A novel role for the integrin-binding III-10 module in fibronectin matrix assembly

Area of Science:

• Cell Biology
• Biochemistry
• Extracellular Matrix Biology

Background:

• Fibronectin matrix assembly is vital for cell adhesion, migration, and differentiation during development and wound repair.
• The alpha 5 beta 1 integrin and specific fibronectin modules (amino terminus, III-1) are known to be important for fibronectin polymerization.
• Previous work identified a conformation-dependent binding site within the III-1 module for fibronectin's amino terminus.

Purpose of the Study:

• To elucidate the precise relationship between the alpha 5 beta 1 integrin and the process of fibronectin polymerization.
• To investigate the role of specific fibronectin modules (III-1 and III-10) in mediating fibronectin self-interactions and matrix assembly.

Main Methods:

• Solid phase binding assays using recombinant fibronectin modules (III-1, III-10).
• Analysis of ternary complex formation involving fibronectin fragments.
• Assessment of fibronectin multimer formation in cell-free systems.
• Inhibition studies using monoclonal antibodies and fibronectin fragments.
• Cell-based assays using fibroblast monolayers and Texas red-conjugated fibronectin fragments to assess matrix assembly and focal adhesion localization.

Main Results:

• The III-10 module of fibronectin contains a conformation-dependent binding site for the III-1 module.
• Fibronectin's III-1 module can simultaneously bind both the III-10 module and the amino-terminal 70-kD fragment.
• Fibronectin III-10 promotes disulfide-bonded multimer formation independently of cells, an effect inhibited by anti-III-1 antibodies and amino-terminal fragments.
• A fibronectin III-10 fragment (III-10/A) blocks matrix assembly via a mechanism independent of integrin binding.
• Fibronectin fragments localize to beta 1-containing focal adhesions, and antibodies against III-1 or III-9,10 modules block this binding without disrupting focal adhesions.

Conclusions:

• The alpha 5 beta 1 integrin plays a role in fibronectin matrix assembly by regulating sequential fibronectin self-interactions.
• These interactions lead to the polymerization of fibronectin, forming the extracellular matrix.
• Specific fibronectin modules, particularly III-10 and III-1, are key mediators of these self-association events.