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To identify the therapeutic targets and improve the efficacy, drug scientists make use of a variety of imaging methods. Part of the drug development process is gaining an understanding of the underlying mechanisms of diseases and how treatment procedures affect these mechanisms. Using imaging methods helps researchers investigate the exact drug targets and this way it can develop better and more efficient medicine.
Fluorescence microscopy is a technique that allows the visibility of insight cells or tissues in three dimensions. This is achieved by marking the structure we want to look at with a special fluorescent molecule. This way a specific structure can be easily distinguished from its surrounding structures. However, any features found very close together are blurred due to the diffraction limit. Diffraction refers to the phenomenon that occurs when a wavelength encounters an obstacle and bends.
In 1994, a method called Stimulated Emission Depletion (STED) microscopy was developed by Stefan Hell. This method offers “super-resolution” beyond the diffraction limit. However, time-lapse images over a larger period could not be made because marker molecule degrades and therefore stops fluorescing under the strong STED beam. The beam refers to the energy in the form of a concentrated visible light that illuminates the targeted structure.
This photobleaching problem is currently being solved by Nagoya University's Institute for Transformative Bio-Molecules (ITbM) research team led by Shigehiro Yamaguchi and Masayasu Taki. They have developed a marker called “MitoPB Yellow” that is absorbed by the mitochondria’s inner membrane, including the fold-like structures called cristae. It has a longer lifespan under the STED beam.
To demonstrate how the STED microscopy works, the researchers put mitochondria under conditions known to cause structural changes using a reagent that suppressed DNA replication and induced dysfunction. This way, researchers could observe mitochondria’s survival and dying processes.
Using images of a 60nm resolution, researchers observed the fusion of neighboring mitochondria’s membranes and other changes that took place within a few minutes, which helped them to understand the timely manner of these processes.
Voltron imaging tool
Voltron is a new fluorescent technique developed by researchers at the Howard Hughes Medical Institute Janelia Research Campus that allows for more precise tracking of neuron activity for a longer period.
The system is composed of a dye molecule and a specifically engineered protein. The protein makes the intensity of the dye change when a specific neuron is active.
This feature allows researchers to detect and track neural signals throughout the brain and distinguish different pathways. This is vital for gaining a thorough understanding of the complex coordination of behavior manifested via the signaling of different neuronal pathways. Even more importantly, Voltron imaging allows the tracking of neuronal activity in real-time. It gives researchers a valuable opportunity to study neurodegenerative diseases in more depth.
Single-particle cryo-electron microscopy
This technique, developed by Case Western Reserve University School of Medicine, US, is used to explore the interactions between the drug molecule and its protein receptor. This approach provides valuable insights into how to improve the efficacy of drug treatments by modifying the molecule of the drug. Better therapeutic effects can be achieved by enhancing the way the drug binds to its receptor.
Single-particle cryo-electron microscopy involves the cooling down of a sample to a very low temperature and using an electron microscope to image the sample. This allows high-resolution imaging of the drug-receptor interactions. The research team of the Case Western Reserve University of Medicine investigates the interactions of setrons and their binding receptor. Setrons are a class of drugs used for nausea and vomiting that bind to serotonin receptors in the gastrointestinal tract. However, setrons do not work on every single patient.
By looking into the way the setrons bind to the serotonin receptors, researchers were able to observe the components of setrons and the receptors that were important for the binding to be accomplished. The researchers adjusted these components to modulate the binding activity. Research and further development of this technique can lead to more effective drugs and a better understanding of the drug-reception interaction dynamics.
Radioactive traces imaging
This is a method developed by Scientists from the University of North Carolina Lineberger Comprehensive Cancer Centre. It involves creating radioactive tracers to track drugs within the body and images of diseases.
This is used in line with positron emission tomography (PET) imaging by attaching radioactive tags to compounds. This was achieved by breaking a specific structure of carbon and hydrogen atoms. Using blue light and a catalyst to speed up the chemical reaction, chemical bonds in the structure could be broken down and replaced with a radioactive molecule called Fluorine-18. This molecule emits gamma rays that are picked up by the PET scanner.
This technique has many applications, including the opportunity to look at the patient’s response to drugs and drug development research.
New imaging tools are vital for the discovery of new drugs and improving current practices. They allow scientists to investigate the dynamics involved in the drug-receptor interaction and provide valuable insight into how our bodies react to drugs.
References and Further Reading
Abdelfattah, A. et al (2019). Bright and photostable chemigenetic indicators for extended in vivo voltage imaging. https://science.sciencemag.org/content/365/6454/699
Basak, S. et al (2019). Molecular mechanism of setron-mediated inhibition of full-length 5-HT3A receptor. https://www.nature.com/articles/s41467-019-11142-8.epdf?author_access_token=78jU7XbBzDayHFYyV-b8DdRgN0jAjWel9jnR3ZoTv0MA_kVujYWA8RHOOE8ff1e1ubqwkxT1GDBcdgzMqXRm49SZHftuCdriRKY9hpttnSJpuquvG-RIJ56Np-U68T34ddbkxD_tiUJwyAQJ2ikr4A%3D%3D
Chen, W. et al. (2019). Direct arene C–H fluorination with 18F− via organic photoredox catalysis. https://science.sciencemag.org/content/364/6446/1170
Wang, C. et al. (2019). A photostable fluorescent marker for the superresolution live imaging of the dynamic structure of the mitochondrial cristae. https://www.pnas.org/content/116/32/15817