Research Reveals Biological Design in the Sensing and Manipulation of Force
A tiny spider has been found that can launch itself at 10 times the speed that would make a test pilot black out. And if that isn’t amazing enough, it builds its own catapult out of web material that can tolerate sustained tension for hours and days. The story of the “slingshot spider” of Peru is told by Georgia Tech. Scientists measured this spider’s acceleration at 100 times that of a cheetah, with a force of 130 Gs. How this one-millimeter spider has mastered physics baffles even the engineers who traveled six hours by boat in Peru to watch it in its native habitat.
Another mystery is how the spider patiently holds the web while waiting for food to fly by. Alexander and Bhamla estimated that stretching the web requires at least 200 dynes, atremendous amount of energy for a tiny spider to generate. Holding that for hours could waste a lot of energy.
“Generating 200 dynes would produce tremendous forces on the tiny legs of the spider,” Bhamla said. “If the reward is a mosquito at the end of three hours, is that worth it? We think the spider must be using some kind of trick to lock its muscles like a latch so it doesn’t need to consume energy while waiting for hours.” [Emphasis added.]
Think of the design requirements for this feat: mastery of materials science, mastery of potential energy to stretch the web and latch it in a cocked position, ability to target fast-moving prey, mastery of ballistics, possessing a body able to withstand exceptional acceleration, and ability to wrap the prey and consume energy from it after a dizzying flight. This is a spider with a PhD in both physics and engineering!
Pushing the Boundaries of Flight
Another example of biological design in the use of force is seen in the flight of the wandering albatross. This bird is “fine tuned to wind conditions,” say engineers at the University of Liverpool.
With a wingspan of over three meters — the largest of any bird alive today — the wandering albatross can fly thousands of miles, even around the world, gliding for long periods in search of fish or squid. Birds search for prey in flight and capture it after landing on the sea surface. Due to their long wings, taking off from the sea surface is by far their most energetically demanding activity, requiring four times more energy than gliding flight.
Using data loggers attached to the birds’ legs, the scientists found that the male albatross, which is 20 percent larger than the female, knows how to take advantage of wind gusts to get launched. On land, the largest wingspan belongs to the California condor. Those birds are known to launch themselves off cliffs or tree branches or run downhill to get airborne. Then they can stay aloft by gliding on thermals.
Scientists can only guess at the mastery of physics of some extinct flyers. Some fossil birds like Pelagonis sandersi had a wingspan almost double that of the wandering albatross — 6.4 meters, almost 24 feet! And then there are pterosaurs (flying reptiles); some were as big as giraffes. Some scientists think Quetzalcoatlus northropi, with a wingspan up to 43 feet, could launch itself up into the air in one second from a standing position (see Live Science). If so, the feat required design not only in the muscles to achieve sufficient acceleration but also in the lightweight but sturdy bones that could withstand forces of flight. Indeed, some of the largest dinosaurs may have had specialized bones that were both strong and lightweight to handle the forces of merely standing and walking on the ground, according to New Scientist.
Force Mastery All the Way Down
Biology has mastered the use of force all the way down to the nano scale. A paper in PNAS on describes the properties of talin, “an adaptor protein that transduces mechanical signals into biochemical cues by recruiting a network of protein ligands in a force-dependent way.” This example complements our earlier article about mechanotransduction. Once again, fine tuning of forces and materials is found, but this time at a scale that is orders of magnitude smaller.
These force cues have a complex nature, oscillate in time with different frequency components, and are often embedded in noise. However, most assays to explore the mechanics of force-sensing proteins rely on simple perturbations, such as constant or ramped forces. Here, we use our magnetic tweezers design to subject single talin domains to oscillatory forces and external mechanical noise. We show that talin ignores random external fluctuations but synchronizes its folding dynamics with force oscillations in a frequency-dependent way. We hypothesize that this finely tuned response could underpin talin force-sensing properties.
Talin’s job as an “exquisite force sensor” is to grab and hold parts together inside the cell.
Talin is a mechanosensing hub protein in focal adhesions, which cross-links transmembrane integrins with the active F-actin filaments and recruits several binding proteins to control the function and fate of this organelle. For example, vinculin binds to cryptic helices in mechanically unfolded talin domains, subsequently recruiting actin filaments that reinforce the cellular junction. Hence, talin transduces mechanical forces through its folding dynamics.
This enzyme senses motions of neighboring cells or the extracellular matrix. Somehow, talin deconvolves this noisy signals of motion into recognizable oscillations at particular frequencies and knows how to respond. Its spring-like domains unfold so that other molecules can attach, and then it binds them together. It is a truly remarkable reaction that differs from other types of mechanosensing, opening the door for more discoveries in biophysics at the molecular scale:
Although initially formulated in the context of nonlinear physics, stochastic resonance has been demonstrated in a broad range of biological systems, with particular emphasis as a sensory mechanism in mechanoreceptors, like the crayfish hair cells, the cricket cercal system, or the vestibular and auditory system. Interestingly, in all of these examples, signal transduction involves the activation of gated ion channels, which convert mechanical perturbations into electrophysiological signals. However, mechanotransduction also involves biochemical signaling, where force stimuli trigger downstream signaling pathways through a complex network of interacting proteins. In this sense, it remains to be explored whether stochastic resonance could also play a role in mechanotransduction pathways that involve ligand binding to force-bearing proteins instead of gating of mechanosensitive channels.
In the case of talin, the implications for design are clear:
Mechanical signal transduction relies on the robust and finely tuned response of molecular force sensors.Mechanical information is encoded in both the amplitude of the signal and its time-dependent evolution. Hence, both components must be accurately deciphered and interpreted by cellular force sensors.
The only evolution spoken of in the paper is the “time-dependent evolution” (unfolding) of the vibrations that talin senses: i.e., the behavior of the oscillations from initiation to damping. That ability implies even more design than a simple response to a vibration. It implies that talin can recognize encoded information both in the signal strength and in its behavior in time, and respond accordingly by unfolding the appropriate domain for binding to other protein parts. The three authors from Columbia University describe the actions of this enzyme as a “tuning fork of cellular mechanotransduction.”
No Miracles Here
These examples of biological mastery of force are not miracles; they are subject to the laws of physics and obey the laws of physics. But wow, do they know how to take advantage of the laws of physics! From the mightiest dinosaur, to the largest birds, to the tiniest spider, to molecules in the cell, biological designs show how to push the limits of the possible. Such exceptional applications of materials and forces rightly excite our wonder and admiration.