Name: Dissection of Third Instar Drosophila Larva for Brain Metabolite Imaging
Uploaded: 2023-10-27T12:00:00
Description: Watch this Scientific Journal Video about Dissection of Third Instar Drosophila Larva for Brain Metabolite Imaging at JoVE.com

dissection of third instar drosophila larva for brain metabolite imaging

Monitoring Brain Metabolite Transport in Drosophila Using FRET Sensors

The brain has high energy requirements due to the high cost of restoring ion gradients in neurons caused by neuronal electric signal generation and transmission, as well as synaptic transmission1,2. This high energy demand has long been thought to be met by the continuous oxidation of glucose to produce ATP3. Specific transporters at the blood-brain barrier transfer the glucose in the blood to the brain. Constant glycemic levels ensure that the brain receives a steady supply of glucose4. Interestingly, growing experimental evidence suggests that molecules derived from glucose metabolism, such as lactate and pyruvate, play an important role in the brain cells&#39; energy production5,6. However, there is still some debate about how important these molecules are for energy production and which cells in the brain produce or use them7,8. The lack of appropriate molecular tools with the high temporal and spatial resolution required for this task is a significant issue that has prevented this controversy from being completely resolved.
The development and application of several engineered fluorescent metabolic sensors have resulted in a remarkable increase in our understanding of where and how metabolites are produced and used, as well as how the metabolic fluxes occur during basal and high neuronal activity9. Genetically encoded metabolic sensors based on F&#246;rster resonance energy transfer (FRET) microscopy, such as ATeam (ATP), FLII12Pglu700&#181;&#948;6 (glucose), Laconic (lactate), and Pyronic (pyruvate), have contributed to our understanding of brain energy metabolism10,11,12,13. However, due to the high costs and sophisticated equipment required to conduct experiments on live animals or tissues, results in vertebrate models are still primarily limited to cell cultures (glial cells and neurons).
The emerging use of the Drosophila model to express these sensors has revealed that key metabolic features are conserved across species and their function can be easily addressed with this tool. More importantly, the Drosophila model has shed light on how glucose and lactate/pyruvate are transported and metabolized in the fly brain, the link between monocarboxylate consumption and memory formation, and the remarkable demonstration of how increases in neural activity and metabolic flux overlap14,15,16,17. The method presented here for measuring monocarboxylate, glucose, and ATP levels using genetically encoded FRET sensors expressed in the larval brain allows researchers to learn more about how the brain of Drosophila uses energy, which can be applied to the brains of other animals.
We show that this method is effective for detecting lactate and glucose in glial cells and neurons, and that a monocarboxylate transporter (Chaski) is involved in lactate import into glial cells. We also demonstrate a simple method for studying metabolite changes during increased neuronal activity, which can be easily induced by bath application of a GABAA receptor antagonist. Finally, we show that this methodology can be used to measure monocarboxylate and glucose transport in other metabolically significant tissues, such as fat bodies.

Author Spotlight: Advances in Brain Energy Metabolism Research Using the Drosophila Model

Visualizing Monocarboxylates and Other Relevant Metabolites in the Ex Vivo Drosophila Larval Brain Using Genetically Encoded Sensors

The brain is a tissue with a high energy consumption that mostly uses glucose as fuel. On the other hand, experimental data suggests that other metabolites like ketone bodies and monocarboxylates might be useful as energy sources. However, there is a still debate on which brain cells are the producers and primary consumers.Lactate has been demonstrated to be a relevant metabolite in vertebrates to drive brain activity in vivo. Our group and others have shown in Drosophila the importance of monocarboxylate molecules to fuel the high energy requirements in neurons, as well as the need of glial-derived lactate and ketone bodies in memory formation. We have described monocarboxylate transporters in the Drosophila brain and determined that the transfer of lactate from glial cells to neurons is necessary to maintain the neural activity during high-demand periods.The main advantage is to have the possibility to determine the intracellular dynamic of glucose and its metabolites, such as lactate, pyruvate, and ATB in glial cells and neurons during basal and high neural activity. Additionally, to measure the transfer of these molecules in other living tissues, such as the fat bodies. The use of the Drosophila as in vivo model for the study of brain energy metabolism using a simple setup.This will allow the modeling of metabolic diseases or to better understand the energy management in the normal brain or during the development of neurodegenerative diseases.

Watch this Scientific Journal Video about Dissection of Third Instar Drosophila Larva for Brain Metabolite Imaging at JoVE.com

Dissection of Third Instar Drosophila Larva for Brain Metabolite Imaging

To begin, place 25 millimeter glass cover slips into the wells of a six-well cell culture plate. Place a drop of polylysine in the center of each cover slip. After incubation is complete, wash each cover slip three times with distilled water.Then wash the cover slips two times with a calcium-free saline solution. Next, install the covers in a recording chamber and fill it with calcium-free saline. Collect the wandering third instar larvae from the desired genetic cross of drosophila.Transfer the larvae to a dish well, then wash the larvae three times with distilled water. Place the larvae in another glass dissection dish well containing 750 microliters of ice-cold, calcium-free saline solution. Now place a larvae under a stereo microscope.With a pair of forceps, make a transverse cut across the back of the abdomen. Push the jaw with the forceps to turn the larvae inside out. Observe the ventral nerve cord next to the jaw.Carefully remove the imaginal discs and remaining brain coddle tissues. Next, separate the ventral nerve cord with the central brain and optic lobes from the rest of the tissue. Transfer the ventral nerve cord to the recording chamber containing calcium-free saline solution.To avoid interference, attach the remaining nerves to the bottom of the cover slip with forceps. To perform experiments on fat bodies, proceed to isolate the fat tissues from the turned-out larva. Place the isolated fat bodies expressing the fret sensors in the recording chamber.To acquire images of tissues expressing sensors, turn on the illumination system of the microscope. Place the recording chamber containing ventral nerve cords or fat bodies on the stage of a fluorescent microscope. To view the GCaMP6f fluorescence, set the excitation wavelength to 488 nanometers.For metabolites such as laconic, pyronic, ATP, or glucose sensors, set the excitation to 440 nanometers. Now set the image acquisition at regular intervals for GCaMP6f and metabolite sensor. Place the water immersion objective, ensuring it remains submerged.Next, connect the recording chamber to a perfusion system. Use a low-flux peristaltic pump to extract the liquid from the chamber and maintain a constant flow of three milliliters per minute. Position the solution containing tubes 25 centimeters above the microscope stage.Before any stimulation, obtain a stable fluorescence baseline by allowing the recording solution to flow for five to 10 minutes. Now, replace the flow solution with the stimulation solution containing glucose, pyruvate, or lactate for five minutes. Capture the images of the stimulated ventral nerve cord that was exposed to 80 micromolar picrotoxin to assess neuronal activity.Laconic sensors in both glial cells and motor neurons respond to one millimolar lactate at a similar rate at the start of the pulse. However, the motor neurons reach a higher increase over the baseline during the five minute pulse. The glucose sensor's signal increased at a similar rate in the glial cells and neurons when exposed to five millimolar glucose.During the glucose pulse, however, the signal in neurons increases more than in glial cells. Knocking out Chaski transporter reduced lactate transport in glial cells. Picrotoxin-induced increases in neuronal activity resulted in a transitory drop in ATP levels in the soma of motor neurons.The laconic sensor was observed to be well-expressed in fat bodies. Increased fluorescence of the glucose sensors was seen when the fat bodies were exposed to increasing glucose concentration. Increased lactate and pyruvate concentrations resulted in a proportional increase in laconic and pyronic fluorescence.