January 31, 2011

When many people think of membrane fusion, they often think of endocytosis and vesicle trafficking. Membrane fusion is a fascinating and dynamic process that is involved in so many processes in the cell, including nuclear envelope formation.

The nuclear envelope is composed of an inner and outer membrane that surrounds DNA and associated proteins. The nuclear envelope contains nuclear pores, which are channels that serve as sites of exchange between the nucleus and cytoplasm of the cell. The double membrane nature of the nucleus presents challenges in nuclear envelope and pore assembly that are overcome by membrane fusion events. A recent paper describes inner/outer membrane fusion during nuclear pore assembly and identifies an intermediate step in the process. By using cold temperatures that slow down nuclear pore formation in frog egg extracts, Fichtman and colleagues were able to clarify when and how fusion occurs between the inner and outer membranes. Specifically, the cold temperature treatment blocks inner/outer membrane fusion while allowing only the nuclear side of the pore to assemble. Images above show nuclear pore formation at room temperature (left set of images) and a cold temperature (right set of images). At room temperature, the nuclear membrane (green) and nuclear pores (red) form around chromatin (blue in merged images) after 15 minutes from the start of nuclear assembly. At cold temperatures, nuclear pores form more slowly and don’t appear on the nuclear envelope until 60 minutes.

Fichtman, B., Ramos, C., Rasala, B., Harel, A., & Forbes, D. (2010). Inner/Outer Nuclear Membrane Fusion in Nuclear Pore Assembly: Biochemical Demonstration and Molecular Analysis Molecular Biology of the Cell, 21 (23), 4197-4211 DOI: g/10.1091/mbc.E10-04-0309">10.1091/mbc.E10-04-0309

January 27, 2011

Just when you think you understand how a gene works…BAM! Alternative splicing shows up and reminds us that there is so much yet to learn, even about a gene as well-studied as formin.

Genes get transcribed into RNA, which gets translated into proteins. After transcription of a gene, different regions of RNA called exons and introns are either connected together (exons) or removed (introns) in a process called splicing. Some genes undergo a process called “alternative splicing” that allows one single gene the ability to splice, or connect, the exons in multiple ways that result in different protein isoforms. A recent paper describes a previously-unidentified isoform of FHOD3 formin, an actin-nucleating protein found at high levels in the heart. This new isoform includes an alternative exon in certain muscle tissues, and this exon contains a phosphorylation site that allows an additional level of regulation of FHOD3. Images above are of neonatal rat heart cells with or without this alternative exon. The presence of the alternative exon (bottom cell) directs formin (left images, green in merged) to myofibrils (middle images, red in merged), which are the repetitive and contractile structures in muscle cells. Without the alternative exon, formin is found in aggregates in the cytoplasm (top cell).

Iskratsch, T., Lange, S., Dwyer, J., Kho, A., Remedios, C., & Ehler, E. (2010). Formin follows function: a muscle-specific isoform of FHOD3 is regulated by CK2 phosphorylation and promotes myofibril maintenance originally published in The Journal of Cell Biology, 191 (6), 1159-1172 DOI: 10.1083/jcb.201005060

January 24, 2011

When we first learned about the cell cycle in high school, we learned about the stunning simplicity of certain proteins that cycle in order to promote progression through the cell cycle. In reality, that picture is quite complex, with many layers of regulation that affect those cycling proteins. A recent paper from the Nurse lab pares down all of that complexity to show us that the simplicity really has been there all along.

The cell cycle is the sequence of events that leads to a cell’s division and is regulated by two classes of molecules – cyclins and cyclin-dependent kinases (CDKs). Progression through the cell cycle is an orderly process, yet the integration of all players involved is complex—different CDKs associate with different cyclins, which are synthesized and degraded at different times, and these associations are regulated by their localization, interaction with inhibitors, checkpoint mechanisms, and complex feedback loops. A recent paper shows that a single cyclin-CDK “engine” is the core mechanism driving cell cycle progression in fission yeast. Coudreuse and Nurse show that without all of the regulatory inputs and feedback loops, a single engineered module containing Cdc2 (a CDK) and Cdc13 (cyclin B) is sufficient to drive cell division. Image above is of a fission yeast cell going through the cell cycle with this engineered module – top row shows the appearance and degradation of the engineered module, while the bottom row shows the DNA.

Adapted by permission from Macmillan Publishers Ltd, copyright 2010.

Coudreuse, D., & Nurse, P. (2010). Driving the cell cycle with a minimal CDK control network Nature, 468 (7327), 1074-1079 DOI: 10.1038/nature09543

January 20, 2011

Not all neurons are created equally…today’s image is from a paper describing a pathway that regulates the differentiation of specialized neurons in the fruit fly Drosophila.

During development, cells are instructed to differentiate into specialized cell types. Although a lot of the signals that participate in this process a known, the exact mechanism for each cell type is not. A recent paper describes the differentiation of Drosophila mechanosensory neurons, which have specialized dendrites with cilia that help mediate sensory perception. The results in this paper show how proneural factors lead to downstream signals that direct the differentiation of these specialized neurons and their ciliary dendrites. Image above shows five of these mechanosensory neurons.

Cachero, S., Simpson, T., zur Lage, P., Ma, L., Newton, F., Holohan, E., Armstrong, J., & Jarman, A. (2011). The Gene Regulatory Cascade Linking Proneural Specification with Differentiation in Drosophila Sensory Neurons PLoS Biology, 9 (1) DOI: 10.1371/journal.pbio.1000568

January 17, 2011

Today’s image is from a paper describing a new microscopy method that allows the best of both worlds – fluorescent AND electron microscopy. The images in this paper are fantastic, but I especially can’t wait to see what images are down the road using this new method.

The strength of electron microscopy lies in its ability to reveal amazing detail about cellular structure, yet transient and dynamic events are difficult to see using this method. Fluorescence microscopy, however, is invaluable in the visualization of dynamic events, even if it cannot provide structural detail. A recent paper has combined the best of both techniques in a correlative fluorescence and electron microscopy method that provides high precision and sensitivity. In the image above, a yeast endocytic protein was imaged using fluorescence microscopy (red mark inside the circle, left-most image). Fiducial markers (numbered circles) in both fluorescence (middle left) and electron micrographs (middle right) help align the images, in short. Finally, a high magnification electron micrograph (right-most image) shows the structural detail of a membrane invagination where the endocytic protein was found in the fluorescence image (marked with a cross).

Kukulski, W., Schorb, M., Welsch, S., Picco, A., Kaksonen, M., & Briggs, J. (2011). Correlated fluorescence and 3D electron microscopy with high sensitivity and spatial precision originally published in The Journal of Cell Biology, 192 (1), 111-119 DOI: 10.1083/jcb.201009037

January 13, 2011

Even if you don’t know what synaptic plasticity is, you should be thankful for it. Synaptic plasticity is the adjustment of a neuron’s response to a signal based on previous signal transmission, and many theorize that synaptic plasticity is the foundation for learning and memory. Today’s image is from a fascinating paper showing the role of myosin in organelle transport within a specific type of neuron, in a process that is important for synaptic plasticity.

Purkinje neurons are found in the cerebellar cortex of our brains, and play a large role in motor control. They are some of the largest neurons in our brain, so the transport of organelles within Purkinje neurons is no small feat. Dendritic spines are regions where the neuron receives its input signals, and the localization of ER (endoplasmic reticulum, a tubular organelle) in the spines is very important. After neuron stimulation, the ER releases calcium ions into the dendritic spine, which facilitates a type of synaptic plasticity in Purkinje neurons. A recent paper describes the role of myosin-Va, an actin motor, in transporting and localizing the ER to dendritic spines in these neurons. Images above are of a mouse Purkinje neuron (top), and a higher magnification view of a region of a Purkinje neuron (bottom). The white dot in the bottom panel is where a form of glutamate was uncaged at the tip of a dendritic spine in order to stimulate that specific spine on the neuron, and the yellow line indicates the line scan of the microscope.

Adapted by permission from Macmillan Publishers Ltd, copyright 2010.

Wagner, W., Brenowitz, S., & Hammer, J. (2010). Myosin-Va transports the endoplasmic reticulum into the dendritic spines of Purkinje neurons Nature Cell Biology, 13 (1), 40-48 DOI: 10.1038/ncb2132

January 10, 2011

My last blog post was about how the immune system gets rid of invaders, but this post is about a paper showing some fascinating results about immune cells actually helping an unwanted resident, a transformed cell. This paper provides a very cool addition to the idea that some cancers look like non-healing wounds.

Zebrafish are a very powerful organism to use in the lab for many reasons, one of which is their transparency during development. Feng and colleagues recently took advantage of this in order to find and image the interactions between immune cells and oncogene-transformed cells as they initiate cancerous growth. These transformed cells recruit leukocytes, which are white blood cells, using H₂O₂ in a similar process that wounds use to recruit immune cells as part of the inflammatory response. When H₂O₂ synthesis was blocked, leukocytes were not recruited to the transformed cells and the number of transformed cells was reduced. The authors’ results suggest that the interactions with leukocytes serve to support proliferation of the transformed cells. Image above shows leukocytes (red) and transformed cells (green) interacting by forming tethers between the two cells.

BONUS!! Very cool video of a tether forming between the cell types can be seen here. And, more cool videos from this paper can be found here.

Feng, Y., Santoriello, C., Mione, M., Hurlstone, A., & Martin, P. (2010). Live Imaging of Innate Immune Cell Sensing of Transformed Cells in Zebrafish Larvae: Parallels between Tumor Initiation and Wound Inflammation PLoS Biology, 8 (12) DOI: 10.1371/journal.pbio.1000562

January 6, 2011

Our immune system has many different types of specialized cells, and macrophages have to be my favorite. One look at our image today, and you’ll see why they are such amazing little workers.

Macrophages are white blood cells that engulf and ingest material as part of the immune response against pathogens. Macrophages have specialized receptors on their surface that identify the target and initiate the pathway leading to engulfment and ingestion. Past models have suggested that these receptors passively find and bind targets, but a recent paper shows that macrophages actively probe the extracellular environment for targets, especially when targets are sparse or moving quickly. In addition, this paper shows that these probing protrusions are actin-rich and are dependent on some known actin regulators. Images above are scanning electron micrographs of macrophages in the presence of beads that are treated to make them targets for macrophages. Protrusions are obvious in control cases (left and middle, arrows), but are absent in cases when actin is depolymerized by a drug called latrunculin B (right).

Flannagan, R., Harrison, R., Yip, C., Jaqaman, K., & Grinstein, S. (2010). Dynamic macrophage "probing" is required for the efficient capture of phagocytic targets originally published in The Journal of Cell Biology, 191 (6), 1205-1218 DOI: 10.1083/jcb.201007056

January 3, 2011

Cancer is a loaded word for many biologists—it is made up of thousands of different diseases when you realize how many different paths can be taken in order for cells to lead to cancer. There are so many biologists investigating cancer, and sometimes there are discoveries that shake up our understanding. These shake-ups are key to making the big steps towards a cure that patients, survivors, and victims all hope for.

Cancer progression involves growth of a tumor as well as metastatic spread of cancerous cells to other tissues. Tumor growth requires the development of a blood supply for the tumor, and it was previously known that outside blood vessels get induced to sprout new vessels at the site of the tumor. A recent set of papers in Nature describe the ability of tumor stem cells in glioblastoma cancer to induce production of endothelial cells used in vessel formation. Images above show endothelial glioblastoma cells forming tubular vascular networks, a key step towards vessel formation, in a three-dimensional culture. DAPI shows nuclei of cells (blue), CD105 indicates dividing angiogenic endothelial cells (green), and DiI-AcLDL labels vascular endothelial cells (red). Image on right shows phase contrast of the tubular network.

Adapted by permission from Macmillan Publishers Ltd, copyright 2010.

Wang, R., Chadalavada, K., Wilshire, J., Kowalik, U., Hovinga, K., Geber, A., Fligelman, B., Leversha, M., Brennan, C., & Tabar, V. (2010). Glioblastoma stem-like cells give rise to tumour endothelium Nature, 468 (7325), 829-833 DOI: 10.1038/nature09624

Accompanying paper:
Ricci-Vitiani, L., Pallini, R., Biffoni, M., Todaro, M., Invernici, G., Cenci, T., Maira, G., Parati, E., Stassi, G., Larocca, L., & De Maria, R. (2010). Tumour vascularization via endothelial differentiation of glioblastoma stem-like cells Nature, 468 (7325), 824-828 DOI: 10.1038/nature09557

Fantastic News and Views paper on these results:
Bautch, V. (2010). Cancer: Tumour stem cells switch sides Nature, 468 (7325), 770-771 DOI: 10.1038/468770a