A DARPin-based molecular toolset to probe gephyrin and inhibitory synapse biology.

Neuroscience currently requires the use of antibodies to study synaptic proteins, where antibody binding is used as a correlate to define the presence, plasticity, and regulation of synapses. Gephyrin is an inhibitory synaptic scaffolding protein used to mark GABAergic and glycinergic postsynaptic sites. Despite the importance of gephyrin in modulating inhibitory transmission, its study is currently limited by the tractability of available reagents. Designed Ankyrin Repeat Proteins (DARPins) are a class of synthetic protein binder derived from diverse libraries by in vitro selection and tested by high-throughput screening to produce specific binders. In order to generate a functionally diverse toolset for studying inhibitory synapses, we screened a DARPin library against gephyrin mutants representing both phosphorylated and dephosphorylated states. We validated the robust use of anti-gephyrin DARPin clones for morphological identification of gephyrin clusters in rat neuron culture and mouse brain tissue, discovering previously overlooked clusters. This DARPin-based toolset includes clones with heterogenous gephyrin binding modes that allowed for identification of the most extensive gephyrin interactome to date and defined novel classes of putative interactors, creating a framework for understanding gephyrin's nonsynaptic functions. This study demonstrates anti-gephyrin DARPins as a versatile platform for studying inhibitory synapses in an unprecedented manner.