Molecular cloning and biochemical characterization of medaka (Oryzias latipes) lysosomal neu4 sialidase.

Glycoconjugates are known to be involved in many physiological events in vertebrates. Sialidase is one of the glycosidases, which removes sialic acid from glycoconjugates. In mammals, the properties and physiological functions of sialidases have been investigated, while there is little understanding of fish sialidase. Here, to investigate the significance of fish neu4 sialidase, neu4 gene was cloned from medaka brain mRNA and identified. Sialidase-specific motifs (GPG, YRVP and Asp-Box) were well conserved in the medaka neu4 polypeptide. Optimal pH of medaka neu4 sialidase was 4.6, but its activity was sustained even at neutral and weak alkaline pH. The neu4 considerably cleaved sialic acid from 4-methylumbelliferyl-N-acetyl-α-D-neuraminic acid and sialyllactose, but not from ganglioside and fetuin, which are good substrates for human NEU4. neu4 activity was mostly detected in mitochondria/lysosome fraction after biochemical fractionation, and indirect immunofluorescence assays revealed neu4 localization in lysosome in neu4 overexpressed cells. Next, developmental change in medaka neu4 and other sialidase mRNA levels were estimated by real-time PCR. Each sialidases showed different expression patterns in embryonic development: neu4 was up-regulated at late developmental stage in embryo, and neu3a mRNA level was quite high in 0.5 dpf. On the other hand, neu3b expression was drastically increased after hatching, suggesting that each sialidase may play a different role in embryonic development.

Suppression of Neu1 sialidase delays the absorption of yolk sac in medaka (Oryzias latipes) accompanied with the accumulation of α2-3 sialo-glycoproteins.

Sialidase catalyzes the removal of sialic acids from glycoconjugates. Recently, medaka sialidase Neu1 has been cloned and its enzymatic properties were investigated. Although enzymatic properties of this sialidase, such as optimal pH and substrate specificity, exhibits high similarity with human NEU1, Neu1 physiological functions in fish are still unclear. Here, to understand Neu1 significance in medaka embryogenesis, sialidase translation knockdown was carried out with one-cell stage fertilized egg using morpholino oligo injection. Neu1 exhibited desialylation of α2-3 sialic acid linkage in vitro and lysosomal localization in medaka caudal fin primary cells. Chloroquine treatment, inhibitor of lysosomal enzymes, caused an accumulation of α2-3 sialo-glycoproteins in the primary cells. During the embryogenesis neu1 mRNA level was elevated until 3.5 day post fertilization (dpf) while an initial decrease of α2-3 sialo-glycoprotein was observed around the same developmental stage. Neu1 knockdown by morpholino oligo induced some abnormal phenotypes such as delay of yolk sac absorption and small embryos. Sialidase-knockdown embryos also showed increase of heart rate in 5.5 and 6.5 dpf. Furthermore, about 37% decrease of hatching rate was observed in Neu1-MO treated embryos compared with control MO. Embryos showing severe phenotypes stopped embryogenesis at the late stage of development. Alteration of embryonic sialo-glycoproteins induced by morpholino injection was examined by lectin blotting to clarify the mechanism of abnormal development. As a result, degradation of several α2-3 sialo-glycoproteins was suppressed in Neu1-MO embryo, possibly induced by the interruption of lysosomal desialylation toward yolk glycoprotein. Our results suggest that medaka Neu1 could be crucial for embryonic development through the degradation of yolk sac nutrition.

Positive regulation of myoblast differentiation by medaka Neu3b sialidase through gangliosides desialylation.

Sialidase Neu3b is an unique enzyme conserved in medaka and tilapia, but not in mammals. Previous study revealed that medaka Neu3b is localized at cytosol and is a ganglioside-specific sialidase. Neu3b functions, however, have not been understood, while Neu3a sialidase, which is widely conserved from human to fish, is known as a regulator of neurite formation. Here, we investigated the biological function of Neu3b for C2C12 myoblast cell differentiation. Bioinformatics analysis using genome browser revealed the presence of neu3b gene in some orders of fish species such as Beloniformes, Perciformes and Cyprinodontiformes. With the treatment of 2% horse serum, Neu3b-overexpression accelerated myoblast cell differentiation to myotubes accompanied with up-regulation of myogenesis biomarkers mRNA, myod and myog. Neu3b altered ganglioside composition in C2C12 cells results showing a decrease in GM2, and the increase of Lac-Cer, while desialylation of glycoproteins were not detected. Contrary to cell differentiation, Neu3b cell proliferation was suppressed in normal growth medium. To understand the mechanism of the alteration of cell differentiation and proliferation, phosphorylation of signal molecules in EGFR/ERK pathway was investigated. Neu3b induced a decline in phosphorylation of ERK and EGFR. Surprisingly, immuno-blot and real-time PCR analysis revealed that down-regulation of egfr gene could be involved in the acceleration of cell differentiation by Neu3b. These results suggested that Neu3b sialidase is a positive regulator for myoblast differentiation, similar with mammalian cytosolic sialidase Neu2.

Lysosomal localization of Japanese medaka (Oryzias latipes) Neu1 sialidase and its highly conserved enzymatic profiles with human.

Desialylation in the lysosome is a crucial step for glycoprotein degradation. The abnormality of lysosomal desialylation by NEU1 sialidase is involved in diseases of mammals such as sialidosis and galactosialidosis. Mammalian Neu1 sialidase is also localized at plasma membrane where it regulates several signaling pathways through glycoprotein desialylation. In fish, on the other hand, the mechanism of desialylation in the lysosome and functions of Neu1 sialidase are still unclear. Here, to understand the significance of fish Neu1 sialidase, neu1 gene was cloned from medaka brain and the profiles of its polypeptides were analyzed. Open reading frame of medaka neu1 consisted 1,182 bp and the similarity of its deduced amino acids with human NEU1 was 57%. As this recombinant polypeptide did not show significant sialidase activity, medaka cathepsin A, known in mammals as protective protein activating Neu1, was cloned and then co-expressed with medaka Neu1 to examine whether medaka cathepsin A activates Neu1 activity. As a result, Neu1/cathepsin A showed a drastic increase of sialidase activity toward MU-NANA. Major substrate of medaka Neu1 was 3-sialyllactose and its optimal pH was 4.0. With immunofluorescence analysis, signal of overexpressed medaka Neu1 was found to coincide with Lysotracker signals (organelle marker of lysosome) and co-localized with medaka cathepsin A in fish hepatic Hepa-T1 cells. Furthermore, part of medaka Neu1 was also detected at plasma membrane. Medaka Neu1 possessed signal peptide sequence at N-terminal and incomplete lysosomal targeting sequence at C-terminus. Medaka neu1 gene was ubiquitously expressed in various medaka tissues, and its expression level was significantly higher than other sialidase genes such as neu3a, neu3b and neu4. The present study revealed the profiles of fish Neu1 sialidase and indicated its high conservation with human NEU1 for the first time, suggesting the presence of similar desialylation system in the medaka lysosome to human. Moreover, the present study showed the possibility of medaka as a model animal of human NEU1 sialidase.