Clicking a Fish: Click Chemistry of Different Biomolecules in Danio rerio.

Click chemistry has found remarkable applications in imaging biomacromolecules in cellular systems and even organisms. Over the past few years, bioorthogonal click reactions have been improved and tailored for specific applications, allowing the selective labeling of biomolecules like DNA, RNA, proteins, glycans, and lipids. The zebrafish, Danio rerio, is a prime model organism for vertebrate development and has gained importance in experimental biology and biochemistry for its ease of handling and appreciable genetic homology to mammals. With respect to biomolecular labeling, the zebrafish represents the next level of complexity relative to in vitro cell line models, challenging researchers to further improve the specificity and targeting of their labeling approaches, and to reduce the level of interference with the host. At the same time, bioorthogonal labeling has facilitated an improved understanding of embryonic zebrafish development. Highlighted in this Perspective are click chemistry approaches that rely on (i) designing analogues of endogenous target molecules, (ii) utilizing the conjugation moiety on these analogues to attach fluorophores by clicking them outside or inside living systems, and finally, (iii) imaging the cells or tissue or, sometimes, the whole organism. While glycans and proteins have been extensively studied in D. rerio using bioorthogonal click chemistry for their intimate involvement in embryonic development, DNA and RNA remain less investigated. Together with the formulation of strategies to perform click reactions with the weakest impact on the host system, bioorthogonal approaches have enormous potential to visualize organisms in all their molecular splendor.