Cellular Footprints: Tracing How Cells Move

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An engineered cell (green) in a fruit fly follicle (red), or egg case, leaves a trail of fluorescent material as it moves across a fruit fly egg chamber, allowing scientists to trace its path and measure how long it took to complete its journey. Credit: David Bilder, University of California, Berkeley.

Cells are the basis of the living world. Our cells make up the tissues and organs of our bodies. Bacteria are also cells, living sometimes alone and sometimes in groups called biofilms. We think of cells mostly as staying in one spot, quietly doing their work. But in many situations, cells move, often very quickly. For example, when you get a cut, infection-fighting cells rally to the site, ready to gobble up bacterial intruders. Then, platelet cells along with proteins from blood gather and form a clot to stop any bleeding. And finally, skin cells surrounding the wound lay down scaffolding before gliding across the cut to close the wound.

This remarkable organization and timing is evident right from the start. Cells migrate within the embryo as it develops so that body tissues and organs end up in the right places. Harmful cells use movement as well, as when cells move and spread (metastasize) from an original cancer tumor to other parts of the body. Learning how and why cells move could give scientists new ways to guide those cells or turn off or slow down the movement when needed.

Glowing Breadcrumbs

Scientists studying how humans and animals form, from a single cell at conception to a complex body at birth, are particularly interested in how and when cells move. They use research organisms like the fruit fly, Drosophila, to watch movements by small populations of cells. Still, watching cells migrate inside a living fly is challenging because the tissue is too dense to see individual cell movement. But moving those cells to a dish in the lab might cause them to behave differently than they do inside the fly. To solve this problem, NIGMS-funded researcher David BilderLink to external web site and colleagues at the University of California, Berkeley, came up with a way to alter fly cells so they could track how the cells behave without removing them from the fly. They engineered the cells to lay down a glowing track of proteins behind them as they moved, leaving a traceable path through the fly’s tissue. The technique, called M-TRAIL (matrix-labeling technique for real-time and inferred location), allows the researchers to see where a cell travels and how long it takes to get there.

Bilder and his team first used M-TRAIL in flies to confirm the results of past studies of Drosophila ovaries in the lab using other imaging techniques. In addition, they found that M-TRAIL could be used to study a variety of cell types. The new technique also could allow a cell’s movement to be tracked over a longer period than other imaging techniques, which become toxic to cells in just a few hours. This is important, because cells often migrate for days to reach their final destinations.

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Genomic Gymnastics of a Single-Celled Ciliate and How It Relates to Humans

Laura Landweber
Credit: Denise Applewhite.
Laura Landweber
Grew up in: Princeton, New Jersey
Job site: Columbia University, New York City
Favorite food: Dark chocolate and dark leafy greens
Favorite music: 1940’s style big band jazz
Favorite hobby: Swing dancing
If I weren’t a scientist I would be a: Chocolatier (see “Experiments in Chocolate” sidebar at bottom of story)

One day last fall, molecular biologist Laura Landweber Link to external web site surveyed the Princeton University lab where she’d worked for 22 years. She and her team members had spent many hours that day laboriously affixing yellow Post-it notes to the laboratory equipment—microscopes, centrifuges, computers—they would bring with them to Columbia University, where Landweber had just been appointed full professor. Each Post-it specified the machinery’s location in the new lab. Items that would be left behind—glassware, chemical solutions, furniture, office supplies—were left unlabeled.

As Landweber viewed the lab, decorated with a field of sunny squares, her thoughts turned to another sorting process—the one used by her primary research subject, a microscopic organism, to sift through excess DNA following mating. Rather than using Post-it notes, the creature, a type of single-celled organism called a ciliate, uses small pieces of RNA to tag which bits of genetic material to keep and which to toss.

Landweber is particularly fond of Oxytricha trifallax, a ciliate with relatives that live in soil, ponds and oceans all over the world. The kidney-shaped cell is covered with hair-like projections called cilia that help it move around and devour bacteria and algae. Oxytricha is not only bizarre in appearance, it’s also genetically creative.

Unlike humans, whose cells are programmed to die rather than pass on genomic errors, Oxytricha cells appear to delight in genomic chaos. During sexual reproduction, the ciliate shatters the DNA in one of its two nuclei into hundreds of thousands of pieces, descrambles the DNA letters, throws most away, then recombines the rest to create a new genome.

Landweber has set out to understand how—and possibly why—Oxytricha performs these unusual genomic acrobatics. Ultimately, she hopes that learning how Oxytricha rearranges its genome can illuminate some of the events that go awry during cancer, a disease in which the genome often suffers significant reorganization and damage.

Oxytricha’s Unique Features

Oxytricha carries two separate nuclei—a macronucleus and a micronucleus. The macronucleus, by far the larger of the two, functions like a typical genome, the source of gene transcription for proteins. The tiny micronucleus only sees action occasionally, when Oxytricha reproduces sexually.

Oxytricha trifallax cells in the process of mating
Two Oxytricha trifallax cells in the process of mating. Credit, Robert Hammersmith.

What really makes Oxytricha stand out is what it does with its DNA during the rare occasions that it has sex. When food is readily available, Oxytricha procreates without a partner, like a plant grown from a cutting. But when food is scarce, or the cell is stressed, it seeks a mate. When two Oxytricha cells mate, the micronuclear genomes in each cell swap DNA, then replicate. One copy of the new hybrid micronucleus remains intact, while the other breaks its DNA into hundreds of thousands of pieces, some of which are tagged, recombined, then copied another thousand-fold to form a new macronucleus. Continue reading

Have Nucleus, Will Travel (in Three Dimensions)

A closeup of two human cells with the cells dyed green and the necleaus dyed red.These two human cells are nearly identical, except that the cell on the left had its nucleus (dyed red) removed. The structures dyed green are protein strands that give cells their shape and coherence. Credit: David Graham, UNC-Chapel Hill.

Both of the cells above can scoot across a microscope slide equally well. But when it comes to moving in 3D, the one without the red blob in the center (the nucleus) stalls out. That’s sort of like an Olympic speed skater who wouldn’t be able to perform even a single leap in a figure skating competition.

Scientists have known for some time that the nucleus is involved in moving cells across a flat surface—it slides to one side of the cell and “pushes” from behind. However, scientists have also shown that cells with their nuclei removed can migrate along a flat surface just as well as their brethren with intact nuclei. But they had no idea that, without a nucleus, a cell could no longer move in three dimensions.

This discovery was made by UNC-Chapel Hill biologists Keith BurridgeLink to external web site and James BearLink to external web site and their colleagues. These NIGMS-funded researchers also observed that cells whose nuclei had been disconnected from the cytoskeleton could not move through 3D matrices. The cytoskeleton is the microscopic network of actin protein filaments and tubules in the cytoplasm of many cells that provides the cell’s shape and coherence. It has also has been thought to play a major role in cell movement.

Two views of cells one on top of the other. The top animation shows a cell moving across the frame while the cells in the bottom box are static.The gray, stringy background of these videos is a 3D jello-like matrix. The cell in the top half of this video has a nucleus and can migrate through the matrix. Both cells in the bottom half have been enucleated (a fancy term for having its nucleus removed) and cannot travel through the matrix. Credit: Graham et al., Journal of Cell Biology, 2018.

The researchers speculate that the reason cells without nuclei (or those whose nuclei have been disconnected from the cytoskeleton) don’t navigate in 3D has to do with complex mechanical interactions between the cytoskeleton and the nucleoskeleton. The nucleoskeleton is a molecular scaffold within the nucleus supporting many functions such as DNA replication and transcription, chromatin remodeling, and mRNA synthesis. The interface between the cytoskeleton and nucleoskeleton consists of interlocking proteins that together provide the physical traction that cells need to push their way through 3D environments. Disrupting this interface is the equivalent of breaking the clutch in a car: the motor revs, but the wheels don’t spin, and the car goes nowhere.

A better understanding of the physical connections between the nucleus and the cytoskeleton and how they influence cell migration may provide additional insight into the role of the nucleus in diseases, such as cancer, in which the DNA-containing organelle is damaged or corrupted.

This research was funded in part by NIGMS grants 5R01GM029860-35, 5P01GM103723-05, and 5R01GM111557-04.

Carole LaBonne: Neural Crest Cells and the Rise of the Vertebrates

The stunning pigmentation of tigers, the massive jaws of sharks, and the hyper-acute vision of eagles. These and other remarkable features of higher organisms (vertebrates) derive from a small group of powerful cells, called neural crest cells, that arose more than 500 million years ago. Molecular biologist Carole LaBonne Exit icon of Northwestern University in Illinois studies how neural crest cells help give rise to these important vertebrate structures throughout development.

Very early during embryonic development, stem cells differentiate into different layers: mesoderm, endoderm, and ectoderm. Each of these layers then gives rise to different cell and tissue types. For example, the ectoderm becomes skin and nerve cells. Mesoderm turns into muscle, bone, fat, blood and the circulatory system. Endoderm forms internal structures such as lungs and digestive organs.

These three layers are present in vertebrates—animals with a backbone and well-defined heads, such as fish, birds, reptiles, and mammals—as well as animals without backbones, such as the marine-dwelling Lancelets and Tunicates (referred to as non-vertebrate chordates). Unlike cells in these layers, neural crest cells, which are found only in vertebrates, don’t specialize until much later in development. The delay gives neural crests cells the extra time and flexibility to sculpt the complex anatomical structures found only in vertebrate animals.

Scientists have long debated how neural crest cells manage to finalize their destiny so much later than all other cell types.

Using the frog Xenopus as a model system, LaBonne and her colleagues performed a series of experiments that revealed the process and identified key genes that control it.

In this video, LaBonne describes the power of neural crest cells and how they can be useful for studies of human health, including how cancer cells can metastasize, or migrate, throughout the body.

Dr. LaBonne’s research is funded in part by NIGMS grant 5R01GM116538.

Computational Geneticist Discusses Genetics of Storytelling at Sundance Film Festival

About 10 years ago, University of Utah geneticist Mark Yandell developed a software platform called VAAST (Variant Annotation, Analysis & Search Tool) to identify rare genes. VAAST, which was funded by NHGRI, was instrumental in pinpointing the genetic cause of a mystery disease that killed four boys across two generations in an Ogden, UT family.

NIGMS has been supporting Yandell’s creation of the next generation of his software, called VAAST 2, for the past few years. The new version incorporates models of how genetic sequences are conserved among different species to improve accuracy with which benign genetic sequences can be differentiated from disease-causing variations. These improvements can help identify novel disease-causing genes responsible for both rare and common diseases.

Yandell and his colleagues in the Utah Genome Project recently took part in a panel at the Sundance Film Festival called the “Genetics of Storytelling” to discuss film’s ability to convey the power of science and medicine. Yandell told the audience his story about his efforts to use VAAST to trace the Ogden boys’ genetic variation back to their great-great-great-great-great grandmother.

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What Zombie Ants Are Teaching Us About Fungal Infections: Q & A with Entomologists David Hughes and Maridel Fredericksen

 

I can still remember that giddy feeling I had seven years ago, when I first read about the “zombie ant.” The story was gruesome and fascinating, and it was everywhere. Even friends and family who aren’t so interested in science knew the basics: in a tropical forest somewhere there’s a fungus that infects an ant and somehow takes control of the ant’s brain, forcing it to leave its colony, crawl up a big leaf, bite down and wait for the sweet relief of death. A grotesque stalk then sprouts from the poor creature’s head, from which fungal spores rain down to infect a new batch of ants.

A fungal fruiting body erupts through the head of a carpenter ant infected by a parasitic fungus in Thailand. Credit: David Hughes, Penn State University.

The problem is, it doesn’t happen quite like that. David Hughes Exit icon , the Penn State University entomologist who reported his extensive field observations of the fungus/ant interactions in BMC Ecology Exit icon, which caused much excitement back in 2011, has continued to study the fungus, Ophiocordyceps unliateralis, and its carpenter ant host, Camponotus leonardi.

In late 2017, Hughes and his colleagues published an article in PNAS Exit icon in which they used sophisticated microscopy and image-processing techniques to describe in great detail how the fungus invades various parts of the ant’s body including muscles in its legs and head.

Although Hughes’s earlier BMC Ecology paper showed fungus in the head of an ant, the new study reveals that the fungus never actually enters the brain.

To me, the new finding somehow made the fungus’ control over the ant even more baffling. What exactly was going on?

To find out, I spoke with Hughes and his graduate student Maridel Fredericksen.

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Interview With a Scientist: Joel Kralj, Electromicist

Every one of our thoughts, emotions, sensations, and movements arise from changes in the flow of electricity in the brain. Disruptions to the normal flow of electricity within and between cells is a hallmark of many diseases, especially neurological and cardiac diseases.

The source of electricity within nerve cells (i.e., neurons) is the separation of charge, referred to as voltage, across neuronal membranes. In the past, scientists weren’t able to identify all the molecules that control neuronal voltage. They simply lacked the tools. Now, University of Colorado biologist Joel Kralj Exit icon has developed a way to overcome this hurdle. His new technique—combining automated imaging tools and genetic manipulation of cells—is designed to measure the electrical contribution of every protein coded by every gene in the human genome. Kralj believes this technology will usher in a new field of “electromics” that will be of enormous benefit to both scientists studying biological processes and clinicians attempting to treat disease.

In 2017, Kralj won a New Innovator Award from the National Institutes of Health for his work on studying voltage in neurons. He is using the grant money to develop a new type of microscope that will be capable of measuring neuronal voltage from hundreds of cells simultaneously. He and his research team will then attempt to identify the genes that encode any of the 20,000 proteins in the human body that are involved in electrical signaling. This laborious process will involve collecting hundreds of nerve cells, genetically removing a single protein from each cell, and using the new microscope to see what happens. If the voltage within a cell is changed in any way when a specific protein is removed, the researchers can conclude that the protein is essential to electrical signaling.

In this video, Kralj discusses how he plans to use his electromics platform to study electricity-generating cells throughout the body, as well as in bacterial cells (see our companion blog post “Feeling Out Bacteria’s Sense of Touch” featuring Kralj’s research for more details).

Dr. Kralj’s work is funded in part by the NIH under grant 1DP2GM123458-01.

Feeling Out Bacteria’s Sense of Touch

Our sense of touch provides us with bits of information about our surroundings that inform the decisions we make. When we touch something, our nervous system transmits signals through nerve endings that feed information to our brain. This enables us to sense the stimulus and take the appropriate action, like drawing back quickly when we touch a hot stovetop.

Bacteria are single cells and lack a nervous system. But like us, they rely on their “sense” of touch to make decisions—or at least change their behavior. For example, bacteria’s sense of touch is believed to trigger the cells to form colonies, called biofilms, on surfaces they make contact with. Bacteria may form biofilms as a way to defend themselves, share limited nutrients, or simply to prevent being washed away in a flowing liquid.

Humans can be harmed by biofilms because these colonies serve as a reservoir of disease-causing cells that are responsible for high rates of human infection. Biofilms can protect at least some cells from being affected by antibiotics. The surviving reservoir of bacteria then have more time to evolve resistance to antibiotics.

At the same time, some biofilms can be valuable; for example, they help to break down waste in water treatment plants and to drive electrical current as part of microbial fuel cells.

Until recently, scientists thought that bacteria formed biofilms and caused infections in response to chemical signals they received from their environments. But research in 2014 showed that the bacterium Pseudomonas aeruginosa could infect a variety of living tissues—from plants to many kinds of animals—simply by making contact with them. In the past year, multiple groups of investigators have learned more about how bacteria sense that they have touched a surface and how that sense translates to changes in their behavior. This understanding could lead to new ways of preventing infections or harmful biofilm formation.

Making Contact

Pili (green) on cells from the bacterium Caulobacter crescentus (orange). Scientists used a fluorescent dye to stain pili so they could watch the structures extend and retract. Credit: Courtney Ellison, Indiana University.

When they first make contact with a surface, bacteria change from free-ranging, swimming cells to stationary ones that secrete a sticky substance, tethering them in one place. To form a biofilm, they begin replicating, creating an organized mass stable enough to resist shaking and to repel potential invaders (see https://biobeat.nigms.nih.gov/2017/01/cool-image-inside-a-biofilm-build-up/).

How do swimming bacteria sense that they have found a potential surface to colonize? Working with the bacterium Caulobacter crescentus, Indiana University Ph.D. student Courtney Ellison and her colleagues, under the direction of professor of biology and NIGMS grantee Yves Brun Exit icon, recently showed that hair-like structures on the cell’s surface, called pili, play a role here. The researchers found that as a bacterial cell swims in a fluid, its pili are constantly stretching out and retracting. When the cell makes contact with a surface, the pili stop moving, start producing a sticky substance and use it to hold onto the surface. Continue reading

Two NIGMS MARC Scholars Receive Prestigious Rhodes Scholarship

Oxford University. Credit: Andrew Shiva, Wikimedia Commons CC BY-SA.

MARC U-STAR Scholars Jasmine Brown and Naomi Mburu were among 32 Americans to recently receive the prestigious Rhodes Scholarship at Oxford University in England. Rhodes Scholars are chosen for their academic and research achievements, as well as their commitment to others and leadership potential.

As current MARC U-STAR Scholars, Brown and Mburu are part of an NIGMS research training program for undergraduate junior and senior honor students. MARC is designed to increase the number of people from groups underrepresented in biomedical sciences by preparing students for high-caliber, doctorate-level training.

Here’s more about these two distinguished women:

Credit: The Source, Washington University in St. Louis.

Jasmine Brown, 21

Brown, of Hillsborough, New Jersey, is a senior at Washington University in St. Louis and works as a research assistant at the Washington University School of Medicine. There, she studies genes that are protective against mental defects that result from West Nile-induced brain inflammation. After she receives her bachelor’s degree in biology, she plans to earn a doctorate degree in neuroscience as a Rhodes Scholar at Oxford University.

In addition to her current training as a MARC Scholar, Brown has spent her summers as an undergraduate research assistant, engaging in the study of these other notable subjects:

  • Lung cancer, at the Broad Institute of MIT and Harvard (2017)
  • Specific drugs’ cough-suppressing effects, at Johns Hopkins University School of Medicine (2015)
  • Long-term neurological effects of cocaine and other stimulants on the teen brain, at the University of Miami Miller School of Medicine (2014)

“What I love about science is that it gives me tools to generate answers and to improve human health. It’s a fun process for me, but also a satisfying one because I can make an impact,” Brown said in a statement.

Equally important to her studies, Brown is a champion for other underrepresented students in the sciences. After her own experience as the target of prejudice, Brown started the Minority Association of Rising Scientists (MARS) to support underrepresented students participating in research and inform faculty members about implicit bias. With the help of the National Science Foundation, Brown is working to expand MARS nationwide.

Brown has given back to the community in other ways. She was a member of The Synapse Project Exit icon, which prepares high school students for a neuroscience competition called Brain Bee Exit icon. She was also a 2014-2015 candidate for Mx. WashU Exit icon, an organization that raises money for a children’s program called City Faces Exit icon.

Naomi Mburu, 21

Credit: Marlayna Desmond for UMBC.

Naomi Mburu, of Ellicott City, Maryland, is the daughter of Kenyan immigrants and the first student in the history of the University of Maryland Baltimore County (UMBC) to receive the Rhodes Scholarship. The senior in chemical engineering plans to complete a doctorate in engineering science and to research heat transfer applications for nuclear fusion reactors.

“I believe the Rhodes Scholarship will allow me to foster a stronger community amongst my fellow scholars because we will all be attending the same institution,” Mburu said in a statement.
Mburu is currently working with Gymama Slaughter Exit icon, UMBC associate professor of computer science and electrical engineering, to develop a machine that ensures human organs remain healthy as they await transplantExit icon.
During her recent summer internship with Intel, Mburu developed an interactive model to estimate the cost of coatings applied to equipment. Her work helped improve pricing negotiations and established additional cost estimates for other chemical processes.

Her other areas of research have included:

  • Assessing phosphate’s effects on the ribosomal protein L4 as a student at Mount Hebron High School
  • Measuring the impurities found in the Large Hadron Collider particle accelerator, at the European Organization for Nuclear Research, Geneva, Switzerland

Mburu’s aspirations involve not just science but education advocacy. Her passion for STEM Exit icon education and increasing diversity in STEM fields led to her current involvement as a MARC trainee, where she’s learned to communicate her desire to make a global impact through her science research and her efforts to remove barriers to education equality.

In her free time, Mburu has helped K-12 students with their homework during her time at UMBC. She continues to mentor youth and helps high school girls on STEM-related research projects.

The Changing Needs of a Cell: No Membrane? No Problem!

Russian nesting dolls. Credit: iStock.

How “membrane-less” organelles help with key cellular functions

Scientists have long known that animal and plant cells have specialized subdivisions called organelles. These organelles are surrounded by a semi-permeable barrier, called a membrane, that both organizes the organelles and insulates them from the rest of the cell’s mix of proteins, salt, and water. This set-up helps to make cells efficient and productive, aiding in energy production and other specialized functions. But, because of their semi-permeable membranes, organelles can’t regroup and reform in response to stress or other outside changes. Cells need a rapid response team working alongside the membrane-bound organelles to meet these fluctuating needs. Until recently, who those rapid responders were and how they worked has been a mystery.

Recent research has led biologists to learn that the inside of a cell or an organelle is not just a lot of different molecules dissolved in water. Instead, we now know that cells contain many pockets of liquid droplets (one type of liquid surrounded by a liquid of different density) with specialized composition and function that are not surrounded by membranes. Because these “membrane-less organelles” are not confined, they can rapidly come together in response to chemical signals, such as those that indicate stress, and equally rapidly fall apart when they are no longer needed, or when the cell is about to divide. This enables membrane-less organelles to be “rapid responders.” They can have complex, multilayered structures that help them to perform many critical cell functions with multiple steps, just like membrane-bound organelles. Scientists even suspect that the way these organelles form as droplets may shed light on how life on Earth first took shape (see sidebar “Could This Be How Life First Took Shape?” at bottom of page).

The Many Membrane-less Organelles

Scientists have identified more than a dozen membrane-less organelles at work in mammalian cells. Several kinds found inside the nucleus—including nuclear speckles, paraspeckles, and Cajal bodies—help with cell growth, stress response, the metabolizing (breaking down) of RNA, and the control of gene expression—the process by which information in a gene is used in the synthesis of a protein. Out in the cytoplasm, P-bodies, germ granules, and stress granules are membrane-less organelles that are involved in metabolizing or protecting messenger RNA (mRNA), controlling which mRNAs are made into proteins, and in maintaining balance, or homeostasis, of the cell’s overall health.

The nucleolus, located inside the nucleus, is probably the largest of the membrane-less organelles. It acts as a factory to assemble ribosomes, the giant molecular machines that “translate” messenger RNAs to make all cellular proteins. Continue reading