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Optical Microchip Tracks Molecules for Research and Clinical Applications: January 31, 2008

Backscattering interferometry relies on a beam of light ricocheting off thousands of molecules in liquid. The light beam that reflects back to a high-speed camera contains an optical pattern called interference fringes. The information gathered from the pattern allows researchers to determine whether two molecules are combining. The technique shows promise for clinical diagnostics, drug development, and biodefense. Photo credit: Daniel Dubois, Vanderbilt University

Cataracts are the leading cause of blindness in the world. Twenty million people age 40 and over in the U.S. have cataracts. By age 80, more than half of all Americans have cataracts. What causes cataracts, or a clouding of the eye’s lens, is unclear, but a new device based on a novel optical technique called backscatter interferometry (BSI) is helping researchers determine the role specialized proteins may play in reversing cataract formation.

Working in collaboration with Hassane Mchaourab, Professor of Molecular Physiology & Biophysics at Vanderbilt University, device developer Darryl Bornhop, Professor of Chemistry at Vanderbilt, and his team are examining the role heat shock proteins play in keeping the lens’s protein in its proper place. Composed mostly of protein and water, the lens helps focus light on the retina. The protein in the lens is arranged in a precise way that keeps the lens clear and allows light to pass through it. As lenses age, some of the protein may clump together—a result of protein folding—and start to cloud a small area of the lens, forming a cataract.

“Heat shock proteins act as chaperones, interfering with the undesirable protein folding mechanism and encouraging the protein back to its native state,” explains Bornhop. As the proteins begin to clump, heat shock proteins jump in to reverse the folding.

This is just one example of the impact Bornhop’s simple yet highly sensitive “micro-chip” device is having on lab research. The device measures a wide range of interactions between molecules, including antibody-antigen for disease detection, DNA-DNA binding, and protein-protein combinations.

Tracking interactions between molecules lies at the heart of drug development, clinical diagnostics, and disease research. But the detection process often requires large quantities of samples and chemical intervention in the form of imaging agents or immobilizing one of the reactants. The BSI device does not require any such specialized chemistry to track the interactions. “This work is potentially revolutionary,” says M.G. Finn, a Chemistry Professor at Scripps Research Institute in LaJolla, California. “It’s cheap, incredibly sensitive, and very much a plug-and-play technology.”

Simple Optics

The BSI device relies on a simple optical setup: a helium-neon laser, a glass or plastic microfluidic chip that holds the sample, an ultrafast camera, and analysis software. A mirror directs laser light onto the sample in the channel within the chip to produce a fan of scattered light. Contained on this light beam are interference fringes, or optical patterns, that change as molecules interact. Analysis software tracks changes in the fringes. Because light bounces through the sample multiple times, the BSI technique is highly sensitive and detects changes to one part per million in picoliter volumes of solution—just a bit more difficult than finding a needle in a haystack.

“The most important aspect of [Bornhop’s] work is the fact that it is label-free and in free solution,” says Robert Flowers, Professor of Chemistry at Lehigh University. “In most analyses, sample preparation is labor intensive. You’re looking at maybe several hours of preparation and then a 2-hour experiment.” With Bornhop’s device, a couple of small injections are made onto the microfluidic chip and the experiment begins. Flowers—who has used the device to study protein interactions—notes that the very small sample concentrations and ease of use make it especially attractive for lab work and the device provides “beautifully reproducible data.”

Advantages Over Current Methods

One of the key tools used to measure the concentration of a molecule in a fluid is Enzyme-Linked ImmunoSorbent Assay, or ELISA. The highly reliable technique can determine the presence of infections, hormones, or illicit drugs, but requires chemical linking of the sample with known molecules. ELISA is one of the standard assays CDC uses. Comparing ELISA and the BSI technique, Bornhop says a CDC researcher described BSI as “ELISA on steroids” minus the chemistry. High sensitivity and ease of use are two key advantages the BSI chip has over other label-free techniques used to track molecular interactions. BSI is 10,000 times more sensitive than isothermal titration calorimetry (ITC), an approach in which a change in temperature indicates binding of one molecule to another. In addition, BSI studies need 100 to 1,000 times less mass or volume of sample as ITC. In surface plasmon resonance one of the molecules under study must be bound to a surface. Changes in the surface indicate a molecular interaction. While this approach is more sensitive than ITC, sample preparation is labor intensive and expensive, and cannot accommodate unknown molecules.

“There are lots of tools available to detect a limited number of variables,” says Finn. “But with Dr. Bornhop’s device, you can rapidly create a detection module to look at a combination of variables.”

What’s Next

To commercialize the BSI device, Bornhop and colleague Scot R. Weinberger founded Molecular Sensing Inc. in 2006. The San Francisco Bay area company will initially focus on systems for lab research but anticipates developing a series of handheld devices for clinical diagnostic, pharmaceutical, and biodefense applications. Bornhop plans to improve the device’s sensitivity even further as well as enhance its ease of use. He anticipates commercial availability by the end of 2008.

This work is supported in part by the National Institute of Biomedical Imaging and Bioengineering.

References

Bornhop DJ, Latham JC, Kussrow A, Markov DA, Jones RD, Sorensen HS. Free-solution, label-free molecular interaction studies by back-scattering interferometry. Science. 2007 Sept 21;317:1732-6.

National Eye Institute. http://www.nei.nih.gov/news/pressreleases/032002.asp.

Darryl Bornhop Photo credit: Daniel Dubois, Vanderbilt University