follicle-stimulating hormone (FSH) and LH release and depressed common alpha and FSHbeta gonadotrophin subunit mRNAs, with no effect on LHbeta subunit mRNA. Hypothalamic GnIH mRNA was higher in incubating (out-of-lay) than in laying hens, but there was no significant difference in the amount of hypothalamic GnIH mRNA in out-of-lay and laying broiler breeder hens at the end of a laying year. It is concluded that avian GnIH may play a role in controlling gonadotrophin synthesis and associated constitutive release in the domestic chicken.

No Evidence for Pituitary Priming to Gonadotropin-Releasing Hormone in Relation to Luteinizing Hormone (LH) Secretion Prior to the Preovulatory LH Surge in Ewes1

The purpose of this study was to determine the occurrence of and the regulatory mechanisms involved in priming of the pituitary to GnRH before the preovulatory LH surge in sheep. Experiment 1: Forty-two ewes had progestagen devices removed after 14 days and were assigned to luteal (Lut) or follicular (Foll) groups. Fifteen days later, blood sampling was initiated either immediately or 36 h after induced luteolysis in groups Lut and Foll, respectively. After 4 h, ewes were administered either saline (n = 5) or 250 ng (n = 8) or 10 microg (n = 8) of GnRH. Five ewes per treatment group were killed 1 h later, while remaining animals were blood sampled for a further 7 h. Experiment 2: Eighteen ewes were allocated to Lut and Foll groups (described above). Blood samples were collected from 2 h before GnRH (10 microg) treatment until 7 h after. Despite up-regulated GnRH-R mRNA levels in Foll ewes, pituitary content and plasma levels of LH and LHbeta mRNA levels were similar between groups. Mean FSHbeta mRNA and plasma FSH levels were elevated in Lut ewes but declined after GnRH treatment. Inversely, plasma estradiol and inhibin-A concentrations were higher in Foll ewes and declined after GnRH treatment. Fewer LH(+ve)/secretogranin II(-ve) (SgII(-ve)) granules were present in gonadotropes of Foll ewes, coincident with increased basal LH levels. Fewer smaller sized granules were present after GnRH treatment. In conclusion, there was no evidence of self-priming before onset of the preovulatory LH surge. Constitutive release of LH(+ve)/SgII(-ve) granules may maintain basal LH levels while smaller sized, presumably mature granules may be preferentially released after GnRH stimulation.

Secretogranin III binds to cholesterol in the secretory granule membrane as an adapter for chromogranin A

Granin-family proteins, including chromogranin A (CgA) and secretogranin III (SgIII), are transported to secretory granules (SGs) in neuroendocrine cells. We previously showed that SgIII binds strongly to CgA in an intragranular milieu and targets CgA to SGs in pituitary and pancreatic endocrine cells. In this study, we demonstrated that with a sucrose density gradient of rat insulinoma-derived INS-1 cell homogenates, SgIII was localized to the SG fraction and was fractionated to the SG membrane (SGM) despite lacking the transmembrane region. With depletion of cholesterol from the SGM using methyl-beta-cyclodextrin, SgIII was impaired to bind to the SGM. Both SgIII and CgA were solubilized from the SGM by Triton X-100 in contrast to the Triton X-100 insolubility of carboxypeptidase E. SgIII and carboxypeptidase E strongly bound to the SGM-type liposome in intragranular conditions, but CgA did not. Instead, CgA bound to the SGM-type liposome only in the presence of SgIII. Immunocytochemical and pulse-chase experiments revealed that SgIII deleting the N-terminal lipid-binding region missorted to the constitutive pathway in mouse corticotroph-derived AtT-20 cells. Thus, we suggest that SgIII directly binds to cholesterol components of the SGM and targets CgA to SGs in pituitary and pancreatic endocrine cells.