March 26, 2015

The actin cytoskeleton at the leading edge of a crawling cell has been a source of both scientific fascination and stunning images for biologists. Today’s image is from a paper that sheds light on how that complex structure is generated. 

The outermost region of a crawling cell’s cortex is called the lamellipodium, and it depends on a complex actin cytoskeleton for its structure and rapid dynamics. The generation of branched actin filaments at lamellipodia requires the activity of the actin-nucleating Arp2/3 protein complex. In a recent study, Henson and colleagues use the very flat and Arp2/3-rich lamellipodia of sea urchin coelomocytes to visualize the actin cytoskeleton. As seen in the images above, treatment of coelomocytes with a drug that inhibits the Arp2/3 complex resulted in drastic changes to the actin cytoskeleton. Specifically, the densely-packed branched filaments of lamellipodia (left, control) were replaced with actin filament arcs that ran diagonal or parallel to the cell’s edge (30 and 60 seconds after drug treatment, middle and right images). These transverse actin arcs may act as mother/scaffold filaments from which more actin filaments are nucleated during organization of the cytoskeleton in lamellipodia.

Henson, J., Yeterian, M., Weeks, R., Medrano, A., Brown, B., Geist, H., Pais, M., Oldenbourg, R., & Shuster, C. (2015). Arp2/3 complex inhibition radically alters lamellipodial actin architecture, suspended cell shape, and the cell spreading process Molecular Biology of the Cell, 26 (5), 887-900 DOI: 10.1091/mbc.E14-07-1244

February 20, 2015

Even the simplest and most elegant song or painting still has a complicated story behind it. That’s what I’m thinking about as I read about cilia today. Cilia are simple and beautiful, but the axonemal structure of cilia is far more complex than one might first appreciate. Today’s image is from a paper describing a protein required for one of the ciliary radial spokes. 

Motile cilia are structures on the surface of some microscopic organisms and certain types of cells, and function in locomotion or the movement of fluid over the cell. Inside each of these cilia is a microtubule-based axoneme structure—9 outer doublets of microtubules form a circle around a central microtubule pair, with radial spokes connecting the center pair with the outer microtubules. These radial spokes are important for regulating ciliary motility. The cilia of most species have three radial spokes, but these spokes are not identical to one another, suggesting that each spoke has a unique functional role. In a recent collaboration between the Nicastro and Gaertig labs, Vasudevan and colleagues found that the ciliary protein FAP206 likely serves as a microtubule docking protein for one of the radial spoke proteins (RS2) and dynein c. In the top images above, FAP206-GFP can be seen exclusively in the cilia of interphase (left) and dividing (right) Tetrahymena cells. In the absence of FAP26, the axoneme lacked proper assembly of the radial spoke RS2. Cryo-electron tomography images (bottom) show a wild-type axoneme (left), with RS2 connecting to the A-tubule of an outer microtubule doublet, compared to an axoneme lacking FAP206, which lacks RS2 (right, arrowhead).

Vasudevan, K., Song, K., Alford, L., Sale, W., Dymek, E., Smith, E., Hennessey, T., Joachimiak, E., Urbanska, P., Wloga, D., Dentler, W., Nicastro, D., & Gaertig, J. (2014). FAP206 is a microtubule-docking adapter for ciliary radial spoke 2 and dynein c Molecular Biology of the Cell, 26 (4), 696-710 DOI: 10.1091/mbc.E14-11-1506

February 12, 2015

Biologists have to wear many hats, and one under-appreciated hat is that of marketing executive. You have to properly name whatever process/protein/structure you just identified so it will be easily remembered. Whoever coined the term “invadopodia” was spot-on….the term is informative, catchy, and ignites my imagination of what it’s like inside a cell. Today’s image is from a fascinating paper on invadopodia formation.

Invadopodia are dynamic protrusions of plasma membrane that locally degrade a cell’s underlying extracellular matrix (ECM). A tumor cell’s invadopodia mediate the invasion of tissue and metastasis. A recent paper describes a study of invadopodia formation within the context of a highly-concentrated collagen matrix, to better mimic the ECM of cancerous tissue. This dense collagen network, Artym and colleagues found, triggers robust invadopodia formation and ECM degradation, in both cancerous and non-cancerous cell lines. This invadopodia formation did not require altered gene or protein expression, but did require phosphorylation of kindlin2, part of a complex integrin regulatory network. As seen in the images, the high-density fibrillary collagen (HDFC, top) network triggered the induction of many more invadopodia (yellow dots) than a gelatin-based matrix (bottom).  

Artym, V., Swatkoski, S., Matsumoto, K., Campbell, C., Petrie, R., Dimitriadis, E., Li, X., Mueller, S., Bugge, T., Gucek, M., & Yamada, K. (2015). Dense fibrillar collagen is a potent inducer of invadopodia via a specific signaling network originally published in the Journal of Cell Biology, 208 (3), 331-350 DOI: 10.1083/jcb.201405099

January 8, 2015

If you are lucky in life, there is at least one person who will always be there for you—a parent, your spouse, maybe even your pooch. As we understand more and more of what goes on inside a cell, it has become clear that actin is always there for the cell’s many organelles. Actin is so supportive and encouraging, and without it our cells would just be puddles of fats and proteins. Today’s images are from a paper describing the role of actin in mitochondrial fission. 

Mitochondria are dynamic organelles that divide by fission. Although a role for the actin cytoskeleton in mitochondrial fission has been suggested, the exact mechanism is unclear. Recent work by Li and colleagues shows a transient association of F-actin (filamentous actin) to mitochondria at the start of fission. Downregulation of the actin regulators cortactin, cofilin, and Arp2/3 caused elongation of mitochondria. Li and colleagues tested the role of Drp1, which is a key player in mitochondrial division, on F-actin assembly during fission. Drp1 inhibition prolonged the localization of F-actin and several actin regulators at mitochondria during fission. In the left group of images above, F-actin (green) and mitochondria (red) are visible in a control mammalian cell (bottom row is at higher magnification). The group of images on the right shows mammalian cells after chemical induction of mitochondrial fission: 2 minutes after drug treatment, many F-actin-rich mitochondria are visible.

Li, S., Xu, S., Roelofs, B., Boyman, L., Lederer, W., Sesaki, H., & Karbowski, M. (2014). Transient assembly of F-actin on the outer mitochondrial membrane contributes to mitochondrial fission The Journal of Cell Biology, 208 (1), 109-123 DOI: 10.1083/jcb.201404050

December 18, 2014

You might not want the dreaded tube socks in your Christmas stocking this year, but you do appreciate the actual tubes that your body depends on in just about every organ system. A recent paper in PLOS Biology describes tube formation in the fly renal system and the signals that regulate it.

Tubes generally start as buds that dramatically elongate during development, but the cell rearrangements that occur during tubulogenesis are not completely understood. Saxena and colleagues recently used the developing fly renal system to track cell movements during tube formation. Tubule elongation primarily occurs through convergent extension, during which cells intercalate along the length of the tube. During these rearrangements, the number of cells around the circumference of the tube drops as the number of cells along the tube increases. Saxena and colleagues show that epidermal growth factor localized at the tip cells of the distal end of the tube guides the polarity of cell rearrangements, via polarization of Myosin II within individual cells. Finally, without proper tube elongation, animals have abnormal excretory function and osmoregulation, leading to lethality. In the images above, the top row shows failure of tube elongation after laser ablation of the distal tip cells (arrowheads). Bottom row shows normal tube elongation without laser ablation of tip cells (arrowheads).

Saxena, A., Denholm, B., Bunt, S., Bischoff, M., VijayRaghavan, K., & Skaer, H. (2014). Epidermal Growth Factor Signalling Controls Myosin II Planar Polarity to Orchestrate Convergent Extension Movements during Drosophila Tubulogenesis PLoS Biology, 12 (12) DOI: 10.1371/journal.pbio.1002013

November 26, 2014

Patterns are soothing for left-brained folks like me, with the exception being those terrible patterned holiday sweaters that will come out of mothball-ridden closets soon (unsettling for everyone, really). Today’s images are from a paper describing a new micropatterning technique to look at plasma membrane proteins. 

The plasma membrane of a cell is riddled with many multi-protein complexes that facilitate communication and transport. These complexes provide a challenge to biologists due to their ubiquitous localization around the cell, their large, complex size, and their transient interactions with proteins. A recent paper describes a technique to study signaling complexes at the plasma membrane, using micropatterns within the plasma membrane. Löchte and colleagues expressed a protein bait in a micropattern in a living cell’s plasma membrane. Dynamics of the interactions between the protein bait and ligand target can then be quantified using live microscopy and on a single-molecule level. Löchte and colleagues used the IFN interferon signaling complex, using one of the receptor units (IFNAR2) as bait. In the top images above, the micropatterned receptor bait (IFNAR2) was able to recruit the other IFN receptor subunit (IFNAR1, green) after the addition of the ligand (red; time represents the addition of the ligand). Bottom image shows a closer view of the high-affinity, micropatterned binding that requires simultaneous interaction of the ligand (red) with both receptor subunits.

Lochte, S., Waichman, S., Beutel, O., You, C., & Piehler, J. (2014). Live cell micropatterning reveals the dynamics of signaling complexes at the plasma membrane originally published in the Journal of Cell Biology, 207 (3), 407-418 DOI: 10.1083/jcb.201406032

November 19, 2014

You might not be able to get rid of the bad guys, but you can still win the battle if you cripple their mobility. Today’s image is from a paper describing how a tumor’s microenvironment can predict the motility of individual tumor cells.

Metastasis is the spread of cancer cells throughout the body. The motility of tumor cells depends on the microenvironment around them, and a recent paper systematically looks at how that microenvironment can predict or alter the behavior of tumor cells. Gligorijevic and colleagues tracked the motility of individual mouse mammary carcinoma cells in vivo using high-resolution multi-photon microscopy, and found that tumor cells exhibited either fast or slow locomotion. Those tumor cells with slow locomotion also exhibited invadopodia, protrusions that Gligorijevic and colleagues directly link to degradation of the underlying extracellular matrix and metastasis. While no single parameter of the tumor’s microenvironment could predict the locomotion of tumor cells, a support vector machine algorithm indicated how combinations of many parameters could predict tumor cell phenotype and behavior. By characterizing the heterogeneous microenvironment of a tumor and predicting the location and behavior of metastatic tumor cells, researchers can better understand treatment of tumors and the varying responses. Images above show protrusions (arrowheads, over 30 minutes) on two different tumor cells with slow locomotion, with protrusions facing collagen fibers (purple).

BONUS!! Check out a movie of these protrusions below. Note that some protrusion face collagen fibers (purple, panels a and b), and some protrude into blood vessels (red, panels c and d).

BONUS!! Check out Bojana Gligorijevic ‘s interview with SciArt about her images, research, and art here.

Gligorijevic, B., Bergman, A., & Condeelis, J. (2014). Multiparametric Classification Links Tumor Microenvironments with Tumor Cell Phenotype PLoS Biology, 12 (11) DOI: 10.1371/journal.pbio.1001995