Identification and function of agrin expressed in rat brain microvessels

Agrin isoforms in the brain microvessels were investigated. Brain microvessels prepared from rat cerebral cortex expressed the short amino terminal (SN) isoform, which contains a transmembrane sequence of SN agrin, and the long amino terminal (LN) isoform, which contains a signal sequence for secretion of LN agrin, were detected in the microvessels. The expression of LN form mRNA in comparison with that of SN form was higher in the brain microvessels than in the cerebral cortex, indicating preferential expression of LN form agrin in the microvessels. In addition, the amino-terminal sequence of agrin expressed in the rat brain microvessels was determined by rapid amplification of 5' cDNA end (5'-RACE). The sequence contained a cluster of hydrophobic amino acids, which may form a signal sequence for secretion. Transfection of expression vector coding amino-terminal sequence of LN agrin fused with green fluorescence protein (GFP) into human embryonic kidney (HEK293) cells caused expression of the protein in the cell as well as in the medium, whereas SN agrin fused with GFP expressed at the cell surface. These results show that the major form of agrin expressed in the brain microvessels is of basal lamina-associated LN agrin and suggest that LN agrin may play a role in molecular organization at the interface between the parenchyma cells and microvessels.