Extracellular ERp57 promotes fibronectin fibril formation during matrix assembly of articular cartilage

Fibronectin 1 (FN1), a general organizer of extracellular matrix (ECM) in various connective tissues, contains disulfide bridges formed by protein disulfide isomerases (PDIs). ERp57 (PDIA3), an ER resident glycoprotein-specific PDI, is also detectable in cartilage extracellular matrix (ECM). Here, we analyzed the extracellular role of ERp57 in FN1 fibrillogenesis in cartilage. ERp57 KO mice exhibited reduced ECM density. Isolated chondrocytes thereof and C28/I2 ERp57 KO chondrocytes formed fewer and shorter FN1 fibrils than WT cells. Significantly, cell membrane-impermeable thiol blockers reduced FN1 assembly in WT cells, while active recombinant ERp57 protein increased it only in the absence of thiol blockers, emphasizing the necessity of ERp57-mediated disulfide bridge formation for FN1 fibrillogenesis. Co-immunofluorescence and proximity ligation assays revealed a direct interaction between ERp57 and FN1. This study highlights a key role for extracellular ERp57 PDI activity in cartilage and further explains phenotypic changes in ERp57 KO animals.