Kinetochores are multi-protein structures that serve as the sites of spindle attachment for each chromosome during mitosis. Kinetochores are assembled on regions of the chromosomes known as the centromere. Although the importance of the centromere has long been appreciated, the exact qualities of centromere-ness remain unresolved. Some past research shows that centromere location in budding yeast is based on DNA sequence, while other organisms rely on repeated DNA sequences or epigenetic marks (meaning not due to DNA sequence) to identify the location of a centromere. A recent paper describes a technique that triggers brief over-expression of one of those epigenetic marks—the protein CID (Drosophila CENP-A or CENH3 for those keeping score). After a pulse of this over-expression, chromosomes are coated with CID signal (green, middle image above), yet after a few generations of cell division, those CID marks are mostly cleared away (right image). The remaining CID marks (arrows), Olszak and colleagues found, became sites of new functional kinetochores. By monitoring the early steps of kinetochore formation, these biologists hope to understand how centromere position is determined.
Agata M. Olszak, Dominic van Essen, António J. Pereira, Sarah Diehl, Thomas Manke, Helder Maiato, Simona Saccani & Patrick Heun (2011). Nature Cell Biology 13,799–808. DOI: 10.1038/ncb2272.
Adapted by permission from Macmillan Publishers Ltd, copyright 2011
They invade...they proliferate...they destroy. It sounds like the tagline for a terrible summer blockbuster starring Samuel L. Jackson and an animated sidekick voiced by one of the Kardashians, but it’s the tagline of something far more sinister and real. I’m talking about tumors. Today’s image is from a paper showing how a membrane protein called caveolin-1 can support tumor invasion.
Caveolin-1 is a membrane protein and a major component of caveolae, which are small membrane invaginations that participate in endocytosis. A recent paper finds that caveolin-1 also functions in cell elongation, migration, and invasion by remodeling a cell’s microenvironment (aka “stroma”). Specifically, Goetz and colleagues found that caveolin-1 affects stromal architecture by regulating the activity of Rho GTPase, a signaling protein frequently involved in actin dynamics. This caveolin-1-inspired remodeling of the stroma is significant for tumor biology, too—the stiffness, contractility, and general architecture of a tumor’s stroma can affect its growth, invasion, and metastasis. In the images above, tumor cells (green) were cultured in a 3D-gels with fibroblast cells (red) that expressed caveolin-1 (top row) or did not express caveolin-1(bottom row). When tumor cells were surrounded by caveolin-1-expressing cells, they were able to invade further into the gel.
Goetz, J., Minguet, S., Navarro-Lérida, I., Lazcano, J., Samaniego, R., Calvo, E., Tello, M., Osteso-Ibáñez, T., Pellinen, T., Echarri, A., Cerezo, A., Klein-Szanto, A., Garcia, R., Keely, P., Sánchez-Mateos, P., Cukierman, E., & Del Pozo, M. (2011). Biomechanical Remodeling of the Microenvironment by Stromal Caveolin-1 Favors Tumor Invasion and Metastasis Cell, 146 (1), 148-163 DOI: 10.1016/j.cell.2011.05.040
Copyright ©2011 Elsevier Ltd. All rights reserved.
When I think of mitochondria, I’m faced with a minor bout of nausea when I remember struggling to memorize all of the steps to oxidative phosphorylation during college. Although my college memories of Napster and the Y2K problem are clearer than those of the citric acid cycle, I know how important mitochondria are. A recent paper describes how mitochondria are anchored throughout the cell.
Mitochondria are organelles that provide metabolic energy to the cell. Depending on the energy needs in different regions of the cell, mitochondria move around using actin- and microtubule-based motors and then anchor themselves in place. A recent paper describes how intermediate filaments bind mitochondria to regulate their distribution and anchor them within the cell. Intermediate filaments provide mechanical strength in many cell types by forming rope-like networks of filaments, and are frequently made of a protein called vimentin. Nekrasova and colleagues found that in cells lacking vimentin, mitochondria were highly mobile within the cell. Images above show the colocalization of mitochondria (purple) and vimentin intermediate filaments (green) in mammalian cells. Middle and right images are higher magnification frames of the boxed regions.Nekrasova, O., Mendez, M., Chernoivanenko, I., Tyurin-Kuzmin, P., Kuczmarski, E., Gelfand, V., Goldman, R., & Minin, A. (2011). Vimentin intermediate filaments modulate the motility of mitochondria Molecular Biology of the Cell, 22 (13), 2282-2289 DOI: 10.1091/mbc.E10-09-0766
Eisosomes are regions on the plasma membrane in budding yeast that are important for plasma membrane organization. They are positioned at the plasma membrane, where they sort membrane proteins and signaling molecules into small membrane invaginations. There is a long list of proteins found at eisosomes, many of which are uncharacterized. A recent paper looks at the functions of two core eisosome proteins – Pil1 and Lsp1. Olivera-Couto and colleagues found that these two proteins contain BAR domains, which are able to sense and change the curvature of membranes. Images above are electron micrographs of liposomes, which are artificially made vesicles. Untreated liposomes (top) are round, while vesicles treated with purified Pil1 or Lsp1 (middle and bottom) had tubules (arrows) deformed from the vesicles, showing that these proteins are capable of bending membranes.
Olivera-Couto, A., Grana, M., Harispe, L., & Aguilar, P. (2011). The eisosome core is composed of BAR domain proteins Molecular Biology of the Cell, 22 (13), 2360-2372 DOI: 10.1091/mbc.E10-12-1021
Cellular imaging is a constantly evolving field made of biologists on a never-ending quest for higher resolution of structures and faster image acquisition of a living cell. There are several challenges to these demands. For example, cells are not pancake-thin. Current techniques use illumination that leads to background noise in an image due to excited out-of-focus light. In addition, these techniques can cause phototoxic effects on cells, and can photobleach the fluorescent tags used to mark structures. Biologists have addressed these problems by using plane-illumination microscopy, which uses a separate excitation lens positioned orthogonally to the detection objective lens, leading to a more confined excitation of the focal plane. Planchon and colleagues recently improved this technique by using thinner sheets of light to illuminate the sample. The images produced using this Bessel beam plane illumination are remarkable, and allow for very fast 3D imaging of living cells. Images above show filopodia on a HeLa cell (left column), and the membrane ruffles on a kidney cell (right group of images). Purple arrowheads point to vacuole formation by macropinocytosis.
BONUS!! For a movie of the filopodia in the image above, click here. For a movie of the membrane ruffles and vacuole formation, click here. For many other knock-your-socks-off movies, click here.
Planchon, T., Gao, L., Milkie, D., Davidson, M., Galbraith, J., Galbraith, C., & Betzig, E. (2011). Rapid three-dimensional isotropic imaging of living cells using Bessel beam plane illumination Nature Methods, 8 (5), 417-423 DOI: 10.1038/nmeth.1586
Adapted by permission from Macmillan Publishers Ltd, copyright 2011
Osteoblasts are the cells responsible for generating bone tissue. They are differentiated from mesenchymal stem cells, and a recent paper identifies a role for pannexin 3 in this process. Pannexins are gap junction proteins, which means they form channels that allow material to be exchanged between cells, or between a cell and its extracellular space. On the cellular level, Ishikawa and colleagues found that pannexin 3 serves many purposes in osteoblasts—as a channel for calcium ions on the ER, as a channel for extracellular release of ATP, and as a channel for the exchange of calcium waves between cells. Image above shows a newborn mouse growth plate (left) that is stained for visualization of pannexin 3 (green) and an osteoblast marker (red).
Ishikawa, M., Iwamoto, T., Nakamura, T., Doyle, A., Fukumoto, S., & Yamada, Y. (2011). Pannexin 3 functions as an ER Ca2+ channel, hemichannel, and gap junction to promote osteoblast differentiation originally published in The Journal of Cell Biology DOI: 10.1083/jcb.201101050
During development, dramatic rearrangements of epithelial sheets results in the formation of branched tubules, as seen in kidney, lung, and mammary gland tissue. As one might expect, these rearrangements require coordination of several cellular events such as cell division, migration, polarization, and adhesion. A recent paper describes the role of two adhesion proteins, E-cadherin and cadherin-6, in tubule formation. Jia and colleagues found that cadherin-6 is important in inhibiting tubule formation, while E-cadherin is important in the formation of a tubule’s lumen (its inside cavity). Images above show the use of cell cysts as a model for epithelial tubule and lumen formation, with fluorescent tags showing a lateral marker (blue) and lumen-facing apical markers (green and red). Samples of control cysts, cysts without cadherin-6, E-cadherin, or both are shown (moving left to right). Although the mutant cysts appear abnormal, polarization was not disrupted in cysts without either cadherin (although multiple lumens are visible in cysts lacking E-cadherin). The polarization of cysts lacking both cadherins, however, was completely disrupted.
Jia, L., Liu, F., Hansen, S., ter Beest, M., & Zegers, M. (2011). Distinct roles of cadherin-6 and E-cadherin in tubulogenesis and lumen formation Molecular Biology of the Cell, 22 (12), 2031-2041 DOI: 10.1091/mbc.E11-01-0038