Author: José R. Pineda got his Ph.D. from University of Barcelona in 2006. Since 2007 he has worked for Institut Curie and The French Alternative Energies and Atomic Energy Commission. Currently he is a researcher of the EHU. He investigates the role of stem cells in physiologic and pathologic conditions.

One of the ideas most of us learn early in biology is that only a few specialized cells can communicate through electrical signals. Neurons do it. Muscle cells do it. Epithelial cells, the cells that form our skin and line most organs, supposedly do not. However, a recent study by Yu and Granick demonstrates that this picture may not be entirely correct. In fact, these seemingly ordinary cells appear capable of generating electrical spikes of their own when tissue integrity is disturbed. Despite  epithelial cells have traditionally been viewed as passive structures whose main functions are to form protective barriers, absorb nutrients, and maintain tissue integrity, Yu and Granick challenges this long-standing idea in a striking way. Their work demonstrates that epithelial cells can generate electrical spikes resembling action potentials, revealing a previously unrecognized form of bioelectrical communication that may help coordinate tissue responses after injury ¹.

The finding is surprising because epithelia are among the most abundant tissues in the body. They line the skin, digestive tract, airways, and many internal organs. Although these tissues are known to participate in wound healing and cellular communication, they were not believed to possess the kind of rapid electrical activity usually associated with the nervous system. The new study suggests that this assumption may have been incomplete. Under certain conditions, epithelial cells appear capable of producing coordinated electrical signals that spread across remarkably large distances. To investigate this phenomenon, the researchers employed two different epithelial models: the first model consisted of primary human skin keratinocytes (the major cellular component of the epidermis). The second model used Madin-Darby Canine Kidney (MDCK) cells (a classical epithelial cell line frequently employed in cell biology research). Both cell types were grown until they formed dense, confluent monolayers resembling miniature sheets of epithelial tissue.

The key experimental tool was a multielectrode array, often abbreviated as “MEA”. This technology can be imagined as a highly sensitive electronic “listening device” containing dozens of microscopic electrodes embedded in a culture surface. When cells generate electrical activity, even extremely small voltage fluctuations can be detected simultaneously at multiple positions. Multielectrode arrays are routinely used to study neurons, but their application to epithelial tissues is relatively uncommon. After establishing epithelial monolayers on these arrays, the researchers created a small and highly localized injury using a focused laser beam. The laser produced controlled damage in a tiny region at the center of the culture while leaving surrounding cells intact. This experimental design allowed the team to observe how cells located at various distances reacted to the injury. Electrical signals were then monitored continuously for one hour. What they observed was unexpected: within the epithelial sheets, electrical spikes appeared shortly after injury and persisted for more than sixty minutes!! These spikes displayed a characteristic sequence of depolarization, repolarization, and hyperpolarization, terms commonly used to describe action potentials in neurons. However, there was one major difference, while neuronal action potentials last a few milliseconds, the epithelial spikes unfolded over one to two seconds, making them roughly one thousand times slower. Despite their slower kinetics, the electrical signals were surprisingly robust, measurements showed that spikes could be detected as far as 740 micrometers from the injury site, a distance corresponding to dozens of cell diameters (Figure 1). Even more remarkably, their amplitude did not progressively decrease with distance as one would expect from a simple wave spreading outward from a central source. In some cases, signals recorded farther away were as large as, or even larger than, those detected closer to the wound. This observation suggested that distant epithelial cells were not merely receiving a weakened signal from injured neighbors. Instead, they may have been generating their own electrical responses as part of a collective tissue-wide process.

Discovering the spikes was only part of the story. The obvious next question was what actually produced them. Were epithelial cells using the same electrical machinery found in neurons, or was something entirely different at work? To answer this, the researchers began dissecting the molecular mechanisms one by one. They began by testing the role of calcium (one of the most important intracellular messengers in biology). Calcium ions regulate a wide range of cellular activities, including contraction, migration, secretion, and gene expression. To determine whether calcium was necessary for epithelial spiking, the researchers added EDTA (a molecule that binds calcium and reduces its availability). Surprisingly, once calcium availability was reduced, the electrical activity virtually vanished!! This immediately suggested that calcium was not just associated with the phenomenon but was an essential part of it. Next, the team examined the contribution of cellular mechanics, because it is known that epithelial cells continuously generate mechanical forces through interactions between actin filaments and myosin motor proteins, a system known as “the actomyosin network”. This machinery allows cells to contract, change shape, and exert forces on neighboring cells. To disrupt this process, the researchers treated cultures with blebbistatin (a selective inhibitor of myosin II activity). Once again, electrical spiking was largely abolished. The similarity between the effects of calcium depletion and myosin inhibition suggested that electrical activity was intimately linked to the mechanical state of the tissue. This observation raised an intriguing possibility: perhaps epithelial electrical signaling is not driven by voltage-gated ion channels (as in neurons), but instead by channels that respond to mechanical forces. To test this hypothesis, the investigators focused on mechanosensitive ion channels, specialized proteins embedded within cell membranes that open when cells are stretched, compressed, or subjected to mechanical stress. Using the inhibitor GsMTx4 (which blocks several mechanosensitive channels including Piezo1), the researchers observed a partial reduction in electrical activity. While spiking was not completely eliminated, both amplitude and coordination decreased substantially. This result implicated mechanosensitive channels as key components of the signaling pathway. To really prove their implication, next the researchers activated them directly using specific pharmacological compounds. One molecule, GSK1016790A, activates the TRPV4 channel. Another, Yoda-1, specifically activates Piezo1. Remarkably, activating these channels was sufficient to generate large electrical spikes even in the absence of any injury. In other words, simply stimulating mechanosensitive channels could reproduce the electrical behavior observed after tissue damage. This provided strong evidence that mechanical sensing lies at the heart of epithelial electrical excitability.

Based on their findings, the authors proposed a positive feedback mechanism: 1) mechanical stress activates mechanosensitive ion channels, allowing calcium to enter the cells. 2) rising calcium levels stimulate actomyosin contraction, generating additional mechanical stress. 3) this new stress then activates more mechanosensitive channels, sustaining the cycle. Such a feedback loop could explain why spiking persists for long periods and why cells distant from the wound remain electrically active. One particularly interesting aspect of the study is its potential relevance to wound healing. Successful tissue repair requires large numbers of cells to coordinate their behavior across considerable distances. Cells must migrate, proliferate, and remodel tissue structures in a synchronized manner. Traditional explanations have emphasized biochemical signaling molecules and calcium waves. The electrical spikes described here introduce an additional layer of communication that may operate on intermediate spatial and temporal scales. According to the authors, these signals could complement chemical and mechanical communication systems, helping tissues organize collective responses after injury. However, the researchers are careful to acknowledge the study’s limitations. Their experiments were performed in cultured epithelial monolayers rather than living organisms. Although the results clearly demonstrate intrinsic electrical excitability in epithelial cells, it remains unknown whether the same phenomenon occurs in intact tissues during natural wound healing. Furthermore, the exact relationship between these electrical spikes and cellular behavior has not yet been determined. It is still unclear whether the spikes actively drive tissue repair or simply accompany mechanical disruption. Additional work involving animal models, voltage-sensitive imaging, and targeted genetic manipulation of mechanosensitive channels will be required to answer these questions.

In conclusion, the study by Yu and Granick reminds us that biology still contains unexpected surprises, even in tissues that have been studied for decades. A simple layer of epithelial cells, once considered electrically silent, may in fact possess its own form of excitability. Rather than transmitting information through millisecond action potentials like neurons, epithelial tissues appear to communicate through slower, mechanically driven electrical spikes capable of coordinating activity across hundreds of micrometers. Whether these signals prove to be a central component of tissue physiology remains to be seen. What is already clear, however, is that the electrical life of cells extends far beyond the nervous system!!

The post When skin cells behave like neurons: The surprising discovery of electrical spikes in epithelia appeared first on Mapping Ignorance.

Scientists have developed remarkably long, precisely patterned molecular threads that could open new possibilities for designing advanced materials. The scientists also discovered these threads — called supramolecular polymers — can dramatically enhance brain cell activity. ¹

Built from self-assembling molecules, the longest threads reach dimensions greater than the diameter of a cell. By molecular standards, they are giant — roughly 100 times more massive than even very large conventional polymers and the largest known proteins in biology. Yet despite their enormous size, scientists can precisely control their lengths and organize them into chemically distinct segments with defined dimensions carrying opposite electrical charges.

That precision also produced an unexpected biological effect. In cell culture studies, the structures enhanced the growth and organization of neurons and increased synapse formation.

The findings introduce a new way to use self-assembly to build highly controlled supramolecular materials with precise physical arrangement of chemical structures. Ultimately, the work could inform materials’ design for a range of applications, including biomaterials for regenerative medicine.

“These structures represent a breakthrough in materials design, and we already discovered one useful application: their superbioactivity toward neurons, which will bring new opportunities in regenerative medicine,” said Northwestern’s Samuel I. Stupp, who led the study. “Typically, when we design a regenerative material, we add biological signals, designed to activate cell receptors. Here, the material itself becomes highly bioactive simply through the precise organization of electrical charges and its dynamic behavior. That introduces a very different way of thinking about how materials can communicate with cells.”

Bringing precision to self-assembly

The new study builds on Stupp’s long history working with supramolecular materials, dynamic structures that form when molecules spontaneously organize through weak, reversible interactions. While conventional polymers, like plastics, hold together using strong, permanent chemical bonds, supramolecular polymers are dynamic, allowing their components to move and reconfigure, which generates life-like properties in synthetic systems. Supramolecular polymers exist in all living organisms, where they perform essential functions and hold promise for future soft matter technologies.

That flexible, dynamic nature can be useful for interacting with living cells in biomedical applications, but it also makes the structures difficult to control. Scientists can design the individual molecular building blocks but then struggle to control how long the resulting structures grow and how they organize their chemical components.

Stupp’s team overcame that limitation by using novel designs of molecules and carefully controlling their assembly process. With this strategy, the researchers could dictate the threads’ lengths and keep them uniform. Once the growth stopped, the structures maintained their precise lengths rather than fusing together or rearranging.

“We let these filaments sit in a test tube for months, and they surprisingly didn’t change,” Stupp said. “They remained the same length and retained their segmented structures.”

Grow, stop, grow again

Stupp and his team traced that unusual ability to a previously unknown process his laboratory calls “self-capping supramolecular polymerization.” As a thread grows, flexible portions at the ends remain highly dynamic and, at proper temperatures, allow new molecules to join in a synchronized manner. Once the available building blocks are consumed, those flexible portions fold over and protect the ends. This effectively shuts down further growth by preventing threads from fusing together or undergoing “Ostwald Ripening,” a process in which smaller structures disappear while larger ones grow.

“To make them longer and longer, we just kept adding new molecules,” Stupp said. “The new molecules stick with strong noncovalent bonds to the growing thread and elongate the assembly, which led to a breakthrough in terms of size control.

“The collective molar mass of their noncovalently bonded monomers reaches billions of daltons, and thus they are ‘giga-assemblies.’ These are the largest supramolecular polymers ever made with this degree of precision. Once the monomer is fully consumed, they finish growing and their dimensions remain stable.”

Approximately the size of one proton or neutron, a dalton is a tiny unit that scientists use to measure the mass of molecules and atoms. More than a billion daltons is enormous by molecular standards. For comparison, the largest common polymers of plastics have molar masses in the range of 10 million daltons, and the largest proteins in nature reach approximately 3 to 5 million daltons.

Researchers can later restart growth by supplying more building blocks. By adding different types of molecules in sequence, the researchers can create precisely defined segments with different chemical properties. In the study, Stupp and his team created threads with positively and negatively charged segments. Because opposites attract, the researchers expected the threads to clump together. Instead, they remained stable in water because the “capping” mechanism prevented the assemblies from fusing together.

Small anchor, big difference

To explore how the pattern of electrical charges might interact with living cells, Stupp and his team turned to neurons, which carry a strong negative charge. The team exposed several threads with different designs to cellular cultures containing cortical neurons.

“The first thing we did was make a structure with only positively charged segments, and it basically killed the neurons,” Stupp said. “Then, we tried a structure with only negatively charged segments, and nothing happened because the neurons repelled the materials.”

When Stupp’s team used supramolecular assemblies with randomly mixed positive and negative charges, the neurons didn’t respond in a notable way. But then the team designed a thread with the breakthrough polymerization strategy comprising a small positively charged segment sandwiched between two negatively charged segments of different lengths. Something interesting and unexpected occurred.

The small positive region anchored the thread to the negatively charged surface of a neuron. Repelled by the neuron’s negative charge, the thread’s negative segments remained highly mobile and in constant motion since they could not escape.

“When we used only positively charged segments, they suffocated the neuron and killed it,” Stupp said. “But with only one positively charged segment surrounded by negative ones, it couldn’t do that anymore. And the negative segments, which ordinarily couldn’t get near the neuron, were forced to stay near it because they were anchored.”

Neurons grow and spring to life

Neurons treated with the precisely segmented threads grew longer neurites — the long projections neurons use to communicate with other cells. The treated neurons also formed more elaborate branches, gaining the ability to connect with neighboring cells.

“The neurons matured to an amazing level,” Stupp said. “They grew long and highly branched neurites, which is how they can start making synaptic connections in the brain and in the spinal cord.”

After just seven days, treated neurons were much more active than untreated neurons. When stimulated, treated neurons produced a calcium response — a sign of neuronal activity. Their overall calcium activity was four times greater. And, after 14 days, treated neurons formed more synapses than untreated neurons.

Stupp and his team saw another striking effect in cultures of human neural progenitor cells, which are immature cells that can develop into neurons. Although these cultures typically grow in flat layers, the cells instead organized into raised, three-dimensional clusters connected by axons. This organization resembled aspects of the brain’s white and gray matter, where somas are separated from axons.

No signals required

Perhaps most surprisingly, the molecular threads produced these effects without carrying biological signals specifically designed to encourage neurons to grow. Stupp posits the negatively charged segments recruit specific proteins already present around the cells that help neurons survive, grow and communicate. These proteins, called neurotrophic factors, have positively charged domains. Stupp’s team also found laminin — an important protein that holds tissues together and has positively charged domains — accumulating along the supramolecular threads near neurons.

Stupp hypothesizes that the interplay between attraction and repulsion causes the threads’ negative segments to rapidly move toward and away from the neurons in a “tapping” motion. Proteins recruited by the negative segments may be drawn toward receptors on the neurons, while electrostatic forces push the segments away. This repeated tapping, occurring on millisecond timescales, could help enhance biological signaling. A live cell movie included in the Science paper has captured this phenomenon.

Beyond neurons, Stupp says the new polymerization strategy could eventually enable scientists to build long molecular structures containing many carefully positioned chemical segments. That could open opportunities to create new materials and devices with features that extend over very long distances.

“We discovered something we had never seen before in chemistry or in biology,” Stupp said. “This work gives us a new level of control over supramolecular materials. In the future, we can imagine creating many segments with different properties. The unknown functions of these structures are likely to be as surprising as the superbioactivity observed on neurons. We’re just beginning to imagine the possibilities.”

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