Thursday, May 2, 2013

a STORM inside a cell

We've been talking about some of the most cutting edge intracellular visualization techniques lately. Array tomography and Serial block-face electron microscopy have been featured. Today we'll talk about STORM imaging.

STORM imaging (Xu et al., 2013)

STORM stands for Stochastic Optical Reconstruction Microscopy. While Array tomography and Serial block-face EM are both revolutionary in that they can combine very high resolution imaging with relatively large volumes of tissue, STORM is an advancement that lets you see tiny tiny little molecules within the cell.

The problem with 'normal' imaging is that molecules are smaller than the diffraction of light.
Example of the STORM resolution (from Zhuang lab's webpage)
In the figure above, imaging some tiny molecules next to each other is impossible with traditional fluorescence microscopy, but with STORM, you can resolve 10s of nanometers (nm).

To do this, STORM uses photoswitchable dyes, which means that the dye can be turned on or off. This allows researchers to turn on tiny little areas and then turn them off. If all the dye is turned on all at once, the image will look like a big mess because the signals will all overlap each other. But turning on only a few at a time allows you to estimate where the actual protein or molecule is.
"The imaging process consists of many cycles during which fluorophores are activated, imaged, and deactivated. In each cycle only a subset of the fluorescent labels are switched on, such that each of the active fluorophores is optically resolvable from the rest. This allows the position of these fluorophores to be determined with nanometer precision." -Zhuang lab webpage
So what amazing things can they do with this STORM?
A recent paper by Xu et al. (2013) found that the actin which plays a huge role in the intracellular structure of a neuron, has a specific ring-like structure along the axons.

Xu et al., 2013 Figure 4F

This is the kind of research that will immediately go into neuroscience and cell biology textbooks. Xu et al. discovered how actin was structured along the axon simply by being able to 'see it'.

Not only did they discover the structure of actin and spectrin (magenta above) in the axon, but they also found some other interesting molecular patterns that appear to relate to the actin ring structure. The sodium channels, which control action potential propagation down the axon, are concentrated about half way between the ends of the spectrin tetramers. The potential for super-resolution microscopy like STORM is huge. The location of molecules with relation to one another probably plays a huge role in the function of cells and now we have the tools to map them.

© TheCellularScale
Xu K, Zhong G, & Zhuang X (2013). Actin, spectrin, and associated proteins form a periodic cytoskeletal structure in axons. Science (New York, N.Y.), 339 (6118), 452-6 PMID: 23239625

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