Materials in Motion: Engineering Bio-Inspired Motile Matter

Life is on the move! Motion is ubiquitous in biology. From the gargantuan steps of an elephant to the tiniest single celled amoeba, movement in biology is a complex phenomenon that originates at the cellular level and involves the organization and regulation of thousands of proteins. These proteins do everything from mixing the cytoplasm to driving cell motility and cell division. Deciphering the origins of motion is no easy feat and scientists have been studying such complex behavior for quite some time. With biology as an inspiration, studying these complex behaviors provides insight into engineering principals which will allow researchers to develop an entirely new category of far-from-equilibrium materials that spontaneously move, flow or swim.

In a recent report in the journal Nature, a team of researchers from Brandeis University consisting of Tim Sanchez, Daniel T. N. Chen, Stephen J. DeCamp, Michael Heymann, and Zvonimir Dogic have constructed a minimal experimental system for studying far-from-equilibrium materials. This system demonstrates the assembly of a simple mixture of proteins that results in a hierarchy of phenomena. This hierarchy begins with extending bundles of bio-filaments, produces networks that mix themselves, and finally culminates in active liquid crystals that impart self-motility to large emulsion droplets.

Their system consists of three basic components: 1) microtubule filaments, 2) kinesin motor proteins which exert forces between microtubule filaments, and 3) a depletion agent which bundles microtubule filaments together. When put together under well-defined conditions, these components form bundled active networks (BANs) that exhibit large-scale spontaneous motion driven by internally generated active stresses. These motions, in turn, drive coherent fluid flows. These features bear a striking resemblance to a biological process called cytoplasmic streaming, in which the cellular cytoskeleton spontaneously mixes its content. Additionally, the system has great potential for testing active matter theories because the researchers can precisely tune the relevant system parameters, such as ATP and protein concentration.

 

The researchers also demonstrate the utility of this biologically-inspired synthetic system by studying materials science topics that have no direct biological analog. Under dense confinement to an oil-water interface, microtubule bundles undergo a spontaneous transition to an aligned state. Soft matter physics describes such materials as liquid crystals, which are the materials used to make liquid crystal displays (LCDs). These active liquid crystals show a rich variety of dynamical behavior that is totally inaccessible to their equilibrium analogs and opens an avenue for studying an entirely new class of materials with highly desirable properties.

Lastly, inspired by streaming flows that occur in cells, the researchers encapsulate the bundled active networks into spherical emulsion droplets. Within the droplet, microtubules again formed a self-organized nematic liquid crystal at the oil-water interface. When the droplets were partially squished between glass plates, the streaming flows generated by the dynamic liquid crystals lead to the emergence of spontaneous self-motility.

This research constitutes several important advances in the studies of the cytoskeleton, non-equilibrium statistical mechanics, soft-condensed matter, active matter, and the hydrodynamics of fluid mixing. The researchers have demonstrated the use of biological materials to produce biomimetic functions ranging from self-motility to spontaneous fluid flows using fundamentally new mechanisms. Additionally, the experimental system of bundled active microtubules is poised to be a model for exploring the physics of gels, liquid crystals, and emulsions under far-from-equilibrium conditions.

To see more videos from the Dogic lab at Brandeis University, check out their YouTube page.

Tenure track faculty position, Biochemistry

The Department of Biochemistry at Brandeis University invites applications for a tenure-track faculty position, to begin Fall, 2013. We are searching for a creative scientist who will establish an independent research program and who in addition will maintain a strong interest in teaching Biochemistry at the undergraduate and graduate levels. The research program should address fundamental questions of biological, biochemical, or biophysical mechanism.

Brandeis University offers the rare combination of a vigorous research institution in a liberal-arts college setting. The suburban campus is located 20 minutes from Boston and Cambridge and is part of the vibrant community of academic and biotechnology centers in the Boston area.

The application should include a cover letter, curriculum vitae, statement of research accomplishments and future plans, and three letters of reference. Applications will be accepted only through https://academicjobsonline.org/ajo/jobs/1813. Additional inquiries may be directed to Chris Miller, Professor of Biochemistry (cmiller@brandeis.edu).

First consideration will be given to applications received by December 1, 2012. Brandeis University is an Equal Opportunity Employer, committed to building a culturally diverse intellectual community. We particularly welcome applications from women and minority candidates.

Hagan to receive Strage Award

On March 26, 2012, Professor Gregory A. Petsko wrote on behalf of the Strage Award Selection Committee:

It is with great pleasure that I announce the recipient of this year’s Strage Award for Aspiring Young Science Faculty, Dr. Michael Hagan of the Physics Department.

Mike is one of Brandeis’ most accomplished young faculty members. His work has focused largely on the factors that govern self-assembly – the ability of macromolecular systems to form organized structures spontaneously. This is at the heart of the development of complexity, not just in living organisms but also in nanotechnology. Please join me in congratulating Mike on winning this award, and bring your students and postdocs to his Strage Award Lecture.

The award ceremony and lecture will take place on Monday, April 16, in Abelson 131, at 12 :30 p.m. The title of the lecture is Mechanisms of Virus Assembly.

Barry awarded Joseph Katz Fellowship from Argonne Natl Lab

Edward Francis Barry (PhD ’11) has recently been awarded the prestigious Argonne Scholar-Joseph Katz Postdoctoral Fellowship at Argonne National Laboratory. Ed began his scientific career studying the self-assembly of fd virus with Zvonimir Dogic, during the latter’s Junior Fellowship at the Rowland Institute at Harvard University. When Dogic joined the physics faculty at Brandeis, Ed also came to Brandeis as a Ph.D. student and helped to start the Dogic lab. Ed published seven papers describing various novel assemblages found in the fd system. Most notably, his 2010 Proceedings of the National Academy of Sciences paper describing the physical properties of colloidal membranes won the 2010 Cozzarelli Prize for scientific excellence. As the Katz fellow, Ed will be working between Argonne National Laboratory and the University of Chicago, where he is working with Experimental Condensed Matter Professor Heinrich Jaeger studying the self-assembly of monolayers composed of nanoparticles.


Escaping the Lattice

The next best thing to seeing real atoms is to mimic them in silico: we assign interactions between the atoms and then — pouf –They’re alive!

The number of particles in a visible sample is on the order of Avogadro’s constant, say ~1023, whereas a fairly muscular computer can only follow ~105-107 atoms at a time. To compensate, computational scientists typically replicate their simulation boxes infinitely in space. This creates a quandary for calculating forces across replication boxes. The simplest option, which is to neglect forces beyond a chosen cut-off, suffices for many interactions, is too crude for the particularly long-range interactions that occur between charges. To accurately account for these interactions, it is customary to use a clever 90-year-old (!) technique, called the Ewald sum.(1)

The problem with the Ewald sum is that it requires imposing a long-range periodicity that is inappropriately short for macromolecules.(2) To avoid artifacts, a number of alternatives have been suggested. One intuitive approach, called “force shifting”, smooths the interaction energy and its first derivative (the force) at the chosen cutoff. However, this creates new artifacts (see figure) when particles have very large or varying charges, as in some ionic liquids. Brandeis scientists Seyit Kale and Judith Herzfeld, have found that this problem can be solved by also smoothing the second derivative of the interaction energy (the acceleration).(3)  This approach performs virtually as well as the Ewald sum in a new reactive force field that they have been developing (see figure).

The neighbor frequencies for bulk water calculated with force shifting at a cutoff of 9 Å (red) and 12 Å (magenta) versus with the authors’ new approach at a 9 Å cutoff (blue) and the Ewald sum (black). The blue and black curves are virtually the same while the red and magenta curves contain artifacts. The inset shows a representation of a water molecule from the force field that the authors are developing.

  1. Ewald P (1921) The Berechnung optischer und elektrostatischer Gitterpotentiale. Ann. Phys. 369: 253-287.
  2. Hunenberger PH, McCammon JA (1999) Effect of artificial periodicity in simulations of biomolecules under Ewald boundary conditions: A continuum electrostatics study. Biophys. Chem. 78: 69-88.
  3. Kale S, Herzfeld J (2011) Pairwise Long-range Compensation for Strongly Ionic Systems. J. Chem. Theory Comput. 7: 3620-3624.

Microtubules and Molecular Motors Do The Wave

Most people are familiar with audiences in crowded arenas performing “the wave,” raising their hands in sync to produce a pattern that propagates around the whole stadium.  This self-organized motion appears seemingly out of nowhere.  It is not produced by any external control, but is rather emerges from thousands of individuals interacting only with their neighbors.  A similar principle of self-organization might also be relevant on length scales that are billion times smaller.  On this scale, nanometer-sized proteins interact with each other to produce dynamical structures and patterns that are essential for life—and some of these processes are reminiscent of waves in crowded stadiums.  For example, thousands of nano-sized molecular motors located within a single eukaryotic flagellum or cilium coordinate their activity to produce wave-like beating patterns.  Furthermore, dense arrays of cilia spontaneously synchronize their beating to produce metachronal waves.

Proper functioning of cilia is essential for health; for example, cilia determine the correct polarity and location of our organs during development.  Defective cilia can cause a serious condition called situs inversus, in which the positions of the heart and lungs are mirrored from the normal state.  In another example, thousands of cilia in our lungs function to clear airways of microscopic debris such as dust or smoke by organizing their beating into coordinated, wave-like patterns.  Despite the importance of ciliar function, the exact mechanisms that lead to spontaneous wave-like patterns within isolated cilia, as well as in dense ciliary fields, is not well understood.

In a paper published in the journal Science this week, an interdisciplinary team consisting of physics graduate student Timothy Sanchez and biochemistry graduate student David Welch working with biophysicist Zvonimir Dogic and biologist Daniela Nicastro present a striking finding: the first example of a simple microscopic system that self-organizes to produce cilia-like beating patterns.  Their experimental system consists of three main components: 1) microtubule filaments; 2) motor proteins called kinesin, which consume chemical fuel to move along microtubules; and (3) a bundling agent that induces assembly of filaments into bundles.  Sanchez et al. found that under a certain set of conditions, these very simple components are able to self-organize into active bundles that spontaneously beat in a periodic manner.  One large spontaneously beating bundle is featured below:

In addition to observing the beating of isolated bundles, the researchers were also able to assemble a dense field of bundles that spontaneously synchronized their beating patterns into traveling waves.  An example of this higher-level organization is shown here:

The significance of these observations is several-fold. First, due to the importance of ciliar function for health, there is great interest in elucidating the mechanism that controls the beating patterns of isolated cilia as well as dense ciliary fields.  However, the complexity of these structures presents a major challenge.  Each eukaryotic flagellum and cilium contains more than 600 different proteins.  For this reason, most previous studies of cilia and flagella have employed a top-down approach; they have attempted to elucidate the beating mechanism by deconstructing the fully functioning organelles through the systematic elimination ­­­of constituent proteins. In this study, the researchers utilize an alternative bottom-up approach and demonstrate for the first time that it is possible to construct artificial cilia-like structures from a “minimal system,” comprised of only three components.  These observations suggest that emergent properties, spontaneously arising when microscopic molecular motors interact with each other, might play a role in formation of ciliary beating patterns.

Second, self-organizing processes in general have recently become the focus of considerable interest in the physics community.  These processes range in scale from microscopic cellular functions and swarms of bacteria to macroscopic phenomena such as flocking of birds and manmade traffic jams. Theoretical models indicate that these vastly different phenomena can be described using similar theoretical formalisms.  However, controllable experiments with flocks of birds or crowds at football stadiums are virtually impossible to conduct.  The experiments described by Sanchez et al. could serve as a model system to test a broad range of theoretical predictions. Third, the reproduction of such an essential biological functionality in a simple in vitro system will be of great interest to the fields of cellular and evolutionary biology. Finally, these findings open the door for the development of one of the major goals of nanotechnology: to design motile nano-scale objects.

These encouraging results are only the first from this very new model system.  The Dogic lab is currently planning refinements to the system to study these topics in greater depth.

UPDATE: Today, this publication was additionally featured on NPR Science Friday as the video pick of the week:

 

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