The actin assembly regulator toca-1 regulates collateral branching in C. elegans

Neuronal branching is essential for establishing complex neural circuits. Many axons and dendrites in the CNS branch, forming complex process arbors. Some of these branches are formed during initial outgrowth and often occur via bifurcation of the growth cones of the extending neuronal processes. However, others are formed de novo by branch extension from a previously existing neurite. This process is often referred to as collateral branching. Here, we investigate the molecular mechanisms underlying this process. We show that in C. elegans, collateral branching of the PLM neuron is mediated by an actin assembly process guided by Transducer of CDC-42-Dependent Actin assembly-1 (TOCA-1). This scaffolding protein is recruited to the branching site at the time of branch formation. cdc-42 and the guanine nucleotide exchange factor dock-11 are also required for branch formation. Biochemically, TOCA-1 has been demonstrated to recruit WASP-1 and activate the Arp2/3 complex to promote actin assembly. In vivo, both wsp-1 and the Arp2/3 complex mutants also disrupt branch formation. While loss of TOCA-1 disrupts branching, it does not influence anterior-posterior (AP) or dorsal-ventral (DV) positioning of the branch, in contrast to previously defined branching regulators. Our data support a model in which TOCA-1 acts downstream of AP and DV positioning factors, directly orchestrating filopodial extension to form the nascent branch.