By Dr Callum Zgierski-Johnston, University of Freiburg, Germany and Dr Rachel Myles, University of Glasgow, UK
In science, questions are posed and hypotheses generated, often well in advance of our ability to answer and test them. One of the most famous examples is the Higgs boson, proposed in 1964 but only with advents in particle colliders, sensing mechanisms and other technologies was it discovered in 2012.
The advent of new technology thus also allows new questions to be formed as we advance our understanding of the basic mechanisms of the world and life itself.
The electrical activity of excitable organs is one of the fundamental bases for life. It ensures the coordinated contraction of muscles, the rhythmic activity of the heart, and the ability to think. Even in non-excitable cells, the difference in charge across a membrane (called the transmembrane voltage) underlies many key processes. Thus, electrophysiology is a major research focus, especially when studying the brain and heart.
Three issues currently limiting our understanding of electrophysiology are: the difficulty measuring absolute transmembrane potential in native tissue, the complexity assessing the role of different cell types, and the challenge of imaging beyond 2D. Our symposium “Seeing, feeling, and hearing electrophysiology in excitable organs: next-generation electrophysiology” at The Physiological Society annual conference, Physiology 2021, will address these issues and novel ways to tackle them.
In the 1960s, a key development in electrophysiology research occurred with the discovery of voltage-sensitives dyes, which allowed visualisation of the electrical properties of cells and tissues. Voltage-sensitive dyes enabled much higher spatial resolutions than pre-existing techniques, enabling conduction to be followed across organs and visualisation of the complex patterns of excitation that can occur, especially in disease, where instead of resembling a wave at a beach, excitation may more resemble the eddies and oscillations that occur in white water rapids. However, a key drawback is the ability to measure only changes rather than the actual values of transmembrane voltages.
Dr Evan Miller from the University of California Berkeley, USA will present his newly developed voltage dyes which allow recording of absolute transmembrane voltage by tracking the lifetime of the emitted fluorescence. Using this technique, absolute transmembrane voltage can be measured in intact cells, tissues, or organs in a non-invasive, non-contact manner.
This research has the potential to transform our understanding in numerous areas including the role of resting transmembrane potential on cellular properties and how cell-cell coupling in tissue alters transmembrane potential.
Another important problem with the use of dyes is the inability to differentiate between different cell types. When studying relatively smaller cell populations, measurements of these populations are often drowned out by the larger populations.
Owing to the advent of optogenetics over the last few decades – where light-sensitive tools for affecting or reporting transmembrane potential are genetically expressed – we can now study the specific electrophysiology of different cell populations.
Dr Franziska Schneider-Warme from the University of Freiburg, Germany will present novel optogenetic tools and how they can be used to tease out the role and interconnectedness of different cell populations in electrophysiology, with a focus on the heart.
A final key issue is how to study electrophysiology in 3D. Even with the advances of multi-photon microscopy increasing the depth into the tissue that can be imaged, we are still limited to near-surface measurements. Plunge electrodes or similar can be used but these damage tissue and offer relatively low spatial resolution.
The development of techniques for imaging deep in tissue is key to understanding of electrophysiology at the organ level. Dr Jan Christoph from University of California San Francisco, USA will present his work linking electrics with mechanics, enabling ultrasound to be used to track cardiac electrophysiology. This technique is well-suited to the heart where electrical activity directly drives mechanical activity, however it does not transfer well to other organ systems.
Professor Daniel Razansky from ETH Zürich will present the alternative option of optoacoustic imaging, which relies on a laser to excite chromophores present in the tissue, which will subsequently produce a sound wave that can be measured with ultrasound transducer. This technique allows much greater penetration depths than optical imaging but retains the ability to spectrally separate probes enabling cell tracking and functional measurements.
The development of technology often drives science, but the converse is also true, with science driving technology development. As new questions are formulated, often new methods need to be investigated to answer these and these new methods will subsequently give rise to new questions. Our understanding of the world and biology is inextricably entwined with our ability to observe and measure. As our technology advances, so do we.