Early Draper-mediated glial refinement of neuropil architecture and synapse number in the Drosophila antennal lobe

Abstract

Glial phagocytic activity refines connectivity, though molecular mechanisms regulating this exquisitely sensitive process are incompletely defined. We developed the Drosophila antennal lobe as a model for identifying molecular mechanisms underlying glial refinement of neural circuits in the absence of injury. Antennal lobe organization is stereotyped and characterized by individual glomeruli comprised of unique olfactory receptor neuronal (ORN) populations. The antennal lobe interacts extensively with two glial subtypes: ensheathing glia wrap individual glomeruli, while astrocytes ramify considerably within them. Phagocytic roles for glia in the uninjured antennal lobe are largely unknown. Thus, we tested whether Draper regulates ORN terminal arbor size, shape, or presynaptic content in two representative glomeruli: VC1 and VM7. We find that glial Draper limits the size of individual glomeruli and restrains their presynaptic content. Moreover, glial refinement is apparent in young adults, a period of rapid terminal arbor and synapse growth, indicating that synapse addition and elimination occur simultaneously. Draper has been shown to be expressed in ensheathing glia; unexpectedly, we find it expressed at high levels in late pupal antennal lobe astrocytes. Surprisingly, Draper plays differential roles in ensheathing glia and astrocytes in VC1 and VM7. In VC1, ensheathing glial Draper plays a more significant role in shaping glomerular size and presynaptic content; while in VM7, astrocytic Draper plays the larger role. Together, these data indicate that astrocytes and ensheathing glia employ Draper to refine circuitry in the antennal lobe before the terminal arbors reach their mature form and argue for local heterogeneity of neuron-glia interactions.

Glia control experience-dependent plasticity in an olfactory critical period

“…Brp is the Drosophila homolog of mammalian ELKS/CAST/ERC and is a structural component of the presynaptic electrondense T-bar (Wagh et al, 2006). We previously demonstrated that active zone size and presynapse number are indistinguishable when quantified by masking endogenous Brp signal or by driving cell type-specific overexpression of Brp-short (Jindal et al, 2023), validating our approach. We focused on defining the lowest odorant concentration leading to robust reductions in VM7 OSN innervation volume and presynaptic number.…”

“…While the response of AL glia to OSN axon injury has been thoroughly investigated (Herrmann et al, 2022;Logan et al, 2012;Ziegenfuss et al, 2012), glial roles in synapse formation, maintenance, and pruning in the AL are less clear. We previously investigated the function of the highly conserved glial engulfment receptor Draper in regulating axon terminal morphology and synapse number of several OSN populations across early and mid-adulthood to test for a role for Draper in shaping circuitry at baseline (Jindal et al, 2023). Intriguingly, the phenotypic and cell-type-specific consequences of losing glial Draper are not constant over time.…”

“…Hence, we wanted to better describe the timing, scope, selectivity, and possible long-term consequences of this striking form of early-life plasticity before interrogating a role for glia. To begin, we employed our Imaris-based pipelines to quantify presynaptic number and OSN terminal arbor volume (Jindal et al, 2023) in young (0-2 DPE) animals across a range of EB concentrations. Briefly, we precisely masked Or4a-mCD8::GFP-labeled OSN terminal arbors using the Surfaces function in Imaris and quantified bruchpilot (Brp) puncta exclusively within the OSN mask.…”

Experience-dependent glial pruning of synaptic glomeruli during the critical period

“…Only the EB-responsive VM7 glomerulus has been tested so far, and studies are needed for other odorant-selective glomeruli to determine the generalization of this mechanism. Glia subclasses differentially refine OSN synaptic architecture in a Draper-dependent mechanism 15 . Three glial classes function as phagocytes and can act cooperatively for neuronal phagocytosis in Drosophila juvenile brains 47 , 75 .…”

“…For example, ethyl butyrate (EB)-responsive Or42a receptor olfactory sensory neurons (OSNs) innervating ventral medial 7 (VM7) glomeruli show extensive synaptic pruning in response to EB experience only during this short, defined critical period 10 , 11 . Importantly, glial cells can function as brain phagocytes to mediate experience-dependent synaptic pruning 13 – 15 , and glial phagocytosis is the primary mechanism of neuronal remodeling in Drosophila injury models and during early Drosophila brain development 16 – 18 .…”