Conserved usage of alternative 5' untranslated exons of the GATA4 gene

BACKGROUND

GATA4 is an essential transcription factor required for the development and function of multiple organs. Despite this important role, our knowledge of how the GATA4 gene is regulated remains limited. To better understand this regulation, we characterized the 5' region of the mouse, rat, and human GATA4 genes.

METHODOLOGY/PRINCIPAL FINDINGS

Using 5' RACE, we identified novel transcription start sites in all three species. GATA4 is expressed as multiple transcripts with varying 5' ends encoded by alternative untranslated first exons. Two of these non-coding first exons are conserved between species: exon 1a located 3.5 kb upstream of the GATA4 ATG site in exon 2, and a second first exon (exon 1b) located 28 kb further upstream. Expression of both mRNA variants was found in all GATA4-expressing organs but with a preference for the exon 1a-containing transcript. The exception was the testis where exon 1a- and 1b-containing transcripts were similarly expressed. In some tissues such as the intestine, alternative transcript expression appears to be regionally regulated. Polysome analysis suggests that both mRNA variants contribute to GATA4 protein synthesis.

CONCLUSIONS/SIGNIFICANCE

Taken together, our results indicate that the GATA4 gene closely resembles the other GATA family members in terms of gene structure where alternative first exon usage appears to be an important mechanism for regulating its tissue- and cell-specific expression.

A comprehensive survey of the laminins and collagens type IV expressed in mouse Leydig cells and their regulation by LH/hCG

Extracellular matrix (ECM) proteins have been shown to alter Leydig cell steroidogenesis in vitro, substantiating the hypothesis that Leydig cell steroidogenic activity and matrix environment are interdependent events. However, the nature of the ECM components synthesized by Leydig cells and their regulation by LH/human chorionic gonadotropin (hCG) remain unknown. Here, we examine the occurrence of the 11 laminin subunits and the 6 alpha chains of collagen IV (COL4A1-6) by RT-PCR in Leydig cells cultured with or without LH/hCG. Leydig cells were a tumor Leydig cell line (mLTC-1) or 8-week-old mice Leydig cells. Based on PCR data, it is suggested that normal Leydig cells may synthesize a maximum of 11 laminin heterotrimers and the 6 alpha chains of collagen IV. They also may synthesize various proteases and inhibitors of the metzincin family. The mLTC-1 cells have a limited repertoire as compared with normal Leydig cells. Interestingly, none of the ten proteases and inhibitors monitored is under LH-hCG regulation whereas every protease and inhibitor of the serine protease family yet identified in Leydig cells is under gonadotropin regulation. In addition, a few laminin and collagen subunit genes are regulated by LH/hCG. These are laminins alpha3 and gamma3 (Lama3 and Lamc3), Col4a3, and Col4a6, which are negatively regulated by LH/hCG in both Leydig cell types, and Col4a4, which was downregulated in primary cultures but not in mLTC-1 cells. Collectively, the present study suggests that Leydig cells modulate in a selective fashion their matrix environment in response to their trophic hormone. This may alter the steroidogenic outcome of Leydig cells.

Deregulation of anti-Mullerian hormone/BMP and transforming growth factor-beta pathways in Leydig cell lesions developed in male heterozygous multiple endocrine neoplasia type 1 mutant mice

Multiple endocrine neoplasia type 1 (MEN1) results from the mutation of the predisposing gene, MEN1. Heterozygous Men1 mutant mice previously generated by several laboratories, including ours, mimic largely MEN1 pathology. Interestingly, our heterozygous Men1 mutant mice exhibit not only the endocrine tumours commonly seen in MEN1 patients, but also Leydig cell tumours (LCT) with high frequency, accompanied systematically by loss of the wild-type Men1 allele. As there exists a similarity of tumour phenotype between these mice and those mutated for the components of anti-Mullerian hormone (AMH)/bone morphogenic protein (BMP) pathway belonging to transforming growth factor-beta (TGF-beta) family, we investigated the expression and the activity of this pathway, known to have an important biological role in Leydig cells. Here, we report that the expression of AMH receptor type 2 is reduced in Men1 LCTs. Both immunostaining and western blot analyses also demonstrate a markedly decreased nuclear expression of Smad1, 3, 4 and 5 in the tumours. More interestingly, we show that the reconstituted menin expression in Men1-deficient Leydig cells derived from LCTs can significantly increase the transcriptional activity of a BMP pathway target promoter, XVent2. Furthermore, we found that the expression of p18, p27 and cyclin dependant kinase 4 (Cdk4), targets of TGF-beta pathways, is altered in the Leydig cell lesions. Our data provide the evidence of the deregulation of AMH/BMP and TGF-beta pathways in mouse Men1 LCTs, highlighting their involvement in tumorigenesis of Leydig cells due to Men1 inactivation.

Role of the GATA family of transcription factors in endocrine development, function, and disease

The WGATAR motif is a common nucleotide sequence found in the transcriptional regulatory regions of numerous genes. In vertebrates, these motifs are bound by one of six factors (GATA1 to GATA6) that constitute the GATA family of transcriptional regulatory proteins. Although originally considered for their roles in hematopoietic cells and the heart, GATA factors are now known to be expressed in a wide variety of tissues where they act as critical regulators of cell-specific gene expression. This includes multiple endocrine organs such as the pituitary, pancreas, adrenals, and especially the gonads. Insights into the functional roles played by GATA factors in adult organ systems have been hampered by the early embryonic lethality associated with the different Gata-null mice. This is now being overcome with the generation of tissue-specific knockout models and other knockdown strategies. These approaches, together with the increasing number of human GATA-related pathologies have greatly broadened the scope of GATA-dependent genes and, importantly, have shown that GATA action is not necessarily limited to early development. This has been particularly evident in endocrine organs where GATA factors appear to contribute to the transcription of multiple hormone-encoding genes. This review provides an overview of the GATA family of transcription factors as they relate to endocrine function and disease.

Consequences of Fetal Irradiation on Follicle Histogenesis and Early Follicle Development in Rat Ovaries1

Follicle histogenesis, in which follicles arise from fragmenting ovigerous cords, is a poorly understood mechanism that is strictly dependent upon the presence of germ cells. Our previous studies have shown that severely germ cell-depleted rat ovaries after fetal gamma-irradiation display modifications of follicular endowment and dynamics during the immature period. The primordial follicle stock was absent and the follicles with primary appearance remained quiescent longer than in control ovaries during the neonatal period. The aim of the present work was to analyze the initial steps of follicle histogenesis, and to investigate the etiology of the alterations observed in the development of irradiated ovaries. Just after birth, we observed, in addition to sterile ovigerous cords, the emergence of the first follicles which exhibited several abnormal features as compared to those of control ovaries. Most of the follicles appeared as primary follicles, as they were composed of a layer of cuboidal-shaped granulosa cells surrounding an enlarged oocyte. Interestingly, the granulosa cells of these primary-like follicles did not proliferate and did not express the genes for anti-Müllerian hormone (Amh) or bone morphogenetic protein receptor type II (Bmpr2), both of which are normally expressed from the primary stage onwards. In contrast, the oocytes strongly expressed the gene for growth and differentiation factor 9 (Gdf9), which is normally upregulated from the primary follicle stage onwards, which suggests an uncoupling of granulosa cell development from oocyte development. In addition, irradiated ovaries displayed a higher frequency of follicles that contained 2 or 3 oocytes, which are also referred to as multi-oocyte follicles (MOFs). Examination at the time of follicle histogenesis indicated that MOFs arise from incomplete ovigerous cord breakdown. Taken together, the results of this study indicate that severe perturbations of follicular histogenesis take place following irradiation and massive germ cell depletion during fetal life. In addition to the classically described sterile cords, we have pointed out the differentiation of MOFs and primary-like quiescent follicles, which finally evolve into growing follicles and participate in ovarian function. We propose that these phenotypes are closely correlated to the proportion of granulosa cells to oocytes at the time of neonatal follicle histogenesis.

The proximal Gata4 promoter directs reporter gene expression to sertoli cells during mouse gonadal development

The GATA4 transcription factor is an important developmental determinant for many organs, such as the heart, gut, and testis. Despite this pivotal role, our understanding of the transcriptional mechanisms that control the proper spatiotemporal expression of the GATA4 gene remains limited. We have generated transgenic mice expressing a green fluorescent protein (GFP) marker under the control of rat Gata4 5' flanking sequences. Several GATA4-expressing organs displayed GFP fluorescence, including the heart, intestine, and pancreas. In the gonads, while GATA4 is expressed in pregranulosa, granulosa, and theca ovarian cells, and Sertoli, Leydig, and peritubular testicular cells, the first 5 kb of Gata4 regulatory sequences immediately upstream of exon 1 were sufficient to direct GFP reporter expression only in testis and, specifically, in Sertoli cells. Onset of GFP expression occurred after Sertoli cell commitment and was maintained in these cells throughout development to adulthood. In vitro studies revealed that the first 118 bp of the Gata4 promoter is sufficient for full basal activity in several GATA4-expressing cell lines. Promoter mutagenesis and DNA-binding experiments identified two GC-box motifs and, particularly, one E-box element within this -118-bp region that are crucial for its activity. Further analysis revealed that members of the USF family of transcription factors, especially USF2, bind to and activate the Gata4 promoter via this critical E-box motif.