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. 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 click chemistry. The small size of UAAs and the possibility of introducing them virtually anywhere into target proteins make this method particularly suitable for labeling of complex and spatially restricted proteins. Using this approach, we labeled two large AIS components, the 186 kDa isoform of neurofascin (NF186; encoded by Nfasc) and the 260 kDa voltage-gated Na+ channel (NaV1.6, encoded by Scn8a) in primary neurons and performed conventional and super-resolution microscopy. We also studied the localization of epilepsy-causing NaV1.6 variants with a loss-of-function effect. Finally, to improve the efficiency of UAA incorporation, we developed adeno-associated viral (AAV) vectors for click labeling in neurons, an achievement that could be transferred to more complex systems such as organotypic slice cultures, organoids, and animal models.

Minimal genetically encoded tags for fluorescent protein labeling in living neurons.

Modern light microscopy, including super-resolution techniques, has brought about a demand for small labeling tags that bring the fluorophore closer to the target. This challenge can be addressed by labeling unnatural amino acids (UAAs) with bioorthogonal click chemistry. The minimal size of the UAA and the possibility to couple the fluorophores directly to the protein of interest with single-residue precision in living cells make click labeling unique. Here, we establish click labeling in living primary neurons and use it for fixed-cell, live-cell, dual-color pulse-chase, and super-resolution microscopy of neurofilament light chain (NFL). We also show that click labeling can be combined with CRISPR/Cas9 genome engineering for tagging endogenous NFL. Due to its versatile nature and compatibility with advanced multicolor microscopy techniques, we anticipate that click labeling will contribute to novel discoveries in the neurobiology field.

Effect of Vectashield-induced fluorescence quenching on conventional and super-resolution microscopy.

Finding the right combination of a fluorescent dye and a mounting medium is crucial for optimal microscopy of fixed samples. It was recently shown that Vectashield, one of the most commonly used mounting media for conventional microscopy, can also be applied to super-resolution direct stochastic optical reconstruction microscopy (dSTORM). dSTORM utilizes conventional dyes and starts with samples in a fluorescent "ON" state. This helps in identifying structures of interest. Subsequently, labelled samples are induced into blinking, which is necessary for determining the position of single molecules and reconstruction of super-resolution images. This is only possible with certain fluorescent dyes and imaging buffers. One of the most widely used dyes for dSTORM, Alexa Fluor 647 (AF647), blinks in Vectashield. However, after preparing immunocytochemical samples in Vectashield, we noticed that the fluorescence intensity of AF647 is quenched. This is particularly evident for dimmer immunostainings, such as stainings of some components of neuronal cytoskeleton and axonal initial segment. Because structures of interest cannot be identified in quenched samples, loss of fluorescence intensity hinders imaging of AF647 in Vectashield. This has consequences for both conventional and dSTORM imaging. To overcome this, we provide: 1) a quantitative analysis of AF647 intensity in different imaging media, 2) a quantitative analysis of the suitability of Vectashield for dSTORM imaging of high and low-abundance AF647-labelled targets. Furthermore, for the first time, we quantitatively analyse the performance of Alexa Fluor Plus 647, a new variant of AF647-conjugated antibody, in dSTORM imaging.