Gonadotropin-releasing hormone (GnRH) gene regulation by N-methyl-D-aspartic acid in GT1-1 neuronal cells: differential involvement of c-fos and c-jun protooncogenes

NMDA Receptor and L-Type Calcium Channel Modulate Prion Formation

Transmissible neurodegenerative prion diseases are characterized by the conversion of the cellular prion protein (PrP^C) to misfolded isoforms denoted as prions or PrP^Sc. Although the conversion can occur in the test tube containing recombinant prion protein or cell lysates, efficient prion formation depends on the integrity of intact cell functions. Since neurons are main targets for prion replication, we asked whether their most specialized function, i.e. synaptic plasticity, could be a factor by which PrP^Sc formation can be modulated.Immortalized gonadotropin-releasing hormone cells infected with the Rocky Mountain Laboratory prion strain were treated with L-type calcium channels (LTCCs) and NMDA receptors (NMDARs) stimulators or inhibitors. Western blotting was used to monitor the effects on PrP^Sc formation in relation to ERK signalling.Infected cells showed enhanced levels of phosphorylated ERK (pERK) compared with uninfected cells. Exposure of infected cells to the LTCC agonist Bay K8644 enhanced pERK and PrP^Sc levels. Although treatment with an LTCC blocker (nimodipine) or an NMDAR competitive antagonist (D-AP5) had no effects, their combination reduced both pERK and PrP^Sc levels. Treatment with the non-competitive NMDAR channel blocker MK-801 markedly reduced pERK and PrP^Sc levels.Our study shows that changes in LTCCs and NMDARs activities can modulate PrP^Sc formation through ERK signalling. During synaptic plasticity, while ERK signalling promotes long-term potentiation accompanied by expansion of post-synaptic lipid rafts, other NMDA receptor-depending signalling pathways, p38-JNK, have opposing effects. Our findings indicate that contrasting intracellular signals of synaptic plasticity can influence time-dependent prion conversion.

Estradiol and progesterone modulate the nitric oxide/cyclic gmp pathway in the hypothalamus of female rats and in GT1-1 cells

Considerable evidence suggests that the nitric oxide (NO)/cGMP signaling pathway plays an important role in the expression of reproductive behavior and in gonadotropin-releasing hormone (GnRH) release from the hypothalamus The effects of the NO/cGMP pathway on GnRH release and gene expression have also been examined in GT1 cells. However, it is still controversial whether NO/cGMP signaling facilitates or inhibits GnRH release in these cells. The current study examined the effects of estradiol and progesterone on neuronal NO synthase (nNOS), soluble guanylyl cyclase (sGC), and NO-dependent cGMP production in the preoptic area (POA) and hypothalamus (HYP) as well as in GT1-1 cells. Ovariectomized female rats received vehicle, estradiol benzoate (48 h) and/or progesterone (3-4 h) before preparation of brain slices. GT1-1 cells were incubated with vehicle, estradiol (48 h), progesterone (3-4 h), or with both hormones. The combination of estradiol and progesterone increased the expression of nNOS protein in the POA and HYP. Hormones had little effect on the abundance of sGC. Estradiol and progesterone together greatly enhanced NO-stimulated sGC activity in HYP-POA slices. In GT1-1 cells, NO-stimulated sGC activity was significantly increased by estradiol and progesterone, alone or in combination, but sGC expression was not altered by hormones.

Jun localization in cytosolic and nuclear compartments in brain-pituitary system of the frog, Rana esculenta: an analysis carried out in parallel with GnRH molecular forms during the annual reproductive cycle

The presence of c-jun like mRNA was assessed in the brain of the frog, Rana esculenta, during the annual sexual cycle. In parallel, Jun protein and GnRH molecular form (mammalian and chicken II also indicated as GnRH1 and GnRH2, respectively) activity was studied in order to establish possible relationships. Northern blot analysis of total RNA reveals the presence of a 2.7 kb c-jun-like mRNA. Western blots, carried out on cytoplasmic and nuclear protein extracts, show the presence of Jun immunoreactive band of 39 kDa in brain and pituitary. Fluctuations of c-jun-like mRNA and Jun immunoreactive protein (cytoplasmic and nuclear) levels in brains during the year indicate relationships among transcription, translation, and nuclear activity. In particular, mRNA levels increase gradually from September until November when Jun protein concentration peaks in cytosolic extracts. Conversely, the nuclear protein reaches highest concentration in July when the cytosolic level shows low values. Immunocytochemical studies confirm the presence of Jun immunoreactivity in both cytoplasmic and nuclear compartments of several brain areas, including those primarily involved in gonadotropin discharge (e.g., anterior preoptic area and preoptic nucleus). GnRH molecular forms and Jun are colocalized in anterior preoptic area and preoptic nucleus. Moreover, during the period characterized by GnRH release, Jun levels strongly decrease in nuclei. Finally, we show that treatments with a GnRH analog (buserelin, Hoechst, Frankfurt) increase Jun levels in brain nuclear extracts.

In VitroParadigms for the Study of GnRH Neuron Function and Estrogen Effects

The elaboration of in vitro paradigms has enabled direct study of GnRH secretion and the regulation of this process. Common findings using different models are the pulsatile nature and calcium-dependency of GnRH secretion, the excitatory effect of glutamate, and the inhibitory or excitatory effect of GABA. Among the different paradigms, the fetal olfactory placode cultures exhibit the unique property of migration in vitro and may retain the capacity to undergo maturational changes in vitro. The short-term incubation of hypothalamic explants obtained at different ages enables one to study developmental changes as well. Estrogens may have important roles in the regulation of GnRH function and can act indirectly via the neighboring neuronal/glial apparatus and directly on GnRH neurons at the cell body and terminal levels. A direct effect is supported by the recent localization of ERalpha and ERbeta transcripts in GnRH neurons using most paradigms. Discrepant effects of estrogens on GnRH neurons were observed since GnRH biosynthesis is inhibited while GnRH secretion can be either stimulated, unaffected, or reduced. It is likely that the regulatory role of sex steroids including estradiol is very complex since it could involve direct and indirect effects on GnRH neurons through genomic and/or non-genomic mechanisms.

Regulation of gene promoters of hypothalamic peptides

In order to fulfill their roles in neuroendocrine regulation, specific hypothalamic neurons are devoted to produce and deliver biologically active peptides to the pituitary gland. The biosynthesis and release of peptides are strictly controlled by afferents to these hypothalamic neurons. Cell-specific expression and biosynthetic regulation largely relies on transcription from the gene promoter for which the 5(')-flanking regions of the peptidergic genes contain essential elements. Cell-specific transcription factors employ these regulatory elements to exert their control over the expression of the peptidergic gene. This article explores the properties of regulatory elements of the major hypothalamic peptides, somatostatin, growth hormone-releasing hormone, gonadotropin-releasing hormone, thyrotropin-releasing hormone, corticotropin-releasing hormone, vasopressin and oxytocin, and the transcription factors acting on them. These transcription factors are often endpoints of signal transduction pathways that can be activated by neurotransmitters or steroid hormones. Others are essential to provide cell-specific expression of the peptidergic gene during development and mature regulation.

Regulation of gonadotropin-releasing hormone secretion: insights from GT1 immortal GnRH neurons

The study of the mammalian GnRH system has been greatly advanced by the development of immortalized cell lines. Of particular relevance are the so-called GT1 cells. Not only do they exhibit many of the known physiologic characteristics of GnRH neurons in situ, but in approximately one decade have yielded new insights regarding the intrinsic physiology of individual cells and networks of GnRH neurons, as well as the nature of central and peripheral signals that directly modulate their function. For instance, valuable information has been generated concerning intrinsic properties of the system such as the inherent pulsatile pattern of secretion displayed by networks of GT1 cells. Concepts regarding feedback regulation and autocrine feedback of GnRH neurons have been dramatically expanded. Likewise, the nature of the receptors and of the proximal and distal signal transduction mechanisms involved in the actions of multiple afferent signals has been identified. Understanding this neuronal system allows a better comprehension of the hypothalamic-pituitary-gonadal axis and of the regulatory influences that ultimately control reproductive competence.

Regulation of Gadd45gamma expression by C/EBP

The Gadd45gamma (growth arrest and DNA damage-inducible) gene is activated transcriptionally by at least two kinds of agents: DNA damaging agent such as methyl methanesulfonate (MMS) and UV radiation, or cytokines such as interleukin (IL)-6, IL-2 and granulocyte colony-stimulating factor (G-CSF). To investigate the sequences and transcription factors involved in induction of Gadd45gamma after treatment with IL-6, the human gene was cloned and sequenced. We found C/EBP (CCAAT/enhancer-binding protein) family proteins, major transcription factors in the IL-6 signal transduction pathway, could regulate the transcriptional activity of the Gadd45gamma promoter. In addition, a noncanonical C/EBP-binding site within the Gadd45gamma promoter where C/EBPbeta and C/EBPdelta could bind, was identified by electrophoretic mobility shift assay (EMSA) and reporter gene analysis. Furthermore, we found a coordinated expression profile between Gadd45gamma mRNA and C/EBPs (beta and delta) protein during the differentiation of M1 cells: the amount of Gadd45gamma transcripts became maximal when both C/EBPbeta and C/EBPdelta levels were high, on day 1 of differentiation of M1 cells after treatment with IL-6. These findings suggest that mitotic growth arrest coupled to M1 cell differentiation is mediated by C/EBPs stimulation of growth arrest-associated genes such as Gadd45gamma.

Glutamatergic regulation of gonadotropin releasing hormone mRNA levels during development in the mouse

The aims of these studies were to investigate the time course of the increase in gonadotropin releasing hormone (GnRH) mRNA levels during sexual development in the mouse, and to test the hypothesis that the neurotransmitter glutamate regulates the GnRH secretory system via actions at the level of GnRH gene expression. GnRH mRNA abundance was estimated by measuring silver grains generated by in situ hybridization of an 35S-labelled oligonucleotide probe. There was a significant increase in GnRH mRNA abundance between the day of birth (P0) and postnatal day 2 (P2) in male mice, but no further increases at later ages when overt pubertal changes are manifest. GnRH mRNA levels also increased significantly between P0 and P2 in female mice. Treatment with the glutamate agonist NMDA caused a significant increase in GnRH mRNA levels in neonatal (P0) mice and adult male mice within 30 min of treatment, which is consistent with previous studies in the rat implicating glutamate in the regulation of GnRH mRNA stability. Treatment with the glutamate antagonist CGP40116 caused an equally rapid decrease in GnRH mRNA levels in adult mice and in mice on P5 after the neonatal increase in GnRH gene expression, but was without effect in mice on P0, prior to the developmental increase. These observations that the effect on GnRH mRNA levels of blocking endogenous glutamatergic signalling depends upon the developmental stage suggest that endogenous glutamate maintains GnRH mRNA levels in adult mouse, and is a potential regulator of the developmental increase seen in the neonatal period.