Syndecans shed their reputation as inert molecules.

The syndecan transmembrane proteoglycans synergize with receptors for extracellular matrix molecules and growth factors to initiate cytoplasmic signals in response to a range of extracellular stimuli. Syndecans influence a wide range of physiological processes, but their contribution is most apparent during wound repair. Aspects of syndecan biology that have attracted research interest include extracellular matrix binding, outside-to-inside plasma membrane signal propagation, activation of cytoplasmic signals, and shedding of the syndecan extracellular domain, but the mechanisms by which syndecan cytoplasmic signals modulate extracellular function remain largely unresolved. Hayashida et al. have now discovered that association between an endocytic regulator, Rab5, and the syndecan-1 cytoplasmic domain controlled the shedding of the syndecan-1 extracellular domain. The work describes a mechanistic investigation into inside-to-outside syndecan signaling and highlights several gaps in our understanding of the relation between cell-surface receptors and proteases. In this Perspective, we summarize the current understanding of receptor interplay and identify the challenges that face investigators of adhesion- and growth factor-dependent signaling.