MiR-8 controls synapse structure by repression of the actin regulator Enabled

Carlos M Loya,Elizabeth M Mcneill,Bing Zhang,Hong Bao,David Van Vactor

doi:10.1242/dev.105791

Carlos M Loya, Elizabeth M Mcneill + Show 3 more

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https://doi.org/10.1242/dev.105791

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Abstract

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play important roles in nervous system development and physiology. However, our understanding of the strategies by which miRNAs control synapse development is limited. We find that the highly conserved miRNA miR-8 regulates the morphology of presynaptic arbors at the Drosophila neuromuscular junction (NMJ) through a postsynaptic mechanism. Developmental analysis shows that miR-8 is required for presynaptic expansion that occurs in response to larval growth of the postsynaptic muscle targets. With an in vivo sensor, we confirm our hypothesis that the founding member of the conserved Ena/VASP (Enabled/Vasodilator Activated Protein) family is regulated by miR-8 through a conserved site in the Ena 3′ untranslated region (UTR). Synaptic marker analysis and localization studies suggest that Ena functions within the subsynaptic reticulum (SSR) surrounding presynaptic terminals. Transgenic lines that express forms of a conserved mammalian Ena ortholog further suggest that this localization and function of postsynaptic Ena/VASP family protein is dependent on conserved C-terminal domains known to mediate actin binding and assembly while antagonizing actin-capping proteins. Ultrastructural analysis demonstrates that miR-8 is required for SSR morphogenesis. As predicted by our model, we find that Ena is both sufficient and necessary to account for miR-8-mediated regulation of SSR architecture, consistent with its localization in this compartment. Finally, electrophysiological analysis shows that miR-8 is important for spontaneous neurotransmitter release frequency and quantal content. However, unlike the structural phenotypes, increased expression of Ena fails to mimic the functional defects observed in miR-8-null animals. Together, these findings suggest that miR-8 limits the expansion of presynaptic terminals during larval synapse development through regulation of postsynaptic actin assembly that is independent of changes in synapse physiology.

Highlights

Synapses are the essential building blocks of neural circuitry
We found that overall neuromuscular junction (NMJ) morphology appeared different from controls (Fig. 1A,B), but quantitative analysis of type 1b bouton and arbor branch addition did not reveal significant abnormality at L1 (Fig. 1E), whereas these parameters were consistently reduced in L2 and L3 compared with genetically matched controls (Fig. 1E)
In wild-type m6/m7 NMJs, subsynaptic reticulum (SSR)-localized Discs large (Dlg) forms a halo around the HRP staining in type Ib boutons (Fig. 2A); boutons deficient in Dlg staining were rare in controls

Summary

Introduction

Synapses are the essential building blocks of neural circuitry. These highly specialized cellular junctions form and remodel under the control of various extrinsic cues and intrinsic regulatory mechanisms. Synaptogenesis is a coordinated process of cellular. It is clear that cell-intrinsic mechanisms are required to control the morphogenesis of synaptic structures. Cellautonomous post-transcriptional regulation by the highly conserved Fragile-X Mental Retardation Protein (FMRP) is essential for limiting the expansion of both postsynaptic dendritic spines in mammals and presynaptic arbors in Drosophila (Gao, 2002)

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