Fibronektiinia

Fibronektiinia, known in English as fibronectin, is a high-molecular-weight glycoprotein of the extracellular matrix. It circulates in plasma as soluble fibronectin and is also produced by cells where it becomes part of the insoluble cellular fibronectin matrix. Fibronectin coordinates cell adhesion, migration, growth, and differentiation and is essential for wound healing and embryonic development.

Structurally, fibronectin is a dimer composed of two nearly identical subunits linked by disulfide bonds near the C-terminus. Each subunit contains modules of type I, II, and III repeats, forming distinct binding domains for collagen, heparin, fibrin, and integrins. Cellular fibronectin isoforms arise from alternative splicing of the FN1 gene and may include extra domains EDA and/or EDB; plasma fibronectin lacks these segments.

Expression: The FN1 gene encodes the fibronectin monomer. Plasma fibronectin is mainly produced by hepatocytes and circulates in blood, while cellular fibronectin is produced by fibroblasts, endothelial cells, and other tissues and assembles into the extracellular matrix.

Function and roles: Binding to integrin receptors, particularly α5β1, via an RGD motif, fibronectin mediates cell adhesion to the extracellular matrix and to other ECM components such as collagen and heparan sulfate. It influences cell survival, migration during development, angiogenesis, and tissue remodeling. It also acts as a provisional matrix during wound repair.

Clinical and research notes: Altered fibronectin expression and isoform composition have been reported in cancer, fibrosis, and inflammatory diseases. Fibronectin is widely used as a model substrate in cell culture and as a component of biomaterials for tissue engineering. In animal models, loss of Fn1 disrupts gastrulation and leads to embryonic lethality, underscoring its developmental importance.