Fibrinogen Activates PAK1/Cofilin Signaling Pathway to Protect Endothelial Barrier Integrity

Area of Science:

• Endothelial biology
• Molecular signaling
• Hemostasis and thrombosis

Background:

• Fibrinogen stabilizes syndecan-1 on endothelial cells, protecting the barrier, but the intracellular pathway is unknown.
• P21 (Rac1) activated kinase 1 (PAK1) is crucial for endothelial barrier integrity via actin cytoskeleton regulation.

Purpose of the Study:

• To investigate if fibrinogen binding to syndecan-1 activates the PAK1 signaling pathway.
• To elucidate the role of PAK1 in fibrinogen-mediated endothelial barrier protection.

Main Methods:

• Primary human lung microvascular endothelial cells were treated with fibrinogen or lactated Ringers (LR).
• Protein phosphorylation, cofilin dephosphorylation, and stress fiber formation were assessed via Western blot and microscopy.
• Endothelial permeability was measured using FITC-dextran.
• PAK1 was silenced using siRNA.
• Mice underwent hemorrhagic shock and resuscitation with LR or fibrinogen, with lung vascular leakage and p-PAK1 levels measured.

Main Results:

• Fibrinogen increased syndecan-1, PAK1 activation, and cofilin dephosphorylation, while decreasing stress fibers and permeability compared to LR.
• PAK1 silencing abolished fibrinogen's protective effects on cofilin and barrier integrity.
• In vivo, fibrinogen resuscitation in hemorrhagic shock prevented lung vascular leakage and restored p-PAK1 levels.

Conclusions:

• Fibrinogen activates PAK1 signaling, leading to cofilin dephosphorylation and stress fiber disassembly.
• This novel pathway reduces endothelial permeability and protects against vascular leakage.
• Fibrinogen represents a potential therapeutic agent for maintaining endothelial barrier integrity.