Syndecan-4 modulates focal adhesion kinase phosphorylation

Area of Science:

• Cell Biology
• Biochemistry
• Molecular Biology

Background:

• Syndecan-4, a cell-surface proteoglycan, and alpha(5)beta(1) integrin cooperate to promote actin stress fiber and focal adhesion formation in fibronectin-adherent cells.
• Fibroblasts on fibronectin's cell-binding domain (CBD) attach but exhibit incomplete spreading and focal adhesion development.
• Syndecan-4 clustering or activation of Rho or protein kinase C can induce focal adhesions and stress fibers in CBD-adherent fibroblasts.

Purpose of the Study:

• To investigate the role of syndecan-4 in regulating focal adhesion formation and focal adhesion kinase (FAK) phosphorylation.
• To determine the signaling pathways, including Rho and protein kinase C, involved in syndecan-4-mediated focal adhesion regulation.
• To elucidate the specific contribution of syndecan-4 to the phosphorylation of FAK at tyrosine 397 (Tyr(397)).

Main Methods:

• Comparison of fibroblast morphology and focal adhesion formation on full-length fibronectin versus its cell-binding domain (CBD).
• Analysis of focal adhesion kinase (FAK) tyrosine 397 (Tyr(397)) phosphorylation in wild-type and syndecan-4-null fibroblasts under various conditions (e.g., LPA, sodium vanadate, PMA treatment).
• Assessment of FAK Tyr(397) phosphorylation rescue upon LPA addition or syndecan-4 re-expression in null fibroblasts.

Main Results:

• Fibroblasts on CBD showed reduced FAK Tyr(397) phosphorylation compared to those on full-length fibronectin, indicating a role for syndecan-4.
• Activation of Rho (via LPA) or inhibition of tyrosine phosphatases (via sodium vanadate) restored FAK Tyr(397) phosphorylation on CBD.
• Syndecan-4-null fibroblasts exhibited lower FAK Tyr(397) phosphorylation on fibronectin, which was rescued by LPA or syndecan-4 re-expression, confirming syndecan-4's specific role.

Conclusions:

• Syndecan-4 ligation directly regulates FAK Tyr(397) phosphorylation, a process dependent on Rho activation but not protein kinase C.
• The pathway involves the negative regulation of a protein-tyrosine phosphatase, highlighting a novel mechanism of focal adhesion control.
• These findings establish a direct role for syndecan-4 activation in the regulation of focal adhesion organization and signaling.