March 29, 2013

I bet a lot of you have that ex-girlfriend or ex-boyfriend who just hangs on….and hangs on tighter even though you’ve driven off to college and totally matured past the desire to want someone who can crush a beer can on his/her forehead.  He/she was totally pulling an endoplasmic reticulum, and the paper that brought us today’s image shows us why.

Endosomes form at the plasma membrane, where they take in material from outside the cell.  They mature, with some cargo recycled back to the plasma membrane and some cargo trafficked to lysosomes for degradation.  The cytoskeletal tracks (microtubules) that endosomes depend on for movement around the cell are also used as tracks for the endoplasmic reticulum (ER), where proteins are synthesized, folded, and sorted.  Recent work has shown interactions between endosomes and the ER, and a more recent paper characterizes these interactions.  Friedman and colleagues used high-resolution three-dimensional electron microscopy and found that ER tubules wrap around maturing endosomes, while both organelles maintain contact with microtubules.  As the endosomes mature, they become more tightly connected to the ER.  Similarly, as an endosome traffics, the interacting ER rearranges its structure in order to maintain endosome contact.  In the images above, early endosomes (Rab5, red) are tightly associated with ER membranes (green).  Higher magnification images (bottom) of the boxed region show three early endosomes (numbers 1,2, and 4) that are and remain in contact with the ER over the course of two minutes, and one endosome (number 3) that becomes associated.

Friedman, J., DiBenedetto, J., West, M., Rowland, A., & Voeltz, G. (2013). Endoplasmic reticulum-endosome contact increases as endosomes traffic and mature Molecular Biology of the Cell, 24 (7), 1030-1040 DOI: 10.1091/mbc.E12-10-0733

March 26, 2013

When Outkast sang that “I like the way you mooove,” I immediately figured they were talking my dancing skills.  Turns out, they were really singing about motile cells…true story*.  Today’s image is from a paper showing a link between the chemical signals that tell a cell to move and how the cell actually moves.

As an axon searches for its synaptic target, it sends out a motile extension called a growth cone.  The chemical cues that initiate mobility in the growth cone trigger major cytoskeletal rearrangements at the leading edge, specifically actin filament polymerization and an engagement with the underlying extracellular substrate.  A recent paper describes the signaling that links the chemical cues to the generation of traction forces that move the growth cone forward.  Toriyama and colleagues show that the protein shootin1 is phosphorylated following signaling from the chemoattractant netrin1.  Phosphorylated shootin1, then, triggers actin-extracellular substrate coupling and generates the forces for axonal outgrowth.  In the images above, phosphorylated shootin1 (green) is enriched at filopodial and lamellipodial extensions in growth cones (actin filaments are in red).

*not really

Toriyama, M., Kozawa, S., Sakumura, Y., & Inagaki, N. (2013). Conversion of a Signal into Forces for Axon Outgrowth through Pak1-Mediated Shootin1 Phosphorylation Current Biology, 23 (6), 529-534 DOI: 10.1016/j.cub.2013.02.017 
Copyright ©2013 Elsevier Ltd. All rights reserved. 

March 22, 2013

If there was an NCAA-type bracket of cool things to image in a cell, I’m pretty sure a spindle would make it pretty far in the tournament—if not take the whole thing.  They are extremely photogenic, and their important role and dynamic nature make them a top seed for sure.  Today’s image is from a paper identifying a new player in spindle positioning.

The position of the mitotic spindle serves as the guide to where the cell will be divided into two daughter cells in cytokinesis.  The spindle is positioned with the help of astral microtubules—microtubules that grow from the centrosomes toward the cell periphery.  The interaction of these microtubules with machinery at the cell’s cortex can generate pulling or pushing forces that position the entire mitotic spindle, yet this interaction isn’t completely understood.  A recent paper identifies a protein called MISP as a player in the microtubule-cortex interaction.  Zhu and colleagues found that depletion of MISP, an actin-binding protein, resulted in shortened astral microtubules and rocking, misoriented spindles, among other mitotic defects.  Zhu and colleagues showed that MISP is phosphorylated by Plk1, a major kinase important in many mitotic processes.  The images above show MISP (gray, red in merged images) localization throughout mitosis.  MISP is localized to cortical actin (green).  Boxed regions are shown in higher magnification in the insets.  

Zhu M, Settele F, Kotak S, Sanchez-Pulido L, Ehret L, Ponting CP, Gönczy P, & Hoffmann I (2013). MISP is a novel Plk1 substrate required for proper spindle orientation and mitotic progression. originally published in the Journal of Cell Biology, 200 (6), 773-87 PMID: 23509069

March 19, 2013

After a lovely Spring Break with my family, HighMag is back in action.  As an oldie, Spring Break takes on a whole new meaning than it did years ago…this pregnant lady didn’t have one single beer!

 Cancer is a series of cellular mistakes, mistakes that are too far gone to fix without the help of the medical field.  Understanding the mistakes at the most basic cellular level is key to fighting the war on cancer, and a recent paper is a fine example of this.

A great model for understanding cell polarity is the intestinal epithelial sheet of cells that provides a barrier between the inside of the intestine and the body.  The different polarized domains – apical and basal – each have a discrete set of adhesion and membrane proteins trafficked to them.  One membrane trafficking protein, Rab25, is a tumor suppressor for colon cancer in both humans and mice.  A recent paper investigates the link between Rab25 and the polarized intestinal cells involved in colon cancer.  Krishnan and colleagues found that a reduction of Rab25 levels in cultured endothelial cells resulted in increased cell invasion and a loss of certain integrins, adhesion proteins, at the plasma membrane.  Rab25 loss also affected the transcription of several genes including the transcription factor ETV4, suggesting that Rab25’s effect on cell polarity is through gene regulation.  The scanning electron images above show brush border microvilli, the fingerlike-projections seen in intestinal epithelial sheets that serve to increase absorption of nutrients from the intestine.  In Rab25-reduced cells (middle row), the brush border is sparsely-packed and disorganized when compared to control cells (top row).  Bottom row shows Rab25-reduced cells in which rabbit Rab25 was reintroduced, and the rescued brush border is more organized.

Krishnan M, Lapierre LA, Knowles BC, & Goldenring JR (2013). Rab25 regulates integrin expression in polarized colonic epithelial cells. Molecular biology of the cell, 24 (6), 818-31 PMID: 23345591

March 7, 2013

DNA is not just shoved into the nucleus of a cell like a college boy’s clothes jammed into his closet (maybe that was just my husband?).  The organization of the nucleus has been getting more attention lately, and the results are pretty fascinating.  Today’s image is from a recent paper showing the association of a promoter to nuclear pores.

The organization of the nucleus may depend on the tethering of chromatin, or packaged DNA, to the nuclear envelope.  While some past results have suggested that actively expressed regions of chromatin are associated with the nuclear envelope in some organisms, other results have shown localization of actively expressed genes at internal nuclear speckles in other organisms.  A recent paper shows a link between the nuclear pore and a promoter, which is a region of DNA that initiates the expression of a nearby gene.  Rohner and colleagues found that the heat shock promoter hsp-16.2 interacts with the nuclear pore after heat shock, a type of stress, in worms.  Without heat shock, the promoter still maintained a perinuclear localization.  Using super-resolution structured illumination microscopy (SR-SIM) to achieve 100-nm resolution, Rohner and colleagues found that after heat shock, the promoter’s localization to the nuclear pore complex increased.  These results suggest that this stress-activated promoter may direct chromatin to the nuclear pores, where genes can be more easily accessed by transcription machinery.  In the image above, a 200-cell stage worm embryo is stained to show the localization of the hsp-16.2 promoter (green) at the nuclear envelope (red, DNA is in blue) under normal circumstances.

BONUS!!  Click here for a video of a 3D reconstruction of super-resolution images showing nuclear pores (green) and nuclear envelope (lamina, red).  

Rohner, S., Kalck, V., Wang, X., Ikegami, K., Lieb, J., Gasser, S., & Meister, P. (2013). Promoter- and RNA polymerase II-dependent hsp-16 gene association with nuclear pores in Caenorhabditis elegans originally published in the Journal of Cell Biology, 200 (5), 589-604 DOI: 10.1083/jcb.201207024

March 4, 2013

To me, science is a world of optimism.  Researchers are constantly looking for cures for countless diseases, and with intelligence, a bit of luck, and grant funding their hard work rolls into results that may lead to new therapies.  I always look forward to seeing how some of the most basic cell biology experiments can lead to something big, and I again fall in love with the whole process.

Medulloblastoma is the most common pediatric brain tumor, yet the treatments are highly toxic and are associated with high morbidity.  A recent paper found that PIGF (placental growth factor) is expressed in most medulloblastomas, regardless of their subtype, and that high expression of the PIGF receptor Nrp1 is associated with poor survival of patients.  Snuderl and colleagues then showed that growth and spread of medulloblastomas are dependent on PIGF and Nrp1, suggesting that PIGF and Nrp1 could serve as targets for future therapies for medulloblastoma.  In the two-photon microscopy images above, the vasculature (green) of medulloblastomas (blue) was less dense in tumors treated with a PIGF-blocking antibody (bottom), compared with control (top) over time, during development.  In the antibody-treated tissue, there was even some vessel regression (white arrows, bottom).

Snuderl, M., Batista, A., Kirkpatrick, N., Ruiz de Almodovar, C., Riedemann, L., Walsh, E., Anolik, R., Huang, Y., Martin, J., Kamoun, W., Knevels, E., Schmidt, T., Farrar, C., Vakoc, B., Mohan, N., Chung, E., Roberge, S., Peterson, T., Bais, C., Zhelyazkova, B., Yip, S., Hasselblatt, M., Rossig, C., Niemeyer, E., Ferrara, N., Klagsbrun, M., Duda, D., Fukumura, D., Xu, L., Carmeliet, P., & Jain, R. (2013). Targeting Placental Growth Factor/Neuropilin 1 Pathway Inhibits Growth and Spread of Medulloblastoma Cell, 152 (5), 1065-1076 DOI: 10.1016/j.cell.2013.01.036
Copyright ©2013 Elsevier Ltd. All rights reserved.

March 1, 2013

There are a handful of major signaling modules in development, and I’m fascinated every time a paper comes out showing a new role for one of these pathways, or how multiple pathways interact.  I admire the resourcefulness of cells in using a limited number of proteins to get an unlimited number of things done.  Today’s image is from a paper showing the role of PCP signaling in primary cilia assembly.

The PCP (polar cell polarity) signaling pathway is important in establishing cell polarity throughout development.  PCP signaling has also been linked to the formation and function of cilia, microtubule-based organelles, and a recent study investigates the mechanism between the two.  Primary cilia are solitary cilia that reside on most vertebrate cells and function in signaling, and Zilber and colleagues found that Fuzzy, a PCP signaling effector protein, is involved in primary cilium formation.  Fuzzy localizes to the basal body of cilia, and is necessary to drive Golgi-derived vesicles to the primary cilium.  Fuzzy regulates the localization of Dishevelled, a core PCP protein, to the basal body, and without Fuzzy, PCP signaling is inhibited (while activating the canonical Wnt pathway).  The images above show migrating mammalian cells at the edge of a wound in control (left) and Fuzzy mutant cells (right).  Wound-healing is controlled by PCP signaling in vertebrates, but the role of Fuzzy was unknown.  In control cells, the Golgi network (green) is polarized in front of the nucleus in most migrating cells.  In Fuzzy mutants, the Golgi network is oriented randomly in most cells (asterisks), and cell migration covered less of the initial wound area than in control cells (direction of migration indicated by white arrow).

Zilber, Y., Babayeva, S., Seo, J., Liu, J., Mootin, S., & Torban, E. (2013). The PCP effector Fuzzy controls cilial assembly and signaling by recruiting Rab8 and Dishevelled to the primary cilium Molecular Biology of the Cell, 24 (5), 555-565 DOI: 10.1091/mbc.E12-06-0437