In the Right Place at the Right Time: miRNAs as Key Regulators in Developing Axons

Abstract

During neuronal circuit formation, axons progressively develop into a presynaptic compartment aided by extracellular signals. Axons display a remarkably high degree of autonomy supported in part by a local translation machinery that permits the subcellular production of proteins required for their development. Here, we review the latest findings showing that microRNAs (miRNAs) are critical regulators of this machinery, orchestrating the spatiotemporal regulation of local translation in response to cues. We first survey the current efforts toward unraveling the axonal miRNA repertoire through miRNA profiling, and we reveal the presence of a putative axonal miRNA signature. We also provide an overview of the molecular underpinnings of miRNA action. Our review of the available experimental evidence delineates two broad paradigms: cue-induced relief of miRNA-mediated inhibition, leading to bursts of protein translation, and cue-induced miRNA activation, which results in reduced protein production. Overall, this review highlights how a decade of intense investigation has led to a new appreciation of miRNAs as key elements of the local translation regulatory network controlling axon development.

Keywords: axonal compartment; local translation; miRNA; miRNA localization; miRNA trafficking; neural development; neuronal circuit formation.

Figure 1

Axonal miRNAs shared among datasets. (a) Venn diagram of all axonal miRNAs profiling. Seven miRNAs are shared by all datasets, and names are reported in the box. (b) Venn diagram of profiling obtained by miRNAs microarray [38] or TaqMan RT-qPCR arrays [39,40]. (c) Venn diagram of miRNAs shared between sequencing datasets. Total number of axonal miRNAs are reported in parenthesis below the references. Venn diagrams are obtained with an open access software [47].

Figure 2

MiRNAs transport in neuronal compartments as inactive pre-miRNAs molecules.

Figure 3

Overview of the broad impact of intra-axonal acting-miRNAs on several steps of neuronal development: polarization (a), outgrowth (b), pathfinding (c), and targeting (d).

Figure 4

Tools for investigating the intra-axonal roles of miRNAs. (a) Schematics for distal axons isolation: Campenot chambers, microfluidic devices or manual removal of the explants in case of organoculture. (b) Techniques for studying miRNA function: by loss of function (LOF) approach blocking miRNA function (1–3), by gain of function (GOF) approach increasing miRNAs level with mimics (4); by blocking miRNA target recognition, masking the miRNA-responsive element (MREs) with a target protector (5); by studying with reporters the translational events potentially regulated by the miRNA under study (6,7); by measuring the miRNA activity using sensor plasmids (8). Abbreviations: ORF, open reading frame; ASOs, antisense oligonucleotides.

Figure 5

Axonal stimulations relieve miRNAs-mediated mRNAs silencing: role in axonal pathfinding. (i) brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) or Slit-2 stimulation induce miRNA de-repression and gate the translation of cytoskeleton regulators (MAP1B, CALM1, Cofilin-1) thereby regulating outgrowth and steering [41,52,73]; (ii) miR-92 keeps Robo1 silent in pre-crossing commissural axons (CAs), and miRNA de-repression at the midline triggers Robo1 translation and concomitant axon sensitivity to cue [75]; (iii) miR-26 de-repression in axons elicits glycogen synthase kinase-3 beta (GSK3β) local synthesis and this leads to the decrease in axonal outgrowth [76].

Figure 6

Proposed model of miRNA-mediated regulation of basal and cue-induced translation. (1) Specific miRNAs keep a subset of mRNAs in an untranslated state, but upon stimulation, the miRNA-mediated silencing is relieved, and this induces a burst of mRNA translation. (2) Inactive pre-miRNAs are transported and stored within the axonal compartment (left), and upon cue exposure (right), distinct pre-miRNAs are locally processed and the associated newly generated miRNAs (NG-miRNAs) inhibit the basal translation of given transcripts. Regardless of the stimulus status, (3) specific transcripts undergo basal translation and (4) others are maintained in an untranslated silent state.