’Cells are really just machines - small, incredibly complex biological machines, but machines nonetheless,’ said Alexander Dunn, assistant of chemical engineering.
If certain types of living cells are placed on a microscope slide, the cells will inch across the glass, find their neighbors and assemble themselves into a simple, if primitive, tissue. A new study at Stanford University may help explain this phenomenon, and then some, about the mechanical structure and behavior of complex living organisms.
In the paper published in the Proceedings of the National Academy of Sciences , Assistant Professor of chemical engineering Alexander Dunn and a multidisciplinary team of researchers in biology, physiology, chemical engineering and mechanical engineering, were able to measure – and to literally see – the mechanical forces at play between and within the living cells.
Pulling back the veil on the exact nature of this mechanism could have a bearing on biological understanding ranging from how tissues and tumors form and grow to the creation of entire complex living organisms.
There are scads of data explaining chemical signaling between cells. "And yet, one of the great roadblocks to a complete knowledge of how cells work together to form tissues, organs and, ultimately, us is an understanding of mechanical forces," said Dunn.
Using a new force-sensing technique, Dunn and team have been able to explore how cells connect to one another and how individual cells control their own shape and movement within larger tissues.
Seeing the force
"Cells are really just machines – small, incredibly complex biological machines, but machines nonetheless," said Dunn. "They rely on thousands of moving parts that give the cell shape and control of its destiny."
The mechanical parts are proteins whose exact functions often remain a mystery, but Dunn and team have helped explain the behaviors of a few.
At its most basic level, a cell is like a balloon filled with saltwater, Dunn said. The exterior of the cell, the balloon part, is known as the membrane. Protruding through the membrane, with portions both inside and outside the cell, are certain proteins called cadherins.
Outside of the cell, cadherins bind one cell to its neighbors like Velcro. The "herin" portion of the name, in fact, shares a Latin root with "adhere."
On the inside of the cell, cadherin is connected to long fibers of actin and myosin that stretch from membrane to nucleus to membrane again. Actin and myosin work together as the muscle of the cell, providing tension that gives the cell shape and the ability to control its own movement. Without this force, the balloon of the cell would be a shapeless, immobile blob.
Puppeteer’s string
"If you watch a cell moving across a glass slide, you can see it attach itself on one side of the cell and detach on the other, which causes a contraction that allows the cell to, bit by bit, pull itself from place to place," said Dunn. "It’s clearly moving itself."
While it was understood that cadherin and actin are connected to one another by other proteins known as catenins , what was not known was how, when and where the cells might be using their muscles (actin and myosin) to tug on the Velcro (cadherin) holding them to other cells.
Dunn and his colleagues have shown for the first time that the actin-catenin-cadherin structure transmits force within the cell and, further, that cadherin can convey mechanical forces from one cell to the next. This is an important problem in the development of organisms, since a cell must somehow control its shape and its attachments to other cells as it grows, divides and migrates from one place to another within the tissue.
Like the strings of a puppeteer controlling a marionette, this mechanism is a form of mechanical communication. Dunn and others in the field believe that these mechanical forces may be important in conveying to a cell how to position itself within a tissue, when to reproduce and when to stop as the tissue reaches its proper size and shape.
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