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Macrophage polarization is a process whereby macrophages acquire distinct effector states (M1 or M2) to carry out multiple and sometimes opposite functions. We show here that translational reprogramming occurs during macrophage polarization and that this relies on the Elongator complex subunit Elp3, an enzyme that modifies the wobble uridine base U34 in cytosolic tRNAs. Elp3 expression is downregulated by classical M1-activating signals in myeloid cells, where it limits the production of pro-inflammatory cytokines via FoxO1 phosphorylation, and attenuates experimental colitis in mice. In contrast, alternative M2-activating signals upregulate Elp3 expression through a PI3K- and STAT6-dependent signaling pathway. The metabolic reprogramming linked to M2 macrophage polarization relies on Elp3 and the translation of multiple candidates, including the mitochondrial ribosome large subunit proteins Mrpl3, Mrpl13, and Mrpl47. By promoting translation of its activator Ric8b in a codon-dependent manner, Elp3 also regulates mTORC2 activation. Elp3 expression in myeloid cells further promotes Wnt-driven tumor initiation in the intestine by maintaining a pool of tumor-associated macrophages exhibiting M2 features. Collectively, our data establish a functional link between tRNA modifications, mTORC2 activation, and macrophage polarization.

Summary Axonal transport is a physiological process adopted by neurons to transport organelles, proteins, and other molecules along their axonal projections. Here, we describe a step-by-step protocol to record the dynamics of axonal transport along the projections of callosal neurons by combining the in utero electroporation technique with the preparation of postnatal organotypic cortical slices. This ex vivo protocol has been developed to investigate axonal transport in a physiological setting closely reproducing the in vivo environment. For complete details on the use and execution of this protocol, please refer to Even et al. (2019)., Graphical Abstract, Highlights • Descriptive method to electroporate DNA plasmids in the embryonic mouse cortex • Step-by-step procedure to generate and mount organotypic brain slices • Protocol to record and analyze axonal transport in callosal projection neurons • Guidelines for protocol troubleshooting and overview on its limitations, Axonal transport is a physiological process adopted by neurons to transport organelles, proteins, and other molecules along their axonal projections. Here, we describe a step-by-step protocol to record the dynamics of axonal transport along the projections of callosal neurons by combining the in utero electroporation technique with the preparation of postnatal organotypic cortical slices. This ex vivo protocol has been developed to investigate axonal transport in a physiological setting closely reproducing the in vivo environment.