Nicholas A Nicholas Kurniawan

Mechanistic adaptability of cancer cells strongly affects anti-migratory drug efficacy

Cancer metastasis involves the dissemination of cancer cells from the primary tumour site and is responsible for the majority of solid tumour-related mortality. Screening of anti-metastasis drugs often includes functional assays that examine cancer cell invasion inside a three-dimensional hydrogel that mimics the extracellular matrix (ECM). Here, we built a mechanically tuneable collagen hydrogel model to recapitulate cancer spreading into heterogeneous tumour stroma and monitored the three-dimensional invasion of highly malignant breast cancer cells, MDA-MB-231. Migration assays were carried out in the presence and the absence of drugs affecting four typical molecular mechanisms involved in cell migration, as well as under five ECMs with different biophysical properties. Strikingly, the effects of the drugs were observed to vary strongly with matrix mechanics and microarchitecture, despite the little dependence of the inherent cancer cell migration on the ECM condition. Specifically, cytoskeletal contractility-targeting drugs reduced migration speed in sparse gels, whereas migration in dense gels was retarded effectively by inhibiting proteolysis. The results corroborate the ability of cancer cells to switch their multiple invasion mechanisms depending on ECM condition, thus suggesting the importance of factoring in the biophysical properties of the ECM in anti-metastasis drug screenings.

Early Stiffening and Softening of Collagen: Interplay of Deformation Mechanisms in Biopolymer Networks

Collagen networks, the main structural/mechanical elements in biological tissues, increasingly serve as biomimetic scaffolds for cell behavioral studies, assays, and tissue engineering, and yet their full spectrum of nonlinear behavior remains unclear. Here, with self-assembled type-I collagen as model, we use metrics beyond those in standard single-harmonic analysis of rheological measurements to reveal strain-softening and strain-stiffening of collagen networks both in instantaneous responses and at steady state. The results show how different deformation mechanisms, such as deformation-induced increase in the elastically active fibrils, nonlinear extension of individual fibrils, and slips in the physical cross-links in the network, can lead to the observed complex nonlinearity. We demonstrate how comprehensive rheological analyses can uncover the rich mechanical properties of biopolymer networks, including the above-mentioned softening as well as an early strain-stiffening, which are important for understanding physiological response of biological materials to mechanical loading.

Factor XIII stiffens fibrin clots by causing fiber compaction

Factor XIII‐induced cross‐linking has long been associated with the ability of fibrin blood clots to resist mechanical deformation, but how FXIII can directly modulate clot stiffness is unknown.

Modulation of surface wettability of superhydrophobic substrates using Si nanowire arrays and capillary-force-induced nanocohesion

We describe a new scalable method to fabricate large-area hybrid superhydrophobic surfaces with selective adhesion properties on silicon (Si) nanowire array substrates by exploiting liquid-medium-dependent capillary-force-induced nanocohesion. Gold (Au) nanoparticles were deposited on Si by glancing angle deposition followed by metal-assisted chemical etching of Si to form Si nanowire arrays. The surfaces were dried in either deionized (DI) water, 2-propanol or methanol to vary the capillary forces exerted on the Si nanowires during the drying process in order to tune the extent of clustering of nanowires and hence the adhesion properties of the resulting superhydrophobic surfaces. Here, we exploit the combined effects of surface tension and Youngs contact angle to modulate the degree of clustering of the Si nanowires during capillary-force-induced nanocohesion. These surfaces were chemically modified and rendered hydrophobic by fluorosilane deposition. Drying in DI water resulted in small clusters of nanowires which produce a low-hysteresis superhydrophobic surface that mimics a lotus leaf. Drying in methanol resulted in large nanowire clusters that lead to a high-hysteresis superhydrophobic surface. Further, we demonstrate the ability to fabricate both small and large nanowire clusters by controlling the drying of the nanowire arrays in order to selectively define and modulate adhesion of water on the same superhydrophobic substrate. The simplicity of our process to tune surface wettability on single substrates paves the way for future applications in lab-on-chip devices and platforms for chemical and biological analyses.

Mechanobiology of cell migration in the context of dynamic two-way cell–matrix interactions

Migration of cells is integral in various physiological processes in all facets of life. These range from embryonic development, morphogenesis, and wound healing, to disease pathology such as cancer metastasis. While cell migratory behavior has been traditionally studied using simple assays on culture dishes, in recent years it has been increasingly realized that the physical, mechanical, and chemical aspects of the matrix are key determinants of the migration mechanism. In this paper, we will describe the mechanobiological changes that accompany the dynamic cell-matrix interactions during cell migration. Furthermore, we will review what is to date known about how these changes feed back to the dynamics and biomechanical properties of the cell and the matrix. Elucidating the role of these intimate cell-matrix interactions will provide not only a better multi-scale understanding of cell motility in its physiological context, but also a more holistic perspective for designing approaches to regulate cell behavior.

Porosity governs normal stresses in polymer gels

When sheared, most elastic solids including metals, rubbers, and polymer gels dilate perpendicularly to the shear plane. This behavior, known as the Poynting effect, is characterized by a positive normal stress. Surprisingly, fibrous biopolymer gels exhibit a negative normal stress under shear. Here we show that this anomalous behavior originates from the open-network structure of biopolymer gels. Using fibrin networks with a controllable pore size as a model system, we show that the normal-stress response to an applied shear is positive at short times, but decreases to negative values with a characteristic time scale set by pore size. Using a two-fluid model, we develop a quantitative theory that unifies the opposite behaviors encountered in synthetic and biopolymer gels.

The role of structure in the nonlinear mechanics of cross-linked semiflexible polymer networks

The microstructural basis of the characteristic nonlinear mechanics of biopolymer networks remains unclear. We present a 3D network model of realistic, cross-linked semiflexible fibers to study strain-stiffening and the effect of fiber volume-occupancy. We identify two structural parameters, namely, network connectivity and fiber entanglements, that fully govern the nonlinear response from small to large strains. The results also reveal distinct deformation mechanisms at different length scales and, in particular, the contributions of heterogeneity at short length scales.

Crowding Alters the Folding Kinetics of a β-Hairpin by Modulating the Stability of Intermediates

Crowded environments inside cells exert significant effects on protein structure, stability, and function, but their effects on (pre)folding dynamics and kinetics, especially at molecular levels, remain ill-understood. Here, we examine the latter for, as an initial candidate, a small de novo β-hairpin using extensive all-atom molecular dynamics simulations for crowder volume fractions φ up to 40%. We find that crowding does not introduce new folding intermediates or misfolded structures, although, as expected, it promotes compact structures and reduces the accessible conformational space. Furthermore, while hydrophobic-collapse-mediated folding is slightly enhanced, the turn-directed zipper mechanism (dominant in crowder-free situations) increases many-fold, becoming even more dominant. Interestingly, φ influences the stability of the folding intermediates (FI(1) and FI(2)) in an apparently counterintuitive manner, which can be understood only by considering specific intrachain interactions and intermediate (and hierarchical) structural transitions. For φ values <20%, native-turn formation is enhanced, and FI(1), characterized by a hairpin structure but slightly mismatched hydrophobic contacts, increases in frequency, thus enhancing eventual folding. However, higher φ values impede native-turn formation, and FI(2), which lacks native turns, re-emerges and increasingly acts as a kinetic trap. The change in the stability of these intermediates with φ strongly correlates with the hierarchical folding stages and their kinetics. The results show that crowding assists intermediate structural changes more by impeding backward transitions than by promoting forward transitions and that a delicate competition between reduction in configuration space and introduction of kinetic traps along the folding route is key to understanding folding kinetics under crowded conditions.

Fibrin Networks Support Recurring Mechanical Loads by Adapting their Structure across Multiple Scales

Tissues and cells sustain recurring mechanical loads that span a wide range of loading amplitudes and timescales as a consequence of exposure to blood flow, muscle activity, and external impact. Both tissues and cells derive their mechanical strength from fibrous protein scaffolds, which typically have a complex hierarchical structure. In this study, we focus on a prototypical hierarchical biomaterial, fibrin, which is one of the most resilient naturally occurring biopolymers and forms the structural scaffold of blood clots. We show how fibrous networks composed of fibrin utilize irreversible changes in their hierarchical structure at different scales to maintain reversible stress stiffening up to large strains. To trace the origin of this paradoxical resilience, we systematically tuned the microstructural parameters of fibrin and used a combination of optical tweezers and fluorescence microscopy to measure the interactions of single fibrin fibers for the first time, to our knowledge. We demonstrate that fibrin networks adapt to moderate strains by remodeling at the network scale through the spontaneous formation of new bonds between fibers, whereas they adapt to high strains by plastic remodeling of the fibers themselves. This multiscale adaptation mechanism endows fibrin gels with the remarkable ability to sustain recurring loads due to shear flows and wound stretching. Our findings therefore reveal a microscopic mechanism by which tissues and cells can balance elastic nonlinearity and plasticity, and thus can provide microstructural insights into cell-driven remodeling of tissues.

Fibrin-fiber architecture influences cell spreading and differentiation

ABSTRACT The mechanical and structural properties of the extracellular matrix (ECM) play an important role in regulating cell fate. The natural ECM has a complex fibrillar structure and shows nonlinear mechanical properties, which are both difficult to mimic synthetically. Therefore, systematically testing the influence of ECM properties on cellular behavior is very challenging. In this work we show two different approaches to tune the fibrillar structure and mechanical properties of fibrin hydrogels. Addition of extra thrombin before gelation increases the protein density within the fibrin fibers without significantly altering the mechanical properties of the resulting hydrogel. On the other hand, by forming a composite hydrogel with a synthetic biomimetic polyisocyanide network the protein density within the fibrin fibers decreases, and the mechanics of the composite material can be tuned by the PIC/fibrin mass ratio. The effect of the changes in gel structure and mechanics on cellular behavior are investigated, by studying human mesenchymal stem cell (hMSC) spreading and differentiation on these gels. We find that the trends observed in cell spreading and differentiation cannot be explained by the bulk mechanics of the gels, but correlate to the density of the fibrin fibers the gels are composed of. These findings strongly suggest that the microscopic properties of individual fibers in fibrous networks play an essential role in determining cell behavior.