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.