Functional Characterization of Fish Sialidases and Their Diversity among Different Orders

Abstract

Sialidase catalyzes the removal of sialic acids from glycoconjugates. While the physiological functions of mammalian sialidases have been elucidated, sialidases in other vertebrates, including those in fish, are not well understood. Recently, increased genomic resources in fish have become available, which will facilitate research on fish sialidase.

Here, three fish orders (Cypriniformes, Beloniformes and Perciformes) have been selected to compare the properties of their sialidases. A Neu1 knockdown in medaka by a morpholino oligo resulted in the accumulation of Sia α2-3 linked sialoglycoproteins, leading to malfunctions in heart beating, embryo growth, hatching, and swimming, indicating that lysosomal Neu1 is a crucial factor for embryogenesis.

Fish Neu3 and Neu4 function varies between fish species. Medaka and tilapia Neu3a localize at the plasma membrane, as is seen in humans. Zebrafish Neu3.1 is an ER sialidase, with an optimal pH (pH 2.6). Neu4 shows high diversity in subcellular localization: medaka and zebrafish Neu4 are detected at the lysosome and ER, respectively. Surprisingly, tilapia Neu4 is detected at nucleus, which makes it the first nuclear sialidase to be identified. Moreover, importin has been identified as a regulator of tilapia Neu4 functions. These results demonstrate the diversity of sialidase functions among fish species and the requirement of further fish research.