Question

Explain the physiological role of the hormones HP AND MIH in relation to zebrafish OOCYTE maturation

Answer 1

Ovarian maturation is regulated by several factors. Hormone action is main factor for the oocyte development and maturation of fishes. Gonadotropins are well characterized in fishes, they are follicle stimulating hormones and luteinizing hormone. Oocyte maturation in fish is initiated by the release of luteinizing hormone secreted from pituitary. LH stimulates the production of 17 α-hydroxyprogesterone (17α-HP), which is converted to 17α, 20β-dihydroxyprogesterone (DHP) the maturation inducing hormone (MIH).

Fish oocytes provide an appropriate experimental system with which to investigate the molecular mechanisms controlling meiosis and the embryonic cell cycle. Several factors responsible for the regulation of meiotic maturation in fish oocytes have been identified. These include the isolation and characterization of a fish maturation-inducing hormone (MIH) and the components of maturation-promoting factor (MPF) (cdc2, the catalytic subunit, and cyclin B, the regulatory subunit).
Oocyte maturation in fish is triggered by MIH, which acts on receptors located on the oocyte membrane and induces the activation of MPF in the oocyte cytoplasm. During the course of maturation, oocytes undergo drastic morphological changes associated with progression of the meiotic cell cycle, among which breakdown of the oocyte nuclear envelope (germinal vesicle breakdown, GVBD) occurring at the prophase/metaphase transition is usually regarded as a hallmark of the progress of oocyte maturation. In a number of teleost species, C21 steroids have been shown to be potent initiators of GVBD in vitro and to be present at high levels in plasma of fish undergoing final oocyte maturation. Among C21 steroids, however, only two were identified as naturally occurring MIH in fish: 17α, 20β-dihydroxy-4-pregnen-3-one (17, 20β-DHP) in amago salmon and 17α, 20β, 21-trihydroxy-4-pregnen-3-one (20β-S), in the Atlantic croaker and spotted sea trout . Testosterone, as well as other C19 steroids, induces GVBD only at high concentrations. Estradiol-17β and other C18 steroids are generally not effective inducers of oocyte maturation in fish. Recently, a strong candidate for the MIH receptor, membrane progestin receptor (mPR), was identified. In zebrafish, two types of mPRs, α and β, were identified. 17α, 20β-DHP has been shown to induce oocyte maturation by stimulating the de novo synthesis of cyclin B, a regulatory subunit of MPF.
Several endocrine-disrupting chemicals or EDCs, Kepon and o,p-DDD, have been reported to antagonize MIH-induced meiotic maturation of fish oocytes in vitro. EDCs such as methoxychlor and ethynyl estradiol also antagonize frog oocyte maturation.

MIH- Maturation inducing hormone HP- hydroxyprogesterone

Microfilament-dependent changes in cyclin B mRNA during zebrafish oocyte maturation. Maturation-inducing hormone (MIH)-induced translational activation of cyclin B mRNA is associated with microfilament-dependent morphological change in the mRNA from the aggregated form to the dispersed form, which may represent the dissociation of masking proteins from the mRNA. Cytochalasin B can mimic the MIH action; it induces the dissociation of masking protein complex and the translational activation of cyclin B mRNA by destroying the architecture of microfilaments, forcing the oocytes to undergo breakdown of the germinal vesicle (GVBD) without MIH. Molecules responsible for the masking and unmasking of cyclin B mRNA in zebrafish oocytes remain to be identified.

In fish ovaries, E2(estrogen) is produced during oocyte growth, while MIH is produced during oocyte maturation. Thus, a dramatic shift in the steroidogenic pathway from E2 to 17a, 20b-DP or 20b-S occurs in fish ovarian follicles immediately prior to oocyte maturation (Fig. 2). This switch in the type of steroid hormone produced during ovarian development is likely to be primarily regulated by changes in the abundance of individual steroidogenic enzymes through changes in expression of genes encoding steroidogenesisrelated proteins in developing ovarian follicles. There are two important stages, the first being the shift in the synthesis of precursor steroids from testosterone to 17a-hydroxyprogesterone, while the other is the shift in the final steroidogenic enzymes from oP450arom (for the production of E2) to 20b-HSD (for the production of 17a,20b-DP or 20b-S).
During the maturation of oocytes, fish postvitellogenic follicles have to synthesize a large amount of MIH, 17a,20b-DP or 20b-S from 17a-hydroxyprogesterone, the immediate precursor of 17a,20b-DP. To achieve this, a shift in precursor steroid production from testosterone (for E2 production) to 17a-hydroxyprogesterone (for 17a,20b-DP production) must occur in fish postvitellogenic follicles immediately prior to oocyte maturation. Because 17a-hydroxyprogesterone is produced from progesterone by the hydroxylase activity of P450c17, its 17,20-lyase activity needs to be downregulated in these postvitellogenic follicles. Sufficient 17ahydroxylase activity must be sustained to supply substrate for 17a,20b-DP synthesis during the oocyte maturation stage. This is complicated because 17ahydroxylyase and 17,20-lyase activities are encoded by a single cytochrome P450c17, which drives sex steroid production by converting pregnenolone to 17a-hydroxypregnenolone and converting progesterone to 17a-hydroxyprogesterone. These 17ahydroxylated steroids may then be converted by 17,20-lyase to dehydroepiandrosterone and androstenedione, respectively. Thus, the P450c17 enzyme is a key branch point in fish steroid hormone synthesis, as these two enzymatic activities distinguishes between synthesis of C21 steroids and C18 and C19 steroids. Until recently no studies were reported in fish to explain the differential regulation of the two enzymatic activities of P450c17.