Direct fluorescent labeling of NF186 and NaV1.6 in living primary neurons using bioorthogonal click chemistry

The axon initial segment (AIS) is a highly specialized neuronal compartment that regulates the generation of action potentials and maintenance of neuronal polarity. Despite its importance, live imaging of the AIS is challenging due to the limited number of suitable labeling methods. To overcome this limitation, we established a novel approach for live labeling of the AIS using unnatural amino acids (UAAs) and bioorthogonal click chemistry. The small size of the UAAs and the possibility of introducing them virtually anywhere into the target proteins make this method particularly suitable for live labeling and imaging of complex and spatially restricted proteins. With this approach, we labeled two large AIS components, the 186 kDa isoform of neurofascin (NF186) and the 260 kDa voltage-gated sodium channel (NaV1.6), and performed widefield and confocal microscopy in fixed and living neurons. Moreover, we demonstrated the applicability of this method by studying the localization of two epilepsy-causing NaV1.6 variants with a loss-of-function effect. Finally, to further improve the efficiency of the UAA incorporation, we developed adeno-associated viral (AAV) vectors for click labeling in primary neurons. The use of AAV vectors will facilitate the transfer of UAA-based click labeling technology to more complex biological systems, such as organotypic slice cultures, organoids, and animal models. ### Competing Interest Statement D.G. is a co-founder of AaviGen GmbH. All the other authors declare no competing interests.