Proopiomelanocortin gene expression, prohormone processing, and secretion during rat pituitary development

Fetal and Neonatal HPA Axis

Stress is an integral part of life. Activation of the hypothalamus-pituitary-adrenal (HPA) axis in the adult can be viewed as mostly adaptive to restore homeostasis in the short term. When stress occurs during development, and specifically during periods of vulnerability in maturing systems, it can significantly reprogram function, leading to pathologies in the adult. Thus, it is critical to understand how the HPA axis is regulated during developmental periods and what are the factors contributing to shape its activity and reactivity to environmental stressors. The HPA axis is not a passive system. It can actively participate in critical physiological regulation, inducing parturition in the sheep for instance or being a center stage actor in the preparation of the fetus to aerobic life (lung maturation). It is also a major player in orchestrating mental function, metabolic, and cardiovascular function often reprogrammed by stressors even prior to conception through epigenetic modifications of gametes. In this review, we review the ontogeny of the HPA axis with an emphasis on two species that have been widely studied-sheep and rodents-because they each share many similar regulatory mechanism applicable to our understanding of the human HPA axis. The studies discussed in this review should ultimately inform us about windows of susceptibility in the developing brain and the crucial importance of early preconception, prenatal, and postnatal interventions designed to improve parental competence and offspring outcome. Only through informed studies will our public health system be able to curb the expansion of many stress-related or stress-induced pathologies and forge a better future for upcoming generations.

Impaired neonatal survival of pro-opiomelanocortin null mutants

Intercrosses of heterozygous pro-opiomelanocortin (POMC) mice result in homozygous null progeny at lower frequencies than expected. Genotyping offspring at pre-, peri-, and postnatal stages revealed that over half of homozygous null mutants die in the early postnatal stages. To investigate the reasons for this early postnatal lethality, we analyzed in detail different parameters in the initial hours after birth. POMC null mutants born to heterozygous dams presented at birth with corticosterone levels no different from wildtype littermates, were euglycemic, and had normal liver glycogen stores. However, already 30 min after birth corticosterone levels dropped by 80% and were undetectable thereafter, while corticosterone levels in wildtype animals increased during postnatal hours. Circulating adrenaline was almost below detection 1h after birth. Blood glucose levels fell sharply in all genotypes within 30 min after birth; however, wildtype and heterozygous pups overcame hypoglycemia within an hour, while mutant pups stayed hypoglycemic. The depletion of liver glycogen stores in mutant pups was significantly less efficient compared to their littermates in the hours after birth. POMC null mutant mice born to POMC null mutant dams completely lack corticosterone and die of the expected respiratory dysfunction. In contrast, POMC null mutant mice born to heterozygous dams do not die of respiratory problems, but rather due to hypoglycemia. Our studies confirm an essential involvement of POMC peptides and of adrenal glucocorticoids and catecholamines on glucose homeostasis critical for early postnatal survival.

Maternal leukemia inhibitory factor (LIF) promotes fetal neurogenesis via a LIF-ACTH-LIF signaling relay pathway

Leukemia inhibitory factor (LIF) promotes the proliferation of neuronal progenitor cells in the cerebrum. However, it remains unclear how fetal LIF level is regulated. Here we show evidence that maternal LIF signals drive fetal LIF levels via the placenta, thereby promoting neurogenesis in the fetal brain in rats. Chronological changes showed that LIF concentration in fetal sera (FS) and fetal cerebrospinal fluid peaked at gestational day (GD) 15.5, after the peak of maternal LIF at GD14.5. LIF injection into rat dams at GD15.5 increased the level of ACTH in FS and subsequently increased LIF levels in FS and fetal cerebrospinal fluid. The elevation of fetal LIF after LIF injection into dams was inhibited by in utero injection of anti-ACTH antibody into fetuses. Cultured syncytiotrophoblasts, which express the LIF receptor and glycoprotein 130, were induced to secrete ACTH and up-regulate Pomc expression by the addition of LIF. Nucleated red blood cells from fetuses at GD15.5, but not GD13.5 or GD17.5, displayed LIF secretion in response to ACTH. Moreover, injection of LIF into dams at GD13.5 or GD17.5 did not result in elevation of ACTH or LIF in fetuses. The labeling index of 5-bromo-2'-deoxyuridine-positive cells in the ventricular zone of the cerebral neocortex increased 24 h after injection of LIF into dams at GD15.5 but not GD13.5 or GD17.5. These results suggest that in rats maternal LIF induces ACTH from the placenta, which in turn induces fetal nucleated red blood cells to secrete LIF that finally increases neurogenesis in fetuses around GD15.

Differentiation of rat hypothalamic dopaminergic neurons is stimulated in vitro by target cells: the melanotrophs

We have investigated in vitro the influence of pituitary intermediate lobe melanotrophs on the differentiation of their afferent hypothalamic dopaminergic neurons. The presence of melanotrophs in primary cultures of foetal hypothalamic neurons induces an increase of the number of dopaminergic neurons (while the total neuronal population remains unchanged) and induces a stimulation of their neuritic outgrowth. These effects are mediated by diffusible factors since they are reproduced by application of conditioned medium issued from co-cultures with intermediate lobe cells from newborn rats. Moreover, by immunoneutralization of alpha-melanocyte-stimulating hormone (alphaMSH) in the co-culture or conditioned medium, or by application of the peptide itself, we demonstrate that the neuritotrophic effect on dopaminergic neurons is mediated by alphaMSH, the main secretory product of melanotrophs, whereas the inductive effect on the number of dopaminergic neurons is attributable to another diffusible neurotrophic factor(s) present in foetal, but not adult, adenohypophysis. Similar effects are observed on cultures of newborn hypothalamic neurons. However, at this stage of neuronal development, alphaMSH also increases the number of dopaminergic neurons, which could be due to a change of neuronal receptivity. We show that the neuritotrophic influence of alphaMSH is restricted to the dopaminergic neurons connected to the melanotrophs, and that in addition, these neurons systematically co-express the tyrosine hydroxylase and glutamate decarboxylase as the neurons innervating the melanotrophs in situ. These findings indicate that the differentiation of dopaminergic hypothalamic neurons is influenced by the target cells, melanotrophs, and that this trophic influence implicates alphaMSH.

Co-culture of hypothalamic neurons and melanotrope cells: a model to study synaptogenesis between central neurons and endocrine cells

As a first step towards elucidating mechanisms involved in neuroendocrine synaptogenesis, we developed a model of co-culture based on hypothalamic-intermediate pituitary interactions. Dissociated hypothalamic neurons from fetal rats at embryonic day 15 were cultured in a defined medium together with melanotrope cells of the pituitary intermediate lobe from neonatal rats. In these co-cultures, establishment of synaptic contacts between GABAergic or dopaminergic neurons and an endocrine target cell the melanotrope cell, was studied by morphofunctional approaches. Using double immunostaining with antibodies directed against glutamate decarboxylase or tyrosine hydroxylase and alpha-melanocyte-stimulating hormone, we demonstrated morphological contacts between GABAergic or dopaminergic neurons and melanotrope cells as early as three days in vitro. Furthermore, using an antibody directed against synapsin I, we showed a modification of synapsin I immunoreactivity from diffuse to punctate distribution correlated with the establishment of contacts and the observation of characteristic neuroendocrine synapses by electron microscopy. These results were further confirmed by electrophysiological studies. Patch-clamp recordings demonstrated that, at six days in vitro, some melanotrope cells displayed GABAergic synaptic currents, which occurred either spontaneously and/or could be evoked chemically by 50 mM KCl or 100 microM kainate. The proportion of the melanotrope cells receiving functional synaptic inputs increased until 10 days in culture, a stage at which virtually all melanotrope cells in contact with neurons possessed functional synapses. The results presented here describe the establishment of neuroendocrine synapses in vitro, studied by combining morphofunctional and electrophysiological approaches.

Ontogeny of glucocorticoid and D2 receptors in the rat pituitary: an in situ hybridization study

The expression of glucocorticoid and D2 dopamine receptors (GR and D2R) during rat pituitary ontogenesis was studied by in situ hybridization (ISH). On early stages, E13-E14, a weak specific signal for GR mRNA was obvious in the whole Rathke's pouch (RP) whereas subsequently, from E17-E18, strong labelling was restricted to the anterior lobe (AL) and the neural lobe (NL). At the same time, D2R mRNAs appeared in the intermediate lobe (IL) and the long isoform of the D2R (D2R 444) was detectable with specific probes. On the postnatal stages, until adult, GR mRNA, if present, was always undetectable in the IL using the conventional ISH technique. These data indicate a possible early regulation of POMC gene expression by glucocorticoid in corticotrophic cells of the AL and by dopamine in the melanotrophic cells of the IL. The possibility of a negative regulation of GR mRNA by dopamine (DA) in the IL as soon as E17 is discussed.

Ontogeny of the prohormone convertases PC1 and PC2 in the mouse hypophysis and their colocalization with corticotropin and alpha-melanotropin

In the adult pituitary, anterior lobe corticotrophs and intermediate lobe melanotrophs differentially process proopiomelanocortin (POMC). Within the corticotrophs, POMC is processed mainly to corticotropin (ACTH) and beta-lipotropin, while alpha-melanotropin (alpha MSH) and beta-endorphin are the major end products in the melanotrophs. The observed transient presence of alpha MSH-like immunoreactivity during ontogeny suggested an age-dependent variation in POMC processing in the adenohypophysis. In this tissue, cell-specific POMC products are likely the result of differential expression of the two known prohormone convertases PC1 and PC2. In the present ontogeny study done in the mouse intermediate and anterior pituitary, we examined how the expression pattern of PC1 and PC2 mRNA transcripts correlates with that of ACTH and alpha MSH-like immunoreactivities. Our data demonstrated that both PC1 and PC2 transcripts can be detected in the presumptive adenohypophysis starting on embryonic day 15 (E15). In the intermediate lobe, PC1 and PC2 mRNAs appear on E18 and E16, respectively, and their levels increased during ontogeny, reaching maximal expression in the adult. Similarly, PC1 expression in the anterior pituitary increased from E15 to adulthood. However, PC2 mRNA expression peaked between postnatal days 1 (P1) and 14 (P14) and then decreased to adult levels. The distribution of PC1 and PC2 immunoreactivity is nicely correlated with the in situ hybridization data. In the anterior lobe, during the P1-P14 postnatal period, PC2 immunoreactivity was detected within cells synthesizing an alpha MSH-like peptide(s). This observation substantiates our earlier biochemical data suggesting that PC2 is the important convertase in the processing of POMC into alpha MSH. Furthermore, the demonstrated variation in the relative ratio of PC1/PC2 expression during ontogeny rationalizes the observed plasticity of POMC processing in the adenohypophysis. It is expected that beta-endorphin processing will follow that of alpha MSH.

Identification of DNA elements cooperatively activating proopiomelanocortin gene expression in the pituitary glands of transgenic mice

The proopiomelanocortin (POMC) gene is highly expressed in adult mouse pituitary anterior lobe corticotrophs and intermediate lobe melanotrophs. To identify the DNA elements important for this tissue-specific expression, we analyzed a series of POMC reporter genes in transgenic mice. A DNA fragment containing rat POMC 5'-flanking sequences from -323 to -34 recapitulated both basal pituitary cell-specific and hormonally stimulated expression in adult mice when fused to a heterologous thymidine kinase promoter. Developmental onset of the reporter gene expression lagged by 1 day but otherwise closely paralleled the normal ontogeny of murine POMC gene expression, including corticotroph activation at embryonic day 14.5 (E14.5) followed by melanotroph activation at E15.5 to E16.5. AtT20 corticotroph nuclear protein extracts interacted with three specific regions of the functional POMC promoter in DNase I protection assays. The positions of these protected sites were -107 to -160 (site 1), -182 to -218 (site 2), and -249 to -281 (site 3). Individual deletions of these footprinted sites did not alter transgene expression; however, the simultaneous deletion of sites 2 and 3 prevented transgene expression in both corticotrophs and melanotrophs. Electrophoretic mobility shift and Southwestern (DNA-protein) assays demonstrated that multiple AtT20 nuclear proteins bound to these footprinted sites. We conclude that the sequences between -323 and -34 of the rat POMC gene promoter are both necessary and sufficient for correct spatial, temporal, and hormonally regulated expression in the pituitary gland. Our data suggest that the three footprinted sites within the promoter are functionally interchangeable and act in combination with promoter elements between -114 and -34. The inability of any reporter gene construction to dissociate basal and hormonally stimulated expression suggests that these DNA elements are involved in both of these two characteristics of POMC gene expression in vivo.

Identification of DNA elements cooperatively activating proopiomelanocortin gene expression in the pituitary glands of transgenic mice

The proopiomelanocortin (POMC) gene is highly expressed in adult mouse pituitary anterior lobe corticotrophs and intermediate lobe melanotrophs. To identify the DNA elements important for this tissue-specific expression, we analyzed a series of POMC reporter genes in transgenic mice. A DNA fragment containing rat POMC 5'-flanking sequences from -323 to -34 recapitulated both basal pituitary cell-specific and hormonally stimulated expression in adult mice when fused to a heterologous thymidine kinase promoter. Developmental onset of the reporter gene expression lagged by 1 day but otherwise closely paralleled the normal ontogeny of murine POMC gene expression, including corticotroph activation at embryonic day 14.5 (E14.5) followed by melanotroph activation at E15.5 to E16.5. AtT20 corticotroph nuclear protein extracts interacted with three specific regions of the functional POMC promoter in DNase I protection assays. The positions of these protected sites were -107 to -160 (site 1), -182 to -218 (site 2), and -249 to -281 (site 3). Individual deletions of these footprinted sites did not alter transgene expression; however, the simultaneous deletion of sites 2 and 3 prevented transgene expression in both corticotrophs and melanotrophs. Electrophoretic mobility shift and Southwestern (DNA-protein) assays demonstrated that multiple AtT20 nuclear proteins bound to these footprinted sites. We conclude that the sequences between -323 and -34 of the rat POMC gene promoter are both necessary and sufficient for correct spatial, temporal, and hormonally regulated expression in the pituitary gland. Our data suggest that the three footprinted sites within the promoter are functionally interchangeable and act in combination with promoter elements between -114 and -34. The inability of any reporter gene construction to dissociate basal and hormonally stimulated expression suggests that these DNA elements are involved in both of these two characteristics of POMC gene expression in vivo.

Differentiation of the melanotrophic cells of rat pituitary primordium in organotypic culture in defined medium

Organotypic cultures, in defined medium, of pituitary primordia obtained from 15-day-old rat fetuses were performed in order to study the in vitro differentiation of melanotrophic cells. The morphological and ultrastructural features of the transplants resembled those of the gland developing in vivo. In situ hybridization on semi-thin sections, using a 35S-labelled oligonucleotide probe, revealed pro-opiomelanocortin-mRNA-containing cells on the first day of culture in the anterior lobe and after 2-3 days in the intermediate lobe. Immunoperoxidase labelling of adjacent sections showed that the same cells reacted with antibodies against alpha-melanocyte-stimulating hormone (alpha MSH), gamma 3MSH and adrenocorticotropic hormone in both lobes. The pro-opiomelanocortin-mRNA-containing cells formed progressively conspicuous areas in the intermediate lobe, which was almost uniformly labelled after 6 days. In the anterior lobe, these cells remained scattered in small cell groups, and colloidal gold immunolabelling showed the progressive disappearance of alpha MSH labelling from the secretory vesicles in cells exhibiting morphological features of adult corticotrophic cells. Both the alpha MSH content of the explants and alpha MSH release into the culture medium increased with time. Treatment with the dopamine agonist bromocriptine induced a strong dose-dependent decrease in alpha MSH secretion, which was significant after 3 days in culture, indicating that dopamine D2 receptors are able to regulate hormonal release of melanotrophic cells at early stages. This system constitutes a suitable model for further studies of factors controlling cell differentiation and cellular interactions involved in histogenesis.

Prenatal processing of pro-opiomelanocortin in the brain and pituitary of mouse embryos

The processing of pro-opiomelanocortin (POMC) to ACTH- (adrenocorticotropin), MSH- (melanotropin) and endorphin-related peptides was studied in mouse embryos with the ultimate aim of determining the role of the POMC-related peptides in early development especially in the CNS. Mouse embryos at gestational days 10.5, 11.5, 12.5 and 14.5 were analyzed for POMC-derived peptides by SDS-PAGE, HPLC and radioimmunoassay using antisera specific for various regions of the prohormone. At embryonic day 10.5 (E 10.5) the prohormone was the major product detected. At E 11.5, POMC was processed to ACTH(1-39), des-acetyl alpha-MSH and beta-endorphin(1-31) and beta-endorphin(1-27). The amounts of these peptides increased at E 12.5, and at E 14.5. At E 14.5, there was a major increase in ACTH(1-39) and beta-endorphin(1-31) peptides. This was attributed to the large increase of corticotrophs in anterior pituitary at this stage. Des-acetyl alpha-MSH levels, however, were similar at E 12.5 and E 14.5 and the peptide was confined mainly to the central nervous system. gamma-MSH was not detected until E 16.5 in the brain. No alpha-MSH or acetylated beta-endorphin was detected between E 11.5 and E 14.5. Thus in early embryonic development, POMC is processed to des-acetyl alpha-MSH, beta-endorphin(1-31), beta-endorphin(1-27) and gamma-MSH in the brain, and primarily to ACTH(1-39) and beta-endorphin(1-31) in the anterior pituitary. Some differences exist in the forms of POMC-derived peptides found in embryonic versus adult brain and pituitary. The embryonic forms of the peptides may be significant in playing a role during development.

Pro-opiomelanocortin mRNA and peptide co-expression in the developing rat pituitary

Pro-opiomelanocortin (POMC) is synthesized in both the pituitary gland and the brain. Various peptide products of this precursor, namely beta-endorphin, ACTH and alpha-MSH are co-localized in the anterior lobe corticotrophs, all intermediate lobe cells and in hypothalamic neurons. Messenger RNA (mRNA) for POMC has further been shown to exist in these tissues. In this study, we have shown that POMC mRNA, and peptide accumulation as detected by in situ hybridization and immunocytochemistry, respectively, occur simultaneously within the rat pituitary gland during ontogeny and that their maturation occurs in parallel during prenatal and early postnatal development.

Ontogeny of proopiomelanocortin (POMC) gene expression in the intermediate lobe of the rat pituitary gland

The ontogeny of proopiomelanocortin (POMC) gene expression in the intermediate lobe of the pituitary was studied in rats of both sexes using quantitative in situ hybridization performed on fixed pituitary sections. Electron microscopic studies revealed in the adult animal the hybridization signal was found in all the secretory cells of the intermediate lobe and only in the POMC cells in the anterior lobe, thus indicating the specificity of the technique used. Hybridization signal was first detected on day 15 of gestation and progressively increased during foetal life. After birth, POMC and mRNA levels markedly increased so that, in one-day old animals, they were 4.5-fold higher than at the end of gestation. Thereafter, mRNA concentrations steadily increased to reach a plateau at 60 days of age in both sexes. No sexual dimorphism was observed at any ages. The results indicate that the development of intermediate lobe cells occurs mostly after birth. They are consistent with previous results indicating a 200-fold increase in the levels of POMC-derived peptides from birth to adulthood.

Ontogenesis of proopiomelanocortin gene expression and regulation in the rat pituitary intermediate lobe

During ontogenesis, proopiomelanocortin (POMC) mRNA appears in the pituitary intermediate lobe (IL) at embryonic day 16 (E16), rather later than in the hypothalamic arcuate nucleus (E13) or the pituitary anterior lobe (E15). POMC mRNA onset in the IL correlates with the appearance of POMC-derived peptides detected by immunocytochemistry (ICC), indicating that there is probably no time lag between POMC mRNA translation. Subsequently, while the IL lobular organization developed progressively, the number of in situ hybridization- (ISH) and ICC-positive cells increased until after birth. During postnatal development, coinciding with innervation of the IL, the POMC mRNA level in the lobe, measured by quantitative ISH, increased about 4-fold to reach the adult value at weaning. The effects of acute or chronic postnatal treatment with a dopamine antagonist (haloperidol) or a dopamine agonist (bromocriptine) show that the physiological dopaminergic inhibitory control of POMC gene expression operates as early as postnatal day 5. The subsequent increase in mRNA levels despite the inhibitory innervation raises the question of the existence of some unknown positive regulation active during postnatal development.

Cell-specific expression of the prolactin gene in transgenic mice is controlled by synergistic interactions between promoter and enhancer elements

Prolactin gene expression is restricted to the lactotrophic and somatomammotrophic cells of the anterior pituitary. In transgenic mice, a fusion gene consisting of 3 kb of prolactin 5'-flanking region fused to a firefly luciferase or human growth hormone (hGH) reporter gene is expressed at high levels with the strict tissue and cell-type specificity that is characteristic of the endogenous prolactin gene. High levels of expression require two cis-acting regions: a distal enhancer (-1.8 to -1.5 kb) and a proximal region (-422 to +33 bp). Each of these regions alone can direct low levels of fusion gene expression to prolactin-producing cell types in transgenic mice, but a synergistic interaction between these regions is necessary for high levels of expression. The ontogeny of the prolactin transgene expression closely follows the appearance of high levels of a POU homeo-domain transcription factor, Pit-1, that has been shown previously to bind structurally related sequences in both the distal enhancer and proximal regions and to activate the expression of the prolactin gene in vitro. Unexpectedly, transgenes containing the distal enhancer removed from its normal context are expressed in both the prolactin-producing lactotrophs and the thyroid-stimulating hormone (TSH)-producing thyrotrophs, thereby suggesting that sequences flanking this enhancer are necessary to restrict expression to the correct cell type within the pituitary. These data indicate that distinct processes of gene activation and restriction are necessary for the fidelity of cell-type-specific expression within an organ.