For people with bipolar disorder, lithium can be a godsend—if it works. The drug, discovered in 1949, is effective for only about one-third of people with the condition. And one of the challenges in using it is its very narrow therapeutic window—too small a dose doesn’t work, whereas too large a dose causes toxicity, resulting in side effects such as tremors and kidney failure.
Now, researchers have developed a DNA-based sensor that measures lithium ion levels in neurons at concentrations close to that therapeutic range, 0.6–1.2 mM (ACS Cent. Sci. 2021, DOI: 10.1021/acscentsci.1c00843). The researchers believe that such a tool could be used both to help adjust lithium doses for people with bipolar disorder and to probe the drug’s mechanism of action, which is poorly understood. “From that mechanistic understanding, we hope to gain insight into either better drugs for bipolar disorder or better use of this drug,” says Yi Lu, a chemist and bioengineer at the University of Illinois at Urbana-Champaign.
Lu launched the lithium sensor project in response to a comment at a conference. During a talk describing how he converted a glucose meter to measure sodium levels, an attendee asked if he could adapt the device for lithium, explaining the clinical need. Few such sensors exist because it’s difficult to detect Li+ among the excess sodium and potassium ions in the human body, Lu says. “The few that do either do not have the selectivity, or their concentration range does not cover this narrow relevant physiologic range,” he explains.
To create a physiologically active sensor with the needed sensitivity and selectivity, Lu turned to DNAzymes—single-stranded stretches of DNA that can act as enzymes. He and his colleagues screened a DNA library for sequences capable of binding and catalyzing reactions with Li+. To ensure specificity, they screened out sequences that also bind Na+, K+, magnesium, calcium, and other monovalent and divalent ions.
The researchers then zeroed in on a DNAzyme sequence that binds strongly to Li+ and cleaves into two smaller pieces once it binds the ion. They paired that DNA with a complementary DNA strand to create a double-stranded segment. The researchers attached a fluorescent dye molecule to one end of the DNAzyme strand and attached a molecule that quenches fluorescence to the same end of the complementary strand.
These two strands separate at a particular temperature, which depends on their lengths—above 37 °C at their full length, and at 37 °C when cleaved into shorter pieces. When the DNAzyme binds Li+ at 37 °C, which is human body temperature, and cleaves, the strands separate from each other, ending the quenching and activating the fluorescence.
They tested the DNAzyme on cells, including neurons derived from cells obtained from people with and without bipolar disorder. To their surprise, they found that Li+ accumulated in the neurons derived from people with bipolar disorder but not in those from people without mental health issues. It’s not clear why, Lu says, but using the sensor to observe how Li+ enters the cell, where in the cell it accumulates, and whether it competes with other ions such as Na+ and K+ may help answer that question.
Peter Cragg, a chemist at the University of Brighton who was not involved with the work, notes that the sensor is close to having the needed specifications for clinical use but requires a bit more optimization. “It has greater than 100-fold selectivity for Li+ over competing metal species,” he says; “however, its limit of detection at 1.1 mM, is currently too high for the accuracy required to adjust medication.”
Lu says this limit was observed in a test tube but in cells grown in a dish, the sensor works across the entire therapeutic range. So “it may not be a barrier of applying this sensor for cellular studies of lithium therapeutics.”
The sensor could also potentially detect the ion in other contexts. The glucose meter that Lu’s team adapted to detect Na+—and many other ions—uses DNAzyme-based test strip technology. They are now tailoring the device to work with DNAzymes to detect Li+ in blood or urine. “This is how [people with diabetes] manage their lives,” Lu says. “We hopefully can use a similar system for people with bipolar disorder.”