HighMag: February 2012

February 27, 2012

Yeast is magical. It gives our bread and beer its deliciousness, and provides biologists with a fantastic tool for understanding cell biology. Many of our monumental cell biology discoveries were due to yeast, so please whisper a heartfelt “Thank you” to yeast the next time you enjoy a beer. Today’s image is from a paper describing the structure of the septin network required for cell division in yeast.

Many yeast species divide by budding – a mother cell replicates its genome within its nucleus while a small bud forms and grows. The nucleus divides and the bud splits off of the mother cell. This split between mother and bud, or cytokinesis, depends on structural proteins called septins. Although the structure and function of septins has been studied for years, exactly how they are arranged at the bud neck of dividing yeast was not clear. Despite the small size of the bud neck, Bertin and colleagues recently imaged septin ultrastructure in dividing yeast cells by using improved techniques of electron microscopy that allow better preservation of membranes, combined with three-dimensional reconstruction of images. Specifically, Bertin and colleagues found septin filaments that ran both parallel and perpendicular to the mother-bud axis. In the images above, a view of the bud neck near the top of the membrane (top) shows circumferential filaments (green arrows). In a deeper view of the bud neck (bottom), filaments that follow the contour of the bud neck (next to red lines) can be seen, as well as a cross-sectional view of the circumferential filaments (green arrows).

Bertin, A., McMurray, M., Pierson, J., Thai, L., McDonald, K., Zehr, E., Garcia, G., Peters, P., Thorner, J., & Nogales, E. (2011). Three-dimensional ultrastructure of the septin filament network in Saccharomyces cerevisiae Molecular Biology of the Cell, 23 (3), 423-432 DOI: 10.1091/mbc.E11-10-0850

February 23, 2012

My bucket list has a few unrealistic things on it, such as “establish a close friendship with an elephant.” When I see how fast imaging technology improves, my goal to “float in a cell with swimmies on my arms” may not be as far off base as finding my BFF pachyderm. Although I may never be able to dive into a cell, I’m willing to bet I can eventually find myself standing in some amazing 3D images of a cell. Today’s image is from a paper describing improved imaging methods that produce super-resolution images that’ll knock your socks off.

Image resolution refers to the ability to distinguish two closely positioned lines or objects. Super-resolution imaging techniques have allowed biologists to see details in small structures, with resolutions down to about 20nm. A group of biologists recently pushed this resolution limit even further, and by doing so, was able to capture images of individual actin filaments with amazing detail. Xu and colleagues improved the 3D STORM (stochastic optical reconstruction microscopy) imaging methods by using two objective lenses to double the amount of fluorescent signal collected off of a cell sample. By imaging individual actin filaments in cells using this dual-objective STORM method, the 3D ultrastructure of the cytoskeleton revealed two layers of actin networks in a sheet-like membrane protrusion. The images above show the distinct organization of each of these two layers, each with a thickness of about 30-40nm. Color coding of the filaments (violet to red) provides a representation of the three-dimensional space (violet actin is closest to the glass coverslip, while red is furthest from the coverslip).

Xu, K., Babcock, H., & Zhuang, X. (2012). Dual-objective STORM reveals three-dimensional filament organization in the actin cytoskeleton Nature Methods, 9 (2), 185-188 DOI: 10.1038/nmeth.1841

February 20, 2012

Full disclosure: today’s image is near and dear to me. Today’s image is from a paper written by members of my former graduate lab, with some very close friends as the co-first authors. This is not to say that this paper isn’t utterly fascinating with sparkling images and fantastic experiments designed by some of the brightest scientists around (enough superlatives for you?), because it is all of the above and more. I’m just disclosing my bias so if today’s post sounds like a love letter, you’ll let it slide.

During development, it is necessary for cells to change their shape—it allows cells and sheets of cells to form into specific structures and tissues. Apical constriction drives cell shape changes in many cell types by contracting the actin-myosin network on the apical side of the cell, causing that side of the cell to shrink. After apical constriction, cells are shaped more like wedges, and a whole sheet of apically constricting cells can result in dramatic changes. For example, apical constriction drives the folding and closing of neural plate cells to form the neural tube, which later becomes our brain and spinal cord. A recent paper looks at apical constriction during gastrulation in the worm embryo, at the start of which two cells on the outside of the embryo apical constrict and are internalized into the middle of the embryo. Roh-Johnson and colleagues found that the actin-myosin networks were dynamic and contracting long before the cells showed any type of shape change, functioning as a molecular “clutch.” The cells were internalized only after the actin-myosin contractions appeared to have been mechanically linked to the cell-cell contact zone. Roh-Johnson and colleagues found this same molecular clutch in apically constricting cells in the developing fruit fly, suggesting that this mechanism might be a key component of apical constriction across the board. The images above show two timepoints of a worm embryo in the process of internalizing the two cells at the start of gastrulation (bottom image is about 6 minutes after the top image). Green is myosin, red is membranes, and blue marks the surfaces of the internalizing cells. Image credit: Chris Higgins and Liang Gao.

BONUS!! Check out a fancy little blurb about this paper in The Scientist here (complete with movie!).

BONUS!! Do you like worms? Of course you do! Mosey over to the Goldstein Lab’s site for more movies here.

Roh-Johnson, M., Shemer, G., Higgins, C., McClellan, J., Werts, A., Tulu, U., Gao, L., Betzig, E., Kiehart, D., & Goldstein, B. (2012). Triggering a Cell Shape Change by Exploiting Preexisting Actomyosin Contractions Science DOI: 10.1126/science.1217869

February 16, 2012

Dynein is a microtubule motor that resides at the cortex of a cell and can position an entire mitotic spindle. To visualize this, it would help if you were aware of one my killer dance moves…I stand in one spot and reel in a friend with my invisible lasso to dance. And, that friend may or may not have a look of total embarrassment (pity?) on his or her face. While dynein will never beat me in a dance-off, it is a pretty spectacular motor protein. Check out today’s image from a paper describing exactly how dynein can generate pulling forces.

Proper positioning of the mitotic spindle is important for cell division, especially when a cell has to divide asymmetrically to result in two cells of different sizes. Dynein is a microtubule motor that resides at the cortex of a dividing cell and can reel in and position an entire spindle. Cortical dynein functions this way in contexts outside of mitosis too—during migration, for example, dynein may help position the microtubule-nucleating centrosome correctly for trafficking of membrane vesicles. Recently, a group of cell biologists looked at exactly how dynein interacts with microtubules to generate a pulling force, and did so by taking dynein and microtubule asters out of cells and into chambers with microfabricated barriers. In this paper, Laan and colleagues looked at how dynein, attached to the fabricated barriers, interacted with microtubules. By capturing microtubules head-on, dynein regulated microtubule dynamics and length. When the microtubule ends were shrinking, dynein generated a pulling force strong enough to center the microtubule aster in the chamber. Images above show microtubule asters in the microchambers with barriers either coated with dynein or not. Without dynein at the barrier, microtubules continued to grow after reaching the barrier then buckled. With dynein-coated barriers to interact with, microtubules were captured by dynein and stopped growing, mostly remaining straight.

Laan, L., Pavin, N., Husson, J., Romet-Lemonne, G., van Duijn, M., López, M., Vale, R., Jülicher, F., Reck-Peterson, S., & Dogterom, M. (2012). Cortical Dynein Controls Microtubule Dynamics to Generate Pulling Forces that Position Microtubule Asters Cell, 148 (3), 502-514 DOI: 10.1016/j.cell.2012.01.007
Copyright ©2012 Elsevier Ltd. All rights reserved.

February 13, 2012

Our nervous system would be in trouble without myelin sheaths and nodes of Ranvier. No, those two things do not refer to some kind of Lord of the Rings-type silliness. They are very important components of our nervous system that ensure fast and efficient signal conduction.

Myelin sheaths are membranes that insulate the axons of many neurons. Myelin sheaths have distinct domains of ion channels and proteins, such as the nodes of Ranvier, along the axon that are required for the high speed and efficiency of signal conduction along the axon. The nodes of Ranvier, for example, are especially important for swift movement of an axon’s action potential, which jumps from node to node in a process termed staltatory conduction. A recent paper describes the importance of a cytoskeletal adaptor protein called 4.1G in regulating the localization of proteins along the axon-sheath interface. Ivanovic and colleagues found that in mice without 4.1G, adhesion proteins and neuronal proteins were mislocalized. Images above show localization of 4.1G at the same sites as two other periaxonal membrane proteins (MAG on left, Necl4 on right) in adult mouse sciatic nerves.

Ivanovic, A., Horresh, I., Golan, N., Spiegel, I., Sabanay, H., Frechter, S., Ohno, S., Terada, N., Mobius, W., Rosenbluth, J., Brose, N., & Peles, E. (2012). The cytoskeletal adapter protein 4.1G organizes the internodes in peripheral myelinated nerves originally published in the Journal of Cell Biology, 196 (3), 337-344 DOI: 10.1083/jcb.201111127

February 9, 2012

We all know that exercise is good for our bodies, and when we hear people talking about it in the media, the benefits are discussed in big-picture terms. A recent paper describes the effects of exercise at the cellular level, and gives me new motivation to get my ass in gear. Well, after I finish this heart-shaped Dunkin’ Donut (don’t give me that smug look…you know it’s delicious).

Autophagy is the process in which a cell metabolizes its own organelles and proteins. Autophagy takes place in the lysosome at a normal rate to rid the cell of old organelles, but is induced at higher rates in response to cellular stress to allow the cell to adjust to changing nutritional needs. A recent study finds that exercise can induce autophagy in muscle cells. In this paper, He and colleagues tracked autophagy in mice after they ran on treadmills. As seen in the images above, the number of autophagosomes (green dots) in the tibialis anterior muscle was higher in mice after 80 minutes of exercise (right), compared to before the exercise (left). Mice with a genetic mutation that prevented exercise-induced autophagy had lower endurance for exercise and had altered glucose metabolism. These fascinating findings provide us with a cellular understanding of how exercise prolongs life and protects our bodies from diseases such as diabetes and cancer.

He, C., Bassik, M., Moresi, V., Sun, K., Wei, Y., Zou, Z., An, Z., Loh, J., Fisher, J., Sun, Q., Korsmeyer, S., Packer, M., May, H., Hill, J., Virgin, H., Gilpin, C., Xiao, G., Bassel-Duby, R., Scherer, P., & Levine, B. (2012). Exercise-induced BCL2-regulated autophagy is required for muscle glucose homeostasis Nature, 481 (7382), 511-515 DOI: 10.1038/nature10758
Adapted by permission from Macmillan Publishers Ltd, copyright ©2012

February 6, 2012

Watching your child learn to crawl, you realize how much coordination she needs to get up on all fours and move forward, working both sides of her body. You are convinced that your child is totally gifted and brilliant. Well, I have news for you…cells have a lot more to sort out in order to crawl. As you sheepishly compare your child’s brilliance to a cell and admit defeat (except for me, of course…my daughter really IS brilliant), take a moment to look at today’s beautiful images from a paper on cell migration.

When a cell is crawling, it first reaches out using membrane protrusions. At the leading edge of these protrusions, the cell will adhere to the underlying matrix. These nascent adhesions serve as anchors to the surface and give the crawling cell traction. Cell-matrix adhesions go through dynamic cycles of formation as nascent adhesions, maturation into focal adhesions, and turnover using a well-studied set of cytoskeletal proteins and regulators, but how these adhesions form and mature is not completely understood. Lawson and colleagues recently published results showing that a protein called FAK (focal adhesion kinase) promotes the recruitment of an adhesion protein called talin to nascent adhesions. Talin binds to integrin, a key adhesion protein, and was previously thought to recruit FAK to nascent adhesions. In the images above, a control cell (left) shows localization of talin (green) to nascent adhesions (red). However, without FAK (right), talin is not recruited to nascent adhesions.

Lawson, C., Lim, S., Uryu, S., Chen, X., Calderwood, D., & Schlaepfer, D. (2012). FAK promotes recruitment of talin to nascent adhesions to control cell motility originally published in the Journal of Cell Biology, 196 (2), 223-232 DOI: 10.1083/jcb.201108078

February 2, 2012

When a cell makes it all the way to cytokinesis, it has already achieved greatness. DNA replication and mitosis are Big Deals, but a cell exits mitosis only to find itself in front of that final all-uphill mile of the New York City Marathon (even as a kid watching it, I thought that was so cruel). There is a lot of regulation and reorganizing that happens for a cell to correctly complete cell division and physically split into two cells, and a recent paper sorts out how membrane trafficking proteins are coordinated during the process.

Cytokinesis is the final step of cell division, when the two daughter cells are physically divided. At the start of cytokinesis, a contractile ring forms around the center of the dividing cell and begins to tighten. These contractions result in a cleavage furrow forming and pinching the dividing cell, after which only an intercellular bridge connects the two new daughter cells. With all of this contracting and pinching of the plasma membrane, it is no surprise that membrane trafficking proteins are important during cytokinesis. A recent paper looks at how endocytosis and membrane trafficking pathways are coordinated during cytokinesis. Specifically, Chesneau and colleagues found that Rab35 GTPase, an endocytic protein known to also be important in cytokinesis, is negatively regulated by ARF GTPase. ARF mutants (ones that are stuck in the activated GTP state, for those paying attention) have cytokinesis defects similar to Rab35 mutants (stuck in the inactive GDP state), including a failure after cleavage furrow progression and an instability of intercellular bridges. A seen in the images above, both the ARF mutant (bottom) and Rab35 mutant (middle) mislocalize a protein called SEPTIN2 (green, arrowheads), which is a cytoskeletal element that provides structure during cytokinesis. In a normal cell (top), SEPTIN2 is localized at the cleavage furrow.

Chesneau, L., Dambournet, D., Machicoane, M., Kouranti, I., Fukuda, M., Goud, B., & Echard, A. (2012). An ARF6/Rab35 GTPase Cascade for Endocytic Recycling and Successful Cytokinesis Current Biology DOI: 10.1016/j.cub.2011.11.058