Identification, expression and regulation of amphioxus G6Pase gene with an emphasis on origin of liver

Vertebrate glucose-6-phosphatase (G6Pase) consists of three isozymes: G6Pase-I, G6Pase-II and G6Pase-III. Despite extensive study on G6Pases in vertebrates, information regarding expression and regulation of G6Pase genes is rather limited in invertebrates. Here we report the identification of G6Pase gene in amphioxus Branchiostoma japonicum, which is abundantly expressed in the digestive diverticulum and ovary in a tissue-specific manner. The phylogenetic and genomic structure analyses reveal that amphioxus G6Pase bears close resemblance to vertebrate G6Pase-III and represents the archetype of vertebrate G6Pase from which the vertebrate G6Pase isoforms may be originated by 2 rounds of genome duplication during vertebrate evolution. We also demonstrate that GH treatment induces a closely similar expression pattern and trend of g6pases in both zebrafish and amphioxus, and that G6Pase activity in amphioxus digestive diverticulum is subjected to regulation of feeding and fasting as observed in vertebrates. Collectively, all these provide functional evidences supporting the notion that the digestive diverticulum is the liver homologue playing a key role in maintaining the glucose homeostasis in amphioxus.