Astrocytes play a key role in Drosophila mushroom body axon pruning

Glia control experience-dependent plasticity in an olfactory critical period

Sensory experience during developmental critical periods has lifelong consequences for circuit function and behavior, but the molecular and cellular mechanisms through which experience causes these changes are not well understood. The Drosophila antennal lobe houses synapses between olfactory sensory neurons (OSNs) and downstream projection neurons (PNs) in stereotyped glomeruli. Many glomeruli exhibit structural plasticity in response to early-life odor exposure, indicating a general sensitivity of the fly olfactory circuitry to early sensory experience. We recently found that glia shape antennal lobe development in young adults, leading us to ask if glia also drive experience-dependent plasticity during this period. Here, we define a critical period for structural and functional plasticity of OSN-PN synapses in the ethyl butyrate (EB)-sensitive glomerulus VM7. EB exposure for the first 2 days post-eclosion drives large-scale reductions in glomerular volume, presynapse number, and post- synaptic activity. Crucially, pruning during the critical period has long-term consequences for circuit function since both OSN-PN synapse number and spontaneous activity of PNs remain persistently decreased following early-life odor exposure. The highly conserved engulfment receptor Draper is required for this critical period plasticity as ensheathing glia upregulate Draper, invade the VM7 glomerulus, and phagocytose OSN presynaptic terminals in response to critical-period EB exposure. Loss of Draper fully suppresses the morphological and physiological consequences of critical period odor exposure, arguing that phagocytic glia engulf intact synaptic terminals. These data demonstrate experience-dependent pruning of synapses and argue that Drosophila olfactory circuitry is a powerful model for defining the function of glia in critical period plasticity.

Astrocyte-dependent local neurite pruning in Beat-Va neurons

Developmental neuronal remodeling is extensive and mechanistically diverse across the nervous system. We sought to identify Drosophila pupal neurons that underwent mechanistically new types of neuronal remodeling and describe remodeling Beat-VaM and Beat-VaL neurons. We show that Beat-VaM neurons produce highly branched neurites in the CNS during larval stages that undergo extensive local pruning. Surprisingly, although the ecdysone receptor (EcR) is essential for pruning in all other cell types studied, Beat-VaM neurons remodel their branches extensively despite cell autonomous blockade EcR or caspase signaling. Proper execution of local remodeling in Beat-VaM neurons instead depends on extrinsic signaling from astrocytes converging with intrinsic and less dominant EcR-regulated mechanisms. In contrast, Beat-VaL neurons undergo steroid hormone-dependent, apoptotic cell death, which we show relies on the segment-specific expression of the Hox gene Abd-B. Our work provides new cell types in which to study neuronal remodeling, highlights an important role for astrocytes in activating local pruning in Drosophila independent of steroid signaling, and defines a Hox gene-mediated mechanism for segment-specific cell elimination.

Glial Regulation of Circuit Wiring, Firing, and Expiring in the Drosophila Central Nervous System

Molecular genetic approaches in small model organisms like Drosophila have helped to elucidate fundamental principles of neuronal cell biology. Much less is understood about glial cells, although interest in using invertebrate preparations to define their in vivo functions has increased significantly in recent years. This review focuses on our current understanding of the three major neuron-associated glial cell types found in the Drosophila central nervous system (CNS)-astrocytes, cortex glia, and ensheathing glia. Together, these cells act like mammalian astrocytes and microglia; they associate closely with neurons including surrounding neuronal cell bodies and proximal neurites, regulate synapses, and engulf neuronal debris. Exciting recent work has shown critical roles for these CNS glial cells in neural circuit formation, function, plasticity, and pathology. As we gain a more firm molecular and cellular understanding of how Drosophila CNS glial cells interact with neurons, it is clear that they share significant molecular and functional attributes with mammalian glia and will serve as an excellent platform for mechanistic studies of glial function.

Glia control experience-dependent plasticity in an olfactory critical period

Sensory experience during developmental critical periods has lifelong consequences for circuit function and behavior, but the molecular and cellular mechanisms through which experience causes these changes are not well understood. The Drosophila antennal lobe houses synapses between olfactory sensory neurons (OSNs) and downstream projection neurons (PNs) in stereotyped glomeruli. Many glomeruli exhibit structural plasticity in response to early-life odor exposure, indicating a general sensitivity of the fly olfactory circuitry to early sensory experience. We recently found that glia shape antennal lobe development in young adults, leading us to ask if glia also drive experience-dependent plasticity during this period. Here we define a critical period for structural and functional plasticity of OSN-PN synapses in the ethyl butyrate (EB)-sensitive glomerulus VM7. EB exposure for the first two days post-eclosion drives large-scale reductions in glomerular volume, presynapse number, and post-synaptic activity. Crucially, pruning during the critical period has long-term consequences for circuit function since both OSN-PN synapse number and spontaneous activity of PNs remain persistently decreased following early-life odor exposure. The highly conserved engulfment receptor Draper is required for this critical period plasticity as ensheathing glia upregulate Draper, invade the VM7 glomerulus, and phagocytose OSN presynaptic terminals in response to critical-period EB exposure. Loss of Draper fully suppresses the morphological and physiological consequences of critical period odor exposure, arguing that phagocytic glia engulf intact synaptic terminals. These data demonstrate experience-dependent pruning of synapses and argue that Drosophila olfactory circuitry is a powerful model for defining the function of glia in critical period plasticity.

Neural Circuit Remodeling: Mechanistic Insights from Invertebrates

As nervous systems mature, neural circuit connections are reorganized to optimize the performance of specific functions in adults. This reorganization of connections is achieved through a remarkably conserved phase of developmental circuit remodeling that engages neuron-intrinsic and neuron-extrinsic molecular mechanisms to establish mature circuitry. Abnormalities in circuit remodeling and maturation are broadly linked with a variety of neurodevelopmental disorders, including autism spectrum disorders and schizophrenia. Here, we aim to provide an overview of recent advances in our understanding of the molecular processes that govern neural circuit remodeling and maturation. In particular, we focus on intriguing mechanistic insights gained from invertebrate systems, such as the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. We discuss how transcriptional control mechanisms, synaptic activity, and glial engulfment shape specific aspects of circuit remodeling in worms and flies. Finally, we highlight mechanistic parallels across invertebrate and mammalian systems, and prospects for further advances in each.

Chemokine-like Orion is involved in the transformation of glial cells into phagocytes in different developmental neuronal remodeling paradigms

During animal development, neurons often form exuberant or inappropriate axons and dendrites at early stages, followed by the refinement of neuronal circuits at late stages. Neural circuit refinement leads to the production of neuronal debris in the form of neuronal cell corpses, fragmented axons and dendrites, and pruned synapses requiring disposal. Glial cells act as predominant phagocytes during neuronal remodeling and degeneration, and crucial signaling pathways between neurons and glia are necessary for the execution of phagocytosis. Chemokine-like mushroom body neuron-secreted Orion is essential for astrocyte infiltration into the γ axon bundle leading to γ axon pruning. Here, we show a role of Orion in debris engulfment and phagocytosis in Drosophila. Interestingly, Orion is involved in the overall transformation of astrocytes into phagocytes. In addition, analysis of several neuronal paradigms demonstrates the role of Orion in eliminating both peptidergic vCrz+ and PDF-Tri neurons via additional phagocytic glial cells like cortex and/or ensheathing glia. Our results suggest that Orion is essential for phagocytic activation of astrocytes, cortex and ensheathing glia, and point to Orion as a trigger of glial infiltration, engulfment and phagocytosis.

The Drivers of Diversity: Integrated genetic and hormonal cues regulate neural diversity

Proper functioning of the nervous system relies not only on the generation of a vast repertoire of distinct neural cell types but also on the precise neural circuitry within them. How the generation of highly diverse neural populations is regulated during development remains a topic of interest. Landmark studies in Drosophila have identified the genetic and temporal cues regulating neural diversity and thus have provided valuable insights into our understanding of temporal patterning of the central nervous system. The development of the Drosophila central complex, which is mostly derived from type II neural stem cell (NSC) lineages, showcases how a small pool of NSCs can give rise to vast and distinct progeny. Similar to the human outer subventricular zone (OSVZ) neural progenitors, type II NSCs generate intermediate neural progenitors (INPs) to expand and diversify lineages that populate higher brain centers. Each type II NSC has a distinct spatial identity and timely regulated expression of many transcription factors and mRNA binding proteins. Additionally, INPs derived from them show differential expression of genes depending on their birth order. Together type II NSCs and INPs display a combinatorial temporal patterning that expands neural diversity of the central brain lineages. We cover advances in current understanding of type II NSC temporal patterning and discuss similarities and differences in temporal patterning mechanisms of various NSCs with a focus on how cell-intrinsic and extrinsic hormonal cues regulate temporal transitions in NSCs during larval development. Cell extrinsic ligands activate conserved signaling pathways and extrinsic hormonal cues act as a temporal switch that regulate temporal progression of the NSCs. We conclude by elaborating on how a progenitor's temporal code regulates the fate specification and identity of distinct neural types. At the end, we also discuss open questions in linking developmental cues to neural identity, circuits, and underlying behaviors in the adult fly.

Drosophila postembryonic nervous system development: a model for the endocrine control of development

During postembryonic life, hormones, including ecdysteroids, juvenile hormones, insulin-like peptides, and activin/TGFβ ligands act to transform the larval nervous system into an adult version, which is a fine-grained mosaic of recycled larval neurons and adult-specific neurons. Hormones provide both instructional signals that make cells competent to undergo developmental change and timing cues to evoke these changes across the nervous system. While touching on all the above hormones, our emphasis is on the ecdysteroids, ecdysone and 20-hydroxyecdysone (20E). These are the prime movers of insect molting and metamorphosis and are involved in all phases of nervous system development, including neurogenesis, pruning, arbor outgrowth, and cell death. Ecdysteroids appear as a series of steroid peaks that coordinate the larval molts and the different phases of metamorphosis. Each peak directs a stereotyped cascade of transcription factor expression. The cascade components then direct temporal programs of effector gene expression, but the latter vary markedly according to tissue and life stage. The neurons read the ecdysteroid titer through various isoforms of the ecdysone receptor, a nuclear hormone receptor. For example, at metamorphosis the pruning of larval neurons is mediated through the B isoforms, which have strong activation functions, whereas subsequent outgrowth is mediated through the A isoform through which ecdysteroids play a permissive role to allow local tissue interactions to direct outgrowth. The major circulating ecdysteroid can also change through development. During adult development ecdysone promotes early adult patterning and differentiation while its metabolite, 20E, later evokes terminal adult differentiation.

Actin-dependent astrocytic infiltration is a key step for axon defasciculation during remodeling

Astrocytes are essential for synapse formation, maturation, and plasticity; however, their function during developmental neuronal remodeling is largely unknown. To identify astrocytic molecules required for axon pruning of mushroom body (MB) γ neurons in Drosophila, we profiled astrocytes before (larva) and after (adult) remodeling. Focusing on genes enriched in larval astrocytes, we identified 12 astrocytic genes that are required for axon pruning, including the F-actin regulators Actin-related protein 2/3 complex, subunit 1 (Arpc1) and formin3 (form3). Interestingly, perturbing astrocytic actin dynamics does not affect their gross morphology, migration, or transforming growth factor β (TGF-β) secretion. In contrast, actin dynamics is required for astrocyte infiltration into the axon bundle at the onset of pruning. Remarkably, decreasing axonal adhesion facilitates infiltration by Arpc1 knockdown (KD) astrocytes and promotes axon pruning. Conversely, increased axonal adhesion reduces lobe infiltration by wild-type (WT) astrocytes. Together, our findings suggest that actin-dependent astrocytic infiltration is a key step in axon pruning, thus promoting our understanding of neuron-glia interactions during remodeling.

Polycomb group genes are required for neuronal pruning in Drosophila

BACKGROUND

Pruning that selectively eliminates unnecessary or incorrect neurites is required for proper wiring of the mature nervous system. During Drosophila metamorphosis, dendritic arbourization sensory neurons (ddaCs) and mushroom body (MB) γ neurons can selectively prune their larval dendrites and/or axons in response to the steroid hormone ecdysone. An ecdysone-induced transcriptional cascade plays a key role in initiating neuronal pruning. However, how downstream components of ecdysone signalling are induced remains not entirely understood.

RESULTS

Here, we identify that Scm, a component of Polycomb group (PcG) complexes, is required for dendrite pruning of ddaC neurons. We show that two PcG complexes, PRC1 and PRC2, are important for dendrite pruning. Interestingly, depletion of PRC1 strongly enhances ectopic expression of Abdominal B (Abd-B) and Sex combs reduced, whereas loss of PRC2 causes mild upregulation of Ultrabithorax and Abdominal A in ddaC neurons. Among these Hox genes, overexpression of Abd-B causes the most severe pruning defects, suggesting its dominant effect. Knockdown of the core PRC1 component Polyhomeotic (Ph) or Abd-B overexpression selectively downregulates Mical expression, thereby inhibiting ecdysone signalling. Finally, Ph is also required for axon pruning and Abd-B silencing in MB γ neurons, indicating a conserved function of PRC1 in two types of pruning.

CONCLUSIONS

This study demonstrates important roles of PcG and Hox genes in regulating ecdysone signalling and neuronal pruning in Drosophila. Moreover, our findings suggest a non-canonical and PRC2-independent role of PRC1 in Hox gene silencing during neuronal pruning.

Astrocytes regulate neuronal network activity by mediating synapse remodeling

Based on experience during our life, neuronal connectivity continuously changes through structural remodeling of synapses. Recent studies have shown that the complex interaction between astrocytes and synapses regulates structural synapse remodeling by inducing the formation and elimination of synapses, as well as their functional maturation. Defects in this astrocyte-mediated synapse remodeling cause problems in not only neuronal network activities but also animal behaviors. Moreover, in various neurological disorders, astrocytes have been shown to play central roles in the initiation and progression of synaptic pathophysiology through impaired interactions with synapses. In this review, we will discuss recent studies identifying the novel roles of astrocytes in neuronal circuit remodeling, focusing on synapse formation and elimination. We will also discuss the potential implication of defective astrocytic function in evoking various brain disorders.

The making of the Drosophila mushroom body

The mushroom body (MB) is a computational center in the Drosophila brain. The intricate neural circuits of the mushroom body enable it to store associative memories and process sensory and internal state information. The mushroom body is composed of diverse types of neurons that are precisely assembled during development. Tremendous efforts have been made to unravel the molecular and cellular mechanisms that build the mushroom body. However, we are still at the beginning of this challenging quest, with many key aspects of mushroom body assembly remaining unexplored. In this review, I provide an in-depth overview of our current understanding of mushroom body development and pertinent knowledge gaps.

Neuron-glia crosstalk in neuronal remodeling and degeneration: Neuronal signals inducing glial cell phagocytic transformation in Drosophila

Neuronal remodeling is a conserved mechanism that eliminates unwanted neurites and can include the loss of cell bodies. In these processes, a key role for glial cells in events from synaptic pruning to neuron elimination has been clearly identified in the last decades. Signals sent from dying neurons or neurites to be removed are received by appropriate glial cells. After receiving these signals, glial cells infiltrate degenerating sites and then, engulf and clear neuronal debris through phagocytic mechanisms. There are few identified or proposed signals and receptors involved in neuron-glia crosstalk, which induces the transformation of glial cells to phagocytes during neuronal remodeling in Drosophila. Many of these signaling pathways are conserved in mammals. Here, we particularly emphasize the role of Orion, a recently identified neuronal CX₃ C chemokine-like secreted protein, which induces astrocyte infiltration and engulfment during mushroom body neuronal remodeling. Although, chemokine signaling was not described previously in insects we propose that chemokine-like involvement in neuron/glial cell interaction is an evolutionarily ancient mechanism.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

Glia-derived temporal signals orchestrate neurogenesis in the Drosophila mushroom body

Intrinsic mechanisms such as temporal series of transcription factors orchestrate neurogenesis from a limited number of neural progenitors in the brain. Extrinsic regulations, however, remain largely unexplored. Here we describe a two-step glia-derived signal that regulates neurogenesis in the Drosophila mushroom body (MB). In a temporal manner, glial-specific ubiquitin ligase dSmurf activates non-cell-autonomous Hedgehog signaling propagation by targeting the receptor Patched to suppress and promote the exit of MB neuroblast (NB) proliferation, thereby specifying the correct α/β cell number without affecting differentiation. Independent of NB proliferation, dSmurf also stabilizes the expression of the cell-adhesion molecule Fasciclin II (FasII) via its WW domains and regulates FasII homophilic interaction between glia and MB axons to refine α/β-lobe integrity. Our findings provide insights into how extrinsic glia-to-neuron communication coordinates with NB proliferation capacity to regulate MB neurogenesis; glial proteostasis is likely a generalized mechanism in orchestrating neurogenesis.

Axonal chemokine-like Orion induces astrocyte infiltration and engulfment during mushroom body neuronal remodeling

The remodeling of neurons is a conserved fundamental mechanism underlying nervous system maturation and function. Astrocytes can clear neuronal debris and they have an active role in neuronal remodeling. Developmental axon pruning of Drosophila memory center neurons occurs via a degenerative process mediated by infiltrating astrocytes. However, how astrocytes are recruited to the axons during brain development is unclear. Using an unbiased screen, we identify the gene requirement of orion, encoding for a chemokine-like protein, in the developing mushroom bodies. Functional analysis shows that Orion is necessary for both axonal pruning and removal of axonal debris. Orion performs its functions extracellularly and bears some features common to chemokines, a family of chemoattractant cytokines. We propose that Orion is a neuronal signal that elicits astrocyte infiltration and astrocyte-driven axonal engulfment required during neuronal remodeling in the Drosophila developing brain.

Astrocytes can engulf axonal debris in the developing brain. However, the mechanisms regulating astrocyte recruitment to the proper axons is unclear. Here, the authors identify Orion as a signal for astrocyte infiltration and engulfment to the mushroom bodies in the Drosophila developing brain.

Neuronal fragile X mental retardation protein activates glial insulin receptor mediated PDF-Tri neuron developmental clearance

Glia engulf and phagocytose neurons during neural circuit developmental remodeling. Disrupting this pruning process contributes to Fragile X syndrome (FXS), a leading cause of intellectual disability and autism spectrum disorder in mammals. Utilizing a Drosophila FXS model central brain circuit, we identify two glial classes responsible for Draper-dependent elimination of developmentally transient PDF-Tri neurons. We find that neuronal Fragile X Mental Retardation Protein (FMRP) drives insulin receptor activation in glia, promotes glial Draper engulfment receptor expression, and negatively regulates membrane-molding ESCRT-III Shrub function during PDF-Tri neuron clearance during neurodevelopment in Drosophila. In this context, we demonstrate genetic interactions between FMRP and insulin receptor signaling, FMRP and Draper, and FMRP and Shrub in PDF-Tri neuron elimination. We show that FMRP is required within neurons, not glia, for glial engulfment, indicating FMRP-dependent neuron-to-glia signaling mediates neuronal clearance. We conclude neuronal FMRP drives glial insulin receptor activation to facilitate Draper- and Shrub-dependent neuronal clearance during neurodevelopment in Drosophila.

The role of astrocyte-mediated plasticity in neural circuit development and function

Neuronal networks are capable of undergoing rapid structural and functional changes called plasticity, which are essential for shaping circuit function during nervous system development. These changes range from short-term modifications on the order of milliseconds, to long-term rearrangement of neural architecture that could last for the lifetime of the organism. Neural plasticity is most prominent during development, yet also plays a critical role during memory formation, behavior, and disease. Therefore, it is essential to define and characterize the mechanisms underlying the onset, duration, and form of plasticity. Astrocytes, the most numerous glial cell type in the human nervous system, are integral elements of synapses and are components of a glial network that can coordinate neural activity at a circuit-wide level. Moreover, their arrival to the CNS during late embryogenesis correlates to the onset of sensory-evoked activity, making them an interesting target for circuit plasticity studies. Technological advancements in the last decade have uncovered astrocytes as prominent regulators of circuit assembly and function. Here, we provide a brief historical perspective on our understanding of astrocytes in the nervous system, and review the latest advances on the role of astroglia in regulating circuit plasticity and function during nervous system development and homeostasis.

Behaviorally consequential astrocytic regulation of neural circuits

Astrocytes are a large and diverse population of morphologically complex cells that exist throughout nervous systems of multiple species. Progress over the last two decades has shown that astrocytes mediate developmental, physiological, and pathological processes. However, a long-standing open question is how astrocytes regulate neural circuits in ways that are behaviorally consequential. In this regard, we summarize recent studies using Caenorhabditis elegans, Drosophila melanogaster, Danio rerio, and Mus musculus. The data reveal diverse astrocyte mechanisms operating in seconds or much longer timescales within neural circuits and shaping multiple behavioral outputs. We also refer to human diseases that have a known primary astrocytic basis. We suggest that including astrocytes in mechanistic, theoretical, and computational studies of neural circuits provides new perspectives to understand behavior, its regulation, and its disease-related manifestations.

Spatiotemporal regulation of developmental neurite pruning: Molecular and cellular insights from Drosophila models

Developmental neurite pruning is a process by which neurons selectively eliminate unnecessary processes of axons and/or dendrites without cell death, which shapes the mature wiring of nervous systems. In this sense, developmental neurite pruning requires spatiotemporally precise control of local degradation of cellular components including cytoskeletons and membranes. The Drosophila nervous system undergoes large-scale remodeling, including axon/dendrite pruning, during metamorphosis. In addition to this unique phenomenon in the nervous system, powerful genetic tools make the Drosophila nervous system a sophisticated model to investigate spatiotemporal regulation of neural remodeling. This article reviews recent advances to our understanding of the molecular and cellular mechanisms of developmental axon/dendrite pruning, mainly focusing on studies in Drosophila sensory neurons and mushroom body neurons.

Independent glial subtypes delay development and extend healthy lifespan upon reduced insulin-PI3K signalling

BACKGROUND

The increasing age of global populations highlights the urgent need to understand the biological underpinnings of ageing. To this end, inhibition of the insulin/insulin-like signalling (IIS) pathway can extend healthy lifespan in diverse animal species, but with trade-offs including delayed development. It is possible that distinct cell types underlie effects on development and ageing; cell-type-specific strategies could therefore potentially avoid negative trade-offs when targeting diseases of ageing, including prevalent neurodegenerative diseases. The highly conserved diversity of neuronal and non-neuronal (glial) cell types in the Drosophila nervous system makes it an attractive system to address this possibility. We have thus investigated whether IIS in distinct glial cell populations differentially modulates development and lifespan in Drosophila.

RESULTS

We report here that glia-specific IIS inhibition, using several genetic means, delays development while extending healthy lifespan. The effects on lifespan can be recapitulated by adult-onset IIS inhibition, whereas developmental IIS inhibition is dispensable for modulation of lifespan. Notably, the effects we observe on both lifespan and development act through the PI3K branch of the IIS pathway and are dependent on the transcription factor FOXO. Finally, IIS inhibition in several glial subtypes can delay development without extending lifespan, whereas the same manipulations in astrocyte-like glia alone are sufficient to extend lifespan without altering developmental timing.

CONCLUSIONS

These findings reveal a role for distinct glial subpopulations in the organism-wide modulation of development and lifespan, with IIS in astrocyte-like glia contributing to lifespan modulation but not to developmental timing. Our results enable a more complete picture of the cell-type-specific effects of the IIS network, a pathway whose evolutionary conservation in humans make it tractable for therapeutic interventions. Our findings therefore underscore the necessity for cell-type-specific strategies to optimise interventions for the diseases of ageing.

Drosophila Glia: Models for Human Neurodevelopmental and Neurodegenerative Disorders

Glial cells are key players in the proper formation and maintenance of the nervous system, thus contributing to neuronal health and disease in humans. However, little is known about the molecular pathways that govern glia-neuron communications in the diseased brain. Drosophila provides a useful in vivo model to explore the conserved molecular details of glial cell biology and their contributions to brain function and disease susceptibility. Herein, we review recent studies that explore glial functions in normal neuronal development, along with Drosophila models that seek to identify the pathological implications of glial defects in the context of various central nervous system disorders.

Extrinsic activin signaling cooperates with an intrinsic temporal program to increase mushroom body neuronal diversity

Temporal patterning of neural progenitors leads to the sequential production of diverse neurons. To understand how extrinsic cues influence intrinsic temporal programs, we studied Drosophila mushroom body progenitors (neuroblasts) that sequentially produce only three neuronal types: γ, then α’β’, followed by αβ. Opposing gradients of two RNA-binding proteins Imp and Syp comprise the intrinsic temporal program. Extrinsic activin signaling regulates the production of α’β’ neurons but whether it affects the intrinsic temporal program was not known. We show that the activin ligand Myoglianin from glia regulates the temporal factor Imp in mushroom body neuroblasts. Neuroblasts missing the activin receptor Baboon have a delayed intrinsic program as Imp is higher than normal during the α’β’ temporal window, causing the loss of α’β’ neurons, a decrease in αβ neurons, and a likely increase in γ neurons, without affecting the overall number of neurons produced. Our results illustrate that an extrinsic cue modifies an intrinsic temporal program to increase neuronal diversity.

Multiple lineages enable robust development of the neuropil-glia architecture in adult Drosophila

Neural remodeling is essential for the development of a functional nervous system and has been extensively studied in the metamorphosis of Drosophila Despite the crucial roles of glial cells in brain functions, including learning and behavior, little is known of how adult glial cells develop in the context of neural remodeling. Here, we show that the architecture of neuropil-glia in the adult Drosophila brain, which is composed of astrocyte-like glia (ALG) and ensheathing glia (EG), robustly develops from two different populations in the larva: the larval EG and glial cell missing-positive (gcm+ ) cells. Whereas gcm+ cells proliferate and generate adult ALG and EG, larval EG dedifferentiate, proliferate and redifferentiate into the same glial subtypes. Each glial lineage occupies a certain brain area complementary to the other, and together they form the adult neuropil-glia architecture. Both lineages require the FGF receptor Heartless to proliferate, and the homeoprotein Prospero to differentiate into ALG. Lineage-specific inhibition of gliogenesis revealed that each lineage compensates for deficiency in the proliferation of the other. Together, the lineages ensure the robust development of adult neuropil-glia, thereby ensuring a functional brain.

With a little help from my friends: how intercellular communication shapes neuronal remodeling

Developmental neuronal remodeling shapes the mature connectivity of the nervous system in both vertebrates and invertebrates. Remodeling often combines degenerative and regenerative events, and defects in its normal progression have been linked to neurological disorders. Here we review recent progress that highlights the roles of cell-cell interactions during remodeling. We propose that these are fundamental to elucidating how spatiotemporal control of remodeling and coordinated circuit remodeling are achieved. We cover examples spanning various neuronal circuits in vertebrates and invertebrates and involving interactions between neurons and different cell types.

Die in pieces: How Drosophila sheds light on neurite degeneration and clearance

Dendrites and axons are delicate neuronal membrane extensions that undergo degeneration after physical injuries. In neurodegenerative diseases, they often degenerate prior to neuronal death. Understanding the mechanisms of neurite degeneration has been an intense focus of neurobiology research in the last two decades. As a result, many discoveries have been made in the molecular pathways that lead to neurite degeneration and the cell-cell interactions responsible for the subsequent clearance of neuronal debris. Drosophila melanogaster has served as a prime in vivo model system for identifying and characterizing the key molecular players in neurite degeneration, thanks to its genetic tractability and easy access to its nervous system. The knowledge learned in the fly provided targets and fuel for studies in other model systems that have further enhanced our understanding of neurodegeneration. In this review, we will introduce the experimental systems developed in Drosophila to investigate injury-induced neurite degeneration, and then discuss the biological pathways that drive degeneration. We will also cover what is known about the mechanisms of how phagocytes recognize and clear degenerating neurites, and how recent findings in this area enhance our understanding of neurodegenerative disease pathology.

Glial Contribution to Excitatory and Inhibitory Synapse Loss in Neurodegeneration

Synapse loss is an early feature shared by many neurodegenerative diseases, and it represents the major correlate of cognitive impairment. Recent studies reveal that microglia and astrocytes play a major role in synapse elimination, contributing to network dysfunction associated with neurodegeneration. Excitatory and inhibitory activity can be affected by glia-mediated synapse loss, resulting in imbalanced synaptic transmission and subsequent synaptic dysfunction. Here, we review the recent literature on the contribution of glia to excitatory/inhibitory imbalance, in the context of the most common neurodegenerative disorders. A better understanding of the mechanisms underlying pathological synapse loss will be instrumental to design targeted therapeutic interventions, taking in account the emerging roles of microglia and astrocytes in synapse remodeling.

Combining Developmental and Perturbation-Seq Uncovers Transcriptional Modules Orchestrating Neuronal Remodeling

Developmental neuronal remodeling is an evolutionarily conserved mechanism required for precise wiring of nervous systems. Despite its fundamental role in neurodevelopment and proposed contribution to various neuropsychiatric disorders, the underlying mechanisms are largely unknown. Here, we uncover the fine temporal transcriptional landscape of Drosophila mushroom body γ neurons undergoing stereotypical remodeling. Our data reveal rapid and dramatic changes in the transcriptional landscape during development. Focusing on DNA binding proteins, we identify eleven that are required for remodeling. Furthermore, we sequence developing γ neurons perturbed for three key transcription factors required for pruning. We describe a hierarchical network featuring positive and negative feedback loops. Superimposing the perturbation-seq on the developmental expression atlas highlights a framework of transcriptional modules that together drive remodeling. Overall, this study provides a broad and detailed molecular insight into the complex regulatory dynamics of developmental remodeling and thus offers a pipeline to dissect developmental processes via RNA profiling.

A stochastic framework to model axon interactions within growing neuronal populations

The confined and crowded environment of developing brains imposes spatial constraints on neuronal cells that have evolved individual and collective strategies to optimize their growth. These include organizing neurons into populations extending their axons to common target territories. How individual axons interact with each other within such populations to optimize innervation is currently unclear and difficult to analyze experimentally in vivo. Here, we developed a stochastic model of 3D axon growth that takes into account spatial environmental constraints, physical interactions between neighboring axons, and branch formation. This general, predictive and robust model, when fed with parameters estimated on real neurons from the Drosophila brain, enabled the study of the mechanistic principles underlying the growth of axonal populations. First, it provided a novel explanation for the diversity of growth and branching patterns observed in vivo within populations of genetically identical neurons. Second, it uncovered that axon branching could be a strategy optimizing the overall growth of axons competing with others in contexts of high axonal density. The flexibility of this framework will make it possible to investigate the rules underlying axon growth and regeneration in the context of various neuronal populations.

Understanding how neuronal cells establish complex circuits with specific functions within a developing brain is a major current challenge. Over the last past years, enormous progress has been done to precisely resolve brain anatomy and to dissect the mechanisms controlling the establishment of precise neuronal networks. However, due to the extreme complexity of the brain, it is still experimentally difficult to investigate in vivo how neurons interact with each other and with their physical environments to innervate target territories during development. Here, we have developed a framework that integrates a dynamic 3D mathematical model of single axonal growth with parameters estimated from neurons grown in vivo and simulations of entire populations of growing axons. The emergent properties of our model enable the study of the mechanistic principles underlying the growth of axonal population in developing brains. Specifically, our results highlight the impact of mechanical interactions on both individual and collective axon growth, and uncover how branching regulate this process.

The Repo Homeodomain Transcription Factor Suppresses Hematopoiesis in Drosophila and Preserves the Glial Fate

Despite their different origins, Drosophila glia and hemocytes are related cell populations that provide an immune function. Drosophila hemocytes patrol the body cavity and act as macrophages outside the nervous system, whereas glia originate from the neuroepithelium and provide the scavenger population of the nervous system. Drosophila glia are hence the functional orthologs of vertebrate microglia, even though the latter are cells of immune origin that subsequently move into the brain during development. Interestingly, the Drosophila immune cells within (glia) and outside (hemocytes) the nervous system require the same transcription factor glial cells deficient/glial cells missing (Glide/Gcm) for their development. This raises the issue of how do glia specifically differentiate in the nervous system, and hemocytes in the procephalic mesoderm. The Repo homeodomain transcription factor and panglial direct target of Glide/Gcm is known to ensure glial terminal differentiation. Here we show that Repo also takes center stage in the process that discriminates between glia and hemocytes. First, Repo expression is repressed in the hemocyte anlagen by mesoderm-specific factors. Second, Repo ectopic activation in the procephalic mesoderm is sufficient to repress the expression of hemocyte-specific genes. Third, the lack of Repo triggers the expression of hemocyte markers in glia. Thus, a complex network of tissue-specific cues biases the potential of Glide/Gcm. These data allow us to revise the concept of fate determinants and help us to understand the bases of cell specification. Both sexes were analyzed.SIGNIFICANCE STATEMENT Distinct cell types often require the same pioneer transcription factor, raising the issue of how one factor triggers different fates. In Drosophila, glia and hemocytes provide a scavenger activity within and outside the nervous system, respectively. While they both require the glial cells deficient/glial cells missing (Glide/Gcm) transcription factor, glia originate from the ectoderm, and hemocytes from the mesoderm. Here we show that tissue-specific factors inhibit the gliogenic potential of Glide/Gcm in the mesoderm by repressing the expression of the homeodomain protein Repo, a major glial-specific target of Glide/Gcm. Repo expression in turn inhibits the expression of hemocyte-specific genes in the nervous system. These cell-specific networks secure the establishment of the glial fate only in the nervous system and allow cell diversification.

Focal adhesion molecules regulate astrocyte morphology and glutamate transporters to suppress seizure-like behavior

Astrocytes are important regulators of neural circuit function and behavior in the healthy and diseased nervous system. We screened for molecules in Drosophila astrocytes that modulate neuronal hyperexcitability and identified multiple components of focal adhesion complexes (FAs). Depletion of astrocytic Tensin, β-integrin, Talin, focal adhesion kinase (FAK), or matrix metalloproteinase 1 (Mmp1), resulted in enhanced behavioral recovery from genetic or pharmacologically induced seizure. Overexpression of Mmp1, predicted to activate FA signaling, led to a reciprocal enhancement of seizure severity. Blockade of FA-signaling molecules in astrocytes at basal levels of CNS excitability resulted in reduced astrocytic coverage of the synaptic neuropil and expression of the excitatory amino acid transporter EAAT1. However, induction of hyperexcitability after depletion of FA-signaling components resulted in enhanced astrocyte coverage and an approximately twofold increase in EAAT1 levels. Our work identifies FA-signaling molecules as important regulators of astrocyte outgrowth and EAAT1 expression under normal physiological conditions. Paradoxically, in the context of hyperexcitability, this pathway negatively regulates astrocytic process outgrowth and EAAT1 expression, and their blockade leading to enhanced recovery from seizure.

Draper-mediated JNK signaling is required for glial phagocytosis of apoptotic neurons during Drosophila metamorphosis

Development of the central nervous system involves elimination of superfluous neurons through apoptosis and subsequent phagocytosis. In Drosophila, this occurs mainly during three developmental stages: embryogenesis, metamorphosis and emerging adult. Two transmembrane glial phagocytic receptors, SIMU (homolog of the mammalian Stabilin-2) and Draper (homolog of the mammalian MEGF10 and Jedi), mediate glial phagocytosis of apoptotic neurons during embryogenesis. However, less is known about the removal of apoptotic neurons during later stages of development. Here we show that during metamorphosis, Draper plays a critical role in apoptotic cell clearance by glia, whereas SIMU, which is mostly expressed in pupal macrophages outside the brain, is not involved in glial phagocytosis. We found that Draper activates Drosophila c-Jun N-terminal kinase (dJNK) signaling predominantly in the ensheathing glia and astrocytes, where it is required for efficient removal of apoptotic neurons. Our data suggest that besides the dJNK pathway, Draper also triggers an additional signaling pathway capable of removing apoptotic neurons in the pupal brain. This study thus reveals that SIMU unexpectedly is not involved in glial phagocytosis of apoptotic neurons during metamorphosis and highlights the novel role of dJNK signaling in developmental apoptotic cell clearance downstream of Draper.

Astroglial vesicular network: evolutionary trends, physiology and pathophysiology

Intracellular organelles, including secretory vesicles, emerged when eukaryotic cells evolved some 3 billion years ago. The primordial organelles that evolved in Archaea were similar to endolysosomes, which developed, arguably, for specific metabolic tasks, including uptake, metabolic processing, storage and disposal of molecules. In comparison with prokaryotes, cell volume of eukaryotes increased by several orders of magnitude and vesicle traffic emerged to allow for communication between distant intracellular locations. Lysosomes, first described in 1955, a prominent intermediate of endo- and exocytotic pathways, operate virtually in all eukaryotic cells including astroglia, the most heterogeneous type of homeostatic glia in the central nervous system. Astrocytes support neuronal network activity in particular through elaborated secretion, based on a complex intracellular vesicle network dynamics. Deranged homeostasis underlies disease and astroglial vesicle traffic contributes to the pathophysiology of neurodegenerative (Alzheimer's disease, Huntington's disease), neurodevelopmental diseases (intellectual deficiency, Rett's disease) and neuroinfectious (Zika virus) disorders. This review addresses astroglial cell-autonomous vesicular traffic network, as well as its into primary and secondary vesicular network defects in diseases, and considers this network as a target for developing new therapies for neurological conditions.

Common and Divergent Mechanisms in Developmental Neuronal Remodeling and Dying Back Neurodegeneration

Cell death is an inherent process that is required for the proper wiring of the nervous system. Studies over the last four decades have shown that, in a parallel developmental pathway, axons and dendrites are eliminated without the death of the neuron. This developmentally regulated 'axonal death' results in neuronal remodeling, which is an essential mechanism to sculpt neuronal networks in both vertebrates and invertebrates. Studies across various organisms have demonstrated that a conserved strategy in the formation of adult neuronal circuitry often involves generating too many connections, most of which are later eliminated with high temporal and spatial resolution. Can neuronal remodeling be regarded as developmentally and spatially regulated neurodegeneration? It has been previously speculated that injury-induced degeneration (Wallerian degeneration) shares some molecular features with 'dying back' neurodegenerative diseases. In this opinion piece, we examine the similarities and differences between the mechanisms regulating neuronal remodeling and those being perturbed in dying back neurodegenerative diseases. We focus primarily on amyotrophic lateral sclerosis and peripheral neuropathies and highlight possible shared pathways and mechanisms. While mechanistic data are only just beginning to emerge, and despite the inherent differences between disease-oriented and developmental processes, we believe that some of the similarities between these developmental and disease-initiated degeneration processes warrant closer collaborations and crosstalk between these different fields.

The diverse actions of astrocytes during synaptic development

In the developing brain, cortical circuits are established through a complex process of synaptogenesis, maturation, and synaptic pruning. Astrocytes carry out diverse functions during each of these stages to facilitate the formation of complex networks. Recent work has begun to demonstrate that these heterogeneous roles during excitatory synaptic development are determined by the astrocyte population, brain region, and neuron type. This review will focus on current findings which highlight cell type specific mechanisms of excitatory synaptogenesis, as well as multiple mechanisms engaged by astrocytes to facilitate synaptic maturation and pruning.

Lapsyn controls branch extension and positioning of astrocyte-like glia in the Drosophila optic lobe

Astrocytes have diverse, remarkably complex shapes in different brain regions. Their branches closely associate with neurons. Despite the importance of this heterogeneous glial cell type for brain development and function, the molecular cues controlling astrocyte branch morphogenesis and positioning during neural circuit assembly remain largely unknown. We found that in the Drosophila visual system, astrocyte-like medulla neuropil glia (mng) variants acquire stereotypic morphologies with columnar and layered branching patterns in a stepwise fashion from mid-metamorphosis onwards. Using knockdown and loss-of-function analyses, we uncovered a previously unrecognized role for the transmembrane leucine-rich repeat protein Lapsyn in regulating mng development. lapsyn is expressed in mng and cell-autonomously required for branch extension into the synaptic neuropil and anchoring of cell bodies at the neuropil border. Lapsyn works in concert with the fibroblast growth factor (FGF) pathway to promote branch morphogenesis, while correct positioning is essential for mng survival mediated by gliotrophic FGF signaling.

How glial cells, such as astrocytes, acquire their characteristic morphology during development is poorly understood. Here the authors describe the morphogenesis of astrocyte-like glia in the Drosophila optic lobe, and through a RNAi screen, they identify a transmembrane LRR protein–Lapsyn–that plays a critical role in this process.

TRAP-seq Profiling and RNAi-Based Genetic Screens Identify Conserved Glial Genes Required for Adult Drosophila Behavior

Although, glial cells have well characterized functions in the developing and mature brain, it is only in the past decade that roles for these cells in behavior and plasticity have been delineated. Glial astrocytes and glia-neuron signaling, for example, are now known to have important modulatory functions in sleep, circadian behavior, memory and plasticity. To better understand mechanisms of glia-neuron signaling in the context of behavior, we have conducted cell-specific, genome-wide expression profiling of adult Drosophila astrocyte-like brain cells and performed RNA interference (RNAi)-based genetic screens to identify glial factors that regulate behavior. Importantly, our studies demonstrate that adult fly astrocyte-like cells and mouse astrocytes have similar molecular signatures; in contrast, fly astrocytes and surface glia-different classes of glial cells-have distinct expression profiles. Glial-specific expression of 653 RNAi constructs targeting 318 genes identified multiple factors associated with altered locomotor activity, circadian rhythmicity and/or responses to mechanical stress (bang sensitivity). Of interest, 1 of the relevant genes encodes a vesicle recycling factor, 4 encode secreted proteins and 3 encode membrane transporters. These results strongly support the idea that glia-neuron communication is vital for adult behavior.

Origins of glial cell populations in the insect nervous system

Glia of vertebrates and invertebrates alike represents a diverse population of cells in the nervous system, divided into numerous classes with different structural and functional characteristics. In insects, glia fall within three basic classes: surface, cell body, and neuropil glia. Due to the glial subclass-specific markers and genetic tools available in Drosophila, it is possible to establish the progenitor origin of these different populations and reconstruct their migration and differentiation during development. We review, and posit when appropriate, recently elucidated aspects of glial developmental dynamics. In particular, we focus on the relationships between mature glial subclasses of the larval nervous system (primary glia), born in the embryo, and glia of the adult (secondary glia), generated in the larva.

A fly's view of neuronal remodeling

Developmental neuronal remodeling is a crucial step in sculpting the final and mature brain connectivity in both vertebrates and invertebrates. Remodeling includes degenerative events, such as neurite pruning, that may be followed by regeneration to form novel connections during normal development. Drosophila provides an excellent model to study both steps of remodeling since its nervous system undergoes massive and stereotypic remodeling during metamorphosis. Although pruning has been widely studied, our knowledge of the molecular and cellular mechanisms is far from complete. Our understanding of the processes underlying regrowth is even more fragmentary. In this review, we discuss recent progress by focusing on three groups of neurons that undergo stereotypic pruning and regrowth during metamorphosis, the mushroom body γ neurons, the dendritic arborization neurons and the crustacean cardioactive peptide peptidergic neurons. By comparing and contrasting the mechanisms involved in remodeling of these three neuronal types, we highlight the common themes and differences as well as raise key questions for future investigation in the field. WIREs Dev Biol 2016, 5:618–635. doi: 10.1002/wdev.241

For further resources related to this article, please visit the WIREs website

Nitric Oxide as a Switching Mechanism between Axon Degeneration and Regrowth during Developmental Remodeling

During development, neurons switch among growth states, such as initial axon outgrowth, axon pruning, and regrowth. By studying the stereotypic remodeling of the Drosophila mushroom body (MB), we found that the heme-binding nuclear receptor E75 is dispensable for initial axon outgrowth of MB γ neurons but is required for their developmental regrowth. Genetic experiments and pharmacological manipulations on ex-vivo-cultured brains indicate that neuronally generated nitric oxide (NO) promotes pruning but inhibits regrowth. We found that high NO levels inhibit the physical interaction between the E75 and UNF nuclear receptors, likely accounting for its repression of regrowth. Additionally, NO synthase (NOS) activity is downregulated at the onset of regrowth, at least partially, by short inhibitory NOS isoforms encoded within the NOS locus, indicating how NO production could be developmentally regulated. Taken together, these results suggest that NO signaling provides a switching mechanism between the degenerative and regenerative states of neuronal remodeling.

Analysis of Glial Distribution in Drosophila Adult Brains

Neurons and glia are the two major cell types in the nervous system and work closely with each other to program neuronal interplay. Traditionally, neurons are thought to be the major cells that actively regulate processes like synapse formation, plasticity, and behavioral output. Glia, on the other hand, serve a more supporting role. To date, accumulating evidence has suggested that glia are active participants in virtually every aspect of neuronal function. Despite this, fundamental features of how glia interact with neurons, and their spatial relationships, remain elusive. Here, we describe the glial cell population in Drosophila adult brains. Glial cells extend and tightly associate their processes with major structures such as the mushroom body (MB), ellipsoid body (EB), and antennal lobe (AL) in the brain. Glial cells are distributed in a more concentrated manner in the MB. Furthermore, subsets of glia exhibit distinctive association patterns around different neuronal structures. Whereas processes extended by astrocyte-like glia and ensheathing glia wrap around the MB and infiltrate into the EB and AL, cortex glia stay where cell bodies of neurons are and remain outside of the synaptic regions structured by EB or AL.

Cdk5 regulates developmental remodeling of mushroom body neurons in Drosophila

BACKGROUND

During metamorphosis, axons and dendrites of the mushroom body (MB) in the Drosophila central brain are remodeled extensively to support the transition from larval to adult behaviors.

RESULTS

We show here that the neuronal cyclin-dependent kinase, Cdk5, regulates the timing and rate of mushroom body remodeling: reduced Cdk5 activity causes a delay in pruning of MB neurites, while hyperactivation accelerates it. We further show that Cdk5 cooperates with the ubiquitin-proteasome system in this process. Finally, we show that Cdk5 modulates the first overt step in neurite disassembly, dissolution of the neuronal tubulin cytoskeleton, and provide evidence that it also acts at additional steps of MB pruning.

CONCLUSIONS

These data show that Cdk5 regulates the onset and extent of remodeling of the Drosophila MB. Given the wide phylogenetic conservation of Cdk5, we suggest that it is likely to play a role in developmental remodeling in other systems, as well. Moreover, we speculate that the well-established role of Cdk5 in neurodegeneration may involve some of the same cellular mechanisms that it uses during developmental remodeling.

Sculpting neural circuits by axon and dendrite pruning

The assembly of functional neural circuits requires the combined action of progressive and regressive events. Regressive events encompass a variety of inhibitory developmental processes, including axon and dendrite pruning, which facilitate the removal of exuberant neuronal connections. Most axon pruning involves the removal of axons that had already made synaptic connections; thus, axon pruning is tightly associated with synapse elimination. In many instances, these developmental processes are regulated by the interplay between neurons and glial cells that act instructively during neural remodeling. Owing to the importance of axon and dendritic pruning, these remodeling events require precise spatial and temporal control, and this is achieved by a range of distinct molecular mechanisms. Disruption of these mechanisms results in abnormal pruning, which has been linked to brain dysfunction. Therefore, understanding the mechanisms of axon and dendritic pruning will be instrumental in advancing our knowledge of neural disease and mental disorders.

Long term ex vivo culturing of Drosophila brain as a method to live image pupal brains: insights into the cellular mechanisms of neuronal remodeling

Holometabolous insects, including Drosophila melanogaster, undergo complete metamorphosis that includes a pupal stage. During metamorphosis, the Drosophila nervous system undergoes massive remodeling and growth, that include cell death and large-scale axon and synapse elimination as well as neurogenesis, developmental axon regrowth, and formation of new connections. Neuronal remodeling is an essential step in the development of vertebrate and invertebrate nervous systems. Research on the stereotypic remodeling of Drosophila mushroom body (MB) γ neurons has contributed to our knowledge of the molecular mechanisms of remodeling but our knowledge of the cellular mechanisms remain poorly understood. A major hurdle in understanding various dynamic processes that occur during metamorphosis is the lack of time-lapse resolution. The pupal case and opaque fat bodies that enwrap the central nervous system (CNS) make live-imaging of the central brain in-vivo impossible. We have established an ex vivo long-term brain culture system that supports the development and neuronal remodeling of pupal brains. By optimizing culture conditions and dissection protocols, we have observed development in culture at kinetics similar to what occurs in vivo. Using this new method, we have obtained the first time-lapse sequence of MB γ neurons undergoing remodeling in up to a single cell resolution. We found that axon pruning is initiated by blebbing, followed by one-two nicks that seem to initiate a more widely spread axon fragmentation. As such, we have set up some of the tools and methodologies needed for further exploration of the cellular mechanisms of neuronal remodeling, not limited to the MB. The long-term ex vivo brain culture system that we report here could be used to study dynamic aspects of neurodevelopment of any Drosophila neuron.

Glial cells in neuronal development: recent advances and insights from Drosophila melanogaster

Glia outnumber neurons and are the most abundant cell type in the nervous system. Whereas neurons are the major carriers, transducers, and processors of information, glial cells, once considered mainly to play a passive supporting role, are now recognized for their active contributions to almost every aspect of nervous system development. Recently, insights from the invertebrate organism Drosophila melanogaster have advanced our knowledge of glial cell biology. In particular, findings on neuron-glia interactions via intrinsic and extrinsic mechanisms have shed light on the importance of glia during different stages of neuronal development. Here, we summarize recent advances in understanding the functions of Drosophila glia, which resemble their mammalian counterparts in morphology and function, neural stem-cell conversion, synapse formation, and developmental axon pruning. These discoveries reinforce the idea that glia are substantial players in the developing nervous system and further advance the understanding of mechanisms leading to neurodegeneration.