Gluconeogenesis in the YSL-like tissue of cloudy catshark ( Scyliorhinus torazame )

Glucose has important roles in the development of hematopoietic stem cells and brain of zebrafish, the vertebrate animal model; however, in most oviparous animals, the amount of maternally provided glucose in the yolk is scarce. For these reasons, developing animals need some ways to supplement glucose. Recently, it was found that developing zebrafish, a teleost fish, undergo gluconeogenesis in the yolk syncytial layer (YSL), an extraembryonic tissue that surrounds the yolk, utilizing yolk nutrients as substrates. However, teleost YSL is evolutionarily unique, and it is not clear how other vertebrates supplement glucose. In this study, we used cloudy catshark, an elasmobranch species which possesses a YSL-like tissue during development, and sought for possible gluconeogenic activities in this tissue. In the catshark yolk sac, an increase in glucose level was found, and our isotope tracking by ¹³ C-labeled substrate combined with LC/MS analysis detected gluconeogenic activities with glycerol most preferred substrate. In addition, expression analysis for gluconeogenic genes showed that many of these were expressed at the YSL-like tissue, suggesting that cloudy catshark engages in gluconeogenesis in this tissue. The gluconeogenesis in teleost YSL and a similar tissue in elasmobranch species implies conservation of the mechanisms of yolk metabolism between these two lineages. Future studies on other vertebrate taxa will be helpful to understand the evolutionary changes in the modes of yolk metabolism that vertebrates have experienced.