Releasing stored lipids to fuel migration and reproduction in the eel, Anguilla australis—a role for 11-ketotestosterone?

Migrating freshwater eels depend on the mobilisation of stored lipids to successfully arrive at their distant spawning locations. As 11-ketotestosterone (11KT) can increase the lipid-transporting capability and enhance gonadal lipid uptake in eel, we hypothesized that this androgen would also regulate lipid mobilisation from its stores. To address this hypothesis, we first sampled residential (yellow) and migrating (silver) short-finned eels from the wild and evaluated the expression of 24 genes encoding lipolytic or lipogenic enzymes, as well as those encoding both nuclear androgen receptors, by NanoString analysis. Plasma 11KT levels in silver eels were dramatically increased, and mRNA levels of more than half of all target genes were higher in silver eel muscle; none of the target genes was significantly downregulated. Gene expression profiles in white muscle from wild-caught eels were subsequently compared with those from yellow and silver eels subjected to implantation with sustained-release implants containing 11KT. Several weeks of exposure resulted in plasma levels of 11KT that resembled those of wild-caught eels and resulted in a dose-dependent increase in gonadosomatic and hepatosomatic index; however, target gene expression profiles in muscle were barely affected. We conclude that lipid physiology in white muscle of silver eels is notably different from that in yellow eels, and that 11KT is not responsible for the differentially expressed gene profile between yellow and silver short-finned eels.