Biosensors detect the concentration or activity of chemicals via natural molecules, such as enzymes or antibodies, which bind other things for a living. Antibodies are expensive to produce, so minimising biosensor costs while maximising selectivity and sensitivity are often difficult goals to achieve simultaneously. To date, highly efficient but expensive antibody tests of blood serum are the gold standard for hormone sensing. With the support of the Marie Skłodowska-Curie Individual Fellowship programme, the SENSHOR project has delivered an inexpensive and effective alternative, replacing the antibodies with a little help from bacteria. In addition, sampling urine rather than blood serum opens the door to cost-effective point-of-care biosensing of all sorts of important molecules.
From bacterial genetics and quantum dots to biosensors
Progesterone is an important steroid hormone that plays a role in regulating the menstrual cycle and in preparing and maintaining the uterus for pregnancy. Serum progesterone tests are used to assess fertility and the state of a pregnancy. However, they rely on antibodies, already expensive to produce, which bind irreversibly to the progesterone, meaning they cannot be recovered and used again. In the context of a strong collaboration with Mark W. Grinstaff, James E. Galagan and colleagues at Boston University, Sébastien Lecommandoux, project coordinator, and fellow Chloé Grazon, both of the University of Bordeaux, had a better idea. “The key innovation in the SENSHOR project was to use proteins called transcription factors (TF) to sense hormones. TF are proteins that reversibly bind specific DNA sequences in the presence of an analyte to control gene expression. For the first time, proteins sensitive to hormones have been produced in bacteria and used ex vivo to develop a sensor,” Lecommandoux explains. To create a detectable signal, Grazon harnessed the uniquely suited phenomenon of fluorescence resonance energy transfer (FRET). FRET is a distance-dependent physical process of energy transfer from an excited molecular fluorophore (the donor) to another fluorophore (the acceptor). In the case of the sensor, the donor-acceptor pair is a fluorescently labelled TF protein and a fluorescent DNA sequence. When there is no hormone present in solution to distance the two, they can exchange energy according to FRET. The presence of the hormone causes them to become unbound, impeding energy transfer and perturbing the fluorescence signal. The fluorescence comes from quantum dots, chosen for their brightness and resistance to photobleaching. Lecommandoux expands: “Within SENSHOR, we improved the stability of the FRET fluorescent quantum dots. Their robustness also allowed us to improve the signal-to-noise ratio and thus the sensor’s detection limit.”
Paving the way to point-of-care biosensors
“We used our pioneering biosensor platform to successfully sense progesterone in urine at physiological concentrations and developed an inexpensive and portable, benchtop device that provides the same fluorescence response to progesterone titration. With the same methodology, we could develop a biosensor for almost anything that is metabolised by bacteria,” states Grazon. These ground-breaking results were published in the esteemed peer-reviewed journal Nature Communications. The team successfully immobilised the biosensor on a surface and also embedded it in hydrogels, important steps toward the development of functional devices for real-world applications. SENSHOR outcomes pave the way to cost effective and portable point-of-care biosensors for hormones and more.
SENSHOR, biosensor, progesterone, FRET, bacteria, antibodies, TF, quantum dots, transcription factor, fluorescence resonance energy transfer