Multifocal two-photon excitation fluorescence microscopy reveals hop diffusion of H-Ras membrane anchors in epidermal cells of zebrafish embryos

Developments in fluorescence microscopy techniques have enabled imaging of individual fluorescently labelled proteins in biological systems, and in the current study, a single-molecule microscopy (SMM) technique has been applied in vivo , using the zebrafish embryo model. We have used multifocal two-photon excitation fluorescence microscopy (2PEFM) to study the dynamics of a GFP-fused H-Ras membrane-anchoring domain, GFP-C10H-Ras, in the epidermal cells of living embryos. In previous studies, a fast and a slow diffusing population of GFP-C10H-Ras molecules had been found. The application of the multifocal 2PEFM technique enabled us to focus on the slow diffusing population, which appears to occur in clusters that diffuse within microdomains of the epidermal cell membranes. Based on their mobility on a short timescale (≤ 1s) we could distinguish between a subpopulation that was diffusing and one that was virtually immobile. Owing to the multifocal 2PEFM imaging mode, we were able to dramatically reduce photobleaching which enabled us to follow the GFP-C10H-Ras particles over a prolonged time (> 3 s) and reconstruct their molecular trajectories of the diffusing subpopulation. These trajectories exhibited that the C10H-Ras particles continuously switch between a diffusing state and brief bursts of increased diffusion. As a result, they display an anomalous mobility pattern that can be referred to as hop diffusion. Taken together, this study demonstrates that multifocal 2PEFM offers a powerful approach to studying individual particles for prolonged periods of time, and that using this approach we were able to uncover the hopping behavior of GFP-C10H-Ras. SUMMARY STATEMENT By application of the two-photon excitation single-molecule microscopy to living zebrafish embryos, anomalous diffusion modes of individual H-Ras membrane anchors in epidermal cells were found. ### Competing Interest Statement The authors have declared no competing interest.