Identification of a novel anti-angiogenic regulatory sequence within the syndecan-3 extracellular core protein.

Syndecans are transmembrane proteoglycans that regulate angiogenesis through both their glycosaminoglycan chains and core proteins. While roles for all four mammalian syndecans in new blood vessel formation are well established, it has more recently emerged that their extracellular core proteins contain discrete bioactive regulatory sequences capable of influencing cellular processes, including angiogenesis. We previously demonstrated that the syndecan-3 (SDC3) ectodomain possesses anti-angiogenic activity independent of its heparan sulphate chains. Here, we identified and characterised a novel anti-angiogenic sequence within the SDC3 ectodomain. Using recombinant truncation mutants, endothelial migration assays and peptide mapping, we localised activity to a discrete region of the extracellular domain and subsequently defined a conserved minimal nine amino acid peptide, QM111, that retained full biological activity. QM111 inhibited endothelial cell migration and angiogenic sprouting in both rat aortic ring and mouse choroidal explant models. Intrinsic disorder analysis revealed that QM111 resides within a region of comparatively reduced disorder, consistent with other syndecan regulatory sequences. This supports the concept that syndecan ectodomains contain conserved functional modules embedded within intrinsically disordered extracellular domains. QM111 did not induce inflammatory chemokine production, exhibited no detectable cytotoxicity, and retained substantial stability in human serum and vitreous humour. Finally, QM111 displayed anti-angiogenic activity comparable to the previously described syndecan-2-derived peptide QM107, with combination treatment producing more robust inhibition of angiogenesis. These findings identify QM111 as a novel endogenous anti-angiogenic peptide and support the concept that syndecan ectodomains are reservoirs of biologically active regulatory sequences with therapeutic potential. The work further establishes syndecan-derived peptides as a promising platform for the development of next-generation anti-angiogenic therapies.