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NEWS FROM THE ACS NATIONAL MEETING Elizabeth Zubritsky reports from San Francisco, CA
Measuring insulin secretion from single cells In type 2 diabetes—the most common form—the body makes insulin, but it is not effective, so the blood sugar level rises. Many researchers think that the insulin does not get released properly, but the coupling of impinging stimuli and insulin release are not well understood. Now, Robert Kennedy and
Montage of images showing Zn2+ release. The images show two cells touching each other, which explains the oblong shape. The red color indicates where Zn2+ is being released. (See http://pubs.acs.org/ac for a movie of Zn2+ release.)
colleagues at the University of Florida have developed a fluorescence microscopy method that can reveal insulin secretion from single cells with spatial and temporal resolution. The researchers had previously used amperometry for direct detection of insulin release from individual b-cells. In a recent application of this method, they showed that the presence of insulin
Why genomics is not enough The human genome may provide the best evidence yet that by gaining knowledge, we begin to realize how little we know. As the sequence nears completion, we hear more comments that this data alone will not answer all of our questions about human development and disease. In fact, Bill Hancock of Agilent Technologies said that more people are now paying attention to the so-called epigenetic model, which holds that “the genome [itself] is essentially featureless. You only get information out of it when you stimulate a pathway.” In a talk advocating the integration of genomics, proteomics, and metabolic studies, Hancock made a simple, yet profound and often overlooked, point: Studies of gene expression often assume that the concentration of mRNA is equal to, or at least proportional to, the concentration of protein. But this is not necessarily true. Citing a paper that found a poor correlation between the amount of mRNA and the amount of protein in samples taken from 60 human cell lines (Anderson, N. L.; Anderson, N. G. Electrophoresis 1998, 19, 1853–1861), Hancock argues that mRNA and protein levels are not necessarily proportional. However, many researchers have preferred to assume a straightforward relationship, he said, but they must now confront the fact that some cases may be considerably more complicated.
outside a cell could trigger the release of more insulin. Other researchers had identified insulin receptors on the surface of b-cells, but no one had linked them to this physiological effect before. However, amperometry is limited because the microelectrodes are single-point sensors. To get high spatial and temporal resolution, the researchers switched to fluorescence microscopy. Because they cannot easily measure insulin fluorescence, they monitor Zn2+ secretion instead, using the fluorescent dye Zinquin (Anal. Chem. 2000, 72, 711– 717). Zn2+ is stored along with insulin in the secretory vesicles inside the b-cell. When the vesicles fuse with the cell membrane, they release the Zn2+–insulin complexes outside the cell. The complexes dissociate, leaving Zn2+ free to bind with Zinquin. Kennedy and colleagues saw that Zn2+—and by extension, insulin— is only released from one region of the cell. Why? The researchers note that b-cells are arranged in rings around blood vessels, and they speculate that this polarity may ensure quick delivery of insulin to the bloodstream. By imaging Zn2+ and Ca2+ simultaneously, the researchers also found that Zn2+ is released in the same region where Ca2+ first enters the cell. Ca2+ eventually enters all over the cell, but the correlation between the site of the initial Ca2+ uptake and the site of Zn2+ release suggests that the Ca2+ entry sites are coupled to the active secretion zone in these cells.
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