A tiny, aquatic, single-celled organism can contract to one quarter of its body length in less than five milliseconds – hundreds of times faster than a human can blink. Researchers have discovered that the organism, Spirostomum ambiguum, uses a calcium-activated protein network in a fishnet-like configuration to power the contraction at much faster speeds than human muscles can contract. The work has implications for designing faster artificial muscles and synthetic cellular machinery.

Spirostomum ambiguum is a giant single-celled ciliate, so-called because of the fringe of hairlike cilia that it uses to swim. It is notable among ciliates for its ability to contract at a rate of around 100 body lengths per second and to quickly repeat the motion – an ability it may use to evade predators or communicate with fellow ciliates. Human muscle fibers, in comparison, can shorten by similar fractions, but it takes about 10 times as long. Spirostomum is of interest to scientists because of the mechanical differences underlying its abilities.

“The difference between what Spirostomum can do and what we can do comes down to what is powering the contraction, and what the machinery behind it looks like,” says Mary Elting, associate professor of biophysics at North Carolina State University. “If we can understand those processes, it could help us build synthetic systems that mimic the speed and power of this single-celled organism.”

The researchers used electron and immunofluorescence microscopy to examine Spirostomum and found that the organism utilizes calcium ions to trigger contraction and a unique fishnet-like structure to complete the movement.

Single-celled organisms like Spirostomum don’t have muscle fibers. Instead, they have myonemes, fibrous structures inside the cell composed of the calcium-binding proteins centrin and Sfi1. In Spirostomum, these myonemes form a fishnet-shaped web across the exterior of the organism. When the contraction is triggered, the net shrinks into itself and then springs back.

“The fishnet geometry is unique because it lets Spirostomum contract uniformly, which protects is internal organelles (single cells’ versions of organs) while it moves so quickly,” Elting says. “It works because the Sfi1 protein in the myoneme can shift from stiff to flexible. In the presence of calcium ions Sfi1 loses its stiffness and clumps up like a ball of wet spaghetti, which causes the fishnet to pull tight, shrinking the organism.”

In humans, adenosine triphosphate, or ATP, stores and releases energy to muscle fibers, triggering contraction.

“Comparing the way our muscles contract to the way Spirostomum works is like comparing gas to electric power,” Elting says. “ATP undergoes a chemical change and gets ‘burned up,’ like gasoline, whereas calcium ions act like an electrical current, although we still don’t know what produces the voltage that starts the current, or how it gets ‘reset’ so contraction can happen again.”

The researchers’ next steps are further research into the calcium trigger and how Spirostomum can reset after each contraction.

“We would expect calcium-triggered reactions to be ‘one shot,’ but Spirostomum can do it repeatedly,” Elting says. “Understanding those aspects of its motion are the keys to building a fast-moving, ATP-independent artificial muscle.”

The research appears in Proceedings of the National Academy of Sciences and was supported by the National Science Foundation under award numbers 1935260, 2313722, 2313724, 1935262, 1817334, 2313727 and 2313725, and by NIGMS of the National Institutes of Health under award numbers R35GM130327 and R35GM142588. Co-corresponding authors of the work are Aaron Dinner, professor of chemistry at the University of Chicago; Jerry Honts, the Marshall and Judith Flapan Professor of Biology at Drake University; and Saad Bhamla, associate professor of chemical and biological engineering at the University of Colorado at Boulder. Other NC State contributors are former Ph.D, student and first author Joseph Lannan, and Research Assistant Professor Peter Thompson. Carlos Floyd and Suriyanarayanan Vaikuntanathan from the University of Chicago; L.X. Xu from the Georgia Institute of Technology; and Connie Yan and Wallace Marshall from the University of California San Francisco also contributed to the work.

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