Fmn2 Regulates Growth Cone Motility by Mediating a Molecular Clutch to Generate Traction Forces

Abstract

Growth cone -mediated axonal outgrowth and accurate synaptic targeting are central to brain morphogenesis. Translocation of the growth cone necessitates mechanochemical regulation of cell -extracellular matrix interactions and the generation of propulsive traction forces onto the growth environment. However, the molecular mechanisms subserving force generation by growth cones remain poorly characterized. The formin family member, Fmn2, has been identified earlier as a regulator of growth cone motility. Here, we explore the mechanisms underlying Fmn2 function in the growth cone. Evaluation of multiple components of the adhesion complexes suggests that Fmn2 regulates point contact stability. Analysis of F-actin retrograde flow reveals that Fmn2 functions as a clutch molecule and mediates the coupling of the actin cytoskeleton to the growth substrate, via point contact adhesion complexes. Using traction force microscopy, we show that the Fmn2-mediated clutch function is necessary for the generation of traction stresses by neurons. Our findings suggest that Fmn2, a protein associated with neurodevelopmental and neurodegenerative disorders, is a key regulator of a molecular clutch activity and consequently motility of neuronal growth cones.

The Formin Fmn2b Is Required for the Development of an Excitatory Interneuron Module in the Zebrafish Acoustic Startle Circuit

“…Fmn2 has previously been shown to be necessary for the axonal outgrowth of spinal neurons in developing chick embryos ( Sahasrabudhe et al, 2016 ; Ghate et al, 2020 ). Similarly, we speculate that the zebrafish spiral fiber neurons have axonal outgrowth defects that result in the loss of synaptic connectivity with the M-cells.…”

“…The molecular mechanism mediating Fmn2b-dependent axonal outgrowth in zebrafish is not known. In chick spinal neurons, Fmn2 mediates growth cone motility by regulating the cell-matrix adhesions necessary to generate traction forces ( Sahasrabudhe et al, 2016 ; Ghate et al, 2020 ). A recent study implicates Fmn2b in regulating growth cone microtubule dynamics in zebrafish Rohon–Beard neurons ( Kundu et al, 2021 ) and highlights another mechanism mediating outgrowth and pathfinding.…”

“…Fmn2 is required for growth cone motility and axonal pathfinding of spinal neurons. Filopodial stability and the dynamics of the adhesion complexes between the growth cone and the substrate are regulated by Fmn2 ( Sahasrabudhe et al, 2016 ; Ghate et al, 2020 ). Another study has identified actin-microtubule cross talk mediated by Fmn2 as a mechanism underlying growth cone turning ( Kundu et al, 2021 ).…”

Unraveling Axon Guidance during Axotomy and Regeneration

“…The F-actin binding protein fmn has been described as one of the families of proteins that can exert the F-actin/MT association. Fmn2 localizes mainly throughout the F-actin bundles, but additionally, in some filopodia tips, and is needed for filopodia formation and elongation [ 41 ], as well as point contact stability and maturation [ 42 ]. This filopodia localization is required for guiding dynamic MTs in the P domain, stabilizing filopodia protrusion and growth cone directional movement [ 43 ].…”

Development of motor neurons and motor activity in zebrafish requires F-actin nucleation by Fmn2b

“…Depletion of Fmn2 causes reduced growth cone motility and mechanotransduction in chick spinal primary neurons (Sahasrabudhe et al, 2016;Ghate et al, 2020). Fmn2 regulates the stabilization of actin patches required for the initiation of axonal branching in cultured chick spinal neurons by insulating the actin patch from actin depolymerizing factors like ADF (Kundu et al, 2020).…”