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

April 24, 2014

I’m still waiting for my miniaturizing spaceship* so I can dive into a cell with my dog and ride in the lamella of a crawling cell. Until then, I will gladly enjoy images like today’s, from the Lippincott-Schwartz lab.

A cell’s shape can change for many reasons, including crawling, tissue regeneration, and cancer progression. Cell shape is dynamic, relying on temporal and spatial coordination of several processes. The three-dimensional nature of cell shape, however, presents a challenge for microscopists. By using structured illumination microscopy (SIM), Burnette and colleagues created images of the three-dimensional organization of actin filaments. From these 3D superresolution analyses in crawling cells, Burnette and colleagues found a contractile network of actin filaments at the top of a crawling cell, organized like muscle sarcomeres. Their model of balanced contraction and adhesion helps to guide further investigations of cell shape changes in healthy and diseased tissue. The image above shows actin filaments at color-coded heights within a crawling cell. The closed arrow points to actin at the top of the cell.

*My birthday is next month, if anyone is wondering about gift ideas.  

Burnette, D., Shao, L., Ott, C., Pasapera, A., Fischer, R., Baird, M., Der Loughian, C., Delanoe-Ayari, H., Paszek, M., Davidson, M., Betzig, E., & Lippincott-Schwartz, J. (2014). A contractile and counterbalancing adhesion system controls the 3D shape of crawling cells originally published in the Journal of Cell Biology, 205 (1), 83-96 DOI: 10.1083/jcb.201311104

April 17, 2014

The endoplasmic reticulum and humans have quite a bit in common. Both are dynamic and constantly changing, but both also need something to ground and stabilize them. Maybe I’m reading too much into the beauty of the ER, but the image today is from a paper that only fuels my fascination.

The endoplasmic reticulum (ER) is a large, complex membrane-bound organelle that spreads throughout the cell and hosts the synthesis, folding, and sorting of membrane and secretory proteins. This network is dynamic and constantly rearranging, with diverse structural and functional domains. A recent paper describes the investigation into the role of the actin cytoskeleton in regulating ER sheet persistence and maintenance, which is important as the stationary domain of the ER. Joensuu and colleagues identified a subset of actin filaments associated with the ER, specifically to polygons defined by ER tubules and sheets. Actin depolymerization caused ER sheet fluctuation and resulted in a defective ER network. The actin motor myosin 1c also localizes to these actin filament arrays. In the images above, actin filament arrays (magenta, arrows) localize to the polygons (asterisks) associated with the ER (green).

Joensuu, M., Belevich, I., Ramo, O., Nevzorov, I., Vihinen, H., Puhka, M., Witkos, T., Lowe, M., Vartiainen, M., & Jokitalo, E. (2014). ER sheet persistence is coupled to myosin 1c-regulated dynamic actin filament arrays Molecular Biology of the Cell, 25 (7), 1111-1126 DOI: 10.1091/mbc.E13-12-0712

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 2, 2014

Nuclear envelope breakdown is far prettier than my own breakdown when I realized that Girl Scout “cookie season” is over. Today’s image is from a paper that describes the importance of SUN proteins in nuclear envelope breakdown.

Early in mitosis, a cell’s nuclear envelope breaks down to allow the attachment of chromosomes to the mitotic spindle. Nuclear envelope breakdown (NEBD) depends on a tearing process, during which microtubules pull the nuclear envelope towards the centrosomes. Turgay and colleagues found that the SUN proteins help to clear membranes from chromatin during NEBD. SUN proteins reside in the inner nuclear membrane and are part of a complex that connects the nucleus to the cytoskeleton. As seen in the images above, simultaneous depletion of both SUN1 and SUN2 (bottom timelapse) delayed removal of the nuclear envelope (green; chromosomes in red), when compared to control (top).  

Turgay, Y., Champion, L., Balazs, C., Held, M., Toso, A., Gerlich, D., Meraldi, P., & Kutay, U. (2014). SUN proteins facilitate the removal of membranes from chromatin during nuclear envelope breakdown The Journal of Cell Biology, 204 (7), 1099-1109 DOI: 10.1083/jcb.201310116