Regulation of polysialic acid expressed on NCAM in neuromuscular junction

NCAM上表达的多聚唾液酸（PSA）和HNK-1糖表位在神经系统发育过程中受到时空调控。有报道称，NCAM上的PSA在鸡运动神经元生长过程中表达，然后在突触发生完成时减少。为了探讨PSA表达调控的分子机制，本研究对NCAM上PSA的生物合成进行了研究，已知PSA表达于NCAM的第五个IG结构域，本研究对NCAM进行了纯化，并对NCAM上HNK-1表位进行了定位，结果表明HNK-1表位也表达于NCAM的第五个IG结构域，提示葡萄糖醛酸转移酶，其是LINK-I表位生物合成的关键酶之一，可被认为是PSA合成的潜在调节剂之一。因为，葡萄糖醛酸转移酶可以催化葡萄糖醛酸转移到NCAM的半乳糖残基，用葡萄糖醛酸残基取代PSA的第一个唾液酸残基，从而阻断聚唾液酸化。为了证明上述可能性，我们从鸡胚中克隆了葡萄糖醛酸转移酶cDNA，并通过原位杂交比较了葡萄糖醛酸转移酶和多聚唾液酸转移酶的分布。

Polysialic acid (PSA) and HNK-1 carbohydrate epitope expressed on NCAM are spatially and temporally regulated during the development of the nervous system. It has been reported that PSA on NCAM is expressed on chick motoneurons during their growth, and then decreases at about the time that synaptogenesis is completed. In order to investigate the molecular mechanism of the regulation of PSA expression, present study focus on the biosynthesis of PSA on NCAM.It is known that PSA is expressed on the fifth Ig domain of NCAM, In the present study, we purified NCAM and determined the position of HNK-1 epitope on NCAM, The result indicated that HNK-I epitope is also expressed on the fifth Ig domain of NCAM, suggesting that glucuronyltransferase, which is one of the key enzyme in the biosynthesis of the LINK-I epitope could be considered as one of potential regulator of PSA synthesis. Because, the glucuronyltransferase can catalyze the transfer of glucuronic acid to galactose residue of NCAM, substituting the first sialic acid residue of PSA by glucuronic acid residue and thus blocking polysialylation. To demonstrate the possibility described above, we cloned the glucuronyltransferase cDNA from chick embryo and the distribution of the glucuronyltransferase are comparing with that of polysialyltransferase by in situ hybridization.