Professors Vladimir Parpura and Umar Mohideen of the University of California, Riverside have used atomic force microscopy (AFM) to investigate the mechanisms by which neurons release neurotransmitters.
Traditional microscopes focus electromagnetic radiation (e.g. light, or electrons) onto a sample and measure resolution only in the plane of the image to produce a two-dimensional image. Instead of using radiation, the atomic force microscope uses a probe to magnify the surface features of a sample. The atomic force microscope has two measures of resolution, one in the plane of the image, and another perpendicular to the plane of the image, enabling researchers to generate three-dimensional images of a sample.
AFM is a type of scanning microscopy which uses a cantilever probe made of a microfabricated silicon nitride membrane. As the probe scans across a sample, it is attracted to or repelled by surface features of the sample, causing deflections in the probe membrane. A laser beam detects and measures the vertical deflections of the probe, and the measurements are used to construct a three-dimensional image of the sample surface. AFM can also be used to measure miniscule forces at work on the sample surface.
Parpura and Mohideen investigated the molecular interactions of, and the strength of bonds between, protein molecules involved in exocytosis, the process by which cells release neurotransmitters into the synapse. They looked at SNARE proteins, which are known to be involved in the final stages of exocytosis, during which the synaptic vesicles containing neurotransmitter molecules fuse with the membrane at the nerve terminal. Their results are published in last month’s issue of the Biophysical Journal.
(L to R) AFM images of vesicles in a lipid bilayer, gap junctions, human chromosomes and breast cancer tissue.
Because it is optimized for measuring surface features, AFM has been widely used in materials scientists and physicists but is increasingly being applied to biology. AFM is just 20 years old, and researchers are only beginning to appreciate the potential of the technique for biological applications.
Using AFM, samples can be magnified up to a million times with atomic resolution. The technique is therefore very useful in studying the shape of proteins such as ion channels and receptors or the surface topography of viral particles; it can also be used to track biomolecular processes such as the enzymatic break down of substrates or the interactions between proteins and DNA strands that occur during transcription. The film clip below shows images of the topography of a network of neurofilaments, also obtained by AFM.
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