IDENTIFICATION OF A NOVEL ALTERNATIVELY SPLICED AGRIN MESSENGER-RNA THAT IS PREFERENTIALLY EXPRESSED IN NONNEURONAL CELLS

A novel agrin isoform was identified based on the isolation of an agri n cDNA from E9 chick brain that lacked 21 base pairs (bp) in the NH2-t erminal encoding region of the agrin mRNA. Reverse transcription-polym erase chain reaction (RT-PGR) of E9 chick brain mRNA confirmed the exi stence of this agrin isoform in brain, although the novel splice varia nt represents a minor fraction of agrin mRNA in brain. However, upon a nalysis of chick brain astrocyte mRNA, smooth muscle mRNA, and cardiac muscle mRNA by RT-PCR, we show that this novel agrin isoform is the p redominant agrin isoform in these non-neuronal cell populations. We ex tended our analyses to examine the expression of this agrin mRNA isofo rm during chick development and show that the agrin mRNA lacking this 21-bp exon is up-regulated with brain development, consistent with the increase in glial number during brain development, while the agrin is oform that does not undergo splicing and thus contains the 21-bp exon is down-regulated in brain development. Because the 21-bp exon is inse rted in the region of chick agrin which encodes the putative signal se quence of agrin, with the signal peptidase site immediately preceding the putative first amino acid of the mature protein being deleted as a result of splicing, these data raise the interesting possibility that the presence or absence of this alternatively spliced exon may differ entially regulate processing of the agrin protein in neuronal and non- neuronal cells, respectively.