I think polar bodies are pretty cute. These little nubbins are products of meiotic division, and a simple testament to how amazing and clever a dividing cell (an oocyte, in this case) can be. Sure, polar bodies aren’t around for long, but I thank my own two long-lost polar bodies that let me have enough nutrients to survive. I hardly knew you, Atticus and Grover. Today’s image is from a paper describing anaphase in mouse oocyte divisions.
Meiosis is a special type of cell division that produces eggs and sperm. In mice, the meiotic spindle in the developing egg, or oocyte, is small and positioned in a very asymmetric location. This helps ensure that the nutrient- and organelle-rich cytoplasm stays with the daughter cell (the egg) that will later be fertilized and support early embryonic development. The other daughter of the division is the polar body, a small round structure that eventually is degraded. A recent paper describes results showing the sequence of anaphase events during mouse meiosis. In most cases of cell division, chromosome separation during anaphase is achieved by the shortening of kinetochore microtubules (termed anaphase A) and the lengthening of the entire meiotic spindle (anaphase B). In most cell divisions, anaphase A precedes anaphase B, yet Greg FitzHarris has shown that the reverse is true in mouse oocytes. Early anaphase B helps to determine the final size of the polar body. In addition, this early anaphase B spindle lengthening is triggered by the loss of tension on kinetochore microtubules, which occurs when cohesion between sister chromatids is lost. The images above are timepoints of anaphase in a mouse oocyte, with microtubules (grey) and chromosomes (green) labeled.
FitzHarris, G. (2012). Anaphase B Precedes Anaphase A in the Mouse Egg Current Biology, 22 (5), 437-444 DOI: 10.1016/j.cub.2012.01.041
Copyright ©2012 Elsevier Ltd. All rights reserved.
In a talk about midbodies while I was in graduate school, a fellow worm biologist* once endearingly described the midbody as a little “turd”. This talk signaled to me that 1) midbodies are totally fascinating, and 2) I can still have my third-grader sense of humor, giggle at the word “turd” AND be a biologist at the same time. Today’s stunning image is from a paper describing a thorough analysis of midbody assembly and maturation.
At the end of cytokinesis, the two resulting daughter cells are separated at the site of the midbody. This structure is derived from the midzone, which is a zone of overlapping microtubules that assembles between the separating chromosomes during anaphase. The midbody is made of this microtubule bundle as well as proteins involved in cytoskeletal regulation and membrane trafficking, and is very compact. In fact, the center of the midbody is so tightly packed that antibodies cannot reach the proteins, in turn preventing immunofluorescent imaging. A recent paper describes how known midbody proteins are rearranged and regulated as the structure assembles and matures. Hu and colleagues found that the proteins from the midzone/midbody fall into three different subgroups that localize to different regions, each subgroup likely having a different function in the mature midbody. In the images above, the proteins CENPE (red) and RacGAP1 (blue) colocalize at the midzone during anaphase (left). After that, the localization of the proteins changes (middle, left)—CENPE flanks RacGAP1 at midbodies starting from furrow ingression in cytokinesis.
*Guesses?
Hu, C., Coughlin, M., & Mitchison, T. (2012). Midbody assembly and its regulation during cytokinesis Molecular Biology of the Cell, 23 (6), 1024-1034 DOI: 10.1091/mbc.E11-08-0721
I love mitotic spindles, so of course I love early fruit fly development and its rapid, synchronized syncytial divisions. Watching these mitotic spindles perform a synchronized swimming routine, complete with tiny little swim caps and nose plugs, is always a treat. Today’s image is from a paper that helps to define the relationship between DNA replication, chromosome condensation, and mitotic progression.
During the cell cycle, DNA strands are replicated. After proper DNA replication, the very long strands are compacted in order to allow efficient and accurate separation of chromosomes during mitosis. When chromosome condensation doesn’t occur correctly, the progress through mitosis is disrupted. This may be due to the well-studied spindle assembly checkpoint, or there may be a checkpoint that monitors chromosome condensation. A recent paper describes results showing the effects of certain inhibitors on chromosome condensation and mitotic progression, marked by entry into anaphase. Fasulo and colleagues found that the inhibitors that severely disrupted chromosome condensation also disrupted anaphase onset. These delays occurred through disruption of the Wee1 kinase, and not due to the spindle assembly checkpoint. By using the early fruit fly embryo, Fasulo and colleagues could track many synchronized cell divisions at once, allowing for fast and direct analysis of the effects of the various inhibitors used. The cartoon and images above show the different steps during mitosis of these divisions. Both chromosomes (top row of images, green in merged) and microtubules (middle row of images, red in merged) are fluorescently tagged.
Fasulo, B., Koyama, C., Yu, K., Homola, E., Hsieh, T., Campbell, S., & Sullivan, W. (2012). Chk1 and Wee1 kinases coordinate DNA replication, chromosome condensation, and anaphase entry Molecular Biology of the Cell, 23 (6), 1047-1057 DOI: 10.1091/mbc.E11-10-0832
If you are a scientist (trained or at heart) reading this blog, you are likely a tinkerer. As a kid, you likely pulled apart all of your toys to figure out how they worked—maybe it was your basic water gun, your Etch-a-Sketch, or poor Teddy Ruxpin. Most biologists still do this today, but without their parents yelling at them about it. Today’s image is from a study identifying the components of primary cilia, which starts out with enough tinkering around to yank the cilia off of cells.
Primary cilia are found on many cells and serve as antenna to extracellular signals. Primary cilia are typically found one to each cell, and are important for many processes. Defects in primary cilia can cause a range of diseases called ciliopathies (polycystic kidney disease, for example). A recent study resulted in the identification of the proteins of primary cilia from mouse kidney cells. In this paper, Ishikawa and colleagues used a calcium-shock method to isolate the cilia from the cells, as seen in the images above. Before shock treatment (top), primary cilia (green) are seen on each cell. After treatment (middle), the isolated cilia (bottom) can be then analyzed for protein identification. From the 195 proteins identified, about 75% were proteins also seen in motile cilia or specialized cilia. About 25% were proteins only found in primary cilia, and will likely provide new insights to primary cilia biology and ciliopathies.
Ishikawa, H., Thompson, J., Yates, J., & Marshall, W. (2012). Proteomic Analysis of Mammalian Primary Cilia Current Biology, 22 (5), 414-419 DOI: 10.1016/j.cub.2012.01.031
Copyright ©2012 Elsevier Ltd. All rights reserved.
You likely know how important it is to maintain, repair, and even remodel your home as it gets older. Hypothetically, maybe your dog has chewed off your bedroom door’s frame in an attempt to greet your dinner guests, or maybe your toddler has drawn an abstract mural on your walls. Whatever the case, you head to your garage to find those necessary, but frequently overlooked, tools. Our cells are no different (except for dog- and toddler-induced damage). Today’s image is from a paper describing structures that are seen during cell-cell junction remodeling.
Endothelial cells line our blood vessels and maintain stable cell-cell junctions between one another to provide a tight barrier for our blood. These adherens junctions are damaged in cases of inflammation, atherosclerosis, and tumor angiogenesis, via endothelial signals and hormones. Vascular endothelial cadherin (VE-cadherin) is the central component of adherens junctions in these cells, and a recent paper by Huveneers and colleagues describes the finding that VE cadherin is found on both stable adherens junctions as well sites of junction remodeling called focal adherens junctions (FAJs). These newly-characterized FAJs are attached to actin bundles and contain Vinculin, which protects the junctions from opening during remodeling. In addition, FAJs were under pulling forces from the actin cytoskeleton during remodeling. Images above show human endothelial cells with FAJs (middle row, box 1) and stable adherens junctions (bottom row, box 2). Vinculin can be seen on FAJs, but not on stable junctions.
BONUS!! Check out some very cool movies of remodeling junctions here and here. All movies from this paper can be found here.
DOUBLE BONUS!! Not surprisingly, the author’s beautiful images impressed JCB so much that they made the cover here.
Huveneers, S., Oldenburg, J., Spanjaard, E., van der Krogt, G., Grigoriev, I., Akhmanova, A., Rehmann, H., & de Rooij, J. (2012). Vinculin associates with endothelial VE-cadherin junctions to control force-dependent remodeling originally published in the Journal of Cell Biology, 196 (5), 641-652 DOI: 10.1083/jcb.201108120
If it walks like a duck and quacks like a duck, it must be a duck….or it could be me doing my best duck impression. Scientists don’t like to make assumptions, but instead are driven to painstakingly test the cellular unknowns. Today’s image is from a paper that serves as a good example of how assumptions may not be accurate, but instead lead to a fascinating story that prompts more questions.
Desmosomes are cell-cell adhesion structures that help tissues resist mechanical stress by connecting with the cells’ intermediate filament networks. The mechanical strength provided by desmosomes is well documented in tissues such as heart muscle and stratified epithelia (skin, esophagus, etc). The intestine is a simple epithelial tissue that contains desmosomes, and as it is under a lot of mechanical stress from the movement and content of digesting food, it can be easily assumed that the desmosomes in intestinal tissue provide mechanical support. Not so, according to a recent paper by Sumigray and Lechler. The desmosomal protein desmoplakin (DP) is not essential for cell adhesion or tissue integrity in intestinal epithelia, nor is it necessary for proper organization of keratin intermediate filaments. However, DP is important for the structure of microvilli, the actin-rich structures that provide surface area for the absorption of nutrients. As seen in the images above, the microvilli of intestinal cells from DP knockout mice (middle, right) are shorter and misshapen when compared to wild-type cells (left).
Sumigray, K., & Lechler, T. (2012). Desmoplakin controls microvilli length but not cell adhesion or keratin organization in the intestinal epithelium Molecular Biology of the Cell, 23 (5), 792-799 DOI: 10.1091/mbc.E11-11-0923
Sometimes we all need a friend to lean on and help us out. If we’re as lucky as myosin IIIB, we have an espin-1 in our lives to help us get to where we need to be, both literally and figuratively. And if we’re super lucky, that espin-1 will share a glass of wine and watch this week’s Parks and Recreation with us. Today’s stunning image is from a paper describing a fascinating relationship between a myosin motor and its cargo.
Myosin is a molecular motor that walks along actin filaments. There are many types of myosins that have different functions, carry different cargoes, and use different mechanisms to perform their task. One myosin that plays a role in human hearing, MYO3A, uses both its motor and actin-binding tail domains to walk like an inchworm to the ends of actin-based membrane protrusions called filopodia. A recent paper finds that another myosin called MYO3B can reach the tips of filopodia, but needs the help of a protein called espin-1. MYO3B does not have a tail domain like MYO3A, but according to Merritt and colleagues, can use espin-1 cargo as a “crutch” to reach filopodia tips. espin-1 does have an actin-binding domain, suggesting that for some modes of myosin motility, the myosin’s actin-binding tail domain can be replaced with cargo containing an actin-binding domain. In addition, both MYO3A and MYO3B can elongate actin protrusions. In the images above, MYO3B and espin-1 together localize to the tips of filopodia (actin is blue).
Merritt, R., Manor, U., Salles, F., Grati, M., Dose, A., Unrath, W., Quintero, O., Yengo, C., & Kachar, B. (2012). Myosin IIIB Uses an Actin-Binding Motif in Its Espin-1 Cargo to Reach the Tips of Actin Protrusions Current Biology, 22 (4), 320-325 DOI: 10.1016/j.cub.2011.12.053
Copyright ©2012 Elsevier Ltd. All rights reserved
Scenario: You’re at my house for a dinner and I make you a mind-blowing chocolate tart. You ask why it is so durn good, and I pass along the fact that Nutella is the ass-kicking ingredient. Next thing you know, you’re trying to add Nutella to everything at home….and by golly, it makes (many) things taste better. Today’s image is from a paper characterizing the relationship between the many cellular changes after treatment by antimitotic drugs. Knowing how these drugs work (by analogy, finding the ass-kicking “ingredient”) can help folks develop improved anti-cancer drugs.
Many anticancer drugs are antimitotic drugs, meaning they function by blocking the progress of cell division during mitosis. In addition to arresting mitosis, these drugs cause apoptosis, DNA damage, and induction of p53 (a tumor suppressor gene), and a recent paper investigates the relationship between all of these events. After treating cells with powerful antimitotic drugs, Orth and colleagues found that the resulting prolonged mitotic arrest (or slippage from that arrest) causes DNA damage, which in turn causes an induction of p53. The DNA damage was inhibited when these treated cells were prevented from launching the pathway for apoptosis, which is programmed cell death. So, Orth and colleagues concluded that the prolonged mitotic arrest caused by antimitotic drugs results in a partial activation of apoptosis. Understanding this partial apoptotic response in the context of treating tumors should help guide development of improved cancer therapies. The images above show increasing DNA damage (red spots) after prolonged treatment with an antimitotic drug, compared with an untreated cell (top left). By 16 and 48 hours of drug treatment (bottom), cells had very high levels of DNA damage. Arrows point to mitotic cells.
Orth, J., Loewer, A., Lahav, G., & Mitchison, T. (2011). Prolonged mitotic arrest triggers partial activation of apoptosis, resulting in DNA damage and p53 induction Molecular Biology of the Cell, 23 (4), 567-576 DOI: 10.1091/mbc.E11-09-0781
Tumors begin when a cell goes rogue. These rogue cells turn against us in a most unforgiving way, even when their environment and neighboring cells try to put the kibosh on such behavior. Today’s image is from a fascinating paper describing how a single mutant cell in a highly organized environment can move out of the tissue, similar to what is seen in some tumors.
The development of a tumor begins with sporadic mutations of oncogenes, but these mutant cells are frequently in a highly organized tissue that limits its growth and movement, two key elements to the spread of cancer. A recent paper describes how single mutant cells are able to overcome their suppressive environment and move out of the tissue. To study this, Leung and Brugge grew mammalian cells in 3D cultures and followed clusters of cells with hollow lumens, called acini, after overexpressing different oncogenes. The overexpression of ERBB2, an oncogene overexpressed in 30% of breast tumors, caused outgrowth of the mutated cells from the cluster’s epithelial layer and into the lumen, a feature commonly seen in carcinoma in situ (non-invasive) breast tumors. In addition, Leung and Brugge found that this is a highly regulated process, as the mutation also causes changes in adhesion between the cell and its underlying matrix. These changes also support the survival and growth of the mutant cells. As seen in the images above, control cells (green, top) remained within the highly organized epithelial layer of the cluster over a course of 56 hours, but a single ERBB2-overexpressing cell (green, bottom) dissociated from the epithelial layer and moved into the hollow lumen.
Leung, C., & Brugge, J. (2012). Outgrowth of single oncogene-expressing cells from suppressive epithelial environments Nature, 482 (7385), 410-413 DOI: 10.1038/nature10826
Adapted by permission from Macmillan Publishers Ltd, copyright ©2012
