Structure, regulation and function of avian glypican

Glypicans are a group of membrane-bound heparan sulfate proteoglycans (HSPG) that are tissue specific and developmentally regulated. Transcripts for avian glypican are found in endocardial cushions, limb buds, somites and forebrain of early chick embryos. Since avian glypican is not well characterized, the cellular localization, regulation of expression, and possible function during cardiac development have been studied. A polyclonal antibody was raised against a 20-amino acid peptide corresponding to an antigenic sequence within avian glypican core protein. The antibody recognized the expressed core protein in bacterial lysates and the endogenous HSPG in the proteoglycan fraction from chick forebrain. Immunolocalization studies indicated that the core protein is associated with cell membranes. The level of mRNA for avian glypican in MEQC (myc embryonic quail cardiomyocytes) grown in medium containing 10% fetal calf serum was compared to the message levels in cells grown without serum for 3 days. By Northern analysis, glypican transcripts were increased markedly after serum starvation. Up-regulation of glypican transcripts by serum withdrawal was partially prevented by addition of TGFbeta-1 and bFGF, suggesting that these growth factors may regulate its expression. MEQC cells deprived of serum migrated into clumps that could be blocked by an antisense OND (oligodeoxynucleotide) to the mRNA encoding the avian glypican. The same antisense OND inhibited the migration of endothelial cells from chick tubular heart explants over the surface of collagen gels. These results indicate that avian glypican may play a role in cell migration during development of endocardial cushions.