$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Axonal mRNA recruitment and local translation enable axons to respond to extracellular stimuli in a temporally and spatially acute manner1. Intra-axonal protein synthesis is best understood in the context of neurodevelopment where it plays crucial roles in growth cone behavior2-8, axon pathfinding9-11 and retrograde signaling12,13. But far less is known about the functional significance of axonal protein synthesis in post-developmental neurons when axonal mRNA and ribosome levels are greatly reduced14,15. Mature vertebrate axons have long been thought to be translationally inactive16. However, recent studies indicate that local translation is reactivated in mature axons under pathological conditions. For instance, a subset of mRNAs is recruited to regenerating axons following nerve injury and intra-axonal protein synthesis is required for the correct regeneration of these axons17. Additionally, our group has demonstrated that specific mRNAs are recruited to axons after local exposure to the Alzheimer’s disease peptide Aβ1-42, and local translation of the transcription factor ATF4 is required to propagate the neurodegenerative effects of Aβ1-42 from axons to the neuronal soma18. Finally, high throughput analyses have revealed that mature axons have a more complex and dynamic transcriptome than expected18-21, especially under pathological conditions. In light of these studies, a highly sensitive and specific method to detect axonally localized mRNAs in the adult nervous system is needed.
Much of the work on mRNA recruitment and local translation in mature axons has been performed on cultured neurons. This is especially true for transcriptome analyses since specialized culturing methods exist that allow the isolation of axons from the somato-dendritic compartment18-20. Although such studies have given valuable insight into the role of local translation in mature axons, the question whether cultured neurons faithfully represent the situation in vivo or if mRNA recruitment is an adaptive response of axons to culturing conditions is still open. Few studies have provided evidence of mRNA recruitment to mature axons in vivo. For example, the transcript coding for the olfactory marker protein has been detected in axons in adult sensory neurons22. A transgene containing the 3’ UTR of β-actin mRNA is transported to axons in peripheral and central nervous system neurons in mice and is locally translated after developmental periods23. Lamin b2 mRNA is localized to retinal axons in Xenopues laevis tadpoles and its depletion affects axon maintenance after axonal development21. Interference with the axonal transport of the mRNA encoding for cytochrome C oxidase IV alters mouse behavior24. Finally, Atf4 mRNA is found in adult axons of mice and human brains in the context of Aβ1-42-induced neurodegeneration18.
High throughput transcriptome analyses have proven to be useful to identify mRNA profiles in isolated axons in vitro but have limitations for in vivo studies since in whole tissues axons are never found in isolation but intermingled with neuronal cell bodies, glial cells and other cell types. Thus, such analyses have to be combined with imaging techniques that confirm the subcellular localization of mRNAs. RNA in situ hybridization (RNA ISH) allows the detection and visualization of specific RNA sequences in cells and tissues. However, original RNA ISH assays were suitable only for the identification of highly abundant RNAs25, which is rarely the case for axonally localized mRNAs. For the last decades increasing efforts have been put into developing novel technologies that allow the detection of mRNAs at the single-molecule level25,26. For instance, Singer and colleagues have developed ISH probes to detect mRNAs in single cells, consisting in 5 non-overlapping fluorescently labelled 50-mers (for details refer to 27). The main difference between the above mentioned techniques and the one here described is that the later uses 20 double Z-structure (not linear) probes that typically target ~ 1 kb region of the RNA of interest ensuring specificity and low background levels. Probes are then hybridized with preamplifier and amplifier sequences that are finally fluorescently labeled or conjugated with enzymes that allow chromogenic reactions. These amplification steps improve the signal-to-noise ratio compared to other ISH technologies28. Here we describe two protocols using RNAscope combined either with fluorescence immunocytochemistry or with histological dyes allowing axonal counterstaining. Both protocols are suitable to visualize axonal localization of Atf4 mRNA in adult mouse and human brains.