Astrocytes Play a Key Role in Drosophila Mushroom Body Axon Pruning

Yaniv Hakim,Oren Schuldiner,Shiri P Yaniv,Efthimios M C Skoulakis

doi:10.1371/journal.pone.0086178

Abstract

Axon pruning is an evolutionarily conserved strategy used to remodel neuronal connections during development. The Drosophila mushroom body (MB) undergoes neuronal remodeling in a highly stereotypical and tightly regulated manner, however many open questions remain. Although it has been previously shown that glia instruct pruning by secreting a TGF-β ligand, myoglianin, which primes MB neurons for fragmentation and also later engulf the axonal debris once fragmentation has been completed, which glia subtypes participate in these processes as well as the molecular details are unknown. Here we show that, unexpectedly, astrocytes are the major glial subtype that is responsible for the clearance of MB axon debris following fragmentation, even though they represent only a minority of glia in the MB area during remodeling. Furthermore, we show that astrocytes both promote fragmentation of MB axons as well as clear axonal debris and that this process is mediated by ecdysone signaling in the astrocytes themselves. In addition, we found that blocking the expression of the cell engulfment receptor Draper in astrocytes only affects axonal debris clearance. Thereby we uncoupled the function of astrocytes in promoting axon fragmentation to that of clearing axonal debris after fragmentation has been completed. Our study finds a novel role for astrocytes in the MB and suggests two separate pathways in which they affect developmental axon pruning.

Highlights

Neuronal remodeling is an evolutionarily conserved process used to refine neuronal circuits during development in both vertebrates and invertebrates
Our results suggest that astrocytes, not ensheathing glia, are the main glial subtype that is responsible for the infiltration and engulfment of mushroom body (MB) c debris following axon fragmentation
We show that astrocytes are one of the major glial subtypes that is responsible for MB c axon pruning, despite the fact that they make up a small subpopulation of glia surrounding the MB during this developmental period

Summary

Introduction

Neuronal remodeling is an evolutionarily conserved process used to refine neuronal circuits during development in both vertebrates and invertebrates. At 24 h APF c neurons start to regrow their axons to a new, adult specific lobe at the same time in which the later born a/b neurons begin to extend their axons It appears that neighboring glia participate in the process in at least two distinct stages. Glia were found to instruct axon fragmentation by secreting myoglianin (myo) [4], a TGF-b ligand, which binds to the TGF-b receptors baboon (babo) and punt/wishful thinking (put; wit) [5] assisted by the TGF-b accessory receptor plum [6] on the membrane of c neurons. We report that astrocytes play several roles throughout MB remodeling and that they, not ensheathing glia, are the major glial subtype involved in clearing MB c axon debris, despite the fact that they are very few in number. At the onset of axon pruning, astrocytes infiltrate the MB lobes and later on engulf most, if not all, of axonal debris in a process that is regulated, at least in part, by Drpr and endocytosis

Results

Discussion

Experimental Procedures