The Dynamics of Wound Healing: Understanding cancer through the physics of cell movement

At some point in their lives, nearly forty percent of all men and women in the United States will be diagnosed with cancer. Across the globe, the number of new annual diagnoses is projected to hit 23.2 million by 2030—an increase of sixty five percent in under two decades. The discovery of a cure for the second-leading killer in the world will represent a prodigious milestone in medicine, and such a finding can only arise from collaborative effort in research across scientific disciplines.

Recent work at the Yale Systems Biology Institute has contributed to this effort by characterizing the relationship between mechanical forces and energy in cellular motion during wound healing processes. Framing cell movement in terms of familiar physical principles allows us to understand how cancer cells spread through the body. In conjunction with ongoing work in biological signaling and cell communication, this research underscores the value of the Systems Biology Institute’s mission to integrate physics, genetics, physiology, biomedical engineering, and synthetic biology into the formulation of new treatments for particularly aggressive forms of cancer.

The Systems Biology Institute prides itself on possessing the interdisciplinary approach essential for innovative problem-solving. “It’s a microcosm of science at Yale,” said Andre Levchenko, John C. Malone Professor of Biomedical Engineering, and the Institute’s director. “We have people from ten different departments and three different schools whose labs are all next to each other. There’s constant encouragement for them not just to discuss the philosophy of biological systems but really to try and see what happens at the interfaces of their respective disciplines.”

Learning from the cellular skeleton

Led by assistant professor of biomedical engineering and physics Michael Murrell and postdoctoral fellow Visar Ajeti, this breakthrough wound healing project studied a fundamental cellular process to understand the dynamical complexities of cancer. Wound healing involves the collective movement of multiple cells, and researchers assessed key physical properties of two types of cell coordination for the first time. At its heart, the experiment was an analysis of the behavior of the cytoskeleton, which provides internal structure and motility to the cell. The cytoskeleton is far more dynamic than analogous skeletal systems in humans or other life forms. “It’s both the equivalent of a skeleton for larger organisms and something that is highly active, fluid, and different from our bones, able to reorganize itself and morph,” Levchenko said.

Actin, a type of protein bundle, enables the cytoskeleton to reconfigure itself into different architectures to provide particular structural or movement capabilities. One of the actin architectures studied by Murrell’s team involves protrusions called lamellipodia, which morph into existence to pull each cell forward and fill the gap created by the wound. Another actin architecture features purse strings, muscle-like bundles of cells that assemble at the wound’s periphery, conjoin the healing cells along their leading faces, and contract to gradually force the cells together and close the wound.

Another critical component of the wound healing process is the substratum or extracellular matrix: the soft, dense base to which cells are attached and across which they advance. Substrata assemble into different forms to provide the structural integrity to hold our bodies together and to transmit information pertaining to cells’ attachment and function. Substrata of varying elasticities, viscosities, and thicknesses maintain unique chemical landscapes. As cells move, they exert forces on the substrata, subtly deforming them. The forces at cell-cell interfaces define the important relationship between migrating cells and the substratum that was investigated by Murrell’s team.

Familiar physics on a smaller scale

The researchers measured different modes of collective cell behavior by making a small hole in a single cell with a laser, allowing the induced wound to widen outward. Using fluorescent particles injected into the substratum, the researchers could visualize the wound’s boundary and identify the two types of actin architectures through the alignment of their filaments. As cells exerted the forces necessary to propel themselves toward the wound, the embedded particles mapped the distortion of the substratum.

The first set of results found that the proportion of each architecture present varied based on both substratum thickness and how far the healing process had progressed. Lamellipodia were observed to be more active for more rigid substrata and in early stages of healing as cells rapidly combatted the widening gulf in the tissue. In later stages and for softer surfaces, purse strings dominated.

The investigators then assessed the impact of each architecture type on wound closure speed. They found that velocity had no dependence on substratum thickness but varied between architectures: lamellipodial movement happened faster than purse string movement by single cells. There appeared to be a critical thickness which marked a shift in the dynamics of architecture-specific wound closure rates. For surfaces stiffer than this value, lamellipodia heal faster; otherwise, purse strings most efficiently aid closure on more flexible surfaces.

The team expanded on these findings by defining local forces and associated energy, which allows one to measure energy expended by cells per unit time and calculate the exerted power. The energy density, or the amount of energy stored in a system per unit volume, was found to be lower for lamellipodia than purse strings. Surprisingly, the disparity in energy density between the architectures precisely counterbalanced the previously demonstrated disparity in velocity. The product of these two values—defined as effective power—is therefore a conserved quantity. This effective power is analogous to the physical work, or the force applied in the direction of motion, transmitted from the leading edge of the wound to the substratum as the cells propel themselves.

A gateway into cancer research

The discovery of mechanical work conservation in these systems represents the most exciting and important insight gleaned from the experiment. Management, sources, and consumption of energy are vital to understanding the operation of any complex life system, including cancerous tumors, which expand through invasive spread and efficiently process huge energetic demands. The existence of a trade-off in different types of cellular movement between energy usage and velocity represents a key constraining principle that will inform future research.

But the discovery also enables more holistic analysis of biological functions in the context of broader scientific progress, leading to identification of basic principles governing cell function. Levchenko explained that an understanding of energetic cycles typically arises from thermodynamic principles applied to systems passively existing at or near equilibrium. Complex biological systems, however, are far more active and function further away from equilibrium, forcing scientists to modify present theories of living systems’ internal processes. This consideration led to the emergence of a new field focused on active matter, which includes forms of matter than can mold themselves while consuming energy.

Levchenko noted that this type of finding could prove to be a fundamental advance with heady ramifications in an accelerating field. “In contrast to physics, there are very few fundamental laws that exist in biology. [Few] biological principles are easily formalized in a definitive way, but this [work] starts formalizing some of them, in a way centered around not just biological but physical concepts,” Levchenko said.

With these insights in hand, the next step in their potential application to cancer cells has already begun. The challenge lies in translating the progress made in wound healing, a relatively normal cellular process, to cancer, which is unpredictable in how cells signal, communicate, and mutate DNA. Cancer, however, is frequently compared to a continuous open wound, expanding from a primary tumor through essentially the same mechanisms as those examined by Murrell’s team. Despite the seemingly crucial difference between contraction and expansion, many of the applicable principles have already been studied in Murrell’s work. The most direct correlation between these processes involves an actin bundle similar to a purse string, which undergoes a fundamental change due to environmental pressures, allowing restrained cancer cells to break free and spread. Levchenko’s lab studies the signaling and gene regulation mechanisms employed by cancer cells to hijack healthy ones into behaving differently. Working with Murrell’s lab, he also wants to know what physical factors drive cancerous expansion, such as energy management or resistive forces potentially akin to surface tension in water beads.

Though an important discovery, insight into wound healing processes is only a small step toward eradicating cancer. Despite the loftiness of its ambitions, the Systems Biology Institute will not shy away from confronting the inevitable obstacles. “There are still lots of gaps in our knowledge about these types of complex, dynamic systems involving cells moving together. But we see challenges as exciting, not depressing,” Levchenko said. “A lot of new insights come from an interdisciplinary approach where you are at the interface of two worlds. I think this is where we will see a lot of discoveries going forward.”