Extracellular vesicles (EVs) are membrane-limited organelles mediating cell-to-cell communication in health and disease. EVs are of high medical interest, but their rational use for diagnostics or therapies is restricted by our limited understanding of the molecular mechanisms governing EV biology. Researchers at KU Leuven tested whether PDZ proteins, molecular scaffolds that support the formation, transport, and function of signal transduction complexes and that coevolved with multicellularity, may represent important EV regulators.
The researchers reveal that the PDZ proteome (ca. 150 proteins in human) establishes a discrete number of direct interactions with the tetraspanins CD9, CD63, and CD81, well-known EV constituents. Strikingly, PDZ proteins interact more extensively with syndecans (SDCs), ubiquitous membrane proteins for which they previously demonstrated an important role in EV biogenesis, loading, and turnover. Nine PDZ proteins were tested in loss-of-function studies. They document that these PDZ proteins regulate both tetraspanins and SDCs, differentially affecting their steady-state levels, subcellular localizations, metabolism, endosomal budding, and accumulations in EVs. Importantly, the researchers also show that PDZ proteins control the levels of heparan sulfate at the cell surface that functions in EV capture.
This study establishes that the extensive networking of SDCs, tetraspanins, and PDZ proteins contributes to EV heterogeneity and turnover, highlighting an important piece of the molecular framework governing intracellular trafficking and intercellular communication.