July 18, 2014

Poor polar bodies typically go the way of that old container of Chinese take-out in your fridge and are eventually dumped. Thanks to a very clever study published in Cell, polar body transfer can prevent the transmission of inherited mitochondrial diseases. Waste not, want not.

The meiotic divisions of an oocyte result in the production of an egg in the extrusion of two very small polar bodies. These polar bodies have the same genetic material as the egg but have only a small number of organelles, including mitochondria. The DNA of mitochondria (mtDNA) can carry mutations that cause a variety of diseases. As mtDNA is maternally inherited due to the abundance of mitochondria in the oocyte, recent studies have focused on the replacement of mutant mtDNA with normal mitochondria to treat these inherited diseases. A recent paper tests the use of polar bodies as the source of donor genomes in a potential new method for mitochondrial replacement. As polar bodies have the same genome as the egg, but does not have mtDNA variants, they can successfully replace the genome in a recipient egg that already has normal mtDNA. Wang and colleagues have shown that polar body genome transfer successfully does just this, and provides a potential new therapy for preventing inherited mitochondrial diseases. The images above show the presence of mitochondria (red) in oocytes and polar bodies. Both polar bodies (PB1 and PB2) have far fewer mitochondria than the ooctyes.

Wang, T., Sha, H., Ji, D., Zhang, H., Chen, D., Cao, Y., & Zhu, J. (2014). Polar Body Genome Transfer for Preventing the Transmission of Inherited Mitochondrial Diseases Cell, 157 (7), 1591-1604 DOI: 10.1016/j.cell.2014.04.042
Copyright ©2014 Elsevier Ltd. All rights reserved.

June 10, 2014

The Life History of a Single Kinetochore Fiber sounds like a book a lot of us would enjoy (well, me at least). It isn’t really a book about a plucky kinetochore fiber who triumphs over a difficult childhood, but rather the focus of a fascinating recent paper. In this paper published in Molecular Biology of the Cell, LaFountain and Oldenbourg present results showing a model for kinetochore microtubule formation that occurs at kinetochores.

Kinetochore fibers link chromosomes to the mitotic spindle, which drives chromosome segregation during anaphase. The prevailing model of kinetochore fiber formation includes a “search and capture” mechanism, in which some dynamic spindle microtubules reach a kinetochore and become stabilized by the interaction. A recent paper by LaFountain and Oldenbourg shows, however, that the maturation of these kinetochore fibers depends on the addition of microtubules at the kinetochore-proximal end, with polymerization towards the spindle pole. In this study, the naturally birefringent microtubules of crane-fly spermatocytes were examined, allowing a quantitative analysis of where microtubules are added. In the images above, kinetochore-proximal addition of microtubules can be seen in the centrosome-free half-spindle (red arrows) of a crane-fly spermatocyte, from early prometaphase to metaphase (top to bottom).

LaFountain, J., & Oldenbourg, R. (2014). Kinetochore-driven outgrowth of microtubules is a central contributor to kinetochore fiber maturation in crane-fly spermatocytes Molecular Biology of the Cell, 25 (9), 1437-1445 DOI: 10.1091/mbc.E14-01-0008

July 30, 2012

It may appear that chromosomes are just floating around inside of the nucleus, but that couldn’t be further from the truth. Inside that double membrane of the nucleus is a lot of chromosome choreography, and this is especially true during meiosis. Today’s image is from a paper showing the interaction between two nuclear envelope proteins and their regulation of chromosome dynamics and pairing.

Meiosis is a type of cell division that reduces the number of chromosomes down to one copy of each in the resulting daughter cells, called gametes (eggs and sperm). During meiosis, the pairing of homologous chromosomes (matching chromosomes) is important for recombination and chromosome segregation later in meiosis. Recombination is the exchange of DNA regions between two paired chromosomes, and is key in generating genetic variation. In yeast and worms, KASH domain proteins and SUN domain proteins of the nuclear envelope interact to ensure proper chromosome pairing and positioning within the nucleus. Mammals were known to have a SUN domain protein, SUN1, and a recent paper identified its KASH domain binding partner, KASH5. Morimoto and colleagues found that KASH5 and SUN1 both localize at telomeres, regions at the ends of chromosomes, during meiosis in mice. In addition, KASH5 interacts with the microtubule-associated dynein-dynactin complex to regulate chromosome movement. In the images above, chromosomes from mouse spermatocytes have both KASH5 and SUN1 localized at telomeres throughout meiosis (chromosomes are blue).

Akihiro Morimoto, Hiroki Shibuya, Xiaoqiang Zhu, Jihye Kim, Kei-ichiro Ishiguro, Min Han, & Yoshinori Watanabe (2012). A conserved KASH domain protein associates with telomeres, SUN1, and dynactin during mammalian meiosis originally published in the Journal of Cell Biology, 198 (2), 165-172 DOI: 10.1083/jcb.201204085

May 21, 2012

Whenever we go on a trip, my long-suffering husband quietly puts our luggage next the car and slinks away, trembling and twitching.  He knows a mad-woman is ready to pack the trunk, playing luggage-Tetris until it all fits and speaking in tongues.  Seriously, though, I’m freaking awesome.  That said, I don’t envy the insane packing that a cell must accomplish to jam all of that DNA into neat little chromosomes ready for their own cell division road trip.  A recent paper helps us understand how that happens at the centromere.

Centromeres are the regions on chromosomes that bind sister chromatids together and serve as the sites of kinetochore assembly during mitosis.  The presence of the protein CENP-A is a hallmark of centromere location, as it is a histone H3 variant that helps package and compact centromeric DNA.  It was previously presumed that CENP-A was passed down to daughter cells epigenetically, inherited from previous cell divisions, but a recent paper shows that this is not the case in the nematode worm C. elegans.  According to Gassmann and colleagues, pre-existing CENP-A is not required for CENP-A localization to centromeres in subsequent divisions.  In fact, CENP-A is unloaded from centromeres at one point in oogenesis, the production of eggs, and later reloaded onto centromeres.  By mapping the location of CENP-A in the genome, Gassmann and colleagues found that regions of transcribed genes are regions where CENP-A is excluded, a pattern that changes when germline gene transcription switches to embryonic gene transcription.  In the images above, the C. elegans germline is labeled to show chromosomes (top image) and the location of CENP-A (bottom).  CENP-A is lost from chromosomes during the pachytene stage of meiosis and later reloaded onto chromosomes during diplotene, and is not found in sperm.  

Gassmann, R., Rechtsteiner, A., Yuen, K., Muroyama, A., Egelhofer, T., Gaydos, L., Barron, F., Maddox, P., Essex, A., Monen, J., Ercan, S., Lieb, J., Oegema, K., Strome, S., & Desai, A. (2012). An inverse relationship to germline transcription defines centromeric chromatin in C. elegans Nature, 484 (7395), 534-537 DOI: 10.1038/nature10973
Adapted by permission from Macmillan Publishers Ltd, copyright ©2012

March 29, 2012

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.

November 10, 2011

Mistakes happen. If you are as special as DNA, then you have someone to point out those mistakes and fix them for you. That sounds like your PI editing your manuscript, doesn’t it? Today’s stunning image is from a paper describing DNA repair in meiotic divisions.

When double-strand breaks happen to DNA, the cell has an efficient pathway that recognizes and repairs the breaks to both DNA strands. Two kinase proteins, ATM and ATR, play pivotal roles by phosphorylating numerous key proteins involved in this process. The roles of these two proteins are well-studied in mitosis, but their function in meiosis is not clear. Meiosis is the reductive cell division that results in gametes (ooctyes or sperm). A recent paper describes the role of ATM and ATR in the meiotic cell divisions of fruit fly ovaries. Both ATM and ATR phosphorylate a protein called histone H2AV at the site of DNA breaks, providing a handy and fluorescently-labeled output of ATM and ATR activity. Joyce and colleagues found that ATR plays a role in regulating the cell cycle checkpoint machinery that halts cell cycle progression in the presence of DNA breaks, while ATM is required for the DNA repair of meiotic double strand breaks. As seen in the images above, H2AV levels (red, phosphorylated and unphosphorylated) decreased in the developing oocyte (green) until it was almost undetectable by stages 4 and 5 of oogenesis (see bottom row for higher magnification). These results suggest that either ATM/ATR does not respond to DNA damage at these stages or that repair occurs before these stages (before the first meiotic division).

Joyce, E., Pedersen, M., Tiong, S., White-Brown, S., Paul, A., Campbell, S., & McKim, K. (2011). Drosophila ATM and ATR have distinct activities in the regulation of meiotic DNA damage and repair originally published in The Journal of Cell Biology, 195 (3), 359-367 DOI: 10.1083/jcb.201104121

July 26, 2010

Meiosis is a special type of cell division that results in the formation of gametes (sperm and eggs). Meiosis is made of two rounds of division that ultimately results in half the chromosomes, so that there is an accurate number of chromosomes after fertilization of the gametes. A recent paper describes an important role for a kinase called Aurora-C in ensuring accurate chromosome segregation in meiosis. Image above shows metaphase of the first meiotic division in control (left) and Aurora-C-deficient (right) mouse oocytes. Microtubules are in green, DNA is in blue, and kinetochores are in red in both whole oocyte (top) and zoomed in images of the spindles (bottom). In the Aurora-C-deficient oocyte, there is aberrant chromosome alignment and attachment to the spindle.

Reference: Kuo-Tai Yang, Shu-Kuei Li, Chih-Chieh Chang, Chieh-Ju C. Tang, Yi-Nan Lin, Sheng-Chung Lee, and Tang K. Tang. Authors’ Molecular Biology of the Cell paper can be found here.