You might know that tiny individual units called cells make up your body. But did you know some of your cells die every day as a part of their normal life cycle? These deaths are balanced by other cells splitting into two identical cells, a process called mitosis.
Claira Sohn credits her grandfather with sparking her interest in science. Although he never studied science at a 4-year university due to financial limitations, he took many community college classes and worked in chemistry labs developing products such as hair dyes and dissolvable stitches. “Every morning, my grandfather would take me to school, and we’d stop to get orange juice and a cookie and talk about science. When I was in elementary school, he bought me a book about quantum mechanics written for kids,” she says. “He inspired me to ask questions and encouraged me to go to college.”
Claira enrolled at Northern Arizona University in Flagstaff after graduating high school. She majored in biomedical sciences and planned to become a medical doctor until her microbiology professor talked to her about the possibility of a research career. “That was an epiphany for me, because while I knew that there was research going on in the world, I didn’t realize there could be a place for me there,” Claira says. During her junior year, she joined the lab of Naomi Lee, Ph.D., where she first experienced what it felt like to be a researcher.Continue reading “Claira Sohn Cultivates Neurons and Diversity in the STEM Community”
The red spray pictured here may look like fireworks erupting across the night sky on July 4th, but it’s actually a rare glimpse of tiny protein strands called microtubules sprouting and growing from one another in a lab. Microtubules are the largest of the molecules that form a cell’s skeleton. When a cell divides, microtubules help ensure that each daughter cell has a complete set of genetic information from the parent. They also help organize the cell’s interior and even act as miniature highways for certain proteins to travel along.
As their name suggests, microtubules are hollow tubes made of building blocks called tubulins. Scientists know that a protein called XMAP215 adds tubulin proteins to the ends of microtubules to make them grow, but until recently, the way that a new microtubule starts forming remained a mystery.
Sabine Petry and her colleagues at Princeton University developed a new imaging method for watching microtubules as they develop and found an important clue to the mystery. They adapted a technique called total internal reflection fluorescence (TIRF) microscopy, which lit up only a tiny sliver of a sample from frog egg (Xenopus) tissue. This allowed the scientists to focus clearly on a few of the thousands of microtubules in a normal cell. They could then see what happened when they added certain proteins to the sample.
The outermost cells that line blood vessels, lungs, and other organs also act like guards, alert and ready to thwart pathogens, toxins, and other invaders that can do us harm. Called epithelial cells, they don’t just lie passively in place. Instead, they communicate with each other and organize their internal structures in a single direction, like a precisely drilled platoon of soldiers lining up together and facing the same way.
Lining up this way is crucial during early development, when tissues and organs are forming and settling into their final positions in the developing body. In fact, failure to line up in the correct way is linked to numerous birth defects. In the lungs, for instance, epithelial cells’ ability to synchronize with one another is important since these cells have special responsibilities such as carrying mucus up and out of lung tissue. When these cells can’t coordinate their functions, disease results.
Some lung epithelial cells are covered in many tiny, hair-like structures called cilia. All the cilia on lung epithelial cells must move uniformly in a tightly choreographed way to be effective in their mucus-clearing job. This is a unique example of a process called planar cell polarity (PCP) that occurs in cells throughout the body. Researchers are seeking to identify the signals cells use to implement PCP. Continue reading “CLAMP Helps Lung Cells Pull Together”
Nerve cells, or neurons, in our brains do amazing work, from telling our hearts to beat to storing our memories. But neurons cannot operate alone. Many kinds of cells support and regulate neurons and—like neurons—they can come under attack due to injuries or disorders, such as stroke or Alzheimer’s disease. Learning what jobs these cells do and how they respond to disease may show researchers new ways to treat central nervous system disorders. One type of support cell, the pericyte, plays some key roles in brain health. These cells are readily adaptable, even in adult brains, and can support a variety of functions.
Pericytes help with blood flow to nerve cells in the brain. They lie wrapped all along the huge networks of capillaries—the tiniest blood vessels—that both feed neurons and form the blood-brain barrier, which filters out certain substances from blood to protect the brain. Pericytes have a body that appears as a bump protruding from a capillary surface. Pericytes also have long thin arms that stretch along each capillary like a snake on a tree branch. These arms, called processes, reach almost to where the next pericyte process begins, without overlapping. This creates a pericyte chain that covers nearly the entire capillary network.
Pericytes are critical for blood vessel stability and blood-brain barrier function. They’re also known to die off as a result of trauma and disease. Andy Shih, Andree-Ann Berthiaume, and colleagues at the Medical University of South Carolina in Charleston, set up an imaging technique in mouse brains that allowed them to see what pericytes do under normal conditions as well as how these cells respond when some are damaged.
Imagine if scientists could zap a single cell (or group of cells) with a pulse of light that makes the cell move, or even turns on or off the cell’s vital functions.
Scientists are working toward this goal using a technology called optogenetics. This tool draws on the power of light-sensitive molecules, called opsins and cryptochromes, which are naturally occurring molecules found in the cell membranes of a wide variety of species, from microscopic bacteria and algae to plants and humans. These light-reacting molecules change their shape or activity when they sense light, so they can be used to trigger cellular activity, such as turning on or off ion flow into the cell and other regulatory pathways. The ability to induce changes in cells has a broad range of practical applications, from enabling scientists to see how cells function to providing the basis for potential therapeutic applications for blindness, cancer, and epilepsy.
Opsins first gained a foothold in research about a decade ago when scientists began using them to study specific electrical networks in the brain. This research relied on channelrhodopsins, opsins that could be used to control the flow of charged ions into and out of the cell. Normally, when a neuron reaches a certain ion concentration, it is triggered to fire, but neuron firing can be changed by inserting opsins in the membrane. Neuroscientists figured out how to incorporate light-sensitive opsin proteins by inserting the opsin gene into the host’s DNA. The genetically encoded opsin proteins in the neuronal membranes could be turned on or off by shining light into the brain itself, using optical fibers or micro-LEDs, to switch on or off the flow of ions and neuron firing.
Since those early studies in the brain, the optogenetics field has come a long way. But the leap from brain cells to other cells has been challenging. Scientists first needed to find a way to deliver light into tissues deep in the body. And, unlike stationary brain cells, they needed a way to target cells that are on the move (such as immune cells). They also needed to develop a way to study not only cell networks but also individual cells and cell parts. The NIGMS-funded researchers highlighted below are among the scientists working to overcome these obstacles and are using optogenetics in new and inventive ways.
Yubin Zhou of Texas A&M is using optogenetics to control the way cells communicate and to study immune cell function. In one line of research, Zhou is using light to make it easier for calcium ions to enter cells. The ions carry instructions for the cell and also help tether small cellular structures (called organelles). Those inter-membrane tethers allow for the movement of proteins and lipids back and forth across the cell, and are critical for sending chemical messengers to communicate information (see illustration). When this process is disrupted, it can lead to extreme changes in cell function and even cell death. Using this technology to “switch on” normal pathways enables the scientists to better understand how such processes can be disrupted.
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.
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 Bilder 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.
One day last fall, molecular biologist Laura Landweber 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.
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 “Genomic Gymnastics of a Single-Celled Ciliate and How It Relates to Humans”
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 Burridge and James Bear 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.
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.
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.
The problem is, it doesn’t happen quite like that. David Hughes, the Penn State University entomologist who reported his extensive field observations of the fungus/ant interactions in BMC Ecology , 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 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.