Molecular cloning of paired related homeobox 2 (prx2) as a novel pituitary transcription factor

Transcription of follicle-stimulating hormone subunit genes is modulated by porcine LIM homeobox transcription factors, LHX2 and LHX3

The LIM-homeobox transcription factors LHX2 and LHX3s (LHX3a and LHX3b) are thought to be involved in regulating the pituitary glycoprotein hormone subunit genes Cga and Fshβ. These two factors show considerable differences in their amino acid sequences for DNA binding and protein-protein interactions and in their vital function in pituitary development. Hence, we compared the DNA binding properties and transcriptional activities of Cga and Fshβ between LHX2 and LHX3s. A gel mobility shift assay for approximately 1.1 kb upstream of Cga and 2.0 kb upstream of Fshβ varied in binding profiles between LHX2 and LHX3s. DNase I footprinting revealed DNA binding sites in 8 regions of the Cga promoter for LHX2 and LHX3s with small differences in the binding range and strength. In the Fshβ promoter, 14 binding sites were identified for LHX2 and LHX3, respectively. There were alternative binding sites to either gene in addition to similar differences observed in the Cga promoter. The transcriptional activities of LHX2 and LHX3s according to a reporter assay showed cell-type dependent activity with repression in the pituitary gonadotrope lineage LβT2 cells and stimulation in Chinese hamster ovary lineage CHO cells. Reactivity of LHX2 and LHX3s was observed in all regions, and differences were observed in the 5'-upstream region of Fshβ. However, immunohistochemistry showed that LHX2 resides in a small number of gonadotropes in contrast to LHX3. Thus, LHX3 mainly controls Cga and Fshβ expression.

Expression studies of neuronatin in prenatal and postnatal rat pituitary

The pituitary gland, an indispensable endocrine organ that synthesizes and secretes pituitary hormones, develops with the support of many factors. Among them, neuronatin (NNAT), which was discovered in the neonatal mouse brain as a factor involved in neural development, has subsequently been revealed to be coded by an abundantly expressing gene in the pituitary gland but its role remains elusive. We analyze the expression profile of Nnat and the localization of its product during rat pituitary development. The level of Nnat expression was high during the embryonic period but remarkably decreased after birth. Immunohistochemistry demonstrated that NNAT appeared in the SOX2-positive stem/progenitor cells in the developing pituitary primordium on rat embryonic day 11.5 (E11.5) and later in the majority of SOX2/PROP1 double-positive cells on E13.5. Thereafter, during pituitary embryonic development, Nnat expression was observed in some stem/progenitor cells, proliferating cells and terminally differentiating cells. In postnatal pituitaries, NNAT-positive cells decreased in number, with most coexpressing Sox2 or Pit1, suggesting a similar role for NNAT to that during the embryonic period. NNAT was widely localized in mitochondria, peroxisomes and lysosomes, in addition to the endoplasmic reticulum but not in the Golgi. The present study thus demonstrated the variability in expression of NNAT-positive cells in rat embryonic and postnatal pituitaries and the intracellular localization of NNAT. Further investigations to obtain functional evidence for NNAT are a prerequisite.

Expression of Krüppel-like factor 6, KLF6, in rat pituitary stem/progenitor cells and its regulation of the PRRX2 gene

Paired-related transcription factors, PRRX1 and PRRX2, which are present in mesenchymal tissues and participate in mesenchymal cell differentiation, were recently found in the stem/progenitor cells of the pituitary gland of ectodermal origin. To clarify the role of PRRX1 and PRRX2 in the pituitary gland, the present study first aimed to identify transcription factors that regulate Prrx1 and Prrx2 expression. A promoter assay for the upstream regions of both genes was performed by co-transfection of the expression vector of several transcription factors, many of which are frequently found in the pituitary stem/progenitor cells. The results for the promoter activity of both genes showed expression in a cell type-dependent manner. Comprehensive comparison of transcriptional activity of several transcription factors was performed with CHO cells, which do not show Prrx1 and Prrx2 expression, and the results revealed the presence of common and distinct factors for both genes. Among them, KLF6 showed specific and remarkable stimulation of Prrx2 expression. In vitro experiments using an electrophoretic mobility shift assay and siRNA interference revealed a potential ability for regulation of Prrx2 expression by KLF6. Finally, immunohistochemistry confirmed the presence of KLF6 in the SOX2/PRRX2 double-positive stem/progenitor cells of the postnatal pituitary gland. Thus, the finding of KLF6 might provide a novel clue to clarify the maintenance of stem/progenitor cells of the postnatal pituitary gland.

Molecular cloning of rat and porcine retina-derived POU domain factor 1 (POU6F2) from a pituitary cDNA library

Homeobox transcription factors are known to play crucial roles in the anterior lobe of the pituitary gland. During molecular cloning with the Yeast One-Hybrid System using a 5'-upstream region of the porcine Fshβ as a bait sequence, we have cloned a cDNA encoding a partial sequence of the retina-derived POU domain factor 1 (RPF1) from the porcine pituitary cDNA library and confirmed its specific binding to the bait sequence. In situ hybridization was performed to examine localization of Rpf1 and showed that this gene is expressed in the stem/progenitor cells of the rat pituitary primordium as well as the diencephalon and retina. In addition, real-time PCR demonstrated that Rpf1 transcripts are abundant in early embryonic periods but that this is followed by a decrease during pituitary development, indicating that this factor plays a role in differentiating cells of the pituitary. The transcriptional activity of RPF1 for genes of Prop1, Prrx1 and Prrx2, which were characterized as genes participating in the pituitary stem/progenitor cells by our group, was then examined with full-length cDNA obtained from the rat pituitary. RPF1 showed regulatory activity for Prop1 and Prrx2, but not for Prrx1. These results indicate the involvement of this retina-derived factor in pituitary development.

Temporospatial gene expression of Prx1 and Prx2 is involved in morphogenesis of cranial placode-derived tissues through epithelio-mesenchymal interaction during rat embryogenesis

Paired-related homeobox transcription factors, PRX1 and PRX2, are verified to play essential roles in limb, heart and craniofacial development by analyses of knockout animals. Their gene expression in the embryonic primordia derived from the mesoderm and neural crest is confirmed by in situ hybridization. Nevertheless, a detailed localization of PRX1 and PRX2 was not carried out because of a lack of specific antibodies for each factor. We have previously confirmed the presence of PRX proteins in rat embryonic pituitary by using an antibody that recognizes both PRX1 and PRX2. However, the pituitary originates in the cranial placodes, not the mesoderm or neural crest. In this study, we analyze the temporospatial distribution of PRX1 and PRX2 with novel antibodies specific for each factor, together with a stem/progenitor marker SOX2 (sex-determining region Y-box 2) in the primordia formed by epithelio-mesenchymal interaction. We observe immunoreactive signals of both PRX proteins in rat embryo, showing a similar pattern to that obtained by in situ hybridization. In early embryogenesis, PRX proteins are not co-localized with SOX2 but PRX2 and/or PRX1-positive cells are present in the border or periphery of SOX2-positive primordia originating in the cranial placode. During advanced embryogenesis, either PRX2-positive cells become condensed in the border of SOX2-positive cells or PRX1 and/or PRX2 become co-localized with SOX2. Our results suggest that PRX proteins, especially PRX2, play a role in the morphogenesis of the primordial tissues formed by the epithelio-mesenchymal interaction and that neural crest cells contribute to the morphogenesis of tissues derived from the cranial placode.

A genome-wide signature of glucocorticoid receptor binding in neuronal PC12 cells

Background

Glucocorticoids, secreted by the adrenals in response to stress, profoundly affect structure and plasticity of neurons. Glucocorticoid action in neurons is mediated by glucocorticoid receptors (GR) that operate as transcription factors in the regulation of gene expression and either bind directly to genomic glucocorticoid response elements (GREs) or indirectly to the genome via interactions with bound transcription factors. These two modes of action, respectively called transactivation and transrepression, result in the regulation of a wide variety of genes important for neuronal function. The objective of the present study was to identify genome-wide glucocorticoid receptor binding sites in neuronal PC12 cells using Chromatin ImmunoPrecipitation combined with next generation sequencing (ChIP-Seq).

Results

In total we identified 1183 genomic binding sites of GR, the majority of which were novel and not identified in other ChIP-Seq studies on GR binding. More than half (58%) of the binding sites contained a GRE. The remaining 42% of the GBS did not harbour a GRE and therefore likely bind GR via an intermediate transcription factor tethering GR to the DNA. While the GRE-containing binding sites were more often located nearby genes involved in general cell functions and processes such as apoptosis, cell motion, protein dimerization activity and vasculature development, the binding sites without a GRE were located nearby genes with a clear role in neuronal processes such as neuron projection morphogenesis, neuron projection regeneration, synaptic transmission and catecholamine biosynthetic process. A closer look at the sequence of the GR binding sites revealed the presence of several motifs for transcription factors that are highly divergent from those previously linked to GR-signaling, including Gabpa, Prrx2, Zfp281, Gata1 and Zbtb3. These transcription factors may represent novel crosstalk partners of GR in a neuronal context.

Conclusions

Here we present the first genome-wide inventory of GR-binding sites in a neuronal context. These results provide an exciting first global view into neuronal GR targets and the neuron-specific modes of GR action and potentially contributes to our understanding of glucocorticoid action in the brain.

Paired-related homeodomain proteins Prx1 and Prx2 are expressed in embryonic pituitary stem/progenitor cells and may be involved in the early stage of pituitary differentiation

We recently cloned a paired-related homeodomain protein Prx2 as a novel factor in the pituitary. In the present study, we investigated the ontogenic profiles of Prx2 and another cognate Prx1 in the rat embryonic pituitary. Quantitative real-time polymerase chain reaction showed low expression of Prx2 and a marked increase of Prx1 on rat embryonic day (E)20.5. Immunohistochemical analyses using an antibody that recognises both proteins, with the aim of investigating their roles in pituitary organogenesis, demonstrated that PRXs first appear in the Rathke's pouch on E13.5 in the pituitary stem/progenitor cells expressing Prop1 and Sox2. After E16.5, the number of Prx-expressing cells was increased in both anterior and intermediate lobes. SOX2(+) stem/progenitor cells in the intermediate lobe started to produce PRXs, and PRX(+) /SOX2(+) /PROP1(+) -cells were present on the anterior side of the marginal cell layer and were scattered in the parenchyma of the anterior lobe. On the other hand, PRX(+) -cells negative for PROP1 and SOX2 were located in the anterior lobe. Analysis of the relationship with pituitary endocrine cells revealed that a part of PRX(+) /PROP1(-) /SOX2(-) -cells in the anterior lobe co-expressed all types of hormones. The proportion of co-localisation of PRXs and hormones was highest on the day each hormone first appeared. These data indicate that PRXs are produced in the pituitary progenitor cells and may play roles in the process of terminal differentiation during early pituitary organogenesis. An in vitro small interfering RNA-knockdown experiment in the pituitary-derived cell line, TtT/GF, revealed that PRX1 and PRX2 play roles in proliferation by different mechanisms because knockdown of Prx2, but not Prx1, induced the p21 expression. Furthermore, immunohistochemical analysis demonstrated that 76% of PRXs(+) cells were positive for a cell proliferation marker Ki67 in the E18.5 pituitary. This is the first report of the involvement of PRX1 and PRX2 in organogenesis of tissue originating from the ectoderm other than the mesoderm.

Molecular cloning of LIM homeodomain transcription factor Lhx2 as a transcription factor of porcine follicle-stimulating hormone beta subunit (FSHβ) gene

We cloned the LIM-homeodomain protein LHX2 as a transcription factor for the porcine follicle-stimulating hormone β subunit gene (Fshβ) by the Yeast One-Hybrid Cloning System using the upstream region of -852/-746 bases (b) from the transcription start site, called Fd2, as a bait sequence. The reporter assay in LβT2 and CHO cells revealed the presence of an LHX2-responsive region other than Fd2. A potential LHX2 binding sequence was confirmed as AATTAAT containing a consensus homeodomain binding core sequence AATT by Systematic Evolution of Ligands by Exponential Enrichment analysis. DNase I footprinting demonstrated three AATTAAT sequences located at regions -835/-829, -818/-812 and -806/-800 b in the Fd2 region and 12 binding sites in the distal and proximal regions mostly containing an AATT-core sequence. RT-PCR analysis of Lhx2 expression during porcine fetal and postnatal pituitary development showed a gradual increase from fetal day (f) 40 to postnatal day (p) 8 followed by a slight decrease to p230, suggesting that LHX2 may play its role largely in the late fetal and postnatal periods. The analyses of Lhx2 expression in pituitary tumor-derived cell lines showed their expressions in cell lines including αT31, LβT2 and others. Since LHX2 was previously identified as a transcription factor for Cga and the in vitro experiments in the present study suggested that LHX2 regulated the expression of Fshβ, it is possible that LHX2 controls the synthesis of FSH at the transcription level.

Pituitary stem/progenitor cells: embryonic players in the adult gland?

The pituitary gland represents the endocrine core of the body, and its hormonal output governs many key physiological processes. Because endocrine demands frequently change, the pituitary has to flexibly remodel its hormone-producing cell compartment. One mechanism of pituitary plasticity may rely on the generation of new hormonal cells from resident stem/progenitor cells. Existence of such 'master' cells in the pituitary has in the past repeatedly been postulated. Only recently, however, very plausible candidates have been identified that express stem cell-associated markers and signalling factors, and display the stem/progenitor cell characteristics of multipotency, efflux capacity (side population phenotype) and niche-like organization. In other adult tissues, stem cells recapitulate the embryonic developmental path on their course towards mature specialized cells. Interestingly, the pituitary stem/progenitor cell compartment shows prominent expression of transcriptional regulators and signalling factors that play a pivotal role during pituitary embryogenesis. This review summarizes the recent progress in pituitary stem/progenitor cell identification, highlights their potential embryonic phenotype, sketches a tentative stem/progenitor cell model, and discusses further research and challenges. Recognizing and scrutinizing the pituitary stem/progenitor cells as embryonic players in the adult gland may profoundly impact on our still poor understanding of the mechanisms underlying pituitary cell turnover and plasticity.

Pituitary homeodomain transcription factors HESX1 and PROP1 form a heterodimer on the inverted TAAT motif

The development and differentiation of the pituitary gland progress through spatial and temporal expressions of many transcription factors. Transcription factor HESX1, which begins to be expressed in the Rathke's pouch at the early stage of pituitary development, acts as a transcription repressor. Another transcription factor, PROP1, which is a pituitary-specific factor and important for the determination of the differentiation of pituitary hormone-producing cells, appears later than HESX1 and is assumed to block the action of HESX1. Both factors are members of the homeodomain family, and the amino acid residue at the 50th position of the homeodomain is glutamine (Gln-50). We recently observed that both factors share the same target sequence through different binding profiles. Hence, using random oligonucleotides and an electrophoretic mobility-shift assay, we have examined the DNA-binding preference of HESX1 by a determination of its binding sequence. HESX1 binds as a monomer to a TAATT motif but not to a TAAT motif. In the presence of PROP1, HESX1 develops to bind to an inverted TAAT motif by forming a heterodimer. Thus, the formation of a heterodimer between HESX1 and PROP1 provides a condition in which, in the early pituitary primordium, HESX1 alters its repressive role to an active one by forming a heterodimer with newly appearing PROP1 so that PROP1 finally replaces HESX1 to advance to the middle stage of pituitary development.