Biomedical Assay: Using a microfluidic chip, a new method analyzes proteins in tears
Tears reveal more than just emotion: The salty drops may harbor signs of disease. Researchers have now developed a speedy microfluidics-based assay that detects specific proteins in tears, which could someday help doctors diagnose and treat eye diseases (Anal. Chem., DOI:10.1021/ac202061v).
Clinics routinely test blood or urine but not tears, says Amy Herr of theUniversity of California, Berkeley, because tears are difficult to analyze. Traditional diagnostic methods, like enzyme-linked immunosorbant assay (ELISA), use an immobilized antibody to capture proteins of interest on a surface. But proteins in tears are notoriously alkaline, which makes them stick to surfaces willy-nilly. The resulting data are difficult to interpret, Herr says.
So Herr and a student, Kelly Karns, developed an assay that doesn’t require participation of any surfaces. Instead, they mix a fluorescently labeled antibody with tear fluid and then load the mixture onto a tiny polacrylamide gel about 1 mm long and 80 µm wide in a channel of a microfluidic chip. The researchers then apply an electric field to the chip, which causes proteins of different sizes to travel at different rates along the gel. They use a fluorescent microscope to record images of the gel, which show a distinct band for the antibody-tear protein complex. The intensity of the fluorescence allows them to quantify the concentration of the tear protein that the antibody binds.
As a test case, the researchers detected a tear protein called lactoferrin. Research has linked this protein to Sjögren’s syndrome, an autoimmune disease that destroys mucus-producing cells in the body and causes, among other ills, dry eye. The researchers washed a tiny amount of tears, less than a microliter, off blotting paper that had been dabbed on the eyes of either people with Sjögren’s syndrome or healthy volunteers. Herr and Karn then mixed the diluted tears with an antibody to lactoferrin and loaded the mixture onto the microfluidic device. The proteins migrated along the gel in less than five seconds. The fluorescence images revealed that people with Sjögren’s Syndrome had around one tenth as much lactoferrin in their tears as those without the disease.
Sjögren’s syndrome is just one cause of dry eye, a problem that affects millions, says Nancy McNamara of the University of California, San Francisco. Herr’s microfluidic device could help researchers diagnose, understand, and treat these conditions, she says, in which tear sources are scarce. “It’s a really interesting technology,” says McNamara. “It’s definitely something I can see having a clinical application in the future.”