Regulation of polysialic acid expressed on NCAM in neuromuscular junction

NCAM上表达的聚唾液酸(PSA)和HNK-1碳水化合物表位在神经系统发育过程中具有时空调节作用。据报道，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下降。为了探讨PSA表达调控的分子机制，本研究对PSA在NCAM上的生物合成进行了研究。已知PSA在NCAM的第五个Ig结构域上表达。在本研究中，我们纯化了NCAM并测定了HNK-1表位在NCAM上的位置，结果表明HNK-I表位也在NCAM的第五个Ig结构域上表达，提示葡萄糖糖醛酸基转移酶作为LINK-I表位生物合成的关键酶之一，可能是PSA合成的潜在调节因素之一。因为，葡萄糖糖醛酸基转移酶可以催化葡萄糖糖醛酸转移到NCAM的半乳糖残基，将PSA的第一个唾液酸残基替换为葡萄糖醛酸残基，从而阻断聚唾液酸化。为了证明上述可能性，我们从鸡胚胎中克隆了葡萄糖醛酸基转移酶基因，并用原位杂交的方法比较了葡萄糖醛酸基转移酶和聚唾液酸基转移酶的分布。

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.