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