Identification and expression analysis of transcripts involved in taurine biosynthesis during early ontogeny of tropical gar Atractosteus tropicus.

In fishes, the availability of taurine is regulated during ontogenetic development, where its endogenous synthesis capacity is species dependent. Thus, different pathways and involved enzymes have been described: pathway I (cysteine sulfinate-dependent pathway), cysteine dioxygenase type 1 (cdo1) and cysteine sulfinic acid decarboxylase (csad); pathway II (cysteic acid pathway), cdo1 and glutamic acid decarboxylase (gad); and pathway III (cysteamine pathway), 2-aminoethanethiol dioxygenase (ado); whereas taurine transporter (taut) is responsible for taurine entry into cells on the cell membrane and the mitochondria. This study determined if the tropical gar (Atractosteus tropicus), an ancient holostean fish model, has the molecular mechanism to synthesize taurine through the identification and analysis expression of transcripts coding for proteins involved in its biosynthesis and transportation, at different embryo-larvae stages and in different organs of juveniles (31 dah). We observed a fluctuating expression of all transcripts involved in the three pathways at all analyzed stages. All transcripts are expressed during the beginning of larval development; however, ado and taut show a peak expression at 9 dah, and all transcripts but csad decreased at 23 dah, when the organism ended the larval period. Furthermore, at 31 dah, we observed taut expression in all examined organs. The transcripts involved in pathways I and III are expressed differently across all organs, whereas pathway II was only observed in the brain, eye, and skin. The results suggested that taurine biosynthesis in tropical gar is regulated during its early development before first feeding, and the pathway might also be organ-type dependent.