Today, I'll point out a speculative line of research on the detection of senescent cells. It is in an early enough stage to make it hard to say whether or not it will go anywhere in the years ahead. The authors of the open access paper linked below propose that the electrical properties of senescent and normal cells are sufficiently different to be used to build an assay for cellular senescence. Even if useful, this may not take off because the present molecular biomarkers for cellular senescence are generally agreed to be good enough for a first pass at the job at hand, meaning efforts to destroy these cells while having a fair idea after the fact as to how many succumbed to the therapy. Since the set of present biomarkers are soon going to used much more widely, given the rapid growth of the senescent cell research field, an entirely different approach to assays will face an uphill battle to gain adoption, whether or not it is better. And electrical measurement is indeed an entirely different approach when compared to the established methods of detecting the levels of a senescence-associated protein such as β-galactosidase, one requiring entirely different tools.
Why do we care about the number of senescent cells found in tissues? Well, to start with these cells are killing you. Ordinary cells become senescent when they reach their evolved replication limit, or in response to damage, or a toxic environment, or as a part of the wound healing process. Most such cells self-destruct or are destroyed by the immune system fairly rapidly. This serves to remove those cells most at risk of developing cancer. Some linger, however, and in growing numbers over the years. These cells secrete a mix of harmful signals that produce chronic inflammation, destroy fine tissue structure, and alter the behavior of surrounding cells for the worse. If just 1% of the cells in a tissue become senescent, and that happens to all of us eventually, they collectively cause significant dysfunction and contribute to the development of ultimately fatal age-related conditions. This is I'm very enthused by progress towards therapies capable of selectively destroying these cells. Senescent cell clearance treatments will be the first legitimate, actual, working rejuvenation therapies: limited in scope, but capable of reverting one cause of aging and all of its immediate consequences on the state of health.
To develop new senescent cell clearance therapies cost-effectively and rapidly, however, it is important to be able to determine how well the prototypes work. A field with a body of reliable, agreed upon tests to determine the quality of therapeutic outcomes is a field that can forge ahead and experiment at low cost. The field of senescent cell clearance is already well equipped on that front. Researchers can make a fair determination of degree of clearance, and have been doing just that. The standard assays have been used in one form or another for the past fifteen years or so. They are simple, but well proven. The need and market for new assays will, I think, be driven by uncertainties over whether or not the established assays are actually finding all of the senescent cells of interest, and whether or not the differences between classes of senescent cell are important in the grand scheme of things. For example, in the past couple of years researchers have started to distinguish senescent immune cells and senescent foam cells in atherosclerosis from the bulk of senescent cells, arguing that these have significantly different characteristics. Fortunately it so far appears that all of the potential senescent cell destruction therapies under development are fairly indiscriminate when it comes to the varieties of cellular senescence. Still, after the first generation of therapies there will be a second and a third, and we want those future treatments to be much improved over those presently in development. That will require a greater understanding of the varieties of senescence, as well as better assays for quantifying the results produced by potential therapies. Whether that will turn out to involve measurement of electrochemical properties of individual cells is an open question, but the prototyping of such an approach makes for interesting reading:
Senescence and disease are the two main contributing factors for the termination of life. Although senescence is one of the major causative factors of disease, senescence can be controlled to extend lifespan. In this context, various biomarkers have been used to measure and analyze senescence. In particular, research on senescence is especially important in cardiovascular research because cardiac myocytes are long-lived postmitotic cells, which need renewal of cellular components as a major ability for lifespan, unlike other short-lived cell types. In general, senescent cells have reduced autophagic activity, reduced telomerase activity, altered contents in mitochondrial phospholipid, increased oxidative stress due to reactive oxygen species (ROS), and increased levels of senescence associated β-galactosidase activity. Additionally, senescence associated changes at various levels of gene transcription and protein translation have also been reported. In all of the aforementioned studies, specific biomarkers have been used to evaluate the potential alterations in cell structure and function. However, such analyses involve complex procedures including chemical modification or tagging. In addition, the acquired data provide only comparative (not absolute) values. Further, given that senescence is a highly complex biological process, it is difficult to assess cellular aging based on the limited number of available biomarkers.
Electrochemical impedance spectroscopy has been utilized to indicate the electrical characteristics of different types of tissues. Even though the measurement of electrical impedance of tissues can provide beneficial information, this method is inconsistent and imprecise owing to the complex structure and composition of tissues. Recently, microelectrochemical impedance spectroscopy has been developed to characterize the electrical properties of cells at the single-cell level owing to the advances in lab on a chip and microfabrication technologies. The electrical impedance measurement at the single-cell level can afford more precise information than that of measurements at the tissue level. This technique contributed to acquire the quantitative information of cells, such as resistance, reactance, capacitance, and conductance, because the electric properties of cells are connected with their physiological states. Therefore, microelectrochemical impedance spectroscopy has been suggested to be a simple, fast, and cost effective diagnostic tool that does not require biomarkers.
Recently, changes in cellular components during senescence were quantitatively analyzed using a new methodology called microelectrochemical impedance spectroscopy for diagnosis of senescence (MEDoS), which involves measurement of electrical impedance of a cell. Since electrical properties of a cell gradually change with changes in the cellular components during senescence, cell impedance can be used to analyze senescence. In addition, cell impedance data can provide quantitative characteristic values for individuals with a higher efficiency than biomarkers. MEDoS was designed to ensure that a captured single cell remains steadily at a certain position during measurement. The MEDoS comprises a microfluidic channel for cell flow, a flexible polymer membrane actuator that functions as a cell trap for capturing, a pair of barriers, and sensing electrodes.
In this study, we investigated age-related changes in cell impedance in cardiac myocytes of zebrafish. MEDoS performed in this study exhibited a high cell-capture rate (90%) for cardiac myocytes from zebrafish hearts. The sequence of cell trapping is as follows. (1) Three groups (3, 6, or 18 months old) of cardiac myocytes in 1% fetal bovine serum solution are injected into the fluidic channel. (2) The membrane actuator is inflated by pneumatic pressure to block the cell flow until a single cell stops in front of the trap. (3) The pressure is reduced so that a single cell can enter the trap in a squeezed state. (4) When a single cell is positioned at the center of the sensing electrodes, the pneumatic pressure is increased again to fix the cell on the central surface of the electrodes. For minimization of cell damage, cardiac myocytes were maintained at 4°C during the experiment, and all experiments were completed within 1 hour.
The resistance of the cytoplasm gradually decreased from the 3-month-old cell group to the 18-month-old cell group. Considering that resistance is inversely proportional to conductance, we reviewed previous aging studies that evaluated changes in cellular components that could affect conductance during senescence. Autophagic activity is especially important in cardiac myocytes, a long-lived postmitotic cell, to maintain homeostasis and longevity. Autophagic activity decreases with senescence, and, accordingly, various reactive oxygen species (ROS) accumulate in the cytoplasm of cardiac myocytes. Thus, accumulated ROS could cause changes in cellular components as well as in electrical impedance. In several studies, an increase in the conductance of induced hypoxic alveolar epithelial cells due to an increase in the ROS level was found. In addition, an increase in the conductivity of hemoglobin caused by high oxidative stress was addressed. In other words, accumulated ROS can increase the conductance of the cytoplasm because of their free-radical characteristics. Therefore, our results could suggest that ROS that accumulate during senescence decrease the resistance of the cytoplasm.
Meanwhile, capacitance, which refers to the cell membrane in the electrical circuit model, gradually increased from the 3-month-old cell group to the 18-month-old cell group. A cell membrane has a phospholipid bilayer, which is composed of different types of molecules such as fatty acids and various proteins. During cell senescence, the level of ROS gradually increases with decreasing autophagic activity. ROS are more soluble in the fluid lipid bilayer than in aqueous solution; thus, the membrane phospholipids and polyunsaturated fatty acids, one of the phospholipid acyl chains, are susceptible to oxidative damage. Peroxidation of polyunsaturated fatty acids in the membrane has been shown to be a cause of senescence. Based on the aforementioned studies, the peroxidizability index (PI) was used to measure the relative age-related susceptibility of fatty acid composition to peroxidative damage in the cell membrane. A high PI value implies that the membrane bilayer is easily affected by lipid peroxidation. Many investigators have found that the PI value and lipoxidation-derived molecular damage increase with aging. In addition, the oxide composition amount increases during the process of lipid peroxidation. These phenomena can be explained by the fact that high PI values are obtained as the oxide composition amount increases. The capacitance of the cell membrane also increases as the oxide composition amount increases in the membrane. Therefore, we hypothesize that the increase in PI values reflects an increase in the capacitance of the cell membrane.