Yoshitaka Nagahama

Bio: Yoshitaka Nagahama is an academic researcher from National Institute for Basic Biology, Japan. The author has contributed to research in topics: Oocyte & Complementary DNA. The author has an hindex of 63, co-authored 272 publications receiving 12523 citations. Previous affiliations of Yoshitaka Nagahama include Graduate University for Advanced Studies.

6 The Functional Morphology of Teleost Gonads

Abstract: Publisher Summary This chapter discusses the functional morphology of teleost gonads. Information presented in this chapter indicates that there are several processes of germ cell development, which are closely associated with changes in cellular activities of somatic cell elements. Recent studies on biochemical aspects of vitellogenesis in teleosts have shown that the hepatic and ovarian yolk proteins are similar to those of amphibian species. An in vitro method involving the separation of the follicular components has facilitated investigations of the detailed mechanism of the production of two major follicular steroid hormones. As a result of the usage of this technique, a two cell-type model involving thecal and granulosa cell layers has been proposed for the production of these two steroids for the first time in lower vertebrates. With further refinements this well characterized incubation procedure should provide an excellent system for studying the molecular basis of the mechanism of gonadotropin action on follicular steroidogenesis.

Foxl2 Up-Regulates Aromatase Gene Transcription in a Female-Specific Manner by Binding to the Promoter as Well as Interacting with Ad4 Binding Protein/Steroidogenic Factor 1

Abstract: Increasing evidence suggests the crucial role of estrogen in ovarian differentiation of nonmammalian vertebrates including fish. The present study has investigated the plausible role of Foxl2 in ovarian differentiation through transcriptional regulation of aromatase gene, using monosex fry of tilapia. Foxl2 expression is sexually dimorphic, like Cyp19a1, colocalizing with Cyp19a1 and Ad4BP/SF-1 in the stromal cells and interstitial cells in gonads of normal XX and sex-reversed XY fish, before the occurrence of morphological sex differentiation. Under in vitro conditions, Foxl2 binds to the sequence ACAAATA in the promoter region of the Cyp19a1 gene directly through its forkhead domain and activates the transcription of Cyp19a1 with its C terminus. Foxl2 can also interact through the forkhead domain with the ligand-binding domain of Ad4BP/SF-1 to form a heterodimer and enhance the Ad4BP/SF-1 mediated Cyp19a1 transcription. Disruption of endogenous Foxl2 in XX tilapia by overexpression of its dominant negative mutant (M3) induces varying degrees of testicular development with occasional sex reversal from ovary to testis. Such fish display reduced expression of Cyp19a1 as well as a drop in the serum levels of 17beta-estradiol and 11-ketotestosterone. Although the XY fish with wild-type tilapia Foxl2 (tFoxl2) overexpression never exhibited a complete sex reversal, there were significant structural changes, such as tissue degeneration, somatic cell proliferation, and induction of aromatase, with increased serum levels of 17beta-estradiol and 11-ketotestosterone. Altogether, these results suggest that Foxl2 plays a decisive role in the ovarian differentiation of the Nile tilapia by regulating aromatase expression and possibly the entire steroidogenic pathway.

Regulation of oocyte growth and maturation in fish

Abstract: Publisher Summary This chapter discusses the current status of the investigations on the hormonal regulation of oocyte growth and maturation in fish. Pituitary gonadotropins are of primary importance in triggering these processes in fish oocytes. In both cases, however, the actions of gonadotropins are not direct but are mediated by the follicular production of steroidal mediators, estradiol-17β (oocyte growth), and 17α,20β-dihydroxy-4-pregnene-3-one (DP) or 4-pregnen-17,20β,21-triol-3-one (20β-S) (oocyte maturation). It has been established that both estradiol- 17β and 17α,20β-DP are biosynthesized by salmonid ovarian follicles through an interaction of two cell layers—namely, the thecal and the granulosa cell layers (two-cell-type model). The granulosa cell layers are the sites of production of these two steroidal mediators, but their production depends on the provision of the precursor steroids by the thecal cell layers.

A Tandem Duplicate of Anti-Müllerian Hormone with a Missense SNP on the Y Chromosome Is Essential for Male Sex Determination in Nile Tilapia, Oreochromis niloticus.

Abstract: Variation in the TGF-β signaling pathway is emerging as an important mechanism by which gonadal sex determination is controlled in teleosts. Here we show that amhy, a Y-specific duplicate of the anti-Mullerian hormone (amh) gene, induces male sex determination in Nile tilapia. amhy is a tandem duplicate located immediately downstream of amhΔ-y on the Y chromosome. The coding sequence of amhy was identical to the X-linked amh (amh) except a missense SNP (C/T) which changes an amino acid (Ser/Leu92) in the N-terminal region. amhy lacks 5608 bp of promoter sequence that is found in the X-linked amh homolog. The amhΔ-y contains several insertions and deletions in the promoter region, and even a 5 bp insertion in exonVI that results in a premature stop codon and thus a truncated protein product lacking the TGF-β binding domain. Both amhy and amhΔ-y expression is restricted to XY gonads from 5 days after hatching (dah) onwards. CRISPR/Cas9 knockout of amhy in XY fish resulted in male to female sex reversal, while mutation of amhΔ-y alone could not. In contrast, overexpression of Amhy in XX fish, using a fosmid transgene that carries the amhy/amhΔ-y haplotype or a vector containing amhy ORF under the control of CMV promoter, resulted in female to male sex reversal, while overexpression of AmhΔ-y alone in XX fish could not. Knockout of the anti-Mullerian hormone receptor type II (amhrII) in XY fish also resulted in 100% complete male to female sex reversal. Taken together, these results strongly suggest that the duplicated amhy with a missense SNP is the candidate sex determining gene and amhy/amhrII signal is essential for male sex determination in Nile tilapia. These findings highlight the conserved roles of TGF-β signaling pathway in fish sex determination.

Mechanisms of Estrogen Action

Abstract: Our appreciation of the physiological functions of estrogens and the mechanisms through which estrogens bring about these functions has changed during the past decade. Just as transgenic mice were produced in which estrogen receptors had been inactivated and we thought that we were about to understand the role of estrogen receptors in physiology and pathology, it was found that there was not one but two distinct and functional estrogen receptors, now called ERα and ERβ. Transgenic mice in which each of the receptors or both the receptors are inactive have revealed a much broader role for estrogens in the body than was previously thought. This decade also saw the description of a male patient who had no functional ERα and whose continued bone growth clearly revealed an important function of estrogen in men. The importance of estrogen in both males and females was also demonstrated in the laboratory in transgenic mice in which the aromatase gene was inactivated. Finally, crystal structures of the estrogen r...

The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature.

Abstract: We provide here a list of 221 P450 genes and 12 putative pseudogenes that have been characterized as of December 14, 1992. These genes have been described in 31 eukaryotes (including 11 mammalian and 3 plant species) and 11 prokaryotes. Of 36 gene families so far described, 12 families exist in all mammals examined to date. These 12 families comprise 22 mammalian subfamilies, of which 17 and 15 have been mapped in the human and mouse genome, respectively. To date, each subfamily appears to represent a cluster of tightly linked genes. This revision supersedes the previous updates [Nebert et al., DNA 6, 1–11, 1987; Nebert et al., DNA 8, 1–13, 1989; Nebert et al., DNA Cell Biol. 10, 1–14 (1991)] in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene and cDNA, we recommend that the italicized root symbol "CYP" for human ("Cyp" for mouse), representing "cytochrome P450," be followed by an Arabic number denoting the family, a letter designating...