Structure-Based Engineering for PTPσ-Targeted Therapeutic Development.

Protein tyrosine phosphatase sigma (PTPσ) is a member of the leukocyte common antigen-related (LAR) family of receptor-type protein tyrosine phosphatases. Its architecture integrates an extracellular proteoglycan-sensing region, a single-pass transmembrane helix, and tandem intracellular phosphatase domains. In neural injury, inhibition of PTPσ signaling can relieve chondroitin sulfate proteoglycan (CSPG)-mediated restriction of axonal growth and plasticity. In rheumatoid arthritis (RA), by contrast, restoration of PTPσ activity in fibroblast-like synoviocytes can suppress pathogenic migration and invasion. Structural studies have established a proteoglycan-switch model in which glycosaminoglycan engagement of the N-terminal immunoglobulin-like domains and extracellular clustering regulate intracellular phosphatase activity. Recent work demonstrated that antibodies to the membrane-proximal fibronectin type III-like domain 9 (Fn9) can regulate PTPσ oligomerization and syndecan-4 association in rheumatoid arthritis. Structure-guided protein engineering has converted a poorly soluble PTPσ Ig1-Ig2 decoy into developable Fc-fusion variants with substantially improved solubility and expression yield while retaining glycosaminoglycan binding and cell-based activity. This review summarizes the structural basis of PTPσ regulation and evaluates therapeutic modalities and engineering strategies for advancing PTPσ-directed biologics toward clinical candidates.