The hexokinase gene family in the zebrafish: structure, expression, functional and phylogenetic analysis

Hexokinase-catalyzed glucose phosphorylation is the first and crucial step for glucose utilization. Although there are reported studies on glucose metabolism in commercial species, knowledge on it is almost nil in zebrafish (Danio rerio), an important model organism for biological research. We have searched these fish hexokinase genes by BLAST analysis; determined their expression in liver, muscle, brain and heart; measured their response to fasting and glucose administration; and performed homology sequences studies to glimpse their evolutionary history. We have confirmed by RT-qPCR studies that the six DNA sequences annotated as possible hexokinases in the NCBI GenBank are transcribed. The organ distribution of the HXK genes is similar in zebrafish as in mammals, to which they are distantly related. Of these, DrGLK and DrSHXK1 are expressed in the fish liver, DrHXK1 in brain and heart, and DrHXK2 in muscle. The only gene responsive to glucose was liver DrGLK. Its expression is induced approximately 1 h after glucose intraperitoneal injection, but not after saline solution injection. The comparison of the fish sequences and the corresponding mammalian ones imply that in both taxa the main muscle and brain isoforms are fusion products of the ancestral gene, their amino halves having separated before than their carboxy ones, followed by the fusion event, whereas fish and mammalian glucokinase genes remained unduplicated.