An alternative gonadotropin-releasing hormone (GnRH) RNA splicing product found in cultured GnRH neurons and mouse hypothalamus

Age-related changes in gonadotropin-releasing hormone (GnRH) splice variants in mouse brain

Gonadotropin-releasing hormone (GnRH) is the primary regulator of the mammalian reproductive axis. We investigated the spatiotemporal expression of GnRH splice variants (V1, V2, and V3) and splicing factors (Srsf7, Srsf9, and Tra-2) in the male mice brain. Further, using in silico tools, we predicted protein structure and the reason for the low translational efficiency of V2 and V3. Messenger RNA levels of GnRH variants and splicing factors were quantified using real-time reverse transcription-polymerase chain reaction at different age groups. Our data show that expression of almost all the variants alters with aging in all the brain regions studied; even in comparison to the hypothalamus, several brain areas were found to have higher expression of these variants. Hypothalamic expression of splicing factors such as Srsf7, Srsf9, and Tra-2 also change with aging. Computational studies have translation repressors site on the V3, which probably reduces its translation efficiency. Also, V2 is an intrinsically disordered protein that might have a regulatory or signaling function. In conclusion, this study provides novel crucial information and multiple starting points for future analysis of GnRH splice variants in the brain.

Development of the gonadotropin-releasing hormone system

This review summarizes the current understanding of the development of the neuroendocrine gonadotropin-releasing hormone (GnRH) system, including discussion on open questions regarding (1) transcriptional regulation of the Gnrh1 gene; (2) prenatal development of the GnRH1 system in rodents and humans; and (3) paracrine and synaptic communication during migration of the GnRH cells.

pHluorin-BACE1-mCherry Acts as a Reporter for the Intracellular Distribution of Active BACE1 In Vitro and In Vivo

β-site APP-cleaving enzyme 1 (BACE1) initiates amyloid precursor protein (APP) cleavage and β-amyloid (Aβ) production, a critical step in the pathogenesis of Alzheimer's disease (AD). It is thus of considerable interest to investigate how BACE1 activity is regulated. BACE1 has its maximal activity at acidic pH and GFP variant-pHluorin-displays pH dependence. In light of these observations, we generated three tandem fluorescence-tagged BACE1 fusion proteins, named pHluorin-BACE1-mCherry, BACE1-mCherry-pHluorin and BACE1-mCherry-EGFP. Comparing the fluorescence characteristics of these proteins in response to intracellular pH changes induced by chloroquine or bafilomycin A1, we found that pHluorin-BACE1-mCherry is a better pH sensor for BACE1 because its fluorescence intensity responds to pH changes more dramatically and more quickly. Additionally, we found that (pro)renin receptor (PRR), a subunit of the v-ATPase complex, which is critical for maintaining vesicular pH, regulates pHluorin's fluorescence and BACE1 activity in pHluorin-BACE1-mCherry expressing cells. Finally, we found that the expression of Swedish mutant APP (APPswe) suppresses pHluorin fluorescence in pHluorin-BACE1-mCherry expressing cells in culture and in vivo, implicating APPswe not only as a substrate but also as an activator of BACE1. Taken together, these results suggest that the pHluorin-BACE1-mCherry fusion protein may serve as a useful tool for visualizing active/inactive BACE1 in culture and in vivo.

Central Precocious Puberty Caused by Novel Mutations in the Promoter and 5'-UTR Region of the Imprinted MKRN3 Gene

Background: Central Precocious Puberty (CPP) is clinically defined by the development of secondary sexual characteristics before the age of 8 years in girls and 9 years in boys. To date, mutations in the coding region of KISS1, KISS1R, PROKR2, DLK1, and MKRN3 genes have been reported as causative for CPP. This study investigated the presence of causative mutations in both the promoter and the 5'-UTR regions of the MKRN3 gene. Methods: Sanger DNA sequencing was used for screening the proximal promoter and 5'-UTR region of the MKRN3 gene in a group of 73 index girls with CPP. Mutations identified were cloned in luciferase reporter gene vectors and transiently transfected in GN11 cells in order to check for changes in the activity of the MKRN3 promoter. GN11 cells were previously checked for Mkrn3 expression using lentivirus mediated knock-down. In silico analysis was implemented for the detection of changes in the mRNA secondary structure of the mutated MKRN3 5'-UTR. Results: Three novel heterozygous mutations (-166, -865, -886 nt upstream to the transcription start site) located in the proximal promoter region of the MKRN3 gene were identified in six non-related girls with CPP. Four of these girls shared the -865 mutation, one the -166, and another one the -886. A 5'-UTR (+13 nt downstream to the transcription start site) novel mutation was also identified in a girl with similar clinical phenotype. Gene reporter assay evaluated the identified promoter mutations and demonstrated a significant reduction of MKRN3 promoter activity in transfected GN11 cells. In silico analysis for the mutated 5'-UTR predicted a significant change of the mRNA secondary structure. The minimum free energy (MFE) of the mutated 5'-UTR was higher when compared to the corresponding wild-type indicating less stable RNA secondary structure. Conclusion: Our findings demonstrated novel genetic alterations in the promoter and 5'-UTR regulatory regions of the MKRN3 gene. These changes add to another region to check for the etiology of CPP.

JAK/STAT signaling pathway gene expression is reduced following Nelf knockdown in GnRH neurons

Hypothalamic gonadotropin releasing hormone (GnRH) is crucial for the proper function of the hypothalamic-pituitary-gonadal (HPG) axis, subsequent puberty, and reproduction. When GnRH neuron migration or GnRH regulation is impaired, hypogonadotropic hypogonadism results. Mutations in the gene for nasal embryonic luteinizing hormone-releasing factor (NELF) have been identified in GnRH-deficient humans. NELF is a predominantly nuclear protein that may participate in gene transcription, but the genes NELF regulates are unknown. To address this question, RNA was extracted from NLT GnRH neuronal cells following either stable Nelf knockdown or scrambled control and subjected to cDNA arrays. Transcription factors and cell migration gene expression was altered most commonly. Members of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway, including Stat1, Stat2, Stat5a, Jak2, Irf7 and Irf9, were significantly down regulated as assessed by RT-qPCR. Protein levels of STAT1, phospho-STAT1, and JAK2 were reduced, but the protein level of phospho-JAK2 was not. These findings suggest a role for NELF in the regulation of the JAK/STAT signaling pathway, which have important functions in GnRH neurons.

WDR11-mediated Hedgehog signalling defects underlie a new ciliopathy related to Kallmann syndrome

WDR11 has been implicated in congenital hypogonadotropic hypogonadism (CHH) and Kallmann syndrome (KS), human developmental genetic disorders defined by delayed puberty and infertility. However, WDR11's role in development is poorly understood. Here, we report that WDR11 modulates the Hedgehog (Hh) signalling pathway and is essential for ciliogenesis. Disruption of WDR11 expression in mouse and zebrafish results in phenotypic characteristics associated with defective Hh signalling, accompanied by dysgenesis of ciliated tissues. Wdr11-null mice also exhibit early-onset obesity. We find that WDR11 shuttles from the cilium to the nucleus in response to Hh signalling. WDR11 regulates the proteolytic processing of GLI3 and cooperates with the transcription factor EMX1 in the induction of downstream Hh pathway gene expression and gonadotrophin-releasing hormone production. The CHH/KS-associated human mutations result in loss of function of WDR11. Treatment with the Hh agonist purmorphamine partially rescues the WDR11 haploinsufficiency phenotypes. Our study reveals a novel class of ciliopathy caused by WDR11 mutations and suggests that CHH/KS may be a part of the human ciliopathy spectrum.

The immune profile induced is crucial to determine the effects of immunocastration over gonadal function, fertility, and GnRH-I expression

PROBLEM

Immunocastration or vaccination against the GnRH-I hormone is a promising alternative to reproductive control in different animal species. Given the low immunogenicity of this hormone, the use of adjuvants becomes necessary.

METHOD OF STUDY

This study evaluated the effects of three adjuvants that induce different immune response profiles over gonadal function, fertility, and expression of GnRH-I. Female mice (n = 6) were vaccinated at days 1 and 30 with a recombinant antigen for immunocastration and different adjuvants that induced preferentially Th1/Th2, Th2, and Th1 immune profiles.

RESULTS

Th1/Th2 response is the most efficient to block reproductive activity in vaccinated animals, reducing the number of luteal bodies and pre-ovulatory follicles. Th2 and Th1/Th2 responses induced an increase in GnRH-I at the hypothalamus.

CONCLUSION

The immune profile induced by different adjuvants is essential on the effects over fertility, gonadal function, and hypothalamic GnRH-I expression in immunocastrated animals.

Generation of kisspeptin-responsive GnRH neurons from human pluripotent stem cells

GnRH neurons are fundamental for reproduction in all vertebrates, integrating all reproductive inputs. The inaccessibility of human GnRH-neurons has been a major impediment to studying the central control of reproduction and its disorders. Here, we report the efficient generation of kisspeptin responsive GnRH-secreting neurons by directed differentiation of human Embryonic Stem Cells and induced-Pluripotent Stem Cells derived from a Kallman Syndrome patient and a healthy family member. The protocol involves the generation of intermediate Neural Progenitor Cells (NPCs) through long-term Bone morphogenetic protein 4 inhibition, followed by terminal specification of these NPCs in media containing Fibroblast Growth Factor 8 and a NOTCH inhibitor. The resulting GnRH-expressing and -secreting neurons display a neuroendocrine gene expression pattern and present spontaneous calcium transients that can be stimulated by kisspeptin. These in vitro generated GnRH expressing cells provide a new resource for studying the molecular mechanisms underlying the development and function of GnRH neurons.

IGF-1 in the Brain as a Regulator of Reproductive Neuroendocrine Function

Given the close relationship among neuroendocrine systems, it Is likely that there may be common signals that coordinate the acquisition of adult reproductive function with other homeo-static processes. In this review, we focus on central nervous system insulin-like growth factor-1 (IGF-1) as a signal controlling reproductive function, with possible links to somatic growth, particularly during puberty. In vertebrates, the appropriate neurosecretion of the decapeptide gonadotropin-releas-ing hormone (GnRH) plays a critical role in the progression of puberty. Gonadotropin-releasing hormone is released in pulses from neuroterminals in the median eminence (ME), and each GnRH pulse triggers the production of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These pituitary hormones in turn stimulate the synthesis and release of sex steroids by the gonads. Any factor that affects GnRH or gonadotropin pulsatility is important for puberty and reproductive function and, among these factors, the neurotrophic factor IGF-1 is a strong candidate. Although IGF-1 is most commonly studied as the tertiary peripheral hormone in the somatotropic axis via its synthesis in the liver, IGF-1 Is also synthesIzed in the brain, within neurons and glia. In neuroendocrine brain regions, central IGF-1 plays roles in the regulation of neuroendocrine functions, including direct actions on GnRH neurons. Moreover, GnRH neurons themselves co-express IGF-1 and the IGF-1 receptor, and this expression is developmentally regulated. Here, we examine the role of IGF-1 acting in the hypothalamus as a critical link between reproductive and other neuroendocrine functions.

A Novel Gonadotropin-Releasing Hormone 1 (Gnrh1) Enhancer-Derived Noncoding RNA Regulates Gnrh1 Gene Expression in GnRH Neuronal Cell Models

Gonadotropin-releasing hormone (GnRH), a neuropeptide released from a small population of neurons in the hypothalamus, is the central mediator of the hypothalamic-pituitary-gonadal axis, and is required for normal reproductive development and function. Evolutionarily conserved regulatory elements in the mouse, rat, and human Gnrh1 gene include three enhancers and the proximal promoter, which confer Gnrh1 gene expression specifically in GnRH neurons. In immortalized mouse hypothalamic GnRH (GT1-7) neurons, which show pulsatile GnRH release in culture, RNA sequencing and RT-qPCR revealed that expression of a novel long noncoding RNA at Gnrh1 enhancer 1 correlates with high levels of GnRH mRNA expression. In GT1-7 neurons, which contain a transgene carrying 3 kb of the rat Gnrh1 regulatory region, both the mouse and rat Gnrh1 enhancer-derived noncoding RNAs (GnRH-E1 RNAs) are expressed. We investigated the characteristics and function of the endogenous mouse GnRH-E1 RNA. Strand-specific RT-PCR analysis of GnRH-E1 RNA in GT1-7 cells revealed GnRH-E1 RNAs that are transcribed in the sense and antisense directions from distinct 5’ start sites, are 3’ polyadenylated, and are over 2 kb in length. These RNAs are localized in the nucleus and have a half-life of over 8 hours. In GT1-7 neurons, siRNA knockdown of mouse GnRH-E1 RNA resulted in a significant decrease in the expression of the Gnrh1 primary transcript and Gnrh1 mRNA. Over-expression of either the sense or antisense mouse GnRH-E1 RNA in immature, migratory GnRH (GN11) neurons, which do not express either GnRH-E1 RNA or GnRH mRNA, induced the transcriptional activity of co-transfected rat Gnrh1 gene regulatory elements, where the induction requires the presence of the rat Gnrh1 promoter. Together, these data indicate that GnRH-E1 RNA is an inducer of Gnrh1 gene expression. GnRH-E1 RNA may play an important role in the development and maturation of GnRH neurons.

Myo10 is required for neurogenic cell adhesion and migration

Myosin X (Myo10), an untraditional member of myosin superfamily, is characterized as an actin-based molecular motor, which plays a critical role in diverse cellular motile events. Previous research by our group has found that Myo10 influenced neuronal radial migration in developing neocortex, but the underlying mechanism is still largely unknown. In this study, we found that knockdown of endogenous Myo10 in a normal gonadotropin-releasing hormone (GnRH) neuronal cell line transfected with the large T antigen (NLT) induced the impairment of cell motility and orientation. In the wound healing assay, with the Golgi complex staining to display cell polarity, Myo10 knockdown cells were randomly oriented compared to the control. Furthermore, suppressing the expression of Myo10 decreased the ability of cell-matrix adhesion. N-cadherin, a calcium-dependent classical cell adhesion molecule, rescued the migration deficiency caused by Myo10 knockdown in cell aggregates and collagen gel assay. These results suggest that Myo10 is required for neurogenic cell migration through N-cadherin mediated cell adhesion.

The metabolite GnRH-(1-5) inhibits the migration of immortalized GnRH neurons

The decapeptide GnRH is an important regulator of reproductive behavior and function. In the extracellular matrix, GnRH is metabolized by the endopeptidase EC3.4.24.15 (EP24.15) to generate the pentapeptide GnRH-(1-5). In addition to its expression in the adult hypothalamus, EP24.15 is expressed along the migratory path of GnRH-expressing neurons during development. Although we have previously demonstrated a role for EP24.15 in the generation of the biologically active pentapeptide GnRH-(1-5) in regulating GnRH expression and mediating sexual behavior during adulthood in rodents, the modulatory role of GnRH-(1-5) in the migration of GnRH neurons during development remains unknown. To address this information gap, we examined the effect of GnRH-(1-5) on the cellular migration of a premigratory GnRH-secreting neuronal cell line, the GN11 cell, using a wound-healing assay. Dose- and time-response studies demonstrated that GnRH-(1-5) significantly delayed wound closure. We then sought to identify the mechanism by which GnRH-(1-5) inhibits migration. Because the cognate GnRH receptor is a G protein-coupled receptor, we examined whether GnRH-(1-5) regulates migration by also activating a G protein-coupled receptor. Using a high-throughput β-arrestin recruitment assay, we identified an orphan G protein-coupled receptor (GPR173) that was specifically activated by GnRH-(1-5). Interestingly, small interfering RNA to GPR173 reversed the GnRH-(1-5)-mediated inhibition on migration of GN11 neurons. Furthermore, we also demonstrate that the GnRH-(1-5)-activated GPR173-dependent signal transduction pathway involves the activation of the signal transducer and activator of transcription 3 in GnRH migration. These findings indicate a potential regulatory role for GnRH-(1-5) in GnRH neuronal migration during development.

Model systems for studying kisspeptin signalling: mice and cells

Kisspeptins are a family of overlapping neuropeptides, encoded by the Kiss1 gene, that are required for activation and maintenance of the mammalian reproductive axis. Kisspeptins act within the hypothalamus to stimulate release of gonadotrophic releasing hormone and activation of the pituitary-gonadal axis. Robust model systems are required to dissect the regulatory mechanisms that control Kiss1 neuronal activity and to examine the molecular consequences of kisspeptin signalling. While studies in normal animals have been important in this, transgenic mice with targeted mutations affecting the kisspeptin signalling pathway have played a significant role in extending our understanding of kisspeptin physiology. Knock-out mice recapitulate the reproductive defects associated with mutations in humans and provide an experimentally tractable model system to interrogate regulatory feedback mechanisms. In addition, transgenic mice with cell-specific expression of modulator proteins such as the CRE recombinase or fluorescent reporter proteins such as GFP allow more sophisticated analyses such as cell or gene ablation or electrophysiological profiling. At a less complex level, immortalized cell lines have been useful for studying the role of kisspeptin in cell migration and metastasis and examining the intracellular signalling events associated with kisspeptin signalling.

SEMA3A, a gene involved in axonal pathfinding, is mutated in patients with Kallmann syndrome

Kallmann syndrome (KS) associates congenital hypogonadism due to gonadotropin-releasing hormone (GnRH) deficiency and anosmia. The genetics of KS involves various modes of transmission, including oligogenic inheritance. Here, we report that Nrp1^sema/sema mutant mice that lack a functional semaphorin-binding domain in neuropilin-1, an obligatory coreceptor of semaphorin-3A, have a KS–like phenotype. Pathohistological analysis of these mice indeed showed abnormal development of the peripheral olfactory system and defective embryonic migration of the neuroendocrine GnRH cells to the basal forebrain, which results in increased mortality of newborn mice and reduced fertility in adults. We thus screened 386 KS patients for the presence of mutations in SEMA3A (by Sanger sequencing of all 17 coding exons and flanking splice sites) and identified nonsynonymous mutations in 24 patients, specifically, a frameshifting small deletion (D538fsX31) and seven different missense mutations (R66W, N153S, I400V, V435I, T688A, R730Q, R733H). All the mutations were found in heterozygous state. Seven mutations resulted in impaired secretion of semaphorin-3A by transfected COS-7 cells (D538fsX31, R66W, V435I) or reduced signaling activity of the secreted protein in the GN11 cell line derived from embryonic GnRH cells (N153S, I400V, T688A, R733H), which strongly suggests that these mutations have a pathogenic effect. Notably, mutations in other KS genes had already been identified, in heterozygous state, in five of these patients. Our findings indicate that semaphorin-3A signaling insufficiency contributes to the pathogenesis of KS and further substantiate the oligogenic pattern of inheritance in this developmental disorder.

Kallmann syndrome is a hereditary developmental disease that affects both the hormonal reproductive axis and the sense of smell. There is a developmental link between the reproductive and olfactory disorders: neuroendocrine cells producing the gonadotropin-releasing hormone that is deficient in the patients normally migrate from the nose to the forebrain along olfactory nerve fibers during embryonic life, and they fail to do so in the patients. Affected individuals usually do not undergo spontaneous puberty. Hormone replacement therapy is the treatment to initiate virilization in males or breast development in females and later to develop fertility in both sexes. This is a genetically heterogeneous disease. Mutations in any of eight causative genes identified so far have been found in approximately 30% of the affected individuals, thus indicating that other genes remain to be discovered. We report on the identification, in 6% of the KS patients, of various loss-of-function mutations in the gene coding for semaphorin-3A, a secreted protein involved in the navigation of olfactory nerve fibers during embryogenesis. The fact that many of these mutations were also detected in clinically unaffected individuals indicates that they must combine with other genetic defects to produce the disease phenotype.

Cell Models for the Study of Sex Steroid Hormone Neurobiology

To date many aspects of neurons and glia biology remain elusive, due in part to the cellular and molecular complexity of the brain. In recent decades, cell models from different brain areas have been established and proven invaluable toward understanding this complexity. In the field of steroid hormone neurobiology, an important question is: what is the profile of steroid hormone receptor expression in these specific cell lines? Currently, a clear summary of such receptor profiling is lacking. For this reason, we summarized in this review the expression of estrogen, progesterone, and androgen receptors in several widely used cell lines (glial and neuronal) derived from the forebrain and midbrain, based on our own data and that from the literature. Such information will aid in the selection of specific cell lines used to test hypotheses related to the biology of estrogens, progestins, and/or androgens.

Phenotypic and molecular characterization of proliferating and differentiated GnRH-expressing GnV-3 cells

GnRH neurons provide the primary driving force upon the neuroendocrine reproductive axis. Here we used GnV-3 cells, a model of conditionally immortalized GnRH-expressing neurons, to perform an analysis of cell cycle and compare the gene expression profile of proliferating cells with differentiated cells. In the proliferation medium, 45 ± 1.5% of GnV-3 cells are in S-phase by FACS analysis. In the differentiation medium, only 9 ± 0.9% of them are in S-phase, and they acquire the characteristic bipolar shape displayed by preoptic GnRH neurons in vivo. In addition, GnV-3 cells in the differentiated state exhibit electrophysiological properties characteristic of neurons. Transcriptomic analysis identified up-regulation of 1931 genes and down-regulation of 1270 genes in cells grown in the differentiation medium compared to cells in the proliferation medium. Subsequent gene ontology study indicated that genes over-expressed in proliferating GnV-3 cells were mainly involved in cell cycle regulations, whereas genes over-expressed in differentiated cells were mainly involved in processes of differentiation, neurogenesis and neuronal morphogenesis. Taken together, these data demonstrate the occurrence of morphological and physiological changes in GnV-3 cells between the proliferating and the differentiated state. Moreover, the genes differentially regulated between these two different states are providing novel pathways potentially important for a better understanding of the physiology of mature GnRH neurons.

Hypothalamic cell lines to investigate neuroendocrine control mechanisms

The hypothalamus is the control center for most physiological processes; yet has been difficult to study due to the inherent heterogeneity of this brain region. For this reason, researchers have turned towards cell models. Primary hypothalamic cultures are difficult to maintain, are heterogeneous neuronal and glial cell populations and often contain a minimal number of viable peptide-secreting neurons. In contrast, immortalized, clonal cell lines represent an unlimited, homogeneous population of neurons that can be manipulated using a number of elegant molecular techniques. Cell line studies and in vivo experimentation are complementary and together provide a powerful tool to drive scientific discovery. This review focuses on three key neuroendocrine systems: energy homeostasis, reproduction, and circadian rhythms; and the use of hypothalamic cell lines to dissect the complex pathways utilized by individual neurons in these systems.

Estrogen regulation of gene expression in GnRH neurons

Estrogen plays an essential role in the regulation of the female reproductive hormone axis, and specifically is a major regulator of GnRH neuronal function in the female brain. GnRH neuronal cell lines were used to explore the direct effects of estradiol on gene expression in GnRH neurons. The presence of estrogen receptor (ER) binding sites was established by a receptor-binding assay, and estrogen receptor alpha and beta mRNA were identified in GN11 cells and ERbeta in GT1-7 cells using RT-PCR analysis of mRNA. ERalpha was more abundantly expressed in GN11 cells than ERbeta as assessed by real-time PCR. Additionally, GN11 cells expressed significantly more of both ERalpha and beta than GT1-7 cells. Functional studies in GN11 and GT1-7 demonstrated estrogen down regulation of endogenous mouse GnRH mRNA levels using quantitative real-time PCR (qRT-PCR). Correspondingly, estradiol also reduced secretion of GnRH from both the GN11 and GT1-7 cell lines. Since estradiol has been shown to regulate progesterone receptor (PR) expression; similar studies were performed demonstrating an estradiol mediated increase in PR in both cell lines. Estradiol regulation of ER expression was also explored and these studies indicated that estradiol decreased ERalpha and ERbeta mRNA levels in a dose-dependent manner in GN11 and GT1-7 cells. These effects were blocked by the addition of the estrogen receptor antagonist ICI 182,780. Both PPT, a specific ERalpha agonist, and DPN, a specific ERbeta agonist, inhibited GnRH gene expression in GN11 cells, but only DPN inhibited GnRH gene expression in GT1-7 cells, consistent with their undetectable levels of ERalpha expression. These studies characterize a direct inhibitory effect of estradiol on GnRH in GnRH neurons, and a direct stimulatory effect of estradiol on PR gene expression. In addition, the agonist studies indicate that there is a functional overlap of ERalpha and ERbeta regulation in GnRH neurons. These studies may give insight into the molecular regulation of estrogen negative feedback in the central reproductive axis.

Nova-1 mediates glucocorticoid-induced inhibition of pre-mRNA splicing of gonadotropin-releasing hormone transcripts

Glucocorticoid (GC) is known to affect the reproductive system by suppressing the gonadotropin-releasing hormone (GnRH) gene expression in the hypothalamus. However, the mechanism of this effect is poorly understood. We show here that the GC-induced reduction of GnRH mRNA is due to attenuation of a post-transcriptional process i.e. splicing of intron A. Treatment of dexamethasone (DEX), a synthetic GC, lowered GnRH mRNA transcripts and was accompanied by reduced excision of the first intron (intron A) from the GnRH pre-mRNA both in vitro and in vivo. While seeking to identify the splicing factors involved in GC-inhibited GnRH pre-mRNA splicing, we found that DEX down-regulated neuro-oncological ventral antigen-1 (Nova-1) mRNA and protein and that knockdown of Nova-1 reduced intron A excision from GnRH pre-mRNA. Nova-1 overexpression reversed the DEX-induced reduction of intron A excision. Nova-1 appears to promote intron A excision by binding to the distal region of exon 1 of the GnRH pre-mRNA. Taken together, our findings indicate that the intron A excision by Nova-1 is a target of GC for down-regulation of GnRH gene expression, and more importantly, we characterized Nova-1, a brain-enriched splicing regulator responsible for GnRH pre-mRNA splicing.

Involvement of headless myosin X in the motility of immortalized gonadotropin-releasing hormone neuronal cells

Myosin X (Myo X), an unconventional myosin with a tail homology 4-band 4.1/ezrin/radixin/moesin (MyTH4-FERM) tail, is expressed ubiquitously in various mammalian tissues. In addition to the full-length Myo X (Myo X FL), a headless form is synthesized in the brain. So far, little is known about the function of this motor-less Myo X. In this study, the role of the headless Myo X was investigated in immortalized gonadotropin-releasing hormone (GnRH) neuronal cells, NLT. NLT cells overexpressing the headless Myo X formed fewer focal adhesions and spread more slowly than the wild-type NLT cells and GFP-expressing NLT cells. In chemomigration assays, the NLT cells overexpressing the headless Myo X migrated shorter distances and had fewer migratory cells compared with the control NLT cells.

Development of an immortalised, post-pubertal gonadotrophin-releasing hormone neuronal cell line

Gonadotrophin-releasing hormone (GnRH) is important in reproduction, although some of the mechanisms for its synthesis and release remain elusive. Progress in understanding the GnRH neurone has been hampered by the limited number and diffuse distribution of the neurone in the mammalian brain. Several stable GnRH-expressing cell lines have been developed using in vivo expression of the simian virus 40 T Antigen (TAg), and they have been helpful for the study of gene expression and neuronal function. However, expression of an immortalising gene may interfere with normal cellular function. We developed a novel GnRH-secreting cell line transgenic mouse model suitable for targeted transformation in post-pubertal mice using a tetracycline-regulated TAg transgene. This clonal cell line, GRT, expresses neuronal markers and GnRH. GRT cells grown in medium containing tetracycline-free serum express increasing mRNA levels of GnRH associated with declining levels of TAg expression. The novelty and ultimately the usefulness of this cell line is that TAg expression, which could affect the GnRH neuronal phenotype, can be regulated by tetracycline.

Temporal and spatial regulation of CRE recombinase expression in gonadotrophin-releasing hormone neurones in the mouse

Gonadotrophin-releasing hormone (GnRH) neurones located within the brain are the final neuroendocrine output regulating the reproductive hormone axis. Their small number and scattered distribution in the hypothalamus make them particularly difficult to study in vivo. The Cre/loxP system is a valuable tool to delete genes in specific cells and tissues. We report the production of two mouse lines that express the CRE bacteriophage recombinase in a GnRH-specific manner. The first line, the GnRH-CRE mouse, contains a transgene in which CRE is under the control of the murine GnRH promoter and targets CRE expression specifically to GnRH neurones in the hypothalamus. The second line, the GnRH-CRETeR mouse, uses the same murine GnRH promoter to target CRE expression to GnRH neurones, but is modified to be constitutively repressed by a tetracycline repressor (TetR) expressed from a downstream tetracycline repressor gene engineered within the transgene. GnRH neurone-specific CRE expression can therefore be induced by treatment with doxycycline which relieves repression by TetR. These GnRH-CRE and GnRH-CRETeR mice can be used to study the function of genes expressed specifically in GnRH neurones. The GnRH-CRETeR mouse can be used to study genes that may have distinct roles in reproductive physiology during the various developmental stages.

Pubertal impairment in Nhlh2 null mice is associated with hypothalamic and pituitary deficiencies

Pubertal development is impaired in mice lacking the basic helix-loop-helix transcription factor Nhlh2. The mechanisms underlying changes in reproduction in Nhlh2-deficient mice (Nhlh2(-/-)) are unclear. Here we show that hypothalamic GnRH-1 content is reduced in adult Nhlh2(-/-) mice as is the number of GnRH-1 neurons localized to mid- and caudal hypothalamic regions. This reduction was detected postnatally after normal migration of GnRH-1 neurons within nasal regions had occurred. Phenotype rescue experiments showed that female Nhlh2(-/-) mice were responsive to estrogen treatment. In contrast, puberty could not be primed in female Nhlh2(-/-) mice with a GnRH-1 regimen. The adenohypophysis of Nhlh2(-/-) mice was hypoplastic although it contained a full complement of the five anterior pituitary cell types. GnRH-1 receptors (GnRHRs) were reduced in Nhlh2(-/-) pituitary gonadotropes as compared with wild type. In vitro assays indicated that Nhlh2 expression is regulated in parallel with GnRHR expression. However, direct transcriptional activity of Nhlh2 on the GnRHR promoter was not found. These results indicate that Nhlh2 plays a role in the development and functional maintenance of the hypothalamic-pituitary-gonadal axis at least at two levels: 1) in the hypothalamus by regulating the number and distribution of GnRH-1 neurons and, 2) in the developing and mature adenohypophysis.

Egr-1 binds the GnRH promoter to mediate the increase in gene expression by insulin

Insulin increases gonadotropin-releasing hormone (GnRH) gene expression in in vitro models of GnRH neurons. Early growth response-1 (Egr-1) is a transcription factor that mediates the effect of insulin on target genes. In the GN11 cell line--an immortalized GnRH-secreting neuronal cell line--insulin maximally increases Egr-1 mRNA after 30min of treatment and Egr-1 protein and GnRH mRNA after 60min of treatment. Egr-1 small interfering RNA blocks the insulin-induced increase in GnRH promoter activity, measured as luciferase expression. Chromatin immunoprecipitation using Egr-1 antibody precipitates DNA in a proximal region of the GnRH promoter but not DNA in a distal region. Mutagenesis of a putative Egr-1 binding site within the proximal region blocks the insulin-induced increase in GnRH promoter activity. Thus, Egr-1 binds the GnRH promoter at a site between -67 and -76bp from the transcriptional start site to mediate the insulin-induced increase in GnRH gene transcription.

Central application of IGF-1 postpones time of vaginal opening in normally fed, but not in food-restricted rats

BACKGROUND/AIMS

Central but also peripheral IGF-1 is suggested to play a role in the initiation of puberty as it directly affects GnRH synthesis and release. A possible intermediate in the effects of IGF-1 on puberty might be the adiposity-signaling hormone leptin, whose plasma levels are decreased in food-restricted (FR) rats.

METHODS

IGF-1 was chronically centrally infused in 23-day-old prepubertal female rats which were either normally fed or 30% FR, and the effects on time of vaginal opening (VO) and plasma leptin levels were monitored.

RESULTS

FR treatment postponed time of VO and decreased plasma leptin levels. In normally fed rats centrally infused with IGF-1, time of VO was found to be postponed to the same extent as FR treatment did. The IGF-1 infusion did not affect plasma leptin levels in normally fed animals but increased leptin levels in the FR group compared to controls. Daily food intake was equal between all groups but body weight course was lower in FR rats. IGF-1 treatment did not significantly affect food intake or body weight course.

CONCLUSION

FR treatment delays the moment of vaginal opening to the same extent as observed in normally fed rats that were centrally infused with IGF-1.

Molecular and evolutionary characterization of the GnRH-II gene in the chicken: Distinctive genomic organization, expression pattern, and precursor sequence

Of all the structural variants of GnRH (gonadotropin-releasing hormone), GnRH-II has been found to be universally present in and uniquely conserved among jawed vertebrates without any sequence substitutions. Our previous study found that the GnRH-II precursor sequences have become divergent in the lineage of eutherian mammals, based on a comparison between reptilian and mammalian GnRH-II. To elucidate the molecular evolution of GnRH-II throughout amniotes, we have performed the first identification of the avian GnRH-II cDNA/gene from the chicken, the species used for the initial discovery of GnRH-II peptide. Gene arrangement around the GnRH-II in the chicken was similar to that in mammals; however, a gene MRPS26 was partly overlapped with the downstream part of the GnRH-II in the chicken. It was identified that the GnRH-II/MRPS26 locus generated at least five distinct types of transcripts with different expression patterns and three of them may produce functional GnRH-II decapeptide. Sequence comparison revealed that the prepro-GnRH-II polypeptide of the chicken was substantially different from those of other species regarding the length and similarity. The present results strongly indicated that considerable variations were generated in the precursor sequence of the evolutionarily conserved GnRH-II during amniote evolution. It was also suggested that the sequence divergence seen in the chicken may have occurred independently of that in the mammalian lineage.

Insulin regulation of GnRH gene expression through MAP kinase signaling pathways

In mammals, reproduction is acutely regulated by metabolic status. Insulin is an important nutritional signal from the periphery that may regulate the reproductive axis. To determine whether insulin acts directly on the GnRH neuron, we performed studies in mouse-derived GnRH-expressing cell lines. Both insulin receptor protein and mRNA were detected in these cells. A saturation radioligand binding assay revealed high affinity, low capacity binding sites for insulin in GnRH neurons. Insulin also stimulated GnRH promoter activity in GnRH neurons. This effect was blocked by pretreatment with the MEK inhibitor, PD98059, indicating a role for MAP kinase signaling. In transient transfection studies, insulin treatment stimulated expression of a 1250 bp mouse GnRH gene promoter fragment four-fold when compared to promoter activity in untreated cells. In contrast, insulin did not stimulate activity of a 587 bp fragment of the mGnRH gene promoter, indicating that the promoter elements mediating insulin stimulation of the GnRH promoter are located between -1250 and -587 bp. Our studies suggest that insulin may regulate reproductive function by direct effects on the GnRH neurons and specifically by stimulating GnRH gene expression.

Expression and differential effects of the activation of glucocorticoid receptors in mouse gonadotropin-releasing hormone neurons

Prenatal exposure of rodents to glucocorticoids (Gc) affects the sexual development of the offspring, possibly interfering with the differentiation of the hypothalamic-pituitary-gonadal axis. Glucocorticoid receptors (GR) are present on gonadotropin-releasing hormone (GnRH) neurons in the rat hypothalamus, suggesting a direct effect of Gc in the control of the synthesis and/or release of the hormone. In this study, we demonstrate the colocalization of immunoreactive GR with GnRH in a subpopulation of mouse hypothalamic GnRH neurons, confirming the possible involvement of Gc in mouse GnRH neuronal physiology. Receptor-binding assay, RT-PCR, immunocytochemistry, and immunoblotting experiments carried out in GN11 immortalized GnRH neurons show the presence of GR even in the more immature mouse GnRH neurons and confirm the expression of GR in GT1-7 mature GnRH cells. In GN11 cells, the activation of GR with dexamethasone produces nuclear translocation, but does not lead to the inhibition of GnRH gene expression already reported in GT1-7 cells. Long-term exposure of GN11 cells to dexamethasone induces an epithelial-like phenotype with a reorganization of F-actin in stress fibers. Finally, we found that Gc treatment significantly decreases the migratory activity in vitro and the levels of phosphorylated focal adhesion kinase of GN11 immature neurons. In conclusion, these data indicate that GR are expressed in mouse hypothalamic GnRH neurons in vivo as well as in the immature GN11 GnRH neurons in vitro. Moreover, the effects of the GR activation in GN11 and in GT1-7 cells may be related to the neuronal maturational stage of the two cell lines, suggesting a differential role of Gc in neuronal development.

Depolarization differentially affects the secretory and migratory properties of two cell lines of immortalized luteinizing hormone-releasing hormone (LHRH) neurons

In this report we studied and compared the biochemical and the electrophysiological characteristics of two cell lines (GT1-7 and GN11) of immortalized mouse LHRH-expressing neurons and the correlation with their maturational stage and migratory activity. In fact, previous results indicated that GN11, but not GT1-7, cells exhibit an elevated motility in vitro. The results show that the two cell lines differ in terms of immunoreactivity for tyrosine hydroxylase and nestin as well as of production and release of 3,4-dihydroxyphenylalanine (DOPA) and of intracellular distribution and release of the LHRH. Patch-clamp recordings in GN11 cells, reveal the presence of a single inward rectifier K+ current indicative of an immature neuronal phenotype (neither firing nor electrical activity). In contrast, as known from previous studies, GT1-7 cells show the characteristics of mature LHRH neurons with a high electrical activity characterized by spontaneous firing and excitatory postsynaptic potentials. K+-induced depolarization induces in GT1-7 cells, but not in GN11 cells, a strong increase in the release of LHRH in the culture medium. However, depolarization of GN11 cells significantly decreases their chemomigratory response. In conclusion, these results indicate that GT1-7 and GN11 cells show different biochemical and electrophysiological characteristics and are representative of mature and immature LHRH neurons, respectively. The early stage of maturation of GN11 cells, as well as the low electrical activity detected in these cells, appears to correlate with their migratory activity in vitro.

Excision of the first intron from the gonadotropin-releasing hormone (GnRH) transcript serves as a key regulatory step for GnRH biosynthesis

The mammalian gonadotropin-releasing hormone (GnRH) gene consists of four short exons (denoted as 1, 2, 3, and 4) and three intervening introns (A, B, and C). Recently, we demonstrated that excision of the first intron (intron A) from the GnRH transcript is regulated in a tissue- and developmental stage-specific fashion and is severely attenuated in hypogonadal (hpg) mouse because of its lack of exonic splicing enhancers (ESE) 3 and 4. In the present study, we examined the influence of intron A on translational efficiency, thereby establishing a post-transcriptional control over GnRH biosynthesis. First, we verified that an intron A-retained GnRH transcript is a splicing variant but not a splicing intermediate. Intron A-retained transcripts can be transported to the cytoplasm in contrast to intron B-containing transcripts, which are restricted to the nucleus. This result implicates the intron A-retained GnRH transcript as a splicing variant; it has a long 5'-untranslated region, as the GnRH prohormone open reading frame (ORF) begins on exon 2. We investigated whether an intron A-retained GnRH transcript can properly initiate translation at the appropriate start codon and found that intron A completely blocks the translation initiation of its downstream reporter ORF both in vivo and in vitro. The inhibition of translation initiation appears to be due to the presence of a tandem repeat of ATG sequences within intron A. Constructs bearing mutations of ATGs to AAGs restored translation initiation at the downstream start codon; the extent of this restoration correlated with the number of mutated ATGs. Besides the failure in the translation initiation of GnRH-coding region in the intron A-containing variant, the present study also suggests that the interference between mature GnRH mRNA and intron A-retained splicing variant could occur to lower the efficiency of GnRH biosynthesis in the GT1-1-immortalized GnRH-producing cell line. Therefore, our results indicate that the precise and efficient excision of intron A and the joining of adjacent exons may be a critical regulatory step for the post-transcriptional regulation of GnRH biosynthesis.

Cell differentiation of gonadotropin-releasing hormone neurons and alternative RNA splicing of the gonadotropin-releasing hormone transcript

Two different, yet related issues regarding gonadotropin-releasing hormone (GnRH), i.e. the development and differentiation of hypothalamic GnRH neurons and the alternative RNA splicing of GnRH gene transcripts, are addressed in the present review. Using the immortalized GnRH-producing GT1 cell line, we found that activation of protein kinase C (PKC) with 12-O-tetradecanoylphorbol-13-acetate induces morphological and functional differentiation of these neurons. Specific isoforms of PKC are involved in neurite growth, cell migration and synaptic contacts and involve different signaling pathways. Using an in vitro splicing assay with HeLa nuclear extract, we found that excision of the first intron of the GnRH primary transcript is attenuated in non-GnRH-producing cells, but not in GnRH-producing cells such as GT1. This attenuation was relieved by exonic splicing enhancers located in the GnRH exons 3 and 4. Interestingly, addition of nuclear extract derived from GT1 cells further increased the excision rate of intron A, indicating that GnRH neurons contain TRANS-acting splicing factors. Extensive biochemical analysis indicates that Tra2alpha, a serine/arginine-rich RNA-binding protein, and other cofactors are likely involved in mediating neuron-specific excision of intron A from the GnRH primary transcript. An understanding of the GnRH neuron-specific splicing machinery provides critical insight into the molecular mechanism of GnRH gene regulation and consequently of mammalian reproductive development.

Evolutionary aspects of cellular communication in the vertebrate hypothalamo-hypophysio-gonadal axis

This review emphasizes the comparative approach for developing insight into knowledge related to cellular communications occurring in the hypothalamus-pituitary-gonadal axis. Indeed, research on adaptive phenomena leads to evolutionary tracks. Thus, going through recent results, we suggest that pheromonal communication precedes local communication which, in turn, precedes communication via the blood stream. Furthermore, the use of different routes of communication by a certain mediator leads to a conceptual change related to what hormones are. Nevertheless, endocrine communication should leave out of consideration the source (glandular or not) of mediator. Finally, we point out that the use of lower vertebrate animal models is fundamental to understanding general physiological mechanisms. In fact, different anatomical organization permits access to tissues not readily approachable in mammals.

Exonic splicing enhancer-dependent splicing of the gonadotropin-releasing hormone premessenger ribonucleic acid is mediated by tra2alpha, a 40-kilodalton serine/arginine-rich protein

In an earlier study, we found that excision of the first intron (intron A) from the rat GnRH primary transcript is attenuated in non-GnRH-producing cells. This attenuation can be partially relieved by exonic splicing enhancers (ESEs) located in GnRH exons 3 and 4. In the present study, we confirmed that intron A of the mouse GnRH pre-mRNA was not excised in a HeLa nuclear extract (NE) in vitro or in COS-7 cells in vivo. Intron A could, however, be partially removed when exon 3 and/or 4 were linked to exon 2. In the presence of an ESE in exon 4 (ESE4), an addition of GT1 NE further increased the excision rate of intron A, whereas the addition of KK1 (a non-GnRH-producing cell) NE decreased it. To define the GnRH neuron-specific splicing activity, GT1 NE was fractionated by ultracentrifugation and ammonium sulfate precipitation. A 50-90% ammonium sulfate pellet (ASP50-90) fraction was further precipitated with 20 mM MgCl(2) to isolate a serine/arginine-rich (SR) protein fraction. Among the ASP fractions, ASP40-50 significantly increased the excision rate of intron A in the presence of HeLa NE or SR protein-rich fraction. However, the ASP40-50 fraction alone could not remove intron A. This result suggests the presence of a cofactor protein(s) in the ASP40-50 fraction that may mediate the interaction between a 3' spliceosome complex and the ESE4-SR protein complex. UV cross-linking and gel mobility shift analysis revealed that Tra2alpha but not other SR proteins tested, specifically binds to ESE4. Moreover, Tra2alpha stimulated intron A excision in a dose-dependent manner. These results imply that Tra2alpha and a cofactor protein in the ASP40-50 fraction are involved in mediating the GnRH neuron-specific excision of intron A from the GnRH primary transcript.

Identification of a discrete promoter region of the human GnRH gene that is sufficient for directing neuron-specific expression: a role for POU homeodomain transcription factors

The human GnRH (hGnRH) gene is expressed, and the GnRH decapeptide produced, primarily in the GnRH neurons of the diencephalon. The molecular elements important for the cell-specific expression and regulation of the hGnRH gene are not well established at this time; therefore, we have used a transgenic mouse model to isolate cis-regulatory elements important for directing gene expression to GnRH neurons in the hypothalamus. Gene constructs consisting of various promoter deletion fragments of the hGnRH gene fused to the luciferase (LUC) reporter gene have been used to create transgenic mouse lines. Cell-specific expression, with the criterion being luciferase expression directed to GnRH neurons of the hypothalamus, was observed when 992 bp, but not 795 bp, of the hGnRH gene promoter were used. Tissue-specific expression was also observed when a deletion construct containing the region from -992 to -763 was fused to a minimal 48-bp promoter fragment fused to LUC. These data indicate that the region between -992 and -795 contains elements both essential and sufficient for targeting gene expression to GnRH neurons. This promoter region was found to contain two DNA-binding sites for the POU class of transcription factors, each of which specifically interacted with the POU homeodomain proteins Brn-2 and Oct-1. Functional studies demonstrated that Brn-2 increased promoter activity of the human and mouse GnRH genes.

Novel mechanism for gonadotropin-releasing hormone neuronal migration involving Gas6/Ark signaling to p38 mitogen-activated protein kinase

Gonadotropin-releasing hormone (GnRH) is the central regulator of the reproductive axis. Normal sexual maturation depends on the migration of GnRH neurons from the olfactory placode to the hypothalamus during development. Previously, we showed restricted expression of the membrane receptor adhesion-related kinase (Ark) in immortalized cell lines derived from migratory but not postmigratory GnRH neurons. In addition, Ark and GnRH transcripts were detected along the GnRH neuron migratory route in the E13 mouse cribriform plate. In the present study, we examined the role of Ark and its ligand, Gas6 (encoded by growth arrest-specific gene 6), in GnRH neuron migration. Gas6 stimulated lamellipodial extension, membrane ruffling, and chemotaxis of immortalized NLT GnRH neuronal cells via the Ark receptor. Gas6/Ark signaling promoted activation of the Rho family GTPase Rac, and adenoviral-mediated expression of dominant negative N17Rac abolished Gas6/Ark-induced actin cytoskeletal reorganization and migration of GnRH neuronal cells. In addition, p38 MAPK was activated downstream of Ark and Rac, and inhibition of p38 MAPK with either SB203580 or adenoviral dominant negative p38alpha also blocked Gas6/Ark-mediated migration. Finally, downstream of Rac and p38 mitogen-activated protein kinase (MAPK), Gas6/Ark signaling promoted activation of MAPK-activated protein kinase 2 and induced phosphorylation of HSP25, a known regulator of cortical actin remodeling. The data are the first to demonstrate a migratory signaling pathway downstream of Ark/Axl family receptors and suggest a previously unidentified role for p38 MAPK in neuronal migration. Furthermore, these studies support a potential role for Ark in the regulation of GnRH neuronal migration.

Molecular and cellular properties of GnRH neurons revealed through transgenics in the mouse

Recent advances in the use of gonadotropin-releasing hormone (GnRH) promoter-driven transgenics in the mouse are beginning to open up the once elusive GnRH neuronal phenotype to detailed molecular and cellular investigation. This review highlights progress in the development of GnRH promoter transgenic constructs and the understanding of murine gene sequences required for the correct temporal and spatial targeting of transgenes to the GnRH phenotype in vivo. Strategies enabling the identification of single, living GnRH neurons in the acute brain slice preparation are allowing gene profiling and electrophysiological experiments to be undertaken. Results so far indicate that, like other neurons, GnRH cells express a variety of sodium, potassium and calcium channels as well as GABAergic and glutamatergic receptors which are responsible for determining the membrane properties and firing characteristics of the GnRH neuron. Many of these receptors and channels appear to be expressed heterogeneously within the GnRH phenotype. Furthermore, several display distinct postnatal developmental expression profiles which are likely to be of consequence to the development of synchronized, pulsatile GnRH secretion in the adult animal.

Luteinizing hormone-releasing hormone (LHRH) neurons: mechanism of pulsatile LHRH release

Many types of neurons and glia exhibit oscillatory changes in membrane potentials and cytoplasmic Ca2+ concentrations. In neurons and neuroendocrine cells an elevation of intracellular Ca2+ concentration is associated with neurosecretion. Since both oscillatory membrane potentials and intracellular Ca2+ oscillations have been described in primary LHRH neurons and in GT1 cells, it is evident that an endogenous pulse-generator/oscillator is present in the LHRH neuron in vitro. The hourly rhythms of LHRH neurosecretion appear to be the synchronization of a population of LHRH neurons. How a network of LHRH neurons synchronizes their activity, i.e., whether by the result of synaptic mechanisms or electrical coupling through gap junctions or through a diffusible substance(s), remains to be clarified. Even though LHRH neurons themselves possess an endogenous pulse-generating mechanism, they may be controlled by other neuronal and nonneuronal elements in vivo. NE, NPY, glutamate, and GABA are neurotransmitters possibly controlling pulsatile LHRH release, and NO, cAMP, and ATP may be diffusible substances involved in pulsatile LHRH release without synaptic input. Although synaptic inputs to the perikarya of LHRH neurons could control the activity of LHRH neurons, a line of evidence suggests that direct neuronal and nonneuronal inputs, especially those from astrocytes to LHRH neuroterminals, appear to be more important for pusatile LHRH release.

Transcription factor activator protein-2 is required for continued luteinizing hormone-releasing hormone expression in the forebrain of developing mice

LHRH is the neuropeptide responsible for reproductive function. Prenatally, LHRH expression begins when neurons are in the olfactory pit and continues as these cells migrate into the brain. Thus, LHRH neurons maintain neuropeptide expression through very distinct environments. The regulatory interactions that control onset and continued expression of the LHRH phenotype are unknown. To begin to address this question primary LHRH neurons were removed from nasal explants at different ages. A complementary DNA (cDNA) subtraction screen was performed comparing a 3.5-days in vitro LHRH neuron [approximately embryonic day 15 (E15) in vivo] to two 10.5-days in vitro LHRH neurons (approximately postnatal day 1 in vivo). The transcription factor activator protein-2 (AP-2alpha) was differentially expressed and was present in the developmentally younger LHRH neuron. In vivo analysis revealed that LHRH neurons expressed AP-2 as they migrated across the cribriform plate and into the forebrain beginning on E13.5, but that coexpression of LHRH and AP-2 was no longer detected in postnatal day 1 animals. This suggested a regulatory role for AP-2 in LHRH neurons. Analysis of animals lacking AP-2alpha revealed a dramatic decrease in forebrain LHRH neurons between E13.5 and E14.5, correlating with normal onset of AP-2 expression in LHRH neurons as they entered the central nervous system. Nasal cells robustly expressing LHRH were still present on E 14.5. The continued presence of forebrain LHRH cells is proposed based on a second marker, galanin, and lack of increased apoptotic/necrotic cells in this region. A decrease in LHRH messenger RNA in forebrain neurons indicates regulation of LHRH occurred at the transcriptional or posttranscriptional level in mutant animals. These results indicate a developmentally restricted involvement of the transcription factor AP-2 in LHRH expression once the LHRH neurons have migrated into the forebrain, but before establishment of an adult-like distribution.

Tissue-specific alternative mRNA splicing of phenylethanolamine N-methyltransferase (PNMT) during development by intron retention

The expression of phenylethanolamine N-methyl transferase (EC 2. 1.1.2.8, PNMT), the final enzyme in the cascade of catecholamine synthesis, is differentially regulated in adrenergic neurons in the brain and in adrenal chromaffin cells. Using reverse transcription-polymerase chain reaction-based techniques, we detected in the prenatal developing rat brainstem, two species of PNMT mRNA which were produced by a rare alternative splicing mechanism known as intron retention. The spliced, intronless message was downregulated postnatally, while the intron-retained mRNA species continued to be constitutively expressed through adulthood. By contrast in the adrenals, at all stages of development examined, only the intronless message was expressed. In line with previous reports on the failure of glucocorticoids to induce PNMT expression in the brain, the pattern of PNMT splicing in brainstem explants was not affected by the presence of the synthetic glucocorticoid dexamethasone. Undifferentiated sympathoadrenal PC12 pheochromocytoma cells expressed very low basal levels of both mRNA variants, accompanied by a very low basal PNMT enzymatic activity. Exposure of PC12 cells to dexamethasone resulted in the upregulation of only the spliced mRNA variant concomitant with a 3-fold increase in PNMT enzymatic activity. In contrast, treatment of PC 12 cells with nerve growth factor (NGF) enhanced the expression of both the intron-retained and the intronless mRNA species without changes in the basal enzyme activity. This latter result suggests that the translation of the intronless mRNA species may be regulated by the intron-retained mRNA species, which by itself may yield a truncated, yet enzymatically functional translational product. Our data suggest that the tissue-specific regulation of PNMT expression is based on a rare alternative splicing mechanism termed intron retention, and that in the adrenal, but not in the brain, this mechanism is sensitive to regulation by glucocorticoids. Thus, this system is uniquely suited for studying the hormonal control of tissue-specific splicing in the nervous system.

Spatiotemporal appearance of developing LHRH neurons in the rat brain

To obtain insight into the development of the heterogeneous intracerebral populations of luteinizing hormone-releasing hormone (LHRH) neurons, their spatiotemporal appearance was examined at different stages in normal rat embryos, in nasal epithelial explants in vitro, and in intrauterine nasal-operated embryos. Following the appearance of nerve cell adhesion molecule in the nasal placode at embryonic day (E) 12.5, LHRH neurons, generated in the nasal placode at E13.5, penetrated the forebrain vesicle (FV) by E14.5-15.5. After E16.5, as the FV elongated to form the olfactory bulb, the migrating neurons traversed posteriorly through the interhemispheric space to penetrate the septopreoptic (S-P) area. By E18.5, LHRH neurons were detected in the preoptic-diagonal band (P-D) area as well as in the S-P region, along with some scattered extrahypothalamic LHRH neurons. To determine the source of these neurons, we separately cultured dissected parts of E12.5 nasal pit epithelium. Neuronal generation was predominantly from the medial wall epithelium (NAP), but some LHRH neurons originated in the roof epithelium. Cocultures of the NAP (E12.5) with the FV, median eminence-arcuate complex, Rathke's pouch, mesencephalon, or medulla oblongata from E14.5 embryos revealed the ability of LHRH cells to penetrate all of these tissues. Uni- or bilateral nasal destruction was conducted at E16.5 or E15.5, respectively, and examined at E18.5 and E21.5. In the operated embryos, most LHRH neurons were present in the P-D system and some in the S-P area. This finding suggests that the neurons generated before E15.5 are primarily predisposed to form the P-D system, whereas those derived afterward form the S-P system.