April 30, 2014

Have you ever driven in the wrong direction on a one-way street. It feels as wrong as a hamburger smoothie and you feel overwhelmed with panic. It’s important to go the right direction on one-way streets, and a neuron understands this. Neurons are polarized so that signals can come and go in the right direction. Today’s stunning image is from a paper describing the cytoskeletal architecture within a region of a neuron that’s important for polarity. 

The axon initial segment (AIS) is the part of an axon closest to the neuron’s cell body, and is the site of action potential initiation. The AIS is crucial for the neuron’s polarity, which facilitates the direction of incoming signals (coming in from dendrites) and outgoing information (out along the axon to the synapse). A recent study from Jones and colleagues investigated how the AIS maintains neuronal polarity. Jones and colleagues used platinum replica electron microscopy (PREM) to image the cytoskeleton in hippocampal neurons, and found that it begins with a bundle of microtubules. A dense fibrillar–globular coat covers this microtubule bundle and contains many proteins as well as actin filaments. Actin filaments are found in two sparse populations—either stable, short filaments or dynamic, long filaments. Jones and colleagues propose that the dynamic actin filaments play a role in the AIS coat, while the stable filaments may play a structural role in the AIS diffusion barrier. This diffusion barrier prevents the mixing of plasma membrane components from dendrites and axons, an important factor in maintaining polarity. The image above shows microtubules within the AIS, with thin fibrils (arrows) and a fibrillar coat over the microtubules (arrowheads) visible.

Jones, S., Korobova, F., & Svitkina, T. (2014). Axon initial segment cytoskeleton comprises a multiprotein submembranous coat containing sparse actin filaments originally published in the Journal of Cell Biology, 205 (1), 67-81 DOI: 10.1083/jcb.201401045

May 6, 2013

Our bodies have multiple fronts for battling viruses, and it’s impressive that any of those suckers manage to invade our bodies at all.  When virus particles do make their way into a cell, it’s important for biologists to understand their pathway through a cell in order to create drug therapies and vaccines.  Today’s image is from a paper describing the use of high resolution imaging to understand this process.

The polarized cells that line our digestive and respiratory tracts form a tight barrier that protects our bodies from viruses.  Understanding how viruses are able to breach these polarized epithelial cells is important in guiding the development of therapeutics and vaccines, yet previous research has focused mainly on in vitro studies of virus entry into nonpolarized cells.  Microscopy advances have recently allowed the high-resolution imaging of virus entry in polarized epithelial cells.  Boulant and colleagues used live-cell spinning-disk confocal microscopy to follow the uptake of single mammalian reovirus (MRV) virions and infectious subvirion particles (ISVPs) in polarized Madin–Darby canine kidney cells.  Both virus particles were internalized by clathrin-mediated endocytosis at the apical surface.  MRV virions reached early and late endosomes, while ISVPs escaped the endocytic pathway prior to reaching early endosomes.  In the images above, the tight-junction protein ZO-1 (red, left) shows the typical belt pattern surrounding the polarized cells (side views are also shown in the top and side strips).  The MRV cell surface receptor JAM-A (red, right) is localized near tight junctions and on the apical side of the cells.

Boulant, S., Stanifer, M., Kural, C., Cureton, D., Massol, R., Nibert, M., & Kirchhausen, T. (2013). Similar uptake but different trafficking and escape routes of reovirus virions and infectious subvirion particles imaged in polarized Madin-Darby canine kidney cells Molecular Biology of the Cell, 24 (8), 1196-1207 DOI: 10.1091/mbc.E12-12-0852

September 4, 2012

Membranes are not just cute little jackets that cells wear to keep them protected. Membranes are no joke—they are dynamic hotspots of action, and their composition plays a key role in many cellular processes. Today’s image is from a paper describing the composition of a cell’s plasma membrane during cell morphogenesis.

The cell’s plasma membrane serves as the entryway and exit for material and plays an important role in cell signaling. Although all plasma membranes are made of a bilayer of lipids, those lipids can vary depending on the type of cell. The local composition of lipids in the plasma membrane can affect cellular processes, as seen in the human pathogenic fungus human C. albicans. C. albicans can grow using a filamentous protrusion called a hypha, which was recently found to have an asymmetric distribution of lipids. According to Vernay and colleagues, the growth of the hyphal filament was accompanied by a gradient of the phospholipid PI(4,5)P2 (phosphoinositide bis-phosphate). PI(4,5)P2 is frequently found within membranes at the sites of polarized growth, trafficking, and actin reorganization. Vernay and colleagues found that the sharp gradient of PI(4,5)P2 is essential for proper cell morphogenesis of C. albicans, and depends on the actin cytoskeleton and PI(4)P synthesis. In the images above, hyphal filaments show a gradient of PI(4,5)P2 using a color-coding of PI(4,5)P2 signal (key shows relative levels). Highest PI(4,5)P2 levels are seen at the tips of the filaments.

Aurélia Vernay, Sébastien Schaub, Isabelle Guillas, Martine Bassilana, & Robert A. Arkowitz (2012). A steep phosphoinositide bis-phosphate gradient forms during fungal filamentous growth originally published in the Journal of Cell Biology, 198 (4), 711-730 DOI: 10.1083/jcb.201203099

May 7, 2012

Phosphorylation serves as a molecular switch, and the combinations of different proteins phosphorylated or dephosphorylated at any given time is mind-numbing. Phosphorylation caused one of many “Holy crap, cells are amazing!” moments in college as I realized how the regulation within a cell can be so complex. Today’s image is from a study that looks at the regulation of phosphorylation of a polarity protein.

Protein kinase C (PKC) is a family of kinases, one of which is the well-studied polarity protein atypical PKC (aPKC). These kinases must be phosphorylated before their catalytic domain is activated. The protein PDK1 serves to phosphorylate and activate newly-made PKC proteins, and a recent paper reveals that it can also phosphorylate older PKC proteins that have been since dephosphorylated, seen as a “rescue” of activity. Mashukova and colleagues also found that PDK1 is localized in membrane compartments on the apical side of some epithelial cells in close proximity to intermediate filaments, suggesting a new signaling function in these membrane compartments (endosomes). In the images above, epithelial cells have PDK1 (red) localized to the apical pole of the cells, where intermediate filaments (green) are found.

Mashukova, A., Forteza, R., Wald, F., & Salas, P. (2012). PDK1 in apical signaling endosomes participates in the rescue of the polarity complex atypical PKC by intermediate filaments in intestinal epithelia Molecular Biology of the Cell, 23 (9), 1664-1674 DOI: 10.1091/mbc.E11-12-0988

November 28, 2011

Asymmetric cell division results in two unequal daughter cells and without it, stem cells would not hold the attention of biologists, sick patients, politicians, and those loud nut-jobs who have more passion than knowledge. Check out today’s image, from a paper identifying a new protein complex important in asymmetric division.

Asymmetric division occurs throughout development, and is when a cell divides to result in two daughter cells that are unequal in size and/or cell fate. Part of this process is the establishment of the axis along which the cell will divide. Once this polarity is established, the mitotic spindle can line up accordingly. Fruit fly neural stem cells, called neuroblasts, divide asymmetrically to result in the birth of another neuroblast and a smaller ganglion mother cell, which eventually gives rise to neurons. A recent paper identifies a new role for a signaling complex in establishing neuroblast polarity. In this paper, Carmena and colleagues found that the Rap1–Rgl–Ral complex of proteins regulates polarity establishment in fruit fly neuroblasts, and works alongside other well-studied polarity proteins such as aPKC, Par6, and Pins. As seen in the images above, Rap1 (bottom row, green in merged; arrows) is enriched on only one side (apical side) of neuroblasts throughout division (DNA is in red).

Carmena, A., Makarova, A., & Speicher, S. (2011). The Rap1-Rgl-Ral signaling network regulates neuroblast cortical polarity and spindle orientation originally published in The Journal of Cell Biology, 195 (4), 553-562 DOI: 10.1083/jcb.201108112

September 22, 2011

I love it when worlds collide. I love the movies where a country boy falls for a city girl. Or a robot develops a friendship with a wookie. Hilarity typically ensues in the movies, but fantastic new ideas and questions result from the discovery of biological processes colliding. So, when I came across a recent paper that revealed new results on the relationship between endocytosis and adhesion, I was all over it.

Cell-cell adhesion is constantly adjusted throughout development, wound healing, and cancer metastasis. E-cadherin is the major adhesion molecule that functions in epithelial cell adhesion and polarity, and is linked to the actin skeleton (via α-catenin) and p120. The level of E-cadherin at the cell surface influences the adhesive strength between two cells, and this strength can be adjusted by internalization (endocytosis) of E-cadherin away from the cell surface. A recent paper discusses results showing how internalization of E-cadherin is regulated by Numb, a protein that interacts with endocytosis adaptor proteins and is important throughout development. Sato and colleagues found that Numb interacts directly with p120, and showed that impairment of Numb prevents E-cadherin internalization. The images above show cysts of epithelial cells. In control cysts (top rows), E-cadherin and p120 (red) were found at the basolateral cell-cell junctions. In cysts with reduced levels of Numb (bottom rows), both E-cadherin and p120 localized to the apical membrane region (blue) too.

Sato, K., Watanabe, T., Wang, S., Kakeno, M., Matsuzawa, K., Matsui, T., Yokoi, K., Murase, K., Sugiyama, I., Ozawa, M., & Kaibuchi, K. (2011). Numb controls E-cadherin endocytosis through p120 catenin with aPKC Molecular Biology of the Cell, 22 (17), 3103-3119 DOI: 10.1091/mbc.E11-03-0274

September 15, 2011

There is something so gratifying about a light switch. My two-year old will pull a chair to our kitchen light switch to turn it on and off. Over. And over. And over again. Maybe that’s why I find phosphorylation so satisfying (and maybe why I have a headache). It’s a molecular switch, and the vast combinations of where, when, and how different proteins are phosphorylated can provide mind-numbing levels of regulation within a cell. Combine my appreciation for phosphorylation with my absolute love for early worm embryos, and you have today’s lovely images.

The one-cell stage worm embryo divides like many cells throughout development—asymmetrically. Asymmetric cell division results in two daughter cells with different developmental fates and frequently different sizes. For asymmetric cell division to take place in the early worm embryo, the entire mitotic spindle apparatus is moved towards one end of the cell, the posterior. A complex of polarity proteins (made of PAR proteins and the aPKC homolog PKC-3) functions upstream of an evolutionary conserved pathway of proteins (made of the NuMA homolog LIN-5 and G-protein signaling), and a recent paper finds the well sought-after link between these two pathways. In Galli and colleagues’ paper, they show that LIN-5 is phosphorylated by PKC-3. The position of PKC-3 at only one side of the cell results in the phosphorylation of LIN-5 only in that region, which in turns allows the mitotic spindle to position itself correctly. In the images above, one-cell stage worm embryos show staining for phosphorylated LIN-5 (top row, red in bottom row) during mitosis (spindle is in green, chromosomes in blue). Phosphorylated LIN-5 is enriched at higher levels at the anterior cortex (the left-hand side in each image) during earlier stages of mitosis.

Galli, M., Muñoz, J., Portegijs, V., Boxem, M., Grill, S., Heck, A., & van den Heuvel, S. (2011). aPKC phosphorylates NuMA-related LIN-5 to position the mitotic spindle during asymmetric division Nature Cell Biology, 13 (9), 1132-1138 DOI: 10.1038/ncb2315
Adapted by permission from Macmillan Publishers Ltd, copyright ©2011

August 11, 2011

“Our country is just so polarized these days,” I say as I shake my fist, then tell those pesky kids to get off of my lawn. “Polarized” doesn’t have to be a dirty word…in fact, if it weren’t for polarized epithelial cells we’d all be big puddles of cells lacking the ability to digest food, reproduce, circulate blood, or breathe. Today’s image is from a paper describing how septins guide microtubule organization in polarized epithelial cells.

Epithelial cells form sheets that line various organs throughout the body. These epithelial sheets are polarized, meaning the cells are not symmetrically organized. For example, our intestine is lined with a polarized epithelial sheet that must absorb nutrients from the intestine’s cavity in one side of the cell sheet, and then eventually guide those nutrients to blood vessels. A key step in the polarization of a cell is the organization of the microtubule cytoskeleton. Microtubules are organized in a radial array in rounded cells, while the microtubules in polarized epithelial cells are arranged along the lateral borders of the cells and in networks under the apical and basal membranes (top and bottom). A recent paper looks at the role of proteins called septins in guiding microtubule organization during the establishment of polarity. Septins are filamentous proteins known for their roles in cell polarity, but Bowen and colleagues were able to show a functional link between septins and microtubule organization. Specifically, two populations of septins allow microtubule growth, bundling, and capture in different regions of the cell. In the image above, thick septin filaments (left, green in merged) are seen colocalized with microtubules (middle, red in merged) in epithelial cells. Bottom images are higher magnification views of the boxed region.

Bowen, J., Hwang, D., Bai, X., Roy, D., & Spiliotis, E. (2011). Septin GTPases spatially guide microtubule organization and plus end dynamics in polarizing epithelia originally published in The Journal of Cell Biology, 194 (2), 187-197 DOI: 10.1083/jcb.201102076

June 20, 2011

As you all vacation this summer at the beach, throw a “Thank you!” out to the sea urchins for their amazing contributions to cell and developmental biology research. Today’s image is from a paper showing a role for a major polarity protein in cilia formation in sea urchin embryos.

After a sea urchin’s initial stages of embryogenesis, it hatches out of its eggshell to become a swimming resident of the marine world. It swims using cilia, which beat to push water around. The cilia on this swimming embryo reside in the outer layer of epithelial cells, and emanate from basal bodies just below each cell’s surface. A recent paper shows a role for the polarity protein aPKC (atypical protein kinase C) in the formation of cilia in the sea urchin embryo. aPKC is a member of a protein complex important in asymmetric cell divisions, and Prulière and colleagues found that it has a very asymmetric localization during ciliogenesis. Images above show cilia (red) of sea urchin embryos in the absence (top left) or presence of an aPKC inhibitor. As the levels of the inhibitor increases (concentrations are indicated), the length of cilia decreases.

Pruliere, G., Cosson, J., Chevalier, S., Sardet, C., & Chenevert, J. (2011). Atypical protein kinase C controls sea urchin ciliogenesis Molecular Biology of the Cell, 22 (12), 2042-2053 DOI: 10.1091/mbc.E10-10-0844

September 2, 2010

The planar cell polarity (PCP) pathway functions to orient cells within the plane of an epithelial tissue. The fruit fly is a great model system for studying the PCP pathway—the bristles on the fly’s back and wing hairs grow in a certain direction as a result of the orientation of the cells, making it easy for researchers to see problems in the pathway. A recent paper has found a role for a transmembrane proton pump protein called VhaPRR in PCP signaling. Images are election micrographs of bristles with (left) or without (right) normal levels of VhaPRR. Without VhaRRR, the bristles are disoriented, as well as the small epithelial hairs underneath.

Reference: Tobias Hermle, Deniz Saltukoglu, Julian Grünewald, Gerd Walz and Matias Simons. Current Biology 20(14): 253-258. ©2010 Elsevier Ltd. All rights reserved. Paper can be found here.

August 5, 2010

Motile cells move in response to environmental cues, and have complex networks that allow them to change their direction accordingly. The rod-shaped bacterium Myxococcus xanthus controls its motility by switching its own polarity from the front (leading) to the back (lagging) end depending on which direction it is headed, meaning that the mechanisms controlling movement are able to switch rapidly. A recent paper finds that the leading side of the cell accumulates a protein called MglA, while MglB is found at the lagging end of the cell where it inactivates MglA. Image above shows MglB (green) on the lagging end of a bacterium, even after it changes direction (middle of image).

Reference: Yong Zhang, Michel Franco, Adrien Ducret, Tâm Mignot. Authors’ PLoS Biology paper can be found here.

April 29, 2010

Epithelial cells form sheets that line cavities and structures. Their organization and structure are polarized, allowing transport of material from one side of the cell to the other. Earlier this year, investigators reported that galectin-3 plays a role in the establishment of epithelial polarity, possibly through its effects on centrosomes, the primary microtubule-organizing centers. Images above show mammalian cells allowed to grow into a polarized epithelial layer. A protein that is typically on the apical side of the cells (top, green) is mislocalized in cells with lower levels of galectin-3 (bottom). DNA is in blue.

Reference: Annett Koch, Francoise Poirier, Ralf Jacob, and Delphine Delacour. Authors’ Molecular Biology of the Cell paper can be found here.

April 22, 2010

During development, polarity proteins play an important role by positioning cells and their machinery in the proper orientation. A recent paper asked how a well-studied set of polarity proteins functions in the oocyte of the fruit fly Drosophila. Image above shows an egg chamber, with microtubules in red and DNA in blue.

Reference: Pierre Fichelson, Marlène Jagut, Sophie Lepanse, Jean-Antoine Lepesant and Jean-René Huynh, 2010. Development: 137, 815-824. doi: 10.1242/dev.045013. Adapted with permission by Development. Paper can be found here.