Chien, C.-T. | bioRxiv Science

bioRxiv2019

Glia-derived exosomal miR-274 targets Sprouty in trachea and synaptic boutons to modulate growth and responses to hypoxia

Secreted exosomal miRNAs mediate inter-organ/tissue communication by downregulating gene expression, thereby modulating developmental and physiological functions. However, the source, route, and function have not been formally established for specific miRNAs. Here, we show that glial miR-274 non-cell autonomously modulates the growth of synaptic boutons and tracheal branches. Whereas precursor miR-274 was expressed in glia, mature miR-274 was secreted. miR-274 secretion to circulating hemolymph was detected in exosomes, a process requiring ESCRT components in exosome biogenesis and Rab11 and Syx1A in exosome release. miR-274 downregulated Sprouty to activate MAPK in synaptic boutons and tracheal branches, thereby promoting their growth. Expression of miR-274 solely in glia of a mir-274 null mutant reset normal levels of Sprouty and MAPK, and hemolymphatic exosomal miR-274. mir-274 mutant larvae were hypersensitive to hypoxia, which was suppressed by increasing tracheal branches. Thus, glia-derived miR-274 coordinates growth of synaptic boutons and tracheal branches to modulate larval hypoxia responses.

developmental biology↗

bioRxiv2019

Glial response to hypoxia in trachealess mutants induces synapse remodeling.

Synaptic structure and activity are sensitive to environmental alterations. Modulation of synaptic morphology and function is often induced by signals from glia. However, the process by which glia mediate synaptic responses to environmental perturbations such as hypoxia remains unknown. Here, we report that, in the Drosophila trachealess (trh) mutant, smaller synaptic boutons form clusters named bunch boutons appear at larval neuromuscular junctions (NMJs), which is induced by the reduction of internal oxygen levels due to defective tracheal branches. Thus, the bunch bouton phenotype in the trh mutant is suppressed by hyperoxia, and recapitulated in wild-type larvae raised under hypoxia. We further show that hypoxia-inducible factor (HIF)-1/Similar (Sima) is critical in mediating hypoxia-induced bunch bouton formation. Sima upregulates the level of the Wnt/Wingless (Wg) signal in glia, leading to reorganized microtubule structures within presynaptic sites. Finally, hypoxia-induced bunch boutons maintain normal synaptic transmission at the NMJs, which is crucial for coordinated larval locomotion. Author summaryOxygen is essential for animals to maintain their life such as growth, metabolism, responsiveness, and movement. It is therefore important to understand how animal cells trigger hypoxia response and adapt to hypoxia thereafter. Both mammalian vascular and insect tracheal branches are induced to enhance the oxygen delivery. However, the study of hypoxia response in the nervous system remains limited. In this study, we assess the morphology of Drosophila neuromuscular junctions (NMJs), a model system to study development and function of synapses, in two hypoxia conditions, one with raising wild-type larvae in hypoxia, and the other in the trachealess (trh) mutant in which the trachea is defective, causing insufficient oxygen supply. Interestingly, glia, normally wrapping the axons of NMJs, invade into synapse and trigger Wg signals to reconstitute the synaptic structure under hypoxia. This synaptic remodeling maintains the synaptic transmission of synapse, which associate the locomotor behavior of larvae.

genetics↗