Activation of ezrin/radixin/moesin mediates attractive growth cone guidance through regulation of growth cone actin and adhesion receptors

The development of a functioning neural network relies on responses of axonal growth cones to molecular guidance cues that are encountered en route to their target tissue. Nerve growth factor (NGF) and neurotrophin-3 serve as attractive cues for chick embryo sensory growth cones in vitro and in vivo, but little is known about the actin-binding proteins necessary to mediate this response. The evolutionarily conserved ezrin/radixin/moesin (ERM) family proteins can tether actin filaments to the cell membrane when phosphorylated at a conserved threonine residue. Here we show that acute neurotrophin stimulation rapidly increases active phospho-ERM levels in chick sensory neuron growth cone filopodia, coincident with an increase in filopodial L1 and β-integrin. Disrupting ERM function with a dominant-negative construct (DN-ERM) results in smaller and less motile growth cones with disorganized actin filaments. Previously, we found that nerve growth factor (NGF) treatment increases ADF/cofilin activity and growth cone F-actin (Marsick et al., 2010). Here, we show this F-actin increase, as well as attractive turning to NGF, is blocked when ERM function is disrupted, despite normal activation of ADF/cofilin. We further show that DN-ERM expression disrupts leading edge localization of active ADF/cofilin and free F-actin barbed ends. Moreover, filopodial phospho-ERM levels are increased by incorporation of active ADF/cofilin, and reduced by knockdown of L1CAM. Taken together, these data suggest that ERM proteins organize actin filaments in sensory neuron growth cones and are crucial for neurotrophin-induced remodeling of F-actin and re-distribution of adhesion receptors.