Nucleation of fibronectin fibril assembly requires binding between heparin and the 13th type III module of fibronectin

Abstract

Fibronectin (FN), a critical component of the extracellular matrix, is assembled into fibrils through a cell-mediated process. Heparan sulfate (HS) binds to the III13 module of FN, and fibroblasts lacking this glycosaminoglycan exhibit reduced FN fibril assembly. To determine if HS depends on III13 to control FN assembly, we deleted both III13 alleles in NIH 3T3 cells using the CRISPR-Cas9 system. ΔIII13cells assembled fewer FN matrix fibrils and less DOC-insoluble FN matrix than wildtype cells. Little if any mutant FN matrix was assembled when purified ΔIII13 FN was provided to Chinese hamster ovary (CHO) cells, showing that lack of III13 caused the deficiency in assembly by ΔIII13cells. Addition of heparin promoted the assembly of wildtype FN by CHO cells, but it had no effect on the assembly of ΔIII13 FN. Furthermore, heparin binding stabilized the folded conformation of III13 and prevented it from self-associating with increasing temperature suggesting that stabilization by HS/heparin binding might regulate interactions between III13 and other FN modules. This effect would be particularly important at matrix assembly sites where our data show that ΔIII13cells require both exogenous wildtype FN and heparin in the culture medium to maximize assembly site formation. Our results show that heparin-promoted growth of fibril nucleation sites is dependent on III13. We conclude that HS/heparin binds to III13 to promote and control the nucleation and development of FN fibrils.