Aggregation-induced association of syndecan-1 with microfilaments mediated by the cytoplasmic domain.
Expression of the transmembrane proteoglycan syndecan-1 in Schwann cells leads to enhanced spreading and cytoskeletal reorganization, but without an apparent stable association of syndecan-1 with cytoskeletal structures such as focal adhesions. Since cell surface oligomerization may be a mechanism for regulating the activities of transmembrane receptors, we wanted to investigate whether antibody-induced aggregation of the proteoglycan would promote its association with the cytoskeleton. When syndecan-1-expressing cells were incubated with anti-syndecan-1 and anti-IgG antibodies, clustering of proteoglycan on the cell surface was observed by immunofluorescence microscopy. The resulting pattern of syndecan-1 distribution was very similar to that of the underlying microfilament network, as visualized by fluorescent-phalloidin staining. In cells that were fixed briefly with paraformaldehyde before addition of the anti-IgG antibodies no such colocalization of syndecan-1 and microfilaments was observed. Additional findings supported the conclusion that this pattern of syndecan-1 distribution reflected an association with microfilaments: aggregated syndecan-1 was resistant to extraction by nonionic detergent; incubation of the cells with cytochalasin b, but not colchicine, altered the pattern of aggregated syndecan-1 distribution; antibody-induced clustering of syndecan-1 led to a reorganization of actin filaments. Syndecan-1 remained on the cell surface following antibody-induced clustering, as revealed by immunogold staining and transmission electron microscopy. A mutant form of syndecan-1 lacking most of the cytoplasmic domain failed to exhibit actin filament association or induce actin reorganization following antibody-mediated aggregation. These results suggest that transient associations of syndecan family proteoglycans with microfilaments may be important aspects of their biological functions.
Carey
D J
DJ
Weis Center for Research, Geisinger Clinic, Danville, Pennsylvania 17822.
Stahl
R C
RC
Tucker
B
B
Bendt
K A
KA
Cizmeci-Smith
G
G
D008841:Actin Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D000199:Actins / Q000378:metabolism; D000818:Animals; D001483:Base Sequence; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D006651:Histocytochemistry; D008562:Membrane Glycoproteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011509:Proteoglycans / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D051381:Rats; D012583:Schwann Cells / Q000378:metabolism* / Q000648:ultrastructure; D013194:Staining and Labeling; D013329:Structure-Activity Relationship; D053668:Syndecan-1; D053667:Syndecans
D000199:Actins; D008562:Membrane Glycoproteins; D011509:Proteoglycans; C508243:Sdc1 protein, rat; D053668:Syndecan-1; D053667:Syndecans