Alpha-actinin interactions with syndecan-4 are integral to fibroblast-matrix adhesion and regulate cytoskeletal architecture.

All cells of the musculoskeletal system possess transmembrane syndecan proteoglycans, notably syndecan-4. In fibroblasts it regulates integrin-mediated adhesion to the extracellular matrix. Syndecan-4 null mice have a complex wound repair phenotype while their fibroblasts have reduced focal adhesions and matrix contraction abilities. Signalling through syndecan-4 core protein to the actin cytoskeleton involves protein kinase Cα and Rho family G proteins but also direct interactions with α-actinin. The contribution of the latter interaction to cell-matrix adhesion is not defined but investigated here since manipulation of Rho GTPase and its downstream targets could not restore a wild type microfilament organisation to syndecan-4 null cells. Microarray and protein analysis revealed no significant alterations in mRNA or protein levels for actin- or α-actinin associated proteins when wild type and syndecan-4 knockout fibroblasts were compared. The binding site for syndecan-4 cytoplasmic domain was identified as spectrin repeat 4 of α-actinin while further experiments confirmed the importance of this interaction in stabilising cell-matrix junctions. However, α-actinin is also present in adherens junctions, these organelles not being disrupted in the absence of syndecan-4. Indeed, co-culture of wild type and knockout cells led to adherens junction-associated stress fibre formation in cells lacking syndecan-4, supporting the hypothesis that the proteoglycan regulates cell-matrix adhesion and its associated microfilament bundles at a post-translational level. These data provide an additional dimension to syndecan function related to tension at the cell-matrix interface, wound healing and potentially fibrosis.

Syndecan signaling: when, where and why?

Syndecans are the only family of transmembrane heparan sulphate proteoglycans. Invertebrates all appear to have one Syndecan core protein, but in mammals there are four. Examination of the core protein sequences shows that the cytoplasmic domains are the most conserved. This suggests that Syndecans make important interactions and/or signalling contributions. It has been established that all syndecans can interact with proteins of the actin-associated cytoskeleton, but details of signalling have been harder to ascertain. However, it appears that Syndecans can signal, primarily to the cytoskeleton, and the details are reviewed here. Only for vertebrate syndecan-4 is there substantial detail, and it remains a challenge to dissect the functions and signalling of other vertebrate and invertebrate syndecans.