There is no better sign that the banana on your counter is ripe than the buzz of a fruit fly, and if that banana were outside, it would likely also have many microscopic worms enjoying the bacteria found on the rotting fruit. They can be creepy. They can be crawly. But C. elegans and Drosophila, or soil nematodes (roundworms) and fruit flies, are actually really helpful in deepening our understanding of the mechanisms that regulate brain structure, function, and aging. These genetic model organisms are truly invaluable to the field of neuroscience. Many of the discoveries made using C. elegans and Drosophila apply throughout the animal kingdom, and this research has led to a broader understanding of human biology. Five C. elegans researchers have won the Nobel Prize, and six Drosophila researchers have also been bestowed the highest honor in science.
KNOWING EACH NEURON
“The fact that the C. elegans’ neuroanatomy is simple is one of the original reasons it was chosen as a model system,” explained Douglas Portman, PhD, professor of Biomedical Genetics, Neuroscience, and Biology at the University of Rochester Medical Center. Scientists have identified all 302 of the adult worm’s neurons. “But C. elegans does have relatively complex behaviors. It can make decisions and react to encountering things like food, a potential predator, or even touching something new. It also has plasticity and can learn.” These behaviors are where neuroscientists come in, they provide researchers the opportunity to understand how genes guide the development and function of the nervous system.
Portman began working with C. elegans during his postdoc under Scott Emmons, PhD, of the Albert Einstein College of Medicine, who trained under Sydney Brenner, PhD, the scientist who established C. elegans as a genetic model system. Portman’s current research has found that biological sex can act to regulate the way that neurons work. His work published in Current Biology found male brains—at least in C. elegans—will suppress the ability to locate food to focus on finding a mate. “The tractability of the C. elegans system has allowed us to understand pretty deeply how that works at the level of changes in gene expression and neuronal properties,” Portman said. “The chromosomal sex of the nervous system gives the primary cue that tells neurons if they should be in a male or hermaphrodite state. One important insight from our work is that it is not a fixed static binary decision but a flexible one. It challenges the way we think about sex as a biological variable.”
It is also an important consideration in human health and disease as scientific discoveries lead to a better understanding of the interactions between genes, hormones, and neurons, the genetic makeup on a neuron could influence its response to hormonal signals.
THE AGING OLD QUESTION
With a three-week lifespan, C. elegans can expedite experiments. Associate professor of Anesthesiology & Perioperative Medicine, and Pharmacology & Physiology Andrew Wojtovich, PhD, finds these to be a beneficial model system for his research around aging and stroke. “C. elegans tell us more about the contextual nature of things. Working with a simple organism allows us to focus on molecular questions rather than technical ones. I like to say that we study the engine while a car is in motion, not while it is up on cinderblocks.”
The Wojtovich Lab studies mitochondrial physiology in the context of hypoxic biology or stroke. In an effort to control the mitochondria’s ability to make energy, they discovered how to create the opposite effect. The lab developed a tool that uses light to recharge the mitochondria, an application that Wojtovich believes has endless potential. “Mitochondrial dysfunction is typical in many neurological diseases. Take, for example, Alzheimer’s disease, which is characterized by impaired energy production and biomarkers of mitochondrial dysfunction. However, the cause-and-effect relationship between mitochondria and pathology progression is unclear. I see our work and genetic tools giving us a better understanding of these disease pathways.”
C. elegans aging is central to work taking place in the Nehrke Lab located in the Nephrology Division in the department of Medicine at the Medical Center. “C. elegans is an unbelievable genetic resource,” professor Keith Nehrke, PhD, said. “We can do things very quickly and effectively, while limiting potential confounds and generating binary (yes or no) answers by asking these questions in a model system. In our case, aging is the main risk factor in most neurodegenerative diseases, and that takes only weeks in C. elegans rather than years.”
More than a decade ago, Nehrke and Anesthesiology & Perioperative Medicine professor Gail Johnson, PhD, began collaborating using C. elegans to ask questions about the underlying mechanisms of Alzheimer’s disease. They are studying how changes to tau—a protein that helps nerve cells maintain this structure and function—impact mitochondrial health and neuronal aging. An impairment in tau function is associated with the development of neurodegenerative diseases like Alzheimer’s. Using a C. elegans model, they are able to leverage the molecular underpinnings of Alzheimer’s together with recent genome-editing technologies such as CRISPR/Cas9 to define the precise mechanism through which toxic tau compromises mitochondrial function and accelerates neuronal aging. The labs’ most recent data indicates that worms expressing even low levels of disease-associated tau show a significant increase in age-dependent neurodegeneration and a suppression of stress-induced mitophagy, a mechanism for recycling damaged mitochondria that is important for maintaining a pool of healthy organelles.
Along with providing insight into underlying mechanisms of disease, the Nehrke and Johnson labs also use the C. elegans model to study a drug’s impact on behavior or neurodegenerative output. Because the molecular targets of pharmaceuticals are often the same in the worms as they are in people, the labs can use pharmacogenetic approaches to see what molecules the drugs interact with to convey a response.
“We know each neuron in C. elegans and working with a small population of neurons gives us better insight into what we are looking at. It is a better first pass at testing a hypothesis because they can be genetically manipulated rapidly,” Johnson said. “We can run experiments in weeks rather than years, like in mice models, and get some real data that tells us where to go next.”
And next—maybe one step up on the model systems chain—from the 302 neurons in C. elegans to ~100,000 neurons in Drosophila melanogaster, or the fruit fly.
WHAT’S ALL THE BUZZ?
“Drosophila have very complex behaviors, similar to larger animals. Flies are trainable in a Pavlovian behavior,” said Matthew Rand, PhD, associate professor of Environmental Medicine. “For example, you can train a fruit fly to go towards a light based on a reward of food being there. Some fundamental neurobiological circuits and processes are coded in this tiny little organism.”
Rand’s lab focuses on mercury toxicity, specifically methyl mercury found in fish and how it impacts the nervous system. His current research intersects with neuroscience in the sensitivity of developmental neurobiology. The fruit fly has been a longstanding model organism that has allowed researchers to identify genes responsible for producing proteins that coordinate building the structure and supporting the functions of our most fundamental tissues and organs. Using this model, his lab has discovered that the developing muscular system is a sensitive target to methyl mercury along with the nervous system.
“It appears that the toxicant attacks a developing organism beyond just a single system, like a neuron or a nervous system, and that there is a higher-level systemic effect that may be attacking muscle and neuron and the communication between the two during development,” Rand said. It begins to allow for the identification of molecular and/or gene candidates. Specifically, themolecules or genes already known to crossover in neurodevelopment and muscle development. These can be targeted to provide insight into how an altered gene expression in a neuron may affect a muscle that could enhance toxicity, or alternatively compensate for injured neurons.
The 100,000 neurons that make up the Drosophila brain are a drop in the bucket compared to the 86 billion neurons in the human brain. Yet do not underestimate the six-legged, two winged insects. “It performs a lot of sophisticated behaviors; it can fly, see, smell, taste, touch, hear, and balance,” said Rajnish Bharadwaj, MBBS, PhD, assistant professor of Pathology & Laboratory Medicine and Neuroscience. “The fruit fly is genetically amenable, allowing us to manipulate genes quickly, examine small subsets of neurons, and more clearly examine certain cell biological processes.” The fruit fly’s fundamental biological make-up is similar to humans. It shares 75 percent of genes that cause disease in people. The Bharadwaj Lab aims to understand the gene involvement in Parkinsonism—a group of neurological disorders that cause movement problems similar to those seen in Parkinson's disease, and neurodegeneration with brain iron accumulation—a rare neurological movement disorder. “We are showing the first evidence that this gene, C19orf12, impacts lipid homeostasis in the brain and other organs,” said Bharadwaj. “We believe understanding more about this gene is important to several neurological disorders. In addition to its known role in the genetic disease NBIA, it has interesting similarities with Parkinson’s disease such as accumulation of the protein synuclein in various parts of the brain.”
NEW LAB TAKES FLIGHT
Gabriella Sterne, PhD, traveled 2,700 miles to begin her assistant faculty position in the Department of Biomedical Genetics at the Medical Center. The collaborative nature of the University and established invertebrate labs attracted the scientist who wrapped up her postdoctoral fellowship at UC Berkeley. “There is this really cool invertebrate community that meets weekly here,” said Sterne. “It is so important to research, it fosters collaboration and breeds new ideas.” Sterne works with Drosophila to understand how the brain encodes feeding experiences and how it uses those experiences to inform memory formation and make future feeding decisions. “Understanding the relationship between memory and feeding decisions will reveal general and fundamental principles of how neural circuits work,” said Sterne. “I think it will offer more specific insights into how feeding is regulated in animals which has implications for human health, disease, and pest control.”
TINY SCIENTIFIC TITANS
The complex questions these model systems can and have answered transforms our understanding of molecular biology and neuroscience. The physical size of Drosophila and C. elegans and their lifespans allows researchers to usher in more data faster than they can with other animal models. Their simple systems are a mecca for scientific discovery. And their similarities to humans are also helpful—and interesting. Did you know fruit flies can taste many of the same things as humans, including sweet, bitter, salty, and sour? It may be why they hang out in your kitchen.
This article originally appeared in NeURoscience | Vol 16.