January 31, 2013

I like to think of early embryos as kicking ass and asking questions later.  Once fertilization happens, embryos undergo rapid, synchronous cell divisions.  Next, the cell divisions slow down as cells begin to move around and form the different germ layers, then organs, within the growing embryo.  Today’s image is from a paper describing this transition in fruit flies, and how gene transcription in the embryo plays a role.

In fruit fly embryos, early development begins with synchronous nuclear divisions, which are divisions in which the nuclei divide without going through cytokinesis.  After 13 rounds of mitosis, the large multinucleate syncytium pauses in cycle 14 and undergoes cellularization to form plasma membranes around nuclei.  During this transition, the transcription of the embryo’s own genes begins as the maternally-contributed genetic material (RNA) is degraded.  A recent paper describes evidence that this switch to zygotic transcription is the trigger for the pause in cycle 14 and cellularization.  Sung and colleagues found a novel mutation in the RNPII215 gene that results in a reduced number of nuclear divisions, as well as premature zygotic transcription and cellularization.  The number of nuclear divisions in this mutant depends on zygotic transcription and Vfl, a transcription factor that controls many early zygotic genes.  In the images above, a mutant early fly embryo (bottom) has fewer cells due to the reduced nuclear divisions, compared to a normal embryo (top).  A nuclear protein is in green, and the pair-rule protein Eve is in red.

Sung, H., Spangenberg, S., Vogt, N., & Großhans, J. (2013). Number of Nuclear Divisions in the Drosophila Blastoderm Controlled by Onset of Zygotic Transcription Current Biology, 23 (2), 133-138 DOI: 10.1016/j.cub.2012.12.013
Copyright ©2013 Elsevier Ltd. All rights reserved. 

January 28, 2013

In my many hours in a dark microscopy suite, I would stare slack-jawed at cells going through mitosis like a perv at a peep show. Beneath the grace of this serious cellular rite of passage is a massive amount of regulation, which just adds to the fascination so many biologists have for the event. Today’s image is from a paper that describes the generation and importance of forces that help align chromosomes on a metaphase plate.

During mitosis, chromosomes must align on the metaphase plate with attachments to opposite spindle poles. Only after this precise alignment can the cell begin a cascade of signals and checkpoints to trigger anaphase, which segregates sister chromatids into their eventual daughter cells. Chromosome alignment gets a hand from polar ejection forces (PEFs), which are forces generated by the microtubule-based motor kinesin found on chromosome arms. These chromokinsesins (kinesin-10 family members) are thought to walk chromosome arms away from spindle poles along microtubules, and are important for timely congression of chromosomes to the metaphase plate. A recent paper describes the relationship between PEFs and the stability of spindle-chromosome attachments. Cane and colleagues manipulated the levels of the fruit fly kinesin-10 protein NOD to similarly manipulate the tension and forces applied to chromosomes. When both kinetochores of a chromosome pair were incorrectly attached to the same spindle pole (called a syntelic attachment), NOD stabilized the attachments by preventing error correction by the protein Aurora B. From their results, Cane and colleagues show that PEFs regulate the stability of kinetochore attachment to spindle microtubules. In the time-lapse images above, NOD overexpression in a fruit fly cell caused an abnormal metaphase plate to form. Despite the syntelic attachments of many chromosomes, the cell still entered anaphase (AO), resulting in daughter cells with too many nuclei (far right image). NOD signal is red, microtubules are green. 

BONUS!!  Beautiful movies from the paper can be found here.

Cane, S., Ye, A., Luks-Morgan, S., & Maresca, T. (2013). Elevated polar ejection forces stabilize kinetochore-microtubule attachments originally published in the Journal of Cell Biology, 200 (2), 203-218 DOI: 10.1083/jcb.201211119

January 25, 2013

Neuromuscular junctions (NMJs) are considered by many to be the business end of our nervous system. NMJs connect nerves with muscle cells, stimulating the muscle contractions that allow you to run from the bear that you just spotted glaring at you with a fork and knife in his paws. A recent paper adds to our understanding of the signaling on both sides of the NMJ. 

The two sides of a NMJ, the presynaptic and postsynaptic structures, are highly coordinated for proper development and plasticity of the junction. As with many cell and developmental processes, this coordination relies on the Wnt signaling pathway. In fruit flies, Wnt/Wingless (Wg) functions both pre- and post-synaptically in larval muscle fibers, and results from a recent paper show how this bidirectional signaling is balanced and regulated. Kamimura and colleagues found that the gene trol, which encodes the protein perlecan (a secreted heparan sulfate proteoglycan, for those down with HSPGs), regulates Wg signaling in fruit fly NMJs. trol mutations causes postsynaptic defects and an overproduction of synaptic boutons, which are button-like presynaptic hotspots of neurotransmitter-containing vesicles. In the images above, normal (top) and trol mutant larval NMJs show immunostaining for a presynaptic marker (magenta) and a postsynaptic marker (green). In mutants, some synaptic boutons lack a postsynaptic structure nearby (“ghost boutons", arrowheads) while some NMJs showed an overproduction of small synaptic boutons (“satellite boutons”, arrow).

Kamimura, K., Ueno, K., Nakagawa, J., Hamada, R., Saitoe, M., & Maeda, N. (2013). Perlecan regulates bidirectional Wnt signaling at the Drosophila neuromuscular junction originally published in the Journal of Cell Biology, 200 (2), 219-233 DOI: 10.1083/jcb.201207036

Celldance at ASCB

In December, the American Society for Cell Biology held their huge annual meeting in San Francisco.  One of the more notable events during the ASCB meetings is the announcement of the Celldance winners.  The Celldance entries feature stunning microscopy images set to clever soundtracks, show a remarkable event in cell biology, or serve as public outreach in educating and exciting laypeople about biology.  

This year's winning entry is a great example of how developmental biology and cell biology can combine to provide breathtaking images and a more complete understanding of what is going on inside an embryo.  It's no shock to me that the winning entry is from a graduate student in Mark Peifer's lab at UNC-Chapel Hill, a lab down a flight of stairs from my old graduate lab.  The Peifer lab is known for awesome developmental biology, with an expertise in capturing the magic in stunning images.  Check out Stephanie Norwotarski's winning entry here.

Please check out all of the Celldance winners here.  For a compilation of Celldance greatest hits from 2009-2011, click here.

January 17, 2013

Membranes have to wear many hats. A cell’s plasma membrane must be sturdy enough to protect the cell, yet fluid enough to support the cell’s dynamic and swingin’ lifestyle. A recent paper hypothesizes how plasma membranes can accomplish both tasks, and provides beautiful images and results to show this fascinating life of a membrane.
 
A cell’s plasma membrane protects and contains the contents of the cell, but is also flexible and fluid enough to allow the many events that take place at or involve the membrane, such as cell migration and changes in cell shape. A recent paper presents a mechanism for how a plasma membrane can accomplish both structure and flexibility. Kapustina and colleagues monitored rounded cells and the periodic membrane protrusions they make, and found compression (folding) and dilation (unfolding) of the plasma membrane and underlying actin cortex during protrusion events. This compression-dilation mechanism allows the cell to make rapid changes in cell shape, and can produce amoeboid-like migration movements under certain conditions. The electron microscopy images above show a cell fixed during membrane oscillations. Membrane folding appears as finger-like or round projections. (The yellow box shows the position of the higher magnification image on the right; red arrows point to dense cortical regions.)

BONUS!!  Beautiful movie of F-actin (green) and myosin (red) during membrane protrusions is below.  More movies of these membrane dynamics can be seen here.

 Kapustina, M., Elston, T., & Jacobson, K. (2013). Compression and dilation of the membrane-cortex layer generates rapid changes in cell shape originally published in the Journal of Cell Biology, 200 (1), 95-108 DOI: 10.1083/jcb.201204157

January 10, 2013

The idea of screening for something valuable is something we’ve all done. When looking for a dog several years ago, I screened through PetFinder to find the exact dog I wanted to bring home based on size, age, scruffiness, etc. Despite the fact that this dog just farted at my feet, my PetFinder screen brought me to my best friend. For a biologist, screening can lead to exciting discoveries about what genes are important in a specific process (minus the gas, I think).

During endocytosis, a cell takes in material from its outside environment. Clathrin-mediated endocytosis involves the inward budding of vesicles, and depends on a lattice, or “coat”, of clathrin molecules that help shape the rounded vesicle. A recent paper describes the results of a screen to find regulators of clathrin-coated vesicle formation at the plasma membrane, one of the earliest steps in clathrin-mediated endocytosis. Kozik and colleagues screened the entire human genome for regulators, and found 92 genes that affect this process. One of these genes encodes the protein V-ATPase, which is found in many membranes and can pump protons across a membrane to regulate pH. In the images above, V-ATPase-inhibited cells (bottom) formed enlarged clathrin-coated structures, compared to the small and uniform clathrin-coated vesicles in control cells (top). Kozik and colleagues found that V-ATPase inhibition blocked the recycling of cholesterol back to the plasma membrane, where it has been suggested that cholesterol aids in membrane bending.

 BONUS!! Electron microscopy image of a heart-shaped vesicle, acquired as part of the screen to find clathrin regulators.

Kozik, P., Hodson, N., Sahlender, D., Simecek, N., Soromani, C., Wu, J., Collinson, L., & Robinson, M. (2012). A human genome-wide screen for regulators of clathrin-coated vesicle formation reveals an unexpected role for the V-ATPase Nature Cell Biology, 15 (1), 50-60 DOI: 10.1038/ncb2652  
Adapted by permission from Macmillan Publishers Ltd, copyright ©2013

January 7, 2013

Happy New Year! To kick off 2013 properly, today I’m featuring an image from a lab whose leader is one of the coolest cats in cell biology (click here for proof). The image is from a paper describing the nanoscale forces within individual focal adhesions, and how these forces guide cell migration.

“Durotaxis” is the directed cell migration along a rigidity gradient—meaning the migration toward a more mechanically stiff extracellular matrix (ECM). Durotaxis occurs throughout development and cancer metastasis, two situations during which the stiffness of a tissue can vary locally or over time. A recent paper describes how cells sample the environment to sense ECM stiffness, and how this sampling guides migration of a cell toward stiff ECM. Using high-resolution time-lapse traction force microscopy, Plotnikov and colleagues documented the forces within individual focal adhesions (FAs) in migrating cells. These individual FAs tug on the ECM to function as local sensors of ECM rigidity. This tugging traction is necessary for durotaxis, but not chemotaxis, another type of cell migration. In the images above, paxillin is fluorescently labeled to show FAs (top, left and zoomed image on right). The corresponding heatmaps (bottom, left and right) show the magnitudes of tugging forces on the ECM (FAs outlined in black), while the stress vector field image (middle right) shows the direction of the forces within the FAs.

Plotnikov, S., Pasapera, A., Sabass, B., & Waterman, C. (2012). Force Fluctuations within Focal Adhesions Mediate ECM-Rigidity Sensing to Guide Directed Cell Migration Cell, 151 (7), 1513-1527 DOI: 10.1016/j.cell.2012.11.034

Copyright ©2013 Elsevier Ltd. All rights reserved.