Senescence is a process in which a cell ages and stops dividing but does not die. In keratinocytes, or skin cells, senescence is marked by depolarization of membrane potential (Vmem), the electrical potential difference across a cell membrane.
Cells maintain an electric gradient via the regulation of charged ions. Generally, there is high potassium on the inside and high sodium on the outside. This unequal distribution of ions across the cell barrier creates a relative negative charge inside the cell and a relative positive charge outside.
Multiple theories have been proposed to explain cellular aging. Historically, researchers turned to the damage-based theory, which holds that individuals age as a result of the accumulation of damage over time. Studies have shown that damage can accumulate in a number of molecules or processes such as RNA, metabolites and proteins. It has also been suggested that reduction of damage accumulation is possible by decreasing generation or increasing repair and clearance systems.
A programmatic-based theory of aging has also been proposed, arguing that molecular algorithms in cells are harmful as they approach maturity, almost as if aging is programmed. Recent studies on mice, monkeys and humans suggest that some mammalian aging follows predetermined patterns encoded in the genome and that aging in mammals is partially programmed.
A third theory, known as the information-loss theory, proposes that accumulated damage leads to information loss in both the genome and epigenome. A senescent cell prohibits it from producing the machinery necessary to maintain homeostasis, form and function.
The epigenome contains all chemical modifications of DNA and histones that regulate gene expression, essentially acting as a system of switches for genes responsible for cell identity, function and form.
The Levin Lab recently published a paper investigating how bioelectric properties change as human keratinocytes undergo senescence. The researchers sought to determine whether these cells became more positive or negatively charged and whether other properties were involved in senescence.
“If you think of a map, certain parts are going to be more hyperpolarized, certain parts are going to be more depolarized in that clustering and that patterning is really important for the development of anatomy and anatomical function,” Hamid Sediqi, a postdoctoral researcher at the Levin Lab and first author of the study, said.
Human keratinocytes underwent replicative senescence, a process in which cells are grown in a culture until they can no longer divide.
Researchers used a voltage sensitive dye — Berkeley Red Sensor of Transmembrane Potential — to compare young, actively dividing cells with cells that had remained in culture for 50 days.
In young cells, the researchers observed distinct clusters formed metaphorical neighborhoods where cells could coordinate behavior. This diversity and “neighborhood” separation is lost as cells become senescent. This discovery suggests that bioelectric patterns in tissues and organs direct their own morphology, important for the development of human anatomy and anatomical function.
“Some of the findings in this paper reaffirms the previous literature, but there’s some more novel, nuanced things that we found,” Sediqi explained.
One finding from the paper in line with previous literature was that cells exhibited a positive charge as they senesced, losing their hyperpolarized state.
The researchers also found that when cells approaching senescence were artificially hyperpolarized, senescence could be delayed. These cells took longer to become non-proliferating and maintained their young phenotype.
“It didn’t fully stop senescence — at least we didn’t continue to study it, in the sense that we didn’t keep growing it out until they achieve senescence. But what we did show is that there was a significant increase in proliferation and delay of senescence,” Sediqi said.
A number of biomarkers, including p16 and cytokines such as IL-6 and IL-8, have traditionally been used to research cell senescence. However, variation in marker usage across studies makes standardization difficult.
Sediqi said bioelectric signatures are universal markers of senescence; whether it’s a skin cell or fibroblast, they depolarize when they achieve senescence.
Using bioelectricity would not only deepen understanding of how cells age, but also serve as markers for senescence in cells as they reach depolarization. Compared to traditional biomarkers, bioelectricity’s universality would allow for standardization that could be utilized in future research.
“People could potentially use wearable devices, like bioelectric devices, that change local bioelectric fields,” Hamid mentioned.
Additionally, as cancer cells tend to be depolarized, administering electric fields may help maintain a hyperpolarized state and inhibit rapid division.
Technical difficulties appear to be inevitable in the field of biology. Dyes may not perform as expected, cells are fragile and require precise handling and care. Despite these challenges, the discoveries gained may drive significant advancements in the field of senescence.
