October 30, 2012

I’ve posted a lot of images from papers on human disease models lately, and that’s a good thing. Despite what you may hear from politicians mocking research using fruit flies, worms, or tiny fish, the power of model systems to study a human problem is undeniable to anyone with half a clue. All science on the continuum from basic research all the way to clinical trials is valuable…so thank your favorite scientist today! Today’s image is from a zebrafish paper that identifies a new pathway that may be a good therapy target for muscular dystrophies.

Muscular dystrophies are fairly common diseases that result in the weakening of the protein complexes that connect muscles to their underlying environment, called the extracellular matrix (ECM). The degeneration of muscle-ECM attachment eventually leads to progressive loss of muscle movement and locomotion for the patient. A recent paper identifies a new pathway in muscle-ECM attachment using zebrafish as a model. Goody and colleagues found that biosynthesis of the small molecule NAD+ can reverse muscle degeneration in certain types of dystrophies in zebrafish and even improve swimming. This pathway improves the organization of laminin, an important ECM protein, suggesting the function of an additional laminin receptor complex and pathway from those already studied. Goody and colleagues suggest that this new pathway may serve as a focal point for new muscular dystrophy therapies. In the images above, normal ECM basement membrane tissue adhered normally to both normal muscle cells (blue) and cells with muscle degeneration mutations (red), even after the stress of swimming. This experiment shows the importance of a healthy ECM in restoring function for dystrophic muscles. 

Goody, M., Kelly, M., Reynolds, C., Khalil, A., Crawford, B., & Henry, C. (2012). NAD+ Biosynthesis Ameliorates a Zebrafish Model of Muscular Dystrophy PLoS Biology, 10 (10) DOI: 10.1371/journal.pbio.1001409

October 26, 2012

Animal models of human diseases allow biologists to move quickly on understanding the physiology behind a disease and make suggestions on developing therapies. Today’s image is from a paper showing the importance of using multiple animal models for the same human disease.

The genetic disorder Usher syndrome (USH) causes hearing and visual impairment. Mice have provided biologists with a fantastic model of hearing impairment in USH patients, specifically those with the most severe form of the syndrome, USH1. The five USH1 proteins (myosin VIIa, harmonin, cadherin-23, protocadherin-15, sans) all play a role in the development of the hair bundle, the structure that drives the conversion of sounds into electrical responses in the ear. Mice with USH1 mutations do not, however, experience the same visual degeneration as human USH1 patients. A recent study finds differences in USH1 protein localization in the photoreceptors of different animal models. Sahly and colleagues found that the photoreceptors of macaques (monkeys) show localization of all USH1 proteins at membrane interfaces, while photoreceptors in mice lack any USH1 network proteins. Sahly and colleagues suggest that the USH1 network found in humans and other primates forms an adhesion belt around the basolateral region of photoreceptors, and this structure may be the key to understanding the visual impairment experienced by USH1 patients. In the images above of macaque retina photoreceptors, the USH1 network protein protocadherin-15 (green) is localized at the junction between the inner (IS) and outer segments (OS) of rods and cones.

Sahly, I., Dufour, E., Schietroma, C., Michel, V., Bahloul, A., Perfettini, I., Pepermans, E., Estivalet, A., Carette, D., Aghaie, A., Ebermann, I., Lelli, A., Iribarne, M., Hardelin, J., Weil, D., Sahel, J., El-Amraoui, A., & Petit, C. (2012). Localization of Usher 1 proteins to the photoreceptor calyceal processes, which are absent from mice originally published in the Journal of Cell Biology, 199 (2), 381-399 DOI: 10.1083/jcb.201202012

October 23, 2012

Understanding how a cell works normally is hard enough for biologists. Understanding how a cancerous cell works is exponentially harder—there are different stages of tumorigenesis and countless different types of cancer and countless different environments within the body. Today’s image is from a study that takes a systematic approach to understanding the interactions between cancerous cells and their environment.

The progression of tumor cells to metastatic cancer cells correlates with poor prognoses for cancer patients. The steps that drive cancer cells to spread (metastasis) are not well understood, but may be effective targets for chemotherapies. For example, tumor cells lose adhesion to their underlying extracellular matrix (ECM) prior to spreading to other regions of the body. Understanding this loss of cell-ECM adhesion may guide the development of new therapies. A recent paper describes the systematic analysis of cell-ECM adhesion in tumor cells by using robotically spotted arrays of 768 paired combinations of ECM molecules. Reticker-Flynn and colleagues monitored the adhesion profiles of lung cancer cell lines at different stages of cancer progression on these arrays of ECM molecules, and found ECM-cell interactions that may be successful therapeutic targets. The images above show an array of spotted ECM protein combinations (visible through immuno- and fluorescent-labeling, top), and examples of cells adhered to the ECM spots (bottom images).

Reticker-Flynn, N., Malta, D., Winslow, M., Lamar, J., Xu, M., Underhill, G., Hynes, R., Jacks, T., & Bhatia, S. (2012). A combinatorial extracellular matrix platform identifies cell-extracellular matrix interactions that correlate with metastasis Nature Communications, 3 DOI: 10.1038/ncomms2128
 Adapted by permission from Macmillan Publishers Ltd, copyright ©2012

October 19, 2012

The next time you see a fruit fly buzzing around your kitchen, take a beat before you smack it with a swatter and remind yourself of the amazing discoveries due to organisms like the (not so) lowly fruit fly. Maybe you’ll offer a thank-you glass of grape juice instead and show the intruder back outside. Today’s image is from a paper describing a fly model of a serious human disease, and serves as a great example of the power in a model organism.

Spinal muscular atrophy (SMA) is a heritable disease that results in infant mortality due motor neuron dysfunction and rapid degeneration of muscle. SMA is caused by the depletion of the SMN (survival motor neuron) protein. A recent paper describes the use of fruit flies in studying SMA, and shows that flies lacking SMN have reduced muscle size and defective motor neuron neurotransmission similar to SMA patients. Imlach and colleagues found that replenishing SMN levels in motor neurons and muscles did not reverse the defects of SMN depletion, yet increasing SMN levels in partner cells (proprioceptive neurons and interneurons) can reverse the defects. These results suggest that SMN depletion primarily affects the sensory-motor network, with secondary effects seen in the motor circuit. In addition, Imlach and colleagues found that increasing motor neural circuit excitability, either genetically or with drugs, could relieve SMN-depletion defects. Using this fly model of SMA, these results suggest that SMA patients may improve by enhancing motor neural network activity. The images above show the differences in muscle size in wild-type (left) and SMN-depleted (right) flies.

Imlach WL, Beck ES, Choi BJ, Lotti F, Pellizzoni L, & McCabe BD (2012). SMN Is Required for Sensory-Motor Circuit Function in Drosophila. Cell, 151 (2), 427-39 PMID: 23063130
Copyright ©2012 Elsevier Ltd. All rights reserved.

 

October 16, 2012

Molecular switches are an elegant way for a cell to regulate countless processes with a limited number of proteins. The layers of regulation that drive switch activation or inactivation allow the cell to drive one very specific process, despite the presence of all of the tools and materials to drive a thousand other processes. Today’s image is from a paper that describes one layer of regulation on a well-studied (and well-loved) set of molecular switches.

Rho GTPases are actin regulators that switch between active and inactive states. Their activation state is regulated by proteins (GEFs) that drive the GTPases into their active state, and others (GAPs) that drive them into their inactive state. RhoGDIs (Rho guanine nucleotide dissociation inhibitors) provide an additional level of control over Rho GTPase activity by sequestering Rho GTPases in inactive complexes. The selective dissociation of individual Rho GTPases from these complexes provides the cell with a context-specific response based on the actin-based structures required. A recent paper describes the function of a protein called diacylglycerol kinase ζ (DGKζ) in the release of two Rho GTPases, Rac1 and RhoA, from RhoGDI complexes. Rac1 regulates membrane ruffling and lamellipodia formation, while RhoA regulates stress fiber and focal adhesion formation. Ard and colleagues found that DGKζ-deficient cells showed signs of faulty RhoA signaling, as seen in the images above. Actin stress fibers (left column, green in merged) and focal adhesions (middle column, red in merged) appear normal in wild-type cells (top). In DGKζ-null cells (bottom row), actin stress fibers appeared condensed (arrow) and less organized, while focal adhesion distribution was impaired.

Ard, R., Mulatz, K., Abramovici, H., Maillet, J., Fottinger, A., Foley, T., Byham, M., Iqbal, T., Yoneda, A., Couchman, J., Parks, R., & Gee, S. (2012). Diacylglycerol kinase regulates RhoA activation via a kinase-independent scaffolding mechanism Molecular Biology of the Cell, 23 (20), 4008-4019 DOI: 10.1091/mbc.E12-01-0026

October 11, 2012

When in a crowded elevator, mall, or football stadium, I panic knowing that I’d be the first person trampled and/or eaten in an emergency. Crowded places give me the willies, but when proteins are over-crowded, interesting things happen. Today’s image is from a paper showing what can happen at the membrane when proteins are over-crowded. 

Membrane bending is an essential part of many cellular events, including endocytosis and filopodia formation. It has been suggested that membrane curvature can stem from two mechanisms—the use of curved proteins to form a curved scaffold for the membrane, or the insertion of wedge-shaped hydrophobic helices into the membrane. A recent paper shows a third mechanism that can drive membrane curvature. Stachowiak and colleagues found that protein-protein crowding drives membrane bending. Protein coverage at 20% is sufficient to drive curvature, by creating lateral pressure of membrane-bound proteins colliding. Interestingly, even proteins unrelated to membrane curvature can induce curvature when overcrowded (GFP, for example). The cartoons and images above show vesicles that contain low (left) or high (right) levels of protein binding at specific domains at the membrane. High levels of protein caused membrane curvature, as seen as long lipid tubules.

Stachowiak JC, Schmid EM, Ryan CJ, Ann HS, Sasaki DY, Sherman MB, Geissler PL, Fletcher DA, & Hayden CC (2012). Membrane bending by protein-protein crowding. Nature cell biology, 14 (9), 944-9 PMID: 22902598 
Adapted by permission from Macmillan Publishers Ltd, copyright ©2012

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

October 4, 2012

What happens when you bake a cake without baking powder? That’s right…you get a flat, dense cake-shaped hockey puck. What about too much baking powder? Kablooey…the cake rises quickly then deflates. Approaching questions in cell biology this same way leads to amazing discoveries about cells, minus the cake. Today’s image is from a paper describing the roles of different dynactin subunits.

Dynactin is a multi-protein complex that is essential for the activity and binding of dynein, the microtubule motor. It has been suggested that dynactin is the key to dynein’s ability to bind countless different cellular cargoes throughout every stage of the cell cycle. Of the 11 subunits in dynein, 4 proteins (Arp11, p62, p27, and p25) make up the “pointed end complex,” a domain believed to be important in cargo specificity. A recent paper shows results on the importance and interactions of these pointed end complex proteins. By depleting levels—or overexpressing—these proteins in tissue culture cells, Yeh and colleagues found that p62 and Arp11 pair up and affect dynactin binding to the nuclear envelope prior to mitosis, while p27 and p25 pair up to regulate membrane binding (early and recycling endosomes). Yeh and colleagues also found that Arp11 and p62 are necessary for dynactin stability. In the images above, mitotic spindles lacking different subunits of the pointed end complex show various defects. The spindles (red, microtubules) of cells lacking Arp11 and p62 are multipolar, as compared to wild-type cells (top row). Spindles of cells lacking p27, however, appeared normal.


Yeh TY, Quintyne NJ, Scipioni BR, Eckley DM, & Schroer TA (2012). Dynactin's pointed-end complex is a cargo-targeting module. Molecular biology of the cell, 23 (19), 3827-37 PMID: 22918948

October 1, 2012

There are a few things in our world that only turn to the right—Derek Zoolander and UPS trucks, for example. Maybe they could be inspired by the images above of myosin IC, which turns in left-hand curves.

Myosins are actin-based motors that function in countless cell processes. Class I myosins link cell membranes with the actin cytoskeleton and bind directly to lipid membranes. Within this class of myosins, myo1c is important in hearing, endocytosis, and membrane trafficking, and binds to the lipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). A recent paper describes the ability of myo1c to drive actin motility on membranes containing PtdIns(4,5)P2. Pyrpassopoulos and colleagues found that myo1c motility occurs along counter-clockwise curved paths (top image above). This curved motility is not typical of other class I myosins—myo1a, for example, drives actin gliding in a straight line (bottom image above). In the images above, the actin filaments’ paths are in green, while the starting position of the filament is in orange.

BONUS!! Additional recent work from this research group describes the kinetic and mechanical properties of myo1c. Based on the results from this work, Greenberg and colleagues propose that myo1c functions as a slow transporter rather than a tension-sensitive anchor. 

Serapion Pyrpassopoulos, Elizabeth A. Feeser, Jessica N. Mazerik, Matthew J. Tyska, & E. Michael Ostap (2012). Membrane-Bound Myo1c Powers Asymmetric Motility of Actin Filaments Current Biology, 22 (18), 1688-1692 DOI: 10.1016/j.cub.2012.06.069 
Copyright ©2012 Elsevier Ltd. All rights reserved.


Michael J. Greenberg, Tianming Lin, Yale E. Goldman, Henry Shuman, & E. Michael Ostap (2012). Myosin IC generates power over a range of loads via a new tension-sensing mechanism Proc Natl Acad Sci USA, 109 (37) DOI: 10.1073/pnas.1207811109