Plasticity of Carbohydrate Transport at the Blood-Brain Barrier

By: McMullen, Ellen, Weiler A, Becker HM, Schirmeier S

Frontiers in Behavioral Neuroscience | Volume: 14 | 271 |

Neuronal function is highly energy demanding, requiring efficient transport of nutrients into the central nervous system (CNS). Simultaneously the brain must be protected from the influx of unwanted solutes. Most of the energy is supplied from dietary sugars, delivered from circulation via the blood-brain barrier (BBB). Therefore, selective transporters are required to shuttle metabolites into the nervous system where they can be utilized. The Drosophila BBB is formed by perineural and subperineurial glial cells, which effectively separate the brain from the surrounding hemolymph, maintaining a constant microenvironment. We identified two previously unknown BBB transporters, MFS3 (Major Facilitator Superfamily Transporter 3), located in the perineurial glial cells, and Pippin, found in both the perineurial and subperineurial glial cells. Both transporters facilitate uptake of circulating trehalose and glucose into the BBB-forming glial cells. RNA interference-mediated knockdown of these transporters leads to pupal lethality. However, null mutants reach adulthood, although they do show reduced lifespan and activity. Here, we report that both carbohydrate transport efficiency and resulting lethality found upon loss of MFS3 or Pippin are rescued via compensatory upregulation of Tret1-1, another BBB carbohydrate transporter, in Mfs3 and pippin null mutants, while RNAi-mediated knockdown is not compensated for. This means that the compensatory mechanisms in place upon mRNA degradation following RNA interference can be vastly different from those resulting from a null mutation.


AMPK-Regulated Astrocytic Lactate Shuttle Plays a Non-Cell-Autonomous Role in Neuronal Survival

By: Muraleedharan, Ranjithmenon, Gawali, Mruniya V , Tiwari, Durgesh, Sukumaran, Abitha , Oatman, Nicole, Anderson, Jane, Nardini, Diana, Bhuiyan, Mohammad Alfrad Nobel, Tkáč, Ivan, Ward, Amber Lynne , Kundu, Mondira, Waclaw, Ronald, Chow, Lionel M, Gross, Christina, Rao, Raghavendra, Schirmeier, Stefanie, Dasgupta, Biplab

Cell Reports | Volume: 32 | Issue: 9

Lactate is used as an energy source by producer cells or shuttled to neighboring cells and tissues. Both glucose and lactate fulfill the bioenergetic demand of neurons, the latter imported from astrocytes. The contribution of astrocytic lactate to neuronal bioenergetics and the mechanisms of astrocytic lactate production are incompletely understood. Through in vivo 1H magnetic resonance spectroscopy, 13C glucose mass spectroscopy, and electroencephalographic and molecular studies, here we show that the energy sensor AMP activated protein kinase (AMPK) regulates neuronal survival in a non-cell-autonomous manner. Ampk-null mice are deficient in brain lactate and are seizure prone. Ampk deletion in astroglia, but not neurons, causes neuronal loss in both mammalian and fly brains. Mechanistically, astrocytic AMPK phosphorylated and destabilized thioredoxin-interacting protein (TXNIP), enabling expression and surface translocation of the glucose transporter GLUT1, glucose uptake, and lactate production. Ampk loss in astrocytes causes TXNIP hyperstability, GLUT1 misregulation, inadequate glucose metabolism, and neuronal loss.

ATF4-Induced Warburg Metabolism Drives Over-Proliferation in Drosophila

By: Sorge, Sebastian, Theelke, Jonas, Yildirim, Kerem, Hertenstein, Helen, McMullen, Ellen, Müller, Stephan, Altbürger, Christian, Schirmeier, Stefanie, Lohmann, Ingrid

Cell Reports | Volume: 31 | Issue: 7 | 107659

Summary The mitochondrial electron transport chain (ETC) enables essential metabolic reactions; nonetheless, the cellular responses to defects in mitochondria and the modulation of signaling pathway outputs are not understood. We show that Notch signaling and ETC attenuation via knockdown of COX7a induces massive over-proliferation. The tumor-like growth is caused by a transcriptional response through the eIF2α-kinase PERK and ATF4, which activates the expression of metabolic enzymes, nutrient transporters, and mitochondrial chaperones. We find this stress adaptation to be beneficial for progenitor cell fitness, as it renders cells sensitive to proliferation induced by the Notch signaling pathway. Intriguingly, over-proliferation is not caused by transcriptional cooperation of Notch and ATF4, but it is mediated in part by pH changes resulting from the Warburg metabolism induced by ETC attenuation. Our results suggest that ETC function is monitored by the PERK-ATF4 pathway, which can be hijacked by growth-promoting signaling pathways, leading to oncogenic pathway activity.

A highly responsive pyruvate sensor reveals pathway-regulatory role of the mitochondrial pyruvate carrier MPC.

By: Arce-Molina R, Cortés-Molina F, Sandoval PY, Galaz A, Alegría K, Schirmeier S, Barros LF, San Martín A

eLife | Volume: 9

Mitochondria generate ATP and building blocks for cell growth and regeneration, using pyruvate as the main substrate. Here we introduce PyronicSF, a user-friendly GFP-based sensor of improved dynamic range that enables real-time subcellular quantitation of mitochondrial pyruvate transport, concentration and flux. We report that cultured mouse astrocytes maintain mitochondrial pyruvate in the low micromolar range, below cytosolic pyruvate, which means that the mitochondrial pyruvate carrier MPC is poised to exert ultrasensitive control on the balance between respiration and anaplerosis/gluconeogenesis. The functionality of the sensor in living tissue is demonstrated in the brain of Drosophila melanogaster larvae. Mitochondrial subpopulations are known to coexist within a given cell, which differ in their morphology, mobility, membrane potential, and vicinity to other organelles. The present tool can be used to investigate how mitochondrial diversity relates to metabolism, to study the role of MPC in disease, and to screen for small-molecule MPC modulators.


Live imaging using a FRET glucose sensor reveals glucose delivery to all cell types in the Drosophila brain

By: Volkenhoff A, Hirrlinger J, Kappel JM, Klämbt C, Schirmeier S

Journal of Insect Physiology |

All complex nervous systems are metabolically separated from circulation by a blood-brain barrier (BBB) that prevents uncontrolled leakage of solutes into the brain. Thus, all metabolites needed to sustain energy homeostasis must be transported across this BBB. In invertebrates, such as Drosophila, the major carbohydrate in circulation is the disaccharide trehalose and specific trehalose transporters are expressed by the glial BBB. Here we analyzed whether glucose is able to contribute to energy homeostasis in Drosophila. To study glucose influx into the brain we utilized a genetically encoded, FRET-based glucose sensor expressed in a cell type specific manner. When confronted with glucose all brain cells take up glucose within two minutes. In order to characterize the glucose transporter involved, we studied Drosophila Glut1, the homologue of which is primarily expressed by the BBB-forming endothelial cells and astrocytes in the mammalian nervous system. In Drosophila, however, Glut1 is expressed in neurons and is not found at the BBB. Thus, Glut1 cannot contribute to initial glucose uptake from the hemolymph. To test whether gap junctional coupling between the BBB forming cells and other neural cells contributes to glucose distribution we assayed these junctions using RNAi experiments and only found a minor contribution of gap junctions to glucose metabolism. Our results provide the entry point to further dissect the mechanisms underlying glucose distribution and offer new opportunities to understand brain metabolism.

Quorum‐sensing regulator RhlR but not its autoinducer RhlI enables Pseudomonas to evade opsonization

By: Haller S, Franchet A, Hakkim A, Chen J, Drenkard E, Yu S, Schirmeier S, Li Z , Martins N , Ausubel FM, Liegeois S, Ferrandon D

EMBO reports | e44880

When Drosophila melanogaster feeds on Pseudomonas aeruginosa, some bacteria cross the intestinal barrier and eventually proliferate in the hemocoel. This process is limited by hemocytes through phagocytosis. P. aeruginosa requires the quorum?sensing regulator RhlR to elude the cellular immune response of the fly. RhlI synthesizes the autoinducer signal that activates RhlR. Here, we show that rhlI mutants are unexpectedly more virulent than rhlR mutants, both in fly and in nematode intestinal infection models, suggesting that RhlR has RhlI-independent functions. We also report that RhlR protects P. aeruginosa from opsonization mediated by the Drosophila thioester-containing protein 4 (Tep4). RhlR mutant bacteria show higher levels of Tep4-mediated opsonization, as compared to rhlI mutants, which prevents lethal bacteremia in the Drosophila hemocoel. In contrast, in a septic model of infection, in which bacteria are introduced directly into the hemocoel, Tep4 mutant flies are more resistant to wild-type P. aeruginosa, but not to the rhlR mutant. Thus, depending on the infection route, the Tep4 opsonin can either be protective or detrimental to host defense.


Glial Cell Evolution: The Origins of a Lipid Store

By: Nave KA, Tzvetanova ID, Schirmeier S

Cell Metabolism | Volume: 26 | Issue: 5 | 701-702

In Drosophila, neuronal mitochondria that lack OXPHOS generate ROS-protective fatty acids and lipid droplets in associated glia. In this issue, Liu et al. (2017) demonstrate that neuronal lipid synthesis is driven by the glial lactate shuttle. This lipoprotein-dependent deposition of lipids may be at the origin of glial specializations evolving in vertebrates.

Insect models of central nervous system energy metabolism and its links to behavior

By: Rittschof CC, Schirmeier S

Glia |

Neuronal activity requires a vast amount of energy. Energy use in the brain is spatially and temporally dynamic, which reflects the changing activity of the neuronal circuits and might be important for modulating neuronal output. Much recent work has focused on understanding how brain glial cells take up nutrients from circulation and subsequently provide metabolic precursors to neurons. However, within the neurons, modulation of cellular metabolic pathway flux also regulates excitability and signaling. A coherent understanding of the links between energy availability and metabolism, neural signaling, and higher-level phenotypes like behavior requires a synthesis of the understanding of glial and neuronal metabolic dynamics. In the current review, we address this synthesis in the context of insect brain metabolism. Insects not only show evidence of a metabolic division of labor and plasticity in neural metabolism that closely resembles that observed in vertebrate species, there also seem to be direct links between brain metabolic dynamics and behavioral phenotypes. We summarize the current knowledge about the metabolic fuels available to the insect nervous system and how they are transported and distributed to the different neural cell types. We discuss the possibility of an ANLS-like metabolic division of labor between glial cells and neurons, and how it is regulated. We then discuss plasticity in flux through energy metabolic pathways in neurons, how flux is regulated, and how it influences neural signaling. We end by discussing how metabolic dynamics in the glia and neurons may interact to impact signaling.

Metabolite transport across the mammalian and insect brain diffusion barriers.

By: Weiler A, Volkenhoff A, Hertenstein H, Schirmeier S

Neurobiology of Disease | Volume: 107 | 15-31 |

The nervous system in higher vertebrates is separated from the circulation by a layer of specialized endothelial cells. It protects the sensitive neurons from harmful blood-derived substances, high and fluctuating ion concentrations, xenobiotics or even pathogens. To this end, the brain endothelial cells and their interlinking tight junctions build an efficient diffusion barrier. A structurally analogous diffusion barrier exists in insects, where glial cell layers separate the hemolymph from the neural cells. Both types of diffusion barriers, of course, also prevent influx of metabolites from the circulation. Because neuronal function consumes vast amounts of energy and necessitates influx of diverse substrates and metabolites, tightly regulated transport systems must ensure a constant metabolite supply. Here, we review the current knowledge about transport systems that carry key metabolites, amino acids, lipids and carbohydrates into the vertebrate and Drosophila brain and how this transport is regulated. Blood-brain and hemolymph-brain transport functions are conserved and we can thus use a simple, genetically accessible model system to learn more about features and dynamics of metabolite transport into the brain.