eLife assessment
Tan and colleagues studied synaptic transmission, presynaptic protein levels, and synaptic ultra-structure in hippocampal cultures of mice lacking the key active-zone proteins RIM (1, 2), ELKS (1, 2), and Munc13 (1, 2). Compared to cultures lacking only RIM and ELKS, additional deletion of Munc13 results in a further decrease of synaptic Munc13-1 levels, a similar reduction of the number of docked synaptic vesicles, and a more pronounced decrease of total synaptic vesicle number. At the physiological level, these RIM-ELKS-Munc13 hextuple knockout cultures display a further decrease in the pool of release-ready synaptic vesicles with largely unchanged release probability compared with RIM-ELKS quadruple KO cultures. The results support the conclusion of the nonredundant role of Munc13 in synaptic vesicle priming. On the other hand, while the genetic removal of all six genes involved clearly require the use of conditional KO mice, the resulting outcome of the experimental design is a hypomorphic phenotype, as neurotransmitter release is still detected and this complicates the interpretation of the findings. Overall, this study reinforces the notion that synapse formation is a remarkably resilient process that occurs even under strong perturbation of presynaptic function.