April 10, 2014

When you host a party at your home, do you hire a caterer to bring in food or do you cook the food right there in your kitchen? One of these options leaves a lot more wiggle room for last-minute changes—a few extra guests, a gluten allergy, a pregnant lady with a disgust for wobbly desserts. A cell recognizes this distinction too. When making certain proteins, a cell will synthesize proteins where and when they’re needed. Today’s image is from Natasha Gutierrez, who recently published a study describing the role of β-actin mRNA and monomer synthesis in adherens junction assembly.

The actin cytoskeleton is made of actin filaments and countless actin-regulating proteins that guide the ever-changing dynamics of the cytoskeleton. Actin filament polymerization is regulated by localized synthesis of β-actin monomers from β-actin mRNA. A recent paper by Gutierrez and colleagues shows that the assembly of adherens junctions (AJs), epithelial cell-cell adhesion structures, requires localized β-actin monomer synthesis, the β-actin 3’ UTR and β-actin mRNA zipcode sequence at cell-cell contact sites. Additionally, active RhoA, which targets zipcode-mediated β-actin mRNA localization, is localized to cell-cell contact sites. In the unpublished images above, mammalian cells were treated with increasing levels (top to bottom) of a Rho inhibitor. The ability to form AJs, seen as the colocalization of actin filaments (left, green in merged) and E-cadherin (middle, red in merged) at cell-cell contact sites, decreased with increasing levels of the Rho inhibitor.

Gutierrez, N., Eromobor, I., Petrie, R., Vedula, P., Cruz, L., & Rodriguez, A. (2014). The B-actin mRNA zipcode regulates epithelial adherens junction assembly but not maintenance RNA DOI: 10.1261/rna.043208.113

April 5, 2013

Cell adhesion is sticky business.  See what I did there?!  Comedy. Gold.  Seriously, though, cell adhesion is complicated, with many types of cell adhesion structures that form at specific regions of the cell at specific times.  As important as it is to understand cell adhesion and its role in development, cancer, and normal cell function, we are all thankful for papers like the one that today’s image comes from.

Cadherins are transmembrane proteins that form cell-cell adhesion structures called adherens junctions.  There are several types of adherens junctions, but they are all composed of clusters of cadherins whose extracellular domains interact with other cells’ cadherins and intracellular domains interact with the cell’s cytoskeleton.  Individual cadherin molecules provide negligible adhesive properties, so understanding how cadherin clusters form is an important question.   A recent paper delves into the details of this process, and finds that actin filaments are indeed necessary for cadherin cluster stability.  Hong and colleagues found that cadherin clusters that were uncoupled from actin were unstable and exhibited random mobility.  When the actin-binding domain of a cadherin-actin adaptor protein called α-catenin (domain called αABD) was coupled to these mutant cadherin structures, the adhesive clusters regained stability and deliberate mobility.  The images above show clusters of this αABD-cadherin chimera (left, green in merged) associated with actin filaments (middle, red in merged; arrows in inset point to colocalization).

Hong, S., Troyanovsky, R., & Troyanovsky, S. (2013). Binding to F-actin guides cadherin cluster assembly, stability, and movement originally published in the Journal of Cell Biology, 201 (1), 131-143 DOI: 10.1083/jcb.201211054

April 30, 2012

One might think that once you’ve won your first Nobel Prize, it’s time to kick back and watch the youngsters do the dirty work of solving all of biology’s riddles. No so. Apparently, Nobel-ers don’t like to kick back at all, but continue to push the limits of our knowledge. I think they’re all wearing superhero capes underneath their biologist uniforms (plaid shirts and old jeans?). Today’s image is from the lab of Eric Wieschaus, in a paper that provides a fascinating alternative mechanism to the prevailing model of epithelial folding.

During development, groups of cells are shaped into tissues and organs in a process called morphogenesis. One of the earliest morphogenesis events is gastrulation, during which the embryo is organized into the three germ layers that will each develop into different tissues and organs. During gastrulation in the fruit fly, the dorsal side of the embryo undergoes two epithelial folds, the anterior and posterior dorsal transverse folds, at precise locations on the embryo. The current model of epithelial folding involves actin-myosin contractions that trigger the initial cell shape changes. Wang and colleagues recently found an alternative mechanism that underlies epithelial folding during fruit fly gastrulation. Specifically, Wang and colleagues found that the adherens junctions that form between epithelial cells relocate to more basal locations in the cells that initiate epithelial folding. The movement of adherens junctions, and in turn dorsal fold initiation, depends on the activity of the polarity proteins Bazooka and Par-1. In the images above, the initiation of anterior (pink arrow) and posterior (blue arrow) folds are visualized by high resolution live imaging. The localization of E-cadherin (white), a key adherens junction protein, drops from the apical surface (top of the cells) to a more basal location in the cells undergoing shape changes.

Wang, Y., Khan, Z., Kaschube, M., & Wieschaus, E. (2012). Differential positioning of adherens junctions is associated with initiation of epithelial folding Nature, 484 (7394), 390-393 DOI: 10.1038/nature10938

Adapted by permission from Macmillan Publishers Ltd, copyright ©2012

March 15, 2012

You likely know how important it is to maintain, repair, and even remodel your home as it gets older. Hypothetically, maybe your dog has chewed off your bedroom door’s frame in an attempt to greet your dinner guests, or maybe your toddler has drawn an abstract mural on your walls. Whatever the case, you head to your garage to find those necessary, but frequently overlooked, tools. Our cells are no different (except for dog- and toddler-induced damage). Today’s image is from a paper describing structures that are seen during cell-cell junction remodeling.

Endothelial cells line our blood vessels and maintain stable cell-cell junctions between one another to provide a tight barrier for our blood. These adherens junctions are damaged in cases of inflammation, atherosclerosis, and tumor angiogenesis, via endothelial signals and hormones. Vascular endothelial cadherin (VE-cadherin) is the central component of adherens junctions in these cells, and a recent paper by Huveneers and colleagues describes the finding that VE cadherin is found on both stable adherens junctions as well sites of junction remodeling called focal adherens junctions (FAJs). These newly-characterized FAJs are attached to actin bundles and contain Vinculin, which protects the junctions from opening during remodeling. In addition, FAJs were under pulling forces from the actin cytoskeleton during remodeling. Images above show human endothelial cells with FAJs (middle row, box 1) and stable adherens junctions (bottom row, box 2). Vinculin can be seen on FAJs, but not on stable junctions.

BONUS!! Check out some very cool movies of remodeling junctions here and here. All movies from this paper can be found here.

DOUBLE BONUS!! Not surprisingly, the author’s beautiful images impressed JCB so much that they made the cover here.

Huveneers, S., Oldenburg, J., Spanjaard, E., van der Krogt, G., Grigoriev, I., Akhmanova, A., Rehmann, H., & de Rooij, J. (2012). Vinculin associates with endothelial VE-cadherin junctions to control force-dependent remodeling originally published in the Journal of Cell Biology, 196 (5), 641-652 DOI: 10.1083/jcb.201108120

January 26, 2012

There are so many images that are in our collective memory…images that mark historic and significant events. There are the photos of Tiananmen Square, the “Migrant Mother” from the Great Depression, Abbey Road, etc. Well, cell biologists have our own images that stick in our collective memory. One of those more recent images is the “Svitkina image” of actin filaments, which I’ve mentioned before. So, when I saw that the Svitkina lab published a paper recently, I knew I had to share!

Cell-cell junctions are crucial for development, tissue structure, and cell-cell communication. One type of cell-cell junction is the adherens junction (AJ), which is a cadherin-based junction that links to the actin cytoskeleton within the cell. Although AJs are well-studied structures, how they assemble is still not completely known. A recent paper looks at the underlying actin filaments in developing AJs. According to Hoelzle and Svitkina, a junction is formed first by neighboring cells’ lamellipodia, sheet-like membrane extensions. Next, the two cells are connected by cadherin on thin bridges that look similar to filopodia, which are finger-like actin projections. Interestingly, these bridges form by actin filament growth from the rear-side of the lamellipodia towards the cell periphery. The images above are transmission electron micrographs of actin filaments in a bridge that connects two different cells (each cell labeled a different color in middle image). Gold beads (yellow, right image) found at the far ends of each cell’s bridge label VASP proteins, which are markers for filopodia.

Hoelzle, M., & Svitkina, T. (2011). The cytoskeletal mechanisms of cell-cell junction formation in endothelial cells Molecular Biology of the Cell, 23 (2), 310-323 DOI: 10.1091/mbc.E11-08-0719

January 12, 2012

For a little protein, the cell is a big place. Many times it’s necessary for proteins to be clustered together in order to get a job done. Today’s image is from a paper describing how E-cadherin gets clustered at adherens junction sites like a flock of 12-year old teeny-bopper girls at a Twilight movie.

Sheets of epithelial cells are polarized—one side of the epithelial sheet faces the inside/lumen of an organ or tissue, while the other attaches to a supportive basement membrane. The establishment and polarization of epithelial cells depends on adherens junctions (AJs), protein complexes that serve as cell-cell junction sites. AJs are composed of a transmembrane protein called E-cadherin that connects the junctions to the cell’s actin cytoskeleton. E-cadherin must be distributed on the cell’s plasma membrane for AJ assembly, but how it is brought to the membrane and/or clustered at certain sites is not fully understood. A recent paper finds an association between E-cadherin and the exocyst protein Exo70. The exocyst is a complex that brings proteins from the Golgi apparatus (where they are sorted) to the plasma membrane. According to Xiong and colleagues, Exo70 is required for E-cadherin clustering at the plasma membrane and for maturation of newly-formed AJs. Exo70 performs this feat through its association with a kinase that can interact directly with E-cadherin (PIPKIγ, for you membrane-junkies out there). As seen in the images above, this kinase (green) and Exo70 (red) both associate at the lateral membranes of epithelial cells, where AJs form. Top row shows the cells as if we are looking down onto the cells, while bottom row shows cells as if we were looking through the plane of cells.

Xiong, X., Xu, Q., Huang, Y., Singh, R., Anderson, R., Leof, E., Hu, J., & Ling, K. (2011). An association between type I PI4P 5-kinase and Exo70 directs E-cadherin clustering and epithelial polarization Molecular Biology of the Cell, 23 (1), 87-98 DOI: 10.1091/mbc.E11-05-0449