A Slipping Clutch in Neuronal Growth Cones Revealed by Transient Single Molecule Interactions between Flowing Actin and N-Cadherin Adhesions

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The molecular clutch model proposes that the actin motile machinery generated inside cells is coupled to adhesion at the cell membrane, thus generating forces on the substrate and allowing cells to move forward. Many reports have provided evidence for clutch-like behaviors in different cell types and adhesive systems, including integrins and cadherins (Giannone et al., Trends Cell Biol 2009). However, the exact mechanisms underlying the dynamic molecular coupling between the actin retrograde flow and adhesion proteins remain elusive. We previously inferred using optical tweezers that a molecular clutch between the actin flow and N-cadherin adhesions could drive growth cone migration (Bard et al., J Neurosci 2008), but did not achieve a direct visualization of the engagement process. Here, to trigger N-cadherin homophilic adhesions at specific locations, we cultured primary neurons on micropatterned substrates comprising arrays of dots coated with purified N-cadherin, surrounded by a cytophobic background. We then tracked the trajectories of single adhesion and cytoskeletal molecules fused with photo-convertible fluorescent proteins (mEOS2), at the ventral surface of growth cones. N-cadherin and α-catenin were immobilized, while the actin retrograde flow was significantly reduced at N-cadherin coated micro-patterns, compared to non-adhesive regions. Normal actin flow on micro-patterns was restored by expression of a dominant negative N-cadherin construct inhibiting the coupling between endogenous N-cadherin and actin, demonstrating specificity of the process. Strikingly, individual actin trajectories exhibited pauses of the order of seconds selectively on N-cadherin-coated micro-patterns. Thus, at the individual molecular level, clutch engagement is characterized by transient interactions between flowing actin filaments and the immobilized N-cadherin/catenin complex. To our knowledge, this study represents the first direct demonstration of the intrinsic molecular coupling underlying the clutch process in growth cone locomotion.