July 31, 2014

Do you ever feel nostalgic for a specific paper? Maybe this paper inspired your own research, or maybe it was a paper you immediately knew would be game-changing. Maybe, like today’s TBT paper, it was a great paper about solidly executed research with a memorable giggle-inducing technique. Thanks to a nostalgic HighMag reader and friend, Omar Quintero, we are being re-introduced to gonad sandwiches. 

In mammals, sex determination refers to the changes during early development that lead to the formation of either the testis or ovary. A gene on the Y chromosome called Sry initiates testis formation from the early bipotential gonad, including organizing Sertoli cells into the testis cord structure. In a 1997 paper, Martineau and colleagues investigated the early cell movements that occur after Sry expression, specifically the movement of nearby mesonephric cells to the genital ridge. To see these cell movements, Martineau and colleagues grafted a “blue” mesonephros from a mouse ubiquitously expressing β-galactosidase next to a “white” gonad from a different mouse. The movement of blue cells into the white gonad in these gonad sandwiches revealed that this movement is dependent on a signal induced by the male (XY) gonad that acts as a chemoattractant. Migration does not occur if an XX gonad is used in the sandwich, yet migration can occur whether an XY or XX mesonophros is used. The images above show the different XX and XY combinations used in these experiments, with XY gonads leading to extensive migration of blue cells. 

Martineau, J., Nordqvist, K., Tilmann, C., Lovell-Badge, R., & Capel, B. (1997). Male-specific cell migration into the developing gonad Current Biology, 7 (12), 958-968 DOI: 10.1016/S0960-9822(06)00415-5

Copyright ©1997 Elsevier Ltd. All rights reserved.

July 24, 2014

How many times can you say the word “gonad” in a sentence without giggling? If the answer is none, then I congratulate you on turning thirteen. If the answer is many, then you must be a biologist. Biologists appreciate the value of a good gonad, and so should you. The gonad of the worm C. elegans serves as an important model in which to study tissue organization and development, as you’ll see in the paper that accompanies today’s image.

At the end of cell division, cytokinesis typically results in two separate daughter cells. Some cytokinesis, though, is incomplete and leads to two daughter cells sharing cytoplasm. This shared cytoplasm, or syncytium, can be found in the germ cells of many species from worms to humans. The germline of the worm C. elegans is a polarized tube in which germ cells are arranged around the shared cytoplasmic core and move along a conveyer belt of oocyte production. Amini and colleagues recently reported on the formation of the syncytial C. elegans germline throughout development, and the role of the short Anillin family scaffold protein ANI-2. ANI-2 is localized to the intercellular bridges that connect the germ cells to the shared cytoplasm, and loss of ANI-2 results in destabilization of intercellular bridges and sterility. The defects seen in worms lacking ANI-2 are likely due to a loss of the stability and elasticity of the intercellular bridges that is required to compensate for the stress of cytoplasmic streaming during oogenesis. Images above show the germlines of wild-type and ani-2(-) worms at different larval stages (membranes in green; nuclei in red). Worms lacking ANI-2 have abnormal multinucleated germ cells (arrowheads).

Amini, R., Goupil, E., Labella, S., Zetka, M., Maddox, A., Labbe, J., & Chartier, N. (2014). C. elegans Anillin proteins regulate intercellular bridge stability and germline syncytial organization originally published in the Journal of Cell Biology, 206 (1), 129-143 DOI: 10.1083/jcb.201310117

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.

July 10, 2014

Do your thoughts and feelings have colors? Do you feel red with rage during traffic, or green with envy when your lady swoons over Ryan Gosling? A recent methods paper introduces a very cool technique that allows the visualization and measurement of voltage within an excited neuron.

Biologists build tools that are ideally accurate, fast, and non-damaging to the cells and organisms on which they are used. In a recent paper in Nature Methods, Hochbaum and colleagues describe the improved technique for simultaneous imaging of neuron stimulation and the resulting action potentials. Hochbaum and colleagues engineered a vector, called Optopatch, that uses their actuator (CheRiff) to induce action potentials and their voltage indicators (QuasAr1 and QuasAr2) to visualize and measure membrane voltage. Optopatch allows the measurement of action potentials on a microsecond timescale, without the need for electrodes. In the images above, a neuron expressing Optopatch shows action potential propagation (left to right, arrow is site of action potential initiation).

Hochbaum, D., Zhao, Y., Farhi, S., Klapoetke, N., Werley, C., Kapoor, V., Zou, P., Kralj, J., Maclaurin, D., Smedemark-Margulies, N., Saulnier, J., Boulting, G., Straub, C., Cho, Y., Melkonian, M., Wong, G., Harrison, D., Murthy, V., Sabatini, B., Boyden, E., Campbell, R., & Cohen, A. (2014). All-optical electrophysiology in mammalian neurons using engineered microbial rhodopsins Nature Methods DOI: 10.1038/nmeth.3000
Adapted by permission from Macmillan Publishers Ltd, copyright ©2014

June 30, 2014

Which came first, the primordial germ cell or the gamete? Unlike the old chicken or egg philosophical dilemma, we know for certain that the primordial germ cell came first. And, thanks to a recent paper about primordial germ cells in sea urchins, we now know that they can migrate across the urchin embryo.

During development, germ cells produce gametes (eggs or sperm). In many organisms, including mammals, primordial germ cells (PGCs) are born far from the eventual location of gametes and must migrate across the embryo while dividing. In sea urchins, small cells called micromeres are PGCs and precisely segregate along the left-right axis of the embryo. A recent paper by Campanale and colleagues describes the use of live-cell imaging of small micromeres in urchin embryos to test whether the precise segregation of these eight micromeres is due to passive translocation or active migration. Images show that the micromeres are, in fact, motile cells with features such as cortical blebs and filopodia that direct migration across the sea urchin embryo, towards the coelomic pouches. In the images above, sea urchin embryos express micromere (red) and apical membrane (green) markers before (left) and during (middle, right) gastrulation.

Campanale, J., Gökirmak, T., Espinoza, J., Oulhen, N., Wessel, G., & Hamdoun, A. (2014). Migration of sea urchin primordial germ cells Developmental Dynamics, 243 (7), 917-927 DOI: 10.1002/dvdy.24133

June 19, 2014

As the widespread therapeutic use of stem cells moves closer to reality, I just fasten my seatbelt a little tighter. An exciting time for stem cells and their scientist stalkers, a recent paper shows the regeneration of damaged monkey hearts by human embryonic stem cell-derived cardiomyocytes.
 
Human embryonic stem cells (hESCs) can be programmed to differentiate into countless different cell types. hESCs are already being tested in humans to treat retinal diseases and spinal cord injuries. hESCs can be differentiated into cardiomyocytes, or heart muscle cells, to potentially repair a damaged heart after injury or failure. In a recent study, Chong and colleagues used hESC-derived cardiomyocytes (hESC-CMs) to repair injured monkey hearts, which are more comparable to human hearts in size and number of cardiomyocytes required. After first developing techniques for producing large, clinical-scale cryopreserved batches of hESC-CMs, Chong and colleagues found that these cells successfully re-muscularized the injured monkey hearts. The electromechanical coupling between host heart tissue and hESC-CM graft tissue was successful, yet non-fatal arrhythmias were observed. In the images above, host vessels (red) extend into graft tissue (white, boxed region and higher magnification below) and are able to successfully perfuse the graft tissue.

Chong, J., Yang, X., Don, C., Minami, E., Liu, Y., Weyers, J., Mahoney, W., Van Biber, B., Cook, S., Palpant, N., Gantz, J., Fugate, J., Muskheli, V., Gough, G., Vogel, K., Astley, C., Hotchkiss, C., Baldessari, A., Pabon, L., Reinecke, H., Gill, E., Nelson, V., Kiem, H., Laflamme, M., & Murry, C. (2014). Human embryonic-stem-cell-derived cardiomyocytes regenerate non-human primate hearts Nature, 510 (7504), 273-277 DOI: 10.1038/nature13233
Adapted by permission from Macmillan Publishers Ltd, copyright ©2014

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