Local Arp2/3-dependent actin assembly modulates applied traction force during apCAM adhesion site maturation

Area of Science:

• Cell Biology
• Neuroscience
• Biophysics

Background:

• Cell adhesion molecules (CAMs) mediate cell-cell interactions and are crucial for neural development.
• Homophilic binding of immunoglobulin superfamily CAMs, like Aplysia cell adhesion molecule (apCAM), triggers actin assembly at adhesion sites.
• The forces generated by this actin assembly are critical for axon growth and guidance but remain poorly characterized.

Purpose of the Study:

• To characterize the forces generated by neuronal growth cones in response to apCAM-mediated adhesion.
• To investigate the role of actin assembly and mechanical restraint in regulating these forces.
• To elucidate the mechanisms underlying traction force generation during axon growth.

Main Methods:

• Utilized apCAM-coated beads as substrates to probe growth cone mechanics.
• Varied the stiffness of mechanical restraint applied to the beads.
• Employed pharmacological inhibitors (Arp2/3 complex, Rac) and F-actin imaging to assess molecular contributions.
• Measured bead propulsion, retrograde flow, and actin cup formation.

Main Results:

• Unrestrained bead propulsion by growth cones matched or exceeded retrograde flow rates, dependent on Arp2/3 complex activity.
• Growth cones exhibited state switching between frictional coupling and Arp2/3-dependent propulsion under low mechanical restraint.
• Stiff restraint induced actin cup formation and forward growth cone translocation.
• Inhibition of Arp2/3 or Rac impaired F-actin assembly, reduced growth responses, and blocked signaling molecule accumulation at adhesions.

Conclusions:

• Introduced a novel model for traction force regulation where local actin assembly buffers nascent adhesions from retrograde flow effects.
• Demonstrated that Arp2/3-dependent actin assembly is essential for generating propulsive forces and mediating growth cone responses.
• Highlighted the interplay between mechanical forces, actin dynamics, and signaling in directed neuronal growth.