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


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 controls the decision between respiration and anaplerosis/gluconeogenesis in an ultrasensitive fashion. 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.


Enterocyte Purge and Rapid Recovery Is a Resilience Reaction of the Gut Epithelium to Pore-Forming Toxin Attack

By: Lee KZ, Lestradet M, Socha C, Schirmeier S, Schmitz A, Spenlé C, Lefebvre O, Keime C, Yamba WM, Bou Aoun R, Liegeois S, Schwab Y, Simon-Assmann P, Dalle F, Ferrandon D

Cell Host and Microbe | Volume: 20 | Issue: 6 | 716-730 |

Besides digesting nutrients, the gut protects the host against invasion by pathogens. Enterocytes may be subjected to damage by both microbial and host defensive responses, causing their death. Here, we report a rapid epithelial response that alleviates infection stress and protects the enterocytes from the action of microbial virulence factors. Intestinal epithelia exposed to hemolysin, a pore-forming toxin secreted by Serratia marcescens, undergo an evolutionarily conserved process of thinning followed by the recovery of their initial thickness within a few hours. In response to hemolysin attack, Drosophila melanogaster enterocytes extrude most of their apical cytoplasm, including damaged organelles such as mitochondria, yet do not lyse. We identify two secreted peptides, the expression of which requires CyclinJ, that mediate the recovery phase in which enterocytes regain their original shape and volume. Epithelial thinning and recovery constitute a fast and efficient response to intestinal infections, with pore-forming toxins acting as alarm signals.

Axon ensheathment and metabolic supply by glial cells in Drosophila.

By: Schirmeier S, Matzat T, Klämbt C

Brain Research | Volume: 1641 | 122–129

Neuronal function requires constant working conditions and a well-balanced supply of ions and metabolites. The metabolic homeostasis in the nervous system crucially depends on the presence of glial cells, which nurture and isolate neuronal cells. Here we review recent findings on how these tasks are performed by glial cells in the genetically amenable model organism Drosophila melanogaster. Despite the small size of its nervous system, which would allow diffusion of metabolites, a surprising division of labor between glial cells and neurons is evident. Glial cells are glycolytically active and transfer lactate and alanine to neurons. Neurons in turn do not require glycolysis but can use the glially provided compounds for their energy homeostasis. Besides feeding neurons, glial cells also insulate neuronal axons in a way similar to Remak fibers in the mammalian nervous system. The molecular mechanisms orchestrating this insulation require neuregulin signaling and resemble the mechanisms controlling glial differentiation in mammals surprisingly well. We hypothesize that metabolic cross talk and insulation of neurons by glial cells emerged early during evolution as two closely interlinked features in the nervous system.


The Drosophila blood-brain barrier as interface between neurons and hemolymph.

By: Schirmeier S, Klämbt C

Mechanisms of Development | Volume: 138 | 50–55

The blood–brain barrier is an evolutionary ancient structure that provides direct support and protection of the nervous system. In all systems, it establishes a tight diffusion barrier that hinders uncontrolled paracellular diffusion into the nervous system. In invertebrates, the blood–brain barrier separates the nervous system from the hemolymph. Thus, the barrier-forming cells need to actively import ions and nutrients into the nervous system. In addition, metabolic or environmental signals from the external world have to be transmitted across the barrier into the nervous system. The first blood–brain barrier that formed during evolution was most likely based on glial cells. Invertebrates as well as primitive vertebrates still have a purely glial-based blood–brain barrier. Here we review the development and function of the barrier forming glial cells at the example of Drosophila.

Glial glycolysis is essential for neuronal survival in Drosophila.

By: Volkenhoff A, Weiler A, Letzel M, Stehling M, Klämbt C, Schirmeier S

Cell Metabolism | Volume: 22 | Issue: 3 | 437-447 |

Neuronal information processing requires a large amount of energy, indicating that sugars and other metabolites must be efficiently delivered. However, reliable neuronal function also depends on the maintenance of a constant microenvironment in the brain. Therefore, neurons are efficiently separated from circulation by the blood-brain barrier, and their long axons are insulated by glial processes. At the example of the Drosophila brain, we addressed how sugar is shuttled across the barrier to nurture neurons. We show that glial cells of the blood-brain barrier specifically take up sugars and that their metabolism relies on glycolysis, which, surprisingly, is dispensable in neurons. Glial cells secrete alanine and lactate to fuel neuronal mitochondria, and lack of glial glycolysis specifically in the adult brain causes neurodegeneration. Our work implies that a global metabolic compartmentalization and coupling of neurons and glial cells is a conserved, fundamental feature of bilaterian nervous systems independent of their size.