FAST BREAKING PAPERS - 2008

Why do you think your paper is highly cited?

Dynamics of the cytoskeleton largely determine the mechanical properties of the living cell that influence a variety of important cellular behaviors such as migration, proliferation, and intercellular communication. Despite extensive studies, however, the dynamics of the cytoskeleton have not been fully understood, due in part to lack of a generally agreed upon physical model that can explain its complex nature and underlying mechanisms.

Recently, increasing evidence from studies on adherent cells in culture demonstrates that the cytoskeleton behaves much like soft glassy materials such as foam, slurry, and colloids. Although the physics underlying the behaviors of such materials remains largely elusive, they are generally regarded as slow dynamics not driven by thermal energy. (Ben Fabry et al., "Scaling the Microrheology of Living Cells," Physical Review Letters 87, [148102]: 2001; Predrag Bursac et al. "Cytoskeletal remodelling and slow dynamics in the living cell" Nature Materials 4: 557-61, 2005).

These findings seem to contradict what has long been known in filamentous systems formed by reconstituted cytoskeletal protein polymers in vitro—that is, that the dynamics of such cytoskeleton-mimicking systems is driven by the thermal fluctuation of semiflexible filaments made of the cytoskeletal protein (Frederick C. MacKintosh et al., "Elasticity of Semiflexible Biopolymer Networks," Physical Review Letters 75: 4425-28, 1995; Margaret L. Gardel et al., "Elastic Behavior of Cross-Linked and Bundled Actin Networks," Science 304 [5675]: 1301-05, 28 May 2004). This contradiction has been a hot debate with regard to whether each type of the observed dynamics is only true to the specific subject under study, and neither represents the behavior of real cells in vivo.

In this paper, we examined the material properties of the cytoskeleton of freshly isolated airway smooth muscle cells, using the same approach as with cultured adherent cells. These freshly isolated cells are one step closer to cells in vivo in terms of their structure and biological properties. But, how would they behave mechanically? As we characterized the material moduli of theses cells as we did with the adherent cells in culture, we found that at low (strain) frequencies (<100 Hz) the cytoskeleton still behaves like a soft glass material as those cultured cells, although softer and more elastic.

However, at high frequencies (>100 Hz), we observed a deviation from glassy behavior towards the elastic behavior associated with a semiflexible filament network. This finding was quite a surprise, and demonstrated for the first time the existence of the semiflexible filament dynamics commonly observed in vitro. Thus, our work has the potential to reconcile observations made in reconstituted cytoskeletal protein filament networks (which are usually characterized as elastic bodies) and in living cells (which behave non-elastically).

Furthermore, the finding that the elastic behavior of the cytoskeleton is observed only at high frequencies (near 1 kHz) suggests that cells may be more appropriately characterized as non-elastic bodies because physiological functions (which require deformation of the cytoskeleton) carried out by the cells normally occur at frequencies much lower than 1 kHz. This is an important conclusion. Taken together, this paper may have provided a more appropriate perspective when we treat the living cell as a complex material. I think this is why our paper is highly cited.

Does it describe a new discovery, methodology, or synthesis of knowledge?

This paper does describe a new finding that the response of the cell to oscillatory strain (0.1-1k Hz) is characterized by two distinct power-law regimes: from 0.1 to 100 Hz a very weak dependence (power-law exponent alpha = 0.05) and from 100-1000 Hz a stronger dependence (power-law exponent beta = 0.75). These are novel findings; in previous rheological studies of cells only one power-law regime had been observed over a very wide range of frequencies/times, with a power-law exponent of 0.1-0.3, and the other power-law regime with a constant power-law exponent of 0.75 had only been observed in reconstituted actin gels. Consequently, these results unify, within a single cell, two rheological models which had been used independently in the past to describe and interpret data from rheological measurements on cells.

From a biological point of view, however, these findings suggest that the semi-flexible polymer dynamics are irrelevant for cellular functions since their influence becomes important at high frequencies (>100 Hz) which are outside of the normal physiological range. This, in turn, implies that the focus of the research in cellular rheology should be shifted from the dynamics of semi-flexible polymer networks, which had been a common approach in the past, to the soft glass rheology that is yet to be fully understood. An important by-product of this new finding is that the observed regime of entropic dynamics in the cell indirectly proved that the magnetic bead-twisting technique we used does probe the actin cytoskeleton.

Would you summarize the significance of your paper in layman’s terms?

This paper may be significant because it demonstrates, for the first time, that the living cell can behave either like a soft glassy material such as Ketchup and soft dough, or like a network system composed of semiflexible filaments, depending on how fast the cell is changing its shape. Although a filamentous network may look like the structure formed by cytoskeletal proteins both in vivo and in vitro, the mechanical behavior of such systems can be elegantly explained in terms of thermal dynamics, and only at a fast rate of shape change can the semiflexible filamentous network account for the mechanical behavior of the living cell.

Within the range of the physiological rate of shape change, the cell behaves, instead, rather like Ketchup or soft dough. In other words, the cell is neither a solid nor a fluid, but something between. What’s more important is that the cell can alter its property from solid-like to fluid-like or vice versa in response to both physical and chemical changes in its environment, much like what glass does when subjected to temperature change. All together, this paper may help us better understand how cells behave both in conditions of health and of disease.

How did you become involved in this research, and were there any problems along the way?

I become involved in this research when I joined the laboratory of Professor Jeffrey Fredberg at the Harvard School of Public Health. The group led by Dr. Fredberg had been studying the mechanical behavior of human airway smooth muscle cells in an attempt to understand its role in pathobiology of asthma. Over the years, the group gradually came to establish that adherent cells passaged in culture resemble soft glassy materials as far as the cell mechanics are concerned. Although soft glassy behavior of the cell has been replicated in other labs using other or similar techniques, the concept that cells are soft glassy materials seems contradictory to what has been learned from gel systems of reconstituted cytoskeletal protein filaments in vitro.

As usual suspects, the cultured cells were blamed for bringing about the peculiar soft glassy behavior that might not portray the true picture of cell mechanics in vivo. Such blame is reasonable due to the apparent structural difference between cultured cells and those observed in tissue or freshly isolated. In order to settle the discrepancy, Dr. Fredberg’s group decided to study the mechanics of the freshly isolated cells using the same technique as with the cultured cells.

The freshly isolated cells are the closest possible to those in vivo, and should more closely represent the cells in vivo with regard to the cell mechanics. However, it proved to be a major challenge to do such studies. The first problem was that it is extremely difficult to prepare freshly isolated cells in a state of attachment to the substrate in the culture dish in order to make mechanical measurement of these cells. There had been unsuccessful attempts to tackle this problem for about two years before I took up the task.

Then I spent about a year solving all aspects of this problem before the first good experiment was achieved. After the initial success in making the measurement, other problems soon followed, including complex issues of statistical analysis and interpretation of the experimental data.

Fortunately, I was surrounded by several geniuses to whom I could turn whenever I encountered a problem. Some of them are my coauthors on this paper. With the help of this resourceful team, all problems were eventually solved, one after another, with each of these individuals contributing an equal effort toward the solution.

Where do you see your research leading in the future?

There are still numerous unknowns regarding cell mechanics and rheology. For example, the physics underlying the soft glass rheology is not fully elucidated; the molecular pathways that regulate the cell’s physical state, whether it is more solid or more fluid, are not clear; how pathological factors affect the soft glassy rheology of the cells; whether there are more regimes of cytoskeleton dynamics, and whether the dynamic regimes are fixed or shiftable, etc. These are primarily the future topics which shall lead my research.

Do you foresee any social or political implications for your research?

My research involves basic sciences only. I don’t see that it has any social or political implications.

Dr. Linhong Deng
Professor and Director
Institute of National 985 Project on Biorheology and Gene Regulations
College of Bioengineering
Chongqing University
Chongqing, PRC
and
Visiting Scientist
Molecular and Integrative Physiological Sciences Program
Department of Environmental Health
Harvard School of Public Health
Boston, MA, USA