model organisms
How Neurons Make Connections
Posted on by Lawrence Tabak, D.D.S., Ph.D.
For many people, they are tiny pests. These fruit flies that sometimes hover over a bowl of peaches or a bunch of bananas. But for a dedicated community of researchers, fruit flies are an excellent model organism and source of information into how neurons self-organize during the insect’s early development and form a complex, fully functioning nervous system.
That’s the scientific story on display in this beautiful image of a larval fruit fly’s developing nervous system. Its subtext is: fundamental discoveries in the fruit fly, known in textbooks as Drosophila melanogaster, provide basic clues into the development and repair of the human nervous system. That’s because humans and fruit flies, though very distantly related through the millennia, still share many genes involved in their growth and development. In fact, 60 percent of the Drosophila genome is identical to ours.
Once hatched, as shown in this image, a larval fly uses neurons (magenta) to sense its environment. These include neurons that sense the way its body presses against the surrounding terrain, as needed to coordinate the movements of its segmented body parts and crawl in all directions.
This same set of neurons will generate painful sensations, such as the attack of a parasitic wasp. Paintbrush-like neurons in the fly’s developing head (magenta, left side) allow the insect to taste the sweetness of a peach or banana.
There is a second subtype of neurons, known as proprioceptors (green). These neurons will give the young fly its “sixth sense” understanding about where its body is positioned in space. The complete collection of developing neurons shown here are responsible for all the fly’s primary sensations. They also send these messages on to the insect’s central nervous system, which contains thousands of other neurons that are hidden from view.
Emily Heckman, now a postdoctoral researcher at the Michigan Neuroscience Institute, University of Michigan, Ann Arbor, captured this image during her graduate work in the lab of Chris Doe, University of Oregon, Eugene. For her keen eye, she received a trainee/early-career BioArt Award from the Federation of American Societies for Experimental Biology (FASEB), which each year celebrates the art of science.
The image is one of many from a much larger effort in the Doe lab that explores the way neurons that will partner find each other and link up to drive development. Heckman and Doe also wanted to know how neurons in the developing brain interconnect into integrated neural networks, or circuits, and respond when something goes wrong. To find out, they disrupted sensory neurons or forced them to take alternate paths and watched to see what would happen.
As published in the journal eLife [1], the system has an innate plasticity. Their findings show that developing sensory neurons instruct one another on how to meet up just right. If one suddenly takes an alternate route, its partner can still reach out and make the connection. Once an electrically active neural connection, or synapse, is made, the neural signals themselves slow or stop further growth. This kind of adaptation and crosstalk between neurons takes place only during a particular critical window during development.
Heckman says part of what she enjoys about the image is how it highlights that many sensory neurons develop simultaneously and in a coordinated process. What’s also great about visualizing these events in the fly embryo is that she and other researchers can track many individual neurons from the time they’re budding stem cells to when they become a fully functional and interconnected neural circuit.
So, the next time you see fruit flies hovering in the kitchen, just remember there’s more to their swarm than you think. Our lessons learned studying them will help point researchers toward new ways in people to restore or rebuild neural connections after devastating disruptions from injury or disease.
Reference:
Presynaptic contact and activity opposingly regulate postsynaptic dendrite outgrowth. Heckman EL, Doe CQ. Elife. 2022 Nov 30;11:e82093.
Links:
Research Organisms (National Institute of General Medical Sciences/NIH)
Doe Lab (University of Oregon, Eugene)
Emily Heckman (University of Michigan, Ann Arbor)
BioArt Awards (Federation of American Societies for Experimental Biology, Rockville, MD)
NIH Support: Eunice Kennedy Shriver National Institute of Child Health and Human Development
Saving Fat for Lean Times
Posted on by Lawrence Tabak, D.D.S., Ph.D.
Humans and all multi-celled organisms, or metazoans, have evolved through millennia into a variety of competing shapes, sizes, and survival strategies. But all metazoans still share lots of intriguing cell biology, including the ability to store excess calories as fat. In fact, many researchers now consider fat-storing cells to be “nutrient sinks,” or good places for the body to stash excess sugars and lipids. Not only can these provide energy needed to survive a future famine, this is a good way to sequester extra molecules that could prove toxic to cells and organs.
Here’s something to think about the next time you skip a meal. Fat-storing cells organize their fat reserves spatially, grouping them into specific pools of lipid types, in order to generate needed energy when food is scarce.
That’s the story behind this striking image taken in a larval fruit fly (Drosophila melanogaster). The image captures fat-storing adipocytes in an organ called a fat body, where a larval fruit fly stores extra nutrients. It’s like the fat tissue in mammals. You can see both large and small lipid droplets (magenta) inside polygon-shaped fat cells, or adipocytes, lined by their plasma membranes (green). But notice that the small lipid droplets are more visibly lined by green, as only these are destined to be saved for later and exported when needed into the fly’s bloodstream.
Working in Mike Henne’s lab at the University of Texas Southwestern Medical Center, Dallas, research associate Rupali Ugrankar discovered how this clever fat-management system works in Drosophila [1]. After either feeding flies high-or-extremely low-calorie diets, Ugrankar used a combination of high-resolution fluorescence confocal microscopy and thin-section transmission electron microscopy to provide a three-dimensional view of adipocytes and their lipid droplets inside.
She observed two distinct sizes of lipid droplets and saw that only the small ones clustered at the cell surface membrane. The adipocytes contorted their membrane inward to grab these small droplets and package them into readily exportable energy bundles.
Ugrankar saw that during times of plenty, a protein machine could fill these small membrane-wrapped fat droplets with lots of triacylglycerol, a high-energy, durable form of fat storage. Their ready access at the surface of the adipocyte allows the fly to balance lipid storage locally with energy release into its blood in times of famine.
Ugrankar’s adeptness at the microscope resulted in this beautiful photo, which was earlier featured in the American Society for Cell Biology’s Green Fluorescent Protein Image and Video Contest. But her work and that of many others help to open a vital window into nutrition science and many critical mechanistic questions about the causes of obesity, insulin resistance, hyperglycemia, and even reduced lifespan.
Such basic research will provide the basis for better therapies to correct these nutrition-related health problems. But the value of basic science must not be forgotten—some of the most important leads could come from a tiny insect in its larval state that shares many aspects of mammalian metabolism.
Reference:
[1] Drosophila Snazarus regulates a lipid droplet population at plasma membrane-droplet contacts in adipocytes. Ugrankar R, Bowerman J, Hariri H, Chandra M, et al. Dev Cell. 2019 Sep 9;50(5):557-572.e5.
Links:
The Interactive Fly (Society for Developmental Biology, Rockville, MD)
Henne Lab (University of Texas Southwestern Medical Center, Dallas)
NIH Support: National Institute of General Medical Sciences
3D Neuroscience at the Speed of Life
Posted on by Dr. Francis Collins
This fluorescent worm makes for much more than a mesmerizing video. It showcases a significant technological leap forward in our ability to capture in real time the firing of individual neurons in a living, freely moving animal.
As this Caenorhabditis elegans worm undulates, 113 neurons throughout its brain and body (green/yellow spots) get brighter and darker as each neuron activates and deactivates. In fact, about halfway through the video, you can see streaks tracking the positions of individual neurons (blue/purple-colored lines) from one frame to the next. Until now, it would have been technologically impossible to capture this “speed of life” with such clarity.
With funding from the NIH-led Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative, Elizabeth Hillman at Columbia University’s Zuckerman Institute, New York, has pioneered the pairing of a 3D live-imaging microscope with an ultra-fast camera. This pairing, showcased above, is a technique called Swept Confocally Aligned Planar Excitation (SCAPE) microscopy.
Since first demonstrating SCAPE in February 2015 [1], Hillman and her team have worked hard to improve, refine, and expand the approach. Recently, they used SCAPE 1.0 to image how proprioceptive neurons in fruit-fly larvae sense body position while crawling. Now, as described in Nature Methods, they introduce SCAPE “2.0,” with boosted resolution and a much faster camera—enabling 3D imaging at speeds hundreds of times faster than conventional microscopes [2]. To track a very wiggly worm, the researchers image their target 25 times a second!
As with the first-generation SCAPE, version 2.0 uses a scanning mirror to sweep a slanted sheet of light across a sample. This same mirror redirects light coming from the illuminated plane to focus onto a stationary high-speed camera. The approach lets SCAPE grab 3D imaging at very high speeds, while also causing very little photobleaching compared to conventional point-scanning microscopes, reducing sample damage that often occurs during time-lapse microscopy.
Like SCAPE 1.0, since only a single, stationary objective lens is used, the upgraded 2.0 system doesn’t need to hold, move, or disturb a sample during imaging. This flexibility enables scientists to use SCAPE in a wide range of experiments where they can present stimuli or probe an animal’s behavior—all while imaging how the underlying cells drive and depict those behaviors.
The SCAPE 2.0 paper shows the system’s biological versatility by also recording the beating heart of a zebrafish embryo at record-breaking speeds. In addition, SCAPE 2.0 can rapidly image large fixed, cleared, and expanded tissues such as the retina, brain, and spinal cord—enabling tracing of the shape and connectivity of cellular circuits. Hillman and her team are dedicated to exporting their technology; they provide guidance and a parts list for SCAPE 2.0 so that researchers can build their own version using inexpensive off-the-shelf parts.
Watching worms wriggling around may remind us of middle-school science class. But to neuroscientists, these images represent progress toward understanding the nervous system in action, literally at the speed of life!
References:
[1] . Swept confocally-aligned planar excitation (SCAPE) microscopy for high speed volumetric imaging of behaving organisms. Bouchard MB, Voleti V, Mendes CS, Lacefield C, et al Nature Photonics. 2015;9(2):113-119.
[2] Real-time volumetric microscopy of in vivo dynamics and large-scale samples with SCAPE 2.0. Voleti V, Patel KB, Li W, Campos CP, et al. Nat Methods. 2019 Sept 27;16:1054–1062.
Links:
Using Research Organisms to Study Health and Disease (National Institute of General Medical Sciences/NIH)
The Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)
Hillman Lab (Columbia University, New York)
NIH Support: National Institute of Neurological Disorders and Stroke; National Heart, Lung, and Blood Institute
Finding Beauty in the Nervous System of a Fruit Fly Larva
Posted on by Dr. Francis Collins
Wow! Click on the video. If you’ve ever wondered where those pesky flies in your fruit bowl come from, you’re looking at it right now. It’s a fruit fly larva. And this 3D movie offers never-before-seen details into proprioception—the brain’s sixth sense of knowing the body’s location relative to nearby objects or, in this case, fruit.
This live-action video highlights the movement of the young fly’s proprioceptive nerve cells. They send signals to the fly brain that are essential for tracking the body’s position in space and coordinating movement. The colors indicate the depth of the nerve cells inside the body, showing those at the surface (orange) and those further within (blue).
Such movies make it possible, for the first time, to record precisely how every one of these sensory cells is arranged within the body. They also provide a unique window into how body positions are dynamically encoded in these cells, as a segmented larva inches along in search of food.
The video was created using a form of confocal microscopy called Swept Confocally Aligned Planar Excitation, or SCAPE. It captures 3D images by sweeping a sheet of laser light back and forth across a living sample. Even better, it does this while the microscope remains completely stationary—no need for a researcher to move any lenses up or down, or hold a live sample still.
Most impressively, with this new high-speed technology, developed with support from the NIH’s BRAIN Initiative, researchers are now able to capture videos like the one seen above in record time, with each whole volume recorded in under 1/10th of a second! That’s hundreds of times faster than with a conventional microscope, which scans objects point by point.
As reported in Current Biology, the team, led by Elizabeth Hillman and Wesley Grueber, Columbia University, New York, didn’t stop at characterizing the structural details and physical movements of nerve cells involved in proprioception in a crawling larva. In another set of imaging experiments, they went a step further, capturing faint flashes of green in individual labeled nerve cells each time they fired. (You have to look very closely to see them.) With each wave of motion, proprioceptive nerve cells light up in sequence, demonstrating precisely when they are sending signals to the animal’s brain.
From such videos, the researchers have generated a huge amount of data on the position and activity of each proprioceptive nerve cell. The data show that the specific position of each cell makes it uniquely sensitive to changes in position of particular segments of a larva’s body. While most of the proprioceptive nerve cells fired when their respective body segment contracted, others were attuned to fire when a larval segment stretched.
Taken together, the data show that proprioceptive nerve cells provide the brain with a detailed sequence of signals, reflecting each part of a young fly’s undulating body. It’s clear that every proprioceptive neuron has a unique role to play in the process. The researchers now will create similar movies capturing neurons in the fly’s central nervous system.
A holy grail of the BRAIN Initiative is to capture the brain in action. With these advances in imaging larval flies, researchers are getting ever closer to understanding the coordinated activities of an organism’s complete nervous system—though this one is a lot simpler than ours! And perhaps this movie—and the anticipation of the sequels to come—may even inspire a newfound appreciation for those pesky flies that sometimes hover nearby.
Reference:
[1] Characterization of Proprioceptive System Dynamics in Behaving Drosophila Larvae Using High-Speed Volumetric Microscopy. Vaadia RD, Li W, Voleti V, Singhania A, Hillman EMC, Grueber WB. Curr Biol. 2019 Mar 18;29(6):935-944.e4.
Links:
Using Research Organisms to Study Health and Disease (National Institute of General Medical Sciences/NIH)
The Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)
Hillman Lab (Columbia University, New York)
Grueber Lab (Columbia University, New York)
NIH Support: National Institute of Neurological Disorders and Stroke; Eunice Kennedy Shriver National Institute of Child Health and Human Development
A Fantastic WALS Lecture
Posted on by Dr. Francis Collins
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