Syndecan-4 cytoplasmic domain regulation of turkey satellite cell focal adhesions and apoptosis

Syndecan-4 is a cell membrane proteoglycan composed of a transmembrane core protein and substituted glycosaminoglycan (GAG) and N-linked glycosylated (N-glycosylated) chains. The core protein has three domains: extracellular, transmembrane and cytoplasmic domains. The GAG and N-glycosylated chains and the cytoplasmic domain of syndecan-4, especially the amino acids: Ser(178) and Tyr(187) are critical in regulation of turkey satellite cell growth and development. How these processes are regulated is still unknown. The objective of the current study was to determine whether the syndecan-4 GAG and N-glycosylated chains and the cytoplasmic domain functions through modulating focal adhesion formation and apoptosis. Twelve mutant clones were generated: a truncated syndecan-4 without the cytoplasmic domain with or without GAG and N-glycosylated chains, and Ser(178) and Tyr(187) mutants with or without GAG and N-glycosylated chains. The wild type syndecan-4 and all of the syndecan-4 mutants were transfected into turkey myogenic satellite cells after which cell apoptosis and focal adhesion formation were measured. Syndecan-4 increased cell membrane localization of β1 integrin and the activity of focal adhesion kinase (FAK) whereas the cytoplasmic domain mutation decreased the phosphorylation of FAK. However, syndecan-4 and syndecan-4 mutants did not influence cell apoptosis. They also had no effect on vinculin or paxillin-containing focal adhesion formation. These results suggested that the syndecan-4 cytoplasmic domain plays an important role in regulating FAK activity and β1 integrin cell membrane localization but not cell apoptosis and vinculin or paxillin-containing focal adhesion formation.