Structural characterization of the chicken SF-1/Ad4BP gene

Development of retroviral vectors for tissue-restricted expression in chicken embryonic gonads

The chicken embryo has long been a useful model organism for studying development, including sex determination and gonadal differentiation. However, manipulating gene expression specifically in the embryonic avian gonad has been difficult. The viral vector RCASBP can be readily used for embryo-wide transgene expression; however global mis-expression using this method can cause deleterious off-target effects and embryo-lethality. In an attempt to develop vectors for the over-expression of sequences in chicken embryonic urogenital tissues, the viral vector RCANBP was engineered to contain predicted promoter sequences of gonadal-expressed genes. Several promoters were analysed and it was found that although the SF1 promoter produced a tissue-restricted expression pattern that was highest in the mesonephros and liver, it was also higher in the gonads compared to the rest of the body. The location of EGFP expression from the SF1 promoter overlapped with several key gonad-expressed sex development genes; however expression was generally low-level and was not seen in all gonadal cells. To further validate this sequence the key testis determinant DMRT1 was over-expressed in female embryos, which due to insufficient levels had no effect on gonad development. The female gene aromatase was then over-expressed in male embryos, which disrupted the testis pathway as demonstrated by a reduction in AMH protein. Taken together, although these data showed that the SF1 promoter can be used for functional studies in ovo, a stronger promoter sequence would likely be required for the functional analysis of gonad genes that require high-level expression.

Chicken LRH-1 gene is transcribed from multiple promoters in steroidogenic organs

Liver receptor homolog-1 (LRH-1) is a homolog of FTZ-F1, a transcription factor of the fruit fly, and belongs to the orphan nuclear receptor family. LRH-1 is expressed in organs derived from the endoderm, including intestine, liver and exocrine pancreas and plays a predominant role in development, bile-acid homeostasis, and reverse cholesterol transport. Recent research has revealed that mammalian LRH-1 is also expressed in the steroidogenic organs and has suggested that LRH-1 shares a role in steroidogenesis with steroidogenic factor-1 (SF-1), which is a paralog of LRH-1. In this study, we determined transcription initiation sites of chicken LRH-1 and showed that LRH-1 is expressed as several splicing variants in chicken steroidogenic organs. From three steroidogenic organs, the adrenal glands, ovaries, and testes, several cDNA fragments including different lengths and sequences were amplified by 5'-RACE and these were mainly classified into five types. Using these sequences, chicken genomic database was searched and four types of first exons were identified in chromosome 8. However, the database sequence of these regions included several gaps. So we cloned gap regions by PCR cloning from chicken genomic DNA and found the other type of first exons in the gaps. Moreover, RT-PCR showed the expression of LRH-1 in chicken steroidogenic organs as many splicing variants. We concluded that the chicken LRH-1 gene is transcribed from at least five different transcription initiation sites and alternative splicing produces several types of mRNA in steroidogenic organs.

Usage of putative chicken U6 promoters for vector-based RNA interference

Gene silencing with short interfering RNA (siRNA) expression vectors is a powerful method for the analysis of gene functions. For the expression of siRNA in mammalian cells, mammalian U6 small nuclear RNA (snRNA) promoters are widely used. However, the mammalian U6 promoter might not function well in other species. In this study, we cloned four putative chicken U6 promoters by PCR and analyzed their functions. First, we screened the chicken genomic database using the human U6 snRNA gene and identified four candidate sequences. The sequences contained some control elements in their promoter regions, but as we could not rule out that they were pseudogenes, we amplified these sequences and used them as promoters for short hairpin RNA (shRNA) expression. Using the firefly luciferase (Luc) gene as a target, transient expression assays were performed with chicken ovary-derived cells. All four putative chicken U6 promoters exhibited suppressive activity toward Luc, and so could act as a promoter for expression of the snRNA gene in the chicken genome. The promoter activity was not as strong as that of a commercially available siRNA expression vector. This probably reflects artificial sequences between the promoters and synthetic DNA encoding shRNA.

Sex-specific and left-right asymmetric expression pattern of Bmp7 in the gonad of normal and sex-reversed chicken embryos

A genetic switch determines whether the indifferent gonad develops into an ovary or a testis. In adult females of many avian species, the left ovary is functional while the right one regresses. In the embryo, bone morphogenetic proteins (BMP) mediate biological effects in many organ developments but their roles in avian sex determination and gonadal differentiation remains largely unknown. Here, we report the sex-specific and left-right (L-R) asymmetric expression pattern of Bmp7 in the chicken gonadogenesis. Bmp7 was L-R asymmetrically expressed at the beginning of genital ridge formation. After sexual differentiation occurred, sex-specific expression pattern of Bmp7 was observed in the ovary mesenchyme. In addition, ovary-specific Bmp7 expression was reduced in experimentally induced female-to-male reversal using the aromatase inhibitor (AI). These dynamic changes of expression pattern of Bmp7 in the gonad with or without AI treatment suggest that BMP may play roles in determination of L-R asymmetric development and sex-dependent differentiation in the avian gonadogenesis.

Regulation of the orphan nuclear receptor steroidogenic factor 1 by Sox proteins

Steroidogenic factor 1 (SF-1) is an essential factor in endocrine proliferation and gene expression. Despite the fact that SF-1 expression is restricted to specialized cells within the endocrine system, the only identified regulatory factors of SF-1 are the ubiquitously expressed E-box proteins (upstream stimulatory factors 1 and 2). Sequence examination of the SF-1 proximal promoter revealed a conserved site of AACAAAG (Sox-BS1), which matches exactly the defined consensus Sox protein binding element. Among the approximately 20 known members of the Sox gene family, we focused on Sox3, Sox8, and Sox9, based on their coexpression with SF-1 in the embryonic testis. Indeed, all three of these Sox proteins were capable of binding the proximal Sox-BS1 within the SF-1 promoter (-110 to -104), albeit with differing affinities. Of the three Sox proteins, Sox9 exhibited high-affinity binding to the Sox-BS1 element and consistently activated SF-1 promoter-reporter constructs. Mutating the Sox-BS1 attenuated SF-1 promoter activity in both embryonic and postnatal Sertoli cells, as well as in the adrenocortical cell line, Y1. Our findings, taken together with the overlapping expression profiles of Sox9 and SF-1, and the similar intersex phenotypes associated with both SOX9 and SF-1 human mutations, suggest that Sox9 up-regulates SF-1 and accounts partially for the sexually dimorphic expression pattern of SF-1 observed during male gonadal differentiation.

Characterization of steroidogenic factor 1 during sexual differentiation in a marsupial

In eutherian mammals, such as mice and humans, steroidogenic factor 1 (SF1) plays important roles in the development of the gonad and in its steroidogenic activity. Marsupial and eutherian mammals have been evolving independently for at least 100 million years and so we were interested in comparing SF1 of a marsupial with that of eutherians. To this end, we have cloned SF1 from an Australian marsupial, the tammar wallaby. Although the amino acid sequence of SF1 is highly conserved among vertebrate species, tammar SF1 appears to have diverged less from the ancestral SF1 than have eutherian SF1 proteins. Tammar SF1 is expressed by both ovaries and testes on the day of birth, just prior to the onset of testicular differentiation, until at least 8 days after birth by which time the ovary also has begun to sexually differentiate. SF1 transcripts are localized predominantly to the pre-granulosa and Sertoli cells of the ovary and testis, respectively. In the testis SF1 transcripts are also present in the interstitial cells, although at a lower level than that which is observed in the Sertoli cells. SF1 is also transcribed in adult testis and ovary. In the adult ovary SF1 is expressed in the interstitial gland, and in the granulosa cells and theca interna of small to medium-sized antral follicles, but is not expressed in large antral follicles. Thus, although the structure of tammar SF1 is divergent from that of eutherians, its expression profile is similar, supporting a conserved role in gonadal development and steroidogenesis.