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Force generation within cells, mediated by motor proteins along cytoskeletal networks, maintains the function of multicellular structures during homeostasis and when generating collective forces. Here, we describe the use of chromatin dynamics to detect cellular force propagation [a technique termed SINK (sensors from intranuclear kinetics)] and investigate the force response of cells to disruption of the monolayer and changes in substrate stiffness. We find that chromatin dynamics change in a substrate stiffness-dependent manner within epithelial monolayers. We also investigate point defects within monolayers to map the impact on the strain field of a heterogeneous monolayer. We find that cell monolayers behave as a colloidal assembly rather than as a continuum since the data fit an exponential decay; the lateral characteristic length of recovery from the mechanical defect is ∼50 µm for cells with a 10 µm spacing. At distances greater than this characteristic length, cells behave similarly to those in a fully intact monolayer. This work demonstrates the power of SINK to investigate diseases including cancer and atherosclerosis that result from single cells or heterogeneities in monolayers.This article has an associated First Person interview with the first author of the paper.
© 2018. Published by The Company of Biologists Ltd.
Cell therapies suffer from poor survival post-transplant due to placement into hostile implant sites characterized by host immune response and innate production of high levels of reactive oxygen species (ROS). We hypothesized that cellular encapsulation within an injectable, antioxidant hydrogel would improve viability of cells exposed to high oxidative stress. To test this hypothesis, we applied a dual thermo- and ROS-responsive hydrogel comprising the ABC triblock polymer poly[(propylene sulfide)-block-(N,N-dimethyl acrylamide)-block-(N-isopropylacrylamide)] (PPS-b-PDMA-b-PNIPAAM, PDN). The PPS chemistry reacts irreversibly with ROS such as hydrogen peroxide (HO), imparting inherent antioxidant properties to the system. Here, PDN hydrogels were successfully integrated with type 1 collagen to form ROS-protective, composite hydrogels amenable to spreading and growth of adherent cell types such as mesenchymal stem cells (MSCs). It was also shown that, using a control hydrogel substituting nonreactive polycaprolactone in place of PPS, the ROS-reactive PPS chemistry is directly responsible for PDN hydrogel cytoprotection of both MSCs and insulin-producing β-cell pseudo-islets against HO toxicity. In sum, these results establish the potential of cytoprotective, thermogelling PDN biomaterials for injectable delivery of cell therapies.
In hovering flight, hummingbirds reverse the angle of attack of their wings through pitch reversal in order to generate aerodynamic lift during both downstroke and upstroke. In addition, the wings may pitch during translation to further enhance lift production. It is not yet clear whether these pitching motions are caused by the wing inertia or actuated through the musculoskeletal system. Here we perform a computational analysis of the pitching dynamics by incorporating the realistic wing kinematics to determine the inertial effects. The aerodynamic effect is also included using the pressure data from a previous three-dimensional computational fluid dynamics simulation of a hovering hummingbird. The results show that like many insects, pitch reversal of the hummingbird is, to a large degree, caused by the wing inertia. However, actuation power input at the root is needed in the beginning of pronation to initiate a fast pitch reversal and also in mid-downstroke to enable a nose-up pitching motion for lift enhancement. The muscles on the wing may not necessarily be activated for pitching of the distal section. Finally, power analysis of the flapping motion shows that there is no requirement for substantial elastic energy storage or energy absorption at the shoulder joint.
Loss of significant functional renal mass results in compensatory structural and hemodynamic adaptations in the nephron. While these changes have been characterized in several injury models, how they affect hemodynamic forces at the glomerular capillary wall has not been adequately characterized, despite their potential physiological significance. Therefore, we used intravital multiphoton microscopy to measure the velocity of red blood cells in individual glomerular capillaries of normal rats and rats subjected to ⅚ nephrectomy. Glomerular capillary blood flow rate and wall shear stress were then estimated using previously established experimental and mathematical models to account for changes in hematocrit and blood rheology in small vessels. We found little change in the hemodynamic parameters in glomerular capillaries immediately following injury. At 2 wk postnephrectomy, significant changes in individual capillary blood flow velocity and volume flow rate were present. Despite these changes, estimated capillary wall shear stress was unchanged. This was a result of an increase in capillary diameter and changes in capillary blood rheology in nephrectomized rats.
A combination of anionic and RAFT polymerization was used to synthesize an ABC triblock polymer poly[(propylenesulfide)-block-(N,N-dimethylacrylamide)-block-(N-isopropylacrylamide)] (PPS-b-PDMA-b-PNIPAAM) that forms physically cross-linked hydrogels when transitioned from ambient to physiologic temperature and that incorporates mechanisms for reactive oxygen species (ROS) triggered degradation and drug release. At ambient temperature (25 °C), PPS-b-PDMA-b-PNIPAAM assembled into 66 ± 32 nm micelles comprising a hydrophobic PPS core and PNIPAAM on the outer corona. Upon heating to physiologic temperature (37 °C), which exceeds the lower critical solution temperature (LCST) of PNIPAAM, micelle solutions (at ≥2.5 wt %) sharply transitioned into stable, hydrated gels. Temperature-dependent rheology indicated that the equilibrium storage moduli (G') of hydrogels at 2.5, 5.0, and 7.5 wt % were 20, 380, and 850 Pa, respectively. The PPS-b-PDMA-b-PNIPAAM micelles were preloaded with the model drug Nile red, and the resulting hydrogels demonstrated ROS-dependent drug release. Likewise, exposure to the peroxynitrite generator SIN-1 degraded the mechanical properties of the hydrogels. The hydrogels were cytocompatible in vitro and were demonstrated to have utility for cell encapsulation and delivery. These hydrogels also possessed inherent cell-protective properties and reduced ROS-mediated cellular death in vitro. Subcutaneously injected PPS-b-PDMA-b-PNIPAAM polymer solutions formed stable hydrogels that sustained local release of the model drug Nile red for 14 days in vivo. These collective data demonstrate the potential use of PPS-b-PDMA-b-PNIPAAM as an injectable, cyto-protective hydrogel that overcomes conventional PNIPAAM hydrogel limitations such as syneresis, lack of degradability, and lack of inherent drug loading and environmentally responsive release mechanisms.
There is a compelling clinical need for bone grafts with initial bone-like mechanical properties that actively remodel for repair of weight-bearing bone defects, such as fractures of the tibial plateau and vertebrae. However, there is a paucity of studies investigating remodeling of weight-bearing bone grafts in preclinical models, and consequently there is limited understanding of the mechanisms by which these grafts remodel in vivo. In this study, we investigated the effects of the rates of new bone formation, matrix resorption, and polymer degradation on healing of settable weight-bearing polyurethane/allograft composites in a rabbit femoral condyle defect model. The grafts induced progressive healing in vivo, as evidenced by an increase in new bone formation, as well as a decrease in residual allograft and polymer from 6 to 12 weeks. However, the mismatch between the rates of autocatalytic polymer degradation and zero-order (independent of time) new bone formation resulted in incomplete healing in the interior of the composite. Augmentation of the grafts with recombinant human bone morphogenetic protein-2 not only increased the rate of new bone formation, but also altered the degradation mechanism of the polymer to approximate a zero-order process. The consequent matching of the rates of new bone formation and polymer degradation resulted in more extensive healing at later time points in all regions of the graft. These observations underscore the importance of balancing the rates of new bone formation and degradation to promote healing of settable weight-bearing bone grafts that maintain bone-like strength, while actively remodeling.
A body with a traveling-wave surface (TWS) is investigated by solving the incompressible Navier-Stokes equation numerically to understand the mechanisms of a novel propulsive strategy. In this study, a virtual model of a foil with a flexible surface which performs a traveling-wave movement is used as a free swimming body. Based on the simulations by varying the traveling-wave Reynolds number and the amplitude and wave number of the TWS, some propulsive properties including the forward speed, the swimming efficiency, and the flow field are analyzed in detail. It is found that the mean forward velocity increases with the traveling-wave Reynolds number, the amplitude, and the wave number of the TWS. A weak wake behind the free swimming body is identified and the propulsive mechanisms are discussed. Moreover, the TWS is a "quiet" propulsive approach, which is an advantage when preying. The results obtained in this study provide a novel propulsion concept, which may also lead to an important design capability for underwater vehicles.
Injectable scaffolds present compelling opportunities for wound repair and regeneration because of their ability to fill irregularly shaped defects and deliver biologics such as growth factors. In this study, we investigated the properties of injectable polyurethane (PUR) biocomposite scaffolds and their application in cutaneous wound repair using a rat excisional model. The scaffolds have a minimal reaction exotherm and clinically relevant working and setting times. Moreover, the biocomposites have mechanical and thermal properties consistent with rubbery elastomers. In the rat excisional wound model, injection of settable biocomposite scaffolds stented the wounds at early time points, resulting in a regenerative rather than a scarring phenotype at later time points. Measurements of wound length and thickness revealed that the treated wounds were less contracted at day 7 compared to blank wounds. Analysis of cell proliferation and apoptosis showed that the scaffolds were biocompatible and supported tissue ingrowth. Myofibroblast formation and collagen fiber organization provided evidence that the scaffolds have a positive effect on extracellular matrix remodeling by disrupting the formation of an aligned matrix under elevated tension. In summary, we have developed an injectable biodegradable PUR biocomposite scaffold that enhances cutaneous wound healing in a rat model.
Copyright © 2011 Wiley Periodicals, Inc.
Although the response of endothelial cells to the disturbed blood flow in the vicinity of atherosclerotic lesions is known to be distinct from that elicited by nonatherogenic laminar flow, the mechanisms involved are poorly understood. Our initial studies confirmed that expression of the endothelial receptor tyrosine kinase Tie1 was evident at regions of atherogenic flow in mature animals. We therefore hypothesized that Tie1 plays a role in the endothelial response to atherogenic shear stress. Consistent with this, we found that Tie1+/- mice bred to the apoE-deficient background displayed a 35% reduction in atherosclerosis relative to Tie1+/+;Apoe-/- mice. Since deletion of Tie1 results in embryonic lethality secondary to vascular dysfunction, we used conditional and inducible mutagenesis to study the effect of endothelial-specific Tie1 attenuation on atherogenesis in Apoe-/- mice and found a dose-dependent decrease in atherosclerotic lesions. Analysis of primary aortic endothelial cells indicated that atheroprotective laminar flow decreased Tie1 expression in vitro. Attenuation of Tie1 was associated with an increase in eNOS expression and Tie2 phosphorylation. In addition, Tie1 attenuation increased IkBα expression while decreasing ICAM levels. In summary, we have found that shear stress conditions that modulate atherogenic events also regulate Tie1 expression. Therefore, Tie1 may play a novel proinflammatory role in atherosclerosis.
OBJECTIVE - Platelets play a dual role in thrombosis by forming aggregates and stimulating coagulation. We investigated the commitment of platelets to these separate functions during collagen-induced thrombus formation in vitro and in vivo.
METHODS AND RESULTS - High-resolution 2-photon fluorescence microscopy revealed that in thrombus formation under flow, fibrin(ogen)-binding platelets assembled into separate aggregates, whereas distinct patches of nonaggregated platelets exposed phosphatidylserine. The latter platelet population had inactivated alphaIIb beta3 integrins and displayed increased binding of coagulation factors. Coated platelets, expressing serotonin binding sites, were not identified as a separate population. Thrombin generation and coagulation favored the transformation to phosphatidylserine-exposing platelets with inactivated integrins and reduced adhesion. Prolonged tyrosine phosphorylation in vitro resulted in secondary downregulation of active alphaIIb beta3.
CONCLUSIONS - These results lead to a new spatial model of thrombus formation, in which aggregated platelets ensure thrombus stability, whereas distinct patches of nonaggregated platelets effectuate procoagulant activity and generate thrombin and fibrin. Herein, the hemostatic activity of a developing thrombus is determined by the balance in formation of proaggregatory and procoagulant platelets. This balance is influenced by antiplatelet and anticoagulant medication.