Animal cells are linked by a structure called the cell cortex – and this structure, the researchers say, looks a bit like a tent.
A tent is made up of a shell with a zippered opening that controls what can get in and out of the tent. This shell is maintained by a system of posts. Likewise, an animal cell cortex is made up of a cell membrane that controls what enters the cell.
The cortex also contains proteins, which help the cell to keep its shape. One of these key proteins, called actin, is a polymer with a linear structure, like a tent pole. But unlike a tent, the cortical proteins in a cell are not stationary. They move along the cell membrane, freely assembling and separating over time, in a process called “cortical excitability.”
When these proteins start to form waves, it is a sign that the cell is preparing to divide. But studying this process within the cell membrane is difficult. Now researchers at the University of Michigan have developed an approach to study these wave patterns outside of a cell by developing an artificial cortex without cells.
When a cell prepares to divide, its proteins in the cell cortex begin to move. First, its cortical proteins form an excitable wave, like spectators performing “the wave” in a football stadium. Second, cortical proteins organize themselves into coherent oscillations, which behave like flashing Christmas lights, associating and dissociating with the membrane at regular intervals. Image credit: Jennifer Landino, A. Miller laboratory
In doing so, they showed that these cortical proteins can self-organize according to two models. First, the proteins form an excitable wave, like spectators performing “the wave” in a stadium. Second, the proteins then organize themselves into coherent oscillations, which behave like blinking Christmas lights. Their study, which examines the Rho and F-actin proteins in frog egg extract, is published in the journal Current Biology.
Almost a hundred years ago, people who studied the cell cortex predicted that it was self-organized, that the patterns and shapes of these proteins were self-determined by the properties of the protein and the properties of the membrane. But you can’t really separate the cortex from the rest of the cell, because then everything just collapses.
Jennifer Landino, senior author, postdoctoral researcher, Department of Molecular, Cellular and Developmental Biology at UM
âIt’s very exciting that we now have a tool to study how these models work outside of cells. At the same time, it also confirms this long-held assumption that the cortex self-organizes and that these patterns simply flow from the properties of the molecules involved.
UM researcher Jennifer Landino extracts cytoplasm from frog eggs to study the behavior of cortical Rho and F-actin proteins as they cross the cell membrane, preparing the cell to divide. Here Rho is shown in cyan and F-actin is shown in magenta. Image credit: Jennifer Landino, A. Miller laboratory
Landino says there’s a need to develop an artificial cortex to study these proteins, because while biologists have tools they can use to manipulate proteins, they have fewer tools to manipulate lipids, the fats that make up the cell membrane. These tools rely on the manipulation of proteins that regulate membrane composition. In the artificial cortex, researchers can directly modify the membrane by mixing different lipids, an approach that is not possible in cells.
Landino uses commercially available lipids to assemble the artificial cortex. She adds them to a flat well, which creates a surface layer that will be closest to the microscope. The researchers use an inverted microscope, which means that the magnification component is under the sample being studied. On top of that, it adds a layer of cytoplasm from frog eggs that contains all of the protein components normally found in the cytoplasm. The moment the cytoplasm is laid on the artificial membrane, the proteins in the cytoplasm begin to self-assemble, just as they would in a natural animal cell, Landino said.
âWhen a cell divides, it pinches itself in the middle and divides into two. We also see these wave patterns forming in cells and we see that they are associated with cell division, âLandino said. âWe think it’s the function of waves, to prepare the cell cortex to undergo a drastic change in shape, but it’s really hard to test in cells. So we hope to use this artificial system to understand both how these wave patterns are formed and what their function might be. “
Ann Miller, associate professor of molecular, cellular and developmental biology, is the lead author of the article. The research, which was funded by the National Science Foundation, is part of a collaborative effort between the Miller Lab and Anthony Vecchiarelli, assistant professor of molecular, cellular, and developmental biology, as well as collaborators at the University of Wisconsin and the University of Edinburgh. .
âThese findings represent a powerful new synthetic platform for cell-free studies of the mechanisms that regulate cortical patterning,â Miller said. “The system that Dr. Landino has developed opens up new possibilities to deepen our understanding of how self-organizing cortical patterns drive essential cellular processes like cell division.”
Landino, J., et al. (2021) Rho and F-actin self-organize within an artificial cellular cortex. Current biology. doi.org/10.1016/j.cub.2021.10.021.