June 28, 2010

The geometric packing of cells within an epithelial sheet is precise, and the hexagonal shape of the cells helps this organization.
A paper from last year has looked at the packing geometry of epithelial fiber cells in the mouse eye lens to determine the role of actin and an associated protein called tropomodulin. The images above are 3D image reconstructions of actin within fiber cells, both in control (left) and tropomodulin mutant (right) lenses. The hexagonal packing of the fiber cells in the mutant is very irregular compared to control cells.

Reference: Roberta B. Nowak, Robert S. Fischer, Rebecca K. Zoltoski, Jerome R. Kuszak, and Velia M. Fowler, .2010. Originally published in Journal of Cell Bioloy. doi: 10.1083/jcb.200905065. Paper can be found here.

June 24, 2010

During mitosis, many structural, regulatory, and checkpoint proteins work towards the goal of accomplishing equal chromosome segregation. Microtubules of the mitotic spindle attach to kinetochores on chromosomes, and this attachment of microtubules to all kinetochores on all chromosomes provides a checkpoint signal that instructs the cell to undergo chromosome segregation. A protein called Spindly was previously known to localize the microtubule motor dynein to kinetochores. A recent paper has found an additional role for Spindly in coordinating microtubule attachment to kinetochores and mitotic checkpoint signaling. Image above shows mitotic spindles (PM, M) in control cells and cells with lower levels of Spindly. In cells with decreased Spindly, mitotic spindles are often abnormal – some are longer (L), some have more than two spindle poles (MP), and some are twisted (T). DNA is in blue, microtubules are in green, and kinetochores are in red.

Reference: Marin Barisic, Bénédicte Sohm, Petra Mikolcevic, Cornelia Wandke, Veronika Rauch, Thomas Ringer, Michael Hess, Günther Bonn, and Stephan Geley. Authors’ Molecular Biology of the Cell paper can be found here.

June 21, 2010

Morphogenesis is the process in which an embryo and its organs are shaped and formed, and is accomplished through organization of groups of cells into compartments. Prior to morphogenesis, cells are programmed to follow a specific cell fate, yet once morphogenesis begins there is typically little plasticity in the identities of the cells. However, a recent paper shows the reprogramming of certain cells during late morphogenesis of the fruit fly Drosophila, dependent on the JNK signaling pathway this is common to many species. Image above is a developing fly embryo undergoing dorsal closure, a well-studied example of morphogenesis during which cells stretch and zip up the tissue. Yellow cells are the “mixer cells” identified as being reprogrammable and able to cross compartment boundaries, while red and green cells intercalate to take their place during (top) and after (bottom) dorsal closure.

Reference: Melanie Gettings, Fanny Serman, Raphaël Rousset, Patrizia Bagnerini, Luis Almeida, Stéphane Noselli. Authors’ PLoS Biology paper can be found here.

June 17, 2010

Mesenchymal stem cells originate in the bone marrow, and are capable of differentiating into bone, cartilage, or fat cells depending on a variety of factors. A recent paper has found that cell shape plays a role in how these stem cells differentiate. When the mesenchymal stem cells are grown in shapes corresponding to higher contractility of the actin cytoskeleton, they are more likely to differentiate into bone cells, while cells in shapes of lower contractility are more likely to become fat cells. Image above shows cells grown in flower (top) or star (shapes). Cells on the left are stained for actin (green), focal adhesion protein vinculin (red), and DNA (blue); cells in the middle are stained for myosin. The heat maps on the right show high contractility in the star shaped cells, at the concave regions between the points of the star shape.

Reference: Kristopher A. Kilian, Branimir Bugarija, Bruce T. Lahn, and Milan Mrksich. Proc Natl Acad Science, 2010. 107(11): 4872–4877. doi: 10.1073/pnas.0903269107. Authors’ PNAS paper can be found here.

June 14, 2010

Sound is processed in our inner ear hair cells as a mechanosensory stimulus to stereocilia, protrusions on the surface of the cells. These stereocilia are made up of a very ordered array of interconnected actin filaments, and our hearing is dependent on this precise structure. TRIOBP, a protein that when mutated causes a form of human deafness, was recently found to play an important role in bundling the actin filaments in stereocilia. Electron microscopy images above show mouse inner ear hair cells lacking TRIOBP, with (left) or without (right) the links that interconnect stereocilia to one another. This experiment was used to test whether or not these extracellular links regulate the stiffness of the stereocilia, which would in turn affect hearing.

Reference: Shin-ichiro Kitajiri, Takeshi Sakamoto, Inna A. Belyantseva, Richard J. Goodyear, Ruben Stepanyan, Ikuko Fujiwara, Jonathan E. Bird, Saima Riazuddin, Sheikh Riazuddin, Zubair M. Ahmed, Jenny E. Hinshaw, James Sellers, James R. Bartles, John A. Hammer III, Guy P. Richardson, Andrew J. Griffith, Gregory I. Frolenkov and Thomas B. Friedman. Cell 141, 786-798. ©2010 Elsevier Ltd All rights reserved. Paper can be found here.

June 10, 2010

In the cell, there are many proteins that function in several different processes. Two of these proteins are caspase-8 and Rab5. Caspase-8 is primarily known for its role in promoting apoptosis (programmed cell death), while Rab5 is known for its involvement in endocytosis. A paper early this year reveals that Rab5 functions downstream of an activated caspase-8 signal to alter adhesion and migration of a cell. Images above show colocalization of Rab5 (red) and actin (blue) at membrane ruffles of migrating cells expressing caspase-8 (bottom), compared to control cells (top).

Reference: Vicente A. Torres, Ainhoa Mielgo, Simone Barbero, Ruth Hsiao, John A. Wilkins, and Dwayne G. Stupack. Authors’ Molecular Biology of the Cell paper can be found here.

June 7, 2010

Endocytosis, the uptake of material into a cell, often involves the structural protein clathrin. Clathrin forms a curved lattice on the plasma membrane that buds inward and eventually pinches off, carrying the material in a clathrin-coated vesicle. Last year, a group carefully measured the assembly and dynamics of clathrin structures and found that there are two distinct clathrin structures that are regulated differently, and interact with different cellular structures. Clathrin coated pits are the rapidly forming and sharply curved canonical structures of clathrin-mediated endocytosis, while clathrin coated plaques are longer-lived and less sharply curved structures. Electron microscopy image above shows both clathrin coated pits (top row) and plaques (bottom) in HeLa cells.

Reference: Saveez Saffarian, Emanuele Cocucci, Tomas Kirchhausen. Authors’ PLoS Biology paper can be found here.

June 3, 2010

Cytokinesis is the division of a cell’s cytoplasmic contents after mitosis. This complex process depends on many proteins that regulate the contractile ring, which constricts until the two new daughter cells are completely separate. A recent paper has looked into the mechanisms of the contractile ring in spermatocytes in the fruit fly Drosophila. The authors found that cytokinesis can be completed by two different complexes of proteins that stabilize actin and myosin at the contractile ring—an anillin-septin complex and a cadherin-catenin complex. Image of spermatocytes above shows colocalization of actin(red) and anillin(green) at sites of cytokinesis (arrows).

Reference: Philip Goldbach, Raymond Wong, Nolan Beise, Ritu Sarpal, William S. Trimble, and Julie A. Brill. Authors’ Molecular Biology of the Cell paper can be found here.