Axonal ribosomes and mRNAs associate with fragile X granules in adult rodent and human brains

Abstract

Local mRNA translation in growing axons allows for rapid and precise regulation of protein expression in response to extrinsic stimuli. However, the role of local translation in mature CNS axons is unknown. Such a mechanism requires the presence of translational machinery and associated mRNAs in circuit-integrated brain axons. Here we use a combination of genetic, quantitative imaging and super-resolution microscopy approaches to show that mature axons in the mammalian brain contain ribosomes, the translational regulator FMRP and a subset of FMRP mRNA targets. This axonal translational machinery is associated with Fragile X granules (FXGs), which are restricted to axons in a stereotyped subset of brain circuits. FXGs and associated axonal translational machinery are present in hippocampus in humans as old as 57 years. This FXGassociated axonal translational machinery is present in adult rats, even when adult neurogenesis is blocked. In contrast, in mouse this machinery is only observed in juvenile hippocampal axons. This differential developmental expression was specific to the hippocampus, as both mice and rats exhibit FXGs in mature axons in the adult olfactory system. Experiments in Fmr1 null mice show that FMRP regulates axonal protein expression but is not required for axonal transport of ribosomes or its target mRNAs. Axonal translational machinery is thus a feature of adult CNS neurons. Regulation of this machinery by FMRP could support complex behaviours in humans throughout life.

Closing the gap: long-term presynaptic plasticity in brain function and disease

“…New work using the Ribotag transgenic mouse identified immunogold labeling of ribosomes in the presynaptic compartment of adult retinal ganglion cells [49]. Additionally, RNA-binding proteins are localized to adult presynaptic terminals, but their role in the regulation of presynaptic protein synthesis in response to activity and experience remains poorly understood [50]. Presumably the presynaptic terminal contains relatively few mRNA species whose translation could act as a molecular switch in pathways involved in plasticity of neurotransmitter release.…”

Cell type‐dependent axonal localization of translational regulators and mRNA in mouse peripheral olfactory neurons

“…As a first step toward understanding potential roles for axonal translation in peripheral olfactory neurons, we explored the potential differential expression of FXGs between axons in the main and accessory olfactory systems. To identify FXGs, we immunostained for FXR2P followed by standardized image processing and analysis (Akins et al, ; Christie et al, ). We have previously shown that FXGs are found throughout life in OSN axons innervating the main olfactory bulb, both in the olfactory nerve layer as well as in their glomerular targets (Akins et al, ; Christie et al, ).…”

“…To identify FXGs, we immunostained for FXR2P followed by standardized image processing and analysis (Akins et al, ; Christie et al, ). We have previously shown that FXGs are found throughout life in OSN axons innervating the main olfactory bulb, both in the olfactory nerve layer as well as in their glomerular targets (Akins et al, ; Christie et al, ). Consistent with these past studies, we found that FXGs were present in OSN axons innervating glomeruli in the main olfactory bulb (Figure a).…”

“…FXGs were not previously identified in axon terminals of VSNs in a broad survey of FXG expression throughout the brain of 15‐day‐old mice, the age of peak FXG expression in most circuits in the mouse brain (Akins et al, ). However, in contrast to axons in the rest of the brain, mouse OSNs exhibit peak FXG expression in 30‐day‐old mice that continues throughout life (Akins et al, ; Christie et al, ). We therefore asked whether VSN axons contain FXGs and similarly exhibit peak expression at ages later than 15 days old.…”

“…This local protein synthesis has been well‐characterized in growing and regenerating axons (Korsak, Mitchell, Shepard, & Akins, ). More recent evidence has revealed that axonal translation also occurs in axons that are integrated into mature circuits, where local translation can support synaptic plasticity (Akins et al, ; Baleriola et al, ; Gumy et al, ; Shigeoka et al, ; Taylor et al, ; Younts et al, ). A key question is how differences in local axonal translation contribute to functional and structural diversity of neurons.…”