Modulation of the PTPRS proteoglycan switch by antibodies binding to the membrane-proximal fibronectin-type III domain

Protein tyrosine phosphatases (PTPs) receptor type II A (R2A) are negatively regulated through oligomerization upon binding of their extracellular domains to glycosaminoglycans (GAGs) on heparan sulfate proteoglycans (HSPGs). Inactivation of receptor PTP sigma (PTPRS) by HSPGs promotes the aggressive behavior of fibroblast-like synoviocytes (FLS) in rheumatoid arthritis (RA). Blocking the binding of its N-terminal, membrane-distal immunoglobulin-like 1 and 2 (Ig1&2) domains to its GAG ligands on the HSPG syndecan-4 (SDC4) promotes PTPRS activity and reverses the pathogenic phenotype of FLS. The potential for therapeutically leveraging other PTPRS ectodomain regions is, however, unknown. We show targeting the membrane-proximal fibronectin type III-like 9 (Fn9) domain offers a novel avenue to activate PTPRS. We mapped PTPRS Fn9 as the binding site of three antibodies (Abs) (13G5, 22H8, 49F2) and characterized their effects on cells. Despite sharing similar epitopes, we found large differences in the ability of these Abs to regulate PTPRS activity. One of these, 13G5, reduced PTPRS-dependent cell migration, PTPRS co-localization with SDC4, and PTPRS oligomerization. Single-chain variable fragment Abs of 13G5 and 22H8 were similarly effective at activating cellular PTPRS as 13G5. Replacing the entire 13G5 constant region enhanced its binding and cellular activity, indicating the Ab's potency can be optimized via isotype engineering. Treatment of cells with recombinant Fn9 protein acted as a decoy, disrupting PTPRS colocalization with SDC4 and oligomerization, and inhibiting FLS migration. Finally, significant disease mitigation in mice using 13G5-derived Abs suggests a viable strategy for the generation of novel drugs for RA therapy.