Ensheathing Glia Function as Phagocytes in the AdultDrosophilaBrain

Abstract

The mammalian brain contains many subtypes of glia that vary in their morphologies, gene expression profiles, and functional roles; however, the functional diversity of glia in the adult Drosophila brain remains poorly defined. Here we define the diversity of glial subtypes that exist in the adult Drosophila brain, show they bear striking similarity to mammalian brain glia, and identify the major phagocytic cell type responsible for engulfing degenerating axons after acute axotomy. We find that neuropil regions contain two different populations of glia: ensheathing glia and astrocytes. Ensheathing glia enwrap major structures in the adult brain, but are not closely associated with synapses. Interestingly, we find these glia uniquely express key components of the glial phagocytic machinery (e.g., the engulfment receptor Draper, and dCed-6), respond morphologically to axon injury, and autonomously require components of the Draper signaling pathway for successful clearance of degenerating axons from the injured brain. Astrocytic glia, in contrast, do not express Draper or dCed-6, fail to respond morphologically to axon injury, and appear to play no role in clearance of degenerating axons from the brain. However, astrocytic glia are closely associated with synaptic regions in neuropil, and express excitatory amino acid transporters, which are presumably required for the clearance of excess neurotransmitters at the synaptic cleft. Together these results argue that ensheathing glia and astrocytes are preprogrammed cell types in the adult Drosophila brain, with ensheathing glia acting as phagocytes after axotomy, and astrocytes potentially modulating synapse formation and signaling. IntroductionGlia are the most abundant cell type in the mammalian nervous system, accounting for ϳ90% of cells in the mature brain. The different subtypes of mammalian glia-astrocytes, oligodendrocytes, microglia, and Schwann cells-have been extensively classified based on morphology, molecular markers, and position within the nervous system and are thought to play largely distinct roles in nervous system development and function. In the CNS, astrocytes provide trophic support for neuronal growth and regulate synapse formation and signaling. Oligodendrocytes ensheath and myelinate axons, thereby protecting axons and providing a stable ionic environment for proper conduction of action potentials. Microglia act as the resident immune cells in the brain, responding to infection and neural trauma, acting as phagocytes, and mediating posttrauma events (for review, see Barres, 2008).The developing Drosophila embryonic and larval nervous systems contain unique subsets of glial cells that are morphologically and molecularly similar to their mammalian counterparts (Ito et al., 1995;Edenfeld et al., 2005; Doherty, 2006, Logan andFreeman, 2007). Drosophila glia play critical roles during the formation of the nervous system, including regulation of axon pathfinding (Hidalgo and Booth, 2000;Poeck et al., 2001), engulfment of apoptotic neurons (Sonnenfel...

Astrocytic glutamate transport regulates a Drosophila CNS synapse that lacks astrocyte ensheathment

“…Levine (University of Arizona). The alrm-GAL4 (Doherty et al, 2009) flies used to drive expression of Green Fluorescent Protein (GFP) in Drosophila astrocytes were a gift from M Freeman (University of Massachusetts). For D42>ChR2-H134R experiments, homozygous animals were used to achieve sufficient expression and subsequent depolarization to trigger neuronal action potentials.…”

“…Fasciclin II-positive axon tracts (magenta) are shown to provide landmarks within the larval VNC neuropil (Landgraf et al, 2003). Drosophila astrocytes were labeled by using the astrocyte-specific driver, alrm-GAL4, to express membrane-targeted GFP (Doherty et al, 2009). In the cross-sectional view of the third thoracic segment in Figure 1A, the cell bodies of 3 of the 12 astrocytes residing in that segment are visible; dorsal (d), lateral (l), and ventral (v) astrocyte cell bodies appear in the left hemisegment.…”

The c-Jun kinase signaling cascade promotes glial engulfment activity through activation of draper and phagocytic function

“…Ensheathing glia are the primary cell type in the Drosophila neuropil that express molecular components of the engulfment machinery, including the Draper receptor and dCed-6 adaptor molecule, and they phagocytose axonal debris after axotomy. 19 To determine whether Bsk functions in ensheathing glia, we drove the expression of bsk RNAi or UAS-puc with the ensheathing glia-specific driver mz0709-Gal4 and assayed glial clearance of degenerating axons. We found that suppression of dJNK signaling in ensheathing glia was sufficient to block the glial engulfment function (Figures 2c and d).…”

Analysis of Glial Distribution in Drosophila Adult Brains

“…All strains were obtained from the Bloomington Stock Center or the Vienna Drosophila Resource Center. NP6520-Gal4, NP577-Gal4, and Alrm-Gal4 were gifts from R. Jackson and M. Freeman [21,22]. All fly crosses were carried out at 25°C under standard laboratory conditions unless noted otherwise.…”