The ASCB/IFCB Meeting in Philadelphia last month saw many new faces in the poster alleys and on panel podiums. These newcomers came to Philadelphia with backgrounds in bioengineering, biophysics, and computer modeling because they are finding that their research paths now run through the heart of cell biology. Still, the classic heart of a cell biology meeting remains the latest data on the secrets of the cytoskeleton and how its parts interact in development and in disease. Philadelphia offered full programs for the newcomers and the classicists.
New Biophysical Approaches to Cell Biology
At the “Organization, Quality Control and Remodeling” minisymposium, Marina Feric, a graduate student in Cliff Brangwynne’s lab at Princeton University, presented her work on understanding how gravity limits cell size. Feric and Brangwynne study the mechanics of the cell nucleus using eggs from the African clawed frog, Xenopus laevis. Feric described how giant Xenopus egg cells manage to support thousands of nuclear bodies, membrane-less compartments inside the nucleus. They are liquid-like drops, made up of RNA and proteins. Inside the nucleus, they act like droplets of vinegar in oil, that is, whenever they get close together, they fuse and sink. And yet unlike the vinegar in salad dressing, all the little nuclear bodies in the nucleus don’t clump into one big pool at the bottom.
Previously Feric and Brangwynne had found that an actin mesh was preventing the nuclear bodies from clumping by keeping them small. But how strong was the mesh? Feric and Brangwynne measured gravity’s pull on nuclear bodies and against the actin mesh that contained them. They injected a tiny magnetic bead into the nucleus and turned on a magnet to add a known force. They found that the actin mesh in the nucleus is softer than jello, but like jello, with some prodding it returns to its original shape. Actin continues to hold up the nuclear bodies against gravity like pieces of fruit in a jello mold, but under rising force, the nuclear jello undergoes sheer thickening, a non-Newtonian property where a liquid becomes more viscous, a property that probably protects the nucleus. At a high enough force though, the actin mesh breaks and can no longer hold up the nuclear bodies. This suggests that actin’s mechanical properties are finely tuned to resist the force of gravity but also allow flexibility and rigidity of the cell nucleus to support life.
Classic Cell Biology at Its Best
Unpacking the Cytoskeleton, One Microtubule at a Time
Our big, bony skeletons are wonders of articulation but they can’t come close to our cytoskeletons for sheer dynamism, says Eva Nogales, a professor at the University of California, Berkeley, and a Howard Hughes Medical Institute investigator. The cytoskeleton is always being rebuilt to help the cell maintain structure, move, and divide through the constant remodeling of microtubules, the building blocks of the cell’s skeleton. Nogales makes atomic resolution models of microtubules using cryo-electron microscopy. She presented her work at the “Cell Structure Across Scales” symposium on December 8. Her lab wants to know how microtubules bound to different cellular molecules affect the stability or instability of the structure. Nogales believes that there are many different factors yet to be discovered that promote what appear to be highly regulated changes in the cell’s skeleton. “We hope what we study will be of relevance to what happens in the cell, and that phenomena that happen in the cell can be explained in this structural framework,” she said in an interview.
The EB family of proteins helps regulate microtubules and can act as a scaffold for other proteins involved in pushing the microtubule chain forward. How these EB proteins function in space and time has remained a mystery. Xuebiao Yao of the Hefei National Laboratory for Physical Sciences at the Nanoscale and University of Science and Technology of China presented his work toward understanding EB protein function in a poster on December 8. Through clever biochemistry combined with superresolution imaging techniques, Yao and colleagues introduced two EB proteins into cells, one with half of a photoactivatable green fluorescent protein (PAGFP), and one with the other half of PAGFP. These complementary PAGFP pieces will only fluoresce if the EB proteins are in a complex together and photoactivated. They can also be switched off with a different wavelength of light. By activating and then bleaching subsets of EB molecules, the researchers could assemble superresolution images of protein complexes.
The cells in your body run a kind of Amazon.com warehouse, sorting, packaging, and shipping items with remarkable speed and efficiency. But the warehouse operations inside neurons have an additional challenge: the packaging stations (the Golgi) are in the cell body while many cargoes need to be delivered to the synapse, which in some neurons can be a meter away. Even the fastest of cellular packages can take two days to travel the meter to their destination at the synapse. Yet the turnover of some cargoes, like lipids, can happen within nanoseconds, so cells must have other ways to transport these goods. Along with others, Pietro de Camilli, a professor at Yale University, has long suspected that there was a “factory direct” route. De Camilli told the “Membrane Trafficking” symposium on December 9 the story of how membrane contact sites likely provide a direct source of lipids.
The cellular factory, the endoplasmic reticulum (ER), has a network of tubes that extend everywhere in the cell, including the synapse, de Camilli explains. Contacts between this network and the cell membrane were observed over 50 years ago but their function was not yet known. But de Camilli believes that these contacts could be a perfect opportunity to swap cargo straight from the factory to the destination. Indeed, numerous proteins had already been implicated in ER-membrane tethering.
One of these tethering proteins is an extended synaptotagmin (E-Syt) protein. But de Camilli and his colleagues thought it might be doing more than just tethering as it has a domain that could act in lipid binding. De Camilli’s colleagues did a structural study and found that E-Syt can dimerize and harbor lipids. In his ASCB talk, de Camilli showed how the researchers observed lipid transfer in vitro using synthetic vesicles made with fluorescent lipids and incubated with E-Syt protein. It seems that membrane contact sites provide a direct source of lipids at the membrane in addition to the traditional packaging route, says de Camilli.