LAUSR

Local translation in presynaptic terminals Proteins carry out most of the functions in cells, including neurons, which are one of the most morphologically complex cell types in the body. This poses challenges for how proteins can be supplied to remote regions where connections (synapses) are made with other neurons. One solution to the neuron protein-supply problem involves the local synthesis of proteins from messenger RNA (mRNA) molecules located at or near synapses. Hafner et al. used RNA sequencing methods and superresolution microscopy to show that axon terminals contain hundreds of mRNA molecules as well as the machinery needed for protein synthesis. Furthermore, the axon terminals were able to use these components to make proteins that participate in synaptic transmission. Science, this issue p. eaau3644 Protein synthesis occurs in all synaptic compartments, including excitatory and inhibitory axon terminals. INTRODUCTION The regulation of synaptic proteins by posttranslational modifications and by ongoing protein synthesis and degradation drives homeostasis and plasticity at synapses. A key question is where synaptic proteins are made. Most of the neuron’s volume comprises axons and dendrites that can be micrometers or millimeters long. Thus, a substantial fraction of proteomic remodeling could potentially occur locally within both axons and dendrites. Whereas there is wealth of data indicating that protein synthesis occurs in mature dendrites, there has been much less evidence in support of local translation in mature axons. RATIONALE Efforts to localize molecules or cell biological events to neuronal pre- or postsynaptic compartments by using fluorescence microscopy are limited by the tight association of the axonal bouton and the dendritic spine; the synaptic cleft, the only clear region of separation, is only ~20 nm wide. In this study, to increase the resolving power to visualize RNA molecules in pre- and postsynaptic compartments, we optimized fluorescence in situ hybridization (FISH) and nascent protein detection methods for use with expansion microscopy. To characterize transcripts and translational machinery in excitatory presynaptic terminals, we used a recently developed platform that couples fluorescence sorting with biochemical fractionation to sort and purify fluorescently labeled synaptosomes. To obtain direct evidence for protein synthesis in synaptic compartments, particularly presynaptic boutons, we adapted the puromycin-based metabolic labeling strategy for detection with electron or expansion microscopy. We also examined the translational signature of these three different forms of local protein synthesis–dependent plasticity in three different synaptic compartments: dendritic spines, excitatory terminals, and inhibitory terminals. RESULTS In adult rodent brain slices and cultured neurons, we found that >75% of both excitatory and inhibitory presynaptic terminals contain the machinery for protein synthesis: rRNA, ribosomes, and polyadenylated [poly (A)+] mRNA. Using mature mouse forebrain synaptosomes that are enriched for vGLUT1+ presynaptic terminals, we identified ~450 transcripts that were enriched (relative to the generic synaptosome transcriptome). These included many mRNAs that code for proteins that regulate neurotransmitter release. Both light microscopy (confocal and super-resolution) and electron microscopy revealed that in the absence of overt stimulation, there was a notably high level of ongoing protein synthesis in both pre- and postsynaptic compartments. After just 5 min of metabolic labeling, ~40% of both excitatory and inhibitory presynaptic terminals and ~60% of dendritic spines exhibited active translation. Three different forms of synaptic plasticity resulted in distinct patterns of protein synthesis stimulation in dendritic spines and in excitatory and inhibitory presynaptic axon terminals. CONCLUSION In this study, we investigated the localization and stimulation of protein synthesis in mature synapses. We unambiguously identified protein synthesis machinery and mRNA translation in individual synaptic compartments. Both excitatory and inhibitory presynaptic boutons (as well as postsynaptic spines) carry out protein synthesis regularly, in the absence of any exogenous stimulation. Synthesis within these three compartments is differentially recruited to modify local proteomes during synaptic plasticity. Local protein synthesis adds spatial and temporal precision for proteome remodeling that can be exploited to rapidly modify synapses in specific subcellular compartments. Plasticity differentially recruits local protein synthesis. Summary scheme indicating how protein synthesis occurs in both post- and presynaptic compartments and how each different form of plasticity examined has a specific translational signature. The three compartments represented, excitatory and inhibitory presynaptic boutons and postsynaptic spines, exhibit different patterns of enhanced protein synthesis (arrows) after the treatments with (S)-3,5-dihydroxyphenylglycine hydrate [metabotropic glutamate receptor long-term depression (mGluR LTD)], brain-derived neurotrophic factor (BDNF) (BDNF potentiation), or arachidonyl-2-chloroethylamide (endocannabinoid receptor activation). There is ample evidence for localization of messenger RNAs (mRNAs) and protein synthesis in neuronal dendrites; however, demonstrations of these processes in presynaptic terminals are limited. We used expansion microscopy to resolve pre- and postsynaptic compartments in rodent neurons. Most presynaptic terminals in the hippocampus and forebrain contained mRNA and ribosomes. We sorted fluorescently labeled mouse brain synaptosomes and then sequenced hundreds of mRNA species present within excitatory boutons. After brief metabolic labeling, >30% of all presynaptic terminals exhibited a signal, providing evidence for ongoing protein synthesis. We tested different classic plasticity paradigms and observed distinct patterns of rapid pre- and/or postsynaptic translation. Thus, presynaptic terminals are translationally competent, and local protein synthesis is differentially recruited to drive compartment-specific phenotypes that underlie different forms of plasticity.