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NEWS FROM THE ACS NATIONAL MEETING

Elizabeth Zubritsky reports from San Francisco, CA

Measuring insulin secretionfrom 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

colleagues at the University of Florida have developed a fluo-

rescence 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

outside a cell could trigger the release of more insulin. Other research-

ers 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 can-

not 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 spec-

ulate that this polarity may ensure quick delivery of insulin to the blood-

stream. 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.

Why genomics is not enough

The human genome may provide the best evi-dence yet that by gaining knowledge, we beginto realize how little we know. As the sequencenears completion, we hear more comments thatthis data alone will not answer all of our ques-tions about human development and disease. Infact, Bill Hancock of Agilent Technologies saidthat more people are now paying attention tothe so-called epigenetic model, which holds that“the genome [itself] is essentially featureless.You only get information out of it when youstimulate a pathway.”In a talk advocating the integration of

genomics, proteomics, and metabolic studies,Hancock made a simple, yet profound andoften overlooked, point: Studies of gene expres-sion often assume that the concentration ofmRNA is equal to, or at least proportional to,the concentration of protein. But this is notnecessarily true. Citing a paper that found apoor correlation between the amount of mRNAand the amount of protein in samples takenfrom 60 human cell lines (Anderson, N. L.;Anderson, N. G. Electrophoresis 1998, 19,1853–1861), Hancock argues that mRNA andprotein levels are not necessarily proportional.However, many researchers have preferred toassume a straightforward relationship, he said,but they must now confront the fact that somecases may be considerably more complicated.

Montage of images showing Zn2+ release. The images show twocells touching each other, which explains the oblong shape. Thered color indicates where Zn2+ is being released. (Seehttp://pubs.acs.org/ac for a movie of Zn2+ release.)