Endothelial colony forming cell rolling and adhesion supported by peptide-grafted hydrogels

The aim of this study was to investigate the ability of peptides and peptide combinations to support circulating endothelial colony forming cell (ECFC) rolling and adhesion under shear flow, informing biomaterial design in moving toward rapid cardiovascular device endothelialization. ECFCs have high proliferative capability and can differentiate into endothelial cells, making them a promising cell source for endothelialization. Both single peptides and peptide combinations designed to target integrins α₄β₁ and α₅β₁ were coupled to poly(ethylene glycol) hydrogels, and their performance was evaluated by monitoring velocity patterns during the ECFC rolling process, in addition to firm adhesion (capture). Tether percentage and velocity fluctuation, a parameter newly defined here, were found to be valuable in assessing cell rolling velocity patterns and when used in combination were able to predict cell capture. REDV-containing peptides binding integrin α₄β₁ have been previously shown to reduce ECFC rolling velocity but not to support firm adhesion. This study finds that the performance of REDV-containing peptides in facilitating ECFC dynamic adhesion and capture can be improved by combination with α₅β₁ integrin-binding peptides, which support ECFC static adhesion. Moreover, when similar in length, the peptide combinations may have synergistic effects in capturing ECFCs. With matching lengths, the peptide combinations including CRRETAWAC(cyclic)+REDV, P_RGDS+KSSP_REDV, and P_RGDS+P_REDV showed high values in both tether percentage and velocity fluctuation and improvement in ECFC capture compared to the single peptides at the shear rate of 20 s^-1. These newly identified peptide combinations have the potential to be used as vascular device coatings to recruit ECFCs. STATEMENT OF SIGNIFICANCE: Restoration of functional endothelium following placement of stents and vascular grafts is critical for maintaining long-term patency. Endothelial colony forming cells (ECFCs) circulating in blood flow are a valuable cell source for rapid endothelialization. Here we identify and test novel peptides and peptide combinations that can potentially be used as coatings for vascular devices to support rolling and capture of ECFCs from flow. In addition to the widely used assessment of final ECFC adhesion, we also recorded the rolling process to quantitatively evaluate the interaction between ECFCs and the peptides, obtaining detailed performance of the peptides and gaining insight into effective capture molecule design. Peptide combinations targeting both integrin α₄β₁ and integrin α₅β₁ showed the highest percentages of ECFC capture.

A general numerical model of leukocyte adhesion in microchannels

Leukocyte adhesion on the vascular endothelium plays an important role in human immune system and reflects the physiological condition of a human body. In this paper, a generally implementable dynamic adhesion model based on the length limit of microvilli was developed to explore the behavior of a suspended leukocyte's adhesion process under microchannel shear flow. Simulations showed that the whole adhesion process can be divided into cell sedimentation, preliminary adhesion and stable dynamic adhesion stages. The cell tumbling kinetics, cell deformation, cell adhesion area and adhesion force were studied under the conditions of various bond strength, cell membrane surface tension, inlet flow velocity and cytoplasmic viscosity. Results showed that the bond strength affects the cell tumbling behaviors differently by changing the adhesion force. The cell with lower membrane surface tension induces a larger adhesion area, and eventually results in a greater adhesion and a lower cell tumbling velocity. The flow velocity changes cell velocity through the flow viscous force during the whole adhesion process. The cytoplasmic viscosity affects adhesion mainly in the preliminary adhesion stage by changing the cell deformation rate but has slight effect on the stabilized dynamic adhesion on cells. This study provides a simple theoretical basis to further clarify the mechanism of cell behaviors under stress and adhesion and becomes one of the prerequisites for study of tissue inflammation, wound healing, and disease treatments.

Escherichia coli Swimming back Toward Stiffer Polyetheylene Glycol Coatings, Increasing Contact in Flow

Bacterial swimming in flow near surfaces is critical to the spread of infection and device colonization. Understanding how material properties affect flagella- and motility-dependent bacteria-surface interactions is a first step in designing new medical devices that mitigate the risk of infection. We report that, on biomaterial coatings such as polyethylene glycol (PEG) hydrogels and end-tethered layers that prevent adhesive bacteria accumulation, the coating mechanics and hydration control the near-surface travel and dynamic surface contact of E. coli cells in gentle shear flow (order 10 s-1). Along relatively stiff (order 1 MPa) PEG hydrogels or end-tethered layers of PEG chains of similar polymer correlation length, run-and-tumble E. coli travel nanometrically close to the coating's surface in the flow direction in distinguishable runs or "engagements" that persist for several seconds, after which cells leave the interface. The duration of these engagements was greater along stiff hydrogels and end-tethered layers compared with softer, more-hydrated hydrogels. Swimming cells that left stiff hydrogels or end-tethered layers proceeded out to distances of a few microns and then returned to engage the surface again and again, while cells engaging the soft hydrogel tended not to return after leaving. As a result of differences in the duration of engagements and tendency to return to stiff hydrogel and end-tethered layers, swimming E. coli experienced 3 times the integrated dynamic surface contact with stiff coatings compared with softer hydrogels. The striking similarity of swimming behaviors near 16-nm-thick end-tethered layers and 100-μm-thick stiff hydrogels argues that only the outermost several nanometers of a highly hydrated coating influence cell travel. The range of material stiffnesses, cell-surface distance during travel, and time scales of travel compared with run-and-tumble time scales suggests the influence of the coating derives from its interactions with flagella and its potential to alter flagellar bundling. Given that restriction of flagellar rotation is known to trigger increased virulence, bacteria influenced by surfaces in one region may become predisposed to form a biofilm downstream.

Engineering nanoscale surface features to sustain microparticle rolling in flow

Nanoscopic features of channel walls are often engineered to facilitate microfluidic transport, for instance when surface charge enables electro-osmosis or when grooves drive mixing. The dynamic or rolling adhesion of flowing microparticles on a channel wall holds potential to accomplish particle sorting or to selectively transfer reactive species or signals between the wall and flowing particles. Inspired by cell rolling under the direction of adhesion molecules called selectins, we present an engineered platform in which the rolling of flowing microparticles is sustained through the incorporation of entirely synthetic, discrete, nanoscale, attractive features into the nonadhesive (electrostatically repulsive) surface of a flow channel. Focusing on one example or type of nanoscale feature and probing the impact of broad systematic variations in surface feature loading and processing parameters, this study demonstrates how relatively flat, weakly adhesive nanoscale features, positioned with average spacings on the order of tens of nanometers, can produce sustained microparticle rolling. We further demonstrate how the rolling velocity and travel distance depend on flow and surface design. We identify classes of related surfaces that fail to support rolling and present a state space that identifies combinations of surface and processing variables corresponding to transitions between rolling, free particle motion, and arrest. Finally we identify combinations of parameters (surface length scales, particle size, flow rates) where particles can be manipulated with size-selectivity.

Dynamic adhesion of eryptotic erythrocytes to immobilized platelets via platelet phosphatidylserine receptors

Glucose depletion of erythrocytes triggers suicidal erythrocyte death or eryptosis, which leads to cell membrane scrambling with phosphatidylserine exposure at the cell surface. Eryptotic erythrocytes adhere to endothelial cells by a mechanism involving phosphatidylserine at the erythrocyte surface and CXCL16 as well as CD36 at the endothelial cell membrane. Nothing has hitherto been known about an interaction between eryptotic erythrocytes and platelets, the decisive cells in primary hemostasis and major players in thrombotic vascular occlusion. The present study thus explored whether and how glucose-depleted erythrocytes adhere to platelets. To this end, adhesion of phosphatidylserine-exposing erythrocytes to platelets under flow conditions was examined in a flow chamber model at arterial shear rates. Platelets were immobilized on collagen and further stimulated with adenosine diphosphate (ADP, 10 μM) or thrombin (0.1 U/ml). As a result, a 48-h glucose depletion triggered phosphatidylserine translocation to the erythrocyte surface and augmented the adhesion of erythrocytes to immobilized platelets, an effect significantly increased upon platelet stimulation. Adherence of erythrocytes to platelets was blunted by coating of erythrocytic phosphatidylserine with annexin V or by neutralization of platelet phosphatidylserine receptors CXCL16 and CD36 with respective antibodies. In conclusion, glucose-depleted erythrocytes adhere to platelets. The adhesive properties of platelets are augmented by platelet activation. Erythrocyte adhesion to immobilized platelets requires phosphatidylserine at the erythrocyte surface and CXCL16 as well as CD36 expression on platelets. Thus platelet-mediated erythrocyte adhesion may foster thromboocclusive complications in diseases with stimulated phosphatidylserine exposure of erythrocytes.