Reduced expression of SMN protein causes spinal muscular atrophy (SMA), a neurodegenerative disorder leading to motor neuron dysfunction and loss. However, the molecular mechanisms by which SMN regulates neuronal dysfunction are not fully understood. Here, we report that reduced SMN protein level alters miRNA expression and distribution in neurons. In particular, miR-183 levels are increased in neurites of SMN-deficient neurons. We demonstrate that miR-183 regulates translation of mTor via direct binding to its 3' UTR. Interestingly, local axonal translation of mTor is reduced in SMN-deficient neurons, and this can be recovered by miR-183 inhibition. Finally, inhibition of miR-183 expression in the spinal cord of an SMA mouse model prolongs survival and improves motor function of Smn-mutant mice. Together, these observations suggest that axonal miRNAs and the mTOR pathway are previously unidentified molecular mechanisms contributing to SMA pathology.

Expansion microscopy1,2 is an increasingly widespread technology for nanoimaging, because its precise physical magnification of biological specimens enables ordinary microscopes to achieve nanoscale effective resolutions. Fluorescent labels such as antibodies can be applied either before or after expansion, with the latter offering the potential for better access to proteins within densely packed environments3–7. We here assess this possibility of epitope decrowding through physical expansion of proteins away from each other, using a 20x expansion protocol that we call expansion revealing (ExR), by labeling, within intact brain circuits, the same set of synaptic proteins both pre- and post-expansion. This comparison shows that post-expansion labeling introduces minimal spatial error and off-target staining relative to pre-expansion staining, while revealing the presence of proteins that are invisible when stained pre-expansion. Using ExR, we show in intact brain tissue the alignment of presynaptic calcium channels with postsynaptic machinery in nanocolumns, which may facilitate precision synaptic transmission, as well as the existence of periodic amyloid-containing nanoclusters containing ion channel proteins in Alzheimer’s model mice, which may help generate novel hypotheses for Alzheimer’s pathology and neural excitability. Thus, the decrowding power of ExR is able to reveal novel nanostructures within intact brain circuitry, and may find broad use in biology and medicine for unmasking nanostructures of importance in normal functions and disease.

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In mouse and human neurons, axonally secreted amyloid precursor protein (APP) fragments are processed in the cell body before being sorted into the axon in a process that requires endocytosis for the processing, but not axonal delivery, of APP.