EVL and MIM/MTSS1 regulate actin cytoskeletal remodeling to promote dendritic filopodia in developing neurons

Dendritic spines are the postsynaptic compartment of a functional neuronal synapse, and are critical for synaptic connectivity and plasticity. The developmental precursor to dendritic spines, dendritic filopodia, are highly motile protrusions that facilitate synapse formation by sampling the environment for suitable axon partners during development and learning. Despite the significance of the actin cytoskeleton in driving these protrusions, the actin remodeling factors involved in this process are not fully characterized. In this work, we identify a critical function for the Ena/VASP protein EVL in the regulation of dendritic filopodia. Amongst the Ena/VASP proteins, EVL is uniquely required for the characteristic morphology and dynamics of dendritic filopodia. Using a combination of genetic and optogenetic manipulations, we demonstrate that EVL promotes protrusive motility through membrane-direct actin polymerization at dendritic filopodia tips. EVL forms a complex at nascent protrusions and dendritic filopodia tips with MIM/MTSS1, an I-BAR protein recently discovered to be important for initiation of dendritic filopodia. We propose a model in which EVL cooperates with MIM to elongate and coalesce branched actin filaments, establishing the dynamic lamellipodia-like architecture of dendritic filopodia in developing neurons.