Filopodia Conduct Target Selection in Cortical Neurons Using Differences in Signal Kinetics of a Single Kinase

Dendritic filopodia select synaptic partner axons by interviewing the cell surface of potential targets, but how filopodia decipher the complex pattern of adhesive and repulsive molecular cues to find appropriate contacts is unknown. Here, we demonstrate in cortical neurons that a single cue is sufficient for dendritic filopodia to reject or select specific axonal contacts for elaboration as synaptic sites. Super-resolution and live-cell imaging reveals that EphB2 is located in the tips of filopodia and at nascent synaptic sites. Surprisingly, a genetically encoded indicator of EphB kinase activity, unbiased classification, and a photoactivatable EphB2 reveal that simple differences in the kinetics of EphB kinase signaling at the tips of filopodia mediate the choice between retraction and synaptogenesis. This may enable individual filopodia to choose targets based on differences in the activation rate of a single tyrosine kinase, greatly simplifying the process of partner selection and suggesting a general principle. [Display omitted] •EphB2 colocalizes with ephrinB1 and VGLUT1 and release sites in stable filopodia•GPhos tyrosine kinase (TK) indicators enable in situ biochemistry of Eph and TKs•Moving and stable filopodia contain EphBs, but their EphB TK signaling is distinct•Differences in the kinetics of EphB signaling guide selection/rejection of contacts Mao et al. provide a mechanism for local control of synaptic specificity. A transfectable indicator of EphB activity reveals that the decision of filopodia to make synapses or to seek new targets is guided by kinetically distinct signals.