September 17, 2014

All good things must end—even the focal adhesions that are so key to cell migration. Today’s notable image is the first live cell visualization of ECM degradation at focal adhesions, in a recent paper that reports the link between CLASPs, exocytosis, and focal adhesion turnover. 

Cell migration depends on the precisely-timed formation of focal adhesions (FAs) that link the crawling cell to the extracellular matrix (ECM). FAs serve as anchor points for the crawling cell, yet must later disassemble in order to allow continued movement of the cell. A recent paper describes how CLASP proteins link FA-associated microtubules, exocytosis, and FA turnover. CLASP proteins are +TIP proteins, which means that they are found on the growing ends of microtubules. Stehbens and colleagues found that the clustering of CLASPs around FAs correlates with the timing of FA disassembly, and that CLASPs are required for ECM degradation. Stehbens and colleagues also found that the tethering of microtubules to FAs, via CLASPs, serve as a transport pathway for exocytic vesicles at FAs. The images above are the first live cell images of ECM degradation (visualized as dark regions, top panel) at FAs (magenta).

BONUS! For more information on the scanning angle interference microscopy used in this paper, check out Matthew Paszek’s Nature Methods paper here.

Stehbens, S., Paszek, M., Pemble, H., Ettinger, A., Gierke, S., & Wittmann, T. (2014). CLASPs link focal-adhesion-associated microtubule capture to localized exocytosis and adhesion site turnover Nature Cell Biology, 16 (6), 561-573 DOI: 10.1038/ncb2975
Adapted by permission from Macmillan Publishers Ltd, copyright ©2014

October 8, 2012

If you are a developmental biologist, there is a high probability that you study Wnt. The Wnt signaling pathway is employed throughout cell and developmental biology in processes ranging from spindle positioning to stem cell fate decisions. Today’s image is from a paper showing how Wnt can be secreted from cells.

Wnt signaling functions by relaying a signal from the cell’s surface to the nucleus, where gene expression is regulated. Active Wnt proteins are secreted out of a cell to induce tissue patterning during development (among many other things), and can travel over a distance of several cells. In trying to understand Wnt secretion, a recent paper describes results showing that active Wnt signals can be secreted from exosomes. Exosomes are vesicles that cells use to secrete various materials into the extracellular space. Gross and colleagues found that Wnt signals are secreted on exosomes in both developing fruit flies and human cells. Wnt signals are trafficked through various endosomal compartments and then to exosomes, with the help of the trafficking protein Ykt6 (an R-SNARE). In the top row of images above, Wnt (left column, red in merged) appears colocalized with an exosomal protein (CD63, green in merged) in developing fly wing discs. Wnt is also colocalized with a marker for multivesicular bodies (LAMP-1, green in merged), vesicles from which exosomes originate.  Insets show higher magnification views of the vesicles. 

Gross JC, Chaudhary V, Bartscherer K, & Boutros M (2012). Active Wnt proteins are secreted on exosomes. Nature cell biology, 14 (10), 1036-45 PMID: 22983114

Adapted by permission from Macmillan Publishers Ltd, copyright ©2012

September 1, 2011

The nuances of the economy are nothing compared with the nuances throughout biology, yet we don’t see our scientists screaming at each other on TV (instead we see researchers versus Jenny McCarthy…ugh!). Researchers take their arguments and evidence to respectable journals and state the facts, which aren’t always in black and white. Today’s stunning image is from a paper that clarifies how actin can prevent AND promote secretion.

Cells use regulated secretion to release certain material outside of the cell at specific times. This multi-stage process involves the production and packaging of the material into secretory granules, trafficking of the material around the cell, and exocytosis (release) of the material out of the cell. Past research complicated the understanding of how important actin is in regulated secretion—some indicates that actin prevents it, while some indicates that actin promotes it. Recently, a research group dove right into the cell to look at regulated secretion in one specific cell type, and clarified the nuances of actin’s prevention/promoting roles. Nightingale and colleagues found that secretory granules are anchored to actin filaments to prevent premature secretion. However, actin later supports secretion by forming a ring-like structure at the site of the secretory granule’s fusion at the plasma membrane. This ring then contracts to help the release of material out of the cell. In the images above, actin (green) is found on the secretory granules (red). The higher magnification images (right) of the boxed region show the actin rings (bottom) found on fused secretory granules (middle).

Nightingale, T., White, I., Doyle, E., Turmaine, M., Harrison-Lavoie, K., Webb, K., Cramer, L., & Cutler, D. (2011). Actomyosin II contractility expels von Willebrand factor from Weibel-Palade bodies during exocytosis originally published in The Journal of Cell Biology, 194 (4), 613-629 DOI: 10.1083/jcb.201011119

June 23, 2011

Life is a balance of giving and taking, and this starts with our cells. I’ve written about cells taking in material (endocytosis) plenty of times, but it’s time to talk about cells secreting material out of the cell. Check out today’s stunning image of salivary gland cells in the fruit fly larva.

All cells undergo some base level of secretion, but there are many cell types with specialized “regulated” secretion. For example, our endocrine cells secrete the hormones that regulate our bodies and throw teenagers into crazed states. Cells with regulated secretion store high concentrations of certain proteins in dense organelles called secretory granules, until there is a signal that triggers the release of these proteins. A recent paper asks how secretory granules are formed, and finds that two vesicle coat proteins, called AP-1 and clathrin, are required. Burgess and colleagues looked at secretory granules in larval fruit fly salivary glands, and found that AP1 and clathrin are localized at newly synthesized secretory proteins, Golgi structures (where the proteins are sorted), and maturing secretory granules. Images show salivary gland cells with AP1 (red) colocalizing with Golgi structures (green).

Burgess, J., Jauregui, M., Tan, J., Rollins, J., Lallet, S., Leventis, P., Boulianne, G., Chang, H., Le Borgne, R., Kramer, H., & Brill, J. (2011). AP-1 and clathrin are essential for secretory granule biogenesis in Drosophila Molecular Biology of the Cell DOI: 10.1091/mbc.E11-01-0054

March 22, 2010

Microscopists are constantly pushing the limits of what microscopes can show and teach us about cell biology. One group recently used TIRF-based polarization techniques to image events at the plasma membrane during exocytosis. Exocytosis is the fusion of secretory vesicles with the plasma membrane to release their contents outside of the cell. The signal in the image above indicates the submicrometer changes in the curvature of the membrane shortly after the vesicle fuses with it (second image in panel is after fusion; cartoon is interpretation of the images).

Reference: Arun Anantharam, Bibiana Onoa, Robert H. Edwards, Ronald W. Holz, and Daniel Axelrod, 2010. Originally published in Journal of Cell Biology. doi:10.1083/jcb.200908010. Paper can be found here.