Stress Fibers Get a Makeover

Stress fibers, often the most prominent cytoskeletal structures in cells growing in tissue culture, are large bundles of actin filaments and associated proteins that typically traverse the ventral surface of a cell and anchor to the cell substratum at focal adhesions. Tension generated within stress fibers is transmitted to the underlying matrix via focal adhesions. Although the forces generated within stress fibers are relatively small, compared with those generated by striated muscles, the structural organization of stress fibers shares many features with striated muscle. In striated muscles, the contractile unit is the sarcomere, consisting of bipolar myosin thick filaments interdigitated with sets of actin filaments that are themselves attached at the Z-disks that define the sarcomeric borders. This organization generates the striations seen by both light and electron microscopy. In a similar way, many of the proteins in stress fibers, such as α-actinin, zyxin, myosin, and tropomyosin also exhibit a striated periodic pattern. However, examining the distribution of any of these proteins shows that the dimensions of stress-fiber sarcomeres vary not only between adjacent stress fibers but also along stress fibers. In contrast, under most situations, sarcomeres within an individual striated muscle fiber exhibit essentially identical dimensions.

Although it has been known for many years that stress fibers are force-generating and load-bearing cytoskeletal structures, relatively little is known about how they remodel in response to external or internal signals. With a focus on zyxin, Beckerle’s lab has studied the dynamics and behavior of stress-fiber sarcomeres in a series of articles. Initially they showed that zyxin is recruited to stress fibers in response to mechanical tension (1,2). Using fluorescently tagged cytoskeletal proteins combined with live cell imaging, they went on to observe that in response to tension, individual sarcomeres stretch and even break (3). Zyxin is targeted to the damaged areas and in turn recruits other cytoskeletal proteins, such as vasodilator-stimulated phosphoprotein and α-actinin, to these regions of stress-fiber strain. In the absence of zyxin, breaks within stress fibers are more frequent and transmission of force to the matrix is impaired (3).

In their most recent study, Chapin and colleagues examined strain dynamics in fibroblast stress fibers using zyxin-green fluorescent protein to track the borders of individual sarcomeres within a given stress fiber (4). Previous work from others showed that in stress fibers stimulated to contract, some sarcomeric units were seen to shorten as expected. However, at the same time, and in the same stress fiber, other sarcomeres were observed to lengthen (5). Unlike this earlier work, Chapin and colleagues did not artificially stimulate contraction of the stress fibers with chemical or physical manipulation. They were surprised to find that the sarcomeric units exhibited spontaneous and dynamic fluctuations in length; this occurred without the whole stress fiber changing its overall length. This behavior led the authors to suggest that there is a tensional homeostasis mechanism such that as one sarcomere shortens or lengthens over time, adjacent sarcomeres adapt their length correspondingly, so that the overall stress-fiber length remains constant. In support of this idea, the authors examined extreme cases where a single stress-fiber sarcomere experiences a strain that increases its length severalfold over the normal average value. In these spontaneously occurring strain events, the sarcomeres adjacent to the strain site were observed to shorten significantly, suggestive of a highly localized compensatory response. Whether the shortening of adjacent sarcomeres is in response to the lengthening of the stretched strain site sarcomere or whether the contraction of these adjacent sarcomeres is what is causing the lengthening remains uncertain and is not mutually exclusive. Regardless of the exact cause-effect relationship in this behavior of adjacent stress-fiber sarcomeres, the work reveals an unexpected degree of internal stress-fiber dynamics and exchange of mechanosensory information between sarcomeres.

One of the most interesting observations to emerge from this work is the authors’ finding that new sarcomeres can be added within the initial strain-site sarcomere even in the middle of stress fibers. Previous work has pointed to the ends of stress fibers where they terminate at focal adhesions as the sites where new sarcomeres are added (6–9). The addition of sarcomeres at strain sites builds on the idea that stress fibers have intrinsic mechanisms to repair damage and can adapt to the mechanical forces that they experience. Spontaneous strain events, where sarcomeres are longer than average, correspond to sites of local tension. The addition of new sarcomeres to these regions will strengthen stress fibers where they are weakest. Understanding how new sarcomeres are added to a preexisting stress fiber is a challenge in cytoskeletal engineering. The study described here, along with previous work from the Beckerle lab, implicates zyxin in this process of sarcomere addition. In the future, it will be very interesting to learn not only how zyxin is recruited to these sites of strain but also how zyxin contributes to the remodeling of preexisting sarcomeres so that new ones can be intercalated into the existing stress fiber while simultaneously maintaining the fiber’s load-bearing properties and mechanical strength.