Previtellogenic ovarian fragments from eel, Anguilla australis, were cultured in vitro in a chemically defined medium containing steroids and/or peptide hormones for 18 days in order to investigate their involvement in control of early oocyte growth. 11-Ketotestosterone (11-KT), but not estradiol-17b, induced a significant 10-20% increase in diameters of previtellogenic oocytes and oocyte nuclei in a dose-dependent manner. Effects were greatest for 100 nM 11-KT, a dose that is within the physiological range seen in very early vitellogenic eels in the wild. The effect was not accompanied by obvious ultrastructural changes in the oocytes other than an apparent increase in nuclear size. Similarly, treatment with recombinant human IGF-I resulted in increased oocyte diameters, whereas no such effect was seen after treatment with heterologous insulin, GH, leptin, or human chorionic gonadotropin. Interestingly, lipid supplementation also resulted in an increase in oocyte diameter, and greater radioactivity in ovarian explants following incubation with 14 C-triglycerides and 11-KT, but not FSH, suggesting that the androgen may play a role in lipid accumulation into the oocyte. Our results implicate hormones from both the reproductive and the metabolic axes in control of previtellogenic oocyte growth in a teleost fish.Reproduction (2007) 133 955-967

This article is available online at http://www.jlr. org Lipoproteins, particles that transport lipids through the blood stream, are distinguished by density and size: from large, low density macromolecules, such as chylomicrons and very low density lipoproteins (VLDL) (high lipid and low protein content), to smaller, high density macromolecules, such as high density lipoproteins and vitellogenin (low lipid and high protein content) ( 7 ). The main lipoprotein component of eel plasma is VLDL ( ‫ف‬ 44% of total plasma lipoproteins), and TAG is its major lipid class (57-66% of VLDL composition ( 8 )). Associated with the lipoproteins is apolipoprotein CII (apoCII), a peptide that has the ability to specifi cally activate the enzyme lipoprotein lipase (LPL) ( 9 ). LPL is attached to the luminal endothelium and located in close proximity to the adipocytes, where it hydrolyses bound TAGs to free fatty acids (FFA). These FFAs, in turn, are sequestered by the surrounding tissue and re-esterifi ed for storage as TAG (e.g., adipose tissue) or oxidized for energy (e.g., muscle tissue ( 10 )). As the catabolism of FFA provides the main source of energy for many species ( 11 ), LPL is considered a key enzyme in whole body lipid metabolism and balance, and the extrahepatic rate-limiting enzyme in the hydrolysis of circulating TAG (12)(13)(14).Aside from their importance in generating energy, the lipid composition of eggs is a key determinant of egg quality in marine fi sh (reviewed in Ref. 15 ); however, there is almost no information about when and how neutral lipids are incorporated into the oocyte or the role that LPL plays in this event. Likewise, the fate and, in particular, the movement of neutral lipid from storage depots to the ovary throughout oogenesis is largely unknown, despite the obvious relevance of these issues for aquaculture operations. Freshwater eels are highly suitable experimental models for the study of lipid movement and uptake as they Abstract To understand the dynamics of lipid uptake into the ovary and the potential role that lipoprotein lipase plays in this event, changes in LPL transcript abundance during oogenesis were measured in both wild-caught and pituitary homogenate-induced artifi cially maturing eels. Also, the effects of 11-ketotestosterone (11-KT) on LPL mRNA levels were investigated in vivo and in vitro. Normalized ovarian LPL transcript abundance increased as oogenesis advanced, and it rose particularly rapidly during midvitellogenesis, corresponding to pronounced increases in ovarian lipid deposits and LPL activity. Furthermore, LPL mRNA levels were dramatically increased following 11-KT treatment in vivo, fi ndings that were reinforced as trends in ovarian tissue incubated in vitro. Ovarian LPL appears to be directly involved in the uptake of lipids into the eel ovary, an involvement that appears to be controlled, at least in part, by the androgen 11-KT. It is well established in teleosts that lipids form the major component of the body nutrient pool in terms of energy storage ( 1...

In order to study the regulation of the growth differentiation factor-9 (gdf9) gene in a primitive teleost with semelparous life history, we cloned a cDNA encoding shortfinned eel Gdf9, expressed a partial peptide in Escherichia coli, and raised an antiserum to evaluate changes in Gdf9 expression during its pituitary homogenate-induced reproductive cycle. The effects of in vivo and in vitro exposure to the androgen 11-ketotestosterone (11-KT), known to affect previtellogenic (PV) oocyte growth, were also determined. Furthermore, we investigated whether Gdf9 expression was metabolically gated by treating PV fish with recombinant GH in vivo. Immunoreactive proteins of ca. 52 and 55 kDa were identified by western blot analysis. Gdf9 message and protein were most abundant in PV oocytes, and peaked slightly earlier for mRNA than for protein. Captivity resulted in reduced gdf9 mRNA levels, which were restored following pituitary homogenate treatment. As oocytes progressed through induced oogenesis, Gdf9 expression decreased. Neither 11-KT nor GH treatment affected gdf9 mRNA levels in PV fish, although GH could partially restore handling-or captivity-induced decreases in gdf9 mRNA levels. Semelparous eels thus show an expression pattern of Gdf9 during oogenesis that is similar to that seen in other vertebrates, that appears responsive to handling or captivity stress, and whose control remains to be elucidated.

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