Glial Glycolysis Is Essential for Neuronal Survival in Drosophila

Association of aerobic glycolysis with the structural connectome reveals a benefit-risk balancing mechanism in the human brain

Aerobic glycolysis (AG), that is, the nonoxidative metabolism of glucose, contributes significantly to anabolic pathways, rapid energy generation, task-induced activity, and neuroprotection; yet high AG is also associated with pathological hallmarks such as amyloid-β deposition. An important yet unresolved question is whether and how the metabolic benefits and risks of brain AG is structurally shaped by connectome wiring. Using positron emission tomography and magnetic resonance imaging techniques as well as computational models, we investigate the relationship between brain AG and the macroscopic connectome. Specifically, we propose a weighted regional distance-dependent model to estimate the total axonal projection length of a brain node. This model has been validated in a macaque connectome derived from tract-tracing data and shows a high correspondence between experimental and estimated axonal lengths. When applying this model to the human connectome, we find significant associations between the estimated total axonal projection length and AG across brain nodes, with higher levels primarily located in the default-mode and prefrontal regions. Moreover, brain AG significantly mediates the relationship between the structural and functional connectomes. Using a wiring optimization model, we find that the estimated total axonal projection length in these high-AG regions exhibits a high extent of wiring optimization. If these high-AG regions are randomly rewired, their total axonal length and vulnerability risk would substantially increase. Together, our results suggest that high-AG regions have expensive but still optimized wiring cost to fulfill metabolic requirements and simultaneously reduce vulnerability risk, thus revealing a benefit-risk balancing mechanism in the human brain.

More Than Mortar: Glia as Architects of Nervous System Development and Disease

Glial cells are an essential component of the nervous system of vertebrates and invertebrates. In the human brain, glia are as numerous as neurons, yet the importance of glia to nearly every aspect of nervous system development has only been expounded over the last several decades. Glia are now known to regulate neural specification, synaptogenesis, synapse function, and even broad circuit function. Given their ubiquity, it is not surprising that the contribution of glia to neuronal disease pathogenesis is a growing area of research. In this review, we will summarize the accumulated evidence of glial participation in several distinct phases of nervous system development and organization-neural specification, circuit wiring, and circuit function. Finally, we will highlight how these early developmental roles of glia contribute to nervous system dysfunction in neurodevelopmental and neurodegenerative disorders.

Mitochondrial Complex I Mutations Predispose Drosophila to Isoflurane Neurotoxicity

BACKGROUND

General anesthetics influence mitochondrial homeostasis, placing individuals with mitochondrial disorders and possibly carriers of recessive mitochondrial mutations at increased risk of perioperative complications. In Drosophila, mutations in the ND23 subunit of complex I of the mitochondrial electron transport chain-analogous to mammalian NDUFS8-replicate key characteristics of Leigh syndrome, an inherited mitochondrial disorder. The authors used the ND23 mutant for testing the hypothesis that anesthetics have toxic potential in carriers of mitochondrial mutations.

METHODS

The authors exposed wild-type flies and ND23 mutant flies to behaviorally equivalent doses of isoflurane or sevoflurane in 5%, 21%, or 75% oxygen. The authors used percent mortality (mean ± SD, n ≥ 3) at 24 h after exposure as a readout of toxicity and changes in gene expression to investigate toxicity mechanisms.

RESULTS

Exposure of 10- to 13-day-old male ND23 flies to isoflurane in 5%, 21%, or 75% oxygen resulted in 16.0 ± 14.9% (n = 10), 48.2 ± 16.1% (n = 9), and 99.2 ± 2.0% (n = 10) mortality, respectively. Comparable mortality was observed in females. In contrast, under the same conditions, mortality was less than 5% for all male and female groups exposed to sevoflurane, except 10- to 13-day-old male ND23 flies with 9.6 ± 8.9% (n = 16) mortality. The mortality of 10- to 13-day-old ND23 flies exposed to isoflurane was rescued by neuron- or glia-specific expression of wild-type ND23. Isoflurane and sevoflurane differentially affected expression of antioxidant genes in 10- to 13-day-old ND23 flies. ND23 flies had elevated mortality from paraquat-induced oxidative stress compared with wild-type flies. The mortality of heterozygous ND23 flies exposed to isoflurane in 75% oxygen increased with age, resulting in 54.0 ± 19.6% (n = 4) mortality at 33 to 39 days old, and the percent mortality varied in different genetic backgrounds.

CONCLUSIONS

Mutations in the mitochondrial complex I subunit ND23 increase susceptibility to isoflurane-induced toxicity and to oxidative stress in Drosophila. Asymptomatic flies that carry ND23 mutations are sensitized to hyperoxic isoflurane toxicity by age and genetic background.

EDITOR’S PERSPECTIVE

Glial Metabolic Rewiring Promotes Axon Regeneration and Functional Recovery in the Central Nervous System

Axons in the mature central nervous system (CNS) fail to regenerate after axotomy, partly due to the inhibitory environment constituted by reactive glial cells producing astrocytic scars, chondroitin sulfate proteoglycans, and myelin debris. We investigated this inhibitory milieu, showing that it is reversible and depends on glial metabolic status. We show that glia can be reprogrammed to promote morphological and functional regeneration after CNS injury in Drosophila via increased glycolysis. This enhancement is mediated by the glia derived metabolites: L-lactate and L-2-hydroxyglutarate (L-2HG). Genetically/pharmacologically increasing or reducing their bioactivity promoted or impeded CNS axon regeneration. L-lactate and L-2HG from glia acted on neuronal metabotropic GABA_B receptors to boost cAMP signaling. Local application of L-lactate to injured spinal cord promoted corticospinal tract axon regeneration, leading to behavioral recovery in adult mice. Our findings revealed a metabolic switch to circumvent the inhibition of glia while amplifying their beneficial effects for treating CNS injuries.

Early lineage segregation of the retinal basal glia in the Drosophila eye disc

The retinal basal glia (RBG) is a group of glia that migrates from the optic stalk into the third instar larval eye disc while the photoreceptor cells (PR) are differentiating. The RBGs are grouped into three major classes based on molecular and morphological characteristics: surface glia (SG), wrapping glia (WG) and carpet glia (CG). The SGs migrate and divide. The WGs are postmitotic and wraps PR axons. The CGs have giant nucleus and extensive membrane extension that each covers half of the eye disc. In this study, we used lineage tracing methods to determine the lineage relationships among these glia subtypes and the temporal profile of the lineage decisions for RBG development. We found that the CG lineage segregated from the other RBG very early in the embryonic stage. It has been proposed that the SGs migrate under the CG membrane, which prevented SGs from contacting with the PR axons lying above the CG membrane. Upon passing the front of the CG membrane, which is slightly behind the morphogenetic furrow that marks the front of PR differentiation, the migrating SG contact the nascent PR axon, which in turn release FGF to induce SGs' differentiation into WG. Interestingly, we found that SGs are equally distributed apical and basal to the CG membrane, so that the apical SGs are not prevented from contacting PR axons by CG membrane. Clonal analysis reveals that the apical and basal RBG are derived from distinct lineages determined before they enter the eye disc. Moreover, the basal SG lack the competence to respond to FGFR signaling, preventing its differentiation into WG. Our findings suggest that this novel glia-to-glia differentiation is both dependent on early lineage decision and on a yet unidentified regulatory mechanism, which can provide spatiotemporal coordination of WG differentiation with the progressive differentiation of photoreceptor neurons.

Amalgam regulates the receptor tyrosine kinase pathway through Sprouty in glial cell development in the Drosophila larval brain

The receptor tyrosine kinase (RTK) pathway plays an essential role in development and disease by controlling cell proliferation and differentiation. Here, we profile the Drosophila larval brain by single-cell RNA-sequencing and identify Amalgam (Ama), which encodes a cell adhesion protein of the immunoglobulin IgLON family, as regulating the RTK pathway activity during glial cell development. Depletion of Ama reduces cell proliferation, affects glial cell type composition and disrupts the blood-brain barrier (BBB), which leads to hemocyte infiltration and neuronal death. We show that Ama depletion lowers RTK activity by upregulating Sprouty (Sty), a negative regulator of the RTK pathway. Knockdown of Ama blocks oncogenic RTK signaling activation in the Drosophila glioma model and halts malignant transformation. Finally, knockdown of a human ortholog of Ama, LSAMP, results in upregulation of SPROUTY2 in glioblastoma cell lines, suggesting that the relationship between Ama and Sty is conserved.

2,2',4,4'-tetrabromodiphenyl ether (BDE-47) induces wide metabolic changes including attenuated mitochondrial function and enhanced glycolysis in PC12 cells

Polybrominated diphenyl ethers (PBDEs) are extensively used as brominated flame retardants in various factory products. As environmental pollutants, the adverse effects of PBDEs on human health have been receiving considerable attention. However, the precise fundamental mechanisms of toxicity induced by PBDEs are still not fully understood. In this study, the mechanism of cytotoxicity induced by 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) was investigated by combining Seahorse XFp analysis and mass spectrometry-based metabolomics and flux approaches in PC12 cells, one of the most widely used neuron-like cell lines for investigating cytotoxic effects. The Seahorse results suggest that BDE-47 significantly attenuated mitochondrial respiration and enhanced glycolysis in PC12 cells. Additionally, metabolomics results revealed the reduction of TCA metabolites such as citrate, succinate, aconitate, malate, fumarate, and glutamate after BDE-47 exposure. Metabolic flux analysis showed that BDE-47 exposure reduced the oxidative metabolic capacity of mitochondria in PC12 cells. Furthermore, various altered metabolites were found in multiple metabolic pathways, especially in glycine-serine-threonine metabolism and glutathione metabolism. A total of 17 metabolic features were determined in order to distinguish potentially disturbed metabolite markers of BDE-47 exposure. Our findings provide possible biomarkers of cytotoxic effects induced by BDE-47 exposure, and elicit a deeper understanding of the intramolecular mechanisms that could be used in further studies to validate the potential neurotoxicity of PBDEs in vivo. Based on our results, therapeutic approaches targeting mitochondrial function and the glycolysis pathway may be a promising direction against PBDE exposure.

Oxidative Metabolism Drives Immortalization of Neural Stem Cells during Tumorigenesis

Metabolic reprogramming is a key feature of many cancers, but how and when it contributes to tumorigenesis remains unclear. Here we demonstrate that metabolic reprogramming induced by mitochondrial fusion can be rate-limiting for immortalization of tumor-initiating cells (TICs) and trigger their irreversible dedication to tumorigenesis. Using single-cell transcriptomics, we find that Drosophila brain tumors contain a rapidly dividing stem cell population defined by upregulation of oxidative phosphorylation (OxPhos). We combine targeted metabolomics and in vivo genetic screening to demonstrate that OxPhos is required for tumor cell immortalization but dispensable in neural stem cells (NSCs) giving rise to tumors. Employing an in vivo NADH/NAD⁺ sensor, we show that NSCs precisely increase OxPhos during immortalization. Blocking OxPhos or mitochondrial fusion stalls TICs in quiescence and prevents tumorigenesis through impaired NAD⁺ regeneration. Our work establishes a unique connection between cellular metabolism and immortalization of tumor-initiating cells.

Wrapping glia regulates neuronal signaling speed and precision in the peripheral nervous system of Drosophila

The functionality of the nervous system requires transmission of information along axons with high speed and precision. Conductance velocity depends on axonal diameter whereas signaling precision requires a block of electrical crosstalk between axons, known as ephaptic coupling. Here, we use the peripheral nervous system of Drosophila larvae to determine how glia regulates axonal properties. We show that wrapping glial differentiation depends on gap junctions and FGF-signaling. Abnormal glial differentiation affects axonal diameter and conductance velocity and causes mild behavioral phenotypes that can be rescued by a sphingosine-rich diet. Ablation of wrapping glia does not further impair axonal diameter and conductance velocity but causes a prominent locomotion phenotype that cannot be rescued by sphingosine. Moreover, optogenetically evoked locomotor patterns do not depend on conductance speed but require the presence of wrapping glial processes. In conclusion, our data indicate that wrapping glia modulates both speed and precision of neuronal signaling.

Polyploidy in the adult Drosophila brain

Long-lived cells such as terminally differentiated postmitotic neurons and glia must cope with the accumulation of damage over the course of an animal's lifespan. How long-lived cells deal with ageing-related damage is poorly understood. Here we show that polyploid cells accumulate in the adult fly brain and that polyploidy protects against DNA damage-induced cell death. Multiple types of neurons and glia that are diploid at eclosion, become polyploid in the adult Drosophila brain. The optic lobes exhibit the highest levels of polyploidy, associated with an elevated DNA damage response in this brain region. Inducing oxidative stress or exogenous DNA damage leads to an earlier onset of polyploidy, and polyploid cells in the adult brain are more resistant to DNA damage-induced cell death than diploid cells. Our results suggest polyploidy may serve a protective role for neurons and glia in adult Drosophila melanogaster brains.

Gut bacteria-derived peptidoglycan induces a metabolic syndrome-like phenotype via NF-κB-dependent insulin/PI3K signaling reduction in Drosophila renal system

Although microbiome-host interactions are usual at steady state, gut microbiota dysbiosis can unbalance the physiological and behavioral parameters of the host, mostly via yet not understood mechanisms. Using the Drosophila model, we investigated the consequences of a gut chronic dysbiosis on the host physiology. Our results show that adult flies chronically infected with the non-pathogenic Erwinia carotorova caotovora bacteria displayed organ degeneration resembling wasting-like phenotypes reminiscent of Metabolic Syndrome associated pathologies. Genetic manipulations demonstrate that a local reduction of insulin signaling consecutive to a peptidoglycan-dependent NF-κB activation in the excretory system of the flies is responsible for several of the observed phenotypes. This work establishes a functional crosstalk between bacteria-derived peptidoglycan and the immune NF-κB cascade that contributes to the onset of metabolic disorders by reducing insulin signal transduction. Giving the high degree of evolutionary conservation of the mechanisms and pathways involved, this study is likely to provide a helpful model to elucidate the contribution of altered intestinal microbiota in triggering human chronic kidney diseases.

Oligodendrocytes Provide Antioxidant Defense Function for Neurons by Secreting Ferritin Heavy Chain

An evolutionarily conserved function of glia is to provide metabolic and structural support for neurons. To identify molecules generated by glia and with vital functions for neurons, we used Drosophila melanogaster as a screening tool, and subsequently translated the findings to mice. We found that a cargo receptor operating in the secretory pathway of glia was essential to maintain axonal integrity by regulating iron buffering. Ferritin heavy chain was identified as the critical secretory cargo, required for the protection against iron-mediated ferroptotic axonal damage. In mice, ferritin heavy chain is highly expressed by oligodendrocytes and secreted by employing an unconventional secretion pathway involving extracellular vesicles. Disrupting the release of extracellular vesicles or the expression of ferritin heavy chain in oligodendrocytes causes neuronal loss and oxidative damage in mice. Our data point to a role of oligodendrocytes in providing an antioxidant defense system to support neurons against iron-mediated cytotoxicity.

Social reprogramming in ants induces longevity-associated glia remodeling

In social insects, workers and queens arise from the same genome but display profound differences in behavior and longevity. In Harpegnathos saltator ants, adult workers can transition to a queen-like state called gamergate, which results in reprogramming of social behavior and life-span extension. Using single-cell RNA sequencing, we compared the distribution of neuronal and glial populations before and after the social transition. We found that the conversion of workers into gamergates resulted in the expansion of neuroprotective ensheathing glia. Brain injury assays revealed that activation of the damage response gene Mmp1 was weaker in old workers, where the relative frequency of ensheathing glia also declined. On the other hand, long-lived gamergates retained a larger fraction of ensheathing glia and the ability to mount a strong Mmp1 response to brain injury into old age. We also observed molecular and cellular changes suggestive of age-associated decline in ensheathing glia in Drosophila.

Dynamic neural and glial responses of a head-specific model for traumatic brain injury in Drosophila

Traumatic brain injury (TBI) is the strongest environmental risk factor for the accelerated development of neurodegenerative diseases. There are currently no therapeutics to address this due to lack of insight into mechanisms of injury progression, which are challenging to study in mammalian models. Here, we have developed and extensively characterized a head-specific approach to TBI in Drosophila, a powerful genetic system that shares many conserved genes and pathways with humans. The Drosophila TBI (dTBI) device inflicts mild, moderate, or severe brain trauma by precise compression of the head using a piezoelectric actuator. Head-injured animals display features characteristic of mammalian TBI, including severity-dependent ataxia, life span reduction, and brain degeneration. Severe dTBI is associated with cognitive decline and transient glial dysfunction, and stimulates antioxidant, proteasome, and chaperone activity. Moreover, genetic or environmental augmentation of the stress response protects from severe dTBI-induced brain degeneration and life span deficits. Together, these findings present a tunable, head-specific approach for TBI in Drosophila that recapitulates mammalian injury phenotypes and underscores the ability of the stress response to mitigate TBI-induced brain degeneration.

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.

The Glial Perspective on Sleep and Circadian Rhythms

While neurons and circuits are almost unequivocally considered to be the computational units and actuators of behavior, a complete understanding of the nervous system must incorporate glial cells. Far beyond a copious but passive substrate, glial influence is inextricable from neuronal physiology, whether during developmental guidance and synaptic shaping or through the trophic support, neurotransmitter and ion homeostasis, cytokine signaling and immune function, and debris engulfment contributions that this class provides throughout an organism's life. With such essential functions, among a growing literature of nuanced roles, it follows that glia are consequential to behavior in adult animals, with novel genetic tools allowing for the investigation of these phenomena in living organisms. We discuss here the relevance of glia for maintaining circadian rhythms and also for serving functions of sleep.

Spatially-resolved metabolic profiling of living Drosophila in neurodegenerative conditions using 1H magic angle spinning NMR

Drosophila flies are versatile animal models for the study of gene mutations in neuronal pathologies. Their small size allows performing in vivo Magic Angle Spinning (MAS) experiments to obtain high-resolution 1H nuclear magnetic resonance (NMR) spectra. Here, we use spatially-resolved 1H high-resolution MAS NMR to investigate in vivo metabolite contents in different segments of the fly body. A comparative study of metabolic changes was performed for three neurodegenerative disorders: two cell-specific neuronal and glial models of Huntington disease (HD) and a model of glutamate excitotoxicity. It is shown that these pathologies are characterized by specific and sometimes anatomically localized variations in metabolite concentrations. In two cases, the modifications of 1H MAS NMR spectra localized in fly heads were significant enough to allow the creation of a predictive model.

Schwann cell interactions during the development of the peripheral nervous system

Schwann cells play a critical role in the development of the peripheral nervous system (PNS), establishing important relationships both with the extracellular milieu and other cell types, particularly neurons. In this review, we discuss various Schwann cell interactions integral to the proper establishment, spatial arrangement, and function of the PNS. We include signals that cascade onto Schwann cells from axons and from the extracellular matrix, bidirectional signals that help to establish the axo-glial relationship and how Schwann cells in turn support the axon. Further, we speculate on how Schwann cell interactions with other components of the developing PNS ultimately promote the complete construction of the peripheral nerve.

A single-cell transcriptomic atlas of the adult Drosophila ventral nerve cord

The Drosophila ventral nerve cord (VNC) receives and processes descending signals from the brain to produce a variety of coordinated locomotor outputs. It also integrates sensory information from the periphery and sends ascending signals to the brain. We used single-cell transcriptomics to generate an unbiased classification of cellular diversity in the VNC of five-day old adult flies. We produced an atlas of 26,000 high-quality cells, representing more than 100 transcriptionally distinct cell types. The predominant gene signatures defining neuronal cell types reflect shared developmental histories based on the neuroblast from which cells were derived, as well as their birth order. The relative position of cells along the anterior-posterior axis could also be assigned using adult Hox gene expression. This single-cell transcriptional atlas of the adult fly VNC will be a valuable resource for future studies of neurodevelopment and behavior.

Evolution of glial wrapping: A new hypothesis

Animals are able to move and react in numerous ways to external stimuli. Thus, environmental stimuli need to be detected, information must be processed and finally an output decision must be transmitted to the musculature to get the animal moving. All these processes depend on the nervous system which comprises an intricate neuronal network and many glial cells. In the last decades, a neurono-centric view on nervous system function channeled most of the scientific interest toward the analysis of neurons and neuronal functions. Neurons appeared early in animal evolution and the main principles of neuronal function from synaptic transmission to propagation of action potentials are conserved during evolution. In contrast, not much is known on the evolution of glial cells that were initially considered merely as static support cells. Although it is now accepted that glial cells have an equally important contribution as their neuronal counterpart to nervous system function, their evolutionary origin is unknown. Did glial cells appear several times during evolution? What were the first roles glial cells had to fulfil in the nervous system? What triggered the formation of the amazing diversity of glial morphologies and functions? Is there a possible mechanism that might explain the appearance of complex structures such as myelin in vertebrates? Here, we postulate a common evolutionary origin of glia and depict a number of selective forces that might have paved the way from a simple supporting cell to a wrapping and myelin forming glial cell.

Neuron-glia interaction in the Drosophila nervous system

Animals are able to move and react in manifold ways to external stimuli. Thus, environmental stimuli need to be detected, information must be processed, and, finally, an output decision must be transmitted to the musculature to get the animal moving. All these processes depend on the nervous system which comprises an intricate neuronal network and many glial cells. Glial cells have an equally important contribution in nervous system function as their neuronal counterpart. Manifold roles are attributed to glia ranging from controlling neuronal cell number and axonal pathfinding to regulation of synapse formation, function, and plasticity. Glial cells metabolically support neurons and contribute to the blood-brain barrier. All of the aforementioned aspects require extensive cell-cell interactions between neurons and glial cells. Not surprisingly, many of these processes are found in all phyla executed by evolutionarily conserved molecules. Here, we review the recent advance in understanding neuron-glia interaction in Drosophila melanogaster to suggest that work in simple model organisms will shed light on the function of mammalian glial cells, too.

Leptin mediates protection of hydrogen sulfide against 6-hydroxydopamine-induced Parkinson's disease: Involving enhancement in Warburg effect

BACKGROUND

Hydrogen sulfide (H₂S) has therapeutic effects on Parkinson's disease (PD). Warburg effect, namely aerobic glycolysis, is benefit to PD. Leptin, a hormone secreted in adipose, plays an important role in the treatment of PD.

OBJECTIVE

To determine whether the mechanism underlying protection of H₂S against PD is involved in promoting Warburg effect via upregulation of leptin.

METHODS

We set a PD model via unilateral intrastriatal injection of 6-hydroxydopamine (6-OHDA) in Sprague Dawley rat. PD-like behavior was analyzed by apomorphine-induced rotations, open field activity test, stepping test and cylinder test. Dopaminergic neurons were detected by immunohistochemistry. The expressions of Hexokinase-2, pyruvate kinase M-2, lactate dehydrogenase, pyruvate dehydrogenase kinase, pyruvate dehydrogenase, and leptin were measured by Western blot. Lactate dehydrogenase (LDHA) activity was monitored by ELISA. The lactate content was measured by lactate assay kit.

RESULTS

We showed that NaHS (a donor of H₂S) prevented 6-OHDA-induced PD-like behaviors as well as the loss of dopaminergic neurons. We also found that NaHS enhanced the Warburg effect and upregulated leptin expression in the substantia nigra of 6-OHDA-exposed rats. While, inhibited leptin signaling by OBR13-A reversed the protections of H₂S against 6-OHDA-exerted PD-like behaviors and the loss of dopaminergic neurons in the substantia nigra, and abolished H₂S-enhanced in the Warburg effect in the substantia nigra.

CONCLUSION

These data indicated that leptin mediates the protection of H₂S against PD, which involves enhancing the Warburg effect of the substantia nigra.

Intertwined ROS and Metabolic Signaling at the Neuron-Astrocyte Interface

Metabolism and redox signalling share critical nodes in the nervous system. In the last years, a series of major findings have challenged the current vision on how neural reactive oxygen species (ROS) are produced and handled in the nervous system. Once regarded as deleterious by-products, ROS are now shown to be essential for a metabolic and redox crosstalk. In turn, this coupling defines neural viability and function to control behaviour or leading to neurodegeneration when compromised. Findings like a different assembly of mitochondrial respiratory supercomplexes in neurons and astrocytes stands behind a divergent production of ROS in either cell type, more prominent in astrocytes. ROS levels are however tightly controlled by an antioxidant machinery in astrocytes, assumed as more efficient than that of neurons, to regulate redox signalling. By exerting this control in ROS abundance, metabolic functions are finely tuned in both neural cells. Further, a higher engagement of mitochondrial respiration and oxidative function in neurons, underpinned by redox equivalents supplied from the pentose phosphate pathway and from glia, differs from the otherwise strong glycolytic capacity of astrocytes. Here, we recapitulate major findings on how ROS and metabolism differ between neural cells but merge to define reciprocal signalling pathways, ultimately defining neural function and fate.

Aerobic Glycolysis in the Brain: Warburg and Crabtree Contra Pasteur

Information processing is onerous. Curiously, active brain tissue does not fully oxidize glucose and instead generates a local surplus of lactate, a phenomenon termed aerobic glycolysis. Why engage in inefficient ATP production by glycolysis when energy demand is highest and oxygen is plentiful? Aerobic glycolysis is associated to classic biochemical effects known by the names of Pasteur, Warburg and Crabtree. Here we discuss these three interdependent phenomena in brain cells, in light of high-resolution data of neuronal and astrocytic metabolism in culture, tissue slices and in vivo, acquired with genetically-encoded fluorescent sensors. These sensors are synthetic proteins that can be targeted to specific cell types and subcellular compartments, which change their fluorescence in response to variations in metabolite concentration. A major site of acute aerobic glycolysis is the astrocyte. In this cell, a Crabtree effect triggered by K⁺ coincides with a Warburg effect mediated by NO, superimposed on a slower longer-lasting Warburg effect caused by glutamate and possibly by NH₄⁺. The compounded outcome is that more fuel (lactate) and more oxygen are made available to neurons, on demand. Meanwhile neurons consume both glucose and lactate, maintaining a strict balance between glycolysis and respiration, commanded by the Na⁺ pump. We conclude that activity-dependent Warburg and Crabtree effects in brain tissue, and the resulting aerobic glycolysis, do not reflect inefficient energy generation but the marshalling of astrocytes for the purpose of neuronal ATP generation. It remains to be seen whether neurons contribute to aerobic glycolysis under physiological conditions.

Cellular Specificity and Inter-cellular Coordination in the Brain Bioenergetic System: Implications for Aging and Neurodegeneration

As an organ with a highly heterogenous cellular composition, the brain has a bioenergetic system that is more complex than peripheral tissues. Such complexities are not only due to the diverse bioenergetic phenotypes of a variety of cell types that differentially contribute to the metabolic profile of the brain, but also originate from the bidirectional metabolic communications and coupling across cell types. While brain energy metabolism and mitochondrial function have been extensively investigated in aging and age-associated neurodegenerative disorders, the role of various cell types and their inter-cellular communications in regulating brain metabolic and synaptic functions remains elusive. In this review, we summarize recent advances in differentiating bioenergetic phenotypes of neurons, astrocytes, and microglia in the context of their functional specificity, and their metabolic shifts upon aging and pathological conditions. Moreover, the metabolic coordination between the two most abundant cell populations in brain, neurons and astrocytes, is discussed regarding how they jointly establish a dynamic and responsive system to maintain brain bioenergetic homeostasis and to combat against threats such as oxidative stress, lipid toxicity, and neuroinflammation. Elucidating the mechanisms by which brain cells with distinctive bioenergetic phenotypes individually and collectively shape the bioenergetic system of the brain will provide rationale for spatiotemporally precise interventions to sustain a metabolic equilibrium that is resilient against synaptic dysfunction in aging and neurodegeneration.

Neuronal lactate levels depend on glia-derived lactate during high brain activity in Drosophila

Lactate/pyruvate transport between glial cells and neurons is thought to play an important role in how brain cells sustain the high-energy demand that neuronal activity requires. However, the in vivo mechanisms and characteristics that underlie the transport of monocarboxylates are poorly described. Here, we use Drosophila expressing genetically encoded FRET sensors to provide an ex vivo characterization of the transport of monocarboxylates in motor neurons and glial cells from the larval ventral nerve cord. We show that lactate/pyruvate transport in glial cells is coupled to protons and is more efficient than in neurons. Glial cells maintain higher levels of intracellular lactate generating a positive gradient toward neurons. Interestingly, during high neuronal activity, raised lactate in motor neurons is dependent on transfer from glial cells mediated in part by the previously described monocarboxylate transporter Chaski, providing support for in vivo glia-to-neuron lactate shuttling during neuronal activity.

The pentose phosphate pathway regulates chronic neuroinflammation and dopaminergic neurodegeneration

BACKGROUND

Metabolic dysfunction and neuroinflammation are increasingly implicated in Parkinson's disease (PD). The pentose phosphate pathway (PPP, a metabolic pathway parallel to glycolysis) converts glucose-6-phosphate into pentoses and generates ribose-5-phosphate and NADPH thereby governing anabolic biosynthesis and redox homeostasis. Brains and immune cells display high activity of glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the PPP. A postmortem study reveals dysregulation of G6PD enzyme in brains of PD patients. However, spatial and temporal changes in activity/expression of G6PD in PD remain undetermined. More importantly, it is unclear how dysfunction of G6PD and the PPP affects neuroinflammation and neurodegeneration in PD.

METHODS

We examined expression/activity of G6PD and its association with microglial activation and dopaminergic neurodegeneration in multiple chronic PD models generated by an intranigral/intraperitoneal injection of LPS, daily subcutaneous injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) for 6 days, or transgenic expression of A53T α-synuclein. Primary microglia were transfected with G6PD siRNAs and treated with lipopolysaccharide (LPS) to examine effects of G6PD knockdown on microglial activation and death of co-cultured neurons. LPS alone or with G6PD inhibitor(s) was administrated to mouse substantia nigra or midbrain neuron-glia cultures. While histological and biochemical analyses were conducted to examine microglial activation and dopaminergic neurodegeneration in vitro and in vivo, rotarod behavior test was performed to evaluate locomotor impairment in mice.

RESULTS

Expression and activity of G6PD were elevated in LPS-treated midbrain neuron-glia cultures (an in vitro PD model) and the substantia nigra of four in vivo PD models. Such elevation was positively associated with microglial activation and dopaminergic neurodegeneration. Furthermore, inhibition of G6PD by 6-aminonicotinamide and dehydroepiandrosterone and knockdown of microglial G6PD attenuated LPS-elicited chronic dopaminergic neurodegeneration. Mechanistically, microglia with elevated G6PD activity/expression produced excessive NADPH and provided abundant substrate to over-activated NADPH oxidase (NOX2) leading to production of excessive reactive oxygen species (ROS). Knockdown and inhibition of G6PD ameliorated LPS-triggered production of ROS and activation of NF-кB thereby dampening microglial activation.

CONCLUSIONS

Our findings indicated that G6PD-mediated PPP dysfunction and neuroinflammation exacerbated each other mediating chronic dopaminergic neurodegeneration and locomotor impairment. Insight into metabolic-inflammatory interface suggests that G6PD and NOX2 are potential therapeutic targets for PD.

Light-driven activation of mitochondrial proton-motive force improves motor behaviors in a Drosophila model of Parkinson's disease

Mitochondrial degeneration is considered one of the major causes of Parkinson's disease (PD). Improved mitochondrial functions are expected to be a promising therapeutic strategy for PD. In this study, we introduced a light-driven proton transporter, Delta-rhodopsin (dR), to Drosophila mitochondria, where the mitochondrial proton-motive force (Δp) and mitochondrial membrane potential are maintained in a light-dependent manner. The loss of the PD-associated mitochondrial gene CHCHD2 resulted in reduced ATP production, enhanced mitochondrial peroxide production and lower Ca2+-buffering activity in dopaminergic (DA) terminals in flies. These cellular defects were improved by the light-dependent activation of mitochondrion-targeted dR (mito-dR). Moreover, mito-dR reversed the pathology caused by the CHCHD2 deficiency to suppress α-synuclein aggregation, DA neuronal loss, and elevated lipid peroxidation in brain tissue, improving motor behaviors. This study suggests the enhancement of Δp by mito-dR as a therapeutic mechanism that ameliorates neurodegeneration by protecting mitochondrial functions.

Rapid metabolic shifts occur during the transition between hunger and satiety in Drosophila melanogaster

Metabolites are active controllers of cellular physiology, but their role in complex behaviors is less clear. Here we report metabolic changes that occur during the transition between hunger and satiety in Drosophila melanogaster. To analyze these data in the context of fruit fly metabolic networks, we developed Flyscape, an open-access tool. We show that in response to eating, metabolic profiles change in quick, but distinct ways in the heads and bodies. Consumption of a high sugar diet dulls the metabolic and behavioral differences between the fasted and fed state, and reshapes the way nutrients are utilized upon eating. Specifically, we found that high dietary sugar increases TCA cycle activity, alters neurochemicals, and depletes 1-carbon metabolism and brain health metabolites N-acetyl-aspartate and kynurenine. Together, our work identifies the metabolic transitions that occur during hunger and satiation, and provides a platform to study the role of metabolites and diet in complex behavior.

The relationship between metabolomic and behavioral changes is not well understood. Here, the authors analyze metabolome changes in D. melanogaster heads and bodies during hunger and satiety, and develop the Flyscape tool to visualize the resulting metabolic networks and integrate them with other omics data.

Genome-wide discovery of the daily transcriptome, DNA regulatory elements and transcription factor occupancy in the monarch butterfly brain

The Eastern North American monarch butterfly, Danaus plexippus, is famous for its spectacular seasonal long-distance migration. In recent years, it has also emerged as a novel system to study how animal circadian clocks keep track of time and regulate ecologically relevant daily rhythmic activities and seasonal behavioral outputs. However, unlike in Drosophila and the mouse, little work has been undertaken in the monarch to identify rhythmic genes at the genome-wide level and elucidate the regulation of their diurnal expression. Here, we used RNA-sequencing and Assay for Transposase-Accessible Chromatin (ATAC)-sequencing to profile the diurnal transcriptome, open chromatin regions, and transcription factor (TF) footprints in the brain of wild-type monarchs and of monarchs with impaired clock function, including Cryptochrome 2 (Cry2), Clock (Clk), and Cycle-like loss-of-function mutants. We identified 217 rhythmically expressed genes in the monarch brain; many of them were involved in the regulation of biological processes key to brain function, such as glucose metabolism and neurotransmission. Surprisingly, we found no significant time-of-day and genotype-dependent changes in chromatin accessibility in the brain. Instead, we found the existence of a temporal regulation of TF occupancy within open chromatin regions in the vicinity of rhythmic genes in the brains of wild-type monarchs, which is disrupted in clock deficient mutants. Together, this work identifies for the first time the rhythmic genes and modes of regulation by which diurnal transcription rhythms are regulated in the monarch brain. It also illustrates the power of ATAC-sequencing to profile genome-wide regulatory elements and TF binding in a non-model organism for which TF-specific antibodies are not yet available.

With a rich biology that includes a clock-regulated migratory behavior and a circadian clock possessing mammalian clock orthologues, the monarch butterfly is an unconventional system with broad appeal to study circadian and seasonal rhythms. While clockwork mechanisms and rhythmic behavioral outputs have been studied in this species, the rhythmic genes that regulate rhythmic daily and seasonal activities remain largely unknown. Likewise, the mechanisms regulating rhythmic gene expression have not been explored in the monarch. Here, we applied genome-wide sequencing approaches to identify genes with rhythmic diurnal expression in the monarch brain, revealing the coordination of key pathways for brain function. We also identified the monarch brain open chromatin regions and provide evidence that regulation of rhythmic gene expression does not occur through temporal regulation of chromatin opening but rather by the time-of-day dependent binding of transcription factors in cis-regulatory elements. Together, our data extend our knowledge of the molecular rhythmic pathways, which may prove important in understanding the mechanisms underlying the daily and seasonal biology of the migratory monarch butterflies.

Alcohol sedation in adult Drosophila is regulated by Cysteine proteinase-1 in cortex glia

Although numerous studies have demonstrated that neuronal mechanisms regulate alcohol-related behaviors, very few have investigated the direct role of glia in behavioral responses to alcohol. The results described here begin to fill this gap in the alcohol behavior and gliobiology fields. Since Drosophila exhibit conserved behavioral responses to alcohol and their CNS glia are similar to mammalian CNS glia, we used Drosophila to begin exploring the role of glia in alcohol behavior. We found that knockdown of Cysteine proteinase-1 (Cp1) in glia increased Drosophila alcohol sedation and that this effect was specific to cortex glia and adulthood. These data implicate Cp1 and cortex glia in alcohol-related behaviors. Cortex glia are functionally homologous to mammalian astrocytes and Cp1 is orthologous to mammalian Cathepsin L. Our studies raise the possibility that cathepsins may influence behavioral responses to alcohol in mammals via roles in astrocytes.

A positive feedback loop between Myc and aerobic glycolysis sustains tumor growth in a Drosophila tumor model

Cancer cells usually exhibit aberrant cell signaling and metabolic reprogramming. However, mechanisms of crosstalk between these processes remain elusive. Here, we show that in an in vivo tumor model expressing oncogenic Drosophila Homeodomain-interacting protein kinase (Hipk), tumor cells display elevated aerobic glycolysis. Mechanistically, elevated Hipk drives transcriptional upregulation of Drosophila Myc (dMyc; MYC in vertebrates) likely through convergence of multiple perturbed signaling cascades. dMyc induces robust expression of pfk2 (encoding 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase; PFKFB in vertebrates) among other glycolytic genes. Pfk2 catalyzes the synthesis of fructose-2,6-bisphosphate, which acts as a potent allosteric activator of Phosphofructokinase (Pfk) and thus stimulates glycolysis. Pfk2 and Pfk in turn are required to sustain dMyc protein accumulation post-transcriptionally, establishing a positive feedback loop. Disruption of the loop abrogates tumorous growth. Together, our study demonstrates a reciprocal stimulation of Myc and aerobic glycolysis and identifies the Pfk2-Pfk governed committed step of glycolysis as a metabolic vulnerability during tumorigenesis.

Cancer arises when cells in the body divide and grow excessively. These cells will often also take up more glucose than normal cells and break it down into another chemical known as lactate faster. This change to the chemical reactions happening within the cell, also called a metabolic change, is required to help produce the extra DNA, proteins and fatty molecules that it needs to grow.

Elevated levels of certain proteins can trigger the changes that lead to the growth of tumors. MYC (called dMyc in fruit flies) is one of these proteins. It controls cell division and increases the production of enzymes that break down glucose. Hipk is another protein that can induce tumor growth in fruit flies, but how it does so was unknown.

Here, Wong et al. show that high levels of Hipk boost glucose metabolism and accumulation of dMyc protein in fruit fly cells. They also describe the link between increased glucose metabolism and uncontrolled cell division.

First, fruit fly cells were fed a fluorescent molecule similar to glucose that cannot be broken down by the cells. This experiment established that glucose uptake increases in cells with too much Hipk. These cells also break down glucose faster, confirming that they have increased glucose metabolism.

Cells with high levels of Hipk also activate the genes that generate the enzymes involved in glucose breakdown, and increase the activity of the gene coding for dMyc. Levels of the dMyc protein are higher in these cells, which was shown by staining the cells with fluorescent molecules that specifically bind the dMyc protein. It is this buildup of dMyc protein that activates the genes coding for the enzymes responsible for glucose breakdown. PFK2 is one of these enzymes.

Finally, Wong et al. inhibited the production of the enzymes that are elevated in cells with high Hipk. Stopping the production of PFK2 prevents both tumor growth and the accumulation of dMyc protein. This shows that high levels of dMyc increase PFK2 levels, leading to further dMyc buildup, and creating a loop that links the uncontrolled cell division caused by too much dMyc and the shift to higher glucose metabolism.

These results highlight new potential targets for cancer therapy, showing that targeting glucose metabolism may reduce, or even stop, tumor growth.

Perineurial Barrier Glia Physically Respond to Alcohol in an Akap200-Dependent Manner to Promote Tolerance

Ethanol is the most common drug of abuse. It exerts its behavioral effects by acting on widespread neural circuits; however, its impact on glial cells is less understood. We show that Drosophila perineurial glia are critical for ethanol tolerance, a simple form of behavioral plasticity. The perineurial glia form the continuous outer cellular layer of the blood-brain barrier and are the interface between the brain and the circulation. Ethanol tolerance development requires the A kinase anchoring protein Akap200 specifically in perineurial glia. Akap200 tightly coordinates protein kinase A, actin, and calcium signaling at the membrane to control tolerance. Furthermore, ethanol causes a structural remodeling of the actin cytoskeleton and perineurial membrane topology in an Akap200-dependent manner, without disrupting classical barrier functions. Our findings reveal an active molecular signaling process in the cells at the blood-brain interface that permits a form of behavioral plasticity induced by ethanol.

Glial Ca2+signaling links endocytosis to K+ buffering around neuronal somas to regulate excitability

Glial-neuronal signaling at synapses is widely studied, but how glia interact with neuronal somas to regulate their activity is unclear. Drosophila cortex glia are restricted to brain regions devoid of synapses, providing an opportunity to characterize interactions with neuronal somas. Mutations in the cortex glial NCKX^zydeco elevate basal Ca²⁺, predisposing animals to seizure-like behavior. To determine how cortex glial Ca²⁺ signaling controls neuronal excitability, we performed an in vivo modifier screen of the NCKX^zydeco seizure phenotype. We show that elevation of glial Ca²⁺ causes hyperactivation of calcineurin-dependent endocytosis and accumulation of early endosomes. Knockdown of sandman, a K_2P channel, recapitulates NCKX^zydeco seizures. Indeed, sandman expression on cortex glial membranes is substantially reduced in NCKX^zydeco mutants, indicating enhanced internalization of sandman predisposes animals to seizures. These data provide an unexpected link between glial Ca²⁺ signaling and the well-known role of glia in K⁺ buffering as a key mechanism for regulating neuronal excitability.

Glial ensheathment of the somatodendritic compartment regulates sensory neuron structure and activity

Sensory neurons perceive environmental cues and are important of organismal survival. Peripheral sensory neurons interact intimately with glial cells. While the function of axonal ensheathment by glia is well studied, less is known about the functional significance of glial interaction with the somatodendritic compartment of neurons. Herein, we show that three distinct glia cell types differentially wrap around the axonal and somatodendritic surface of the polymodal dendritic arborization (da) neuron of the Drosophila peripheral nervous system for detection of thermal, mechanical, and light stimuli. We find that glial cell-specific loss of the chromatin modifier gene dATRX in the subperineurial glial layer leads to selective elimination of somatodendritic glial ensheathment, thus allowing us to investigate the function of such ensheathment. We find that somatodendritic glial ensheathment regulates the morphology of the dendritic arbor, as well as the activity of the sensory neuron, in response to sensory stimuli. Additionally, glial ensheathment of the neuronal soma influences dendritic regeneration after injury.

From Early to Late Neurogenesis: Neural Progenitors and the Glial Niche from a Fly's Point of View

Drosophila melanogaster is an important model organism used to study the brain development of organisms ranging from insects to mammals. The central nervous system in fruit flies is formed primarily in two waves of neurogenesis, one of which occurs in the embryo and one of which occurs during larval stages. In order to understand neurogenesis, it is important to research the behavior of progenitor cells that give rise to the neural networks which make up the adult nervous system. This behavior has been shown to be influenced by different factors including interactions with other cells within the progenitor niche, or local tissue microenvironment. Glial cells form a crucial part of this niche and play an active role in the development of the brain. Although in the early years of neuroscience it was believed that glia were simply scaffolding for neurons and passive components of the nervous system, their importance is nowadays recognized. Recent discoveries in progenitors and niche cells have led to new understandings of how the developing brain shapes its diverse regions. In this review, we attempt to summarize the distinct neural progenitors and glia in the Drosophila melanogaster central nervous system, from embryo to late larval stages, and make note of homologous features in mammals. We also outline the recent advances in this field in order to define the impact that glial cells have on progenitor cell niches, and we finally emphasize the importance of communication between glia and progenitor cells for proper brain formation.

Perturbation of IIS/TOR signaling alters the landscape of sex-differential gene expression in Drosophila

Background

The core functions of the insulin/insulin-like signaling and target of rapamycin (IIS/TOR) pathway are nutrient sensing, energy homeostasis, growth, and regulation of stress responses. This pathway is also known to interact directly and indirectly with the sex determination regulatory hierarchy. The IIS/TOR pathway plays a role in directing sexually dimorphic traits, including dimorphism of growth, metabolism, stress and behavior. Previous studies of sexually dimorphic gene expression in the adult head, which includes both nervous system and endocrine tissues, have revealed variation in sex-differential expression, depending in part on genotype and environment. To understand the degree to which the environmentally responsive insulin signaling pathway contributes to sexual dimorphism of gene expression, we examined the effect of perturbation of the pathway on gene expression in male and female Drosophila heads.

Results

Our data reveal a large effect of insulin signaling on gene expression, with greater than 50% of genes examined changing expression. Males and females have a shared gene expression response to knock-down of InR function, with significant enrichment for pathways involved in metabolism. Perturbation of insulin signaling has a greater impact on gene expression in males, with more genes changing expression and with gene expression differences of larger magnitude. Primarily as a consequence of the response in males, we find that reduced insulin signaling results in a striking increase in sex-differential expression. This includes sex-differences in expression of immune, defense and stress response genes, genes involved in modulating reproductive behavior, genes linking insulin signaling and ageing, and in the insulin signaling pathway itself.

Conclusions

Our results demonstrate that perturbation of insulin signaling results in thousands of genes displaying sex differences in expression that are not differentially expressed in control conditions. Thus, insulin signaling may play a role in variability of somatic, sex-differential expression. The finding that perturbation of the IIS/TOR pathway results in an altered landscape of sex-differential expression suggests a role of insulin signaling in the physiological underpinnings of trade-offs, sexual conflict and sex differences in expression variability.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5308-3) contains supplementary material, which is available to authorized users.

The Drosophila Blood-Brain Barrier Adapts to Cell Growth by Unfolding of Pre-existing Septate Junctions

The blood-brain barrier is crucial for nervous system function. It is established early during development and stays intact during growth of the brain. In invertebrates, septate junctions are the occluding junctions of this barrier. Here, we used Drosophila to address how septate junctions grow during larval stages when brain size increases dramatically. We show that septate junctions are preassembled as long, highly folded strands during embryonic stages, connecting cell vertices. During subsequent cell growth, these corrugated strands are stretched out and stay intact during larval life with very little protein turnover. The G-protein coupled receptor Moody orchestrates the continuous organization of junctional strands in a process requiring F-actin. Consequently, in moody mutants, septate junction strands cannot properly stretch out during cell growth. To compensate for the loss of blood-brain barrier function, moody mutants form interdigitating cell-cell protrusions, resembling the evolutionary ancient barrier type found in primitive vertebrates or invertebrates such as cuttlefish.

Maintenance mechanisms of circuit-integrated axons

Adult, circuit-integrated neurons must be maintained and supported for the life span of their host. The attenuation of either maintenance or plasticity leads to impaired circuit function and ultimately to neurodegenerative disorders. Over the last few years, significant discoveries of molecular mechanisms were made that mediate the formation and maintenance of axons. Here, we highlight intrinsic and extrinsic mechanisms that ensure the health and survival of axons. We also briefly discuss examples of mutations associated with impaired axonal maintenance identified in specific neurological conditions. A better understanding of these mechanisms will therefore help to define targets for therapeutic interventions.

The sulfite oxidase Shopper controls neuronal activity by regulating glutamate homeostasis in Drosophila ensheathing glia

Specialized glial subtypes provide support to developing and functioning neural networks. Astrocytes modulate information processing by neurotransmitter recycling and release of neuromodulatory substances, whereas ensheathing glial cells have not been associated with neuromodulatory functions yet. To decipher a possible role of ensheathing glia in neuronal information processing, we screened for glial genes required in the Drosophila central nervous system for normal locomotor behavior. Shopper encodes a mitochondrial sulfite oxidase that is specifically required in ensheathing glia to regulate head bending and peristalsis. shopper mutants show elevated sulfite levels affecting the glutamate homeostasis which then act on neuronal network function. Interestingly, human patients lacking the Shopper homolog SUOX develop neurological symptoms, including seizures. Given an enhanced expression of SUOX by oligodendrocytes, our findings might indicate that in both invertebrates and vertebrates more than one glial cell type may be involved in modulating neuronal activity.

Differential expression of Öbek controls ploidy in the Drosophila blood-brain barrier

During development, tissue growth is mediated by either cell proliferation or cell growth, coupled with polyploidy. Both strategies are employed by the cell types that make up the Drosophila blood-brain barrier. During larval growth, the perineurial glia proliferate, whereas the subperineurial glia expand enormously and become polyploid. Here, we show that the level of ploidy in the subperineurial glia is controlled by the N-terminal asparagine amidohydrolase homolog Öbek, and high Öbek levels are required to limit replication. In contrast, perineurial glia express moderate levels of Öbek, and increased Öbek expression blocks their proliferation. Interestingly, other dividing cells are not affected by alteration of Öbek expression. In glia, Öbek counteracts fibroblast growth factor and Hippo signaling to differentially affect cell growth and number. We propose a mechanism by which growth signals are integrated differentially in a glia-specific manner through different levels of Öbek protein to adjust cell proliferation versus endoreplication in the blood-brain barrier.

Metabolic Analysis of Drosophila melanogaster Larval and Adult Brains

This protocol describes a method for measuring the metabolism in Drosophila melanogaster larval and adult brains. Quantifying metabolism in whole organs provides a tissue-level understanding of energy utilization that cannot be captured when analyzing primary cells and cell lines. While this analysis is ex vivo, it allows for the measurement from a number of specialized cells working together to perform a function in one tissue and more closely models the in vivo organ. Metabolic reprogramming has been observed in many neurological diseases, including neoplasia, and neurodegenerative diseases. This protocol was developed to assist the D. melanogaster community's investigation of metabolism in neurological disease models using a commercially available metabolic analyzer. Measuring metabolism of whole brains in the metabolic analyzer is challenging due to the geometry of the brain. This analyzer requires samples to remain at the bottom of a 96-well plate. Cell samples and tissue punches can adhere to the surface of the cell plate or utilize spheroid plates, respectively. However, the spherical, three-dimensional shape of D. melanogaster brains prevents the tissue from adhering to the plate. This protocol requires a specially designed and manufactured micro-tissue restraint that circumvents this problem by preventing any movement of the brain while still allowing metabolic measurements from the analyzer's two solid-state sensor probes. Oxygen consumption and extracellular acidification rates are reproducible and sensitive to a treatment with metabolic inhibitors. With a minor optimization, this protocol can be adapted for use with any whole tissue and/or model system, provided that the sample size does not exceed the chamber generated by the restraint. While basal metabolic measurements and an analysis after a treatment with mitochondrial inhibitors are described within this protocol, countless experimental conditions, such as energy source preference and rearing environment, could be interrogated.

Regulation of Carbohydrate Energy Metabolism in Drosophila melanogaster

Carbohydrate metabolism is essential for cellular energy balance as well as for the biosynthesis of new cellular building blocks. As animal nutrient intake displays temporal fluctuations and each cell type within the animal possesses specific metabolic needs, elaborate regulatory systems are needed to coordinate carbohydrate metabolism in time and space. Carbohydrate metabolism is regulated locally through gene regulatory networks and signaling pathways, which receive inputs from nutrient sensors as well as other pathways, such as developmental signals. Superimposed on cell-intrinsic control, hormonal signaling mediates intertissue information to maintain organismal homeostasis. Misregulation of carbohydrate metabolism is causative for many human diseases, such as diabetes and cancer. Recent work in Drosophila melanogaster has uncovered new regulators of carbohydrate metabolism and introduced novel physiological roles for previously known pathways. Moreover, genetically tractable Drosophila models to study carbohydrate metabolism-related human diseases have provided new insight into the mechanisms of pathogenesis. Due to the high degree of conservation of relevant regulatory pathways, as well as vast possibilities for the analysis of gene-nutrient interactions and tissue-specific gene function, Drosophila is emerging as an important model system for research on carbohydrate metabolism.

The Axon-Myelin Unit in Development and Degenerative Disease

Axons are electrically excitable, cable-like neuronal processes that relay information between neurons within the nervous system and between neurons and peripheral target tissues. In the central and peripheral nervous systems, most axons over a critical diameter are enwrapped by myelin, which reduces internodal membrane capacitance and facilitates rapid conduction of electrical impulses. The spirally wrapped myelin sheath, which is an evolutionary specialisation of vertebrates, is produced by oligodendrocytes and Schwann cells; in most mammals myelination occurs during postnatal development and after axons have established connection with their targets. Myelin covers the vast majority of the axonal surface, influencing the axon's physical shape, the localisation of molecules on its membrane and the composition of the extracellular fluid (in the periaxonal space) that immerses it. Moreover, myelinating cells play a fundamental role in axonal support, at least in part by providing metabolic substrates to the underlying axon to fuel its energy requirements. The unique architecture of the myelinated axon, which is crucial to its function as a conduit over long distances, renders it particularly susceptible to injury and confers specific survival and maintenance requirements. In this review we will describe the normal morphology, ultrastructure and function of myelinated axons, and discuss how these change following disease, injury or experimental perturbation, with a particular focus on the role the myelinating cell plays in shaping and supporting the axon.

Axonal Ensheathment in the Nervous System of Lamprey: Implications for the Evolution of Myelinating Glia

In the nervous system, myelination of axons enables rapid impulse conduction and is a specialized function of glial cells. Myelinating glia are the last cell type to emerge in the evolution of vertebrate nervous systems, presumably in ancient jawed vertebrates (gnathostomata) because jawless vertebrates (agnathans) lack myelin. We have hypothesized that, in these unmyelinated species, evolutionary progenitors of myelinating cells must have existed that should still be present in contemporary agnathan species. Here, we used advanced electron microscopic techniques to reveal axon-glia interactions in the sea lamprey Petromyzon marinus By quantitative assessment of the spinal cord and the peripheral lateral line nerve, we observed a marked maturation-dependent growth of axonal calibers. In peripheral nerves, all axons are ensheathed by glial cells either in bundles or, when larger than the threshold caliber of 3 μm, individually. The ensheathing glia are covered by a basal lamina and express SoxE-transcription factors, features of mammalian Remak-type Schwann cells. In larval lamprey, the ensheathment of peripheral axons leaves gaps that are closed in adults. CNS axons are also covered to a considerable extent by glial processes, which contain a high density of intermediate filaments, glycogen particles, large lipid droplets, and desmosomes, similar to mammalian astrocytes. Indeed, by in situ hybridization, these glial cells express the astrocyte marker Aldh1l1 Specimens were of unknown sex. Our observations imply that radial sorting, ensheathment, and presumably also metabolic support of axons are ancient functions of glial cells that predate the evolutionary emergence of myelin in jawed vertebrates.SIGNIFICANCE STATEMENT We used current electron microscopy techniques to examine axon-glia units in a nonmyelinated vertebrate species, the sea lamprey. In the PNS, lamprey axons are fully ensheathed either individually or in bundles by cells ortholog to Schwann cells. In the CNS, axons associate with astrocyte orthologs, which contain glycogen and lipid droplets. We suggest that ensheathment, radial sorting, and metabolic support of axons by glial cells predate the evolutionary emergence of myelin in ancient jawed vertebrates.

Role of Glial Immunity in Lifespan Determination: A Drosophila Perspective

Increasing body of evidence indicates that proper glial function plays an important role in neuroprotection and in organismal physiology throughout lifespan. Work done in the model organism Drosophila melanogaster has revealed important aspects of glial cell biology in the contexts of longevity and neurodegeneration. In this mini review, we summarize recent findings from work done in the fruit fly Drosophila about the role of glia in maintaining a healthy status during animal’s life and discuss the involvement of glial innate immune pathways in lifespan and neurodegeneration. Overactive nuclear factor kappa B (NF-κB) pathways and defective phagocytosis appear to be major contributors to lifespan shortening and neuropathology. Glial NF-κB silencing on the other hand, extends lifespan possibly through an immune–neuroendocrine axis. Given the evolutionary conservation of NF-κB innate immune signaling and of macrophage ontogeny across fruit flies, rodents, and humans, the above observations in glia could potentially support efforts for therapeutic interventions targeting to ameliorate age-related pathologies.