Cell Adhesion-Dependent Biphasic Axon Outgrowth Elucidated by Femtosecond Laser Impulse

Axon outgrowth is promoted by the mechanical coupling between the dynamic actin cytoskeleton and adhesive substrates via clutch and adhesion molecules in the axonal growth cone. In this study, we utilized a femtosecond laser-induced impulse to break the coupling between an axonal growth cone and an adhesive substrate, enabling us to evaluate the strength of the binding between proteins in the growth cone and a laminin substrate, and also determine the contribution of adhesion strength to axon outgrowth. We found that the adhesion strength of axonal L1 cell adhesion molecule (L1CAM)-laminin binding increased with the density of the laminin substrate. In addition, fluorescent speckle microscopy revealed that the retrograde flow of actin filaments in the axonal growth cone was dependent on the laminin density such that the flow speed reduced with increasing L1CAM-laminin binding. However, axon outgrowth did not increase monotonically with increased L1CAM-laminin binding but rather exhibited biphasic behavior, in which the outgrowth was suppressed by excessive L1CAM-laminin binding. Our quantitative evaluations of the adhesion strength suggest that the biphasic outgrowth is regulated by the balance between traction force and adhesion strength as a result of changes in the number of L1CAM-laminin interactions. These results imply that adhesion modulation is key to the regulation of axon guidance. Significance StatementThere is a lack of a method to evaluate an adhesion strength of axonal growth cone. We evaluated the adhesion strength of axonal growth cones to a substrate by utilizing the force applied from a femtosecond laser impulse. This study shows that the adhesion strength between the growth cone and substrate is strengthened by L1CAM-laminin binding. Axon outgrowth did not increase monotonically with increased L1CAM-laminin binding but rather exhibited biphasic behavior, indicating that existence of suitable adhesion strength for axonal growth. Our findings suggest that the balance between growth cone adhesion strength and the traction force transmitted via cytoskeletal flow is a key factor in axon guidance.