June 18, 2013

The study of how cells move in development is not just about development.  Understanding cell migration can also help researchers understand how tumors spread and invade other tissues.  So, the next time you see someone roll their eyes at your fruit fly egg chambers (or worm vulva, or culture dishes), take pity at their ignorance and explain to them how they should thank you instead.

The movement of cells during development drives the shape changes and organization of an embryo.  In the fruit fly ovary, a small cluster of border cells migrates across a region of the egg chamber in order to reach the oocyte.  This collective migration of these border cells depends on polarization of the actin cytoskeleton.  A recent paper describes the role of the Hippo signaling pathway in driving the polarization of actin to the outer rim of the migrating border cell cluster.  Lucas and colleagues found that upstream Hippo pathway components localize to the contacts between border cells within the cluster in order to link polarity signaling with actin cytoskeleton organization.  In the images above, the actin cytoskeleton (red) can be seen at the outer rim of the migrating cluster of border cells (arrows) as it moves across the egg chamber towards the oocyte (top to bottom, chronologically).  Higher magnification views of the cluster are on the right.

 Lucas, E., Khanal, I., Gaspar, P., Fletcher, G., Polesello, C., Tapon, N., & Thompson, B. (2013). The Hippo pathway polarizes the actin cytoskeleton during collective migration of Drosophila border cells originally published in the Journal of Cell Biology, 201 (6), 875-885 DOI: 10.1083/jcb.201210073

June 11, 2013

 I’m willing to bet that most scientists were pretty destructive as kids.  Not intentionally destructive, though…I bet we all liked taking things apart to see what each part of a toy did.  Maybe your Teddy Ruxpin eventually sounded like a demonic doll after your experiments, or you finally pulled apart your Etch-A-Sketch to uncover the magic.  Either way, it was early training for what scientists do every day to understand cells better.  Today’s image is from a paper that takes this route to understand cell adhesion better.

Integrins are transmembrane proteins that attach a cell to another cell or its surrounding extracellular matrix, so their role in regulating cell adhesion in normal cell function, development, and disease is of serious interest to biologists.  There are several types of integrins, frequently expressed together at the same time.  A recent paper describes a system to investigate how specific integrins function in cell adhesion, and found notable differences.  Schiller and colleagues expressed either β1- or αv-class integrins in fibroblasts lacking all other integrins, and found that β1-class integrins are important in small peripheral adhesions, while αv-class integrins are important for large focal adhesions.  In the images above, activity of the actin-based motor myosin and presence of the focal adhesion protein paxillin are shown in cells plated on micropatterned shapes.  Cells with both β1- and αv-class integrins (middle row) have the highest myosin activity (pMLC) and paxillin signal, while cells with either β1- or αv-integrin alone have lower myosin activity and paxillin signal.  

Schiller, H., Hermann, M., Polleux, J., Vignaud, T., Zanivan, S., Friedel, C., Sun, Z., Raducanu, A., Gottschalk, K., Théry, M., Mann, M., & Fässler, R. (2013). β1- and αv-class integrins cooperate to regulate myosin II during rigidity sensing of fibronectin-based microenvironments Nature Cell Biology, 15 (6), 625-636 DOI: 10.1038/ncb2747
Adapted by permission from Macmillan Publishers Ltd, copyright ©2013 

June 4, 2013

HighMag is back from an early summer vacation at the beach, and ready to get our microscopic groove on.  Happy Summer, everyone!

Our world is the same size it’s always been, but so many advances in technology have made the world seem a lot smaller.  We can call our brother across the country and video chat with our sister on the other side of the world, all while searching the internet for the lyrics to Snow’s “Informer” (side note…knowing the lyrics won’t help you understand the song AT ALL).  Similarly, a neuron’s axons make the nervous system seem a lot smaller too, with their amazing ability to grow very long.  Today’s image is from a paper showing how microtubule sliding is involved.

Neurons can transmit signals to far away neurons, thanks to the ability of their axons to grow insanely long.  Both actin filaments and microtubules participate in axon growth, with microtubules pushing the growing axon out and actin filaments working directly at the tip of the growing axon, called the growth cone.  A recent paper shows the role of microtubule sliding in the initial growth of neurites in fruit fly neurons.  Lu and colleagues found that the microtubule motor kinesin-1 drives the sliding of microtubules past one other in order to push out the neuron’s growing projection.  This mechanism does not require actin filaments, and is suppressed during maturation of neurons.  In the images above, microtubules (green) can be seen pushing against the tip of a growing neurite in a young neuron (membrane of neuron in red and bottom right series).  The whole neuron is on the left, and the neurite in the boxed region is shown in the time-lapse images on the right.

Lu, W., Fox, P., Lakonishok, M., Davidson, M., & Gelfand, V. (2013). Initial Neurite Outgrowth in Drosophila Neurons Is Driven by Kinesin-Powered Microtubule Sliding Current Biology, 23 (11), 1018-1023 DOI: 10.1016/j.cub.2013.04.050 
Copyright ©2013 Elsevier Ltd. All rights reserved.

May 24, 2013

There isn’t a cell biologist out there who doesn’t fantasize about reaching her hands into a cell and physically manipulating whatever protein or structure that she obsesses over.  While we can’t do that with our own hands, optical tweezers can…and the information we learn is invaluable.  Today’s image is from a paper that uses optical tweezers to measure the forces within a mitotic spindle.

The mechanics within a mitotic spindle are complicated, and cannot be fully understood until all of the forces that exist within it are determined.  A recent paper measures the forces of chromosome and spindle pole movements using optical tweezers to trap either kinetochores or spindle poles.  Optical tweezers use refracted light to trap small objects within a cell, and are able to stop the movement of the object as the laser power of the trap approaches (or exceeds) the cell’s own force on the object.  With this technology, Ferraro-Gideon and colleagues measured the forces in mitotic spindles in several samples—a flatworm, a crane fly, and a mammalian spindle.  The force used to stop chromosome movement was closer to the theoretical values of chromosome force, much less than the force values calculated by experiments performed by Bruce Nicklas in the 1980s using needles to manipulate chromosome movement.  In the DIC (left) and fluorescence images (right) above, a trap (square) is applied to kinetochores in mammalian cells.  The trap did not damage the microtubules of the spindle (right image).

Ferraro-Gideon, J., Sheykhani, R., Zhu, Q., Duquette, M., Berns, M., & Forer, A. (2013). Measurements of forces produced by the mitotic spindle using optical tweezers Molecular Biology of the Cell, 24 (9), 1375-1386 DOI: 10.1091/mbc.E12-12-0901

Check out an extended report on these experiments, and others from this lab:
Sheykhani, R., Baker, N., Gomez-Godinez, V., Liaw, L., Shah, J., Berns, M., & Forer, A. (2013). The role of actin and myosin in PtK2 spindle length changes induced by laser microbeam irradiations across the spindle Cytoskeleton, 70 (5), 241-259 DOI: 10.1002/cm.21104

May 21, 2013

“LET THERE BE LIGHT!” said the microscopist.  Light plays a crucial role in microscopy and cell biology, and a recent paper describes the use of light to understand protein secretion.

Light is used in microscopy in countless ways—to illuminate a sample, excite a fluorophore, and signal the localization or dynamics of a protein.  Light can also be used to manipulate cellular events through the use of “caged” compounds that become active after illumination by certain wavelengths of light.  This technology gives biologists the ability to spatially and temporally control cellular events in order to understand them better.  Recent advances in this technology use illumination of plant photoreceptors to control protein-protein interactions, but some cellular processes such as protein secretion have been difficult to manipulate.  A recent paper describes the use of the plant photoreceptor UVR8 in the first light-triggered protein secretion system developed.  Chen and colleagues have shown that the recently described UVR8 can conditionally sequester proteins bound for secretion in the ER, and then upon illumination with UV light releases these proteins to the plasma membrane.  In the images above, a neuron before (left) and after (right) UV illumination with this UVR8 system shows the movement of proteins known to be secreted from the soma and dendritic processes (arrowheads), where the ER is distributed, and into the Golgi (arrow), a necessary step in protein secretion.

Chen, D., Gibson, E., & Kennedy, M. (2013). A light-triggered protein secretion system originally published in the Journal of Cell Biology, 201 (4), 631-640 DOI: 10.1083/jcb.201210119

May 13, 2013

Molecular motors are some of the raddest things in a cell.  They can walk along cytoskeletal elements such as microtubules and actin filaments, and the list of cellular events that they participate in is a long, long list.  Today’s image is from a paper showing a beautiful pattern of nonmuscle myosin II in epithelial cells.

Epithelial cells assemble junctions to adhere to one another, and the actin motor nonmuscle myosin II (NMII) is a major component of these epithelial apical junctions.  NMII helps the epithelial sheet respond to morphogenesis and changes in tissue homeostasis, and a recent paper describes how the network of NMII motors does so.  Ebrahim and colleagues have found that NMII in the apical junctional complex of epithelial cells assembles into precise muscle-like sarcomere units that form a belt around each cell.  The sarcomeres of neighboring cells are aligned, in turn assembling into a contractile network that can result in changes in cell shape.  In the images above, NMII (green) is seen in repeated sarcomere units around each cell (actin is in red).  NMII puncta are paired together in neighboring cells.  Arrows (middle) point to the junctions between three cells, seen at higher magnification on the right.

Ebrahim, S., Fujita, T., Millis, B., Kozin, E., Ma, X., Kawamoto, S., Baird, M., Davidson, M., Yonemura, S., Hisa, Y., Conti, M., Adelstein, R., Sakaguchi, H., & Kachar, B. (2013). NMII Forms a Contractile Transcellular Sarcomeric Network to Regulate Apical Cell Junctions and Tissue Geometry Current Biology, 23 (8), 731-736 DOI: 10.1016/j.cub.2013.03.039 
Copyright ©2013 Elsevier Ltd. All rights reserved.

May 9, 2013

When we think of wounds, we don’t typically think of them as part of normal, healthy function.  Micro-wounds, however, form when white blood cells have to cross the barrier in our blood vessels to get to an injury or infection.  These micro-wounds happen all the time, and our cells heal these wounds efficiently and elegantly.

One of the most important barriers in our body is that created by the vascular endothelium.  Vascular endothelial cells line all of our blood vessels—from the largest vessels to the smallest capillaries—and function in fluid filtration, hormone trafficking, and recruitment and trafficking of blood and stem cells.  The movement of cells, for example white blood cells, across the vascular endothelium and out of circulation creates “micro-wounds” that can compromise the integrity of the tissue.  A recent paper describes how these micro-wounds are healed, based on a model in which the vascular endothelium senses a loss of tension upon micro-wounding and triggers its own repair.  Martinelli and colleagues tracked micro-wounds that were created by either transmigrating white blood cells or by mechanical disruption by a probe, and found that ventral lamellipodia are generated by endothelial cells to close the micro-wounds.  These lamellipodia are enriched in Rac1 effector proteins, and require reactive oxygen species (ROS) and Arp2/3 for efficient wound closing.  Images above show probe-induced wounding of an endothelial cell, followed by wound healing.  The wound initially expanded to 20um across (80 seconds), with multiple nodes of ventral lamellipodia (blue arrowheads) and ventral F-actin waves forming around the wound and closing it.

Martinelli, R., Kamei, M., Sage, P., Massol, R., Varghese, L., Sciuto, T., Toporsian, M., Dvorak, A., Kirchhausen, T., Springer, T., & Carman, C. (2013). Release of cellular tension signals self-restorative ventral lamellipodia to heal barrier micro-wounds originally published in the Journal of Cell Biology, 201 (3), 449-465 DOI: 10.1083/jcb.201209077