Cell surface modulation of the neural cell adhesion molecule resulting from alternative mRNA splicing in a tissue-specific developmental sequence.

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Abstract
The neural cell adhesion molecule N-CAM is an intrinsic membrane glycoprotein that is expressed in the embryonic chicken nervous system as two different polypeptide chains encoded by alternatively spliced transcripts of a single gene. Because they differ by the presence or absence of approximately 250 amino acids in their cytoplasmic domains, these polypeptides are designated ld and sd, for large and small cytoplasmic domain, respectively. We report here that the ld-specific sequences comprise a single exon in the chicken N-CAM gene and that developmental expression of the ld and sd chains occurs in a tissue-specific fashion, with the ld chain restricted to the nervous system. Comparison of the nucleotide sequences from an N-CAM genomic clone with cDNA sequences showed that a single exon of 783 base pairs corresponded to the unique cytoplasmic domain of the ld polypeptide. Sequences from this exon were absent from the single N-CAM mRNA detected in several non-neural tissues by RNA blot hybridization, and immunoblot analysis confirmed that antigenic determinants unique to the ld-specific domain were not expressed in these tissues. Immunohistochemical experiments indicated that only the sd chain was expressed on cell surfaces of non-neural tissues throughout embryonic development. The ld chain was found on cell bodies and neurites of differentiated neurons; it first appeared as neurons began to extend neurites and to express the neuron-glia cell adhesion molecule (Ng-CAM) and it was restricted to definite layers in laminar tissues such as the retina and cerebellum. These results suggest that the control of mRNA splicing may affect the regulation of N-CAM function at specific sites within the nervous system and thus influence the control of neural morphogenesis and histogenesis.