Mechanisms underlying formin 2 function in the motility of neuronal growth cones

Abstract

Stereotyped wiring of the nervous system during development is accomplished by guidance cues which tightly control and shape neuronal trajectories and instruct the growth cone to make appropriate synaptic contacts Tight regulation of growth cone steering is achieved by remodelling the underlying growth cone cytoskeleton that regulates polarity protrusion substrate adhesion and generation of coordinated traction forces However exact mechanisms of this regulation still remain less characterized in growth cones In the following thesis we focus on Formin 2 Fmn2 a member of the formin family of actin binding proteins Earlier work from our group has shown that perturbation of Fmn2 in the developing chick spinal cord results in defective trajectories of spinal commissural neurons in vivo In this study we show that Fmn2 transcript is not only enriched in the spinal cord but also higher expression coincides with the developmental window of commissural interneuron pathfinding in the spinal cord Depletion of Fmn2 affects growth cone motility in vitro and results in a reduced lifetime of growth cone filopodia On the other hand reduction in Fmn2 does not affect the filopodial elongation and initiation rates suggesting a role in substrate adhesion based stability of filopodia To understand the slow growth cone movement in this study we have investigated growth cone substrate dependent interactions in neurons that suggested the importance of Fmn2 in regulating growth cone point contacts Earlier work has suggested that force based maturation of point contacts is compromised upon Fmn2 depletion implicating Fmn2 in the molecular clutch mechanism Supporting this hypothesis the current study reveals a regulatory role for Fmn2 in the generation of traction forces by growth cones Additionally retrograde flow of F actin was found to be elevated upon knockdown of Fmn2 indicating slipping of the molecular clutch and further underscoring Fmn2 function as a component of the molecular clutch Supporting studies in mouse newline newline