Neuron-restricted expression of the rat gonadotropin-releasing hormone gene is conferred by a cell-specific protein complex that binds repeated CAATT elements

Differential CRE Expression in Lhrh-cre and GnRH-cre Alleles and the Impact on Fertility in Otx2-Flox Mice

There is an increasing trend in studies utilizing cell-specific deletion of genes through conditional gene deletion by CRE recombination. Despite numerous advantages, this strategy also has limitations such as ectopic CRE-expression and germline recombination. Two commonly used gonadotropin-releasing hormone (Gnrh)-driven CRE-expressing mice both target GnRH neurons. However, a direct comparison of the cells targeted and their phenotypic outcome have not yet been presented. To compare where recombination takes place, we crossed the Gnrh-cre and Lhrh-cre lines with the Rosa26-LacZ reporter mouse. Lhrh-cre allowed recombination of the Rosa26-LacZ gene in ∼700 cells, which is comparable to the GnRH neuronal population. Surprisingly, there were > 20 times more LacZ expressing cells in the adult Gnrh-cre:Rosa26-LacZ than the Lhrh-cre:Rosa26-LacZ brain. The greatest differences in targeting of the Gnrh-cre and Lhrh-cre lines were found in the septum, the suprachiasmatic nucleus, and the septohypothalamic area. This difference in cells targeted was present from embryonic day 12. A prior study using the Gnrh-cre to delete the transcription factor Otx2 found fewer GnRH neurons, leading to male and female subfertility. To recapitulate this study, we performed a fertility assay in Otx2:Lhrh-cre mice. We confirmed the requirement for Otx2 in GnRH neuron development, fertility and correct gonadotropin hormone release in Otx2:Lhrh-cre males, but the subfertility was more modest than in Otx2:Gnrh-cre and absent in female Otx2:Lhrh-cre. This suggests that ectopic expression of Gnrh-cre contributes to the reproductive phenotype observed. Finally, the Cre alleles caused germline recombination of the flox allele when transmitted from either parent, generating embryonic lethal knock-out offspring, producing smaller live litters.

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.

Molecular regulation of hypothalamus-pituitary-gonads axis in males

The hypothalamic-pituitary-gonadal axis (HPG) plays vital roles in reproduction and steroid hormone production in both sexes. The focus of this review is upon gene structures, receptor structures and the signaling pathways of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The hormones' functions in reproduction as well as consequences resulting from mutations are also summarized. Specific characteristics of hormones such as the pulsatile secretions of GnRH are also covered. The different regulators of the HPG axis are introduced including kisspeptin, activin, inhibin, follistatin, androgens and estrogen. This review includes not only their basic information, but also their unique function in the HPG axis. Here we view the HPG axis as a whole, so relations between ligands and receptors are well described crossing different levels of the HPG axis. Hormone interactions and transformations are also considered. The major information of this article is depicted in three figures summarizing the current discoveries on the HPG axis. This article systematically introduces the basic knowledge of the HPG axis and provides information of the current advances relating to reproductive hormones.

Disrupted kisspeptin signaling in GnRH neurons leads to hypogonadotrophic hypogonadism

Landmark studies have shown that mutations in kisspeptin and the kisspeptin receptor (Kiss1r) result in reproductive dysfunction in humans and genetically altered mouse models. However, because kisspeptin and its receptor are present in target cells of the central and peripheral reproductive axis, the precise location(s) for the pathogenic signal is unknown. The study described herein shows that the kisspeptin-Kiss1r signaling pathway in the GnRH neuron is singularly critical for both the onset of puberty as well as the attainment of normal reproductive function. In this study, we directly test the hypothesis that kisspeptin neurons regulate GnRH secretion through the activation of Kiss1r on the plasma membrane of GnRH neurons. A GnRH neuron-specific Kiss1r knockout mouse model (GKirKO) was generated, and reproductive development and phenotype were assessed. Both female and male GKirKO mice were infertile, having low serum LH and FSH levels. External abnormalities such as microphallus and decreased anogenital distance associated with failure of preputial gland separation were present in GKirKO males. A delay in pubertal onset and abnormal estrous cyclicity were observed in female GKirKO mice. Taken together, these data provide in vivo evidence that Kiss1r in GnRH neurons is critical for reproductive development and fertility.

Neurokinin B causes acute GnRH secretion and repression of GnRH transcription in GT1-7 GnRH neurons

Genetic studies in human patients with idiopathic hypogonadotropic hypogonadism (IHH) identified mutations in the genes that encode neurokinin B (NKB) and the neurokinin 3 receptor (NK3R). However, determining the mechanism whereby NKB regulates gonadotropin secretion has been difficult because of conflicting results from in vivo studies investigating the luteinizing hormone (LH) response to senktide, a NK3R agonist. NK3R is expressed in a subset of GnRH neurons and in kisspeptin neurons that are known to regulate GnRH secretion. Thus, one potential source of inconsistency is that NKB could produce opposing direct and indirect effects on GnRH secretion. Here, we employ the GT1-7 cell model to elucidate the direct effects of NKB on GnRH neuron function. We find that GT1-7 cells express NK3R and respond to acute senktide treatment with c-Fos induction and increased GnRH secretion. In contrast, long-term senktide treatment decreased GnRH secretion. Next, we focus on the examination of the mechanism underlying the long-term decrease in secretion and determine that senktide treatment represses transcription of GnRH. We further show that this repression of GnRH transcription may involve enhanced c-Fos protein binding at novel activator protein-1 (AP-1) half-sites identified in enhancer 1 and the promoter, as well as chromatin remodeling at the promoter of the GnRH gene. These data indicate that NKB could directly regulate secretion from NK3R-expressing GnRH neurons. Furthermore, whether the response is inhibitory or stimulatory toward GnRH secretion could depend on the history or length of exposure to NKB because of a repressive effect on GnRH transcription.

Mutational analysis of the necdin gene in patients with congenital isolated hypogonadotropic hypogonadism

CONTEXT

Necdin activates GNRH gene expression and is fundamental for the development, migration, and axonal extension of murine GNRH neurons. In humans, necdin plays a potential role in the hypogonadotropic hypogonadism phenotype in patients with Prader-Willi syndrome.

AIM

To investigate necdin gene (NDN) variants in patients with isolated hypogonadotropic hypogonadism (IHH).

PATIENTS AND METHODS

We studied 160 Brazilian patients with IHH, which includes 92 with Kallmann syndrome and 68 with normosmic IHH. Genomic DNA was extracted and the single NDN exon was amplified and sequenced. To measure GNRH transcriptional activity, luciferase reporter plasmids containing GNRH regulatory regions were transiently transfected into GT1-7 cells in the presence and absence of overexpressed wild-type or mutant necdin.

RESULTS

A heterozygous variant of necdin, p.V318A, was identified in a 23-year-old male with Kallmann syndrome. The p.V318A was also present in affected aunt and his father and was absent in 100 Brazilian control subjects. Previous FGFR1 gene analysis revealed a missense mutation (p.P366L) in this family. Functional studies revealed a minor difference in the activation of GNRH transcription by mutant protein compared with wild type in that a significant impairment of the necdin protein activity threshold was observed.

CONCLUSION

A rare variant of necdin (p.V318A) was described in a family with Kallmann syndrome associated with a FGFR1 mutation. Familial segregation and in vitro analysis suggested that this non-synonymous variant did not have a direct causative role in the hypogonadism phenotype. NDN mutations are not a frequent cause of congenital IHH.

The gonadotropin-releasing hormone cell-specific element is required for normal puberty and estrous cyclicity

Appropriate tissue-specific gene expression of gonadotropin-releasing hormone (GnRH) is critical for pubertal development and maintenance of reproductive competence. In these studies, a common element in the mouse GnRH (mGnRH) promoter, between -2806 and -2078 bp, is shown to mediate differential regulation of hypothalamic and ovarian mGnRH expression. To further characterize this region, we generated a knock-out mouse (GREKO(-/-)) with a deletion of the mGnRH promoter fragment between -2806 and -2078 bp. GnRH mRNA expression in the brain of GREKO(-/-) was less than the expression in wild-type mice; however, immunohistochemical analysis revealed no difference between the numbers of GnRH neurons among groups. GnRH mRNA expression in the ovary was fivefold higher in GREKO(-/-). The immunohistochemical staining for GnRH in the ovary increased in surface epithelial and granulosa cells and also in the corpora lutea of GREKO(-/-) mice. The reproductive phenotype revealed that the mean day of vaginal opening was delayed, and additionally, there was a significant decrease in the length of proestrus and diestrus-metestrus phases of the estrous cycle, resulting in a shortened estrous cycle in GREKO(-/-) mice. This work supports the hypothesis that the region of the GnRH promoter contained between -2806 and -2078 bp acts as a cell-specific enhancer in the GnRH neuron and as a repressor in the ovary. Deletion of this region in vivo implicates the GnRH promoter in mediating pubertal development and periodic reproductive cycling, and forms the foundation to define the nuclear proteins important for puberty and estrous cycling in mammals.

Hypothalamic dysregulation and infertility in mice lacking the homeodomain protein Six6

The hypothalamus, pituitary, and gonads coordinate to direct the development and regulation of reproductive function in mammals. Control of the hypothalamic-pituitary-gonadal axis is dependent on correct migration of gonadotropin-releasing hormone (GnRH) neurons from the nasal placode to the hypothalamus, followed by proper synthesis and pulsatile secretion of GnRH, functions absent in patients with hypogonadal hypogonadism. In this study, we identify sine oculis-related homeobox 6 (Six6) as a novel factor necessary for proper targeting of GnRH expression to the limited population of GnRH neurons within the adult mouse hypothalamus and demonstrate that it is required for proper reproductive function in both male and female mice. Female Six6-null mice exhibit a striking decrease in fertility, failing to progress through the estrous cycle normally, show any signs of successful ovulation, or produce litters. Although basal gonadotropin production in these mice is relatively normal, analysis of GnRH expression reveals a dramatic decrease in total GnRH neuron numbers. We show that expression of Six6 is dramatically increased during GnRH neuronal maturation and that overexpression of Six6 induces GnRH transcription in neuronal cells. Finally, we demonstrate that this induction in GnRH expression is mediated via binding of Six6 to evolutionarily conserved ATTA sites located within the GnRH proximal promoter. Together, these data indicate that Six6 plays an important role in the regulation of GnRH expression and hypothalamic control of fertility.

Enhancers of GnRH transcription embedded in an upstream gene use homeodomain proteins to specify hypothalamic expression

GnRH, the central regulator of reproductive function, is produced by only approximately 800 highly specialized hypothalamic neurons. Previous studies identified a minimal promoter [GnRH minimal promoter (GnRH-P)] (-173/+1) and a neuron-specific enhancer [GnRH-enhancer (E)1] (-1863/-1571) as regulatory regions in the rat gene that confer this stringent specificity of GnRH expression to differentiated GnRH neurons. In transgenic mice, these two elements target only GnRH neurons but fail to drive expression in the entire population, suggesting the existence of additional regulatory regions. Here, we define two novel, highly conserved, upstream enhancers in the GnRH gene termed GnRH-E2 (-3135/-2631) and GnRH-E3 (-4199/-3895) that increase neuron-specific GnRH expression through interactions with GnRH-E1 and GnRH-P. GnRH-E2 and GnRH-E3 regulate GnRH expression through similar mechanisms via Oct-1, Msx1, and Dlx2, which bind both GnRH-E2 and the GnRH-E3 critical region at -3952/-3895. Overexpression of Dlx2 increases transcription through GnRH-E2 and GnRH-E3. Remarkably, these novel elements are contained within the 3' untranslated region of the neighboring upstream gene, yet are marked endogenously by histone modification signatures consistent with those of enhancers. Thus, GnRH-E2 and GnRH-E3 are novel regulatory elements that, together with GnRH-E1 and GnRH-P, confer the specificity of GnRH expression to differentiated and mature GnRH neurons.

Leptin increases L-type Ca2+ channel expression and GnRH-stimulated LH release in LbetaT2 gonadotropes

Leptin, a mediator of long-term regulation of energy balance, has been implicated in the release of adenohypophyseal gonadotropins by regulating gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus. However, a direct effect of leptin on hormone release from gonadotropes remains virtually unexplored. In the current report, we assessed the long-term (48 h) actions of leptin on voltage-gated channel activity and luteinizing hormone (LH) production in mouse pituitary gonadotrope LbetaT2 cells. Electrophysiological recordings showed that leptin treatment significantly increased whole-cell patch-clamp Ba(2+) current through L-type Ca(2+) channels. Quantitative RT-PCR analysis revealed increased levels of L-type (alpha(1D)) Ca(2+) channel mRNA. Likewise, radioimmunoassays using specific antibodies provided evidence that leptin alone had no effect on LH release but did enhance GnRH-induced secretion of the hormone. Leptin had no apparent effects on LH gene transcription in absence of GnRH, as measured by transient transfection assays using a LH promoter-reporter gene and real-time RT-PCR. These observations suggest that leptin might affect LH release by acting directly on the gonadotropes, favoring hormone production by enhancing responsiveness to GnRH as a result of increased Ca(2+) channel expression.

Mammalian comparative sequence analysis of the Agrp locus

Agouti-related protein encodes a neuropeptide that stimulates food intake. Agrp expression in the brain is restricted to neurons in the arcuate nucleus of the hypothalamus and is elevated by states of negative energy balance. The molecular mechanisms underlying Agrp regulation, however, remain poorly defined. Using a combination of transgenic and comparative sequence analysis, we have previously identified a 760 bp conserved region upstream of Agrp which contains STAT binding elements that participate in Agrp transcriptional regulation. In this study, we attempt to improve the specificity for detecting conserved elements in this region by comparing genomic sequences from 10 mammalian species. Our analysis reveals a symmetrical organization of conserved sequences upstream of Agrp, which cluster into two inverted repeat elements. Conserved sequences within these elements suggest a role for homeodomain proteins in the regulation of Agrp and provide additional targets for functional evaluation.

The role of Dlx homeogenes in early development of the olfactory pathway

Development of the olfactory pathway requires interaction between cells and signals of different origin. Olfactory receptor neurons (ORN) in the olfactory placodes (OP) extend axons towards the forebrain (FB); with innervation taking place at a later time following degradation of the basement membrane. Cells from the OP migrate along ORN axons and differentiate into various elements, including ensheathing and Gonadotropin Releasing Hormone (GnRH)+ cells. The importance of the olfactory connection and migration is highlighted by the severe endocrine phenotype in Kallmann's patients who lack this migratory pathway. Little is known about the genetic control of intrinsic ORN properties. Inactivation of the distalless-related Dlx5 prevents connections between ORNs and FB. Using a grafting approach we show that misguidance and lack of connectivity is due to intrinsic defects in ORN neurites and migratory cells (MgC), and not to environmental factors. These data point to a cell-autonomous function of Dlx5 in providing ORN axons with their connectivity properties. Dlx5 also marks a population of early MgC that partly overlaps with the GnRH+ population. In the absence of Dlx5 MgCs of the Dlx5+ lineage migrate, associated with PSA-NCAM+ axons, but fail to reach the FB as a consequence of the lack of axonal connection and not an inability to migrate. These data suggests that Dlx5 is not required to initiate migration and differentiation of MgCs.

Synthesis and secretion of GnRH

Comprehensive studies have provided a clear understanding of the effects of gonadal steroids on the secretion of gonadotropin releasing hormone (GnRH), but some inconsistent results exist with regard to effects on synthesis. It is clear that regulation of both synthesis and the secretion of GnRH are effected by neurotransmitter systems in the brain. Thus, steroid regulation of GnRH synthesis and secretion can be direct, but the predominant effects are transmitted through steroid-responsive neuronal systems in various parts of the brain. There is also emerging evidence of direct effects on GnRH cells. Overriding effects on synthesis and secretion of GnRH can be observed during aging, in undernutrition and under stressful situations; these involve various neuronal systems, which may have serial or parallel effects on GnRH cells. The effect of aging is accompanied by changes in GnRH synthesis, but comprehensive studies of synthesis during undernutrition and stress are less well documented. Altered GnRH and gonadotropin secretion that occurs in seasonal breeding animals and during the pubertal transition is not generally accompanied by changes in GnRH synthesis. Secretion of GnRH from the brain is a reflection of the inherent function of GnRH cells and the inputs that integrate all of the central regulatory elements. Ultimately, the pattern of secretion dictates the reproductive status of the organism. In order to fully understand the central mechanisms that control reproduction, more extensive studies are required on the neuronal circuitry that provides input to GnRH cells.

The Groucho-related gene family regulates the gonadotropin-releasing hormone gene through interaction with the homeodomain proteins MSX1 and OCT1

Gonadotropin-releasing hormone (GnRH) is exclusively expressed in a unique population of hypothalamic neurons that controls reproductive function. GnRH gene expression is highly dynamic. Its transcriptional activity is regulated in a complex spatiotemporal manner during embryonic development and postnatal life. Although a variety of transcription factors have been identified as regulators of GnRH transcription, most are promiscuous in their DNA-binding requirements, and none are solely expressed in GnRH neurons. Their specific activity is probably determined by interactions with distinct cofactors. Here we find that the Groucho-related gene (GRG) family of co-repressors is expressed in a model cell line for the GnRH neuron and co-expresses with GnRH during prenatal development. GRG proteins associate in vivo with the GnRH promoter. Furthermore, GRG proteins interact with two regulators of GnRH transcription, the homeodomain proteins MSX1 and OCT1. Co-transfection experiments indicate that GRG proteins regulate GnRH promoter activity. The long GRG forms enhance MSX1 repression and counteract OCT1 activation of the GnRH gene. In contrast, the short form, GRG5, has a dominant-negative effect on MSX1-dependent repression. Taken together, these data suggest that the dynamic switch between activation and repression of GnRH transcription is mediated by recruitment of the GRG co-regulators.

The protein kinase C pathway acts through multiple transcription factors to repress gonadotropin-releasing hormone gene expression in hypothalamic GT1-7 neuronal cells

The GnRH gene uses two well-defined regions to target expression to a small population of hypothalamic GnRH neurons: a 173-bp proximal promoter and a 300-bp enhancer localized at approximately -1800 to -1500 bp from the start site. Interaction of multiple factors with the GnRH enhancer and promoter is required to confer neuron-specific expression in vivo and in cells in culture. In addition, the expression of the GnRH gene is regulated by numerous neurotransmitters and hormones. Several of these effectors act through membrane receptors to trigger the protein kinase C pathway, and 12-O-tetradecanoyl phorbol-13-acetate (TPA), a modulator of this pathway, has been shown to suppress GnRH gene expression through the promoter. We find that TPA suppresses expression through the GnRH enhancer as well as the promoter. In the enhancer, an Oct-1 binding site, a Pbx/Prep binding site, Msx/Dlx binding sites, and a previously unidentified protein-binding element at -1793, all contribute to TPA suppression. TPA treatment leads to decreased binding of Oct-1 and Pbx1a/Prep to their sites. However, a complex formed by GT1-7 nuclear extracts on the -1793 site is not affected by TPA treatment. It is known that cooperative interaction among multiple factors is necessary for GnRH gene expression; thus, one mechanism by which TPA suppresses GnRH gene expression is to disengage some of these factors from their cis-regulatory elements.

Developmental regulation of gonadotropin-releasing hormone gene expression by the MSX and DLX homeodomain protein families

Gonadotropin-releasing hormone (GnRH) is the central regulator of the hypothalamic-pituitary-gonadal axis, controlling sexual maturation and fertility in diverse species from fish to humans. GnRH gene expression is limited to a discrete population of neurons that migrate through the nasal region into the hypothalamus during embryonic development. The GnRH regulatory region contains four conserved homeodomain binding sites (ATTA) that are essential for basal promoter activity and cell-specific expression of the GnRH gene. MSX and DLX are members of the Antennapedia class of non-Hox homeodomain transcription factors that regulate gene expression and influence development of the craniofacial structures and anterior forebrain. Here, we report that expression patterns of the Msx and Dlx families of homeodomain transcription factors largely coincide with the migratory route of GnRH neurons and co-express with GnRH in neurons during embryonic development. In addition, MSX and DLX family members bind directly to the ATTA consensus sequences and regulate transcriptional activity of the GnRH promoter. Finally, mice lacking MSX1 or DLX1 and 2 show altered numbers of GnRH-expressing cells in regions where these factors likely function. These findings strongly support a role for MSX and DLX in contributing to spatiotemporal regulation of GnRH transcription during development.

Phylogenetic footprinting reveals evolutionarily conserved regions of the gonadotropin-releasing hormone gene that enhance cell-specific expression

Reproductive function is controlled by the hypothalamic neuropeptide, GnRH, which serves as the central regulator of the hypothalamic-pituitary-gonadal axis. GnRH expression is limited to a small population of neurons in the hypothalamus. Targeting this minute population of neurons (as few as 800 in the mouse) requires regulatory elements upstream of the GnRH gene that remain to be fully characterized. Previously, we have identified an evolutionarily conserved promoter region (-173 to +1) and an enhancer (-1863 to -1571) in the rat gene that targets a subset of the GnRH neurons in vivo. In the present study, we used phylogenetic sequence comparison between human and rodents and analysis of the transcription factor clusters within conserved regions in an attempt to identify additional upstream regulatory elements. This approach led to the characterization of a new upstream enhancer that regulates expression of GnRH in a cell-specific manner. Within this upstream enhancer are nine binding sites for Octamer-binding transcription factor 1 (OCT1), known to be an important transcriptional regulator of GnRH gene expression. In addition, we have identified nuclear factor I (NF1) binding to multiple elements in the GnRH-regulatory regions, each in close proximity to OCT1. We show that OCT1 and NF1 physically and functionally interact. Moreover, the OCT1 and NF1 binding sites in the regulatory regions appear to be essential for appropriate GnRH gene expression. These findings indicate a role for this upstream enhancer and novel OCT1/NF1 complexes in neuron-restricted expression of the GnRH gene.

TALE homeodomain proteins regulate gonadotropin-releasing hormone gene expression independently and via interactions with Oct-1

Gonadotropin-releasing hormone (GnRH) is the central regulator of reproductive function. Expression of the GnRH gene is confined to a rare population of neurons scattered throughout the hypothalamus. Restricted expression of the rat GnRH gene is driven by a multicomponent enhancer and an evolutionarily conserved promoter. Oct-1, a ubiquitous POU homeodomain transcription factor, was identified as an essential factor regulating GnRH transcription in the GT1-7 hypothalamic neuronal cell line. In this study, we conducted a two-hybrid interaction screen in yeast using a GT1-7 cDNA library to search for specific Oct-1 cofactors. Using this approach, we isolated Pbx1b, a TALE homeodomain transcription factor that specifically associates with Oct-1. We show that heterodimers containing Pbx/Prep1 or Pbx/Meis1 TALE homeodomain proteins bind to four functional elements within the GnRH regulatory region, each in close proximity to an Oct-1-binding site. Cotransfection experiments indicate that TALE proteins are essential for GnRH promoter activity in the GT1-7 cells. Moreover, Pbx1 and Oct-1, as well as Prep1 and Oct-1, form functional complexes that enhance GnRH gene expression. Finally, Pbx1 is expressed in GnRH neurons in embryonic as well as mature mice, suggesting that the associations between TALE homeodomain proteins and Oct-1 regulate neuron-specific expression of the GnRH gene in vivo.