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.

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.

Metabolite transport across the mammalian and insect brain diffusion barriers.

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

Neurobiology of Disease |

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.


The Drosophila blood-brain barrier: Development and function of a glial endothelium.

By: Limmer S, Weiler A, Volkenhoff A, Babatz F, Klämbt C

Frontiers in Neuroscience | Volume: 8 | 365 |

The efficacy of neuronal function requires a well-balanced extracellular ion homeostasis and a steady supply with nutrients and metabolites. Therefore, all organisms equipped with a complex nervous system developed a so-called blood-brain barrier, protecting it from an uncontrolled entry of solutes, metabolites or pathogens. In higher vertebrates, this diffusion barrier is established by polarized endothelial cells that form extensive tight junctions, whereas in lower vertebrates and invertebrates the blood-brain barrier is exclusively formed by glial cells. Here, we review the development and function of the glial blood-brain barrier of Drosophila melanogaster. In the Drosophila nervous system, at least seven morphologically distinct glial cell classes can be distinguished. Two of these glial classes form the blood-brain barrier. Perineurial glial cells participate in nutrient uptake and establish a first diffusion barrier. The subperineurial glial (SPG) cells form septate junctions, which block paracellular diffusion and thus seal the nervous system from the hemolymph. We summarize the molecular basis of septate junction formation and address the different transport systems expressed by the blood-brain barrier forming glial cells.

Closing the gap between glia and neuroblast proliferation.

By: Limmer S, Klämbt C

Developmental Cell | Volume: 30 | 249–250

Reporting in this issue of Developmental Cell, Spéder and Brand (2014) show that gap junctions are required in blood-brain barrier glial cells to reactivate proliferation of quiescent neuroblasts. Gap junctions allow synchronous Ca2+ waves and control insulin-like protein Dipl6 expression and secretion to trigger neuroblast division.


Pseudomonas aeruginosa RhlR is required to neutralize the cellular immune response in a Drosophila melanogaster oral infection model

By: Limmer S, Haller S, Lee J, Yu S, Kocks C, Ausubel FM, Ferrandon D

Proceedings of the National Academy of… | Volume: 108 | Issue: 42 | 17378–17383

An in-depth mechanistic understanding of microbial infection necessitates a molecular dissection of host–pathogen relationships. Both Drosophila melanogaster and Pseudomonas aeruginosa have been intensively studied. Here, we analyze the infection of D. melanogaster by P. aeruginosa by using mutants in both host and pathogen. We show that orally ingested P. aeruginosa crosses the intestinal barrier and then proliferates in the hemolymph, thereby causing the infected flies to die of bacteremia. Host defenses against ingested P. aeruginosa included an immune deficiency (IMD) response in the intestinal epithelium, systemic Toll and IMD pathway responses, and a cellular immune response controlling bacteria in the hemocoel. Although the observed cellular and intestinal immune responses appeared to act throughout the course of the infection, there was a late onset of the systemic IMD and Toll responses. In this oral infection model, P. aeruginosa PA14 did not require its type III secretion system or other well-studied virulence factors such as the two-component response regulator GacA or the protease AprA for virulence. In contrast, the quorum-sensing transcription factor RhlR, but surprisingly not LasR, played a key role in counteracting the cellular immune response against PA14, possibly at an early stage when only a few bacteria are present in the hemocoel. These results illustrate the power of studying infection from the dual perspective of host and pathogen by revealing that RhlR plays a more complex role during pathogenesis than previously appreciated.