6th Annual Symposium
Physics of Cancer
Leipzig, Germany
September 7-9, 2015
Contributed Talk
Programming biological systems through synthetic nanoscale building blocks
David Smith
Fraunhofer Institute for Cell Therapy and Immunology, Perlickstr. 1, Leipzig, Germany
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Biologically evolved systems are often used as inspiration in the design and development of new materials. For instance, the biopolymer constituents of the highly dynamic cellular cytoskeleton have inspired a deep understanding of soft polymer-based materials on both the experimental and theoretical levels. Meanwhile, collagen-based scaffolds remain one of the preferred substrates for investigating mechanically directed cell programming behavior. However, the molecular toolbox provided by biological systems has been evolutionarily optimized to carry out the necessary functions of cells. The resulting inability to systematically modify basic properties such as biopolymer stiffness or crosslink affinity in experimentally available model systems hinders a meticulous examination of parameter space. This is increasingly crucial in, e.g., regenerative medicine where external mechanical cues such as matrix stiffness are increasingly shown to play an important role in directing stem cell differentiation. We circumvent these limitations using model systems and components assembled from programmable nanomaterials such as DNA and peptides.

Micrometer-long nanotubes with tunable diameters and rigidity can be constructed from small sets of synthetically produced DNA strands. By systematically varying the rigidity of these synthetic, semiflexible filaments in low-density entangled networks, we for the first time experimentally determine the dependence of bulk stiffness on the previously inaccessible parameter of persistence length. Curiously, we uncover a linear dependence, which stands in stark contrast to the sublinear behavior predicted by long-accepted theoretical treatments. Furthermore, introducing crosslinks into the entangled system through inter-filament DNA hybridization interactions stiffens the network, providing a density-independent control parameter for modulating mechanical behavior.

We also take a similar approach to program the properties of natural biological materials such as entangled actin networks or even living cells. Synthetic constructs for crosslinking actin filaments are fabricated de novo from DNA strands which have been conjugated to actin-binding peptides. Crosslinkers that are programmed to have weaker (Kd ~ 2 µM) or stronger (Kd ~ 100 nM) affinity between binding peptides and actin filaments, similar to the naturally occurring crosslinkers alpha-actinin or fascin, cause a clear increase of bulk network elasticity in accordance with increasing pairwise crosslinking strength. Furthermore, introduction of these synthetic constructs into metastatic cancer cells both arrests motility and decreases whole-cell deformability, indicating a temporary and controllable rigidification of the internal actin network. These simple, protein-sized (~15-40kDa) components can be easily modified to precisely vary parameters such as size, crosslink valency or binding strength, and offer the unique possibility to program cellular behaviors while entirely bypassing the genetic machinery.
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